METHOD FOR PRODUCING NONAQUEOUS ELECTROLYTE SECONDARY BATTERY SEPARATOR

Provided is a nonaqueous electrolyte secondary battery including a porous film containing polyolefin as a main component, the nonaqueous electrolyte secondary battery separator having a parameter X of not more than 20, the parameter X being calculated based on the following equation: X=100×|MD tan δ−TD tan δ|/{(MD tan δ+TD tan δ)/2}, where MD tan δ is tan δ in a machine direction of the porous film and TD tan δ is tan δ in a transverse direction of the porous film, MD tan δ and TD tan δ each being obtained by viscoelasticity measurement carried out with respect to the porous film at a frequency of 10 Hz and a temperature of 90° C. the nonaqueous electrolyte secondary battery separator making it possible to reduce an increase in internal resistance which increase is caused by repeated charge and discharge.