NONAQUEOUS ELECTROLYTE POWER STORAGE ELEMENT, AND POWER STORAGE DEVICE

An aspect of the present invention is a nonaqueous electrolyte energy storage device including: a positive electrode having a positive composite layer including a positive active material; a negative electrode having a negative composite layer including a negative active material; and a nonaqueous electrolyte including a nonaqueous solvent, in which the positive active material includes a lithium-transition metal composite oxide that contains nickel as a transition metal and has a layered α-NaFeO2-type crystal structure, a ratio (N/P) between mass (N) per unit area of the negative active material and mass (P) per unit area of the positive active material is 0.30 or more and 0.45 or less, the nonaqueous solvent contains PC, DEC, and EMC, a content of the PC in the nonaqueous solvent is 25% by volume or more and 55% by volume or less, a total content of the DEC and the EMC in the nonaqueous solvent is 45% by volume or more and 75% by volume or less, and a content of the DEC is equal to or higher than the content of the EMC.