Negative plate and preparation method thereof, battery, battery pack and electric equipment

Abstract

The invention provides a negative plate and a preparation method thereof, a battery, a battery pack and electric equipment, the negative plate comprises a current collector, and a stress buffer layer and an active layer which are sequentially arranged on at least one functional surface of the current collector, the stress buffer layer comprises graphite, an elastic polymer, a conductive agent and a binder; the active layer comprises a silicon-based material and a graphite material. As the negative plate comprises the stress buffer layer, the stress distribution of the silicon negative electrode in the charging and discharging process can be effectively dispersed, so that the expansion rate of the negative plate is remarkably reduced, and the cycle service life of the negative plate is prolonged.

Description

Technical Field

[0001] The present invention relates to the technical field of batteries, and in particular to a negative electrode sheet and a preparation method thereof, a battery, a battery pack, and an electrical device. Background Art

[0002] Traditional graphite negative electrode materials have a low theoretical capacity, which limits the improvement of the energy density of lithium-ion batteries. Silicon-carbon composite materials are considered to be strong candidates for the next generation of high-capacity negative electrode materials due to their higher theoretical capacity and good conductivity.

[0003] However, when lithium ions are embedded in and extracted from silicon particles, they cause a huge expansion of the silicon volume, leading to stress within the electrode, resulting in the fragmentation of silicon particles, loss of electrical contact, and the continuous breakdown and reformation of the solid electrolyte interface (SEI). This leads to rapid decay of the electrode capacity and difficulty maintaining cycling stability. Therefore, how to design a silicon anode structure that can effectively suppress volume expansion and improve cycling stability is one of the current focuses of lithium-ion battery research. Summary of the Invention

[0004] The present invention provides a negative electrode sheet, which includes a stress buffer layer. The layer can effectively disperse the stress distribution of the silicon negative electrode during the charge and discharge process, thereby significantly reducing the expansion rate of the negative electrode sheet and improving its cycle service life.

[0005] The present invention also provides a method for preparing the above-mentioned negative electrode sheet, which can be used to prepare the above-mentioned negative electrode sheet, is simple to operate, and is energy-saving and environmentally friendly.

[0006] The present invention also provides a battery comprising the negative electrode sheet, which has high energy density and long cycle life.

[0007] The present invention also provides a battery pack comprising the battery, which has the advantages of high energy density and long cycle life.

[0008] The present invention also provides an electrical device. Since the electrical device includes the battery or battery pack, the service life of the electrical device is relatively long.

[0009] In detail, in the first aspect, the present invention provides a negative electrode sheet, comprising a current collector and a stress buffer layer and an active layer sequentially arranged on at least one functional surface of the current collector, wherein the stress buffer layer comprises graphite, an elastic polymer and a binder; the active layer comprises a silicon-based material and a graphite material.

[0010] Furthermore, the elastic polymer includes at least one of polyetheresteramide elastomer, polyetherimide elastomer, polystyrene elastomer, polyurethane elastomer, polyamide elastomer, and styrene-butadiene rubber.

[0011] Furthermore, the binder contains an R group, and the R group is bonded to the current collector through a chemical bond.

[0012] Furthermore, the R group is at least one of a thiocarbonyl group, a thioester group, a sulfhydryl group, and an amino group;

[0013] The binder containing R groups is formed by cross-linking a first raw material and a second raw material; the first raw material includes a polyacrylic acid polymer; and the second raw material includes the R groups.

[0014] Furthermore, the second raw material includes at least one of thiourea, 2-mercaptobenzothiazole, 2-mercaptopyridine, and 2-mercaptoethanesulfonic acid;

[0015] And / or, the mass ratio of the first raw material to the second raw material is 1.5-3:1;

[0016] and / or, the molecular weight of the polyacrylic acid polymer is 400,000 g / mol-800,000 g / mol;

[0017] And / or, the first raw material includes a polyacrylic acid polymer having a molecular weight of 200,000 g / mol-400,000 g / mol and a polyacrylic acid polymer having a molecular weight of 600,000 g / mol-800,000 g / mol.

[0018] Furthermore, in the stress buffer layer, the graphite is secondary particles of graphite.

[0019] Furthermore, in the stress buffer layer, the mass ratio of the graphite, the binder and the elastic polymer is 75-85:5-8:8-12;

[0020] And / or, the stress buffer layer further comprises a dispersant and a conductive agent, and the mass ratio of the binder, the dispersant and the conductive agent is 5-8:1-3:1-5.

[0021] Furthermore, in the active layer, the silicon-based material is selected from at least one of pure silicon, silicon carbon, silicon oxygen, and silicon alloy; and / or the graphite material is secondary particles of graphite.

[0022] Furthermore, the silicon carbon includes one or more of: carbon-coated nano- or micron silicon materials, nano- or micron SiOx and carbon composite materials, and nano- or micron silicon carbon wire materials, wherein 0 <x≤2;

[0023] And / or, in the silicon carbon, the mass proportion of silicon is 10 wt% to 50 wt%.

[0024] Furthermore, in the active layer, the D50 of the secondary graphite particles is 12 μm-15 μm;

[0025] And / or, the silicon carbon is carbon-coated nano-silicon, the particle size of the nano-silicon is 50nm to 200nm, and the thickness of the carbon coating layer is 2nm to 5nm;

[0026] And / or, the mass ratio of the carbon-coated nano-silicon and graphite secondary particles is 1:4-1.

[0027] Furthermore, the thickness of the stress buffer layer is 30 μm to 60 μm, and the single-side density is 40 mg / cm 3 -60mg / cm 3 ;

[0028] And / or, the thickness of the active layer is 30 μm to 60 μm, and the single-surface density of the active layer is 40 mg / cm 3 -60mg / cm 3 .

[0029] In a second aspect, the present invention provides a method for preparing a negative electrode sheet, comprising the following steps:

[0030] A first slurry containing graphite, a binder, an elastic polymer, and a conductive agent is coated on at least one functional surface of the current collector and heat-treated at 100° C.-140° C.; a second slurry containing a silicon-based material and a graphite material is then coated, dried and rolled to obtain the negative electrode sheet.

[0031] In a third aspect, the present invention provides a battery comprising the negative electrode sheet described in the first aspect.

[0032] In a fourth aspect, the present invention provides a battery pack comprising the battery described in the third aspect.

[0033] In a fifth aspect, the present invention provides an electrical device comprising the battery described in the third aspect or the battery pack described in the fourth aspect.

[0034] The negative electrode sheet provided by the present invention can effectively disperse the stress distribution of the silicon negative electrode during the charge and discharge process by arranging a stress buffer layer on the lower surface of the active layer, thereby significantly reducing the expansion rate of the negative electrode sheet and improving its cycle life. BRIEF DESCRIPTION OF THE DRAWINGS

[0035] To more clearly illustrate the technical solutions in the embodiments of the present invention or related technologies, the following briefly introduces the drawings required for use in the embodiments of the present invention or related technologies. Obviously, the drawings described below are only some embodiments of the present invention. Those skilled in the art can also derive other drawings based on these drawings without inventive effort.

[0036] Figure 1 A negative electrode sheet according to a specific embodiment of the present invention is

[0037] In the figure, 1-current collector; 2-stress buffer layer; 3-active layer. DETAILED DESCRIPTION

[0038] The following will clearly and completely describe the technical solutions in the embodiments of the present invention in conjunction with the accompanying drawings. Obviously, the described embodiments are only part of the embodiments of the present invention, not all of the embodiments. Based on the embodiments of the present invention, all other embodiments obtained by ordinary technicians in this field without making creative efforts are within the scope of protection of the present invention.

[0039] In the first aspect, the present invention provides a negative electrode sheet, Figure 1 , comprising a current collector 1 and a stress buffer layer 2 and an active layer 3 sequentially arranged on at least one functional surface of the current collector, wherein the stress buffer layer comprises graphite, an elastic polymer and a binder; the active layer comprises a silicon-based material and a graphite material.

[0040] The present invention can effectively disperse the stress distribution of the silicon negative electrode during the charge and discharge process by arranging a stress buffer layer on the surface of the negative electrode sheet, thereby significantly reducing the expansion rate of the negative electrode sheet and improving its cycle life. In detail, the silicon particles undergo significant volume changes during the charge and discharge process. This volume expansion and contraction can lead to poor contact between the silicon particles and the current collector, and even cause the silicon particles to break and fall off. The stress buffer layer has a certain elasticity and flexibility, and can effectively absorb and relieve the stress caused by the volume change of the silicon particles during the charge and discharge process, thereby reducing the mechanical stress inside the silicon negative electrode and alleviating the volume change of the pole piece caused by the expansion and contraction of the silicon particles during the charge and discharge process, thereby improving the cycle stability of the battery. In addition, the graphite of the stress buffer layer can not only construct a conductive network and improve the conductivity of the stress buffer layer, but also provide a certain adaptation space for the silicon particles in the active layer when they expand, thereby further dispersing the stress distribution of the silicon negative electrode during the charge and discharge process.

[0041] The elastic polymer refers to a polymer that deforms when subjected to an external force and quickly returns to its original shape and size after the external force is removed. In an optional embodiment, the elastic polymer includes at least one of polyetheresteramide elastomer, polyetherimide elastomer, polystyrene elastomer, polyurethane elastomer, polyamide elastomer, and styrene-butadiene rubber.

[0042] In an optional embodiment, the binder contains an R group, and the R group is bonded to the current collector through a chemical bond.

[0043] In the above embodiment, the R group is bonded to the current collector through a chemical bond, and the binder can also be bonded to the current collector through van der Waals force due to its own adhesion. Therefore, there are both chemical bonds (for example, metal-S bonds, metal-N bonds, etc.) and van der Waals forces between the stress buffer layer and the current collector. Compared with ordinary binders that are only bonded to the current collector through van der Waals forces, chemical bonds have higher bonding strength, which makes the peeling strength between the negative electrode sheet and the current collector higher and more stable during the cycle process, thereby achieving the purpose of improving the cycle life of the negative electrode sheet without affecting the capacity.

[0044] For example, when the current collector includes Cu, the chemical bonds include, but are not limited to, Cu-S bonds, Cu-N bonds, etc. In some embodiments, the chemical bonds can be detected by conventional means such as XPS.

[0045] In an alternative embodiment, the R group is at least one of a thiocarbonyl group, a thioester group, a sulfhydryl group, and an amino group;

[0046] And / or, the binder containing R groups is formed by cross-linking a first raw material and a second raw material; the first raw material includes a polyacrylic acid polymer; and the second raw material includes the R groups.

[0047] Among them, polyacrylic acid polymers are rich in carboxyl groups, which can undergo a cross-linking reaction with at least part of the R groups, thereby tightly combining the first raw material and the second raw material into an R-containing binder, which in turn helps to tightly combine the stress buffer layer and the current collector, as well as the stress buffer components, thereby making the negative electrode sheet more stable during the cycle.

[0048] Illustratively, the polyacrylic acid polymer includes at least one of polyacrylic acid, polyacrylonitrile-polyacrylic acid, polyvinyl alcohol grafted modified polyacrylic acid, glycine amide grafted modified polyacrylic acid, alkali lignin grafted modified polyacrylic acid, and ureido pyrimidone grafted modified polyacrylic acid.

[0049] In an optional embodiment, the second raw material includes at least one of thiourea, 2-mercaptobenzothiazole, 2-mercaptopyridine, and 2-mercaptoethanesulfonic acid;

[0050] And / or, the mass ratio of the first raw material to the second raw material is 1.5-3:1;

[0051] And / or, the molecular weight of the polyacrylic acid polymer is 400,000 g / mol-800,000 g / mol.

[0052] The mass ratio of the first raw material to the second raw material within the above-described embodiment can further improve the peel strength between the negative electrode sheet and the current collector. If the mass of the first raw material is too high relative to the first raw material, the chemical bond density between the first transition layer and the current collector can be low, resulting in a minimal improvement in the peel strength between the negative electrode sheet and the current collector. If the mass of the first raw material is too low relative to the first raw material, the first transition layer has insufficient adhesion, similarly resulting in a minimal improvement in the peel strength between the negative electrode sheet and the current collector. Furthermore, the molecular weight of the polyacrylic acid polymer is within the range of 400,000 g / mol to 800,000 g / mol, further ensuring a high level of adhesion of the first transition layer itself.

[0053] Illustratively, the molecular weight of the polyacrylic acid polymer is any value in the range of 400,000 g / mol-500,000 g / mol, 600,000 g / mol-700,000 g / mol, 50-800,000 g / mol, etc.

[0054] In an alternative embodiment, the first raw material includes a polyacrylic acid polymer having a molecular weight of 200,000 g / mol-400,000 g / mol and a polyacrylic acid polymer having a molecular weight of 600,000 g / mol-800,000 g / mol. The combination of the high-molecular-weight polyacrylic acid polymer and the low-molecular-weight polyacrylic acid polymer can simultaneously improve the peel strength of the adhesive and adjust the viscosity of the slurry during preparation to meet process requirements.

[0055] In an optional embodiment, in the stress buffer layer, the graphite is secondary particles of graphite.

[0056] Secondary graphite particles are larger particles formed by the aggregation of many primary particles. By limiting graphite to secondary graphite particles, the above embodiment can further provide a buffer for silicon expansion, absorbing the volume expansion of silicon during the charge and discharge process, thereby reducing the overall expansion effect of the electrode.

[0057] In an optional embodiment, in the stress buffer layer, the mass ratio of the graphite, the binder and the elastic polymer is 75-85:5-8:8-12.

[0058] Among them, the graphite within the above-mentioned proportion range can enable the stress buffer layer to form a good conductive network, which helps to improve the overall conductivity of the negative electrode sheet and improve the rate performance of the battery. The elastic polymer within the above-mentioned proportion range can provide sufficient buffer space for silicon expansion without causing negative impact on the energy density of the negative electrode sheet. If the relative mass of the elastic polymer is too high, it will affect the integrity and density of the overall conductive network of the negative electrode sheet, thereby affecting the energy density of the negative electrode sheet. If the relative mass of the elastic polymer is too low, the stress buffer layer will provide insufficient buffer space for the silicon expansion of the active layer, which is not conducive to effectively reducing the expansion effect of the negative electrode sheet and affects the degree of improvement of the battery's cycle life.

[0059] In an optional embodiment, the stress buffer layer further includes a dispersant and a conductive agent, and the mass ratio of the binder, the dispersant and the conductive agent is 5-8:1-3:1-5.

[0060] The conductive agent includes but is not limited to: one or more of carbon black, carbon tubes, and graphene.

[0061] In an optional embodiment, in the active layer, the silicon-based material is selected from at least one of pure silicon, silicon carbon, silicon oxygen, and silicon alloy; and / or the graphite material is secondary particles of graphite.

[0062] In order to further improve the gram capacity of the negative electrode sheet, in a specific embodiment, the silicon carbon includes: one or more of carbon-coated nano- or micro-silicon materials, nano- or micro-SiOx and carbon composite materials, and nano- or micro-silicon carbon wire materials, wherein 0 <x≤2;

[0063] And / or, in the silicon carbon, the mass proportion of silicon is 10% to 50%.

[0064] Exemplarily, the mass proportion of silicon in the silicon carbon is 12%, 15%, 17%, 20%, 25%, 27%, 30%, 32%, 35%, 37%, 40%, 42%, 45%, 47%, 50%, etc.

[0065] In an optional embodiment, the D50 of the secondary particles of graphite is 12 μm-15 μm;

[0066] And / or, the silicon carbon is carbon-coated nano-silicon, the particle size of the nano-silicon is 50nm to 200nm, and the thickness of the carbon coating layer is 2nm to 5nm;

[0067] And / or, the mass ratio of the carbon-coated nano-silicon and graphite secondary particles is 1:4-1.

[0068] Silicon carbon is carbon-coated nano-silicon, which can further alleviate the problem of volume expansion of silicon during charging and discharging. The carbon coating layer has good conductivity and mechanical stability, can form a stable conductive network, and improve the conductivity of the composite material. At the same time, the carbon coating layer can protect the nano-silicon and reduce the direct contact between the electrolyte and the nano-silicon, thereby reducing side reactions and the formation of solid electrolyte interface (SEI) film, and improving cycle stability.

[0069] The above embodiment can further reduce the volume expansion effect of silicon by limiting the D50 of the secondary particles of graphite, the particle size of nano-silicon and the thickness of the carbon coating layer, thereby further improving the cycle life of the negative electrode sheet while taking into account the energy density of the negative electrode sheet. The reason (taking into account both cycle life and energy density) is that: on the one hand, the particle size of nano-silicon is controlled in the range of 50nm to 200nm, which can reduce the volume expansion of silicon particles during the charge and discharge process, thereby reducing damage to the negative electrode structure and improving the cycle stability of the battery; and a carbon coating layer with a thickness of 2nm to 5nm is formed outside the nano-silicon particles, which not only improves the conductivity of the silicon particles, but also provides a buffering effect when the silicon particles expand, reducing damage to the negative electrode sheet structure. In addition, the carbon coating helps to form a stable SEI film, reducing the continuous consumption of electrolyte, thereby improving the cycle life of the battery. The composite use of carbon-coated nano-silicon (Si / C-NP) and artificial graphite secondary particles can balance the high capacity of silicon-based materials and the high stability of graphite. While maintaining the high energy density of the negative electrode sheet, the stability of graphite is used to buffer the volume change of silicon particles and reduce the capacity decay during the battery cycle. On the other hand, the mass proportion of silicon in the silicon-carbon is 10wt% to 50wt%. The appropriate silicon content can balance the energy density and cycle stability of the battery.

[0070] In the present invention, D50 represents the particle size value corresponding to 50% (by volume) of the cumulative amount in the particle size cumulative distribution curve, generally determined by testing with a laser diffraction particle size distribution analyzer. For example, the D50 of the secondary graphite particles is 12 μm, 13 μm, 14 μm, 15 μm, etc.; the particle size of the nano-silicon is 60 nm, 70 nm, 80 nm, 90 nm, 100 nm, 110 nm, 120 nm, 130 nm, 140 nm, 150 nm, 160 nm, 170 nm, 180 nm, 190 nm, etc.; and the thickness of the carbon coating layer is 2 nm, 3 nm, 4 nm, 5 nm, etc.

[0071] In an optional embodiment, the thickness of the stress buffer layer is 30 μm to 60 μm, and the single-side density is 40 mg / cm 3 -60mg / cm 3 ;

[0072] And / or, the thickness of the active layer is 30 μm to 60 μm, and the single-surface density of the active layer is 40 mg / cm 3 -60mg / cm 3 .

[0073] The thickness and surface density of the stress buffer layer within the above range can effectively reduce the expansion rate of the negative electrode sheet and ensure the volatilization of the energy density of the negative electrode sheet. If the thickness of the stress buffer layer is less than 30μm or the single-sided surface density is less than 40mg / cm 3 , it cannot effectively disperse the stress distribution of the silicon negative electrode during the charge and discharge process. If the thickness of the stress buffer layer is higher than 60μm and the single-sided surface density is higher than 60mg / cm 3 , which will affect the energy density of the negative electrode sheet. The thickness and surface density of the active layer within the above range can further ensure the balance between the gram capacity of the negative electrode sheet and the expansion rate of the negative electrode sheet. If the thickness and surface density are higher, although the gram capacity of the negative electrode sheet is higher, it will also lead to a greater expansion rate of the negative electrode sheet.

[0074] Exemplarily, the thickness of the stress buffer layer is any value among 32 μm, 35 μm, 37 μm, 40 μm, 42 μm, 45 μm, 47 μm, 50 μm, 52 μm, 55 μm, 57 μm, 60 μm, etc., or a range consisting of any two thereof; and / or, and / or, the thickness of the active layer is any value among 32 μm, 35 μm, 37 μm, 40 μm, 42 μm, 45 μm, 47 μm, 50 μm, 52 μm, 55 μm, 57 μm, 60 μm, etc., or a range consisting of any two thereof; the single-sided surface density of the stress buffer layer is 42 mg / cm 3 , 45mg / cm 3 , 47mg / cm 3 , 50mg / cm 3 , 52mg / cm 3 、55mg / cm 3 , 57mg / cm 3 Any value or any range between the two; the single surface density of the active layer is 40 mg / cm 3 , 45mg / cm 3 , 47mg / cm 3 , 50mg / cm 3 , 52mg / cm 3 , 55mg / cm 3 , 57mg / cm 3 Any value or any range between them.

[0075] It can be understood that the above-mentioned active layer also includes a binder, and the binder can be selected from at least one of carboxymethyl cellulose, styrene-butadiene rubber, polyvinyl chloride, carboxylated polyvinyl chloride, polyvinyl fluoride, polymers containing ethylene oxide, polyvinyl pyrrolidone, polyurethane, polytetrafluoroethylene, polyvinylidene fluoride, polyethylene, polypropylene, polyamide-imide, polyvinyl alcohol, and sodium polyacrylate; preferably, at least one of polyurethane, polyvinylidene fluoride, polyacrylonitrile, polytetrafluoroethylene, polyvinyl alcohol, polyacrylic acid, polystyrene sulfonate, sodium carboxymethylate, epoxy resin and vinyl acetate.

[0076] The material of the current collector is not specifically limited in the present invention. For example, the current collector may be selected from any one or more of copper foil, titanium foil, tin foil, chromium foil, and composite foils of the above metals.

[0077] In order to improve the energy density of the battery, preferably, the first transition layer, the second transition layer and the active layer are sequentially arranged on the two functional surfaces of the current collector.

[0078] In a second aspect, the present invention provides a method for preparing a negative electrode sheet, comprising the following steps:

[0079] A first slurry containing graphite, a binder, an elastic polymer, and a conductive agent is coated on at least one functional surface of the current collector and heat-treated at 100° C.-140° C.; a second slurry containing a silicon-based material and a graphite material is then coated, dried and rolled to obtain the negative electrode sheet.

[0080] The above preparation method first applies the first slurry and heat treats it at 100℃-140℃ to ensure that more metal-S bonds and / or metal-N bonds are formed between the binder and the current collector. The second slurry is then applied to the functional surface of the current collector and then dried at the same time. This can strengthen the adhesion between the stress buffer layer and the active layer, make the particles contact more closely, enhance the peeling force, and reduce the solid-phase diffusion impedance of the electrode.

[0081] The present invention does not particularly limit the heat treatment time, and it can be any value within the range of 0.5-2 hours.

[0082] The first slurry and the second slurry further include a solvent. The present invention does not specifically limit the type of the solvent. Any known solvent with a dispersing effect can be selected, and N-methylpyrrolidone or water is generally selected.

[0083] In a third aspect, the present invention provides a battery comprising the negative electrode sheet described in the first aspect.

[0084] In a fourth aspect, the present invention provides a battery pack comprising the battery described in the third aspect.

[0085] In addition, it can be understood that the battery of the present invention includes the negative electrode sheet, the positive electrode sheet, the separator and the electrolyte.

[0086] The positive electrode sheet includes a current collector and a positive electrode coating provided on at least one functional surface of the current collector, wherein the positive electrode coating includes a positive electrode active material, a binder, and a conductive agent. The present invention does not specifically limit the positive electrode active material. For example, the positive electrode active material can be selected from one or more commonly used positive electrode active materials in the art, such as lithium cobalt oxide, lithium manganese oxide, lithium nickel manganese oxide, lithium nickel cobalt manganese oxide, lithium nickel cobalt aluminum oxide, lithium manganese phosphate, and lithium iron phosphate. Lithium manganese oxide can be selected from LiMnO2 and LiMn2O4; lithium nickel cobalt manganese oxide material can be selected from LiNi 1 / 3 Co 1 / 3 Mn 1 / 3 O2、LiNi 0.5 Co 0.2 Mn 0.3 O2、LiNi 0.6 Co 0.2 Mn 0.2 O2、LiNi 0.8 Co 0.1 Mn 0.1 O2 one or more; the binder can be selected from at least one of carboxymethyl cellulose, styrene-butadiene rubber, polyvinyl chloride, carboxylated polyvinyl chloride, polyvinyl fluoride, polymers containing ethylene oxide, polyvinyl pyrrolidone, polyurethane, polytetrafluoroethylene, polyvinylidene fluoride, polyethylene, polypropylene, polyamide-imide, polyvinyl alcohol, and sodium polyacrylate; preferably at least one of polyurethane, polyvinylidene fluoride, polyacrylonitrile, polytetrafluoroethylene, polyvinyl alcohol, polyacrylic acid, polystyrene sulfonate, sodium carboxymethylate, epoxy resin and vinyl acetate; the conductive agent can be selected from one or more of carbon black, graphite, carbon tubes, and graphene.

[0087] The battery of the present invention can be manufactured according to conventional methods in the field. For example, the positive electrode sheet, the separator, and the negative electrode sheet can be stacked in sequence, and then assembled into a battery cell through a winding process or a stacking process. Then, after packaging and baking, the electrolyte is injected, and the battery is manufactured after processes such as hot pressing.

[0088] The present invention does not impose any particular limitation on the aforementioned separator. Any known porous separator with electrochemical and chemical stability may be selected. For example, it may be at least one of glass fiber, non-woven fabric, polyethylene, polypropylene, or polyvinylidene fluoride. The separator may be a single layer or multiple layers.

[0089] The present invention does not particularly limit the above-mentioned electrolyte. For example, an electrolyte comprising an organic solvent and an electrolyte salt may be selected. The organic solvent, as a medium for transporting ions in the electrochemical reaction, may be an organic solvent known in the art for battery electrolytes, such as one or more of fluorocarbonates, fluorocarboxylates, non-fluorocarbonates, fluorocarbonates, non-fluorocarboxylates, fluorocarboxylates, fluoroethers, non-fluoroethers, and tetrahydrofuran. The electrolyte salt, as a source of ions, may be an electrolyte salt known in the art for battery electrolytes, such as one or more of lithium hexafluorophosphate, bistrifluoromethylsulfonyl imide, and lithium bis(fluorosulfonyl)imide.

[0090] In a fifth aspect, the present invention provides an electrical device comprising the battery described in the third aspect or the battery pack described in the fourth aspect.

[0091] It should be noted that the above-mentioned electrical equipment can be any conventional equipment that requires electricity, such as but not limited to computers, electric cars, air conditioners, refrigerators, washing machines, microwave ovens, printers, fax machines, etc.

[0092] The technical solutions of the present invention are further illustrated below with reference to specific examples. All parts, percentages, and ratios described in the following examples are based on weight. All reagents used in the examples are commercially available or synthesized according to conventional methods and can be used directly without further treatment. The instruments used in the examples are commercially available.

[0093] The following tests involve some raw material sources and / or parameters:

[0094] PAA binder specifically for silicon-based negative electrodes: BOBSive 290S3, provided by Blue Ocean Black Stone, with a solid content of 10±0.3% and a viscosity ≥15000mPa.S (12rpm / 25°C).

[0095] Carbon-coated nanosilicon: SL650B1-SC, provided by Tianmu Pioneer Battery Materials Technology Co., Ltd.

[0096] Secondary particles of artificial graphite: FSN-1, produced by Shanghai Shanshan Technology Co., Ltd.

[0097] Polyetheresteramide elastomer: Pebax, produced by Arkema, with an average molecular weight (weight average molecular weight (Mw)) of about 192,000 g / mol.

[0098] Polystyrene elastomer: Sigma-Aldrich-182427 was provided by Sigma-Aldrich, and the average molecular weight (weight average molecular weight (Mw)) was about 280,000 g / mol.

[0099] 2-Mercaptoethanesulfonic acid: CAS number: 3375-50-6, Tokyo Chemical Industry (Shanghai) Co., Ltd.

[0100] Glycine amide grafted modified polyacrylic acid: S59106, provided by Yuanye brand, with an average molecular weight (weight average molecular weight (Mw)) of 450,000 g / mol.

[0101] Styrene-butadiene rubber (SBR): SBR 1502, produced by Zeon Co., Ltd. of Japan.

[0102] Example 1

[0103] This example provides a negative electrode sheet, combined with Figure 1 , comprising a current collector 1 and a stress buffer layer 2 and an active layer 3 sequentially arranged on at least one functional surface of the current collector, wherein the stress buffer layer comprises graphite, an elastic polymer, a conductive agent and a binder; the active layer comprises a silicon-based material and a graphite material.

[0104] The stress buffer layer includes secondary particles of artificial graphite in a mass ratio of 80:6:2:10:2, an R-group-containing binder (formed by polyacrylonitrile-polyacrylic acid copolymer and thiourea, with the R groups being amino and thiocarbonyl), sodium carboxymethyl cellulose, polyether ester amide elastomer (PEEA), and acetylene black (AB);

[0105] The active layer includes carbon-coated nano-silicon, secondary particles of artificial graphite and PAA binder for silicon-based negative electrode in a mass ratio of 10:80:10, wherein the thickness of the carbon coating layer is 5nm, the diameter of the nano-silicon particles is 80nm, and the D 50 =13.0±1.0μm, the particle size of acetylene black is 30nm;

[0106] The thickness of the stress buffer layer and active layer are 50 μm and 50 μm respectively, and the single-side density is 60 mg / cm 3 、60mg / cm 3 The molecular weight of polyacrylonitrile-polyacrylic acid copolymer is 400,000 g / mol, and the single-surface density of the negative electrode is 120 g / m 2 ;Negative electrode compaction density 1.7g / cm 3 .

[0107] The preparation method thereof comprises the following steps:

[0108] 1) Pretreatment of copper foil: First, pretreat the copper foil with oxalic acid solution and dry it thoroughly;

[0109] 2) Preparation of an R-group-containing binder: 30 g of polyacrylonitrile-polyacrylic acid copolymer and 10 g of thiourea were stirred and dissolved in 200 g of deionized water to obtain a uniform solution, which was then vacuum-dried at 40° C. to obtain an R-group-containing binder.

[0110] 3) weighing secondary particles of artificial graphite, an R-group-containing binder, sodium carboxymethyl cellulose, a polyether ester amide elastomer, and acetylene black, and dispersing them in pure water. After stirring, the mixture is uniformly mixed to obtain a first slurry. The first slurry is uniformly coated on a 6 μm thick copper foil using a coater, and heated at 100° C. for 1 h to obtain a stress buffer layer with a thickness of approximately 50 μm.

[0111] 4) Weigh carbon-coated nano-silicon, secondary particles of artificial graphite, and PAA binder for silicon-based negative electrodes, disperse them in N-methylpyrrolidone, stir evenly to obtain a second slurry, and then use a doctor blade coating process to evenly cover the second slurry on the stress buffer layer. Dry and solidify the slurry at 110°C, and then dry and roll press to obtain the negative electrode sheet.

[0112] Example 2

[0113] The difference from Example 1 is that the formula of the stress buffer layer is changed to: graphite: R-group-containing binder: sodium carboxymethyl cellulose: elastic polymer: conductive agent = 85:5:2:6:2.

[0114] Example 3

[0115] The difference from Example 1 is that the formula of the stress buffer layer is changed to: graphite: composite binder: sodium carboxymethyl cellulose: elastic polymer: conductive agent = 85:8:2:3:2.

[0116] Example 4

[0117] The difference from Example 1 is that the polyetheresteramide elastomer of the stress buffer layer is changed to polystyrene elastomer.

[0118] Example 5

[0119] The difference from Example 1 is that in the stress buffer layer, thiourea is replaced by 2-mercaptoethanesulfonic acid.

[0120] Example 6

[0121] The first transition layer does not contain thiourea, but directly uses polyacrylonitrile-polyacrylic acid copolymer.

[0122] Example 7

[0123] The difference from Example 1 is that in the active layer, the secondary particles of artificial graphite are replaced by D 50 = Primary particles of 13.0±1.0 μm.

[0124] Example 8

[0125] The difference from Example 1 is that in the active layer, the carbon-coated nano-silicon is replaced by a composite material of nano-SiOx and carbon (the mass ratio of SiOx to carbon is 1:1, and the particle size D50 is 100 nm).

[0126] Example 9

[0127] The difference from Example 1 is that: in the active layer, the carbon-coated nano-silicon has a particle size of 200 nm and a thickness of the carbon coating layer of 2 nm.

[0128] Example 10

[0129] The difference from Example 1 is that in the active layer, the D50 of the secondary graphite particles is 10 μm.

[0130] Comparative Example 1

[0131] The difference from Example 1 is that the stress buffer layer does not contain elastic polymer.

[0132] Comparative Example 2

[0133] The difference from Example 1 is that no stress buffer layer is provided.

[0134] Application Examples

[0135] Preparation of lithium-ion battery: The negative electrode sheets prepared in the examples and comparative examples were cut into 7.5 cm × 6.4 cm sheets, and the corresponding ion exchange membranes and positive electrode sheets were prepared. The positive electrode sheet contained 96.2% NCM lithium nickel cobalt manganese oxide LiNi 0.6 Co 0.24 Mn 0.16 The O2 positive electrode active material comprises NCM, binder PVDF, and conductive carbon black in a ratio of 96.2:1.5:2.3. The electrolyte consists of LiPF6 dissolved in a ternary solvent consisting of EC, EMC, and DMC in a 1:1:1 ratio. VC and FEC additives are added to form the electrolyte. The LiPF6 concentration is 1 mol / L, VC accounts for 5% by weight, and FEC accounts for 1% by weight. Both the positive and negative electrodes are coated on both sides with a NP ratio of 1.08. A 7+8-layer laminated cell is assembled using PP film as the ion exchange membrane and aluminum-plastic film as the outer shell. The battery is vacuum dried, the electrolyte is injected into the battery shell, the battery is packaged, allowed to stand, formed, and then volumetrically divided to produce a lithium-ion battery.

[0136] Performance Testing

[0137] Peeling force test: Take the negative electrode prepared in the embodiment and the comparative example, and ensure that its surface is clean and has no obvious damage. Cut the electrode into standard size, generally 100mm in length and 40mm in width. Take a flat thin steel plate and stick a double-sided tape in the center of the steel plate. The length is greater than the test length of the sample and the same width as the electrode. Stick the electrode on the tape to ensure that the electrode fits tightly to the tape. Use a tensile testing machine to test, insert the steel plate with the fixed electrode into the lower clamp and fix it vertically. Insert the un-glued electrode into the upper clamp and fix it so that the electrode attached to the tape forms a 180° angle with the electrode fixed by the upper clamp. Set the test width, the electrode peeling length is 50mm, and the peeling speed is set to 50mm / min. Start the tensile testing machine, record the force value changes during the peeling process, and obtain the peeling strength curve and average value.

[0138] Electrode Expansion Rate Test: Sample Preparation: Take the negative electrode sheets prepared in the Examples and Comparative Examples and use an in-situ expansion test system. Use constant pressure mode to apply constant pressure to the cell surface, maintaining this pressure in real time during charge and discharge, while simultaneously capturing thickness curves. Set the starting and ending voltages to 2.8V and 4.2V, respectively. Charge at 0.5C to 4.2V, then charge at 4.2V constant voltage until the current drops to 0.05C, and then discharge at 0.2C to 2.8V. During the test, record the capacity, voltage, and thickness change data for each test. Calculate the expansion rate after the test is complete.

[0139] Cycling performance test: At room temperature of 25°C, use a fixture to fix the soft-pack battery, with the starting and ending voltages of 2.8V and 4.2V, respectively. Charge to 4.2V at 0.5C, then charge at 4.2V constant voltage until the current drops to 0.05C, and discharge to 2.8V at 0.2C. Record the capacity retention rate under different cycle numbers. The results are shown in Table 1.

[0140] Table 1:

[0141]

[0142]

[0143] As can be seen from Table 1, Examples 1 to 4 demonstrate the effects of electrode formulations with different ratios on the battery cycle life. The experimental results show that with the increase in the amount of elastic polymer and binder added, the capacity retention rate of the battery is significantly improved. This phenomenon can be attributed to the filling and bonding effect of the elastic polymer and composite binder in the electrode. They not only enhance the structural stability of the electrode material, but also improve the adhesion between the electrode and the current collector, thereby reducing the shedding and breakage of the electrode material during the charge and discharge cycle.

[0144] Since the negative electrode sheet without the addition of elastic polymer in Comparative Example 1 showed a poor cycle life, this experimental result showed that the elastic polymer played a key role in alleviating the volume expansion and stress concentration of the electrode material during the charge and discharge process.

[0145] Comparative Example 2 shows that when no stress buffer layer is set, the cycle life of the silicon-carbon negative electrode is significantly lower than that of other formulations. The experimental results show the importance of the stress buffer layer in improving the cycle life of the silicon-carbon negative electrode.

[0146] Finally, it should be noted that the above embodiments are only used to illustrate the technical solutions of the present invention, rather than to limit it. Although the present invention has been described in detail with reference to the above embodiments, those skilled in the art should understand that they can still modify the technical solutions described in the above embodiments, or replace some or all of the technical features therein with equivalents. However, these modifications or replacements do not cause the essence of the corresponding technical solutions to deviate from the scope of the technical solutions of the embodiments of the present invention.

Claims

1. A negative electrode sheet, characterized in that: It includes a current collector and a stress buffer layer and an active layer sequentially arranged on at least one functional surface of the current collector, wherein the stress buffer layer includes graphite, an elastic polymer and a binder; and the active layer includes a silicon-based material and a graphite material.

2. The negative electrode sheet according to claim 1, characterized in that: The elastic polymer includes at least one of polyetheresteramide elastomer, polyetherimide elastomer, polystyrene elastomer, polyurethane elastomer, polyamide elastomer and styrene-butadiene rubber.

3. The negative electrode sheet according to claim 1, characterized in that: The binder contains an R group, and the R group is combined with the current collector through a chemical bond.

4. The negative electrode sheet according to claim 3, characterized in that: The R group is at least one of a thiocarbonyl group, a thioester group, a sulfhydryl group, and an amino group; The binder containing R groups is formed by cross-linking a first raw material and a second raw material; the first raw material includes a polyacrylic acid polymer; and the second raw material includes the R groups.

5. The negative electrode sheet according to claim 4, characterized in that: The second raw material includes at least one of thiourea, 2-mercaptobenzothiazole, 2-mercaptopyridine, and 2-mercaptoethanesulfonic acid; And / or, the mass ratio of the first raw material to the second raw material is 1.5-3:1; and / or, the molecular weight of the polyacrylic acid polymer is 400,000 g / mol-800,000 g / mol; And / or, the first raw material includes a polyacrylic acid polymer having a molecular weight of 200,000 g / mol-400,000 g / mol and a polyacrylic acid polymer having a molecular weight of 600,000 g / mol-800,000 g / mol.

6. The negative electrode sheet according to any one of claims 1 to 5, characterized in that: In the stress buffer layer, the graphite is secondary graphite particles.

7. The negative electrode sheet according to any one of claims 1 to 6, characterized in that: In the stress buffer layer, the mass ratio of the graphite, the binder and the elastic polymer is 75-85:5-8:8-12; And / or, the stress buffer layer further comprises a dispersant and a conductive agent, and the mass ratio of the binder, the dispersant and the conductive agent is 5-8:1-3:1-5.

8. The negative electrode sheet according to any one of claims 1 to 7, characterized in that: In the active layer, the silicon-based material is selected from at least one of pure silicon, silicon carbon, silicon oxygen, and silicon alloy; and / or the graphite material is secondary particles of graphite.

9. The negative electrode sheet according to claim 8, characterized in that: The silicon carbon includes one or more of carbon-coated nano- or micron-silicon materials, nano- or micron-SiOx and carbon composite materials, and nano- or micron-silicon carbon wire materials, wherein 0 <x≤2; And / or, in the silicon carbon, the mass proportion of silicon is 10 wt% to 50 wt%.

10. The negative electrode sheet according to claim 8 or 9, characterized in that: In the active layer, the D50 of the secondary graphite particles is 12 μm-15 μm; And / or, the silicon carbon is carbon-coated nano-silicon, the particle size of the nano-silicon is 50nm to 200nm, and the thickness of the carbon coating layer is 2nm to 5nm; And / or, the mass ratio of the carbon-coated nano-silicon and graphite secondary particles is 1:4-1.

11. The negative electrode sheet according to any one of claims 1 to 10, characterized in that: The thickness of the stress buffer layer is 30 μm to 60 μm, and the single-side density is 40 mg / cm 3 -60mg / cm 3 ; And / or, the thickness of the active layer is 30 μm to 60 μm, and the single-surface density of the active layer is 40 mg / cm 3 -60mg / cm 3 .

12. A method for preparing a negative electrode sheet according to any one of claims 1 to 11, characterized in that: The following steps are involved: A first slurry containing graphite, a binder, an elastic polymer, and a conductive agent is coated on at least one functional surface of the current collector and heat-treated at 100° C.-140° C.; a second slurry containing a silicon-based material and a graphite material is then coated, dried and rolled to obtain the negative electrode sheet.

13. A battery, characterized in that: A negative electrode sheet comprising any one of claims 1 to 11.

14. A battery pack, characterized in that: A battery comprising the battery of claim 13.

15. An electrical device, characterized in that: Comprising the battery according to claim 13 or the battery pack according to claim 14.

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