Removing the flammable organic solvent is the headline safety pitch for solid-state batteries, and it’s real: there’s no liquid to vaporise, pressurise a cell and vent as flammable gas. It’s also led to a common shorthand in the coverage of the technology: that solid-state cells are simply immune to thermal runaway. They aren’t. Recent reviews are explicit that all-solid-state batteries remain susceptible to thermal runaway under thermal, mechanical and electrical abuse (Kim et al., Adv. Energy Mater. 2025), and the failure modes involved are different enough from a conventional liquid-electrolyte cell that they need their own treatment.
Solid-solid exothermic reactions. The electrolyte itself doesn’t need to be flammable for the cell to release a dangerous amount of heat, it can react exothermically with a charged cathode in direct solid-solid contact. Charged (delithiated) LiNi₀.₈Co₀.₁Mn₀.₁O₂ in contact with argyrodite Li₊PS₅Cl has been shown to burn explosively from an onset around 150°C, even under an inert argon atmosphere (Chem. Mater. 2022). This is a temperature a cell can reach from a fairly modest abuse event, not just an extreme one. The same charged-cathode contact also pulls forward the electrolyte’s own decomposition: Li₊PS₅Cl is thermally stable to around 800°C on its own, but that decomposition onset drops to roughly 300°C once it’s in contact with a delithiated Ni-rich cathode. Recent work attributes the more severe failures specifically to instability at the cathode/sulfide interface rather than to the bulk solid electrolyte or the lithium metal anode (dual-stage thermal runaway study, 2025), which is a useful place to focus mitigation: it’s the same cathode/electrolyte interface covered on the Cathode-Electrolyte Interface & Catholyte Engineering page, and a well-executed protective coating that limits interdiffusion also limits how much of this exothermic pathway is available.
H₂S generation. Sulfide electrolytes hydrolyse on contact with moisture and the reaction releases hydrogen sulfide. H₂S is toxic and is itself flammable. This is a manufacturing and handling hazard rather than an in-service one (a sealed, dry cell isn’t hydrolysing), but it shapes the plant: industrial dry rooms for sulfide processing are more commonly specified below roughly −45 to −60°C dew point, compared with around −40°C for conventional lithium-ion manufacturing (Randrema et al. 2024). Not all sulfides are equal here: argyrodite compositions evolve roughly eight times less H₂S than Li₄P₃S₁₁ under equivalent moisture exposure (ACS Appl. Energy Mater.), which is one of the practical reasons argyrodite has become the more common commercial choice despite LGPS’s higher peak conductivity (see the Solid Electrolyte Comparative Matrix).
Pinhole and dendrite short circuits. A liquid-electrolyte cell that develops an internal short has a pressure-relief path: heat vaporises electrolyte, pressure rises, and (in a well-designed cell) a vent or CID acts before the reaction runs away completely. A solid electrolyte separator has no equivalent pressure-relief mechanism. Lithium dendrites and metal filaments can still find their way through a solid separator. Not by displacing solid material the way they’d displace a liquid, but by growing through pre-existing structural defects: pinholes, grain boundaries, and cracks introduced during pressing or by the electrolyte’s own porosity (see Grain Boundary Dendrite Growth in Solid-State Electrolytes). Once a filament bridges the separator, the short concentrates Joule heating at that single point rather than dissipating it, and with no vapour-pressure relief valve to interrupt the process, a localised short can drive local temperatures high enough to trigger the cathode/electrolyte exothermic reaction described above. It’s also worth distinguishing a hard short from a soft short: intermittent, partial-contact “soft” shorts are a recognised and sometimes misdiagnosed failure mode in solid-state cell testing in their own right (“Dynamic Soft-Shorts in Solid-State Battery Research,” DOE), and can precede a full hard short by an unpredictable margin.
Needs review: the exothermic onset and decomposition figures above are from specific argyrodite/Ni-rich NMC material combinations in the cited studies. Different sulfide compositions, cathode coatings, and Ni content will shift these numbers — treat them as illustrative of the mechanism, not as a rating for any specific commercial cell.
See also: Solid State Battery, Grain Boundary Dendrite Growth in Solid-State Electrolytes, Physics of Thermal Runaway, Solid Electrolyte Comparative Matrix