Lithium iron phosphate positive electrode material, preparation method thereof and lithium ion battery

Abstract

This application provides a lithium iron phosphate cathode material, its preparation method, and a lithium-ion battery. The preparation method includes the following steps: (1) preparing a two-dimensional precursor of iron hydroxymethyl acid salt; (2) mixing the two-dimensional precursor of iron hydroxymethyl acid salt with a nitrogen-containing carbon source and reacting to obtain a two-dimensional precursor material; (3) mixing the two-dimensional precursor material, a lithium source, and a phosphorus source and sintering to obtain the lithium iron phosphate cathode material. This application prepares a lithium iron phosphate cathode material using a simple process and operation method. The cathode material has a large electrode / electrolyte contact interface, which is beneficial for the complete wetting of the electrolyte and the cathode material. At the same time, the microstructure of the cathode material inherits the two-dimensional plate-like morphology of the iron hydroxymethyl acid salt precursor. Its large specific surface area provides sufficient active sites for the storage of lithium ions and electrons, promotes the rapid diffusion and transport of lithium ions, and is beneficial for improving the electrochemical performance of lithium-ion batteries.

Description

Technical Field

[0001] This application relates to the field of battery materials technology, such as a lithium iron phosphate cathode material, its preparation method, and a lithium-ion battery. Background Technology

[0002] With the expanding demand in the portable electronic devices and electric vehicle markets, rechargeable lithium-ion batteries have gained widespread attention due to their high energy density, relatively long lifespan, and environmental friendliness. Lithium iron phosphate (LiFePO4), as a cathode material for lithium-ion batteries, boasts advantages such as low cost, safety, environmental friendliness, high capacity, and stable cycle performance, making it one of the preferred choices for power batteries. However, due to its structural limitations, LiFePO4 suffers from low electronic conductivity, low lithium-ion diffusion rate at the FePO4 / LiFePO4 two-phase interface, and low tap density, which restricts its application in large-scale energy storage and high-rate power batteries. To address this issue, particle surface coating modification, ion doping, morphology control, and nano-sizing techniques have been used to modify lithium iron phosphate, thereby improving the material's electrochemical properties.

[0003] Two-dimensional sheet-like structures, due to their large specific surface area and ultra-thin sheet thickness, can provide abundant electrochemical active sites, thus effectively shortening the lithium-ion insertion / extraction channels and expanding their migration range, which is beneficial for improving the rate capability and low-temperature performance of lithium-ion batteries. Introducing this structure into the design of lithium iron phosphate cathode materials would be significant for improving the overall electrical performance of batteries. Furthermore, most existing lithium iron phosphate preparation methods, such as solid-phase, liquid-phase, and sol-gel methods, suffer from incomplete reactions, irregular crystallization, complex and demanding processes, and high energy consumption; moreover, the electrical performance of the obtained lithium iron phosphate also falls short of the theoretical capacity, requiring further improvement.

[0004] Therefore, developing a simple and easy-to-operate method to prepare new lithium iron phosphate cathode materials with excellent performance parameters is of great significance for improving the overall electrochemical performance of lithium-ion batteries. Summary of the Invention

[0005] The following is an overview of the subject matter described in detail herein. This overview is not intended to limit the scope of the claims.

[0006] The purpose of this application is to provide a lithium iron phosphate cathode material, its preparation method, and a lithium-ion battery. This application describes a simple and easy-to-operate method for preparing a nitrogen-doped carbon-coated lithium iron phosphate cathode material with a two-dimensional porous structure. This cathode material has a large electrode / electrolyte interface, which is beneficial for complete wetting of the electrolyte and the cathode material. Simultaneously, the microstructure of this cathode material inherits the two-dimensional lamellar morphology of the iron hydroxymethyl phosphate precursor, and its large specific surface area provides ample active sites for lithium-ion and electron storage, promoting rapid diffusion and transport of lithium ions, and thus improving the electrochemical performance of lithium-ion batteries. This preparation method has good prospects for industrial application.

[0007] To achieve the purpose of this application, the following technical solution is adopted:

[0008] In a first aspect, this application provides a method for preparing a lithium iron phosphate cathode material, the method comprising the following steps:

[0009] (1) Preparation of two-dimensional precursors of iron hydroxymethyl acid salt;

[0010] (2) The iron hydroxymethyl salt two-dimensional precursor and a nitrogen-containing carbon source are mixed and reacted to obtain a two-dimensional precursor material;

[0011] (3) The two-dimensional precursor material, lithium source and phosphorus source are mixed and sintered to obtain the lithium iron phosphate cathode material.

[0012] This application involves coating a nitrogen-containing carbon layer onto the surface of a two-dimensional iron hydroxymethyl phosphate precursor sheet. After mixing with a lithium source and a phosphorus source and sintering, the organic groups Fe(OH)(OCH3) in the two-dimensional precursor material undergo pyrolysis, forming a porous structure on the two-dimensional sheet. This yields a nitrogen-doped carbon-coated lithium iron phosphate cathode material with a two-dimensional porous structure. This cathode material has a large electrode / electrolyte interface, which is beneficial for the complete wetting of the electrolyte and the cathode material. At the same time, the microstructure of this cathode material inherits the two-dimensional sheet-like morphology of the iron hydroxymethyl phosphate precursor. Its large specific surface area provides sufficient active sites for the storage of lithium ions and electrons, promotes the rapid diffusion and transport of lithium ions, and is beneficial for improving the electrochemical performance of lithium-ion batteries.

[0013] The preparation process provided in this application is simple, easy to operate, and has low energy consumption, showing good prospects for industrial application.

[0014] As an optional technical solution of this application, the preparation method of the two-dimensional precursor of iron hydroxymethyl acid salt in step (1) includes:

[0015] Ferrous acetate tetrahydrate and an alcohol solvent were mixed and subjected to a solvothermal reaction to obtain the two-dimensional precursor of the iron hydroxymethyl acid salt.

[0016] It should be noted that solvothermal reaction refers to a synthesis method in which an original mixture reacts in a closed system, such as an autoclave, using organic or non-aqueous solvents as solvents, under certain temperature and autogenous pressure of the solution.

[0017] This application involves mixing ferrous acetate tetrahydrate with an alcohol solvent and performing a solvothermal reaction to prepare a two-dimensional precursor of ferric hydroxymethyl salt, which yields a product with high purity and superior crystal structure.

[0018] In one embodiment, the alcohol solvent includes at least one of methanol, ethanol, and propanol.

[0019] In one embodiment, the concentration of the solution obtained by mixing ferrous acetate tetrahydrate and an alcohol solvent is (0.17-0.5) mol / L, for example, it can be 0.17 mol / L, 0.2 mol / L, 0.25 mol / L, 0.3 mol / L, 0.35 mol / L, 0.4 mol / L, 0.45 mol / L, or 0.5 mol / L, etc.

[0020] In this application, if the concentration of the solution obtained by mixing ferrous acetate tetrahydrate and an alcohol solvent is too low, it may lead to incomplete crystal growth of the precursor material, affecting its crystallinity; if the concentration of the solution obtained by mixing ferrous acetate tetrahydrate and an alcohol solvent is too high, it may lead to agglomeration of the precursor material. Both of these factors will affect the electrical properties of the synthesized lithium iron phosphate material.

[0021] In one embodiment, the temperature of the solvothermal reaction is 150-200°C, for example, 150°C, 160°C, 170°C, 180°C, 190°C, or 200°C, and the time is 24-36 hours, for example, 24 hours, 26 hours, 28 hours, 30 hours, 32 hours, 34 hours, or 36 hours.

[0022] In this application, a solvothermal reaction is carried out at 150-200℃ for 24-36 hours, which helps to accelerate the reaction rate, increase the reaction yield, and generate precursor materials with good uniformity and high crystallinity.

[0023] As an optional technical solution of this application, the nitrogen-containing carbon source in step (2) includes dopamine hydrochloride.

[0024] In this application, dopamine hydrochloride, as a nitrogen-containing carbon source, can undergo a self-polymerization reaction in a buffer solution to form polydopamine.

[0025] In one embodiment, the mass concentration of dopamine hydrochloride in the solution obtained by mixing in step (2) is 0.5-0.8 g / L, for example, it can be 0.5 g / L, 0.6 g / L, 0.7 g / L or 0.8 g / L.

[0026] In this application, the concentration of dopamine hydrochloride affects the thickness of the nitrogen-doped carbon layer. If the concentration is too low, the nitrogen-doped carbon layer will be thin, which will result in a lack of significant improvement in the conductivity of the lithium iron phosphate material. If the concentration is too high, the nitrogen-doped carbon layer will be thick, which will affect the migration of lithium ions to some extent, thereby affecting the electrical performance of the lithium iron phosphate material.

[0027] As an optional technical solution of this application, the mixing method in step (2) includes:

[0028] The two-dimensional precursor of iron hydroxymethyl acid salt was dispersed in a buffer solution, and then a nitrogen-containing carbon source was added for blending.

[0029] In one embodiment, the buffer solution comprises a Tris buffer solution.

[0030] In this application, the Tris buffer solution is also called tris(hydroxymethyl)aminomethane hydrochloride buffer solution, with an effective buffering range of pH = 7.0-9.0. Using this substance can effectively maintain the acid-base balance of the solution and keep the pH value of the solution relatively stable.

[0031] In one embodiment, the mixing temperature in step (2) is room temperature, and the time is 12-24h, for example, 12h, 18h or 24h.

[0032] It should be noted that this application does not limit the specific temperature of room temperature. For example, it can be 25±5℃, including 20℃, 25℃ or 30℃, etc.

[0033] In this application, mixing the two-dimensional precursor of iron hydroxymethyl salt and the nitrogen-containing carbon source at room temperature for 12-24 hours allows the nitrogen-containing carbon source to fully adhere to the surface of the two-dimensional precursor of iron hydroxymethyl salt.

[0034] As an optional technical solution of this application, the lithium source in step (3) includes at least one of lithium carbonate, lithium hydroxide and lithium acetate.

[0035] In one embodiment, the phosphorus source in step (3) includes at least one of ammonium dihydrogen phosphate, ammonium hydrogen phosphate, and sodium hydrogen phosphate.

[0036] In one embodiment, the molar ratio of the two-dimensional precursor material, phosphorus source and lithium source in step (3) is 1:(0.98-1.05):1, for example, it can be 1:0.98:1, 1:0.99:1, 1:1:1, 1:1.01:1, 1:1.02:1, 1:1.03:1, 1:1.04:1 or 1:1.05:1, etc.

[0037] In this application, if the molar ratio of the two-dimensional precursor material, phosphorus source, and lithium source is too small or too large, it will affect the crystal structure stability of the lithium iron phosphate material, which will lead to the difficulty of lithium ion extraction and insertion, and thus reduce the electrical performance of the material. As an optional technical solution of this application, the mixing method in step (3) includes ball milling.

[0038] In this application, ball milling can be used to obtain a uniform particle distribution.

[0039] In one implementation, the mixing time in step (3) is 2-5 hours, for example, 2 hours, 3 hours, 4 hours or 5 hours.

[0040] In one embodiment, the sintering in step (3) is carried out in a protective atmosphere.

[0041] In this application, sintering is carried out in a protective atmosphere, which can effectively prevent the oxidation of ferrous ions, reduce impurity contamination, and improve the purity of the material.

[0042] In one embodiment, the gas in the protective atmosphere includes nitrogen and / or argon.

[0043] In one embodiment, the sintering method in step (3) is multi-stage sintering, which includes primary sintering and secondary sintering.

[0044] In this application, a multi-stage sintering method is used to obtain materials with a denser and more uniform structure and better crystallinity.

[0045] In one embodiment, the temperature of the first-stage sintering is 400-550℃, for example, 400℃, 450℃, 500℃ or 550℃, and the time is 2-5h, for example, 2h, 3h, 4h or 5h.

[0046] In this application, primary sintering at 400-550℃ will improve the crystallinity of lithium iron phosphate materials.

[0047] In one embodiment, the temperature of the secondary sintering is 650-750℃, for example, 650℃, 675℃, 700℃, 725℃ or 750℃, and the time is 6-10h, for example, 6h, 7h, 8h, 9h or 10h.

[0048] In this application, if the secondary sintering temperature is too low, the lithium iron phosphate material will not undergo complete crystal transformation; if the secondary sintering temperature is too high, the material crystals will grow too large, resulting in problems such as weakened grain boundary bonding. Both of these factors will degrade the electrical properties of the material.

[0049] As an optional technical solution of this application, the preparation method includes the following steps:

[0050] (1) Ferrous acetate tetrahydrate and an alcohol solvent to remove oxygen from the solution are stirred and mixed, and a solvothermal reaction is carried out at 150-200℃ for 24-36h. After the reaction is completed, the mixture is cooled to room temperature, and then centrifuged and washed to obtain the two-dimensional precursor of iron hydroxymethyl acid salt.

[0051] The concentration of the solution obtained by mixing ferrous acetate tetrahydrate and an alcohol solvent is (0.17-0.5) mol / L.

[0052] (2) The iron hydroxymethyl salt two-dimensional precursor was ultrasonically dispersed in a buffer solution, and then dopamine hydrochloride was added and stirred. The reaction was carried out at room temperature for 12-24 hours. After the reaction was completed, the precursor material was centrifuged and washed to obtain the two-dimensional precursor material, namely Fe(OH)(OCH3)@polydopamine material.

[0053] The mass concentration of dopamine hydrochloride in the resulting solution is 0.5-0.8 g / L.

[0054] (3) Under a protective atmosphere, the two-dimensional precursor material, phosphorus source and lithium source are ball-milled and mixed at a molar ratio of 1:(0.98-1.05):1 for 2-5 hours. After drying, primary sintering and secondary sintering, the lithium iron phosphate cathode material is obtained.

[0055] The temperature for the first-stage sintering is 400-550℃ and the time is 2-5 hours, while the temperature for the second-stage sintering is 650-750℃ and the time is 6-10 hours.

[0056] It should be noted that the reaction equation for the solvothermal reaction is as follows:

[0057] Fe(CH3CO2)2·4H2O+3CH3OH=Fe(OH)(OCH3)+2CH3CO2CH3+5H2O.

[0058] In this application, with the synergistic effect of multiple parameters, a two-dimensional iron hydroxymethyl salt precursor sheet is first prepared by a solvothermal method. Then, a polydopamine layer is coated on its surface. After mixing with a lithium source and a phosphorus source, it is calcined at high temperature. The pyrolysis of Fe(OH)(OCH3) organic groups forms a porous structure on the two-dimensional sheet, thus obtaining a nitrogen-doped carbon layer coated with a two-dimensional porous structure of lithium iron phosphate sheet. The microstructure of lithium iron phosphate inherits the two-dimensional sheet-like morphology of the iron hydroxymethyl salt precursor, which can promote the rapid diffusion and transport of lithium ions and improve the electrochemical performance of the material. This is of great significance for improving the overall electrochemical performance of lithium-ion batteries.

[0059] It should be noted that Fe(OH)(OCH3)@polydopamine material refers to Fe(OH)(OCH3) coated with polydopamine.

[0060] In a second aspect, this application provides a lithium iron phosphate cathode material prepared by the preparation method described in the first aspect, wherein the lithium iron phosphate cathode material includes a LiFePO4 core and a nitrogen-doped carbon layer coated on the surface of the LiFePO4 core.

[0061] The LiFePO4 core has a two-dimensional sheet-like porous structure.

[0062] In this application, the two-dimensional sheet-like porous LiFePO4 core has a large electrode / electrolyte contact interface, which is conducive to the complete wetting of the electrolyte and the cathode material; at the same time, the large specific surface area of ​​this structure provides sufficient active sites for the storage of lithium ions and electrons, shortens the transport path of lithium ions and electrons, and is beneficial to improving the rate performance of lithium batteries.

[0063] In this application, the presence of a nitrogen-doped carbon layer can improve the electronic conductivity and mechanical strength of the electrode material, mitigate volume changes during cycling, and enhance the stability and uniformity of the material.

[0064] The lithium iron phosphate cathode material synthesized in this application is uniformly dispersed and has high purity. Furthermore, the layered structure helps to reduce the aggregation of the material during the charging and discharging process.

[0065] As an optional technical solution of this application, the porosity of the LiFePO4 core is 26-32%, for example, it can be 26%, 28%, 30% or 32%, etc., and the average pore size is 8.3-11.5nm, for example, it can be 8.3nm, 9.6nm, 10.7nm or 11.5nm, etc.

[0066] In one embodiment, the thickness of the nitrogen-doped carbon layer is 5.5-6.7 nm, for example, it can be 5.5 nm, 5.8 nm, 6.2 nm or 6.7 nm.

[0067] In this application, a nitrogen-doped carbon layer of a certain thickness can not only improve the electronic conductivity and mechanical strength of the electrode material, but also mitigate the volume change of the material during cycling and enhance its structural stability. In addition, the nitrogen-doped carbon layer coating of a certain thickness can effectively avoid the phenomenon of sheet aggregation caused by secondary crystallization during solid-state sintering, significantly improving the integrity and uniformity of the product.

[0068] Thirdly, this application provides a lithium-ion battery, wherein the positive electrode of the lithium-ion battery includes the lithium iron phosphate positive electrode material as described in the second aspect.

[0069] The lithium-ion battery provided in this application uses the aforementioned lithium iron phosphate cathode material, which is beneficial for improving the rate performance of the lithium battery.

[0070] The numerical range described in this application includes not only the point values ​​listed above, but also any point values ​​between the above numerical ranges that are not listed. Due to space limitations and for the sake of brevity, this application will not exhaustively list the specific point values ​​included in the range.

[0071] Compared with related technologies, this application has the following advantages:

[0072] (1) The preparation process provided in this application is simple, easy to operate and has low energy consumption, showing good prospects for industrial application.

[0073] (2) In this application, a nitrogen-containing carbon layer is coated on the surface of a two-dimensional iron hydroxymethyl salt precursor sheet. After mixing with a lithium source and a phosphorus source and sintering, the organic groups Fe(OH)(OCH3) in the two-dimensional precursor material undergo pyrolysis, forming a porous structure on the two-dimensional sheet, thereby obtaining a nitrogen-doped carbon layer coated with a two-dimensional porous structure lithium iron phosphate cathode material. This cathode material has a large electrode / electrolyte contact interface, which is conducive to the complete wetting of the electrolyte and the cathode material. At the same time, the microstructure of this cathode material inherits the two-dimensional sheet-like morphology of the iron hydroxymethyl salt precursor. Its large specific surface area provides sufficient active sites for the storage of lithium ions and electrons, promotes the rapid diffusion and transport of lithium ions, and is beneficial to improving the electrochemical performance of lithium-ion batteries.

[0074] (3) In this application, the presence of a nitrogen-doped carbon layer not only improves the electronic conductivity and mechanical strength of the electrode material, but also alleviates the volume change of the material during cycling, enhancing its structural stability. Furthermore, the nitrogen-doped carbon layer effectively avoids the phenomenon of sheet aggregation caused by secondary crystallization during solid-state sintering, significantly improving the integrity and uniformity of the product. In addition, the lithium iron phosphate cathode material synthesized in this application is uniformly dispersed and has high purity, and the layered structure also helps reduce material agglomeration during charging and discharging.

[0075] After reading and understanding the accompanying diagrams and detailed descriptions, the other aspects can be understood. Attached Figure Description

[0076] The accompanying drawings are used to provide a further understanding of the technical solutions in this paper and form part of the specification. They are used together with the embodiments of this application to explain the technical solutions in this paper and do not constitute a limitation on the technical solutions in this paper.

[0077] Figure 1 This is a process flow diagram for preparing lithium iron phosphate cathode material in Example 1 of this application.

[0078] Figure 2This is a scanning electron microscope image of the lithium iron phosphate cathode material prepared in Example 1 of this application. Detailed Implementation

[0079] The technical solution of this application will be further described below through specific embodiments. Those skilled in the art should understand that the embodiments described are merely to help understand this application and should not be regarded as specific limitations on this application.

[0080] It should be noted that the room temperature in the following embodiments is 25°C.

[0081] Example 1

[0082] This embodiment provides a method for preparing lithium iron phosphate cathode material, and its process flow diagram is shown below. Figure 1 As shown, the preparation method includes the following steps:

[0083] (1) Take 60 mL of methanol and place it in a beaker. Under stirring conditions, purge with nitrogen for 1 h to remove oxygen from the solution. Then add 0.02 mol of ferrous acetate tetrahydrate to the methanol after the above treatment. After stirring and mixing for 30 min, place the resulting mixture in a 100 mL polytetrafluoroethylene-lined high-pressure reactor and carry out a solvothermal reaction in an oven at 180 °C for 24 h. After the reaction is completed, cool to room temperature and then collect the product by centrifugation. Wash with deionized water and anhydrous ethanol. Finally, place the obtained two-dimensional precursor of iron hydroxymethyl salt, namely Fe(OH)(OCH3) two-dimensional precursor material, in a vacuum oven at 70 °C and dry for 12 h.

[0084] The concentration of the mixed solution was 0.33 mol / L;

[0085] (2) The iron hydroxymethyl salt two-dimensional precursor was ultrasonically dispersed in 100 mL of Tris buffer solution (10 mmol, pH = 8.5), and 0.05 g of dopamine hydrochloride (DA) was added and stirred. The reaction was carried out at room temperature for 12 h under stirring conditions to allow dopamine to polymerize on the Fe(OH)(OCH3) surface. After the reaction was completed, the Fe(OH)(OCH3)@polydopamine (PDA) product was collected by centrifugation and washed with deionized water and anhydrous ethanol in sequence. The obtained two-dimensional precursor material, namely Fe(OH)(OCH3)@PDA material, was placed in a vacuum oven at 70 °C and dried for 12 h.

[0086] The mass concentration of dopamine hydrochloride in the solution after mixing with dopamine hydrochloride is 0.5 g / L.

[0087] (3) The two-dimensional precursor material, ammonium dihydrogen phosphate and lithium carbonate are dispersed in anhydrous ethanol at a molar ratio of 1:1.02:1 and ball-milled for 3 hours at a speed of 3000 rpm. Then, the mixture is spray-dried to obtain precursor powder. The precursor powder is then subjected to multi-stage sintering under an argon atmosphere. First, the temperature is increased to 400℃ at a heating rate of 5℃ / min for 3 hours for primary sintering, and then the temperature is increased to 700℃ for 8 hours for secondary sintering. After the sintering is completed, the lithium iron phosphate cathode material, namely LiFePO4 / NC two-dimensional porous material (NC is the nitrogen-doped carbon layer formed after multi-stage sintering of PDA), is obtained.

[0088] This embodiment also provides a lithium iron phosphate cathode material prepared by the above preparation method. The lithium iron phosphate cathode material includes a LiFePO4 core and a nitrogen-doped carbon layer coated on the surface of the LiFePO4 core. The LiFePO4 core has a two-dimensional sheet-like porous structure.

[0089] The LiFePO4 core has a porosity of 29.7% and an average pore size of 10.7 nm, and the nitrogen-doped carbon layer has a thickness of 5.5 nm.

[0090] Figure 2 The image shows a scanning electron microscope (SEM) image of the lithium iron phosphate cathode material prepared in this embodiment. As can be seen from the image, the synthesized lithium iron phosphate material has a two-dimensional sheet-like structure.

[0091] Example 2

[0092] This embodiment provides a method for preparing lithium iron phosphate cathode material, the method comprising the following steps:

[0093] (1) Take 60 mL of methanol and place it in a beaker. Under stirring conditions, purge with nitrogen for 1 h to remove oxygen from the solution. Then add 0.025 mol of ferrous acetate tetrahydrate to the methanol after the above treatment. After stirring and mixing for 30 min, place the resulting mixture in a 100 mL polytetrafluoroethylene-lined high-pressure reactor and carry out a solvothermal reaction in an oven at 180 °C for 36 h. After the reaction is completed, cool to room temperature and then collect the product by centrifugation. Wash with deionized water and anhydrous ethanol. Finally, dry the obtained two-dimensional precursor of iron hydroxymethyl salt, namely Fe(OH)(OCH3), in a vacuum oven at 70 °C for 12 h.

[0094] The concentration of the mixture was 0.42 mol / L.

[0095] (2) The iron hydroxymethyl salt two-dimensional precursor was ultrasonically dispersed in 100 mL of Tris buffer solution (10 mmol, pH = 8.5), and 0.05 g of dopamine hydrochloride was added and stirred. The reaction was carried out at room temperature for 12 h under stirring conditions to allow dopamine to polymerize on the Fe(OH)(OCH3) surface. After the reaction was completed, the Fe(OH)(OCH3)@polydopamine (PDA) product was collected by centrifugation and washed with deionized water and anhydrous ethanol in sequence. The obtained two-dimensional precursor material, namely Fe(OH)(OCH3)@PDA material, was placed in a vacuum oven at 70 °C and dried for 12 h.

[0096] The mass concentration of dopamine hydrochloride in the solution after mixing with dopamine hydrochloride is 0.5 g / L.

[0097] (3) The two-dimensional precursor material, ammonium dihydrogen phosphate and lithium carbonate were dispersed in anhydrous ethanol at a molar ratio of 1:1.02:1 and ball-milled for 3 hours at a speed of 3000 rpm. Then, the mixture was spray-dried to obtain precursor powder. The precursor powder was then subjected to primary sintering at 450°C for 3 hours under an argon atmosphere at a heating rate of 5°C / min. The temperature was then increased to 700°C for secondary sintering for 10 hours. After the sintering was completed, the lithium iron phosphate cathode material was obtained.

[0098] This embodiment also provides a lithium iron phosphate cathode material prepared by the above preparation method. The lithium iron phosphate cathode material includes a LiFePO4 core and a nitrogen-doped carbon layer coated on the surface of the LiFePO4 core. The LiFePO4 core has a two-dimensional sheet-like porous structure.

[0099] The LiFePO4 core has a porosity of 31.3% and an average pore size of 11.5 nm, and the nitrogen-doped carbon layer has a thickness of 5.5 nm.

[0100] Example 3

[0101] This embodiment provides a method for preparing lithium iron phosphate cathode material, the method comprising the following steps:

[0102] (1) Take 60 mL of methanol and place it in a beaker. Under stirring conditions, purge with nitrogen for 1 h to remove oxygen from the solution. Then add 0.025 mol of ferrous acetate tetrahydrate to the methanol after the above treatment. After stirring and mixing for 30 min, place the resulting mixture in a 100 mL polytetrafluoroethylene-lined high-pressure reactor and carry out a solvothermal reaction in an oven at 180 °C for 24 h. After the reaction is completed, cool to room temperature and then collect the product by centrifugation. Wash with deionized water and anhydrous ethanol. Finally, dry the obtained two-dimensional precursor of iron hydroxymethyl salt, namely Fe(OH)(OCH3), in a vacuum oven at 70 °C for 12 h.

[0103] The concentration of the mixture was 0.42 mol / L.

[0104] (2) The iron hydroxymethyl salt two-dimensional precursor was ultrasonically dispersed in 100 mL of Tris buffer solution (10 mmol, pH = 8.5), and 0.07 g of dopamine hydrochloride was added and stirred. The reaction was carried out at room temperature for 24 h under stirring conditions to allow dopamine to polymerize on the Fe(OH)(OCH3) surface. After the reaction was completed, the Fe(OH)(OCH3)@polydopamine (PDA) product was collected by centrifugation and washed with deionized water and anhydrous ethanol in sequence. The obtained two-dimensional precursor material, namely Fe(OH)(OCH3)@PDA material, was placed in a vacuum oven at 70 °C and dried for 12 h.

[0105] The mass concentration of dopamine hydrochloride in the solution after mixing with dopamine hydrochloride was 0.7 g / L.

[0106] (3) The two-dimensional precursor material, ammonium dihydrogen phosphate and lithium carbonate were dispersed in anhydrous ethanol at a molar ratio of 1:0.99:1 and ball-milled for 3 hours at a speed of 3000 rpm. Then, the mixture was spray-dried to obtain precursor powder. The precursor powder was then subjected to primary sintering at 450°C for 3 hours under an argon atmosphere at a heating rate of 5°C / min. The temperature was then increased to 750°C for secondary sintering for 10 hours. After the sintering was completed, the lithium iron phosphate cathode material was obtained.

[0107] This embodiment also provides a lithium iron phosphate cathode material prepared by the above preparation method. The lithium iron phosphate cathode material includes a LiFePO4 core and a nitrogen-doped carbon layer coated on the surface of the LiFePO4 core. The LiFePO4 core has a two-dimensional sheet-like porous structure.

[0108] The LiFePO4 core has a porosity of 30.5% and an average pore size of 11 nm, and the nitrogen-doped carbon layer has a thickness of 6.2 nm.

[0109] Example 4

[0110] The difference between this embodiment and embodiment 1 is that the amount of ferrous acetate tetrahydrate added in step (1) is adjusted to 0.01 mol, so that the concentration of the mixture is 0.17 mol / L; and the concentration of dopamine hydrochloride solution in step (2) is adjusted to 0.8 g / L.

[0111] The remaining preparation methods and parameters are consistent with those in Example 1.

[0112] Example 5

[0113] The difference between this embodiment and Embodiment 1 is that the amount of ferrous acetate tetrahydrate added in step (1) is adjusted to 0.005 mol, so that the concentration of the mixture is 0.08 mol / L.

[0114] The remaining preparation methods and parameters are consistent with those in Example 1.

[0115] Example 6

[0116] The difference between this embodiment and Embodiment 1 is that the amount of ferrous acetate tetrahydrate added in step (1) is adjusted to 0.05 mol, so that the concentration of the mixture is 0.83 mol / L.

[0117] The remaining preparation methods and parameters are consistent with those in Example 1.

[0118] Example 7

[0119] The difference between this embodiment and embodiment 1 is that the mass concentration of dopamine hydrochloride in step (2) is 0.2 g / L.

[0120] The remaining preparation methods and parameters are consistent with those in Example 1.

[0121] Example 8

[0122] The difference between this embodiment and embodiment 1 is that the mass concentration of dopamine hydrochloride in step (2) is 1.0 g / L.

[0123] The remaining preparation methods and parameters are consistent with those in Example 1.

[0124] Example 9

[0125] The difference between this embodiment and embodiment 1 is that the molar ratio of the two-dimensional precursor material, ammonium dihydrogen phosphate and lithium carbonate in step (3) is 1:0.9:1.

[0126] The remaining preparation methods and parameters are consistent with those in Example 1.

[0127] Example 10

[0128] The difference between this embodiment and embodiment 1 is that the molar ratio of the two-dimensional precursor material, ammonium dihydrogen phosphate and lithium carbonate in step (3) is 1:1.2:1.

[0129] The remaining preparation methods and parameters are consistent with those in Example 1.

[0130] Example 11

[0131] The difference between this embodiment and embodiment 1 is that primary sintering is not performed in step (3).

[0132] The remaining preparation methods and parameters are consistent with those in Example 1.

[0133] Example 12

[0134] The difference between this embodiment and embodiment 2 is that the amount of ferrous acetate tetrahydrate added in step (1) is adjusted to 0.03 mol, so that the concentration of the mixture is 0.5 mol / L; and the concentration of dopamine hydrochloride solution in step (2) is adjusted to 0.7 g / L.

[0135] The remaining preparation methods and parameters are consistent with those in Example 2.

[0136] Example 13

[0137] The difference between this embodiment and embodiment 2 is that the temperature of the secondary sintering in step (3) is 600℃.

[0138] The remaining preparation methods and parameters are consistent with those in Example 1.

[0139] Example 14

[0140] The difference between this embodiment and embodiment 2 is that the temperature of the secondary sintering in step (3) is 800℃.

[0141] The remaining preparation methods and parameters are consistent with those in Example 1.

[0142] Comparative Example 1

[0143] The difference between this comparative example and Example 2 is that steps (1) and (2) are omitted, and instead ferrous sulfate, ammonium dihydrogen phosphate and lithium carbonate are directly mixed.

[0144] The remaining preparation methods and parameters are consistent with those in Example 2.

[0145] Comparative Example 2

[0146] The difference between this comparative example and Example 3 is that step (2) is omitted, and instead the iron hydroxymethyl salt two-dimensional precursor, ammonium dihydrogen phosphate and lithium carbonate obtained in step (1) are directly mixed.

[0147] The remaining preparation methods and parameters are consistent with those in Example 3.

[0148] Performance testing

[0149] The lithium iron phosphate cathode materials prepared in the above examples and comparative examples were formulated into coin cells. The specific steps included: uniformly mixing the lithium iron phosphate cathode material, conductive agent acetylene black, and adhesive polyvinylidene fluoride in N-methylpyrrolidone at a mass ratio of 92:4:4 to form a slurry, then coating it onto aluminum foil, and drying it in a vacuum drying oven. Then, pressing it into a positive electrode sheet using a tablet press, with a negative electrode sheet being a lithium metal sheet, and the electrolyte being 1 mol / L LiPF6-EC:DMC (volume ratio of 1:1). A polypropylene porous membrane was used as the separator, and the battery assembly was carried out in an argon glove box.

[0150] The electrochemical performance of the above batteries was tested (with the charge and discharge voltage controlled between 2.5 and 4.5V), and the test results are shown in Table 1.

[0151] Table 1

[0152]

[0153]

[0154] analyze:

[0155] As shown in the table above, the lithium iron phosphate cathode material prepared in this application is a two-dimensional porous sheet structure coated with a nitrogen-doped carbon layer. This structure has a large specific surface area and high conductivity, and the contact area between the electrolyte and the cathode material is large. Therefore, the diffusion path of lithium ions is shortened and the rate is accelerated, and the electrochemical performance of the lithium battery is significantly improved.

[0156] As can be seen from Examples 1 and 5-6, if the concentration of the solution obtained by mixing ferrous acetate tetrahydrate and alcohol solvent is too low, it may lead to incomplete crystal growth of the precursor material and affect its crystallinity; if the concentration of the solution obtained by mixing ferrous acetate tetrahydrate and alcohol solvent is too high, it may lead to agglomeration of the precursor material. These factors will affect the electrical performance of the synthesized lithium iron phosphate material.

[0157] As can be seen from Examples 1 and 7-8, if the concentration of dopamine hydrochloride is too low, the nitrogen-doped carbon layer formed will be thin, which will result in an insignificant improvement in the conductivity of lithium iron phosphate material; if the concentration of dopamine hydrochloride is too high, the nitrogen-doped carbon layer formed will be thick, which will affect the migration of lithium ions to a certain extent, and thus affect the electrical performance of lithium iron phosphate material.

[0158] As can be seen from Examples 1 and 9-10, if the molar ratio of the two-dimensional precursor material, ammonium dihydrogen phosphate, and lithium carbonate is too low or too high, it will affect the crystal structure stability of the lithium iron phosphate material. This will make it difficult for lithium ions to be extracted and inserted, thereby reducing the electrical performance of the material.

[0159] As can be seen from Examples 1 and 11, if primary sintering is not performed, the crystallinity of lithium iron phosphate material may be low, and the crystal structure may have defects, which in turn will affect its electrical performance.

[0160] As can be seen from Examples 2 and 13-14, if the secondary sintering temperature is too low, the lithium iron phosphate material will not undergo complete crystal transformation; if the secondary sintering temperature is too high, the material crystals will grow too large, resulting in problems such as weakened grain boundary bonding. Both of these factors will cause a decrease in the electrical properties of the material.

[0161] As can be seen from Example 2 and Comparative Example 1, if ferrous sulfate, ammonium dihydrogen phosphate and lithium carbonate are directly mixed, an irregular lithium iron phosphate bulk without nitrogen-doped carbon layer coating will be obtained. This will result in the synthesized lithium iron phosphate material having a small specific surface area, low lithium ion migration rate and conductivity, and poor electrochemical performance.

[0162] As can be seen from Example 3 and Comparative Example 2, if the two-dimensional precursor of iron hydroxymethyl salt, ammonium dihydrogen phosphate and lithium carbonate are directly mixed, a lithium iron phosphate sheet coated with a carbon layer without nitrogen doping will be obtained. This will result in low electrical conductivity of the lithium iron phosphate material and may cause agglomeration, which will affect its electrical performance.

[0163] The applicant declares that this application illustrates the process method through the above embodiments, but this application is not limited to the above process steps, that is, it does not mean that this application must rely on the above process steps to be implemented. Those skilled in the art should understand that any improvements to this application, equivalent substitutions of the raw materials used in this application, addition of auxiliary components, selection of specific methods, etc., all fall within the protection scope and disclosure scope of this application.

Claims

1. A method for preparing a lithium iron phosphate cathode material, comprising the following steps: (1) Preparation of two-dimensional precursors of iron hydroxymethyl acid salt; (2) The iron hydroxymethyl salt two-dimensional precursor and the nitrogen-containing carbon source are mixed and reacted to obtain a two-dimensional precursor material; (3) The two-dimensional precursor material, lithium source and phosphorus source are mixed and sintered to obtain the lithium iron phosphate cathode material; The preparation method of the two-dimensional precursor of iron hydroxymethyl acid salt in step (1) includes: Ferrous acetate tetrahydrate and an alcohol solvent were mixed and subjected to a solvothermal reaction to obtain the two-dimensional precursor of the iron hydroxymethyl acid salt. The nitrogen-containing carbon source in step (2) includes dopamine hydrochloride; The mass concentration of dopamine hydrochloride in the solution obtained by mixing in step (2) is 0.5-0.8 g / L; The molar ratio of the two-dimensional precursor material, phosphorus source, and lithium source in step (3) is 1:(0.98-1.05):1; The sintering method described in step (3) is multi-stage sintering, which includes primary sintering and secondary sintering; The temperature of the first-stage sintering is 400-550℃, and the time is 2-5 hours. The secondary sintering temperature is 650-750℃, and the time is 6-10h.

2. The preparation method according to claim 1, characterized in that, The concentration of the solution obtained by mixing ferrous acetate tetrahydrate and an alcohol solvent is (0.17-0.5) mol / L.

3. The preparation method according to claim 1, characterized in that, The solvothermal reaction is carried out at a temperature of 150-200℃ for 24-36 hours.

4. The preparation method according to claim 1, characterized in that, The mixing method described in step (2) includes: The two-dimensional precursor of iron hydroxymethyl acid salt was dispersed in a buffer solution, and then a nitrogen-containing carbon source was added for blending.

5. The preparation method according to claim 4, characterized in that, The buffer solution includes a Tris buffer solution.

6. The preparation method according to claim 1, characterized in that, The mixing temperature in step (2) is room temperature, and the time is 12-24 hours.

7. The preparation method according to claim 1, characterized in that, The mixing method described in step (3) includes ball milling.

8. The preparation method according to claim 1, characterized in that, The sintering in step (3) is carried out in a protective atmosphere.

9. The preparation method according to claim 1, characterized in that, It includes the following steps: (1) Ferrous acetate tetrahydrate and an alcohol solvent to remove oxygen from the solution are stirred and mixed, and a solvothermal reaction is carried out at 150-200℃ for 24-36h. After the reaction is completed, the mixture is cooled to room temperature, and then centrifuged and washed to obtain the two-dimensional precursor of iron hydroxymethyl acid salt. The concentration of the solution obtained by mixing ferrous acetate tetrahydrate and an alcohol solvent is (0.17-0.5) mol / L. (2) The iron hydroxymethyl salt two-dimensional precursor was ultrasonically dispersed in a buffer solution, and then dopamine hydrochloride was added and stirred and mixed. The reaction was carried out at room temperature for 12-24 hours. After the reaction was completed, the precursor material was centrifuged and washed to obtain the two-dimensional precursor material, namely Fe(OH)(OCH3)@polydopamine material. The mass concentration of dopamine hydrochloride in the resulting solution is 0.5-0.8 g / L. (3) Under a protective atmosphere, the two-dimensional precursor material, phosphorus source and lithium source are ball-milled and mixed at a molar ratio of 1:(0.98-1.05):1 for 2-5 hours. After drying, primary sintering and secondary sintering, the lithium iron phosphate cathode material is obtained. The temperature for the first-stage sintering is 400-550℃ and the time is 2-5 hours, while the temperature for the second-stage sintering is 650-750℃ and the time is 6-10 hours.

10. A lithium iron phosphate cathode material prepared by the preparation method according to any one of claims 1-9, characterized in that, The lithium iron phosphate cathode material includes a LiFePO4 core and a nitrogen-doped carbon layer coating the surface of the LiFePO4 core. The LiFePO4 core has a two-dimensional sheet-like porous structure.

11. The lithium iron phosphate cathode material according to claim 10, characterized in that, The LiFePO4 core has a porosity of 26-32% and an average pore size of 8.3-11.5 nm.

12. The lithium iron phosphate cathode material according to claim 10, characterized in that, The thickness of the nitrogen-doped carbon layer is 5.5-6.7 nm.

13. A lithium-ion battery, wherein, The positive electrode of the lithium-ion battery includes the lithium iron phosphate positive electrode material as described in any one of claims 10-12.

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