Laminated separator including inorganic particle and polyolefin layer for electricity storage device

1. A laminated separator comprising

a first polyolefin microporous layer and

a second polyolefin microporous layer which is laminated on the first polyolefin microporous layer and which is different from the first polyolefin microporous layer,

wherein at least one of the first polyolefin microporous layer and the second polyolefin microporous layer comprises

an inorganic filler (i) having a primary particle size of 10 nm or more and 80 nm or less, (ii) having a concentration of more than 60% by mass and 95% by mass or less in a total amount of the polyolefin and the inorganic filler in the at least one of the first polyolefin microporous layer and the second polyolefin microporous layer, and (iii) comprising silica and/or alumina.

2. The laminated separator according to claim 1 ,

wherein a difference between concentration C1 of the inorganic filler in a total amount of the polyolefin and the inorganic filler in the first polyolefin microporous layer and concentration C2 of the inorganic filler in a total amount of the polyolefin and the inorganic filler in the second polyolefin microporous layer is 10% by mass or more and 95% by mass or less.

3. The laminated separator according to claim 1 , wherein the second polyolefin microporous layer consists of a polyolefin resin.

4. The laminated separator according to claim 1 , wherein the primary particle size is 13 nm or more and 80 nm or less.

5. The laminated separator according to claim 1 , wherein the thickness of at least one of the first and second microporous layers has a thickness of 0.5 μm to 15 μm.

6. The laminated separator according to claim 1 , wherein the primary particle size is 10 nm or more and 63 nm or less.

7. The laminated separator according to claim 1 , wherein the polyolefin is selected from the group consisting of polyethylene, polypropylene, polybutene, ethylene-propylene rubber, propylene-ethylene copolymer, and propylene-α-olefin copolymer.

8. The laminated separator according to claim 1 , wherein the polyolefin comprises polyethylene and/or polypropylene.

9. The laminated separator according to claim 1 , wherein

a difference between concentration C1 of the inorganic filler in a total amount of the polyolefin and the inorganic filler in the first polyolefin microporous layer and concentration C2 of the inorganic filler in a total amount of the polyolefin and the inorganic filler in the second polyolefin microporous layer is 60% by mass or more and 95% by mass or less.

10. The laminated separator according to claim 1 , wherein

the laminated separator has a two-type three-layer structure comprising the first polyolefin microporous layers as surface layers and the second polyolefin microporous layer as an intermediate layer,

a difference between the concentration of C1 of the inorganic filler in a total amount of the polyolefin resin and the inorganic filler in the first polyolefin microporous layer and concentration C2 of the inorganic filler in a total amount of the polyolefin resin and the inorganic filler in the second polyolefin microporous layer is 10% by mass or more and 95% by mass or less, and

concentration C1 is more than 60% by mass and 95% by mass or less.

11. The laminated separator according to claim 1 , wherein

the laminated separator has a two-type three-layer structure comprising the second polyolefin microporous layers as surface layers and the first polyolefin microporous layer as an intermediate layer,

a difference between the concentration of C1 of the inorganic filler in a total amount of the polyolefin resin and the inorganic filler in the first polyolefin microporous layer and concentration C2 of the inorganic filler in a total amount of the polyolefin resin and the inorganic filler in the second polyolefin microporous layer is 10% by mass or more and 95% by mass or less, and

concentration C1 is more than 60% by mass and 95% by mass or less.

12. The laminated separator according to claim 1 , wherein

the laminated separator has a two-type three-layer structure comprising the first polyolefin microporous layers as surface layers and the second polyolefin microporous layer as an intermediate layer,

a difference between the concentration of C1 of the inorganic filler in a total amount of the polyolefin resin and the inorganic filler in the first polyolefin microporous layer and concentration C2 of the inorganic filler in a total amount of the polyolefin resin and the inorganic filler in the second polyolefin microporous layer is 20% by mass or more and 95% by mass or less,

concentration C1 is more than 60% by mass and 95% by mass or less, and

concentration C2 is 0% by mass or more and 40% by mass or less.

13. The laminated separator according to claim 1 , wherein

the laminated separator has a two-type three-layer structure comprising the second polyolefin microporous layers as surface layers and the first polyolefin microporous layer as an intermediate layer,

a difference between the concentration of C1 of the inorganic filler in a total amount of the polyolefin resin and the inorganic filler in the first polyolefin microporous layer and concentration C2 of the inorganic filler in a total amount of the polyolefin resin and the inorganic filler in the second polyolefin microporous layer is 20% by mass or more and 95% by mass or less,

concentration C1 is more than 60% by mass and 95% by mass or less, and

concentration C2 is 0% by mass or more and 40% by mass or less.

14. A method of producing the laminated separator according to claim 1 or 2 , the method comprising:

(1) a kneading step of forming (i) a first kneaded mixture by kneading a first polyolefin, a plasticizer and, where necessary, the inorganic filler, and (ii) a second kneaded mixture by kneading a second polyolefin, a plasticizer and, where necessary, the inorganic filler;

(2) a sheet molding step, following the kneading step, of processing the first and second kneaded mixtures into a sheet-like molded body by extruding the first and second kneaded mixtures into a sheet shape according to an intended layer structure and by cooling and solidifying the sheet-like molded body;

(3) a stretching step, following the sheet molding step, of forming a stretched product by biaxially stretching the sheet-like molded body with an area magnification of 20× or more and 200× or less;

(4) a porous body forming step, following the stretching step, of forming a porous body by extracting the plasticizer from the stretched product; and

(5) a heat treatment step, following the porous body forming step, of heat treating the porous body at a temperature equal to or lower than a melting point of the polyolefin and widthwise stretching the porous body.

15. The method according to claim 14 , wherein at least one of the first kneaded mixture and the second kneaded mixture further comprises a low molecular weight resin having a viscosity average molecular weight of 800 to 10,000.