To provide an electrolyte easily manufactured at low cost, and a power storage device including such an electrolyte. The power storage device includes a positive electrode having a positive electrode current collector and a positive electrode active material, a negative electrode having a negative electrode current collector and a negative electrode active material, and an electrolyte having 1-piperidine-1-propanesulfonic acid or 1-piperidine-1-butanesulfonic acid, which is provided between the positive electrode and the negative electrode. The capacitance can be increased when water is added to the obtained electrolyte and the temperature of the power storage device rises.