A modified aqueous negative electrode, its preparation method and application

Abstract

This invention provides a modified aqueous negative electrode, its preparation method, and its application. The modified aqueous negative electrode includes a negative electrode current collector and a negative electrode active material layer disposed on the surface of the current collector. The negative electrode active material layer includes a negative electrode active material, a conductive agent, sodium alginate, waterborne polyurethane, and a pore-forming agent. The modified aqueous negative electrode of this invention solves both the raw material shortage problem of oil-based negative electrode formulations and the pain points of current aqueous negative electrode formulations, improving the slurry solids content, electrode porosity, and flexibility, while reducing electrode resistivity. Furthermore, this formulation is environmentally friendly, requiring no control of ambient humidity, and can significantly reduce production costs.

Description

Technical Field

[0001] This invention belongs to the field of lithium-ion battery technology, and relates to a modified aqueous negative electrode, its preparation method, and its application. Background Technology

[0002] Binders are among the most important auxiliary materials in lithium-ion batteries. Typically, negative electrode formulations use a mixture of sodium carboxymethyl cellulose (CMCNa) and styrene-butadiene rubber (SBR) as the binder. However, with the market's increasing pursuit of higher energy density and the significant increase in negative electrode surface area, the drawbacks of the CMCNa+SBR mixture system are becoming increasingly apparent, such as poor electrode flexibility and susceptibility to cracking. Therefore, the development of new negative electrode binder formulations is urgently needed.

[0003] Porous electrode structure design is one of the important means to improve battery performance. The electronic conduction network and ion transport network are affected by the structural parameters of porous electrodes, namely porosity, pore size and distribution, tortuosity, and electrode composition distribution. Among these, porosity has been the most studied. Most researchers have simply added pore-forming agents such as NaCl and PMMA to increase the porosity of the electrode, ignoring the fact that excessively high porosity can directly affect the energy density of the battery.

[0004] CN115763814A discloses a negative electrode slurry and its preparation method, a negative electrode sheet, and a lithium-ion battery. The negative electrode slurry comprises a negative electrode active material, a conductive agent, a thickener, a binder, a material with cationic groups, and a solvent. The thickener includes sodium carboxymethyl cellulose (CMC), and the binder includes styrene-butadiene rubber (SBR).

[0005] CN105449220A discloses an aqueous binder for lithium-ion batteries, wherein the binder is a modified polyacrylate solution with a mass content of 1%-6% and deionized water as the solvent.

[0006] One of the above solutions is to use polyacrylic acid binders to replace PVDF, but the electrode sheets produced by this binder have poor flexibility and are very easy to break and fall off; the other solution is to use a CMCNa+SBR mixed binder, where CMCNa mainly plays a dispersing and thickening role and SBR mainly plays a binding role, but the slurry produced by this solution has too low solids content, requiring a higher coating thickness and easily causing the electrode sheets to crack.

[0007] Therefore, addressing the technical shortcomings of water-based adhesives and rationally adding and selecting pore-forming agents becomes particularly important. Summary of the Invention

[0008] The purpose of this invention is to provide a modified aqueous anode, its preparation method, and its application. The modified aqueous anode of this invention solves both the raw material shortage problem of oil-based anode formulations and the pain points of current aqueous anode formulations, improving slurry solids content, electrode porosity, and flexibility, while simultaneously reducing electrode resistivity. Furthermore, this formulation is environmentally friendly, requiring no control of ambient humidity, and can significantly reduce production costs.

[0009] To achieve this objective, the present invention employs the following technical solution:

[0010] In a first aspect, the present invention provides a modified aqueous negative electrode, the modified aqueous negative electrode comprising a negative electrode current collector and a negative electrode active material layer disposed on the surface of the negative electrode current collector, the negative electrode active material layer comprising a negative electrode active material, a conductive agent, sodium alginate, waterborne polyurethane and a pore-forming agent.

[0011] This invention uses a sodium alginate + water-based polyurethane mixed binder, which not only improves the problems of poor electrode flexibility and low peel strength, but also increases the solid content of the slurry. Simultaneously, it creatively incorporates an organic ammonium salt as a pore-forming agent. The addition of the pore-forming agent effectively increases the porosity of the electrode, thereby reducing the electrode resistivity and consequently lowering the battery's internal resistance.

[0012] Sodium alginate, as described in this invention, is a natural polysaccharide that possesses the stability, solubility, viscosity, and safety required for battery additives. During homogenization, sodium alginate resists the van der Waals forces between negative electrode material particles through its electrostatic repulsion and steric hindrance, thereby preventing slurry agglomeration and increasing the solid content of the slurry.

[0013] The waterborne polyurethane of this invention is a novel polyurethane system that uses water instead of organic solvents as the dispersion medium. Compared to oil-based polyurethane, waterborne polyurethane has the following advantages: it uses water as a solvent, resulting in no pollution, safety and reliability, excellent mechanical properties, good compatibility, and ease of modification. During the homogenization process, waterborne polyurethane effectively acts as a binder.

[0014] Preferably, the total mass percentage of sodium alginate + waterborne polyurethane in the negative electrode active material layer is 'a', and the compaction density of the modified aqueous negative electrode is 's', with units of g / cm³. 3 FL=(a÷s)×100, the mass percentage of the conductive agent in the negative electrode active material layer is b, the mass percentage of the pore-forming agent in the negative electrode active material layer is c, the porosity of the modified aqueous negative electrode is p, EC=(b+c)×p×100, 0.5≤FL×EC≤1.2.

[0015] This invention links the ratio of waterborne polyurethane to the compaction density of the negative electrode sheet, thereby reflecting the flexibility of the electrode sheet to a certain extent. Simultaneously, it correlates the ratio of conductive agent to pore-forming agent with the porosity of the negative electrode sheet, reflecting its conductivity. A negative electrode sheet that satisfies this formula exhibits good flexibility and excellent conductivity.

[0016] Preferably, the peel strength of the modified aqueous negative electrode is 21.8 to 23.2 N / m, for example: 21.8 N / m, 22 N / m, 22.5 N / m, 23 N / m or 23.2 N / m, etc.

[0017] Preferably, the resistivity of the modified aqueous negative electrode is 0.32 to 0.36 Ω·cm, for example: 0.32 Ω·cm, 0.33 Ω·cm, 0.34 Ω·cm, 0.35 Ω·cm or 0.36 Ω·cm, etc.

[0018] Preferably, the porosity of the modified aqueous negative electrode is 36.9% to 38.3%, for example: 36.9%, 37%, 37.5%, 38% or 38.5%, etc.

[0019] Preferably, the compacted density of the modified aqueous negative electrode is 1.4–1.8 g / cm³. 3 For example: 1.4g / cm 3 1.55g / cm 3 1.65g / cm 3 1.7g / cm 3 Or 1.8g / cm 3 wait.

[0020] Preferably, the aqueous polyurethane includes any one or a combination of at least two of polyurethane emulsion, polyurethane dispersion, or aqueous polyurethane solution.

[0021] The pore-forming agent includes azo compounds, sulfonyl hydrazides, preferably p-toluenesulfonyl hydrazide or azodicarbonamide.

[0022] Pore-forming agents are additives that increase the porous structure of materials, and are generally substances that easily decompose into gases. Due to the limitations of electrode baking temperature, this invention uses p-toluenesulfonyl hydrazine and azodicarbonamide, which are two organic ammonium salts with low thermal decomposition temperatures. The pore-forming agent described in this invention undergoes thermal decomposition at 80-105℃, which can effectively increase the porosity of the electrode, thereby reducing the electrode resistivity and thus reducing the internal resistance of the battery.

[0023] Preferably, the negative electrode active material includes graphite.

[0024] Preferably, the conductive agent includes acetylene black.

[0025] Preferably, the mass ratio of the negative electrode active material, conductive agent, sodium alginate, aqueous polyurethane, and pore-forming agent is (94-98.5):(0.5-2):(0.5-2):(0.5-1.5):(0.03-0.05), for example: 95:2:2:1.45:0.05, 95.5:1.8:1.7:0.96:0.04, 96:1:1.5:1.45:0.05, or 97:1:1:0.97:0.03, etc.

[0026] Secondly, the present invention provides a method for preparing a modified aqueous negative electrode as described in the first aspect, the method comprising the following steps:

[0027] (1) Sodium alginate and water are mixed and dispersed to obtain a dispersion. The dispersion is mixed and stirred with a conductive agent and a pore-forming agent. Then, a negative electrode active material is added. After adjusting the viscosity of the slurry, waterborne polyurethane is added to obtain a negative electrode slurry.

[0028] (2) The negative electrode slurry is coated on the surface of the negative electrode current collector and dried to obtain the modified aqueous negative electrode.

[0029] Preferably, the solid content of the dispersion in step (1) is 0.6% to 2%, for example: 0.6%, 0.8%, 1%, 1.5% or 2%, etc.

[0030] Preferably, the solid content of the negative electrode slurry is 54-55.5%, for example: 54%, 54.55%, 54.85%, 55%, 55.13% or 55.5%, etc.

[0031] Preferably, the drying temperature in step (2) is 40 to 80°C, for example: 40°C, 50°C, 60°C, 70°C or 80°C.

[0032] Preferably, the drying time is 2 to 5 minutes, for example: 2 minutes, 3 minutes, 4 minutes or 5 minutes.

[0033] Thirdly, the present invention provides a lithium-ion battery comprising a modified aqueous negative electrode as described in the first aspect.

[0034] Compared with the prior art, the present invention has the following beneficial effects:

[0035] (1) This invention does not use two commercially available water-based binder systems (polyacrylic acid binder and CMC+SBR mixed system), but instead uses sodium alginate + water-based polyurethane as the binder. Sodium alginate can improve the adhesion of the negative electrode slurry, while water-based polyurethane can improve the flexibility of the electrode sheet. A pore-forming agent is used by introducing organic ammonium salts into the slurry and increasing the baking temperature of the electrode sheet, causing the pore-forming agent to undergo thermal decomposition at 80-105℃. This method can effectively increase the porosity of the electrode sheet, thereby reducing the electrode sheet resistivity and thus reducing the internal resistance of the battery.

[0036] (2) The modified aqueous negative electrode of the present invention can solve the problem of raw material shortage in oil-based negative electrode formulations and address the pain points of current aqueous negative electrode formulations, improving the slurry solid content, electrode porosity and flexibility, while reducing electrode resistivity. In addition, this formulation is environmentally friendly, requiring no control of ambient humidity, which can greatly reduce production costs.

[0037] (3) The modified aqueous negative electrode of the present invention improves the electrode peeling force by 20-25%, reduces the electrode resistivity by 15-25%, increases the electrode porosity by 10-15%, reduces HPPC at both room temperature and low temperature, and increases the number of cycles at 3C / 1C room temperature by 12-18%. Detailed Implementation

[0038] The technical solution of the present invention will be further illustrated below through specific embodiments. Those skilled in the art should understand that the embodiments described are merely illustrative of the present invention and should not be construed as limiting the invention in any way.

[0039] Example 1

[0040] This embodiment provides a modified aqueous negative electrode, and the preparation method of the modified aqueous negative electrode is as follows:

[0041] (1) Weigh graphite, acetylene black, sodium alginate, waterborne polyurethane and pore-forming agent (p-toluenesulfonyl hydrazine and azodicarbonamide 1:1) according to a mass ratio of 95.5:1.8:1.7:0.96:0.04. Add sodium alginate and deionized water to a double planetary stirrer and disperse at medium speed for 4 hours. Control the solid content at 1%. Add the prepared solution, acetylene black and pore-forming agent to the double planetary stirrer and disperse at medium speed for 1 hour. Then add artificial graphite and an appropriate amount of deionized water and disperse at high speed for 3.5 hours. During this period, add an appropriate amount of deionized water to adjust the viscosity. Then add waterborne polyurethane and disperse at low speed for 0.8 hours to obtain a negative electrode slurry with a solid content of 55.06%.

[0042] (2) The negative electrode slurry is coated on the surface of the negative electrode current collector and dried at 50°C for 3 minutes to obtain the modified aqueous negative electrode.

[0043] The modified aqueous negative electrode has a peel strength of 23.2 N / m, a resistivity of 0.32 Ω·cm, and a compaction density of 1.55 g / cm³. 3 The total mass percentage of sodium alginate + waterborne polyurethane is 1.65%, FL = (a ÷ s) × 100 = 1.06, the mass percentage of conductive agent is 1.8%, the mass percentage of pore-forming agent is 0.04%, the porosity is 37.9%, EC = (b + c) × p × 100 = 0.7, and FL × EC = 0.74.

[0044] Example 2

[0045] This embodiment provides a modified aqueous negative electrode, and the preparation method of the modified aqueous negative electrode is as follows:

[0046] (1) Weigh graphite, acetylene black, sodium alginate, waterborne polyurethane and pore-forming agent (p-toluenesulfonyl hydrazine and azodicarbonamide 1:1) according to a mass ratio of 96:1:1.5:1.45:0.05. Add sodium alginate and deionized water to a double planetary stirrer and disperse at medium speed for 4.5 h. Control the solid content at 1.2%. Add the prepared solution, acetylene black and pore-forming agent to the double planetary stirrer at the same time and disperse at medium speed for 1 h. Then add artificial graphite and an appropriate amount of deionized water and disperse at high speed for 3.8 h. During this period, add an appropriate amount of deionized water again to adjust the viscosity. Then add waterborne polyurethane and disperse at low speed for 0.8 h to obtain a negative electrode slurry with a solid content of 54.42%.

[0047] (2) The negative electrode slurry is coated on the surface of the negative electrode current collector and dried at 60°C for 3 minutes to obtain the modified aqueous negative electrode.

[0048] The modified aqueous negative electrode has a peel strength of 22.8 N / m, a resistivity of 0.35 Ω·cm, and a compaction density of 1.65 g / cm³. 3 The total mass percentage of sodium alginate + waterborne polyurethane is 1.65%, FL = (a ÷ s) × 100 = 1, the mass percentage of conductive agent is 1.8%, the mass percentage of pore-forming agent is 0.03%, the porosity is 36.8%, EC = (b + c) × p × 100 = 0.67, FL × EC = 0.67.

[0049] Example 3

[0050] The only difference between this embodiment and Embodiment 1 is that the total mass percentage (a) of sodium alginate + waterborne polyurethane is 1%. All other conditions and parameters are exactly the same as in Embodiment 1, with FL×EC = 0.45.

[0051] Example 4

[0052] The only difference between this embodiment and Embodiment 1 is that the total mass percentage (a) of sodium alginate + waterborne polyurethane is 4%. All other conditions and parameters are exactly the same as in Embodiment 1, with FL×EC = 1.81.

[0053] Example 5

[0054] The only difference between this embodiment and Example 1 is that the mass ratio of the pore-forming agent is 0.01%, the porosity of the electrode is only 33.4%, and the other conditions and parameters are exactly the same as in Example 1, with FL×EC=0.64.

[0055] Example 6

[0056] The only difference between this embodiment and Example 1 is that the mass ratio of the pore-forming agent is 0.1%, the porosity of the electrode is 39.2%, and the other conditions and parameters are exactly the same as in Example 1, with FL×EC=0.79.

[0057] Comparative Example 1

[0058] This comparative example provides a conventional negative electrode sheet, the preparation method of which is as follows:

[0059] 1% CMC (by weight of the slurry) was added to 50% deionized water and dispersed in a dual planetary mixer for 4 hours. Then, 1.5% acetylene black (by weight of the slurry) was added to the solvent and dispersed at medium speed for 1.5 hours. Subsequently, 46.5% artificial graphite (by weight of the slurry) was added and stirred at high speed for 3 hours. Finally, 1% SBR was added and dispersed at low speed for 0.8 hours to obtain a control negative electrode slurry. The prepared control slurry was coated onto one or both sides of a copper foil or carbon-coated copper foil using transfer coating or extrusion coating. The negative electrode sheet was then baked in an oven at 95°C for 3 minutes.

[0060] The negative electrode sheet has a peel strength of 18.8 N / m, a resistivity of 0.42 Ω·cm, and a porosity of 32.7%.

[0061] Performance testing:

[0062] ① The negative electrode sheet prepared in Examples 1-6 was used, and the positive electrode was a lithium iron phosphate positive electrode with a specific formula of LFP, SP, CNT, and PVDF weight ratio of 96.5:1.0:0.8:1.7. The electrolyte was LiPF6 organic solvent, and the separator was a polyethylene film separator. Finally, the positive electrode sheet, separator, and negative electrode sheet obtained in steps 4 were stacked in sequence, with the separator acting as a separator between the positive and negative electrode sheets. After stacking and winding, a bare cell was obtained. The bare cell was placed in an outer packaging shell, dried, and then injected with electrolyte. After vacuum sealing, standing, formation, and shaping processes, a lithium-ion battery was obtained. The battery was subjected to the following performance tests:

[0063] 1) Basic electrical performance test: At 25℃, the lithium-ion batteries prepared in the examples and comparative examples were first pre-charged with a small current for 180 min, then fully charged with a constant current of 0.33C to 3.65V, and then fully discharged with a constant voltage of 0.05C to 2.5V. The ratio of discharge capacity to charge capacity was calculated to obtain the first effect. Then, the batteries were fully charged with a constant current of 1C to 3.65V, and then fully discharged with a constant voltage of 0.05C to 2.5V. Finally, the batteries were disassembled to measure the rebound rate of the negative electrode.

[0064] 2) Dynamic performance test: The lithium-ion batteries prepared in the examples and comparative examples were tested at 25°C and -20°C respectively using the standard HPPC test method to measure the charge-discharge DCR at different SOCs. The main evaluation was the charge-discharge DCR at 50%.

[0065] 3) Cyclic performance test: At 25°C, the lithium-ion batteries prepared in the examples and comparative examples were charged at a 3C rate and discharged at a 1C rate for a full charge-discharge cycle test until the capacity of the lithium-ion batteries decayed to 80% of the initial capacity. The number of cycles was recorded, and the test results are shown in Table 1.

[0066] Table 1

[0067]

[0068]

[0069] As can be seen from Table 1, and from Examples 1-2, the modified aqueous negative electrode of the present invention can achieve an initial efficiency of over 88.1%, an electrode peel strength of over 22.8 N / m, and a cycle count of over 1180 at 25°C.

[0070] A comparison of Examples 1 and 3-4 shows that the total mass ratio of sodium alginate and aqueous polyurethane in the modified aqueous negative electrode of this invention affects its performance. Controlling the total mass ratio of the two binders to 1-3% results in better performance of the modified aqueous negative electrode. If the mass ratio of the binder is too low, the viscosity of the slurry will be low, thus reducing the peel strength of the electrode sheet; if the mass ratio of the binder is too high, the slurry will gel and the coating surface density will be uneven, thus increasing the DCR of the battery.

[0071] A comparison of Examples 1 and 5-6 shows that the mass ratio of the pore-forming agent in the modified aqueous negative electrode of the present invention affects its performance. When the mass ratio of the pore-forming agent is controlled at 0.03-0.05%, the performance of the modified aqueous negative electrode is better. If the mass ratio of the pore-forming agent is too low, the porosity of the electrode sheet will be low, thereby reducing the DCR of the battery. If the mass ratio of the pore-forming agent is too high, although it can increase the porosity of the electrode sheet, the cost of pore forming is high, and it will also have a certain impact on the cycle life of the battery.

[0072] Comparing Example 1 and Comparative Example 1, it can be seen that, compared with conventional negative electrode sheets (Comparative Example 1), the modified aqueous negative electrode of the present invention improves the electrode sheet peeling force by 20-25%, reduces the electrode sheet resistivity by 15-25%, increases the electrode sheet porosity by 10-15%, reduces HPPC at both room temperature and low temperature, and increases the number of cycles at 3C / 1C room temperature by 12-18%.

[0073] ② The relationship between different formulation ratios of sodium alginate + waterborne polyurethane and cell performance: Lithium-ion batteries were fabricated using different formulation ratios, and the relationship between these ratios and cell performance was verified. The mass ratios of artificial graphite, acetylene black, sodium alginate, waterborne polyurethane powder, and pore-forming agent were kept constant at 100. The types of artificial graphite, acetylene black, and pore-forming agent were fixed, and only the ratio of sodium alginate to waterborne polyurethane powder was changed. The test results are shown in Table 2-3.

[0074] Table 2

[0075]

[0076] By fixing the content of 1.0% aqueous polyurethane dry powder and varying the proportion of sodium alginate, as shown in Table 2, the mass percentage of sodium alginate in the active material layer is between 0.5% and 2% (inclusive). There is no processing risk, and the battery performance remains largely unchanged. A slurry content less than 0.5% poses a risk of sedimentation, while a content greater than 2.0% will cause cracking in the electrode coating. Therefore, the optimal sodium alginate proportion is 0.5%–2.0%.

[0077] Table 3

[0078]

[0079] By fixing the sodium alginate content at 1.5% and varying the proportion of waterborne polyurethane powder, the results showed that there was no processing risk when the waterborne polyurethane content was between 0.5% and 1.5% (inclusive). The peel strength increased with the increase of the proportion of waterborne polyurethane powder. There was little difference between room temperature and low temperature HPPC. Coating with a content less than 0.5% had a risk of cracking, while coating with a content greater than 1.5% showed a significant decrease in cycle performance and was not recommended.

[0080] The applicant declares that the above description is only a specific embodiment of the present invention, but the protection scope of the present invention is not limited thereto. Those skilled in the art should understand that any changes or substitutions that can be easily conceived by those skilled in the art within the technical scope disclosed in the present invention fall within the protection and disclosure scope of the present invention.

Claims

1. A modified aqueous negative electrode, characterized in that, The modified aqueous negative electrode includes a negative electrode current collector and a negative electrode active material layer disposed on the surface of the negative electrode current collector. The negative electrode active material layer includes a negative electrode active material, a conductive agent, sodium alginate, waterborne polyurethane, and a pore-forming agent. The mass ratio of sodium alginate and waterborne polyurethane in the negative electrode active material layer is 'a', and the compaction density of the modified aqueous negative electrode is 's', with units of g / cm³. 3 FL=(a÷s)×100, the mass percentage of the conductive agent in the negative electrode active material layer is b, the mass percentage of the pore-forming agent in the negative electrode active material layer is c, the porosity of the modified aqueous negative electrode is p, EC=(b+c)×p×100, 0.5 ≤ FL×EC ≤ 1.

2.

2. The modified aqueous negative electrode as described in claim 1, characterized in that, The electrode peel strength of the modified aqueous negative electrode is 21.8~23.2 N / m.

3. The modified aqueous negative electrode as described in claim 1, characterized in that, The resistivity of the modified aqueous negative electrode is 0.32~0.36 Ω·cm.

4. The modified aqueous negative electrode as described in claim 1, characterized in that, The porosity of the modified aqueous negative electrode is 36.9~38.3%.

5. The modified aqueous negative electrode as described in claim 1, characterized in that, The compacted density of the modified aqueous negative electrode is 1.4~1.8 g / cm³. 3 .

6. The modified aqueous negative electrode as described in claim 1, characterized in that, The aqueous polyurethane includes any one or a combination of at least two of polyurethane emulsion, polyurethane dispersion or polyurethane aqueous solution. The pore-forming agent includes azo compounds and sulfonyl hydrazide compounds.

7. The modified aqueous negative electrode as described in claim 6, characterized in that, The pore-forming agent is p-toluenesulfonyl hydrazine and azodicarbonamide.

8. The modified aqueous negative electrode as described in claim 1, characterized in that, The negative electrode active material includes graphite.

9. The modified aqueous negative electrode as described in claim 1, characterized in that, The conductive agent includes acetylene black.

10. The modified aqueous negative electrode as described in claim 1, characterized in that, The mass ratio of the negative electrode active material, conductive agent, sodium alginate, aqueous polyurethane and pore-forming agent is (94~98.5):(0.5~2):(0.5~2):(0.5~1.5):(0.03~0.05).

11. A method for preparing a modified aqueous negative electrode as described in any one of claims 1-10, characterized in that, The preparation method includes the following steps: (1) Sodium alginate and water are mixed and dispersed to obtain a dispersion. The dispersion is mixed and stirred with conductive agent and pore-forming agent, and then the negative electrode active material is added. After adjusting the viscosity of the slurry, waterborne polyurethane is added to obtain the negative electrode slurry. (2) The negative electrode slurry is coated on the surface of the negative electrode current collector and dried to obtain the modified aqueous negative electrode.

12. The preparation method according to claim 11, characterized in that, The solid content of the dispersion in step (1) is 0.6-2%.

13. The preparation method according to claim 11, characterized in that, The solid content of the negative electrode slurry is 54-55.5%.

14. The preparation method according to claim 11, characterized in that, The drying temperature in step (2) is 40~80℃.

15. The preparation method according to claim 11, characterized in that, The drying time is 2-5 minutes.

16. A lithium-ion battery, characterized in that, The lithium-ion battery comprises a modified aqueous negative electrode as described in any one of claims 1-10.

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