US Patent 8927127 Square lithium secondary battery

A square lithium secondary battery includes a wound body in which a collective sheet in which a positive electrode sheet and a negative electrode sheet overlap each other with a first separator interposed therebetween is wound while a second separator is put inside the collective sheet. An active material mixture layer on one or both surfaces of at least one of the positive electrode sheet and the negative electrode sheet includes a region with a plurality of openings and a region with no opening. At least a bent portion of the collective sheet is covered with the region with the plurality of openings.