Mechanical and Thermal Failure Induced by Contact between a Li1.5Al0.5Ge1.5(PO4)3 Solid Electrolyte and Li Metal in an All Solid-State Li Cell

Chemical reactions at the solid electrolyte (SE) and Li metal interface form an interphase before electrochemical reactions occur. This study investigates the effects of the chemically formed interphase between Li metal and Li 1.5 Al 0.5 Ge 1.5 (PO 4 ) 3 (LAGP) on cell failures under various experimental conditions. LAGP forms a black interphase by chemically reacting with Li metal. The interphase comprises a stoichiometrically changed LAGP and Li-related oxides and behaves as a mixed ionic and electronic conductor with the electronic conductivity dominating. Thus, upon application of an electrical current to Li metal anode, most of the Li ions can be reduced at the SE side surface of the interphase rather than the Li metal side, causing a local volumetric increase that triggers cracks in the SE. This crack formation process continues the pulverization of SE, leading to a gradual increase in cell resistance. Under cell operating conditions, electrochemical reactions with the chemically formed interphase can lead to the mechanical deterioration of the SE, leading to cell failure. Furthermore, the chemically formed interphase between melted Li and LAGP above 200 °C induces a rigorous chemical reaction with Li that leads to a thermal runaway. The chemical stability of the SE against Li metal can strongly affect the solid-state cell’s electrical properties, mechanical integrity, and thermal stability.