A lithium manganese iron phosphate positive electrode material, a preparation method and application thereof
By embedding carbon nanotubes into lithium manganese iron phosphate cathode material and forming a three-dimensional conductive network structure, the problem of low conductivity of lithium manganese iron phosphate is solved, and the battery performance of lithium-ion batteries is improved.
Patent Information
- Authority / Receiving Office
- CN · China
- Patent Type
- Patents(China)
- Current Assignee / Owner
- SVOLT ENERGY TECHNOLOGY CO LTD
- Filing Date
- 2024-05-09
- Publication Date
- 2026-07-14
AI Technical Summary
In existing technologies, the conductivity improvement of lithium manganese iron phosphate cathode materials is limited, which affects their performance in lithium-ion batteries.
Lithium manganese iron phosphate material was grown in situ on the surface of oxidized carbon nanotubes using the sol-gel method, and a three-dimensional conductive network structure was formed by segmented heat treatment to improve the conductivity of carbon nanotubes.
It significantly improves the conductivity of lithium manganese iron phosphate cathode material, thereby enhancing the cycle performance, rate performance, and initial efficiency of lithium-ion batteries.
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Figure CN118495493B_ABST
Abstract
Description
Technical Field
[0001] This invention belongs to the field of lithium-ion battery technology, and relates to a lithium manganese iron phosphate cathode material, its preparation method and application. Background Technology
[0002] Since its initial invention in the 1990s, lithium-ion batteries have permeated all aspects of people's lives. With increased government support for new energy sources in recent years, electric vehicles have become commonplace. Against this backdrop, power batteries for electric vehicles are attracting increasing market attention. Among them, lithium-ion batteries, developed over decades, have proven their reliability and high energy density. Lithium iron manganese phosphate (LFP), a lithium-ion cathode material with advantages such as high capacity, stable charge / discharge voltage, low price, good safety, good thermal stability, and no environmental pollution, has recently gained popularity among battery manufacturers. However, the electron transition energy in LFP is much higher than in lithium iron phosphate (LiFePO4). LiFePO4 can be compared to a semiconductor, while LFP is essentially an insulator. Therefore, the conductivity of LFP electrodes has always been a challenge hindering its application.
[0003] Studies have shown that combining carbon materials with lithium manganese iron phosphate can effectively improve the electrical conductivity of the material. For example, CN109888205A discloses a nano-spherical carbon-coated lithium manganese iron phosphate composite material and its preparation method, a lithium battery cathode material, and a lithium battery. The composite material includes lithium manganese iron phosphate and an outer carbon layer coating the lithium manganese iron phosphate. The chemical composition of the lithium manganese iron phosphate is LiMn. 1-x Fe x PO4, wherein 0.1≤x≤1, the particle size D50 of the composite material is 1 to 10 μm, and the carbon content in the lithium manganese iron phosphate is 1% to 10% by mass. For example, CN115632120A discloses a method for preparing rapidly conductive modified lithium manganese iron phosphate, a battery positive electrode, and a battery. The method includes coating the surface of the ground lithium manganese iron phosphate with a layer of graphene, which improves the conductivity of the material to some extent. However, the surface-coated carbon layer or surface-coated graphene provided in the above-mentioned literature has very limited effect on improving the conductivity of lithium manganese iron phosphate materials.
[0004] Therefore, how to improve the conductivity of lithium manganese iron phosphate cathode material is a technical problem that urgently needs to be solved. Summary of the Invention
[0005] To address the shortcomings of existing technologies, the present invention aims to provide a lithium manganese iron phosphate cathode material, its preparation method, and its applications. The preparation method provided by this invention yields a lithium manganese iron phosphate material with carbon nanotubes embedded within the bulk phase. These carbon nanotubes form a three-dimensional conductive network structure, significantly improving the conductivity of the lithium manganese iron phosphate cathode material, thereby enhancing the battery's cycle performance, rate performance, and initial efficiency.
[0006] To achieve this objective, the present invention adopts the following technical solution:
[0007] In a first aspect, the present invention provides a method for preparing a lithium manganese iron phosphate cathode material, the method comprising the following steps:
[0008] A sol-gel precursor solution is obtained by mixing an iron source, a manganese source, a phosphorus source, a lithium source, a chelating agent, a solvent, and carbon nanotubes after oxidation treatment, followed by heat treatment to obtain the lithium manganese iron phosphate cathode material.
[0009] The preparation method provided by this invention grows lithium manganese iron phosphate cathode material in situ on the surface of oxidized carbon nanotubes using a sol-gel method. The heat treatment not only achieves the generation of lithium manganese iron phosphate material, but also realizes the high-temperature reduction of oxidized carbon nanotubes, removing the adverse effects of the modified groups after oxidation on the carbon nanotubes, and improving the conductivity of the carbon nanotubes themselves. This results in lithium manganese iron phosphate material with carbon nanotubes embedded in the bulk phase, and the carbon nanotubes form a three-dimensional conductive network structure, which greatly improves the conductivity of the lithium manganese iron phosphate cathode material, thereby improving the cycle performance, rate performance and first-time efficiency of the battery.
[0010] In this invention, oxidized carbon nanotubes serve as a carrier for the in-situ growth of lithium manganese iron phosphate (LMP), enabling uniform crystallization of LMP. Without oxidation, uniform dispersion of carbon nanotubes in the solution is impossible, leading to uneven crystallization of LMP on the carbon nanotube surface. Furthermore, this invention avoids the problem that, without the sol-gel method for in-situ growth of LMP on carbon nanotubes, perfectly crystalline LMP cannot be formed. Additionally, if carbon nanotubes are not generated in-situ within LMP, the enhanced electronic conductivity of the composite material cannot be achieved.
[0011] Preferably, the oxidation treatment method includes:
[0012] A mixed oxidant solution and the carbon nanotubes to be treated are subjected to modification treatment to obtain oxidized carbon nanotubes.
[0013] Preferably, the amount of oxidant solution added to each 1g of carbon nanotube to be treated is 25 to 50mL, such as 25mL, 30mL, 35mL, 40mL, 45mL or 50mL, but is not limited to the listed values. Other unlisted values within this range are also applicable.
[0014] In this invention, if too much oxidant solution is added to each 1g of carbon nanotubes to be treated, exceeding 50mL, the carbon nanotubes will be over-oxidized, resulting in insufficient reduction during subsequent high-temperature sintering and affecting the electronic conductivity of the material; while if too little oxidant solution is added, less than 25mL, it will not be conducive to the dispersion of carbon nanotubes in the reaction solution.
[0015] Preferably, the oxidant solution comprises concentrated sulfuric acid and / or concentrated nitric acid.
[0016] Preferably, the volume ratio of concentrated sulfuric acid to concentrated nitric acid is (1.5 to 4):1, for example, 1.5:1, 2:1, 2.5:1, 3:1, 3.5:1 or 4:1, but it is not limited to the listed values. Other unlisted values within this range are also applicable.
[0017] Preferably, the modification treatment is followed by solid-liquid separation and drying.
[0018] Preferably, during the primary mixing process, the mixed material further includes a carbon source.
[0019] In the preparation method provided by the present invention, an additional carbon source can be added during the mixing process, thereby achieving the internal embedding of carbon nanotubes and the dual coating of carbon layers, which further improves the conductivity of the cathode material.
[0020] Preferably, the chelating agent comprises citric acid and / or ethylene glycol.
[0021] Preferably, the amount of the chelating agent added is 0.1 to 50 wt% of the total mass of the non-solvent raw materials in a single mixing process, such as 0.1 wt%, 1 wt%, 5 wt%, 10 wt%, 15 wt%, 20 wt%, 25 wt%, 30 wt%, 35 wt%, 40 wt%, 45 wt%, or 50 wt%, but is not limited to the listed values; other unlisted values within this range are also applicable.
[0022] Preferably, the carbon source is any one or a combination of at least two of glucose, sucrose, or starch.
[0023] Preferably, the solution after one mixing is heated and stirred to obtain a sol-gel precursor solution.
[0024] Preferably, the heating and stirring temperature is 20 to 100°C, such as 20°C, 30°C, 40°C, 50°C, 60°C, 70°C, 80°C, 90°C, or 100°C, but is not limited to the listed values. Other unlisted values within this range are also applicable.
[0025] Preferably, the heating and stirring time is 5 to 20 hours, such as 5 hours, 8 hours, 10 hours, 13 hours, 15 hours, 18 hours or 20 hours, but it is not limited to the listed values. Other unlisted values within this range are also applicable.
[0026] Preferably, the sol-gel precursor solution is baked at high temperature and then subjected to heat treatment.
[0027] Preferably, the high-temperature baking is carried out in a vacuum environment.
[0028] Preferably, the high-temperature baking temperature is 60-200℃, such as 60℃, 70℃, 80℃, 90℃, 100℃, 110℃, 120℃, 130℃, 140℃, 150℃, 160℃, 170℃, 180℃, 190℃ or 200℃, but is not limited to the listed values. Other unlisted values within this range are also applicable.
[0029] Preferably, the high-temperature baking time is 5 to 20 hours, such as 5 hours, 8 hours, 10 hours, 13 hours, 15 hours, 18 hours or 20 hours, but it is not limited to the listed values. Other unlisted values within this range are also applicable.
[0030] Preferably, the heat treatment includes performing a first heat treatment and a second heat treatment in sequence.
[0031] Preferably, the heat treatment is performed under a protective atmosphere.
[0032] The heat treatment provided by this invention, carried out in stages, improves the crystallization efficiency of lithium manganese iron phosphate. The first heat treatment achieves high-temperature thermal reduction of carbon nanotubes and causes preliminary decomposition of the raw materials. The second heat treatment yields well-crystallized olivine-type lithium manganese iron phosphate. If a single-stage heat treatment is used, it is difficult to generate well-crystallized lithium manganese iron phosphate on the surface of carbon nanotubes.
[0033] Preferably, the temperature of the first heat treatment is 200 to 500°C, such as 200°C, 250°C, 300°C, 350°C, 400°C, 450°C, or 500°C, but it is not limited to the listed values. Other unlisted values within this range are also applicable.
[0034] In this invention, if the temperature of the heat treatment is too low, it will affect the decomposition of the raw materials and the reduction of carbon nanotubes, while if the temperature is too high, it will cause the raw materials to decompose too quickly and crystallize prematurely.
[0035] Preferably, the duration of the heat treatment is 2 to 10 hours, such as 2 hours, 3 hours, 4 hours, 5 hours, 6 hours, 7 hours, 8 hours, 9 hours, or 10 hours, but it is not limited to the listed values. Other unlisted values within this range are also applicable.
[0036] Preferably, the temperature of the secondary heat treatment is 500 to 1000°C, such as 500°C, 550°C, 600°C, 650°C, 700°C, 750°C, 800°C, 850°C, 900°C, 950°C, or 1000°C, but it is not limited to the listed values. Other unlisted values within this range are also applicable.
[0037] Preferably, the duration of the secondary heat treatment is 5 to 10 hours, such as 5 hours, 6 hours, 7 hours, 8 hours, 9 hours, or 10 hours, but it is not limited to the listed values. Other unlisted values within this range are also applicable.
[0038] As a preferred technical solution, the preparation method includes the following steps:
[0039] A mixed solution of concentrated sulfuric acid and concentrated nitric acid in a volume ratio of (1.5–4):1 was mixed with the carbon nanotubes to be treated. The amount of the mixed solution of concentrated sulfuric acid and concentrated nitric acid added to each 1g of carbon nanotubes to be treated was 25–50mL. After modification treatment, solid-liquid separation and drying, the oxidized carbon nanotubes were obtained.
[0040] Iron source, manganese source, phosphorus source, lithium source, solvent, chelating agent and oxidized carbon nanotubes are mixed and heated and stirred at 20-100℃ for 5-20h to obtain sol-gel precursor solution, which is then baked at 60-200℃ in a vacuum environment for 5-20h.
[0041] The material after high-temperature baking is heated to 200-500℃ for a first heat treatment of 2-10 hours, and then heated to 500-1000℃ for a second heat treatment of 5-10 hours to obtain the lithium manganese iron phosphate cathode material.
[0042] It should be noted that the specific types of raw materials (such as iron source, manganese source, phosphorus source, lithium source and carbon source) in the preparation method provided by the present invention are all conventional technical selections. Those skilled in the art can select the types according to actual needs.
[0043] Optionally, the iron source includes but is not limited to iron phosphate, iron hydrogen phosphate, iron dihydrogen phosphate, etc.; the manganese source includes but is not limited to manganese phosphate, manganese hydrogen phosphate, manganese dihydrogen phosphate, etc.; the phosphorus source includes but is not limited to phosphoric acid; the carbon source includes but is not limited to glucose, composite carbon source, agarose, lactose, fructose, etc.; and the mass ratio or molar ratio between the iron source, manganese source, phosphorus source, lithium source and carbon source can be adjusted according to the required stoichiometric ratio.
[0044] In a second aspect, the present invention provides a lithium iron manganese phosphate cathode material, which is prepared by the preparation method as described in the first aspect; the lithium iron manganese phosphate cathode material includes lithium iron manganese phosphate particles and carbon nanotubes embedded inside the lithium iron manganese phosphate particles.
[0045] For the lithium iron manganese phosphate cathode material provided by the present invention, the carbon nanotubes are embedded inside the bulk phase of the lithium iron manganese phosphate particles and interpenetrate with each other, forming a three-dimensional conductive network structure, which greatly improves the conductivity of the cathode material and has a high conductivity, resulting in a significant improvement in the cycle stability and rate performance of the battery.
[0046] Preferably, the chemical general formula of the lithium iron manganese phosphate particles is LiFe x Mn 1-x PO4, where 0 < x < 1, such as 0.05, 0.1, 0.2, 0.3, 0.4, 0.5, 0.6, 0.7, 0.8, 0.9 or 0.95, etc., but not limited to the listed values, and other unlisted values within this numerical range are equally applicable.
[0047] Preferably, the mass proportion of carbon in the lithium iron manganese phosphate cathode material is 0.1 - 2 wt%, such as 0.1, 0.2 wt%, 0.3 wt%, 0.4 wt%, 0.5 wt%, 0.6 wt%, 0.7 wt%, 0.8 wt%, 0.9 wt%, 1 wt%, 1.1 wt%, 1.2 wt%, 1.3 wt%, 1.4 wt%, 1.5 wt%, 1.6 wt%, 1.7 wt%, 1.8 wt%, 1.9 wt% or 2 wt%, etc., but not limited to the listed values, and other unlisted values within this numerical range are equally applicable.
[0048] In a third aspect, the present invention further provides a lithium ion battery, which includes the lithium iron manganese phosphate cathode material as described in the second aspect.
[0049] Compared with the prior art, the present invention has the following beneficial effects:
[0050] The preparation method provided by this invention grows lithium manganese iron phosphate cathode material in situ on the surface of oxidized carbon nanotubes using a sol-gel method. The heat treatment not only achieves the generation of lithium manganese iron phosphate material, but also realizes the high-temperature reduction of oxidized carbon nanotubes, removing the adverse effects of the modified groups after oxidation on the carbon nanotubes, and improving the conductivity of the carbon nanotubes themselves. This results in lithium manganese iron phosphate material with carbon nanotubes embedded in the bulk phase, and the carbon nanotubes interpenetrate to form a three-dimensional conductive network structure, which greatly improves the conductivity of the lithium manganese iron phosphate cathode material, thereby improving the cycle performance, rate performance and first-time efficiency of the battery. Attached Figure Description
[0051] Figure 1 This is a schematic diagram of the structure of the lithium manganese iron phosphate cathode material provided in Example 1. Detailed Implementation
[0052] The technical solution of the present invention will be further illustrated below through specific embodiments. Those skilled in the art should understand that the embodiments described are merely illustrative of the present invention and should not be construed as limiting the invention in any way.
[0053] Example 1
[0054] This embodiment provides a lithium iron phosphate cathode material, such as... Figure 1 As shown, the lithium manganese iron phosphate cathode material includes lithium manganese iron phosphate particles and carbon nanotubes (interpenetrating each other) embedded inside the lithium manganese iron phosphate particles. The chemical formula of the lithium manganese iron phosphate cathode material is LiFe. 3 / 4 Mn 1 / 4 PO4 contains 0.45% carbon by mass in the cathode material.
[0055] The preparation method of the lithium manganese iron phosphate cathode material is as follows:
[0056] S1: Take 30 mL of concentrated sulfuric acid and 10 mL of concentrated nitric acid to prepare a mixed solution, add 1 g of neutral carbon nanotubes (carbon nanotubes to be treated) to it, stir at 60 °C for 10 h, then dialyze the obtained solution, filter and dry to obtain F-CNT material (carbon nanotubes after oxidation treatment).
[0057] S2: Disperse 50 mg of F-CNT material in 100 mL of deionized water, and add 0.718 g of lithium hydroxide, 4.499 g of iron phosphate, 2.987 g of manganese dihydrogen phosphate, 2.940 g of phosphoric acid and 6.917 g of citric acid to it;
[0058] S3: Stir the mixed solution at 90℃ for 10h to obtain a sol-gel precursor solution, and then bake it in a vacuum oven at 100℃ for 12h to obtain the precursor material (sol-gel precursor).
[0059] S4: The precursor obtained above was first heat-treated at 350℃ for 5 hours (first heat treatment), and then heat-treated at 700℃ for 10 hours (second heat treatment). Both heat treatments were carried out in an argon atmosphere. After natural cooling, the final product LFMP@CNT-1 was obtained.
[0060] Example 2
[0061] This embodiment provides a lithium manganese iron phosphate cathode material, which includes lithium manganese iron phosphate particles and carbon nanotubes (interpenetrating each other) embedded inside the lithium manganese iron phosphate particles. The chemical formula of the lithium manganese iron phosphate cathode material is LiFe. 3 / 4 Mn 1 / 4 PO4, with carbon accounting for 0.9 wt% of the cathode material.
[0062] The preparation method of the lithium manganese iron phosphate cathode material is as follows:
[0063] S1: Take 15 mL of concentrated sulfuric acid and 10 mL of concentrated nitric acid to prepare a mixed solution, add 1 g of neutral carbon nanotubes to it, stir at 60 °C for 10 h, then dialyze the resulting solution, filter and dry to obtain F-CNT material;
[0064] S2: Disperse 100mg of F-CNT material in 100mL of deionized water, and add 0.718g of lithium hydroxide, 4.499g of ferric phosphate, 2.987g of manganese dihydrogen phosphate, 2.940g of phosphoric acid and 6.917g of citric acid to it;
[0065] S3: Stir the mixed solution at 50℃ for 15h to obtain a sol-gel precursor solution, and then bake it in a vacuum oven at 150℃ for 8h to obtain the precursor material (sol-gel precursor).
[0066] S4: The precursor obtained above was first heat-treated at 200℃ for 10h, and then heat-treated at 500℃ for 10h. Both heat treatments were carried out in an argon atmosphere. After natural cooling, the final product LFMP@CNT-2 was obtained.
[0067] Example 3
[0068] This embodiment provides a lithium manganese iron phosphate cathode material, which includes lithium manganese iron phosphate particles and carbon nanotubes (interpenetrating each other) embedded inside the lithium manganese iron phosphate particles. The chemical formula of the lithium manganese iron phosphate cathode material is LiFe. 3 / 4 Mn 1 / 4 PO4, with carbon accounting for 1.35 wt% of the cathode material.
[0069] The preparation method of the lithium manganese iron phosphate cathode material is as follows:
[0070] S1: Take 40 mL of concentrated sulfuric acid and 10 mL of concentrated nitric acid to prepare a mixed solution, add 1 g of neutral carbon nanotubes to it, stir at 60 °C for 10 h, then dialyze the resulting solution, filter and dry to obtain F-CNT material;
[0071] S2: Disperse 150mg of F-CNT material in 100mL of deionized water, and add 0.718g of lithium hydroxide, 2.999g of iron phosphate, 2.987g of manganese dihydrogen phosphate, 4.480g of phosphoric acid and 6.917g of citric acid to it;
[0072] S3: Stir the mixed solution at 90℃ for 10h, and then bake it in a vacuum oven at 100℃ for 12h to obtain the precursor material;
[0073] S4: The precursor obtained above was first heat-treated at 500℃ for 2 hours, and then at 1000℃ for 5 hours. Both heat treatments were carried out in an argon atmosphere. After natural cooling, the final product LFMP@CNT-3 was obtained.
[0074] Example 4
[0075] The difference between this embodiment and embodiment 1 is that fructose is added as a carbon source in step S2 of this embodiment. The mass of fructose added is 1.35 wt% (based on the total mass of F-CNT material, lithium hydroxide, iron phosphate, manganese dihydrogen phosphate, phosphoric acid and citric acid being 100 wt%, an additional 1.35 wt% of fructose is added).
[0076] The remaining preparation methods and parameters are consistent with those in Example 1.
[0077] Example 5
[0078] The difference between this embodiment and Embodiment 1 is that in this embodiment, the mixed solution in step S1 is a mixture of 45 mL of concentrated sulfuric acid and 15 mL of concentrated nitric acid (the oxidizing agent solution is 60 mL).
[0079] The remaining preparation methods and parameters are consistent with those in Example 1.
[0080] Example 6
[0081] The difference between this embodiment and Embodiment 1 is that the mixed solution in step S1 of this embodiment is a mixed solution of 15 mL concentrated sulfuric acid and 5 mL concentrated nitric acid (the oxidizing agent solution is 20 mL).
[0082] The remaining preparation methods and parameters are consistent with those in Example 1.
[0083] Example 7
[0084] The difference between this embodiment and embodiment 1 is that in step S4 of this embodiment, the precursor is directly heated to 700°C for 10 hours for heat treatment, without performing the first heat treatment process.
[0085] The remaining preparation methods and parameters are consistent with those in Example 1.
[0086] Example 8
[0087] The difference between this embodiment and Embodiment 1 is that the temperature of the first heat treatment in step S4 is 600°C.
[0088] The remaining preparation methods and parameters are consistent with those in Example 1.
[0089] Example 9
[0090] The difference between this embodiment and Embodiment 1 is that the temperature of the first heat treatment in step S4 is 150°C.
[0091] The remaining preparation methods and parameters are consistent with those in Example 1.
[0092] Comparative Example 1
[0093] The difference between this comparative example and Example 1 is that step S1 is omitted, and carbon nanotubes are not added in step S2. That is, the lithium manganese iron phosphate cathode material in this example does not contain carbon nanotubes.
[0094] The remaining preparation methods and parameters are the same as in Example 1.
[0095] Comparative Example 2
[0096] The difference between this comparative example and Example 1 is that the modification treatment of carbon nanotubes in step S1 is not performed; that is, neutral carbon nanotubes are directly added in step S2.
[0097] The remaining preparation methods and parameters are consistent with those in Example 1.
[0098] Lithium-ion batteries were prepared using lithium manganese iron phosphate provided in Examples 1-9 and Comparative Examples 1-2, respectively.
[0099] (a): Preparation of positive electrode sheet
[0100] Weigh out the three materials according to the mass ratio of active material: conductive agent (super-p) and polyvinylidene fluoride (PVDF) = 8:1:1, and dry grind them in an agate mortar for 15 minutes to mix them evenly. Then transfer them to a glass bottle, add an appropriate amount of N-methylpyrrolidone (NMP) to adjust the viscosity of the slurry, seal it, and stir continuously at 600 rpm for 6 hours to form a uniform slurry. Then coat it onto the rough side of a pre-cleaned and dried copper foil using a scraper method, with a coating thickness of about 200 μm. Then pre-dry it in a 110℃ oven for 2 hours, and then place it in a 100℃ vacuum drying oven for 12 hours. After natural cooling, cut it into round pieces with a diameter of 12 mm using a slicer, weigh them, seal them, and place them in a desiccator for later use.
[0101] (b): Assembling batteries
[0102] CR2032 coin cells were assembled in a glove box filled with a high-purity argon atmosphere, where the contents of H2O and O2 were both below 0.1 ppm. The electrolyte was 1 M LiPF6 (EC:EMC:DMC = 1:1:1 vol%), and the separator was a PP membrane.
[0103] (c): Electrochemical performance testing and analysis
[0104] Constant current charge / discharge tests were conducted using the LANDCT2001A battery testing system at an ambient temperature of 25℃. The charge / discharge voltage window was 2–4.3V, and 1C = 155mAh / g. The test results are shown in Table 1.
[0105] Table 1
[0106]
[0107] Electrochemical impedance spectroscopy (EIS) was performed using a PARSTATPMC-1000 electrochemical workstation to measure the interfacial transfer resistance, charge transfer resistance, and total resistance of the lithium manganese iron phosphate cathode materials provided in Examples 1-9 and Comparative Examples 1-2, respectively. The electrochemical impedance measurement frequency ranged from 0.01 to 100 kHz, and the test amplitude was 5 mV. The test results are shown in Table 2.
[0108] Table 2
[0109]
[0110] From Tables 1 and 2, we can obtain:
[0111] The data from Examples 1, 5, and 6 show that if too much silica solution is added to 1g of carbon nanotubes to be treated, over-oxidation will occur, which will increase the internal resistance of the battery; while if the mass ratio is too small, i.e. the degree of oxidation is too low, the carbon nanotubes will be unevenly dispersed, affecting the formation of the conductive network.
[0112] The data results from Examples 1 and 7 show that if segmented sintering is not performed during the heat treatment process, a good crystallization effect cannot be achieved, which leads to a decrease in cycle stability.
[0113] The data from Examples 8 and 9 show that if the temperature of the first heat treatment is too high, the reactants will decompose excessively, resulting in uneven crystallization and thus affecting the cycle stability. If the temperature of the first heat treatment is too low, the reactants will not decompose sufficiently, which will also affect the cycle stability.
[0114] The data results from Example 1 and Comparative Example 1 show that without in-situ growth of lithium manganese iron phosphate on carbon nanotubes and without the intercalation and cross-intercalation of carbon nanotubes within the bulk phase of the particles, the problem of low electrical conductivity of lithium manganese iron phosphate materials cannot be solved.
[0115] The data from Example 1 and Comparative Example 2 show that the goal of improving electrical conductivity cannot be achieved without oxidizing the carbon nanotubes.
[0116] In summary, the preparation method provided by this invention grows lithium manganese iron phosphate cathode material in situ on the surface of oxidized carbon nanotubes using the sol-gel method. The heat treatment not only achieves the generation of lithium manganese iron phosphate material but also realizes the high-temperature reduction of oxidized carbon nanotubes, removing the adverse effects of the modified groups after oxidation on the carbon nanotubes and improving the conductivity of the carbon nanotubes themselves. This results in lithium manganese iron phosphate material with carbon nanotubes embedded in the bulk phase, and the carbon nanotubes interpenetrate to form a three-dimensional conductive network structure, which greatly improves the conductivity of the lithium manganese iron phosphate cathode material, thereby improving the cycle performance, rate performance, and first-efficiency of the battery.
[0117] The applicant declares that the above description is only a specific embodiment of the present invention, but the protection scope of the present invention is not limited thereto. Those skilled in the art should understand that any changes or substitutions that can be easily conceived by those skilled in the art within the technical scope disclosed in the present invention fall within the protection and disclosure scope of the present invention.
Claims
1. A method for preparing a lithium manganese iron phosphate cathode material, characterized in that, The preparation method includes the following steps: A sol-gel precursor solution is obtained by mixing an iron source, a manganese source, a phosphorus source, a lithium source, a chelating agent, a solvent, a carbon source, and oxidized carbon nanotubes in one step, followed by heat treatment to obtain the lithium manganese iron phosphate cathode material. The carbon source is any one or a combination of at least two of glucose, fructose, sucrose, or starch; The chelating agent includes citric acid and / or ethylene glycol; The heat treatment includes performing a first heat treatment and a second heat treatment in sequence; The temperature of the first heat treatment is 200~500℃; The temperature of the secondary heat treatment is 500~1000℃; The oxidation treatment method includes: mixing an oxidant solution and carbon nanotubes to be treated, and then modifying them to obtain oxidized carbon nanotubes. The amount of oxidant solution added to each 1g of carbon nanotubes to be treated is 25~50mL; The oxidizing agent solution includes concentrated sulfuric acid and / or concentrated nitric acid; The volume ratio of concentrated sulfuric acid to concentrated nitric acid is (1.5~4):1; The modified material is then subjected to solid-liquid separation and drying.
2. The method for preparing lithium manganese iron phosphate cathode material according to claim 1, characterized in that, The amount of the chelating agent added is 0.1 to 50 wt% of the total mass of the non-solvent raw materials in a single mixing process.
3. The method for preparing lithium manganese iron phosphate cathode material according to claim 1, characterized in that, The solution after one mixing is heated and stirred to obtain a sol-gel precursor solution.
4. The method for preparing lithium manganese iron phosphate cathode material according to claim 3, characterized in that, The heating and stirring temperature is 20~100℃.
5. The method for preparing lithium manganese iron phosphate cathode material according to claim 3, characterized in that, The heating and stirring time is 5 to 20 hours.
6. The method for preparing lithium manganese iron phosphate cathode material according to claim 1, characterized in that, The sol-gel precursor solution is baked at high temperature and then subjected to heat treatment.
7. The method for preparing lithium manganese iron phosphate cathode material according to claim 6, characterized in that, The high-temperature baking is carried out in a vacuum environment.
8. The method for preparing lithium manganese iron phosphate cathode material according to claim 6, characterized in that, The high-temperature baking temperature is 60~200℃.
9. The method for preparing lithium manganese iron phosphate cathode material according to claim 6, characterized in that, The high-temperature baking time is 5~20 hours.
10. The method for preparing lithium manganese iron phosphate cathode material according to claim 1, characterized in that, The duration of the heat treatment is 2 to 10 hours.
11. The method for preparing lithium manganese iron phosphate cathode material according to claim 1, characterized in that, The duration of the secondary heat treatment is 5-10 hours.
12. The method for preparing lithium manganese iron phosphate cathode material according to claim 1, characterized in that, The preparation method includes the following steps: A mixed solution of concentrated sulfuric acid and concentrated nitric acid in a volume ratio of (1.5~4):1 was mixed with the carbon nanotubes to be treated. The amount of the mixed solution of concentrated sulfuric acid and concentrated nitric acid added to each 1g of carbon nanotubes to be treated was 25~50mL. After modification treatment, solid-liquid separation and drying, carbon nanotubes after oxidation treatment were obtained. Iron source, manganese source, phosphorus source, lithium source, solvent, chelating agent, carbon source and oxidized carbon nanotubes are mixed and heated and stirred at 20~100℃ for 5~20h to obtain sol-gel precursor solution, which is then baked at 60~200℃ in a vacuum environment for 5~20h. The carbon source is any one or a combination of at least two of glucose, fructose, sucrose, or starch; The material after high-temperature baking is heated to 200~500℃ for a first heat treatment for 2~10 hours, and then heated to 500~1000℃ for a second heat treatment for 5~10 hours to obtain the lithium manganese iron phosphate cathode material.
13. A lithium iron phosphate cathode material, characterized in that, The lithium manganese iron phosphate cathode material is prepared by the preparation method according to any one of claims 1-12; the lithium manganese iron phosphate cathode material includes lithium manganese iron phosphate particles and carbon nanotubes embedded inside the lithium manganese iron phosphate particles.
14. The lithium iron phosphate cathode material according to claim 13, characterized in that, The general chemical formula of the lithium manganese iron phosphate particles is LiFe. x Mn 1-x PO4, where 0 <x<1。 15. The lithium iron phosphate cathode material according to claim 13, characterized in that, The mass percentage of carbon in lithium manganese iron phosphate cathode material is 0.1~2 wt%.
16. A lithium-ion battery, characterized in that, The lithium-ion battery includes the lithium manganese iron phosphate cathode material as described in any one of claims 13-15.