A positive electrode material, a preparation method thereof, a positive electrode sheet, and a lithium ion battery

Abstract

The application discloses a positive electrode material and a preparation method thereof, a positive electrode sheet and a lithium ion battery; the positive electrode material comprises a lithium cobaltate material, a first coating layer coated on the surface of the lithium cobaltate material and a second coating layer coated on the surface of the first coating layer; the material of the first coating layer comprises LiPO3-Al(PO3)3, and the material of the second coating layer comprises TiO2-LiF. Wherein, the LiPO3-Al(PO3)3 as the first coating layer realizes effective stability of the surface structure of the LiCoO2 through the covalent bond effect between the LiPO3-Al(PO3)3 and the lithium cobaltate, and further provides a smoother transmission path for the electron, thereby effectively improving the cycle stability and the rate performance of the LiCoO2 material; the TiO2-LiF as the second coating layer not only enhances the resistance of the lithium cobaltate material to the electrolyte corrosion, but also further stabilizes the surface structure of the LiCoO2, thereby providing the battery with higher performance stability and service life.

Description

TECHNICAL FIELD

[0001] The present application relates to the field of lithium ion batteries, in particular to a positive electrode material, a preparation method thereof, a positive electrode sheet and a lithium ion battery. BACKGROUND

[0002] Lithium cobalt oxide (LiCoO2) is one of the earliest commercialized materials in the field of lithium ion battery positive electrode materials, and occupies an important position in many fields such as mobile electronic devices, electric vehicles and energy storage systems due to its high theoretical specific capacity and good electrochemical performance.

[0003] However, with the increasing demand for lithium ion battery performance, lithium cobalt oxide material has exposed several limiting factors in commercial applications, especially the performance degradation problem in high-rate charging and discharging and long-term cycling is increasingly prominent. These problems mainly manifest that in the process of continuous charging and discharging, lithium cobalt oxide material will undergo irreversible structural phase change, resulting in gradual attenuation of battery capacity. At the same time, a series of complex side reactions will occur on the surface of lithium cobalt oxide material, such as electrolyte decomposition, unstable formation of solid electrolyte interface (SEI) layer, etc. These problems not only further aggravate the performance degradation of the material, but also affect the safety and service life of the battery.

[0004] Therefore, it is necessary to develop a positive electrode material that can balance the rate performance of the battery while ensuring the cycle stability. SUMMARY

[0005] The present application aims at: in view of the shortcomings of the prior art, a positive electrode material can effectively improve the rate performance and cycle stability of the battery.

[0006] To achieve the above-mentioned purpose, the technical scheme adopted by the present application is as follows:

[0007] A positive electrode material, comprising a lithium cobalt oxide material, a first coating layer coated on the surface of the lithium cobalt oxide material, and a second coating layer coated on the surface of the first coating layer.

[0008] The first coating layer comprises LiPO3-Al(PO3)3, and the second coating layer comprises TiO2-LiF.

[0009] The first coating layer LiPO3-Al(PO3)3 and lithium cobalt oxide can effectively stabilize the surface structure of LiCoO2 through the action of covalent bond, optimize the electron transport path, and thus improve the cycle stability and rate performance of LiCoO2. The second coating layer TiO2-LiF has fewer holes that can be connected in series with each other, and the flow of electrolyte between the holes is hindered, thereby resisting the corrosion of electrolyte, stabilizing the surface structure of LiCoO2, and further improving the cycle stability and rate performance of lithium cobalt oxide in cooperation with the first coating layer.

[0010] Preferably, the mass ratio of the lithium cobalt oxide material to the first coating layer is 1:0.0004-0.0006. When the mass ratio is too small, the side reaction cannot be reduced, and the interface stability cannot be maintained; when the mass ratio is too large, the impedance will be increased, which is not conducive to long cycle.

[0011] Preferably, the mass ratio of the lithium cobalt oxide material to the second coating layer is 1:0.0001-0.0004. When the mass ratio is too small, the ability to resist electrolyte corrosion will be weakened; when the mass ratio is too large, the transmission of lithium ions will be affected, thereby affecting the electrochemical performance.

[0012] In addition, the application also provides a preparation method of the positive electrode material, comprising the following steps:

[0013] Step S1, mixing LiH2PO4 and Al(H2PO4)3 to obtain a mixture A, and adding the mixture A into water to obtain a solution A;

[0014] Step S2, dispersing LiCoO2 into the solution A, and after ultrasonic treatment, heating, drying and calcination, a pretreated lithium cobalt oxide is obtained;

[0015] Step S3, dispersing the pretreated lithium cobalt oxide in step S2 and TiF4 in a solution, and after ultrasonic treatment, heating, drying and calcination, the positive electrode material is obtained.

[0016] Preferably, in step S1, in order to obtain a suitable coating amount of the material, the mass ratio of LiH2PO4 to Al(H2PO4)3 is (1-2):1.

[0017] Preferably, in step S2, in order to obtain a suitable coating amount of the material, the mass ratio of the mixture A to LiCoO2 is (0.01-0.05):1.

[0018] Preferably, in step S2, two-stage temperature is adopted for calcination to make the coating more uniform, and the specific steps of calcination are as follows: first, high-temperature calcination at 280-350℃ for 2-3h, and then high-temperature calcination at 550-700℃ for 4-6h.

[0019] Preferably, in step S3, in order to obtain a suitable coating amount of the material, the mass ratio of the pretreated lithium cobalt oxide to TiF4 is 1:(0.01-0.05).

[0020] Preferably, in step S3, in order to provide a good coating temperature, the calcination temperature is 700-900℃, and the time is 7-9h.

[0021] In addition, the application also provides a positive electrode sheet comprising the above-mentioned positive electrode material.

[0022] Furthermore, the application also provides a lithium ion battery, which comprises an electric core wound by a negative electrode sheet, a positive electrode sheet and a diaphragm, and an electrolyte, and a shell for packaging the electric core and the electrolyte, wherein the positive electrode sheet is the positive electrode sheet.

[0023] Compared with the prior art, the application has the beneficial effects that: the application cooperates the surface structure of LiCoO2 by coating two layers of coating layers outside the lithium cobalt oxide, optimizes the transmission path of electrons, and thus improves the cycle stability and rate performance of lithium cobalt oxide. Among them, LiPO3-Al(PO3)3 as the first coating layer, through the covalent bond between lithium cobalt oxide, realizes the effective stability of the surface structure of LiCoO2, and also provides a more smooth transmission path for electrons, and thus effectively improves the cycle stability and rate performance of LiCoO2 material; TiO2-LiF as the second coating layer, further optimizes the surface structure and electron transmission path of the positive electrode material on the basis of LiPO3-Al(PO3)3; the TiO2-LiF coating layer not only enhances the resistance of the material to electrolyte corrosion, but also further stabilizes the surface structure of LiCoO2, and provides higher performance stability and service life for the battery. Therefore, through the synergistic effect of the two layers of coating layers, the lithium cobalt oxide positive electrode material of the application exhibits excellent cycle stability and rate performance. BRIEF DESCRIPTION OF DRAWINGS

[0024] Figure 1 It is a structure schematic diagram of the positive electrode material of an experimental example of the application.

[0025] Among them, 1-lithium cobalt oxide material; 2-first coating layer; 3-second coating layer. DETAILED DESCRIPTION

[0026] In order to make the technical scheme and advantages of the application clearer, the technical scheme of the application will be described clearly and completely below in combination with specific examples. Obviously, the described examples are some examples of the application, rather than all the examples. Based on the examples in the application, all other examples obtained by those skilled in the art without creative labor fall within the protection scope of the application.

[0027] According to the first aspect of the application, the application provides a positive electrode material, which comprises a lithium cobalt oxide material, a first coating layer coated on the surface of the lithium cobalt oxide material, and a second coating layer coated on the surface of the first coating layer; wherein the first coating layer comprises LiPO3-Al(PO3)3, and the second coating layer comprises TiO2-LiF.

[0028] In some examples, the mass ratio of the lithium cobalt oxide material to the first coating layer material is 1:0.0004-0.0006.

[0029] In some embodiments, the mass ratio of the lithium cobaltate material to the second coating layer material is 1:0.0001-0.0004.

[0030] In some embodiments, the mass ratio of the first coating layer material to the second coating layer material is 1:0.0001-0.0004.

[0031] According to a second aspect of the present application, the present application provides a preparation method of a positive electrode material, comprising the following steps:

[0032] Step S1, mixing LiH2PO4 and Al(H2PO4)3 to obtain a mixture A, and adding the mixture A into water to obtain a solution A;

[0033] Step S2, dispersing LiCoO2 into the solution A, and after ultrasonic treatment, heating, drying and calcination, a pretreated lithium cobaltate is obtained;

[0034] Step S3, dispersing the pretreated lithium cobaltate in step S2 and TiF4 into a solution, and after ultrasonic treatment, heating, drying and calcination, the positive electrode material is obtained.

[0035] In some embodiments, in step S1, the mass ratio of LiH2PO4 to Al(H2PO4)3 is (1-2):1, for example, it can be 1:1, 1.1:1, 1.2:1, 1.3:1, 1.4:1, 1.5:1, 1.6:1, 1.7:1, 1.8:1, 1.9:1, 2.0:1.

[0036] In some embodiments, in step S2, the mass ratio of the mixture A to LiCoO2 is (0.01-0.05):1, for example, it can be 0.01:1, 0.02:1, 0.03:1, 0.04:1, 0.05:1.

[0037] In some embodiments, in step S2, the specific step of calcination is: first calcination at a high temperature of 280-350℃ for 2-3h, and then calcination at a high temperature of 550-700℃ for 4-6h.

[0038] In some embodiments, in step S3, the mass ratio of the pretreated lithium cobaltate to TiF4 is 1:(0.01-0.05), for example, it can be 1:0.01, 1:0.02, 1:0.03, 1:0.04, 1:0.05.

[0039] In some embodiments, in step S3, the calcination temperature is 700-900℃, for example, it can be 700℃, 750℃, 800℃, 850℃, 900℃, and the time is 7-9h, for example, it can be 7h, 8h, 9h.

[0040] According to a third aspect of the present application, the present application provides a positive electrode sheet, comprising a positive electrode current collector and a positive electrode active material layer coated on at least one surface of the positive electrode current collector, wherein the positive electrode active material layer comprises the positive electrode material described above.

[0041] According to a fourth aspect of the present application, the present application provides a lithium ion battery, comprising an electric core wound by a negative electrode sheet, a positive electrode sheet and a separator, and an electrolyte, and a shell encapsulating the electric core and the electrolyte, wherein the positive electrode sheet is the positive electrode sheet described above.

[0042] The negative electrode sheet comprises a negative electrode current collector and a negative electrode active material layer coated on at least one surface of the negative electrode current collector. The negative electrode active material layer can be one or more of, but not limited to, graphite, soft carbon, hard carbon, carbon fiber, mesocarbon microbeads, silicon-based material, tin-based material, lithium titanate or other metals capable of forming alloys with lithium. The graphite can be selected from one or more of artificial graphite, natural graphite and modified graphite; the silicon-based material can be selected from one or more of elemental silicon, silicon oxide compound, silicon-carbon composite and silicon alloy; and the tin-based material can be selected from one or more of elemental tin, tin oxide compound and tin alloy. The negative electrode current collector is usually a structure or part for collecting current, and can be various materials suitable for use as a negative electrode current collector of a lithium ion battery in the art, for example, the negative electrode current collector can be one or more of, but not limited to, metal foil and the like, and more specifically can be one or more of, but not limited to, copper foil and the like.

[0043] The separator can be various materials suitable for use as a separator of a lithium ion battery in the art, for example, can be one or more of, but not limited to, a combination of polyethylene, polypropylene, polyvinylidene fluoride, aramid, polyethylene terephthalate, polytetrafluoroethylene, polyacrylonitrile, polyimide, polyamide, polyester and natural fiber and the like.

[0044] The lithium ion battery further comprises an electrolyte, which comprises an organic solvent, an electrolyte lithium salt and an additive. The electrolyte lithium salt can be LiPF6 and / or LiBOB used in high-temperature electrolyte; can be at least one of LiBF4, LiBOB and LiPF6 used in low-temperature electrolyte; can be at least one of LiBF4, LiBOB, LiPF6 and LiTFSI used in overcharge-preventing electrolyte; and can be at least one of LiClO4, LiAsF6, LiCF3SO3 and LiN(CF3SO2)2. The organic solvent can be cyclic carbonate, including PC and EC; can be chain carbonate, including DFC, DMC or EMC; and can be carboxylic acid ester, including MF, MA, EA and MP. The additive includes at least one of, but not limited to, film-forming additive, conductive additive, flame-retardant additive, overcharge-preventing additive, additive for controlling the content of H2O and HF in electrolyte, additive for improving low-temperature performance and multifunctional additive.

[0045] To make the technical solution and advantages of the present invention clearer, the present invention and its beneficial effects will be described in further detail below with reference to specific embodiments and accompanying drawings, but the embodiments of the present invention are not limited thereto.

[0046] Experimental Example 1

[0047] (1) Preparation of cathode materials:

[0048] like Figure 1 As shown, the positive electrode material provided in this embodiment includes a lithium cobalt oxide material 1 and a first coating layer 2 covering the outer surface of the lithium cobalt oxide material 1, and a second coating layer 3 covering the surface of the first coating layer 2, wherein the first coating layer 2 is LiPO3-Al(PO3)3 and the second coating layer 3 is TiO2-LiF.

[0049] The preparation method of the positive electrode material includes the following steps:

[0050] Step S1: Mix LiH2PO4 and Al(H2PO4)3 at a weight ratio of 1:1 to obtain mixture A, and add mixture A to water to obtain solution A;

[0051] Step S2: Disperse LiCoO2 into solution A, with a mass ratio of LiCoO2 to mixture A of 1:0.01. Sonicate for 35 min, transfer to an oil bath and heat at 85°C until viscous. Remove and dry completely in a vacuum drying oven. Then place in a muffle furnace and calcine at 320°C for 2.5 h, followed by calcination at 720°C for 5.5 h to obtain pretreated lithium cobalt oxide.

[0052] Step S3: Add the pretreated lithium cobalt oxide and TiF4 from step S2 to anhydrous ethanol solution at a weight ratio of 1:0.01, sonicate for 35 min, transfer to an oil bath and heat at 85°C with stirring until a viscous state is reached. Remove and dry completely in a vacuum drying oven, then calcine at 720°C for 8.5 h in a muffle furnace to obtain the cathode material.

[0053] The mass ratio of lithium cobalt oxide material to the first coating layer material is 1:0.0003, and the mass ratio of lithium cobalt oxide material to the second coating layer material is 1:0.0001.

[0054] (2) Preparation of positive electrode

[0055] The prepared positive electrode material, conductive agent acetylene black, and binder polyvinylidene fluoride (PVDF) are stirred in an N-methyl methylpyrrolidone solvent system in a mass ratio of 95.5:2.5:2. After uniform stirring, a positive electrode slurry is obtained, which is coated on an Al foil, baked, rolled, and slitted to obtain a positive electrode sheet.

[0056] (3) Preparation of the negative electrode sheet

[0057] The graphite, conductive agent carbon nanotube, binder styrene butadiene rubber, and thickening agent sodium carboxymethyl cellulose are stirred in deionized water in a mass ratio of 97.2:1.2:0.8:0.8. After uniform mixing, a negative electrode slurry is coated on a Cu foil, baked, rolled, and slitted to obtain a negative electrode sheet.

[0058] (4) Preparation of the separator

[0059] A polyethylene (PE) polymer film is used as the isolation film.

[0060] (5) Preparation of the electrolyte

[0061] EC, PP, VC, DEC, and PC are mixed in the following components and mass ratio to obtain an organic solution: ethylene carbonate (EC): propyl propionate (PP): vinylene carbonate (VC): diethyl carbonate (DEC): propylene carbonate (PC) = 22:25:3:28:22. Lithium salt LiPF6 is added to the organic solution in a mass ratio of 9.5:90.5 to the mass of the organic solution. After uniform mixing, an electrolyte is obtained.

[0062] (6) Preparation of the battery

[0063] The positive electrode sheet, isolation film, and negative electrode sheet are sequentially stacked and wound to obtain an electrode assembly. The electrode assembly is placed in a packaging shell, electrolyte is injected into the packaging shell, and the packaging shell is sealed to obtain a lithium ion battery.

[0064] Experimental Example 2

[0065] The difference from Experimental Example 1 is that the mass ratio of lithium cobaltate to the first coating layer in this experimental example is 1:0.0004.

[0066] The rest is the same as Experimental Example 1, which is not repeated here.

[0067] Experimental Example 3

[0068] The difference from Experimental Example 1 is that the mass ratio of lithium cobaltate to the first coating layer in this experimental example is 1:0.0005.

[0069] The rest is the same as Experimental Example 1, which is not repeated here.

[0070] Experimental Example 4

[0071] The only difference from Experimental Example 1 is that in this experimental example, the mass ratio of the lithium cobalt oxide to the first coating layer is 1 : 0.0006.

[0072] The rest is the same as Experimental Example 1, which will not be repeated here.

[0073] Experimental Example 5

[0074] The only difference from Experimental Example 1 is that in this experimental example, the mass ratio of the lithium cobalt oxide to the first coating layer is 1 : 0.0007.

[0075] The rest is the same as Experimental Example 1, which will not be repeated here.

[0076] Experimental Example 6

[0077] The only difference from Experimental Example 1 is that in this experimental example, the mass ratio of the lithium cobalt oxide to the first coating layer is 1 : 0.0005, and the mass ratio of the lithium cobalt oxide to the second coating layer is 1 : 0.00005.

[0078] The rest is the same as Experimental Example 1, which will not be repeated here.

[0079] Experimental Example 7

[0080] The only difference from Experimental Example 1 is that in this experimental example, the mass ratio of the lithium cobalt oxide to the first coating layer is 1 : 0.0005, and the mass ratio of the lithium cobalt oxide to the second coating layer is 1 : 0.0002.

[0081] The rest is the same as Experimental Example 1, which will not be repeated here.

[0082] Experimental Example 8

[0083] The only difference from Experimental Example 1 is that in this experimental example, the mass ratio of the lithium cobalt oxide to the first coating layer is 1 : 0.0005, and the mass ratio of the lithium cobalt oxide to the second coating layer is 1 : 0.0003.

[0084] The rest is the same as Experimental Example 1, which will not be repeated here.

[0085] Experimental Example 9

[0086] The only difference from Experimental Example 1 is that in this experimental example, the mass ratio of the lithium cobalt oxide to the first coating layer is 1 : 0.0005, and the mass ratio of the lithium cobalt oxide to the second coating layer is 1 : 0.0004.

[0087] The rest is the same as Experimental Example 1, which will not be repeated here.

[0088] Experimental Example 10

[0089] The difference from Experimental Example 1 is only that the mass ratio of lithium cobaltate to the first coating layer is 1:0.0005 and the mass ratio of lithium cobaltate to the second coating layer is 1:0.0005 in this experimental example.

[0090] The rest is the same as Experimental Example 1, which will not be repeated here.

[0091] Experimental Example 11

[0092] The difference from Experimental Example 1 is only that this experimental example does not contain the second coating layer and the mass ratio of lithium cobaltate to the first coating layer is 1:0.0005.

[0093] The rest is the same as Experimental Example 1, which will not be repeated here.

[0094] Experimental Example 12

[0095] The difference from Experimental Example 1 is only that this experimental example does not contain the first coating layer.

[0096] The rest is the same as Experimental Example 1, which will not be repeated here.

[0097] Experimental Example 13

[0098] The difference from Experimental Example 1 is only that this experimental example does not contain the first coating layer and the second coating layer.

[0099] The rest is the same as Experimental Example 1, which will not be repeated here.

[0100] Performance test

[0101] The lithium ion batteries prepared in Experimental Examples 1-13 above were subjected to electrical performance tests at room temperature, and the test conditions were 0.1C for charging and discharging, and the charging and discharging test voltage range was 3.0V-4.60V. The test results are shown in Table 1 below.

[0102] Table 1

[0103]

[0104]

[0105] From the comparison of the test data of Experimental Examples 1-13 in Table 1 above, it can be seen that when the mass ratios of single-layer coating and double-layer coating are different, the discharge capacity and the initial efficiency have no obvious difference, but after multiple 100 cl normal temperature discharge cycles, there is a significant difference, and the capacity retention rate of lithium cobaltate coated by double layers is higher.

[0106] Among them, the experimental example 3 of the lithium cobalt oxide coated by double layers is the optimal experimental example, which still has a good capacity retention rate after 100 cycles. Mainly, the coating amount of the experimental example 3 effectively improves the cycle stability and the rate performance of the lithium cobalt oxide, and too much coating amount will affect the deintercalation and transmission of lithium ions, and then affect the capacity retention rate after long time cycle.

[0107] It can be seen from the test data of the experimental examples 1-10 that too large mass ratio will cause the coating layer to be too thick, and then affect the lithium ion transmission path of the positive electrode material, and reduce the capacity retention rate of long cycle.

[0108] According to the disclosure and teaching of the above description, those skilled in the art of the present application can also make changes and modifications to the above embodiments. Therefore, the present application is not limited to the above specific embodiments, and any obvious improvements, replacements or modifications made by those skilled in the art on the basis of the present application all belong to the protection scope of the present application. In addition, although some specific terms are used in the present specification, these terms are only for convenience of description and do not constitute any limitation on the present application.

Claims

1. A positive electrode material, characterized in that, It includes a lithium cobalt oxide material, a first coating layer covering the surface of the lithium cobalt oxide material, and a second coating layer covering the surface of the first coating layer; The first coating layer comprises LiPO3-Al(PO3)3, and the second coating layer comprises TiO2-LiF; The preparation of cathode materials includes the following steps: Step S1: Mix LiH2PO4 and Al(H2PO4)3 to obtain mixture A, and add mixture A to water to obtain solution A; Step S2: Disperse LiCoO2 into solution A, and obtain pretreated lithium cobalt oxide after ultrasonic treatment, heating, drying and calcination; Step S3: Disperse the pretreated lithium cobalt oxide and TiF4 in a solution, and obtain the cathode material by ultrasonic treatment, heating, drying and calcination.

2. The cathode material according to claim 1, characterized in that, The mass ratio of the lithium cobalt oxide material to the first coating layer is 1:0.0004-0.0006.

3. The cathode material according to claim 1, characterized in that, The mass ratio of the lithium cobalt oxide material to the second coating layer is 1:0.0001-0.0004.

4. The cathode material according to claim 1, characterized in that, In step S1, the mass ratio of LiH2PO4 to Al(H2PO4)3 is (1-2):

1.

5. The positive electrode material according to claim 1, characterized in that, In step S2, the mass ratio of mixture A to LiCoO2 is (0.01-0.05):

1.

6. The cathode material according to claim 1, characterized in that, In step S3, the mass ratio of pretreated lithium cobalt oxide to TiF4 is 1:(0.01-0.05).

7. The cathode material according to claim 1, characterized in that, In step S2, the specific steps of calcination are as follows: first, calcinate at a high temperature of 280-350℃ for 2-3 hours, and then raise the temperature to 550-700℃ for 4-6 hours. And / or, in step S3, the calcination temperature is 700-900℃ and the time is 7-9h.

8. A positive electrode plate, characterized in that, Includes the cathode material as described in any one of claims 1-7.

9. A lithium-ion battery, characterized in that, Includes the positive electrode sheet as described in claim 8.

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