A surface-modified cathode material, its preparation method and application

Abstract

The present invention provides a surface-modified cathode material, a preparation method thereof, and an application. The cathode material of the present invention includes a cathode active material and a functional coating material coated on the surface of the cathode active material; the functional coating material contains antioxidant functional groups, which can capture the active oxygen generated during the charge and discharge process of the cathode active material. The cathode material of the present invention can not only effectively inhibit the dissolution of transition metal ions, but also avoid direct contact between the cathode material and the electrolyte, reduce interfacial side reactions, inhibit the release of active oxygen, and improve the structural stability of the material. The lithium metal secondary battery prepared by using the cathode material of the present invention can effectively improve the cycle performance and structural stability of the battery, and has good application prospects.

Description

Technical Field

[0001] The present invention belongs to the field of electrochemical power sources, and particularly relates to a surface-modified cathode material, a preparation method thereof, and an application thereof. Background Art

[0002] With the rapid development of portable electronic devices, electric vehicles, and smart grids, people have put forward higher requirements for energy storage systems with higher capacity and longer life. Lithium secondary batteries have received extensive attention from scientific researchers in recent years due to their advantages such as long cycle life, high energy density, and environmental friendliness.

[0003] As one of the core components of lithium secondary batteries, the cathode material is an important factor determining the battery voltage, energy density, and safety. Currently, relatively representative cathode materials include layered structure metal oxides (LiCoO2, LiNiO2, LiMnO2, Li[Ni 1-x-y Co x Mn y O2 (NCM)), olivine structure phosphates (LiFePO4, LiMnPO4, LiCoPO4), and spinel structure oxides (LiMn2O4). However, these cathode materials also have some problems: (1) the dissolution of transition metal ions during charge and discharge, resulting in irreversible capacity loss of the cathode, accelerating the attenuation of battery capacity; (2) cation mixing and the accompanying release of active oxygen, leading to battery swelling and increased impedance; (3) cracks and pulverization caused by anisotropic stress of particles during cycling, exacerbating the side reaction between the electrode and the electrolyte.

[0004] Research shows that surface coating modification is an effective method to improve the performance of battery cathode materials. On the one hand, the coating layer serves as a physical barrier between the cathode surface and the electrolyte, which can effectively reduce the effective contact area between the active material and the electrolyte, inhibit interfacial side reactions and the dissolution of metal ions; on the other hand, surface coating can also reduce the structural damage of the material during charge and discharge, improve the structural stability of the cathode during cycling, thereby enhancing the overall performance of the battery. However, currently, surface coating is mainly carried out using inorganic substances (Al2O3, ZrO2, MgF2, AlF3, AlPO4, etc.) or organic substances with fixed cross-linking points (polypyrrole, polythiophene, etc.). Such coating materials are relatively brittle, have poor elasticity, and are prone to agglomeration, resulting in uneven coating layers, and cannot effectively solve the problem of structural degradation caused by the formation of molecular oxygen in the bulk lattice and the large volume change of the cathode during cycling. Summary of the Invention

[0005] The purpose of the present invention is to provide a surface-modified cathode material, a preparation method thereof, and an application thereof.

[0006] The present invention provides a positive electrode material, which includes a positive electrode active material and a functional coating material, and the functional coating material is coated on the surface of the positive electrode active material;

[0007] The functional coating material contains antioxidant functional groups and can capture the active oxygen generated by the positive electrode active material during charge and discharge.

[0008] According to an embodiment of the present invention, the positive electrode active material is selected from layered structure metal oxides, olivine structure phosphates or spinel structure oxides.

[0009] Preferably, the layered structure metal oxides are selected from at least one of LiCoO2, LiNiO2, LiMnO2, Li2MnO3, LiCo x Ni 1- x O2 (0 < x < 1), LiCo x Mn 1-x O2 (0 < x < 1), Li[Ni 1-x-y Co x Mn y O2 (0 < x < 1, 0 < y < 1), LiNi x Co y Al 1-x-y O2 (0 < x < 1, 0 < y < 1).

[0010] Preferably, the olivine structure phosphates are selected from at least one of LiFePO4, LiMnPO4, LiCoPO4, LiNiPO4.

[0011] Preferably, the spinel structure oxides are selected from at least one of LiMn2O4, Li4Mn5O 12 , LiCoMnO4.

[0012] According to an exemplary embodiment of the present invention, the positive electrode active material is selected from Li[Ni 1-x-y Co x Mn y O2 (0 < x < 1, 0 < y < 1), LiFePO4, Li2MnO3, for example, Li[Ni 0.6 Co 0.2 Mn 0.2 O2 (trade name NCM622).

[0013] According to an embodiment of the present invention, the functional coating material forms a uniform and continuous coating layer on the particle surface of the positive electrode active material.

[0014] Preferably, the functional coating material has good elasticity and flexibility, and can adapt to the large volume change of the cathode active material during the charge and discharge cycle.

[0015] According to an embodiment of the present invention, the antioxidant functional group is selected from at least one of a hydroxyl group, a carbonyl group, and an amino group.

[0016] Preferably, the functional coating material is selected from an antioxidant and / or a highly elastic polymer.

[0017] Preferably, the antioxidant is selected from substances containing a hydroxyl group and / or an amino group, such as at least one selected from hydroquinone, polyvinyl alcohol, chitosan, fucoidan, polyaniline, diaryl secondary amine, p-phenylenediamine, aldehyde amine, ketone amine, N-phenyl-α-aniline, N-phenyl-β-naphthylamine, N-phenyl-N'-isopropyl-p-phenylenediamine, N-N'-diphenyl-p-phenylenediamine, 4, 4'-bis(2, 2-dimethylbenzyl) diphenylamine.

[0018] Preferably, the highly elastic polymer contains at least one of a hydroxyl group, a carbonyl group, and an amino group. Exemplarily, the highly elastic polymer is selected from at least one of polyrotaxane crosslinked polyacrylic acid, polyethylene glycol, polyacrylate, and polycarbonate.

[0019] According to an exemplary embodiment of the present invention, the functional coating material is preferably polyrotaxane crosslinked polyacrylic acid. For example, the number average molecular weight of the polyrotaxane crosslinked polyacrylic acid is 450,000 - 500,000, such as 470,000 - 480,000, preferably 472,000 - 480,000.

[0020] According to an embodiment of the present invention, the thickness of the coating layer is 5 - 50 nm, preferably 10 - 20 nm, for example 10 nm.

[0021] According to an embodiment of the present invention, the coating ratio of the functional coating material to the cathode active material is (1 - 5):100, such as 1:100, 3:100, 5:100. The coating ratio in the present invention is the feeding mass ratio of the functional coating material to the cathode active material.

[0022] According to an exemplary embodiment of the present invention, the cathode material includes Li[Ni 0.6 Co 0.2 Mn 0.2 O2 and polyrotaxane crosslinked polyacrylic acid, and the polyrotaxane crosslinked polyacrylic acid forms a uniform and continuous coating layer on the particle surface of Li[Ni 0.6 Co 0.2 Mn 0.2 O2;

[0023] The coating ratio of the polyrotaxane cross-linked polyacrylic acid and Li[Ni 0.6 Co 0.2 Mn 0.2 O2 is (1 - 5):100. According to an exemplary embodiment of the present invention, the positive electrode material includes Li[Ni 0.6 Co 0.2 Mn 0.2 O2 and polyethylene glycol, and the polyethylene glycol forms a uniform and continuous coating layer on the particle surface of Li[Ni 0.6 Co 0.2 Mn 0.2 O2;

[0024] The coating ratio of polyethylene glycol and Li[Ni 0.6 Co 0.2 Mn 0.2 O2 is (1 - 5):100.

[0025] The present invention also provides a preparation method of the above positive electrode material, which includes the following steps:

[0026] (1) Dissolve the above functional coating material in a solvent to obtain a functional coating material solution;

[0027] (2) Add the functional coating material solution in step (1) to the above positive electrode active material, and after dispersion, dissolution, removal of the solvent, and drying, the positive electrode material is obtained;

[0028] The functional coating material and the positive electrode active material have the meanings as described above.

[0029] According to an embodiment of the present invention, in step (1), the mass - volume ratio of the functional coating material to the solvent is (50 - 100) mg:(10 - 20) mL, for example, 50 mg:10 mL.

[0030] According to an embodiment of the present invention, in step (1), the solvent is selected from solvents with a boiling point in the range of 70 - 100 °C, for example, ethanol, and preferably anhydrous ethanol.

[0031] According to an embodiment of the present invention, the dissolution temperature in step (1) is in the range of 50 - 60 °C, for example, 50 °C, 55 °C, 60 °C.

[0032] According to an embodiment of the present invention, in step (2), the mass ratio of the functional coating material to the positive electrode active material is (5 - 25):500, preferably (15 - 20):500, for example, 15:500.

[0033] According to an embodiment of the present invention, in step (2), the dispersion can be selected from dispersion methods known in the art, such as ultrasonic dispersion. Preferably, the dispersion time is 20 - 60 min, more preferably 30 - 60 min, for example 30 min, 40 min, or 50 min.

[0034] According to an embodiment of the present invention, in step (2), the dissolution can be selected from dispersion methods known in the art, such as stirring dissolution. Preferably, the dissolution temperature is 50 - 70 °C. The dissolution time is 0.5 - 2 h, 1 - 2 h, for example, stirring for 1 h at 60 °C.

[0035] According to an embodiment of the present invention, in step (2), the removal of the solvent can be selected from methods known in the art to remove the solvent, such as using a rotary evaporator to remove the solvent.

[0036] According to an embodiment of the present invention, in step (2), the drying can be selected from drying methods known in the art, such as air drying or vacuum drying. Preferably, the drying temperature is 50 - 70 °C, and the drying time is 12 - 24 h. For example, air drying at 60 °C for 24 h.

[0037] According to an exemplary embodiment of the present invention, the preparation method specifically comprises the following steps:

[0038] (1) Dissolve the functional coating material in a solvent, stir to dissolve, and obtain a functional coating material solution;

[0039] (2) Add the functional coating material solution in step (1) to the cathode active material, disperse by ultrasonic treatment, stir to dissolve, remove anhydrous ethanol using a rotary evaporator, and dry to obtain the cathode material;

[0040] Preferably, the functional coating material is polyrotaxane crosslinked polyacrylic acid, the cathode active material is Li[Ni 0.6 Co 0.2 Mn 0.2 O2, and the solvent is anhydrous ethanol.

[0041] Preferably, in step (1), the mass - volume ratio of the functional coating material to anhydrous ethanol is 50 mg:10 mL.

[0042] Preferably, in step (1), the dissolution temperature is 60 °C.

[0043] Preferably, in step (2), the mass ratio of the functional coating material to the cathode active material is 15:500.

[0044] Preferably, the ultrasonic dispersion time is 30 min.

[0045] Preferably, the dissolution temperature is 60 °C and the stirring time is 1 h.

[0046] Preferably, the drying temperature is 60 °C and the drying time is 24 h.

[0047] The present invention also provides a positive electrode plate, which contains the above positive electrode material.

[0048] According to an embodiment of the present invention, the positive electrode plate further contains a conductive additive and / or a binder.

[0049] Preferably, the conductive additive can be selected from the conductive additives known in the technical field, preferably acetylene black and Ketjen black.

[0050] Preferably, the binder can be selected from the binders known in the technical field, preferably polyvinylidene fluoride, sodium alginate, and sodium carboxymethyl cellulose.

[0051] According to an embodiment of the present invention, the mass ratio of the positive electrode active material, the conductive additive, and the binder is (7.5 - 8.5):(0.5 - 1.5):(0.5 - 1.5), for example, 80:10:10.

[0052] According to an embodiment of the present invention, the positive electrode plate further contains a current collector, and the current collector can be selected from the current collectors known in the technical field, such as aluminum foil.

[0053] The present invention also provides a method for preparing the above positive electrode plate, which includes: coating the above positive electrode material on a current collector and drying to obtain the positive electrode plate.

[0054] According to an embodiment of the present invention, before coating, the positive electrode material needs to be ground evenly with a conductive additive and / or a binder, and the conductive additive and the binder have the meanings as described above. Exemplarily, the conductive additive is acetylene black. Exemplarily, the binder is polyvinylidene fluoride.

[0055] According to an embodiment of the present invention, the drying conditions are: the temperature is 50 - 70 °C and the drying time is 24 - 36 h. Exemplarily, the drying conditions are: drying at 60 °C for 24 h. Preferably, the drying is vacuum drying.

[0056] The present invention also provides the application of the above positive electrode material or positive electrode plate in energy storage devices.

[0057] The present invention also provides an energy storage device, which contains the above positive electrode material or the above positive electrode plate.

[0058] According to an embodiment of the present invention, the energy storage device can be a lithium secondary battery.

[0059] Preferably, the lithium secondary battery can be selected from any one of a lithium ion battery and a lithium metal battery.

[0060] The present invention also provides a lithium secondary battery, which includes the above-mentioned positive electrode material or the above-mentioned positive electrode sheet.

[0061] According to an embodiment of the present invention, the lithium secondary battery further includes: a negative electrode, a separator, and an electrolyte. In the present invention, the negative electrode, the separator, or the electrolyte can be selected from the negative electrodes, separators, or electrolytes known in the art.

[0062] According to an exemplary solution of the present invention, the negative electrode is selected from a lithium metal sheet.

[0063] According to an exemplary solution of the present invention, the separator is selected from a polypropylene separator and a polyethylene separator.

[0064] According to an exemplary solution of the present invention, the electrolyte is selected from LiPF6 + EC:DMC:DEC = 1:1:1 (V:V:V) + 2% VC.

[0065] Advantages of the present invention:

[0066] The present invention provides a surface-modified positive electrode material, which includes a positive electrode active material and a functional coating material coated on its surface. The functional coating material forms a continuous and uniform coating layer on the surface of the positive electrode active material, which can not only effectively inhibit the dissolution of transition metal ions as a protective layer, but also avoid the direct contact between the positive electrode active material and the electrolyte, reduce the interfacial side reaction, inhibit the release of active oxygen, and improve the structural stability of the material. For example, polyrotaxane-crosslinked polyacrylic acid (PR-PAA) is coated on the surface of the positive electrode active material. The dynamic slippage of α-cyclodextrin can disperse the stress in the polymer network, relieve the anisotropic stress and distortion during the cycling process, prevent the material from cracking or pulverizing, and maintain the original structure to a certain extent. In addition, a large number of hydroxyl functional groups are contained in the structure of PR-PAA, which can inhibit the release of surface active oxygen of the electrode active material during charge and discharge, and avoid problems such as electrolyte oxidation and serious gas generation in the battery. In addition, PR-PAA has good elasticity and flexibility, can better adapt to the large volume change of the positive electrode during cycling, avoid delamination of the coating layer and the positive electrode active material, and thus improve the cycling performance and structural stability of the battery.

[0067] Compared with the modification methods of coating the surface with inorganic compounds (such as Al2O3, ZrO2, MgF2, AlF3, AlPO4, etc.) or organic compounds (such as polypyrrole, polythiophene, etc.), the present invention introduces a functional coating material on the surface of the cathode active material, which can not only effectively inhibit the dissolution of transition metal ions in the cathode active material, but also avoid the direct contact area between the cathode active material and the electrolyte, reducing the interfacial side reactions. In addition, the structure of the functional coating material contains rich antioxidant functional groups, which can effectively capture the active oxygen generated during charge and discharge, solve the serious gas generation problem of the battery, and thus exhibit better structural stability and cycling performance, improving the electrochemical performance of the lithium secondary battery. The surface-modified cathode material prepared by the method of the present invention provides a basis for the practical application of lithium secondary batteries and has good application prospects. Description of the Drawings

[0068] Figure 1 Shows the metal dissolution amounts of the samples in Example 1 and Comparative Example 1 after 300 charge-discharge cycles.

[0069] Figure 2 Scanning electron microscope photograph of the sample in Comparative Example 1 before cycling.

[0070] Figure 3 Scanning electron microscope photograph of the sample in Example 1 before cycling.

[0071] Figure 4 Transmission scanning electron microscope photograph of the sample in Example 1 before cycling.

[0072] Figure 5 Cycling curves of the batteries in Example 1 and Comparative Example 1 at a rate of 0.5 C.

[0073] Figure 6 Cycling curves of the batteries in Example 2 and Comparative Example 1 at a rate of 0.5 C.

[0074] Figure 7 Cycling curves of the batteries in Example 3 and Comparative Example 1 at a rate of 0.5 C. Detailed Description of the Invention

[0075] The technical solutions of the present invention will be further described in detail below with reference to specific embodiments. It should be understood that the following embodiments are only for illustrative and explanatory purposes of the present invention and should not be construed as limiting the protection scope of the present invention. All technologies implemented based on the above content of the present invention are covered within the scope of protection intended by the present invention.

[0076] Unless otherwise specified, the raw materials and reagents used in the following embodiments are all commercially available products or can be prepared by known methods.

[0077] In the present invention, Li[Ni 0.6 Co 0.2 Mn 0.2 O2 is abbreviated as NCM622; polyrotaxane crosslinked polyacrylic acid is abbreviated as PR-PAA; polyethylene glycol is abbreviated as PEG.

[0078] Preparation Example

[0079] Preparation of PR-PAA polymer

[0080] (1) Synthesis of carboxylated PEG: Weigh 5 g of PEG, 50 mg of TEMPO, and 50 mg of NaBr in 50 mL of deionized water, and stir to dissolve at room temperature. Slowly add dropwise 5 mL of NaClO solution. During the reaction, control the pH of the solution ≤ 11. Add 5 mL of absolute ethanol, stir for half an hour, then add HCl to make the pH of the solution ≤ 2, and continue to stir for half an hour. Extract with CH2Cl2 in small amounts multiple times. Rotate and evaporate the lower colorless and transparent liquid at 30 °C for 1 h until a small amount of white solid precipitates. Transfer it to a beaker, add 100 mL of hot absolute ethanol, let it stand and cool at room temperature, and then stand in the refrigerator (4 °C) for 12 h to fully precipitate the precipitate. Filter, separate, and wash under normal pressure, and dry in a vacuum oven at 50 °C for 24 h to obtain a white solid powder, which is carboxylated PEG.

[0081] (2) Synthesis of PR: Dissolve 2 g of carboxylated PEG and 8 g of α-CD in 100 mL of deionized water, stir to dissolve, and then stand in the refrigerator (4 °C) for 12 h, and freeze-dry with a freeze dryer to obtain a white solid powder. Weigh 1.4 g of the powder, 16 mg of adamantane ammonium salt, 48 mg of BOP, and 19 mg of EDIPA in 100 mL of DMF, react for 1 h until completely dissolved, and stand in the refrigerator (4 °C) for 12 h. Slowly add dropwise absolute methanol until the white solid powder completely precipitates, centrifuge at a speed of 8000 revolutions per minute for 5 min, wash with deionized water, and then dry in a vacuum oven at 50 °C for 24 h to obtain a white solid powder PR.

[0082] (3) Grafting of PR with propylene oxide: Under an ice-water bath (0 °C), weigh 1 g of PR and dissolve it in 100 mL of 1 mol L -1 NaOH solution, and stir until completely dissolved. Under an ice-water bath and vigorous stirring, slowly add dropwise 9 g of 1,2-propylene oxide solution. After the reaction is complete, let it stand at room temperature for 12 h to obtain a colorless and transparent solution. Transfer the reaction solution to a dialysis bag, dialyze and purify with deionized water for 3 days, and then freeze-dry with a freeze dryer to obtain a white solid powder. Dissolve the solid powder in an appropriate amount of CH2Cl2 solution, and dry in a vacuum oven at 50 °C for 12 h to obtain a light yellow solid powder, which is PR grafted with propylene oxide.

[0083] (4) PAA cross-linking: PAA solution (0.05 g mL -1 ), CDI solution (0.1 g mL -1 ) and PR solution of grafted propylene oxide (0.1 g mL -1 ) for use. 19 mL of PAA solution and 0.113 mL of CDI solution were stirred at room temperature for 30 min under argon atmosphere to make them evenly mixed, and then stirred at 50 °C for 12 h. Then cooled to room temperature, 0.5 mL of PR solution grafted with propylene oxide was added, and after sufficient stirring for 30 min, the temperature was raised to 70 °C and reacted for 72 h to obtain a colorless, transparent, viscous solution. 100 mL of THF was added to the above solution to obtain a milky white colloidal polymer precipitate, which was dissolved in 20 mL of anhydrous methanol, and then an appropriate amount of anhydrous ether was added to completely precipitate the polymer. Finally, the polymer was dissolved in an appropriate amount of deionized water and freeze-dried in a freeze dryer to obtain PR-PAA.

[0084] The test shows that the number average molecular weight of PR-PAA is 47,2000-48,0000.

[0085] Example 1

[0086] (I) Preparation of positive electrode material (coating ratio is 3 wt%): PR-PAA coated NCM622 positive electrode active material

[0087] 50 mg PR-PAA was dissolved in 10 mL anhydrous ethanol and stirred at 60 °C to obtain 5 mg mL -1 PR-PAA solution. Weigh 500 mg of NCM622 positive electrode active material, add 3 mL of PR-PAA solution, and then add anhydrous ethanol to make the total volume of the mixed solution 10 mL. After the mixed solution was ultrasonically dispersed for 30 min, it was stirred at 60 °C for 1 h to make the NCM622 positive electrode active material completely dispersed in the PR-PAA solution. The solvent was removed by rotary evaporation at room temperature and dried in a forced air oven at 60 °C for 12 h to obtain a positive electrode material of PR-PAA coated NCM622 (coating ratio of 3 wt%), which was recorded as sample 1.

[0088] (II) Lithium secondary battery assembly and charge-discharge cycle performance testing

[0089] The above positive electrode material, acetylene black and poly(plyfluoroacetic acid) were accurately weighed in a mass ratio of 8:1:1, and a uniform black slurry was obtained after sufficient grinding. The slurry was spread evenly on the surface of the carbon-coated aluminum foil with a scraper, and dried in a vacuum oven at 60°C for 24 h to obtain a positive electrode sheet.

[0090] The above positive electrode sheet was cut into a diameter of 10 mm as the positive electrode, the negative electrode was a lithium metal sheet, Celgard 2400 was used as the separator, and LiPF6 + EC:DMC:DEC = 1:1:1 (V:V:V) + 2% VC was used as the electrolyte to assemble a lithium metal secondary battery, denoted as Battery 1.

[0091] Charge and discharge cycle performance test: The above battery was subjected to constant current charge and discharge test using a charge and discharge instrument. The test temperature was 25 °C, the test current density was 0.5 C, and the charge and discharge voltage range was 2.5 - 4.3 V.

[0092] Comparative Example 1

[0093] The NCM622 positive electrode active material in Example 1 was used as Comparative Sample 1, and the battery assembly and charge and discharge cycle performance test were carried out according to the steps (ii) of Example 1, denoted as Comparative Battery 1.

[0094] Figure 1 are the graphs of metal dissolution amounts of Sample 1 and Comparative Sample 1 after 300 cycles at a rate of 2 C. It can be seen from Figure 1 that the battery pack 1 assembled with the coated Sample 1 has a lower ion dissolution amount and can significantly inhibit the dissolution of transition metal ions. Figure 2 and Figure 3 are the scanning electron microscope photos of Comparative Sample 1 and Sample 1 (3 wt% coating) before the charge and discharge cycle test respectively. It can be seen from Figure 2 and 3 that there are obvious boundaries and voids between the particles of the NCM622 positive electrode active material, which is likely to exacerbate the interfacial side reactions between the electrolyte and the active material during charge and discharge, resulting in a decline in battery performance. However, for Sample 1 in Example 1, by coating on the surface of the NCM622 positive electrode active material particles, PR-PAA can effectively fill the voids between the primary particles of the positive electrode active material. For Sample 1 with a coating amount of 3 wt%, a continuous and uniform coating layer is formed on the surface of the positive electrode active material particles, effectively suppressing the side reactions. Figure 4 is the transmission scanning electron microscope photo of the sample in Example 1 before cycling. It can be seen from Figure 4 that there is a uniform and continuous light gray polymer coating layer on the edge of the surface of the positive electrode active material particles of Sample 1, and its thickness is about 10 nm. The ultra-thin polymer coating layer can improve the interfacial stability of the positive electrode material without hindering the lithium ion transmission.

[0095] Figure 5 are the charge and discharge cycle curves of Battery 1 and Comparative Battery 1 prepared in Example 1 and Comparative Example 1 at a rate of 0.5 C. After 100 cycles, the capacity retention rate of the battery pack 1 is 85.5%, and the specific capacity is 132.2 mA h•g-1 ; After 100 charge-discharge cycles, the capacity retention rate of Comparative Battery 1 is 51.5%, and the specific capacity is 75.3 mA h•g -1 . This is because there is no protective effect of PR-PAA on the surface of the particles of Comparative Sample 1, and the side reaction between the positive electrode active material and the electrolyte is significantly aggravated, resulting in a significant decline in the cycling performance of Comparative Battery 1. It can be seen from this that Battery 1 of Example 1 has a higher capacity retention rate and more excellent battery performance.

[0096] Example 2

[0097] (I) Preparation of the positive electrode material (coating ratio is 1 wt%): The difference between this example and Example 1 is only that the amount of PR-PAA added is different. The specific steps are as follows:

[0098] 50 mg of PR-PAA is dissolved in 10 mL of absolute ethanol and stirred to dissolve at 60 °C to obtain a PR-PAA solution of 5 mg mL -1 . Weigh 500 mg of NCM622 positive electrode active material, add 1 mL of the PR-PAA solution, and then add absolute ethanol to make the total volume of the mixed solution 10 mL. After ultrasonic dispersion of the mixed solution for 30 min, continue to stir at 60 °C for 1 h to completely disperse the NCM622 positive electrode active material in the PR-PAA solution. Rotate and evaporate to remove the solvent at room temperature, and dry in a blast oven at 60 °C for 12 h to obtain a positive electrode material with NCM622 coated with PR-PAA (coating ratio is 1 wt%), denoted as Sample 2, and the thickness of its coating layer is 7 nm.

[0099] (II) Assembly of the lithium secondary battery and testing of the charge-discharge cycling performance: The battery is assembled according to the steps (II) of Example 1 and denoted as Battery 2.

[0100] Figure 6 Fig. is the charge-discharge cycling curve diagram of Battery 2 and Comparative Battery 1 prepared in Example 2 and Comparative Example 1 at a rate of 0.5 C. After 100 charge-discharge cycles of Battery 2 at a rate of 0.5 C, its capacity retention rate is 74.3%, and the specific capacity is 112.8 mAh•g -1 . This is because, since Sample 2 reduces the coating ratio of the PR-PAA polymer, the coating layer formed on the surface of the positive electrode active material particles is less continuous and uniform than that of Sample 1. Therefore, the contact area between the positive electrode active material and the electrolyte increases, and the effect of suppressing side reactions weakens. Although the cycling performance decreases slightly compared with Example 1, it is still better than that of Comparative Example 1.

[0101] Example 3

[0102] (1) Preparation of the positive electrode material (coating ratio: 5 wt%): The difference between this example and Example 1 is only that the amount of PR-PAA added is different. Specifically:

[0103] 50 mg of PR-PAA was dissolved in 10 mL of absolute ethanol and stirred at 60 °C until dissolved to obtain a PR-PAA solution with a concentration of 5 mg / mL. -1 500 mg of the NCM622 positive electrode active material was weighed, 5 mL of the PR-PAA solution was added, and then absolute ethanol was added to make the total volume of the mixed solution 10 mL. After ultrasonic dispersing the mixed solution for 30 min, stirring was continued at 60 °C for 1 h to completely disperse the NCM622 positive electrode active material in the PR-PAA solution. The solvent was removed by rotary evaporation at room temperature and dried in a blast drying oven at 60 °C for 12 h to obtain the positive electrode material with NCM622 coated with PR-PAA (coating ratio: 5 wt%), denoted as Sample 3, and the thickness of its coating layer was 22 nm.

[0104] (2) Assembly of the lithium secondary battery and testing of charge-discharge cycle performance: The battery was assembled according to the steps (2) of Example 1 and denoted as Battery 3.

[0105] Figure 7 FIG. 3 is the charge-discharge cycle curve diagram of Battery 3 and Comparative Battery 1 prepared in Example 3 and Comparative Example 1 at a rate of 0.5 C. After 100 charge-discharge cycles of Battery 3 at a rate of 0.5 C, its capacity retention rate was 80.5%, and the specific capacity was 108.3 mAh•g -1 . This is because when the coating ratio is 5%, the coating layer on the surface of the positive electrode is relatively thick, which has a certain hindering effect on the migration of lithium ions, and adhesion is likely to occur between the active particles, resulting in uneven dispersion of the positive electrode material. Therefore, the cycle performance of Battery 3 is slightly lower than that of Example 1, but still better than that of Comparative Example 1.

[0106] Example 4

[0107] (1) Preparation of the positive electrode material (coating ratio: 3 wt%): The difference between this example and Example 1 is that the coating material is polyethylene glycol (PEG, average molecular weight: 20,000). Specifically:

[0108] 50 mg of PEG was dissolved in 10 mL of absolute ethanol and stirred thoroughly at 60 °C until dissolved to obtain a PEG solution with a concentration of 5 mg / mL. -1PEG solution. Weigh 500 mg of NCM622 cathode active material, add 3 mL of PEG solution, and then add an appropriate amount of absolute ethanol to make the total volume of the mixed solution 10 mL. After ultrasonic dispersing the mixed solution for 30 min, continue to stir at 60 °C for 1 h to completely disperse the NCM622 cathode active material in the PEG solution. Rotate and evaporate to remove the solvent at room temperature, and dry in a 60 °C blast drying oven for 12 h to obtain the cathode material with PEG-coated NCM622 (coating ratio is 3 wt%), denoted as Sample 4, and the thickness of its coating layer is 15 nm.

[0109] (II) Assembly of lithium secondary battery and charge-discharge cycle performance test: The battery was assembled according to the steps (II) of Example 1, denoted as Battery 4. The difference is that the charge-discharge interval for testing is 2.5 - 4.3 V.

[0110] After 100 charge-discharge cycles at 0.5 C rate, the capacity retention rate of Battery 4 is 75.2%, and the specific capacity is 110.4 mAh•g -1 , which is better than the performance of Comparative Battery 1.

[0111] Example 5

[0112] (I) Preparation of cathode material (coating ratio is 3wt%): The difference between this example and Example 1 is that the cathode active material is LiFePO4, specifically:

[0113] 50 mg of PR-PAA in 10 mL of absolute ethanol, stir and dissolve at 60 °C to obtain a 5 mg mL -1 PR-PAA solution. Weigh 500 mg of LiFePO4 cathode active material, add 3 mL of PR-PAA solution, and then add absolute ethanol to make the total volume of the mixed solution 10 mL. After ultrasonic dispersing the mixed solution for 30 min, continue to stir at 60 °C for 1 h to completely disperse the LiFePO4 cathode active material in the PR-PAA solution. Rotate and evaporate to remove the solvent at room temperature, and dry in a 60 °C blast drying oven for 12 h to obtain the cathode material with PR-PAA-coated LiFePO4 (coating ratio is 3wt%), denoted as Sample 5, and the thickness of its coating layer is 17 nm.

[0114] (II) Assembly of lithium secondary battery and charge-discharge cycle performance test: The battery was assembled according to the steps (II) of Example 1, denoted as Battery 5. The difference is that the electrolyte is LiTFSI + DOL:DME = 1:1 (V:V) + 1% LiNO3, and the charge-discharge interval for testing is 2.5 - 4.0 V.

[0115] Comparative Example 2

[0116] The LiFePO4 cathode active material in Example 5 was used as Comparative Sample 2, and battery assembly and charge-discharge cycle performance tests were carried out according to step (2) of Example 5, denoted as Comparative Battery 2.

[0117] After 100 charge-discharge cycles at a rate of 0.5 C, the capacity retention rate of Battery 5 was 79%, and the specific capacity was 122.7 mAh•g -1 ; the capacity retention rate of Comparative Battery 2 was only 73%, and the specific capacity was 109.2 mAh•g -1 .

[0118] Example 6

[0119] (1) Preparation of the cathode material (coating ratio: 3 wt%): The difference between this example and Example 1 is that the cathode active material is a lithium-rich layered cathode Li2MnO3, specifically:

[0120] 50 mg of PR-PAA was dissolved in 10 mL of absolute ethanol and stirred at 60 °C until dissolved to obtain a 5 mg mL -1 PR-PAA solution. 500 mg of lithium-rich layered cathode active material Li2MnO3 was weighed, 3 mL of the PR-PAA solution was added, and an appropriate amount of absolute ethanol was added to make the total volume of the mixed solution 10 mL. After ultrasonic dispersion of the mixed solution for 30 min, stirring was continued at 60 °C for 1 h to fully disperse the cathode active material in the PR-PAA solution. The solvent was removed by rotary evaporation at room temperature and dried in a blast oven at 60 °C for 12 h to obtain a cathode material with PR-PAA-coated lithium-rich layered cathode Li2MnO3 (coating ratio: 3 wt%), denoted as Sample 6, and the thickness of its coating layer was 14 nm.

[0121] (2) Lithium secondary battery assembly and charge-discharge cycle performance test: Battery assembly was carried out according to step (2) of Example 1, denoted as Battery 6, except that the charge-discharge interval for testing was 2.0 - 4.8 V.

[0122] Comparative Example 3

[0123] The lithium-rich layered cathode Li2MnO3 active material in Example 6 was used as Comparative Sample 3, and battery assembly and charge-discharge cycle performance tests were carried out according to step (2) of Example 6, denoted as Comparative Battery 3.

[0124] After 100 charge-discharge cycles at a rate of 0.5 C, the capacity retention rate of Battery 6 was 68%, and the specific capacity was 102.6 mAh•g -1 ; the capacity retention rate of Comparative Battery 3 was only 59%, and the specific capacity was 86 mAh•g -1 .

[0125] As described above, exemplary embodiments of the present invention have been illustrated. However, the present invention is not limited to the above-described embodiments. Any modifications, equivalent substitutions, improvements, etc. made by those skilled in the art within the spirit and principles of the present invention shall be included within the protection scope of the present invention.

Claims

1. A method for preparing a cathode material, characterized in that The preparation method includes the following steps: (1) Dissolve the functional coating material in a solvent to obtain a functional coating material solution; (2) Add the functional coating material solution in step (1) to the cathode active material, disperse, dissolve, remove the solvent using a rotary evaporator, and dry to obtain the cathode material; the drying temperature is 50-70 °C, and the drying time is 12-24 h; The cathode material includes a cathode active material and a functional coating material, and the functional coating material forms a uniform and continuous coating layer on the surface of the particles of the cathode active material; the thickness of the coating layer is 10-20 nm; The functional coating material is selected from polyrotaxane cross-linked polyacrylic acid; The cathode active material is selected from layered structure metal oxides, olivine structure phosphates, or spinel structure oxides.

2. The preparation method according to claim 1, characterized in that, The layered structured metal oxide is selected from at least one of the following materials: LiCoO2, LiNiO2, LiMnO2, Li2MnO3, LiCo x Ni 1-x O2 where 0 < x < 1, LiCo x Mn 1-x O2 where 0 < x < 1, Li[Ni 1-x-y Co x Mn y O2 where 0 < x < 1 and 0 < y < 1, LiNi x Co y Al 1-x-y O2 where 0 < x < 1 and 0 < y < 1; The olivine structure phosphates are selected from at least one of LiFePO4, LiMnPO4, LiCoPO4, and LiNiPO4; The spinel-structured oxide is selected from at least one of LiMn2O4, Li4Mn5O 12 , and LiCoMnO4; The functional coating material has good elasticity and flexibility and can adapt to the large volume change of the cathode active material during the charge-discharge cycle.

3. The preparation method according to claim 1, wherein The positive electrode active material is selected from at least one of the following materials: Li[Ni 1-x-y Co x Mn y O2 where 0 < x < 1 and 0 < y < 1, LiFePO4.

4. The preparation method according to claim 1, characterized in that, The coating mass ratio of the functional coating material to the cathode active material is 1-5:

100.

5. The preparation method according to claim 1, characterized in that, In step (1), the mass-volume ratio of the functional coating material to the solvent is 50-100 mg:10-20 mL; In step (1), the solvent is selected from solvents with a boiling point of 70-100 °C; The dissolution temperature in step (1) is 50-60 °C.

6. The preparation method according to claim 1, wherein In step (2), the mass ratio of the functional coating material to the cathode active material is 5-25:500; In step (2), the dispersion time is 20-60 min; In step (2), the dissolution temperature is 50-70 °C, and the dissolution time is 0.5-2 h.

7. The preparation method according to claim 1, characterized in that In step (2), the mass ratio of the functional coating material to the cathode active material is 15-20:

500.

8. The preparation method according to claim 1, wherein The preparation method includes the following steps: (1) Dissolve the functional coating material in a solvent, stir and dissolve to obtain a functional coating material solution; (2) Add the functional coating material solution in step (1) to the cathode active material, ultrasonically disperse, stir and dissolve, remove the solvent with a rotary evaporator, and dry to obtain the cathode material; The positive electrode active material is Li[Ni 0.6 Co 0.2 Mn 0.2 O2, and the solvent is anhydrous ethanol.

9. A cathode material, which is obtained by the preparation method according to any one of claims 1-8.

10. A positive electrode sheet, characterized in that, The cathode electrode sheet contains the cathode material according to claim 9.

11. The method for preparing the positive electrode sheet according to claim 10, characterized in that, The preparation method includes: coating the cathode material according to claim 9 on a current collector and drying in vacuum to obtain the cathode electrode sheet.

12. The application of the cathode material according to claim 9 or the cathode electrode sheet according to claim 10 in an energy storage device.

13. An energy storage device, characterized in that, The energy storage device includes the cathode material according to claim 9 or the cathode electrode sheet according to claim 10; The energy storage device is a lithium secondary battery.

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