Negative electrode sheet, electrochemical device, and preparation method for negative electrode sheet

Abstract

A negative electrode sheet, an electrochemical device, and a preparation method for the negative electrode sheet, relating to the technical field of secondary batteries. The negative electrode sheet comprises a current collector and a negative electrode active material layer. The negative electrode active material layer is located on at least one side of the current collector, and the negative electrode active material layer comprises a first active material layer and a second active material layer; the first active material layer and the second active material layer are stacked in the direction close to the current collector; and the structural stability parameter ratio S1 / S2 of the first active material layer to the second active material layer satisfies 0.01-10.0. According to the negative electrode sheet, the electrochemical device, and the preparation method for the negative electrode sheet, by adjusting structural stability parameters of a double-layer structure in the negative electrode active material layer, the negative electrode active material layer forms a structure having a gradually decreasing hardness in the direction close to the current collector, so as to effectively optimize the pore distribution and the structural stability of the negative electrode active material layer, thereby improving the fast charging, cycle and storage performance of a lithium ion battery.

Description

Negative electrode sheet, electrochemical device and method for preparing negative electrode sheet Technical Field

[0001] The present invention relates to the technical field of secondary batteries, and in particular to a negative electrode sheet, an electrochemical device, and a method for preparing the negative electrode sheet. Background Art

[0002] In traditional lithium-ion battery systems, the battery's charging capacity, cycling performance, processing performance, and safety are largely determined by the anode. Current optimization strategies for anode pole pieces typically focus on factors such as compaction density, surface density, and active material ratio. However, as battery cell performance approaches bottlenecks, we need to further refine the impact of the pole piece layer on cell performance, thereby effectively improving the performance of the corresponding battery cell at an intrinsic level.

[0003] At present, there is no room for simply adjusting the compaction density, surface density, and active material ratio of the negative electrode material to optimize the fast charging and cycling performance of the battery cell. It is necessary to reasonably consider the physical and chemical characteristics of the negative electrode plate, and optimize the distribution and orientation of particles and pores in the negative electrode active material layer by comprehensively adjusting the material ratio, hardness distribution, and compaction density of the negative electrode active material layer, thereby improving the kinetic balance of the negative electrode plate and optimizing the fast charging and cycling performance of the lithium-ion battery.

[0004] Therefore, it is necessary to design a negative electrode sheet, an electrochemical device and a method for preparing the negative electrode sheet to solve the above problems.

[0005] Summary of the Invention

[0006] In view of the above shortcomings of the prior art, the present invention provides a negative electrode sheet, an electrochemical device and a method for preparing a negative electrode sheet, which comprehensively adjusts the material ratio, hardness distribution and compaction density of the negative electrode active material layer in the negative electrode sheet, thereby improving the technical problem that the prior art is difficult to comprehensively optimize the distribution and orientation of particles and pores in the negative electrode active material layer and cannot effectively balance the dynamic performance of the negative electrode sheet.

[0007] To achieve the above-mentioned object and other related objects, the present invention provides a negative electrode plate, which includes a current collector and a negative electrode active material layer.

[0008] In which, the negative electrode active material layer is located on at least one side of the current collector, and the negative electrode active material layer includes a first active material layer and a second active material layer, and the first active material layer and the second active material layer are stacked in a direction close to the current collector; and the structural stability parameter ratio S1 / S2 of the first active material layer and the second active material layer satisfies 0.01-10.0.

[0009] In an example of the present invention, the negative electrode material in the negative electrode active material layer includes graphite material.

[0010] In an example of the present invention, the structural stability parameter ratio S1 / S2 of the first active material layer and the second active material layer satisfies 0.1-5.0.

[0011] In an example of the present invention, the structural stability parameter S1 of the first active material layer is 3-16.

[0012] In one embodiment of the present invention, the negative electrode active material layer satisfies Wherein, τ is the tortuosity of the negative electrode active material layer, ε is the porosity of the negative electrode active material layer, P is the compaction density of the negative electrode active material layer, V OI It is the stacking orientation degree of the negative electrode material particles in the negative electrode active material layer.

[0013] In one embodiment of the present invention, the negative electrode active material layer satisfies

[0014] In one embodiment of the present invention, the density of the negative electrode active material layer is 1.65-1.75 g / cm 3 .

[0015] On the other hand, the present invention provides a method for preparing a negative electrode plate, which comprises the following steps: mixing a first negative electrode material with a binder, a conductive agent and a thickener to form a slurry to obtain a first active material layer slurry; mixing a second negative electrode material with a binder, a conductive agent and a thickener to form a slurry to obtain a second active material layer slurry; coating the second active material layer slurry and the first active material layer slurry on at least one side of a current collector in sequence, and after coating, drying, compacting, die-cutting and punching to obtain a negative electrode plate; wherein the hardness of the first negative electrode material is higher than that of the second negative electrode material.

[0016] In an example of the present invention, the structural stability parameter ratio S1 / S2 of the first negative electrode material and the second negative electrode material satisfies 0.01-10.0.

[0017] In an example of the present invention, the average particle size Dv50 of the first negative electrode material is 11-17 μm, and the average particle size Dv50 of the second negative electrode material is 16-19 μm.

[0018] The present invention also provides an electrochemical device, which includes a positive electrode plate, a separator, an electrolyte, and a negative electrode plate described in any one of the above examples.

[0019] The negative electrode sheet, electrochemical device, and method for preparing the negative electrode sheet of the present invention adjust the structural stability parameters of the double-layer structure in the negative electrode active material layer to form a structure with a decreasing hardness gradient along the direction close to the current collector. This effectively optimizes the pore distribution and structural stability of the negative electrode active material layer, and forms a negative electrode sheet with low tortuosity, high orientation, and abundant porosity during compaction, thereby further improving the fast charging, cycling, and storage performance of lithium-ion batteries. Therefore, the present invention effectively overcomes some practical problems in the prior art and has high utilization value and practical significance. BRIEF DESCRIPTION OF THE DRAWINGS

[0020] In order to more clearly illustrate the embodiments of the present invention or the technical solutions in the prior art, the following briefly introduces the drawings required for use in the embodiments or the description of the prior art. Obviously, the drawings described below are only some embodiments of the present invention. For ordinary technicians in this field, other drawings can be obtained based on these drawings without paying any creative work.

[0021] FIG1 is a schematic structural diagram of a negative electrode plate in one embodiment of the present invention.

[0022] FIG2 is a schematic flow chart of a method for preparing a negative electrode sheet according to an embodiment of the present invention.

[0023] Component number description

[0024] 100 , current collector; 200 , negative electrode active material layer; 210 , first active material layer; 220 , second active material layer. DETAILED DESCRIPTION

[0025] The following describes the embodiments of the present invention through specific examples. Those skilled in the art can easily understand other advantages and effects of the present invention from the contents disclosed in this specification. The present invention can also be implemented or applied through other different specific embodiments, and the details in this specification can also be modified or changed based on different viewpoints and applications without departing from the spirit of the present invention. It should be noted that the following examples and the features in the examples can be combined with each other unless they conflict. It should also be understood that the terms used in the examples of the present invention are for the purpose of describing specific embodiments, not for the purpose of limiting the scope of protection of the present invention. The test methods for which specific conditions are not specified in the following examples are generally carried out under conventional conditions or under the conditions recommended by the manufacturers.

[0026] It should be noted that the terms such as "upper", "lower", "left", "right", "middle" and "one" cited in this specification are only for the convenience of description and are not used to limit the scope of implementation of the present invention. Changes or adjustments to their relative relationships should be regarded as the scope of implementation of the present invention without substantially changing the technical content.

[0027] In this specification, the structural stability parameter S of the negative electrode material is an index of the structural stability of the secondary particles of the material (a single particle is defined as 100.0). For specific definitions, please refer to the test method of the structural stability parameter below.

[0028] The tortuosity τ of the negative electrode sheet is defined as the ratio of the actual length of the pores in the negative electrode active material layer of the negative electrode sheet to the shortest distance in the direction perpendicular to the current collector of the negative electrode active material layer.

[0029] The porosity ε of the negative electrode sheet is defined as: the ratio of the pore volume in the porous negative electrode material to the apparent volume (or total volume) of the porous negative electrode material in the negative electrode sheet, generally expressed as a percentage.

[0030] Orientation degree V of negative electrode OI It indicates the stacking orientation degree of the negative electrode material particles in the negative electrode sheet, and is defined as the ratio of the intensities of the I004 / I110 peaks in the XRD diffraction of the negative electrode material in the negative electrode sheet.

[0031] The average particle size Dv50 of a material can be defined as the particle size corresponding to 50% of the cumulative volume in the particle size distribution curve. The average particle size Dv50 can be measured, for example, by laser diffraction. Laser diffraction methods are generally capable of measuring particle sizes from the submicron range to several millimeters, thereby achieving highly reproducible and high-resolution results.

[0032] The present invention provides a negative electrode plate, which is used to participate in the production of an electrochemical device. An electrochemical device generally includes a positive electrode plate, a negative electrode plate, an electrolyte, a diaphragm, and corresponding connecting accessories and circuits. The positive and negative electrode materials in the positive and negative electrode plates can intercalate and deintercalate lithium ions to achieve energy storage and release. The electrolyte is a carrier for the transmission of lithium ions between the positive and negative electrodes. The diaphragm is permeable to lithium ions but is non-conductive, thereby separating the positive and negative electrodes to prevent short circuits. The positive and negative electrode plates generally play a decisive role in key performance factors such as the energy storage function of the lithium battery, the energy density of its battery cell, the cycle performance and safety.

[0033] As shown in Figure 1, the negative electrode sheet provided by the present invention includes a current collector 100 and a negative electrode active material layer 200, wherein the negative electrode active material layer 200 is located on at least one surface of the current collector 100. Specifically, the current collector 100 has two surfaces that are opposite to each other in its thickness direction, and the negative electrode active material layer 200 is disposed on either or both of the two surfaces of the current collector 100. The current collector 100 can be made of a material with good electrical conductivity and mechanical strength, serving as a conductor and current collector. In some embodiments, the current collector 100 can be made of copper foil.

[0034] As shown in FIG1 , the negative electrode active material layer 200 includes a first active material layer 210 and a second active material layer 220. In the negative electrode active material layer 200, the first active material layer 210 and the second active material layer 220 are stacked in a direction approaching the current collector 100, with the second active material layer 220 disposed on the surface of the current collector 100 and the first active material layer 210 disposed on the second active material layer 220.

[0035] In particular, because the first active material layer 210 and the second active material layer 220 of the negative electrode active material layer 200 are stacked in a direction approaching the current collector 100, by adjusting the ratio of the structural stability parameter S of the first active material layer 210 and the second active material layer 220, the material hardness of the negative electrode active material layer 200 after compaction can be made to decrease gradually in the direction approaching the current collector 100. The material structure of the negative electrode active material layer 200, where the hardness increases as it approaches the current collector 100 and then decreases as it approaches the current collector 100, is more conducive to forming a reasonable distribution of particles and pores during the compaction process, thereby increasing the structural stability and pore richness of the negative electrode sheet.

[0036] In some embodiments, in the negative electrode active material layer, the structural stability parameter ratio S1 / S2 of the first active material layer and the second active material layer materials satisfies 0.01-10.0, so that a negative electrode active material layer with appropriate overall hardness and rich porosity can be formed on the negative electrode sheet.

[0037] When the structural stability parameter ratio S1 / S2 of the first active material layer and the second active material layer materials satisfies a range of 0.01-10.0, the negative electrode sheet exhibits a reasonable pore distribution and high structural stability, with a porosity ε of 15%-30% and a tortuosity τ of 3-6. Excessively high S1 / S2 ratios improve the pore richness and hardness of the negative electrode sheet, but significantly reduce its structural stability and compaction density. Excessively low S1 / S2 ratios reduce the pore richness of the negative electrode sheet, causing its porosity ε to drop below 15% and its tortuosity τ to rise above 6, leading to deterioration in the negative electrode sheet's kinetic performance and a significant decline in the secondary battery's charge and discharge performance.

[0038] In some embodiments, in the negative electrode active material layer, the structural stability parameter ratio S1 / S2 of the materials of the first active material layer and the second active material layer satisfies: 0.01≤S1 / S2≤3, 0.1≤S1 / S2≤3, 0.1≤S1 / S2≤5, 1≤S1 / S2≤7, 3≤S1 / S2≤7, 3≤S1 / S2≤8, 3≤S1 / S2≤10, 5≤S1 / S2≤7 or 5≤S1 / S2≤10, etc.

[0039] In some preferred embodiments, the structural stability parameter ratio S1 / S2 of the first active material layer and the second active material layer in the negative electrode active material layer satisfies a range of 0.01-5.0. In the negative electrode active material layer of the negative electrode sheet, a structural stability parameter ratio S1 / S2 of the first active material layer and the second active material layer within an appropriate range is more conducive to improving the porosity, compaction density, and structural stability of the negative electrode sheet while reducing the pore tortuosity of the negative electrode sheet.

[0040] In some embodiments, on the basis that the negative electrode plate meets the above conditions, the structural stability parameter S1 of the first active material layer material is 3.0-16.0, for example, 3.0, 3.7, 5.0, 7.4, 8.0, 8.3, 10.0, 10.4, 13.4, 15.1, 15.6 or 16.0; the structural stability parameter S2 of the second active material layer material can be any value that satisfies the structural stability parameter ratio range (0.01≤S1 / S2≤10.0), for example, the structural stability parameter S2 of the second active material layer material can be any value that satisfies the stability parameter ratio range of 1.0-100.0.

[0041] In some embodiments, by adjusting the overall compaction density and interlayer structure parameters of the negative electrode sheet, the negative electrode active material layer satisfies: Within this range, the negative electrode sheet forms a negative electrode active material layer with low tortuosity, high orientation and high porosity, thereby improving the fast charging capability of the double-layer negative electrode active material layer in the negative electrode sheet; wherein τ is the tortuosity of the negative electrode active material layer, ε is the porosity of the negative electrode active material layer, P is the compaction density of the negative electrode active material layer, V OI It is the stacking orientation degree of the negative electrode material particles in the negative electrode active material layer.

[0042] In some embodiments, the negative electrode active material layer satisfies: or

[0043] In some preferred embodiments, the negative electrode active material layer of the negative electrode sheet satisfies Among them, when the negative electrode active material layer of the negative electrode sheet When the parameters are within the above range, it is more conducive to improving both the fast charging and cycle performance of the secondary battery.

[0044] After in-depth research by the inventors, it was found that when the negative electrode plate of the present application satisfies the above conditions and optionally satisfies one or more of the following design conditions, the performance of the secondary battery can be further improved.

[0045] In some preferred embodiments, the compaction density P of the negative electrode sheet is 1.65-1.75 g / cm 3 , for example, 1.65 g / cm 3 , 1.67g / cm 3 , 1.70g / cm 3 , 1.73g / cm 3 or 1.75g / cm 3 .

[0046] In some preferred embodiments, the porosity ε of the negative electrode sheet is 15%-30%, for example, 15%, 17%, 20%, 22%, 25%, 27%, 29% or 30%.

[0047] In some preferred embodiments, the tortuosity τ of the negative electrode sheet is 3-5, for example, 3.0, 3.3, 3.37, 3.5, 3.6, 3.64, 3.8, 4.0, 4.2, 4.5, 4.7 or 5.0.

[0048] In some preferred embodiments, the orientation degree V of the negative electrode sheet OI is 14-24, for example, 14, 16, 18, 21, 22, 23 or 24.

[0049] In some embodiments, the negative electrode material in the negative electrode active material layer includes a core and a coating layer, the core includes graphite material, the coating layer is coated on the surface of the core, and the coating layer includes amorphous carbon.

[0050] In the above embodiment, the graphite material includes natural graphite, artificial graphite and graphitized carbon. Preferably, the graphite material can be artificial graphite.

[0051] In the above embodiment, the amorphous carbon coating layer can be formed by carbonizing an organic carbon source. For example, the organic carbon source can be selected from polymers such as coal tar, petroleum asphalt, phenolic resin and other polymer materials.

[0052] Amorphous carbon has a disordered structure and a large interlayer spacing. Therefore, coating the core material with amorphous carbon allows for faster diffusion of active ions within the anode material particles, thereby improving the anode material's fast charging capability. Furthermore, the amorphous carbon coating protects the core, significantly reducing the exfoliation of the graphite layers caused by solvent co-intercalation, thus imparting greater structural stability to the anode material. Consequently, the anode material exhibits a higher capacity and cycle life.

[0053] It should also be noted that the negative electrode active material layer also includes a binder and a conductive agent. The binder is selected from at least one of polyimide, styrene-butadiene rubber, polyacrylic acid, polyvinylidene fluoride, sodium carboxymethyl cellulose, and lithium carboxymethyl cellulose. The conductive agent is selected from at least one of carbon black, SP, single-walled carbon nanotubes, multi-walled carbon nanotubes, and graphene.

[0054] Referring to FIG. 2 , the present invention provides a method for preparing a negative electrode sheet, comprising the following steps:

[0055] S1, mixing a first negative electrode material with a binder, a conductive agent, and a thickener to prepare a slurry to obtain a first active material layer slurry;

[0056] S2, mixing the second negative electrode material with a binder, a conductive agent, and a thickener to form a slurry to obtain a second active material layer slurry;

[0057] S3. The second active material layer slurry and the first active material layer slurry are sequentially coated on at least one side of the current collector to form a negative electrode active material layer on the current collector (in the negative electrode active material layer, the second active material layer is coated on the current collector, and the first active material layer is coated on the second active material layer). After coating, the negative electrode sheet is obtained by drying, compacting, die-cutting, and punching.

[0058] In this negative electrode sheet preparation method, a first negative electrode material and a second negative electrode material with suitable physical parameters are selected to prepare a first active material layer slurry and a second active material layer slurry. The interlayer structure of the negative electrode active material layer is designed by sequentially coating the second active material layer slurry and the first active material layer slurry on the current collector. After drying and compaction, the interlayer structure of the negative electrode active material layer can be rationally spaced and arranged within the material particles of different particle sizes, thereby forming a large number of highly oriented pore structures between the particles. This further improves the structural stability and rationality of the pore distribution of the negative electrode active material layer based on the dynamics of the selected negative electrode materials.

[0059] In steps S1 and S2, the first and second negative electrode materials used are both negative electrode materials having a core of a graphite material and a coating of amorphous carbon. The graphite material is selected from one or more of natural graphite, artificial graphite, and graphitized carbon. The amorphous carbon can be formed by carbonizing an organic carbon source on the graphite material. For example, the organic carbon source can be selected from a polymer, such as coal tar, petroleum asphalt, phenolic resin, or other polymer materials.

[0060] In step S1 and step S2, the binder is selected from at least one of polyimide, styrene-butadiene rubber, polyacrylic acid, polyvinylidene fluoride, sodium carboxymethyl cellulose, and lithium carboxymethyl cellulose, among which styrene-butadiene rubber is preferred; the conductive agent is selected from at least one of carbon black, SP, single-walled carbon nanotubes, multi-walled carbon nanotubes, and graphene, among which carbon black is preferred; and the thickener is sodium carboxymethyl cellulose (CMC).

[0061] In some embodiments, in steps S1 and S2, the structural stability parameter ratio S1 / S2 of the selected first and second negative electrode materials satisfies 0.01-10.0. Under this structural parameter, the hardness of the first negative electrode material is higher than that of the second negative electrode material. The second negative electrode material and the first negative electrode material within this structural stability parameter ratio range form a double-layer structure of the negative electrode active material layer on the current collector in a direction away from the current collector, and the internal hardness of the negative electrode active material layer after compaction processing can be made to show a gradient downward trend in the direction close to the current collector, thereby increasing the structural stability of the negative electrode sheet and the rationality of the pore distribution.

[0062] Furthermore, in some preferred embodiments, the structural stability parameter ratio S1 / S2 of the selected first negative electrode material and the second negative electrode material satisfies 0.1-5.0.

[0063] In some embodiments, in step S1 and step S2, a first negative electrode material with an average particle size Dv50 of 11-17 μm is selected to prepare a first active material layer slurry, for example, a first negative electrode material with an average particle size Dv50 of 11 μm, 11.2 μm, 12.2 μm, 13 μm, 13.3 μm, 13.7 μm, 14 μm, 14.5 μm, 15 μm, 15.6 μm, 16 μm, 16.2 μm or 17 μm is selected. A negative electrode material; a second negative electrode material having an average particle size Dv50 of 16-19 μm is selected to prepare a second active material layer slurry, for example, a second negative electrode material having an average particle size Dv50 of 16 μm, 16.7 μm, 17 μm, 17.5 μm, 18 μm, 18.3 μm, 18.7 μm or 19 μm is selected; and the average particle size Dv50 of the second negative electrode material is selected to be higher than the average particle size Dv50 of the first negative electrode material.

[0064] In this embodiment, in step S3, a double-layer negative electrode active material layer is formed by coating the second active material layer slurry and the first active material layer slurry onto the current collector. The negative electrode active material layer has a double-layer structure with a similar average particle size Dv50, and the average particle size Dv50 of the particles of the outer first active material layer in the double-layer structure is relatively low, and the average particle size Dv50 of the particles of the inner second active material layer is relatively high.

[0065] Because the interlayer structure of the negative electrode active material layer generally exhibits a low average outer layer particle size (Dv50) and a high inner layer particle size (Dv50), the gaps between the inner layer particles are larger, while the gaps between the outer layer particles are smaller. Therefore, during the compaction process of the negative electrode sheet, the particles of the first active material are compacted and embedded between the particles of the second active material layer, resulting in a spaced arrangement of particles in the negative electrode active material layer with high spatial utilization. This large number of spaced particles facilitates improving both the orientation and porosity of the negative electrode active material layer. Furthermore, because the average particle size (Dv50) of the particles in the two-layer structure of the negative electrode active material layer is relatively close, the pore structure formed between the particles after compaction has high orientation and low tortuosity.

[0066] Therefore, in step S3, on the basis of coating the interlayer structure of the negative electrode active material layer on the current collector, the compaction density of the negative electrode active material layer is adjusted by compaction to make the negative electrode sheet The range is 6.5-80 to form a negative electrode sheet with low tortuosity, high orientation and rich pores, thereby effectively improving the fast charging and cycle performance of the negative electrode sheet.

[0067] In step S3, a double-cavity coating die is used to pass the first active material layer slurry into the upper cavity of the double-cavity coating die, and the second active material layer slurry is passed into the lower cavity of the double-cavity coating die. Then, the first active material layer slurry and the second active material layer slurry in the upper cavity and the lower cavity of the double-cavity coating die are mixed in a weight ratio of 1:1, with a coating weight of 105 g / m 2 The parameters are evenly coated on the surfaces of both sides of the current collector, thereby forming a negative electrode active material layer with a double-layer structure of the second active material layer and the first active material layer stacked in the direction away from the current collector on the surface of the current collector; after coating, the negative electrode active material layer on the current collector is dried, compacted, die-cut and punched to obtain the negative electrode sheet.

[0068] The present invention also provides an electrochemical device comprising the negative electrode sheet described in any of the above embodiments of the present invention, or the negative electrode sheet prepared by the preparation method described in any of the above embodiments. The electrochemical device, for example, is a secondary lithium-ion battery, comprising a positive electrode sheet, a negative electrode sheet, a separator disposed between the positive and negative electrode sheets, and an electrolyte. The positive and negative electrode materials on the positive and negative electrode sheets can intercalate and deintercalate lithium ions to achieve energy storage and release. The electrolyte is a carrier for lithium ions to be transferred between the positive and negative electrodes. The separator is permeable to lithium ions but is non-conductive, thereby separating the positive and negative electrodes to prevent short circuits.

[0069] The positive electrode sheet, separator and electrolyte can be made of conventional materials and prepared using conventional methods in the art. For example:

[0070] The preparation process of secondary lithium-ion batteries is as follows:

[0071] Preparation of positive electrode sheet: ternary positive electrode material (LiNi 0.8 Co 0.1 Mn 0.1 A positive electrode slurry is prepared by mixing a conductive carbon black (Super P) and a binder (polyvinylidene fluoride) in a mass ratio of 97:1.5:1.5. N-methylpyrrolidone (NMP) is added as a solvent and the mixture is thoroughly stirred to obtain a positive electrode slurry. The positive electrode slurry is then coated onto a positive electrode current collector aluminum foil. The positive electrode sheet is prepared through drying, cold pressing, and slitting. The conductive agent may also be selected from at least one of acetylene black, carbon nanotubes, graphene, and VGCF. The binder may also be selected from at least one of PVDF and PTFE.

[0072] The negative electrode sheet preparation process is as mentioned above.

[0073] Preparation of the diaphragm: A PE porous membrane with a thickness of 11 μm was selected as the diaphragm. The thickness of the diaphragm ranged from 9 μm to 18 μm, the air permeability ranged from 180 s / 100 mL to 380 s / 100 mL, and the porosity ranged from 30% to 50%.

[0074] Preparation of electrolyte: Ethylene carbonate (EC), ethyl methyl carbonate (EMC), and diethyl carbonate (DEC) are uniformly mixed in a volume ratio of 1:1:1 to obtain an organic solvent. Then, fully dried lithium salt LiPF6 is dissolved in the mixed organic solvent to prepare an electrolyte with a concentration of 1 mol / L.

[0075] Battery assembly: The prepared positive electrode sheet, separator, and negative electrode sheet are stacked in sequence, with the separator positioned between the positive and negative electrodes to act as a barrier. They are then wrapped in aluminum-plastic film, dried, and filled with the prepared electrolyte. After packaging, resting, and forming, a 1Ah soft-pack battery (i.e., lithium-ion battery) is finally produced.

[0076] The technical solutions of the present invention are described in detail below through several specific examples and comparative examples. Unless otherwise specified, the raw materials and reagents used in the following examples are commercially available or can be prepared by conventional methods in the art.

[0077] Example 1

[0078] This embodiment provides a negative electrode plate. The structural stability parameter ratio S1 / S2 of the first active material layer and the second active material layer in the negative electrode plate is 1.6. It is 37.0.

[0079] The method for preparing the negative electrode sheet includes:

[0080] S1. A first negative electrode material is mixed with a binder of styrene-butadiene rubber, sodium carboxymethyl cellulose, and a conductive agent of carbon black (purchased from Swiss Graphite and Carbon Company, model Super P, referred to as SP) in a ratio of 94.7 parts by weight: 2.5 parts by weight: 1.8 parts by weight: 1. To 100 parts by weight of the mixture, 82 parts by weight of water are added, and the mixture is stirred to obtain a first active material layer slurry; wherein the first negative electrode material is an artificial graphite material having a structural stability parameter S1 of 3.7 and an average particle size Dv50 of 15.6 μm;

[0081] S2. The second negative electrode material is mixed with a binder of styrene-butadiene rubber, sodium carboxymethyl cellulose, and a conductive agent of carbon black (purchased from Swiss Graphite and Carbon Company, model number Super P, referred to as SP) in a ratio of 94.7 parts by weight: 2.5: 1.8: 1. To 100 parts by weight of the mixture, 82 parts by weight of water are added, and the mixture is stirred to obtain a second active material layer slurry; wherein the second negative electrode material is an artificial graphite material having a structural stability parameter S2 of 2.3 and an average particle size Dv50 of 16.7 μm;

[0082] S3, using a double-cavity coating die (manufactured by Mannster, model BG01A-400-30B), the first active material layer slurry was passed into the upper cavity of the double-cavity coating die, and the second active material layer slurry was passed into the lower cavity of the double-cavity coating die. Then, the first active material layer slurry and the second active material layer slurry in the upper cavity and the lower cavity of the double-cavity coating die were mixed in a weight ratio of 1:1, with a coating weight of 105 g / m 2 The parameters are evenly coated on both sides of the 8μm copper foil current collector; then, the current collector is dried and roll-pressed so that the compaction density of the negative electrode active material layer on the current collector is 1.65g / m 3 Finally, the negative electrode sheet is obtained through die cutting and punching.

[0083] This embodiment also uses the above-mentioned negative electrode sheet to prepare a secondary lithium-ion battery. The preparation method of the secondary lithium-ion battery is as follows:

[0084] (1) Preparation of positive electrode sheet:

[0085] The ternary cathode material (LiNi 0.8 Co 0.1 Mn 0.1 A mixture of 1,000 molten aluminum (O2), conductive carbon black (Super P), and binder polyvinylidene fluoride (PVDF) was prepared in a mass ratio of 97:1.5:1.5. N-methylpyrrolidone (NMP) was added as a solvent and thoroughly stirred to produce a positive electrode slurry. The mixture was stirred in a vacuum mixer until the mixture became homogeneous and transparent. The slurry was evenly coated onto a 16μm aluminum foil current collector. The aluminum foil current collector was then air-dried at room temperature and then dried in an oven at 80°C to 120°C for 6 hours. The positive electrode sheets were then cold-pressed and slit.

[0086] (2) The negative electrode sheet preparation process is shown in this embodiment.

[0087] (3) Preparation of electrolyte:

[0088] In an argon atmosphere glove box with a water content of <10ppm, ethylene carbonate (EC), ethyl methyl carbonate (EMC), and diethyl carbonate (DEC) were uniformly mixed in a volume ratio of 1:1:1 to obtain an organic solvent. Then, fully dried lithium salt LiPF6 was dissolved in the mixed organic solvent to prepare an electrolyte with a concentration of 1 mol / L.

[0089] (4) Diaphragm preparation:

[0090] A PE porous membrane with a thickness of 11 μm was selected as the separator.

[0091] (5) Assembling the battery:

[0092] The positive electrode sheet, separator, and negative electrode sheet prepared above were stacked in sequence, with the separator positioned between the positive and negative electrode sheets to act as a barrier. The sheets were then wrapped with aluminum-plastic film and dried in a vacuum oven at 120°C. 3.0g / Ah of electrolyte was injected and sealed. After standing, hot and cold pressing, formation, clamping, and capacity grading, a soft-pack battery (i.e., lithium-ion battery) with a capacity of 1Ah was finally produced.

[0093] The specific steps and conditions for electrolyte formation are as follows: after the electrolyte is injected, maintain a hot press environment of 0.1MPa, charge at 0.02C for 17 minutes at 45°C in a static state, stand for 5 minutes and then charge to 0.3Ah at 0.02C, then cut off the air bag and vacuum seal, and stand at room temperature for 48 hours to complete the electrolyte formation.

[0094] Example 2

[0095] The difference between this embodiment and embodiment 1 is that when preparing the negative electrode sheet, in step S1 , a first negative electrode material with a structural stability parameter S1 of 7.4 and an average particle size Dv50 of 13.7 μm is used.

[0096] In the negative electrode sheet obtained in this embodiment, the structural stability parameter ratio S1 / S2 of the first active material layer and the second active material layer in the negative electrode active material layer is 3.2, and the negative electrode active material layer It is 30.4.

[0097] Example 3

[0098] The difference between this embodiment and embodiment 1 is that when preparing the negative electrode sheet, in step S1 , a first negative electrode material with a structural stability parameter S1 of 10.4 and an average particle size Dv50 of 12.2 μm is used.

[0099] In the negative electrode sheet obtained in this embodiment, the structural stability parameter ratio S1 / S2 of the first active material layer and the second active material layer in the negative electrode active material layer is 3.2, and the negative electrode active material layer It is 26.7.

[0100] Example 4

[0101] The difference between this embodiment and embodiment 1 is that: when preparing the negative electrode sheet, in step S1 and step S2, a first negative electrode material having a structural stability parameter S1 of 8.3 and an average particle size Dv50 of 13.3 μm and a second negative electrode material having a structural stability parameter S2 of 1.2 and an average particle size Dv50 of 18.3 μm are used; in step S3, after the negative electrode active material layer is coated on the current collector, the compaction density of the negative electrode active material layer on the current collector is adjusted to 1.75 g / m 3 .

[0102] In the negative electrode sheet obtained in this embodiment, the structural stability parameter ratio S1 / S2 of the first active material layer and the second active material layer in the negative electrode active material layer is 6.9, and the negative electrode active material layer It is 49.0.

[0103] Example 5

[0104] The difference between this embodiment and embodiment 1 is that: when preparing the negative electrode sheet, in step S1 and step S2, a first negative electrode material with a structural stability parameter S1 of 8.0 and an average particle size Dv50 of 16.2 μm and a second negative electrode material with a structural stability parameter S2 of 100 and an average particle size Dv50 of 18.0 μm are used; in step S3, after the negative electrode active material layer is coated on the current collector, the compaction density of the negative electrode active material layer on the current collector is adjusted to 1.75 g / m 3 .

[0105] In the negative electrode sheet obtained in this embodiment, the structural stability parameter ratio S1 / S2 of the first active material layer and the second active material layer in the negative electrode active material layer is 6.9, and the negative electrode active material layer It is 71.2.

[0106] Example 6

[0107] The difference between this embodiment and embodiment 1 is that: when preparing the negative electrode sheet, in step S1 and step S2, a first negative electrode material having a structural stability parameter S1 of 15.6 and an average particle size Dv50 of 11.2 μm and a second negative electrode material having a structural stability parameter S2 of 1.2 and an average particle size Dv50 of 18.3 μm are used; in step S3, after the negative electrode active material layer is coated on the current collector, the compaction density of the negative electrode active material layer on the current collector is adjusted to 1.75 g / m 3 .

[0108] In the negative electrode sheet obtained in this embodiment, the structural stability parameter ratio S1 / S2 of the first active material layer and the second active material layer in the negative electrode active material layer is 13.0, and the negative electrode active material layer It is 64.7.

[0109] Example 7

[0110] The difference between this embodiment and embodiment 6 is that when preparing the negative electrode sheet, in step S1, a first negative electrode material with a structural stability parameter S1 of 13.4 and an average particle size Dv50 of 12.6 μm is used.

[0111] In the negative electrode sheet obtained in this embodiment, the structural stability parameter ratio S1 / S2 of the first active material layer and the second active material layer in the negative electrode active material layer is 11.2, and the negative electrode active material layer It is 56.8.

[0112] Example 8

[0113] The difference between this embodiment and embodiment 1 is that: when preparing the negative electrode sheet, in step S1 and step S2, a first negative electrode material with a structural stability parameter S1 of 9.7 and an average particle size Dv50 of 13.1 μm and a second negative electrode material with a structural stability parameter S2 of 2.3 and an average particle size Dv50 of 16.7 μm are used; in step S3, after the negative electrode active material layer is coated on the current collector, the compaction density of the negative electrode active material layer on the current collector is adjusted to 1.75 g / m 3 .

[0114] In the negative electrode sheet obtained in this embodiment, the structural stability parameter ratio S1 / S2 of the first active material layer and the second active material layer in the negative electrode active material layer is 4.2, and the negative electrode active material layer It is 86.3.

[0115] Example 9

[0116] The difference between this embodiment and embodiment 1 is that: when preparing the negative electrode sheet, in step S1 and step S2, a first negative electrode material having a structural stability parameter S1 of 1.51 and an average particle size Dv50 of 13.7 μm and a second negative electrode material having a structural stability parameter S2 of 1.2 and an average particle size Dv50 of 18.3 μm are used; in step S3, after the negative electrode active material layer is coated on the current collector, the compaction density of the negative electrode active material layer on the current collector is adjusted to 1.75 g / m 3 .

[0117] In the negative electrode sheet obtained in this embodiment, the structural stability parameter ratio S1 / S2 of the first active material layer and the second active material layer in the negative electrode active material layer is 12.6, and the negative electrode active material layer It is 91.2.

[0118] Material parameter tests were performed on the negative electrode sheets prepared in Examples 1 to 9, and fast charge time tests, cycle capacity retention rate tests, and storage performance tests were performed on secondary lithium-ion batteries equipped with the negative electrode sheets prepared in Examples 1 to 10.

[0119] (1) Structural stability parameter S test of negative electrode material:

[0120] Referring to the national standard GB / T24533-2019, the powder sample was pressed into a tablet. Before the test, the standard pressure of 2T was maintained for 20s. The formal operation began after a pressure relief interval of 10 minutes. The powder pressure of 5T was measured as P0. Then the powder was tested at 2.5T, 3T, 3.5T...2+0.5n T...pressures. The compactions were recorded as P1, P2, P3...Pn... respectively. When Pn / Pn-1≥1.05, this Pn was called the particle structure failure compaction, and the structural stability parameter S=2+0.5n / 5 was recorded.

[0121] (2) Porosity ε test of negative electrode sheet:

[0122] Referring to the test method of national standard GB21650.1-2008, a small number of negative electrodes were selected for testing.

[0123] (3) Test of the tortuosity τ of the negative electrode:

[0124] The test was performed with reference to the image method used in the literature Ebner M, Wood V. Tool for Tortuosity Estimation in Lithium Ion Battery Porous Electrodes[J]. Journal of the Electrochemical Society, 2014, 162(2): A3064-A3070.

[0125] (4) Orientation degree V of negative electrode OI test:

[0126] According to the national standard GB / T23442-2009, a small number of negative electrode pieces were selected for testing, and the intensity ratio of the I004 / I110 peak in the XRD diffraction of the negative electrode pieces was taken as the orientation degree V OI .

[0127] (5) Fast charging time test:

[0128] The battery cell is directly charged to 8% SOC state with 0.33C current, and then the charging windows of 10%, 20%, 30%, 40%, 50%, 60%, 70%, and 80% are tested according to the three-electrode window of the battery cell, namely C1, C2, C3, C4, C5, C6, C7, and C8, and step charge is used to charge to 80%, that is, 8%-10% is charged with C1, 10% to 20% is charged with C2, and so on. The charging time from 8% to 80% SOC state is recorded as the fast charging capability measurement standard. The calculation formula is

[0129] T=(0.02 / C1+0.1 / C2+0.1 / C3+0.1 / C4+0.1 / C5+0.1 / C6+0.1 / C7+0.1 / C8)×60.

[0130] (6) Cycle capacity retention test:

[0131] At 25°C, charge and discharge the cell at 0.21 A / g (calculated based on the mass of the positive electrode material) and 2.8-4.25V. Record the number of cycles until 80% of the initial capacity is reached, which is used as the evaluation number of the material's cycling capability.

[0132] (7) Storage performance test:

[0133] At 25°C, the capacity is fixed with a current of 0.33C and recorded as C0. The battery cell is then stored at a high temperature of 60°C. After that, the battery cell is taken out every 7 days and the capacity is tested at room temperature and recorded as C1, C2...Cn. The number of days when Cn deteriorates to 80% of C0 is recorded as a measure of storage capacity.

[0134] The preparation parameters of the negative electrode sheets in Examples 1 to 9 are shown in Tables 1 and 2, and the performance test results of the negative electrode sheets in secondary lithium batteries are shown in Table 3.

[0135] Table 1: Parameters of negative electrode materials used in preparing negative electrode sheets in Examples 1 to 9

[0136] Table 2: Material parameters of the negative electrode sheets prepared in Examples 1 to 9

[0137] Table 3: Fast charge performance, cycle discharge performance and storage performance of secondary lithium-ion batteries prepared with negative electrode sheets prepared in Examples 1 to 9

[0138] From the test results of Examples 1 to 9, it can be seen that the structural parameters of the negative electrode active material layer in the negative electrode sheet are adjusted so that the structural stability parameter ratio S1 / S2 of the first active material layer and the second active material layer in the negative electrode active material layer is 0.01-10.0 and meets At the same time, it can improve the fast charging performance, cycle performance and storage performance of secondary lithium-ion batteries.

[0139] From the test results of Examples 1 to 3, it can be seen that when the structural stability parameter ratio S1 / S2 of the first active material layer and the second active material layer in the negative electrode active material layer is 0.1-5.0 and meets The secondary lithium-ion battery has good fast charging performance, cycle performance and storage performance at the same time. The fast charging time of the secondary lithium-ion battery to 80% SOC can be reduced to less than 15 minutes, the number of cycles to maintain 80% capacity is more than 3,000 cycles, and the number of days to maintain 80% storage capacity is more than 210 days.

[0140] Comparing the test results of Examples 1 to 3 and Examples 4 to 5, it can be seen that when the structural stability parameter ratio S1 / S2 of the first active material layer and the second active material layer in the negative electrode active material layer is lower than 0.1 or higher than 5.0, it will lead to the negative electrode plate being adjusted by compaction and plate design. The effect of improving the fast charging performance, cycle performance and storage performance of secondary lithium-ion batteries is not obvious. As shown in the test results, the fast charging time of secondary lithium-ion batteries to 80% SOC is increased to more than 15 minutes, the number of cycles to maintain 80% capacity is reduced to 1700 to 2100 cycles, and the number of days to maintain 80% storage capacity is also reduced to less than 210 days.

[0141] By comparing the test results of Examples 1 to 5 and Examples 6 to 7, it can be seen that when the structural stability parameter ratio S1 / S2 of the first active material layer and the second active material layer materials in the negative electrode active material layer is higher than 10.0, the fast charging performance, cycle performance and storage performance of the secondary lithium ion battery will be further reduced. As shown in the test results, with the increase of S1 / S2 in the negative electrode plate, the fast charging time of the secondary lithium ion battery to 80% SOC is further increased on the basis of 15 minutes, the number of cycles to maintain 80% capacity is further reduced on the basis of 1700 cycles, and the number of days at which 80% storage capacity is maintained is further reduced to less than 170 days.

[0142] Comparing the test results of Examples 1 to 4 and Example 8, it can be seen that when the negative electrode active material layer When the structure stability parameter ratio S1 / S2 of the materials of the first active material layer and the second active material layer is within the range of 0.1-5.0, the fast charging performance, cycle performance and storage performance of the secondary lithium-ion battery will be greatly reduced. As shown in the test results, the fast charging time of the secondary lithium-ion battery to 80% SOC is increased to more than 18 minutes, the number of cycles to maintain 80% capacity is reduced to less than 1400 cycles, and the number of days to maintain 80% storage capacity is reduced to less than 140 days.

[0143] Comparing the test results of Example 8 and Example 9, it can be seen that when the negative electrode active material layer When the structural stability parameter ratio S1 / S2 of the materials of the first active material layer and the second active material layer is higher than 10.0, the fast charging performance, cycle performance and storage performance of the secondary lithium-ion battery will further decline significantly. As shown in the test results, the fast charging time of the secondary lithium-ion battery to 80% SOC is further increased to more than 20 minutes, the number of cycles to maintain 80% capacity is further reduced to less than 1,000 cycles, and the number of days to maintain 80% storage capacity is further reduced to less than 110 days.

[0144] In summary, the present invention provides a negative electrode sheet, which adjusts the double-layer material selection and overall compaction density of the negative electrode active material layer to make the negative electrode active material layer meet the structural stability parameter ratio S1 / S2 of the first active material layer and the second active material layer of 0.01-10.0, and The value is 6.5-80.0, so as to adjust the structural stability of the negative electrode and the rationality of the pore distribution, and use the negative electrode to form a negative electrode active material layer with low tortuosity, high orientation and rich pores during compression, thereby improving the fast charging, cycling and storage performance of the secondary lithium-ion battery.

[0145] The above embodiments are merely illustrative of the principles and effects of the present invention and are not intended to limit the present invention. Anyone skilled in the art may modify or alter the above embodiments without departing from the spirit and scope of the present invention. Therefore, all equivalent modifications or alterations made by one of ordinary skill in the art without departing from the spirit and technical principles disclosed herein are intended to be covered by the claims of the present invention.

Claims

1. A negative electrode plate, characterized in that: include: current collector; A negative electrode active material layer, the negative electrode active material layer is at least located on one side of the current collector, the negative electrode active material layer includes a first active material layer and a second active material layer, the first active material layer and the second active material layer are stacked in a direction close to the current collector; Wherein, the structural stability parameter ratio S1 / S2 of the first active material layer and the second active material layer satisfies 0.01-10.

0.

2. The negative electrode sheet according to claim 1, characterized in that: The negative electrode material in the negative electrode active material layer includes a graphite material.

3. The negative electrode sheet according to claim 1, characterized in that: The structural stability parameter ratio S1 / S2 of the first active material layer and the second active material layer satisfies 0.1-5.

0.

4. The negative electrode sheet according to claim 1, characterized in that: The structural stability parameter S1 of the first active material layer is 3-16.

5. The negative electrode sheet according to claim 1, characterized in that: The negative electrode active material layer satisfies Wherein, τ is the tortuosity of the negative electrode active material layer, ε is the porosity of the negative electrode active material layer, P is the compaction density of the negative electrode active material layer, V OI It is the stacking orientation degree of the negative electrode material particles in the negative electrode active material layer.

6. The negative electrode sheet according to claim 5, characterized in that: The negative electrode active material layer satisfies 7. The negative electrode sheet according to claim 1, characterized in that: The compaction of the negative electrode active material layer is 1.65-1.75 g / cm 3 .

8. A method for preparing a negative electrode sheet according to any one of claims 1 to 7, characterized in that: include: Mixing the first negative electrode material with a binder, a conductive agent and a thickener to form a slurry to obtain a first active material layer slurry; Mixing the second negative electrode material with a binder, a conductive agent and a thickener to form a slurry to obtain a second active material layer slurry; The second active material layer slurry and the first active material layer slurry are sequentially coated on at least one side of the current collector, and after coating, the negative electrode sheet is obtained by drying, compacting, die-cutting and punching; Wherein, the hardness of the first negative electrode material is higher than that of the second negative electrode material.

9. The method for preparing a negative electrode sheet according to claim 8, characterized in that: The structural stability parameter ratio S1 / S2 of the first negative electrode material and the second negative electrode material satisfies 0.01-10.

0.

10. The method for preparing a negative electrode sheet according to claim 8, characterized in that: The average particle size Dv50 of the first negative electrode material is 11-17 μm, and the average particle size Dv50 of the second negative electrode material is 16-19 μm.

11. An electrochemical device, characterized in that: It comprises a positive electrode sheet, a separator, an electrolyte and a negative electrode sheet as claimed in any one of claims 1 to 7.

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