NOVELTY - A high-temperature electrolyte comprises a mixed lithium salt solution prepared by mixing electrolyte salt with nonaqueous organic solvent, a high-temperature film-forming additive and a high-temperature inhibiting gas-production additive mixed in the mixed lithium salt solution. The high-temperature film-forming additive is lithium borate dioxalate. The content of lithium borate dioxalate with respect to the mixed lithium salt solution is 0.2-3 %mass. The high-temperature inhibiting gas-production additive is ionic liquid and phthalic anhydride.    USE - High-temperature electrolyte is used for lithium-ion battery e.g. square soft package lithium-ion battery (both claimed).    ADVANTAGE - The high-temperature electrolyte is prepared using simple method. The lithium-ion battery formed using the electrolyte has excellent comprehensive property.    DETAILED DESCRIPTION - A high-temperature electrolyte comprises a mixed lithium salt solution prepared by mixing electrolyte salt with nonaqueous organic solvent, a high-temperature film-forming additive and a high-temperature inhibiting gas-production additive mixed in the mixed lithium salt solution. The high-temperature film-forming additive is lithium borate dioxalate. The content of lithium borate dioxalate with respect to the mixed lithium salt solution is 0.2-3 %mass. The high-temperature inhibiting gas-production additive is ionic liquid and phthalic anhydride. The content of high-temperature inhibiting gas-production additive with respect to the mixed lithium salt solution is 0.8-0.9 %mass. The ionic liquid is imidazole-containing ionic liquid of formula (I). The ionic liquid is 1-ethyl-3-methylimidazolium-bis(trifluoromethane sulfonyl)imide (EMI-TFSI) ionic liquid, 3-methyl-1-propylimidazolium-bis(trifluoromethane sulfonyl)imide (PMI-TFSI) ionic liquid or 1-butyl-3-methylimidazolium-bis(trifluoromethane sulfonyl)imide (BMI-TFSI) ionic liquid. The total content of high-temperature film-forming additive and high-temperature inhibiting gas-production additive with respect to the mixed lithium salt solution is 1-11 %mass.    R=CH2CH3, CH2CH2CH3 or CH2CH2CH2CH3.    An INDEPENDENT CLAIM is included for lithium-ion battery, which comprises a positive electrode, a negative electrode, a diaphragm, and a high-temperature lithium-ion battery electrolyte.