(Invited) Wetting Mechanism of Molten Lithium on Garnet-Type Solid Electrolytes

Abstract

Garnet-type solid electrolytes are one of key materials towards oxide-based all-solid-state batteries. Their superb electrochemical stability against lithium metal have driven ambition to employ lithium metal as large-capacity anode. However, poor contact between the garnet-type solid electrolytes and lithium metal causes large area specific resistance on lithium plating/stripping reaction, which is supposed to be one of reasons of lithium growth through the solid electrolytes. To achieve good contact, various processes have been proposed: polishing, vacuum deposition and/or interface layers.[1] Wetting the surface of garnet-type solid electrolytes with molten lithium is a simple but certain process to form dense interface between Li and garnet-type solid electrolyte. In this work, we report a mechanism of wetting of molten lithium on the garnet-type solid electrolytes. X-ray photoelectron spectroscopy of surface and interface of garnet-type solid electrolytes indicated a three-step wetting mechanism: (1) formation of Li2CO3 on garnet-type solid electrolytes, (2) reduction of Li2CO3 to form C by molten lithium, and (3) wetting of molten lithium on carbon formed on the solid electrolyte. Lithium ions are inserted to the carbon layer, which play a role of mixed ionic and electronic conductor. This mechanism can also explain the novel short-circuit mechanism without current.