An iron-based polyanion sodium ion battery positive electrode material and preparation method thereof

Abstract

This patent application discloses an iron-based polyanion type sodium ion battery positive electrode material, its preparation method and application. Specifically, an iron-based polyanion type sodium ion battery positive electrode material, the iron-based polyanion type sodium ion battery positive electrode material is a carbon-coated modified Na4Fe3(PO4)2(P2O7) sodium ion battery positive electrode material. In the preparation method provided in this patent application, the polyanion material is first prepared by a solvent combustion method (i.e., step S1), and then the carbon coating is completed by ball milling with a carbon source (such as expanded graphite). The design includes two calcination processes. The first time, the organic acid therein is carbonized to perform an initial trace carbon coating, and the second calcination is a sintering process for the expanded graphite and the polyanion material. The two carbon coatings make the polyanion type sodium ion battery positive electrode material have more excellent electrochemistry.

Description

Technical Field

[0001] This patent application relates to the technical field of sodium ion battery materials, and more specifically, to an iron-based polyanion sodium ion battery positive electrode material and a preparation method thereof. Background Art

[0002] The development of clean and renewable energy sources, such as solar and wind power, has spurred the development of energy storage technologies. Secondary batteries, represented by lithium-ion batteries, have conquered the portable electronics and new energy vehicle markets. With the rapid expansion of the electric vehicle market and the construction of large-scale energy storage stations to support power generation systems such as photovoltaics and wind power, the low abundance of lithium in the Earth's crust, 0.0065%, has made lithium-ion batteries increasingly expensive. As researchers strive to find alternatives to lithium-ion batteries for energy storage, sodium-ion batteries (Na-ion batteries) offer a promising alternative due to their similar electrochemical properties and the high abundance of sodium, making them a low-cost alternative.

[0003] The main challenges to the commercialization of sodium-ion batteries are the low energy density and unstable cycle life of electrode materials. The performance of sodium-ion batteries depends largely on the selection of their positive and negative electrode materials. The positive electrode materials currently under research can be divided into four categories: transition metal oxides, polyanionic compounds, Prussian blue analogs, and others such as sulfates and fluorides. Compared with other positive electrode materials, polyanionic compounds have attracted much attention due to their stable structure, good thermal stability, and small volume change during cycling. The polyanionic positive electrode of sodium-ion batteries is mainly composed of vanadium-based compounds and iron-based compounds. Considering the cost and environmental impact, iron-based electrode materials are the most ideal choice for sodium-ion batteries.

[0004] Among the iron-based polyanion sodium ion battery cathode materials, Na4Fe3(PO4)2(P2O7) contains both PO4 and P2O7 units in its structure, taking advantage of the advantages of phosphate and pyrophosphate cathodes, and thus has many superior properties, including high stability, good theoretical capacity (129 mAh g -1 ), low cost, environmental friendliness and easy synthesis. However, the inherent low electronic conductivity and poor ionic conductivity of this material limit its application. Summary of the Invention

[0005] To overcome at least one of the problems of the prior art, this patent application provides a modified iron-based polyanion sodium-ion battery cathode material and a method for preparing the same. The objective is to provide a modified iron-based polyanion sodium-ion battery cathode material that is cost-effective, environmentally friendly, and mass-producible.

[0006] In order to solve the above technical problems, the technical solution adopted in this patent application is:

[0007] The invention discloses an iron-based polyanion type sodium ion battery positive electrode material, wherein the iron-based polyanion type sodium ion battery positive electrode material is a carbon-coated modified Na4Fe3(PO4)2(P2O7) sodium ion battery positive electrode material.

[0008] This patent application also provides a method for preparing the above-mentioned iron-based polyanion sodium ion battery positive electrode material, which comprises the following steps:

[0009] S1, in a container, dissolving an iron source compound, a sodium source compound, a phosphorus source compound, and an organic acid in water, stirring uniformly, heating and stirring until the water evaporates, and grinding the solid product obtained after evaporation to obtain a phosphate precursor powder;

[0010] S2, placing the phosphate precursor powder described in step S1 in a tube furnace for calcination to obtain Na4Fe3(PO4)2(P2O7);

[0011] S3, ball milling the Na4Fe3(PO4)2(P2O7) and carbon source described in step S2 to obtain NFPP@EG black powder;

[0012] S4, placing the NFPP@EG black powder obtained in step S3 into a tube furnace for sintering to obtain carbon-coated modified Na4Fe3(PO4)2(P2O7).

[0013] Compared with the prior art, the beneficial effects of this patent application are:

[0014] (1) In the preparation method provided in this patent application, a polyanion material is first prepared by a solvent combustion method (i.e., step S1), and then carbon-coated with a carbon source (e.g., expanded graphite) by ball milling. The design includes two calcination processes. The first calcination process carbonizes the organic acid therein to perform a preliminary trace carbon coating, and the second calcination process is a sintering process for the expanded graphite and the polyanion material. The two carbon coatings give the polyanion-type sodium ion battery positive electrode material more excellent electrochemical properties, namely, higher charge and discharge capacity, higher rate performance, and higher cycle performance.

[0015] (2) The iron source compound, sodium source compound, phosphorus source compound and organic acid used in this patent application are all green and environmentally friendly, and no harmful waste liquid is generated during the synthesis process; the heat treatment is carried out under an inert atmosphere, and the calcination temperature is generally not higher than 650°C. The production process is safe and very simple. BRIEF DESCRIPTION OF THE DRAWINGS

[0016] Figure 1 This is an SEM image of the expanded graphite-coated sodium ion battery positive electrode material prepared in Example 1 of this patent application;

[0017] Figure 2XRD patterns of the expanded graphite-coated sodium ion battery positive electrode material prepared in Example 1 of this patent application and the sodium ion positive electrode material prepared in the comparative example;

[0018] Figure 3 XRD patterns of the expanded graphite-coated sodium ion battery positive electrode material prepared in Example 2 of this patent application and the sodium ion positive electrode material prepared in the comparative example;

[0019] Figure 4 The charge-discharge curve of the sodium ion positive electrode material prepared in the comparative example of this patent application at a current of 0.1C and a voltage range of 1.7-4.3V for 1-10 cycles;

[0020] Figure 5 The charge-discharge curve of the expanded graphite-coated sodium ion positive electrode material prepared in Example 1 of the present patent application at a current of 0.1C and a voltage range of 1.7-4.3V for 1-10 cycles;

[0021] Figure 6 The performance curve of the expanded graphite-coated sodium ion positive electrode material prepared in Example 1 of this patent application in the voltage range of 1.7-4.3V and the rate of 0.1-50C;

[0022] Figure 7 This is a graph showing the performance of the expanded graphite-coated sodium ion cathode material prepared in Example 2 of the present patent application in the voltage range of 1.7-4.3V and the rate of 0.1-50C;

[0023] Figure 8 This is a cycling performance diagram of the expanded graphite-coated sodium ion positive electrode material prepared in Example 1 of this patent application at a current of 1C and a voltage range of 1.7-4.3V;

[0024] Figure 9 This is a cycling performance diagram of the expanded graphite-coated sodium ion positive electrode material prepared in Example 1 of this patent application at a current of 5C and a voltage range of 1.7-4.3V;

[0025] Figure 10 This is a cycling performance diagram of the expanded graphite-coated sodium ion positive electrode material prepared in Example 1 of this patent application at a current of 10C and a voltage range of 1.7-4.3V;

[0026] Figure 11 This is a cycling performance diagram of the expanded graphite-coated sodium ion positive electrode material prepared in Example 1 of this patent application at a current of 20C and a voltage range of 1.7-4.3V. DETAILED DESCRIPTION

[0027] The embodiments of the present application will be described in detail below in conjunction with the examples, but it will be appreciated by those skilled in the art that the following examples are merely illustrative of the present application and should not be construed as limiting the scope of the present application. The unspecified conditions in the examples are carried out according to conventional conditions or manufacturer recommendations. The unspecified manufacturers for reagents used or instruments are conventional products that can be purchased commercially. In the following examples, the term XRD art-specific terminology that appears refers to an X-ray diffractometer. The iron source, sodium source, phosphorus source, and organic reducing agent used are conventional materials commonly used in the art and are not particularly limited herein.

[0028] It should be noted that:

[0029] In this patent application, unless otherwise specified, all the embodiments and preferred implementation methods mentioned herein can be combined with each other to form a new technical solution.

[0030] In this patent application, unless otherwise stated, each reaction or operation step can be carried out sequentially or in accordance with the sequence. Preferably, the reaction method herein is carried out sequentially.

[0031] Unless otherwise indicated, the professional and scientific terms used herein are the same as those familiar to those skilled in the art. In addition, any method or material similar or equivalent to the described content may also be applied to this patent application.

[0032] In the preparation method of the iron-based polyanion sodium ion battery positive electrode material provided in this patent application, the polyanion material is first prepared by a solvent combustion method (i.e., step S1), and then the carbon source (e.g., expanded graphite) is ball-milled with it to complete carbon coating. The design includes two calcination processes. The first calcination is to carbonize the organic acid therein and perform a preliminary trace carbon coating on it. The second calcination is a sintering process for the expanded graphite and the polyanion material. The two carbon coatings give the polyanion sodium ion battery positive electrode material more excellent electrochemical properties, that is, the electronic conductivity and ionic conductivity are improved by carbon coating, thereby achieving more excellent electrochemical performance.

[0033] This patent application provides an iron-based polyanion type sodium ion battery cathode material. The iron-based polyanion type sodium ion battery cathode material is a carbon-coated modified Na4Fe3(PO4)2(P2O7) sodium ion battery cathode material.

[0034] In some preferred embodiments, the carbon coating is expanded graphite coating.

[0035] This patent application provides a method for preparing the above-mentioned iron-based polyanion sodium ion battery positive electrode material, which comprises the following steps:

[0036] S1, in a container, dissolving an iron source compound, a sodium source compound, a phosphorus source compound, and an organic acid in water, stirring uniformly, heating and stirring until the water evaporates, and grinding the solid product obtained after evaporation to obtain a phosphate precursor powder;

[0037] S2, placing the phosphate precursor powder described in step S1 in a tube furnace for calcination to obtain Na4Fe3(PO4)2(P2O7);

[0038] S3, ball milling the Na4Fe3(PO4)2(P2O7) and carbon source described in step S2 to obtain NFPP@EG black powder;

[0039] S4, placing the NFPP@EG black powder obtained in step S3 into a tube furnace for sintering to obtain carbon-coated modified Na4Fe3(PO4)2(P2O7).

[0040] This method first prepares the polyanion material using a solvent combustion method, then carbon-coates it with expanded graphite by ball milling. The design incorporates two calcination processes: the first to carbonize the organic acid in the material and perform a trace amount of carbon coating, and the second calcination to sinter the expanded graphite and polyanion material. This double carbon coating gives the polyanion-based sodium-ion battery cathode material superior electrochemical performance.

[0041] In addition, the iron source compounds, sodium source compounds, phosphorus source compounds and organic acids used in the preparation method are all green and environmentally friendly, and no harmful waste liquid is generated during the synthesis process; the heat treatment is carried out under an inert atmosphere, and the calcination temperature is generally not higher than 650°C, and the production process is safe and very simple.

[0042] In some embodiments, the iron source compound includes any one of ferric nitrate, ferric phosphate, and ferric oxalate, or a combination of at least two thereof.

[0043] In some preferred embodiments, the sodium source compound includes any one of sodium dihydrogen phosphate, sodium phosphate, disodium hydrogen phosphate, sodium carbonate, sodium acetate, and sodium citrate, or a combination of at least two thereof.

[0044] In some preferred embodiments, the phosphorus source compound includes any one of sodium dihydrogen phosphate, sodium phosphate, disodium hydrogen phosphate, ammonium dihydrogen phosphate, and diammonium hydrogen phosphate, or a combination of at least two thereof.

[0045] In some preferred embodiments, the organic acid includes any one of citric acid, ascorbic acid, and oxalic acid, or a combination of at least two of them.

[0046] In some preferred embodiments, the carbon source is expanded graphite, and the mixing ratio of Na4Fe3(PO4)2(P2O7) to expanded graphite in step S3 is (8:2) to (9.9:0.1). This configuration maintains the original structure of the material and prevents it from falling off after coating. However, excessive carbon content can cause the active material to fall off the carrier, degrading battery performance.

[0047] In some preferred embodiments, the molar ratio of the iron in the iron source compound, the sodium in the sodium source compound, and the phosphorus in the phosphorus source compound is 3:4:4.

[0048] In some preferred embodiments, the calcination in step S2 and the sintering in step S4 are both divided into two stages of heat treatment, with the first stage calcination temperature being 200-400°C and the calcination time being 1-6 hours, and the second stage calcination temperature being 500-600°C and the calcination time being 8-24 hours. In this arrangement, the first stage calcination serves as a pre-oxidation process, and the second stage calcination serves as a sintering process.

[0049] In some preferred embodiments, the ball milling process in step S3 is dry ball milling, the ball-to-material ratio is 1:10 to 1:50, the ball milling speed is 200 to 500 revolutions per minute, and the ball milling time is 2 to 10 hours.

[0050] In some preferred embodiments, the molar ratio of the iron source, sodium source, and phosphorus source is a stoichiometric ratio, that is, the molar ratio of the iron source, sodium source, and phosphorus source in Na4Fe3(PO4)2(P2O7) is 3:4:4, and the molar number of the organic acid is 0.4 to 2 times the total molar number of the iron source, sodium source, and phosphorus source.

[0051] This patent application also provides the application of the above-mentioned iron-based polyanion sodium ion battery positive electrode material in the field of sodium ion batteries.

[0052] Next, the preparation method of the iron-based polyanion sodium ion battery positive electrode material in this patent application is described in detail with specific examples.

[0053] Example 1 Preparation of Carbon-coated Modified Iron-based Polyanion Sodium Ion Battery Cathode Material NFPP@EG (9.5:0.5)

[0054] The preparation method comprises the following steps:

[0055] S1. In a beaker, weigh 3.03 g of Fe(NO3)3·9H2O, 1.56 g of NaH2PO4·2H2O, and 2.64 g of ascorbic acid and dissolve them in 50 ml of water. Stir in an oil bath at 120°C until the water evaporates. Grind the solid product obtained after evaporation to obtain a phosphate precursor powder.

[0056] S2, placing the phosphate precursor powder obtained in step S1 into a tube furnace, and under an argon protective atmosphere, first heating the temperature to 300°C at a rate of 3K / min and holding the temperature for 3 hours, then heating the temperature to 600°C at the same rate and holding the temperature for 12 hours, and grinding the powder to obtain a black powder of Na4Fe3(PO4)2(P2O7) (abbreviated as NFPP);

[0057] S3, weighing 0.95g of the NFPP black powder from step S2 and 0.05g of expanded graphite, placing them in a zirconia ball mill with a ball-to-material ratio of 50:1, and running them in a planetary ball mill at a speed of 300r / min for 4h to obtain NFPP@EG black powder.

[0058] S4, placing the NFPP@EG black powder obtained in step S3 into a tube furnace, and under an argon protective atmosphere, first heating the temperature to 300°C at a rate of 3K / min and keeping it for 3 hours, then heating the temperature to 600°C at the same rate and keeping it for 12 hours. After grinding, NFPP@EG (9.5:0.5) powder is obtained, where the ratio of 9.5:0.5 is the mass ratio of NFPP and EG (expanded graphite).

[0059] Figure 1 This is a SEM image of the NFPP@EG (9.5:0.5) material prepared in this example. The SEM image clearly shows the external morphology of the NFPP@EG (9.5:0.5) material, including a strip of expanded graphite, indicating that the expanded graphite has been ball-milled into the material, indicating that the material is a Na4Fe3(PO4)2(P2O7) cathode material for sodium-ion batteries coated with expanded graphite.

[0060] Figure 2 The XRD patterns of the NFPP@EG (9.5:0.5) material prepared in this example and the NFPP material prepared in the following comparative example are shown in FIG. Figure 2 It can be seen that the NFPP@EG (9.5:0.5) material prepared in this embodiment has a higher peak intensity of the diffraction peak than the NFPP material prepared in the comparative example and has better crystallinity.

[0061] Example 2 Preparation of Carbon-coated Modified Iron-based Polyanion Sodium Ion Battery Cathode Material NFPP@EG (9:1)

[0062] The preparation method comprises the following steps:

[0063] S1. In a beaker, weigh 2.67 g of Fe(NO3)3·9H2O, 1.37 g of NaH2PO4·2H2O, and 2.32 g of ascorbic acid and dissolve them in 50 ml of water. Stir in an oil bath at 130°C until the water evaporates. Grind the solid product obtained after evaporation to obtain a phosphate precursor powder.

[0064] S2, placing the phosphate precursor powder obtained in step S1 into a tube furnace, heating it to 300°C at a rate of 3K / min under an argon atmosphere and holding it for 3 hours, then heating it to 600°C at the same rate and holding it for 12 hours, and grinding it to obtain a black powder of Na4Fe3(PO4)2(P2O7) (abbreviated as NFPP);

[0065] S3, weighing 0.9 g of the NFPP black powder and 0.1 g of expanded graphite from step S2, placing them in a zirconia ball mill with a ball-to-material ratio of 50:1, and running them in a planetary ball mill at a speed of 300 r / min for 4 h to obtain NFPP@EG black powder;

[0066] S4, placing the NFPP@EG black powder obtained in step S3 into a tube furnace, heating the mixture to 300°C at a heating rate of 3K / min under an argon protective atmosphere and holding the mixture for 3 hours, then heating the mixture to 600°C at the same heating rate and holding the mixture for 12 hours. After grinding, NFPP@EG (9:1) powder was obtained, where the ratio 9:1 is the mass ratio of NFPP to EG (expanded graphite).

[0067] Figure 3 The XRD patterns of the NFPP@EG (9:1) material and the NFPP material prepared in this example are shown. As can be seen from the figure, the NFPP@EG (9:1) material prepared in this example has a higher peak intensity of diffraction peaks than the NFPP material prepared in the comparative example, indicating better crystallinity.

[0068] Example 3 Preparation of Carbon-coated Modified Iron-based Polyanion Sodium Ion Battery Cathode Material NFPP@EG Powder

[0069] The preparation method comprises the following steps:

[0070] S1. In a beaker, weigh 2.82 g of Fe2(C2O4)3, 1.56 g of NaH2PO4·2H2O, and 2.88 g of citric acid and dissolve them in 60 ml of water. Stir in an oil bath at 110°C until the water evaporates. Grind the solid product obtained after evaporation to obtain a phosphate precursor powder.

[0071] S2, placing the phosphate precursor powder obtained in step S1 into a tube furnace, and under an argon protective atmosphere, first heating the temperature to 300°C at a rate of 5K / min and holding it for 2 hours, then heating the temperature to 550°C at the same rate and holding it for 12 hours, and grinding it to obtain a black powder of Na4Fe3(PO4)2(P2O7) (abbreviated as NFPP);

[0072] S3, weighing 0.97g of the NFPP black powder in step S2 and 0.03g of expanded graphite, placing them in a zirconia ball mill with a ball-to-material ratio of 45:1, and running them in a planetary ball mill at a speed of 200r / min for 5h to obtain NFPP@EG black powder. At this time, the black powder obtained is a powder in which the expanded graphite and NFPP powder are in contact with each other after simple ball milling.

[0073] In step S4, the NFPP@EG black powder obtained in step S3 is placed in a tube furnace. Under an argon atmosphere, the temperature is first increased to 300°C at a rate of 5 K / min and held for 2 hours. The temperature is then increased to 550°C at the same rate and held for 12 hours. After grinding, the NFPP@EG powder is obtained. This powder is sintered to coat the NFPP with expanded graphite.

[0074] The raw materials used in Example 3 differ only in the iron source and the organic acid from those in Examples 1 and 2. This was done to explore the effects of different organic acids on the material after synthesis, such as achieving a more complete reaction and reducing impurities. Fewer impurities can lead to better performance and demonstrate the versatility of the synthesis process, utilizing cheaper and more readily available raw materials.

[0075] Example 4 Preparation of Carbon-Coated Modified Iron-Based Polyanion Sodium Ion Battery Cathode Material NFPP@EG Powder

[0076] The preparation method comprises the following steps:

[0077] S1. In a beaker, weigh 3.03 g of Fe(NO3)3·9H2O, 0.82 g of CH3COONa, 1.15 g of NH4H2PO4, and 2.64 g of ascorbic acid and dissolve them in 50 ml of water. Stir the mixture in an oil bath at 120°C until the water evaporates. Grind the solid product obtained after evaporation to obtain a phosphate precursor powder.

[0078] S2, placing the phosphate precursor powder obtained in step S1 into a tube furnace, heating the temperature to 300°C at a rate of 3K / min under an argon atmosphere and holding the temperature for 3 hours, then heating the temperature to 600°C at the same rate and holding the temperature for 12 hours, and grinding the resulting black powder of Na4Fe3(PO4)2(P2O7) (abbreviated as NFPP);

[0079] S3, weighing 0.8 g of the NFPP black powder from step S2 and 0.2 g of expanded graphite, placing them in a zirconia ball mill with a ball-to-material ratio of 40:1, and running them in a planetary ball mill at a speed of 250 r / min for 4.5 h to obtain NFPP@EG black powder.

[0080] S4. The NFPP@EG black powder obtained in step S3 is placed in a tube furnace. Under an argon protective atmosphere, the temperature is increased to 300°C at a rate of 3K / min and kept at this temperature for 3 hours. Then, the temperature is increased to 500°C at the same rate and kept at this temperature for 12 hours. After grinding, NFPP@EG powder is obtained.

[0081] The preparation method in Example 4 is different from that in Example 1 only in that the sodium source used in Example 4 is different. This is also to illustrate the versatility of the synthesis process, and cheaper and more readily available raw materials can be used to prepare the products in this application.

[0082] Example 5 Preparation of Carbon-coated Modified Iron-based Polyanion Sodium Ion Battery Cathode Material NFPP@EG Powder The preparation method comprises the following steps:

[0083] S1. In a beaker, weigh 2.25 g FePO4, 1.64 g CH3COONa, 0.575 g NH4H2PO4, and 4.32 g citric acid and dissolve them in 50 ml water. Stir in an oil bath at 100°C until the water evaporates. Grind the solid product obtained after evaporation to obtain a phosphate precursor powder.

[0084] S2, placing the phosphate precursor powder obtained in step S1 into a tube furnace, heating the temperature to 300°C at a rate of 5K / min under an argon atmosphere and holding the temperature for 2 hours, then heating the temperature to 500°C at the same rate and holding the temperature for 8 hours, and grinding the powder to obtain a black powder of Na4Fe3(PO4)2(P2O7) (abbreviated as NFPP);

[0085] S3, weighing 0.85g of the NFPP black powder from step S2 and 0.15g of expanded graphite, placing them in a zirconia ball mill with a ball-to-material ratio of 35:1, and running them in a planetary ball mill at a speed of 350r / min for 3h to obtain NFPP@EG black powder.

[0086] S4. The NFPP@EG black powder obtained in step S3 is placed in a tube furnace. Under an argon protective atmosphere, the temperature is increased to 300°C at a rate of 5K / min and kept at this temperature for 2 hours. Then, the temperature is increased to 500°C at the same rate and kept at this temperature for 8 hours. After grinding, NFPP@EG powder is obtained.

[0087] The only difference between the preparation method in Example 5 and that in Example 4 is the iron source and the organic acid, which also illustrates the versatility of the synthesis process. Cheaper and more readily available raw materials can be used to prepare the carbon-coated modified iron-based polyanion sodium ion battery positive electrode material NFPP@EG powder in this application.

[0088] Example 6 Preparation of Carbon-coated Modified Iron-based Polyanion Sodium Ion Battery Cathode Material NFPP@EG Powder The preparation method comprises the following steps:

[0089] S1. In a beaker, weigh 2.25 g FePO4, 1.64 g CH3COONa, 0.575 g NH4H2PO4, and 1.44 g citric acid and dissolve them in 50 ml water. Stir in an oil bath at 100°C until the water evaporates. Grind the solid product obtained after evaporation to obtain a phosphate precursor powder.

[0090] S2, placing the phosphate precursor powder obtained in step S1 into a tube furnace, heating the temperature to 300°C at a rate of 5K / min under an argon atmosphere and holding the temperature for 2 hours, then heating the temperature to 500°C at the same rate and holding the temperature for 8 hours, and grinding the powder to obtain a black powder of Na4Fe3(PO4)2(P2O7) (abbreviated as NFPP);

[0091] S3, weighing 0.85g of the NFPP black powder from step S2 and 0.15g of expanded graphite, placing them in a zirconia ball mill with a ball-to-material ratio of 35:1, and running them in a planetary ball mill at a speed of 350r / min for 3h to obtain NFPP@EG black powder.

[0092] S4. The NFPP@EG black powder obtained in step S3 is placed in a tube furnace. Under an argon protective atmosphere, the temperature is increased to 300°C at a rate of 5K / min and kept at this temperature for 2 hours. Then, the temperature is increased to 500°C at the same rate and kept at this temperature for 8 hours. After grinding, NFPP@EG powder is obtained.

[0093] The only difference between the preparation method in Example 6 and Example 5 is the different quality of citric acid. By studying the different qualities of organic acids, the influence on the valence state of Fe is explored, which is also an exploration of the reduction of impurities.

[0094] Example 7 Preparation of Carbon-coated Modified Iron-based Polyanion Sodium Ion Battery Cathode Material NFPP@EG Powder The preparation method comprises the following steps:

[0095] S1. In a beaker, weigh 4.5 g FePO4, 1.64 g C6H5Na3O7, 1.56 g NaH2PO4, and 1.44 g citric acid and dissolve them in 50 ml water. Stir in an oil bath at 100°C until the water evaporates. Grind the solid product obtained after evaporation to obtain a phosphate precursor powder.

[0096] S2, placing the phosphate precursor powder obtained in step S1 into a tube furnace, heating the temperature to 300°C at a rate of 5 K / min under an argon protective atmosphere and holding the temperature for 2 h, then heating the temperature to 550°C at the same rate and holding the temperature for 8 h, and grinding the resulting black powder of Na4Fe3(PO4)2(P2O7) (abbreviated as NFPP);

[0097] S3, weighing 0.87 g of the NFPP black powder from step S2 and 0.13 g of expanded graphite, placing them in a zirconia ball mill with a ball-to-material ratio of 30:1, and running them in a planetary ball mill at a speed of 500 r / min for 2 h to obtain NFPP@EG black powder.

[0098] S4. The NFPP@EG black powder obtained in step S3 is placed in a tube furnace. Under an argon protective atmosphere, the temperature is increased to 300°C at a rate of 5K / min and kept at this temperature for 2 hours. Then, the temperature is increased to 550°C at the same rate and kept at this temperature for 8 hours. After grinding, NFPP@EG powder is obtained.

[0099] Compared with the preparation method in Example 6, the only difference in the quality of the iron source and the phosphorus source is that the quality of the iron source is improved, but the ratio of the iron source to the phosphorus source remains unchanged, in order to explore whether the high-amount reaction affects the performance of the material.

[0100] There is basically no difference between the five NFPP@EG powders prepared in Examples 3 to 7 above. The only difference is that the different qualities of the raw materials, different organic acids, and different raw materials in the preparation method are adjusted to explore the success factors of the synthesis process and whether the impurities are reduced to improve the performance.

[0101] Comparative Example Preparation of NFPP Black Powder, a Positive Electrode Material for Iron-Based Polyanion Sodium Ion Batteries

[0102] The preparation method comprises the following steps:

[0103] S1. In a beaker, weigh 3.03 g of Fe(NO3)3·9H2O, 1.56 g of NaH2PO4·2H2O, and 2.64 g of ascorbic acid and dissolve them in 50 ml of water. Stir in an oil bath at 120°C until the water evaporates. Grind the solid product obtained after evaporation to obtain a phosphate precursor powder.

[0104] S2. Place the phosphate precursor powder obtained in step S1 into a tube furnace. Under an argon protective atmosphere, heat the temperature to 300°C at a rate of 3K / min and keep it for 3 hours. Then heat the temperature to 600°C at the same rate and keep it for 12 hours. After grinding, a black powder of Na4Fe3(PO4)2(P2O7) (abbreviated as NFPP) is obtained.

[0105] Compared with Example 1, this comparative example does not coat the Na4Fe3(PO4)2(P2O7) black powder with expanded graphite. Figure 4 The charge and discharge curve of the sodium ion positive electrode material prepared in the comparative example of this patent application at 0.1C current and 1.7-4.3V voltage range for 1-10 cycles. Figure 4 It can be seen that the charge and discharge specific capacities of NFPP without expanded graphite are 81 and 78 respectively. Figure 5 The charge and discharge specific capacity of NFPP coated with expanded graphite has been significantly improved, which means that the electrical conductivity of the iron-based polyanion sodium ion battery positive electrode material NFPP@EG (9.5:0.5) coated with expanded graphite is significantly improved compared with the NFPP material not coated with expanded graphite.

[0106] Application example: Preparation of sodium ion button batteries

[0107] 0.4g of positive electrode material (selected from the iron-based polyanion sodium ion battery positive electrode materials prepared in Examples 1, 2, and Comparative Example 1), 0.05g of conductive carbon material (super C45), and 0.05g of binder (polyvinylidene fluoride) were weighed in a mass ratio of 8:1:1 and dispersed in N-methylpyrrolidone. The resulting mixed slurry was evenly coated on aluminum foil and vacuum-dried at 110°C for 12 hours to obtain a positive electrode sheet. A sodium metal sheet was used as the counter electrode. The spring, gasket, counter electrode, separator, and positive electrode sheet were placed in a CR2025 button cell in that order. An electrolyte solution with a concentration of 1 mol / L, consisting of sodium perchlorate as the solute and polycarbonate and ethylene carbonate (in a 1:1 volume ratio) plus 5% by volume of fluoroethylene carbonate as the solvent, was added. The battery was then packaged to obtain a sodium ion button cell.

[0108] Test conditions: voltage window is 1.7~4.3V.

[0109] After the above application experiments, the test results are as follows Figures 5 to 11 Now combined with the attached Figures 5-11 , the experimental test results are analyzed and explained as follows:

[0110] Figure 5 The charge-discharge curve of the expanded graphite-coated sodium ion positive electrode material prepared in Example 1 of this patent application at a current of 0.1C and a voltage range of 1.7-4.3V for 1-10 cycles. Figure 5 It can be seen that the charge and discharge platforms of the button battery using the expanded graphite-coated iron-based polyanion sodium ion battery positive electrode material NFPP@EG (9.5:0.5) as the positive electrode and the NFPP button battery without expanded graphite as the positive electrode are the same, and there is also a first cycle activation process. Figure 5 contrast Figure 4 It can be seen that the charge and discharge specific capacity has been significantly improved, from 78 mAh g-1 to about 120 mAh g-1, which means that the electrical conductivity of the iron-based polyanion sodium ion battery positive electrode material NFPP@EG (9.5:0.5) coated with expanded graphite is significantly improved compared with the NFPP material not coated with expanded graphite.

[0111] Figure 6 This is a performance curve of the expanded graphite-coated sodium ion positive electrode material prepared in Example 1 of this patent application in the voltage range of 1.7-4.3V and the rate of 0.1-50C. Figure 7 This is a performance curve of the expanded graphite-coated sodium ion positive electrode material prepared in Example 2 of this patent application in the voltage range of 1.7-4.3V and the rate of 0.1-50C.

[0112] Figure 6 and Figure 7 This is a comparative test graph of carbon content. Figure 6 This is a rate performance test, and it can be seen that at a high rate of 50C, there is still a specific capacity of 30. Figure 7 contrast, Figure 6 It can be clearly shown that 5wt.% expanded graphite coating has better rate performance than 10wt.%.

[0113] Figure 8 This is a cycling performance diagram of the expanded graphite-coated sodium ion positive electrode material prepared in Example 1 of this patent application at a current of 1C and a voltage range of 1.7-4.3V;

[0114] Figure 9 This is a cycling performance diagram of the expanded graphite-coated sodium ion positive electrode material prepared in Example 1 of this patent application at a current of 5C and a voltage range of 1.7-4.3V;

[0115] Figure 10 This is a cycling performance diagram of the expanded graphite-coated sodium ion positive electrode material prepared in Example 1 of this patent application at a current of 10C and a voltage range of 1.7-4.3V;

[0116] Figure 11 Cyclic performance diagram of the sodium ion positive electrode material coated with expanded graphite prepared in Example 1 of this patent application at a current of 20C and a voltage range of 1.7-4.3V

[0117] from Figures 8-11As can be seen, at different rates (e.g., currents of 1C, 5C, 10C, and 20C), the expanded graphite-coated polyanion sodium-ion battery cathode material prepared in this patent application exhibits excellent cycling performance, maintaining a 90% retention rate after 200 cycles. This significantly improves battery life and reduces operating costs.

[0118] The carbon coating in this patent application enables the polyanion sodium ion battery positive electrode material, especially the expanded graphite-coated polyanion sodium ion battery positive electrode material, to have higher electronic conductivity and ionic conductivity when used in sodium ion batteries. It is speculated that the reason is that carbon coating (such as expanded graphite coating) can increase the sodium ion diffusion coefficient, thereby improving the conductivity.

[0119] In summary, the preparation method of the iron-based polyanion sodium ion battery positive electrode material in this patent application has the following advantages:

[0120] (1) In the preparation method provided in this patent application, a polyanion material is first prepared by a solvent combustion method (i.e., steps S1 and S2), and then carbon-coated with a carbon source (e.g., expanded graphite) by ball milling. The design includes two calcination processes. The first calcination process carbonizes the organic acid therein to perform a preliminary trace carbon coating, and the second calcination process is a sintering process for the expanded graphite and the polyanion material. The two carbon coatings give the polyanion-type sodium ion battery positive electrode material more excellent electrochemical performance.

[0121] (2) The iron source compound, sodium source compound, phosphorus source compound and organic acid used in this patent application are all green and environmentally friendly, and no harmful waste liquid is generated during the synthesis process; the heat treatment is carried out under an inert atmosphere, and the calcination temperature is generally not higher than 650°C. The production process is safe and very simple.

[0122] In the description of this specification, reference to the terms "one embodiment," "some embodiments," "illustrative embodiments," "examples," "specific examples," or "some examples" means that the specific features, structures, materials, or characteristics described in conjunction with the embodiment or example are included in at least one embodiment or example of this patent application. In this specification, the schematic representation of the above terms does not necessarily refer to the same embodiment or example. Moreover, the specific features, structures, materials, or characteristics described may be combined in any appropriate manner in any one or more embodiments or examples.

[0123] Although several embodiments of the present patent application have been shown and described, those skilled in the art will appreciate that various changes, modifications, substitutions, and variations may be made to these embodiments without departing from the principles and spirit of the present patent application, and the scope of the present patent application is defined by the claims and their equivalents.

Claims

1. A method for preparing an iron-based polyanion sodium ion battery cathode material, characterized in that: The following steps are involved: S1, in a container, dissolving an iron source compound, a sodium source compound, a phosphorus source compound, and an organic acid in water, stirring the mixture, and then heating and stirring the mixture until the water evaporates, and grinding the solid product obtained after evaporation to obtain a phosphate precursor powder; S2, placing the phosphate precursor powder described in step S1 in a tube furnace for calcining to carbonize the organic acid to form a primary trace carbon coating to obtain Na4Fe3(PO4)2(P2O7); S3, ball milling the Na4Fe3(PO4)2(P2O7) described in step S2 with a carbon source to obtain NFPP@EG black powder, wherein the carbon source is expanded graphite, and the mixing ratio of the Na4Fe3(PO4)2(P2O7) to the expanded graphite is (8:2) to (9.9:0.1); S4, placing the NFPP@EG black powder obtained in step S3 into a tube furnace for sintering to obtain double carbon-coated Na4Fe3(PO4)2(P2O7).

2. The method for preparing the iron-based polyanion sodium ion battery positive electrode material according to claim 1, wherein: The iron source compound includes any one of ferric nitrate, ferric phosphate, and ferric oxalate, or a combination of at least two of them.

3. The method for preparing the iron-based polyanion sodium ion battery cathode material according to claim 1, wherein: The sodium source compound includes any one of sodium dihydrogen phosphate, sodium phosphate, disodium hydrogen phosphate, sodium carbonate, sodium acetate, and sodium citrate, or a combination of at least two thereof.

4. The method for preparing the iron-based polyanion sodium ion battery cathode material according to claim 1, wherein: The phosphorus source compound includes any one of sodium dihydrogen phosphate, sodium phosphate, disodium hydrogen phosphate, ammonium dihydrogen phosphate, and diammonium hydrogen phosphate, or a combination of at least two thereof, and / or the organic acid includes any one of citric acid, ascorbic acid, and oxalic acid, or a combination of at least two thereof.

5. The method for preparing the iron-based polyanion sodium ion battery positive electrode material according to claim 1, wherein: The molar ratio of the iron in the iron source compound, the sodium in the sodium source compound, and the phosphorus in the phosphorus source compound is 3:4:

4.

6. The method for preparing the iron-based polyanion sodium ion battery cathode material according to claim 1, characterized in that: The calcination in step S2 and the sintering in step S4 are both divided into two stages of heat treatment, the first stage calcination temperature is 200-400°C and the calcination time is 1-6 hours, and the second stage calcination temperature is 500-600°C and the calcination time is 8-24 hours.

7. The method for preparing the iron-based polyanion sodium ion battery cathode material according to claim 3, characterized in that: The ball milling process in step S3 is dry ball milling, the ball-to-material ratio is 1:10-1:50, the ball milling speed is 200-500 revolutions per minute, and the ball milling time is 2-10 hours.

Images