A negative electrode binder, a negative electrode sheet, and a secondary battery

Abstract

Aiming at the problems of low stripping strength and low cycle life of the existing negative electrode binder coating, the present invention provides a negative electrode binder, a negative electrode sheet and a secondary battery. The negative electrode binder includes a copolymer, and the copolymer includes acrylate units, vinyl aromatic units, acrylamide units, acrylate-polyol units and crosslinking units. The negative electrode binder provided by the present invention enables the negative electrode active material layer to have high adhesion, low electrode sheet rebound and small full-charge rebound, ensuring that the battery has a long cycle life.

Description

Technical Field

[0001] The present invention belongs to the technical field of secondary batteries, and particularly relates to a negative electrode binder, a negative electrode sheet and a secondary battery. Background Art

[0002] Lithium-ion batteries have become the main power source choice for modern portable electronic devices, electric vehicles and energy storage systems due to their advantages such as high energy density, long cycle life and low self-discharge rate, and higher requirements have been placed on their electrical properties including low-temperature cycle performance, rate performance and storage performance.

[0003] At present, those skilled in the art have done a lot of work to improve the electrochemical performance of batteries, improve the working efficiency and reliability of lithium-ion batteries, and meet the requirements of most application scenarios. At present, there have been studies to improve the low-temperature performance and rate performance of batteries by optimizing the electrolyte formula, improving the electrode material, optimizing the electrode structure, improving the binder composition, etc. Among them, the negative electrode binder improves the low-temperature cycle performance and rate performance of lithium-ion batteries by enhancing the adhesion between the negative electrode active layer and the negative electrode current collector, ensuring the structural stability, conductivity of the negative electrode sheet, and reducing the probability of lithium dendrite precipitation.

[0004] Conventional negative electrode binders are mainly styrene-butadiene rubber (SBR), which is a polymer copolymerized from 1,3-butadiene and styrene. The SBR emulsion itself is a product with a balance of hydrophilicity and hydrophobicity. On the one hand, it organically combines graphite through hydrophobicity, and on the other hand, it undergoes a condensation reaction with the surface oxide or hydroxyl group of the copper foil through hydrophilic groups to form chemical bonds, providing the adhesion between the negative electrode active material particles and between the active material layer and the current collector. Its main advantages are stable performance, moderate adhesion, low price and wide application in the battery cell. However, because its water emulsion is a small molecule linear chain emulsion with good elasticity after film formation, after rolling, the negative electrode sheet will have a large degree of thickness rebound, and the battery cell has a large full-charge rebound, affecting the cycle life of the battery cell; and the adhesion of the SBR binder to the foil material is insufficient.

[0005] Therefore, how to improve the coating peel strength and cycle life of the thick electrode system is an urgent problem to be solved. Summary of the Invention

[0006] Aiming at the problems of low coating peel strength and cycle life of the existing negative electrode binder, the present invention provides a negative electrode binder, a negative electrode sheet and a secondary battery.

[0007] To solve the above technical problems, the present invention provides a negative electrode binder, the negative electrode binder includes a copolymer, and the copolymer includes acrylate units, vinyl aromatic units, acrylamide units, acrylate-polyol units and crosslinking units.

[0008] Preferably, the mass ratio of the acrylate unit, vinyl aromatic unit, acrylamide unit, acrylate-polyol unit and crosslinked structural unit is (30-70):(10-50):(5-30):(8-25):(0.5-2).

[0009] Preferably, the copolymer further comprises a cationic unit, which is a structural unit obtained by polymerization of a cationic monomer containing a double bond. The cationic monomer includes one or more of dimethyldodecylammonium methacryloyloxyethyl bromide, dimethylhexadecylammonium methacryloyloxyethyl bromide, trimethylammonium methacryloyloxyethyl bromide, dimethybutylammonium methacryloyloxyethyl bromide, dimethylpentylammonium methacryloyloxyethyl bromide, trimethylammonium chloride methacryloyloxyethyl, trimethylammonium chloride acryloyloxyethyl, and dimethyldiallylammonium chloride.

[0010] Preferably, the mass ratio of the acrylate unit to the cationic unit is (30-70):(5-30).

[0011] Preferably, the acrylate unit is a structural unit obtained by polymerization of an acrylate monomer. The acrylate monomer includes one or more of methyl acrylate, ethyl acrylate, isobutyl acrylate, tert-butyl acrylate, butyl acrylate, 2-ethylhexyl acrylate, lauryl acrylate, stearyl acrylate, methyl methacrylate, butyl methacrylate, phenyl methacrylate, and n-hexyl acrylate.

[0012] Preferably, the vinyl aromatic unit is a structural unit obtained by polymerization of a vinyl aromatic monomer. The vinyl aromatic monomer includes one or more of styrene, β-methylstyrene, 4-methoxystyrene, and 2-methylstyrene.

[0013] Preferably, the acrylamide unit is a structural unit obtained by polymerization of an acrylamide monomer. The acrylamide monomer includes one or more of hydroxyethyl acrylamide, isobutoxymethyl acrylamide, and diacetone acrylamide.

[0014] Preferably, the acrylate-polyol unit is a structural unit obtained by polymerization of an acrylate-polyol monomer. The acrylate-polyol monomer includes one or more of poly(ethylene glycol) diacrylate, polyethylene glycol dimethacrylate, polyethylene glycol-block-polypropylene glycol-block-polyethylene glycol diacrylate, and poly(propylene glycol) dimethacrylate;

[0015] And / or, the crosslinking unit is a structural unit obtained by the polymerization of a crosslinking agent, and the crosslinking agent includes one or more of diethylene glycol dimethacrylate, ethylene glycol diacrylate, tetra(ethylene glycol) diacrylate, ethylene glycol dimethacrylate, tetraethylene glycol dimethacrylate, 1,3-butanediol dimethacrylate, neopentyl glycol diacrylate, 1,6-hexanediol dimethacrylate, and pentaerythritol tetraacrylate.

[0016] In a second aspect, the present application provides a negative electrode sheet, including a negative electrode current collector and a negative electrode active material layer disposed on at least one surface of the negative electrode current collector, and the negative electrode active material layer is composed of the negative electrode binder described in any one of the above.

[0017] In a third aspect, the present application provides a secondary battery, including a positive electrode sheet, an electrolyte, and the negative electrode sheet as described above.

[0018] In the present application, by introducing acrylate units, vinyl aromatic units, acrylamide units, acrylate-polyol units, and crosslinking units into the negative electrode binder, the negative electrode binder has an electrolyte mass swelling degree of 35% to 85%, and the negative electrode active material layer has a high adhesive force, low electrode sheet rebound, and small full charge rebound, ensuring that the battery has a long cycle life. Detailed embodiments

[0019] In order to make the technical problems, technical solutions, and beneficial effects solved by the present invention clearer, the present invention will be further described in detail below with reference to embodiments. It should be understood that the specific embodiments described herein are only used to explain the present invention and are not used to limit the present invention.

[0020] An embodiment of the present application provides a negative electrode binder, and the negative electrode binder includes a copolymer, and the copolymer includes acrylate units, vinyl aromatic units, acrylamide units, acrylate-polyol units, and crosslinking units.

[0021] By introducing acrylate units, vinyl aromatic units, acrylamide units, acrylate-polyol units, and crosslinking units into the negative electrode binder of this embodiment, the negative electrode binder has an electrolyte mass swelling degree of 35% to 85%, and the negative electrode active material layer has a high adhesive force, low electrode sheet rebound, and small full charge rebound, ensuring that the battery has a long cycle life.

[0022] In some embodiments, the mass ratio of the acrylate unit, vinyl aromatic unit, acrylamide unit, acrylate-polyol unit, and crosslinked structure unit is (30 - 70):(10 - 50):(5 - 30):(8 - 25):(0.5 - 2). By optimizing the mass ratio of the acrylate unit, vinyl aromatic unit, acrylamide unit, acrylate-polyol unit, and crosslinked unit, the negative electrode binder has a moderate electrolyte mass swelling degree, and the negative electrode active material layer has a high adhesive force, low electrode sheet rebound, and small full charge rebound, ensuring that the battery has a long cycle life. The acrylate unit is used to adjust the electrolyte swelling degree of the copolymer and provide the adhesive force with the active material and the foil. The vinyl aromatic unit is mainly used to stabilize the structure of the copolymer and maintain a certain glass transition temperature, improving the cracking problem during the electrode sheet processing. The polar functional group of the acrylamide unit is mainly used to further increase the adhesive force with the active material and the foil. The crosslinked network structure formed by the acrylate-polyol unit and the crosslinking agent is partly used to control the swelling degree of the copolymer and partly to control the rebound of the binder itself, thereby effectively suppressing the rebound of the active coating.

[0023] If there are too many acrylate units, although the adhesive force of the coating can be improved to a certain extent, the acrylate has good affinity with the electrolyte, resulting in large swelling of the binder, increasing the full charge rebound of the battery cell. The large rebound causes partial detachment of the negative electrode active material layer and the foil in the later stage of the cycle, resulting in a reduction in the cycle life. Fewer acrylate units will lead to low swelling, reduced lithium ion migration rate, and reduced adhesive force with the active material and the foil. If there are too many vinyl aromatic units and acrylamide units, the glass transition temperature of the binder will increase, and the electrode sheet is prone to cracking during the processing and baking process, resulting in brittle and hard electrode sheets and problems such as material loss during rolling. Too few will lead to unstable binder structure. Too many acrylate-polyol units will also cause an increase in the glass transition temperature of the binder. Too few acrylate-polyol units cannot ensure that the binder has a certain degree of swelling and control the rebound of the binder itself.

[0024] Specifically, the mass ratio of the acrylate unit, vinyl aromatic unit, acrylamide unit, acrylate-polyol unit, and crosslinked structure unit includes but is not limited to 30:50:30:25:2, 70:10:5:8:0.5, 50:30:20:15:1, or 40:20:20:15:1.

[0025] In a preferred embodiment, the mass ratio of the acrylate unit, vinyl aromatic unit, acrylamide unit, acrylate-polyol unit, and crosslinked structure unit is (40 - 55):(20 - 35):(15 - 25):(13 - 20):(1 - 1.5).

[0026] In some embodiments, the copolymer further includes cationic units, which are structural units obtained by the polymerization of cationic monomers containing double bonds. The cationic monomers include one or more of dimethyldodecyl(2-methacryloyloxyethyl)ammonium bromide, dimethylhexadecyl(2-methacryloyloxyethyl)ammonium bromide, trimethyl(2-methacryloyloxyethyl)ammonium bromide, dimethybutyl(2-methacryloyloxyethyl)ammonium bromide, dimethylpentyl(2-methacryloyloxyethyl)ammonium bromide, trimethyl(2-methacryloyloxyethyl)ammonium chloride, acryloyloxyethyltrimethylammonium chloride, and dimethyldiallylammonium chloride.

[0027] By introducing cationic units into the binder to form a cationic copolymer, a positive charge layer is formed on the surface of the graphite material by the binder in the negative electrode slurry, and mutual repulsion makes the slurry stably suspended, which greatly improves the dispersibility of the graphite material and the stability of the slurry. If the content of the cationic units is too low, they cannot effectively participate in the polymerization reaction and cannot achieve a good effect of dispersing graphite; if the content of the cationic units is too high, they cannot fully participate in the polymerization, resulting in side reactions between the monomer residues and the electrolyte, reducing the battery cycle life.

[0028] In some embodiments, the mass ratio of the acrylate units to the cationic units is (30 - 70):(5 - 30). By controlling the content of the cationic units, the positive charge amount carried by the binder is adjusted to improve the dispersibility.

[0029] Specifically, the mass ratio of the acrylate units to the cationic units includes but is not limited to 30:30, 70:5, 40:15, or 50:30.

[0030] In a preferred embodiment, the mass ratio of the acrylate units to the cationic units is (40 - 55):(8 - 15).

[0031] In some embodiments, the acrylate units are structural units obtained by the polymerization of acrylate monomers. The acrylate monomers include one or more of methyl acrylate, ethyl acrylate, isobutyl acrylate, tert-butyl acrylate, butyl acrylate, 2-ethylhexyl acrylate, lauryl acrylate, stearyl acrylate, methyl methacrylate, butyl methacrylate, phenyl methacrylate, and n-hexyl acrylate. By selecting the above acrylate monomers, the negative electrode binder has an appropriate electrolyte swelling degree.

[0032] In some embodiments, the vinyl aromatic units are structural units obtained by the polymerization of vinyl aromatic monomers. The vinyl aromatic monomers include one or more of styrene, β-methylstyrene, 4-methoxystyrene, and 2-methylstyrene. By selecting the above vinyl aromatic monomers, the negative electrode binder has an appropriate glass transition temperature, improving the cracking problem during the processing of the electrode sheet.

[0033] In some embodiments, the acrylamide unit is a structural unit obtained by the polymerization of acrylamide monomers, and the acrylamide monomers include one or more of hydroxyethyl acrylamide, isobutoxymethyl acrylamide, and diacetone acrylamide. By selecting the above acrylamide monomers, the adhesion between the negative electrode binder and the active material and the current collector foil is further increased.

[0034] In some embodiments, the acrylate-polyol unit is a structural unit obtained by the polymerization of acrylate-polyol monomers, and the acrylate-polyol monomers include one or more of poly(ethylene glycol) diacrylate, polyethylene glycol dimethacrylate, polyethylene glycol-block-polypropylene glycol-block-polyethylene glycol diacrylate, and poly(propylene glycol) dimethacrylate.

[0035] The crosslinking unit is a structural unit obtained by the polymerization of a crosslinking agent, and the crosslinking agent includes one or more of diethylene glycol dimethacrylate, ethylene glycol diacrylate, tetra(ethylene glycol) diacrylate, ethylene glycol dimethacrylate, tetraethylene glycol dimethacrylate, 1,3-butanediol dimethacrylate, neopentyl glycol diacrylate, 1,6-hexanediol dimethacrylate, and pentaerythritol tetraacrylate. By selecting the above acrylate-polyol monomers and crosslinking agent, the negative electrode binder has a crosslinked network structure to control its swelling degree and rebound. If the proportion of the crosslinking agent is too low, the effect of controlling the swelling degree and suppressing the self-rebound of the binder cannot be achieved. If the proportion is too high, the crosslinking agent is likely to remain and react with the electrolyte to deteriorate the battery performance.

[0036] Specifically, the preparation method of the negative electrode binder includes the following steps:

[0037] Mix a polymeric emulsifier, a cationic emulsifier, a crosslinking agent with water, and then mix with acrylate monomers, vinyl aromatic monomers, acrylamide monomers, acrylate-polyol monomers, and cationic monomers to obtain a pre-emulsion.

[0038] Mix water, a buffer, an initiator, and the pre-emulsion for reaction, the reaction temperature is 40~80 °C, and the time is 1~10 h to obtain a cationic binder emulsion.

[0039] Among them, the mass ratio of the polymeric emulsifier to the copolymer is (0.1-1.0):(40-60).

[0040] The mass ratio of the acrylate monomers, cationic emulsifier, buffer, and initiator is (30-70):(0.01-0.2):(0.01-10):(0.1-1.5).

[0041] In the present invention, there are no special restrictions on the above-mentioned polymeric emulsifier, cationic emulsifier, buffer, and initiator, and various substances commonly used in the field of chemical synthesis can be adopted.

[0042] Specifically, the cationic emulsifier includes one or more of dodecyl ammonium chloride, cetyl trimethyl ammonium chloride, octadecyl trimethyl ammonium chloride, cetyl trimethyl ammonium bromide, cetylpyridinium bromide, and octadecylamine polyoxyethylene ether bisquaternary ammonium salt.

[0043] The polymeric emulsifier includes one or more of allyl polyoxyethylene ether, ammonium allyl polyoxyethylene ether sulfonate, sodium salt of ether sulfonic acid containing propenyl, sodium vinyl sulfonate, and double bond alkyl polyoxyethylene phosphate.

[0044] The buffer includes one or more of sodium bicarbonate, sodium dihydrogen phosphate, disodium hydrogen phosphate, and N-(2-hydroxyethyl)iminodiacetic acid. The initiator includes one or more of azobisisobutyronitrile, azobis(2-methylpropionamidine) dihydrochloride, 2,2'-azobis(2-isopropylimidazoline) dihydrochloride, dimethyl 2,2'-azobis(2-methylpropionate), ammonium persulfate, sodium persulfate, and potassium persulfate.

[0045] One embodiment of the present application provides a negative electrode sheet, which includes a negative electrode current collector and a negative electrode active material layer provided on at least one surface of the negative electrode current collector, and the negative electrode active material layer is composed of the negative electrode binder described in any one of the above.

[0046] One embodiment of the present application provides a secondary battery, which includes a positive electrode sheet, an electrolyte, and the negative electrode sheet as described above.

[0047] The present invention will be further described below through examples.

[0048] Specifically, the negative electrode binder, negative electrode, and secondary battery disclosed in the present invention are described.

[0049] Example 1

[0050] Negative electrode binder

[0051] 0.5 part of polymeric emulsifier (allyl polyoxyethylene ether), 40 parts of acrylate monomer (methyl acrylate), 20 parts of vinyl aromatic (styrene), 10 parts of cationic monomer (methacryloyloxyethyl dimethyl dodecyl ammonium bromide), 20 parts of acrylamide monomer (hydroxyethyl acrylamide), 15 parts of acrylate-polyol (poly(ethylene glycol) diacrylate), 1 part of crosslinking agent (tetra(ethylene glycol) diacrylate), 0.5 part of initiator (azobisisobutyronitrile), 0.05 part of cationic emulsifier (dodecyl ammonium chloride), 1 part of buffer (sodium bicarbonate), and 70 parts of water.

[0052] Mix the polymeric emulsifier, cationic emulsifier, crosslinking agent and water, and then mix with acrylate monomers, vinyl aromatic monomers, acrylamide monomers, acrylate-polyol monomers, and cationic monomers to obtain a pre-emulsion;

[0053] Mix water, buffer, initiator and pre-emulsion for reaction. The reaction temperature is 80 °C and the time is 5 h to obtain a cationic binder emulsion. The solid content of the binder emulsion is 45%.

[0054] Negative electrode sheet

[0055] Stir 96% graphite, 1.5% thickener sodium carboxymethyl cellulose, and 1% conductive carbon black at a low speed of 15 rpm for 15 min, add water to adjust the solid content to 66%, pre-stir at 40 rpm for 15 min and then knead. After stirring at 40 rpm for 40 min, disperse at a high speed of 15 m / s for 40 min, add 1.2% cationic binder, disperse at 5 m / s for 20 min, and then discharge under vacuum to remove bubbles. Then coat the negative electrode paste on both sides of the negative electrode current collector Cu foil, and the single-sided coating surface density is 120 g / m 2 , after drying, cold pressing (compacted density 1.7 g / cm 3 ), slitting and other processes, the negative electrode sheet is obtained.

[0056] Positive electrode sheet

[0057] Mix the positive electrode active material lithium iron phosphate, conductive carbon black, and positive electrode binder in a mass ratio of 90:3:7, and add NMP to make a positive electrode paste. Coat the positive electrode paste on the aluminum foil and dry it to obtain the positive electrode sheet.

[0058] Preparation of secondary battery

[0059] Wind the negative electrode sheet, separator (PE film), and positive electrode sheet in sequence, so that the separator is in the middle of the positive electrode and the negative electrode to play a role in isolation, to make an electrode assembly. Place the electrode assembly in the outer package, inject a commercially available electrolyte and encapsulate it, and then go through processes such as liquid injection, formation, and exhaust to obtain a secondary battery.

[0060] Examples 2 - 12

[0061] Most of the steps in Examples 2 - 12 are the same as those in Example 1. The difference is that the formulations in Table 1 are used.

[0062] Comparative Examples 1 - 5

[0063] Most of the steps in Comparative Examples 1 - 5 are the same as those in Example 1. The difference is that the formulations in Table 1 are used.

[0064] Table 1

[0065]

[0066] The binders, anode slurries, anode sheets, and secondary batteries prepared in the above examples and comparative examples were tested as follows.

[0067] 1) Test for the mass swelling degree of the electrolyte:

[0068] Under standard atmospheric pressure and at 25 °C, 1 M lithium hexafluorophosphate was added to a mixed solution of ethylene carbonate and ethyl methyl carbonate with a mass ratio of 1:1 as the electrolyte. Take two containers and add the electrolyte. Make the wetting agent into a film and dry it and weigh it to get Winitial. The film specification is a circular sheet with a diameter of 20 mm and a thickness of 20 μm. Immerse the wetting agent film in the electrolyte, take it out after 72 h, wipe the electrolyte on the surface dry, and weigh it to get Wequilibrium. The mass swelling degree = (Wequilibrium - Winitial) / Winitial * 100%.

[0069] 2) Test for the viscosity stability of the anode slurry:

[0070] After the anode slurry was prepared, a rotor of model 3# was used to rotate at a speed of 12 revolutions per minute. The viscosity of the anode slurry measured under this condition was the initial viscosity. After the slurry was left standing for 24 h, its viscosity was measured again.

[0071] 3) Test for the rebound of the electrode sheet:

[0072] The coated electrode sheet was roll-pressed to ensure that the compaction density was 1.7 g / cm 3 , and the thickness of the electrode sheet measured at this time was the initial thickness. After the electrode sheet was placed in a moisture-proof cabinet in a constant-temperature room for 24 h, the thickness measured with a micrometer was the cold-pressed thickness after 24 h. The electrode sheet rebound rate = (the thickness of the electrode sheet after 24 h - the initial thickness) / the initial thickness × 100%. Take 10 points for each electrode sheet and calculate the average value of the 10 points.

[0073] 4) Test for the peel strength of the anode sheet:

[0074] After the coated single-sided anode sheet was compacted at 2.4 g / cm 3 , a tensile machine with a measuring range of 20 N was used. The electrode sheet was cut into a length of 20 cm × a width of 3 cm. The 3M double-sided tape was pasted on the steel plate. The coated surface of the electrode sheet was fixed face-down on the tape on the steel plate. After rolling back and forth 6 times with a 2.5 kg roller, then the coating and the copper foil were torn apart. The upper plate clamped the copper foil side and stretched at a speed of 50 mm / min and at 180 °C. The data in the stable tensile section were recorded as the peel strength (N / m).

[0075] 5) Room temperature cycle test

[0076] The specific method is as follows: After leaving the battery to stand for 1 h at 25°C ± 2°C in a constant-temperature test chamber, charge it at a constant current and voltage of 1C until 3.65V, with a cut-off current of 0.05C; discharge it at a constant current of 1C until 2.5V, and record the discharge capacity; repeat the above steps 500 times and calculate the capacity retention rate.

[0077] 6) Full-charge rebound test of the negative electrode sheet:

[0078] For the fully-capacity-divided battery cells, charge them at a constant current and voltage of 0.5C until fully charged, leave the battery cells to stand in a drying room (relative humidity 0.5%) and perform full-charge disassembly. Immediately measure the thickness of the disassembled negative electrode sheet after disassembly. The full-charge rebound rate = (thickness of the electrode sheet after full-charge disassembly - thickness of the electrode sheet after 24 h) / thickness of the electrode sheet after 24 h × 100%. Take 10 points for each electrode sheet and calculate the average value of the 10 points.

[0079] The above test results are shown in Table 2 below.

[0080] Table 2

[0081]

[0082] It can be concluded from the above results that:

[0083] Comparing Examples 1, 2, 11, and 12, it can be seen that without the cationic unit to reduce the dispersibility of graphite, it will lead to a decrease in the peel strength, slight material loss during full-charge disassembly, and a decrease in the battery cycle life; when the content of the cationic unit shows a downward trend and is lower than the scope of this application, it will not be able to effectively participate in the polymerization reaction, and the effect of dispersing graphite well cannot be achieved, resulting in poor viscosity stability after 24 h, low peel strength of the electrode sheet, high full-charge rebound, and a decrease in the battery cycle life; when the content of the cationic unit shows an upward trend, although the dispersibility of graphite is good and the peel strength is high, there are residual cationic monomers that cannot fully participate in the polymerization, and inevitably will react with the electrolyte side reaction, reducing the battery cycle life.

[0084] Comparing Examples 1, 3, 4, and Comparative Example 1, it can be seen that when the proportion of the acrylate unit shows a decreasing trend, the mass swelling degree is low, which is not conducive to the insertion and extraction of lithium ions during the charge and discharge process, affecting the battery cycle life; and the low peel strength of the electrode sheet leads to material loss during full charge, and the battery cycle life decreases; when the proportion of the acrylate unit shows an upward trend, although the peel strength of the electrode sheet can be guaranteed to a certain extent, the mass swelling degree of the binder is large, resulting in a large full-charge rebound, and local shedding of the active material in the later stage of battery cycling, affecting the cycle life of the battery cell; when there is no acrylate unit, the mass swelling degree of the binder is low, and the peel strength of the electrode sheet is low, resulting in a decrease in the battery cycle life.

[0085] It can be seen from Comparative Examples 1, 5, 6 and Comparative Example 2 that when the content of vinyl aromatic units shows an increasing trend, the swelling is small, which is not conducive to the insertion and extraction of lithium ions; when the content of vinyl aromatic units shows a decreasing trend, the content of hard monomer components is small, resulting in large swelling of the binder, large pole piece rebound and large full charge rebound, and the battery cycle life decreases; if there are no vinyl aromatic units, the mass swelling degree of the binder is even greater, resulting in unstable binder structure, large pole piece rebound, and large full charge rebound of the battery, thus reducing the battery cycle life.

[0086] It can be seen from Comparative Examples 1, 7, 8 and Comparative Example 3 that when the proportion of acrylamide units shows an increasing trend, although the bonding force is high, it will increase the glass transition temperature of the binder, making the pole piece brittle and not conducive to processing; when the proportion shows a decreasing trend, it will reduce the bonding force between the active material and the foil, resulting in high full charge rebound of the pole piece and decreased battery cycle life; when acrylamide units are not added, the peel strength of the pole piece is low, and a large area of material drops off during full charge disassembly, thus reducing the battery cycle life.

[0087] It can be seen from Comparative Examples 1, 9, 10 and Comparative Example 4 that when the proportion of acrylate-polyol units shows a decreasing trend, it will reduce the swelling degree of the binder, which is not conducive to the insertion and extraction of lithium ions during the charge and discharge process, and the low peel strength will affect the battery cycle life; when the proportion shows an increasing trend, it will increase the swelling degree of the binder, resulting in an increase in full charge rebound and a decrease in the cycle life of the battery cell; when acrylate-polyol units are not added, both the mass swelling degree and the peel strength of the binder are low, reducing the battery cycle life.

[0088] It can be seen from Comparative Example 1 and Comparative Example 5 that the viscosity stability of the slurry for 24 hours, the pole piece rebound, the peel strength, the full charge rebound, and the cycle life of the conventional SBR system are far worse than those of the binder of the present invention.

[0089] The above are only the preferred embodiments of the present invention and are not intended to limit the present invention. Any modifications, equivalent replacements, and improvements made within the spirit and principle of the present invention shall be included within the protection scope of the present invention.

Claims

1. A negative electrode binder, characterized in that: The negative electrode binder comprises a copolymer, which is composed of an acrylate unit, a vinyl aromatic unit, an acrylamide unit, an acrylate-polyol unit, a cationic unit and a crosslinking unit; the cationic unit is a structural unit obtained by polymerization of a cationic monomer containing a double bond; The mass ratio of the acrylic acid ester units, vinyl aromatic units, acrylamide units, acrylic acid ester-polyol units and cross-linked structural units is (30-70): (10-50): (5-30): (8-25): (0.5-2); The mass ratio of the acrylic acid ester unit to the cationic unit is (30-70): (5-30); The acrylamide units are structural units obtained by polymerization of acrylamide monomers, and the acrylamide monomers are selected from one or more of hydroxyethyl acrylamide, isobutoxymethyl acrylamide and diacetone acrylamide; The acrylate-polyol unit is a structural unit obtained by polymerization of acrylate-polyol monomers, and the acrylate-polyol monomers are selected from one or more of poly(ethylene glycol) diacrylate, polyethylene glycol dimethacrylate, polyethylene glycol-block-polypropylene glycol-block-polyethylene glycol diacrylate, and poly(propylene glycol) dimethacrylate; The cross-linking unit is a structural unit obtained by polymerization with the participation of a cross-linking agent, and the cross-linking agent is selected from one or more of diethylene glycol dimethacrylate, ethylene glycol diacrylate, tetra(ethylene glycol) diacrylate, ethylene glycol dimethacrylate, tetraethylene glycol dimethacrylate, 1,3-butanediol dimethacrylate, neopentyl glycol diacrylate, 1,6-hexanediol dimethacrylate, and pentaerythritol tetraacrylate.

2. The negative electrode binder according to claim 1, characterized in that The cationic monomer includes one or more of methacryloyloxyethyl dimethyl dodecyl ammonium bromide, methacryloyloxyethyl dimethyl hexadecyl ammonium bromide, methacryloyloxyethyl trimethyl ammonium bromide, methacryloyloxyethyl dimethyl butyl ammonium bromide, methacryloyloxyethyl dimethyl pentyl ammonium bromide, methacryloyloxyethyl trimethyl ammonium chloride, acryloyloxyethyl trimethyl ammonium chloride and dimethyl diallyl ammonium chloride.

3. The negative electrode binder according to claim 1, characterized in that The acrylic acid ester unit is a structural unit obtained by polymerization of acrylic acid ester monomers, and the acrylic acid ester monomers include one or more of methyl acrylate, ethyl acrylate, isobutyl acrylate, tert-butyl acrylate, butyl acrylate, 2-isooctyl acrylate, lauryl acrylate, octadecyl acrylate, methyl methacrylate, butyl methacrylate, phenyl methacrylate, and n-hexyl acrylate.

4. The negative electrode binder according to claim 1, characterized in that The vinyl aromatic unit is a structural unit obtained by polymerization of a vinyl aromatic monomer, and the vinyl aromatic monomer includes one or more of styrene, β-methylstyrene, 4-methoxystyrene, and 2-methylstyrene.

5. A negative electrode sheet, characterized in that: The invention comprises a negative electrode current collector and a negative electrode active material layer arranged on at least one side of the negative electrode current collector, wherein the negative electrode active material layer comprises the negative electrode binder according to any one of claims 1 to 4.

6. A secondary battery, characterized in that: It comprises a positive electrode sheet, an electrolyte and the negative electrode sheet as claimed in claim 5.

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