PURPOSE: To improve the charge/discharge efficiency and cycle life by using an electrolyte added with dimethyl sulfoxide and a derivative of thiophene to an organic solvent solved with lithium salt.

CONSTITUTION: An electrolyte added with dimethyl sulfoxide to an organic solvent solved with lithium salt is used. The added quantity of dimethyl sulfoxide is set to 0.01-35vol.% against the organic solvent, and the added quantity of a derivative of thiophene is set to 0.01-10vol.% against the organic solvent. The state of the boundary surface between a lithium electrode and the electrolyte is changed, and the reaction between the solvent and lithium and the growth of a dendrite can be suppressed.