Patent ID: 8354188

Claim:
A lithium ion secondary battery comprising a positive electrode, a negative electrode, a separator disposed between said positive electrode and said negative electrode and an electrolyte solution, wherein; a polymer is disposed between said positive electrode or said negative electrode and said separator, said polymer being obtained by cationic polymerization of a first polymer: comprising (A) a monomer unit having cationic polymerization ability, (B) a monomer unit imparting affinity to said electrolyte solution, (C) a monomer unit imparting poor solubility in said electrolyte solution and (D) a monomer unit containing an anionic or nonionic hydrophilic group; being obtained by radical polymerization based on emulsion polymerization or suspension polymerization; and having an elution rate of 10% by weight or less in a mixture solvent of ethylene carbonate (EC) and diethyl carbonate (DEC) having a weight ratio of EC:DEC=5:5, the elution rate being defined by the following equation: Elution rate (wt %)=(Weight of the first polymer eluted in the mixture solvent/Weight of the total first polymer)×100.