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The distinction between sulfide and halide solid electrolytes begins at the level of their fundamental bonding chemistry, and this difference propagates directly into their interfacial behavior against lithium metal. Sulfides rely on relatively polarizable sulfur anions, which softens the lattice and lowers the barrier for lithium ion hopping, explaining the liquid-like conductivities that have made argyrodites and LGPS-type compositions so attractive. That same softness, however, means the sulfur sublattice is chemically reactive, and thermodynamic calculations consistently place most sulfide compositions outside the stability window of lithium metal. When contact is made, reduction cascades produce lithium sulfide, lithium phosphide, and lithium chloride in proportions that depend on the parent composition and on whether the reaction proceeds under open circuit or under applied bias. Halides, by contrast, are built on chloride, bromide, or iodide anions whose lower polarizability and higher electronegativity yield wider electrochemical stability at the oxidative end, which is precisely why they pair so well with uncoated high-voltage oxide cathodes.

The asymmetry in stability windows creates a natural division of labor that the review’s hybrid roadmap exploits. A halide electrolyte can sit against a cathode operating above 4.5 volts without the protective oxide coatings that sulfide systems demand, while a sulfide layer can provide the high-conductivity, mechanically compliant medium adjacent to the lithium anode. This bilayer architecture, in which each electrolyte is placed on the side of the cell where it is intrinsically more stable, sidesteps the weakest interface of each material class. The remaining challenge is the sulfide-halide junction itself, where interdiffusion, space charge effects, and possible formation of mixed-conducting interphases must be managed. Recent work suggests that brief annealing treatments or deliberately introduced interfacial dopants can suppress mutual diffusion while preserving low contact resistance, but the long-term evolution of such internal junctions under thousands of cycles remains an open question that operando studies are only beginning to address.

Critical current density, the current above which lithium filaments penetrate the electrolyte, serves as the most widely cited single metric for anode-side stability, yet its interpretation requires care because it is not an intrinsic material property. Measured values depend on stack pressure, temperature, electrolyte thickness, grain size, surface roughness, and the electrochemical history of the interface. The same argyrodite pellet can exhibit critical current densities varying by an order of magnitude depending on how it was densified and how well it wets the lithium electrode. This sensitivity is not a nuisance but a diagnostic opportunity: the dependence of critical current density on pressure and microstructure reveals which failure pathway dominates in a given system. A strong pressure dependence points to void-driven contact loss, whereas insensitivity to pressure combined with temperature dependence suggests electronic leakage through the electrolyte bulk or along grain boundaries as the rate-limiting step for filament nucleation.

Void formation during stripping deserves particular attention because it initiates a feedback loop that can destroy an otherwise stable cell. As lithium is extracted, the metal retreats from contact points, and if the local stripping rate exceeds the ability of lithium to creep and maintain contact, pores nucleate at the interface. These voids concentrate current on their rims, raising local current density by factors that scale with the void geometry. The elevated current accelerates stripping around the void perimeter, enlarging it, while the concentrated flux during the subsequent plating half-cycle preferentially deposits lithium into the void base, where it can bridge into the electrolyte as a filament. This chemo-mechanical coupling explains why stack pressure is such a powerful lever: modest pressures in the range of a few megapascals can suppress void nucleation entirely, but excessive pressure risks fracture of brittle electrolytes, particularly the halides, whose fracture toughness is generally lower than that of dense sulfide ceramics.

Grain boundaries have emerged as double-edged features in this failure landscape. In sulfide electrolytes, grain boundaries can be either faster or slower ion conduction pathways than the grain interiors depending on composition and dopant segregation, and their electronic character varies similarly. When grain boundaries carry appreciable electronic conductivity, whether intrinsic or induced by partial decomposition, they become preferential highways for lithium nucleation inside the ceramic, producing the characteristic trans-granular and inter-granular filament patterns observed in post-mortem cross-sections. In halides, the picture is complicated by the tendency of some compositions to form electronically conductive reduced phases at low potentials, meaning that the anode-side interface of a halide must be protected even when the bulk electrolyte is an excellent ionic conductor. This is why bilayer designs typically interpose a thin sulfide or lithium-compatible interlayer between lithium metal and the halide layer rather than exposing the halide directly.

Space charge layers, arising from the mismatch in lithium chemical potential between the metal and the electrolyte, redistribute mobile ions and vacancies over nanometer distances at the contact. Although the affected region is thin, its consequences are disproportionate: lithium ion flux can be locally concentrated or depleted, interfacial resistance can rise substantially, and the altered defect population can change the local electronic conductivity. In sulfide systems, the space charge layer at the lithium contact often manifests as a lithium-rich, electronically leaky region that seeds inhomogeneous plating. Mitigation through ultrathin artificial interphases, such as lithium fluoride or lithium nitride layers deposited by evaporation or formed in situ by controlled reaction, works partly by flattening the chemical potential gradient and partly by providing a phase whose decomposition products are themselves ionically conductive and electronically insulating. The design rule that emerges is that an ideal interphase must be simultaneously ion-conducting, electron-blocking, mechanically soft enough to accommodate volume changes, and thermodynamically stable against both lithium and the electrolyte.

The role of electronic leakage deserves emphasis because it reframes the traditional figure of merit for electrolytes. A material can possess outstanding ionic conductivity and still fail catastrophically if its electronic conductivity, even at the level of parts per billion relative to its ionic transport, allows parasitic lithium deposition within the bulk. Over long times, the product of electronic conductivity and operating time determines the extent of internal plating, which is why cells that perform well in short-term tests can still short after hundreds of hours of float or cycling. This time-dependent degradation mode favors compositions and dopants that widen the electronic band gap or introduce electron traps, and it explains the observed benefit of certain aliovalent substitutions that pin the Fermi level away from the conduction band edge. Screening electrolytes by electronic conductivity, not merely ionic conductivity, is therefore becoming standard practice in the field.

Manufacturability considerations increasingly shape which laboratory strategies can survive translation. Sulfides offer the advantage of cold-press densification and room-temperature processability, but their moisture sensitivity demands dry-room handling at industrial scale, since hydrogen sulfide evolution upon air exposure poses both safety and material-loss problems. Halides are generally more tolerant of ambient handling, though many still require controlled atmospheres, and their synthesis often involves expensive indium or yttrium precursors that pressure cost targets. Coating strategies that work in the laboratory, such as atomic layer deposition of conformal alumina or lithium niobate layers, face throughput constraints when scaled to the areas required for automotive cells, motivating research into solution-based and roll-to-roll coating alternatives. The review’s staged roadmap, moving from simple protective coatings toward AI-guided, application-specific interface designs by the mid-2030s, reflects the reality that no single interfacial solution will satisfy both high-energy-density EV packs and lower-cost stationary storage simultaneously.

Operando diagnostics are transforming the field’s ability to distinguish among competing failure hypotheses. Synchrotron X-ray computed tomography now resolves filament growth in three dimensions in real time, revealing that penetration often initiates far from the geometric center of current distribution, at defects invisible to surface characterization. Neutron depth profiling quantifies lithium distribution through the electrolyte thickness, directly imaging internal plating. Electrochemical impedance spectroscopy, when interpreted with appropriate equivalent circuits and combined with distribution of relaxation times analysis, can separate interfacial resistance growth from bulk degradation. Coupling these measurements with acoustic emission and dilatometry links mechanical events, such as cracking or delamination, to specific points in the charge-discharge cycle. The convergence of evidence from these techniques supports the review’s central claim that interface failure is inherently multifactorial, and it provides the empirical basis for the coupled chemical-electrochemical-mechanical framework that organizes the comparative analysis.

Looking forward, the most promising research directions combine the complementary strengths of the two material classes rather than seeking a single universal electrolyte. Composite designs that embed halide phases within a sulfide matrix, or vice versa, aim to harvest halide oxidative stability and sulfide reductive stability in one continuous medium, though percolation, phase compatibility, and interfacial ion transport between the constituent phases must be engineered deliberately. Doping strategies that simultaneously suppress electronic leakage and enhance grain boundary ionic transport offer another path, particularly for sulfides where rare-earth and halogen substitutions have shown synergistic effects. On the mechanical side, designing electrolytes with controlled porosity or introducing compliant secondary phases may allow cells to tolerate volume changes without the high stack pressures that complicate pack-level engineering. Ultimately, the practical success of lithium-metal solid-state batteries will rest not on any single breakthrough but on the disciplined, material-class-specific integration of chemical, electrochemical, and mechanical interface engineering, validated under the realistic pressures, currents, and lifetimes that commercial deployment demands.

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

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

Article References: Tron, A. (2026). Interface failure mechanisms and engineering strategies in sulfide and halide solid electrolytes for lithium metal solid-state batteries. Frontiers in Batteries and Electrochemistry, 5, Article 1844424. https://doi.org/10.3389/fbael.2026.1844424

Image Credits: AI Generated

DOI: 10.3389/fbael.2026.1844424

Keywords: Interface, failure, mechanisms, engineering, strategies, sulfide, halide, solid, electrolytes, lithium, metal, solid-state