Interface failure mechanisms and engineering strategies in sulfide and halide solid electrolytes for lithium metal solid-state batteries

Highlights

• A comparative deep dive into failure mechanisms at the lithium-electrolyte interface in sulfide versus halide solid electrolytes, backed by the latest experimental and theoretical insights.
• Targeted mitigation strategies ranging from protective coatings and interlayers to composite solid-state electrolyte design and stack-pressure optimization, linked directly to specific degradation modes.
• Performance trade-offs are visualized via a radar chart that highlights the complementary strengths and weaknesses informing hybrid electrolyte design.
• A roadmap outlining staged advancements in interface engineering from early coatings to AI-guided, application-specific solutions by 2035.
• A hybrid/composite pathway is proposed to integrate sulfide conductivity with halide stability, bridging the gap between current lab successes and scalable, manufacturable solid-state battery technologies.

1 Introduction

Nowadays, the transition to carbon-neutral energy systems requires energy storage systems that combine very high energy density, intrinsic safety, and a long calendar and cycle life. Lithium metal is the most attractive negative electrode material for next-generation batteries due to its exceptionally high theoretical specific capacity (3,860 mAh g^-1) and lowest practical electrochemical potential among common anode materials (Janek and Zeier, 2016). When paired with inorganic solid electrolytes (SEs), lithium-metal solid-state batteries (LMSBs) eliminate the flammability and leakage risks associated with organic liquid electrolytes. LMSBs also enable simplified cell architectures with the potential for higher energy density at the cell level (Banerjee et al., 2020; Zhang et al., 2018).

Among inorganic SE chemistries, sulfides (e.g., argyrodite Li₆PS₅Cl and LGPS type Li₁₀GeP₂S₁₂) and halides (e.g., Li₆MX₆ or Li₃MCl₆, X = Cl, Br, I) have emerged as leading candidates because they combine high room temperature ionic conductivity with processing routes that can be compatible with practical cell fabrication. Sulfide electrolytes have been shown to exhibit liquid-like ionic conductivities (>10^–3 S cm^-1) and are amenable to low-temperature densification and cold press processing, a property that renders them well-suited for scalable manufacturing (Kato et al., 2016; Kamaya et al., 2011; Doux et al., 2020). However, many sulfides exhibit high reactivity with lithium and moisture, resulting in the formation of resistive decomposition products (e.g., Li₂S, Li₃P, LiCl) and gaseous byproducts (H₂S) (Kato et al., 2016; Kamaya et al., 2011; Doux et al., 2020; Zou et al., 2021; Guo et al., 2023; Asano et al., 2018; Gao et al., 2019; Machín et al., 2025). These reactions complicate the handling and long-term stability of these materials. Halide electrolytes, which have been re-discovered and rapidly developed in recent years, frequently display greater electrochemical stability windows and reduced moisture sensitivity in comparison to many sulfides. Moreover, several halide compositions have demonstrated direct compatibility with high voltage cathodes, thereby obviating the necessity for extensive cathode coatings (Guo et al., 2023; Asano et al., 2018; Gao et al., 2019; Machín et al., 2025). However, halides have been observed to demonstrate mechanical brittleness and specific reductive decomposition pathways that constrain their intrinsic capacity to impede lithium penetration and regulate filamentary growth under certain operating conditions (Asano et al., 2018; Cheng et al., 2020; Han et al., 2019).

It should be noted that failures at the Li/SE interface are inherently multifactorial and are conveniently grouped into three interacting classes: chemical, electrochemical, and mechanical. Chemical failures are the result of spontaneous reactions that form resistive or mixed conducting interphases. While electrochemical failures may include reductive or oxidative decomposition under applied bias and local electronic leakage that drives parasitic plating. Last one, mechanical failures encompass void formation during stripping, fracture under stack pressure, and shear-driven penetration or crack-assisted filament growth (Zhao et al., 2019; Richards et al., 2016; Wang et al., 2021; Brahmbhatt et al., 2020). These modes exhibit a high degree of coupling, which can result in adverse effects such as increased electronic leakage, localized lithium plating, and the initiation of mechanical fracture and dendrite propagation (Han et al., 2019; Richards et al., 2016).

Thanks to recent operando and post-mortem studies emphasize additional, often underappreciated, contributors to interfacial instability such as microstructure and grain boundary networks, space charge layers arising from defect chemistry and chemical potential mismatch, and local electronic conductivity within the SE. Moreover, the presence of high local electronic conductivity, whether intrinsic or induced by decomposition, has been directly implicated in internal lithium deposition and shorting in several SE families. This finding indicates that reducing electronic leakage is as important as maintaining high ionic conductivity (Cheng et al., 2020; Han et al., 2019). The effects of space charge at heterogeneous interfaces have the capacity to impede or concentrate Li ion flux locally, thereby altering the propensity for localized failure and the effective interfacial resistance (Cheng et al., 2020).

The present mini-review has three objectives: (1) The dominant chemical, electrochemical, and mechanical failure mechanisms in each material class must be elucidated. (2) The state-of-the-art mitigation strategies must be surveyed, including, but not limited to, conformal interphase coatings, electrolyte chemistry and doping, microstructure and grain boundary control, mechanical reinforcement and stack pressure management, and advanced in situ/operando diagnostics. (3) A comparative analysis and roadmap must be provided that highlights hybrid and material class-specific approaches to interface stabilization for practical LMSBs (Banerjee et al., 2020; Doux et al., 2020; Wang et al., 2021; Zou et al., 2021; Ye et al., 2021). The scope of this study is constrained to inorganic sulfide and halide electrolytes in conjunction with lithium metal, polymer and oxide SEs which are addressed exclusively in instances where they contribute to hybrid strategies or mitigation concepts.

To accentuate the significance of translation, we underscore metrics that are pertinent to practical cells, including critical current density, interfacial resistance evolution, mechanical robustness under realistic stack pressures, and manufacturability (air sensitivity, processing temperature, and scalability). By doing so, we facilitate a comprehensive evaluation of the tradeoffs between ionic performance and practical stability (Doux et al., 2020; Ye et al., 2021; Fraunhofer, 2022). Finally, recent roadmap and industry analyses emphasize that interface engineering, scalable coating methods, and operando diagnostics are the imminent obstacles to the commercialization of LMSBs, underscoring the necessity for material class-specific strategies that balance performance and manufacturability (Fraunhofer, 2022; Amici et al., 2022). A graphical overview of the principal interface parameters and engineering strategies is shown in Figure 1.

2 Failure mechanisms at the lithium-electrolyte interface

The Li-solid electrolyte interface is a dynamic, heterogeneous region where electrochemistry, ion transport, and mechanics interact on length and time scales that are concurrent. Instabilities originating at the interface are the predominant cause of performance degradation and catastrophic failure in lithium metal solid-state batteries. These instabilities stem from coupled chemo-mechanical and electrochemical processes, necessitating their treatment as multifactorial phenomena (Banerjee et al., 2020). This section involves a reorganization and tightening of the original text, incorporation of key mechanistic points and metrics used to quantify interfacial stability, and establishment of links between failure modes and measurable observables and mitigation levers. A comparative schematic of the dominant failure pathways is shown in Figure 2.

2.1 Lithium dendrite formation

Metallic filaments (dendrites) or internal Li deposits nucleate at the anode and propagate into the SE, producing electronic shorting or local loss of ionic continuity. The initiation of dendrites is governed by three interacting factors: local current density inhomogeneity, local electronic leakage, and mechanical constraint. Local current hotspots emerge due to imperfect contact, interfacial voids, grain boundary networks, and microstructural heterogeneity. These factors concentrate plating and favor protrusion growth along weak paths, such as pores and grain boundaries (Doux et al., 2020; Cheng et al., 2020). The presence of high local electronic conductivity within the SE or within a chemically formed interphase provides the electrons necessary for internal Li nucleation and growth. Therefore, it is as important to lower electronic leakage as it is to raise ionic conductivity for the purpose of preventing internal deposition. A significant number of sulfide electrolytes have been found to possess relatively low shear moduli, which renders them mechanically incapable of impeding filament growth under conditions of elevated local current densities. For example, the presence of reductive decomposition products, such as Li₂S, Li₃P and LiCl, has been observed to result in the formation of mixed conducting interphases. These MCIs have been shown to facilitate the formation of electronically percolating pathways, thereby accelerating the process of localized plating (Guo et al., 2023; Han et al., 2019; Gao et al., 2019; Machín et al., 2025). While, halides frequently exhibit broader intrinsic electrochemical windows and form thinner passivation layers (YCl, YBr, LiCl, LiBr, Li₃Y), which curtails continuous MCI growth. However, halide electrolytes grain boundaries, low-density regions, or mechanically damaged zones can still provide low-resistance penetration pathways. Aggressive cycling (high current density, repeated cycling) can trigger filamentary penetration even in halides (Asano et al., 2018; Wang et al., 2021). Therefore, the practical stability of a given cell stack is thus determined by the critical current density (CCD) and the evolution of the CCD with cycling and pressure operation. In halide electrolytes, long-term cycling stability is strongly influenced by microstructural relaxation and grain-boundary behavior. Although halides form thinner and more stable passivation layers than sulfides, their lower shear modulus enables localized creep under sustained stack pressure, which can gradually modify interfacial contact and promote current constriction. Defect-rich or partially densified grain-boundary regions may act as preferential penetration pathways during extended cycling, even when bulk stability is high. These observations highlight that halide stability is governed not only by chemical passivation but also by microstructural evolution under practical cycling and pressure conditions (Guo et al., 2023; Asano et al., 2018; Wang et al., 2021; Tron et al., 2025).

2.2 Electrochemical instability

Electrochemical instability at the Li-solid electrolyte interface is a central determinant of long-term cell performance because it governs whether the interface evolves toward protection or degradation. Therefore, it is imperative to comprehend how reaction thermodynamics and product-formation kinetics dictate the interphase that ultimately forms. These formation of a stable, ionically conductive, electronically insulating passivation layer or the development of a continuously evolving, resistive MCI which is contingent upon the thermodynamic driving force for reaction with Li and the kinetics of product formation and transport. It should be noted that sulfide electrolytes are often thermodynamically unstable when exposed to lithium (Li), undergoing decomposition to form sulfides and phosphides (e.g., Li₂S, Li₃P, LiCl). This process results in the formation of chemically heterogeneous, resistive metal-cored intermetallics, which consume active lithium (Li) and solid electrolyte during cycling (Guo et al., 2023; Richards et al., 2016). In contrast, many halide chemistries demonstrate wider oxidative and reductive stability windows and are capable of forming thinner, more ionically conductive passivation layers (YCl, YBr, LiCl, LiBr, Li₃Y) at the Li interface. However, partial reduction of cations (e.g., reduction of multivalent metal centres under extreme local overpotentials) can still occur and degrade long-term stability in some halides (Asano et al., 2018; Tron et al., 2025). Therefore, the kinetic factors are of consequence such as the rapid formation of a thin, electronically insulating yet Li ion conductive interphase can be advantageous, whereas the slow, continuous decomposition that produces electronically percolating phases is detrimental. Interphase composition, thickness, porosity, and electronic conductivity are therefore critical descriptors that correlate with impedance growth, critical current density decline, and capacity fade (Han et al., 2019; Richards et al., 2016).

2.3 Mechanical degradation

Mechanical degradation is an equally critical dimension of interfacial instability, as electrochemical cycling continuously drives volumetric changes, stress accumulation, and evolving contact conditions at the Li-solid electrolyte interface. These stresses have the potential to exceed the fracture toughness of brittle ceramic electrolytes, which are prevalent for sulfide- and halide-based electrolytes. This phenomenon can lead to the initiation and propagation of cracks. Once formed, these cracks expose fresh reactive surfaces and create interconnected pathways that accelerate further degradation. Electrochemical cycling produces volumetric changes, stress accumulation, and evolving contact conditions at the Li/SE interface. Brittle fracture of ceramics (a common occurrence in many sulfides and some halides) exposes fresh reactive surfaces and creates networks of cracks. These networks increase local current density and provide mechanical pathways for filament propagation (Zhao et al., 2019; Richards et al., 2016). Plastic deformation, otherwise known as creep, has been observed in softer SEs or under sustained stack pressure. This phenomenon can serve to alleviate stress; however, it can also result in non-uniform contact and creeping. This, in turn, can concentrate current in localized regions, thereby promoting uneven plating and dead Li formation (Wang et al., 2021). The interplay between elastic modulus, fracture toughness, and applied stack pressure determines whether the interface fails by cracking, creeping/contact loss, or a mixed mode. Classical chemo-mechanical models (e.g., elastic deposition coupling) and fracture mechanics approaches provide useful design rules for matching SE mechanical properties to expected stack pressures and current densities (Ye et al., 2021; Monroe and Newman, 2005).

While both sulfide and halide electrolytes are often categorized as brittle ceramics, their mechanical responses under practical stack pressures differ in important ways. Sulfides typically exhibit lower fracture toughness and limited plastic accommodation, making them more prone to catastrophic cracking during cycling (Zhang et al., 2018; Kato et al., 2016; Doux et al., 2020). Halides, in contrast, possess slightly higher plasticity and lower shear modulus, allowing them to undergo limited creep or densification under pressure (Guo et al., 2023; Asano et al., 2018). This plastic accommodation can mitigate crack formation but may also promote gradual contact loss or interfacial deformation over long-term cycling (Guo et al., 2023; Asano et al., 2018; Tron et al., 2025). Thus, halides are mechanically “brittle” in an absolute sense, yet comparatively more plastically accommodating than sulfides under typical operating pressures a distinction that clarifies their intermediate position between sulfides and oxides in terms of mechanical failure modes (Banerjee et al., 2020; Zhang et al., 2018; Sharafi et al., 2017).

2.4 Void and pore formation

Another critical interfacial degradation pathway is void and pore formation, which emerges when lithium is stripped faster than it can redistribute. This results in localized voids that disrupt ionic contact and sharply increase impedance (interfacial resistance). During the process of lithium stripping, Li can be removed more rapidly at the local level than it can redistribute, thereby producing interfacial voids that disrupt ionic contact and significantly increase impedance. Void formation is particularly deleterious because it converts a nominally continuous interface into a heterogeneous contact map with current constrictions. These constrictions become sites for subsequent localized plating and dendrite nucleation. In sulfide systems, void-induced impedance rises are frequently precipitous, with the potential to swiftly curtail usable capacity, unless mitigated by stack pressure, compliant interlayers, or engineered interphases that maintain conformal contact (Brahmbhatt et al., 2020). In halide systems, voids may evolve more gradually, yet they still act as nucleation sites for dendrites if they are not promptly refilled. Maintaining conformal contact under realistic, application-relevant stack pressures is therefore critical for both classes (Kato et al., 2016).

2.5 Inactive (“dead”) lithium accumulation

In addition, inactive lithium accumulation has been identified as another significant degradation pathway. This process emerges as a result of repeated cycles of uneven stripping/plating and mechanical disruption, which generate lithium fragments that are electronically isolated and no longer participate in charge transfer. The accumulation of dead Li has been shown to have several deleterious effects on the cell, including a reduction in Coulombic efficiency and an increase in cell impedance. Furthermore, the presence of dead Li has been observed to result in mechanical abrasion or chemical alteration of the interphase. The presence of dead Li has been found to be closely associated with unstable interphases, contact loss, and local electronic leakage. In addition, quantifying dead Li using operando neutron or NMR methods, or by post-mortem mass balance, has been identified as a crucial diagnostic tool for assessing the health of interfaces (Han et al., 2019; Lian et al., 2023; Maslyn et al., 2018).

2.6 Thermal instability

Thermal instability represents an additional significant acceleration pathway for interfacial degradation, as elevated temperatures intensify the chemical, electrochemical, and mechanical processes that destabilize the Li-solid-electrolyte interface. Elevated temperatures can accelerate the decomposition kinetics of sulfide deposits, promote phase transformations, and increase gas evolution in reactive sulfides. They can also increase creep rates and morphological coarsening at the Li interface, which degrades contact quality and accelerates impedance growth (Sharafi et al., 2017; Monroe and Newman, 2005). While thermal runaway is less prevalent in inorganic SEs compared to liquid systems, localized thermal events associated with internal shorting or rapid decomposition remain a safety concern and must be addressed in cell-level designs.

2.7 Microstructure, grain boundaries, and space-charge layers

Microstructural heterogeneity has been identified as a fundamental driver of interfacial instability. The presence of features such as grain boundaries, pores, secondary phases, and compositional gradients gives rise to localized variations in transport and reactivity, thereby concentrating electrochemical activity. The behavior of grain boundaries can be classified as either fast or slow Li ion pathways, contingent upon chemistry and segregation. Additionally, these boundaries can function as preferential sites for reduction or mechanical failure. The presence of space charge layers (SCLs) at heterogeneous interfaces has been demonstrated to induce alterations in the local concentration and potential of lithium ions (Li ions). These alterations, in turn, result in the modulation of local overpotentials and the propensity for localized plating or depletion. The engineering of grain boundary chemistry and the control of SCLs through the use of doping, interfacial coatings, or compositional grading represent effective strategies for the mitigation of local hotspots and the delay of failure (Cheng et al., 2020; Richards et al., 2016).

2.8 Measurable metrics and diagnostics

Measurable metrics and operando diagnostics are essential for assessing interfacial health, as they provide quantitative links between observed failure modes and the underlying electrochemical and mechanical processes. These metrics include, but are not limited to, initial interfacial resistance, the rate of interfacial resistance growth with cycling, critical current density, dead Li accumulation per cycle, and mechanical integrity under target stack pressure. Operando techniques, such as neutron depth profiling, synchrotron X-ray tomography, operando TEM/cryogenic TEM, and operando impedance spectroscopy, when combined with post-mortem surface analysis (e.g., XPS, TOF SIMS, cryo FIB TEM), are essential to establish a link between observed failure modes and the underlying chemistry and mechanics (Cheng et al., 2020; Han et al., 2019; Richards et al., 2016).

Interface failure in sulfide and halide SEs is not a single mechanism but rather a coupled set of chemical, electrochemical, and mechanical processes that amplify one another. Consequently, effective mitigation necessitates concurrent regulation of electronic leakage, interphase chemistry, microstructure, and mechanical boundary conditions.

3 Strategies to mitigate interface failure

The practical lifetime and safety of lithium metal solid state batteries are contingent upon the stabilization of the Li/SE interface against coupled chemical, electrochemical, and mechanical degradation. The effective mitigation of these issues necessitates the integration of chemical selectivity, mechanical compliance, and manufacturing compatibility. This integration ensures the durability of protection under realistic stack pressures, current densities, and cell processing routes (Figure 3).

3.1 Interface engineering of chemistry, mechanics, and manufacturability

The objective of interface engineering is to establish a thin, Li⁺ conductive, electronically insulating, and mechanically compliant barrier that preserves conformal contact and prevents parasitic chemistry at the Li/SE boundary. The following practical approaches have been identified: (1) The subject of this investigation is ultrathin inorganic and hybrid coatings. The atomic layer deposition (ALD), molecular layer deposition (MLD), sputter, and solution routes deposit oxides and phosphates (LiNbO₃, Li₃PO₄, Al₂O₃), fluorides (LiF), and halide-rich layers. These layers have been shown to block electron transport while permitting Li⁺ flux. Conformal ALD/MLD films have been shown to minimize the presence of pinholes on rough SE surfaces, making them a valuable solution for complex geometries (Kamaya et al., 2011; Doux et al., 2020; Zou et al., 2021). (2) The composition gradient and composite interlayers are of particular interest in this study. Graded stacks (oxide → halide → polymer) or blended interlayers (halide/sulfide composites) serve as chemical buffers, mitigating reactive species and reducing modulus mismatch. This, in turn, enhances adhesion to both Li and the SE. By aligning interlayer ionic conductivity with the bulk SE within approximately one order of magnitude, the formation of a transport bottleneck can be circumvented (Asano et al., 2018). (3) Reactive and in situ formed interphases. Controlled in situ lithiation or reactive wetting can form thin, self-passivating layers that lower initial interfacial resistance and promote uniform nucleation during plating. These approaches can be more straightforward to scale than vacuum ALD for powder-based electrodes (Cheng et al., 2020). (4) The present study will examine the factors of adhesion and mechanical compliance. It is imperative to adhere to Li and to the SE; delamination converts a protective film into a source of voids and hotspots. The incorporation of thin, crack-tolerant films or compliant polymeric tie layers has been demonstrated to enhance durability under cyclic strain. It is imperative to report adhesion metrics, including peel and nanoindentation, in conjunction with electrochemical data (Doux et al., 2020; Zou et al., 2021). Key trade-offs: The application of thicker films has been demonstrated to enhance mechanical robustness; however, it has also been shown to increase interfacial resistance. Conversely, the utilization of ultrathin films has been demonstrated to minimize resistance, yet it is imperative that these films be deposited without defects. The quantification of benefit can be achieved by reporting initial interfacial resistance, resistance growth per 100 cycles, and CCD before and after coating (Kamaya et al., 2011; Doux et al., 2020).

3.2 Electrolyte design and hybrid architectures

Electrolyte design and hybrid architectures play a central role in suppressing interfacial instability, as tuning bulk chemistry and microstructure directly targets the intrinsic drivers of failure: (1) The processes of halogen doping and aliovalent substitution in sulfides, as well as cation substitution in halides, have been demonstrated to enhance electrochemical stability and reduce electronic leakage, while maintaining high levels of ionic conductivity. It has been demonstrated that reducing electronic conductivity is often more effective at suppressing internal Li deposition than marginal gains in ionic conductivity (Han et al., 2019; Zhao et al., 2019). (2) The utilization of composite and hybrid electrolytes is imperative in this context. Sulfide-halide blends, polymer inorganic composites, and filler-reinforced matrices exhibit a combination of high conductivity, improved chemical robustness, and mechanical compliance. Strategically positioning the chemically benign phase adjacent to Li while preserving a high conductivity bulk has been demonstrated to be an effective motif. Recent hybrid examples demonstrate enhanced CCD and cycle life in comparison to single phase electrolytes (Richards et al., 2016; Wang et al., 2021; Lian et al., 2023). (3) The focus of this study is twofold: firstly, the microstructure of the subject will be examined, and secondly, the engineering of grain boundaries will be analyzed. The presence of high density, the control of grain size, and the passivation of grain boundaries have been demonstrated to reduce low resistance penetration paths and limit void formation during cycling (Gao et al., 2019; Machín et al., 2025). The chemistry of grain boundaries can be modified through the introduction of dopants or the application of thin boundary coatings, with the objective of impeding rapid electronic pathways (Brahmbhatt et al., 2020). The following conductivities must be provided for the candidate compositions: bulk ionic and electronic conductivities, grain boundary resistance, CCD, and impedance evolution (Han et al., 2019; Richards et al., 2016).

3.3 Mechanical reinforcement and stack-pressure design

Measurable metrics and operando diagnostics are essential for assessing interfacial health, as they provide quantitative links between observed failure modes and the underlying electrochemical and mechanical processes: (1) The optimization of stack pressure is a critical component of the system’s design. Moderate sustained pressures (application dependent; many studies use MPa level ranges) have been shown to improve contact and suppress voiding. However, excessive pressure can fracture brittle SEs. It is imperative that pressure be optimized for the SE’s modulus and fracture toughness, and that these values be reported explicitly in all tests (Doux et al., 2020; Wang et al., 2021; Brahmbhatt et al., 2020). (2) The mechanical layers were designed with particular attention to architectural considerations. Gradient modulus stacks, fiber-reinforced composites, and multilayer arrest layers have been demonstrated to distribute strain, arrest cracks, and create mechanical barriers to filament propagation. Thin ductile buffers or lithiophilic scaffolds have been demonstrated to accommodate Li volume change and preserve electrical continuity (Zou et al., 2021; Lian et al., 2023).

3.4 Dendrite suppression: combined electrochemical and structural approaches

Measurable metrics and operando diagnostics are essential for assessing interfacial health, as they provide quantitative links between observed failure modes and the underlying electrochemical and mechanical processes: (1) The process under consideration involves two fundamental mechanisms: mechanical blocking and nucleation control. Beyond mechanical and structural considerations, differences in space-charge layer (SCL) formation at the Li/SE interface play a decisive role in dendrite suppression. Sulfide electrolytes generally exhibit stronger SCL formation due to their higher electronic polarizability and greater propensity for interfacial redox activity, which can locally enhance electronic leakage and promote non-uniform Li⁺ flux (Cheng et al., 2020; Han et al., 2019). In contrast, halide electrolytes possess wider band gaps and more stable anion frameworks, resulting in weaker SCLs and reduced interfacial charge accumulation. These distinctions influence interface-engineering strategies: sulfides benefit from dopants or coatings that suppress electronic conductivity and stabilize the interphase, whereas halides respond more favorably to densification, defect control, and microstructural homogenization, which help maintain uniform Li transport and mitigate void formation (Guo et al., 2023; Tron et al., 2025). Conformal high modulus films (garnet oxides, LiF-rich layers) in combination with lithiophilic hosts (porous Ag/Cu/Au scaffolds, structured carbon) have been shown to homogenize Li deposition and raise CCD. The LiF-rich interphases are attractive due to their high mechanical strength and wide electrochemical window; however, deposition quality and adhesion are critical factors (Wang et al., 2025; Sharafi et al., 2017). (2) The following operational controls are in place: The driving force for filament initiation is reduced by limiting the areal current density, employing pulsed or waveform charging, and regulating plated capacity per cycle. Standardized CCD protocols and reporting facilitate fair comparison (Ye et al., 2021). The report of CCD should be submitted in accordance with standardized protocols, and the real capacity per half cycle should be specified. Additionally, the post-mortem filament morphology should be documented (Ye et al., 2021).

3.5 Thermal management and CTE matching

Temperature is a critical accelerator of interfacial degradation, as it strongly influences reaction kinetics, creep behavior, and the mechanical stress state at the Li-solid-electrolyte interface: (1) The necessity of thermal control is paramount in this context. The utilization of passive cooling methods, such as phase change materials and thermally conductive fillers, in conjunction with active cooling techniques, serves to mitigate transient heating and the formation of local hotspots. These phenomena have been observed to expedite interfacial degradation. Thermal design must be integrated with electrochemical and mechanical constraints (Monroe and Newman, 2005; Lin et al., 2016). (2) The subject of this text is CTE matching. The selection of layers exhibiting compatible coefficients of thermal expansion has been demonstrated to effectively mitigate cyclic thermal stress and delamination, a critical consideration in the context of multilayer stacks and cells operating across broad temperature ranges (Maslyn et al., 2018).

3.6 Advanced diagnostics and integrated modelling

Early detection and physics-based prediction are critical for intervening before catastrophic failure, enabling targeted mitigation informed by real-time observation of interfacial evolution: (1) Operando imaging and spectroscopy. Synchrotron X-ray tomography, neutron radiography, operando (cryo)TEM, and operando Raman/IR have been demonstrated to reveal void growth, interphase evolution, and dendrite initiation in real time. Furthermore, the combination of imaging with time-resolved EIS and reference electrode measurements has been shown to link morphology to electrochemical signatures (Cheng et al., 2020; Han et al., 2019; Ortiz et al., 2024). (2) The quantification of coupled instabilities is facilitated by phase field, continuum, and finite element chemo-mechanical models. These models also guide the design of interlayer thickness, modulus gradients, and microstructure. In order to be both predictive and actionable, models must be validated against operando observables (Wang et al., 2022). The utilization of operando diagnostics is imperative for the extraction of void volume fraction, interphase thickness, and local overpotential maps, thereby facilitating model validation (Ortiz et al., 2024; Paul et al., 2025; Wang et al., 2022).

3.7 Practical research and testing recommendations

Standardized and reproducible testing protocols are imperative for ensuring fair comparison across studies and enabling reliable translation of interfacial performance metrics: (1) The benchmark stack pressure and CCD protocol were utilized in this study. It is imperative to report the nominal stack pressure and a clear CCD protocol. The protocol should specify the fixed capacity per half cycle or fixed time, step size, and rest conditions (Doux et al., 2020; Ye et al., 2021). The following data should be included: the real capacity per step and the total plated thickness. (2) In order to achieve a comprehensive understanding of the underlying mechanisms and factors contributing to failure, it is essential to integrate operando tomography (void/dendrite mapping), operando spectroscopy (interphase chemistry), and time-resolved EIS on a unified cell architecture. This approach facilitates the correlation of mechanical, chemical, and electrochemical failure signatures, thereby providing a multifaceted perspective on the failure process (Ortiz et al., 2024; Paul et al., 2025). (3) The operational envelope, wherein low impedance and dendrite suppression coexist, is mapped by systematically varying modulus gradients, interlayer thickness (5–200 nm), and stack pressure (Brahmbhatt et al., 2020; Ye et al., 2021). (4) The following metrics are relevant for the analysis of manufacturing and sustainability. In addition to electrochemical performance, the report includes moisture uptake, gas evolution (H₂S for sulfides), ambient handling windows, and simple recovery/leaching tests for critical elements. This comprehensive approach provides a foundation for informed decisions regarding lifecycle and facility requirements. Roadmaps underscore the significance of scalable coatings, interface engineering, and operando diagnostics as imminent obstacles to commercialization (Fraunhofer, 2022; Amici et al., 2022).

A combination of interface engineering (coatings and graded interlayers), electrolyte chemistry and microstructure tuning, and mechanical design (stack pressure and architected reinforcement) is required to interrupt the coupled failure pathways. The implementation of these mechanisms in conjunction, along with their validation under controlled test conditions, is imperative for the translation of laboratory advancements into enduring cellular outcomes.

3.8 Practical research and testing recommendations

In order to promote reproducible progress and facilitate fair comparison across solid-state electrolyte chemistries, it is recommended that the following standardized experimental and reporting practices be implemented: (1) A standardized low-pressure cycling protocol has been adopted as a community benchmark for stack pressure (e.g., 5 MPa nominal, with controlled ramps to 15 MPa) and a defined current-density schedule (e.g., 0.1 → 1.0 → 2.0 mA cm^-2, 10 cycles per step). (2) A recent study combined interfacial diagnostics on the same cell architecture to correlate mechanical, chemical, and electrochemical failure signatures. This approach entailed operando synchrotron X-ray tomography (void/dendrite mapping), in-situ Raman/IR (interphase chemistry), and time-resolved EIS (interfacial resistance evolution). (3) A systematic variation of the modulus gradient (e.g., stiff/ductile/stiff stacks), interlayer thickness (5–200 nm), and stack pressure is employed in the mechanics-electrochemistry test matrix to map the operational envelope where dendrite suppression and low impedance coexist. (4) A hybrid electrolyte screening workflow is employed to screen sulfide/halide blends across the composition space (e.g., 80:20 → 20:80) using high-throughput impedance and CCD tests. Then, the top candidates are validated in full-cell cycling with NCM cathodes under reduced pressure. It is imperative that composition, processing, and densification details are provided for the purpose of ensuring reproducibility. (5) The following manufacturing readiness metrics have been developed such as moisture uptake, H₂S evolution (for sulfides) and ambient handling windows (for halides). These metrics are intended to quantify processing cost and facility requirements. (6) The principles of sustainability and end-of-life management encompass the implementation of straightforward leaching and recovery assessments for critical elements, including S, P, Y, In, and halides. These evaluations are conducted to estimate material recovery yields, thereby providing crucial insights for the early stages of materials selection and informing the subsequent lifecycle assessments.

The implementation of standardized protocols, diagnostics, and reporting metrics, as recommended, has the potential to mitigate interstudy variability, expedite the identification of robust material/process combinations, and establish a uniform framework for the comparison of sulfide, halide, and hybrid electrolytes.

4 Comparative analysis: sulfide vs. halide electrolytes

Sulfide and halide solid electrolytes represent two leading inorganic classes for lithium metal solid state batteries. Each class offers distinct transport, chemical, mechanical, and processing attributes that determine interface behavior and the most effective engineering strategies. The subsections that follow compare key properties, link them to dominant failure modes, and highlight practical design implications and metrics for selection or hybridization (Table 1).

TABLE 1. Key differences between sulfide and halide solid electrolytes in ionic transport, chemical stability, mechanical response, and processing behavior, illustrating how these intrinsic attributes shape their characteristic interfacial failure mechanisms in Li-metal solid-state batteries. Adapted from and informed by Banerjee et al. (2020), Zhang et al. (2018), Guo et al. (2023), Asano et al. (2018), Tron et al. (2025), Gao et al. (2019), Machín et al. (2025).

4.1 Ionic conductivity and transport

Ionic conductivity and bulk transport properties are foundational to electrolyte performance, as they directly govern the rate at which Li⁺ can be delivered to the interface and thereby limit or enable stable cycling. It should be noted that the sulfide electrolytes (e.g., Li₆PS₅Cl, Li₁₀GeP₂S₁₂) frequently attain remarkably elevated room-temperature Li⁺ conductivities (>10^–3 S cm^-1) due to the presence of soft anion frameworks and open diffusion channels, which serve to reduce migration barriers and promote defect formation (Zhang et al., 2018; Kato et al., 2016; Kamaya et al., 2011). While, halides (e.g., Li₆MX₆ or Li₃MCl₆, X = Cl, Br, I) can reach comparable conductivities in optimized compositions, however, their conduction is often more defect-mediated within denser halide lattices and is more sensitive to composition and processing (Asano et al., 2018; Tron et al., 2025; Manthiram et al., 2017). The microstructure, defined by parameters such as grain size and porosity, along with the grain boundary chemistry and the space charge layers, exerts a significant influence on the effective transport properties in both classes. These local transport variations, in turn, determine the spatial distribution of the current density, which, in turn, governs the initiation of failure. Quantitative metrics are to be reported, including bulk ionic conductivity, grain boundary resistance, and effective in-plane/out-of-plane conductivity for pelletized stacks (Cheng et al., 2020; Richards et al., 2016).

4.2 Electrochemical stability

Electrochemical stability is a defining constraint for solid electrolytes, as their intrinsic redox reactivity with lithium and limited oxidative tolerance dictate the voltage window in which they can operate safely. Solid electrolytes, such as sulfides, are often prone to reduction by Li due to their thermodynamic reactivity. They can exhibit limited oxidative stability at high voltages. Consequently, direct pairing with Ni-rich cathodes typically requires cathode coatings or engineered interlayers to prevent continuous decomposition (Richards et al., 2016). While halides generally exhibit broader practical electrochemical windows and diminished oxidative reactivity, thereby facilitating enhanced compatibility with high voltage cathodes and reducing the necessity for aggressive cathode surface engineering in numerous instances (Asano et al., 2018; Tron et al., 2025). The kinetics and local conditions remain decisive factors in this regard. For instance, even halides with wide nominal windows can degrade under high local overpotentials or when electronic leakage is introduced by interfacial reactions. Consequently, operando measures of interfacial impedance growth and CCD under realistic cathode potentials are essential comparators (Han et al., 2019; Tron et al., 2025).

4.3 Interfacial reactivity with lithium

Interfacial reactivity with lithium represents a pivotal constraint for sulfide electrolytes, as their thermodynamic instability propels the formation of mixed-conducting decomposition products, thereby undermining interphase integrity. Typically, sulfides react with Li to form sulfide/phosphide decomposition products (e.g., Li₂S, Li₃P, LiCl), which in turn produce chemically heterogeneous, often mixed-conducting interphases. These MCIs consume active Li, raise impedance, and can create electronic pathways that accelerate localized plating and filament growth (Guo et al., 2023; Han et al., 2019; Richards et al., 2016). Halides, on the other hand, more frequently form thinner, more ionically conductive passivation layers (YCl, YBr, LiCl, LiBr, Li₃Y) at the Li interface. These layers can be intrinsically less electronically percolating and, as a result, less prone to runaway decomposition. However, under conditions of severe cycling, certain halides may undergo partial cation reduction or other reductive pathways, resulting in the degradation of passivation and compromising long-term stability (Asano et al., 2018; Tron et al., 2025). The design implications are as follows: electronic insulation at the interface for sulfides should be prioritized in order to prevent MCI percolation. Additionally, mechanical robustness and adhesion for halides should be prioritized to ensure the preservation of thin passivation under stress. The composition of the interphase (XPS/TOF SIMS), as well as its thickness and electronic conductivity, must be reported in order to facilitate a comparison of the chemistries (Han et al., 2019; Richards et al., 2016). The trade-offs can be conceptualized using a radar chart (Figure 4).

4.4 Mechanical properties

The mechanical properties of the material under consideration have been demonstrated to exert a significant influence on the interfacial stability of the material. The balance between stiffness and brittleness is a critical factor in determining whether the electrolyte will resist filament penetration or whether it will fracture and expose fresh reactive surfaces. Sulfides are often mechanically stiff and brittle. While a higher elastic modulus can help resist immediate filament penetration, the increased brittleness can lead to heightened susceptibility to microcracking under tensile or shear stress, particularly at grain boundaries or under excessive stack pressure which exposes fresh reactive surfaces and accelerates chemical degradation (Zhao et al., 2019; Richards et al., 2016). Halides are generally softer and more deformable, which improves initial conformal contact with Li and reduces catastrophic fracture. However, their lower shear strength and tendency to creep can permit contact loss and localized current focusing unless mechanically reinforced (Wang et al., 2021; Lian et al., 2023). Sulfide-based cells benefit from fracture-tolerant architectures and compliant interlayers. In contrast, halide-based cells require reinforcement strategies, such as modulus gradients or arrest layers, to preserve beneficial contact without negating ductility. The subsequent analysis will entail the comparison of Young’s modulus, fracture toughness, and CCD as a function of applied pressure (Ye et al., 2021; Monroe and Newman, 2005).

4.5 Environmental stability and processing

It is imperative to consider environmental stability and processing sensitivity when assessing sulfide and halide electrolytes, as their marked reactivity with moisture has the potential to compromise safety, handling, and long-term performance (Gao et al., 2019; Machín et al., 2025). For example, sulfide electrolytes exhibit heightened sensitivity to moisture, a characteristic that can result in the release of H₂S upon exposure to ambient conditions. This inherent sensitivity necessitates the implementation of stringent protocols for dry room handling, a process that is known to increase processing costs and complexity (Guo et al., 2023). While halides exhibit a high degree of tolerance to ambient conditions and are more straightforward to manage, thereby facilitating manufacturing processes and reducing facility requirements. This enhanced ambient processability constitutes a significant practical advantage for scale-up operations, notwithstanding the necessity for mechanical reinforcement in the final cell (Asano et al., 2018; Tron et al., 2025). The following metrics must be reported in order to assess manufacturability: ambient handling window, moisture uptake (ppm H₂O), H₂O evolution rates, and required dry room class. These metrics directly impact cost and facility design trade-offs, especially for implementation of these electrolytes in the production of small and large cell formats (Fraunhofer, 2022; Amici et al., 2022).

4.6 Hybrid and composite electrolytes

Hybrid strategies, such as graded interlayers, sulfide/halide blends, or mixed phase composites, have been developed to leverage the high conductivity of sulfides with the electrochemical robustness and ambient processability of halides. Examples include sulfide-rich bulks with halide-rich interlayers or mixed-phase pellets. In the former, the Li-facing surface is halide-dominated, while the bulk remains sulfide-rich. It has been demonstrated that the formulation of hybrids in a precise manner has been shown to enhance CCD, extend the symmetric cell lifetime, and improve full cell Coulombic efficiency under conditions of reduced stack pressure when compared with single phase electrolytes (Richards et al., 2016; Tron et al., 2025; Manthiram et al., 2017). The following design principles must be observed in order to ensure the successful integration of hybrid components: The following four points must be considered in order to ensure the successful implementation of the aforementioned methodology (Gao et al., 2019; Machín et al., 2025): (1) The chemically benign phase must be placed adjacent to Li. (2) Continuous high conductivity pathways must be ensured through the bulk. (3) Interfacial adhesion and CTE compatibility must be controlled. (4) Percolation thresholds for both ionic and electronic transport must be characterized (Table 2).

TABLE 2. Distills these comparisons into a radar chart, visually highlighting the complementary nature of the two electrolyte classes. Adapted from and informed by Banerjee et al. (2020), Zhang et al. (2018), Guo et al. (2023), Asano et al. (2018), Tron et al. (2025).

Sulfides offer superior ionic transport and mechanical stiffness, but they require aggressive interface stabilization and dry room processing. Halides provide wider electrochemical windows and easier handling, but they need mechanical reinforcement to prevent creep and contact loss. Hybrid or composite electrolytes that combine sulfides and halides can shift the trade-offs toward higher CCD, improved full-cell performance, and more manufacturable processes.

5 Conclusion and outlook

A comparative evaluation of sulfide and halide-based solid electrolytes confirms a central conclusion: no single electrolyte chemistry yet delivers the full set of properties required for practical lithium metal solid-state batteries. These properties include high ionic transport, durable interfacial stability, mechanical resilience, manufacturability, and environmental robustness. Sulfides exhibit remarkable room temperature ionic conductivity and desirable mechanical stiffness, however, they are encumbered by narrow electrochemical windows, severe reactivity with lithium (Li) that yields mixed conducting interphases, and heightened moisture sensitivity, necessitating dry room processing (Kato et al., 2016; Kamaya et al., 2011; Guo et al., 2023; Richards et al., 2016). Halides offer broader practical stability windows, enhanced ambient tolerance, and augmented cathode compatibility. However, their diminished shear strength and proclivity for creep necessitate mechanical reinforcement and meticulous contact engineering to circumvent contact loss and localized plating (Asano et al., 2018; Tron et al., 2025; Wang et al., 2021). Hybrid and composite electrolyte strategies offer a pragmatic approach to address these trade-offs. These strategies involve the placement of halide-rich, chemically benign phases at the Li interface, while retaining sulfide-rich, high-conductivity bulk regions. This approach has the potential to enhance critical current density, extend the symmetric cell lifetime, and improve full cell coulombic efficiency under reduced stack pressure (Richards et al., 2016; Tron et al., 2025; Manthiram et al., 2017). The manufacturing advantages of halide content or protective interlayers, including lower temperature densification and reduced dry room dependence, render mixed phase approaches appealing for scale-up. Nevertheless, long-term capacity retention and interfacial degradation after extended cycling remain unresolved in many demonstrations (Doux et al., 2020; Wang et al., 2021; Tron et al., 2025). Figure 5 presents a comprehensive overview of the major milestones and projected directions in the domain of interface engineering.

5.1 Priority research directions

The focus of research in this field has shifted towards the establishment of interface-specific design rules. This shift has led to the need for diagnostics capable of resolving interphase chemistry, electronic transport, and mechanical evolution at the relevant length and time scales: (1) The implementation of interface-specific design rules is facilitated by multimodal operando diagnostics. It is imperative to resolve interphase chemistry, electronic conductivity, and mechanical evolution at the pertinent length and time scales to establish a correlation between spectroscopic signatures and failure modes (Cheng et al., 2020; Han et al., 2019; Ortiz et al., 2024). (2) SE architectures have demonstrated a high degree of mechanical robustness. The development of multilayer, fiber-reinforced, and gradient modulus designs is imperative to arrest crack propagation in sulfides and limit creep in halides while preserving ionic percolation (Zou et al., 2021; Lian et al., 2023; Ye et al., 2021). (3) The following investigation will address the matter of electronic insulation strategies at the Li interface. The objective is to identify target coatings and interlayers that suppress electronic leakage (preventing MCI percolation) without introducing transport bottlenecks. The quantification of trade-offs can be achieved through the use of CCD and interfacial resistance growth metrics (Kamaya et al., 2011; Han et al., 2019; Wang et al., 2025). (4) The composition and defect engineering processes are guided by artificial intelligence and machine learning algorithms. The acceleration of the discovery of dopants and phase combinations that optimize the conductivity-stability-mechanics triad, in addition to the prediction of promising hybrid formulations, is of the utmost importance (Wang et al., 2022). (5) The utilization of scalable, low-temperature processing routes is imperative. It is imperative to prioritize deposition and densification methods that are compatible with powder processing and roll-to-roll assembly. This approach is crucial to minimize reliance on dry room conditions and reduce manufacturing costs (Fraunhofer, 2022; Amici et al., 2022). (6) The utilization of standardized, low-pressure testing and the implementation of long-duration full-cell benchmarks are essential components in this field. The adoption of community protocols for CCD, stack pressure, and diagnostics is imperative to ensure the comparability and predictability of results, thereby facilitating the estimation of pack level performance (Doux et al., 2020; Ye et al., 2021; Fraunhofer, 2022). (7) The final stage of the lifecycle of an object, as well as the processes involved in the reuse, repurposing, or discarding of that object. The incorporation of recovery tests and lifecycle metrics for critical elements (S, P, Y, In, halides) at the outset is imperative to facilitate informed decisions regarding sustainable material selection and the enhancement of supply chain resilience (Fraunhofer, 2022; Amici et al., 2022; IDTechEx, 2025; McKinsey and Co, 2023).

5.2 Translational challenges and near-term milestones

The demonstration of translational progress is contingent upon the substantiation of performance under practical operating conditions. Consequently, the attainment of stable full-cell cycling at realistic areal capacities and stack pressures constitutes a critical near-term milestone: (1) It is imperative to demonstrate stable full cell cycling at application-relevant areal capacities and low to moderate stack pressures (e.g., ≥3 mAh·cm^-2 at ≤10 MPa) for several hundred cycles. In addition, there is a need to report CCD, interfacial resistance evolution, and post-mortem interphase composition (Doux et al., 2020; Ye et al., 2021). (2) The process of scale coating and interlayer deposition to electrode scale formats must be carried out with the objective of retaining adhesion and uniformity. It is imperative to demonstrate that the benefits observed at the laboratory scale persist after calendaring and cell assembly (Kamaya et al., 2011; Fraunhofer, 2022). (3) The validation of predictive models with operando observables (void fraction, interphase thickness, and local overpotential maps) is essential for ensuring that design rules translate into robust engineering choices (Ortiz et al., 2024; Wang et al., 2022).

The path to practical lithium-metal solid-state batteries (SSBs) lies in the synergistic integration of chemical passivation, mechanical resilience, and scalable processing. This integration is guided by standardized testing and predictive modeling. Prioritizing interface-specific design rules, pressure-tolerant architectures, and diagnostics-driven materials discovery will determine the speed at which these technologies transition from promising prototypes to commercial products.

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Footnotes

Edited by: Raghvendra Pandey, University of Delhi, India

Reviewed by: Liqun Guo, University of Houston, United States

Yi Peng, Southeast University, China