Separator for power storage device and power storage device

A separator for a power storage device, comprising a composite material which comprises inorganic microparticles and an organic binder, wherein

• (a) the composite material has a pigment volume concentration (PVC) which is determined by the following formula of 55% or more,
• PVC=(Volume of Inorganic Microparticles)÷(Volume of Inorganic Microparticles+Volume of Organic Binder)×100 (1)
• in which the Volume of Inorganic Microparticles is the weight of the inorganic microparticles divided by the density of the inorganic microparticles, and the Volume of Organic Binder is the weight of the organic binder divided by the density of the organic binder;
• and
• (b) the inorganic microparticles have an average particle size in a range of 0.2 to 3.0 μm, and a general particle shape index in a range of 0.50 to 0.85, wherein the general particle shape index is the product of the degree of circularity for the inorganic microparticles of the formula (2), and the degree of smoothness for the inorganic microparticles of formula (3),
• Degree of Circularity=Equivalent Circle Diameter÷Equivalent Ellipse Major Axis (2)
• Degree of Smoothness=Equivalent Ellipse Outer Perimeter÷Outer Perimeter (3).