A sodium-ion battery cathode material Na 2-x FePO4F 1-x , its preparation method and application

Abstract

The present invention belongs to the technical field of sodium-ion batteries, and more specifically, relates to a sodium-ion battery cathode material Na 2‑ x FePO4F 1‑x , its preparation method and application. The present invention provides a sodium-ion battery cathode material Na 2‑x FePO4F 1‑x , where the value range of x is 0.02 ≤ x ≤ 0.08. By dissolving a phosphorus source, an iron source, a sodium source, a structure regulator, and a carbon source in water to form a mixed solution, performing hydrothermal reaction after sanding, drying to obtain a precursor material, and finally sintering to obtain the Na 2‑x FePO4F 1‑x cathode material. The present invention provides a preparation technology for a cathode material with low cost, high safety, long cycle life, and high initial efficiency. The prepared cathode material has extremely wide application value and is a battery material with great market prospects, especially bringing hope to the market of energy storage materials.

Description

Technical Field

[0001] The present invention belongs to the technical field of sodium ion batteries, and more specifically, relates to a sodium ion battery positive electrode material Na 2- x FePO4F 1-x , its preparation method and application. Background Art

[0002] In 2022, global lithium mines continued to rise, reaching as high as 600,000 yuan / ton. Although lithium mines have experienced a parabolic plunge since 2023 and have now fallen to nearly 100,000 yuan / ton, they are still much higher than sodium ore resources represented by sodium carbonate. It is worth mentioning that as elements of the same main group, sodium and lithium have very similar physical and chemical properties, but their resource distribution is very different. The crustal abundance of sodium resources is 2.75%, while lithium resources are only 0.0065%. With the rapid development of new energy vehicles and energy storage materials, the resource-rich sodium-ion battery industry chain will usher in new opportunities and challenges. At the same time, many studies have found that sodium-ion batteries and lithium-ion batteries are very similar in working principles, key technologies, etc. Therefore, under the policy guidance of the country's strong support and development of energy storage materials, sodium-ion batteries with abundant resources, low costs and higher safety performance have attracted much attention. In particular, in the past two years, many scientific researchers have focused on sodium-ion batteries with abundant resources, lower costs, stable electrochemical properties and higher safety performance. It is expected that sodium-ion batteries will be widely used in energy storage batteries, base station backup power supplies, low-speed four-wheel vehicles, electric two-wheel vehicles and other fields, forming a complementary pattern with lithium batteries.

[0003] However, although sodium-ion batteries have received much attention, their larger ionic radius will inevitably inhibit the sodium ion deintercalation rate during the charge and discharge process, thereby reducing the chemical reaction kinetic rate. In addition, in order to further improve the cycle stability of sodium-ion batteries, it is necessary not only to consider the effect of the larger ionic radius on the ion transport of battery materials, but also to analyze the phase change problem of battery materials during the charge and discharge process, as well as structural deformation, etc. As we all know, the positive electrode materials of sodium-ion batteries mainly include layered oxide compounds, Prussian blue compounds and polyanion compounds. Although layered oxide positive electrode materials have a significant advantage of high specific capacity, poor cycle performance is their pain point; as Prussian blue compounds with higher specific capacity, they are extremely toxic, which also restricts the application of this type of battery materials. In comparison, polyanion positive electrode materials are slightly insufficient in specific capacity, but because of their long cycle life and safety and stability, they will definitely have a place in the energy storage market.

[0004] By consulting a large number of literatures, it has been reported in existing literatures that problems such as specific capacity, rate performance, and cycle stability can be solved by methods such as cation doping and carbon coating. Although sodium vanadate phosphate, which has attracted much attention from researchers, has good cycle performance, the toxicity of the material itself cannot be ignored. In recent years, sodium iron phosphate, which has low raw material cost and extremely wide sources, has become the focus of research. For example, the sodium iron phosphate cathode material synthesized by Zhou et al. using the high-temperature solid-state method was coated with sucrose as a carbon source. The first discharge capacity of the material after carbon coating reached 121 mAh / g at 0.05. In addition, Rui et al. reported a carbon-coated rod-shaped Na2FePO4F / C cathode material synthesized by the solvothermal method, and the initial discharge specific capacity at 0.1 C was 114.3 mAh / g. At the same time, Gui et al. also reported a Na2FePO4F material synthesized by the rheological phase method, and the discharge specific capacity at 0.1 C was 110 mAh / g. At the same time, there is a publicly available patent literature on the cathode material Na2FePO4F, and the first-cycle discharge specific capacity at a rate of 0.1 C is 120 mAh / g. It can be seen that most of the reported Na2FePO4F cathode materials are the charge-discharge specific capacities at 0.1 C. It can be seen that its synthesis technology still has some deficiencies, and the specific capacity, rate performance, cycle stability, and safety all need to be improved. More importantly, the most prominent problems of the reported Na2FePO4F cathode materials are low first efficiency and unstable structure. And these problems are crucial factors affecting the cycle stability and specific capacity of the cathode material. Therefore, it is urgent to develop a technology that can regulate the structure of battery materials to solve the problems of low first efficiency of charge and discharge, unstable structure, and poor cycle performance of the materials. Furthermore, a green, low-cost and long-cycle sodium-ion battery cathode material is synthesized and applied to energy storage materials, low-speed electric vehicles and other fields. Summary of the Invention

[0005] Aiming at the defects of the existing technology, the purpose of the present invention is to provide a sodium-ion battery cathode material Na 2- x FePO4F 1-x , its preparation method and application, aiming to solve the technical problems of low first efficiency, poor cycle stability and specific capacity of the existing Na2FePO4F cathode material.

[0006] To achieve the above purpose, the present invention provides a sodium-ion battery cathode material, and its chemical formula is Na 2- x FePO4F 1-x , where the value range of x is 0.02 ≤ x ≤ 0.08.

[0007] According to another aspect of the present invention, a preparation method of the above-mentioned cathode material is provided, including the following steps:

[0008] (1) Mix a phosphorus source, an iron source, a sodium source, a carbon source and a structure regulator with deionized water to form a mixed solution, and obtain a mixed salt system after sand milling; the structure regulator is sodium fluoride; the molar ratio of iron element in the iron source to fluorine element in the structure regulator is 1:0.92 - 0.98;

[0009] (2) Carry out a hydrothermal reaction on the mixed salt system described in step (1);

[0010] (3) Dry the mixed salt system after the hydrothermal reaction described in step (2) to obtain a precursor of the positive electrode material for a sodium ion battery;

[0011] (4) Sinter the precursor of the positive electrode material for a sodium ion battery described in step (3) to obtain the positive electrode material for a sodium ion battery.

[0012] Preferably, the particle size of the mixed salt in the mixed salt system described in step (1) is 0.1 - 10 μm.

[0013] Preferably, the molar ratio of the sodium source, the iron source, and the phosphorus source described in step (1) is 2:1:1 - 1:1:1, and the molar ratio of iron element in the iron source to fluorine element in the structure regulator is 1:0.94 - 0.96.

[0014] According to another aspect of the present invention, a sodium ion battery is provided, which includes a positive electrode, a negative electrode and a separator, wherein the active material of the positive electrode includes the positive electrode material Na 2-x FePO4F 1-x .

[0015] Generally speaking, compared with the prior art through the above technical solutions conceived by the present invention, the following technical advantages are mainly possessed:

[0016] (1) The present invention provides a positive electrode material Na 2-x FePO4F 1-x for a sodium ion battery, where the value range of x is 0.02 ≤ x ≤ 0.08. By regulating the ratio of fluorine atoms to iron atoms in the chemical formula of the positive electrode material, the structure of the positive electrode material is regulated. Experiments prove that when the ratio of fluorine atoms to iron atoms is less than 1, within a certain range, the first reversible capacity and cycle stability of the positive electrode material are both improved to a large extent.

[0017] (2) The present invention dissolves a phosphorus source, an iron source, a sodium source and a carbon source in water in a certain proportion, and prepares a precursor material of the positive electrode for a sodium ion battery through sand milling and then hydrothermal reaction, and finally obtains Na 2-x FePO4F 1-xCathode material. During the preparation process, by controlling the fluorine source within an appropriate range, the obtained cathode material has significantly improved initial reversible capacity and cycle stability compared to the existing NaFePO4F.

[0018] (3) The present invention mainly improves the initial efficiency and cycle stability by regulating the structure of the sodium-ion battery cathode material Na 2-x FePO4F 1-x . The sodium-ion battery cathode material prepared in the preferred embodiment of the present invention is assembled into a battery. At 1C, the initial reversible discharge capacity can reach 123.04 mAh / g, and the initial cycle efficiency is 96.03%, which is close to the theoretical specific capacity. After 100 cycles, the specific capacity is still 113.02 mAh / g, and the capacity retention rate is 91.86%. Thus, both the initial efficiency and cycle performance are significantly improved. Therefore, the prepared sodium-ion battery cathode material Na 2-x FePO4F 1-x of the present invention has great application value, will be an excellent energy storage material, and is expected to become the leader of the new generation of energy storage materials.

[0019] (4) The prepared adjustable structure cathode material Na 2-x FePO4F 1-x of the present invention has a wide range of raw material sources, a simple synthesis technical route, and is more conducive to industrial production.

[0020] In summary, the prepared adjustable structure sodium-ion battery cathode material Na 2-x FePO4F 1-x of the present invention has the advantages of a simple synthesis technical route, low cost, high initial charge-discharge efficiency, stable structure, good safety, good cycle stability, and can be applied to an aqueous solution system, etc. In view of this, it can be speculated that the technology of the adjustable battery material structure proposed in this patent is expected to be popularized and applied in the field of chemical power source technology. Brief Description of the Drawings

[0021] Figure 1 is the structural diagram obtained from the unit cell parameters of the sodium-ion battery cathode material Na 2-x FePO4F 1-x obtained in Example 1 of the present invention;

[0022] Figure 2 is the X-ray phase analysis diagram of the sodium-ion battery cathode material Na 2-x FePO4F 1-x obtained in Example 1 of the present invention;

[0023] Figure 3 is the sodium-ion battery cathode material Na 2-x FePO4F 1-xSEM images;

[0024] Figure 4 The cathode materials Na 2-x FePO4F 1-x obtained in Example 1 and Comparative Example 1 of the present invention are the charge-discharge curves at a rate of 1C;

[0025] Figure 5 The cathode materials Na 2-x FePO4F 1-x obtained in Example 1 and Comparative Example 1 of the present invention are the cycle stability diagrams at a rate of 1C. Detailed implementation manners

[0026] The present invention focuses on regulating the structure of battery materials, aiming to improve the initial efficiency and cycle stability of polyanion cathode materials represented by Na 2-x FePO4F 1-x . Existing research results show that for the iron-based sodium-ion battery cathode material with fluorophosphate, its theoretical charge-discharge platform is 3.0 V. Based on the Fe 2+ / Fe 3+ redox reaction, its highest discharge specific capacity can reach 124 mAh / g through calculation. However, the highest discharge specific capacity of the sodium iron fluorophosphate cathode material prepared by the existing technology is still far lower than this theoretical maximum value. For example, the highest initial reversible capacity of the Na2FePO4F cathode material prepared by the inventor's prior patent application CN116081594A is only 110 mAh / g. Although the volume change rate of such materials is relatively low, with the increase of the number of cycles, the specific capacity decays, and the initial efficiency is low. But if a series of problems of the initial efficiency and cycle performance of these materials can be solved, the iron-based cathode material with fluoropolyphosphate will be one of the most potential energy storage materials to replace the current lithium battery system.

[0027] Therefore, the present invention provides a sodium battery cathode material with the chemical formula Na 2-x FePO4F 1-x , where the value range of x is 0.02 ≤ x ≤ 0.08. A technology for regulating the structure of the fluoropolyphosphate-based iron-based sodium-ion battery cathode material Na 2- x FePO4F 1-x provided by the present invention includes the following steps:

[0028] (1) Mix a phosphorus source, an iron source, a sodium source, a carbon source and a structure regulator with deionized water to form a mixed solution, and obtain a mixed salt system after sanding; the structure regulator is sodium fluoride; the molar ratio of iron element in the iron source to fluorine element in the structure regulator is 1:0.92 - 0.98,

[0029] (2) Hydrothermally react the mixed salt system described in step (1) at a certain temperature;

[0030] (3) Dry the mixed salt after the hydrothermal reaction described in step (2) to obtain a sodium-ion battery cathode precursor;

[0031] (4) Sinter the sodium-ion battery cathode precursor described in step (3) to obtain a cathode material Na 2-x FePO4F 1-x ..

[0032] The sodium salt described in step (1) is sodium acetate, sodium carbonate, sodium oxalate, sodium chloride, sodium nitrate or sodium citrate; the iron source is iron phosphate, iron oxalate or ferrous oxalate; the carbon source is sucrose, oxalic acid, citric acid, maltose, fructose or glucose; the phosphorus source is iron phosphate, diammonium hydrogen phosphate, ammonium dihydrogen phosphate, ammonium phosphate or phosphoric acid; the mass ratio of the solute to deionized water in the mixed salt system is 30-70:100, preferably 30-50:100, where the solute includes the phosphorus source, iron source, sodium source, carbon source and structure regulator.

[0033] In some embodiments, the molar ratio of the sodium source, iron source, and phosphorus source in step (1) is 2:1:1 to 1:1:1, and the molar ratio of the iron element in the iron source to the fluorine element in the structure regulator is 1:0.92 to 0.98, more preferably 1:0.94 to 0.96.

[0034] In a preferred embodiment, the iron source is iron phosphate dihydrate, the structure regulator is sodium fluoride, and the molar ratio of the iron element in the iron source to the fluorine element in the structure regulator is 1:0.94 to 0.96.

[0035] In some embodiments of the present invention, iron phosphate dihydrate is used as the iron source. Iron phosphate dihydrate can be obtained by commercial purchase or self-preparation. The FePO4·2H2O powder used in the present invention has a particle size of 0.1~10 μm.

[0036] In some embodiments, in step (1), the particle size of the mixed salt in the mixed salt system is 0.1~10 μm by sand grinding.

[0037] In a preferred embodiment, in order to promote the dissolution of the sodium source, carbon source, etc., the mixed solution obtained by mixing is left standing overnight to promote its dissolution for subsequent reactions.

[0038] In some embodiments, the hydrothermal reaction temperature in step (2) is 60~120 °C, the reaction time is 10~30 h. For a large-volume stirring kettle, a stirring device is also set, and the stirring rate is 100~1500 rpm.

[0039] In some embodiments, the drying in step (3) is spray drying; spray drying has the effect of granulation and can also control the particle size of the powder. The particle size of the dried cathode material precursor of the present invention is 0.1 - 10 μm. In some embodiments, the outlet temperature during spray drying is 100 - 180 °C, the air intake is 1 - 10 m 3 / min, and the feed rate is 200 - 1000 ml / h. In some embodiments, the precursor obtained by spray drying has a D 50 of 3 - 5 μm.

[0040] In some embodiments, the sintering conditions in step (4) are: sintering at 200 - 900 °C for 4 - 25 h in an inert atmosphere to obtain the sodium ion battery cathode material. Preferably, pre-sintering is first performed at 200 - 450 °C for 1 - 8 h in an inert atmosphere, and then secondary sintering is performed at 500 - 900 °C for 3 - 15 h to obtain the sodium ion battery cathode material.

[0041] In some embodiments, the inert atmosphere is nitrogen, argon, nitrogen-hydrogen mixed atmosphere, argon-hydrogen mixed atmosphere or vacuum atmosphere.

[0042] The synthesis of the present invention obtains a cathode material Na 2-x FePO4F 1-x with a controllable structure, which can be used as the positive active material of a sodium ion battery. The present invention also provides a sodium ion battery, comprising a positive electrode, a negative electrode and a separator, wherein the active material of the positive electrode comprises the cathode material Na 2-x FePO4F 1-x of the present invention.

[0043] The present invention discloses a technology for improving the initial efficiency and cycle stability of the cathode material Na 2-x FePO4F 1-x by regulating the structure. The use of this technology can promote the rapid development of China's electrochemical energy storage technology, greatly alleviate the problem of the lag in the development of energy storage batteries caused by the shortage of lithium resources, and is expected to be a useful supplement to lithium ion batteries. During the preparation process of the cathode material of the present invention, the addition of sodium fluoride not only introduces fluoride ions with strong electronegativity and uses its induction effect to improve the electrochemical performance of the prepared active material; but also adds it as a structure regulator to the reaction system, and by changing the dosage of sodium fluoride, the bond length of Na2-O in the cathode material is regulated and increased to improve the sodium ion transport rate and solve the problems of low initial efficiency and poor cycle performance during the charge and discharge process.

[0044] Specific implementation process of some embodiments: Add a certain amount of deionized water into a plastic bucket, and then sequentially add a certain amount of FePO4·2H2O, Na2CO3, NaF and sucrose. Add the materials while stirring, mix evenly, and let stand overnight. Grind under certain conditions, and after grinding, place it in a hydrothermal reactor for constant-temperature hydrothermal reaction to obtain the cathode material Na 2-x FePO4F 1-x precursor; then dilute the precursor with deionized water in a certain proportion and spray-dry it under certain conditions. Then, place the dried precursor in a box furnace and sinter it at different temperatures in an inert atmosphere to obtain the cathode material Na 2- x FePO4F 1-x . The preparation method in the present invention has a simple synthesis process and is easy to industrialize production.

[0045] To make the purpose, technical solution and advantages of the present invention clearer, the technical solutions in the embodiments of the present invention will be clearly and completely described below in conjunction with the embodiments of the present invention. Those skilled in the art should understand that the embodiments are only for helping to understand the present invention and should not be regarded as specific limitations on the present invention. Based on the embodiments of the present invention, all other embodiments obtained by those of ordinary skill in the art without creative efforts fall within the scope of protection of the present invention.

[0046] The embodiments of the present invention are implemented on the premise of the technical solution of the present invention, and the detailed implementation methods and processes are given. However, the protection scope of the present invention is not limited to the following embodiments. For the process parameters without specific conditions noted in the following embodiments, they are usually in accordance with conventional conditions.

[0047] In the present invention, unless otherwise specified and / or described, throughout, all numerical values related to the component dosages are "parts by weight". For the process parameters without specific conditions noted in the following embodiments, they are usually in accordance with conventional conditions.

[0048] In the present invention, unless otherwise specified and / or described, throughout, all numerical values related to the component dosages are "parts by weight". For the process parameters without specific conditions noted in the following embodiments, they are usually in accordance with conventional conditions.

[0049] The following are the embodiments:

[0050] Example 1

[0051] A cathode material Na 2-x FePO4F 1-xPreparation method, the specific steps are as follows:

[0052] (1) Weigh 286.0 g of NaF, 1080.0 g of FePO4·2H2O (D 50 is 2 μm, the same below), 290.0 g of sucrose, 380.0 g of Na2CO3, add an appropriate amount of deionized water to obtain a mixed solution, and water accounts for 60% of the total mass of the mixed solution, and let it stand overnight. Place the mixed materials in a sand mill and stir well, and grind for 5 h at 2800 rpm to obtain a mixed salt that is uniformly mixed and has a particle size of 1-3 μm;

[0053] (2) Place the mixed salt prepared in step (1) in a hydrothermal reaction kettle, set the reaction temperature to 80 °C, the time to 16 h, and the stirring rate to 800 rpm to obtain the mixed salt after hydrothermal treatment;

[0054] (3) Spray-dry the mixed salt after the hydrothermal reaction described in step (2), with an inlet temperature of 220 °C, an outlet temperature of 120 °C, and a feed rate of 300 ml / h to obtain a positive electrode precursor for a sodium-ion battery;

[0055] (4) Place the Na 2-x FePO4F 1-x precursor in a vacuum box furnace and pre-sinter at 300 °C for 3 h and then secondary-sinter at 600 °C for 8 h to obtain the Na 2-x FePO4F 1-x positive electrode material.

[0056] Example 2

[0057] A preparation method of a positive electrode material Na 2-x FePO4F 1-x for a sodium-ion battery, the specific steps are as follows:

[0058] (1) Weigh 290.0 g of NaF, 1080.0 g of FePO4·2H2O, 330.0 g of glucose, 580.0 g of CH3COONa, add an appropriate amount of deionized water to obtain a mixed solution, and water accounts for 60% of the total mass of the mixed solution, and let it stand overnight. Place the mixed materials in a sand mill and stir well, and grind for 6 h at 2500 rpm to obtain a mixed salt that is uniformly mixed and has a particle size of 1-3 μm;

[0059] (2) Place the mixed salt prepared in step (1) in a hydrothermal reaction kettle, set the reaction temperature to 100 °C, the time to 14 h, and the stirring rate to 800 rpm to obtain the mixed salt after hydrothermal treatment;

[0060] (3) Spray dry the mixed salt after the hydrothermal reaction described in step (2) again, with an inlet temperature of 180 °C, an outlet temperature of 100 °C, and a feed rate of 280 ml / h to obtain the positive electrode precursor of the sodium-ion battery;

[0061] (4) Place the Na 2-x FePO4F 1-x precursor prepared in step (3) in a rotary tube furnace, and pre-sinter at 350 °C for 2 h and then secondary-sinter at 650 °C for 6 h under an argon atmosphere to obtain the Na 2-x FePO4F 1-x positive electrode material of the sodium-ion battery.

[0062] Example 3

[0063] A preparation method of a positive electrode material Na 2-x FePO4F 1-x of a sodium-ion battery is as follows:

[0064] (1) Weigh 280.0 g of NaF, 1660.0 g of Fe2(C2O4)3·5H2O, 940.0 g of (NH)2HPO4, 360.0 g of citric acid, and 580.0 g of CH3COONa, add an appropriate amount of deionized water to obtain a mixed solution, where water accounts for 60% of the total mass of the mixed solution, and let it stand overnight. Place the mixed materials in a sand mill and stir well, and mill at 3000 rpm for 5 h to obtain a uniformly mixed mixed salt with a particle size of 1-3 μm;

[0065] (2) Place the mixed salt prepared in step (1) in a hydrothermal reaction kettle, set the reaction temperature to 120 °C, the time to 12 h, and the stirring rate to 800 rpm to obtain the mixed salt after hydrothermal treatment;

[0066] (3) Spray dry the mixed salt after the hydrothermal reaction described in step (2) again, with an inlet temperature of 200 °C, an outlet temperature of 110 °C, and a feed rate of 250 ml / h to obtain the positive electrode precursor of the sodium-ion battery;

[0067] (4) Place the Na 2-x FePO4F 1-x precursor prepared in step (3) in a rotary tube furnace, and pre-sinter at 250 °C for 4 h and then secondary-sinter at 550 °C for 6 h under an argon atmosphere to obtain the Na 2-x FePO4F 1-x positive electrode material of the sodium-ion battery.

[0068] Example 4

[0069] A positive electrode material Na 2-xFePO4F 1-x The preparation method is as follows:

[0070] (1)Weigh 300.0 g of NaF, 1280.0 g of FeC2O4·2H2O, 330.0 g of glucose, 940.0 g of (NH)2HPO4, and 480.0 g of Na2C2O4, add appropriate amount of deionized water to obtain a mixed solution, where water accounts for 60% of the total mass of the mixed solution, and let it stand overnight. Place the mixed materials in a sand mill and stir well, grind at 1000 rpm for 8 h to obtain a uniformly mixed mixed salt with a particle size of 1 - 3 μm;

[0071] (2)Place the mixed salt prepared in step (1) in a hydrothermal reaction kettle, set the reaction temperature to 90 °C, the time to 10 h, and the stirring rate to 1000 rpm to obtain the hydrothermally treated mixed salt;

[0072] (3)Spray - dry the mixed salt after the hydrothermal reaction in step (2) again, with an inlet temperature of 240 °C, an outlet temperature of 125 °C, and a feeding rate of 350 ml / h to obtain the precursor of the positive electrode of the sodium - ion battery;

[0073] (4)Place the Na 2-x FePO4F 1-x precursor prepared in step (3) in a rotary tube furnace, pre - sinter at 200 °C for 4 h and then secondary - sinter at 500 °C for 10 h under an argon atmosphere to obtain the positive electrode material of the sodium - ion battery Na 2-x FePO4F 1-x for sodium - ion batteries.

[0074] Example 5

[0075] A preparation method of a positive electrode material Na 2-x FePO4F 1-x for sodium - ion batteries is as follows:

[0076] (1)Weigh 290.0 g of NaF, 1280.0 g of FeC2O4·2H2O, 290.0 g of sucrose, 820.0 g of NH4H2PO4, and 380.0 g of Na2CO3, add appropriate amount of deionized water to obtain a mixed solution, where water accounts for 60% of the total mass of the mixed solution, and let it stand overnight. Place the mixed materials in a sand mill and stir well, grind at 2000 rpm for 7 h to obtain a uniformly mixed mixed salt with a particle size of 1 - 3 μm;

[0077] (2)Place the mixed salt prepared in step (1) in a hydrothermal reaction kettle, set the reaction temperature to 60 °C, the time to 16 h, and the stirring rate to 800 rpm to obtain the hydrothermally treated mixed salt;

[0078] (3) Spray-dry the mixed salt after the hydrothermal reaction described in step (2) again, with an inlet temperature of 230 °C, an outlet temperature of 120 °C, and a feed rate of 320 ml / h to obtain the precursor of the positive electrode of the sodium-ion battery;

[0079] (4) Place the Na 2-x FePO4F 1-x precursor prepared in step (3) in a rotary tube furnace, and pre-sinter at 400 °C for 1 h and then secondary-sinter at 700 °C for 6 h under an argon atmosphere to obtain the 2-x FePO4F 1-x positive electrode material of the sodium-ion battery.

[0080] Example 6

[0081] A preparation method of a positive electrode material Na 2-x FePO4F 1-x of a sodium-ion battery is as follows:

[0082] (1) Weigh 280.0 g of NaF, 1080.0 g of FePO4·2H2O, 330.0 g of glucose, and 480.0 g of Na2C2O4, add an appropriate amount of deionized water to obtain a mixed solution, where water accounts for 60% of the total mass of the mixed solution, and let it stand overnight. Place the mixed materials in a sand mill and stir well, and sand mill at 1500 rpm for 7.5 h to obtain a uniformly mixed mixed salt with a particle size of 1 - 3 μm;

[0083] (2) Place the mixed salt prepared in step (1) in a hydrothermal reaction kettle, set the reaction temperature to 100 °C, the time to 12 h, and the stirring rate to 1200 rpm to obtain the mixed salt after hydrothermal treatment;

[0084] (3) Spray-dry the mixed salt after the hydrothermal reaction described in step (2) again, with an inlet temperature of 160 °C, an outlet temperature of 100 °C, and a feed rate of 240 ml / h to obtain the precursor of the positive electrode of the sodium-ion battery;

[0085] (4) Place the Na 2-x FePO4F 1-x precursor prepared in step (3) in a rotary tube furnace, and pre-sinter at 350 °C for 2 h and then secondary-sinter at 650 °C for 6 h under an argon atmosphere to obtain the 2-x FePO4F 1-x positive electrode material of the sodium-ion battery.

[0086] Comparative Example 1

[0087] (1) Weigh 300.7 g of NaF, 1080.0 g of FePO4·2H2O, 290.0 g of sucrose, and 380.0 g of Na2CO3, add an appropriate amount of deionized water to obtain a mixed solution, where water accounts for 60% of the total mass of the mixed solution, and let it stand overnight. Place the mixed materials in a sand mill and stir well. Grind at 2800 rpm for 5 h to obtain a uniformly mixed mixed salt with a particle size of 1 - 3 μm;

[0088] (2) Place the mixed salt prepared in step (1) in a hydrothermal reaction kettle, set the reaction temperature to 80 °C, the time to 16 h, and the stirring rate to 800 rpm to obtain the Na 2-x FePO4F 1-x precursor;

[0089] (3) Spray-dry the mixed salt after the hydrothermal reaction in step (2) again, with an inlet temperature of 220 °C, an outlet temperature of 120 °C, and a feeding rate of 300 ml / h to obtain the cathode precursor of the sodium-ion battery.

[0090] (4) Place the Na 2-x FePO4F 1-x precursor in a vacuum box furnace and pre-sinter it at 300 °C for 3 h and then sinter it at 600 °C for 8 h to obtain the Na 2-x FePO4F 1-x cathode material of the sodium-ion battery.

[0091] Keep other conditions the same as in Comparative Example 1, only change the amount of sodium fluoride, which is adjusted from 300.7 g to 315.7 g, 330.8 g, 345.8 g, 360.8 g, 270.6 g, 276.6 g, 282.6 g, 288.7 g, 294.7 g, 255.6 g, and 150.3 g in turn, to obtain Comparative Example 2, Comparative Example 3, Comparative Example 4, Comparative Example 5, Comparative Example 6, Example 7, Example 8, Example 9, Example 10, Comparative Example 7, Comparative Example 8, and Comparative Example 9. It should be noted here that since Comparative Example 9 has many impurity peaks and no refinement was done, there is no data corresponding to this comparative example in the following table.

[0092] After XRD refinement, the unit cell parameters of the cathode materials obtained in the examples and comparative examples before charge and discharge are specifically listed in Table 1 below.

[0093]

[0094] As can be seen from Table 1, the unit cell parameters of Example 1 and each comparative example are different. Among them, the volume of Example 1 is the largest, and the Na2-O distance is also the largest. This further indicates that different amounts of fluorine result in different Na2-O distances. The larger the Na2-O distance, the more favorable the deintercalation and intercalation of sodium ions, and the higher the specific capacity of the material.

[0095] In addition, in order to further illustrate the volume change of the cathode material after structural regulation during the charge-discharge process, in this patent, in-situ XRD tests were also carried out on the cathode materials prepared in Example 1 and Comparative Example 1, that is, the XRD of the cathode materials in Example 1 and Comparative Example 1 was tested after one charge-discharge cycle, and then the unit cell parameters before and after charge-discharge were obtained through refinement. See Table 2 for details.

[0096]

[0097] It can be easily seen from Table 2 that the unit cell parameters of Example 1 and Comparative Example 1 have changed after one week of charge-discharge. Among them, the volume change rate of Example 1 is 2.80%, and this result is significantly smaller than the volume change rate of 3.4% of the Na2Fe 0.95 Mg 0.05 PO4F@C material reported in the existing literature. The volume change rate of Comparative Example 1 is 3.68%, and this result is basically the same as the 3.70% reported in the literature. Obviously, the volume change rate of Example 1 is smaller. It can be speculated from this that the material will have better cycle stability during the charge-discharge process.

[0098] The cathode material Na 2-x FePO4F 1-x prepared in Example 1 was placed on an X-ray diffractometer, and the X-ray diffraction phase analysis of the cathode material Na 2-x FePO4F 1-x can be seen in the Figure 2 content. It can be seen that a single peak appears at 2 = 12.8°, a clearly split doublet appears at 2 = 29.9° and 2 = 30.3°. Especially, a clearly split triplet appears at 2 = 12.8°, 2 = 34.2°, 2 = 34.5° and 2 = 34.8°. These three groups of peaks are consistent with the characteristic peaks of Na2FePO4F, indicating that the Na 2-x FePO4F 1-x cathode material is obtained by this method. At the same time, by refining the XRD of this material, Na 2-x FePO4F 1-xThe structure is as Figure 1 shown in the structure, and it can be clearly seen from Figure 1 that by controlling the dosage of the structure regulator sodium fluoride, the distance between Na2-O in the structure of the cathode material is significantly increased.

[0099] The cathode material of the sodium-ion battery prepared in Example 1 was observed and processed by scanning electron microscopy, and the Figure 3 resulting SEM image is shown. It can be seen from the figure that the 2-x Na 1-x FePO4F

[0100] The cathode materials prepared in different examples and comparative examples were analyzed by XRF. By calculating the elemental stoichiometric ratio from the test results, the chemical formulas of the cathode materials corresponding to different fluorine dosages were obtained, and the results are shown in Tables 3 and 4.

[0101]

[0102]

[0103] As can be seen from Tables 3 and 4, for the cathode materials prepared in Example 1 and Examples 7 to 10, it can be known from XRF analysis that the ratio of F atoms to Fe atoms in their chemical formulas is less than 1, corresponding to 2-x Na 1-x FePO4F

[0104] The cathode material Na 2-x FePO4F 1-x prepared in Example 1 was subjected to electrochemical performance testing. Using the Na 2-x FePO4F 1-x prepared in Example 1 as the cathode active material, the electrochemical performance was tested with a blue battery. Figure 4 is the charge-discharge curve of the cathode materials of the sodium-ion batteries prepared in Example 1 and Comparative Example 1 at a 1C rate. It can be clearly seen from the figure that the initial discharge specific capacity of Example 1 is significantly higher than that of Comparative Example 1, and more importantly, the discharge platform of Example 1 is more stable. This result shows that adding an appropriate amount of structure regulator can better maintain the stability of the structure. Figure 5 is the cathode material Na 2-x FePO4F 1-xCycling stability graph at 1C rate. Tables 5 and 6 are the cathode materials for sodium-ion batteries prepared in different examples and comparative examples, Na 2-x FePO4F 1-x Cycling performance data and first-cycle efficiency data at 1C rate. It can be seen from the figure and table that the cathode material for sodium-ion battery, Na 2- x FePO4F 1-x prepared and synthesized in Example 1 has a first reversible charge and discharge specific capacity of 128.12 mAh / g and 123.04 mAh / g respectively at 1C rate, and the first-cycle efficiency is 96.03%; in Examples 7 to 10, controlling the amount of sodium fluoride within a certain range can also obtain relatively high first reversible charge and discharge specific capacities and cycling performance. The first-cycle charge and discharge specific capacities of Comparative Example 1 at 1C rate are described as 117.22 mAh / g and 109.90 mAh / g respectively, and the first-cycle efficiency is 93.76%. It can be seen from this that the first-cycle discharge capacity and first-cycle efficiency of the cathode material prepared in Example 1 are significantly better than those of Comparative Example 1, and the first-cycle discharge specific capacity is relatively close to the theoretical specific capacity. At the same time, it can also be seen from the figure that after cycling 100 times at 1C, the discharge specific capacity of Example 1 is still 113.02 mAh / g, and the capacity retention rate is 91.86%; while that of Comparative Example 1 is 93.28 mAh / g, and the capacity retention rate is 84.88%. The cycling stability data of other examples and comparative examples are listed in Tables 5 and 6 one by one. It can also be seen from the table that the cycling stability of Examples 1, 7, 8, 9 and 10 is better than that of the comparative examples. This result further proves the conclusion that an appropriate amount of structure regulator can better maintain the structural stability. Based on this, we can say that by regulating the material structure, the first-cycle efficiency of the material during charge and discharge can be significantly improved and long cycling can be achieved. It should be particularly noted here that the charge and discharge of the cathode material obtained in the present invention are all tested at 1C rate, and its so high first-cycle efficiency and long cycling performance are at the leading level in the industry. Through comparison of the test results, it is not difficult to find that the Na 2-x FePO4F 1-x cathode material prepared in the present invention has better performance in terms of specific capacity, first-cycle efficiency, cycling stability, safety and rate performance.

[0105]

[0106]

[0107] In summary, a kind of sodium-ion battery cathode material with adjustable structure, Na 2- x FePO4F 1-x, not only has the characteristics of low cost, high specific capacity and good safety, but also has the advantages of high initial efficiency and good cycle stability. Therefore, the technology for regulating the structure of battery materials provided by the present invention can be popularized and applied in the field of chemical power sources.

[0108] It is easy for those skilled in the art to understand that the above are only the preferred embodiments of the present invention and are not intended to limit the present invention. Any modifications, equivalent replacements, and improvements made within the spirit and principle of the present invention shall be included within the protection scope of the present invention.

Claims

1. A sodium ion battery positive electrode material, characterized in that: Its chemical formula is Na 2-x FePO4F 1-x , the value range of x is 0.04≤x≤0.05; The method for preparing the positive electrode material comprises the following steps: (1) A phosphorus source, an iron source, a sodium source, a carbon source and a structure regulator are mixed with deionized water to form a mixed solution, and then sand-milled to obtain a mixed salt system; the structure regulator is sodium fluoride; the molar ratio of the iron element in the iron source to the fluorine element in the structure regulator is 1:0.92 to 0.98; the molar ratio of the sodium source, the iron source and the phosphorus source is 2:1:1 to 1:1:1; (2) subjecting the mixed salt system of step (1) to a hydrothermal reaction; the hydrothermal reaction temperature is 60 to 120° C., and the reaction time is 10 to 30 h; (3) drying the mixed salt system after the hydrothermal reaction in step (2) to obtain a precursor of a positive electrode material for a sodium ion battery; (4) Sintering the sodium ion battery positive electrode material precursor of step (3) to obtain the sodium ion battery positive electrode material; the sintering conditions are: pre-sintering at 200-450°C for 1-8 h under an inert atmosphere, and then secondary sintering at 500-900°C for 3-15 h to obtain the sodium ion battery positive electrode material.

2. The method for preparing a positive electrode material for a sodium ion battery according to claim 1, wherein: The steps include: (1) A phosphorus source, an iron source, a sodium source, a carbon source and a structure regulator are mixed with deionized water to form a mixed solution, and then sand-milled to obtain a mixed salt system; the structure regulator is sodium fluoride; the molar ratio of the iron element in the iron source to the fluorine element in the structure regulator is 1:0.92 to 0.98; the molar ratio of the sodium source, the iron source and the phosphorus source is 2:1:1 to 1:1:1; (2) subjecting the mixed salt system of step (1) to a hydrothermal reaction; the hydrothermal reaction temperature is 60 to 120° C., and the reaction time is 10 to 30 h; (3) drying the mixed salt system after the hydrothermal reaction in step (2) to obtain a precursor of a positive electrode material for a sodium ion battery; (4) Sintering the sodium ion battery positive electrode material precursor of step (3) to obtain the sodium ion battery positive electrode material; the sintering conditions are: pre-sintering at 200-450°C for 1-8 h under an inert atmosphere, and then secondary sintering at 500-900°C for 3-15 h to obtain the sodium ion battery positive electrode material.

3. The preparation method according to claim 2, characterized in that: The particle size of the mixed salt in the mixed salt system of step (1) is 0.1 to 10 μm.

4. The preparation method according to claim 2, characterized in that: The sodium source in step (1) is sodium acetate, sodium carbonate, sodium oxalate, sodium chloride, sodium nitrate or sodium citrate; and / or; The iron source is ferric phosphate, ferric oxalate or ferrous oxalate; and / or; The carbon source is sucrose, oxalic acid, citric acid, maltose, fructose or glucose; and / or; The phosphorus source is ferric phosphate, diammonium hydrogen phosphate, ammonium dihydrogen phosphate, ammonium phosphate or phosphoric acid; and / or; The mass ratio of the solute to the deionized water in the mixed salt system is 30-70:100, wherein the solute includes the phosphorus source, the iron source, the sodium source, the carbon source and the structure regulator.

5. The preparation method according to claim 2, characterized in that: The stirring rate of the hydrothermal reaction in step (2) is 100 to 1500 rpm.

6. The preparation method according to claim 2, characterized in that: The drying in step (3) is spray drying.

7. The preparation method according to claim 2, characterized in that: The inert atmosphere in step (4) is nitrogen, argon, a nitrogen-hydrogen mixed atmosphere, or an argon-hydrogen mixed atmosphere.

8. A sodium ion battery, characterized in that: The invention comprises a positive electrode, a negative electrode and a separator, wherein the active material of the positive electrode comprises the positive electrode material Na as claimed in claim 1 2-x FePO4F 1-x .

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