POWER STORAGE ELEMENT

Provided are an energy storage device sufficiently having both durability performance and high-rate input/output performance and also a method for producing the energy storage device. The energy storage device of the present embodiment includes a positive electrode having a positive active material layer containing an active material in the form of particles. The positive active material layer contains primary particles of the active material and secondary particles formed by aggregation of a plurality of primary particles. The proportion of primary particles relative to all particles of the active material in the positive active material layer is 5% or more and 40% or less. The method for producing an energy storage device of the present embodiment includes forming a positive active material layer from a composite containing at least secondary particles of an active material to prepare a positive electrode having the positive active material layer, and assembling an energy storage device using the prepared positive electrode. In the preparation of the positive electrode, the positive active material layer is pressed to deagglomerate some of the secondary particles into primary particles, and the proportion of primary particles relative to all particles of the active material in the positive active material layer is adjusted to 5% or more and 40% or less.