Battery

A battery, comprising:

• a positive electrode including a positive electrode current collector and a positive electrode active material layer provided on the positive electrode current collector,
• the positive electrode active material layer including a lithium nickel composite oxide and a lithium manganese composite oxide;
• wherein a mass ratio of (a mass of the lithium nickel composite oxide):(a mass of the lithium manganese composite oxide) in the positive electrode active material layer is in a range of from 5:5 to 10:0,
• a negative electrode;
• and
• a separator at least including a porous film,
• the porous film having a porosity ε[%] and an air permeability t[sec/100 cc] which satisfy formula of:
• t=a×Ln(ε)−4.02a+100
• and
• −1.87×1010×S4.96≦a≦−40
• where S is an area density of the positive electrode active material layer [mg/cm2] and Ln is natural logarithm.