Separator for electricity storage device
A separator for electricity storage devices which comprises: a base comprising a porous film; and a thermoplastic polymer arranged on at least one surface of the base. The thermoplastic polymer has a dispersion (σ 2 ), defined by the following numerical equation using the areas (Si) of Voronoi polygons obtained by Voronoi tessellation, of 0.01-0.7. (In the equation, Si is the measured area of each Voronoi polygon, m is an average of the measured areas of the Voronoi polygons, and n is the total number of the Voronoi polygons).
1. A separator for an electricity storage device, comprising a base material containing at least a porous film, and a thermoplastic polymer arranged on at least one surface of the base material, wherein
the thermoplastic polymer contains an acrylic polymer which contains a crosslinked acrylic polymer containing a crosslinking monomer as a monomer unit, and the content ratio of the crosslinking monomer in the acrylic polymer is from 0.01 to 10 mass % per 100 mass % of the acrylic polymer,
the thermoplastic polymer has a dispersion (σ 2 ) of 0.01 to 0.7 as defined by the following mathematical equation using the area (Si) of a Voronoi polygon obtained by Voronoi tessellation,
the dispersion (σ 2 ) is defined as an average value of 95 dispersions (σ 2 ) calculated respectively in measurement visual fields set from an image captured by photographing the thermoplastic polymer arranged on the base material by a scanning electron microscope,
each of the measurement visual fields is a captured image taken at a magnification set such that the number of thermoplastic polymer particles observed in one visual field is from 80 to 200, and
the measurement visual fields are set as follows:
i) each measurement visual field: an image captured by a scanning electron microscope,
ii) method for setting the visual field:
a) an initial visual field is set,
b) 19 visual fields consisting of 9 visual fields composed of regions sequentially adjoining the initial visual field in the transverse direction, 9 visual fields composed of regions sequentially adjoining in the longitudinal direction, and the initial visual field are set,
c) a region defined by the 19 visual fields is set as an initial section,
d) 4 sections composed of regions sequentially adjoining the initial section in the uniaxial direction at intervals of 10 mm are set,
e) in each of the 4 sections, 19 visual fields are set at positions similar to those of 19 visual fields in the initial section, and
f) a total of 95 visual fields (19 visual fields ×5 sections) in the 4 sections and the initial section are set as the measurement visual field;
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(wherein Si is the measured area of a Voronoi polygon, m is an average value of the measured areas of Voronoi polygons, and n is the total number of Voronoi polygons).
2. The separator for an electricity storage device according to claim 1 , wherein the dispersion (σ 2 ) is from 0.01 to 0.6.
3. The separator for an electricity storage device according to claim 1 , wherein the dispersion (σ 2 ) is from 0.01 to 0.5.
4. The separator for an electricity storage device according to claim 1 , wherein the thermoplastic polymer is particulate.
5. The separator for an electricity storage device according to claim 1 , wherein the average particle diameter of the thermoplastic polymer is from 10 to 2,000 nm.
6. The separator for an electricity storage device according to claim 1 , wherein the average particle diameter of the thermoplastic polymer is from 50 to 1,500 nm.
7. The separator for an electricity storage device according to claim 1 , wherein the average particle diameter of the thermoplastic polymer is from 100 to 1,000 nm.
8. The separator for an electricity storage device according to claim 1 , wherein the average particle diameter of the thermoplastic polymer is from 130 to 800 nm.
9. The separator for an electricity storage device according to claim 1 , wherein the area density of the thermoplastic polymer is from 30 to 80%.
10. The separator for an electricity storage device according to claim 1 , wherein the thermoplastic polymer is present substantially without overlapping.
11. The separator for an electricity storage device according to claim 1 , wherein the ratio of an infrared absorption peak intensity at a wavelength of 1,720 to 1,750 cm −1 to an infrared absorption peak intensity at a wavelength of 740 to 770 cm −1 , of the thermoplastic polymer, is from 1 to 18.
12. The separator for an electricity storage device according to claim 1 , wherein the ratio of an infrared absorption peak intensity at a wavelength of 2,220 to 2,260 cm −1 to an infrared absorption peak intensity at a wavelength of 1,720 to 1,750 cm −1 , of the thermoplastic polymer, is from 0.001 to 0.320.
13. The separator for an electricity storage device according to 1 , wherein the thermoplastic polymer contains at least one member selected from the group consisting of the following (1) to (3):
(1) a copolymer having a (meth)acrylic acid ester as a monomer unit (with a proviso excluding the following copolymer (2) and copolymer (3)),
(2) a copolymer having a cyano group-containing monomer and a (meth)acrylic acid ester monomer as monomer units, and
(3) a copolymer having an aromatic vinyl monomer and a (meth)acrylic acid ester monomer as monomer units.
14. The separator for an electricity storage device according to claim 13 , wherein the cyano group-containing monomer is (meth)acrylonitrile.
15. The separator for an electricity storage device according to claim 13 , wherein the aromatic vinyl monomer is styrene.
16. An electricity storage device having mounted therein the separator for an electricity storage device according to claim 1 .
17. The method for producing the separator for an electricity storage device of claim 1 , comprising:
a step of adjusting the viscosity of a solution containing the thermoplastic polymer to 20 mPa·s or more, and
a step of coating a base material containing a porous film with the prepared thermoplastic polymer solution while applying a shear force.
18. The separator for an electricity storage device according to claim 1 , wherein the crosslinking monomer is selected from a monomer having two or more radical polymerizable double bonds, and a monomer having a functional group that provides a self-crosslinked structure during polymerization or after polymerization.
19. The separator for an electricity storage device according to claim 1 , wherein the acrylic polymer contains a (meth)acrylic compound as a monomer unit in the content of from 5 to 95 mass % per 100 mass % of the acrylic polymer.
20. The separator for an electricity storage device according to claim 1 , wherein the acrylic polymer layer contains the thermoplastic polymer in an amount of 60 mass % or more, and the thermoplastic polymer is the acrylic polymer.
21. The separator for an electricity storage device according to claim 1 , wherein the thermoplastic polymer has at least two glass transition temperatures, at least one of these glass transition temperatures is present in a region of less than 20° C., and at least one of these glass transition temperatures is present in a region of 20° C. or more.
22. The separator for an electricity storage device according to claim 20 , wherein the thermoplastic polymer contains a copolymer having a cyano group-containing monomer and a (meth)acrylic acid ester monomer as monomer units, and glass transition temperature thereof is present in a region of 20° C. or more.