SECONDARY BATTERY

A secondary battery includes a positive electrode, a negative electrode, and an electrolytic solution. The positive electrode includes a positive electrode active material layer. The electrolytic solution includes an aqueous solvent. The positive electrode active material layer includes a positive electrode active material and a positive electrode binder. The positive electrode active material includes a lithium-containing compound which a lithium ion is to be inserted into and extracted from. The positive electrode binder includes a polymer compound. The polymer compound includes a repeating unit represented by Formula (1), a repeating unit represented by Formula (2), or both. A solubility of the polymer compound in 100 g of water is less than or equal to 1 g. A ratio of a volume density of the positive electrode active material layer to a true density of the positive electrode active material is greater than or equal to 30% and less than or equal to 70%.