A positive electrode sheet, a preparation method thereof and an application

Abstract

The present invention provides a positive electrode plate, a preparation method thereof and an application. The positive electrode plate includes a current collector and an electrode layer located on at least one side of the current collector. The electrode layer at least includes a first active material layer and a second active material layer which are stacked, and the first active material layer is in direct contact with the surface of the current collector; wherein, the active material in the first active material layer includes a layered oxide cathode material, and the active material in the second active material layer includes a manganese-based cathode material. The positive electrode plate provided by the present invention improves the electronic conductivity of the manganese-based cathode material, and at the same time cooperates with the layered oxide cathode material, improves the safety performance of the battery, improves the long cycle performance of the battery, gives full play to the respective advantages of the manganese-based cathode material and the layered oxide cathode material, makes the charge-discharge curve smoother, and helps the BMS calculation.

Description

Technical Field

[0001] The present invention belongs to the technical field of lithium-ion batteries, and relates to a positive electrode sheet, a preparation method thereof and an application thereof. Background Art

[0002] The positive electrode material plays a crucial role in lithium-ion batteries, such as manganese-based positive electrode materials and layered oxide positive electrode materials.

[0003] The lithium iron manganese phosphate positive electrode material has poor conductivity, and its polarization will continuously increase during the cycling process, thereby affecting the cycle life of the battery; in order to improve its electronic conductivity and reduce the ohmic impedance, the lithium iron manganese phosphate positive electrode coating generally needs to use carbon-coated aluminum foil, and the use of carbon-coated aluminum foil will lead to an increase in cost and cannot meet the requirements of large-scale production. For example, CN113410427A provides a positive electrode sheet, a preparation method thereof and an application thereof, including a positive electrode current collector, a carbon-coated layer and a positive electrode active material layer; the carbon-coated layer is coated on at least one surface of the positive electrode current collector; the positive electrode active material layer is coated on the side of the carbon-coated layer away from the positive electrode current collector, and the positive electrode active material includes lithium iron manganese phosphate, lithium manganate material and lithium-rich manganese-based positive electrode material, etc.

[0004] The layered oxide material has become the most promising positive electrode material for lithium-ion batteries due to its high charge and discharge voltage, high specific capacity and low cost. However, the layered oxide material inevitably has many defects, such as low thermal stability caused by cation mixing and increasing nickel content, generation of microcracks during the charge and discharge process, high surface residual alkali content, poor cycle performance and other problems.

[0005] At present, some researchers mix lithium iron manganese phosphate and layered oxide materials to prepare a mixed slurry of the two for coating to obtain a lithium iron manganese phosphate-layered oxide mixed electrode. For example, CN108777298A discloses a positive electrode material, including lithium iron manganese phosphate and a ternary material; the ternary material is lithium nickel cobalt manganese oxide and / or lithium nickel cobalt aluminate. The purpose is to give full play to the advantages of lithium iron manganese phosphate and layered oxide materials to improve the conductivity of lithium iron manganese phosphate and enhance the safety of layered oxide materials. However, the lithium iron manganese phosphate-layered oxide mixed electrode obtained by the above method will have problems such as poor coating effect, uneven distribution of lithium iron manganese phosphate and layered oxide materials, and unsatisfactory safety improvement effect.

[0006] At the same time, aiming at the problem of the failure of the layered oxide material during long cycling, it is necessary to develop a high-safety layered oxide material application system to improve the thermal stability of the layered oxide material and reduce problems such as thermal runaway caused by its thermal failure.

[0007] Therefore, how to improve the long-cycle performance and conductivity of the lithium-ion battery positive electrode system while improving its safety performance without increasing the production cost too much is a technical problem that needs to be solved urgently. Summary of the Invention

[0008] The present invention aims to provide a positive electrode plate, a preparation method, and applications thereof. The positive electrode plate provided by the present invention enhances the electronic conductivity of manganese-based positive electrode materials and, in conjunction with layered oxide positive electrode materials, improves battery safety and long-cycle performance. It leverages the respective advantages of manganese-based and layered oxide positive electrode materials, resulting in smoother charge and discharge curves and facilitating battery management system (BMS) calculations.

[0009] In order to achieve the purpose of the invention, the present invention adopts the following technical solutions:

[0010] In a first aspect, the present invention provides a positive electrode plate, comprising a current collector and an electrode layer located on at least one side of the current collector, wherein the electrode layer comprises at least a first active material layer and a second active material layer stacked together, and the first active material layer is in direct contact with the surface of the current collector; wherein the active material in the first active material layer comprises a layered oxide positive electrode material, and the active material in the second active material layer comprises a manganese-based positive electrode material.

[0011] The positive electrode plate provided by the present invention improves the electronic conductivity of the manganese-based positive electrode material, and at the same time cooperates with the layered oxide positive electrode material to improve the safety performance of the battery, enhance the long cycle performance of the battery, give full play to the respective advantages of the manganese-based positive electrode material and the layered oxide positive electrode material, make the charge and discharge curve smoother, and facilitate BMS calculation.

[0012] Preferably, the current collector includes a plain foil and a coating located on the surface of the plain foil.

[0013] The current collector provided in the present invention may also not use a structure containing a coating, which can save costs. The use of a coated current collector can further improve the conductivity of the electrode.

[0014] Preferably, the coating layer comprises a carbon coating layer, and the material in the carbon coating layer comprises any one of conductive carbon black, graphene or carbon nanotubes, or a combination of at least two of them.

[0015] Preferably, the chemical formula of the layered oxide positive electrode material is Li a Ni x Co y M 1-x-yAny one or a combination of at least two of O2 or zLi2MnO3·(1 - z)LiBO2, M includes any one or a combination of at least two of Al, Mn, Fe, Cu, Zn, Zr, Mg, W, V, Nb, Sr, K, Na or Ti, 1 ≤ a ≤ 2, 0.2 ≤ x ≤ 1, 0 ≤ y < 1; B includes any one or a combination of at least two of Mn, Ni or Co, 0 < z < 1. For example, a can be 1, 1.1, 1.2, 1.3, 1.4, 1.5, 1.6, 1.7, 1.8, 1.9 or 2, etc., x can be 0.2, 0.3, 0.4, 0.5, 0.6, 0.7, 0.8, 0.9 or 1, etc., y can be 0.1, 0.2, 0.3, 0.4, 0.5, 0.6, 0.7, 0.8 or 0.9, etc., z can be 0.2, 0.3, 0.4, 0.5, 0.6, 0.7, 0.8 or 0.9, etc. Preferably, the D50 of the layered oxide cathode material is 1 - 30 μm, such as 1 μm, 1.1 μm, 1.5 μm, 2 μm, 5 μm, 10 μm, 15 μm, 20 μm, 25 μm or 30 μm, etc.

[0016] Preferably, the manganese - based cathode material includes LiMn x Fe y A (1-x-y) PO4, any one or a combination of at least two of zLi2MnO3·(1 - z)LiCO2 or LiMn2O4, where 0 ≤ x ≤ 1, 0 ≤ y ≤ 1, A includes any one or a combination of at least two of Al, Cu, Zn, Zr, Mg, W, V, Nb, Sr, K, Na, Ti, Y, S, F, N or Cl, C includes any one or a combination of at least two of Mn, Ni or Co, 0 < z < 1. For example, x can be 0, 0.1, 0.2, 0.3, 0.4, 0.5, 0.6, 0.7, 0.8, 0.9 or 1, etc., y can be 0, 0.1, 0.2, 0.3, 0.4, 0.5, 0.6, 0.7, 0.8, 0.9 or 1, etc., z can be 0.2, 0.3, 0.4, 0.5, 0.6, 0.7, 0.8 or 0.9, etc.

[0017] In the present invention, the chemical formula zLi2MnO3·(1 - z)LiBO2 in the layered oxide and the chemical formula zLi2MnO3·(1 - z)LiCO2 in the manganese - based cathode material are only adjusted for distinction. Essentially, they can represent the same substance, and the expressions of other chemical formulas are also adjusted only for distinction. The letters therein are only for referring to elements and there are no any other objections.

[0018] Preferably, the electrode layer is a type-I electrode layer, wherein the active material in the first active material layer is a layered oxide cathode material, and the active material in the second active material layer is a manganese-based cathode material.

[0019] Preferably, in the type-I electrode layer, the ratio of the areal density of the active material in the second active material layer to the areal density of the active material in the first active material layer is 0.01 to 100, such as 0.01, 0.05, 0.1, 0.5, 1, 5, 10, 15, 20, 25, 30, 35, 40, 45, 50, 55, 60, 65, 70, 75, 80, 85, 90, 95 or 100, etc., and preferably 0.1 to 20.

[0020] In the present invention, in the type-I electrode layer, when the ratio of the areal density of the active material in the second active material layer to the areal density of the active material in the first active material layer is within a further preferred range (0.1 to 20), the problem of poor ionic conductivity of the manganese-based cathode material is solved by the layered oxide cathode material, and at the same time, there is no need to use carbon-coated aluminum foil, saving costs; the manganese-based cathode material in the second active material layer realizes the coating effect on the layered oxide and reduces the heat diffusion, while reducing the kinetic characteristics of the first-layer active material, avoiding the structural deterioration of the layered oxide cathode material during the cycling process. However, when the ratio of its areal density exceeds 20, it will affect the ionic conductance characteristics of the second-layer layered oxide cathode material, thus unable to achieve the reduction of the electrode impedance and the improvement of the electrode ionic conductance, and it is not conducive to the stability of the electronic conductivity in the later stage of cycling. When the areal density value is less than 0.1, it will cause poor insulation heat diffusion and coating effect of the manganese-based cathode material on the layered oxide, affecting the overall safety performance.

[0021] Preferably, in the type-I electrode layer, the ratio of the thickness of the second active material layer to the thickness of the first active material layer is 0.01 to 200, such as 0.01, 0.05, 0.1, 0.5, 1, 5, 10, 15, 20, 25, 30, 35, 40, 45, 50, 55, 60, 65, 70, 75, 80, 85, 90, 95, 100, 110, 120, 130, 140, 150, 160, 170, 180, 190 or 200, etc., and preferably 0.1 to 40.

[0022] In the Class I electrode layer provided by the present invention, the ratio of the thickness of the second active material layer to the thickness of the first active material layer is within a further preferred range (0.1 - 40), which is conducive to giving full play to the good ionic conductivity characteristics of the layered oxide cathode material and the safety characteristics of the manganese-based cathode material, ensuring good specific capacity performance, long cycle life and safety performance of the electrode. Exceeding the preferred range is not conducive to the exertion of the good ionic conductivity characteristics of the first-layer layered oxide cathode material, and will affect the specific capacity and long cycle performance of the active material. Below the preferred range, the coating and isolation effect of the second-layer manganese-based cathode material on the first-layer layered oxide cathode material will be poor, affecting the safety performance.

[0023] Preferably, in the Class I electrode layer, the ratio of the BET specific surface area BET2 of the active material in the second active material layer to the BET specific surface area BET1 of the active material in the first active material layer is 0.01 ≤ BET2 / BET1 ≤ 150, such as 0.01, 0.05, 0.1, 0.5, 1, 5, 10, 15, 20, 25, 30, 35, 40, 45, 50, 55, 60, 65, 70, 75, 80, 85, 90, 95, 100, 110, 120, 130, 140 or 150, etc., and preferably 2.1 ≤ BET2 / BET1 ≤ 60.

[0024] Preferably, the electrode layer is a Class II electrode layer, wherein the active material in the first active material layer includes a layered oxide cathode material, and the active material in the second active material layer includes a manganese-based cathode material and a layered oxide cathode material.

[0025] In the structure of the Class II electrode layer in the present invention, when the second active material layer includes a manganese-based cathode material and a layered oxide cathode material, good synergistic effects of the manganese-based cathode material and the layered oxide cathode material can be achieved. The layered oxide cathode material evenly distributed in the manganese-based cathode material can form a good ionic conductivity transmission network, which can significantly improve the ionic conductivity characteristics of the manganese-based cathode material, and the evenly distributed manganese-based cathode material has a good wrapping effect on the layered oxide cathode material therein, thereby improving the overall safety performance, and thus better improving the specific capacity, cycle performance and safety performance of the cathode material.

[0026] Preferably, in the Class II electrode layer, the ratio of the surface density of the active material in the second active material layer to the surface density of the active material in the first active material layer is 0.01 - 100, such as 0.01, 0.05, 1, 5, 10, 15, 20, 25, 30, 35, 40, 45, 50, 55, 60, 65, 70, 75, 80, 85, 90, 95 or 100, etc., and preferably 0.1 - 15.

[0027] In the Class II electrode layer provided by the present invention, the ratio of the thickness of the second active material layer to the areal density of the first active material layer is within a further preferred range (0.1 to 15), which is beneficial to fully exerting the good ionic conductivity characteristics of the layered oxide cathode material and the safety characteristics of the manganese-based cathode material, ensuring good specific capacity performance, long cycle life and safety performance of the electrode. If it exceeds the preferred range, it is not conducive to the exertion of the good ionic conductivity characteristics of the first-layer layered oxide cathode material, and will affect the specific capacity and long cycle performance of the active material. If it is lower than the preferred range, it will lead to poor coating and isolation effects of the second active material layer on the first-layer layered oxide cathode material, affecting the safety performance.

[0028] Preferably, in the Class II electrode layer, the ratio of the thickness of the second active material layer to the thickness of the first active material layer is 0.01 to 200, such as 0.01, 0.05, 0.1, 0.5, 1, 5, 10, 15, 20, 25, 30, 35, 40, 45, 50, 55, 60, 65, 70, 75, 80, 85, 90, 95, 100, 110, 120, 130, 140, 150, 160, 170, 180, 190 or 200, etc., and preferably 0.1 to 50.

[0029] In the Class II electrode layer provided by the present invention, the ratio of the thickness of the second active material layer to the thickness of the first active material layer is within a further preferred range (0.1 to 50), which is beneficial to fully exerting the good ionic conductivity characteristics of the layered oxide cathode material and the safety characteristics of the manganese-based cathode material, ensuring good specific capacity performance, long cycle life and safety performance of the electrode. If it exceeds the preferred range, it is not conducive to the exertion of the good ionic conductivity characteristics of the first-layer layered oxide cathode material, and will affect the specific capacity and long cycle performance of the active material. If it is lower than the preferred range, it will lead to poor coating and isolation effects of the second active material layer on the first-layer layered oxide cathode material, affecting the safety performance.

[0030] Preferably, in the Class II electrode layer, the mass ratio of the manganese-based cathode material to the layered oxide cathode material in the second active material layer is (0.01 to 200):1, such as 0.01:1, 0.05:1, 0.06:1, 0.1:1, 0.5:1, 1:1, 3:1, 5:1, 8:1, 10:1, 13:1, 15:1, 18:1 or 20:1, etc., and preferably (0.05 to 20):1.

[0031] In the Class II electrode layer provided by the present invention, when the mass ratio of the manganese-based cathode material to the layered oxide cathode material in the second active material layer is within a further preferred range ((0.05 to 20):1), the synergistic effect of the manganese-based cathode material and the layered oxide cathode material can be better realized, enabling the advantageous characteristics of both to be fully exerted. However, if the mass ratio is too large, that is, too much manganese-based cathode material is added, it will lead to sparse distribution of the layered oxide cathode material in the second active material layer and an incomplete ion conduction network, affecting the ionic conductivity characteristics of the electrode. On the contrary, if the mass ratio is too small, it will affect the isolation and coating effect of the manganese-based cathode material on the layered oxide cathode material, thereby affecting the safety performance.

[0032] Preferably, the electrode layer is a Class III electrode layer, wherein the active materials in the first active material layer include a layered oxide cathode material and a manganese-based cathode material, and the active materials in the second active material layer include a manganese-based cathode material and a layered oxide cathode material.

[0033] In the structure of the Class III electrode layer of the present invention, both the first active material layer and the second active material layer include a manganese-based cathode material and a layered oxide cathode material. Under this electrode structure, the synergistic effect of the manganese-based cathode material and the layered oxide cathode material can be realized, enabling the advantageous characteristics of both to be fully exerted, thereby better improving the electrochemical performance and safety performance of the active materials. The first active material layer includes a layered oxide cathode material and a manganese-based cathode material. The layered oxide cathode material has good ionic conductivity characteristics, and its uniform distribution can improve the conductivity of the electrode and reduce the ohmic impedance. The addition of the low-cost manganese-based cathode material has an isolation and coating effect on the layered oxide cathode material, which is beneficial to improving the safety of the first active material layer and reducing the raw material cost, showing an obvious cost advantage. When the second active material layer includes a manganese-based cathode material and a layered oxide cathode material, the good synergistic effect of the manganese-based cathode material and the layered oxide cathode material can be realized; the layered oxide cathode material is uniformly distributed in the manganese-based cathode material and forms a good ionic conductivity transmission network, which can significantly improve the ionic conductivity characteristics of the manganese-based cathode material. The uniformly distributed manganese-based cathode material also has a good wrapping effect on the layered oxide cathode material in the first active material layer and the second active material layer, thereby improving the overall safety performance and better improving the specific capacity, cycle performance and safety performance of the cathode material.

[0034] Preferably, in the Class III electrode layer, the ratio of the areal density of the active materials in the second active material layer to the areal density of the active materials in the first active material layer is 0.01 to 80, such as 0.01, 0.05, 1, 5, 10, 15, 20, 25, 30, 35, 40, 45 or 50, etc., and preferably 0.1 to 25.

[0035] In the Class III electrode layer provided by the present invention, the ratio of the areal density of the second active material layer to the areal density of the first active material layer is within a further preferred range (0.1 - 25), which is beneficial to improving the electrochemical performance of the active material while taking into account the safety performance. If it exceeds the preferred range, it is not conducive to the good ionic conductivity characteristics of the first active material layer, and it will affect the specific capacity and long-cycle performance of the active material. If it is lower than the preferred range, the coating and isolation effect of the second active material layer on the first-layer layered oxide cathode material will be poor, affecting the safety performance.

[0036] Preferably, in the Class III electrode layer, the ratio of the thickness of the second active material layer to the thickness of the first active material layer is 0.01 - 200, such as 0.01, 0.05, 0.1, 0.5, 1, 5, 10, 15, 20, 25, 30, 35, 40, 45, 50, 55, 60, 65, 70, 75, 80, 85, 90, 95, 100, 110, 120, 130, 140, 150, 160, 170, 180, 190 or 200, etc., and preferably 0.1 - 40.

[0037] In the Class III electrode layer provided by the present invention, the ratio of the thickness of the second active material layer to the thickness of the first active material layer is within a further preferred range (0.1 - 40), which is beneficial to improving the electrochemical performance of the active material while taking into account the safety performance. If it exceeds the preferred range, it is not conducive to the good ionic conductivity characteristics of the first-layer layered oxide cathode material, and it will affect the specific capacity and long-cycle performance of the active material. If it is lower than the preferred range, the coating and isolation effect of the second active material layer on the first-layer layered oxide cathode material will be poor, affecting the safety performance.

[0038] Preferably, in the Class III electrode layer, the mass ratio of the manganese-based cathode material to the layered oxide cathode material in the second active material layer is (0.01 - 200):1, such as 0.01:1, 0.05:1, 0.1:1, 0.5:1, 1:1, 3:1, 5:1, 8:1, 10:1, 13:1, 15:1, 18:1 or 20:1, etc., and preferably (0.05 - 20):1.

[0039] Preferably, in the Class III electrode layer, the mass proportion of the manganese-based cathode material in the second active material in the active material is greater than the mass proportion of the manganese-based cathode material in the first active material in the active material.

[0040] In the Class III electrode layer provided by the present invention, the distribution of the manganese-based cathode material in the second active material layer and the distribution of the manganese-based cathode material in the first active material layer achieve directional distribution, that is, the mass ratio of the manganese-based cathode material in the first active material layer in the active material is lower than the mass ratio of the manganese-based cathode material in the second active material layer in the active material, which is more conducive to improving the ionic conductivity characteristics of the electrode, reducing the impedance, and taking into account the safety performance of the electrode while improving the electrochemical performance of the active material; at the same time, by adopting the method of mixing two active materials, the ionic conductivities of the first layer and the second layer can be set directionally, so as to optimize the kinetic characteristics of the first layer and the second layer, enabling the two-layer active material system to have directionally designable kinetics, thereby realizing the improvement of the overall cycle performance of the battery. If the addition amounts of the manganese-based materials in the two layers are kept the same, problems such as an increase in the ohmic impedance and polarization of the electrode will occur, which will further affect the performance of the active material's electrochemical performance. On the contrary, if the second active material layer is less than the first active material layer, it will affect the good ionic conductivity characteristics of the layered oxide cathode material, resulting in the inability to solve the problem of poor ionic conductivity of the lithium iron phosphate manganese material only by using a light foil, and the cost advantage will also disappear.

[0041] It should be noted that the positive electrode sheet provided by the present invention can have electrode layers on both sides with the same structure, or can have only an electrode layer structure on one side; at the same time, in the active material layer of the electrode layer, in addition to the active material, it also includes a conductive agent and a binder, and their mass ratios are all conventional technical means, that is, the ratios conventionally used for the preparation of the positive electrode sheet of the battery, and the present invention is applicable to all of them.

[0042] In a second aspect, the present invention provides a method for preparing a positive electrode sheet as described in the first aspect, and the preparation method includes the following steps:

[0043] Prepare the slurry of the first active material layer and the slurry of the second active material layer respectively, and coat the slurry of the first active material layer and the slurry of the second active material layer on the surface of at least one side of the current collector to obtain the positive electrode sheet.

[0044] According to the structure of the electrode layer of the positive electrode sheet provided in the first aspect, the preparation method provided by the present invention only needs to prepare and coat the slurry, and can be coated simultaneously, or the first active material layer can be coated first, and then the second active material layer can be coated.

[0045] In a third aspect, the present invention further provides a lithium-ion battery, and the lithium-ion battery includes the positive electrode sheet as described in the first aspect.

[0046] Compared with the prior art, the present invention has the following beneficial effects:

[0047] The positive electrode sheet provided by the present invention saves costs, improves the ionic conductivity of the manganese-based positive electrode material, and at the same time cooperates with the layered oxide positive electrode material to improve the safety performance of the battery, enhance the long cycle performance of the battery, give full play to the respective advantages of the manganese-based positive electrode material and the layered oxide positive electrode material, make the charge and discharge curve smoother, and contribute to the BMS calculation. Detailed implementation manners

[0048] The technical solutions of the present invention will be further described below through specific implementation manners. Those skilled in the art should understand that the embodiments are only for helping to understand the present invention and should not be regarded as specific limitations on the present invention.

[0049] In a specific implementation manner, the present invention provides a method for preparing a slurry of an active material layer in a positive electrode sheet, and the preparation method includes:

[0050] Mix the active material, conductive carbon black and polyvinylidene fluoride in a mass ratio of 94:2:4 into NMP to obtain the slurry of the active material layer.

[0051] The preparation methods and ratios of the slurries of the active material layers provided in the following examples and comparative examples all adopt the methods provided in the specific implementation manner, and only change the types of the active materials.

[0052] Examples 1-7

[0053] This example provides a positive electrode sheet, which includes a current collector and an electrode layer on the surface of the current collector (obtained by double-sided coating on both sides of the current collector, and the electrode layer structures on both sides are the same). The electrode layer is composed of a first active material layer and a second active material layer arranged in a stacked manner, and the first active material layer is in direct contact with the surface of the current collector; wherein, the ratio of the areal density of the active material in the second active material layer to the areal density of the active material in the first active material layer and the ratio of the thickness of the second active material layer to the thickness of the first active material layer are both shown in Table 1 (the electrode layer is a type I electrode layer).

[0054] The preparation methods of Examples 1-7 are as follows:

[0055] The slurries of the first active material layer and the second active material layer are prepared based on the specific implementation method;

[0056] Double-sided coat the slurry of the first active material layer on the surface of the aluminum foil (bright foil), dry, then double-sided coat the slurry of the second active material layer, dry, and roll to obtain the positive electrode sheet.

[0057] Comparative Examples 1-3

[0058] Comparative Examples 1-3 provide a positive electrode sheet, the positive electrode sheet includes a current collector and an electrode layer on the surface of the current collector (coated on both sides of the current collector, that is, obtained by double-sided coating, and the electrode layer structures on both sides are the same), and the types of active materials in the electrode layer are shown in Table 1.

[0059] The preparation methods of Comparative Examples 1-3 are as follows:

[0060] The preparation of the slurry of the active material layer is carried out based on the specific implementation method;

[0061] The slurry of the active material layer is double-sided coated on the surface of the aluminum foil (bright foil), dried, and roll-pressed to obtain the positive electrode sheet.

[0062] Table 1

[0063]

[0064] After rolling and cutting the positive electrode sheets provided in Examples 1-7 and Comparative Examples 1-2, they are used as positive electrode plates, and lithium sheets are used as negative electrodes to prepare button cells.

[0065] The batteries provided in Examples 1-3 and Comparative Examples 1-3 were subjected to electrochemical performance tests, and the test conditions were as follows: gram capacity test: the battery was charged at a constant current and constant voltage to 4.3V with 0.1C, the cut-off current was 0.02C, and then discharged to 2.5V with 0.1C to calculate the discharge gram capacity of the battery; cycle test: the battery was charged at a constant current and constant voltage to 4.3V with 1C, the cut-off current was 0.02C, and then discharged at a constant current of 1C to 2.5V for 50 cycles, and then the capacity retention rate of the battery was compared and analyzed; the test results are shown in Table 2.

[0066] Table 2

[0067] Capacity (mAh / g) Long cycle performance (%) Example 1 167.3 100.72 Example 2 205.4 100.56 Example 3 162.3 100.52 Example 4 205.6 100.21 Example 5 160.9 99.24 Example 6 161.5 99.20 Example 7 159.8 98.54 Comparative Example 1 205.1 98.42 Comparative Example 2 152.4 98.54 Comparative Example 3 158.7 97.86

[0068] Combining Table 1 and Table 2, it can be seen that:

[0069] From the data results of Example 1 and Examples 4 and 5, it can be known that in the I-type electrode layer, if the ratio of the surface density is too low, it will lead to poor heat diffusion isolation and coating effect of the manganese-based positive electrode material on the layered oxide, and it cannot effectively inhibit the corrosion of the free acid in the electrolyte on the first-layer layered oxide positive electrode material, resulting in side reactions such as interfacial oxidation, which will further affect the overall cycle performance and safety performance; while if the ratio of the surface density is too large, it will lead to poor ionic conductivity of the electrode, which is not conducive to the stability of the electronic conductivity in the later stage of the cycle, and further affects the overall cycle stability.

[0070] From the data results of Example 1 and Examples 6 and 7, it can be seen that in the Class I electrode layer, if the ratio of the thickness is too low, it will lead to poor coating and isolation effects of the second manganese-based cathode material on the first-layer layered oxide cathode material, and cannot effectively inhibit the interfacial oxidation of the layered oxide cathode material to the electrolyte, affecting the safety performance and cycle performance; while if the ratio of the thickness is too large, it will cause the first-layer layered oxide cathode material to not play its ionic conductivity characteristics well, affecting the specific capacity of the active material and the long-cycle performance.

[0071] From the data results of Example 1 and Comparative Example 1 and Comparative Example 2, it can be seen that when the positive electrode sheet is only a single-layered oxide cathode material or a single manganese-based cathode material, neither can effectively inhibit side reactions and reduce the electrode contact impedance, thereby affecting its cycle life and safety performance.

[0072] From the data results of Example 1 and Comparative Example 3, it can be seen that when the layered oxide cathode material and the manganese-based cathode material in the positive electrode sheet are mixed into one layer and there is only one layer in the electrode sheet, it will cause the advantages of the two to not be fully exerted, affecting the overall specific capacity performance, cycle performance and safety performance.

[0073] Examples 8 - 16

[0074] This example provides a positive electrode sheet, the positive electrode sheet includes a current collector and an electrode layer on the surface of the current collector (obtained by double-sided coating on both sides of the current collector, and the electrode layer structures on both sides are the same). The electrode layer is composed of a first active material layer and a second active material layer arranged in a stacked manner, and the first active material layer is in direct contact with the surface of the current collector; wherein, the active materials in the first active material layer, the active materials in the second active material layer, the ratio of the surface density of the active materials in the second active material layer to the surface density of the active materials in the first active material layer, the ratio of the thickness of the second active material layer to the thickness of the first active material layer, and the ratio of the mass ratio of the manganese-based cathode material to the layered oxide cathode material in the second active material layer are all shown in Table 3 (the electrode layer is a Class II electrode layer).

[0075] The preparation methods of Examples 8 - 16 are as follows:

[0076] The preparation of the slurry of the first active material layer and the slurry of the second active material layer is carried out based on the specific implementation method;

[0077] Double-sided coat the slurry of the first active material layer on the surface of the aluminum foil (bright foil), after drying, double-sided coat the slurry of the second active material layer, dry, and roll to obtain the positive electrode sheet.

[0078] Table 3

[0079]

[0080]

[0081] The button cells were prepared in the same preparation method as in Examples 1-7 for Examples 8-16.

[0082] The batteries provided in Examples 8-16 and Comparative Examples 1-3 were subjected to electrochemical performance tests, and the test conditions were the same as those in the Class I electrode layer. The results are shown in Table 4.

[0083] Table 4

[0084] Capacity (mAh / g) Long cycle performance (%) Example 8 169.6 100.75 Example 9 190.91 100.42 Example 10 165.3 100.57 Example 11 207.5 100.34 Example 12 163.4 100.23 Example 13 167.6 100.12 Example 14 167.2 100.05 Example 15 206.5 99.87 Example 16 165.0 99.42 Comparative Example 1 205.1 98.42 Comparative Example 2 152.4 98.54 Comparative Example 3 161.2 97.52

[0085] Combining Table 3 and Table 4, it can be seen that:

[0086] From the data results of Example 8 and Examples 11 and 12, it can be known that in the Class II electrode layer, when the ratio of the areal density is too low, the coating and isolation effect of the second active material layer on the first-layer layered oxide cathode material is poor, and the corrosion effect of free acid in the electrolyte on the first-layer layered oxide cathode material cannot be effectively inhibited, affecting the safety performance and cycle performance. And when the ratio of the areal density is too large, it is not conducive to the exertion of the good ionic conductivity characteristics of the first-layer layered oxide cathode material, affecting the specific capacity of the active material and the long cycle performance.

[0087] From the data results of Example 8 and Examples 13 and 14, it can be known that in the Class II electrode layer, when the ratio of the thickness is too low, the coating and isolation effect of the second active material layer on the first-layer layered oxide cathode material is poor, affecting the overall safety performance and cycle performance. And when the ratio of the thickness is too large, it is not conducive to the exertion of the good ionic conductivity characteristics of the first-layer layered oxide cathode material, affecting the specific capacity of the active material and the long cycle performance.

[0088] From the data results of Example 8 and Examples 15 and 16, it can be known that in the Class II electrode layer, when the ratio of the mass ratio of the manganese-based cathode material to the layered oxide cathode material in the second active material layer is too low, the isolation and coating effect of the manganese-based cathode material on the layered oxide cathode material will be affected, and the corrosion effect of free acid in the electrolyte on the first-layer layered oxide cathode material cannot be effectively inhibited, thereby affecting the safety performance and cycle performance. And when the ratio is too large, it will cause the layered oxide cathode material in the second active material layer to be sparsely distributed and the ion conduction network to be incomplete, affecting the ionic conductivity characteristics of the electrode, and thus affecting the overall specific capacity exertion and long cycle performance.

[0089] From the data results of Example 8 and Comparative Examples 1 and 2, it can be seen that when the positive electrode sheet contains only a single layered oxide positive electrode material or a single manganese-based positive electrode material, the effective suppression of side reactions and the reduction of electrode contact impedance cannot be achieved, thereby affecting its specific capacity, cycle life, and safety performance.

[0090] From the data results of Example 8 and Comparative Example 3, it can be seen that when the layered oxide positive electrode material and the manganese-based positive electrode material in the positive electrode sheet are mixed into one layer and there is only one layer in the electrode sheet, the advantages of the two cannot be fully exerted, affecting the overall specific capacity, cycle performance, and safety performance.

[0091] Examples 17 - 25

[0092] This example provides a positive electrode sheet, which includes a current collector and an electrode layer on the surface of the current collector (coated on both sides of the current collector, that is, obtained by double-sided coating, and the electrode layer structures on both sides are the same). The electrode layer is composed of a first active material layer and a second active material layer arranged in a stacked manner, and the first active material layer is in direct contact with the surface of the current collector; wherein, the active materials in the first active material layer, the active materials in the second active material layer, the ratio of the areal density of the active materials in the second active material layer to the areal density of the active materials in the first active material layer, the ratio of the thickness of the second active material layer to the thickness of the first active material layer, and the ratio of the mass ratio of the manganese-based positive electrode material to the layered oxide positive electrode material in the second active material layer are all shown in Table 5 (the electrode layer is a Class II electrode layer).

[0093] The preparation methods of Examples 8 - 16 are as follows:

[0094] The preparation of the slurry of the first active material layer and the slurry of the second active material layer is carried out based on the specific implementation method;

[0095] The slurry of the first active material layer is double-sided coated on the surface of the aluminum foil (bright foil), dried, then the slurry of the second active material layer is double-sided coated, dried, and roll-pressed to obtain the positive electrode sheet.

[0096] Table 5

[0097]

[0098]

[0099] The coin cells are prepared by using the same preparation method as that of Examples 1 - 7 for Examples 17 - 25.

[0100] The batteries provided by Examples 8 - 16 and Comparative Examples 1 - 3 are subjected to electrochemical performance tests, and the test conditions are the same as those in the Class I electrode layer. The results are shown in Table 6.

[0101] Table 6

[0102] Capacity (mAh / g) Long cycle performance (%) Example 17 168.3 100.67 Example 18 193.25 100.31 Example 19 162.4 100.47 Example 20 167.3 100.24 Example 21 163.6 100.35 Example 22 200.8 99.87 Example 23 161.8 99.65 Example 24 164.8 99.32 Example 25 165.8 98.71 Comparative Example 1 205.1 98.42 Comparative Example 2 152.4 98.54 Comparative Example 3 159.7 97.86

[0103] It can be seen from Table 5 and Table 6 that:

[0104] From the data results of Example 17 and Examples 20 and 21, it can be known that in the Class III electrode layer, if the ratio of the areal density is too low, it will lead to poor coating and isolation effects of the second active material layer on the first-layer layered oxide cathode material, resulting in the layered oxide material being more likely to transform into the Ni-O layer during charge and discharge, and the formed Ni-O rock salt phase has no electrochemical activity and ionic conductivity, which will enhance the kinetic barrier of lithium-ion diffusion, leading to an increase in battery impedance and a decrease in electrochemical performance, and further affecting the overall safety performance and cycle performance. If the ratio of the areal density is too large, it will be unfavorable for the good ionic conductivity characteristics of the first active material layer to play, reducing its kinetic characteristics, and further affecting the specific capacity of the active material and the long-cycle performance.

[0105] From the data results of Example 17 and Examples 22 and 23, it can be known that in the Class III electrode layer, if the ratio of the thickness is too low, it will lead to poor coating and isolation effects of the second active material layer on the first-layer layered oxide cathode material, and it cannot effectively inhibit the interfacial oxidation of the layered oxide cathode material to the electrolyte, affecting the safety performance and cycle performance. If the ratio of the thickness is too large, it will be unfavorable for the good ionic conductivity characteristics of the first-layer layered oxide cathode material to play, reducing its kinetic characteristics, and affecting the specific capacity of the active material and the long-cycle performance.

[0106] From the data results of Example 17 and Examples 24 and 25, it can be known that in the Class III electrode layer, when the mass ratio of the manganese-based cathode material in the second active material layer to that in the first active material layer remains the same, problems such as an increase in the electrode ohmic impedance and polarization will occur, and the phase transformation of the layered oxide in the second layer will be aggravated, enhancing the kinetic barrier of lithium-ion diffusion, leading to an increase in battery impedance and a decrease in electrochemical performance, and further affecting the overall safety performance and cycle performance. If it is less than that of the first layer, it will affect the good ionic conductivity characteristics of the layered oxide cathode material to play, reducing its kinetic characteristics, resulting in the inability to solve the problem of poor ionic conductivity of lithium iron phosphate manganese material only by using light foil, and the cost advantage will also not exist.

[0107] From the data results of Example 17 and Comparative Examples 1 and 2, it can be known that when the positive electrode plate only contains a single layered oxide cathode material or a single manganese-based cathode material, the effective suppression of side reactions and the reduction of electrode contact impedance cannot be achieved, and further affect the performance of its specific capacity, cycle life and safety performance.

[0108] From the data results of Example 17 and Comparative Example 3, it can be seen that when the layered oxide cathode material and the manganese-based cathode material in the cathode electrode are mixed into one layer and there is only one layer in the electrode, the advantages of the two cannot be fully exerted, which affects the overall specific capacity, cycle performance and safety performance.

[0109] In summary, the cathode electrode provided by the present invention does not require the use of carbon-coated aluminum foil, which saves costs and improves the electronic conductivity of the manganese-based cathode material. At the same time, it cooperates with the layered oxide cathode material to improve the safety performance of the battery, enhance the long-cycle performance of the battery, and fully exert the respective advantages of the manganese-based cathode material and the layered oxide cathode material, making the charge-discharge curve smoother and contributing to BMS calculation.

[0110] The applicant declares that the above description is only the specific implementation manner 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 thought of within the technical scope disclosed by the present invention fall within the protection scope and the disclosure scope of the present invention.

Claims

1. A positive electrode sheet, characterized in that, The positive electrode sheet includes a current collector and an electrode layer located on at least one side of the current collector. The electrode layer at least includes a first active material layer and a second active material layer which are stacked, and the first active material layer is in direct contact with the surface of the current collector; The electrode layer is a type-II electrode layer. Among them, the active material in the first active material layer includes a layered oxide cathode material, and the active material in the second active material layer includes a manganese-based cathode material and a layered oxide cathode material; Alternatively, the electrode layer is a type-III electrode layer. Among them, the active material in the first active material layer includes a layered oxide cathode material and a manganese-based cathode material, and the active material in the second active material layer includes a manganese-based cathode material and a layered oxide cathode material; In the type-II electrode layer, the ratio of the areal density of the active material in the second active material layer to the areal density of the active material in the first active material layer is 0.1 to 15; in the type-III electrode layer, the ratio of the areal density of the active material in the second active material layer to the areal density of the active material in the first active material layer is 0.1 to 25; In the type-II electrode layer, the ratio of the thickness of the second active material layer to the thickness of the first active material layer is 0.1 to 50; in the type-III electrode layer, the ratio of the thickness of the second active material layer to the thickness of the first active material layer is 0.1 to 40; In the type-II electrode layer, the mass ratio of the manganese-based cathode material to the layered oxide cathode material in the second active material layer is (0.05 to 20):1; in the type-III electrode layer, the mass ratio of the manganese-based cathode material to the layered oxide cathode material in the second active material layer is (0.05 to 20):1; In the type-III electrode layer, the mass fraction of the manganese-based cathode material in the active material in the second active material layer is greater than the mass fraction of the manganese-based cathode material in the active material in the first active material layer; The chemical general formula of the layered oxide cathode material is Li a Ni x Co y Mn 1-x-y O2, where 1 ≤ a ≤ 2, 0.2 ≤ x ≤ 1, 0 ≤ y < 1; The manganese-based cathode material is LiMnFePO4.

2. The positive electrode sheet according to claim 1, wherein The current collector includes a light foil and a coating located on the surface of the light foil.

3. The positive electrode sheet according to claim 2, wherein The coating includes a carbon-coated layer, and the material in the carbon-coated layer includes any one or a combination of at least two of conductive carbon black, graphene or carbon nanotubes.

4. The positive electrode sheet according to claim 1, wherein The D50 of the layered oxide cathode material is 1 to 30 μm.

5. A method for preparing a positive electrode sheet according to any one of claims 1-4, characterized in that, The preparation method includes the following steps: Prepare the slurry of the first active material layer and the slurry of the second active material layer respectively, and coat the slurry of the first active material layer and the slurry of the second active material layer on the surface of at least one side of the current collector to obtain the positive electrode sheet.

6. A lithium-ion battery, characterized in that, The lithium-ion battery includes the positive electrode sheet according to any one of claims 1-4.

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