Modified positive electrode material and preparation method thereof, positive electrode and lithium ion battery

Abstract

The invention provides a modified positive electrode material and a preparation method thereof, a positive electrode and a lithium ion battery. The modified positive electrode material comprises an inner core and a porous carbon coating layer arranged on the surface of the inner core, the inner core has a general formula of LiMnxFe1-x-yCoyPO4, x is greater than or equal to 0.3 and less than or equal to 0.8, and y is greater than or equal to 0.01 and less than or equal to 0.03; the material of the porous carbon coating layer comprises nitrogen-doped porous carbon and reduced graphene oxide. Porous carbon is combined with reduced graphene oxide, so that a three-dimensional conductive framework is linked with a two-dimensional conductive sheet layer to form a three-dimensional conductive network, the conductivity of the modified positive electrode material is remarkably improved, and the volume expansion of the modified positive electrode material in the charge-discharge cycle process is relieved; the doping of Co ions can shorten the M-P bond (M = Mn and / or Fe, and P is phosphorus in phosphate radical), reduce the unit cell volume and broaden the lithium ion transmission path, thereby improving the rate capability and cycle performance of the lithium ion battery.

Description

Technical Field

[0001] This application relates to the field of lithium-ion battery technology, and more specifically, to a modified cathode material and its preparation method, a cathode, and a lithium-ion battery. Background Technology

[0002] Lithium-ion batteries have become a core component of current new energy technology research due to their environmental friendliness, wide range of applications, and efficient energy storage capabilities. Among numerous cathode materials, lithium iron phosphate (LiFePO4) has become the preferred choice for commercial applications due to its excellent structural stability, high energy density, outstanding environmental compatibility and safety performance, and long cycle life. Compared to lithium iron phosphate, lithium manganese iron phosphate (LiMnFePO4), which also has an olivine structure, has a higher discharge voltage plateau (the discharge voltage plateau of lithium iron phosphate is about 3.4V, while that of lithium manganese iron phosphate is about 4.1V) and a higher theoretical specific capacity (reaching 170mAh / g). Therefore, lithium manganese iron phosphate materials are also gradually attracting the attention of researchers.

[0003] However, due to the inherent defects of its olivine structure, namely low electronic conductivity and limited lithium-ion diffusion paths, lithium manganese iron phosphate (LMP) materials suffer from certain negative impacts on their performance, limiting the potential of their finished products. In the olivine structure, electrons can only be transported through a point-to-point route, while lithium ions only have a one-dimensional curved channel along the (010) direction. Therefore, in practical applications, to improve the electronic conductivity and lithium-ion diffusion kinetics of LMP materials, researchers typically adopt the following strategies: first, coating with highly conductive materials such as carbon materials or lithium salts to improve the electronic conductivity of LMP materials; second, promoting lithium-ion diffusion through elemental doping, such as cobalt and nickel; and third, adjusting the primary particle size and morphology of LMP materials to shorten the lithium-ion transport distance, thereby improving the lithium-ion transport efficiency.

[0004] Existing literature (publication number CN115893363A) discloses a composite cathode material and its preparation method and application. The preparation method includes the following steps: (1) mixing a first lithium source, a manganese source, an iron source and a phosphorus source with a solvent to obtain a lithium iron manganese phosphate precursor, heat-treating the lithium iron manganese phosphate to obtain lithium iron manganese phosphate powder, mixing nickel cobalt manganese hydroxide and a second lithium source, and sintering to obtain a lithium nickel cobalt manganese oxide cathode material; (2) mixing the lithium iron manganese phosphate powder and the lithium nickel cobalt manganese oxide cathode material obtained in step (1) with MOF material and stirring to obtain a mixed powder; (3) calcining the mixed powder obtained in step (2) to obtain a composite cathode material with MOF coating LMFP and NCM.

[0005] Existing literature (publication number CN116344762A) discloses a method for preparing MOF-derived porous carbon thin-layer coated lithium manganese iron phosphate material. Nitrogen-doped metal-organic framework material and lithium manganese iron phosphate material are prepared separately. The two materials are mixed in phosphate buffer solution at low temperature by ultrasonication to obtain a homogeneous slurry. The slurry is then milled, spray-dried, and sintered under an inert atmosphere to obtain the MOF-derived porous carbon thin-layer coated carbon-nitrogen co-doped lithium manganese iron phosphate material.

[0006] Existing literature (CN117038936A) discloses a modified lithium manganese iron phosphate material, its preparation method, cathode, and lithium battery. This modified lithium manganese iron phosphate material includes a ZIF-67 matrix and a lithium manganese iron phosphate coating layer on the surface of the ZIF-67 matrix. The preparation method includes: step S1, subjecting raw materials including the ZIF-67 matrix, lithium source, iron source, manganese source, phosphorus source, and iridium source to a hydrothermal reaction followed by cooling to obtain a precursor precipitate; step S2, sintering the precursor precipitate under an inert atmosphere to obtain the modified lithium manganese iron phosphate material.

[0007] Existing literature (CN118173765A) discloses a composite lithium manganese iron phosphate cathode material, its preparation method, and its application. The preparation method includes: dispersing an iron source and a manganese source in a solvent, adding an organic ligand and performing ultrasonic treatment, then adding a gelation aid to perform a gelation reaction, and freeze-drying to obtain MnFe-MOF aerogel; ball milling the MnFe-MOF aerogel with a lithium source and a phosphorus source, and then calcining it under a protective atmosphere to obtain the composite lithium manganese iron phosphate cathode material.

[0008] However, the above-mentioned methods for preparing lithium manganese iron phosphate cathode materials have problems such as complex production conditions and high costs. Furthermore, by simply combining them with organic ligands such as organometallic framework compounds and zeolite imidazole metal framework compounds through physical means, it is difficult to maximize their effect. The conductivity and lithium-ion transport efficiency of the prepared lithium manganese iron phosphate materials still need to be improved.

[0009] Therefore, researching and developing a modified cathode material and its preparation method is of great significance for improving the conductivity and lithium-ion transport efficiency of lithium manganese iron phosphate cathode materials. Summary of the Invention

[0010] The main objective of this application is to provide a modified cathode material and its preparation method, a cathode, and a lithium-ion battery, in order to solve the problems of low conductivity and lithium-ion transport efficiency of lithium manganese iron phosphate cathode materials in the prior art, and the resulting poor rate performance and cycle performance of lithium-ion batteries.

[0011] To achieve the above objectives, this application provides a modified cathode material comprising a core and a porous carbon coating layer disposed on the surface of the core; the core has the general formula LiMnx Fe 1-x-y Co y PO4, where 0.3≤x≤0.8, 0.01≤y≤0.03; the porous carbon coating material includes nitrogen-doped porous carbon and reduced graphene oxide.

[0012] Furthermore, the specific surface area of ​​the modified cathode material is 16.5–24 m². 2 / g, preferably 20-23m 2 / g; compacted density is 2.25~2.45g / cm³ 3 .

[0013] Furthermore, the carbon content in the porous carbon coating layer is 1.2 to 2.5 wt%, based on the weight percentage of the modified cathode material.

[0014] Furthermore, the weight ratio of porous carbon to reduced graphene oxide is (1-5):(5-9), preferably (1-3):(7-9).

[0015] Furthermore, based on the weight percentage of reduced graphene oxide, the content of oxygen-containing functional groups in reduced graphene oxide is ≤30wt%.

[0016] To achieve the above objectives, another aspect of this application provides a method for preparing the modified cathode material provided in this application. The method includes: step S1, mixing a manganese source, an iron source, a phosphorus source, a cobalt source, 2-methylimidazole, and a solvent, and sequentially subjecting them to a solvothermal reaction and a first drying to obtain a composite precursor containing a metal-organic framework material; step S2, mixing a lithium source, the composite precursor, and a graphene oxide suspension, and sequentially subjecting them to grinding and a second drying to obtain a material to be calcined; wherein the graphene oxide suspension is a mixture of graphene oxide and water; and step S3, calcining the material to be calcined under a protective gas atmosphere, and obtaining the modified cathode material after cooling.

[0017] Further, in step S1, the molar ratio of manganese in the manganese source, iron in the iron source, cobalt in the cobalt source, and phosphorus in the phosphorus source is (0.3-0.8):(0.17-0.69):(0.01-0.03):1.

[0018] Furthermore, the molar ratio of cobalt to 2-methylimidazole in the cobalt source is 1:(2-4).

[0019] Further, a manganese source, an iron source, a phosphorus source, a cobalt source, 2-methylimidazole, and a solvent are mixed to obtain a mixture; preferably, the molar concentration of 2-methylimidazole in the mixture is 0.2 to 0.4 mol / L.

[0020] Further, the manganese source is selected from one or more of the group consisting of manganese acetate, manganese carbonate, and manganese dioxide; and / or, the iron source is selected from one or more of the group consisting of ferric phosphate, ferric acetate, ferric acetate tetrahydrate, ferric nitrate nonahydrate, and ferrous sulfate heptahydrate; and / or, the phosphorus source is selected from one or more of the group consisting of phosphoric acid, lithium dihydrogen phosphate, and ammonium dihydrogen phosphate; and / or, the cobalt source is selected from cobalt nitrate hexahydrate and / or cobalt chloride; and / or, the solvent is selected from a mixture of water and an organic solvent, or an organic solvent, wherein the organic solvent is selected from one or more of the group consisting of methanol, ethanol, ethylene glycol, and N,N-dimethylformamide.

[0021] Furthermore, the temperature of the solvothermal reaction is 150–250 °C, and the time is 12–72 h.

[0022] Furthermore, the first drying temperature is 60–90°C, and the time is 8–24 hours.

[0023] Further, in step S2, the method for preparing the graphene oxide suspension includes: mixing graphene oxide with water and then treating it with ultrasound to obtain the graphene oxide suspension; preferably, the ultrasound treatment time is 1 to 3 hours and the frequency is 30 to 50 kHz; preferably, the solid content of the graphene oxide suspension is 0.5 to 2 wt%.

[0024] Furthermore, in step S2, the weight ratio of graphene oxide to cobalt source in the graphene oxide suspension is 1:(0.1~0.35).

[0025] Furthermore, the molar ratio of lithium in the lithium source to phosphorus in the phosphorus source is 1:(0.9 to 1.1).

[0026] Furthermore, the lithium source is selected from one or more of the group consisting of lithium carbonate, lithium hydroxide, and lithium dihydrogen phosphate.

[0027] Furthermore, during the mixing process in step S2, stirring is carried out. Preferably, the stirring rate is 200-400 rpm and the stirring time is 1-3 hours.

[0028] Furthermore, the grinding speed is 1500-2200 rpm, and the time is 0.5-2.5 h; preferably, a sand mill is used for grinding.

[0029] Furthermore, the temperature for the second drying is 100–110°C, and the time is 0.5–1.5 h; preferably, spray drying is used for the second drying.

[0030] Further, in step S3, the calcination process includes a heating stage and a holding stage; preferably, the heating rate in the heating stage is 2-5℃ / min; preferably, the temperature in the holding stage is 500-750℃ and the time is 6-12h; preferably, the rate of introduction of the protective gas is 0.5-1L / min; preferably, the protective gas is selected from nitrogen and / or inert gas.

[0031] Another aspect of this application provides a positive electrode, including a positive electrode current collector and a positive electrode active material layer stacked together, wherein the positive electrode active material layer includes the modified positive electrode material provided in this application.

[0032] A fourth aspect of this application also provides a lithium-ion battery, including a positive electrode, a negative electrode, an electrolyte, and a separator disposed between the positive electrode and the negative electrode, wherein the positive electrode is the aforementioned positive electrode provided in this application.

[0033] Applying the technical solution of this application, the modified cathode material provided by this application, by combining porous carbon with reduced graphene oxide, forms a composite porous carbon coating structure. The porous carbon is attached to the sheet structure of reduced graphene oxide, thereby linking the three-dimensional conductive framework with the two-dimensional conductive sheets to form a three-dimensional conductive network, significantly improving the conductivity of the modified cathode material. Simultaneously, the porous carbon coating structure can also alleviate the volume expansion of the modified cathode material during charge-discharge cycles, improving the rate performance and cycle performance of lithium-ion batteries. The introduction of porous carbon can, on the one hand, increase the porosity and specific surface area of ​​the modified cathode material, increase its contact efficiency with the electrolyte, and provide more lithium-ion diffusion channels, thereby improving the lithium-ion insertion / extraction process during battery charge-discharge, increasing lithium-ion transport efficiency, and thus improving the rate performance and cycle performance of lithium-ion batteries. On the other hand, the nitrogen element in the porous carbon can regulate the charge balance inside the modified cathode material, thereby improving the electrochemical activity of the modified cathode material. By employing a core with the aforementioned specific general formula and limiting the values ​​of x and y to the aforementioned specific range, the electrochemical stability and cycle performance of the modified cathode material can be improved. Doping Co into lithium manganese iron phosphate material can shorten the MP bond (where M = Mn and / or Fe, and P is phosphorus in phosphate) by introducing Co ions with smaller ionic radii, thereby reducing the cell volume, widening the lithium ion transport path, improving the lithium ion transport efficiency and the stability of the modified cathode material, and thus improving the rate performance and cycle performance of lithium-ion batteries. Attached Figure Description

[0034] The accompanying drawings, which form part of this application, are used to provide a further understanding of this application. The illustrative embodiments and descriptions of this application are used to explain this application and do not constitute an undue limitation of this application. In the drawings:

[0035] Figure 1The infrared spectrum of graphene oxide prepared by the Hummers method in Example 1 of this application is shown.

[0036] Figure 2 The SEM image of the modified cathode material prepared in Example 1 of this application is shown.

[0037] Figure 3 The EDS diagram of the modified cathode material prepared in Example 1 of this application is shown;

[0038] Figure 4 The XRD patterns of the modified cathode materials prepared in Examples 1 to 3 of this application are shown.

[0039] Figure 5 The diagram shows a comparison of the capacity retention rates of lithium-ion batteries assembled in Example 1 and Comparative Example 1 after 200 cycles at 1C rate. Detailed Implementation

[0040] It should be noted that, unless otherwise specified, the embodiments and features described in this application can be combined with each other. The present application will now be described in detail with reference to the embodiments.

[0041] As described in the background section, existing lithium manganese iron phosphate cathode materials suffer from low conductivity and lithium-ion transport efficiency, resulting in poor rate performance and cycle performance of lithium-ion batteries. To address these technical problems, this application provides a modified cathode material comprising a core and a porous carbon coating layer disposed on the surface of the core; the core has the general formula LiMn x Fe 1-x-y Co y PO4, where 0.3≤x≤0.8, 0.01≤y≤0.03; the porous carbon coating material is nitrogen-doped porous carbon and reduced graphene oxide.

[0042] The modified cathode material provided in this application combines porous carbon with reduced graphene oxide to form a composite porous carbon coating structure. The porous carbon adheres to the sheet structure of the reduced graphene oxide, thereby linking the three-dimensional conductive framework with the two-dimensional conductive sheets to form a three-dimensional conductive network. This significantly improves the conductivity of the modified cathode material. Simultaneously, the porous carbon coating structure can alleviate the volume expansion of the modified cathode material during charge-discharge cycles, improving the rate performance and cycle performance of the lithium-ion battery. The introduction of porous carbon increases the porosity and specific surface area of ​​the modified cathode material, increasing its contact efficiency with the electrolyte and providing more lithium-ion diffusion channels. This improves the lithium-ion insertion / extraction process during battery charge-discharge, enhancing lithium-ion transport efficiency and thus improving the rate performance and cycle performance of the lithium-ion battery. Furthermore, the uniformly distributed nitrogen element in the porous carbon can regulate the charge balance within the modified cathode material, thereby improving its electrochemical activity. By employing a core with the aforementioned specific general formula and limiting the values ​​of x and y to the aforementioned specific range, the electrochemical stability and cycle performance of the modified cathode material can be improved. Doping Co into lithium manganese iron phosphate material can shorten the MP bond (where M = Mn and / or Fe, and P is phosphorus in phosphate) by introducing Co ions with smaller ionic radii, thereby reducing the cell volume, widening the lithium ion transport path, improving the lithium ion transport efficiency and the stability of the modified cathode material, and thus improving the rate performance and cycle performance of lithium-ion batteries.

[0043] In a preferred embodiment, the specific surface area of ​​the modified cathode material is 16.5–24 m². 2 / g, preferably 20-23m 2 / g; compacted density is 2.15~2.35g / cm³ 3 The specific surface area and compaction density of the modified cathode material include, but are not limited to, the ranges mentioned above. Limiting them to these ranges is beneficial for improving the conductivity of the modified cathode material, increasing its contact efficiency with the electrolyte, and enhancing its electrochemical activity. It also helps to provide more diffusion channels for lithium ions, thereby improving the lithium ion transport efficiency and ultimately enhancing the rate performance and cycle performance of lithium-ion batteries.

[0044] In a preferred embodiment, the carbon content in the porous carbon coating layer is 1.2–2.5 wt%, based on the weight percentage of the modified cathode material. The carbon content in the porous carbon coating layer includes, but is not limited to, the above range. Limiting it within this range is beneficial for forming a continuous conductive network, improving the conductivity of the modified cathode material, and also helps maintain the content of the cathode active material, the specific surface area of ​​the modified cathode material, and the compaction density within a more suitable range, thereby improving the rate performance and cycle performance of the lithium-ion battery.

[0045] In a preferred embodiment, the weight ratio of porous carbon to reduced graphene oxide is (1-5):(5-9). This weight ratio includes, but is not limited to, the range described above. Limiting it to this range enhances the bonding between the three-dimensional conductive framework and the two-dimensional conductive sheets, resulting in a more stable and conductive porous carbon coating structure. This, in turn, improves the conductivity and structural stability of the modified cathode material, promotes rapid electron and lithium-ion transport, and ultimately enhances the rate performance and cycle performance of lithium-ion batteries.

[0046] To further enhance the combination of the three-dimensional conductive framework and the two-dimensional conductive sheet, construct a more stable and conductive porous carbon coating structure, further improve the conductivity and structural stability of the modified cathode material, promote the rapid transport of electrons and lithium ions, and thus further improve the rate performance and cycle performance of lithium-ion batteries, preferably, the weight ratio of porous carbon to reduced graphene oxide is (1-3):(7-9).

[0047] The oxygen-containing functional groups in reduced graphene oxide include hydroxyl, carboxyl, and ether bonds. In a preferred embodiment, the content of oxygen-containing functional groups in the reduced graphene oxide is ≤30 wt%, preferably 20-25 wt%, based on the weight percentage of the reduced graphene oxide. The content of oxygen-containing functional groups in the reduced graphene oxide includes, but is not limited to, the above range. Limiting it to the above range is beneficial to improving the interfacial properties of the modified cathode material, enhancing its electrochemical stability, and thus improving the cycle stability of lithium-ion batteries.

[0048] The second aspect of this application provides a method for preparing the modified cathode material provided in this application. The method includes: step S1, mixing a manganese source, an iron source, a phosphorus source, a cobalt source, 2-methylimidazole and a solvent, and sequentially subjecting the mixture to a solvothermal reaction and a second drying to obtain a composite precursor containing a metal-organic framework material; step S2, mixing a lithium source, the composite precursor and a graphene oxide suspension, and sequentially subjecting the mixture to grinding and a second drying to obtain a material to be calcined; wherein the graphene oxide suspension is a mixture of graphene oxide and water; step S3, calcining the material to be calcined under a protective gas atmosphere, and obtaining the modified cathode material after cooling.

[0049] In step S1 of this application, a solvothermal reaction is carried out by dispersing manganese, iron, phosphorus, cobalt, and 2-methylimidazole in a solvent. Cobalt ions and 2-methylimidazole first generate a composite precursor of metal-organic framework material (ZIF-67, which belongs to the zeolite imidazole ester framework material) and manganese iron phosphate material in situ through coordination. After a first drying, the composite precursor is obtained. Step S1 can obtain a composite precursor with uniformly dispersed manganese and iron elements. On the other hand, the unique porous structure and nitrogen-containing carbon network of ZIF-67 formed in situ can be transformed into nitrogen-containing porous carbon particles with rich pores during subsequent calcination, which can regulate the charge balance inside the modified cathode material and form a three-dimensional conductive network, thereby improving the electrochemical activity of the modified cathode material. In step S2, the lithium source, composite precursor, and graphene oxide suspension are mixed, and after grinding and a second drying, the material to be calcined is obtained, so that the three-dimensional metal-organic framework and two-dimensional graphene oxide sheets in the mixed system are interconnected to form a three-dimensional network. Step S3 involves calcining the material to be calcined under a protective gas atmosphere. During calcination, the organic components of ZIF-67 decompose, forming a porous structure in situ, resulting in three-dimensional nitrogen-containing porous carbon particles. Cobalt ions replace some iron sites in these particles, doping them into lithium manganese iron phosphate, thereby improving the electrochemical stability and cycle performance of the modified cathode material. Doping lithium manganese iron phosphate with Co ions, which have smaller ionic radii, shortens MP bonds, reduces cell volume, and widens lithium-ion transport paths, thus improving lithium-ion transport efficiency and the stability of the modified cathode material, ultimately enhancing the rate performance and cycle performance of the lithium-ion battery. Simultaneously, reduced graphene oxide combines with the nitrogen-containing porous carbon particles formed by ZIF-67 conversion, forming a composite porous carbon coating layer. This improves the conductivity and lithium-ion diffusion capacity of the modified cathode material, mitigating volume expansion during charge-discharge cycles, thereby further enhancing the rate performance and cycle performance of the lithium-ion battery.

[0050] In summary, the modified cathode material prepared by the above-mentioned preparation method provided in this application has high electrical conductivity and lithium-ion transport efficiency. Its application in lithium-ion batteries can effectively improve the rate performance and cycle performance of lithium-ion batteries.

[0051] In order to obtain modified cathode materials with better electrochemical performance, promote the effective transport of electrons and ions, and further improve the rate performance and cycle performance of lithium-ion batteries, in a preferred embodiment, in step S1, the molar ratio of manganese in the manganese source, iron in the iron source, cobalt in the cobalt source and phosphorus in the phosphorus source is (0.3~0.8):(0.17~0.69):(0.01~0.03):1.

[0052] In a preferred embodiment, the molar ratio of cobalt to 2-methylimidazole in the cobalt source is 1:(2-4). This molar ratio includes, but is not limited to, the range described above. Limiting it to this range facilitates the in-situ growth of ZIF-67, forming a metal-organic framework with a high specific surface area and porous structure, thus facilitating the subsequent formation of a porous carbon coating layer. Simultaneously, it also helps to control the subsequent doping amount of cobalt in the lithium manganese iron phosphate material, thereby improving the lithium-ion transport path, increasing the conductivity of the modified cathode material, and ultimately improving the rate performance and cycle performance of the lithium-ion battery.

[0053] In order to improve the dispersibility of the raw materials in the hydrothermal reaction and improve the efficiency of the hydrothermal reaction, in a preferred embodiment, manganese source, iron source, phosphorus source, cobalt source, 2-methylimidazole and water are mixed to obtain a mixture; preferably, the molar concentration of 2-methylimidazole in the mixture is 0.2 to 0.4 mol / L.

[0054] In order to obtain modified cathode materials with better electrochemical performance, thereby further improving the rate performance and cycle performance of lithium-ion batteries, in a preferred embodiment, the manganese source includes, but is not limited to, one or more of the group consisting of manganese acetate, manganese carbonate and manganese dioxide; the iron source includes, but is not limited to, one or more of the group consisting of iron phosphate, iron acetate, iron acetate tetrahydrate, iron nitrate nonahydrate and ferrous sulfate heptahydrate; and the phosphorus source includes, but is not limited to, one or more of the group consisting of phosphoric acid, lithium dihydrogen phosphate and ammonium dihydrogen phosphate.

[0055] In a preferred embodiment, the cobalt source includes, but is not limited to, cobalt nitrate hexahydrate and / or cobalt chloride. The types of cobalt sources include, but are not limited to, those described above. Limiting the source to these ranges facilitates the coordination reaction between cobalt ions and 2-methylimidazole, generating a stable metal-organic framework compound ZIF-67, thereby improving the conductivity and lithium-ion transport efficiency of the modified cathode material.

[0056] In order to improve the dispersibility of raw materials in solvothermal reactions and further improve the efficiency of solvothermal reactions, in a preferred embodiment, the solvent includes, but is not limited to, a mixture of water and organic solvents, or an organic solvent, wherein the organic solvent includes, but is not limited to, one or more of the group consisting of methanol, ethanol, ethylene glycol and N,N-dimethylformamide.

[0057] In a preferred embodiment, the solvothermal reaction temperature is 150–250°C, and the time is 12–72 h. The temperature and time of the solvothermal reaction include, but are not limited to, the above ranges. Limiting them to the above ranges is beneficial to improving the efficiency of the solvothermal reaction, promoting the in-situ growth of ZIF-67, and suppressing the overgrowth of ZIF-67 caused by excessively high temperature or excessively long time. This is beneficial to improving the conductivity and lithium-ion transport efficiency of the modified cathode material.

[0058] In order to improve the solvent removal efficiency and obtain a fully dried composite precursor, in a preferred embodiment, the first drying temperature is 60-90°C and the time is 8-24 hours.

[0059] In order to improve the dispersion uniformity of graphene oxide and form a stable graphene oxide suspension, thereby facilitating the improvement of subsequent coating effect, in a preferred embodiment, step S2, the preparation method of graphene oxide suspension includes: mixing graphene oxide with water and then treating it with ultrasound to obtain graphene oxide suspension.

[0060] To further improve the dispersion uniformity of graphene oxide and form a more stable graphene oxide suspension, thereby further improving the subsequent coating effect, preferably, the solid content of the graphene oxide suspension is 0.5 to 2 wt%.

[0061] In a preferred embodiment, the ultrasonic treatment time is 1–3 hours, and the frequency is 30–50 kHz. The ultrasonic treatment time and frequency include, but are not limited to, the above ranges. Limiting them to the above ranges is beneficial to improving the dispersion uniformity of graphene oxide and reducing its agglomeration, thereby facilitating the formation of a more uniform and stable graphene oxide suspension.

[0062] The graphene oxide described in this application is prepared using methods commonly used in the art. In a preferred embodiment, graphene oxide is prepared using the Hummers method. The Hummers method mainly involves oxidizing graphite with potassium permanganate in a strong acid (usually sulfuric acid) medium, thereby converting graphite into graphene oxide.

[0063] In a preferred embodiment, in step S2, the weight ratio of graphene oxide to cobalt source in the graphene oxide suspension is 1:(0.1 to 0.35). The weight ratio of graphene oxide to cobalt source includes, but is not limited to, the above range. Limiting it to the above range is beneficial to improving the interaction between the above raw materials, thereby improving the overall performance of the modified cathode material, and further beneficial to improving the rate performance and cycle performance of lithium-ion batteries.

[0064] In a preferred embodiment, the molar ratio of lithium in the lithium source to phosphorus in the phosphorus source is 1:(0.9 to 1.1). The molar ratio of lithium in the lithium source to phosphorus in the phosphorus source includes, but is not limited to, the range described above. Limiting it to this range is beneficial for forming a modified cathode material with superior electrochemical performance during subsequent calcination, thereby improving the rate performance and cycle performance of lithium-ion batteries.

[0065] In order to obtain modified cathode materials with better electrochemical performance, thereby further improving the rate performance and cycle performance of lithium-ion batteries, in a preferred embodiment, the lithium source includes, but is not limited to, one or more of the group consisting of lithium carbonate, lithium hydroxide and lithium dihydrogen phosphate.

[0066] In order to improve the mixing efficiency of lithium source, composite precursor and graphene oxide suspension and facilitate subsequent processes, in a preferred embodiment, stirring is carried out during the mixing process in step S2. Preferably, the stirring rate is 200-400 rpm and the time is 1-3 hours.

[0067] In order to improve the grinding effect and enhance the dispersibility and uniformity of the lithium source, composite precursor and graphene oxide suspension, thereby further improving the electrochemical performance of the modified cathode material, in a preferred embodiment, the grinding speed is 1500-2200 rpm and the grinding time is 0.5-2.5 h.

[0068] To further improve the grinding effect and enhance the dispersibility and uniformity of the lithium source, composite precursor, and graphene oxide suspension, sand milling is preferably used.

[0069] In a preferred embodiment, the second drying temperature is 100–110°C, and the time is 0.5–1.5 h. The temperature and time of the second drying include, but are not limited to, the above range. Limiting them to the above range is beneficial for removing water from the material to be calcined, ensuring that the material is fully dried, and facilitating subsequent calcination.

[0070] In order to further remove water from the material to be calcined and to dry the material thoroughly, and to improve the pelletizing properties of the material to be calcined, spray drying is preferably used for the second drying process. During the spray drying process, the frequency of the feed peristaltic pump is 5 to 40 Hz, preferably 15 to 25 Hz.

[0071] In order to improve the crystallinity, grain size and coating effect of the modified cathode material, in a preferred embodiment, the calcination process in step S3 includes a heating stage and a holding stage.

[0072] In a preferred embodiment, the heating rate during the heating stage is 2–5 °C / min. The heating rate during the heating stage includes, but is not limited to, the above range. Limiting it to the above range is beneficial for obtaining modified cathode materials with more regular morphology and higher particle crystallinity, thereby improving the electronic conductivity and lithium-ion diffusion efficiency of the modified cathode material, and further improving the rate performance and cycle performance of lithium-ion batteries.

[0073] In a preferred embodiment, the temperature during the heat preservation stage is 500–750°C, and the time is 6–12 hours. The temperature and time of the heat preservation stage include, but are not limited to, the above ranges. Limiting them to these ranges is beneficial for forming modified cathode materials with more regular morphology, more suitable grain size, and higher particle crystallinity. It also helps improve the coating effect, resulting in a uniform and continuous porous carbon coating layer. Furthermore, it helps suppress the agglomeration and excessive growth of the modified cathode material caused by excessively high temperatures, thereby improving the conductivity and lithium-ion transport efficiency of the modified cathode material, and consequently, improving the rate performance and cycle performance of the lithium-ion battery.

[0074] In order to provide a stable protective gas atmosphere during calcination, suppress side reactions such as oxidation, and further improve the purity and structural integrity of the modified cathode material, and at the same time to further promote the reduction process of graphene oxide and suppress the reaction between graphene oxide and oxygen, so as to facilitate the obtaining of reduced graphene oxide, in a preferred embodiment, the protective gas introduction rate is 0.5 to 1 L / min; preferably, the protective gas includes, but is not limited to, nitrogen and / or inert gas, more preferably nitrogen and / or argon.

[0075] A third aspect of this application also provides a positive electrode, comprising a positive electrode current collector and a positive electrode active material layer stacked together, wherein the positive electrode active material layer comprises the modified positive electrode material provided in this application. The modified positive electrode material provided in this application has high electrical conductivity and lithium-ion transport efficiency. Using it as a positive electrode active material in a positive electrode, and applying this positive electrode in a lithium-ion battery, can improve the rate performance and cycle performance of the lithium-ion battery.

[0076] A fourth aspect of this application also provides a lithium-ion battery, including a positive electrode, a negative electrode, an electrolyte, and a separator disposed between the positive and negative electrodes, wherein the positive electrode is the aforementioned positive electrode provided in this application. The aforementioned positive electrode provided in this application includes the aforementioned modified positive electrode material, which has high conductivity and lithium-ion transport efficiency. Using the aforementioned modified positive electrode material as a positive electrode active material in a lithium-ion battery can improve the rate performance and cycle performance of the lithium-ion battery.

[0077] The present application will be further described in detail below with reference to specific embodiments, which should not be construed as limiting the scope of protection claimed in the present application.

[0078] It should be noted that the modified cathode materials prepared in all the following embodiments and comparative examples of this application were subjected to the following tests:

[0079] (1) The carbon content of the porous carbon coating in the modified cathode material was measured using a high-frequency infrared carbon-sulfur analyzer (Wanlianda Xinke, CS901B).

[0080] (2) The specific surface area of ​​the modified cathode material was measured according to the BET method and using a specific surface area and pore size analyzer (Anton Pacanta, Nova800BET);

[0081] (3) The compaction density of the modified cathode material was measured by referring to the method described in the national standard GB / T 44330-2024 "Determination of compaction density of lithium-ion battery cathode material powder";

[0082] (4) The resistivity of the modified cathode material was measured using a four-probe tester (Yuaneng, PCD2000) at a pressure of 30 MPa.

[0083] The test results are shown in Table 1.

[0084] Example 1

[0085] A method for preparing a modified cathode material specifically includes the following steps:

[0086] (1) Manganese carbonate, ferric acetate tetrahydrate, phosphoric acid and cobalt nitrate hexahydrate were dissolved in ethylene glycol to obtain a first mixture. 2-methylimidazole was dissolved in ethylene glycol to obtain a second mixture. The second mixture was mixed with the first mixture to obtain a third mixture, wherein the concentration of 2-methylimidazole was 0.3 mol / L. The third mixture was transferred to a reaction vessel and subjected to a solvothermal reaction at 200°C for 24 h. After the reaction was completed, the mixture was filtered to obtain a filter cake. The filter cake was then dried at 85°C for 12 h to obtain a composite precursor.

[0087] The molar ratios of Mn in manganese carbonate, Fe in ferric acetate tetrahydrate, Co in cobalt nitrate hexahydrate, and P in phosphoric acid are 0.6:0.38:0.02:1; the molar ratio of Co in cobalt nitrate hexahydrate to 2-methylimidazole is 1:3.

[0088] (2) Graphene oxide was prepared by the Hummers method. The specific steps included: adding 3.2 g of sodium nitrate to 46 mL of concentrated sulfuric acid and stirring until dissolved; adding 2 g of graphite at 0 °C with stirring; maintaining the temperature below 5 °C and slowly adding 6 g of potassium permanganate; stirring continuously for 60 min and then transferring to a 40 °C oil bath and stirring for another 90 min; then slowly adding deionized water to the reaction system and transferring it to a 100 °C oil bath, stirring for 15 min and then removing it; and lowering the temperature. Deionized water was added at a temperature as low as 60°C, and hydrogen peroxide solution was added dropwise until no bubbles were generated in the system. After standing for 12 hours, the centrifugation and washing process was repeated 5 times with 5wt% hydrochloric acid solution. Then, the centrifugation and washing process was repeated with deionized water until the pH of the supernatant was 7. The lower solid layer was taken and dried in an oven at 60°C for 24 hours to obtain graphene oxide with a monolayer thickness of 0.8–1.2 nm. The obtained graphene oxide was characterized by Fourier transform infrared spectroscopy, and the infrared spectrum of the graphene oxide is shown below. Figure 1 As shown, by Figure 1 It can be seen that this graphene oxide at 3350 cm⁻¹ -1 and 1400cm -1 An OH absorption peak is observed at 1734 cm⁻¹. -1 The peak of C=O stretching vibration is observed at 1250 cm⁻¹. -1 and 1070cm -1 The graphene oxide exhibits a vibrational absorption peak of COC, indicating that the prepared graphene oxide contains hydroxyl, carbonyl, and ether bonds.

[0089] Graphene oxide was mixed with water and ultrasonically treated at 25°C and 50 kHz for 2 hours to obtain a graphene oxide suspension with a solid content of 1 wt%.

[0090] (3) Lithium carbonate, the composite precursor obtained in step (1) and the graphene oxide suspension obtained in step (2) are mixed at a stirring rate of 250 rpm for 1 h. The weight ratio of graphene oxide to cobalt source in the graphene oxide suspension is 1:0.23, and the molar ratio of Li element in lithium carbonate to P element in phosphoric acid in step (1) is 1:1. Then, it is sand-milled at a speed of 2000 rpm for 2 h, and then spray-dried at 110℃ and a feeding frequency of 25 Hz for 45 min to obtain the material to be calcined.

[0091] (4) Place the material to be calcined obtained in step (3) into a box furnace, set the gas flow rate of protective gas N2 to 0.5 L / min, and then raise the temperature from 25℃ to 650℃ at a heating rate of 2℃ / min, and keep it at 650℃ for 10h. After natural cooling, the modified cathode material is obtained.

[0092] The modified cathode material prepared in Example 1 has a core-shell structure, including a core and a porous carbon coating layer covering its surface, wherein the core is LiMn. 0.6 Fe 0.38 Co 0.02 PO4, as a percentage of the modified cathode material by weight, accounts for 1.8 wt% of the carbon content in the porous carbon coating layer; the weight ratio of porous carbon to reduced graphene oxide in the porous carbon coating layer is 2:8; the specific surface area of ​​the modified cathode material is 21.6 m². 2 / g, compacted density is 2.28g / cm³ 3 .

[0093] SEM image of the modified cathode material prepared in Example 1 is shown below. Figure 2 As shown, by Figure 2 It can be seen that the modified cathode material prepared in Example 1 has a distinct spherical morphology and the particle size is between 10 and 20 μm.

[0094] The elemental distribution of the modified cathode material prepared in Example 1 was measured by EDS (energy-dispersive X-ray spectroscopy), and the resulting elemental distribution diagram is shown below. Figure 3 As shown, by Figure 3 It can be seen that the Mn, Fe, P and Co elements are uniformly distributed in the modified cathode material prepared in Example 1.

[0095] Example 2

[0096] A method for preparing a modified cathode material specifically includes the following steps:

[0097] (1) Manganese acetate, ferric nitrate nonahydrate, ammonium dihydrogen phosphate and cobalt nitrate hexahydrate were dissolved in methanol to obtain a first mixture. 2-methylimidazole was dissolved in methanol to obtain a second mixture. The second mixture was mixed with the first mixture to obtain a third mixture, wherein the concentration of 2-methylimidazole was 0.25 mol / L. The third mixture was transferred to a reaction vessel and subjected to a solvothermal reaction at 150 °C for 48 h. After the reaction was completed, the mixture was filtered to obtain a filter cake. The filter cake was then dried at 80 °C for 12 h to obtain a composite precursor.

[0098] The molar ratios of Mn in manganese acetate, Fe in ferric nitrate nonahydrate, Co in cobalt nitrate hexahydrate, and P in ammonium dihydrogen phosphate are 0.8:0.19:0.01:1; the molar ratio of Co in cobalt nitrate hexahydrate to 2-methylimidazole is 1:4.

[0099] (2) Graphene oxide was prepared by the Hummers method (the preparation method is the same as in Example 1). The prepared graphene oxide was mixed with water and ultrasonically treated at 25°C and 30 kHz for 1 h to obtain a graphene oxide suspension with a solid content of 0.5 wt%.

[0100] (3) Lithium hydroxide, the composite precursor obtained in step (1) and the graphene oxide suspension obtained in step (2) are mixed at a stirring rate of 250 rpm for 1 h. The weight ratio of graphene oxide to cobalt source in the graphene oxide suspension is 1:0.1, and the molar ratio of Li element in lithium hydroxide to P element in ammonium dihydrogen phosphate in step (1) is 1.1:1. Then, it is sand-milled at a speed of 1500 rpm for 2.5 h. After completion, it is spray-dried at 100°C for 1 h to obtain the material to be calcined.

[0101] (4) Place the material to be calcined obtained in step (3) into a box furnace, set the gas flow rate of protective gas N2 to 0.6 L / min, and then raise the temperature from 25°C to 700°C at a heating rate of 3°C / min, and keep it at 700°C for 8 hours. After natural cooling, the modified cathode material is obtained.

[0102] The modified cathode material prepared in Example 2 has a core-shell structure, including a core and a porous carbon coating layer covering its surface, wherein the core is LiMn. 0.8 Fe 0.19 Co 0.01 PO4, as a weight percentage of the modified cathode material, accounts for 1.61 wt% of the carbon content in the porous carbon coating layer; the weight ratio of porous carbon to reduced graphene oxide in the porous carbon coating layer is 1:9; the specific surface area of ​​the modified cathode material is 20.17 m². 2 / g, compacted density is 2.34g / cm³ 3 .

[0103] Example 3

[0104] A method for preparing a modified cathode material specifically includes the following steps:

[0105] (1) Manganese dioxide, ferrous sulfate heptahydrate, phosphoric acid and cobalt chloride were dissolved in ethanol to obtain a first mixture. 2-methylimidazole was dissolved in ethanol to obtain a second mixture. The second mixture was mixed with the first mixture to obtain a third mixture, wherein the concentration of 2-methylimidazole was 0.4 mol / L. The third mixture was transferred to a reaction vessel and subjected to a solvothermal reaction at 230°C for 18 h. After the reaction was completed, the mixture was filtered to obtain a filter cake. The filter cake was then dried at 75°C for 18 h to obtain a composite precursor.

[0106] The molar ratios of Mn in manganese dioxide, Fe in ferrous sulfate heptahydrate, Co in cobalt chloride, and P in phosphoric acid are 0.5:0.47:0.03:1; the molar ratio of Co in cobalt chloride to 2-methylimidazole is 1:2.

[0107] (2) Graphene oxide was prepared by the Hummers method (the preparation method is the same as in Example 1). The prepared graphene oxide was mixed with water and ultrasonically treated at 25°C and 30 kHz for 3 h to obtain a graphene oxide suspension with a solid content of 1.5 wt%.

[0108] (3) Lithium carbonate, the composite precursor obtained in step (1) and the graphene oxide suspension obtained in step (2) are mixed at a stirring rate of 250 rpm for 1 h. The weight ratio of graphene oxide to cobalt source in the graphene oxide suspension is 1:0.35, and the molar ratio of Li element in lithium carbonate to P element in phosphoric acid in step (1) is 0.9:1. Then, it is sand-milled at a speed of 2200 rpm for 1.5 h. After completion, it is spray-dried at 105℃ for 50 min to obtain the material to be calcined.

[0109] (4) Place the material to be calcined obtained in step (3) into a box furnace, set the gas flow rate of protective gas N2 to 0.8 L / min, and then raise the temperature from 25°C to 600°C at a heating rate of 5°C / min, and keep it at 600°C for 12 hours. After natural cooling, the modified cathode material is obtained.

[0110] The modified cathode material prepared in Example 3 has a core-shell structure, including a core and a porous carbon coating layer covering its surface, wherein the core is LiMn. 0.5 Fe 0.47 Co 0.03 PO4, as a percentage of the modified cathode material by weight, accounts for 1.91 wt% of the carbon content in the porous carbon coating layer; the weight ratio of porous carbon to reduced graphene oxide in the porous carbon coating layer is 3:7; the specific surface area of ​​the modified cathode material is 22.57 m². 2 / g, compacted density is 2.16g / cm³ 3 .

[0111] The modified cathode materials prepared in Examples 1 to 3 were subjected to crystal form testing using an X-ray diffractometer (Brook, D8 Discovery). The testing conditions were: CuKα radiation, and the data collection range was 10°–80°. The XRD characterization results are as follows: Figure 4 As shown, by Figure 4It can be seen that the characteristic peaks of the modified cathode materials prepared in Examples 1 to 3 match the characteristic peaks of LiFePO4 with an olivine structure, indicating that the modified cathode materials prepared in Examples 1 to 3 belong to an orthorhombic crystal system compound with an ordered olivine structure, and Figure 4 The absence of obvious impurity phase peaks and carbon peaks indicates that the modified cathode materials prepared in Examples 1 to 3 have high purity, no impurity phases, and low carbon content.

[0112] Example 4

[0113] The difference from Example 1 is that in step (1), the molar ratio of Mn in manganese carbonate, Fe in ferric acetate tetrahydrate, Co in cobalt nitrate hexahydrate and P in phosphoric acid is 0.3:0.68:0.02:1; the remaining steps are the same as in Example 1.

[0114] Example 5

[0115] The difference from Example 1 is that in step (1), the molar ratio of Mn in manganese carbonate, Fe in ferric acetate tetrahydrate, Co in cobalt nitrate hexahydrate and P in phosphoric acid is 0.2:0.75:0.05:1; the remaining steps are the same as in Example 1.

[0116] Example 6

[0117] The difference from Example 1 is that in step (3), the weight ratio of cobalt source to graphene oxide is 0.1:1, and the molar ratio of Li element in lithium carbonate to P element in phosphoric acid in step (1) is 1:1.1; the remaining steps are the same as in Example 1.

[0118] Example 7

[0119] The difference from Example 1 is that in step (3), the weight ratio of cobalt source to graphene oxide is 0.35:1, and the molar ratio of Li element in lithium carbonate to P element in phosphoric acid in step (1) is 1:1.02; the remaining steps are the same as in Example 1.

[0120] Example 8

[0121] The difference from Example 1 is that in step (3), the weight ratio of cobalt source to graphene oxide is 0.45:1, and the molar ratio of Li element in lithium carbonate to P element in phosphoric acid in step (1) is 0.9:1; the remaining steps are the same as in Example 1.

[0122] Example 9

[0123] The difference from Example 1 is that in step (4), the material to be calcined obtained in step (3) is placed in a box furnace and heated from 25°C to 500°C at a heating rate of 5°C / min, and kept at 500°C for 12 hours; the remaining steps are the same as in Example 1.

[0124] Example 10

[0125] The difference from Example 1 is that in step (4), the material to be calcined obtained in step (3) is placed in a box furnace and heated from 25°C to 750°C at a heating rate of 2°C / min, and kept at 750°C for 6 hours; the remaining steps are the same as in Example 1.

[0126] Example 11

[0127] The difference from Example 1 is that in step (4), the material to be calcined obtained in step (3) is placed in a box furnace and heated from 25°C to 650°C at a heating rate of 7°C / min, and kept at 650°C for 10 hours; the remaining steps are the same as in Example 1.

[0128] Example 12

[0129] The difference from Example 1 is that in step (4), the material to be calcined obtained in step (3) is placed in a tube furnace and heated from 25°C to 450°C at a heating rate of 2°C / min, and kept at 450°C for 10 hours; the remaining steps are the same as in Example 1.

[0130] Comparative Example 1

[0131] The difference from Example 1 is that: in step (1), no cobalt source and 2-methylimidazole are introduced, and the molar ratio of Mn element in manganese carbonate, Fe element in ferric acetate tetrahydrate and P element in phosphoric acid is 0.6:0.4:1; in step (3), the weight ratio of lithium carbonate, the composite precursor obtained in step (1) and the graphene oxide obtained in step (2) is 10:1; the remaining steps are the same as in Example 1.

[0132] Comparative Example 2

[0133] The difference from Example 1 is that step (2) is omitted, and in step (3), the graphene oxide suspension is not introduced; the remaining steps are the same as in Example 1.

[0134] The chemical formula of the core of the modified cathode materials prepared in Examples 1 to 12 and Comparative Examples 1 and 2, the weight percentage of the porous carbon coating layer in the modified cathode material (i.e., carbon content), and the specific surface area, compaction density and resistivity of the modified cathode materials are shown in Table 1.

[0135] Table 1

[0136]

[0137] The lithium-ion battery was assembled as follows: (1) Preparation of positive electrode sheet: The positive electrode active material (the modified positive electrode material prepared in Examples 1 to 12 and Comparative Examples 1 and 2 of this application, respectively) was dispersed in 1-methyl-2-pyrrolidone (NMP) in a weight ratio of 8:1:1 to obtain a positive electrode slurry; the positive electrode slurry was coated on the surface of a 14 μm thick aluminum foil, and then dried at 80°C for 1 h, and then vacuum dried at 100°C for 12 h to obtain a positive electrode sheet semi-finished product. The semi-finished product was cut into a circular piece with a diameter of 12 mm to obtain a positive electrode sheet. In the positive electrode sheet, the loading of the modified positive electrode material was 5 mg / cm. 2 (2) Lithium-ion battery assembly using standard CR2016 button mold: assembly is carried out in a glove box filled with argon gas, with lithium metal as the negative electrode of the standard half cell; the positive electrode sheet prepared above is used as the positive electrode; LiPF6 is dissolved in a mixture of ethylene carbonate (EC), dimethyl carbonate (DMC) and methyl acetate (EMC) (volume ratio of EC, DMC and EMC 1:1:1) to prepare a 1 mol / L LiPF6 electrolyte; polypropylene (PP) film is used as the separator; and lithium-ion battery is assembled.

[0138] The lithium-ion batteries assembled in all the above embodiments and comparative examples were subjected to charge-discharge tests and cycle performance tests at 25°C. The specific test conditions are as follows:

[0139] (1) Charge and discharge test: At 25°C, charge and discharge tests were conducted at 0.2C and 1C rates in the voltage range of 2.0 to 4.5V. The specific capacity of lithium-ion batteries assembled in Examples 1 to 12 and Comparative Examples 1 and 2 at 25°C were measured at 0.2C charge, 0.2C discharge, 0.2C first-cycle coulombic efficiency and 1C discharge specific capacity. The test results are shown in Table 2.

[0140] At 25°C, charge and discharge tests were conducted at 2C and 5C rates in the voltage range of 2.0 to 4.5V. The 2C discharge specific capacity and 5C discharge specific capacity of the lithium-ion batteries assembled in Examples 1 to 3 and Comparative Example 1 at 25°C were measured. The test results are shown in Table 3.

[0141] Table 2

[0142]

[0143] Table 3

[0144]

[0145] (2) Cyclic performance test: The lithium-ion batteries assembled in Example 1 and Comparative Example 1 were subjected to 200 cycles at 25°C, 1C rate, and a voltage range of 2.0 to 4.5V. The capacity retention rate of Example 1 after 200 cycles was 97.7%, while that of Comparative Example 1 was 87.1%. Figure 5 This paper presents a comparison chart showing the capacity retention of lithium-ion batteries assembled in Example 1 and Comparative Example 1 after 200 cycles at 1C rate. Figure 5 It can be seen that the capacity retention rate of Example 1 is significantly higher than that of Comparative Example 1, indicating that the modified cathode material prepared in Example 1 has better cycle stability.

[0146] As can be seen from the above description, the embodiments of this application achieve the following technical effects:

[0147] Comparing Example 1 and Comparative Example 1, in conjunction with Tables 1, 2, and 3, and... Figure 5 It can be seen that the lithium-ion battery assembled in Example 1 exhibits significantly better electrochemical performance and cycle stability than that of Comparative Example 1. Therefore, this application, by introducing a cobalt source and 2-methylimidazole, enables the in-situ generation of the metal-organic framework compound ZIF-67 through coordination during a solvothermal reaction. Furthermore, ZIF-67 is subsequently transformed into porous nitrogen-containing carbon particles during calcination, effectively regulating the charge balance within the modified cathode material, promoting the formation of a three-dimensional conductive network, and thus effectively improving the electrochemical activity of the modified cathode material, reducing resistivity, and enhancing the rate performance and cycle performance of the resulting lithium-ion battery.

[0148] Comparing Example 1 and Comparative Example 2, and referring to Tables 1 and 2, it can be seen that the modified cathode material prepared in Example 1 has a lower resistivity than that of Comparative Example 2, and the lithium-ion battery assembled from it exhibits better rate performance. This indicates that introducing a graphene oxide suspension during the preparation of the modified cathode material, and combining the reduced graphene oxide formed by calcination with nitrogen-containing porous carbon particles converted from ZIF-67, forms a composite porous carbon coating layer. This effectively improves the conductivity and lithium-ion diffusion capacity of the modified cathode material, alleviates the volume expansion of the modified cathode material during charge-discharge cycles, and thus improves the rate performance of the lithium-ion battery.

[0149] Comparing Examples 1, 4, and 5, it can be seen that, compared to other ranges, limiting the molar ratio of manganese in the manganese source, iron in the iron source, cobalt in the cobalt source, and phosphorus in the phosphorus source to the above-mentioned preferred range of this application is beneficial to obtaining modified cathode materials with better electrochemical performance, promoting the effective transport of electrons and ions, thereby improving the rate performance and cycle performance of lithium-ion batteries.

[0150] Comparing Examples 1, 6 to 8, it can be seen that, compared to other ranges, limiting the weight ratio of graphene oxide to cobalt source in the graphene oxide suspension to the preferred range described above in this application is beneficial to promoting the formation of a more uniform and continuous porous carbon coating layer, thereby improving the conductivity and lithium-ion transport efficiency of the modified cathode material, and thus improving the rate performance and cycle performance of the lithium-ion battery.

[0151] Comparing Examples 1, 9 to 12, it can be seen that, compared with other ranges, limiting the heating rate of the heating stage and the temperature of the holding stage during the calcination process to the above-mentioned preferred ranges in this application is beneficial to suppressing abnormal grain growth and promoting the formation of a more uniform and continuous porous carbon coating layer, thereby improving the conductivity and lithium-ion transport efficiency of the modified cathode material, and thus improving the rate performance and cycle performance of the lithium-ion battery.

[0152] It should be noted that the terms "first," "second," etc., used in the specification and claims of this application are used to distinguish similar objects and are not necessarily used to describe a specific order or sequence. It should be understood that such terms can be used interchangeably where appropriate so that the embodiments of this application described herein can be implemented, for example, in a sequence other than those described herein.

[0153] The above description is merely a preferred embodiment of this application and is not intended to limit this application. Various modifications and variations can be made to this application by those skilled in the art. Any modifications, equivalent substitutions, improvements, etc., made within the spirit and principles of this application should be included within the protection scope of this application.

Claims

1. A modified cathode material, characterized in that, The modified cathode material comprises a core and a porous carbon coating layer disposed on the surface of the core; the core has the general formula LiMn x Fe 1-x-y Co y PO4, wherein 0.3≤x≤0.8, 0.01≤y≤0.03; the porous carbon coating material includes nitrogen-doped porous carbon and reduced graphene oxide.

2. The modified cathode material according to claim 1, characterized in that, The specific surface area of ​​the modified cathode material is 16.5–24 m². 2 / g, preferably 20-23m 2 / g, compacted density is 2.15~2.35g / cm³ 3 ; Preferably, the carbon content in the porous carbon coating layer is 1.2 to 2.5 wt%, based on the weight percentage of the modified cathode material.

3. The modified cathode material according to claim 1 or 2, characterized in that, The weight ratio of the porous carbon to the reduced graphene oxide is (1-5):(5-9), preferably (1-3):(7-9). Preferably, the content of oxygen-containing functional groups in the reduced graphene oxide is ≤30wt%, based on the weight percentage of the reduced graphene oxide.

4. A method for preparing a modified cathode material according to any one of claims 1 to 3, characterized in that, The preparation method includes: Step S1: Manganese source, iron source, phosphorus source, cobalt source, 2-methylimidazole and solvent are mixed and subjected to solvothermal reaction and first drying in sequence to obtain a composite precursor containing metal-organic framework material; Step S2: The lithium source, the composite precursor, and the graphene oxide suspension are mixed and then ground and dried sequentially to obtain the material to be calcined; wherein the graphene oxide suspension is a mixture of graphene oxide and water. Step S3: Under a protective gas atmosphere, the material to be calcined is calcined, and after cooling, the modified cathode material is obtained.

5. The method for preparing the modified cathode material according to claim 4, characterized in that, In step S1, the molar ratio of manganese in the manganese source, iron in the iron source, cobalt in the cobalt source, and phosphorus in the phosphorus source is (0.3-0.8):(0.17-0.69):(0.01-0.03):1; Preferably, the molar ratio of cobalt in the cobalt source to 2-methylimidazole is 1:(2-4). Preferably, the manganese source, the iron source, the phosphorus source, the cobalt source, the 2-methylimidazole, and the solvent are mixed to obtain a mixture; more preferably, the molar concentration of the 2-methylimidazole in the mixture is 0.2 to 0.4 mol / L. Preferably, the manganese source is selected from one or more of the group consisting of manganese acetate, manganese carbonate, and manganese dioxide; and / or, the iron source is selected from one or more of the group consisting of ferric phosphate, ferric acetate, ferric acetate tetrahydrate, ferric nitrate nonahydrate, and ferrous sulfate heptahydrate; and / or, the phosphorus source is selected from one or more of the group consisting of phosphoric acid, lithium dihydrogen phosphate, and ammonium dihydrogen phosphate; and / or, the cobalt source is selected from cobalt nitrate hexahydrate and / or cobalt chloride; and / or, the solvent is selected from a mixture of water and an organic solvent, or an organic solvent, wherein the organic solvent is selected from one or more of the group consisting of methanol, ethanol, ethylene glycol, and N,N-dimethylformamide. Preferably, the temperature of the solvothermal reaction is 150–250°C, and the time is 12–72 h; Preferably, the temperature of the first drying process is 60–90°C, and the time is 8–24 hours.

6. The method for preparing the modified cathode material according to claim 4, characterized in that, In step S2, the method for preparing the graphene oxide suspension includes: mixing the graphene oxide with water and then treating it with ultrasound to obtain the graphene oxide suspension. Preferably, the ultrasonic treatment lasts for 1 to 3 hours and the frequency is 30 to 50 kHz. Preferably, the solid content of the graphene oxide suspension is 0.5 to 2 wt%.

7. The method for preparing the modified cathode material according to claim 6, characterized in that, In step S2, the weight ratio of graphene oxide to cobalt source in the graphene oxide suspension is 1:(0.1~0.35). Preferably, the molar ratio of lithium in the lithium source to phosphorus in the phosphorus source is 1:(0.9 to 1.1). Preferably, the lithium source is selected from one or more of the group consisting of lithium carbonate, lithium hydroxide, and lithium dihydrogen phosphate; Preferably, stirring is performed during the mixing process in step S2; more preferably, the stirring rate is 200-400 rpm and the stirring time is 1-3 hours. Preferably, the grinding speed is 1500-2200 rpm and the time is 0.5-2.5 h; more preferably, a sand mill is used for the grinding. Preferably, the second drying temperature is 100-110°C and the time is 0.5-1.5 hours; more preferably, spray drying is used for the second drying.

8. The method for preparing the modified cathode material according to any one of claims 4 to 7, characterized in that, In step S3, the calcination process includes a heating stage and a heat preservation stage; Preferably, the heating rate during the heating stage is 2–5 °C / min; Preferably, the temperature during the heat preservation stage is 500–750°C, and the time is 6–12 hours. Preferably, the protective gas is introduced at a rate of 0.5 to 1 L / min; more preferably, the protective gas is selected from nitrogen and / or an inert gas.

9. A positive electrode, comprising a positive electrode current collector and a positive electrode active material layer stacked together, characterized in that, The positive electrode active material layer comprises the modified positive electrode material according to any one of claims 1 to 3.

10. A lithium-ion battery, comprising a positive electrode, a negative electrode, an electrolyte, and a separator disposed between the positive electrode and the negative electrode, characterized in that, The positive electrode is the positive electrode as described in claim 9.

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