A dual-halide solid electrolyte, Li₃YCl₃Br₃, was synthesized using a wet-chemistry route instead of the conventional mechanical ball-milling route. Li₃YCl₃B_r3 exhibits an ion conductivity of 2.08 mS/cm and an electro- chemical stability window of 3.8 V. Additionally, an all-solid-state lithium-ion battery using Li₃YCl₃Br₃ and LiNi_0.83Co_0.11Mn_0.06O₂ (NCM811) as the cathode material achieves a capacity retention of 93% after 200 cycles at 0.3C and maintains a specific capacity of 115 mA·h/g during 2C cycling. This exceptional performance is attributed to the high oxidative stability of Li₃YCl₃Br₃ and the in-situ formation of Y₂O₃ inert protective layer on the NCM811 surface under high voltage. Consequently, the study demonstrates the feasibility of a simple, cost-effective wet-chemistry route for synthesizing multi-component halides, highlighting its potential for large-scale production of halide solid electrolytes for practical applications.