Electrochemical initiation and chemical reaction cascades in dual-stage thermal runaway in sulfide-based all-solid-state batteries

Domain: chemistry
Published: 2026-02-19
Editorial outcome: major_revision

Abstract
Sulfide all solid-state batteries represent a promising next generation energy storage technology. However, their presumed safety is challenged by the risk of thermal runaway initiating at unexpectedly low temperatures. This critical issue stems from the unstable chemical interface between the positive electrode and thiophosphate solid electrolyte, a factor often overlooked in favor of electrochemical studies. Here we demonstrate that this electrochemically formed interphase is the primary trigger for catastrophic failure, not the bulk materials. Our investigation reveals a universal two stage degradation mechanism. The first stage involves intense exothermic reactions at the interface below 160°C, releasing heat and gases. This initiates a second stage of propagating reactions leading to thermal runaway. Crucially, we show this hazardous process can be suppressed by interface engineering. We design a stable interfacial layer using a germanium sulfur chemistry, specifically lithium germanium sulfide. This modification delivers improved thermal safety without sacrificing battery performance. Our findings have the potential to establish a forward-looking safety paradigm, shifting the focus from bulk material compatibility to interfacial stability, and provide a vital design principle for future safe solid-state batteries.

Introduction
While modern lithium-ion batteries (LIBs) have attained high energy densities, their intrinsic safety limitations remain unresolved stemming from flammable electrolyte decomposition cascades that initiate thermal runaway (TR) chain reactions. This critical vulnerability has shifted the paradigm towards inorganic solid electrolytes (SEs), which fundamentally bypass the Arrhenius-driven pyrolysis pathways inherent to volatile organic solvents1. Recent advancements in sulfide-based solid electrolytes, particularly argyrodite Li6PS5Cl (LPSC), have positioned this material family at the forefront of all-solid-state battery (ASSB) research due to its high ionic conductivity and favorable mechanical deformability2–4. Particularly, this decade has witnessed substantial progress in LPSC-based ASSBs with synergistic developments in electrochemical stability and scalable processing developments, enabling them as leading candidates for next-generation energy storage commercialization5–7. While the significantly enhanced thermal safety is widely acknowledged as a hallmark merit of LPSC-based ASSBs, increasingly emerging evidence of paradoxical thermal failure behaviors challenges this presumed superiority. Contrary to initial expectations, LPSC-based ASSBs composed of thermally stable components exhibit exacerbated TR performance under abuse conditions, with heat propagation rates exceeding those of liquid electrolyte systems8–11. Accelerating calorimeter (ARC) testing of a home-made 3.8Ah Li|LPSC|LiNi0.5Co0.2Mn0.3O2 pouch cell demonstrates an initial self-heating temperature at 163°C, and then culminates into explosive TR at 275°C9. This acceleration effect magnifies at pack level, where the 3Ah ASSB configurations demonstrate TR propagation velocities 10-fold higher than commercially available LIB equivalents10. Unlike the well-characterized negative electrode-triggering TR pathways in conventional liquid-electrolyte LIBs, the safety paradox of ASSBs originates from positive electrode-electrolyte interfacial degradation dynamics, where incompatible phase boundaries initiate self-sustaining exothermic reactions at relatively low temperatures. This mechanistic inconsistency becomes particularly evident when examining delithiated oxide positive electrode|SE interfaces at abused conditions, which demonstrated divergent thermal decomposition pathways across previously reported literature. For instance, Ouyang et al. identified solid-solid interfacial reactions between delithiated NCM811 and LPSC as the primary TR trigger, where self-ignition occurs at 325°C and the whole exothermic process is free of detectable sulfurous gas (e.g., SO2)12. Lee’s team investigated exothermic and gas-evolution behaviors in composite positive electrodes under abuse conditions, demonstrating that mechanical grinding or thermal exposure to 150°C in argon triggers TR accompanied by substantial SO2 emissions13,14. Wu et al. identified that in the LiCoO2|LPSC system, bulk chemical reactions between delithiated positive electrode and sulfide electrolyte dominated thermal decomposition above 300°C15. Such contrasting findings spanning reaction thresholds, gaseous byproducts, and failure modes, underscore fundamental knowledge gaps in solid-state interfacial degradation mechanisms.

The thermal runaway mechanisms in sulfide-based ASSBs are governed by intertwined thermochemical and electrochemical processes at oxide positive electrode|SE interfaces. Thermochemically, bulk S-O interdiffusion directly drives interfacial reconstruction, forming metastable phosphate-sulfate phases through elemental exchange reactions16,17. Meanwhile, electrochemically-driven interfacial dynamics, particularly charge transfer and anion redox activities, critically influence thermal degradation pathways. Sulfide SEs such as LPSC, constrained by narrow electrochemical stability windows, undergo progressive oxidation during conditioning. This process generates metastable sulfur-bridged intermediates and decomposition byproducts (e.g., -S-S-, -P-S-P-, Li3PS4) that progressively destabilize interfacial integrity18,19. Fundamental band alignment analyses further reveal that the valence band maximum (VBM) of sulfide SEs lies energetically above the Fermi level of oxide positive electrodes. This interfacial energy offset drives electron transferring from positive electrode to SE and accelerates electrochemical degradation20. Under thermal abuse conditions, these electrochemically preconditioned interfaces affect thermal runaway initiation via dual pathways: (1) interfacial decomposition of sulfur species and their subsequent exothermic reactions with delithiated positive electrodes, and (2) thermally driven lattice oxygen release from delithiated oxide positive electrodes, which synergistically amplifies interfacial instability. This process further triggers self-sustaining cascades that integrate bulk-phase redox reactions, gaseous byproduct evolution (e.g., SO2, O2), and exothermic interfacial recombination, a multifaceted degradation dynamic absent in conventional liquid-electrolyte systems. Crucially, a fundamental challenge persists in quantifying the thermodynamic-kinetic interplay between bulk S-O interdiffusion and electrochemically driven interfacial decomposition, mechanistic competitors whose relative contributions remain unresolved due to the field’s disproportionate focus on bulk thermochemistry over preconditioned interface dynamics. Additionally, the sequence of gaseous emissions (SO2, O2), solid-solid reactions, and their respective heat release ratios also need to be further mapped. Establishing decoupled thermochemical/electrochemical dominance across thermal degradation stages is therefore imperative, not only to resolve mechanistic contradictions in literature but to guide interface engineering strategies that intrinsically suppress exothermic cascades.

Here, leveraging multiscale calorimetry and online gassing techniques, we systematically elucidated the specific exothermic reaction pathway induced from chemical and electrochemical reactions in NCM811|LPSC composite, revealing a two-stage exothermic cascade. Crucially, the electrochemically conditioning evolved interphase, characterized by disulfide bridges (-S-S-), -P-S-P-, and metastable Li3PS4 phases, governs early-stage self-heating through interfacial exothermic decomposition accompanied by SO2 evolution. At high temperatures, solid-solid reactions across positive electrode|SE interface triggers sulfide oxidation and layered positive electrode disintegration, amplifying heat generation rates and leading to uncontrollable TR. This interfacial failure paradigm extends to Li10GeP2S12 (LGPS) and Li9.54Si1.74P1.44S11.7Cl0.3 (LSPSC) systems, demonstrating universal disulfide bridge induced thermal-electrochemical coupling cascade reaction. To address this, we engineer NCM811|Li4GeS4 composite positive electrodes via Ge-S bond stabilization to elevate the thermal thresholds. Onset temperature of self-heating (Tonset) and thermal runaway temperature (Ttr) of the as-assembled ASSBs are increased from 168°C, 228°C to 223°C, 312°C compared to the counterpart of LPSC, respectively. These findings advance the fundamental understanding of thermal runaway mechanisms and establish interface thermodynamic-kinetic mapping as a critical toolkit for designing fail-safe sulfide-based ASSBs.

Results
Thermal runaway characteristics of ASSBs
To mechanistically correlate the single-cell safety characteristic with material-level thermal stabilities, a thorough assessment of the thermal runaway behavior in full ASSBs is essential. Here, LPSC-based (Supplementary Fig. 1) sulfide ASSBs were assembled employing NCM811 and Li-In as electrodes, and their charge/discharge profiles showcased good electrochemical performance (Supplementary Fig. 2). The thermal safety characteristics of both fully charged (Fig. S3, 4.5 V vs. Li+/Li, 100% state of charge (SOC)) and fully discharged (2.8 V vs. Li+/Li, 0% SOC) ASSBs were analyzed using an ARC under typical heat-wait-search (HWS) conditions. Key parameters including onset temperature of self-heating (Tonset), self-heating rate (SHR), and thermal runaway temperature (Ttr, defined as the temperature point when SHR reaches 1°C·min−1) were recorded. Apparently, violent thermal runaway was observed for the 100% SOC ASSB, with the Tonset and Ttr being 163.0±8.5°C and 231.7±2.6°C (Fig. 1a and Supplementary Fig. 4). These findings indicate that ASSBs, while demonstrating improved safety characteristics, do not entirely mitigate thermal runaway risks and remain susceptible to severe thermal propagation under abuse conditions, consistent with prior studies9. However, the fully discharged ASSB also underwent severe thermal runaway with Tonset of 239±8.5°C and Ttr of 304.7±14.7°C (Fig. 1a and Supplementary Fig. 5), further emphasizing that high thermal stability of battery materials does not necessarily equate to high safety of full cell. To pinpoint the origins of thermal instability, we systematically investigate the thermochemical compatibility and interfacial exothermic interactions between electrode materials and SE. Negative electrode material and composite positive electrode (NMC811, LPSC, and vapor-grown carbon fiber (VGCF) in a weight ratio of 70:30:3) were carefully disassembled from ASSBs in an Ar-filled glove box for ARC analysis.

Initial results showed that no exothermic behavior is detected for Li-In negative electrode with LPSC up to 350°C, indicating their good thermal stability and limited involvement of the exothermic chain reactions during elevated temperatures (Fig. 1b and Supplementary Fig. 6). However, 100% SOC composite positive electrode exhibited Tonset and Ttr of 169.3±7.8°C, and 255.3±14.7°C, which aligned with the onset self-heating of the full cell, mirroring that positive electrode/SE reactions were mainly responsible for the observed exothermic behavior of the full cell (Fig. 1b and Supplementary Fig. 7). The differing thermal runaway behaviors between isolated positive electrode materials and full cells are primarily due to electrochemical crosstalk, where reactive species like oxygen released from the positive electrode migrate to the negative electrode and trigger intense secondary reactions, combined with contrasting experimental conditions, specifically the closed system of material-level tests versus the potentially open system in cell-level tests, which significantly alters gas release and heat accumulation dynamics. Additionally, it was unexpected to find that 0% SOC composite positive electrode also generated three discontinuous exothermic processes at 228°C, 276°C, and 340°C in the HWS plot (Fig. 1b and Supplementary Fig. 8), indicating complex reaction mechanisms that differed from phase transition-induced interfacial reactions observed in delithiated states. The electrode|SE reactions were further examined by ramp heating test at ARC with a heating rate of 5°C min-1. Severe self-heating was observed in the 100% SOC composite positive electrode, with the temperature rapidly rising from 203°C to 363°C within just 3.9 seconds (Supplementary Fig. 9). Moreover, grinding the 100% SOC composite positive electrode in the Ar-filled glove box led to violent burning (Supplementary Fig. 10). These findings raise significant safety concerns regarding the 100% SOC composite positive electrode, as it can trigger thermal runaway under both thermal and mechanical abuse conditions. Overall, these results unveil that it is the positive electrode|SE reactions that initiate and accelerate the self-heating of the full cell, thus the following investigation will focus on decoding the specific reaction evolution path between the layered oxide positive electrode and SE.

Material-level interfacial reactions
To unravel the complex interfacial reactions governing thermal instability in solid-state batteries, the thermochemical compatibility between pristine oxide positive electrode, SE and VGCF was firstly investigated. Utilizing a customized online accelerating rate calorimetry-mass spectrometry (ARC-MS) system and differential scanning calorimetry (DSC), we precisely tracked exothermic events and gas evolution profiles of the composite positive electrode under heating condition. Initial characterization confirmed the individual stability of NCM811 layered oxide, LPSC, and VGCF additives, with no detectable thermal decomposition below 500°C when tested in isolation (Supplementary Fig. 11). However, when formulated into an uncycled composite positive electrode (70:30:3 mass ratio), this ostensibly stable system underwent dramatic interfacial reactions, exhibiting two distinct exothermic peaks at 373°C and 465°C (Fig. 1c and Supplementary Fig. 12). ARC-MS results indicated the absence of gaseous byproducts, mirroring their solid-solid reaction pathways. To further elucidate this emergent thermal sensitivity, staged heat treatments followed by microstructural analysis were conducted. Scanning electron microscopy with energy-dispersive spectroscopy (SEM-EDS) of the composite heated to 400°C revealed nascent sulfur diffusion from LPSC into NCM811 surfaces, accompanied by oxygen migration in the reverse direction (Fig. 1c). Upon reaching 500°C, these interdiffusion processes culminated in substantial sulfur and oxygen depletion, coinciding with the formation of nickel-sulfur and phosphorus-oxygen bonds, a transformation that aligned systematically with the hard-soft acid-base principle. Specifically, the hard acid character of phosphorus (P5+) drives preferential coordination with oxygen (O2-, hard base), while softer nickel cations (Ni3+/4+) selectively interact with sulfur anions (S2-, soft base). This chemically selective bonding behavior, rooted in Lewis acid-base interaction energetics, provides a fundamental thermodynamic rationale for the inherent incompatibility between NCM811 and LPSC components, ultimately governing their interfacial degradation dynamics under thermal abuse conditions21,22. Complementary in situ heating X-ray diffraction (XRD) analysis revealed the progressive disappearance of NCM811 (003) and (104) diffraction peaks near 373°C, concurrent with the emergence of a (220) peak attributed to NiO formation, confirming phase transformation in non-delithiated NCM811 when thermally coupled with LPSC (Fig. 1d). Distinctive phase evolution was observed at elevated temperatures, where XRD patterns at 400°C indicated co-formation of NiO, Li2S, and Li3PO4, attributed to the reaction between NCM811 and LPSC (Fig. 1e). Upon reaching 500°C, complete LPSC decomposition coincided with appearance of Ni3S2 and CoS, confirming transition metal sulfide formation through sulfur interaction with oxide components, thereby accounting for sulfur migration into NCM811. To validate the secondary exothermic mechanism involving transition metal oxide|LPSC interactions, stoichiometric NiO was mixed with LPSC mixtures and then subjected to 500°C thermal treatment, the yielding Li2S, NiS, and Li3PO4 as reaction products convinced the critical role of rocksalt transition metal oxide on contributing the second stage exothermic reactions (Supplementary Fig. 13). Concurrently, X-ray photoelectron spectroscopy (XPS), time-of-flight secondary ion mass spectrometry (TOF-SIMS) profiles detected sequential phosphate, hypophosphite, sulfate, and sulfite species generation during thermal excursions, with sulfite/sulfate signatures at 400°C quantitatively matching sulfur diffusion trajectories (Fig. 1f, Supplementary Fig. 14). Raman spectral evolution further corroborated this interfacial degradation pathway, showing Li2S/LiS and PO43- formation at 400°C, followed by LPSC signal attenuation and SO42- emergence at 500°C (Fig. 1g). These multimodal characterizations collectively confirm the solid-solid interfacial degradation mechanisms. Despite inherent thermal stability of individual components, the thermodynamic incompatibility between pristine NCM811 and LPSC is conclusively demonstrated to initiate exothermic interfacial reactions under thermal abuses.

Beyond bulk chemical interactions, electrochemically driven interfacial reactions are also critical factors that must be carefully considered. The inherent electrochemical instability of LPSC, manifested through its narrow stability window and valence band misalignment with NCM811, induces formation of passivating interphase comprising sulfurous species under operating potentials. Concurrently, structural destabilization of the delithiated layered oxide positive electrode tends to exacerbate interfacial reactions. To systematically evaluate these coupled phenomena, composite positive electrodes harvested from ASSBs after formation cycling, including fully charged (4.5 V vs. Li+/Li, 100% SOC) and discharged (2.8 V vs. Li+/Li, 0% SOC) states, were subjected to in situ thermal characterizations. For the 0% SOC sample, two exothermic events were recorded at 302°C (ΔH=81.9±4.9 J·g-1,) and 379°C (ΔH=333.9±56.2 J·g-1) (Fig. 2a and Supplementary Fig. 15). Structural evolution monitored via in situ heating XRD spectra revealed minimal crystallographic changes below 310°C, followed by abrupt disappearance of NCM811|LPSC diffraction peaks and concurrent NiO phase emergence at 379°C, confirming bulk material interactions during the second exothermic phase. The absence of gaseous products throughout heating demonstrated their solid-solid reaction pathways. XPS analysis of the 310°C-treated composite revealed diminished -P-S-P- bond intensity and complete -S-S- bond dissociation in S 2p/P 2p spectra, directly implicating electrochemical interphase decomposition as the origin of the initial exothermic event (Fig. 2b). Subsequent Ni3S2 and PO43- formation detected at 450°C validated the reaction between NCM811 and LPSC. These findings collectively establish a bifurcated thermal degradation paradigm: the first exothermic peak arises from electrochemically formed metastable interphase breakdown, while the second originates from intrinsic NCM811|LPSC incompatibility. The depressed decomposition threshold of interphase species (relative to bulk material reactions) underscores electrochemical preconditioning as a critical thermal risk accelerator. Complementary postmortem analyses through SEM, ex situ XRD, and Raman spectroscopy systematically corroborated these mechanistic interpretations (Supplementary Figs. 16-18). Noteworthily, the 100% SOC composite positive electrode displayed significantly enhanced interfacial reactivity under thermal stress, characterized by two sharp exothermic peaks at 209°C and 299°C (Fig. 2c and Supplementary Fig. 19). This process was accompanied by a markedly greater heat release (ΔH1=381.8±16.4 J·g⁻¹, ΔH2=869.7±89.5 J·g⁻¹) compared to the pristine and 0% SOC counterparts (Supplementary Table S1). The initial exothermic event was critically noted to align with severe self-heating behavior identified in prior ramp tests (Supplementary Fig. 9), with its early onset temperature and correlation to thermal runaway mechanisms necessitating focused investigation. In situ heating XRD analysis revealed structural evolution during these events: the first peak corresponded to phase transformation of delithiated NCM811 from layered to spinel structure, while the second marked transition to rock-salt phase alongside progressive LPSC signal attenuation. The formation of NiO occurred alongside the initial reaction of LPSC, suggesting a synergistic promotion driven by the combined effects of lattice oxygen release from the positive electrode and destabilization by sulfur species (Fig. 2c). These observations confirm delithiated positive electrode-driven initiation of thermal decomposition, with bulk solid electrolyte involvement restricted to later stages (280°C-350°C). Concurrent gas evolution profiles detected via ARC-MS identify SO2 (m/z=64), CO2 (m/z=44), CO (m/z=28), and O2 (m/z=32) release during the first exothermic event, implicating lattice oxygen from delithiated NCM811 in oxidative side reactions (Fig. 2c). Post-reaction characterization of samples quenched at 250°C and 400°C revealed mechanistic details. XPS analysis at 250°C showed depletion of -S-S- bonds, reduced -P-S-P- intensity, and emergence of SO42-/SO32-/PO43- species, confirming electrochemical interphase participation in early exothermic processes (Fig. 2d). Conversely, 400°C-treated samples demonstrated LPSC decomposition with NiS/CoS formation, signifying direct NCM811|LPSC interactions. SEM-EDS mapping at 250°C verified the reaction between NCM811 and interphase species, while 400°C analysis revealed Ni-S and P-O bonding indicative of complete interfacial reactions (Fig. 2e). XRD phase evolution further corroborated this mechanism. (003) peak weakening/shifting below 250°C reflected positive electrode restructuring without LPSC participation, whereas LPSC signal disappearance and NiO/NiS/Li3PO4 emergence at 400°C confirmed bulk material interactions. Raman spectra exhibited PS43- depletion and SO42-/PO43- formation at 400°C, consistent with the reaction between NCM811 and LPSC (Supplementary Fig. 20). Electron energy loss spectroscopy (EELS) analysis elucidated dynamic redox evolution of nickel species during thermal degradation. At 250°C, concurrent shifts in the O K-edge pre-edge (527–534 eV) toward higher energy loss and Ni L-edge (852–858 eV) toward lower energy loss were observed, directly evidencing Ni3+/Ni4+ reduction coupled with reactive lattice oxygen liberation. This dual electronic-state modulation mechanistically accounted for oxygen-containing gas evolution during the initial exothermic phase (Supplementary Fig. 21). Subsequent O K-edge disappearance and continued Ni L-edge shifts at 400°C verified rock-salt phase formation. Collectively, these results establish a dual-path thermal degradation mechanism: low-temperature exothermicity arises from electrochemical interphase|lattice oxygen interactions, while high-temperature events originate from intrinsic NCM811|LPSC incompatibility.

To further clarify the proposed mechanisms, cross-sectional specimens of thermally treated NCM811 were prepared via focused ion beam (FIB) milling and cross-sectional polishing (CP). Comprehensive microstructural characterization was performed through SEM, electron backscatter diffraction (EBSD), and transmission electron microscopy (TEM) to map elemental distribution and phase evolution. At 250°C, SEM-EDS analysis confirmed NCM811 particles retained Ni-O dominance without sulfur penetration into bulk regions (Fig. 3a), while EBSD mapping identified spinel-phase M3O4 (M=Ni/Co/Mn) formation within positive electrode particles (Fig. 3c). These observations collectively demonstrate that the initial exothermic event is governed by positive electrode phase transformation. Thermal escalation to 400°C revealed substantial sulfur infiltration into NCM811 particles, evidenced by intensified S signals and attenuated O intensity in SEM-EDS (Fig. 3b). EBSD quantification detected interfacial reaction products, NiS (79.4%), Li2SO4 (18.1%), and Li3PO4 (2.5%), distributed across positive electrode cross-sections, confirming S migration from LPSC and subsequent sulfide formation (Fig. 3d). High-angle annular dark-field transmission electron microscopy (HAADF-TEM) imaging coupled with selected area electron diffraction (SAED) patterns conclusively verified spinel M3O4 at 250°C and crystalline NiS at 400°C (Fig. 3e, f), corroborating SEM/EBSD findings. Simulation calculations were conducted to explore the interfacial incompatibility between NCM811 and LPSC at atomic scales. According to solid-state band theory, the calculated VBM of LPSC was 1.47 eV higher than the Fermi level of NCM811, indicating that electron transfer from LPSC to NCM811 was energetically favorable (Supplementary Fig. 22). This transfer triggers the oxidative decomposition of LPSC, highlighting the inherent incompatibility between NCM811 and LPSC. Furthermore, density of states (DOS) calculations revealed significant overlap between the S 3p orbitals of LPSC and the O 2p/Ni eg orbitals of NCM811 near the Fermi level (Supplementary Fig. 23). This overlap indicates a lower energy barrier for electron exchange, further supporting the hypothesis of strong chemical incompatibility between them. Moreover, the P 2p orbitals of LPSC also showed considerable overlap with the O 2p/Ni eg orbitals of NCM811, suggesting a tendency for spontaneous chemical bonding23,24. In addition, ab initio molecular dynamics (AIMD) simulations provided critical insights into the interfacial dynamics. At 250°C, the simulations showed that delithiated NCM811 and LPSC maintain their structural integrity, with PS43- units remaining intact (Fig. 3g and Supplementary Fig. 24). However, at 400°C, LPSC degradation becomes evident, marked by the cleavage of PS43- bonds and the formation of P-O bonds. This convergence between multiscale computational results and experimental data strongly corroborates the proposed thermal decomposition pathways.

At this point, the specific reaction pathway between VGCF and electrochemically-derived interphase components and the delithiated positive electrode becomes critically interrogated. To decouple individual interaction mechanisms of the interphase components, strategically synthesized electrochemical degradation products were systematically investigated. Electrochemically decomposed LPSC (4.5 V-(LPSC/VGCF), LPSC and VGCF in 9:1 mass ratio) was synthesized by charging an In|LPSC|(LPSC/VGCF) half-cell to 4.5 V vs. Li+/Li, with XPS and Raman spectroscopic characterization confirming the presence of -S-S- and -P-S-P- species mirroring the 100% SOC composite positive electrode’s interphase compositions (Supplementary Figs. 25 and 26). Neat delithiated NCM811 (De-NCM) collected from fully-charged liquid-electrolyte pouch cells exhibited identical structural characteristics to ASSB-derived counterparts (Supplementary Fig. 27), ensuring comparability across experimental systems. Different combinations of De-NCM mixed with other interphase components were tested under DSC and ARC-MS. Clearly, isolated De-NCM demonstrated singular exothermic initiation at 221°C accompanied by O2 evolution (Figs. 3h-3I, Supplementary Fig. 28). The De-NCM/pristine-(LPSC/VGCF) system exhibited analogous DSC onset temperatures and oxygen release patterns, suggesting that their exothermic reactions start with phase transition of delithiated NCM while the LPSC takes part in the interaction at latter stage. Strikingly, the De-NCM/4.5 V-(LPSC/VGCF) system containing electrochemical degradation products displayed accelerated thermal response, with exothermic onset at 207°C and 263% enthalpy increase (495.8±47.6 J·g-1 vs 149.2 J·g-1), concurrently evolving SO2 and O2 (Supplementary Fig. 29). Given the good thermal stability of 4.5 V-(LPSC/VGCF) below 400°C, these observations conclusively attribute early-stage heat generation and sulfurous gas evolution to reactive interphase between electrochemically-degraded LPSC and delithiated NCM811 (Supplementary Fig. 30). To probe the interfacial reaction mechanisms, individual decomposition products of LPSC, including Li2S, S, β-Li3PS4 (β-LPS, Supplementary Fig. 31), and -P-S-P- species, were mixed and heated with De-NCM. DSC analysis revealed minimal influence of Li2S on primary exothermic events (no SO2 detection, Supplementary Fig. 32), although Li2S did react with delithiated NCM811 at high temperatures to produce Ni3S2 (Supplementary Fig. 33). Conversely, De-NCM/S and De-NCM/β-LPS mixtures exhibited intensified exothermicity 490.6±87.1/441.4±32.3 J·g-1 with predominant SO2 emission and attenuated O2 release (Fig. 3h, i, Supplementary Figs. 34 and 35). Furthermore, electrochemically decomposed LPS (4.5 V-(LPS/VGCF)) was further obtained to examine the exothermic behavior of -P-S-P- species. DSC result reveals large exothermic signals of delithiated-LPS/delithiated-NCM811 (903.3 J·g-1) and P2S5/delithiated-NCM811 (2072.1 J·g-1), confirming -P-S-P- species significantly contribute to the early-stage exothermic reactions (Supplementary Fig. 36). The Fermi levels and electronic properties of the key reactive intermediates provide insight into electron transfer processes (Supplementary Table S2). Upon electrochemical oxidation (e.g., during cycling or at high voltage), LPSC decomposes into products such as elemental sulfur, Li3PS4, and polysulfides. These degradation products have distinctly different sulfur chemical environments and are more reactive with oxygen. The systematic evidence establishes electrochemical LPSC decomposition products, particularly sulfurous species, as critical initiators of early-stage exothermic cascades and exclusive SO2 sources in fully charged positive electrodes during thermal runaway propagation.

Furthermore, the influence of VGCF on thermal propagation was systematically investigated. Its relevance stems from a dual functionality within the composite positive electrode: reacting with delithiated NCM811 to generate CO and CO2, and catalyzing the decomposition of the solid electrolyte into sulfur-containing substances. In the 100% SOC configuration, CO/CO2 evolution detected during initial exothermic events (Fig. 2c) is attributed to interfacial reactions between delithiated NCM811. Quantitative calorimetric analysis revealed this carbon-positive electrode interaction contributes 90.8 J·g-1 (16.8% of total 541.9 J·g-1), establishing its secondary thermal role (Supplementary Fig. 37). The catalytic role of VGCF in decomposing the sulfide electrolyte was further confirmed. In cycled positive electrodes, increasing the carbon content from 0.6 wt.% to 13.0 wt.% led to a marked increase in the formation of -S-S- and -P-S-P- species as well as Li3PS4 (Supplementary Figs. 38–39), in agreement with previous reports of carbon-mediated SE degradation25–27. Thermal analysis of these systems with varied carbon content demonstrated a proportional escalation in exothermic enthalpy (169.2, 399.6, and 631.2 J·g⁻¹) corresponding to increased VGCF loading (0.6, 2.9, and 13.0 wt%; Supplementary Fig. 40). This correlation confirms that heat release is driven by two synergistic mechanisms: (1) direct exothermic carbon-electrode reactions producing CO/CO2, and (2) catalytically enhanced LPSC decomposition that amplifies subsequent interfacial heat generation. This dual mechanism establishes conductive carbon as both reactant and reaction promoter in thermal runaway initiation.

Thus, the specific thermal runaway pathway in NCM811|LPSC systems could be summarized into a staged progression: electrochemical incompatibility initiates self-heating while chemical incompatibility accelerates it. Reactive oxygen species from delithiated NCM811 preferentially react with cycling-induced interphase components (-S-S-/-P-S-P-/Li3PS4) rather than LPSC itself to start the exothermic chain reactions, producing SO2 and sulfate/phosphate species alongside CO/CO2 from VGCF oxidation. This interfacial reaction dominates initial heat generation, driving rapid exothermic escalation. Subsequent temperature rise activates the reaction between LPSC and NCM811, forming NiS, Li3PO4, and sulfate/phosphate compounds with substantial exothermic contribution. The sequential energy release, electrochemical initiation at interfaces followed by chemical amplification in bulk materials, establishes a chain reaction mechanism where electrochemical triggering and chemical acceleration synergistically drive system failure (Fig. 4a).

Furthermore, we also elucidated the thermal reaction mechanism of the composite positive electrode at 0% SOC to contrast with its decomposition pathway at 100% SOC. The first exothermic event at 0% SOC is attributable to the reaction between NCM and phosphorus sulfide intermediates (such as -P-S-P- bridged structures) generated from LPSC decomposition (Supplementary Figs. 41-42). Conversely, the second exothermic event is attributed to the reaction between the LPSC and NCM.

The universal applicability of the thermal degradation mechanism across different sulfide solid electrolytes was systematically evaluated (Fig. 4b-d). Specifically, we extended our investigation to Li10GeP2S12 (LGPS) and Li9.54Si1.74P1.44S11.7Cl0.3 (LSPSC) using the same methodology applied to LPSC. Composite positive electrodes were prepared in both the pristine state (De-NCM mixed with pristine SE) and the electrochemically decomposed state (De-NCM mixed with SE charged to 4.5 V). The thermal behavior of these systems was subsequently characterized using adiabatic calorimetry and gas analysis.

Both fully charged LGPS- and LSPSC-based composites exhibit significant self-heating initiating near 202°C and 199°C, respectively, closely matching the behavior of the LPSC system (199°C) (Supplementary Figs. 43-46). The first exothermic peak releases 395.5 J·g-1 for the charged LGPS|VGCF composite and 419.9 J·g-1 for the LSPSC-based counterpart. Crucially, the majority of this heat (246.3 J·g-1 and 270.7 J·g-1, respectively) originates from the reaction between De-NCM and the electrochemically generated interphase species (e.g., oxidized sulfur and phosphorus compounds), rather than with the pristine electrolytes (Supplementary Fig. 47). This initial reaction stage is accompanied by significant SO2 evolution, as confirmed by mass spectrometry (Supplementary Fig. 48). In stark contrast, control samples containing De-NCM mixed with the pristine (uncharged) LGPS or LSPSC powders showed negligible exothermic activity or gas release. These results universally confirm that the primary trigger for thermal runaway is the reaction between the delithiated positive electrode and the metastable decomposition products formed at the interface during high-voltage charging, not the reaction with the bulk, pristine solid electrolyte. This validates the generalizability of our proposed degradation roadmap across different sulfide electrolytes.

Interface regulation towards safer sulfide ASSBs
The aforementioned findings, which demonstrate the critical role of interfacial interactions in initiating heat generation during early-stage thermal runaway, underscore the necessity of interfacial stabilization between NCM811 and LPSC to improve ASSB thermal safety. Positive electrode surface coating has been established as an effective mitigation strategy through the introduction of inert interfacial barriers to suppress positive electrode/SE decomposition, as documented in prior researches28,29. To evaluate this approach, ASSBs incorporating Al2O3-coated NCM811 positive electrodes (designated C-NCM, 4 nm coating thickness, Supplementary Fig. 49) were systematically investigated. Initial thermal characterization of uncycled composite positive electrodes (70:30:3 mass ratio of C-NCM, LPSC, and VGCF) revealed two elevated exothermic peaks at 408°C and 485°C in DSC profiles (Supplementary Fig. 50), demonstrating temperature increments compared to uncoated counterparts (373°C and 465°C, Fig. 1c). In situ heating XRD analysis further confirmed enhanced stability, with no phase transitions observed in coated positive electrodes below 400°C (Supplementary Fig. 51), contrasting with prominent NiO signatures detected at 373°C in uncoated systems (Fig. 1d). These observations indicate improved interfacial compatibility through Al2O3 modification. Post-cycling analysis of C-NCM-based ASSBs (Supplementary Fig. 52) revealed attenuated XPS signals for -S-S-, -P-S-P-, and PO43- species compared to uncoated positive electrodes at full SOC (Supplementary Fig. 53), confirming reduced electrochemical degradation. Thermal evaluation of cycled C-NCM positive electrodes through DSC, ARC-MS demonstrated partial heat release mitigation, with initial exothermic onset at 213°C (ΔH=318.8 J·g-1) versus 209°C (ΔH=399.6 J·g-1) for unmodified positive electrodes (Fig. 2c and Supplementary Fig. 54). However, persistent SO2 evolution was detected at elevated temperatures despite reduced emission quantities, accompanied by comparable self-heating profiles during ARC ramp tests (200°C onset, Supplementary Fig. 55). These findings indicate that while surface coating partially suppresses parasitic interfacial reactions, complete thermal runaway prevention remains unachievable due to the intrinsic chemical incompatibility between layered oxide positive electrodes and sulfide SEs.

The oxidative decomposition tendency of LPSC under high-voltage conditions has been fundamentally attributed to its VBM positioning above the Fermi level of NCM811, creating inherent energy level misalignment with layered oxide positive electrodes as per solid-state band theory20. The complete resolution of this thermo-electrochemical decomposition phenomenon necessitates intrinsic material-level interfacial compatibility regulation. Phosphorus-containing sulfide SEs (LPSC, LPS, LGPS, LSPSC) universally exhibit analogous thermal safety limitations due to shared P-S bonding characteristics. According to hard-soft acid base principles, the strong affinity between hard acid phosphorus and hard base oxygen in NCM811 drives interfacial incompatibility. Compared with phosphorus, germanium (Ge) demonstrates softer acid properties with reduced thermodynamic propensity for oxygen bonding, theoretically enabling enhanced positive electrode|SE compatibility through P to Ge substitution in LGS synthesis21,30. Experimentally synthesized Li4GeS4 demonstrated room-temperature ionic conductivity of 1×10-4 S cm-1 (Supplementary Fig. 56). First-principles calculations revealed improved chemical stability between NCM811 and LGS (-0.201 eV/atom) compared with NCM811-LPSC (-0.357 eV/atom) (Fig. 5a and Supplementary Fig. 57). Energy band analysis further confirmed improved LGS/NCM811 alignment through reduced VBM-Fermi level gap versus LPSC/NCM811 (Supplementary Fig. 58). These computational predictions validate the Ge-substitution strategy for interfacial optimization. Despite these advantages, Ge-containing SEs like LGPS exhibit lithium metal incompatibility through Li-Ge alloy formation (Supplementary Fig. 59)31,32, prompting the development of LGS as ionic conductor additives rather than bulk SE replacements. XPS and DSC analysis of both De-NCM/pristine LGS and De-NCM/4.5 V-(LGS-VGCF) composites revealed that LGS undergoes less electrochemical decomposition and ΔH than LPSC (Supplementary Figs. 60 and 61). The fabricated Li2.5Si|LPSC|NCM811-LGS ASSBs demonstrated enhanced electrochemical performance with 200.8 mAh·g-1 initial discharge capacity (0.05C, specific current: 1C=200 mA g-1, 45°C) and 87.4% capacity retention after 100 cycles (0.5C, 45°C) (Fig. 5b, Supplementary Fig. 62). Thermal safety evaluation utilized composite positive electrodes (NMC811, LGS, and VGCF in a weight ratio of 70:30:3) extracted from fully charged cells (Supplementary Fig. 63). S 2p XPS analysis revealed substantially reduced -S-S- species at LGS interfaces versus LPSC-based positive electrodes, experimentally confirming decomposition mitigation (Fig. 5c). For thermal characterizations, pristine LGS composites showed no exothermic activity below 450°C versus LPSC’s 373°C decomposition (Fig. 5d vs 1c, Supplementary Fig. 64). Post-cycling 0% SOC LGS composites maintained high thermal stability to 450°C, contrasting with LPSC’s 302°C exothermic initiation (Fig. 5d vs 2a, Supplementary Fig. 65). Charged-state LGS positive electrodes exhibited thermal runaway onset ΔH reduction (381.8±16.4 to 99.9±9.3 J·g⁻¹), indicating suppressed parasitic reactions (Fig. 5d vs 2c, Supplementary Fig. 19 vs 66,). These observations were further corroborated by in situ MS detection of minimal SO2 emissions from LGS systems (Fig. 5e). The ultimate validation of this interface regulation strategy emerged from full-cell thermal runaway analysis. ARC testing revealed complete elimination of severe self-heating in LGS-based systems (Supplementary Fig. 67), attributable to blocked reaction pathways between stabilized interphases and delithiated positive electrodes. Most significantly, whole-cell evaluations demonstrated substantial safety parameter improvements with Tonset/Ttr elevated to 204.0±15.9/324.0±9.1°C versus baseline 163.0±8.5/231.7±2.6°C (Fig. 5f, Supplementary Fig. 68). The interlinked experimental evidence collectively demonstrates that strategic manipulation of interfacial chemistry through Ge-substitution effectively decouples the chain reactions driving thermal runaway, providing critical insights for developing thermally stable ASSBs.

This study systematically elucidates the thermal runaway mechanisms in sulfide-based all-solid-state batteries through comprehensive in situ and ex situ investigations of LPSC electrolyte|NCM811 positive electrode interactions. The experimental findings systematically demonstrate that positive electrode|electrolyte interfacial reactions govern ASSB thermal instability, with electrochemically generated interphase components (formed during cycling) reacting preferentially with delithiated NCM811 to drive initial heat generation and sulfur dioxide evolution during early-stage thermal runaway. As temperatures escalate, bulk chemical interactions between NCM811 and LPSC dramatically accelerate exothermic processes, ultimately triggering uncontrolled thermal propagation. These interfacial degradation pathway extensions extend across other sulfide systems (LGPS, LSPSC). Moreover, by stabilizing the electrochemically formed interface through Ge-S bond engineering, the Ge-substituted LiIn|LPSC|Li4GeS4-NCM811 ASSBs demonstrate significantly enhanced safety with Tonset/Ttr increasing from 168/228°C to 223/312°C, validating interfacial stabilization as a critical strategy for thermally robust ASSBs. The findings provide both mechanistic insights into interface-driven failure and practical solutions for safety-enhanced solid-state batteries.

Methods
Electrode preparation
For the preparation of composite positive electrode powders, positive electrode materials such as LiNi0.8Co0.1Mn0.1O2 and Al2O3-coated LiNi0.8Co0.1Mn0.1O2, SEs (such as LPSC, LGPS, LSPSC, and LGS), and VGCF powders are mixed in a weight ratio of 70:30:3. The mixing process involves manual grinding for 30 minutes in a glove box under an argon atmosphere. SE/VGCF (LPSC/VGCF, LGPS/VGCF, and LSPSC/VGCF) electrode composite powders are prepared through a mechanochemical method. A stoichiometric mixture of SE and VGCF in a weight ratio of 9:1 is ball-milled at 300 rpm for 2 h. The Li-In alloy is stored after the lithium foil (3 mm in diameter) and indium foil (8 mm in diameter) are pressed under a force of 0.5 tons for 5 minutes in a PEEK die (10 mm diameter). The Li2.5Si alloy is prepared by manually grinding the stoichiometric mixture of Si powders and lithium foil for 30 minutes in a glove box filled with an Ar atmosphere.

Electrochemical performance
Li-In|LPSC|LiNi0.8Co0.1Mn0.1O2 ASSBs are assembled in a PEEK die (10 mm diameter). 100 mg LPSC powders are first pressed under a force of 1.6 tons as the electrolyte layer, with a thickness of 0.68 mm. Then the prepared composite positive electrode powders are added homogeneously on the top of the electrolyte layer and compressed under a force of 3.2 tons (the electrode active material loading is 17.83 mg/cm2). In the end, Li-In alloy negative electrode is added on the bottom of the LPSC layer and compressed for 2 min under a force of 0.4 tons. The stack pressure is 50 MPa during battery operation. The cycle performances are evaluated using a Neware battery test system (CT-4008Q, China) in the voltage range of 2.8−4.2 V (vs. Li+/Li) at 0.1 C (specific current: 1C=200 mA g-1) and 30°C.

Li-In|LPSC|LiNi0.8Co0.1Mn0.1O2 ASSBs are assembled for thermal runaway investigation. The ASSBs are assembled in a PEEK die (20 mm diameter). 400 mg LPSC powders are first pressed under a force of 4 tons as the electrolyte layer, with a thickness of 0.74 mm. Then the prepared composite positive electrode powders are added homogeneously on the top of the electrolyte layer and compressed under a force of 10 tons (the electrode active material loading is 36.78 mg/cm2). In the end, Li-In alloy negative electrode is added on the bottom of the LPSC layer and compressed for 2 min under a force of 1.6 tons. The stack pressure is 50 MPa during battery operation. The cycle performances are evaluated using a Neware battery test system (CT-4008Q, China) in the voltage range of 2.8−4.5 V (vs. Li+/Li) at 0.05 C (specific current: 1C=200 mA g-1) and 30°C.

SE/VGCF (LPSC/VGCF, LGPS/VGCF, and LSPSC/VGCF)/LPSC/In half-cells are assembled to obtain the SEs after electrochemical decomposition. The half-cells are assembled in a PEEK die (20 mm diameter). 400 mg LPSC powders are first pressed under a force of 4 tons as the electrolyte layer (0.74 mm in thickness). Then the prepared composite positive electrode powders are added homogeneously on the top of the electrolyte layer and compressed under a force of 10 tons (the electrode active material loading is 11.82 mg/cm2). In the end, indium negative electrode is added on the bottom of the LPSC layer and compressed for 2 min under a force of 1.6 tons. The stack pressure is 50 MPa during battery operation. The cycle performances are evaluated using a Neware battery test system (CT-4008Q, China) after charging to 4.5 V (vs. Li+/Li) at 0.05 C and 30°C.

Li2.5Si|LPSC|LGS+LiNi0.8Co0.1Mn0.1O2 ASSB is assembled for investigating the cycling performance of the battery with LGS as an ionic conductor additive in the composite positive electrode. The ASSB is assembled in a PEEK die (10 mm diameter). 100 mg LPSC powders are first pressed under a force of 1.6 tons as the electrolyte layer (0.68 mm in thickness). Then the prepared composite positive electrode powders are added homogeneously on the top of the electrolyte layer and compressed under a force of 3.2 tons (the electrode active material loading was 9.8 mg/cm2). In the end, Li2.5Si alloy negative electrode is added on the bottom of the LPSC layer and compressed for 2 min under a force of 4 tons. The stack pressure is 50 MPa during battery operation. The cycle performances are evaluated using a Neware battery test system (CT-4008Q, China). The assembled ASSBs are activated for 2 cycles in the voltage range of 2.4−4.2 V at 0.05 C and 45°C, and then undergo charge-discharge cycling in the voltage range of 2.4−4.2 V at 0.5 C and 45°C.

Materials characterization
Accelerating Rate Calorimeter (ARC) and Ramp test of pouch cell and electrode-electrolyte are conducted with a BTC130 (HEL, England), equipped with a mass spectrum (MS, HPR-20, Hiden Analytical Ltd). The “Heat-Wait-Search” model starting temperature is 100°C with a heating step of 10°C. The self-heating rate detection limit is 0.02°C min-1 and thermal runaway criteria is 1°C min-1. The “Ramp” model is heated with a heating rate of 5°C min-1 up to 400°C. The electrode materials and SEs are carefully disassembled from ASSBs in an Ar-filled glove box. 200 mg electrode materials are transferred into the small bomb chamber made of Hastelloy alloy for Ramp test, and the released gases are analyzed by MS. All MS experiments were performed under an Ar flow at a rate of 5 mL/min. The Differential Scanning Calorimeter (DSC) measurement is conducted with a Netzsch DSC 214 system from 20°C to 500°C with a heating rate of 5°C min-1. Samples (~10.0 mg) with an NCM-to-SE weight ratio of 7:3, prepared under the same pressure as the composite positive electrode, were placed in a 100 μL stainless steel high-pressure sealed crucible (NETZSCH, Germany) for testing. The complementary use of ARC (with large sample mass to simulate bulk adiabatic runaway) and DSC (with small sample mass to resolve individual reactions) provides a complete picture of the thermal degradation mechanisms. Scanning electron microscopy (SEM) images of positive electrode are taken by S-4800 field emission SEM (Hitachi, Japan), operated at 10 kV and element mapping test is conducted by 7593-H (Horiba, Japan). Lab-scale X-ray diffraction (XRD) measurements are conducted using a Rigaku SmartLab X-ray diffractometer equipped with Cu Kα radiation. Temperature-resolved X-ray diffraction with an external direct heating component from room temperature up to 400°C at a heating rate of 1°C min-1, is tested from 10° to 70° with a scan speed of 10° min-1. Raman measurements are conducted on a Thermo Scientific spectrometer (Raman Microscope DXR) equipped with a 532 nm laser. The surface chemistry of the materials is detected by X-ray photoelectron spectroscopy (XPS, Thermo Scientific ESCALab 250Xi). Time-of-flight secondary ion mass spectrometry (TOF-SIMS) measurements are performed in a negative mode with a 30 keV Bi3+ primary ion beam source. Transmission electron microscope (TEM, Talos F200X) and electron energy loss spectroscopy (EELS) are taken for a detailed structural analysis. The electron backscattered diffraction (EBSD) images are acquired with Oxford C-nano. The focused ion beam (FIB) technique can cut samples and expose the cross sections of samples for TEM. The cross section polisher (CP) technique can cut samples and expose the cross sections of samples for EBSD.

Calculation method
First-principles calculations were performed using the projector-augmented wave plane-wave method, as implemented in the Vienna ab initio simulation package (VASP) via the MedeA platform33,34. The Perdew–Burke–Ernzerhof (PBE) functional under the generalized gradient approximation (GGA) was adopted for the exchange–correlation potential35. PBE+U approach is performed to calculate accurate structural properties and electronic properties with spin polarization. The effective U value was set to 6.7, 4.2, and 4.9 eV for Ni, Mn, and Co, respectively36. The plane-wave energy cutoff of 520 eV was used for structural relaxation. The conjugated gradient method is utilized to optimize the geometry with the convergence threshold of 10-5 eV in energy and 0.01 eV/Å in force, respectively. The potential decomposition reactions at the interfaces were examined along the pseudo-binary phase diagrams with varying fractions of reactants37. The crystal structures of Li6PS5Cl, Li2S, Li3PS4, P2S5 and Li4GeS4 were sourced from the Materials Project database. The NCM model for DFT calculations was constructed based on the LiNiO2 structure from the Materials Project by substituting Ni atoms with Co and Mn in the appropriate stoichiometric ratio. To closely mirror the highly delithiated state of the NCM positive electrode observed in our experiments, we modeled a delithiated NCM structure with an 80% degree of delithiation. This was achieved by removing the corresponding number of lithium atoms from the pristine model. The final structure for the density of states (DOS) calculations was the lowest-energy configuration obtained after DFT relaxation.

The interfacial reaction of the positive electrode with the electrolyte at elevated temperature were simulated by ab initio molecular dynamics (AIMD). The equations of motion were integrated with the Verlet algorithm using a time step of 2 fs, and micro canonical ensemble (NVE) was used to simulate the interfacial reaction from room temperature to 500°C. A plane-wave basis set with an energy cut-off of 520 eV and a 1×1×1 k-point mesh was used for the AIMD simulations. The AIMD simulations were performed for 2 ps. The Fermi levels of two materials obtained through VASP calculations cannot be directly compared due to the absence of a unified reference point. We employed a vacuum slab model to calculate the energy difference between the vacuum level and the Fermi level (i.e., the work function), selecting crystallographic planes with the lowest surface energy as our computational models. By aligning the vacuum levels of both materials under this vacuum slab model as a common reference benchmark and setting them as the energy zero point, we obtained and compared the numerical values of the materials’ Fermi levels relative to the vacuum level. This approach enables a meaningful comparison of the Fermi levels or VBM between the two materials.

Peer review rounds
[{"round": 1, "reviewer_comments": {"reviewer_1": "This study investigates the influence of electrochemically formed LPSC decomposition products on the thermal stability of sulfide all-solid-state batteries. The fully charged NCM exhibits thermal instability with solid electrolytes containing electrochemical decomposition products, whereas composites consisting of the charged NCM and pristine solid electrolytes show less exothermic reactions. These indicate that the decomposition of solid electrolytes during charge also contributes to thermal runaway mechanisms. Furthermore, the authors introduce Li4GeS4, which has improved chemical stability with NCM, and demonstrate its higher thermal stability compared to other sulfide solid electrolytes. These findings provide novel insights into the thermal instability induced by the electrochemically formed interfacial species. Therefore, I would recommend the publication of this paper after addressing minor comments.\n1. Rui et al. reported that thermal runaway between NCM and LPSC follows a solid-solid reaction (Energy Environ. Sci., 2023, 16, 3552-3563). This implies that the degree of contact between NCM and solid electrolytes has influence on thermal behavior. In addition, hand grinding of fully charged NCM and LPSC leads to violent burning even under inert atmospheres. How did the authors prepare the De-NCM/SE composites? Were the samples pelletized or loosely contacted?\n2. Adiabatic ramp tests showed a rapid temperature increase of the 100% SOC composite cathode near 200 °C without further temperature rises, while several exothermic peaks appeared in DSC. Why is there a discrepancy between the two methods? In addition, the amounts of sample used for ARC and DSC were significantly different: 200 mg for ARC and 10 mg for DSC. Does this difference have influence on the thermal behavior?\n3. The authors claimed that thermal runaway mechanisms are divided into two stages. The initial heat generation is driven by reactions between delithiated NCM and electrochemically formed interfacial species. At elevated temperatures, sulfur-oxygen interdiffusion leads to bulk exothermic reactions. Although Li4GeS4 showed improved thermal stability, it is still unclear in which stage Li4GeS4 mitigated the detrimental reactions. Therefore, it would be helpful to clarify the role of Li4GeS4 by performing DSC and XPS on De-NCM/pristine LGS and De-NCM/4.5V-(LGS-VGCF) samples. In addition, NCM811-LGS ASSBs were cycled at 45 °C, which is higher than the operating temperature of NCM811-LPSC cells. This increased operating temperature could influence the electrochemical decomposition. Figure 5c shows a higher content of sulfate and sulfite products in the 100% SOC composite cathode compared to those in Figure 2d.\n4. When the valence band edge of the solid electrolyte is above the Fermi level of the cathode, electron transfer from the solid electrolyte to the cathode leads to the oxidation of the solid electrolyte (ACS Appl. Energy Mater. 2022, 5, 2, 2045–2053).\n5. The sentences from line 346 to 352 need references to support it.\n6. Please add a scale bar in DSC Figures and specify the carrier gas used for MS measurements.", "reviewer_2": "This paper investigates thermal runaway mechanisms in sulfide-based all-solid-state batteries, focusing on NCM811/LPSC composite systems. It proposes a two-stage thermal degradation mechanism where early exothermic reactions arise from electrochemically formed interphase species, followed by bulk S-O interdiffusion at higher temperatures. Systematic thermal characterization reveals that charged cathodes (100% SOC) show greater thermal instability than discharged ones due to reactive interphase decomposition products like -S-S-, -P-S-P-, and Li3PS4. The study extends this to other sulfide electrolytes (LGPS, LSPSC) and suggests Li4GeS4 (LGS) for interface engineering, enhancing thermal safety by reducing thermodynamic driving forces for S-O exchange. DFT calculations support improved interfacial compatibility and experimental results indicate elevated thermal runaway temperatures, rising from 228°C to 312°C.\nHowever, this paper has significant theoretical inconsistencies and methodological limitations that undermine its scientific rigor. The computational analysis focuses on pristine materials (0% SOC), while the critical experimental work centers on delithiated states (100% SOC), creating a mismatch between theory and experiment. The band theory explanation contains errors in the direction of electron transfer and inaccurately compares electronic properties without proper interface analysis. The electronic structure calculations overlook the actual reactive species crucial to the proposed mechanism, indicating a major gap in understanding. Experimental reproducibility is questionable, as thermal safety data are presented without statistical analysis, error bars, or multiple trials, which are vital for safety applications. The novelty of the Ge substitution approach is limited due to prior work by the same group, and practical validation is insufficient for real-world implementation. Furthermore, the paper exhibits numerical inconsistencies, inadequate figure descriptions, and unclear reaction energetics that further undermine its reliability. Due to these deficiencies and lack of completeness, I do not recommend this manuscript in its current form, published in Nature Communications.\nComment 1) Figures 1a–1c present the ARC HWS and DSC traces for only one specimen, leaving the reproducibility of your thermal-safety metrics unclear. Please repeat the ARC-HWS and DSC tests at least three times under identical conditions (sample mass, cell pressure, HWS step length, purge gas, etc.) and report the mean ± standard deviation for the key parameters (Tonset, Ttr, and ΔH).\nComment 2) There is a fundamental inconsistency between the computational and experimental approaches. The DFT calculations are performed on pristine materials representing 0% SOC conditions, while the critical thermal runaway analysis focuses on 100% SOC delithiated states. This mismatch severely undermines the theoretical justification for experimental observations, as the electronic properties of delithiated NCM811 differ significantly from the pristine state used in calculations.\nComment 3) Similar to Comment 2, there is ambiguity regarding the lithiation state of NCM811 used in the reaction energy calculations shown in Figure 5a. The authors do not specify whether the computational analysis employs delithiated NCM811 (relevant to 100% SOC conditions) or lithiated NCM811 (relevant to 0% SOC conditions). Given that the thermal runaway mechanism is critically dependent on the delithiation state, this specification is essential for the proper interpretation of the calculated energetics and their relevance to experimental conditions.\nComment 4) The electronic structure analysis is incomplete and fails to address the actual reactive species driving the thermal runaway mechanism. While the authors calculate band alignments between bulk NCM811 and LPSC, they ignore the Fermi levels and electronic properties of the key reactive intermediates (Li3PS4, Li2S, Li2Sx, -S-S-, -P-S-P-) that they identify as primary thermal runaway initiators. For a mechanism centered on electrochemical interphase decomposition, the electronic structure of these species is crucial for understanding electron transfer processes.\nComment 5) The band theory analysis contains fundamental conceptual errors in explaining electron transfer mechanisms. The authors state that \"electron transfer from NCM811 to LPSC was energetically favorable\" based on LPSC's VBM being 1.47 eV higher than NCM811's Fermi level. However, this interpretation is incorrect: electrons naturally flow from higher to lower energy levels, so if LPSC's VBM is higher than NCM811's Fermi level, electron transfer should occur from LPSC to NCM811, not the reverse. Furthermore, the authors inappropriately compare VBM (a material property of LPSC) with the Fermi level (a material property of NCM811) without a proper band alignment analysis that accounts for interface formation, band bending, and equilibrium conditions.\nComment 6) The novelty of the proposed Ge substitution strategy is questionable given that the role of Ge in improving sulfide electrolyte stability has been previously established by the same research group's earlier publications, Kim et al. [13] The authors should clearly articulate what new insights or improvements their Li4GeS4 approach provides beyond existing Ge-based solutions and provide adequate differentiation from prior work.\nComment 7) There is a numerical inconsistency in the reported Tonset. The authors state \"Post-cycling 0% SOC LGS composites maintained stability to 450°C, contrasting with LPSC's 303°C exothermic initiation (Figure 5d vs 2a)\" and \"For the 0% SOC sample, two exothermic events were recorded at 303°C (ΔH=75.1 J/g) and 379°C (ΔH=268.8 J/g) (Figure 2a).\" However, Figure 2a shows the onset temperature as 302°C, not 303°C as reported in the text. This discrepancy requires correction for accuracy.\nComment 8) The manuscript contains inconsistent abbreviation usage. The authors define \"valence band maximum (VBM)\" on page 4, but subsequently use the full term again in \"the calculated valence band maximum (VBM) of LPSC was 1.47 eV higher than the Fermi level of NCM811\" (Figure S14). According to standard scientific writing conventions, once an abbreviation is defined, it should be used consistently throughout the manuscript rather than alternating between full terms and abbreviations.\nComment 9) Figure 4b lacks any description or discussion in the manuscript text. The authors present this figure without adequate explanation of its content, methodology, or relevance to the overall mechanism. This omission leaves readers unable to interpret the significance of the data presented, representing an incomplete presentation of the results.", "reviewer_3": "This manuscript investigates the thermal stability of sulfide-based all-solid-state batteries and elucidates the origin of thermal runaway, which is triggered by the interaction between the delithiated cathode and electrochemically decomposed LPSC. Furthermore, the authors provide insightful analysis on the role of VGCF in enhancing thermal safety, which is particularly meaningful for understanding how each component contributes to the overall thermal stability of ASSBs.\nThe study offers valuable insight into the safety aspects of sulfide-based solid-state batteries and makes a significant contribution by demonstrating that electrochemical decomposition products of the solid electrolyte at the cathode interface play a critical role in determining thermal stability. I believe this work is highly meaningful and recommend it for publication in Nature Communications, pending satisfactory responses to the following comments and questions.\nQ1. If the thermal stability is mainly dependent on the cathode interfacial reaction as authors suggested, then, what is the origin of the difference of Ttr of SOC 100 between full cell and composite cathode as shown in Figure 1a and 1b. Additionally, even the HWS curve of SOC0 between full cell and composite cathode is seems to be different. Is there any interaction between cathode or anode (like cross-talk)?\nQ2. According to the author’s insists, the first exothermic reaction is due to the decomposition of interphase. Given the chemical composition of interphase is nearly same regardless of SOC (0 or 100) as shown in XPS (Fig 2b, d), can you explain the reason of decomposition temperature composite with SOC 100 is lower than composite with SOC 0? If the delithiated cathode participate in decomposition of interphase, what is the exact reaction mechanism? According to the references, SOC 100 cathode can also evolve the O2 gas from the cathode when it converts to the spinel phase from layered structure. Then, is the reaction between O2 gas and interphase? In addition, the reason should be explained that gaseous product is evolved only at the first step of exothermic reaction not the second step of rock salt phase transition where the much more amount of O2 gas can be evolved.\nQ3. The evaluation of the interaction between Li2S, S, Li3PS4 and delithiated NCM is quite reasonable to investigate the effect of electrochemically degraded LPSCl on thermal stability as interphase. Can you explain why the S or Li3PS4 can evolve the SO2 gas evolution through the interaction with delithiated NCM? (in contrast to pristine LPSC).\nQ4. To emphasize the effect of interphase on thermal stability depending on the interaction with cathode, it is recommended to note the interfacial decomposition mechanism of SOC 0 also for comparison with SOC 100 (like as shown in Figure 4a).", "reviewer_4": "The manuscript presents a comprehensive investigation into the thermal runaway behavior of sulfide-based all-solid-state batteries (ASSBs), emphasizing the role of electrochemically formed interphases and chemical cascade reactions at the cathode-electrolyte interface. The use of multiscale calorimetry, gas analysis, and advanced structural characterization to support a two-stage decomposition mechanism is novel. The authors correlate material-level interfacial reactions with cell-level thermal behavior and propose a paradigm shift from bulk stability to interfacial thermodynamics as the primary safety determinant. The following comments should be addressed.\n1. While the authors attribute the first exothermic peak at 0% SOC to electrochemical interphase breakdown, it is unclear how such interphases persist or form under discharged conditions. Can the authors clarify whether residual sulfur species or kinetically trapped decomposition products contribute, and how this behavior is decoupled from active redox at 100% SOC?\n2. The author states that there is a sequential progression from interphase breakdown to bulk NCM-LPSC reactions but does not define the temperature boundaries or timing clearly. Could the authors specify more definitively when LPSC becomes actively involved?\n3. While the authors report multiple exothermic peaks, there are no normalized quantitative comparisons of reaction enthalpies (ΔH) across pristine, uncycled, and cycled cathodes. This makes it difficult to rank or compare thermal risk attributed to specific interfacial chemistries across states of charge. Can the authors elaborate on this to allow a clearer comparison?\n4. The authors present in-situ heating XRD and identify NiO formation, but it is unclear whether NiO arises from lattice oxygen evolution, sulfur-induced destabilization, or both. Can the authors clarify this? NiO is typically associated with the oxidative decomposition of NCM cathodes, whereas sulfur-rich environments tend to reduce transition metals.\n5. In section 2.3, the authors use NCM811 cathodes with a 4 nm coating, but do not explore whether this thickness is optimal. Given prior literature showing non-linear effects of coating thickness on both ionic conductivity and interfacial stability, can the authors comment on why 4 nm was chosen and whether thicker or thinner coatings were evaluated?\n6. While LGS shows good interfacial stability with NCM811 on the cathode side, the authors employ a Li-Si anode in full-cell validation without evaluating its interfacial compatibility with LGS or LPSC. Given known issues of volume expansion and potential interfacial reactivity in Si-based alloys, the absence of any post-mortem analysis or thermal characterization at the anode-SE interface limits the comprehensiveness of the system-level safety claims. Can the authors provide a discussion on whether the LGS impacts long-term anode stability or interacts chemically?"}, "author_response": "We sincerely appreciate the reviewer's careful reading and positive evaluation of our work.\n1. Rui et al. reported that thermal runaway between NCM and LPSC follows a solid-solid reaction (Energy Environ. Sci., 2023, 16, 3552-3563). This implies that the degree of contact between NCM and solid electrolytes has influence on thermal behavior. In addition, hand grinding of fully charged NCM and LPSC leads to violent burning even under inert atmospheres. How did the authors prepare the De-NCM/SE composites? Were the samples pelletized or loosely contacted?\nResponse: We sincerely thank the reviewer for raising this constructive question. The influence of interfacial contact between the cathode and solid electrolyte (SE) is indeed a crucial factor in the thermal behavior of the composite, as also highlighted in the work by Rui et al. Our findings further support the significant role of solid-solid interfacial reactions in the thermal runaway mechanism.\nIn this study, the delithiated NCM (De-NCM) material was scraped off from the current collector in an argon-filled glove box and then mixed with the solid electrolyte (SE). Specifically, De-NCM and SE were mixed at a mass ratio of 7:3, thoroughly hand-ground to ensure homogeneity, and then pressed under 312 MPa—the same pressure applied during the full cell assembly process. All compared samples in this study were prepared and tested under consistent pelleting conditions to ensure scientific rigor and comparability.\nWe have added a sentence in the Materials characterization section (paragraph 1, line 12-15) in Supplementary Information to clarify this point: “Samples (~10.0 mg) with an NCM-to-SE weight ratio of 7:3, prepared under the same pressure as the composite cathode, were placed in a 100 μL stainless steel high-pressure sealed crucible (NETZSCH, Germany) for testing.”\n2. Adiabatic ramp tests showed a rapid temperature increase of the 100% SOC composite cathode near 200 °C without further temperature rises, while several exothermic peaks appeared in DSC. Why is there a discrepancy between the two methods? In addition, the amounts of sample used for ARC and DSC were significantly different: 200 mg for ARC and 10 mg for DSC. Does this difference have influence on the thermal behavior?\nResponse: We thank the reviewer for these insightful questions regarding the differences between adiabatic ramp test at ARC and DSC results, as well as the influence of sample mass. These points are crucial for interpreting thermal runaway data correctly.\nThe apparent discrepancy between the ARC and DSC results is not due to inconsistent data, but rather arises from the fundamentally different operating principles and detection capabilities of the two techniques. The following table summarizes the key differences:\nTable R1-1. Comparison of features between ARC and DSC tests.\nFeature Adiabatic Ramp test (ARC) DSC\nIsothermal or Dynamic scanning with\nAdiabatic (Tries to hold ΔT = 0, no heat\nPrinciple constant heat sink. Measures heat\nloss). Measures self-heating rate (SHR)\nflow.\nSmall-scale, ideal for deconvoluting\n\"Worst-case\" scenario, simulating a large,\nCondition complex reactions under controlled\ninsulated cell where heat cannot escape.\nheating.\nSample mass Large (100 mg - several grams) Small (~1-10 mg)\nKey Identifies thermal runaway initiation and Identifies individual reaction steps and\noutcome tracks its autocatalytic acceleration. their onset temperatures & enthalpies.\nBelow are some specific reasons for the observed differences in our data:\nAutocatalytic vs. Stepwise Behavior: In the ARC test, once the main exothermic reaction initiates near 200 °C, it generates heat so rapidly under adiabatic conditions that the temperature skyrockets in a single, uncontrollable thermal runaway event. The instrument's primary output is the temperature rise from this single catastrophic event; it does not resolve finer details once runaway begins. Conversely, the DSC, with its high sensitivity to heat flow and non-adiabatic condition, excellently resolves the multiple, individual exothermic reactions that collectively contribute to the overall runaway process seen in the ARC. The several DSC peaks represent the deconvoluted reaction mechanism that is lumped together in the ARC trace.\nHeat Accumulation: The large sample mass in the ARC promotes heat accumulation. The heat from one initial reaction (which might appear as a small, separate peak in DSC) is not lost and immediately contributes to heating the adjacent material, triggering the next reaction faster in a feedback loop. This leads to the observed single, sharp temperature rise.\nIn summary, the ARC result shows how thermal runaway propagates in a realistic, bulk scenario, while the DSC reveals the sequence of fundamental chemical reactions that cause it. The two methods are profoundly complementary, not contradictory.\nBesides, the reviewer correctly identifies that the sample mass difference has a significant and intentional influence on the observed thermal behavior, and it is a critical factor in the design of these experiments.\nARC Requires a Large Sample Mass: For an adiabatic calorimeter like the ARC to function correctly and detect a self-heating rate (SHR), the heat generated by the sample must be sufficient to overcome inevitable minor heat losses to the sample container and thermocouples (phi-factor). A larger sample mass provides a higher thermal inertia and increases the total heat output, making it possible to accurately detect the onset of adiabatic thermal runaway. A 10 mg sample would be too small to be detected by most ARCs.\nDSC Requires a Small Sample Mass: The DSC is designed for high-resolution thermal analysis with a small crucible. A small sample mass (~10 mg) ensures: Excellent thermal conductivity throughout the sample, minimizing internal temperature gradients. The recorded temperature is highly representative of the entire sample. The resolution of closely spaced thermal events is maximized.\nUsing a large amount sample in a DSC would likely result in poor resolution, broad peaks, and shifted onset temperatures due to significant thermal lag.\nTherefore, the difference in sample mass is not an oversight but a deliberate choice to leverage the strengths of each instrument. The ARC with a large mass tells us if and when a full-scale runaway will occur, while the DSC with a small mass tells us why it occurs by breaking down the chemistry.\nWe have added a sentence in the Materials characterization section (paragraph 1, line 15-18) to clarify this point:\n\"The complementary use of ARC (with large sample mass to simulate bulk adiabatic runaway) and DSC (with small sample mass to resolve individual reactions) provides a complete picture of the thermal degradation mechanisms.\"\n3. The authors claimed that thermal runaway mechanisms are divided into two stages. The initial heat generation is driven by reactions between delithiated NCM and electrochemically formed interfacial species. At elevated temperatures, sulfur-oxygen interdiffusion leads to bulk exothermic reactions. Although Li4GeS4 showed improved thermal stability, it is still unclear in which stage Li4GeS4 mitigated the detrimental reactions. Therefore, it would be helpful to clarify the role of Li4GeS4 by performing DSC and XPS on De-NCM/pristine LGS and De-NCM/4.5V-(LGS-VGCF) samples. In addition, NCM811-LGS ASSBs were cycled at 45 °C, which is higher than the operating temperature of NCM811-LPSC cells. This increased operating temperature could influence the electrochemical decomposition. Figure 5c shows a higher content of sulfate and sulfite products in the 100% SOC composite cathode compared to those in Figure 2d.\nResponse: We thank the reviewer for this insightful comment, which allows us to clarify the specific stage at which Li₄GeS₄ (LGS) improves thermal stability and to address the concerns regarding operating temperature and interfacial products.\nAs summarized in our study, thermal runaway occurs in two distinct stages:\nThe first exothermic stage is dominated by reactions between delithiated NCM and electrochemically formed interfacial species (e.g., –S–S–, thiophosphates), accompanied by O₂ and SO₂ release. The second stage involves direct solid–solid reactions between decomposed NCM and the bulk solid electrolyte.\nOur results demonstrate that LGS primarily suppresses the first stage. As shown in Figure R1-1, the first exothermic peak is significantly reduced in the LGS-based composite (110.7 J/g) compared to the LPSC-based system (399.6 J/g). In contrast, the second-stage reaction was not markedly improved, indicating that LGS does not enhance the stability against bulk solid–solid reactions at higher temperatures.\nFigure R1-1. DSC curves of 100% SOC LPSC and LGS composite cathodes.\nTo further elucidate the function of LGS, we conducted additional experiments as suggested: XPS analysis of both De-NCM/pristine LGS and De-NCM/4.5V-(LGS-VGCF) composites (Figures R1-2) revealed that LGS undergoes notably less electrochemical decomposition than LPSC. Importantly, the formation of highly reactive species such as –S–S– and –Ge–S–Ge– was significantly suppressed.\nFigure R1-2. S 2p and P 2p XPS spectra for the LPSC/VGCF and LGS/VGCF before and after being charged to 4.5 V (vs. Li+/Li).\nDSC measurements of these composites (Figure R1-3) further confirmed a marked reduction in heat release during the first stage in the LGS system, corroborating its role in inhibiting interfacial decomposition.\nFigure R1-3. DSC curves of De-NCM/LGS and De-NCM/4.5V-(LGS/VGCF).\nAs to the difference in sulfate/sulfite content between Figure 5c and Figure 2d, we appreciate the reviewer’s insightful observation. We agree that the elevated cycling temperature (45 °C) applied to the NCM811-LGS-based cells could promote electrochemical decomposition, leading to an increased amount of sulfate/sulfite species compared to the NCM811-LPSC cells cycled at room temperature. As rightly noted by the reviewer, we conducted additional XPS analyses on the cathode material from the NCM811-LGS cell cycled at room temperature. The results confirm that the LGS-based system exhibits a lower amount of decomposition products relative to the LPSC-based reference, which aligns with its improved thermal stability as demonstrated in our DSC and ARC tests.\nFigure R1-4. S 2p XPS spectra of the 100% SOC LGS and LPSC composite cathodes\nRegarding the operating temperature used in different experiments:\nAll thermal safety tests (DSC and ARC) were conducted using materials harvested from cells that were cycled under identical conditions (30 °C) and employing equivalent cell configurations (NCM-LPSC/LPSC/Li-In or NCM-LGS/LPSC/Li-In) to ensure comparability. The 45 °C cycling mentioned in the manuscript refers only to long-term electrochemical performance evaluations of full cells, and these conditions did not affect the thermal runaway tests conducted under standardized settings.\nIn summary, LGS enhances thermal stability mainly by suppressing the electrochemically driven interfacial decomposition at the first stage. We have incorporated these clarifications and new figures (S60-S61) into the revised manuscript (Section 2.3, Page 21): “XPS and DSC analysis of both De-NCM/pristine LGS and De-NCM/4.5V-(LGS-VGCF) composites revealed that LGS undergoes notably less electrochemical decomposition and smaller ΔH than LPSC (Figures S60-61).”\n4. When the valence band edge of the solid electrolyte is above the Fermi level of the cathode, electron transfer from the solid electrolyte to the cathode leads to the oxidation of the solid electrolyte (ACS Appl. Energy Mater. 2022, 5, 2, 2045–2053).\nResponse: We apologize for the typographical error, it has been revised.\nManuscript (Section 2.2, Page 13): “According to solid-state band theory, the calculated valence band maximum (VBM) of LPSC was 1.47 eV higher than the Fermi level of NCM811, indicating that electron transfer from LPSC to NCM811 was energetically favorable (Figure S22).”\nSupplementary Information (FigureS22): “This interfacial energy offset drives electron transferring from SE to cathode, leading to the oxidative degradation of LPSC.”\n5. The sentences from line 346 to 352 need references to support it.\nResponse: We agree with the reviewer that the statements between lines 346 and 352 require supporting references. Appropriate citations have now been added to this section to substantiate our claims regarding the pDOS analysis.\n“Furthermore, density of states (DOS) calculations revealed significant overlap between the S 3p orbitals of LPSC and the O 2p/Ni eg orbitals of NCM811 near the Fermi level (Figure S23). This overlap indicates a lower energy barrier for electron exchange, further supporting the hypothesis of strong chemical incompatibility between them. Moreover, the P 2p orbitals of LPSC also showed considerable overlap with the O 2p/Ni eg orbitals of NCM811, suggesting a tendency for spontaneous chemical bonding (23, 24).” (Section 2.2, Page 13)\nReferences\n[23] Yanhao Dong and Ju Li. Oxide Cathodes: Functions, Instabilities, Self Healing, and Degradation Mitigations. Chemical Reviews 2023 123 (2), 811-833.\n[24] Yizhou Zhu, Xingfeng He, Yifei Mo. Origin of Outstanding Stability in the Lithium Solid Electrolyte Materials: Insights from Thermodynamic Analyses Based on First-Principles Calculations. ACS Appl. Mater. Interfaces 2015, 7, 42, 23685–23693\n6. Please add a scale bar in DSC Figures and specify the carrier gas used for MS measurements.\nResponse: We apologize for these omissions. Scale bars have been added to all DSC figures (as shown in the representative figure listed below, others are revised in the manuscript and Supplementary Information).\nFigure R1-5. DSC of the De-NCM, repeated three times under identical conditions.\n(ΔH=87.8±1.9 J/g)\nFurthermore, the carrier gas used for the MS measurements has now been clearly stated in the Materials characterization section (paragraph 1, line 9-10): “All MS experiments were performed under an Ar flow at a rate of 5 mL/min.”\nReviewer #2 (Remarks to the Author):\nThis paper investigates thermal runaway mechanisms in sulfide-based all-solid-state batteries, focusing on NCM811/LPSC composite systems. It proposes a two-stage thermal degradation mechanism where early exothermic reactions arise from electrochemically formed interphase species, followed by bulk S-O interdiffusion at higher temperatures. Systematic thermal characterization reveals that charged cathodes (100% SOC) show greater thermal instability than discharged ones due to reactive interphase decomposition products like -S-S-, -P-S-P-, and Li3PS4. The study extends this to other sulfide electrolytes (LGPS, LSPSC) and suggests Li4GeS4 (LGS) for interface engineering, enhancing thermal safety by reducing thermodynamic driving forces for S-O exchange. DFT calculations support improved interfacial compatibility and experimental results indicate elevated thermal runaway temperatures, rising from 228°C to 312°C.\nHowever, this paper has significant theoretical inconsistencies and methodological limitations that undermine its scientific rigor. The computational analysis focuses on pristine materials (0% SOC), while the critical experimental work centers on delithiated states (100% SOC), creating a mismatch between theory and experiment. The band theory explanation contains errors in the direction of electron transfer and inaccurately compares electronic properties without proper interface analysis. The electronic structure calculations overlook the actual reactive species crucial to the proposed mechanism, indicating a major gap in understanding. Experimental reproducibility is questionable, as thermal safety data are presented without statistical analysis, error bars, or multiple trials, which are vital for safety applications. The novelty of the Ge substitution approach is limited due to prior work by the same group, and practical validation is insufficient for real-world implementation. Furthermore, the paper exhibits numerical inconsistencies, inadequate figure descriptions, and unclear reaction energetics that further undermine its reliability. Due to these deficiencies and lack of completeness, I do not recommend this manuscript in its current form, published in Nature Communications.\nResponse: We sincerely thank the reviewer for their highly constructive suggestions. We acknowledge that our original manuscript did not clearly emphasize that both the AIMD and DOS calculations were performed on the delithiated NCM, same to the critical experimental situations, which may have led to misunderstanding. This point has now been clarified in the revised text. Furthermore, we have supplemented the band structure calculations with data for the delithiated NCM to better corroborate our experimental findings. The inaccuracies regarding the direction of electron transfer have been corrected, and additional electronic structure calculations have been incorporated in response to the reviewer’s comments. All relevant experiments have been repeated to ensure reproducibility. We have also included further comparative analysis regarding the Ge-substitution strategy to illustrate the differences between the two systems. Additionally, issues such as numerical inconsistencies, insufficient figure descriptions, and ambiguities in reaction energetics have been thoroughly addressed and corrected.\nBelow, we provide detailed responses to each specific point raised.\nComment 1) Figures 1a–1c present the ARC HWS and DSC traces for only one specimen, leaving the reproducibility of your thermal-safety metrics unclear. Please repeat the ARC-HWS and DSC tests at least three times under identical conditions (sample mass, cell pressure, HWS step length, purge gas, etc.) and report the mean ± standard deviation for the key parameters (T , T , and ΔH).\nonset tr\nResponse: We thank the reviewer for raising this important point regarding the reproducibility of our thermal safety data. We fully agree that demonstrating the repeatability of the ARC and DSC measurements is essential for validating the reported thermal-runaway characteristics.\nIn direct response to this comment, we have repeated both the ARC-HWS and DSC tests three times under identical experimental conditions—including sample mass, cell pressure, HWS step length, and purge gas. The new replicate data are listed below and provided in Supplementary (Figure S4-8, S12), which clearly displays all individual runs together with the original dataset.\nFigure R2-1. HWS curves of the 100% SOC NCM811/LPSC/Li-In ASSBs. (T =163.0±8.5℃;\nonset\nT =231.7±2.6℃)\ntr\nFigure R2-2. HWS curves of the 0% SOC NCM811/LPSC/Li-In ASSBs. (T =239±8.5℃;\nonset\nT =304.7±14.7℃)\ntr\nFigure R2-3. HWS curves of the 100% SOC LPSC composite cathode. (T =169.3±7.8℃;\nonset\nT =255.3±14.7℃)\ntr\nFigure R2-4. HWS curves of the 0% SOC LPSC composite cathode.\nFigure R2-5. HWS curves of Li-In+LPSC.\nFigure R2-6. DSC of the pristine LPSC composite cathode. (ΔH=978.3±44.5 J/g)\nMoreover, beyond Figure 1, we have also conducted repeat experiments for all other key thermal characterizations presented in the study. These results, some of which are exemplarily shown in the figures below, consistently demonstrate high reproducibility and support the reliability of our conclusions.\nAll key thermal parameters—including onset temperature (T ), thermal runaway temperature (T ), and enthalpy change (ΔH)—are now reported as mean ± standard deviation derived from three independent measurements. These updates have been incorporated throughout the main text at the relevant locations (S15, S19, S28-29, S32, S34-35, S37, S63-S65, and S67).\nFigure R2-7. HWS curves of the 100% SOC NCM811-LGS/LPSC/Li-In ASSBs.\n(T =204.0±15.9℃; T =324.0±9.1℃)\nonset tr\nFigure R2-8. DSC of the 0% SOC LPSC composite cathode. ( ΔH =81.9±4.9 J/g; ΔH =333.9±56.2\n1 2\nJ/g)\nFigure R2-9. DSC of the 0% SOC LPSC composite cathode. (ΔH =381.8±16.4 J/g;\n1\nΔH =869.7±89.5 J/g)\nFigure R2-11. DSC of the De-NCM/LPSC. (ΔH =87.8±1.9 J/g)\n2 1\nFigure R2-16. DSC of the pristine LGS composite cathode.\nFigure R2-17. DSC of the 100% SOC composite cathode. (ΔH =99.9±9.3 J/g)\nComment 2) There is a fundamental inconsistency between the computational and experimental approaches. The DFT calculations are performed on pristine materials representing 0% SOC conditions, while the critical thermal runaway analysis focuses on 100% SOC delithiated states. This mismatch severely undermines the theoretical justification for experimental observations, as the electronic properties of delithiated NCM811 differ significantly from the pristine state used in calculations.\nResponse: We thank the reviewer for raising this important concern regarding the consistency between our computational and experimental approaches. We sincerely apologize for the lack of clarity in the original manuscript, which may have led to a misunderstanding.\nWe would like to clarify that all AIMD and DOS calculations were in fact performed using structures representative of the delithiated NCM and LPSC, consistent with the composite cathode system studied in our thermal runaway analysis. The electronic structure analysis was conducted specifically for these delithiated states to ensure direct relevance to the experimental results.\nWe recognize that the figure captions did not explicitly state the delithiated state of NCM, which likely caused the misunderstanding. This has been corrected as shown below and corrected in the revised manuscript (Figure 3g, Figure S23).\nFigure 3g. AIMD simulations of the interfacial reactions between delithiated NCM811 and LPSC at 250℃, and 400℃.\nFigure R2-19. Densities of states (DOSs) of LPSC and De-NCM811.\nFurthermore, to enhance consistency and clarity, we have supplemented the band alignment analysis with explicit inclusion of De-NCM, as shown in Figure R2-20 and R2-21. The results clearly indicate that the ΔE between LPSC and the delithiated cathode is significantly larger than that between LPSC and NCM, suggesting more severe interfacial instability between LPSC and the delithiated cathode. Moreover, the valence band maximum of LGS is lower than that of LPCL, and the corresponding ΔE values with both NCM and De-NCM are smaller than those of LPSC. This demonstrates that LGS exhibits improved interfacial stability compared to LPSC with both lithiated and delithiated cathodes, thus providing a more coherent connection between computational predictions and experimental observations. The new data are provided in Supporting Information (Figure S22 and S58).\nFigure R2-20. Schematic of oxidative degradation mechanism of a solid electrolyte in contact with a cathode active material based on band theory of solids.\nFigure R2-21. Schematic of oxidative degradation mechanism of sulfide SEs (LPSC and LGS) in contact with cathode active material (NCM811) based on band theory of solids.\nThese revisions affirm that our electronic structure calculations are well-aligned with the electrochemical state central to the thermal degradation study. Thank you again for prompting us to improve the presentation of our computational methodology.\nComment 3) Similar to Comment 2, there is ambiguity regarding the lithiation state of NCM811 used in the reaction energy calculations shown in Figure 5a. The authors do not specify whether the computational analysis employs delithiated NCM811 (relevant to 100% SOC conditions) or lithiated NCM811 (relevant to 0% SOC conditions). Given that the thermal runaway mechanism is critically dependent on the delithiation state, this specification is essential for the proper interpretation of the calculated energetics and their relevance to experimental conditions.\nResponse: We thank the reviewer for this important comment regarding the lithiation state of NCM811 in the reaction energy calculations in Figure 5a. We agree that clearly specifying the computational models is essential for interpreting the results in the context of state-of-charge-dependent thermal stability.\nIn the original Figure 5a, the reaction energy calculations were performed using lithiated NCM811 (0% SOC) to evaluate the intrinsic chemical compatibility between the pristine cathode and different solid electrolytes (LGS vs. LPSC). The results indicate that LGS exhibits superior interfacial stability with NCM compared to LPSC in the lithiated state.\nTo directly address the reviewer’s question and eliminate any ambiguity, we have performed additional reaction energy calculations using delithiated NCM811 (100% SOC).\nThe reaction energies were analyzed using a convex hull diagram, which quantitatively evaluates the thermodynamic stability of interfaces between the cathode and solid electrolyte [1, 2]. In this approach, each interface is modeled as a pseudobinary system composed of end members A (electrolyte) and B (cathode). The x-axis represents the compositional ratio between A and B, denoted as x A + (1-x) B, while the y-axis represents the reaction energy (ΔE) of potential reaction products at various compositions. The convex hull (solid line) connects the most thermodynamically stable states across all compositions. The minimum point on each convex hull line reflects the most energetically favorable reaction pathway, and its value quantifies the thermodynamic driving force for interfacial reactions—more negative values indicate a stronger tendency for decomposition.\nAs shown in the new Figure R2-22, the results confirm that the reaction energy between delithiated NCM and LGS is significantly less exothermic (i.e., the reaction energy is less negative) than that between delithiated NCM and LPSC. This clearly indicates enhanced interfacial compatibility when LPSC is replaced by LGS, even under delithiated conditions.\nFigure R2-22. Reaction energies of De-NCM811/LPSC and De-NCM811/LGS by first-principles calculations.\nWe have updated this figure in the Supplementary Information (S57) to include both lithiated and delithiated state calculations, providing a complete picture across relevant electrochemical conditions. Thank you again for prompting us to improve the clarity and comprehensiveness of our computational analysis.\nReferences\n[1] Nolan, A. M.; Liu, Y.; Mo, Y. Solid-state chemistries stable with high-energy cathodes for lithium-ion batteries. ACS Energy Lett. 2019, 4 (10), 2444–2451.\n[2] Zhu, Y.; He, X.; Mo, Y. First principles study on electrochemical and chemical stability of solid electrolyte–electrode interfaces in all-solid-state Li-ion batteries. J. Mater. Chem. A 2016, 4 (9), 3253–3266.\nComment 4) The electronic structure analysis is incomplete and fails to address the actual reactive species driving the thermal runaway mechanism. While the authors calculate band alignments between bulk NCM811 and LPSC, they ignore the Fermi levels and electronic properties of the key reactive intermediates (Li3PS4, Li2S, Li2Sx, -S-S-, -P-S-P-) that they identify as primary thermal runaway initiators. For a mechanism centered on electrochemical interphase decomposition, the electronic structure of these species is crucial for understanding electron transfer processes.\nResponse: We are deeply grateful to the reviewer for this critical and insightful comment. We fully agree that a thorough electronic structure analysis of the key reactive intermediates is essential to elucidate the electron transfer processes governing interphase decomposition and thermal runaway. Our initial submission overlooked this crucial aspect, and we sincerely apologize for the omission.\nIn direct response to this comment, we have conducted comprehensive first-principles calculations to determine the Fermi levels (E), electronic density of states (DOS), and band gaps of the major interfacial decomposition products identified in our study. The Fermi levels and band gaps of these species are summarized below:\nFigure R2-23. Density of states for varied reactive intermediates.\nThermodynamic Driving Force and Fermi Level Analysis\nThe Fermi level represents the highest occupied electronic energy level of a material. A higher E (vs. vacuum level set as 0 eV) indicates a stronger tendency to donate electrons, while a lower E signifies a stronger tendency to accept electrons. Our calculations reveal a clear hierarchy in E (from high to low): LPSC > Li₃PS₄ > Li₂S > NCM811 > –S–S– > P₂S₅ > De-NCM811.\nThis ordering is critically important: The highly oxidized sulfur intermediates (e.g., polysulfides, –P–S–P– motifs) possess higher E than both the delithiated cathode (De-NCM811) and stable Li₃PS₄. This creates a strong thermodynamic driving force for spontaneous electron transfer from these intermediates into the empty states of De-NCM811.\nNotably, De-NCM811 has the lowest E (-6.13 eV), indicating it is a potent electron acceptor. This property, acquired upon delithiation, makes it prone to extracting electrons from other species, directly driving interfacial degradation.\nFurthermore, the degradation product P₂S₅ has an extremely low E (-6.07 eV), making it a strong electron sink. Its formation from LPSC decomposition could create a vicious cycle, further accelerating interfacial reactions by \"capturing\" electrons.\nTable R2-1. Summary of Fermi Level and Band Gap for key reactive intermediates\nSpecies Fermi Level (eV vs. Vacuum Level) Band Gap (eV)\nNCM -5.2950 0.454\nDe-NCM -6.1345 0.078\nLPSC -3.8207 2.320\nLi₃PS₄ -4.1658 2.803\nLi₂S -4.9028 3.348\nSulfur -5.5304 2.434\nP S -6.0734 2.378\n2 5\nBand Gap Analysis\nThe band gap (E ), the energy difference between the valence band maximum and conduction band minimum, directly determines a material's electronic conductivity. The calculated E values (from small to large) are: De-NCM811 (0.078 eV) < NCM811 (0.454 eV) < LPSC (2.32 eV) < P₂S₅ (2.38 eV) < –S–S– (2.43 eV) < Li₃PS₄ (2.80 eV) < Li₂S (3.35 eV).\nDe-NCM811 exhibits a vanishingly small band gap (0.078 eV), indicating metallic character and facilitating rapid electron transport. This explains the high electronic conductivity of the decomposed cathode interface. In contrast, stable final products like Li₂S and Li₃PS₄ are wide-bandgap insulators, which would severely hinder electron flow and eventually passivate the interface. The initiating intermediates (e.g., polysulfides) have moderate band gaps, but their higher E and the conductive De-NCM811 surface enable rapid electron transfer kinetics.\nThe interfacial instability can be understood from the alignment: the large E difference between LPSC (-3.82 eV) and De-NCM811 (-6.13 eV), amounting to 2.31 eV, is a primary driver for electron transfer from the electrolyte to the cathode, leading to LPSC oxidation. The degradation of LPSC into intermediates with lower E (e.g., Li₃PS₄, P₂S₅) and the simultaneous formation of a metallic De-NCM811 surface create a perfect storm for a self-accelerating decomposition loop.\nWe acknowledge that the actual interface is a complex system influenced by factors like morphology, nanocrystallinity, and ion transport, which may cause deviations from our idealized calculations. However, by identifying the key electronic descriptors—Fermi level and band gap (thermodynamic driving force)—our analysis robustly identifies the species most likely to initiate catastrophic failure. This significantly strengthens our mechanistic proposal by directly linking material identity to electronic function.\nWe have added these new results, the table, and a detailed discussion to the revised manuscript (Section 2.2, Page 16) and Supporting Information (Table S2).\nTable S2 Summary of Fermi Level and Band Gap for key reactive intermediates\nSpecies Fermi Level (eV vs. Vacuum Level) Band Gap (eV)\nNCM -5.2950 0.454\nDe-NCM -6.1345 0.078\nLPSC -3.8207 2.320\nLi₃PS₄ -4.1658 2.803\nLi₂S -4.9028 3.348\nSulfur -5.5304 2.434\nP S -6.0734 2.378\n2 5\nThe Fermi level (E) shows how easily a material gives or takes electrons. A high E means it likes to donate electrons, while a low E means it accepts electrons. E (from highest to lowest): LPSC > Li₃PS₄ > Li₂S > NCM811 > –S–S– > P₂S₅ > De-NCM811. Highly oxidized sulfur intermediates have high E, while the delithiated cathode (De-NCM811) has the lowest E (–6.13 eV). This large difference drives electrons to spontaneously flow from the intermediates into De-NCM811, causing degradation. The band gap (E ) shows how well a material conducts electricity. De-NCM811 has a very small band gap (0.078 eV), making it highly conductive. This helps electrons move quickly and speeds up reactions. In contrast, stable products like Li₂S have large band gaps and act as insulators, which would normally slow down reactions.\nIn short: the big difference in E drives electron transfer, and the small band gap of De-NCM811 allows fast electron movement. From a theoretical perspective, they together create a self-accelerating degradation cycle at the interface.\nComment 5) The band theory analysis contains fundamental conceptual errors in explaining electron transfer mechanisms. The authors state that \"electron transfer from NCM811 to LPSC was energetically favorable\" based on LPSC's VBM being 1.47 eV higher than NCM811's Fermi level. However, this interpretation is incorrect: electrons naturally flow from higher to lower energy levels, so if LPSC's VBM is higher than NCM811's Fermi level, electron transfer should occur from LPSC to NCM811, not the reverse. Furthermore, the authors inappropriately compare VBM (a material property of LPSC) with the Fermi level (a material property of NCM811) without a proper band alignment analysis that accounts for interface formation, band bending, and equilibrium conditions.\nResponse: We thank the reviewer for catching this critical conceptual error in our explanation of the electron transfer mechanism. The reviewer is absolutely correct: electrons transfer from a higher energy level to a lower one. We apologize for this error and have carefully revised the text to ensure the electron transfer direction is described accurately.\nManuscript (Section 2.2, Page 13): “According to solid-state band theory, the calculated valence band maximum (VBM) of LPSC was 1.47 eV higher than the Fermi level of NCM811, indicating that electron transfer from LPSC to NCM811 was energetically favorable (Figure S22).”\nSupplementary Information (FigureS22): “This interfacial energy offset drives electron transferring from SE to cathode, leading to the oxidative degradation of LPSC.”\nAs to the band alignment, we apologize for not clarifying in the article that the comparison of their valence band maxima (VBM) and Fermi levels was conducted after band alignment using the vacuum slab model [1, 2]. The Fermi levels of two materials obtained through VASP calculations cannot be directly compared due to the absence of a unified reference point. We employed a vacuum slab model to calculate the energy difference between the vacuum level and the Fermi level (i.e., the work function), selecting crystallographic planes with the lowest surface energy as our computational models. By aligning the vacuum levels of both materials under this vacuum slab model as a common reference benchmark and setting them as the energy zero point, we obtained and compared the numerical values of the materials' Fermi levels relative to the vacuum level. This approach enables a meaningful comparison of the Fermi levels or VBM between the two materials.\nThis clarification has been added to Supporting Information (1.5. Calculation method).\nReferences\n[1] Swift, M. W.; Qi, Y. First-Principles Prediction of Potentials and Space-Charge Layers in All-Solid-State Batteries. Phys. Rev. Lett. 2019, 122 (16), 167701(167701–167706).\n[2] Swift, M. W.; Jagad, H.; Park, J.; Qie, Y.; Wu, Y.; Qi, Y. Predicting low-impedance interfaces for solid-state batteries. Curr. Opin. Solid State Mater. Sci. 2022, 26 (3), 100990.\nComment 6) The novelty of the proposed Ge substitution strategy is questionable given that the role of Ge in improving sulfide electrolyte stability has been previously established by the same research group's earlier publications, Kim et al. [13] The authors should clearly articulate what new insights or improvements their Li4GeS4 approach provides beyond existing Ge-based solutions and provide adequate differentiation from prior work.\nResponse: We thank the reviewer for raising this important point regarding the novelty of our Ge-substitution strategy. We fully acknowledge that previous studies have indicated the beneficial role of Ge in enhancing the stability of sulfide electrolytes. Nevertheless, our study introduces several conceptual and practical advances that extend significantly beyond prior Ge-containing systems such as LGPS (Li₁₀GeP₂S₁₂).\nAs the reviewer rightly noted, Kim et al. previously investigated Ge substitution within the LGPS structure, which still contains redox-active PS₄³⁻ units that are prone to electrochemical oxidation and thermal degradation. Although LGPS exhibits high ionic conductivity, it does not fundamentally address the thermochemical instability at the cathode interface. This is corroborated by our experimental results, which show that the LGPS system still undergoes violent thermal runaway at the cathode/solid electrolyte interface.\nIn contrast, our approach utilizes Li₄GeS₄ (LGS), a phosphorus-free compound. Based on Hard-Soft Acid-Base principles, Ge⁴⁺ acts as a softer acid compared to P⁵⁺, resulting in reduced reactivity toward oxygen released from delithiated NCM cathodes. This leads to markedly improved interfacial compatibility with oxide cathodes. Moreover, both our experimental results and theoretical calculations (Figure 5a, FigureS43) demonstrate that LGS effectively suppresses the initial exothermic reactions between the cathode and solid electrolyte.\nCritically, we have developed an innovative cell architecture: LGS is deployed solely as a stabilizing additive within the composite cathode, while LPSC is retained as the bulk solid electrolyte. This design harnesses the high ionic conductivity of LPSC while avoiding the poor anode compatibility of LGS. It is well established that certain sulfide solid electrolytes (e.g., LGPS) react severely with lithium metal, forming mixed-conducting interphases (MCI) that lead to continuous decomposition and accelerated thermal failure [1-3]. Although LGS shows improved stability against oxide cathodes, it exhibits high impedance and poor compatibility when in direct contact with lithium metal anodes, as confirmed by our XPS analysis (Figure R2-24). The reduction of Ge species upon contact with lithium results in high interfacial resistance and rapid degradation. Precisely for this reason, we designed a hybrid architecture that confines LGS to the cathode side—preventing direct contact with the anode—while leveraging its ability to suppress cathode-side parasitic reactions.\nFigure R2-24. Ge 3d XPS spectra of LGS and LGS/Li pressed into tablets after 24 hours.\nThus, the novelty of our work lies not simply in the use of Ge, but in: (1) The application of phosphorus-free Li₄GeS₄ as an interfacial stabilizer; (2) A hybrid electrolyte design that decouples interface stabilization from bulk ion transport; (3) The explicit demonstration of suppressed early-stage exothermic reactions and enhanced overall safety, without compromising room-temperature electrochemical performance.\nMoreover, the primary contribution of this work is not merely another Ge-substituted electrolyte, but a fundamental and detailed understanding of interfacial thermal failure mechanisms in sulfide-based all-solid-state batteries. This understanding informed a rational material and cell design strategy. The coating and electrolyte modifications presented serve as validations and applications of these insights, rather than constituting the central focus.\nWe have revised the manuscript to clarify these points and better emphasize the conceptual and practical advancements beyond earlier Ge-based systems (Figure S59).\nThank you once again for this insightful comment.\nReferences\n[1] Zhao, J.; Zhao, C.; Zhu, J. et al. Size-Dependent Chemomechanical Failure of Sulfide Solid Electrolyte Particles during Electrochemical Reaction with Lithium. Nano Lett., 2022, 22(1): 411-418.\n[2] Wenzel, S.; Randau, S.; Leichtweiß, T. et al. Direct Observation of the Interfacial Instability of the Fast Ionic Conductor Li₁₀GeP₂S₁₂ at the Lithium Metal Anode. Chem. Mater., 2016, 28(7): 2400-2407.\n[3] Yan, J.; Yao, J.; Zhao, J. et al. Revealing the Thermal Stability of the Li/Sulfide Solid Electrolyte Interface at Atomic Scale via Cryogenic Electron Microscopy. Adv. Funct. Mater., 2025, 35: 2421918.\nComment 7) There is a numerical inconsistency in the reported Tonset. The authors state \"Post-cycling 0% SOC LGS composites maintained stability to 450°C, contrasting with LPSC's 303°C exothermic initiation (Figure 5d vs 2a)\" and \"For the 0% SOC sample, two exothermic events were recorded at 303°C (ΔH=75.1 J/g) and 379°C (ΔH=268.8 J/g) (Figure 2a).\" However, Figure 2a shows the onset temperature as 302°C, not 303°C as reported in the text. This discrepancy requires correction for accuracy.\nResponse: Thank you for highlighting this discrepancy. The value of 303°C mentioned in the text was a typographical error. The correct onset temperature, as clearly indicated in Figure 2a, is 302°C. This has been corrected in the revised manuscript and listed below to ensure full consistency between the text and the figure. We have further conducted a full-text review to ensure that no similar errors are present. We appreciate your careful attention to the details.\n“For the 0% SOC sample, two exothermic events were recorded at 302 ℃ (ΔH=75.1 J·g-1) and 379 ℃ (ΔH=268.8 J·g-1) (Figure 2a).” (Section 2.2, Page 9-10)\n“Post-cycling 0% SOC LGS composites maintained stability to 450 ℃, contrasting with LPSC's 302 ℃ exothermic initiation (Figure 5d vs 2a).” (Section 2.2, Page 21)\nComment 8) The manuscript contains inconsistent abbreviation usage. The authors define \"valence band maximum (VBM)\" on page 4, but subsequently use the full term again in \"the calculated valence band maximum (VBM) of LPSC was 1.47 eV higher than the Fermi level of NCM811\" (Figure S14). According to standard scientific writing conventions, once an abbreviation is defined, it should be used consistently throughout the manuscript rather than alternating between full terms and abbreviations.\nResponse: We thank the reviewer for this careful reading and for pointing out the inconsistent use of abbreviations. The reviewer is absolutely correct to highlight this deviation from standard scientific writing conventions.\nWe have now carefully reviewed the entire manuscript and standardized the terminology. The abbreviation \"VBM\" (valence band maximum) is now used consistently after its initial definition.\nThe specific sentence in the caption of Figure S22 (or the relevant section in the main text) has been corrected to: “...the calculated VBM of LPSC was 1.47 eV higher than the Fermi level of NCM811.”\nWe apologize for this oversight and are grateful to the reviewer for helping us improve the clarity and professionalism of our manuscript.\nComment 9) Figure 4b lacks any description or discussion in the manuscript text. The authors present this figure without adequate explanation of its content, methodology, or relevance to the overall mechanism. This omission leaves readers unable to interpret the significance of the data presented, representing an incomplete presentation of the results.\nResponse: We thank the reviewer for pointing out this significant omission. We sincerely apologize for the lack of explanation for Figure 4b in the original manuscript, which hindered the readers' ability to interpret its significance.\nIn the revised manuscript, we have now added a comprehensive description and discussion of Figure 4b in the revised manuscript to fully explain its methodology, content, and critical role in supporting the proposed mechanism, as presented below. (Section 2.2 Page 17-18)\nThe universal applicability of the thermal degradation mechanism across different sulfide solid electrolytes was systematically evaluated (Figure 4b). Specifically, we extended our investigation to Li₁₀GeP₂S₁₂ (LGPS) and Li₉.₅₄Si₁.₇₄P₁.₄₄S₁₁.₇Cl₀.₃ (LSPSC) using the same methodology applied to LPSC. Composite cathodes were prepared in both the pristine state (De-NCM mixed with pristine SE) and the electrochemically decomposed state (De-NCM mixed with SE charged to 4.5 V within a VGCF composite). The thermal behavior of these systems was subsequently characterized using adiabatic calorimetry and gas analysis.\nBoth fully charged LGPS- and LSPSC-based composites exhibit significant self-heating initiating near 202 °C and 199 °C, respectively, closely matching the behavior of the LPSC system (199 °C) (Figure S43-S46). The first exothermic peak releases 395.5 J/g for the charged LGPS-VGCF composite and 419.9 J/g for the LSPSC-based counterpart. Crucially, the majority of this heat (246.3 J/g and 270.7 J/g, respectively) originates from the reaction between De-NCM and the electrochemically generated interphase species (e.g., oxidized sulfur and phosphorus compounds), rather than with the pristine electrolytes (Figure S47). This initial reaction stage is accompanied by significant SO₂ evolution, as confirmed by mass spectrometry (Figure S48). In stark contrast, control samples containing De-NCM mixed with the pristine (uncharged) LGPS or LSPSC powders showed negligible exothermic activity or gas release. These results universally confirm that the primary trigger for thermal runaway is the reaction between the delithiated cathode and the metastable decomposition products formed at the interface during high-voltage charging, not the reaction with the bulk, pristine solid electrolyte. This validates the generalizability of our proposed degradation roadmap across different sulfide electrolytes.\nFigure 4b. Thermal behaviors between delithiated NCM811 (De-NCM) and sulfide SEs\nReviewer #3 (Remarks to the Author):\nThis manuscript investigates the thermal stability of sulfide-based all-solid-state batteries and elucidates the origin of thermal runaway, which is triggered by the interaction between the delithiated cathode and electrochemically decomposed LPSC. Furthermore, the authors provide insightful analysis on the role of VGCF in enhancing thermal safety, which is particularly meaningful for understanding how each component contributes to the overall thermal stability of ASSBs.\nThe study offers valuable insight into the safety aspects of sulfide-based solid-state batteries and makes a significant contribution by demonstrating that electrochemical decomposition products of the solid electrolyte at the cathode interface play a critical role in determining thermal stability. I believe this work is highly meaningful and recommend it for publication, pending satisfactory responses to the following comments and questions.\nResponse: We sincerely thank the reviewer for their positive evaluation of our work and their supportive comments regarding the insights into thermal runaway mechanisms of sulfide-based all-solid-state batteries. Below we provide point-by-point responses to all comments raised.\nQ1. If the thermal stability is mainly dependent on the cathode interfacial reaction as authors suggested, then, what is the origin of the difference of Ttr of SOC 100 between full cell and composite cathode as shown in Figure 1a and 1b. Additionally, even the HWS curve of SOC 0 between full cell and composite cathode is seems to be different. Is there any interaction between cathode or anode (like cross-talk)?\nResponse: We thank the reviewer for this insightful question regarding the differences in thermal behavior between the full cell and the composite cathode. We fully agree that the thermal stability of a full cell is influenced by multiple interrelated factors.\nThe observed differences in the thermal runaway trigger temperature (T ) and the Heat-Wait-Search (HWS) profiles between the full cell and the cathode composite (at both 0% and 100% SOC) can be attributed to several effects, with electrode crosstalk being a primary factor. As correctly noted by the reviewer and previously elucidated in liquid electrolyte systems [1,2], oxygen species (O₂) released from the delithiated NCM at elevated temperatures can migrate through solid state electrolyte to the anode and participate in highly exothermic reactions, thereby intensifying the severity and altering the thermal runaway behavior in full cells, as also reported by Zhang Qiang et al in thermal runaways of solid state batteries [3]. Furthermore, our preliminary studies suggest that elemental sulfur present at the interface may also participate in cross-reactions with the anode [4]. Additional contributors include interfacial reactions between the anode and solid electrolyte, self-heating due to micro-shorts, and overall cell architecture.\nMoreover, differences in experimental conditions between material-level and cell-level tests play a significant role. For material-level tests, the composite cathode was sealed in a small, confined vessel, representing a closed system. In contrast, during cell-level tests, the battery casing may rupture at high temperatures, effectively converting the system into an open one, which further influences gas release and heat accumulation.\nOur study focuses specifically on the cathode interface because our data indicate that exothermic reactions originating from the cathode/electrolyte interface occur earlier and are more intense, serving as the primary trigger for the thermal runaway cascade. While crosstalk and anode-related reactions modulate the overall process, understanding and mitigating this initial cathode-driven exothermic event are crucial for improving overall cell safety. Therefore, we concentrate on decoupling the reaction pathways at the cathode to develop targeted safety strategies.\nReference\n[1] L. Huang, G. Xu, X. Du, et al., Uncovering LiH triggered thermal runaway mechanism of a high-energy LiNi0.5Co0.2Mn0.3 O2/graphite pouch cell. Advanced Science, 2021, 8: 2100676.\n[2] X. Liu, D. Ren, H. Hsu, et al., Thermal runaway of lithium-ion batteries without internal short circuit. Joule, 2018, 2(10): 2047-2064.\n[3] Y. Li, X. Liu, L. Wang, et al., Thermal runaway mechanism of lithium-ion battery with LiNi0.8Mn0.1Co0.1O2 cathode materials. Nano Energy, 2021, 85: 105878.\n[4] L. Huang, T. Lu, G. Xu, X. et al., Thermal runaway routes of large-format lithium-sulfur pouch cell batteries. Joule, 2022, 6(4): 906-922.\nQ2. According to the author’s insists, the first exothermic reaction is due to the decomposition of interphase. Given the chemical composition of interphase is nearly same regardless of SOC (0 or 100) as shown in XPS (Fig 2b, d), can you explain the reason of decomposition temperature composite with SOC 100 is lower than composite with SOC 0? If the delithiated cathode participate in decomposition of interphase, what is the exact reaction mechanism? According to the references, SOC 100 cathode can also evolve the O2 gas from the cathode when it converts to the spinel phase from layered structure. Then, is the reaction between O2 gas and interphase? In addition, the reason should be explained that gaseous product is evolved only at the first step of exothermic reaction not the second step of rock salt phase transition where the much more amount of O2 gas can be evolved.\nResponse: We thank the reviewer for this insightful question regarding the difference in decomposition temperature between the 0% and 100% SOC composites and the specific role of the delithiated cathode.\nThe key reason for the lower decomposition temperature of the 100% SOC composite lies in the thermodynamic instability of the highly delithiated NCM cathode, which actively participates in the interfacial reactions. Although the chemical composition of the interphase is similar at both SOCs (as shown by XPS in Fig. 2b, d), the delithiated NCM811 at 100% SOC acts as a strong oxidant, significantly reducing the activation energy required for the decomposition of interphase species.\nFigure 2b. S 2p and P 2p XPS spectra of the 0% SOC composite cathode at room temperature (RT, 25 ℃), 310 ℃, and 450 ℃.\nFigure 2d. S 2p and P 2p XPS spectra of the 100% SOC composite cathode at RT, 250 ℃, and 400 ℃.\nThe exact reaction mechanism involves reactive oxygen species (O) released from the delithiated NCM reacting with interphase compounds such as –S–S–, Li₃PS₄, and –P–S–P–. This leads to the formation of sulfates, phosphates, and the release of SO₂, as supported by our DSC and MS data (Figs. 3h–3i and R3-1).\nFigure 3h-3i. (h) DSC curves of different materials with delithiated NCM811 (De-NCM). (i) MS curves of different materials with delithiated NCM811.\nFigure R3-1. DSC profiles of β-Li3PS4 (LPS) and delithiated NCM811 (LPS and delithiated NCM811 in a weight ratio of 3:7); delithiated-β-Li3PS4 (De-LPS, including -P-S-P- and -S-S-) and delithiated NCM811 (De-LPS and delithiated NCM811 in a weight ratio of 3:7); P2S5 and delithiated NCM811 (P2S5 and delithiated NCM811 in a weight ratio of 3:7).\nTo unequivocally demonstrate that the oxygen source is lattice oxygen from NCM rather than gaseous O₂, we conducted an isotope labeling experiment: the 100% SOC composite was heated under an ¹⁸O₂ atmosphere, and the evolved gases were monitored in situ. The results showed that the SO₂ produced contained almost exclusively ¹⁶O, confirming that the oxygen originates from the NCM lattice and not from atmospheric O₂ (Fig. R3-2).\nFigure R3-2. MS curves of the 100% SOC LPSC composite cathode in 18O2.\nIn contrast, the lithiated NCM (0% SOC) does not release reactive oxygen species and therefore does not participate in such reactions. For the 0% SOC composite, we further explored the reaction mechanism through additional DSC tests. The results indicate that the exothermic reactions are initiated by the interaction between –P–S–P– species and lithiated NCM, as detailed in our response to Comment 4 below.\nRegarding gas evolution during the two exothermic stages:\nThe first stage involves the oxidation of degraded sulfur species (e.g., disulfides (–S–S–) in the interphase) by atomic oxygen, leading to SO₂ release.\nIn the second stage, after the reactive sulfur species are depleted, the process transitions to a solid–solid interaction between decomposed NCM and LPSC, accompanied by sulfur–oxygen interdiffusion. This stage consumes lattice oxygen to form non-gaseous products such as sulfates and sulfites, explaining the absence of significant gas release. The difference in gas evolution between the two stages stems from the distinct chemical nature of the sulfur-containing reactants involved.\nThis mechanism aligns with the findings of Ouyang et al. [1], who also reported that crystalline sulfides primarily undergo solid–solid reactions at elevated temperatures. We have added these clarifications to the revised manuscript (Section 2.2, Page 17) and Supporting Information (S41-S42) to provide a deeper mechanistic explanation.\nReference\n[1] X. Rui, D. Ren, X. Liu, et al., Distinct thermal runaway mechanisms of sulfide-based all-solid-state batteries. Energy Environ. Sci. 2023, 16, 3552–3563.\nQ3. The evaluation of the interaction between Li2S, S, Li3PS4 and delithiated NCM is quite reasonable to investigate the effect of electrochemically degraded LPSC on thermal stability as interphase. Can you explain why the S or Li3PS4 can evolve the SO2 gas evolution through the interaction with delithiated NCM? (in contrast to pristine LPSC).\nResponse: We thank the reviewer for this insightful question regarding the mechanism of SO₂ evolution during the interfacial reaction between degraded LPSC products (such as S and Li₃PS₄) and delithiated NCM. The difference in reactivity indeed stems from the thermodynamic instability and distinct chemical environments of sulfur in these species compared to pristine LPSC.\nIn pristine LPSC, sulfur exists primarily in the form of PS₄³⁻ tetrahedral units, where sulfur is in a reduced state and is stabilized by strong P–S bonds within a rigid anionic framework. This structure is both chemically and electrochemically stable within the operating window of the solid electrolyte, and does not readily release SO₂ under normal conditions.\nHowever, upon electrochemical oxidation (e.g., during cycling or at high voltage), LPSC decomposes into products such as elemental sulfur (S⁰), Li₃PS₄, and polysulfides. These degradation products have distinctly different sulfur chemical environments: Elemental sulfur (S₈) contains S–S bonds that are highly susceptible to attack by nucleophiles or oxidants. Li₃PS₄ still contains PS₄³⁻ units but in a less structurally constrained environment compared to the crystalline LPSC, making it more reactive. When these species come into contact with delithiated NCM811, which is a strong oxidant and can release atomic oxygen (O) at elevated temperatures, their interaction becomes thermodynamically favorable:\nThe driving force for SO₂ release is the high oxidation tendency of sulfur in S⁰ and weakly bound PS₄³⁻ in decomposition products, coupled with the strong oxidizing power of delithiated NCM.\nBy contrast, in pristine LPSC, the sulfur is already in a stable tetrahedral coordination with phosphorus. The energy required to break P–S bonds and oxidize S²⁻ to S⁴⁺ (in SO₂) is significantly higher, making SO₂ evolution less favorable both kinetically and thermodynamically. This was demonstrated in the work of Ouyang et al. [1], the thermal reaction mechanisms between De-NCM and sulfide electrolytes depend significantly on the crystallinity and composition of the electrolyte.\nWe have added a paragraph in the revised manuscript (Section 2.2, Page 16) to include this explanation and cited relevant literature on sulfur oxidation and solid electrolyte decomposition:\n“Upon electrochemical oxidation (e.g., during cycling or at high voltage), LPSC decomposes into products such as elemental sulfur, Li PS , and polysulfides. These degradation products have\n3 4\ndistinctly different sulfur chemical environments and are more reactive with oxygen.”\nThank you again for encouraging us to clarify this important mechanistic detail.\nReference\n[1] X. Rui, D. Ren, X. Liu, et al., Distinct thermal runaway mechanisms of sulfide-based all-solid-state batteries. Energy Environ. Sci. 2023, 16, 3552–3563.\nQ4. To emphasize the effect of interphase on thermal stability depending on the interaction with cathode, it is recommended to note the interfacial decomposition mechanism of SOC 0 also for comparison with SOC 100 (like as shown in Figure 4a).\nResponse: We sincerely thank the reviewer for this valuable suggestion. We agree that a comparative analysis of the interfacial decomposition mechanisms at both 0% and 100% SOC is essential for a comprehensive understanding of the state-of-charge-dependent thermal stability.\nIn direct response to this comment, we performed additional experiments to further elucidate the thermal reaction mechanism of the 0% SOC composite cathode. Specifically, we conducted DSC measurements on a mixture of electrochemically decomposed LPSC (obtained at 2.8 V vs Li+/Li) and pristine NCM. A distinct exothermic peak was observed at 302 °C, which aligns perfectly with the first exothermic peak of the 0% SOC composite cathode (Figs. R3-3 and 2a). In contrast, no such exothermic behavior was detected at this temperature for the individual components (decomposed LPSC alone, pristine NCM, or a mixture of pristine NCM and pristine LPSC, Figs. R3-3 and R3-4).\nFigure R3-3. DSC profiles of NCM/2.8V-(LPSC/VGCF), NCM/LPSC, and 2.8V-(LPSC/VGCF).\nFigure 2a. In-situ heating DSC-XRD-MS analysis of the 0% SOC composite cathode.\nFigure R3-4. DSC profiles of LPSC, VGCF, and NCM811 samples.\nFurthermore, systematic DSC tests combining pristine NCM with various electrochemical decomposition products (Li₂S, S, Li₃PS₄, and P₂S₅) revealed that only the mixture of NCM and P₂S₅ produced a sharp exothermic peak near 300 °C (Figs. R3-5). This strongly suggests that the first exothermic event at 0% SOC is attributable to the reaction between NCM and phosphorus sulfide intermediates (such as -P-S-P- bridged structures) generated from LPSC decomposition.\nFigure R3-5. DSC profiles of NCM/S, NCM/P2S5, NCM/Li3PS4, and NCM/Li2S.\nTo probe the specific reaction pathway, we employed XPS to analyze the NCM/2.8V-(LPSC/VGCF) mixture before and after heating to 310 °C (Figs. R3-6). The spectra confirmed the consumption of -P-S-P- species and the simultaneous formation of oxidation products including SO₃²⁻, SO₄²⁻, and PO₄³⁻. This evidence supports the proposed reaction pathway for the first stage:\n-P-S-P- + NCM → SO₃²⁻ + SO₄²⁻ + PO₄³⁻.\nFigure R3-6. S 2p and P 2p XPS spectra of the NCM/2.8V-(LPSC/VGCF) at room temperature (RT, 25 ℃) and 310 ℃.\nRegarding the second exothermic stage, in situ XRD results indicated the simultaneous disappearance of characteristic diffraction peaks from both NCM and LPSC, confirming that a direct solid-solid reaction between the bulk materials occurs. The overall reaction can be summarized as:\nLPSC + NCM → Li₂S + NiO + Ni₃S₂ + SO₃²⁻ + SO₄²⁻ + PO₄³⁻.\nThese new findings, which have been added to the revised manuscript (Section 2.2, Page 17) and Supporting Information (S41-S42), provide a mechanistic parallel to the 100% SOC decomposition analysis already presented in Figure 4a. This comparison substantially strengthens our discussion on how the interfacial decomposition mechanism evolves with the state of charge.\nThank you again for this suggestion, which has helped us provide a more complete and mechanistic picture of the thermal degradation pathways in our system.\nReviewer #4 (Remarks to the Author):\nThe manuscript presents a comprehensive investigation into the thermal runaway behavior of sulfide-based all-solid-state batteries (ASSBs), emphasizing the role of electrochemically formed interphases and chemical cascade reactions at the cathode-electrolyte interface. The use of multiscale calorimetry, gas analysis, and advanced structural characterization to support a two-stage decomposition mechanism is novel. The authors correlate material-level interfacial reactions with cell-level thermal behavior and propose a paradigm shift from bulk stability to interfacial thermodynamics as the primary safety determinant. The following comments should be addressed.\nResponse: Thank you for this positive assessment of our work and for acknowledging its novelty and comprehensive nature. We greatly appreciate your constructive comments and have prepared a detailed point-by-point response to each of the specific points raised below.\n1. While the authors attribute the first exothermic peak at 0% SOC to electrochemical interphase breakdown, it is unclear how such interphases persist or form under discharged conditions. Can the authors clarify whether residual sulfur species or kinetically trapped decomposition products contribute, and how this behavior is decoupled from active redox at 100% SOC?\nResponse: We thank the reviewer for raising this important question regarding the persistence and origin of reactive interphases under discharged (0% SOC) conditions. We agree that this point requires careful clarification.\nAs correctly noted by the reviewer and supported by prior literature [1,2], LPSC undergoes reversible electrochemical redox reactions during cycling. At high potentials, it is oxidized to species such as Li₃PS₄, elemental S (or disulfide -S-S-), and phosphorus sulfides (e.g., P₂S₅, containing -P-S-P- motifs). At low potentials, it is reduced to Li₂S and Li₃P. Part of this decomposition is reversible and contributes to the cycling capacity. However, not all decomposition products are electrochemically reversible. Kinetically trapped, metastable species—particularly phosphorus-rich sulfides like P₂S₅ (and its polymeric forms with -P-S-P- linkages)—persist at the interface even after full discharge. These residues are key to explaining the first exothermic peak at 302 °C at 0% SOC (Figs. 2a).\nFigure 2a. In-situ heating DSC-XRD-MS analysis of the 0% SOC composite cathode.\nTo confirm that these residual species drive the exothermic reaction, we performed additional DSC experiments:\nA mixture of electrochemically decomposed LPSC and pristine NCM exhibited a clear exothermic peak at 302 °C (Figs. R4-1). No such peak was observed for decomposed LPSC alone, pristine NCM alone, or a mixture of pristine NCM and pristine LPSC (Figs. R4-1 and R-2).\nFurthermore, systematic mixing of pristine NCM with individual decomposition products (Li₂S, S, Li₃PS₄, and P₂S₅) revealed that only P₂S₅ produced a sharp exotherm near 300 °C (Figs. R4-3).\nFigure R4-1. DSC profiles of NCM/2.8V-(LPSC/VGCF), NCM/LPSC, and 2.8V-(LPSC/VGCF).\nFigure R4-2. DSC profiles of LPSC, VGCF, and NCM811 samples.\nFigure R4-3. DSC profiles of NCM/S, NCM/P S , NCM/Li PS , and NCM/Li S.\n2 5 3 4 2\nThis indicates that the thermal decomposition of these irreversible, kinetically trapped phosphorus-sulfide intermediates is responsible for the initial heat release at 0% SOC.\nWe further probed the reaction pathway via XPS analysis of the NCM/decomposed-LPSC mixture before and after heating to 310 °C (Figs. R4-4). The spectra showed the disappearance of the -P-S-P- signal and the formation of SO₃²⁻, SO₄²⁻, and PO₄³⁻, supporting the following pathway:\n-P-S-P- (from residues) + NCM → SO₃²⁻ + SO₄²⁻ + PO₄³⁻.\nFigure R4-4. S 2p and P 2p XPS spectra of the NCM/2.8V-(LPSC/VGCF) at room temperature (RT, 25 ℃) and 310 ℃.\nIn contrast, at 100% SOC, the delithiated NCM is highly oxidizing and actively drives the decomposition of LPSC, generating a thicker interphase containing -S-S-, Li₃PS₄, and more -P-S-P- species. These can all react with oxygen released from NCM, leading to more intense heat release and SO₂ generation.\nThus, the behavior is decoupled: at 0% SOC, the exotherm is caused by thermal breakdown of irreversibly formed interfacial residues; at 100% SOC, it is intensified by combined electrochemical and thermal instability.\nWe have added this discussion and the supporting data to the revised manuscript (Section 2.2 Page 17) and Supporting Information (S41-S42) to provide a clearer mechanistic picture. Thank you for this insightful comment.\nReference\n[1] Tan, D. H. S.; Wu, E. A.; Nguyen, H. et al. Elucidating Reversible Electrochemical Redox of Li₆PS₅Cl Solid Electrolyte. ACS Energy Lett., 2019, 4(10): 2418-2427.\n[2] Schwietert, T. K.; Arszelewska, V. A.; Wang, C. et al. Clarifying the Relationship between Redox Activity and Electrochemical Stability in Solid Electrolytes. Nat. Mater., 2020, 19: 428–435.\n2. The author states that there is a sequential progression from interphase breakdown to bulk NCM-LPSC reactions but does not define the temperature boundaries or timing clearly. Could the authors specify more definitively when LPSC becomes actively involved?\nResponse: We thank the reviewer for this comment, which helps us clarify the temperature boundaries that define the transition from interphase-driven to bulk electrolyte-driven reactions.\nBased on our in situ high-temperature XRD results of the 100% SOC composite cathode (Figure R4-5), the active involvement of bulk LPSC in the decomposition reaction begins at approximately 280 °C, coinciding with the onset of the phase transition of NCM from a spinel to a rock-salt structure (NiO) and the gradual attenuation of LPSC diffraction peaks. The reaction continues until around 350 °C, at which point all characteristic peaks of LPSC completely disappear, indicating full consumption of the solid electrolyte.\nFigure R4-5. In-situ heating XRD analysis of the 100% SOC composite cathode.\nThis temperature range is strongly supported by our DSC analysis (see Figure R4-6). The second exothermic peak, which we have previously attributed—via a combination of EBSD, MS, and XPS—to the reaction between bulk LPSC and delithiated NCM, is observed to start at approximately 280 °C and conclude near 350 °C. The excellent agreement between the DSC and in situ XRD data provides robust and consistent evidence for the temperature window (~280–350 °C) within which bulk LPSC becomes actively involved in the exothermic solid-solid reaction.\nFigure R4-6. DSC of the 100% SOC LPSC composite cathode.\nTherefore, the sequential progression is clearly demarcated: the first exothermic stage (lower temperature) corresponds to the decomposition of the electrochemically formed interphase, while the second stage (initiating around 280 °C) marks the participation of bulk LPSC.\nWe have added this clarification along with the corresponding references to Figure R4-4 and the DSC result (Figure R4-5) in the revised manuscript (Section 2.2, Page 10-11) to more precisely delineate the reaction stages.\n“In-situ heating XRD analysis revealed structural evolution during these events: the first peak corresponded to phase transformation of delithiated NCM811 from layered to spinel structure, while the second marked transition to rock-salt phase alongside progressive LPSC signal attenuation (Figure 2c). These observations confirm delithiated cathode-driven initiation of thermal decomposition, with bulk solid electrolyte involvement restricted to later stages (280 ℃-350 ℃).”\n3. While the authors report multiple exothermic peaks, there are no normalized quantitative comparisons of reaction enthalpies (ΔH) across pristine, uncycled, and cycled cathodes. This makes it difficult to rank or compare thermal risk attributed to specific interfacial chemistries across states of charge. Can the authors elaborate on this to allow a clearer comparison?\nResponse: We thank the reviewer for raising this important point. We agree that a quantitative comparison of the reaction enthalpies (ΔH) is essential for objectively evaluating the thermal hazards associated with different interfacial states. We apologize for this omission in the original manuscript.\nIn response to this comment, we have now calculated and normalized the total exothermic heat release (ΔH, in J g⁻¹) for all tested conditions—including the pristine, uncycled, and cycled cathodes at various states of charge (SOC). The quantitative results are summarized in the table below and have been added to the revised manuscript as a new table (Figure R4-7). The results clearly indicate that the delithiated (100% SOC) cathode composites exhibit the highest exothermic heat release, confirming the critical role of the degraded cathode-electrolyte interphase in accelerating thermal decomposition. In contrast, the pristine material and composites at 0% SOC show notably lower enthalpies.\nFigure R4-7. DSC of the pristine, 0% SOC and 100% SOC LPSC composite cathodes.\nTable R4-1. DSC Parameters of the Pristine, 0% SOC, and 100% SOC LPSC Composite Cathodes\nComposite 1st peak ΔH of 1st peak 2nd peak ΔH of 2nd peak\nsample (ΔH1) (ΔH2)\nPristine 348-389 ℃ 61.9 J/g 389-500 ℃ 894.3 J/g\n0%SOC 280-322 ℃ 75.1 J/g 335-400 ℃ 268.8 J/g *\n100%SOC 171-246 ℃ 399.6 J/g 263-383 ℃ 863.9 J/g\n* The reduced heat release in the 0% SOC sample compared to the pristine sample may be attributed to the presence of inert substances in its interface layer, which hinder the solid-solid reaction between NCM and SE.\nFurthermore, to ensure data reliability, the thermal stability of each sample was tested three times using the identical method. As shown in the figure below, the DSC curves exhibit excellent reproducibility, thus confirming the repeatability and reliability of our measurements.\nFigure R4-8 DSC of the pristine LPSC composite cathode (ΔH=978.3±44.5 J/g).\nFigure 4-9. DSC of the 0% SOC LPSC composite cathode (ΔH =81.9±4.9 J/g; ΔH =333.9±56.2 J/g).\n1 2\nFigure R4-10. DSC of the 100% SOC LPSC composite cathode (ΔH =381.8±16.4 J/g；\n1\nΔH =869.7±89.5 J/g).\n4. The authors present in-situ heating XRD and identify NiO formation, but it is unclear whether NiO arises from lattice oxygen evolution, sulfur-induced destabilization, or both. Can the authors clarify this? NiO is typically associated with the oxidative decomposition of NCM cathodes, whereas sulfur-rich environments tend to reduce transition metals.\nResponse: We thank the reviewer for this insightful question regarding the origin of NiO formation during the thermal decomposition process. We agree that the formation of NiO involves a complex interplay between oxygen release from the cathode and reduction by sulfur species, and our experimental data provide clear evidence for both mechanisms occurring concurrently.\nOur in-situ heating XRD results reveal that the formation of NiO is synchronized with the disappearance of crystalline LPSC, indicating a direct thermochemical reaction between the delithiated NCM and the solid electrolyte (Figure R4-11). Several key observations help clarify the dual origin of NiO:\nFigure R4-11. In-situ heating XRD analysis of the 100% SOC composite cathode.\nSulfur-Induced Destabilization: 0% SOC NCM alone remains stable and does not form NiO below 400 °C (Figure R4-12). However, when mixed with LPSC (Figure R4-13), NiO formation is observed near 350 °C, confirming that sulfur-containing species from LPSC destabilize the NCM structure and facilitate its reduction to NiO.\nFigure R4-12. In-situ heating XRD analysis of the 0% SOC NCM811.\nFigure R4-13. In-situ heating XRD analysis of the 0% SOC composite cathode.\nLattice Oxygen Participation: Post-reaction characterizations (XPS and Raman spectroscopy) confirm the presence of sulfite (SO₃²⁻), sulfate (SO₄²⁻), phosphate (PO₄³⁻), and nickel sulfide (NiS). The increase in sulfur oxidation state (e.g., S²⁻ → S⁴⁺/S⁶⁺) and the reduction of nickel (e.g., Ni³⁺ → Ni²⁺) strongly suggest that sulfur-containing species derived from LPSC act as reducing agents, facilitating the reduction of transition metals in NCM and leading to NiO formation.\nLattice Oxygen Release: Concurrently, the generation of oxygen-containing anions (SO₃²⁻, SO₄²⁻, PO₄³⁻) implies that lattice oxygen released during the phase transition of NCM (from a layered/spinel structure to a rock-salt NiO phase) participates in the oxidation of sulfur and phosphorus species.\nThus, NiO formation in our system arises from a cooperative process:\n(1) Sulfur-induced reduction of nickel, destabilizing the NCM structure, and\n(2) Subsequent release and involvement of lattice oxygen in oxidizing sulfur/phosphorus species.\nThis mechanism has been clarified in the revised manuscript (“The formation of NiO occurred alongside the initial reaction of LPSC, suggesting a synergistic promotion driven by the combined effects of lattice oxygen release from the cathode and destabilization by sulfur species” (Section 2.2, page 10-11) to provide a more definitive explanation. We thank the reviewer for prompting this important discussion.\n5. In section 2.3, the authors use NCM811 cathodes with a 4 nm coating, but do not explore whether this thickness is optimal. Given prior literature showing non-linear effects of coating thickness on both ionic conductivity and interfacial stability, can the authors comment on why 4 nm was chosen and whether thicker or thinner coatings were evaluated?\nResponse: We sincerely thank the reviewer for raising this critical point regarding coating thickness optimization. We agree that the thickness of a coating layer can non-linearly influence both ionic transport and interfacial stability, and we appreciate the opportunity to clarify our experimental design and expand on the implications of our findings.\nIn this study, a 4 nm Al₂O₃ coating was selected not as an optimized thickness, but as a representative and widely reported benchmark for constructing a physical barrier between NCM811 and LPSC. As correctly noted by the reviewer, the primary role of such coating is to mitigate (electro)chemical reactions and suppress the space-charge layer effect at the cathode–sulfide electrolyte interface. Numerous coating materials—such as LiNbO₃, Li₂ZrO₃, and LiAlO₂—have been investigated with the aim of enhancing interfacial stability while maintaining satisfactory ionic conductivity, as highlighted in the literature [1,2].\nAlthough our results indicate that the 4 nm Al₂O₃ coating delayed the initial exothermic reaction and reduced heat release to some extent, it could not fully prevent the electrochemical degradation of LPSC. This is consistent with the inherent limitations of coating strategies: even an ideally uniform nanoscale coating may exhibit pinholes or insufficient electronic insulation, and it cannot entirely suppress all parasitic reactions under high voltages.\nIn direct response to the reviewer’s comment, we conducted additional experiments using a thinner Al₂O₃ coating (2 nm) to preliminarily probe the thickness dependence. The results confirm that the 2 nm coating offers inferior suppression of the first exothermic peak compared to the 4 nm version (heat reduction of 80.8 J/g vs. 69.4 J/g), supporting the role of coating thickness in moderating interfacial reactions.\nFigure R4-14. Typical surface TEM images of 2nm-Al O and 4nm- Al O coated\n2 3 2 3\nLiNi Co Mn O (named as C-NCM) samples.\n0.8 0.1 0.1 2\nFigure R4-15. S 2p and P 2p XPS spectra of the 100% SOC composite cathode (layered oxide cathode, LPSC, and VGCF in a weight ratio of 70:30:3) with 2nm-Al2O3 and 4nm-Al2O3 coated NCM811 and uncoated NCM (bare NCM811).\nFigure R4-16. DSC of the 100% SOC composite cathode (layered oxide cathode, LPSC, and VGCF in a weight ratio of 70:30:3) with 2nm-Al O and 4nm-Al O coated NCM811 and NCM\n2 3 2 3\n(bare NCM811).\nNevertheless, both coated samples eventually underwent significant thermal decomposition, underscoring a fundamental limitation: extrinsic coating layers may improve interfacial stability only within a narrow operational window. Moreover, practical challenges—such as uniformity, scalability, and cost—further complicate the large-scale application of coated cathodes.\nTherefore, while coating can offer partial improvement, we concur that future efforts should focus not only on optimizing coating materials and thickness but, more importantly, on developing intrinsically more stable sulfide electrolytes. Such materials would alleviate the need for complex interfacial engineering and facilitate the realization of high-energy, long-life all-solid-state batteries.\nWe thank the reviewer again for this insightful suggestion, which has significantly enriched our analysis.\nReference\n[1] Banerjee A, Tang H, Wang X, et al. Revealing nanoscale solid-solid interfacial phenomena for long-life and high-energy all-solid-state batteries. ACS Applied Materials & Interfaces, 2019, 11(46): 43138-43145\n[2] Liu B, Wang D, Avdeev M, et al. High-throughput computational screening of Li containing fluorides for battery cathode coatings. ACS Sustainable Chemistry & Engineering, 2020, 8(2): 948-957.\n6. While LGS shows good interfacial stability with NCM811 on the cathode side, the authors employ a Li-Si anode in full-cell validation without evaluating its interfacial compatibility with LGS or LPSC. Given known issues of volume expansion and potential interfacial reactivity in Si-based alloys, the absence of any post-mortem analysis or thermal characterization at the anode-SE interface limits the comprehensiveness of the system-level safety claims. Can the authors provide a discussion on whether the LGS impacts long-term anode stability or interacts chemically?\nResponse: We thank the reviewer for this insightful comment regarding the anode-side interfacial compatibility of LGS. We agree that a comprehensive safety evaluation must consider all cell components, and we appreciate the opportunity to clarify our experimental design and provide further justification.\nFirstly, we would like to emphasize that all thermal safety evaluations—including ARC and DSC tests for both LPSC and LGS-based cells—were performed using Li-In anodes. This ensures consistency in the variable under investigation (the cathode/electrolyte interface) and allows for a direct and fair comparison of the thermal behavior between different solid electrolytes. The Li-Si anode was employed only in long-term electrochemical cycling tests of the LGS-based full cell, as it offers superior cycling performance compared to Li-In, thereby enabling a more meaningful evaluation of the cell’s capacity retention over time.\nWe acknowledge the well-known challenges of Si-based anodes, including substantial volume changes. To mitigate this, a constant stack pressure was applied throughout the cycling tests to maintain mechanical integrity and interfacial contact, which contributed to the stable cycling performance observed.\nIt is well documented in the literature that certain sulfide solid electrolytes (such as LGPS) exhibit severe reactivity with lithium metal, forming mixed-conducting interphases that lead to continuous decomposition and accelerated thermal failure [1-2]. Although LGS shows improved stability with oxide cathodes, it demonstrates high impedance and poor compatibility when in direct contact with lithium-based anodes, as confirmed by our own XPS analysis (Figure R4-17). LGS reacts with lithium metal to form reduced Ge species, leading to high impedance and rapid degradation. This is precisely why we designed a hybrid cell architecture: LGS is used only as a stabilizing additive within the composite cathode, while LPSC is retained as the bulk separator. This strategy successfully prevents direct contact between LGS and the anode, thereby circumventing its inherent instability while still leveraging its beneficial role in suppressing cathode-side parasitic reactions.\nFigure R4-17. Ge 3d XPS spectra of LGS and LGS/Li pressed into tablets after 24 hours.\nMoreover, regarding the thermal stability at the anode interface: we conducted supplementary DSC tests of LPSC with both Li-In and Li-Si. The results exhibit their excellent thermal compatibility with LPSC, showing no exothermic reactions below 350 °C (Figure R4-18). This indicates that the anode is not the primary source of thermal instability in our system.\nFigure R4-18. DSC of LPSC/LiSi and LPSC/LiIn.\nIn conclusion, while LGS is unstable against lithium metal, our cell design isolates it from the anode. We have added a discussion on this design rationale and the supporting anode compatibility data to the revised Supporting Information (S59) to provide a more comprehensive perspective. We thank the reviewer for raising this important point regarding anode-electrolyte interfacial compatibility, which is indeed critical for evaluating the overall safety and stability of all-solid-state batteries.\nReference\n[1] Wenzel, S.; Randau, S.; Leichtweiß, T. et al. Direct Observation of the Interfacial Instability of the Fast Ionic Conductor Li₁₀GeP₂S₁₂ at the Lithium Metal Anode. Chem. Mater., 2016, 28(7): 2400-2407.\n[2] Yan, J.; Yao, J.; Zhao, J. et al. Revealing the Thermal Stability of the Li/Sulfide Solid Electrolyte Interface at Atomic Scale via Cryogenic Electron Microscopy. Adv. Funct. Mater., 2025, 35: 2421918."}, {"round": 2, "reviewer_comments": {"reviewer_1": "The manuscript has been revised based on the reviewers’ previous comments. However, the interpretation of the computational and experimental results still remains questionable. The authors argue that the strong overlap of S and O in their DOS calculations indicates the feasibility of a bond exchange. Nevertheless, the XRD results after heat treatment suggest that it is not simply the delithiated NCM reacting with LPSCl to form NiS while maintaining the oxidation state of Ni. 2 Instead, the results imply that Ni4+ is reduced and NiS is formed. This suggests that the reaction is not a simple S–O exchange, but rather that S2- transfers electrons to Ni and becomes oxidized, which could also result in the formation of gaseous species such as SO . 2 In addition, the crystallographic data used for the DOS calculations need to be clearly provided. To perform DOS calculations, the initial crystal structures are required, but it is not specified whether these were obtained from databases such as Materials Project or derived from experimental XRD results. If refined XRD structures were used, then the corresponding refined XRD patterns together with the crystallographic information tables should be included to ensure reproducibility and reliability of the computational results.", "reviewer_2": "The author addressed the reviewers' comments to improve the completeness of the research. First, they clarified ambiguities in the experimental conditions by thoroughly analyzing the reactivity of NCM811 under 100% SOC conditions, which means fully delithiated NCM811, and is directly linked to thermal runaway phenomena. Additionally, by examining the Fermi levels of key reaction intermediates, the author strengthened the explanation of the electron transfer mechanism at the interface. They also corrected a conceptual error in the description comparing the VBM of LPSC and the Fermi level of NCM811 and provided the accurate direction of electron transfer based on energy levels, increasing the analysis's reliability. Through these revisions, the author has carefully addressed the reviewers' questions and further bolstered the paper's main arguments.\nDespite revisions to several sections, this is an additional comment that a critical issue remains unresolved. The author's DFT calculations were based on pristine materials in their initial state (0% SOC), but the thermal runaway phenomenon, the main focus of analysis, occurs in a fully delithiated state corresponding to 100% SOC. This fundamental mismatch between the material state used in the calculations and the actual conditions of analysis makes the theoretical basis supporting the experimental results insufficient. The electronic properties of delithiated NCM811 differ significantly from those in the initial state, so the current calculation results alone have limitations in explaining the actual phenomenon.\nComment 1) The fundamental discrepancy between calculations and experimental conditions remains unresolved. While DFT calculations seem to assume the material is in its initial state (0% SOC), the critical thermal runaway analysis focuses on a fully delithiated state at 100% SOC. This inconsistency risks undermining the theoretical basis needed to support the experimental findings. Specifically, the cathode shown in Figure 3g appears to contain a considerable amount of lithium, raising doubt on the claim that it is fully delithiated. If the structure used by the authors is not a fully delithiated but instead represents a state where lithium is removed to the maximum extent possible while maintaining structural stability, then the exact chemical formula for this state must be clearly stated in the Computational Method section to ensure the model's accuracy and reproducibility.", "reviewer_3": "Authors successfully addressed the questions and comments that I have raised. Regarding the reply for the question 1 (difference of Ttr of SOC100 for full cell and composite cathode), it is recommended to add the discussion. After that, I would recommend the publication of this manuscript in Nature Communications.", "reviewer_4": "The authors have addressed the comments satisfactorily. The revised version is in excellent scientific standing."}, "author_response": "Reviewer #1:\nComment: The manuscript has been revised based on the reviewers’ previous comments.\nHowever, the interpretation of the computational and experimental results still remains questionable. The authors argue that the strong overlap of S and O in their DOS calculations indicates the feasibility of a bond exchange. Nevertheless, the XRD results after heat treatment suggest that it is not simply the delithiated NCM reacting with LPSCl to form NiS while maintaining the oxidation state of Ni. Instead, the results imply that Ni4+ is reduced and NiS is formed. This suggests that the reaction is not a simple S–O exchange, but rather that S2- transfers electrons to Ni and becomes oxidized, which could also result in the formation of gaseous species such as SO .\nResponse: We sincerely thank the reviewer for this insightful comment, which has drawn our attention to a key aspect of the reaction mechanism between delithiated NCM and LPSCl. We agree that the process is more complex than a simple S–O exchange, and the reduction of Ni⁴⁺ along with NiS formation, as revealed by XRD, is a critical observation.\nIn our original discussion, the pronounced S and O orbital overlap identified in the DOS was interpreted as an indicator of favorable chemical interaction that may facilitate interfacial co-diffusion. We acknowledge, however, that these calculations do not fully capture the complexity of the interfacial process, particularly the subsequent redox reactions that occur under high-temperature conditions.\nDuring thermal treatment, multi-stage reactions take place between delithiated NCM and LPSCl. The structural collapse of delithiated NCM results in oxygen release and reduction of Ni⁴⁺. The resulting NiO could further react with LPSCl to form NiS, as experimentally confirmed in Figure S13 in our original Supporting Information.\nSimultaneously, under high-temperature conditions, S²⁻ in the PS₄³⁻ tetrahedral units can also be oxidized through reaction pathways governed by the local chemical environment. In the presence of metal oxides (e.g., NCM) and lithium sources, this process preferentially yields sulfate species, as supported by prior studies [1].\nCollectively, these findings confirm that the interfacial reactions involve complex multi-step chemistry beyond a simple S–O exchange, encompassing both anion diffusion and coupled redox processes.\nFigure S13. XRD patterns of LPSC with NiO in a 1:1 mass ratio after heating at 500 ℃.\nTo address this point, we have revised the relevant descriptions in the manuscript to more accurately reflect the complexity of the process. The term \"S–O exchange\" has been replaced with \"cathode-SE interaction involving both anion exchange and redox chemistry\" to avoid ambiguity and to better represent the multi-step nature of the reaction. We believe this revised interpretation better aligns the computational and experimental results and provides a more accurate description of the interfacial reaction.\nBesides, the crystal structures of Li₆PS₅Cl, Li₂S, Li₃PS₄, P₂S₅ and Li GeS were sourced\n4 4\nfrom the Materials Project database. For NCM811, its crystal structure was constructed based on our experimental XRD results (Figure S27), which confirm that all diffraction peaks correspond to the α-NaFeO₂-type hexagonal structure with the R-3m space group, a well-established model widely reported in the literatures [2-6]. The NCM model for DFT calculations was constructed based on the LiNiO₂ structure from the Materials Project by substituting Ni atoms with Co and Mn in the appropriate stoichiometric ratio. The 80% delithiated structure was then generated by removing a corresponding number of Li atoms from the pristine model. The final structure used for DOS calculations was determined as the lowest-energy configuration after DFT relaxation.\nWe have revised the computational methods section (1.5. Calculation method) to explicitly state the data source for all compounds studied. This amendment will allow other researchers to easily retrieve the same structures and verify our calculations.\nReference\n[1] Rui, X.; Ren, D.; Liu, X.; Wang, X.; Wang, K.; Lu, Y.; Li, L.; Wang, P.; Zhu, G.; Mao, Y.; et al. Distinct Thermal Runaway Mechanisms of Sulfide-Based All-Solid-State Batteries. Energy Environ. Sci. 2023, 16, 3552–3563.\n[2] Dixit, M.; Kosa, M.; Lavi, O. S.; Markovsky, B.; Aurbach, D.; Major, D. T. Thermodynamic and Kinetic Studies of LiNi0.5Co0.2Mn0.3O2 as a Positive Electrode Material for Li-Ion Batteries Using First Principles. Phys. Chem. Chem. Phys. 2016, 18, 6799–6812.\n[3] Kuo, L.-Y.; Guillon, O.; Kaghazchi, P. Origin of Structural Phase Transitions in Ni-Rich LixNi0.8Co0.1Mn0.1O2 with Lithiation/Delithiation: A First-Principles Study. ACS Sustainable Chem. Eng. 2021, 9, 7437–7446.\n[4] Dai, Z.; Zhao, H.; Chen, W.; Zhang, Q.; Song, X.; He, G.; Zhao, Y.; Lu, X.; Bai, Y. In Situ Construction of Gradient Oxygen Release Buffer and Interface Cation Self-Accelerator Stabilizing High-Voltage Ni-Rich Cathode. Adv. Funct. Mater. 2022, 32, 2209075.\n[5] Hsieh, I. T.; Wu, Y.; Li, B.; Qi, Y. First-Principles Study of the Structures and Redox Mechanisms of Ni-Rich Lithium Nickel Manganese Cobalt Oxides. Solid State Ion. 2024, 411, 116562.\n[6] Lowe, J. S.; Hafiz, H.; Jr, L. G. H. First Principles Method for Predicting Lithiation Induced Volume Change and Open Circuit Voltage of NCM Cathode Materials. J. Electrochem. Soc. 2025, 172, 083504.\nReviewer #2:\nThe author addressed the reviewers' comments to improve the completeness of the research. First, they clarified ambiguities in the experimental conditions by thoroughly analyzing the reactivity of NCM811 under 100% SOC conditions, which means fully delithiated NCM811, and is directly linked to thermal runaway phenomena.\nAdditionally, by examining the Fermi levels of key reaction intermediates, the author strengthened the explanation of the electron transfer mechanism at the interface. They also corrected a conceptual error in the description comparing the VBM of LPSC and the Fermi level of NCM811 and provided the accurate direction of electron transfer based on energy levels, increasing the analysis's reliability. Through these revisions, the author has carefully addressed the reviewers' questions and further bolstered the paper's main arguments.\nDespite revisions to several sections, this is an additional comment that a critical issue remains unresolved. The author's DFT calculations were based on pristine materials in their initial state (0% SOC), but the thermal runaway phenomenon, the main focus of analysis, occurs in a fully delithiated state corresponding to 100% SOC. This fundamental mismatch between the material state used in the calculations and the actual conditions of analysis makes the theoretical basis supporting the experimental results insufficient. The electronic properties of delithiated NCM811 differ significantly from those in the initial state, so the current calculation results alone have limitations in explaining the actual phenomenon.\nComment 1) The fundamental discrepancy between calculations and experimental conditions remains unresolved. While DFT calculations seem to assume the material is in its initial state (0% SOC), the critical thermal runaway analysis focuses on a fully delithiated state at 100% SOC. This inconsistency risks undermining the theoretical basis needed to support the experimental findings. Specifically, the cathode shown in Figure 3g appears to contain a considerable amount of lithium, raising doubt on the claim that it is fully delithiated. If the structure used by the authors is not a fully delithiated but instead represents a state where lithium is removed to the maximum extent possible while maintaining structural stability, then the exact chemical formula for this state must be clearly stated in the Computational Method section to ensure the model's accuracy and reproducibility.\nResponse: We sincerely thank the reviewer for this insightful comment regarding the alignment between our computational models and experimental conditions. We acknowledge that the original description lacked precision and have now provided the following clarification.\nThe reviewer correctly points out that a fully delithiated state is neither experimentally attainable nor computationally stable. In our work, the focus is on the fully charged state (defined as 100% SOC at 4.5 V, Result and Discussion section, line 6), which corresponds to a highly delithiated, but not completely lithium-free, state of the NCM cathode. Our DFT calculations were therefore conducted on a structural model that closely mirrors this realistic, highly delithiated experimental state. Specifically, the model used corresponds to an 80% degree of delithiation, as referenced in our original Supporting Information (Figure S24). This choice was based on the following experimental and practical considerations:\nFigure S24. Initial atomic configuration of the 80% delithiated NCM811 | LPSCl interface from AIMD simulation at 25 ℃.\n(1) Experimental Capacity Analysis: When charged to 4.5 V, the full cell delivered a specific capacity of 243.5 mAh/g (Figure S3). Based on the theoretical capacity of NCM811 (~280 mAh/g), this would correspond to a theoretical delithiation degree of ~87%. However, the measured capacity includes a minor contribution from the electrochemical decomposition of the sulfide solid-state electrolyte. Therefore, the actual degree of delithiation from the NCM cathode is slightly lower than 87%. To accurately reflect the realistic delithiation level while avoiding the severe structural degradation associated with higher voltages, we adopted a conservative and representative 80% delithiation model for our simulations. This approach ensures that our computational results are pertinent to the actual interfacial state under investigation.\nFigure S3. Typical charge-discharge profiles of NCM811/LPSC/Li-In cells in the voltage range of 2.8−4.5 V (vs. Li+/Li) at 0.05C.\n(2) Computational Stability: Furthermore, this 80% delithiation degree represents a highly delithiated yet structurally stable configuration, making it suitable for meaningful and reliable DFT simulations of the high-SOC reactivity relevant to thermal runaway scenarios.\nWe agree with the reviewer on the importance of clarity and reproducibility. In response, we have explicitly stated the specific delithiation degree of 80% in the revised computational methods section (1.5. Calculation method).\nRegarding the presence of lithium in the cathode in Figure 3g, it originates from two sources: (1) Residual lithium in the delithiated cathode, consistent with the 80% delithiation level. (2) Lithium migration from the Li₆PS₅Cl solid electrolyte into the cathode vacancies during heating, as part of the interfacial reaction, a phenomenon also supported by previous studies (e.g., Adv. Funct. Mater., 2404806, 2024).\nIn response to the reviewer’s comment on clarity and reproducibility, we have explicitly provided the chemical formula of the 80% delithiated model (Li Ni Co Mn O ) in the revised computational methods section (1.5. Calculation method), along with the rationale for selecting this specific delithiation degree. We believe these clarifications resolve the apparent discrepancy and strengthen the alignment between our computational models and experimental conditions. Thank you again for this valuable suggestion.\nReference\n[1]. T. Kim, H. Chang, G. Song, S. Lee, K. Kim, S. Lee, J. Moon, K. T. Lee, Critical Factors Contributing to the Thermal Runaway of Thiophosphate Solid Electrolytes for All‐Solid‐State Batteries. Adv Funct Materials, 2404806 (2024).\nReviewer #3:\nAuthors successfully addressed the questions and comments that I have raised.\nRegarding the reply for the question 1 (difference of Ttr of SOC100 for full cell and composite cathode), it is recommended to add the discussion. After that, I would recommend the publication of this manuscript in Nature Communications.\nResponse: We sincerely thank the reviewer for their positive feedback and for acknowledging our responses. As recommended, we have now added the discussion in the Discussion section of the revised manuscript as shown below:\n“The differing thermal runaway behaviors between full cells and cathode composites are primarily attributed to electrode crosstalk, where reactive species from the cathode migrate to the anode and undergo exothermic reactions, alongside other architectural factors.”"}, {"round": 3, "reviewer_comments": {"reviewer_1": "The authors have addressed all the comments appropriately. The revised manuscript provides a thorough and insightful discussion on the thermal stability in sulfide-based all-solid-state batteries.", "reviewer_2": "The authors addressed the final comments that I raised. I recommend the publication of this manuscript in Nature Communications when all other reviewers agree."}, "author_response": ""}]

Decision letter