Ionic conductivity and electro/chemical compatibility of Li₁₀SnP₂S₁₂ electrolytes play crucial roles in achieving superior electrochemical performances of the corresponding solid-state batteries. However, the relatively low Li-ion conductivity and poor stability of Li₁₀SnP₂S₁₂ toward high-voltage layered oxide cathodes limit its applications. Here, a Br-substituted strategy has been applied to promote Li-ion conductivity. The optimal composition of Li_9.9SnP₂S_11.9Br_0.1 delivers high conductivity up to 6.0 mS cm⁻¹. ⁷Li static spin-lattice relaxation (T₁) nuclear magnetic resonance (NMR) and density functional theory simulation are combined to unravel the improvement of Li-ion diffusion mechanism for the modified electrolytes. To mitigate the interfacial stability between the Li_9.9SnP₂S_11.9Br_0.1 electrolyte and the bare LiNi_0.7Co_0.1Mn_0.2O₂ cathode, introducing Li₂ZrO₃ coating layer and Li₃InCl₆ isolating layer strategies has been employed to fabricate all-solid-state lithium batteries with excellent electrochemical performances. The Li₃InCl₆-LiNi_0.7Co_0.1Mn_0.2O₂/Li₃InCl₆/Li_9.9SnP₂S_11.9Br_0.1/Li-In battery delivers much higher discharge capacities and fast capacity degradations at different charge/discharge C rates, while the Li₂ZrO₃@LiNi_0.7Co_0.1Mn_0.2O₂/Li_9.9SnP₂S_11.9Br_0.1/Li-In battery shows slightly lower discharge capacities at the same C rates and superior cycling performances. Multiple characterization methods are conducted to reveal the differences of battery performance. The poor electrochemical performance of the latter battery configuration is associated with the interfacial instability between the Li₃InCl₆ electrolyte and the Li_9.9SnP₂S_11.9Br_0.1 electrolyte. This work offers an effective strategy to constructing Li₁₀SnP₂S₁₂-based all-solid-state lithium batteries with high capacities and superior cyclabilities.