Water based lithium secondary battery

The present invention provides a water based lithium secondary battery that can inhibit deteriorations in capacity owing to charge-and-discharge operations and maintain a high capacity even after it is charged and discharged repeatedly. The water based lithium secondary battery includes a positive electrode, a negative electrode, a separator 4 sandwiched between these, and an aqueous electrolyte solution obtained by dissolving an electrolyte made of a lithium salt in a water based solvent. As the water based solvent, a pH buffer solution is employed. The buffer solution is obtained by dissolving an acid and its conjugate base's salt, a base and its conjugate acid's salt, a salt made from a weak acid and a strong base, a salt made from a weak base and a strong acid, or a salt made from a weak acid and a weak base in water.