Stage-Dominated Thermal Runaway in Sulfide ASSBs: Decoupled Electrochemical Ignition and Chemical Cascades

Stage-Dominated Thermal Runaway in Sulfide ASSBs: Decoupled Electrochemical Ignition and Chemical Cascades | Research Square window.SnipcartSettings = { analytics: { enabled: false } }; (function() { var accessVector = localStorage.getItem('access_vector') || ''; window.dataLayer = window.dataLayer || []; if (accessVector) { window.dataLayer.push({ user: { profile: { profileInfo: { snid: accessVector } } } }); } })(); (function(w,d,s,l,i){w[l]=w[l]||[];w[l].push({'gtm.start':new Date().getTime(),event:'gtm.js'});var f=d.getElementsByTagName(s)[0],j=d.createElement(s),dl=l!='dataLayer'?'&l='+l:'';j.async=true;j.src='https://www.googletagmanager.com/gtm.js?id='+i+dl;f.parentNode.insertBefore(j,f);})(window,document,'script','dataLayer','GTM-K279D39R'); Browse Preprints In Review Journals COVID-19 Preprints AJE Video Bytes Research Tools Research Promotion AJE Professional Editing AJE Rubriq About Preprint Platform In Review Editorial Policies Our Team Advisory Board Help Center Sign In Submit a Preprint Cite Share Download PDF Article Stage-Dominated Thermal Runaway in Sulfide ASSBs: Decoupled Electrochemical Ignition and Chemical Cascades Guanglei Cui, Yuhan Wu, Shu Zhang, Youlong Sun, Lang Huang, Jiahao Xu, and 8 more This is a preprint; it has not been peer reviewed by a journal. https://doi.org/ 10.21203/rs.3.rs-6428540/v1 This work is licensed under a CC BY 4.0 License Status: Published Journal Publication published 19 Feb, 2026 Read the published version in Nature Communications → Version 1 posted You are reading this latest preprint version Abstract Sulfide all-solid-state batteries (ASSBs) are poised to revolutionize energy storage with unparalleled energy densities and reduced flammability risks, yet emerging studies reveal a critical safety paradox where cathode-electrolyte interactions induce thermal runaway at unexpectedly low temperatures, challenging the assumption of inherent thermal stability. While current research prioritizes electrochemical performance, the thermochemical degradation dynamics between oxide cathode and thiophosphate electrolyte remain poorly understood, with even conflicting mechanistic interpretations. Herein, leveraging multiscale calorimetry and in-situ gaseous analytical techniques, it is noted that the metastable interphase between nickel-rich cathodes and thiophosphate electrolytes, formed through electrochemically preconditioned but persistently overlooked, serve as primary triggers for exothermic cascades, a phenomenon starkly distinct from liquid electrolyte counterparts. Thermal degradation in composite cathodes evolves through dual mechanistic phases: First, delithiated cathode materials react with electrochemical precondition generated sulfur-rich species (-S-S-, -P-S-P-, and Li 3 PS 4 ), driving rapid heat accumulation below 160 ℃ by interphase-dominated chemistry, accompanied by gaseous emissions (SO 2 , CO 2 , O 2 ); subsequently, sulfur-oxygen interdiffusion in bulk phases accelerates solid-solid exothermic reactions, driving thermal propagation and eventual runaway. This dual-stage mechanism generalizes across other sulfide systems. Crucially, it is demonstrated that the electrochemically formed interface through Ge-S bond engineering effectively suppresses thermal cascades, where the as-designed Li 4 GeS 4 -modified interface system achieves unprecedented thermal safety without compromising electrochemical performance. Our study establishes a new paradigm for thermal runaway causality by prioritizing interfacial thermodynamics over bulk material compatibility as the primary determinant of thermal safety—a framework fundamentally divergent from conventional liquid electrolyte batteries. Physical sciences/Energy science and technology/Energy storage/Batteries Physical sciences/Materials science/Materials for energy and catalysis/Batteries Figures Figure 1 Figure 2 Figure 3 Figure 4 Figure 5 Introduction While modern lithium-ion batteries (LIBs) have attained record-breaking 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 solvents. [ 1 ] Recent advancements in sulfide-based solid electrolytes, particularly argyrodite Li 6 PS 5 Cl (LPSC), have positioned this material family at the forefront of all-solid-state battery (ASSB) research due to its exceptional ionic conductivity and favorable mechanical deformability. [ 2 – 4 ] Notably, this decade has witnessed substantial progress in LPSC-based ASSBs with synergistic breakthroughs in electrochemical stability and scalable processing developments, enabling them as leading candidates for next-generation energy storage commercialization. [ 5 – 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 surpassing liquid electrolyte systems. [ 8 – 11 ] ARC testing of a home-made 3.8 Ah LiNi 0.5 Co 0.2 Mn 0.3 O 2 /LPSC/Li pouch cell demonstrates an initial self-heating temperature at 163 ℃, and then culminates into explosive TR at 275 ℃. [ 9 ] This acceleration effect magnifies at pack level, where the 3 Ah ASSB configurations demonstrate TR propagation velocities 10-fold higher than commercially available LIB equivalents. [ 10 ] Unlike the well-characterized anode-triggering TR pathways in conventional liquid-electrolyte LIBs, the safety paradox of ASSBs originates from cathode-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 cathode-SE interfaces at abused conditions, which demonstrate 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 ℃ and the whole exothermic process is free of detectable sulfurous gas (e.g., SO 2 ). [ 12 ] Lee’s team investigated exothermic and gas-evolution behaviors in composite cathodes under abuse conditions, demonstrating that mechanical grinding or thermal exposure to 150 ℃ in argon triggers TR accompanied by substantial SO 2 emissions. [ 13 , 14 ] Wu et al. identified that in the LiCoO 2 /LPSC system, bulk chemical reactions between delithiated cathode and sulfide electrolyte dominated thermal decomposition above 300 ℃. [ 15 ] 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 cathode/SE interfaces. Thermochemically, bulk S-O interdiffusion directly drives interfacial reconstruction, forming metastable phosphate-sulfate phases through elemental exchange reactions. [ 16 , 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-, Li 3 PS 4 ) that progressively destabilize interfacial integrity. [ 18 , 19 ] Fundamental band alignment analyses further reveal that the valence band maximum of sulfide SEs lies energetically above the Fermi level of oxide cathodes. This interfacial energy offset drives electron transferring from cathode to SE and accelerates electrochemical degradation. [ 20 ] 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 cathodes, and (2) thermally driven lattice oxygen release from delithiated oxide cathodes, which synergistically amplifies interfacial instability. This process further triggers self-sustaining cascades that integrate bulk-phase redox reactions, gaseous byproduct evolution (e.g., SO 2 , O 2 ), 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 (SO 2 , O 2 ), 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 Li 3 PS 4 phases, governs early-stage self-heating through interfacial exothermic decomposition accompanied by SO 2 evolution. At high temperatures, sulfur/oxygen co-diffusive solid-solid reactions across cathode-SE interface triggers sulfide oxidation and layered cathode disintegration, amplifying heat generation rates and leading to uncontrollable TR. This interfacial failure paradigm extends to Li 10 GeP 2 S 12 and Li 9.54 Si 1.74 P 1.44 S 11.7 Cl 0.3 systems, demonstrating universal disulfide bridge induced thermal-electrochemical coupling cascade reaction. To address this, we engineer NCM811/Li 4 GeS 4 composite cathodes via Ge-S bond stabilization to elevate the thermal thresholds. T onset and T tr of the as-assembled ASSBs are increased from 168 ℃, 228 ℃ to 223 ℃, 312 ℃ 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 failsafe sulfide-based ASSBs. Result and Discussion 2.1 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 ( Figure S1 ) sulfide ASSBs were assembled employing NCM811 and Li-In as electrodes, and their charge/discharge profiles showcased impressive electrochemical performance ( Figure S2 ). The thermal safety characteristics of both fully charged ( Figure 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 accelerating rate calorimeter (ARC) under typical heat-wait-search (HWS) conditions. Key parameters including onset temperature of self-heating ( T onset ), self-heating rate (SHR), and thermal runaway temperature ( T tr , defined as the temperature point when SHR reaches 1 ℃·min − 1 ) were recorded. Apparently, violent thermal runaway was observed for the 100% SOC ASSB, with the T onset and T tr being 168 ℃ and 228 ℃ (Fig. 1 a). 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 extreme abuse conditions, consistent with prior studies. [ 9 ] Surprisingly, the fully discharged ASSB also underwent severe thermal runaway with T onset of 234 ℃ and T tr of 313 ℃, 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. Anode material and composite cathode (NMC811, LPSC, and 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 anode with LPSC up to 350 ℃, indicating their good thermal stability and limited involvement of the exothermic chain reactions during elevated temperatures (Fig. 1 b). However, 100% SOC composite cathode exhibited T onset and T tr of 160 ℃ and 274 ℃, which aligned with the onset self-heating of the full cell, mirroring that cathode/SE reactions were mainly responsible for the observed exothermic behavior of the full cell. Additionally, it was unexpected to find that 0% SOC composite cathode also generated three discontinuous exothermic processes at 228 ℃, 276 ℃, and 340 ℃ in the HWS plot, 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 test at ARC with a heating rate of 5 ℃ min − 1 . Severe self-heating was observed in the 100% SOC composite cathode, with the temperature rapidly rising from 203 ℃ to 363 ℃ within just 3.9 seconds ( Figure S4 ). Moreover, grinding the 100% SOC composite cathode in the Ar-filled glove box led to violent burning ( Figure S5 ). These findings raise significant safety concerns regarding the 100% SOC composite cathode, as it can trigger thermal runaway under both thermal and mechanical abuse conditions. Overall, these results unveil that it is the cathode-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 cathode and SE. 2.2 Material-level interfacial reactions To unravel the complex interfacial reactions governing thermal instability in solid-state batteries, the thermochemical compatibility between pristine oxide cathode, 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 cathode under heating condition. Initial characterization confirmed the exceptional individual stability of NCM811 layered oxide, LPSC, and VGCF additives, with no detectable thermal decomposition below 500 ℃ when tested in isolation ( Figure S6 ). However, when formulated into an uncycled composite cathode (70:30:3 mass ratio), this ostensibly stable system underwent dramatic interfacial reactions, exhibiting two distinct exothermic peaks at 373 ℃ and 465 ℃ (Fig. 1 c). 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 ℃ revealed nascent sulfur diffusion from LPSC into NCM811 surfaces, accompanied by oxygen migration in the reverse direction (Fig. 1 c). Upon reaching 500 ℃, 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 (HSAB) principle. Specifically, the hard acid character of phosphorus (P 5+ ) drives preferential coordination with oxygen (O 2− , hard base), while softer nickel cations (Ni 3+/4+ ) selectively interact with sulfur anions (S 2− , 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 conditions. [ 21 , 22 ] Complementary in-situ heating XRD analysis revealed the progressive disappearance of NCM811 (003) and (104) diffraction peaks near 373 ℃, 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. 1 d). Distinctive phase evolution was observed at elevated temperatures, where XRD patterns at 400°C indicated co-formation of NiO, Li 2 S, and Li 3 PO 4 , attributed to oxygen migration from NCM811 reacting with LPSC, a phenomenon corroborated by oxygen diffusion observed through SEM-EDS mapping (Fig. 1 e). Upon reaching 500°C, complete LPSC decomposition coincided with appearance of Ni 3 S 2 and CoS 2 , 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 ℃ thermal treatment, the yielding Li 2 S, Ni 3 S 2 , and Li 3 PO 4 as reaction products convinced the critical role of rock salt transition metal oxide on contributing the second stage exothermic reactions ( Figure S7 ). Concurrently, XPS and TOF-SIMS profiles detected sequential phosphate, hypophosphite, sulfate, and sulfite species generation during thermal excursions, with sulfite/sulfate signatures at 400 ℃ quantitatively matching sulfur diffusion trajectories (Fig. 1 f, S8). Raman spectral evolution further corroborated this interfacial degradation pathway, showing Li 2 S/Li 2 S x and PO 4 3− formation at 400 ℃, followed by LPSC signal attenuation and SO 4 2− emergence at 500 ℃ (Fig. 1 g). These multimodal characterizations collectively confirm the solid-solid interfacial degradation mechanisms wherein LPSC-derived sulfur and NCM811-sourced oxygen undergo reciprocal diffusion, driving transition metal sulfide and oxyanion formation. 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 cathode tends to exacerbate interfacial reactions. To systematically evaluate these coupled phenomena, composite cathodes 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 303 ℃ ( ΔH = 75.1 J·g − 1 ) and 379 ℃ ( ΔH = 268.8 J·g − 1 ) (Fig. 2 a). Structural evolution monitored via in-situ heating XRD spectra revealed minimal crystallographic changes below 310 ℃, followed by abrupt disappearance of NCM811/LPSC diffraction peaks and concurrent NiO phase emergence at 379 ℃, confirming bulk material interactions during the second exothermic phase. Notably, the absence of gaseous products throughout heating demonstrated their solid-solid reaction pathways. XPS analysis of the 310 ℃-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. 2 b). Subsequent Ni 3 S 2 and PO 4 3− formation detected at 450 ℃ validated the established sulfur-oxygen interdiffusion mechanism. 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 ( Figures S9-S11 ). Notably, the 100% SOC composite cathode was observed to exhibit intensified interfacial reactivity under thermal stress, characterized by two distinct exothermic peaks at 209 ℃ ( ΔH = 399.6 J·g − 1 ) and 299 ℃ ( ΔH = 863.9 J·g − 1 ), respectively (Fig. 2 c). The initial exothermic event was critically noted to align with severe self-heating behavior identified in prior ramp tests ( Figure S4 ), 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 (Fig. 2 c). These observations confirm delithiated cathode-driven initiation of thermal decomposition, with bulk solid electrolyte involvement restricted to later stages. Concurrent gas evolution profiles detected via ARC-MS identify SO 2 (m/z = 64), CO 2 (m/z = 44), CO (m/z = 28), and O 2 (m/z = 32) release during the first exothermic event, implicating lattice oxygen from delithiated NCM811 in oxidative side reactions (Fig. 2 c). Post-reaction characterization of samples quenched at 250 ℃ and 400 ℃ revealed mechanistic details: XPS analysis at 250 ℃ showed depletion of -S-S- bonds, reduced -P-S-P- intensity, and emergence of SO 4 2− /SO 2 2− /PO 4 3− species, confirming electrochemical interphase participation in early exothermic processes (Fig. 2 d). Conversely, 400 ℃-treated samples demonstrated LPSC decomposition with NiS/CoS formation, signifying direct NCM811-LPSC interactions. SEM-EDS mapping at 250 ℃ verified S/O interdiffusion between NCM811 and interphase species, while 400 ℃ analysis revealed Ni-S and P-O bonding indicative of complete interfacial reactions (Fig. 2 e). XRD phase evolution further corroborated this mechanism. (003) peak weakening/shifting below 250 ℃ reflected cathode restructuring without LPSC participation, whereas LPSC signal disappearance and NiO/NiS/Li 3 PO 4 emergence at 400 ℃ confirmed bulk material interactions. Raman spectra exhibited PS 4 3− depletion and SO 4 2− /PO 4 3− formation at 400 ℃, consistent with sulfur-oxygen exchange dynamics ( Figure S12 ). Electron energy loss spectroscopy (EELS) analysis elucidated dynamic redox evolution of nickel species during thermal degradation. At 250 ℃, 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 Ni 3+ / Ni 4+ reduction coupled with reactive lattice oxygen liberation. This dual electronic-state modulation mechanistically accounted for oxygen-containing gas evolution during the initial exothermic phase ( Figure S13 ). Subsequent O K-edge disappearance and continued Ni L-edge shifts at 400 ℃ 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 ℃, SEM-EDS analysis confirmed NCM811 particles retained Ni-O dominance without sulfur penetration into bulk regions (Fig. 3 a), while EBSD mapping identified spinel-phase M 3 O 4 (M = Ni/Co/Mn) formation within cathode particles (Fig. 3 c). These observations collectively demonstrate that the initial exothermic event is governed by cathode phase transformation rather than sulfur interdiffusion. Thermal escalation to 400 ℃ revealed substantial sulfur infiltration into NCM811 particles, evidenced by intensified S signals and attenuated O intensity in SEM-EDS (Fig. 3 b). EBSD quantification detected interfacial reaction products—NiS (79.4%), Li 2 SO 4 (18.1%), and Li 3 PO 4 (2.5%)—distributed across cathode cross-sections, confirming S migration from LPSC and subsequent sulfide formation (Fig. 3 d). High-angle annular dark-field transmission electron microscopy (HAADF-TEM) imaging coupled with selected area electron diffraction (SAED) patterns conclusively verified spinel M 3 O 4 at 250 ℃ and crystalline NiS at 400 ℃ (Fig. 3 e- 3 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 valence band maximum (VBM) of LPSC was 1.47 eV higher than the Fermi level of NCM811, indicating that electron transfer from NCM811 to LPSC was energetically favorable ( Figure S14 ). 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 e g orbitals of NCM811 near the Fermi level ( Figure S15 ). 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 e g orbitals of NCM811, suggesting a tendency for spontaneous chemical bonding. In addition, ab initio molecular dynamics (AIMD) simulations provided critical insights into the interfacial dynamics. At 250 ℃, the simulations showed that delithiated NCM811 and LPSC maintain their structural integrity, with PS 4 3− units remaining intact ( Figures S16 and 3g ). However, at 400 ℃, LPSC degradation becomes evident, marked by the cleavage of PS 4 3− bonds and the formation of P-O bonds. These findings are in good agreement with experimental observations, offering a consistent explanation for the thermal degradation processes. This convergence between multiscale computational results and experimental data strongly corroborates the proposed thermal decomposition pathways. At this point, the specific reaction pathway between electrochemically-derived interphase components and the delithiated cathode becomes critically interrogated. To decouple individual interaction mechanisms of the interphase components, strategically synthesized electrochemical degradation products were systematically investigated. Electrochemically decomposed LPSC (4.5V-(LPSC/VGCF), LPSC and VGCF in 9:1 mass ratio) was synthesized by charging an (LPSC/VGCF)/LPSC/In 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 cathode's interphase compositions ( Figures S17-S18 ). Neat delithiated NCM811 (De-NCM) collected from fully-charged liquid-electrolyte pouch cells (reference methodology [12]) exhibited identical structural characteristics to ASSB-derived counterparts ( Figure S19 ), 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 ℃ accompanied by O 2 evolution (Figs. 3 h-i). 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/4.5V-(LPSC/VGCF) system containing electrochemical degradation products displayed accelerated thermal response, with exothermic onset at 207 ℃ and 263% enthalpy increase (541.9 vs 149.2 J·g − 1 ), concurrently evolving SO 2 and O 2 . Given the excellent thermal stability of 4.5V-(LPSC/VGCF) below 400 ℃ ( Figure S20 ), these observations conclusively attribute early-stage heat generation and sulfurous gas evolution to reactive interphase between electrochemically-modified LPSC and delithiated NCM811. Mechanistic investigations extended to individual LPSC decomposition products (Li 2 S, S, β-Li 3 PS 4 (β-LPS, Figure S21 )) through controlled thermal coupling with De-NCM. Calorimetric analysis revealed minimal influence of Li 2 S on primary exothermic events (no SO 2 detection), although Li 2 S did react with delithiated NCM811 at high temperatures to produce Ni 3 S 2 ( Figure S22 ). Conversely, De-NCM/S and De-NCM/β-LPS mixtures exhibited intensified exothermicity (590.7/445.7 J·g − 1 ) with predominant SO 2 emission and attenuated O 2 release (Figs. 3 h-i). This systematic evidence establishes electrochemical LPSC decomposition products, particularly sulfurous species, as critical initiators of early-stage exothermic cascades and exclusive SO 2 sources in fully charged cathodes during thermal runaway propagation. Moreover, the influence of conductive carbon additives (VGCF) on thermal propagation mechanisms is systematically examined, given their dual functionality in composite cathode systems, reacting with de-lithiated NCM811 to produce CO and CO 2 , and promoting the decomposition of SE into sulfur-containing substances. In the 100% SOC configuration, CO/CO 2 evolution detected during initial exothermic events (Fig. 2 c) is attributed to interfacial reactions between VGCF and delithiated NCM811. Quantitative calorimetric analysis revealed this carbon-cathode interaction contributes 90.8 J·g − 1 (16.8% of total 541.9 J·g − 1 ), establishing its secondary thermal role ( Figure S23 ). Complementary investigations confirmed VGCF's catalytic effect on sulfide electrolyte decomposition, as evidenced by increased -S-S-/-P-S-P-/Li 3 PS 4 formation in electrochemically cycled cathodes with elevated carbon content (0.6 wt% to 13.0 wt%, Figures S24-S25 ), phenomena consistent with prior reports of carbon-mediated SE degradation. [ 23 – 25 ] Thermal characterization of these carbon-varied systems demonstrated proportional enthalpy escalation (169.2, 399.6 and 631.2 J·g − 1 ) with VGCF loading (0.6 wt%, 2.9 wt%, 13.0 wt%, Figure S26 ), confirming two synergistic thermal pathways: (1) direct exothermic carbon-cathode reactions generating CO/CO 2 , and (2) catalytic acceleration of LPSC decomposition that amplifies subsequent interfacial exothermicity. 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-/Li 3 PS 4 ) rather than LPSC itself to start the exothermic chain reactions, producing SO 2 and sulfate/phosphate species alongside CO/CO 2 from VGCF oxidation. This interfacial reaction dominates initial heat generation, driving rapid exothermic escalation. Subsequent temperature rise activates bulk S-O interdiffusion reactions between LPSC and NCM811, forming NiS, Li 3 PO 4 , and sulfate/phosphate compounds with substantial exothermic contribution (68.4% total heat). 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. 4 a). To further explore the universal validity of this thermal degradation roadmap, extended experimental investigations employing identical methodology were conducted on alternative sulfide SEs including Li 10 GeP 2 S 12 (LGPS) and Li 9.54 Si 1.74 P 1.44 S 11.7 Cl 0.3 (LSPSC). Consistent with observations in LPSC-based systems, fully charged composite cathodes incorporating LGPS and LSPSC demonstrated significant self-heating phenomena accompanied by sulfur dioxide evolution near 200 ℃, as evidenced by thermal analysis data ( Figure S27-S28 ). The formation of analogous electrochemically generated interphase species was confirmed through XPS spectral analysis ( Figure S29 ). Contrastingly, control systems containing De-NCM combined with pristine SE/VGCF composites without electrochemical interphase exhibited negligible thermal activity or gas emission, while substantial self-heating and SO 2 release were consistently observed in De-NCM/4.5V-(SE/VGCF) configurations ( Figure S30-S31 ). These experimental observations were corroborated by DSC thermal analysis results ( Figure S32 ), thereby confirming the pivotal involvement of electrochemically derived interphase decomposition products in initiating the primary exothermic reactions characteristic of early-stage thermal runaway mechanisms. 2.3 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. Cathode surface coating has been established as an effective mitigation strategy through the introduction of inert interfacial barriers to suppress cathode/SE decomposition, as documented in prior research. [ 26 , 27 ] To evaluate this approach, ASSBs incorporating Al 2 O 3 -coated NCM811 cathodes (designated C-NCM, 4 nm coating thickness, Figure S33 ) were systematically investigated. Initial thermal characterization of uncycled composite cathodes (70:30:3 mass ratio of C-NCM, LPSC, and VGCF) revealed two elevated exothermic peaks at 408 ℃ and 485 ℃ in DSC profiles ( Figure S34 ), demonstrating temperature increments compared to uncoated counterparts (373 ℃ and 465 ℃, Fig. 1 c). In-situ heating XRD analysis further confirmed enhanced stability, with no phase transitions observed in coated cathodes below 400 ℃ ( Figure S35 ), contrasting with prominent NiO signatures detected at 373 ℃ in uncoated systems (Fig. 1 d). These observations indicate improved interfacial compatibility through Al 2 O 3 modification. Post-cycling analysis of C-NCM-based ASSBs ( Figure S36 ) revealed attenuated XPS signals for -S-S-, -P-S-P-, and PO 4 3− species compared to uncoated cathodes at full SOC ( Figure S37 ), confirming reduced electrochemical degradation. Thermal evaluation of cycled C-NCM cathodes through DSC, ARC- MS demonstrated partial heat release mitigation, with initial exothermic onset at 213 ℃ ( ΔH = 318.8 J·g − 1 ) versus 209 ℃ ( ΔH = 399.6 J·g − 1 ) for unmodified cathodes ( Figure S38, 2c ). However, persistent SO 2 evolution was detected at elevated temperatures despite reduced emission quantities, accompanied by comparable self-heating profiles during ARC ramp tests (200 ℃ onset, Figure S39 ). 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 cathodes and sulfide SEs. The oxidative decomposition tendency of LPSC under high-voltage conditions has been fundamentally attributed to its valence band maximum positioning above the Fermi level (Ɛ F ) of NCM811, creating inherent energy level misalignment with layered oxide cathodes as per solid-state band theory. [ 20 ] The complete resolution of this thermoelectrochemical decomposition phenomenon necessitates intrinsic material-level interfacial compatibility regulation. Notably, 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 oxide/SE compatibility through P→Ge substitution in LGS synthesis. [ 21 , 28 ] Experimentally synthesized Li 4 GeS 4 demonstrated room-temperature ionic conductivity of 1×10 − 4 S cm − 1 ( Figure S40 ). First-principles calculations revealed improved chemical stability between NCM811 and LGS (-0.201 eV/atom) compared with NCM811-LPSC (-0.357 eV/atom) (Fig. 5 a). Energy band analysis further confirmed superior LGS/NCM811 alignment through reduced valence band maximum-Fermi level gap versus LPSC/NCM811 ( Figure S41 ). 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, [ 29 , 30 ] prompting the development of LGS as ionic conductor additives rather than bulk SE replacements. The fabricated NCM811-LGS/LPSC/LiSi ASSBs demonstrated enhanced electrochemical performance with 200.8 mAh·g − 1 initial discharge capacity (0.05C, 45 ℃) and 87.4% capacity retention after 100 cycles (0.5C, 45 ℃) (Fig. 5 b). Thermal safety evaluation utilized composite cathodes (70:30:3 NCM811:LGS:VGCF) extracted from fully charged cells under standardized conditions ( Figure S42 ). S 2p XPS analysis revealed substantially reduced -S-S- species at LGS interfaces versus LPSC-based cathodes, experimentally confirming decomposition mitigation (Fig. 5 c). Thermal characterization demonstrated remarkable stability: pristine LGS composites showed no exothermic activity below 450 ℃ versus LPSC's 373 ℃ decomposition (Fig. 5 d vs 1c ). Post-cycling 0% SOC LGS composites maintained stability to 450 ℃, contrasting with LPSC's 303 ℃ exothermic initiation (Fig. 5 d vs 2a ). Charged-state LGS cathodes exhibited thermal runaway onset ΔH reduction (399.6 to 110.7 J·g⁻¹), indicating suppressed parasitic reactions (Fig. 5 d vs 2c ). These observations were further corroborated by in-situ MS detection of minimal SO 2 emissions from LGS systems, contrasting sharply with LPSC's substantial gas evolution (Fig. 5 e vs 2c ). 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 ( Figure S43 ), attributable to blocked reaction pathways between stabilized interphases and delithiated cathodes. Most significantly, whole-cell evaluations demonstrated substantial safety parameter improvements with T onset /T tr elevated to 223/312 ℃ versus baseline 168/228 ℃ (Fig. 5 f vs 1a ). 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. Conclusion 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 cathode interactions. The experimental findings systematically demonstrate that cathode-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 through sulfur-oxygen interdiffusion mechanisms dramatically accelerate exothermic processes, ultimately triggering uncontrolled thermal propagation. These interfacial degradation pathways extend across other sulfide systems (LGPS, LSPSC). Moreover, by stabilizing the electrochemically formed interface through Ge-S bond engineering, the Ge-substituted Li 4 GeS 4 -NCM811/LPSC/Li-In ASSBs demonstrate significantly enhanced safety with T onset /T tr increasing from 168/228 ℃ to 223/312 ℃, 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. References P. Lu, L. Liu, S. Wang, J. Xu, J. Peng, W. Yan, Q. Wang, H. Li, L. Chen, F. Wu, Advanced Materials 2021 , 33 , 2100921. Y. Kato, S. Hori, R. 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Supplementary Files SupportingInformation.pdf Stage-Dominated Thermal Runaway in Sulfide ASSBs: Decoupled Electrochemical Ignition and Chemical Cascades Cite Share Download PDF Status: Published Journal Publication published 19 Feb, 2026 Read the published version in Nature Communications → Version 1 posted You are reading this latest preprint version Research Square lets you share your work early, gain feedback from the community, and start making changes to your manuscript prior to peer review in a journal. As a division of Research Square Company, we’re committed to making research communication faster, fairer, and more useful. We do this by developing innovative software and high quality services for the global research community. Our growing team is made up of researchers and industry professionals working together to solve the most critical problems facing scientific publishing. Also discoverable on Platform About Our Team In Review Editorial Policies Advisory Board Help Center Resources Author Services Accessibility API Access RSS feed Manage Cookie Preferences © Research Square 2026 | ISSN 2693-5015 (online) Privacy Policy Terms of Service Do Not Sell My Personal Information {"props":{"pageProps":{"initialData":{"identity":"rs-6428540","acceptedTermsAndConditions":true,"allowDirectSubmit":false,"archivedVersions":[],"articleType":"Article","associatedPublications":[],"authors":[{"id":448242327,"identity":"8214d237-34aa-497e-a6ff-c7229db2ad52","order_by":0,"name":"Guanglei 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12:56:04","currentVersionCode":1,"declarations":"","doi":"10.21203/rs.3.rs-6428540/v1","doiUrl":"https://doi.org/10.21203/rs.3.rs-6428540/v1","draftVersion":[],"editorialEvents":[{"content":"https://doi.org/10.1038/s41467-026-69472-3","type":"published","date":"2026-02-19T05:00:00+00:00"}],"editorialNote":"","failedWorkflow":false,"files":[{"id":83660093,"identity":"44a36f76-2c9a-48a1-a823-3c2c3bc81313","added_by":"auto","created_at":"2025-05-30 09:41:51","extension":"png","order_by":1,"title":"Figure 1","display":"","copyAsset":false,"role":"figure","size":599827,"visible":true,"origin":"","legend":"\u003cp\u003eThermal runaway characteristics of ASSBs, and thermal reactions of the pristine composite cathode (NMC811, LPSC, and VGCF in a weight ratio of 70:30:3). (a) Heat-wait-search (HWS) curves of the fully charged (100% SOC) and fully discharged (0% SOC) NCM811/LPSC/Li-In ASSBs. (b) HWS curves of the electrolyte with electrodes collected from fully charged and fully discharged ASSBs. (c) DSC and MS curves of the pristine composite cathode. Inset of (c) displays typical SEM and element mapping images of the pristine composite cathode at room temperature (RT, 25℃), 400 ℃, and 500 ℃. (d) In-situ heating XRD patterns of the pristine composite cathode upon heating from 40 ℃ to 400 ℃. (e) XRD patterns of the pristine composite cathode at RT, 400 ℃, and 500 ℃. (f) S 2p and P 2p XPS spectra of the pristine composite cathode at RT, 400 ℃, and 500 ℃. (d) Raman spectra of the pristine composite cathode at RT, 400 ℃, and 500 ℃.\u003c/p\u003e","description":"","filename":"floatimage1.png","url":"https://assets-eu.researchsquare.com/files/rs-6428540/v1/14dcb463a316b872051777e7.png"},{"id":83659095,"identity":"1abd63af-a2ab-476c-91e6-d62d787c32a5","added_by":"auto","created_at":"2025-05-30 09:33:51","extension":"png","order_by":2,"title":"Figure 2","display":"","copyAsset":false,"role":"figure","size":634772,"visible":true,"origin":"","legend":"\u003cp\u003eThermal reactions of the composite cathodes (NMC811, LPSC, and VGCF in a weight ratio of 70:30:3) at different SOCs. (a) In-situ heating DSC-XRD-MS analysis of the 0% SOC composite cathode. (b) S 2p and P 2p XPS spectra of the 0% SOC composite cathode at room temperature (RT, 25℃), 310 ℃, and 450 ℃. (c) In-situ heating DSC-XRD-MS analysis of the 100% SOC composite cathode. (d) S 2p and P 2p XPS spectra of the 100% SOC composite cathode at RT, 250 ℃, and 400 ℃. (e) SEM and element mapping images of the 100% SOC composite cathode at RT, 250 ℃, and 400 ℃. (f) XRD patterns of the 100% SOC composite cathode at RT, 250 ℃, and 400 ℃.\u003c/p\u003e","description":"","filename":"floatimage2.png","url":"https://assets-eu.researchsquare.com/files/rs-6428540/v1/68fd129019b37e8098f8d41a.png"},{"id":83660094,"identity":"9284c446-bcc8-4d06-8fa4-f51f48c816a4","added_by":"auto","created_at":"2025-05-30 09:41:51","extension":"png","order_by":3,"title":"Figure 3","display":"","copyAsset":false,"role":"figure","size":987921,"visible":true,"origin":"","legend":"\u003cp\u003eThermal reactions of the 100% SOC composite cathode (NMC811, LPSC, and VGCF in a weight ratio of 70:30:3). Cross-sectional SEM and element mapping images of the 100% SOC composite cathode at (a) 250 ℃, and (b) 400 ℃. EBSD maps of the 100% SOC composite cathode at (c) 250 ℃, and (d) 400 ℃. HAADF-STEM images and the corresponding SAED patterns of the 100% SOC composite cathode at (e) 250 ℃, and (f) 400 ℃. (g) AIMD simulations of the interfacial reactions between delithiated NCM811 and LPSC at 250 ℃, and 400 ℃. (h) DSC curves of different materials with delithiated NCM811 (De-NCM). (i) MS curves of different materials with delithiated NCM811.\u003c/p\u003e","description":"","filename":"floatimage3.png","url":"https://assets-eu.researchsquare.com/files/rs-6428540/v1/7abbbf9b3d4509bf7699dc27.png"},{"id":83659108,"identity":"38e7ab7f-eb3f-4ddc-ba99-921d19c3c593","added_by":"auto","created_at":"2025-05-30 09:33:51","extension":"png","order_by":4,"title":"Figure 4","display":"","copyAsset":false,"role":"figure","size":580155,"visible":true,"origin":"","legend":"\u003cp\u003eThermal reactions of the fully charged NCM811/sulfide SE systems. (a) Thermal runaway route map for 100% SOC LPSC-NCM811 composite cathode. (b) Thermal behaviors between delithiated NCM811 (De-NCM) and sulfide SEs.\u003c/p\u003e","description":"","filename":"floatimage4.png","url":"https://assets-eu.researchsquare.com/files/rs-6428540/v1/5613aecb7d27987238547ef9.png"},{"id":83659109,"identity":"a5934838-cec9-4880-b310-3a165fa37a21","added_by":"auto","created_at":"2025-05-30 09:33:51","extension":"png","order_by":5,"title":"Figure 5","display":"","copyAsset":false,"role":"figure","size":181848,"visible":true,"origin":"","legend":"\u003cp\u003eStrategy for improving interfacial compatibility and thermal safety performance by Li\u003csub\u003e4\u003c/sub\u003eGeS\u003csub\u003e4\u003c/sub\u003e. (a) Reaction energies of NCM811-LPSC and NCM811-LGS by first-principles calculations. (b) Cycle performance and coulombic efficiency of NCM811-LGS/LPSC/Li-Si ASSBs. (c) S 2p XPS spectra of the LGS composite cathode before and after charging to 4.5 V vs. Li\u003csup\u003e+\u003c/sup\u003e/Li. (d) DSC curves of the pristine, 0% SOC, and 100% SOC LGS composite cathodes. (e) MS curves of the 100% SOC LGS composite cathode. (f) HWS curves of the fully charged (100% SOC) NCM811-LGS/LPSC/Li-In ASSBs.\u003c/p\u003e","description":"","filename":"floatimage5.png","url":"https://assets-eu.researchsquare.com/files/rs-6428540/v1/3ca785629967507844b70cda.png"},{"id":105618274,"identity":"4709d409-ec85-4b64-b437-a3864a0bd08d","added_by":"auto","created_at":"2026-03-28 07:12:20","extension":"pdf","order_by":0,"title":"","display":"","copyAsset":false,"role":"manuscript-pdf","size":3598609,"visible":true,"origin":"","legend":"","description":"","filename":"manuscript.pdf","url":"https://assets-eu.researchsquare.com/files/rs-6428540/v1/7319c739-ebac-462f-8eee-77ed3801d2db.pdf"},{"id":83660221,"identity":"51f4ff24-87d2-4001-892f-6a244aa8da4e","added_by":"auto","created_at":"2025-05-30 09:49:51","extension":"pdf","order_by":1,"title":"","display":"","copyAsset":false,"role":"supplement","size":5844301,"visible":true,"origin":"","legend":"Stage-Dominated Thermal Runaway in Sulfide ASSBs: Decoupled Electrochemical Ignition and Chemical Cascades","description":"","filename":"SupportingInformation.pdf","url":"https://assets-eu.researchsquare.com/files/rs-6428540/v1/cb00c1e9403cd97be95258dc.pdf"}],"financialInterests":"There is \u003cb\u003eNO\u003c/b\u003e Competing Interest.","formattedTitle":"Stage-Dominated Thermal Runaway in Sulfide ASSBs: Decoupled Electrochemical Ignition and Chemical Cascades","fulltext":[{"header":"Introduction","content":"\u003cp\u003eWhile modern lithium-ion batteries (LIBs) have attained record-breaking 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 solvents.\u003csup\u003e[\u003cspan citationid=\"CR1\" class=\"CitationRef\"\u003e1\u003c/span\u003e]\u003c/sup\u003e Recent advancements in sulfide-based solid electrolytes, particularly argyrodite Li\u003csub\u003e6\u003c/sub\u003ePS\u003csub\u003e5\u003c/sub\u003eCl (LPSC), have positioned this material family at the forefront of all-solid-state battery (ASSB) research due to its exceptional ionic conductivity and favorable mechanical deformability.\u003csup\u003e[\u003cspan additionalcitationids=\"CR3\" citationid=\"CR2\" class=\"CitationRef\"\u003e2\u003c/span\u003e\u0026ndash;\u003cspan citationid=\"CR4\" class=\"CitationRef\"\u003e4\u003c/span\u003e]\u003c/sup\u003e Notably, this decade has witnessed substantial progress in LPSC-based ASSBs with synergistic breakthroughs in electrochemical stability and scalable processing developments, enabling them as leading candidates for next-generation energy storage commercialization.\u003csup\u003e[\u003cspan additionalcitationids=\"CR6\" citationid=\"CR5\" class=\"CitationRef\"\u003e5\u003c/span\u003e\u0026ndash;\u003cspan citationid=\"CR7\" class=\"CitationRef\"\u003e7\u003c/span\u003e]\u003c/sup\u003e 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 surpassing liquid electrolyte systems.\u003csup\u003e[\u003cspan additionalcitationids=\"CR9 CR10\" citationid=\"CR8\" class=\"CitationRef\"\u003e8\u003c/span\u003e\u0026ndash;\u003cspan citationid=\"CR11\" class=\"CitationRef\"\u003e11\u003c/span\u003e]\u003c/sup\u003e ARC testing of a home-made 3.8 Ah LiNi\u003csub\u003e0.5\u003c/sub\u003eCo\u003csub\u003e0.2\u003c/sub\u003eMn\u003csub\u003e0.3\u003c/sub\u003eO\u003csub\u003e2\u003c/sub\u003e/LPSC/Li pouch cell demonstrates an initial self-heating temperature at 163 ℃, and then culminates into explosive TR at 275 ℃.\u003csup\u003e[\u003cspan citationid=\"CR9\" class=\"CitationRef\"\u003e9\u003c/span\u003e]\u003c/sup\u003e This acceleration effect magnifies at pack level, where the 3 Ah ASSB configurations demonstrate TR propagation velocities 10-fold higher than commercially available LIB equivalents.\u003csup\u003e[\u003cspan citationid=\"CR10\" class=\"CitationRef\"\u003e10\u003c/span\u003e]\u003c/sup\u003e Unlike the well-characterized anode-triggering TR pathways in conventional liquid-electrolyte LIBs, the safety paradox of ASSBs originates from cathode-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 cathode-SE interfaces at abused conditions, which demonstrate 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 ℃ and the whole exothermic process is free of detectable sulfurous gas (e.g., SO\u003csub\u003e2\u003c/sub\u003e).\u003csup\u003e[\u003cspan citationid=\"CR12\" class=\"CitationRef\"\u003e12\u003c/span\u003e]\u003c/sup\u003e Lee\u0026rsquo;s team investigated exothermic and gas-evolution behaviors in composite cathodes under abuse conditions, demonstrating that mechanical grinding or thermal exposure to 150 ℃ in argon triggers TR accompanied by substantial SO\u003csub\u003e2\u003c/sub\u003e emissions.\u003csup\u003e[\u003cspan citationid=\"CR13\" class=\"CitationRef\"\u003e13\u003c/span\u003e, \u003cspan citationid=\"CR14\" class=\"CitationRef\"\u003e14\u003c/span\u003e]\u003c/sup\u003e Wu et al. identified that in the LiCoO\u003csub\u003e2\u003c/sub\u003e/LPSC system, bulk chemical reactions between delithiated cathode and sulfide electrolyte dominated thermal decomposition above 300 ℃.\u003csup\u003e[\u003cspan citationid=\"CR15\" class=\"CitationRef\"\u003e15\u003c/span\u003e]\u003c/sup\u003e Such contrasting findings spanning reaction thresholds, gaseous byproducts, and failure modes, underscore fundamental knowledge gaps in solid-state interfacial degradation mechanisms.\u003c/p\u003e \u003cp\u003eThe thermal runaway mechanisms in sulfide-based ASSBs are governed by intertwined thermochemical and electrochemical processes at oxide cathode/SE interfaces. Thermochemically, bulk S-O interdiffusion directly drives interfacial reconstruction, forming metastable phosphate-sulfate phases through elemental exchange reactions.\u003csup\u003e[\u003cspan citationid=\"CR16\" class=\"CitationRef\"\u003e16\u003c/span\u003e, \u003cspan citationid=\"CR17\" class=\"CitationRef\"\u003e17\u003c/span\u003e]\u003c/sup\u003e Meanwhile, electrochemically-driven interfacial dynamics\u0026mdash;particularly charge transfer and anion redox activities\u0026mdash;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-, Li\u003csub\u003e3\u003c/sub\u003ePS\u003csub\u003e4\u003c/sub\u003e) that progressively destabilize interfacial integrity.\u003csup\u003e[\u003cspan citationid=\"CR18\" class=\"CitationRef\"\u003e18\u003c/span\u003e, \u003cspan citationid=\"CR19\" class=\"CitationRef\"\u003e19\u003c/span\u003e]\u003c/sup\u003e Fundamental band alignment analyses further reveal that the valence band maximum of sulfide SEs lies energetically above the Fermi level of oxide cathodes. This interfacial energy offset drives electron transferring from cathode to SE and accelerates electrochemical degradation.\u003csup\u003e[\u003cspan citationid=\"CR20\" class=\"CitationRef\"\u003e20\u003c/span\u003e]\u003c/sup\u003e 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 cathodes, and (2) thermally driven lattice oxygen release from delithiated oxide cathodes, which synergistically amplifies interfacial instability. This process further triggers self-sustaining cascades that integrate bulk-phase redox reactions, gaseous byproduct evolution (e.g., SO\u003csub\u003e2\u003c/sub\u003e, O\u003csub\u003e2\u003c/sub\u003e), and exothermic interfacial recombination\u0026mdash;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\u0026mdash;mechanistic competitors whose relative contributions remain unresolved due to the field\u0026rsquo;s disproportionate focus on bulk thermochemistry over preconditioned interface dynamics. Additionally, the sequence of gaseous emissions (SO\u003csub\u003e2\u003c/sub\u003e, O\u003csub\u003e2\u003c/sub\u003e), 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.\u003c/p\u003e \u003cp\u003eHere, 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 Li\u003csub\u003e3\u003c/sub\u003ePS\u003csub\u003e4\u003c/sub\u003e phases, governs early-stage self-heating through interfacial exothermic decomposition accompanied by SO\u003csub\u003e2\u003c/sub\u003e evolution. At high temperatures, sulfur/oxygen co-diffusive solid-solid reactions across cathode-SE interface triggers sulfide oxidation and layered cathode disintegration, amplifying heat generation rates and leading to uncontrollable TR. This interfacial failure paradigm extends to Li\u003csub\u003e10\u003c/sub\u003eGeP\u003csub\u003e2\u003c/sub\u003eS\u003csub\u003e12\u003c/sub\u003e and Li\u003csub\u003e9.54\u003c/sub\u003eSi\u003csub\u003e1.74\u003c/sub\u003eP\u003csub\u003e1.44\u003c/sub\u003eS\u003csub\u003e11.7\u003c/sub\u003eCl\u003csub\u003e0.3\u003c/sub\u003e systems, demonstrating universal disulfide bridge induced thermal-electrochemical coupling cascade reaction. To address this, we engineer NCM811/Li\u003csub\u003e4\u003c/sub\u003eGeS\u003csub\u003e4\u003c/sub\u003e composite cathodes via Ge-S bond stabilization to elevate the thermal thresholds. T\u003csub\u003eonset\u003c/sub\u003e and T\u003csub\u003etr\u003c/sub\u003e of the as-assembled ASSBs are increased from 168 ℃, 228 ℃ to 223 ℃, 312 ℃ 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 failsafe sulfide-based ASSBs.\u003c/p\u003e"},{"header":"Result and Discussion","content":"\u003cdiv id=\"Sec3\" class=\"Section2\"\u003e \u003ch2\u003e2.1 Thermal Runaway Characteristics of ASSBs\u003c/h2\u003e \u003cp\u003eTo 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 (\u003cb\u003eFigure \u003cspan refid=\"MOESM1\" class=\"InternalRef\"\u003eS1\u003c/span\u003e\u003c/b\u003e) sulfide ASSBs were assembled employing NCM811 and Li-In as electrodes, and their charge/discharge profiles showcased impressive electrochemical performance (\u003cb\u003eFigure S2\u003c/b\u003e). The thermal safety characteristics of both fully charged (\u003cb\u003eFigure S3\u003c/b\u003e, 4.5 V vs. Li\u003csup\u003e+\u003c/sup\u003e/Li, 100% state of charge (SOC)) and fully discharged (2.8 V vs. Li\u003csup\u003e+\u003c/sup\u003e/Li, 0% SOC) ASSBs were analyzed using an accelerating rate calorimeter (ARC) under typical heat-wait-search (HWS) conditions. Key parameters including onset temperature of self-heating (\u003cem\u003eT\u003c/em\u003e\u003csub\u003e\u003cem\u003eonset\u003c/em\u003e\u003c/sub\u003e), self-heating rate (SHR), and thermal runaway temperature (\u003cem\u003eT\u003c/em\u003e\u003csub\u003e\u003cem\u003etr\u003c/em\u003e\u003c/sub\u003e, defined as the temperature point when SHR reaches 1 ℃\u0026middot;min\u003csup\u003e\u0026minus;\u0026thinsp;1\u003c/sup\u003e) were recorded. Apparently, violent thermal runaway was observed for the 100% SOC ASSB, with the \u003cem\u003eT\u003c/em\u003e\u003csub\u003e\u003cem\u003eonset\u003c/em\u003e\u003c/sub\u003e and \u003cem\u003eT\u003c/em\u003e\u003csub\u003e\u003cem\u003etr\u003c/em\u003e\u003c/sub\u003e being 168 ℃ and 228 ℃ (Fig.\u0026nbsp;\u003cspan refid=\"Fig1\" class=\"InternalRef\"\u003e1\u003c/span\u003ea). 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 extreme abuse conditions, consistent with prior studies.\u003csup\u003e[\u003cspan citationid=\"CR9\" class=\"CitationRef\"\u003e9\u003c/span\u003e]\u003c/sup\u003e Surprisingly, the fully discharged ASSB also underwent severe thermal runaway with \u003cem\u003eT\u003c/em\u003e\u003csub\u003e\u003cem\u003eonset\u003c/em\u003e\u003c/sub\u003e of 234 ℃ and \u003cem\u003eT\u003c/em\u003e\u003csub\u003e\u003cem\u003etr\u003c/em\u003e\u003c/sub\u003e of 313 ℃, further emphasizing that high thermal stability of battery materials does not necessarily equate to high safety of full cell.\u003c/p\u003e \u003cp\u003e \u003c/p\u003e \u003cp\u003eTo pinpoint the origins of thermal instability, we systematically investigate the thermochemical compatibility and interfacial exothermic interactions between electrode materials and SE. Anode material and composite cathode (NMC811, LPSC, and 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 anode with LPSC up to 350 ℃, indicating their good thermal stability and limited involvement of the exothermic chain reactions during elevated temperatures (Fig.\u0026nbsp;\u003cspan refid=\"Fig1\" class=\"InternalRef\"\u003e1\u003c/span\u003eb). However, 100% SOC composite cathode exhibited \u003cem\u003eT\u003c/em\u003e\u003csub\u003e\u003cem\u003eonset\u003c/em\u003e\u003c/sub\u003e and \u003cem\u003eT\u003c/em\u003e\u003csub\u003e\u003cem\u003etr\u003c/em\u003e\u003c/sub\u003e of 160 ℃ and 274 ℃, which aligned with the onset self-heating of the full cell, mirroring that cathode/SE reactions were mainly responsible for the observed exothermic behavior of the full cell. Additionally, it was unexpected to find that 0% SOC composite cathode also generated three discontinuous exothermic processes at 228 ℃, 276 ℃, and 340 ℃ in the HWS plot, 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 test at ARC with a heating rate of 5 ℃ min\u003csup\u003e\u0026minus;\u0026thinsp;1\u003c/sup\u003e. Severe self-heating was observed in the 100% SOC composite cathode, with the temperature rapidly rising from 203 ℃ to 363 ℃ within just 3.9 seconds (\u003cb\u003eFigure S4\u003c/b\u003e). Moreover, grinding the 100% SOC composite cathode in the Ar-filled glove box led to violent burning (\u003cb\u003eFigure S5\u003c/b\u003e). These findings raise significant safety concerns regarding the 100% SOC composite cathode, as it can trigger thermal runaway under both thermal and mechanical abuse conditions. Overall, these results unveil that it is the cathode-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 cathode and SE.\u003c/p\u003e \u003c/div\u003e\n\u003ch3\u003e2.2 Material-level interfacial reactions\u003c/h3\u003e\n\u003cp\u003eTo unravel the complex interfacial reactions governing thermal instability in solid-state batteries, the thermochemical compatibility between pristine oxide cathode, 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 cathode under heating condition. Initial characterization confirmed the exceptional individual stability of NCM811 layered oxide, LPSC, and VGCF additives, with no detectable thermal decomposition below 500 ℃ when tested in isolation (\u003cb\u003eFigure S6\u003c/b\u003e). However, when formulated into an uncycled composite cathode (70:30:3 mass ratio), this ostensibly stable system underwent dramatic interfacial reactions, exhibiting two distinct exothermic peaks at 373 ℃ and 465 ℃ (Fig.\u0026nbsp;\u003cspan refid=\"Fig1\" class=\"InternalRef\"\u003e1\u003c/span\u003ec). 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 ℃ revealed nascent sulfur diffusion from LPSC into NCM811 surfaces, accompanied by oxygen migration in the reverse direction (Fig.\u0026nbsp;\u003cspan refid=\"Fig1\" class=\"InternalRef\"\u003e1\u003c/span\u003ec). Upon reaching 500 ℃, 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 (HSAB) principle. Specifically, the hard acid character of phosphorus (P\u003csup\u003e5+\u003c/sup\u003e) drives preferential coordination with oxygen (O\u003csup\u003e2\u0026minus;\u003c/sup\u003e, hard base), while softer nickel cations (Ni\u003csup\u003e3+/4+\u003c/sup\u003e) selectively interact with sulfur anions (S\u003csup\u003e2\u0026minus;\u003c/sup\u003e, 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 conditions.\u003csup\u003e[\u003cspan citationid=\"CR21\" class=\"CitationRef\"\u003e21\u003c/span\u003e, \u003cspan citationid=\"CR22\" class=\"CitationRef\"\u003e22\u003c/span\u003e]\u003c/sup\u003e Complementary in-situ heating XRD analysis revealed the progressive disappearance of NCM811 (003) and (104) diffraction peaks near 373 ℃, 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.\u0026nbsp;\u003cspan refid=\"Fig1\" class=\"InternalRef\"\u003e1\u003c/span\u003ed). Distinctive phase evolution was observed at elevated temperatures, where XRD patterns at 400\u0026deg;C indicated co-formation of NiO, Li\u003csub\u003e2\u003c/sub\u003eS, and Li\u003csub\u003e3\u003c/sub\u003ePO\u003csub\u003e4\u003c/sub\u003e, attributed to oxygen migration from NCM811 reacting with LPSC, a phenomenon corroborated by oxygen diffusion observed through SEM-EDS mapping (Fig.\u0026nbsp;\u003cspan refid=\"Fig1\" class=\"InternalRef\"\u003e1\u003c/span\u003ee). Upon reaching 500\u0026deg;C, complete LPSC decomposition coincided with appearance of Ni\u003csub\u003e3\u003c/sub\u003eS\u003csub\u003e2\u003c/sub\u003e and CoS\u003csub\u003e2\u003c/sub\u003e, 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 ℃ thermal treatment, the yielding Li\u003csub\u003e2\u003c/sub\u003eS, Ni\u003csub\u003e3\u003c/sub\u003eS\u003csub\u003e2\u003c/sub\u003e, and Li\u003csub\u003e3\u003c/sub\u003ePO\u003csub\u003e4\u003c/sub\u003e as reaction products convinced the critical role of rock salt transition metal oxide on contributing the second stage exothermic reactions (\u003cb\u003eFigure S7\u003c/b\u003e). Concurrently, XPS and TOF-SIMS profiles detected sequential phosphate, hypophosphite, sulfate, and sulfite species generation during thermal excursions, with sulfite/sulfate signatures at 400 ℃ quantitatively matching sulfur diffusion trajectories (Fig.\u0026nbsp;\u003cspan refid=\"Fig1\" class=\"InternalRef\"\u003e1\u003c/span\u003ef, S8). Raman spectral evolution further corroborated this interfacial degradation pathway, showing Li\u003csub\u003e2\u003c/sub\u003eS/Li\u003csub\u003e2\u003c/sub\u003eS\u003csub\u003ex\u003c/sub\u003e and PO\u003csub\u003e4\u003c/sub\u003e\u003csup\u003e3\u0026minus;\u003c/sup\u003e formation at 400 ℃, followed by LPSC signal attenuation and SO\u003csub\u003e4\u003c/sub\u003e\u003csup\u003e2\u0026minus;\u003c/sup\u003e emergence at 500 ℃ (Fig.\u0026nbsp;\u003cspan refid=\"Fig1\" class=\"InternalRef\"\u003e1\u003c/span\u003eg). These multimodal characterizations collectively confirm the solid-solid interfacial degradation mechanisms wherein LPSC-derived sulfur and NCM811-sourced oxygen undergo reciprocal diffusion, driving transition metal sulfide and oxyanion formation. 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.\u003c/p\u003e \u003cp\u003eBeyond 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 cathode tends to exacerbate interfacial reactions. To systematically evaluate these coupled phenomena, composite cathodes harvested from ASSBs after formation cycling, including fully charged (4.5 V vs. Li\u003csup\u003e+\u003c/sup\u003e/Li, 100% SOC) and discharged (2.8 V vs. Li\u003csup\u003e+\u003c/sup\u003e/Li, 0% SOC) states, were subjected to in-situ thermal characterizations. For the 0% SOC sample, two exothermic events were recorded at 303 ℃ (\u003cem\u003eΔH\u003c/em\u003e\u0026thinsp;=\u0026thinsp;75.1 J\u0026middot;g\u003csup\u003e\u0026minus;\u0026thinsp;1\u003c/sup\u003e) and 379 ℃ (\u003cem\u003eΔH\u003c/em\u003e\u0026thinsp;=\u0026thinsp;268.8 J\u0026middot;g\u003csup\u003e\u0026minus;\u0026thinsp;1\u003c/sup\u003e) (Fig.\u0026nbsp;\u003cspan refid=\"Fig2\" class=\"InternalRef\"\u003e2\u003c/span\u003ea). Structural evolution monitored via in-situ heating XRD spectra revealed minimal crystallographic changes below 310 ℃, followed by abrupt disappearance of NCM811/LPSC diffraction peaks and concurrent NiO phase emergence at 379 ℃, confirming bulk material interactions during the second exothermic phase. Notably, the absence of gaseous products throughout heating demonstrated their solid-solid reaction pathways. XPS analysis of the 310 ℃-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.\u0026nbsp;\u003cspan refid=\"Fig2\" class=\"InternalRef\"\u003e2\u003c/span\u003eb). Subsequent Ni\u003csub\u003e3\u003c/sub\u003eS\u003csub\u003e2\u003c/sub\u003e and PO\u003csub\u003e4\u003c/sub\u003e\u003csup\u003e3\u0026minus;\u003c/sup\u003e formation detected at 450 ℃ validated the established sulfur-oxygen interdiffusion mechanism. 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 (\u003cb\u003eFigures S9-S11\u003c/b\u003e).\u003c/p\u003e \u003cp\u003eNotably, the 100% SOC composite cathode was observed to exhibit intensified interfacial reactivity under thermal stress, characterized by two distinct exothermic peaks at 209 ℃ (\u003cem\u003eΔH\u003c/em\u003e\u0026thinsp;=\u0026thinsp;399.6 J\u0026middot;g\u003csup\u003e\u0026minus;\u0026thinsp;1\u003c/sup\u003e) and 299 ℃ (\u003cem\u003eΔH\u003c/em\u003e\u0026thinsp;=\u0026thinsp;863.9 J\u0026middot;g\u003csup\u003e\u0026minus;\u0026thinsp;1\u003c/sup\u003e), respectively (Fig.\u0026nbsp;\u003cspan refid=\"Fig2\" class=\"InternalRef\"\u003e2\u003c/span\u003ec). The initial exothermic event was critically noted to align with severe self-heating behavior identified in prior ramp tests (\u003cb\u003eFigure S4\u003c/b\u003e), 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 (Fig.\u0026nbsp;\u003cspan refid=\"Fig2\" class=\"InternalRef\"\u003e2\u003c/span\u003ec). These observations confirm delithiated cathode-driven initiation of thermal decomposition, with bulk solid electrolyte involvement restricted to later stages. Concurrent gas evolution profiles detected via ARC-MS identify SO\u003csub\u003e2\u003c/sub\u003e (m/z\u0026thinsp;=\u0026thinsp;64), CO\u003csub\u003e2\u003c/sub\u003e (m/z\u0026thinsp;=\u0026thinsp;44), CO (m/z\u0026thinsp;=\u0026thinsp;28), and O\u003csub\u003e2\u003c/sub\u003e (m/z\u0026thinsp;=\u0026thinsp;32) release during the first exothermic event, implicating lattice oxygen from delithiated NCM811 in oxidative side reactions (Fig.\u0026nbsp;\u003cspan refid=\"Fig2\" class=\"InternalRef\"\u003e2\u003c/span\u003ec). Post-reaction characterization of samples quenched at 250 ℃ and 400 ℃ revealed mechanistic details: XPS analysis at 250 ℃ showed depletion of -S-S- bonds, reduced -P-S-P- intensity, and emergence of SO\u003csub\u003e4\u003c/sub\u003e\u003csup\u003e2\u0026minus;\u003c/sup\u003e/SO\u003csub\u003e2\u003c/sub\u003e\u003csup\u003e2\u0026minus;\u003c/sup\u003e/PO\u003csub\u003e4\u003c/sub\u003e\u003csup\u003e3\u0026minus;\u003c/sup\u003e species, confirming electrochemical interphase participation in early exothermic processes (Fig.\u0026nbsp;\u003cspan refid=\"Fig2\" class=\"InternalRef\"\u003e2\u003c/span\u003ed). Conversely, 400 ℃-treated samples demonstrated LPSC decomposition with NiS/CoS formation, signifying direct NCM811-LPSC interactions. SEM-EDS mapping at 250 ℃ verified S/O interdiffusion between NCM811 and interphase species, while 400 ℃ analysis revealed Ni-S and P-O bonding indicative of complete interfacial reactions (Fig.\u0026nbsp;\u003cspan refid=\"Fig2\" class=\"InternalRef\"\u003e2\u003c/span\u003ee). XRD phase evolution further corroborated this mechanism. (003) peak weakening/shifting below 250 ℃ reflected cathode restructuring without LPSC participation, whereas LPSC signal disappearance and NiO/NiS/Li\u003csub\u003e3\u003c/sub\u003ePO\u003csub\u003e4\u003c/sub\u003e emergence at 400 ℃ confirmed bulk material interactions. Raman spectra exhibited PS\u003csub\u003e4\u003c/sub\u003e\u003csup\u003e3\u0026minus;\u003c/sup\u003e depletion and SO\u003csub\u003e4\u003c/sub\u003e\u003csup\u003e2\u0026minus;\u003c/sup\u003e/PO\u003csub\u003e4\u003c/sub\u003e\u003csup\u003e3\u0026minus;\u003c/sup\u003e formation at 400 ℃, consistent with sulfur-oxygen exchange dynamics (\u003cb\u003eFigure S12\u003c/b\u003e). Electron energy loss spectroscopy (EELS) analysis elucidated dynamic redox evolution of nickel species during thermal degradation. At 250 ℃, concurrent shifts in the O K-edge pre-edge (527\u0026ndash;534 eV) toward higher energy loss and Ni L-edge (852\u0026ndash;858 eV) toward lower energy loss were observed, directly evidencing Ni\u003csup\u003e3+\u003c/sup\u003e/ Ni\u003csup\u003e4+\u003c/sup\u003e reduction coupled with reactive lattice oxygen liberation. This dual electronic-state modulation mechanistically accounted for oxygen-containing gas evolution during the initial exothermic phase (\u003cb\u003eFigure S13\u003c/b\u003e). Subsequent O K-edge disappearance and continued Ni L-edge shifts at 400 ℃ 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.\u003c/p\u003e \u003cp\u003e \u003c/p\u003e \u003cp\u003eTo 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 ℃, SEM-EDS analysis confirmed NCM811 particles retained Ni-O dominance without sulfur penetration into bulk regions (Fig.\u0026nbsp;\u003cspan refid=\"Fig3\" class=\"InternalRef\"\u003e3\u003c/span\u003ea), while EBSD mapping identified spinel-phase M\u003csub\u003e3\u003c/sub\u003eO\u003csub\u003e4\u003c/sub\u003e (M\u0026thinsp;=\u0026thinsp;Ni/Co/Mn) formation within cathode particles (Fig.\u0026nbsp;\u003cspan refid=\"Fig3\" class=\"InternalRef\"\u003e3\u003c/span\u003ec). These observations collectively demonstrate that the initial exothermic event is governed by cathode phase transformation rather than sulfur interdiffusion. Thermal escalation to 400 ℃ revealed substantial sulfur infiltration into NCM811 particles, evidenced by intensified S signals and attenuated O intensity in SEM-EDS (Fig.\u0026nbsp;\u003cspan refid=\"Fig3\" class=\"InternalRef\"\u003e3\u003c/span\u003eb). EBSD quantification detected interfacial reaction products\u0026mdash;NiS (79.4%), Li\u003csub\u003e2\u003c/sub\u003eSO\u003csub\u003e4\u003c/sub\u003e (18.1%), and Li\u003csub\u003e3\u003c/sub\u003ePO\u003csub\u003e4\u003c/sub\u003e (2.5%)\u0026mdash;distributed across cathode cross-sections, confirming S migration from LPSC and subsequent sulfide formation (Fig.\u0026nbsp;\u003cspan refid=\"Fig3\" class=\"InternalRef\"\u003e3\u003c/span\u003ed). High-angle annular dark-field transmission electron microscopy (HAADF-TEM) imaging coupled with selected area electron diffraction (SAED) patterns conclusively verified spinel M\u003csub\u003e3\u003c/sub\u003eO\u003csub\u003e4\u003c/sub\u003e at 250 ℃ and crystalline NiS at 400 ℃ (Fig.\u0026nbsp;\u003cspan refid=\"Fig3\" class=\"InternalRef\"\u003e3\u003c/span\u003ee-\u003cspan refid=\"Fig3\" class=\"InternalRef\"\u003e3\u003c/span\u003ef), 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 valence band maximum (VBM) of LPSC was 1.47 eV higher than the Fermi level of NCM811, indicating that electron transfer from NCM811 to LPSC was energetically favorable (\u003cb\u003eFigure S14\u003c/b\u003e). 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 e\u003csub\u003eg\u003c/sub\u003e orbitals of NCM811 near the Fermi level (\u003cb\u003eFigure S15\u003c/b\u003e). 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 e\u003csub\u003eg\u003c/sub\u003e orbitals of NCM811, suggesting a tendency for spontaneous chemical bonding. In addition, ab initio molecular dynamics (AIMD) simulations provided critical insights into the interfacial dynamics. At 250 ℃, the simulations showed that delithiated NCM811 and LPSC maintain their structural integrity, with PS\u003csub\u003e4\u003c/sub\u003e\u003csup\u003e3\u0026minus;\u003c/sup\u003e units remaining intact (\u003cb\u003eFigures S16 and 3g\u003c/b\u003e). However, at 400 ℃, LPSC degradation becomes evident, marked by the cleavage of PS\u003csub\u003e4\u003c/sub\u003e\u003csup\u003e3\u0026minus;\u003c/sup\u003e bonds and the formation of P-O bonds. These findings are in good agreement with experimental observations, offering a consistent explanation for the thermal degradation processes. This convergence between multiscale computational results and experimental data strongly corroborates the proposed thermal decomposition pathways.\u003c/p\u003e \u003cp\u003e \u003c/p\u003e \u003cp\u003eAt this point, the specific reaction pathway between electrochemically-derived interphase components and the delithiated cathode becomes critically interrogated. To decouple individual interaction mechanisms of the interphase components, strategically synthesized electrochemical degradation products were systematically investigated. Electrochemically decomposed LPSC (4.5V-(LPSC/VGCF), LPSC and VGCF in 9:1 mass ratio) was synthesized by charging an (LPSC/VGCF)/LPSC/In half-cell to 4.5 V vs. Li\u003csup\u003e+\u003c/sup\u003e/Li, with XPS and Raman spectroscopic characterization confirming the presence of -S-S- and -P-S-P- species mirroring the 100% SOC composite cathode's interphase compositions (\u003cb\u003eFigures S17-S18\u003c/b\u003e). Neat delithiated NCM811 (De-NCM) collected from fully-charged liquid-electrolyte pouch cells (reference methodology [12]) exhibited identical structural characteristics to ASSB-derived counterparts (\u003cb\u003eFigure S19\u003c/b\u003e), 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 ℃ accompanied by O\u003csub\u003e2\u003c/sub\u003e evolution (Figs.\u0026nbsp;\u003cspan refid=\"Fig3\" class=\"InternalRef\"\u003e3\u003c/span\u003eh-i). 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/4.5V-(LPSC/VGCF) system containing electrochemical degradation products displayed accelerated thermal response, with exothermic onset at 207 ℃ and 263% enthalpy increase (541.9 vs 149.2 J\u0026middot;g\u003csup\u003e\u0026minus;\u0026thinsp;1\u003c/sup\u003e), concurrently evolving SO\u003csub\u003e2\u003c/sub\u003e and O\u003csub\u003e2\u003c/sub\u003e. Given the excellent thermal stability of 4.5V-(LPSC/VGCF) below 400 ℃ (\u003cb\u003eFigure S20\u003c/b\u003e), these observations conclusively attribute early-stage heat generation and sulfurous gas evolution to reactive interphase between electrochemically-modified LPSC and delithiated NCM811. Mechanistic investigations extended to individual LPSC decomposition products (Li\u003csub\u003e2\u003c/sub\u003eS, S, β-Li\u003csub\u003e3\u003c/sub\u003ePS\u003csub\u003e4\u003c/sub\u003e (β-LPS, \u003cb\u003eFigure S21\u003c/b\u003e)) through controlled thermal coupling with De-NCM. Calorimetric analysis revealed minimal influence of Li\u003csub\u003e2\u003c/sub\u003eS on primary exothermic events (no SO\u003csub\u003e2\u003c/sub\u003e detection), although Li\u003csub\u003e2\u003c/sub\u003eS did react with delithiated NCM811 at high temperatures to produce Ni\u003csub\u003e3\u003c/sub\u003eS\u003csub\u003e2\u003c/sub\u003e (\u003cb\u003eFigure S22\u003c/b\u003e). Conversely, De-NCM/S and De-NCM/β-LPS mixtures exhibited intensified exothermicity (590.7/445.7 J\u0026middot;g\u003csup\u003e\u0026minus;\u0026thinsp;1\u003c/sup\u003e) with predominant SO\u003csub\u003e2\u003c/sub\u003e emission and attenuated O\u003csub\u003e2\u003c/sub\u003e release (Figs.\u0026nbsp;\u003cspan refid=\"Fig3\" class=\"InternalRef\"\u003e3\u003c/span\u003eh-i). This systematic evidence establishes electrochemical LPSC decomposition products, particularly sulfurous species, as critical initiators of early-stage exothermic cascades and exclusive SO\u003csub\u003e2\u003c/sub\u003e sources in fully charged cathodes during thermal runaway propagation.\u003c/p\u003e \u003cp\u003eMoreover, the influence of conductive carbon additives (VGCF) on thermal propagation mechanisms is systematically examined, given their dual functionality in composite cathode systems, reacting with de-lithiated NCM811 to produce CO and CO\u003csub\u003e2\u003c/sub\u003e, and promoting the decomposition of SE into sulfur-containing substances. In the 100% SOC configuration, CO/CO\u003csub\u003e2\u003c/sub\u003e evolution detected during initial exothermic events (Fig.\u0026nbsp;\u003cspan refid=\"Fig2\" class=\"InternalRef\"\u003e2\u003c/span\u003ec) is attributed to interfacial reactions between VGCF and delithiated NCM811. Quantitative calorimetric analysis revealed this carbon-cathode interaction contributes 90.8 J\u0026middot;g\u003csup\u003e\u0026minus;\u0026thinsp;1\u003c/sup\u003e (16.8% of total 541.9 J\u0026middot;g\u003csup\u003e\u0026minus;\u0026thinsp;1\u003c/sup\u003e), establishing its secondary thermal role (\u003cb\u003eFigure S23\u003c/b\u003e). Complementary investigations confirmed VGCF's catalytic effect on sulfide electrolyte decomposition, as evidenced by increased -S-S-/-P-S-P-/Li\u003csub\u003e3\u003c/sub\u003ePS\u003csub\u003e4\u003c/sub\u003e formation in electrochemically cycled cathodes with elevated carbon content (0.6 wt% to 13.0 wt%, \u003cb\u003eFigures S24-S25\u003c/b\u003e), phenomena consistent with prior reports of carbon-mediated SE degradation.\u003csup\u003e[\u003cspan additionalcitationids=\"CR24\" citationid=\"CR23\" class=\"CitationRef\"\u003e23\u003c/span\u003e\u0026ndash;\u003cspan citationid=\"CR25\" class=\"CitationRef\"\u003e25\u003c/span\u003e]\u003c/sup\u003e Thermal characterization of these carbon-varied systems demonstrated proportional enthalpy escalation (169.2, 399.6 and 631.2 J\u0026middot;g\u003csup\u003e\u0026minus;\u0026thinsp;1\u003c/sup\u003e) with VGCF loading (0.6 wt%, 2.9 wt%, 13.0 wt%, \u003cb\u003eFigure S26\u003c/b\u003e), confirming two synergistic thermal pathways: (1) direct exothermic carbon-cathode reactions generating CO/CO\u003csub\u003e2\u003c/sub\u003e, and (2) catalytic acceleration of LPSC decomposition that amplifies subsequent interfacial exothermicity. This dual mechanism establishes conductive carbon as both reactant and reaction promoter in thermal runaway initiation.\u003c/p\u003e \u003cp\u003eThus, 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-/Li\u003csub\u003e3\u003c/sub\u003ePS\u003csub\u003e4\u003c/sub\u003e) rather than LPSC itself to start the exothermic chain reactions, producing SO\u003csub\u003e2\u003c/sub\u003e and sulfate/phosphate species alongside CO/CO\u003csub\u003e2\u003c/sub\u003e from VGCF oxidation. This interfacial reaction dominates initial heat generation, driving rapid exothermic escalation. Subsequent temperature rise activates bulk S-O interdiffusion reactions between LPSC and NCM811, forming NiS, Li\u003csub\u003e3\u003c/sub\u003ePO\u003csub\u003e4\u003c/sub\u003e, and sulfate/phosphate compounds with substantial exothermic contribution (68.4% total heat). 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.\u0026nbsp;\u003cspan refid=\"Fig4\" class=\"InternalRef\"\u003e4\u003c/span\u003ea).\u003c/p\u003e \u003cp\u003e \u003c/p\u003e \u003cp\u003eTo further explore the universal validity of this thermal degradation roadmap, extended experimental investigations employing identical methodology were conducted on alternative sulfide SEs including Li\u003csub\u003e10\u003c/sub\u003eGeP\u003csub\u003e2\u003c/sub\u003eS\u003csub\u003e12\u003c/sub\u003e (LGPS) and Li\u003csub\u003e9.54\u003c/sub\u003eSi\u003csub\u003e1.74\u003c/sub\u003eP\u003csub\u003e1.44\u003c/sub\u003eS\u003csub\u003e11.7\u003c/sub\u003eCl\u003csub\u003e0.3\u003c/sub\u003e (LSPSC). Consistent with observations in LPSC-based systems, fully charged composite cathodes incorporating LGPS and LSPSC demonstrated significant self-heating phenomena accompanied by sulfur dioxide evolution near 200 ℃, as evidenced by thermal analysis data (\u003cb\u003eFigure S27-S28\u003c/b\u003e). The formation of analogous electrochemically generated interphase species was confirmed through XPS spectral analysis (\u003cb\u003eFigure S29\u003c/b\u003e). Contrastingly, control systems containing De-NCM combined with pristine SE/VGCF composites without electrochemical interphase exhibited negligible thermal activity or gas emission, while substantial self-heating and SO\u003csub\u003e2\u003c/sub\u003e release were consistently observed in De-NCM/4.5V-(SE/VGCF) configurations (\u003cb\u003eFigure S30-S31\u003c/b\u003e). These experimental observations were corroborated by DSC thermal analysis results (\u003cb\u003eFigure S32\u003c/b\u003e), thereby confirming the pivotal involvement of electrochemically derived interphase decomposition products in initiating the primary exothermic reactions characteristic of early-stage thermal runaway mechanisms.\u003c/p\u003e\n\u003ch3\u003e2.3 Interface regulation towards safer sulfide ASSBs\u003c/h3\u003e\n\u003cp\u003eThe 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. Cathode surface coating has been established as an effective mitigation strategy through the introduction of inert interfacial barriers to suppress cathode/SE decomposition, as documented in prior research.\u003csup\u003e[\u003cspan citationid=\"CR26\" class=\"CitationRef\"\u003e26\u003c/span\u003e, \u003cspan citationid=\"CR27\" class=\"CitationRef\"\u003e27\u003c/span\u003e]\u003c/sup\u003e To evaluate this approach, ASSBs incorporating Al\u003csub\u003e2\u003c/sub\u003eO\u003csub\u003e3\u003c/sub\u003e-coated NCM811 cathodes (designated C-NCM, 4 nm coating thickness, \u003cb\u003eFigure S33\u003c/b\u003e) were systematically investigated. Initial thermal characterization of uncycled composite cathodes (70:30:3 mass ratio of C-NCM, LPSC, and VGCF) revealed two elevated exothermic peaks at 408 ℃ and 485 ℃ in DSC profiles (\u003cb\u003eFigure S34\u003c/b\u003e), demonstrating temperature increments compared to uncoated counterparts (373 ℃ and 465 ℃, Fig.\u0026nbsp;\u003cspan refid=\"Fig1\" class=\"InternalRef\"\u003e1\u003c/span\u003ec). In-situ heating XRD analysis further confirmed enhanced stability, with no phase transitions observed in coated cathodes below 400 ℃ (\u003cb\u003eFigure S35\u003c/b\u003e), contrasting with prominent NiO signatures detected at 373 ℃ in uncoated systems (Fig.\u0026nbsp;\u003cspan refid=\"Fig1\" class=\"InternalRef\"\u003e1\u003c/span\u003ed). These observations indicate improved interfacial compatibility through Al\u003csub\u003e2\u003c/sub\u003eO\u003csub\u003e3\u003c/sub\u003e modification. Post-cycling analysis of C-NCM-based ASSBs (\u003cb\u003eFigure S36\u003c/b\u003e) revealed attenuated XPS signals for -S-S-, -P-S-P-, and PO\u003csub\u003e4\u003c/sub\u003e\u003csup\u003e3\u0026minus;\u003c/sup\u003e species compared to uncoated cathodes at full SOC (\u003cb\u003eFigure S37\u003c/b\u003e), confirming reduced electrochemical degradation. Thermal evaluation of cycled C-NCM cathodes through DSC, ARC- MS demonstrated partial heat release mitigation, with initial exothermic onset at 213 ℃ (\u003cem\u003eΔH\u003c/em\u003e\u0026thinsp;=\u0026thinsp;318.8 J\u0026middot;g\u003csup\u003e\u0026minus;\u0026thinsp;1\u003c/sup\u003e) versus 209 ℃ (\u003cem\u003eΔH\u003c/em\u003e\u0026thinsp;=\u0026thinsp;399.6 J\u0026middot;g\u003csup\u003e\u0026minus;\u0026thinsp;1\u003c/sup\u003e) for unmodified cathodes (\u003cb\u003eFigure S38, 2c\u003c/b\u003e). However, persistent SO\u003csub\u003e2\u003c/sub\u003e evolution was detected at elevated temperatures despite reduced emission quantities, accompanied by comparable self-heating profiles during ARC ramp tests (200 ℃ onset, \u003cb\u003eFigure S39\u003c/b\u003e). 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 cathodes and sulfide SEs.\u003c/p\u003e \u003cp\u003eThe oxidative decomposition tendency of LPSC under high-voltage conditions has been fundamentally attributed to its valence band maximum positioning above the Fermi level (Ɛ\u003csub\u003eF\u003c/sub\u003e) of NCM811, creating inherent energy level misalignment with layered oxide cathodes as per solid-state band theory.\u003csup\u003e[\u003cspan citationid=\"CR20\" class=\"CitationRef\"\u003e20\u003c/span\u003e]\u003c/sup\u003e The complete resolution of this thermoelectrochemical decomposition phenomenon necessitates intrinsic material-level interfacial compatibility regulation. Notably, 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 oxide/SE compatibility through P\u0026rarr;Ge substitution in LGS synthesis.\u003csup\u003e[\u003cspan citationid=\"CR21\" class=\"CitationRef\"\u003e21\u003c/span\u003e, \u003cspan citationid=\"CR28\" class=\"CitationRef\"\u003e28\u003c/span\u003e]\u003c/sup\u003e Experimentally synthesized Li\u003csub\u003e4\u003c/sub\u003eGeS\u003csub\u003e4\u003c/sub\u003e demonstrated room-temperature ionic conductivity of 1\u0026times;10\u003csup\u003e\u0026minus;\u0026thinsp;4\u003c/sup\u003e S cm\u003csup\u003e\u0026minus;\u0026thinsp;1\u003c/sup\u003e (\u003cb\u003eFigure S40\u003c/b\u003e). First-principles calculations revealed improved chemical stability between NCM811 and LGS (-0.201\u0026nbsp;eV/atom) compared with NCM811-LPSC (-0.357\u0026nbsp;eV/atom) (Fig.\u0026nbsp;\u003cspan refid=\"Fig5\" class=\"InternalRef\"\u003e5\u003c/span\u003ea). Energy band analysis further confirmed superior LGS/NCM811 alignment through reduced valence band maximum-Fermi level gap versus LPSC/NCM811 (\u003cb\u003eFigure S41\u003c/b\u003e). 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,\u003csup\u003e[\u003cspan citationid=\"CR29\" class=\"CitationRef\"\u003e29\u003c/span\u003e, \u003cspan citationid=\"CR30\" class=\"CitationRef\"\u003e30\u003c/span\u003e]\u003c/sup\u003e prompting the development of LGS as ionic conductor additives rather than bulk SE replacements. The fabricated NCM811-LGS/LPSC/LiSi ASSBs demonstrated enhanced electrochemical performance with 200.8 mAh\u0026middot;g\u003csup\u003e\u0026minus;\u0026thinsp;1\u003c/sup\u003e initial discharge capacity (0.05C, 45 ℃) and 87.4% capacity retention after 100 cycles (0.5C, 45 ℃) (Fig.\u0026nbsp;\u003cspan refid=\"Fig5\" class=\"InternalRef\"\u003e5\u003c/span\u003eb). Thermal safety evaluation utilized composite cathodes (70:30:3 NCM811:LGS:VGCF) extracted from fully charged cells under standardized conditions (\u003cb\u003eFigure S42\u003c/b\u003e). S 2p XPS analysis revealed substantially reduced -S-S- species at LGS interfaces versus LPSC-based cathodes, experimentally confirming decomposition mitigation (Fig.\u0026nbsp;\u003cspan refid=\"Fig5\" class=\"InternalRef\"\u003e5\u003c/span\u003ec). Thermal characterization demonstrated remarkable stability: pristine LGS composites showed no exothermic activity below 450 ℃ versus LPSC's 373 ℃ decomposition (Fig.\u0026nbsp;\u003cspan refid=\"Fig5\" class=\"InternalRef\"\u003e5\u003c/span\u003ed \u003cb\u003evs 1c\u003c/b\u003e). Post-cycling 0% SOC LGS composites maintained stability to 450 ℃, contrasting with LPSC's 303 ℃ exothermic initiation (Fig.\u0026nbsp;\u003cspan refid=\"Fig5\" class=\"InternalRef\"\u003e5\u003c/span\u003ed \u003cb\u003evs 2a\u003c/b\u003e). Charged-state LGS cathodes exhibited thermal runaway onset ΔH reduction (399.6 to 110.7 J\u0026middot;g⁻\u0026sup1;), indicating suppressed parasitic reactions (Fig.\u0026nbsp;\u003cspan refid=\"Fig5\" class=\"InternalRef\"\u003e5\u003c/span\u003ed \u003cb\u003evs 2c\u003c/b\u003e). These observations were further corroborated by in-situ MS detection of minimal SO\u003csub\u003e2\u003c/sub\u003e emissions from LGS systems, contrasting sharply with LPSC's substantial gas evolution (Fig.\u0026nbsp;\u003cspan refid=\"Fig5\" class=\"InternalRef\"\u003e5\u003c/span\u003ee \u003cb\u003evs 2c\u003c/b\u003e). 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 (\u003cb\u003eFigure S43\u003c/b\u003e), attributable to blocked reaction pathways between stabilized interphases and delithiated cathodes. Most significantly, whole-cell evaluations demonstrated substantial safety parameter improvements with T\u003csub\u003eonset\u003c/sub\u003e/T\u003csub\u003etr\u003c/sub\u003e elevated to 223/312 ℃ versus baseline 168/228 ℃ (Fig.\u0026nbsp;\u003cspan refid=\"Fig5\" class=\"InternalRef\"\u003e5\u003c/span\u003ef \u003cb\u003evs 1a\u003c/b\u003e). 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.\u003c/p\u003e \u003cp\u003e \u003c/p\u003e"},{"header":"Conclusion","content":"\u003cp\u003eThis 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 cathode interactions. The experimental findings systematically demonstrate that cathode-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 through sulfur-oxygen interdiffusion mechanisms dramatically accelerate exothermic processes, ultimately triggering uncontrolled thermal propagation. These interfacial degradation pathways extend across other sulfide systems (LGPS, LSPSC). Moreover, by stabilizing the electrochemically formed interface through Ge-S bond engineering, the Ge-substituted Li\u003csub\u003e4\u003c/sub\u003eGeS\u003csub\u003e4\u003c/sub\u003e-NCM811/LPSC/Li-In ASSBs demonstrate significantly enhanced safety with T\u003csub\u003eonset\u003c/sub\u003e/T\u003csub\u003etr\u003c/sub\u003e increasing from 168/228 ℃ to 223/312 ℃, 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.\u003c/p\u003e"},{"header":"References","content":"\u003col\u003e\n\u003cli\u003eP. Lu, L. Liu, S. Wang, J. Xu, J. Peng, W. Yan, Q. Wang, H. Li, L. Chen, F. Wu, \u003cem\u003eAdvanced Materials\u003c/em\u003e \u003cstrong\u003e2021\u003c/strong\u003e, \u003cem\u003e33\u003c/em\u003e, 2100921.\u003c/li\u003e\n\u003cli\u003eY. Kato, S. Hori, R. 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Mater.\u003c/em\u003e\u003cstrong\u003e2016\u003c/strong\u003e, \u003cem\u003e28\u003c/em\u003e, 2400.\u003c/li\u003e\n\u003c/ol\u003e"}],"fulltextSource":"","fullText":"","funders":[],"hasAdminPriorityOnWorkflow":false,"hasManuscriptDocX":true,"hasOptedInToPreprint":true,"hasPassedJournalQc":"","hasAnyPriority":true,"hideJournal":false,"highlight":"","institution":"","isAcceptedByJournal":true,"isAuthorSuppliedPdf":false,"isDeskRejected":"","isHiddenFromSearch":false,"isInQc":false,"isInWorkflow":false,"isPdf":false,"isPdfUpToDate":true,"isWithdrawnOrRetracted":false,"journal":{"display":true,"email":"[email protected]","identity":"nature-portfolio","isNatureJournal":true,"hasQc":false,"allowDirectSubmit":false,"externalIdentity":"","sideBox":"","snPcode":"","submissionUrl":"","title":"Nature Portfolio","twitterHandle":"","acdcEnabled":false,"dfaEnabled":false,"editorialSystem":"ejp","reportingPortfolio":"","inReviewEnabled":true,"inReviewRevisionsEnabled":false},"keywords":"","lastPublishedDoi":"10.21203/rs.3.rs-6428540/v1","lastPublishedDoiUrl":"https://doi.org/10.21203/rs.3.rs-6428540/v1","license":{"name":"CC BY 4.0","url":"https://creativecommons.org/licenses/by/4.0/"},"manuscriptAbstract":"\u003cp\u003eSulfide all-solid-state batteries (ASSBs) are poised to revolutionize energy storage with unparalleled energy densities and reduced flammability risks, yet emerging studies reveal a critical safety paradox where cathode-electrolyte interactions induce thermal runaway at unexpectedly low temperatures, challenging the assumption of inherent thermal stability. While current research prioritizes electrochemical performance, the thermochemical degradation dynamics between oxide cathode and thiophosphate electrolyte remain poorly understood, with even conflicting mechanistic interpretations. Herein, leveraging multiscale calorimetry and in-situ gaseous analytical techniques, it is noted that the metastable interphase between nickel-rich cathodes and thiophosphate electrolytes, formed through electrochemically preconditioned but persistently overlooked, serve as primary triggers for exothermic cascades, a phenomenon starkly distinct from liquid electrolyte counterparts. Thermal degradation in composite cathodes evolves through dual mechanistic phases: First, delithiated cathode materials react with electrochemical precondition generated sulfur-rich species (-S-S-, -P-S-P-, and Li\u003csub\u003e3\u003c/sub\u003ePS\u003csub\u003e4\u003c/sub\u003e), driving rapid heat accumulation below 160 ℃ by interphase-dominated chemistry, accompanied by gaseous emissions (SO\u003csub\u003e2\u003c/sub\u003e, CO\u003csub\u003e2\u003c/sub\u003e, O\u003csub\u003e2\u003c/sub\u003e); subsequently, sulfur-oxygen interdiffusion in bulk phases accelerates solid-solid exothermic reactions, driving thermal propagation and eventual runaway. This dual-stage mechanism generalizes across other sulfide systems. Crucially, it is demonstrated that the electrochemically formed interface through Ge-S bond engineering effectively suppresses thermal cascades, where the as-designed Li\u003csub\u003e4\u003c/sub\u003eGeS\u003csub\u003e4\u003c/sub\u003e-modified interface system achieves unprecedented thermal safety without compromising electrochemical performance. Our study establishes a new paradigm for thermal runaway causality by prioritizing interfacial thermodynamics over bulk material compatibility as the primary determinant of thermal safety\u0026mdash;a framework fundamentally divergent from conventional liquid electrolyte batteries.\u003c/p\u003e","manuscriptTitle":"Stage-Dominated Thermal Runaway in Sulfide ASSBs: Decoupled Electrochemical Ignition and Chemical Cascades","msid":"","msnumber":"","nonDraftVersions":[{"code":1,"date":"2025-05-30 09:33:46","doi":"10.21203/rs.3.rs-6428540/v1","editorialEvents":[],"status":"published","journal":{"display":true,"email":"[email protected]","identity":"nature-communications","isNatureJournal":true,"hasQc":false,"allowDirectSubmit":false,"externalIdentity":"NCOMMS","sideBox":"Learn more about [Nature Communications](http://www.nature.com/ncomms/)","snPcode":"","submissionUrl":"https://mts-ncomms.nature.com/","title":"Nature Communications","twitterHandle":"","acdcEnabled":true,"dfaEnabled":true,"editorialSystem":"ejp","reportingPortfolio":"Nature Communications","inReviewEnabled":true,"inReviewRevisionsEnabled":false}}],"origin":"","ownerIdentity":"60a34038-5a2c-47d3-9de6-5b36734a0f06","owner":[],"postedDate":"May 30th, 2025","published":true,"recentEditorialEvents":[],"rejectedJournal":[],"revision":"","amendment":"","status":"published-in-journal","subjectAreas":[{"id":47712295,"name":"Physical sciences/Energy science and technology/Energy storage/Batteries"},{"id":47712296,"name":"Physical sciences/Materials science/Materials for energy and catalysis/Batteries"}],"tags":[],"updatedAt":"2026-03-28T07:12:14+00:00","versionOfRecord":{"articleIdentity":"rs-6428540","link":"https://doi.org/10.1038/s41467-026-69472-3","journal":{"identity":"nature-communications","isVorOnly":false,"title":"Nature Communications"},"publishedOn":"2026-02-19 05:00:00","publishedOnDateReadable":"February 19th, 2026"},"versionCreatedAt":"2025-05-30 09:33:46","video":"","vorDoi":"10.1038/s41467-026-69472-3","vorDoiUrl":"https://doi.org/10.1038/s41467-026-69472-3","workflowStages":[]},"version":"v1","identity":"rs-6428540","journalConfig":"researchsquare"},"__N_SSP":true},"page":"/article/[identity]/[[...version]]","query":{"redirect":"/article/rs-6428540","identity":"rs-6428540","version":["v1"]},"buildId":"8U1c8b4HqxoKbykW_rLl7","isFallback":false,"isExperimentalCompile":false,"dynamicIds":[84888],"gssp":true,"scriptLoader":[]}