Positive active material, positive slurry, positive plate and lithium ion battery

Abstract

The invention relates to a positive active material, positive slurry, a positive plate and a lithium ion battery, and belongs to the technical field of lithium ion batteries. The positive active material comprises the lithium iron phosphate and the lithium vanadium phosphate, has the advantages of the lithium iron phosphate and the lithium vanadium phosphate, and can not only improve the energy density of the battery, but also improve the cycle performance of the battery when being used for the lithium ion battery. By screening the particle sizes of the lithium vanadium phosphate and the lithium iron phosphate, the large-particle lithium vanadium phosphate can reduce the occurrence of side reactions on the surface of the material and reduce agglomeration; the small-particle lithium iron phosphate can shorten the transmission path of lithium ions, and the rate capability is improved while the cycle performance is improved. The lithium vanadium phosphate and the lithium iron phosphate with specific particle sizes can generate a synergistic effect, so that the agglomeration of the lithium vanadium phosphate in the charge-discharge process is reduced, and the cycle and rate performance is improved, thereby having relatively high energy density and relatively good comprehensive performance.

Description

Technical Field

[0001] The invention relates to a positive electrode active material, a positive electrode slurry, a positive electrode sheet and a lithium ion battery, belonging to the technical field of lithium ion batteries. Background Art

[0002] Energy crisis has become an important problem facing mankind in the 21st century, and it is also one of the problems that must be solved. Since its introduction, lithium-ion batteries have received widespread attention and become a hot topic of research due to their high energy density, high output voltage, low self-discharge, no memory effect, long service life, and green environmental protection. In particular, in 1990, Sony Corporation of Japan first launched commercial lithium-ion batteries. Since then, lithium-ion batteries have entered an era of rapid development.

[0003] Lithium-ion batteries mainly include positive electrodes, negative electrodes, electrolytes, and separators, among which the positive electrode is one of the main factors affecting the performance of lithium-ion batteries, such as the battery's energy density, safety performance, and cycle performance. In addition, since the positive electrode accounts for a large proportion of the battery, its cost also directly determines the cost of the battery. Therefore, the choice of the positive electrode is crucial to the performance and cost of the battery.

[0004] Positive electrode materials are mainly divided into three categories according to their crystal structure: layered oxide LiMO2 (M is transition metal oxide Ni, Co, Mn, etc.); spinel structure materials, such as LiMn2O4, etc.; polyanion structure materials, mainly olivine-type LiFePO4, LiMnPO4, etc. and NASCION-type Li2V3(PO4)3 and other materials.

[0005] At present, the most widely used positive electrode materials are layered materials LiCoO2, nickel-cobalt-manganese ternary materials, and olivine-type LiFePO4 materials. As the earliest layered positive electrode material, LiCoO2 has the characteristics of high operating voltage, high compaction density, and mature preparation process. It is currently widely used in portable 3C products. However, LiCoO2 has a low capacity, high cobalt toxicity, and is expensive. Compared with LiCoO2, nickel-cobalt-manganese ternary layered materials have higher reversible capacity, better cycle performance, and lower cost, but the biggest problem with nickel-cobalt-manganese ternary layered materials is safety, and the safety issues that exist cannot be completely solved at present.

[0006] Olivine-type LiFePO4 is widely favored because of its advantages such as stable structure, good cyclability, low price and high safety. However, the platform voltage and gram capacity of lithium iron phosphate materials are low, which seriously affects the energy density of the battery and limits the application of lithium iron phosphate materials. Monoclinic Li2V3(PO4)3 with NASCION configuration, which is also a polyanion structure, can completely release 3 lithium ions in the voltage range of 3.0-4.8V, corresponding to four voltage platforms. The release of the first lithium corresponds to two voltage platforms of 3.60V and 3.68V, the voltage platform corresponding to the release of the second lithium is 4.08V, and the voltage platform corresponding to the release of the third lithium is 4.53V. Compared with lithium iron phosphate, the voltage platform of lithium vanadium phosphate is much higher than that of lithium iron phosphate, which can greatly improve the energy density of the battery. However, lithium vanadium phosphate is prone to agglomeration during the charge and discharge process, and the cycle performance is poor. Summary of the invention

[0007] The object of the present invention is to provide a positive electrode active material, which can solve the problem that lithium vanadium phosphate positive electrode materials are prone to agglomeration during charge and discharge and have poor cycle performance.

[0008] The second object of the present invention is to provide a positive electrode slurry.

[0009] The third object of the present invention is to provide a positive electrode sheet.

[0010] A fourth object of the present invention is to provide a lithium ion battery.

[0011] In order to achieve the above objectives, the technical solution adopted by the positive electrode active material of the present invention is:

[0012] A positive electrode active material comprises lithium iron phosphate and lithium vanadium phosphate; the particle size of the lithium iron phosphate is 0.5-1.5 μm, and the particle size of the lithium vanadium phosphate is 3-6 μm.

[0013] The positive electrode active material of the present invention includes lithium iron phosphate and lithium vanadium phosphate, which have the advantages of both lithium iron phosphate and lithium vanadium phosphate. When used in lithium-ion batteries, it can not only improve the energy density of the battery, but also improve the cycle performance of the battery. By screening the particle size of lithium vanadium phosphate and lithium iron phosphate, large particles of lithium vanadium phosphate can reduce the occurrence of side reactions on the surface of the material and reduce agglomeration; small particles of lithium iron phosphate can shorten the lithium ion transmission path, improve the cycle performance and improve the rate performance. Lithium vanadium phosphate and lithium iron phosphate of specific particle size can produce a synergistic effect, reduce the agglomeration of lithium vanadium phosphate during the charge and discharge process, and improve the cycle and rate performance, thereby having both high energy density and good comprehensive performance.

[0014] Preferably, the mass ratio of the lithium iron phosphate to the lithium vanadium phosphate is (1-9):(1-9). Further preferably, the mass ratio of the lithium iron phosphate to the lithium vanadium phosphate is (3-7):(3-7).

[0015] Preferably, the positive electrode active material is a mixture of lithium iron phosphate and lithium vanadium phosphate.

[0016] Preferably, the mixing is ball milling mixing.

[0017] The technical solution adopted by the positive electrode slurry of the present invention is:

[0018] A positive electrode slurry comprises a conductive agent, a binder and the positive electrode active material as described above.

[0019] When the positive electrode slurry of the present invention is used to prepare a lithium ion battery, the energy density of the battery can be increased and the cycle performance of the battery can be improved.

[0020] Preferably, the conductive agent is selected from one or any combination of acetylene black, conductive carbon black, graphite, Kogen black, carbon nanotubes, carbon nanofibers, and graphene. For example, the conductive carbon black is SUPERLI; the graphite is KS-6; and the carbon nanofiber is VGCF.

[0021] Preferably, the binder is a polyvinylidene fluoride binder. Preferably, the weight average molecular weight of the polyvinylidene fluoride binder is 500,000 to 1.5 million.

[0022] Preferably, the mass ratio of the positive electrode active material, the conductive agent and the binder is (90-98):(1-5):(1-5). Further preferably, the mass ratio of the positive electrode active material, the conductive agent and the binder is 95.5:2:2.5. When the mass ratio of the positive electrode active material, the conductive agent and the binder meets the above range, the requirements of the electrode sheet adhesion can be met and the conductivity of the electrode sheet can be ensured. On this basis, the high active material content helps to improve the battery energy density.

[0023] Preferably, the positive electrode slurry further comprises a solvent. Preferably, the solvent is N-methylpyrrolidone.

[0024] A positive electrode sheet comprises a current collector and a positive electrode material layer arranged on the current collector, wherein the positive electrode material layer is made of the positive electrode slurry.

[0025] When the positive electrode sheet of the present invention is used to prepare a lithium ion battery, the energy density and the cycle performance of the battery can be improved simultaneously.

[0026] Preferably, the positive electrode material layer is obtained by coating the positive electrode slurry on a positive electrode current collector, followed by drying and roll pressing. Preferably, the positive electrode current collector is Al foil.

[0027] A lithium ion battery comprises a negative electrode sheet, a separator, an electrolyte and the positive electrode sheet as described above.

[0028] The lithium ion battery of the present invention has higher energy density and better cycle performance.

[0029] Preferably, the negative electrode sheet comprises a negative electrode current collector and a negative electrode material layer disposed on the negative electrode current collector, the negative electrode material layer is made of negative electrode slurry, and the negative electrode slurry comprises a negative electrode active material, a conductive agent, a thickener and a binder.

[0030] Preferably, the negative electrode active material is selected from one or any combination of artificial graphite, natural graphite, soft carbon, hard carbon, and silicon carbon.

[0031] Preferably, the conductive agent in the negative electrode slurry is conductive carbon black.

[0032] Preferably, the thickener in the negative electrode slurry is sodium carboxymethyl cellulose thickener.

[0033] Preferably, the binder in the negative electrode slurry is a styrene-butadiene rubber binder.

[0034] Preferably, in the negative electrode slurry, the mass ratio of the negative electrode active material, the conductive agent, the thickener and the binder is 95.3:1.5:1.4:1.8. Preferably, the negative electrode current collector is Cu foil.

[0035] Preferably, the diaphragm is selected from one or any combination of polypropylene diaphragm, polyethylene diaphragm, ceramic diaphragm, non-woven diaphragm, polypropylene and polyethylene composite ceramic diaphragm. The diaphragm can be obtained commercially, for example, the manufacturers of ceramic diaphragm include Cangzhou Mingzhu, Xingyuan Materials, etc., and the product model can be determined according to the specific width and thickness; the manufacturers of non-woven diaphragm include Mitsubishi Paper, etc.; the manufacturers of polypropylene and polyethylene composite ceramic diaphragm include Cangzhou Mingzhu, Xingyuan Materials, Yi Teng, etc.

[0036] Preferably, the electrolyte is composed of an organic solvent, an electrolyte salt and a functional additive, and the functional additive is selected from one or any combination of vinylene carbonate, fluoroethylene carbonate, 1,3-propane sultone, vinyl sulfate, propylene sultone, lithium bis(oxalate borate), lithium difluoro(oxalate borate), lithium difluorobis(oxalate phosphate), lithium tetrafluoro(oxalate phosphate), lithium tetrafluoroborate, methylene disulfonate, tris(trimethylsilane) borate, and tris(trimethylsilane) phosphate. Further preferably, the functional additive is composed of vinylene carbonate, vinyl sulfate and lithium difluoro(oxalate borate). Preferably, the mass ratio of vinylene carbonate, vinyl sulfate and lithium difluoro(oxalate borate) is 2:1:1. Preferably, the mass fraction of the functional additive in the electrolyte is 2 to 6%. For example, the mass fraction of the functional additive in the electrolyte is 4%.

[0037] Preferably, the organic solvent is selected from one or any combination of ethylene carbonate, diethyl carbonate, dimethyl carbonate, propylene carbonate, ethyl methyl carbonate, methyl formate, methyl acetate, ethyl acetate, ethyl butyrate, and methyl butyrate. Further preferably, the organic solvent consists of ethylene carbonate, ethyl methyl carbonate, and diethyl carbonate. Preferably, the volume ratio of ethylene carbonate, ethyl methyl carbonate, and diethyl carbonate is (2-4):(4-6):(1-3). For example, the volume ratio of ethylene carbonate, ethyl methyl carbonate, and diethyl carbonate is 3:5:2.

[0038] Preferably, the electrolyte salt is lithium hexafluorophosphate. Preferably, the concentration of the electrolyte salt in the electrolyte is 1.2 mol / L. DETAILED DESCRIPTION

[0039] The technical solution of the present invention is further described below in conjunction with specific embodiments.

[0040] 1. Specific embodiments of the positive electrode active material of the present invention are as follows:

[0041] Example 1

[0042] The positive electrode active material of this embodiment is composed of lithium iron phosphate and lithium vanadium phosphate in a mass ratio of 1:9. The particle size of the lithium iron phosphate is 0.5-1.5 μm, and the particle size of the lithium vanadium phosphate is 3-6 μm.

[0043] Example 2

[0044] The positive electrode active material of this embodiment is composed of lithium iron phosphate and lithium vanadium phosphate in a mass ratio of 3:7. The particle size of the lithium iron phosphate is 0.5-1.5 μm, and the particle size of the lithium vanadium phosphate is 3-6 μm.

[0045] Example 3

[0046] The positive electrode active material of this embodiment is composed of lithium iron phosphate and lithium vanadium phosphate in a mass ratio of 5:5. The particle size of the lithium iron phosphate is 0.5-1.5 μm, and the particle size of the lithium vanadium phosphate is 3-6 μm.

[0047] Example 4

[0048] The positive electrode active material of this embodiment is composed of lithium iron phosphate and lithium vanadium phosphate in a mass ratio of 7:3. The particle size of the lithium iron phosphate is 0.5-1.5 μm, and the particle size of the lithium vanadium phosphate is 3-6 μm.

[0049] Example 5

[0050] The positive electrode active material of this embodiment is composed of lithium iron phosphate and lithium vanadium phosphate in a mass ratio of 9:1. The particle size of the lithium iron phosphate is 0.5-1.5 μm, and the particle size of the lithium vanadium phosphate is 3-6 μm.

[0051] 2. The specific embodiments of the positive electrode slurry of the present invention are as follows:

[0052] Example 6

[0053] The positive electrode slurry of this embodiment is composed of N-methylpyrrolidone, the positive electrode active material of Example 1, a conductive agent and a binder; the conductive agent is conductive carbon black; the binder is a polyvinylidene fluoride binder; the mass ratio of the positive electrode active material, the conductive agent, and the binder is 95.5:2:2.5. The viscosity of the positive electrode slurry of this embodiment is 5000mPa·s.

[0054] Example 7

[0055] The difference between the positive electrode slurry of this embodiment and the positive electrode slurry of Embodiment 6 is that the positive electrode active material used in the positive electrode slurry of this embodiment is the positive electrode active material of Embodiment 2.

[0056] Example 8

[0057] The difference between the positive electrode slurry of this embodiment and the positive electrode slurry of Embodiment 6 is that the positive electrode active material used in the positive electrode slurry of this embodiment is the positive electrode active material of Embodiment 3.

[0058] Example 9

[0059] The positive electrode slurry of this embodiment is different from the positive electrode slurry of Embodiment 6 only in that the positive electrode active material used in the positive electrode slurry of this embodiment is the positive electrode active material of Embodiment 4.

[0060] Example 10

[0061] The positive electrode slurry of this embodiment is different from the positive electrode slurry of Embodiment 6 only in that the positive electrode active material used in the positive electrode slurry of this embodiment is the positive electrode active material of Embodiment 5.

[0062] Embodiment 11

[0063] The positive electrode slurry of this embodiment is different from the positive electrode slurry of Embodiment 6 only in that the mass ratio of the positive electrode active material, the conductive agent, and the binder of this embodiment is 90:5:5.

[0064] Example 12

[0065] The positive electrode slurry of this embodiment is different from the positive electrode slurry of Embodiment 6 only in that the mass ratio of the positive electrode active material, the conductive agent, and the binder of this embodiment is 98:1:1.

[0066] 3. The specific embodiments of the positive electrode sheet of the present invention are as follows:

[0067] Embodiment 13

[0068] The positive electrode sheet of this embodiment includes a positive electrode current collector and a positive electrode material layer disposed on the current collector. The positive electrode material layer is obtained by uniformly coating the positive electrode slurry of Example 6 on the positive electrode current collector, followed by drying and roll pressing. The positive electrode current collector is Al foil.

[0069] Embodiment 14

[0070] The positive electrode sheet of this embodiment includes a positive electrode current collector and a positive electrode material layer arranged on the current collector. The positive electrode material layer is obtained by uniformly coating the positive electrode slurry of Example 7 on the positive electrode current collector, followed by drying and roll pressing. The positive electrode current collector is Al foil.

[0071] Embodiment 15

[0072] The positive electrode sheet of this embodiment includes a positive electrode current collector and a positive electrode material layer arranged on the current collector. The positive electrode material layer is obtained by uniformly coating the positive electrode slurry of Example 8 on the positive electrode current collector, followed by drying and roll pressing. The positive electrode current collector is Al foil.

[0073] Example 16

[0074] The positive electrode sheet of this embodiment includes a positive electrode current collector and a positive electrode material layer arranged on the current collector. The positive electrode material layer is obtained by uniformly coating the positive electrode slurry of Example 9 on the positive electrode current collector, followed by drying and roll pressing. The positive electrode current collector is Al foil.

[0075] Embodiment 17

[0076] The positive electrode sheet of this embodiment includes a positive electrode current collector and a positive electrode material layer arranged on the current collector. The positive electrode material layer is obtained by uniformly coating the positive electrode slurry of Example 10 on the positive electrode current collector, followed by drying and roll pressing. The positive electrode current collector is Al foil.

[0077] Embodiment 18

[0078] The positive electrode sheet of this embodiment includes a positive electrode current collector and a positive electrode material layer arranged on the current collector. The positive electrode material layer is obtained by uniformly coating the positive electrode slurry of Example 11 on the positive electrode current collector, followed by drying and roll pressing. The positive electrode current collector is Al foil.

[0079] Embodiment 19

[0080] The positive electrode sheet of this embodiment includes a positive electrode current collector and a positive electrode material layer arranged on the current collector. The positive electrode material layer is obtained by uniformly coating the positive electrode slurry of Example 12 on the positive electrode current collector, followed by drying and roll pressing. The positive electrode current collector is Al foil.

[0081] 4. The specific embodiments of the lithium ion battery of the present invention are as follows:

[0082] Embodiment 20

[0083] The lithium-ion battery of this embodiment includes a positive electrode sheet, a negative electrode sheet, a separator and an electrolyte;

[0084] The positive electrode sheet is the positive electrode sheet of Example 13;

[0085] The negative electrode sheet is obtained by uniformly coating the negative electrode slurry on the negative electrode current collector, followed by drying and roller pressing. The negative electrode slurry consists of a negative electrode active material, a conductive agent, a thickener and a binder. The negative electrode active material is artificial graphite, the conductive agent is conductive carbon black, the thickener is sodium carboxymethyl cellulose thickener (manufacturer is Daisiro, model is CMC2200), the binder is styrene-butadiene rubber binder, and the mass ratio of the negative electrode active material, the conductive agent, the thickener and the binder is 95.3:1.5:1.4:1.8; the negative electrode current collector is Cu foil;

[0086] The diaphragm is a ceramic diaphragm;

[0087] The electrolyte is composed of an organic solvent, an electrolyte salt and a functional additive, wherein the organic solvent is composed of ethylene carbonate, ethyl methyl carbonate and diethyl carbonate in a volume ratio of 3:5:2, the electrolyte salt is lithium hexafluorophosphate, and the functional additive is composed of vinylene carbonate, vinyl sulfate and lithium difluorooxalatoborate in a mass ratio of 2:1:1, the concentration of the electrolyte salt in the electrolyte is 1.2 mol / L, and the mass fraction of the functional additive in the electrolyte is 4%;

[0088] The lithium ion battery of this embodiment is prepared by a method comprising the following steps:

[0089] The positive electrode sheet, the separator and the negative electrode sheet are stacked to form a battery cell assembly, which is then placed in a shell and then injected with liquid electrolyte.

[0090] Embodiment 21

[0091] The only difference between the lithium ion battery of this embodiment and the lithium ion battery of Example 20 is that the positive electrode sheet used in the lithium ion battery of this embodiment is the positive electrode sheet of Example 14.

[0092] Embodiment 22

[0093] The only difference between the lithium ion battery of this embodiment and the lithium ion battery of Example 20 is that the positive electrode sheet used in the lithium ion battery of this embodiment is the positive electrode sheet of Example 15.

[0094] Embodiment 23

[0095] The only difference between the lithium ion battery of this embodiment and the lithium ion battery of Example 20 is that the positive electrode sheet used in the lithium ion battery of this embodiment is the positive electrode sheet of Example 16.

[0096] Embodiment 24

[0097] The only difference between the lithium ion battery of this embodiment and the lithium ion battery of Example 20 is that the positive electrode sheet used in the lithium ion battery of this embodiment is the positive electrode sheet of Example 17.

[0098] Embodiment 25

[0099] The only difference between the lithium ion battery of this embodiment and the lithium ion battery of embodiment 20 is that the positive electrode sheet used in the lithium ion battery of this embodiment is the positive electrode sheet of embodiment 18.

[0100] Embodiment 26

[0101] The only difference between the lithium ion battery of this embodiment and the lithium ion battery of embodiment 20 is that the positive electrode sheet used in the lithium ion battery of this embodiment is the positive electrode sheet of embodiment 19.

[0102] Comparative Example 1

[0103] The lithium ion battery of this comparative example includes a positive electrode sheet, a negative electrode sheet, a separator and an electrolyte; the negative electrode sheet is the same as the negative electrode sheet used in the lithium ion battery of Example 13, the separator is the same as the separator used in the lithium ion battery of Example 20, and the electrolyte is the same as the electrolyte used in the lithium ion battery of Example 20; the positive electrode sheet includes a positive electrode collector and a positive electrode material layer arranged on the current collector, the positive electrode material layer is obtained by uniformly coating the positive electrode slurry on the positive electrode collector and then drying and rolling in sequence, the positive electrode slurry is composed of N-methylpyrrolidone, a positive electrode active material, a conductive agent and a binder, the positive electrode active material is lithium vanadium phosphate, the particle size of the lithium vanadium phosphate is 3 to 6 μm, the conductive agent is conductive carbon black (the same as the conductive carbon black in Example 6), the binder is polyvinylidene fluoride binder (the same as the polyvinylidene fluoride binder in Example 6), and the mass ratio of the positive electrode active material, the conductive agent and the binder is 95.5:2:2.5.

[0104] Comparative Example 2

[0105] The lithium ion battery of this comparative example is different from the lithium ion battery of comparative example 1 only in that the positive electrode active material used in the lithium ion battery of this comparative example is lithium iron phosphate, and the particle size of the lithium iron phosphate is 0.5 to 1.5 μm.

[0106] Comparative Example 3

[0107] The only difference between the lithium ion battery of this comparative example and the lithium ion battery of comparative example 1 is that the positive electrode active material used in the lithium ion battery of this comparative example is composed of lithium iron phosphate and lithium vanadium phosphate in a mass ratio of 1:9, the particle size of the lithium iron phosphate is 0.5-1.5 μm, and the particle size of the lithium vanadium phosphate is 1-3 μm.

[0108] Comparative Example 4

[0109] The only difference between the lithium ion battery of this comparative example and the lithium ion battery of comparative example 1 is that the positive electrode active material used in the lithium ion battery of this comparative example is composed of lithium iron phosphate and lithium vanadium phosphate in a mass ratio of 1:9, the particle size of the lithium iron phosphate is 0.5-1.5 μm, and the particle size of the lithium vanadium phosphate is 6-9 μm.

[0110] Comparative Example 5

[0111] The only difference between the lithium ion battery of this comparative example and the lithium ion battery of comparative example 1 is that the positive electrode active material used in the lithium ion battery of this comparative example is composed of lithium iron phosphate and lithium vanadium phosphate in a mass ratio of 1:9, the particle size of the lithium iron phosphate is 0.5-1.5 μm, and the particle size of the lithium vanadium phosphate is 1-8 μm.

[0112] Comparative Example 6

[0113] The only difference between the lithium ion battery of this comparative example and the lithium ion battery of comparative example 1 is that the positive electrode active material used in the lithium ion battery of this comparative example is composed of lithium iron phosphate and lithium vanadium phosphate in a mass ratio of 1:9, the particle size of the lithium iron phosphate is 0.1-0.5 μm, and the particle size of the lithium vanadium phosphate is 3-6 μm.

[0114] Comparative Example 7

[0115] The only difference between the lithium ion battery of this comparative example and the lithium ion battery of comparative example 1 is that the positive electrode active material used in the lithium ion battery of this comparative example consists of lithium iron phosphate and lithium vanadium phosphate in a mass ratio of 1:9, the particle size of the lithium iron phosphate is 1.5-2μm, and the particle size of the lithium vanadium phosphate is 3-6μm.

[0116] Comparative Example 8

[0117] The only difference between the lithium ion battery of this comparative example and the lithium ion battery of comparative example 1 is that the positive electrode active material used in the lithium ion battery of this comparative example consists of lithium iron phosphate and lithium vanadium phosphate in a mass ratio of 1:9, the particle size of the lithium iron phosphate is 0.8-1.2 μm, and the particle size of the lithium vanadium phosphate is 3-6 μm.

[0118] Comparative Example 9

[0119] The only difference between the lithium ion battery of this comparative example and the lithium ion battery of comparative example 1 is that the positive electrode active material used in the lithium ion battery of this comparative example consists of lithium iron phosphate and lithium vanadium phosphate in a mass ratio of 1:9, the particle size of the lithium iron phosphate is 0.1-2 μm, and the particle size of the lithium vanadium phosphate is 3-6 μm.

[0120] Comparative Example 10

[0121] The lithium ion battery of this comparative example is different from the lithium ion battery of comparative example 1 only in that the positive electrode active material used in the lithium ion battery of this comparative example is prepared according to Example 2 in patent document CN114300661A.

[0122] Experimental example

[0123] The lithium ion batteries of Examples 20-26 and Comparative Examples 1-10 were subjected to a room temperature cycle test of 1C charge / 1C discharge at 2.5-4.3V, and the initial discharge capacity and 100 cycle capacity retention rate of each lithium ion battery were recorded. The experimental results are shown in Table 1.

[0124] Table 1 Initial discharge capacity and 100 cycle capacity retention rate of lithium ion batteries of Examples 20-26 and Comparative Examples 1-10

[0125]

[0126]

[0127] The results show that by mixing lithium vanadium phosphate and lithium iron phosphate positive electrode materials of specific particle size to form a lithium vanadium phosphate / lithium iron phosphate composite positive electrode material, lithium vanadium phosphate and lithium iron phosphate play a synergistic role. Large particles of lithium vanadium phosphate can reduce the occurrence of side reactions on the surface of the material and reduce agglomeration. Small particles of lithium iron phosphate can shorten the lithium ion transmission path, which is beneficial to improving the cycle performance. It can overcome the shortcomings of a single material and obtain good comprehensive performance.

Claims

1. A positive electrode active material, characterized in that: It comprises lithium iron phosphate and lithium vanadium phosphate; the particle size of the lithium iron phosphate is 0.5-1.5 μm, and the particle size of the lithium vanadium phosphate is 3-6 μm.

2. The positive electrode active material according to claim 1, characterized in that The mass ratio of the lithium iron phosphate to the lithium vanadium phosphate is (1-9):(1-9).

3. The positive electrode active material according to claim 1 or 2, characterized in that The mass ratio of the lithium iron phosphate to the lithium vanadium phosphate is (3-7):(3-7).

4. A positive electrode slurry, characterized in that: The invention comprises a conductive agent, a binder and a positive electrode active material as claimed in any one of claims 1 to 3.

5. The positive electrode slurry according to claim 4, characterized in that The conductive agent is selected from one or any combination of acetylene black, conductive carbon black, graphite, Kogen black, carbon nanotubes, carbon nanofibers, and graphene; and the binder is a polyvinylidene fluoride binder.

6. The positive electrode slurry according to claim 5, characterized in that The mass ratio of the positive electrode active material, the conductive agent and the binder is (90-98):(1-5):(1-5).

7. A positive electrode sheet, characterized in that: The invention comprises a current collector and a positive electrode material layer arranged on the current collector, wherein the positive electrode material layer is made of the positive electrode slurry according to any one of claims 4 to 6.

8. A lithium ion battery, characterized in that: The invention comprises a negative electrode sheet, a separator, an electrolyte and the positive electrode sheet as claimed in claim 7.

9. The lithium ion battery according to claim 8, characterized in that The negative electrode sheet comprises a negative electrode current collector and a negative electrode material layer arranged on the negative electrode current collector, wherein the negative electrode material layer is made of a negative electrode slurry, and the negative electrode slurry comprises a negative electrode active material, a conductive agent, a thickener and a binder; the electrolyte is composed of an organic solvent, an electrolyte salt and a functional additive, wherein the functional additive is selected from one or any combination of vinylene carbonate, fluoroethylene carbonate, 1,3-propane sultone, vinyl sulfate, propylene sultone, lithium bis(oxalatoborate), lithium difluorooxalatoborate, lithium difluorobis(oxalatophosphate), lithium tetrafluorooxalatophosphate, lithium tetrafluoroborate, methylene methanedisulfonate, tris(trimethylsilane)borate and tris(trimethylsilane)phosphate; the organic solvent is selected from one or any combination of vinyl carbonate, diethyl carbonate, dimethyl carbonate, propylene carbonate, ethyl methyl carbonate, methyl formate, methyl acetate, ethyl acetate, ethyl butyrate and methyl butyrate.

10. The lithium ion battery according to claim 9, characterized in that The negative electrode active material is selected from one or any combination of artificial graphite, natural graphite, soft carbon, hard carbon, and silicon carbon; the functional additive is composed of vinylene carbonate, vinyl sulfate and lithium difluorooxalatoborate; the organic solvent is composed of vinyl carbonate, ethyl methyl carbonate and diethyl carbonate, and the volume ratio of vinyl carbonate, ethyl methyl carbonate and diethyl carbonate is (2-4):(4-6):(1-3); the electrolyte salt is lithium hexafluorophosphate.

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