Positive electrode active material, method for preparing the same, and positive electrode sheet, secondary battery, and electric device comprising the same

Abstract

The present application provides a positive electrode active material, a preparation method thereof, a positive electrode sheet comprising the same, a secondary battery, and an electric device. The positive electrode active material comprises an inner core and a shell covering the inner core, the inner core comprises Li m A x Mn I‑y B y P 1‑z C z O 4‑n D n , and the shell comprises a crystalline pyrophosphate Li a MP2O7 and / or M b (P2O7). The positive electrode active material of the present application can make the secondary battery have a higher energy density, and meanwhile improve the cycle performance, safety performance, and / or rate performance. p B q O r and carbon, and the shell comprises one or more coating layers, each coating layer independently comprises one or more of a crystalline pyrophosphate Li a MP2O7 and / or M b (P2O7) c , a crystalline phosphate XPO4, a crystalline borate Y p BqO r and carbon. The positive electrode active material of the present application can make the secondary battery have a higher energy density, and meanwhile improve the cycle performance, safety performance, and / or rate performance.

Description

Technical Field

[0001] This application belongs to the field of battery technology, specifically relating to a positive electrode active material, its preparation method, and a positive electrode sheet containing the same, a secondary battery, and an electrical device. Background Technology

[0002] In recent years, rechargeable batteries have been widely used in energy storage power systems such as hydropower, thermal power, wind power, and solar power plants, as well as in power tools, electric bicycles, electric motorcycles, electric cars, military equipment, aerospace, and many other fields. With the application and promotion of rechargeable batteries, their safety performance has received increasing attention. Lithium manganese phosphate has become one of the most popular cathode active materials due to its advantages such as high capacity, good safety performance, and abundant raw material sources. However, lithium manganese phosphate is prone to manganese ion dissolution during charging, leading to rapid capacity decay. Summary of the Invention

[0003] The purpose of this application is to provide a positive electrode active material, a method for preparing the same, and a positive electrode sheet, a secondary battery, and an electrical device comprising the same, which enables the secondary battery using the positive electrode active material to have a high energy density while also improving cycle performance, safety performance, and / or rate performance.

[0004] The first aspect of this application provides a positive electrode active material with a core-shell structure, comprising a core and a shell covering the core.

[0005] The kernel includes Li m A x Mn 1-y B y P 1-z C z O 4-n D nThe A comprises one or more elements selected from Group IA, Group IIA, Group IIIA, Group IIB, Group VB, and Group VIB, and optionally includes one or more elements selected from Zn, Al, Na, K, Mg, Nb, Mo, and W. The B comprises one or more elements selected from Group IA, Group IIA, Group IIIA, Group IVA, Group VA, Group IIB, Group IVB, Group VB, Group VIB, and Group VIIIB, and optionally includes one or more elements selected from Ti, V, Zr, Fe, Ni, Mg, Co, Ga, Sn, Sb, Nb, and Ge. The C comprises one or more elements selected from Group IIIA, Group IVA, Group VA, and Group VIA, and optionally includes... The core comprises one or more elements selected from B (boron), S, Si, and N, wherein D comprises one or more elements selected from groups VIA and VIIA, and optionally comprises one or more elements selected from S, F, Cl, and Br, wherein m is selected from the range of 0.900 to 1.100, and optionally from the range of 0.995 to 1.002, wherein x is selected from the range of 0 to 0.100, and optionally from the range of 0.001 to 0.005, wherein y is selected from the range of 0.001 to 0.500, wherein z is selected from the range of 0.001 to 0.100, wherein n is selected from the range of 0 to 0.100, and optionally from the range of 0.001 to 0.005, and wherein the core is electrically neutral.

[0006] The shell comprises crystalline pyrophosphate Li a MP2O7 and / or M b (P2O7) c , Crystalline phosphate XPO4, Crystalline borate Y p B q O r And carbon, and the shell comprises one or more coating layers, each coating layer independently comprising crystalline pyrophosphate Li. a MP2O7 and / or M b (P2O7) c , Crystalline phosphate XPO4, Crystalline borate Y p B q O r And one or more of carbon, crystalline pyrophosphate Li a MP2O7 and / or M b (P2O7) cEach M in the series independently includes one or more metallic elements selected from transition metals, Group IA, Group IIA, Group IIIA, Group IVA, Group VA, and the lanthanides, and optionally includes one or more elements selected from Fe, Ni, Mg, Mn, Co, Cu, Zn, Ti, Ag, Zr, Nb, Sb, and Al, where 0 ≤ a ≤ 2, 1 ≤ b ≤ 4, and 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 To maintain electrical neutrality, X comprises one or more metallic elements selected from transition metals, Group IA, Group IIA, Group IIIA, Group IVA, Group VA, and the lanthanides, optionally including one or more elements selected from Li, Fe, Ni, Mg, Mn, Co, Cu, Zn, Ti, Ag, Zr, Nb, Sb, and Al; Y comprises one or more metallic elements selected from transition metals, Group IA, Group IIA, Group IIIA, Group IVA, Group VA, and the lanthanides, optionally including one or more elements selected from Li, Fe, Ni, Mg, Mn, Co, Cu, Zn, Ti, Ag, Zr, Nb, Sb, and Al, 1≤p≤4, 1≤q≤7, 2≤r≤12, and the values ​​of p, q, and r satisfy the following condition: to make crystalline borate Y... p B q O r Maintain electrical neutrality.

[0007] After extensive research, the inventors discovered that by modifying and coating lithium manganese phosphate, a novel core-shell structured positive electrode active material can be obtained. This positive electrode active material can significantly reduce manganese ion dissolution and lattice change rate. When used in secondary batteries, it can improve the cycle performance, rate performance, safety performance, and capacity utilization of secondary batteries.

[0008] In any embodiment of this application, the crystalline pyrophosphate Li a MP2O7 and / or M b (P2O7) c The interplanar spacing ranges from 0.293 nm to 0.470 nm, and the included angle of the crystal orientation (111) ranges from 18.00° to 32.00°.

[0009] In any embodiment of this application, the interplanar spacing of the crystalline phosphate XPO4 ranges from 0.244 nm to 0.425 nm, and the included angle of the crystal orientation (111) ranges from 20.00° to 37.00°.

[0010] 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 the dissolution of manganese ions during lithium insertion / extraction, thereby improving the high-temperature cycle performance and high-temperature storage performance of secondary batteries.

[0011] In any embodiment of this application, the carbon is a mixture of SP2 carbon and SP3 carbon. Optionally, the molar ratio of SP2 carbon to SP3 carbon is any value within the range of 0.1 to 10, and more preferably any value within the range of 2.0 to 3.0. By limiting the molar ratio of SP2 carbon to SP3 carbon to the above range, this application improves the overall performance of the secondary battery.

[0012] In any embodiment of this application, 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.

[0013] In any embodiment of this application, in the core, the ratio of z to 1-z is 1:9 to 1:999, optionally 1:499 to 1:249. This further improves the cycle performance and rate performance of the secondary battery.

[0014] In any embodiment of this application, in the core, B comprises one or more elements selected from Fe, Ti, V, Ni, Co, and Mg, and optionally includes at least two elements selected from Fe, Ti, V, Ni, Co, and Mg. Simultaneous doping of two or more of the above elements at the Mn site is beneficial for enhancing the doping effect, further reducing the lattice change rate on the one hand, and further reducing surface oxygen activity on the other.

[0015] In any embodiment of this application, the C in the core includes an element selected from B (boron), S, Si, and N. This can further improve the rate performance of the secondary battery.

[0016] In any embodiment of this application, q:r is 1:3.

[0017] In any embodiment of this application, the shell includes a first coating layer covering the core, a second coating layer covering the first coating layer, a third coating layer covering the second coating layer, and a fourth coating layer covering the third coating layer. The fourth coating layer comprises carbon, and the first coating layer, the second coating layer, and the third coating layer each independently comprise crystalline pyrophosphate Li. a MP2O7 and / or M b (P2O7) c crystalline phosphate XPO4 and crystalline borate Y p Bq O r Any one of them.

[0018] In any embodiment of this application, the first coating layer comprises crystalline pyrophosphate Li a MP2O7 and / or M b (P2O7) c The fourth coating layer comprises carbon, and the second and third coating layers each independently comprise crystalline phosphate XPO4 or crystalline borate Y. p B q O r Optionally, 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 comprises crystalline borate Y. p B q O r The fourth coating layer comprises carbon. This can further improve the cycle performance, safety performance, and / or rate performance of the secondary battery.

[0019] In any embodiment of this application, the coating amount of the first coating layer is greater than 0 and less than or equal to 6% by weight, and optionally greater than 0 and less than or equal to 5.5% by weight, based on the weight of the core.

[0020] In any embodiment of this application, the coating amount of the second coating layer is greater than 0 and less than or equal to 6% by weight, and optionally greater than 0 and less than or equal to 5.5% by weight, based on the weight of the core.

[0021] In any embodiment of this application, the coating amount of the third coating layer is greater than 0 and less than or equal to 6% by weight, and optionally greater than 0 and less than or equal to 5.5% by weight, based on the weight of the core.

[0022] In any embodiment of this application, the coating amount of the fourth 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.

[0023] The coating amount of the four coating layers is preferably within the above range, thereby enabling sufficient coating of the core and further improving the cycle performance, safety performance, and / or rate performance of the secondary battery without sacrificing the specific capacity of the positive electrode active material.

[0024] In any embodiment of this application, the thickness of the first coating layer is 1 nm to 15 nm.

[0025] In any embodiment of this application, the thickness of the second coating layer is 1 nm to 15 nm.

[0026] In any embodiment of this application, the thickness of the third coating layer is 1 nm to 15 nm.

[0027] In any embodiment of this application, the thickness of the fourth coating layer is 2 nm to 25 nm.

[0028] The thickness of the four coating layers is preferably within the above-mentioned range, thereby enabling sufficient coating of the core and further improving the cycle performance, safety performance, and / or rate performance of the secondary battery without sacrificing the specific capacity of the positive electrode active material.

[0029] In any embodiment of this application, based on the weight of the positive electrode active material, the manganese content is in the range of 10% to 35% by weight, preferably in the range of 15% to 30% by weight, and more preferably in the range of 17% to 20% by weight. This effectively avoids problems such as decreased structural stability and reduced density of the positive electrode active material that may be caused by excessive manganese content, thereby improving the cycle life, storage, and compaction density performance of the secondary battery; and also avoids problems such as a low voltage plateau that may be caused by insufficient manganese content, thereby improving the energy density of the secondary battery.

[0030] In any embodiment of this application, based on the weight of the positive electrode active material, the phosphorus content is in the range of 12% to 25% by weight, and optionally in the range of 15% to 20% by weight. This effectively avoids the following situations: if the phosphorus content is too high, it may lead to excessive covalentity of PO, affecting the conductivity of small polarons and thus the conductivity of the positive electrode active material; if the phosphorus content is too low, it may reduce the stability of the pyrophosphate and / or phosphate lattice structure in the core and shell, thereby affecting the overall stability of the positive electrode active material.

[0031] In any embodiment of this application, based on the weight of the positive electrode active material, the weight ratio of manganese to phosphorus ranges from 0.90 to 1.25, and can be optionally from 0.95 to 1.20. This effectively avoids the following situations: if the weight ratio is too high, it may lead to increased manganese ion dissolution, affecting the stability of the positive electrode active material and the cycle performance and storage performance of the secondary battery; if the weight ratio is too low, it may cause a decrease in the discharge voltage plateau of the positive electrode active material, thereby reducing the energy density of the secondary battery.

[0032] In any embodiment of this application, the lattice change rate of the 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% to 3.8%. In this case, the positive electrode active material can improve the capacity utilization and rate performance of the secondary battery.

[0033] In any embodiment of this application, the Li / Mn antisite defect concentration of the positive electrode active material is 4% or less, optionally 2.2% or less, and more preferably 1.5% to 2.2%. By keeping the Li / Mn antisite defect concentration within the above range, Mn... 2+ Hinder Li + This improves the transmission efficiency and enhances the capacity utilization and rate performance of the positive electrode active material.

[0034] In any embodiment of this application, the compaction density of the positive electrode active material at 3T is 2.2 g / cm³. 3 The above can be selected as 2.2g / cm. 3 Above and 2.8g / cm 3 The following is a summary. Therefore, this is beneficial for improving the volumetric energy density of secondary batteries.

[0035] In any embodiment of this application, the surface oxygen valence state of the positive electrode active material is below -1.90, and can be selected as -1.90 to -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 active material and the electrolyte can be reduced, thereby improving the cycle performance and storage performance of the secondary battery.

[0036] The second aspect of this application provides a method for preparing a positive electrode active material, comprising the following steps of providing a core material and a coating step.

[0037] Steps for providing kernel material: The kernel includes Li m A x Mn 1-y B y P 1-z C z O 4-n D nThe A comprises one or more elements selected from Group IA, Group IIA, Group IIIA, Group IIB, Group VB, and Group VIB, and optionally includes one or more elements selected from Zn, Al, Na, K, Mg, Nb, Mo, and W. The B comprises one or more elements selected from Group IA, Group IIA, Group IIIA, Group IVA, Group VA, Group IIB, Group IVB, Group VB, Group VIB, and Group VIIIB, and optionally includes one or more elements selected from Ti, V, Zr, Fe, Ni, Mg, Co, Ga, Sn, Sb, Nb, and Ge. The C comprises one or more elements selected from Group IIIA, Group IVA, Group VA, and Group VIA, and optionally includes... The core comprises one or more elements selected from B (boron), S, Si, and N, wherein D comprises one or more elements selected from groups VIA and VIIA, and optionally comprises one or more elements selected from S, F, Cl, and Br, wherein m is selected from the range of 0.900 to 1.100, and optionally from the range of 0.995 to 1.002, wherein x is selected from the range of 0 to 0.100, and optionally from the range of 0.001 to 0.005, wherein y is selected from the range of 0.001 to 0.500, wherein z is selected from the range of 0.001 to 0.100, wherein n is selected from the range of 0 to 0.100, and optionally from the range of 0.001 to 0.005, and wherein the core is electrically neutral.

[0038] Coating steps: Provide Li pyrophosphate separately. a MP2O7 and / or M b (P2O7) c Phosphate XPO4, borate Y p B q O r A coating solution containing a carbon source is prepared, and the core material is added to and mixed with the coating solution. The mixture is then sintered to obtain a positive electrode active material. The positive electrode active material has a core-shell structure, comprising the core and a shell coating the core. The shell comprises crystalline pyrophosphate Li. a MP2O7 and / or M b (P2O7) c , Crystalline phosphate XPO4, Crystalline borate Y p B q O r And carbon, and the shell comprises one or more coating layers, each coating layer independently comprising crystalline pyrophosphate Li. a MP2O7 and / or M b (P2O7) c , Crystalline phosphate XPO4, Crystalline borate Y p B q O rAnd one or more of carbon, crystalline pyrophosphate Li a MP2O7 and / or M b (P2O7) c Each of the M elements independently includes one or more metallic elements selected from transition metals, Group IA, Group IIA, Group IIIA, Group IVA, Group VA, and the lanthanides, and optionally includes one or more elements selected from Fe, Ni, Mg, Mn, Co, Cu, Zn, Ti, Ag, Zr, Nb, Sb, and Al, where 0 ≤ a ≤ 2, 1 ≤ b ≤ 4, and 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 To maintain electrical neutrality, X comprises one or more metallic elements selected from transition metals, Group IA, Group IIA, Group IIIA, Group IVA, Group VA, and the lanthanides, optionally including one or more elements selected from Li, Fe, Ni, Mg, Mn, Co, Cu, Zn, Ti, Ag, Zr, Nb, Sb, and Al; Y comprises one or more metallic elements selected from transition metals, Group IA, Group IIA, Group IIIA, Group IVA, Group VA, and the lanthanides, optionally including one or more elements selected from Li, Fe, Ni, Mg, Mn, Co, Cu, Zn, Ti, Ag, Zr, Nb, Sb, and Al, 1≤p≤4, 1≤q≤7, 2≤r≤12, and the values ​​of p, q, and r satisfy the following condition: to make crystalline borate Y... p B q O r Maintain electrical neutrality.

[0039] In any embodiment of this application, the step of providing the core material includes the following steps: Step (1): mixing and stirring a manganese source, a dopant of element B and an acid in a container to obtain manganese salt particles doped with element B; Step (2): mixing the manganese salt particles doped with element B with a lithium source, a phosphorus source, a dopant of element C, optionally a dopant of element A and optionally a dopant of element D in a solvent to obtain a slurry, and sintering it under an inert gas atmosphere to obtain the core material.

[0040] In any embodiment of this application, step (1) is performed at a temperature of 20°C to 120°C, optionally 40°C to 120°C.

[0041] In any embodiment of this application, the stirring in step (1) is carried out at 400 rpm to 700 rpm for 1 hour to 9 hours, or optionally 3 hours to 7 hours.

[0042] 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 the dissolution of manganese ions, reduce the 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.

[0043] In any embodiment of this application, step (2) is performed at a temperature of 20°C to 120°C, optionally 40°C to 120°C, for 1 hour to 12 hours.

[0044] In any embodiment of this application, the sintering in step (2) is sintering at 600°C to 950°C for 4 to 10 hours in an inert gas or a mixture of inert gas and hydrogen.

[0045] In any embodiment of this application, the coating step includes coating crystalline pyrophosphate Li a MP2O7 and / or M b (P2O7) c Steps for coating crystalline phosphate XPO4, steps for coating crystalline borate Y p B q O r The steps and the steps of coating carbon.

[0046] Optionally, in any embodiment of this application, the first coating step is to coat crystalline pyrophosphate Li. a MP2O7 and / or M b (P2O7) c The first step is to coat crystalline phosphate XPO4, and the second step is to coat crystalline borate Y. p B q O r The fourth coating step is the carbon coating step, thereby obtaining a positive electrode active material with a core-shell structure, which includes the 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, a third coating layer covering the second coating layer, and a fourth coating layer covering the third coating layer. The first coating layer includes crystalline pyrophosphate Li. a MP2O7 and / or M b (P2O7) c The second coating layer comprises crystalline phosphate XPO4, and the third coating layer comprises crystalline borate Y. p B q O r The fourth coating layer comprises carbon.

[0047] In any embodiment of this application, the coated crystalline pyrophosphate Lia MP2O7 and / or M b (P2O7) c The steps include: adding a source of element M, a phosphorus source, an acid, and optionally a lithium source to a solvent to obtain a coating solution; thoroughly mixing the material to be coated with the coating solution; drying; and then sintering to obtain crystalline pyrophosphate Li. a MP2O7 and / or M b (P2O7) c The covering material.

[0048] In any embodiment of this application, the step of coating crystalline phosphate XPO4 includes the following steps: adding a source of element X, a phosphorus source, and an acid to a solvent to obtain a coating solution; thoroughly mixing the material to be coated with the coating solution; drying; and then sintering to obtain a material coated with crystalline phosphate XPO4.

[0049] In any embodiment of this application, the coated crystalline borate Y p B q O r The steps include: adding a source of element Y and a source of boron to a solvent to obtain a coating solution; thoroughly mixing the material to be coated with the coating solution; drying; and then sintering to obtain crystalline borate Y. p B q O r The covering material.

[0050] In any embodiment of this application, the carbon coating step includes the following steps: adding a carbon source to a solvent to obtain a coating solution, adding the material to be coated to the coating solution, mixing evenly, drying, and then sintering to obtain a carbon-coated material.

[0051] In any embodiment of this application, the coated crystalline pyrophosphate Li a MP2O7 and / or M b (P2O7) c In the steps, 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 to 6.5. The solution is then stirred and reacted for 1 to 5 hours. The solution is then heated to 50°C to 120°C and maintained at that temperature for 2 to 10 hours.

[0052] In any embodiment of this application, the coated crystalline pyrophosphate Li a MP2O7 and / or M b (P2O7) c The sintering process in this step is carried out at 650°C to 800°C for 2 to 6 hours.

[0053] In any embodiment of this application, in the step of coating crystalline phosphate XPO4, the source of element X, the phosphorus source and the acid are dissolved in a solvent, stirred and reacted for 1 to 10 hours, and then the solution is heated to 60°C to 150°C and maintained at that temperature for 2 to 10 hours.

[0054] In any embodiment of this application, the sintering in the step of coating crystalline phosphate XPO4 is carried out at 500°C to 700°C for 6 to 10 hours.

[0055] In any embodiment of this application, the coated crystalline borate Y p B q O r The sintering process in this step is carried out at 300°C to 500°C for 2 to 10 hours.

[0056] In any embodiment of this application, the sintering in the carbon coating step is carried out at 700°C to 800°C for 6 to 10 hours.

[0057] By controlling the conditions of the above coating steps within the aforementioned range, it is possible to guarantee or even improve the capacity performance, cycle performance, high-temperature storage performance, and rate performance of secondary batteries prepared using the aforementioned positive electrode active material.

[0058] 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.

[0059] A third aspect of this application provides a positive electrode sheet, which includes 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 includes the positive electrode active material of the first aspect of this application or the positive electrode active material prepared by the method of the second aspect of this application, and the content of the positive electrode active material in the positive electrode film layer is more than 10% by weight, optionally from 90% to 99.5% by weight, based on the total weight of the positive electrode film layer.

[0060] The positive electrode sheet of this application, when used in secondary batteries, can improve the energy density, cycle performance, safety performance, and / or rate performance of secondary batteries.

[0061] The fourth aspect of this application provides a secondary battery, including the positive electrode active material of the first aspect of this application, or the positive electrode active material prepared by the method of the second aspect of this application, or the positive electrode sheet of the third aspect of this application.

[0062] The fifth aspect of this application provides an electrical device, including the secondary battery of the fourth aspect of this application.

[0063] The positive electrode sheet, secondary battery, and electrical device of this application include the positive electrode active material of this application, and therefore have at least the same advantages as the positive electrode active material. Attached Figure Description

[0064] To more clearly illustrate the technical solutions of the embodiments of this application, the accompanying drawings used in the embodiments of this application will be briefly described below. Obviously, the drawings described below are only some embodiments of this application. For those skilled in the art, other drawings can be obtained based on the drawings without creative effort.

[0065] Figure 1 This is a schematic diagram of one embodiment of the battery cell of this application.

[0066] Figure 2 This is an exploded view of one embodiment of the battery cell of this application.

[0067] Figure 3 This is a schematic diagram of one embodiment of the battery module of this application.

[0068] Figure 4 This is a schematic diagram of one embodiment of the battery pack of this application.

[0069] Figure 5 yes Figure 4 An exploded view of an embodiment of the battery pack shown.

[0070] Figure 6 This is a schematic diagram of one embodiment of an electrical device that uses a secondary battery as a power source, as described in this application.

[0071] The accompanying drawings are not necessarily drawn to scale. The reference numerals are explained as follows: 1 Battery pack, 2 Upper casing, 3 Lower casing, 4 Battery module, 5 Individual battery cell, 51 Housing, 52 Electrode assembly, 53 Cover plate. Detailed Implementation

[0072] The following detailed description, with appropriate reference to the accompanying drawings, discloses the positive electrode active material, its preparation method, and embodiments of the positive electrode sheet, secondary battery, and power-consuming device comprising the same. 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 to enable those skilled in the art to fully understand this application and are not intended to limit the subject matter of the claims.

[0073] 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 particular 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 included. Furthermore, if minimum range values ​​of 1 and 2 are listed, and if maximum range values ​​of 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.

[0074] Unless otherwise specified, all embodiments and optional embodiments of this application may be combined with each other to form new technical solutions, and such technical solutions should be considered to be included in the disclosure of this application.

[0075] Unless otherwise specified, all technical features and optional technical features of this application may be combined to form new technical solutions, and such technical solutions shall be deemed to be included in the disclosure of this application.

[0076] Unless otherwise specified, all steps in this application may be performed sequentially or randomly, preferably 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 mention that the method may also include step (c) indicates 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.

[0077] 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.

[0078] 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).

[0079] In this article, the terms "multiple", "various", and "multi-layered" refer to two, two kinds, or more than two layers.

[0080] In this article, “about” refers to a range of values, specifically the range of ±10% of that value.

[0081] In this document, the term "coating layer" refers to a layer of material coating the lithium manganese phosphate core, which may completely or partially coat the lithium manganese phosphate core. The use of "coating layer" is for descriptive purposes only and is not intended to limit the invention. Furthermore, each coating layer may be a complete or partial coating. Similarly, the term "coating layer thickness" refers to the thickness of the material coating the lithium manganese phosphate core in the radial direction of the lithium manganese phosphate core.

[0082] In this document, the median particle size Dv50 refers to the particle size corresponding to a cumulative volume distribution percentage of 50% in 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.

[0083] The inventors of this application discovered in practical work that existing lithium manganese phosphate (LiMnPO4) cathode active materials suffer from significant manganese ion dissolution 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 dissolved manganese ions from migrating further into the electrolyte. After migrating to the negative electrode, the dissolved manganese ions are 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, which also has an irreversible impact on the capacity retention rate of the secondary battery.

[0084] After extensive research, the inventors discovered that by doping and modifying lithium manganese phosphate and coating it, a novel positive electrode active material with a core-shell structure can be obtained. This positive electrode active material can significantly reduce manganese ion dissolution and reduce lattice change rate. When used in secondary batteries, it can improve the cycle performance, rate performance, safety performance, and capacity utilization of secondary batteries.

[0085] Positive electrode active material

[0086] Specifically, the first aspect of this application proposes a positive electrode active material with a core-shell structure, comprising a core and a shell covering the core.

[0087] The kernel includes Li m A x Mn 1-y B y P 1-z C z O 4-n D n The A includes one or more elements selected from families IA, IIA, IIIA, IIB, VB, and VIB; the B includes one or more elements selected from families IA, IIA, IIIA, IVA, VA, IIB, IVB, VB, VIB, and VIIIB; the C includes one or more elements selected from families IIIA, IVA, VA, and VIA; the D includes one or more elements selected from families VIA and VIIA; the m is selected from the range of 0.900 to 1.100; the x is selected from the range of 0 to 0.100; the y is selected from the range of 0.001 to 0.500; the z is selected from the range of 0.001 to 0.100; the n is selected from the range of 0 to 0.100; and the kernel is electrically neutral.

[0088] The shell comprises crystalline pyrophosphate Li a MP2O7 and / or M b (P2O7) c , Crystalline phosphate XPO4, Crystalline borate Y p B q O r And carbon, and the shell comprises one or more coating layers, each coating layer independently comprising crystalline pyrophosphate Li. a MP2O7 and / or M b (P2O7) c , Crystalline phosphate XPO4, Crystalline borate Y p B q O rAnd one or more of carbon. Crystalline pyrophosphate Li a MP2O7 and / or M b (P2O7) c Each of the M elements independently comprises one or more metallic elements selected from transition metals, Group IA, Group IIA, Group IIIA, Group IVA, Group VA, and the lanthanides, where 0 ≤ a ≤ 2, 1 ≤ b ≤ 4, and 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. X comprises one or more metallic elements selected from transition metals, Group IA, Group IIA, Group IIIA, Group IVA, Group VA, and the lanthanides. Y comprises one or more metallic elements selected from transition metals, Group IA, Group IIA, Group IIIA, Group IVA, Group VA, and the lanthanides, 1≤p≤4, 1≤q≤7, 2≤r≤12, and the values ​​of p, q, and r satisfy the following condition: making the crystalline borate Y... p B q O r Maintain electrical neutrality.

[0089] Unless otherwise stated, in the above chemical formulas, when A consists of two or more elements, the limitation on the range of x values ​​described above 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 consists of two or more elements A1, A2...An, the stoichiometric coefficients x1, x2...xn of each of A1, A2...An must each fall within the range of x values ​​defined in this application, and the sum of x1, x2...xn must also fall within this range. Similarly, for the case where B, C, and D consist of two or more elements, the limitation on the range of stoichiometric coefficients of B, C, and D in this application also has the above meaning.

[0090] In an optional implementation, when B is one, two, three, or four elements selected from the above range, B y For Q n1 D n2 E n3 K n4 Let 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 the above range. 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 m Ax Mn 1-y B y P 1-z C z O 4-n D n In this process, it is advantageous to dope one, two, three or four of the aforementioned B elements at the Mn site, and optionally, one, two or three of the aforementioned B elements are doped. In addition, it is advantageous to dope one or two C elements at the P site, which is beneficial for making the doped elements uniformly distributed.

[0091] The kernel Li m A x Mn 1-y B y P 1-z C z O 4-n D n In the process, the value of m is affected by the valence state and doping amount of dopants A, B, C, and D to ensure the overall core system is electrically neutral. If the value of m is too small, the lithium content of the entire core system will decrease, affecting the capacity of the cathode active material. Therefore, the value of m is limited to 0.900 to 1.100. The value of y limits the total amount of all dopants at the Mn site. If y is too small, i.e., the doping amount is too low, 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 cathode active material. Therefore, the value of y is limited to 0.001 to 0.500. The C element is doped at the P site. Since the PO tetrahedron is relatively stable, and the z value is too large, it will affect the stability of the cathode active material. Therefore, the z value is limited to 0.001 to 0.100.

[0092] In the kernel, m is selected from the range of 0.900 to 1.100, for example, m can be 0.900, 0.991, 0.992, 0.993, 0.994, 0.995, 0.996, 0.997, 0.998, 1.000, 1.001, 1.002, 1.003, 1.004, 1.005, or 1.006. Optionally, m is selected from the range of 0.995 to 1.002.

[0093] In the kernel, x is selected from the range of 0 to 0.100, for example, x can be 0, 0.001, 0.005, 0.08, or 0.1. Optionally, x is selected from the range of 0.001 to 0.100 or 0.001 to 0.005.

[0094] In the kernel, y is selected from the range of 0.001 to 0.500, for example, y can be 0.001, 0.100, 0.200, 0.250, 0.300, 0.350, 0.400, 0.450, or 0.500.

[0095] In the kernel, z is selected from the range of 0.001 to 0.100, for example, z can be 0.001, 0.002, 0.003, 0.004, 0.005, or 0.100.

[0096] In the kernel, n is selected from the range of 0 to 0.100, for example, n can be 0, 0.001, 0.005, 0.08, or 0.1. Optionally, n is selected from the range of 0.001 to 0.100 or 0.001 to 0.005. Optionally, n is selected from the range of 0.001 to 0.100 or 0.001 to 0.005.

[0097] The x is selected from the range of 0 to 0.100, and the n is selected from the range of 0 to 0.100, meaning that the Li and O sites of lithium manganese phosphate may or may not be doped.

[0098] In some embodiments, x may optionally be selected from the range of 0.001 to 0.100, i.e., element A is doped at the Li site of lithium manganese phosphate.

[0099] In some embodiments, n may optionally be selected from the range of 0.001 to 0.100, i.e., element D is doped at the O site of lithium manganese phosphate.

[0100] In some embodiments, x may optionally be selected from the range of 0.001 to 0.100, and n may be selected from the range of 0.001 to 0.1, meaning that both the Li and O sites of lithium manganese phosphate are simultaneously doped.

[0101] The positive electrode active material of this application can improve the capacity utilization, cycle performance, and safety performance of secondary batteries. Although the mechanism is not yet clear, it is speculated that the positive electrode active material of this application may have a special core-shell structure. By doping the lithium manganese phosphate core, not only can the dissolution of manganese ions be effectively reduced, thereby reducing the number of manganese ions migrating to the negative electrode and reducing the electrolyte consumption due to SEI film decomposition, thus improving the cycle performance and safety performance of the secondary battery, but it can 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. The shell comprises crystalline pyrophosphate Li a MP2O7 and / or M b (P2O7) c , Crystalline phosphate XPO4, Crystalline borate Y p B q O rThe secondary battery comprises carbon, and the shell includes one or more coating layers. Crystalline pyrophosphate can further increase the migration resistance of manganese ions, reduce their dissolution, and reduce the surface lithium content and the contact between the core and the electrolyte, thereby reducing interfacial side reactions, reducing gas production, and improving the high-temperature storage performance, cycle performance, and safety performance of the secondary battery. Crystalline phosphate can effectively reduce the interfacial side reactions between the positive electrode active material and the electrolyte, thereby improving the high-temperature cycle and storage performance of the secondary battery. Crystalline borate has excellent lithium-ion and electron conduction capabilities, which can further reduce manganese ion dissolution and reduce the surface lithium content, thereby further reducing the interfacial side reactions between the positive electrode active material and the electrolyte, while further improving the high-temperature cycle and storage performance of the secondary battery. Carbon can effectively improve the conductivity and desolvation capability of the positive electrode active material, thereby further improving the safety and kinetic performance of the secondary battery.

[0102] Furthermore, in the core, element B, doped at the Mn site of lithium manganese phosphate, helps to reduce the lattice change rate of lithium manganese phosphate during lithium insertion / extraction, improves the structural stability of the lithium manganese phosphate cathode active material, greatly reduces the dissolution of manganese ions, and reduces the oxygen activity on the particle surface; element C, doped at the P site, 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; element A, doped at the Li site, also helps to reduce the lattice change rate of lithium manganese phosphate during lithium insertion / extraction; and element D, doped at the O site, helps to reduce interfacial side reactions.

[0103] 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 or form impurities within the crystal lattice. Maintaining electrical neutrality minimizes such impurities. Additionally, ensuring system electrical neutrality can, in some cases, generate lithium vacancies in the cathode active material, thereby improving its kinetic performance.

[0104] Therefore, positive electrode sheets and electrical devices such as secondary batteries using the positive electrode active material of this application can have high energy density and improve cycle performance, safety performance, and / or rate performance.

[0105] In some embodiments, optionally, A includes one or more elements selected from Zn, Al, Na, K, Mg, Nb, Mo, and W. By selecting the Li-site doping element within the above range, the lattice change rate can be further reduced, thereby further improving the rate performance of the secondary battery.

[0106] In some embodiments, optionally, B includes one or more elements selected from Ti, V, Zr, Fe, Ni, Mg, Co, Ga, Sn, Sb, Nb, and Ge; more preferably, B includes one or more elements selected from Fe, Ti, V, Ni, Co, and Mg; and even more preferably, B includes at least two elements selected from Fe, Ti, V, Ni, Co, and Mg. Simultaneous doping of two or more of the above-mentioned elements at the Mn site in the lithium manganese phosphate cathode active material is beneficial to enhancing the doping effect. On the one hand, it further reduces the lattice change rate, thereby suppressing the dissolution of manganese ions and reducing the consumption of electrolyte and active lithium ions. On the other hand, it also helps to further reduce surface oxygen activity, reducing interfacial side reactions between the cathode active material and the electrolyte, thereby improving the cycle performance and high-temperature storage performance of the secondary battery.

[0107] In some embodiments, optionally, the C includes one or more elements selected from B (boron), S, Si, and N; more preferably, the C includes one element selected from B (boron), S, Si, and N. By selecting the p-site doping element within the above range, the rate performance of the secondary battery can be further improved.

[0108] In some embodiments, the D may optionally include one or more elements selected from S, F, Cl and Br. By selecting the doping element at the O site within the above range, the side reactions at the interface can be further reduced and the high-temperature stability of the secondary battery can be improved.

[0109] In some embodiments, the M may optionally include one or more elements selected from Fe, Ni, Mg, Mn, Co, Cu, Zn, Ti, Ag, Zr, Nb, Sb, and Al.

[0110] In some embodiments, X may optionally include one or more elements selected from Li, Fe, Ni, Mg, Mn, Co, Cu, Zn, Ti, Ag, Zr, Nb, Sb, and Al.

[0111] In some embodiments, the Y may optionally include one or more elements selected from Li, Fe, Ni, Mg, Mn, Co, Cu, Zn, Ti, Ag, Zr, Nb, Sb, and Al.

[0112] 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 B dopants at the Mn sites. 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.

[0113] 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 C elements at the p-site dopant sites. 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.

[0114] In some implementations, q:r is 1:3.

[0115] The cores prepared in this application have an average particle size ranging from 50 nm to 500 nm, and a Dv50 ranging from 200 nm to 300 nm. The primary particle size of the cores is consistently within the range of 50 nm to 500 nm, with a Dv50 of 200 nm to 300 nm. If the average particle size of the cores is too large (exceeding 500 nm), the 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.

[0116] 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 X-ray diffraction (XRD) pattern of the doped lithium manganese phosphate of this application are consistent with those of undoped LiMnPO4, indicating that no impurity phase was introduced during the doping process. Therefore, the improvement in core performance mainly comes from elemental doping, rather than impurity phases. After preparing the cathode active material described in this application, the inventors used focused ion beam (FIB) to cut the middle region (core region) of the prepared cathode 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.

[0117] In this application, "crystalline" refers to a crystallinity of 50% or higher, i.e., 50% to 100%. Crystallinity less than 50% is referred to as glassy (or amorphous). The crystallinity of the crystalline pyrophosphate, crystalline phosphate, and crystalline borate described in this application is 50% to 100%. Pyrophosphate, phosphate, and borate with a certain degree of crystallinity not only fully utilize the functions of the pyrophosphate coating layer in hindering manganese ion dissolution, the phosphate coating layer in providing excellent lithium-ion conduction, reducing interfacial side reactions, and the borate coating layer in providing excellent lithium-ion and electron conduction, but also enable better lattice matching of the coating layer, thereby achieving a tighter bond.

[0118] In this application, the crystallinity of crystalline pyrophosphate, crystalline phosphate, and crystalline borates can be tested using conventional techniques in the art, such as density methods, infrared spectroscopy, differential scanning calorimetry, and nuclear magnetic resonance absorption methods, or, for example, X-ray diffraction. A specific X-ray diffraction test method may include the following steps: taking a certain amount of positive electrode active material powder, measuring the total scattering intensity using X-rays, which is the sum of the scattering intensities of all matter in space, and is only related to the intensity of primary rays, the chemical structure of the positive electrode active material powder, and the total number and mass of electrons participating in diffraction, but not to the order state of the sample; then separating crystalline scattering and non-crystalline scattering from the diffraction pattern, and the crystallinity is the ratio of the crystalline scattering to the total scattering intensity. It should be noted that, in this application, the crystallinity of the pyrophosphate, phosphate, and borates 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 some embodiments, the crystalline pyrophosphate Li in the shell a MP2O7 and / or M b (P2O7) c The interplanar spacing ranges from 0.293 nm to 0.470 nm, and the included angle of the crystal orientation (111) ranges from 18.00° to 32.00°.

[0120] In some embodiments, the interplanar spacing of the crystalline phosphate XPO4 in the shell ranges from 0.244 nm to 0.425 nm, and the included angle of the crystal orientation (111) ranges from 20.00° to 37.00°.

[0121] Crystalline pyrophosphates and crystalline phosphates can be characterized using conventional techniques in the art, or, for example, by transmission electron microscopy (TEM). Under TEM, the core and cladding layers can be distinguished by measuring the interplanar spacing.

[0122] The specific testing method for the interplanar spacing and angle of crystalline pyrophosphate and crystalline phosphate in the coating layer can include the following steps: Take a certain amount of the coated positive electrode active material sample powder into a test tube, inject a solvent such as alcohol into the test tube, and 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 to obtain the original TEM image. Open the original image obtained from the above TEM test in the diffractometer software, and perform 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 angle can be calculated according to the Bragg equation.

[0123] 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.

[0124] 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 the dissolution of manganese ions during lithium insertion / extraction, thereby improving the high-temperature cycle performance and high-temperature storage performance of secondary batteries.

[0125] In some embodiments, the carbon in the shell 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 to 10, and more preferably any value in the range of 2.0 to 3.0.

[0126] 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.

[0127] By selecting the form of carbon in the shell, the overall electrochemical performance of secondary batteries can be improved. Specifically, by using a mixture of SP2 and SP3 carbon forms and limiting the ratio of SP2 to SP3 carbon within a certain range, the following situations can be effectively avoided: if the carbon in the shell 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-ion insertion and 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 improving the kinetic and cycle performance of secondary batteries.

[0128] The mixing ratio of the SP2 and SP3 forms of carbon can be controlled by sintering conditions such as sintering temperature and sintering time. For example, when using sucrose as a carbon source, pyrolysis of the sucrose at high temperature, followed by deposition under high temperature, will produce carbon in both SP2 and SP3 forms. The ratio of SP2 to SP3 carbon can be adjusted by selecting high-temperature pyrolysis and sintering conditions.

[0129] The structure and characteristics of carbon can be determined by Raman spectroscopy. The specific testing method is as follows: by separating the peaks of the Raman spectrum, I0 can be obtained. d / I g (I d For the peak intensity of SP3 carbon, I g(The peak intensity of the SP2 form of carbon) was used to confirm the molar ratio between the two.

[0130] In some embodiments, the shell comprises multiple coating layers, and each coating layer independently comprises crystalline pyrophosphate Li. a MP2O7 and / or M b (P2O7) c , Crystalline phosphate XPO4, Crystalline borate Y p B q O r And one or more of carbon.

[0131] In some embodiments, the shell includes a first coating layer covering the core, a second coating layer covering the first coating layer, a third coating layer covering the second coating layer, and a fourth coating layer covering the third coating layer, the fourth coating layer comprising carbon, and the first coating layer, the second coating layer, and the third coating layer each independently comprising crystalline pyrophosphate Li. a MP2O7 and / or M b (P2O7) c crystalline phosphate XPO4 and crystalline borate Y p B q O r Any one of them.

[0132] For example, the first coating layer includes crystalline pyrophosphate Li a MP2O7 and / or M b (P2O7) c The second coating layer comprises crystalline phosphate XPO4, and the third coating layer comprises crystalline borate Y. p B q O r Alternatively, the first coating layer comprises crystalline pyrophosphate Li. a MP2O7 and / or M b (P2O7) c The second coating layer comprises crystalline borate Y p B q O r The third coating layer comprises crystalline phosphate XPO4. Alternatively, the first coating layer comprises crystalline phosphate XPO4, and the second coating layer comprises crystalline borate Y. p B q O r The third coating layer comprises crystalline pyrophosphate Li a MP2O7 and / or M b (P2O7) cAlternatively, the first coating layer comprises crystalline phosphate XPO4, and the second coating layer comprises crystalline pyrophosphate Li. a MP2O7 and / or M b (P2O7) c The third coating layer comprises crystalline borate Y p B q O r Alternatively, the first coating layer comprises crystalline borate Y. p B q O r The second coating layer comprises crystalline phosphate XPO4, and the third coating layer comprises crystalline pyrophosphate Li. a MP2O7 and / or M b (P2O7) c Alternatively, the first coating layer comprises crystalline borate Y. p B q O r The second coating layer comprises crystalline pyrophosphate Li a MP2O7 and / or M b (P2O7) c The third coating layer comprises crystalline phosphate XPO4.

[0133] In some embodiments, the first coating layer may optionally comprise crystalline pyrophosphate Li. a MP2O7 and / or M b (P2O7) c The fourth coating layer comprises carbon, and the second and third coating layers each independently comprise crystalline phosphate XPO4 or crystalline borate Y. p B q O r .

[0134] In some implementations, the coating amount of the first coating layer is greater than 0 and less than or equal to 6% by weight, or optionally greater than 0 and less than or equal to 5.5% by weight, based on the weight of the core.

[0135] In some embodiments, the coating amount of the second coating layer is greater than 0 and less than or equal to 6% by weight, or optionally greater than 0 and less than or equal to 5.5% by weight, based on the weight of the core.

[0136] In some embodiments, the coating amount of the third coating layer is greater than 0 and less than or equal to 6% by weight, or optionally greater than 0 and less than or equal to 5.5% by weight, based on the weight of the core.

[0137] In some embodiments, the coating amount of the fourth 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.

[0138] In this application, the coverage of each of the four coating layers is not zero.

[0139] In the core-shell structured positive electrode active material described in this application, the coating amount of the four coating layers is preferably within the above-mentioned range, thereby enabling sufficient coating of the core and further improving the cycle performance, safety performance, and / or rate performance of the secondary battery without sacrificing the specific capacity of the positive electrode active material.

[0140] In some embodiments, the thickness of the first coating layer is 1 nm to 15 nm.

[0141] In some embodiments, the thickness of the second coating layer is 1 nm to 15 nm.

[0142] In some embodiments, the thickness of the third coating layer is 1 nm to 15 nm.

[0143] In some embodiments, the thickness of the fourth coating layer is 2 nm to 25 nm.

[0144] In the core-shell structured positive electrode active material described in this application, the thickness of the four coating layers is preferably within the above-mentioned range, thereby enabling sufficient coating of the core and further improving the cycle performance, safety performance, and / or rate performance of the secondary battery without sacrificing the specific capacity of the positive electrode active material.

[0145] 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.

[0146] In some embodiments, the first coating layer may optionally comprise crystalline pyrophosphate Li. a MP2O7 and / or M b (P2O7) c The second coating layer comprises crystalline phosphate XPO4, and the third coating layer comprises crystalline borate Y. p B q O r The fourth coating layer comprises carbon.

[0147] Further research by the inventors unexpectedly revealed that by coating the core with a first coating layer including crystalline pyrophosphate, the migration resistance of manganese ions can be further increased, reducing their dissolution, and decreasing the surface lithium content and contact between the core and the electrolyte. This reduces interfacial side reactions, gas generation, and improves the high-temperature storage performance, cycle performance, and safety performance of the secondary battery. Further coating with a crystalline phosphate coating layer with excellent lithium-ion conductivity can effectively reduce interfacial side reactions between the positive electrode active material and the electrolyte, thereby improving the high-temperature cycle and storage performance of the secondary battery. Further coating with a crystalline borate coating layer with excellent lithium-ion and electron conductivity can further reduce manganese ion dissolution and surface lithium content, further reducing interfacial side reactions between the positive electrode active material and the electrolyte, while further improving the high-temperature cycle and storage performance of the secondary battery. Finally, further coating with a carbon layer as a fourth coating layer can further enhance the safety and kinetic performance of the secondary battery.

[0148] 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.

[0149] 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 cathode active 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 B and C 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.

[0150] 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 stability of the positive electrode active material. Crystalline phosphate has a very stable structure and excellent lithium-ion conductivity; therefore, using crystalline phosphate for coating can effectively reduce interfacial side reactions between the positive electrode active material and the electrolyte, thereby improving the high-temperature cycle performance and high-temperature 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.

[0151] The main reason for choosing borate as the third coating layer is that it has lower surface activity compared to phosphate, which can further reduce the surface lithium content, reduce electrolyte decomposition, and further inhibit manganese ion dissolution. Therefore, using crystalline borate for coating can further reduce the interfacial side reactions between the positive electrode active material and the electrolyte, thereby further improving the high-temperature cycle performance and high-temperature storage performance of the secondary battery.

[0152] The primary reason for using carbon as the fourth 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.

[0153] Therefore, positive electrode sheets and electrical devices such as secondary batteries using the positive electrode active material with the above-described coating sequence of the present application can have further improved cycle performance, safety performance, and / or rate performance.

[0154] In some embodiments, the first coating layer comprises crystalline pyrophosphate Li a MP2O7 and / or M b (P2O7) c 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, and more preferably greater than 0 and less than or equal to 2% by weight, based on the weight of the core.

[0155] In some embodiments, the second coating layer comprises crystalline phosphate XPO4, and 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% to 4% by weight, based on the weight of the core.

[0156] In some embodiments, the third coating layer comprises crystalline borate Y p B q O r 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 2% to 5% by weight, based on the weight of the core.

[0157] In some embodiments, the fourth coating layer comprises carbon, and the coating amount of the fourth 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.

[0158] In the core-shell structured positive electrode active material described in this application, the coating amount of the four coating layers with the above-mentioned coating order is preferably within the above-mentioned range, thereby enabling sufficient coating of the core and further improving the cycle performance, safety performance, and / or rate performance of the secondary battery without sacrificing the specific capacity of the positive electrode active material.

[0159] For the first coating layer, keeping the coating amount within the aforementioned range effectively avoids the following situations: insufficient coating amount means a thin coating layer, which may not effectively hinder the migration of transition metals; excessive coating amount means an excessively thick coating layer, which may affect Li... + The migration of these molecules affects the rate performance of the positive electrode active material.

[0160] For the second coating layer, by keeping the coating amount within the above range, the following situations can be effectively avoided: too much coating amount may affect the overall plateau voltage of the positive electrode active material; too little coating amount may not achieve sufficient coating effect.

[0161] For the third coating layer, by keeping the coating amount within the above range, the following situations can be effectively avoided: too much coating amount may result in an excessively thick coating layer, increasing battery impedance; too little coating amount may result in insufficient suppression of manganese ion dissolution, and at the same time, the improvement of lithium ion and electron transport performance will not be significant.

[0162] For the fourth 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.

[0163] In some embodiments, the first coating layer comprises crystalline pyrophosphate Li a MP2O7 and / or M b (P2O7) cThe thickness of the first coating layer is from 1 nm to 15 nm. 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, about 10 nm, about 11 nm, about 12 nm, about 13 nm, about 14 nm, or about 15 nm, or any range of the above values. Optionally, the thickness of the first coating layer is from 1 nm to 10 nm. This avoids the adverse effects on the kinetic performance of the positive electrode active material that may occur when the coating layer is too thick, and avoids the problem that it may not effectively hinder the migration of transition metal ions when the coating layer is too thin.

[0164] In some embodiments, the second coating layer comprises crystalline phosphate XPO4, and the thickness of the second coating layer is from 1 nm to 15 nm. 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, or about 15 nm, or any range of the above values. Optionally, the thickness of the second coating layer is from 2 nm to 15 nm. In this case, the surface structure of the second coating layer is stable, and the side reactions with the electrolyte are small, thus effectively reducing interfacial side reactions and improving the high-temperature cycle performance and high-temperature storage performance of the secondary battery.

[0165] In some embodiments, the third coating layer comprises crystalline borate Y p B q O r The thickness of the third coating layer is from 1 nm to 15 nm. 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, or about 15 nm, or any range of the above values. Optionally, the thickness of the third coating layer is from 1 nm to 10 nm. This can further suppress the dissolution of manganese ions and the decomposition of the electrolyte, while further promoting the transport of lithium ions and electrons.

[0166] In some embodiments, the fourth coating layer comprises carbon, and the thickness of the fourth coating layer is from 2 nm to 25 nm. In some embodiments, the thickness of the fourth 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. This can improve the conductivity of the positive electrode active material and increase the compaction density of the positive electrode sheet prepared using the positive electrode active material.

[0167] In some embodiments, based on the weight of the positive electrode active material, the manganese content is in the range of 10% to 35% by weight, preferably in the range of 15% to 30% by weight, and more preferably in the range of 17% to 20% by weight.

[0168] In some embodiments, the phosphorus content, based on the weight of the positive electrode active material, is in the range of 12% to 25% by weight, and optionally in the range of 15% to 20% by weight.

[0169] In some embodiments, based on the weight of the positive electrode active material, the weight ratio of manganese to phosphorus ranges from 0.90 to 1.25, and is optionally from 0.95 to 1.20.

[0170] 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.

[0171] In this application, limiting the content of manganese element within the above-mentioned range can effectively avoid problems such as poor structural stability and decreased density of the positive electrode active material 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 problems such as low voltage platform that may be caused by excessive manganese element content, thereby improving the energy density of the secondary battery.

[0172] 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, thereby affecting the conductivity of the positive electrode active material; if the phosphorus content is too low, it may reduce the stability of the pyrophosphate and / or phosphate lattice structure in the core and the shell, thereby affecting the overall stability of the positive electrode active material.

[0173] 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 the dissolution of manganese ions, affecting the stability and capacity of the positive electrode active material, and 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 impurities, which will cause the discharge voltage plateau of the positive electrode active material to drop, thereby reducing the energy density of the secondary battery.

[0174] 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.

[0175] In some embodiments, the lattice change rate of the 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% to 3.8%.

[0176] 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 cathode 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) patterns.

[0177] In some embodiments, the Li / Mn antisite defect concentration of the positive electrode active material is below 4%, optionally below 2.2%, and more preferably from 1.5% to 2.2%. The Li / Mn antisite defect mentioned in this application refers to the Li / Mn antisite 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.

[0178] The 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 transmission efficiency and enhances the capacity utilization and rate performance of the positive electrode active material.

[0179] In some embodiments, the compaction density of the positive electrode active material at 3T is 2.2 g / cm³. 3 The above can be selected as 2.2g / cm. 3 Above and 2.8g / cm 3 The higher the compaction density, the greater the weight of the positive electrode active material per unit volume. Therefore, increasing the compaction density is beneficial for improving the volumetric energy density of secondary batteries. Compaction density can be measured according to GB / T 24533-2009.

[0180] In some embodiments, the surface oxygen valence state of the positive electrode active material is below -1.90, optionally between -1.90 and -1.98. The stable valence state of oxygen is -2; the closer the valence state is to -2, the stronger its electron-accepting ability, i.e., the stronger its oxidizing power. Typically, 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 range as described above, can mitigate interfacial side reactions between the positive electrode active material and the electrolyte, thereby improving the cycle performance and storage performance of the secondary battery. The surface oxygen valence state can be measured by methods known in the art, such as electron energy loss spectroscopy (EELS).

[0181] In some embodiments, the average particle size of the primary particles of the positive electrode active material ranges from 50 nm to 500 nm, and the volume median particle size (Dv50) is in the range of 200 nm to 300 nm. Since particle agglomeration can occur, the actual measured size of the agglomerated secondary particles may be from 500 nm 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 effectively 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.

[0182] The above scheme can effectively suppress the lattice change rate of lithium manganese phosphate and the dissolution of manganese ions during the lithium insertion / extraction process, thereby improving the high-temperature cycle performance and high-temperature storage performance of secondary batteries.

[0183] Preparation method

[0184] The second aspect of this application provides a method for preparing a positive electrode active material, which can prepare the positive electrode active material of the first aspect of this application.

[0185] Specifically, the preparation method includes the following steps: providing core material and coating.

[0186] Steps for providing kernel material: The kernel includes Li m A x Mn 1-y B y P 1-z C z O 4-n D nThe A comprises one or more elements selected from Group IA, Group IIA, Group IIIA, Group IIB, Group VB, and Group VIB, and optionally includes one or more elements selected from Zn, Al, Na, K, Mg, Nb, Mo, and W. The B comprises one or more elements selected from Group IA, Group IIA, Group IIIA, Group IVA, Group VA, Group IIB, Group IVB, Group VB, Group VIB, and Group VIIIB, and optionally includes one or more elements selected from Ti, V, Zr, Fe, Ni, Mg, Co, Ga, Sn, Sb, Nb, and Ge. The C comprises one or more elements selected from Group IIIA, Group IVA, Group VA, and Group VIA, and optionally includes... The core comprises one or more elements selected from B (boron), S, Si, and N, wherein D comprises one or more elements selected from groups VIA and VIIA, and optionally comprises one or more elements selected from S, F, Cl, and Br, wherein m is selected from the range of 0.900 to 1.100, and optionally from the range of 0.995 to 1.002, wherein x is selected from the range of 0 to 0.100, and optionally from the range of 0.001 to 0.005, wherein y is selected from the range of 0.001 to 0.500, wherein z is selected from the range of 0.001 to 0.100, wherein n is selected from the range of 0 to 0.100, and optionally from the range of 0.001 to 0.005, and wherein the core is electrically neutral.

[0187] Coating steps: Provide Li pyrophosphate separately. a MP2O7 and / or M b (P2O7) c Phosphate XPO4, borate Y p B q O r A coating solution containing a carbon source is prepared, and the core material is added to and mixed with the coating solution. The mixture is then sintered to obtain a positive electrode active material. The positive electrode active material has a core-shell structure, comprising the core and a shell coating the core. The shell comprises crystalline pyrophosphate Li. a MP2O7 and / or M b (P2O7) c , Crystalline phosphate XPO4, Crystalline borate Y p B q O r And carbon, and the shell comprises one or more coating layers, each coating layer independently comprising crystalline pyrophosphate Li. a MP2O7 and / or M b (P2O7) c , Crystalline phosphate XPO4, Crystalline borate Y p B q O rAnd one or more of carbon, crystalline pyrophosphate Li a MP2O7 and / or M b (P2O7) c Each M in the series independently includes one or more metallic elements selected from transition metals, Group IA, Group IIA, Group IIIA, Group IVA, Group VA, and the lanthanides, and optionally includes one or more elements selected from Fe, Ni, Mg, Mn, Co, Cu, Zn, Ti, Ag, Zr, Nb, Sb, and Al, where 0 ≤ a ≤ 2, 1 ≤ b ≤ 4, and 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 To maintain electrical neutrality, X comprises one or more metallic elements selected from transition metals, Group IA, Group IIA, Group IIIA, Group IVA, Group VA, and the lanthanides, optionally including one or more elements selected from Li, Fe, Ni, Mg, Mn, Co, Cu, Zn, Ti, Ag, Zr, Nb, Sb, and Al; Y comprises one or more metallic elements selected from transition metals, Group IA, Group IIA, Group IIIA, Group IVA, Group VA, and the lanthanides, optionally including one or more elements selected from Li, Fe, Ni, Mg, Mn, Co, Cu, Zn, Ti, Ag, Zr, Nb, Sb, and Al, 1≤p≤4, 1≤q≤7, 2≤r≤12, and the values ​​of p, q, and r satisfy the following condition: to make crystalline borate Y... p B q O r Maintain electrical neutrality.

[0188] In some implementations, the step of providing the core material includes steps (1) and (2).

[0189] Step (1): Mix and stir the manganese source, element B dopant and acid in a container to obtain manganese salt particles doped with element B.

[0190] Step (2): The manganese salt particles doped with element B are mixed with lithium source, phosphorus source, dopant of element C, optional dopant of element A and optional dopant of element D in a solvent to obtain a slurry, which is then sintered under an inert gas atmosphere to obtain the core material.

[0191] 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.

[0192] In some embodiments, the dopant of element A is selected from one or more of the following: element A in its elemental form, carbonate, sulfate, chloride, nitrate, organic acid salt, oxide, and hydroxide.

[0193] In some embodiments, the dopant of element B is one or more selected from element B, carbonates, sulfates, chlorides, nitrates, organic acid salts, oxides, and hydroxides.

[0194] In some embodiments, the dopant of element C is one or more selected from inorganic acids, organic acids, sulfates, chlorides, nitrates, organic acid salts, oxides, and hydroxides of element C.

[0195] In some embodiments, the dopant of element D is selected from one or more of element D in its elemental form and ammonium salt.

[0196] 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.

[0197] 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.

[0198] In this application, the lithium source may be any lithium-containing material known in the art that can be used to prepare lithium manganese phosphate. As an example, the lithium source may be one or more selected from lithium carbonate, lithium hydroxide, lithium phosphate, and lithium dihydrogen phosphate.

[0199] 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.

[0200] In this application, the amount of dopant added to each of the elements A, B, C, and D depends on the target doping amount.

[0201] In some embodiments, after the manganese source, the dopant of element B, and the acid are reacted in a solvent to obtain a suspension of manganese salt doped with element B, the suspension is filtered, dried, and milled to obtain manganese salt particles doped with element B with a particle size of 50-200 nm.

[0202] In some embodiments, the slurry in step (2) is dried to obtain powder, and then the powder is sintered to obtain core material.

[0203] In some embodiments, step (1) is performed at a temperature of 20°C to 120°C, optionally 40°C to 120°C.

[0204] In some embodiments, the stirring in step (1) is carried out at 400 rpm to 700 rpm for 1 hour to 9 hours, or optionally 3 hours to 7 hours.

[0205] 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.

[0206] In some embodiments, step (2) is carried out at a temperature of 20°C to 120°C, optionally 40°C to 120°C, for 1 hour to 12 hours. Optionally, the reaction temperature in step (2) can be carried out at 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 mixing in step (2) is carried out 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, about 10 hours, about 11 hours, or about 12 hours; optionally, the reaction temperature and mixing time in step (2) can be within any range of the above values.

[0207] 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 the dissolution of manganese ions, reduce the 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.

[0208] In some embodiments, during the preparation of the core material, the solution pH is controlled to be 3.5 to 6; optionally, the solution pH is controlled to be 4 to 6; more preferably, the solution pH is controlled to be 4 to 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.

[0209] In some embodiments, optionally, in step (2), the molar ratio of the manganese salt particles doped with element B to the lithium source and the phosphorus source is 1:(0.5-2.1):(0.5-2.1), and more preferably, the molar ratio of the manganese salt particles doped with element B to the lithium source and the phosphorus source is about 1:1:1.

[0210] In some embodiments, the sintering conditions during the preparation of the core material are: sintering at 600°C to 950°C for 4 to 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.

[0211] In the process of preparing core materials, if the sintering temperature is too low or the sintering time is too short, the crystallinity of the 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 core, which will also affect the overall performance. If the sintering time is too long, the core particles will be too large, which will affect the capacity, compaction density and rate performance.

[0212] In some embodiments, the protective atmosphere may optionally be a mixture of 70-90% by volume nitrogen and 10-30% by volume hydrogen.

[0213] In some embodiments, the coating step includes coating crystalline pyrophosphate Li a MP2O7 and / or M b (P2O7) c Steps for coating crystalline phosphate XPO4, steps for coating crystalline borate Y p B q O r The steps include the coating process and the carbon coating process. The specific coating sequence of the above coating steps is not limited and can be adaptively adjusted according to the specific structure of the shell of the required positive electrode active material.

[0214] Optionally, the coated crystalline pyrophosphate Li a MP2O7 and / or M b (P2O7) c The steps include: adding a source of element M, a phosphorus source, an acid, and optionally a lithium source to a solvent to obtain a coating solution; thoroughly mixing the material to be coated with the coating solution; drying; and then sintering to obtain crystalline pyrophosphate Li. a MP2O7 and / or M b (P2O7) c The coating material. The coating material may be a core material, a material with one coating layer, a material with two coating layers, or a material with three coating layers, depending on the actual situation.

[0215] Optionally, the step of coating crystalline phosphate XPO4 includes the following steps: adding a source of element X, a phosphorus source, and an acid to a solvent to obtain a coating solution; thoroughly mixing the material to be coated with the coating solution; drying; and then sintering to obtain a material coated with crystalline phosphate XPO4. The material to be coated can be a core material, a material coated with one layer, a material coated with two layers, or a material coated with three layers, depending on the actual situation.

[0216] Optionally, the coated crystalline borate Y p B q O r The steps include: adding a source of element Y and a source of boron to a solvent to obtain a coating solution; thoroughly mixing the material to be coated with the coating solution; drying; and then sintering to obtain crystalline borate Y. p B q O r The coating material. The coating material may be a core material, a material with one coating layer, a material with two coating layers, or a material with three coating layers, depending on the actual situation.

[0217] Optionally, the carbon coating step includes the following steps: adding a carbon source to a solvent to obtain a coating solution, adding the material to be coated to the coating solution, mixing evenly, drying, and then sintering to obtain a carbon-coated material. The material to be coated can be a core material, a material coated with one layer, a material coated with two layers, or a material coated with three layers, depending on the actual situation.

[0218] In some embodiments, optionally, the first coating step is to coat crystalline pyrophosphate Li a MP2O7 and / or M b (P2O7) c The first step is to coat crystalline phosphate XPO4, and the second step is to coat crystalline borate Y. p B q O r The fourth coating step is the carbon coating step, thereby obtaining a positive electrode active material with a core-shell structure, which includes the 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, a third coating layer covering the second coating layer, and a fourth coating layer covering the third coating layer. The first coating layer includes crystalline pyrophosphate Li. a MP2O7 and / or M b (P2O7) c The second coating layer comprises crystalline phosphate XPO4, and the third coating layer comprises crystalline borate Y. p B q Or The fourth coating layer comprises carbon.

[0219] In some embodiments, the source of element M is one or more selected from elemental form, carbonate, sulfate, chloride, nitrate, organic acid salt, oxide, and hydroxide.

[0220] In some embodiments, the source of element X is one or more selected from elemental form, carbonate, sulfate, chloride, nitrate, organic acid salt, oxide, and hydroxide of element X.

[0221] In some embodiments, the source of element Y is one or more selected from elemental form of element Y, carbonate, sulfate, chloride, nitrate, organic acid salt, oxide, and hydroxide.

[0222] In this application, the amount of source added to each of the elements M, X, and Y depends on the target coverage amount.

[0223] As an example, the carbon source is one or more selected from starch, sucrose, glucose, polyvinyl alcohol, polyethylene glycol, and citric acid.

[0224] In this application, the amount of carbon source added can be determined based on the residual carbon value and the target coating amount.

[0225] In some embodiments, the coated crystalline pyrophosphate Li a MP2O7 and / or M b (P2O7) c In the steps, 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 to 6.5. The solution is then stirred and reacted for 1 to 5 hours. The solution is then heated to 50°C to 120°C and maintained at that temperature for 2 to 10 hours.

[0226] Optionally, in the coating of crystalline pyrophosphate Li a MP2O7 and / or M b (P2O7) c In this step, the reaction proceeds fully. Optionally, 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, the reaction time can be within any range of the above values.

[0227] Optionally, in the coating of crystalline pyrophosphate Li a MP2O7 and / or M b (P2O7) c In this step, the pH of the solution is controlled to be between 4 and 6.

[0228] Optionally, in the coating of crystalline pyrophosphate Lia MP2O7 and / or M b (P2O7) c In the steps, 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, the heating temperature and holding time can be within any range of the above values.

[0229] In some embodiments, the coated crystalline pyrophosphate Li a MP2O7 and / or M b (P2O7) c The sintering process in the step is carried out at 650°C to 800°C for 2 to 6 hours. Optionally, the sintering can be carried out 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.

[0230] Coated crystalline pyrophosphate Li a MP2O7 and / or M b (P2O7) c In the process, by controlling the sintering temperature and time within the above range, the following situation can be effectively avoided: when the sintering temperature is too low and the sintering time is too short, it will lead to the formation of crystalline pyrophosphate Li. a MP2O7 and / or M b (P2O7) c The low crystallinity and high content of amorphous substances in the coating layer reduce its effectiveness in inhibiting metal dissolution, thus affecting the cycle and storage performance of the secondary battery. Conversely, excessively high sintering temperatures can lead to the formation of impurities in the coating layer, also impacting its ability to inhibit metal dissolution and consequently affecting the cycle and storage performance of the secondary battery. Furthermore, excessively long sintering times increase the thickness of the coating layer, affecting the Li... + The migration of these molecules affects the capacity utilization and rate performance of the positive electrode active material.

[0231] In some embodiments, in the step of coating crystalline phosphate XPO4, the source of element X, the phosphorus source and the acid are dissolved in a solvent, stirred and reacted for 1 to 10 hours, and then the solution is heated to 60°C to 150°C and maintained at that temperature for 2 to 10 hours.

[0232] Optionally, in the step of coating crystalline phosphate XPO4, the reaction is carried out to a sufficient extent. Optionally, the reaction is carried out for about 1.5 hours, about 2 hours, about 3 hours, about 4 hours, about 4.5 hours, about 5 hours, about 6 hours, about 7 hours, about 8 hours, about 9 hours, or about 10 hours. Optionally, the reaction time can be within any range of the above values.

[0233] Optionally, in the step of coating crystalline phosphate XPO4, the solution is heated to about 65°C, about 70°C, about 80°C, about 90°C, about 100°C, about 110°C, about 120°C, about 130°C, about 140°C, or about 150°C, and held at this temperature 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 heating temperature and holding time can be within any range of the above values.

[0234] In the step of providing the core material, the coated crystalline pyrophosphate Li a MP2O7 and / or M b (P2O7) c In the steps of coating crystalline phosphate XPO4, before sintering, i.e., in the preparation of the core material where the chemical reaction occurs (i.e., steps (1) and (2)) and in the preparation of the coating liquid for each coating layer, by selecting appropriate reaction temperature and reaction time as described above, the following situations can be effectively 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 impurities; 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.

[0235] In some embodiments, the sintering in the step of coating crystalline phosphate XPO4 is performed at 500°C to 700°C for 6 to 10 hours. Optionally, the sintering can be performed 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; alternatively, the sintering temperature and sintering time can be within any range of the above values.

[0236] In the process of coating crystalline phosphate XPO4, by controlling the sintering temperature and time within the above range, the following situations can be effectively avoided: When the sintering temperature is too low or the sintering time is too short, the crystallinity of crystalline phosphate XPO4 will be low, with more amorphous phases, reducing the surface reactivity of the positive electrode active material and thus affecting the cycle performance and storage performance of the secondary battery; when the sintering temperature is too high, impurities will appear in the formed coating layer, which will also affect its effect on reducing the surface reactivity of the positive electrode active material, thus affecting the cycle performance and storage performance of the secondary battery; when the sintering time is too long, the thickness of the formed coating layer will increase, affecting the voltage plateau of the positive electrode active material, thereby reducing the energy density of the secondary battery.

[0237] In some embodiments, the coated crystalline borate Y p B q O r The sintering process in the step is carried out at 300°C to 500°C for 2 to 10 hours. Optionally, the sintering can be carried out at about 300°C, about 350°C, about 400°C, about 450°C, or about 500°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.

[0238] Coated crystalline borate Y p B q O r In the process, by controlling the sintering temperature and time within the above range, the following situation can be avoided: when the sintering temperature is too low and the sintering time is too short, it will lead to the formation of crystalline borate Y. p B q O r Its low crystallinity and high amorphous content result in poor coating effect, insufficient inhibition of manganese ion dissolution, and insignificant improvement on lithium ion and electron transport performance. Furthermore, excessively high sintering temperature and long sintering time will increase the thickness of the coating layer, thereby increasing battery impedance and affecting the kinetic performance and energy density of the secondary battery.

[0239] In some embodiments, the sintering in the carbon coating step is carried out at 700°C to 800°C for 6 to 10 hours. Optionally, the sintering can be carried out 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.

[0240] In the carbon coating step, by controlling the sintering temperature and time within the above range, the following situations can be effectively avoided: when the sintering temperature is too low, the graphitization degree of the carbon material decreases, affecting its conductivity and thus the capacity of the positive electrode active material; when the sintering temperature is too high, the graphitization degree of the carbon material becomes too high, affecting the Li... + The transmission of the cathode material can affect its capacity utilization, etc.; if the sintering time is too short, the coating layer of the formed layer will be too thin, affecting its conductivity and thus affecting the capacity utilization of the cathode active material; if the sintering time is too long, the coating layer will be too thick, affecting the compaction density of the cathode active material, etc.

[0241] The above-mentioned coated crystalline pyrophosphate Li a MP2O7 and / or M b (P2O7) c Steps for coating crystalline phosphate XPO4, steps for coating crystalline borate Y p B q O r In the steps of drying and coating with carbon, the drying can be 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. The drying time can be 3 hours to 9 hours, optionally 4 hours to 8 hours, more preferably 5 hours to 7 hours, and most preferably about 6 hours.

[0242] The positive electrode active material prepared by the method described in this application results in a reduced amount of manganese ion dissolution in the secondary battery after cycling, and improved high-temperature storage performance, cycle performance, and rate performance. Furthermore, the raw materials are widely available, inexpensive, and the process is simple, facilitating industrialization.

[0243] Positive electrode sheet

[0244] 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 the positive electrode active material of the first aspect of this application or a positive electrode active material prepared by the method of the second aspect of this application, and the content of the positive electrode active material in the positive electrode film layer is 10% by weight or more, optionally from 90% by weight to 99.5% by weight, based on the total weight of the positive electrode film layer. The positive current collector has two surfaces opposite each other in its thickness direction, and the positive electrode film layer is disposed on either or both of the two opposite surfaces of the positive current collector.

[0245] The positive electrode film layer does not exclude other positive electrode active materials besides the positive electrode active material of the first aspect of this application or the positive electrode active material prepared by the method of the second aspect of this application. For example, the positive electrode film layer may also include other positive electrode active materials besides the four-layer coated positive electrode active materials mentioned above. Optionally, the other positive electrode active materials may include at least one of lithium transition metal oxides and their modified compounds. As an example, the other positive electrode active materials may include at least one of lithium cobalt oxide, lithium nickel oxide, lithium manganese oxide, lithium nickel cobalt oxide, lithium manganese cobalt oxide, lithium nickel manganese oxide, lithium nickel cobalt manganese oxide, lithium nickel cobalt aluminum oxide, and their respective modified compounds.

[0246] In some embodiments, the positive electrode film may optionally include a positive electrode conductive agent. This application does not impose any particular limitation on the type of positive electrode conductive agent. As an example, the positive electrode conductive agent includes at least one of superconducting carbon, conductive graphite, acetylene black, carbon black, Ketjen black, carbon dots, carbon nanotubes, graphene, and carbon nanofibers.

[0247] In some embodiments, the positive electrode film layer may optionally include a positive electrode binder. This application does not impose any particular limitation on the type of positive electrode binder. As an example, the positive electrode binder may include at least one of polyvinylidene fluoride (PVDF), polytetrafluoroethylene (PTFE), PVDF-tetrafluoroethylene-propylene terpolymer, PVDF-hexafluoropropylene-tetrafluoroethylene terpolymer, tetrafluoroethylene-hexafluoropropylene copolymer, and fluorinated acrylate resins.

[0248] In some embodiments, the positive current collector may be a metal foil or a composite current collector. An example of a metal foil is aluminum foil. The composite current collector may include a polymer substrate and a metal layer formed on at least one surface of the polymer substrate. As an example, the metal material may be selected from at least one of aluminum, aluminum alloys, nickel, nickel alloys, titanium, titanium alloys, silver, and silver alloys. As an example, the polymer substrate may be selected from polypropylene (PP), polyethylene terephthalate (PET), polybutylene terephthalate (PBT), polystyrene (PS), polyethylene (PE), etc.

[0249] The positive electrode film is typically formed by coating a positive electrode slurry onto a positive electrode current collector, followed by drying and cold pressing. The positive electrode slurry is typically formed by dispersing the positive electrode active material, optional conductive agent, optional binder, and any other components in a solvent and stirring until homogeneous. The solvent may be N-methylpyrrolidone (NMP), but is not limited to this.

[0250] Secondary batteries

[0251] A fourth aspect of this application provides a secondary battery that includes the positive electrode of the third aspect of this application.

[0252] A secondary battery, also known as a rechargeable battery or accumulator, is a battery that can be recharged after discharge to reactivate its active materials and continue to be used. Typically, a secondary battery consists of an electrode assembly and an electrolyte. The electrode assembly includes a positive electrode, a negative electrode, and a separator. The separator is positioned between the positive and negative electrodes, primarily preventing short circuits while allowing active ions to pass through. The electrolyte, located between the positive and negative electrodes, conducts the active ions.

[0253] The secondary battery mentioned in the embodiments or implementations of this application refers to a single physical module comprising one or more battery cells to provide higher voltage and capacity. For example, the secondary battery mentioned in this application may include battery cells, battery modules, or battery packs. A battery cell is the smallest unit constituting a secondary battery, capable of charging and discharging independently. This application does not impose any particular limitation on the shape of the battery cell; it can be cylindrical, square, or any other arbitrary shape. Figure 1 The example shown is a square-structured battery cell 5.

[0254] In some embodiments, the battery cell includes an electrode assembly, and the battery cell may also include an outer packaging. The electrode assembly is made from positive electrode sheets, negative electrode sheets, and a separator through a winding process and / or a stacking process, and the outer packaging is used to encapsulate the electrode assembly. The outer packaging can be a rigid shell, such as a hard plastic shell, an aluminum shell, or a steel shell. The outer packaging can also be a flexible package, such as a pouch. The material of the flexible package can be plastic, such as one or more of polypropylene (PP), polybutylene terephthalate (PBT), and polybutylene succinate (PBS).

[0255] In some implementations, such as Figure 2 As shown, 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 enclosing a receiving cavity. The housing 51 has an opening communicating with the receiving cavity, and the cover 53 is used to cover the opening to close the receiving cavity. Electrode assemblies 52 are encapsulated in the receiving cavity. The number of electrode assemblies 52 contained in the battery cell 5 may be one or more, and can be adjusted as needed.

[0256] In some embodiments of this application, individual battery cells can be assembled into a battery module. The number of individual battery cells contained in a battery module can be multiple, and the specific number can be adjusted according to the application and capacity of the battery module. Figure 3 This is a schematic diagram of battery module 4 as an example. Figure 3As shown, in battery module 4, multiple battery cells 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 battery cells 5 can be fixed in place using fasteners.

[0257] Optionally, the battery module 4 may also include a housing with a receiving space in which multiple battery cells 5 are received.

[0258] 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 adjusted according to the application and capacity of the battery pack. Figure 4 and Figure 5 This is a schematic diagram of battery pack 1 as an example. Figure 4 and Figure 5 As shown, 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. The upper body 2 covers the lower body 3, forming a closed space for accommodating the battery modules 4. The multiple battery modules 4 can be arranged in any manner within the battery box.

[0259] [Positive electrode plate]

[0260] The positive electrode used in the secondary battery of this application is the positive electrode described in any embodiment of the third aspect of this application.

[0261] [Negative electrode plate]

[0262] In some embodiments, 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 and comprising a negative electrode active material. For example, the negative current collector has two surfaces opposite each other in its thickness direction, and the negative electrode film layer is disposed on either or both of the two opposite surfaces of the negative current collector.

[0263] The negative electrode active material may be any negative electrode active material known in the art for use in secondary batteries. As an example, the negative electrode active material includes, but is not limited to, at least one of natural graphite, artificial graphite, soft carbon, hard carbon, silicon-based materials, tin-based materials, and lithium titanate. The silicon-based material may include at least one of elemental silicon, silicon oxide, silicon-carbon composite, silicon-nitrogen composite, and silicon alloy. The tin-based material may include at least one of elemental tin, tin oxide, and tin alloy. This application is not limited to these materials, and other conventionally known materials that can be used as negative electrode active materials for secondary batteries may also be used. These negative electrode active materials may be used alone or in combination of two or more.

[0264] In some embodiments, the negative electrode film may optionally include a negative electrode conductive agent. This application does not impose any particular limitation on the type of negative electrode conductive agent. As an example, the negative electrode conductive agent may include at least one of superconducting carbon, conductive graphite, acetylene black, carbon black, Ketjen black, carbon dots, carbon nanotubes, graphene, and carbon nanofibers.

[0265] In some embodiments, the negative electrode film layer may optionally include a negative electrode binder. This application does not impose any particular limitation on the type of negative electrode binder. As an example, the negative electrode binder may include at least one of styrene-butadiene rubber (SBR), water-soluble unsaturated resin SR-1B, waterborne acrylic resins (e.g., polyacrylic acid PAA, polymethacrylic acid PMAA, sodium polyacrylate PAAS), polyacrylamide (PAM), polyvinyl alcohol (PVA), sodium alginate (SA), and carboxymethyl chitosan (CMCS).

[0266] In some embodiments, the negative electrode film may optionally include other additives. As an example, other additives may include thickeners, such as sodium carboxymethyl cellulose (CMC), PTC thermistor materials, etc.

[0267] In some embodiments, the negative electrode current collector may be a metal foil or a composite current collector. As an example of a metal foil, copper foil may be used. The composite current collector may include a polymer base material and a metal material layer formed on at least one surface of the polymer base material. As an example, the metal material may be selected from at least one of copper, copper alloys, nickel, nickel alloys, titanium, titanium alloys, silver, and silver alloys. As an example, the polymer base material may be selected from polypropylene (PP), polyethylene terephthalate (PET), polybutylene terephthalate (PBT), polystyrene (PS), polyethylene (PE), etc.

[0268] The negative electrode film layer is typically formed by coating a negative electrode slurry onto a negative electrode current collector, followed by drying and cold pressing. The negative electrode slurry is typically formed by dispersing a negative electrode active material, optional conductive agent, optional binder, and other optional additives in a solvent and stirring until homogeneous. The solvent can be N-methylpyrrolidone (NMP) or deionized water, but is not limited to these.

[0269] The negative electrode sheet does not exclude other additional functional layers besides the negative electrode film layer. For example, in some embodiments, the negative electrode sheet of this application further includes a conductive undercoat layer (e.g., composed of a conductive agent and an adhesive) sandwiched between the negative electrode current collector and the negative electrode film layer and disposed on the surface of the negative electrode current collector. In other embodiments, the negative electrode sheet of this application further includes a protective layer covering the surface of the negative electrode film layer.

[0270] [Electrolytes]

[0271] This application does not impose specific limitations on the type of electrolyte, which can be selected according to requirements. For example, the electrolyte can be selected from at least one of solid electrolytes and liquid electrolytes (i.e., electrolyte solutions).

[0272] In some embodiments, the electrolyte is an electrolyte solution comprising an electrolyte salt and a solvent.

[0273] The type of electrolyte salt is not specifically limited and can be selected according to actual needs. In some embodiments, as an example, the electrolyte salt may include at least one of lithium hexafluorophosphate (LiPF6), lithium tetrafluoroborate (LiBF4), lithium perchlorate (LiClO4), lithium hexafluoroarsenate (LiAsF6), lithium bis(fluorosulfonyl)imide (LiFSI), lithium bis(trifluoromethanesulfonyl)imide (LiTFSI), lithium trifluoromethanesulfonate (LiTFS), lithium difluorooxalate borate (LiDFOB), lithium dioxalate borate (LiBOB), lithium difluorophosphate (LiPO2F2), lithium difluorodioxalate phosphate (LiDFOP), and lithium tetrafluorooxalate phosphate (LiTFOP).

[0274] The type of solvent is not specifically limited and can be selected according to actual needs. In some embodiments, as an example, the solvent may include at least one of ethylene carbonate (EC), propylene carbonate (PC), ethyl methyl carbonate (EMC), diethyl carbonate (DEC), dimethyl carbonate (DMC), dipropyl carbonate (DPC), methyl propyl carbonate (MPC), ethyl propyl carbonate (EPC), butyl carbonate (BC), fluoroethylene carbonate (FEC), methyl formate (MF), methyl acetate (MA), ethyl acetate (EA), propyl acetate (PA), methyl propionate (MP), ethyl propionate (EP), propyl propionate (PP), methyl butyrate (MB), ethyl butyrate (EB), 1,4-butyrolactone (GBL), sulfolane (SF), dimethyl sulfone (MSM), methyl ethyl sulfone (EMS), and diethyl sulfone (ESE).

[0275] In some embodiments, the electrolyte may optionally include additives. For example, the additives may include negative electrode film-forming additives, positive electrode film-forming additives, and additives that can improve certain battery performance, such as additives that improve battery overcharge performance, additives that improve battery high-temperature performance, and additives that improve battery low-temperature power performance.

[0276] [Isolation membrane]

[0277] Secondary batteries using electrolytes, as well as some secondary batteries using solid electrolytes, also include a separator. The separator is disposed between the positive and negative electrodes, primarily serving to prevent short circuits between the positive and negative electrodes while allowing active ions to pass through. 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.

[0278] In some embodiments, the material of the separator may include 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. When the separator is a multi-layer composite film, the materials of each layer may be the same or different.

[0279] The method for preparing the secondary battery described in this application is well known. In some embodiments, a positive electrode, a separator, a negative electrode, and an electrolyte can be assembled to form a secondary battery. As an example, the positive electrode, separator, and negative electrode can be formed into an electrode assembly through a winding or stacking process. The electrode assembly is then placed in an outer package, dried, and injected with electrolyte. After vacuum sealing, settling, formation, and shaping, a battery cell is obtained. Multiple battery cells can be further connected in series, parallel, or a combination thereof to form a battery module. Multiple battery modules can also be connected in series, parallel, or a combination thereof to form a battery pack. In some embodiments, multiple battery cells can also be directly assembled into a battery pack.

[0280] Electrical appliances

[0281] The fifth aspect of this application provides an electrical device comprising a secondary battery as described in this application. The secondary battery 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 be, but is not limited to, mobile devices (e.g., mobile phones, tablets, 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.

[0282] The electrical device can select the specific type of secondary battery according to its usage requirements, such as a battery cell, battery module, or battery pack.

[0283] Figure 6 This is a schematic diagram illustrating an example of an electrical device. This device could be a pure electric vehicle, a hybrid electric vehicle, or a plug-in hybrid electric vehicle, etc. To meet the device's requirements for high power and high energy density, a battery pack or battery module can be used as the power source.

[0284] Another example of an electrical device could be a mobile phone, tablet, or laptop. These devices typically require a slim and lightweight design and can use a single battery cell as their power source.

[0285] Example

[0286] The following embodiments describe the disclosure of this application in more detail. These embodiments are merely illustrative, as various modifications and variations will be apparent to those skilled in the art within the scope of the disclosure of this application. Unless otherwise stated, all parts, percentages, and ratios reported in the following embodiments are based on mass, and all reagents used in the embodiments are commercially available or synthesized by conventional methods and can be used directly without further processing, and the instruments used in the embodiments are commercially available.

[0287] The sources of raw materials involved in the embodiments of this application are as follows:

[0288] name Chemical formula factory Specification manganese carbonate <![CDATA[MnCO3]]> Shandong Xiya Chemical Industry Co., Ltd. 1Kg lithium carbonate <![CDATA[Li2CO3]]> Shandong Xiya Chemical Industry Co., Ltd. 1Kg Magnesium carbonate <![CDATA[MgCO3]]> Shandong Xiya Chemical Industry Co., Ltd. 1Kg Zinc carbonate <![CDATA[ZnCO3]]> Wuhan Xinru Chemical Co., Ltd. 25Kg Ferrous carbonate <![CDATA[FeCO3]]> Xi'an Lanzhiguang Fine Materials Co., Ltd. 1Kg Nickel sulfate <![CDATA[NiCO3]]> Shandong Xiya Chemical Industry Co., Ltd. 1Kg Titanium sulfate <![CDATA[Ti(SO4)2]]> Shandong Xiya Chemical Industry Co., Ltd. 1Kg Cobalt sulfate <![CDATA[CoSO4]]> Xiamen Zhixin Chemical Co., Ltd. 500g Vanadium dichloride <![CDATA[VCl2]]> Shanghai Jinjinle Industrial Co., Ltd. 1Kg Oxalic acid dihydrate <![CDATA[C2H2O4.2(H2O)]]> Shanghai Jinjinle Industrial Co., Ltd. 1Kg Ammonium dihydrogen phosphate <![CDATA[NH4H2PO4]]> Shanghai Chengshao Biotechnology Co., Ltd. 500g sucrose <![CDATA[C 12 H 22 O 11 ]]> Shanghai Yuanye Biotechnology Co., Ltd. 100g dilute sulfuric acid <![CDATA[H2SO4]]> Shenzhen Haisi'an Biotechnology Co., Ltd. Quality score 60% dilute nitric acid <![CDATA[HNO3]]> Anhui Lingtian Fine Chemical Co., Ltd. Quality score 60% Silicate <![CDATA[H2SiO3]]> Shanghai Yuanye Biotechnology Co., Ltd. 100g, mass fraction 99.8%

[0289] I. Battery Manufacturing

[0290] Example 1

[0291] Step 1: Preparation of positive electrode active material

[0292] Step S1: Preparation of co-doped manganese oxalate

[0293] 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 and the reaction was terminated with no more bubbles, yielding a 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.

[0294] Step S2: Preparation of core Li 0.997 Mn 0.60 Fe 0.393 V 0.004 Co 0.003 P 0.997 S 0.003 O4

[0295] 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°C for 10 hours to obtain a slurry. Transfer the slurry to a spray drying equipment for spray drying and granulation, and dry at 250°C to obtain a powder. In a protective atmosphere (90% nitrogen and 10% hydrogen), sinter the powder in a roller kiln at 700°C for 4 hours to obtain the above-mentioned core material.

[0296] Step S3: Preparation of the coating solution for the first coating layer

[0297] 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 hours to obtain the first coating layer solution. The solution was then heated to 80 °C and maintained at this temperature for 4 hours to obtain the first coating layer solution.

[0298] Step S4: Coating with the first coating layer

[0299] The 1571.9g of doped lithium manganese phosphate core material obtained in step S2 was added to the first coating layer coating liquid (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.

[0300] Step S5: Preparation of the coating solution for the second coating layer

[0301] 3.7g lithium carbonate, 11.6g ferrous carbonate, 11.5g ammonium dihydrogen phosphate and 12.6g oxalic acid dihydrate were dissolved in 1500mL deionized water, stirred and reacted for 6 hours to obtain a solution. The solution was then heated to 120℃ and maintained at this temperature for 6 hours to obtain the second coating layer solution.

[0302] Step S6: Coating with the second coating layer

[0303] The 1586.8g of pyrophosphate-coated material obtained in step S4 was added to the second coating liquid (coating material content of 47.1g) obtained in step S5. 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 700℃ for 8 hours to obtain the two-layer coated material.

[0304] Step S7: Preparation of the coating solution for the third coating layer

[0305] 28.3 g of lithium hydroxide and 13.7 g of boron oxide were added to 500 mL of deionized water to obtain the third coating layer solution.

[0306] Step S8: Covering with the third coating layer

[0307] 1633.9g of the two-layer coated material obtained in step S6 was added to the third coating liquid (coating substance content of 31.4g) obtained in step S7. After mixing evenly, the mixture was dried in an oven at 120℃ for 6 hours and then sintered at 400℃ for 10 hours to obtain the three-layer coated material.

[0308] Step S9: Preparation of the fourth coating layer aqueous solution

[0309] Dissolve 37.3g of sucrose in 500g of deionized water, then stir and dissolve completely to obtain a sucrose aqueous solution.

[0310] Step S10: Coating with the fourth coating layer

[0311] 1665.3g of the three-layer coated material obtained in step S8 was added to the sucrose solution obtained in step S9 and stirred together for 6 hours. After mixing evenly, the mixture was placed in a 150°C oven and dried for 6 hours. Then, it was sintered at 700°C for 10 hours to obtain the four-layer coated material, which is the positive electrode active material.

[0312] Step 2: Preparation of the positive electrode sheet

[0313] The positive electrode active material, conductive agent acetylene black, and binder polyvinylidene fluoride (PVDF) were added to N-methylpyrrolidone (NMP) at a weight ratio of 97:1.2:1.8 and stirred until homogeneous to obtain the positive electrode slurry. Then, the positive electrode slurry was coated with a density of 0.018 g / cm³. 2 The coating is evenly applied to aluminum foil, and then dried, cold-pressed, and slit to obtain the positive electrode sheet.

[0314] Step 3: Preparation of the negative electrode sheet

[0315] 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 coated with a solution to achieve a surface density of 0.0075 g / cm³. 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.

[0316] Step 4: Preparation of electrolyte

[0317] 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.

[0318] Step 5: Preparation of the separating membrane

[0319] 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).

[0320] Step 6: Preparation of the full cell

[0321] 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 electrode assembly is then wound up. The electrode assembly is placed in an outer package, injected with the electrolyte, and sealed to obtain a full cell (hereinafter also referred to as "full cell").

[0322] [Preparation of button cells]

[0323] The positive electrode active material, conductive agent acetylene black, and binder polyvinylidene fluoride (PVDF) 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 surface density was 0.015 g / cm³. 2 The compacted density is 2.0 g / cm³. 3 .

[0324] 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.

[0325] Example 2-33

[0326] The positive electrode active materials of Examples 2-33 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 to 7. All other processes are the same as those of Example 1.

[0327] Examples 34-51

[0328] The positive electrode active materials of Examples 34-51 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 8 to 10. All other processes are the same as those of Example 1.

[0329] Comparative Examples 1-18

[0330] The positive electrode active materials of Comparative Examples 1-18 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 to 7. Comparative Examples 1-2, 4-10 and 12 were not coated with the first coating layer, so steps S3 and S4 were omitted; Comparative Examples 1-11 were not coated with the second coating layer, so steps S5 and S6 were omitted; and Comparative Examples 1-18 were not coated with the third coating layer, so steps S7 and S8 were omitted.

[0331] In addition, in all embodiments and comparative examples of this application, unless otherwise specified, the first coating layer material, the second coating layer material, and the third coating layer material used are all assumed to be crystalline.

[0332]

[0333]

[0334]

[0335] Table 2: Preparation of the coating solution for the first coating layer (step S3)

[0336]

[0337] Table 3: Coverage of the first coating layer (step S4)

[0338]

[0339] Table 4: Preparation of the coating solution for the second coating layer (step S5)

[0340]

[0341] Table 5: Covering of the second coating layer (step S6)

[0342]

[0343] Table 6: Covering with the third coating layer (step S8)

[0344]

[0345] Table 7: Covering of the fourth coating layer (step S10)

[0346]

[0347]

[0348] Table 8: Investigation of the material of the first coating layer

[0349]

[0350] Table 9: Investigation of the Second Coating Material

[0351]

[0352] Table 10: Investigation of the third coating material

[0353]

[0354] II. Performance Evaluation

[0355] 1. Methods for measuring lattice change rate

[0356] Under a constant temperature environment of 25℃, the positive electrode active material sample was placed in an X-ray powder diffractometer (model Bruker D8 Discover) and tested at a rate of 1° / minute. 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 the unit cell in each direction, and v0 represents the volume of the unit cell, which can be directly obtained from the XRD refinement results).

[0357] 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.

[0358] 2. Li / Mn antisite defect concentration

[0359] 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.

[0360] 3. Surface oxygen valence state

[0361] 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.

[0362] 4. Compacted density

[0363] Take 5g of the prepared positive electrode active material powder and place it in a compaction mold (CARVER mold, model 13mm, USA). Then place the mold on a compaction density instrument. Apply a pressure of 3T (tons) and read the thickness of the powder under pressure (thickness after depressurization; the area of ​​the container used for testing is 1540.25mm²) on the instrument. 2 The compaction density is calculated using ρ = m / v.

[0364] 5. Method for measuring the initial specific capacity of button cells

[0365] The coin cells prepared in the above embodiments and comparative examples were charged to 4.3V at 0.1C, 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.

[0366] 6. 3C charging constant current ratio

[0367] Under a constant temperature of 25°C, the fresh full batteries 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. The batteries were then discharged at 1 / 3C to 2.5V, allowed to stand for 5 minutes, and then charged at 3C to 4.3V. After standing for 5 minutes, the charging capacity at this point was recorded as C1. The 3C charging constant current ratio is C1 / C0 × 100%. A higher 3C charging constant current ratio indicates better rate performance of the secondary battery.

[0368] 7. Battery swelling test after 30 days of storage at 60°C.

[0369] The 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. After every 48 hours of storage, the full cells were removed, allowed to stand for 1 hour, and then the open-circuit voltage (OCV) and internal resistance (IMP) were tested. 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 experiment, the weight F2 of the battery at this moment is measured, and the buoyant force F on the battery is measured. 浮 That is, F1-F2, and then according to Archimedes' principle, F 浮 =ρ×g×V 排 The battery volume V is calculated to be V = (F1 - F2) / (ρ × g).

[0370] Based on the OCV and IMP test results, the batteries in all embodiments maintained a SOC of over 99% throughout the entire testing process until the end of storage.

[0371] After 30 days of storage, the battery volume was measured, and the percentage increase in battery volume after storage was calculated relative to the battery volume before storage.

[0372] 8. Cyclic performance test of the entire battery at 45°C

[0373] Under a constant temperature environment of 45℃, the full battery is charged at 1C to 4.3V, then charged at 4.3V at a constant voltage until the current is ≤0.05mA. After resting for 5 minutes, it is discharged at 1C to 2.5V, and the capacity is recorded as D0. The above process is repeated until the capacity decays to 80% of D0. The number of repetitions at this point is recorded, which is the number of cycles corresponding to 80% capacity retention at 45℃.

[0374] 9. Dissolution test of transition metal Mn (and Fe doped at Mn sites)

[0375] 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.

[0376] 10. Measurement of manganese and phosphorus elements in positive electrode active materials

[0377] Dissolve 5g of the prepared positive electrode active material in 100mL of aqua regia (concentrated hydrochloric acid: concentrated nitric acid = 1:3). Use ICP to test the content of each element in the solution. 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.

[0378] 11. Coating thickness test

[0379] 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, and then performing TEM testing on the slice to obtain the original TEM image.

[0380] 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.

[0381] Measure the thickness of the selected particle at three locations and take the average value.

[0382] 12. Interplanar spacing and angle testing

[0383] 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. Stir and disperse 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 sample chamber of a TEM (Talos F200sG2) for testing and obtain the original TEM test image.

[0384] 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.

[0385] By comparing the obtained interplanar spacing and corresponding angle data with their standard values, different materials in the coating layer can be identified.

[0386] 13. Determination of the molar ratio of SP2 and SP3 forms in the third coating layer of carbon

[0387] This test was performed using Raman spectroscopy. I was obtained by peak separation of the Raman spectrum.d / I g I d For the peak intensity of SP3 carbon, I g The peak intensity of the SP2 carbon was used to confirm the molar ratio between the two.

[0388] 14. Determination of core chemical formula and composition of different coating layers

[0389] High spatial resolution characterization of the internal microstructure and surface structure of the positive electrode active material was performed using spherical aberration electron microscopy (ACSTEM). Combined with three-dimensional reconstruction technology, the core chemical formula and the composition of different coating layers of the positive electrode active material were obtained.

[0390] Table 11 shows the performance data of the positive electrode active material, coin cell or full cell in Examples 1-51 and Comparative Examples 1-18, measured according to the above performance test methods.

[0391] Table 12 shows the thickness of each coating layer and the weight ratio of manganese and phosphorus in the positive electrode active materials prepared in Examples 1-17 and Comparative Examples 3-4 and 12.

[0392] Table 13 shows the interplanar spacing and angle between the first and second coating layers in the positive electrode active materials prepared in Examples 1, 34-46.

[0393] Table 11

[0394]

[0395]

[0396] Table 12

[0397]

[0398] Table 13

[0399]

[0400] As shown in Table 11, compared with the comparative example, the embodiment achieves a smaller lattice change rate, a smaller Li / Mn antisite defect concentration, a larger compaction density, a surface oxygen valence state closer to -2, less Mn and Fe dissolution after cycling, and better battery performance, such as better high-temperature storage performance and high-temperature cycling performance.

[0401] As can be seen from Table 12, by doping and coating the Mn and P sites of lithium manganese iron phosphate (containing 35% manganese and about 20% phosphorus), the manganese content and the weight ratio of manganese to phosphorus in the positive electrode active material are significantly reduced. In addition, comparing Examples 1-17 with Comparative Examples 3, 4, and 12, and referring to Table 11, it can be seen that the reduction of manganese and phosphorus in the positive electrode active material will reduce the amount of manganese and iron ions dissolved and improve the performance of the battery prepared from it.

[0402] As shown in Table 13, the interplanar spacing and included angle of the first and second coating layers in the positive electrode active material of this application are both within the range described in this application. As shown in Table 11, using the first and second coating layers containing other elements within the scope of this application also yielded a positive electrode active material with good performance and achieved good battery performance results.

[0403] The inventors then investigated the effect of the coating sequence on battery performance. The positive electrode active materials of Examples 52-56 were prepared in a manner similar to Example 1. The differences in the preparation of the positive electrode active materials are shown in Table 14; all other processes were the same as in Example 1.

[0404] Table 15 shows the performance data of the positive electrode active material, coin cell, or full cell in Examples 52-56, measured according to the above performance test methods.

[0405] Table 14

[0406] Serial Number First coating layer Second coating layer Third coating layer Fourth coating layer Example 1 <![CDATA[1%Li2FeP2O7]]> <![CDATA[3%LiFePO4]]> <![CDATA[2%Li3BO3]]> 1% carbon Example 52 <![CDATA[1%Li2FeP2O7]]> <![CDATA[2%Li3BO3]]> <![CDATA[3%LiFePO4]]> 1% carbon Example 53 <![CDATA[3%LiFePO4]]> <![CDATA[1%Li2FeP2O7]]> <![CDATA[2%Li3BO3]]> 1% carbon Example 54 <![CDATA[3%LiFePO4]]> <![CDATA[2%Li3BO3]]> <![CDATA[1%Li2FeP2O7]]> 1% carbon Example 55 <![CDATA[2%Li3BO3]]> <![CDATA[1%Li2FeP2O7]]> <![CDATA[3%LiFePO4]]> 1% carbon Example 56 <![CDATA[2%Li3BO3]]> <![CDATA[3%LiFePO4]]> <![CDATA[1%Li2FeP2O7]]> 1% carbon

[0407] Table 15

[0408]

[0409] As shown in Tables 14 and 15, when the first coating layer includes crystalline pyrophosphate Li a MP2O7 and / or M b (P2O7) c The second coating layer comprises crystalline phosphate XPO4, and the third coating layer comprises crystalline borate Y. p B q O r When the fourth coating layer includes carbon, the battery has better overall performance.

[0410] 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 kernel includes Li m A x Mn 1-y B y P 1-z C z O 4-n D n The A includes one or more elements selected from families IA, IIA, IIIA, IIB, VB, and VIB; the B includes one or more elements selected from families IA, IIA, IIIA, IVA, VA, IIB, IVB, VB, VIB, and VIIIB; the C includes one or more elements selected from families IIIA, IVA, VA, and VIA; the D includes one or more elements selected from families VIA and VIIA; the m is selected from the range of 0.900 to 1.100; the x is selected from the range of 0 to 0.100; the y is selected from the range of 0.001 to 0.500; the z is selected from the range of 0.001 to 0.100; the n is selected from the range of 0 to 0.100; and the kernel is electrically neutral. The shell comprises crystalline pyrophosphate Li a MP2O7 and / or M b (P2O7) c , Crystalline phosphate XPO4, Crystalline borate Y p B q O r And carbon, and the shell comprises one or more coating layers, each coating layer independently comprising crystalline pyrophosphate Li. a MP2O7 and / or M b (P2O7) c , Crystalline phosphate XPO4, Crystalline borate Y p B q O r And one or more of carbon, crystalline pyrophosphate Li a MP2O7 and / or M b (P2O7) c Each of the M elements independently comprises one or more metallic elements selected from transition metals, Group IA, Group IIA, Group IIIA, Group IVA, Group VA, and the lanthanides, where 0 ≤ a ≤ 2, 1 ≤ b ≤ 4, and 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 To maintain electrical neutrality, X comprises one or more metallic elements selected from transition metals, Group IA, Group IIA, Group IIIA, Group IVA, Group VA, and the lanthanides; Y comprises one or more metallic elements selected from transition metals, Group IA, Group IIA, Group IIIA, Group IVA, Group VA, and the lanthanides; 1 ≤ p ≤ 4, 1 ≤ q ≤ 7, 2 ≤ r ≤ 12; and the values ​​of p, q, and r satisfy the following condition: to make crystalline borate Y... p B q O r Maintain electrical neutrality.

2. The positive electrode active material according to claim 1, wherein, The A element comprises one or more elements selected from Zn, Al, Na, K, Mg, Nb, Mo, and W; the B element comprises one or more elements selected from Group IA, Group IIA, Group IIIA, Group IVA, Group VA, Group IIB, Group IVB, Group VB, Group VIB, and Group VIIIB; the C element comprises one or more elements selected from B, S, Si, and N; and the D element comprises one or more elements selected from S, F, Cl, and Br.

3. The positive electrode active material according to claim 1, wherein, The m is selected from the range of 0.995 to 1.002, the x is selected from the range of 0.001 to 0.005, and the n is selected from the range of 0.001 to 0.

005.

4. The positive electrode active material according to claim 1, wherein, The crystalline pyrophosphate Li a MP2O7 and / or M b (P2O7) c Each of the M elements independently includes one or more elements selected from Fe, Ni, Mg, Mn, Co, Cu, Zn, Ti, Ag, Zr, Nb, Sb, and Al, and / or, In the crystalline phosphate XPO4, X comprises one or more elements selected from Li, Fe, Ni, Mg, Mn, Co, Cu, Zn, Ti, Ag, Zr, Nb, Sb, and Al, and / or, The crystalline borate Y p B q O r In this context, Y includes one or more elements selected from Li, Fe, Ni, Mg, Mn, Co, Cu, Zn, Ti, Ag, Zr, Nb, Sb, and Al.

5. The positive electrode active material according to claim 1, wherein, The crystalline pyrophosphate Li a MP2O7 and / or M b (P2O7) c The interplanar spacing ranges from 0.293 nm to 0.470 nm, and the included angle of the crystal orientation (111) ranges from 18.00° to 32.00°; and / or, The interplanar spacing of the crystalline phosphate XPO4 ranges from 0.244 nm to 0.425 nm, and the included angle of the crystal orientation (111) ranges from 20.00° to 37.00°.

6. The positive electrode active material according to any one of claims 1-5, wherein, The carbon is a mixture of SP2 and SP3 carbon.

7. The positive electrode active material according to claim 6, wherein, The molar ratio of SP2 carbon to SP3 carbon is any value within the range of 0.1 to 10.

8. The positive electrode active material according to claim 6, wherein, The molar ratio of SP2 carbon to SP3 carbon is any value within the range of 2.0 to 3.

0.

9. The positive electrode active material according to any one of claims 1-5, wherein, In the kernel, the ratio of y to 1-y is 1:10 to 1:1; and / or, In the kernel, the ratio of z to 1-z is 1:9 to 1:999; and / or, In the core, B comprises one or more elements selected from Fe, Ti, V, Ni, Co, and Mg; and / or, In the kernel, C includes an element selected from B, S, Si, and N.

10. The positive electrode active material according to any one of claims 1-5, wherein, In the kernel, the ratio of y to 1-y is 1:4 to 1:1; and / or, In the kernel, the ratio of z to 1-z is 1:499 to 1:249; and / or, In the core, B comprises at least two elements selected from Fe, Ti, V, Ni, Co, and Mg.

11. The positive electrode active material according to any one of claims 1-5, wherein, The ratio of q:r is 1:

3.

12. The positive electrode active material according to any one of claims 1-5, wherein, The shell includes a first coating layer covering the core, a second coating layer covering the first coating layer, a third coating layer covering the second coating layer, and a fourth coating layer covering the third coating layer. The fourth coating layer comprises carbon, and the first, second, and third coating layers each independently comprise crystalline pyrophosphate Li. a MP2O7 and / or M b (P2O7) c crystalline phosphate XPO4 and crystalline borate Y p B q O r Any one of them.

13. The positive electrode active material according to claim 12, wherein, The first coating layer comprises crystalline pyrophosphate Li a MP2O7 and / or M b (P2O7) c The fourth coating layer comprises carbon, and the second and third coating layers each independently comprise crystalline phosphate XPO4 or crystalline borate Y. p B q O r .

14. The positive electrode active material according to claim 12, wherein, 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 comprises crystalline borate Y. p B q O r The fourth coating layer comprises carbon.

15. The positive electrode active material according to claim 12, 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; And / or, The coating amount of the fourth coating layer is greater than 0 and less than or equal to 6% by weight, based on the weight of the core.

16. The positive electrode active material according to claim 12, 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; And / or, The coating amount of the fourth coating layer is greater than 0 and less than or equal to 5.5% by weight, based on the weight of the core.

17. The positive electrode active material according to claim 12, wherein, The coating amount of the fourth coating layer is greater than 0 and less than or equal to 2% by weight, based on the weight of the core.

18. The positive electrode active material according to claim 12, wherein, The thickness of the first coating layer is 1 nm to 15 nm; and / or, The thickness of the second coating layer is 1 nm to 15 nm; and / or, The thickness of the third coating layer is 1 nm to 15 nm; and / or, The thickness of the fourth coating layer is 2 nm to 25 nm.

19. The positive electrode active material according to any one of claims 1-5, wherein, Based on the weight of the positive electrode active material, the manganese content ranges from 10% to 35% by weight; and / or, Based on the weight of the positive electrode active material, the phosphorus content ranges from 12% to 25% by weight; and / or, Based on the weight of the positive electrode active material, the weight ratio of manganese to phosphorus ranges from 0.90 to 1.

25.

20. The positive electrode active material 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 15% to 30% by weight; and / or, Based on the weight of the positive electrode active material, the phosphorus content is in the range of 15% to 20% by weight; and / or, Based on the weight of the positive electrode active material, the weight ratio of manganese to phosphorus ranges from 0.95 to 1.

20.

21. The positive electrode active material 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 17% to 20% by weight.

22. The positive electrode active material according to any one of claims 1-5, wherein, The positive electrode active material satisfies at least one of the following conditions (1) to (4): (1) The lattice change rate of the positive electrode active material before and after complete lithium insertion / extraction is less than 4%; (2) The concentration of Li / Mn antisite defects in the positive electrode active material is less than 4%; (3) The compaction density of the positive electrode active material at 3T is 2.2 g / cm³. 3 above; (4) The surface oxygen valence state of the positive electrode active material is below -1.

90.

23. The positive electrode active material according to any one of claims 1-5, wherein, The positive electrode active material satisfies at least one of the following conditions (1) to (4): (1) The lattice change rate of the positive electrode active material before and after complete lithium insertion / extraction is less than 3.8%; (2) The concentration of Li / Mn antisite defects in the positive electrode active material is below 2.2%; (3) The compaction density of the positive electrode active material at 3T is 2.2 g / cm³. 3 Above and 2.8g / cm 3 the following; (4) The surface oxygen valence state of the positive electrode active material is -1.90 to -1.

98.

24. The positive electrode active material according to any one of claims 1-5, wherein, The positive electrode active material satisfies at least one of the following conditions (1) to (4): (1) The lattice change rate of the positive electrode active material before and after complete lithium insertion / extraction is 2.0% to 3.8%; (2) The concentration of Li / Mn antisite defects in the positive electrode active material is 1.5% to 2.2%.

25. A method for preparing a positive electrode active material, comprising the following steps: Steps for providing kernel material: The kernel includes Li m A x Mn 1-y B y P 1-z C z O 4-n D n The A includes one or more elements selected from families IA, IIA, IIIA, IIB, VB, and VIB; the B includes one or more elements selected from families IA, IIA, IIIA, IVA, VA, IIB, IVB, VB, VIB, and VIIIB; the C includes one or more elements selected from families IIIA, IVA, VA, and VIA; the D includes one or more elements selected from families VIA and VIIA; the m is selected from the range of 0.900 to 1.100; the x is selected from the range of 0 to 0.100; the y is selected from the range of 0.001 to 0.500; the z is selected from the range of 0.001 to 0.100; the n is selected from the range of 0 to 0.100; and the kernel is electrically neutral. Coating steps: Provide Li pyrophosphate separately. a MP2O7 and / or M b (P2O7) c Phosphate XPO4, borate Y p B q O r A coating solution containing a carbon source is prepared, and the core material is added to and mixed with the coating solution. The mixture is then sintered to obtain a positive electrode active material. The positive electrode active material has a core-shell structure, comprising the core and a shell coating the core. The shell comprises crystalline pyrophosphate Li. a MP2O7 and / or M b (P2O7) c , Crystalline phosphate XPO4, Crystalline borate Y p B q O r And carbon, and the shell comprises one or more coating layers, each coating layer independently comprising crystalline pyrophosphate Li. a MP2O7 and / or M b (P2O7) c , Crystalline phosphate XPO4, Crystalline borate Y p B q O r And one or more of carbon, crystalline pyrophosphate Li a MP2O7 and / or M b (P2O7) c Each of the M elements independently comprises one or more metallic elements selected from transition metals, Group IA, Group IIA, Group IIIA, Group IVA, Group VA, and the lanthanides, where 0 ≤ a ≤ 2, 1 ≤ b ≤ 4, and 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 To maintain electrical neutrality, X comprises one or more metallic elements selected from transition metals, Group IA, Group IIA, Group IIIA, Group IVA, Group VA, and the lanthanides; Y comprises one or more metallic elements selected from transition metals, Group IA, Group IIA, Group IIIA, Group IVA, Group VA, and the lanthanides; 1 ≤ p ≤ 4, 1 ≤ q ≤ 7, 2 ≤ r ≤ 12; and the values ​​of p, q, and r satisfy the following condition: to make crystalline borate Y... p B q O r Maintain electrical neutrality.

26. The preparation method according to claim 25, wherein, The A element comprises one or more elements selected from Zn, Al, Na, K, Mg, Nb, Mo, and W; the B element comprises one or more elements selected from Group IA, Group IIA, Group IIIA, Group IVA, Group VA, Group IIB, Group IVB, Group VB, Group VIB, and Group VIIIB; the C element comprises one or more elements selected from B, S, Si, and N; and the D element comprises one or more elements selected from S, F, Cl, and Br.

27. The preparation method according to claim 25, wherein, The m is selected from the range of 0.995 to 1.002, the x is selected from the range of 0.001 to 0.005, and the n is selected from the range of 0.001 to 0.

005.

28. The preparation method according to claim 25, wherein, The crystalline pyrophosphate Li a MP2O7 and / or M b (P2O7) c Each of the M elements independently includes one or more elements selected from Fe, Ni, Mg, Mn, Co, Cu, Zn, Ti, Ag, Zr, Nb, Sb, and Al, and / or, In the crystalline phosphate XPO4, X comprises one or more elements selected from Li, Fe, Ni, Mg, Mn, Co, Cu, Zn, Ti, Ag, Zr, Nb, Sb, and Al, and / or, The crystalline borate Y p B q O r In this context, Y includes one or more elements selected from Li, Fe, Ni, Mg, Mn, Co, Cu, Zn, Ti, Ag, Zr, Nb, Sb, and Al.

29. The preparation method according to any one of claims 25 to 28, wherein, The steps for providing the core material include the following steps: Step (1): Mix and stir the manganese source, dopant of element B and acid in a container to obtain manganese salt particles doped with element B; Step (2): Mix the manganese salt particles doped with element B with lithium source, phosphorus source, dopant of element C, optional dopant of element A and optional dopant of element D in a solvent to obtain a slurry, and sinter it under an inert gas atmosphere to obtain the core material.

30. The preparation method according to any one of claims 25 to 28, wherein, Step (1) is performed at a temperature between 20°C and 120°C; and / or, The stirring in step (1) is carried out at 400 rpm to 700 rpm for 1 hour to 9 hours; and / or, Step (2) involves mixing at a temperature of 20°C to 120°C for 1 to 12 hours; and / or, The sintering in step (2) is carried out at 600°C to 950°C for 4 to 10 hours in an inert gas atmosphere or a mixture of inert gas and hydrogen.

31. The preparation method according to any one of claims 25 to 28, wherein, Step (1) is performed at a temperature of 40°C to 120°C; and / or, The stirring in step (1) is carried out at 400 rpm to 700 rpm for 3 to 7 hours; and / or, Step (2) involves mixing at a temperature of 40°C to 120°C for 1 to 12 hours; and / or, The sintering in step (2) is carried out at 600°C to 950°C for 4 to 10 hours in an inert gas atmosphere or a mixture of inert gas and hydrogen.

32. The preparation method according to any one of claims 25 to 28, wherein, The coating step includes coating crystalline pyrophosphate Li a MP2O7 and / or M b (P2O7) c Steps for coating crystalline phosphate XPO4, steps for coating crystalline borate Y p B q O r The steps and the steps of coating carbon.

33. The preparation method according to any one of claims 25 to 28, wherein, The first coating step is to coat crystalline pyrophosphate Li. a MP2O7 and / or M b (P2O7) c The first step is to coat crystalline phosphate XPO4, and the second step is to coat crystalline borate Y. p B q O r The fourth coating step is the carbon coating step, thereby obtaining a positive electrode active material with a core-shell structure, which includes the 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, a third coating layer covering the second coating layer, and a fourth coating layer covering the third coating layer. The first coating layer includes crystalline pyrophosphate Li. a MP2O7 and / or M b (P2O7) c The second coating layer comprises crystalline phosphate XPO4, and the third coating layer comprises crystalline borate Y. p B q O r The fourth coating layer comprises carbon.

34. The preparation method according to claim 33, wherein, The coated crystalline pyrophosphate Li a MP2O7 and / or M b (P2O7) c The steps include: adding a source of element M, a phosphorus source, an acid, and optionally a lithium source to a solvent to obtain a coating solution; thoroughly mixing the material to be coated with the coating solution; drying; and then sintering to obtain crystalline pyrophosphate Li. a MP2O7 and / or M b (P2O7) c The covering material.

35. The preparation method according to claim 33, wherein, The steps of coating crystalline phosphate XPO4 include the following steps: adding the source of element X, the phosphorus source and the acid to a solvent to obtain a coating solution; thoroughly mixing the material to be coated with the coating solution; drying; and then sintering to obtain a material coated with crystalline phosphate XPO4.

36. The preparation method according to claim 33, wherein, The coated crystalline borate Y p B q O r The steps include: adding a source of element Y and a source of boron to a solvent to obtain a coating solution; thoroughly mixing the material to be coated with the coating solution; drying; and then sintering to obtain crystalline borate Y. p B q O r The covering material.

37. The preparation method according to claim 33, wherein, The carbon coating process includes the following steps: adding a carbon source to a solvent to obtain a coating solution, adding the material to be coated to the coating solution, mixing evenly, drying, and then sintering to obtain a carbon-coated material.

38. The preparation method according to claim 33, wherein, The coated crystalline pyrophosphate Li a MP2O7 and / or M b (P2O7) c In the following steps, the pH of the solution containing the dissolved source of element M, phosphorus source, and acid, and optionally lithium source, is controlled to be 3.5 to 6.5, then stirred and reacted for 1 to 5 hours, then the solution is heated to 50°C to 120°C and maintained at that temperature for 2 to 10 hours; and / or, The coated crystalline pyrophosphate Li a MP2O7 and / or M b (P2O7) c The sintering process in this step is carried out at 650°C to 800°C for 2 to 6 hours; and / or, In the step of coating crystalline phosphate XPO4, the source of element X, the phosphorus source, and the acid are dissolved in a solvent, stirred, and reacted for 1 to 10 hours. Then, the solution is heated to 60°C to 150°C and maintained at this temperature for 2 to 10 hours; and / or, The sintering process in the step of coating crystalline phosphate XPO4 is carried out at 500°C to 700°C for 6 to 10 hours; and / or, The coated crystalline borate Y p B q O r The sintering process in this step is carried out at 300°C to 500°C for 2 to 10 hours; and / or, The sintering process in the carbon coating step is carried out at 700°C to 800°C for 6 to 10 hours.

39. 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 according to any one of claims 1-24 or a positive electrode active material prepared by any one of claims 25-38, and the positive electrode active material having a content of 10% by weight or more in the positive electrode film layer based on the total weight of the positive electrode film layer.

40. The positive electrode sheet according to claim 39, wherein, The positive electrode active material has a content of 90% to 99.5% by weight in the positive electrode film, based on the total weight of the positive electrode film.

41. A secondary battery comprising the positive electrode active material according to any one of claims 1-24, or the positive electrode active material prepared by any one of claims 25-38, or the positive electrode sheet according to claim 39 or 40.

42. An electrical device comprising the secondary battery of claim 41.

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