SEPARATOR, ELECTRODE GROUP, SECONDARY BATTERY, BATTERY PACK, VEHICLE, AND STATIONARY POWER SUPPLY

According to one embodiment, a separator (4) is provided. The separator (4) includes a composite membrane (53). The composite membrane (53) includes a substrate layer (530), a first composite layer (531), and a second composite layer (532). The first composite layer (531) is located on one surface (S1) of the substrate layer (530). The second composite layer (531) is located on the other surface (S2) of the substrate layer (530). The composite membrane (53) has a coefficient of air permeability of 1 × 10-14 m2 or less. The first composite layer (531) has a first surface (FSC1) and a second surface (SSC1). The first surface (FSC1) is in contact with the substrate layer (530). The second surface (SSC1) is located on an opposite side to the first surface (FSci) . Denseness of a portion including the first surface (FSC1) is lower than denseness of a portion including the second surface (SSC1) in the first composite layer (531).