Abstract: A lithium-ion secondary battery includes: a single cell that includes a first electrode, a separator stacked on the first electrode, and a second electrode stacked on the separator. The first electrode includes a porous body that includes at least one LLZ-based solid electrolyte of a lithium lanthanum zirconate or the lithium lanthanum zirconate doped with an atom other than a Li atom, a La atom, and a Zr atom, and has a pore, and an active material held in the pore. The separator has a relative density of 80% or more, and includes the at least one LLZ-based solid electrolyte of the lithium lanthanum zirconate or the lithium lanthanum zirconate doped with the atom other than the Li atom, the La atom, and the Zr atom, and at least one of a B atom, a P atom, or a Si atom.