A lithium ion secondary battery includes a positive electrode, a negative electrode, and an electrolyte disposed between the positive electrode and the negative electrode. The electrolyte includes a polymeric electrolyte. The polymeric electrolyte is composed of a polymer and a lithium salt of a fluorosulfonyl-group-containing compound, and the polymer is a polymer of a vinylene carbonate species. The lithium salt of a fluorosulfonyl-group-containing compound includes lithium bis(fluorosulfonyl)imide.