Lithium ion battery negative electrode and preparation method and application thereof

Abstract

The invention relates to the technical field of lithium ion batteries, in particular to a high-stability lithium ion battery negative electrode, a preparation method of the negative electrode and application of the negative electrode in a lithium ion battery. The negative electrode contains a negative electrode active material and an interface modifier, the interface modifier is a random copolymer which is subjected to chemical pretreatment by a lithiation agent and contains double acidic groups, and the double acidic groups are a carboxylic acid group and a sulfonic acid group. The pre-lithiated polymer forms a stable functional interface layer rich in lithium ions on the surface of the negative electrode active material, and the layer has efficient lithium ion selective transmission capability and excellent chemical stability, can effectively block the erosion of electrolyte solvent molecules, inhibits interface side reaction under a high-temperature cycle condition, and improves the service life of the negative electrode active material. And rapid desolvation and migration of lithium ions are promoted. Experiments show that when the lithium ion battery adopting the negative electrode material circulates at high temperatures of 45 DEG C and 55 DEG C, the capacity retention ratio is remarkably improved compared with that of an unmodified system.

Description

TECHNICAL FIELD

[0001] The present application relates to the technical field of lithium ion batteries, in particular to a high-stability lithium ion battery negative electrode and a preparation method thereof, and application of the negative electrode in a lithium ion battery. BACKGROUND

[0002] Lithium ion batteries are widely used in portable electronic devices, electric vehicles and energy storage systems due to their high energy density, long cycle life and excellent reversibility. However, the cycle life and safety under high working temperature (>45℃) environment face serious challenges. High temperature will exacerbate the decomposition side reaction of electrolyte at the interface of graphite, silicon-carbon and other negative electrode materials, leading to continuous thickening of the solid electrolyte interface (SEI) film, rising of the interface impedance and irreversible loss of active lithium, ultimately causing rapid decay of battery capacity. Therefore, improving the cycle stability of lithium ion batteries under high temperature conditions has become a technical problem to be solved in this field.

[0003] In the prior art, one of the common strategies to improve the high-temperature performance of the battery is to introduce film-forming additives such as vinylene carbonate and fluorinated carbonate into the electrolyte (Journal of The Electrochemical Society, 2020, 167, 130543; CN202111634458.9). Such additives are usually preferentially decomposed at the electrode / electrolyte interface to form a protective film, thereby inhibiting side reactions and improving cycle performance. However, this strategy has inherent defects: the additives are irreversibly consumed in the film-forming process and cannot continuously repair and stabilize the interface in long-term cycling; at high temperatures, their decomposition products may be unstable and even catalyze the generation of gas, posing a safety hazard; in addition, the additives are uniformly distributed in the electrolyte, lacking directional anchoring to the negative electrode surface, and the protection efficiency is limited.

[0004] Another type of technical solution focuses on modifying the negative electrode material itself, such as constructing a polymer coating layer on the surface of the negative electrode active material, or dispersing a functional polymer as an additive in the negative electrode slurry (CN202111390102.5). This method aims to build a physical barrier to block the direct contact between the electrolyte and the negative electrode. However, existing polymer modifiers generally have the problem of single function: most polymers cannot simultaneously have excellent ionic conductivity and stable interface chemical properties. Some polymers are prone to swelling or degradation in high-temperature electrolyte, losing their protective function; while some polymers with good stability have low intrinsic ionic conductivity, which will seriously hinder the transmission of lithium ions and increase the polarization of the battery. Therefore, developing a new type of polymer modification strategy that can simultaneously solve the contradiction between interface stability and ion transport dynamics is a technical bottleneck that needs to be broken through in this field. SUMMARY

[0005] The primary object of the present application is to overcome the above-mentioned deficiencies of the prior art, and to provide a lithium-ion battery negative electrode which can maintain an extremely stable interface structure under high-temperature environments and significantly inhibit capacity decay. Another object of the present application is to provide a preparation method for the negative electrode, which is simple in process and has good compatibility with existing battery manufacturing processes. Still another object of the present application is to illustrate the application of a lithium-ion battery containing the negative electrode, which is particularly suitable for application scenarios requiring high temperature stability.

[0006] To achieve the above-mentioned objects, the present application proposes the following technical solutions: A lithium-ion battery negative electrode, the negative electrode containing a negative electrode active material and an interface modifier, the interface modifier being a random copolymer containing double acidic groups chemically pretreated by a lithiation agent, the double acidic groups being carboxylic acid groups and sulfonic acid groups.

[0007] The interface modifier is obtained by pretreating a double-acid-group-containing polymer in an aqueous solvent or an alcohol solvent with a lithiation agent; wherein the degree of pretreatment is 50% to 100% of the total number of moles of acidic groups in the polymer, and the lithiation agent is lithium hydroxide, lithium carbonate or lithium ethoxide.

[0008] The double-acid-group-containing polymer is a copolymer of acrylic monomers and sulfonic monomers, wherein the molar ratio of carboxylic acid groups to sulfonic acid groups in the copolymerization system is 1:9 to 9:1, so as to realize the improvement of interface stability and lithium ion migration ability through the synergistic effect of interface anchoring by carboxylic acid groups and lithium-rich domains formed by sulfonic acid groups.

[0009] The acrylic monomers are one or a combination of several of acrylic acid, methacrylic acid, itaconic acid, maleic acid and fumaric acid.

[0010] The sulfonic monomers are one or a combination of several of 2-acrylamide-2-methylpropane sulfonic acid, styrene sulfonic acid, sodium styrene sulfonate, allyl sulfonic acid and methacrylic sulfonic acid.

[0011] A preparation method of the lithium-ion battery negative electrode: After the negative electrode sheet is formed, the interface modifier is introduced in the form of soaking, coating or spraying to coat the surface of the negative electrode active material, and then dried to obtain a battery negative electrode with an interface protection layer; Or, during the preparation of the negative electrode slurry, the interface modifier is added to the negative electrode slurry and uniformly dispersed, and then the slurry is coated on the surface of the current collector and dried to obtain a battery negative electrode containing the interface modifier.

[0012] The negative electrode sheet or negative electrode slurry usually further contains a conductive agent and a binder. The conductive agent is used to improve the electronic conductivity of the negative electrode sheet and build a stable electron transmission network. The conductive agent is selected from carbon black type conductive materials, one-dimensional or two-dimensional carbon materials, and combinations thereof. Preferably, the carbon black type conductive material includes but is not limited to acetylene black, superconducting carbon black, and ketchen black; the one-dimensional or two-dimensional carbon material includes but is not limited to carbon nanotubes, graphene and its derivatives. The binder is used to enhance the adhesion strength between the negative active material, the conductive agent and the current collector, and to improve the overall stability of the electrode structure. The binder is selected from high molecular adhesive materials in water-based or organic solvent systems. Preferably, the binder includes but is not limited to polyvinylidene fluoride, polyvinylidene fluoride-hexafluoropropylene copolymer, carboxymethyl cellulose, butadiene rubber, polyacrylic acid and its derivatives, sodium alginate and combinations thereof.

[0013] When the interface modifier exists in the form of a coating layer, the thickness of the coating layer is 10 nm to 500 nm.

[0014] The interface modifier is dispersed in the slurry, and accounts for 0.05 wt% to 5 wt% of the mass of the negative active material.

[0015] The negative active material is at least one of graphite, hard carbon, soft carbon, silicon-based material, tin-based material or silicon-carbon composite material.

[0016] A lithium ion battery comprising the lithium ion battery negative electrode as described.

[0017] Compared with the prior art, the present application has the following remarkable advantages and beneficial effects: 1. Synergistic interface regulation mechanism: The present application innovatively uses pre-lithiated double acidic group polymer as an interface modifier. The carboxylic acid group provides strong adhesion to the surface of the negative active material, ensuring the stability of the modified layer during the cycle process; and the sulfonic acid group endows the modified layer with extremely high lithium ion transference number, building a high-efficiency ion transmission channel. The two work together to achieve the dual functions of "stable anchoring" and "high-speed ion guiding" of the negative electrode interface.

[0018] 2. Excellent high-temperature cycle stability: The lithiated polymer of the present application can effectively block the contact between the electrolyte solvent molecules and the negative electrode at high temperature, fundamentally inhibiting the occurrence of side reactions. Experimental data show that the capacity retention rate of the battery using the present application is significantly higher than that of the unmodified comparative battery when cycled at 45℃ and 55℃, showing excellent high-temperature stability.

[0019] 3. Unique "lithiophilic and solvent-phobic" characteristics: after pre-lithiation, the polymer backbone is negatively charged, which has high selectivity for lithium ions, and can repel solvent molecules and electrolyte anions in the solvation sheath, greatly promoting the desolvation process of lithium ions at the interface and reducing the interface impedance.

[0020] 4. Pre-lithiation function: the interface modifier of the present application can play a protective role while its own lithium can compensate for part of the active lithium loss during the first charge and discharge process, which helps to improve the first cycle coulombic efficiency and the recyclable lithium inventory of the battery throughout its life cycle.

[0021] 5. The polymer of the present application can form a nano-coating layer on the surface of the active material through a simple liquid phase method, or can be directly used as an additive and blended with the negative electrode slurry, which has a simple process flow, is highly compatible with existing mass production lines, and has high potential for industrial application. BRIEF DESCRIPTION OF DRAWINGS

[0022] Table 1. Effect of different polymers and treatment methods on capacity retention rate.

[0023] Figure 1 Capacity retention rate curve of the battery in Example 1 at 45℃ and 55℃ for 100 cycles.

[0024] Figure 2 Capacity retention rate curve of the battery in Comparative Example 1 at 45℃ and 55℃ for 100 cycles. DETAILED DESCRIPTION

[0025] The present application will be further described in detail by specific examples. Obviously, the described examples are only part of the embodiments of the present application, but not all the embodiments. Based on the examples in the present application, all other embodiments obtained by those skilled in the art without creative labor are within the scope of protection of the present application.

[0026] The negative electrode material comprises a chemically pre-lithiated random copolymer interface modifier with both carboxylic acid groups and sulfonic acid groups, and the molar ratio of the carboxylic acid groups to the sulfonic acid groups is 1:9 to 9:1. The interface modifier can modify the surface of the negative electrode active material in the form of a nano-coating layer, or be uniformly dispersed in the negative electrode slurry as a functional additive. The pre-lithiated polymer forms a stable and lithium ion-rich functional interface layer at the negative electrode interface, which has high lithium ion selective transport capacity and excellent chemical stability, can effectively block the invasion of electrolyte solvent molecules, inhibit the interface side reaction under high temperature cycling conditions, and promote the rapid desolvation and migration of lithium ions. Experiments show that the capacity retention rate of the lithium ion battery using the negative electrode material is significantly improved at 45°C and 55°C high temperature, and the battery has excellent high temperature cycling stability and interface durability.

[0027] Example 1 Acrylic acid and 2-acrylamide-2-methylpropane sulfonic acid (AMPS) are copolymerized at a molar ratio of 1:1 to obtain a polymer containing carboxylic acid groups and sulfonic acid groups. The polymer is dissolved in water, and lithium hydroxide is added in an amount equal to the molar amount of the acidic groups of the polymer. The mixture is stirred for 1 hour to obtain a lithiumated polymer solution, and the degree of lithiumation is 100%.

[0028] The solution is coated on the surface of a graphite negative electrode material, and a composite negative electrode is obtained after drying at 60°C.

[0029] A soft pack battery is assembled using the above-obtained negative electrode, a lithium iron phosphate positive electrode, and a conventional carbonate electrolyte (1 mol / L LiPF6 in DMC:EC:EMC=1:1:1 Vol%).

[0030] After 100 cycles at 45°C, the battery capacity retention rate is 97%. After 100 cycles at 55°C, the capacity retention rate is 90% (. Figure 1 ).

[0031] Example 2 The lithiumated polymer obtained in Example 1 is added to a graphite negative electrode slurry (graphite and styrene-butadiene rubber are dispersed in water to form a slurry) at a proportion of 1 wt% of the negative electrode active material to prepare a negative electrode sheet.

[0032] A soft pack battery is assembled using the above-obtained negative electrode, a lithium iron phosphate positive electrode, and a conventional carbonate electrolyte (1 mol / L LiPF6 in DMC:EC:EMC=1:1:1 Vol%).

[0033] After 100 cycles at 45°C, the battery capacity retention rate is 95%. After 100 cycles at 55°C, the capacity retention rate is 89%.

[0034] Example 3 The lithiated polymer obtained in Example 1 was added to a graphite negative electrode slurry (graphite and styrene-butadiene rubber dispersed in water) at a proportion of 3 wt% of the negative electrode active material to prepare a negative electrode sheet.

[0035] A soft pack battery was assembled using this negative electrode, a lithium iron phosphate positive electrode, and a conventional carbonate electrolyte (1 mol / L LiPF6 in DMC:EC:EMC = 1:1:1 Vol%).

[0036] After 100 cycles at 45°C, the battery capacity retention rate was 94%. After 100 cycles at 55°C, the capacity retention rate was 90%.

[0037] Example 4 An acrylic acid and AMPS copolymer was obtained at a molar ratio of 1:2, and a polymer containing carboxylic acid groups and sulfonic acid groups was obtained. The polymer was dissolved in water, lithium hydroxide was added in an equimolar amount to the acidic groups in the polymer, and stirring was performed for 1 hour to obtain a lithiated polymer solution, and the degree of lithiation was 100%.

[0038] The solution was coated on the surface of a graphite negative electrode material, and a composite negative electrode was obtained after drying at 60°C. A soft pack battery was assembled using this negative electrode, a lithium iron phosphate positive electrode, and a conventional carbonate electrolyte (1 mol / L LiPF6 in DMC:EC:EMC = 1:1:1 Vol%).

[0039] After 100 cycles at 45°C, the battery capacity retention rate was 93%. After 100 cycles at 55°C, the capacity retention rate was 87%.

[0040] Example 5 An acrylic acid and 4-vinylbenzenesulfonic acid copolymer was obtained at a molar ratio of 1:1, and a polymer containing carboxylic acid groups and sulfonic acid groups was obtained.

[0041] The polymer was dissolved in water, lithium carbonate was added in an amount of half (molar ratio) of the acidic groups in the polymer, and stirring was performed for 1 hour to obtain a lithiated polymer solution, and the degree of lithiation was 100%.

[0042] The solution was coated on the surface of a graphite negative electrode material, and a composite negative electrode was obtained after drying at 60°C. A soft pack battery was assembled using this negative electrode, a lithium iron phosphate positive electrode, and a conventional carbonate electrolyte (1 mol / L LiPF6 in DMC:EC:EMC = 1:1:1 Vol%).

[0043] After 100 cycles at 45°C, the battery capacity retention rate was 95%. After 100 cycles at 55°C, the capacity retention rate was 88%.

[0044] Example 6 The lithiated polymer solution obtained in Example 1 was coated on the surface of the graphite negative electrode material, and a composite negative electrode was obtained after drying at 60°C. A soft pack battery was assembled using this negative electrode, a lithium cobalt oxide positive electrode, and a conventional carbonate electrolyte (1 mol / L LiPF6 in DMC:EC:EMC = 1:1:1 Vol%).

[0045] After 100 cycles at 45°C, the capacity retention rate of the battery was 94%. After 100 cycles at 55°C, the capacity retention rate was 87%.

[0046] Example 7 The lithiated polymer solution obtained in Example 1 was coated on the surface of the hard carbon negative electrode material, and a composite negative electrode was obtained after drying at 60°C. A soft pack battery was assembled using this negative electrode, a lithium iron phosphate positive electrode, and a conventional carbonate electrolyte (1 mol / L LiPF6 in DMC:EC:EMC = 1:1:1 Vol%).

[0047] After 100 cycles at 45°C, the capacity retention rate of the battery was 95%. After 100 cycles at 55°C, the capacity retention rate was 89%.

[0048] Example 8 An acrylic acid-AMPS copolymer containing carboxylic acid groups and sulfonic acid groups was obtained by copolymerization of acrylic acid and AMPS at a molar ratio of 1:1. The polymer was dissolved in water, lithium hydroxide was added at half the molar ratio of the acidic groups in the polymer, and the mixture was stirred for 1 hour to obtain a lithiated polymer solution, with a lithiation degree of 50%.

[0049] The solution was coated on the surface of the graphite negative electrode material, and a composite negative electrode was obtained after drying at 60°C.

[0050] A soft pack battery was assembled using the above-obtained negative electrode, a lithium iron phosphate positive electrode, and a conventional carbonate electrolyte (1 mol / L LiPF6 in DMC:EC:EMC = 1:1:1 Vol%).

[0051] After 100 cycles at 45°C, the capacity retention rate of the battery was 95%. After 100 cycles at 55°C, the capacity retention rate was 88% Figure 1 .

[0052] Comparative Example 1 A soft pack battery was prepared using a graphite negative electrode without any added polymer, a lithium iron phosphate positive electrode, and a conventional carbonate electrolyte (1 mol / L LiPF6 in DMC:EC:EMC = 1:1:1 Vol%). After 100 cycles at 45°C, the capacity retention rate was only 77%, and after 100 cycles at 55°C, the capacity retention rate was 65%.

[0053] Comparative Example 2 The polymer obtained in Example 1 was added to a graphite negative electrode slurry without lithiation at a proportion of 1 wt% of the negative electrode active material to prepare a negative electrode sheet. A soft pack battery was assembled with this negative electrode, a lithium iron phosphate positive electrode, and a conventional carbonate electrolyte (1 mol / L LiPF6 in DMC:EC:EMC = 1:1:1 Vol%).

[0054] The capacity retention rate was 82% after 100 cycles at 45°C and 72% after 100 cycles at 55°C.

[0055] Comparative Example 3 A lithiated polymer containing only carboxylic acid groups (lithium polyacrylate, degree of lithiation 100%) was used as a negative electrode additive, added to a graphite negative electrode slurry at a proportion of 1 wt% of the negative electrode active material to prepare a negative electrode sheet. A soft pack battery was assembled with this negative electrode, a lithium iron phosphate positive electrode, and a conventional carbonate electrolyte (1 mol / L LiPF6 in DMC:EC:EMC = 1:1:1 Vol%).

[0056] The capacity retention rate was 83% after 100 cycles at 45°C and 70% after 100 cycles at 55°C. The capacity retention rates at both of the above temperatures were lower than the double acidic group polymer of the present application.

[0057] Comparative Example 4 A lithiated polymer containing only sulfonic acid groups (lithium polystyrene sulfonate, degree of lithiation 100%) was used as a negative electrode additive, added to a graphite negative electrode slurry at a proportion of 1 wt% of the negative electrode active material to prepare a negative electrode sheet. A soft pack battery was assembled with this negative electrode, a lithium iron phosphate positive electrode, and a conventional carbonate electrolyte (1 mol / L LiPF6 in DMC:EC:EMC = 1:1:1 Vol%).

[0058] The capacity retention rate was 82% after 100 cycles at 45°C and 72% after 100 cycles at 55°C. The capacity retention rates at both of the above temperatures were lower than the double acidic group polymer of the present application.

[0059] Comparative Example 5 A polymer containing a mercapto group and a phosphoric acid group was obtained by copolymerizing 3-mercaptopropyl methacrylate and 2-hydroxyethyl methacrylate phosphate at a molar ratio of 1:1. The polymer was dissolved in water, and lithium hydroxide was added in an amount equivalent to the acidic groups of the polymer. The mixture was stirred for 1 hour to obtain a lithiated polymer solution, and the degree of lithiation was 100%.

[0060] The solution was coated on the surface of a graphite negative electrode material, and a composite negative electrode was obtained after drying at 60°C.

[0061] The above obtained negative electrode, lithium iron phosphate positive electrode and conventional carbonate electrolyte (1 mol / L LiPF6 in DMC:EC:EMC = 1:1:1 Vol%) were assembled into a soft pack battery.

[0062] After 100 cycles at 45°C, the capacity retention rate was 84%. After 100 cycles at 55°C, the capacity retention rate was 73%.

[0063] Comparative Example 6 A soft pack battery was prepared using a graphite negative electrode, lithium cobalt oxide positive electrode and conventional carbonate electrolyte (1 mol / L LiPF6 in DMC:EC:EMC = 1:1:1 Vol%) without adding any lithiated polymer, and the capacity retention rate was only 75% after 100 cycles at 45°C, and the capacity retention rate was 64% after 100 cycles at 55°C.

[0064] Comparative Example 7 A soft pack battery was prepared using a hard carbon negative electrode, lithium iron phosphate positive electrode and conventional carbonate electrolyte (1 mol / L LiPF6 in DMC:EC:EMC = 1:1:1 Vol%) without adding any lithiated polymer, and the capacity retention rate was only 76% after 100 cycles at 45°C, and the capacity retention rate was 62% after 100 cycles at 55°C.

[0065] Table 1: Effect of different polymers and treatment methods on capacity retention rate

Claims

1. A lithium-ion battery negative electrode, characterized in that, The negative electrode contains a negative electrode active material and an interface modifier. The interface modifier is a random copolymer containing dual acid groups that has been chemically pretreated with a lithium-ionizing agent. The dual acid groups are carboxylic acid groups and sulfonic acid groups.

2. The lithium-ion battery negative electrode according to claim 1, characterized in that, The interface modifier is obtained by pre-lithiation of a polymer containing dual acid groups in an aqueous or alcoholic solvent using a lithiation agent; wherein the degree of pre-lithiation is 50% to 100% of the total molar number of acid groups in the polymer, and the lithiation agent is lithium hydroxide, lithium carbonate, or lithium ethoxide.

3. The lithium-ion battery negative electrode according to claim 1 or 2, characterized in that, The polymer containing dual acid groups is a random copolymer of acrylic acid monomers and sulfonic acid monomers, wherein the molar ratio of carboxylic acid groups to sulfonic acid groups in the copolymer system is 1:9 to 9:

1.

4. The lithium-ion battery negative electrode according to claim 3, characterized in that, The acrylic monomers are one or a combination of acrylic acid, methacrylic acid, itaconic acid, maleic acid and fumaric acid.

5. The lithium-ion battery negative electrode according to claim 3, characterized in that, The sulfonic acid monomers are one or a combination of several of 2-acrylamido-2-methylpropanesulfonic acid, styrene sulfonic acid, sodium styrene sulfonate, allyl sulfonic acid, and methacrylic sulfonic acid.

6. A method for preparing a lithium-ion battery negative electrode according to claim 1, characterized in that, After the negative electrode sheet is formed, the interface modifier is introduced by immersion, coating or spraying to coat the surface of the negative electrode active material, and then dried to obtain a battery negative electrode with an interface protective layer. Alternatively, during the preparation of the negative electrode slurry, the interface modifier is added to the negative electrode slurry and uniformly dispersed, and then the slurry is coated on the surface of the current collector and dried to obtain a battery negative electrode containing the interface modifier.

7. The method for preparing the lithium-ion battery negative electrode according to claim 6, characterized in that, When the interface modifier exists in the form of a coating layer, the coating layer thickness is from 10 nm to 500 nm.

8. The method for preparing the lithium-ion battery negative electrode according to claim 6, characterized in that, The interface modifier is dispersed in the slurry, accounting for 0.05 wt% to 5 wt% of the mass of the negative electrode active material.

9. The method for preparing the lithium-ion battery negative electrode according to claim 6, characterized in that, The negative electrode active material is at least one of graphite, hard carbon, soft carbon, silicon-based materials, tin-based materials, or silicon-carbon composite materials.

10. A lithium-ion battery, characterized in that, It includes a lithium-ion battery negative electrode as described in any one of claims 1-8.

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