A doped multilayer coated iron phosphate-based sodium-ion battery cathode material and a preparation method thereof

Abstract

The application provides a doped multilayer coated iron phosphate-based sodium-ion battery positive electrode material and a preparation method thereof, and comprises the following contents: iron phosphate dihydrate, a carbon source, a dispersing agent and a solvent are ball-milled to obtain a precursor slurry; after drying and grinding, high-temperature sintering is performed under an inert atmosphere to obtain Fe3(PO4)2@C; the Fe3(PO4)2@C is mixed with Ti3AlC2, a sodium source, a lithium source, a fluorine source and an acid, Ti3C2 and AlF3 are in-situ grown on the Fe3(PO4)2@C to form a coating; after drying and grinding, sintering is performed under an inert atmosphere, and the material is obtained after grinding. In the method, the iron phosphate dihydrate and the carbon source are pre-sintered at high temperature, the graphitization degree of the coated carbon is increased, the in-situ grown Ti3C2 and AlF3 nanosheets are used for coating, and part of lithium elements are doped, a three-dimensional structure is formed after sintering, the electronic conductivity of the material can be improved, the material can be effectively prevented from agglomeration during drying and sintering, ion transmission can be accelerated, the loss of sodium ions during charging and discharging can be reduced, and the electrochemical performance can be improved.

Description

Technical Field

[0001] This invention belongs to the field of electrode material technology, specifically relating to a doped multilayer coated iron phosphate-based sodium-ion battery cathode material and its preparation method. Background Technology

[0002] Currently, lithium-ion batteries are ubiquitous in secondary batteries, found in small consumer electronics, power tools, and large power sources driving (plug-in) hybrid electric transportation and grid systems. This diverse consumption of lithium has led to rising prices and raised concerns that it might become the new gold standard. To avoid this, sodium-ion batteries can play a crucial role. Unlike lithium, sodium (Na) is abundant in natural resources and evenly distributed geographically. Besides being the fifth most abundant element in the Earth's crust, sodium is also the second lightest alkali element in the periodic table. Given this context, a growing number of researchers are working to develop highly efficient sodium-ion batteries.

[0003] Currently, research on sodium-ion battery cathode materials mainly falls into three categories: polyanionic, Prussian blue, and oxide-based. In summary, while oxide-based sodium-ion battery cathode materials offer high capacity, the use of rare metals leads to higher costs; Prussian blue-based materials suffer from pollution during preparation and structural instability. Therefore, in the long run, polyanionic sodium-ion battery cathode materials appear to be a more advantageous choice. Among polyanionic sodium-ion battery cathode materials, vanadium-based materials offer long lifespan and good rate performance, but are costly and highly toxic. In contrast, iron-based sodium-ion battery cathode materials have lower cost and toxicity, and offer moderate capacity and voltage plateaus. Sodium iron fluorophosphate (Na2FePO4F) has a relatively high theoretical capacity (124 mAh*g). -1 It exhibits a stable structure, is not prone to volume changes, and possesses excellent electrochemical performance. However, some problems exist. When using ferric phosphate as the iron source, high-temperature sintering of ferric phosphate dihydrate is required to remove the water of crystallization, increasing the complexity and cost of the process. Using conventional solid-state preparation methods, particle size control is difficult, grinding time is long, and the carbon coating process is prone to inhomogeneity. To obtain a purer phase, a low sintering temperature is required, resulting in a low degree of graphitization of the coated carbon. In addition, the intrinsic electronic conductivity and sodium ion diffusion coefficient of sodium fluorophosphate are relatively low, affecting the material's performance. Summary of the Invention

[0004] In view of the technical problems existing in the prior art, the present invention provides a doped multilayer coated iron phosphate-based sodium-ion battery cathode material and its preparation method. The present invention pre-calcines iron phosphate dihydrate with a carbon source at high temperature, eliminating the need for separate high-temperature drying of iron phosphate dihydrate to prepare iron phosphate, increasing the graphitization degree of the coated carbon, introducing partial lithium doping, and utilizing in-situ grown Ti3C2 and AlF3 nanosheets for coating. After sintering, a three-dimensional structure is formed, which can improve the electronic conductivity of the material, effectively prevent material agglomeration during drying and sintering, accelerate ion transport, and reduce sodium ion loss during charge and discharge, thereby improving electrochemical performance.

[0005] To solve the above-mentioned technical problems, the present invention adopts the following technical solution:

[0006] A method for preparing a doped multilayer coated iron phosphate-based sodium-ion battery cathode material includes the following steps:

[0007] a) Ball milling and mixing ferric phosphate dihydrate, carbon source, dispersant and solvent to obtain precursor slurry A;

[0008] b) After vacuum drying and grinding of the precursor slurry A, it is sintered at high temperature under an inert atmosphere, and then ground to obtain material B, namely Fe3(PO4)2@C;

[0009] c) The material B is mixed with Ti3AlC2, sodium source, lithium source, fluorine source and acid, and after ultrasonication and stirring, Ti3C2 and AlF3 are grown in situ on Fe3(PO4)2@C to form a coating by acid etching, so as to obtain slurry C;

[0010] d) After vacuum drying and grinding of the material C, sintering is carried out under an inert atmosphere. After grinding, a sodium iron fluoride phosphate cathode material with Ti3C2 nanosheets, AlF3 and carbon multilayer coating is obtained, which is the doped multilayer coated sodium iron phosphate-based cathode material.

[0011] In step a) of the above method, the ferric phosphate dihydrate serves as both a phosphorus source and an iron source. The iron source used in this invention is ferric phosphate dihydrate, which eliminates the need for the high-temperature sintering process in preparing ferric phosphate dihydrate, allowing direct application of ferric phosphate dihydrate to the preparation of sodium-ion battery cathode materials.

[0012] The carbon source is selected from at least one of glucose, citric acid, sucrose, ascorbic acid, etc.; the dispersant is selected from at least one of sodium hexametaphosphate, polyvinylpyrrolidone, polyacrylamide, etc.; the solvent is at least one of water, anhydrous ethanol, acetone, etc.; the solid content in the precursor slurry A is between 20% and 50%; the amount of dispersant added is 0.1% to 1% of the mass of ferric phosphate dihydrate; the ball milling is carried out in a ball mill, the ball-to-material mass ratio is between 5 and 10; the rotation speed of the ball mill is between 300 and 800 rpm, and the ball milling time is between 4 and 18 hours; the ball milling medium can be zirconium balls.

[0013] In step b) of the above method, the drying temperature is between 50 and 90°C, and the drying time is between 8 and 24 hours; the grinding is carried out once using a mortar and pestle or a grinder; the grinding time is between 20 and 40 minutes; the inert gas can be nitrogen, argon, etc.; the sintering temperature is between 650 and 800°C, and the sintering time is between 10 and 18 hours.

[0014] In step b) of the above method, ferric phosphate dihydrate, when sintered at high temperature in the presence of a carbon source, will be reduced to ferrous phosphate by carbon, thus obtaining Fe3(PO4)2@C.

[0015] In step c) of the above method, the sodium source includes sodium fluoride, sodium oxalate, sodium carbonate, sodium citrate, sodium acetate, etc.; the lithium source is lithium fluoride.

[0016] The fluorine source includes sodium fluoride and lithium fluoride. In this invention, lithium fluoride and sodium fluoride are used together as fluorine sources. The F in lithium fluoride mainly participates in the Ti3AlC2 reaction to generate AlF3, and the F in sodium fluoride mainly participates in the generation of Na2FePO4F.

[0017] The acid may be hydrochloric acid, and the molar concentration of the hydrochloric acid is in the range of 8-10 mol / L; the molar ratio of sodium ions in Ti3AlC2 and sodium source is 1:60-100; the molar ratio of Ti3AlC2 and LiF is 1:3-10; the molar ratio of sodium fluoride to ferric phosphate dihydrate in step a) is 0.8-1:1; the molar ratio of Ti3AlC2 to ferric phosphate dihydrate in step a) is between 1:30-50; the temperature for ultrasonication and stirring is 30-40℃; the frequency and duration of ultrasonication are 30 Hz and 1 h, respectively; the frequency and duration of stirring are 200-300 rpm and 24 h, respectively.

[0018] In step d) of the above method, the drying temperature is between 50 and 90°C, and the drying time is between 8 and 24 hours; the grinding is carried out once using a mortar and pestle or a grinder; the grinding time is between 20 and 40 minutes; the inert gas can be nitrogen, argon, etc.; the sintering temperature is between 500 and 650°C (preferably between 600 and 650°C), and the sintering time is between 10 and 18 hours.

[0019] The preparation method of the above-mentioned doped multilayer coated iron phosphate-based sodium-ion battery cathode material is as follows:

[0020] 1) Calculate and weigh different raw materials according to the target product and the molar ratio of ferric phosphate dihydrate to carbon source of 12:5. The raw materials include ferric phosphate dihydrate and carbon source.

[0021] 2) Add the weighed raw materials from step 1) into the solvent according to a certain solid content and mix evenly. Add a certain proportion of dispersant, stir to remove the gas in the slurry, add ball milling media (such as zirconium balls) according to a certain ball-to-material ratio, control the ball mill speed and ball milling time, so that the slurry particle size reaches D50=500±50nm, and obtain precursor slurry A.

[0022] 3) The precursor slurry A obtained in step 2) is dried in a vacuum oven, ground, and then placed in a tube furnace for sintering under an inert atmosphere. A certain sintering program and sintering time are set, and after grinding, material B (Fe3(PO4)2@C) is obtained.

[0023] 4) The material B obtained in step 3) is mixed with Ti3AlC2, sodium source, lithium source, fluorine source and acid, and then subjected to ultrasonication and stirring to obtain slurry C;

[0024] 5) Dry the slurry C from step 4) in a vacuum oven, grind it, and then place it in a tube furnace for sintering under an inert atmosphere. Set a certain sintering program and sintering time to obtain material D.

[0025] 6) Grind the material D from step 5) to obtain a sodium iron fluorophosphate sodium-ion battery cathode material with Ti3C2 nanosheets, AlF3 and carbon multilayer coating.

[0026] The doped multilayer coated iron phosphate-based sodium-ion battery cathode material prepared by the above method also falls within the scope of protection of this invention.

[0027] This invention also protects a positive electrode sheet, including the doped multilayer coated iron phosphate-based sodium-ion battery positive electrode material described above.

[0028] This invention also protects a sodium-ion battery.

[0029] The sodium-ion battery includes the positive electrode sheet described above in this invention.

[0030] The present invention provides a doped multilayer coated iron phosphate-based sodium-ion battery cathode material and its preparation method, which, compared with the prior art, have the following beneficial effects:

[0031] 1. The method for preparing doped multilayer coated iron phosphate-based sodium-ion battery cathode material provided by the present invention involves pre-calcining iron phosphate dihydrate with a carbon source at high temperature, eliminating the need for separate high-temperature drying of iron phosphate dihydrate to prepare iron phosphate. This method removes water of crystallization while increasing the graphitization degree of the coated carbon. Adding an appropriate amount of dispersant during sand milling not only improves the grinding efficiency but also ensures more uniform mixing of the carbon source and iron phosphate, avoiding problems such as impure phase and uneven carbon coating in the finished product caused by uneven mixing in conventional methods.

[0032] 2. The preparation method of the doped multilayer coated iron phosphate-based sodium-ion battery cathode material provided by the present invention utilizes in-situ grown Ti3C2 and AlF3 nanosheets for coating, and forms a three-dimensional structure after sintering. On the one hand, Ti3C2 can improve the electronic conductivity of the material, effectively prevent the material from agglomerating during drying and sintering, and accelerate ion transport. On the other hand, the coating of AlF3 can reduce the loss of sodium ions during charging and discharging. In addition, the doping of lithium further improves the capacity, cycle life, rate performance and other properties of the material.

[0033] 3. The preparation method of the doped multilayer coated iron phosphate-based sodium-ion battery cathode material provided by the present invention has high reproducibility, low raw material cost, and environmental friendliness, and has good application prospects in the fields of power supply and chemical energy storage for electric two-wheeled, three-wheeled and four-wheeled vehicles. Attached Figure Description

[0034] Figure 1 The image shows a SEM image of the sodium fluorophosphate cathode material with single carbon coating prepared in Comparative Example 1.

[0035] Figure 2 The image shows a SEM image of the Ti3C2 nanosheets, AlF3, and carbon multilayer coated sodium fluorophosphate cathode material prepared in Example 2.

[0036] Figure 3 The images show the XRD patterns of sodium fluorophosphate cathode materials prepared in Examples 1, 2, 1, and 2.

[0037] Figure 4 The figures show the charge-discharge curves of batteries obtained using sodium iron difluorophosphate cathode materials in Examples 1, 2, 1, and 2. Detailed Implementation

[0038] This invention provides a doped multilayer coated iron phosphate-based sodium-ion battery cathode material and its preparation method, comprising the following steps: ball milling and mixing an iron source, a carbon source, a dispersant, and a solvent to obtain a precursor slurry; vacuum drying and grinding followed by high-temperature sintering in a tube furnace under an inert atmosphere to obtain Fe3(PO4)2@C; mixing Fe3(PO4)2@C with Ti3AlC2, a sodium source, a lithium source, a fluorine source, and an acid, and using acid etching to grow Ti3C2 and AlF3 in situ on Fe3(PO4)2@C to form a coating; vacuum drying and grinding followed by sintering in a tube furnace under an inert atmosphere; the powder obtained after grinding is the multilayer coated iron phosphate sodium-ion battery cathode material consisting of Ti3C2 nanosheets, AlF3, and carbon. This method involves high-temperature pre-calcining of iron phosphate dihydrate with a carbon source to increase the graphitization degree of the coated carbon. In-situ grown Ti3C2 and AlF3 nanosheets are used for coating, and some lithium doping is introduced. After sintering, a three-dimensional structure is formed, which can improve the electronic conductivity of the material, effectively prevent the material from agglomerating during drying and sintering, accelerate ion transport, and reduce the loss of sodium ions during charging and discharging, thereby improving electrochemical performance.

[0039] The present invention will now be described in further detail with reference to specific embodiments. The given embodiments are merely illustrative of the invention and not intended to limit its scope. The embodiments provided below can serve as a guide for further improvements by those skilled in the art and do not constitute a limitation on the invention in any way.

[0040] Unless otherwise specified, the experimental methods used in the following examples are conventional methods, performed according to the techniques or conditions described in the literature in this field or according to the product instructions. Unless otherwise specified, the materials and reagents used in the following examples are commercially available.

[0041] Example 1, Na 1.9 Li 0.1 Preparation of FePO4F@C / Ti3C2 / AlF3

[0042] This invention provides an iron-based polyanionic sodium-ion battery cathode material and its preparation method, the preparation process of which includes the following steps:

[0043] 1) Weigh out 0.12 mol of ferric phosphate dihydrate and 0.05 mol of glucose;

[0044] 2) Add the weighed raw materials from step 1) to deionized water at a solid content of 30% and mix evenly. Add 0.5% polyvinylpyrrolidone dispersant (based on the mass of ferric phosphate dihydrate), stir to remove gas from the slurry, add zirconium balls at a ball-to-material ratio of 6:1, and mill the ball at 600 rpm for 8 hours to obtain slurry A.

[0045] 3) Place the slurry A from step 2) in a vacuum oven at 80°C for 12 hours, grind it in a mortar for 30 minutes, place it in a tube furnace, and sinter it under a nitrogen atmosphere. The sintering temperature and sintering time are 650°C and 15 hours, respectively. Grind it in a mortar for 30 minutes to obtain material B.

[0046] 4) Disperse material B from step 3) with 0.00288 mol Ti3AlC2, 0.11664 mol NaF, 0.012 mol LiF, and 0.05568 mol Na2CO3 into 50 ml of 9M hydrochloric acid, and obtain slurry C by sonication (the frequency and time of the sonication are 30 Hz and 1 h, respectively) and stirring (the frequency and time of the stirring are 200-300 rpm and 24 h, respectively).

[0047] 5) Place the slurry C from step 4) in a vacuum oven at 80°C for 12 hours, grind it in a mortar for 30 minutes, place it in a tube furnace, and sinter it under a nitrogen atmosphere. The heating rate is 2°C / min, and the sintering temperature and time are 600°C and 15 hours, respectively. Grind it in a mortar for 30 minutes to obtain material D, which is Na. 1.9 Li 0.1 FePO4F@C / Ti3C2 / AlF3.

[0048] Figure 3 The image shows the XRD pattern of the target phase. As can be seen from the image, the target phase is relatively pure, with a strong main peak, no obvious impurity peaks, and good crystallinity.

[0049] According to the prepared Na 1.9 Li 0.1 FePO4F@C / Ti3C2 / AlF3 positive electrode material, SP conductive agent, and PVDF binder were weighed and mixed in a mass ratio of 8:1:1. At the same time, N-methyl-2-pyrrolidone (NMP) was used as a dispersant to prepare the slurry. The slurry was coated on aluminum foil to form a positive electrode sheet, a sodium metal sheet as the negative electrode sheet, a polypropylene microporous membrane as the separator, and a 1 mol / L NaClO4 solution as the electrolyte. The CR2032 coin cell was fabricated in an argon protective atmosphere.

[0050] The prepared button cells were subjected to charge-discharge tests at room temperature, with a voltage range of 1.5 to 4.0V.

[0051] Figure 4 This is the initial charge-discharge curve, where Na... 1.9 Li 0.1The FePO4F@C / Ti3C2 / AlF3 cathode material exhibits an initial charge capacity of 99.3 mAh / g at 0.2C, an initial discharge capacity of 100.0 mAh / g, a discharge capacity of 100.68 mAh / g at 0.2C, a discharge capacity of 95 mAh / g at 0.5C, and a discharge capacity of 91.5 mAh / g at 1C. This indicates that the introduction of lithium and the coating of Ti3C2 and AlF3 improve the initial charge and rate performance of the material.

[0052] Example 2, Na 1.8 Li 0.2 Preparation of FePO4F@C / Ti3C2 / AlF3-650

[0053] 1) Weigh out 0.12 mol of ferric phosphate dihydrate and 0.05 mol of glucose;

[0054] 2) Add the weighed raw materials from step 1) to deionized water at a solid content of 30% and mix evenly. Add 0.5% polyvinylpyrrolidone dispersant (based on the mass of ferric phosphate dihydrate), stir to remove gas from the slurry, add zirconium balls at a ball-to-material ratio of 6:1, and mill the ball at 600 rpm for 8 hours to obtain slurry A.

[0055] 3) Place the slurry A from step 2) in a vacuum oven at 80°C for 12 hours, grind it in a mortar for 30 minutes, place it in a tube furnace, and sinter it under a nitrogen atmosphere. The heating rate is 2°C / min, and the sintering temperature and sintering time are 650°C and 15 hours, respectively. Grind it in a mortar for 30 minutes to obtain material B.

[0056] 4) Disperse material B from step 3) with 0.00288 mol Ti3AlC2, 0.10464 mol NaF, 0.024 mol LiF, and 0.05568 mol Na2CO3 into 50 ml of 9M hydrochloric acid, and obtain slurry C by sonication (the frequency and time of the sonication are 30 Hz and 1 h, respectively) and stirring (the frequency and time of the stirring are 200-300 rpm and 24 h, respectively).

[0057] 5) Place the slurry C from step 4) in a vacuum oven at 80°C for 12 hours, grind it in a mortar for 30 minutes, place it in a tube furnace, and sinter it under a nitrogen atmosphere. The sintering temperature and time are 650°C and 15 hours, respectively. Grind it in a mortar for 30 minutes to obtain material D, which is Na. 1.8 Li 0.2 FePO4F@C / Ti3C2 / AlF3-650.

[0058] According to the prepared Na 1.8 Li 0.2FePO4F@C / Ti3C2 / AlF3-650 positive electrode material, SP conductive agent, and PVDF binder were weighed and mixed in a mass ratio of 8:1:1. At the same time, N-methyl-2-pyrrolidone (NMP) was used as a dispersant to prepare the slurry. The mixture was coated on aluminum foil to form a positive electrode sheet, a sodium metal sheet as the negative electrode sheet, a polypropylene microporous membrane as the separator, and a 1 mol / L NaClO4 solution as the electrolyte. The CR2032 coin cell was fabricated in an argon protective atmosphere.

[0059] The prepared button cells were subjected to charge-discharge tests at room temperature, with a voltage range of 1.5 to 4.0V.

[0060] Figure 2 Na prepared for this example 1.8 Li 0.2 The SEM image of the FePO4F@C / Ti3C2 / AlF3-650 cathode material shows that the particles are all spherical, uniform in size, and without obvious agglomeration. Figure 3 The image shows the XRD pattern of the target phase, which has no impurity peaks and exhibits good crystallinity. Figure 4 This is the initial charge-discharge curve, where Na... 1.8 Li 0.2 The FePO4F@C / Ti3C2 / AlF3-650 cathode material exhibits an initial charge capacity of 102.1 mAh / g at 0.2C, an initial discharge capacity of 101.5 mAh / g, a discharge capacity of 101.1 mAh / g at 0.2C, a discharge capacity of 102.1 mAh / g at 0.5C, and a discharge capacity of 101.1 mAh / g at 1C. This indicates that appropriately increasing the introduction of lithium elements, along with the coating of Ti3C2 and AlF3, further improves the material's initial charge and rate performance.

[0061] Comparative Example 1: Preparation of Na2FePO4F@C

[0062] This invention provides a single carbon-coated sodium fluorophosphate cathode material and its preparation method, the preparation process of which includes the following steps:

[0063] 1) Weigh out 0.12 mol of ferric phosphate dihydrate and 0.05 mol of glucose;

[0064] 2) Add the weighed raw materials from step 1) to deionized water at a solid content of 30% and mix evenly. Add 0.5% polyvinylpyrrolidone dispersant (based on the mass of ferric phosphate dihydrate), stir to remove gas from the slurry, add zirconium balls at a ball-to-material ratio of 6:1, and mill the ball at 600 rpm for 8 hours to obtain slurry A.

[0065] 3) Place the slurry A from step 2) in a vacuum oven at 80°C for 12 hours, grind it in a mortar for 30 minutes, place it in a tube furnace, and sinter it under a nitrogen atmosphere. The sintering temperature and sintering time are 650°C and 15 hours, respectively. Grind it in a mortar for 30 minutes to obtain material B.

[0066] 4) Disperse material B from step 3) with 0.12 mol NaF and 0.06 mol Na2CO3 into deionized water, and obtain slurry C by ultrasonication (the frequency and time of the ultrasonication are 30 Hz and 1 h, respectively) and stirring (the frequency and time of the stirring are 200-300 rpm and 24 h, respectively);

[0067] 5) Place the slurry C from step 4) in a vacuum oven at 80°C for 12 hours, grind it in a mortar for 30 minutes, place it in a tube furnace, and sinter it under a nitrogen atmosphere. The heating rate is 2°C / min, and the sintering temperature and sintering time are 600°C and 15 hours, respectively. Grind it in a mortar for 30 minutes to obtain material D, which is Na2FePO4F@C.

[0068] The prepared Na2FePO4F@C positive electrode material, SP conductive agent, and PVDF binder were weighed and mixed in a mass ratio of 8:1:1. At the same time, N-methyl-2-pyrrolidone (NMP) was used as a dispersant to prepare the slurry. The slurry was coated on aluminum foil to form a positive electrode sheet, a negative electrode sheet of metallic sodium, a polypropylene microporous membrane as the separator, and a 1 mol / L NaClO4 solution as the electrolyte. The CR2032 type coin cell was fabricated in an argon protective atmosphere.

[0069] The prepared button cells were subjected to charge-discharge tests at room temperature, with a voltage range of 1.5 to 4.0V.

[0070] Figure 1 The SEM image of the Na2FePO4F@C cathode material prepared in this example shows that its morphology is irregular, its distribution is uneven, and it has obvious agglomeration. Figure 3 The XRD pattern shows obvious impurity peaks, indicating that uneven mixing may have led to incomplete reaction during sintering, resulting in the formation of impurity phases. Figure 4 The first charge / discharge curves show the following values: 91.4 mAh / g for the first charge at 0.2C, 102.9 mAh / g for the first discharge, 102.6 mAh / g for the discharge at 0.2C, 90.4 mAh / g for the discharge at 0.5C, and 78.0 mAh / g for the discharge at 1C.

[0071] Comparative Example 2, Na 1.8 Li 0.2 Preparation of FePO4F@C / Ti3C2 / AlF3-700

[0072] 1) Weigh out 0.12 mol of ferric phosphate dihydrate and 0.05 mol of glucose;

[0073] 2) Add the weighed raw materials from step 1) to deionized water at a solid content of 30% and mix evenly. Add 0.5% polyvinylpyrrolidone dispersant (based on the mass of ferric phosphate dihydrate), stir to remove gas from the slurry, add zirconium balls at a ball-to-material ratio of 6:1, and mill the ball at 600 rpm for 8 hours to obtain slurry A.

[0074] 3) Place the slurry A from step 2) in a vacuum oven at 80°C for 12 hours, grind it in a mortar for 30 minutes, place it in a tube furnace, and sinter it under a nitrogen atmosphere. The heating rate is 2°C / min, and the sintering temperature and sintering time are 700°C and 15 hours, respectively. Grind it in a mortar for 30 minutes to obtain material B.

[0075] 4) Disperse material B from step 3) with 0.00288 mol Ti3AlC2, 0.10464 mol NaF, 0.024 mol LiF, and 0.05568 mol Na2CO3 into 50 ml of 9M hydrochloric acid, and obtain slurry C by sonication (the frequency and time of the sonication are 30 Hz and 1 h, respectively) and stirring (the frequency and time of the stirring are 200-300 rpm and 24 h, respectively).

[0076] 5) Place the slurry C from step 4) in a vacuum oven at 80°C for 12 hours, grind it in a mortar for 30 minutes, place it in a tube furnace, and sinter it under a nitrogen atmosphere. The sintering temperature and time are 700°C and 15 hours, respectively. Grind it in a mortar for 30 minutes to obtain material D, which is Na. 1.8 Li 0.2 FePO4F@C / Ti3C2 / AlF3-700.

[0077] According to the prepared Na 1.8 Li 0.2 FePO4F@C / Ti3C2 / AlF3-700 positive electrode material, SP conductive agent, and PVDF binder were weighed and mixed in a mass ratio of 8:1:1. At the same time, N-methyl-2-pyrrolidone (NMP) was used as a dispersant to prepare the slurry. The mixture was coated on aluminum foil to form a positive electrode sheet, a sodium metal sheet as the negative electrode sheet, a polypropylene microporous membrane as the separator, and a 1 mol / L NaClO4 solution as the electrolyte. The CR2032 coin cell was fabricated in an argon protective atmosphere.

[0078] The prepared button cells were subjected to charge-discharge tests at room temperature, with a voltage range of 1.5 to 4.0V.

[0079] Figure 3 The XRD pattern of the target phase shows that the impurity peaks are significantly stronger. Figure 4This is the initial charge-discharge curve, where Na... 1.8 Li 0.2 The FePO4F@C / Ti3C2 / AlF3-700 cathode material exhibits an initial charge capacity of 65.8 mAh / g at 0.2C, an initial discharge capacity of 63.5 mAh / g, a discharge capacity of 60.6 mAh / g at 0.2C, a discharge capacity of 53.7 mAh / g at 0.5C, and a discharge capacity of 44.9 mAh / g at 1C. This indicates that increasing the secondary sintering temperature significantly affects the phase composition and coin cell performance of the material, and the secondary sintering temperature should not be too high.

[0080] Table 1 shows the charge-discharge data of batteries obtained from sodium iron fluorophosphate cathode materials in Examples 1, 2, 3, and 4. In the table below: Initial efficiency % = Initial discharge capacity / Initial charge capacity × 100%; 0.2C discharge specific capacity: the discharge specific capacity obtained by performing a charge-discharge test at 0.2C after the initial 0.2C test; 0.5C discharge specific capacity: the discharge specific capacity obtained by performing a charge-discharge test at 0.5C after the initial 0.2C test; 1C discharge specific capacity: the discharge specific capacity obtained by performing a charge-discharge test at 1C after the initial 0.5C test.

[0081] Table 1

[0082]

[0083] The above description of the preferred embodiments is quite detailed, but it should not be considered as a limitation on the scope of protection of this invention. Those skilled in the art, under the guidance of this invention, can make substitutions or modifications without departing from the scope of protection of the claims of this invention, and all such substitutions or modifications fall within the scope of protection of this invention. The scope of protection of this invention should be determined by the appended claims.

Claims

1. A method for preparing a doped multilayer coated iron phosphate-based sodium-ion battery cathode material, comprising the following steps: a) Ball milling and mixing ferric phosphate dihydrate, carbon source, dispersant and solvent to obtain precursor slurry A; b) After vacuum drying and grinding of the precursor slurry A, it is sintered at high temperature under an inert atmosphere, and after grinding, material B, namely Fe3(PO4)2@C, is obtained. c) The material B is mixed with Ti3AlC2, sodium source, lithium source, fluorine source and acid, and after ultrasonication and stirring, Ti3C2 and AlF3 are grown in situ on Fe3(PO4)2@C to form a coating by acid etching, so as to obtain slurry C; d) After vacuum drying and grinding of the material C, sintering is carried out under an inert atmosphere. After grinding, Ti3C2 nanosheets, AlF3 and carbon multilayer coated sodium fluorophosphate are obtained, which is the doped multilayer coated sodium phosphate-based cathode material.

2. The preparation method according to claim 1, characterized in that: In step a), the carbon source is selected from at least one of glucose, citric acid, sucrose, and ascorbic acid; Alternatively, the dispersant is selected from at least one of sodium hexametaphosphate, polyvinylpyrrolidone, and polyacrylamide; Alternatively, the solvent may be selected from at least one of water, anhydrous ethanol, and acetone.

3. The preparation method according to claim 1 or 2, characterized in that: In step a), the solid content of the precursor slurry A is between 20% and 50%. Alternatively, the amount of the dispersant added is 0.1% to 1% of the mass of ferric phosphate dihydrate; Alternatively, the ball milling is carried out in a ball mill, the ball-to-material mass ratio is between 5 and 10, the rotation speed of the ball mill is between 300 and 800 rpm, and the ball milling time is between 4 and 18 hours.

4. The preparation method according to claim 1, characterized in that: In step b), the drying temperature is between 50 and 90°C, and the drying time is between 8 and 24 hours. Alternatively, in step b), the grinding is performed once using a mortar and pestle or a grinder; the grinding time is between 20 and 40 minutes. Alternatively, in step b), the sintering temperature is between 650 and 800°C, and the sintering time is between 10 and 18 hours.

5. The preparation method according to claim 1, characterized in that: In step c), the sodium source includes at least one of sodium fluoride, sodium oxalate, sodium carbonate, sodium dihydrogen phosphate, sodium phosphate, sodium citrate, and sodium acetate. Alternatively, the lithium source may be lithium fluoride; Alternatively, the fluorine source may include sodium fluoride and lithium fluoride; Alternatively, the acid is hydrochloric acid, and the molar concentration of the hydrochloric acid is in the range of 8 to 10 mol / L; Alternatively, the molar ratio of sodium ions in Ti3AlC2 to sodium source is 1:60 to 100; the molar ratio of Ti3AlC2 to ferric phosphate dihydrate is between 1:30 and 50. Alternatively, the temperature for both ultrasound and stirring is 30–40°C; the frequency and duration of the ultrasound used are 30 Hz and 1 h, respectively. Alternatively, the stirring frequency and duration are 200–300 rpm and 24 h, respectively.

6. The preparation method according to claim 5, characterized in that: The molar ratio of Ti3AlC2 to lithium fluoride is 1:3 to 10; the molar ratio of sodium fluoride to iron phosphate dihydrate in step a) is 0.8 to 1:

1.

7. The preparation method according to claim 1, characterized in that: In step d), the sintering temperature is between 500 and 650°C, and the sintering time is between 10 and 18 hours. Alternatively, in step d), the drying temperature is between 50 and 90°C, and the drying time is between 8 and 24 hours. Alternatively, the grinding may be carried out in one pass using a mortar and pestle or a grinder; the grinding time may be between 20 and 40 minutes.

8. The doped multilayer coated iron phosphate-based sodium-ion battery cathode material prepared by the method of any one of claims 1-7.

9. A positive electrode sheet comprising the doped multilayer coated iron phosphate-based sodium-ion battery positive electrode material as described in claim 8.

10. A sodium-ion battery, comprising the positive electrode sheet as described in claim 9.

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