NONAQUEOUS ELECTROLYTE POWER STORAGE ELEMENT AND POWER STORAGE DEVICE

A nonaqueous electrolyte energy storage device according to one aspect of the present invention includes an electrode assembly including a positive electrode containing positive active material particles, and a negative electrode, in which a void index A represented by the following Formula (1) in the positive active material particles is 5 or more, a boron element is present on at least a part of a surface of the positive active material particles, and at least a part of the electrode assembly is in a pressed state: A=S1/S2 in Formula (1), S1 is a BET specific surface area of the positive active material particles, and S2 is a specific surface area obtained when the positive active material particles are assumed to be spheres each having a diameter equal to an average particle size thereof.