Gradient positive plate of lithium ion battery and preparation method of gradient positive plate

Abstract

The invention discloses a lithium ion battery gradient positive electrode plate and a preparation method thereof, the lithium ion battery gradient positive electrode plate comprises a positive electrode current collector, a first positive electrode layer coated on the surface of the positive electrode current collector, and a second positive electrode layer coated on the surface of the first positive electrode layer, wherein a first positive electrode active material in the first positive electrode layer is a ternary positive electrode material with relatively high nickel content, and a second positive electrode active material in the second positive electrode layer is a ternary positive electrode material with relatively low nickel content. According to the lithium ion battery gradient positive electrode plate provided by the invention, the first positive electrode layer with relatively high nickel content can provide relatively high energy density, and the second positive electrode layer with relatively low nickel content is relatively high in cycling stability and covers the surface of the first positive electrode layer with relatively high nickel content, so that the stability of the whole electrode plate in a charge-discharge cycle can be maintained; and the cycle performance and the compensation capacity are improved.

Description

Technical Field

[0001] This invention belongs to the field of lithium-ion battery technology and relates to a gradient positive electrode sheet for lithium-ion batteries and its preparation method. Background Technology

[0002] With the large-scale application of lithium-ion batteries in power batteries and energy storage, consumers are placing higher demands on the energy density and safety of lithium-ion batteries. Currently, there are two main methods to improve the energy density of lithium-ion batteries: increasing the operating voltage and increasing the nickel content in the cathode material.

[0003] Increasing the operating voltage not only tests the structural stability of the cathode material, but also places higher demands on the electrolyte. Currently, this field is limited in the short term because electrolyte components are prone to severe interfacial side reactions with the active materials of the cathode material under high voltage, resulting in gas accumulation and thermal runaway of lithium-ion batteries, which can easily lead to safety accidents.

[0004] Increasing the nickel content in the active material to prepare ultra-high nickel ternary cathode materials can improve the capacity density of the cathode material. However, the more active high-nickel surface can lead to interfacial side reactions with the electrolyte, causing the active material to dissolve and forming an inactive rock salt phase on the surface of the cathode material particles. This hinders lithium-ion migration and diffusion, thereby reducing the battery's cycle performance and rate performance. Compared to ultra-high nickel ternary cathode materials, high-nickel ternary cathode materials have a relatively lower nickel content, which improves their stability during charge-discharge cycles. Furthermore, compared to low-nickel materials, their higher nickel content also provides higher charge-discharge capacity.

[0005] Existing lithium-ion batteries typically use transition metal oxides, lithium iron phosphate, lithium manganese iron phosphate, organic polymers, etc., as active materials. Conductive agents and binders are added in a certain proportion and mixed evenly to form a positive electrode paste. This paste is then coated onto a current collector and subjected to processes such as rolling, drying, and slicing to obtain the lithium-ion battery positive electrode sheet. For example, CN105552306A discloses a lithium transition metal oxide battery, including a negative electrode sheet and a positive electrode sheet. The positive electrode sheet includes a positive active material layer, a positive current collector Al foil, and a positive conductive coating. The conductive coating is coated on the positive current collector Al foil, and the positive active material layer is located on the conductive coating. The positive active material layer includes lithium transition metal oxide, a positive binder, and a positive conductive agent. CN114400300A discloses a lithium iron phosphate positive electrode sheet, its preparation method, and a battery. The lithium iron phosphate positive electrode sheet includes a positive current collector and a positive electrode coating. The positive electrode coating includes carbon-coated lithium iron phosphate positive electrode material, a positive conductive additive, and a positive binder. When a cathode sheet made of a single-layer ultra-high nickel ternary cathode material mixture is used as the cathode of a lithium-ion battery, the battery has a high energy density but poor cycle performance and safety. On the other hand, when a cathode sheet made of a single-layer high nickel ternary cathode material mixture is used as the cathode of a lithium-ion battery, the battery has good cycle performance and safety, but relatively low energy density.

[0006] Therefore, there is an urgent need to develop a positive electrode that can provide high battery energy density and ensure stability during charge and discharge cycles. Summary of the Invention

[0007] To address the shortcomings of existing technologies, the present invention aims to provide a gradient cathode sheet for lithium-ion batteries and its preparation method. By employing gradient two-layer electrode technology and through the synergistic effect of ultra-high nickel ternary materials and high nickel ternary materials, the stability and safety of the battery during charge-discharge cycles can be improved while maintaining the high capacity characteristics of the cathode material.

[0008] To achieve this objective, the present invention adopts the following technical solution:

[0009] In a first aspect, the present invention provides a gradient positive electrode sheet for a lithium-ion battery, the gradient positive electrode sheet for a lithium-ion battery comprising a positive current collector, a first positive electrode layer coated on the surface of the positive current collector, and a second positive electrode layer coated on the surface of the first positive electrode layer;

[0010] The raw materials for the first positive electrode layer include the first positive electrode active material, with the general chemical formula LiNi. x Co y Mn z Where x + y + z = 1, 0.8 ≤ x ≤ 0.99, 0 < y ≤ 0.1, and 0 < z ≤ 0.1;

[0011] The raw materials for the second positive electrode layer include a second positive electrode active material with the general chemical formula LiNi. x Co y Mn z Where x + y + z = 1, 0.6 ≤ x ≤ 0.9, 0 < y ≤ 0.2, and 0 < z ≤ 0.2;

[0012] The nickel content of the first positive electrode active material is higher than that of the second positive electrode active material;

[0013] Based on a total mass percentage of 100wt%, the first positive electrode active material comprises 20-80wt%, and the remainder is the second positive electrode active material.

[0014] The lithium-ion battery gradient cathode sheet provided by this invention employs a gradient two-layer electrode technology. The first cathode layer and the second cathode layer each contain cathode active materials with different transition metal ratios. The first cathode active material in the first cathode layer is a ternary cathode material with a high nickel content, while the second cathode active material in the second cathode layer is a ternary cathode material with a low nickel content. The high nickel content of the first cathode layer provides higher energy density, while the low nickel content of the second cathode layer offers higher cycle stability. Covering the surface of the high nickel content first cathode layer helps maintain the stability of the entire electrode sheet during charge-discharge cycles, improving cycle performance and compensation capacity.

[0015] In the chemical formula of the first positive electrode active material, 0.8 ≤ x ≤ 0.99, for example, it can be 0.8, 0.82, 0.85, 0.9, 0.96 or 0.99, but is not limited to the listed values. Other unlisted values ​​within the range are also applicable.

[0016] In the chemical formula of the first positive electrode active material, 0 < y ≤ 0.1, for example, it can be 0.005, 0.01, 0.02, 0.05, 0.08 or 0.1, but is not limited to the listed values. Other unlisted values ​​within the range are also applicable.

[0017] In the chemical formula of the first positive electrode active material, 0 < z ≤ 0.1, for example, it can be 0.005, 0.01, 0.02, 0.05, 0.08 or 0.1, but is not limited to the listed values. Other unlisted values ​​within the range are also applicable.

[0018] In the general chemical formula of the second positive electrode active material, 0.6 ≤ x ≤ 0.9, for example, it can be 0.6, 0.65, 0.7, 0.8 or 0.9, but is not limited to the listed values. Other unlisted values ​​within the range are also applicable.

[0019] In the chemical formula of the second positive electrode active material, 0 < y ≤ 0.2, for example, it can be 0.01, 0.05, 0.1, 0.15 or 0.2, but is not limited to the listed values. Other unlisted values ​​within the range are also applicable.

[0020] In the chemical formula of the second positive electrode active material, 0 < z ≤ 0.2, for example, it can be 0.01, 0.05, 0.1, 0.15 or 0.2, but is not limited to the listed values. Other unlisted values ​​within the range are also applicable.

[0021] In the first positive electrode active material and the second positive electrode active material, the mass percentage of the first positive electrode active material is 20-80wt%, for example, it can be 20wt%, 30wt%, 40wt%, 60wt% or 80wt%, but is not limited to the listed values. Other unlisted values ​​within the range are also applicable.

[0022] Preferably, the raw materials of the first positive electrode layer further include a first conductive agent and a first binder.

[0023] Preferably, based on a total mass percentage of 100wt%, the raw materials of the first positive electrode layer include: 80-99wt% of the first positive electrode active material, 0.5-10wt% of the first conductive agent, and 0.5-10wt% of the first binder.

[0024] The mass percentage of the first positive electrode active material in the raw material of the first positive electrode layer is 80-99 wt%, for example, it can be 80 wt%, 85 wt%, 90 wt%, 95 wt% or 99 wt%, but is not limited to the listed values. Other unlisted values ​​within the range are also applicable.

[0025] The mass percentage of the first conductive agent in the raw material of the first positive electrode layer is 0.5-10 wt%, for example, it can be 0.5 wt%, 1 wt%, 3 wt%, 5 wt%, 8 wt% or 10 wt%, but is not limited to the listed values. Other unlisted values ​​within the range are also applicable.

[0026] The mass percentage of the first binder in the raw material of the first positive electrode layer is 0.5-10 wt%, for example, it can be 0.5 wt%, 1 wt%, 3 wt%, 5 wt%, 8 wt% or 10 wt%, but is not limited to the listed values. Other unlisted values ​​within the range are also applicable.

[0027] Preferably, the raw materials of the second positive electrode layer further include a second conductive agent and a second binder.

[0028] Preferably, based on a total mass percentage of 100wt%, the raw materials of the second positive electrode layer include: 80-99wt% of the second positive electrode active material, 0.5-10wt% of the second conductive agent, and 0.5-10wt% of the second binder.

[0029] The mass percentage of the second positive electrode active material in the raw material of the second positive electrode layer is 80-99 wt%, for example, it can be 80 wt%, 85 wt%, 90 wt%, 95 wt% or 99 wt%, but is not limited to the listed values. Other unlisted values ​​within the range are also applicable.

[0030] The mass percentage of the second conductive agent in the raw material of the second positive electrode layer is 0.5-10 wt%, for example, it can be 0.5 wt%, 1 wt%, 3 wt%, 5 wt%, 8 wt% or 10 wt%, but is not limited to the listed values. Other unlisted values ​​within the range are also applicable.

[0031] The mass percentage of the second binder in the raw material of the second positive electrode layer is 0.5-10 wt%, for example, it can be 0.5 wt%, 1 wt%, 3 wt%, 5 wt%, 8 wt% or 10 wt%, but is not limited to the listed values. Other unlisted values ​​within the range are also applicable.

[0032] Preferably, the first conductive agent and the second conductive agent each independently comprise any one or a combination of at least two of conductive carbon black, Ketjen black, graphite, graphene, carbon nanotubes or carbon fibers. Typical but non-limiting combinations include a combination of conductive carbon black and Ketjen black, a combination of graphite, graphene and carbon nanotubes, or a combination of conductive carbon black, Ketjen black, graphite, graphene, carbon nanotubes and carbon fibers.

[0033] Preferably, the first adhesive and the second adhesive each independently comprise any one or a combination of at least two of polyvinylidene fluoride, polyaniline, sodium carboxymethyl cellulose, styrene-butadiene rubber, polyacrylic acid, polyvinylpyrrolidone, or polysiloxane. Typical but non-limiting combinations include combinations of polyvinylidene fluoride and polyaniline, combinations of sodium carboxymethyl cellulose, styrene-butadiene rubber, and polyacrylic acid, combinations of styrene-butadiene rubber, polyacrylic acid, polyvinylpyrrolidone, and polysiloxane, or combinations of polyvinylidene fluoride, polyaniline, sodium carboxymethyl cellulose, styrene-butadiene rubber, polyacrylic acid, polyvinylpyrrolidone, and polysiloxane.

[0034] Preferably, the thickness of the first positive electrode layer and the second positive electrode layer are independently 20-200 μm, for example, 20 μm, 50 μm, 80 μm, 100 μm, 150 μm or 200 μm, but not limited to the listed values, and other unlisted values ​​within the range are also applicable.

[0035] Preferably, the areal density of the first positive electrode layer is 200-350 g / m³. 2 The areal density of the second cathode layer is 200-350 g / m³. 2 .

[0036] The areal density of the first positive electrode layer is 200-350 g / m³ 2 For example, it could be 200g / m 2 220g / m 2 250g / m 2 300g / m 2 Or 350g / m 2 However, this does not limit the listed values; other unlisted values ​​within the range are also applicable.

[0037] The areal density of the second positive electrode layer is 200-350 g / m³. 2 For example, it could be 200g / m 2 240g / m 2 250g / m 2 300g / m 2 Or 350g / m 2 However, this does not limit the listed values; other unlisted values ​​within the range are also applicable.

[0038] In a second aspect, the present invention provides a method for preparing a gradient cathode sheet for a lithium-ion battery as described in the first aspect, the method comprising the following steps:

[0039] (1) Mix the raw materials of the first positive electrode layer in the formula amount evenly, and then degas them under vacuum to obtain the first positive electrode slurry; mix the raw materials of the second positive electrode layer in the formula amount evenly, and then degas them under vacuum to obtain the second positive electrode slurry;

[0040] (2) The first positive electrode slurry obtained in step (1) is coated on the surface of the positive electrode current collector and dried in the first step to form the first positive electrode layer; then the second positive electrode slurry obtained in step (1) is coated on the surface of the first positive electrode layer and dried in the second step to form the second positive electrode layer; the resulting composite structure is dried in the third step to obtain the lithium-ion battery gradient positive electrode sheet.

[0041] The method for preparing a gradient cathode sheet for lithium-ion batteries provided by this invention involves combining ultra-high nickel ternary materials and high nickel ternary materials, and then using a double-layer coating technique to sequentially coat the ultra-high nickel ternary layer and the high nickel ternary layer onto the cathode current collector before drying to obtain a gradient double-layer cathode sheet. The synergistic effect of the ultra-high nickel ternary cathode material and the high nickel ternary cathode material is achieved through the double-layer active cathode material, which can improve the stability and safety of the battery in charge-discharge cycles while maintaining the high capacity characteristics of the ultra-high nickel ternary cathode material.

[0042] Preferably, the raw materials for the first positive electrode layer in step (1) include an organic solution of a first positive electrode active material, a first conductive agent, and a first binder.

[0043] Preferably, the organic solution of the first adhesive is obtained by mixing the first adhesive with a first organic solvent.

[0044] Preferably, the mass of the first organic solvent is 30-60% of the mass of the first positive electrode slurry, for example, it can be 30%, 35%, 40%, 50% or 60%, but is not limited to the listed values. Other unlisted values ​​within the range are also applicable.

[0045] Preferably, the raw materials for the second positive electrode layer in step (1) include an organic solution of a second positive electrode active material, a second conductive agent, and a second binder.

[0046] Preferably, the organic solution of the second adhesive is obtained by mixing the second adhesive with a second organic solvent.

[0047] Preferably, the mass of the second organic solvent is 30-60% of the mass of the second positive electrode slurry, for example, it can be 30%, 35%, 40%, 50% or 60%, but is not limited to the listed values. Other unlisted values ​​within the range are also applicable.

[0048] Preferably, the first organic solvent and the second organic solvent each independently comprise any one or a combination of at least two of N-methyl-2-pyrrolidone, dibutyl ether, tetraethylene glycol dimethyl ether, diethylene glycol dimethyl ether, formamide, dimethylformamide, acetonitrile, trimethyl phosphate, diethyl carbonate, dimethyl carbonate, dipropyl carbonate, methyl propyl carbonate, ethyl propyl carbonate, methyl ethyl carbonate, ethylene carbonate, 1,2-difluoroethylene carbonate, 1,1-difluoroethylene carbonate, methyl acetate, ethyl acetate, n-propyl acetate, tert-butyl acetate, methyl propionate, ethoxymethoxyethane, 2-methyltetrahydrofuran, or tetrahydrofuran, typically but not limitingly. Combinations include combinations of N-methyl-2-pyrrolidone and dibutyl ether, combinations of tetraethylene glycol dimethyl ether, diethylene glycol dimethyl ether and formamide, or combinations of N-methyl-2-pyrrolidone, dibutyl ether, tetraethylene glycol dimethyl ether, diethylene glycol dimethyl ether, formamide, dimethylformamide, acetonitrile, trimethyl phosphate, diethyl carbonate, dimethyl carbonate, dipropyl carbonate, methyl propyl carbonate, ethyl propyl carbonate, methyl ethyl carbonate, ethyl carbonate, 1,2-difluoroethylene carbonate, 1,1-difluoroethylene carbonate, methyl acetate, ethyl acetate, n-propyl acetate, tert-butyl acetate, methyl propionate, ethoxymethoxyethane, 2-methyltetrahydrofuran and tetrahydrofuran.

[0049] Preferably, the temperature of the first drying step (2) is 85-95℃, for example, it can be 85℃, 88℃, 90℃, 92℃ or 95℃, but is not limited to the listed values. Other unlisted values ​​within the range are also applicable.

[0050] Preferably, the drying temperature in step (2) is 85-95°C, for example, it can be 85°C, 88°C, 90°C, 92°C or 95°C, but is not limited to the listed values. Other unlisted values ​​within the range are also applicable.

[0051] Preferably, the temperature of the third drying step (2) is 100-110℃, for example, it can be 100℃, 102℃, 105℃, 108℃ or 110℃, but is not limited to the listed values. Other unlisted values ​​within the range are also applicable.

[0052] Preferably, step (2) after the third drying process further includes slicing and weighing.

[0053] The numerical range described in this invention includes not only the point values ​​listed above, but also any point values ​​within the numerical ranges not listed above. Due to space limitations and for the sake of brevity, this invention will not exhaustively list all the specific point values ​​included in the range.

[0054] Compared with the prior art, the present invention has the following beneficial effects:

[0055] (1) The lithium-ion battery gradient cathode sheet provided by the present invention adopts a gradient two-layer electrode technology. The first cathode layer and the second cathode layer respectively contain cathode active materials with different transition metal ratios. The first cathode active material in the first cathode layer is a ternary cathode material with a high nickel content, and the second cathode active material in the second cathode layer is a ternary cathode material with a low nickel content. The first cathode layer with a high nickel content can provide a higher energy density, while the second cathode layer with a low nickel content has higher cycle stability. Covering the surface of the first cathode layer with a high nickel content can maintain the stability of the entire electrode sheet in charge-discharge cycles, improve cycle performance and compensation capacity.

[0056] (2) Compared with the technology of modifying cathode materials, the preparation method provided by the present invention has lower requirements for the performance of cathode materials, does not require modification of battery production equipment, and can be quickly promoted for mass production. In addition, the preparation method provided by the present invention can also be used in conjunction with cathode material modification technologies such as coating, doping, and microstructure engineering to further improve the performance of batteries on the basis of existing technologies and meet the diversified market demands. Detailed Implementation

[0057] The technical solution of the present invention will be further illustrated below through specific embodiments. Those skilled in the art should understand that the embodiments described are merely illustrative of the present invention and should not be construed as limiting the invention in any way.

[0058] Example 1

[0059] This embodiment provides a gradient cathode sheet for a lithium-ion battery. The gradient cathode sheet includes a positive electrode current collector, a first positive electrode layer coated on the surface of the positive electrode current collector, and a second positive electrode layer coated on the surface of the first positive electrode layer. The first positive electrode layer has a thickness of 80 μm and an areal density of 300 g / m³. 2 The second cathode layer has a thickness of 80 μm and an areal density of 240 g / m³. 2 .

[0060] Based on a total mass percentage of 100 wt%, the raw materials of the first positive electrode layer include: the first positive electrode active material LiNi 0.96 Co 0.02 Mn 0.02 90wt%, conductive carbon black 5wt%, polyvinylidene fluoride 5wt%; based on a total mass percentage of 100wt%, the raw materials for the second positive electrode layer include: the second positive electrode active material LiNi 0.8 Co 0.1 Mn 0.1 90wt%, conductive carbon black 5wt%, polyvinylidene fluoride 5wt%; based on a total mass percentage of 100wt%, the first positive electrode active material LiNi 0.96 Co 0.02 Mn 0.02 With the second positive electrode active material LiNi 0.8 Co 0.1 Mn 0.1 In the middle, the first positive electrode active material LiNi 0.96 Co 0.02 Mn 0.02 The content is 60 wt%, with the balance being the second positive electrode active material LiNi. 0.8 Co 0.1 Mn 0.1 .

[0061] The method for preparing the gradient cathode sheet for the lithium-ion battery includes the following steps:

[0062] (1) Polyvinylidene fluoride (PVDF) and N-methyl-2-pyrrolidone (N-Methyl-2-pyrrolidone) are mixed to obtain a PVDF N-methyl-2-pyrrolidone solution. The first positive electrode active material LiNi is then added in the specified amount. 0.96 Co 0.02 Mn 0.02A solution of conductive carbon black and polyvinylidene fluoride in N-methyl-2-pyrrolidone was mixed evenly and then vacuum degassed to obtain a first positive electrode slurry; the mass of N-methyl-2-pyrrolidone was 40% of the mass of the first positive electrode slurry.

[0063] A polyvinylidene fluoride (PVDF) and N-methyl-2-pyrrolidone (N-Methyl-2-pyrrolidone) solution was obtained by mixing PVDF with N-methyl-2-pyrrolidone. The formulated amount of the second positive electrode active material, LiNi, was then added. 0.8 Co 0.1 Mn 0.1 A solution of conductive carbon black and polyvinylidene fluoride in N-methyl-2-pyrrolidone was mixed evenly and then vacuum degassed to obtain a second positive electrode slurry; the mass of N-methyl-2-pyrrolidone was 40% of the mass of the second positive electrode slurry.

[0064] (2) The first positive electrode slurry obtained in step (1) is coated on the surface of the positive electrode current collector and dried at 90°C to form the first positive electrode layer; then the second positive electrode slurry obtained in step (1) is coated on the surface of the first positive electrode layer and dried at 90°C to form the second positive electrode layer; the resulting composite structure is dried at 105°C, sliced ​​and weighed to obtain the lithium-ion battery gradient positive electrode sheet.

[0065] Example 2

[0066] This embodiment provides a gradient cathode sheet for a lithium-ion battery. The gradient cathode sheet includes a positive electrode current collector, a first positive electrode layer coated on the surface of the positive electrode current collector, and a second positive electrode layer coated on the surface of the first positive electrode layer. The first positive electrode layer has a thickness of 20 μm and an areal density of 220 g / m³. 2 The second cathode layer has a thickness of 20 μm and an areal density of 200 g / m³. 2 .

[0067] Based on a total mass percentage of 100 wt%, the raw materials of the first positive electrode layer include: the first positive electrode active material LiNi 0.8 Co 0.1 Mn 0.1 99wt%, graphite 0.5wt%, sodium carboxymethyl cellulose 0.5wt%; based on a total mass percentage of 100wt%, the raw materials for the second positive electrode layer include: the second positive electrode active material LiNi 0.6 Co 0.2 Mn 0.2 99wt%, graphite 0.5wt%, sodium carboxymethyl cellulose 0.5wt%; based on a total mass percentage of 100wt%, the first positive electrode active material LiNi 0.8 Co 0.1 Mn 0.1 With the second positive electrode active material LiNi0.6 Co 0.2 Mn 0.2 In the middle, the first positive electrode active material LiNi 0.8 Co 0.1 Mn 0.1 20 wt%, with the balance being the second positive electrode active material LiNi. 0.6 Co 0.2 Mn 0.2 .

[0068] The method for preparing the gradient cathode sheet for the lithium-ion battery includes the following steps:

[0069] (1) Sodium carboxymethyl cellulose and dibutyl ether are mixed to obtain a sodium carboxymethyl cellulose dibutyl ether solution, and the first positive electrode active material LiNi is added in the specified amount. 0.8 Co 0.1 Mn 0.1 A solution of graphite and sodium carboxymethyl cellulose in dibutyl ether is mixed evenly and then degassed under vacuum to obtain a first positive electrode slurry; the mass of the dibutyl ether is 30% of the mass of the first positive electrode slurry.

[0070] Sodium carboxymethyl cellulose was mixed with tetraethylene glycol dimethyl ether to obtain a sodium carboxymethyl cellulose solution in tetraethylene glycol dimethyl ether. The formulated amount of the second positive electrode active material, LiNi, was then added. 0.6 Co 0.2 Mn 0.2 A solution of graphite and sodium carboxymethyl cellulose in tetraethylene glycol dimethyl ether is mixed evenly and then degassed under vacuum to obtain a second positive electrode slurry; the mass of the tetraethylene glycol dimethyl ether is 30% of the mass of the second positive electrode slurry.

[0071] (2) The first positive electrode slurry obtained in step (1) is coated on the surface of the positive electrode current collector and dried at 85°C to form the first positive electrode layer; then the second positive electrode slurry obtained in step (1) is coated on the surface of the first positive electrode layer and dried at 85°C to form the second positive electrode layer; the resulting composite structure is dried at 100°C, sliced ​​and weighed to obtain the lithium-ion battery gradient positive electrode sheet.

[0072] Example 3

[0073] This embodiment provides a gradient cathode sheet for a lithium-ion battery. The gradient cathode sheet includes a positive current collector, a first positive electrode layer coated on the surface of the positive current collector, and a second positive electrode layer coated on the surface of the first positive electrode layer. The first positive electrode layer has a thickness of 200 μm and an areal density of 350 g / m³. 2 The second cathode layer has a thickness of 200 μm and an areal density of 300 g / m³. 2 .

[0074] Based on a total mass percentage of 100 wt%, the raw materials of the first positive electrode layer include: the first positive electrode active material LiNi 0.99 Co 0.005 Mn 0.005 80wt%, carbon nanotubes 10wt%, styrene-butadiene rubber 10wt%; based on a total mass percentage of 100wt%, the raw materials for the second cathode layer include: the second cathode active material LiNi 0.9 Co 0.05 Mn 0.05 80wt%, graphene 10wt%, styrene-butadiene rubber 10wt%; based on a total mass percentage of 100wt%, the first positive electrode active material LiNi 0.99 Co 0.005 Mn 0.005 With the second positive electrode active material LiNi 0.9 Co 0.05 Mn 0.05 In the middle, the first positive electrode active material LiNi 0.99 Co 0.005 Mn 0.005 80 wt%, with the balance being the second positive electrode active material LiNi. 0.9 Co 0.05 Mn 0.05 .

[0075] The method for preparing the gradient cathode sheet for the lithium-ion battery includes the following steps:

[0076] (1) Styrene-butadiene rubber and acetonitrile are mixed to obtain a styrene-butadiene rubber acetonitrile solution, and the first positive electrode active material LiNi is added in the specified amount. 0.99 Co 0.005 Mn 0.005 A solution of carbon nanotubes and styrene-butadiene rubber in acetonitrile is mixed evenly and then degassed under vacuum to obtain a first positive electrode slurry; the mass of the acetonitrile is 60% of the mass of the first positive electrode slurry.

[0077] Styrene-butadiene rubber (SBR) and acetonitrile are mixed to obtain an acetonitrile solution of SBR. The formulated amount of the second positive electrode active material, LiNi, is then added. 0.9 Co 0.05 Mn 0.05 A solution of graphene and styrene-butadiene rubber in acetonitrile is mixed evenly and then vacuum degassed to obtain a second positive electrode slurry; the mass of the acetonitrile is 60% of the mass of the second positive electrode slurry.

[0078] (2) The first positive electrode slurry obtained in step (1) is coated on the surface of the positive electrode current collector and dried at 95°C to form the first positive electrode layer; then the second positive electrode slurry obtained in step (1) is coated on the surface of the first positive electrode layer and dried at 95°C to form the second positive electrode layer; the resulting composite structure is dried at 110°C, sliced ​​and weighed to obtain the lithium-ion battery gradient positive electrode sheet.

[0079] Example 4

[0080] This embodiment provides a gradient cathode sheet for a lithium-ion battery. The difference from Embodiment 1 is that, except that the thickness of both the first cathode layer and the second cathode layer is adjusted to 10 μm, everything else is the same as in Embodiment 1.

[0081] Example 5

[0082] This embodiment provides a gradient cathode sheet for a lithium-ion battery. The difference from Embodiment 1 is that, except that the thickness of both the first cathode layer and the second cathode layer is adjusted to 220 μm, everything else is the same as in Embodiment 1.

[0083] Example 6

[0084] This embodiment provides a gradient cathode sheet for a lithium-ion battery. The difference from Embodiment 1 is that the first cathode active material, LiNi, is used. 0.96 Co 0.02 Mn 0.02 The mass percentage content of the first positive electrode material (LiNi) was adjusted to 75 wt%, the mass percentage content of conductive carbon black was adjusted to 10 wt%, and the mass percentage content of polyvinylidene fluoride was adjusted to 15 wt%. 0.8 Co 0.1 Mn 0.1 The mass percentage of the conductive carbon black was adjusted to 75 wt%, the mass percentage of the polyvinylidene fluoride was adjusted to 10 wt%, and the mass percentage of the polyvinylidene fluoride was adjusted to 15 wt%. All other contents were the same as in Example 1.

[0085] Comparative Example 1

[0086] This comparative example provides a gradient cathode sheet for a lithium-ion battery. The difference from Example 1 is that the chemical formula of the first cathode active material is adjusted to LiNi. 0.8 Co 0.1 Mn 0.1 The chemical formula of the second positive electrode active material is adjusted to LiNi. 0.96 Co 0.02 Mn 0.02 Everything else is the same as in Example 1.

[0087] Comparative Example 2

[0088] This comparative example provides a gradient cathode sheet for a lithium-ion battery. The difference from Example 1 is that the chemical formula of the first cathode active material is adjusted to LiNi. 0.6 Co 0.2 Mn 0.2 Everything else is the same as in Example 1.

[0089] Comparative Example 3

[0090] This comparative example provides a gradient cathode sheet for a lithium-ion battery. The difference from Example 1 is that the first cathode active material, LiNi, is used. 0.96 Co 0.02 Mn 0.02 The mass percentage content is adjusted to 10wt%, with the balance being the second positive electrode active material LiNi. 0.8 Co 0.1 Mn 0.1 Everything else is the same as in Example 1.

[0091] Comparative Example 4

[0092] This comparative example provides a gradient cathode sheet for a lithium-ion battery. The difference from Example 1 is that the first cathode active material, LiNi, is used. 0.96 Co 0.02 Mn 0.02 The mass percentage content is adjusted to 90wt%, with the balance being the second positive electrode active material LiNi. 0.8 Co 0.1 Mn 0.1 Everything else is the same as in Example 1.

[0093] Comparative Example 5

[0094] This comparative example provides a single-layer positive electrode sheet for a lithium-ion battery. The difference from Example 1 is that it lacks a second positive electrode layer, and the chemical formula of the positive electrode active material in the first positive electrode layer is adjusted to LiNi. 0.9 Co 0.05 Mn 0.05 Everything else is the same as in Example 1.

[0095] The positive electrode sheets prepared in all the above examples and comparative examples were matched with lithium metal to assemble lithium-ion batteries. At 25°C and a voltage range of 2.7-4.3V, the batteries were first activated by charging and discharging at a rate of 0.1C for 3 cycles, and then charged and discharged at 1C for 100 cycles. The electrochemical performance was then tested, and the test results are shown in Table 1.

[0096] Table 1

[0097]

[0098] As can be seen from Table 1, the lithium-ion battery prepared by the gradient cathode sheet provided by the present invention has high capacity and rate performance, as well as good charge-discharge cycle stability.

[0099] A comparison of Examples 1 with Examples 4 and 5 shows that if the thickness of the first and second positive electrode layers is too low, the protective effect of the second positive electrode layer on the first positive electrode layer will be limited, and the volumetric energy density and gravimetric energy density of the lithium-ion battery assembled with this positive electrode sheet will also be low. If the thickness is too high, the mechanical stress generated by the internal volume change during charging and discharging will be greater, which will easily lead to contact failure between particles, damage to the conductive network, and even detachment from the current collector. In addition, the degree of reaction of the electrode from the surface to the inside is uneven, and the surface may have been overcharged / over-discharged while the inside has not yet fully reacted. This non-uniformity will accelerate local failure. All of these factors lead to severe capacity decay of the overly thick positive electrode sheet during long-term charge-discharge cycles. A comparison of Examples 1 with Example 6 shows that if the content of positive active material in the raw materials of the first and second positive electrode layers is too low, the volumetric energy density and gravimetric energy density of the lithium-ion battery assembled with this positive electrode sheet will drop sharply. Furthermore, binders and conductive agents (especially carbon materials) are usually more prone to side reactions with the electrolyte than active materials. Furthermore, increased content of these substances may lead to more electrolyte decomposition and gas generation, which in turn consumes active lithium, increases impedance, and impairs long-term cycling performance.

[0100] A comparison of Example 1 and Comparative Example 1 shows that the nickel content of the second positive electrode active material is too high, exceeding that of the first positive electrode active material. This leads to severe interfacial side reactions between the highly active surface of the second positive electrode active material and the electrolyte, and more severe lithium-nickel mixing causes a more severe irreversible phase transition in the deep delithiation state. Strain accumulation causes microcracks to propagate, and the exposed high-nickel fresh interface then undergoes severe interfacial side reactions with the electrolyte, resulting in the dissolution of transition metals and the formation of inactive phases such as salt rock layers. This reduces the capacity retention and cycle life of the lithium-ion battery during long-term charge-discharge cycles. A comparison of Example 1 and Comparative Example 2 shows that the nickel content of the second positive electrode active material is higher than that of the first positive electrode active material, while the nickel content of the first positive electrode active material is too low. This results in the first positive electrode layer failing to provide the expected high charge-discharge capacity, and the second positive electrode layer experiencing severe capacity decay due to severe interfacial side reactions with the electrolyte caused by its high nickel content. Ultimately, this leads to a lithium-ion battery with not only low charge-discharge capacity but also... Furthermore, the capacity retention rate during long-term charge-discharge cycles is also low. A comparison of Example 1 with Comparative Examples 3 and 4 shows that if the proportion of the first positive electrode active material in the total amount of the first and second positive electrode active materials is too low, the nickel content of the active material in the overall positive electrode sheet will decrease, resulting in a lower charge-discharge capacity. Conversely, if the proportion of the first positive electrode active material is too high, the protective effect of the second positive electrode sheet on the first positive electrode sheet will be significantly reduced, thus decreasing the cycle stability of the lithium-ion battery during long-term charge-discharge cycles. A comparison of Example 1 with Comparative Example 5 shows that, compared to gradient positive electrode sheets, the use of a single-layer positive electrode sheet results in a lower capacity retention rate during long-term charge-discharge cycles because the second positive electrode sheet, with its lower nickel content, stronger structural stability, and weaker surface reactivity, does not provide protection for the first positive electrode sheet, which has a higher nickel content, weaker structural stability, and stronger surface reactivity. This demonstrates the positive effect of this invention on improving the charge-discharge capacity and cycle stability of the positive electrode sheet and the lithium-ion battery assembled with it.

[0101] In summary, the lithium-ion battery gradient cathode sheet provided by this invention employs a gradient two-layer electrode technology. The first cathode layer and the second cathode layer each contain cathode active materials with different transition metal ratios. The first cathode active material in the first cathode layer is a ternary cathode material with a high nickel content, while the second cathode active material in the second cathode layer is a ternary cathode material with a low nickel content. The first cathode layer with a high nickel content can provide higher energy density, while the second cathode layer with a low nickel content offers higher cycle stability. Covering the surface of the first cathode layer with a high nickel content helps maintain the stability of the entire electrode sheet during charge-discharge cycles, improving cycle performance and compensation capacity.

[0102] Compared with technologies that modify cathode materials, the preparation method provided by this invention has lower performance requirements for cathode materials, requires no modification to battery production equipment, and can be rapidly promoted for mass production. In addition, the preparation method provided by this invention can also be used in conjunction with cathode material modification technologies such as coating, doping, and microstructure engineering to further improve battery performance on the basis of existing technologies and meet diverse market demands.

[0103] The above description is only a specific embodiment of the present invention, but the protection scope of the present invention is not limited thereto. Those skilled in the art should understand that any changes or substitutions that can be easily conceived by those skilled in the art within the technical scope disclosed in the present invention fall within the protection and disclosure scope of the present invention.

Claims

1. A gradient cathode sheet for lithium-ion batteries, characterized in that, The lithium-ion battery gradient cathode includes a cathode current collector, a first cathode layer coated on the surface of the cathode current collector, and a second cathode layer coated on the surface of the first cathode layer; The raw materials for the first positive electrode layer include the first positive electrode active material, with the general chemical formula LiNi. x Co y Mn z Where x + y + z = 1, 0.8 ≤ x ≤ 0.99, 0 < y ≤ 0.1, and 0 < z ≤ 0.1; The raw materials for the second positive electrode layer include a second positive electrode active material with the general chemical formula LiNi. x Co y Mn z Where x + y + z = 1, 0.6 ≤ x ≤ 0.9, 0 < y ≤ 0.2, and 0 < z ≤ 0.2; The nickel content of the first positive electrode active material is higher than that of the second positive electrode active material; Based on a total mass percentage of 100wt%, the first positive electrode active material comprises 20-80wt%, and the remainder is the second positive electrode active material.

2. The lithium-ion battery gradient cathode sheet according to claim 1, characterized in that, The raw materials for the first positive electrode layer also include a first conductive agent and a first binder; Preferably, based on a total mass percentage of 100wt%, the raw materials of the first positive electrode layer include: 80-99wt% of the first positive electrode active material, 0.5-10wt% of the first conductive agent, and 0.5-10wt% of the first binder.

3. The lithium-ion battery gradient cathode sheet according to claim 2, characterized in that, The raw materials for the second positive electrode layer also include a second conductive agent and a second binder; Preferably, based on a total mass percentage of 100wt%, the raw materials of the second positive electrode layer include: 80-99wt% of the second positive electrode active material, 0.5-10wt% of the second conductive agent, and 0.5-10wt% of the second binder.

4. The lithium-ion battery gradient cathode sheet according to claim 3, characterized in that, The first conductive agent and the second conductive agent each independently include any one or a combination of at least two of conductive carbon black, Ketjen black, graphite, graphene, carbon nanotubes or carbon fibers. Preferably, the first adhesive and the second adhesive each independently comprise any one or a combination of at least two of polyvinylidene fluoride, polyaniline, sodium carboxymethyl cellulose, styrene-butadiene rubber, polyacrylic acid, polyvinylpyrrolidone, or polysiloxane.

5. The lithium-ion battery gradient cathode sheet according to any one of claims 1-4, characterized in that, The thicknesses of the first positive electrode layer and the second positive electrode layer are independently 20-200 μm; Preferably, the areal density of the first positive electrode layer is 200-350 g / m³. 2 The areal density of the second cathode layer is 200-350 g / m³. 2 .

6. A method for preparing a lithium-ion battery gradient cathode sheet as described in any one of claims 1-5, characterized in that, The preparation method includes the following steps: (1) Mix the raw materials of the first positive electrode layer in the formula amount evenly, and then degas them under vacuum to obtain the first positive electrode slurry; mix the raw materials of the second positive electrode layer in the formula amount evenly, and then degas them under vacuum to obtain the second positive electrode slurry; (2) The first positive electrode slurry obtained in step (1) is coated on the surface of the positive electrode current collector and dried to form the first positive electrode layer; Then the second positive electrode slurry obtained in step (1) is coated on the surface of the first positive electrode layer, and the second positive electrode layer is formed after a second drying; the resulting composite structure is dried in a third drying process to obtain the lithium-ion battery gradient positive electrode sheet.

7. The preparation method according to claim 6, characterized in that, Step (1) The raw materials of the first positive electrode layer include an organic solution of a first positive electrode active material, a first conductive agent, and a first binder; Preferably, the organic solution of the first adhesive is obtained by mixing the first adhesive with a first organic solvent; Preferably, the mass of the first organic solvent is 30-60% of the mass of the first positive electrode slurry.

8. The preparation method according to claim 7, characterized in that, Step (1) The raw materials for the second positive electrode layer include an organic solution of a second positive electrode active material, a second conductive agent, and a second binder; Preferably, the organic solution of the second adhesive is obtained by mixing the second adhesive with a second organic solvent; Preferably, the mass of the second organic solvent is 30-60% of the mass of the second positive electrode slurry.

9. The preparation method according to claim 8, characterized in that, The first organic solvent and the second organic solvent each independently comprise any one or a combination of at least two of the following: N-methyl-2-pyrrolidone, dibutyl ether, tetraethylene glycol dimethyl ether, diethylene glycol dimethyl ether, formamide, dimethylformamide, acetonitrile, trimethyl phosphate, diethyl carbonate, dimethyl carbonate, dipropyl carbonate, methyl propyl carbonate, ethyl propyl carbonate, methyl ethyl carbonate, ethylene carbonate, 1,2-difluoroethylene carbonate, 1,1-difluoroethylene carbonate, methyl acetate, ethyl acetate, n-propyl acetate, tert-butyl acetate, methyl propionate, ethoxymethoxyethane, 2-methyltetrahydrofuran, or tetrahydrofuran.

10. The preparation method according to any one of claims 6-9, characterized in that, Step (2) The temperature of the first drying step is 85-95℃; Preferably, the drying temperature in step (2) is 85-95°C; Preferably, the temperature of the third drying step (2) is 100-110℃.

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