Electrode sheet and battery

Abstract

An electrode sheet and a battery. The electrode sheet comprises a current collector and active material layers, wherein the active material layers comprise a first active material layer and a second active material layer in the thickness direction, the first active material layer is located on at least one functional surface of the current collector, and the second active material layer is located on the surface of the side of the first active material layer distant from the current collector; and the absolute difference between the porosity of the first active material layer and the porosity of the second active material layer is less than 5%. By controlling the uniformity of the porosities of the active material layers, a transmission path of lithium ions and an electrolyte is shortened, and the transmission speed of the lithium ions and the electrolyte into the electrode sheet is increased, thereby improving the dynamics performance of the electrode sheet, reducing the risk of lithium precipitation, and improving the rate capability, cycle performance and safety performance of the battery.

Description

Electrode sheet and battery

[0001] This application claims the priority of the Chinese patent application filed with the China Patent Office on August 29, 2023, with application number 202311096094.2 and application name “An Electrode Sheet and Battery”, the entire contents of which are incorporated herein by reference. Technical Field

[0002] The present application belongs to the field of lithium-ion batteries, and specifically relates to an electrode sheet and a battery. Background Art

[0003] With the rapid development of new energy vehicles and large-scale energy storage industries, higher requirements are placed on the energy density, rate performance, cycle performance and other properties of lithium-ion batteries. The content of active materials in the electrode sheet determines the energy density of the battery. In order to improve the energy density of the battery, the thickness and compaction density of the active material layer in the positive and negative electrode sheets are usually increased, which can effectively improve the volume energy density of the battery. However, as the thickness and compaction density of the active material layer increase, the porosity decreases, especially during the rolling process of the active material layer surface, the force is large, the compaction is higher, and the pores are smaller, resulting in a longer electrolyte transmission path, increased internal resistance, poor wettability of the electrolyte to the electrode sheet, reduced lithium ion transmission efficiency, poor kinetic performance of the electrode sheet, increased risk of lithium plating, and poor rate performance and cycle performance of the battery.

[0004] Summary of the Invention

[0005] The electrode sheet provided in the present application can shorten the transmission path of lithium ions and electrolyte, increase the transmission speed of lithium ions and electrolyte into the interior of the electrode sheet by controlling the uniformity of the porosity of the first active material layer and the second active material layer, and jointly improve the kinetic performance of the electrode sheet, reduce the risk of lithium plating, and thus improve the rate performance, cycle performance, and safety performance of the battery.

[0006] The battery provided in this application, due to the use of the above-mentioned electrode sheet, performs excellent in terms of rate performance, cycle performance, safety performance, etc.

[0007] In a first aspect of the present application, an electrode sheet is provided, comprising a current collector and an active material layer, wherein the active material layer comprises a first active material layer and a second active material layer along a thickness direction.

[0008] The first active material layer is located on at least one functional surface of the current collector, and the second active material layer is located on a side of the first active material layer away from the current collector;

[0009] An absolute difference between the porosity of the first active material layer and the porosity of the second active material layer is less than 5%.

[0010] The electrode sheet as described above, wherein the absolute difference between the porosity of the first active material layer and the porosity of the second active material layer is less than 3%;

[0011] Preferably, the absolute difference between the porosity of the first active material layer and the porosity of the second active material layer is less than 1%.

[0012] The electrode sheet as described above, wherein the ratio of the pore tortuosity T of the active material layer to the electrolyte permeation flow rate K is in the range of 0.3 to 1.3.

[0013] The electrode sheet as described above, wherein the porosity of the first active material layer is 17% to 33%, and / or

[0014] The porosity of the second active material layer is 15% to 28%.

[0015] The electrode sheet as described above, wherein the electrolyte permeation flow rate K of the electrode sheet is 1.4 to 3.5 μm / s;

[0016] The electrolyte permeation flow rate K of the electrode sheet is the ratio of the transmission path length Lt of the electrolyte through the pores of the active material layer to the permeation time.

[0017] The electrode sheet as described above, wherein the pore tortuosity T of the active material layer is 1.1 to 1.5, wherein the pore tortuosity T is the ratio of the transmission path length Lt of the electrolyte passing through the pores of the active material layer to the thickness L0 of the active material layer.

[0018] The electrode sheet as described above, wherein the transmission path length Lt satisfies: 80 μm ≤ Lt ≤ 110 μm; and / or,

[0019] The compacted density PD of the active material layer satisfies: 1.50 g / cm 3 ≤PD≤1.75g / cm 3 and / or,

[0020] The Dv99 of the active material particles in the active material layer satisfies: 30 μm ≤ Dv99 ≤ 60 μm; and / or,

[0021] The thickness L0 of the active material layer satisfies: 65 μm ≤ L0 ≤ 80 μm; and / or,

[0022] The overall porosity ε of the active material layer satisfies: 16%≤ε≤31%.

[0023] The electrode sheet as described above, wherein the active material layer comprises at least an active material and a block copolymer, and the block copolymer comprises at least a polyacrylic acid block and a polyethylene oxide block.

[0024] The electrode sheet as described above, wherein the first active material layer comprises the following components by mass fraction: 96% to 98% of a first active material, 0.5% to 3% of a first conductive agent, 1% to 2% of a thickener, and 1% to 3% of a first binder; and / or,

[0025] The second active material layer includes the following components in terms of mass percentage: 94% to 97.5% of a second active material, 0.5% to 3.5% of a second conductive agent, 1% to 3% of a second binder, and 0.6% to 3% of a block copolymer.

[0026] The electrode sheet as described above, wherein the block copolymer has the following structure:

[0027] Where m>0, n>0.

[0028] The electrode sheet as described above, wherein the molecular weight of the block copolymer is 100,000-4,000,000, and the particle size of the block copolymer is 90-220 nm.

[0029] In a second aspect of the present application, a battery is provided, comprising the electrode sheet as described in the first aspect above.

[0030] The implementation of this application has at least the following beneficial effects:

[0031] The electrode sheet provided in the present application improves the uniformity of the overall porosity of the active material layer by controlling the absolute difference between the first active material layer and the second active material layer to be less than 3%, thereby shortening the transmission path of lithium ions and electrolyte and increasing the transmission speed of lithium ions and electrolyte to the interior of the electrode sheet, thereby improving the kinetic performance of the electrode sheet, reducing the risk of lithium plating, and thereby improving the rate performance, cycle performance, and safety performance of the battery. BRIEF DESCRIPTION OF THE DRAWINGS

[0032] FIG1 is a cross-sectional SEM image of an electrode sheet in one embodiment of the present application;

[0033] FIG2 is a cross-sectional SEM image of an electrode sheet in another embodiment of the present application;

[0034] FIG3 is an optical image of the effect of the negative electrode sheet after electrolyte penetration in Example 4 of the present application;

[0035] FIG4 is an optical image of the effect of the negative electrode sheet after electrolyte penetration in Comparative Example 1 of the present application.

[0036] Description of reference numerals:

[0037] a-first active material layer; b-second active material layer; 1-negative electrode current collector; 2-negative electrode active material layer; 201-first surface; c-third active material layer; d-fourth active material layer. DETAILED DESCRIPTION

[0038] To make the purpose, technical solutions, and advantages of this application more clear, the technical solutions in the embodiments of this application will be clearly and completely described below in conjunction with the embodiments of this application. Obviously, the described embodiments are part of the embodiments of this application, not all of the embodiments. Based on the embodiments of this application, all other embodiments obtained by ordinary technicians in this field without making creative efforts are within the scope of protection of this application.

[0039] In a first aspect of the present application, an electrode sheet is provided, which includes a current collector and an active material layer. The active material layer includes a first active material layer and a second active material layer along the thickness direction. The first active material layer is located on at least one functional surface of the current collector, and the second active material layer is located on the side surface of the first active material layer away from the current collector; the absolute difference between the porosity of the first active material layer and the porosity of the second active material layer is less than 5%.

[0040] The current collector includes two functional surfaces arranged opposite to each other, and the two functional surfaces are used to coat the active material layer. In the present application, the active material layer can be coated on only one functional surface of the current collector, or on both functional surfaces of the current collector at the same time.

[0041] The present application does not limit the specific material of the current collector, for example, it can be aluminum, copper, etc.

[0042] The active material layer on a functional surface is divided into a first active material layer and a second active material layer along the thickness direction of the current collector. The first active material layer is located on at least one functional surface of the current collector, and the second active material layer is located on the side surface of the first active material layer away from the current collector.

[0043] By controlling the uniformity of the porosity of the first active material layer and the second active material layer, it is beneficial to promote the rapid embedding and rapid removal of lithium ions on the surface of the electrode sheet while ensuring the permeability of the electrolyte to the electrode sheet, thereby reducing the risk of lithium plating during fast charging and cycling of the battery. The absolute difference between the porosity of the second active material layer and the porosity of the first active material layer is less than 5%. In this way, it is beneficial to achieve uniform overall pores of the active material layer and reduce the risk of lithium plating while ensuring the permeability of the electrolyte. When the electrode sheet is applied to the battery, it can shorten the transmission path of lithium ions and electrolyte, increase the transmission speed of lithium ions and electrolyte to the inside of the electrode sheet, thereby improving the battery's rate performance, cycle performance, and safety performance.

[0044] The absolute difference between the porosity of the second active material layer and the porosity of the first active material layer refers to the absolute value of the difference between the porosity of the second active material layer and the porosity of the first active material layer. For example, when the porosity of the first active material layer is P1 and the porosity of the second active material layer is P2, the absolute difference is the absolute value of P1-P2.

[0045] It should be noted that pores refer to the distance between particles in each active material layer, and porosity refers to the percentage of the pore volume in each active material layer to the total volume of each active material layer. The porosity of each layer can be measured using the gas displacement method.

[0046] By improving the uniformity of the overall pores of the first active material layer and the second active material layer, the risk of lithium plating is reduced while ensuring the permeability of the electrolyte. The higher the uniformity, the more conducive it is to improving the permeability and reducing the risk of lithium plating. In addition, the inventors believe that when the absolute difference between the porosity of the second active material layer and the porosity of the first active material layer is less than 3%, in addition to reducing the risk of lithium plating while ensuring the permeability of the electrolyte, it is also beneficial to reduce the thickness expansion rate of the battery assembled by the electrode sheet; when the absolute difference between the porosity of the second active material layer and the porosity of the first active material layer is not higher than 2.5%, in addition to reducing the thickness expansion rate of the battery, it can also improve the rate performance of the battery, so that the 3C discharge capacity retention rate of the battery is ≥65%, and the 4C discharge capacity retention rate is ≥53%. Furthermore, when the absolute difference between the porosity of the first active material layer and the porosity of the second active material layer is less than 1%, it is possible to reduce the risk of lithium plating and the thickness expansion rate of the battery while ensuring the permeability of the electrolyte, and at the same time improve the rate performance of the battery, so that the thickness expansion rate of the battery is not higher than 5.5%, the 3C discharge capacity retention rate is ≥70%, and the 4C discharge capacity retention rate is ≥60%.

[0047] In some embodiments, the ratio of the pore tortuosity T of the active material layer to the electrolyte permeation flow rate K is in the range of 0.3 to 1.3. The pore tortuosity T of the active material layer is the length L of the transmission path of the electrolyte through the pores of the active material layer. t The electrolyte penetration flow rate K is the ratio of the length of the transmission path Lt of the electrolyte through the pores of the active material layer to the penetration time, L t The unit of L0 is μm, and the unit of penetration time is s.

[0048] The porosity of the active material layer determines the diffusion and transport of lithium ions. The higher the porosity, the more conducive it is to the penetration of the electrolyte and the transport of lithium ions, promoting the increase in the speed of lithium ions and electrolyte transport into the interior of the electrode sheet. At the same time, it is conducive to the increase in the channels and directions for the embedding and extraction of lithium ions in the active material layer, thereby enabling the battery to have excellent high-rate charge and discharge performance. However, the higher the porosity, the lower the volume density of the active material layer, and the lower the energy density of the battery. In some embodiments of the present application, the porosity of the first active material layer is 17% to 33%, and the porosity of the second active material layer is 15% to 28%, which is conducive to taking into account both the energy density and rate performance of the battery.

[0049] The prior art usually requires a long period of rest after the battery is injected to ensure that the electrolyte completely penetrates the electrode sheet. This long time seriously affects the battery production speed. The above-mentioned electrode sheet provided by the present application can improve the dynamic performance of the electrode sheet by shortening the transmission path of lithium ions and electrolyte and increasing the transmission speed of lithium ions and electrolyte to the inside of the electrode sheet. Among them, the transmission path of lithium ions and electrolyte is related to the tortuosity of the pore structure in the active material layer. The inventors have found through research that by further limiting the tortuosity of the pore structure in the active material layer, the rest time after injection can be further reduced and the low-temperature cycle performance of the battery can be improved. In some embodiments, the pore tortuosity T of the active material layer is 1.1 to 1.5, wherein the pore tortuosity T is the length L of the transmission path through which the electrolyte passes through the pores of the active material layer. t The ratio of the thickness of the active material layer L0 to the transmission path length L t It can be understood as the shortest path that the electrolyte passes through the active material layer (the entire active material layer on one side of the current collector). The pore tortuosity T of the active material layer refers to the pore tortuosity of the entire active material layer on one side of the current collector, and the thickness L0 of the active material layer refers to the thickness of the entire active material layer on one side of the current collector.

[0050] Among them, the transmission path length L t Available formula L t =2×(PD×Dv99)×(1-ε), in μm, L0 is the vertical distance from the surface of the active material layer away from the current collector to the current collector, in μm, PD is the compacted density of the active material layer, in g / cm 3 , Dv99 represents the value at which 99% of the measured particle volumes of the active material layer are smaller than this value, and the unit is μm, and ε is the porosity of the active material layer.

[0051] This application does not select specific values ​​of the above parameters, for example, the length L of the transmission path through which the electrolyte passes in the overall pores of the active material layer t Meet: 80μm≤L t≤110μm; and / or, the compacted density PD of the active material layer satisfies: 1.50g / cm 3 ≤PD≤1.75g / cm 3 ; and / or, the Dv99 of the active material particles in the active material layer satisfies: 30μm≤Dv99≤60μm; and / or, the thickness L0 of the active material layer satisfies: 65μm≤L0≤80μm; and / or, the electrolyte permeation flow rate K of the electrode sheet is 1.4~3.5μm / s, wherein the electrolyte permeation flow rate K of the active material layer is the transmission path length L of the electrolyte through the pores of the active material layer t The ratio of penetration time.

[0052] The active material layer at least includes an active material, a conductive agent, a binder and a block copolymer, and the block copolymer at least includes a polyacrylic acid block and a polyethylene oxide block.

[0053] The present application does not limit the specific types of active materials, conductive agents, and adhesives in the active material layer, and they can be conventional active materials, conductive agents, and adhesives. For example, the active material is a negative electrode active material such as a carbon-based material or a silicon-based material, or it can be a positive electrode active material such as a ternary material or lithium iron phosphate. The carbon-based material includes at least one of artificial graphite, natural graphite, and mixed graphite; the conductive agent includes one or more of carbon black, natural graphite, artificial graphite, acetylene black, carbon fiber, and carbon nanotubes; the adhesive includes one or more of polyvinylidene fluoride, vinylidene fluoride-hexafluoropropylene, polyamide, polyacrylonitrile, polyacrylate, polyacrylic acid, polyacrylate, polyvinylpyrrolidone, polyvinyl ether, and polymethyl methacrylate.

[0054] The block copolymer includes at least a polyacrylic acid block and a polyethylene oxide block. That is, the block copolymer may contain only the polyacrylic acid block and the polyethylene oxide block, or may contain other blocks in addition to the polyacrylic acid block and the polyethylene oxide block.

[0055] This application does not limit the arrangement of the individual blocks in the block copolymer. When the block copolymer contains only polyacrylic acid blocks and polyethylene oxide blocks, the two blocks are sequentially bonded by hydrogen bonds, van der Waals forces, etc. The polyacrylic acid blocks are formed by the polymerization of polyacrylic acid, and the polyethylene oxide blocks are formed by the polymerization of polyethylene oxide.

[0056] When the block copolymer further contains other blocks, the polyacrylic acid blocks, polyethylene oxide blocks and other blocks may be bonded alternately, sequentially, or randomly. This application does not impose any further restrictions on this.

[0057] According to the research of this application, the application of the electrode sheet provided by this application in a battery can effectively improve the battery's rate performance, cycle performance, and safety performance. This is because, by controlling the uniformity of the porosity of the active material layer, it helps to improve the porosity of the active material layer, reduce the pore tortuosity, shorten the transmission path of lithium ions and electrolyte, and help to accelerate the rate at which the electrolyte penetrates the electrode sheet, improving permeability. In addition, the block copolymer also has a high ionic conductivity, which plays a positive role in promoting the transmission of lithium ions.

[0058] The present application does not limit the specific structure of the block copolymer, as long as it includes two blocks formed by the polymerization monomers of polyacrylic acid and polyethylene oxide. In some embodiments, the block copolymer has the following structure:

[0059] Where m>0, n>0.

[0060] On the one hand, the block copolymer has a three-dimensional network structure, which helps to improve the porosity of the active material layer, reduces the pore tortuosity, and shortens the transmission path of lithium ions and electrolytes. On the other hand, the block copolymer has electrolyte affinity, which can accelerate the rate at which the electrolyte penetrates the electrode sheet and improve permeability. In addition, the block copolymer also has a high ionic conductivity, which plays a positive role in promoting the transmission of lithium ions. It can be seen that the present application utilizes block copolymers to improve the kinetic performance of the electrode sheet by shortening the transmission path of lithium ions and electrolytes and increasing the transmission speed of lithium ions and electrolytes to the inside of the electrode sheet, thereby reducing the risk of lithium precipitation, thereby improving the rate performance, cycle performance, and safety performance of the battery. The present application does not limit the molecular weight ratio of each block, and the molecular weight ratio of each block can be adjusted by controlling the addition amount of each polymerized monomer according to actual needs. For example, the molecular weight of the polyacrylic acid block accounts for 20 to 80% of the total molecular weight of the block copolymer, and the molecular weight of the polyethylene oxide block accounts for 20 to 80% of the total molecular weight of the block copolymer.

[0061] Furthermore, this application does not limit the molecular weight of the block copolymer. The specific molecular weight of the block copolymer can be adjusted according to actual needs by controlling the amount of monomer added. For example, the molecular weight of the block copolymer is 100,000 to 4,000,000, preferably 100,000 to 600,000. The molecular weight refers to the number-average molecular weight. If the molecular weight of the block copolymer is too large, it can easily lead to pore blockage, which in turn can cause severe lithium precipitation.

[0062] In this application, the block copolymer is added during the preparation of the active material layer slurry, without the need to increase the process and equipment, thus avoiding increased costs. By controlling the particle size of the block copolymer, the dispersibility of the block copolymer in the active material layer slurry can be improved, which is conducive to maximizing the performance of the block copolymer. In some embodiments, the particle size of the block copolymer is 90 to 220 μm, and the particle size range refers to the particle size corresponding to 50% to 97% of the volume distribution of the material. If the particle size is too large or too small, it will directly affect the dispersion of the block copolymer, and severe agglomeration will easily occur, which will have a certain impact on the uniformity of the pore structure of the active material layer, thereby affecting the permeability of the electrolyte.

[0063] This application does not limit the preparation process of the active material layer, as long as the above-mentioned active material layer is prepared. For example, the active material layer is prepared by the following method: dissolving the active material, adhesive, conductive agent and block copolymer in a solvent according to the required component ratio, and stirring them uniformly by a stirring device to obtain an active material layer slurry, wherein the solid content of the active material layer slurry is 48% to 52%, and then coating the stirred active material layer slurry on at least one functional surface of the current collector to obtain the active material layer.

[0064] By adding a compound that increases viscosity to the active material layer, the adhesion of the active material layer to the current collector can be further enhanced, preventing the active material layer from peeling off from the current collector. Specifically, the active material layer of the present application also includes a thickener, and the thickener includes one or more of sodium carboxymethyl cellulose and lithium carboxymethyl cellulose, to ensure reliable bonding strength between the active material layer and the current collector, to prevent the active material layer from falling off from the current collector, and to ensure the stability of the electrode sheet structure, thereby ensuring the electrochemical performance of the battery. The present application does not limit the content of the thickener, which can be adjusted according to actual needs. For example, in the active material layer, the content of the thickener is 0.6% to 2%.

[0065] In the present application, the second active material layer is further away from the current collector than the first active material layer, and the second active material layer is closer to the electrolyte than the first active material layer. The present application does not limit the types of the components in the first active material layer, and it can include conventional active materials, conductive agents, binders, and thickeners in the field. It is sufficient for the second active material layer to only include a block copolymer, which is beneficial for improving production efficiency and reducing costs.

[0066] When the above electrode sheet is applied to a battery, under the same compaction condition, the second active material layer is subject to greater force. By making the second active material layer contain a block copolymer, it is beneficial to increase the porosity of the second active material layer, and can avoid the problem that the second active material layer has a higher compaction density and smaller pores during rolling, thereby reducing the pore tortuosity and shortening the transmission paths of lithium ions and electrolyte. In addition, the block copolymer in the second active material layer has high electrolyte affinity and ionic conductivity. When the second active material layer is closer to the electrolyte than the first active material layer, it can further accelerate the rate of electrolyte penetration into the electrode sheet, improve the transmission of lithium ions, thereby improving the kinetic performance of the electrode sheet, reducing the risk of lithium deposition, and thus improving the rate performance, cycle performance and safety performance of the battery.

[0067] Under the same compaction density, by adjusting the component contents in the first active material layer and the second active material layer, it is beneficial to make the absolute difference in porosity between the second active material layer and the first active material layer less than 3%. For example, the first active material layer includes the following components by mass fraction: 96% - 98% of the first active material, 0.5% - 3% of the first conductive agent, 1% - 2% of the thickening agent, and 1% - 3% of the first binder; and / or, the second active material layer includes the following components by mass percentage: 94% - 97.5% of the second active material, 0.5% - 3.5% of the second conductive agent, 1% - 3% of the second binder, and 0.6 - 3% of the block copolymer. By adjusting the addition amount of the block copolymer, the absolute difference in porosity between the second active material layer and the first active material layer can be further adjusted to be less than 1%.

[0068] Under the condition of meeting the above porosity condition, by reasonably adjusting the thickness ratio of the first active material layer and the second active material layer, it is beneficial to take into account the capacity while ensuring the electrochemical performance. As shown in Figure 2, in some embodiments, the thickness of the first active material layer a is d1 μm, and the thickness of the second active material layer b is d2 μm, satisfying the following relationship: 0.25 < d2 / d1 < 2.5. If the d2 / d1 ratio is too small, the thickness of the second active material layer is too thin, and the improvement effect of the transmission speed of lithium ions and electrolyte into the electrode sheet is not obvious, which is not conducive to input and output at high rates; if the d2 / d1 ratio is too large, the thickness of the second active material layer is too thick, and the proportion of the active material is low, resulting in a low battery capacity.

[0069] This application does not limit the arrangement of the active material layers on both sides of the current collector, which can be a single-sided arrangement or a double-sided arrangement. For example, as shown in Figure 2, the first functional surface of the current collector 1 is respectively provided with a first active material layer a and a second active material layer b, and the second functional surface of the current collector 1 is respectively provided with a third active material layer c and a fourth active material layer d. At this time, the pore tortuosity T of the active material layer refers to the pore tortuosity of the first active material layer a and the second active material layer b as a whole, and may also refer to the pore tortuosity of the third active material layer c and the fourth active material layer d as a whole. The third active material layer c can be consistent with the components and parameters of the first active material layer a, and the fourth active material layer d can be consistent with the components and parameters of the second active material layer b. This application does not make too many restrictions here.

[0070] The electrode sheet of the present application can be a negative electrode sheet or a positive electrode sheet, which can be adjusted according to actual needs. When the electrode sheet is a negative electrode sheet, the current collector is copper foil, and the material of the active material layer is graphite, silicon-based or other negative electrode active materials; when the electrode sheet is a positive electrode sheet, the current collector is aluminum foil, and the material of the active material layer is a ternary material or a positive electrode active material such as lithium iron phosphate. It is preferably a negative electrode sheet, because the lithium plating problem is mainly on the negative electrode sheet. When only the kinetic performance of the positive electrode sheet is improved, it means that the rate of lithium ion extraction from the positive electrode is increased. At this time, if the kinetic performance of the negative electrode sheet cannot keep up, lithium plating will be more serious. Therefore, in one embodiment of the present application, the above-mentioned electrode sheet is a negative electrode sheet, and the positive electrode sheet can be a conventional positive electrode sheet in the field.

[0071] In a second aspect of the present application, a battery is provided, comprising the electrode sheet according to the first aspect.

[0072] Specifically, the electrode sheets include a negative electrode sheet and a positive electrode sheet. The negative electrode sheet, the positive electrode sheet, and the separator can form a battery cell, wherein at least one of the negative electrode sheet and the positive electrode sheet is the electrode sheet of the first aspect described above. By installing the battery cell and the protection circuit together inside the battery housing, a battery for charge / discharge can be formed. The quality of the battery cell directly determines the quality of the battery. Due to the use of the above-mentioned electrode sheets, the battery of the present application performs excellently in terms of rate performance, cycle performance, safety performance, etc.

[0073] The present application will be further described below through specific examples and comparative examples.

[0074] Example 1

[0075] 1. Preparation of positive electrode sheet

[0076] Lithium cobalt oxide (LiCoO2), conductive agent SP, PVDF and solvent NMP are evenly mixed to obtain a positive electrode slurry with a solid content of 76%. The obtained positive electrode slurry is evenly coated on the surface of the positive electrode collector, and the positive electrode sheet is obtained after drying, rolling and cutting.

[0077] 2. Preparation of negative electrode sheet

[0078] (1) First active material layer slurry: 96.4% graphite, 1% conductive agent, 1.1% thickener, and 1.5% binder are uniformly mixed to obtain the first active material layer slurry;

[0079] (2) Second active material layer slurry: 95.7% graphite, 1% conductive carbon, 1.2% thickener, 1.5% binder, 0.6% block copolymer, and H2O are uniformly mixed to obtain a second active material layer slurry; wherein the block copolymer is composed of polyacrylic acid block and polyethylene oxide block, the number average molecular weight of the block copolymer is 200,000 to 600,000, and the particle size of the block copolymer is 100 to 180 μm;

[0080] (3) The first active material layer slurry and the second active material layer slurry are coated on the negative electrode current collector in sequence by double-layer coating, and after rolling and cutting, a negative electrode sheet is obtained, wherein the compaction density of the second active material layer and the first active material layer is 1.7 g / cm 3 , the ratio of the thickness of the second active material layer to the thickness of the first active material layer is 2:8.

[0081] 3. Preparation of lithium-ion batteries

[0082] The positive electrode sheet, separator and negative electrode sheet are stacked in sequence and wound to obtain a bare cell, which is then placed in an outer packaging foil aluminum-plastic film. The electrolyte is injected into the bare cell, and after vacuum packaging, standing, formation, secondary sealing, sorting, shaping and testing processes, a soft-pack lithium-ion battery is obtained. The electrolyte is purchased from Xinzhoubang LBC450A23 and the separator is purchased from Enjie.

[0083] Example 2

[0084] The preparation process is basically the same as that of Example 1, except that in step (2) of the preparation of the negative electrode sheet, "95.7% graphite, 1% conductive carbon, 1.2% thickener, 1.5% binder and 0.6% block copolymer" is replaced with "95.5% graphite, 1% conductive carbon, 0.8% thickener, 1.5% binder and 1.2% block copolymer". Other conditions remain unchanged, and the negative electrode sheet of Example 1 is replaced with the negative electrode sheet of this example to obtain a lithium ion battery.

[0085] Example 3

[0086] The preparation process is basically the same as that of Example 1, except that in step (2) of the preparation of the negative electrode sheet, "95.7% graphite, 1% conductive carbon, 1.2% thickener, 1.5% binder and 0.6% block copolymer" is replaced with "95.1% graphite, 1% conductive carbon, 0.6% thickener, 1.5% binder and 1.8% block copolymer". Other conditions remain unchanged. The negative electrode sheet of Example 1 is replaced with the negative electrode sheet of this example to obtain a lithium ion battery.

[0087] Example 4

[0088] The preparation process is basically the same as that of Example 1, except that in step (2) of the preparation of the negative electrode sheet, "95.7% graphite, 1% conductive carbon, 1.2% thickener, 1.5% binder and 0.6% block copolymer" is replaced by "95.9% graphite, 1% conductive carbon, 0.6% thickener, 1.3% binder and 1.2% block copolymer", and "the ratio of the thickness of the second active material layer to the thickness of the first active material layer is 2:8" is replaced by "the ratio of the thickness of the second active material layer to the thickness of the first active material layer is 3:7". Other conditions remain unchanged, the negative electrode sheet of Example 1 is replaced by the negative electrode sheet of this example, and a lithium ion battery is obtained.

[0089] Example 5

[0090] The preparation process is basically the same as that of Example 1, except that in step (2) of the preparation of the negative electrode sheet, "95.7% graphite, 1% conductive carbon, 1.2% thickener, 1.5% binder and 0.6% block copolymer" is replaced by "95.5% graphite, 1% conductive carbon, 0.8% thickener, 1.5% binder and 1.2% block copolymer", and "the ratio of the thickness of the second active material layer to the thickness of the first active material layer is 2:8" is replaced by "the ratio of the thickness of the second active material layer to the thickness of the first active material layer is 5:5". Other conditions remain unchanged, and the negative electrode sheet of Example 1 is replaced by the negative electrode sheet of this example to obtain a lithium ion battery.

[0091] Example 6

[0092] The preparation process is basically the same as that of Example 1, except that in step (2) of the preparation of the negative electrode sheet, "95.7% graphite, 1% conductive carbon, 1.2% thickener, 1.5% binder and 0.6% block copolymer" is replaced by "95.5% graphite, 1% conductive carbon, 0.8% thickener, 1.5% binder and 1.2% block copolymer", and "the ratio of the thickness of the second active material layer to the thickness of the first active material layer is 2:8" is replaced by "the ratio of the thickness of the second active material layer to the thickness of the first active material layer is 7:3". Other conditions remain unchanged, and the negative electrode sheet of Example 1 is replaced by the negative electrode sheet of this example to obtain a lithium ion battery.

[0093] Example 7

[0094] The preparation process is basically the same as that of Example 1, except that step (1) is omitted, and in the preparation of the negative electrode sheet, step (2) replaces "95.7% graphite, 1% conductive carbon, 1.2% thickener, 1.5% binder and 0.6% block copolymer" with "95.5% graphite, 1% conductive carbon, 0.8% thickener, 1.5% binder and 1.2% block copolymer", and in step (3), only the second active material layer slurry is coated on the negative electrode current collector, and other conditions remain unchanged to obtain a negative electrode sheet;

[0095] The negative electrode sheet of Example 1 was replaced with the negative electrode sheet of this example, and a lithium-ion battery was assembled.

[0096] Example 8

[0097] The preparation process is basically the same as that of Example 1, except that in the preparation of the negative electrode sheet, step (2) replaces "95.7% graphite, 1% conductive carbon, 1.2% thickener, 1.5% binder and 0.6% block copolymer" with "94.7% graphite, 1.3% conductive carbon, 0.6% thickener, 1% binder and 2.4% block copolymer".

[0098] Comparative Example 1

[0099] The preparation process is basically the same as that of Example 1, except that in the preparation of the negative electrode sheet, the "0.6% block copolymer" is removed in step (2). Other conditions remain unchanged. The negative electrode sheet of Example 1 is replaced by the negative electrode sheet of this comparative example to obtain a lithium ion battery.

[0100] Test example

[0101] 1. Porosity determination

[0102] The negative electrode sheets were cut into rectangular blocks of the same size. Twenty samples were tested for each embodiment, and the volume of each sample was about 2 cm. 3 The porosity P1 of the first active material layer, the porosity P2 of the second active material layer, and the porosity ε of the active material layer as a whole were measured in accordance with the standard "GB / T 24586-2009 Iron Ore - Determination of Apparent Density, True Density, and Porosity." Helium was used as the test gas. The results are shown in Table 1.

[0103] 2. Pore tortuosity determination

[0104] Pore ​​tortuosity T = L t / L0,L t The shortest path of the electrolyte in the negative electrode active material layer (including the first active material layer and the second active material layer); the transmission path L of the electrolyte in the negative electrode sheet t The formula L t=2×(PD×Dv99)×(1-ε), in μm. As shown in FIG1 , L0 is the vertical distance from the first surface 201 of the negative electrode active material layer 2 away from the negative electrode current collector 1 to the negative electrode current collector 1, in μm. PD is the compacted density of the active material layer, in g / cm 3 The compacted density of the active material layer of the above examples and comparative examples is 1.7 g / cm 3 Dv99 represents the value below which 99% of the measured graphite volume is smaller than, expressed in μm. The Dv99 values ​​of the graphite used in the above examples and comparative examples were both 41 μm. ε represents the overall porosity of the coating on the negative electrode sheet. The results are shown in Table 1.

[0105] 3. Electrolyte permeability test

[0106] Take a 30 mm long and 30 mm wide negative electrode sheet. Under dry conditions, drop 1 mL of electrolyte onto the sample surface. Start timing and stop when the electrolyte completely disappears from the sample surface. The time is recorded as the penetration time. For each example, 50 samples were tested and the average value was taken. The electrolyte was purchased from Xinzhoubang LBC450A23.

[0107] 4. Rate discharge test

[0108] (1) Let the lithium-ion battery stand for 10 minutes; (2) discharge at 0.2C to the lower limit voltage and let it stand for 10 minutes; (3) charge at 0.7C to the upper limit voltage in a constant temperature room, with a cutoff current of 0.05C, and let it stand for 10 minutes; (4) discharge at a certain rate (the rate is as follows) to the lower limit voltage in a constant temperature room or a constant temperature box environment; (5) let it stand for 10 minutes; (3) to (5) cycle until all rate discharge tests are completed, where the discharge rate is: 3C / 4C.

[0109] 5. DC resistance (DCIR) test of lithium-ion batteries

[0110] At 0°C, charge the lithium-ion battery at a constant current of 0.7C to full charge, then cut off at 0.05C and let it rest for 30 minutes. Discharge at 0.1C for 10 seconds, recording the voltage as U1. Discharge at 1C for 360 seconds, recording the voltage as U2. Repeat the charge and discharge steps five times. DC resistance (DCIR) = (V0 - V1) / (1C - 0.1C). "1C" is the current required to fully discharge the lithium-ion battery in 1 hour.

[0111] 6. Electrochemical impedance spectroscopy (EIS) test

[0112] The three-electrode battery with copper wire was connected to the electrochemical workstation for testing. The test temperature was 25°C, the test frequency range was 70mHz to 20kHz, and the amplitude was 5mV. After collecting the data, the data was analyzed through the impedance complex plane diagram to obtain the impedance RSEI data.

[0113] 7. Lithium deposition test

[0114] (1) The lithium-ion battery was left at rest at 0℃±2℃ for 4 hours; (2) the battery was discharged at 0.2C to the lower voltage limit and left at rest for 10 minutes; (3) the battery was charged at 2C to the upper voltage limit, then cut off at 0.05C and left at rest for 10 minutes; (4) the battery was discharged at 0.5C to the lower voltage limit and left at rest for 10 minutes; (3) to (4) were cycled 20 times. After 20 cycles, the fully charged battery was dissected. See Figures 3 and 4.

[0115] Determination of the degree of lithium deposition: The degree of lithium deposition is determined based on the state of the negative electrode sheet when it is fully charged and disassembled. When the entire negative electrode sheet appears golden yellow and the area displayed in gray is less than 2%, it is determined that there is no lithium deposition; when most of the negative electrode is golden yellow, but gray can be observed in some positions, and the gray area is between 2% and 20%, it is determined to be mild lithium deposition; when the negative electrode is partially gray, but some golden yellow can still be observed, and the gray area is between 20% and 60%, it is determined to be moderate lithium deposition; when most of the negative electrode appears gray, and the gray area is greater than 60%, it is determined to be severe lithium deposition.

[0116] 8. Cyclic test and expansion test

[0117] (1) Place the lithium-ion battery at 25℃±2℃ for 10 minutes; (2) Place it at 0℃±2℃ for 4 hours; discharge it at 0.2C to the lower limit voltage and place it at rest for 10 minutes; (3) Charge it at 0.7C to the upper limit voltage, cut off at 0.05C, and place it at rest for 10 minutes; (4) Discharge it at 0.2C to the lower limit voltage (for initial capacity test); (5) Place it at rest for 10 minutes; (6) Charge it at 0.7C to the upper limit voltage, cut off at 0.05C, and test the thickness (7) Stand at 0℃±2℃ for 10min; (8) Discharge at 0.5C to the lower limit voltage; Stand for 10min; (9) Charge at 0.7C to the upper limit voltage, cut off at 0.05C, Stand for 10min, and cycle 800 times in steps 8 to 9. Test the thickness of the battery at full charge every 50 times for the first 200 times and every 100 times after 200 times; Repeat steps (3) to (4) at 0℃ for capacity test every 100 times, and measure the thickness of the battery at full charge after the cycle is completed.

[0118] The cycle thickness expansion rate was calculated according to (PPG thickness after cycle - PPG thickness of sample) / PPG thickness of sample×100%.

[0119] Table 1

[0120] In Table 1, the thickness ratio refers to the ratio of the thickness of the second active material layer to the thickness of the first active material layer. Example 7 is a single-layer active layer, and there is no thickness ratio value.

[0121] Table 2

[0122] In Table 2, the permeation flow rate K is L t The ratio of the electrolyte to the penetration time is as follows: the larger the ratio, the higher the electrolyte penetration rate.

[0123] According to Tables 1 and 2, under the same compaction, the pore tortuosity of the active material layer of the embodiment of the present application is smaller than that of the active material layer of the comparative example, indicating that the addition of the block copolymer in the present application helps to form a rich pore structure; the charge transfer impedance and electrolyte penetration time of the embodiment of the present application are much smaller than those of the comparative example, and the electrolyte penetration flow rate of the embodiment of the present application is much greater than that of the comparative example, indicating that the electrode sheet of the present application can increase the electrolyte infiltration speed and promote Li + Rapid transmission; In addition, the lithium deposition in the examples of the present application is better than that in the comparative examples. According to Figures 3 and 4, the electrolyte penetration effect of Example 4 of the present application is good, and the electrolyte in Comparative Example 1 is difficult to diffuse and gathers in the center. In summary, the electrode sheet provided by the present application provides sufficient transfer channels for lithium ions and electrolyte, which is conducive to improving the penetration performance of the electrolyte, reducing the charge transfer impedance and slowing down lithium deposition. The rate performance and cycle performance of the electrode sheet provided by the present application are significantly improved.

[0124] The above describes in detail the preferred embodiments and experimental verifications of the present application. It should be understood that ordinary skill in the art can make numerous modifications and variations based on the concepts of the present application without inventive effort. Therefore, any technical solutions that can be derived by those skilled in the art based on the concepts of the present application through logical analysis, reasoning, or limited experimentation on the basis of the existing technology should be within the scope of protection defined by the claims.

Claims

1. An electrode sheet, wherein: The electrode sheet includes a current collector and an active material layer, wherein the active material layer includes a first active material layer and a second active material layer along a thickness direction. The first active material layer is located on at least one functional surface of the current collector, and the second active material layer is located on a side surface of the first active material layer away from the current collector; An absolute difference between the porosity of the first active material layer and the porosity of the second active material layer is less than 5%.

2. The electrode sheet according to claim 1, wherein: An absolute difference between the porosity of the first active material layer and the porosity of the second active material layer is less than 1%.

3. The electrode sheet according to claim 1, wherein: The ratio of the pore tortuosity T of the active material layer to the electrolyte permeation flow rate K is in the range of 0.3 to 1.

3.

4. The electrode sheet according to claim 1, wherein: The porosity of the first active material layer is 17% to 33%, and / or The porosity of the second active material layer is 15% to 28%.

5. The electrode sheet according to claim 3, wherein: The electrolyte permeation flow rate K of the electrode sheet is 1.4 to 3.5 μm / s; The electrolyte penetration velocity of the electrode sheet is the length L of the transmission path through which the electrolyte passes in the pores of the active material layer. t Ratio to penetration time.

6. The electrode sheet according to any one of claims 1 to 5, wherein: The pore tortuosity T of the active material layer is 1.1 to 1.5, wherein the pore tortuosity T is the transmission path length L of the electrolyte passing through the pores of the active material layer. t The ratio of the active material layer thickness L0.

7. The electrode sheet according to claim 5 or 6, wherein: The transmission path length L t Satisfy: 80μm≤L t ≤110μm; and / or, The compaction density PD of the active material layer satisfies: 1.50 g / cm 3 ≤PD≤1.75g / cm 3 and / or, The Dv99 of the active material particles in the active material layer satisfies: 30 μm ≤ Dv99 ≤ 60 μm; and / or, The thickness L0 of the active material layer satisfies: 65 μm≤L0≤80 μm; and / or, The overall porosity ε of the active material layer satisfies: 16%≤ε≤31%.

8. The electrode sheet according to any one of claims 1 to 7, wherein: The active material layer contains at least an active material and a block copolymer, wherein the block copolymer includes at least a polyacrylic acid block and a polyethylene oxide block.

9. The electrode sheet according to claim 8, wherein: The first active material layer comprises the following components by mass fraction: 96% to 98% of a first active material, 0.5% to 3% of a first conductive agent, 1% to 2% of a thickener, and 1% to 3% of a first binder; and / or, The second active material layer includes the following components in terms of mass percentage: 94% to 97.5% of a second active material, 0.5% to 3.5% of a second conductive agent, 1% to 3% of a second binder, and 0.6% to 3% of a block copolymer.

10. The electrode sheet according to claim 8 or 9, wherein: The block copolymer has the structure shown below: Wherein m>0, n>0.

11. The electrode sheet according to claim 10, wherein: The molecular weight of the block copolymer is 100,000-4,000,000, and the particle size of the block copolymer is 90-220 nm.

12. A battery, wherein: The electrode sheet comprises the electrode sheet according to any one of claims 1 to 11.

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