Fast‐Kinetic and Stable Li‐CO₂ Batteries Driven by an Oxygen‐Defective Cu₂O‐ZnNb₂O₆ Catalyst Through d‐p‐d Orbital Hybridization

ABSTRACT

Lithium‐carbon dioxide (Li‐CO₂) batteries provide a distinctive energy storage route by directly capturing and converting CO₂. However, their development is plagued by sluggish reaction kinetics and poor cycling stability. Although catalysts are indispensable to mitigate these issues, achieving both low polarization and durable high‐rate cycling remains a longstanding challenge. Here, we engineer a cost‐effective copper (I) oxide (Cu₂O)‐zinc niobate (ZnNb₂O₆) heterostructured catalyst featuring oxygen vacancies and strong d‐p‐d interfacial orbital hybridization to overcome these bottlenecks. Multimodal in situ spectroscopies, synchrotron characterization, and computational studies reveal that this configuration constructs enriched active sites and continuous interfacial charge‐transfer channels, promoting the formation of finely dispersed lithium carbonate (Li₂CO₃) with elongated Li─O bonds. These synergetic effects accelerate CO₂ conversion kinetics and reduce the decomposition barrier of discharge products, enabling highly reversible cycling even under high rates. The resulting Li‐CO₂ battery thus operates stably for 5400 h at 100 mA g⁻¹ and maintains a low overpotential of 1.6 V at 1000 mA g⁻¹, with a pouch cell retaining robust cyclability over 484 cycles at the same high rate, demonstrating record performance that surpasses prior reported results. This work opens a promising pathway for designing high‐performance catalysts for sustainable batteries with robust performance.

A strong d‐p‐d orbital hybridized V_O‐Cu₂O‐ZnNb₂O₆ catalyst is designed to simultaneously modulate interfacial active sites and discharged products, enabling Li‐CO₂ battery to deliver long‐term cycling stability of 5400 h, maintain low voltage polarization at 1000 mA g⁻¹, and sustains 484 stable cycles in a pouch cell at the same high rate, which offers a promising strategy for designing high‐performance catalysts toward sustainable battery systems.

1. Introduction

Carbon capture and utilization (CCU) has emerged as a pivotal strategy for reducing carbon dioxide (CO₂) emissions and mitigating global warming [1, 2]. Among various CCU routes including electrochemical reduction, photocatalytic reduction, and metal‐CO₂ batteries, lithium‐CO₂ (Li‐CO₂) batteries serve as an integrated platform that combines energy storage with CO₂ utilization, enabling a direct link between carbon cycling and electrochemical processes [3, 4, 5, 6, 7, 8, 9, 10, 11, 12]. Despite a high specific energy (theoretically up to 1876 Wh kg⁻¹), practical Li‐CO₂ batteries face critical hurdles. The sluggish kinetics of the complex solid–liquid–gas triphasic reactions at the electrode severely limit rate capability [13, 14]. Moreover, the widely accepted reaction pathway (4Li⁺ + 3CO₂ + 4e⁻ ↔ 2Li₂CO₃ + C) involves the formation of aggregated, chemically and electrochemically inert discharged products, primarily lithium carbonate (Li₂CO₃), which subsequently block the active sites of the catalytic cathode [15, 16]. Additionally, the decomposition of these products faces a high thermodynamic barrier, often requiring charging voltages above 4.3 V [17, 18]. Such high operating voltages induce severe detrimental effects, including electrolyte decomposition, aggravated side reactions, and electrode degradation. Consequently, the battery system suffers from low Coulombic efficiency (CE), significant overpotential, poor reversibility, and a markedly shortened cycle life.

Despite the promising potential of diverse catalysts, including noble metals, carbon‐based materials, and transition metal‐based compounds, in addressing the key problems faced by Li‐CO₂ batteries, their practical development remains constrained by inherent limitations [19, 20, 21]. Noble‐metal catalysts are expensive and often suffer from inadequate cycling stability (typically fewer than 100 cycles). Carbon‐based catalysts generally lack sufficient activity to drive efficient and reversible CO₂ conversion and Li₂CO₃ decomposition [22, 23]. Transition metal‐based catalysts, although cost‐effective and tunable, frequently operate with large overpotentials under high‐rate conditions. Recently, niobate‐based materials such as zinc niobate (ZnNb₂O₆, denoted as ZNO) have attracted attention in fast‐charging Li⁺‐ion batteries and CO₂ conversion reaction due to their stable oxygen polyhedral frameworks and modifiable surface electronic properties [24]. First, Nb sites exhibit strong oxophilicity, providing active centers for CO₂ adsorption and polarization [25, 26]. Second, their Lewis acidic sites and adjustable band structures can be tailored via doping and surface engineering to enhance catalytic activity and interfacial charge transfer [27]. Third, ZNO crystallizes in an open columbite structure with alternating [ZnO₆] and [NbO₆] octahedron, which supports Li⁺ migration and could facilitate CO₂ transport with rational nanostructure design [28, 29]. Nevertheless, ZNO has rarely been studied in Li‐CO₂ batteries. Its low intrinsic electronic conductivity and limited active sites utilization may restrict catalytic performance in such systems. More importantly, its specific roles in CO₂ reduction and Li₂CO₃ formation/decomposition remain unclear. Since the discharge products critically govern battery kinetics, developing efficient strategies to regulate them through tailored catalyst electronic structures is essential for high‐performance Li‐CO₂ batteries. To date, however, systematic studies in this direction remain scarce.

In this study, we present the design of an oxygen vacancy‐enriched, Cu⁺ stabilized ZNO (denoted as V_O‐CZNO heterostructure) with strong d‐p‐d orbital hybridization, which simultaneously enables interfacial active site engineering and discharged products regulation. The combination of heterostructure engineering and vacancy introduction facilitates accelerated charge transfer and an increased density of active sites, thereby collectively enhancing catalytic activity. Through a synergistic approach combining in situ spectroscopies, synchrotron characterization, and theoretical simulations, we identify that the hybridized Cu‐O‐Nb interface establishes an efficient d‐p‐d charge‐transfer pathway. This interfacial electronic hybridization reconstructs Nb 4d electronic states and induces favorable charge redistribution at the catalyst‐product interface, thereby enhancing CO₂ activation, weakening the Li─O bonds in discharge products, and accelerating Li⁺ transport, all of which facilitate reversible Li₂CO₃ decomposition even under high current density. As a result, the V_O‐CZNO‐based Li‐CO₂ battery sustains stable operation for 5400 h at 100 mA g⁻¹. Even at a high rate of 1000 mA g⁻¹, a low overpotential of 1.6 V can be retained and a corresponding Li‐CO₂ pouch cell achieves 484 cycles, confirming its outstanding kinetics and practical viability. Moreover, the designed catalyst is successfully extended to Na‐CO₂ systems, demonstrating the broad applicability of the d‐p‐d orbital hybridization strategy across other metal‐CO₂ batteries.

2. Results and Discussion

2.1. Formation Mechanism and Characterizations of the V_O‐CZNO Catalyst

The formation mechanism involves a multistep coupled process centered on the synergistic transformation and interfacial reconstruction between Cu⁺‐zeolitic imidazolate framework‐8 (Cu‐ZIF‐8) and Nb₂O₅. Figure S1 schematically illustrates the preparation of the V_O‐CZNO catalyst via a template‐confined, diffusion‐driven transformation under a reducing atmosphere. First, Cu‐ZIF‐8 was synthesized using ZIF‐8 as a template and retained a similar morphology (Figure S2), indicating that Cu doping did not alter the crystal habit or induce structural degradation. The observed increase in particle size is attributed to the suppression of ZIF‐8 nucleation by Cu²⁺ introduction, which promotes crystal growth [30]. X‐ray diffraction (XRD) patterns of Cu‐ZIF‐8 exhibit no peak shifts and additional phases compared to pristine ZIF‐8 (Figure S3), verifying the preservation of the parent sodalite (SOD) topology [30, 31]. The absence of detectable lattice distortion by XRD, along with the similar ionic radii of Cu²⁺ (0.73 Å) and Zn²⁺ (0.74 Å), strongly suggests that Cu²⁺ substitutes Zn²⁺ ions at the tetrahedral metal sites [32, 33]. Subsequently, the Cu‐ZIF‐8 precursor was mixed with Nb₂O₅. Under a high‐temperature reducing atmosphere, Nb₂O₅ undergoes partial surface reduction, converting Nb⁵⁺ to Nb⁴⁺ and generating abundant V_o, thereby enhancing surface reactivity [34]. Meanwhile, Zn²⁺ species volatilized from Cu‐ZIF‐8 diffuse toward the activated Nb₂O₅ and undergo an in situ solid‐state reaction, crystallizing into ZNO micro‐rod frameworks [35]. In this process, Zn participates in forming ZnNb₂O₆, while the Nb₂O₅ matrix stabilizes Cu⁺ as Cu₂O by preventing excessive reduction, ultimately enabling the construction of the V_O‐CZNO heterointerface [36]. Figure 1a presents a schematic structure of the V_O‐CZNO. For comparison, a ZNO catalyst (Figure S4) was prepared under identical conditions using pristine ZIF‐8 instead of Cu‐ZIF‐8.

FIGURE 1.

Formation mechanism and structural characterizations of V_O‐CZNO. (a) Schematic illustration of the V_O‐CZNO. (b) SEM images of V_O‐CZNO and EDS elemental mapping of (c) Zn, (d) Nb, (e) O, and (f) Cu. (g) TEM image and (h) HRTEM images of V_O‐CZNO. (i) Rietveld refinement of V_O‐CZNO. (j) FTIR spectra and (k) Raman spectra of V_O‐CZNO and ZNO. (l) N₂ adsorption–desorption isotherms and pore size distribution (inset) of V_O‐CZNO.

The SEM image (Figure 1b) shows that the heterostructured catalyst largely retains the rod‐like morphology of the Nb₂O₅ precursor with increased surface roughness and residual ZIF‐8 frameworks, likely due to intense interfacial reactions and Cu incorporation. EDS elemental mapping (Figure 1c–f) confirms the homogeneous distribution of Zn, Nb, Cu, and O. TEM images further verify the rod‐like structure (Figures 1g and S5a). High‐resolution (HR)TEM images (Figure S5b,c, taken from the purple‐ and orange‐boxed regions in Figure 1g, respectively) reveal distinct lattice fringes corresponding to the (131) plane of ZnNb₂O₆ and the (200) plane of Cu₂O, confirming the successful formation of the heterostructured composite. Additionally, a residual N‐doped carbon (NC) framework derived from the carbonization of the 2‐methylimidazole organic ligands remains intimately interconnected with the V_O‐CZNO structure (inset in Figure 1g, marked by the white‐boxed region), contributing to the preservation of the overall morphology even at high temperatures (Figure S6) [33]. Figure 1h shows a disordered interfacial region between Cu₂O and ZnNb₂O₆ (marked in yellow), indicative of the heterojunction formation, which may provide additional active sites for Li⁺‐storage and facilitate charge transfer. Notably, compared with the precursor, V_O‐CZNO exhibits a roughened surface, disordered interfacial regions, and an enriched porous structure. These features provide abundant active sites and accelerate the interphase charge transfer, which are believed to facilitate the electrochemical reaction kinetics and enhance battery performance [19, 21].

The phase structures of the as‐synthesized samples were first characterized by XRD. As shown in Figure S7, all diffraction peaks of both V_O‐CZNO and ZNO correspond well to the ZnNb₂O₆ phase (PDF#00‐037‐1371). Notably, an additional weak peak at 2θ = 42.3° appears in the V_O‐CZNO pattern, which is indexed to the (200) plane of Cu₂O (PDF#98‐000‐0186). Rietveld refinement of the XRD pattern for V_O‐CZNO (Figure 1i) yielded excellent reliability factors (Rwp = 6.89%; Rp = 5.21%), indicating a high‐quality structural fit [37]. Based on the refinement, the phase contents are estimated to be 84.4 wt% for ZNO and 15.6 wt% for Cu₂O. Importantly, refinement also quantitatively confirms the presence of V_O within the ZnNb₂O₆ lattice at a concentration of 4.3%, which may provide abundant active sites conductive to CO₂ adsorption and Li⁺ migration [38]. A schematic model of the Vo‐containing ZnNb₂O₆ structure is presented in Figure S8. Fourier‐transform infrared (FTIR) spectroscopy (Figure 1j) further confirms the characteristic Nb─O bond vibrations (blue region) and mixed Zn─O/Nb─O vibrations (green region) in both V_O‐CZNO and ZNO [29]. The broad spectral features observed between 1800 and 1300 cm⁻¹ are attributed to organic residues or oxygen‐containing functional groups adsorbed on or formed at the sample surface during measurement, rather than intrinsic lattice vibrations. In addition, the peak at 1629.5 cm⁻¹, corresponds to the H─O─H bending vibration (δH₂O) of adsorbed water, showing significantly higher intensity in V_O‐CZNO than in ZNO, likely due to enhanced water adsorption promoted by oxygen vacancies [39]. Raman spectroscopy (Figure 1k) provides further evidence for the formation of a ZnNb₂O₆‐type mixed oxide. The strong and sharp band near 900 cm⁻¹ (purple region) is attributed to the symmetric bridging Nb═O vibration. Bands around 483.9 and 529.0 cm⁻¹ (blue region) are assigned to Nb─O stretching modes, while features below 400 cm⁻¹ (green region) arise from Zn─O and Nb─O vibrations [40]. All characteristic Raman peaks of the V_O‐CZNO catalyst exhibit a clear red shift relative to pure ZNO, which originates from lattice expansion induced by interfacial charge transfer and reduced bond covalency due to the occupation of Nb⁴⁺ anti‐bonding orbitals [41]. Such electronic structural modification facilitates CO₂ adsorption and reduces the energy required for V_O formation. Additionally, both V_O‐CZNO and ZNO display two peaks at 1332.8 cm⁻¹ (D band) and 1587.7 cm⁻¹ (G band) (Figure S9), confirming the presence of carbon species. The I_D/I_G ratio of 1.01 in Vo‐CZNO indicates the presence of structural defects, possibly related to disorder or carbonaceous species introduced during synthesis [42]. These defects may serve as active sites, potentially enhancing the catalytic performance. Thermogravimetric analysis (TGA) (Figure S10) estimates the carbon content at ca. 1.6 wt%, which facilitates enhancing the electrical conductivity of the catalyst [43, 44, 45]. N₂ adsorption–desorption measurements (Figures 1l and S11) reveal that V_O‐CZNO possesses a specific surface area comparable to that of ZNO, along with slit‐like mesopores induced by Cu species [9, 46]. This distinct pore structure confirms the dual role of Cu as both an interfacial promoter and a structural modifier, generating a pore network with enhanced mass transport properties.

2.2. Interfacial Electronic Structure of Catalysts

To gain deeper insight into how the introduced Cu species modulate the interfacial electronic structure of ZNO and influence its reaction behavior, density functional theory (DFT) calculations combined with multimodal spectroscopic analyses were performed. Projected density of states (PDOS) analysis (Figure 2a) reveals a distinct electronic reconstruction in the V_O‐CZNO heterostructure. Notably, the Cu‐3d, O‐2p, and Nb‐4d states show a clear energy‐level ordering and strong overlap in the range of −3.5 to 0 eV, which is characteristic of interfacial d‐p‐d electronic hybridization [47, 48]. The introduction of Cu species plays a decisive role in this process by forming interfacial Cu─O─Nb linkages, which enable strong coupling among Cu 3d, O 2p, and Nb 4d orbitals, thereby inducing the formation of a d‐p‐d hybridized electronic structure. The Cu‐3d states are located at higher energies compared to the O‐2p band, with a band center at −1.85 eV. In contrast, ZNO and V_O‐ZNO exhibit only Nb‐4d and O‐2p hybridization. Here, V_O introduction shifts the O‐2p band center upward toward the Fermi level (from −2.35 to −2.15 eV). In V_O‐CZNO, however, strong interfacial Cu─O─Nb hybridization shifts the O‐2p band center downward to −3.43 eV and markedly reconstructs the Nb‐4d states, driving its band center upward across the Fermi level (from −5.00 eV in ZNO to −4.91 eV in V_O‐ZNO, and finally to +0.11 eV in V_O‐CZNO). These synergistic changes thereby establish a continuous energy pathway along the Cu─O─Nb bridge. Furthermore, the emergence of Nb‐4d states near the Fermi level enhances their electron‐accepting capability, satisfying the prerequisites for efficient interfacial charge transport [48]. This predicted hybridization drives specific charge redistribution, visualized in real space by the charge density difference plot (Figure 2b). The plot shows clear electron depletion at both Cu and Nb sites (yellow), and pronounced electron accumulation around the bridging O atoms (blue), providing direct spatial evidence for the charge transfer through the oxygen bridge [47]. Quantitatively, Bader charge analysis (Figure 2c) further confirms the calculated charges of +0.22 e⁻ for Cu1, +0.21 e⁻ for Cu2, +2.35 e⁻ for Nb1 and +2.33 e⁻ for Nb2, demonstrating the electron‐transfer path [49]. Moreover, these results provide compelling evidence for an interfacial Cu─O─Nb configuration in which both Cu and Nb act as electron donors toward the bridging oxygen atoms, consistent with the PDOS analysis and charge‐density mapping. In conclusion, PDOS, charge density difference, and Bader analysis collectively demonstrate that the interfacial electronic reconstruction establishes an efficient d‐p‐d hybridized transport channel [50]. This channel, defined by the Cu─O─Nb charge redistribution pathway, plays a pivotal role in promoting superior reaction reversibility and accelerating electrochemical kinetics (Figure 2d), the robust d–d interaction between Cu 3d and Nb 4d, mediated by O 2p orbitals, kinetically “locks” Cu⁺ in a metastable state in the presence of Nb₂O₅. This stabilization imposes a substantial kinetic barrier for the reduction of Cu⁺ to Cu⁰, as the transitions must overcome not only the redox energy but also the energy required to break the established d–d hybridization, thereby effectively trapping Cu⁺ species [46].

FIGURE 2.

Interfacial‐hybridization investigation by DFT calculations and multi‐scale spectra. (a) Projected density of states (PDOS) of V_O‐CZNO showing Cu 3d, O 2p, and Nb 4d contributions. (b) Atomic structure model and charge density difference at the V_O‐CZNO interface, illustrating electron depletion (yellow) and electron accumulation (blue). (c) Two‐dimensional slice of the differential charge density corresponding to Bader charge values. (d) Schematic illustration of the d–p–d orbital hybridization. High‐resolution XPS spectra of (e) Nb 3d and (f) O 1s. XANES spectra of the V_O‐CZNO and ZNO at (g) Nb k‐edge and (h) the corresponding FT curves of the EXAFS data (FT‐EXAFS) of the V_O‐CZNO and ZNO with the reference spectra of Nb foil. (i) The WT spectra of Nb foil, V_O‐CZNO, and ZNO, showing the k and R space data simultaneously.

To experimentally corroborate the electronic interactions inferred from PDOS analysis, x‐ray photoelectron spectroscopy (XPS) was employed to probe the electronic structure differences between ZNO and V_O‐CZNO. The XPS survey spectra (Figure S12a) confirm the presence of Cu, Zn, C, and O in V_O‐CZNO. Deconvolution of the Cu 2p spectrum (Figure S12b) indicates the coexistence of Cu⁺ and Cu²⁺ states [20]. The presence of Cu²⁺ likely originates from a superficial oxide layer formed during ambient exposure [47]. As shown in Figure 2e, the Nb 3d spectrum of V_O‐CZNO exhibits a new peak that emerged at 205.1 eV, which is assigned to Nb⁴⁺ 3d_5/2, while the main peak at 207.5 eV can be deconvoluted into overlapping contributions from Nb⁵⁺ 3d_5/2 and Nb⁴⁺ 3d_3/2 [26, 50]. Compared with ZNO, the entire Nb 3d spectrum shifts to higher binding energy, directly evidencing the partial reduction of Nb⁵⁺ to Nb⁴⁺. In the O 1s spectrum (Figure 2f), a peak at 530.3 eV (20.6 at%) emerges, consistent with the V_O characteristic signals and associated charge‐compensation mechanisms [39]. V_O sites act as electron donors, and the localized electrons are redistributed through the lattice via the O‐2p mediated network, leading to partial reduction of Nb⁵⁺ to Nb⁴⁺ at the interfacial sites. Thus, V_O not only acts as an adsorption site but also stabilizes Cu⁺ species by inducing charge redistribution and strengthening Cu─O─Nb interfacial coupling via d‐p‐d orbital hybridization. Additionally, the peak area of ─OH (24.0 at%) increases significantly in V_O‐CZNO compared to ZNO (16.0 at%), possibly because V_O at the interface serves as a strong hydrophilic site, enhancing H₂O adsorption [39]. Simultaneously, the negative binding energy shift of the Zn 2p peaks (Figure S12c) suggests increased electron density around Zn, resulting from the local charge redistribution after removal of negatively charged O²⁻ ligands [39]. Additionally, the exclusive peak at 395.3 eV in the N 1s spectrum (Figure S12d), which can be attributed to strongly coupled N species induced by pronounced interfacial electronic interactions between Cu species and the residual ZIF‐8‐derived NC framework after Cu doping.

To further determine the atomic‐scale structure, x‐ray absorption near‐edge structure (XANES) and extended x‐ray absorption fine structure (EXAFS) experiments were performed to elucidate the chemical states and the atomic coordination structures, respectively. Nb K‐edge XANES analysis (Figure 2g) shows nearly overlapping edge positions and line shapes for ZNO and V_O‐CZNO, indicating similar average Nb valence states [41, 50, 51]. Only minor spectral variations are observed in the inset region, reflecting subtle modifications of the Nb 4d electronic structure upon Vo introduction and interfacial hybridization [48]. To clarify how these perturbations manifest in the coordination structure, the EXAFS signals were Fourier transformed (FT) (Figure 2h). ZNO exhibits a strong and sharp first‐shell Nb─O peak at approximately 1.2–2.0 Å, whereas the corresponding peak in V_O‐CZNO is significantly attenuated and slightly broadened, indicating a weakened or distorted short‐range Nb─O coordination. In the second‐shell region (approximately 2.5–3.5 Å), ZNO shows a distinct peak corresponding to Nb/Zn‐Nb scattering, while the signal in V_O‐CZNO is markedly diminished, suggesting a further reduction in long‐range coordination order. The FT results clearly reveal that both short‐range and long‐range local structures in V_O‐CZNO are looser and more asymmetric. Based on these observations, the wavelet‐transform (WT) analysis was performed to differentiate the contributions of various scattering paths in the k–R space (Figure 2i). ZNO exhibits strong and concentrated energy‐intensity distributions in both the low‐R/high‐k region (Nb─O scattering) and the higher‐R region (Nb/Zn‐Nb scattering), whereas the corresponding signals in V_O‐CZNO are significantly weaker and accompanied by diffused contour boundaries, showing broader and more blurred scattering features. This further indicates a higher degree of structural disorder around Nb in V_O‐CZNO, consistent with the shell‐amplitude attenuation observed in the FT spectra. These results consistently demonstrate that the local electronic structure and coordination environment of Nb in V_O‐CZNO are more perturbed and disordered than in ZNO, reflecting pronounced structural relaxation and weakened coordination. Overall, the experimental data confirm that the V_O‐CZNO contains oxygen vacancy defects and Nb⁴⁺ species. The presence of V_O in CZNO was further verified by electron paramagnetic resonance (EPR) spectroscopy. As shown in Figure S13, an asymmetric signal centered at a g‐factor of 2.02 was detected. This value is distinctly higher than the typical g ≈ 2.002 for isolated V_O, signifying a modified local environment around the V_O. The shift can be ascribed to the coordinated influence of the constituent transition metals (Cu, Zn, Nb) [52]. Specifically, the coexistence of Cu⁺/Cu²⁺ and Nb⁵⁺/Nb⁴⁺ ions alters the crystal field and enhances the spin‐orbit hybridization near the V_O, thereby affecting the g‐factor of the trapped electron. This electronic perturbation originates from the efficient d‐p‐d hybridization among Cu (3d), O (2p), and Nb (4d) orbitals. In summary, the engineered d‐p‐d hybridized electronic network not only stabilizes the V_O but also provides an efficient pathway for interfacial charge transfer, leading to a remarkable enhancement of the intrinsic electronic conductivity of the catalyst.

2.3. Electrochemical Performance in Li‐CO₂ Batteries

To elucidate the influence of the unique d‐p‐d orbital hybridization on catalytic performance, a series of electrochemical tests was conducted on V_O‐CZNO and ZNO as catalytic cathodes for Li‐CO₂ batteries. The electrochemically active surface areas (ECSAs) were first determined from double‐layer capacitance (C_dl) derived from cyclic voltammetry (CV) measurements in a non‐Faradaic region (Figure S14). The ECSA values for ZNO and V_O‐CZNO are 1.23 and 1.44 mF cm⁻², respectively. Despite their differing pore structures, the comparable ECSAs suggest that the superior chemical properties of V_O‐CZNO depend on its intrinsic activity, rather than surface area, likely due to the enhanced electronic structure or increased active sites imparted by the Cu incorporation. Under an Ar atmosphere, both V_O‐CZNO and ZNO exhibit significantly suppressed current responses without distinct redox peaks (Figure S15a,b), demonstrating that CO₂ is essential as a reactant in the redox reaction processes [19]. However, in a CO₂ atmosphere (Figure 3a), V_O‐CZNO exhibits higher cathodic and anodic peak currents than ZNO, indicating enhanced electrochemical activity and accelerated charge‐transfer kinetics for both the CO₂ reduction reaction (CO₂RR) and CO₂ evolution reaction (CO₂ER) in Li‐CO₂ batteries. Linear sweep voltammetry (LSV) results (Figure S16) further demonstrate its superior catalytic activity toward CO₂RR and CO₂ER. At 2.0 V (vs. Li/Li⁺), current densities reached 5.33 and 3.72 A g⁻¹ for V_O‐CZNO and ZNO, respectively. Notably, during CO₂ER at 6.0 V, V_O‐CZNO delivers a significantly higher current density of 6.70 A g⁻¹, approximately 7 times that of ZNO (0.98 A g⁻¹), indicating its superior catalytic activity attributable to the distinct d‐p‐d hybridization [46]. Full discharge/charge tests (Figure 3b) show that V_O‐CZNO exhibits a maximum discharge capacity of 20995 mAh g⁻¹ at 500 mA g⁻¹ with a CE of 97.8% (calculated based on the cutoff voltage between 2 and 4.56 V), while the ZNO delivers a lower discharge capacity of 9611 mAh g⁻¹ (Figure S17) and shows an abnormal charge curve with the voltage increasing under the same condition to that of Vo‐CZNO. The ultrahigh capacity arises from the excellent Li⁺ transport properties of the niobate structure and its unique electronic properties are enabled by the Vo‐CZNO catalyst [40]. Rate capability tests (Figures 3c–e and S18) reveal that as the current density increases from 100 to 1000 mA g⁻¹, the voltage gaps of ZNO widen from 1.53 to 1.74 V. In contrast, V_O‐CZNO exhibits lower voltage gaps of 1.47, 1.49, 1.50, 1.54, and 1.67 V, corresponding to energy efficiencies (EE) of 65.37%, 64.87%, 64.86%, 63.95%, and 61.47%, respectively. Upon returning to 100 mA g⁻¹, the voltage gap of V_O‐CZNO decreases to 1.33 V with an improved EE of 68.16%. The reduced overpotential and enhanced efficiency after high‐rate cycling suggest an electrochemical activation effect, likely due to more uniform deposition of discharge products [53]. To further probe the origin of this activation, electrochemical impedance spectroscopy (EIS) was performed on the V_O‐CZNO‐based battery (Figure S19) after 30 cycles (10 cycles per rate at 100, 500, and 1000 mA g⁻¹) under different current densities. The results show that the impedance recovered to a lower value after recharging at 100 mA g⁻¹, remained nearly unchanged at 500 mA g⁻¹, and even further decreased after cycling at 1000 mA g⁻¹. Combined with rate capability and EIS results, this behavior under high current density suggests an “electrochemical activation” effect at the electrode interface, likely arising from favorable surface reconstruction of the catalyst that exposes additional active sites [52]. Long‐term cycling stability tests (Figure 3f) demonstrate that V_O‐CZNO could cycle stably for nearly 350 cycles at 1000 mA g⁻¹ under a limited capacity of 500 mAh g⁻¹, maintaining a voltage gap of ca. 1.6 V. Remarkably, at 100 mA g⁻¹, the battery operates for 5400 h (Figure 3g), significantly surpassing ZNO (ca.1500 h). Furthermore, under 300 mA g^{− 1} (Figure S20) and 500 mA g^{− 1} (Figure S21), V_O‐CZNO achieves cycle lives of 1100 h (320 cycles, overpotential ≈ 1.45 V, EE ≈ 65.7%) and 950 h (450 cycles, overpotential ≈ 1.50 V, EE ≈ 64.0%), respectively. These results confirm the significant advantage of Cu incorporation in enhancing cycle life and reducing overpotential. Compared with previously reported systems under similar conditions, the V_O‐CZNO‐based system delivers outstanding cycling stability (Figure 3h and Table S1). To further evaluate practical applicability, a higher loading of ca.0.5 mg cm⁻² was also tested (Figure S22). The V_O‐CZNO‐based cell can sustain stable cycling for over 100 h with a low overpotential of 1.46 V, outperforming ZNO (1.82 V) and pure carbon paper, confirming the intrinsic catalytic superiority of V_O‐CZNO. Notably, stable cycling is also retained when the V_O‐CZNO catalyst is applied in an analogous Na‐CO₂ battery (Figure S23). This observation indicates that the V_O‐CZNO catalyst possesses good universality and can be effectively extended to other metal‐CO₂ batteries, and potentially to broader metal–air battery systems. As a proof of concept, a Li‐CO₂ pouch cell employing a V_O‐CZNO catalyst was fabricated (Figure S24). The open‐circuit voltage (OCV) of the cell (7.5 cm × 4 cm in size) was recorded before and after CO₂ gas injection, as shown in Figure S25. Upon introducing CO₂, the OCV rapidly rose from 2.54 to 2.92 V within a short period, confirming the effective involvement of CO₂ as an active species in the electrochemical process. It is worth noting that the Li‐CO₂ pouch battery achieved a high energy density of 317.3 Wh kg⁻¹ based on the cathode mass (Figure S26a), and exhibited excellent cycling stability of over 120 h at 100 mA g⁻¹ (Figure S26b). Even more remarkably, it operated stably for 484 cycles at an ultrahigh current density of 1000 mA g⁻¹ (Figure 3i). After electrochemical activation during the first 50 cycles, the EE stabilized at around 67.8%, showing great competitiveness with the state‐of‐the‐art Li‐CO₂ pouch cells (Figure 3j and Table S2). Furthermore, the V_O‐CZNO‐based Li‐CO₂ pouch cell demonstrates the capability to power a mobile phone (Figure 3k).

FIGURE 3.

Electrochemical performance of Li‐CO₂ batteries. (a) CV curves of V_O‐CZNO and ZNO under a CO₂ atmosphere. (b) Fully discharged and charged curves of V_O‐CZNO‐based cell at 500 mA g⁻¹ selected within 2.0–4.56 V. For comparison, the Coulombic efficiency was calculated using the charge capacity up to 4.56 V. (c) Rate capabilities with a capacity limit of 500 mAh g⁻¹ for V_O‐CZNO and ZNO cathodes. (d) Galvanostatic charge–discharge (GDC) profiles with a limited capacity of 500 mAh g⁻¹ at different current densities for the Li‐CO₂ battery with the V_O‐CZNO catalyst. (e) Energy efficiency at different current densities for the Li‐CO₂ battery with the V_O‐CZNO‐based cathodes. (f) The median potential of the two cells for long‐term cycling at 1000 mA g⁻¹. (g) Cycling performance of the cell using the two catalysts at 100 mA g⁻¹ with a limited capacity of 500 mAh g⁻¹. (h) Performance comparison for Li‐CO₂ batteries with V_O‐CZNO and other reported catalysts. (i) Li‐CO₂ pouch cell performance of V_O‐CZNO at the median potential and EE for long‐term cycling at 1000 mA g⁻¹ with a limited capacity of 200 mAh g⁻¹. (j) Performance comparison for Li‐CO₂ pouch cell with V_O‐CZNO and other reported catalysts. (k) Photograph of the V_O‐CZNO‐based Li‐CO₂ pouch cell powering a mobile phone.

2.4. Characterizations of the Discharge Products Li₂CO₃

To explore the influence of d‐p‐d orbital hybridization on the formation of discharged products, comprehensive characterizations of the cycled electrodes were conducted. The V_O‐CZNO was analyzed by XRD and SEM at different charge and discharge states under current densities of 100, 500, and 1000 mA g⁻¹. As shown in Figure 4a, the voltage gap increased only slightly after one cycle at each current density, indicating significantly optimized interfacial reaction kinetics. The high reversibility of V_O‐CZNO at high current densities arises from the synergistic roles of Nb⁴⁺ and V_O. Nb⁴⁺ in the d‐p‐d hybridized network facilitates electron transfer, while V_O sites serve as electron donors, provide fast Li⁺ transport pathways and adsorb CO₂, together enabling efficient and reversible CO₂RR/CO₂ER [39, 40, 41]. Combined XRD and Raman spectroscopy analyses (Figure S27) confirmed that the discharge products on deeply discharged V_O‐CZNO‐based electrodes consist of Li₂CO₃. These products were efficiently decomposed during subsequent charging, leading to the recovery of the electrode to its pristine state. The decomposition of Li₂CO₃ is a key criterion for evaluating the catalysts in Li‐CO₂ batteries. Incomplete decomposition of Li₂CO₃ leads to its accumulation on the cathode surface, blocking mass transport and active sites, passivating the catalyst, and resulting in high overpotential and poor cycling stability. Ex situ XRD patterns (Figure 4b) show weak diffraction peaks assignable to Li₂CO₃ (PDF#22‐1141) after discharge, which nearly vanish after recharging, demonstrating that the product can be efficiently oxidized and decomposed even under high current density conditions. The electrochemical reaction kinetics of V_O‐CZNO at different current densities were further studied using EIS combined with distribution of relaxation times (DRT) analysis. In the DRT spectra (Figure 4c), the middle peak corresponds to the charge transfer process. Due to Li₂CO₃ formation, the charge transfer resistance (Rct) of V_O‐CZNO generally increases after discharge and recovers after recharging. It is noteworthy that the Rct exhibits a slight and progressive increase as the current density rises from 100 to 1000 mA g⁻¹. This trend may be attributed to the faster formation and accumulation of insulating Li₂CO₃ at higher discharge rates, which partially impede charge transfer. Nevertheless, the relatively small increase in Rct implies that the spatial distribution and morphology of Li₂CO₃ remain largely favorable for electron transport even under high current densities [53]. To elucidate the relationship between current density and Li₂CO₃ morphology, we performed SEM characterization of electrodes discharged and recharged at 100 and 1000 mA g⁻¹. As shown in Figure 4d, the discharge products are uniformly dispersed as fine nanoparticles on the catalyst surface. Compared with the 100 mA g⁻¹ condition, a greater amount of discharge product forms at 1000 mA g⁻¹, likely due to the accelerated reaction kinetics and enhanced CO₂ conversion efficiency at higher current density. Moreover, these products fully disappear after recharging, demonstrating that V_O‐CZNO promotes effective decomposition of Li₂CO₃ through enhanced d‐p‐d interfacial hybridization, which improves local electron transfer and reaction reversibility [53]. In contrast, the ZNO (Figure S28) forms larger discharge products at both 100 and 1000 mA g⁻¹, which may hinder charge transfer kinetics. Nearly full decomposition of the discharge product in ZNO is achieved after recharging, reflecting the good electronic conductivity of the ZNO framework. Further in situ Raman spectroscopy conducted during 1‐h discharge/charge cycles at 1000 mA g⁻¹ (Figure 4e) demonstrates the reversible appearance and disappearance of the characteristic Li₂CO₃ peak at ca. 1100 cm⁻¹, providing direct evidence of the structural reversibility under high current density reactions and the stability of the electron cycle in the catalytic process. C 1s XPS spectra of electrodes discharged and recharged at 1000 mA g⁻¹ further confirm the complete decomposition of Li₂CO₃ (Figure 4f). Moreover, S 2p XPS spectra (Figure S29) exhibit no detectable changes, indicating the absence of electrolyte decomposition during cycling. The good stability of the electrolyte at the testing conditions is also verified by linear sweep voltammetry (LSV) (Figure S30a) and the molecular orbital energy levels analysis based on DFT calculation (Figure S30b). This observation verifies that the superior electrochemical performance originates from the rational design of the catalyst. As illustrated in Figure 4g, the exceptional reversibility of V_O‐CZNO under high energy density is fundamentally attributed to its synergistically modulated interfacial electronic structure. The d‐p‐d hybridized orbitals in the Cu‐O‐Nb bridge provide an efficient pathway for reversible electron transfer between Nb⁵⁺ and Nb⁴⁺, with O‐2p orbitals serving as a local mediator and electron reservoir, while the V_O not only enhances CO₂ adsorption but also accelerates Li⁺ migration. The synergistic interaction between the two components guides the uniform nucleation of Li₂CO₃ into ultrafine nanoparticles during discharge and promotes their rapid and complete decomposition during recharging, thereby enabling highly reversible CO₂ conversion and stable interfacial reaction kinetics. Therefore, the modulation of the interfacial electronic structure via Cu‐induced d‐p‐d orbital hybridization and V_O acts as a key regulator in achieving a refined discharge product morphology, which is essential for the enhanced reversibility of the Li‐CO₂ battery.

FIGURE 4.

Characterizations of discharged products. (a) Discharge and charge profiles of the V_O‐CZNO‐based cell at current densities of 100, 500, and 1000 mA g⁻¹. (b) Ex situ XRD patterns of V_O‐CZNO at different states. (c) EIS and DRT spectra of V_O‐CZNO after discharge and recharge at different current densities. (d) SEM images of discharged and recharged V_O‐CZNO at 100 and 1000 mA g⁻¹. (e) Charge–discharge curves at 1000 mA g⁻¹ and the corresponding in situ Raman spectra. (f) The C 1s XPS spectra of discharged and recharged V_O‐CZNO at 1000 mA g⁻¹. (g) Schematic diagram of the discharge products formation in Li‐CO₂ batteries.

2.5. Mechanism Investigation

To investigate the reaction mechanism and verify the superior catalytic performance of the V_O‐CZNO catalyst in Li‐CO₂ batteries, the conversion reaction was investigated by in situ differential electrochemical mass spectrometry (DEMS) at a current of 100 µA (corresponding to a current density of 1000 mA g⁻¹) (Figure 5a,b). The DEMS results indicate that only CO₂ consumption and evolution occur during the charge–discharge process, confirming that the primary electrochemical reaction follows: 4Li⁺ + 3CO₂ + 4e⁻ → 2Li₂CO₃ + C (E = 2.8 V vs. Li/Li⁺), with an electron‐to‐CO₂ ratio of 4e⁻/3CO₂. The corresponding theoretical CO₂ consumption flux is 0.777 nmol s⁻¹, and the total theoretical molar consumption is 2.798 × 10⁻⁶ mol. Based on the integrated peak area, the actual CO₂ molar consumption for the V_O‐CZNO is calculated as 2.764 × 10⁻⁶ mol, corresponding to a CO₂ reduction efficiency of 98.78%. Furthermore, the V_O‐CZNO‐based system maintains stable discharge and charge voltages, indicating that the rationally designed catalyst structure endows V_O‐CZNO with excellent electrocatalytic activity for the CO₂RR [54]. To gain deeper mechanistic insights into the experimentally observed high CO₂ conversion efficiency revealed by DEMS, DFT calculations were further carried out to evaluate the adsorption behaviors of Li, CO₂, and Li₂CO₃ on the catalyst surface. A strong adsorption capability for Li, CO₂ and Li₂CO₃ plays a critical role in initiating the CO₂RR. High adsorption energies for these species facilitate their effective capture and utilization during the reaction process. As shown in Figures 5c and S31, benefitting from the introduced oxygen vacancies, V_O‐CZNO exhibits markedly higher adsorption energies for both Li and CO₂ compared to ZNO. This reflects the superior affinity of V_O‐CZNO toward reactant molecules, which originates from electron‐rich V_O and the associated low‐coordination metal centers [54]. The enhanced adsorption leads to an increased local concentration of the reactants on the catalyst surface, thereby promoting the progression of the CO₂RR.

FIGURE 5.

CO₂ reduction and Li₂CO₃ decomposition mechanism. The in situ DEMS spectra of V_O‐CZNO tested during (a) discharge and (b) charge process. (c) Li, CO₂, and Li₂CO₃ adsorption energy on V_O‐CZNO and ZNO. (d) PDOS and (e) charge density difference after Li₂CO₃ adsorption in V_O‐CZNO and ZNO. (f) Bond length (Å) changes and (g) configurations after Li₂CO₃ adsorption in V_O‐CZNO and ZNO. (h) Decomposition energy barriers and corresponding detailed decomposition paths of Li₂CO₃ on V_O‐CZNO and ZNO. (i) Schematic illustration of the interfacial electronic structure and reaction mechanism on V_O‐CZNO.

To elucidate the origin of the enhanced Li₂CO₃ decomposition activity on V_O‐CZNO, we analyzed the electronic structure of the adsorbed Li₂CO₃ by the PDOS (Figure 5d). Compared with ZNO, Li₂CO₃ on V_O‐CZNO exhibits pronounced electronic perturbation, characterized by a downward shift of the O‐2p band center from −5.47 to −5.57 eV, accompanied by significant peak broadening, indicating enhanced orbital delocalization and stronger interfacial hybridization [26]. Meanwhile, the Nb‐4d states near the Fermi level exhibit intensified interaction with O‐2p states, reflecting strengthened d‐p‐d orbital coupling across the Cu─O─Nb interface. This hybridization promotes substantial charge redistribution toward O atoms and disrupts the intrinsic localized bonding configuration of Li₂CO₃. The charge‐density‐difference maps provide direct confirmation (Figure 5e). In contrast to ZNO, V_O‐CZNO displays a markedly more pronounced electron transfer from Nb to O at the interface, leading to extensive charge redistribution within Li₂CO₃. This theoretical observation is strongly supported by Nb 4d XPS analysis (Figure S32). After discharge, Nb remains predominantly in the +5 oxidation state, whereas a reversible Nb⁴⁺ component emerges upon charging. This behavior indicates that Nb centers function as both electron buffers and electron acceptors during Li₂CO₃ decomposition, in full agreement with the interface charge‐transfer mechanism predicted by DFT. Such electronic polarization also structurally manifests. Bond‐length analysis (Figure 5f,g) shows that Li─O bonds on ZNO undergo only slight elongation (1.85–2.01 Å), largely preserving the Li₂CO₃ geometry. However, on V_O─CZNO, these bonds are markedly stretched to 2.13 and 2.21 Å, accompanied by mild distortion of the C─O framework. Given that Li─O bond breakage is recognized as the rate‐determining step in Li₂CO₃ decomposition, these elongated bonds signify a more activated, dissociative configuration on V_O‐CZNO. Kinetic evidence further reinforces this conclusion. The decomposition barrier (Figure 5h) of Li₂CO₃ decreases from 0.85 eV on ZNO to 0.78 eV on V_O‐CZNO, demonstrating that interfacial electronic reconstruction substantially lowers the transition‐state energy. This result is consistent with in situ Raman observation of the rapid disappearance of the characteristic peak of Li₂CO₃ during charging, further confirming the promoting role of Cu‐mediated d‐p‐d orbital hybridization in the efficient decomposition of Li₂CO₃. The reduced barrier facilitates the formation of ultrafine, highly reversible Li₂CO₃ nanoparticles during cycling. Therefore, V_O induces local electronic redistribution through the d‐p‐d hybridized network, enhancing the activity of nearby Nb centers as electron buffers and facilitating the formation of a multicenter cooperative region, ultimately enabling rapid and reversible Li₂CO₃ decomposition on V_O‐CZNO (Figure 5i).

3. Conclusion

This work presents a highly efficient V_O‐CZNO catalyst for Li‐CO₂ batteries and clarifies the relationship between its catalytic structure and the properties of the discharge products. Through electrochemical testing and in situ spectroscopic analysis, combined with DFT calculations, we demonstrate that the tailored interface induces a unique, Cu‐mediated d‐p‐d orbital hybridization, which promotes electron transfer from Nb sites in V_O‐CZNO to O atoms in Li₂CO₃. This interaction elongates Li─O bonds and guides Li₂CO₃ nucleation into finely dispersed nanoparticles. Within these domains, the weakened Li─O bonds facilitate efficient decomposition of Li₂CO₃ during charging, thereby inhibiting the accumulation of such insulating products and sustaining rapid reaction kinetics. Benefiting from these synergistic effects, the Li‐CO₂ battery exhibits outstanding reversibility under high‐rate operation and remarkable long‐term stability, achieving stable cycling for 5400 h at 100 mA g⁻¹ and a low overpotential of 1.6 V at 1000 mA g⁻¹. The robust performance is further validated in practical pouch cell configurations, highlighting the structural stability and viability of the catalyst design. Notably, the V_O‐CZNO catalyst also enables stable cycling in an analogous Na‐CO₂ battery, demonstrating that the core strategy of interfacial orbital engineering transcends a single chemistry. This work establishes a versatile blueprint for designing durable, high‐performance catalysts, paving the way for the development of sustainable metal‐CO₂ battery systems.

Author Contributions

Shasha Xiao: conceptualization, investigation, methodology, data curation, formal analysis, writing – original draft, and validation. Ying Xiao: conceptualization, funding acquisition, supervision, and writing – review and editing. Yu Yang: formal analysis and investigation. Xinyu Wang: formal analysis. Yiming Fang: formal analysis. Shimou Chen: funding acquisition, supervision, and writing – review and editing.

Conflicts of Interest

The authors declare no conflicts of interest.

Supporting information

Supporting File 1: anie72738‐sup‐0001‐SuppMat.docx.

Data Availability Statement

The data that support the findings of this study are available from the corresponding author upon reasonable request.