A positive electrode active material having a core-shell structure, a method for manufacturing the same, a positive electrode sheet, a secondary battery, a battery module, a battery pack, and a power tool

Abstract

The application provides a positive electrode active material, a preparation method thereof, a positive electrode sheet, a secondary battery, a battery module, a battery pack and a power utilization device. The positive electrode active material comprises a core containing Li m A x Mn 1‑y B y P 1‑ z C z O 4‑n D n a core containing crystalline pyrophosphate Li a MP2O7 and / or M b (P2O7) c a first coating layer containing oxide M′ d O e a second coating layer, and a third coating layer containing carbon. The positive electrode active material can reduce the generation of Li / Mn antisite defects, reduce manganese elution and reduce the lattice change rate, improve the capacity of the secondary battery, and improve the cycle performance, high-temperature storage performance and safety performance of the secondary battery.

Description

Technical Field

[0001] This application relates to the field of secondary battery technology, and in particular to a positive electrode active material with a core-shell structure, a method for preparing the positive electrode active material, a positive electrode sheet, a secondary battery, a battery module, a battery pack, and an electrical device. Background Technology

[0002] In recent years, with the increasingly widespread application of rechargeable batteries, they 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 vehicles, military equipment, aerospace, and many other fields. Due to the significant development of rechargeable batteries, higher requirements have been placed on their energy density, cycle performance, and safety performance. As an existing positive electrode active material for rechargeable batteries, lithium manganese phosphate is prone to Li / Mn antisite defects during charge and discharge, resulting in severe manganese dissolution, which affects the specific capacity of the rechargeable battery and leads to poor safety and cycle performance. Summary of the Invention

[0003] This application is made in view of the above-mentioned problems, and its purpose is to provide a positive electrode active material with a core-shell structure, a method for preparing the positive electrode active material, a positive electrode sheet, a secondary battery, a battery module, a battery pack, and an electrical device, so as to solve the problem that existing lithium manganese phosphate positive electrode active materials are prone to Li / Mn antisite defects and severe manganese dissolution during charging and discharging, thereby solving the problems of low capacity, poor safety performance, and poor cycle performance of secondary batteries.

[0004] To achieve the above objectives, 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 nWherein, m is selected from any value in the range of 0.9-1.1, x is selected from any value in the range of 0.001-0.1, y is selected from any value in the range of 0.001-0.6, optionally from any value in the range of 0.001-0.5, z is selected from any value in the range of 0.001-0.1, n is selected from any value in the range of 0.001-0.1, and A is one or more elements selected from Zn, Al, Na, K, Mg, Nb, Mo and W, optionally from Al, Mg, Nb, M One or more elements selected from o and W, B is one or more elements selected from Ti, V, Zr, Fe, Ni, Mg, Co, Ga, Sn, Sb, Nb and Ge, optionally one or more elements selected from Ti, V, Fe, Ni, Mg and Co, C is one or more elements selected from B, S, Si and N, optionally one or more elements selected from S, Si and N, and D is one or more elements selected from S, F, Cl and Br, optionally one or more elements selected from F, Cl and Br;

[0006] The shell includes a first covering layer that covers the core, a second covering layer that covers the first covering layer, and a third covering layer that covers the second covering layer, wherein,

[0007] The first coating layer contains crystalline pyrophosphate Li. a MP2O7 and / or M b (P2O7) c Where a is greater than 0 and less than or equal to 2, b is any value in the range of 1-4, and c is any value in the range of 1-3, crystalline pyrophosphate Li a MP2O7 and M b (P2O7) c Each of the M elements is independently selected from one or more elements chosen from Fe, Ni, Mg, Co, Cu, Zn, Ti, Ag, Zr, Nb, and Al, and optionally from one or more elements chosen from Fe, Ni, Mg, Co, Cu, Zn, Ti, Ag, Zr, and Al.

[0008] The second coating layer contains oxide M′ d O eWherein, d is greater than 0 and less than or equal to 2, e is greater than 0 and less than or equal to 5, and M′ is one or more elements selected from alkali metals, alkaline earth metals, transition metals, Group IIIA elements, Group IVA elements, lanthanides and Sb, optionally selected from Li, Be, B, Na, Mg, Al, Si, P, S, K, Ca, Sc, Ti, V, Cr, Mn, Fe, Co, Ni, Cu, Zn, Ga, Ge, As, Se, Sr, Y, Zr, Nb, Mo, Tc, Ru, Rh, Pd, Ag, Cd, In, Sn, Sb, Te, W, La and Ce, more preferably selected from Mg, Al, Ca, Ti, V, Co, Ni, Cu, Zn and Zr;

[0009] The third coating layer contains carbon.

[0010] The inventors of this application discovered in practical operation that lithium manganese phosphate cathode active materials are prone to Li / Mn antisite defects during deep charge-discharge processes, resulting in significant manganese dissolution. The dissolved manganese migrates to the anode and is reduced to metallic manganese. This generated metallic manganese acts as a "catalyst," catalyzing the decomposition of the SEI film (solid electrolyte interphase) on the anode surface. Some of the byproducts are gases, which can easily cause battery expansion, affecting the safety performance of the secondary battery. Others deposit on the anode surface, hindering the passage of lithium ions in and out of the anode, increasing the impedance of the secondary battery, and affecting its kinetic and cycle performance. Furthermore, to replenish the lost SEI film, the electrolyte and active lithium inside the battery are continuously consumed, irreversibly impacting the capacity retention rate of the secondary battery.

[0011] Consequently, the applicant unexpectedly discovered that by doping lithium manganese phosphate with elements A, B, C, and D at the Li, manganese, phosphorus, and O sites to obtain a doped lithium manganese phosphate core, and then sequentially coating the core surface with three layers, a novel core-shell structured positive electrode active material is provided. This material can significantly reduce the generation of Li / Mn antisite defects, significantly reduce manganese dissolution and lattice change rate, and increase compaction density. When applied to secondary batteries, it can improve the capacity, cycle performance, high-temperature storage performance, and safety performance of the secondary battery. The oxide in the second coating layer possesses high structural stability and low surface activity; therefore, coating with the second coating layer can effectively reduce interfacial side reactions, thereby improving the battery's high-temperature cycle and high-temperature storage performance.

[0012] In this article, "crystalline state" refers to a crystallinity of 50% or higher, i.e., 50%-100%. Crystallinity less than 50% is called "glassy state".

[0013] The crystallinity of the crystalline pyrophosphate in this application is 50% to 100%. Pyrophosphate with a certain degree of crystallinity not only helps to fully utilize the ability of the pyrophosphate coating layer to inhibit manganese dissolution and reduce interfacial side reactions, but also enables the pyrophosphate coating layer and the oxide coating layer to achieve better lattice matching, thereby achieving a tight bond between the coating layers.

[0014] Unless otherwise stated, the chemical formula Li m A x Mn 1-y B y P 1-z C z O 4-n D n In this context, when A comprises two or more elements, the aforementioned limitation on the range of x values ​​applies not only to the stoichiometric coefficient of each element representing A, but also to the sum of the stoichiometric coefficients of all elements representing A. For example, when A comprises 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 comprise two or more elements, the limitation on the range of stoichiometric coefficients of B, C, and D in this application also has the aforementioned meaning.

[0015] Unless otherwise stated, the chemical formula M b (P2O7) c In this context, when M consists of two or more elements, the aforementioned limitation on the range of values ​​for b applies not only to the stoichiometric coefficient of each element as M, but also to the sum of the stoichiometric coefficients of all elements as M. For example, when M consists of two or more elements M1, M2...Mn, the stoichiometric coefficients b1, b2...bn of each element must fall within the range of values ​​for b defined in this application, and the sum of b1, b2...bn must also fall within this range. Similarly, for the chemical formula M′... d O e When M′ contains two or more elements, the limitation on the numerical range of the stoichiometric coefficient d of M′ in this application also has the above meaning.

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

[0017] Optionally, the interplanar spacing of the crystalline pyrophosphate in the first coating layer ranges from 0.300 to 0.310 nm; and / or,

[0018] The included angle of the crystal orientation (111) of the crystalline pyrophosphate in the first coating layer ranges from 29.00° to 30.00°.

[0019] The first coating layer of the positive electrode active material in this application uses crystalline materials, and their interplanar spacing and angles are within the aforementioned range. This effectively avoids impurity phases in the coating layer, thereby improving the specific capacity of the material and enhancing the cycle performance and rate performance of the secondary battery.

[0020] In any implementation of the first aspect, x is any value in the range of 0.001-0.005; and / or,

[0021] y is any value in the range of 0.01-0.5, optionally any value in the range of 0.25-0.5; and / or,

[0022] z is any value in the range of 0.001-0.005; and / or,

[0023] n is any value in the range of 0.001 to 0.005.

[0024] Therefore, within the above ranges, x, y, z, and n can further reduce the generation of Li / Mn antisite defects, reduce manganese dissolution, and lower the lattice change rate, thereby further improving the capacity of the secondary battery and enhancing its cycle performance, high-temperature storage performance, and safety performance.

[0025] In any embodiment of the first aspect, in the core, the ratio of 1-y to y is 0.67-999, optionally 1 to 4, and more preferably 1.5 to 3. This further improves the cycle performance and rate performance of the secondary battery.

[0026] In any implementation of the first aspect, the ratio of m to x in the kernel is between 9 and 1100, optionally between 190 and 998. This further improves the cycle performance and rate performance of the secondary battery.

[0027] In any embodiment of the first aspect, the carbon in the third coating layer is a mixture of SP2 carbon and SP3 carbon; optionally, the molar ratio of SP2 carbon to SP3 carbon is any value in the range of 0.07-13, more preferably any value in the range of 0.1-10, and even more preferably any value in the range of 2.0-3.0.

[0028] This application improves the overall performance of the secondary battery by limiting the molar ratio of SP2 carbon to SP3 carbon within the above-mentioned range.

[0029] In any embodiment of the first aspect, 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 further optionally greater than 0 and less than or equal to 2% by weight, based on the kernel's weight; and / or

[0030] 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, and further optionally 2% to 4% by weight, based on the kernel's weight; and / or

[0031] 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 further optionally greater than 0 and less than or equal to 2% by weight, based on the kernel weight.

[0032] In the core-shell structured positive electrode active material of this application, the coating amount of the three coating layers is preferably within the above-mentioned range, thereby enabling sufficient coating of the core and further improving the kinetic performance and safety performance of the secondary battery without sacrificing the specific capacity of the positive electrode active material.

[0033] In any embodiment of the first aspect, the thickness of the first coating layer is 1-10 nm. In this application, when the thickness of the first coating layer is in the range of 1-10 nm, the dissolution and migration of transition metal ions can be further reduced, thereby improving the kinetic performance of the secondary battery.

[0034] In any embodiment of the first aspect, the thickness of the second coating layer is 2-25 nm, optionally 2-15 nm. When the thickness of the second coating layer is within the above range, 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 performance of the secondary battery.

[0035] In any embodiment of the first aspect, the thickness of the third coating layer is 2-25 nm. When the thickness of the third coating layer is in the range of 2-25 nm, the electrical conductivity of the material can be improved and the compaction density performance of the battery electrode prepared using the positive electrode active material can be improved.

[0036] In any embodiment of the first aspect, a gravimeter based on the positive electrode active material,

[0037] The manganese content is in the range of 10% to 35% by weight, preferably in the range of 13.3% to 33.2% by weight, more preferably in the range of 15% to 30% by weight, and even more preferably in the range of 17% to 20% by weight, and / or

[0038] The phosphorus content is in the range of 12%-25% by weight, preferably in the range of 15%-20% by weight, and even more preferably in the range of 16.8%-19.5% by weight;

[0039] Optionally, the weight ratio of manganese to phosphorus is 0.71-1.85, more preferably in the range of 0.90-1.25, and even more preferably in the range of 0.95-1.20.

[0040] In the core-shell structured positive electrode active material of this application, the content of manganese is within the above-mentioned range, which can effectively improve the structural stability and density of the positive electrode active material, thereby improving the cycle, storage and compaction density performance of the secondary battery; and can maintain a certain voltage plateau height, thereby improving the energy density of the secondary battery.

[0041] In the core-shell structured positive electrode active material of this application, the phosphorus content is within the above-mentioned range, which can effectively improve the conductivity of the positive electrode active material and improve the structural stability of the positive electrode active material.

[0042] In the core-shell structured positive electrode active material of this application, the weight ratio of manganese to phosphorus is within the above-mentioned range, which can reduce the dissolution of transition metals, thereby improving the stability of the positive electrode active material and the cycle and storage performance of the secondary battery, and maintaining a certain discharge voltage plateau height, thereby improving the energy density of the secondary battery.

[0043] In any embodiment of the first aspect, the lattice change rate of the positive electrode active material before and after complete lithium insertion / extraction is 8.1% or less, optionally 4% or less, more preferably 3.8% or less, and even more preferably 2.0%-3.8%.

[0044] The core-shell structured positive electrode active material of this application can achieve a lattice change rate of less than 8.1% (optionally less than 4%) before and after lithium insertion / extraction. Therefore, the use of the positive electrode active material can improve the specific capacity and rate performance of secondary batteries.

[0045] In any embodiment of the first aspect, 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%-2.2%. By keeping the Li / Mn antisite defect concentration within the above range, the Li... + This improves the transmission capacity of the cathode active material and the rate performance of the secondary battery.

[0046] In any embodiment of the first aspect, the compaction density of the positive electrode active material at 3T is 1.98 g / cm³. 3 The above can be selected as 2.2g / cm. 3 The above is optional, 2.2 g / cm³. 3Above and 2.8g / cm 3 Therefore, increasing the compaction density increases the weight of the positive electrode active material per unit volume, which is beneficial for improving the volumetric energy density of the secondary battery.

[0047] In any embodiment of the first aspect, the surface oxygen valence state of the positive electrode active material is -1.90 or lower, optionally from -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 material and the electrolyte can be reduced, thereby improving the cell's cycle life, high-temperature storage gas generation, and other performance characteristics.

[0048] A second aspect of this application provides a method for preparing a positive electrode active material, comprising the following steps:

[0049] Steps for providing kernel materials: Kernel materials contain Li m A x Mn 1-y B y P 1-z C z O 4-n D n Wherein, m is selected from any value in the range of 0.9-1.1, x is selected from any value in the range of 0.001-0.1, y is selected from any value in the range of 0.001-0.6, optionally from any value in the range of 0.001-0.5, z is selected from any value in the range of 0.001-0.1, n is selected from any value in the range of 0.001-0.1, and A is one or more elements selected from Zn, Al, Na, K, Mg, Nb, Mo and W, optionally from Al, Mg, Nb, M One or more elements selected from o and W, B is one or more elements selected from Ti, V, Zr, Fe, Ni, Mg, Co, Ga, Sn, Sb, Nb and Ge, optionally one or more elements selected from Ti, V, Fe, Ni, Mg and Co, C is one or more elements selected from B, S, Si and N, optionally one or more elements selected from S, Si and N, and D is one or more elements selected from S, F, Cl and Br, optionally one or more elements selected from F, Cl and Br;

[0050] First coating step: Providing Li containing pyrophosphate a MP2O7 and / or M b (P2O7) c The first mixture is used to mix the core material with the first mixture, dry it, and sinter it to obtain the material coated by the first coating layer; wherein, a is greater than 0 and less than or equal to 2, b is any value in the range of 1-4, and c is any value in the range of 1-3; Li pyrophosphate a MP2O7 and Mb (P2O7) c Each of the M elements is independently selected from one or more elements chosen from Fe, Ni, Mg, Co, Cu, Zn, Ti, Ag, Zr, Nb, and Al, and optionally from one or more elements chosen from Fe, Ni, Mg, Co, Cu, Zn, Ti, Ag, Zr, and Al.

[0051] Second coating step: Providing oxide M′ d O e The second mixture is obtained by mixing the material coated by the first coating layer with the second mixture, drying, and sintering to obtain a material coated by two coating layers; wherein, d is greater than 0 and less than or equal to 2, e is greater than 0 and less than or equal to 5, and M′ is one or more elements selected from alkali metals, alkaline earth metals, transition metals, Group IIIA elements, Group IVA elements, lanthanides, and Sb, optionally one or more elements selected from Li, Be, B, Na, Mg, Al, Si, P, S, K, Ca, Sc, Ti, V, Cr, Mn, Fe, Co, Ni, Cu, Zn, Ga, Ge, As, Se, Sr, Y, Zr, Nb, Mo, Tc, Ru, Rh, Pd, Ag, Cd, In, Sn, Sb, Te, W, La, and Ce, and more preferably one or more elements selected from Mg, Al, Ca, Ti, V, Co, Ni, Cu, Zn, and Zr;

[0052] The third coating step: a third mixture containing a carbon source is provided, the material coated by the two coating layers is mixed with the third mixture, dried, and sintered to obtain the positive electrode active material;

[0053] The positive electrode active material has a core-shell structure, comprising a core and a shell covering the core, wherein the core contains Li. m A x Mn 1-y B y P 1-z C z O 4-n D n The shell includes a first coating layer covering the core, a second coating layer covering the first coating layer, and a third coating layer covering the second coating layer. The first coating layer contains crystalline pyrophosphate Li. a MP2O7 and / or M b (P2O7) c The second coating layer contains oxide M′ d O e The third coating layer contains carbon.

[0054] Therefore, this application provides a novel core-shell structured positive electrode active material by doping lithium manganese phosphate with elements A, B, C, and D at the Li, manganese, phosphorus, and O sites, and then sequentially coating the core surface with three layers. This material can significantly reduce the generation of Li / Mn antisite defects, significantly reduce manganese dissolution and lattice change rate, and increase compaction density. When applied to secondary batteries, it can improve the capacity of secondary batteries, and enhance their cycle performance, high-temperature storage performance, and safety performance.

[0055] In any embodiment of the second aspect of this application, the step of providing the kernel material includes the following steps:

[0056] Step (1): Mix the manganese source, the source of element B, the acid, and an optional solvent to obtain a mixture;

[0057] Step (2): Mix the mixture with a lithium source, a phosphorus source, a source of element A, a source of element C, and a source of element D, and optionally a solvent, dry, and sinter to obtain a mixture containing Li. m A x Mn 1-y B y P 1-z C z O 4-n D n The core material.

[0058] In any embodiment of the second aspect of this application, step (1) is performed at 60°C-120°C; and / or, in step (1), the mixture is performed by stirring at a speed of 200-800 rpm.

[0059] In any embodiment of the second aspect of this application, in step (2), the mixing is carried out for 8-15 hours.

[0060] In any embodiment of the second aspect of this application, in step (2), sintering is carried out at 600°C-900°C for 6-14 hours.

[0061] In any embodiment of the second aspect of this application,

[0062] In the first coating step, a first mixture is obtained by mixing a source of element M, a phosphorus source, an acid, an optional lithium source, and an optional solvent; and / or,

[0063] In the second coating step, a second mixture is obtained by mixing the source of element M′ with a solvent; and / or,

[0064] In the third coating step, a third mixture is obtained by mixing the carbon source with a solvent.

[0065] In any embodiment of the second aspect of this application, in the first coating step, the source of element M, the phosphorus source, the acid, the optional lithium source, and the optional solvent are mixed at room temperature for 1-5 hours, then heated to 50°C-120°C and maintained at that temperature for 2-10 hours, and the above mixing is carried out under conditions of pH 3.5-6.5.

[0066] In any embodiment of the second aspect of this application, in the second coating step, the source of element M′ and the solvent are mixed at room temperature for 1-10 hours, then heated to 60°C-150°C and kept at that temperature for 2-10 hours.

[0067] In any embodiment of the second aspect of this application, the source of element A is one or more selected from the elemental form, oxide, phosphate, oxalate, carbonate, and sulfate of element A; and / or,

[0068] The source of element B is one or more selected from elemental form, oxide, phosphate, oxalate, carbonate, and sulfate of element B; and / or,

[0069] The source of element C is one or more selected from sulfates, borates, nitrates, and silicates of element C; and / or,

[0070] The source of element D is one or more selected from element D in its simple form and ammonium salt.

[0071] In any embodiment of the second aspect of this application, in the first coating step, sintering is carried out at 650-800°C for 2-6 hours; and / or,

[0072] In the second coating step, sintering is carried out at 500-700℃ for 6-10 hours; and / or,

[0073] In the third coating step, sintering is carried out at 700-800℃ for 6-10 hours.

[0074] 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 preparation method of the second aspect of this application. Optionally, the content of the positive electrode active material in the positive electrode film layer is 90-99.5% by weight, more preferably 95-99.5% by weight, based on the total weight of the positive electrode film layer.

[0075] 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 preparation method of the second aspect of this application, or the positive electrode sheet of the third aspect of this application.

[0076] The fifth aspect of this application provides a battery module, including the secondary battery of the fourth aspect of this application.

[0077] A sixth aspect of this application provides a battery pack that includes the battery module of the fifth aspect of this application.

[0078] A seventh aspect of this application provides an electrical device comprising at least one selected from the fourth aspect of this application, the fifth aspect of this application, and the sixth aspect of this application. Attached Figure Description

[0079] Figure 1 This is a schematic diagram of a three-layer coated positive electrode active material according to an embodiment of this application.

[0080] Figure 2 This is a schematic diagram of a secondary battery according to one embodiment of this application.

[0081] Figure 3 yes Figure 2 An exploded view of a secondary battery according to one embodiment of this application is shown.

[0082] Figure 4 This is a schematic diagram of a battery module according to one embodiment of this application.

[0083] Figure 5 This is a schematic diagram of a battery pack according to one embodiment of this application.

[0084] Figure 6 yes Figure 5 An exploded view of a battery pack according to one embodiment of this application is shown.

[0085] Figure 7 This is a schematic diagram of an electrical device that uses a secondary battery as a power source according to one embodiment of this application.

[0086] Explanation of reference numerals in the attached figures:

[0087] 1 Battery pack; 2 Upper housing; 3 Lower housing; 4 Battery module; 5 Secondary battery; 51 Housing; 52 Electrode assembly; 53 Top cover assembly. Detailed Implementation

[0088] The following detailed description, with appropriate reference to the accompanying drawings, discloses embodiments of the positive electrode active material, the method for preparing the positive electrode active material, the positive electrode sheet, the secondary battery, the battery module, the battery pack, and the power supply device of this application. However, unnecessary detailed descriptions may be omitted. For example, detailed descriptions of well-known matters and repetitive descriptions of practically identical structures may be omitted. This is to avoid unnecessarily lengthy descriptions and to facilitate understanding by those skilled in the art. Furthermore, the accompanying drawings and the following description are provided for those skilled in the art to fully understand this application and are not intended to limit the subject matter of the claims.

[0089] 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.

[0090] In this application, the terms "below a certain value" and "above a certain value" refer to the range defined by a certain value as the upper or lower limit.

[0091] Unless otherwise specified, all embodiments and optional embodiments of this application can be combined to form new technical solutions.

[0092] Unless otherwise specified, all technical features and optional technical features of this application may be combined to form new technical solutions.

[0093] Unless otherwise specified, all steps of this application may be performed sequentially or randomly, preferably sequentially. For example, if the method includes steps (a) and (b), it means that the method may include steps (a) and (b) performed sequentially, or it may include steps (b) and (a) performed sequentially. For example, if the method may also include step (c), it means 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.

[0094] 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.

[0095] 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).

[0096] Unless otherwise specified, in this application, the term "cladding layer" refers to a material layer covering the core, which may completely or partially cover the core. The use of "cladding layer" is for ease of description only and is not intended to limit the invention.

[0097] Unless otherwise specified, in this application, the term "thickness of the coating layer" refers to the thickness of the material layer covering the core in the radial direction of the core.

[0098] [Rechargeable Battery]

[0099] Secondary batteries, also known as rechargeable batteries or storage batteries, are batteries that can be recharged after being discharged to activate the active materials and continue to be used.

[0100] Typically, a secondary battery consists of a positive electrode, a negative electrode, a separator, and an electrolyte. During charging and discharging, active ions (such as lithium ions) repeatedly insert and extract between the positive and negative electrodes. The separator, positioned between the positive and negative electrodes, primarily prevents short circuits while allowing active ions to pass through. The electrolyte, also positioned between the positive and negative electrodes, mainly serves to conduct active ions.

[0101] [Positive electrode active material]

[0102] This application provides a positive electrode active material with a core-shell structure, comprising a core and a shell covering the core.

[0103] The kernel includes Li m A x Mn 1-y B y P 1-z C z O 4-n D nWhere m is selected from any value in the range of 0.9-1.1, x is selected from any value in the range of 0.001-0.1 (e.g., 0.002, 0.003, 0.005, 0.05), y is selected from any value in the range of 0.001-0.6, optionally from any value in the range of 0.001-0.5, z is selected from any value in the range of 0.001-0.1 (e.g., 0.005, 0.04, 0.05, 0.08), n is selected from any value in the range of 0.001-0.1 (e.g., 0.002, 0.005, 0.01, 0.05, 0.08), and A is selected from Zn, Al, N a) one or more elements selected from K, Mg, Nb, Mo, and W, optionally one or more elements selected from Al, Mg, Nb, Mo, and W; b) one or more elements selected from Ti, V, Zr, Fe, Ni, Mg, Co, Ga, Sn, Sb, Nb, and Ge, optionally one or more elements selected from Ti, V, Fe, Ni, Mg, and Co; c) one or more elements selected from B, S, Si, and N, optionally one or more elements selected from S, Si, and N; and d) one or more elements selected from S, F, Cl, and Br, optionally one or more elements selected from F, Cl, and Br.

[0104] The shell includes a first covering layer that covers the core, a second covering layer that covers the first covering layer, and a third covering layer that covers the second covering layer, wherein,

[0105] The first coating layer contains crystalline pyrophosphate Li. a MP2O7 and / or M b (P2O7) c Where a is greater than 0 and less than or equal to 2, b is any value in the range of 1-4, and c is any value in the range of 1-3, crystalline pyrophosphate Li a MP2O7 and M b (P2O7) c Each of the M elements is independently selected from one or more elements chosen from Fe, Ni, Mg, Co, Cu, Zn, Ti, Ag, Zr, Nb, and Al, and optionally from one or more elements chosen from Fe, Ni, Mg, Co, Cu, Zn, Ti, Ag, Zr, and Al.

[0106] The second coating layer contains oxide M′ d O e (Optional: crystalline oxide M′) d O e), wherein d is greater than 0 and less than or equal to 2, e is greater than 0 and less than or equal to 5, and M′ is one or more elements selected from alkali metals, alkaline earth metals, transition metals, Group IIIA elements, Group IVA elements, lanthanides and Sb, optionally selected from Li, Be, B, Na, Mg, Al, Si, P, S, K, Ca, Sc, Ti, V, Cr, Mn, Fe, Co, Ni, Cu, Zn, Ga, Ge, As, Se, Sr, Y, Zr, Nb, Mo, Tc, Ru, Rh, Pd, Ag, Cd, In, Sn, Sb, Te, W, La and Ce, more preferably selected from Mg, Al, Ca, Ti, V, Co, Ni, Cu, Zn and Zr;

[0107] The third coating layer contains carbon.

[0108] The positive electrode active material of this application can improve the specific capacity, cycle performance, and safety performance of secondary batteries. Although the mechanism is not yet clear, it is speculated that the positive electrode active material of this application has a core-shell structure, in which elements A, B, C, and D are doped at the Li, manganese, phosphorus, and O sites of the lithium manganese phosphate core, respectively. This not only effectively reduces manganese dissolution, thereby reducing the number of manganese ions migrating to the negative electrode and reducing the electrolyte consumed due to SEI film decomposition, thus improving the cycle performance and safety performance of the secondary battery, but also promotes Mn-O bond adjustment, lowers the lithium ion migration barrier, promotes lithium ion migration, and improves the rate performance of the secondary battery. Furthermore, by coating the core with a first layer of crystalline pyrophosphate... The coating layer can further increase the migration resistance of manganese, reduce its dissolution, and reduce the content of surface lithium impurities 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. By further coating with a highly stable oxide coating layer, the interfacial side reactions on the surface of the positive electrode active material can be effectively reduced, thereby improving the high-temperature cycle and storage performance of the secondary battery. By further coating with a carbon layer as a third coating layer, the safety performance and kinetic performance of the secondary battery can be further improved. Furthermore, in the core, lattice change rate is reduced through Li and Mn site doping; Mn site doping also effectively reduces surface activity, thereby suppressing Mn dissolution and interfacial side reactions between the positive electrode active material and the electrolyte; P site doping makes the Mn-O bond length change rate faster, reducing the small polaron migration barrier of the material, which is beneficial to electronic conductivity; O site doping has a good effect on reducing interfacial side reactions; P and O site doping also affects the Mn dissolution and kinetic properties of antisite defects; therefore, doping reduces the concentration of antisite defects in the material, improves the kinetic properties and specific capacity of the material, and can also change the morphology of the particles, thereby increasing the compaction density.

[0109] Unless otherwise stated, in the chemical formula Li mA x Mn 1-y B y P 1-z C z O 4-n D n In this context, when A comprises two or more elements, the aforementioned limitation on the range of x values ​​applies not only to the stoichiometric coefficient of each element representing A, but also to the sum of the stoichiometric coefficients of all elements representing A. For example, when A comprises 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 comprise two or more elements, the limitation on the range of stoichiometric coefficients of B, C, and D in this application also has the aforementioned meaning.

[0110] In some implementations, when A is one, two, three, or four elements selected from Zn, Al, Na, K, Mg, Nb, Mo, and W, A x For Q x1 H x2 E x3 K x4 Where x1 + x2 + x3 + x4 = x, and x1, x2, x3, and x4 are all positive numbers and not all zero simultaneously. Q, H, E, and K are each independently selected from Zn, Al, Na, K, Mg, Nb, Mo, and W. Optionally, one of x1, x2, x3, and x4 is zero, and the others are not zero; more alternatively, two of x1, x2, x3, and x4 are zero, and the others are not zero; still alternatively, three of x1, x2, x3, and x4 are zero, and the others are not zero. Kernel Li m A x 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 A elements at the Li site, and optionally, to dope one, two or three of the aforementioned A elements; in addition, it is advantageous to dope one or two B elements at the Mn site, one or two C elements at the phosphorus site, and one or two D elements at the O site, which is beneficial to make the doped elements uniformly distributed.

[0111] In some implementations, the values ​​of m, x, y, z, and n satisfy the condition that the entire kernel remains electrically neutral.

[0112] Kernel Li m A x Mn 1-y By P 1-z C z O 4-n D n In this system, the value of m is influenced by the valence states of A, B, C, and D, as well as the values ​​of x, y, z, and n, to ensure the overall system remains electrically neutral. If the value of m is too small, the lithium content of the entire core system will decrease, affecting the specific capacity of the material. The values ​​of x, y, z, and n limit the total amount of all dopants. If the doping amount is too low, the dopants will not have an effect; if y exceeds 0.6, the Mn content in the system will be low, affecting the voltage plateau of the material. For C doping at P sites, since the PO tetrahedron is relatively stable, and a large z value would affect the stability of the material, the z value is limited to 0.001-0.1.

[0113] Furthermore, maintaining the electrical neutrality of the entire core system ensures that defects and impurities in the cathode active material are minimized. If an excess of transition metal (such as manganese) exists in the cathode active material, due to the relatively stable structure of the material system itself, the excess transition metal is likely to precipitate as elemental form or form impurities within the crystal lattice. Maintaining electrical neutrality minimizes such impurities. In addition, ensuring the electrical neutrality of the system can, in some cases, generate lithium vacancies in the material, thereby improving the material's kinetic performance.

[0114] By controlling the process (e.g., thoroughly mixing and grinding materials from various sources), it is possible to ensure that each element is uniformly distributed in the crystal lattice and that no aggregation occurs. The main characteristic peak positions in the XRD pattern of element-doped lithium manganese phosphate 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 element doping, rather than impurity phases. After preparing the cathode active material, the inventors of this application used focused ion beam (FIB) to cut the middle 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.

[0115] In some implementations, the values ​​of a, b, c, d, and e satisfy the following condition: making crystalline pyrophosphate Li a MP2O7 or M b (P2O7) c and M′ d O e Maintain electrical neutrality.

[0116] In some embodiments, "crystalline" refers to a crystallinity of 50% or higher, i.e., 50%-100%. Crystallinity less than 50% is referred to as "glassy." The crystalline pyrophosphate of this application has a crystallinity of 50% to 100%. Pyrophosphate with a certain degree of crystallinity not only facilitates the full utilization of the pyrophosphate coating's ability to inhibit manganese dissolution and reduce interfacial side reactions, but also allows for better lattice matching between the pyrophosphate coating and the oxide coating, thereby achieving a tighter bond between the coating layers.

[0117] In some embodiments, the crystallinity of the first coating layer material of the positive electrode active material, crystalline pyrophosphate, can be tested by conventional techniques in the art, such as density method, infrared spectroscopy, differential scanning calorimetry and nuclear magnetic resonance absorption method, or by, for example, X-ray diffraction.

[0118] A specific X-ray diffraction method for testing the crystallinity of the first coating layer crystalline pyrophosphate of a positive electrode active material may include the following steps:

[0119] A certain amount of positive electrode active material powder is taken, and the total scattering intensity is measured by X-rays. It is the sum of the scattering intensities of all matter in space. It is only related to the intensity of the primary rays, the chemical structure of the positive electrode active material powder, and the total number of electrons participating in diffraction, i.e., the mass, and is independent of the order state of the sample. Then, crystalline scattering and non-crystalline scattering are separated from the diffraction pattern. The crystallinity is the ratio of the scattering of the crystalline part to the total scattering intensity.

[0120] It should be noted that, in some embodiments, the crystallinity of the pyrophosphate 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.

[0121] In some embodiments, since metal ions are difficult to migrate in pyrophosphate, pyrophosphate, as a first coating layer, can effectively isolate 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.

[0122] In some embodiments, the bonding between the first coating layer and the core resembles 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. This tight bonding ensures that the coating layer will not detach during subsequent cycling, thus contributing to the long-term stability of the material. The degree of bonding between the first coating layer and the core is primarily measured by calculating the mismatch between the lattice constants of the core and the coating. In this application, after doping the core with elements, the matching degree between the core and the first coating layer is improved compared to undoped elements, allowing for a tighter bonding between the core and the pyrophosphate coating layer.

[0123] The oxide was chosen as the second coating layer primarily because of its high lattice matching degree with the first coating layer, crystalline pyrophosphate (mismatch of only 3%). Secondly, the oxide itself is more stable than pyrophosphate, and coating pyrophosphate with it helps improve the material's stability. The oxide structure is very stable; therefore, using oxides for coating can effectively reduce interfacial side reactions on the surface of the positive electrode active material, thereby improving the high-temperature cycling and storage performance of the secondary battery. The lattice matching between the second and first coating layers is similar to the bonding between the first coating layer and the core; when the lattice mismatch is below 5%, the lattice matching is good, and the two easily bond tightly.

[0124] The primary reason for using carbon as the third coating layer is its excellent electronic conductivity. Since electrochemical reactions occur in secondary batteries, requiring electrons, carbon, with its superior conductivity, is used for coating 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.

[0125] Figure 1 This is a schematic diagram of an ideal three-layer coated cathode active material. As shown in the figure, the innermost circle represents the core, and from the inside out are the first coating layer, the second coating layer, and the third coating layer. This diagram represents the ideal state where each layer is fully coated. In practice, each coating layer can be fully coated or partially coated.

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

[0127] Optionally, the interplanar spacing of the crystalline pyrophosphate in the first coating layer ranges from 0.300 to 0.310 nm (e.g., 0.303 nm); and / or,

[0128] The included angle of the crystal orientation (111) of the crystalline pyrophosphate in the first coating layer ranges from 29.00° to 30.00° (e.g., 29.496°).

[0129] The crystalline pyrophosphate in the first coating layer can be characterized using conventional techniques in the art, or for example, by transmission electron microscopy (TEM). Under TEM, the core and coating layer can be distinguished by measuring the interplanar spacing.

[0130] The specific testing methods for the interplanar spacing and angle of crystalline pyrophosphate in the first coating layer may include the following steps:

[0131] Take a certain amount of the coated positive electrode active material sample powder into a test tube, and inject a solvent such as alcohol into the test tube. Then, stir and disperse it thoroughly. Then, use a clean disposable plastic pipette to take an appropriate amount of the above solution and drop it onto a 300-mesh copper grid. At this time, some powder will remain on the copper grid. Transfer the copper grid along with the sample to the TEM sample chamber for testing, obtain the original TEM test image, and save the original image.

[0132] Open the original image obtained from the TEM test in the diffractometer software and perform a Fourier transform to obtain the diffraction pattern. Measure the distance from the diffraction spot to the center position in the diffraction pattern to obtain the interplanar spacing. The included angle is calculated according to the Bragg equation.

[0133] Crystalline pyrophosphates within the aforementioned interplanar spacing and angle range can more effectively suppress the lattice change rate of lithium manganese phosphate and Mn dissolution during lithium insertion / extraction, thereby improving the high-temperature cycle performance, cycle stability, and high-temperature storage performance of secondary batteries.

[0134] In some implementations, x is any value in the range of 0.001-0.005; and / or,

[0135] y is any value in the range of 0.01-0.5, optionally any value in the range of 0.25-0.5; and / or,

[0136] z is any value in the range of 0.001-0.005; and / or,

[0137] n is any value in the range of 0.001 to 0.005.

[0138] Therefore, within the above ranges, x, y, z, and n can further reduce the generation of Li / Mn antisite defects, reduce manganese dissolution, and lower the lattice change rate, thereby further improving the capacity of the secondary battery and enhancing its cycle performance, high-temperature storage performance, and safety performance.

[0139] In some implementations, the ratio of 1-y to y in the core is 0.67-999, optionally 1 to 4, and more preferably 1.5 to 3. This further improves the cycle performance and rate performance of the secondary battery. Here, y represents the sum of the stoichiometric coefficients of the B dopant elements 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.

[0140] In some implementations, the ratio of m to x in the core is between 9 and 1100, optionally between 190 and 998. 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 secondary batteries using positive electrode active materials can be further improved.

[0141] In some embodiments, the carbon in the third coating layer is a mixture of SP2 carbon and SP3 carbon; optionally, the molar ratio of SP2 carbon to SP3 carbon is any value in the range of 0.07-13, more preferably any value in the range of 0.1-10, and even more preferably any value in the range of 2.0-3.0.

[0142] 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.

[0143] In this application, "about" for a certain value represents a range, specifically a range of ±10% of that value.

[0144] By selecting the form of carbon in the carbon coating layer, the overall electrical performance of the secondary battery can be improved. Specifically, by using a mixture of SP2 and SP3 carbon forms and limiting the ratio of SP2 to SP3 carbon within a certain range, good conductivity can be achieved while ensuring the lithium-ion pathway, thus benefiting the realization of the secondary battery's function and its cycle performance.

[0145] The mixing ratio of SP2 and SP3 carbon forms in the third coating layer can be controlled by sintering conditions such as sintering temperature and sintering time. For example, when using sucrose as a carbon source to prepare the third coating layer, after the sucrose is pyrolyzed at high temperature and deposited on the second coating layer under high temperature, a carbon coating layer with both SP2 and SP3 forms will be produced. The ratio of SP2 to SP3 carbon can be adjusted by selecting high-temperature pyrolysis and sintering conditions.

[0146] The structure and characteristics of the third coating carbon can be determined by Raman spectroscopy. The specific testing method is as follows: by dividing the energy spectrum of the Raman test, the Id / Ig ratio is obtained (where Id is the peak intensity of SP3 carbon and Ig is the peak intensity of SP2 carbon), thereby confirming the molar ratio between the two.

[0147] In some implementations, 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 further optionally greater than 0 and less than or equal to 2% by weight, based on the kernel's weight; and / or

[0148] 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, and further optionally 2% to 4% by weight, based on the kernel's weight; and / or

[0149] 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 further optionally greater than 0 and less than or equal to 2% by weight, based on the kernel weight.

[0150] In this application, the coverage of each layer is not zero.

[0151] In the core-shell structured positive electrode active material of this application, the coating amount of the three coating layers is preferably within the above-mentioned range, thereby enabling sufficient coating of the core and further improving the kinetic performance and safety performance of the secondary battery without sacrificing the specific capacity of the positive electrode active material.

[0152] For the first coating layer, by keeping the coating amount within the above range, the dissolution of transition metals can be reduced, ensuring the smooth migration of lithium ions, thereby improving the rate performance of the positive electrode active material.

[0153] For the second coating layer, by keeping the coating amount within the above range, the positive electrode active material can maintain a certain plateau voltage and ensure the coating effect.

[0154] For the third coating layer, the carbon coating mainly plays the role of enhancing electron transport between particles. However, since the structure also contains a large amount of amorphous carbon, the carbon density is low. By keeping the coating amount within the above range, the compaction density of the electrode can be guaranteed.

[0155] In some embodiments, the thickness of the first coating layer is 1-10 nm; and / or

[0156] The thickness of the second coating layer is 2-25 nm, optionally 2-15 nm; and / or

[0157] The thickness of the third coating layer is 2-25 nm.

[0158] In some embodiments, the thickness of the first coating layer may be about 2 nm, about 3 nm, about 4 nm, about 5 nm, about 6 nm, about 7 nm, about 8 nm, about 9 nm, or about 10 nm, or any range of any of the above values.

[0159] In some embodiments, the thickness of the second coating layer may be about 2 nm, about 3 nm, about 4 nm, about 5 nm, about 6 nm, about 7 nm, about 8 nm, about 9 nm, about 10 nm, about 11 nm, about 12 nm, about 13 nm, about 14 nm, about 15 nm, or any range of the above values.

[0160] In some embodiments, the thickness of the third coating layer may be about 2 nm, about 3 nm, about 4 nm, about 5 nm, about 6 nm, about 7 nm, about 8 nm, about 9 nm, about 10 nm, about 11 nm, about 12 nm, about 13 nm, about 14 nm, about 15 nm, about 16 nm, about 17 nm, about 18 nm, about 19 nm, about 20 nm, about 21 nm, about 22 nm, about 23 nm, about 24 nm, or about 25 nm, or any range of any of the above values.

[0161] When the thickness of the first coating layer is in the range of 1-10 nm, it can effectively reduce the dissolution of transition metals and ensure the kinetic performance of the secondary battery.

[0162] When the thickness of the second coating layer is within the above range, the surface structure of the second coating layer is stable and the side reactions with the electrolyte are small. Therefore, it can effectively reduce the interfacial side reactions and thus improve the high-temperature performance of the secondary battery.

[0163] When the thickness of the third coating layer is in the range of 2-25 nm, it can improve the electrical conductivity of the material and improve the compaction performance of the battery electrode prepared using the positive electrode active material.

[0164] 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.

[0165] In some embodiments, based on the weight of the positive electrode active material, the manganese content is in the range of 10 wt% to 35 wt%, preferably in the range of 13.3 wt% to 33.2 wt%, more preferably in the range of 15 wt% to 30 wt%, and even more preferably in the range of 17 wt% to 20 wt%; and / or, the phosphorus content is in the range of 12 wt% to 25 wt%, preferably in the range of 15 wt% to 20 wt%, and more preferably in the range of 16.8 wt% to 19.5 wt%; and / or, the weight ratio of manganese to phosphorus ranges from 0.71 to 1.85, more preferably from 0.90 to 1.25, and optionally from 0.95 to 1.20.

[0166] 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.

[0167] In this application, limiting the content of manganese within the above-mentioned range can ensure the stability and density of the positive electrode active material, thereby improving the cycle, storage and compaction performance of the secondary battery, and maintaining a certain voltage plateau height, thereby improving the energy density of the secondary battery.

[0168] In this application, limiting the phosphorus content within the aforementioned range can effectively improve the electrical conductivity of the material and enhance its overall stability.

[0169] When the weight ratio of manganese to phosphorus is within the above range, the leaching of the transition metal manganese can be effectively reduced, the stability and specific capacity of the positive electrode active material can be improved, thereby enhancing the cycle performance and storage performance of the secondary battery. At the same time, it helps to reduce impurities in the material and maintain the discharge voltage plateau height of the material, thus increasing the energy density of the secondary battery.

[0170] 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.

[0171] In some embodiments, the lattice change rate of the core-shell structured positive electrode active material before and after complete lithium insertion / extraction is less than 8.1%, preferably less than 4%, more preferably less than 3.8%, and even more preferably 2.0-3.8%.

[0172] The lithium intercalation / deintercalation process of lithium manganese phosphate (LiMnPO4) is a two-phase reaction. The interfacial stress between the two phases is determined by the rate of lattice change before and after lithium intercalation / deintercalation; the smaller the rate of lattice change, the smaller the interfacial stress. + The easier the transmission, the better. Therefore, reducing the lattice change rate of the core will be beneficial for enhancing Li. + The core-shell structured positive electrode active material of this application achieves a lattice change rate of less than 8.1% (optionally 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 by methods known in the art, such as X-ray diffraction (XRD).

[0173] In some embodiments, the Li / Mn antisite defect concentration of the core-shell structured positive electrode active material is less than 4%, preferably less than 2.2%, and more preferably 1.5-2.2%.

[0174] The Li / Mn inversion defect in this application refers to the Li / Mn inversion defect in the LiMnPO4 lattice. + With Mn 2+The positions of the Li / Mn antisite defects are interchanged. Accordingly, the Li / Mn antisite defect concentration refers to the concentration relative to the Mn concentration. 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.

[0175] The core-shell structured positive electrode active material of this application achieves 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 + The transport of Mn. Therefore, the core-shell structured positive electrode active material of this application, due to its low Li / Mn antisite defect concentration within the aforementioned range, can reduce Mn. 2+ Hinder Li + This improves the transport efficiency and enhances the specific capacity and rate performance of the positive electrode active material.

[0176] In some embodiments, the compaction density of the positive electrode active material at 3T is 1.98 g / cm³. 3 The above can be selected as 2.2g / cm. 3 The above is optional, 2.2 g / cm³. 3 Above and 2.8g / cm 3 The higher the compaction density, the greater the weight of active material per unit volume. Therefore, increasing the compaction density is beneficial for improving the volumetric energy density of the battery cell. Compaction density can be measured according to GB / T24533-2009.

[0177] In some embodiments, the surface oxygen valence state of the positive electrode active material is below -1.90, and optionally from -1.90 to -1.98.

[0178] Oxygen's stable valence state is -2. The closer the valence state is to -2, the stronger its ability to gain electrons, i.e., the stronger its oxidizing power. Under normal circumstances, its surface valence state is below -1.7. This application limits the surface oxygen valence state of the positive electrode active material to the above-mentioned range, which can reduce the interfacial side reactions between the positive electrode material and the electrolyte, thereby improving the cell's cycle life, high-temperature storage gas generation, and other performance characteristics.

[0179] The surface oxygen valence state can be measured by methods known in the art, such as by electron energy loss spectroscopy (EELS).

[0180] In some implementations, m can be any value selected from the range of 0.97 to 1.01.

[0181] In some implementations, y can be any value selected from the range of 0.25 to 0.5.

[0182] In some implementations, 'a' can be any value selected from the range 1-2, such as 1 or 2.

[0183] In some implementations, b can be, for example, 1, 2, 3, or 4.

[0184] In some implementations, c can be, for example, 1, 2, or 3.

[0185] In some implementations, d can be any value selected from the range 1-2, such as 1 or 2.

[0186] In some implementations, e can be any value selected from the range of 1-5, such as 1, 2, 3, 4, 5.

[0187] [Preparation method of positive electrode active material]

[0188] This application provides a method for preparing a positive electrode active material, including the following steps:

[0189] Steps for providing kernel materials: Kernel materials contain Li m A x Mn 1-y B y P 1-z C z O 4-n D n Wherein, m is selected from any value in the range of 0.9-1.1, x is selected from any value in the range of 0.001-0.1, y is selected from any value in the range of 0.001-0.6, optionally from any value in the range of 0.001-0.5, z is selected from any value in the range of 0.001-0.1, n is selected from any value in the range of 0.001-0.1, and A is one or more elements selected from Zn, Al, Na, K, Mg, Nb, Mo and W, optionally from Al, Mg, Nb, M One or more elements selected from o and W, B is one or more elements selected from Ti, V, Zr, Fe, Ni, Mg, Co, Ga, Sn, Sb, Nb and Ge, optionally one or more elements selected from Ti, V, Fe, Ni, Mg and Co, C is one or more elements selected from B, S, Si and N, optionally one or more elements selected from S, Si and N, and D is one or more elements selected from S, F, Cl and Br, optionally one or more elements selected from F, Cl and Br;

[0190] First coating step: Providing Li containing pyrophosphate a MP2O7 and / or M b (P2O7) c The first mixture is used to mix the core material with the first mixture, dry it, and sinter it to obtain the material coated by the first coating layer; wherein, a is greater than 0 and less than or equal to 2, b is any value in the range of 1-4, and c is any value in the range of 1-3; Li pyrophosphate a MP2O7 and M b (P2O7) c Each of the M elements is independently selected from one or more elements chosen from Fe, Ni, Mg, Co, Cu, Zn, Ti, Ag, Zr, Nb, and Al, and optionally from one or more elements chosen from Fe, Ni, Mg, Co, Cu, Zn, Ti, Ag, Zr, and Al.

[0191] Second coating step: Providing oxide M′ d O e The second mixture is obtained by mixing the material coated by the first coating layer with the second mixture, drying, and sintering to obtain a material coated by two coating layers; wherein, d is greater than 0 and less than or equal to 2, e is greater than 0 and less than or equal to 5, and M′ is one or more elements selected from alkali metals, alkaline earth metals, transition metals, Group IIIA elements, Group IVA elements, lanthanides, and Sb, optionally one or more elements selected from Li, Be, B, Na, Mg, Al, Si, P, S, K, Ca, Sc, Ti, V, Cr, Mn, Fe, Co, Ni, Cu, Zn, Ga, Ge, As, Se, Sr, Y, Zr, Nb, Mo, Tc, Ru, Rh, Pd, Ag, Cd, In, Sn, Sb, Te, W, La, and Ce, and more preferably one or more elements selected from Mg, Al, Ca, Ti, V, Co, Ni, Cu, Zn, and Zr;

[0192] The third coating step: a third mixture containing a carbon source is provided, the material coated by the two coating layers is mixed with the third mixture, dried, and sintered to obtain the positive electrode active material;

[0193] The positive electrode active material has a core-shell structure, comprising a core and a shell covering the core, wherein the core contains Li. m A x Mn 1-y B y P 1-z C z O 4-n D nThe shell includes a first coating layer covering the core, a second coating layer covering the first coating layer, and a third coating layer covering the second coating layer. The first coating layer contains crystalline pyrophosphate Li. a MP2O7 and / or M b (P2O7) c The second coating layer contains oxide M′ d O e The third coating layer contains carbon. The definitions of A, B, C, D, M, M′, m, x, y, z, n, a, b, c, d, and e are as described above.

[0194] Therefore, this application provides a novel core-shell structured positive electrode active material by doping lithium manganese phosphate with elements A, B, C, and D at the Li, manganese, phosphorus, and O sites, and then sequentially coating the core surface with three layers. This material can significantly reduce the generation of Li / Mn antisite defects, significantly reduce manganese dissolution and lattice change rate, and increase compaction density. When applied to secondary batteries, it can improve the capacity of secondary batteries, and enhance their cycle performance, high-temperature storage performance, and safety performance.

[0195] In some implementations, the step of providing the core material includes the following steps:

[0196] Step (1): Mix the manganese source, the source of element B, the acid, and an optional solvent to obtain a mixture;

[0197] Step (2): Mix the mixture with a lithium source, a phosphorus source, a source of element A, a source of element C, and a source of element D, and optionally a solvent, dry, and sinter to obtain a mixture containing Li. m A x Mn 1-y B y P 1-z C z O 4-n D n The core material.

[0198] In some embodiments, step (1) is performed at 60°C-120°C (e.g., about 70°C, about 80°C, about 90°C, about 100°C, about 110°C or about 120°C); and / or, in step (1), the mixing is performed by stirring at a speed of 200-800 rpm (e.g., 300 rpm, 400 rpm, 500 rpm, 600 rpm, 700 rpm).

[0199] In some embodiments, in step (2), the mixture is mixed at a temperature of 20-120°C, optionally 40-120°C (e.g., 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).

[0200] In some implementations, in step (2), the mixing is carried out for 8-15 hours (e.g., about 8 hours, about 9 hours, about 10 hours, about 11 hours, about 12 hours, about 13 hours, or about 14 hours).

[0201] When the temperature and time during the core particle preparation process are within the above range, the obtained core and the positive electrode active material made from it have fewer lattice defects, which is beneficial to suppress manganese dissolution, reduce interfacial side reactions between the positive electrode active material and the electrolyte, and thus improve the cycle performance and safety performance of the secondary battery.

[0202] In some embodiments, in step (2), mixing is performed at a pH of 3.5-6, optionally at a pH of 4-6, and more preferably at a pH of 4-5. It should be noted that the pH can be adjusted in this application using methods commonly used in the art, for example, by adding an acid or a base.

[0203] In some alternative embodiments, the mixture obtained in step (1) is filtered, dried, and milled to obtain element B-doped manganese salt particles with a particle size of 50-200 nm. The element B-doped manganese salt particles are used in step (2) to mix with lithium source, phosphorus source, source of element A, source of element C and source of element D and optional solvent.

[0204] In some embodiments, optionally, in step (2), the molar ratio of the mixture or manganese salt particles doped with element B to the lithium source and phosphorus source is 1:0.4-2.1:0.1-2.1, optionally about 1:0.4-0.5:0.1-1.

[0205] In some embodiments, in step (2), sintering is performed at 600-900°C for 6-14 hours; optionally, sintering can be performed 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, sintering is performed in an inert gas or a mixture of inert gas and hydrogen; more preferably, the protective atmosphere is a mixture of 70-90 vol% nitrogen and 10-30 vol% hydrogen; optionally, the sintering temperature and sintering time can be within any range of the above values, which can improve the crystallinity of the core, reduce the generation of impurity phases, maintain a certain particle size of the core, thereby improving the specific capacity and compaction density of the positive electrode active material, and improving the overall performance of the secondary battery, including rate performance.

[0206] In some alternative implementations, in step (2), drying is performed using a spray drying device.

[0207] In some alternative implementations, in step (2), grinding is performed simultaneously with mixing.

[0208] In some embodiments, in the first coating step, a first mixture is obtained by mixing a source of element M, a phosphorus source, an acid, an optional lithium source, and an optional solvent; and / or,

[0209] In the second coating step, a second mixture is obtained by mixing the source of element M′ with a solvent; and / or,

[0210] In the third coating step, a third mixture is obtained by mixing the carbon source with a solvent.

[0211] In some embodiments, in the first coating step, the source of element M, the phosphorus source, the acid, the optional lithium source, and the optional solvent are mixed at room temperature for 1-5 hours (e.g., about 1.5 hours, about 2 hours, about 3 hours, about 4 hours, about 4.5 hours, or about 5 hours), then heated to 50°C-120°C (e.g., 55°C, about 60°C, about 70°C, about 80°C, about 90°C, about 100°C, about 110°C, or about 120°C) and kept at this temperature for 2-10 hours (e.g., 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), all of which are carried out under conditions of pH 3.5-6.5 (e.g., 4-6).

[0212] In some embodiments, in the second coating step, the source of element M′ is mixed with the solvent at room temperature for 1-10 hours (e.g., 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), then heated to 60°C-150°C (e.g., 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 kept at that temperature for 2-10 hours (e.g., 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).

[0213] 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 and hydroxide, provided that the source can achieve the purpose of the preparation method of this application.

[0214] In some embodiments, the source of element A is one or more selected from elemental form, oxide, phosphate, oxalate, carbonate, and sulfate of element A; and / or,

[0215] The source of element B is one or more selected from elemental form, oxide, phosphate, oxalate, carbonate, and sulfate of element B; and / or,

[0216] The source of element C is one or more selected from sulfates, borates, nitrates, and silicates of element C; and / or,

[0217] The source of element D is one or more selected from element D in its simple form and ammonium salt.

[0218] In some embodiments, the source of element M is one or more selected from elemental form, carbonate, sulfate, halide, nitrate, organic acid salt, oxide and hydroxide of element M.

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

[0220] The amount of source added for each of elements A, B, C, D, M, and M' depends on the target doping amount, and the ratio of the amount of lithium source, manganese source, and phosphorus source used conforms to the stoichiometric ratio.

[0221] 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.

[0222] 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.

[0223] 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.

[0224] 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.

[0225] In this application, as an example, the carbon source is one or more selected from starch, sucrose, glucose, polyvinyl alcohol, polyethylene glycol, and citric acid.

[0226] In some embodiments, during the first coating step, sintering is carried out at 650-800°C (e.g., about 650°C, about 700°C, about 750°C, or about 800°C) for 2-6 hours (about 2 hours, about 3 hours, about 4 hours, about 5 hours, or about 6 hours); and / or,

[0227] In the second coating step, sintering is carried out at 500-700°C (e.g., about 550°C, about 600°C, or about 700°C) for 6-10 hours (e.g., about 6 hours, about 7 hours, about 8 hours, about 9 hours, or about 10 hours); and / or,

[0228] In the third coating step, sintering is carried out at 700-800°C (e.g., about 700°C, about 750°C, or about 800°C) for 6-10 hours (e.g., about 6 hours, about 7 hours, about 8 hours, about 9 hours, or about 10 hours).

[0229] In the first, second, and third coating steps described above, the drying is carried out at a 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 is 3-9 hours, optionally 4-8 hours, more preferably 5-7 hours, and most preferably about 6 hours.

[0230] [Positive electrode plate]

[0231] 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 aforementioned positive electrode active material or a positive electrode active material prepared by the aforementioned preparation method. Optionally, the content of the positive electrode active material in the positive electrode film layer is more than 10% by weight, based on the total weight of the positive electrode film layer.

[0232] In some embodiments, the content of the positive electrode active material in the positive electrode film is 90-99.5% by weight, based on the total weight of the positive electrode film. This ensures that the secondary battery has high capacity, good cycle performance, high-temperature storage performance, and safety performance.

[0233] In some embodiments, the positive current collector may be a metal foil or a composite current collector. For example, aluminum foil may be used as the metal foil. The composite current collector may include a polymer substrate and a metal layer formed on at least one surface of the polymer substrate. The composite current collector may be formed by forming a metal material (aluminum, aluminum alloy, nickel, nickel alloy, titanium, titanium alloy, silver and silver alloy, etc.) on a polymer substrate (such as a substrate of polypropylene (PP), polyethylene terephthalate (PET), polybutylene terephthalate (PBT), polystyrene (PS), polyethylene (PE), etc.).

[0234] In some embodiments, the positive electrode film may also include other positive electrode active materials known in the art for use in batteries. As an example, the positive electrode active material may include at least one of the following: lithium-containing phosphates with an olivine structure and their modified compounds. However, this application is not limited to these materials, and other conventional materials that can be used as positive electrode active materials for batteries may also be used. These positive electrode active materials may be used alone or in combination of two or more. Examples of lithium-containing phosphates with an olivine structure include, but are not limited to, at least one of lithium iron phosphate (such as LiFePO4 (also referred to as LFP)), lithium iron phosphate and carbon composites, lithium manganese phosphate (such as LiMnPO4), lithium manganese phosphate and carbon composites, lithium iron manganese phosphate, and lithium iron manganese phosphate and carbon composites.

[0235] In some embodiments, the positive electrode film layer may optionally include a binder. As an example, the 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 resin.

[0236] In some embodiments, the positive electrode film may optionally include a conductive agent. As an example, the conductive agent may include at least one selected from superconducting carbon, acetylene black, carbon black, Ketjen black, carbon dots, carbon nanotubes, graphene, and carbon nanofibers.

[0237] [Negative electrode plate]

[0238] The negative electrode sheet includes a negative current collector and a negative electrode film layer disposed on at least one surface of the negative current collector, the negative electrode film layer including a negative electrode active material.

[0239] As an example, the negative electrode current collector has two surfaces opposite each other in its own thickness direction, and the negative electrode film layer is disposed on either or both of the two opposite surfaces of the negative electrode current collector.

[0240] In some embodiments, the negative electrode current collector may be a metal foil or a composite current collector. For example, copper foil may be used as the metal foil. The composite current collector may include a polymer material substrate and a metal layer formed on at least one surface of the polymer material substrate. The composite current collector may be formed by forming a metal material (copper, copper alloy, nickel, nickel alloy, titanium, titanium alloy, silver and silver alloy, etc.) on a polymer material substrate (such as a substrate of polypropylene (PP), polyethylene terephthalate (PET), polybutylene terephthalate (PBT), polystyrene (PS), polyethylene (PE), etc.).

[0241] In some embodiments, the negative electrode active material may be a negative electrode active material known in the art for use in batteries. As an example, the negative electrode active material may include at least one of the following materials: artificial graphite, natural graphite, soft carbon, hard carbon, silicon-based materials, tin-based materials, and lithium titanate, etc. Silicon-based materials may be selected from at least one of elemental silicon, silicon oxide compounds, silicon-carbon composites, silicon-nitrogen composites, and silicon alloys. Tin-based materials may be selected from at least one of elemental tin, tin oxide compounds, and tin alloys. However, this application is not limited to these materials, and other conventional materials that can be used as negative electrode active materials for batteries may also be used. These negative electrode active materials may be used alone or in combination of two or more.

[0242] In some embodiments, the negative electrode film layer may optionally include a binder. As an example, the binder may be selected from at least one of styrene-butadiene rubber (SBR), polyacrylic acid (PAA), sodium polyacrylate (PAAS), polyacrylamide (PAM), polyvinyl alcohol (PVA), sodium alginate (SA), polymethacrylic acid (PMAA), and carboxymethyl chitosan (CMCS).

[0243] In some embodiments, the negative electrode film may optionally include a conductive agent. As an example, the conductive agent may be selected from at least one of superconducting carbon, acetylene black, carbon black, Ketjen black, carbon dots, carbon nanotubes, graphene, and carbon nanofibers.

[0244] In some embodiments, the negative electrode film may optionally include other additives, such as thickeners (e.g., sodium carboxymethyl cellulose (CMC-Na)).

[0245] In some embodiments, the negative electrode sheet can be prepared by dispersing the components used to prepare the negative electrode sheet, such as the negative electrode active material, conductive agent, binder and any other components, in a solvent (e.g., deionized water) to form a negative electrode slurry; coating the negative electrode slurry onto the negative electrode current collector, and then obtaining the negative electrode sheet after drying, cold pressing and other processes.

[0246] [Electrolytes]

[0247] The electrolyte acts as a conductor of ions between the positive and negative electrodes. This application does not impose specific restrictions on the type of electrolyte; it can be selected according to requirements. For example, the electrolyte can be liquid, gel, or entirely solid.

[0248] In some embodiments, the electrolyte is liquid and includes an electrolyte salt and a solvent.

[0249] In some embodiments, the electrolyte salt may be selected from at least one of lithium hexafluorophosphate, lithium tetrafluoroborate, lithium perchlorate, lithium hexafluoroarsenate, lithium bis(fluorosulfonyl)imide, lithium bis(trifluoromethanesulfonyl)imide, lithium trifluoromethanesulfonate, lithium difluorophosphate, lithium difluorooxalate borate, lithium dioxalate borate, lithium difluorodioxalate phosphate, and lithium tetrafluorooxalate phosphate.

[0250] In some embodiments, the solvent may be selected from at least one of ethylene carbonate, propylene carbonate, methyl ethyl carbonate, diethyl carbonate, dimethyl carbonate, dipropyl carbonate, methyl propyl carbonate, ethyl propyl carbonate, butyl carbonate, fluoroethylene carbonate, methyl formate, methyl acetate, ethyl acetate, propyl acetate, methyl propionate, ethyl propionate, propyl propionate, methyl butyrate, ethyl butyrate, 1,4-butyrolactone, sulfolane, dimethyl sulfone, methyl ethyl sulfone, and diethyl sulfone.

[0251] In some embodiments, the electrolyte may optionally include additives. As examples, additives may include negative electrode film-forming additives, positive electrode film-forming additives, and may also include additives that can improve certain battery performance, such as additives that improve battery overcharge performance, additives that improve battery high-temperature or low-temperature performance, etc.

[0252] [Isolation membrane]

[0253] In some embodiments, the secondary battery also includes a separator. This application does not impose any particular limitation on the type of separator; any known porous separator with good chemical and mechanical stability can be selected.

[0254] In some embodiments, the material of the separator can be selected from at least one of glass fiber, nonwoven fabric, polyethylene, polypropylene, and polyvinylidene fluoride. The separator can be a single-layer film or a multi-layer composite film, without particular limitation. When the separator is a multi-layer composite film, the materials of each layer can be the same or different, without particular limitation.

[0255] In some implementations, the positive electrode, negative electrode, and separator can be fabricated into an electrode assembly using a winding or stacking process.

[0256] In some embodiments, the secondary battery may include an outer packaging. This outer packaging may be used to encapsulate the electrode assembly and electrolyte described above.

[0257] In some embodiments, the outer packaging of the secondary battery can be a hard shell, such as a hard plastic shell, an aluminum shell, or a steel shell. The outer packaging of the secondary battery can also be a soft pack, such as a pouch. The material of the soft pack can be plastic; examples of plastics include polypropylene, polybutylene terephthalate, and polybutylene succinate.

[0258] This application does not impose any particular limitation on the shape of the secondary battery; it can be cylindrical, square, or any other arbitrary shape. For example, Figure 2 This is an example of a square-structured secondary battery 5.

[0259] In some implementations, refer to Figure 3 The outer packaging may include a housing 51 and a cover 53. The housing 51 may include a base plate and side plates connected to the base plate, the base plate and side plates forming a receiving cavity. The housing 51 has an opening communicating with the receiving cavity, and the cover 53 can be placed over the opening to close the receiving cavity. The positive electrode, negative electrode, and separator may be formed into an electrode assembly 52 by a winding process or a stacking process. The electrode assembly 52 is encapsulated within the receiving cavity. Electrolyte is immersed in the electrode assembly 52. ​​The secondary battery 5 may contain one or more electrode assemblies 52, which can be selected by those skilled in the art according to specific practical needs.

[0260] In some implementations, the secondary batteries can be assembled into a battery module, and the number of secondary batteries contained in the battery module can be one or more, the specific number of which can be selected by those skilled in the art according to the application and capacity of the battery module.

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

[0262] Optionally, the battery module 4 may also include a housing with a receiving space in which a plurality of secondary batteries 5 are received.

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

[0264] Figure 5 and Figure 6 This is battery pack 1 as an example. (See reference...) Figure 5 and Figure 6 The battery pack 1 may include a battery box and multiple battery modules 4 disposed within the battery box. The battery box includes an upper body 2 and a lower body 3, with the upper body 2 covering the lower body 3 to form a closed space for accommodating the battery modules 4. The multiple battery modules 4 can be arranged in any manner within the battery box.

[0265] In addition, this application also provides an electrical device, which includes at least one of the secondary battery, battery module, or battery pack provided in this application. The secondary battery, battery module, or battery pack can be used as the power source of the electrical device or as the energy storage unit of the electrical device. The electrical device may include, but is not limited to, mobile devices (e.g., mobile phones, laptops, etc.), electric vehicles (e.g., pure electric vehicles, hybrid electric vehicles, plug-in hybrid electric vehicles, electric bicycles, electric scooters, electric golf carts, electric trucks, etc.), electric trains, ships and satellites, energy storage systems, etc.

[0266] As an electrical device, a secondary battery, battery module, or battery pack can be selected according to its usage requirements.

[0267] Figure 7 This is an example of an electrical device. The device could be a pure electric vehicle, a hybrid electric vehicle, or a plug-in hybrid electric vehicle. To meet the high power and high energy density requirements of the secondary battery for this device, a battery pack or battery module can be used.

[0268] [Example]

[0269] The following describes embodiments of this application. The embodiments described below are exemplary and are only used to explain this application, and should not be construed as limiting this application. Where specific techniques or conditions are not specified in the embodiments, they are performed according to the techniques or conditions described in the literature in this field or according to the product instructions. Reagents or instruments used, unless otherwise specified, are all conventional products that can be obtained commercially.

[0270] The sources of raw materials involved in the embodiments of this application are as follows:

[0271]

[0272]

[0273] Preparation of positive active material and slurry thereof

[0274] Example 1

[0275] Step S1: Preparation of doped manganese oxalate

[0276] 1.3 mol MnSO4·H2O and 0.7 mol FeSO4·H2O were thoroughly mixed in a mixer for 6 hours. The mixture was then transferred to a reaction vessel, and 10 L of deionized water and 2 mol oxalic acid dihydrate were added. The mixture was heated to 80 °C and then stirred at 600 rpm for 6 hours until the reaction was terminated (no more bubbles were generated), yielding a suspension of Fe-doped manganese oxalate. The suspension was filtered, and the filter cake was dried at 120 °C and ground to obtain a particle size Dv. 50The particles are Fe-doped manganese oxalate particles with a diameter of approximately 100 nm.

[0277] Step S2: Preparation of Li 0.994 Mo 0.001 Mn 0.65 Fe 0.35 P 0.999 Si 0.001 O 3.999 F 0.001 kernel

[0278] 1 mol of Fe-doped manganese oxalate particles, 0.497 mol of lithium carbonate, 0.001 mol of Mo(SO4)3, an 85% phosphoric acid aqueous solution containing 0.999 mol of phosphoric acid, 0.001 mol of H4SiO4, 0.0005 mol of NH4HF2, and 0.005 mol of sucrose were added to 20 L of deionized water. The mixture was then transferred to a sand mill and thoroughly ground and stirred for 10 hours to obtain a slurry. The slurry was then transferred to a spray dryer for spray drying and granulation at 250 °C for 4 hours to obtain particles. The particles were sintered at 700 °C for 10 hours under a nitrogen (90% v / v) + hydrogen (10% v / v) protective atmosphere to obtain the core material. The elemental composition of the core material was determined using inductively coupled plasma atomic emission spectrometry (ICP), and the core chemical formula is as shown above.

[0279] Step S3: Preparation of the first coating layer suspension

[0280] Preparation of Li2FeP2O7 solution: 7.4g lithium carbonate, 11.6g ferrous carbonate, 23.0g ammonium dihydrogen phosphate and 12.6g oxalic acid dihydrate were dissolved in 500mL deionized water, and the pH was controlled at 5. The mixture was then stirred and reacted at room temperature for 2h to obtain a solution. The solution was then heated to 80℃ and maintained at this temperature for 4h to obtain the first coating layer suspension.

[0281] Step S4: Coating with the first coating layer

[0282] The 157.2g of doped lithium manganese phosphate core material obtained in step S2 was added to the first coating layer suspension (coating material content of 1.572g) 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.

[0283] Step S5: Preparation of the second coating layer suspension

[0284] 4.71g of nano-Al2O3 (particle size of about 20nm) was dissolved in 1500mL of deionized water and stirred for 2h to obtain a second coating layer suspension.

[0285] Step S6: Coating with the second coating layer

[0286] The 158.772g of pyrophosphate-coated material obtained in step S4 was added to the second coating suspension (coating material content of 4.71g) obtained in step S5. The mixture was stirred and mixed thoroughly for 6 hours. After mixing evenly, the mixture was dried in an oven at 120℃ for 6 hours and then sintered at 700℃ for 8 hours to obtain the two-layer coated material.

[0287] Step S7: Preparation of the third coating layer aqueous solution

[0288] Dissolve 37.3g of sucrose in 500g of deionized water, then stir and dissolve completely to obtain a sucrose aqueous solution.

[0289] Step S8: Coating with the third coating layer

[0290] 1633.9g of the two-layer coated material obtained in step S6 was added to the sucrose solution obtained in step S7 and stirred together for 6 hours. After mixing evenly, the mixture was placed in an oven at 150°C and dried for 6 hours. Then, it was sintered at 700°C for 10 hours to obtain the three-layer coated material.

[0291] Examples 2 to 89 and Comparative Examples 1 to 12

[0292] The positive electrode active materials of Examples 2 to 89 and Comparative Examples 1 to 12 were prepared using a method similar to that of Example 1. The differences in the preparation of the positive electrode active materials are shown in Tables 1-6.

[0293] Among them, Comparative Examples 1-9 and Comparative Examples 11 did not cover the first layer, so there were no steps S3-S4; Comparative Examples 1-10 did not cover the second layer, so there were no steps S5-S6.

[0294]

[0295]

[0296]

[0297]

[0298]

[0299]

[0300]

[0301]

[0302]

[0303]

[0304]

[0305]

[0306]

[0307]

[0308]

[0309]

[0310]

[0311]

[0312]

[0313]

[0314]

[0315]

[0316]

[0317]

[0318]

[0319]

[0320]

[0321]

[0322]

[0323]

[0324] Preparation of positive electrode sheet

[0325] The three-layer coated positive electrode active material, conductive agent acetylene black, and binder polyvinylidene fluoride (PVDF) prepared above were added to N-methylpyrrolidone (NMP) at a weight ratio of 97.0:1.2:1.8, and stirred until homogeneous to obtain the positive electrode slurry. Then, the positive electrode slurry was prepared at a ratio of 0.280 g / 1540.25 mm.2 The material is evenly coated onto aluminum foil, then dried, cold-pressed, and slit to obtain the positive electrode sheet.

[0326] Preparation of negative electrode sheet

[0327] Artificial graphite (anode active material), superconducting carbon black (Super-P) (conductive agent), styrene-butadiene rubber (SBR) (binder), and sodium carboxymethyl cellulose (CMC-Na) (thickener) were dissolved in deionized water at a mass ratio of 95%:1.5%:1.8%:1.7%. After thorough mixing, a negative electrode slurry with a viscosity of 3000 mPa·s and a solid content of 52% was obtained. The negative electrode slurry was coated onto a 6 μm copper foil for the negative electrode current collector, and then baked at 100℃ for 4 hours to dry it. After rolling, a negative electrode sheet with a compacted density of 1.75 g / cm3 was obtained.

[0328] Separator film

[0329] Polypropylene film is used.

[0330] Preparation of electrolyte

[0331] Ethylene carbonate, dimethyl carbonate, and 1,2-propanediol carbonate were mixed in a volume ratio of 1:1:1, and then LiPF6 was uniformly dissolved in the mixture to obtain an electrolyte. The concentration of LiPF6 in the electrolyte was 1 mol / L.

[0332] Preparation of full cell

[0333] The obtained positive electrode, separator, and negative electrode are stacked in sequence, with the separator positioned between the positive and negative electrodes to provide isolation. The electrodes are then wound to obtain a bare cell. The bare cell is placed in an outer packaging, injected with the electrolyte, and sealed to obtain a full battery (hereinafter also referred to as "full battery").

[0334] Preparation of button cell

[0335] The positive electrode, negative electrode, and electrolyte are assembled together in a button cell to form a button cell (hereinafter also referred to as "button cell").

[0336] I. Performance test of positive active material

[0337] 1. Method for testing lattice change rate:

[0338] Under a constant temperature environment of 25℃, the positive electrode active material sample was placed in an XRD (model Bruker D8 Discover) and tested at 1° / min. The test data were then organized and analyzed. Referring to the standard PDF card, the lattice constants a0, b0, c0 and v0 were calculated (a0, b0 and c0 represent the length of each aspect of the unit cell, and v0 represents the volume of the unit cell, which can be directly obtained from the XRD refinement results).

[0339] Using the above-described method for preparing coin cells, 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. The positive electrode sheet was then removed from the coin cell 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 for testing fresh samples. (v0-v1) / v0×100% was taken as the lattice change rate (cell volume change rate) before and after complete lithium insertion / extraction.

[0340] 2. Determination of Li / Mn antisite defect concentration:

[0341] 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.

[0342] 3. Determination of compacted density:

[0343] Take 5g of the prepared positive electrode active material powder and place it in a compaction mold (CARVER mold, model 13mm). Then place the mold on a compaction density instrument. Apply a pressure of 3T and read the thickness of the powder under pressure (thickness after depressurization) on the instrument. Calculate the compaction density using ρ = m / v. The area value used is the standard small image area of ​​1540.25mm². 2 .

[0344] 4.3C charging constant current ratio determination:

[0345] Under a constant temperature environment of 25°C, the fresh full cells prepared in the above embodiments and comparative examples were allowed to stand for 5 minutes, then discharged at 1 / 3C to 2.5V. After standing for 5 minutes, they were charged at 1 / 3C to 4.3V, and then charged at a constant voltage of 4.3V until the current was less than or equal to 0.05mA. After standing for 5 minutes, the charging capacity at this point was recorded as C0. After discharging at 1 / 3C to 2.5V, standing for 5 minutes, and then charging at 3C to 4.3V, and standing for 5 minutes, the charging capacity at this point was recorded as C1. The constant current ratio for 3C charging is C1 / C0 × 100%.

[0346] The higher the constant current ratio during 3C charging, the better the rate performance of the secondary battery.

[0347] 5. Dissolution test of transition metal Mn (and Fe doped at Mn sites):

[0348] The full cells made from the positive electrode active materials of the above-mentioned 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. Then, the batteries were disassembled, the negative electrode was removed, and 30 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.

[0349] 6. Determination of surface oxygen valence state:

[0350] Take 5g of the positive electrode active material sample prepared above and prepare a button cell according to the above preparation method. Charge the button cell at a low rate of 0.05C until the current decreases to 0.01C. Then, remove the positive electrode sheet from the button cell and soak it in DMC for 8 hours. Then, dry it, scrape off the powder, and screen out particles with a particle size of less than 500nm. Measure the obtained particles 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, calculate the number of occupied electrons by integrating the valence band density of states data, and thus deduce the valence state of the surface oxygen after charging.

[0351] 7. Measurement of manganese and phosphorus elements in positive electrode active materials:

[0352] Dissolve 5g of the prepared positive electrode active material in 100ml of aqua regia (concentrated hydrochloric acid: concentrated nitric acid = 1:3) (concentrated hydrochloric acid concentration ~37%, concentrated nitric acid concentration ~65%). Use ICP to test the content of each element in the solution, and then measure and convert the content of manganese or phosphorus (amount of manganese or phosphorus / amount of positive electrode active material * 100%) to obtain its weight percentage.

[0353] 8. Method for measuring the initial specific capacity of button cells:

[0354] At 2.5-4.3V, the coin cells prepared in the above embodiments and comparative examples were charged to 4.3V at 0.1C, and then charged at 4.3V at a constant voltage until the current was less than or equal to 0.05mA. After standing for 5 minutes, they were discharged to 2.0V at 0.1C. The discharge capacity at this time is the initial specific capacity, denoted as D0.

[0355] 9. Cell expansion test after 30 days of storage at 60℃:

[0356] Full cells prepared in the above-described embodiments and comparative examples were stored at 60°C at 100% state of charge (SOC). The open-circuit voltage (OCV) and internal resistance (IMP) of the cells were measured before, during, and after storage to monitor SOC, and the cell volume was also measured. The full cells were removed after every 48 hours of storage, allowed to stand for 1 hour, and then the open-circuit voltage (OCV) and internal resistance (IMP) were measured. After cooling to room temperature, the cell volume was measured using the water displacement method. The water displacement method involves first measuring the cell's weight F1 separately using a balance with automatic unit conversion of dial data, and then completely immersing the cell in deionized water (with a known density of 1 g / cm³). 3 In the process, measure the weight F2 of the battery cell at this moment, and the buoyancy F of the battery cell. 浮 That is, F1-F2, and then according to Archimedes' principle, F 浮 =ρ×g×V 排 The cell volume V is calculated to be V = (F1 - F2) / (ρ × g).

[0357] Based on the OCV and IMP test results, the batteries in all embodiments maintained a SOC of over 99% throughout the entire experiment until the end of storage.

[0358] After 30 days of storage, the cell volume was measured, and the percentage increase in cell volume after storage was calculated relative to the cell volume before storage.

[0359] 10. Cyclic performance test of the entire battery at 45°C:

[0360] Under constant temperature conditions of 45℃, the capacitor is charged at 1C to 4.3V within a range of 2.5-4.3V, then charged at a constant voltage of 4.3V until the current is ≤0.05mA. After resting for 5 minutes, it is discharged at 1C to 2.5V. The capacitance is denoted as D. n (n = 0, 1, 2, ...). Repeat the above process until the capacity decays to 80%, and record the number of repetitions at this point. This number of cycles corresponds to 80% capacity retention at 45°C.

[0361] 11. Interplanar spacing and angle test:

[0362] Take 1g of each of the above-prepared positive electrode active material powders into a 50mL test tube, and inject 10mL of 75% alcohol into the test tube. Then, stir and disperse the mixture thoroughly for 30 minutes. Then, use a clean disposable plastic pipette to take an appropriate amount of the above solution and drop it onto a 300-mesh copper grid. At this time, some powder will remain on the copper grid. Transfer the copper grid along with the sample to the TEM (Talos F200s G2) sample chamber for testing, obtain the original TEM test image, and save the original image format (xx.dm3).

[0363] 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.

[0364] By comparing the obtained interplanar spacing and corresponding angle data with their standard values, the substances and crystal states of different coating layers can be identified.

[0365] 12. Coating thickness test:

[0366] The thickness of the coating layer was tested by cutting a thin slice of about 100 nm thickness from the middle of a single particle of the positive electrode active material prepared above using FIB. Then, the slice was subjected to TEM testing to obtain the original TEM test image, which was saved in the original image format (xx.dm3).

[0367] 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.

[0368] Measure the thickness of the selected particle at three locations and take the average value.

[0369] 13. Determination of the molar ratio of SP2 and SP3 forms in the third coating layer of carbon:

[0370] This test was performed using Raman spectroscopy. By splitting the energy spectrum from the Raman test, the Id / Ig ratio was obtained, where Id is the peak intensity of SP3 carbon and Ig is the peak intensity of SP2 carbon, thus confirming the molar ratio between the two.

[0371] 14. Determination of the core chemical formula and composition of different coating layers:

[0372] 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.

[0373] The performance test results of the positive electrode active materials in the examples and comparative examples are shown in the table below.

[0374]

[0375]

[0376]

[0377]

[0378]

[0379]

[0380]

[0381]

[0382]

[0383]

[0384]

[0385]

[0386]

[0387]

[0388]

[0389]

[0390]

Claims

1. A positive electrode active material with a core-shell structure, comprising a core and a shell covering the core, The kernel contains Li m A x Mn 1-y B y P 1-z C z O 4-n D n ,in, The m is selected from any value in the range of 0.9-1.1, the x is selected from any value in the range of 0.001-0.1, the y is selected from any value in the range of 0.001-0.6, the z is selected from any value in the range of 0.001-0.1, the n is selected from any value in the range of 0.001-0.1, A is one or more elements selected from Zn, Al, Na, K, Mg, Nb, Mo and W, B is one or more elements selected from Ti, V, Zr, Fe, Ni, Mg, Co, Ga, Sn, Sb, Nb and Ge, C is one or more elements selected from B, S, Si and N, and D is one or more elements selected from S, F, Cl and Br. The shell includes a first covering layer covering the core, a second covering layer covering the first covering layer, and a third covering layer covering the second covering layer, wherein... The first coating layer contains crystalline pyrophosphate Li a MP2O7 and / or M b (P2O7) c Wherein, a is greater than 0 and less than or equal to 2, b is any value in the range of 1-4, c is any value in the range of 1-3, and the crystalline pyrophosphate Li a MP2O7 and M b (P2O7) c Each of the M elements is independently selected from one or more elements selected from Fe, Ni, Mg, Co, Cu, Zn, Ti, Ag, Zr, Nb, and Al; The second coating layer comprises oxide M′ d O e Wherein, d is greater than 0 and less than or equal to 2, e is greater than 0 and less than or equal to 5, and M′ is one or more elements selected from alkali metals, alkaline earth metals, transition metals, Group IIIA elements, Group IVA elements, lanthanides and Sb; The third coating layer contains carbon.

2. The positive electrode active material according to claim 1, wherein, The value of y is selected from any value in the range of 0.001-0.

5.

3. The positive electrode active material according to claim 1, wherein, A is one or more elements selected from Al, Mg, Nb, Mo and W.

4. The positive electrode active material according to claim 1, wherein, The element B is selected from one or more elements chosen from Ti, V, Fe, Ni, Mg, and Co.

5. The positive electrode active material according to claim 1, wherein, The C is one or more elements selected from S, Si, and N.

6. The positive electrode active material according to claim 1, wherein, D is one or more elements selected from F, Cl and Br.

7. The positive electrode active material according to claim 1, wherein, The crystalline pyrophosphate Li a MP2O7 and M b (P2O7) c Each M in the M is independently selected from one or more elements selected from Fe, Ni, Mg, Co, Cu, Zn, Ti, Ag, Zr and Al.

8. The positive electrode active material according to claim 1, wherein, M′ is one or more elements selected from Li, Be, B, Na, Mg, Al, Si, P, S, K, Ca, Sc, Ti, V, Cr, Mn, Fe, Co, Ni, Cu, Zn, Ga, Ge, As, Se, Sr, Y, Zr, Nb, Mo, Tc, Ru, Rh, Pd, Ag, Cd, In, Sn, Sb, Te, W, La, and Ce.

9. The positive electrode active material according to claim 1, wherein, M′ is one or more elements selected from Mg, Al, Ca, Ti, V, Co, Ni, Cu, Zn and Zr.

10. The positive electrode active material according to claim 1, wherein, The ratio of 1-y to y is 0.67-999; and / or, The ratio of m to x ranges from 9 to 1100.

11. The positive electrode active material according to claim 1, wherein, The ratio of 1-y to y is between 1 and 4; and / or, The ratio of m to x is 190-998.

12. The positive electrode active material according to claim 1, wherein, The ratio of 1-y to y is between 1.5 and 3.

13. The positive electrode active material according to claim 1, wherein, The x is any value in the range of 0.001-0.005; and / or, The y is any value in the range of 0.01-0.5; and / or, The z is any value in the range of 0.001-0.005; and / or, The value of n is any value in the range of 0.001 to 0.

005.

14. The positive electrode active material according to claim 1, wherein, The value of y is any value in the range of 0.25-0.

5.

15. The positive electrode active material according to claim 1, wherein, The carbon in the third coating layer is a mixture of SP2 and SP3 carbon.

16. The positive electrode active material according to claim 15, wherein, The molar ratio of SP2 carbon to SP3 carbon is any value within the range of 0.07-13.

17. The positive electrode active material according to claim 15, wherein, The molar ratio of SP2 carbon to SP3 carbon is any value within the range of 0.1-10.

18. The positive electrode active material according to claim 15, wherein, The molar ratio of SP2 carbon to SP3 carbon is any value within the range of 2.0-3.

0.

19. The positive electrode active material according to claim 1, 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.

20. The positive electrode active material according to claim 1, 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 second coating layer has a coating amount 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.

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

22. The positive electrode active material according to claim 1, wherein, The thickness of the first coating layer is 1-10 nm; and / or, The thickness of the second coating layer is 2-25 nm; and / or, The thickness of the third coating layer is 2-25 nm.

23. The positive electrode active material according to claim 1, wherein, The thickness of the second coating layer is 2-15 nm.

24. The positive electrode active material according to claim 1, wherein, Based on the weight of the positive electrode active material, the manganese content is in the range of 10%-35% by weight; and / or, Based on the weight of the positive electrode active material, the phosphorus content is in the range of 12%-25% by weight; and / or, The weight ratio of manganese to phosphorus is 0.71-1.

85.

25. The positive electrode active material according to claim 1, wherein, Based on the weight of the positive electrode active material, the manganese content is between 13.3 wt% and 33.2 wt%, and / or, Based on the weight of the positive electrode active material, the phosphorus content is in the range of 15%-20% by weight; and / or, The weight ratio of manganese to phosphorus is in the range of 0.90-1.

25.

26. The positive electrode active material according to claim 1, wherein, Based on the gravimetric analysis of the positive electrode active material, the manganese content is in the range of 15%-30% by weight, and / or, Based on the weight of the positive electrode active material, the phosphorus content is between 16.8 wt% and 19.5 wt%; and / or, The weight ratio of manganese to phosphorus is in the range of 0.95-1.

20.

27. The positive electrode active material according to claim 1, wherein, The interplanar spacing of the crystalline pyrophosphate in the first coating layer ranges from 0.293 to 0.470 nm, and the included angle of the crystal orientation [111] ranges from 18.00° to 32.00°.

28. The positive electrode active material according to claim 1, wherein, The interplanar spacing of the crystalline pyrophosphate in the first coating layer ranges from 0.300 to 0.310 nm; and / or, The crystal orientation [111] of the crystalline pyrophosphate in the first coating layer has an angle range of 29.00°-30.00°.

29. The positive electrode active material according to claim 1, wherein, The lattice change rate of the positive electrode active material before and after complete lithium insertion / extraction is less than 8.1%.

30. The positive electrode active material according to claim 1, wherein, The lattice change rate of the positive electrode active material before and after complete lithium insertion / extraction is less than 4%.

31. The positive electrode active material according to claim 1, wherein, The lattice change rate of the positive electrode active material before and after complete lithium insertion / extraction is less than 3.8%.

32. The positive electrode active material according to claim 1, wherein, The lattice change rate of the positive electrode active material before and after complete lithium insertion / extraction is 2.0%-3.8%.

33. The positive electrode active material according to claim 1, wherein, The concentration of Li / Mn antisite defects in the positive electrode active material is below 4%.

34. The positive electrode active material according to claim 1, wherein, The concentration of Li / Mn antisite defects in the positive electrode active material is below 2.2%.

35. The positive electrode active material according to claim 1, wherein, The concentration of Li / Mn antisite defects in the positive electrode active material is 1.5%-2.2%.

36. The positive electrode active material according to claim 1, wherein, The compaction density of the positive electrode active material at 3T is 1.98 g / cm³. 3 above.

37. The positive electrode active material according to claim 1, wherein, The compaction density of the positive electrode active material at 3T is 2.2 g / cm³. 3 above.

38. The positive electrode active material according to claim 1, wherein, The compaction density of the positive electrode active material at 3T is 2.2 g / cm³. 3 Above and 2.8 g / cm 3 the following.

39. The positive electrode active material according to any one of claims 1-38, wherein, The surface oxygen valence state of the positive electrode active material is below -1.

90.

40. The positive electrode active material according to any one of claims 1-38, wherein, The surface oxygen valence state of the positive electrode active material is -1.90 to -1.

98.

41. A method for preparing a positive electrode active material, comprising the following steps: The step of providing the core material: the core material contains Li m A x Mn 1-y B y P 1-z C z O 4-n D n Wherein, m is selected from any value in the range of 0.9-1.1, x is selected from any value in the range of 0.001-0.1, y is selected from any value in the range of 0.001-0.6, z is selected from any value in the range of 0.001-0.1, n is selected from any value in the range of 0.001-0.1, A is one or more elements selected from Zn, Al, Na, K, Mg, Nb, Mo and W, B is one or more elements selected from Ti, V, Zr, Fe, Ni, Mg, Co, Ga, Sn, Sb, Nb and Ge, C is one or more elements selected from B, S, Si and N, and D is one or more elements selected from S, F, Cl and Br; First coating step: Providing Li containing pyrophosphate a MP2O7 and / or M b (P2O7) c The first mixture is used to mix the core material with the first mixture, dry it, and sinter it to obtain the material coated by the first coating layer; wherein, a is greater than 0 and less than or equal to 2, b is any value in the range of 1-4, and c is any value in the range of 1-3; the pyrophosphate Li a MP2O7 and M b (P2O7) c Each of the M elements is independently selected from one or more elements selected from Fe, Ni, Mg, Co, Cu, Zn, Ti, Ag, Zr, Nb, and Al; Second coating step: Providing oxide M′ d O e The second mixture is obtained by mixing the material coated by the first coating layer with the second mixture, drying, and sintering to obtain a material coated by two coating layers; wherein, d is greater than 0 and less than or equal to 2, e is greater than 0 and less than or equal to 5, and M′ is one or more elements selected from alkali metals, alkaline earth metals, transition metals, Group IIIA elements, Group IVA elements, lanthanides, and Sb; The third coating step: a third mixture containing a carbon source is provided, the material coated by the two coating layers is mixed with the third mixture, dried, and sintered to obtain the positive electrode active material; The positive electrode active material has a core-shell structure, comprising a core and a shell enclosing the core, wherein the core contains Li. m A x Mn 1-y B y P 1-z C z O 4-n D n The shell includes a first coating layer covering the core, a second coating layer covering the first coating layer, and a third coating layer covering the second coating layer. The first coating layer comprises crystalline pyrophosphate Li. a MP2O7 and / or M b (P2O7) c The second coating layer comprises oxide M′ d O e The third coating layer contains carbon.

42. The preparation method according to claim 41, wherein, The value of y is selected from any value in the range of 0.001-0.

5.

43. The preparation method according to claim 41, wherein, A is one or more elements selected from Al, Mg, Nb, Mo and W.

44. The preparation method according to claim 41, wherein, The element B is selected from one or more elements chosen from Ti, V, Fe, Ni, Mg, and Co.

45. The preparation method according to claim 41, wherein, The C is one or more elements selected from S, Si, and N.

46. ​​The preparation method according to claim 41, wherein, D is one or more elements selected from F, Cl and Br.

47. The preparation method according to claim 41, wherein, The crystalline pyrophosphate Li a MP2O7 and M b (P2O7) c Each M in the M is independently selected from one or more elements selected from Fe, Ni, Mg, Co, Cu, Zn, Ti, Ag, Zr and Al.

48. The preparation method according to claim 41, wherein, M′ is one or more elements selected from Li, Be, B, Na, Mg, Al, Si, P, S, K, Ca, Sc, Ti, V, Cr, Mn, Fe, Co, Ni, Cu, Zn, Ga, Ge, As, Se, Sr, Y, Zr, Nb, Mo, Tc, Ru, Rh, Pd, Ag, Cd, In, Sn, Sb, Te, W, La, and Ce.

49. The preparation method according to claim 41, wherein, M′ is one or more elements selected from Mg, Al, Ca, Ti, V, Co, Ni, Cu, Zn and Zr.

50. The preparation method according to claim 41, wherein the step of providing the core material comprises the following steps: Step (1): Mix the manganese source, the source of element B, and the acid, or mix the manganese source, the source of element B, the acid, and the solvent to obtain a mixture; Step (2): Mix the mixture with a lithium source, a phosphorus source, a source of element A, a source of element C, and a source of element D, or mix the mixture with a lithium source, a phosphorus source, a source of element A, a source of element C, and a source of element D, and a solvent; dry and sinter to obtain a mixture containing Li. m A x Mn 1-y B y P 1-z C z O 4-n D n The core material.

51. The preparation method according to claim 50, wherein, The step (1) is carried out at 60℃-120℃; and / or, in the step (1), the mixing is carried out by stirring at a speed of 200-800 rpm.

52. The preparation method according to claim 50, wherein, In step (2), the mixing process takes 8-15 hours.

53. The preparation method according to claim 50, wherein, In step (2), the sintering is carried out at 600℃-900℃ for 6-14 hours.

54. The preparation method according to claim 41, wherein, In the first coating step, the first mixture is obtained by mixing a source of element M, a phosphorus source, an acid, an optional lithium source, and an optional solvent; and / or, In the second coating step, a second mixture is obtained by mixing the source of element M′ with a solvent; and / or, In the third coating step, a third mixture is obtained by mixing the carbon source with a solvent.

55. The preparation method according to claim 54, wherein, In the first coating step, the source of element M, phosphorus source, acid, optional lithium source and optional solvent are mixed at room temperature for 1-5 h, then heated to 50℃-120℃ and kept at this temperature for 2-10 h. All the above mixing is carried out under the condition of pH 3.5-6.

5.

56. The preparation method according to claim 54, wherein, In the first coating step, and in the second coating step, the source of element M′ is mixed with the solvent at room temperature for 1-10 h, and then heated to 60℃-150℃ and kept at that temperature for 2-10 h.

57. The preparation method according to claim 50, wherein, The source of element A is one or more selected from elemental form, oxide, phosphate, oxalate, carbonate, and sulfate of element A; and / or, The source of element B is one or more selected from elemental form, oxide, phosphate, oxalate, carbonate, and sulfate of element B; and / or, The source of element C is one or more selected from sulfates, borates, nitrates, and silicates of element C; and / or, The source of element D is one or more selected from elemental form and ammonium salt of element D.

58. The preparation method according to any one of claims 41 to 57, wherein, In the first coating step, the sintering is carried out at 650-800°C for 2-6 hours; and / or, In the second coating step, the sintering is carried out at 500-700°C for 6-10 hours; and / or, In the third coating step, the sintering is carried out at 700-800℃ for 6-10 hours.

59. A positive electrode sheet comprising a positive current collector and a positive electrode film disposed on at least one surface of the positive current collector, said positive electrode film comprising the positive electrode active material according to any one of claims 1-40 or the positive electrode active material prepared by the preparation method according to any one of claims 41-58.

60. The positive electrode sheet according to claim 59, wherein, The content of the positive electrode active material in the positive electrode film is 90-99.5% by weight, based on the total weight of the positive electrode film.

61. The positive electrode sheet according to claim 59, wherein, The content of the positive electrode active material in the positive electrode film is 95-99.5% by weight, based on the total weight of the positive electrode film.

62. A secondary battery comprising the positive electrode active material according to any one of claims 1-40, or the positive electrode active material prepared by any one of claims 41-58, or the positive electrode sheet according to any one of claims 59-61.

63. A battery module comprising the secondary battery of claim 62.

64. A battery pack comprising the battery module of claim 63.

65. An electrical device comprising at least one selected from the secondary battery of claim 62, the battery module of claim 63, and the battery pack of claim 64.

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