Designing solid electrolytes for all-solid-state lithium batteries that can be compatible with high-voltage cathodes is challenging. Here we report that the nanocomposite Li₂B₁₂H₁₂-Al₂O₃ mechano-chemically treated for 120 h achieves an electrochemical stability window up to ∼ 3.8 V owing to the interfacial interaction via B-O bonds and a high lithium ionic conductivity, σ(Li⁺) = 2.73 × 10⁻⁵ S cm⁻¹ at 30 °C. Moreover, this electrolyte shows high compatibility with the lithium metal anode as demonstrated by stable Li plating/stripping for 300 cycles. These electrolyte properties enable the first demonstration of a 3.8 V all-solid-state battery In_1.3Li_0.3|Li₂B₁₂H₁₂-Al₂O₃ (25 vol%)|LiNi_0.8Co_0.1Mn_0.1O₂ (NCM811) operating for 120 cycles with a rate of 0.2 C and at 30 °C, with a reversible specific capacity of 77.5 mAh g⁻¹ and Coulombic efficiency of 99.8 %. The results demonstrate the potential of hydroborates as electrolyte for all-solid-state lithium batteries.