Electronic Structure Modulation of Heterogeneous Cathode Catalysts for Reversible Aprotic Li‐CO₂ Batteries: Mechanisms and Strategies

ABSTRACT

Lithium‐carbon dioxide batteries (Li‐CO₂), featuring a high discharge voltage (∼2.8 V) and a high theoretical energy density (1876 Wh kg^{− 1}), have garnered significant attention for their dual capability in energy storage and CO₂ fixation. However, the complex reaction pathways across multiphase interfaces and sluggish discharge‐charge kinetics result in poor reversibility, which severely hinders their practical application. Addressing these challenges necessitates the development of efficient cathode catalysts, whose activity is fundamentally governed by their electronic structure. In this review, we systematically elucidate the structure‐performance‐mechanism relationships of cathode catalysts in Li‐CO₂ batteries by first examining the underlying reaction mechanisms at the electrode‐electrolyte interface. We then provide a detailed analysis of how the electronic structures of heterogeneous catalysts influence discharge‐charge processes. Particular emphasis is placed on specific electronic structure modulation methods or their combinations, through strategies targeting active sites, surface morphology, and interface structure, as a pivotal route for constructing high‐performance catalysts. Subsequently, we also discuss the underlying atomic‐level origins of these modulation effects. Finally, we propose several future research directions aimed at advancing the fundamental understanding of Li‐CO₂ electrochemistry, optimizing electrocatalytic performance, and accelerating the practical implementation of Li‐CO₂ batteries.

This review systematically deciphers the interfacial reaction pathways, highlighting atomic‐scale electronic modulation mechanisms and strategies for heterogeneous cathode catalysts in Li‐CO₂ batteries.

1. Introduction

The increased depletion of finite hydrocarbon resources and the consequent increase in carbon emissions have led to critical energy shortages and ecological disasters. To align with the urgent “carbon neutrality” initiative, rapid advancement of sustainable technologies on CO₂ utilization has taken place [1, 2]. Among them, Li‐CO₂ batteries, which employ CO₂ gas as the cathode reactant, offer dual functionality in energy storage and CO₂ fixation, with a theoretical energy density far exceeding that of conventional intercalation‐based lithium‐ion batteries. This provides a good prospect of dealing with problems like excessive CO₂ emissions and the inability of the current lithium‐ion batteries to satisfy the demand for high energy density and power [3, 4]. When compared with other metal‐CO₂ batteries (e.g., Na, K, and Zn), metallic Li possesses the most negative standard reduction potential as well as the highest specific capacity. Therefore, for the typical charge‐discharge reaction: $4\mathrm{Li}+3{\mathrm{CO}}_2\rightleftharpoons 2{\mathrm{Li}}_2{\mathrm{CO}}_3+\mathrm{C}$, Li‐CO₂ batteries possess the highest theoretical equilibrium potential (2.8 V vs. Li/Li⁺) and the highest energy density (1876 Wh kg⁻¹). These advantages give Li‐CO₂ batteries a prominent position among various metal‐CO₂ battery systems.

The evolution of Li‐CO₂ batteries can be traced back to investigations into Li‐O₂/CO₂ systems. In 2011, Takechi et al. demonstrated that introducing CO₂ into Li‐O₂ batteries could effectively eliminate superoxide radicals, thereby yielding substantial improvements in discharge capacity [5]. Building on this foundation, in 2013, Xu et al. pioneered the development of Li‐CO₂ batteries operating in a pure CO₂ atmosphere, where ex situ characterization confirmed Li₂CO₃ as the predominant discharge product [6]. A series of subsequent studies indicated that, as illustrated in Figure 1a, the charge‐discharge reaction mechanism of Li‐CO₂ batteries is closely related to the cathode catalyst. For the discharge reaction (CO₂ reduction reaction, CO₂RR), in addition to the typical Li₂CO₃ and C, other discharge products, including Li₂C₂O₄ [7], Li₂CO₃ & CO [8], and Li₂O & C [9], have been detected in different catalytic systems. For the charge reaction (CO₂ evolution reaction, CO₂ER), Li₂CO₃ undergoes self‐decomposition on specific substrates. However, fundamentally, thermodynamic reaction pathways merely serve as the basis for electrochemically active electrodes. In recent years, with advancements in in situ characterization techniques and theoretical calculations, the focus of mechanistic research has gradually shifted from the charge‐discharge reactions of different cathode catalysts to specific reaction pathways occurring at the electrode‐electrolyte interface. Key intermediates (such as *CO₂ ⁻, *CO, and *Li₂CO₃) have obtained direct spectroscopic evidence and support from theoretical calculations [10, 11, 12]. Previous reviews have primarily summarized charge‐discharge reactions based on the types of discharge products [13, 14, 15]. Therefore, we believe it is imperative to systematically consolidate recent research findings on the charge‐discharge reaction mechanism of Li‐CO₂ batteries.

FIGURE 1.

Major advances in (a) mechanistic studies and (b) heterogeneous cathode catalysts for Li‐CO₂ batteries.

On the other hand, the commercial viability of Li‐CO₂ batteries is currently hindered by excessive overpotentials, poor cyclability, and inferior rate performance. These problems are primarily from the thermodynamic inertness of the CO₂ (BDE(C═O) = 806 kJ mol⁻¹), the insulating nature of the discharge product Li₂CO₃ with a wide bandgap (5.206 eV), and the sluggish electron and mass transfer processes at multiphase interfaces (especially for solid catalysts). Such factors collectively inhibit the efficiency of both the CO₂RR and CO₂ER processes [16, 17]. To mitigate these issues, as depicted in Figure 1b, the scope of cathode catalyst research has been broadened to encompass metals [18, 19], metal oxides [20, 21, 22], carbon materials [23, 24, 25], and various organic frameworks [26, 27, 28, 29], aiming to accelerate the kinetics of the discharge/charge process and enhance electrochemical performance. Although many reviews have summarized the material types and design strategies of cathode catalysts for Li‐CO₂ batteries, providing useful guidance for researchers [1, 14, 30, 31, 32, 33, 34], the role of intrinsic electronic structure in the catalytic mechanism has often been overlooked. Consequently, an in‐depth understanding of the correlation between electronic structure and the associated charge/discharge processes remains lacking. Electrocatalytic reactions essentially occur on the catalyst surface or interface, accompanied by multi‐electron transfer processes involving reaction intermediates. Therefore, the electronic structure can be considered a critical descriptor for explaining corresponding catalytic behaviors. Considering the complex reaction pathways and diverse catalyst materials described above, to further deepen the understanding of the structure‐performance‐mechanism relationship, it is necessary to systematically discuss the electronic modulation mechanisms of electrocatalysts from the perspective of rational atomic‐level structural design.

In this review, we first overview the specific reaction mechanisms of CO₂RR/CO₂ER in Li‐CO₂ batteries, providing a comprehensive analysis of various reaction pathways on heterogeneous catalyst surfaces as well as the critical role of the electrolyte in the discharge/charge processes. Simultaneously, we summarized the influence of the electronic structure of heterogeneous catalysts on the discharge/charge processes from three perspectives: the nucleation and morphology of discharge products, the existence forms of Li₂CO₃, and the modification of discharge products. This established the primary objectives for electronic regulation. Subsequently, from the perspective of atomic‐level material design, we highlighted a series of electronic structure regulation strategies, including the design of active sites, surface morphological structures, and interface structures, along with the corresponding mechanisms of electronic structure effects (Figure 2). Finally, we emphasized the challenges and prospects for developing catalysts that balance high efficiency with low cost. This comprehensive analysis contributes to deepening the mechanistic understanding of Li‐CO₂ systems, provides foundational principles for designing advanced electrocatalysts, and facilitates the practical implementation of this emerging energy storage technology.

FIGURE 2.

Schematic illustration of electronic structure modulation for cathode catalysts in Li‐CO₂ batteries.

2. Reaction Mechanisms

In Li‐CO₂ batteries, the discharge process at the cathode involves the reduction of CO₂ at the catalyst/electrolyte interface. With the participation of Li⁺ and electrons, CO₂ is reduced to various products, including Li₂CO₃ & C, Li₂C₂O₄, Li₂CO₃ & CO, and Li₂O & C (Equations (1), (2), (3), (4)) [13]. The charge process is defined by the oxidative breakdown of these discharge species, effectively regenerating CO₂ gas and Li⁺. Herein, we outline specific reaction mechanisms to decode the catalytic origin of heterogeneous catalysts and the regulatory impact of the electrolyte in governing the discharge/charge processes. It should be noted that this work specifically focuses on heterogeneous catalytic systems. Regarding soluble homogeneous catalysts (redox mediators, RMs), although they can optimize the electrochemical reaction interface and significantly enhance the discharge/charge voltages by modifying reaction pathways, inherent challenges such as dissolution, the shuttle effect, and sluggish kinetics arising from concentration polarization remain difficult to resolve completely [1, 13]. They will not be discussed further here.

$$4\mathrm{L}{\mathrm{i}}^{+}+3\mathrm{C}{\mathrm{O}}_2+4{\mathrm{e}}^{-}\leftrightarrow 2\mathrm{L}{\mathrm{i}}_2\mathrm{C}{\mathrm{O}}_3+\mathrm{C}{E}^{\ominus }=2.80\mathrm{V}$$ (1)

$$2\mathrm{L}{\mathrm{i}}^{+}+2\mathrm{C}{\mathrm{O}}_2+2{\mathrm{e}}^{-}\leftrightarrow \mathrm{L}{\mathrm{i}}_2{\mathrm{C}}_2{\mathrm{O}}_4{E}^{\ominus }=3.01\mathrm{V}$$ (2)

$$2\mathrm{L}{\mathrm{i}}^{+}+2\mathrm{C}{\mathrm{O}}_2+2{\mathrm{e}}^{-}\to \mathrm{L}{\mathrm{i}}_2\mathrm{C}{\mathrm{O}}_3+\mathrm{CO}{E}^{\ominus }=2.50\mathrm{V}$$ (3)

$$4\mathrm{L}{\mathrm{i}}^{+}+\mathrm{C}{\mathrm{O}}_2+4{\mathrm{e}}^{-}\to 2\mathrm{L}{\mathrm{i}}_2\mathrm{O}+\mathrm{C}{E}^{\ominus }=1.89\mathrm{V}$$ (4)

2.1. CO₂RR

The electrocatalytic CO₂RR is a complex process involving various intermediates. However, within Li‐CO₂ battery systems, the CO₂RR pathway involving Li⁺ participation differs significantly from the electrocatalytic route. Differences in the stability of reaction intermediates result in distinct reaction pathways for CO₂ and Li⁺ during the discharge process, which in turn governs the cathode's selectivity toward the predominant formation of Li₂CO₃ and Li₂C₂O₄. As a two‐electron reduction product, Li₂C₂O₄ is thermodynamically unstable and tends to decompose further into Li₂CO₃. Consequently, the discharge pathways can be classified into two distinct types based on whether Li₂C₂O₄ stably exists as an intermediate/discharge product during the nucleation of discharge products [35]. Furthermore, since the CO₂RR occurs at the electrode/electrolyte interface, reaction intermediates may adsorb onto the catalyst surface or dissolve within the electrolyte [36]. Therefore, the nucleation pathway and selectivity of discharge products are critically dependent on the specific properties of both the electrolyte and the catalyst.

2.1.1. Surface‐Mediated Pathway

2.1.1.1. Li₂CO₃ + C Pathway (Without Li₂C₂O₄ Intermediate)

The process is typically divided into two stages, as illustrated in Figure 3a. The first stage involves the formation of the intermediates *Li₂CO₃ and *CO. According to previous reports, five potential pathways lead to these intermediates [38, 39]. In principle, the initial step involves the activation of either CO₂ or Li on the catalyst surface, followed by a lithiation reaction (coupled e ⁻/Li⁺ transfer) or CO₂ incorporation, leading to the formation of *LiCO₂ or *CO + *CO₃ intermediates. Subsequent further lithiation or CO₂ incorporation then yields *Li₂CO₂ or *CO + *LiCO₃ intermediates; ultimately generating *CO and *Li₂CO₃. The stability of these intermediates is determined by the stability of their adsorption configuration on the catalyst surface [10]. Generally, the strong coordination of CO often hinders its desorption, leading to further reduction on the catalyst surface. Consequently, gaseous CO is typically not detected as a byproduct [11]. The second stage entails the formation of Li₂CO₃ and C. Since the disproportionation of *CO into *C and *CO₂ is energetically unfavorable, typically requiring high temperatures or plasma activation, the species undergoes further reduction to yield Li₂CO₃ and C [40]. Although the specific reaction mechanism remains debated, a widely accepted simplified pathway involves the reaction of *CO with CO₂ to directly form *CO₃ and *C, followed by the reaction of *CO₃ with e⁻ /Li⁺ to produce a second Li₂CO₃.

FIGURE 3.

(a) Schematic illustration of the surface‐mediated reaction mechanism in Li‐CO₂ batteries. (b) CO₂RR mechanism on Au(hkl) surfaces and its modulation by low‐DN and high‐DN solvents [36]. Copyright 2025, American Chemical Society. (c) Dissociation free energy of Li⁺ dissolved in TEGDME and DMSO. (d) Schematic illustration of CO₂RR mechanisms in different electrolytes [37]. Copyright 2023, American Chemical Society.

2.1.1.2. Li₂C₂O₄ as Product/Intermediate

For surface‐mediated pathways, the involvement of the Li₂C₂O₄ route is primarily determined by the initial two‐electron‐transfer steps. In the first stage, the process initiates with a lithiation reaction or CO₂ adsorption to form the *LiCO₂ intermediate. Distinct from previously described pathways, CO₂ can also undergo dimerization to form the *C₂O₄ intermediate. This is followed by the generation of *LiC₂O₄/*Li₂CO₂ intermediates, ultimately producing *Li₂C₂O₄. Generally, *Li₂C₂O₄ is thermodynamically unstable and undergoes further decomposition into *CO and *Li₂CO₃, which then proceeds to the second stage described earlier. Through rational catalyst design, the Li₂C₂O₄ intermediate can be stabilized as the final discharge product. Shifting the discharge product from the thermodynamically stable Li₂CO₃ to more easily decomposable Li₂C₂O₄ may represent a more effective strategy for enhancing Li‐CO₂ battery performance; this will be further discussed later [13, 41]. The rate‐determining step (RDS) of the second stage typically possesses a high energy barrier that impedes the reaction. Recently, Zhang et al. investigated this mechanism using density functional theory (DFT) calculations and observed that on the Au(111) surface, the *CO produced by *Li₂C₂O₄ decomposition undergoes preferential lithiation to form *LiCO rather than disproportionation. Subsequently, *LiCO can further lithiate into the intermediate *Li₂O after C nucleation, ultimately forming Li₂CO₃ in the presence of sufficient CO₂. This theoretically explains the formation of the Li₂O discharge product [40].

2.1.2. Solvent‐Mediated Pathway

In liquid electrolyte systems, the relative interaction strengths of cation‐solvent and cation‐anion typically dictate the solvation structure of Li⁺ (Li⁺‐(solvent)_n). Considering that the overpotential is not excessively high and the electrolyte is sufficiently stable to avoid side reactions [12], the dissociation of Li⁺ from the solvent and its subsequent coordination with intermediates play a pivotal role during the discharge process of non‐aqueous Li‐CO₂ batteries, further dictating the reaction pathways and the growth of discharge products. The solvating power of an electrolyte is generally evaluated using the dielectric constant (DC, denoted by ε) or the Gutmann donor number (DN), which measures the strength of the solvent as a Lewis base [42, 43]. For example, Gao et al. investigated the CO₂RR pathways on Au(hkl) single‐crystal electrodes in electrolytes with varying DN values (dimethyl sulfoxide (DMSO), propylene carbonate (PC)) [36]. Direct spectroscopic evidence of key surface/intermediate species (including *CO₂ ⁻, *CO, and *Li₂CO₃) was obtained via in situ shell‐isolated nanoparticle‐enhanced Raman spectroscopy. Combined with theoretical calculations, this revealed the influence of different DN electrolytes on the CO₂RR pathways. As shown in Figure 3b, in high‐DN electrolytes (DMSO), Li⁺ easily forms a stable solvation structure with solvent molecules. According to Hard and Soft Acids and Bases (HSAB) theory, this “softened” Li⁺ enhances the interaction with the intermediate *CO₂ ⁻, thereby promoting a solution‐mediated pathway to generate CO and Li₂CO₃. Conversely, in low‐DN electrolytes (PC), the limited solvation capability facilitates the direct interaction between Li⁺ and *CO₂ ⁻ on the electrode surface. This surface‐mediated reduction process leads to the formation of dense Li₂CO₃ on the electrode, which subsequently covers the active sites and passivates the surface. Furthermore, Wang et al. discovered that under conditions of Li⁺ unavailability, the solution‐mediated process facilitates the growth of C₂O₄ ²⁻ from the solution phase [37]. In the highly polar, low‐concentration DMSO‐based electrolyte, the desolvation of Li⁺ is thermodynamically unfavorable (Figure 3c). This signifies that Li⁺ remains largely unavailable for electrochemical reactions, inhibiting its direct interaction with intermediates to form solid, insoluble Li₂CO₃. As illustrated in Figure 3d, CO₂ in this environment tends to undergo a two‐electron reduction to generate C₂O₄ ²⁻ intermediates, which diffuse into the electrolyte and subsequently react with solvated Li⁺ to reversibly form Li₂C₂O₄. Because the decomposition kinetics of soluble intermediates (C₂O₄ ²⁻) are more favorable during the charging process, this system significantly reduces electrode interfacial polarization and improves the overall reaction kinetics.

Furthermore, the interactions between reaction intermediates and the solvent also influence the reaction pathways. For example, the electrolyte can modulate the diffusion and dissolution behavior of Li₂C₂O₄ initially formed on the electrode surface, which ultimately determines whether the two‐electron discharge reaction can proceed continuously. Under these conditions, if the newly generated Li₂C₂O₄ species can rapidly diffuse away from the inner Helmholtz plane (IHP), the catalytically active sites are re‐exposed, enabling the further formation of new Li₂C₂O₄ until equilibrium is established. Conversely, if the formed Li₂C₂O₄ species remain bound to the electrode surface without timely detachment, they will continue to accept electrons and convert into Li₂CO₃, leading to an elevated charging voltage. To quantitatively evaluate the capacity of solvents to solubilize Li₂C₂O₄, Dai et al. introduced Hansen solubility parameters (HSP) as a theoretical guideline to screen electrolyte solvents capable of inducing the dissolution of discharge products, thereby altering the reaction pathway [44]. By measuring the absorbance spectra of each Li₂C₂O₄/solvent suspension, the δD (dispersion), δP (polarity), and δH (hydrogen bonding) parameters of the solute Li₂C₂O₄ can be preliminarily determined. By substituting these values alongside the corresponding parameters of known solvents into the theoretical equation, the HSP distance (R _a), which represents the interaction radius between the solvent and the solute, can be calculated. A smaller R _a value signifies a stronger solvating power of the solvent for the solute. Combining HSP predictions with theoretical calculations, tetraethylene glycol dimethyl ether (G4) was ultimately identified as the optimal solvent for generating and stabilizing Li₂C₂O₄, which was then coupled with a catalyst consisting of Pd nanoparticles supported on oxidized carbon nanotubes. Ex situ mechanistic studies confirmed that the G4 solvent effectively dissolves and stabilizes the in situ generated metastable intermediate Li₂C₂O₄, promoting its desorption into the electrolyte. This “solution‐mediated discharge” mechanism effectively circumvents the premature nucleation and passivation of Li₂CO₃ on the electrode surface, significantly reducing the charging overpotential and achieving a low charging plateau voltage of 3.15 V.

2.2. CO₂ER

For rechargeable energy storage systems, the coulombic and energy efficiencies are directly governed by the capacity and potential during charging. As with the discharge mechanism, the charging mechanism is also analyzed primarily in terms of reaction pathways, whose onset is determined by the varying final discharge product from the previous cycle [45]. Ideally, the charging reaction should be the reverse of the discharge process. However, for typical discharge products like Li₂CO₃ and C, charging is only partially reversible. Li₂CO₃ is characterized by a wide bandgap and high thermodynamic stability, which leads to a high energy barrier for decomposition and sluggish reaction kinetics. Furthermore, the direct decomposition of Li₂CO₃ at high voltages typically leads to poor battery reversibility. Nevertheless, through rational design of specific cathode materials, relevant charging pathways can achieve the co‐oxidation of Li₂CO₃ and C at relatively low potentials. On the other hand, alternative discharge products such as Li₂C₂O₄, exhibit weaker covalent C–C bonds within the C₂O₄ group, which directly accounts for the faster kinetics and lower charging voltage of Li₂C₂O₄, thereby facilitating its reversible decomposition [46]. Furthermore, compared to the synergistic oxidation between C and Li₂CO₃, the coupled oxidation of CO and Li₂CO₃ exhibits superior thermodynamic feasibility (ΔG ^⊖ = 480.49 vs. 1081.04 kJ mol⁻¹) and enhanced mass transport kinetics [47, 48]. However, the specific charging reaction mechanisms for these processes remain less explored. Therefore, the subsequent section will discuss two approaches of (i) electrochemical self‐decomposition of Li₂CO₃ and (ii) its reversible interaction with C for Li₂CO₃ charging.

2.2.1. Stepwise Decomposition of Li₂CO₃

Regarding the self‐decomposition process, relevant discussions have mainly focused on oxygen species. Yang et al. investigated the gas evolution of pre‐filled Li₂CO₃/¹²C and Li₂CO₃/¹³C electrodes during the charging process by employing in situ gas chromatography‐mass spectrometry (GC‐MS) combined with an isotopic tracing method [51]. The primary charging product detected was CO₂ (m/z = 44), and no gaseous O₂ was generated throughout the process. Since the fragment peak at m/z = 45 was observed on both ¹²C and ¹³C substrates, the possibility that it originated from ¹³CO₂ (due to decomposition of the carbon substrate) was ruled out. By further correlating this specific signal with the peak shape characteristics of other fragment signals, as well as the Fourier‐transform infrared (FT‐IR) spectroscopy results before and after charging, this peak was attributed to the decomposition of the electrolyte solvent. Based on this hypothesis, alongside simulation experiments involving the superoxide radical (O₂ ^{· −}), the formation of superoxide species was proposed: Li₂CO₃ decomposes into CO₂, O₂ ^{· −}, and O₂ (formed by the loss of an electron from O₂ ^{· −}). Both the O₂ and O₂ ^{· −} are consumed in subsequent reactions with the tetraglyme electrolyte solvent, rendering them undetectable. This proposed decomposition mechanism is also highly consistent with the observed fragment peaks. Using in situ surface‐enhanced Raman spectroscopy, Qiao et al. found that Au‐based electrocatalysts decompose Li₂CO₃ through a single pathway (Equation 5), which requires a high charging potential [9]. During discharge, DMSO₂ was observed, attributed to the nucleophilic attack of reduced oxygen species (O₂ ^{· −}, etc.) on the DMSO solvent (Figure 4a). Additionally, it was found that kinetic factors (current density) may play a significant role in altering the decomposition pathway of Li₂CO₃. Differential electrochemical mass spectrometry (DEMS) (Figure 4b–c) demonstrated distinct behaviors depending on the current density. Under a lower charging rate of 500 mA g⁻¹, the reaction exclusively yields CO₂ (corresponding to ≈ 3e ⁻/2CO₂). In contrast, increasing the current to 2000 mA g⁻¹ triggers the simultaneous evolution of CO₂ (2e ⁻/CO₂) and O₂ (4e ⁻/O₂). Therefore, the breakdown of Li₂CO₃ follows Equation (6) at low rates and shifts to Equation (5) under high‐rate conditions.

$$2\mathrm{L}{\mathrm{i}}_2\mathrm{C}{\mathrm{O}}_3\to 4\mathrm{L}{\mathrm{i}}^{+}+4{\mathrm{e}}^{-}+2\mathrm{C}{\mathrm{O}}_2+{\mathrm{O}}_2{E}^{\ominus }=3.82\mathrm{V}$$ (5)

$$2\mathrm{L}{\mathrm{i}}_2\mathrm{C}{\mathrm{O}}_3\to 4\mathrm{L}{\mathrm{i}}^{+}+3{\mathrm{e}}^{-}+2\mathrm{C}{\mathrm{O}}_2+{{\mathrm{O}}_2}^{·-}$$ (6)

FIGURE 4.

Mechanistic investigation of CO₂ER in Li‐CO₂ batteries. (a) Capacity‐correlated in situ Raman spectra collected during constant‐current charging at 5 µA after discharging to 10 µAh. (b, c) DEMS results of CO₂ and O₂ gas evolution during charging after discharging to point B at current densities of (b) 500 mA g⁻¹ and (c) 2000 mA g⁻¹ [9]. Copyright 2017, Elsevier Inc. (d) HPLC analysis and (e) ¹H‐NMR spectra of the electrolyte after potentiostatic polarization at specified potentials for a carbon black/Li₂CO₃/PTFE composite electrode with a capacity of 0.064 mAh [49]. Copyright 2018, Wiley‐VCH Verlag GmbH & Co. KGaA, Weinheim. (f) Schematic representation of Li₂CO₃ decomposition during charging [50]. Copyright 2022, Springer Nature.

Despite the aforementioned decomposition mechanisms, these pathways have not been directly verified. To address this uncertainty, Mahne et al. employed 9,10‐dimethylanthracene (DMA) as a chemical probe to detect singlet oxygen (¹O₂), a possible charged species formed during Li₂CO₃ oxidation [49]. Within the charging potential range of 3.8–4.2 V, the transformation of DMA into its endoperoxide (DMA‐O₂) was confirmed by high‐performance liquid chromatography (HPLC) (Figure 4d) and ¹H NMR analysis (Figure 4e). Since DMA reacts exclusively with ¹O₂ and remains inert to superoxide (another possible reactive oxygen species), these experimental results provide evidence for a charging pathway involving ¹O₂. To probe the specific influence of conductive carbon substrates on Li₂CO₃ oxidation, Cao et al. utilized an in situ differential electrochemical mass spectrometry‐gas chromatography (DEMS‐GC) coupled system to quantify gas evolution during the electrochemical oxidation of Li₂CO₃ on carbon substrates [50]. By selectively employing isotope labeling (¹³C) on Li₂CO₃, the C substrate, and both, they accurately distinguished the CO₂ and CO originating from different sources (i.e., Li₂CO₃ itself, the C substrate, and electrolyte). Similarly, the presence of ¹O₂ was confirmed using a DMA probe, and ex situ experiments were conducted on ¹³C substrates under ¹O₂ and ¹O₂/O₂ ⁻ _(sol) environments, respectively. Since ¹³CO and ¹³CO₂ were simultaneously detected in the ¹O₂ environment, whereas no ¹³CO₂ was observed in the O₂ ⁻ _(sol) environment, the contribution of superoxide was ruled out. Subsequently, the role of ¹O₂ was further explored. The results indicated that the decomposition of Li₂CO₃ in the tetraethylene glycol bis(trifluoromethanesulfonyl)imide lithium electrolyte is a multi‐step process (Equations (7), (8), (9)). In the first step, Li₂CO₃ is oxidized to CO₂ and ¹O₂. Subsequently, the ¹O₂ simultaneously oxidizes the carbon substrate and the electrolyte, forming CO₂ and CO as gaseous byproducts. Approximately 80% of the cumulative CO₂ evolution originates from the decomposition of Li₂CO₃, while the remaining 20% is derived from the decomposition of the carbon substrate and the electrolyte (Figure 4f).

(7)

(8)

(9)

2.2.2. Co‐Decomposition of Li₂CO₃ & C

As discussed earlier, the direct decomposition of Li₂CO₃ not only generates aggressive oxygen species (O₂ ^{· −}, ¹O₂, etc.) but also requires high charging voltages, leading to electrolyte decomposition and electrode corrosion. These factors collectively hinder the overall performance of highly efficient Li‐CO₂ batteries. It should be noted that the discussions herein predominantly focus on liquid electrolyte systems. As for solid‐state electrolyte systems, although sufficiently stable electrolytes and salts can theoretically circumvent parasitic side reactions and reduce aggressive oxygen species back to triplet oxygen (³O₂), the discharge‐charge process remains fundamentally irreversible [52, 53]. Furthermore, due to the inferior mobility of solid‐state electrolytes compared to their liquid counterparts, the evacuation of discharge products is significantly impeded. This exacerbates the potential accumulation of “dead Li₂CO₃”, consequently leading to severe capacity decay [54]. Consequently, the key to achieving a better charging process lies in designing catalysts to improve the reversibility of the oxidation process, thereby fundamentally transforming the high‐energy‐consumption CO₂ fixation strategy into a novel renewable energy storage technology. Under the ideal condition of complete reversibility, the decomposition pathway can be regarded as the reverse of the discharge pathway (Section 2.1.1), while the energy of adsorbed intermediates varies correspondingly with voltage [10, 55, 56]. Nevertheless, the inherent unfavorable properties of Li₂CO₃ continue to pose significant challenges to the charging process, which highlights the necessity of investigating more efficient catalysts from both performance and mechanistic perspectives.

3. Influence of Electronic Structure on Discharge/Charge Processes

Currently, the primary impediment to the practical implementation of Li‐CO₂ batteries is the excessive charging potential, which is largely dictated by the specific type and morphology of the discharge products. As described above, the cycling process involves the irreversible decomposition of wide‐bandgap insulating Li₂CO₃ and the accumulation of solid carbonate species on the cathode. This results in severe electrochemical degradation, rapid impedance rise, and eventual “sudden death”. Consequently, modulating the electronic structure of cathode catalysts to facilitate Li₂CO₃ decomposition or regulate the reaction pathway has become a major research objective. For the former strategy, research efforts focus on regulating the morphology of Li₂CO₃ discharge products or disrupting their highly stable crystal structure to lower crystallinity and enhance decomposition kinetics [57]. Furthermore, if the discharge pathway could be altered to bypass the formation of thermodynamically stable C and Li₂CO₃, the charging voltage could be substantially reduced, thereby boosting the round‐trip energy efficiency of Li‐CO₂ batteries [47].

3.1. Nucleation Pathway and Morphology

Similar to Li‐O₂ batteries, the adsorption capacity of the catalyst for reactants and intermediates in Li‐CO₂ battery systems serves as a critical factor in determining the morphology of discharge products, which subsequently dictates the electrochemical performance. The discharge products, Li₂CO₃ and C, primarily grow via two pathways, including the solvent‐mediated growth pathway and the surface‐adsorption growth pathway [41]. Generally, if the binding affinity between the reactants/intermediates and the active sites is weak, Li₂CO₃ preferentially nucleates and grows within the electrolyte in close proximity to the cathode surface, and subsequently self‐assembles and deposits onto the cathode surface driven by van der Waals forces. Conversely, when the binding energy with the active sites is strong, the discharge products are more inclined to grow directly on the cathode surface via the surface‐adsorption pathway. These divergent growth mechanisms exert a significant influence on the morphology and spatial distribution of the discharge products, functioning as a decisive factor for the resulting electrochemical performance.

Previous investigations have demonstrated that a strong adsorption affinity for reactants and intermediates facilitates the surface‐adsorbed growth pathway, leading to the formation of thin and uniformly distributed discharge products. This mechanism ensures intimate contact between the discharge products and the active sites, promoting electron transport through the product layer and circumventing the high charging overpotentials and parasitic side reactions associated with elevated voltages [61, 62]. In contrast, the formation of cluster‐like discharge products via the solvent‐mediated pathway is generally more conducive to achieving capacity retention. Based on morphological characterization and DFT analysis, Wang et al. found that compared to Ru and IrRu nanoalloys, Ir nanoparticles exhibit weaker CO₂ adsorption energy. Consequently, the reduction of CO₂ and the subsequent formation of Li₂CO₃ occur primarily at the outer Helmholtz plane (OHP), resulting in the development of flower‐like discharge products. While this structure facilitates an increase in discharge capacity relative to Ru, the extended electron diffusion pathways lead to an elevated charging overpotential. As a result, Li₂CO₃ is decomposed predominantly through the self‐decomposition pathway [63]. Furthermore, Shen et al. directly observed the CO₂ conversion process in Li‐CO₂ batteries at the nanoscale using in situ atomic force microscopy (AFM), laser confocal microscopy‐differential interference contrast microscopy (LCM‐DIM), and in situ Raman spectroscopy, which further revealed a laser‐tuned reaction pathway [58]. During discharge without laser irradiation, the intermediates primarily dissolve into the electrolyte, resulting in the formation of cluster‐like Li₂CO₃/C through a solvent‐mediated pathway (Figure 5a). These products can be reversibly decomposed within the bulk electrolyte, which is conducive to capacity retention (Figure 5b). Conversely, laser irradiation (λ = 405 nm) promotes the formation of the less soluble intermediate Li₂C₂O_4, reducing the concentration of soluble intermediates during discharge. This shift facilitates rapid nucleation of Li₂CO₃/C via the surface‐mediated pathway and its deposition as dense flake structures (Figure 5c). During charging, the decomposition of these films occurs preferentially at the bottom interface, leading to the formation of isolated voids. The coalescence and expansion of these voids cause the flakes to detach from the electrode surface, leading to irreversible capacity decay (Figure 5d). In conclusion, optimizing the cathode design for Li‐CO₂ batteries necessitates the precise regulation of discharge product growth mechanisms to achieve a balance between capacity and efficiency.

FIGURE 5.

(a) Schematic illustration of the (a) growth and (b) decomposition of Li₂CO₃/C cluster, and the corresponding (c) growth of Li₂CO₃/C flake under laser irradiation (λ = 405 nm) and (d) its decomposition [58]. Copyright 2023, Wiley‐VCH GmbH. (e) Charge density difference and adsorption energy of Li₂CO₃ adsorbed on Te_AC@NCNS and NCNS [59]. Copyright 2023, Wiley‐VCH GmbH. (f) Schematic illustration of Li₂CO₃ decomposition induced by symmetric and over‐coordinated asymmetric sites [60]. Copyright 2026, Wiley‐VCH GmbH. (g) Schematic diagram of chemical reactions during discharge and charge processes in Li‐CO₂ batteries using Mo₂C/CNT [7]. Copyright 2017, Wiley‐VCH Verlag GmbH & Co. KGaA, Weinheim. (h) Free energy profiles for *Li₂CO₃ and CO formation pathways on Cu_ML@Au surfaces at the equilibrium potential U₀(Li₂CO₃) = 2.87 V [11]. Copyright 2022, American Chemical Society.

3.2. Existence Forms of Li₂CO₃

Beyond the morphology of the discharge products, the existence form of Li₂CO₃ also affects its reversible decomposition. Typically, depending on the electrochemical environment, Li₂CO₃ exists in crystalline and/or amorphous states. For solid Li₂CO₃, its valence and conduction bands are predominantly composed of the frontier orbitals of CO₃ ²⁻ ions, with minimal contribution from Li⁺. This wide‐bandgap characteristic corresponds to the large energy gap between the highest occupied molecular orbital (HOMO) and lowest unoccupied molecular orbital (LUMO) [64]. In CO₃ ²⁻, the 2p orbital of the C atom, which primarily constitutes the frontier orbitals, hybridizes with the 2p orbitals of three O atoms to form a stable trigonal planar conjugated structure, which subsequently binds with Li⁺. Generally, carbonates adsorb onto the catalyst surface via physical or chemical interactions (including covalent and ionic interactions). During the stronger chemisorption process, Li₂CO₃ undergoes chemical bonding with the active sites (typically via metal–O or heteroatom–Li coordination), ultimately covering the catalyst surface. By fully leveraging such chemical bonding, charge transfer between the catalyst and Li₂CO₃ can be facilitated, effectively breaking its symmetric charge distribution. Simultaneously, this interaction promotes the formation of low‐crystallinity/amorphous discharge products, thereby accelerating the decomposition of the carbonate.

Mechanistic investigations primarily focus on the disruption of the stable trigonal structure of CO₃ ²⁻ via metal–O coordination interactions [59, 65, 66]. As exemplified, Zhang et al. prepared W₂C nanoparticles uniformly embedded in carbon nanotubes (W₂C‐CNTs). They attributed the low polarization to the electron‐rich W atoms, which disrupt the stable triangular structure of CO₃ ²⁻ via W–O bonds. The resulting amorphous discharge products undergo facile reversible decomposition during charging, as confirmed by Raman and X‐ray absorption spectroscopy (XAS). DFT calculations indicate that the interaction between Li₂CO₃ and W₂C surface is chemical adsorption, in contrast to the physical adsorption on CNTs. This chemical coupling facilitates electron transfer and lowers the decomposition energy barrier of Li₂CO₃ [66]. The resulting discharge products, characterized by their large surface areas and low crystallinity, facilitate reversible formation and decomposition. Additionally, Wang et al. designed and synthesized a catalyst comprising isolated Te atom clusters supported on nitrogen‐doped carbon nanosheets (Te_AC@NCNS). Acting as active centers, these Te clusters transfer substantial electrons to Li₂CO₃ via the formation of Te–O bonds (Figure 5e) [59]. This process suppresses the crystallization of Li₂CO₃, leading to the formation of an amorphous, film‐like Li₂CO₃ during discharge, especially at relatively low depths of discharge. The resulting discharge product, characterized by its large surface areas and low crystallinity, enables reversible formation and decomposition. Concurrently, the efficient electron transfer between Te active sites and reaction intermediates lowers the energy barriers for both nucleation and decomposition of Li₂CO₃.

In addition to activating the C–O bond by disrupting the stable trigonal planar conjugated structure of CO₃ ²⁻, the activation of the Li–O bond is equally critical for facilitating the decomposition of Li₂CO₃. Given that the cleavage of the Li–O bond is the initial/rate‐determining step in Li₂CO₃ decomposition, regulating the electron exchange between metal sites and Li₂CO₃ can effectively weaken the Li–O interaction, thereby promoting Li₂CO₃ decomposition [60, 67]. As a typical study, Zhai et al. constructed over‐coordinated asymmetric W–W pairs by introducing dislocations into layered tungsten boride (WB) [60]. The over‐coordinated W atoms, acting as ligands, polarize the originally symmetric W─W pairs, thereby transforming them into asymmetric W─W sites (Figure 5f). This configuration not only breaks the inherent charge symmetry, thereby generating a local dipole moment, but also incorporates geometric constraints. The polarized charge distribution induced by the asymmetric sites strengthens the W–O bond and polarizes the charge distribution on the O atoms. This promotes electron occupation in the Li–O antibonding orbitals, consequently weakening the Li–O interaction. Consequently, compared to pristine WB, the over‐coordinated tungsten boride catalyst significantly lowers the decomposition energy barrier of Li₂CO₃ (0.53 eV vs. 0.69 eV) and achieves an ultra‐low charging voltage of 2.99 V at a current density of 20 µA cm⁻².

In short, the electronic structure of the catalyst surface exerts a profound influence on the interfacial interactions with discharge products. It not only governs the adsorption affinity for reaction intermediates but also profoundly determines their final existence form (e.g., crystallinity). Clarifying the sites and mechanisms of such interfacial interactions will contribute to a deeper understanding of catalyst structural design.

3.3. Modifying Discharge Products

As indicated in the preceding discussion, the decomposition of Li₂CO₃ during charging is a multi‐step process. The aggressive oxygen species generated during high‐rate operation readily overflow and erode carbon additives/electrolytes, leading to rapid battery failure. In contrast, Li₂C₂O₄ avoids such issues due to its theoretically faster reaction kinetics and lower charging voltage [68]. Unfortunately, the instability of Li₂C₂O₄ renders it highly susceptible to disproportionation into C and Li₂CO₃ [47]. Beyond the role of the solvent (discussed earlier), disproportionation can be suppressed if C₂O₄ ²⁻ species can be stabilized by the catalyst, allowing the Li₂C₂O₄ phase to persist as the final divalent discharge product instead of Li₂CO₃.

Hou et al. were the first to identify the stabilizing effect of Mo–O bonding on Li₂C₂O₄ by utilizing Mo₂C/CNT as the cathode catalyst [7]. X‐ray diffraction (XRD) analysis indicated the discharge product was amorphous and non‐crystalline (neither Li₂CO₃ nor Li₂O). Combined with Raman and X‐ray photoelectron spectroscopy (XPS) analyses, the product was ultimately identified as amorphous Li₂C₂O₄ stabilized by Mo–O bonds (Figure 5g). However, due to its amorphous nature, direct proof of Li₂C₂O₄’s existence remained elusive. Yang et al. employed first‐principles DFT calculations to systematically investigate the Gibbs free energy changes of different intermediates during the nucleation of Li₂C₂O₄ and Li₂CO₃ on Mo₂C surfaces. This theoretically confirmed the thermodynamic stability of Li₂C₂O₄ [10]. Surface charge distribution analysis revealed electron transfer from the C atoms of Li₂C₂O₄ and the Mo atoms of the Mo₂C surface layer toward the O atoms. This aligns with prior experimental findings that Mo in low oxidation states (Mo²⁺, Mo³⁺, Mo⁵⁺) was oxidized to higher states (Mo⁵⁺, Mo⁶⁺) by the end of discharge, accompanied by the emergence of Mo–O coupling. Further, Xie et al. revealed the reasons for the distinct product selectivity of homologous metal carbides (Mo₂C vs. W₂C) from the perspective of electronic structure, based on a surface model covered by a CO₂ molecular layer [69]. For Mo₂C(101), the localized d‐orbitals were further activated by the upshift of the d‐band center after adsorbing CO₂, promoting strong electron transfer to the key intermediate *C₂O₄. This mechanism stabilized the oxalates as the final discharge products by enhancing the ionic interactions with the metastable oxalate intermediate. In contrast, the delocalized d‐orbitals of W₂C(101) tended to hybridize with the p‐orbitals of C and O in the CO₂ molecule to form covalent bonds, significantly weakening the C═O bond of CO₂ and thus inhibiting dimerization to generate oxalate species. Furthermore, since the interaction with *C₂O₄ was mainly covalent, the stability of the C═C bond in *C₂O₄ was further reduced, leading to the final formation of carbonate products.

Beyond Mo₂C, this stabilizing effect on the two‐electron discharge product Li₂C₂O₄ via Mo–O bonding has also been observed on other Mo‐based catalysts (e.g., MoN [70], Mo₃N₂ [71], MoB [72]). However, due to relatively limited stabilization, the long‐term cyclability of Li₂C₂O₄ remains poor. A deeper understanding of the interactions between catalysts and intermediates is required to rationally design efficient pathway‐selective catalysts. Additionally, analogous phenomena characterized by electron transfer and enhanced adsorption via coordination interactions, thereby suppressing the decomposition of Li₂C₂O₄, have been observed on non‐Mo‐based catalysts (e.g., Te–O [73], Ti–O [74]). Nevertheless, reports of such cases remain scarce, suggesting that metal elements may exert a more dominant influence on product selectivity.

Compared to solid carbon (C), gaseous CO facilitates Li₂CO₃ breakdown through better mass transfer properties. The polar covalent bond of CO makes it chemically easier to activate than the nonpolar structure of solid carbon. Meanwhile, CO possesses extensive industrial applications, including its use in Fischer‐Tropsch synthesis, pharmaceuticals, and ore reduction [8]. In 2013, Xu et al. theoretically proposed a discharge reaction at elevated temperatures that yields CO and Li₂CO₃ as the discharge products [6]. Subsequently, Xie et al. were the first to detect CO as a discharge product on a 3D porous fractal zinc cathode using gas chromatography [8]. Nevertheless, the fundamental determinants of CO selectivity related to the electronic configuration of the catalyst and the precise reaction pathways remain unclear. To clarify this, Zhao et al. obtained direct spectroscopic evidence of intermediates and products (CO₂ ⁻, CO, and Li₂CO₃) formed during the CO₂RR on Cu monolayer‐coated Au (Cu_ML@Au) using surface‐enhanced Raman spectroscopy (SERS) [11]. By combining these findings with DFT calculations, they proposed a specific reaction pathway. Compared to the parent Au and Cu electrocatalysts, the electronic structure of the Cu_ML@Au electrocatalyst undergoes significant modification, characterized by a substantial increase in the density of 3d states between −2 and 0 eV below the Fermi level (E _F). Among the three model electrocatalysts (Au, Cu, Cu_ML@Au), Cu_ML@Au exhibits the most favorable formation energy and adsorption energy for the key intermediate *LiCO₂. The reaction pathway on Cu_ML@Au at U₀(Li₂CO₃) = 2.87 V (Figure 5h) demonstrates that all elementary steps in Pathway I’ (involving *CO₂ and *LiCO₃ intermediates) exhibit downhill energy changes, rendering it more favorable. This finding corroborates the CO₂RR pathway proposed by in situ characterization results. Further, Liu et al. proposed that promoting CO desorption by modulating the catalyst's d‐electrons can inhibit the further reduction of CO to C [47]. By incorporating Co atoms into cubic Pt to form a CoPt alloy, the d‐band center of Pt shifts negatively. The electronic modulation promotes CO desorption during CO₂RR, resulting in Li₂CO₃ & CO as the primary discharge products. This further illustrates the profound influence of the catalyst's electronic structure on the selectivity of discharge products.

4. Electronic Structure Modulation Strategies

4.1. Active Sites

Catalytic active sites typically possess specific chemical properties and electronic structures, enabling strong interactions with reactant molecules to promote adsorption, activation, and reaction progression. The regulation of catalytically active sites can be categorized into two types: increasing the concentration of active sites and enhancing the intrinsic catalytic activity of individual sites [75]. Empirical evidence indicates that the latter usually exerts a more pronounced impact on overall catalytic performance. To modulate the surface electronic structure, introducing defects (such as point defects, planar defects, and volume defects, depending on the material) is considered one of the most effective surface modification strategies [39, 76]. However, most studies still attribute the enhancement of catalytic activity to changes in the overall electronic structure of the catalyst, while paying less attention to the precise identification and targeted regulation of active sites that directly interact with reactants. On the other hand, the advancement of in situ characterization techniques has provided powerful tools for directly observing surface catalytic sites, enabling the revelation of deep relationships between the dynamic evolution of active sites and catalytic performance. This has gradually shifted the research focus toward the construction of unified and identifiable sites.

Understanding the specific structure and electronic states of catalyst active sites is helpful for deepening the knowledge of material design. This section starts with the construction of active sites via fundamental defect engineering and further systematically summarizes a series of modulation and synergistic strategies based on structurally well‐defined metal catalytic active centers. These insights emphasize the necessity of research based on active site design and intrinsic catalytic mechanisms for designing efficient cathode catalysts.

4.1.1. Doping and Vacancy Defects

As revealed by the second law of thermodynamics, the disorder (entropy) within crystals makes the existence of absolutely defect‐free structures impossible. In catalyst design, besides artificially introducing intrinsic defects (containing no dopants, such as edges and vacancies), the further introduction of extrinsic defects (such as heteroatom doping) can more precisely regulate catalyst surface properties [76, 77]. On the one hand, these defects themselves often serve as efficient catalytic centers. By forming coordinatively unsaturated sites with higher reactivity, they directly act as adsorption sites to significantly improve catalytic activity. For example, as the most common defect type in transition metal oxides, oxygen vacancies play a very important role in catalysts [75]. On the other hand, defects may not directly adsorb reactant molecules but instead modulate the electronic structure of the catalyst to change the chemical state of neighboring atoms, thereby enhancing the ability to adsorb and activate reaction intermediates [39, 76]. Considering the decisive influence of the intrinsic characteristics of substrate materials on defect mechanisms, and based on previous achievements in the Li‐CO₂ battery field, this section will systematically discuss the construction strategies of defect sites and their regulation mechanisms on catalytic activity for carbon‐based materials and transition metal compounds, respectively.

4.1.1.1. Carbon‐Based Materials

For carbon‐based materials, including CNTs, graphene, and graphdiyne, relevant strategies mainly focus on the construction of surface defects. Approaches involving intrinsic defects (e.g., topological, edge defects), extrinsic defects (e.g., heteroatom doping sites), and synergistic combined sites have been extensively investigated as powerful tools for introducing defect‐derived active sites onto the material surface. Introducing heteroatoms (such as N, S, B, P) into carbon materials induces charge transfer that modulates the surface electronic states of the carbon skeleton, which in turn alters the adsorption behavior toward reactants and enhances catalytic activity. Among these, nitrogen doping has been the most extensively studied. As illustrated in Figure 6a, N‐6, N‐5, N‐Q, and N‐O represent four common active nitrogen configurations in electrocatalysis: pyridinic‐N, pyrrolic‐N, graphitic‐N (or quaternary‐N), and oxidized‐N, respectively. The chemical properties of these active sites are heavily dependent on their specific atomic configuration [14]. For instance, Li et al. combined experimental and theoretical results to reveal that for N‐doped CNTs anchored on metal wires, the N‐6 active site plays a decisive role in promoting the kinetics of both CO₂RR and CO₂ER [82]. Regarding N‐doped graphene (NG), Chen et al. utilized CO₂ activation to significantly elevate the ratio of N‐6 and N‐5 species (reaching 72.65%), with theoretical calculations confirming the superior intrinsic activity of these sites [83]. In addition to single‐atom doping, co‐doping strategies (N, S co‐doping [84], B, N co‐doping [85], N, P, F co‐doping [86], etc.) have also been used to utilize the synergistic effects of heteroatoms and increase the number of doped heteroatoms, achieving multifunctionality [30, 87]. However, as structural complexity increases, precisely constructing and distinguishing the specific functions of various doping sites becomes increasingly challenging. Liu et al. introduced abundant BC₂O sites into CNTs, capitalizing on the electronegativity difference between B and O to successfully construct B–O electron donor–acceptor pairs [78]. The presence of these BC₂O sites was confirmed by XPS and B K‐edge XAS analyses. The electrophilic B atoms and nucleophilic O atoms facilitated the co‐adsorption of CO₂ and Li⁺, respectively. This shifts the Li₂CO₃ growth mode from the solvent‐mediated pathway to the surface‐adsorption pathway, ultimately resulting in the formation of uniformly distributed, small‐sized discharge products with a higher degree of structural distortion (Figure 6b). Nevertheless, the catalytic activity of pure carbon‐based materials remains relatively limited. Further strategies, such as utilizing modified catalysts or anchoring metal sites via defects, will be discussed in subsequent sections.

FIGURE 6.

(a) Schematic diagram of four types of nitrogen doping configurations [14]. Copyright 2022, Elsevier Ltd. (b) Schematic illustration of the Li₂CO₃ growth mechanism on CNT and BCNT [78]. Copyright 2024, Wiley‐VCH GmbH. (c) Configurations of C5, C7, C5 + C7, pyridinic‐N, and graphitic‐N [79]. Copyright 2021, Wiley‐VCH. (d) Schematic diagram of CO₂RR and CO₂ER kinetics on nucleophilic and electrophilic dual active sites on the ReS₂ surface [38]. Copyright 2022, American Chemical Society. (e) Schematic illustration of the interaction in the valence layers of pristine NiO and NiO‐Vc [80]. Copyright 2021, Elsevier B.V. (f) Charge density distribution of Li₂C₂O₄ on MGA and MGA (without vacancies). (g) Reaction pathway of the CO₂RR process on MGA [81]. Copyright, 2025 Wiley‐VCH GmbH.

On the other hand, doping heteroatoms as a means to introduce extrinsic defects inevitably induces intrinsic defects at the same time [14, 88]. Among them, topological defects (such as the substitution of hexagonal rings in the carbon skeleton with pentagons and heptagons) are important intrinsic active sites. Compared to the conventional basal hexagonal carbon rings, they have been proven to possess higher energy states and exist as active sites. To distinguish the true active sites, Ye et al. prepared topologically defective graphene (TDG) by removing “N” dopants at high temperatures (e.g., 1000°C) [79]. DFT calculations indicated that negatively charged C atoms in topological defects can serve as adsorption and activation sites for CO₂ molecules. Among the potential configurations in Figure 6c, including two N‐doping defects (pyridinic‐N and graphitic‐N) and three topological defects (C5, C7, and C5 + C7), the pentagonal ring (C5) exhibited the lowest theoretical overpotential gap of 1.01 V, thereby delivering the highest catalytic activity for CO₂RR/ER.

4.1.1.2. Transition Metal Compounds

For transition metal compounds (mainly oxides and sulfides), defects can be classified into anion vacancies (oxygen or sulfur vacancies) and cation vacancies (metal vacancies) based on the charge of the atoms in the compound. Besides these intrinsic defects, heteroatom doping is also an important means to optimize electrocatalytic activity and can be categorized into cation or anion doping based on the dopant type. When some non‐equivalent heteroions enter the lattice, extrinsic vacancies (distinct from intrinsic vacancies) are introduced near the doping sites based on the charge compensation mechanism, thereby further affecting charge distribution and influencing the material's conductivity and electronic structure [76, 89]. As mentioned earlier, these surface defect sites can directly serve as active sites for reactions. Especially for two‐dimensional (2D) transition metal compounds (TMCs), most internal atoms are exposed or close to the surface, providing convenience for introducing defects [89]. The synergistic effect of surface dual‐component sites also provides possibilities for constructing CO₂RR/ER bifunctional active sites. Chen et al. controllably introduced nucleophilic N‐dopants and electrophilic S‐vacancies on the ReS₂ basal plane by regulating the precursor N:S ratio (0, 8/7, 3, 5, and 7), which enhanced the adsorption of intermediate Li atoms and C/O atoms, respectively (Figure 6d) [38]. Among them, NSvReS₂(5) (with a precursor N:S ratio of 5) exhibited the best electrochemical performance. DFT calculation results showed that the 3N3S_V‐ReS₂ model (incorporating 3 N atoms and 3 S vacancies), constructed based on the optimal catalyst composition, exhibited the highest adsorption energy for Li₂CO₃ through Li–S and Li–N bonds and weakened the strength of the Li–O bond, thereby promoting CO₂ER kinetics. For the CO₂RR process, Gibbs free energy calculations indicated that 3N3S_V‐ReS₂ had lower RDS barriers in both stages. These findings are also corroborated by electrochemical performance tests. Consequently, this design strategy utilizing electrophilic and nucleophilic dual active centers offers a new paradigm for the development of catalysts.

Alternatively, defect sites can operate by modulating the local electronic structure of neighboring atoms, thereby tuning the overall electronic landscape of the catalyst to potentiate the adsorption and activation of reaction intermediates. Wang et al. exemplified this by introducing oxygen vacancies (V_o) into NiO via Ar plasma engraving. Upon the release of lattice oxygen, two electrons are donated to the adjacent Ni cations [80]. During CO₂RR, the unpaired electrons from Ni orbitals undergo spontaneous injection into the C 2p orbitals of CO₂, significantly facilitating its activation. Analysis of projected density of states (PDOS) demonstrates the appearance of new electronic states near the E _F (Figure 6e), which simultaneously optimizes the adsorption energetics of CO₂ and Li₂CO₃ and enhances electrical conductivity. To further amplify the stabilizing effect of Mo‐based catalysts on Li₂C₂O₄ via Mo–O interactions, Tian et al. recently engineered a Mo_1.33C@rGO aerogel (MGA) tailored for Li‐CO₂ batteries [81]. The presence of ordered Mo vacancies induces abundant mobile electrons, which reinforce the coupling between Mo atoms and the intermediate C₂O₄ ²⁻, thereby ensuring the long‐term cyclability. As illustrated in Figure 6f, the higher electron density observed between defective MGA and Li₂C₂O₄ confirms a more robust binding affinity compared to the vacancy‐free counterpart. This conclusion was quantified by Crystal Orbital Hamiltonian Population (COHP) analysis, where the COHP value for the Mo–O bond between MGA and Li₂C₂O₄ was 1.02, higher than that of the vacancy‐free catalyst (0.56), indicating stronger chemical bonding. Gibbs free energy calculations revealed that the formation of Li₂C₂O₄ on the MGA surface is thermodynamically favorable across all elementary steps, with the specific reaction pathway depicted in Figure 6g. Moreover, the MGA's oriented 3D MGA framework provides ordered active sites and well‐organised electronic and ionic transport pathways, reducing charge transfer resistance and effectively stabilising the formation of the discharge product Li₂C₂O₄. The assembled Li‐CO₂ battery achieved a low overpotential of 0.46 V and superior cycling stability (330 cycles at 20 µA cm⁻²).

In this section, we have underscored the pivotal role of defects in boosting CO₂RR/ER activity. Nevertheless, the complexity of defects renders precise control over defect site location and concentration, identification of catalytic functions across different defect sites, and targeted construction of specific active sites as enduring challenges. Furthermore, discerning the precise origin of activity enhancement induced by dopants, whether they function as direct active sites or merely as electronic modulators of adjacent atoms, often remains elusive and warrants further investigation.

4.1.2. Local Metal Catalytic Site Modulation

The intrinsic catalytic activity of active sites acts as the primary determinant governing both the reaction kinetics and product selectivity. In the context of metal active sites, the local reactivity is fundamentally dictated by the electronic structure of the metal center, where intermediate binding occurs, and its immediate local coordination environment. In contrast to defect‐based active site construction strategies, the focus here lies on the influence of the local structure of the metallic active site itself upon its electronic structure and corresponding catalytic performance.

In electrocatalytic reactions, the metal center typically dictates the activity and selectivity of the reaction [94]. For instance, Zhang et al. prepared a single‐metal‐site catalyst based on a porphyrin‐based covalent organic framework (TTCOF) [90]. DFT calculations revealed that the Mn‐TAPP site exhibits strong adsorption toward CO₂. Compared to other TAPP‐M (M = Co, Ni, Cu) catalytic sites, the uniqueness of TAPP‐Mn lies in its resistance to reduction to Mn(I). This enables an effective four‐electron conversion of inert CO₂, forming Li₂CO₃ and C. In contrast, a two‐electron pathway likely occurs on the TAPP‐Co site, yielding Li₂CO₃ and CO as discharge products (Figure 7a). This work provides the first molecular‐level elucidation of catalytically active sites and a well‐defined reaction pathway in Li‐CO₂ batteries. Further, Liu et al. employed DFT calculations to screen a series of SACs on N‐doped graphene (SAMe@NG, Me = Cr, Mn, Fe, Co, Ni, Cu) [95]. It is identified that Cr–N₄ possesses the highest CO₂ adsorption energy, the most favorable d‐band center, and a lower decomposition energy barrier for Li₂CO₃. Electrochemical testing further confirmed significantly enhanced reversibility compared to other metals, achieving a cycling capability exceeding 720 h at 100 µA cm⁻². Beyond the type of metals in the active center, the local electronic structure (spin state, oxidation state, etc.) of the catalytic center also closely influences the adsorption strength between catalytic sites and reactive species. For the spinel structure (AB₂X₄, where A and B occupy tetrahedral (T _d) and octahedral (O _h) sites, respectively, connected via A_Td–O–B_Oh geometric configurations), Liu et al. regulated the local spin state by controlling the content of Ni doping in Co₃O₄ (Ni_xCo_3−xO₄, where Ni occupies octahedral sites; x = 0.1, 0.5, and 1) to enhance the catalytic ability of Co‐based spinels [21]. It is indicated that only a trace amount of Ni doping (x = 0.1) is needed to effectively activate Co₃O₄. As shown in Figure 7b, according to DFT results, the CO₂ adsorption energy is maximum when x = 0.1. By comparing the changes in magnetic moments before and after adsorption (Figure 7c), it was confirmed that at x = 0.1, the octahedral Ni³⁺ doping site is in a high‐spin state, and the magnetic moment changes significantly before and after CO₂ adsorption; while at x = 0.5 and 1, it is in a low‐spin state, with basically unchanged magnetic moments. This result indicates that the high‐spin state facilitates the transfer of 3d electrons to the unoccupied π* orbitals of CO₂, thereby enhancing both the adsorption of CO₂ and the kinetics of its reduction during the discharge process.

FIGURE 7.

(a) The four‐electron pathway on TAPP‐Mn sites and the two‐electron pathway on TAPP‐Co sites [90]. Copyright 2021, American Chemical Society. (b) CO₂ adsorption energy of Ni_xCo_3‐xO₄, and (c) changes in magnetic moments before and after CO₂ adsorption [21]. Copyright 2024, Wiley‐VCH GmbH. (d) Schematic diagram of the structural evolution of CoS₂ during the cycling process [91]. Copyright 2024, Springer Nature. (e) Structural schematics of Co₃O₄ and its Zn‐ and Al‐substituted forms [22]. Copyright 2024, American Chemical Society. (f) Schematic diagram of d−p orbital hybridization [92]. Copyright 2025 Wiley‐VCH GmbH. (g) Orbital‐level mechanism of Li₂CO₃ activation by low‐symmetry catalysts [93]. Copyright 2025 Wiley‐VCH GmbH.

From the perspective of coordination chemistry, the catalytic performance of active sites is highly sensitive to their local coordination environment (such as the type, number, and geometric configuration of coordinating atoms) [96, 97]. For SACs, Wang et al. designed atomically dispersed Mn–N₄ sites embedded in bowl‐like mesoporous carbon particles. Epoxy groups in the second coordination sphere caused a downshift of the d‐band center of the Mn sites. This optimized the Li₂CO₃ nucleation pathway via a “surface adsorption pathway”, forming thin and uniformly distributed discharge products [98]. For transition metal compounds, Liu et al. investigated the reconstruction of the local geometric ligand environment of cobalt during the cycling of CoS_x (x = 8/9, 1.097, and 2) using XAS. They found that the different oxidation states after reconstruction played a decisive role in battery performance [91]. During cycling, the complete oxidation of CoS_1.097 and Co₉S₈ (structurally similar to CoO in Li‐CO₂ batteries) led to a decline in electrochemical performance. In contrast, CoS₂ evolved into a Co–S₄–O₂ structure as the endpoint (Figure 7d), which increased local polarity and upshifted the d‐band center, which consequently enhances the catalytic ability of CoS₂. To elucidate the specific contributions of different geometric configurations (T _d and O _h sites) of the spinel structure to catalytic activity and further explore the underlying mechanisms, Liu et al. substituted Co ions at the tetrahedral and octahedral sites of Co₃O₄ with inert Zn²⁺ (d ¹⁰) and Al³⁺ (d ⁰), respectively (forming ZnCo₂O₄ and CoAl₂O₄) (Figure 7e) [22]. Both computational and experimental results indicated that octahedral cobalt (Co³⁺ _Oh) interacts strongly with reactants (CO₂, Li), effectively promoting reaction kinetics. Furthermore, by constructing catalysts with different e _g orbital filling numbers at octahedral sites (ZnCo₂O₄, ZnMn₂O₄, NiO; with e _g filling numbers of 0, 1, and 2, respectively), they found that low e _g filling (ZnCo₂O₄) favored CO₂ and Li adsorption, showing a positive correlation with CO₂RR activity. This work highlights the role of local electronic and geometric structures in catalysis.

On the other hand, recent studies indicate that disrupting local coordination symmetry can break the degeneracy of d‐orbitals under high‐symmetry crystal fields (octahedral, tetrahedral, etc.), thereby optimizing the orbital energy level matching between active sites and reaction intermediates. For the spinel structure, Qiu et al. proposed a strategy utilizing local asymmetric structures to induce d‐orbital spin splitting [92]. By introducing heteroatoms (Cu, Fe) into the octahedral sites of the symmetric Co₃O₄ spinel lattice, and utilizing the A_Td–O–B_Oh corner‐sharing connection, they induced strong local structural distortion and asymmetry around the tetrahedral active center Co atoms, accompanied by enhanced electron delocalization around the tetrahedral sites. According to electronic structure analysis results, the structural asymmetry broke the degenerate state of the original Co_Td sites, causing significant electron spin splitting of the d‐orbitals. This splitting significantly increased the electron occupancy of d _xz and d _yz orbitals while decreasing that of d _xy orbitals, accompanied by charge loss at Co_Td sites and an upshift of the d‐band center. As shown in Figure 7f, based on orbital symmetry, the d _xz/yz orbitals of Co_Td hybridize with the p _x/y orbitals of reactants/intermediate O to form π and π^* bonds, whereas the d _xy orbital is non‐bonding due to symmetry mismatch limitations. Moreover, this change was more significant in Cu‐Co₃O₄ compared to Fe‐Co₃O₄, indicating that the d _xz and d _yz orbitals of Co_Td formed stronger π‐bonding with the O p‐orbitals, thereby enhancing adsorption. Benefiting from the precise modulation of the active site electronic states by the asymmetric structure, Cu‐Co₃O₄ catalyst achieved superior electrochemical performance compared to symmetric Co₃O₄ and weakly distorted Fe‐Co₃O₄. Furthermore, to selectively regulate the effective coupling between metal d‐orbitals and the non‐bonding orbital (HOMO‐1) of the Li₂CO₃ molecule (composed of two O‐p _z orbitals), Zhang et al. proposed a strategy to control orbital splitting via regulating the local coordination geometry of metal active sites to achieve energy matching, thereby activating inert Li₂CO₃ [93]. The researchers synthesized metastable square pyramidal NiS₅ (tp‐NiS) with low symmetry. Compared to the traditional thermodynamically stable octahedral NiS₆ structure (o‐NiS), the absence of axial ligands in tp‐NiS led to reduced crystal field symmetry (Figure 7g). This distortion of geometric structure effectively broke the degenerate state of Ni 3d orbitals, causing the z‐component orbitals perpendicular to the plane (especially $d_{{\mathrm{z}}^2}$) to destabilize and shift toward E _F, greatly enhancing the directional orbital overlap between the metal center and O‐p _z orbitals. In situ spectroscopic characterization indicated that this optimized orbital coupling induced the dynamic and reversible formation of Ni–O chemical bonds during cycling. This facilitated the uniform deposition and efficient decomposition of Li₂CO₃ on the catalyst surface. Conversely, the octahedral NiS₆ coordination in o‐NiS imposed spatial and orbital constraints that hindered Ni–O bond formation, resulting in the bulk deposition of Li₂CO₃.

In brief, the construction of well‐defined metal active sites is crucial for developing high‐performance Li‐CO₂ cathode catalysts and elucidating the origins of their intrinsic activity (e.g., electronic structures, spin states). However, for atomic metal site catalysts, their mass loading and long‐term cycling stability remain a challenge. Furthermore, the precise construction of active sites at the atomic scale and their dynamic structural evolution during charge‐discharge cycles require further investigation.

4.1.3. Metal‐Substrate Interaction

The role of the catalytic support extends beyond providing stable anchoring sites to prevent the agglomeration and dissolution of metal nanoparticles. The electronic effects at the interface between the active sites and the support are equally crucial and can play a decisive role. This is because the electronic modulation of the metallic sites by the support influences the adsorption strength of reactant molecules, thereby determining the overall catalytic performance [99]. Next, we will summarize the impact of electronic effects between metal sites and the support on the catalytic performance in CO₂RR and CO₂ER process.

Leveraging the low standard electrode potential of liquid metal (LM), Chai et al. in situ generated Ru on LM particles (LMPs) via a galvanic replacement reaction (GRR), forming an LM@Ru core‐shell structure anchored on reduced graphene oxide (rGO@LM@Ru) [100]. Owing to its electron‐rich surface, LM enhances cathode conductivity, improves charge transfer efficiency, and plays a dominant role in Ru formation and stabilisation. At a current density of 40 µA cm⁻², rGO@LM@Ru composite exhibited a smaller voltage gap (1.00 V) compared to rGO@LM (1.61 V) and rGO (1.73 V), along with improved reversible capacity, cycling stability, and rate capability. By altering the central metal in metal‐organic framework (MOF) precursors, Deng et al. achieved highly dispersed and anchored Ru–O–Zr/Ce active sites at the heterogeneous interface of Zr/CeO₂(111) [101]. XRD analysis indicated that introducing Ce into the MOF precursor promoted the transformation of ZrO₂ from monoclinic to a cubic bimetallic oxide, suggesting a possible phase transformation or rearrangement between CeO₂ and ZrO₂. DFT calculations indicated that Ru supported on cubic bimetallic oxides exhibited an optimized interfacial electronic structure. In contrast to the electron density accumulation observed for Ru loaded on monoclinic or tetragonal monometallic oxides, the opposite trend was identified for bimetallic oxide. The Ru–O–Zr/Ce sites demonstrated higher CO₂ adsorption energy and facilitated electron transfer toward Ru via the C═O bond of CO₂. This interaction promoted CO₂ adsorption and reduction during the discharge process and created favorable conditions for breaking the stable C═O bond during charging. Benefiting from the optimised physical and electronic structure of the heterocatalytic interface Zr/CeO₂(111), the assembled Li‐CO₂ battery exhibited significantly superior electrochemical performance compared to control samples based on single‐metal oxide supports and those without Ru loading.

Therefore, in addition to focusing on the metal sites themselves, optimisation of the substrate should receive equal attention. As research progresses, investigating the impact of different substrate types and structures on the catalytic performance of metal sites, along with their underlying mechanisms, will also become a significant direction in cathode catalyst design.

4.1.4. Multi‐Metal Catalytic Site Synergy

Beyond intrinsic catalytic activity, synergistic effects between metal sites present a significant avenue for designing multifunctional CO₂RR/CO₂ER catalysts. Due to the structural complexity of metal catalysts, acquiring a fundamental understanding of the synergistic interplay between their active sites at the atomic and molecular level remains highly challenging [102]. Integrating the types of metal sites (e.g., single atoms, clusters, and nanoparticles) with their local structural characteristics will facilitate a deeper understanding of the cooperative regulation mechanisms among metal active sites.

4.1.4.1. Different Chemical State Sites

Metal sites with distinct chemical states (e.g., valence states of active centers, coordination structures) may offer an effective strategy to overcome the limitation of pristine metal sites, which typically adsorb or activate only a single type of substrate and thus struggle to achieve bidirectional catalytic activity. This enables the independent manifestation of activity toward both CO₂RR and CO₂ER. In this regard, Dong et al. exemplified this by constructing a series of bimetallic MOF catalysts via the self‐assembly of porphyrin ligands and transition metal ions, featuring metalloporphyrin sites (M‐Por, M = Mn, Co) and coordination metal nodes (M‐Pz) [108]. Comparative electrochemical analyses revealed that all variants containing Mn‐Por moieties displayed identical discharge plateaus, establishing that the Mn‐Por site dominates the CO₂RR process. In contrast, during charging, MnTPzP‐Mn exhibited a markedly reduced potential compared to CoTPzP‐Mn (4.00 V vs. 4.56 V), providing compelling evidence that the Mn‐Pz site facilitates Li₂CO₃ decomposition. This work revealed the intrinsic catalytic sites for CO₂RR/ER in Li‐CO₂ batteries using structurally defined crystal materials. Further, Liu et al. reported that a novel manganese dual‐active‐site cathode catalyst supported on N‐doped carbon nanofibers (MOC@NCNF) exhibited a distinct activation process during cycling [103]. As illustrated in Figure 8a, compared to the NCNF control, MOC@NCNF showed a gradual increase in discharge potential and a decrease in charge potential during the initial cycles, eventually stabilizing after a certain number of cycles. Notably, this activation phenomenon was also observed at a higher current density (50 µA cm⁻²). The Mn 3s XPS spectra (Figure 8b) showed that, relative to the pristine MOC@NCNF, ΔE(3s) increased to 6.10 eV after the 10th discharge and decreased to 5.98 eV after the 10th charge. Corroborated by X‐ray absorption fine structure (XAFS) characterization, this activation process was confirmed to be associated with the in situ electrochemical reconstruction of Mn(II) and Mn(III) species (Figure 8c): Mn(II) was electrochemically oxidized to Mn(III) during charging, generating abundant Mn(III) active sites. DFT calculations indicated that Mn(II) sites exhibit stronger affinity for CO₂, whereas Li₂CO₃ preferentially adsorbs on Mn(III) sites; the differentiated electronic configurations and coordination environments of Mn(II) and Mn(III) sites respectively, promote the kinetics of CO₂RR and CO₂ER. This work offers novel perspectives for the design of multivalent manganese‐based catalysts.

FIGURE 8.

(a) Comparison of electrochemical activation processes for MOC@NCNF and NCNF. (b) XPS characterization of the MOC@NCNF cathode after ten cycles. (c) Schematic illustration of manganese site reconstruction on MOC@NCNF surfaces [103]. Copyright 2024, Wiley‐VCH GmbH. (d) Design principle of CoCuCN catalysts, and (e) DOS for CO₂ adsorption on the (400) crystal plane of CoCN and CoCuCN [104]. Copyright 2025, Wiley‐VCH GmbH. (f) Decomposition mechanism of Li₂CO₃ on Ir‐Ru nanoalloy surfaces during charging [63]. (g) Adjacent coexisting sites of Li₂C₂O₄ and Li₂CO₃ with parallel and crossed orientations on the PIFCCZ surface, and the corresponding charge density distributions [105]. Copyright 2026, American Chemical Society. (h) Schematic illustration of the HEAOs structure, and (i) schematic illustration of the electronic coupling between M–O–M units [106]. Copyright 2026, Wiley‐VCH GmbH. (j) Schematic illustration of the mechanism for high‐entropy PBAs during the discharge‐charge process [107]. Copyright 2025, Wiley‐VCH GmbH.

4.1.4.2. Adjacent Site Interactions

For binuclear metal sites, when the distance between two catalytically active centers decreases to a certain extent, the interaction between active centers becomes a factor that must be considered for influencing intrinsic activity. Besides homonuclear metal atomic sites (e.g., adjacent Co sites on graphene oxide [109]), this synergistic effect can also exist between different structural forms. Lin et al. designed a catalyst incorporating Ru atomic clusters (Ru_AC) and Ru–N₄ sites (Ru_SA) on nitrogen‐doped carbon nanoboxes(Ru_AC+SA@NCB) [56]. DFT calculations indicated that Ru_AC+SA@NCB possesses a lower work function than Ru_SA@NCB, implying facilitated electron emission that is conducive to enhanced electrocatalytic activity. Moreover, significant differences in the d‐band center and density of states (DOS) confirmed that the electronic structure of the Ru–N₄ active sites is modulated by the adjacent Ru_AC. Gibbs free energy calculations for CO₂RR/ER processes revealed that the energy barriers for most steps are lower at Ru_SA sites than at Ru_AC sites, identifying Ru–N₄ as the primary active center. Benefiting from the regulation by adjacent Ru_AC, the Ru–N₄ site significantly reduced the energy barrier for the RDSs of CO₂RR/ER. The assembled battery exhibits overpotentials as low as 1.65 and 1.86 V at current densities of 1 A g⁻¹ and 2 A g^−1, respectively.

On the other hand, introducing a second metal can further enhance catalytic activity and selectivity through synergistic effects between different metals, effectively overcoming the limitations of single‐metal sites. For instance, Huang et al. utilized chemical vapour deposition to construct atomically dispersed Cr–N₄, Cu–N₄, and bridged N₃–Cu–N–Cr–N₃ sites on hollow bimodal porous carbon nanocages [110]. DFT calculations indicated charge transfer from Cr to Cu at the N₃–Cu–N–Cr–N₃ site. The Cu/Cr dual single‐atom sites with modulated electronic structure significantly optimized adsorption strengths for the key intermediate Li₂C₂O₄ and the discharge product Li₂CO₃. Charge density analysis further elucidated this synergistic mechanism: the N₃–Cu–N site exhibits stronger electronic interactions with Li₂C₂O₄, thereby dominating the CO₂RR process; conversely, the N–Cr–N₃ site displays higher electron density with Li₂CO₃, rendering it decisive for Li₂CO₃ adsorption. As another example, Hu et al. designed a cyanide‐bridged heterometallic active site catalyst (Cu₃ [Co(CN)₆]₂, abbreviated as CuCoCN). Due to the lower d‐band center and stronger electron‐withdrawing ability of Cu, electrons transfer from Co to Cu via the d(Co)–Π(CN)–d(Cu) pathway (Figure 8d). The d‐Π feedback effect further weakens the electron delocalization of the Co 3d orbitals [104]. According to the comparison of PDOS between the CuCoCN(400) plane and its single‐metal counterpart (CoCN) (Figure 8e), the dominant proportion of Co 3d orbitals indicates that its electrons are highly active, serving as a key factor enhancing catalytic activity. Moreover, the Co 3d orbitals in CoCN show a wider dispersion in the range of 0 to −2 eV compared to CuCoCN (0 to −1 eV), suggesting that the Co 3d orbitals in CuCoCN are more concentrated near E _F. This facilitates alignment between the Co 3d orbital and the Π* orbital of CO₂, enhancing CO₂ adsorption. Meanwhile, the moderate downshift of the d‐band center at the Cu‐adjusted Co active site ensures appropriate adsorption strength for intermediates. Therefore, the asymmetric electron distribution in the cyanide bridge promotes electron transfer, and the Co active sites, assisted by Cu active sites, significantly improve CO₂ adsorption capacity and catalytic activity.

4.1.4.3. Alloying

Compared to binuclear metal sites or clusters, alloyed bimetallic nanoparticles possess increased geometric complexity, but the ordered geometric features of their crystal structure exhibit higher stability [102]. Moreover, the formation of solid‐solution phases in bimetallic alloys facilitates the realization of synergistic interactions between dual metallic components. For instance, Wang et al. prepared uniform ultrafine Ir‐Ru alloy nanoparticles modified N‐doped carbon nanotube composites (IrRu/N‐CNT) [63]. In the CO₂RR process, the Ir‐Ru alloy showed the lowest free energy barrier for the RDS compared to single‐component metals. For the CO₂ER process, CV results showed that the oxidation peaks of the IrRu/N‐CNTs cathode at 4.05 and 4.26 V coincided with those of Ir/N‐CNTs and Ru/N‐CNTs cathodes, respectively. Combined with DFT calculations and discharge product morphology characterization, it was confirmed that the dense film‐like Li₂CO₃ first undergoes self‐decomposition at Ir‐dominated sites; as the reaction voltage increases, the Li₂CO₃ formed at Ir‐dominated active centers is gradually depleted. Meanwhile, at Ru‐dominated active centers, the accumulated Li₂CO₃ decomposes via a reversible reaction with C (Figure 8f). This unique two‐step Li₂CO₃ decomposition mechanism facilitates the complete removal of discharge products during charging. Benefiting from this, the assembled battery achieved nearly 100% coulombic efficiency and a cycle life exceeding 7660 h, demonstrating unprecedented cycling stability. On the other hand, the electronic structure of the main metal active sites can also be adjusted via alloying. Representatively, Liu et al. incorporated Co atoms into cubic Pt to form a CoPt alloy, effectively downshifting the d‐band center of Pt [47]. This electronic modulation reduced the CO adsorption energy, thereby preventing deep reduction to solid C and promoting CO desorption during the CO₂RR process, resulting in Li₂CO₃ & CO as the primary discharge products. Leveraging the kinetic and thermodynamic advantages of the gas‐solid co‐oxidation reaction between CO and Li₂CO₃, electrochemical tests revealed that the battery employing CoPt alloy cathode catalyst achieved a low charging potential of only 2.78 V at 20 µA cm⁻². This value is significantly lower than that of single‐metal Co and Pt catalysts, while the battery also exhibits exceptional cycling stability and energy efficiency.

4.1.4.4. Entropy Effect

As anticipated, with the further incorporation of metallic elements, the corresponding structure‐activity relationships become increasingly complex. In recent years, high‐entropy materials have exhibited unique physicochemical properties originating from the exceptionally high configurational entropy (ΔS) generated by the mixing of multiple elements [111]. For example, Wen et al. designed a series of FeCoNiCuRu alloy catalysts with varying ∆S _mix values by regulating the heat treatment temperature. Both theoretical calculations and electrochemical measurements confirmed a positive correlation between ∆S _mix and catalytic activity [112]. Furthermore, multimetallic sites with distinct electronic structures are randomly distributed at the atomic scale. Their mutual interactions can synergistically modulate the electronic environment of the active centers, providing an ideal platform for constructing diversified catalytically active sites. Chen et al. employed a six‐element high‐entropy alloy, PtRuZnCoNiCu (PRZCNC‐HEA), as a cathode catalyst [113]. The coexistence of the discharge products, Li₂C₂O₄ and Li₂CO₃, was confirmed via XRD pattern, Raman spectroscopy, and in situ FT‐IR spectroscopy. Adsorption energy calculations revealed that different metal sites exhibit distinct adsorption preferences for Li₂CO₃ and Li₂C₂O₄, consequently driving the simultaneous occurrence of both reaction pathways on the surface. The charge density distributions visualized a synergistic coupling phenomenon among the metal, carbonate, and oxalate species when Li₂C₂O₄ and Li₂CO₃ coexisted on the PRZCNC‐HEA surface, thereby facilitating the stable formation of Li₂C₂O₄. This coupling mechanism of coexisting Li₂CO₃ and Li₂C₂O₄ has also been observed in other HEA systems [105, 114]. For instance, Wang et al. synthesized a PtIrFeCoCuZn (PIFCCZ) high‐entropy intermetallic (HEI) with L1₂‐type atomic ordering to serve as a cathode catalyst. Unlike solid‐solution alloys, intermetallic compounds (IMCs) feature precise atomic arrangements owing to their long‐range ordering at the atomic scale, thereby exhibiting unique local electronic configurations and enhanced catalytic stability. It is theoretically demonstrated that both Li₂C₂O₄ and Li₂CO₃ preferentially adsorb onto specifically oriented metal bridge sites (Figure 8g). Upon co‐adsorption in a crossed orientation, the adsorption energy of Li₂C₂O₄ is further decreased to −11.03 eV, indicating that Li₂CO₃ preferentially occupies adsorption sites adjacent to Li₂C₂O₄ to provide further stabilization. Subsequent adsorption energy calculations on the pre‐adsorbed surfaces revealed that newly arriving Li₂C₂O₄ molecules preferentially stabilize near the pre‐adsorbed Li₂CO₃ rather than near existing Li₂C₂O₄, while newly introduced Li₂CO₃ molecules preferentially adsorb at sites adjacent to the pre‐adsorbed Li₂CO₃. Consequently, this molecular‐level spatial isolation hinders the formation of an extended Li₂C₂O₄ lattice, ultimately resulting in the separate nucleation and growth of fine‐grained Li₂CO₃ and amorphous Li₂C₂O₄ characterized by a narrow bandgap. Benefiting from the precise coupling between the catalytic surface and the discharge products, the assembled Li‐CO₂ batteries delivered a high discharge voltage of 3.08 V and achieved stable operation for over 1000 h.

Furthermore, the concept of high entropy has also been extended to a broader range of material compositions. Regarding metal oxides, to alleviate the detrimental irreversible structural transformations caused by local charge accumulation during discharge‐charge cycles, Liu et al. designed high‐entropy amorphous oxides (HEAOs, composed of Cr, Fe, Co, Ni, Cu, and O) featuring a high degree of structural freedom and adaptive topological disorder as a model catalytic system (Figure 8h) [106]. The increase in entropy provides the thermodynamic basis for the transition from the crystalline phase to the amorphous state. The flexible M–O–M structure of HEAOs triggers d–d electron transfer between transition metals and d–p orbital coupling between transition metals and O, facilitating a unique variation characterized by single‐site charge transfer and adjacent structural stretching compensation (Figure 8i). According to the XAS and XPS results, compared to other elements whose valence states remain almost unchanged, the valence state of Cr increases during the discharge process and remains stable after charging, thus serving as the primary active site. The variation in the Cr–O bond length is realized by the contraction of M–O bonds within the adjacent coordination polyhedra, thereby compensating for the valence change of Cr. This dynamic adaptive mechanism effectively alleviates the localized stress accumulated during discharge‐charge cycles, thereby significantly enhancing the long‐term cycling capability of the battery. As for Prussian blue analogues (PBAs), which connect MN₆ octahedra and FeC₆ octahedra into a three‐dimensional framework via the M–N≡C–M’ coordination structure, Zhu et al. successfully synthesized a series of PBAs with varying entropy values by regulating the types and quantities of metal precursors at the M sites [107]. By ex situ XPS characterization of high‐entropy PBA (HE‐PBA (PBA‐FeMnNiCoCu)) before and after discharge‐charge cycles, it is revealed that the valence states of Co and Cu change significantly, acting as the primary catalytic sites. During the discharge process, Co²⁺ is oxidized to Co³⁺, while Cu²⁺ is reduced to Cu⁺. This Co–Cu redox system balances electron transfer, thereby avoiding the detrimental impact of localized charge accumulation of a single component on cycling stability. In contrast, Fe³⁺, Mn²⁺, and Ni²⁺ maintain their oxidation states throughout the discharge‐charge processes. They primarily participate in the charge compensation network to facilitate electron transfer, modulate the local electronic environment of Cu/Co, and further enhance lattice stability. Concomitant with the increase in entropy, the d‐band center of PBA‐FeMnNiCoCu also shifts closer to the E _F. These applications further substantiate the immense potential of high‐entropy strategies in modulating the electronic structures of materials.

In short, multi‐metal active sites have demonstrated immense potential for harnessing the “synergistic effect” to achieve bifunctional catalysis for CO₂RR and CO₂ER. Nevertheless, as the geometric complexity increases, the construction of well‐defined, identifiable multi‐metal sites and the elucidation of the interactions between these active sites have become a significant challenge in the design of multi‐metal site catalysts. This area will require extensive further research in the future.

4.2. Surface Morphology and Structure

As the primary interface interacting with reactants, the surface morphology and structure of catalysts critically influence catalytic performance. Tailored morphological architectures not only enhance specific surface area to expose more catalytically active sites but also modulate the electronic structure of catalyst surfaces, thereby regulating material adsorption properties [2]. Furthermore, rational pore structure design in cathode materials governs mass transport of Li⁺ and CO₂. Hence, this section systematically reviews recent surface engineering strategies for optimizing cathode catalytic performance.

4.2.1. Different Dimensions

Carbon materials with diverse morphologies have demonstrated superior electrochemical performance compared to conventional pure carbon materials (e.g., Ketjen black (KB), Super P) [1]. Among them, one‐dimensional carbon architectures, including CNTs [23], CNFs [118], bamboo‐like nanotubes [119], and two‐dimensional graphene [24] have been extensively employed as cathode catalysts or supports in Li‐CO₂ batteries. Strategies for introducing active sites and related modifications were detailed in the preceding section. Recently, graphdiyne (GDY), a novel 2D material comprising sp‐hybridized carbon atoms, has emerged as a promising candidate due to its extensive π‐conjugation, uniform porosity, and exceptional gas adsorption capabilities. Typically, Zhang et al. pioneered the use of monolayer graphdiyne as a cathode catalyst, achieving outstanding long‐term stability (158 cycles at 400 mA g⁻¹) [120]. Subsequent fluorine substitution (FGDY) enhanced electronic conductivity, while the ionic liquid 1‐butyl‐3‐methyl‐imidazoliumbis (trifluoromethosulfonimide) (BMImTFSI) addressed the high‐temperature volatility issues of conventional electrolytes (e.g., TEGDME) while maintaining high CO₂ solubility and electrochemical stability [121]. The resulting Li‐CO₂ battery operated at 80°C realized a high discharge capacity of 29 050 mAh g⁻¹ at 5.0 A g⁻¹ and sustained 204 cycles at 1 A g⁻¹. Conversely, three‐dimensional carbon frameworks with structural diversity, high specific surface area, and tunable 3D porous networks show equal promise. For instance, Xiao et al. constructed a freestanding N‐doped bilayer 3D NCNT/G cathode (Figure 9a), featuring vertically aligned N‐doped CNTs sandwiched between two N‐doped graphene layers [115]. This creates a continuous interface that enables efficient CO₂ diffusion and rapid electron/electrolyte transport, while also delivering a high surface area and robust mechanical stability. Further, Jiang et al. proposed a strategy combining electrode architecture with metallic catalytic centers, developing a wood‐derived, self‐supporting hierarchical porous carbon electrode functionalized with Ni nanoparticles and Ru clusters [116]. Carbon nanotubes grown in situ on the wood substrate establish decoupled gas and ion diffusion channels (Figure 9b): the natural vertical channels of the carbonized wood serve as conduits for gas transport and accommodation spaces for discharge products, while the in situ NCNT network facilitates ion transport and electron conduction. Furthermore, interestingly, the authors discovered that dual Ni‐Ru active centers with a moderate degree of alloying most effectively optimize the electronic structure and regulate intermediate adsorption energies, thereby significantly lowering the energy barrier of RDS. Benefiting from the synergy between this hierarchical architecture and the bimetallic active sites, the battery achieved an ultralow overpotential gap of 0.619 V and a prolonged cycle life exceeding 1100 cycles (2200 h).

FIGURE 9.

(a) Schematic illustration of 3D NCNT/G serving as a cathode for Li‐CO₂ batteries [115]. Copyright 2020, American Chemical Society. (b) Schematic diagram of Ni‐Ru‐NCNT/CW_x as a cathode for Li‐CO₂ batteries [116]. Copyright 2025, Wiley‐VCH GmbH. (c) Reaction mechanism schematic of atomic Ir‐Te catalysts during discharge [117]. Copyright 2021, Elsevier B.V. (d) Structure and catalytic mechanism of HOF‐FJU‐1 [28]. Copyright 2023, Wiley‐VCH GmbH. (e) XRD patterns of Ti₃AlC₂, Ti₃C₂T_x, and Ex‐Ti₃C₂T_x‐m (m = 1, 2, 3, 4) films, and (f) derived interlayer structure of Ex‐Ti₃C₂T_x‐3 based on XRD analysis. (g) Cross‐section SEM images of Ti₃C₂T_x (left) and Ex‐Ti₃C₂T_x‐3 (right) films [74]. Copyright 2025, Wiley‐VCH GmbH.

For metal‐based catalysts, zero‐dimensional (0D) nanoparticle structures often exhibit a limited number of catalytic sites and restricted catalytic activity. Designing catalysts with specific morphologies can effectively increase the specific surface area. Simultaneously, modulating the coordination number of surface atoms facilitates the increased exposure of catalytically active sites and optimizes the surface electronic structure. Zhai et al. uniformly super‐assembled an approx. 3 nm thick amorphous Ir‐Te surface layer composed of Ir atomic clusters on Te nanowires (NWs) via a surface atomic replacement strategy. This unique structure maximized the catalytic capability of the active Ir sites while providing a refined reaction pathway between Ir active sites and the Te substrate (Figure 9c) [117]. DFT calculations and in situ analysis indicated that on Ir sites, the discharge process begins with Li⁺ adsorption, proceeding through the Li₂C₂O₄ intermediate to form the discharge product Li₂CO₃. Notably, the charge process does not follow the simple reverse trajectory of discharge. Instead, it first forms the Li₂C₂O₄ intermediate, followed by C₂O₄, and finally completes decomposition via delithiation and CO₂ desorption. Acting as a P‐type semiconductor, the Te substrate plays a pivotal role by leveraging its superior electron‐accepting capability. Its high adsorption energy for Li⁺ stabilizes and accelerates the generation of the key intermediate Li₂C₂O₄ during the discharge process. Furthermore, the positive adsorption energy of CO₂ on Te not only optimizes the CO₂RR pathway on Ir active sites but also promotes CO₂ desorption during CO₂ER process. Based on this, the Ir‐Te NWs catalyst exhibits excellent catalytic performance, demonstrating stable operation for over 350 cycles at 1000 mA g⁻¹ and over 200 cycles at 2000 mA g⁻¹. Regarding the design of 2D structure catalysts, Wang et al. successfully prepared a series of ultrathin 2D Ru‐M (M = Co, Ni, Cu) alloy nanosheets (NSs) using a one‐pot solvothermal method, with Ru₃(CO)₁₂ and metal acetylacetonates as precursors and CO as a shape‐directing agent [122]. Ru K‐edge extended X‐ray absorption fine structures (EXAFS) analysis confirmed the successful construction of 2D in‐plane alloy sites (Ru–Co). Compared to 0D RuCo nanoparticles (NPs), RuCo NSs showed a higher Ru–Co coordination number (4.0 vs. 1.7) and a lower Ru–Ru coordination number (4.8 vs. 5.8). DFT calculations showed that on the RuCo(0001) facet, the adsorption energies for CO₂ and Li₂CO₃ on RuCo–Ru/RuCo–Co sites were significantly higher than on CNTs. Simultaneously, the alloy sites interacted strongly with the O atoms of Li₂CO₃, weakening the C–O bond and effectively reducing its decomposition barrier. Consequently, RuCo NSs exhibited exceptional electrochemical performance, with the charging plateau voltage reduced to 3.74 V and an overpotential of only 0.94 V, which are significantly lower than those of RuCo NPs and CNTs.

In recent years, crystalline organic frameworks with well‐defined porous network structures and multifunctional sites, including MOFs [123, 124], COFs [27, 125], and hydrogen‐bonded organic frameworks (HOFs) [28], have emerged as ideal platforms for developing stable Li‐CO₂ batteries. Their high specific surface area and ordered porous structure facilitate CO₂ capture and Li⁺ transport, while providing distinct pathways for gas, ions, and electrons, thereby enhancing reaction kinetics. For example, Li et al. designed tailor‐made 1D channels based on COFs, which function as both CO₂ collectors and channels for ion transport and gas diffusion. Through coordination interactions with Ru loaded on CNTs (Ru@CNT), they enabled the rapid reversible formation and decomposition of Li₂CO₃/C at the interface between the COF and Ru@CNT [27]. Furthermore, the functional modification of pore structures can significantly optimize catalytic activity. Cheng et al. prepared a cyano‐functionalized HOF (HOF‐FJU‐1) encapsulating Ru⁰ nanoparticles supported on CNTs (HOF‐FJU‐1‐Ru@CNT) as a cathode catalyst [28]. As shown in Figure 9d, cyano groups are periodically distributed within the channels of HOF‐FJU‐1, inducing uniform deposition of discharge products. The structure exhibits high stability via π···π stacking coupled with CN···H–C hydrogen‐bonding networks. The regularly arranged porous channels also provide pathways for rapid CO₂ and Li⁺ diffusion. More importantly, the strong interaction between the abundant cyano groups of HOF‐FJU‐1 and the Ru⁰ nanoparticles not only formed efficient catalytic sites but also caused an upshift in the d‐band center of the Ru(111) plane (from −1.731 to −1.491 eV), thereby significantly optimizing CO₂RR/ER kinetics. Benefiting from this, the assembled battery achieves electrochemical performance on par with crystalline catalysts, broadening the application prospects of HOF‐based catalytic materials in the field of Li‐CO₂ batteries. On the other hand, 2D ultrathin materials possess layered structures conducive to ion insertion/extraction, along with abundant surface functional groups and high redox reactivity, making them equally promising for applied research. To better leverage the advantages of COFs, Jiang et al. employed a KMnO₄ chemical exfoliation method to delaminate a quinone‐based COF (DQTP‐COF) into ultrathin nanosheets (MnO₂/DQTP‐COF‐NS), which feature a well‐developed porous structure, abundant quinone groups, and uniformly loaded MnO₂ [125]. By adjusting the amount of KMnO₄, the resulting MnO₂/DQTP‐COF‐NS‐3 (with an MnO₂ loading of 18.9 wt%) exhibits an ultrathin morphology, measuring only 1.87 nm in thickness and 500 nm in width. DFT calculations further reveal the quinone‐assisted CO₂ activation mechanism. The quinone groups in DQTP‐COF first acquire electrons and transform into an intermediate reduced state, which subsequently interacts with CO₂ to form stable quinone‐CO₂ adducts. Compared to direct conversion of CO₂ → CO₂ ^{· −}, this pathway entails a significantly lower Gibbs free energy, thereby greatly enhancing the reaction kinetics.

MXene (M_n+1X_nT_x (n = 1–3)) is a novel two‐dimensional layered nanomaterial composed of transition metal carbides, carbonitrides, or nitrides. It features a hexagonal close‐packed structure (space group P6₃/mmc), in which the M atoms (transition metals) are closely arranged in a two‐dimensional honeycomb lattice, while the X atoms (C/N) occupy the octahedral interstitial sites between the M atom planes, and T_x represents the surface terminations (e.g., –O, –F) [126]. Leveraging its rich surface chemistry, superior electrical conductivity, and exceptional charge transfer efficiency, MXene also exhibits immense application potential in the field of Li‐CO₂ batteries. Compared to multilayer MXene substrate structures, monolayer or few‐layer architectures demonstrate remarkable mechanical flexibility and a larger specific surface area, which significantly facilitates ion transport and CO₂ adsorption. To weaken the strong interlayer Van der Waals forces of Ti₃C₂T_x, Li et al. employed an MgO‐templated chemical exfoliation method [74]. They inserted MgO nanoparticles into the negatively charged Ti₃C₂T_x interlayers via electrostatic self‐assembly, thereby introducing significant steric hindrance, weakening the interlayer interaction forces, and expanding the interlayer spacing. XRD results (Figure 9e–f) indicate that with increasing MgO content, the (002) interplanar spacing of the Ex‐Ti₃C₂T_x‐3 film (interlayer‐expanded Ti₃C₂T_x, corresponding to a Ti₃C₂T_x to MgO mass ratio of 1:6) expanded from 12.41 Å (untreated) to 15.04 Å, with a thickness of only 0.7 nm. As shown in Figure 9g, scanning electron microscopy (SEM) images similarly demonstrated the expansion of the interlayer spacing. FT‐IR measurements demonstrated that Ex‐Ti₃C₂T_x‐3 possessed the minimum amount of surface –OH groups, which can maximally mitigate H₂O interference and suppress interfacial oxidation, thereby enhancing chemical stability. Furthermore, XPS analysis indicated that the reduction in –OH groups led to a partial loss of chemical coordination for the adjacent Ti atoms, exposing more uncoordinated Ti atoms or vacancy defects as active sites. Notably, the structure of Ex‐Ti₃C₂T_x, rich in surface defect sites, can stabilize the two‐electron product Li₂C₂O₄ via a Ti³⁺/Ti²⁺ coupling bridge, effectively preventing disproportionation into Li₂CO₃. This was supported by both experimental and theoretical calculations. Based on these properties, the Li‐CO₂ battery employing an exfoliated few‐layer Ex‐Ti₃C₂T_x‐3 film cathode achieved a full discharge capacity as high as 3452.33 µAh cm⁻², with a low charging voltage and a cycle life exceeding 1600 h.

It is evident that for Li‐CO₂ batteries, surface morphologies and structures of varying dimensions, ranging from particles (0D), nanowires (1D), and nanosheets (2D) to porous architectures (3D), endow materials with unique catalytic properties. As research deepens, a more precise and comprehensive understanding of the structure‐function relationship will undoubtedly be attained.

4.2.2. Surface Facets

Since catalytic reactions mainly occur on the material surface, the exposed crystal facets of catalyst particles directly determine the surface atomic composition and coordination structure, serving as a pivotal determinant influencing catalytic performance. To construct catalytic materials with specific preferred orientations, Chen et al. utilized a Joule heating‐driven high‐temperature shock (HTS) technique to convert disordered platinum into a 3D porous Pt catalyst with a predominant (111) orientation [127]. Surface energy calculations revealed the thermodynamic driving force for this orientation transformation: compared to the (200) and (220) facets, the Pt(111) facet has the lowest surface energy (1.44 J m⁻²), indicating it is the most thermodynamically stable facet and thus tends to be exposed during the quenching process. As confirmed by XRD results, after HTS treatment, the intensity of the Pt(111) peak increased significantly, corresponding to a lattice spacing of 0.225 nm, and the narrowing of the full width at half maximum (FWHM) also indicated improved crystallinity. Further calculations of the adsorption energies of reactants on different Pt facets (Figure 10a) showed that the adsorption energies of Pt(111) for Li⁺ and CO₂ are −1.12 and −0.43 eV, respectively, which are higher than those of other facets. Moreover, it has the lowest barrier for Li₂CO₃ decomposition (1.09 eV). Therefore, owing to the improved CO₂ conversion kinetics on the (111) facet, the developed porous electrocatalyst with specifically oriented Pt(111) achieved outstanding electrochemical performance at the minimal loading, markedly outperforming the initial Pt catalyst.

FIGURE 10.

(a) Comparison of the adsorption energies of CO₂, Li, and Li₂CO₃ on different Pt crystal planes [127]. Copyright 2023, Elsevier Ltd. (b) Schematic comparison of spin‐polarized models with a single d‐band center (dashed line) and a double d‐band center (solid line). (c) PDOS plots of different crystal facets for CoSe₂ and P‐CoSe₂ [128]. Copyright 2025, American Chemical Society. (d) Schematic illustration of Zn and Cl co‐modulated MXene catalyst. (e) CO₂ adsorption energies corresponding to different surface groups on MXene, and (f) the dband centers [129]. Copyright 2024, American Chemical Society. (g) Schematic illustration of the synthesis process and atomic arrangement of the 4H/fcc Ru‐Ni heterostructure. (h) Atomic‐resolution HAADF‐STEM image clarifying the lattice fringes near the edge of 4H Ni. (i) Schematic illustration of the distinct Li‐CO₂ electrochemical mechanisms on 4H/fcc RuNi and 2H Ru [130]. Copyright 2024, Wiley‐VCH GmbH.

On the other hand, lattice distortion induced by heteroatom doping can simultaneously generate tensile and compressive strains on different crystal facets, offering a strategy to construct dual‐functional reaction sites within a unitary system. In light of this, Zhang et al. constructed strain‐complementary active facets, specifically, the tensile‐strained (111) facet and the compressive‐strained (120) facet, by introducing P‐doped cobalt diselenide on a nitrogen‐doped honeycomb carbon framework (P‐CoSe₂@NC), utilizing the anisotropic lattice distortion generated by phosphorus doping [128]. Both experimental and theoretical results indicated that this distortion significantly increased the proportion of the high‐spin state of Co sites (46.7% vs. 17.8%), thereby increasing the electron occupancy of e _g orbitals ($d_{{\mathrm{z}}^2}$ and $d_{{\mathrm{x}}^2-{\mathrm{y}}^2}$). To address the limitations of conventional d‐band center theory in describing highly spin‐polarized transition metal systems, a dual d‐band center model (spin‐up ε _d↑ and spin‐down ε _d↓) (Figure 10b) was employed, revealing the competition mechanism between spin orbitals caused by the high‐spin state change induced by P‐doping. By analyzing the contribution of split t _2g and e _g orbitals to the d‐band center shift (where the vertical solid line represents the d‐band center), it was found that for P‐CoSe₂(111), the electron occupancy of e _g orbitals increased only slightly, and the overall upshift of the d‐band center mainly originated from the upshift of the d _yz and d _xz orbital centers. In contrast, on P‐CoSe₂(120), although the d _xz orbital center shifted up slightly, the significant downshift of the $d_{{\mathrm{x}}^2-{\mathrm{y}}^2}$ orbital center dominated the negative shift of ε _d↑ and ε _d↓. As shown in Figure 10c, this differentiated regulation shifted the d‐band center of P‐CoSe₂(111) upward from −2.73 (ε _d↑)/ −1.21 (ε _d↓) to −2.34 (ε _d↑)/ −0.95 (ε _d↓), alleviating the strong repulsion of the pristine surface and enhancing adsorption. Conversely, the downshift of the d‐band center in P‐CoSe₂(120) weakened the excessively strong adsorption. Consequently, a precise balance was achieved between the adsorption behaviors for CO₂RR and CO₂ER. This work offers deeper insights into modulating catalytic performance via nonmetal doping within spin‐polarized systems.

Therefore, as a core concept in the field of catalysis, crystal facet engineering serves as a critical entry point for deeply understanding the origins of electrocatalytic performance on highly active surfaces. It also offers an important direction for the precise regulation of CO₂RR/ER performance. Currently, research concerning the potential selectivity of different crystal facets toward discharge products, as well as the identification of actual catalytic reaction sites, remains relatively limited and urgently requires further in‐depth exploration in future work.

4.2.3. Surface Modifications

Surface modification engineering of catalysts, by introducing specific atoms, molecules, or coatings onto the catalytic surface, can modulate the electronic structure and adsorption properties of the catalyst via surface chemistry without altering its bulk composition, effectively tailoring the interfacial interactions between the catalyst and the reactants [2]. In this regard, owing to its unique two‐dimensional structure and abundant surface terminations, MXene emerges as an ideal candidate for tuning surface electronic structures. Recently, related applications in the field of Li‐CO₂ batteries have also been reported [35, 129, 131, 132, 133]. For instance, Liu et al. employed a thermal annealing strategy to decrease the content of oxygen‐containing surface terminations (–O, –OH) on Ti₃C₂T_x MXene, thereby minimizing the generation of the LiOH byproduct, which is detrimental to the cycling performance of the battery [131]. Simultaneously, benefiting from the modulation of the electronic structure, the particle size of the Li₂CO₃ is reduced. Furthermore, Tian et al. modulated the adsorption of discharge product Li₂CO₃ and the electron transfer between the catalyst and Li₂CO₃ by introducing Zn and Cl surface groups onto a Ti₃C₂ MXene catalyst (Zn‐Ti₃C₂Cl₂) (Figure 10d) [129]. DFT calculations revealed that the surface groups of MXene influence CO₂ adsorption (Figure 10e). Ti₃C₂Cl₂ exhibited the strongest adsorption energy (−4.75 eV), which is fundamental for achieving rapid CO₂RR kinetics. This was followed by Ti₃C₂F₂ (−4.52 eV), Ti₃C₂Br₂ (−4.51 eV), Ti₃C₂I₂ (−4.46 eV), and Ti₃C₂O₂ (−3.81 eV). Furthermore, the introduction of Zn not only provided uniformly distributed active sites but also shifted the d‐band center of Ti closer to the E _F (Figure 10f), facilitating electron transfer between the catalyst and discharge products. This change was corroborated by the analysis of Li–O bond lengths and COHP values. Notably, during cycling performance testing, a sudden drop in energy efficiency was observed in the Zn‐Ti₃C₂Cl₂ cathode at the 33rd cycle. Analysis of XPS C 1s spectra revealed that CO₂ was initially reduced to form the intermediate product C₂O₄ ²⁻, which was stabilized by Zn‐Ti₃C₂Cl₂ under limited discharge capacity (i.e., without full discharge). Subsequently, C₂O₄ ²⁻ underwent disproportionation to form Li₂CO₃, leading to the observed reduction in energy efficiency. Benefiting from this surface engineering and band structure modulation strategy, Li‐CO₂ battery employing the Zn‐Ti₃C₂Cl₂ catalyst showed an overpotential as low as 0.72 V and a cycling stability up to 1500 h at 200 mA g⁻¹.

4.2.4. Phase Transition

Crystal phase, as a key structural parameter for regulating the intrinsic activity of metals, can effectively tune the surface adsorption properties and charge states of metals by adjusting atomic arrangements. Certain unconventional crystal phases (e.g., 2H, 1T) may exhibit enhanced electrical conductivity and higher surface activity [134]. However, guidance on material crystal phase/composition for designing bidirectional catalysts remains scarce. Based on this, Zhou et al. reported a theoretically guided 4H/fcc Ru‐Ni heterostructure as a high‐performance cathode catalyst [130]. DFT simulation screening results indicated that among five representative noble metals (Ru, Ir, Pd, Pt, and Rh) and four non‐noble metals (Fe, Co, Ni, and Zn), Ru and Ni are the optimal metal components for promoting CO₂RR and CO₂ER, respectively. More crucially, compared to the thermodynamically stable 2H or fcc phases, the unconventional 4H phase of Ru‐Ni not only effectively lowers the CO₂RR/ER reaction barriers but also plays a significant role in stabilizing the key intermediate Li₂C₂O₄. Compared to the conventional 2H (002) and fcc (111) facets, the unique phase and microstructure of 4H/fcc Ru‐Ni endow it with superior (004)/(110) facets, which exhibit higher adsorption energies and lower reaction barriers for reaction intermediates. Furthermore, combined with dynamic Li migration simulations, a relay migration mechanism during charge‐discharge cycles was proposed: . Given the difficulty in avoiding the formation of the fcc phase during the actual preparation of the 4H phase, a two‐step epitaxial growth method was employed using 4H/fcc Au nanorods as sacrificial templates to obtain ultrathin Ru‐Ni heterostructure nanotubes with a hollow structure (Figure 10g). Transmission electron microscope (TEM) and high‐angle annular dark‐field scanning transmission electron microscopy (HAADF‐STEM) characterizations showed that the synthesized 4H/fcc Ru‐Ni presents a hollow nanotube morphology with a diameter of 20–40 nm, with numerous dendrites (average length 8–10 nm) anchored on the nanotube surface and abundant nanopores embedded in the walls. Corresponding fast Fourier transform (FFT) patterns confirmed the formation of the 4H phase in both Ru and Ni regions. HAADF‐STEM images (Figure 10h) observed “ABCB” and “ABC” atomic stacking sequences along the close‐packed [001]_4H/ [111]_fcc direction, confirming the coexistence of 4H and fcc phases. Electrochemical testing and discharge product analysis revealed its unique mechanism: the Ni component in 4H/fcc Ru‐Ni stabilizes Li₂C₂O₄ via metal‐oxygen coupling, while the Ru component mainly acts to enhance adsorption. Consequently, the discharge product was successfully adjusted to be dominated by Li₂C₂O₄ (Figure 10i), effectively solving the problem of electrolyte/catalyst failure caused by corrosive singlet oxygen generated from Li₂CO₃ decomposition. In contrast, Li₂C₂O₄ generated on 2H Ru spontaneously converts to Li₂CO₃ via a disproportionation reaction. Benefiting from this, Li‐CO₂ battery constructed with 4H/fcc RuNi offered a low overpotential of 0.65 V and achieved long‐term cycling stability of over 220 cycles at 250 mA g⁻¹. This work demonstrates the high feasibility and unique advantages of unconventional phase metal nanomaterials in enhancing metal‐gas electrochemistry for practical applications.

4.3. Interface Structure

Beyond atomic‐level catalytic modifications, interface engineering is a feasible and effective strategy to regulate the electronic structure and surface properties of catalysts. Interface structures are typically constructed between two or more distinct materials or phases. Generally, interfacial effects can be summarized as electronic synergistic effects and ensemble effects between components, as shown in Figure 11a. The formation of this region has a significant impact on electrocatalytic activity [135, 136, 137]. Specifically, the electronic synergistic effect (Case A) originates from electronic interactions caused by the energy band differences between components on both sides of the hetero‐interface. This establishes an electron transport bridge between A and B, regulating the adsorption energy of reaction intermediates. In contrast, for the ensemble effect (Case B), components A and B are responsible for reactant adsorption/intermediate formation and intermediate conversion/product desorption, respectively. In Li‐CO₂ batteries, this design philosophy is also applied to the construction of bifunctional catalysts (see below). It is worth emphasizing that ensemble effects and electronic synergistic effects often coexist and function synergistically at the interface. Rational design via atomic interface engineering can maximize these effects. Given the widespread application of metal‐based and metal compound (oxides, nitrides, sulfides, etc.) catalysts in interface engineering, this section will systematically elaborate on the research progress of interface structure design in the Li‐CO₂ battery field in recent years. The discussion is categorized by interface type, including metal‐metal interfaces, metal‐metal compound interfaces, metal compound‐metal compound interfaces, and hierarchical hetero‐interfaces constructed with other conductive materials (e.g., graphene, MXene).

FIGURE 11.

(a) Schematic diagram of heterogeneous interface catalysis mechanism composed of element A and element B [137]. Copyright 2018, Wiley‐VCH Verlag GmbH & Co. KGaA, Weinheim. (b) Schematic diagram of the formation process of Ni/RuHNPs. (c) Magnified HRTEM images of (10−10) atomic layers and (d) (0002) atomic layers for RuNPs (left) and Ni/RuHNPs (right), providing direct evidence of the ordered compressed arrangement of the bottom atomic layers [138]. Copyright 2022, Wiley‐VCH GmbH. (e) Charge and discharge reaction pathways of NP‐Ni₃Al/Ni [139]. Copyright 2023, Elsevier B.V. (f) Stereogram of the electron density difference for Ru_fcc+hcp clusters, and (g) schematic diagram of the proposed electrocatalytic process [140]. Copyright 2025, American Chemical Society.

4.3.1. Metal/Metal Interface

Interface metal/metal (alloy) interface engineering is typically constructed in alloys, heterojunctions, and core‐shell structures [137]. Among them, core‐shell nanostructures can rationally regulate the electronic structure of surface metals via lattice mismatch between the core and shell. As exemplified, Fan et al. designed and synthesized Ni/Ru core/shell hexagonal nanoplates (Ni/Ru HNPs) featuring biaxial compressive strain (Figure 11b) [138]. Visualized and quantified by high‐resolution TEM (HRTEM) and HAADF‐STEM, the lattice spacings of the Ni/Ru HNPs on the side {01−10} and basal (0002) planes were found to contract to approximately 0.223 and 0.210 nm, respectively, corresponding to compressive strains of ≈ 5.1% and ≈ 3.2% compared to pure Ru NPs. As depicted in Figure 11c–d, upon stacking multiple atomic layers on the (10−10) and (0002) planes, the top atomic layer of the Ni/Ru HNP is positioned significantly lower than that of the Ru NP, directly confirming the presence of lattice compression. Due to this strain effect, Ru 3p XPS spectra exhibited a positive shift in the binding energy of Ru⁰ in Ni/Ru HNPs relative to Ru NPs. DFT calculations further revealed that the introduced compressive strain enhances d‐orbital overlap, leading to a downshift of the Ru dband center. This enhances the diffusion of CO₂ away from the cathode surface, facilitating the flake‐like growth of Li₂CO₃ (during discharge) and its efficient decomposition (during charge). Regarding alloy architectures, in addition to atomically disordered solid‐solution alloys, IMCs also emerge as a class of highly promising candidate materials [139, 141]. For instance, Jian et al. prepared nanoporous Ni₃Al IMC/Ni heterojunctions (NP‐Ni₃Al/Ni) by selectively etching Al‐rich Ni‐Al alloy precursors in NaOH aqueous solution via a tailored chemical dealloying strategy [139]. In this structure, the unique interaction between Al and Ni in the Ni₃Al phase significantly optimized the adsorption characteristics of the catalyst surface for reactants/products, thereby promoting the reaction kinetics of Li and CO₂ during discharge and the reversible co‐decomposition of Li₂CO₃ and C during charging (Figure 11e). XPS characterization showed corresponding shifts in the binding energies of Ni and Al in Ni₃Al IMC, confirming that the strong electronic interaction between Ni and Al promoted electron transfer from Al to Ni, giving the Ni sites in Ni₃Al a higher electron density. Theoretically, it further elucidates the catalytic mechanism: during discharge, NP‐Ni₃Al/Ni exhibits higher CO₂ adsorption energy and maintains a stronger interaction with Li compared to nanoporous nickel (NP‐Ni). During charging, the in‐plane electron‐rich Ni sites of Ni₃Al have appropriate interaction strengths with intermediates, and NP‐Ni₃Al/Ni has a lower reaction barrier for the RDS, thereby promoting the co‐oxidation of Li₂CO₃ & C.

Beyond interfaces formed between distinct materials, heterophase materials have garnered significant attention due to their unique interfacial properties arising from a chemically homogeneous yet structurally multiphasic composition. Synergistic effects across phase boundaries can generate distinctive physicochemical characteristics. For example, Sun et al. developed ∼5.5 nm Ru heterophase nanoparticles on a KB matrix (Ru_fcc+hcp/KB) via a facile wet‐chemical method [140]. Bader charge analysis (Figure 11f) and X‐ray absorption near edge structure (XANES) confirmed electron transfer and charge redistribution between the polycrystalline phases. EXAFS fitting results revealed that Ru_fcc+hcp/KB exhibited a significantly lower average first‐shell Ru coordination number (5.2 ± 1.0) compared to Ru_fcc/KB (7.3 ± 1.0), Ru_hcp/KB (6.2 ± 1.0), and Ru foil (12), thereby providing a greater number of active sites. Furthermore, the Ru_fcc+hcp/KB heterophase demonstrated enhanced thermal stability. XRD results indicated that both fcc and hcp phases were retained even at 510 °C, whereas the Ru_fcc phase is typically only stable at moderate temperatures (around 300 °C). DFT investigations revealed that, compared to monophasic fcc or hcp counterparts, the heterophasic fcc+hcp structure weakens the adsorption strength of Li₂CO₃ and Li₂C₂O₄ (Figure 11g). During discharge, it exhibits the lowest energy barrier for the RDS (*Li₂CO₃ → Li₂CO₃), thereby promoting Li₂CO₃ nucleation and vertical growth. During the charging process, this structure facilitates the diffusion of discharge products into the electrolyte for decomposition while significantly lowering the decomposition barriers of intermediate species (*Li₂CO₃, *Li₂C₂O₄). Benefiting from these advantages, the battery demonstrated a low overpotential of only 0.75 V and a long cycle life exceeding 2260 h.

4.3.2. Metal/Metal Compound Interface

Another strategy involves constructing hetero‐interfaces between metal compounds (including metal oxides, sulfides, phosphides, etc.) and metals. Among them, transition metal oxides (TMOs), as previously mentioned, are characterized by poor electronic conductivity and low catalytic activity, leading to sluggish electrochemical reaction kinetics [75]. Based on interface engineering, coupling TMOs with alloy structures has been utilized as an effective approach to improve electrochemical performance. Ma et al. formed nanoporous Cu₂O/RuAl heterojunctions (MP‐Cu₂O/RuAl) by dealloying a RuCuAl alloy. This process selectively leached Al while synchronously driving the oxidation of Cu to Cu₂O and the phase reconstruction of Ru‐Al into RuAl intermetallic compounds [147]. Selected area electron diffraction (SAED) and HRTEM characterizations confirmed that the RuAl IMC and Cu₂O are closely stacked, creating abundant nano‐heterointerfaces. XPS analysis revealed strong electron interactions at the interface, with both Ru and Cu binding energies exhibiting significant negative shifts in MP‐Cu₂O/RuAl. This indicates that Al readily donates electrons to Ru and Cu, thereby enhancing the electron density and catalytic activity of the latter species. DFT calculations demonstrate that MP‐Cu₂O/RuAl enhances the adsorption of Li, CO₂, and Li₂CO₃ while substantially lowering the charge reaction energy barrier. This effectively accelerates reaction kinetics and improves battery reversibility.

Furthermore, Mott‐Schottky heterojunctions constructed from metal‐semiconductor materials can regulate reaction pathways and enhance catalytic activity by inducing interfacial energy band bending and charge transfer based on the E _F differences [134]. As mentioned earlier, some Mo‐based catalysts can stabilize the two‐electron discharge product Li₂C₂O₄ during the CO₂RR process. To address the poor conductivity inherent in Mo‐based compounds due to their semiconductor nature, Wu et al. synthesized Mo₃P/Mo nanorods Mott‐Schottky heterojunction catalysts featuring a rich porous structure. Controlled synthesis of the Mo₃P/Mo ratio was achieved by varying phosphorus content and reaction time [142]. Driven by the difference in E _F, electron redistribution occurs at the interface. Both Bader charge analysis (Figure 12a) and differential charge density calculations indicate that electrons spontaneously migrate from Mo to Mo₃P and accumulate on the Mo₃P side of the interface, establishing an internal electric field. The construction of the Mott–Schottky heterojunction significantly accelerates electron transport. The formation of Mo–O coupling bridges between the catalyst and Li₂C₂O₄ effectively promotes the reversible formation and decomposition of discharge products. As a result, the assembled Li‐CO₂ battery exhibited a low charging potential of 2.92 V at 50 mA g⁻¹, with a polarization voltage of only 0.11 V.

FIGURE 12.

(a) Structural diagrams and atomic Bader charge analysis of Mo, Mo₃P, and the Mo₃P/Mo Mott‐Schottky heterojunction [142]. Copyright 2023, Wiley‐VCH GmbH. (b) Schematic diagram of the hetero‐interface between NiS₂ and FeS₂ in the hollow NiS₂/FeS₂‐NSGA cathode catalyst [143]. Copyright 2022, Elsevier B.V. (c) Schematic diagram of the catalytic mechanism of Mn₂O₃‐Mn₃O₄ [144]. Copyright 2020, American Chemical Society. (d) Schematic diagram of the electrochemical mechanism of Mo₂N‐ZrO₂@NCNF [145]. Copyright 2023, Wiley‐VCH GmbH. (e) Cross‐section SEM image of CPM‐0.1MnO₂. (f) Optimized structures and corresponding adsorption energies of Li₂CO₃ on M‐MnO₂ (left) and MnO₂ (right) [57]. Copyright 2023, Elsevier B.V. (g) Schematic diagram of the CO₂RR and CO₂ER processes on the Co/NSCDs@CNTs cathode [146]. Copyright 2025, Wiley‐VCH GmbH.

4.3.3. Metal Compound/Metal Compound Interface

For heterointerfaces composed of metal compound‐metal compound systems, similarly, charge density redistribution at the interface can effectively enhance the anchoring of CO₂ and intermediates [143, 148, 149]. As a representative case, Jin et al. employed electrostatic stacking to construct a hollow NiS₂/FeS₂ heterostructure catalyst supported on nitrogen/sulfur co‐doped graphene aerogel (NiS₂/FeS₂‐NSGA) [143]. The hierarchical 3D porous architecture of NSGA facilitates mass transport and enhances surface area, while the hollow NiS₂/FeS₂ heterostructures (approximately 80–100 nm in size) provide abundant interfacial active sites (Figure 12b). Analysis of the interfacial electronic structure indicates a charge density rearrangement, with electrons transferring from FeS₂ to NiS₂, which facilitates interfacial charge transport. Additionally, the d‐band center of the NiS₂(210)/FeS₂(100) interface (−2.83 eV) shifts toward the E _F compared to FeS₂(100) (−3.37 eV) and NiS₂(210) (−3.05 eV), promoting chemical bonding with the intermediate product. Furthermore, by leveraging the orbital coupling of asymmetric active units in metal compounds, in addition to constructing active sites as previously described, this cascade orbital hybridization strategy has also been employed to design atomic‐scale asymmetric heterointerfaces. Deng et al. introduced Ce into the spinel structure of Co₃O₄ to occupy the octahedral sites. Through the atomic‐scale asymmetric Co–O–Ce active units, a continuous 3d–2p–4f gradient orbital coupling was established at the interface [150]. HRTEM results confirmed the formation of coherent/semi‐coherent interfaces at the phase boundary between CeO₂ and Co₃O₄. Subsequent experimental characterizations and theoretical calculations consistently demonstrated that electron delocalization at the interface facilitates highly efficient electron transfer from Ce to Co, increasing the oxidation state of surface Co species and realizing efficient interfacial charge transfer. Consequently, compared to pristine Co₃O₄, the adsorption energies of key intermediates (*CO₂, *Li₂C₂O₄, *Li₂CO₃) are significantly optimized.

Based on the ensemble effect of heterogeneous interfaces, engineered interfacial structures in Li‐CO₂ batteries enable distinct reaction steps to occur at adjacent catalytic sites, synergistically accelerating the overall catalytic process. This cooperative mechanism is particularly advantageous for the multi‐step electrochemical reactions involved in CO₂RR and CO₂ER. As exemplified, Liu et al. designed and in situ synthesized sea urchin‐like Mn₂O₃‐Mn₃O₄ nanocomposites to explore the synergistic effects between Mn₂O₃ and Mn₃O₄ during battery operation [144]. HRTEM images revealed interplanar spacings of 0.492 and 0.271 nm, corresponding to the Mn₃O₄(101) and Mn₂O₃(222) planes, respectively. The intimately distributed Mn₂O₃ and Mn₃O₄ nanoparticles formed close biphasic interfaces, effectively combining their respective catalytic advantages to enhance CO₂RR and CO₂ER kinetics. Adsorption energy calculations demonstrated that Mn₃O₄ efficiently promotes CO₂ molecule adsorption and activation. The subsequently formed *CO₃ intermediate stabilizes more readily on the Mn₂O₃ surface, further inducing the nucleation of Li₂CO₃. This deposition behavior offers the following advantages (Figure 12c): (i) Inducing Li₂CO₃ deposition on Mn₂O₃ fully exposes the CO₂RR catalytic sites on Mn₃O₄. (ii) Compared to Mn₃O₄, Mn₂O₃ exhibits stronger interaction with *CO₃ while destabilizing CO₂ adsorption, favoring Li₂CO₃ decomposition. (iii) The uniform distribution of Mn₂O₃ within Mn₂O₃‐Mn₃O₄ promotes homogeneous Li₂CO₃ deposition across the catalyst surface, preventing aggregation and pore blockage. A similar design strategy was applied to optimize Mo‐based catalysts for stabilizing Li₂C₂O₄. For instance, while MoN exhibits relatively poor CO₂ adsorption capability, ZrO₂ effectively enhances CO₂ adsorption, enabling smooth CO₂ conversion. Building on this, Cheng et al. engineered a bifunctional Mo₂N‐ZrO₂ heterostructure within freestanding conductive carbon nanofibers (Mo₂N‐ZrO₂@NCNF) [145]. TEM analysis confirmed the uniform distribution of Mo₂N/ZrO₂ heterojunction nanoparticles (average size: 4.13 ± 0.20 nm) on the carbon nanofiber surfaces. This architecture integrates dual advantages (Figure 12d) of stabilizing intermediate discharge products (via Mo₂N) and catalyzing CO₂ conversion (via ZrO₂). The assembled Li‐CO₂ battery achieved an ultralow overpotential of 0.32 V, high energy efficiency (89.8%), and stable discharge/charge cycling exceeding 400 cycles at 50 µA cm⁻².

4.3.4. Other Interface Configurations

Some non‐metallic conductive materials (such as CNTs [151], MXene [152]) have also been used as support substrates to construct hetero‐interface composites. Such structures can promote rapid electron transport from the catalyst surface to Li₂CO₃, thereby improving electrode reaction kinetics, and are also beneficial for enhancing the stability of catalytic materials. For example, to solve the problems of poor conductivity and unfavorable electron/ion transport associated with the transition metal oxide MoO₃, Chen et al. constructed MoO₃‐coated CNT (MoO₃@CNT) core/shell array catalysts [151]. The high‐conductivity network of CNTs provided abundant electron transport channels, and the highly uniform deposition of MoO₃ improved the catalytic activity and stability of the entire electrode. Additionally, Hu et al. prepared 1D parallel‐aligned nanotubes (MNT) formed by 2D Ti₃C₂T_x MXene/carbon heterostructures via a self‐sacrificial template method [152]. DFT calculations revealed the interaction between Ti₃C₂T_x MXene and graphene, in which the E _F of graphene is much higher than that of MXene, leading to electron migration from graphene to the MXene surface, thereby optimizing the local charge distribution and promoting catalytic reactions. The nanotube morphology of MNT also endows it with a high specific surface area, which is conducive to catalysis at the interface between gas, electrolyte, and electrode.

Moreover, engineering hierarchical architectures to leverage the synergistic coupling among multiple interfacial components has been demonstrated as a potent strategy for the design of catalysts. For instance, Wang et al. constructed a hierarchical architecture by uniformly distributing MnO₂ nanosheets on carbon paper (CP)‐supported MXene (CPM‐MnO₂) [57]. Here, CP acts as a 1D substrate/core, MXene as an internal dielectric layer, and MnO₂ as both a catalyst and an antioxidative outer coating. Cross‐section SEM imaging (Figure 12e) reveals CPM‐0.1MnO₂ exhibiting a distinct gradient morphology. According to FT‐IR results, the –OH characteristic peak at 1350 cm⁻¹ in the CPM‐0.1MnO₂ material exhibits a blue shift, indicating strong hydrogen bonding interactions between MXene and MnO₂ components. Adsorption energy calculations (Figure 12f) demonstrate that M‐MnO₂ possesses a stronger adsorption affinity for Li₂CO₃ (E _ads = −6.044 eV) than bare MnO₂ (E _ads = −5.709 eV). Further electronic structure analysis indicates that efficient charge transfer is facilitated by –OH⋯O hydrogen bridging originating from Mxene's –OH groups to MnO₂. This charge injection significantly narrows the insulating Li₂CO₃ bandgap, imparting metal‐like conductivity and substantially lowering the decomposition barrier. Consequently, the Li‐CO₂ battery with a CPM‐MnO₂ cathode achieved exceptional long‐term stability of 1087 cycles (4348 h) at 200 µA cm⁻², along with a low polarization voltage of approximately 0.47 V. As another example, utilizing the thiophilicity of Co and the catalytic efficacy of S, Pan et al. proposed an in situ interface programming strategy [146]. This design used N, S‐doped carbon dots (N, S‐CDs) as a sulfur source and structure‐directing agent to induce the directional enrichment of sulfur on the Co surface. This process constructed a carbon coating outer layer and a Co–S rich interface layer in situ on the Co surface, finally forming a clear hierarchical structure (Co/NSCDs@CNTs) consisting of an inner metallic Co layer, an intermediate Co–S rich interface, and an outer amorphous carbon/CNT network. Combined XAS/EXAFS characterization results indicate that the formation of the Co–S interface layer reconstructed the local geometric coordination environment, effectively lowering both the coordination number and coordination symmetry of Co due to local sulfidation. The introduction of the Co–S interface layer caused an upshift of the Co d‐band center, enhancing the adsorption capacity for CO₂, Li⁺, and Li₂CO₃. DOS analysis confirmed significant orbital overlap between S and adsorbed Li₂CO₃, indicating that the sulfur‐rich interface layer promotes electron transfer between the catalyst and Li₂CO₃ molecules, thereby significantly lowering its decomposition barrier. Benefiting from this dual regulation of geometric and electronic structures (Figure 12g), the catalyst showed superior electrochemical performance, including a high discharge capacity of 33 767 mAh g⁻¹, while maintaining excellent cycling stability with a charge potential consistently below 3.42 V.

In short, the engineering of heterostructures represents a pivotal strategy for constructing electrocatalytically active interfaces. It can create new catalytic sites and enhance interfacial charge transfer kinetics by tailoring the interfacial electronic structure and lattice strain. Beyond merely expanding the material systems used to construct heterojunction interfaces, the synergistic coupling and precision design of multi‐interface architectures are garnering increasing research focus. Furthermore, how to maximize the exposure of interfacial active sites through nanostructure engineering is also an important development direction in this field. A complex interfacial environment implies stronger ensemble effects and greater electronic synergism among multiple components, which requires further in‐depth analysis of each catalyst component and the interface structure.

5. Conclusions and Perspectives

Amid global carbon neutrality initiatives, Li‐CO₂ batteries, as a prominent member of metal‐air batteries, hold substantial application potential owing to their ultrahigh theoretical energy density and unique CO₂ fixation capability. This review focuses on the influence of electronic structure on the reaction mechanisms and conversion pathways during discharge‐charge processes in Li‐CO₂ batteries, as well as the role of various atomic structures in regulating the electronic structure and catalytic performance of cathode catalysts. The intrinsic catalytic activity of materials is typically determined by analyzing the structure‐activity relationship between their practical catalytic efficiency and electronic properties. An in‐depth understanding of the connection between the electronic structure of catalysts, adsorption characteristics, and CO₂RR/CO₂ER pathways is essential for designing cathodes with high selectivity and stability, and for elucidating the correlations among structure, performance, and reaction mechanism. On the other hand, the often indescribable “synergistic effects” arising from the intrinsic structural complexity of heterogeneous catalysts make the construction of cathodic catalysts with well‐defined structural features and uniform active sites equally imperative for developing high‐efficiency electrocatalysts. Table 1 summarizes the specific micro‐electronic structure modulation methods employed in recent representative advanced catalysts, alongside their corresponding macro‐electrochemical metrics and design strategies. Although significant progress has been made in both material development and mechanistic understanding, a considerable gap remains before efficient and cost‐effective cathode catalysts can be realized. Here, we propose several promising directions for future research (Figure 13a–d).

TABLE 1.

Performance of state‐of‐the‐art Li‐CO₂ cathode catalysts.

Regulation strategy	Catalyst	Electronic structure criteria	Discharge capacity/current density	Cycle performance/current density	Overpotential /current density	Refs.
Active Sites	BCNT	p–π conjugation electron redistribution	1311 µAh cm−2/40 µA cm−2	>1300 h/20 µA cm−2	—	[78]
	NiO‐Vo NAs/CT	New electronic states near E f	11 231.5 mAh g−1/100 mA g−1	159 cycles/100 mA g−1	1.3 V/100 mA g−1	[80]
	SV‐CoS	Lower S 2p band center	7790.6 µAh cm−2/50 µA cm−2	> 400 h/20 µA cm−2	0.43 V/20 µA cm−2	[67]
	Vs‐Co2CuS4	Elevate S 2p band center	1948 µAh cm−2/50 µA cm−2	> 600 h/20 µA cm−2	0.73 V/20 µA cm−2	[39]
	SACr@NG	Higher d‐band center	> 1500 µAh cm−2/50 µA cm−2	> 350 cycles/100 µA cm−2	< 1.5 V/(from 20 to 100 µA cm−2)	[95]
	Ni0.1Co2.9O4	High‐spin state	2824 µAh cm−2/50 µA cm−2	> 400 h/50 µA cm−2	0.72 V/20 µA cm−2	[21]
	MnS	d–p orbital hybridization	19 782 mAh g−1/100 mA g−1	> 430 cycles/500 mA g−1	1.28 V/100 mA g−1	[153]
	Co3O4	Low e g orbital occupancy	2407 µAh cm−2/50 µA cm−2	> 600 h/20 µA cm−2	1.0 V/20 µA cm−2	[22]
	Cu‐Co3O4	d‐orbital spin splitting	6951.0 µAh cm−2/50 µA cm−2	> 800 h/20 µA cm−2	0.73 V/20 µA cm−2	[92]
	tp‐NiS	d‐orbital spin splitting	—	> 1800 h/20 µA cm−2	0.95 V/10 µA cm−2	[93]
	Cu0.9Cr0.1/N‐HCNs	Electron transfer from Cr to Cu	23 928 mAh g−1/100 mA g−1	338 cycles/100 mA g−1	0.8 V/ 100 mA g−1	[110]
	Cu3 [Co(CN)6]2	Electron transfer from the Co to Cu through CN bridge	14 858 mAh g−1/300 mA g−1	> 1480 h/300 mA g−1	1.18 V/300 mA g−1	[104]
	CoPt	Downshift d‐band center	2.41 mAh cm−2/20 µA cm−2	218 cycles/50µA cm−2	0.26 V/20 µA cm−2	[47]
	HEAOs	d–d/d–p orbital hybridization via M−O−M	6764.82 µAh cm−2/40 µA cm−2	> 430 cycles/100 µA cm−2	∼0.55 V/20 µA cm−2	[106]
	HE‐PBA	Upshift d‐band center	19 317 mAh g−1/100 mA g−1	> 1500 h/100 mA g−1	—	[107]
Surface Morphology and Structure	HOF‐FJU‐1‐Ru@CNT	Upshift d‐band center	24 245 mAh g−1/100 mA g−1	> 1800 h/400 mA g−1	1.09 V/100 mA g−1	[28]
	P‐CoSe2@NC	Increased high‐spin state fraction; Shifted d‐band center on different facets	17 395.6 mAh g−1/200 mA g−1	> 600 cycles/1000 mA g−1	1.53 V/200 mA g−1	[128]
	Ti3C2Tx ‐500	Upshift d‐band center	15 740.38 mA h g−1/100 mA g−1	> 700 h/200 mA g−1	1.98 V/200 mA g−1	[131]
	Zn‐Ti3C2Cl2	Upshift d‐band center	18 529.7 mA h g−1/200 mA g−1	> 1500 h/200 mA g−1	0.72 V/200 mA g−1	[129]
	Nb2CClx	Upshift d‐band center	15 521.2 mAh g−1/200 mA g−1	> 1570 h/200 mA g−1	1.50 V/200 mA g−1	[133]
Interface Structure	Ni/Ru HNPs	Downshift d‐band center	∼9700 mA h g−1/200 mA g−1	> 1200 h/200 mA g−1	0.88 V/200 mA g−1	[138]
	Rufcc+hcp/KB	Electron transfer from fcc phase to hcp phase	23 517 mA h g−1/100 mA g−1	> 2260 h/100 mA g−1	0.73 V/100 mA g−1	[140]
	Mo3P/Mo	Electron transfer from Mo to Mo3P	10 577 mAh g−1/50 mA g−1	> 300 h/250 mA g−1	0.11 V/50 mA g−1	[142]
	NiS2/FeS2‐NSGA	Electron transfer from FeS2 to NiS2; Upshift d‐band center	21 178 mAh g−1/0.3 A g−1	> 127 cycles/1 A g−1	1.10 V/0.1 A g−1	[143]
	Ce‐Co3O4	4f–2p–3d orbital coupling; Electron transfer from Ce to Co	14 659 mAh g−1/100 mA g−1	> 183 cycles/100 mA g−1	1.37 V/100 mA g−1	[150]
	MNT	Electron transfer from graphene to the MXene surface	11 458 mAh g−1/500 mA g−1	—	1.38 V/0.2 A g−1	[152]
	CPM‐0.1MnO2	Electron transfer from MXene to MnO2 via hydrogen bonds	4500 µAh cm−2/40 µA cm−2	> 1087 cycles/200 µA cm−2	∼0.47 V/200 µA cm−2	[57]
	Co/NSCDs@CNTs	Upshift d‐band center	33 767 mAh g−1/100 mA g−1	> 400 h/200 mA g−1	0.49 V/50 mA g−1	[146]

FIGURE 13.

Perspectives on heterogeneous cathode catalysts for highly efficient Li‐CO₂ batteries.

5.1. Material Fundamentals Remain Paramount

Despite the continuous emergence of novel materials, such as various high‐entropy materials, 2D MXenes, graphdiyne, and organic framework materials, that have been explored as cathode catalysts for Li‐CO₂ batteries, it is equally vital to persist in understanding their intrinsic electrocatalytic enhancement mechanisms. This entails deliberate construction of active sites and rational structural design. Unlike isolated electrocatalytic systems, battery operation involves cycling between low discharge and high charge voltages, inevitably inducing reconstruction of catalytic sites and structural evolution during practical cycling. The impact of such dynamic transformations (beneficial or detrimental) on electrochemical performance warrants rigorous investigation. For instance, Liu et al. observed that partial oxidation of CoS₂ during cycling enhanced its catalytic activity [91]. Liu et al. demonstrated that electrochemical reconstruction of Mn(II)/Mn(III) redox couples created distinct active sites for CO₂RR and CO₂ER, respectively [103]. Beyond activity considerations, enhancing the inherent electrochemical stability of materials constitutes a crucial consideration for developing practical electrocatalysts.

5.2. The Application of In Situ/Operando Characterization Techniques

The discharge‐charge reactions of Li‐CO₂ batteries involve multiple intermediates and products. In the initial stages, due to the lack of effective in situ/Operando characterization techniques, only the overall reaction could be elucidated. While theoretical calculations predicted the existence of various intermediates, direct evidence remained scarce. In recent years, advances in in situ/Operando characterizations have enabled the direct identification of key intermediates and discharge products formed during Li‐CO₂ battery reactions. For example, SERS has provided direct evidence for the presence of intermediates, including *CO₂ ⁻, *CO, and *Li₂CO₃ on material surfaces [11, 36]. Similarly, the existence of the discharge product Li₂C₂O₄ was proven using in situ IR spectroscopy [113], and the coexistence of Li₂C₂O₄ and Li₂CO₃ was quantitatively delineated through time‐of‐flight secondary ion mass spectrometry (TOF‐SIMS) [105]. For the charging process, techniques such as GC‐MS [9] and DEMS‐GC [50] have been employed to monitor gas evolution. Additionally, characterization techniques such as in situ AFM and LCM‐DIM have also been employed to investigate conversion processes at the electrode‐electrolyte interface [58]. The synergistic combination of advanced characterization techniques with theoretical calculations will facilitate the identification of key intermediates in the CO₂RR and CO₂ER processes. This integrated approach will also advance research into material structures and mechanisms, offering new insights into catalytic mechanisms.

5.3. Establishing Universal Descriptors for CO₂RR/CO₂ER

Research on catalytic processes in this field still lags behind material synthesis. Currently, most synthesis work is not guided by catalytic principles, and consequently, catalyst modifications do not invariably enhance battery performance. Although adsorption energy can effectively describe the activity for specific reactions on given materials, its computational complexity hinders its utility in guiding the rational design of new catalysts, particularly in the context of machine learning and AI‐driven high‐throughput screening. Therefore, it is imperative to establish criteria for rapidly evaluating the catalytic properties of specific structures. Beyond the standard d‐band center, refined models applicable to specific systems, such as the double d‐band center model incorporating electron spin [128], as well as novel descriptors, including ${\phi }_{{\mathrm{x}}^2-{\mathrm{y}}^2}$ [55], $\phi ={\epsilon }_{{\mathrm{d}}_{\mathrm{xy}}}+{\epsilon }_{{\mathrm{d}}_{\mathrm{yz}}}$ [154], ξ [155] and Φ_TMC [156] have been proposed for certain material structural systems. These descriptors demonstrate superior linear correlations with Li₂CO₃ decomposition energy. Their relatively accessible parameter values significantly enhance the efficiency of material screening. Research focused on these descriptors facilitates the design of highly selective electrocatalysts and provides a theoretical foundation for the practical synthesis of materials. Furthermore, many potential influencing factors, such as multi‐species adsorption on catalytic surfaces, solvent effects, and electrochemical reaction kinetics, are often overlooked in activity predictions, which may lead to considerable discrepancies between computational and experimental results. Effectively integrating theoretical calculations with experimental validation is also crucial for improving computational accuracy and identification efficiency.

5.4. Advancing Practical Applications

Although laboratory‐scale experiments have demonstrated the immense potential of Li‐CO₂ batteries, translating these research achievements to a practical, application‐level scale remains a formidable challenge. Specifically, when scaling up laboratory‐level atomic‐scale design strategies to practical systems, issues such as cost, manufacturing processes, and mass transfer under high mass loadings become indispensable factors that must be taken into consideration. Furthermore, the majority of laboratory evaluations involve mixing catalysts with inactive binders and conductive additives, which are subsequently coated onto brittle carbon paper. The incorporation of binders and conductive additives inevitably conceals a portion of the active sites, thereby restricting the intrinsic catalytic activity and specific energy density of the catalysts. This conventional fabrication method also renders the resulting electrodes rigid and undeformable, failing to satisfy the structural requirements of flexible electronic devices. With the gradual shift of consumer electronics such as mobile phones and wearable devices toward flexible form factors, the market demand for flexible batteries is steadily increasing. Binder‐free, self‐supporting electrodes, owing to their robust mechanical properties, superior energy density, and enhanced cycling stability, are increasingly being reported. Additionally, the deployment of liquid‐free electrolyte systems (e.g., solid‐state or quasi‐solid‐state electrolytes) can theoretically circumvent the risks of leakage, volatilization, and decomposition associated with liquid electrolytes operating in semi‐open systems. However, compared to liquid electrolytes, the newly established solid‐solid interface between the electrolyte and the electrode intrinsically exacerbates charge transfer resistance. Moreover, unlike in liquid electrolyte systems, the existential states and transport mechanisms of Li⁺ and intermediates undergo a fundamental transformation. This paradigm shift likely exerts a decisive influence on the reaction mechanism and interfacial evolution within solid‐state batteries; unfortunately, research in this domain remains relatively limited [54, 157]. Concurrently, existing literature predominantly focuses on room‐temperature environments. The development of Li‐CO₂ batteries tailored for diverse operating environments will unlock broader potential in aerospace exploration and other demanding scenarios. This will pave the way for broader adoption of Li‐CO₂ batteries.

To sum up, clarifying the reaction mechanisms and understanding the intrinsic properties of catalysts are paramount for advancing the development of high‐efficiency Li‐CO₂ batteries. This review offers insights and inspiration for the design of highly active electrocatalysts and mechanistic studies for Li‐CO₂ batteries, thereby contributing to the advancement of Li‐CO₂ battery technology.

Conflicts of Interest

There are no conflicts of interest to declare.

Data Availability Statement

No primary research results, software or code have been included and no new data were generated or analysed as part of this review.