Abstract: To achieve a power storage unit that can be repeatedly bent without a large decrease in charge and discharge capacity. In the flexible power storage unit, the content of a binder in an active material layer containing an active material is greater than or equal to 1 wt % and less than or equal to 10 wt %, preferably greater than or equal to 2 wt % and less than or equal to 8 wt %, and more preferably greater than or equal to 3 wt % and less than or equal to 5 wt %.

Abstract: An object is to provide an electrochemical device in which lithium deposition and reduction in battery capacity can be inhibited even when the concentration of a lithium salt in an electrolytic solution is lower than 1.0 M. Lithium deposition can be inhibited and lithium whiskers can be dissolved by applying an inversion pulse current for a short time more than once in a charging period of a secondary battery which deteriorates. By applying the inversion pulse current more than once, deterioration of a lithium-ion secondary battery due to repeated charging can be suppressed even when it is a secondary battery in which the concentration of a lithium salt in an electrolytic solution is lower than 1.0 M and therefore lithium is easily deposited.