Surface Coordination of Garnet Fillers Improves the Organic–Inorganic Interfacial Compatibility of Composite Solid Electrolyte

Abstract Composite solid electrolytes (CSEs) have become one of the most promising solid‐state electrolytes due to their favorable safety and flexibility. However, the weak interaction between inorganic fillers and polymer matrix leads to poor organic–inorganic interfacial compatibility, which degrades the electrochemical performance of CSEs. Herein, it is demonstrated that Li 6.4 La 3 Zr 1.4 Ta 0.6 O 12 (LLZTO) can be chemically bonded to the polymer matrix by surface coordination of the 1,2‐dithiolane group of lipoic acid (LA) with metal atoms on the surface of LLZTO through a combination of experimental investigations and theoretical calculations. The surface coordination not only enhances the interfacial compatibility between LLZTO and the polymer matrix, but also facilitates rapid Li + transport, which leads to the ionic conductivity of the prepared CSE (P‐V‐M@LLZTO) as high as 6.1 × 10 −4 S cm −1 at 30 °C. The excellent interface compatibility ensures a stable cycle of Li/P‐V‐M@LLZTO/Li symmetrical cell for more than 3500 h. As a result, LiFePO 4 /P‐V‐M@LLZTO/Li cell delivers the discharge capacity of 161 mAh g −1 after 5 cycles with a capacity retention of 81% after 500 cycles at 0.5C under 30 °C. This work demonstrates that surface coordination is an effective strategy to solve the inherent interfacial incompatibility problem in CSEs.