Interphase Morphology between a Solid-State Electrolyte and Lithium Controls Cell Failure

The interfaces between many solid-state electrolytes (SSEs) and lithium metal are (electro)chemically unstable, and improved understanding of how interfacial transformations influence electrochemical degradation is necessary to stabilize these interfaces and therefore enable a wider range of viable SSEs for batteries. Here, the (electro)chemical reaction processes that occur at the interface between Li _1.4 Al _0.4 Ge _1.6 (PO ₄ ) ₃ (LAGP) electrolyte and lithium are studied using in situ transmission electron microscopy and ex situ techniques. The reaction of lithium with LAGP causes amorphization and volume expansion, which induce mechanical stress and fracture of the SSE along with a massive increase in impedance. The evolved interphase has a nonuniform morphology at high currents, which causes accelerated chemo-mechanical failure. This work demonstrates that the current-dependent nature of the reaction at the SSE/Li interface plays a crucial role in determining chemo-mechanical degradation mechanisms, with implications for understanding and controlling degradation in a wide variety of SSE materials with unstable interfaces.