An electrode sheet and a battery

Abstract

The present invention provides an electrode sheet and a battery. The electrode sheet 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 a 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%. By controlling the uniformity of the porosity of the active material layer, the transmission paths of lithium ions and electrolyte are shortened, and the transmission speed of lithium ions and electrolyte into the electrode sheet is increased to 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.

Description

Technical Field

[0001] The present invention belongs to the field of lithium-ion batteries, and particularly relates to an electrode sheet and a battery. Background Art

[0002] With the rapid development of new energy vehicles and large-scale energy storage industries, higher requirements are put forward for the performance of lithium-ion batteries, such as energy density, rate performance, cycle performance, etc. The content of the active material in the electrode sheet determines the energy density of the battery. In order to improve the energy density of the battery, usually the thickness and compaction density of the active material layer in the positive and negative electrode sheets are increased, which can effectively improve the volume energy density of the battery. However, with the increase of the thickness and compaction density of the active material layer, the porosity decreases. Especially during the rolling process on the surface of the active material layer, the force is large, the compaction is higher, and the pores are smaller, resulting in a longer electrolyte transmission path, an increase in internal resistance, a poorer wettability of the electrolyte to the electrode sheet, a decrease in the lithium-ion transmission efficiency, a deterioration in the kinetic performance of the electrode sheet, an increase in the risk of lithium deposition, and a deterioration in the rate performance and cycle performance of the battery. Summary of the Invention

[0003] The electrode sheet provided by the present invention can shorten the transmission paths of lithium ions and electrolyte and improve the transmission speeds of lithium ions and electrolyte into the electrode sheet by controlling the uniformity of the porosities of the first active material layer and the second active material layer, jointly improve the kinetic performance of the electrode sheet, reduce the risk of lithium deposition, and thus improve the rate performance, cycle performance, and safety performance of the battery.

[0004] The battery provided by the present invention, due to adopting the above electrode sheet, exhibits excellent performance in terms of rate performance, cycle performance, safety performance, etc.

[0005] In the first aspect of the present invention, an electrode sheet is provided. The electrode sheet 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.

[0006] 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 first active material layer away from the current collector.

[0007] 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%.

[0008] For the electrode sheet as described above, 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%.

[0009] 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%.

[0010] The electrode sheet as described above, wherein the ratio range of the tortuosity T of the pores in the active material layer to the electrolyte penetration flow rate K is 0.3 to 1.3.

[0011] The electrode sheet as described above, wherein the porosity of the first active material layer is 17% to 33%, and / or

[0012] the porosity of the second active material layer is 15% to 28%.

[0013] The electrode sheet as described above, wherein the electrolyte penetration flow rate K of the electrode sheet is 1.4 to 3.5 μm / s;

[0014] wherein the electrolyte penetration flow rate of the electrode sheet is the ratio of the transmission path length Lt of the electrolyte passing through the pores in the active material layer to the penetration time.

[0015] The electrode sheet as described above, wherein the tortuosity T of the pores in the active material layer is 1.1 to 1.5, wherein the tortuosity T is the ratio of the transmission path length Lt of the electrolyte passing through the pores in the active material layer to the thickness L0 of the active material layer.

[0016] The electrode sheet as described above, wherein the transmission path length Lt satisfies: 80 μm ≤ Lt ≤ 110 μm; and / or

[0017] the tap density PD of the active material layer satisfies: 1.50 g / cm 3 ≤ PD ≤ 1.75 g / cm 3 ; and / or

[0018] the Dv99 of the active material particles in the active material layer satisfies: 30 μm ≤ Dv99 ≤ 60 μm; and / or

[0019] the thickness L0 of the active material layer satisfies: 65 μm ≤ L0 ≤ 80 μm; and / or

[0020] the overall porosity ε of the active material layer satisfies: 16% ≤ ε ≤ 31%.

[0021] The electrode sheet as described above, wherein the active material layer at least comprises an active material and a block copolymer, and the block copolymer at least comprises a polyacrylic acid block and a polyethylene oxide block.

[0022] 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 thickening agent, and 1% to 3% of a first binder; and / or

[0023] The second active material layer comprises the following components by mass percentage: 94% - 97.5% of a second active material, 0.5% - 3.5% of a second conductive agent, 1% - 3% of a second binder, and 0.6 - 3% of a block copolymer.

[0024] The electrode sheet as described above, wherein the block copolymer has the structure shown below:

[0025]

[0026] Where m > 0 and n > 0.

[0027] The electrode sheet as described above, wherein the molecular weight of the block copolymer is 1,000,000 - 4,000,000, and the particle size of the block copolymer is 90 - 220 nm.

[0028] In the second aspect of the present invention, a battery is provided, comprising the electrode sheet as described in the first aspect above.

[0029] The implementation of the present invention has at least the following beneficial effects:

[0030] For the electrode sheet provided by the present invention, by controlling the absolute difference between the first active material layer and the second active material layer to be less than 3%, the uniformity of the overall porosity of the active material layer is improved, so that the kinetic performance of the electrode sheet can be improved by shortening the transmission path of lithium ions and electrolyte and increasing the transmission speed of lithium ions and electrolyte into the electrode sheet, reducing the risk of lithium deposition, and further improving the rate performance, cycle performance, and safety performance of the battery. Description of the Drawings

[0031] Figure 1 is a cross-sectional SEM diagram of the electrode sheet in an embodiment of the present invention;

[0032] Figure 2 is a cross-sectional SEM diagram of the electrode sheet in another embodiment of the present invention;

[0033] Figure 3 is an optical diagram of the effect after the negative electrode sheet in Example 4 of the present invention is penetrated by the electrolyte;

[0034] Figure 4 is an optical diagram of the effect after the negative electrode sheet in Comparative Example 1 of the present invention is penetrated by the electrolyte.

[0035] Description of the Reference Numerals:

[0036] 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 Embodiments

[0037] To make the objectives, technical solutions, and advantages of the present invention clearer, the technical solutions in the embodiments of the present invention will be clearly and completely described below in conjunction with the embodiments of the present invention. Obviously, the described embodiments are some, but not all, of the embodiments of the present invention. All other embodiments obtained by those of ordinary skill in the art based on the embodiments of the present invention without creative efforts shall fall within the scope of protection of the present invention.

[0038] In a first aspect of the present invention, an electrode sheet is provided. The electrode sheet 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 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 a 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%.

[0039] The current collector includes two relatively arranged functional surfaces for coating the active material layer. In the present invention, the active material layer can be coated only on one functional surface of the current collector or on both functional surfaces of the current collector at the same time.

[0040] The present invention does not limit the specific material of the current collector. For example, it can be aluminum, copper, etc.

[0041] Along the thickness direction of the current collector, the active material layer on one functional surface is divided into a first active material layer and a second active material layer. 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 surface of the first active material layer away from the current collector.

[0042] 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 insertion and rapid extraction of lithium ions on the surface of the electrode sheet while ensuring the permeability of the electrolyte to the electrode sheet, and reduce the risk of lithium precipitation 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 the overall porosity uniformity of the active material layer and reduce the risk of lithium precipitation while ensuring the penetration performance of the electrolyte. Applying this electrode sheet to a battery can shorten the transmission path of lithium ions and the electrolyte, improve the transmission speed of lithium ions and the electrolyte into the electrode sheet, thereby improving the rate performance, cycle performance, and safety performance of the battery.

[0043] Wherein, 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.

[0044] It should be noted that the pores are the particle spacings in each layer of the active material layer, and the porosity refers to the percentage of the pore volume in each layer of the active material layer to the total volume of each layer of the active material layer. Among them, the porosity of each layer can be measured by the gas replacement method.

[0045] 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 electrolyte penetration performance. The higher the uniformity, the more beneficial it is to improve the penetration performance and reduce the risk of lithium plating. In addition, the inventors have studied and 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 electrolyte penetration performance, it is also beneficial to reduce the thickness expansion rate of the battery assembled from this 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 ≥ 65%, and the 4C discharge capacity retention rate ≥ 53%. Further, 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 electrolyte penetration performance, and at the same time take into account the improvement of 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 ≥ 70%, and the 4C discharge capacity retention rate ≥ 60%.

[0046] In some embodiments, the ratio range of the pore tortuosity T of the active material layer to the electrolyte penetration flow rate K is 0.3 to 1.3. Among them, the pore tortuosity T of the active material layer is the ratio of the transmission path length L t of the electrolyte passing through the pores of the active material layer to the thickness L0 of the active material layer, and the electrolyte penetration flow rate K is the ratio of the transmission path length Lt of the electrolyte passing through the pores of the active material layer to the penetration time. The units of Lt and L0 are μm, and the unit of the penetration time is s.

[0047] The porosity of the active material layer determines the diffusion and transmission of lithium ions. The higher the porosity, the more beneficial it is to the penetration of the electrolyte and the transmission of lithium ions, promoting the improvement of the transmission speed of lithium ions and the electrolyte into the electrode sheet, and at the same time being beneficial to the increase of the insertion and extraction channels and directions of lithium ions in the active material layer, so that the battery has 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 invention, 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 beneficial to taking into account the energy density and rate performance of the battery.

[0048] In the prior art, after the battery is filled with electrolyte, it usually needs to stand still for a long time to ensure that the electrolyte completely penetrates the electrode sheet, which takes a long time and seriously affects the battery production speed. However, the above-mentioned electrode sheet provided by the present invention can improve the kinetic performance of the electrode sheet by shortening the transmission paths of lithium ions and electrolyte and increasing the transmission speed of lithium ions and electrolyte into the electrode sheet. Among them, the transmission paths of lithium ions and electrolyte are related to the tortuosity of the pore structure in the active material layer. The inventor has found through research that by further defining the tortuosity of the pore structure in the active material layer, the standing time after liquid 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, where the pore tortuosity T is the ratio of the transmission path length L t of the electrolyte passing through the pores in the active material layer to the thickness L0 of the active material layer. The transmission path length L t can be understood as the shortest path of the electrolyte passing through the entire 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.

[0049] Among them, the transmission path length L t can be calculated by the formula L t = 2×(PD×Dv99)×(1 - ε), with the unit of μm. L0 is the value of the perpendicular distance from the surface of the active material layer far from the current collector side to the current collector, with the unit of μm. PD is the compaction density of the active material layer, with the unit of g / cm 3 , Dv99 represents the value at which 99% of the measured particle volume of the active material layer is less than this value, with the unit of μm, and ε is the porosity of the active material layer.

[0050] The present invention does not specify the specific values of the above parameters. For example, the transmission path length L t of the electrolyte passing through the overall pores in the active material layer satisfies: 80μm ≤ L t ≤ 110μm; and / or, the compaction density PD of the active material layer satisfies: 1.50g / cm 3 ≤ PD ≤ 1.75g / cm 3 ; and / or, 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 penetration flow rate K of the electrode sheet is 1.4 to 3.5μm / s, where the electrolyte penetration flow rate K of the active material layer is the ratio of the transmission path length L t of the electrolyte passing through the pores in the active material layer to the penetration time.

[0051] The active material layer at least comprises an active material, a conductive agent, a binder, and a block copolymer, and the block copolymer at least comprises a polyacrylic acid block and a polyethylene oxide block.

[0052] The present invention does not limit the specific types of the active material, the conductive agent, and the binder in the active material layer, and they can be the conventional active materials, conductive agents, and binders in the art. For example, the active material is a negative electrode active material such as a carbon-based material or a silicon-based material, or can also 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 nanotube; the binder includes one or more of polyvinylidene fluoride, vinylidene fluoride-hexafluoropropylene, polyamide, polyacrylonitrile, polyacrylate, polyacrylic acid, polyacrylate salt, polyvinylpyrrolidone, polyethylene ether, and polymethyl methacrylate.

[0053] The block copolymer at least comprises a polyacrylic acid block and a polyethylene oxide block. That is to say, the block copolymer can only contain a polyacrylic acid block and a polyethylene oxide block, and can also contain other blocks in addition to the polyacrylic acid block and the polyethylene oxide block.

[0054] The present invention does not limit the arrangement structure of each block in the block copolymer. When the block copolymer only contains a polyacrylic acid block and a polyethylene oxide block, these two blocks are sequentially bonded under the action of hydrogen bonds, van der Waals forces, etc. Among them, the polyacrylic acid block is a block formed by polymerization of polyacrylic acid, and the polyethylene oxide block is a block formed by polymerization of polyethylene oxide.

[0055] When the block copolymer also contains other blocks, the polyacrylic acid block, the polyethylene oxide block, and the other blocks can be alternately bonded, sequentially bonded, or randomly bonded. The present invention does not make too many limitations here.

[0056] According to the research of the present invention, applying the electrode sheet provided by the present invention to a battery can effectively improve the rate performance, cycle performance, and safety performance of the battery. This is because by controlling the uniformity of the porosity of the active material layer, it helps to increase the porosity of the active material layer, reduce the pore tortuosity, shorten the transmission path of lithium ions and electrolyte, help to accelerate the rate of electrolyte penetration into the electrode sheet, improve the permeability. In addition, the block copolymer also has a high ionic conductivity, which plays a positive promoting role in the transmission of lithium ions.

[0057] The present invention does not limit the specific structure of the block copolymer, as long as it includes a diblock formed by polymerization monomers of polyacrylic acid and polyethylene oxide. In some embodiments, the block copolymer has the following structure:

[0058]

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

[0060] On the one hand, the above-mentioned block copolymer has a three-dimensional network structure, which helps to increase the porosity of the active material layer, reduce the pore tortuosity, and shorten the transmission paths of lithium ions and electrolytes. On the other hand, the above-mentioned block copolymer has electrolyte affinity, which can accelerate the rate of electrolyte penetration into the electrode sheet and improve the 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 invention uses a block copolymer to improve the kinetic performance of the electrode sheet by shortening the transmission paths of lithium ions and electrolytes and increasing the transmission speed of lithium ions and electrolytes into the electrode sheet, reducing the risk of lithium deposition, thereby improving the rate performance, cycle performance, and safety performance of the battery. The present invention 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 polymer monomer according to actual needs. For example, the molecular weight of the polyacrylic acid block accounts for (20-80%) of the total molecular weight of the block copolymer, and the molecular weight of the polyethylene oxide block accounts for 20-80% of the total molecular weight of the block copolymer.

[0061] In addition, the present invention does not limit the molecular weight of the block copolymer, and the specific molecular weight of the block copolymer can be adjusted by controlling the addition amount of the polymer monomer according to actual needs. For example, the molecular weight of the block copolymer is 100,000-4,000,000, preferably 100,000-600,000. This molecular weight refers to the number average molecular weight. If the molecular weight of the block copolymer is too large, it is easy to cause the pores to be blocked, resulting in serious lithium deposition.

[0062] In the present invention, the block copolymer is added during the preparation of the active material layer slurry, without the need to increase the process and equipment, avoiding cost increase. 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 beneficial to maximizing the performance of the block copolymer. In some embodiments, the particle size of the block copolymer is 90-220 μm, and this particle size range refers to the particle size corresponding to 50%-97% in 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, easily causing serious agglomeration phenomena, having a certain impact on the uniformity of the pore size structure of the active material layer, and further affecting the electrolyte permeability.

[0063] The present invention does not limit the preparation process of the active material layer, as long as the above-mentioned active material layer can be obtained. For example, the active material layer is prepared by the following method: dissolving an active material, a binder, a conductive agent, and a block copolymer in a solvent according to the required component ratio, and uniformly stirring the mixture by a stirring device to obtain an active material layer slurry with a solid content of 48% to 52%. Subsequently, the uniformly stirred active material layer slurry is coated 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, and the active material layer can be prevented from peeling off from the current collector. Specifically, the active material layer of the present invention further includes a thickening agent, and the thickening agent includes one or more of sodium carboxymethyl cellulose and lithium carboxymethyl cellulose, ensuring a reliable bonding strength between the active material layer and the current collector, preventing the active material layer from peeling off from the current collector, being beneficial to ensuring the stability of the electrode sheet structure, and thus ensuring the electrochemical performance of the battery. The present invention does not limit the content of the thickening agent, which can be specifically adjusted according to actual needs. For example, in the active material layer, the content of the thickening agent is 0.6% to 2%.

[0065] In the present invention, the second active material layer is farther 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 invention does not limit the types of components in the first active material layer, which may include conventional active materials, conductive agents, binders, and thickening agents in the art. It is only necessary to make the second active material layer contain a block copolymer, which is beneficial to improving production efficiency and reducing costs.

[0066] When the above-mentioned electrode sheet is applied to a battery, under the same compaction working conditions, the second active material layer is subjected to greater force. By making the second active material layer contain a block copolymer, the present invention is beneficial to increasing 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 when the above-mentioned electrode sheet structure is roll-pressed, thereby reducing the pore tortuosity and shortening the transmission path of lithium ions and the 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] At 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 achieve that the absolute difference in porosity between the second active material layer and the first active material layer is 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 conditions, 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. For example Figure 2 As shown, 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 the 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] The present invention does not limit the setting of the active material layers on both sides of the current collector, which can be set on one side or both sides. For example, as Figure 2 shown, on the first functional surface of the current collector 1, the first active material layer a and the second active material layer b are respectively provided, and on the second functional surface of the current collector 1, the third active material layer c and the fourth active material layer d are respectively provided. At this time, the tortuosity T of the pores of the active material layer refers to the tortuosity of the pores when the first active material layer a and the second active material layer b are taken as a whole, or can also refer to the tortuosity of the pores when the third active material layer c and the fourth active material layer d are taken 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. The present invention does not make too many limitations here.

[0070] The electrode sheet of the present invention can be a negative electrode sheet or a positive electrode sheet, which can be specifically 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 a negative electrode active material such as graphite or silicon-based material; 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 positive electrode active material such as ternary material or lithium iron phosphate. It is preferably a negative electrode sheet because the problem of lithium deposition mainly occurs on the negative electrode sheet. When only the kinetic performance of the positive electrode sheet is improved, it means that the rate of lithium ions escaping from the positive electrode increases. At this time, if the kinetic performance of the negative electrode sheet cannot keep up, lithium deposition will be even more serious. Therefore, in an embodiment of the present invention, the above-mentioned electrode sheet is a negative electrode sheet, and the positive electrode sheet can be a conventional positive electrode sheet in the art.

[0071] In the second aspect of the present invention, a battery is provided, including the electrode sheet of the first aspect above.

[0072] Specifically, the electrode sheet includes a negative electrode sheet and a positive electrode sheet. The negative electrode sheet, the positive electrode sheet and the separator can form the battery core in the battery, and at least one of the negative electrode sheet and the positive electrode sheet is the electrode sheet of the first aspect above. By installing the battery core and the protection circuit together inside the battery case, a battery for charging / discharging can be formed. The quality of the battery core directly determines the quality of the battery. Since the above-mentioned electrode sheet is adopted, the battery of the present invention has excellent performance in terms of rate performance, cycle performance, safety performance, etc.

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

[0074] Example 1

[0075] I. Preparation of the positive electrode sheet

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

[0077] II. Preparation of the negative electrode sheet

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

[0079] (2) The 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 the second active material layer slurry; wherein, the block copolymer is composed of a polyacrylic acid block and a 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 slurry of the first active material layer and the slurry of the second active material layer are sequentially coated on the negative electrode current collector by double-layer coating. After rolling and slitting, a negative electrode sheet is obtained. Among them, the compaction density of the second active material layer and the first active material layer is 1.7 g / cm 3 , and the ratio of the thickness of the second active material layer to the thickness of the first active material layer is 2:8.

[0081] III. Preparation of Lithium-Ion Batteries

[0082] The positive electrode sheet, the separator, and the negative electrode sheet are stacked in sequence. After winding, a bare battery core is obtained. The bare battery core is placed in an outer packaging aluminum-plastic film, and the electrolyte is injected into the bare battery core. After vacuum packaging, standing, formation, secondary sealing, sorting, shaping, and testing processes, a soft-pack lithium-ion battery is obtained. Among them, the electrolyte is purchased from Shinjoo Bang LBC450A23, and the separator is purchased from Enjie.

[0083] Example 2

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

[0089] Example 5

[0090] The preparation process is basically the same as that of Example 1, except that in the preparation of the negative electrode sheet, in step (2), "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", 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 with "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. The negative electrode sheet of Example 1 is replaced with 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 the preparation of the negative electrode sheet, in step (2), "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", 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 with "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. The negative electrode sheet of Example 1 is replaced with 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. In the preparation of the negative electrode sheet, in step (2), "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". In step (3), only the slurry of the second active material layer is coated on the negative electrode current collector. Other conditions remain unchanged to obtain a negative electrode sheet;

[0095] Replace the negative electrode sheet of Example 1 with the negative electrode sheet of this example to assemble a lithium-ion battery.

[0096] Example 8

[0097] It is basically the same as the preparation process 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 "94.7% graphite, 1.3% conductive carbon, 0.6% thickener, 1% binder and 2.4% block copolymer".

[0098] Comparative Example 1

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

[0100] Test Example

[0101] 1. Porosity measurement

[0102] Cut the negative electrode sheet into cuboids of the same size. Test 20 samples for each example, and the volume of each sample is about 2 cm 3 . According to the standard of "GB / T24586-2009 Determination of Apparent Density, True Density and Porosity of Iron Ore", test the porosity P1 of the first active material layer, the porosity P2 of the second active material layer and the overall porosity ε of the active material layer in each example and comparative example. The test gas is helium. The results are shown in Table 1.

[0103] 2. Porosity tortuosity measurement

[0104] The porosity tortuosity T = L t / L0, L t is the shortest path for the electrolyte to pass through 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 is calculated by the formula L t = 2×(PD×Dv99)×(1 - ε), with the unit of μm. As Figure 1 shown, L0 is the value of the perpendicular distance from the first surface 201 on the side of the negative electrode active material layer 2 away from the negative electrode current collector 1 to the negative electrode current collector 1, with the unit of μm, PD is the compaction density of the active material layer, with the unit of g / cm 3 , and the compaction density of the active material layer in the above examples and comparative examples is 1.7 g / cm 3, Dv99 represents the value at which 99% of the measured graphite volume is less than this value, with the unit of μm. The Dv99 of the graphite used in the above embodiments and comparative examples is 41 μm; ε is 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 negative electrode sheet with a length of 30 mm and a width of 30 mm. Under dry conditions, drop 1 mL of electrolyte on the surface of the sample and start timing. Stop timing when the electrolyte on the surface of the sample completely disappears. The timing time is recorded as the penetration time. Average values are taken from 50 samples tested for each example. The electrolyte is purchased from CAPCHEM LBC450A23.

[0107] 4. Rate discharge test

[0108] (1) Let the lithium-ion battery stand for 10 min; (2) Discharge at 0.2C to the lower limit voltage and stand for 10 min; (3) Charge at 0.7C to the upper limit voltage in a constant temperature chamber, with a cut-off current of 0.05C, and stand for 10 min; (4) Discharge at a certain rate (the rates are as follows) to the lower limit voltage in a constant temperature chamber or incubator environment; (5) Stand for 10 min; Steps (3) to (5) are cycled until all rate discharge tests are completed, where the discharge rates are: 3C / 4C.

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

[0110] At 0 °C, charge the lithium-ion battery at a constant current of 0.7C to full charge, with a cut-off of 0.05C, and stand for 30 minutes. Discharge at 0.1C for 10 seconds and record the voltage value as U1. Discharge at 1C for 360 seconds and record the voltage value as U2. Repeat the charge and discharge steps 5 times. The DC resistance DCIR = (V0 - V1) / (1C - 0.1C), where "1C" is the current value that completely discharges the lithium-ion battery capacity within 1 hour.

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

[0112] Connect the three-electrode battery with copper wire plated to an electrochemical workstation for testing. The test temperature is 25 °C, the test frequency range is from 70 mHz to 20 kHz, the amplitude is 5 mV. After collecting data, analyze the data through an impedance complex plane diagram to obtain the impedance R SEI data.

[0113] 7. Lithium plating test

[0114] (1) Let the lithium-ion battery stand still at 0°C ± 2°C for 4 h; (2) Discharge at 0.2C to the lower limit voltage and stand still for 10 min; (3) Charge at 2C to the upper limit voltage, cut off at 0.05C, and stand still for 10 min; (4) Discharge at 0.5C to the lower limit voltage and stand still for 10 min; Repeat steps (3) to (4) 20 times. After 20 cycles, dissect the battery in the fully charged state. As Figure 3 and Figure 4 shown.

[0115] Determination of the degree of lithium deposition: It is determined according to the state of the fully charged and disassembled negative electrode sheet. When the whole negative electrode sheet shows golden yellow and the area showing gray is <2%, it is determined that there is no lithium deposition; when most of the negative electrode is golden yellow, but gray can be observed at some positions and the gray area is between 2% and 20%, it is determined that there is slight lithium deposition; when part of the negative electrode is gray, but part of the golden yellow can still be observed and the gray area is between 20% and 60%, it is determined that there is moderate lithium deposition; when most of the negative electrode shows gray and the gray area >60%, it is determined that there is severe lithium deposition.

[0116] 8. Cycling test and swelling test

[0117] (1) Let the lithium-ion battery stand still at 25°C ± 2°C for 10 min; (2) Stand still at 0°C ± 2°C for 4 h; Discharge at 0.2C to the lower limit voltage and stand still for 10 min; (3) Charge at 0.7C to the upper limit voltage, cut off at 0.05C, and stand still for 10 min; (4) Discharge at 0.2C to the lower limit voltage (for initial capacity test); (5) Stand still for 10 min; (6) Charge at 0.7C to the upper limit voltage, cut off at 0.05C, and measure the thickness; (7) Stand still at 0°C ± 2°C for 10 min; (8) Discharge at 0.5C to the lower limit voltage; Stand still for 10 min; (9) Charge at 0.7C to the upper limit voltage, cut off at 0.05C, and stand still for 10 min. Repeat steps 8 - 9 800 times. For the first 200 times, measure the thickness every 50 times, and after 200 times, measure the thickness at full charge every 100 times during the test process; Repeat steps (3) - (4) at 0°C every 100 times for capacity test. After cycling, measure the thickness in the fully charged state.

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

[0119] Table 1

[0120]

[0121] 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. In Example 7, it is a single active layer and there is no thickness ratio value.

[0122] Table 2

[0123]

[0124]

[0125] In Table 2, the seepage flow rate K is L t The ratio to the seepage time. The larger the ratio, the higher the seepage flow rate of the electrolyte.

[0126] It can be seen from Table 1 to Table 2 that under the same compaction, the pore tortuosity of the active material layer of the embodiments of the present invention is less than that of the active material layer of the comparative example, indicating that the addition of the block copolymer in the present invention helps to form a rich pore structure; the charge transfer resistance and electrolyte seepage time of the embodiments of the present invention are much smaller than those of the comparative example, and the electrolyte seepage flow rate of the embodiments of the present invention is much larger than that of the comparative example, indicating that the electrode sheet of the present invention can improve the electrolyte infiltration speed and promote Li + Fast transmission; in addition, the lithium deposition situation of the embodiments of the present invention is better than that of the comparative example. According to Figure 3 and Figure 4 It can be known that the electrolyte seepage effect of Embodiment 4 of the present invention is good, and the electrolyte of Comparative Example 1 is difficult to diffuse and accumulates at the central position. In summary, the electrode sheet provided by the present invention provides sufficient transfer channels for lithium ions and electrolyte, which is beneficial to improving the electrolyte seepage performance, reducing the charge transfer resistance and slowing down lithium deposition, and the rate performance and cycle performance of the electrode sheet provided by the present invention are significantly improved.

[0127] The preferred specific embodiments of the present invention and the experimental verification have been described in detail above. It should be understood that those of ordinary skill in the art can make many modifications and variations according to the concept of the present invention without creative labor. Therefore, all technical solutions that can be obtained by those skilled in the art in the technical field based on the concept of the present invention through logical analysis, reasoning or limited experiments on the basis of the prior art should be within the protection scope determined by the claims.

Claims

1. An electrode sheet, characterized in that, The electrode sheet 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 a 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%; the ratio of the pore tortuosity T to the electrolyte penetration flow rate K of the active material layer ranges from 0.3 to 1.3; the electrolyte penetration flow rate K of the electrode sheet is 1.4 to 3.5 μm / s. Wherein, the electrolyte penetration flow rate of the electrode sheet is the ratio of the length L of the transmission path through which the electrolyte passes in the pores of the active material layer t to the penetration time; The tortuosity T of the pores in the active material layer is 1.1 to 1.5, where the tortuosity T is the ratio of the transport path length L t of the electrolyte passing through the pores in the active material layer to the thickness L0 of the active material layer; the transport path length L t satisfies: 80 μm ≤ L t ≤ 110 μm.

2. The electrode sheet according to claim 1, characterized in that 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%.

3. 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%.

4. The electrode sheet according to claim 1, wherein The tap density PD of the active material layer satisfies: 1.50 g / cm 3 ≤ PD ≤ 1.75 g / cm 3 ; and / or, 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%.

5. The electrode sheet according to any one of claims 1-4, wherein The active material layer at least contains an active material and a block copolymer, and the block copolymer at least includes a polyacrylic acid block and a polyethylene oxide block.

6. The electrode sheet according to claim 5, characterized in that, The first active material layer includes the following components by mass fraction: 96% - 98% of a first active material, 0.5% - 3% of a first conductive agent, 1% - 2% of a thickening agent, and 1% - 3% of a first binder; and / or The second active material layer includes the following components by mass percentage: 94% - 97.5% of a second active material, 0.5% - 3.5% of a second conductive agent, 1% - 3% of a second binder, and 0.6 - 3% of a block copolymer.

7. The electrode sheet according to claim 6, wherein The block copolymer has the following structure: ； Where m > 0, n > 0.

8. The electrode sheet according to claim 7, 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.

9. A battery, characterized in that, Including the electrode sheet according to any one of claims 1-8.

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