Mastering the interface for advanced all-solid-state lithium rechargeable batteries

A solid electrolyte with a high Li-ion conductivity and a small interfacial resistance against a Li metal anode is a key component in all-solid-state Li metal batteries, but there is no ceramic oxide electrolyte available for this application except the thin-film Li-P oxynitride electrolyte; ceramic electrolytes are either easily reduced by Li metal or penetrated by Li dendrites in a short time. Here, we introduce a solid electrolyte LiZr₂(PO4)₃ with rhombohedral structure at room temperature that has a bulk Li-ion conductivity σ_Li = 2 × 10^-4 S·cm^-1 at 25°C, a high electrochemical stability up to 5.5 V versus Li⁺/Li, and a small interfacial resistance for Li⁺ transfer. It reacts with a metallic lithium anode to form a Li⁺-conducting passivation layer (solid-electrolyte interphase) containing Li₃P and Li₈ZrO₆ that is wet by the lithium anode and also wets the LiZr₂(PO₄)₃ electrolyte. An all-solid-state Li/LiFePO₄ cell with a polymer catholyte shows good cyclability and a long cycle life.