Abstract: A lithium ion secondary battery containing a positive electrode mixture layer, a separation membrane, and a negative electrode mixture layer, in this 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 different from the first solvent, and the separation membrane contains at least one resin selected from the group consisting of a resin containing, as a monomer unit, at least one monomer having a (meth)acryloyl group, and a resin containing, as a monomer unit, at least one olefin containing fluorine.