Provided is a lithium ion secondary battery that has improved cycle characteristics and employs a silicon material as a negative electrode active material. The lithium ion secondary battery according to the present invention comprises a negative electrode comprising at least a copolymer and a material comprising silicon as a constituent element, wherein the copolymer comprises a monomer unit based on an ethylenically unsaturated carboxylic acid alkali metal salt and a monomer unit based on an aromatic vinyl and the copolymer comprises an alkali metal constituting the alkali metal salt in an amount of 1000 mass ppm or more.