Lithium ion secondary battery, separator, battery pack and charging method

Abstract

A lithium ion secondary battery including a positive electrode, a negative electrode, a separator and a nonaqueous electrolyte, where the separator essentially includes a porous sheet. The positive electrode active material and the negative electrode active material can be reversibly doped and dedoped such that, where Qp (mAh) is an electric charge necessary for causing total lithium contained in the positive electrode to be dedoped and Qn (mAh) is an electric charge necessary for causing lithium to fully dope the negative electrode, Qp>Qn. Further, when the battery is charged at a charging current Ic (mA) in a range of 0.2 Qn/h<Ic<2 Qn/h, in a range of an electric charge for charging Qc (mAh) of 1<Qc/Qn<Qp/Qn, doping of the positive electrode by lithium is initiated by producing lithium particles on the negative electrode by charging of the battery until Qc>Qp.

1 - 21 . (canceled)  
     
     
         22 . A lithium ion secondary battery separator comprising a sheet (A) with an average film thickness of 10-35 μm, a basis weight of 6-20 g/m 2 , a gas permeability (JIS P8117) of no longer than 100 seconds, a MacMullin number of 10 or smaller at 25° C. and a MacMullin number×average film thickness value of no greater than 200 μm.  
     
     
         23 . A separator according to  claim 22 , wherein the sheet is composed of fibers and the average diameter of the fibers composing the sheet is ½ to {fraction (1/10)} of the average film thickness of the sheet (A).  
     
     
         24 . A separator according to  claim 23 , wherein the sheet (A) is a nonwoven fabric.  
     
     
         25 . A separator according to  claim 22 , wherein the sheet (A) is composed of at least one of a polyester, an aromatic polyamide, a polyphenylene sulfide and a polyolefin.  
     
     
         26 . A lithium ion secondary battery separator comprising a porous film with an average film thickness of 10-35 μm and a basis weight of 10-25 g/m 2 , which contains a porous organic polymer film (B) that surrounds a sheet (A) according to  claim 22 , and is swelled with the electrolyte solution and retains it.  
     
     
         27 . A separator according to  claim 26 , wherein the porous organic polymer film (B) is composed mainly of polyvinylidene fluoride (PVdF).  
     
     
         28 . A separator according to  claim 27 , wherein the porous organic polymer film (B) is composed of a polyvinylidene fluoride copolymer containing 92-98% mole percent of vinylidene fluoride (VdF).  
     
     
         29 . A separator according to  claim 28 , wherein the porous organic polymer film (B) is composed of a terpolymer of vinylidene fluoride, hexafluoropropylene (HFP) and chlorotrifluoroethylene (CTFE).  
     
     
         30 . A separator according to  claim 29 , wherein the copolymer composition of the terpolymer is VdF/HFP(a)/CTFE(b), where (a)=2-8 wt % and (b)=1-6 wt %.