Lithium halide electrolytes with high ion conductivity and good cathode compatibility have shown great potential for solid-state batteries. Li3YBr6, with a conductivity of 0.39 mS/cm at room temperature, synthesized by mechanical milling (BM-Li3YBr6), which can be further increased by heat treatment. The annealing parameters are tailored to obtain pure Li3YBr6 (AN-Li3YBr6) with a higher conductivity of 3.31 mS/cm by annealing the BM-Li3YBr6 at 500 °C for 5 h. The higher conductivity of AN-Li3YBr6 compared to the previously-reported results is due to the lower activation energy. NMR and simulation results show that the lithium ion migration between Li-1 and Li-2 sites along the [001] direction is the major obstacle for lithium diffusion in AN-Li3YBr6. The K- and L3-edge X-ray absorption near-edge structure (XANES) of Y for BM-Li3YBr6 and AN-Li3YBr6 showed that Y exists with similar local structures. The increased vibrations of AN-Li3YBr6 due to increased temperatures increase the rate of lithium jumping from one site to another, yielding higher lithium ion mobility. Lithium nuclear density maps prove that the mobile lithium on the 4g(Li) site is more sensitive to the varying temperatures. Both BM- and AN-Li3YBr6 are incompatible with Li, however, an annealing process can improve the electrochemical stability. Both the experimental and simulation results confirm the anode incompatibility between In and AN-Li3YBr6. To mitigate the cathode and anode incompatibility with AN-Li3YBr6, a LiNbO3 coating layer and a Li5.7PS4.7Cl1.3 buffer layer are introduced at the cathode side and anode side, respectively, to assemble all-solid-state batteries with improved capacity and cyclability.@en