A control valve 11 (safety valve) is arranged in the sealing plate 10. If the internal pressure of the battery cell is raised by a gas generated by electrolysis of the aqueous solvent, the generated gas can be released from the control valve 11 to the outside. As the control valve 11, for example, a return type valve that operates when the internal pressure exceeds a set value and functions as a sealing plug when the internal pressure lowers can be used. Alternatively, a non-return type valve that cannot recover the function as a sealing plug once it operates can be used. In
Additionally, a liquid pouring port 12 is provided in the sealing plate 10. The aqueous electrolyte can be poured via the liquid pouring port 12. The liquid pouring port 12 can be closed by a sealing plug 13 after the aqueous electrolyte is poured. The liquid pouring port 12 and the sealing plug 13 may be omitted.