Abstract: In this study, composite solid-state electrolytes (CSEs) were fabricated by incorporating 20%, 50%, and 80% of Li_1.3Al_0.3Ti_1.7(PO₄)₃ <LATP> solid electrolyte powder into a poly (ethylene oxide) (PEO)-based solid polymer electrolyte (SPE) matrix via a solution casting method. The microstructural characteristics, ionic conductivity, Young’s modulus, and interfacial electrochemical stability with lithium metal anodes of the LATP/SPE composite electrolytes were systematically investigated. Experimental results demonstrated that the CSE containing 50% LATP exhibited optimal electrochemical performance, achieving stable cycling for 1 600 h at a current density of 0.5 mA/cm². This performance significantly surpassed that of composites with 0, 20%, and 80% LATP additions. Furthermore, the interfacial failure mechanisms between lithium metal and CSEs with varying LATP contents were thoroughly elucidated.