High interfacial compatibility between electrolytes and electrodes is of great importance for the stable operation of all-solid-state batteries (ASSBs). Here, we report that the introduction of LiBH₄ into Li₂B₁₂H₁₂-5Li₂B₁₀H₁₀ improves the electrochemical window to ∼3.0 V and Li-ion conductivity to 1.0 × 10^-4 S cm^-1 at room temperature (RT). Moreover, the Li₂B₁₂H₁₂-5Li₂B₁₀H₁₀-6LiBH₄ electrolyte exhibits good compatibility with a metallic Li anode and TiS₂ cathode, allowing the stable operation of the all-solid-state In_1.3Li_0.3||TiS₂ cell for 120 cycles at 0.1 C and RT, with 117.8 mAh g^-1 of capacity and ∼100% of the coulombic efficiency. This work illustrates that a hydroborate electrolyte with high ionic conductivity and large electrochemical stability can enable the development of an ASSB with a voltage up to 2.7 V.