An LMFP / CNF carbon nanofiber composite material, preparation method and application

Abstract

The present invention relates to the technical field of new energy battery, and particularly to an LMFP / CNF carbon nanofiber composite material, a preparation method and an application thereof. In this method, a lithium source material, a manganese source material, an iron source material, a phosphate root source material and a polymer are added into an organic solvent to obtain a spinning solution, and the spinning solution is electrospun to obtain precursor fibers; the precursor fibers are sequentially subjected to preheating oxidation and carbonization treatment to obtain the LMFP / CNF carbon nanofiber composite material; the phosphate root source includes one or more of 1-butyl-3-methylimidazolium dihydrogen phosphate, phosphoric acid, potassium dihydrogen phosphate, and ammonium dihydrogen phosphate, and the polymer includes one or more of polyacrylonitrile, polyvinyl alcohol, and polylactic acid. This method enables the polymer to cover the active substances, and a continuous carbon coating is formed on the surface of the active material; when the composite material is used as a cathode material of a battery, it has good electrochemical performance.

Description

Technical Field

[0001] The present invention relates to the technical field of new energy batteries, and more particularly, to an LMFP / CNF carbon nanofiber composite material, a preparation method and an application thereof. Background Art

[0002] New energy refers to an energy form that has lower environmental impact, is cleaner and more renewable compared to traditional fossil fuels. Globally, the development of new energy has become the focus of attention for the general public. Lithium-ion battery products have been fully integrated into people's lives, powering mobile phones and laptops that have brought revolutionary changes to modern society, and are now entering the transportation industry, such as plug-in hybrid vehicles and electric vehicles. Researchers are committed to discovering and developing 3D porous free-standing electrodes that can achieve a continuous conductive network for electron transport and interconnected hierarchical porosity for ion transport. This effectively improves the electrochemical performance of electrode materials, thus promoting the development of electrochemical energy storage technologies.

[0003] Lithium manganese iron phosphate (LMFP, LiMn x Fe 1-x PO4) is formed by doping iron on the basis of modifying LiMnPO4. It has a stable structure and a high voltage platform, and is a very promising new cathode material. However, since the commonly used inorganic phosphorus sources (ammonium dihydrogen phosphate and phosphoric acid) for synthesizing LMFP are difficult to dissolve in PAN solution to form a uniform and stable electrospinning precursor solution, most of the reported electrospun LMFP is based on hydrophilic polymer systems, such as polyvinyl alcohol (PVA) and polyvinylpyrrolidone (PVP), and the mechanical properties of these systems are poor. Polymers with good mechanical properties, such as polyacrylonitrile, polyvinyl alcohol, and polylactic acid, have a high carbonization rate and structural stability, and are ideal polymers for forming self-supporting carbon nano-networks through electrospinning. However, these high molecular polymers have strong selectivity for solvents, which limits their complete coverage of active substances, resulting in the inability to form a continuous carbon coating on the surface of active materials.

[0004] These problems seriously affect the popularization and application of LMFP. Summary of the Invention

[0005] The technical problem to be solved by the present invention is to provide an LMFP / CNF carbon nanofiber composite material, a preparation method and an application thereof to solve the problems of poor solubility and poor conductivity of LMFP in the prior art.

[0006] The technical solution of the present invention for solving the above technical problems is as follows:

[0007] The present invention provides a method for preparing an LMFP / CNF carbon nanofiber composite material. A lithium source material, a manganese source material, an iron source material, a phosphate source material, and a polymer are added to an organic solvent to obtain a spinning solution. The spinning solution is subjected to electrospinning to obtain precursor fibers. The precursor fibers are sequentially subjected to preheating oxidation and carbonization treatment to obtain the LMFP / CNF carbon nanofiber composite material.

[0008] Among them, the phosphate source includes one or more of 1-butyl-3-methylimidazolium dihydrogen phosphate, phosphoric acid, potassium dihydrogen phosphate, and ammonium dihydrogen phosphate, and the polymer includes one or more of polyacrylonitrile, polyvinyl alcohol, and polylactic acid.

[0009] On the basis of the above technical solution, the present invention can also be improved as follows.

[0010] Further, in the spinning solution, the mass percentage of the polymer is 10% - 12%.

[0011] Further, in the spinning solution, the molar ratio of lithium, manganese, iron, and phosphate is 1:(0.6 - 0.8):(0.4 - 0.2):1, and the mass ratio of the polymer to the lithium source is 4.56 - 15.27:1.

[0012] Further, the lithium source material includes one or more of anhydrous lithium acetate, lithium bromide, and lithium fluoride, the manganese source material includes one or more of anhydrous manganese acetate, manganese nitrate, and manganese chloride, and the iron source material includes one or more of iron(III) acetylacetonate, iron nitrate, and iron chloride.

[0013] Further, the organic solvent includes one or more of dimethylformamide, dimethylacetamide, and acetonitrile.

[0014] Further, the step of preheating oxidation includes preheating the precursor fibers in an air atmosphere, the temperature of the preheating oxidation is 230°C - 250°C, and the time of the preheating oxidation is 2h - 3h.

[0015] Further, the step of carbonization treatment includes carbonizing the precursor fibers that have undergone the preheating oxidation in a mixed atmosphere of an inert gas and hydrogen, the temperature of the carbonization treatment is 700°C - 750°C, and the time of the carbonization treatment is 2h - 6h.

[0016] The present invention also provides an LMFP / CNF carbon nanofiber composite material, which is prepared by the above method.

[0017] Furthermore, the composite material includes LMFP grains, a carbon fiber structure, and a thin carbon layer. The LMFP grains are embedded in or coated with the carbon fiber structure, and the thin carbon layer uniformly covers the surface of the LMFP grains and / or the outer layer of the carbon fiber structure. The particle size of the LMFP grains is 50 - 150 nm.

[0018] The present invention also provides an application of the LMFP / CNF carbon nanofiber composite material as described above in a new energy battery, characterized in that the composite material is used to prepare a positive electrode material.

[0019] The beneficial effects of the present invention are as follows:

[0020] (1) In the preparation method of the LMFP / CNF carbon nanofiber composite material of the present invention, an ionic liquid is used as the source material of phosphoric acid. Combining preheating oxidation and carbonization treatment enables the decomposition of the polymer (especially polyacrylonitrile) to well cover the active substances, so that a continuous carbon coating can be formed on the surface of the active material.

[0021] (2) The preparation method of the LMFP / CNF carbon nanofiber composite material of the present invention has the advantages of cost saving, energy consumption reduction, optimized preparation process, and easy scale-up preparation.

[0022] (3) The LMFP / CNF carbon nanofiber composite material of the present invention has the characteristic of good solubility, which improves the feasibility of preparing electrode materials in a polymer-based electrospinning system.

[0023] (4) The LMFP / CNF carbon nanofiber composite material of the present invention can be used as a structure guiding agent to induce the formation of LMFP nanocubes, providing a convenient path for the transmission of lithium ions.

[0024] (5) The LMFP / CNF carbon nanofiber composite material of the present invention can also form an N-doped biphasic carbon coating during the heat treatment process, further improving the electronic conductivity of the entire electrode. Description of the Drawings

[0025] Figure 1 This is the performance characterization test diagram of the LMFP / CNF carbon nanofiber composite material of the present invention and the materials obtained in Comparative Example 1. Figure 1 In it, a is the SEM diagram of LMFP / CNF-BP(6%); Figure 1 In it, b is the SEM diagram of LMFP / CNF-BP(8%); Figure 1 In it, c is the SEM diagram of LMFP / CNF-BP(10%); Figure 1 In it, d is the SEM diagram of LMFP / CNF-BP(12%);

[0026] Figure 2 For the LMFP / CNF carbon nanofiber composite of the present invention, the performance characterization test diagrams of the materials obtained in Comparative Example 2 Figure 2 In which, a is the SEM diagram of LMFP / CNF-BP (700 °C); Figure 2 In which, b is the SEM diagram of LMFP / CNF-BP (750 °C); Figure 2 In which, c is the SEM diagram of LMFP / CNF-BP (800 °C);

[0027] Figure 3 For the LMFP / CNF carbon nanofiber composite of the present invention, the performance characterization test diagrams of the materials obtained in Comparative Example 3 Figure 3 In which, a is the FESEM diagram of LMFP / CNF-BP (2 h); Figure 3 In which, b is the FESEM diagram of LMFP / CNF-BP (4 h); Figure 3 In which, c is the FESEM diagram of LMFP / CNF-BP (6 h); Figure 3 In which, d is the FESEM diagram of LMFP / CNF-BP (8 h);

[0028] Figure 4 For the LMFP / CNF carbon nanofiber composite of the present invention, the performance characterization test diagrams of the materials obtained in Comparative Example 4 Figure 4 In which, a is the FESEM diagram of LMFP / CNF-BP; Figure 4 In which, b is the FESEM diagram of LMFP / CNF-HP; Figure 4 In which, c is the FESEM diagram of LMFP / C-BP;

[0029] Figure 5 For the LMFP / CNF carbon nanofiber composite of the present invention, the charge-discharge curves of the materials obtained in Comparative Example 1 Figure 5 In which, a is the first charge-discharge curve at 0.1 C; Figure 5 In which, b is the cyclic stability performance diagram;

[0030] Figure 6 For the LMFP / CNF carbon nanofiber composite of the present invention, the charge-discharge curves of the materials obtained in Comparative Example 2 Figure 6 In which, a is the first charge-discharge curve at 0.1 C; Figure 6 In which, b is the cyclic stability performance diagram;

[0031] Figure 7 For the LMFP / CNF carbon nanofiber composite of the present invention, the charge-discharge curves of the materials obtained in Comparative Example 3 Figure 7 In which, a is the first charge-discharge curve at 0.1 C; Figure 7 In which, b is the cyclic stability performance diagram;

[0032] Figure 8 For the LMFP / CNF carbon nanofiber composite of the present invention, the charge-discharge curves of the materials obtained in Comparative Example 4 Figure 8 where a is the first charge-discharge curve at 0.1C, Figure 8 and b is the cyclic stability performance diagram Detailed implementation manners

[0033] The principles and features of the present invention will be described below in conjunction with the accompanying drawings. The examples given are only used to explain the present invention and are not intended to limit the scope of the present invention.

[0034] The preparation method of the LMFP / CNF carbon nanofiber composite of the present invention is as follows: a lithium source material, a manganese source material, an iron source material, a phosphate source material, and a polymer are added to an organic solvent to obtain a spinning solution, and the spinning solution is subjected to electrospinning to obtain precursor fibers; the precursor fibers are sequentially subjected to preheating oxidation and carbonization treatments to obtain the LMFP / CNF carbon nanofiber composite. Among them, the phosphate source includes one or more of 1-butyl-3-methylimidazolium dihydrogen phosphate, phosphoric acid, potassium dihydrogen phosphate, and ammonium dihydrogen phosphate, and the polymer includes one or more of polyacrylonitrile, polyvinyl alcohol, and polylactic acid.

[0035] In the preparation method of the present invention, an ionic liquid (1-butyl-3-methylimidazolium dihydrogen phosphate) is used as the phosphate source material. Combining preheating oxidation and carbonization treatments, after the polymer (especially polyacrylonitrile) is thermally decomposed, it can well cover the active substances, so that a continuous carbon coating can be formed on the surface of the active material. The composite material obtained in this way has good electrochemical performance when used as a cathode material.

[0036] In addition, in the preparation method of the present invention, due to the selection of a suitable organic solvent, when using a non-ionic liquid phosphate source material (phosphoric acid, potassium dihydrogen phosphate, ammonium dihydrogen phosphate), it can also be well mixed and dissolved with polyacrylonitrile, polyvinyl alcohol, and polylactic acid, so that good coverage of the polymer can also be achieved.

[0037] Preferably, in the spinning solution, the mass percentage of the polymer is 10% - 12%; the composite material prepared using the polymer within this mass percentage range has the best electrochemical performance.

[0038] More preferably, the polymer is polyacrylonitrile, and its mass percentage is 10%, 11%, 12%, etc.

[0039] Preferably, in the spinning solution, the molar ratio of lithium, manganese, iron, and phosphate is 1:(0.6 - 0.8):(0.4 - 0.2):1, and the mass ratio of the high molecular polymer to the lithium source is 4.56 - 15.27:1.

[0040] Specifically, in the material prepared with the above ratio, the specific chemical formula of LMFP is LiMn 0.8 Fe 0.2 PO4 or LiMn 0.7 Fe 0.3 PO4 or others.

[0041] Preferably, the lithium source material includes one or more of lithium acetate anhydrous, lithium bromide, and lithium fluoride; the manganese source material includes one or more of manganese acetate anhydrous, manganese nitrate, and manganese chloride; the iron source material includes one or more of iron(III) acetylacetonate, iron nitrate, and iron chloride.

[0042] Preferably, the organic solvent includes one or more of dimethylformamide, dimethylacetamide, and acetonitrile.

[0043] In the preparation method of the present invention, the preheating oxidation step includes preheating the precursor fiber in an air atmosphere. The preheating oxidation temperature is 230°C - 250°C, and the preheating oxidation time is 2h - 3h.

[0044] More preferably, the preheating oxidation temperature is 230°C, 235°C, 240°C, 245°C, 250°C, etc., and the preheating oxidation time is 2h, 2.5h, 3h, etc.

[0045] Preferably, the heating rate of preheating is 1°C - 4°C / min; more preferably, the heating rate of preheating is 2°C / min.

[0046] In the preparation method of the present invention, the carbonization treatment step includes carbonizing the preheated and oxidized precursor fiber in a mixed atmosphere of inert gas and hydrogen. The carbonization treatment temperature is 700°C - 750°C, and the carbonization treatment time is 2h - 6h.

[0047] Preferably, the carbonization treatment temperature is 700°C, 710°C, 720°C, 730°C, 740°C, 750°C, etc., and the carbonization treatment time is 2h, 4h, 6h, etc.

[0048] Preferably, the heating rate of carbonization treatment is 1°C - 7°C / min; more preferably, the heating rate of carbonization treatment is 5°C / min.

[0049] Preferably, the mixed atmosphere of inert gas and hydrogen is specifically an argon - hydrogen mixture, wherein the volume percentage of hydrogen is 2 - 6%; more preferably, the volume percentage of hydrogen is 5%.

[0050] The preparation method of the present invention is specifically as follows:

[0051] 1) Prepare lithium source material, manganese source material, iron source material, and phosphate source material respectively according to the ratio, and then mix them with a polymer. After adding an appropriate amount of organic solvent, magnetically stir for a period of time under the water bath condition of 60 °C to 80 °C until a homogeneous spinning solution is formed. Before electrospinning, ultrasonically treat the orange-red spinning solution first.

[0052] Preferably, the ultrasonic treatment time is 10 min to 20 min.

[0053] 2) Place the spinning solution obtained in step 1) in a 10 ml plastic syringe with a 20G (inner diameter 0.6 mm) stainless steel needle, then fix the syringe on a micro-injection pump, connect one end of the needle to the positive electrode of a high-voltage electrostatic generator, and connect the collection device to the negative electrode of the high-voltage electrostatic generator; the distance between the needle with a DC high voltage of 20 kV and the drum collection device covered with aluminum foil is 12 cm, the flow rate of the spinning solution is 0.4 ml / h, the relative environmental humidity is 25 - 30%, and the temperature is 24 - 28 °C.

[0054] 3) Place the precursor fibers prepared by electrospinning at room temperature overnight, then transfer the precursor fibers to a tubular furnace, first perform preheating oxidation, and then perform carbonization treatment to obtain the LMFP / CNF material.

[0055] The LMFP / CNF carbon nanofiber composite material of the present invention is prepared by the above method.

[0056] The composite material of the present invention includes LMFP crystal grains, carbon fiber structures, and thin carbon layers. The LMFP crystal grains are embedded in or coated on the carbon fiber structures, and the thin carbon layers evenly cover the surface of the LMFP crystal grains and / or the outer layer of the carbon fiber structures; the particle size of the LMFP crystal grains is 50 - 150 nm.

[0057] The composite material of the present invention has the characteristic of good solubility, which improves the feasibility of preparing electrode materials in a PAN-based electrospinning system.

[0058] The application of the LMFP / CNF carbon nanofiber composite material of the present invention in new energy batteries is specifically used for preparing a positive electrode material.

[0059] Specifically, the composite material of the present invention can be used as a structure guiding agent to induce the formation of LMFP nanocubes, providing a convenient path for the transmission of lithium ions.

[0060] The composite material of the present invention can also form an N-doped biphasic carbon coating during the heat treatment process, further improving the electronic conductivity of the entire electrode.

[0061] The present invention will be specifically described below through specific embodiments.

[0062] Example 1

[0063] The method of the present invention is used to prepare the LMFP / CNF carbon nanofiber composite material. Specifically, the polymer used in this example is polyacrylonitrile (PAN), and the phosphate source material is the ionic liquid 1-butyl-3-methylimidazolium dihydrogen phosphate ([Bmim]H2PO4). Therefore, the composite material of this example is the LMFP / CNF-BP composite cathode material. At the same time, according to the molar ratio of each element, the LMFP of the present invention is specifically LiMn 0.8 Fe 0.2 PO4.

[0064] The preparation method of this example includes the following steps:

[0065] 1) First, weigh 0.6 g of polyacrylonitrile powder, and then weigh 1.2 mmol of anhydrous manganese acetate, 0.3 mmol of ferric acetylacetonate, 1.5 mmol of anhydrous lithium acetate, and 1.5 mmol of 1-butyl-3-methylimidazolium dihydrogen phosphate. Add an appropriate amount of DMF as a solvent, and magnetically stir at 70 °C in a water bath for 6 h until a homogeneous spinning solution is formed. Before electrospinning, ultrasonically treat the orange-red spinning solution for 15 min and set it aside for use.

[0066] 2) Electrospin the electrospinning precursor solution obtained in step 1). Place the precursor solution in a 10 ml plastic syringe with a 20G (inner diameter 0.6 mm) stainless steel needle, then fix the syringe on a micro-injection pump. One end of the needle is connected to the positive electrode of a high-voltage electrostatic generator, and the collection device is connected to the negative electrode of the high-voltage electrostatic generator; the DC high voltage between the needle and the drum collection device covered with aluminum foil is 20 kV, the distance is 12 cm, the flow rate of the spinning solution is 0.4 ml / h, the ambient relative humidity is 25-30%, and the temperature is 24-28 °C.

[0067] 3) Place the precursor fibers prepared by electrospinning at room temperature overnight, and then transfer the precursor fibers to a tubular furnace. First, pre-oxidize in air at 230 °C for 2 h, and then carry out carbonization treatment in an argon-hydrogen mixed gas atmosphere. The temperature of the carbonization treatment is 750 °C, the heating rate is 5 °C / min, and the holding time is 4 h. After the carbonization treatment is completed, the LMFP / CNF-BP composite cathode material is obtained. Among them, in the argon-hydrogen mixed gas, the volume percentage of hydrogen is 5%.

[0068] It should be noted that when potassium dihydrogen phosphate or ammonium dihydrogen phosphate is used as the phosphorus source and polyvinyl alcohol or polylactic acid is used as the polymer, the properties of the prepared composite material are similar to those of Example 1. Although the present invention does not continue to list them specifically, these specific embodiments are still within the protection scope of the present invention.

[0069] Comparative Example 1

[0070] In this comparative example, LiMn 0.8 Fe 0.2 PO4 / CNF-BP composite cathode materials were prepared using different mass percentages of polyacrylonitrile. The specific preparation process is as follows:

[0071] 1) Prepare 4 portions of 1.2 mmol of anhydrous manganese acetate, 0.3 mmol of ferric acetylacetonate, 1.5 mmol of anhydrous lithium acetate, and 1.5 mmol of 1-butyl-3-methylimidazolium dihydrogen phosphate, and then mix them with polyacrylonitrile powders with mass percentages of 6%, 8%, 10%, and 12% respectively. Add an appropriate amount of DMF as a solvent, and magnetically stir for 6 h at 70 °C in a water bath until a homogeneous spinning solution is formed. Before electrospinning, ultrasonically treat the orange-red spinning solution for 15 min and set it aside.

[0072] 2) Electrospin the electrospinning precursor solution obtained in step 1). Place the precursor solution in a 10 ml plastic syringe with a 20G (inner diameter 0.6 mm) stainless steel needle, then fix the syringe on a micro-injection pump. One end of the needle is connected to the positive electrode of a high-voltage electrostatic generator, and the collection device is connected to the negative electrode of the high-voltage electrostatic generator; the distance between the needle with a DC high voltage of 20 kV and the drum collection device covered with aluminum foil is 12 cm, the flow rate of the spinning solution is 0.4 ml / h, the environmental relative humidity is 25 - 30%, and the temperature is 24 - 28 °C.

[0073] 3) Place the precursor fibers prepared by electrospinning at room temperature overnight, and then transfer the precursor fibers to a tube furnace. First, pre-oxidize in air at 230 °C with a heating rate of 2 °C / min for 2 h, and then perform carbonization treatment in an argon-hydrogen mixed gas atmosphere. The temperature of the carbonization treatment is 750 °C, the heating rate is 5 °C / min, and the holding time is 4 h. Among them, in the argon-hydrogen mixed gas, the volume percentage of hydrogen is 5%.

[0074] After the carbonization treatment, LMFP / CNF-BP(6%), LMFP / CNF-BP(8%), LMFP / CNF-BP(10%), and LMFP / CNF-BP(12%) composite cathode materials are obtained respectively.

[0075] Use a scanning electron microscope (TM3030) to respectively perform SEM analysis and testing on the precursor nanomaterials of the above four composite cathode materials.Figure 1 Among them, a to d respectively correspond to composite materials of LMFP / CNF-BP (6%), LMFP / CNF-BP (8%), LMFP / CNF-BP (10%), and LMFP / CNF-BP (12%).

[0076] According to Figure 1 It can be seen that when the mass percentage of PAN is 6% and 8%, no carbon fiber structure is found in the prepared composite cathode materials; in addition, according to Figure 1 In c, when the mass fraction of PAN is 10%, the presence of carbon fiber structure can be seen, and the size of the material is significantly smaller than Figure 1 In a and b; from Figure 1 In d, it can be seen that when the mass fraction of PAN is 12%, the carbon fiber network structure of the material is obvious, and the particle agglomeration phenomenon is greatly improved.

[0077] The first charge-discharge tests and cyclic stability tests of the above four composite materials were carried out at 0.1C respectively.

[0078] The results of the first charge-discharge test at 0.1C are as Figure 5 shown in a. According to Figure 5 In a, it can be seen that as the mass percentage of PAN increases, the initial discharge specific capacity increases significantly. When the mass percentage of PAN reaches 12%, the initial charge-discharge specific capacity of the composite material is 96.2 mAh·g -1 , when the mass percentage of PAN is 6%, the initial discharge specific capacity of the composite material is the lowest, and the iron discharge platform of the material is not obvious. From Figure 5 Analysis of b shows that as the polymer mass fraction increases, the rate performance also improves significantly.

[0079] It can be seen that when the mass percentage of PAN in the electrospinning solution is 12%, the electrochemical performance of the prepared LMFP / CNF-BP composite cathode material is optimal.

[0080] Comparative Example 2

[0081] In this comparative example, corresponding LMFP / CNF-BP composite cathode materials were prepared at different carbonization temperatures. The specific preparation process is as follows:

[0082] 1) First, weigh polyacrylonitrile powder with a concentration of 12%, then weigh 1.2 mmol of anhydrous manganese acetate, 0.3 mmol of ferric acetylacetonate, 1.5 mmol of anhydrous lithium acetate, and 1.5 mmol of 1-butyl-3-methylimidazolium dihydrogen phosphate. Add an appropriate amount of DMF as a solvent, and magnetically stir at 70°C in a water bath for 6 h until a homogeneous spinning solution is formed. Before electrospinning, the orange-red spinning solution is ultrasonically treated for 15 min and set aside.

[0083] 2) Electrospin the electrospinning precursor solution obtained in step 1). Place the precursor solution in a 10 ml plastic syringe with a 20G (inner diameter 0.6 mm) stainless steel needle, then fix the syringe on a micro-injection pump. One end of the needle is connected to the positive electrode of a high-voltage electrostatic generator, and the collection device is connected to the negative electrode of the high-voltage electrostatic generator; the distance between the needle with a DC high voltage of 20 kV and the drum collection device covered with aluminum foil is 12 cm, the flow rate of the spinning solution is 0.4 ml / h, the relative humidity of the environment is 25 - 30%, and the temperature is 24 - 28 °C.

[0084] 3) Leave the precursor fibers prepared by electrospinning at room temperature overnight, then transfer the precursor fibers to a tubular furnace and pre-oxidize them at 230 °C in air for 2 h.

[0085] Then carry out carbonization treatment under the atmosphere of argon-hydrogen mixture. The carbonization treatment temperatures are 700 °C, 750 °C, and 800 °C respectively, the heating rate is 5 °C / min for all, and the holding time is 4 h for all. Among them, in the argon-hydrogen mixture, the volume percentage of hydrogen is 5%.

[0086] After the carbonization treatment is completed, LMFP / CNF-BP(700 °C), LMFP / CNF-BP(750 °C), and LMFP / CNF-BP(800 °C) are obtained respectively.

[0087] Use a scanning electron microscope (TM3030) to respectively conduct SEM analysis and testing on the precursor nanomaterials of the above three composite materials. The SEM analysis results are as Figure 2 shown.

[0088] According to Figure 2 a and b, it can be seen that when heat treatment is carried out at lower temperatures (700 °C, 750 °C), the carbon fiber structure is obvious and more carbon fiber structures are retained. From Figure 2 c, it can be seen that when the carbonization temperature is 800 °C, a certain degree of aggregation appears and the carbon fiber structure is not obvious.

[0089] Conduct electrochemical tests on the above three obtained composite materials respectively.

[0090] Figure 6 a is the first charge-discharge curve of the composite materials prepared at different carbonization temperatures at 0.1C. As the carbonization temperature increases, the discharge specific capacity of the LMFP / CNF-BP composite cathode material does not show a corresponding increasing trend, but reaches the optimal value at 750 °C, which is 118.4 mAh·g -1 . Figure 6 b is the cyclic stability performance diagram of the composite materials prepared at different carbonization temperatures. From Figure 6It can be seen that when the carbonization temperature is 750 °C, the discharge specific capacity reaches the optimal value and the overall rate performance of the material is well maintained. The discharge specific capacity at 5C is 80.3 mAh·g -1 .

[0091] Comparative Example 3

[0092] In this comparative example, corresponding LiMn 0.8 Fe 0.2 PO4 / CNF-BP composite cathode materials were prepared using different carbonization times. The specific preparation process is as follows:

[0093] 1) First, weigh 0.6 g of polyacrylonitrile powder, and then weigh 1.2 mmol of anhydrous manganese acetate, 0.3 mmol of iron(III) acetylacetonate, 1.5 mmol of anhydrous lithium acetate, and 1.5 mmol of 1-butyl-3-methylimidazolium dihydrogen phosphate. Add an appropriate amount of DMF as a solvent and magnetically stir at 70 °C in a water bath for 6 h until a homogeneous spinning solution is formed. Before electrospinning, ultrasonically treat the orange-red spinning solution for 15 min and set it aside for use.

[0094] 2) Electrospin the electrospinning precursor solution obtained in step 1). Place the precursor solution in a 10 ml plastic syringe with a 20G (inner diameter 0.6 mm) stainless steel needle, then fix the syringe on a micro-injection pump. One end of the needle is connected to the positive electrode of a high-voltage electrostatic generator, and the collection device is connected to the negative electrode of the high-voltage electrostatic generator; the distance between the needle with a DC high voltage of 20 kV and the drum collection device covered with aluminum foil is 12 cm, the flow rate of the spinning solution is 0.4 ml / h, the ambient relative humidity is 25 - 30%, and the temperature is 24 - 28 °C.

[0095] 3) Place the precursor fibers prepared by electrospinning at room temperature overnight, and then transfer the precursor fibers to a tube furnace. First, pre-oxidize in air at 230 °C for 2 h.

[0096] Then, carry out carbonization treatment in an argon-hydrogen mixed gas atmosphere. The carbonization temperature is 750 °C, the heating rate is 5 °C / min, and the holding times are 2 h, 4 h, 6 h, and 8 h respectively. Among them, in the argon-hydrogen mixed gas, the volume percentage of hydrogen is 5%.

[0097] After the carbonization treatment is completed, LMFP / CNF-BP(2h), LMFP / CNF-BP(4h), LMFP / CNF-BP(6h), and LMFP / CNF-BP(8h) are obtained respectively.

[0098] Field emission scanning electron microscopy analysis (Gemini SEM 300) was used to perform FESEM analysis and testing on the above four composite cathode materials respectively. The FESEM analysis results are as Figure 3 shown.

[0099] Figure 3 Among them, a to d correspond to LMFP / CNF-BP(2h), LMFP / CNF-BP(4h), LMFP / CNF-BP(6h), and LMFP / CNF-BP(8h) respectively. From Figure 3 As can be seen from a to c in it, within a relatively short carbonization time (2h, 4h, and 6h), the carbon fiber structure of the composite material is relatively intact. From Figure 3 As can be seen from d in it, when the carbonization time increases to 8h, most of the particles aggregate together, and only a few residual carbon fiber structures remain.

[0100] Electrochemical tests were respectively carried out on the above four obtained composite materials.

[0101] Figure 7 a is the first charge-discharge curve of the composite materials prepared with different carbonization times at 0.1C. From Figure 7 It can be seen that when the carbonization time is 6h, the discharge specific capacity of the composite material reaches the optimal value, which is 164.1 mAh·g at 0.1C -1 . Figure 7 b is the cyclic stability performance diagram of the composite materials prepared with different carbonization times. From Figure 7 As can be seen from b, when the carbonization time is 6h, the rate performance of the overall material is significantly higher than that under other conditions, and the discharge specific capacities at 0.5C and 5C are 147.1 and 93.8 mAh·g respectively -1 .

[0102] Comparative Example 4

[0103] In this comparative example, H3PO4 was used as the phosphoric acid source material to replace the ionic liquid ([Bmim]H2PO4), and the prepared composite material was LMFP / CNF-HP. In addition, in this comparative example, the LMFP / C-BP material was prepared by the sol-gel method, where LMFP was specifically LiMn 0.8 Fe 0.2 PO4.

[0104] The phosphorus source for preparing LMFP / CNF-HP was phosphoric acid, and the carbonization temperature was 750°C. The rest of the specific processes and conditions were the same as those in Example 1.

[0105] The specific preparation process of the LiMn 0.8 Fe 0.2 PO4 / C-BP material by the sol-gel method is as follows:

[0106] 1) First, a certain mass of citric acid (C6H8O7·H2O) was weighed and used as a reducing agent and a carbon source, dissolved in an appropriate amount of DMF, and a uniform solution was formed under magnetic stirring.

[0107] 2) Then, according to the stoichiometric ratio of lithium source: manganese source: iron source: phosphate source (Li:Mn:Fe:PO4) = 1:0.8:0.2:1, 4.8 mmol of manganese acetate ((CH3COO)2Mn·4H2O), 1.2 mmol of iron(III) acetylacetonate (Fe(acac)3), 6.0 mmol of lithium acetate (CH3COOLi·2H2O), and 6.0 mmol of 1-butyl-3-methylimidazolium dihydrogen phosphate ([Bmim]H2PO4) were weighed respectively.

[0108] 3) Then, the mixed solution was heated in a water bath at 70 °C in a fume hood under magnetic stirring to slowly evaporate the solvent until a wet gel was formed, obtaining the LMFP / C-BP material.

[0109] Field emission scanning electron microscopy analysis (Gemini SEM 300) was used to perform FESEM analysis tests on LMFP / CNF-BP (750 °C) prepared in Comparative Example 2, LMFP / CNF-HP prepared in this comparative example, and LMFP / C-BP.

[0110] Figure 4 In which a to c correspond to LMFP / CNF-BP, LMFP / CNF-HP, and LMFP / C-BP respectively. From Figure 4 a and b in it, it can be seen that the ionic liquid, as a structure-directing agent, can largely induce the formation of LMFP nanoparticles. Taking Figure 4 c in it as a comparison, it can be seen that under non-electrospinning conditions, the composite material prepared by using only the ionic liquid shows a larger particle size.

[0111] The above are only the preferred embodiments of the present invention and are not intended to limit the present invention. Any modifications, equivalent replacements, improvements, etc. made within the spirit and principle of the present invention shall be included within the protection scope of the present invention.

Claims

1. A preparation method of an LMFP / CNF carbon nanofiber composite material, characterized in that A spinning solution is obtained by adding a lithium source material, a manganese source material, an iron source material, a phosphate source material, and a polymer into an organic solvent. The spinning solution is subjected to electrospinning to obtain precursor fibers. The precursor fibers are sequentially subjected to preheating oxidation and carbonization treatment to obtain the LMFP / CNF carbon nanofiber composite material. Among them, the phosphate source is 1-butyl-3-methylimidazolium dihydrogen phosphate, and the polymer is polyacrylonitrile. In the spinning solution, the mass percentage of the polyacrylonitrile is 12%. The steps of the carbonization treatment include carbonizing the precursor fibers after the preheating oxidation in a mixed atmosphere of an inert gas and hydrogen. The temperature of the carbonization treatment is 750 °C, and the time of the carbonization treatment is 6 h.

2. The preparation method of an LMFP / CNF carbon nanofiber composite material according to claim 1, characterized in that, In the spinning solution, the molar ratio of lithium, manganese, iron, and phosphate is 1:0.6-0.8:0.4-0.2:1, and the mass ratio of the polymer to the lithium source material is 4.56-15.27:

1.

3. The preparation method of an LMFP / CNF carbon nanofiber composite material according to claim 1, wherein, The lithium source material includes one or more of lithium acetate anhydrous, lithium bromide, and lithium fluoride. The manganese source material includes one or more of manganese acetate anhydrous, manganese nitrate, and manganese chloride. The iron source material includes one or more of iron(III) acetylacetonate, iron nitrate, and iron chloride.

4. The preparation method of an LMFP / CNF carbon nanofiber composite material according to claim 1, characterized in that, The organic solvent includes one or more of dimethylformamide, dimethylacetamide, and acetonitrile.

5. A method for preparing an LMFP / CNF carbon nanofiber composite according to any one of claims 1 to 4, characterized in that, The steps of the preheating oxidation include preheating the precursor fibers in an air atmosphere. The temperature of the preheating oxidation is 230 °C to 250 °C, and the time of the preheating oxidation is 2 h to 3 h.

6. A LMFP / CNF carbon nanofiber composite material, characterized in that, The composite material is prepared by the method according to any one of claims 1 to 5.

7. A kind of LMFP / CNF carbon nanofiber composite material according to claim 6, characterized in that, The composite material includes LMFP crystal grains, a carbon fiber structure, and a thin carbon layer. The LMFP crystal grains are embedded in or coated on the carbon fiber structure. The thin carbon layer uniformly covers the surface of the LMFP crystal grains and / or the outer layer of the carbon fiber structure. The particle size of the LMFP crystal grains is 50 to 150 nm.

8. Use of the LMFP / CNF carbon nanofiber composite material according to claim 6 or 7 in a new energy battery, characterized in that, The composite material is used for preparing a cathode material.

Images