Nonaqueous electrolyte secondary battery separator and use thereof

Abstract

A nonaqueous electrolyte secondary battery separator, which includes a porous film containing a polyolefin-based resin as a main component, has a difference of not more than 2.5 between (a) a white index measured on a surface of the porous film which has not been irradiated with ultraviolet light having 255 W/m 2 and (b) a white index measured on the surface of the porous film which has been irradiated, for 75 hours, with the ultraviolet light having 255 W/m 2 .

1 . A nonaqueous electrolyte secondary battery separator comprising a porous film containing not less than 50% by volume of a polyolefin-based resin having a weight-average molecular weight of 5×10 5  to 15×10 6 ,
 the nonaqueous electrolyte secondary battery separator having ΔWI of not more than 2.5, ΔWI being defined by the following Equation (1):
     ΔWI=WI   1   −WI   0   Equation (1)
 
 
 where WI is a white index defined by American Standard Test Method (ASTM) E313, WI 0  is WI that is measured, by use of a spectrocolorimeter, on a surface of the porous film which has not been irradiated with ultraviolet light having 255 W/m 2 , and With is WI that is measured, by use of the spectrocolorimeter, on the surface of the porous film which has been irradiated, for 75 hours, with the ultraviolet light having 255 W/m 2 . 
 
     
     
         2 . A nonaqueous electrolyte secondary battery laminated separator comprising:
 a nonaqueous electrolyte secondary battery separator recited in  claim 1 ; and   a porous layer.   
     
     
         3 . A nonaqueous electrolyte secondary battery member comprising:
 a cathode;   a nonaqueous electrolyte secondary battery separator recited in  claim 1 ; and   an anode,   the cathode, the nonaqueous electrolyte secondary battery separator, and the anode being provided in this order.   
     
     
         4 . A nonaqueous electrolyte secondary battery member comprising:
 a cathode;   a nonaqueous electrolyte secondary battery laminated separator recited in  claim 2 ; and   an anode,   the cathode, the nonaqueous electrolyte secondary battery laminated separator, and the anode being provided in this order.   
     
     
         5 . A nonaqueous electrolyte secondary battery comprising:
 a nonaqueous electrolyte secondary battery separator recited in  claim 1 .   
     
     
         6 . A nonaqueous electrolyte secondary battery comprising:
 a nonaqueous electrolyte secondary battery laminated separator recited in  claim 2 .   
     
     
         7 . A method for producing a nonaqueous electrolyte secondary battery separator that comprises a porous film containing a polyolefin-based resin as a main component,
 the method comprising the steps of:   extruding a resin from an outlet of an extrusion machine; and   causing a pair of rollers to roll the resin so as to form the resin into a thin film,   the resin being extruded from the outlet at a temperature of not lower than 200° C. and not higher than 250° C., and   the resin being exposed to air for not longer than 3.6 seconds after being extruded from the outlet and then brought into contact with the pair of rollers,   the nonaqueous electrolyte secondary battery separator having ΔWI of not more than 2.5, ΔWI being defined by the following Equation (1):
     ΔWI=WI   1   −WI   0   Equation (1)
 
   where WI is a white index defined by American Standard Test Method (ASTM) E313; WI 0  is WI that is measured, by use of a spectrocolorimeter, on a surface of the porous film which has not been irradiated with h ultraviolet light having 2 55 W/m 2 ; and WI 1  is WI that is measured, by use of the spectrocolorimeter, on the surface of the porous film which has been irradiated, for 75 hours, with the ultraviolet light having 255 W/m 2 .