Li-ion conductive polycrystalline Li₄B₇O₁₂Cl to be used as a solid-state electrolyte was successfully synthesized by an ionothermal route at a low temperature. The reaction of lithium acetate dihydrate, boric acid, and copper(II) chloride dihydrate in trihexyltetradecylphosphonium chloride at 130 °C led to single-phase Li₄B₇O₁₂Cl within 10 h. Li₄B₇O₁₂Br was also successfully synthesized in a similar way. The ionic conductivity, activation energy, and electrochemical window of Li₄B₇O₁₂Cl electrolyte were measured. A relatively high ionic conductivity of 3 × 10^-4 S cm^-1 at room temperature was detected. A linear sweep voltammogram of the Li₄B₇O₁₂Cl electrolyte exhibited an electrochemical stability of up to 4.3 V vs. Li/Li⁺ under the measured conditions. The fabricated {Li ∥ Li₄B₇O₁₂Cl ∥ Li} symmetrical cell exhibits a relatively low voltage hysteresis of 120 mV for 60 cycles.