Battery, battery separator and method for producing battery separator

Abstract

A battery includes a positive electrode, a negative electrode, a separator interposed therebetween, and an electrolyte. The separator includes a plurality of nanofibers and has a form of a sheet having a first surface and a second surface opposite thereto. When the average maximum fiber diameter of the nanofibers in the plane direction of the separator is compared between in vicinities of the first and second surfaces and at a center portion in the thickness direction of the separator, average maximum fiber diameters Ds 1 and Ds 2 of the nanofibers in the vicinity of the first and second surfaces are smaller than an average maximum fiber diameter Dc of the nanofibers at the center portion in the thickness direction of the separator.

1 . A nanofiber sheet comprising a plurality of nanofibers and having a form of a sheet having a first surface and a second surface opposite to the first surface, and
 when an average maximum fiber diameter of the plurality of nanofibers in a plane direction of the nanofiber sheet is compared between in vicinities of the first surface and the second surface and at a center portion in a thickness direction of the nanofiber sheet, an average maximum fiber diameter Ds 1  of the plurality of nanofibers in the vicinity of the first surface and an average maximum fiber diameter Ds 2  of the plurality of nanofibers in the vicinity of the second surface being smaller than an average maximum fiber diameter Dc of the plurality of nanofibers at the center portion in the thickness direction of the nanofiber sheet.   
     
     
         2 . The nanofiber sheet according to  claim 1 , wherein relationships 1.1≦Dc/Ds 1 ≦2.5 and 1.1≦Dc/Ds 2 ≦2.5 are satisfied. 
     
     
         3 . The nanofiber sheet according to  claim 1 , wherein the nanofiber sheet has an average porosity P of 50 to 95%. 
     
     
         4 . The nanofiber sheet according to  claim 1 , wherein the average maximum fiber diameter Dc is 60 nm to 2 μm. 
     
     
         5 . The nanofiber sheet according to  claim 1 , wherein the average maximum fiber diameter Ds 1  and the average maximum fiber diameter Ds 2  satisfy 0.9≦Ds 1 /Ds 2 ≦1.1, and a porosity Ps 1  of the nanofiber sheet in the vicinity of the first surface and a porosity Ps 2  of the nanofiber sheet in the vicinity of the second surface satisfy 0.9≦Ps 1 /Ps 2 ≦1.1. 
     
     
         6 . The nanofiber sheet according to  claim 1 , wherein provided that the nanofiber sheet is divided into a first region including the first surface and a second region including the second surface, with the center in the thickness direction of the nanofiber sheet being a boundary between the first region and the second region, the first region and the second region are substantially symmetric in structure with respect to the boundary. 
     
     
         7 . The nanofiber sheet according to  claim 1 , wherein the plurality of nanofibers are randomly welded to one another. 
     
     
         8 . The nanofiber sheet according to  claim 1 , wherein the plurality of nanofibers include at least one selected from the group consisting of olefin resins, fluorocarbon resins, polyamide resins, and polyimide resins. 
     
     
         9 . The nanofiber sheet according to  claim 1 , wherein the nanofiber sheet has a matrix structure of the plurality of nanofibers formed by electrospinning. 
     
     
         10 . A battery separator comprising a plurality of nanofibers and having a form of a sheet having a first surface and a second surface opposite to the first surface,
 when an average maximum fiber diameter of the plurality of nanofibers in a plane direction of the separator is compared between in vicinities of the first surface and the second surface and at a center portion in a thickness direction of the separator, an average maximum fiber diameter Ds 1  of the plurality of nanofibers in the vicinity of the first surface and an average maximum fiber diameter Ds 2  of the plurality of nanofibers in the vicinity of the second surface being smaller than an average maximum fiber diameter Dc of the plurality of nanofibers at the center portion in the thickness direction of the separator.   
     
     
         11 . A battery comprising a positive electrode, a negative electrode, a separator interposed between the positive electrode and the negative electrode, and an electrolyte,
 the separator comprising a plurality of nanofibers and having a form of a sheet having a first surface and a second surface opposite to the first surface,   when an average maximum fiber diameter of the plurality of nanofibers in a plane direction of the separator is compared between in vicinities of the first surface and the second surface and at a center portion in a thickness direction of the separator, an average maximum fiber diameter Ds 1  of the plurality of nanofibers in the vicinity of the first surface and an average maximum fiber diameter Ds 2  of the plurality of nanofibers in the vicinity of the second surface being smaller than an average maximum fiber diameter De of the plurality of nanofibers at the center portion in the thickness direction of the separator.   
     
     
         12 . The battery according to  claim 11 , wherein
 the positive electrode includes a positive electrode active material capable of absorbing and releasing lithium ions,   the negative electrode includes a negative electrode active material capable of absorbing and releasing lithium ions, and   the electrolyte is a non-aqueous electrolyte with lithium ion conductivity.