Positive electrode sheet, lithium-ion battery and energy storage device

Abstract

Disclosed in the present disclosure are a positive electrode sheet, a lithium-ion battery and an energy storage device. The positive electrode sheet comprises a positive electrode current collector and a positive electrode active material layer, wherein the positive electrode active material layer is arranged on at least part of the surface of the positive electrode current collector and comprises a positive electrode active material, and the positive electrode active material is doped with a metal element M; and in the thickness direction away from the positive electrode current collector, the doping amount of the metal element M of the positive electrode active material in the positive electrode active material layer sequentially decreases, and the ratio of the doping amount of the metal element M of the positive electrode active material in the positive electrode active material layer at a location closest to the positive electrode current collector to the doping amount of the metal element M of the positive electrode active material in the positive electrode active material layer at a location farthest from the positive electrode current collector is less than or equal to 8.

Description

Positive electrode sheets, lithium-ion batteries and energy storage devices

[0001] Priority information

[0002] This application claims priority and benefits of patent application No. 2023107180220 filed with the State Intellectual Property Office of China on June 16, 2023, and the entire text of which is incorporated herein by reference. Technical Field

[0003] The present disclosure belongs to the technical field of lithium-ion batteries, and particularly relates to a positive electrode sheet, a lithium-ion battery, and an energy storage device. Background Art

[0004] Currently, fossil fuels remain the primary source of global energy consumption, accounting for 85%. Renewable energy accounts for only 10% of global energy consumption. However, Western countries must achieve carbon neutrality by 2050, requiring renewable energy consumption to reach over 30%. Wind and solar power, considered key means of achieving carbon neutrality in the future, have been slow to commercialize due to their instability and susceptibility to grid disruptions. Energy storage, therefore, will address these drawbacks on the power generation side. Energy storage is an essential option for ensuring the safe and stable operation of power systems when a high proportion of renewable energy is integrated.

[0005] Lithium-ion batteries, due to their high operating voltage and specific energy density, are increasingly being used in energy storage applications such as mobile devices, power tools, and electric vehicles. Of course, as user demands continue to increase, the lifespan and energy efficiency of lithium-ion batteries have become key factors in meeting these needs.

[0006] The existing lithium-ion batteries have low discharge energy efficiency during use, resulting in incomplete discharge and insufficient utilization of the lithium-ion batteries during application. In view of this, it is necessary to further improve the existing lithium-ion batteries to improve the discharge energy efficiency of lithium-ion batteries.

[0007] Public content

[0008] The present disclosure aims to address, at least to some extent, one of the technical problems in the related art. To this end, the present disclosure provides a positive electrode sheet, a lithium-ion battery, and an energy storage device. The present disclosure significantly reduces the resistance to lithium ion diffusion in the positive electrode sheet, while also reducing the polarization of lithium ions during the diffusion process in the positive electrode sheet, thereby improving the cycle performance and energy efficiency of the lithium-ion battery.

[0009] In one aspect of the present disclosure, a positive electrode sheet is provided. According to an embodiment of the present disclosure, the positive electrode sheet includes: a positive electrode current collector;

[0010] A positive electrode active material layer, the positive electrode active material layer being disposed on at least a portion of the surface of the positive electrode current collector, the positive electrode active material layer comprising a positive electrode active material doped with a metal element M, the metal element M being used to enhance the ability of the positive electrode active material to intercalate and deintercalate lithium ions;

[0011] Assume that the direction from the positive electrode current collector to the positive electrode active material layer is the thickness direction of the positive electrode sheet. Along the thickness direction away from the positive electrode current collector, the doping amount of the metal element M of the positive electrode active material in the positive electrode active material layer decreases successively, and along the thickness direction of the positive electrode sheet, the ratio of the doping amount of the metal element M of the positive electrode active material in the positive electrode active material layer closest to the positive electrode current collector to the doping amount of the metal element M of the positive electrode active material in the positive electrode active material layer farthest from the positive electrode current collector is less than or equal to 8.

[0012] According to the positive electrode sheet of the embodiment of the present disclosure, doping the metal element M in the positive electrode active material improves the lithium ion deintercalation ability of the positive electrode active material in the positive electrode active material layer as a whole, and the improvement in the lithium ion deintercalation ability of the positive electrode active material in the positive electrode active material layer closest to the positive electrode current collector (i.e., the lower layer) is the largest. Along the thickness direction away from the positive electrode current collector, the improvement in the lithium ion deintercalation ability of the positive electrode active material in the positive electrode active material layer by the doped metal element M decreases successively, thereby significantly reducing the difference in lithium ion deintercalation ability of the active material in the upper and lower layers of the positive electrode sheet, improving the utilization of the active material in the active material layer (i.e., the lower layer) closer to the positive electrode current collector, and reducing the failure risk of the active material in the active material layer (i.e., the upper layer) farther from the positive electrode current collector. The discharge energy is improved, and its cycle performance and energy efficiency are improved by about 5% to 20%, thereby improving the cycle performance and energy efficiency of the lithium ion battery as a whole.

[0013] In a second aspect of the present disclosure, a lithium-ion battery is provided. According to an embodiment of the present disclosure, the lithium-ion battery comprises the positive electrode sheet of the above embodiment, thereby improving the cycle performance and energy efficiency of the lithium-ion battery.

[0014] In a third aspect of the present disclosure, an energy storage device is provided. According to an embodiment of the present disclosure, the energy storage device comprises a lithium-ion battery as described above. Thus, the energy storage device has all the advantages of the lithium-ion battery, which will not be elaborated here.

[0015] Additional aspects and advantages of the present disclosure will be set forth in part in the following description and, in part, will be obvious from the following description, or may be learned through practice of the present disclosure. BRIEF DESCRIPTION OF THE DRAWINGS

[0016] The above and / or additional aspects and advantages of the present disclosure will become apparent and readily understood from the description of the embodiments with reference to the following drawings, in which:

[0017] FIG1 is a schematic structural diagram of a positive electrode sheet according to an embodiment of the present disclosure.

[0018] In the accompanying drawings, 100 is the positive electrode current collector, 200 is the first positive electrode active material layer, and 300 is the second positive electrode active material layer.

[0019] Detailed Description of the Invention

[0020] The embodiments of the present disclosure are described in detail below, examples of which are shown in the accompanying drawings, wherein the same or similar reference numerals throughout represent the same or similar elements or elements having the same or similar functions. The embodiments described below with reference to the accompanying drawings are exemplary and are intended to be used to explain the present disclosure, and should not be understood as limiting the present disclosure.

[0021] In one aspect of the present disclosure, a positive electrode sheet is provided. According to an embodiment of the present disclosure, referring to Figures 1-3, a positive electrode current collector; a positive electrode active material layer, the positive electrode active material layer being disposed on at least a portion of the surface of the positive electrode current collector, the positive electrode active material layer comprising a positive electrode active material, the positive electrode active material being doped with a metal element M, the metal element M being used to enhance the positive electrode active material's ability to deintercalate and deintercalate lithium ions; assuming that the direction from the positive electrode current collector to the positive electrode active material layer is the thickness direction of the positive electrode sheet, along the thickness direction away from the positive electrode current collector, the doping amount of the metal element M of the positive electrode active material in the positive electrode active material layer decreases successively, and along the thickness direction of the positive electrode sheet, the ratio of the doping amount of the metal element M of the positive electrode active material in the positive electrode active material layer closest to the positive electrode current collector to the doping amount of the metal element M of the positive electrode active material in the positive electrode active material layer farthest from the positive electrode current collector is less than or equal to 8. Therefore, doping the metal element M in the positive electrode active material improves the lithium ion deintercalation ability of the positive electrode active material in the positive electrode active material layer as a whole, and the improvement in the lithium ion deintercalation ability of the positive electrode active material in the positive electrode active material layer (i.e., the lower layer) closest to the positive electrode current collector is the greatest. Along the thickness direction away from the positive electrode current collector, the improvement in the lithium ion deintercalation ability of the positive electrode active material in the positive electrode active material layer by the doped metal element M decreases successively, thereby significantly reducing the difference in the lithium ion deintercalation ability of the active material in the upper and lower layers of the positive electrode sheet, improving the utilization of the active material in the active material layer (i.e., the lower layer) closer to the positive electrode current collector, and reducing the failure risk of the active material in the active material layer (i.e., the upper layer) farther away from the positive electrode current collector, the discharge energy is improved, and its cycle performance and energy efficiency are improved by about 5% to 20%, thereby improving the cycle performance and energy efficiency of the lithium ion battery as a whole.

[0022] The following is a detailed description of the principle by which the positive electrode sheet proposed in this disclosure can achieve the above beneficial effects:

[0023] The inventors discovered that, particularly in thickly coated positive electrode sheets, due to the thick coating of the active material layer, during the diffusion and transmission of lithium ions, lithium ions in the active material layer farther from the positive current collector are easily deintercalated (i.e., lithium ions in the active material in the upper layer of the electrode sheet are easily deintercalated), while lithium ions in the active material layer closer to the positive current collector are not easily deintercalated (i.e., lithium ions in the active material in the lower layer of the electrode sheet are not easily deintercalated), which easily leads to polarization during the charge and discharge process. During long-term cycling, the active material in the active material layer farther from the positive current collector releases more lithium ions, causing the active material in the active material layer farther from the positive current collector to easily fail, while the capacity of the active material in the active material layer closer to the positive current collector cannot be fully utilized, resulting in reduced energy efficiency and cycle life of the lithium-ion battery.

[0024] To address this issue, the present disclosure enhances the lithium ion deintercalation capability of the positive electrode active material by doping the positive electrode active material in the positive electrode active material layer with a metal element M. The doping amount of the metal element M in the positive electrode active material in the positive electrode active material layer decreases gradually as the thickness moves away from the positive electrode current collector. It should be noted that, within a reasonable doping range, the higher the doping amount of the metal element M in the positive electrode active material, the greater the improvement in the lithium ion deintercalation capability of the positive electrode active material. Therefore, doping the metal element M in the positive electrode active material improves the deintercalation ability of the positive electrode active material in the positive electrode active material layer for lithium ions as a whole, reduces the diffusion resistance of lithium ions in the positive electrode sheet, and improves the lithium ion deintercalation ability of the positive electrode active material in the positive electrode active material layer (i.e., the lower layer) closest to the positive electrode current collector. The improvement is the largest, and along the direction away from the positive electrode current collector, the improvement of the lithium ion deintercalation ability of the positive electrode active material in the positive electrode active material layer by the doped metal element M decreases successively, thereby significantly reducing the difference in the deintercalation ability of the active material in the upper and lower layers of the positive electrode sheet for lithium ions, reducing the polarization of lithium ions during the diffusion process of the positive electrode sheet, improving the utilization of the active material in the active material layer (i.e., the lower layer) closer to the positive electrode current collector, and reducing the failure risk of the active material in the active material layer (i.e., the upper layer) farther away from the positive electrode current collector, the discharge energy is improved, and its cycle performance and energy efficiency are improved by about 5% to 20%, thereby improving the cycle performance and energy efficiency of the lithium ion battery as a whole.

[0025] The inventors discovered that after doping the positive electrode active material (such as lithium iron phosphate) with a metal element M, the Li-O bond energy in the positive electrode active material (such as lithium iron phosphate) is weakened, vacancies appear within the structure, the diffusion channel becomes wider, and the lithium ion diffusion resistance is reduced, thereby making it easier for lithium ions to be deintercalated. Taking lithium iron phosphate as an example, after doping lithium iron phosphate with a metal element M, the metal element M replaces part of the iron element in the lithium iron phosphate lattice, weakening the Li-O bond energy in the lithium iron phosphate lattice, creating vacancies within the structure, widening the diffusion channel, and reducing the lithium ion diffusion resistance, thereby making it easier for lithium ions to be deintercalated.

[0026] The inventors found that by limiting the ratio of the doping amount of the metal element M of the positive electrode active material in the positive electrode active material layer (i.e., the lower layer) closest to the positive electrode current collector to the doping amount of the metal element M of the positive electrode active material in the positive electrode active material layer (upper layer) farthest from the positive electrode current collector to a range of less than or equal to 8, the difference in the ability of the active materials in the upper and lower layers of the positive electrode sheet to deintercalate and deintercalate lithium ions can be effectively reduced. If the ratio is greater than 8, it is very likely that the doping amount of the metal element M of the lower layer positive electrode active material is too high, resulting in the lower layer positive electrode active material having a greater deintercalation and deintercalation ability of the upper layer positive electrode active material, thereby increasing the risk of failure of the lower layer positive electrode active material, which is not conducive to improving its cycle performance and energy efficiency.

[0027] According to some specific embodiments of the present disclosure, referring to FIG1 , the positive electrode active material layer includes: a first positive electrode active material layer 200, the first positive electrode active material layer 200 being disposed on at least a portion of the surface of a positive electrode current collector 100; and a second positive electrode active material layer 300, the second positive electrode active material layer 300 being disposed on at least a portion of the surface of the first positive electrode active material layer 200 away from the positive electrode current collector 100. Furthermore, the ratio of the doping amount of the metal element M of the positive electrode active material in the first positive electrode active material layer 200 to the doping amount of the metal element M of the positive electrode active material in the second positive electrode active material layer 300 is less than or equal to 8. This reduces the difference in lithium ion intercalation and deintercalation capabilities of the active materials in the first and second positive electrode active material layers, improves the utilization of the active materials in the first positive electrode active material layer, and reduces the risk of failure of the active materials in the second positive electrode active material layer. Cycling performance and energy efficiency are improved by approximately 5% to 20%, thereby improving the cycling performance and energy efficiency of the lithium-ion battery overall. It should be noted that the number of layers of each sub-positive electrode active material layer is not particularly limited, and can be two, three, four, five, etc. Preferably, the content of positive electrode active material in each sub-positive electrode active material layer is the same.

[0028] As some specific examples, in Figure 1, the direction from the positive current collector to the positive active material layer is the direction from A to B, and the direction from A to B is the thickness direction of the positive electrode sheet. Along the thickness direction of the positive electrode sheet, the positive electrode active material layer closest to the positive current collector 100 is the first positive electrode active material layer 200, and the positive electrode active material layer farthest from the positive current collector 100 is the second positive electrode active material layer 300.

[0029] Furthermore, by limiting the ratio of the doping amount of the metal element M of the positive electrode active material in the first positive electrode active material layer to the doping amount of the metal element M of the positive electrode active material in the second positive electrode active material layer to a range of less than or equal to 8, the difference in the lithium ion intercalation and deintercalation ability of the active materials in the first and second positive electrode active material layers can be effectively reduced. If the ratio is greater than 8, the lithium ion intercalation and deintercalation ability of the positive electrode active material in the first positive electrode active material layer is likely to be greater than that of the positive electrode active material in the second positive electrode active material layer due to the excessive doping amount of the metal element M in the positive electrode active material in the first positive electrode active material layer, thereby increasing the risk of failure of the positive electrode active material in the first positive electrode active material layer, which is detrimental to the improvement of its cycle performance and energy efficiency. Preferably, the ratio of the doping amount of the metal element M of the positive electrode active material in the first positive electrode active material layer to the doping amount of the metal element M of the positive electrode active material in the second positive electrode active material layer is 3-5.

[0030] Furthermore, the doping amount of the metal element M in the positive electrode active material in the first positive electrode active material layer is 2000ppm-7000ppm; and / or the doping amount of the metal element M in the positive electrode active material in the second positive electrode active material layer is 500ppm-2000ppm. By limiting the doping amount of the metal element M in the positive electrode active material in the first positive electrode active material layer and the doping amount of the metal element M in the positive electrode active material in the second positive electrode active material layer to the above range, the difference in the lithium ion intercalation and deintercalation ability of the active material in the first positive electrode active material layer and the second positive electrode active material layer is further reduced, the utilization of the active material in the first positive electrode active material layer is improved, and the failure risk of the active material in the second positive electrode active material layer is reduced. The cycle performance and energy efficiency are improved by approximately 5% to 20%, thereby improving the cycle performance and energy efficiency of the lithium-ion battery as a whole.

[0031] It should be noted that the doping amount of the metal element M refers to the content of the metal element M based on the total mass of the positive electrode active material doped with the metal element M.

[0032] In the embodiments of the present disclosure, the specific type of the above-mentioned positive electrode active material is not particularly limited. Those skilled in the art can flexibly select it according to actual needs. As some specific examples, the positive electrode active material includes at least one of lithium iron phosphate, lithium nickelate, lithium manganate, lithium cobaltate, lithium manganese phosphate, lithium vanadium phosphate, nickel cobalt aluminum lithium oxide, and nickel cobalt manganese lithium oxide. Lithium iron phosphate is preferred. In addition, the type of positive electrode active material in the first positive electrode active material layer and the type of positive electrode active material in the second positive electrode active material layer may be the same or different. Preferably, the positive electrode active material in the first positive electrode active material layer and the positive electrode active material in the second positive electrode active material layer are both lithium iron phosphate.

[0033] In the embodiment of the present disclosure, the specific type of the above-mentioned doped metal element M is not particularly limited and can be selected according to the specific type of the positive electrode active material. For example, the metal element M doped in lithium iron phosphate should have an atomic radius that is not much different from that of the iron element, so that the metal element M can replace part of the iron element in the lithium iron phosphate lattice. When the positive electrode active material is lithium iron phosphate, the metal element M can be selected from at least one of Ti, V, Mg and Al, preferably Ti. It should be noted that the type of the metal element M doped in the positive electrode active material in the first positive electrode active material layer and the type of the metal element M doped in the positive electrode active material in the second positive electrode active material layer can be the same or different. Preferably, the positive electrode active material in the first positive electrode active material layer and the positive electrode active material in the second positive electrode active material layer are both doped with the metal element Ti.

[0034] As before, in a thick-coated positive electrode sheet, for example, the area density of the positive electrode active material layer is 300 mg / 1540.12 mm 2 -500mg / 1540.12mm 2 When the active material layer is applied thickly, the lithium ions in the active material layer farther from the positive electrode current collector are easily deintercalated (i.e., the lithium ions in the active material of the upper layer of the electrode sheet are easily deintercalated), while the lithium ions in the active material layer closer to the positive electrode current collector are not easily deintercalated (i.e., the lithium ions in the active material of the lower layer of the electrode sheet are not easily deintercalated), thereby easily causing polarization during the charge and discharge process. The present disclosure provides a multi-layer active material layer, wherein the doping amount of the metal element M in the active material layer closer to the positive electrode current collector is higher, while the doping amount of the metal element M in the active material layer farther from the positive electrode current collector is lower, thereby reducing the difference in the deintercalation ability of the active materials of the upper and lower layers of the positive electrode sheet to lithium ions.

[0035] Furthermore, the surface density of the first positive electrode active material layer is 150 mg / 1540.12 mm 2 -250mg / 1540.12mm 2and / or, the surface density of the second positive electrode active material layer is 150mg / 1540.12mm 2 -250mg / 1540.12mm 2 As a result, the difference in lithium ion intercalation and deintercalation capabilities between the active materials in the first and second positive electrode active material layers is further reduced, the utilization of the active materials in the first positive electrode active material layer is improved, and the failure risk of the active materials in the second positive electrode active material layer is reduced. The cycle performance and energy efficiency are improved by about 5% to 20%, thereby improving the cycle performance and energy efficiency of the lithium-ion battery as a whole.

[0036] In an embodiment of the present disclosure, the positive electrode active material layer includes, in addition to the positive electrode active material, a conductive agent, a binder, and a dispersant, and the mass ratio of the positive electrode active material, the conductive agent, the binder, and the dispersant is (94.5-98.5): (0.5-2.0): (1-3.0): (0-1). The conductive agent includes, but is not limited to, at least one of acetylene black, Super P, carbon nanotubes, graphene, and conductive carbon fibers. The binder includes, but is not limited to, at least one of PVDF, PTFE, and NBR.

[0037] In the embodiment of the present disclosure, the ICP test method may be used to test the doping amount of the metal element M in the positive electrode active material in the positive electrode active material layer.

[0038] In an embodiment of the present disclosure, the preparation method of the above-mentioned positive electrode sheet is as follows:

[0039] (1) According to conventional methods for preparing positive electrode sheets in the art, positive electrode active materials, conductive agents, binders, and dispersants with different metal element doping amounts are weighed according to predetermined mass ratios, and solvents are added to each of them and stirred and mixed in a certain manner to form positive electrode slurries. The solid content of the positive electrode slurry is not particularly limited, for example, the solid content is 40% to 80%. The specific type of solvent is also not particularly limited, for example, at least one of NMP (N-methylpyrrolidone), DMF (dimethylformamide), and DMSO can be selected.

[0040] For example, a positive electrode active material with a metal element doping amount of 2000ppm to 3000ppm, a conductive agent, a binder, a dispersant and a solvent are mixed to form a first positive electrode slurry, and a positive electrode active material with a metal element doping amount of 500ppm to 1500ppm, a conductive agent, a binder, a dispersant and a solvent are mixed to form a second positive electrode slurry.

[0041] It should be noted that the preparation method of the positive electrode active material with different metal element doping amounts belongs to the conventional technical means in this field and will not be described in detail here.

[0042] (2) Each positive electrode slurry prepared in step (1) is applied to the positive electrode current collector in descending order of metal element doping, and dried to form a positive electrode sheet. Finally, the positive electrode sheet is cut into a specific shape according to the different battery shells for use.

[0043] For example, a first positive electrode slurry with a metal element doping amount of 2000ppm to 3000ppm is first applied to the positive electrode current collector, and a first positive electrode active material layer is formed after drying. Then, a second positive electrode slurry with a metal element doping amount of 500ppm to 1500ppm is applied to the surface of the first positive electrode active material layer away from the positive electrode current collector, and a second positive electrode active material layer is formed after drying.

[0044] Preferably, the coating area density of the first positive electrode active material layer is about 150 mg / 1540.12 mm 2 ~250mg / 1540.12mm 2 , and / or, the coating area density of the second positive electrode active material layer is about 150 mg / 1540.12 mm 2 ~250mg / 1540.12mm 2 .

[0045] In a second aspect of the present disclosure, a lithium-ion battery is provided. According to an embodiment of the present disclosure, the lithium-ion battery comprises the positive electrode sheet of the above embodiment, thereby improving the cycle performance and energy efficiency of the lithium-ion battery.

[0046] Specifically, the lithium-ion battery includes the positive electrode sheet, the negative electrode sheet, and a separator disposed between the positive electrode sheet and the negative electrode sheet. The separator includes at least one of a PP separator, a PE separator, a single-sided ceramic separator, a double-sided ceramic separator, a non-woven fabric separator, and a glass fiber separator.

[0047] The negative electrode sheet includes a negative electrode current collector and a negative electrode active material layer formed on the negative electrode current collector. The negative electrode active material layer includes a negative electrode active material (e.g., graphite), a negative electrode dispersant, a conductive agent, a negative electrode binder, and a plasticizer. Furthermore, the mass ratio of the negative electrode active material, the negative electrode dispersant, the conductive agent, the negative electrode binder, and the plasticizer is (97-98.5):(1.2-1.6):(0.4-2.0):(1.3-2.3):(1-2).

[0048] The negative electrode sheet preparation method includes: uniformly mixing the negative electrode active material, negative electrode dispersant, conductive agent, negative electrode binder, and plasticizer according to a predetermined ratio; adding a solvent and stirring to form a negative electrode slurry; then applying the mixture to a current collector, drying, and finally cutting the negative electrode sheets into a specific shape based on the battery casing for use. Furthermore, the solids content of the negative electrode slurry is 40% to 60%.

[0049] Preparation of battery cells: Add separators to the positive and negative electrode sheets and wind them up. After winding, the positive and negative electrode tabs are welded. Then the bare battery cells are encapsulated in aluminum-plastic film. After encapsulation, the battery cells are vacuum-baked for 10 to 20 hours. Then, after liquid injection, standing, high-temperature and high-pressure formation, degassing and packaging, and capacity separation, a lithium-ion battery with a double-layer coated positive electrode is obtained.

[0050] In a third aspect of the present disclosure, an energy storage device is provided. According to an embodiment of the present disclosure, the energy storage device comprises a lithium-ion battery as described above. Thus, the energy storage device has all the advantages of a lithium-ion battery, which will not be elaborated here.

[0051] Specifically, the energy storage device may include at least one of an electric energy storage device for the power generation side of the power system, an electric energy storage device (such as an electrochemical energy storage device) for the distribution side of the power system, and an electric energy storage device for the user side of the power system.

[0052] The following examples of the present disclosure are described in detail. It should be noted that the following examples are illustrative and are intended only to explain the present disclosure and are not to be construed as limiting the present disclosure. In addition, unless otherwise expressly stated, all reagents used in the following examples are commercially available or can be synthesized according to methods described herein or known methods. Reaction conditions not listed are also readily available to those skilled in the art.

[0053] Example 1

[0054] This embodiment provides a lithium-ion battery, the preparation method of which includes:

[0055] (1) Preparation of positive electrode slurry:

[0056] a. Preparation of the First Cathode Slurry: Weigh lithium iron phosphate, conductive agent C, binder PVDF, and dispersant PVP in the appropriate mass ratio. The Ti doping level in the lithium iron phosphate is 2500 ppm, and the mass ratio of lithium iron phosphate, conductive agent, binder, and dispersant is 96.5:1.25:2:0.5. Add NMP solvent and stir to form a first cathode slurry with a solids content of 60%.

[0057] b. Preparation of the Second Positive Electrode Slurry: Weigh lithium iron phosphate, a conductive agent, a binder, and a dispersant by mass. The Ti doping level in the lithium iron phosphate is 750 ppm, and the mass ratio of lithium iron phosphate, conductive agent, binder, and dispersant is 96.5:1.25:2:0.5. Add the solvent and stir to form a second positive electrode slurry with a solids content of 60%.

[0058] (2) Preparation of negative electrode slurry: Graphite, negative electrode dispersant sodium carboxymethyl cellulose, conductive agent SP, negative electrode binder SBR, and plasticizer propylene glycol were weighed according to the mass ratio, wherein the mass ratio of graphite, negative electrode dispersant, conductive agent, negative electrode binder, and plasticizer was 97.75:1.4:1.2:1.8:1.5. Aqueous solvent was added and stirred to form a negative electrode slurry having a solid content of 50%.

[0059] (3) Preparation of positive and negative electrodes:

[0060] Preparation of positive electrode sheet: Apply the first positive electrode slurry formed in step a to the first layer of positive electrode current collector with a coating density of about 200 mg / 1540.12 mm 2 , drying; the second positive electrode slurry formed in step b is applied to the second layer of the positive electrode current collector, with a coating surface density of about 200mg / 1540.12mm 2 , drying; and rolling and cutting the obtained positive electrode sheet to obtain the final positive electrode sheet.

[0061] Preparation of negative electrode sheet: The negative electrode slurry obtained in step (2) is evenly coated on the negative electrode current collector, dried, and the obtained electrode sheet is rolled and cut to obtain the negative electrode sheet.

[0062] (4) Cell Preparation: The positive and negative electrode sheets are wound with separators, and the positive and negative electrode tabs are welded after winding. The bare cell is then encapsulated in an aluminum-plastic film. After encapsulation, the cell is vacuum-baked for 15 hours. After liquid injection, standing, high-temperature and high-pressure formation, degassing, packaging, and volume separation, a lithium-ion battery with a double-layer coated positive electrode is obtained. The electrolyte of the lithium-ion battery includes lithium hexafluorophosphate and dimethyl carbonate, wherein the concentration of lithium hexafluorophosphate is 1 mol / L.

[0063] Example 2

[0064] This embodiment provides a lithium-ion battery, which differs from the first embodiment in that:

[0065] In step a, the doping amount of Ti element in lithium iron phosphate is 3500 ppm;

[0066] In step b, the doping amount of Ti element in lithium iron phosphate is 1500 ppm.

[0067] The rest of the contents are the same as those in Example 1.

[0068] Example 3

[0069] This embodiment provides a lithium-ion battery, which differs from the first embodiment in that:

[0070] In step a, the doping amount of Ti element in lithium iron phosphate is 4500 ppm;

[0071] In step b, the doping amount of Ti element in lithium iron phosphate is 1500 ppm.

[0072] The rest of the contents are the same as those in Example 1.

[0073] Example 4

[0074] This embodiment provides a lithium-ion battery, which differs from the first embodiment in that:

[0075] In step a, the doping amount of Ti element in lithium iron phosphate is 5500 ppm;

[0076] In step b, the doping amount of Ti element in lithium iron phosphate is 1500 ppm.

[0077] The rest of the contents are the same as those in Example 1.

[0078] Example 5

[0079] This embodiment provides a lithium-ion battery, which differs from the first embodiment in that:

[0080] In step a, the doping amount of Ti element in lithium iron phosphate is 6500 ppm;

[0081] In step b, the doping amount of Ti element in lithium iron phosphate is 1500 ppm.

[0082] The rest of the contents are the same as those in Example 1.

[0083] Comparative Example 1

[0084] This comparative example provides a lithium ion battery, which differs from Example 1 in that:

[0085] In step a, the doping amount of Ti element in lithium iron phosphate is 750 ppm;

[0086] In step b, the doping amount of Ti element in lithium iron phosphate is 750 ppm.

[0087] The rest of the contents are the same as those in Example 1.

[0088] Comparative Example 2

[0089] This comparative example provides a lithium ion battery, which differs from Example 1 in that:

[0090] In step a, the doping amount of Ti element in lithium iron phosphate is 1500 ppm;

[0091] In step b, the doping amount of Ti element in lithium iron phosphate is 1500 ppm.

[0092] The rest of the contents are the same as those in Example 1.

[0093] Cycling performance test: At 25°C, the batteries prepared in Examples 1-5 and Comparative Examples 1-2 were charged to 3.65V at a 1P rate and discharged to 2.5V at a 1P rate. Full charge and discharge cycle tests were performed until the capacity of the lithium-ion battery was less than 80% of the initial capacity. The number of cycles and energy efficiency were recorded. The test results are shown in Table 1.

[0094] Table 1

[0095] It can be seen from Table 1 that the energy efficiency of the lithium ion batteries of Examples 1-5 is significantly higher than that of Comparative Examples 1 and 2.

[0096] In the description of this specification, the description with reference to the terms "one embodiment", "some embodiments", "example", "specific example", or "some examples" means that the specific features, structures, materials or characteristics described in conjunction with the embodiment or example are included in at least one embodiment or example of the present disclosure. In this specification, the schematic representations of the above terms do not necessarily refer to the same embodiment or example. Moreover, the specific features, structures, materials or characteristics described may be combined in any one or more embodiments or examples in a suitable manner. In addition, those skilled in the art may combine and combine different embodiments or examples described in this specification and features of different embodiments or examples, unless they are mutually inconsistent.

[0097] Although the embodiments of the present disclosure have been shown and described above, it is understood that the above embodiments are illustrative and are not to be construed as limitations on the present disclosure. A person skilled in the art may change, modify, replace and vary the above embodiments within the scope of the present disclosure.

Claims

1. A positive electrode sheet, wherein: include: Positive electrode current collector; A positive electrode active material layer, the positive electrode active material layer is arranged on at least a portion of the surface of the positive electrode current collector, the positive electrode active material layer includes a positive electrode active material, the positive electrode active material is doped with a metal element M, and the metal element M is used to enhance the ability of the positive electrode active material to deintercalate lithium ions; Assuming that the direction from the positive current collector to the positive active material layer is the thickness direction of the positive electrode sheet, along the thickness direction away from the positive current collector, the doping amount of the metal element M of the positive active material in the positive active material layer decreases successively, and along the thickness direction of the positive electrode sheet, the ratio of the doping amount of the metal element M of the positive active material in the positive active material layer closest to the positive current collector to the doping amount of the metal element M of the positive active material in the positive active material layer farthest from the positive current collector is less than or equal to 8.

2. The positive electrode sheet according to claim 1, wherein: The positive electrode active material layer comprises: a first positive electrode active material layer, wherein the first positive electrode active material layer is disposed on at least a portion of the surface of the positive electrode current collector; a second positive electrode active material layer, the second positive electrode active material layer being disposed on at least a portion of a surface of the first positive electrode active material layer away from the positive electrode current collector; And the ratio of the doping amount of the metal element M of the positive electrode active material in the first positive electrode active material layer to the doping amount of the metal element M of the positive electrode active material in the second positive electrode active material layer is less than or equal to 8.

3. The positive electrode sheet according to claim 2, wherein: The ratio of the doping amount of the metal element M of the positive electrode active material in the first positive electrode active material layer to the doping amount of the metal element M of the positive electrode active material in the second positive electrode active material layer is 3-5.

4. The positive electrode sheet according to claim 2 or 3, wherein: The doping amount of the metal element M in the positive electrode active material of the first positive electrode active material layer is 2000ppm-7000ppm; And / or, the doping amount of the metal element M in the positive electrode active material of the second positive electrode active material layer is 500 ppm to 2000 ppm.

5. The positive electrode sheet according to any one of claims 1 to 4, wherein: The positive electrode active material includes at least one of lithium iron phosphate, lithium nickelate, lithium manganate, lithium cobaltate, lithium manganese phosphate, lithium vanadium phosphate, nickel cobalt aluminum lithium oxide and nickel cobalt manganese lithium oxide.

6. The positive electrode sheet according to any one of claims 1 to 5, wherein: The metal element M includes at least one of Ti, V, Mg and Al.

7. The positive electrode sheet according to any one of claims 1 to 6, wherein: The surface density of the positive electrode active material layer is 300 mg / 1540.12 mm 2 -500mg / 1540.12mm 2 .

8. The positive electrode sheet according to any one of claims 2 to 7, wherein: The surface density of the first positive electrode active material layer is 150 mg / 1540.12 mm 2 -250mg / 1540.12mm 2 ; And / or, the surface density of the second positive electrode active material layer is 150 mg / 1540.12 mm 2 -250mg / 1540.12mm 2 .

9. The positive electrode sheet according to any one of claims 1 to 8, wherein: The positive electrode active material layer also includes: a conductive agent, a binder and a dispersant; The mass ratio of the positive electrode active material, the conductive agent, the binder and the dispersant is (94.5-98.5):(0.5-2.0):(1-3.0):(0-1).

10. A lithium ion battery, wherein: A positive electrode sheet according to any one of claims 1 to 9.

11. An energy storage device, wherein: A lithium ion battery according to claim 10.

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