Nonaqueous electrolyte secondary battery separator

A nonaqueous electrolyte secondary battery separator comprising a polyolefin porous film,

• the nonaqueous electrolyte secondary battery separator having ion permeability barrier energy of not less than 300 J/mol/μm and not more than 900 J/mol/μm per unit film thickness,
• wherein the nonaqueous electrolyte secondary battery separator causes an electrode resistance increasing rate after 100 charge-discharge cycles of a nonaqueous electrolyte secondary battery containing the nonaqueous electrolyte secondary battery separator to be lower than an electrode resistance increasing rate after 100 charge-discharge cycles of a nonaqueous electrolyte secondary battery containing a reference nonaqueous electrolyte secondary battery separator comprising a polyolefin porous film and having an ion permeability battery energy of less than 300 J/mol/μm or more than 900 J/mol/μm per unit film thickness,
• wherein the electrode resistance increasing rate after 100 charge-discharge cycles is measured by subjecting the nonaqueous electrolyte secondary battery to 100 charge-discharge cycles each performed at 55° C. at a voltage ranging from 2.7 V to 4.2 V with CC-CV charge at a charge current value of 1 C, terminal current condition 0.02 C and with CC discharge at a discharge current value of 10 C,
• wherein CC-CV is a charging method comprising charging the battery at a constant electric current to a certain voltage and maintaining the certain voltage while the electric current is reduced;
• and
• wherein CC discharge is a discharging method comprising discharging the battery at a constant electric current until the certain voltage is reached.