This lithium ion secondary battery (1) has a negative electrode (12), a positive electrode (10), an aqueous electrolyte (16) containing a lithium salt, and a separation layer (14) arranged between the negative electrode (12) and the positive electrode (10). The aqueous electrolyte (16) is in contact with the positive electrode (10), and the separation layer (14) includes a non-aqueous solid electrolyte A in which a matrix polymer, the lithium salt and a hydrophobic ionic liquid are compounded.