LITHIUM-ION BATTERY SEPARATOR COATED WITH SURFACE TREATED ALUMINA

Abstract

A separator for a lithium-ion battery contains an organic substrate coated with a coating layer, containing a binder and alumina particles. The alumina particles are surface treated with a silane of general formula (I) or (Ia). A method can be used for synthesis of the separator, which can be used in lithium-ion batteries.

Claims

1: A separator for a lithium-ion battery, comprising an organic substrate coated with a coating layer comprising a binder and a surface treated alumina, wherein the surface treated alumina is prepared by surface treatment of alumina with a compound of general formula (I) or (Ia): ##STR00002## wherein R═H or CH.sub.3, 0≤h≤2, A is H or a branched or unbranched C.sub.1 to C.sub.4 alkyl residue, B is a branched or unbranched, aliphatic, aromatic or mixed aliphatic-aromatic C.sub.1 to C.sub.30 carbon-based group, and X is selected from the group consisting of Cl; a group OY, wherein Y is H or a C.sub.1 to C.sub.30 branched or unbranched alkyl-, alkenyl-, aryl-, or aralkyl-group; a branched or unbranched C.sub.2 to C.sub.30 alkylether-group; a branched or unbranched C.sub.2 to C.sub.30 alkylpolyether-group; and a mixture thereof.

2: The separator according to claim 1, wherein the compound of general formula (I) is selected from the group consisting of 3-(triethoxysilyl)propyl methacrylate, 3-(triethoxysilyl)propyl methacrylate, 3-(trichlorosilyl)propyl methacrylate, and a mixture thereof.

3: The separator according to claim 1, wherein the compound of general formula (Ia) is selected from the group consisting, of (dichlorosilanediyl)bis(propane-3,1-diyl) bis(2-methylacrylate), (dimethoxysilanediyl)bis(propane-3,1-diyl) bis(2-methylacrylate), (diethoxysilanediyl)bis(propane-3,1-diyl) bis(2-methylacrylate), and a mixture thereof.

4: The separator according to claim 1, wherein the surface treated alumina is fumed alumina.

5: The separator according to claim 1, wherein the surface treated alumina has a specific BET surface area of 50 m.sup.2/g-150 m.sup.2 g.

6: The separator according to claim 1, wherein the surface treated alumina has a carbon content of 0.5%-5.0% by weight.

7: The separator according to claim 1, wherein the surface treated alumina has a number mean particle size d.sub.50 of 20 nm 400 nm.

8: The separator according to claim 1, wherein the organic substrate comprises a polyolefin resin, a fluorinated polyolefin resin, a polyester resin, a polyacrylonitrile resin, a cellulose resin, a non-woven fabric, or a mixture thereof.

9: The separator according to claim 1, wherein the binder is selected from the group consisting of poly(vinylidene fluoride), a copolymer of vinylidene fluoride and hexafluoropropylene, poly(vinyl acetate), polyethylene oxide), poly(methyl methacrylate), poly(ethyl acrylate), polyvinyl chloride), poly(urethane), poly(acrylonitrile), a copolymer of ethylene and vinyl acetate, carboxyl methyl cellulose, poly(imide), and a mixture thereof.

10: The separator according to claim 1, wherein a total thickness of the separator is 5 μm-200 μm.

11: The separator according to claim 1, wherein a thickness of the coating layer is 0.1 μm-20 μm.

12: A process for producing the separator according to claim 1, the process comprising: 1) preparing the surface treated alumina by surface treatment of alumina with a surface treatment agent; 2) preparing a coating mixture comprising the surface treated alumina and the binder; and 3) coating a surface of the organic substrate with the coating mixture to form the coating layer comprising the surface treated alumina and the binder.

13: A method, comprising: assembling a lithium-ion battery comprising the separator according to claim 1.

14: The method according to claim 13, wherein the lithium-ion battery comprises a gel electrolyte.

15: A lithium-ion battery comprising the separator according to claim 1.

16: The lithium-ion battery according to claim 15, wherein the lithium-ion battery comprises a gel electrolyte.

Description

EXPERIMENTAL PART

[0090] Inorganic Particles

[0091] Fumed Alumina 1

[0092] Preparation of fumed alumina 1 surface treated with 3-(trimethoxysilyl)propyl methacrylate was carried out according to example 11 of EP 1628916B1. Alumina 1 had a BET of 93 m.sup.2/g and C-content of 4.0 wt. %.

[0093] AEROXIDE® Alu C 805

[0094] Fumed alumina surface treated with n-octyl trimethoxysilane:

[0095] AEROXIDE® Alu C 805, manufacturer: Evonik Resource Efficiency GmbH. According to the data sheet, AEROXIDE® Alu C 805 had a BET of 75-105 m.sup.2/g and a C-content of 3.5-4.5 wt. %.

[0096] AEROSIL® R 711

[0097] Fumed silica surface treated with 3-(trimethoxysilyl)propyl methacrylate:

[0098] AEROSIL® R711, manufacturer: Evonik Resource Efficiency GmbH. According to the data sheet, AEROSIL® R711 had a BET of 125-175 m.sup.2/g and a C-content of 4.5-6.5 wt. %.

[0099] Separator

[0100] Polyethylene film of 9 μm thickness (manufacturer: SK Innovation).

[0101] Binder

[0102] Copolymer of polyvinylidene fluoride and hexafluoropropylene (Kynar Flex® 2801-00, manufacturer: Arkema).

[0103] Coating of Separator: General Procedure

[0104] The polyethylene separator film was coated with a mixture of inorganic particles and the binder diluted with N-methyl-2-pyrrolidone as a solvent (inorganic particles:

[0105] binder:NMP=5:5:90 by weight) using a Doctor blade device SA-202 (manufacturer: Tester Sangyo) to achieve a total thickness of coated polyethylene separator of 15 μm.

[0106] Lithium-Ion Battery

[0107] The lithium-ion battery with a ratio of designed areal capacity of negative and positive electrode (N/P ratio)=1.175; areal capacity: 2.0 mAh/cm.sup.2, containing an anode electrode and a cathode electrode which were purchased from Bexel in Korea, a separator and an electrolyte, was assembled using the following materials:

[0108] Anode electrode: 90 wt % of artificial graphite (loading level: 6.86 mg/cm.sup.2) from Showa Denko+3 wt % of conductive carbon+7 wt % of PVdF binder KF9130 (Kureha).

[0109] Cathode electrode: 95 wt % of NCM 622, LiNi.sub.0.6Co.sub.0.2Mn.sub.0.2O.sub.2 (loading level: 12.0 mg/cm.sup.2)+3 wt % of conductive carbon+2 wt % of PVdF binder KF7208 (Kureha).

[0110] Separator: polyethylene film coated with inorganic particles as described above.

[0111] Electrolyte:

[0112] Liquid electrolyte (LE): A mixture of 100 wt. % of 1.15 M solution of LiPF.sub.6 in ethylene carbonate (EC)/ethyl methylcarbonate (EMC)/diethyl carbonate (DEC) (3:5:2 vol:vol:vol) (manufacturer: Panax Etec) with 5 wt. % fluoroethylene carbonate (FEC, manufacturer: Panax Etec) and 1 wt. % vinyl ethylene carbonate (VEC, manufacturer: Panax Etec).

[0113] Gel electrolyte (GE): A mixture of 100 wt. % of the above described liquid electrolyte with 6 wt. % tetra(ethylene glycol) diacrylate (TEGDA, manufacturer: Sigma-Aldrich) and 0.06 wt. % of 2,2′-azobis(isobutyronitrile) (AIBN, manufacturer: Sigma-Aldrich).

[0114] Assembly of the Lithium-Ion Battery (Coin Cells): [0115] (1) Cut the cathode electrode with 14 mm in diameter, the anode electrode with 16 mm in diameter and separator with 18 mm in diameter. [0116] (2) Place the circle shape cathode, anode and separator into the glovebox for assembly. [0117] (3) Prepare a coin cell (CR2032) as coin cell parts, consisting of case, gasket, disk (1 mm in thickness), spring and cap, respectively. [0118] (4) Place the case and cathode electrode with coating side up in the centre. [0119] (5) Use a micro pipette to quantify 40 μL of the gel electrolyte precursor solution or liquid electrolyte and drop it on the cathode electrode well. [0120] (6) Place the separator on the cathode electrode. Prevent forming air bubbles between the cathode electrode and the separator. Use a Teflon forceps to move out bubbles in-between if any are present. In case of coated separator, the coating surface is facing up for contacting anode electrode. [0121] (7) Insert the gasket in the right direction, so that the cathode and anode electrode cannot move. [0122] (8) Use a micro pipette to quantify 40 μL of the gel electrolyte precursor solution or liquid electrolyte, and drop it over the centre of the separator fixed with gasket. [0123] (9) Place the anode electrode with coating surface down, and place the disk, spring and cap parts on the anode copper foil in sequence. [0124] (10) Press the assembled cells by the top face of Teflon forceps to ensure all the parts fit together well, and then use the crimping machine to complete the CR2032 coin cells. [0125] (11) Place the coin cells in 25° C. oven for 12 hours as aging process for separator and electrodes to be well wetted with liquid electrolyte or gel electrolyte precursor. [0126] (12) Put coin cells in an oven at 70° C. for 2 hours to induce chemical cross-linking of gel electrolyte precursor after aging process. [0127] (13) Remove coin cells from the oven and proceed further tests.

[0128] Lithium-Ion Battery Tests

[0129] Alternating Current (AC) Impedance

[0130] Measurement of AC Impedance was carried out at 25° C. or 55° C. using the AC impedance analyser CHI 660D (manufacturer: CH instruments). The values measured with one coin cell are presented in the following Table 1 and Table 2:

TABLE-US-00001 TABLE 1 AC impedance at 25° C. Inorganic particles Fumed for Separator/GE LIB No R711 C805 alumina 1 AC impedance directly after 20.0 17.4 20.6 16.9 formation of LIB, R.sub.tot, [Ω] AC impedance after 300 105.0 88.4 108.3 84.9 recharge cycles, R.sub.tot, [Ω] AC impedance increase 525 508 526 502 after 300 cycles, [%]

TABLE-US-00002 TABLE 2 AC impedance at 55° C. Inorganic particles Fumed for Separator/GE LIB No R711 C805 alumina 1 AC impedance directly after 14.0 15.6 14.9 15.5 formation of LIB, R.sub.tot, [Ω] AC impedance after 100 79.8 80.8 79.0 72.4 recharge cycles, R.sub.tot, [Ω] AC impedance increase 570 518 530 467 after 100 cycles, [%]

[0131] Table 1 and Table 2 show that lithium-ion batteries with the inventive separators coated with surface treated alumina particles demonstrate lower AC impedance increase both after 300 recharge cycles at 25° C. and after 100 cycles at 55° C., when compared with the same separator without any coating or separators coated with other inorganic particles.

[0132] Cycle Performance

[0133] Cycle performance was measured at 25° C. or at 55° C. using battery cycler PEBC 50.2 (manufacturer: PNE solutions) at cut-off voltage of 3.0-4.3 V, charge rate: 0.5 C CC/CV and discharge rate: 0.5 C CC/CV (0.5 C rate corresponds to current density of 1.0 mAh/cm.sup.2). At least three to five cells were assembled and tested in each case to ensure the reproducibility of the results. The average values of these tests are presented in the following Table 3 and Table 4:

TABLE-US-00003 TABLE 3 Cycle performance at 25° C. Inorganic particles Fumed for Separator/GE LIB No R711 C805 alumina 1 Charge first cycle, [mAh/g] 167.7 167.9 168.0 167.2 Discharge first cycle, [mAh/g] 162.5 163.5 164.3 162.9 Efiiciency first cycle, [%] 96.9 97.4 97.8 97.4 Charge 300.sup.th cycle, [mAh/g] 116.0 125.0 115.5 130.8 Discharge 300.sup.th cycle, [mAh/g] 115.8 125.0 115.3 130.5 Efiiciency 300.sup.th cycle, [%] 99.8 100.0 99.8 99.8 Retention after 300 cycles, [%] 71.3 76.5 70.2 80.1

TABLE-US-00004 TABLE 4 Cycle performance at 55° C. Inorganic particles Fumed for Separator/GE LIB No R711 C805 alumina 1 Charge first cycle, [mAh/g] 177.1 167.2 168.0 166.6 Discharge first cycle, [mAh/g] 169.4 166.8 167.4 166.4 Efiiciency first cycle, [%] 95.7 99.8 99.6 99.8 Charge 100.sup.th cycle, [mAh/g] 124.2 122.3 123.1 130.2 Discharge 100.sup.th cycle, [mAh/g] 123.0 122.2 122.9 129.8 Efiiciency 100.sup.th cycle, [%] 99.0 99.9 99.8 99.7 Retention after 100 cycles, [%] 72.6 73.3 73.4 78.0

[0134] Table 3 and Table 4 show that lithium-ion batteries with the inventive separators coated with surface treated alumina particles demonstrate higher retention both after 300 recharge cycles at 25° C. and after 100 cycles at 55° C., when compared with the same separator without any coating or separators coated with other inorganic particles.

[0135] Hydrofluoric Acid (HF) Content Measurement

[0136] Measurement of the HF content of the mixture of the electrolyte (89.4 wt %), cross-linker (5.7 wt %), initiator (0.1 wt %) and inorganic particles (4.8 wt %) was carried out by acid-base titration with a 0.01 M solution of triethylamine in dimethyl carbonate (DMC) using methyl orange as an indicator, after storage of this mixture at 55° C. for 7 days.

TABLE-US-00005 TABLE 5 HF content Inorganic particles Fumed for Separator/GE LIB No R711 alumina 1 HF content, [ppm] 576 9104 4680

[0137] The results of HF content measurement (Table 5) show increased HF content in both samples with separators coated with hydrophobic silica and alumina, when compared with non-coated separator, though the system with silica particles shows twice as much HF as the system with alumina particles. Both silica and alumina powders inevitably contain water traces, which lead to partial hydrolysis of LiPF.sub.6 in the electrolyte.