Cation–Anion Redox Co‐Modulation: Unlocking the Potential of All‐Electrochem‐Active Sulfur‐Based Solid‐State Batteries

ABSTRACT

The capacity utilization of all‐solid‐state sulfur cathodes reveals a significant disparity between material and electrode levels due to the high proportion of inactive components required for electro‐ionic transport. While the all‐electrochem‐active (AEA) electrode concept seeks to bridge this gap, fully realizing the energy‐density potential of sulfur‐based cathodes remains challenging. Here, we report a new strategy for co‐modulating the redox of the transition‐metal cation/sulfur anion to unlock the potential of the sulfur‐based electrode. By carefully adjusting the coordination between S anions and Ti cations, we constructed the AEA electrode with S‐anion (TiS_x, x > 2)/Ti‐cation (amorphous TiS₂) co‐redox, where TiS_x activates the redox activity of sulfur‐rich phases with narrower bandgaps through the reversible cleavage and recombination of S–S bonds, thereby enhancing the capacity utilization of anion‐redox in the electrode level, and amorphous TiS₂ serves as an electrochemically active matrix facilitating mixed ionic‐electronic conduction. This design eliminates inactive components and enables synergistic anion‐cation redox chemistry. Consequently, this designed cathode achieves an unprecedented electrode‐level energy density of 1829 Wh/kg, sustains an areal capacity of 11.6 mAh/cm², and exhibits long‐term stability over 10 000 h. Device‐level demonstrations validate this synergistic approach as an effective design principle for realizing high‐energy‐density, long‐life all‐solid‐state battery cathodes under practical conditions.

We propose an all‐electrochemical‐active sulfur‐based (AEA‐S‐TM) cathode by simultaneously modulating the redox of Ti cations and S anions. By optimizing coordination and sulfur stoichiometry, we form a sulfur‐rich anion‐redox phase (TM[S]) with superior kinetics for high anion‐redox capacity and a cation‐redox disulfide phase ([TM]S) as an efficient ion‐electron network, achieving record energy density, long cyclability, and scalability.

1. Introduction

With the rapid development of electrification in vehicles, marine shipping, and aviation, the demand for batteries with simultaneously high energy density and high safety has increased significantly [1, 2, 3]. According to the targets proposed by the United States Advanced Battery Consortium (USABC) and the Advanced Research Projects Agency‐Energy (ARPA‐E), cell‐level energy densities of approximately 700 and 1000 Wh/kg are required for electric vehicles and aviation applications [4]. However, current transition‐metal oxide cathodes, such as LiCoO₂ and LiNi_0.8Co_0.1Mn_0.1O₂, are fundamentally constrained by intercalation‐based redox chemistry, which imposes a ceiling on energy density and prevents them from meeting the requirements of emerging high‐energy applications [5, 6]. In contrast, sulfur cathodes operate via multi‐electron conversion chemistry rather than single‐electron intercalation with a theoretical specific capacity of 1672 mAh/g and an energy density of 2600 Wh/kg, fundamentally breaking the energy‐density ceiling of transition‐metal oxide cathodes [7, 8], widely recognized as a highly attractive cathode material for high‐energy‐density battery applications [9, 10]. Moreover, its moderate operating voltage (approximately 2 V vs. Li/Li⁺) and oxygen‐free nature ensure good chemical compatibility with sulfide‐based solid electrolytes (SEs), which exhibit high lithium‐ion conductivity and favorable mechanical deformability [11, 12, 13]. Collectively, these features highlight sulfur cathodes as an ideal platform for realizing stable and safe all‐solid‐state Li‐S batteries (ASSLSBs).

Although sulfur‐based materials promise high theoretical capacity and energy density, their practical cathode‐level performance is limited by the low sulfur content commonly employed, due to their inherently poor lithium‐ion and electronic conductivity (Figure 1aI) [14, 15, 16]. As a result, traditional ASSLSB cathodes require a high percentage of inactive conductive additives and a solid‐state electrolyte to maintain continuous charge and carrier transport across the three‐phase interface throughout the triple‐phase traditional S₈ electrode (Figure 1b and Figure S1) [17, 18, 19, 20], which is fundamentally limited by interdependent constraints stemming from their architecture: (i) reliance on conductive networks by high‐fraction inactive conductors creates a trade‐off with energy density [10, 18, 19, 21]; (ii) phase mutual obstruction among sulfur, SE, and C leads to tortuous ion/electron transport, hindering high‐mass‐loading [18, 22, 23].

FIGURE 1.

Design process of the AEA‐S‐TM cathode. (a) Qualitative evolution of the redox centers and bandgap width in transition metal sulfides with different sulfur content ([TM]S and TM[S] denote sulfides where the primary redox center is the transition‐metal (TM) and sulfur (S), respectively; E _F is aligned for comparison; this trend is not universal). (b) Schematic illustrations of the microscopic pathways for charge/carrier transport in the traditional S₈, the AEA‐S/[TM]S, and the AEA‐S‐TM cathodes. (c) Comparison of the capacities for the traditional S₈, the AEA‐S/[TM]S, and the AEA‐S‐TM cathodes. The assessment of the AEA‑S‑TM cathode takes into account the distinct theoretical capacities (represented by circle sizes) and different electrode‑mass fractions of the TM[S]. Details are provided in Note S1.

Given their kinetic advantages, transition‐metal sulfides ([TM]S, Figure 1aIII) are often studied as sulfur‐equivalent electrodes. However, the one‐electron cation‐redox chemistry imposes a capacity ceiling, resulting in capacity and transport characteristics that are fundamentally different from those of sulfur yet equally unbalanced. Therefore, neither sulfur nor transition‐metal sulfides alone can strike a balance between high capacity and superior transport as the sole active materials.

To address the above issue, previously, we proposed a new class of the all‐electrochem‐active (AEA) sulfur‐based electrodes, where the electrochemically active mixed ionic‐electronic conductors (MIECs)—typically transition‐metal sulfides ([TM]S)—are composited with sulfur to form AEA‐S/[TM]S cathodes without SEs or carbon [24, 25], thereby eliminating the triple‐phase interfaces and integrating ionic and electronic transport within a single phase (Figure 1b and Figure S1) [23, 26]. Despite our previous AEA design, the electrode‐level energy density remains far below its achievable potential, primarily because introducing a large amount of [TM]S to ensure dispersion of the conductive network constrains the sulfur content to below 35 wt.%. As a result, the capacity contribution from sulfur anion redox is diluted [24].

2. Results and Discussion

2.1. Cation–Anion Redox Co‐Modulation for AEA Sulfur Cathodes

To unlock higher energy densities, it is imperative to increase the fraction of high‐capacity anion‐redox materials in the cathode, which requires fundamental improvements in the ionic and electronic transport properties of sulfur‐based materials and further optimization of the cathode architecture.

Transition‐metal polysulfides typically exhibit superior electronic conductivity and lithium‐ion diffusion coefficients compared to sulfur [4, 27]. More importantly, compared to transition‐metal sulfides, they also retain high‐capacity anion redox, offering the potential to achieve a balance between high capacity and superior transport properties [28]. For certain transition metals, such as Ti [29, 30] and Mo [24, 31, 32], variations in sulfur stoichiometry during bonding lead to distinct coordination environments and electronic configurations, thereby generating different redox‐active centers and bandgap widths (Figure 1a). This coordination tunability between transition‐metal (TM) and sulfur atoms provides a coordination‐driven route to reconcile transport efficiency with active capacity.

Specifically, increasing the S/TM molar ratio to form sulfur‐rich phases (TM[S], Figure 1aII) leads to the emergence of strongly covalent S–S structural units [28, 33, 34]. In such systems, the anion p‐band of the less electronegative sulfur shifts upward, approaching or even overlapping the TM d‐band, leading to strong orbital hybridization. This interaction gives rise to a conduction band minimum (CBM) dominated by antibonding states of the S–S units, which defines the lowest unoccupied molecular orbital (LUMO) and underpins enhanced electronic transport and anion‐redox activity [35, 36]. In this electronic regime, sulfur anions act as the primary redox centers, enabling multi‐electron transfer and thereby affording a high specific capacity. Notably, most of these sulfur‐rich compounds exist in an amorphous state (e.g., TiS₄ [30], MoS₅ [31], and MoS₆ [32]), and their open‐framework structures further facilitate Li‐ion diffusion.

In contrast, at low S/TM molar ratios, only low‐order sulfides ([TM]S, Figure 1aIII) with exclusive TM–S coordination are formed, such as the aforementioned transition‐metal sulfides. In this regime, the anion p‐band shifts below the TM d‐band, such that the d‐band dominates both the valence band maximum (VBM) and the conduction band minimum (CBM), rendering TM cations the primary redox centers with limited capacity [28]. Meanwhile, the bandgap progressively narrows, transitioning semiconducting behavior and even approaching a semimetallic state, accompanied by continuously improved charge‐transport kinetics [37].

Guided by this principle, we propose a cathode design based on a single transition‑metal element bonded with sulfur at different stoichiometries, forming two distinct classes of sulfides with complementary capacity and kinetic characteristics. Together, they constitute a novel all‐electrochemical‐active sulfur‐metal (AEA‐S‐TM) cathode (Figure 1b and Figure S1). The first class comprises high‑order sulfides (TM[S]), in which sulfur anions serve as redox centers, enabling multi‐electron transfer and providing a high specific capacity that defines the upper limit of cathode‐level energy density. Compared to sulfur, these phases preserve sufficient ionic and electronic conductivity, along with high capacity, alleviating dependence on external conductive additives and permitting a higher mass fraction in the cathode. The second class comprises low‑order sulfides ([TM]S), which exhibit significantly higher ionic and electronic conductivities, thereby ensuring efficient charge transport throughout the AEA framework. Meanwhile, their transition‑metal centers contribute additional capacity through conventional single‑electron redox, further boosting the overall electrode energy density.

As illustrated in Figure 1c, the low fraction of high‐capacity anion‐redox‐active species severely limits the achievable capacity in both the traditional S₈ cathode and the previously reported AEA‐S/[TM]S cathode. In contrast, the TM[S] material possesses a favorable balance between theoretical capacity and transport properties. When operating within the highlighted blue region, the AEA‐S‐TM cathode accesses a substantially expanded window for capacity increase, revealing significant potential for high‐energy‐density electrode design (Figure 1c). Additionally, this AEA‐S‐TM architecture achieves nearly full coverage of electrochemically active sites while minimizing the tortuosity of ionic and electronic transport pathways, thus enhancing rate capability and allowing for the development of high‐areal‐loading electrodes.

Motivated by these advantages, we next designed an AEA‐S‐TM cathode system. An ideal transition‐metal element for the AEA‐S‐TM cathode should satisfy the following criteria: (i) it forms multiple chemical‐stable sulfide phases with sulfur, giving rise to distinct redox‐active centers; (ii) the corresponding anion‐redox phases deliver a competitive theoretical energy density ceiling (≥ 1000 Wh/kg); (iii) the resulting transition‐metal‐redox phases exhibit sufficiently high ionic and electronic conductivity; and (iv) the average discharge voltage remains above 1.0 V versus Li/Li⁺ to ensure practical applicability.

Guided by these criteria, we systematically surveyed the literature to identify transition‐metal sulfide cathode systems in which a single transition‐metal element can form multiple distinct sulfide phases by bonding with sulfur. For each system, we compiled the discharge voltage plateaus (≥ 1.0 V vs. Li/Li⁺) and the corresponding specific capacities (Figure 2a and Table S1). Notably, sulfide cathodes derived from the same transition‐metal element exhibit comparable discharge plateau potentials, consistent with element‐specific redox energetics.

FIGURE 2.

Confirmation of composition and structure of the AEA‐S‐TM cathode. (a) Reported gravimetric discharge capacities and average discharge voltages for various transition metal sulfide cathodes. (b) Corresponding estimated maximum energy densities and reaction types; energy densities were estimated by multiplying the gravimetric discharge capacity by the average discharge voltage. (c) Comparison of ionic diffusion coefficients of c‐TS and a‐TS at different SOC. d, XRD patterns of c‐TS and a‐TS, and TEM image with corresponding FFT pattern of a‐TS. (e) First‐cycle galvanostatic discharge‐charge curves for c‐TS and a‐TS at 0.1 C (1 C = 239 mA/g). (f) Schematic of the synthesis of the AEA‐S‐TM cathode material.

Moreover, by excluding electrochemically inactive components from the electrode formulation, we estimated the upper bound of electrode‐level energy density attainable for each candidate system (Figure 2b). This representation reveals that V, Ti, Nb, and Mo can all access sulfur‐anion‐dominated redox regimes with gravimetric energy densities exceeding 1000 Wh/kg, identifying them as promising candidates for high‑capacity TM[S] compounds. Notably, however, when these high‑capacity anion‑redox materials are employed as cathode active materials in prior studies, they still predominantly rely on substantial fractions of electrochemically inactive conductive additives, owing to their insufficient intrinsic ionic and electronic conductivity [30, 31, 32]. This observation underscores the necessity of incorporating [TM]S phases with high electronic and ionic conductivity to establish an effective conduction network within the AEA‐S‐TM cathode.

Among the low‐order sulfides, TiS₂ is an ideal candidate as an electrochemically active mixed ionic‐electronic conductor (MIEC), owing to its semiconducting electronic conductivity, high lithium‐ion diffusivity, and prior validation as an AEA cathode in the literature [24, 38]. In addition, TiS₂ offers a relatively high theoretical specific capacity of 239 mAh/g, while titanium‐based sulfides generally exhibit higher discharge voltage plateaus than many other sulfides—both attributes being advantageous for the construction of high‐energy‐density cathodes. Accordingly, titanium was selected as the transition‐metal host for the target cathode, yielding an AEA‐S‐TM architecture composed of sulfur‐rich TiS_x (x > 2) and TiS₂ phases.

To validate this design concept, we began by probing the transport characteristics and electrochemical response of TiS₂ as an AEA cathode. Using the galvanostatic intermittent titration technique (GITT), we quantified the Li‐ion diffusion coefficient of crystalline TiS₂ (c‐TS) over different states of charge (SOC). The result reveals substantial fluctuations in the diffusion coefficient during discharge/charge, ranging from 1.10 × 10⁻¹³ to 5.74 × 10⁻¹¹ cm²/s across three orders of magnitude (Figure 2c and Figure S2). Such large fluctuations preclude the formation of a stable ionic conduction network during cycling, unlike solid electrolytes, which maintain relatively consistent ionic conductivity.

Given their grain‐boundary‐free structure and defect‐rich nature, amorphous materials typically demonstrate fast and stable ionic diffusion kinetics during electrochemical energy storage and conversion [30, 39, 40]. Accordingly, amorphous TiS₂ (a‐TS) was prepared by ball milling. GITT measurements show that a‐TS exhibits the enhanced Li‐ion diffusion coefficient in the range of 6.34 × 10⁻¹² to 5.87 × 10⁻¹¹ cm²/s, with variations confined to within one order of magnitude over discharge and charge, indicating a markedly more stable ionic conduction network (Figure 2c and Figure S2).

Amorphization disrupts the layered framework of TiS₂, introducing numerous defect sites and fostering more isotropic ion‐transport pathways, which together enhance and stabilize the Li‐ion diffusion (Figure S3). X‐ray diffraction (XRD) shows the absence of sharp Bragg reflections, confirming the predominantly amorphous nature of a‐TS and broad humps centered at 34.1° and 53.8° indicate the presence of minute amounts of nanocrystalline TiS₂ impurities resulting from inhomogeneities during the ball‐milling process (Figure 2d). The amorphous nature of the material is confirmed by transmission electron microscopy (TEM), which shows diffuse rings in the fast Fourier transform (FFT) pattern (Figure 2d). Ball‐milling effectively reduces the particle size from approximately 10 µm to around 500 nm (Figure S4). Notably, DC polarization measurements indicate that the electronic conductivity of amorphous a‐TS decreases only slightly, from 0.674 to 0.251 S/cm. This high level of conductivity persists throughout cycling, demonstrating that amorphization maintains an efficient electronic conduction network (Figure S5). Consequently, a‐TS achieves a robust and stable mixed ion‐electron conducting network.

This reinforced mixed‐conduction network translates directly into improved electrochemical performance. As an AEA cathode, a‐TS delivers an initial discharge—124 mAh/g higher than c‐TS and increases the initial Coulombic efficiency (CE) from 79.16% to 86.98%, reflecting enhanced charge storage and kinetic reversibility (Figure 2e). To elucidate the origin of this improvement, we compared the cyclic voltammetry (CV) responses of c‐TS and a‐TS (Figure S6 and Figure S7). In contrast to the predominantly diffusion‐controlled kinetics of c‐TS, a‐TS delivers a pseudocapacitive contribution exceeding 80%, evidencing a markedly accelerated kinetic response.

Beyond kinetic enhancement, amorphization also elevates the thermodynamic potential [41], enabling capacity that is confined to lower voltages in the crystalline phase to be accessed above 1 V. This voltage uplift directly translates into higher specific capacity and enhanced charge‐storage capability (Figure S8). Furthermore, due to the stable amorphous structure (Figure S9), these combined benefits in kinetics and capacity are reliably maintained over extended cycling. Compared with c‐TS, a‐TS not only retains an additional 116 mAh/g after 150 cycles with stable cycling performance but also demonstrates significantly enhanced long‐term cycling stability under high areal loading (10.32 mg/cm²) and at the temperature of 30°C (Figures S10 to S12). Together, the enhanced charge‐storage capability and the robust, self‐sustained mixed ionic‐electronic conducting network establish a‐TS as an ideal electrochemically active conductive matrix for high‐capacity electrodes, thereby facilitating the realization of high‐energy‐density electrode architectures.

Building on established synthesis routes for titanium polysulfides, we employed S₈ and TiS₂ as direct precursors to fabricate the AEA‐S‐TM cathode via solid‑state ball‑milling (Figure 2f) [39, 40]. During milling, chemical interactions between S₈ and TiS₂ occur via bond cleavage and reorganization, thereby driving the formation of titanium polysulfide species. By tuning the milling parameters, the reaction pathway can be regulated to yield a mixed phase comprising sulfur‐rich titanium polysulfides and residual TiS₂, thereby achieving the desired AEA‐S‐TM cathode architecture.

2.2. Coordination‐Driven Construction of High‐Energy‐Density AEA‐S‐TM Cathodes

To systematically tailor the sulfur content of the AEA‐S‐TM cathode, a series of electrodes was prepared by progressively increasing the S₈‐to‐TiS₂ volumetric ratio in the precursor mixture. The resulting cathodes are denoted as xS‐T, where x represents the molar ratio of S/Ti. The optimal sulfur content was subsequently determined by evaluating both the initial discharge energy density and the corresponding Coulombic efficiency, achieving a balance between high energy density and electrochemical reversibility.

Meanwhile, to further demonstrate the advantages of a‐TS in enabling high‐energy‐density cathodes, we fabricated AEA‐S‐TM cathodes using both c‐TS (c‐T‐S) and a‐TS (a‐T‐S) as precursors and evaluated their performance at 0.1 C (1 C = 550 mA/g) with areal loadings of 3–4 mg/cm². For both crystalline‐ and amorphous‐derived AEA‐S‐TM cathodes, the cathode‐level energy density initially increases with sulfur content and then declines at higher sulfur loadings, accompanied by a similar evolution in initial Coulombic efficiency (Figure 3a and Table S2). At moderate sulfur contents, the increasing fraction of high‐order polysulfides introduces additional anion‐redox capacity, thereby boosting the cathode energy density. However, excessive sulfur enrichment leads to a predominance of high‐order polysulfides, which have intrinsically poor electronic conductivity, thereby compromising electrode kinetics beyond a critical threshold. This kinetic degradation ultimately manifests as concurrent reductions in both energy density and coulombic efficiency.

FIGURE 3.

Construction of the high‐energy‐density AEA‐S‐TM cathodes. (a) Initial discharge gravimetric energy density (E _g) and Coulombic efficiency (CE) of AEA‐S‐TM cathodes at various sulfur contents. (b) Ionic and electronic conductivities of 3.1S‐T, 3.7S‐T, and 4.5S‐T measured by DC polarization at 60°C. (c) Comparison of ionic diffusion coefficients for 3.1S‐T, 3.7S‐T, and 4.5S‐T at different SOCs.

The c‐T‐S cathode reaches a maximum energy density of 1008 Wh/kg at 3.1S‐T with an initial Coulombic efficiency of 76%. In a remarkable contrast, the a‐T‐S cathode surpasses 1000 Wh/kg across a substantially broader sulfur‐composition window, delivering 1258 Wh/kg (86.47%) at 3.1S‐T, 1415 Wh/kg (88.45%) at 3.7S‐T, and 1829 Wh/kg (85.05%) at 4.5S‐T, revealing an ideal region for designing high‐energy‐density AEA‐S‐TM cathodes with sulfur contents ranging from 3.1 to 4.5. This pronounced performance enhancement originates from the markedly reduced particle size of a‐TS, which promotes more homogeneous dispersion during electrode fabrication. Together with its higher intrinsic capacity and robust mixed ionic‐electronic conducting network, these features synergistically enable exceptional capacity utilization and high reversibility in the AEA‐S‐TM cathode. Additionally, discharge‐charge curve testing revealed that the 3.1S‐T, 3.7S‐T, and 4.5S‐T cathodes exhibit similar average discharge voltages (around 2.09 V vs. Li/Li⁺), while their initial discharge specific capacity reaches 593.5, 677.7, and 874.1 mAh/g, respectively, serving as key factors in achieving high energy density (Figure S14).

As discussed above, a higher sulfur content significantly increases the specific capacity but also progressively deteriorates overall electrode kinetics. Therefore, to elucidate the kinetic differences arising from varying sulfur contents and thus enable the rational selection of AEA‐S‐TM cathodes for diverse application scenarios, we conducted electronic and ionic conductivity measurements on the 3.1S‐T, 3.7S‐T, and 4.5S‐T cathodes within the ideal region. While ionic conductivities of three cathodes are similar, the electronic conductivities decrease by two orders of magnitude with rising sulfur content (Figure 3b and Figures S16–S18). This indicates that higher sulfur content primarily impairs electronic conduction due to an increased proportion of low‐conductivity, high‐order titanium polysulfides, consistent with our earlier analysis. Overall, the AEA‐S‐TM cathode exhibits the highest combined ionic (3.5 × 10⁻⁵ S/cm) and electronic conductivity at 3.1S‐T. Meanwhile, it exhibits the highest diffusion coefficient (2.8 × 10⁻¹² to 1.2 × 10⁻¹⁰ cm²/s) and the least variation during discharge/charge, reflecting superior kinetic stability (Figure 3c). These kinetic advantages translate into the superior overall electrochemical performance under more practical operating conditions (Figure S19 and Figure S20). Consequently, AEA‐S‐TM cathodes with varying sulfur contents exhibit distinct energy densities and kinetic characteristics, endowing our AEA‐S‐TM cathodes with versatility to meet diverse practical requirements within the ideal region.

2.3. Characterizing the AEA‐S‐TM Cathodes

Because the performance of cathodes is intrinsically governed by structure and composition, we conducted a series of characterizations to further verify the structural and compositional consistency of the 3.1S‐T, 3.7S‐T, and 4.5S‐T cathodes with the targeted electrode design. SEM images reveal that all cathode powders are predominantly composed of submicron‐sized particles (Figure S21). The diffuse rings in fast Fourier‐transform TEM images, along with the absence of sharp Bragg peaks in XRD patterns, confirm the amorphous nature of the three cathodes (Figure 4a and Figure S22).

FIGURE 4.

Characterization of the AEA‐S‐TM cathodes. (a) TEM images and corresponding FFT patterns of the 3.1S‐T, 3.7S‐T, and 4.5S‐T materials. (b) XPS S 2p spectra of 3.1S‐T, 3.7S‐T, 4.5S‐T, and S₈. (c) XPS S 2p spectra of the AEA‐S‐TM cathode at 4.5S‐T in different SOC. (d) XPS Ti 2p spectra of the AEA‐S‐TM cathode at 4.5S‐T in different SOC.

Furthermore, the characteristic signal of sulfur disappears in both differential scanning calorimetry (DSC) and X‐ray photoelectron spectroscopy (XPS) tests, indicating the complete consumption of free sulfur precursors (Figure S23 and Figure 4b). During ball milling, the S₈ ring structure is disrupted and reorganized into covalent S–S units characteristic of titanium polysulfides, giving rise to S₂ ²⁻ species that constitute the targeted high‐capacity anion‐redox motifs, as evidenced by the S 2p doublet at 162.6 and 163.7 eV in the S 2p spectra (Figure 4b) [42, 43, 44]. In addition, minor contributions assigned to S^d− (0 < d < 1) are also observed in all three cathodes, likely associated with residual polysulfide chain segments generated during milling [42]. Meanwhile, both titanium polysulfides and titanium disulfide contain S²⁻ species originating from Ti‐S ionic bonds, giving rise to consistent peaks near 161.1 and 162.3 eV across all samples [43, 44]. By deconvoluting the polysulfide contribution, the maximum TiS₂ fractions in the 3.1S‐T, 3.7S‐T, and 4.5S‐T cathodes are estimated to be 32.7%, 16.1%, and 27.2%, respectively. Ti 2p spectra further confirm that titanium shows no detectable redox change throughout ball milling (Figure S24).

The reversibility of S–S bond cleavage and recombination is confirmed by operando S 2p XPS of the 4.5S‐T cathode at different states of charge (Figure 4c). During discharge, the S₂ ²⁻ signal diminishes concomitant with the growth of S²⁻ species, with the trend fully reversed upon charging, evidencing a multi‐electron anion‐redox process. In parallel, a reversible Ti³⁺ feature in the Ti 2p spectra indicates additional capacity contribution from TiS₂ via transition‐metal redox (Figure 4d). Together, these observations demonstrate the coexistence and synergy of anionic and cationic redox reactions, directly validating the AEA‐S‐TM design principle.

2.4. Evaluating Charge Transport and Mechanical Stability

AEA‐S‐TM cathodes exhibit fundamentally different compositions and structures, which significantly impact both the charge and carrier transport dynamics and the structural integrity of the cathode during operation. To validate these effects, we selected 3.1S‐T, which exhibits the optimal kinetic characteristics within the ideal region, for a series of characterization and comparative analyses that clearly demonstrate the advanced design of the AEA‐S‐TM cathode.

To evaluate the kinetic transport performance across various cathode compositions and structures, a traditional S₈ cathode (S‐SE‐C) and hybrid cathodes made of amorphous (a‐TS/S) or crystalline (c‐TS/S) TiS₂ mixed with sulfur were fabricated, maintaining the same sulfur content as the precursor. The ionic and electronic conductivities of the S‐SE‐C cathode are two and one order of magnitude lower than those of the 3.1S‐T cathode, respectively (Figure 5a and Figures S27, S28). The mutually obstructive transport pathways among S₈, SE, and C degrade overall electrode kinetics, underscoring the advantage of homogeneous charge‑carrier transport in AEA‐S‐TM cathodes. Notably, although both a‐TS/S and c‐TS/S exhibit excellent electronic conductivity, their overall conductivity remains inferior to that of the 3.1S‐T electrode due to the poorer ion‐diffusion of sulfur compared to polysulfides (Note S2). This kinetic difference enables the 3.1S‐T cathode to achieve a higher capacity delivery compared to the a‐TS/S and c‐TS/S cathodes (Figure S29). In summary, the AEA‐S‐TM electrode attains optimal conductivity through a carefully crafted structural and material‐kinetic design.

FIGURE 5.

Comparison between AEA‐S‐TM and S‐SE‐C cathodes. (a) Ionic and electronic conductivities of the 3.1ST cathode, the a‐TS/S cathode, the c‐TS/S cathode, and the S‐SE‐C cathode measured by DC polarization at 60°C. (b) In situ EIS‐DRT analysis of the AEA‐S‐TM cathode at 3.1S‐T, and (c) the S‐SE‐C cathode during the first cycle. (d) Top‐view and cross‐sectional SEM images of two electrodes before and after cycling. (e) Schematic of charge and carrier transport in the AEA‐S‐TM electrode under high areal capacity. (f) Cycling performance of two cathodes at an areal capacity of 6 mAh/cm². (g) Schematic of charge and carrier transport in the S‐SE‐C electrode under high areal capacity.

Furthermore, to investigate the structural integrity and stability of the electrodes during operation, we performed operando electrochemical impedance spectroscopy combined with distribution of relaxation times (EIS‐DRT) analysis on the 3.1S‐T (Figure 5b) and the S‐SE‐C cathodes (Figure 5c). Three distinct peaks exhibiting notable variation during testing were identified and labeled as D1, D2, and D3. Their peak intensities continuously decrease during discharge and increase during charge. This behavior likely correlates with the expansion and contraction of the cathode during cycling, which reduces and increases the interfacial resistance via modulation of the solid‐solid contact (Note S3). Notably, the peak intensities for the AEA‐S‐TM cathode are consistently lower than those of the S‐SE‐C cathode throughout cycling, and its maximum resistance at the end of charging is approximately 2.5 times lower, indicating significantly smaller volume changes during lithiation/delithiation.

This is further confirmed by SEM images (Figure 5d). After cycling, the AEA‐S‐TM cathode showed significantly reduced pore size, whereas the S‐SE‐C cathode developed large‐scale cracks and exposed electrode particles. Moreover, the thickness of the S‐SE‐C cathode increased from 93 to 130 µm, corresponding to an expansion of 40%, while the AEA‐S‐TM cathode expanded by merely 3 µm in thickness, yielding a significantly lower expansion rate of 19%. The reduced volume expansion facilitates superior interfacial contact and more efficient connectivity within the conductive network during long‐term cycling, ultimately enhancing the electrode's long‐term stability.

Furthermore, to investigate their potential for developing more practical high‐areal‐capacity electrodes, we fabricated and tested cathodes with an areal capacity of 6 mAh/cm². Notably, the AEA‐S‐TM cathode features a smaller thickness due to the elimination of low‐density inactive conductive additives, which enhances the overall compactness of the electrode (Figures S30 and S31). The inherent heterogeneity in charge‐carrier transport in S‐SE‐C electrodes is further exacerbated at larger thicknesses, leading to increased transport tortuosity (Figure 5g), whereas the homogeneous conducting network and reduced thickness of the AEA‐S‐TM electrode substantially lower transport tortuosity (Figure 5e), thereby enhancing electrode kinetics and overall electrochemical performance (Figure 5f).

2.5. Demonstrating in All‐Solid‐State Batteries

To assess the practical potential of the AEA‐S‐TM cathode, we assembled all‐solid‐state batteries (ASSBs) with a cathode | LPSC | In_xLi configuration and evaluated their electrochemical performance under application‐relevant conditions within the ideal S/Ti region. To further probe the high‐energy‐density advantage and the persistence during extended cycling, we compared the cathode‐level energy density of the sulfur‐rich 4.5S‐T cathode with that of a traditional S₈ cathode (S‐SE‐C) containing the same sulfur weight fraction, evaluated between 1.0 and 3.0 V (Figure 6b and Figure S32). The AEA‐S‐TM cathode achieved an initial energy density of up to 1829 Wh/kg, which stabilized after a brief initial decline and thereafter exhibited surprisingly long‐term stability, retaining a stable reversible energy density of 1154 Wh/kg after 10 000 h.

FIGURE 6.

Electrochemical performance of the AEA‐S‐TM cathode. (a) Initial galvanostatic discharge‐charge curves of the AEA‐S‐TM cathode at 4.5S‐T. (b) Comparison of the energy density (E _g) during cycling between the AEA‐S‐TM cathode at 4.5S‐T and the traditional S₈ cathode (S‐SE‐C) with the same precursor sulfur content (wt.%). (c) First cycle galvanostatic discharge‐charge curves of the AEA‐S‐TM cathode at different areal loadings. (d) Cycling performance of the AEA‐S‐TM cathode at an areal loading of 20 mg/cm². (e) Rate performance of the AEA‐S‐TM cathode. (f) First‐cycle galvanostatic discharge‐charge curves of the AEA‐S‐TM cathode at different areal loadings measured at 30 °C. (g) Energy density comparison of the AEA‐S‐TM cathodes with various cathode systems reported in the literature.

Composed of only two elements, the cathode intrinsically forms a chemically compatible and stable internal interface. The initial energy density and capacity decay observed during early cycling are attributed to transient contact loss within the active reaction zones induced by volume expansion. With continued cycling, interfacial contacts progressively stabilize, leading to a sustained high‐capacity plateau with negligible further decay. In contrast, the S‐SE‐C cathode consistently exhibited lower energy density, undergoing a sharp, nearly linear decay and falling below 1000 Wh/kg prematurely. Together, these results establish the AEA‐S‐TM cathode as a platform capable of sustaining high energy density with durable reversibility under prolonged cycling.

Owing to the superior charge‐transport kinetics of the AEA‐S‐TM cathode at 3.1S‐T, this cathode exhibits excellent scalability toward high‐areal‐loading electrodes (Figure 6c). At areal loadings of 10.2 and 15.4 mg/cm², the cathode delivers areal capacity of 6.1 and 8.6 mAh/cm², respectively. Even when the loading is further increased to 20 mg/cm², a high areal capacity of 11.6 mAh/cm² is maintained. Notably, the AEA‐S‐TM cathode retains its long‐term cycling stability across various mass loadings (Figure S33). Even under more practical high‐areal‐loading conditions of 15.4 (Figure S34) and 20 (Figure 6d) mg/cm², the cathode retains areal capacities of 4.03 mAh/cm² after 100 cycles and 5.6 mAh/cm² after 40 cycles, respectively.

Simultaneously, the AEA‐S‐TM cathode exhibits excellent rate capability and reversibility, delivering stable capacities from 447.4 to 239.8 mAh/g at 0.2 to 2 C, with full capacity recovery upon returning to 0.1 C and subsequent stable cycling (Figure 6e). Notably, the rate‐dependent response reveals a pronounced asymmetry between discharge and charge, with slower capacity decay during discharge, indicative of more favorable discharge kinetics. Accordingly, we assembled ASSBs operating at a constant 2 C discharge rate with charging performed at 0.1 C and 2 C for comparison (Figure S35). After 150 cycles, capacities of 383.16 and 196.5 mAh/g are retained, respectively, confirming the AEA‐S‑TM cathode's superior capacity retention under high‐rate discharge and operational flexibility. Even at a reduced operating temperature of 30°C, the AEA‐S‐TM cathode still achieves discharge/charge capacities of 609.8/479.2, 595.3/413.1, 589.5/389.1, and 573.2/352 mAh/g at areal loadings of 3.18, 4.2, 6.11, and 8.3 mg/cm², respectively (Figure 6f). This result demonstrates the feasibility of room‐temperature operation and further underscores the practical relevance of the AEA‐S‐TM cathode (Figure S36).

To benchmark the energy‐density advantage of the AEA‐S‐TM cathode within an all‐electrochemical‐active architecture, we compared its cathode‐level energy density with those of representative cathode systems reported in the literature (Figure 6g and Table S3). The AEA‐S‐TM cathode clearly outperforms existing systems, delivering a maximum energy density of 1829 Wh/kg, which places it at the current state of the art. We anticipate that such exceptionally high cathode‐level energy density can be effectively translated to pouch‐cell configurations, offering a viable pathway toward all‐solid‐state batteries that simultaneously achieve high areal loading, long cycle life, and genuinely high practical energy density.

3. Conclusion

In conclusion, we establish a transition‐metal cation/sulfur anion redox co‐modulated design paradigm for all‐electrochemical‐active sulfur‐based cathodes, where sulfur–metal coordination and ligand modulation are deliberately employed to reconcile high capacity with robust reaction kinetics. By integrating amorphous TiS₂ as an all‐electrochemical‐active mixed ionic‐electronic‐conducting matrix and simultaneously generating sulfur‐rich titanium polysulfide phases with abundant S–S units, the cathode architecture enables efficient multi‐electron anion redox while maintaining a stable, self‐supported, cation‐redox‐conductive network. This balanced regulation markedly enhances the capacity utilization of sulfur anions and fully removes the constraints imposed by electrochemically inactive components.

As a result, the cathode achieves a record cathode‐level energy density of up to 1829 Wh/kg, together with outstanding long‐term stability and scalability to high areal loadings. The homogeneous ion‐electron transport network, near‐complete coverage of active reaction sites, reduced electrode thickness, and mitigated volumetric expansion collectively underpin its exceptional electrochemical performance in all‐solid‐state batteries, including a reversible energy density of 1154 Wh/kg after 10 000 h and an areal capacity of 11.6 mAh/cm² at a loading of 20 mg/cm². Beyond the specific sulfur–titanium system demonstrated here, this work highlights the transition‐metal cation/sulfur‐anion redox co‐modulation as a general and powerful strategy for designing high‐energy‐density, long‐life all‐solid‐state battery cathodes, offering a viable pathway toward practical next‐generation energy storage.

Author Contributions

Guoliang Jiang: conceptualization, methodology, data curation, investigation, validation, visualization, project administration, writing – original draft. Xiaolin Xiong: methodology, validation, conceptualization. Xiqian Yu: writing – review and editing, project administration, resources, supervision. Liquan Chen: resources, writing – review and editing, supervision. Hong Li: supervision, resources, writing – review and editing, project administration. Liumin Suo: conceptualization, supervision, resources, project administration, funding acquisition, writing – review and editing. Weiping Li: methodology, validation.

Conflicts of Interest

The authors declare no conflicts of interest.

Supporting information

Supporting File: anie73265‐sup‐0001‐SuppMat.docx

Data Availability Statement

The data that supports the findings of this study are available in the Supporting Information of this article.