High-rate lithium transition metal phosphate material and secondary battery

Abstract

The present application provides a lithium-ion secondary battery, a lithium transition metal phosphate material, a method for preparing the lithium transition metal phosphate material, and an electric device, and specifically relates to a lithium-ion secondary battery. The lithium-ion secondary battery comprises a positive electrode sheet, wherein the positive electrode sheet comprises a positive electrode current collector and a positive electrode film layer arranged on at least one surface of the positive electrode current collector. The positive electrode film layer comprises a lithium transition metal phosphate material, and the lithium transition metal phosphate material comprises a lithium transition metal phosphate inner core, a carbon coating layer, and a conductive agent distributed on the surface of the carbon coating layer, wherein the carbon coating layer at least covers part of the surface of the inner core; the conductive agent comprises a linear conductive agent and / or a flaked conductive agent; and at least part of the conductive agent is embedded in the carbon coating layer. In the present invention, the lithium transition metal phosphate is modified by a high-electron-conductivity material, such that the electronic conductivity of the lithium transition metal phosphate is effectively improved, thereby improving the high-rate charge and discharge capability of a battery cell.

Description

High-rate lithium transition metal phosphate material and secondary battery

[0001] Cross-reference to related applications

[0002] This application is based on and claims priority to the application with CN application number 202411258817.9 and filing date of September 9, 2024, the contents of which are hereby incorporated by reference in their entirety. TECHNICAL FIELD

[0003] The present application relates to the technical field of batteries, in particular to a high-rate lithium transition metal phosphate material and a lithium ion secondary battery. BACKGROUND

[0004] Lithium manganese iron phosphate (LiMn x Fe 1-x PO4) is a new type of phosphate lithium battery positive electrode material formed by doping a certain proportion of manganese on lithium iron phosphate (LiFePO4). Through doping, on the one hand, the advantages of iron and manganese can be effectively combined, and on the other hand, manganese and iron are adjacent in the periodic table and have similar ionic radii, so doping does not significantly affect the original structure. Compared with lithium iron phosphate, the high-voltage characteristics of manganese make lithium manganese iron phosphate have a higher voltage platform, which also leads to a higher energy density at the same specific capacity. Under the same conditions, the energy density is 10%-20% higher than that of lithium iron phosphate, but the disadvantage is that the introduction of manganese significantly reduces the electrical conductivity of the material.

[0005] Lithium manganese iron phosphate batteries have poor high-rate charge and discharge performance due to their poor intrinsic electronic conductivity, which limits the development of lithium manganese iron phosphate batteries in the fast charging field.

[0006] Current solutions to improve the electronic conductivity of lithium manganese iron phosphate include introducing carbon nanotubes (CNT), graphene and other conductive agents during electrode coating, or growing CNT or graphene and other high electronic conductivity materials on the surface of lithium transition metal phosphate materials by introducing catalysts to improve their rate performance. The above methods have little effect on lithium manganese iron phosphate or are difficult to be widely applied. Among them, introducing CNT, graphene and other conductive agents during electrode coating, due to the low intrinsic electronic conductivity of lithium manganese iron phosphate, the simple physical mixing method of introducing high conductivity conductive agents does not significantly improve the rate performance; generating a CNT layer with high electrical conductivity on the surface of lithium manganese iron phosphate in situ, on the one hand, the introduced transition metal catalyst may deteriorate the electrochemical performance of lithium manganese iron phosphate batteries, and on the other hand, this solution is difficult to implement on a large scale, and has low value in industrial production. SUMMARY

[0007] To solve the above problems, the application provides a lithium ion secondary battery, a lithium iron manganese phosphate material, a method for preparing the lithium iron manganese phosphate material, and an electric device.

[0008] The first aspect of the application provides a lithium ion secondary battery, comprising a positive electrode sheet, wherein the positive electrode sheet comprises a positive electrode current collector and a positive electrode film layer arranged on at least one surface of the positive electrode current collector; the positive electrode film layer comprises a lithium transition metal phosphate material, wherein the lithium transition metal phosphate material comprises a lithium transition metal phosphate core, a carbon coating layer, and a conductive agent distributed on the surface of the carbon coating layer; the carbon coating layer covers at least part of the surface of the core; the conductive agent comprises linear conductive agents and / or sheet-shaped conductive agents; and at least part of the conductive agent is embedded in the carbon coating layer.

[0009] The application modifies lithium iron manganese phosphate with high electronic conductivity materials such as carbon nanotubes, graphene, and carbon nanofibers, effectively improves the electronic conductivity of lithium iron manganese phosphate, and thus improves the high-rate charge and discharge capability of the battery.

[0010] In some embodiments, the lithium transition metal phosphate material comprises a conductive agent comprising at least one of carbon nanotubes, graphene, and carbon nanofibers.

[0011] In some embodiments, the lithium transition metal phosphate material has a powder resistivity of 0.1-100 Ω / cm; the powder resistivity is measured by a powder resistivity tester at 50 MPa. The method for testing the powder resistivity can be performed according to the method recommended by the manufacturer of the powder resistivity tester.

[0012] In some embodiments, the conductive agent is carbon nanotubes. The carbon nanotubes used in the application can be single-walled carbon nanotubes or multi-walled carbon nanotubes, and the difference in the effect of the two on the material properties is small. Considering the availability of the material, in some embodiments, multi-walled carbon nanotubes can be used to implement the application. In some embodiments, the carbon nanotubes have a diameter of 1 nm-100 nm (for example, 1 nm-10 nm, 10 nm-20 nm, 20 nm-30 nm, 30 nm-40 nm, 40 nm-50 nm, 50 nm-60 nm, 60 nm-70 nm, 70 nm-80 nm, 80 nm-90 nm, or 90 nm-100 nm). In some embodiments, the carbon nanotubes have a length of 1 nm-100 nm (for example, 1 nm-10 nm, 10 nm-20 nm, 20 nm-30 nm, 30 nm-40 nm, 40 nm-50 nm, 50 nm-60 nm, 60 nm-70 nm, 70 nm-80 nm, 80 nm-90 nm, or 90 nm-100 nm). ≤ 10 nm (for example, 10 nm-20 nm, 20 nm-30 nm, 30 nm-40 nm, 40 nm-50 nm, 50 nm-60 nm, 60 nm-70 nm, 70 nm-80 nm, 80 nm-90 nm, or 90 nm-100 nm). ≤1 nm, 1 nm to 2 nm, 2 nm to 3 nm, 3 nm to 4 nm, 4 nm to 5 nm, 5 nm to 6 nm, 6 nm to 7 nm, 7 nm to 8 nm, 8 nm to 9 nm, or 9 nm to 10 nm. In some embodiments, the carbon nanotube has an aspect ratio of 100 to 200 (e.g., 100 to 120, 120 to 140, 140 to 160, 160 to 180, 180 to 200). Carbon nanotubes with the above dimensions can better improve the rate performance of the material.

[0013] To obtain better rate performance, the degree of carbon coating can be adjusted. In some embodiments, the mass of the carbon coating layer is 1.0 wt% to 2.5 wt% of the sum of the mass of the inner core and the mass of the carbon coating layer.

[0014] In some embodiments, the lithium transition metal phosphate inner core has a molecular formula of Li (1+a) Fe (1-x) Mn x M y (PO 4-b )X b , wherein M comprises one or more metal elements from Group IVB, Group VB, Group VIII, Group IIA, Group IIIA, Group IVA, and Group VA, except Mn, X comprises one or more elements from F, S, and N, -0.5≤a≤0.5, 0≤x≤1, 0≤y≤0.1, and 0≤b≤0.1.

[0015] In some embodiments, the lithium transition metal phosphate inner core has a molecular formula of Li (1+a) Fe (1-x) Mn x M y PO4, wherein M comprises one metal element from Group IVB, Group VB, Group VIII, Group IIA, Group IIIA, Group IVA, and Group VA, except Mn, -0.5≤a≤0.5, 0≤x≤1, and 0≤y≤0.1.

[0016] In some embodiments, the x is selected from 0 to 0.1, 0.1 to 0.2, 0.2 to 0.3, 0.3 to 0.4, 0.4 to 0.5, 0.5 to 0.6, 0.6 to 0.7, 0.7 to 0.8, 0.8 to 0.9, or 0.9 to 1.0.

[0017] In some embodiments, y is selected from 0 to 0.001, 0.001 to 0.003, 0.003 to 0.005, 0.005 to 0.007, 0.007 to 0.008, 0.008 to 0.01, 0.01 to 0.012, 0.012 to 0.015, 0.015 to 0.02, 0.02 to 0.023, 0.023 to 0.025, 0.025 to 0.03, 0.03 to 0.035, 0.035 to 0.04, 0.04 to 0.045, 0.045 to 0.05, 0.05 to 0.056, 0.056 to 0.06, 0.06 to 0.065, 0.065 to 0.07, 0.07 to 0.075, 0.075 to 0.08, 0.08 to 0.085, 0.085 to 0.09, 0.09 to 0.095, 0.095 to 0.098, or 0.098 to 0.1.

[0018] In some embodiments, -0.1 < a < 0.1. In some embodiments, a is 0.

[0019] In some embodiments, 0.001 < b < 0.1.

[0020] In some embodiments, M comprises at least one of Al, Mg, Ni, Co, Ti, Ga, Cu, V, Nb, Zr, Ce, In, Zn, Y.

[0021] In some embodiments, the lithium transition metal phosphate core has a formula of Li 0.9～1.1 Fe 0.3～1 Mn 0～0.7 M 0～0.01 PO4.

[0022] In some embodiments, M is Ti.

[0023] In some embodiments, the core has a formula selected from LiFe 0.4 Mn 0.6 PO4, LiFe 0.4 Mn 0.6 Ti 0.01 PO4, LiFe 0.5 Mn 0.5 PO4, LiFe 0.8 Mn 0.2 PO4, LiFe 0.3 Mn 0.7 PO4, LiFePO4.

[0024] The second aspect of the present application provides a lithium transition metal phosphate material, comprising a lithium transition metal phosphate core, a carbon coating layer, and a conductive agent distributed on the surface of the carbon coating layer; the carbon coating layer covers at least part of the surface of the core; the conductive agent comprises linear conductive agents and / or sheet conductive agents; at least part of the conductive agent is embedded in the carbon coating layer.

[0025] In some embodiments, the conductive agent comprises at least one of carbon nanotubes, graphene, and carbon nanofibers.

[0026] In some embodiments, it has a powder resistivity of 0.1-100 Ω / cm; the powder resistivity is measured by a powder resistance tester at 50 MPa.

[0027] In some embodiments, the mass of the carbon coating layer accounts for 1.0wt%-2.5wt% of the sum of the mass of the core and the carbon coating layer.

[0028] In some embodiments, the lithium transition metal phosphate core has a molecular formula of Li (1+a) Fe (1-x) Mn x M y (PO 4-b )X b , wherein M comprises one or more metal elements from Group IVB, Group VB, Group VIII, Group IIA, Group IIIA, Group IVA, and Group VA except Mn, X comprises one or more elements from F, S, and N, -0.5≤a≤0.5, 0≤x≤1, 0≤y≤0.1, and 0≤b≤0.1.

[0029] In some embodiments, the lithium transition metal phosphate core has a molecular formula of Li (1+a) Fe (1-x) Mn x M y PO4, wherein M comprises one metal element from Group IVB, Group VB, Group VIII, Group IIA, Group IIIA, Group IVA, and Group VA except Mn, -0.5≤a≤0.5, 0≤x≤1, and 0≤y≤0.1.

[0030] In some embodiments, the x is selected from 0-0.1, 0.1-0.2, 0.2-0.3, 0.3-0.4, 0.4-0.5, 0.5-0.6, 0.6-0.7, 0.7-0.8, 0.8-0.9, or 0.9-1.0.

[0031] In some embodiments, y is selected from 0 to 0.001, 0.001 to 0.003, 0.003 to 0.005, 0.005 to 0.007, 0.007 to 0.008, 0.008 to 0.01, 0.01 to 0.012, 0.012 to 0.015, 0.015 to 0.02, 0.02 to 0.023, 0.023 to 0.025, 0.025 to 0.03, 0.03 to 0.035, 0.035 to 0.04, 0.04 to 0.045, 0.045 to 0.05, 0.05 to 0.056, 0.056 to 0.06, 0.06 to 0.065, 0.065 to 0.07, 0.07 to 0.075, 0.075 to 0.08, 0.08 to 0.085, 0.085 to 0.09, 0.09 to 0.095, 0.095 to 0.098, or 0.098 to 0.1.

[0032] In some embodiments, -0.1 < a < 0.1. In some embodiments, a is 0.

[0033] In some embodiments, 0.001 < b < 0.1.

[0034] In some embodiments, M comprises at least one of Al, Mg, Ni, Co, Ti, Ga, Cu, V, Nb, Zr, Ce, In, Zn, Y.

[0035] In some embodiments, the lithium transition metal phosphate core has a formula of Li 0.9～1.1 Fe 0.3～1 Mn 0～0.7 M 0～0.01 PO4.

[0036] In some embodiments, M is Ti.

[0037] In some embodiments, the core has a formula selected from LiFe 0.4 Mn 0.6 PO4, LiFe 0.4 Mn 0.6 Ti 0.01 PO4, LiFe 0.5 Mn 0.5 PO4, LiFe 0.8 Mn 0.2 PO4, LiFe 0.3 Mn 0.7 PO4, LiFePO4.

[0038] A third aspect of the present application provides a method of preparing a lithium transition metal phosphate material, comprising the steps of:

[0039] S1: providing a conductive agent dispersion liquid comprising a conductive agent uniformly dispersed in a solvent; the conductive agent comprises a linear conductive agent and / or a flaky conductive agent;

[0040] S2: mixing the carbon-coated lithium transition metal phosphate powder with the dispersion liquid to obtain a slurry;

[0041] S3: removing the liquid in the slurry to obtain a mixed powder, and sintering the mixed powder in a protective atmosphere to obtain the lithium transition metal phosphate material, the protective atmosphere comprising one or more of nitrogen and inert gas.

[0042] In some embodiments, the sintering is performed at a temperature of 550-750°C.

[0043] In some embodiments, in step S1, the conductive agent comprises at least one of carbon nanotubes, graphene, carbon nanofibers.

[0044] In some embodiments, the solvent comprises at least one of water, a lower alcohol (such as methanol, ethanol, n-propanol, isopropanol), NMP.

[0045] In some embodiments, the solvent comprises alcohol.

[0046] In some embodiments, the conductive agent dispersion liquid is prepared by dispersing the conductive agent in the solvent by ultrasonic.

[0047] In some embodiments, in step S2, the lithium transition metal phosphate has a molecular formula of Li (1+a) Fe (1-x) Mn x M y (PO 4-b )X b , wherein M comprises one or more metal elements from Group IVB, Group VB, Group VIII, Group IIA, Group IIIA, Group IVA and Group VA except Mn, X comprises one or more elements from F, S and N, -0.5≤a≤0.5, 0≤x≤1, 0≤y≤0.1, 0≤b≤0.1.

[0048] In some embodiments, the lithium transition metal phosphate has a molecular formula of Li (1+a) Fe (1- x) Mn x M y PO4, wherein M comprises one metal element from Group IVB, Group VB, Group VIII, Group IIA, Group IIIA, Group IVA and Group VA except Mn, -0.5≤a≤0.5, 0≤x≤1, 0≤y≤0.1.

[0049] In some embodiments, x is selected from 0-0.1, 0.1-0.2, 0.2-0.3, 0.3-0.4, 0.4-0.5, 0.5-0.6, 0.6-0.7, 0.7-0.8, 0.8-0.9, or 0.9-1.0.

[0050] In some embodiments, y is selected from 0-0.001, 0.001-0.003, 0.003-0.005, 0.005-0.007, 0.007-0.008, 0.008-0.01, 0.01-0.012, 0.012-0.015, 0.015-0.02, 0.02-0.023, 0.023-0.025, 0.025-0.03, 0.03-0.035, 0.035-0.04, 0.04-0.045, 0.045-0.05, 0.05-0.056, 0.056-0.06, 0.06-0.065, 0.065-0.07, 0.07-0.075, 0.075-0.08, 0.08-0.085, 0.085-0.09, 0.09-0.095, 0.095-0.098, or 0.098-0.1.

[0051] In some embodiments, -0.1

[0052] In some embodiments, 0.001

[0053] In some embodiments, M comprises at least one of Al, Mg, Ni, Co, Ti, Ga, Cu, V, Nb, Zr, Ce, In, Zn, Y.

[0054] In some embodiments, the lithium transition metal phosphate has a formula of Li 0.9～1.1 Fe 0.3～1 Mn 0～0.7 M 0～0.01 PO4.

[0055] In some embodiments, M is Ti.

[0056] In some embodiments, the lithium transition metal phosphate has a formula selected from LiFe 0.4 Mn 0.6 PO4, LiFe 0.4 Mn 0.6 Ti 0.01 PO4, LiFe 0.5 Mn 0.5 PO4, LiFe 0.8 Mn 0.2 PO4, LiFe0.3 Mn 0.7 PO4, LiFePO4.

[0057] In some embodiments, the mass percentage of the carbon coating layer in the carbon-coated lithium transition metal phosphate is 1.0wt%-2.5wt%.

[0058] In some embodiments, the mass ratio of the conductive agent to the carbon-coated lithium transition metal phosphate is Qwt%, and Q% is 0.1wt%-5wt%.

[0059] In some embodiments, in step S2, the stirring time ranges from 1h to 10h.

[0060] In some embodiments, in step S3, the liquid in the slurry is removed by one or more of suction filtration, vacuum drying, and spray drying.

[0061] In some embodiments, in step S3, the protective gas is a mixture of one or more of N2, Ar, and He.

[0062] In some embodiments, the sintering is performed in a tube furnace.

[0063] In some embodiments, the sintering time ranges from 2h to 10h.

[0064] In some embodiments, the lithium transition metal phosphate material prepared by the method is the lithium transition metal phosphate material provided in the second aspect of the present application. In some embodiments, the lithium transition metal phosphate material prepared by the method is included in the lithium ion secondary battery provided in the first aspect of the present application.

[0065] The fourth aspect of the present application provides a device powered by electricity, comprising a lithium ion secondary battery, wherein the lithium ion secondary battery comprises the lithium ion secondary battery provided in the first aspect of the present application. BRIEF DESCRIPTION OF DRAWINGS

[0066] In order to more clearly illustrate the technical solutions of the embodiments of the present application, the drawings needed in the embodiments of the present application will be briefly introduced as follows. Obviously, the drawings described below are only some embodiments of the present application, and other drawings can be obtained by those skilled in the art without creative labor on the basis of the drawings.

[0067] FIG. 1 is a schematic diagram of a lithium ion secondary battery according to an embodiment of the present application.

[0068] FIG. 2 is an exploded view of the lithium ion secondary battery according to an embodiment of the present application shown in FIG. 1.

[0069] FIG. 3 is a schematic view of a battery module according to an embodiment of the present application.

[0070] FIG. 4 is a schematic view of a battery pack according to an embodiment of the present application.

[0071] FIG. 5 is an exploded view of the battery pack according to an embodiment of the present application shown in FIG. 4.

[0072] FIG. 6 is a schematic view of an electric device using a lithium ion secondary battery as a power source according to an embodiment of the present application.

[0073] In the drawings, the drawings are not drawn to scale.

[0074] BRIEF DESCRIPTION OF DRAWINGS

[0075] FIGS. 7 and 8 are SEM photographs of the positive active material prepared in Example 1 and Comparative Example 3, respectively.

[0076] FIGS. 9 and 10 are SEM photographs of two positive electrode sheets. FIG. 10 is a photograph of the positive electrode sheet prepared using the positive active material prepared in Example 1 (CNT high-temperature coating) and further prepared, and shows that the CNTs are uniformly distributed on the surface of the lithium iron manganese phosphate and partially embedded in the carbon coating layer. FIG. 10 is a photograph of the positive electrode sheet prepared using the positive active material prepared in Comparative Example 2 (CNTs added to the slurry and physically mixed), and shows that the CNTs are severely agglomerated with SPs and not embedded in the carbon coating layer. DETAILED DESCRIPTION

[0077] The embodiments of the present application will be described in further detail below with reference to the accompanying drawings and examples. The detailed description of the following examples and the accompanying drawings are intended to exemplarily illustrate the principles of the present application, but cannot be used to limit the scope of the present application, i.e., the present application is not limited to the described examples.

[0078] Hereinafter, various embodiments of the present application are specifically disclosed in detail with appropriate reference to the accompanying drawings. However, there can be cases where unnecessary detailed descriptions are omitted. For example, there can be cases where detailed descriptions of matters that are well known, repeated descriptions of substantially the same structures are omitted. This is to avoid the following description from becoming unnecessarily lengthy and to facilitate understanding by those skilled in the art. In addition, the accompanying drawings and the following description are provided to enable those skilled in the art to fully understand the present application, and are not intended to limit the subject matter recited in the claims.

[0079] The ranges disclosed herein are meant to be inclusive of the endpoints and include the end values in the range. Ranges can be combined to form new ranges, e.g., a range of "60-120 and 80-110" is understood to include 60-110 and 80-120. Further, if a minimum range value is listed as 1 and a maximum range value is listed as 3, 4, and 5, then the following ranges are all contemplated: 1-3, 1-4, 1-5, 2-3, 2-4, and 2-5. In this application, unless otherwise indicated, a numerical range "a-b" means a range of any combination of the numbers a and b, wherein both a and b are real numbers. For example, the numerical range "0-5" means that all real numbers between 0 and 5 have been listed herein, and "0-5" is merely a shorthand for listing all of those numbers. Also, when a parameter is stated to be an integer > 2, it is equivalent to state that the parameter is, for example, an integer 2, 3, 4, 5, 6, 7, 8, 9, 10, 11, 12, etc.

[0080] All embodiments and optional embodiments of the present application can be combined with each other to form new technical solutions, unless otherwise specified.

[0081] All technical features and optional technical features of the present application can be combined with each other to form new technical solutions, unless otherwise specified.

[0082] All steps of the present application can be performed in sequence or randomly, preferably in sequence, unless otherwise specified. For example, the method comprises steps (a) and (b), which means that the method can comprise steps (a) and (b) in sequence, or steps (b) and (a) in sequence. For example, the method can further comprise step (c), which means that step (c) can be added to the method in any order, for example, the method can comprise steps (a), (b) and (c), or steps (a), (c) and (b), or steps (c), (a) and (b), etc.

[0083] If not otherwise specified, "including" and "containing" mentioned in the present application are open-ended. For example, "including" and "containing" can mean that other components not listed can also be included or contained.

[0084] If not specifically stated, the term "or" in this application is inclusive. For example, any of the following satisfy the condition "A or B": 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).

[0085] As used herein, the term "one or more" means 1 or more than 1, for example 2, 3, 4, 5, or 10, under reasonable conditions.

[0086] Unless specified, as used herein, the connection point of a substituent can come from any suitable position of the substituent.

[0087] As described in the background, due to the poor intrinsic electronic conductivity of the material of the lithium iron manganese phosphate battery, the large-rate charge-discharge performance of the battery is poor. Adding CNT as a conductive agent to the positive electrode slurry to coat the positive electrode sheet does not have a significant effect on the rate performance of the battery cell. The inventors found that uniformly dispersing high electronic conductivity materials such as carbon nanotubes, graphene, carbon nanofibers, etc. on the surface of lithium iron manganese phosphate particles and then performing high-temperature treatment can effectively improve the rate performance.

[0088] [Secondary battery]

[0089] The first aspect of the present application provides a lithium ion secondary battery.

[0090] Generally, a lithium ion secondary battery includes a positive electrode sheet, a negative electrode sheet, an electrolyte, and a separator. During the charge and discharge of the battery, active ions are inserted and extracted between the positive electrode sheet and the negative electrode sheet. The electrolyte plays a role in conducting ions between the positive electrode sheet and the negative electrode sheet. The separator is disposed between the positive electrode sheet and the negative electrode sheet and mainly plays a role in preventing short circuiting between the positive and negative electrodes while allowing ions to pass through.

[0091] [Positive electrode sheet]

[0092] The lithium ion secondary battery provided by the present application comprises a positive electrode sheet, wherein the positive electrode sheet comprises a positive electrode current collector and a positive electrode film layer arranged on at least one surface of the positive electrode current collector; the positive electrode film layer comprises a lithium transition metal phosphate material, the lithium transition metal phosphate material comprises a lithium transition metal phosphate core, a carbon coating layer, and a conductive agent distributed on the surface of the carbon coating layer; the carbon coating layer covers at least part of the surface of the core; the conductive agent comprises linear conductive agents and / or sheet-shaped conductive agents; and at least part of the conductive agent is embedded in the carbon coating layer.

[0093] The present application modifies lithium iron manganese phosphate with high electronic conductivity materials such as carbon nanotubes, graphene, and carbon nanofibers, effectively improves the electronic conductivity performance of lithium iron manganese phosphate, and thus improves the high-rate charge-discharge capability of the battery cell.

[0094] In some embodiments, the lithium transition metal phosphate material comprises an electrically conductive agent including at least one of carbon nanotubes, graphene, carbon nanofibers.

[0095] The lithium transition metal phosphate material used in the present application has good electrical conductivity. In some embodiments, the lithium transition metal phosphate material has a powder resistivity of 0.1-100 Ω / cm (e.g., 0.1-0.5 Ω / cm, 0.5-1 Ω / cm, 1-5 Ω / cm, 5-10 Ω / cm, 10-20 Ω / cm, 20-30 Ω / cm, 30-40 Ω / cm, 40-50 Ω / cm, 50-60 Ω / cm, 60-70 Ω / cm, 70-80 Ω / cm, 80-90 Ω / cm, or 90-100 Ω / cm), measured using a powder resistivity tester at 50 MPa.

[0096] In some embodiments, the mass of the carbon coating layer is 1.0wt%-2.5wt% (e.g., 1.0wt%-1.3wt%, 1.3wt%-1.5wt%, 1.5wt%-1.7wt%, 1.7wt%-1.9wt%, 1.9wt%-2.1wt%, 2.1wt%-2.3wt%, or 2.3wt%-2.5wt%) of the sum of the mass of the inner core and the mass of the carbon coating layer. The mass of the carbon coating layer in the above range can allow the lithium iron manganese phosphate to be completely coated without exposing the surface, and also without causing the powder capacity to decrease due to too much carbon coating, so that the material has sufficient energy density.

[0097] The inner core of the lithium transition metal phosphate material can be doped or undoped lithium iron phosphate, and doped or undoped lithium iron manganese phosphate. In some embodiments, the lithium transition metal phosphate inner core has a molecular formula of Li (1+a) Fe (1-x) Mn x M y (PO 4-b )X b , wherein M includes one or more metal elements from Group IVB, Group VB, Group VIII, Group IIA, Group IIIA, Group IVA, and Group VA, except Mn, X includes one or more elements from F, S, and N, -0.5≤a≤0.5, 0≤x≤1, 0≤y≤0.1, and 0≤b≤0.1.

[0098] In some embodiments, the lithium transition metal phosphate inner core has a molecular formula of Li (1+a) Fe (1-x) Mn x M yPO4, wherein M comprises one metal element selected from the group consisting of Group IVB, Group VB, Group VIII, Group IIA, Group IIIA, Group IVA and Group VA except Mn, -0.5≤a≤0.5, 0≤x≤1, 0≤y≤0.1.

[0099] In some embodiments, the x is selected from 0-0.1, 0.1-0.2, 0.2-0.3, 0.3-0.4, 0.4-0.5, 0.5-0.6, 0.6-0.7, 0.7-0.8, 0.8-0.9, or 0.9-1.0.

[0100] In some embodiments, the y is selected from 0-0.001, 0.001-0.003, 0.003-0.005, 0.005-0.007, 0.007-0.008, 0.008-0.01, 0.01-0.012, 0.012-0.015, 0.015-0.02, 0.02-0.023, 0.023-0.025, 0.025-0.03, 0.03-0.035, 0.035-0.04, 0.04-0.045, 0.045-0.05, 0.05-0.056, 0.056-0.06, 0.06-0.065, 0.065-0.07, 0.07-0.075, 0.075-0.08, 0.08-0.085, 0.085-0.09, 0.09-0.095, 0.095-0.098, or 0.098-0.1.

[0101] In some embodiments, the M comprises at least one of Al, Mg, Ni, Co, Ti, Ga, Cu, V, Nb, Zr, Ce, In, Zn, Y.

[0102] In some embodiments, -0.1≤a≤0.1. In some embodiments, a is 0.

[0103] In some embodiments, 0.001≤b≤0.1.

[0104] In some embodiments, the lithium transition metal phosphate core has a formula of Li 0.9～1.1 Fe 0.3～1 Mn 0～0.7 M 0～0.01 PO4.

[0105] In some embodiments, the M is Ti.

[0106] In some embodiments, the lithium transition metal phosphate core has a formula selected from the group consisting of: LiFe 0.4 Mn 0.6 PO4, LiFe 0.4 Mn0.6 Ti 0.01 PO4, LiFe 0.5 Mn 0.5 PO4, LiFe 0.8 Mn 0.2 PO4, LiFe 0.3 Mn 0.7 PO4, LiFePO4.

[0107] As an example, the positive electrode current collector has two surfaces opposite in the thickness direction thereof, and the positive electrode film layer is provided on either one or both of the two opposite surfaces of the positive electrode current collector.

[0108] In some embodiments, the positive electrode current collector can be a metal foil or a composite current collector. For example, as the metal foil, an aluminum foil can be used. The composite current collector can include a polymer material base layer and a metal layer formed on at least one surface of the polymer material base layer. The composite current collector can be formed by forming a metal material (aluminum, aluminum alloy, nickel, nickel alloy, titanium, titanium alloy, silver, and silver alloy, etc.) on a polymer material base material such as a base material of polypropylene (PP), polyethylene terephthalate (PET), polybutylene terephthalate (PBT), polystyrene (PS), polyethylene (PE), etc.

[0109] In the positive electrode film layer, the lithium iron manganese phosphate material of the first aspect of the present application, or the lithium iron manganese phosphate material prepared by the method of the second aspect of the present application, is used as a positive electrode active material.

[0110] In some embodiments, the positive electrode film layer can further include other conventional materials that can be used as a battery positive electrode active material. These positive electrode active materials can be used alone only one or in combination of two or more. Among them, examples of lithium transition metal oxides can include, but are not limited to, lithium cobalt oxide (such as LiCoO2), lithium nickel oxide (such as LiNiO2), lithium manganese oxide (such as LiMnO2, LiMn2O4), lithium nickel cobalt oxide, lithium manganese cobalt oxide, lithium nickel manganese oxide, lithium nickel cobalt manganese oxide (such as LiNi 1 / 3 Co 1 / 3 Mn 1 / 3 O2 (also referred to as NCM 333 ), LiNi 0.5 Co 0.2 Mn 0.3 O2 (also referred to as NCM 523 ), LiNi 0.5 Co 0.25 Mn 0.25 O2 (also referred to as NCM 211 ), LiNi 0.6 Co 0.2 Mn0.2 O2(also can be referred to as NCM 622 ), LiNi 0.8 Co 0.1 Mn 0.1 O2(also can be referred to as NCM 811 ), lithium nickel cobalt aluminum oxide (such as LiNi 0.85 Co 0.15 Al 0.05 O2) and modified compounds thereof. Examples of lithium-containing phosphates of olivine structure can include, but are not limited to, one or more of lithium iron phosphate (such as LiFePO4(also can be referred to as LFP)), a composite of lithium iron phosphate and carbon, lithium manganese phosphate (such as LiMnPO4), a composite of lithium manganese phosphate and carbon, lithium manganese iron phosphate, a composite of lithium manganese iron phosphate and carbon.

[0111] In some embodiments, the positive electrode film layer can also optionally include a binder. As an example, the binder can include one or more of polyvinylidene fluoride (PVDF), polytetrafluoroethylene (PTFE), a vinylidene fluoride-tetrafluoroethylene-propylene terpolymer, a vinylidene fluoride-hexafluoropropylene-tetrafluoroethylene terpolymer, a tetrafluoroethylene-hexafluoropropylene copolymer, and a fluorine-containing acrylate resin. In some embodiments, the binder is PVDF.

[0112] In some embodiments, the positive electrode film layer can also optionally include an electrically conductive agent added separately from the electrically conductive agent contained in the lithium transition metal phosphate material. As an example, the electrically conductive agent can include one or more of super P, acetylene black, carbon black, ketjen black, carbon dots, carbon nanotubes, graphene, and carbon nanofibers. Microscopically, the electrically conductive agent added separately to the positive electrode film layer can be separate from the electrically conductive agent (such as carbon nanotubes, graphene, or carbon nanofibers) on the surface of the lithium transition metal phosphate material employed in the present application. This is different from the positive electrode film layer obtained by introducing the electrically conductive agent through simple physical mixing as mentioned in the background art.

[0113] In some embodiments, the positive electrode sheet can be prepared by dispersing the components for preparing the positive electrode sheet, such as the positive electrode active material, the binder, and any other components, in a solvent (such as N-methylpyrrolidone) to form a positive electrode slurry; coating the positive electrode slurry on a positive electrode current collector, and after processes such as drying, cold pressing, etc., the positive electrode sheet can be obtained.

[0114] [Positive electrode sheet]

[0115] The negative electrode sheet includes a negative electrode current collector and a negative electrode active material coating layer disposed on at least one surface of the negative electrode current collector, the negative electrode active material coating layer including a negative electrode active material.

[0116] As an example, the negative electrode current collector has two surfaces opposite in the thickness direction thereof, and the negative electrode film layer is provided on either one or both of the two surfaces of the negative electrode current collector.

[0117] In some embodiments, the negative electrode current collector can employ a metal foil or a composite current collector. For example, as the metal foil, a copper foil can be employed. The composite current collector can include a polymer material base layer and a metal layer formed on at least one surface of the polymer material base layer. The composite current collector can be formed by forming a metal material (copper, copper alloy, nickel, nickel alloy, titanium, titanium alloy, silver, silver alloy, etc.) on a polymer material base layer such as a base layer of polypropylene (PP), polyethylene terephthalate (PET), polybutylene terephthalate (PBT), polystyrene (PS), polyethylene (PE), etc.

[0118] In some embodiments, the negative electrode active material can employ a negative electrode active material for a battery known in the art. As an example, the negative electrode active material can include at least one of artificial graphite, natural graphite, soft carbon, hard carbon, mesocarbon microbeads, carbon fiber, carbon nanotube, silicon-based material, tin-based material, and lithium titanate, etc. The silicon-based material can be selected from at least one of elemental silicon, silicon oxide compound, silicon-carbon composite, silicon-nitrogen composite, and silicon alloy. The tin-based material can be selected from at least one of elemental tin, tin oxide compound, and tin alloy. However, the present application is not limited to these materials, and other conventional materials that can be used as a negative electrode active material for a battery can also be used. These negative electrode active materials can be used alone or in combination of two or more.

[0119] In some embodiments, the negative electrode active material coating layer can further optionally include a binder. As an example, the binder can 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).

[0120] In some embodiments, the negative electrode active material coating layer can further optionally include a conductive agent. As an example, the conductive agent can be selected from at least one of super conductive carbon, acetylene black, carbon black, ketjen black, carbon dot, carbon nanotube, graphene, and carbon nanofiber.

[0121] In some embodiments, the negative electrode active material coating layer can further optionally include other auxiliary agents, such as a thickening agent (e.g., sodium carboxymethyl cellulose (CMC-Na)) and the like.

[0122] In some embodiments, the negative electrode sheet can be prepared by dispersing the above-mentioned components for preparing the negative electrode sheet, such as the negative electrode active material, the conductive agent, the binder, and any other components, in a solvent (e.g., deionized water) to form a negative electrode slurry; coating the negative electrode slurry on a negative electrode current collector, and then drying, cold-pressing, or the like to obtain the negative electrode sheet.

[0123] [Electrolyte]

[0124] The electrolyte functions to conduct ions between the positive electrode sheet and the negative electrode sheet. The type of electrolyte is not particularly limited in the present application, and can be selected as needed. For example, the electrolyte can be liquid, gel, or all-solid.

[0125] In some embodiments, the electrolyte is an electrolyte solution. The electrolyte solution includes an electrolyte salt and a solvent.

[0126] In some embodiments, the electrolyte salt can be selected from one or more of lithium hexafluorophosphate, lithium tetrafluoroborate, lithium perchlorate, lithium hexafluoroarsenate, lithium bisfluorosulfonylimide, lithium bistrifluoromethylsulfonylimide, lithium trifluoromethanesulfonate, lithium difluorophosphate, lithium difluorobisoxalate borate, lithium bisoxalate borate, lithium difluorodioxalate phosphate, and lithium tetrafluorodioxalate phosphate.

[0127] In some embodiments, the solvent can be selected from one or more of ethylene carbonate, propylene carbonate, methyl ethyl carbonate, diethyl carbonate, dimethyl carbonate, dipropyl carbonate, methyl propyl carbonate, ethyl propyl carbonate, butylene 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.

[0128] In some embodiments, the electrolyte solution can optionally further include an additive. For example, the additive can include a negative electrode film-forming additive, a positive electrode film-forming additive, and can further include an additive that can improve certain properties of the battery, such as an additive that improves overcharge performance of the battery, an additive that improves high-temperature or low-temperature performance of the battery, and the like.

[0129] [Separator]

[0130] In some embodiments, the lithium ion secondary battery further includes a separator. The type of separator is not particularly limited in the present application, and any known porous structure separator having good chemical stability and mechanical stability can be used.

[0131] In some embodiments, the material of the separator film can be selected from at least one of glass fiber, non-woven fabric, polyethylene, polypropylene, and polyvinylidene fluoride. The separator film can be a single layer film or a multi-layer composite film, and is not particularly limited. When the separator film is a multi-layer composite film, the materials of the layers can be the same or different, and are not particularly limited.

[0132] In some embodiments, the positive electrode tab, the negative electrode tab, and the separator film can be used to form an electrode assembly through a winding process or a stacking process.

[0133] In some embodiments, the lithium ion secondary battery can include a lithium ion secondary battery cell, or include a battery module and a battery pack.

[0134] In some embodiments, the lithium ion secondary battery can include an outer package. The outer package can be used to encapsulate the electrode assembly and the electrolyte described above.

[0135] In some embodiments, the outer package of the lithium ion secondary battery can be a hard shell, such as a hard plastic shell, an aluminum shell, a steel shell, etc. The outer package of the lithium ion secondary battery can also be a soft package, such as a pouch soft package. The material of the soft package can be plastic, and as plastic, polypropylene, polybutylene terephthalate, and polybutylene succinate, etc. can be listed.

[0136] The shape of the lithium ion secondary battery cell is not particularly limited in the present application, and can be cylindrical, square, or any other shape. For example, FIG. 1 is a square structure of a lithium ion secondary battery cell 5 as an example.

[0137] In some embodiments, referring to FIG. 2, the outer package can include a shell 51 and a top cover assembly 53. The shell 51 can include a bottom plate and a side plate connected to the bottom plate, and the bottom plate and the side plate enclose a receiving cavity. The shell 51 has an opening communicating with the receiving cavity, and the top cover assembly 53 can be arranged on the opening to close the receiving cavity. The positive electrode tab, the negative electrode tab, and the separator film can be used to form an electrode assembly 52 through a winding process or a stacking process. The electrode assembly 52 is encapsulated in the receiving cavity. The electrolyte is impregnated in the electrode assembly 52. The number of electrode assemblies 52 contained in the lithium ion secondary battery cell 5 can be one or more, and can be selected by those skilled in the art according to specific actual needs.

[0138] In some embodiments, the lithium ion secondary battery cell can be assembled into a battery module, and the number of lithium ion secondary battery cells contained in the battery module can be one or more, and the specific number can be selected by those skilled in the art according to the application and capacity of the battery module.

[0139] FIG. 3 is a battery module 4 as an example. Referring to FIG. 3, in the battery module 4, a plurality of lithium ion secondary battery cells 5 can be arranged in series along the length direction of the battery module 4. Of course, the arrangement can be made in any other manner. Further, the plurality of lithium ion secondary battery cells 5 can be fixed by fasteners.

[0140] Optionally, the battery module 4 can further include a housing having an accommodation space, and the plurality of lithium ion secondary battery cells 5 can be accommodated in the accommodation space.

[0141] In some embodiments, the above-described battery module can be assembled into a battery pack, and the number of battery modules contained in the battery pack can be one or more, and the specific number can be selected by those skilled in the art according to the application and capacity of the battery pack.

[0142] FIGS. 4 and 5 are a battery pack 1 as an example. Referring to FIGS. 4 and 5, the battery pack 1 can include a battery box and a plurality of battery modules 4 arranged in the battery box. The battery box includes an upper box body 2 and a lower box body 3, and the upper box body 2 can be arranged on the lower box body 3 to form a closed space for accommodating the battery modules 4. The plurality of battery modules 4 can be arranged in the battery box in any manner.

[0143] [Lithium manganese iron phosphate material]

[0144] In one embodiment of the present application, a lithium transition metal phosphate material is provided, which includes a lithium transition metal phosphate core, a carbon coating layer, and a conductive agent distributed on the surface of the carbon coating layer; the carbon coating layer covers at least part of the surface of the core; the conductive agent includes linear conductive agents and / or sheet-shaped conductive agents; and at least part of the conductive agent is embedded in the carbon coating layer.

[0145] The present application modifies lithium transition metal phosphates such as carbon-coated lithium manganese iron phosphate using high electronic conductive materials, effectively improves the electronic conductive performance of lithium transition metal phosphates, and thus improves the high-rate charge and discharge capability of the battery cell. Through transmission electron microscopy (TEM), scanning electron microscopy (SEM), and powder resistivity, it can be found that the ports or partial defect sites of the high electronic conductive materials will bond with the amorphous carbon on the surface of the lithium transition metal phosphate, thereby greatly reducing the energy barrier for the transfer of electrons from the conductive network to the surface of the lithium transition metal phosphate, i.e., improving the ability to transfer electrons during electrochemical reaction.

[0146] In some embodiments, the lithium transition metal phosphate material contains a conductive agent including at least one of carbon nanotubes, graphene, and carbon nanofibers.

[0147] The lithium transition metal phosphate material of the present invention exhibits good electrical conductivity. In some embodiments, the lithium transition metal phosphate material has a powder resistivity of 0.1-100 Ω / cm (e.g., 0.1-0.5 Ω / cm, 0.5-1 Ω / cm, 1-5 Ω / cm, 5-10 Ω / cm, 10-20 Ω / cm, 20-30 Ω / cm, 30-40 Ω / cm, 40-50 Ω / cm, 50-60 Ω / cm, 60-70 Ω / cm, 70-80 Ω / cm, 80-90 Ω / cm, or 90-100 Ω / cm); said powder resistivity is measured using a powder resistance meter at 50 MPa.

[0148] In some embodiments, the carbon coating layer accounts for 1.0wt%-2.5wt% of the sum of the core and the carbon coating layer (e.g., 1.0wt%-1.3wt%, 1.3wt%-1.5wt%, 1.5wt%-1.7wt%, 1.7wt%-1.9wt%, 1.9wt%-2.1wt%, 2.1wt%-2.3wt%, or 2.3wt%-2.5wt%). A carbon coating layer within these ranges ensures complete coating of the lithium manganese iron phosphate without surface exposure, and avoids reducing the powder's specific capacity due to excessive carbon coating, thus providing the material with sufficient energy density.

[0149] The core of the lithium transition metal phosphate material can be doped or undoped lithium iron phosphate, or doped or undoped lithium manganese iron phosphate. In some embodiments, the lithium transition metal phosphate core has the molecular formula Li. (1+a) Fe (1-x) Mn x M y (PO 4-b )X b Where M includes one or more metallic elements other than Mn from Groups IVB, VB, VIII, IIA, IIIA, IVA and VA, and X includes one or more elements from F, S and N, and -0.5≤a≤0.5, 0≤x≤1, 0≤y≤0.1, 0≤b≤0.1.

[0150] In some embodiments, the lithium transition metal phosphate core has the molecular formula Li (1+a) Fe (1-x) Mn x M y PO4, wherein M includes one metallic element other than Mn from Groups IVB, VB, VIII, IIA, IIIA, IVA and VA, and -0.5≤a≤0.5, 0≤x≤1, 0≤y≤0.1.

[0151] In some embodiments, x is selected from 0 to 0.1, 0.1 to 0.2, 0.2 to 0.3, 0.3 to 0.4, 0.4 to 0.5, 0.5 to 0.6, 0.6 to 0.7, 0.7 to 0.8, 0.8 to 0.9, or 0.9 to 1.0.

[0152] In some embodiments, y is selected from 0 to 0.001, 0.001 to 0.003, 0.003 to 0.005, 0.005 to 0.007, 0.007 to 0.008, 0.008 to 0.01, 0.01 to 0.012, 0.012 to 0.015, 0.015 to 0.02, 0.02 to 0.023, 0.023 to 0.025, 0.025 to 0.03, 0.03 to 0.035, 0.035 to 0.04, 0.04 to 0.045, 0.045 to 0.05, 0.05 to 0.056, 0.056 to 0.06, 0.06 to 0.065, 0.065 to 0.07, 0.07 to 0.075, 0.075 to 0.08, 0.08 to 0.085, 0.085 to 0.09, 0.09 to 0.095, 0.095 to 0.098, or 0.098 to 0.1.

[0153] In some embodiments, -0.1 < a < 0.1. In some embodiments, a is 0.

[0154] In some embodiments, 0.001 < b < 0.1.

[0155] In some embodiments, M comprises at least one of Al, Mg, Ni, Co, Ti, Ga, Cu, V, Nb, Zr, Ce, In, Zn, Y.

[0156] In some embodiments, the lithium transition metal phosphate core has a formula of Li 0.9～1.1 Fe 0.3～1 Mn 0～0.7 M 0～0.01 PO4.

[0157] In some embodiments, M is Ti.

[0158] In some embodiments, the core has a formula selected from LiFe 0.4 Mn 0.6 PO4, LiFe 0.4 Mn 0.6 Ti 0.01 PO4, LiFe 0.5 Mn 0.5 PO4, LiFe 0.8 Mn 0.2 PO4, LiFe0.3 Mn 0.7 PO4, LiFePO4.

[0159] [Method for preparing lithium transition metal phosphate material]

[0160] The present application provides a method for preparing lithium transition metal phosphate material, which is simple and feasible, easy to mass production, and the obtained lithium transition metal phosphate material has improved rate performance. The method comprises the following steps:

[0161] S1: providing a conductive agent dispersion liquid comprising a conductive agent uniformly dispersed in a solvent; the conductive agent comprises linear conductive agent and / or sheet conductive agent;

[0162] S2: mixing the carbon-coated lithium transition metal phosphate powder with the dispersion liquid to obtain a slurry;

[0163] S3: removing the liquid in the slurry to obtain a mixed powder, and sintering the mixed powder in a protective atmosphere to obtain the lithium transition metal phosphate material, wherein the protective gas comprises one or more of nitrogen and inert gas.

[0164] Carbon coating as a modification means of lithium transition metal phosphate material can effectively build a fast conductive network, provide an effective channel for Li + diffusion, and improve the electrical conductivity of the material. The method of the present application further modifies the carbon-coated lithium transition metal phosphate material, mixes the carbon-coated lithium transition metal phosphate material with the conductive agent uniformly, and then sintering, so that the conductive agent forms a chemical bond with the carbon layer on the surface of the lithium transition metal phosphate under high temperature conditions, further reduces the energy barrier for the conduction of electrons from the conductive agent to the surface of the lithium transition metal phosphate, and can significantly improve the rate performance of the lithium transition metal phosphate material, and the effect is significantly better than the simple physical mixing method mentioned in the background art.

[0165] To further improve the rate performance of the positive electrode material, the sintering temperature can be adjusted. In some embodiments, the sintering is performed at a temperature of 550-750°C (e.g., 550-600°C, 600-650°C, 650-700°C, or 700-750°C). In some embodiments, the sintering is performed at a temperature of 500-550°C. High temperature sintering is beneficial for the bonding of the conductive agent with the lithium transition metal phosphate on one hand, but on the other hand, it can also promote the crystal growth of the lithium transition metal phosphate and the reaction between the carbon coating layer and the lithium transition metal phosphate to generate impurities. Crystal growth and reaction impurities can cause an increase in impedance, which is not conducive to the rate performance. The higher the temperature, the more serious the related effects. By selecting the above sintering temperature, on one hand, the bonding of the conductive agent with the lithium transition metal phosphate is facilitated, and on the other hand, the crystal size can be prevented from being too large and the generation of impurities can be reduced, which is conducive to obtaining better rate performance.

[0166] In some embodiments, the conductive agent comprises at least one of carbon nanotubes, graphene, and carbon nanofibers.

[0167] In some embodiments, the solvent comprises at least one of water, a lower alcohol, and NMP. In the present application, a lower alcohol includes an alcohol having a carbon atom number less than or equal to 6, including methanol, ethanol, n-propanol, isopropanol, etc.

[0168] In some embodiments, the solvent comprises alcohol.

[0169] In some embodiments, the conductive agent dispersion is self-made, for example, the conductive agent can be dispersed in the solvent by ultrasonic to obtain the conductive agent dispersion. In other embodiments, the conductive agent dispersion can be obtained by commercial purchase.

[0170] In some embodiments, the conductive agent is carbon nanotubes. In some embodiments, the carbon nanotubes have a diameter of 1-100 nm (e.g., 1-10 nm, 10-20 nm, 20-30 nm, 30-40 nm, 40-50 nm, 50-60 nm, 60-70 nm, 70-80 nm, 80-90 nm, or 90-100 nm). In some embodiments, the carbon nanotubes have a length of 1-1000 nm (e.g., 1-10 nm, 10-20 nm, 20-30 nm, 30-40 nm, 40-50 nm, 50-60 nm, 60-70 nm, 70-80 nm, 80-90 nm, 90-100 nm, 100-200 nm, 200-300 nm, 300-400 nm, 400-500 nm, 500-600 nm, 600-700 nm, 700-800 nm, 800-900 nm, or 900-1000 nm). ≤ 10 nm (e.g., ≤1 nm, 1 nm-2 nm, 2 nm-3 nm, 3 nm-4 nm, 4 nm-5 nm, 5 nm-6 nm, 6 nm-7 nm, 7 nm-8 nm, 8 nm-9 nm, or 9 nm-10 nm). In some embodiments, the carbon nanotubes have an aspect ratio of 100-200 (e.g., 100-120, 120-140, 140-160, 160-180, 180-200). Carbon nanotubes of the above sizes can better improve the rate performance of the material.

[0171] In step S2, the carbon-coated lithium transition metal phosphate powder is mixed with the dispersion liquid to uniformly disperse the conductive agent between the lithium iron manganese phosphate particles, and a slurry is obtained.

[0172] The carbon-coated lithium transition metal phosphate powder can be commercially available or can be prepared by any method disclosed in the prior art. In some embodiments, the carbon-coated lithium transition metal phosphate can be prepared by a method comprising the steps of: providing a lithium transition metal phosphate core; and forming a carbon coating layer on at least part of the surface of the core.

[0173] In some embodiments, forming the carbon coating layer on at least part of the surface of the core comprises: forming a pre-carbon coating layer on the surface of the core by a carbon source to obtain a pre-coated positive electrode active material; and performing a sintering treatment on the pre-coated positive electrode active material in an inert gas atmosphere to form the carbon coating layer, to obtain the carbon-coated lithium transition metal phosphate. The carbon source includes, but is not limited to, starch, sucrose, glucose, citric acid, polyethylene glycol, and other carbon sources for carbon coating of lithium transition metal phosphate active materials.

[0174] In some embodiments, the lithium transition metal phosphate in step S2 has a molecular formula of Li (1+a) Fe (1-x) Mn x M y (PO 4-b )X b , wherein M comprises one or more metal elements from Group IVB, Group VB, Group VIII, Group IIA, Group IIIA, Group IVA, and Group VA, except Mn, X comprises one or more elements from F, S, and N, -0.5≤a≤0.5, 0≤x≤1, 0≤y≤0.1, and 0≤b≤0.1.

[0175] In some embodiments, the lithium transition metal phosphate has a molecular formula of Li (1+a) Fe (1- x) Mn x M yPO4, wherein M comprises one metal element selected from the group consisting of Group IVB, Group VB, Group VIII, Group IIA, Group IIIA, Group IVA and Group VA except Mn, -0.5≤a≤0.5, 0≤x≤1, 0≤y≤0.1.

[0176] In some embodiments, the x is selected from 0-0.1, 0.1-0.2, 0.2-0.3, 0.3-0.4, 0.4-0.5, 0.5-0.6, 0.6-0.7, 0.7-0.8, 0.8-0.9, or 0.9-1.0.

[0177] In some embodiments, the y is selected from 0-0.001, 0.001-0.003, 0.003-0.005, 0.005-0.007, 0.007-0.008, 0.008-0.01, 0.01-0.012, 0.012-0.015, 0.015-0.02, 0.02-0.023, 0.023-0.025, 0.025-0.03, 0.03-0.035, 0.035-0.04, 0.04-0.045, 0.045-0.05, 0.05-0.056, 0.056-0.06, 0.06-0.065, 0.065-0.07, 0.07-0.075, 0.075-0.08, 0.08-0.085, 0.085-0.09, 0.09-0.095, 0.095-0.098, or 0.098-0.1.

[0178] In some embodiments, -0.1≤a≤0.1. In some embodiments, a is 0.

[0179] In some embodiments, 0.001≤b≤0.1.

[0180] In some embodiments, the M comprises at least one of Al, Mg, Ni, Co, Ti, Ga, Cu, V, Nb, Zr, Ce, In, Zn, Y.

[0181] In some embodiments, the lithium transition metal phosphate has a molecular formula of: Li 0.9～1.1 Fe 0.3～1 Mn 0～0.7 M 0～0.01 PO4.

[0182] In some embodiments, the M is Ti.

[0183] In some embodiments, the lithium transition metal phosphate has a molecular formula selected from: LiFe 0.4 Mn 0.6 PO4, LiFe 0.4 Mn 0.6Ti 0.01 PO4, LiFe 0.5 Mn 0.5 PO4, LiFe 0.8 Mn 0.2 PO4, LiFe 0.3 Mn 0.7 PO4, LiFePO4.

[0184] In some embodiments, the carbon-coated lithium transition metal phosphate has a carbon-coating layer with a mass percentage of 1.0wt%-2.5wt% (e.g. 1.0wt%-1.3wt%, 1.3wt%-1.5wt%, 1.5wt%-1.7wt%, 1.7wt%-1.9wt%, 1.9wt%-2.1wt%, 2.1wt%-2.3wt% or 2.3wt%-2.5wt%). The mass of the carbon-coating layer within the above range can ensure that the lithium iron manganese phosphate is completely coated without exposing the surface, and also prevent the powder from having a reduced capacity due to too much carbon coating, thus ensuring that the material has sufficient energy density.

[0185] The rate performance of the material can be improved by adjusting the amount of conductive agent added. In some embodiments, the mass ratio of the added conductive agent to the carbon-coated lithium transition metal phosphate is Q wt%, and Q wt% is 0.1wt%-5wt% (e.g. 0.1wt%-0.5wt%, 0.5wt%-1wt%, 1wt%-1.5wt%, 1.5wt%-2wt%, 2wt%-2.5wt%, 2.5wt%-3wt%, 3wt%-3.5wt%, 3.5wt%-4wt%, 4wt%-4.5wt% or 4.5wt%-5wt%). The amount of conductive agent added within the above range can improve the rate performance of the material, and on the other hand, the slurry has moderate fluidity, which is beneficial for slurry coating, and the active material coating load is moderate, and the electrode sheet mass distribution is uniform.

[0186] The conductive agent can be better dispersed among the lithium iron manganese phosphate particles by long-time and fast stirring. In some embodiments, the stirring time in step S2 is 1h-10h (e.g. 1h-2h, 2h-4h, 4h-6h, 6h-8h or 8h-10h). Longer stirring time has less effect on the dispersion effect, which is specifically manifested in that the rate performance changes less. In some embodiments, the stirring rate is 200r / min-500r / min (e.g. 200r / min-300r / min, 300r / min-400r / min or 500r / min-500r / min). Faster stirring rate has less effect on the dispersion effect, which is specifically manifested in that the rate performance changes less.

[0187] In some embodiments, in step S3, the liquid in the slurry is removed by one or more of filtration, vacuum drying, and spray drying.

[0188] In some embodiments, in step S3, the protective gas is a mixture of one or more of N2, Ar, and He.

[0189] In some embodiments, the sintering is performed in a tube furnace.

[0190] In some embodiments, the sintering time is 2-10 h, for example, 2-3 h, 3-6 h, or 6-10 h.

[0191] According to the above embodiments, a lithium transition metal phosphate material with improved rate performance can be prepared.

[0192] In some embodiments, the lithium transition metal phosphate material prepared by the method is the lithium transition metal phosphate material according to the second aspect of the present application. In some embodiments, the lithium transition metal phosphate material prepared by the method is used in the lithium ion secondary battery according to the first aspect of the present application.

[0193] [Power-Consuming Device]

[0194] In addition, the present application also provides a power-consuming device, which comprises the lithium ion secondary battery provided by the present application. The lithium ion secondary battery can be used as a power source of the power-consuming device, or can be used as an energy storage unit of the power-consuming device. The power-consuming device can include a mobile device (such as a mobile phone, a notebook computer, etc.), an electric vehicle (such as a pure electric vehicle, a hybrid electric vehicle, a plug-in hybrid electric vehicle, an electric bicycle, an electric scooter, an electric golf cart, an electric truck, etc.), an electric train, a ship and a satellite, an energy storage system, etc., but is not limited thereto.

[0195] As the power-consuming device, a lithium ion secondary battery cell, a battery module, or a battery pack can be selected according to the use requirements thereof.

[0196] FIG. 6 is a power-consuming device as an example. The power-consuming device is a pure electric vehicle, a hybrid electric vehicle, or a plug-in hybrid electric vehicle, etc. In order to meet the high power and high energy density requirements of the lithium ion secondary battery for the power-consuming device, a battery pack or a battery module can be used.

[0197] Embodiments

[0198] Hereinafter, the examples of the present application will be described. The examples described below are illustrative and are intended to explain the present application, and are not to be understood as limiting the present application. In the examples, unless a specific technique or condition is mentioned, the technique or condition described in the literature in the art or according to the product manual is used. The reagents or instruments used, unless the manufacturer is mentioned, are all conventional products that can be obtained commercially.

[0199] Method for preparing the positive electrode active material of each example:

[0200] Step 1: 1) Lithium carbonate and phosphoric acid were weighed according to the molar ratio Li:P = 1:1, slowly added into a stirring tank, and stirred to dissolve. 2) Ferrous oxalate and manganese oxalate were added to the lithium-phosphorus solution according to the molar ratio Fe+Mn:P = 1:1, and sand milling was performed until the particle size was about 0.5 μm to obtain a precursor slurry. 3) The precursor slurry was spray dried, and then sintered at 650°C for 8h under nitrogen protection to obtain lithium manganese iron phosphate. 4) Lithium manganese iron phosphate n kg was added into a sand mill, and 2n water and 0.15n glucose were added for sand milling and dispersion. 5) The mixed solution of lithium manganese iron phosphate and carbon source was spray dried, and then sintered at 720°C for 10h under nitrogen protection to obtain carbon-coated lithium manganese iron phosphate material with a carbon content of about 1.5%.

[0201] Step 2: Preparation of conductive agent dispersion: carbon nanotubes were mixed with a solvent, and ultrasonic treatment was performed to uniformly disperse the carbon nanotubes; the length of the carbon nanotubes used in the examples and comparative examples was 0.2-10 μm, the tube wall thickness was within 10 nm, and the aspect ratio was 100-200;

[0202] Step 3: The carbon-coated lithium manganese iron phosphate powder was added into the conductive agent dispersion, and stirring was performed to uniformly disperse the conductive agent between the lithium manganese iron phosphate particles to obtain a slurry;

[0203] Step 4: The liquid in the slurry was removed by vacuum drying at 110°C for 8h, and the obtained powder was sintered in a nitrogen atmosphere to obtain the lithium manganese iron phosphate material.

[0204] Method for characterizing the positive electrode active material:

[0205] (1) Test of CNT distribution:

[0206] The surface of the material was tested by SEM-EDS to confirm the presence and distribution of CNTs in the material.

[0207] A Gemini 360 field emission scanning electron microscope of Carl Zeiss Company, Germany was used to determine the distribution of CNTs on the carbon-coated lithium manganese iron phosphate by morphology characterization.

[0208] The material was analyzed by TEM (X-Max EDS of Oxford Instruments Group, UK, combined with TEM of Thermo Scientific-Talos F200S G2 of Thermo Fisher Scientific, USA) to determine whether the connection mode of CNT and carbon-coated lithium iron manganese phosphate was physical contact or chemical bonding.

[0209] (2) Determination of carbon content of positive electrode material

[0210] An appropriate amount of sample was weighed in a special crucible, an appropriate amount of fluxing agent was added and mixed uniformly, the sample was burned in oxygen to convert carbon and sulfur into CO2 and SO2, which entered the absorption cell and was converted into corresponding signals by the detector. This signal was sampled by the computer, linearly corrected and converted into a value proportional to the concentration of CO2 and SO2. Then the values of the entire analysis process were added, after the analysis was completed, the cumulative value was divided by the weight value in the computer, multiplied by the correction coefficient, and the blank was deducted to obtain the percentage of carbon and sulfur in the sample.

[0211] Battery preparation method

[0212] (1) Preparation of positive electrode sheet: the positive electrode active material, the binder polyvinylidene fluoride (PVDF), and the conductive agent acetylene black were dissolved in the solvent N-methyl pyrrolidone (NMP) according to the weight ratio of 92:5.5:2.5, and were fully stirred and mixed uniformly to prepare a positive electrode slurry; the positive electrode slurry was uniformly coated on the positive electrode current collector aluminum foil, and then was dried, cold-pressed, and cut to obtain the positive electrode sheet.

[0213] (2) Negative electrode sheet: a lithium metal sheet was used as the negative electrode sheet.

[0214] (3) Separator film: a polypropylene film was used, one side of which was coated with an aluminum oxide coating.

[0215] (4) Preparation of electrolyte: ethylene carbonate (EC), diethyl carbonate (DEC), and dimethyl carbonate (DMC) were mixed according to a volume ratio of 1:1:1, and then LiPF6 was uniformly dissolved in the above solution to obtain the electrolyte. In the electrolyte, the concentration of LiPF6 was 1 mol / L.

[0216] (5) Preparation of button cell: the above positive electrode sheet, separator film, and negative electrode sheet were assembled in a button cell box and injected with the electrolyte to obtain the button cell.

[0217] Comparative Example 1: carbon-coated LiFe 0.4 Mn 0.6 PO4 was sintered at 650°C for 6h as a positive electrode active material, and a battery was prepared according to the above method.

[0218] Comparative Example 2: carbon-coated LiFe 0.4 Mn 0.6PO4as the positive active material, carbon nanotubes were added in the positive slurry, and no sintering was performed. The mass of carbon nanotubes accounted for 1.5% of the mass of carbon-coated LiFe 0.4 Mn 0.6 PO4. The carbon nanotube dispersion liquid was mixed with the carbon-coated LiFe 0.4 Mn 0.6 PO4after stirring to obtain a powder by vacuum drying. The powder resistance of the powder was measured.

[0219] Comparative Example 3: Glucose and carbon nanotubes were used as carbon sources, and LiFe 0.4 Mn 0.6 PO4was mixed and sintered to obtain a positive active material, wherein the total carbon content was 3%, the ball milling speed was 400 r / min, the ball milling time was 8 h, and the ball-to-powder ratio was 10:1. Comparative Example 4: Carbon-coated LiFe 0.4 Mn 0.6 PO4was used as the positive active material, and a battery was prepared according to the above method.

[0220] Next, the test methods of the physical parameters and performance parameters mentioned in the embodiments of the present application are briefly introduced.

[0221] 1. Test of electronic conductivity of the positive material:

[0222] The powder resistance tester (PRCD1100 model of Yuan Neng Technology Co., Ltd.) was used to test the powder resistivity of the positive material at 50 MPa. The reciprocal of the powder resistivity was the electronic conductivity of the material.

[0223] 2. Test of specific capacity of the battery:

[0224] At 25°C, the battery was charged at 0.1C to 4.3V, then charged at 4.3V to 0.01C, rested for 5 min, and then discharged at 0.1C to 2.0V. The obtained discharge capacity was denoted as c1, and the discharge capacity obtained after repeating the cycle once was denoted as c2. Three parallel samples were prepared, the average value of c2 of the three parallel samples was taken, and then divided by the weight of the positive material to obtain the specific capacity of the battery (0.1c capacity in Table 1).

[0225] 3. Test of charge-discharge rate performance of the battery:

[0226] At 25°C, first constant current charge to 4.3V at 0.1C, then constant voltage charge to 0.01C at 4.3V, rest for 5min, then discharge to 2.0V at 0.1C, so cycle 5 times; then change the charge and discharge rate, cycle 5 times at 0.5C, 1C, 2C, 3C, 5C rate respectively, after 5C rate cycle, then cycle 5 times at 0.1C rate again. Compare the ratio of capacity at high rate to capacity at 0.1C rate to judge the rate performance of the material.

[0227] Characterization and test results:

[0228] Fig. 7 and Fig. 8 show SEM photos of positive active materials prepared in Example 1 and Comparative Example 3, respectively. From Fig. 1, it can be seen that the nanowires are wrapped around the surface of lithium manganese iron phosphate; from Fig. 2, it can be seen that the CNT is coated with carbon after pyrolysis of the carbon source, and the morphology and function have changed, which is not conducive to the conductivity.

[0229] Fig. 9 and Fig. 10 are SEM photos of two positive electrode sheets, Fig. 10 is a positive active material prepared in Example 1 (CNT high-temperature coating) and further prepared into an electrode sheet, which shows that the CNT is uniformly distributed on the surface of lithium manganese iron phosphate, and part of the CNT is embedded in the carbon coating layer; Fig. 10 is an electrode sheet prepared in Comparative Example 2 (adding CNT in the slurry and physically mixing), which shows that the CNT and SP are severely agglomerated, and are not embedded in the carbon coating layer. The preparation parameters of the positive active material and the corresponding test results are shown in Tables 1-4.

Claims

1. A lithium-ion secondary battery, characterized by comprising: The positive electrode sheet comprises a positive electrode current collector and a positive electrode film layer arranged on at least one surface of the positive electrode current collector; the positive electrode film layer comprises a lithium transition metal phosphate material, the lithium transition metal phosphate material comprises a lithium transition metal phosphate core, a carbon coating layer, and a conductive agent distributed on the surface of the carbon coating layer; the carbon coating layer covers at least part of the surface of the core; the conductive agent comprises linear conductive agent and / or sheet conductive agent; and at least part of the conductive agent is embedded in the carbon coating layer.

2. The lithium-ion secondary battery according to claim 1, characterized by The lithium transition metal phosphate material comprises at least one of carbon nanotubes, graphene, and carbon nanofibers.

3. The lithium-ion secondary battery according to claim 1 or 2, characterized by The lithium transition metal phosphate material has a powder resistivity of 0.1-100 Ω / cm, which is measured by a powder resistivity tester at 50 MPa.

4. The lithium-ion secondary battery according to any one of claims 1 to 3, characterized by The mass of the carbon coating layer accounts for 1.0wt%-2.5wt% of the total mass of the core and the carbon coating layer.

5. The lithium-ion secondary battery according to any one of claims 1 to 4, characterized by The lithium transition metal phosphate core has a molecular formula of Li (1+a) Fe (1-x) Mn x M y (PO 4- b )X b , wherein M includes one or more metal elements from Groups IVB, VB, VIII, IIA, IIIA, IVA and VA, except Mn, X includes one or more elements from F, S and N, -0.5≤a≤0.5, 0≤x≤1, 0≤y≤0.1, 0≤b≤0.

1.

6. The lithium-ion secondary battery according to any one of claims 1 to 5, characterized by The lithium transition metal phosphate core has a molecular formula of Li (1+a) Fe (1-x) Mn x M y PO4, wherein M includes one metal element from group IVB, group VB, group VIII, group IIA, group IIIA, group IVA and group VA, except Mn, -0.5≤a≤0.5, 0≤x≤1, 0≤y≤0.

1.

7. The lithium-ion secondary battery according to claim 5 or 6, characterized by The M comprises at least one of Al, Mg, Ni, Co, Ti, Ga, Cu, V, Nb, Zr, Ce, In, Zn, Y.

8. The lithium-ion secondary battery according to any one of claims 1 to 7, characterized by The molecular formula of the lithium transition metal phosphate core is: Li 0.9～1.1 Fe 0.3～1 Mn 0～0.7 M 0～0.01 PO4.

9. A lithium transition metal phosphate material characterized in that, The lithium transition metal phosphate material comprises a lithium transition metal phosphate core, a carbon coating layer, and a conductive agent distributed on the surface of the carbon coating layer; the carbon coating layer covers at least part of the surface of the core; the conductive agent comprises linear conductive agent and / or sheet conductive agent; and at least part of the conductive agent is embedded in the carbon coating layer.

10. The lithium transition metal phosphate material of claim 9, wherein, The conductive agent comprises at least one of carbon nanotubes, graphene, and carbon nanofibers.

11. The lithium transition metal phosphate material of claim 9 or 10, wherein, The lithium transition metal phosphate material has a powder resistivity of 0.1-100 Ω / cm, which is measured by a powder resistivity tester at 50 MPa.

12. A method of making a lithium transition metal phosphate material, characterized in that, The method comprises the following steps: S1: providing a conductive agent dispersion liquid comprising a conductive agent uniformly dispersed in a solvent; the conductive agent comprises linear conductive agent and / or sheet conductive agent; S2: mixing carbon-coated lithium transition metal phosphate powder with the dispersion liquid to obtain a slurry; S3: removing the liquid in the slurry to obtain a mixed powder, and sintering the mixed powder in a protective atmosphere to obtain the lithium transition metal phosphate material; the protective gas comprises one or more of nitrogen and inert gas.

13. The method of claim 12, wherein, The sintering is performed at a temperature of 550-750°C.

14. The method according to claim 12 or 13, characterized in that, In step S1, the conductive agent comprises at least one of carbon nanotubes, graphene, and carbon nanofibers.

15. The method according to any one of claims 12-14, characterized in that, In the carbon-coated lithium transition metal phosphate, the mass percentage of the carbon coating layer is 1.0wt%-2.5wt%; and / or The mass ratio of the conductive agent to the carbon-coated lithium transition metal phosphate is Qwt%, and Q% is 0.1wt%-5wt%.

16. An electrical device, comprising: The lithium ion secondary battery comprises the lithium ion secondary battery according to any one of claims 1-8.

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