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

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_6.4La₃Zr_1.4Ta_0.6O₁₂ (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, LiFePO₄/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.