LITHIUM ION SECONDARY BATTERY AND SEPARATION MEMBRANE

One aspect of the present invention provides a lithium ion secondary battery which sequentially comprises a positive electrode mixture layer, a separation membrane, and a negative electrode mixture layer in the stated order, wherein: the positive electrode mixture layer contains a positive electrode active material, a first lithium salt, and a first solvent; the negative electrode mixture layer contains a negative electrode active material, a second lithium salt, and a second solvent that is different from the first solvent; and the separation membrane contains a third lithium salt and a polymer that comprises, as a monomer unit, a monomer represented by formula (1).[wherein, in formula (1), R1 represents a hydrogen atom or a methyl group, R2 represents a divalent organic group, R3, R4 and R5 each independently represents a hydrogen atom or a monovalent organic group, and X− represents a counter anion.]