Lithium secondary batteries and nonaqueous electrolyte for use in the same

Abstract

A lithium secondary battery comprising a battery case and an electrode group is provided. The electrode group comprises a positive electrode, a negative electrode, a microporous film separator and a nonaqueous electrolyte comprising a lithium salt. The electrode group and the nonaqueous electrolyte are held in the battery case and the electrodes each comprise a current collector and an active-material layer containing an active material capable of occluding/releasing a lithium ion. The nonaqueous electrolyte comprises a difluorophosphoric acid salt in an amount of 10 ppm or more of the whole nonaqueous electrolyte. The battery case comprises a casing material in which at least part of the battery inner side comprises a sheet formed from a thermoplastic resin and with which the electrode group can be enclosed by placing the electrode group in the casing material and heat sealing layers of the thermoplastic resin to each other.

1 . (canceled) 
     
     
         2 . A lithium secondary battery comprising:
 a battery case; and   an electrode group, comprising:
 a positive electrode, 
 a negative electrode, 
 a microporous film separator interposed between the positive electrode and the negative electrode; and 
 a nonaqueous electrolyte comprising a nonaqueous solvent and a lithium salt, 
 wherein 
 the electrode group and the nonaqueous electrolyte are held in the battery case, 
 and the positive electrode and the negative electrode each comprise a current collector and, formed thereon, an active-material layer containing an active material capable of occluding/releasing a lithium ion,
 wherein 
 the nonaqueous electrolyte is a nonaqueous electrolyte which comprises a difluorophosphoric acid salt in an amount of 10 ppm or more of the whole nonaqueous electrolyte; and 
 
   the battery case comprises a casing material in which at least part of the battery inner side comprises a sheet formed from a thermoplastic resin and with which the electrode group can be enclosed by placing the electrode group in the casing material and heat sealing layers of the thermoplastic resin to each other.   
     
     
         3 . The lithium secondary battery according to  claim 2 , wherein the content of the difluorophosphoric acid salt is from 0.01% by mass to 5% by mass. 
     
     
         4 . The lithium secondary battery according to  claim 2 , wherein a proportion of the volume of the electrode group to the volume of the battery case of the lithium secondary battery which excludes any projection is 0.7 or lower. 
     
     
         5 . The lithium secondary battery according to  claim 2 , wherein the shape of the battery case is a bottomed polygonal shape, wherein a proportion of double the area S 1  of the largest face of this battery case (product of the width and height of the outer dimensions excluding the terminal parts; unit, cm 2 ) to the thickness T (unit, cm) of the battery outer shape, i.e., the value of (2×S 1 /T), is 100 or larger. 
     
     
         6 . The lithium secondary battery according to  claim 2 , which is a secondary battery having a multilayered electrode group, wherein in each of the positive electrode and the negative electrode, a ratio of the overall length L (unit, cm) of the periphery of the electrode to double the electrode area S 2  (unit, cm 2 ), i.e., (L/(2×S 2 )), is 1 or smaller. 
     
     
         7 . The lithium secondary battery according to  claim 2 , which is a secondary battery of a bottomed cylindrical shape having a wound electrode group, wherein a proportion of the length-direction dimension to the width-direction dimension in the positive electrode is from 15 to 200. 
     
     
         8 . The lithium secondary battery according to  claim 2 , wherein an overall electrode area of the positive electrode is at least 15 times the surface area of the case of the lithium secondary battery. 
     
     
         9 . The lithium secondary battery according to  claim 2 , wherein the microporous film separator has a property of closing pores upon heating and the battery case is made of a casing material in which at least part of the battery inner side comprises a sheet formed from a thermoplastic resin having a melting point higher than the pore closing initiation temperature of the microporous film separator. 
     
     
         10 . The lithium secondary battery according to  claim 2 , wherein a casing material of the battery case comprises a multilayered sheet having an aluminum layer and a thermoplastic-resin layer laminated. 
     
     
         11 . A battery assembly comprising five or more serially connected batteries which each are the lithium secondary battery according to  claim 2 . 
     
     
         12 . A battery assembly comprising two or more connected batteries which each are the lithium secondary battery according to  claim 2 , wherein the battery assembly has a cooling mechanism for keeping the batteries at or below a certain temperature. 
     
     
         13 . The lithium secondary battery according to  claim 2 , which is mounted on a vehicle and supplies power at least to the driving system of the vehicle. 
     
     
         14 . The battery assembly according to  claim 12 , which is mounted on a vehicle and supplies power at least to the driving system of the vehicle.