Provided is a heat-sensitive layer for a lithium ion secondary battery that can efficiently block rising temperature during abnormal heating by increasing battery internal resistance and thereby reducing current. The heat-sensitive layer for a lithium ion secondary battery is formed from a heat-sensitive layer composition containing particles in each of which a second component is substantially partially disposed at the outside of a particle formed from a first component. The heat-sensitive layer composition has a storage modulus at 60° C. of 10 kPa or more and a storage modulus at 150° C. of 1 kPa or less.