Mechanism of Lithium Metal Penetration through Inorganic Solid Electrolytes

Li deposition is observed and measured on a solid electrolyte in the vicinity of a metallic current collector. Four types of ion‐conducting, inorganic solid electrolytes are tested: Amorphous 70/30 mol% Li₂S‐P₂S₅, polycrystalline β‐Li₃PS₄, and polycrystalline and single‐crystalline Li₆La₃ZrTaO₁₂ garnet. The nature of lithium plating depends on the proximity of the current collector to defects such as surface cracks and on the current density. Lithium plating penetrates/infiltrates at defects, but only above a critical current density. Eventually, infiltration results in a short circuit between the current collector and the Li‐source (anode). These results do not depend on the electrolytes shear modulus and are thus not consistent with the Monroe–Newman model for “dendrites.” The observations suggest that Li‐plating in pre‐existing flaws produces crack‐tip stresses which drive crack propagation, and an electrochemomechanical model of plating‐induced Li infiltration is proposed. Lithium short‐circuits through solid electrolytes occurs through a fundamentally different process than through liquid electrolytes. The onset of Li infiltration depends on solid‐state electrolyte surface morphology, in particular the defect size and density.