Reversibly cross-linked polymer-coated positive electrode material, method of preparation, positive electrode and use

Abstract

The application provides a reversible cross-linked polymer-coated positive electrode material. The reversible cross-linked polymer is formed by one or more branched polymers through the mutual reaction between the branched groups thereof to form one or more reversible cross-linked structures. At room temperature, the polymer is in a cross-linked state, reducing the contact between the electrolyte and the positive electrode and improving the cycle stability. When the battery is subjected to external high temperature, the cross-linked polymer depolymerizes, generating a large amount of groups containing unsaturated double bonds. The depolymerization reaction absorbs part of the heat, and the generated unsaturated double bonds can react with the oxygen radicals released by the positive electrode to prevent the diffusion of the oxygen radicals to the negative electrode and cause more severe battery thermal runaway.

Description

Technical Field

[0001] This application relates to a cathode material, specifically to a cathode material coated with a reversible cross-linked polymer and its preparation method, the prepared cathode and its uses, and the use of the reversible cross-linked polymer in the preparation of cathode materials or cathodes. Background Technology

[0002] Lithium-ion batteries are widely used in electronic products, electric vehicles, and large-scale energy storage due to their high energy density and good cycle performance. However, with the expansion of their applications, safety issues related to lithium-ion batteries have gradually emerged, especially in large-scale modules. Thermal runaway in one cell can trigger a chain reaction in adjacent cells at high temperatures, leading to a large-scale explosion. Currently, it is generally believed that the key factor in lithium-ion battery thermal runaway is the decomposition of the positive electrode material at high temperatures, releasing oxygen free radicals. These oxygen free radicals then react violently with the negative electrode lithium intercalation material and the electrolyte, releasing enormous energy and causing the cell to explode.

[0003] Surface modification of cathode materials through coating is an important method to suppress oxygen release during material decomposition and improve cell safety. For example, patent document CN 113955813 A describes coating a fluorine-containing disordered rock salt lithium-rich oxide onto the surface of the cathode material, alleviating the problem of lattice oxygen release under high operating voltage and improving the cell's cycle life and safety. Patent document CN109921000 A describes coating high-voltage resistant materials such as barium titanate, lithium niobate, sodium niobate, lithium tantalate, potassium sodium metaniobate, and barium strontium metaniobate onto the surface of the cathode material, finding that the thermal stability of the coated material is significantly improved. Patent document CN 111769267 B describes coating the material surface with both nickel-rich cathode material and nano-lithium manganese iron phosphate. This two-layer coating significantly reduces the exothermic reaction between the electrolyte and the cathode material under overcharge, short circuit, heating, and needle penetration conditions, thereby improving cell safety. Patent document CN112201791 A mixes a cathode material with phenolic and amine derivatives, introduces oxygen to form an in-situ bonded nanofilm, which can absorb oxygen atoms released from the cathode material, thereby improving interface stability and battery safety.

[0004] However, the current method of coating the positive electrode mainly adopts inorganic coating to reduce the contact between the positive electrode and the electrolyte. Inorganic layers often do not have high conductivity, and this method is based on the principle of physical isolation. During charging and discharging, the material often undergoes large volume expansion. After long-term cycling, the coating layer will develop defects and gradually lose its barrier effect. Summary of the Invention

[0005] This application addresses the shortcomings of the prior art, where the volume of electrode materials often expands during the charging and discharging process of batteries, and the inorganic surface coating layer may crack during long-term cycling, leading to the loss of its inhibition function. The present invention relates to a positive electrode material coated with a reversible cross-linked polymer, a positive electrode, and the use of the reversible cross-linked polymer.

[0006] The specific plan is as follows:

[0007] A positive electrode material coated with a reversible crosslinked polymer, wherein the reversible crosslinked polymer is formed by the interaction of one or more branched polymers to form one or more reversible crosslinked structures.

[0008] Furthermore, the reversible crosslinking structure is formed by thermal addition of branched groups with end groups containing electron-rich conjugated diene groups and branched groups with end groups containing electron-deficient dienophilic groups.

[0009] Furthermore, the reversible crosslinking structure is R1 or R2, wherein R1 or R2 is as follows:

[0010]

[0011] Furthermore, the branched polymer is selected from one or more of polyester, polyether, or polyolefin.

[0012] Furthermore, the branched polymer is selected from one or more of the following formulas: P1, P2 and P3.

[0013]

[0014] Where a and b are independently selected from any one of -CH3, -H, -F, -OH and -NH2, 1≤m≤45, n≥2, m and n are positive integers, and R represents the branched end group.

[0015] Furthermore, in the branched polymer, 1 ≤ m ≤ 20, n ≥ 5.

[0016] Furthermore, the cathode material is selected from one or more of lithium iron phosphate, lithium cobalt oxide, lithium manganese oxide, lithium nickel oxide, binary lithium nickel cobalt oxide, binary lithium nickel manganese oxide, ternary lithium nickel cobalt manganese oxide, and ternary lithium nickel cobalt aluminum oxide.

[0017] Furthermore, relative to 100 parts by weight of the cathode material, the amount of the reversible crosslinking polymer is 0.05 to 20 parts by weight;

[0018] Preferably,

[0019] The amount of the reversible crosslinked polymer relative to 100 parts by weight of the cathode material is 0.1-12.5 parts by weight, preferably 5-10 parts by weight.

[0020] Furthermore, the reversible cross-linked polymer-coated cathode material is obtained by mixing and drying a reversible cross-linked polymer, a lithium salt, and a cathode material.

[0021] A method for preparing the above-mentioned reversible crosslinked polymer-coated cathode material, wherein the reversible crosslinked polymer and the cathode material are mixed and dried to obtain the reversible crosslinked polymer-coated cathode material.

[0022] A positive electrode includes a positive electrode material coated with the above-described reversible crosslinked polymer or a positive electrode material coated with a polymer prepared by the above-described method.

[0023] Use of the above-mentioned reversible crosslinked polymer in the preparation of cathode materials or cathodes.

[0024] A lithium-ion battery comprising a positive electrode made of a positive electrode material coated with the above-described reversible cross-linked polymer or a positive electrode material coated with a reversible cross-linked polymer prepared by the above-described method.

[0025] Beneficial effects

[0026] (1) Using the reversible cross-linked polymer described in this application to encapsulate the positive electrode material is beneficial to reduce the contact between the electrolyte and the positive electrode and reduce material corrosion, thereby improving cycle stability;

[0027] (2) When the battery is subjected to high external temperatures, the cross-linked polymer undergoes a depolymerization reaction, producing a large number of groups containing unsaturated double bonds. This process is endothermic, and the internal temperature of the battery is reduced through the above reaction. When the battery is subjected to continuous high heat, and the internal temperature continues to rise to the oxygen evolution temperature of the positive electrode, oxygen free radicals will be generated at the positive electrode. The unsaturated structure formed by the depolymerization of the cross-linked polymer can combine with the oxygen free radicals released from the positive electrode to undergo an addition reaction, absorbing them and preventing oxygen from freely diffusing to the negative electrode and causing thermal runaway.

[0028] (3) Compared with other methods, the positive electrode coating preparation method provided in this application is simple and easy to scale up production. Attached Figure Description

[0029] Figure 1 This is a diagram showing the effect of particle surface coating in Example 1;

[0030] Figure 2 This is a diagram showing the effect of the uncoated particle surface in Comparative Example 1.

[0031] Figure 3 The oxygen release results of the ternary material in Comparative Example 1, which was packaged in an aluminum-plastic bag;

[0032] Figure 4 Figure showing the oxygen release results of the ternary material encapsulated in Example 1 of aluminum-plastic bag packaging;

[0033] Figure 5 The graphs show the DSC test results for Example 1 and Comparative Example 2.

[0034] Figure 6 The cell heating box test temperature curves for Example 1 and Comparative Example 1 are shown.

[0035] Figure 7 This is a synthetic route diagram for a branched polymer with furan end groups.

[0036] Figure 8 A schematic diagram of the reaction process for preparing crosslinked polymer 1;

[0037] Figure 9 A schematic diagram of the reaction process for preparing crosslinked polymer 11. Detailed Implementation

[0038] Although the present invention has been disclosed above with reference to embodiments, it is not intended to limit the present invention. Anyone skilled in the art may make some modifications and refinements without departing from the spirit and scope of the present invention. Therefore, the scope of protection of the present invention shall be determined by the scope of the appended patent application.

[0039] This application provides a positive electrode material coated with a reversible crosslinked polymer, which is formed by one or more branched polymers through the mutual reaction between their branched groups to form one or more reversible crosslinked structures.

[0040] In some embodiments of this application, the reversible crosslinked structure is formed by thermal addition of a branched group with an end group containing an electron-rich conjugated diene group and a branched group with an end group containing an electron-deficient dienophile group.

[0041] In this application, an electron-rich conjugated diene group refers to a diene containing two carbon-carbon double bonds separated by a single bond, i.e., a conjugated system. The presence of the conjugated system in conjugated dienes gives them a special interatomic interaction—the conjugation effect. Compared to isolated dienes, the bond lengths are averaged, the molecular refractive index increases, and the internal energy decreases.

[0042] In this application, the dienophile group is typically a derivative of ethylene or acetylene, and one or both reacting atoms may be heteroatoms. The reactivity of dienophile groups varies and is structure-dependent. Generally, the more electron-withdrawing substituents on the double or triple bond, the more reactive the dienophile, because the substituents lower the energy of the dienophile's LUMO orbital. When the HOMO orbital of the diene and the LUMO orbital of the dienophile have similar energies, interaction is more likely to occur.

[0043] Some Diels-Alder reactions occur between an electron-rich conjugated dienophile group and an electron-deficient dienophile group. The most commonly used dienophiles are α,β-unsaturated carbonyl compounds. Typical examples are acrolein, acrylic acid and its esters, maleic acid and its anhydrides, fumaric acid or fumarate esters, 2-butyn-1,4-diacid (ethynyl dicarboxylic acid), and 2-cyclohexenone derivatives.

[0044] In some embodiments of this application, the reversible crosslinked structure formed is as shown in R1 or R2:

[0045]

[0046] Due to the reversibility of the DA reaction, the material can be transformed into a flowable linear polymer through thermally triggered crosslinking and depolymerization, and is often used as a self-healing material and a recyclable rubber.

[0047] In this application, the thermal runaway of lithium-ion batteries is mainly caused by the release of oxygen from the positive electrode material at high temperatures, which is then transferred to the negative electrode, causing a violent chemical reaction. The current solution is mainly through coating the positive electrode. Through research, the inventors of this application found that unsaturated double bond groups are prone to react with oxygen free radicals. However, substances containing unsaturated double bonds are often too reactive and react with the electrodes during charging and discharging. Through rigorous experiments, the inventors of this application discovered that introducing a reversible cross-linked polymer into the battery results in a cross-linked polymer state at low temperatures, which has stable electrochemical performance. Once triggered by heat, the polymer depolymerizes and generates unsaturated bonds to absorb the oxygen released from the positive electrode, which can significantly improve electrical performance and safety performance.

[0048] In this application, maleimide and furan can undergo depolymerization at temperatures below 100°C, which meets the temperature requirements before thermal runaway in lithium batteries.

[0049] In some embodiments of this application, the branched polymer is selected from one or more of polyester, polyether, or polyolefin.

[0050] In some embodiments of this application, the branched polymer is selected from one or more of the following formulas: P1, P2 and P3.

[0051]

[0052] Where a and b are independently selected from any one of -CH3, -H, -F, -OH and -NH2, 1≤m≤45, n≥2, and m and n are positive integers; R represents the branched end group.

[0053] For example, m can be 1, 2, 3, 4, 5, 6, 7, 8, 9, 10, 11, 12, 13, 14, 15, 16, 17, 18, 19, 20; n can be 5, 6, 7, 8, 9, 10, 11, 12, 13, 14, 15, 16, 17, 18, 19, 20, 21, 22, 23, 24, 25, 26, 27, 28, 29, 30 or a larger positive integer; further, preferably, 1≤m≤20, n≥5.

[0054] In some embodiments of this application, the number of the reversible crosslinked structures formed is less than or equal to the number of the branched polymer repeating units.

[0055] In one specific embodiment of this application, the reversible crosslinked polymer is formed by the interaction of two polymers, such as P1, to form an R1 structure.

[0056] In one specific embodiment of this application, the reversible crosslinked polymer is formed by the interaction of two polymers, such as P2, to form an R1 structure.

[0057] In one specific embodiment of this application, the reversible crosslinked polymer is formed by the interaction of two polymers, such as P3, to form an R1 structure.

[0058] In one specific embodiment of this application, the reversible crosslinked polymer is formed by the interaction of two polymers, such as P1, to form an R2 structure.

[0059] In one specific embodiment of this application, the reversible crosslinked polymer is formed by the interaction of two polymers, such as P2, to form an R2 structure.

[0060] In one specific embodiment of this application, the reversible crosslinked polymer is formed by the interaction of two polymers, such as P3, to form an R2 structure.

[0061] In one specific embodiment of this application, the reversible crosslinking polymer is formed by the mutual reaction between the branched groups of a polymer as shown in P1 and a polymer as shown in P2 to form an R1 structure.

[0062] In one specific embodiment of this application, the reversible crosslinking polymer is formed by the interaction between a polymer as shown in P1 and a polymer as shown in P3 to form an R1 structure.

[0063] In one specific embodiment of this application, the reversible crosslinking polymer is formed by the interaction between a polymer as shown in P3 and a polymer as shown in P2 to form an R1 structure.

[0064] In one specific embodiment of this application, the reversible crosslinking polymer is formed by the mutual reaction between the branched groups of a polymer as shown in P1 and a polymer as shown in P2 to form an R2 structure.

[0065] In one specific embodiment of this application, the reversible crosslinking polymer is formed by the mutual reaction between the branched groups of a polymer as shown in P1 and a polymer as shown in P3 to form an R2 structure.

[0066] In one specific embodiment of this application, the reversible crosslinking polymer is formed by the interaction between a polymer as shown in P3 and a polymer as shown in P2 to form an R2 structure.

[0067] In one specific embodiment of this application, the reversible crosslinking polymer is formed by the interaction of the branched groups of a polymer as shown in P1, a polymer as shown in P2, and a polymer as shown in P3 to form an R1 structure.

[0068] In one specific embodiment of this application, the reversible crosslinking polymer is formed by the interaction of the branched groups of a polymer as shown in P1, a polymer as shown in P2, and a polymer as shown in P3 to form an R2 structure.

[0069] In one specific embodiment of this application, the reversible crosslinking polymer is formed by the mutual reaction between the branched groups of a polymer as shown in P1, a polymer as shown in P2, and a polymer as shown in P3 to form structures R1 and R2.

[0070] In some embodiments of this application, during the formation of the reversible crosslinked polymer, the branched end groups of the branched polymers react with each other to form a reversible crosslinked structure. That is, the branched polymers crosslink with each other through groups that form the reversible crosslinked structure. The branched end groups are maleimide groups, furan groups, or cyclopentadiene groups. Multiple branched polymers are linked together to form the reversible crosslinked polymer through a Diels-Alder reaction between maleimide and furan groups, or a Diels-Alder reaction between two cyclopentadiene groups.

[0071] In some embodiments of this application, the cathode material is selected from one or more of lithium iron phosphate, lithium cobalt oxide, lithium manganese oxide, lithium nickel oxide, binary lithium nickel cobalt oxide, binary lithium nickel manganese oxide, ternary lithium nickel cobalt manganese oxide, and ternary lithium nickel cobalt aluminum oxide.

[0072] In some embodiments of this application, the amount of the reversible crosslinked polymer is 0.05-20 parts by weight, preferably 0.1-12.5 parts by weight, and more preferably 5-10 parts by weight, relative to 100 parts by weight of the cathode material.

[0073] For example, relative to 100 parts by weight of the cathode material, the amount of the reversible crosslinking polymer can be 0.1 parts by weight, 0.2 parts by weight, 0.3 parts by weight, 0.4 parts by weight, 0.5 parts by weight, 0.6 parts by weight, 0.7 parts by weight, 0.8 parts by weight, 0.9 parts by weight, 1.0 parts by weight, 1.1 parts by weight, 1.2 parts by weight, 1.3 parts by weight, 1.4 parts by weight, 1.5 parts by weight, 1.6 parts by weight, 1.7 parts by weight, 1.8 parts by weight, 1.9 parts by weight, 2.0 parts by weight, 2.1 parts by weight, 2.2 parts by weight, 2.3 parts by weight, 2.4 parts by weight, 2.5 parts by weight, 2.6 parts by weight, 2.7 parts by weight, 2.8 parts by weight, or 2.9 parts by weight. 3.0 parts by weight, 3.1 parts by weight, 3.2 parts by weight, 3.3 parts by weight, 3.4 parts by weight, 3.5 parts by weight, 3.6 parts by weight, 3.7 parts by weight, 3.8 parts by weight, 3.9 parts by weight, 4.0 parts by weight, 4.1 parts by weight, 4.2 parts by weight, 4.3 parts by weight, 4.4 parts by weight, 4.5 parts by weight, 4.6 parts by weight, 4.7 parts by weight, 4.8 parts by weight, 4.9 parts by weight, 5.0 parts by weight, 5.1 parts by weight, 5.2 parts by weight, 5.3 parts by weight, 5.4 parts by weight, 5.5 parts by weight, 5.6 parts by weight, 5.7 parts by weight, 5.8 parts by weight, 5.9 parts by weight, 6.0 parts by weight, 6.1 parts by weight, 6.2 parts by weight, 6.3 parts by weight 6.4 parts by weight, 6.5 parts by weight, 6.6 parts by weight, 6.7 parts by weight, 6.8 parts by weight, 6.9 parts by weight, 7.0 parts by weight, 7.1 parts by weight, 7.2 parts by weight, 7.3 parts by weight, 7.4 parts by weight, 7.5 parts by weight, 7.6 parts by weight, 7.7 parts by weight, 7.8 parts by weight, 7.9 parts by weight, 8.0 parts by weight, 8.1 parts by weight, 8.2 parts by weight, 8.3 parts by weight, 8.4 parts by weight, 8.5 parts by weight, 8.6 parts by weight, 8.7 parts by weight, 8.8 parts by weight, 8.9 parts by weight, 9.0 parts by weight, 9.1 parts by weight, 9.2 parts by weight, 9.3 parts by weight, 9.4 parts by weight, 9.5 parts by weight, 9.6 parts by weight, 9. 7 parts by weight, 9.8 parts by weight, 9.9 parts by weight, 10.0 parts by weight, 10.1 parts by weight, 10.2 parts by weight, 10.3 parts by weight, 10.4 parts by weight, 10.5 parts by weight, 10.6 parts by weight, 10.7 parts by weight, 10.8 parts by weight, 10.9 parts by weight, 11.0 parts by weight, 11.1 parts by weight, 11.2 parts by weight, 11.3 parts by weight, 11.4 parts by weight, 11.5 parts by weight, 11.6 parts by weight, 11.7 parts by weight, 11.8 parts by weight, 11.9 parts by weight, 12.0 parts by weight, 12.1 parts by weight, 12.2 parts by weight, 12.3 parts by weight, 12.4 parts by weight, 12.5 parts by weight, or any range thereof.

[0074] In some embodiments of this application, the reversible cross-linked polymer-coated cathode material is obtained by mixing and drying a reversible cross-linked polymer, a lithium salt, and a cathode material.

[0075] In some embodiments of this application, the lithium salt is selected from any one of lithium hexafluorophosphate (LiPF6), lithium perchlorate (LiClO4), lithium bis(trifluoromethanesulfonyl)imide (LiTFSI), lithium fluoroalkyl phosphate, lithium bis(trifluoromethanesulfonyl)methylation, lithium dioxolaneborate, lithium hexafluoroarsenate, lithium tetrafluoroborate, lithium trifluoromethanesulfonylimide, and lithium difluorosulfonylimide.

[0076] This application provides a method for preparing the above-mentioned reversible cross-linked polymer-coated cathode material, wherein the reversible cross-linked polymer and the cathode material are mixed and dried to obtain the reversible cross-linked polymer-coated cathode material.

[0077] In some embodiments of this application, a reversible crosslinked polymer material, a cathode material, and a lithium salt are mixed, heated to the depolymerization temperature of the material's reversible structure, and then stirred and mixed. After being mixed evenly, the mixture is cooled to room temperature and polymerization crosslinking occurs, with the reversible crosslinked polymer coating the surface of the cathode material.

[0078] In some embodiments of this application, a reversible crosslinked polymer material, a cathode material, a lithium salt, and a solvent are uniformly mixed to obtain a slurry with a total solid content of 10wt%-80wt%. After uniform mixing, the slurry is heated and dried above the depolymerization temperature of the reversible crosslinked polymer structure, and then cooled to room temperature to obtain a cathode material encapsulated by the reversible crosslinked polymer. The solvent is N-methylpyrrolidone (NMP), dimethylformamide (DMF), dimethyl sulfoxide (DMSO), or acetone.

[0079] For a description of reversible crosslinked polymers and cathode materials, please refer to the previous text; it will not be repeated here.

[0080] This application provides a positive electrode, including the positive electrode material coated with the above-described reversible cross-linked polymer or the positive electrode material coated with the above-described reversible cross-linked polymer.

[0081] This application provides a lithium-ion battery, including a positive electrode made of the above-described reversible cross-linked polymer coated positive electrode material or the above-described reversible cross-linked polymer coated positive electrode material.

[0082] In some embodiments of this application, the preparation method of the reversible crosslinked polymer is as follows: a branched polymer with one or more maleimide groups and furan groups as branched end groups is dissolved in a solvent, and the maleimide groups and furan groups undergo a Diels-Alder reaction to form a thermally reversible structure R1, thereby forming a polymer with R1 as the reversible crosslinked structure.

[0083] In some embodiments of this application, the preparation method of the reversible crosslinked polymer is as follows: one or more branched polymers with cyclopentadienyl end groups are dissolved in a solvent, and the cyclopentadienyl groups undergo a Diels-Alder reaction with each other to form a thermally reversible structure R2, thereby forming a reversible crosslinked polymer with a reversible crosslinking structure of R2.

[0084] In some embodiments of this application, the reversible crosslinked polymer is prepared as follows: a branched polymer with one or more branched end groups being cyclopentadienyl groups, maleimide groups, and furan groups is dissolved in a solvent. The thermally reversible structure formed by the Diels-Alder reaction between the maleimide groups and furan groups is R1, and the thermally reversible structure formed by the Diels-Alder reaction between the cyclopentadienyl groups is R2.

[0085] In one specific embodiment, a branched polyester polymer with furan or maleimide end groups is prepared as follows: Fumaric acid, 1,4-butanediol, zinc chloride, and hydroquinone are added to a three-necked flask in a molar ratio of 1:3:0.01:0.006, introducing hydroquinone as a free radical crosslinking inhibitor. The esterification reaction is carried out at 150°C under a nitrogen atmosphere for 5 hours until the theoretical amount of water is separated. The product is heated and dissolved in tetrachloroethane, then poured into cold methanol for precipitation, drying, resolution, secondary precipitation, filtration, and drying to obtain the precursor polymer PBF. 7.5 g of PBF and 150 mL of a mixed solvent of CHCl2CHCl2 / CH3CN (v / v = 1 / 1) were added to a three-necked flask. After stirring under nitrogen, 7.8 mL of furfural (FA) and 1.25 mL of triethylamine (Et3N) were added. After reacting at 80 °C for 24 h, a branched polyester PBF-FA with furan groups at the end of the branched chain was obtained (P1' was tested by gel permeation chromatography with PDI = 2.5, and its main chain repeating unit n = 80.2 was tested by 1H NMR spectroscopy. For ease of comparison, it will be described as n = 80 thereafter). Alternatively, 7.5 g of PBF and 150 mL of a mixed solvent of CHCl2CHCl2 / CH3CN (v / v = 1 / 1) were added to a three-necked flask. After stirring under nitrogen, 7.8 mL of N-(5-aminopentyl)maleimide (NA) and 1.25 mL of triethylamine (Et3N) were added. After reacting at 80 °C for 24 h, a branched polyester PBF-NA with maleimide end groups was obtained (P1 was tested by gel chromatography with PDI = 2.3, and its main chain repeating unit n = 79.6 by 1H NMR spectroscopy, and it is described as n = 80 for easy comparison).

[0086] In one specific embodiment, a branched polyether polymer with furan or maleimide end groups is prepared as follows: 100g of glycidyl ether styrene and 300g of ethylene glycol dimethyl ether are added to a three-necked flask and stirred evenly. After adding 1g of lithium hexafluorophosphate, the mixture is reacted at 40°C for 2 hours to initiate the ring-opening polymerization of glycidyl ether styrene. The solution is then introduced into water for precipitation, filtered, and washed repeatedly three times to obtain a branched polyether PEO-FA with furan end groups (P2' was tested by gel electrophoresis, PDI = 3.2, and its main chain repeating unit n = 79.8 by 1H NMR spectroscopy, and is described as n = 80 for ease of comparison). Alternatively, 100g of glycidyl ether maleimide and 300g of ethylene glycol dimethyl ether are added to a three-necked flask and stirred until homogeneous. After adding 1g of lithium hexafluorophosphate, the mixture is reacted at 40°C for 2 hours to initiate the ring-opening polymerization of glycidyl ether maleimide. The solution is then introduced into water for precipitation, filtration, and washing three times to obtain a branched polyether PEO-NA with maleimide end groups (P2 was tested by gel chromatography with PDI = 3.4, and its main chain repeating unit n = 80.3 by 1H NMR spectroscopy; for ease of comparison, it will be described as n = 80 thereafter).

[0087] In one specific embodiment, a branched polyolefin polymer with furan or maleimide end groups is prepared as follows: Polymerization is carried out in a 1L polymerization reactor equipped with a mechanical stirrer and temperature control device. The reactor is heated to 80°C, evacuated, and purged with ethylene, repeated three times. Then, 300 mL of n-hexane, 10 mL of triethylaluminum n-hexane solution, 100 g of 2-allylfuran, and 100 mg of Ziegler-Natta catalyst are added sequentially. Hydrogen and ethylene are then introduced to a pressure of 0.1 MPa. After polymerization for 1 hour, stirring is stopped to terminate the polymerization. The polymer is washed and dried to obtain a branched polyolefin PE-FA (P3') with furan end groups. Alternatively, the polymerization reaction is carried out in a 1L polymerization reactor equipped with a mechanical stirrer and temperature control device. The reactor is heated to 80°C, evacuated, and purged with ethylene, repeated 3 times. Then, 300mL of n-hexane, 10mL of triethylaluminum n-hexane solution, 100g of N-allyl maleimide, and 100mg of catalyst (Ziegler-Natta catalyst) are added sequentially. Hydrogen and ethylene are then introduced to 0.1MPa. After polymerization for 1 hour, stirring is stopped to terminate the polymerization. The product is washed and dried to obtain a branched polyolefin PE-NA (P3) with maleimide end groups.

[0088] In one specific embodiment, the branched polyester polymer PBF-CP (P1”) with cyclopentadienyl end groups is prepared as follows: the monomer is replaced with cyclopentadienylethylamine with amine groups, in the same manner as the preparation of the branched maleimide polymer P1.

[0089] In one specific embodiment, the branched polyether polymer PEO-CP with cyclopentadiene end groups is prepared as follows: the monomer is replaced with cyclopentadiene glycidyl ether in the same manner as the preparation of the maleimide polymer P2 with side chains.

[0090] In one specific embodiment, the branched polyolefin polymer PE-CP, whose end-branch groups are cyclopentadiene groups, is prepared as follows: the monomer is replaced with allylcyclopentadiene in the same manner as the polymer P2 with side-chain maleimide.

[0091] Preparation Example

[0092] Example 1 of preparation of reversible crosslinked polymers:

[0093] A branched polymer P1, with maleimide end groups and n=80, m=1, and a and b being -CH3 groups; and a branched polymer P1', with furan end groups and n=80, m=1, and a and b being -CH3 groups, were dissolved in DMF in a 1:1 molar ratio in a three-necked flask and reacted with stirring at 60°C for 2 hours. The maleimide and furan groups underwent Diels-Alder reaction to form a thermally reversible structure, R1, thus forming a reversible crosslinked polymer. The reaction product was then poured into an evaporating dish and dried at 80°C for 24 hours. Unreacted products were washed away with dimethyl carbonate (DMC), and the product was dried again to obtain crosslinked polymer 1 (attached). Figure 8 ).

[0094] Example 2 of preparation of reversible crosslinked polymers:

[0095] In a three-necked flask, 50g of a branched polymer P1” with cyclopentadienyl end groups was dissolved in 200g of DMF, where n=80, m=1, and a and b are -CH3. The mixture was stirred at 40°C for 1h, resulting in a Diels-Alder reaction between the cyclopentadienyl groups to form a branched crosslinked polymer with a thermally reversible structure R2. The reaction product was then poured into an evaporating dish and dried at 80°C for 24h. Unreacted products were washed away with dimethyl carbonate (DMC), and the product was dried to obtain crosslinked polymer 11 (see attached image). Figure 9 ).

[0096] Example 3: Preparation of reversible crosslinked polymers

[0097] In a three-necked flask, polymers P1 (n=80, m=1, a and b are -CH3), P1' (n=80, m=1, a and b are -CH3), and P1'' (n=80, m=1, a and b are -CH3), were dissolved in DMF in a molar ratio of 0.5 mol: 0.5 mol: 1 mol. After stirring at 50°C for 2 hours, a thermally reversible structure R1 was formed by the Diels-Alder reaction between maleimide and furan groups, and a thermally reversible structure R2 was formed by the Diels-Alder reaction between cyclopentadiene groups, thus forming a polymer with both reversible crosslinking structures R1 and R2. The polymer was then poured into an evaporating dish and dried at 80°C for 24 hours. Unreacted products were then washed away with dimethyl carbonate (DMC), and the polymer was dried again to obtain reversible crosslinked polymer 10.

[0098] Example

[0099] Example 1: Prepared reversible crosslinked polymer 1 from Preparation Example 1

[0100] Coating process: 5g of crosslinked polymer 1 from Preparation Example 1, 5g of LiTFSI as lithium salt, and 90g of NCM811 as positive electrode material were used, wherein the mass ratio of crosslinked polymer 1, positive electrode material and lithium salt was 5:90:5. After initial dry mixing for 20 minutes, a solid mixture was formed. The solid mixture was heated to 100°C and stirred for 30 minutes. Then, while stirring, the temperature was lowered to 50°C, and then cooled to room temperature to obtain the coated positive electrode material.

[0101] Battery preparation and testing: 95g of coated positive electrode material, 3g of conductive agent, and 2g of PVDF were added to 80g of NMP, stirred evenly, coated onto the current collector, and dried to obtain the positive electrode sheet with a single-sided areal density of 200g / m². 2 96g of graphite, 1.2g of CMC, 1.5g of SBR, and 1.3g of conductive agent were dissolved in 150g of pure water, stirred, coated onto a current collector, and dried to obtain a negative electrode sheet with a single-sided areal density of 114g / cm2. The obtained positive electrode sheet, negative electrode sheet, positive and negative electrode sheets, and commercial separator were used to prepare a 10Ah soft-pack battery cell. After injecting electrolyte (EC / EMC / DMC = 1 / 1 / 1 3% VC 1M LiPF6) and soaking for 24h, formation and capacity testing were performed to obtain the final soft-pack battery cell.

[0102] The above-mentioned soft-pack battery cells were subjected to 0.1C charge-discharge and hot box tests (heating rate 5℃ / min, cutoff temperature 200℃).

[0103] Example 2

[0104] The difference between Example 2 and Example 1 is that the crosslinked polymer 2 is prepared according to the method of Preparation Example 1, except that furfurylamine and N-(5-aminopentyl)maleimide are replaced with aminofuran and aminomaleimide with longer alkyl chains. The rest is the same as in Example 1. A branched polyester P1 with maleimide end groups and a branched polyester P1' with furan end groups are formed. In P1, n = 80, m = 10, a and b are -CH3, and in P1', n = 80, m = 10, a and b are -CH3, and R group is R1.

[0105] Example 3

[0106] The difference between Example 3 and Example 1 is that the crosslinked polymer 3 is prepared according to the method of Preparation Example 1, except that furfurylamine and N-(5-aminopentyl)maleimide are replaced with aminofuran and aminomaleimide with longer alkyl chains. The rest is the same as in Example 1. A branched polyester P1 with maleimide end groups and a branched polyester P1' with furan end groups are formed. In P1, n = 80, m = 20, a and b are -CH3, and in P1', n = 80, m = 20, a and b are -CH3, and R group is R1.

[0107] Example 4

[0108] The only difference between Example 4 and Example 1 is that the crosslinked polymer 4 is prepared according to the method of Preparation Example 1, except that the molar ratio of fumaric acid, 1,4-butanediol, zinc chloride and hydroquinone in the preparation process of P1 is changed to 1:5:0.01:0.006 to reduce its main chain molecular weight. The rest of the process is the same. In P1, n=50, m=1, and a and b are -CH3; in P1', n=50, m=1, a and b are -CH3, and the R group is R1.

[0109] Example 5

[0110] The difference between Example 5 and Example 1 is that the crosslinked polymer 5 is prepared according to the method of Preparation Example 1. The branched polymer of crosslinked polymer 5 is a polyether, and the branched polyether P2 has maleimide end groups, where n = 80, m = 1, a and b are -CH3. It reacts with the branched polyether P2' has furan end groups, where n = 80, m = 1, a and b are -CH3, and R group is R1.

[0111] Example 6

[0112] The difference between Example 6 and Example 1 is that the crosslinked polymer 6 is prepared according to the method of Preparation Example 1. The branched polymer of crosslinked polymer 6 is a polyolefin, and the branched polymer P3 is a maleimide group with branched end groups, where n = 80, m = 1, a and b are -CH3. It reacts with a branched polyolefin P3' with furan group with branched end groups, where n = 80, m = 1, a and b are -CH3, and R group is R1.

[0113] Example 7

[0114] The difference between Example 7 and Example 1 is that the crosslinked polymer 7 is prepared according to the method of Preparation Example 1. A branched polyester P1 with maleimide end groups, where n = 80, m = 1, and a and b are -CH3, is reacted with a branched polyether P2' with furan end groups, where n = 80, m = 1, a and b are -CH3, and R group is R1; and the molar ratio of P1 to P2' is 1:1.

[0115] Example 8

[0116] The difference between Example 8 and Example 1 is that the crosslinked polymer 8 is prepared according to the method of Preparation Example 1, and the branched polyester P1 with maleimide end groups, where n = 80, m = 1, a and b are -CH3, reacts with the branched polyolefin P3' with furan end groups, where n = 80, m = 1, a and b are -CH3, and R group is R1; and the molar ratio of P1 to P3' is 1:1.

[0117] Example 9

[0118] The difference between Example 9 and Example 1 is that the crosslinked polymer 9 is prepared according to the method of Preparation Example 1, and the branched polyether P2 with maleimide end groups, where n = 80, m = 1, a and b are -CH3, reacts with the branched polyolefin P3' with furan end groups, where n = 80, m = 1, a and b are -CH3, and R group is R1; and the molar ratio of P2 to P3' is 1:1.

[0119] Example 10

[0120] Crosslinked polymer 1 was used. The only difference between Example 10 and Example 1 was that the mass ratio of crosslinked polymer 1, positive electrode material and lithium salt was 0.1:99.8:0.1. The rest was the same as in Example 1.

[0121] Example 11

[0122] Crosslinked polymer 1 was used. The only difference between Example 11 and Example 1 was that the mass ratio of crosslinked polymer 1, positive electrode material and lithium salt was 10:80:10. The rest was the same as in Example 1.

[0123] Example 12

[0124] The difference between Example 12 and Example 1 is that the crosslinked polymer 10 is prepared according to the method of Preparation Example 3. The three branched polyesters P1 with maleimide end groups, P1' with furan end groups, and P1" with cyclopentadienyl end groups are blended and then reacted. n = 80, m = 1, a and b are -CH3, and the R group contains both R1 and R2 with a molar ratio of 1:1.

[0125] Example 13

[0126] The only difference between Example 13 and Example 1 is that the crosslinked polymer 11 of Preparation Example 2 was used, n=80, m=1, a and b are -CH3, and R group is R2; the rest is the same as in Example 1.

[0127] Comparative Example 1

[0128] Using uncoated cathode material, the battery preparation and testing were the same as in Example 1.

[0129] Comparative Example 2

[0130] The only difference between Comparative Example 2 and Example 1 is that the coating polymer is a non-branched polymer PBF, where n = 80a and b is -CH3; the rest is the same as Example 1, that is, the positive electrode coating material is a polymer that has not formed a reversible cross-linked structure.

[0131] Example 14

[0132] The difference between Example 14 and Example 1 is only that the crosslinked polymer 13 is prepared according to the method of Preparation Example 1, except that the molar ratio of fumaric acid, 1,4-butanediol, zinc chloride, and hydroquinone in the preparation of P1 is changed to 1:6:0.01:0.006, the reaction temperature is lowered to 100°C, and its main chain molecular weight is reduced; the rest of the process is the same. Polyester P1 in which n=20, m=1, a and b are -CH3 groups reacts with a branched polyester P1' whose end group is a furan group, where n=20, m=1, a and b are -CH3 groups, and the R group is R1.

[0133] Example 15

[0134] The difference between Example 15 and Example 1 is that the crosslinked polymer 14 is prepared according to the method of Preparation Example 1, except that furfurylamide and N-(5-aminopentyl)maleimide are replaced with aminofuran and aminomaleimide with longer alkyl chains. The rest is the same as in Example 1, forming a branched polyester P1 with maleimide end groups. This polyester P1 is then reacted with a branched polyester P1' with furan end groups. In P1, n = 80, m = 25, and a and b are -CH3. In P1', n = 80, m = 25, a and b are -CH3, and the R group is R1.

[0135] Example 16

[0136] The difference between Example 16 and Example 1 is that the crosslinked polymer 15 is prepared according to the method of Preparation Example 1, except that furfurylamide and N-(5-aminopentyl)maleimide are replaced with aminofuran and aminomaleimide with longer alkyl chains. The rest is the same as in Example 1, forming a branched polyester P1 with maleimide end groups. This polyester P1 is then reacted with a branched polyester P1' with furan end groups. In P1, n = 80, m = 45, and a and b are -CH3; in P1', n = 80, m = 45, a and b are -CH3, and the R group is R1.

[0137] Example 17

[0138] The only difference between Example 17 and Example 1 is that the mass ratio of crosslinked polymer 1, cathode material and lithium salt is 0.05:99.9:0.05, and the rest is the same as in Example 1.

[0139] Example 18

[0140] The only difference between Example 18 and Example 1 is that the mass ratio of crosslinked polymer 1, cathode material and lithium salt is 20:60:20, and the rest is the same as in Example 1.

[0141] Example Test Results

[0142] Test method:

[0143] Depolymerization temperature test: DSC, temperature range 25-300℃, heating rate 10℃ / min;

[0144] Thermal runaway temperature: The battery is placed in a special heating oven for lithium-ion batteries and heated to 130°C at a rate of 5°C / min. Then, the temperature is increased by 10°C and held for 1 hour, with the maximum temperature reaching 200°C. The ambient temperature of the battery near the explosion point in the oven is the thermal runaway temperature of the lithium-ion battery.

[0145] Cyclic testing: The battery was charged and discharged using Wuhan Landian Equipment, with a voltage range of 2.75-4.25V and 0.1C / 0.1C charge and discharge. The capacity of the first discharge cycle was taken as the capacity at 0.1C, and the number of cycles when the capacity decayed to 80% was counted.

[0146] Oxygen release test: The positive electrode material is sealed in an aluminum-plastic bag under nitrogen atmosphere. After sealing, the aluminum-plastic bag is placed in a special heating oven for lithium-ion batteries and heated to 200°C at a rate of 5°C / min for 1 hour. Then, the gas in the aluminum-plastic bag is tested by gas chromatography.

[0147] In terms of materials analysis Figure 1 and Figure 2 The SEM images show that the surface of the cathode material prepared using this method is coated with a cross-linked polymer, which helps to reduce the contact between the electrolyte and the particles and reduce material corrosion.

[0148] from Figure 3 , Figure 4 GC-MS test results show that the cathode material coated with cross-linked polymer releases less oxygen after high temperature, indicating that the introduction of cross-linked polymer can reduce oxygen release from the cathode, thereby improving safety.

[0149] Figure 5 The DSC test results for Example 1 and Comparative Example 2 show that, in the presence of reversible crosslinking groups, the polymer exhibits a depolymerization endothermic peak. During the early stage of battery thermal runaway, the polymer depolymerizes after being heated, which can absorb some heat.

[0150] The application effect in battery cells can be seen from Figure 6 see, Figure 6 The temperature curves of the cell test chambers for Example 1 and Comparative Example 1 are shown. The cell using the cross-linked polymer coated with the positive electrode material of this application has a significantly higher thermal runaway temperature.

[0151] Table 1. Effects of m and n on performance

[0152]

[0153]

[0154] Note: n in the table is a rounded value.

[0155] As can be seen from Examples 1-3, 15, and 16, there is an optimal value for the range of m. When the range of m is too large, the thermal runaway temperature of the battery is not significantly improved compared to the liquid cell. This is because the side chain is too long, which leads to a decrease in the proportion of R groups, thus the improvement in thermal safety is not significant.

[0156] As can be seen from the comparison between Example 1 and Example 14, the value range of n cannot be too small. This is because n is the degree of polymerization of the molecular backbone. If the degree of polymerization is too low, the activity of the monomer will increase, resulting in too fast cycle decay.

[0157] Table 2. Impact of P-structure on performance

[0158]

[0159] Comparing Examples 1, 5, and 6, it can be seen that when the three polymers with different structures (polyester, polyether, and polyolefin) are used to construct cross-linked polymers as coating layers, there are slight differences in electrical properties. In terms of capacity performance, polyether > polyester > polyolefin. This is because ethers and esters have better affinity for electrolytes, which is conducive to electrolyte wetting and the construction of fast ion channels. Moreover, the oxidation state of ester bonds is higher than that of ether bonds, resulting in better high-voltage resistance, thermal stability, and cycling performance.

[0160] As can be seen from Examples 7, 8, and 9, a coating layer with better performance can be obtained by crosslinking ester-containing segments and ether-containing segments; the introduction of ether segments improves the flexibility of the polyester, which is more conducive to improving ionic conductivity, thereby reducing concentration polarization and improving cycle stability.

[0161] Table 3 Effect of polymer coating amount on performance

[0162]

[0163]

[0164] As can be seen from Examples 1, 10, and 11, with the increase of coating amount, i.e., the amount of reversible cross-linked polymer used for the cathode material, the capacity of the battery prepared with it gradually decreases, while the thermal runaway temperature increases. Selecting a certain range of coating amount can improve the safety performance of the cell while ensuring electrical performance. When the coating amount exceeds the reasonable range: as in Example 17, if the coating amount is too small, there is basically no significant improvement in thermal safety; as in Example 18, if the coating amount is too large, the kinetics decrease and the battery capacity is low. Neither of these is the preferred range.

[0165] Table 4. Effect of R group on performance

[0166] R 0.1C Capacity Utilization (Ah) Capacity decays to 80% of cycle count Thermal runaway temperature (°C) Example 1 <![CDATA[R1]]> 9.5 450 180 Example 12 <![CDATA[R1 and R2]]> 9.5 453 180 Example 13 <![CDATA[R2]]> 9.5 455 180 Comparative Example 1 / 10.0 345 155 Comparative Example 2 / 9.4 399 155

[0167] As can be seen from Examples 1, 12, and 13, both R1 and R2 structures provided in this application can help with battery thermal safety. This is because both structures are thermally reversible cross-linked structures, which will undergo depolymerization reaction to absorb heat when heated, and the unsaturated structure formed can combine with oxygen free radicals released from the positive electrode, preventing them from diffusing to the negative electrode and causing thermal runaway. As can be seen from Comparative Example 2, when reversible cross-linked groups are not formed in the polymer, ordinary polymer matrix cannot improve the thermal safety of the cell.

[0168] In summary, in this application, the surface of the positive electrode material is coated with a cross-linked polymer, which helps to reduce the contact between the electrolyte and the particles and reduce material corrosion, and releases less oxygen after high temperature. R1 and R2 are thermally reversible cross-linked structures, meaning that when a reversible cross-linked structure exists in the polymer, the polymer will undergo a depolymerization reaction to absorb heat when heated, and the resulting unsaturated structure can combine with the oxygen free radicals released from the positive electrode, preventing them from diffusing to the negative electrode and causing thermal runaway; if there are no reversible cross-linked groups in the polymer, relying solely on an ordinary polymer matrix cannot improve the thermal safety of the battery cell.

[0169] When three polymers with different structures (polyester, polyether, and polyolefin) are used to construct crosslinked polymers for coating layers, slight differences in the electrical properties of the cathode are observed. This is because ethers and esters have better affinity for the electrolyte, which is beneficial for electrolyte wetting and the formation of fast ion channels. Furthermore, the oxidized state of ester bonds is higher than that of ether bonds, resulting in better high-voltage resistance, thermal stability, and cycling performance. Simultaneously, copolymerization of ester-containing and ether-containing segments can yield coating layers with even better performance. The introduction of ether segments improves the flexibility of the polyester, which is more conducive to improving ionic conductivity, thereby reducing concentration polarization and improving cycling stability.

Claims

1. A positive electrode material coated with a reversibly cross-linked polymer, characterized in that, The reversible crosslinked polymer is formed by the mutual reaction between the branched groups of one or more branched polymers to form one or more reversible crosslinked structures; the reversible crosslinked structure is formed by thermal addition of branched groups with electron-rich conjugated diene groups at the end and branched groups with electron-deficient dienophilic groups at the end. The reversible crosslinking structure is R1 or R2, wherein R1 or R2 is as follows: R1R2.

2. The cathode material according to claim 1, characterized in that, The branched polymer is selected from one or more of polyester, polyether, or polyolefin.

3. The cathode material according to claim 2, characterized in that, The branched polymer is selected from one or more of the following formulas: P1, P2 and P3. Formula P1 Formula P2 Formula P3 Where a and b are independently selected from any one of -CH3, -H, -F, -OH and -NH2, 1≤m≤45, n≥2, m and n are positive integers, and R represents the branched end group.

4. The cathode material according to claim 3, characterized in that... In the branched polymer, 1 ≤ m ≤ 20, n ≥ 5.

5. The positive electrode material according to claim 1, characterized in that, The cathode material is selected from one or more of lithium iron phosphate, lithium cobalt oxide, lithium manganese oxide, lithium nickel oxide, binary lithium nickel cobalt oxide, binary lithium nickel manganese oxide, ternary lithium nickel cobalt manganese oxide, and ternary lithium nickel cobalt aluminum oxide.

6. The cathode material according to claim 1, characterized in that, The amount of the reversible crosslinked polymer is 0.05 to 20 parts by weight relative to 100 parts by weight of the cathode material.

7. The cathode material according to claim 6, characterized in that, The amount of the reversible crosslinked polymer is 0.1-12.5 parts by weight relative to 100 parts by weight of the cathode material.

8. The cathode material according to claim 7, characterized in that, The amount of the reversible crosslinked polymer is 5-10 parts by weight relative to 100 parts by weight of the cathode material.

9. The positive electrode material according to claim 1, characterized in that, The reversible cross-linked polymer-coated cathode material is obtained by mixing and drying a reversible cross-linked polymer, a lithium salt, and a cathode material.

10. A method for preparing a cathode material coated with a reversible crosslinked polymer according to any one of claims 1-9, wherein the reversible crosslinked polymer and the cathode material are mixed and dried to obtain the cathode material coated with the reversible crosslinked polymer.

11. A positive electrode comprising a positive electrode material coated with a reversibly crosslinked polymer as described in any one of claims 1-9 or a positive electrode material coated with a polymer as prepared by the method of claim 10.

12. Use of the reversible crosslinked polymer according to any one of claims 1-9 in the preparation of a cathode material or cathode.

13. A lithium-ion battery comprising a positive electrode made of a positive electrode material coated with a reversible cross-linked polymer as described in any one of claims 1-9 or a positive electrode material coated with a reversible cross-linked polymer as described in claim 10.

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