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

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