SECONDARY BATTERY

This invention relates to a highly safe secondary battery. In the secondary battery of this invention, a positive electrode is formed of an oxide which adsorbs/desorbs lithium ions; a negative electrode is formed of a carbon material which adsorbs/desorbs lithium ions; and an electrolyte solution is formed of an ion liquid and a phosphoric acid ester derivative. Consequently, the secondary battery can be highly safe. Since a phosphate ester and an ion liquid are contained at the same time, high discharge capacity can be maintained even when the phosphate ester is used at a high concentration.