To provide a power storage device having a high energy density, rarely suffering an internal short circuit under low-temperature environments, having stable cycle characteristics under high-temperature environments, and capable of offering a high battery voltage.

The power storage device at least includes a positive electrode and a negative electrode, the positive electrode contains a positive electrode active material represented by the formula: Li_xMn_yO_z, where 1≤x≤2.2, 1≤y≤2, and 2≤z≤5, and the negative electrode contains a negative electrode active material composed of an oxide material at least containing Sn, and P and/or B as constituent elements.