Interfacial Chemistry Enables Stable Cycling of All-Solid-State Li Metal Batteries at High Current Densities

The application of flexible, robust, and low-cost solid polymer electrolytes in next-generation all-solid-state lithium metal batteries has been hindered by the low room-temperature ionic conductivity of these electrolytes and the small critical current density of the batteries. Both issues stem from the low mobility of Li⁺ ions in the polymer and the fast lithium dendrite growth at the Li metal/electrolyte interface. Herein, Mg(ClO₄)₂ is demonstrated to be an effective additive in the poly(ethylene oxide) (PEO)-based composite electrolyte to regulate Li⁺ ion transport and manipulate the Li metal/electrolyte interfacial performance. By combining experimental and computational studies, we show that Mg²⁺ ions are immobile in a PEO host due to coordination with ether oxygen and anions of lithium salts, which enhances the mobility of Li⁺ ions; more importantly, an in-situ formed Li⁺-conducting Li₂MgCl₄/LiF interfacial layer homogenizes the Li⁺ flux during plating and increases the critical current density up to a record 2 mA cm^-2. Each of these factors contributes to the assembly of competitive all-solid-state Li/Li, LiFePO₄/Li, and LiNi_0.8Mn_0.1Co_0.1O₂/Li cells, demonstrating the importance of surface chemistry and interfacial engineering in the design of all-solid-state Li metal batteries for high-current-density applications.