Positive electrode active material, method for manufacturing the same, positive electrode sheet, secondary battery, battery module, battery pack, and power tool

Abstract

The application provides a positive electrode active material with a core-shell structure, a preparation method of the positive electrode active material, a positive electrode sheet containing the positive electrode active material, a secondary battery, a battery module, a battery pack and a power utilization device. The positive electrode active material comprises an inner core and a shell covering the inner core. The inner core comprises Li 1+x Mn 1‑y A y P 1‑z R z O4. The shell comprises a first coating layer covering the inner core, a second coating layer covering the first coating layer and a third coating layer covering the second coating layer. The first coating layer comprises a crystalline pyrophosphate Li a MP2O7 and / or M b (P2O7) c P2O7 is electrically neutral. The second coating layer comprises a crystalline phosphate XPO4. The third coating layer is carbon.

Description

Technical Field

[0001] This application relates to the field of lithium battery technology, and in particular to positive electrode active materials and their preparation methods, positive electrode sheets containing the same, secondary batteries, battery modules, battery packs and electrical devices. Background Technology

[0002] With the rapid development of the new energy field, lithium-ion batteries, with their excellent electrochemical performance, lack of memory effect, and low environmental pollution, are widely used in various large-scale power devices, energy storage systems, and consumer products, especially in the field of new energy vehicles such as pure electric vehicles and hybrid electric vehicles. Among them, lithium manganese phosphate cathode active material has advantages such as high operating voltage, wide availability of raw materials, and low environmental pollution, and is considered to be a promising candidate to replace lithium iron phosphate as the cathode active material for power lithium-ion batteries.

[0003] However, in existing technologies, the cycle performance, high-temperature storage performance, and safety performance of secondary batteries using lithium manganese phosphate cathode active materials have not been comprehensively improved, which greatly limits the wider application of lithium manganese phosphate batteries. Therefore, the industry is eager to design a lithium manganese phosphate cathode active material that combines high specific capacity, good cycle performance, and safety performance. Summary of the Invention

[0004] This application is made in view of the above-mentioned issues, and its purpose is to provide a novel core-shell structured doped lithium manganese phosphate cathode active material, so that the secondary battery using the cathode active material has high specific capacity, good cycle performance and safety performance.

[0005] To achieve the above objectives, a first aspect of this application provides a positive electrode active material with a core-shell structure, comprising a core and a shell enclosing the core.

[0006] The chemical formula of the core is Li 1+x Mn 1-y A y P 1-z R z O4, where x is any value in the range of -0.100 to 0.100, y is any value in the range of 0.001 to 0.500, z is any value in the range of 0.001 to 0.100, A is one or more elements selected from Zn, Al, Na, K, Mg, Mo, W, Ti, V, Zr, Fe, Ni, Co, Ga, Sn, Sb, Nb and Ge, and may be one or more elements selected from Fe, Ti, V, Ni, Co and Mg, R is one or more elements selected from B, Si, N and S, and optionally, R is one element selected from B, Si, N and S;

[0007] The values ​​of x, y, and z satisfy the following condition: keeping the entire kernel electrically neutral;

[0008] The shell includes a first covering layer covering the core, a second covering layer covering the first covering layer, and a third covering layer covering the second covering layer, wherein...

[0009] The first coating layer comprises crystalline pyrophosphate Li a MP2O7 and / or M b (P2O7) c ,in,

[0010] 0≤a≤2, 1≤b≤4, 1≤c≤6, where the values ​​of a, b, and c satisfy the following condition: This makes crystalline pyrophosphate Li... a MP2O7 or M b (P2O7) c Maintain electrical neutrality

[0011] The crystalline pyrophosphate Li a MP2O7 and M b (P2O7) c Each of the M elements is independently selected from Fe, Ni, Mg, Co, Cu, Zn, Ti, Ag, Zr, Nb, or Al.

[0012] The second coating layer comprises crystalline phosphate XPO4, wherein X is one or more elements selected from Li, Fe, Ni, Mg, Co, Cu, Zn, Ti, Ag, Zr, Nb or Al;

[0013] The third coating layer is carbon.

[0014] In this document, "crystalline state" refers to a crystallinity of 50% or higher, i.e., 50%-100%. Crystallinity less than 50% is referred to as the glassy state. The crystalline pyrophosphate and crystalline phosphate described in this application have a crystallinity of 50% to 100%. Pyrophosphate and phosphate with a certain degree of crystallinity not only fully utilize the pyrophosphate coating's ability to inhibit manganese dissolution and the phosphate coating's excellent lithium-ion conduction capabilities, reducing interfacial side reactions, but also enable better lattice matching between the pyrophosphate and phosphate coatings, thus achieving a tighter bond between the coatings.

[0015] This application provides a novel core-shell structured lithium manganese phosphate cathode active material by doping element A at the manganese site and element R at the phosphorus site of lithium manganese phosphate to obtain a doped lithium manganese phosphate core, and then sequentially coating the surface of the core with three layers. When the cathode active material is applied to a secondary battery, it can significantly improve the high-temperature cycle performance, cycle stability and high-temperature storage performance of the secondary battery.

[0016] In any embodiment, the interplanar spacing of the crystalline pyrophosphate in the first coating layer ranges from 0.293 to 0.470 nm, and the included angle of the crystal orientation (111) ranges from 18.00° to 32.00°; the interplanar spacing of the crystalline phosphate in the second coating layer ranges from 0.244 to 0.425 nm, and the included angle of the crystal orientation (111) ranges from 20.00° to 37.00°.

[0017] The first and second coating layers in the positive electrode active material described in this application both use crystalline materials, and their interplanar spacing and angles are within the aforementioned ranges. This effectively avoids impurity phases in the coating layers, thereby improving the specific capacity, cycle performance, and rate performance of the material.

[0018] In any embodiment, in the core, the ratio of y to 1-y is 1:10 to 1:1, optionally 1:4 to 1:1. This further improves the cycle performance and rate performance of the secondary battery.

[0019] In any embodiment, in the core, the ratio of z to 1-z is 1:999 to 1:9, optionally 1:499 to 1:249. This further improves the cycle performance and rate performance of the secondary battery.

[0020] In any embodiment, the carbon in the third coating layer is a mixture of SP2 carbon and SP3 carbon. Optionally, the molar ratio of SP2 carbon to SP3 carbon is any value in the range of 0.1-10, and can be any value in the range of 2.0-3.0.

[0021] This application improves the overall performance of the secondary battery by limiting the molar ratio of SP2 carbon to SP3 carbon within the above-mentioned range.

[0022] In any embodiment, the coating amount of the first coating layer is greater than 0 and less than or equal to 6% by weight, optionally greater than 0 and less than or equal to 5.5% by weight, more preferably greater than 0 and less than or equal to 2% by weight, based on the weight of the core; and / or

[0023] The coating amount of the second coating layer is greater than 0 and less than or equal to 6% by weight, optionally greater than 0 and less than or equal to 5.5% by weight, more preferably 2-4% by weight, based on the weight of the core; and / or

[0024] The coating amount of the third coating layer is greater than 0 and less than or equal to 6% by weight, optionally greater than 0 and less than or equal to 5.5% by weight, and more preferably greater than 0 and less than or equal to 2% by weight, based on the weight of the core.

[0025] In the core-shell structured positive electrode active material described in this application, the coating amount of the three coating layers is preferably within the above-mentioned range, thereby enabling sufficient coating of the core and further improving the kinetic performance and safety performance of the secondary battery without sacrificing the specific capacity of the positive electrode active material.

[0026] In any embodiment, the thickness of the first coating layer is 1-10 nm; and / or

[0027] The thickness of the second coating layer is 2-15 nm; and / or

[0028] The thickness of the third coating layer is 2-25 nm.

[0029] In this application, when the thickness of the first coating layer is in the range of 1-10 nm, it can avoid the adverse effects on the dynamic properties of the material that may occur when it is too thick, and it can also avoid the problem that it cannot effectively hinder the migration of transition metal ions when it is too thin.

[0030] When the thickness of the second coating layer is in the range of 2-15nm, the surface structure of the second coating layer is stable and the side reactions with the electrolyte are small. Therefore, it can effectively reduce the interface side reactions and thus improve the high-temperature performance of the secondary battery.

[0031] When the thickness of the third coating layer is in the range of 2-20 nm, it can improve the electrical conductivity of the material and improve the compaction density performance of the battery electrode sheet prepared using the positive electrode active material.

[0032] In any embodiment, based on the weight of the core-shell structured positive electrode active material, the manganese content is in the range of 10 wt%-35 wt%, optionally in the range of 15 wt%-30 wt%, more preferably in the range of 17 wt%-20 wt%, the phosphorus content is in the range of 12 wt%-25 wt%, optionally in the range of 15 wt%-20 wt%, and the weight ratio of manganese to phosphorus is in the range of 0.90-1.25, optionally in the range of 0.95-1.20.

[0033] In the core-shell structured positive electrode active material described in this application, the content of manganese element is within the above-mentioned range, which can effectively avoid the problems of poor material structure stability and density reduction that may be caused by excessive manganese element content, thereby improving the cycle, storage and compaction density performance of the secondary battery; and can also avoid the problems of low voltage platform that may be caused by excessive manganese element content, thereby improving the energy density of the secondary battery.

[0034] In the core-shell structured positive electrode active material described in this application, the phosphorus content is within the above-mentioned range, which can effectively avoid the following situations: if the phosphorus content is too high, it may cause the covalent nature of PO to be too strong, affecting the conductivity of small polarons, thereby affecting the conductivity of the material; if the phosphorus content is too low, it may reduce the stability of the pyrophosphate lattice structure in the core, the first coating layer and / or the phosphate lattice structure in the second coating layer, thereby affecting the overall stability of the material.

[0035] In the core-shell structured positive electrode active material described in this application, the weight ratio of manganese to phosphorus is within the above-mentioned range, which can effectively avoid the following situations: if the weight ratio is too large, it may lead to increased dissolution of transition metals, affecting the stability of the material and the cycle and storage performance of the secondary battery; if the weight ratio is too small, it may cause the discharge voltage plateau of the material to drop, thereby reducing the energy density of the secondary battery.

[0036] In any embodiment, the lattice change rate of the core-shell structured positive electrode active material before and after complete lithium insertion / extraction is less than 4%, preferably less than 3.8%, and more preferably 2.0-3.8%.

[0037] The core-shell structured cathode active material described in this application can achieve a lattice change rate of less than 4% before and after lithium insertion / extraction. Therefore, using this cathode active material can improve the specific capacity and rate performance of secondary batteries.

[0038] In any embodiment, the Li / Mn antisite defect concentration of the core-shell structured positive electrode active material is 4% or less, preferably 2.2% or less, and more preferably 1.5-2.2%. By keeping the Li / Mn antisite defect concentration within the above range, Mn... 2+ Hinder Li + This improves the transport efficiency and simultaneously enhances the specific capacity and rate performance of the positive electrode active material.

[0039] In any embodiment, the compaction density of the core-shell structured positive electrode active material at 3T (tons) is 2.2 g / cm³. 3 The above is optional, 2.2 g / cm³. 3 Above and 2.8g / cm 3 Therefore, increasing the compaction density increases the weight of the active material per unit volume, which is more conducive to improving the volumetric energy density of the secondary battery.

[0040] In any embodiment, the surface oxygen valence state of the core-shell structured positive electrode active material is below -1.90, optionally between -1.90 and -1.98. Therefore, by limiting the surface oxygen valence state of the positive electrode active material to the above range as described above, the interfacial side reactions between the positive electrode material and the electrolyte can be reduced, thereby improving the cell's cycle life, high-temperature storage gas generation, and other performance characteristics.

[0041] A second aspect of this application provides a method for preparing a positive electrode active material, comprising the following steps:

[0042] The steps for providing the core material: the core has the chemical formula Li 1+x Mn 1-y A y P 1-z R z O4, where x is any value in the range of -0.100 to 0.100, y is any value in the range of 0.001 to 0.500, z is any value in the range of 0.001 to 0.100, A is one or more elements selected from Zn, Al, Na, K, Mg, Mo, W, Ti, V, Zr, Fe, Ni, Co, Ga, Sn, Sb, Nb and Ge, and may be one or more elements selected from Fe, Ti, V, Ni, Co and Mg, R is one or more elements selected from B, Si, N and S, and optionally, R is one element selected from B, Si, N and S;

[0043] Coating steps: Provide Li separately a MP2O7 and / or M b (P2O7) c And an XPO4 suspension, the core material is added to the above suspension and mixed, and then sintered to obtain a positive electrode active material, wherein 0≤a≤2, 1≤b≤4, 1≤c≤6, and the values ​​of a, b and c satisfy the following condition: making crystalline pyrophosphate Li a MP2O7 or M b (P2O7) c Maintaining electrical neutrality; each of M is independently selected from one or more elements selected from Fe, Ni, Mg, Co, Cu, Zn, Ti, Ag, Zr, Nb, or Al; and X is selected from one or more elements selected from Li, Fe, Ni, Mg, Co, Cu, Zn, Ti, Ag, Zr, Nb, or Al.

[0044] The positive electrode active material has a core-shell structure, comprising a core and a shell covering the core. The shell includes a first coating layer covering the core, a second coating layer covering the first coating layer, and a third coating layer covering the second coating layer. The first coating layer comprises crystalline pyrophosphate Li. aMP2O7 and / or M b (P2O7) c The second coating layer comprises crystalline phosphate XPO4, and the third coating layer is carbon.

[0045] In any implementation, the step of providing the core material includes the following steps:

[0046] Step (1): Mix and stir the manganese source, dopant of element A and acid in a container to obtain manganese salt particles doped with element A;

[0047] Step (2): The manganese salt particles doped with element A are mixed with a lithium source, a phosphorus source, and a dopant of element R in a solvent to obtain a slurry. After sintering under an inert gas atmosphere, a core doped with elements A and R is obtained, wherein the core doped with elements A and R is Li. 1+x Mn 1-y A y P 1-z R z O4, where x is any value in the range of -0.100 to 0.100, y is any value in the range of 0.001 to 0.500, z is any value in the range of 0.001 to 0.100, A is one or more elements selected from Zn, Al, Na, K, Mg, Mo, W, Ti, V, Zr, Fe, Ni, Co, Ga, Sn, Sb, Nb and Ge, and may be one or more elements selected from Fe, Ti, V, Ni, Co and Mg, R is one or more elements selected from B, Si, N and S, and optionally, R is one element selected from B, Si, N and S.

[0048] In any embodiment, the dopant of element A is one or more elements selected from the following: elemental form, carbonate, sulfate, chloride, nitrate, organic acid salt, oxide, and hydroxide.

[0049] In any embodiment, the dopant of element R is one or more of the inorganic acids, flavonoids, organic acids, sulfates, chlorides, nitrates, organic acid salts, oxides, and hydroxides of one or more elements selected from B, Si, N, and S.

[0050] In any embodiment, step (1) is performed at a temperature of 20-120°C, optionally 40-120°C; and / or

[0051] The stirring in step (1) is carried out at 400-700 rpm for 1-9 hours, optionally for 3-7 hours.

[0052] When the heating temperature and stirring time during the core particle preparation process are within the above range, the obtained core and the positive electrode active material made from it have fewer lattice defects, which is beneficial to suppress manganese dissolution, reduce interfacial side reactions between the positive electrode active material and the electrolyte, and thus improve the cycle performance and safety performance of the secondary battery.

[0053] In any embodiment, step (2) is performed at a temperature of 20-120°C, optionally 40-120°C, for 1-12 hours.

[0054] In any implementation, the coating step includes:

[0055] First coating step: The source of element M, phosphorus source, acid, and optionally lithium source are used to obtain a first coating layer suspension; the core obtained in the core step is thoroughly mixed with the first coating layer suspension obtained in the first coating step, dried, and then sintered to obtain the material coated by the first coating layer.

[0056] Second coating step: Dissolve the source of element X, phosphorus source and acid in solvent to obtain a second coating layer suspension; mix the material coated by the first coating layer obtained in the first coating step with the second coating layer suspension obtained in the second coating step, dry, and then sinter to obtain a material coated by two coating layers.

[0057] The third coating step: Dissolve the carbon source in a solvent to obtain a third coating layer solution; then add the material coated by the two coating layers obtained in the second coating step to the third coating layer solution, mix evenly, dry, and then sinter to obtain a material coated by the three coating layers, i.e., the positive electrode active material.

[0058] In any embodiment, the source of element M is one or more elements selected from Fe, Ni, Mg, Co, Cu, Zn, Ti, Ag, Zr, Nb or Al, including their elemental form, carbonate, sulfate, chloride, nitrate, organic acid salt, oxide, and hydroxide.

[0059] In any embodiment, the source of element X is one or more elements selected from the following: elemental form, carbonate, sulfate, chloride, nitrate, organic acid salt, oxide, hydroxide, etc., of one or more elements selected from Li, Fe, Ni, Mg, Co, Cu, Zn, Ti, Ag, Zr, Nb, or Al.

[0060] In this application, the amount of each source of elements A, R, M, and X added depends on the target doping amount, and the ratio of the amount of lithium source, manganese source, and phosphorus source used conforms to the stoichiometric ratio.

[0061] In any embodiment, in the first coating step, the pH of the solution containing the source of element M, the phosphorus source, and the acid, and optionally the lithium source, is controlled to be 3.5-6.5, then stirred and reacted for 1-5 hours, then the solution is heated to 50-120°C and held at that temperature for 2-10 hours, and / or sintering is carried out at 650-800°C for 2-6 hours.

[0062] By controlling the conditions of the first coating step within the above range, it is possible to guarantee or even improve the specific capacity, cycle and high-temperature storage performance, specific capacity and rate performance of the secondary battery prepared using the positive electrode active material.

[0063] In any embodiment, in the second coating step, after dissolving the source of element X, the phosphorus source and the acid in the solvent, the mixture is stirred and reacted for 1-10 hours. Then, the solution is heated to 60-150°C and held at that temperature for 2-10 hours, and / or, sintering is carried out at 500-700°C for 6-10 hours.

[0064] In the steps of providing the core material and the first coating step and the second coating step, before sintering, that is, in the preparation of the core material in which the chemical reaction occurs (steps (1)-(2)) and in the preparation of the first coating layer suspension and the second coating layer suspension, by selecting the reaction temperature and reaction time as described above, the following situations can be avoided: when the reaction temperature is too low, the reaction cannot occur or the reaction rate is slow; when the temperature is too high, the product decomposes or forms an impurity phase; when the reaction time is too long, the product particle size is large, which may increase the time and difficulty of subsequent processes; when the reaction time is too short, the reaction is incomplete and less product is obtained.

[0065] In any embodiment, the sintering in the third coating step is carried out at 700-800°C for 6-10 hours.

[0066] By controlling the conditions of the third coating step within the above range, it is possible to ensure or even improve the specific capacity and compaction density of the secondary battery prepared using the positive electrode active material.

[0067] The method for preparing the positive electrode active material described in this application uses widely available and inexpensive raw materials, and has a simple process, which is conducive to industrialization.

[0068] A third aspect of this application provides a positive electrode sheet comprising a positive current collector and a positive electrode film layer disposed on at least one surface of the positive current collector. The positive electrode film layer comprises a positive electrode active material with a core-shell structure as described in the first aspect of this application or a positive electrode active material prepared by the method described in the second aspect of this application. The content of the positive electrode active material in the positive electrode film layer is 90-99.5% by weight, preferably 95-99.5% by weight, based on the total weight of the positive electrode film layer.

[0069] The positive electrode sheet described in this application is used in batteries to improve the high-temperature cycle performance, rate performance, and safety performance of secondary batteries.

[0070] A fourth aspect of this application provides a secondary battery comprising the positive electrode plate described in the third aspect of this application. The secondary battery provided by this application has high rate performance, good cycle performance, good safety performance, and large battery capacity.

[0071] The fifth aspect of this application provides a battery module that includes the secondary battery described in the fourth aspect of this application.

[0072] A sixth aspect of this application provides a battery pack that includes the battery module described in the fifth aspect of this application.

[0073] A seventh aspect of this application provides an electrical device comprising at least one of the secondary battery described in the fourth aspect of this application, the battery module described in the fifth aspect of this application, or the battery pack described in the sixth aspect of this application.

[0074] The positive electrode sheet, secondary battery, battery module, battery pack, and electrical device described in this application include the positive electrode active material described in this application, and therefore have high specific capacity, good cycle performance, and safety performance. Attached Figure Description

[0075] Figure 1 This is a schematic diagram of the positive electrode active material with a core-shell structure described in this application.

[0076] Figure 2 This is a schematic diagram of a secondary battery according to one embodiment of this application.

[0077] Figure 3 for Figure 2 An exploded view of a secondary battery according to one embodiment of this application is shown.

[0078] Figure 4 This is a schematic diagram of a battery module according to one embodiment of this application.

[0079] Figure 5 This is a schematic diagram of a battery pack according to one embodiment of this application.

[0080] Figure 6 for Figure 5 An exploded view of a battery pack according to one embodiment of this application is shown.

[0081] Figure 7 This is a schematic diagram of an electrical device that uses a secondary battery as a power source according to an embodiment of this application.

[0082] Explanation of reference numerals in the attached figures:

[0083] 1 Battery pack; 2 Upper housing; 3 Lower housing; 4 Battery module; 5 Secondary battery; 51 Casing; 52 Electrode assembly; 53 Top cover assembly Detailed Implementation

[0084] The following detailed description, with appropriate reference to the accompanying drawings, discloses embodiments of the positive electrode active material, its manufacturing method, positive electrode sheet, secondary battery, battery module, battery pack, and power-consuming device of this application. However, unnecessary detailed descriptions may be omitted. For example, detailed descriptions of well-known matters and repetitive descriptions of practically identical structures may be omitted. This is to avoid unnecessarily lengthy descriptions and to facilitate understanding by those skilled in the art. Furthermore, the accompanying drawings and the following description are provided for the purpose of enabling those skilled in the art to fully understand this application and are not intended to limit the subject matter of the claims.

[0085] The "range" disclosed in this application is defined by a lower limit and an upper limit. A given range is defined by selecting a lower limit and an upper limit, which define the boundaries of a specific range. Ranges defined in this way can include or exclude endpoints and can be arbitrarily combined; that is, any lower limit can be combined with any upper limit to form a range. For example, if ranges of 60-120 and 80-110 are listed for a specific parameter, it is expected that ranges of 60-110 and 80-120 are also expected. Furthermore, if minimum range values ​​1 and 2 are listed, and if maximum range values ​​3, 4, and 5 are listed, then the following ranges are all expected: 1-3, 1-4, 1-5, 2-3, 2-4, and 2-5. In this application, unless otherwise stated, the numerical range "ab" represents a shortened representation of any combination of real numbers between a and b, where a and b are real numbers. For example, the numerical range "0-5" indicates that all real numbers between "0-5" have been listed in this article; "0-5" is simply a shortened representation of these numerical combinations. Furthermore, when a parameter is stated as an integer ≥2, it is equivalent to disclosing that the parameter is, for example, an integer such as 2, 3, 4, 5, 6, 7, 8, 9, 10, 11, 12, etc.

[0086] Unless otherwise specified, all embodiments and optional embodiments of this application can be combined to form new technical solutions.

[0087] Unless otherwise specified, all technical features and optional technical features of this application may be combined to form new technical solutions.

[0088] Unless otherwise specified, all steps in this application may be performed sequentially or randomly, optionally sequentially. For example, the method includes steps (a) and (b), indicating that the method may include steps (a) and (b) performed sequentially, or it may include steps (b) and (a) performed sequentially. For example, the method may also include step (c), indicating that step (c) may be added to the method in any order. For example, the method may include steps (a), (b), and (c), or it may include steps (a), (c), and (b), or it may include steps (c), (a), and (b), etc.

[0089] Unless otherwise specified, the terms "comprising" and "including" as used in this application can be open-ended or closed-ended. For example, "comprising" and "including" can mean that other components not listed may also be included, or that only the listed components may be included.

[0090] The terms “above” and “below” used in this application include the number itself. For example, “above one” means one or more, and “above one of A and B” means “A”, “B” or “A and B”.

[0091] Unless otherwise specified, the term "or" is inclusive in this application. For example, the phrase "A or B" means "A, B, or both A and B". More specifically, the condition "A or B" is satisfied by any of the following conditions: A is true (or exists) and B is false (or does not exist); A is false (or does not exist) and B is true (or exists); or both A and B are true (or exist).

[0092] It should be noted that, in this document, the term "coating layer" refers to a material layer covering the lithium manganese phosphate core, which may completely or partially cover the lithium manganese phosphate core. The use of "coating layer" is for ease of description only and is not intended to limit the invention. Similarly, the term "coating layer thickness" refers to the thickness of the material layer covering the lithium manganese phosphate core in the radial direction of the lithium manganese phosphate core.

[0093] The inventors of this application discovered in practical operation that existing lithium manganese phosphate cathode active materials suffer from significant manganese leaching during deep charge-discharge processes. Although existing technologies have attempted to coat lithium manganese phosphate with lithium iron phosphate to reduce interfacial side reactions, this coating cannot prevent the leached manganese from migrating further into the electrolyte. After migrating to the negative electrode, the leached manganese is reduced to metallic manganese. This metallic manganese acts as a "catalyst," catalyzing the decomposition of the SEI (solid electrolyte interphase) film on the negative electrode surface, producing byproducts. Some of these byproducts are gaseous, causing the secondary battery to expand and affecting its safety performance. Additionally, another portion of these byproducts deposits on the negative electrode surface, obstructing the channels for lithium ions to enter and exit the negative electrode, increasing the secondary battery impedance and thus affecting its kinetic performance. Furthermore, to replenish the lost SEI film, the electrolyte and the active lithium inside the battery are continuously consumed, irreversibly impacting the capacity retention rate of the secondary battery.

[0094] After extensive research, the inventors discovered that by modifying lithium manganese phosphate and coating it with multiple layers, a novel positive electrode active material with a core-shell structure can be obtained. This positive electrode active material can achieve significantly reduced manganese dissolution and reduced lattice change rate. When used in secondary batteries, it can improve the battery's cycle performance, rate performance, safety performance, and increase battery capacity.

[0095] [Positive electrode active material]

[0096] The first aspect of this application provides a novel positive electrode active material with a core-shell structure, comprising a core and a shell enclosing the core, wherein the core has the chemical formula Li. 1+x Mn 1-y A y P 1-z R z O4, where x is any value in the range of -0.100 to 0.100, y is any value in the range of 0.001 to 0.500, z is any value in the range of 0.001 to 0.100, A is one or more elements selected from Zn, Al, Na, K, Mg, Mo, W, Ti, V, Zr, Fe, Ni, Co, Ga, Sn, Sb, Nb and Ge, and may be one or more elements selected from Fe, Ti, V, Ni, Co and Mg, R is one or more elements selected from B, Si, N and S, and optionally, R is one element selected from B, Si, N and S;

[0097] The values ​​of x, y, and z satisfy the following condition: keeping the entire kernel electrically neutral;

[0098] The shell includes a first covering layer covering the core, a second covering layer covering the first covering layer, and a third covering layer covering the second covering layer, wherein...

[0099] The first coating layer comprises crystalline pyrophosphate Li a MP2O7 and / or M b (P2O7) c ,in,

[0100] 0≤a≤2, 1≤b≤4, 1≤c≤6, where the values ​​of a, b, and c satisfy the following condition: This makes crystalline pyrophosphate Li... a MP2O7 or M b (P2O7) c Maintain electrical neutrality

[0101] The crystalline pyrophosphate Li a MP2O7 and M b (P2O7) c Each of the M elements is independently selected from Fe, Ni, Mg, Co, Cu, Zn, Ti, Ag, Zr, Nb, or Al.

[0102] The second coating layer comprises crystalline phosphate XPO4, wherein,

[0103] X is one or more elements selected from Li, Fe, Ni, Mg, Co, Cu, Zn, Ti, Ag, Zr, Nb, or Al;

[0104] The third coating layer is carbon.

[0105] Unless otherwise stated, in the above-described core chemical formula, when A comprises two or more elements, the limitation on the numerical range of y applies not only to the stoichiometric coefficient of each element as A, but also to the sum of the stoichiometric coefficients of all elements as A. For example, when A comprises two or more elements A1, A2...An, the stoichiometric coefficients y1, y2...yn of each of A1, A2...An must each fall within the numerical range of y defined in this application, and the sum of y1, y2...yn must also fall within this numerical range. Similarly, for the case where R comprises two or more elements, the limitation on the numerical range of the stoichiometric coefficient of R in this application has the same meaning.

[0106] In an optional implementation, when A is one, two, three, or four elements selected from Zn, Al, Na, K, Mg, Mo, W, Ti, V, Zr, Fe, Ni, Co, Ga, Sn, Sb, Nb, and Ge, A y For Q n1 D n2 E n3 Kn4 Where n1 + n2 + n3 + n4 = y, and n1, n2, n3, and n4 are all positive numbers and not all zero simultaneously. Q, D, E, and K are each independently selected from Zn, Al, Na, K, Mg, Mo, W, Ti, V, Zr, Fe, Ni, Co, Ga, Sn, Sb, Nb, and Ge. Optionally, at least one of Q, D, E, and K is Fe. Optionally, one of n1, n2, n3, and n4 is zero, and the others are not zero; more preferably, two of n1, n2, n3, and n4 are zero, and the others are not zero; even more preferably, three of n1, n2, n3, and n4 are zero, and the others are not zero. The kernel Li 1+x Mn 1-y A y P 1-z R z In O4, it is advantageous to dope one, two, three or four of the aforementioned A elements at the manganese sites, and optionally, one, two or three of the aforementioned A elements are doped. In addition, it is advantageous to dope one or two R elements at the phosphorus sites, which is beneficial to make the doped elements uniformly distributed.

[0107] The kernel Li 1+x Mn 1-y A y P 1-z R z In O4, the value of x is influenced by the valence states of A and R, as well as the values ​​of y and z, to ensure the overall system remains electrically neutral. If the value of x is too small, the lithium content of the entire core system will decrease, affecting the specific capacity of the material. The value of y limits the total amount of all dopants. If y is too small, the doping amount is too low, and the dopants will not play a role. If y exceeds 0.5, the Mn content in the system will be low, affecting the voltage plateau of the material. The R element is doped at the P site. Since the PO tetrahedron is relatively stable, and a large z value would affect the stability of the material, the z value is limited to 0.001-0.100.

[0108] The positive electrode active material of this application can improve the specific capacity, cycle performance, and safety performance of secondary batteries. Although the mechanism is not yet clear, it is speculated that the lithium manganese phosphate positive electrode active material of this application has a core-shell structure. By doping the manganese and phosphorus sites of the lithium manganese phosphate core with elements A and R respectively, it can not only effectively reduce manganese dissolution, thereby reducing the number of manganese ions migrating to the negative electrode and reducing the electrolyte consumed due to SEI film decomposition, thus improving the cycle performance and safety performance of the secondary battery, but also promote Mn-O bond adjustment, lower the lithium ion migration barrier, promote lithium ion migration, and improve the rate performance of the secondary battery. By coating the core with a first coating layer including crystalline pyrophosphate, it can... Further increasing the migration resistance of manganese reduces its dissolution, and also reduces the content of surface impurities lithium and the contact between the core and the electrolyte, thereby reducing interfacial side reactions and gas generation, and improving the high-temperature storage performance, cycle performance and safety performance of the secondary battery; by further coating with a crystalline phosphate coating layer with excellent lithium-ion conductivity, the interfacial side reactions on the surface of the positive electrode active material can be effectively reduced, thereby improving the high-temperature cycle and storage performance of the secondary battery; by further coating with a carbon layer as a third coating layer, the safety performance and kinetic performance of the secondary battery can be further improved.

[0109] Furthermore, in the core, the element A doped at the manganese site of lithium manganese phosphate helps to reduce the lattice change rate of lithium manganese phosphate during the lithium insertion / extraction process, improves the structural stability of the lithium manganese phosphate cathode material, greatly reduces the dissolution of manganese and reduces the oxygen activity on the particle surface; the element R doped at the phosphorus site also helps to change the ease of Mn-O bond length change, thereby improving electronic conductivity and reducing the lithium ion migration barrier, promoting lithium ion migration and improving the rate performance of the secondary battery.

[0110] Furthermore, maintaining the electrical neutrality of the entire core system ensures that defects and impurities in the cathode active material are minimized. If an excess of transition metal (such as manganese) exists in the cathode active material, due to the relatively stable structure of the material system itself, the excess transition metal is likely to precipitate as elemental form or form impurities within the crystal lattice. Maintaining electrical neutrality minimizes such impurities. In addition, ensuring the electrical neutrality of the system can, in some cases, generate lithium vacancies in the material, thereby improving the material's kinetic performance.

[0111] The cores prepared in this application have an average particle size range of 50-500 nm and a Dv50 of 200-300 nm. The primary particle size of the cores is consistently within the 50-500 nm range, with a Dv50 of 200-300 nm. If the average particle size of the cores is too large (exceeding 500 nm), the specific capacity of the secondary battery using this material will be affected; if the average particle size of the cores is too small, its specific surface area is too large, making it prone to aggregation and difficult to achieve uniform coating.

[0112] In this application, the median particle size Dv50 refers to the particle size corresponding to a cumulative volume distribution percentage of 50% for the material. In this application, the median particle size Dv50 of the material can be determined using laser diffraction particle size analysis. For example, it can be determined using a laser particle size analyzer (e.g., Malvern Master Size 3000) in accordance with standard GB / T19077-2016.

[0113] By controlling the process (e.g., thoroughly mixing and grinding materials from various sources), it is possible to ensure that each element is uniformly distributed in the crystal lattice and that no aggregation occurs. The main characteristic peak positions in the XRD pattern of lithium manganese phosphate doped with elements A and R are consistent with those of undoped LiMnPO4, indicating that no impurity phase was introduced during the doping process. Therefore, the improvement in the core performance mainly comes from elemental doping, rather than from impurity phases. After preparing the positive electrode active material described in this application, the inventors used focused ion beam (FIB) to cut the middle region of the prepared positive electrode active material particles. Testing using transmission electron microscopy (TEM) and energy dispersive X-ray spectroscopy (EDS) revealed that the elements were uniformly distributed and no aggregation occurred.

[0114] In this application, "crystalline state" refers to a crystallinity of 50% or higher, i.e., 50%-100%. Crystallinity less than 50% is referred to as the glassy state. The crystallinity of the crystalline pyrophosphate and crystalline phosphate described in this application is 50% to 100%. Pyrophosphate and phosphate with a certain degree of crystallinity not only fully utilize the pyrophosphate coating's ability to inhibit manganese dissolution and the phosphate coating's excellent lithium-ion conduction capabilities, reducing interfacial side reactions, but also enable better lattice matching between the pyrophosphate and phosphate coatings, thus achieving a tighter bond between the coatings.

[0115] In this application, the crystallinity of the first coating layer material crystalline pyrophosphate and the second coating layer material crystalline phosphate of the positive electrode active material can be tested by conventional technical means in the art, such as by density method, infrared spectroscopy, differential scanning calorimetry and nuclear magnetic resonance absorption method, or by, for example, X-ray diffraction.

[0116] A specific X-ray diffraction method for testing the crystallinity of the first coating layer crystalline pyrophosphate and the second coating layer crystalline phosphate of the positive electrode active material may include the following steps:

[0117] A certain amount of positive electrode active material powder is taken, and the total scattering intensity is measured by X-rays. It is the sum of the scattering intensities of all matter in space. It is only related to the intensity of the primary rays, the chemical structure of the positive electrode active material powder, and the total number of electrons participating in diffraction, i.e., the mass, and is independent of the order state of the sample. Then, crystalline scattering and non-crystalline scattering are separated from the diffraction pattern. The crystallinity is the ratio of the scattering of the crystalline part to the total scattering intensity.

[0118] It should be noted that, in this application, the crystallinity of pyrophosphate and phosphate in the coating layer can be adjusted, for example, by adjusting the process conditions of the sintering process, such as sintering temperature and sintering time.

[0119] In this application, since metal ions are difficult to migrate in pyrophosphate, pyrophosphate, as the first coating layer, can effectively isolate the doped metal ions from the electrolyte. Crystalline pyrophosphate has a stable structure; therefore, coating with crystalline pyrophosphate can effectively suppress the dissolution of transition metals and improve cycle performance.

[0120] The bonding between the first coating layer and the core is similar to that of a heterojunction, and the strength of this bonding is limited by the degree of lattice matching. When the lattice mismatch is below 5%, the lattice matching is good, and the two easily bond tightly. A tight bonding ensures that the coating layer will not detach during subsequent cycling, which is beneficial for ensuring the long-term stability of the material. The degree of bonding between the first coating layer and the core is mainly measured by calculating the mismatch between the lattice constants of the core and the coating. In this application, after doping the core with A and R elements, the matching degree between the core and the first coating layer is improved compared to undoped elements, and the core and the pyrophosphate coating layer can bond more tightly.

[0121] Crystalline phosphate was chosen as the second coating layer primarily because of its high lattice matching (mismatch of only 3%) with the first coating layer, crystalline pyrophosphate. Secondly, phosphate itself is more stable than pyrophosphate, and coating pyrophosphate with phosphate helps improve the material's stability. Crystalline phosphate has a very stable structure and excellent lithium-ion conductivity; therefore, using crystalline phosphate for coating can effectively reduce interfacial side reactions on the surface of the positive electrode active material, thereby improving the high-temperature cycling and storage performance of the secondary battery. The lattice matching between the second and first coating layers is similar to the bonding between the first coating layer and the core; when the lattice mismatch is below 5%, the lattice matching is good, and the two easily bond tightly.

[0122] The primary reason for using carbon as the third coating layer is its excellent electronic conductivity. Since electrochemical reactions occur in secondary batteries, requiring electrons, carbon, with its superior conductivity, can be used to coat the positive electrode active material to increase electron transport between particles and between different locations on the particles. Carbon coating effectively improves the conductivity and desolvation capability of the positive electrode active material.

[0123] Figure 1 This is a schematic diagram of an ideal three-layer coated cathode active material. As shown in the figure, the innermost circle represents the core, and from the inside out are the first coating layer, the second coating layer, and the third coating layer. This diagram represents the ideal state where each layer is fully coated. In practice, each coating layer can be fully coated or partially coated.

[0124] In some embodiments, the average particle size of the primary particles of the positive electrode active material ranges from 50 to 500 nm, and the volume median particle size (Dv50) is in the range of 200 to 300 nm. Since particle agglomeration can occur, the actual measured size of the agglomerated secondary particles may be 500 to 40,000 nm. The size of the positive electrode active material particles affects the processing of the material and the compaction density performance of the electrode sheet. By selecting an average particle size of the primary particles within the above range, the following situations can be avoided: if the average particle size of the primary particles of the positive electrode active material is too small, it may cause particle agglomeration, making dispersion difficult and requiring more binder, resulting in poor electrode sheet brittleness; if the average particle size of the primary particles of the positive electrode active material is too large, it may result in larger gaps between particles, reducing the compaction density.

[0125] The above scheme can effectively suppress the lattice change rate of lithium manganese phosphate and the dissolution of Mn during the lithium insertion / extraction process, thereby improving the high-temperature cycle stability and high-temperature storage performance of secondary batteries.

[0126] In some embodiments, the interplanar spacing of the crystalline pyrophosphate in the first coating layer ranges from 0.293 to 0.470 nm, and the included angle of the crystal orientation (111) ranges from 18.00° to 32.00°; the interplanar spacing of the crystalline phosphate in the second coating layer ranges from 0.244 to 0.425 nm, and the included angle of the crystal orientation (111) ranges from 20.00° to 37.00°.

[0127] The first and second coating layers of the positive electrode active material described in this application both use crystalline materials. The crystalline pyrophosphate and crystalline phosphate in the coating layers can be characterized using conventional techniques in the art, or for example, by transmission electron microscopy (TEM). Under TEM, the core and coating layers can be distinguished by measuring the interplanar spacing.

[0128] The specific testing methods for the interplanar spacing and angle of crystalline pyrophosphate and crystalline phosphate in the coating layer may include the following steps:

[0129] Take a certain amount of the coated positive electrode active material sample powder into a test tube, and inject a solvent such as alcohol into the test tube. Then stir and disperse it thoroughly. Then, use a clean disposable plastic pipette to take an appropriate amount of the above solution and drop it onto a 300-mesh copper grid. At this time, some powder will remain on the copper grid. Transfer the copper grid along with the sample to the TEM sample chamber for testing, obtain the original TEM test image, and save the original image.

[0130] Open the original image obtained from the TEM test in the diffractometer software and perform a Fourier transform to obtain the diffraction pattern. Measure the distance from the diffraction spot to the center position in the diffraction pattern to obtain the interplanar spacing. The included angle is calculated according to the Bragg equation.

[0131] The interplanar spacing range of crystalline pyrophosphates differs from that of crystalline phosphates, and can be directly determined by the value of the interplanar spacing.

[0132] Crystalline pyrophosphate and crystalline phosphate within the aforementioned interplanar spacing and angle range can more effectively suppress the lattice change rate of lithium manganese phosphate and Mn dissolution during lithium insertion / extraction, thereby improving the high-temperature cycle performance, cycle stability and high-temperature storage performance of secondary batteries.

[0133] In some embodiments, the ratio of y to 1-y in the core is 1:10 to 1:1, optionally 1:4 to 1:1. Here, y represents the sum of the stoichiometric coefficients of the Mn-doped element A. When the above conditions are met, the energy density and cycle performance of the secondary battery using the positive electrode active material can be further improved.

[0134] In some embodiments, the ratio of z to 1-z in the core is from 1:9 to 1:999, optionally from 1:499 to 1:249. Here, z represents the sum of the stoichiometric coefficients of the p-site doping elements R. When the above conditions are met, the energy density and cycle performance of the secondary battery using the positive electrode active material can be further improved.

[0135] In some embodiments, the carbon in the third coating layer is a mixture of SP2 carbon and SP3 carbon. Optionally, the molar ratio of SP2 carbon to SP3 carbon is any value in the range of 0.1-10, and can be any value in the range of 2.0-3.0.

[0136] In some embodiments, the molar ratio of SP2 carbon to SP3 carbon may be about 0.1, about 0.2, about 0.3, about 0.4, about 0.5, about 0.6, about 0.7, about 0.8, about 0.9, about 1, about 2, about 3, about 4, about 5, about 6, about 7, about 8, about 9 or about 10, or any range of the above values.

[0137] In this application, "about" for a certain value represents a range, specifically a range of ±10% of that value.

[0138] By selecting the morphology of carbon in the carbon coating layer, the overall electrical performance of the secondary battery can be improved. Specifically, by using a mixture of SP2 and SP3 carbon morphologies and limiting the ratio of SP2 to SP3 carbon within a certain range, the following situations can be avoided: if the carbon in the coating layer is all amorphous SP3, the conductivity is poor; if it is all graphitized SP2, although the conductivity is good, there are few lithium-ion pathways, which is not conducive to lithium insertion / extraction. In addition, limiting the molar ratio of SP2 to SP3 carbon within the above-mentioned range can achieve both good conductivity and ensure lithium-ion pathways, thus benefiting the realization of secondary battery functions and cycle performance.

[0139] The mixing ratio of SP2 and SP3 carbon forms in the third coating layer can be controlled by sintering conditions, such as sintering temperature and sintering time. For example, when using sucrose as a carbon source to prepare the third coating layer, after the sucrose is pyrolyzed at high temperature and deposited on the second coating layer under high temperature, a carbon coating layer with both SP2 and SP3 forms will be produced. The ratio of SP2 to SP3 carbon can be adjusted by selecting high-temperature pyrolysis and sintering conditions.

[0140] The structure and characteristics of the third coating carbon can be determined by Raman spectroscopy. The specific testing method is as follows: by dividing the energy spectrum of the Raman test, Id / Ig (where Id is the peak intensity of SP3 carbon and Ig is the peak intensity of SP2 carbon) is obtained, thereby confirming the molar ratio of the two.

[0141] In some embodiments, the coating amount of the first coating layer is greater than 0 and less than or equal to 6% by weight, optionally greater than 0 and less than or equal to 5.5% by weight, more preferably greater than 0 and less than or equal to 2% by weight, based on the weight of the core; and / or

[0142] The coating amount of the second coating layer is greater than 0 and less than or equal to 6% by weight, optionally greater than 0 and less than or equal to 5.5% by weight, more preferably 2-4% by weight, based on the weight of the core; and / or

[0143] The coating amount of the third coating layer is greater than 0 and less than or equal to 6% by weight, optionally greater than 0 and less than or equal to 5.5% by weight, and more preferably greater than 0 and less than or equal to 2% by weight, based on the weight of the core.

[0144] In this application, the coverage of each layer is not zero.

[0145] In the core-shell structured positive electrode active material described in this application, the coating amount of the three coating layers is preferably within the above-mentioned range, thereby enabling sufficient coating of the core and further improving the kinetic performance and safety performance of the secondary battery without sacrificing the specific capacity of the positive electrode active material.

[0146] For the first coating layer, by keeping the coating amount within the above range, the following situations can be avoided: too little coating amount means the coating layer is too thin, which may not effectively hinder the migration of transition metals; too much coating amount means the coating layer is too thick, which may affect Li + The migration of these molecules affects the rate performance of the material.

[0147] For the second coating layer, by keeping the coating amount within the above range, the following situations can be avoided: too much coating amount may affect the overall plateau voltage of the material; too little coating amount may not achieve sufficient coating effect.

[0148] For the third coating layer, the carbon coating mainly plays the role of enhancing electron transport between particles. However, since the structure also contains a large amount of amorphous carbon, the carbon density is low. Therefore, if the coating amount is too large, it will affect the compaction density of the electrode.

[0149] In some embodiments, the thickness of the first coating layer is 1-10 nm; and / or

[0150] The thickness of the second coating layer is 2-15 nm; and / or

[0151] The thickness of the third coating layer is 2-25 nm.

[0152] In some embodiments, the thickness of the first coating layer may be about 2 nm, about 3 nm, about 4 nm, about 5 nm, about 6 nm, about 7 nm, about 8 nm, about 9 nm, or about 10 nm, or any range of the above values.

[0153] In some embodiments, the thickness of the second coating layer may be about 2 nm, about 3 nm, about 4 nm, about 5 nm, about 6 nm, about 7 nm, about 8 nm, about 9 nm, about 10 nm, about 11 nm, about 12 nm, about 13 nm, about 14 nm, about 15 nm, or any range of the above values.

[0154] In some embodiments, the thickness of the third coating layer may be about 2 nm, about 3 nm, about 4 nm, about 5 nm, about 6 nm, about 7 nm, about 8 nm, about 9 nm, about 10 nm, about 11 nm, about 12 nm, about 13 nm, about 14 nm, about 15 nm, about 16 nm, about 17 nm, about 18 nm, about 19 nm, about 20 nm, about 21 nm, about 22 nm, about 23 nm, about 24 nm, or about 25 nm, or any range of any of the above values.

[0155] When the thickness of the first coating layer is in the range of 1-10 nm, it can avoid the adverse effects on the dynamic properties of the material that may occur when it is too thick, and it can also avoid the problem that it may not be able to effectively hinder the migration of transition metal ions when it is too thin.

[0156] When the thickness of the second coating layer is in the range of 2-15nm, the surface structure of the second coating layer is stable and the side reactions with the electrolyte are small. Therefore, it can effectively reduce the interface side reactions and thus improve the high-temperature performance of the secondary battery.

[0157] When the thickness of the third coating layer is in the range of 2-25 nm, it can improve the electrical conductivity of the material and improve the compaction performance of the battery electrode sheet prepared using the positive electrode active material.

[0158] The thickness of the coating layer is mainly tested by FIB. The specific method may include the following steps: randomly select a single particle from the positive electrode active material powder to be tested, cut a thin slice with a thickness of about 100 nm from the middle position or near the middle position of the selected particle, and then perform TEM test on the thin slice to measure the thickness of the coating layer. Measure 3-5 positions and take the average value.

[0159] In some embodiments, based on the weight of the positive electrode active material, the manganese content is in the range of 10 wt% to 35 wt%, optionally in the range of 15 wt% to 30 wt%, and more preferably in the range of 17 wt% to 20 wt%, the phosphorus content is in the range of 12 wt% to 25 wt%, optionally in the range of 15 wt% to 20 wt%, and the weight ratio of manganese to phosphorus is in the range of 0.90 to 1.25, optionally in the range of 0.95 to 1.20.

[0160] In this application, when only the core of the positive electrode active material contains manganese, the manganese content can correspond to the content of the core.

[0161] In this application, limiting the content of manganese element within the above-mentioned range can effectively avoid problems such as poor material structure stability and decreased density that may be caused by excessive manganese element content, thereby improving the cycle, storage and compaction performance of the secondary battery; and can also avoid problems such as low voltage platform that may be caused by excessive manganese element content, thereby improving the energy density of the secondary battery.

[0162] In this application, limiting the phosphorus content within the above-mentioned range can effectively avoid the following situations: if the phosphorus content is too high, it may cause the covalent nature of PO to be too strong, affecting the conductivity of small polarons and thus affecting the conductivity of the material; if the phosphorus content is too low, it may reduce the stability of the pyrophosphate lattice structure in the core, the first coating layer and / or the phosphate lattice structure in the second coating layer, thereby affecting the overall stability of the material.

[0163] The weight ratio of manganese to phosphorus has the following effects on the performance of secondary batteries: If the weight ratio is too high, it means that there is too much manganese, which increases manganese dissolution and affects the stability and specific capacity of the positive electrode active material, thus affecting the cycle performance and storage performance of the secondary battery; if the weight ratio is too low, it means that there is too much phosphorus, which is prone to forming impurity phases, which will lower the discharge voltage plateau of the material and thus reduce the energy density of the secondary battery.

[0164] The measurement of manganese and phosphorus can be performed using conventional techniques in the field. In particular, the content of manganese and phosphorus is determined by the following method: the material is dissolved in dilute hydrochloric acid (concentration 10-30%), the content of each element in the solution is tested by ICP, and then the content of manganese is measured and converted to obtain its weight percentage.

[0165] In some embodiments, the lattice change rate of the core-shell structured positive electrode active material before and after complete lithium insertion / extraction is less than 4%, preferably less than 3.8%, and more preferably 2.0-3.8%.

[0166] The lithium intercalation / deintercalation process of lithium manganese phosphate (LiMnPO4) is a two-phase reaction. The interfacial stress between the two phases is determined by the rate of lattice change before and after lithium intercalation / deintercalation; the smaller the rate of lattice change, the smaller the interfacial stress. + The easier the transmission, the better. Therefore, reducing the lattice change rate of the core will be beneficial for enhancing Li. + The core-shell structured positive electrode active material described in this application achieves a lattice change rate of less than 4% before and after lithium insertion / extraction, thus improving the rate performance of the secondary battery. The lattice change rate can be measured using methods known in the art, such as X-ray diffraction (XRD).

[0167] In some embodiments, the Li / Mn antisite defect concentration of the core-shell structured positive electrode active material is less than 4%, optionally less than 2.2%, and more preferably 1.5-2.2%.

[0168] The Li / Mn inversion defect described in this application refers to the Li / Mn inversion defect in the LiMnPO4 lattice. + With Mn 2+ The positions of the Li and Mn antisite defects have been interchanged. Accordingly, the Li / Mn antisite defect concentration refers to the concentration relative to Mn. 2+ Interchangeable Li + Zhan Li + Percentage of the total amount. In this application, the concentration of Li / Mn antisite defects can be tested, for example, according to JIS K 0131-1996.

[0169] The core-shell structured positive electrode active material described in this application can achieve the aforementioned low Li / Mn antisite defect concentration. Although the mechanism is not yet fully understood, the inventors of this application speculate that due to the presence of Li in the LiMnPO4 lattice... + With Mn 2+ The positions will be swapped, and Li + The transmission channel is a one-dimensional channel, therefore Mn 2+ In Li + Migration will be difficult within the channel, thus hindering Li + Therefore, the core-shell structured positive electrode active material described in this application, due to its low Li / Mn antisite defect concentration within the aforementioned range, can avoid Mn transport. 2+ Hinder Li + This improves the transport efficiency and enhances the specific capacity and rate performance of the positive electrode active material.

[0170] In some embodiments, the compaction density of the positive electrode active material at 3T is 2.2 g / cm³. 3 The above is optional, 2.2 g / cm³. 3 Above and 2.8g / cm 3 The higher the compaction density, the greater the weight of active material per unit volume. Therefore, increasing the compaction density is beneficial for improving the volumetric energy density of the battery cell. Compaction density can be measured according to GB / T 24533-2009.

[0171] In some embodiments, the surface oxygen valence state of the positive electrode active material is below -1.90, optionally from -1.90 to -1.98.

[0172] Oxygen's stable valence state is -2. The closer the valence state is to -2, the stronger its electron-accepting ability, i.e., the stronger its oxidizing power. Under normal circumstances, its surface valence state is below -1.7. This application, by limiting the surface oxygen valence state of the positive electrode active material to the above-mentioned range as described above, can reduce the interfacial side reactions between the positive electrode material and the electrolyte, thereby improving the cell's cycle life, high-temperature storage gas generation, and other performance characteristics.

[0173] The surface oxygen valence state can be measured by methods known in the art, such as by electron energy loss spectroscopy (EELS).

[0174] A second aspect of this application provides a method for preparing a positive electrode active material, comprising the following steps:

[0175] The steps for providing the core material: the core has the chemical formula Li 1+x Mn 1-y A y P 1-z R z O4, where x is any value in the range of -0.100 to 0.100, y is any value in the range of 0.001 to 0.500, z is any value in the range of 0.001 to 0.100, A is one or more elements selected from Zn, Al, Na, K, Mg, Mo, W, Ti, V, Zr, Fe, Ni, Co, Ga, Sn, Sb, Nb and Ge, and may be one or more elements selected from Fe, Ti, V, Ni, Co and Mg, R is one or more elements selected from B, Si, N and S, and optionally, R is one element selected from B, Si, N and S;

[0176] Coating steps: Provide Li separately a MP2O7 and / or M b (P2O7) c And an XPO4 suspension, the core material is added to the above suspension and mixed, and then sintered to obtain a positive electrode active material, wherein 0≤a≤2, 1≤b≤4, 1≤c≤6, and the values ​​of a, b and c satisfy the following condition: making crystalline pyrophosphate Li a MP2O7 or M b (P2O7) c Maintaining electrical neutrality; M is each independently one or more elements selected from Fe, Ni, Mg, Co, Cu, Zn, Ti, Ag, Zr, Nb, or Al; X is one or more elements selected from Li, Fe, Ni, Mg, Co, Cu, Zn, Ti, Ag, Zr, Nb, or Al;

[0177] The positive electrode active material has a core-shell structure, comprising a core and a shell covering the core. The shell includes a first coating layer covering the core, a second coating layer covering the first coating layer, and a third coating layer covering the second coating layer. The first coating layer comprises crystalline pyrophosphate Li. a MP2O7 and / or M b (P2O7) c The second coating layer comprises crystalline phosphate XPO4, and the third coating layer is carbon.

[0178] In some implementations, the step of providing the core material includes the following steps:

[0179] Step (1): Mix and stir the manganese source, dopant of element A and acid in a container to obtain manganese salt particles doped with element A;

[0180] Step (2): The manganese salt particles doped with element A are mixed with a lithium source, a phosphorus source, and a dopant of element R in a solvent to obtain a slurry. After sintering under an inert gas atmosphere, a core doped with elements A and R is obtained, wherein the core doped with elements A and R is Li. 1+x Mn 1-y A y P 1-z R z O4, where x is any value in the range of -0.100 to 0.100, y is any value in the range of 0.001 to 0.500, z is any value in the range of 0.001 to 0.100, A is one or more elements selected from Zn, Al, Na, K, Mg, Mo, W, Ti, V, Zr, Fe, Ni, Co, Ga, Sn, Sb, Nb and Ge, and may be one or more elements selected from Fe, Ti, V, Ni, Co and Mg, R is one or more elements selected from B, Si, N and S, and optionally, R is one element selected from B, Si, N and S.

[0181] The preparation method of this application does not have any particular restrictions on the source of materials. The source of a certain element may include one or more of the element's elemental form, sulfate, halide, nitrate, organic acid salt, oxide or hydroxide. The precursor is from this source to achieve the purpose of the preparation method of this application.

[0182] In some embodiments, the dopant of element A is one or more elements selected from the following: elemental form, carbonate, sulfate, chloride, nitrate, organic acid salt, oxide, and hydroxide.

[0183] In some embodiments, the dopant of element R is one or more of the inorganic acids, flavonoids, organic acids, sulfates, chlorides, nitrates, organic acid salts, oxides, and hydroxides of one or more elements selected from B, Si, N, and S.

[0184] In this application, the manganese source can be any manganese-containing substance known in the art that can be used to prepare lithium manganese phosphate. As an example, the manganese source can be one or more selected from elemental manganese, manganese dioxide, manganese phosphate, manganese oxalate, and manganese carbonate.

[0185] In this application, the acid may be one or more organic acids selected from hydrochloric acid, sulfuric acid, nitric acid, phosphoric acid, silicic acid, siliceous acid, and organic acids such as oxalic acid. In some embodiments, the acid is a dilute organic acid with a concentration of 60% by weight or less.

[0186] In this application, the lithium source can be any lithium-containing material known in the art that can be used to prepare lithium manganese phosphate. As an example, the lithium source is one or more selected from lithium carbonate, lithium hydroxide, lithium phosphate, and lithium dihydrogen phosphate.

[0187] In this application, the phosphorus source may be any phosphorus-containing substance known in the art that can be used to prepare lithium manganese phosphate. As an example, the phosphorus source may be one or more selected from diammonium hydrogen phosphate, ammonium dihydrogen phosphate, ammonium phosphate, and phosphoric acid.

[0188] In some alternative embodiments, after the manganese source, the dopant of element A and the acid are reacted in a solvent to obtain a suspension of manganese salt doped with element A, the suspension is filtered, dried and sand-milled to obtain manganese salt particles doped with element A with a particle size of 50-200 nm.

[0189] In some alternative embodiments, the slurry in step (2) is dried to obtain powder, and then the powder is sintered to obtain a core doped with elements A and R.

[0190] In some embodiments, step (1) is performed at a temperature of 20-120°C, optionally 40-120°C; and / or

[0191] The stirring in step (1) is carried out at 400-700 rpm for 1-9 hours, optionally for 3-7 hours.

[0192] Optionally, the reaction temperature in step (1) can be about 30°C, about 50°C, about 60°C, about 70°C, about 80°C, about 90°C, about 100°C, about 110°C, or about 120°C; the stirring in step (1) can be carried out for about 2 hours, about 3 hours, about 4 hours, about 5 hours, about 6 hours, about 7 hours, about 8 hours, or about 9 hours; optionally, the reaction temperature and stirring time in step (1) can be within any range of the above values.

[0193] In some embodiments, step (2) is performed at a temperature of 20-120°C, optionally 40-120°C, for 1-12 hours. Optionally, the reaction temperature in step (2) can be performed at approximately 30°C, approximately 50°C, approximately 60°C, approximately 70°C, approximately 80°C, approximately 90°C, approximately 100°C, approximately 110°C, or approximately 120°C; the mixing in step (2) can be performed for approximately 2 hours, approximately 3 hours, approximately 4 hours, approximately 5 hours, approximately 6 hours, approximately 7 hours, approximately 8 hours, approximately 9 hours, approximately 10 hours, approximately 11 hours, or approximately 12 hours; optionally, the reaction temperature and mixing time in step (2) can be within any range of the above values.

[0194] When the temperature and time during the core particle preparation process are within the above range, the obtained core and the positive electrode active material made from it have fewer lattice defects, which is beneficial to suppress manganese dissolution, reduce interfacial side reactions between the positive electrode active material and the electrolyte, and thus improve the cycle performance and safety performance of the secondary battery.

[0195] In some embodiments, optionally, during the preparation of dilute manganese acid particles doped with elements A and R, the pH of the solution is controlled to be 3.5-6; alternatively, the pH of the solution is controlled to be 4-6; more preferably, the pH of the solution is controlled to be 4-5. It should be noted that the pH of the resulting mixture can be adjusted using methods commonly used in the art, for example, by adding an acid or a base.

[0196] In some embodiments, optionally, in step (2), the molar ratio of the manganese salt particles to the lithium source and the phosphorus source is 1:0.5-2.1:0.5-2.1. More preferably, the molar ratio of the manganese salt particles doped with element A to the lithium source and the phosphorus source is about 1:1:1.

[0197] In some embodiments, optionally, the sintering conditions in the preparation of A and R-doped lithium manganese phosphate are: sintering at 600-950°C for 4-10 hours in an inert gas or a mixture of inert gas and hydrogen; optionally, the sintering can be carried out at about 650°C, about 700°C, about 750°C, about 800°C, about 850°C, or about 900°C for about 2 hours, about 3 hours, about 4 hours, about 5 hours, about 6 hours, about 7 hours, about 8 hours, about 9 hours, or about 10 hours; optionally, the sintering temperature and sintering time can be within any range of the above values. In the preparation of lithium manganese phosphate doped with elements A and R, if the sintering temperature is too low or the sintering time is too short, the crystallinity of the material core will be low, which will affect the overall performance. If the sintering temperature is too high, impurity phases are likely to appear in the material core, which will also affect the overall performance. If the sintering time is too long, the material core particles will be too large, which will affect the specific capacity, compaction density and rate performance.

[0198] In some alternative embodiments, the protective atmosphere may optionally be a mixture of 70-90 vol% nitrogen and 10-30 vol% hydrogen.

[0199] In some embodiments, the coating step includes:

[0200] First coating step: Dissolve the source of element M, phosphorus source, acid, and optionally lithium source in a solvent to obtain a first coating layer suspension; thoroughly mix the core obtained in the core step with the first coating layer suspension obtained in the first coating step, dry, and then sinter to obtain the material coated by the first coating layer.

[0201] Second coating step: Dissolve the source of element X, phosphorus source and acid in solvent to obtain a second coating layer suspension; mix the material coated by the first coating layer obtained in the first coating step with the second coating layer suspension obtained in the second coating step, dry, and then sinter to obtain a material coated by two coating layers.

[0202] The third coating step: Dissolve the carbon source in a solvent to obtain a third coating layer solution; then add the material coated by the two coating layers obtained in the second coating step to the third coating layer solution, mix evenly, dry, and then sinter to obtain a material coated by the three coating layers, i.e., the positive electrode active material.

[0203] In some embodiments, the source of element M is one or more elements selected from the following: elemental form, carbonate, sulfate, chloride, nitrate, organic acid salt, oxide, hydroxide, etc., of one or more elements selected from Fe, Ni, Mg, Co, Cu, Zn, Ti, Ag, Zr, Nb, or Al.

[0204] In some embodiments, the source of element X is one or more elements selected from the group consisting of the elemental form, carbonate, sulfate, chloride, nitrate, organic acid salt, oxide, and hydroxide of Li, Fe, Ni, Mg, Co, Cu, Zn, Ti, Ag, Zr, Nb, or Al.

[0205] The amount of each of the elements A, R, M, and X sources added depends on the target doping amount, and the ratio of the amounts of lithium source, manganese source, and phosphorus source used conforms to the stoichiometric ratio.

[0206] As an example, the carbon source is one or more selected from starch, sucrose, glucose, polyvinyl alcohol, polyethylene glycol, and citric acid.

[0207] In some embodiments, in the first coating step, the pH of the solution containing the source of element M, the phosphorus source, and the acid, and optionally the lithium source, is controlled to be 3.5-6.5, then stirred and reacted for 1-5 hours, then the solution is heated to 50-120°C and held at that temperature for 2-10 hours, and / or sintering is carried out at 650-800°C for 2-6 hours.

[0208] Optionally, in the first coating step, the reaction proceeds fully. Optionally, in the first coating step, the reaction proceeds for approximately 1.5 hours, approximately 2 hours, approximately 3 hours, approximately 4 hours, approximately 4.5 hours, or approximately 5 hours. Optionally, in the first coating step, the reaction time can be within any range of the above-mentioned values.

[0209] Optionally, in the first coating step, the pH of the solution is controlled to be 4-6. Optionally, in the first coating step, the solution is heated to approximately 55°C, approximately 60°C, approximately 70°C, approximately 80°C, approximately 90°C, approximately 100°C, approximately 110°C, or approximately 120°C, and held at this temperature for approximately 2 hours, approximately 3 hours, approximately 4 hours, approximately 5 hours, approximately 6 hours, approximately 7 hours, approximately 8 hours, approximately 9 hours, or approximately 10 hours; Optionally, in the first coating step, the heating temperature and holding time can be within any range of the above values.

[0210] Optionally, in the first coating step, the sintering can be performed at about 650°C, about 700°C, about 750°C, or about 800°C for about 2 hours, about 3 hours, about 4 hours, about 5 hours, or about 6 hours; optionally, the sintering temperature and sintering time can be within any range of the above values.

[0211] In the first coating step, by controlling the sintering temperature and time within the above range, the following situations can be avoided: When the sintering temperature in the first coating step is too low and the sintering time is too short, the crystallinity of the first coating layer will be low, and there will be more amorphous substances. This will reduce the effect of inhibiting metal dissolution, thereby affecting the cycle performance and high-temperature storage performance of the secondary battery. When the sintering temperature is too high, impurities will appear in the first coating layer, which will also affect its effect of inhibiting metal dissolution, thereby affecting the cycle performance and high-temperature storage performance of the secondary battery. When the sintering time is too long, the thickness of the first coating layer will increase, affecting the Li + The migration of these molecules affects the material's specific capacity and rate performance.

[0212] In some embodiments, in the second coating step, after dissolving the source of element X, the phosphorus source and the acid in a solvent, the mixture is stirred and reacted for 1-10 hours, and then the solution is heated to 60-150°C and held at that temperature for 2-10 hours, and / or sintering is carried out at 500-700°C for 6-10 hours.

[0213] Optionally, in the second coating step, the reaction proceeds fully. Optionally, in the second coating step, the reaction proceeds for approximately 1.5 hours, approximately 2 hours, approximately 3 hours, approximately 4 hours, approximately 4.5 hours, approximately 5 hours, approximately 6 hours, approximately 7 hours, approximately 8 hours, approximately 9 hours, or approximately 10 hours. Optionally, in the second coating step, the reaction time can be within any range of the above-mentioned values.

[0214] Optionally, in the second coating step, the solution is heated to approximately 65°C, approximately 70°C, approximately 80°C, approximately 90°C, approximately 100°C, approximately 110°C, approximately 120°C, approximately 130°C, approximately 140°C, or approximately 150°C, and held at this temperature for approximately 2 hours, approximately 3 hours, approximately 4 hours, approximately 5 hours, approximately 6 hours, approximately 7 hours, approximately 8 hours, approximately 9 hours, or approximately 10 hours; optionally, in the second coating step, the heating temperature and holding time can be within any range of the above values.

[0215] In the steps of providing the core material and the first coating step and the second coating step, before sintering, that is, in the preparation of the core material in which the chemical reaction occurs (steps (1)-(2)) and in the preparation of the first coating layer suspension and the second coating layer suspension, by selecting appropriate reaction temperature and reaction time as described above, the following situations can be avoided: when the reaction temperature is too low, the reaction cannot occur or the reaction rate is slow; when the temperature is too high, the product decomposes or forms an impurity phase; when the reaction time is too long, the product particle size is large, which may increase the time and difficulty of subsequent processes; when the reaction time is too short, the reaction is incomplete and less product is obtained.

[0216] Optionally, in the second coating step, the sintering can be carried out at about 550°C, about 600°C, or about 700°C for about 6 hours, about 7 hours, about 8 hours, about 9 hours, or about 10 hours; optionally, the sintering temperature and sintering time can be within any range of the above values.

[0217] In the second coating step, by controlling the sintering temperature and time within the above range, the following situations can be avoided: When the sintering temperature in the second coating step is too low and the sintering time is too short, the crystallinity of the second coating layer will be low, with more amorphous phases, reducing the performance of the material surface reactivity and thus affecting the cycle and high-temperature storage performance of the secondary battery; when the sintering temperature is too high, impurity phases will appear in the second coating layer, which will also affect its effect of reducing the material surface reactivity and thus affect the cycle and high-temperature storage performance of the secondary battery; when the sintering time is too long, the thickness of the second coating layer will increase, affecting the voltage plateau of the material and thus reducing the energy density of the material.

[0218] In some embodiments, the sintering in the third coating step is performed at 700-800°C for 6-10 hours. Optionally, in the third coating step, the sintering can be performed at about 700°C, about 750°C, or about 800°C for about 6 hours, about 7 hours, about 8 hours, about 9 hours, or about 10 hours; alternatively, the sintering temperature and sintering time can be within any range of the above values.

[0219] In the third coating step, by controlling the sintering temperature and time within the above range, the following situations can be avoided: when the sintering temperature in the third coating step is too low, the graphitization degree of the third coating layer will decrease, affecting its conductivity and thus affecting the specific capacity of the material; when the sintering temperature is too high, the graphitization degree of the third coating layer will be too high, affecting the Li + The transmission of electrical conductivity can affect the specific capacity of the material. If the sintering time is too short, the coating layer will be too thin, affecting its conductivity and thus the specific capacity of the material. If the sintering time is too long, the coating layer will be too thick, affecting the compaction density of the material.

[0220] In the first, second, and third coating steps described above, the drying is carried out at a drying temperature of 100°C to 200°C, optionally 110°C to 190°C, more preferably 120°C to 180°C, even more preferably 120°C to 170°C, and most preferably 120°C to 160°C, and the drying time is 3-9 hours, optionally 4-8 hours, more preferably 5-7 hours, and most preferably about 6 hours.

[0221] The positive electrode active material prepared by the method described in this application results in a reduced dissolution of Mn and Mn-site dopant elements in the secondary battery after cycling, and improved high-temperature stability, high-temperature cycling performance, and rate performance. Furthermore, the raw materials are widely available, inexpensive, and the process is simple, facilitating industrialization.

[0222] [Positive electrode plate]

[0223] A third aspect of this application provides a positive electrode sheet, comprising a positive current collector and a positive electrode film layer disposed on at least one surface of the positive current collector. The positive electrode film layer comprises a positive electrode active material with a core-shell structure as described in the first aspect of this application or a positive electrode active material prepared by the method described in the second aspect of this application. The content of the positive electrode active material in the positive electrode film layer is more than 10% by weight, optionally 90-99.5% by weight, and more preferably 95-99.5% by weight, based on the total weight of the positive electrode film layer.

[0224] In this application, as an example, the positive current collector has two surfaces opposite each other in its own thickness direction, and the positive electrode film layer is disposed on either or both of the two opposite surfaces of the positive current collector.

[0225] In some embodiments, the positive current collector may be a metal foil or a composite current collector. For example, aluminum foil may be used as the metal foil. The composite current collector may include a polymer substrate and a metal layer formed on at least one surface of the polymer substrate. The composite current collector may be formed by forming a metal material (aluminum, aluminum alloy, nickel, nickel alloy, titanium, titanium alloy, silver and silver alloy, etc.) on a polymer substrate (such as a substrate of polypropylene (PP), polyethylene terephthalate (PET), polybutylene terephthalate (PBT), polystyrene (PS), polyethylene (PE), etc.).

[0226] In this application, the positive electrode active material is the positive electrode active material described in this application.

[0227] In some embodiments, the positive electrode film layer may optionally include a binder. As an example, the binder may include at least one selected from polyvinylidene fluoride (PVDF), polytetrafluoroethylene (PTFE), PVDF-tetrafluoroethylene-propylene terpolymer, PVDF-hexafluoropropylene-tetrafluoroethylene terpolymer, tetrafluoroethylene-hexafluoropropylene copolymer, and fluorinated acrylate resin.

[0228] In some embodiments, the positive electrode film may optionally include a conductive agent. As an example, the conductive agent may include at least one selected from superconducting carbon, acetylene black, carbon black, Ketjen black, carbon dots, carbon nanotubes, graphene, and carbon nanofibers.

[0229] In some embodiments, the binder accounts for 0.4-5.5% by weight, optionally 0.4-4.5% by weight, the conductive agent accounts for 0.1-2.5% by weight, optionally 0.1-0.5% by weight, and other additives account for 0.001-1% by weight, based on the total weight of the positive electrode film.

[0230] In some embodiments, the coating weight of the positive electrode sheet is 0.28-0.45 g / 1540.25 mm. 2 The compaction density reached 2.0 g / cm³. 3 The above values ​​can be selected as 2.2-2.8 g / cm³. 3 .

[0231] In some embodiments, the positive electrode sheet can be prepared by dispersing the above-mentioned components for preparing the positive electrode sheet, such as positive active material, conductive agent, binder and any other components, in a solvent (e.g., N-methylpyrrolidone) to form a positive electrode slurry; coating the positive electrode slurry onto the positive electrode current collector, and then obtaining the positive electrode sheet after drying, cold pressing and other processes.

[0232] [Negative electrode plate]

[0233] The negative electrode sheet includes a negative current collector and a negative electrode film layer disposed on at least one surface of the negative current collector, the negative electrode film layer including a negative electrode active material.

[0234] As an example, the negative electrode current collector has two surfaces opposite each other in its own thickness direction, and the negative electrode film layer is disposed on either or both of the two opposite surfaces of the negative electrode current collector.

[0235] In some embodiments, the negative electrode current collector may be a metal foil or a composite current collector. For example, copper foil may be used as the metal foil. The composite current collector may include a polymer substrate and a metal layer formed on at least one surface of the polymer substrate. The composite current collector may be formed by forming a metal material (copper, copper alloy, nickel, nickel alloy, titanium, titanium alloy, silver and silver alloy, etc.) on a polymer substrate (such as a substrate of polypropylene (PP), polyethylene terephthalate (PET), polybutylene terephthalate (PBT), polystyrene (PS), polyethylene (PE), etc.).

[0236] In some embodiments, the negative electrode active material may be a negative electrode active material known in the art for use in secondary batteries. As an example, the negative electrode active material may include at least one of the following materials: artificial graphite, natural graphite, soft carbon, hard carbon, silicon-based materials, tin-based materials, and lithium titanate, etc. The silicon-based material may be at least one selected from elemental silicon, silicon oxide compounds, silicon-carbon composites, silicon-nitrogen composites, and silicon alloys. The tin-based material may be at least one selected from elemental tin, tin oxide compounds, and tin alloys. However, this application is not limited to these materials, and other conventional materials that can be used as battery negative electrode active materials may also be used. These negative electrode active materials may be used alone or in combination of two or more.

[0237] In some embodiments, the negative electrode film layer may optionally include a binder. The binder may be at least one selected from styrene-butadiene rubber (SBR), polyacrylic acid (PAA), sodium polyacrylate (PAAS), polyacrylamide (PAM), polyvinyl alcohol (PVA), sodium alginate (SA), polymethacrylic acid (PMAA), and carboxymethyl chitosan (CMCS).

[0238] In some embodiments, the negative electrode film may optionally include a conductive agent. The conductive agent may be at least one selected from superconducting carbon, acetylene black, carbon black, Ketjen black, carbon dots, carbon nanotubes, graphene, and carbon nanofibers.

[0239] In some embodiments, the negative electrode film may optionally include other additives, such as thickeners (e.g., sodium carboxymethyl cellulose (CMC-Na)).

[0240] In some embodiments, the negative electrode sheet can be prepared by dispersing the components used to prepare the negative electrode sheet, such as the negative electrode active material, conductive agent, binder and any other components, in a solvent (e.g., deionized water) to form a negative electrode slurry; coating the negative electrode slurry onto the negative electrode current collector, and then obtaining the negative electrode sheet after drying, cold pressing and other processes.

[0241] [Electrolytes]

[0242] The electrolyte acts as a conductor of ions between the positive and negative electrodes. This application does not impose specific restrictions on the type of electrolyte; it can be selected according to requirements. For example, the electrolyte can be liquid, gel, or entirely solid.

[0243] In some embodiments, the electrolyte is an electrolyte solution. The electrolyte solution includes an electrolyte salt and a solvent.

[0244] In some embodiments, the electrolyte salt may be at least one selected from lithium hexafluorophosphate, lithium tetrafluoroborate, lithium perchlorate, lithium hexafluoroarsenate, lithium bis(fluorosulfonyl)imide, lithium bis(trifluoromethanesulfonyl)imide, lithium trifluoromethanesulfonate, lithium difluorophosphate, lithium difluorooxalate borate, lithium dioxalate borate, lithium difluorodioxalate phosphate, and lithium tetrafluorooxalate phosphate.

[0245] In some embodiments, the solvent may be at least one selected from ethylene carbonate, propylene carbonate, methyl ethyl carbonate, diethyl carbonate, dimethyl carbonate, dipropyl carbonate, methyl propyl carbonate, ethyl propyl carbonate, butyl carbonate, fluoroethylene carbonate, methyl formate, methyl acetate, ethyl acetate, propyl acetate, methyl propionate, ethyl propionate, propyl propionate, methyl butyrate, ethyl butyrate, 1,4-butyrolactone, sulfolane, dimethyl sulfone, methyl ethyl sulfone, and diethyl sulfone.

[0246] In some embodiments, the electrolyte may optionally include additives. For example, additives may include negative electrode film-forming additives, positive electrode film-forming additives, and may also include additives that can improve certain battery performance, such as additives that improve battery overcharge performance, additives that improve battery high-temperature or low-temperature performance, etc.

[0247] [Isolation membrane]

[0248] In some embodiments, the secondary battery also includes a separator. This application does not impose any particular limitation on the type of separator; any known porous separator with good chemical and mechanical stability can be selected.

[0249] In some embodiments, the material of the separator may be at least one selected from glass fiber, nonwoven fabric, polyethylene, polypropylene, and polyvinylidene fluoride. The separator may be a single-layer film or a multi-layer composite film, without particular limitation. When the separator is a multi-layer composite film, the materials of each layer may be the same or different, without particular limitation.

[0250] In some implementations, the positive electrode, negative electrode, and separator can be fabricated into an electrode assembly using a winding or stacking process.

[0251] [Rechargeable Battery]

[0252] The fourth aspect of this application provides a secondary battery comprising a positive electrode active material with a core-shell structure as described in the first aspect of this application, a positive electrode active material prepared by the method described in the second aspect of this application, or a positive electrode sheet as described in the third aspect of this application.

[0253] The secondary battery includes a positive electrode, a negative electrode, an electrolyte, and a separator as described in this application. During charging and discharging, active ions repeatedly insert and extract between the positive and negative electrodes. The electrolyte acts as a conductor of ions between the positive and negative electrodes. The separator, positioned between the positive and negative electrodes, primarily prevents short circuits between the positive and negative electrodes while allowing ions to pass through.

[0254] In some embodiments, the lithium-ion secondary battery may include an outer packaging. This outer packaging can be used to encapsulate the electrode assembly and electrolyte described above.

[0255] In some implementations, the outer packaging of a lithium-ion secondary battery can be a rigid shell, such as a hard plastic shell, aluminum shell, or steel shell. The outer packaging of a lithium-ion secondary battery can also be a soft pack, such as a pouch. The material of the soft pack can be plastic; examples of plastics include polypropylene (PP), polybutylene terephthalate (PBT), and polybutylene succinate (PBS).

[0256] The secondary battery, battery module, battery pack, and power supply device of this application will be described below with appropriate reference to the accompanying drawings.

[0257] This application does not impose any particular limitation on the shape of the secondary battery; it can be cylindrical, square, or any other arbitrary shape. For example, Figure 2 This is an example of a square-structured secondary battery 5.

[0258] In some implementations, refer to Figure 3 The outer packaging may include a housing 51 and a cover 53. The housing 51 may include a base plate and side plates connected to the base plate, the base plate and side plates forming a receiving cavity. The housing 51 has an opening communicating with the receiving cavity, and the cover 53 can be placed over the opening to close the receiving cavity. A positive electrode, a negative electrode, and a separator can be formed into an electrode assembly 52 using a winding or stacking process. The electrode assembly 52 is encapsulated within the receiving cavity. Electrolyte is immersed in the electrode assembly 52. ​​The secondary battery 5 may contain one or more electrode assemblies 52, which can be selected by those skilled in the art according to specific practical needs.

[0259] In some implementations, lithium-ion secondary batteries can be assembled into battery modules, and the number of lithium-ion batteries contained in a battery module can be one or more, the specific number of which can be selected by those skilled in the art according to the application and capacity of the battery module.

[0260] Therefore, a fifth aspect of this application provides a battery module comprising a secondary battery as described in the fourth aspect of this application.

[0261] Figure 4 This is battery module 4, used as an example. (See reference...) Figure 4 In battery module 4, multiple lithium-ion batteries 5 can be arranged sequentially along the length of battery module 4. Of course, they can also be arranged in any other manner. Furthermore, these multiple lithium-ion batteries 5 can be fixed in place using fasteners.

[0262] Optionally, the battery module 4 may also include a housing with a receiving space in which multiple lithium-ion batteries 5 are housed.

[0263] In some embodiments, the battery modules described above can also be assembled into a battery pack, and the number of battery modules contained in the battery pack can be one or more, the specific number of which can be selected by those skilled in the art according to the application and capacity of the battery pack.

[0264] Therefore, a sixth aspect of this application provides a battery pack that includes the battery module described in the fifth aspect of this application.

[0265] Figure 5 and Figure 6 This is battery pack 1 as an example. (See reference...) Figure 5 and Figure 6 The battery pack 1 may include a battery box and multiple battery modules 4 disposed within the battery box. The battery box includes an upper body 2 and a lower body 3, with the upper body 2 covering the lower body 3 to form a closed space for accommodating the battery modules 4. The multiple battery modules 4 can be arranged in any manner within the battery box.

[0266] A seventh aspect of this application provides an electrical device comprising at least one of the following: a secondary battery as described in the fourth aspect of this application, a battery module as described in the fifth aspect of this application, or a battery pack as described in the sixth aspect of this application. The secondary battery, battery module, or battery pack can be used as a power source for the electrical device or as an energy storage unit for the electrical device. The electrical device may include, but is not limited to, mobile devices (e.g., mobile phones, laptops, etc.), electric vehicles (e.g., pure electric vehicles, hybrid electric vehicles, plug-in hybrid electric vehicles, electric bicycles, electric scooters, electric golf carts, electric trucks, etc.), electric trains, ships and satellites, energy storage systems, etc.

[0267] As the electrical device, a secondary battery, battery module, or battery pack can be selected according to its usage requirements.

[0268] Figure 7 This is an example of an electrical device. The device could be a pure electric vehicle, a hybrid electric vehicle, or a plug-in hybrid electric vehicle. To meet the high power and high energy density requirements of the secondary battery for this device, a battery pack or battery module can be used.

[0269] Another example device could be a mobile phone, tablet, or laptop. These devices typically require a slim and lightweight design and can use a rechargeable battery as their power source.

[0270] The positive electrode sheet, secondary battery, battery module, battery pack, and electrical device described in this application include the positive electrode active material described in this application, and therefore have high specific capacity, good cycle performance, and safety performance.

[0271] Example

[0272] The following describes embodiments of this application. The embodiments described below are exemplary and are only used to explain this application, and should not be construed as limiting this application. Where specific techniques or conditions are not specified in the embodiments, they are performed according to the techniques or conditions described in the literature in the art or according to the product instructions. Reagents or instruments used, unless otherwise specified, are all conventional products that can be obtained commercially. Unless otherwise specified, the content of each component in the embodiments of this invention is based on the mass of the component excluding crystallization water.

[0273] The sources of raw materials involved in the embodiments of this application are as follows:

[0274]

[0275]

[0276] I. Battery Manufacturing

[0277] Example 1:

[0278] Step 1: Preparation of positive electrode active material

[0279] Step S1: Preparation of Fe, Co, V and S co-doped manganese oxalate

[0280] 689.6 g of manganese carbonate, 455.27 g of ferrous carbonate, 4.65 g of cobalt sulfate, and 4.87 g of vanadium dichloride were added to a mixer and mixed thoroughly for 6 hours. The resulting mixture was then transferred to a reaction vessel, and 5 L of deionized water and 1260.6 g of oxalic acid dihydrate were added. The mixture was heated to 80°C and stirred thoroughly at 500 rpm for 6 hours until homogeneous mixing and the reaction was terminated without bubble formation, yielding a Fe, Co, and V co-doped manganese oxalate suspension. The suspension was then filtered, dried at 120°C, and milled to obtain manganese oxalate particles with a particle size of 100 nm.

[0281] Step S2: Preparation of core Li 0.997 Mn 0.60 Fe 0.393 V 0.004 Co 0.003 P 0.997 S 0.003O4

[0282] Take 1793.1g of manganese oxalate, 368.3g of lithium carbonate, 1146.6g of ammonium dihydrogen phosphate, and 4.9g of dilute sulfuric acid prepared in (1), add them to 20L of deionized water, stir thoroughly, and react uniformly at 80℃ for 10h to obtain a slurry. Transfer the slurry to a spray drying equipment for spray drying and granulation, and dry at 250℃ to obtain a powder. In a protective atmosphere (90% nitrogen and 10% hydrogen), sinter the powder in a roller kiln at 700℃ for 4h to obtain the above-mentioned core material.

[0283] Step S3: Preparation of the first coating layer suspension

[0284] To prepare a Li2FeP2O7 solution, 7.4 g of lithium carbonate, 11.6 g of ferrous carbonate, 23.0 g of ammonium dihydrogen phosphate and 12.6 g of oxalic acid dihydrate were dissolved in 500 mL of deionized water, and the pH was controlled at 5. The solution was then stirred and reacted at room temperature for 2 h to obtain a solution. The solution was then heated to 80 °C and maintained at this temperature for 4 h to obtain the first coating layer suspension.

[0285] Step S4: Coating with the first coating layer

[0286] The 1571.9g of doped lithium manganese phosphate core material obtained in step S2 was added to the first coating layer suspension (coating material content of 15.7g) obtained in step S3. The mixture was stirred and mixed thoroughly for 6 hours. After being mixed evenly, the mixture was dried in an oven at 120℃ for 6 hours and then sintered at 650℃ for 6 hours to obtain the pyrophosphate coated material.

[0287] Step S5: Preparation of the second coating layer suspension

[0288] 3.7 g lithium carbonate, 11.6 g ferrous carbonate, 11.5 g ammonium dihydrogen phosphate and 12.6 g oxalic acid dihydrate were dissolved in 1500 mL deionized water, stirred and reacted for 6 h to obtain a solution. The solution was then heated to 120 °C and maintained at this temperature for 6 h to obtain a second coating layer suspension.

[0289] Step S6: Coating with the second coating layer

[0290] The 1586.8g of pyrophosphate-coated material obtained in step S4 was added to the second coating suspension (coating material content of 47.1g) obtained in step S5. The mixture was stirred and mixed thoroughly for 6 hours. After mixing evenly, the mixture was dried in an oven at 120℃ for 6 hours and then sintered at 700℃ for 8 hours to obtain the two-layer coated material.

[0291] Step S7: Preparation of the third coating layer aqueous solution

[0292] Dissolve 37.3g of sucrose in 500g of deionized water, then stir and dissolve completely to obtain a sucrose aqueous solution.

[0293] Step S8: Coating with the third coating layer

[0294] 1633.9g of the two-layer coated material obtained in step S6 was added to the sucrose solution obtained in step S7 and stirred together for 6 hours. After mixing evenly, the mixture was placed in an oven at 150°C and dried for 6 hours. Then, it was sintered at 700°C for 10 hours to obtain the three-layer coated material.

[0295] Step 2: Preparation of the positive electrode sheet

[0296] The three-layer coated positive electrode active material, conductive agent acetylene black, and binder polyvinylidene fluoride (PVDF) prepared above were added to N-methylpyrrolidone (NMP) at a weight ratio of 97.0:1.2:1.8, and stirred until homogeneous to obtain the positive electrode slurry. Then, the positive electrode slurry was prepared at a ratio of 0.280 g / 1540.25 mm. 2 The material is evenly coated onto aluminum foil, then dried, cold-pressed, and slit to obtain the positive electrode sheet.

[0297] Step 3: Preparation of the negative electrode sheet

[0298] A negative electrode slurry was prepared by dissolving artificial graphite (anode active material), hard carbon, acetylene black (conductive agent), styrene-butadiene rubber (SBR) (binder), and sodium carboxymethyl cellulose (CMC) (thickener) in deionized water at a weight ratio of 90:5:2:2:1 and stirring until homogeneous. The negative electrode slurry was then prepared at a concentration of 0.117 g / 1540.25 mm. 2 The coating is uniformly applied to the copper foil of the negative electrode current collector, and then dried, cold-pressed, and slit to obtain the negative electrode sheet.

[0299] Step 4: Preparation of electrolyte

[0300] In an argon atmosphere glove box (H2O < 0.1 ppm, O2 < 0.1 ppm), ethylene carbonate (EC) / ethyl methyl carbonate (EMC) organic solvents were mixed evenly at a volume ratio of 3 / 7. 12.5% ​​by weight (based on the weight of ethylene carbonate / ethyl methyl carbonate solvent) of LiPF6 was added and dissolved in the above organic solvents and stirred evenly to obtain the electrolyte.

[0301] Step 5: Preparation of the separating membrane

[0302] The material used was a commercially available PP-PE copolymer microporous film with a thickness of 20 μm and an average pore size of 80 nm (from Zogo Electronics Technology Co., Ltd., model 20).

[0303] Step 6: Preparation of the full cell

[0304] The obtained positive electrode, separator, and negative electrode are stacked in sequence, with the separator positioned between the positive and negative electrodes to provide isolation. The electrodes are then wound to obtain a bare cell. The bare cell is placed in an outer packaging, injected with the electrolyte, and sealed to obtain a full battery (hereinafter also referred to as "full battery").

[0305] [Preparation of button cells]

[0306] The prepared positive electrode active material, polyvinylidene fluoride (PVDF), and acetylene black were added to N-methylpyrrolidone (NMP) in a weight ratio of 90:5:5, and stirred in a drying chamber to form a slurry. The slurry was then coated onto aluminum foil, dried, and cold-pressed to form a positive electrode sheet. The coating amount was 0.2 g / cm³. 2 The compacted density is 2.0 g / cm³. 3 .

[0307] A lithium sheet is used as the negative electrode, and a solution of 1 mol / L LiPF6 in ethylene carbonate (EC), diethyl carbonate (DEC) and dimethyl carbonate (DMC) in a volume ratio of 1:1:1 is used as the electrolyte. Together with the positive electrode sheet prepared above, they are assembled into a coin cell (hereinafter also referred to as "coin cell") in a coin cell box.

[0308] Examples 2 to 27 and Comparative Examples 1 to 19

[0309] The positive electrode active materials and batteries of Examples 2 to 27 and Comparative Examples 1 to 19 were prepared in a manner similar to that of Example 1. The differences in the preparation of the positive electrode active materials are shown in Tables 1-6. Comparative Examples 1-2, 4-10 and 12 were not coated with the first layer, so steps S3 and S4 were not included; Comparative Examples 1-11 were not coated with the second layer, so steps S5-S6 were not included.

[0310] Note: In all embodiments and comparative examples of this application, unless otherwise specified, the first coating layer material and / or the second coating layer material used are assumed to be crystalline.

[0311] Table 1: Raw materials for kernel preparation

[0312]

[0313]

[0314]

[0315]

[0316]

[0317]

[0318]

[0319]

[0320]

[0321]

[0322] II. Performance Evaluation

[0323] 1. Method for testing lattice change rate:

[0324] Under a constant temperature environment of 25℃, the positive electrode active material sample was placed in an XRD (model Bruker D8 Discover) and tested at 1° / min. The test data were then organized and analyzed. Referring to the standard PDF card, the lattice constants a0, b0, c0 and v0 were calculated (a0, b0 and c0 represent the length of each aspect of the unit cell, and v0 represents the volume of the unit cell, which can be directly obtained from the XRD refinement results).

[0325] Using the coin cell preparation method described in the above embodiments, the positive electrode active material sample was prepared into a coin cell, and the coin cell was charged at a low rate of 0.05C until the current decreased to 0.01C. Then, the positive electrode sheet of the coin cell was removed and immersed in dimethyl carbonate (DMC) for 8 hours. After drying, the powder was scraped off, and particles with a diameter less than 500 nm were screened out. Samples were taken, and their cell volume v1 was calculated in the same manner as the fresh samples tested above. The lattice change rate (cell volume change rate) before and after complete lithium insertion / extraction is shown in the table.

[0326] 2. Li / Mn antisite defect concentration

[0327] The XRD results obtained from the "lattice change rate measurement method" are compared with the PDF (Powder Diffraction File) card of the standard crystal to determine the Li / Mn antisite defect concentration. Specifically, the XRD results obtained from the "lattice change rate measurement method" are imported into the General Structure Analysis System (GSAS) software to automatically obtain refined results, which include the occupancy of different atoms. The Li / Mn antisite defect concentration is then obtained by reading the refined results.

[0328] 3. Compacted density

[0329] Take 5g of the prepared positive electrode active material powder and place it in a compaction mold (CARVER mold, model 13mm). Then place the mold on a compaction density instrument. Apply a pressure of 3T and read the thickness of the powder under pressure (thickness after depressurization) on the instrument. Calculate the compaction density using ρ = m / v. The area value used is the standard small image area of ​​1540.25mm². 2 .

[0330] 4. 3C charging constant current ratio

[0331] Under a constant temperature environment of 25°C, the fresh full cells prepared in the above embodiments and comparative examples were allowed to stand for 5 minutes, then discharged at 1 / 3C to 2.5V. After standing for 5 minutes, they were charged at 1 / 3C to 4.3V, and then charged at a constant voltage of 4.3V until the current was less than or equal to 0.05mA. After standing for 5 minutes, the charging capacity at this point was recorded as C0. After discharging at 1 / 3C to 2.5V, standing for 5 minutes, and then charging at 3C to 4.3V, and standing for 5 minutes, the charging capacity at this point was recorded as C1. The constant current ratio for 3C charging is C1 / C0 × 100%.

[0332] The higher the constant current ratio during 3C charging, the better the rate performance of the secondary battery.

[0333] 5. Dissolution test of transition metal Mn (and Fe doped at Mn sites)

[0334] The full cells prepared in the above-described embodiments and comparative examples, after being cycled at 45°C until their capacity decayed to 80%, were discharged at a rate of 0.1C to a cutoff voltage of 2.0V. The batteries were then disassembled, and the negative electrode was removed. Thirty unit areas (1540.25 mm²) were randomly selected from the negative electrode. 2 The discs were tested using an Agilent ICP-OES730 inductively coupled plasma emission spectrometry (ICP). The amounts of Fe (if the Mn site of the positive electrode active material is doped with Fe) and Mn were calculated based on the ICP results, thus determining the amount of Mn (and Mn-doped Fe) dissolved after cycling. The testing standard was based on EPA-6010D-2014.

[0335] 6. Surface oxygen valence state

[0336] 5g of the positive electrode active material sample prepared above was used to prepare a coin cell according to the coin cell preparation method described in the above embodiments. The coin cell was charged at a low rate of 0.05C until the current decreased to 0.01C. Then, the positive electrode sheet of the coin cell was removed and immersed in DMC for 8 hours. Then, it was dried, scraped off, and particles with a particle size of less than 500nm were screened out. The obtained particles were measured using electron energy loss spectroscopy (EELS, using a Talos F200S instrument) to obtain the energy loss near-edge structure (ELNES), which reflects the density of states and energy level distribution of the element. Based on the density of states and energy level distribution, the number of occupied electrons was calculated by integrating the valence band density of states data, thereby deducing the valence state of the surface oxygen after charging.

[0337] 7. Measurement of manganese and phosphorus elements in positive electrode active materials

[0338] Dissolve 5g of the prepared positive electrode active material in 100ml of aqua regia (concentrated hydrochloric acid: concentrated nitric acid = 1:3) (concentrated hydrochloric acid concentration ~37%, concentrated nitric acid concentration ~65%). Use ICP to test the content of each element in the solution, and then measure and convert the content of manganese or phosphorus (amount of manganese or phosphorus / amount of positive electrode active material * 100%) to obtain its weight percentage.

[0339] 8. Method for measuring the initial specific capacity of button cells

[0340] At 2.5-4.3V, the coin cells prepared in the above embodiments and comparative examples were charged to 4.3V at 0.1C, and then charged at 4.3V at a constant voltage until the current was less than or equal to 0.05mA. After standing for 5 minutes, they were discharged to 2.0V at 0.1C. The discharge capacity at this time is the initial specific capacity, denoted as D0.

[0341] 10. Cell expansion test after 30 days of storage at 60℃:

[0342] Full cells prepared in the above-described embodiments and comparative examples were stored at 60°C at 100% state of charge (SOC). The open-circuit voltage (OCV) and internal resistance (IMP) of the cells were measured before, during, and after storage to monitor SOC, and the cell volume was also measured. The full cells were removed after every 48 hours of storage, allowed to stand for 1 hour, and then the open-circuit voltage (OCV) and internal resistance (IMP) were measured. After cooling to room temperature, the cell volume was measured using the water displacement method. The water displacement method involves first measuring the cell's weight F1 separately using a balance with automatic unit conversion of dial data, and then completely immersing the cell in deionized water (with a known density of 1 g / cm³). 3 In the process, measure the weight F2 of the battery cell at this moment, and the buoyancy F of the battery cell. 浮 That is, F1-F2, and then according to Archimedes' principle, F 浮 =ρ×g×V排 The cell volume V is calculated to be V = (F1 - F2) / (ρ × g).

[0343] Based on the OCV and IMP test results, the batteries in all embodiments maintained a SOC of over 99% throughout the entire experiment until the end of storage.

[0344] After 30 days of storage, the cell volume was measured, and the percentage increase in cell volume after storage was calculated relative to the cell volume before storage.

[0345] 11. Cyclic performance test of the full battery at 45°C

[0346] Under constant temperature conditions of 45℃, the capacitor is charged at 1C to 4.3V within a range of 2.5-4.3V, then charged at a constant voltage of 4.3V until the current is ≤0.05mA. After resting for 5 minutes, it is discharged at 1C to 2.5V. The capacitance is denoted as D. n (n=0,1,2,……). Repeat the above process until the capacity decays to 80%, and record the number of repetitions at this point, which is the number of cycles corresponding to 80% capacity retention at 45℃.

[0347] 12. Interplanar spacing and angle testing

[0348] Take 1g of each of the above-prepared positive electrode active material powders into a 50mL test tube, and inject 10mL of 75% alcohol into the test tube. Then, stir and disperse the mixture thoroughly for 30 minutes. Then, use a clean disposable plastic pipette to take an appropriate amount of the above solution and drop it onto a 300-mesh copper grid. At this time, some powder will remain on the copper grid. Transfer the copper grid along with the sample to the TEM (Talos F200s G2) sample chamber for testing, obtain the original TEM test image, and save the original image format (xx.dm3).

[0349] Open the original image obtained from the TEM test in DigitalMicrograph software and perform a Fourier transform (the software will automatically complete this step after clicking) to obtain the diffraction pattern. Measure the distance from the diffraction spot to the center position in the diffraction pattern to obtain the interplanar spacing. The included angle is calculated according to the Bragg equation.

[0350] By comparing the obtained interplanar spacing and corresponding angle data with their standard values, different materials in the coating layer can be identified.

[0351] 13. Coating thickness test

[0352] The thickness of the coating layer was tested by cutting a thin slice of about 100 nm thickness from the middle of a single particle of the positive electrode active material prepared above using FIB. Then, the slice was subjected to TEM testing to obtain the original TEM test image, which was saved in the original image format (xx.dm3).

[0353] Open the original images obtained from the TEM test in DigitalMicrograph software, identify the cladding layer using the lattice spacing and angle information, and measure the thickness of the cladding layer.

[0354] Measure the thickness of the selected particle at three locations and take the average value.

[0355] 14. Determination of the molar ratio of SP2 and SP3 forms in the third coating layer of carbon

[0356] This test was performed using Raman spectroscopy. By splitting the energy spectrum from the Raman test, the Id / Ig ratio was obtained, where Id is the peak intensity of SP3 carbon and Ig is the peak intensity of SP2 carbon, thus confirming the molar ratio between the two.

[0357] The performance test results for all embodiments and comparative examples are shown in the table below.

[0358]

[0359]

[0360]

[0361]

[0362]

[0363]

[0364]

[0365]

[0366]

[0367]

[0368]

[0369] Both the compaction density and the performance of the prepared battery (capacity, high-temperature cycling performance, and high-temperature storage performance) are excellent.

[0370] It should be noted that this application is not limited to the above-described embodiments. The above embodiments are merely examples, and any embodiments with the same structure and effect as the technical concept within the scope of this application are included in the technical scope of this application. Furthermore, various modifications that can be conceived by those skilled in the art to the embodiments, and other ways of constructing by combining some of the constituent elements of the embodiments, without departing from the spirit of this application, are also included in the scope of this application.

Claims

1. A positive electrode active material with a core-shell structure, comprising a core and a shell covering the core, The chemical formula of the core is Li 1+x Mn 1-y A y P 1-z R z O4, where x is any value in the range of -0.100 to 0.100, y is any value in the range of 0.001 to 0.500, z is any value in the range of 0.001 to 0.100, A is one or more elements selected from Zn, Al, Na, K, Mg, Mo, W, Ti, V, Zr, Fe, Ni, Co, Ga, Sn, Sb, Nb and Ge, and R is one or more elements selected from B, Si, N and S; The values ​​of x, y, and z satisfy the following condition: keeping the entire kernel electrically neutral; The shell includes a first covering layer covering the core, a second covering layer covering the first covering layer, and a third covering layer covering the second covering layer, wherein... The first coating layer comprises crystalline pyrophosphate Li a MP2O7 and / or M b (P2O7) c The crystallinity of the crystalline pyrophosphate is 50%-100%, wherein 0≤a≤2, 1≤b≤4, and 1≤c≤6, and the values ​​of a, b, and c satisfy the following condition: making the crystalline pyrophosphate Li... a MP2O7 or M b (P2O7) c Maintain electrical neutrality The crystalline pyrophosphate Li a MP2O7 and M b (P2O7) c Each of the M elements is independently selected from Fe, Ni, Mg, Co, Cu, Zn, Ti, Ag, Zr, Nb, or Al. The second coating layer comprises crystalline phosphate XPO4, wherein the crystallinity of the crystalline phosphate is 50%-100%, and wherein X is one or more elements selected from Li, Fe, Ni, Mg, Co, Cu, Zn, Ti, Ag, Zr, Nb or Al; The third coating layer is carbon.

2. The positive electrode active material with a core-shell structure according to claim 1, wherein, A is one or more elements selected from Fe, Ti, V, Ni, Co, and Mg; and / or R is an element selected from B, Si, N, and S.

3. The positive electrode active material with a core-shell structure according to claim 1, wherein, The interplanar spacing of the crystalline pyrophosphate in the first coating layer ranges from 0.293 to 0.470 nm, and the included angle of the crystal orientation (111) ranges from 18.00° to 32.00°; the interplanar spacing of the crystalline phosphate in the second coating layer ranges from 0.244 to 0.425 nm, and the included angle of the crystal orientation (111) ranges from 20.00° to 37.00°.

4. The positive electrode active material with a core-shell structure according to claim 1, wherein, In the kernel, the ratio of y to 1-y is between 1:10 and 1:

1.

5. The positive electrode active material with a core-shell structure according to claim 1, wherein, In the kernel, the ratio of y to 1-y is 1:4 to 1:

1.

6. The positive electrode active material having a core-shell structure according to any one of claims 1-5, wherein, In the kernel, the ratio of z to 1-z is between 1:9 and 1:

999.

7. The positive electrode active material having a core-shell structure according to any one of claims 1-5, wherein, In the kernel, the ratio of z to 1-z is between 1:499 and 1:

249.

8. The positive electrode active material having a core-shell structure according to any one of claims 1-5, wherein, The carbon in the third coating layer is a mixture of SP2 and SP3 carbon.

9. The positive electrode active material with a core-shell structure according to claim 8, wherein, The molar ratio of SP2 carbon to SP3 carbon is any value within the range of 0.1-10.

10. The positive electrode active material with a core-shell structure according to claim 8, wherein, The molar ratio of SP2 carbon to SP3 carbon is any value within the range of 2.0-3.

0.

11. The positive electrode active material having a core-shell structure according to any one of claims 1-5, wherein, The coating amount of the first coating layer is greater than 0 and less than or equal to 6% by weight, based on the weight of the core; and / or The coating amount of the second coating layer is greater than 0 and less than or equal to 6% by weight, based on the weight of the core; and / or The coating amount of the third coating layer is greater than 0 and less than or equal to 6% by weight, based on the weight of the core.

12. The positive electrode active material having a core-shell structure according to any one of claims 1-5, wherein, The coating amount of the first coating layer is greater than 0 and less than or equal to 5.5% by weight, based on the weight of the core; and / or The coating amount of the second coating layer is greater than 0 and less than or equal to 5.5% by weight, based on the weight of the core; and / or The coating amount of the third coating layer is greater than 0 and less than or equal to 5.5% by weight, based on the weight of the core.

13. The positive electrode active material having a core-shell structure according to any one of claims 1-5, wherein, The coating amount of the first coating layer is greater than 0 and less than or equal to 2% by weight, based on the weight of the core; and / or The second coating layer has a coating amount of 2-4% by weight, based on the weight of the core; and / or The coating amount of the third coating layer is greater than 0 and less than or equal to 2% by weight, based on the weight of the core.

14. The positive electrode active material having a core-shell structure according to any one of claims 1-5, wherein, The thickness of the first coating layer is 1-10 nm; and / or The thickness of the second coating layer is 2-15 nm; and / or The thickness of the third coating layer is 2-25 nm.

15. The positive electrode active material having a core-shell structure according to any one of claims 1-5, wherein, Based on the weight of the positive electrode active material, the manganese content is in the range of 10%-35% by weight, the phosphorus content is in the range of 12%-25% by weight, and the weight ratio of manganese to phosphorus is in the range of 0.90-1.

25.

16. The positive electrode active material having a core-shell structure according to any one of claims 1-5, wherein, Based on the gravimetric analysis of the positive electrode active material, the manganese content is in the range of 15%-30% by weight; and / or Based on the weight of the positive electrode active material, the phosphorus content is in the range of 15%-20% by weight, and the weight ratio of manganese to phosphorus is in the range of 0.95-1.

20.

17. The positive electrode active material with a core-shell structure according to claim 15 or 16, wherein, Based on the weight of the positive electrode active material, the manganese content is in the range of 17%-20% by weight.

18. The positive electrode active material having a core-shell structure according to any one of claims 1-5, wherein, The lattice change rate of the core-shell structured positive electrode active material before and after complete lithium insertion / extraction is less than 4%.

19. The positive electrode active material having a core-shell structure according to any one of claims 1-5, wherein, The lattice change rate of the core-shell structured positive electrode active material before and after complete lithium insertion / extraction is less than 3.8%.

20. The positive electrode active material having a core-shell structure according to any one of claims 1-5, wherein, The lattice change rate of the core-shell structured positive electrode active material before and after complete lithium insertion / extraction is 2.0-3.8%.

21. The positive electrode active material having a core-shell structure according to any one of claims 1-5, wherein, The concentration of Li / Mn antisite defects in the core-shell structured positive electrode active material is below 4%.

22. The positive electrode active material having a core-shell structure according to any one of claims 1-5, wherein, The concentration of Li / Mn antisite defects in the core-shell structured positive electrode active material is below 2.2%.

23. The positive electrode active material having a core-shell structure according to any one of claims 1-5, wherein, The Li / Mn antisite defect concentration of the core-shell structured positive electrode active material is 1.5-2.2%.

24. The positive electrode active material having a core-shell structure according to any one of claims 1-5, wherein, The core-shell structured positive electrode active material has a compaction density of 2.2 g / cm³ at 3T. 3 above.

25. The positive electrode active material having a core-shell structure according to any one of claims 1-5, wherein, The core-shell structured positive electrode active material has a compaction density of 2.2 g / cm³ at 3T. 3 Above and 2.8 g / cm 3 the following.

26. The positive electrode active material having a core-shell structure according to any one of claims 1-5, wherein, The surface oxygen valence state of the core-shell structured positive electrode active material is below -1.

90.

27. The positive electrode active material having a core-shell structure according to any one of claims 1-5, wherein, The surface oxygen valence state of the core-shell structured positive electrode active material is -1.90 to -1.

98.

28. A method for preparing a positive electrode active material, comprising the following steps: The steps for providing the core material: the core has the chemical formula Li 1+x Mn 1-y A y P 1-z R z O4, where x is any value in the range of -0.100 to 0.100, y is any value in the range of 0.001 to 0.500, z is any value in the range of 0.001 to 0.100, A is one or more elements selected from Zn, Al, Na, K, Mg, Mo, W, Ti, V, Zr, Fe, Ni, Co, Ga, Sn, Sb, Nb and Ge, and R is one or more elements selected from B, Si, N and S; Coating steps: Provide Li separately a MP2O7 and / or M b (P2O7) c And an XPO4 suspension, the core material is added to the above suspension and mixed, and then sintered to obtain a positive electrode active material, wherein 0≤a≤2, 1≤b≤4, 1≤c≤6, and the values ​​of a, b and c satisfy the following condition: making crystalline pyrophosphate Li a MP2O7 or M b (P2O7) c Maintaining electrical neutrality; each of M is independently selected from one or more elements selected from Fe, Ni, Mg, Co, Cu, Zn, Ti, Ag, Zr, Nb, or Al; and X is selected from one or more elements selected from Li, Fe, Ni, Mg, Co, Cu, Zn, Ti, Ag, Zr, Nb, or Al. The positive electrode active material has a core-shell structure, comprising a core and a shell covering the core. The shell includes a first coating layer covering the core, a second coating layer covering the first coating layer, and a third coating layer covering the second coating layer. The first coating layer comprises crystalline pyrophosphate Li. a MP2O7 and / or M b (P2O7) c The second coating layer comprises crystalline phosphate XPO4, the third coating layer is carbon, the crystallinity of the crystalline pyrophosphate is 50%-100%, and the crystallinity of the crystalline phosphate is 50%-100%.

29. The method for preparing the positive electrode active material according to claim 28, wherein the step of providing the core material includes the following steps: Step (1): Mix and stir the manganese source, dopant of element A and acid in a container to obtain manganese salt particles doped with element A; Step (2): The manganese salt particles doped with element A are mixed with a lithium source, a phosphorus source, and a dopant of element R in a solvent to obtain a slurry. After sintering under an inert gas atmosphere, a core doped with elements A and R is obtained, wherein the core doped with elements A and R is Li. 1+x Mn 1-y A y P 1-z R z O4, where x is any value in the range of -0.100 to 0.100, y is any value in the range of 0.001 to 0.500, z is any value in the range of 0.001 to 0.100, A is one or more elements selected from Zn, Al, Na, K, Mg, Mo, W, Ti, V, Zr, Fe, Ni, Co, Ga, Sn, Sb, Nb and Ge, and R is one or more elements selected from B, Si, N and S.

30. The method for preparing the positive electrode active material according to claim 29, wherein the step of providing the core material includes the following steps: A is one or more elements selected from Fe, Ti, V, Ni, Co, and Mg; and / or R is an element selected from B, Si, N, and S.

31. The method for preparing the positive electrode active material according to claim 29, wherein, Step (1) is performed at a temperature of 20-120°C; and / or The stirring in step (1) is carried out at 400-700 rpm for 1-9 h.

32. The method for preparing the positive electrode active material according to claim 29, wherein, The step (1) is carried out at a temperature of 40-120°C; and / or The stirring in step (1) is carried out at 400-700 rpm for 3-7 h.

33. The method for preparing the positive electrode active material according to claim 29, wherein, Step (2) involves mixing at a temperature of 20-120°C for 1-10 hours.

34. The method for preparing the positive electrode active material according to claim 29, wherein, The step (2) is carried out at a temperature of 40-120℃ for 1-10 h.

35. The method for preparing the positive electrode active material according to any one of claims 29-34, wherein, The dopant of element A is one or more elements selected from the following: elemental form, carbonate, sulfate, chloride, nitrate, organic acid salt, oxide, and hydroxide; and / or, The dopant of element R is one or more of the inorganic acids, organic acids, sulfates, chlorides, nitrates, organic acid salts, oxides, and hydroxides of one or more elements selected from B, Si, N, and S.

36. The method for preparing the positive electrode active material according to any one of claims 28-34, wherein, The coating step includes: First coating step: Dissolve the source of element M, phosphorus source, acid, and optionally lithium source in a solvent to obtain a first coating layer suspension; thoroughly mix the core obtained in the core step with the first coating layer suspension obtained in the first coating step, dry, and then sinter to obtain the material coated by the first coating layer. Second coating step: Dissolve the source of element X, phosphorus source and acid in solvent to obtain a second coating layer suspension; mix the material coated by the first coating layer obtained in the first coating step with the second coating layer suspension obtained in the second coating step, dry, and then sinter to obtain a material coated by two coating layers. The third coating step: Dissolve the carbon source in a solvent to obtain a third coating layer solution; then add the material coated by the two coating layers obtained in the second coating step to the third coating layer solution, mix evenly, dry, and then sinter to obtain a material coated by the three coating layers, i.e., the positive electrode active material.

37. The method for preparing the positive electrode active material according to claim 36, wherein, In the first coating step, the pH of the solution containing the dissolved source of element M, phosphorus source, acid, and optionally lithium source is controlled to be 3.5-6.5, then stirred and reacted for 1-5 hours, then the solution is heated to 50-120°C and maintained at that temperature for 2-10 hours, and / or, The sintering is carried out at 650-800℃ for 2-6 hours.

38. The method for preparing the positive electrode active material according to claim 36, wherein, In the second coating step, the source of element X, the phosphorus source, and the acid are dissolved in a solvent, stirred, and reacted for 1-10 h. Then, the solution is heated to 60-150°C and maintained at that temperature for 2-10 h, and / or... Sintering is carried out at 500-700℃ for 6-10 hours.

39. The method for preparing the positive electrode active material according to claim 36, wherein, The sintering in the third coating step is carried out at 700-800℃ for 6-10 hours.

40. A positive electrode sheet comprising a positive current collector and a positive electrode film layer disposed on at least one surface of the positive current collector, the positive electrode film layer comprising a positive electrode active material having a core-shell structure as described in any one of claims 1-27 or a positive electrode active material prepared by the method for preparing a positive electrode active material as described in any one of claims 28-39, and the positive electrode active material having a content of 90-99.5% by weight in the positive electrode film layer based on the total weight of the positive electrode film layer.

41. The positive electrode sheet according to claim 40, wherein, The content of the positive electrode active material is 95-99.5% by weight, based on the total weight of the positive electrode film.

42. A secondary battery comprising a positive electrode active material having a core-shell structure as described in any one of claims 1-27, or a positive electrode active material prepared by the method for preparing a positive electrode active material as described in any one of claims 28-39, or a positive electrode sheet as described in claim 40.

43. A battery module comprising the secondary battery of claim 42.

44. A battery pack comprising the battery module of claim 43.

45. An electrical device comprising at least one selected from the secondary battery of claim 42, the battery module of claim 43, or the battery pack of claim 44.

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