Positive electrode plate, secondary battery and electrical device

Abstract

A positive electrode sheet, a secondary battery, and an electrical device are disclosed. The positive electrode sheet includes a positive current collector, a positive electrode film layer disposed on at least one surface of the positive current collector, and a conductive undercoating layer located between the positive current collector and the positive electrode film layer. The positive electrode film layer includes a positive electrode active material with a core-shell structure. The positive electrode active material includes a core and a shell covering the core. The shell includes a first coating layer covering the core, a second coating layer covering the first coating layer, and a third coating layer covering the second coating layer. The conductive undercoating layer includes a first polymer, a first aqueous binder, and a first conductive agent.

Description

TECHNICAL FIELD

[0001] The present application relates to the technical field of batteries, in particular to a positive electrode sheet, a secondary battery and an electric device. BACKGROUND

[0002] In recent years, with the application range of secondary batteries becoming more and more extensive, secondary batteries are widely used in energy storage power supply systems such as hydropower, thermal power, wind power and solar power stations, and in many fields such as electric tools, electric bicycles, electric motorcycles, electric vehicles, military equipment, aerospace, etc. Due to the great development of secondary batteries, higher requirements are put forward for their energy density, cycle performance, etc.

[0003] The related art improves one or more performances of a secondary battery by setting a conductive primer layer between an active material and a current collector of a positive electrode sheet.

[0004] In order to further improve the performance of the battery, the prior art needs a more optimal positive electrode sheet. SUMMARY

[0005] In view of the above problems, the present application provides a new type of positive electrode sheet, a secondary battery and an electric device, which are described below respectively.

[0006] The first aspect of the present application provides a positive electrode sheet, comprising a positive electrode current collector, a positive electrode film layer arranged on at least one surface of the positive electrode current collector, and a conductive primer layer between the positive electrode current collector and the positive electrode film layer, wherein,

[0007] The positive electrode film layer comprises a positive electrode active material with a core-shell structure, the positive electrode active material comprising an inner core and a shell covering the inner core,

[0008] The chemical formula of the inner core is Li 1+x Mn 1-y A y P 1-z R z O4, wherein x is any value in the range of -0.100-0.100, y is any value in the range of 0.001-0.500, z is any value in the range of 0.001-0.100, A is one or more elements selected from Zn, Al, Na, K, Mg, Mo, W, Ti, V, Zr, Fe, Ni, Co, Ga, Sn, Sb, Nb and Ge, which can be one or more elements selected from Fe, Ti, V, Ni, Co and Mg, R is one or more elements selected from B, Si, N and S, which can be one element selected from B, Si, N and S, and the values of x, y and z satisfy the following conditions: to maintain the overall inner core electrically neutral;

[0009] The shell includes a first cladding layer cladding the inner core, a second cladding layer cladding the first cladding layer, and a third cladding layer cladding the second cladding layer, wherein

[0010] The first cladding layer includes a crystalline pyrophosphate Li a MP2O7 and / or M b (P2O7) c wherein 0≤a≤2, 1≤b≤4, 1≤c≤6, the values of a, b and c satisfy the following condition: the crystalline pyrophosphate Li a MP2O7 or M b (P2O7) c maintains electrical neutrality, the crystalline pyrophosphate Li a MP2O7 and M b (P2O7) c each independently is one or more elements selected from Fe, Ni, Mg, Co, Cu, Zn, Ti, Ag, Zr, Nb or Al,

[0011] The second cladding layer includes a crystalline phosphate XPO4, wherein the X is one or more elements selected from Li, Fe, Ni, Mg, Co, Cu, Zn, Ti, Ag, Zr, Nb or Al,

[0012] The third cladding layer is carbon;

[0013] The conductive primer layer includes a first polymer, a first aqueous binder, and a first conductive agent,

[0014] The first polymer includes:

[0015] a first monomer unit represented by Formula 1;

[0016] a second monomer unit selected from at least one of a group consisting of a monomer unit represented by Formula 2 and a monomer unit represented by Formula 3;

[0017] a third monomer unit selected from at least one of a group consisting of a monomer unit represented by Formula 4 and a monomer unit represented by Formula 5; and

[0018] a fourth monomer unit represented by Formula 6, R 1 , R 2 , R 3 each independently represents H, a carboxyl group, an ester group, and a substituted or unsubstituted group of C1 to C10 alkyl, C1 to C10 alkoxy, C2 to C10 alkenyl, C6 to C10 aryl, R 4 represents H, and a substituted or unsubstituted group of C1 to C10 alkyl, C1 to C10 alkoxy, C2 to C10 alkenyl, C6 to C10 aryl;

[0019]

[0020]

[0021] In some embodiments, the first polymer comprises one or more selected from the group consisting of hydrogenated nitrile butadiene rubber, hydrogenated carboxyl nitrile butadiene rubber; and / or,

[0022] the first monomer unit has a mass percentage of M1, M1 is 10% to 55%, optionally 25% to 55%; and / or,

[0023] the second monomer unit has a mass percentage of M2, M2 is 40% to 80%, optionally 50% to 70%; and / or,

[0024] the third monomer unit has a mass percentage of M3, M3 is 0% to 10%, optionally 0.001% to 2%; and / or,

[0025] the fourth monomer unit has a mass percentage of M4, M4 is 0% to 10%, optionally 0.1% to 1%.

[0026] In some embodiments, M3 / (M2+M3) is 0% to 5%, optionally 0.001% to 1%.

[0027] In some embodiments, the first polymer comprises one or more selected from the group consisting of hydrogenated nitrile butadiene rubber, hydrogenated carboxyl nitrile butadiene rubber; and / or,

[0028] the first polymer has a weight average molecular weight of 500,000 to 1,500,000, optionally 2,000,000 to 4,000,000.

[0029] In some embodiments, the first water-based binder comprises one or more selected from the group consisting of water-based polyacrylic acid resin and derivatives thereof, water-based amino-modified polyacrylic resin and derivatives thereof, polyvinyl alcohol and derivatives thereof, optionally comprises one or more selected from the group consisting of water-based acrylic acid-acrylate copolymer; and / or,

[0030] the first water-based binder has a weight average molecular weight of 2,000,000 to 15,000,000, optionally 3,000,000 to 4,000,000.

[0031] In some embodiments, the first conductive agent comprises one or more selected from the group consisting of superconducting carbon, conductive graphite, acetylene black, carbon black, ketjen black, carbon dots, carbon nanotubes, graphene, carbon nanofibers, optionally comprises one or more selected from the group consisting of carbon nanotubes, graphene, carbon nanofibers.

[0032] In some embodiments, the conductive primer layer comprises one or more selected from the group consisting of hydrogenated nitrile butadiene rubber, hydrogenated carboxyl nitrile butadiene rubber; and / or,

[0033] a mass percentage of the first polymer is X1, X1 is 5% to 20%, optionally 5% to 10%; and / or,

[0034] a mass percentage of the first aqueous binder is X2, X2 is 30% to 80%, optionally 40% to 50%; and / or,

[0035] a mass percentage of the first conductive agent is X3, X3 is 10% to 50%, optionally 40% to 50%.

[0036] In some embodiments, the thickness of the conductive primer layer is 1 μm to 20 μm, optionally 3 μm to 10 μm.

[0037] In some embodiments, the positive electrode film layer further comprises one or more selected from a wetting agent, a dispersant, and optionally, the positive electrode film layer further comprises both a wetting agent and a dispersant.

[0038] In some embodiments, the surface tension of the wetting agent is 20 mN / m to 40 mN / m, and optionally, the wetting agent comprises at least one of the following functional groups: -CN, -NH2, -NH-, N-, -OH, -COO-, -C(=O)-O-C(=O)-.

[0039] In some embodiments, the wetting agent comprises one or more selected from a small molecule organic solvent, a low molecular weight polymer,

[0040] Optionally, the small molecule organic solvent comprises one or more selected from an alcohol amine compound, an alcohol compound, a nitrile compound, and optionally, the alcohol amine compound has a number of carbon atoms of 1 to 16, optionally 2 to 6.

[0041] Optionally, the low molecular weight polymer comprises one or more selected from a maleic anhydride-styrene copolymer, a polyvinylpyrrolidone, a polysiloxane, and optionally, the low molecular weight polymer has a weight average molecular weight of 6000 or less, optionally 3000 to 6000.

[0042] In some embodiments, the dispersant comprises a second polymer, and the second polymer comprises:

[0043] a fifth monomer unit represented by Formula 7;

[0044] a sixth monomer unit selected from at least one of a monomer unit represented by Formula 8 and a monomer unit represented by Formula 9; and

[0045] a seventh monomer unit selected from at least one of a monomer unit represented by Formula 10 and a monomer unit represented by Formula 11;

[0046]

[0047] In some embodiments, based on the total mass of the second polymer,

[0048] The fifth monomer unit has a mass percentage content of M5, where M5 is 10% to 55%, optionally 25% to 55%; and / or,

[0049] The sixth monomer unit has a mass percentage content of M6, where M6 is 40%–80%, optionally 50%–70%; and / or,

[0050] The seventh monomer unit has a mass percentage content of M7, which is 0% to 10%, and optionally 0.001% to 2%.

[0051] In some implementations, M7 / (M6+M7) is 0% to 5%, and optionally 0.001% to 1%.

[0052] In some embodiments, the second polymer is hydrogenated nitrile butadiene rubber; and / or,

[0053] The weight-average molecular weight of the second polymer is 50,000 to 500,000, and optionally 150,000 to 350,000.

[0054] In some implementations, based on the total mass of the positive electrode film,

[0055] The dispersant has a mass percentage content of Y1, where Y1 is 0.05% to 1%, optionally 0.1% to 0.5%; and / or,

[0056] The wetting agent has a mass percentage content of Y2, which is 0.05% to 2%, and optionally 0.2% to 0.8%.

[0057] In some implementations, Y1 / Y2 is 0.05 to 20, optionally 0.1 to 1, and further 0.3 to 0.8.

[0058] In some embodiments, the mass ratio of the first polymer to the second polymer in the positive electrode sheet is 1.5 to 5, and optionally 2 to 3.

[0059] In some embodiments, the interplanar spacing of the crystalline pyrophosphate in the first coating layer ranges from 0.293 to 0.470 nm, and the included angle of the crystal orientation (111) ranges from 18.00° to 32.00°; the interplanar spacing of the crystalline phosphate in the second coating layer ranges from 0.244 to 0.425 nm, and the included angle of the crystal orientation (111) ranges from 20.00° to 37.00°.

[0060] In some implementations, in the kernel, the ratio of y to 1-y is 1:10 to 1:1, optionally 1:4 to 1:1; and / or,

[0061] In the kernel, the ratio of z to 1-z is 1:9 to 1:999, and can be selected as 1:499 to 1:249.

[0062] In some embodiments, the carbon in the third coating layer is a mixture of SP2 carbon and SP3 carbon. Optionally, the molar ratio of SP2 carbon to SP3 carbon is any value in the range of 0.1-10, and can be any value in the range of 2.0-3.0.

[0063] In some embodiments, the coating amount of the first coating layer is greater than 0 and less than or equal to 6% by weight, optionally greater than 0 and less than or equal to 5.5% by weight, more preferably greater than 0 and less than or equal to 2% by weight, based on the weight of the core; and / or

[0064] The coating amount of the second coating layer is greater than 0 and less than or equal to 6% by weight, optionally greater than 0 and less than or equal to 5.5% by weight, more preferably 2-4% by weight, based on the weight of the core; and / or

[0065] The coating amount of the third coating layer is greater than 0 and less than or equal to 6% by weight, optionally greater than 0 and less than or equal to 5.5% by weight, and more preferably greater than 0 and less than or equal to 2% by weight, based on the weight of the core.

[0066] In some embodiments, the thickness of the first coating layer is 1-10 nm; and / or

[0067] The thickness of the second coating layer is 2-15 nm; and / or

[0068] The thickness of the third coating layer is 2-25 nm.

[0069] In some embodiments, based on the weight of the positive electrode active material, the manganese content is in the range of 10 wt% to 35 wt%, optionally in the range of 15 wt% to 30 wt%, and more preferably in the range of 17 wt% to 20 wt%, the phosphorus content is in the range of 12 wt% to 25 wt%, optionally in the range of 15 wt% to 20 wt%, and the weight ratio of manganese to phosphorus is in the range of 0.90 to 1.25, optionally in the range of 0.95 to 1.20.

[0070] In some embodiments, the lattice change rate of the core-shell structured positive electrode active material before and after complete lithium insertion / extraction is less than 4%, preferably less than 3.8%, and more preferably 2.0-3.8%.

[0071] In some embodiments, the Li / Mn antisite defect concentration of the core-shell structured positive electrode active material is less than 4%, optionally less than 2.2%, and more preferably 1.5-2.2%.

[0072] In some embodiments, the core-shell structured positive electrode active material has a compaction density of 2.2 g / cm³ at 3T. 3 The above is optional, 2.2 g / cm³. 3 Above and 2.8g / cm 3 the following.

[0073] In some embodiments, the surface oxygen valence state of the core-shell structured positive electrode active material is below -1.90, optionally from -1.90 to -1.98.

[0074] In some implementations, the specific surface area of ​​the positive electrode active material is 15 m². 2 / g~25m 2 / g, the coating weight on one side of the positive electrode current collector is 20mg / cm³. 2 ~40mg / cm 2 When the specific surface area of ​​the positive electrode active material is 15m² 2 / g~25m 2 / g, the coating weight on one side of the positive electrode current collector is 20mg / cm³. 2 ~40mg / cm 2 During the coating process, film peeling is prone to occur. This application employs a novel conductive undercoat to increase the adhesion strength between the positive electrode active material layer and the current collector.

[0075] A second aspect of this application provides a secondary battery comprising the positive electrode sheet described in any of the preceding claims.

[0076] A third aspect of this application provides an electrical device including the aforementioned secondary battery.

[0077] Regarding novel core-shell structured doped lithium manganese phosphate cathode active materials

[0078] This application provides a novel core-shell structured doped lithium manganese phosphate cathode active material, which enables secondary batteries using the cathode active material to have high specific capacity, good cycle performance, and safety performance.

[0079] To achieve the above objectives, this application provides a positive electrode active material with a core-shell structure, comprising a core and a shell enclosing the core.

[0080] The chemical formula of the core is Li 1+x Mn 1-y A y P 1-zR z O4, where x is any value in the range of -0.100 to 0.100, y is any value in the range of 0.001 to 0.500, z is any value in the range of 0.001 to 0.100, A is one or more elements selected from Zn, Al, Na, K, Mg, Mo, W, Ti, V, Zr, Fe, Ni, Co, Ga, Sn, Sb, Nb and Ge, and may be one or more elements selected from Fe, Ti, V, Ni, Co and Mg, R is one or more elements selected from B, Si, N and S, and optionally, R is one element selected from B, Si, N and S;

[0081] The values ​​of x, y, and z satisfy the following condition: keeping the entire kernel electrically neutral;

[0082] 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...

[0083] The first coating layer comprises crystalline pyrophosphate Li a MP2O7 and / or M b (P2O7) c ,in,

[0084] 0≤a≤2, 1≤b≤4, 1≤c≤6, where the values ​​of a, b, and c satisfy the following condition: This makes crystalline pyrophosphate Li... a MP2O7 or M b (P2O7) c Maintain electrical neutrality

[0085] The crystalline pyrophosphate Li a MP2O7 and M b (P2O7) c Each of the M elements is independently selected from Fe, Ni, Mg, Co, Cu, Zn, Ti, Ag, Zr, Nb, or Al.

[0086] The second coating layer comprises crystalline phosphate XPO4, wherein X is one or more elements selected from Li, Fe, Ni, Mg, Co, Cu, Zn, Ti, Ag, Zr, Nb or Al;

[0087] The third coating layer is carbon.

[0088] In this document, "crystalline state" refers to a crystallinity of 50% or higher, i.e., 50%-100%. Crystallinity less than 50% is referred to as the glassy state. The crystalline pyrophosphate and crystalline phosphate described in this application have a crystallinity of 50% to 100%. Pyrophosphate and phosphate with a certain degree of crystallinity not only fully utilize the pyrophosphate coating's ability to inhibit manganese dissolution and the phosphate coating's excellent lithium-ion conduction capabilities, reducing interfacial side reactions, but also enable better lattice matching between the pyrophosphate and phosphate coatings, thus achieving a tighter bond between the coatings.

[0089] This application provides a novel core-shell structured lithium manganese phosphate cathode active material by doping element A at the manganese site and element R at the phosphorus site of lithium manganese phosphate to obtain a doped lithium manganese phosphate core, and then sequentially coating the surface of the core with three layers. When the cathode active material is applied to a secondary battery, it can significantly improve the high-temperature cycle performance, cycle stability and high-temperature storage performance of the secondary battery.

[0090] In any embodiment, the interplanar spacing of the crystalline pyrophosphate in the first coating layer ranges from 0.293 to 0.470 nm, and the included angle of the crystal orientation (111) ranges from 18.00° to 32.00°; the interplanar spacing of the crystalline phosphate in the second coating layer ranges from 0.244 to 0.425 nm, and the included angle of the crystal orientation (111) ranges from 20.00° to 37.00°.

[0091] The first and second coating layers in the positive electrode active material described in this application both use crystalline materials, and their interplanar spacing and angles are within the aforementioned ranges. This effectively avoids impurity phases in the coating layers, thereby improving the specific capacity, cycle performance, and rate performance of the material.

[0092] In any embodiment, in the core, the ratio of y to 1-y is 1:10 to 1:1, optionally 1:4 to 1:1. This further improves the cycle performance and rate performance of the secondary battery.

[0093] In any embodiment, in the core, the ratio of z to 1-z is 1:999 to 1:9, optionally 1:499 to 1:249. This further improves the cycle performance and rate performance of the secondary battery.

[0094] In any embodiment, the carbon in the third coating layer is a mixture of SP2 carbon and SP3 carbon. Optionally, the molar ratio of SP2 carbon to SP3 carbon is any value in the range of 0.1-10, and can be any value in the range of 2.0-3.0.

[0095] 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.

[0096] In any embodiment, the coating amount of the first coating layer is greater than 0 and less than or equal to 6% by weight, optionally greater than 0 and less than or equal to 5.5% by weight, more preferably greater than 0 and less than or equal to 2% by weight, based on the weight of the core; and / or

[0097] The coating amount of the second coating layer is greater than 0 and less than or equal to 6% by weight, optionally greater than 0 and less than or equal to 5.5% by weight, more preferably 2-4% by weight, based on the weight of the core; and / or

[0098] The coating amount of the third coating layer is greater than 0 and less than or equal to 6% by weight, optionally greater than 0 and less than or equal to 5.5% by weight, and more preferably greater than 0 and less than or equal to 2% by weight, based on the weight of the core.

[0099] In the core-shell structured positive electrode active material described in this application, the coating amount of the three coating layers is preferably within the above-mentioned range, thereby enabling sufficient coating of the core and further improving the kinetic performance and safety performance of the secondary battery without sacrificing the specific capacity of the positive electrode active material.

[0100] In any embodiment, the thickness of the first coating layer is 1-10 nm; and / or

[0101] The thickness of the second coating layer is 2-15 nm; and / or

[0102] The thickness of the third coating layer is 2-25 nm.

[0103] In this application, when the thickness of the first coating layer is in the range of 1-10 nm, it can avoid the adverse effects on the dynamic properties of the material that may occur when it is too thick, and it can also avoid the problem that it cannot effectively hinder the migration of transition metal ions when it is too thin.

[0104] When the thickness of the second coating layer is in the range of 2-15nm, the surface structure of the second coating layer is stable and the side reactions with the electrolyte are small. Therefore, it can effectively reduce the interface side reactions and thus improve the high-temperature performance of the secondary battery.

[0105] When the thickness of the third coating layer is in the range of 2-20 nm, it can improve the electrical conductivity of the material and improve the compaction density performance of the battery electrode sheet prepared using the positive electrode active material.

[0106] In any embodiment, based on the weight of the core-shell structured positive electrode active material, the manganese content is in the range of 10 wt%-35 wt%, optionally in the range of 15 wt%-30 wt%, more preferably in the range of 17 wt%-20 wt%, the phosphorus content is in the range of 12 wt%-25 wt%, optionally in the range of 15 wt%-20 wt%, and the weight ratio of manganese to phosphorus is in the range of 0.90-1.25, optionally in the range of 0.95-1.20.

[0107] In the core-shell structured positive electrode active material described in this application, the content of manganese element is within the above-mentioned range, which can effectively avoid the problems of poor material structure stability and density reduction that may be caused by excessive manganese element content, thereby improving the cycle, storage and compaction density performance of the secondary battery; and can also avoid the problems of low voltage platform that may be caused by excessive manganese element content, thereby improving the energy density of the secondary battery.

[0108] In the core-shell structured positive electrode active material described in this application, the phosphorus content is within the above-mentioned range, which can effectively avoid the following situations: if the phosphorus content is too high, it may cause the covalent nature of PO to be too strong, affecting the conductivity of small polarons, thereby affecting the conductivity of the material; if the phosphorus content is too low, it may reduce the stability of the pyrophosphate lattice structure in the core, the first coating layer and / or the phosphate lattice structure in the second coating layer, thereby affecting the overall stability of the material.

[0109] In the core-shell structured positive electrode active material described in this application, the weight ratio of manganese to phosphorus is within the above-mentioned range, which can effectively avoid the following situations: if the weight ratio is too large, it may lead to increased dissolution of transition metals, affecting the stability of the material and the cycle and storage performance of the secondary battery; if the weight ratio is too small, it may cause the discharge voltage plateau of the material to drop, thereby reducing the energy density of the secondary battery.

[0110] In any embodiment, the lattice change rate of the core-shell structured positive electrode active material before and after complete lithium insertion / extraction is less than 4%, preferably less than 3.8%, and more preferably 2.0-3.8%.

[0111] The core-shell structured positive electrode active material described in this application can achieve a lattice change rate of less than 4% before and after lithium insertion / extraction. Therefore, using this positive electrode active material can improve the specific capacity and rate performance of secondary batteries.

[0112] In any embodiment, the Li / Mn antisite defect concentration of the core-shell structured positive electrode active material is 4% or less, preferably 2.2% or less, and more preferably 1.5-2.2%. By keeping the Li / Mn antisite defect concentration within the above range, Mn...2+ Hinder Li + This improves the transport efficiency and simultaneously enhances the specific capacity and rate performance of the positive electrode active material.

[0113] In any embodiment, the compaction density of the core-shell structured positive electrode active material at 3T (tons) is 2.2 g / cm³. 3 The above is optional, 2.2 g / cm³. 3 Above and 2.8g / cm 3 Therefore, increasing the compaction density increases the weight of the active material per unit volume, which is more conducive to improving the volumetric energy density of the secondary battery.

[0114] In any embodiment, the surface oxygen valence state of the core-shell structured positive electrode active material is below -1.90, optionally between -1.90 and -1.98. Therefore, by limiting the surface oxygen valence state of the positive electrode active material to the above range as described above, the interfacial side reactions between the positive electrode material and the electrolyte can be reduced, thereby improving the cell's cycle life, high-temperature storage gas generation, and other performance characteristics.

[0115] A second aspect of this application provides a method for preparing a positive electrode active material, comprising the following steps:

[0116] The steps for providing the core material: the core has the chemical formula Li 1+x Mn 1-y A y P 1-z R z O4, where x is any value in the range of -0.100 to 0.100, y is any value in the range of 0.001 to 0.500, z is any value in the range of 0.001 to 0.100, A is one or more elements selected from Zn, Al, Na, K, Mg, Mo, W, Ti, V, Zr, Fe, Ni, Co, Ga, Sn, Sb, Nb and Ge, and may be one or more elements selected from Fe, Ti, V, Ni, Co and Mg, R is one or more elements selected from B, Si, N and S, and optionally, R is one element selected from B, Si, N and S;

[0117] Coating steps: Provide Li separately a MP2O7 and / or M b (P2O7) c And an XPO4 suspension, the core material is added to the above suspension and mixed, and then sintered to obtain a positive electrode active material, wherein 0≤a≤2, 1≤b≤4, 1≤c≤6, and the values ​​of a, b and c satisfy the following condition: making crystalline pyrophosphate Li a MP2O7 or M b (P2O7)c Maintaining electrical neutrality; each of M is independently selected from one or more elements selected from Fe, Ni, Mg, Co, Cu, Zn, Ti, Ag, Zr, Nb, or Al; and X is selected from one or more elements selected from Li, Fe, Ni, Mg, Co, Cu, Zn, Ti, Ag, Zr, Nb, or Al.

[0118] The positive electrode active material has a core-shell structure, comprising a core and a shell covering the core. The shell includes a first coating layer covering the core, a second coating layer covering the first coating layer, and a third coating layer covering the second coating layer. The first coating layer comprises crystalline pyrophosphate Li. a MP2O7 and / or M b (P2O7) c The second coating layer comprises crystalline phosphate XPO4, and the third coating layer is carbon.

[0119] In any implementation, the step of providing the core material includes the following steps:

[0120] Step (1): Mix and stir the manganese source, dopant of element A and acid in a container to obtain manganese salt particles doped with element A;

[0121] Step (2): The manganese salt particles doped with element A are mixed with a lithium source, a phosphorus source, and a dopant of element R in a solvent to obtain a slurry. After sintering under an inert gas atmosphere, a core doped with elements A and R is obtained, wherein the core doped with elements A and R is Li. 1+x Mn 1-y A y P 1-z R z O4, where x is any value in the range of -0.100 to 0.100, y is any value in the range of 0.001 to 0.500, z is any value in the range of 0.001 to 0.100, A is one or more elements selected from Zn, Al, Na, K, Mg, Mo, W, Ti, V, Zr, Fe, Ni, Co, Ga, Sn, Sb, Nb and Ge, and may be one or more elements selected from Fe, Ti, V, Ni, Co and Mg, R is one or more elements selected from B, Si, N and S, and optionally, R is one element selected from B, Si, N and S.

[0122] In any embodiment, the dopant of element A is one or more elements selected from the following: elemental form, carbonate, sulfate, chloride, nitrate, organic acid salt, oxide, and hydroxide.

[0123] In any embodiment, the dopant of element R is one or more of the inorganic acids, flavonoids, organic acids, sulfates, chlorides, nitrates, organic acid salts, oxides, and hydroxides of one or more elements selected from B, Si, N, and S.

[0124] In any embodiment, step (1) is performed at a temperature of 20-120°C, optionally 40-120°C; and / or

[0125] The stirring in step (1) is carried out at 400-700 rpm for 1-9 hours, or optionally 3-7 hours.

[0126] When the heating temperature and stirring time during the core particle preparation process are within the above range, the obtained core and the positive electrode active material made from it have fewer lattice defects, which is beneficial to suppress manganese dissolution, reduce interfacial side reactions between the positive electrode active material and the electrolyte, and thus improve the cycle performance and safety performance of the secondary battery.

[0127] In any embodiment, step (2) is performed at a temperature of 20-120°C, optionally 40-120°C, for 1-12 hours.

[0128] In any implementation, the coating step includes:

[0129] First coating step: The source of element M, phosphorus source, acid, and optionally lithium source are used to obtain a first coating layer suspension; the core obtained in the core step is thoroughly mixed with the first coating layer suspension obtained in the first coating step, dried, and then sintered to obtain the material coated by the first coating layer.

[0130] Second coating step: Dissolve the source of element X, phosphorus source and acid in solvent to obtain a second coating layer suspension; mix the material coated by the first coating layer obtained in the first coating step with the second coating layer suspension obtained in the second coating step, dry, and then sinter to obtain a material coated by two coating layers.

[0131] The third coating step: Dissolve the carbon source in a solvent to obtain a third coating layer solution; then add the material coated by the two coating layers obtained in the second coating step to the third coating layer solution, mix evenly, dry, and then sinter to obtain a material coated by the three coating layers, i.e., the positive electrode active material.

[0132] In any embodiment, the source of element M is one or more elements selected from Fe, Ni, Mg, Co, Cu, Zn, Ti, Ag, Zr, Nb or Al, including their elemental form, carbonate, sulfate, chloride, nitrate, organic acid salt, oxide, and hydroxide.

[0133] In any embodiment, the source of element X is one or more elements selected from the following: elemental form, carbonate, sulfate, chloride, nitrate, organic acid salt, oxide, hydroxide, etc., of one or more elements selected from Li, Fe, Ni, Mg, Co, Cu, Zn, Ti, Ag, Zr, Nb, or Al.

[0134] In this application, the amount of each source of elements A, R, M, and X added depends on the target doping amount, and the ratio of the amount of lithium source, manganese source, and phosphorus source used conforms to the stoichiometric ratio.

[0135] In any embodiment, in the first coating step, the pH of the solution containing the source of element M, the phosphorus source, and the acid, and optionally the lithium source, is controlled to be 3.5-6.5, then stirred and reacted for 1-5 hours, then the solution is heated to 50-120°C and held at that temperature for 2-10 hours, and / or sintering is carried out at 650-800°C for 2-6 hours.

[0136] By controlling the conditions of the first coating step within the above range, it is possible to guarantee or even improve the specific capacity, cycle and high-temperature storage performance, specific capacity and rate performance of the secondary battery prepared using the positive electrode active material.

[0137] In any embodiment, in the second coating step, after dissolving the source of element X, the phosphorus source and the acid in the solvent, the mixture is stirred and reacted for 1-10 hours. Then, the solution is heated to 60-150°C and held at that temperature for 2-10 hours, and / or, sintering is carried out at 500-700°C for 6-10 hours.

[0138] In the steps of providing the core material and the first coating step and the second coating step, before sintering, that is, in the preparation of the core material in which the chemical reaction occurs (steps (1)-(2)) and in the preparation of the first coating layer suspension and the second coating layer suspension, by selecting the reaction temperature and reaction time as described above, the following situations can be avoided: when the reaction temperature is too low, the reaction cannot occur or the reaction rate is slow; when the temperature is too high, the product decomposes or forms an impurity phase; when the reaction time is too long, the product particle size is large, which may increase the time and difficulty of subsequent processes; when the reaction time is too short, the reaction is incomplete and less product is obtained.

[0139] In any embodiment, the sintering in the third coating step is carried out at 700-800°C for 6-10 hours.

[0140] By controlling the conditions of the third coating step within the above range, it is possible to ensure or even improve the specific capacity and compaction density of the secondary battery prepared using the positive electrode active material.

[0141] The method for preparing the positive electrode active material described in this application uses widely available and inexpensive raw materials, and has a simple process, which is conducive to industrialization.

[0142] The present application provides a positive electrode sheet comprising a positive current collector and a positive electrode film layer disposed on at least one surface of the positive current collector. The positive electrode film layer comprises a positive electrode active material with a core-shell structure as described in the first aspect of the present application or a positive electrode active material prepared by the method described in any aspect of the present application. The content of the positive electrode active material in the positive electrode film layer is 90-99.5% by weight, preferably 95-99.5% by weight, based on the total weight of the positive electrode film layer.

[0143] The positive electrode sheet of any of the embodiments in this application is used in the battery, which improves the high-temperature cycle performance, rate performance and safety performance of the secondary battery.

[0144] The positive electrode sheet, secondary battery, battery module, battery pack, and electrical device of any embodiment in this application include the positive electrode active material described in this application, and therefore have high specific capacity, good cycle performance, and safety performance.

[0145] Beneficial effects

[0146] One or more embodiments of this application have one or more of the following beneficial effects:

[0147] (1) This application provides a novel doped positive electrode active material with a core-shell structure by doping element A at the manganese site of lithium manganese phosphate and doping element R at the phosphorus site to obtain a doped lithium manganese phosphate core and sequentially coating the surface of the core with three layers. When the positive electrode active material is applied to a secondary battery, it can significantly improve the high-temperature cycle performance, cycle stability and high-temperature storage performance of the secondary battery.

[0148] (2) When the BET specific surface area of ​​the positive electrode active material is large and there are many small particles, the bonding strength between the positive electrode active material and the current collector (aluminum foil) is weak, and the film delamination phenomenon occurs during the coating process. This application adopts a novel conductive undercoating to increase the bonding strength between the positive electrode active material layer and the current collector.

[0149] (3) During the process of coating the positive electrode active material slurry (containing solvent N-methylpyrrolidone, abbreviated as NMP) on the conductive base coating surface, the first polymer in the conductive base coating will dissolve again after contacting the solvent NMP, thereby diffusing with the positive electrode active material slurry. After curing, the active material layer can be integrated with the base coating, thereby effectively increasing the bonding strength between the positive electrode film layer and the positive electrode current collector.

[0150] (4) When the first water-based binder in the conductive base coating is an acrylic-acrylate copolymer (weight average molecular weight: 200,000 to 1,500,000), the binder has strong polarity and can achieve good adhesion with the current collector (aluminum foil). In addition, the acrylic-acrylate copolymer has good stability in the electrolyte, high temperature resistance, corrosion resistance, and low electrolyte absorption efficiency (low swelling degree).

[0151] (5) When the conductive agent in the conductive base coating is selected from one or two of carbon black, acetylene black, carbon fiber, graphite and carbon nanotubes, it can reduce the interface resistance, improve the charge and discharge rate performance of the battery and extend the cycle life of the battery. Attached Figure Description

[0152] Figure 1 This is a schematic diagram of the positive electrode sheet according to an embodiment of this application.

[0153] Figure 2 This is a schematic flowchart illustrating the measurement of electrode adhesion force according to an embodiment of this application;

[0154] Figure 3 This is a schematic diagram of a secondary battery according to one embodiment of this application.

[0155] Figure 4 yes Figure 3 An exploded view of a secondary battery according to one embodiment of this application is shown.

[0156] Figure 5 This is a schematic diagram of a battery module according to one embodiment of this application.

[0157] Figure 6 This is a schematic diagram of a battery pack according to one embodiment of this application.

[0158] Figure 7 yes Figure 6 An exploded view of a battery pack according to one embodiment of this application is shown.

[0159] Figure 8 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.

[0160] Figure 9 This is a schematic diagram of a positive electrode active material with an ideal three-layer coating structure according to an embodiment of this application.

[0161] Explanation of reference numerals in the attached figures:

[0162] 1 Battery pack; 2 Upper casing; 3 Lower casing; 4 Battery module; 5 Secondary battery; 51 Housing; 52 Electrode assembly; 53 Top cover assembly; 11 Positive current collector; 112 Surface; 12 Conductive base coating; 13 Positive film layer; 510 Steel plate; 520 Double-sided adhesive; 530 Electrode sheet; Detailed Implementation

[0163] The following detailed description, with appropriate reference to the accompanying drawings, discloses embodiments of the negative electrode active material and its manufacturing method, positive electrode sheet, negative electrode sheet, secondary battery, battery module, battery pack, and 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 to enable those skilled in the art to fully understand this application and are not intended to limit the subject matter of the claims.

[0164] 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.

[0165] Unless otherwise specified, all embodiments and optional embodiments of this application can be combined to form new technical solutions.

[0166] Unless otherwise specified, all technical features and optional technical features of this application may be combined to form new technical solutions.

[0167] Unless otherwise specified, all steps in this application may be performed sequentially or randomly, preferably sequentially. For example, the method includes steps (a) and (b), indicating that the method may include steps (a) and (b) performed sequentially, or it may include steps (b) and (a) performed sequentially. For example, the mention that the method may also include step (c) indicates that step (c) may be added to the method in any order. For example, the method may include steps (a), (b), and (c), or it may include steps (a), (c), and (b), or it may include steps (c), (a), and (b), etc.

[0168] 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.

[0169] 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).

[0170] [Rechargeable Battery]

[0171] 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.

[0172] 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.

[0173] [Positive electrode plate]

[0174] In some embodiments, this application provides a positive electrode sheet, including a positive current collector, a positive electrode film layer disposed on at least one surface of the positive current collector, and a conductive undercoat layer located between the positive current collector and the positive electrode film layer, wherein,

[0175] The positive electrode film layer includes a positive electrode active material with a core-shell structure, wherein the positive electrode active material includes a core and a shell covering the core.

[0176] The chemical formula of the core is Li 1+x Mn 1-y A y P 1-z R z O4, where x is any value in the range of -0.100 to 0.100, y is any value in the range of 0.001 to 0.500, z is any value in the range of 0.001 to 0.100, A is one or more elements selected from Zn, Al, Na, K, Mg, Mo, W, Ti, V, Zr, Fe, Ni, Co, Ga, Sn, Sb, Nb and Ge, and may be one or more elements selected from Fe, Ti, V, Ni, Co and Mg, R is one or more elements selected from B, Si, N and S, and optionally R is one element selected from B, Si, N and S, and the values ​​of x, y and z satisfy the following condition: keeping the entire core electrically neutral;

[0177] 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...

[0178] The first coating layer comprises crystalline pyrophosphate Li a MP2O7 and / or M b (P2O7) c Where 0≤a≤2, 1≤b≤4, 1≤c≤6, and the values ​​of a, b, and c satisfy the following condition: making crystalline pyrophosphate Li a MP2O7 or M b (P2O7) c Maintaining electrical neutrality, the crystalline pyrophosphate Li a MP2O7 and M b (P2O7) c Each of the M elements is independently selected from Fe, Ni, Mg, Co, Cu, Zn, Ti, Ag, Zr, Nb, or Al.

[0179] The second coating layer comprises crystalline phosphate XPO4, wherein X is one or more elements selected from Li, Fe, Ni, Mg, Co, Cu, Zn, Ti, Ag, Zr, Nb, or Al.

[0180] The third coating layer is carbon;

[0181] The conductive undercoat includes a first polymer, a first water-based binder, and a first conductive agent.

[0182] The first polymer comprises:

[0183] Equation 1 represents the first single-unit cell;

[0184] A second monomer unit selected from at least one of the monomer units represented by Formula 2 and the monomer units represented by Formula 3;

[0185] A third monomer unit selected from the group consisting of monomer units represented by Formula 4 and monomer units represented by Formula 5; and

[0186] The fourth monomer unit represented by Equation 6, R 1 R 2 R 3 Each group independently represents H, carboxyl, ester, and the following substituted or unsubstituted groups: C1–C10 alkyl, C1–C10 alkoxy, C2–C10 alkenyl, C6–C10 aryl, R 4 The following groups, whether substituted or unsubstituted, represent H: alkyl (C1-C10), alkoxy (C1-C10), alkenyl (C2-C10), and aryl (C6-C10).

[0187]

[0188] In the positive electrode sheet based on the above scheme, the positive electrode film layer and the positive electrode current collector have enhanced bonding strength. Without theoretical limitations, during the process of coating the positive electrode active material slurry (containing the solvent N-methylpyrrolidone, abbreviated as NMP) onto the conductive undercoat surface, the first polymer in the conductive undercoat will dissolve again upon contact with the solvent NMP, thereby interdiffusion with the positive electrode active material slurry. After curing, the active material layer can fuse with the undercoat layer, thus effectively increasing the bonding strength between the positive electrode film layer and the positive electrode current collector.

[0189] In some embodiments, the first polymer is a random copolymer.

[0190] Nitrile butadiene rubber (NBR) is a random copolymer formed by the polymerization (e.g., emulsion polymerization) of acrylonitrile and butadiene monomers, with the following general structural formula:

[0191]

[0192] In nitrile rubber, the linkage between butadiene (B) and acrylonitrile (A) units is generally a ternary combination of BAB, BBA, or ABB, ABA, and BBB. However, with increasing acrylonitrile content, a pentagonal linkage of AABAA can also occur, and it can even become the bulk polymer of acrylonitrile. In nitrile rubber, the sequence distribution of butadiene is mainly trans-1,4 structure, and its microstructure is related to the polymerization conditions.

[0193] Hydrogenated nitrile butadiene rubber (HNBR) is a product obtained by adding hydrogen to the carbon-carbon double bonds in the molecular chain of nitrile butadiene rubber until it is partially or completely saturated. The chemical formula of fully saturated hydrogenated nitrile butadiene rubber is as follows:

[0194]

[0195] There are three main methods for preparing hydrogenated nitrile butadiene rubber (HNBR): ethylene-acrylonitrile copolymerization, NBR solution hydrogenation, and NBR emulsion hydrogenation.

[0196] Hydrogenated carboxylated butyl rubber (HXNBR) is a polymer obtained by selectively hydrogenating C=C bonds in a copolymer formed by copolymerizing nitrile (e.g., acrylonitrile), conjugated diene (e.g., butadiene) and unsaturated carboxylic acid. Hydrogenated carboxylated butyl rubber is essentially hydrogenated nitrile butadiene rubber with the addition of carboxyl groups.

[0197] Esters of unsaturated carboxylic acids are, for example, esters of α,β-unsaturated monocarboxylic acids. Alkyl esters and alkoxyalkyl esters of α,β-unsaturated monocarboxylic acids are acceptable. Alkyl esters of α,β-unsaturated monocarboxylic acids, such as C1-C18 alkyl esters, are also acceptable. Alkyl esters of acrylic acid or methacrylic acid, such as C1-C18 alkyl esters, include methyl acrylate, ethyl acrylate, propyl acrylate, n-butyl acrylate, tert-butyl acrylate, 2-ethylhexyl acrylate, n-dodecyl acrylate, methyl methacrylate, ethyl methacrylate, butyl methacrylate, and 2-ethylhexyl methacrylate. Alkoxyalkyl esters of α,β-unsaturated monocarboxylic acids are also acceptable. Alkoxyalkyl esters of acrylic acid or methacrylic acid, such as C2-C12-alkoxyalkyl esters of acrylic acid or methacrylic acid, are further acceptable. Methoxymethyl acrylate, methoxyethyl (meth)acrylate, ethoxyethyl (meth)acrylate, and methoxyethyl (meth)acrylate are also acceptable. Mixtures of alkyl esters (such as those mentioned above) and alkoxyalkyl esters (such as those in the forms mentioned above) may also be used. Hydroxyalkyl acrylates and hydroxyalkyl methacrylates wherein the hydroxyalkyl group has 1-12 carbon atoms may also be used, such as 2-hydroxyethyl acrylate, 2-hydroxyethyl methacrylate, and 3-hydroxypropyl acrylate. Similarly, epoxy-containing esters, such as glycidyl methacrylate, may be used. Cyanoalkyl acrylates and cyanoalkyl methacrylates wherein the cyanoalkyl group has 2-12 carbon atoms may also be used, such as α-cyanoethyl acrylate, β-cyanoethyl acrylate, and cyanobutyl methacrylate. Fluorine-substituted benzyl acrylates or methacrylates may also be used, such as fluorobenzyl acrylate and fluorobenzyl methacrylate. Fluoroalkyl acrylates and methacrylates may also be used, such as trifluoroethyl acrylate and tetrafluoropropyl methacrylate. Amino-containing α,β-unsaturated carboxylic acid esters, such as dimethylaminomethyl acrylate and diethylaminoethyl acrylate, may also be used.

[0198] In some embodiments, based on the total mass of the first polymer,

[0199] The mass percentage content of the first monomer unit is M1, where M1 is 10% to 55%, optionally 25% to 55%; and / or,

[0200] The second monomer unit has a mass percentage content of M2, where M2 is 40%–80%, optionally 50%–70%; and / or,

[0201] The third monomer unit has a mass percentage content of M3, where M3 is 0% to 10%, optionally 0.001% to 2%; and / or,

[0202] The fourth monomer unit has a mass percentage content of M4, where M4 is 0% to 10%, and optionally 0.1% to 1%.

[0203] The conductive undercoat layer based on this scheme can dissolve moderately during the coating process, thereby forming an enhanced bond with the positive electrode film layer.

[0204] In some embodiments, based on the total mass of the first polymer,

[0205] The first monomer unit has a mass percentage content of M1, which is 10% to 55%, optionally 10%-15%, 15%-20%, 20%-25%, 25%-30%, 30%-35%, 35%-40%, 40%-45%, 45%-50%, or 50%-55%; and / or,

[0206] The second monomer unit has a mass percentage content of M2, which is 40% to 80%, optionally 40%-45%, 45%-50%, 50%-55%, 55%-60%, 60%-65%, 65%-70%, 70%-75%, or 75%-80%; and / or,

[0207] The third monomer unit has a mass percentage content of M3, where M3 is 0% to 10%, optionally 0.001%-1%, 1%-2%, 2%-3%, 3%-4%, 4%-5%, 5%-6%, 6%-7%, 7%-8%, 8%-9%, or 9%-10%; and / or,

[0208] The fourth monomer unit has a mass percentage of M4, which is 0%–10%, optionally 0.01%–1%, 1%–2%, 2%–3%, 3%–4%, 4%–5%, 5%–6%, 6%–7%, 7%–8%, 8%–9%, ​​or 9%–10%. The positive electrode sheet based on this scheme is used in a secondary battery, and one or more performance characteristics of the secondary battery are significantly improved. The conductive undercoating layer based on this scheme can dissolve moderately during the coating process, thereby forming a reinforced bond with the positive electrode film layer.

[0209] In some implementations, M3 / (M2+M3) is 0% to 5%, optionally 0.001% to 1%. Positive electrode sheets based on this scheme are used in secondary batteries, resulting in significant improvements in one or more performance characteristics of the secondary battery.

[0210] In some implementations, M3 / (M2+M3) is 0.01%-1%, 1%-2%, 2%-3%, 3%-4%, or 4%-5%.

[0211] In some embodiments, the first polymer comprises one or more selected from hydrogenated nitrile butadiene rubber and hydrogenated carboxylated nitrile butadiene rubber; and / or, the weight-average molecular weight of the first polymer is 50,000 to 1,500,000, optionally 200,000 to 400,000. Positive electrode sheets based on this design are used in secondary batteries, resulting in significant improvements in one or more performance characteristics of the secondary battery.

[0212] In some embodiments, the weight-average molecular weight of the first polymer is 100,000-300,000, 300,000-500,000, 500,000-700,000, 700,000-900,000, 900,000-1,100,000, 1,100,000-1,300,000, or 1,300,000-1,500,000.

[0213] In some embodiments, the first waterborne adhesive comprises one or more selected from waterborne polyacrylic resins and their derivatives, waterborne amino-modified polypropylene resins and their derivatives, and polyvinyl alcohol and its derivatives, optionally including waterborne acrylic-acrylate copolymers; and / or,

[0214] The first aqueous binder has a weight-average molecular weight of 200,000 to 1,500,000, optionally 300,000 to 400,000. Positive electrode sheets based on this design are used in secondary batteries, resulting in significant improvements in one or more performance characteristics of the secondary battery.

[0215] In some embodiments, the weight-average molecular weight of the first aqueous adhesive is 100,000-300,000, 300,000-500,000, 500,000-700,000, 700,000-900,000, 900,000-1,100,000, or 1,100,000-1,300,000.

[0216] In some embodiments, the first conductive agent comprises one or more selected from superconducting carbon, conductive graphite, acetylene black, carbon black, Ketjen black, carbon dots, carbon nanotubes, graphene, and carbon nanofibers, and optionally includes one or more selected from carbon nanotubes, graphene, and carbon nanofibers. A positive electrode based on this scheme is used in a secondary battery, and one or more performance characteristics of the secondary battery are significantly improved.

[0217] In some embodiments, the total mass of the conductive undercoating is used as the basis for measurement.

[0218] The mass percentage of the first polymer is X1, where X1 is 5% to 20%, optionally 5% to 10%; and / or,

[0219] The first water-based adhesive has a mass percentage content of X2, where X2 is 30% to 80%, optionally 40% to 50%; and / or,

[0220] The first conductive agent has a mass percentage of X3, where X3 is 10% to 50%, and optionally 40% to 50%. The positive electrode sheet based on this scheme is used in a secondary battery, resulting in a significant improvement in one or more performance characteristics of the secondary battery.

[0221] In some embodiments, the thickness of the conductive undercoat is 1 μm to 20 μm, optionally 3 μm to 10 μm. Positive electrode sheets based on this design are used in secondary batteries, resulting in significant improvements in one or more performance characteristics of the secondary battery.

[0222] In some embodiments, the positive electrode film layer further includes one or more selected from wetting agents and dispersants; optionally, the positive electrode film layer also includes both a wetting agent and a dispersant. Positive electrode sheets based on this design are used in secondary batteries, resulting in significant improvements in one or more performance characteristics of the secondary battery.

[0223] In some embodiments, the surface tension of the wetting agent is 20 mN / m to 40 mN / m. Optionally, the wetting agent includes at least one of the following functional groups: -CN, -NH2, -NH-, N-, -OH, -COO-, -C(=O)-OC(=O)-. Positive electrode sheets based on this scheme are used in secondary batteries, and one or more performance characteristics of the secondary battery are significantly improved.

[0224] In some implementations, surface tension can be measured using the Wilhelmy Plate Method. Specific test procedures can be found in commonly used standards in the field, such as GB / T / 22237-2008 Surfactants—Determination of surface tension, and ASTM D1331-14. Standard test methods for surface tension and interfacial tension of coating solutions, solvents, surfactant solutions and related materials.

[0225] In some embodiments, the wetting agent comprises one or more selected from small molecule organic solvents and low molecular weight polymers. Optionally, the small molecule organic solvent comprises one or more selected from alkanolamines, alcohols, and nitrile compounds. Optionally, the alkanolamine has 1 to 16 carbon atoms, and optionally 2 to 6. Optionally, the low molecular weight polymer comprises one or more selected from maleic anhydride-styrene copolymers, polyvinylpyrrolidone, and polysiloxanes. Optionally, the weight-average molecular weight of the low molecular weight polymer is below 6000, and optionally 3000 to 6000. The positive electrode sheet based on this scheme is used in secondary batteries, and one or more performance characteristics of the secondary battery are significantly improved.

[0226] In some embodiments, the dispersant comprises a second polymer, and the second polymer comprises:

[0227] The fifth monomer unit represented by Equation 7;

[0228] A sixth monomeric unit selected from at least one of the monomeric units represented by Formula 8 and the monomeric units represented by Formula 9; and

[0229] A seventh monomer unit selected from at least one of the monomer units represented by Formula 10 and the monomer units represented by Formula 11.

[0230]

[0231] The positive electrode sheet based on this scheme is used in secondary batteries, and one or more of the performance characteristics of the secondary battery are significantly improved.

[0232] In some embodiments, based on the total mass of the second polymer,

[0233] The fifth monomer unit has a mass percentage content of M5, where M5 is 10% to 55%, optionally 25% to 55%; and / or,

[0234] The sixth monomer unit has a mass percentage content of M6, where M6 is 40%–80%, optionally 50%–70%; and / or,

[0235] The seventh single-cell unit has a mass percentage of M7, which is 0% to 10%, and optionally 0.001% to 2%. The positive electrode sheet based on this scheme is used in a secondary battery, resulting in a significant improvement in one or more performance characteristics of the secondary battery.

[0236] In some embodiments, based on the total mass of the second polymer,

[0237] The fifth monomer unit has a mass percentage content of M5, which is 10% to 55%, optionally 10%-15%, 15%-20%, 20%-25%, 25%-30%, 30%-35%, 35%-40%, 40%-45%, 45%-50%, or 50%-55%; and / or,

[0238] The sixth monomer unit has a mass percentage content of M6, where M6 is 40%–80%, optionally 40%–45%, 45%–50%, 50%–55%, 55%–60%, 60%–65%, 65%–70%, 70%–75%, or 75%–80%; and / or,

[0239] The seventh monomer unit has a mass percentage content of M7, which is 0% to 10%, and optionally 0.01%-1%, 1%-2%, 2%-3%, 3%-4%, 4%-5%, 5%-6%, 6%-7%, 7%-8%, 8%-9%, or 9%-10%.

[0240] In some implementations, M7 / (M6+M7) is 0% to 5%, optionally 0.001% to 1%. Positive electrode sheets based on this scheme are used in secondary batteries, resulting in significant improvements in one or more performance characteristics of the secondary battery.

[0241] In some embodiments, the second polymer is hydrogenated nitrile butadiene rubber; and / or, the weight-average molecular weight of the second polymer is 50,000 to 500,000, optionally 150,000 to 350,000. Positive electrode sheets based on this design are used in secondary batteries, resulting in significant improvements in one or more performance characteristics of the secondary battery.

[0242] In some implementations, based on the total mass of the positive electrode film,

[0243] The dispersant has a mass percentage content of Y1, where Y1 is 0.05% to 1%, optionally 0.1% to 0.5%; and / or,

[0244] The wetting agent has a mass percentage of Y2, which is 0.05% to 2%, optionally 0.2% to 0.8%. The positive electrode sheet based on this scheme is used in a secondary battery, and one or more performance characteristics of the secondary battery are significantly improved.

[0245] In some embodiments, Y1 / Y2 is 0.05 to 20, optionally 0.1 to 1, and further 0.3 to 0.8. Positive electrode sheets based on this scheme are used in secondary batteries, resulting in significant improvements in one or more performance characteristics of the secondary battery.

[0246] In some embodiments, the mass ratio of the first polymer to the second polymer in the positive electrode is 1.5 to 5, optionally 2 to 3. Positive electrode sheets based on this design are used in secondary batteries, resulting in significant improvements in one or more performance characteristics of the secondary battery.

[0247] In some embodiments, the interplanar spacing of the crystalline pyrophosphate in the first coating layer ranges from 0.293 to 0.470 nm, and the included angle of the crystal orientation (111) ranges from 18.00° to 32.00°; the interplanar spacing of the crystalline phosphate in the second coating layer ranges from 0.244 to 0.425 nm, and the included angle of the crystal orientation (111) ranges from 20.00° to 37.00°. The positive electrode sheet based on this scheme is used in secondary batteries, and one or more performance characteristics of the secondary battery are significantly improved.

[0248] In some embodiments, in the core, the ratio of y to 1-y is 1:10 to 1:1, optionally 1:4 to 1:1; and / or, in the core, the ratio of z to 1-z is 1:9 to 1:999, optionally 1:499 to 1:249. Positive electrode sheets based on this scheme are used in secondary batteries, resulting in significant improvements in one or more performance characteristics of the secondary battery.

[0249] In some embodiments, the carbon in the third coating layer is a mixture of SP2 and SP3 carbon. Optionally, the molar ratio of SP2 to SP3 carbon can be any value within the range of 0.1-10, and can be selected as any value within the range of 2.0-3.0. The positive electrode sheet based on this scheme is used in a secondary battery, and one or more performance characteristics of the secondary battery are significantly improved.

[0250] In some embodiments, the coating amount of the first coating layer is greater than 0 and less than or equal to 6% by weight, optionally greater than 0 and less than or equal to 5.5% by weight, more preferably greater than 0 and less than or equal to 2% by weight, based on the weight of the core; and / or

[0251] The coating amount of the second coating layer is greater than 0 and less than or equal to 6% by weight, optionally greater than 0 and less than or equal to 5.5% by weight, more preferably 2-4% by weight, based on the weight of the core; and / or

[0252] The coating amount of the third coating layer is greater than 0 and less than or equal to 6% by weight, optionally greater than 0 and less than or equal to 5.5% by weight, and more preferably greater than 0 and less than or equal to 2% by weight, based on the weight of the core. The positive electrode sheet based on this scheme is used in a secondary battery, and one or more performance characteristics of the secondary battery are significantly improved.

[0253] In some embodiments, the thickness of the first coating layer is 1-10 nm; and / or

[0254] The thickness of the second coating layer is 2-15 nm; and / or

[0255] The thickness of the third coating layer is 2-25 nm. Positive electrode sheets based on this design are used in secondary batteries, resulting in significant improvements in one or more performance characteristics of the secondary battery.

[0256] In some embodiments, based on the weight of the positive electrode active material, the manganese content is in the range of 10 wt%-35 wt%, optionally in the range of 15 wt%-30 wt%, and more preferably in the range of 17 wt%-20 wt%; the phosphorus content is in the range of 12 wt%-25 wt%, optionally in the range of 15 wt%-20 wt%; and the weight ratio of manganese to phosphorus is in the range of 0.90-1.25, optionally in the range of 0.95-1.20. Positive electrode sheets based on this scheme are used in secondary batteries, resulting in significant improvements in one or more performance characteristics of the secondary battery.

[0257] In some embodiments, the lattice change rate of the core-shell structured positive electrode active material before and after complete lithium insertion / extraction is less than 4%, preferably less than 3.8%, and more preferably 2.0-3.8%. Positive electrode sheets based on this scheme are used in secondary batteries, resulting in significant improvements in one or more performance characteristics of the secondary battery.

[0258] In some embodiments, the Li / Mn antisite defect concentration of the core-shell structured positive electrode active material is below 4%, optionally below 2.2%, and more preferably 1.5-2.2%. Positive electrode sheets based on this scheme are used in secondary batteries, resulting in significant improvements in one or more performance characteristics of the secondary battery.

[0259] In some embodiments, the core-shell structured positive electrode active material has a compaction density of 2.2 g / cm³ at 3T. 3 The above is optional, 2.2 g / cm³. 3 Above and 2.8g / cm 3 The following describes how positive electrode plates based on this scheme are used in secondary batteries, resulting in significant improvements in one or more performance characteristics of the secondary batteries.

[0260] In some embodiments, the surface oxygen valence state of the core-shell structured positive electrode active material is below -1.90, optionally between -1.90 and -1.98. Positive electrode sheets based on this design are used in secondary batteries, resulting in significant improvements in one or more performance characteristics of the secondary battery.

[0261] In some embodiments, this application provides a secondary battery including the positive electrode sheet described in any of the above embodiments.

[0262] In some embodiments, this application provides an electrical device including the aforementioned secondary battery.

[0263] [Positive electrode active material]

[0264] The first aspect of this application provides a novel positive electrode active material with a core-shell structure, comprising a core and a shell enclosing the core, wherein the core has the chemical formula Li. 1+x Mn 1-y Ay P 1-z R z O4, where x is any value in the range of -0.100 to 0.100, y is any value in the range of 0.001 to 0.500, z is any value in the range of 0.001 to 0.100, A is one or more elements selected from Zn, Al, Na, K, Mg, Mo, W, Ti, V, Zr, Fe, Ni, Co, Ga, Sn, Sb, Nb and Ge, and may be one or more elements selected from Fe, Ti, V, Ni, Co and Mg, R is one or more elements selected from B, Si, N and S, and optionally, R is one element selected from B, Si, N and S;

[0265] The values ​​of x, y, and z satisfy the following condition: keeping the entire kernel electrically neutral;

[0266] 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...

[0267] The first coating layer comprises crystalline pyrophosphate Li a MP2O7 and / or M b (P2O7) c ,in,

[0268] 0≤a≤2, 1≤b≤4, 1≤c≤6, where the values ​​of a, b, and c satisfy the following condition: This makes crystalline pyrophosphate Li... a MP2O7 or M b (P2O7) c Maintain electrical neutrality

[0269] The crystalline pyrophosphate Li a MP2O7 and M b (P2O7) c Each of the M elements is independently selected from Fe, Ni, Mg, Co, Cu, Zn, Ti, Ag, Zr, Nb, or Al.

[0270] The second coating layer comprises crystalline phosphate XPO4, wherein,

[0271] X is one or more elements selected from Li, Fe, Ni, Mg, Co, Cu, Zn, Ti, Ag, Zr, Nb, or Al;

[0272] The third coating layer is carbon.

[0273] The inventors of this application discovered in practical operation that existing lithium manganese phosphate cathode active materials suffer from significant manganese leaching during deep charge-discharge processes. Although existing technologies have attempted to coat lithium manganese phosphate with lithium iron phosphate to reduce interfacial side reactions, this coating cannot prevent the leached manganese from migrating further into the electrolyte. After migrating to the negative electrode, the leached manganese is reduced to metallic manganese. This metallic manganese acts as a "catalyst," catalyzing the decomposition of the SEI (solid electrolyte interphase) film on the negative electrode surface, producing byproducts. Some of these byproducts are gaseous, causing the secondary battery to expand and affecting its safety performance. Additionally, another portion of these byproducts deposits on the negative electrode surface, obstructing the channels for lithium ions to enter and exit the negative electrode, increasing the secondary battery impedance and thus affecting its kinetic performance. Furthermore, to replenish the lost SEI film, the electrolyte and the active lithium inside the battery are continuously consumed, irreversibly impacting the capacity retention rate of the secondary battery.

[0274] After extensive research, the inventors discovered that by modifying lithium manganese phosphate and coating it with multiple layers, a novel positive electrode active material with a core-shell structure can be obtained. This positive electrode active material can achieve significantly reduced manganese dissolution and reduced lattice change rate. When used in secondary batteries, it can improve the battery's cycle performance, rate performance, safety performance, and increase battery capacity.

[0275] It should be noted that, in this document, the term "coating layer" refers to a material layer covering the lithium manganese phosphate core, which may completely or partially cover the lithium manganese phosphate core. The use of "coating layer" is for ease of description only and is not intended to limit the invention. Similarly, the term "coating layer thickness" refers to the thickness of the material layer covering the lithium manganese phosphate core in the radial direction of the lithium manganese phosphate core.

[0276] Unless otherwise stated, in the above-described core chemical formula, when A comprises two or more elements, the limitation on the numerical range of y applies not only to the stoichiometric coefficient of each element as A, but also to the sum of the stoichiometric coefficients of all elements as A. For example, when A comprises two or more elements Al, A2…An, the stoichiometric coefficients y1, y2…yn of each of Al, A2…An must each fall within the numerical range of y defined in this application, and the sum of y1, y2…yn must also fall within this numerical range. Similarly, for the case where R comprises two or more elements, the limitation on the numerical range of the stoichiometric coefficient of R in this application has the same meaning.

[0277] In an optional implementation, when A is one, two, three, or four elements selected from Zn, Al, Na, K, Mg, Mo, W, Ti, V, Zr, Fe, Ni, Co, Ga, Sn, Sb, Nb, and Ge, A y For Q n1 D n2 E n3 K n4 Where n1 + n2 + n3 + n4 = y, and n1, n2, n3, and n4 are all positive numbers and not all zero simultaneously. Q, D, E, and K are each independently selected from Zn, Al, Na, K, Mg, Mo, W, Ti, V, Zr, Fe, Ni, Co, Ga, Sn, Sb, Nb, and Ge. Optionally, at least one of Q, D, E, and K is Fe. Optionally, one of n1, n2, n3, and n4 is zero, and the others are not zero; more preferably, two of n1, n2, n3, and n4 are zero, and the others are not zero; even more preferably, three of n1, n2, n3, and n4 are zero, and the others are not zero. The kernel Li 1+x Mn 1-y A y P 1-z R z In O4, it is advantageous to dope one, two, three or four of the aforementioned A elements at the manganese sites, and optionally, one, two or three of the aforementioned A elements are doped. In addition, it is advantageous to dope one or two R elements at the phosphorus sites, which is beneficial to make the doped elements uniformly distributed.

[0278] The kernel Li 1+x Mn 1-y A y P 1-z R z In O4, the value of x is influenced by the valence states of A and R, as well as the values ​​of y and z, to ensure the overall system remains electrically neutral. If the value of x is too small, the lithium content of the entire core system will decrease, affecting the specific capacity of the material. The value of y limits the total amount of all dopants. If y is too small, the doping amount is too low, and the dopants will not play a role. If y exceeds 0.5, the Mn content in the system will be low, affecting the voltage plateau of the material. The R element is doped at the P site. Since the PO tetrahedron is relatively stable, and a large z value would affect the stability of the material, the z value is limited to 0.001-0.100.

[0279] The positive electrode active material of this application can improve the specific capacity, cycle performance, and safety performance of secondary batteries. Although the mechanism is not yet clear, it is speculated that the lithium manganese phosphate positive electrode active material of this application has a core-shell structure. By doping the manganese and phosphorus sites of the lithium manganese phosphate core with elements A and R respectively, it can not only effectively reduce manganese dissolution, thereby reducing the number of manganese ions migrating to the negative electrode and reducing the electrolyte consumed due to SEI film decomposition, thus improving the cycle performance and safety performance of the secondary battery, but also promote Mn-O bond adjustment, lower the lithium ion migration barrier, promote lithium ion migration, and improve the rate performance of the secondary battery. By coating the core with a first coating layer including crystalline pyrophosphate, it can... Further increasing the migration resistance of manganese reduces its dissolution, and also reduces the content of surface impurities lithium and the contact between the core and the electrolyte, thereby reducing interfacial side reactions and gas generation, and improving the high-temperature storage performance, cycle performance and safety performance of the secondary battery; by further coating with a crystalline phosphate coating layer with excellent lithium-ion conductivity, the interfacial side reactions on the surface of the positive electrode active material can be effectively reduced, thereby improving the high-temperature cycle and storage performance of the secondary battery; by further coating with a carbon layer as a third coating layer, the safety performance and kinetic performance of the secondary battery can be further improved.

[0280] Furthermore, in the core, the element A doped at the manganese site of lithium manganese phosphate helps to reduce the lattice change rate of lithium manganese phosphate during the lithium insertion / extraction process, improves the structural stability of the lithium manganese phosphate cathode material, greatly reduces the dissolution of manganese and reduces the oxygen activity on the particle surface; the element R doped at the phosphorus site also helps to change the ease of Mn-O bond length change, thereby improving electronic conductivity and reducing the lithium ion migration barrier, promoting lithium ion migration and improving the rate performance of the secondary battery.

[0281] 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.

[0282] The cores prepared in this application have an average particle size range of 50-500 nm and a Dv50 of 200-300 nm. The primary particle size of the cores is also within the range of 50-500 nm, with a Dv50 of 200-300 nm. If the average particle size of the cores is too large (exceeding 500 nm), the specific capacity of the secondary battery using this material will be affected; if the average particle size of the cores is too small, its specific surface area is too large, making it prone to aggregation and difficult to achieve uniform coating.

[0283] In this application, the median particle size Dv50 refers to the particle size corresponding to a cumulative volume distribution percentage of 50% for the material. In this application, the median particle size Dv50 of the material can be determined using laser diffraction particle size analysis. For example, it can be determined using a laser particle size analyzer (e.g., Malvem Master Size 3000) in accordance with standard GB / T19077-2016.

[0284] By controlling the process (e.g., thoroughly mixing and grinding materials from various sources), it is possible to ensure that each element is uniformly distributed in the crystal lattice and that no aggregation occurs. The main characteristic peak positions in the XRD pattern of lithium manganese phosphate doped with elements A and R are consistent with those of undoped LiMnPO4, indicating that no impurity phase was introduced during the doping process. Therefore, the improvement in the core performance mainly comes from elemental doping, rather than from impurity phases. After preparing the positive electrode active material described in this application, the inventors used focused ion beam (FIB) to cut the middle region of the prepared positive electrode active material particles. Testing using transmission electron microscopy (TEM) and energy dispersive X-ray spectroscopy (EDS) revealed that the elements were uniformly distributed and no aggregation occurred.

[0285] In this application, "crystalline state" refers to a crystallinity of 50% or higher, i.e., 50%-100%. Crystallinity less than 50% is referred to as the glassy state. The crystallinity of the crystalline pyrophosphate and crystalline phosphate described in this application is 50% to 100%. Pyrophosphate and phosphate with a certain degree of crystallinity not only fully utilize the pyrophosphate coating's ability to inhibit manganese dissolution and the phosphate coating's excellent lithium-ion conduction capabilities, reducing interfacial side reactions, but also enable better lattice matching between the pyrophosphate and phosphate coatings, thus achieving a tighter bond between the coatings.

[0286] In this application, the crystallinity of the first coating layer material crystalline pyrophosphate and the second coating layer material crystalline phosphate of the positive electrode active material can be tested by conventional technical means in the art, such as by density method, infrared spectroscopy, differential scanning calorimetry and nuclear magnetic resonance absorption method, or by, for example, X-ray diffraction.

[0287] A specific X-ray diffraction method for testing the crystallinity of the first coating layer crystalline pyrophosphate and the second coating layer crystalline phosphate of the positive electrode active material may include the following steps:

[0288] 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.

[0289] It should be noted that, in this application, the crystallinity of pyrophosphate and phosphate in the coating layer can be adjusted, for example, by adjusting the process conditions of the sintering process, such as sintering temperature and sintering time.

[0290] In this application, since metal ions are difficult to migrate in pyrophosphate, pyrophosphate, as the first coating layer, can effectively isolate the doped metal ions from the electrolyte. Crystalline pyrophosphate has a stable structure; therefore, coating with crystalline pyrophosphate can effectively suppress the dissolution of transition metals and improve cycle performance.

[0291] The bonding between the first coating layer and the core is similar to that of a heterojunction, and the strength of this bonding is limited by the degree of lattice matching. When the lattice mismatch is below 5%, the lattice matching is good, and the two easily bond tightly. A tight bonding ensures that the coating layer will not detach during subsequent cycling, which is beneficial for ensuring the long-term stability of the material. The degree of bonding between the first coating layer and the core is mainly measured by calculating the mismatch between the lattice constants of the core and the coating. In this application, after doping the core with A and R elements, the matching degree between the core and the first coating layer is improved compared to undoped elements, and the core and the pyrophosphate coating layer can bond more tightly.

[0292] Crystalline phosphate was chosen as the second coating layer primarily because of its high lattice matching (mismatch of only 3%) with the first coating layer, crystalline pyrophosphate. Secondly, phosphate itself is more stable than pyrophosphate, and coating pyrophosphate with phosphate helps improve the material's stability. Crystalline phosphate has a very stable structure and excellent lithium-ion conductivity; therefore, using crystalline phosphate for coating can effectively reduce interfacial side reactions on the surface of the positive electrode active material, thereby improving the high-temperature cycling and storage performance of the secondary battery. The lattice matching between the second and first coating layers is similar to the bonding between the first coating layer and the core; when the lattice mismatch is below 5%, the lattice matching is good, and the two easily bond tightly.

[0293] The primary reason for using carbon as the third coating layer is its excellent electronic conductivity. Since electrochemical reactions occur in secondary batteries, requiring electrons, carbon, with its superior conductivity, can be used to coat the positive electrode active material to increase electron transport between particles and between different locations on the particles. Carbon coating effectively improves the conductivity and desolvation capability of the positive electrode active material.

[0294] Figure 9 This is a schematic diagram of a cathode active material with an ideal three-layer coating structure in some embodiments. 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 figure represents the ideal state where each layer is fully coated. In practice, each coating layer can be fully coated or partially coated.

[0295] In some embodiments, the average particle size of the primary particles of the positive electrode active material ranges from 50 to 500 nm, and the volume median particle size (Dv50) is in the range of 200 to 300 nm. Since particle agglomeration can occur, the actual measured size of the agglomerated secondary particles may be 500 to 40,000 nm. The size of the positive electrode active material particles affects the processing of the material and the compaction density performance of the electrode sheet. By selecting an average particle size of the primary particles within the above range, the following situations can be avoided: if the average particle size of the primary particles of the positive electrode active material is too small, it may cause particle agglomeration, making dispersion difficult and requiring more binder, resulting in poor electrode sheet brittleness; if the average particle size of the primary particles of the positive electrode active material is too large, it may result in larger gaps between particles, reducing the compaction density.

[0296] The above scheme can effectively suppress the lattice change rate of lithium manganese phosphate and the dissolution of Mn during the lithium insertion / extraction process, thereby improving the high-temperature cycle stability and high-temperature storage performance of secondary batteries.

[0297] In some embodiments, the interplanar spacing of the crystalline pyrophosphate in the first coating layer ranges from 0.293 to 0.470 nm, and the included angle of the crystal orientation (111) ranges from 18.00° to 32.00°; the interplanar spacing of the crystalline phosphate in the second coating layer ranges from 0.244 to 0.425 nm, and the included angle of the crystal orientation (111) ranges from 20.00° to 37.00°.

[0298] The first and second coating layers of the positive electrode active material described in this application both use crystalline materials. The crystalline pyrophosphate and crystalline phosphate in the coating layers can be characterized using conventional techniques in the art, or for example, by transmission electron microscopy (TEM). Under TEM, the core and coating layers can be distinguished by measuring the interplanar spacing.

[0299] The specific testing methods for the interplanar spacing and angle of crystalline pyrophosphate and crystalline phosphate in the coating layer may include the following steps:

[0300] 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.

[0301] 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.

[0302] The interplanar spacing range of crystalline pyrophosphates differs from that of crystalline phosphates, and can be directly determined by the value of the interplanar spacing.

[0303] Crystalline pyrophosphate and crystalline phosphate within the aforementioned interplanar spacing and angle range can more effectively suppress the lattice change rate of lithium manganese phosphate and Mn dissolution during lithium insertion / extraction, thereby improving the high-temperature cycle performance, cycle stability and high-temperature storage performance of secondary batteries.

[0304] In some embodiments, the ratio of y to 1-y in the core is 1:10 to 1:1, optionally 1:4 to 1:1. Here, y represents the sum of the stoichiometric coefficients of the Mn-doped elements A. When the above conditions are met, the energy density and cycle performance of the secondary battery using the positive electrode active material can be further improved.

[0305] In some embodiments, the ratio of z to 1-z in the core is from 1:9 to 1:999, optionally from 1:499 to 1:249. Here, z represents the sum of the stoichiometric coefficients of the p-site doping elements R. When the above conditions are met, the energy density and cycle performance of the secondary battery using the positive electrode active material can be further improved.

[0306] In some embodiments, the carbon in the third coating layer is a mixture of SP2 carbon and SP3 carbon. Optionally, the molar ratio of SP2 carbon to SP3 carbon is any value in the range of 0.1-10, and can be any value in the range of 2.0-3.0.

[0307] 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.

[0308] In this application, "about" for a certain value represents a range, specifically a range of ±10% of that value.

[0309] By selecting the morphology of carbon in the carbon coating layer, the overall electrical performance of the secondary battery can be improved. Specifically, by using a mixture of SP2 and SP3 carbon morphologies and limiting the ratio of SP2 to SP3 carbon within a certain range, the following situations can be avoided: if the carbon in the coating layer is all amorphous SP3, the conductivity is poor; if it is all graphitized SP2, although the conductivity is good, there are few lithium-ion pathways, which is not conducive to lithium insertion / extraction. In addition, limiting the molar ratio of SP2 to SP3 carbon within the above-mentioned range can achieve both good conductivity and ensure lithium-ion pathways, thus benefiting the realization of secondary battery functions and cycle performance.

[0310] 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.

[0311] The structure and characteristics of the third coating carbon can be determined by Raman spectroscopy. The specific testing method is as follows: by dividing the energy spectrum of the Raman test, Id / Ig (where Id is the peak intensity of SP3 carbon and Ig is the peak intensity of SP2 carbon) is obtained, thereby confirming the molar ratio of the two.

[0312] In some embodiments, the coating amount of the first coating layer is greater than 0 and less than or equal to 6% by weight, optionally greater than 0 and less than or equal to 5.5% by weight, more preferably greater than 0 and less than or equal to 2% by weight, based on the weight of the core; and / or

[0313] The coating amount of the second coating layer is greater than 0 and less than or equal to 6% by weight, optionally greater than 0 and less than or equal to 5.5% by weight, more preferably 2-4% by weight, based on the weight of the core; and / or

[0314] The coating amount of the third coating layer is greater than 0 and less than or equal to 6% by weight, optionally greater than 0 and less than or equal to 5.5% by weight, and more preferably greater than 0 and less than or equal to 2% by weight, based on the weight of the core.

[0315] In this application, the coverage of each layer is not zero.

[0316] In the core-shell structured positive electrode active material described in this application, the coating amount of the three coating layers is preferably within the above-mentioned range, thereby enabling sufficient coating of the core and further improving the kinetic performance and safety performance of the secondary battery without sacrificing the specific capacity of the positive electrode active material.

[0317] For the first coating layer, by keeping the coating amount within the above range, the following situations can be avoided: too little coating amount means the coating layer is too thin, which may not effectively hinder the migration of transition metals; too much coating amount means the coating layer is too thick, which may affect Li + The migration of these molecules affects the rate performance of the material.

[0318] For the second coating layer, by keeping the coating amount within the above range, the following situations can be avoided: too much coating amount may affect the overall plateau voltage of the material; too little coating amount may not achieve sufficient coating effect.

[0319] For the third coating layer, the carbon coating mainly plays the role of enhancing electron transport between particles. However, since the structure also contains a large amount of amorphous carbon, the carbon density is low. Therefore, if the coating amount is too large, it will affect the compaction density of the electrode.

[0320] In some embodiments, the thickness of the first coating layer is 1-10 nm; and / or

[0321] The thickness of the second coating layer is 2-15 nm; and / or

[0322] The thickness of the third coating layer is 2-25 nm.

[0323] In some embodiments, the thickness of the first coating layer may be about 2 nm, about 3 mm, about 4 nm, about 5 nm, about 6 nm, about 7 nm, about 8 nm, about 9 nm, or about 10 nm, or any range of the above values.

[0324] 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.

[0325] 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.

[0326] When the thickness of the first coating layer is in the range of 1-10 nm, it can avoid the adverse effects on the dynamic properties of the material that may occur when it is too thick, and it can also avoid the problem that it may not be able to effectively hinder the migration of transition metal ions when it is too thin.

[0327] When the thickness of the second coating layer is in the range of 2-15nm, the surface structure of the second coating layer is stable and the side reactions with the electrolyte are small. Therefore, it can effectively reduce the interface side reactions and thus improve the high-temperature performance of the secondary battery.

[0328] When the thickness of the third coating layer is in the range of 2-25 nm, it can improve the electrical conductivity of the material and improve the compaction performance of the battery electrode sheet prepared using the positive electrode active material.

[0329] 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.

[0330] In some embodiments, based on the weight of the positive electrode active material, the manganese content is in the range of 10 wt% to 35 wt%, optionally in the range of 15 wt% to 30 wt%, and more preferably in the range of 17 wt% to 20 wt%, the phosphorus content is in the range of 12 wt% to 25 wt%, optionally in the range of 15 wt% to 20 wt%, and the weight ratio of manganese to phosphorus is in the range of 0.90 to 1.25, optionally in the range of 0.95 to 1.20.

[0331] 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.

[0332] In this application, limiting the content of manganese element within the above-mentioned range can effectively avoid problems such as poor material structure stability and decreased density that may be caused by excessive manganese element content, thereby improving the cycle, storage and compaction performance of the secondary battery; and can also avoid problems such as low voltage platform that may be caused by excessive manganese element content, thereby improving the energy density of the secondary battery.

[0333] In this application, limiting the phosphorus content within the above-mentioned range can effectively avoid the following situations: if the phosphorus content is too high, it may cause the covalent nature of PO to be too strong, affecting the conductivity of small polarons and thus affecting the conductivity of the material; if the phosphorus content is too low, it may reduce the stability of the pyrophosphate lattice structure in the core, the first coating layer and / or the phosphate lattice structure in the second coating layer, thereby affecting the overall stability of the material.

[0334] The weight ratio of manganese to phosphorus has the following effects on the performance of secondary batteries: If the weight ratio is too high, it means that there is too much manganese, which increases manganese dissolution and affects the stability and specific capacity of the positive electrode active material, thus affecting the cycle performance and storage performance of the secondary battery; if the weight ratio is too low, it means that there is too much phosphorus, which is prone to forming impurity phases, which will lower the discharge voltage plateau of the material and thus reduce the energy density of the secondary battery.

[0335] 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.

[0336] In some embodiments, the lattice change rate of the core-shell structured positive electrode active material before and after complete lithium insertion / extraction is less than 4%, preferably less than 3.8%, and more preferably 2.0-3.8%.

[0337] The lithium intercalation / deintercalation process of lithium manganese phosphate (LiMnPO4) is a two-phase reaction. The interfacial stress between the two phases is determined by the rate of lattice change before and after lithium intercalation / deintercalation; the smaller the rate of lattice change, the smaller the interfacial stress. + The easier the transmission, the better. Therefore, reducing the lattice change rate of the core will be beneficial for enhancing Li. + The core-shell structured positive electrode active material described in this application achieves a lattice change rate of less than 4% before and after lithium insertion / extraction, thus improving the rate performance of the secondary battery. The lattice change rate can be measured using methods known in the art, such as X-ray diffraction (XRD).

[0338] In some embodiments, the Li / Mn antisite defect concentration of the core-shell structured positive electrode active material is less than 4%, optionally less than 2.2%, and more preferably 1.5-2.2%.

[0339] The Li / Mn inversion defect described in this application refers to the Li / Mn inversion defect in the LiMnPO4 lattice. + With Mn2 + The positions of the Li and Mn antisite defects have been interchanged. Accordingly, the Li / Mn antisite defect concentration refers to the concentration relative to Mn. 2+ Interchangeable Li + Zhan Li + Percentage of the total amount. In this application, the concentration of Li / Mn antisite defects can be tested, for example, according to JIS K 0131-1996.

[0340] The core-shell structured positive electrode active material described in this application can achieve the aforementioned low Li / Mn antisite defect concentration. Although the mechanism is not yet fully understood, the inventors of this application speculate that due to the presence of Li in the LiMnPO4 lattice... + With Mn 2+ The positions will be swapped, and Li + The transmission channel is a one-dimensional channel, therefore Mn 2+ In Li + Migration will be difficult within the channel, thus hindering Li + Therefore, the core-shell structured positive electrode active material described in this application, due to its low Li / Mn antisite defect concentration within the aforementioned range, can avoid Mn transport. 2+ Hinder Li + This improves the transport efficiency and enhances the specific capacity and rate performance of the positive electrode active material.

[0341] In some embodiments, the compaction density of the positive electrode active material at 3T is 2.2 g / cm³. 3 The above is optional, 2.2 g / cm³. 3 Above and 2.8g / cm 3 The higher the compaction density, the greater the weight of active material per unit volume. Therefore, increasing the compaction density is beneficial for improving the volumetric energy density of the battery cell. Compaction density can be measured according to GB / T 24533-2009.

[0342] In some embodiments, the surface oxygen valence state of the positive electrode active material is below -1.90, optionally from -1.90 to -1.98.

[0343] Oxygen's stable valence state is -2. The closer the valence state is to -2, the stronger its electron-accepting ability, i.e., the stronger its oxidizing power. Under normal circumstances, its surface valence state is below -1.7. This application, by limiting the surface oxygen valence state of the positive electrode active material to the above-mentioned range as described above, can reduce the interfacial side reactions between the positive electrode material and the electrolyte, thereby improving the cell's cycle life, high-temperature storage gas generation, and other performance characteristics.

[0344] The surface oxygen valence state can be measured by methods known in the art, such as by electron energy loss spectroscopy (EELS).

[0345] A second aspect of this application provides a method for preparing a positive electrode active material, comprising the following steps:

[0346] The steps for providing the core material: the core has the chemical formula Li 1+x Mn 1-y A y P 1-z R z O4, where x is any value in the range of -0.100 to 0.100, y is any value in the range of 0.001 to 0.500, z is any value in the range of 0.001 to 0.100, A is one or more elements selected from Zn, Al, Na, K, Mg, Mo, W, Ti, V, Zr, Fe, Ni, Co, Ga, Sn, Sb, Nb and Ge, and may be one or more elements selected from Fe, Ti, V, Ni, Co and Mg, R is one or more elements selected from B, Si, N and S, and optionally, R is one element selected from B, Si, N and S;

[0347] Coating steps: Provide Li separately a MP2O7 and / or M b (P2O7) c And an XPO4 suspension, the core material is added to the above suspension and mixed, and then sintered to obtain a positive electrode active material, wherein 0≤a≤2, 1≤b≤4, 1≤c≤6, and the values ​​of a, b and c satisfy the following condition: making crystalline pyrophosphate Li a MP2O7 or M b (P2O7) c Maintaining electrical neutrality; M is each independently one or more elements selected from Fe, Ni, Mg, Co, Cu, Zn, Ti, Ag, Zr, Nb, or Al; X is one or more elements selected from Li, Fe, Ni, Mg, Co, Cu, Zn, Ti, Ag, Zr, Nb, or Al;

[0348] The positive electrode active material has a core-shell structure, comprising a core and a shell covering the core. The shell includes a first coating layer covering the core, a second coating layer covering the first coating layer, and a third coating layer covering the second coating layer. The first coating layer comprises crystalline pyrophosphate Li. a MP2O7 and / or M b (P2O7) c The second coating layer comprises crystalline phosphate XPO4, and the third coating layer is carbon.

[0349] In some implementations, the step of providing the core material includes the following steps:

[0350] Step (1): Mix and stir the manganese source, dopant of element A and acid in a container to obtain manganese salt particles doped with element A;

[0351] Step (2): The manganese salt particles doped with element A are mixed with a lithium source, a phosphorus source, and a dopant of element R in a solvent to obtain a slurry. After sintering under an inert gas atmosphere, a core doped with elements A and R is obtained, wherein the core doped with elements A and R is Li. 1+x Mn 1-y A y P 1-z R z O4, where x is any value in the range of -0.100 to 0.100, y is any value in the range of 0.001 to 0.500, z is any value in the range of 0.001 to 0.100, A is one or more elements selected from Zn, Al, Na, K, Mg, Mo, W, Ti, V, Zr, Fe, Ni, Co, Ga, Sn, Sb, Nb and Ge, and may be one or more elements selected from Fe, Ti, V, Ni, Co and Mg, R is one or more elements selected from B, Si, N and S, and optionally, R is one element selected from B, Si, N and S.

[0352] The preparation method of this application does not have any particular restrictions on the source of materials. The source of a certain element may include one or more of the element's elemental form, sulfate, halide, nitrate, organic acid salt, oxide or hydroxide. The precursor is from this source to achieve the purpose of the preparation method of this application.

[0353] In some embodiments, the dopant of element A is one or more elements selected from the following: elemental form, carbonate, sulfate, chloride, nitrate, organic acid salt, oxide, and hydroxide.

[0354] In some embodiments, the dopant of element R is one or more of the inorganic acids, flavonoids, organic acids, sulfates, chlorides, nitrates, organic acid salts, oxides, and hydroxides of one or more elements selected from B, Wi, N, and S.

[0355] 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.

[0356] 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.

[0357] 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.

[0358] 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.

[0359] In some alternative embodiments, after the manganese source, the dopant of element A and the acid are reacted in a solvent to obtain a suspension of manganese salt doped with element A, the suspension is filtered, dried and sand-milled to obtain manganese salt particles doped with element A with a particle size of 50-200 nm.

[0360] In some alternative embodiments, the slurry in step (2) is dried to obtain powder, and then the powder is sintered to obtain a core doped with elements A and R.

[0361] In some embodiments, step (1) is performed at a temperature of 20-120°C, optionally 40-120°C; and / or

[0362] The stirring in step (1) is carried out at 400-700 rpm for 1-9 hours, or optionally 3-7 hours.

[0363] Optionally, the reaction temperature in step (1) can be about 30°C, about 50°C, about 60°C, about 70°C, about 80°C, about 90°C, about 100°C, about 110°C, or about 120°C; the stirring in step (1) can be carried out for about 2 hours, about 3 hours, about 4 hours, about 5 hours, about 6 hours, about 7 hours, about 8 hours, or about 9 hours; optionally, the reaction temperature and stirring time in step (1) can be within any range of the above values.

[0364] In some embodiments, step (2) is performed at a temperature of 20-120°C, optionally 40-120°C, for 1-12 hours. Optionally, the reaction temperature in step (2) can be performed at approximately 30°C, approximately 50°C, approximately 60°C, approximately 70°C, approximately 80°C, approximately 90°C, approximately 100°C, approximately 110°C, or approximately 120°C; the mixing in step (2) can be performed for approximately 2 hours, approximately 3 hours, approximately 4 hours, approximately 5 hours, approximately 6 hours, approximately 7 hours, approximately 8 hours, approximately 9 hours, approximately 10 hours, approximately 11 hours, or approximately 12 hours; optionally, the reaction temperature and mixing time in step (2) can be within any range of the above values.

[0365] 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.

[0366] In some embodiments, optionally, during the preparation of dilute manganese acid particles doped with elements A and R, the pH of the solution is controlled to be 3.5-6; alternatively, the pH of the solution is controlled to be 4-6; more preferably, the pH of the solution is controlled to be 4-5. It should be noted that the pH of the resulting mixture can be adjusted using methods commonly used in the art, for example, by adding an acid or a base.

[0367] In some embodiments, optionally, in step (2), the molar ratio of the manganese salt particles to the lithium source and the phosphorus source is 1:0.5-2.1:0.5-2.1. More preferably, the molar ratio of the manganese salt particles doped with element A to the lithium source and the phosphorus source is about 1:1:1.

[0368] In some embodiments, optionally, the sintering conditions in the preparation of A and R-doped lithium manganese phosphate are: sintering at 600-950°C for 4-10 hours in an inert gas or a mixture of inert gas and hydrogen; optionally, the sintering can be carried out at about 650°C, about 700°C, about 750°C, about 800°C, about 850°C, or about 900°C for about 2 hours, about 3 hours, about 4 hours, about 5 hours, about 6 hours, about 7 hours, about 8 hours, about 9 hours, or about 10 hours; optionally, the sintering temperature and sintering time can be within any range of the above values. In the preparation of lithium manganese phosphate doped with elements A and R, if the sintering temperature is too low or the sintering time is too short, the crystallinity of the material core will be low, which will affect the overall performance. If the sintering temperature is too high, impurity phases are likely to appear in the material core, which will also affect the overall performance. If the sintering time is too long, the material core particles will be too large, which will affect the specific capacity, compaction density and rate performance.

[0369] In some alternative embodiments, the protective atmosphere may optionally be a mixture of 70-90 vol% nitrogen and 10-30 vol% hydrogen.

[0370] In some embodiments, the coating step includes:

[0371] First coating step: Dissolve the source of element M, phosphorus source, acid, and optionally lithium source in a solvent to obtain a first coating layer suspension; thoroughly mix the core obtained in the core step with the first coating layer suspension obtained in the first coating step, dry, and then sinter to obtain the material coated by the first coating layer.

[0372] Second coating step: Dissolve the source of element X, phosphorus source and acid in solvent to obtain a second coating layer suspension; mix the material coated by the first coating layer obtained in the first coating step with the second coating layer suspension obtained in the second coating step, dry, and then sinter to obtain a material coated by two coating layers.

[0373] The third coating step: Dissolve the carbon source in a solvent to obtain a third coating layer solution; then add the material coated by the two coating layers obtained in the second coating step to the third coating layer solution, mix evenly, dry, and then sinter to obtain a material coated by the three coating layers, i.e., the positive electrode active material.

[0374] In some embodiments, the source of element M is one or more elements selected from the following: elemental form, carbonate, sulfate, chloride, nitrate, organic acid salt, oxide, hydroxide, etc., of one or more elements selected from Fe, Ni, Mg, Co, Cu, Zn, Ti, Ag, Zr, Nb, or Al.

[0375] In some embodiments, the source of element X is one or more elements selected from the group consisting of the elemental form, carbonate, sulfate, chloride, nitrate, organic acid salt, oxide, and hydroxide of Li, Fe, Ni, Mg, Co, Cu, Zn, Ti, Ag, Zr, Nb, or Al.

[0376] The amount of each of the elements A, R, M, and X sources added depends on the target doping amount, and the ratio of the amounts of lithium source, manganese source, and phosphorus source used conforms to the stoichiometric ratio.

[0377] As an example, the carbon source is one or more selected from starch, sucrose, glucose, polyvinyl alcohol, polyethylene glycol, and citric acid.

[0378] In some embodiments, in the first coating step, the pH of the solution containing the source of element M, the phosphorus source, and the acid, and optionally the lithium source, is controlled to be 3.5-6.5, then stirred and reacted for 1-5 hours, then the solution is heated to 50-120°C and held at that temperature for 2-10 hours, and / or sintering is carried out at 650-800°C for 2-6 hours.

[0379] Optionally, in the first coating step, the reaction proceeds fully. Optionally, in the first coating step, the reaction proceeds for approximately 1.5 hours, approximately 2 hours, approximately 3 hours, approximately 4 hours, approximately 4.5 hours, or approximately 5 hours. Optionally, in the first coating step, the reaction time can be within any range of the above-mentioned values.

[0380] Optionally, in the first coating step, the pH of the solution is controlled to be 4-6. Optionally, in the first coating step, the solution is heated to approximately 55°C, approximately 60°C, approximately 70°C, approximately 80°C, approximately 90°C, approximately 100°C, approximately 110°C, or approximately 120°C, and held at this temperature for approximately 2 hours, approximately 3 hours, approximately 4 hours, approximately 5 hours, approximately 6 hours, approximately 7 hours, approximately 8 hours, approximately 9 hours, or approximately 10 hours; Optionally, in the first coating step, the heating temperature and holding time can be within any range of the above values.

[0381] Optionally, in the first coating step, the sintering can be performed at about 650°C, about 700°C, about 750°C, or about 800°C for about 2 hours, about 3 hours, about 4 hours, about 5 hours, or about 6 hours; optionally, the sintering temperature and sintering time can be within any range of the above values.

[0382] In the first coating step, by controlling the sintering temperature and time within the above range, the following situations can be avoided: When the sintering temperature in the first coating step is too low and the sintering time is too short, the crystallinity of the first coating layer will be low, and there will be more amorphous substances. This will reduce the effect of inhibiting metal dissolution, thereby affecting the cycle performance and high-temperature storage performance of the secondary battery. When the sintering temperature is too high, impurities will appear in the first coating layer, which will also affect its effect of inhibiting metal dissolution, thereby affecting the cycle performance and high-temperature storage performance of the secondary battery. When the sintering time is too long, the thickness of the first coating layer will increase, affecting the Li + The migration of these molecules affects the material's specific capacity and rate performance.

[0383] In some embodiments, in the second coating step, after dissolving the source of element X, the phosphorus source and the acid in a solvent, the mixture is stirred and reacted for 1-10 hours, and then the solution is heated to 60-150°C and held at that temperature for 2-10 hours, and / or sintering is carried out at 500-700°C for 6-10 hours.

[0384] Optionally, in the second coating step, the reaction proceeds fully. Optionally, in the second coating step, the reaction proceeds for approximately 1.5 hours, approximately 2 hours, approximately 3 hours, approximately 4 hours, approximately 4.5 hours, approximately 5 hours, approximately 6 hours, approximately 7 hours, approximately 8 hours, approximately 9 hours, or approximately 10 hours. Optionally, in the second coating step, the reaction time can be within any range of the above-mentioned values.

[0385] Optionally, in the second coating step, the solution is heated to approximately 65°C, approximately 70°C, approximately 80°C, approximately 90°C, approximately 100°C, approximately 110°C, approximately 120°C, approximately 130°C, approximately 140°C, or approximately 150°C, and held at this temperature for approximately 2 hours, approximately 3 hours, approximately 4 hours, approximately 5 hours, approximately 6 hours, approximately 7 hours, approximately 8 hours, approximately 9 hours, or approximately 10 hours; optionally, in the second coating step, the heating temperature and holding time can be within any range of the above values.

[0386] In the steps of providing the core material and the first coating step and the second coating step, before sintering, that is, in the preparation of the core material in which the chemical reaction occurs (steps (1)-(2)) and in the preparation of the first coating layer suspension and the second coating layer suspension, by selecting appropriate reaction temperature and reaction time as described above, the following situations can be avoided: when the reaction temperature is too low, the reaction cannot occur or the reaction rate is slow; when the temperature is too high, the product decomposes or forms an impurity phase; when the reaction time is too long, the product particle size is large, which may increase the time and difficulty of subsequent processes; when the reaction time is too short, the reaction is incomplete and less product is obtained.

[0387] Optionally, in the second coating step, the sintering can be carried out at about 550°C, about 600°C, or about 700°C for about 6 hours, about 7 hours, about 8 hours, about 9 hours, or about 10 hours; optionally, the sintering temperature and sintering time can be within any range of the above values.

[0388] In the second coating step, by controlling the sintering temperature and time within the above range, the following situations can be avoided: When the sintering temperature in the second coating step is too low and the sintering time is too short, the crystallinity of the second coating layer will be low, with more amorphous phases, reducing the performance of the material surface reactivity and thus affecting the cycle and high-temperature storage performance of the secondary battery; when the sintering temperature is too high, impurity phases will appear in the second coating layer, which will also affect its effect of reducing the material surface reactivity and thus affect the cycle and high-temperature storage performance of the secondary battery; when the sintering time is too long, the thickness of the second coating layer will increase, affecting the voltage plateau of the material and thus reducing the energy density of the material.

[0389] In some embodiments, the sintering in the third coating step is performed at 700-800°C for 6-10 hours. Optionally, in the third coating step, the sintering can be performed at about 700°C, about 750°C, or about 800°C for about 6 hours, about 7 hours, about 8 hours, about 9 hours, or about 10 hours; alternatively, the sintering temperature and sintering time can be within any range of the above values.

[0390] In the third coating step, by controlling the sintering temperature and time within the above range, the following situations can be avoided: when the sintering temperature in the third coating step is too low, the graphitization degree of the third coating layer will decrease, affecting its conductivity and thus affecting the specific capacity of the material; when the sintering temperature is too high, the graphitization degree of the third coating layer will be too high, affecting the Li + The transmission of electrical conductivity can affect the specific capacity of the material. If the sintering time is too short, the coating layer will be too thin, affecting its conductivity and thus the specific capacity of the material. If the sintering time is too long, the coating layer will be too thick, affecting the compaction density of the material.

[0391] In the first, second, and third coating steps described above, the drying is carried out at a drying temperature of 100°C to 200°C, optionally 110°C to 190°C, more preferably 120°C to 180°C, even more preferably 120°C to 170°C, and most preferably 120°C to 160°C, and the drying time is 3-9 hours, optionally 4-8 hours, more preferably 5-7 hours, and most preferably about 6 hours.

[0392] The positive electrode active material prepared by the method described in this application results in a reduced dissolution of Mn and Mn-site dopant elements in the secondary battery after cycling, and improved high-temperature stability, high-temperature cycling performance, and rate performance. Furthermore, the raw materials are widely available, inexpensive, and the process is simple, facilitating industrialization.

[0393] In some implementations, the positive electrode typically 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 including a positive electrode active material.

[0394] Figure 1 A schematic diagram of a positive electrode sheet according to an embodiment is shown. As shown, a positive electrode sheet includes a positive current collector 11, a positive electrode film layer 13 disposed on at least one surface 112 of the positive current collector 11, and a conductive undercoat layer 12 located between the positive current collector 11 and the positive electrode film layer 13.

[0395] As an example, the positive current collector has two surfaces opposite each other in its own thickness direction, and the positive electrode film layer is disposed on either or both of the two opposite surfaces of the positive current collector.

[0396] 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.).

[0397] 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.

[0398] 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.

[0399] In some embodiments, the positive electrode sheet can be prepared by dispersing the above-mentioned components for preparing the positive electrode sheet, such as positive active material, conductive agent, binder and any other components, in a solvent (e.g., N-methylpyrrolidone) to form a positive electrode slurry; coating the positive electrode slurry onto the positive electrode current collector, and then obtaining the positive electrode sheet after drying, cold pressing and other processes.

[0400] [Negative electrode plate]

[0401] 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.

[0402] 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.

[0403] 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.).

[0404] 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.

[0405] 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).

[0406] 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.

[0407] In some embodiments, the negative electrode film may optionally include other additives, such as thickeners (e.g., sodium carboxymethyl cellulose (CMC-Na)).

[0408] 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.

[0409] [Electrolytes]

[0410] 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.

[0411] In some embodiments, the electrolyte is liquid and includes an electrolyte salt and a solvent.

[0412] 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.

[0413] 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.

[0414] 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.

[0415] [Isolation membrane]

[0416] 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.

[0417] 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.

[0418] In some implementations, the positive electrode, negative electrode, and separator can be fabricated into an electrode assembly using a winding or stacking process.

[0419] 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.

[0420] 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.

[0421] 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 3 This is an example of a square-structured secondary battery 5.

[0422] In some implementations, refer to Figure 4 The outer packaging may include a housing 51 and a cover plate 53. The housing 51 may include a bottom plate and side plates connected to the bottom plate, the bottom plate and side plates forming a receiving cavity. The housing 51 has an opening communicating with the receiving cavity, and the cover plate 53 can be placed over the opening to close the receiving cavity. The positive electrode, negative electrode, and separator can 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.

[0423] 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.

[0424] Figure 5 This is battery module 4, used as an example. (See reference...) Figure 5In 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.

[0425] Optionally, the battery module 4 may also include a housing with a receiving space in which a plurality of secondary batteries 5 are received.

[0426] 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.

[0427] Figure 6 and Figure 7 This is battery pack 1 as an example. (See reference...) Figure 6 and Figure 7 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.

[0428] 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 a power source for the electrical device, or as an energy storage unit for the electrical device. The electrical device may include, but is not limited to, mobile devices (e.g., mobile phones, laptops, etc.), electric vehicles (e.g., pure electric vehicles, hybrid electric vehicles, plug-in hybrid electric vehicles, electric bicycles, electric scooters, electric golf carts, electric trucks, etc.), electric trains, ships and satellites, energy storage systems, etc.

[0429] As the electrical device, a secondary battery, battery module, or battery pack can be selected according to its usage requirements.

[0430] Figure 8 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.

[0431] Specific embodiments of novel positive electrode active materials

[0432] The following describes embodiments of this application. The embodiments described below are exemplary and are only used to explain this application, and should not be construed as limiting this application. Where specific techniques or conditions are not specified in the embodiments, they are performed according to the techniques or conditions described in the literature in the art or according to the product instructions. Reagents or instruments used, unless otherwise specified, are all conventional products that can be obtained commercially. Unless otherwise specified, the content of each component in the embodiments of this invention is based on the mass of the component excluding crystallization water.

[0433] The sources of raw materials involved in the embodiments of this application are as follows:

[0434]

[0435] I. Battery Manufacturing

[0436] Example 1:

[0437] Step 1: Preparation of positive electrode active material

[0438] Step S1: Preparation of Fe, Co, V and S co-doped manganese oxalate

[0439] 689.6 g of manganese carbonate, 455.27 g of ferrous carbonate, 4.65 g of cobalt sulfate, and 4.87 g of vanadium dichloride were added to a mixer and mixed thoroughly for 6 hours. The resulting mixture was then transferred to a reaction vessel, and 5 L of deionized water and 1260.6 g of oxalic acid dihydrate were added. The mixture was heated to 80°C and stirred thoroughly at 500 rpm for 6 hours until homogeneous mixing and the reaction was terminated without bubble formation, yielding a Fe, Co, and V co-doped manganese oxalate suspension. The suspension was then filtered, dried at 120°C, and milled to obtain manganese oxalate particles with a particle size of 100 nm.

[0440] Step S2: Preparation of core Li 0.997 Mn 0.60 Fe 0.393 V 0.004 Co 0.003 P 0.997 S 0.003 O4

[0441] Take 1793.1g of manganese oxalate, 368.3g of lithium carbonate, 1146.6g of ammonium dihydrogen phosphate, and 4.9g of dilute sulfuric acid prepared in (1), add them to 20L of deionized water, stir thoroughly, and react uniformly at 80℃ for 10h to obtain a slurry. Transfer the slurry to a spray drying equipment for spray drying and granulation, and dry at 250℃ to obtain a powder. In a protective atmosphere (90% nitrogen and 10% hydrogen), sinter the powder in a roller kiln at 700℃ for 4h to obtain the above-mentioned core material.

[0442] Step S3: Preparation of the first coating layer suspension

[0443] To prepare a Li2FeP2O7 solution, 7.4 g of lithium carbonate, 11.6 g of ferrous carbonate, 23.0 g of ammonium dihydrogen phosphate and 12.6 g of oxalic acid dihydrate were dissolved in 500 mL of deionized water, and the pH was controlled at 5. The solution was then stirred and reacted at room temperature for 2 h to obtain a solution. The solution was then heated to 80 °C and maintained at this temperature for 4 h to obtain the first coating layer suspension.

[0444] Step S4: Coating with the first coating layer

[0445] The 1571.9g of doped lithium manganese phosphate core material obtained in step S2 was added to the first coating layer suspension (coating material content of 15.7g) obtained in step S3. The mixture was stirred and mixed thoroughly for 6 hours. After being mixed evenly, the mixture was dried in an oven at 120℃ for 6 hours and then sintered at 650℃ for 6 hours to obtain the pyrophosphate coated material.

[0446] Step S5: Preparation of the second coating layer suspension

[0447] 3.7 g lithium carbonate, 11.6 g ferrous carbonate, 11.5 g ammonium dihydrogen phosphate and 12.6 g oxalic acid dihydrate were dissolved in 1500 mL deionized water, stirred and reacted for 6 h to obtain a solution. The solution was then heated to 120 °C and maintained at this temperature for 6 h to obtain a second coating layer suspension.

[0448] Step S6: Coating with the second coating layer

[0449] The 1586.8g of pyrophosphate-coated material obtained in step S4 was added to the second coating suspension (coating material content of 47.1g) obtained in step S5. The mixture was stirred and mixed thoroughly for 6 hours. After mixing evenly, the mixture was dried in an oven at 120℃ for 6 hours and then sintered at 700℃ for 8 hours to obtain the two-layer coated material.

[0450] Step S7: Preparation of the third coating layer aqueous solution

[0451] Dissolve 37.3g of sucrose in 500g of deionized water, then stir and dissolve completely to obtain a sucrose aqueous solution.

[0452] Step S8: Covering with the third coating layer

[0453] 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.

[0454] Step 2: Preparation of the positive electrode sheet

[0455] 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.

[0456] Step 3: Preparation of the negative electrode sheet

[0457] A negative electrode slurry was prepared by dissolving artificial graphite (anode active material), hard carbon, acetylene black (conductive agent), styrene-butadiene rubber (SBR) (binder), and sodium carboxymethyl cellulose (CMC) (thickener) in deionized water at a weight ratio of 90:5:2:2:1 and stirring until homogeneous. The negative electrode slurry was then prepared at a concentration of 0.117 g / 1540.25 mm. 2 The coating is uniformly applied to the copper foil of the negative electrode current collector, and then dried, cold-pressed, and slit to obtain the negative electrode sheet.

[0458] Step 4: Preparation of electrolyte

[0459] In an argon atmosphere glove box (H2O < 0.1 ppm, O2 < 0.1 ppm), the organic solvents ethylene carbonate (EC) and ethyl methyl carbonate (EMC) are mixed evenly at a volume ratio of 3 / 7. 12.5% ​​by weight (based on the weight of the ethylene carbonate / ethyl methyl carbonate solvent) of LiPF6 is added and dissolved in the organic solvent. The mixture is stirred evenly to obtain the electrolyte.

[0460] Step 5: Preparation of the separating membrane

[0461] The material used was a commercially available PP-PE copolymer microporous film with a thickness of 20 μm and an average pore size of 80 nm (from Zogo Electronics Technology Co., Ltd., model 20).

[0462] Step 6: Preparation of the full cell

[0463] 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").

[0464] [Preparation of button cells]

[0465] The prepared positive electrode active material, polyvinylidene fluoride (PVDF), and acetylene black were added to N-methylpyrrolidone (NMP) in a weight ratio of 90:5:5, and stirred in a drying chamber to form a slurry. The slurry was then coated onto aluminum foil, dried, and cold-pressed to form a positive electrode sheet. The coating amount was 0.02 g / cm³. 2 The compacted density is 2.0 g / cm³. 3 .

[0466] A lithium sheet is used as the negative electrode, and a solution of 1 mol / L LiPF6 in ethylene carbonate (EC), diethyl carbonate (DEC) and dimethyl carbonate (DMC) in a volume ratio of 1:1:1 is used as the electrolyte. The lithium sheet and the positive electrode prepared above are assembled into a coin cell in a coin cell box to form a coin cell (hereinafter also referred to as "coin cell").

[0467] Examples 2 to 27 and Comparative Examples 1 to 19

[0468] The positive electrode active materials and batteries of Examples 2 to 27 and Comparative Examples 1 to 19 were prepared in a manner similar to that of Example 1. The differences in the preparation of the positive electrode active materials are shown in Tables 1-6. Comparative Examples 1-2, 4-10 and 12 were not coated with the first layer, so steps S3 and S4 were not performed. Comparative Examples 1-11 were not coated with the second layer, so steps S5 and S6 were not performed.

[0469] Note: In all embodiments and comparative examples of this application, unless otherwise specified, the first coating layer material and / or the second coating layer material used are assumed to be crystalline.

[0470]

[0471]

[0472]

[0473]

[0474]

[0475]

[0476]

[0477]

[0478]

[0479]

[0480]

[0481]

[0482]

[0483]

[0484] II. Performance Evaluation

[0485] 1. Method for testing lattice change rate:

[0486] 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).

[0487] Using the coin cell preparation method described in the above embodiments, the positive electrode active material sample was prepared into a coin cell, and the coin cell was charged at a low rate of 0.05C until the current decreased to 0.01C. Then, the positive electrode sheet of the coin cell was removed and immersed in dimethyl carbonate (DMC) for 8 hours. After drying, the powder was scraped off, and particles with a diameter less than 500 nm were screened out. Samples were taken, and their cell volume v1 was calculated in the same manner as the fresh samples tested above. The lattice change rate (cell volume change rate) before and after complete lithium insertion / extraction is shown in the table.

[0488] 2. Li / Mn antisite defect concentration

[0489] 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.

[0490] 3. Compacted density

[0491] 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 .

[0492] 4.3C charging constant current ratio

[0493] 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 of 3C charging is C1 / C0 x 100%.

[0494] The higher the constant current ratio during 3C charging, the better the rate performance of the secondary battery.

[0495] 5. Dissolution test of transition metal Mn (and Fe doped at Mn sites)

[0496] The full cells prepared in the above-described embodiments and comparative examples, after being cycled at 45°C until their capacity decayed to 80%, were discharged at a rate of 0.1C to a cutoff voltage of 2.0V. The batteries were then disassembled, and the negative electrode was removed. Thirty unit areas (1540.25 mm²) were randomly selected from the negative electrode. 2 The discs were tested using an Agilent ICP-OES730 inductively coupled plasma emission spectrometry (ICP). The amounts of Fe (if the Mn site of the positive electrode active material is doped with Fe) and Mn were calculated based on the ICP results, thus determining the amount of Mn (and Mn-doped Fe) dissolved after cycling. The testing standard was based on EPA-6010D-2014.

[0497] 6. Surface oxygen valence state

[0498] 5g of the positive electrode active material sample prepared above was used to prepare a coin cell according to the coin cell preparation method described in the above embodiments. The coin cell was charged at a low rate of 0.05C until the current decreased to 0.01C. Then, the positive electrode sheet of the coin cell was removed and immersed in DMC for 8 hours. Then, it was dried, scraped off, and particles with a particle size of less than 500nm were screened out. The obtained particles were measured using electron energy loss spectroscopy (EELS, using a Talos F200S instrument) to obtain the energy loss near-edge structure (ELNES), which reflects the density of states and energy level distribution of the element. Based on the density of states and energy level distribution, the number of occupied electrons was calculated by integrating the valence band density of states data, thereby deducing the valence state of the surface oxygen after charging.

[0499] 7. Measurement of manganese and phosphorus elements in positive electrode active materials

[0500] 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.

[0501] 8. Method for measuring the initial specific capacity of button cells

[0502] 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.

[0503] 10. Cell expansion test after 30 days of storage at 60℃:

[0504] 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).

[0505] 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.

[0506] 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.

[0507] 11. Cyclic performance test of the full battery at 45°C

[0508] 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 an 80% capacity retention rate at 45°C.

[0509] 12. Interplanar spacing and angle testing

[0510] 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).

[0511] 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.

[0512] By comparing the obtained interplanar spacing and corresponding angle data with their standard values, different substances in the coating layer can be identified.

[0513] 13. Coating thickness test

[0514] 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).

[0515] 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.

[0516] Measure the thickness of the selected particle at three locations and take the average value.

[0517] 14. Determination of the molar ratio of SP2 and SP3 forms in the third coating layer of carbon

[0518] 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.

[0519] The performance test results for all embodiments and comparative examples are shown in the table below.

[0520]

[0521]

[0522]

[0523]

[0524]

[0525]

[0526]

[0527]

[0528]

[0529]

[0530]

[0531]

[0532]

[0533]

[0534]

[0535]

[0536]

[0537]

[0538] Specific embodiments of the novel conductive undercoating

[0539] 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.

[0540] To distinguish them from the specific embodiments of the novel cathode material mentioned above, the specific embodiments of the novel conductive undercoating are numbered with a suffix ['].

[0541] Example 1' (Positive electrode active material of Example 1)

[0542] 1. Provide the first polymer

[0543] In the following embodiments, the first polymer is a hydrogenated carboxylated nitrile butadiene rubber containing a first monomer unit, a second monomer unit, a third monomer unit, and a fourth monomer unit. The weight percentages of the first monomer unit, the second monomer unit, the third monomer unit, and the fourth monomer unit in the polymer, as well as the weight-average molecular weight of the first polymer, are shown in Table 1P.

[0544] The first monomer unit is the monomer unit represented by Equation 1;

[0545]

[0546] The second monomer unit is selected from at least one of the groups consisting of monomer units represented by Equation 2 and monomer units represented by Equation 3.

[0547] One kind

[0548]

[0549] The third monomer unit is selected from at least one of the groups consisting of monomer units represented by Equation 4 and monomer units represented by Equation 5.

[0550] A sort of;

[0551]

[0552] The fourth monomer unit is the monomer unit represented by Equation 6:

[0553]

[0554] In this embodiment, R 1 R 2 and R 3 Both are H, R 4 It is n-butyl.

[0555] Table 1P

[0556]

[0557] 2. Preparation of aluminum foil with conductive undercoating

[0558] The first polymer, the first water-based binder (polyacrylic acid-acrylate copolymer, weight average molecular weight 340,000) and the first conductive agent (SP) are mixed in a weight ratio of 15:40:45, dissolved / dispersed in deionized water to prepare a conductive base coating slurry.

[0559] A conductive undercoating slurry is applied to both sides of an aluminum foil, and after drying, a conductive undercoating with a thickness of 5 μm is formed on each side. An aluminum foil with a conductive undercoating is obtained.

[0560] 3) Preparation of positive electrode sheet

[0561] The three-layer coated lithium manganese phosphate positive electrode active material of Example 1' above was mixed evenly with the conductive agent acetylene black and the binder polyvinylidene fluoride (PVDF) in an N-methylpyrrolidone solvent system at a weight ratio of 92:2.5:5.5 to obtain a positive electrode slurry. The positive electrode slurry was coated on both sides of an aluminum foil with a conductive undercoat, dried, and cold-pressed to form a positive electrode film, thus obtaining a positive electrode sheet. The density of the positive electrode film on one side was 0.025 g / cm³. 2 The compacted density is 2.4 g / cm³. 3 .

[0562] 4) Preparation of negative electrode sheet

[0563] Artificial graphite (negative electrode active material), hard carbon, acetylene black (conductive agent), styrene-butadiene rubber (SBR) (binder), and sodium carboxymethyl cellulose (CMC) (thickener) were mixed evenly in deionized water at a weight ratio of 90:5:2:2:1. The mixture was then coated onto copper foil, dried, and cold-pressed to form a positive electrode film, thus obtaining the negative electrode sheet. The one-sided density of the negative electrode film was 0.013 g / cm³. 2 The compacted density is 1.7 g / cm³. 3 .

[0564] 5) Assembly of the full battery

[0565] Using a porous polyethylene (PE) polymer film as a separator, the positive electrode, separator, and negative electrode are stacked in sequence, with the separator positioned between the positive and negative electrodes to provide isolation. The resulting bare cell is then wound up. The bare cell is placed in an outer packaging, infused with electrolyte, and sealed to obtain a full battery (hereinafter referred to as "full battery").

[0566] The weight of the positive electrode active material in a single full cell is 565.66g; the weight of the negative electrode active material is 309.38g.

[0567] Examples 2' to 40' (Positive electrode active materials of Examples 2 to 40)

[0568] The difference between Examples 2' to 40' and Example 1' lies in step 3). The other steps and parameters are the same as in Example 1'.

[0569] The positive electrode active materials used in step 3) of Examples 2' to 40' are the same as those in Examples 2 to 40 above.

[0570] Comparative Examples 1' to 18' (without conductive undercoat)

[0571] The difference between Comparative Examples 1' to 18' and Example 1' lies in steps 2) and 3). The other step parameters are the same as those in Example 1'.

[0572] In Comparative Examples 1' to 18', in steps 2) and 3), instead of preparing an aluminum foil with a conductive undercoat, the positive electrode slurry was directly coated onto the aluminum foil and dried and cold-pressed to form a positive electrode film, thus obtaining a positive electrode sheet.

[0573] The positive electrode active materials used in Comparative Examples 1' to 17' in step 3) are the same as those in Comparative Examples 1 to 17 above.

[0574] Comparative Example 18' uses the same positive electrode active material as in Example 1 above in step 3).

[0575] Comparative Example 19' (excluding the first polymer)

[0576] The difference between Comparative Example 19' and Example 1' lies in step 2). The other steps and parameters are the same as in Example 1'.

[0577] In Comparative Example 19', in step 2), a first aqueous binder (polyacrylic acid-acrylate copolymer) and a first conductive agent (SP) were mixed in a weight ratio of 40:45, dissolved / dispersed in deionized water, and a conductive primer slurry was prepared. The conductive primer slurry was coated onto an aluminum foil, and after drying, a conductive primer coating with a thickness of 5 μm was formed. An aluminum foil with a conductive primer coating was obtained.

[0578] Comparative Example 20' (replacing the first polymer with the I polymer)

[0579] The difference between Comparative Example 20' and Example 1' lies in step 2). The other steps and parameters are the same as in Example 1'.

[0580] In Comparative Example 20', in step 2), the first polymer, the first aqueous binder (polyacrylic acid-acrylate copolymer), and the first conductive agent (SP) were mixed in a weight ratio of 15:40:45 and dissolved / dispersed in deionized water to prepare a conductive undercoating slurry. The conductive undercoating slurry was coated onto an aluminum foil, and after drying, a conductive undercoating with a thickness of 5 μm was formed. An aluminum foil with a conductive undercoating was obtained.

[0581] The difference between polymer I and polymer I lies in their composition. The composition of polymer I and its weight-average molecular weight are shown in Table 2P below.

[0582] Table 2P

[0583]

[0584] Comparative Example 21' (replacing the first water-based adhesive with the first adhesive)

[0585] The difference between Comparative Example 21' and Example 1' lies in step 2). The other steps and parameters are the same as in Example 1'.

[0586] In Comparative Example 21', in step 2), the first polymer, the first binder (polyacrylic acid, weight average molecular weight 350,000), and the first conductive agent (SP) were mixed in a weight ratio of 15:40:45 and dissolved / dispersed in deionized water to prepare a conductive undercoating slurry. The conductive undercoating slurry was coated onto an aluminum foil, and after drying, a conductive undercoating with a thickness of 5 μm was formed. An aluminum foil with a conductive undercoating was obtained.

[0587] Analysis and testing

[0588] 1. Adhesion test of positive electrode sheet

[0589] Figure 2 (a) through (d) show the flowchart of the peel test. Figure 2 As shown in (a), a steel plate 510 is first provided, with dimensions of 30mm wide × 100mm long. Figure 2 As shown in (b), a double-sided tape 520 is then provided, with dimensions of 20mm wide × 30mm long. The double-sided tape 520 is attached to the steel plate 510, with one wide edge of the double-sided tape 520 aligned with one wide edge of the steel plate 510. Figure 2 As shown in (c), a test electrode 530 is then provided, with dimensions of 20mm wide × 180mm long. The test electrode 530 is placed over the double-sided adhesive 520 (aligned on both sides), with the coated side of the electrode 530 facing the double-sided adhesive 520. Because the length of the test electrode 530 is greater than the length of the double-sided adhesive 520, a portion of the test electrode 530 is not bonded to the double-sided adhesive. Figure 2As shown in (d), the steel plate 510 is fixed on the base of the tensile testing machine. A clamp holds the end of the electrode 530 to be tested that is not bonded to the double-sided adhesive. The clamp is then stretched towards the other end (as indicated by the arrow). The direction of the stretching force is perpendicular to the steel plate 510 and maintains a certain distance from its surface. While stretching the electrode outwards from the paper, the steel plate moves upwards to keep the stretching direction perpendicular to the electrode peeling position. During the stretching process, the electrode 530 is gradually peeled off the steel plate. The stretching speed of the clamp is 50 mm / min. The tension of the clamp is recorded during the stretching process. After the tension stabilizes, a further 40 mm peeling length is performed, and the average tension at this peeling length is taken as the adhesive force (in N).

[0590] 2. Battery DC resistance test

[0591] At 25℃, the battery was charged to 4.3V using a constant current and constant voltage at 1.0C (1.0C refers to the nominal capacity); the battery charge was adjusted to 50% SOC at a 1.0C rate, and after resting for 5 minutes, it was discharged at a constant current (Im) of 4C for 30 seconds (voltage data was collected every 1 second). The initial voltage U0 and the voltage U after 30 seconds of discharge were recorded. 30 The DC impedance (DCR) value is calculated using the following formula.

[0592] DC impedance value = (U0 - U 30 ) / I m

[0593] In Example 1, the DC resistance value of the battery is 100%. The changes in other examples and comparative examples relative to Example 1' are expressed as percentages.

[0594] 3. Number of cycles in which the battery retains 80% capacity at 45°C (hereinafter referred to as "80% capacity cycles")

[0595] Under constant temperature conditions of 45℃, the full battery was charged at 1C to 4.3V within a range of 2.5V 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 resting for 5 minutes, it was discharged at 1C to 2.5V, and the discharge capacity at this point was recorded as D0. This charge-discharge cycle was repeated until the discharge capacity decreased to 80% of D0. The number of cycles completed at this point was recorded.

[0596] According to the above detection and analysis methods, the bonding force of the positive electrode sheets prepared in Examples 1' to 40' and Comparative Examples 1' to 21' were tested, the DC impedance value of the battery and the number of cycles with 80% capacity retention at 45°C were tested, and the results are shown in Table 3P below.

[0597] Table 3P

[0598]

[0599]

[0600]

[0601] As shown in Table 3P, the positive electrode sheets of Examples 1' to 40' exhibited improved adhesion, and the batteries of Examples 1' to 40' exhibited reduced DC resistance and improved cycle capacity retention.

[0602] Comparative Examples 18' (without conductive primer), 19' (without first polymer), 20' (with first polymer I replaced first polymer), and 21' (with first adhesive I replaced first water-based adhesive) failed to achieve the above-mentioned improved effect.

[0603] Examples 3-1' to 3-7' (Compositional variations of the first polymer)

[0604] The difference between Examples 3-1' to 3-7' and Example 1' lies in step 2). The other steps and parameters are the same as in Example 1'.

[0605] In step 2), the composition of the first polymer used in Examples 3-1' to 3-7' differs from that in Example 1', specifically in the weight percentages of the second and third monomer units. The composition of the first polymer in Examples 3-1' to 3-7' is shown in Table 4P below.

[0606] Table 4P

[0607]

[0608] Examples 3-8' to 3-12' (Variations on the thickness of the conductive undercoat)

[0609] The difference between Examples 3-8' to 3-12' and Example 1' lies in step 2). The other steps and parameters are the same as in Example 1'.

[0610] In step 2), the thickness of the conductive undercoat in Examples 3-8' to 3-12' is different from that in Example 1', as detailed in Table 5P.

[0611] Table 5P

[0612]

[0613] Examples 3-13' to 3-18' (Variations on the composition of the conductive undercoat)

[0614] The difference between Examples 3-13' to 3-18' and Example 1' lies in step 2). The other steps and parameters are the same as in Example 1'.

[0615] In step 2), the conductive base coating composition (the ratio of the first polymer, the first water-based binder, and the first conductive agent) of Examples 3-13' to 3-18' is different from that of Example 1', as detailed in Table 6P.

[0616] Table 6P

[0617]

[0618] According to the above detection and analysis methods, the bonding force of the positive electrode sheet prepared in Examples 1', 3-1' to 3-18' above, the DC impedance value of the battery, and the number of cycles with 80% capacity retention at 45°C were tested. The results are shown in Table 7P below.

[0619] Table 7P

[0620]

[0621]

[0622] As shown in Table 7P, the positive electrode sheets of Examples 1', 3-1' to 3-18' exhibited improved adhesion, and the batteries of Examples 1', 3-1' to 3-18' exhibited reduced DC resistance and improved cycle capacity retention. When the value of M3 / (M2+M3) was 0% to 5%, the mass impedance of the battery showed a significant reduction.

[0623] Examples 4-1' to 4-9'

[0624] The difference between Examples 4-1 to 4-9 and Example 1 is step 3). The other steps and parameters are the same as in Example 1.

[0625] In step 3) of Examples 4-1' to 4-9', the three-layer coated lithium manganese phosphate positive electrode active material of Example 1 above is mixed evenly with conductive agent acetylene black, binder polyvinylidene fluoride (PVDF), dispersant and wetting agent in an N-methylpyrrolidone solvent system at a weight ratio of (92-Y1-Y2):2.5:5.5:Y1:Y2 to obtain a positive electrode slurry. The positive electrode slurry is coated on both sides of an aluminum foil with a conductive undercoat and dried and cold-pressed to form a positive electrode film, thus obtaining a positive electrode sheet. The single-sided density of the positive electrode film is 0.025 g / cm³. 2 The compacted density is 2.4 g / cm³. 3 .

[0626] The wetting agent in Examples 4-1' to 4-9' was a maleic anhydride-styrene copolymer (molecular weight 5000). The dispersant in Examples 4-1' to 4-9' was a second polymer.

[0627] The second polymer is a hydrogenated nitrile butadiene rubber containing a fifth monomer unit, a sixth monomer unit, and a seventh monomer unit. The weight percentages of the fifth, sixth, and seventh monomer units in the polymer, as well as the weight-average molecular weight of the second polymer, are shown in Table 8P.

[0628] The fifth monomer unit is the monomer unit represented by Equation 1;

[0629]

[0630] The sixth monomer unit is selected from at least one of the monomer units represented by Formula 8 and the monomer units represented by Formula 9.

[0631]

[0632] The seventh monomer unit is selected from at least one of the monomer units represented by Formula 10 and the monomer units represented by Formula 11.

[0633]

[0634] Table 8P

[0635]

[0636] In the positive electrode sheets of Examples 4-1' to 4-9', the mass ratio of the first polymer (from the conductive undercoat) to the second polymer (from the positive electrode film) is 2:1.

[0637] The proportions of dispersant (second polymer) Y1 and wetting agent (maleic anhydride-styrene copolymer) Y2 used in step 3) in Examples 4-1' to 4-9', as well as their ratio Y1 / Y2, are shown in Table 9P below.

[0638] Table 9P

[0639]

[0640]

[0641] According to the above detection and analysis methods, the bonding force of the positive electrode sheet prepared in Examples 1' and 4-1' to 4-9' above, the DC impedance value of the battery, and the number of cycles with 80% capacity retention at 45°C were tested. The results are shown in Table 10P below.

[0642] Table 10P

[0643] Pole piece adhesion Direct current resistance Cycles to failure Example 1' 15 100％ 1470 Example 4-1' 78 93％ 1562 Example 4-2' 60 105％ 1577 Example 4-3' 144 134％ 1516 Example 4-4' 189 167％ 1600 Example 4-5' 103 100％ 1684 Example 4-6' 103 99％ 1469 Example 4-7' 115 98％ 1504 Example 4-8' 108 101％ 1536 Example 4-9' 110 110％ 1488

[0644] As shown in Table 10P, based on the above-mentioned novel conductive undercoating, the combination of a novel positive electrode film containing dispersants and wetting agents can further improve the adhesion of the electrode sheets and / or reduce the DC impedance of the battery and / or improve the cycle performance of the battery.

[0645] Based on the experimental data above, this application provides a novel positive electrode sheet, a secondary battery, and an electrical device. The positive electrode sheet includes a novel positive electrode active material and a novel conductive undercoating.

[0646] The new positive electrode active materials have achieved better performance in one or all of the following aspects: cycle performance, high-temperature storage performance, and safety performance.

[0647] The new conductive undercoat coating achieves superior performance in one or all of the following aspects: improving electrode adhesion, reducing battery DC impedance, and improving battery cycle performance.

[0648] It should be noted that this application is not limited to the above-described embodiments. The above embodiments are merely examples, and any embodiments with the same structure and effect as the technical concept within the scope of this application are included in the technical scope of this application. Furthermore, various modifications that can be conceived by those skilled in the art to the embodiments, and other ways of constructing by combining some of the constituent elements of the embodiments, without departing from the spirit of this application, are also included in the scope of this application.

Claims

1. A positive electrode sheet, comprising a positive electrode current collector, a positive electrode film layer disposed on at least one surface of the positive electrode current collector, and a conductive primer layer between the positive electrode current collector and the positive electrode film layer, wherein the positive electrode film layer comprises a positive electrode active material having a core-shell structure, the positive electrode active material comprising an inner core and a shell covering the inner core, the shell comprising a first coating layer covering the inner core, a second coating layer covering the first coating layer, and a third coating layer covering the second coating layer, wherein the second coating layer comprises a crystalline phosphate XPO 4, wherein the X is one or more elements selected from Li, Fe, Ni, Mg, Co, Cu, Zn, Ti, Ag, Zr, Nb, or Al, and the third coating layer is carbon; the conductive primer layer comprises a first polymer, a first aqueous binder, and a first conductive agent, the first polymer comprises: a first monomer unit represented by Formula 1; at least one of a second monomer unit selected from the group consisting of a monomer unit represented by Formula 2 and a monomer unit represented by Formula 3; at least one of a third monomer unit selected from the group consisting of a monomer unit represented by Formula 4 and a monomer unit represented by Formula 5; and at least one of a fourth monomer unit selected from the group consisting of a monomer unit represented by Formula 6 and a monomer unit represented by Formula 7; the A is one or more elements selected from Fe, Ti, V, Ni, Co, and Mg; the R is one element selected from B, Si, N, and S; based on the total mass of the first polymer, the mass percentage of the first monomer unit is M 1, M 1 is 10% to 55%; and / or, the mass percentage of the second monomer unit is M 2, M 2 is 40% to 80%; and / or, the mass percentage of the third monomer unit is M 3, M 3 is 0% to 10%; and / or, the mass percentage of the fourth monomer unit is M 4, M 4 is 0% to 10%; M 1 is 25% to 55%; and / or, M 2 is 50% to 70%; and / or, M 3 is 0.001% to 2%; and / or, M 4 is 0.1% to 1%; M 3 / (M 2+M 3) is 0% to 5%; and / or, M 3 / (M 2+M 3) is 0.001% to 1%. 8.The positive electrode sheet of claim 1, wherein the first polymer comprises one or more of hydrogenated nitrile butadiene rubber, hydrogenated carboxyl nitrile butadiene rubber; and / or, the weight average molecular weight of the first polymer is 500,000 to 1,500,000. The chemical formula of the core is Li 1+x Mn 1-y A y P 1-z R z O4, wherein x is any value within the range of -0.100-0.100, y is any value within the range of 0.001-0.500, z is any value within the range of 0.001-0.100, the A is one or more elements selected from Zn, Al, Na, K, Mg, Mo, W, Ti, V, Zr, Fe, Ni, Co, Ga, Sn, Sb, Nb, and Ge, the R is one or more elements selected from B, Si, N, and S, and the values of x, y, and z satisfy the following condition: to maintain the entire core electrically neutral; The weight average molecular weight of the first polymer is 2,000,000 to 4,000,000. said first cladding layer comprising a crystalline pyrophosphate Li a MP2O7 and / or M b (P2O7) c wherein 0≤a≤2, 1≤b≤4, 1≤c≤6, the values of a, b and c satisfying the condition that the crystalline pyrophosphate Li a MP2O7 or M b (P2O7) c is electrically neutral, the crystalline pyrophosphate Li a MP2O7 and M b (P2O7) c M in each case independently is one or more elements selected from Fe, Ni, Mg, Co, Cu, Zn, Ti, Ag, Zr, Nb or Al, 10.The positive electrode sheet of claim 1, wherein the first aqueous binder comprises one or more of an aqueous polyacrylic acid resin and derivatives thereof, an aqueous amino-modified polypropylene resin and derivatives thereof, polyvinyl alcohol and derivatives thereof; and / or, the weight average molecular weight of the first aqueous binder is 2,000,000 to 1,500,000. The first aqueous binder comprises an aqueous acrylic acid-acrylate copolymer; and / or, the weight average molecular weight of the first aqueous binder is 3,000,000 to 4,000,000. The first conductive agent comprises one or more of super conductive carbon, conductive graphite, acetylene black, carbon black, ketjen black, carbon dots, carbon nanotubes, graphene, carbon nanofibers. ​ ​ ​ ​ a fourth monomer unit represented by Formula 6, R 1 , R 2 , R 3 each independently represent H, a carboxyl group, an ester group, and a substituted or unsubstituted group selected from the group consisting of a C1-C10 alkyl group, a C1-C10 alkoxy group, a C2-C10 alkenyl group, and a C6-C10 aryl group, R 4 represents H, and a substituted or unsubstituted group selected from the group consisting of a C1-C10 alkyl group, a C1-C10 alkoxy group, a C2-C10 alkenyl group, and a C6-C10 aryl group; Formula 1 Formula 2 Formula 3 Formula 4 Formula 5 Formula 6.

2. The cathode sheet of claim 1, wherein, ​ 3. The cathode sheet of claim 1, wherein, ​ 4. The cathode sheet of Claim 1, wherein, ​ ​ ​ ​ ​ 5. The cathode sheet of claim 4, wherein, ​ ​ ​ ​ 6. The cathode sheet of claim 4, wherein, ​ 7. The cathode sheet of Claim 4, wherein, ​ ​ ​ ​ 9. The cathode sheet of Claim 1, wherein, ​ ​ ​ ​ 11. The cathode sheet of Claim 1, wherein, ​ ​ 12. The cathode sheet of Claim 1, wherein, ​ 13. The cathode sheet of Claim 1, wherein, The first conductive agent comprises one or more of carbon nanotubes, graphene, carbon nanofibers.

14. The cathode sheet of Claim 1, wherein, The first conductive agent comprises one or more of carbon nanotubes, graphene, carbon nanofibers. The mass percentage of the first polymer is X1, X1 is 5% to 20%; and / or, The mass percentage of the first aqueous binder is X2, X2 is 30% to 80%; and / or, The mass percentage of the first conductive agent is X3, X3 is 10% to 50%.

15. The cathode sheet of Claim 14, wherein, X1 is 5% to 10%; and / or, X2 is 40% to 50%; and / or, X3 is 40% to 50%.

16. The cathode sheet of Claim 1, wherein, The thickness of the conductive primer layer is 1 μm to 20 μm.

17. The cathode sheet of Claim 1, wherein, The thickness of the conductive primer layer is 3 μm to 10 μm.

18. The cathode sheet of Claim 1, wherein, The positive electrode film layer further comprises one or more of a wetting agent and a dispersant.

19. The cathode sheet of Claim 18, wherein, The positive electrode film layer further comprises one or more of a wetting agent and a dispersant.

20. The cathode sheet of Claim 18, wherein, The surface tension of the wetting agent is 20 mN / m to 40 mN / m.

21. The cathode sheet of Claim 18, wherein, The wetting agent comprises at least one of the following functional groups: -CN, -NH2, -NH-, -N-, -OH, -COO-, -C(=O)-O-C(=O)-.

22. The positive electrode web of claim 18, wherein, The wetting agent comprises one or more of a small molecule organic solvent and a low molecular weight polymer.

23. The cathode sheet of Claim 22, wherein, The small molecule organic solvent comprises one or more of an alcohol amine compound, an alcohol compound, and a nitrile compound.

24. The cathode sheet of Claim 23, wherein, The alcohol amine compound has a number of carbon atoms of 1 to 16.

25. The cathode sheet of Claim 23, wherein, The alcohol amine compound has a number of carbon atoms of 2 to 6.

26. The cathode sheet of Claim 22, wherein, The low molecular weight polymer comprises one or more of a maleic anhydride-styrene copolymer, polyvinylpyrrolidone, and polysiloxane.

27. The cathode sheet of Claim 22 wherein, The low molecular weight polymer has a weight average molecular weight of 6000 or less.

28. The cathode sheet of Claim 22 wherein, The low molecular weight polymer has a weight average molecular weight of 3000 to 6000.

29. The cathode sheet of Claim 18, wherein, The dispersant comprises a second polymer, and the second polymer comprises: a fifth monomer unit represented by Formula 7; a sixth monomer unit selected from at least one of a monomer unit represented by Formula 8 and a monomer unit represented by Formula 9; and a seventh monomer unit selected from at least one of a monomer unit represented by Formula 10 and a monomer unit represented by Formula 11; Formula 7 Formula 8 Formula 9 Formula 10 Formula 11.

30. The cathode sheet of Claim 29, wherein, The mass percentage of the fifth monomer unit is M5, M5 is 10% to 55%; and / or, The mass percentage of the sixth monomer unit is M6, M6 is 40% to 80%; and / or, The mass percentage of the seventh monomer unit is M7, M7 is 0% to 10%. M5 is 25% to 55%; and / or, 31. The cathode sheet of Claim 30, wherein, M6 is 50% to 70%; and / or, M7 is 0.001% to 2%. M7 / (M6+M7) is 0% to 5%.

32. The cathode sheet of Claim 30, wherein, M7 / (M6+M7) is 0.001% to 1%.

33. The cathode sheet of Claim 30, wherein, 34. The positive electrode web of claim 29, wherein, The second polymer is hydrogenated nitrile rubber; and / or, The second polymer has a weight average molecular weight of 500,000 to 5000,000. The second polymer has a weight average molecular weight of 1,500,000 to 3,500,000.

35. The cathode sheet of Claim 29, wherein, The mass percentage of the second polymer is M2, M2 is 10% to 80%; and / or, 36. The cathode sheet of Claim 18 wherein, The mass percentage of the second polymer is M2, M2 is 10% to 80%; and / or, The mass percentage content of the dispersant is Y1, Y1 is 0.05-1%; and / or, The mass percentage content of the infiltrating agent is Y2, Y2 is 0.05-2%.

37. The cathode sheet of Claim 36, wherein, Y1 is 0.1%-0.5%; and / or, Y2 is 0.2%-0.8%.

38. The cathode sheet of Claim 36, wherein, Y1 / Y2 is 0.05-20.

39. The cathode sheet of Claim 36, wherein, Y1 / Y2 is 0.1-1.

40. The cathode sheet of Claim 36, wherein, Y1 / Y2 is 0.3-0.

8.

41. The cathode sheet of Claim 29 wherein, In the positive electrode sheet, the mass ratio of the first polymer to the second polymer is 1.5-5.

42. The cathode sheet of Claim 29 wherein, In the positive electrode sheet, the mass ratio of the first polymer to the second polymer is 2-3.

43. The cathode sheet of Claim 1 wherein, The interplanar spacing of the crystalline pyrophosphate in the first coating layer ranges from 0.293 nm to 0.470 nm, and the included angle of the crystal direction (111) ranges from 18.00° to 32.00°; the interplanar spacing of the crystalline phosphate in the second coating layer ranges from 0.244 nm to 0.425 nm, and the included angle of the crystal direction (111) ranges from 20.00° to 37.00°.

44. The positive electrode sheet of claim 1, wherein, In the core, the ratio of y to 1-y is 1:10 to 1:1; and / or, In the core, the ratio of z to 1-z is 1:9 to 1:

999.

45. The positive electrode sheet of claim 1, wherein, In the core, the ratio of y to 1-y is 1:4 to 1:1; and / or, In the core, the ratio of z to 1-z is 1:499 to 1:

249.

46. The cathode sheet of Claim 1 wherein, The carbon of the third coating layer is a mixture of SP2 form carbon and SP3 form carbon.

47. The cathode sheet of Claim 46, wherein, The molar ratio of the SP2 form carbon to the SP3 form carbon is any value in the range of 0.1-10.

48. The cathode sheet of Claim 46, wherein, The molar ratio of the SP2 form carbon to the SP3 form carbon is any value in the range of 2.0-3.

0.

49. The positive electrode sheet of 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.

50. The positive electrode sheet of 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 coating amount of the second coating layer is greater than 0 and less than or equal to 5.5% by weight based on the weight of the core; and / or The coating amount of the third coating layer is greater than 0 and less than or equal to 5.5% by weight based on the weight of the core.

51. The positive electrode sheet of 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 coating amount of the second coating layer is 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.

52. The positive electrode plate of claim 1, wherein, the first coating layer has a thickness of 1-10 nm; and / or the second coating layer has a thickness of 2-15 nm; and / or the third coating layer has a thickness of 2-25 nm.

53. The cathode sheet of Claim 1 wherein, the content of manganese element is in the range of 10 wt% to 35 wt%, the content of phosphorus element is in the range of 12 wt% to 25 wt%, and the weight ratio of manganese element to phosphorus element is in the range of 0.90 to 1.25, based on the weight of the positive electrode active material.

54. The cathode sheet of Claim 1 wherein, the content of manganese element is in the range of 15 wt% to 30 wt%, based on the weight of the positive electrode active material.

55. The cathode sheet of Claim 1 wherein, the content of manganese element is in the range of 17 wt% to 20 wt%, based on the weight of the positive electrode active material.

56. The cathode sheet of Claim 1 wherein, the content of phosphorus element is in the range of 15 wt% to 20 wt%, based on the weight of the positive electrode active material.

57. The cathode sheet of Claim 1, wherein, the weight ratio of manganese element to phosphorus element is in the range of 0.95 to 1.20, based on the weight of the positive electrode active material.

58. The cathode sheet of Claim 1 wherein, the lattice change rate of the positive electrode active material with core-shell structure before and after complete deintercalation of lithium is 4% or less.

59. The cathode sheet of Claim 1, wherein, the lattice change rate of the positive electrode active material with core-shell structure before and after complete deintercalation of lithium is 3.8% or less.

60. The cathode sheet of Claim 1 wherein, the lattice change rate of the positive electrode active material with core-shell structure before and after complete deintercalation of lithium is 2.0-3.8%.

61. The cathode sheet of Claim 1 wherein, the Li / Mn anti-site defect concentration of the positive electrode active material with core-shell structure is 4% or less.

62. The cathode sheet of Claim 1 wherein, the Li / Mn anti-site defect concentration of the positive electrode active material with core-shell structure is 2.2% or less.

63. The cathode sheet of Claim 1 wherein, the Li / Mn anti-site defect concentration of the positive electrode active material with core-shell structure is 1.5-2.2%.

64. The cathode sheet of Claim 1 wherein, The positive electrode active material having a core-shell structure has a compaction density of 2.2 g / cm 3 The above.

65. The cathode sheet of Claim 1, wherein, The positive electrode active material having a core-shell structure has a compaction density of 2.2 g / cm 3 above and 2.8 g / cm 3 below.

66. The cathode sheet of any one of claims 1-65, wherein, the surface oxygen valence state of the positive electrode active material with core-shell structure is -1.90 or less.

67. The cathode sheet of any one of claims 1-65, wherein, the surface oxygen valence state of the positive electrode active material with core-shell structure is -1.90 to -1.

98.

68. A secondary battery comprising the positive electrode plate of any one of claims 1-67.

69. An electric device comprising the secondary battery of claim 68.

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