Dispersing agent for negative plate, preparation method of dispersing agent and lithium secondary battery

Abstract

The invention provides a dispersing agent for a negative plate, the dispersing agent comprises a structural unit as shown in formula 1, in the formula 1, R is independently selected from at least one of H, R1 and R2; wherein R1 and R2 are respectively and independently selected from at least two of-O-,-Si-O-,-S-,-COO-,-NH-CO-O-, C1-C12 alkyl of which at least one H is substituted by halogen or not substituted by halogen, C2-C12 alkenyl, and C1-C12 alkylene of which at least one H is substituted by halogen or not substituted by halogen; m is in the range of 200 to 6000; m is selected from at least one of Li < + > and Na < + >; the substitution degree of the dispersing agent is 0.7 to 0.95. According to the dispersing agent for the negative electrode plate, the flexibility of the dispersing agent is improved, the dispersing agent shrinks in the drying process of the electrode plate, and the elasticity is increased and the rigidity is weakened, so that the cracking of the electrode plate is relieved, the cycle performance of a battery is improved, the rebound of the electrode plate in the cycle process is reduced, and the expansion rate of a battery cell in the cycle process of the electrode plate is reduced.

Description

TECHNICAL FIELD

[0001] The present application relates to the technical field of batteries, in particular to a dispersant for negative electrode sheets, a preparation method thereof and a lithium secondary battery. BACKGROUND

[0002] The existing lithium battery includes a negative electrode sheet, which includes a negative electrode active material, a binder, a conductive agent, a dispersant, etc. The existing negative electrode active material has a graphite system, a silicon-carbon system, a hard carbon system, etc. No matter which system of the negative electrode active material, the obtained negative electrode sheet has the problem of sheet cracking during the negative electrode sheet coating process.

[0003] The main reason for the negative electrode sheet cracking is that sodium carboxymethyl cellulose (CMC-Na) is added in the preparation process of the negative electrode sheet. CMC-Na is a high tensile modulus product, and its substance is relatively hard and brittle, which can cause the CMC-Na glue film to shrink during the drying process of the negative electrode sheet, resulting in sheet cracking and affecting the cycle performance of the battery.

[0004] To solve the above problem, the existing technology adds 1.3-butanediol and other small molecule alcohol compounds. Such molecules belong to water retaining agents and have certain water absorption performance, which can delay the drying rate of the negative electrode sheet and improve the sheet cracking. However, this method can cause the residual of alcohol small molecules in the negative electrode sheet, thereby deteriorating the cycle performance of the battery. SUMMARY

[0005] To solve the problems of the existing dispersant sodium carboxymethyl cellulose, such as glue film shrinkage during the drying process of the negative electrode sheet, sheet cracking and cycle performance deterioration of the battery, the present application provides a dispersant for negative electrode sheets, a preparation method thereof and a lithium secondary battery.

[0006] In one aspect, the present application provides a dispersant for negative electrode sheets, which includes a structural unit shown in formula 1, Formula 1 wherein R is independently selected from at least one of H, R1, R2; wherein R1, R2 is independently selected from at least two of -O-, -Si-O-, -S-, -COO-, -NH-CO-O-, C1-C12 alkyl with at least one H substituted by halogen or unsubstituted by halogen, C2-C12 alkenyl, C1-C12 alkylene with at least one H substituted by halogen or unsubstituted by halogen; m is 200-6000; M is selected from at least one of Li + , Na + ; The degree of substitution of the dispersant is 0.7-0.95.

[0007] Preferably, R is independently selected from one or more of -R3-OH, -Si-(CH3)2, -NH-CO-O-R4; wherein R3 is selected from C2-C12 alkyl, and R4 is selected from C2-C12 alkyl.

[0008] Preferably, the molecular weight of the dispersant is 200000-800000 g / mol.

[0009] Preferably, the elongation at break of the adhesive film formed by the dispersant is 0.3-3%. Preferably, the tensile strength of the adhesive film of the dispersant is 5-20 MPa.

[0010] Preferably, the mass content of Na in the dispersant is 1%-8%. Preferably, the mass content of Li in the dispersant is 0%-8%.

[0011] In a second aspect, the present application provides a preparation method of the dispersant described above, comprising the following steps: obtaining a carboxymethyl cellulose salt; mixing deionized water, the carboxymethyl cellulose salt and an acidic compound uniformly to obtain a first mixed solution; adding an acidic catalyst and a reactant to the first mixed solution under a protective atmosphere to perform a first reaction, and purifying to obtain the dispersant after the first reaction; the reactant comprises at least one of ethylene oxide, tetrahydrofuran, a siloxane compound and a carbamate; the carboxymethyl cellulose salt comprises a structural unit shown in formula 2 formula 2, wherein m is 200-6000, and is selected from at least one of Li + , Na + .

[0012] Preferably, mixing deionized water, the carboxymethyl cellulose salt and an acidic compound uniformly to obtain a first mixed solution comprises the following steps: adding deionized water and the carboxymethyl cellulose salt into a reaction container, adding an acidic compound into the reaction container to adjust the pH value to 4-6, stirring and dissolving at 50-60°C, the stirring time is 30-90 min, after the stirring is completed, cooling to below 30°C, stirring at a rotation speed of 500-1000 rpm, the stirring time is 30-60 min, and the first mixed solution is obtained after the stirring is completed.

[0013] Preferably, the temperature of the first reaction is 65-90°C, and the first reaction time is 2-4 h. The acidic compound includes one or more of hydrochloric acid, dilute sulfuric acid, phosphoric acid, and citric acid; The acidic catalyst includes at least one of p-toluenesulfonic acid, sulfuric acid, lactic acid, and fluorosulfonic acid; The molar ratio of the carboxymethyl cellulose salt to the reactant is 1:(1-5); The mass ratio of the carboxymethyl cellulose salt to the acidic catalyst is 1:0.015-1:0.1; In the first mixed solution, the mass ratio of the deionized water to the carboxymethyl cellulose salt is 5:1-70:1.

[0014] Preferably, obtaining the carboxymethyl cellulose salt includes the following steps: The cellulose is soaked in an alkaline solution to perform an alkalization reaction to form alkali cellulose; The organic solvent, chloroacetic acid, and alkali cellulose are mixed to perform an etherification reaction under the condition that the pH value is 9-12, the pH value is adjusted to 7-9 using a first alkaline compound after the etherification reaction is completed, and then washing is performed to obtain the carboxymethyl cellulose salt; The alkaline solution includes a second alkaline compound, and the mass concentration of the second alkaline compound in the alkaline solution is 15%-20%; The first alkaline compound contains Li, and the second alkaline compound contains Na; The reaction temperature of the alkalization reaction is 20-30°C, and the reaction time of the alkalization reaction is 0.5-2h; The reaction temperature of the etherification reaction is 50-70°C, and the reaction time of the etherification reaction is 2-4h; The organic solvent includes an alcohol solvent; The molar ratio of the cellulose to the second alkaline compound is 1.2:1-4.5:1; The molar ratio of the chloroacetic acid to the alkali cellulose is (2-16):1; The mass ratio of the organic solvent to the alkali cellulose is (5-30):1.

[0015] In a second aspect, the present application provides a lithium secondary battery, which includes a negative electrode sheet, the negative electrode sheet includes a negative electrode material layer, the negative electrode material layer contains a dispersing agent, and the dispersing agent is the dispersing agent for a negative electrode described above or is prepared by the preparation method of the dispersing agent for a negative electrode described above.

[0016] The dispersant for the negative electrode sheet provided by the application comprises a structural unit shown in formula 1, the flexible groups of R1 and R2 are grafted in the long chain of carboxymethyl cellulose salt (such as CMC-Na), the flexibility of the dispersant is improved, the dispersant shrinks during the drying process of the electrode sheet, and due to the increase of elasticity and the decrease of rigidity, the cracking of the electrode sheet is alleviated, the cycle performance of the battery is improved, the rebound of the electrode sheet during the cycle is reduced, and the expansion rate of the battery cell during the cycle of the electrode sheet is reduced. DETAILED DESCRIPTION

[0017] In order to make the purpose, technical scheme and advantages of the present application clearer, the present application will be further described in detail below in combination with embodiments. It should be understood that the specific embodiments described herein are only used to explain the present application and do not limit the present application.

[0018] In order to illustrate the technical scheme of the present application, the following will be described through specific embodiments.

[0019] In the first aspect, the present application provides a dispersant for a negative electrode sheet, the dispersant is a compound shown in formula 1, Formula 1 wherein each R is independently selected from at least one of H, R1 and R2; wherein each R1 and R2 is independently selected from at least two of -O-, -Si-O-, -S-, -COO-, -NH-CO-O-, C1-C12 alkyl having at least one H substituted by halogen or not substituted by halogen, C2-C12 alkenyl, C1-C12 alkylene having at least one H substituted by halogen or not substituted by halogen; m is 200-6000; M is selected from at least one of Li + , Na + ; The degree of substitution of the dispersant is 0.7-0.95.

[0020] Specifically, each R is independently selected from at least one of H, R1 and R2, and the degree of substitution of the dispersant is 0.7-0.95, which indicates that in the structure of the dispersant, there is at least one R selected from R1 or R selected from R2 structural unit.

[0021] R1 and R2 are each independently selected from at least two of the following: -O-, -Si-O-, -S-, -COO-, -NH-CO-O-, at least one H-substituted or unsubstituted halogenated C1-C12 alkyl groups, and C2-C12 alkenyl groups. The flexible groups of R1 and R2 are grafted onto the long chain of carboxymethyl cellulose salt (such as CMC-Na). This not only improves cracking but also serves as a dispersant in the negative electrode. When added in appropriate amounts, the original dispersibility can be maintained without the residue of small molecule alcohols, thus preserving the original electrical performance of the battery.

[0022] Halogens include F, Cl, Br, etc. At least one H is replaced by a halogen. Understandably, this can be one or two Hs in a C1-C12 alkyl group, or more than two Hs can be replaced by a halogen.

[0023] The C1-C12 alkyl groups include C1-C12 straight-chain alkyl groups or C1-C12 branched alkyl groups, such as ethyl, propyl, butyl, isobutyl, etc. The C2-C12 alkenyl groups include C2-C12 straight-chain alkenyl groups or C2-C12 branched alkenyl groups, such as vinyl, propenyl, -CH(CH3)CH=CH2, etc.

[0024] C1~C12 alkylene groups, i.e., those with the molecular formula C n H 2n It does not contain unsaturated bonds and can be a straight-chain C1~C12 alkylene group or a branched C1~C12 alkylene group, such as -CH2-, -CH2-CH2-, -CH(CH3)CH2CH2-, etc.

[0025] The dispersant for negative electrode provided in this application includes the structural unit shown in Formula 1, with flexible groups R1 and R2 grafted into the long chain of carboxymethyl cellulose salt (such as CMC-Na) to improve the flexibility of the dispersant. During the electrode drying process, the dispersant shrinks, and due to the increase in elasticity, the rigidity is weakened, thereby alleviating electrode cracking, improving battery cycle performance, reducing electrode rebound during cycle, and reducing cell expansion rate during electrode cycle.

[0026] In specific embodiments, the degree of substitution of the dispersant is 0.7 to 0.95.

[0027] Specifically, the degree of substitution here is an average value, which means that in the repeating units of the carboxymethyl cellulose salt, 0.7-0.95 hydrogen atoms in each unit of the hydroxyl group are successfully replaced by R1 and R2. The dispersant has a degree of substitution in the range of 0.7-0.95, and the obtained dispersant has good water solubility and dispersibility, which can be used for the preparation of negative electrode slurry to prevent agglomeration and improve the stability of the slurry. In addition, the dispersant also has good strength, which can alleviate the cracking of the negative electrode sheet, improve the cycle performance of the battery, reduce the rebound of the negative electrode sheet, and reduce the expansion rate during the cycle process of the battery.

[0028] The degree of substitution of the dispersant can be in the following ranges: 0.7-0.8, 0.8-0.9, or 0.9-0.95.

[0029] In some preferred embodiments, R is independently selected from one or more of -R3-OH, -Si-(CH3)2, -NH-CO-O-R4; wherein R3 is selected from C2-C12 alkyl, and R4 is selected from C2-C12 alkyl.

[0030] Further preferably, R is selected from one or more of -CH2CH2OH, -CH2CH2CH2CH2OH, -Si-(CH3)2, -NH-CO-O-CH3, -NH-CO-O-CH2CH3.

[0031] In some embodiments, the molecular weight of the dispersant is 200,000-800,000 g / mol.

[0032] Specifically, the molecular weight of the dispersant is in the range of 200,000-800,000 g / mol, which has good dispersing effect during the preparation of the negative electrode slurry, ensures the stability of the negative electrode slurry, reduces the sedimentation of the negative active material, improves the processability of the negative electrode sheet, alleviates the cracking of the negative electrode sheet, improves the cycle performance of the battery, reduces the rebound of the negative electrode sheet, and reduces the expansion rate during the cycle process of the battery.

[0033] The molecular weight of the dispersant can be in the following ranges: 200,000-300,000 g / mol, 300,000-400,000 g / mol, 400,000-500,000 g / mol, 500,000-700,000 g / mol, or 700,000-800,000 g / mol.

[0034] In some embodiments, the elongation at break of the adhesive film formed by the dispersant is 0.3-3%.

[0035] The adhesive film formed by the dispersant refers to dissolving the dispersant in a solvent to obtain a mixed solution, coating the mixed solution on the surface of a substrate, drying to form an adhesive film on the surface of the substrate, and testing the elongation at break of the adhesive film. It should be noted that the method of obtaining the adhesive film and testing the elongation at break of the adhesive film both belong to the prior art.

[0036] The solvent is a solvent capable of dissolving the dispersant, including water.

[0037] In the process of preparing the negative electrode sheet, the dispersant can form a continuous gel film network with the negative electrode conductive agent and the negative electrode active material. The breaking elongation of the gel film formed by the dispersant is in the range of 0.3-3%, which improves the flexibility of the negative electrode sheet, ensures that the negative electrode sheet has good breaking elongation, and improves the processability of the negative electrode sheet. At the same time, during the charging and discharging of the battery, it also has the effect of reducing the rebound of the negative electrode sheet and reducing the fracture of the negative electrode sheet.

[0038] Specifically, the breaking elongation of the dispersant gel film can be 0.3%, 0.5%, 0.8%, 1.0%, 1.5%, 2.0%, 2.3%, 2.5%, 2.6%, 3.0%, etc., as long as the breaking elongation of the gel film formed by the dispersant is in the range of 0.3-3%.

[0039] In some embodiments, the tensile strength of the dispersant gel film is 5-20 MPa.

[0040] The tensile strength of the dispersant gel film is in the range of 5-20 MPa, and the dispersant has high mechanical strength, which can alleviate the expansion of the negative electrode active material during the battery cycle, improve the battery cycle performance, reduce the rebound of the negative electrode sheet, and reduce the expansion rate during the cycle.

[0041] The tensile strength of the dispersant gel film can be in the following ranges: 5-8 MPa, 8-10 MPa, 10-15 MPa, or 15-20 MPa.

[0042] In some embodiments, the mass content of Na in the dispersant is 1-8%; and / or, the mass content of Li in the dispersant is 0-8%.

[0043] Specifically, M in the structural unit represented by Formula 1 is selected from one or both of Li or Na. When the reactant used to prepare the dispersant is CMC-Na, M in the corresponding structural unit represented by Formula 1 is selected from Na, and the mass content of Na in the corresponding dispersant is in the range of 1-8%, and the mass content of Li in the dispersant is 0%.

[0044] When the reactant used to prepare the dispersant is CMC-Li, M in the corresponding structural unit represented by Formula 1 is selected from Na and Li, and the mass content of Na in the corresponding dispersant is in the range of 1-8%, and the mass content of Li in the dispersant is greater than 0 and less than 8%.

[0045] The mass content of Na in the dispersant is in the range of 1-8%, and / or the mass content of Li is in the range of 0-8%, and the metal elements contained are beneficial to the dissolution of the dispersant in the solvent, improve the dispersibility in the negative electrode slurry, improve the stability of the negative electrode slurry, improve the processing performance of the negative electrode sheet, and slow down the cracking problem of the negative electrode sheet.

[0046] In specific embodiments, the mass content of Na in the dispersant can be 1%, 2%, 3%, 4%, 5%, 6%, 7%, 8%, etc., or a range of any two of the above.

[0047] In specific embodiments, the mass content of Li in the dispersant can be 0, 0.5%, 1%, 2%, 3%, 4%, 5%, 6%, 7%, 8%, etc., or a range of any two of the above.

[0048] In some preferred embodiments, the mass content of Li in the dispersant is 0.5%-8%.

[0049] In some embodiments, the viscosity of a mixed solution of water and the dispersant is 2000-25000 mPa·s at 25°C, and the mass content of the dispersant in the mixed solution is 1%.

[0050] In a second aspect, the application provides a preparation method of the above-mentioned dispersant, comprising the following steps: obtaining a carboxymethyl cellulose salt; mixing deionized water, the carboxymethyl cellulose salt and an acidic compound uniformly to obtain a first mixed solution; adding an acidic catalyst and a reactant to the first mixed solution to perform a first reaction under a protective atmosphere, and purifying to obtain the dispersant after the first reaction; The reactant includes at least one of ethylene oxide, tetrahydrofuran, a siloxane compound and a urethane compound. The carboxymethyl cellulose salt is a structural unit shown in formula 2 Formula 2, wherein m is 200-6000, and is selected from at least one of Li + , Na + .

[0051] The preparation method of the dispersant provided by the application adds an acidic catalyst and a reactant to a first mixed solution to perform a first reaction with a carboxymethyl cellulose salt, the reactant is grafted into the carboxymethyl cellulose salt, thereby forming a dispersant containing a structural unit shown in formula 1, the preparation process is simple, and the cost is low.

[0052] Specifically, siloxane compounds include dimethylsiloxane. Carbamates include methyl carbamate, ethyl carbamate, etc.

[0053] In some embodiments, deionized water, the carboxymethyl cellulose salt, and the acidic compound are mixed evenly to obtain a first mixed solution, comprising the following steps: Deionized water and carboxymethyl cellulose salt were added to the reaction vessel. An acidic compound was added to the reaction vessel to adjust the pH to 4-6. The mixture was stirred at 50-60℃ and 500-1500 rpm for 30-90 minutes. After stirring, the temperature was lowered to below 30℃ and stirred at 500-1000 rpm for 30-60 minutes. The first mixed solution was obtained after stirring.

[0054] Specifically, deionized water and the carboxymethyl cellulose salt are added to the reaction vessel. The carboxymethyl cellulose salt dissolves in the deionized water, and then an acidic compound is added to adjust the pH to 4-6, making the entire system weakly acidic. The hydroxyl group (-OH) itself has weak nucleophilicity, and the carboxymethyl cellulose salt has numerous intramolecular / intermolecular hydrogen bonds, resulting in a tightly coiled main chain and encapsulated active sites, making it difficult for it to contact the reactants and acidic catalyst. Under acidic conditions, H… + It will combine with the oxygen atom of the hydroxyl group to form a protonated hydroxyl group (-OH2). + ), -OH2 + It is a better leaving group than -OH, and readily undergoes substitution reactions under the action of nucleophiles. + It can break the hydrogen bonds between CMC molecules, allowing the main chain to unfold from its coiled state, exposing more hydroxyl active sites, increasing the probability of contact between monomers and active sites, and thus improving grafting efficiency.

[0055] Dissolve the acidic compound by stirring at 50-60°C and 500-1500 rpm for 30-90 minutes. This step is mainly to promote uniform mixing of the acidic compound and carboxymethyl cellulose salt. In specific embodiments, the stirring speed can be in the following ranges: 500-700 rpm, 700-800 rpm, 800-1000 rpm, 1000-1200 rpm, and 1200-1500 rpm.

[0056] Cool the mixture to below 30°C and stir at 500-1000 rpm for 30-60 minutes. The low temperature allows the already substituted sites to continue reacting, which facilitates control of the grafting rate between the reactants and carboxymethyl cellulose salt, increases the success rate of grafting between the reactants and carboxymethyl cellulose salt, and thus obtains a dispersant with a degree of substitution of 0.7-0.95.

[0057] In specific embodiments, the stirring speed can be 500 rpm, 550 rpm, 600 rpm, 650 rpm, 700 rpm, 750 rpm, 800 rpm, 850 rpm, 900 rpm, 950 rpm, 1000 rpm, or any two of the above. The stirring time can be 30 min, 35 min, 40 min, 45 min, 50 min, 55 min, 60 min, or any two of the above.

[0058] In some embodiments, the acid catalyst and the reactant are added to the first mixed solution to perform the first reaction under a protective atmosphere, and the compound represented by Formula 1 is obtained by purification after the first reaction. The water and the acid catalyst are mixed to obtain a second mixed solution, and the second mixed solution and the reactant are added to the first mixed solution to perform the first reaction under a protective atmosphere. The temperature of the first reaction is 65-90°C, and the first reaction time is 2-4 h. After the first reaction, the temperature is reduced to below 40°C, and the product, the compound represented by Formula 1, is obtained by washing with deionized water and drying.

[0059] Specifically, the protective atmosphere refers to performing the reaction in a protective gas atmosphere to prevent the presence of oxygen from affecting the polymerization of free radicals. The protective gas includes one of nitrogen and a noble gas. The noble gas includes helium, neon, argon, etc.

[0060] First, the water and the acid catalyst are mixed to fully dissolve the acid catalyst in the water and improve the reaction rate.

[0061] The second mixed solution is added to the first mixed solution under a protective atmosphere. The second mixed solution can be added to the first mixed solution in batches or all at once, and is preferably added in batches.

[0062] The second mixed solution and the reactant are added to the first mixed solution to perform the first reaction, which includes the following steps: the second mixed solution is first added to the first mixed solution in batches, and the reactant is added to the first mixed solution to perform the first reaction at 65-90°C.

[0063] The reactant reacts with the carboxymethyl cellulose salt represented by Formula 2 under the action of the acid catalyst to obtain the dispersant of the compound represented by Formula 1. During the reaction, the reaction is exothermic, and the temperature gradually rises. Cooling water can be used to cool the reaction temperature to 65-90°C.

[0064] Specifically, the first reaction temperature can be 65°C, 70°C, 75°C, 78°C, 80°C, 82°C, 85°C, 88°C, 90°C, etc., as long as the first reaction temperature is in the range of 65-90°C. The first reaction time can be 2, 2.5, 3, 3.5, 4h, etc., as long as the first reaction time is in the range of 2-4h.

[0065] After the first reaction is completed, the temperature of the reaction container is lowered to below 40°C, and deionized water is used for washing and impurity removal, with the number of washing times being greater than or equal to 1, to remove residual suspending agent and salt.

[0066] In some embodiments, the acidic compound includes one or more of hydrochloric acid, dilute sulfuric acid, phosphoric acid, and citric acid.

[0067] In some embodiments, the acidic catalyst includes at least one of p-toluenesulfonic acid, sulfuric acid, lactic acid, and fluorosulfonic acid.

[0068] In some embodiments, the molar ratio of the carboxymethyl cellulose salt to the reactant is 1:(1-5).

[0069] Specifically, the molar ratio of the carboxymethyl cellulose salt to the reactant is in the range of 1:(1-5), the collision rate of the reactant with the active carbon chain of the carboxymethyl cellulose salt is high, the chain has more opportunities to increase, and thus the degree of substitution of the compound represented by Formula 1 formed is in the range of 0.7-0.95, the molecular weight is in the range of 200000-800000 g / mol, the tensile strength is in the range of 5-20 MPa, the elasticity is increased, the rigidity is weakened, the electrode sheet cracking is alleviated, the battery cycle performance is improved, the electrode sheet rebound during the cycle is reduced, and the cell swelling rate during the electrode sheet cycle is reduced.

[0070] In specific embodiments, the molar ratio of the carboxymethyl cellulose salt to the reactant can be 1:1, 1:1.5, 1:2, 1:2.5, 1:3, 1:3.5, 1:4, 1:4.5, 1:5, or a range between any two of the foregoing.

[0071] In some embodiments, the mass ratio of the carboxymethyl cellulose salt to the acidic catalyst is 1:0.015-1:0.1.

[0072] Specifically, the mass ratio of the carboxymethyl cellulose salt to the acidic catalyst is in the range of 1:0.015-1:0.1, the reaction rate is improved, and the compound represented by Formula 1 formed has a molecular weight of 200000-800000.

[0073] In specific embodiments, the mass ratio of the carboxymethyl cellulose salt to the acidic catalyst can be 1:0.015, 1:0.02, 1:0.05, 1:0.07, 1:0.08, 1:0.1, or a range between any two of the foregoing.

[0074] In some embodiments, in the first mixed solution, the mass ratio of the deionized water to the carboxymethyl cellulose salt is 5:1 to 70:1.

[0075] Specifically, in the first mixed solution, the mass ratio of the deionized water to the carboxymethyl cellulose salt is in the range of 5:1 to 70:1, the deionized water mainly plays a role in dissolving the carboxymethyl cellulose salt, and at the same time facilitates the subsequent controllable polymerization of the reactants and the carboxymethyl cellulose salt under the action of the initiator.

[0076] In specific embodiments, the mass ratio of the deionized water to the carboxymethyl cellulose salt can be 5:1, 10:1, 15:1, 20:1, 30:1, 40:1, 50:1, 60:1, 70:1, or a range between any two of the above.

[0077] In some embodiments, obtaining the carboxymethyl cellulose salt includes the following steps: immersing the cellulose in an alkaline solution to perform an alkalization reaction to form alkali cellulose; mixing the organic solvent, chloroacetic acid, and alkali cellulose, and then performing an etherification reaction under the condition that the pH value is 9 to 12, adjusting the pH value to 7 to 9 using a first alkaline compound after the etherification reaction is completed, and then washing to obtain the carboxymethyl cellulose salt; The alkaline solution includes a second alkaline compound, and in the alkaline solution, the mass concentration of the second alkaline compound is 15% to 20%; The first alkaline compound contains Li, and the second alkaline compound contains Na; The reaction temperature of the alkalization reaction is 20 to 30°C, and the reaction time of the alkalization reaction is 0.5 to 2h; The reaction temperature of the etherification reaction is 50 to 70°C, and the reaction time of the etherification reaction is 2 to 4h.

[0078] Specifically, the cellulose is immersed in an alkaline solution and stirred uniformly to perform an alkalization reaction, the reaction temperature of the alkalization reaction is in the range of 20 to 30°C, and the reaction time of the alkalization reaction is in the range of 0.5 to 2h, the alkaline solution destroys the dense structure of the cellulose, and the cellulose is fully swollen to form alkali cellulose with reactivity, which is conducive to the efficiency of the subsequent etherification reaction and the quality of the product. The alkalization reaction temperature can be 20°C, 22°C, 23°C, 25°C, 27°C, 29°C, 30°C, etc. The alkalization reaction time can be 0.5h, 0.6h, 0.7h, 0.8h, 1.0h, 1.2h, 1.5h, 1.6h, 1.8h, 2.0h.

[0079] Chloroacetic acid introduces carboxymethyl groups. The organic solvent serves to dissolve the chloroacetic acid and inhibit side reactions of the chloroacetic acid. The organic solvent also penetrates into the interior of the swollen fibers, fully stretching the sodium cellulose gum film, and ensuring that each hydroxyl site can contact the dissolved chloroacetic acid.

[0080] In some embodiments, the organic solvent comprises an alcohol solvent.

[0081] The alcohol solvent comprises ethanol, propanol, isopropanol, butanol, etc.

[0082] In some embodiments, the molar ratio of the cellulose to the second basic compound is 1.2:1-4.5:1.

[0083] The molar ratio of the cellulose to the second basic compound in the range of 1.2:1-4.5:1 is conducive to the alkalization reaction of the second basic compound, destroys the dense structure of the cellulose, fully swells the cellulose, and forms an alkali cellulose with reactivity, which is conducive to the efficiency of the subsequent etherification reaction and the quality of the product.

[0084] In specific embodiments, the molar ratio of the cellulose to the second basic compound can be 1.2:1, 1.5:1, 1.8:1, 2.0:1, 2.5:1, 3.0:1, 3.5:1, 4.5:1, etc.

[0085] In some embodiments, the molar ratio of the chloroacetic acid to the alkali cellulose is (2-16):1.

[0086] Specifically, the molar ratio of the chloroacetic acid to the alkali cellulose is in the range of (2-16):1, which is conducive to the nucleophilic substitution reaction of the alkali cellulose and the chloroacetic acid, generating the compound shown in Formula 2.

[0087] In specific embodiments, the molar ratio of the chloroacetic acid to the alkali cellulose can be 2:1, 4:1, 5:1, 8:1, 10:1, 12:1, 14:1, 15:1, 16:1, etc.

[0088] In some embodiments, the mass ratio of the organic solvent to the alkali cellulose is (5-30):1.

[0089] The mass ratio of the organic solvent to the alkali cellulose in the range of (5-30):1 is conducive to the organic solvent fully penetrating into the interior of the swollen fibers, fully stretching the sodium cellulose gum film, and ensuring that each hydroxyl site can contact the dissolved chloroacetic acid.

[0090] In specific embodiments, the mass ratio of the organic solvent to the alkali cellulose can be 5:1, 10:1, 12:1, 15:1, 18:1, 20:1, 25:1, 30:1, etc.

[0091] In some embodiments, the cellulose includes cotton linter, wood pulp, etc.

[0092] In some embodiments, the etherification reaction step is performed after mixing the organic solvent, chloroacetic acid, and alkali cellulose at a pH of 9-12, wherein a third basic compound is used to adjust the pH to 9-12, and the molar ratio of chloroacetic acid to the third basic compound is (1.5-2): 1.

[0093] The etherification reaction of chloroacetic acid and alkali cellulose can be performed at a pH of 9-12.

[0094] The first basic compound includes one of lithium hydroxide and lithium carbonate.

[0095] The second basic compound and the third basic compound include sodium hydroxide.

[0096] In the step of adjusting the pH to 7-9 after the etherification reaction using the first basic compound, the reaction equation is CMC-Na + LiOH → CMC-Li + NaOH.

[0097] In specific embodiments, the molar ratio of chloroacetic acid to the third basic compound can be 1.5:1, 1.6:1, 1.7:1, 1.8:1, 1:1.9, 1:2, or any two of the above.

[0098] In some embodiments, the carboxymethyl cellulose salt obtained after washing includes the following steps: washing with a washing liquid to remove impurities, and drying to obtain the carboxymethyl cellulose salt, wherein the washing liquid includes at least one of an alcohol solvent and deionized water.

[0099] Specifically, the alcohol solvent includes ethanol. Washing with an alcohol solvent or deionized water removes impurities, including residual salts such as sodium chloride and sodium bicarbonate, and unreacted reactants. In the drying step to obtain the carboxymethyl cellulose salt, the drying method is vacuum drying, and the drying temperature is 60-80°C.

[0100] CMC is a water-soluble polymer, but has very low solubility in high-concentration alcohols such as ethanol, while unreacted small molecule reactants (still soluble in alcohol-water mixed solution. By increasing the alcohol concentration in a gradient, the washing effect of "product precipitation, impurity dissolution" is achieved.

[0101] The washing with a washing liquid includes the following steps: After the etherification reaction, the pH is adjusted to 7-9 using the first basic compound to obtain a first mixture, and the following washing steps are performed: First step: using 30%-50wt% first alcohol water mixture to wash the first mixture, to obtain the second mixture. At this time, the CMC part swells but does not dissolve, and the small molecule reactants are preferentially dissolved in the mixture, and most of the easily soluble impurities are preliminarily removed.

[0102] Second step: using 70%-95wt% second alcohol water mixture to wash the second mixture, to obtain the third mixture. The CMC is completely precipitated (forming flocculent or granular precipitate), and a small amount of residual small molecule impurities continue to dissolve in the high concentration alcohol solution, realizing deep washing.

[0103] Third step: using anhydrous alcohol to wash the third mixture 1-2 times to remove residual water and alcohol-soluble impurities, and facilitate subsequent drying.

[0104] It should be noted that the molecular weight of the compound represented by formula 1 can be obtained by GPC test.

[0105] In a third aspect, the present application provides a lithium secondary battery, comprising a negative electrode sheet, wherein the negative electrode sheet comprises a negative electrode material layer, and the negative electrode material layer contains a dispersing agent, and the dispersing agent is the dispersing agent described above or is prepared by the preparation method of the dispersing agent described above.

[0106] The lithium secondary battery provided by the present application contains the dispersing agent of the compound represented by formula 1 in the negative electrode sheet, grafts the flexible groups of R1 and R2 in the long chain of carboxymethyl cellulose salt (such as CMC-Na), improves the flexibility of the compound represented by formula 1, and shrinks the compound represented by formula 1 during the drying process of the electrode sheet. Due to the increase in elasticity and the decrease in rigidity, the cracking of the electrode sheet is alleviated, the cycle performance of the battery is improved, the rebound of the electrode sheet during the cycle process is reduced, and the swelling rate of the battery cell during the cycle process of the electrode sheet is reduced.

[0107] It should be noted that, in addition to the dispersing agent in the negative electrode sheet using the compound represented by formula 1 provided by the present application, the positive electrode sheet and the negative electrode sheet in the lithium secondary battery are prepared by using the existing technology, and the preparation method of the lithium secondary battery also refers to the existing technology. The lithium secondary battery includes a lithium ion battery.

[0108] The present application is further illustrated by the following examples.

[0109] Example 1 1) Preparation of dispersing agent: S1: Preparation of sodium carboxymethyl cellulose S11: Soak the cellulose cotton wool in a sodium hydroxide solution with a mass concentration of 18%, stir uniformly, perform alkalization reaction, and make the cellulose swell sufficiently to form alkali cellulose. The reaction temperature of the alkalization reaction is 25℃, the reaction time of the alkalization reaction is 1h, and the molar ratio of cellulose to sodium hydroxide is 1:2.

[0110] S12: After mixing ethanol, chloroacetic acid and alkali cellulose obtained in step S11, sodium hydroxide is added, wherein the molar ratio of sodium hydroxide to chloroacetic acid is 1.8:1, the pH value is adjusted to 10, and then etherification reaction is carried out, the temperature of etherification reaction is 60°C, the time of etherification reaction is 3h, after the etherification reaction is completed, a solution containing CMC-Na is obtained, the solution containing CMC-Na is neutralized by using lithium hydroxide solution, the pH value is adjusted to 7, and a first mixture is obtained.

[0111] The molar ratio of chloroacetic acid to alkali cellulose is 5:1, and the mass ratio of ethanol to alkali cellulose is 20:1.

[0112] S13: First step: the first mixture in step S12 is washed with a 30wt% first alcohol water mixture to obtain a second mixture, Second step: the second mixture is further washed with a second alcohol water mixture with an alcohol concentration of 80wt% to obtain a third mixture.

[0113] Third step: the third mixture is washed with anhydrous alcohol for 1-2 times to remove residual water and alcohol-soluble impurities, and facilitate subsequent drying.

[0114] After washing, CMC-Na is obtained by vacuum drying at 60-80°C, and the structural formula of CMC-Na is wherein M is selected from Na and Li.

[0115] S2: Preparation of dispersant S21: Deionized water and CMC-Na powder obtained in step S13 are added to a reaction kettle, then hydrochloric acid is added to the reaction kettle to adjust the pH value to 6, stirring is carried out at 50-60°C for 30 minutes, after stirring is completed, the temperature is lowered to below 30°C, then high-speed stirring is carried out at a speed of 800rpm, the stirring time is 40min, and a first mixed solution is obtained after uniform mixing.

[0116] The mass ratio of deionized water to CMC-Na is 20:1.

[0117] S22: Nitrogen is introduced to replace the air in the kettle, the acidic catalyst p-toluenesulfonic acid is mixed with deionized water to obtain a second mixed solution, the second mixed solution is added to the reaction kettle in batches, the temperature is raised to 80°C, and the reactant ethylene oxide is slowly added to carry out the first reaction, the reaction temperature of the first reaction is 80°C, the reaction time of the first reaction is 3h, after the first reaction is completed, the temperature is lowered to below 40°C, deionized water is washed for 2-3 times to remove residual suspending agent and salt, and a dispersant product is obtained, and the structural formula of the product is , wherein M is selected from Na and Li, R is selected from -CH2CH2OH, and m is 200-6000; the mass content of Na in the dispersant product is 4%, and the mass content of Li is 2%.

[0118] The molar ratio of CMC-Na to ethylene oxide is 1:2, and the mass ratio of CMC-Na to the acidic catalyst p-toluenesulfonic acid is 1:0.03.

[0119] Example 2 Most of the steps in this example and Example 1 are the same, except that the S21 step is different, and the stirring speed in Example 2 is 500 rpm. The rest is the same as Example 1.

[0120] Example 3 Most of the steps in this example and Example 1 are the same, except that the S21 step is different, and the stirring speed in Example 3 is 1000 rpm. The rest is the same as Example 2.

[0121] Example 4 Most of the steps in this example and Example 1 are the same, except that the S21 step is different, and the stirring speed in Example 4 is 200 rpm. The rest is the same as Example 1.

[0122] Example 5 Most of the steps in this example and Example 1 are the same, except that the S21 step is different, and the stirring speed in Example 5 is 1200 rpm. The rest is the same as Example 1.

[0123] Example 6 Most of the steps in this example and Example 1 are the same, except that the S22 step is different, and the reactant in Example 6 is tetrahydrofuran, the acidic catalyst is fluorosulfonic acid, the molar ratio of CMC-Na to tetrahydrofuran is 1:3, and the mass ratio of CMC-Na to the acidic catalyst p-toluenesulfonic acid is 1:0.05; the first reaction temperature is 70°C, and the first reaction time is 4h, and the rest is the same as Example 1.

[0124] Example 7 Most of the steps in this example and Example 1 are the same, except that the S22 step is different, and the reactant in Example 7 is dimethylsiloxane, the catalyst is p-toluenesulfonic acid, the molar ratio of CMC-Na to dimethylsiloxane is 1:4; the mass ratio of CMC-Na to the acidic catalyst p-toluenesulfonic acid is 1:0.08; the first reaction temperature is 65°C, and the first reaction time is 2.5h, and the rest is the same as Example 1.

[0125] Example 8 The steps of this embodiment and most of the steps of Example 1 are the same, except that the step S21 is different, and in Example 8, the pH value is adjusted to 4; in the step S22, the molar ratio of CMC-Na to ethylene oxide added is 1:5; the reaction temperature of the first reaction is 65°C, and the reaction time of the first reaction is 4h. The rest is the same as Example 1.

[0126] Example 9 The steps of this embodiment and most of the steps of Example 1 are the same, except that there is no high-speed stirring step in the step S21, and the rest is the same as Example 1. The step S21 in Example 9 is as follows.

[0127] S21: Deionized water and CMC-Na powder obtained in the step S13 are added to a reaction kettle, then hydrochloric acid is added to the reaction kettle to adjust the pH value to 6, and a first mixed solution is obtained after mixing.

[0128] Example 10 The steps of this embodiment and most of the steps of Example 1 are the same, except that the step S12 is different, which is as follows.

[0129] S12: After mixing ethanol, chloroacetic acid and alkali cellulose obtained in the step S11, sodium hydroxide is added, wherein the molar ratio of sodium hydroxide to chloroacetic acid is 1.5:1, the pH value is adjusted to 9, then etherification reaction is carried out, the etherification reaction temperature is 50°C, the etherification reaction time is 4h, after the etherification reaction is completed, a CMC-Na-containing solution is obtained, the CMC-Na-containing solution is neutralized using lithium hydroxide solution, and the pH value is adjusted to 8.

[0130] Example 11 The steps of this embodiment and most of the steps of Example 1 are the same, except that the step S12 is different, which is as follows.

[0131] S12: After mixing ethanol, chloroacetic acid and alkali cellulose obtained in the step S11, sodium hydroxide is added, wherein the molar ratio of sodium hydroxide to chloroacetic acid is 2:1, the pH value is adjusted to 12, then etherification reaction is carried out, the etherification reaction temperature is 70°C, the etherification reaction time is 2h, after the etherification reaction is completed, a CMC-Na-containing solution is obtained, the CMC-Na-containing solution is neutralized using lithium hydroxide solution, and the pH value is adjusted to 9.

[0132] Example 12 The steps of this embodiment and most of the steps of Example 1 are the same, except that the step S12 is different, which is as follows.

[0133] S12: After mixing ethanol, chloroacetic acid and the alkali cellulose obtained in step S11, sodium hydroxide is added, wherein the molar ratio of sodium hydroxide to chloroacetic acid is 1.3:1, the pH value is adjusted to 8, and then etherification reaction is carried out, the temperature of etherification reaction is 50°C, the time of etherification reaction is 2h, after the etherification reaction is completed, a solution containing CMC-Na is obtained, the solution containing CMC-Na is neutralized using lithium hydroxide solution, and the pH is adjusted to 7.

[0134] Example 13 Most of the steps of this example and example 12 are the same, the difference is that the step S12 is different, specifically as follows.

[0135] S12: After mixing ethanol, chloroacetic acid and the alkali cellulose obtained in step S11, sodium hydroxide is added, wherein the molar ratio of sodium hydroxide to chloroacetic acid is 1.3:1, the pH value is adjusted to 8, and then etherification reaction is carried out, the temperature of etherification reaction is 40°C, the time of etherification reaction is 2h, after the etherification reaction is completed, a solution containing CMC-Na is obtained, the solution containing CMC-Na is neutralized using lithium hydroxide solution, and the pH is adjusted to 7.

[0136] Comparative Example 1 The dispersant in Comparative Example 1 is the product CMC-Na obtained in step S1 of Example 1.

[0137] Physical property test of dispersant The dispersants prepared in the above examples and comparative examples are tested for molecular weight, tensile strength, and elongation at break of the dispersant film.

[0138] 1. The molecular weight of the dispersant is tested by GPC.

[0139] 2. Preparation method of dispersant film: (1) Fold the 10cm*5cm mold box with aluminum foil or copper foil; (2) Dissolve CMC in water to obtain a solution with a concentration of 1.3wt%; (3) Pour 300ml of the above CMC solution into the mold; (4) The above mold is placed in a 60°C oven for drying.

[0140] The elongation at break and tensile strength of the dispersant film are tested using a high-iron tension machine, and the test method is as follows: (1) Turn on the power of the tension machine, and start the main machine and the control software; (2) Select the clamp according to the characteristics of the film, and recommend using a pneumatic flat push clamp; (3) Input the actual thickness, width, and original gauge length of the sample; (4) Set the tensile speed according to the standard or product requirements, and the film is usually 50mm / min; (5) Set "sample break automatic stop", and then start the test, get the elongation at break, tensile strength of the dispersant film.

[0141] The test results are shown in Table 1.

[0142] Table 1 The dispersants prepared in the above examples and comparative examples are used as negative electrode dispersants, and lithium ion batteries are prepared according to the prior art, as follows.

[0143] 1) Preparation of negative electrode sheet The negative electrode active material graphite, conductive agent carbon black, binder styrene-butadiene emulsion, and the above dispersant are mixed in a mass ratio of 97.7:1:0.7:0.5, and deionized water is added and mixed uniformly to obtain a negative electrode slurry. The above negative electrode slurry is coated on both sides of a copper foil, dried, rolled, and cut to obtain a negative electrode sheet.

[0144] 2) Preparation of positive electrode sheet: Lithium cobaltate, conductive carbon black and binder polyvinylidene fluoride are mixed in a mass ratio of 97.4:1.4:1.2, dispersed in N-methyl-2-pyrrolidone to obtain a positive electrode slurry. The positive electrode slurry is uniformly coated on both sides of an aluminum foil, dried, rolled, and cut to obtain a positive electrode sheet.

[0145] 3) Battery assembly: The positive electrode sheet, negative electrode sheet and separator are wound into a sandwich structure, and then the wound body is flattened and placed in an aluminum plastic shell. After welding the tabs, the aluminum plastic shell is sealed to obtain an electrode core ready for liquid injection. Commercial electrolyte is injected into the electrode core, sealed, aged, and formed to prepare a lithium ion battery.

[0146] Performance test Cycle performance test The test temperature is 25°C, and the capacity obtained by the above steps is the initial discharge capacity C1. The thickness H1 of the full battery is tested using a spiral micrometer.

[0147] The cycle test is carried out at 1C charge / 1C discharge, and after 300 cycles, the discharge capacity C2 is obtained, and the cycle capacity retention rate is calculated. The thickness H2 of the full battery after the cycle is tested using a spiral micrometer.

[0148] Cycle thickness expansion rate = (H2-H1) / H1*100%.

[0149] Cycle capacity retention rate = C2 / C1*100%.

[0150] After the above cycle, each disassembled 1 pcs full battery, observe whether there is a crack in the negative plate, if there is a crack, record the number of cracks, and record the length of the longest crack.

[0151] The corresponding test results are shown in Table 2.

[0152] Table 2 As can be seen from Table 1 and Table 2, by comparing Example 1 and Comparative Example 1, it is shown that grafting flexible groups of R1 and R2 in the long chain of carboxymethyl cellulose salt (such as CMC-Na) improves the flexibility of the dispersant, and the dispersant shrinks during the drying process of the pole piece. Due to the increase in elasticity and the decrease in rigidity, the pole piece cracking is alleviated, the battery cycle performance is improved, the pole piece rebound during the cycle is reduced, and the cell expansion rate during the pole piece cycle is reduced.

[0153] Comparing Example 1-Example 3 and Example 4-5, Example 9, there is no high-speed stirring step in Example 9, the stirring speed in Example 4 is lower than 500-1000 rpm, and the stirring speed in Example 5 is higher than 500-1000 rpm. The tensile strength of the dispersant film obtained in Example 4 and Example 5 is lower than 5-20 MPa, and the elongation at break of the film formed by the dispersant is low. It is shown that stirring at a speed of 500-1000 rpm for 30-60 min improves the grafting success rate of the reactants and the carboxymethyl cellulose salt. The dispersant obtained has a degree of substitution of 0.7-0.95, the tensile strength of the dispersant film is in the range of 5-20 MPa, the elongation at break of the film formed by the dispersant is 0.3-3%, the flexibility of the dispersant is improved, the pole piece cracking is alleviated, and the battery cycle performance is improved.

[0154] Comparing Example 1 and Example 6-7, changing the type of reactant, the obtained dispersant satisfies the structural unit shown in Formula 1, grafting flexible groups of R1 and R2 in the long chain of carboxymethyl cellulose salt (such as CMC-Na) improves the flexibility of the dispersant, alleviates the pole piece cracking, improves the battery cycle performance, reduces the pole piece rebound during the cycle, and reduces the cell expansion rate during the pole piece cycle.

[0155] Comparing Example 1, 10, 11 and Example 12-13, in the step of preparing carboxymethyl cellulose salt, the pH value adjusted in the etherification reaction step is in the range of 9-12, the etherification reaction temperature is 50-70°C, and the etherification reaction time is in the range of 2-4h. The molecular weight of the obtained dispersant is in the range of 200-800 thousand. If the pH value adjusted in the etherification reaction step is less than 9, the lower the etherification reaction temperature, the smaller the molecular weight of the obtained dispersant, the lower the cycle capacity retention rate of the prepared battery, the higher the thickness expansion rate, the more cracks during the cycle, and the longer the crack length.

[0156] The above embodiments are only used to illustrate the technical solutions of the present application, but not limit the present application. Although the present application has been described in detail with reference to the foregoing embodiments, it is to be understood that modifications or equivalent arrangements can be made to the technical solutions described in the foregoing embodiments by those skilled in the art without departing from the spirit and scope of the embodiments of the present application. Such modifications or equivalent arrangements should be included in the scope of the protection of the present application.

Claims

1. A dispersant for negative electrode sheets, characterized in that, The dispersant comprises the structural unit shown in Formula 1. Formula 1 Wherein, R is independently selected from at least one of H, R1, and R2; wherein R1 and R2 are independently selected from at least two of -O-, -Si-O-, -S-, -COO-, -NH-CO-O-, at least one H is a C1-C12 alkyl group or not substituted with a halogen, a C2-C12 alkenyl group, and at least one H is a C1-C12 alkylene group or not substituted with a halogen; m is 200~6000; M is selected from Li + Na + At least one of them; The degree of substitution of the dispersant is 0.7 to 0.

95.

2. The dispersant for negative electrode sheets according to claim 1, characterized in that, R is independently selected from one or more of -R3-OH, -Si-(CH3)2, and -NH-CO-O-R4; wherein R3 is selected from C2~C12 alkyl groups and R4 is selected from C2~C12 alkyl groups.

3. The dispersant for negative electrode sheets according to claim 1, characterized in that, The molecular weight of the dispersant is 200,000 to 800,000 g / mol.

4. The dispersant for negative electrode sheets according to claim 1, characterized in that, The elongation at break of the film formed by the dispersant is 0.3-3%; And / or, the tensile strength of the dispersant film is 5~20MPa.

5. The dispersant for negative electrode sheets according to claim 1, characterized in that, The dispersant contains 1% to 8% Na by mass. And / or, the mass content of Li in the dispersant is 0%~8%.

6. A method for preparing a dispersant for a negative electrode according to any one of claims 1-5, characterized in that, Includes the following steps: Obtain carboxymethyl cellulose salt; Deionized water, the carboxymethyl cellulose salt, and the acidic compound are mixed evenly to obtain a first mixed solution; Under a protective atmosphere, an acidic catalyst and reactants are added to the first mixed solution to carry out a first reaction. After the first reaction is completed, the dispersant is purified to obtain the dispersant. The reactants include at least one of ethylene oxide, tetrahydrofuran, siloxane compounds, and carbamates; The carboxymethyl cellulose salt comprises the structural unit shown in Formula 2. Equation 2, Where m is 200~6000, selected from Li + Na + At least one of them.

7. The method for preparing the dispersant for the negative electrode sheet according to claim 6, characterized in that, The process of mixing deionized water, the carboxymethyl cellulose salt, and the acidic compound to obtain a first mixed solution includes the following steps: Deionized water and carboxymethyl cellulose salt were added to a reaction vessel. An acidic compound was added to the reaction vessel to adjust the pH to 4-6. The mixture was stirred at 50-60°C and 500-1500 rpm for 30-90 minutes. After stirring, the temperature was lowered to below 30°C and stirred at 500-1000 rpm for 30-60 minutes. The first mixed solution was obtained after stirring.

8. The method for preparing the dispersant for the negative electrode sheet according to claim 6, characterized in that, The temperature of the first reaction is 65~90℃, and the reaction time is 2~4h; The acidic compound includes one or more of hydrochloric acid, dilute sulfuric acid, phosphoric acid, and citric acid; The acidic catalyst includes at least one of p-toluenesulfonic acid, sulfuric acid, lactic acid, and fluorosulfonic acid; The molar ratio of the carboxymethyl cellulose salt to the reactant is 1:(1~5); The mass ratio of the carboxymethyl cellulose salt to the acidic catalyst is 1:0.015 to 1:0.1; In the first mixed solution, the mass ratio of the deionized water to the carboxymethyl cellulose salt is 5:1 to 70:

1.

9. The method for preparing the dispersant for the negative electrode sheet according to claim 6, characterized in that, Obtaining carboxymethyl cellulose salts includes the following steps: Cellulose is soaked in an alkaline solution to undergo an alkalization reaction, forming alkali cellulose; An organic solvent, chloroacetic acid, and alkali cellulose are mixed and subjected to an etherification reaction at a pH of 9-12. After the etherification reaction is completed, the pH is adjusted to 7-9 using a first alkaline compound, and then the mixture is washed to obtain the carboxymethyl cellulose salt. The alkaline solution includes a second alkaline compound, wherein the mass concentration of the second alkaline compound in the alkaline solution is 15% to 20%. The first basic compound contains Li, and the second basic compound contains Na; The alkalization reaction is carried out at a temperature of 20-30°C for 0.5-2 hours. The etherification reaction is carried out at a temperature of 50-70°C for 2-4 hours. The organic solvent includes alcohol solvents; The molar ratio of cellulose to the second basic compound is 1.2:1 to 4.5:1; The molar ratio of chloroacetic acid to alkali cellulose is (2~16):1; The mass ratio of the organic solvent to the alkali cellulose is (5~30):

1.

10. A lithium secondary battery, characterized in that, The invention includes a negative electrode sheet, the negative electrode sheet comprising a negative electrode material layer, the negative electrode material layer containing a dispersant, the dispersant being the dispersant for a negative electrode sheet as described in any one of claims 1-5 or prepared by the method for preparing the dispersant for a negative electrode sheet as described in any one of claims 6-9.

Images