Preparation method of binder, composition, slurry and pole piece for battery negative electrode

Abstract

The present invention discloses a preparation method of a binder, composition, slurry and pole piece for a battery negative electrode. Citrate ester is creatively combined with polyacrylic acid to effectively solve the problem of low bonding strength between active materials and pole pieces in lithium batteries, easy shedding of active materials, and thus reduced battery cycle performance. Polyacrylic acid and triethyl citrate are configured in the binder composition at 97wt% and 3wt%, respectively, which can significantly improve the bonding strength between the active materials and pole pieces. The breaking and regeneration of hydrogen bonds can effectively adapt to the volume change of the active materials during charging and discharging. The peeling force after rolling reaches a maximum of 9.21N / m, the shedding of active materials is reduced, and the battery cycle performance is improved, achieving unexpected technical effects.

Description

Technical Field

[0001] The present invention belongs to the technical field of lithium-ion batteries, and in particular relates to a preparation method of a binder, a composition, a slurry and a pole piece for a battery negative electrode. Background Art

[0002] In recent years, with the continuous development of the new energy industry, the technical requirements for lithium batteries have continued to increase. The variety of positive and negative electrode active materials has also increased, and each material has different properties and compatibility with other lithium battery materials. The application of a new material will also drive the development of other materials. The performance of lithium batteries is mainly influenced by the positive electrode material, electrolyte, separator, and negative electrode material. Battery electrode performance is affected not only by the properties of the active material on the electrode, but also by the binder that disperses and binds the active material particles.

[0003] Binders are one of the important functional auxiliary materials in lithium-ion batteries. They have no capacity themselves and account for a very small proportion in the battery, but they play a key role in the mechanical properties of the entire electrode. For example, silicon-based or silicon-carbon composite negative electrode materials have a larger theoretical capacity than graphite negative electrode materials, but during the charge and discharge process, they are more likely to expand and fall off than graphite negative electrodes, so the role of binders is particularly important. In addition to providing good bonding properties, binders also need to withstand electrolyte swelling, be less prone to electrochemical corrosion during battery charge and discharge, and remain stable at the operating voltage of the electrode. However, there are not many materials that can be used as binders in lithium-ion batteries.

[0004] Currently, the main aqueous binders used in lithium-ion battery negative electrode plates are styrene-butadiene emulsions and carboxymethyl cellulose. Polyacrylonitrile, polyacrylic acid, and polyacrylates also hold a significant market share. However, styrene-butadiene emulsions have poor dispersibility, carboxymethyl cellulose is brittle and has poor elasticity, and polyacrylic acid also has poor flexibility. Furthermore, during actual use, they are unable to effectively restrain the expansion of the negative electrode active material over long periods of time and prevent the active material from shedding, thus affecting battery performance. Therefore, improving the performance of lithium-ion battery binders and ensuring the stability of lithium battery negative electrode materials remains a challenge for those skilled in the art. Summary of the Invention

[0005] In view of the above-mentioned deficiencies in the prior art, the present invention aims to provide a binder, composition, slurry, lithium battery negative electrode plate and preparation method thereof for battery negative electrode, so as to solve the problem that the active material in the lithium battery has low adhesion between itself and the electrode plate, and the active material is easy to fall off, thereby reducing the battery cycle performance.

[0006] To achieve the above object, the technical solution of the present invention is as follows:

[0007] A binder for battery negative electrodes comprises polyacrylic acid and citrate, wherein the polyacrylic acid accounts for 95% to 99% of the total mass of the polyacrylic acid and the citrate accounts for 1% to 5% of the total mass of the polyacrylic acid.

[0008] Furthermore, polyacrylic acid accounts for 96% to 98% of the total mass of the two, and citrate accounts for 2% to 4% of the total mass of the two.

[0009] Furthermore, polyacrylic acid accounts for 97% of the total mass of the two, and citrate accounts for 3% of the total mass of the two.

[0010] Furthermore, the citrate ester is at least one of triethyl citrate, tri-n-butyl citrate, and trioctyl citrate.

[0011] Furthermore, the citrate ester is triethyl citrate.

[0012] A composition for preparing battery negative electrode slurry comprises, by weight, 0.5-5 parts of a binder and 0.5-2.5 parts of a thickener; the binder is the above-mentioned binder for battery negative electrode; the thickener is at least one of carboxymethyl cellulose, lithium carboxymethyl cellulose and sodium carboxymethyl cellulose.

[0013] A negative electrode slurry for a battery comprises, in parts by mass: 90 to 99 parts of an active material, 0.5 to 5 parts of a binder, 0.5 to 2.5 parts of a thickener, and 0 to 2.5 parts of a conductive agent; the binder is the above-mentioned binder for a negative electrode of a battery; the thickener is at least one of carboxymethyl cellulose, lithium carboxymethyl cellulose, and sodium carboxymethyl cellulose.

[0014] A battery negative electrode plate is made from the above-mentioned battery negative electrode slurry.

[0015] A method for preparing a negative electrode sheet of a battery, comprising the following steps:

[0016] Material preparation: preparing the negative electrode slurry for the above-mentioned battery;

[0017] Stirring: Use deionized water as a dispersant and stir the above negative electrode slurry in the dispersant evenly;

[0018] Coating: The dispersed negative electrode slurry is coated on the current collector, dried and rolled to obtain the negative electrode sheet.

[0019] A lithium battery comprises the above-mentioned negative electrode plate or the negative electrode plate prepared by the above-mentioned method.

[0020] Compared with the prior art, the present invention has the following beneficial effects:

[0021] The present invention effectively addresses the problem of weak adhesion between the active material and the electrode in lithium batteries, resulting in easy active material shedding and reduced battery cycle performance. The binder composition, in which polyacrylic acid and triethyl citrate are configured at 97% and 3% by mass, helps improve the adhesion between the active material and the electrode. The breaking and regeneration of hydrogen bonds effectively adapts to volume changes during the charge and discharge process. The peel force after rolling reaches a maximum of 9.21 N / m, reducing active material shedding and improving battery cycle performance, achieving unexpected technical results. BRIEF DESCRIPTION OF THE DRAWINGS

[0022] Figure 1 This is a molecular structure diagram of polyacrylic acid and triethyl citrate provided in Example 3 of the present invention.

[0023] It should be noted that Figure 1 This diagram only illustrates a possible spatial state of coexistence of polyacrylic acid and triethyl citrate. In other specific embodiments, there may be other spatial relationships between the two. DETAILED DESCRIPTION

[0024] In order to enable those skilled in the art to better understand the technical solutions of the present invention, the present invention will be further described below in conjunction with specific examples, but the embodiments of the present invention are not limited thereto.

[0025] The numerical ranges herein are understood to specifically disclose every intervening value between the upper and lower limits of the range. Each smaller range between any stated value or intervening value in a stated range and any other stated value or intervening value in that stated range is also encompassed within the invention. The upper and lower limits of these smaller ranges may independently be included or excluded in the range.

[0026] Unless otherwise indicated, all technical and scientific terms used herein have the same meanings as commonly understood by those skilled in the art. Although the present invention has only described preferred methods and materials, any methods and materials similar or equivalent to those described herein may also be used in the implementation or testing of the present invention. All documents mentioned in this specification are incorporated by reference to disclose and describe the methods and / or materials related to the documents. In the event of any conflict with any incorporated document, the contents of this specification shall prevail. As used herein, "comprising", "including", "having", "containing", etc. are all open-ended terms, meaning including but not limited to.

[0027] Unless otherwise specified, the experimental methods used in the present invention are all conventional methods.

[0028] Unless otherwise specified, the materials, reagents, etc. used in the present invention can be purchased or synthesized by known methods.

[0029] The quantitative tests in the present invention were repeated three times and the results were averaged.

[0030] The present invention will be further described below through specific examples and comparative examples.

[0031] 1. A binder for battery negative electrode

[0032] A binder for battery negative electrodes comprises polyacrylic acid and citrate, wherein the polyacrylic acid accounts for 90% to 99% of the total mass of the two, and the citrate accounts for 1% to 10% of the total mass of the two.

[0033] Wherein, the average molecular weight of the polyacrylic acid is 100,000 to 800,000.

[0034] The citric acid ester is one or more of tri-n-butyl citrate (TBC), acetyl triethyl citrate (ATEC), acetyl tributyl citrate (ATBC), triethyl citrate (TEC), trioctyl citrate (TOC), and acetyl trioctyl citrate. For example, in some possible embodiments, two citric acid esters, tri-n-butyl citrate and acetyl triethyl citrate, can be used. Alternatively, in other possible embodiments, triethyl citrate, trioctyl citrate, and acetyl tributyl citrate can be used simultaneously. It should be understood that in one embodiment of the technical solution of the present invention, different types of citric acid esters can be added. Of course, only one type of citric acid ester can also be used.

[0035] It should be understood that in possible implementations of the technical solution of the present invention, other components may also be present, such as some impurities, solvents or other components, which all fall within the scope of protection of the present invention.

[0036] See also Figure 1 The results show that the small molecule citrate ester cross-links with polyacrylic acid in situ to form hydrogen bonds. Through these hydrogen bonds, the citrate ester and polyacrylic acid form an elastic, self-healing structure, which can effectively provide a cross-linked network structure to cover the active material and has good adaptability to the volume changes of the active material during the charge and discharge process.

[0037] Example 1

[0038] Polyacrylic acid and triethyl citrate are prepared into an adhesive composition according to the following mass percentages: 99% and 1%.

[0039] Example 2

[0040] The difference between this embodiment and embodiment 1 is that the polyacrylic acid and triethyl citrate in the adhesive composition are prepared according to the following mass percentages: 98% and 2%.

[0041] Example 3

[0042] The difference between this embodiment and embodiment 1 is that the polyacrylic acid and triethyl citrate in the adhesive composition are prepared according to the following mass percentages: 97% and 3%.

[0043] Example 4

[0044] The difference between this embodiment and embodiment 1 is that the polyacrylic acid and triethyl citrate in the adhesive composition are prepared according to the following mass percentages: 96% and 4%.

[0045] Example 5

[0046] The difference between this embodiment and embodiment 1 is that the polyacrylic acid and triethyl citrate in the adhesive composition are prepared according to the following mass percentages: 95% and 5%.

[0047] Example 6

[0048] The difference between this embodiment and embodiment 1 is that the polyacrylic acid and triethyl citrate in the adhesive composition are prepared according to the following mass percentages: 90% and 10%.

[0049] Example 7

[0050] This embodiment differs from embodiment 3 in that triethyl citrate is replaced with tri-n-butyl citrate.

[0051] Example 8

[0052] This embodiment differs from embodiment 3 in that triethyl citrate is replaced with acetyl triethyl citrate.

[0053] Example 9

[0054] This embodiment differs from embodiment 3 in that triethyl citrate is replaced with trioctyl citrate.

[0055] Comparative Example

[0056] The difference between the comparative example and Example 1 is that the polyacrylic acid and triethyl citrate in the adhesive composition are prepared according to the following mass percentages: 100%, 0%.

[0057] 2. A composition for preparing battery negative electrode slurry

[0058] In terms of parts by mass, it includes 0.5-5 parts of binder and 0.5-2.5 parts of thickener.

[0059] The binder is the above-mentioned binder for battery negative electrode.

[0060] The thickener is at least one of carboxymethyl cellulose, lithium carboxymethyl cellulose and sodium carboxymethyl cellulose.

[0061] 3. A negative electrode slurry for batteries

[0062] According to the parts by mass, it includes 90-99 parts of active material, 0-2.5 parts of conductive agent, 0.5-2.5 parts of thickener, and 0.5-5 parts of binder;

[0063] Wherein, the binder is any of the above-mentioned binders for battery negative electrodes, that is, in the binder, the mass proportion of polyacrylic acid is 90% to 99%, and the mass proportion of citrate is 1% to 10%.

[0064] Examples 10-18

[0065] The negative electrode active material, conductive agent, thickener, and binder composition were prepared into a negative electrode slurry according to the mass fractions shown in Table 1.

[0066] Table 1 Negative electrode slurry ratios for Examples 10-18 and Comparative Examples

[0067]

[0068] Among them, the citrate ester in Examples 10-15 is triethyl citrate; the citrate ester in Example 16 is tri-n-butyl citrate; the citrate ester in Example 17 is acetyl triethyl citrate; and the citrate ester in Example 18 is trioctyl citrate.

[0069] The active material is a mixture of any one of mesophase carbon microsphere artificial graphite, needle coke artificial graphite, petroleum coke graphite, composite graphite and silicon dioxide.

[0070] The mass ratio of the graphite to silicon dioxide is 9:1.

[0071] The conductive agent is conductive carbon black (Super-P).

[0072] The thickener is carboxymethyl cellulose.

[0073] 4. A method for preparing a negative electrode sheet for a lithium battery

[0074] Prepare the materials according to the ingredients and proportions in Table 1, including the following steps:

[0075] S1, stirring and mixing the active material and the conductive agent;

[0076] S2. Weigh a certain amount of thickener and binder composition according to the ratio and stir and mix them;

[0077] S3, using deionized water as a dispersant, and stirring the mixed substances obtained in step S1 and step S2 in the dispersant uniformly;

[0078] S4. Spread the mixture obtained in step S3 on the current collector, dry it, and roll-press it to obtain a negative electrode sheet.

[0079] The specific steps are:

[0080] Pulping process: Add deionized water and carboxymethyl cellulose mixed colloid into a double planetary mixer, add the ground conductive carbon black during the stirring process, and disperse it at 1500 rpm and 15 rpm for 45 minutes; after the mixture is evenly mixed, add the active material and rotate at 2000 rpm and 20 rpm, and stir for 4 hours to complete the dispersion process of the conductive carbon black and the active material; add the binder composition and rotate at 2000 rpm and 20 rpm, and stir for 25 minutes, and vacuum to remove bubbles, and filter with a 300-mesh stainless steel screen to complete the preparation of the negative electrode slurry;

[0081] The negative electrode slurry was applied to the surface of the current collector using a transfer coater at a coating speed of 15 m / min. After coating, it was dried in a 5-stage oven at a temperature of 100-130°C. After drying, it was pressurized using a roller press to obtain the negative electrode sheet. The coating thickness was 125 μm and the compaction density was 1.80 g / cm 3 .

[0082] 5. A lithium battery

[0083] The positive electrode sheet is prepared using a conventional formula. The positive electrode materials include lithium iron phosphate, conductive carbon black, and polyvinylidene fluoride. The lithium iron phosphate, conductive carbon black, polyvinylidene fluoride, and methyl pyrrolidone (NMP) solvent are stirred into a positive electrode slurry, and the positive electrode slurry is coated on the positive electrode collector to obtain the positive electrode sheet.

[0084] The diaphragm adopts ordinary single-layer 12μm PE diaphragm.

[0085] The conventional electrolyte formula is adopted: 1.0MLiF6in ethylene carbonate (EC): dimethyl carbonate (DMC): ethyl methyl carbonate (EMC) = 1:1:1 Vol%.

[0086] The positive electrode sheet, negative electrode sheet and separator are wound into a battery cell and encapsulated with aluminum-plastic film; after baking under vacuum for 48 hours to remove moisture, the electrolyte is injected; the battery is formed and sorted to obtain a square soft-pack lithium-ion battery with a thickness of 4mm, width of 60mm and length of 72mm respectively.

[0087] 6. Performance Testing

[0088] The batteries prepared in Examples 10-18 and the comparative example were tested, and the comparison results are as follows:

[0089] Table 2 Performance test data comparison table

[0090]

[0091]

[0092] According to the experimental data of Comparative Examples 10, 11, 12, 13, and 14 and the comparative example, it can be found that with the gradual increase of triethyl citrate, the peeling force and cycle retention rate of the pole piece show a trend of first increasing and then decreasing, indicating that the appropriate addition of triethyl citrate helps to improve the performance of the pole piece. In the appropriate ratio system of triethyl citrate and polyacrylic acid, the hydrogen bond formed by the two has a good effect on improving the bonding force. Through the disassembly experiment after 20 cycles, it can be seen that the reproducibility of hydrogen bonds makes the blend system more suitable for the more obvious volume change in the graphite-silicon composite system, so that the pole piece can better maintain the integrity of the active material during the charge and discharge process.

[0093] The data from Example 15 show that excessive addition of triethyl citrate will reduce the performance of the electrode. Excessive addition of triethyl citrate will reduce the use of binder, reduce the adhesive effect, and cause a significant decrease in peel force and cohesion. During the cycle, due to the poor bonding effect, the graphite-silicon composite system cannot be effectively restrained during expansion, resulting in severe powder loss. The first coulombic efficiency of Example 15 and the cycle retention rate at different temperatures indicate that triethyl citrate will also participate in the reaction during the cycle, causing lithium precipitation and affecting the cycle performance of the battery.

[0094] A 180° peel test demonstrated that an appropriate amount of triethyl citrate contributes to the formation of better cohesion in the blended system, as corroborated by the peel force after roller pressing and the disassembly test results after cycling. Examples 3 and 12 demonstrate that a binder composition containing polyacrylic acid and triethyl citrate at 97% and 3% by weight improves the adhesion between the active material and the electrode. The breaking and regeneration of hydrogen bonds effectively adapts to the volume changes of the active material during charge and discharge, reducing active material shedding and improving battery cycling performance.

[0095] The thickness growth rates after the two cycles indicate that the addition of triethyl citrate helps improve the thickness variation of the battery. The appropriate addition helps inhibit the expansion of the silicon-carbon electrode and the shedding of the active material. This suggests that the three-dimensional network of hydrogen bonds formed between polyacrylic acid and triethyl citrate helps stabilize the active material in the electrode, adapting to the significant volume changes of the silicon-carbon electrode during charge and discharge, and improving the battery's cycling performance.

[0096] It can be seen from Examples 16, 17 and 18 that when tri-n-butyl citrate, acetyl triethyl citrate or trioctyl citrate is used as the citrate ester in the battery negative electrode binder, the peeling force test membrane integrity and cycle capacity retention rate of the electrode are still good, indicating that in addition to triethyl citrate, tri-n-butyl citrate, acetyl triethyl citrate or trioctyl citrate can also be used as the citrate ester in the binder of the present invention.

[0097] In summary, the inventors believe that the present invention, through the introduction of a small molecule citrate ester and polyacrylic acid, forms a network of hydrogen-bonded crosslinks. This is achieved by hydrogen bonding the oxygen groups of the citrate ester, which are abundant in the ester groups, with the side chains of the polyacrylic acid. Due to the multidirectional spatial distribution of the citrate ester groups, a large number of hydrogen bonds can be formed between the molecules, forming a network crosslinking structure, thereby enhancing the binder's adhesion to the active material. This also provides improved adhesion and flexibility to the electrode sheet, achieving certain self-healing properties, adapting to the volume changes of the active material during charging, optimizing the coating performance of the slurry, and improving the overall cycling performance of the battery. Hydrogen bonds can also be repeatedly formed to adapt to the volume changes of the active material during charging and discharging, improving the flexibility of the electrode sheet. Furthermore, the presence of a hydroxyl group in triethyl citrate makes it more soluble in aqueous solution than other citrate esters, resulting in superior performance. Furthermore, the addition of the citrate ester significantly reduces defects such as fisheyes and pinholes on the electrode sheet, and the surface is smoother than that of the electrode sheet without the addition of the citrate ester. The results of this implementation have exceeded expectations!

[0098] Furthermore, the present invention found that when carboxymethyl cellulose is used as a thickener in combination with two adhesives, polyacrylic acid and citrate, the flexibility of the electrode is improved compared to using one of them alone. The inventor analyzed the reason and found that it may be because the hydrogen atoms or oxygen atoms on the carboxyl groups on the carboxymethyl cellulose can also form hydrogen bonds with the oxygen atoms or hydrogen atoms on the citrate, which strengthens the intermolecular force of the carboxymethyl cellulose and thus improves the flexibility of the electrode. The thickener is not limited to carboxymethyl cellulose. It should be understood that lithium carboxymethyl cellulose and sodium carboxymethyl cellulose are formed by replacing the hydrogen atoms on some of the carboxyl groups of carboxymethyl cellulose with lithium or sodium. There are still independent carboxyl groups on lithium carboxymethyl cellulose and sodium carboxymethyl cellulose. Therefore, the thickener is not limited to carboxymethyl cellulose. The use of lithium carboxymethyl cellulose or sodium carboxymethyl cellulose can achieve a synergistic effect.

[0099] Finally, it should be noted that the above embodiments are only used to illustrate the technical solutions of the present invention rather than to limit the technical solutions. Those skilled in the art should understand that modifications or equivalent replacements of the technical solutions of the present invention that do not depart from the purpose and scope of the technical solutions of the present invention should be included in the scope of the claims of the present invention.

Claims

1. A binder for a battery negative electrode, characterized in that: Comprising polyacrylic acid and citrate; wherein polyacrylic acid accounts for 95% to 99% of the total mass of the two, and citrate accounts for 1% to 5% of the total mass of the two; The citrate ester is one or more of tri-n-butyl citrate, acetyl triethyl citrate, acetyl tributyl citrate, triethyl citrate, trioctyl citrate, and acetyl trioctyl citrate.

2. The battery negative electrode binder according to claim 1, wherein Polyacrylic acid accounts for 96% to 98% of the total mass of the two, and citrate accounts for 2% to 4% of the total mass of the two.

3. The battery negative electrode binder according to claim 2, wherein Polyacrylic acid accounts for 97% of the total mass of the two, and citrate accounts for 3% of the total mass of the two.

4. A composition for preparing a negative electrode slurry for a battery, characterized in that: In parts by mass, it comprises 0.5-5 parts of a binder and 0.5-2.5 parts of a thickener; the binder is the binder for a negative electrode of a battery according to any one of claims 1 to 3; the thickener is at least one of carboxymethyl cellulose, lithium carboxymethyl cellulose and sodium carboxymethyl cellulose.

5. A negative electrode slurry for a battery, characterized in that: The invention comprises, in parts by mass: 90 to 99 parts of active material, 0.5 to 5 parts of binder, 0.5 to 2.5 parts of thickener, and 0 to 2.5 parts of conductive agent; the binder is the binder for battery negative electrode according to any one of claims 1 to 3; the thickener is at least one of carboxymethyl cellulose, lithium carboxymethyl cellulose, and sodium carboxymethyl cellulose.

6. A battery negative electrode plate, characterized in that: The battery is manufactured by the negative electrode slurry according to claim 5.

7. A method for preparing a negative electrode sheet of a battery, characterized in that: The specific steps include: Preparation: preparing the negative electrode slurry for the battery according to claim 5; Stirring: Use deionized water as a dispersant and stir the above negative electrode slurry in the dispersant evenly; Coating: The dispersed negative electrode slurry is coated on the current collector, dried and rolled to obtain the negative electrode sheet.

8. A lithium battery, characterized in that: Including the negative electrode sheet according to claim 6 or the negative electrode sheet prepared by the preparation method according to claim 7.

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