Water-based adhesive system, Si-C composite negative electrode and preparation method of Si-C composite negative electrode

Abstract

The invention belongs to the technical field of binders, and provides a water-based adhesive system, a Si-C composite negative electrode and a preparation method of the Si-C composite negative electrode. The water-based adhesive system comprises polyacrylic acid, a water-based composition, a polymer and water, the water-based composition comprises carboxymethyl cellulose and butadiene styrene rubber in a mass ratio of 1: (1-2), and the polymer comprises a polyurethane elastomer and / or a dynamic covalent bond polymer. The PAA is used as a main bonding matrix, hydrogen bonds or weak chemical bonds are easily formed on the surfaces of silicon particles, and Si expansion can be restrained; cMC / SBR provides certain flexibility and dispersity, and the overall uniformity and flexibility of the electrode coating are improved; the dynamic covalent bond polymer or the polyurethane elastomer can realize reversible crosslinking when electrode microcracks are generated, and the self-repairing capability is provided; the Si-C composite negative electrode prepared by the water-based adhesive system shows higher capacity retention ratio and longer cycle life in a half-battery or full-battery test.

Description

Technical Field

[0001] The present invention relates to the technical field of binders, and particularly to an aqueous binder system, a Si-C composite negative electrode and a preparation method thereof. Background Art

[0002] In high-energy-density lithium-ion batteries, silicon-based negative electrodes (such as Si, SiO x and Si-C composite materials) have attracted much attention due to their ultra-high theoretical specific capacity. However, the volume expansion of silicon during charge and discharge may lead to the rupture of the electrode structure and the shedding of active particles, thereby reducing the cycle life and capacity retention rate. In view of this problem, the reasonable selection and optimization of the binder system are crucial for improving the mechanical properties of the electrode interface and maintaining the integrity of the electrode.

[0003] Traditional binders such as PVDF usually need to be dissolved in organic solvents such as NMP, which is not conducive to environmental friendliness and the safety of large-scale production. In terms of aqueous binders, CMC (carboxymethyl cellulose) and SBR (styrene-butadiene rubber) are widely used due to their good elasticity and aqueous dispersion properties; PAA (polyacrylic acid) has strong adhesion and affinity for the surface functional groups of silicon, and can better constrain silicon particles.

[0004] Therefore, it is of great significance to provide an aqueous binder system suitable for high specific capacity Si-C composite negative electrodes. Summary of the Invention

[0005] The purpose of the present invention is to provide an aqueous binder system, a Si-C composite negative electrode and a preparation method thereof in view of the deficiencies of the prior art.

[0006] To achieve the above-mentioned invention purpose, the present invention provides the following technical solutions:

[0007] The present invention provides an aqueous binder system, which comprises polyacrylic acid, an aqueous composition, a polymer and water;

[0008] The aqueous composition comprises carboxymethyl cellulose and styrene-butadiene rubber, and the mass ratio of carboxymethyl cellulose to styrene-butadiene rubber is 1:1 to 2;

[0009] The polymer comprises a polyurethane elastomer and / or a dynamic covalent bond polymer;

[0010] In the aqueous binder system, the mass fraction of polyacrylic acid is 0.5 to 2%, the mass fraction of the aqueous composition is 1 to 3%, and the mass fraction of the polymer is 0.1 to 1%.

[0011] Preferably, the weight-average molecular weight of polyacrylic acid is 200000 to 500000.

[0012] Preferably, the dynamic covalent bond polymer has imide bonds, thioether bonds or reversible Diels-Alder bonds;

[0013] The polyurethane elastomer is an aqueous polyurethane dispersion, and the particle size of the polyurethane elastomer is ≤100 nm.

[0014] Preferably, the degree of substitution of the carboxymethyl cellulose is 0.7 to 0.9, the styrene-butadiene rubber is added in the form of a styrene-butadiene rubber emulsion, and the solid content of the styrene-butadiene rubber emulsion is 45 to 55%.

[0015] The present invention also provides a method for preparing the aqueous adhesive system, comprising the following steps:

[0016] 1) Mix polyacrylic acid and water to obtain a polyacrylic acid solution, and mix the polyacrylic acid solution and the aqueous composition to obtain a mixture;

[0017] 2) Mix the mixture and the polymer to obtain an aqueous adhesive system.

[0018] Preferably, in step 1), the mixing time of the polyacrylic acid and water is 0.5 to 2 h, stirring is carried out during the mixing process, and the stirring rate is independently 300 to 1000 rpm; in step 2), the mixing is stirring and / or ultrasonic mixing, and the mixing time is 10 to 30 min.

[0019] The present invention also provides a method for preparing a Si-C composite negative electrode using the aqueous adhesive system, comprising the following steps:

[0020] Mix the negative electrode active material, the conductive agent and the aqueous binder system to form a slurry; coat the slurry on a substrate and dry it to obtain a Si-C composite negative electrode;

[0021] The negative electrode active material is one or more of Si-C, SiO x and metal alloys; the conductive agent is conductive carbon black.

[0022] Preferably, the substrate is a carbon fiber substrate or a metal current collector; the drying temperature is 80 to 120 °C.

[0023] The present invention also provides a Si-C composite negative electrode prepared by the method.

[0024] The beneficial effects of the present invention include the following points:

[0025] 1) The present invention provides an aqueous binder system for the high-stress expansion problem of Si-C composite anode materials. By optimizing the ratio of PAA, CMC / SBR and introducing dynamic covalent bond polymers or PU elastomers, an electrode layer with high adhesion, flexibility and self-healing ability is formed on the carbon fiber substrate for the anode material, thereby achieving long-term cycle stability and high specific capacity retention rate.

[0026] 2) PAA in the present invention serves as the main binding matrix, which is easy to form hydrogen bonds or weak chemical bonds with the surface of silicon particles, helping to restrain the expansion of Si; CMC / SBR provides certain flexibility and dispersibility, improving the overall uniformity and flexibility of the electrode coating; dynamic covalent bond polymers or polyurethane elastomers can achieve reversible cross-linking when microcracks occur in the electrode, providing self-healing ability; the Si-C composite anode prepared by the aqueous binder system of the present invention shows higher capacity retention rate and cycle life in half-cell or full-cell tests.

[0027] 3) The aqueous binder system of the present invention significantly improves the structural stability of the Si-C anode layer during cycling, reduces the shedding of active materials; provides a self-healing electrode interface, improving the long-term cycle life and rate performance; the binder system of the present invention is a fully aqueous system, taking into account environmental friendliness and safety, and providing convenience for industrial production. Detailed embodiments

[0028] The present invention provides an aqueous binder system, which comprises polyacrylic acid, an aqueous composition, a polymer and water;

[0029] The aqueous composition comprises carboxymethyl cellulose (CMC) and styrene-butadiene rubber (SBR), and the mass ratio of carboxymethyl cellulose to styrene-butadiene rubber is 1:1 to 2;

[0030] The polymer comprises a polyurethane elastomer and / or a dynamic covalent bond polymer;

[0031] In the aqueous binder system, the mass fraction of polyacrylic acid is 0.5 to 2%, the mass fraction of the aqueous composition is 1 to 3%, and the mass fraction of the polymer is 0.1 to 1%.

[0032] In the present invention, the mass ratio of carboxymethyl cellulose to styrene-butadiene rubber is preferably 1:1.2 to 1.8, and more preferably 1:1.4 to 1.5; in the aqueous binder system, the mass fraction of polyacrylic acid is preferably 0.8 to 1.6%, more preferably 1.2 to 1.5%, the mass fraction of the aqueous composition is preferably 1.5 to 2.5%, more preferably 2%, and the mass fraction of the polymer is preferably 0.3 to 0.8%, more preferably 0.5 to 0.6%.

[0033] In the present invention, the polymer has elastic or reversible cross-linking characteristics.

[0034] In the present invention, the weight-average molecular weight (M w ) of polyacrylic acid (PAA) is preferably 200,000 to 500,000, more preferably 300,000 to 400,000, and even more preferably 350,000.

[0035] In the present invention, the dynamic covalent bond polymer preferably has imide bonds, thioether bonds or reversible Diels-Alder bonds;

[0036] The polyurethane elastomer (PU elastomer) is preferably an aqueous polyurethane dispersion, and the particle size of the polyurethane elastomer is preferably ≤100 nm, more preferably ≤90 nm.

[0037] In the present invention, the weight-average molecular weight (M w ) of the aqueous polyurethane dispersion is preferably 100,000 to 200,000, more preferably 130,000 to 170,000, and even more preferably 150,000.

[0038] In the present invention, the dynamic covalent bond polymer can achieve a reversible process of bonding / fracture when slightly heated or under chemical environmental changes, and spontaneously repair microcracks during the charge and discharge process of the electrode; the PU elastomer has excellent elasticity and fatigue resistance; the dynamic covalent bond polymer or polyurethane elastomer can provide the electrode with self-repair ability, buffer stress and improve cycle stability; by introducing a polymer with imide dynamic bonds, a self-repair mechanism can be generated in the negative electrode film layer, effectively buffering the large volume change of the Si-based material, reducing the shedding of active particles and interface rupture, and improving the cycle life and capacity retention rate of the electrode; when microcracks or damage occur in the electrode under charge and discharge cycles and mechanical stress, the imide (C=N) bond energy can be partially broken and react with surrounding groups again to form new bonds, thereby realizing the "self-repair" of the electrode layer and a longer cycle life; the number-average molecular weight (Mn) of the polymer with imide dynamic bonds is preferably 50,000 to 100,000, more preferably 70,000 to 80,000.

[0039] In the present invention, the control of the weight-average molecular weight of PAA can ensure good adhesion strength and functional group interaction.

[0040] In the present invention, the degree of substitution of the carboxymethyl cellulose is preferably 0.7 to 0.9, more preferably 0.75 to 0.85, and even more preferably 0.8. Styrene-butadiene rubber is preferably added in the form of a styrene-butadiene rubber emulsion, and the solid content of the styrene-butadiene rubber emulsion is preferably 45 to 55%, more preferably 48 to 52%, and even more preferably 50%.

[0041] The present invention also provides a preparation method of the aqueous adhesive system, comprising the following steps:

[0042] 1) Mix polyacrylic acid and water to obtain a polyacrylic acid solution, and then mix the polyacrylic acid solution with the aqueous composition to obtain a mixture;

[0043] 2) Mix the mixture with a polymer to obtain an aqueous adhesive system.

[0044] In the present invention, the mixing time of the polyacrylic acid and water in step 1) is preferably 0.5 - 2 h, more preferably 1 - 1.5 h; stirring is preferably carried out during the mixing process, and the stirring rate is independently preferably 300 - 1000 rpm, more preferably 500 - 800 rpm, and even more preferably 600 - 700 rpm; the mixing in step 2) is preferably stirring and / or ultrasonic mixing, and the mixing time is preferably 10 - 30 min, more preferably 15 - 25 min, and even more preferably 20 min.

[0045] The present invention also provides a method for preparing a Si - C composite negative electrode using the aqueous adhesive system described above, comprising the following steps:

[0046] Mix the negative electrode active material, conductive agent, and aqueous binder system to form a slurry; coat the slurry on a substrate and dry it to obtain a Si - C composite negative electrode;

[0047] The negative electrode active material is one or more of Si - C, SiO x and metal alloys; the conductive agent is conductive carbon black.

[0048] In the present invention, the conductive carbon black is preferably KetjenBlack.

[0049] In the present invention, the substrate is preferably a carbon fiber substrate or a metal current collector; the drying temperature is preferably 80 - 120 °C, more preferably 90 - 110 °C, and even more preferably 100 °C.

[0050] The present invention also provides a Si - C composite negative electrode prepared by the method described above.

[0051] The technical solutions provided by the present invention will be described in detail below with reference to the examples, but they should not be construed as limiting the protection scope of the present invention.

[0052] In the example, the process for surface treatment of T700 carbon fiber cloth is as follows: Plasma treatment is carried out on the surface of T700 carbon fiber cloth to increase the surface energy. Fix the T700 carbon fiber cloth (unidirectional prepreg) flat on the sample stage of the vacuum chamber, pump the chamber pressure down to 0.1 Torr (13.3 Pa), introduce high-purity oxygen and maintain it at 0.1 Torr, start the radio frequency power supply (50 - 100 W), and the treatment time is 5 min. Turn off the radio frequency power supply and the gas flow, slowly increase the pressure to atmospheric pressure, and take out the T700 carbon fiber cloth. At this time, the oxygen-containing functional groups on the fiber surface increase, the surface energy increases by 30 - 40%, and the wettability and adhesion to the aqueous slurry are significantly improved.

[0053] The separator is Celgard 2400 or PP / PE composite membrane, and the electrolyte is a conventional carbonate system.

[0054] Example 1

[0055] The aqueous binder system consists of PAA (M w is 450000), CMC (high-purity CMC, degree of substitution 0.8), SBR emulsion (solid content 50%), aqueous PU elastomer (M w is 150000, particle size is 90 nm) and water; in the aqueous binder system, the mass fraction of PAA is 1%, the mass fractions of CMC and SBR are 1% respectively, and the mass fraction of the aqueous PU elastomer is 0.5%.

[0056] Dissolve PAA in deionized water and stir at a speed of 600 rpm for 1 h, then slowly add CMC and SBR emulsion, and continue to stir at a speed of 600 rpm until a uniform emulsion is formed; the uniform emulsion and the aqueous PU elastomer are stirred (at a speed of 1500 rpm) and ultrasonically dispersed (power 300 W) for 20 min to obtain a uniformly dispersed aqueous binder system.

[0057] Mix SiO x and Si - C to obtain Si - C - SiO x Mix the negative electrode active material (the mass ratio of Si, C and SiO x is 1:3:0.5), KetjenBlack with the aqueous binder system by stirring. The mass ratio of the Si - C - SiO x mixed negative electrode active material, KetjenBlack to the binder (dry basis) is 92:5:3 to obtain a slurry with a viscosity of 2500 mPa·S. The slurry is uniformly coated on the surface-treated T700 carbon fiber cloth substrate, and the dry film (negative electrode active layer) thickness is 20 μm. After drying at 100 °C, the negative electrode film layer is obtained. Assemble the coated negative electrode sheet with a lithium metal sheet, a separator, and an electrolyte into a coin-type half-cell under an argon atmosphere in a glove box.

[0058] Comparative Example 1

[0059] Omit the aqueous PU elastomer in Example 1, and the other process conditions are the same as those in Example 1.

[0060] Example 2

[0061] The aqueous adhesive system consists of PAA (M w is 450000), CMC (high-purity CMC, degree of substitution 0.8), SBR emulsion (solid content is 50%), polymer with imide dynamic bonds (polyimide) and water; in the aqueous adhesive system, the mass fraction of PAA is 1%, and the mass fractions of CMC and SBR are 1% respectively, and the mass fraction of the polymer with imide dynamic bonds is 0.5%.

[0062] Dissolve PAA in deionized water and stir at a speed of 600 rpm for 1 h, then slowly add CMC and SBR, and continue to stir at a speed of 600 rpm until a uniform emulsion is formed; the uniform emulsion and the polymer with imide dynamic bonds are ultrasonically dispersed (power 300 W) for 20 min to obtain a uniformly dispersed aqueous adhesive system.

[0063] Mix the Si-C anode active material (mass ratio of Si to C is 1:4), KetjenBlack with the aqueous adhesive system to obtain a slurry. The slurry is coated on the surface-treated T700 carbon fiber cloth substrate, dried at 100 °C and then assembled into a coin-type half-cell. The dry film (anode active layer) thickness is 25 μm, and the anode film layer is obtained after drying at 100 °C. Assemble the coated anode sheet with a lithium metal sheet, a separator, and an electrolyte into a coin-type half-cell.

[0064] Comparative Example 2

[0065] Omit the polymer with imide dynamic bonds in Example 2, and the other process conditions are the same as those in Example 2.

[0066] Test the coin-type half-cells of Examples 1-2 and Comparative Examples 1-2. The coin-type half-cell of Example 1 has a capacity retention rate of more than 80% after 200 cycles at a 0.2C rate, which is 5% higher than that of the electrode without adding the aqueous PU elastomer (Comparative Example 1), and the cracks on the electrode surface are significantly reduced. The coin-type half-cell of Example 2 has a capacity retention rate increased to 88% after 150 cycles, significantly reducing the electrode cracking and delamination phenomena. The coin-type half-cell of Comparative Example 2 has an initial discharge specific capacity of 1100 mAh / g at a 0.1C rate, and a capacity retention rate of 85% after 100 cycles, without obvious delamination.

[0067] Example 3

[0068] The aqueous adhesive system consists of PAA (M wPAA (molecular weight is 400,000), CMC (high-purity CMC, degree of substitution 0.75), SBR emulsion (solid content is 50%), aqueous PU elastomer (M w is 150,000, particle size is 90 nm), and water; in the aqueous adhesive system, the mass fraction of PAA is 1.5%, the mass fractions of CMC and SBR are 1% respectively, and the mass fraction of the aqueous PU elastomer is 0.3%.

[0069] Dissolve PAA in deionized water and stir at a speed of 800 rpm for 1 h, then slowly add CMC and SBR emulsion, and continuously stir at a speed of 800 rpm until a homogeneous emulsion is formed; the homogeneous emulsion and the aqueous PU elastomer are stirred (at a speed of 1500 rpm) and ultrasonically dispersed (power 300 W) for 20 min to obtain a uniformly dispersed aqueous adhesive system.

[0070] Mix the Si-C (mass ratio of Si and C is 1:4) and Sn-based alloy (Sn-Co alloy, particle size 50 nm) mixed anode active material (in the mixed anode active material, the mass fraction of Si-C is 95%, and the mass fraction of the Sn-based alloy is 5%), KetjenBlack and the aqueous adhesive system to obtain a slurry, and the slurry is coated on the surface-treated T700 carbon fiber cloth (width 10 cm, length 50 cm) substrate by a roll-to-roll process, and the total coating area is 500 cm 2 and dried at 100 °C. After cutting the large-area anode material into several single pieces, it is assembled with lithium metal sheets, separators and electrolytes into a stacked full cell (such as a small soft-pack battery).

[0071] The large-area electrode in Example 3 showed in the full cell test that it could still maintain a specific capacity of more than 70% at a high rate (1C), and there was no obvious peeling or interfacial cracking after 300 cycles, indicating that reliable performance comparable to that verified in small-scale tests could still be obtained at a larger size, reflecting the potential of the aqueous adhesive of the present invention in large-scale applications.

[0072] The aqueous adhesive system of the present invention has a significant synergistic effect on the high specific capacity anode material, and can provide better interfacial binding force and flexible self-healing ability under the conditions of stress accumulation and active material expansion. In the assembly of the Si-C composite anode into a half cell test, the capacity retention rate can be increased by 5-10% after 100 cycles, and at the same time, the peeling rate and crack density are significantly reduced; it can still maintain a high specific capacity and stable cycling performance at a high rate (≥1C), showing the elasticity and self-healing ability of the adhesive system; the composite anode of the present invention has high capacity, long life and excellent structural stability while maintaining mechanical strength.

[0073] The above are only the preferred embodiments of the present invention. It should be noted that for those of ordinary skill in the art, without departing from the principle of the present invention, several improvements and refinements can be made, and these improvements and refinements should also be regarded as the protection scope of the present invention.

Claims

1. A water-based adhesive system, characterized in that: The aqueous adhesive system comprises polyacrylic acid, an aqueous composition, a polymer and water; The aqueous composition comprises carboxymethyl cellulose and styrene-butadiene rubber, wherein the mass ratio of the carboxymethyl cellulose to the styrene-butadiene rubber is 1:1-2; The polymer comprises a polyurethane elastomer and / or a dynamic covalent bond polymer; In the aqueous binder system, the mass fraction of polyacrylic acid is 0.5-2%, the mass fraction of the aqueous composition is 1-3%, and the mass fraction of the polymer is 0.1-1%.

2. The aqueous adhesive system according to claim 1, characterized in that: The weight average molecular weight of polyacrylic acid is 200,000 to 500,000.

3. The aqueous adhesive system according to claim 1 or 2, characterized in that: The dynamic covalent bond polymer has an imide bond, a thioether bond or a reversible Diels-Alder bond; The polyurethane elastomer is an aqueous polyurethane dispersion, and the particle size of the polyurethane elastomer is ≤100nm.

4. The aqueous adhesive system according to claim 3, characterized in that: The substitution degree of the carboxymethyl cellulose is 0.7-0.9, the styrene-butadiene rubber is added in the form of styrene-butadiene rubber emulsion, and the solid content of the styrene-butadiene rubber emulsion is 45-55%.

5. The method for preparing the aqueous adhesive system according to any one of claims 1 to 4, characterized in that: The following steps are included: 1) mixing polyacrylic acid and water to obtain a polyacrylic acid solution, and mixing the polyacrylic acid solution and an aqueous composition to obtain a mixture; 2) The mixture is mixed with a polymer to obtain a water-based adhesive system.

6. The preparation method according to claim 5, characterized in that: In step 1), the polyacrylic acid and water are mixed for 0.5 to 2 hours, and stirring is performed during the mixing process, and the stirring rate is independently 300 to 1000 rpm; in step 2), the mixing is stirring and / or ultrasonic mixing, and the mixing time is 10 to 30 minutes.

7. The method for preparing a Si-C composite negative electrode using an aqueous binder system according to any one of claims 1 to 4, characterized in that: The following steps are included: The negative electrode active material, the conductive agent and the aqueous binder system are mixed to form a slurry; the slurry is coated on a substrate and dried to obtain a Si-C composite negative electrode; The negative electrode active material is Si-C, SiO x and one or more of metal alloys; the conductive agent is conductive carbon black.

8. The method according to claim 7, characterized in that The substrate is a carbon fiber substrate or a metal current collector; the drying temperature is 80-120°C.

9. The Si-C composite negative electrode prepared by the method according to claim 7 or 8.