ENERGY STORAGE DEVICE

One aspect of the present invention is an energy storage device satisfying the following formula 1, in which a flat electrode assembly obtained by winding a positive electrode and a negative electrode with a separator interposed therebetween and having two curved surface portions facing each other, and a case that houses the electrode assembly, and at least one of the positive electrode and the negative electrode includes an active material layer containing active material particles and a fibrous conductive agent. In the formula 1, X is a distance from a distal end of one of the curved surface portions to the inner surface of the case facing the distal end of the curved surface portion as viewed in the winding axis direction of the electrode assembly. R is a length of a periphery of the one curved surface portion as viewed in the winding axis direction of the electrode assembly. D is an average particle size D50 of the active material particles. A is a surface roughness Ra of the active material layer.(R/D)A≥2X 1