Negative electrode sheet, method for manufacturing negative electrode sheet, and battery

Abstract

This invention provides a negative electrode sheet, a method for preparing the negative electrode sheet, and a battery, comprising a current collector and a negative electrode active material layer. The negative electrode active material layer includes a negative electrode active material, a composite binder, and a conductive agent. The composite binder includes a main binder, an auxiliary binder, and a crosslinking agent. The main binder and the auxiliary binder are crosslinked with each other through the crosslinking agent to form a network structure. In the negative electrode sheet provided by this invention, the main binder and the auxiliary binder are crosslinked with the crosslinking agent to form a continuous network structure, which strengthens the adhesion between the active material particles. At the same time, the siloxane group of the modifier can undergo a condensation reaction with the hydroxyl groups on the surface of the active material and the current collector, tightly connecting the crosslinked network of the active material particles, the current collector, and the composite binder into one through chemical bonds, thereby improving the interfacial bonding strength within the active material layer and between the active material layer and the current collector.

Description

Technical Field

[0001] This invention relates to the field of battery technology, and in particular to a negative electrode, a method for preparing the negative electrode, and a battery. Background Technology

[0002] In the development of high-energy-density, long-cycle lithium-ion battery systems, optimizing the performance of the negative electrode has become a key issue to be addressed. Currently, the industry primarily uses a binary adhesive system centered on a combination of CMC (carboxymethyl cellulose) and SBR (styrene-butadiene rubber). This system leverages the complementary properties of CMC's dispersibility and SBR's flexibility to achieve basic bonding. Other methods include using PVDF (polyvinylidene fluoride) or PAA (polyacrylic acid) alone, or incorporating biomimetic adhesives or hyperbranched polyether composites with PVDF. Active materials are mainly pure graphite. Silicon-carbon composites still utilize traditional adhesive systems. Conductive agents are often single SuperP (superconducting carbon black) or carbon nanotubes, relying on point-to-point bonding between particles to form a conductive network. In terms of processes, coating employs a single-coat method, primarily using single-stage or simple three-stage hot air drying. Rolling is mostly done at room temperature with single-stage compaction, while some special systems use fixed-pressure hot rolling. Current collectors undergo only basic corona treatment or no pretreatment. Regarding interface improvement, only a few solutions modify the current collector surface by adding a single silane coupling agent or use ALD (atomic layer deposition) to coat the active material and enhance polarity.

[0003] However, with the increasing demand for high energy density and long cycle life in batteries from new energy vehicles and high-end electronic devices, the application of high-performance active materials such as silicon-carbon has become inevitable. However, existing technologies are unable to solve the problem of electrode stability caused by volume expansion. Furthermore, simply increasing the amount of binder will reduce the electrode conductivity and energy density. While improving adhesion, lithium-ion transport efficiency is sacrificed. Further optimization of process parameters cannot fundamentally improve interface bonding and structural stability. At the same time, the low peel strength, poor cycle stability, and powder shedding of the electrode cannot meet the usage requirements. Summary of the Invention

[0004] To address the issues of powder shedding and delamination that easily occur in existing negative electrode materials, affecting battery energy density and cycle performance, this paper provides a negative electrode material, a method for preparing the negative electrode material, and a battery.

[0005] The technical solution adopted by the present invention to solve the above-mentioned technical problems is as follows: On one hand, the present invention provides a negative electrode sheet, including a current collector and a negative electrode active material layer, wherein the negative electrode active material layer includes a negative electrode active material, a composite binder and a conductive agent, and the composite binder includes a main binder, an auxiliary binder and a crosslinking agent, wherein the main binder and the auxiliary binder are crosslinked with each other through the crosslinking agent to form a network structure; The negative electrode active material includes a modifier and an active material. The modifier has siloxane groups and reactive groups for reacting and connecting with the composite binder. The reactive groups include one or more of amine groups and epoxy groups.

[0006] Optionally, the mass ratio of the main adhesive, auxiliary adhesive and crosslinking agent is (6~8):(1~2):(0.5~1.5).

[0007] Optionally, the primary adhesive comprises one or more of styrene-butadiene rubber and polyvinylidene fluoride; and / or, The auxiliary adhesive includes PAMAM; and / or, The crosslinking agent includes one or more of GMA and MBA.

[0008] Optionally, the modifier includes a compound of γ-aminopropyltriethoxysilane and γ-glycidoxypropyltrimethoxysilane; The mass ratio of the compounded γ-aminopropyltriethoxysilane to γ-glycidoxypropyltrimethoxysilane is 1:(0.8~1.2).

[0009] Optionally, the active material further includes a mixture of graphite and silicon-carbon composite material, wherein the mass ratio of graphite to silicon-carbon composite material is (7~9):1.

[0010] Optionally, the surface of the silicon-carbon composite material is coated with amorphous carbon with a coating thickness of 5~15nm.

[0011] Optionally, the conductive agent includes a composite of carbon nanotubes and graphene, wherein the mass ratio of carbon nanotubes to graphene is (3-5):1; The carbon nanotubes have a diameter of 8~15nm and a length of 1~5μm; The graphene has 3 to 10 layers.

[0012] Optionally, the porosity of the negative electrode sheet is 35% to 45%.

[0013] Optionally, the method for preparing the negative electrode includes the following operations: The active material and the modifier are stirred and mixed at high temperature to obtain the negative electrode active material. The adhesive, auxiliary adhesive, and crosslinking agent are dissolved in water and kept at a certain temperature to obtain a composite adhesive solution. The negative electrode active material and conductive agent are added to the composite binder solution and stirred to disperse evenly to obtain the negative electrode slurry; The negative electrode slurry is coated onto the surface of the current collector, dried and rolled, then kept at a temperature in a protective atmosphere. After cross-linking and curing, the negative electrode sheet is obtained.

[0014] Optionally, the preparation of the negative electrode also includes the following operations: First, a negative electrode slurry is coated onto the surface of the negative electrode current collector. After initial drying, a second coating of the negative electrode slurry is applied, followed by a second drying process to obtain the negative electrode sheet.

[0015] Optionally, the coating thickness of the first coating is 5~10μm, and the coating thickness of the second coating is 80~120μm of the total thickness of the negative electrode sheet; The initial drying temperature is 60~70℃, and the drying time is 10~15min; The secondary drying is a gradient drying process, with drying temperatures of 70~80℃ for 15~20 min, 90~100℃ for 15~20 min, and 80-90℃ for 15~20 min.

[0016] Optionally, in the preparation of the negative electrode active material, the high temperature is 80~100℃, the stirring speed is 1500~2000r / min, and the stirring time is 30~60min.

[0017] Optionally, in the preparation of the composite adhesive solution, the dissolution temperature is 50~60℃ and the holding time is 20~30min.

[0018] Optionally, in the preparation of the negative electrode slurry, the stirring time is 30-40 min at low speed, 15-20 min at ultrasonic dispersion, the ultrasonic power is 300-500 W, the stirring speed is 20-30 min at high speed, and the stirring speed is 8000-10000 r / min.

[0019] Optionally, the protective atmosphere is maintained at a temperature of 2-4 hours, and the protective atmosphere includes one or more of nitrogen and argon.

[0020] Optionally, the solid content of the negative electrode slurry is 55%~65%, and the viscosity is 3000~5000 mPa·s.

[0021] Optionally, the negative electrode slurry comprises the following components by mass: The composition includes 89-96 parts of negative electrode active material, 3-8 parts of composite binder, and 1-3 parts of conductive agent.

[0022] On the other hand, the present invention provides a battery comprising the aforementioned negative electrode sheet, or a negative electrode sheet prepared by the method for preparing the aforementioned negative electrode sheet.

[0023] The beneficial effects of this application are as follows: The negative electrode sheet provided in this application includes a current collector and a negative electrode active material layer. The negative electrode active material layer includes a negative electrode active material, a composite binder, and a conductive agent. The composite binder includes a main binder, an auxiliary binder, and a crosslinking agent. The main binder and the auxiliary binder are crosslinked together by the crosslinking agent to form a network structure. The negative electrode active material includes a modifier and an active material. The modifier has siloxane groups and reactive groups for reacting and connecting with the composite binder. The composite binder is formed by the main binder and the auxiliary binder being crosslinked by the crosslinking agent to form a continuous network structure. This structure can serve as a skeletal support for the active material layer, strengthening the adhesion between the active material particles. Simultaneously, the modifier in the negative electrode active material... It possesses both siloxane groups and reactive groups such as amine and epoxy groups. The siloxane groups can undergo condensation reactions with the hydroxyl groups on the surface of the active material and the current collector, thus anchoring the modifier at the interface between the active material and the current collector. Meanwhile, the reactive groups such as amine and epoxy groups can chemically react with the active sites in the composite binder network structure, tightly connecting the cross-linked network of the active material particles, the current collector, and the composite binder into one unit through chemical bonds. This significantly improves the interfacial bonding strength within the active material layer and between the active material layer and the current collector, effectively resisting the internal stress generated by the volume expansion and contraction of the negative electrode during charging and discharging, preventing the active material particles from falling off and the active material layer from peeling off from the current collector, ultimately ensuring the energy density and cycle performance stability of the battery. Detailed Implementation

[0024] 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.

[0025] The present invention provides a negative electrode sheet, comprising a current collector and a negative electrode active material layer, wherein the negative electrode active material layer comprises a negative electrode active material, a composite binder and a conductive agent, and the composite binder comprises a main binder, an auxiliary binder and a crosslinking agent, wherein the main binder and the auxiliary binder are crosslinked with each other via the crosslinking agent to form a network structure; The negative electrode active material includes a modifier and an active material. The modifier has siloxane groups and reactive groups for reacting and connecting with the composite binder. The reactive groups include one or more of amine groups and epoxy groups.

[0026] It should be noted that the siloxane groups of the modifier are used for dehydration condensation bonding with hydroxyl groups and other reactive groups on the surface of the active material, and the reactive groups are used for bonding with the composite adhesive.

[0027] Specifically, the main binder and the auxiliary binder are cross-linked with each other via the cross-linking agent to form a network structure. The negative electrode active material includes a modifier and an active material. The modifier has siloxane groups and reactive groups for reacting and bonding with the composite binder. The composite binder is formed by the cross-linking of the main binder and the auxiliary binder with the cross-linking agent to form a continuous network structure. This structure can serve as a skeletal support for the active material layer, strengthening the adhesion between the active material particles. Simultaneously, the modifier in the negative electrode active material possesses both siloxane groups and reactive groups such as amine groups and epoxy groups. The siloxane groups can react with the active material and the surface of the current collector. The hydroxyl groups undergo a condensation reaction, anchoring the modifier at the interface between the active material and the current collector. Meanwhile, reactive groups such as amine and epoxy groups can chemically react with the active sites in the composite binder network structure, tightly connecting the active material particles, current collector, and composite binder network through chemical bonds. This significantly improves the interfacial bonding strength within the active material layer and between the active material layer and the current collector, effectively resisting the internal stress generated by the volume expansion and contraction of the negative electrode during charging and discharging. This prevents the active material particles from falling off and the active material layer from peeling off from the current collector, ultimately ensuring the energy density and cycle performance stability of the battery.

[0028] In some embodiments, the mass ratio of the main adhesive, auxiliary adhesive, and crosslinking agent is (6~8):(1~2):(0.5~1.5).

[0029] Specifically, the mass ratio of the main binder, auxiliary binder, and crosslinking agent is set between (6~8):(1~2):(0.5~1.5). The main binder has the highest proportion, serving as the main body of the binder and providing basic bonding performance and flexible support. The auxiliary binder has a moderate proportion, which can introduce more reaction sites and promote the crosslinking reaction with the crosslinking agent. The crosslinking agent has a reasonable proportion, which can ensure that the main and auxiliary binders are fully crosslinked to form a dense network structure, and will not cause the binder to become brittle due to excessive crosslinking agent, thus avoiding the shedding of active materials caused by brittle fracture of the binder during charging and discharging.

[0030] The composite binder with this ratio has excellent adhesion, flexibility and resistance to deformation, which can effectively adapt to the volume expansion characteristics of negative electrode active materials (especially silicon-carbon composite materials) and further improve the cycle stability of the negative electrode sheet.

[0031] In some embodiments, the primary binder comprises one or more of styrene-butadiene rubber and polyvinylidene fluoride; and / or, The auxiliary adhesive includes PAMAM; and / or, The crosslinking agent includes one or more of GMA and MBA.

[0032] Specifically, the auxiliary binder is PAMAM. PAMAM molecules have abundant end functional groups (such as amine groups), which can act as a bridge for crosslinking reactions and fully react with the active sites of the crosslinking agent and the main binder to improve the density and uniformity of the crosslinking network. The crosslinking agents are GMA (glycidyl methacrylate) and MBA (N,N'-methylenebisacrylamide). GMA contains epoxy groups, which can undergo ring-opening reactions with amine and hydroxyl groups. MBA contains bisacrylamide groups, which can undergo free radical crosslinking reactions with polymer chains. The two can be used alone or in combination to achieve efficient crosslinking and quickly form a stable network structure.

[0033] In some embodiments, the modifier comprises a compound of γ-aminopropyltriethoxysilane and γ-glycidoxypropyltrimethoxysilane; The mass ratio of the compounded γ-aminopropyltriethoxysilane to γ-glycidoxypropyltrimethoxysilane is 1:(0.8~1.2).

[0034] Specifically, the modifier is a compound of γ-aminopropyltriethoxysilane (KH550) and γ-glycidoxypropyltrimethoxysilane (KH560). KH550 contains amino reactive groups, and KH560 contains epoxy reactive groups. The compound provides two different types of reaction sites, which are more compatible with the network structure of the composite adhesive and can more fully form chemical bonds with the adhesive. At the same time, the siloxane groups of the two silane coupling agents can form a denser siloxane cross-linked layer after hydrolysis, which enhances the interfacial anchoring effect of the active material-current collector.

[0035] The mass ratio of γ-aminopropyltriethoxysilane to γ-glycidoxypropyltrimethoxysilane 1:(0.8~1.2) ensures uniform dispersion of the two silane coupling agents, avoids the problem of insufficient interfacial bonding caused by the single reaction site of a single silane coupling agent, and further improves the modification effect of the modifier on the active material.

[0036] Furthermore, the mass ratio of the compounded γ-aminopropyltriethoxysilane to γ-glycidoxypropyltrimethoxysilane includes, but is not limited to, 1:0.8, 1:0.9, 1:1.0, 1:1.1 or 1:2.

[0037] In some embodiments, the active material further includes a mixture of graphite and silicon-carbon composite material, wherein the mass ratio of graphite to silicon-carbon composite material is (7~9):1.

[0038] Specifically, graphite has the advantages of good conductivity and low volume expansion rate, which can ensure the cycle stability and rate performance of the negative electrode. The theoretical specific capacity of silicon-carbon composite material (about 4200 mAh / g) is higher than that of graphite, which can significantly improve the energy density of the battery. However, silicon-carbon composite material has a high volume expansion rate (about 300%). In this application, a mixture of graphite and silicon-carbon composite material is used as the active material, and the mass ratio is (7~9):1. This can improve the energy density while using the low expansion characteristics of graphite to alleviate the volume expansion stress of silicon-carbon composite material and reduce the risk of cracking of the active material layer. At the same time, the synergistic effect of composite binder and modifier can further suppress the expansion of silicon-carbon composite material, and achieve a balance between energy density and cycle stability.

[0039] In some embodiments, the surface of the silicon-carbon composite material is coated with amorphous carbon with a coating thickness of 5-15 nm.

[0040] Specifically, the amorphous carbon coating layer can serve as a buffer layer for silicon-carbon composite materials, absorbing the volume expansion stress of silicon particles during charging and discharging, and preventing silicon particles from breaking. Amorphous carbon has good electrical conductivity, which can improve the electronic conduction efficiency of silicon-carbon composite materials and improve the rate performance of the negative electrode. Setting the coating thickness in the range of 5~15nm can achieve better results. It avoids the problem that if the coating is too thin, it will not be able to effectively buffer the volume expansion, while if it is too thick, it will reduce the energy density of the negative electrode. This thickness can achieve the optimal balance between buffering effect and energy density.

[0041] Furthermore, the coating thickness includes, but is not limited to, 5nm, 8nm, 10nm, 12nm or 15nm.

[0042] In some embodiments, the conductive agent comprises a composite of carbon nanotubes and graphene, wherein the mass ratio of carbon nanotubes to graphene is (3-5):1. The carbon nanotubes have a diameter of 8~15nm and a length of 1~5μm; The graphene has 3 to 10 layers.

[0043] Specifically, the carbon nanotubes can serve as one-dimensional materials to connect dispersed active material particles, while graphene, a two-dimensional nanomaterial, coats the surface of the active material. The combination of the two can form a three-dimensional interwoven conductive network, which significantly improves the electronic conduction efficiency of the negative electrode active material layer and reduces the internal resistance of the battery.

[0044] The mass ratio of carbon nanotubes to graphene (3-5):1 ensures the synergistic dispersion of carbon nanotubes and graphene, avoiding graphene sheet agglomeration or carbon nanotube entanglement. The setting of carbon nanotubes with a diameter of 8~15nm and a length of 1~5μm, and graphene layers of 3~10 layers, allows the conductive agent to be evenly distributed in the active material layer, maximizing the coverage of the conductive network, while not affecting the compaction density of the negative electrode sheet due to the excessively large particle size of the conductive agent.

[0045] In some embodiments, the porosity of the negative electrode sheet is 35% to 45%.

[0046] Specifically, the porosity of the negative electrode sheet of 35% to 45% can provide sufficient storage space for the electrolyte, ensuring that the electrolyte fully wets the active material layer and improves the lithium ion transport efficiency. At the same time, this porosity can reserve an appropriate amount of expansion space to accommodate the volume expansion of the silicon-carbon composite material during charging and discharging, avoid the pore blockage caused by the volume expansion of the negative electrode sheet, and maintain the smooth flow of ion conduction channels during cycling.

[0047] If the porosity is too low, the electrolyte will not wet sufficiently, and ion conduction will be hindered; if the porosity is too high, the compaction density of the negative electrode will decrease, and the energy density will decrease. This range is the optimal range for porosity.

[0048] Specifically, the porosity of the negative electrode sheet can be 35%, 38%, 40%, 42%, or 45%.

[0049] In some embodiments, the method for preparing the negative electrode sheet includes the following operations: The active material and the modifier are stirred and mixed at high temperature to obtain the negative electrode active material. The adhesive, auxiliary adhesive, and crosslinking agent are dissolved in water and kept at a certain temperature to obtain a composite adhesive solution. The negative electrode active material and conductive agent are added to the composite binder solution and stirred to disperse evenly to obtain the negative electrode slurry; The negative electrode slurry is coated onto the surface of the current collector, dried and rolled, then kept at a temperature in a protective atmosphere. After cross-linking and curing, the negative electrode sheet is obtained.

[0050] The method for preparing the negative electrode sheet specifically includes the following operations: Graphite and silicon-carbon composite materials (15% silicon by mass, with a 10nm amorphous carbon layer on the surface) were mixed at a mass ratio of 8:1, and 0.3% of modifier (KH-550 and KH-560 by mass ratio of 1:1) were added. The mixture was then mixed for 45 minutes at 90℃ and 1800r / min under nitrogen protection to obtain the negative electrode active material.

[0051] SBR, PAMAM, and MBA were mixed in a mass ratio of 7:1.5:1, then added to deionized water. The mixture was stirred and dissolved at 55°C and kept warm for 25 minutes to obtain a composite adhesive solution.

[0052] 92 parts of negative electrode active material and 2 parts of conductive agent (carbon nanotubes to graphene in a mass ratio of 4:1) were added to 5.7 parts of composite binder solution. The mixture was stirred at low speed for 35 min, ultrasonically treated at 400W for 18 min, and sheared and stirred at 9000 r / min for 25 min. The solid content of the slurry was adjusted to 60% and the viscosity to 4000 mPa·s to obtain the negative electrode slurry.

[0053] Copper foil with a surface tension of 40 mN / m was corona-treated. First, an 8 μm base coating was applied and pre-dried at 65°C for 12 min. Then, a surface slurry was applied to a total thickness of 100 μm. The foil was then dried in three stages at 75°C, 95°C and 85°C, with each stage lasting 18 min. Using a 45℃ heated compaction roller, compaction is carried out at a pressure gradient of 12MPa→22MPa→16MPa to control the porosity of the electrode sheets at 40%.

[0054] The electrode was placed in an argon atmosphere at 120℃ and 0.8L / min, and a vacuum of -0.06MPa was applied for 3 hours to obtain the negative electrode.

[0055] The active material and the modifier are mixed at high temperature: the high temperature condition can promote the hydrolysis and condensation reaction of the siloxane groups of the modifier, so that the modifier is uniformly coated on the surface of the active material, and the active material is fully modified. Insulation preparation of composite binder solution: The insulation process can promote the pre-crosslinking reaction of the main and auxiliary binders and crosslinking agents, forming a uniform binder precursor solution and avoiding binder agglomeration during subsequent slurry preparation; Slurry mixing and dispersion process: The active materials and conductive agents are added to the binder solution and dispersed to ensure that the components are uniformly mixed and form a stable negative electrode slurry; Crosslinking and curing under a protective atmosphere: The protective atmosphere (nitrogen / argon) can prevent the binder and modifier from being oxidized during the high-temperature crosslinking process, ensuring the smooth progress of the crosslinking reaction and forming a dense and stable composite binder network structure, ultimately ensuring the interfacial bonding strength and cycle performance of the negative electrode sheet.

[0056] In some embodiments, the preparation of the negative electrode sheet further includes the following operations: First, a negative electrode slurry is coated onto the surface of the negative electrode current collector. After initial drying, a second coating of the negative electrode slurry is applied, followed by a second drying process to obtain the negative electrode sheet.

[0057] Specifically, after the initial drying of the first coating (thin coating), a uniform base coating layer can be formed on the surface of the current collector, which enhances the interfacial bonding between the subsequent second coating slurry and the current collector, and avoids problems such as poor slurry leveling and uneven drying caused by a single thick coating. Secondary coating can achieve the target thickness of the negative electrode sheet, meeting the requirements of high-energy-density batteries for negative electrode thickness. At the same time, the two coating processes can reduce the internal stress of the active material layer and reduce the risk of cracking caused by uneven thickness during the drying process.

[0058] In some embodiments, the coating thickness of the first coating is 5~10μm, and the coating thickness of the second coating is 80~120μm of the total thickness of the negative electrode sheet; The initial drying temperature is 60~70℃, and the drying time is 10~15min; The secondary drying is a gradient drying, with drying temperatures of 70~80℃ for 15~20 min, 90~100℃ for 15~20 min, and 80~90℃ for 15~20 min.

[0059] Specifically, the first coating thickness of 5~10μm ensures a tight bond between the base coating and the current collector; the second coating thickness of 80~120μm enables high load capacity and improves battery energy density. The initial drying temperature is 60~70℃ and the time is 10~15min, which can quickly remove some of the solvent from the base coating and avoid mixing of the two layers of slurry during the second coating. The gradient drying process (70~80℃→90~100℃→80~90℃) can gradually remove the solvent in the slurry, avoiding the formation of pores and cracks in the active material layer due to excessively fast drying rate, and ensuring the density and uniformity of the active material layer.

[0060] In some embodiments, during the preparation of the negative electrode active material, the high temperature is 80~100℃, the stirring speed is 1500~2000r / min, and the stirring time is 30~60min.

[0061] Specifically, a temperature of 80~100℃ can accelerate the hydrolysis reaction of the siloxane groups of the modifier, and a high rotation speed of 1500~2000r / min can make the modifier uniformly dispersed on the surface of the active material and avoid the agglomeration of the modifier. A stirring time of 30 to 60 minutes ensures that the modifier and the active material are in full contact, achieving complete coating of the active material particles and maximizing the interfacial modification effect of the modifier.

[0062] In some embodiments, the dissolution temperature in the preparation of the composite adhesive solution is 50~60℃, and the holding time is 20~30min.

[0063] Specifically, a temperature of 50-60℃ can increase the dissolution rate of the main and auxiliary binders in water, ensuring that the binders are fully dissolved; a holding time of 20-30 minutes can allow the binder and crosslinking agent to undergo a preliminary pre-crosslinking reaction, forming a uniform binder solution, providing a stable binder precursor for subsequent slurry preparation.

[0064] In some embodiments, the preparation of the negative electrode slurry includes low-speed stirring for 30-40 minutes, ultrasonic dispersion for 15-20 minutes, ultrasonic power of 300-500W, high-speed stirring for 20-30 minutes, and stirring speed of 8000-10000r / min.

[0065] Specifically, stir at low speed for 30-40 minutes: This allows the active materials and conductive agents to be initially dispersed in the binder solution, avoiding the breakage of binder molecular chains caused by high-speed stirring; Ultrasonic dispersion for 15-20 minutes (power 300-500W): The ultrasonic cavitation effect is used to break up the agglomerates of active materials and conductive agents, achieving nanoscale dispersion; Stir at high speed for 20-30 minutes (8000-10000 r / min): This further improves the dispersion uniformity of the slurry, ensures that the conductive agent forms a continuous conductive network in the active material layer, and at the same time makes the binder uniformly coat the surface of the active material, thus improving the bonding effect.

[0066] In some embodiments, the protective atmosphere is maintained at a temperature of 2-4 hours, and the protective atmosphere includes one or more of nitrogen and argon.

[0067] Specifically, a heat preservation time of 2-4 hours can ensure that the composite binder and modifier undergo a full cross-linking reaction to form a dense and stable network structure; the nitrogen / argon protective atmosphere can isolate oxygen and prevent the binder and modifier from oxidizing and degrading at high temperatures, thus ensuring the interfacial bonding strength and electrochemical performance of the negative electrode sheet.

[0068] In some embodiments, the solid content of the negative electrode slurry is 55% to 65%, and the viscosity is 3000 to 5000 mPa·s.

[0069] Specifically, a solid content of 55% to 65% can ensure that the slurry has good leveling properties, which is convenient for coating operations, and at the same time, the compaction density of the negative electrode sheet will not decrease due to too low a solid content. A viscosity of 3000~5000 mPa.s ensures that the slurry will not sag during the coating process, forming a wet film of uniform thickness, and ultimately obtaining an active material layer with a uniform microstructure.

[0070] In some embodiments, the negative electrode slurry comprises the following components by mass: The composition includes 89-96 parts of negative electrode active material, 3-8 parts of composite binder, and 1-3 parts of conductive agent.

[0071] Specifically, the active material accounts for a relatively high proportion, ensuring the high specific capacity of the negative electrode sheet; The composite adhesive accounts for 3 to 8 parts, which can ensure the bonding performance and avoid the reduction in energy density caused by excessive adhesive. A conductive agent content of 1-3 parts can form a highly efficient conductive network, while the amount of conductive agent will not affect the loading of active materials.

[0072] Another embodiment of the present invention provides a battery including the aforementioned negative electrode sheet, or a negative electrode sheet prepared by the method for preparing the aforementioned negative electrode sheet.

[0073] Specifically, the battery includes the negative electrode sheet provided in this application. The composite binder of the negative electrode sheet is formed by cross-linking a main binder and an auxiliary binder through a cross-linking agent to form a continuous network structure. This structure can serve as a skeleton support for the active material layer, strengthening the adhesion between the active material particles. At the same time, the modifier in the negative electrode active material has both siloxane groups and reactive groups such as amine groups and epoxy groups. The siloxane groups can undergo condensation reactions with the hydroxyl groups on the surface of the active material and the current collector, realizing the anchoring of the modifier at the interface between the active material and the current collector. The reactive groups such as amine groups and epoxy groups can chemically react with the active sites in the network structure of the composite binder, tightly connecting the active material particles, the current collector, and the cross-linking network of the composite binder into one through chemical bonds. This significantly improves the interfacial bonding strength within the active material layer and between the active material layer and the current collector, effectively resisting the internal stress generated by the volume expansion and contraction of the negative electrode during charging and discharging, preventing the active material particles from falling off and the active material layer from peeling off from the current collector, and ultimately ensuring the energy density and cycle performance stability of the battery.

[0074] The present invention will be further illustrated by the following examples.

[0075] Table 1 Example 1 This embodiment illustrates the negative electrode sheet, negative electrode sheet preparation method, and battery disclosed in this invention, and includes the following operational steps: Graphite and silicon-carbon composite materials (15% silicon by mass, with a 10nm amorphous carbon layer on the surface) were mixed at a mass ratio of 8:1, and 0.3% of modifier (KH-550 and KH-560 by mass ratio of 1:1) were added. The mixture was then mixed for 45 minutes at 90℃ and 1800r / min under nitrogen protection to obtain the negative electrode active material.

[0076] SBR, PAMAM, and MBA were mixed in a mass ratio of 7:1.5:1, then added to deionized water. The mixture was stirred and dissolved at 55°C and kept warm for 25 minutes to obtain a composite adhesive solution.

[0077] 92 parts of negative electrode active material and 2 parts of conductive agent (carbon nanotubes to graphene in a mass ratio of 4:1) were added to 5.7 parts of composite binder. The mixture was stirred at low speed for 35 minutes, ultrasonically treated at 400W for 18 minutes, and sheared and stirred at 9000r / min for 25 minutes. The solid content of the slurry was adjusted to 60% and the viscosity to 4000mPa.s to obtain the negative electrode slurry.

[0078] Copper foil with a surface tension of 40 mN / m was corona-treated. First, an 8 μm base coating was applied and pre-dried at 65°C for 12 min. Then, a surface slurry was applied to a total thickness of 100 μm. The foil was then dried in three stages at 75°C, 95°C and 85°C, with each stage lasting 18 min. Using a 45℃ heated compaction roller, compaction is carried out at a pressure gradient of 12MPa→22MPa→16MPa to control the porosity of the electrode sheets at 40%.

[0079] The electrode was placed in an argon atmosphere at 120℃ and 0.8L / min, and a vacuum of -0.06MPa was applied for 3 hours to obtain the negative electrode.

[0080] The positive electrode can be any type of positive electrode conventionally used in the field, and this application does not impose any special restrictions on the positive electrode.

[0081] A battery is obtained by assembling the negative electrode, positive electrode, separator, and casing, and then injecting electrolyte.

[0082] Examples 2-8 Examples 2-8 illustrate the negative electrode sheet, negative electrode sheet preparation method, and battery disclosed in this invention, and include most of the operations in Example 1, except that: The mass ratios of graphite and silicon-carbon composite materials, main binder, auxiliary binder, and crosslinking agent, the mass ratio of KH-550 to KH-560, the mass ratio of carbon nanotubes to graphene, the percentage of negative electrode active material, the percentage of conductive agent, the percentage of composite binder, the obtained solid content (%), and the porosity (%) of the core negative electrode sheet are all recorded in Table 1.

[0083] Comparative Example 1 This comparative example is used to illustrate the negative electrode sheet, negative electrode sheet preparation method, and battery disclosed in this invention, and includes the following operations: Graphite and silicon-carbon composite materials (15% silicon by mass, with a 10nm amorphous carbon layer on the surface) were mixed at a mass ratio of 8:1, and 0.3% of modifier (KH-550 and KH-560 by mass ratio of 1:1) were added. The mixture was then mixed for 45 minutes at 90℃ and 1800r / min under nitrogen protection to obtain the negative electrode active material.

[0084] SBR and PAMAM were mixed at a mass ratio of 7:1.5, then added to deionized water, stirred and dissolved at 55°C and kept warm for 25 minutes to obtain a composite adhesive solution; that is, no crosslinking agent was added to the composite adhesive.

[0085] 92 parts of negative electrode active material and 2 parts of conductive agent (carbon nanotubes to graphene in a mass ratio of 4:1) were added to 5.7 parts of composite binder. The mixture was stirred at low speed for 35 minutes, ultrasonically treated at 400W for 18 minutes, and sheared and stirred at 9000r / min for 25 minutes. The solid content of the slurry was adjusted to 60% and the viscosity to 4000mPa.s to obtain the negative electrode slurry.

[0086] Copper foil with a surface tension of 40 mN / m was corona-treated. First, an 8 μm base coating was applied and pre-dried at 65°C for 12 min. Then, a surface slurry was applied to a total thickness of 100 μm. The foil was then dried in three stages at 75°C, 95°C and 85°C, with each stage lasting 18 min. Using a 45℃ heated compaction roller, compaction is carried out at a pressure gradient of 12MPa→22MPa→16MPa to control the porosity of the electrode sheets at 40%.

[0087] The electrode was placed in an argon atmosphere at 120℃ and 0.8L / min, and a vacuum of -0.06MPa was applied for 3 hours to obtain the negative electrode.

[0088] Comparative Example 2 This comparative example is used to illustrate the negative electrode sheet, negative electrode sheet preparation method, and battery disclosed in this invention, and includes the following operations: Graphite and silicon-carbon composite materials (15% silicon by mass, with a 10nm amorphous carbon layer on the surface) were mixed at a mass ratio of 8:1, and 0.3% of modifier (KH-550 and KH-560 by mass ratio of 1:1) were added. The mixture was then mixed for 45 minutes at 90℃ and 1800r / min under nitrogen protection to obtain the negative electrode active material.

[0089] SBR was added to deionized water, stirred and dissolved at 55°C and kept warm for 25 minutes to obtain a composite adhesive solution; that is, no PAMAM and crosslinking agent were added to the adhesive.

[0090] 92 parts of negative electrode active material and 2 parts of conductive agent (carbon nanotubes to graphene in a mass ratio of 4:1) were added to 5.7 parts of composite binder. The mixture was stirred at low speed for 35 minutes, ultrasonically treated at 400W for 18 minutes, and sheared and stirred at 9000r / min for 25 minutes. The solid content of the slurry was adjusted to 60% and the viscosity to 4000mPa.s to obtain the negative electrode slurry.

[0091] Copper foil with a surface tension of 40 mN / m was corona-treated. First, an 8 μm base coating was applied and pre-dried at 65°C for 12 min. Then, a surface slurry was applied to a total thickness of 100 μm. The foil was then dried in three stages at 75°C, 95°C and 85°C, with each stage lasting 18 min. Using a 45℃ heated compaction roller, compaction is carried out at a pressure gradient of 12MPa→22MPa→16MPa to control the porosity of the electrode sheets at 40%.

[0092] The electrode was placed in an argon atmosphere at 120℃ and 0.8L / min, and a vacuum of -0.06MPa was applied for 3 hours to obtain the negative electrode.

[0093] Comparative Example 3 This comparative example is used to illustrate the negative electrode sheet, negative electrode sheet preparation method, and battery disclosed in this invention, and includes the following operations: Graphite and silicon-carbon composite materials (15% silicon by mass, with a 10nm amorphous carbon layer on the surface) were mixed at a mass ratio of 8:1, and 0.3% of modifier (KH-550 and KH-560 by mass ratio of 1:1) were added. The mixture was then mixed for 45 minutes at 90℃ and 1800r / min under nitrogen protection to obtain the negative electrode active material.

[0094] SBR and MBA were added to deionized water, stirred and dissolved at 55°C and kept warm for 25 minutes to obtain a composite adhesive solution; that is, no PAMAM was added to the adhesive.

[0095] 92 parts of negative electrode active material and 2 parts of conductive agent (carbon nanotubes to graphene in a mass ratio of 4:1) were added to 5.7 parts of composite binder. The mixture was stirred at low speed for 35 minutes, ultrasonically treated at 400W for 18 minutes, and sheared and stirred at 9000r / min for 25 minutes. The solid content of the slurry was adjusted to 60% and the viscosity to 4000mPa.s to obtain the negative electrode slurry.

[0096] Copper foil with a surface tension of 40 mN / m was corona-treated. First, an 8 μm base coating was applied and pre-dried at 65°C for 12 min. Then, a surface slurry was applied to a total thickness of 100 μm. The foil was then dried in three stages at 75°C, 95°C and 85°C, with each stage lasting 18 min. Using a 45℃ heated compaction roller, compaction is carried out at a pressure gradient of 12MPa→22MPa→16MPa to control the porosity of the electrode sheets at 40%.

[0097] The electrode was placed in an argon atmosphere at 120℃ and 0.8L / min, and a vacuum of -0.06MPa was applied for 3 hours to obtain the negative electrode.

[0098] Performance testing The following performance tests were performed on Examples 1-8 and Comparative Examples 1-3 prepared above: Loop testing: Cyclic performance testing involved placing the aforementioned lithium-ion batteries in a 25°C environment for 8 hours, charging at 1C, discharging at 1.5C, and maintaining a voltage of 3.0~4.2V. The cycle ended when the capacity retention reached 80% SOC.

[0099] Cell expansion rate: 1) After the battery cell leaves the factory, the initial thickness H1 is the 50% SOC group at the set capacity; 2) The speed positioning H2 after the battery cell has undergone 150 cycles and is fully charged; 3) Expansion rate = [(H2-H1) / H1]*100%.

[0100] Electrode peeling force: 1) Stick the double-sided tape onto the steel plate. The steel plate dimensions are (150mm long, 25mm wide, and 1mm thick). 2) Cut the electrode sheet into 20mm*100mm pieces and attach them to the other side of the double-sided tape; 3) Place the above electrode assembly on a tensile testing machine, peel the electrode at 90°, set the parameters, start the test, and take the readings.

[0101] The test results are entered into Table 2.

[0102] Table 2 As can be seen from the test results in Table 2, the negative electrode sheets obtained in Examples 1-8 and their application in lithium-ion batteries have significantly better overall performance (peeling force, cycle life, cell expansion) than Comparative Examples 1-3. Specifically, the electrode peeling force of the embodiments all reached 20 N / m and above, with the highest being 23 N / m in Example 7, while the peeling force of Comparative Examples 1 to 3 was only 8-13 N / m. This indicates that the present invention significantly improves the interfacial bonding strength between the active material layer and the current collector through the network structure formed by the main binder, auxiliary binder and crosslinking agent, as well as the chemical bonding effect of the modifier. Cyclic performance tests show that all examples can reach more than 390 cycles, with Example 7 reaching 450 cycles, which is much higher than the 150-220 cycles of Comparative Examples 1-3. This demonstrates that the negative electrode can effectively resist the internal stress generated by the volume expansion of the silicon-carbon composite material during charging and discharging, and avoid the active material from falling off and delaminating. The cell expansion rate test showed that the expansion rate of the examples was at a low level of 15.4% to 16.0%, while the expansion rate of the comparative examples was as high as 22% to 25%. This further confirms that the composite bonding system of the negative electrode sheet provided by the present invention can effectively suppress the volume expansion during battery cycling and ensure the stability of the battery structure. Furthermore, Examples 7 (KH-550 to KH-560 mass ratio 1:1.2) and 8 (carbon nanotube to graphene mass ratio 3:1) exhibited the best overall performance, with higher peel strength, cycle life, and expansion rate than other examples. This indicates that by adjusting the ratio of modifier compounding and conductive agent mixing, the overall performance of the negative electrode sheet can be further optimized.

[0103] Comparing the test results of the above embodiments and comparative examples, it can be seen that the negative electrode sheet provided in this application has a composite binder formed by cross-linking a main binder and an auxiliary binder through a cross-linking agent to form a continuous network structure. This structure can serve as a skeleton support for the active material layer, strengthening the adhesion between active material particles. At the same time, the modifier in the negative electrode active material has both siloxane groups and reactive groups such as amine groups and epoxy groups. The siloxane groups can undergo condensation reactions with the hydroxyl groups on the surface of the active material and the current collector, realizing the anchoring of the modifier at the interface between the active material and the current collector. The reactive groups such as amine groups and epoxy groups can chemically react with the active sites in the network structure of the composite binder, tightly connecting the cross-linking network of the active material particles, the current collector, and the composite binder into one through chemical bonds. This significantly improves the interfacial bonding strength within the active material layer and between the active material layer and the current collector, effectively resisting the internal stress generated by the volume expansion and contraction of the negative electrode during charging and discharging, preventing the active material particles from falling off and the active material layer from peeling off from the current collector, and ultimately ensuring the energy density and cycle performance stability of the battery.

[0104] 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 sheet, characterized in that, It includes a current collector and a negative electrode active material layer. The negative electrode active material layer includes a negative electrode active material, a composite binder, and a conductive agent. The composite binder includes a main binder, an auxiliary binder, and a crosslinking agent. The main binder and the auxiliary binder are crosslinked with each other through the crosslinking agent to form a network structure. The negative electrode active material includes a modifier and an active material. The modifier has siloxane groups and reactive groups for reacting and connecting with the composite binder. The reactive groups include one or more of amine groups and epoxy groups.

2. The negative electrode sheet according to claim 1, characterized in that, The mass ratio of the main adhesive, auxiliary adhesive, and crosslinking agent is (6~8):(1~2):(0.5~1.5).

3. The negative electrode sheet according to claim 1, characterized in that, The primary adhesive comprises one or more of styrene-butadiene rubber and polyvinylidene fluoride; and / or The auxiliary adhesive includes PAMAM; and / or, The crosslinking agent includes one or more of GMA and MBA.

4. The negative electrode sheet according to claim 1, characterized in that, The modifier includes a compound of γ-aminopropyltriethoxysilane and γ-glycidoxypropyltrimethoxysilane; The mass ratio of the compounded γ-aminopropyltriethoxysilane to γ-glycidoxypropyltrimethoxysilane is 1:(0.8~1.2).

5. The negative electrode sheet according to claim 1, characterized in that, The active material also includes a mixture of graphite and silicon-carbon composite material, wherein the mass ratio of graphite to silicon-carbon composite material is (7~9):

1.

6. The negative electrode sheet according to claim 5, characterized in that, The surface of the silicon-carbon composite material is coated with amorphous carbon, with a coating thickness of 5~15nm.

7. The negative electrode sheet according to claim 1, characterized in that, The conductive agent includes a compound of carbon nanotubes and graphene, wherein the mass ratio of carbon nanotubes to graphene is (3-5):

1. The carbon nanotubes have a diameter of 8~15nm and a length of 1~5μm; The graphene has 3 to 10 layers.

8. The negative electrode sheet according to claim 1, characterized in that, The porosity of the negative electrode is 35%~45%.

9. The method for preparing the negative electrode sheet according to any one of claims 1 to 7, characterized in that, Includes the following operations: The active material and the modifier are stirred and mixed at high temperature to obtain the negative electrode active material. The adhesive, auxiliary adhesive, and crosslinking agent are dissolved in water and kept at a certain temperature to obtain a composite adhesive solution. The negative electrode active material and conductive agent are added to the composite binder solution and stirred to disperse evenly to obtain the negative electrode slurry; The negative electrode slurry is coated onto the surface of the current collector, dried and rolled, then kept at a temperature in a protective atmosphere. After cross-linking and curing, the negative electrode sheet is obtained.

10. The method for preparing the negative electrode sheet according to claim 9, characterized in that, The preparation of the negative electrode also includes the following operations: First, a negative electrode slurry is coated onto the surface of the negative electrode current collector. After initial drying, a second coating of the negative electrode slurry is applied, followed by a second drying process to obtain the negative electrode sheet.

11. The method for preparing the negative electrode sheet according to claim 10, characterized in that, The thickness of the first coating is 5~10μm, and the thickness of the second coating is 80~120μm of the total thickness of the negative electrode sheet; The initial drying temperature is 60~70℃, and the drying time is 10~15min; The secondary drying is a gradient drying process, with drying temperatures of 70~80℃ for 15~20 min, 90~100℃ for 15~20 min, and 80-90℃ for 15~20 min.

12. The method for preparing the negative electrode sheet according to claim 9, characterized in that, In the preparation of the negative electrode active material, the high temperature is 80~100℃, the stirring speed is 1500~2000r / min, and the stirring time is 30~60min.

13. The method for preparing the negative electrode sheet according to claim 9, characterized in that, In the preparation of the composite adhesive solution, the dissolution temperature is 50~60℃ and the holding time is 20~30min.

14. The method for preparing the negative electrode sheet according to claim 9, characterized in that, In the preparation of the negative electrode slurry, the stirring time is 30-40 min at low speed, 15-20 min at ultrasonic dispersion, the ultrasonic power is 300-500 W, the stirring time is 20-30 min at high speed, and the stirring speed is 8000-10000 r / min.

15. The method for preparing the negative electrode sheet according to claim 9, characterized in that, The protective atmosphere is maintained at a temperature of 2-4 hours, and the protective atmosphere includes one or more of nitrogen and argon.

16. The method for preparing the negative electrode sheet according to claim 9, characterized in that, The negative electrode slurry has a solid content of 55% to 65% and a viscosity of 3000 to 5000 mPa·s.

17. The method for preparing the negative electrode sheet according to claim 9, characterized in that, The negative electrode slurry comprises the following components by mass: The composition includes 89-96 parts of negative electrode active material, 3-8 parts of composite binder, and 1-3 parts of conductive agent.

18. A battery, characterized in that, Includes the negative electrode sheet according to any one of claims 1 to 8, or the negative electrode sheet prepared by the method for preparing the negative electrode sheet according to any one of claims 9 to 17.

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