A binder, a negative electrode sheet, a battery

Abstract

To address the problem that existing binders cannot effectively suppress the expansion of silicon anodes, this invention provides an binder, an anode sheet, and a battery. The binder comprises a core and a shell. The core comprises a first polymer, and the shell comprises a second polymer. The glass transition temperature of the first polymer is higher than that of the second polymer. The first polymer comprises rigid structural units, flexible structural units, ion-conducting structural units, and cross-linked structural units. The second polymer comprises rigid structural units, flexible structural units, ion-conducting structural units, acrylic structural units, and cross-linked structural units. The surface of the shell has an uneven structure, and the roughness Sa of the shell is 0.1 μm ≤ Sa ≤ 0.3 μm. The binder provided by this invention improves the adhesive performance by controlling the types of structural units and the structure of the shell, thereby suppressing the expansion of the active material in the anode.

Description

Technical Field

[0001] This invention belongs to the field of secondary battery technology, specifically relating to an adhesive, a negative electrode sheet, and a battery. Background Technology

[0002] Lithium-ion batteries, with their advantages of high energy density, long cycle life, and environmental friendliness, have been widely used in new energy vehicles, portable electronic devices, and energy storage systems. Their performance is closely related to the stability and reliability of the manufacturing process. The theoretical specific capacity of graphite-based anode materials is only 372 mAh / g, which is insufficient to meet the development needs of high-energy-density lithium-ion batteries.

[0003] Currently, nano-silicon materials (~4200 mAh / g), silicon-carbon composites (>1700 mAh / g), and silicon suboxide materials (>1500 mAh / g) are gradually being developed and applied as anode materials. However, silicon anodes experience significant volume expansion during charge and discharge (up to 300% or more of their original volume). This massive expansion leads to structural breakage, loss of electrical contact between the active material and the current collector, and between active materials themselves. This disrupts the lithium-ion insertion / extraction process, resulting in a large, irreversible capacity and rapid capacity decay. Therefore, effectively controlling the volume expansion of silicon and improving its cycle stability is a key research focus in the field of silicon-based anodes. One approach is to use anode binders to buffer the expansion / contraction of the active material during charge and discharge, thereby improving battery cycle stability. However, traditional anode material binders such as sodium carboxymethyl cellulose (CMC) and styrene-butadiene rubber (SBR) are not well-suited to handle the significant volume changes caused by silicon-based anodes, leading to electrode structure damage during cycling. However, binders such as polyacrylic acid (PAA) and polyacrylonitrile (PAN) have high glass transition temperatures and high hardness, resulting in brittle negative electrode sheets. During electrode sheet cutting, powder is easily shed, which affects battery performance.

[0004] Therefore, there is an urgent need for a new type of anode binder to suppress the volume expansion of silicon anodes, maintain the integrity of the electrode structure, achieve excellent electrochemical performance, and improve the cycle stability of the anode. Summary of the Invention

[0005] This invention addresses the problem that existing binders cannot effectively suppress the volume expansion of silicon anodes, and provides an binder, anode sheet, and a battery.

[0006] To solve the above-mentioned technical problems, the present invention provides an adhesive comprising a core and a shell, wherein the shell is disposed on the outer surface of the core and at least partially covers the core, the core comprises a first polymer, the shell 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 rigid structural units, flexible structural units, ion-conducting structural units, and cross-linked structural units; the second polymer comprises rigid structural units, flexible structural units, ion-conducting structural units, acrylic acid structural units, and cross-linked structural units. The surface of the shell has an uneven structure, and the roughness Sa of the shell is 0.1μm≤Sa≤0.3μm.

[0007] Preferably, the ion-conducting structural unit includes at least one of the following: a sulfonic acid group-containing structural unit, a carboxyl group-containing structural unit, an amide-containing structural unit, a polyether-containing structural unit, and a polyethylene glycol-containing structural unit.

[0008] Preferably, in the first polymer, the mass ratio of the rigid structural unit, the flexible structural unit, the ion-conducting structural unit, and the cross-linking structural unit is (50~90):(10~50):(1~5):(1~8). And / or, in the second polymer, the mass ratio of the rigid structural unit, the flexible structural unit, the ion-conducting structural unit, the olefinic structural unit and the crosslinking structural unit is (5~10):(40~60):(6~10):(6~16):(4~8).

[0009] Preferably, the second polymer further includes partially capped vinyl silane structural units; in the second polymer, the mass ratio of the rigid structural unit, flexible structural unit, ion-conducting structural unit, olefinic structural unit, partially capped vinyl silane structural unit and crosslinking structural unit is (5~10):(40~60):(6~10):(6~16):(0.5~3):(4~8).

[0010] Preferably, the partially capped vinylsilane structural unit comprises vinyl and silane, wherein the molar ratio of vinyl to silane is 1:0.5 to 2.8.

[0011] Preferably, the rigid structural unit includes at least one of vinyl aromatic structural units, acrylonitrile structural units, and acrylate structural units.

[0012] And / or, the flexible structural unit includes acrylate structural units.

[0013] Preferably, the glass transition temperature of the first polymer is 60~100℃; And / or, the glass transition temperature of the second polymer is 0~25°C.

[0014] Preferably, the volume average particle size of the adhesive is 300~600nm.

[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 including the binder as described in any of the preceding claims.

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

[0017] In this application, in the core-shell structure of the binder, the glass transition temperature of the core is higher than that of the shell, which provides better core support and higher cohesive strength for the binder to cope with the expansion of the silicon anode. The shell provides better adhesion and elongation for the binder, promotes the adhesion between the binder and the anode active material, and forms a synergistic effect with the core to prevent the binder film from becoming hard and brittle.

[0018] By controlling the types of structural units in the core and shell, the synergistic effect of each component is ensured, resulting in a binder with high tensile strength and elongation at break, improving the binder's bonding performance, and suppressing the expansion of the negative electrode active material.

[0019] Specifically, by introducing ion conduction structural units into the core and shell respectively, the ion conduction structural units can form "micro-ion channels" in the first polymer of the core and the second polymer of the shell. During the charging and discharging process of the negative electrode, lithium ions can migrate rapidly through these micro-ion channels, reducing the accumulation of lithium ions on the surface of the negative electrode active material, reducing interfacial polarization, and solving the problems of low electronic conductivity and severe polarization of the negative electrode.

[0020] By designing an uneven structure on the shell surface and limiting its roughness range, the contact area between the binder and the active material and current collector is increased, providing more contact sites and further enhancing the adhesion of the binder. At the same time, the uneven structure on the shell surface can form a porous structure between the interface with the negative electrode active material, providing a physical channel for ion transport and further improving the electrochemical performance of the battery. Detailed Implementation

[0021] To make the technical problems solved, the technical solutions, and the beneficial effects of this invention clearer, the 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 and not intended to limit the invention.

[0022] One embodiment of this application provides an adhesive 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, wherein the glass transition temperature of the first polymer is higher than that of the second polymer; The first polymer comprises rigid structural units, flexible structural units, ion-conducting structural units, and cross-linked structural units; The second polymer comprises rigid structural units, flexible structural units, ion-conducting structural units, olefinic structural units, and cross-linked structural units; The surface of the shell has an uneven structure, and the roughness Sa of the shell is 0.1μm≤Sa≤0.3μm.

[0023] In this embodiment, in the core-shell structure of the binder, the glass transition temperature of the core is higher than that of the shell, which provides better core support and higher cohesive strength for the binder to cope with the expansion of the silicon anode. The shell provides better adhesion and elongation for the binder, promotes the adhesion between the binder and the anode active material, and forms a synergistic effect with the core to prevent the binder film from becoming hard and brittle.

[0024] By controlling the types of structural units in the core and shell, the synergistic effect of each component is ensured, resulting in a binder with high tensile strength and elongation at break, improving the binder's bonding performance, and suppressing the expansion of the negative electrode active material.

[0025] In particular, by introducing ion conduction structural units into the core and shell, these units can form "micro-ion channels" in the first polymer of the core and the second polymer of the shell. During the charging and discharging process of the negative electrode, lithium ions can migrate rapidly through these micro-ion channels, reducing the accumulation of lithium ions on the surface of the negative electrode active material, reducing interfacial polarization, and solving the problems of low electronic conductivity and severe polarization of the negative electrode.

[0026] By designing an uneven structure on the shell surface and limiting its roughness range, the contact area between the binder and the active material and current collector is increased, providing more contact sites and further enhancing the adhesion of the binder. At the same time, the uneven structure on the shell surface can form a porous structure between the interface with the negative electrode active material, improving the physical channels for ion transport and further enhancing the electrochemical performance of the battery.

[0027] If the roughness is less than 0.1 μm, the shell is a relatively smooth surface and cannot form an effective gap with the main material, which is not conducive to ion transport. If the roughness is greater than 0.3 μm, the shell surface is too rough, the gap with the main material increases, which is not conducive to the bonding of the main material and may result in material loss during processing.

[0028] In the first polymer, rigid structural units enhance the structural strength and adhesion of the core, while flexible structural units improve the toughness of the core, preventing it from becoming brittle due to excessive rigidity. Crosslinking structural units form a three-dimensional crosslinked network, enhancing the mechanical strength and bonding stability of the binder and further improving the integrity of the electrode structure. In the second polymer, rigid structural units ensure the basic bonding performance of the shell, flexible structural units strengthen the shell's buffering capacity, ion-conducting structural units further improve lithium-ion conduction efficiency, and acrylic structural units enhance the adhesion between the binder and the active material and current collector. The crosslinking structural units synergize with the crosslinked network of the first polymer to form a more stable overall structure, further suppressing electrode structure damage.

[0029] In some embodiments, the ion-conducting structural units in the first polymer and the second polymer independently include at least one of the following: sulfonic acid group-containing structural units, carboxyl group-containing structural units, amide group-containing structural units, polyether group-containing structural units, and polyethylene glycol group-containing structural units. The main function of these ion-conducting structural units is to promote ion transport and reduce internal resistance. The amide bond has moderate polarity and excellent electrolyte affinity, exhibiting affinity for lithium salt systems. Its nitrogen and oxygen lone pair electrons can coordinate and complex lithium ions, promoting ion dissociation and migration. The ether bond's oxygen atom lone pair electrons efficiently solubilize lithium ions, allowing for rapid desolvation and migration. The sulfonic acid group has a very high degree of ionization, readily adsorbing lithium ions in the electrolyte to form ion pairs and activate them. The carboxyl group has weak ionization, which can weakly bind lithium ions and assist ion transitions. Multiple ion-conducting structural units can also be used in combination to achieve a synergistic improvement in ion conduction performance.

[0030] In some embodiments, the ion-conducting structural unit comprises a structural unit formed by the polymerization of ion-conducting monomers. The ion-conducting monomers include monomers containing sulfonic acid groups, carboxyl groups, amides, polyethers, and ethylene glycol. The monomers containing sulfonic acid groups include at least one of sodium methacrylate sulfonate, lithium 2-acrylamido-2-methylpropanesulfonate, sodium 3-allyloxy-2-hydroxypropanesulfonate, sodium vinyl sulfonate, and sodium monobutyl maleate sulfonate. The monomers containing carboxyl groups include at least one of methacrylic acid, acrylic acid, and itaconic acid. The monomers containing amides include at least one of acrylamide, methacrylamide, and N,N-dimethylacrylamide. The monomers containing polyethylene glycol include at least one of polyethylene glycol acrylate, polyethylene glycol monomethyl ether methacrylate, and polyethylene glycol monomethyl ether acrylate.

[0031] By selecting ion-conducting monomers with good ionic conductivity and the ability to polymerize effectively with other monomers, it is ensured that the ion-conducting structural units can exist stably in the polymer molecular chain of the binder, while further improving the polymerization stability and overall performance of the binder.

[0032] In some embodiments, the mass ratio of the rigid structural unit, flexible structural unit, ion-conducting structural unit and cross-linking structural unit in the first polymer is (50~90):(10~50):(1~5):(1~8). In some embodiments, the mass ratio of the rigid structural unit, flexible structural unit, ion-conducting structural unit, olefinic structural unit and crosslinking structural unit in the second polymer is (5~10):(40~60):(6~10):(6~16):(4~8).

[0033] It should be noted that the rigid structural units, flexible structural units, ion-conducting structural units, and cross-linking structural units in the first polymer and the second polymer may be the same or different.

[0034] Specifically, the mass ratio of rigid structural units, flexible structural units, ion-conducting structural units, and cross-linking structural units in the first polymer includes, but is not limited to, 50:10:1:1, 90:50:5:8, 50:25:1:1, 50:50:1:1, 50:10:3:1, 50:10:5:1, 50:10:1:4, 50:10:1:8, 70:10:1:1, 90:10:1:1, and 70:30:3:4.

[0035] The mass ratios of rigid structural units, flexible structural units, ion-conducting structural units, olefinic structural units, and crosslinking structural units in the second polymer include, but are not limited to, 5:40:6:6:4, 10:60:10:16:8, 7:50:6:6:4, 10:40:6:6:4, 5:50:6:6:4, 5:60:6:6:4, 5:40:8:6:4, 5:40:10:6:4, 5:40:6:11:4, 5:40:6:16:4, 5:40:6:6:6, 5:40:6:6:8, and 10:50:8:11:6.

[0036] In some embodiments, the second polymer further includes partially capped vinyl silane structural units, wherein the mass ratio of the rigid structural unit, flexible structural unit, ion-conducting structural unit, alkenoic acid structural unit, partially capped vinyl silane structural unit and crosslinking structural unit in the second polymer is (5~10):(40~60):(6~10):(6~16):(0.5~3):(4~8).

[0037] The vinylsilane structural unit contains siloxane groups, which can undergo condensation reactions with hydroxyl groups on the surface of silicon-based active materials to form a stable chemical bond. This significantly enhances the compatibility and adhesion between the binder and the silicon-based active material, reduces interfacial defects between the active material and the binder, and avoids poor ion conduction and capacity decay caused by interfacial separation. Furthermore, by partially end-capping the vinylsilane structural unit, the degree of hydrolysis of the vinylsilane structural unit can be controlled, preventing the self-condensation of the hydrolyzed vinylsilane structural unit.

[0038] Specifically, in the second polymer, the mass ratio of rigid structural units, flexible structural units, ion-conducting structural units, acrylic acid structural units, partially end-capped vinyl silane structural units, and crosslinked structural units includes, but is not limited to, 5:40:6:6:0.5:4, 10:60:10:16:0.5:8, 7:40:6:6:0.5:4, 10:40:6:6:0.5:4, and 5:50:6:6:0.5. :4、5:60:6:6:0.5:4、5:40:8:6:0.5:4、5:40:10:6:0.5:4、5:40:6:11:0.5:4、5:40:6:16:0.5:4、5:40:6:6:2:4、5:40:6:6:3:4、5:40:6:6:0.5:6、5:40:6:6:0.5:8、7:50:8:11:2:6、

[0039] In some embodiments, the partially end-capped vinylsilane structural unit comprises vinyl and silane groups in a molar ratio of 1:0.5 to 2.8. This molar ratio of vinyl to silanes within this range is advantageous in ensuring sufficient silane is retained in the emulsion, allowing for stronger interaction with the foil or graphite substrate during slurry processing, thereby improving adhesion.

[0040] Specifically, the molar ratio of the vinyl group to the silane group includes, but is not limited to, 1:0.5, 1:0.8, 1:2, 1:2.5, and 1:2.8.

[0041] The partially capped vinyl silane structural unit comprises a structural unit formed by the polymerization of partially capped vinyl silane monomers. The partially capped vinyl silane monomers are formed by the reaction of a capping agent and a vinyl silane monomer, and the mass ratio of the capping agent to the vinyl silane monomer is (0.5~2):1.

[0042] By limiting the mass ratio of the end-capping agent to the vinylsilane monomer to (0.5~2):1, partial end-capping of the vinylsilane monomer can be achieved, retaining an appropriate amount of siloxane that can be hydrolyzed to generate silanol groups. Insufficient end-capping will cause the vinylsilane structural units to undergo extensive hydrolysis in the aqueous environment during subsequent free radical polymerization, generating a large number of silanol groups, resulting in self-condensation and affecting the adhesiveness of the binder; excessive end-capping will result in insufficient silanol content after hydrolysis of the vinylsilane during subsequent free radical polymerization, leading to a decrease in the binding effect with the negative electrode active material.

[0043] Specifically, the mass ratio of the capping agent to the vinyl silane monomer includes, but is not limited to, 0.5:1, 1:1, 1.5:1 or 2:1.

[0044] In some embodiments, the capping agent includes at least one selected from methyltriethoxysilane, methyltrimethoxysilane, ethyltriethoxysilane, propyltriethoxysilane, and dimethyldimethoxysilane; The vinyl silane monomer includes at least one of vinyltrimethoxysilane (A-171), vinyltriethoxysilane (A-151), vinyltri(2-methoxyethoxy)silane (A-172), vinyltriisopropoxysilane (A-173), and 3-methacryloyloxypropyltrimethoxysilane (KH570).

[0045] The selected end-capping agent and vinyl silane monomer both have good reactivity and compatibility, enabling them to undergo end-capping and polymerization reactions smoothly, ensuring that the partially end-capped vinyl silane structural units are stably present in the second polymer.

[0046] Specifically, the method for preparing partially capped vinylsilane monomers includes the following steps: (1) Mix vinyl silane monomer with ethanol-water mixture at a volume ratio of 3~5:0.1~2 to obtain silane mixture. The ethanol-water mixture is an equal volume mixture of anhydrous ethanol and deionized water. (2) Add dilute acetic acid dropwise to the silane mixture to adjust the pH of the system to 4.0~5.0, and stir at 200~300 r / min at 25~30℃ for 10~20 min to hydrolyze the siloxane groups of the vinyl silane monomer to -Si-OH, so as to obtain a monodisperse hydrolyzed silane system; the vinyl silane monomer is a silane coupling agent containing double bonds, epoxy groups or amino groups, which is suitable for the -COOH, -OH or -C=C- active sites on the surface of the binder particles; (3) Add 0.5% to 2.0% of the mass of vinyl silane monomer as a capping agent to the hydrolyzed silane system, stir at 25 to 30°C for 5 to 8 minutes to cap the -Si-OH of the hydrolyzed silane, and obtain a partially capped vinyl silane monomer.

[0047] In some embodiments, the rigid structural unit includes at least one of vinyl aromatic structural units, nitrile structural units, and acrylate structural units.

[0048] The rigid structural unit is a structural unit formed by the polymerization of rigid monomers, and the rigid monomers include at least one of vinyl aromatic monomers, acrylonitrile monomers, and acrylate monomers. Vinyl aromatic monomers include, but are not limited to, styrene, methylstyrene, isopropylstyrene, 4-methoxystyrene, and 2,5-dimethylstyrene; acrylonitrile monomers include, but are not limited to, acrylonitrile; and acrylate monomers include, but are not limited to, methyl methacrylate, hydroxypropyl methacrylate, and hydroxyethyl methacrylate.

[0049] Rigid monomers correspond to homopolymers with glass transition temperatures ranging from 45 to 130°C, exhibiting strong rigidity and structural stability. The rigid structural units formed by their polymerization can improve the cohesiveness and mechanical strength of the binder, enhance its adhesion, and prevent the electrodes from losing electrical contact due to structural collapse during charging and discharging. The selected rigid monomers, such as styrene, acrylonitrile, and methyl methacrylate, all possess good polymerization activity and can effectively polymerize with other monomers to form structurally stable polymer chains. Temperatures below the lower limit of 45°C are detrimental to maintaining cohesiveness and mechanical strength.

[0050] In some embodiments, the flexible structural unit includes acrylate structural units.

[0051] The flexible structural unit is a structural unit formed by the polymerization of flexible monomers, and the flexible monomers include acrylate monomers.

[0052] The flexible monomers correspond to homopolymers with glass transition temperatures ranging from -90°C to 44°C, exhibiting excellent flexibility and elasticity. The flexible structural units formed by their polymerization can improve the electrolyte affinity and elongation at break of the binder. The selected flexible monomers can synergistically polymerize with rigid monomers and ion-conducting monomers to balance the rigidity and flexibility of the binder. If the temperature exceeds the upper limit of 44°C, the provided flexibility is insufficient, and the elongation will be significantly reduced; if the temperature is below -90°C, the shell is too soft, resulting in poor shear resistance during processing.

[0053] By selecting rigid and flexible monomers with significant differences in glass transition temperature, and by adjusting the proportion of each structural unit in the first and second polymers, the synergy between the binder core and shell can be achieved. This ensures that the binder has sufficient rigidity to maintain the stability of the electrode structure, while also possessing good flexibility to adapt to the volume changes of the silicon-based anode, effectively suppressing anode expansion.

[0054] Further, the flexible monomer includes at least one of methyl acrylate, ethyl acrylate, glycidyl methacrylate, butyl acrylate, butyl methacrylate, hydroxypropyl acrylate, hydroxyethyl acrylate, isooctyl acrylate, isooctyl methacrylate, lauryl acrylate, and lauryl methacrylate.

[0055] In some embodiments, the crosslinked structural unit is a structural unit obtained after the crosslinking monomer participates in the polymerization reaction, and the crosslinking monomer includes a monomer with crosslinking function containing at least two double bonds. The addition of the crosslinking monomer can promote the polymerization to form a crosslinked structure, increase the degree of crosslinking of the adhesive, control the swelling and dissolution of the adhesive in the electrolyte, improve the stability of the adhesive in the electrolyte, and at the same time, the network structure formed can further balance the elongation and tensile strength of the adhesive film.

[0056] Further, the crosslinking monomer includes at least one of trimethacrylate, pentaerythritol triacrylate, triallyl isocyanurate, trimethylolpropane triacrylate, ethylene glycol diacrylate, diethylene glycol diacrylate, 1,6-hexanediol diacrylate, divinylbenzene, ethylene glycol dimethacrylate, and allyl methacrylate.

[0057] In some embodiments, the acrylic structural unit is a structural unit obtained after the acrylic monomer participates in the polymerization reaction, and the acrylic monomer includes at least one of methacrylic acid, acrylic acid, and itaconic acid.

[0058] In some embodiments, the glass transition temperature of the first polymer is 60-100°C, and the glass transition temperature of the second polymer is 0-25°C. The glass transition temperature of the first polymer in the core layer is 60-100°C, providing better core support and higher cohesive strength for the binder to cope with the expansion of the silicon anode. The glass transition temperature of the second polymer in the shell layer is 0-25°C, providing better adhesion and elongation for the binder, promoting the adhesion between the binder and the anode active material, and simultaneously forming a synergistic effect with the core layer to prevent the binder film from becoming hard and brittle. This ensures that the binder can better adapt to the volume changes of the silicon-based anode, maintain the integrity of the electrode structure, and improve the cycle stability of the battery.

[0059] It should be noted that the glass transition temperature Tg in the above embodiments was obtained by differential scanning calorimetry.

[0060] In some embodiments, the volume average particle size D of the adhesive 50The particle size should be 300-600 nm. This range ensures good dispersibility of the binder, generating sufficient adhesive force to prevent material loss during the die-cutting process of the negative electrode. Excessively large particle sizes lead to uneven dispersion, making it difficult to fill the spaces between the active materials in the negative electrode and form an effective bond. Furthermore, the smaller specific surface area results in a smaller effective contact area, leading to poor peeling. Conversely, excessively small particle sizes cause the binder to float too quickly, resulting in uneven distribution within the electrode and poor peeling of the negative electrode.

[0061] Specifically, the volume average particle size D of the binder 50 This includes, but is not limited to, any single value or a range between any two values ​​from 300nm, 330nm, 360nm, 390nm, 420nm, 450nm, 480nm, 510nm, 540nm, 570nm, or 600nm. Specifically, the particle size of the binder is controlled by adjusting the content of the emulsifier and the concentration of the monomer.

[0062] In some embodiments, the adhesive has a tensile strength of 15-30 MPa and a tensile elongation of 100%-500%.

[0063] The tensile strength of the binder is limited to 15-30 MPa to ensure sufficient mechanical strength to withstand the pressure from the volume expansion of the silicon-based anode and prevent binder fracture that could damage the electrode structure. The elongation at break is limited to 100%-500% to ensure good toughness and deformation capacity, effectively buffering the volume expansion and contraction of the silicon-based anode, reducing stress concentration, and improving the electrode's processing performance. If the tensile strength is <15 MPa, the binder's mechanical strength is insufficient and it is prone to fracture; if the tensile strength is >30 MPa, the binder's brittleness increases and the elongation at break decreases; if the elongation at break is <100%, the buffering effect is poor; if the elongation at break is >500%, the binder's strength is insufficient and it is prone to deformation.

[0064] In some embodiments, the mass ratio of the core to the shell is 1:0.8 to 4. Within this range, the volume ratio of the core to the shell can be kept within a suitable range, and the support of the core and the extension of the shell are in a relatively balanced state.

[0065] Furthermore, one embodiment of this application provides a method for preparing an adhesive, comprising the following steps: Preparation of nuclei: (1) Mix 0.5 to 5 parts of emulsifier, 0.5 to 3 parts of buffer and water evenly and stir thoroughly. This mixture is called A1. (2) Mix 50-90 parts of rigid monomer, 10-50 parts of flexible monomer, 1-5 parts of ion-conducting monomer, 1-8 parts of crosslinking monomer, and 0.5-3 parts of regulator in a pre-emulsification tank until homogeneous. Then add 2 / 3 of A1 to the pre-emulsification tank and stir at 1000-1500 rpm for 1-2 hours until a milky white emulsion is formed and there is no separation. This is recorded as A2. (3) In another pre-emulsification vessel, the initiator and water are mixed and stirred evenly, and this mixture is recorded as A3; (4) Add 1 / 3 of A1 into the reactor, turn on the heating, set the temperature to 75~78℃, and the rotation speed to 100rpm; (5) After the temperature of the reactor is constant, slowly add A2 and A3, with the addition time being 3~3.5h and 3.5~4h respectively.

[0066] (6) After the reaction is complete, the temperature is lowered to 60~65℃, and the reactor is kept under negative pressure of -1.0~-1.5MPa for 2~4h; (7) After the reaction is complete, the emulsion is collected, filtered through a filter screen, and the filtrate is collected to finally obtain the M1 emulsion of the nucleus.

[0067] Preparation of adhesive: (1) Add 20-60 parts of the core and 20-30 parts of water to the reactor, stir for 3-4 hours at a speed of 100-120 rpm, and turn on the nitrogen protection. (2) In another container, stir and mix 1-5 parts of initiator and 10-20 parts of water until homogeneous to obtain a mixed solution. Take 3 / 4 of the mixed solution and record it as A4, and the remaining solution as A5. (3) Add 0.5-2 parts of emulsifier and 10-40 parts of water to a pre-emulsification vessel and stir at 300-400 rpm for 1-2 hours. Add 10-30 parts of rigid monomer, 3-5 parts of ion-conducting monomer, 1-3 parts of crosslinking monomer and 1-8 parts of acrylic monomer to the pre-emulsification vessel, mix and stir evenly, and stir at 1000-1500 rpm for 1-2 hours until a white emulsion is formed and there is no separation, which is recorded as A6; (4) Add 0.5-2 parts of emulsifier and 10-40 parts of water, 10-65 parts of flexible monomer, 5-8 parts of acrylic monomer, 3-5 parts of ion-conducting monomer, 0.5-3 parts of partially end-capped vinyl silane monomer, and 3-5 parts of crosslinking monomer to another pre-emulsification vessel. Mix and stir evenly at 1000-1500 rpm for 1-2 hours until a white emulsion is formed and no layering occurs. This is recorded as A7. (5) Set the temperature of the reactor to 75~78℃. After the temperature is constant, add A6 and A7 dropwise at a rate of 0.2~20mL / min. After an interval of 1~2h, start adding A4 dropwise and continue the reaction for 6~7h. (6) Then raise the temperature to 80~83℃, add A5 and continue the reaction for 2~3 hours; (7) After the reaction is complete, the temperature is lowered to 60~65℃, and the reactor is kept under negative pressure of -1.0~-1.5MPa for 2~3 hours; (8) After the reaction is complete, the emulsion is collected, filtered with a filter screen, and the filtrate is collected to obtain the binder.

[0068] Adding A5 in the later stages of the reaction and using negative pressure conditions can eliminate residual small-molecule monomers, reduce its own dissolution rate, and improve the stability of the binder in the electrolyte. Adding A6 and A7 separately, due to their different degrees of polymerization and poor compatibility, results in a heterogeneous shell layer, causing an uneven structure to form on the shell surface. The more A6 and A7 coexist, and the higher the mass ratio of A6 / A7 and the slower the dropping rate, the more pronounced this uneven structure becomes.

[0069] Further, in some embodiments, the emulsifier includes at least one of sodium dodecyl sulfate, sodium dodecylbenzene sulfonate, sodium vinyl sulfonate, sodium allyl sulfonate, sodium 3-allyloxy-2-hydroxy-1-propanesulfonate, sodium alkyl alcohol ether sulfosuccinate, alkyl alcohol ether sulfate, octylphenol polyoxyethylene ether, polyethylene glycol monooctyl ether, polyethylene glycol dioctyl ether, ethylene oxide and propylene oxide block polyethers, fatty alcohol polyoxyethylene ethers, lauryl alcohol polyoxyethylene ether, isooctyl alcohol polyoxyethylene ether, allyloxynonylphenol polyoxyethylene ether, and polyoxyethylene sorbitan ester. By adding the emulsifier, the surface tension of the emulsion is reduced, allowing the monomers and water to form a stable emulsion, preventing the aggregation of oily monomers, thereby improving the stability of the emulsion.

[0070] The initiator includes at least one of persulfate compounds and peroxide compounds, preferably selected from at least one of ammonium persulfate (APS), potassium persulfate (KPS), and sodium persulfate.

[0071] Buffers include sodium bicarbonate or disodium hydrogen phosphate, which help maintain the pH stability of the reaction system and prevent the emulsifier from becoming ineffective due to excessively low pH.

[0072] The regulator includes at least one of tert-dodecyl mercaptan, n-dodecyl mercaptan, isooctyl mercaptoacetate, and methyl 3-mercaptopropionate. The polymerization rate is controlled by the regulator, thereby controlling the molecular weight of the binder to be within a narrow distribution.

[0073] 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.

[0074] As those skilled in the art will know, the reactions described above are conventional free radical polymerization reactions, and the specific methods and reaction conditions are common free radical polymerization methods in the prior art, which will not be elaborated upon in this invention.

[0075] One embodiment of the present invention also provides a negative electrode sheet, comprising 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 comprises the binder described in any of the above embodiments. By selecting the above-mentioned binder, problems such as volume expansion, active material shedding, and electrode powder shedding of silicon-based negative electrode sheets are solved, thereby improving the structural stability and electrochemical performance of the negative electrode sheet and providing a reliable negative electrode structure for high-energy-density lithium-ion batteries.

[0076] Furthermore, the negative electrode active material layer also includes a negative electrode active material and a conductive agent, wherein the negative electrode active material includes at least one of graphite, silicon, silicon carbide, and silicon oxide. The conductive agent includes at least one of carbon nanotubes, graphene, spherical carbon, and conductive carbon black.

[0077] Furthermore, the mass ratio of graphite, silicon, conductive agent, thickener and binder is (60~80):(10~30)(0.5~2):(0.5~2):(1~3).

[0078] An embodiment of the present invention also provides a battery, including the negative electrode sheet as described above. Applying the above-described high-performance negative electrode sheet to battery assembly improves the overall performance of the battery by enhancing the performance of the negative electrode sheet, thus solving the problems of poor cycle stability and short lifespan in existing batteries.

[0079] The present invention will be further illustrated by the following examples.

[0080] Specifically, the adhesive, negative electrode sheet, and battery disclosed in this invention are described.

[0081] Example 1 1. The steps for synthesizing partially capped vinylsilane monomers include: (1) 3-methacryloyloxypropyltrimethoxysilane (KH570) was mixed with an ethanol-water mixture at a volume ratio of 3:0.5 to obtain a silane mixture, wherein the ethanol-water mixture was an equal volume mixture of anhydrous ethanol and deionized water; (2) Add dilute acetic acid dropwise to the silane mixture to adjust the pH of the system to 4.0, and stir at 200 r / min for 10 min at 25°C to partially hydrolyze the siloxane group of the silane coupling agent to -Si-OH, thereby obtaining a monodisperse hydrolyzed silane system. (3) Add 0.5% by mass of methyltriethoxysilane as a silane coupling agent to the hydrolyzed silane system, stir at room temperature for 5 min, and end-cap the -Si-OH of the hydrolyzed silane to obtain a solution containing partially end-capped vinyl silane monomer E-1.

[0082] 2. The adhesive preparation steps include: Nucleosome synthesis: (1) Mix 0.5 parts sodium dodecyl sulfate, 0.5 parts sodium bicarbonate and 30 parts water evenly and stir thoroughly. This mixture is recorded as A1. (2) Mix 66 parts styrene, 30 parts glycidyl methacrylate, 2 parts sodium monobutyl maleate sulfonate, 2 parts allyl methacrylate, and 0.5 parts n-dodecyl mercaptan evenly in a pre-emulsification tank. Then take 2 / 3 of A1 and add it to the pre-emulsification tank. Stir at 1000~1500 rpm for 2 hours until a milky white emulsion is formed and there is no separation. This is recorded as A2. (3) In another pre-emulsification vessel, mix 2 parts of potassium persulfate and 7.5 parts of water, stir well and record as A3; (4) Add 1 / 3 of A1 into the reactor, turn on the heating, set the temperature to 75℃ and the rotation speed to 100 rpm; (5) After the temperature of the reactor is constant, slowly add A2 and A3, with a dropping time of 3h and 3.5h respectively.

[0083] (6) After the reaction is complete, the temperature is lowered to 60°C, and the reactor is kept under negative pressure of -1.0MPa for 2 hours; (7) After the reaction is complete, the emulsion is collected, filtered through a 200-mesh filter, and the filtrate is collected to finally obtain the nucleus M1; Preparation of adhesive: (1) Add 20 parts of the core and 30 parts of water to the reactor, stir at 100 rpm for 3 hours, and turn on the nitrogen protection. (2) In another container, stir and mix 1 part potassium persulfate and 10 parts water until they are evenly mixed to obtain a mixed solution. Take 3 / 4 of the mixed solution and record it as A4, and the remaining solution as A5. (3) Add 1 part sodium dodecyl sulfate and 10 parts water to the pre-emulsification vessel and stir at 300 rpm for 1 hour until the emulsifier is completely dissolved. Add 5 parts styrene, 1 part ethylene glycol dimethacrylate, 5 parts sodium methacrylate sulfonate, and 2 parts methacrylic acid to the pre-emulsification vessel and stir at 1500 rpm for 2 hours until a white emulsion is formed and does not separate into layers, and record this as A6; (4) Add 1 part sodium dodecyl sulfate and 15 parts water to the pre-emulsification vessel and stir at 300 rpm for 1 h until the emulsifier is completely dissolved. Add 47 parts butyl acrylate, 2 parts partially capped vinyl silane monomer, 5 parts sodium methacrylate sulfonate, 7 parts methacrylic acid and 6 parts ethylene glycol dimethacrylate to the pre-emulsification vessel and mix and stir evenly. Stir at 1500 rpm for 2 h until a white emulsion is formed and there is no separation. Record it as A7. (5) Set the temperature of the reactor to 75℃. After the temperature is constant, add A6 and A7 dropwise at a rate of 3 mL / min. After 1 hour, start adding A4 dropwise and react for 6 hours. (6) Then raise the temperature to 80°C, add A5 and continue the reaction for 2 hours; (7) After the reaction is complete, the temperature is lowered to 60°C, and the reactor is kept under negative pressure of -1.0MPa for 2 hours; (8) After the reaction is complete, the emulsion is collected and filtered through a 200-mesh filter to obtain the binder.

[0084] 3. Negative electrode plate A mixture of 3 wt% binder, 74.5 wt% graphite (model PQ1-X3), 20% silicon (model SO332), 1 wt% conductive carbon black (model SP conductive carbon black), and 1.5 wt% thickener sodium carboxymethyl cellulose (model 2300) is prepared by adding deionized water and stirring, controlling the solid content of the slurry to 50% and the viscosity to 3000~4000cp. The negative electrode slurry is then coated onto both surfaces of the Cu foil negative electrode current collector. After drying, cold pressing, and slitting, the final negative electrode sheet is obtained.

[0085] 4. Battery After assembling the negative electrode, separator (model: SW16), and positive electrode (model: NCM 523 series S701C) into a dry cell, the electrolyte is injected to obtain a lithium battery. The electrolyte model is LBC435E53. After packaging, the lithium-ion battery is obtained through processes such as formation and degassing.

[0086] Examples 2-31, Comparative Examples 1-6 Examples 2-31, Comparative Examples 1-6 are largely the same as Example 1, except that the first polymer uses the formulation in Table 1, and the second polymer uses the formulations in Tables 2 and 3. In these examples, the ion-conducting monomers are distributed in A6 and A7 at a mass ratio of 1:1, and the crosslinking monomers are distributed in A6 and A7 at a mass ratio of 1:6.

[0087] In Example 10, the preparation method of E-2 is the same as most of the steps in Example 1, except that the capping agent is methyltrimethoxysilane and the vinylsilane monomer is A-171.

[0088] In Example 11, the preparation method of E-3 is the same as most of the steps in Example 1, except that the capping agent is dimethyldimethoxysilane and the vinylsilane monomer is A-172.

[0089] In Example 12, the preparation method of E-4 is the same as most of the steps in Example 1, except that the capping agent is propyltriethoxysilane and the vinylsilane monomer is A-173.

[0090] In Example 13, the preparation method of E-5 is the same as most of the steps in Example 1, except that the mass ratio of the capping agent to the vinyl silane monomer is 1:1.

[0091] In Example 14, the preparation method of E-6 is the same as most of the steps in Example 1, except that the mass ratio of the capping agent to the vinyl silane monomer is 2:1.

[0092] Comparative Example 7 Comparative Example 7 and Example 1 follow most of the same steps, except that in the preparation of the adhesive, A6 and A7 are mixed and emulsified to obtain a mixed solution, which is then dropped into the reaction vessel with a roughness Sa of 0.06 μm.

[0093] Table 1 Table 2 Table 3 The binders, negative electrode sheets, and batteries prepared in the above embodiments and comparative examples were tested as follows.

[0094] 1. Tensile test: Pour the adhesive emulsion into a silicone mold and place it at 25°C for 5 hours, then dry it at 90°C for 2 hours. After that, let it stand at 25°C, cut the sample into strips, and then perform a tensile test on a universal testing machine at a tensile speed of 50 mm / min to test the breaking strength and elongation at break.

[0095] 2. Volume average particle size test of binder: The test was conducted using a Malvern 3000 laser particle size analyzer with water as the medium and a refractive index of 1.33; the sample absorptivity was 0.01 and the refractive index was 1.596.

[0096] 3. Peel strength test of negative electrode: The peel strength of the negative electrode is measured using a tensile testing machine. The test method is in accordance with GB / T2792—2014.

[0097] 4. Glass transition temperature test: The glass transition temperature of the first polymer is tested according to the test standard GB / T19466.2-2004. The glass transition temperature of the second polymer is tested under the above standard after being synthesized separately according to the method of the second polymer.

[0098] 5. Cyclic Test: The battery is cyclically tested using a battery testing system. At 25℃, it is charged at a constant current of 1C to 3.65V, then charged at a constant voltage with a cutoff current of 0.15A. After resting for 10 minutes, it is discharged at a constant current of 1C to 2.5V and rested for 10 minutes. The discharge capacity at this point is taken as the initial discharge capacity Qt. This cycle is repeated 500 times. Then, it is discharged at a constant current of 1C to 2.5V, and the discharge capacity Qret is recorded. This capacity is used to calculate the capacity retention rate. Cyclic capacity retention rate = (Qret / Qt) × 100%.

[0099] 6. Full-charge rebound test: After the negative electrode sheet is rolled, the thickness is measured with a micrometer and recorded as D1; ​​during battery manufacturing, select the electrode sheet with the above-tested thickness for battery assembly; let it stand until the temperature of the soft-pack lithium-ion battery reaches 25℃±2℃, charge it at a constant current of 0.5C until the voltage is 4.25V, cut off at 0.05C, after the cell is fully charged, disassemble it in a low humidity room (humidity <10%), take the disassembled negative electrode sheet and measure the thickness with a micrometer, recorded as D2, calculate the full-charge rebound = [ (D2-D1) / D1]×100%.

[0100] 7. Adhesive shell roughness Sa: Tested using Keyence VK-X1000 according to the methods of ISO 25178 and ASTM D7127 Appendix X1. The surface roughness is judged by the magnitude of Sa (arithmetic mean deviation). The larger the value, the rougher the surface.

[0101] The test results are shown in Table 1-4.

[0102] Table 4 As can be seen from the test results of the examples and Comparative Example 1 in Table 4, the glass transition temperature of the core in Comparative Example 1 is relatively low, resulting in a decrease in the tensile strength of the encapsulated film, which in turn leads to a decrease in peel strength and elongation, an increase in full-charge rebound, and a reduction in cycle performance. In Comparative Example 2, the core lacks ion conduction structural units, and the ion conduction channels connected to the shell are blocked, resulting in a decrease in the cycle performance of the battery. In Comparative Example 3, the shell contains flexible structural units but lacks the support of rigid structural units, resulting in a significant decrease in the cohesion of the shell, which leads to a decrease in peel strength and tensile strength of the encapsulated film. Furthermore, the roughness of the shell decreases and the pore channels shrink, thus the cycle performance of the battery deteriorates. In Comparative Example 4, the shell contains rigid structural units but lacks flexible structural units to provide toughness and adhesion, resulting in a significant decrease in the elongation of the shell film. At the same time, the surface roughness of the shell decreases, which reduces the cycle performance of the battery. In Comparative Examples 5 and 6, when the mass ratio of the rigid structural units to the flexible structural units of the shell is in the range of (5~10):(40~60), the balance between the cohesive force and the adhesive force of the shell is disrupted, leading to a decrease in adhesive strength, an increase in full-charge rebound, and a decrease in the cycle performance of the battery.

[0103] The test results of Examples 1 and 28 show that introducing partially end-capped vinyl silane structural units into the second polymer can further improve the peel strength of the adhesive.

[0104] The test results of Examples 1 and 17-31 show that when the mass ratio of rigid structural units, flexible structural units, ion-conducting structural units, olefinic structural units, partially end-capped vinylsilane structural units, and cross-linked structural units is in the range of (5~10):(40~60):(6~10):(6~16):(0.5~3):(4~8), the mechanical properties of the binder can be further improved, the full-charge rebound rate can be reduced, and the cycle performance of the battery can be improved.

[0105] 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. An adhesive, characterized in that, The binder includes 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 including a first polymer, the shell including a second polymer, and the glass transition temperature of the first polymer being higher than that of the second polymer. The first polymer comprises rigid structural units, flexible structural units, ion-conducting structural units, and cross-linked structural units; the second polymer comprises rigid structural units, flexible structural units, ion-conducting structural units, acrylic acid structural units, and cross-linked structural units. The surface of the shell has an uneven structure, and the roughness Sa of the shell is 0.1μm≤Sa≤0.3μm.

2. The adhesive according to claim 1, characterized in that, The ion-conducting structural unit includes at least one of the following: a structural unit containing a sulfonic acid group, a structural unit containing a carboxyl group, a structural unit containing an amide group, a structural unit containing a polyether group, and a structural unit containing a polyethylene glycol group.

3. The adhesive according to claim 1, characterized in that, In the first polymer, the mass ratio of the rigid structural unit, the flexible structural unit, the ion-conducting structural unit, and the cross-linking structural unit is (50~90):(10~50):(1~5):(1~8). And / or, in the second polymer, the mass ratio of the rigid structural unit, the flexible structural unit, the ion-conducting structural unit, the olefinic structural unit and the crosslinking structural unit is (5~10):(40~60):(6~10):(6~16):(4~8).

4. The adhesive according to any one of claims 1-3, characterized in that, The second polymer further includes partially capped vinyl silane structural units; in the second polymer, the mass ratio of the rigid structural unit, flexible structural unit, ion-conducting structural unit, alkenoic acid structural unit, partially capped vinyl silane structural unit and crosslinking structural unit is (5~10):(40~60):(6~10):(6~16):(0.5~3):(4~8).

5. The adhesive according to claim 4, characterized in that, The partially capped vinylsilane structural unit comprises vinyl and silane, wherein the molar ratio of vinyl to silane is 1:0.5 to 2.

8.

6. The adhesive according to claim 1, characterized in that, The rigid structural unit includes at least one of vinyl aromatic structural units, nitrile structural units, and acrylate structural units; And / or, the flexible structural unit includes acrylate structural units.

7. The adhesive according to claim 1, characterized in that, The glass transition temperature of the first polymer is 60~100℃; And / or, the glass transition temperature of the second polymer is 0~25°C.

8. The adhesive according to any one of claims 1-7, characterized in that, The volume average particle size of the binder is 300~600nm.

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, wherein the negative electrode active material layer includes the binder according to any one of claims 1-8.

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

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