Preparation method of crosslinking-type aqueous binder for lithium-ion batteries incorporating slurry coating and drying process

Abstract

A preparation method of a crosslinking-type aqueous binder for lithium-ion batteries. An organic carboxylic group-, amino group- or hydroxyl group-containing hydrophilic polymer, and a hydroxyl group-, amine group- or carboxyl group-containing water-soluble small-molecule crosslinker, both serve as starting materials of the aqueous binder, and can be crosslinked by esterification or amidation under coating and drying conditions of lithium-ion battery electrode slurry. The preparation method of the crosslinking-type aqueous binder is simple, without the need of modifying the current process or conditions for lithium-ion battery manufacture. The obtained electrodes have excellent binding capacity, flexibility, and elasticity.

Claims

1. A crosslinking-type aqueous binder for lithium-ion batteries, comprising: an organic carboxylic group-, amino group- or hydroxyl group-containing hydrophilic polymer, and a hydroxyl group-, amine group- or carboxyl group-containing water-soluble small-molecule crosslinker, wherein the organic carboxylic group-, amino group- or hydroxyl group-containing hydrophilic polymer has a molecular weight of 70000-1000000; the organic carboxylic group-containing hydrophilic polymer is prepared through a free radical graft copolymerization or a Michael addition reaction of an acrylic monomer and the hydroxyl group-containing hydrophilic polymer; the hydroxyl group-containing hydrophilic polymer is one or more selected from the group consisting of arabic gum, cyclodextrin, cellulose derivatives, xanthan gum, pectin, gelatin, starch, sesbania gum, polyvinyl alcohol, polyethylene glycol and polyhydroxy polybutadiene; the amino group-containing hydrophilic polymer is one or more selected from the group consisting of arabic gum, chitosan and its derivatives, linear polyethyleneimine and branched polyethyleneimine; a method of preparing the crosslinking-type aqueous binder comprises the following steps: (1) dissolving the organic carboxylic group-, amino group- or hydroxyl group-containing hydrophilic polymer in deionized water, and stirring under a protective gas atmosphere for 0.5-2.5 hours to remove oxygen and obtain a solution; (2) adding the hydroxyl group-, amine group- or carboxyl group-containing water-soluble small-molecule crosslinker to the solution prepared in the step (1), stirring to obtain the crosslinking-type aqueous binder; wherein a molar ratio of the hydroxyl group-, amine group- or carboxyl group-containing water-soluble small-molecule crosslinker and the organic carboxylic group-, amino group-or hydroxyl group-containing hydrophilic polymer is 0.01:10 to 1:10; wherein the hydroxyl group-containing water-soluble small-molecule crosslinker is at least one monomer selected from the group consisting of a saturated diol having a carbon number more than or equal to 2, a saturated polyol having a carbon number more than or equal to 3, a saturated dihydroxyalkylamine NH—(C.sub.mH.sub.2m—OH).sub.2, and a saturated polyhydroxyalkylamine N—(C.sub.m′H.sub.2m—OH).sub.3, wherein m≥1 and m′≥1; the amine group-containing water-soluble small-molecule crosslinker is at least one monomer selected from the group consisting a saturated diamine C.sub.nH.sub.2n(NH.sub.2).sub.2 and a saturated polyamine C.sub.n′H.sub.2n′+2−x(NH.sub.2).sub.n, wherein n≥2, n′≥3, x ≥3; the carboxyl group-containing water-soluble small-molecule crosslinker is at least one monomer selected from the group consisting of a saturated diacid, and a saturated polyacid having a carbon number more than or equal to 3.

2. The crosslinking-type aqueous binder according to claim 1, wherein, in the step (1), the protective gas atmosphere is nitrogen gas and/or argon gas, and the a stirring rate is 100-500 rpm; in the step (2), the molar ratio of the hydroxyl group-, amine group- or carboxyl group-containing water-soluble small-molecule crosslinker and the organic carboxylic group-, amino group- or hydroxyl group-containing hydrophilic polymer is 0.5:10 to 1:10.

3. A crosslinking-type aqueous binder for lithium-ion batteries, comprising: an organic carboxylic group-, amino group- or hydroxyl group-containing hydrophilic polymer, and a hydroxyl group-, amine group-or carboxyl group-containing water-soluble small-molecule crosslinker, wherein the organic carboxylic group-, amino group- or hydroxyl group-containing hydrophilic polymer has a molecular weight of 70000-1000000; the organic carboxylic group-containing hydrophilic polymer is prepared through a free radical graft copolymerization or a Michael addition reaction of an acrylic monomer and the hydroxyl group-containing hydrophilic polymer; the hydroxyl group-containing hydrophilic polymer is one or more selected from the group consisting of arabic gum, cyclodextrin, cellulose derivatives, xanthan gum, pectin, gelatin, starch, sesbania gum, polyvinyl alcohol, polyethylene glycol and polyhydroxy polybutadiene; the amino group-containing hydrophilic polymer is one or more selected from the group consisting of arabic gum, chitosan and its derivatives, linear polyethyleneimine and branched polyethyleneimine; a method of preparing the crosslinking-type aqueous binder comprises the following steps: (1) dissolving the organic carboxylic group-, amino group- or hydroxyl group-containing hydrophilic polymer in deionized water, and stirring under a protective gas atmosphere for 0.5-2.5 hours to remove oxygen and obtain a solution; (2) adding the hydroxyl group-, amine group- or carboxyl group-containing water-soluble small-molecule crosslinker to the solution prepared in the step (1), stirring to obtain the crosslinking-type aqueous binder; wherein a molar ratio of the hydroxyl group-, amine group- or carboxyl group-containing water-soluble small-molecule crosslinker and the organic carboxylic group-, amino group- or hydroxyl group-containing hydrophilic polymer is 0.01:10 to 1:10; wherein, the hydroxyl group-, amine group- or carboxyl group-containing water-soluble small-molecule crosslinker is at least one selected from the group consisting of ethylene glycol, glycerol, pentaerythritol, diethanolamine, triethanolamine, hexamethoxymethyl melamine, hexanedioic acid, propanedioic acid, and tricarballylic acid.

Description

BRIEF DESCRIPTION OF THE DRAWINGS

(1) FIGS. 1 to 3 are the infrared spectra showing the crosslinking of GA-PAA with crosslinkers having hydroxyl groups, amine groups, etc, in embodiments 4 to 6.

(2) FIG. 4 shows the DSC analysis of the interaction between GA-PAA and crosslinkers having hydroxyl groups, amine groups, etc, in embodiments 7 to 9.

(3) FIG. 5 shows the DSC analysis showing the crosslinking of PVA-PAA-MA with crosslinkers having hydroxyl groups, amine groups, etc, in embodiment 14.

(4) FIG. 6 shows the cycling-performance test where GA-PAA binders crosslinked by crosslinkers having hydroxyl groups, amine groups, etc, were applied to a silicon anode active material in embodiment 16.

(5) FIG. 7 shows electrochemical-impedance curves of the silicon anodes and the comparative electrode in embodiment 16.

(6) FIG. 8 shows the cycling-performance test where GA-PAA binders crosslinked by crosslinkers having hydroxyl groups, amine groups, etc, were applied to an LFP cathode active material in embodiment 17.

(7) FIG. 9 shows electrochemical-impedance curves of the lithium iron phosphate electrodes and the comparative electrode in embodiment 17.

(8) Arabic gum is denoted as GA, polyvinyl alcohol is denoted as PVA, pentaerythritol is denoted as PER, triethanolamine is denoted as TEOA, hexamethoxymethyl melamine is denoted as HMMM, and methyl acrylate is denoted as MA.

DETAILED DESCRIPTION OF THE EMBODIMENTS

(9) The following embodiments are intended to further illustrate the invention but not to limit the invention.

(10) Arabic gum is denoted as GA. The organic carboxylic group-containing hydrophilic polymer prepared by a free radical graft copolymerization or a Michael addition reaction of arabic gum and acrylic acid is denoted as GA-PAA. Polyvinyl alcohol is denoted as PVA. The organic carboxylic group-containing hydrophilic prepared by a free radical graft copolymerization or a Michael addition reaction of acrylic acid and polyvinyl alcohol is denoted as PVA-PAA. The organic carboxylic group-containing hydrophilic polymer prepared by a free radical graft copolymerization or a Michael addition reaction of methyl acrylate, acrylic acid and polyvinyl alcohol is denoted as PVA-PAA-MA. Pentaerythritol is denoted as PER. Triethanolamine is denoted as TEOA. Hexamethoxymethyl melamine is denoted as HMMM. Methyl acrylate is denoted as MA.

(11) Taking PVA-PAA as an example, the preparation method is as follows: Adding PVA into a two-neck flask, followed by the addition of DI-water. Under an argon gas atmosphere, stirring the mixture until PVA is completely dissolved, then raising the temperature to 65° C., and sequentially adding a solution of Na2S2O8 and NaHSO3 and acrylic acid. The materials in the solution undergo a reaction at 65° C. under the argon gas atmosphere for 48 hours, so as to obtain the hydrophilic polymer PVA-PAA. An amount of the initiator is 0.01-5 wt % of a total mass of the monomer.

(12) Taking hydroxyl group-containing GA and carboxyl group-containing GA-PAA modified with acrylic acid as the examples, hydroxyl group- containing GA is crosslinked by esterification with the carboxyl group-containing water-soluble small-molecule crosslinker, which is illustrated by the following equation:

(13) ##STR00001##

(14) Carboxyl group-containing GA-PAA modified by acrylic acid is crosslinked by esterification with the hydroxyl group-containing water-soluble small-molecule crosslinker, which is illustrated by the following equation:

(15) ##STR00002##

Embodiment 1

Solubility Test of GA-PAA Crosslinked With Hydroxyl Group-Containing Small-Molecule Crosslinker

(16) (1) First, 1 g of GA-PAA (a molecular weight of 400000-1000000) was pre-dissolved in 10 ml of deionized water (DI-Water), thoroughly stirred under an argon gas atmosphere for 0.5-2.5 hours to remove oxygen and obtain a uniform and well-dispersed solution; the stirring rate was 100-500 rpm.

(17) (2) One or more of ethylene glycol, glycerol and pentaerythritol (OH:GA-PAA=1:10, molar ratio) was added into the solution obtained in step (1), thoroughly stirred to obtain a mixed solution. The solution was then placed in a vacuum to allow esterification at 45° C., 90° C. or 110° C. for 1-24 hours. The products were dissolved in a certain mass of water for observations on their solubilities.

Embodiment 2

(18) Embodiment 2 was similar to embodiment 1 but different in step (2): an amine group-containing small-molecule crosslinker such as triethanolamine was added into the solution obtained in step (1); after amidation under various temperatures, observations on the products' solubilities were performed.

Embodiment 3

(19) Embodiment 3 was similar to embodiment 1 but different in step (2): a crosslinker hexamethoxymethyl melamine (hmmm) was added into the solution obtained in step (1); after esterification under various temperatures, observations on the products' solubilities were performed.

(20) As can be concluded from embodiments 1 to 3, GA-PAA could be completely dissolved at 45-90° C., but would undergo crosslinking above 110° C. where swelling was observed. The GA-PAA system added with pentaerythritol (PER), triethanolamine (TEOA) or hexamethoxymethyl melamine (hmmm) could be completely dissolved at 45° C., but would undergo crosslinking above 90° C. where swelling was observed.

Embodiment 4

(21) (1) First, 1 g of GA-PAA (a molecular weight of 400000-1000000) was pre-dissolved in 10 ml of deionized water (DI-Water), thoroughly stirred under an argon gas atmosphere for 0.5-2.5 hours to remove oxygen and obtain a uniform and well-dispersed solution; the stirring rate was 100-500 rpm.

(22) (2) One or more of ethylene glycol, glycerol and pentaerythritol (OH:GA-PAA=1:10, molar ratio) was added into the solution obtained in step (1), thoroughly stirred to obtain a mixed solution. The solution was then placed in a vacuum to allow esterification at 45° C., 90° C. or 110° C. for 24 hours. See FIG. 1 for the infrared spectrum.

Embodiment 5

(23) Embodiment 5 was similar to embodiment 4 but different in step (2): an amine group-containing small-molecule crosslinker such as triethanolamine was added into the solution obtained in step (1). See FIG. 2 for the infrared spectrum.

Embodiment 6

(24) Embodiment 6 was similar to embodiment 4 but different in step (2): a crosslinker hexamethoxymethyl melamine (hmmm) was added into the solution obtained in step (1), and the solution was treated under various temperatures. See FIG. 3 for the infrared spectrum.

(25) As can be concluded from FIG. 1 to FIG. 3, a crosslinking reaction of GA-PAA with pentaerythritol (PER), triethanolamine (TEOA) or hexamethoxymethyl melamine (hmmm) would occur above 110° C.

Embodiment 7

(26) (1) First, 1 g of GA-PAA (a molecular weight of 400000-1000000) was pre-dissolved in 10 ml of deionized water (DI-Water), thoroughly stirred under an argon gas atmosphere for 0.5-2.5 hours to remove oxygen and obtain a uniform and well-dispersed solution; the stirring rate was 100-500 rpm.

(27) (2) One or more of ethylene glycol, glycerol and pentaerythritol (OH:GA-PAA=1:10, molar ratio) was added into the solution obtained in step (1), thoroughly stirred to obtain a mixed solution. The solution was then placed in a vacuum to allow esterification at 45° C. for 24 hours. See FIG. 4 for the DSC analysis of the interaction between GA-PAA and hydroxyl group-containing crosslinkers.

Embodiment 8

(28) Embodiment 8 was similar to embodiment 7 but different in step (2): an amine group-containing small-molecule crosslinker such as triethanolamine was added into the solution obtained in step (1). See FIG. 4 for the DSC analysis of the interaction between GA-PAA and amine group-containing crosslinkers.

Embodiment 9

(29) Embodiment 9 was similar to embodiment 7 but different in that: a methoxyl group-containing crosslinker such as hexamethoxymethyl melamine was added into the solution obtained in step (1). See FIG. 4 for the DSC analysis of the interaction between GA-PAA and methoxyl group-containing crosslinkers.

(30) As can be concluded from FIG. 4, an endothermic crosslinking reaction of GA-PAA with pentaerythritol (PER), triethanolamine (TEOA) or hexamethoxymethyl melamine (hmmm) would occur above 90° C.

Embodiment 10

Peel Strength Experiment of the Crosslinking-Type Binder

(31) (1) First, 1 g of polyethyleneimine (PEI, a molecular weight of 70000) was pre-dissolved in 10 ml of deionized water (DI-Water), thoroughly stirred under an argon gas atmosphere for 0.5-2.5 hours to remove oxygen and obtain a uniform and well-dispersed solution; the stirring rate was 100-500 rpm.

(32) (2) One or more of propanedioic acid, tricarballylic acid and hexanedioic acid was added into the solution obtained in step (1), thoroughly stirred to obtain a mixed solution. The solution was then placed in a vacuum to allow reaction at 45° C. or 90° C. for 1-24 hours. The products were dissolved in a certain mass of water for observations on their solubilities. It was observed that, after heated at 90° C., PEI underwent a crosslinking reaction with the above various crosslinkers.

(33) (3) The mixed binders obtained in step (2) were spread on aluminum foils with a thickness of 100 μm, and subjected to force-air drying at 110° C. The peel strengths of the binders were measured and determined to be 0.08 N/mm.

Embodiment 11

(34) Embodiment 11 was similar to embodiment 10 but different in that: in step (1), polyethyleneimine was replaced with polyvinyl alcohol (PVA), a hydroxyl group-containing hydrophilic polymer; in step (2), the solution was placed in a vacuum to allow reaction at 45° C. or 110° C. for 1-24 hours for later observations on solubilities. It was observed that, after heated at 110° C., PVA underwent a crosslinking reaction with the above various crosslinkers. The mixed binders obtained in step (2) were spread on aluminum foils with a thickness of 100 μm, and subjected to force-air drying at 110° C. The peel strengths of the binders were measured and determined to be 0.10 N/mm.

Embodiment 12

(35) Embodiment 12 was similar to embodiment 1 but different in that: in step (1), GA-PAA was replaced with PVA-PAA; in step (2), the crosslinker was pentaerythritol (PER), triethanolamine (TEOA) or hexamethoxymethyl melamine (hmmm). Observations on the products' solubilities were performed.

Embodiment 13

(36) Embodiment 13 was similar to embodiment 12 but different in that: in step (1), PVA-PAA was replaced with PVA-PAA-MA. Observations on the products' solubilities were performed.

Embodiment 14

(37) Embodiment 14 was similar to embodiment 5 but different in that: in step (1), the substrate was PVA-PAA-MA. See FIG. 5 for the DSC analysis. As can be concluded from FIG. 5, PVA-PAA-MAA underwent an endothermic crosslinking reaction with pentaerythritol (PER) above 90° C.

Embodiment 15

Preparation of Electrodes

(38) The lithium-ion battery anode comprised a current collector and a lithium-ion battery anode slurry loaded on the current collector. The lithium-ion battery anode slurry comprised an anode active material, a conductive agent, and a binder. A weight ratio of the anode active material, the conductive agent and the binder was 70:20:10. The solvent was water. The anode active material was silicon. The conductive agent was acetylene black. The current collector was a copper foil current collector. A solids content of the lithium-ion battery anode slurry was 30% and a viscosity of the lithium-ion battery anode slurry was 3000 mPa.Math.s. The peel strengths of the prepared GA-PAA-Si anodes were as shown in Table 1. As can be seen from the table, the silicon electrodes obtained by crosslinking with various crosslinkers showed relatively high peel strengths, wherein the GA-PAA-HMMM-Si electrode had the highest peel strength of up to 0.28 N/mm.

(39) TABLE-US-00001 TABLE 1 Electrode Peel strength (N/mm) GA-PAA-Si 0.18 GA-PAA-PER-Si 0.2 GA-PAA-HMMM-Si 0.28 GA-PAA-TEOA-Si 0.19

Embodiment 16

1. Preparation of Test Electrode

(40) As one embodiment of the lithium-ion battery anode of the present invention, the electrode comprised a current collector and a lithium-ion battery anode slurry loaded on the current collector. The lithium-ion battery anode slurry comprised an anode active material, a conductive agent, and a binder. A weight ratio of the anode active material, the conductive agent and the binder was 70:20:10. The solvent was water. The anode active material was silicon. The conductive agent was acetylene black. The current collector was a copper foil current collector. A solids content of the lithium-ion battery anode slurry was 75% and a viscosity of the lithium-ion battery anode slurry was 8000 mPa.Math.s.

(41) GA-PAA, as the starting material of the aqueous binder, was dissolved in deionized water, and thoroughly stirred under an argon gas atmosphere for 0.5-2.5 hours to remove oxygen and obtain a uniform and well-dispersed solution. Then, any one of pentaerythritol (PER), triethanolamine (TEOA) and hexamethoxymethyl melamine (hmmm) was added into the solution and thoroughly stirred, wherein a molar ratio of GA-PAA and the water-soluble small-molecule crosslinker was 10:1. Silicon and the conductive agent were mixed and stirred to a uniform mixture which was then added to the above system, an appropriate amount of water was added to adjust the viscosity, and thereby the electrode slurry was obtained. The slurry was evenly spread over the copper foil, and vacuum-dried at 110° C. to initiate a crosslinking reaction, so as to obtain a silicon anode. The vacuum-dried electrode was weighed and installed into a 2025 battery case in a glove box, with a lithium sheet as the counter electrode, a polyethylene film as the separator and 1 M LiPF6 EC/DMC/DEC (v/v/v=1/1) as the electrolyte, to assemble a battery which was then subjected to a constant current charge-discharge test.

2. Electrochemical Test

(42) A constant current charge-discharge test was performed on the test electrodes to determine their electrochemical performance and electrochemical impedances.

4. Results Analysis

(43) FIG. 6 shows the cycling-performance test curves of the silicone electrodes of the present embodiment at 400 mA/g. As can be seen from the figure, the GA-PAA-TEOA-Si and GA-PAA-HMMM-Si anodes, which were prepared with the three-dimensional crosslinking-type polymer binders, had shown excellent cycling performances. After 60 charging-discharging cycles, the electrodes still exhibited a charging specific capacity above 1000 mAh/g, far higher than that of the GA-PAA-Si electrode under the same condition, indicating that they had excellent cycling performance and electrochemical stability. In summary, the addition of crosslinker could largely improve the cyclic stability of silicon electrode.

(44) FIG. 7 shows the electrochemical impedance curves of the silicon anodes and the comparative electrode. As can be seen from the figure, after 60 charging-discharging cycles, the GA-PAA-TEOA-Si and GA-PAA-HMMM-Si anodes, which were prepared with three-dimensional crosslinking-type polymer binders, exhibited an electrochemical impedance lower than that of the GA-PAA-Si electrode under the same condition. In summary, the addition of crosslinkers TEOA and HMMM could reduce the electrochemical impedance of silicon electrode.

Embodiment 17

1. Preparation of Test Electrode

(45) The lithium-ion battery cathode comprised a current collector and a lithium-ion battery cathode slurry loaded on the current collector. The lithium-ion battery cathode slurry comprised a cathode active material, a conductive agent, and a binder. A weight ratio of the cathode active material, the conductive agent and the binder was 95:2:3. The solvent was water. The cathode active material was LFP. The conductive agent was acetylene black. The current collector was an aluminum foil current collector. A solids content of the lithium-ion battery cathode slurry was 50% and a viscosity of the lithium-ion battery cathode slurry was 5000 mPa.Math.s.

(46) GA-PAA, as the starting material of the aqueous binder, was dissolved in deionized water, and thoroughly stirred under an argon gas atmosphere for 0.5-2.5 hours to remove oxygen and obtain a uniform and well-dispersed solution. Then, any one of pentaerythritol (PER), triethanolamine (TEOA) and hexamethoxymethyl melamine (hmmm) was added into the solution and thoroughly stirred, wherein a molar ratio of GA-PAA and the water-soluble small-molecule crosslinker was 10:0.1. LFP and the conductive agent were mixed and stirred to a uniform mixture which was then added to the above system, an appropriate amount of water was added to adjust the viscosity, and thereby the LFP electrode slurry was obtained. The slurry was evenly spread over the aluminum foil, and vacuum-dried at 110° C. to initiate a crosslinking reaction, so as to obtain an LFP electrode. The vacuum-dried electrode was weighed and installed into a 2025 battery case in a glove box, with a lithium sheet as the counter electrode, a polyethylene film as the separator and 1 M LiPF6 EC/DMC/DEC (v/v/v=1/1) as the electrolyte, to assemble a battery which was then subjected to a constant current charge-discharge test.

2. Preparation of Comparative Electrode

(47) A comparative electrode was prepared by the same method with GA-PAA as the binder.

3. Electrochemical Test

(48) A test was performed on the test electrodes and the comparative electrode to determine their electrochemical performance and electrochemical impedances.

4. Results Analysis

(49) FIG. 8 shows the cycling-performance test curves of the LFP electrodes and the comparative electrode of the present embodiment at a charging and discharging current density of 0.5 C. The LFP electrodes, which were prepared with the three-dimensional crosslinking-type GA-PAA polymer binders, had shown excellent cyclic stability. Compared with the GA-PAA-LFP electrode, the GA-PAA-PER-LFP electrode exhibited a higher specific capacity. After 50 charging-discharging cycles, the specific capacities of the GA-PAA-LFP, GA-PAA-PER-LFP, GA-PAA-HMMM-LFP and GA-PAA-TEOA-LFP electrodes were respectively 135.6, 142.9, 127 and 132 mAh/g. In summary, the addition of crosslinker PER could improve the specific capacity and cyclic stability of LFP electrode.

(50) FIG. 9 shows the electrochemical-impedance curves of the LFP electrodes and the comparative electrode. As can be seen from the figure, compared with the GA-PAA-LFP electrode, the GA-PAA-PER-LFP electrode exhibited a lower electrochemical impedance after 50 charging-discharging cycles, indicating that the addition of crosslinker PER could improve the electrochemical performance of LFP electrode.