Negative electrode binder, negative electrode plate and secondary battery

Abstract

The invention provides a negative electrode binder, a negative electrode plate and a secondary battery, aiming at the problem that the existing binder cannot compatibly inhibit the expansion of the negative electrode plate and ensure the electrical property of the battery. The negative electrode binder comprises a core body and a shell layer, the shell layer is arranged on the outer surface of the core body and at least partially covers the core body, the core body comprises a first polymer, the shell layer comprises a second polymer, and the glass transition temperature of the first polymer is higher than that of the second polymer; the first polymer comprises an acrylate structural unit, an aromatic vinyl structural unit and a first reaction structural unit, and the second polymer comprises an alkene nitrile structural unit, an acrylate structural unit, an alkenyl siloxane structural unit and a second reaction structural unit capable of reacting with the first reaction structural unit; the second reaction structure unit comprises an epoxy olefin structure unit. The negative electrode binder can inhibit the expansion of the negative electrode sheet and ensure the electrical performance of the battery at the same time.

Description

TECHNICAL FIELD

[0001] The application 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

[0002] Lithium ion batteries have become the main power source selection for modern portable electronic devices, electric vehicles and energy storage systems due to their high energy density, long cycle life and low self-discharge rate.

[0003] A lithium ion battery is usually mainly composed of a positive electrode, a negative electrode, a separator, an electrolyte and a battery shell. In order to improve the energy density of the battery, the compaction density of the positive and negative electrodes is also improved to be close to the limit. In the charging and discharging cycle process, the expansion of the negative electrode will cause the deformation of the battery, especially the silicon-carbon negative electrode will expand more than 300%, which seriously affects the safety and reliability of the battery. On the other hand, with the increase of the capacity of the battery, the area, volume and weight of the battery are also increasing. In the battery production process, the internal structure of the battery is often loose and the mechanical strength is too low, causing the battery to deform and become fluffy, and even leading to the serious problem of being unable to enter the shell.

[0004] In order to solve the above problems, the current solution is to use a polyacrylic resin as a negative electrode binder. However, because the polyacrylic chain is relatively rigid, it can be bound when the silicon negative electrode expands during lithium intercalation, but it is easy to cause interface separation between the polyacrylic chain and the silicon negative electrode when the silicon negative electrode is deintercalated, resulting in failure in the later cycle.

[0005] Therefore, there is an urgent need for a new type of negative electrode binder to reduce the expansion of the negative electrode sheet while improving the electrical performance of the battery. SUMMARY

[0006] The application provides a negative electrode binder, a negative electrode sheet and a secondary battery to solve the problem that the existing binder cannot inhibit the expansion of the negative electrode sheet and guarantee the electrical performance of the battery.

[0007] To solve the above technical problems, the application provides a negative electrode binder, which comprises a core and a shell layer, the shell layer is arranged on the outer surface of the core and at least partially covers the core, the core comprises a first polymer, and the shell layer comprises a second polymer, the glass transition temperature of the first polymer is higher than that of the second polymer. The first polymer comprises an acrylate structural unit, an aromatic vinyl structural unit and a first reaction structural unit. The second polymer comprises an acrylonitrile structural unit, an acrylate structural unit, an alkenyl siloxane structural unit and a second reaction structural unit which can react with the first reaction structural unit; the second reaction structural unit comprises an epoxy alkenyl structural unit.

[0008] Preferably, the first reactive structural unit contains a first reactive group, which is selected from at least one of carboxyl and amino groups; The first reactive group has a mass content of 0.2%-0.9% in the negative electrode binder, and the epoxy group in the second reactive structural unit has a mass content of 0.9%-2.0% in the negative electrode binder.

[0009] Preferably, in the negative electrode binder core, the mass ratio of the aromatic ethylene structural unit, the acrylate structural unit, and the first reactive structural unit is (35-45):(5-10):(1-3).

[0010] Preferably, in the negative electrode binder shell layer, the mass ratio of the acrylonitrile structural unit, the acrylate structural unit, the alkenylsiloxane structural unit and the epoxy olefin structural unit is (1-20):(25-45):(1-3):(3-6).

[0011] Preferably, the glass transition temperature of the first polymer is 40-95°C, and the glass transition temperature of the second polymer is -50-0°C.

[0012] Preferably, the negative electrode binder has a mass swelling rate of 30%-70% in the electrolyte.

[0013] Preferably, the mass ratio of the core to the shell is (50-70):(30-50).

[0014] Preferably, the particle size D50 of the negative electrode binder is 450-900 nm, and the particle size D50 of the core is 400-700 nm.

[0015] Secondly, this application provides a negative electrode sheet, including a negative electrode current collector and a negative electrode active material layer disposed on at least one side surface of the negative electrode current collector, the negative electrode active material layer being formed by coating with a negative electrode slurry, the negative electrode slurry including the negative electrode binder as described in any of the preceding claims.

[0016] Thirdly, this application provides a secondary battery, including the negative electrode sheet as described above.

[0017] In this application, the first reactive structural unit of the first polymer and the second reactive structural unit of the second polymer react after the negative electrode sheet is hot-rolled (e.g., under conditions of temperature ≥100℃ and pressure ≥10000kgf) to form stable chemical bonds, thus tightly bonding the core-shell structure of the negative electrode binder. The second polymer, with a lower glass transition temperature, can achieve strong adhesion with the negative electrode active material, while the first polymer, with a higher glass transition temperature, has higher mechanical properties. This allows the negative electrode binder to deform with the expansion of the negative electrode active material without interfacial detachment, and to generate a rebound force during lithium removal, restoring the negative electrode active material to its original shape. This suppresses the full-charge rebound of the negative electrode sheet, improves the peel strength and cohesive strength of the negative electrode sheet, and further improves the cycle performance of the battery. Detailed Implementation

[0018] To make the technical problems solved, technical solutions, and beneficial effects of 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 merely illustrative of the present invention and are not intended to limit the present invention.

[0019] One embodiment of this application provides a negative electrode binder, the negative electrode binder comprising a core and a shell, the shell being disposed on the outer surface of the core and at least partially covering the core, the core comprising a first polymer, the shell comprising a second polymer, the glass transition temperature of the first polymer being higher than the glass transition temperature of the second polymer; The first polymer comprises acrylate structural units, aromatic vinyl structural units, and a first reactive structural unit; The second polymer comprises an acrylonitrile structural unit, an acrylate structural unit, an alkenylsiloxane structural unit, and a second reactive structural unit that can react with the first reactive structural unit; the second reactive structural unit comprises an epoxy olefin structural unit.

[0020] In this embodiment, the first reactive structural unit of the first polymer and the second reactive structural unit of the second polymer react after the negative electrode sheet is hot-rolled, forming a stable chemical bond connection, thus tightly bonding the core-shell structure of the negative electrode binder. The second polymer, with a lower glass transition temperature, can achieve strong adhesion with the negative electrode active material. In particular, the epoxy groups in the second reactive structural unit can chemically react with the hydroxyl groups on the silicon negative electrode active material to form a chemical bond connection, further increasing the connection strength between the shell layer and the negative electrode active material. The first polymer, with a higher glass transition temperature, has higher mechanical properties, allowing the negative electrode binder to deform with the expansion of the negative electrode active material without interfacial detachment, and to generate a rebound force during lithium removal, restoring the negative electrode active material to its original shape. This suppresses the full-charge rebound of the negative electrode sheet, improves the peel strength and cohesive strength of the negative electrode sheet, and further improves the cycle performance of the battery.

[0021] In some embodiments, the temperature during hot rolling of the negative electrode sheet is greater than or equal to 100°C, and the pressure is greater than or equal to 10000 kgf.

[0022] In some embodiments, the first reactive structural unit contains a first reactive group, which is selected from at least one of carboxyl and amino groups; the mass content of the first reactive group in the negative electrode binder is 0.2%-0.9%, and the mass content of the epoxy group in the second reactive structural unit in the negative electrode binder is 0.9%-2.0%, all of which are calculated based on the mass content of the groups. By controlling the mass content of the first reactive group and epoxy group in the negative electrode binder within the above range, it can be ensured that the first reactive group and epoxy group form sufficient chemical bonds, so that the core-shell structure of the negative electrode binder is tightly bonded, allowing the negative electrode binder to deform with the expansion of the negative electrode active material without interfacial detachment. If the carboxyl group is excessive, it will increase the water content of the electrode, resulting in an increase in negative electrode side reactions. If the epoxy group is excessive, it will cause the negative electrode binder to become over-crosslinked and brittle, thereby affecting the mechanical properties of the negative electrode and causing the battery's electrical performance to deteriorate.

[0023] Specifically, the mass content of the first reactive group in the negative electrode binder includes, but is not limited to, 0.2%, 0.3%, 0.4%, 0.5%, 0.6%, 0.7%, 0.8%, or 0.9%. The mass content of the epoxy group in the second reactive structural unit in the negative electrode binder includes, but is not limited to, 0.9%, 1.2%, 1.4%, 1.6%, 1.8%, or 2.0%.

[0024] In some embodiments, the first reactive structural unit is a structural unit formed by the polymerization of a first reactive monomer, and the first reactive monomer includes at least one of β-acryloyloxypropionic acid, itaconic acid, itaconic acid monobutyl ester, 2-(tert-butylamino)ethyl methacrylate, amino(meth)acrylate, and N-methylallylamine.

[0025] In some embodiments, the epoxy olefin structural unit is a structural unit formed by the polymerization of epoxy olefin monomers, and the epoxy olefin monomers include at least one of glycidyl methacrylate (GMA), allyl glycidyl ether (AGE), and 3,4-epoxycyclohexyl methacrylate (ECHMA).

[0026] In some embodiments, the mass ratio of the aromatic vinyl structural unit, the acrylate structural unit, and the first reactive structural unit is (35-45):(5-10):(1-3), where the mass ratio is calculated based on the monomer feed ratio. By limiting the mass ratio of the acrylate structural unit, the aromatic vinyl structural unit, and the first reactive structural unit to the above range, the first polymer exhibits better mechanical strength and flexibility.

[0027] Specifically, the mass ratio of the aromatic ethylene structural unit, the acrylate structural unit, and the first reactive structural unit includes, but is not limited to, 41:7:2, 39:10:1, 38:10:2, 37:10:3, 35:10:1, or 43:5:2.

[0028] Furthermore, the acrylate structural unit is a structural unit formed by the polymerization of acrylate monomers, and the acrylate monomers include at least one of isooctyl acrylate, butyl acrylate, ethyl acrylate, methyl methacrylate, methyl acrylate, hydroxypropyl methacrylate, and hydroxyethyl acrylate.

[0029] Aromatic ethylene structural units are structural units formed by the polymerization of aromatic monomers, including but not limited to at least one of styrene and methylstyrene.

[0030] In some embodiments, the mass ratio of the acrylonitrile structural unit, the acrylate structural unit, the alkenylsiloxane structural unit, and the epoxy olefin structural unit is (1-20):(25-45):(1-3):(3-6), where the mass ratio is calculated based on the monomer feed ratio. By limiting the mass ratio of the acrylonitrile structural unit, the acrylate structural unit, the alkenylsiloxane structural unit, and the epoxy olefin structural unit in the second polymer to within the above range, the electrolyte mass swelling rate of the second polymer is controlled to improve the electrolyte wettability of the negative electrode sheet. Simultaneously, by introducing acrylonitrile structural units into the second polymer, the ion-conducting ability of the negative electrode binder can be improved, the internal resistance of the negative electrode sheet can be reduced, and the electrical performance of the battery can be further improved. In particular, by introducing alkenylsiloxane structural units into the second polymer, the bonding strength with the silicon negative electrode active material is enhanced, allowing the negative electrode binder to deform with the expansion of the negative electrode active material without interfacial detachment.

[0031] Specifically, the mass ratio of the acrylonitrile structural unit, the acrylate structural unit, the alkenylsiloxane structural unit, and the epoxy olefin structural unit includes, but is not limited to, 5:40:1:4, 10:35:1:4, 15:30:2:3, 10:31:3:6, or 15:28:1:6.

[0032] Furthermore, the acrylonitrile structural unit is a structural unit formed by the polymerization of acrylonitrile monomers, and the acrylonitrile monomers include, but are not limited to, at least one of acrylonitrile and butadiene acrylonitrile.

[0033] The acrylate structural unit is a structural unit formed by the polymerization of acrylate monomers. The acrylate monomers include at least one of isooctyl acrylate, butyl acrylate, ethyl acrylate, methyl methacrylate, methyl acrylate, hydroxypropyl methacrylate, and hydroxyethyl acrylate.

[0034] The alkenylsiloxane structural unit is a structural unit formed by the polymerization of alkenylsiloxane monomers, including at least one of γ-methacryloyloxypropyltrimethoxysilane (KH570) and vinyltrimethoxysilane (KH171).

[0035] In some embodiments, the glass transition temperature (Tg) of the first polymer is 40-95°C, and the glass transition temperature (Tg) of the second polymer is -50-0°C. By limiting the glass transition temperatures of the first and second polymers to the above ranges, the shell layer of the negative electrode binder can form a strong interfacial bond with the negative electrode active material after hot rolling of the negative electrode sheet. The core with a high glass transition temperature can maintain good mechanical properties and suppress expansion when the silicon negative electrode active material expands or recovers. After hot rolling, the first and second reaction structural units react, causing the core and shell to be connected by chemical bonds, forming a strong bonding force, thereby improving the peel strength and cohesive strength of the negative electrode sheet, and further improving the kinetics and cycle performance of the battery.

[0036] Specifically, the glass transition temperature of the first polymer includes, but is not limited to, 40°C, 45°C, 50°C, 55°C, 60°C, 65°C, 70°C, 75°C, 80°C, 85°C, 90°C, or 95°C. The glass transition temperature of the second polymer includes, but is not limited to, -50°C, -45°C, -40°C, -35°C, -30°C, -25°C, -20°C, -15°C, -10°C, -5°C, or 0°C.

[0037] It should be noted that the glass transition temperatures of the first polymer and the second polymer were obtained by differential scanning calorimetry.

[0038] In some embodiments, the mass swelling rate of the negative electrode binder in the electrolyte is 30%-70%. By limiting the electrolyte mass swelling rate of the negative electrode binder to the above range, the electrolyte wettability of the electrode can be significantly improved, lithium-ion transport can be accelerated, and moderate swelling will not cause the negative electrode binder structure to collapse, thus balancing kinetic performance and structural stability.

[0039] Specifically, the mass swelling rate of the negative electrode binder in the electrolyte includes, but is not limited to, 30%, 35%, 40%, 45%, 50%, 55%, 60%, 65%, or 70%.

[0040] In some embodiments, the mass ratio of the core to the shell is (50-70):(30-50), where the mass ratio is calculated based on the monomer feed ratio. By limiting the mass ratio of the shell to the core within the above range, the negative electrode binder exhibits better bonding strength and the ability to suppress the expansion of the negative electrode active material.

[0041] If the shell content is too high, the rebound force generated by the core in the negative electrode binder is weak, which can easily lead to material loss in the later stages of the negative electrode cycle. If the shell content is too low, the interfacial adhesion between the shell and the negative electrode active material is weak after the negative electrode active material is deformed, which can easily cause the negative electrode binder to detach from the negative electrode active material, leading to failure in the later stages of the cycle.

[0042] Specifically, the mass ratio of the core to the shell includes, but is not limited to, 50:50, 55:45, 60:40, 65:35, or 70:30.

[0043] In some embodiments, the particle size D50 of the negative electrode binder is 450-900 nm, and the particle size D50 of the core is 400-700 nm. If the particle size D50 of the negative electrode binder is greater than 900 nm, the number of negative electrode binder particles added to the negative electrode slurry is too small, resulting in fewer bonding sites and a decrease in bonding strength. If the particle size D50 of the negative electrode binder is less than 450 nm, the number of negative electrode binder particles added to the negative electrode slurry is too large, which can easily cause the negative electrode binder to coat the main material, affecting lithium-ion transport. By limiting the particle size D50 of the negative electrode binder and the core to the above ranges, the interfacial adhesion between the negative electrode binder and the negative electrode active material is improved.

[0044] Specifically, the particle size D50 of the negative electrode binder includes, but is not limited to, 450 nm, 500 nm, 550 nm, 600 nm, 650 nm, 700 nm, 750 nm, 800 nm, 850 nm, or 900 nm. The particle size D50 of the core includes, but is not limited to, 400 nm, 450 nm, 500 nm, 550 nm, 600 nm, 650 nm, or 700 nm.

[0045] In this invention, each of the aforementioned structural units represents the structural portion of the corresponding monomer present in the resulting polymer after the monomer participates in the polymerization reaction. The mass ratio of each structural unit is based on the mass content of the corresponding monomer in the total amount of monomers participating in the polymerization.

[0046] Furthermore, one embodiment of this application provides a method for preparing a negative electrode binder, comprising the following steps: Emulsifier, acrylonitrile monomer, acrylate monomer, first reaction monomer and initiator are added to a pre-emulsification vessel and mixed evenly to obtain M1; react at 70-80℃ for 8-12 hours to obtain the core; In a pre-emulsification reactor, an emulsifier, aromatic vinyl monomers, acrylate monomers, alkenyl siloxane monomers, epoxy olefin monomers, and an initiator are added and stirred until homogeneous to form M2. In a reaction vessel, M1 is added, followed by M2, and the mixture is heated to 70-80°C and reacted for 8-12 seconds to obtain negative electrode binder particles. At this temperature, the first reacting monomer and the epoxy olefin monomer do not undergo a chemical reaction.

[0047] The initiator includes one or more of the following: ammonium persulfate, potassium persulfate, sodium persulfate, sodium bisulfite, azobisisobutylamidine hydrochloride, azobisisobutylimidazoline hydrochloride, azobisisopropylimidazoline, and azobiscyanopentanoic acid. The amount of initiator added is 0.2%-1.2% of the total mass of the monomers.

[0048] The emulsifier includes one or more of sodium dodecylbenzenesulfonate, sodium dodecyl sulfate, sodium stearate, and OP-10. The amount of emulsifier added is 0.2-1.2% of the total mass of the monomers.

[0049] As those skilled in the art know, the reactions in the above steps are conventional free radical polymerization, etc., and the specific methods and reaction conditions are common free radical polymerization methods in the prior art, which will not be described in detail in this invention.

[0050] This application provides a negative electrode sheet, including a negative electrode current collector and a negative electrode active material layer disposed on at least one side surface of the negative electrode current collector, wherein the negative electrode active material layer is formed by coating with a negative electrode slurry, and the negative electrode slurry includes a negative electrode binder as described in any of the preceding claims.

[0051] Furthermore, the negative electrode slurry also includes a negative electrode active material and a negative electrode conductive agent.

[0052] Specifically, the mass ratio of the negative electrode active material, negative electrode conductive agent, thickener, and negative electrode binder is 95.6:1:1.6:1.8.

[0053] Preferably, when the active material in the negative electrode active material layer is silicon-carbon or silicon, the binder composition of the above embodiments can provide sufficient adhesive force to buffer and suppress volume changes of silicon negative electrode material particles.

[0054] This application provides a secondary battery, including the negative electrode sheet as described above.

[0055] The present invention will be further illustrated by the following examples.

[0056] Specifically, this invention discloses the negative electrode binder, negative electrode sheet, and secondary battery.

[0057] Example 1 Negative electrode binder: 0.4 parts of sodium dodecylbenzenesulfonate, 41 parts of styrene, 7 parts of isooctyl acrylate, 2 parts of itaconic acid butyl ester, and 0.15 parts of potassium persulfate were added to a pre-emulsification vessel and mixed evenly to obtain M1. The reaction was carried out at 75°C for 8 hours to obtain a core with a particle size D50 of 612 nm and a Tg of 61 °C for the first polymer.

[0058] In a pre-emulsification vessel, 0.4 parts of sodium dodecylbenzenesulfonate, 5 parts of acrylonitrile, 39 parts of isooctyl acrylate, 2 parts of γ-methacryloyloxypropyltrimethoxysilane, 4 parts of glycidyl methacrylate, and 0.15 parts of potassium persulfate were added and stirred until homogeneous to form M2. In a reaction vessel, M1 was added, followed by M2, and the mixture was heated to 75°C and reacted for 8 hours to obtain a negative electrode binder with a particle size D50 of 762 nm. The Tg of the second polymer was -45°C. The mass ratio of the first polymer to the second polymer was 50:50.

[0059] Negative electrode plate: Silicon-carbon anode (80 / 20), conductive agent Super P, CMC2300, and anode binder were blended in a mass ratio of 95.6:1:1.6:1.8 to obtain a blended powder. The powder was then mixed with deionized water to prepare a slurry.

[0060] The slurry was coated onto copper foil with an areal density of 110 g / m². 2 The negative electrode sheet was rolled at 10000 kgf and 100°C, resulting in a compacted density of 1.6 g / cm³. 3 The negative electrode sheet is cut into a fixed size.

[0061] Secondary batteries: After stacking negative electrode sheets and NCM positive electrode sheets to assemble a dry cell, a lithium battery is obtained by injecting electrolyte. The electrolyte is made by mixing ethylene carbonate (EC) and diethyl carbonate (DEC) in a 1:1 mass ratio, and then adding lithium hexafluorophosphate (LiPF6) to a molar concentration of 1.0 mol / L.

[0062] Examples 2 to 25 Examples 2 through 19 are largely the same as Example 1, except that they use the formulations in Table 1.

[0063] Examples 20-25 are mostly the same as Example 1, except that the formulation in Table 1 is used and the proportion of emulsifier is adjusted. When the emulsifier is increased, the particle size D50 of the negative electrode binder increases, and when the emulsifier is decreased, the particle size D50 of the negative electrode binder decreases.

[0064] Comparative Examples 1 to 3 The steps of Comparative Examples 1 to 3 are mostly the same as those of Example 1, except that the formulations in Table 1 are used.

[0065] Comparative Example 4 Most of the steps in Comparative Example 4 and Example 1 are the same, except that the negative electrode binder is polyacrylic acid.

[0066] Comparative Example 5 Most of the steps in Comparative Example 5 and Example 1 are the same, except that the negative electrode is rolled at 25°C.

[0067] Comparative Example 6 Most of the steps in Comparative Example 6 and Example 1 are the same, except that the negative electrode binder is prepared as follows: 0.4 parts of sodium dodecylbenzenesulfonate, 41 parts of styrene, 7 parts of isooctyl acrylate, 2 parts of itaconic acid butyl ester, and 0.15 parts of potassium persulfate were added to a pre-emulsification vessel and mixed evenly to obtain M1. The reaction was carried out at 75°C for 8 hours to obtain a core with a particle size D50 of 611 nm and a Tg of 61 °C for the first polymer.

[0068] In a pre-emulsifying reactor, 0.4 parts of sodium dodecylbenzenesulfonate, 5 parts of acrylonitrile, 39 parts of isooctyl acrylate, 2 parts of γ-methacryloyloxypropyltrimethoxysilane, 4 parts of glycidyl methacrylate, and 0.15 parts of potassium persulfate were added and stirred until homogeneous to form M2. The mixture was heated to 75°C and reacted for 8 hours to obtain a negative electrode binder with a particle size D50 of 606 nm. The Tg of the second polymer was -45°C. The mass ratio of the first polymer to the second polymer was 50:50.

[0069] Table 1 Note: In Table 1, the particle size D50 of the core is D1, and the particle size D50 of the negative electrode binder is D2.

[0070] The negative electrode sheets and secondary batteries prepared in the above embodiments and comparative examples were tested as follows.

[0071] 1. Negative electrode binder mass swelling rate test: Place the negative electrode binder in a 2cm*5cm*1cm container and dry it in an oven at 60℃ for 24 hours. After drying, cut each piece to a weight of 0.3-0.5g and weigh and record the weight M1. Immerse the piece in a lithium salt electrolyte at room temperature for 24 hours. After immersion, wipe off the electrolyte on the surface and weigh and record the weight M2. Swelling rate = (M2-M1) / M1.

[0072] 2. Negative electrode peeling force test: The coated single-sided negative electrode sheet was prepared at a ratio of 2.4 g / cm². 3 After compaction, a tensile testing machine with a range of 20N is used. The electrode is cut into pieces 20cm long and 3cm wide. 3M double-sided tape is attached to the steel plate. The coated side of the electrode is fixed to the tape on the steel plate with the coated side facing down. After rolling back and forth 6 times with a 2.5kg roller, the coating and copper foil are peeled off. The upper plate clamps the copper foil side. The electrode is stretched at a speed of 50mm / min and at 180°. The data of the stable tensile section is recorded as the peel strength (N / m).

[0073] 3. Negative electrode flexibility test: The negative electrode sheet is wound onto the surface of a winding needle of a fixed diameter, and the cracking state of the electrode surface is observed under a microscope. 4. Electrode full-charge rebound rate test: Test the cell thickness L1. At room temperature, maintain a constant current and voltage of 1C to 4.2V, then test the thickness L2. Fully charged thickness rebound = (L2 - L1) / L1; 5. The test method for DCIR testing is as follows: ① Test temperature: 25℃; ② SOC: 0, 10%, 20%, 30%, 40%, 50%, 60%, 70%, 80%, 90%, 100%; ③ DCIR test method: 3C discharge for 30s.

[0074] 6. Room temperature cycling test: After placing the battery in a constant temperature test chamber at 25℃±2℃ for 1 hour, charge it to 4.2V with a constant current and constant voltage of 1C and cut off the current at 0.05C; discharge it to 3.0V with a constant current of 1C and record the discharge capacity; repeat the above steps 500 times and calculate the capacity retention rate.

[0075] The test results are shown in Table 2 below.

[0076] Table 2 As shown in Table 2, the test results of Examples 1-5 indicate that when the first reaction structural unit is below the range of this application, the electrode rebound rate and capacity retention rate deteriorate. When the first reaction structural unit is above the range of this application, the emulsion is unstable, resulting in a decrease in the peel strength of the negative electrode and a deterioration in the electrode rebound rate and capacity retention rate. As shown in the test results of Examples 1 and Examples 6-9, when the mass ratio of aromatic ethylene structural units, acrylate structural units, and the first reaction structural unit in the core is within the range of this application, the electrode rebound rate and capacity retention rate are both good. When the mass ratio of aromatic ethylene structural units, acrylate structural units, and the first reaction structural unit is outside the range, the Tg of the first polymer is also outside the range of this application, and the electrode rebound rate and capacity retention rate deteriorate.

[0077] The test results of Examples 1 and 10-13 show that when the second reaction structural unit is below the range of this application, the mass swelling rate of the negative electrode binder increases, and the electrode rebound rate and capacity retention rate deteriorate. When the second reaction structural unit is above the range of this application, the flexibility of the negative electrode decreases, and the electrode rebound rate and capacity retention rate deteriorate. The test results of Examples 1 and 14-19 show that when the mass ratio of acrylonitrile structural units, acrylate structural units, alkenylsiloxane structural units, and epoxy olefin structural units in the shell is within the range of this application, the electrode rebound rate and capacity retention rate are both good. When acrylonitrile, isooctyl acrylate, and epoxy units are outside the range of this application, the peel strength and flexibility of the negative electrode decrease, and the electrode rebound rate and capacity retention rate deteriorate. When the siloxane monomer exceeds the range of this application, the emulsion is unstable, and the peel strength and capacity retention rate of the negative electrode decrease.

[0078] The test results of Examples 1 and 20-25 show that the larger the particle size D50 of the negative electrode binder, the lower the flexibility and peel strength; the smaller the particle size D50 of the negative electrode binder, the higher the DCIR; when the particle size D50 of the negative electrode binder is not within the range of this application, the full charge rebound rate, DCIR, or capacity retention rate of the electrode deteriorates. The test results of Examples 1 and Comparative Examples 1-3 show that when the negative electrode binder does not have the first or second reactive groups, the peel strength decreases and the capacity retention rate decreases significantly. When the negative electrode binder lacks alkenyl siloxane structural units, the electrode peel strength decreases and the capacity retention rate declines. Compared with Comparative Example 4, Example 1 shows that the negative electrode using the negative electrode binder of the present invention exhibits excellent peel strength, flexibility, and full-charge rebound performance, and the battery's DCIR and capacity retention rate are superior to Comparative Example 4. Compared with Comparative Example 5, Example 1 shows that the first and second reactive groups did not react under room temperature rolling, resulting in a slight decrease in peel strength, but the full-charge rebound and capacity retention rate of the electrode deteriorated significantly. Compared with Comparative Example 6, Example 1 shows that without the core-shell structure combination of the negative electrode binder, the peel strength of the negative electrode decreased significantly, the flexibility decreased, the full-charge rebound of the electrode increased, and the capacity retention rate of the battery deteriorated significantly.

[0079] The above description is only a preferred embodiment of the present invention and is not intended to limit the present invention. Any modifications, equivalent substitutions, and improvements made within the spirit and principles of the present invention should be included within the protection scope of the present invention.

Claims

1. A negative electrode binder, characterized in that, The negative electrode binder includes a core and a shell. The shell is disposed on the outer surface of the core and at least partially covers the core. The core includes a first polymer, and the shell includes a second polymer. The glass transition temperature of the first polymer is higher than that of the second polymer. The first polymer comprises acrylate structural units, aromatic vinyl structural units, and a first reactive structural unit; The second polymer comprises an acrylonitrile structural unit, an acrylate structural unit, an alkenylsiloxane structural unit, and a second reactive structural unit that can react with the first reactive structural unit; the second reactive structural unit comprises an epoxy olefin structural unit.

2. The negative electrode binder according to claim 1, characterized in that, The first reactive structural unit contains a first reactive group, which is selected from at least one of carboxyl and amino groups; The first reactive group has a mass content of 0.2%-0.9% in the negative electrode binder, and the epoxy group in the second reactive structural unit has a mass content of 0.9%-2.0% in the negative electrode binder.

3. The negative electrode binder core according to claim 1, characterized in that, The mass ratio of the aromatic ethylene structural unit, the acrylate structural unit, and the first reactive structural unit is (35-45):(5-10):(1-3).

4. The negative electrode binder shell layer according to claim 1, characterized in that, The mass ratio of the acrylonitrile structural unit, the acrylate structural unit, the alkenylsiloxane structural unit and the epoxy olefin structural unit is (1-20):(25-45):(1-3):(3-6).

5. The negative electrode binder according to claim 1, characterized in that, The glass transition temperature of the first polymer is 40-95℃, and the glass transition temperature of the second polymer is -50-0℃.

6. The negative electrode binder according to claim 1, characterized in that, The mass swelling rate of the negative electrode binder in the electrolyte is 30%-70%.

7. The negative electrode binder according to claim 1, characterized in that, The mass ratio of the first polymer to the second polymer is (50-70):(30-50).

8. The negative electrode binder according to claim 1, characterized in that, The particle size D50 of the negative electrode binder is 450-900nm, and the particle size D50 of the core is 400-700nm.

9. A negative electrode sheet, characterized in that, It includes a negative electrode current collector and a negative electrode active material layer disposed on at least one side surface of the negative electrode current collector, the negative electrode active material layer being formed by coating with a negative electrode slurry, the negative electrode slurry including the negative electrode binder according to any one of claims 1 to 8.

10. A secondary battery, characterized in that, Includes the negative electrode sheet as described in claim 9.

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