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

Abstract

Provided are a lithium iron phosphate positive electrode material and a preparation method therefor, and a lithium-ion battery. The preparation method comprises the following steps: (1) preparing an iron hydroxymethyl acid salt two-dimensional precursor; (2) mixing the iron hydroxymethyl acid salt two-dimensional precursor with a nitrogen-containing carbon source for reaction to obtain a two-dimensional precursor material; and (3) mixing the two-dimensional precursor material, a lithium source, and a phosphorus source and sintering the mixture to obtain the lithium iron phosphate positive electrode material. The lithium iron phosphate positive electrode material is prepared by a method that is simple in process and simple to operate. The positive electrode material has a large electrode / electrolyte contact interface, so that complete infiltration of the electrolyte and the positive electrode material is facilitated; in addition, the microstructure of the positive electrode material inherits the two-dimensional sheet-like appearance of the iron hydroxymethyl acid salt precursor, and a large specific surface area of the positive electrode material provides sufficient active sites for the storage of lithium ions and electrons, thereby promoting rapid diffusion and transmission of lithium ions, and facilitating the improvement of the electrochemical performance of the lithium-ion battery.

Description

A lithium iron phosphate positive electrode material and preparation method thereof and lithium ion battery Technical Field

[0001] The present application relates to the technical field of battery materials, for example, a lithium iron phosphate positive electrode material and a preparation method thereof, and a lithium-ion battery. Background Art

[0002] With the ever-expanding market demand for portable electronic devices and electric vehicles, rechargeable lithium-ion batteries have attracted widespread attention due to their high energy density, relatively long life, and environmental friendliness. Lithium iron phosphate (LiFePO4), as a cathode material for lithium-ion batteries, has become one of the top choices for power batteries due to its advantages such as low price, safety, environmental protection, high capacity, and cycle stability. However, due to its structural limitations, LiFePO4 has disadvantages such as low electronic conductivity, low lithium ion diffusion rate at the FePO4 / LiFePO4 two-phase interface, and low tap density, which restricts its application in scenarios such as large-scale energy storage and high-rate power batteries. To address this issue, methods such as particle surface coating, ion doping, morphology control, and nano-sizing have been used to modify lithium iron phosphate, thereby improving the electrochemical properties of the material.

[0003] Among them, materials with two-dimensional sheet-like morphology can provide abundant electrochemical active sites due to their large specific surface area and ultra-thin sheet thickness. Therefore, they can effectively shorten the lithium ion deintercalation / embedding channels and expand their migration range, which is beneficial to improving the rate and low-temperature resistance of lithium-ion batteries. If this structure is introduced into the design of lithium iron phosphate positive electrode materials, it will be of great significance to improving the comprehensive electrical performance of the battery. In addition, most of the related lithium iron phosphate preparation methods such as solid-phase method, liquid-phase method and sol-gel method have defects such as incomplete reaction, irregular crystallization, complex and harsh process and high energy consumption; and the electrical performance of the obtained lithium iron phosphate is also somewhat different from the theoretical capacity, which needs to be further improved.

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

[0005] Summary of the Invention

[0006] The following is a summary of the subject matter described in detail herein. This summary is not intended to limit the scope of the claims.

[0007] The purpose of this application is to provide a lithium iron phosphate cathode material, a preparation method thereof, and a lithium ion battery. This application uses a method that is simple in process and easy to operate to prepare a lithium iron phosphate cathode material coated with a nitrogen-doped carbon layer having a two-dimensional porous structure. The cathode material has a large electrode / electrolyte contact interface, which is conducive to the complete infiltration of the electrolyte and the cathode material. At the same time, the microstructure of the cathode material inherits the two-dimensional flaky morphology of the iron hydroxymethylate precursor. Its large specific surface area provides sufficient active sites for the storage of lithium ions and electrons, promotes the rapid diffusion and transmission of lithium ions, and is conducive to improving the electrochemical performance of lithium ion batteries. This preparation method has good prospects for industrial application.

[0008] To achieve the purpose of this application, this application adopts the following technical solutions:

[0009] In a first aspect, the present application provides a method for preparing a lithium iron phosphate positive electrode material, the preparation method comprising the following steps:

[0010] (1) preparing a two-dimensional precursor of iron hydroxymethylate;

[0011] (2) mixing the iron hydroxymethylate two-dimensional precursor and the nitrogen-containing carbon source, and reacting them to obtain a two-dimensional precursor material;

[0012] (3) Mixing the two-dimensional precursor material, a lithium source, and a phosphorus source, and sintering the mixture to obtain the lithium iron phosphate positive electrode material.

[0013] In the present application, a nitrogen-containing carbon layer is coated on the surface of a two-dimensional iron hydroxymethylate precursor sheet. After mixing with a lithium source and a phosphorus source and sintering, the organic group Fe(OH)(OCH3) in the two-dimensional precursor material undergoes thermal decomposition, forming a porous structure on the two-dimensional sheet, thereby preparing a lithium iron phosphate positive electrode material coated with a nitrogen-doped carbon layer having a two-dimensional porous structure. The positive electrode material has a large electrode / electrolyte contact interface, which is conducive to the complete infiltration of the electrolyte and the positive electrode material. At the same time, the microstructure of the positive electrode material inherits the two-dimensional lamellae morphology of the iron hydroxymethylate precursor, and its large specific surface area provides sufficient active sites for the storage of lithium ions and electrons, promotes the rapid diffusion and transmission of lithium ions, and is beneficial to improving the electrochemical performance of lithium-ion batteries.

[0014] The preparation process provided in this application is simple, easy to operate and has low energy consumption, showing good prospects for industrial application.

[0015] As an optional technical solution of the present application, the method for preparing the iron hydroxymethylate two-dimensional precursor in step (1) includes:

[0016] Ferrous acetate tetrahydrate and an alcohol solvent are mixed and subjected to a solvothermal reaction to obtain the iron hydroxymethylate two-dimensional precursor.

[0017] It should be noted that solvothermal reaction refers to a synthesis method in which the original mixture reacts in a closed system such as an autoclave, using organic matter or non-aqueous solvent as solvent, at a certain temperature and autogenous pressure of the solution.

[0018] In the present application, ferrous acetate tetrahydrate and an alcohol solvent are mixed and a solvent thermal reaction is performed to prepare an iron hydroxymethylate two-dimensional precursor, thereby obtaining a product with high purity and excellent crystal structure.

[0019] In one embodiment, the alcohol solvent includes at least one of methanol, ethanol and propanol.

[0020] In one embodiment, the concentration of the solution obtained by mixing the ferrous acetate tetrahydrate and the 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.

[0021] In this application, if the concentration of the solution obtained by mixing ferrous acetate tetrahydrate and an alcohol solvent is too low, it may cause 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 cause agglomeration of the precursor material. The above factors will affect the electrical properties of the synthesized lithium iron phosphate material.

[0022] In one embodiment, the solvent thermal reaction temperature is 150-200°C, for example, 150°C, 160°C, 170°C, 180°C, 190°C or 200°C, and the time is 24-36h, for example, 24h, 26h, 28h, 30h, 32h, 34h or 36h, etc.

[0023] In the present application, a solvent thermal reaction is carried out at 150-200° C. for 24-36 hours, which helps to accelerate the reaction rate, improve the reaction yield, and generate a precursor material with good uniformity and high crystallinity.

[0024] As an optional technical solution of the present application, the nitrogen-containing carbon source in step (2) includes dopamine hydrochloride.

[0025] In the present application, dopamine hydrochloride is used as a nitrogen-containing carbon source, which can undergo self-polymerization in a buffer solution to form polydopamine.

[0026] 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.

[0027] In the present application, the concentration of dopamine hydrochloride will affect the thickness of the nitrogen-doped carbon layer. If the concentration is too low, the thickness of the nitrogen-doped carbon layer formed will be thin, which will result in an insignificant improvement in the conductivity of the lithium iron phosphate material; if the concentration is too high, the nitrogen-doped carbon layer formed will be thicker, which will affect the migration of lithium ions to a certain extent, and thus affect the electrical properties of the lithium iron phosphate material.

[0028] As an optional technical solution of the present application, the mixing method in step (2) includes:

[0029] The iron hydroxymethylate two-dimensional precursor is dispersed in a buffer solution, and then a nitrogen-containing carbon source is added for blending.

[0030] In one embodiment, the buffer solution comprises a Tris buffer solution.

[0031] In the present application, Tris buffer solution is also called tris (hydroxymethyl)aminomethane hydrochloride buffer solution, and has an effective buffer range of pH = 7.0-9.0. By using this substance, the acid-base balance of the solution can be effectively maintained, and the pH value of the solution can be kept relatively stable.

[0032] In one embodiment, the mixing temperature in step (2) is room temperature, and the mixing time is 12-24 hours, for example, 12 hours, 18 hours, or 24 hours.

[0033] It should be noted that the present application does not limit the specific temperature of room temperature. For example, it can be 25±5°C, including 20°C, 25°C or 30°C.

[0034] In the present application, the iron hydroxymethylate two-dimensional precursor and the nitrogen-containing carbon source are mixed at room temperature for 12-24 hours, so that the nitrogen-containing carbon source can be fully attached to the surface of the iron hydroxymethylate two-dimensional precursor.

[0035] As an optional technical solution of the present application, the lithium source in step (3) includes at least one of lithium carbonate, lithium hydroxide and lithium acetate.

[0036] In one embodiment, the phosphorus source in step (3) includes at least one of ammonium dihydrogen phosphate, ammonium hydrogen phosphate and sodium hydrogen phosphate.

[0037] In one embodiment, the molar ratio of the two-dimensional precursor material, the phosphorus source and the 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.

[0038] In the present application, if the molar ratio of the two-dimensional precursor material, the phosphorus source, and the lithium source is too small or too large, the crystal structure stability of the lithium iron phosphate material will be affected, which will make it difficult for lithium ions to be released and embedded, thereby reducing the electrical properties of the material. As an optional technical solution of the present application, the mixing method described in step (3) includes ball milling.

[0039] In the present application, ball milling is used to obtain uniform particle distribution.

[0040] In one embodiment, the mixing time in step (3) is 2-5 hours, for example, 2 hours, 3 hours, 4 hours or 5 hours.

[0041] In one embodiment, the sintering in step (3) is performed in a protective atmosphere.

[0042] In the present 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.

[0043] In one embodiment, the gas in the protective atmosphere includes nitrogen and / or argon.

[0044] In one embodiment, the sintering method in step (3) is multi-stage sintering, and the multi-stage sintering includes primary sintering and secondary sintering.

[0045] In this application, a multi-stage sintering method is adopted to obtain a material with a denser, more uniform structure and better crystallinity.

[0046] In one embodiment, the primary sintering temperature is 400-550°C, such as 400°C, 450°C, 500°C or 550°C, and the sintering time is 2-5h, such as 2h, 3h, 4h or 5h.

[0047] In the present application, primary sintering is performed at 400-550° C. to improve the crystallinity of the lithium iron phosphate material.

[0048] In one embodiment, the secondary sintering temperature is 650-750°C, such as 650°C, 675°C, 700°C, 725°C or 750°C, and the time is 6-10h, such as 6h, 7h, 8h, 9h or 10h.

[0049] In this application, if the secondary sintering temperature is too low, the lithium iron phosphate material will not be fully transformed; if the secondary sintering temperature is too high, the material crystals will grow too large, resulting in problems such as weakened grain boundary bonding. These factors will degrade the electrical properties of the material.

[0050] As an optional technical solution of the present application, the preparation method comprises the following steps:

[0051] (1) mixing ferrous acetate tetrahydrate and an alcohol solvent from which oxygen has been removed from the solution, performing a solvothermal reaction at 150-200° C. for 24-36 hours, cooling to room temperature after the reaction, and then centrifuging and washing to obtain the iron hydroxymethylate two-dimensional precursor;

[0052] The concentration of the solution obtained by mixing ferrous acetate tetrahydrate and an alcohol solvent is (0.17-0.5) mol / L;

[0053] (2) ultrasonically dispersing the iron hydroxymethylate two-dimensional precursor in a buffer solution, adding dopamine hydrochloride and stirring, and reacting at room temperature for 12-24 hours. After the reaction is completed, centrifugation and washing are performed to obtain a two-dimensional precursor material, namely, Fe(OH)(OCH3)@polydopamine material;

[0054] The mass concentration of dopamine hydrochloride in the mixed solution is 0.5-0.8 g / L;

[0055] (3) Under a protective atmosphere, the two-dimensional precursor material, the phosphorus source, and the lithium source are ball-milled and mixed at a molar ratio of 1:(0.98-1.05):1 for 2-5 hours, and after drying, primary sintering, and secondary sintering, the lithium iron phosphate positive electrode material is obtained;

[0056] The primary sintering temperature is 400-550°C and the time is 2-5 hours, and the secondary sintering temperature is 650-750°C and the time is 6-10 hours.

[0057] It should be noted that the reaction equation of the solvothermal reaction is as follows: Fe(CH3CO2)2·4H2O+3CH3OH=Fe(OH)(OCH3)+2CH3CO2CH3+5H2O.

[0058] In this application, with the coordinated cooperation of multiple parameters, a two-dimensional iron hydroxymethylate precursor sheet is first prepared by a solvent thermal method, and then a layer of polydopamine is coated on its surface. After mixing with a lithium source and a phosphorus source and calcined at high temperature, the thermal decomposition of the Fe(OH)(OCH3) organic group forms a porous structure on the two-dimensional sheet, and a nitrogen-doped carbon layer-coated lithium iron phosphate sheet with a two-dimensional porous structure is obtained. The microstructure of lithium iron phosphate inherits the two-dimensional lamellae morphology of the iron hydroxymethylate precursor, which can promote the rapid diffusion and transport of lithium ions and improve the electrochemical properties of the material, which 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 polydopamine coated on the surface of Fe(OH)(OCH3).

[0060] In a second aspect, the present application provides a lithium iron phosphate positive electrode material prepared by the preparation method according to the first aspect, wherein the lithium iron phosphate positive electrode material comprises a LiFePO4 core and a nitrogen-doped carbon layer coated on the surface of the LiFePO4 core;

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

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

[0063] In the present application, the presence of the nitrogen-doped carbon layer can improve the electronic conductivity and mechanical strength of the electrode material, alleviate the volume change of the material during the cycle, and enhance the stability and uniformity of the material.

[0064] The lithium iron phosphate cathode material synthesized in the present application is evenly dispersed and has high purity, and its lamellar structure is also beneficial for reducing the agglomeration of the material during the charge and discharge process.

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

[0066] In one embodiment, the thickness of the nitrogen-doped carbon layer is 5.5-6.7 nm, for example, 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 not only improves the electronic conductivity and mechanical strength of the electrode material, but also mitigates the material's volume changes during cycling, enhancing its structural stability. Furthermore, a nitrogen-doped carbon layer of a certain thickness effectively prevents sheet aggregation caused by secondary crystallization during solid-phase sintering, significantly improving the integrity and uniformity of the product.

[0068] In a third aspect, the present 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 adopts the above-mentioned lithium iron phosphate positive electrode material, which is beneficial to 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 limited space and for the sake of simplicity, this application no longer exhaustively lists the specific point values ​​included in the range.

[0071] Compared with the related art, this application has the following beneficial effects:

[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 the present application, a nitrogen-containing carbon layer is coated on the surface of a two-dimensional iron hydroxymethylate precursor sheet. After mixing with a lithium source and a phosphorus source and sintering, the organic group Fe(OH)(OCH3) in the two-dimensional precursor material undergoes thermal decomposition, forming a porous structure on the two-dimensional sheet, thereby preparing a lithium iron phosphate positive electrode material coated with a nitrogen-doped carbon layer having a two-dimensional porous structure. The positive electrode material has a large electrode / electrolyte contact interface, which is conducive to the complete infiltration of the electrolyte and the positive electrode material. At the same time, the microstructure of the positive electrode material inherits the two-dimensional sheet morphology of the iron hydroxymethylate precursor. Its large specific surface area provides sufficient active sites for the storage of lithium ions and electrons, promotes the rapid diffusion and transmission of lithium ions, and is conducive to improving the electrochemical performance of lithium-ion batteries.

[0074] (3) In the present application, the presence of the nitrogen-doped carbon layer can not only improve the electronic conductivity and mechanical strength of the electrode material, but also alleviate the volume change of the material during the cycle and enhance its structural stability. In addition, the nitrogen-doped carbon layer can effectively avoid the phenomenon of sheet aggregation caused by secondary crystallization during the solid-phase sintering process, significantly improving the integrity and uniformity of the product. In addition, the lithium iron phosphate positive electrode material synthesized in the present application is evenly dispersed and has a high purity, and the lamellar structure is also conducive to reducing the agglomeration of the material during the charge and discharge process.

[0075] Still other aspects will become apparent upon reading and understanding the accompanying drawings and detailed description. BRIEF DESCRIPTION OF THE DRAWINGS

[0076] The accompanying drawings are used to provide a further understanding of the technical solution of this article and constitute a part of the specification. Together with the embodiments of this application, they are used to explain the technical solution of this article and do not constitute a limitation on the technical solution of this article.

[0077] FIG1 is a process flow chart of preparing lithium iron phosphate positive electrode material in Example 1 of this application.

[0078] FIG2 is a scanning electron microscope image of the lithium iron phosphate positive electrode material prepared in Example 1 of the present application. DETAILED DESCRIPTION

[0079] The technical solution of the present application is further described below through specific implementation methods. Those skilled in the art should understand that the embodiments are only used to help understand the present application and should not be regarded as specific limitations of the present 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 a lithium iron phosphate positive electrode material, the process flow chart of which is shown in FIG1 . The preparation method includes the following steps:

[0083] (1) 60 mL of methanol was placed in a beaker and purged with nitrogen for 1 h under stirring to remove oxygen from the solution. 0.02 mol of ferrous acetate tetrahydrate was then added to the treated methanol. After stirring for 30 min, the resulting mixture was placed in a 100 mL polytetrafluoroethylene-lined autoclave and subjected to a solvent thermal reaction in an oven at 180 ° C for 24 h. After the reaction was completed, the mixture was cooled to room temperature and then centrifuged to collect the product. The product was washed with deionized water and anhydrous ethanol in turn. Finally, the obtained iron hydroxymethylate two-dimensional precursor, i.e., Fe(OH)(OCH3) two-dimensional precursor material, was placed in a vacuum oven at 70 ° C and dried for 12 h.

[0084] Among them, the concentration of the mixed solution is 0.33 mol / L;

[0085] (2) ultrasonically dispersing the iron hydroxymethylate two-dimensional precursor in 100 mL of Tris buffer solution (10 mmol, pH = 8.5), adding 0.05 g of dopamine hydrochloride (DA) and stirring and mixing, and reacting at room temperature for 12 hours under stirring conditions to polymerize dopamine on the Fe(OH)(OCH3) surface. After the reaction, the Fe(OH)(OCH3)@polydopamine (PDA) product was collected by centrifugation, washed with deionized water and anhydrous ethanol in turn, and the obtained two-dimensional precursor material, i.e., Fe(OH)(OCH3)@PDA material, was placed in a vacuum oven at 70°C and dried for 12 hours;

[0086] Wherein, the mass concentration of dopamine hydrochloride in the mixed solution after adding 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, ball milled for 3 hours at a rotation speed of 3000 rpm, and then spray dried to obtain a precursor powder. The precursor powder is then multi-stage sintered in an argon atmosphere, that is, firstly sintered to 400°C at a heating rate of 5°C / min for 3 hours, and then heated to 700°C for 8 hours for secondary sintering. After the sintering, the lithium iron phosphate positive electrode material, namely LiFePO4 / NC two-dimensional porous material, is obtained. (NC is a nitrogen-doped carbon layer formed after multi-stage sintering of PDA).

[0088] This embodiment also provides a lithium iron phosphate positive electrode material prepared by the above preparation method, wherein the lithium iron phosphate positive electrode material comprises a LiFePO4 core and a nitrogen-doped carbon layer coated on the surface of the LiFePO4 core, wherein the LiFePO4 core has a two-dimensional sheet-like porous structure;

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

[0090] FIG2 shows a scanning electron microscope image of the lithium iron phosphate positive electrode material prepared in this embodiment. As can be seen from the image, the synthesized lithium iron phosphate material has a two-dimensional sheet structure.

[0091] Example 2

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

[0093] (1) 60 mL of methanol was placed in a beaker and purged with nitrogen for 1 h under stirring to remove oxygen from the solution. 0.025 mol of ferrous acetate tetrahydrate was then added to the treated methanol. After stirring for 30 min, the resulting mixture was placed in a 100 mL polytetrafluoroethylene-lined autoclave and subjected to a solvent thermal reaction in an oven at 180 ° C for 36 h. After the reaction was completed, the mixture was cooled to room temperature and then centrifuged to collect the product. The product was washed with deionized water and anhydrous ethanol in succession. Finally, the obtained iron hydroxymethylate two-dimensional precursor, i.e., Fe(OH)(OCH3), was placed in a vacuum oven at 70 ° C and dried for 12 h.

[0094] Among them, the concentration of the mixed solution is 0.42 mol / L;

[0095] (2) ultrasonically dispersing the iron hydroxymethylate two-dimensional precursor in 100 mL of Tris buffer solution (10 mmol, pH = 8.5), adding 0.05 g of dopamine hydrochloride and stirring, and reacting at room temperature for 12 hours under stirring to polymerize dopamine on the Fe(OH)(OCH3) surface. After the reaction, the Fe(OH)(OCH3)@polydopamine (PDA) product was collected by centrifugation, washed with deionized water and anhydrous ethanol in turn, and the obtained two-dimensional precursor material, i.e., Fe(OH)(OCH3)@PDA material, was placed in a vacuum oven at 70°C and dried for 12 hours;

[0096] Wherein, the mass concentration of dopamine hydrochloride in the mixed solution after adding dopamine hydrochloride is 0.5 g / L;

[0097] (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, ball milled for 3 hours at a rotation speed of 3000 rpm, and then spray dried to obtain a precursor powder. The precursor powder is then subjected to primary sintering at a heating rate of 5°C / min to 450°C for 3 hours under an argon atmosphere, and then heated to 700°C for 10 hours. Secondary sintering, the lithium iron phosphate positive electrode material is obtained.

[0098] This embodiment also provides a lithium iron phosphate positive electrode material prepared by the above preparation method, wherein the lithium iron phosphate positive electrode material comprises a LiFePO4 core and a nitrogen-doped carbon layer coated on the surface of the LiFePO4 core, wherein the LiFePO4 core has a two-dimensional sheet-like porous structure;

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

[0100] Example 3

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

[0102] (1) 60 mL of methanol was placed in a beaker and purged with nitrogen for 1 h under stirring to remove oxygen from the solution. Subsequently, 0.025 mol of ferrous acetate tetrahydrate was added to the treated methanol and stirred for 30 min. The resulting mixture was placed in a 100 mL polytetrafluoroethylene-lined autoclave and subjected to a solvent thermal reaction in an oven at 180 ° C for 24 h. After the reaction was completed, the mixture was cooled to room temperature and then centrifuged to collect the product. The product was washed with deionized water and anhydrous ethanol in succession. Finally, the obtained iron hydroxymethylate two-dimensional precursor, i.e., Fe(OH)(OCH3), was placed in a vacuum oven at 70 ° C and dried for 12 h.

[0103] Among them, the concentration of the mixed solution is 0.42 mol / L;

[0104] (2) ultrasonically dispersing the iron hydroxymethylate two-dimensional precursor in 100 mL of Tris buffer solution (10 mmol, pH = 8.5), adding 0.07 g of dopamine hydrochloride and stirring, and reacting at room temperature for 24 hours under stirring to polymerize dopamine on the Fe(OH)(OCH3) surface. After the reaction, the Fe(OH)(OCH3)@polydopamine (PDA) product was collected by centrifugation, washed with deionized water and anhydrous ethanol in turn, and the obtained two-dimensional precursor material, i.e., Fe(OH)(OCH3)@PDA material, was placed in a vacuum oven at 70°C and dried for 12 hours;

[0105] Wherein, the mass concentration of dopamine hydrochloride in the mixed solution after adding dopamine hydrochloride is 0.7 g / L;

[0106] (3) The two-dimensional precursor material, ammonium dihydrogen phosphate and lithium carbonate are dispersed in anhydrous ethanol at a molar ratio of 1:0.99:1, ball milled for 3 hours at a rotation speed of 3000 rpm, and then spray dried to obtain a precursor powder. The precursor powder is then subjected to primary sintering at a heating rate of 5°C / min to 450°C for 3 hours under an argon atmosphere, and then heated to 750°C for 10 hours. Secondary sintering, the lithium iron phosphate positive electrode material is obtained.

[0107] This embodiment also provides a lithium iron phosphate positive electrode material prepared by the above preparation method, wherein the lithium iron phosphate positive electrode material comprises a LiFePO4 core and a nitrogen-doped carbon layer coated on the surface of the LiFePO4 core, wherein the LiFePO4 core has a two-dimensional sheet-like porous structure;

[0108] The porosity of the LiFePO4 core is 30.5%, the average pore diameter is 11 nm, and the thickness of the nitrogen-doped carbon layer is 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 mixed solution is 0.17 mol / L; and the concentration of the dopamine hydrochloride solution in step (2) is adjusted to 0.8 g / L.

[0111] The rest of the preparation methods and parameters remained the same as 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 mixed solution is 0.08 mol / L.

[0114] The rest of the preparation methods and parameters remained the same as 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 mixed solution is 0.83 mol / L.

[0117] The rest of the preparation methods and parameters remained the same as 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 rest of the preparation methods and parameters remained the same as 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 rest of the preparation methods and parameters remained the same as 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 rest of the preparation methods and parameters remained the same as 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 rest of the preparation methods and parameters remained the same as in Example 1.

[0130] Example 11

[0131] The difference between this embodiment and embodiment 1 is that no primary sintering is performed in step (3).

[0132] The rest of the preparation methods and parameters remained the same as 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 mixed solution is 0.5 mol / L; and the concentration of the dopamine hydrochloride solution in step (2) is adjusted to 0.7 g / L.

[0135] The rest of the preparation methods and parameters remained the same as 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°C.

[0138] The rest of the preparation methods and parameters remained the same as 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°C.

[0141] The rest of the preparation methods and parameters remained the same as in Example 1.

[0142] Comparative Example 1

[0143] The difference between this comparative example and Example 2 is that steps (1) and (2) are not performed, but ferrous sulfate, ammonium dihydrogen phosphate and lithium carbonate are directly mixed.

[0144] The rest of the preparation methods and parameters remained the same as in Example 2.

[0145] Comparative Example 2

[0146] The difference between this comparative example and Example 3 is that step (2) is not performed, but the iron hydroxymethylate two-dimensional precursor obtained in step (1), ammonium dihydrogen phosphate and lithium carbonate are directly mixed.

[0147] The rest of the preparation methods and parameters remained the same as in Example 3.

[0148] Performance Testing

[0149] The lithium iron phosphate positive electrode materials prepared in the above examples and comparative examples were assembled into button batteries. The specific steps included: uniformly mixing the lithium iron phosphate positive electrode material, the conductive agent acetylene black, and the adhesive polyvinylidene fluoride in a mass ratio of 92:4:4 in N-methylpyrrolidone to form a slurry, which was then coated on aluminum foil and dried in a vacuum drying oven. The slurry was then pressed into a positive electrode sheet using a tablet press. The negative electrode sheet was a metal lithium sheet, the electrolyte was 1 mol / L LiPF6-EC:DMC (volume ratio of 1:1), and a polypropylene porous membrane was used as a separator. The battery was assembled in an argon glove box.

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

[0151] Table 1

[0152] analyze:

[0153] As can be seen from the above table, the lithium iron phosphate positive electrode material prepared in this application is a two-dimensional porous thin sheet structure coated with a nitrogen-doped carbon layer. The material of this structure has a large specific surface area and high conductivity, and the contact area between the electrolyte and the positive electrode 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.

[0154] It can be seen from Example 1 and Examples 5-6 that if the concentration of the solution obtained by mixing ferrous acetate tetrahydrate and an alcohol solvent is too low, it may cause incomplete crystal growth of the precursor material and affect its crystallinity; if the concentration of the solution obtained by mixing ferrous acetate tetrahydrate and an alcohol solvent is too high, it may cause agglomeration of the precursor material. These factors will affect the electrical properties of the synthesized lithium iron phosphate material.

[0155] It can be seen from Example 1 and Examples 7-8 that if the concentration of dopamine hydrochloride is too low, the thickness of the nitrogen-doped carbon layer formed is relatively thin, which will result in an insignificant improvement in the conductivity of the lithium iron phosphate material; if the concentration of dopamine hydrochloride is too high, the nitrogen-doped carbon layer formed is relatively thick, which will affect the migration of lithium ions to a certain extent, and thus affect the electrical properties of the lithium iron phosphate material.

[0156] It can be seen from Examples 1 and 9-10 that 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, which will make it difficult for lithium ions to escape and embed, and thus reduce the electrical properties of the material.

[0157] It can be seen from Example 1 and Example 11 that if the primary sintering is not performed, the crystallinity of the lithium iron phosphate material may be low and the crystal structure may have defects, which in turn affects its electrical properties.

[0158] As shown in Examples 2 and 13-14, if the secondary sintering temperature is too low, the lithium iron phosphate material will not be fully transformed; if the secondary sintering temperature is too high, the material crystals will grow too large, resulting in a weakened grain boundary bonding force and other problems. These factors will degrade the electrical properties of the material.

[0159] It can be seen from Example 2 and Comparative Example 1 that if ferrous sulfate, ammonium dihydrogen phosphate and lithium carbonate are directly mixed, an irregular lithium iron phosphate block without a nitrogen-doped carbon layer will be obtained, which 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.

[0160] It can be seen from Example 3 and Comparative Example 2 that if the iron hydroxymethylate two-dimensional precursor, ammonium dihydrogen phosphate and lithium carbonate are directly mixed, a lithium iron phosphate sheet coated with a nitrogen-free doped carbon layer will be obtained, which will result in a low electrical conductivity of the lithium iron phosphate material and may cause agglomeration, thereby affecting its electrical properties.

[0161] The applicant declares that while the above-mentioned embodiments are used to illustrate the process of the present application, the present application is not limited to the above-mentioned process steps, which does not mean that the present application must rely on the above-mentioned process steps in order to be implemented. Those skilled in the art should understand that any improvements to the present application, equivalent replacements for the raw materials used in the present application, addition of auxiliary components, and selection of specific methods, etc., fall within the scope of protection and disclosure of the present application.

Claims

1. A preparation method of a lithium iron phosphate cathode material, which comprises the following steps: (1) Prepare a two-dimensional precursor of iron hydroxymethylate; (2) Mix the two-dimensional precursor of iron hydroxymethylate and a nitrogen-containing carbon source, and carry out a reaction to obtain a two-dimensional precursor material; (3) Mix the two-dimensional precursor material, a lithium source and a phosphorus source, and carry out sintering to obtain the lithium iron phosphate cathode material.

2. The preparation method according to claim 1, wherein, The preparation method of the two-dimensional precursor of iron hydroxymethylate in step (1) comprises: Mix ferrous acetate tetrahydrate and an alcohol solvent, and carry out a solvothermal reaction to obtain the two-dimensional precursor of iron hydroxymethylate.

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

4. The preparation method according to claim 2 or 3, wherein, The temperature of the solvothermal reaction is 150 - 200 °C, and the time is 24 - 36 h.

5. The preparation method according to any one of claims 1-4, wherein, The nitrogen-containing carbon source in step (2) includes dopamine hydrochloride; Optionally, the mass concentration of dopamine hydrochloride in the solution obtained by mixing in step (2) is 0.5 - 0.8 g / L.

6. The preparation method according to any one of claims 1-5, wherein, The mixing method in step (2) includes: Disperse the two-dimensional precursor of iron hydroxymethylate in a buffer solution, and then add a nitrogen-containing carbon source for co-blending; Optionally, the buffer solution includes Tris buffer solution; Optionally, the mixing temperature in step (2) is room temperature, and the time is 12 - 24 h.

7. The preparation method according to any one of claims 1-6, wherein, The molar ratio of the two-dimensional precursor material, the phosphorus source and the lithium source in step (3) is 1∶(0.98 - 1.05)∶1.

8. The preparation method according to any one of claims 1-7, wherein, The mixing method in step (3) includes ball milling; Optionally, the sintering in step (3) is carried out in a protective atmosphere.

9. The preparation method according to any one of claims 1-8, wherein, The sintering method in step (3) is multi-stage sintering, and the multi-stage sintering includes primary sintering and secondary sintering.

10. The preparation method according to claim 9, wherein, The temperature of the primary sintering is 400 - 550 °C, and the time is 2 - 5 h.

11. The preparation method according to claim 9 or 10, wherein The temperature of the secondary sintering is 650 - 750 °C, and the time is 6 - 10 h.

12. The preparation method according to any one of claims 1 - 11, which comprises the following steps: (1) Stir and mix ferrous acetate tetrahydrate and an alcohol solvent with removed solution oxygen, carry out a solvothermal reaction at 150 - 200 °C for 24 - 36 h, cool to room temperature after the reaction, and then carry out centrifugation and washing to obtain the two-dimensional precursor of iron hydroxymethylate; wherein, the concentration of the solution obtained by mixing ferrous acetate tetrahydrate and the alcohol solvent is (0.17 - 0.5) mol / L; (2) Ultrasonically disperse the two-dimensional precursor of iron hydroxymethylate in a buffer solution, add dopamine hydrochloride and stir and mix, and carry out a reaction at room temperature for 12 - 24 h. After the reaction, carry out centrifugation and washing to obtain a two-dimensional precursor material, namely Fe(OH)(OCH3)@polydopamine material; wherein, the mass concentration of dopamine hydrochloride in the mixed solution is 0.5 - 0.8 g / L; (3) Under a protective atmosphere, ball mill and mix the two-dimensional precursor material, the phosphorus source and the lithium source according to a molar ratio of 1∶(0.98 - 1.05)∶1 for 2 - 5 h, and after drying, primary sintering and secondary sintering, obtain the lithium iron phosphate cathode material; Among them, the temperature of the first-stage sintering is 400 - 550 °C, the time is 2 - 5 h, the temperature of the second-stage sintering is 650 - 750 °C, and the time is 6 - 10 h.

13. A lithium iron phosphate cathode material prepared by the preparation method according to any one of claims 1-12, 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; The LiFePO4 core is a two-dimensional flaky porous structure.

14. The lithium iron phosphate cathode material according to claim 13, wherein, The porosity of the LiFePO4 core is 26 - 32%, and the average pore diameter is 8.3 - 11.5 nm; Optionally, the thickness of the nitrogen-doped carbon layer is 5.5 - 6.7 nm.

15. A lithium-ion battery, wherein, The positive electrode of the lithium-ion battery includes the lithium iron phosphate cathode material as described in claim 13 or 14.

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