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The interphase structure and Li-ion transport across the interface are essential for high-performance composite solid electrolytes (CPEs). Tailoring the interface between rigid inorganic solid electrolyte fillers and the polymer electrolyte matrix is a promising way. A Li-ion conductive filler such as garnet Li7La3Zr2O12 (LLZO) is proposed for the high performance of CPEs, which suffered from an unavoidable insulating contamination layer and incomplete contact with the polymer matrix. In this study, a wet chemical method was applied for converting the insulation layer on the LLZO surface to LiF with the formation of oxygen vacancies by NH4F solution etching at moderate temperature. The equally dispersed in situ formed LiF in CPEs improves the electrochemical stability. Removal of resistive barriers accompanied by firm contact with the polymer matrix induced by bonding between PEO chains and oxygen vacancies forms a CPE with homogeneous fast Li-ion transport. Owing to these advantageous properties, the ionic conductivity of CPEs with etched LLZO increases from 6.45 × 10−4 S cm−1 to 1.28 × 10−3 S cm−1 at 60 °C, and the electrochemical stable window is extended. An all-solid-state battery with etched powder exhibits extraordinary cycle stability at 2C with an initial discharge capacity of 136 mA h g−1, and capacity retention remains at 90.4% after 300 cycles. [ABSTRACT FROM AUTHOR]