NOVELTY - An aqueous lithium secondary battery (1) consists of a positive electrode (2), a negative electrode (3), and an aqueous electrolyte solution (5) formed by dissolving an electrolyte containing lithium salt in an aqueous medium. The fluctuation in pH of the electrolyte solution after repeating 20 cycles of charging/discharging at 20 degrees C, is controlled within plus minus 0.5.    USE - Aqueous lithium secondary battery is used in information communication apparatuses, such as personal computers and mobile telephones, and as power supply for electric vehicles and hybrid electric vehicles.    ADVANTAGE - The aqueous lithium secondary battery has suppressed deterioration in capacitance upon charging/discharging, and exhibits high capacitance even after repetition of charge/discharge.    DETAILED DESCRIPTION - An aqueous lithium secondary battery consists of a positive electrode, a negative electrode, and an aqueous electrolyte solution formed by dissolving an electrolyte containing lithium salt in an aqueous medium. The fluctuation in pH of the electrolyte solution after repeating 20 cycles of charging/discharging at 20 degrees C, is controlled within plus minus 0.5. A buffer solution obtained by dissolving an acid, salt of conjugate base, salt and base and its conjugate acid, salt of weak acid and strong base, salt of weak base and strong acid, or salt of weak acid and weak base in water, is used as the aqueous medium. The buffer solution contains potassium acetate and acetic acid in a molar ratio of 7-10:3-0. The lithium salt is lithium nitrate. The positive electrode contains a compound having olivine structure with lithium-iron phosphate as basic composition, as active material. The negative electrode contains a compound having spinel structure with lithium vanadate as basic composition, as active material.    DESCRIPTION OF DRAWING(S) - The drawing shows an explanatory view of the aqueous lithium secondary battery.    Aqueous lithium secondary battery (1)    Positive electrode (2)    Negative electrode (3)    Separator (4)    Aqueous electrolyte solution (5)