A negative electrode material, a preparation method thereof, a negative electrode sheet and a secondary battery

Abstract

The present invention belongs to the technical field of secondary batteries, and particularly relates to a negative electrode material, graphite, and a manganese pyrophosphate material layer coated on the surface of the graphite. In the negative electrode material of the present invention, a manganese pyrophosphate material layer is coated on the surface of the graphite. Manganese pyrophosphate has multi-dimensional lithium ion diffusion channels, enabling the graphite to achieve fast charge and discharge. Moreover, manganese pyrophosphate has a stable lattice structure, enabling the graphite to perform multiple charge and discharge cycles under the conditions of fast charge and discharge.

Description

Technical Field

[0001] The present invention belongs to the technical field of secondary batteries, and particularly relates to a negative electrode material, a preparation method thereof, a negative electrode sheet and a secondary battery. Background Art

[0002] The length of charge and discharge time is the main evaluation index for evaluating the kinetic performance of lithium-ion secondary batteries. Among all the main materials of lithium-ion secondary batteries, the negative electrode material and the electrolyte are the main factors affecting the kinetic performance of lithium-ion secondary batteries, and the negative electrode material has the greatest correlation. Currently, the most widely used and successful negative electrode material for lithium-ion secondary batteries in commercial applications is graphite. Graphite has the advantages of high capacity, low cost, and small swelling, and is highly sought after by major manufacturers. It is widely used in the field of 3C consumer electronics products. With the rapid development of hybrid and electric vehicles, lithium-ion batteries are also widely used in the fields of power and energy storage, and higher requirements are put forward for the performance of graphite negative electrode materials.

[0003] At present, the bottlenecks encountered in the development of power batteries or soft-pack batteries are not only limited by the working voltage of the positive electrode material, but also restricted by the charging ability of the graphite negative electrode material. If the charging speed is too fast, the impedance of lithium ions when inserted into the graphite interlayer increases, making it difficult to insert lithium, and the negative electrode polarization will increase, resulting in the reduction reaction of lithium ions with electrons on the graphite surface, causing the occurrence of lithium plating phenomenon, which brings potential safety hazards to the battery cells. Therefore, it is urgent to develop a graphite negative electrode material with strong charging ability under the condition of ensuring the energy density of the graphite negative electrode material.

[0004] In response to these problems, corresponding research has been carried out at home and abroad. Therefore, there is an urgent need for a technical solution to solve the above problems. Summary of the Invention

[0005] One of the purposes of the present invention is to provide, in view of the deficiencies of the prior art, a negative electrode material with a layer of manganese pyrophosphate material coated on the surface of graphite. Manganese pyrophosphate has a multi-dimensional lithium ion diffusion channel, enabling graphite to achieve fast charge and discharge, and manganese pyrophosphate has a stable crystal lattice structure, enabling graphite to perform multiple charge and discharge cycles under the condition of fast charge and discharge.

[0006] To achieve the above purpose, the present invention adopts the following technical solutions:

[0007] A negative electrode material includes graphite and a layer of manganese pyrophosphate material coated on the surface of graphite.

[0008] Preferably, the coating rate of the manganese pyrophosphate material layer is 60% - 90%.

[0009] Preferably, the thickness of the manganese pyrophosphate material layer is 10 - 120 nm.

[0010] The second object of the present invention is to provide a preparation method of a negative electrode material in view of the deficiencies of the prior art. By using liquid-phase mixing and heat sintering, manganese pyrophosphate is in-situ generated on the surface of graphite to form a manganese pyrophosphate material layer, so that the manganese pyrophosphate material layer can be evenly dispersed on the surface of graphite, making the performance more stable.

[0011] To achieve the above object, the present invention adopts the following technical solutions:

[0012] A preparation method of a negative electrode material includes the following steps:

[0013] Step S1: Add a manganese source and a phosphorus source to a solvent, mix and stir to obtain a mixed solution;

[0014] Step S2: Add graphite to the above mixed solution, stir and disperse, vacuum filter, heat, and vacuum dry to obtain a pre-product;

[0015] Step S3: Heat and sinter the pre-product in an inert atmosphere, cool down, and crush to obtain the negative electrode material.

[0016] Preferably, in step S1, the weight ratio of the manganese source to the phosphorus source is 1-3:1-5.

[0017] Preferably, the temperature of the vacuum drying is 70-80 °C, and the vacuum drying time is 5-15 h.

[0018] Preferably, in step S3, the heating process is specifically to first increase the temperature at a rate of 5-10 °C / min to 150-300 °C, and then increase the temperature at a rate of 2-5 °C / min to 900-1000 °C, and maintain the temperature for 4-12 h.

[0019] Preferably, the particle size after crushing in step S3 is 100-250 mesh.

[0020] The third object of the present invention is to provide a negative electrode sheet with good rate performance and cycle performance in view of the deficiencies of the prior art.

[0021] To achieve the above object, the present invention adopts the following technical solutions:

[0022] A negative electrode sheet includes the above negative electrode material.

[0023] The fourth object of the present invention is to provide a secondary battery with good rate performance and cycle performance in view of the deficiencies of the prior art.

[0024] To achieve the above object, the present invention adopts the following technical solutions:

[0025] A secondary battery includes the above negative electrode sheet.

[0026] Compared with the prior art, the beneficial effects of the present invention are as follows: A negative electrode material of the present invention has a manganese pyrophosphate material layer coated on the surface of graphite. Manganese pyrophosphate has multi-dimensional lithium ion diffusion channels, enabling graphite to achieve fast charge and discharge. Moreover, manganese pyrophosphate has a stable lattice structure, enabling graphite to perform multiple charge and discharge cycles under the conditions of fast charge and discharge. Specific Embodiments

[0027] The following combines specific embodiments to further describe the present invention in detail, but the embodiments of the present invention are not limited thereto.

[0028] A negative electrode material includes graphite and a manganese pyrophosphate material layer coated on the surface of graphite. A negative electrode material of the present invention has a manganese pyrophosphate material layer coated on the surface of graphite. Manganese pyrophosphate has multi-dimensional lithium ion diffusion channels, enabling graphite to achieve fast charge and discharge. Moreover, manganese pyrophosphate has a stable lattice structure, enabling graphite to perform multiple charge and discharge cycles under the conditions of fast charge and discharge.

[0029] The present invention uses in-situ manganese pyrophosphate liquid-phase coating of graphite negative electrode material. Manganese pyrophosphate (Mn2P2O7), as a polyanionic inorganic salt material, has excellent lattice structure and multi-dimensional lithium ion diffusion channels, and can inhibit the reduction reaction of the electrolyte, reducing the degree of loss of the first efficiency due to the formation of the SEI film and resulting in capacity loss of the battery cell. In addition, since the manganese source participating in the reaction is an organic substance, which itself contains -C-H bonds, dehydrogenation condensation reaction occurs with the -C-H bonds on the surface of graphite at high temperature to form -C-C bonds, making the coating material and the matrix material combine more tightly, and the coating layer is stable and not easy to fall off during the cycling process, thereby improving the cycling performance of the graphite negative electrode material.

[0030] In some embodiments, the coating rate of the manganese pyrophosphate material layer is 60% - 90%. The manganese pyrophosphate material layer is coated on the surface of graphite. Setting a certain carbon coating rate can not only exert the fast charging function and excellent stability of manganese pyrophosphate, but also avoid the influence of too high carbon coating rate on the performance of graphite. Preferably, the coating rate of the manganese pyrophosphate material layer is 60% - 70%, 70% - 80%, 80% - 90%. Specifically, the coating rate of the manganese pyrophosphate material layer is 60%, 63%, 65%, 68%, 69%, 70%, 75%, 78%, 80%, 82%, 85%, 86%, 87%, 89%, 90%.

[0031] In some embodiments, the thickness of the manganese pyrophosphate material layer is 10 to 120 nm. The thickness of the manganese pyrophosphate material layer has a certain influence on the performance of graphite. If the thickness of the manganese pyrophosphate material layer is too thick, the ion movement distance is increased. If the thickness of the manganese pyrophosphate material layer is too thin, it is easy to fall off during multiple charge and discharge processes, affecting the cycle performance. The thickness of the manganese pyrophosphate material layer is 10 to 50 nm, 50 to 90 nm, 90 to 100 nm, 100 to 120 nm. Specifically, the thickness of the manganese pyrophosphate material layer is 50 nm, 53 nm, 54 nm, 55 nm, 58 nm, 60 nm, 64 nm, 65 nm, 68 nm, 70 nm, 72 nm, 74 nm, 75 nm, 78 nm, 80 nm, 82 nm, 84 nm, 85 nm, 88 nm, 90 nm, 93 nm, 95 nm, 98 nm, 100 nm, 110 nm, 113 nm, 115 nm, 118 nm, 120 nm.

[0032] A method for preparing a negative electrode material uses liquid phase mixing and heating and sintering to generate manganese pyrophosphate in situ on the graphite surface to form a manganese pyrophosphate material layer, so that the manganese pyrophosphate material layer can be evenly dispersed on the graphite surface, making the performance more stable.

[0033] A method for preparing a negative electrode material comprises the following steps:

[0034] Step S1, adding a manganese source and a phosphorus source to a solvent, mixing and stirring to obtain a mixed solution;

[0035] Step S2, adding graphite to the above mixed solution, stirring and dispersing, vacuum filtering, heating, and vacuum drying to obtain a pre-product;

[0036] Step S3: heating and sintering the pre-product in an inert atmosphere, cooling it, and breaking it up to obtain a negative electrode material.

[0037] The present invention uses a liquid method to form a manganese pyrophosphate material layer on the graphite surface raw material, which is more uniform, has a better coverage rate, and has good thickness controllability, facilitating the reaction and production of the corresponding negative electrode material. Ultrasound can be used during mixing and stirring to accelerate the dissolution rate. The gas in the inert atmosphere can be one or both of nitrogen and argon.

[0038] In some embodiments, the weight ratio of the manganese source to the phosphorus source in step S1 is 1-3:1-5. The weight ratio of the manganese source to the phosphorus source is 1-2:1-3, 2-3:3-5, 2-3:2-4, 2-3:4-5. Specifically, the weight ratio of the manganese source to the phosphorus source is 1:1, 1:2, 1:3, 1:4, 1:5, 2:3, 2:5, 2:3.5, 3:1, 3:2.3, 3:5, 2:3.5. A certain amount of manganese source and phosphorus source is set to make the reaction more uniform.

[0039] In some embodiments, the temperature of the vacuum drying in step S2 is 70 - 80°C, and the vacuum drying time is 5 - 15 h. The temperatures of the vacuum drying are 70°C, 72°C, 75°C, 76°C, 78°C, 80°C, and the vacuum drying times are 5 - 8 h, 8 - 10 h, 10 - 12 h, 12 - 15 h. Specifically, the vacuum drying times are 5 h, 6 h, 7 h, 8 h, 9 h, 10 h, 11 h, 12 h, 13 h, 14 h, 15 h.

[0040] In some embodiments, the heating process in step S3 is specifically as follows: first, the temperature is raised at a heating rate of 5 - 10°C / min to 150 - 300°C, and then the temperature is raised at a heating rate of 2 - 5°C / min to 900 - 1000°C, and the temperature is maintained for 4 - 12 h. The heating process uses two-stage heating. First, the temperature is rapidly raised to a certain temperature to shorten the heating time and at the same time quickly dry the raw materials. Then, the temperature is buffered and raised to a higher temperature for sintering to enable the raw materials to react under high-temperature conditions. Preferably, the heating rate in the first stage is 5 - 8°C / min, 8 - 10°C / min. Specifically, the heating rates in the first stage are 5°C / min, 6°C / min, 7°C / min, 8°C / min, 9°C / min, 10°C / min, and the temperatures reached in the first stage are 150°C, 180°C, 200°C, 220°C, 250°C, 280°C, 300°C. The heating rate in the second stage is 2 - 5°C / min. Specifically, the heating rates in the second stage are 2°C / min, 3°C / min, 4°C / min, 5°C / min, and the temperatures reached in the second stage are 900°C, 910°C, 920°C, 930°C, 940°C, 950°C, 960°C, 970°C, 980°C, 990°C, 1000°C.

[0041] In some embodiments, the particle size after comminution in step S3 is 100 - 250 mesh. The particle sizes are 100 - 150 mesh, 150 - 200 mesh, 200 - 250 mesh. Specifically, the particle sizes are 100 mesh, 130 mesh, 150 mesh, 180 mesh, 190 mesh, 200 mesh, 250 mesh. Setting the anode material with a certain particle size affects the thickness and performance of the anode material after it is made into a slurry.

[0042] A negative electrode sheet of the present invention has good rate performance and cycling performance. Specifically, a negative electrode sheet includes the above-mentioned anode material.

[0043] A secondary battery of the present invention has good rate performance and cycling performance. Specifically, a secondary battery includes the above-mentioned anode material.

[0044] A secondary battery can be a lithium-ion battery, a sodium-ion battery, a magnesium-ion battery, a calcium-ion battery, a potassium-ion battery, etc. Preferably, taking the lithium-ion battery as an example among the following secondary batteries, the lithium-ion battery includes a positive electrode sheet, a negative electrode sheet, a separator, an electrolyte, and a housing. The separator separates the positive electrode sheet and the negative electrode sheet, and the housing is used to accommodate the positive electrode sheet, the negative electrode sheet, the separator, and the electrolyte. The negative electrode sheet is the above-mentioned negative electrode plate.

[0045] Positive electrode

[0046] The positive electrode sheet includes a positive electrode current collector and a positive electrode active material layer provided on at least one surface of the positive electrode current collector. The positive electrode active material layer includes a positive electrode active material. The positive electrode active material can be one or more combinations of compounds represented by, but not limited to, the chemical formula such as Li a Ni x Co y M z O 2-b N b (where 0.95 ≤ a ≤ 1.2, x > 0, y ≥ 0, z ≥ 0, and x + y + z = 1, 0 ≤ b ≤ 1, M is selected from one or more combinations of Mn and Al, and N is selected from one or more combinations of F, P, and S), and the positive electrode active material can also be one or more combinations of, but not limited to, LiCoO2, LiNiO2, LiVO2, LiCrO2, LiMn2O4, LiCoMnO4, Li2NiMn3O8, LiNi 0.5 Mn 1.5 O4, LiCoPO4, LiMnPO4, LiFePO4, LiNiPO4, LiCoFSO4, CuS2, FeS2, MoS2, NiS, TiS2, etc. The positive electrode active material can also be subjected to modification treatment. The methods for modifying the positive electrode active material should be known to those skilled in the art. For example, methods such as coating and doping can be used to modify the positive electrode active material, and the materials used for the modification treatment can be one or more combinations of, but not limited to, Al, B, P, Zr, Si, Ti, Ge, Sn, Mg, Ce, W, etc. The positive electrode current collector is usually a structure or component that collects current, and the positive electrode current collector can be various materials suitable for use as the positive electrode current collector of a lithium-ion battery in the art. For example, the positive electrode current collector can be, but not limited to, a metal foil, and more specifically, it can be, but not limited to, an aluminum foil, etc.

[0047] Negative electrode

[0048] The negative electrode sheet includes a negative electrode current collector and a negative electrode active material layer provided on the surface of the negative electrode current collector. The negative electrode current collector is generally a structure or component for collecting current, and the negative electrode current collector can be various materials suitable for use as the negative electrode current collector of a lithium-ion battery in the art. For example, the negative electrode current collector can include, but is not limited to, metal foils, etc., and more specifically can include, but is not limited to, copper foils, etc.

[0049] Electrolyte

[0050] The lithium-ion battery further includes an electrolyte, which includes an organic solvent, an electrolyte lithium salt, and an additive. Among them, the electrolyte lithium salt can be LiPF6 and / or LiBOB used in high-temperature electrolytes; it can also be at least one of LiBF4, LiBOB, and LiPF6 used in low-temperature electrolytes; it can also be at least one of LiBF4, LiBOB, LiPF6, and LiTFSI used in overcharge-preventing electrolytes; it can also be at least one of LiClO4, LiAsF6, LiCF3SO3, and LiN(CF3SO2)2. The organic solvent can be a cyclic carbonate, including PC and EC; it can also be a chain carbonate, including DFC, DMC, or EMC; it can also be a carboxylic acid ester, including MF, MA, EA, MP, etc. The additive includes at least one of, but is not limited to, a film-forming additive, a conductive additive, a flame-retardant additive, an overcharge-preventing additive, an additive for controlling the content of H2O and HF in the electrolyte, an additive for improving low-temperature performance, and a multifunctional additive.

[0051] The separator can be various materials suitable for use as the separator of a lithium-ion battery in the art. For example, it can be a combination of one or more of, but is not limited to, polyethylene, polypropylene, polyvinylidene fluoride, aramid, polyethylene terephthalate, polytetrafluoroethylene, polyacrylonitrile, polyimide, polyamide, polyester, and natural fibers, etc.

[0052] Preferably, the material of the housing is one of stainless steel and aluminum plastic film. More preferably, the housing is an aluminum plastic film.

[0053] Example 1

[0054] The first step: Preparation of manganese pyrophosphate pre-coated graphite negative electrode;

[0055] Weigh the required masses of manganese disodium ethylene diamine tetraacetate (organic manganese source) and ammonium dihydrogen phosphate (phosphorus source) according to the molar ratio of manganese element to phosphorus element of 1:1 and place them in a certain amount of deionized water. Use ultrasound to accelerate their dissolution rate; weigh 5 g of graphite according to the mass ratio of graphite: manganese pyrophosphate = 1:0.03 and pour it into the above solution to obtain a mixed solution. At room temperature, stir at a speed of 320 rpm / min for 11 h to evenly disperse the graphite material; conduct vacuum filtration on the mixture through a polytetrafluoroethylene membrane to obtain a mixture, and then vacuum dry it in an oven at 78 °C for 10 h.

[0056] Step 2: Prepare a graphite anode coated with manganese pyrophosphate;

[0057] Transfer the pre-coated graphite dried above into a tube furnace. Under a N2 (non-oxidizing gas) atmosphere of 5 L / min, raise the temperature to 200 °C at a heating rate of 8 °C / min, and then raise the temperature to 950 °C at a heating rate of 3 °C / min, hold for 8 h, and cool down naturally. After crushing the obtained sample, perform 150-mesh screening to finally obtain a graphite anode material coated with manganese pyrophosphate. The coating rate of the manganese pyrophosphate material layer is 85%, and the thickness of the manganese pyrophosphate material layer is 80 nm.

[0058] Step 3: Make a slurry from the manganese pyrophosphate-coated graphite anode material prepared above, conductive agent Super-P, thickening agent sodium carboxymethyl cellulose (CMC), and binder styrene-butadiene rubber (SBR) according to the mass ratio of 96:2.0:1.0:1.0. Coat the slurry on the current collector copper foil and dry it at 85 °C, then perform edge cutting, slicing, and striping. After striping, dry it at 110 °C for 4 h under vacuum conditions, and weld the electrode tabs to make a negative electrode sheet.

[0059] Step 4: Mix lithium cobaltate, conductive agent Super-P, and binder polyvinylidene fluoride (PVDF) evenly according to the mass ratio of 97:1.5:1.5 to make a lithium-ion battery positive electrode slurry with a certain viscosity. Coat the slurry on the current collector aluminum foil, dry it at 85 °C and then perform cold pressing; then perform edge cutting, slicing, and striping. After striping, dry it at 110 °C for 4 h under vacuum conditions, and weld the electrode tabs to make a positive electrode sheet.

[0060] Step 5: Dissolve lithium hexafluorophosphate (LiPF6) in a mixed solvent composed of ethylene carbonate (EC), dimethyl carbonate (DMC), and ethyl methyl carbonate (EMC) (the mass ratio of the three is 1:2:1) to obtain an electrolyte with a concentration of 1 mol / L.

[0061] Step 6: Wind the above positive electrode sheet, separator, and negative electrode sheet into an electrode core, with the separator located between the positive electrode sheet and the negative electrode sheet. The positive electrode is led out by spot welding with an aluminum tab, and the negative electrode is led out by spot welding with a nickel tab. Then, place the electrode core in an aluminum-plastic packaging bag, inject the above electrolyte, and through processes such as encapsulation, formation, and capacity measurement, a lithium-ion battery is manufactured.

[0062] Example 2

[0063] The difference from Example 1 is that the coating rate of the manganese pyrophosphate material layer is 60%.

[0064] The rest is the same as in Example 1 and will not be elaborated here.

[0065] Example 3

[0066] The difference from Example 1 is that the coating rate of the manganese pyrophosphate material layer is 70%.

[0067] The rest is the same as in Example 1 and will not be elaborated here.

[0068] Example 4

[0069] The difference from Example 1 is that the coating rate of the manganese pyrophosphate material layer is 80%.

[0070] The rest is the same as in Example 1 and will not be elaborated here.

[0071] Example 5

[0072] The difference from Example 1 is that the coating rate of the manganese pyrophosphate material layer is 90%.

[0073] The rest is the same as in Example 1 and will not be elaborated here.

[0074] Example 6

[0075] The difference from Example 1 is that the coating rate of the manganese pyrophosphate material layer is 95%.

[0076] The rest is the same as in Example 1 and will not be elaborated here.

[0077] Example 7

[0078] The difference from Example 1 is that the thickness of the manganese pyrophosphate material layer is 10 nm.

[0079] The rest is the same as in Example 1 and will not be elaborated here.

[0080] Example 8

[0081] The difference from Example 1 is that the thickness of the manganese pyrophosphate material layer is 60 nm.

[0082] The rest is the same as in Example 1 and will not be elaborated here.

[0083] Example 9

[0084] The difference from Example 1 is that the thickness of the manganese pyrophosphate material layer is 100 nm.

[0085] The rest is the same as in Example 1 and will not be elaborated here.

[0086] Example 10

[0087] The difference from Example 1 is that the thickness of the manganese pyrophosphate material layer is 120 nm.

[0088] The rest is the same as in Example 1 and will not be elaborated here.

[0089] Comparative Example 1

[0090] The difference from Example 1 is that the negative electrode material is graphite and there is no manganese pyrophosphate material layer on the surface.

[0091] The rest is the same as in Example 1 and will not be elaborated here.

[0092] Performance test: The batteries of Examples 1-10 and Comparative Example 1 were subjected to performance tests, and the test results are shown in Table 1.

[0093] Table 1

[0094]

[0095]

[0096] It can be seen from Table 1 above that the negative electrode material prepared by the present invention has a better capacity retention rate compared with the secondary battery of the prior art. The initial capacity retention rate is as high as 93.5%, and the capacity retention rate remains at 81.3% after 500 charge and discharge cycles. Moreover, it can be concluded from the comparison of Examples 1-6 that when the coating rate of the manganese pyrophosphate material layer is set to 85%, the performance of the negative electrode material prepared for use in a secondary battery is better. The manganese pyrophosphate material layer can maintain and improve the stability of graphite, so that the negative electrode material will not undergo structural changes during rapid charge and discharge, resulting in poor performance. Moreover, setting a certain coating rate of the manganese pyrophosphate material layer can avoid too high a coating rate from affecting the performance of the graphite material, and also avoid too low a coating rate where the manganese pyrophosphate does not provide enough ion channels and the rate performance improvement is limited.

[0097] It can be concluded from the comparison of Examples 1, 7-10 that when the thickness of the manganese pyrophosphate material layer is set to 80 nm, the performance of the negative electrode material prepared for use in a secondary battery is better. If the thickness of the manganese pyrophosphate material layer is too thick, it will affect the performance of graphite. If the thickness of the manganese pyrophosphate material layer is too thin, it is likely to fall off when in-situ generated on the surface of graphite, and the structure is not very stable after multiple cycles.

[0098] Based on the disclosure and teachings of the above specification, those skilled in the art to which the present invention pertains are also able to make changes and modifications to the above embodiments. Therefore, the present invention is not limited to the above specific embodiments, and any obvious improvements, substitutions, or variations made by those skilled in the art based on the present invention fall within the protection scope of the present invention. In addition, although some specific terms are used in this specification, these terms are only for convenience of description and do not constitute any limitation to the present invention.

Claims

1. A negative electrode material, characterized in that, It includes graphite and a manganese pyrophosphate material layer coated on the surface of the graphite. The coating rate of the manganese pyrophosphate material layer is 60% - 95%, and the thickness of the manganese pyrophosphate material layer is 10 - 120 nm; The preparation of the negative electrode material includes the following steps: Step S1: Add a manganese source and a phosphorus source to a solvent and mix and stir to obtain a mixed solution. The manganese source is an organic substance and contains C - H bonds; Step S2: Add graphite to the above - mentioned mixed solution, stir and disperse the mixed solution, filter it under vacuum, heat it, and dry it under vacuum to obtain a pre - product; Step S3: Heat - sinter the pre - product in an inert atmosphere. During heating, the C - H bonds of the manganese source react with the C - H bonds on the surface of the graphite to generate C - C bonds, and a manganese pyrophosphate material layer is in - situ generated on the surface of the graphite. The manganese pyrophosphate material layer is evenly dispersed on the surface of the graphite. Cool down and crush to obtain a manganese pyrophosphate - coated graphite negative electrode material.

2. The negative electrode material according to claim 1, characterized in that, The weight part ratio of the manganese source to the phosphorus source is 1 - 3:1 - 5.

3. The negative electrode material according to claim 1, characterized in that, The pre - product is obtained by vacuum drying at a temperature of 70 - 80 °C for 5 - 15 h.

4. The negative electrode material according to claim 1, wherein In step S3, the heating process is specifically to first increase the temperature at a rate of 5 - 10 °C / min to 150 - 300 °C, and then increase the temperature at a rate of 2 - 5 °C / min to 900 - 1000 °C, and maintain the temperature for 4 - 12 h.

5. The negative electrode material according to claim 1, characterized in that, The particle size of the crushed negative electrode material is 100 - 250 mesh.

6. A negative electrode plate, characterized in that, It includes the negative electrode material according to any one of claims 1 - 5.

7. A secondary battery, characterized in that, It includes the negative electrode plate according to claim 6.

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