Preparation method of sodium iron phosphate pyrophosphate and application thereof in sodium ion battery

Abstract

The application discloses a preparation method of sodium iron pyrophosphate phosphate and application thereof in sodium ion batteries, and belongs to the technical field of battery material preparation. The preparation method of the battery positive electrode material sodium iron pyrophosphate phosphate Na4Fe3(PO4)2P2O7 is as follows: sodium pyrophosphate, iron phosphate dihydrate and iron powder are dry-mixed in proportion to obtain a precursor, the precursor is heated by microwaves to 440-460 DEG C for insulation for a certain time, and sodium iron pyrophosphate phosphate is synthesized. According to the characteristics of the target product, the phase transformation of phosphate and pyrophosphate in the sintering process, the metering ratio, the reduction of trivalent iron into divalent iron and the like, a new synthesis mode is designed, including the reselection of raw materials and the redesign of the sintering mode. The sodium iron pyrophosphate phosphate material is synthesized accurately, efficiently and at low cost through solid-phase mixing and then microwave heating, the low-cost synthesis process problem of the sodium iron pyrophosphate phosphate positive electrode of the sodium ion battery is solved, and the application prospect of the method is very wide in industrial production.

Description

Technical Field

[0001] This invention relates to a method for preparing sodium iron pyrophosphate and its application in sodium-ion batteries, belonging to the field of battery technology. Background Technology

[0002] With the continuous expansion of global demand for large-scale energy storage, the scarcity of lithium resources on Earth will become a bottleneck restricting the future large-scale application of lithium-ion batteries. Sodium belongs to the same group as lithium, has similar chemical properties, and accounts for 2.64% of the Earth's crust (far exceeding lithium's 0.006%). Therefore, developing resource-rich and environmentally friendly sodium-ion battery technology is of great significance. However, many technical challenges remain to be overcome in the field of sodium-ion batteries, especially in developing low-cost, high-performance cathode materials. Among various cathode material systems, iron-based materials are considered the most commercially promising sodium-ion battery cathode material system due to their readily available, widely sourced, and environmentally friendly advantages. However, sodium iron phosphate (NaFePO4), a material with a chemical composition similar to LiFePO4, exists in a stable sodium phosphate iron ore structure, lacking unobstructed sodium ion channels and thus lacking electrochemical activity. The bi-anionic sodium iron pyrophosphate Na4Fe3(PO4)2P2O7 structure has an open sodium ion diffusion channel and a moderate operating voltage, making it a promising and inexpensive iron-based polyanionic cathode material.

[0003] The main synthesis method for sodium iron pyrophosphate (SO4) as a positive electrode polyanionic material for sodium-ion batteries involves uniformly mixing an iron source, phosphate, and sodium source, followed by high-temperature solid-state sintering. The sintering process directly uses trivalent iron as the raw material, requiring high-temperature carbon reduction of trivalent iron to divalent iron. However, the phase transition temperatures of iron reduction and phosphate conversion to pyrophosphate are inconsistent, leading to unstable conversion ratios, the presence of numerous impurity phases, and unstable and poor sintered capacity. Furthermore, the synthesis process is complex, with high burn-off rates and high energy consumption during sintering. Therefore, developing a low-cost method for preparing sodium iron pyrophosphate as a positive electrode material for sodium-ion batteries that overcomes these shortcomings is of great significance. Summary of the Invention

[0004] To address the aforementioned issues, this invention provides a novel preparation process for sodium iron pyrophosphate material. This process precisely controls the stoichiometric ratio of pyrophosphate to phosphate and performs dry mixing of the selected raw materials. It also utilizes microwave heating to achieve uniform heating at the molecular level and spontaneous redox reactions. The process is characterized by low reaction temperature and simple steps, significantly reducing energy consumption and thus lowering costs. Furthermore, it enhances the electrochemical performance of the battery.

[0005] The first objective of this invention is to provide a method for preparing sodium iron pyrophosphate (Na4Fe3(PO4)2P2O7), a battery cathode material, comprising the following steps:

[0006] S1. Sodium pyrophosphate, ferric phosphate dihydrate, and iron powder are dry-mixed in a certain proportion to obtain a precursor; the molar ratio of sodium pyrophosphate, ferric phosphate dihydrate, and iron powder is 0.8-1.2:1.8-2.2:0.8-1.2.

[0007] S2. The precursor obtained in step S1 is microwave-heated to 440-460℃ and kept at that temperature to synthesize sodium iron pyrophosphate Na4Fe3(PO4)2P2O7.

[0008] Furthermore, in step S1, the particle size of sodium pyrophosphate is between 5 and 20 μm. For example, 5 μm, 7 μm, 10 μm, 13 μm, 15 μm, 17 μm, 20 μm, etc., including but not limited to the particle size values ​​listed above.

[0009] Furthermore, in step S1, the particle size of ferric phosphate dihydrate is between 5 and 20 μm. For example, 5 μm, 7 μm, 10 μm, 13 μm, 15 μm, 17 μm, 20 μm, etc., including but not limited to the particle size values ​​listed above.

[0010] Further, in step S1, the iron powder is micron-sized iron powder, nano-sized iron powder, or a mixture of both. Preferably, the D50 of the iron powder is 0.5-5 μm. For example, 0.5 μm, 1.0 μm, 1.5 μm, 2.0 μm, 2.5 μm, 3.0 μm, 3.5 μm, 4.0 μm, 4.5 μm, 5.0 μm, etc., including but not limited to the particle size values ​​listed above.

[0011] Furthermore, in step S1, the dry mixing is airflow mixing, preferably using a high-speed mixer with high-speed rotating blades to generate airflow, so that different powders evaporate into the air to come into contact and mix.

[0012] Furthermore, during airflow mixing, the airflow velocity is 5-20 m / s, the stirrer speed is 500-3000 rpm, the mixing time is 30-60 min, and the temperature is 20-25℃.

[0013] Further, in step S1, the molar ratio of sodium pyrophosphate, ferric phosphate dihydrate, and iron powder is 0.8-1.2:1.8-2.2:0.8-1.2, such as 0.8:1.8:0.8, 0.9:1.9:0.9, 1:2:1, 1.1:2.1:1.1, 1.2:2.2:1.2, etc., including but not limited to the ratios listed above.

[0014] Furthermore, in step S2, the microwave power is 1-10 kW, and the temperature is controlled to reach 440-460°C within 2-5 minutes. For example, 440°C, 445°C, 450°C, 455°C, 460°C, etc., including but not limited to the temperature values ​​listed above.

[0015] Furthermore, in step S2, the temperature is maintained for 1-3 hours. For example, 1 hour, 1.2 hours, 1.5 hours, 1.7 hours, 2 hours, 2.3 hours, 2.5 hours, 2.8 hours, 3 hours, etc., including but not limited to the time values ​​listed above.

[0016] A second objective of this invention is to provide sodium iron pyrophosphate Na4Fe3(PO4)2P2O7 obtained by the above preparation method.

[0017] The third objective of this invention is to provide a positive electrode sheet, comprising a positive active layer, wherein the positive active layer comprises a positive active material, a positive conductive agent, and a positive binder, and the positive active material is sodium iron pyrophosphate Na4Fe3(PO4)2P2O7.

[0018] Furthermore, the positive electrode active layer comprises the following components by mass percentage: 75-95% positive electrode active material, 5-15% positive electrode conductive agent and 5-15% positive electrode binder.

[0019] Furthermore, the positive electrode conductive agent is selected from one or more of conductive carbon black, conductive graphite, carbon nanotubes, and graphene.

[0020] Furthermore, the positive electrode binder can be any positive electrode binder well known to those skilled in the art, without any special limitations, such as chitosan, xanthan gum, gellan gum, gum arabic, polyacrylonitrile, polyvinylidene fluoride, polyvinyl alcohol, carboxymethyl cellulose, lithium carboxymethyl cellulose, sodium carboxymethyl cellulose, polymethacrylamide, polyacrylic acid, lithium polyacrylate, sodium polyacrylate, polyacrylamide, styrene-butadiene rubber, sodium alginate, polyethylene glycol, guar gum, guar gum polymers and guar gum copolymers, or one or more of these.

[0021] Furthermore, the thickness range of the positive electrode active layer is preferably 100-300μm, such as 150μm, 160μm, 170μm, 180μm, 190μm, 200μm, 210μm, 220μm, 230μm, 240μm, 250μm, etc., including but not limited to the thickness values ​​listed above.

[0022] A fourth objective of this invention is to provide a method for preparing the above-described positive electrode sheet, comprising the following steps:

[0023] S01. Add the formulated amount of positive electrode active material, conductive agent, and binder to water and stir evenly to obtain positive electrode active slurry;

[0024] S02. The positive electrode active slurry is coated on at least one side of the current collector along the thickness direction, dried to obtain the positive electrode active layer, and pressed to obtain the positive electrode sheet.

[0025] Furthermore, the current collector is aluminum foil, or other materials that can be used as current collectors; preferably, the thickness of the current collector is in the range of 5-14 μm.

[0026] A fifth object of the present invention is to provide a sodium-ion battery comprising a negative electrode, a separator and an electrolyte, and importantly, comprising the aforementioned positive electrode material or positive electrode.

[0027] Furthermore, the negative electrode sheet includes a negative electrode active layer; the negative electrode active layer includes a negative electrode active material, a negative electrode conductive agent, and a negative electrode binder.

[0028] Furthermore, the negative electrode active layer comprises the following components by mass percentage: 75-95% negative electrode active material, 5-15% negative electrode conductive agent and 5-15% negative electrode binder.

[0029] Furthermore, the negative electrode active material is selected from one or more of artificial graphite, natural graphite, elemental silicon, silicon oxide, and titanium phosphate oxide (including sodium titanium phosphate, sodium titanium manganese phosphate, and titanium phosphate).

[0030] Furthermore, the negative electrode conductive agent is selected from one or more of conductive carbon black, conductive graphite, carbon nanotubes, and graphene.

[0031] Furthermore, the negative electrode binder can be any negative electrode binder well known to those skilled in the art, without any special limitations, such as chitosan, xanthan gum, gellan gum, gum arabic, polyacrylonitrile, polyvinylidene fluoride, polyvinyl alcohol, carboxymethyl cellulose, lithium carboxymethyl cellulose, sodium carboxymethyl cellulose, polymethacryloyl, polyacrylic acid, lithium polyacrylate, sodium polyacrylate, polyacrylamide, styrene-butadiene rubber, sodium alginate, polyethylene glycol, guar gum, guar gum polymers and guar gum copolymers, or one or more of these.

[0032] Furthermore, in this invention, the type of diaphragm can be any diaphragm known to those skilled in the art, and there are no special limitations, such as one or more of polyethylene, polypropylene, polyacrylonitrile, polyacrylonitrile fiber, polyamic acid, polyimide, polyamide, polydopamine, polyarylethersulfone, polyvinylidene fluoride, polyethylene oxide, polyethylene terephthalate, polyester, nonwoven fabric membrane, and cellulose paper-based separator membrane.

[0033] Furthermore, the electrolyte can be any electrolyte well known to those skilled in the art, and there are no special restrictions. The solvent is at least one of methyl ethyl carbonate, diethyl carbonate, propylene carbonate, ethylene carbonate, dimethyl carbonate, and diethylene glycol diethanol ether; the electrolyte salt is at least one of sodium perchlorate, sodium bis(oxalate)borate, and sodium hexafluorophosphate.

[0034] The beneficial effects of this invention are:

[0035] This invention selects specific raw materials: sodium pyrophosphate (Na4P2O7) and ferric phosphate dihydrate (FePO4). 4· The precursor is prepared by dry mixing of nano-iron powder (2H2O). Subsequently, sodium iron pyrophosphate (Na4Fe3(PO4)2P2O7) is prepared as a sodium-ion battery cathode material using a microwave heating process. This heating method utilizes the absorption of microwaves by the nano-iron powder to activate active sites and achieve molecular-level heating of the polar molecules of the water of crystallization in ferric phosphate dihydrate. This triggers a spontaneous redox reaction at a lower temperature, allowing the trivalent iron in ferric phosphate dihydrate to undergo a spontaneous redox reaction with the nano-iron powder, replacing the high-temperature carbon reduction sintering process. Simultaneously, this synthesis method precisely controls the stoichiometric ratio of pyrophosphate to phosphate, resulting in a low reaction temperature, no second-step reaction, and no further conversion of phosphate to pyrophosphate. This precise control of the target product, compared to the uncontrollable two-stage high-temperature sintering of phosphate to pyrophosphate in conventional methods, offers more precise control and a purer target product. Attached Figure Description

[0036] Figure 1 The image shows the XRD phase diffraction pattern of the material in Example 1.

[0037] Figure 2 The charge-discharge curves of button batteries prepared from the sample materials of Example 1 and Comparative Example 1 are compared.

[0038] Figure 3 The charge-discharge curves of button batteries prepared from the sample materials of Example 1 and Comparative Example 2 are compared.

[0039] Figure 4 The charge-discharge curves of button batteries prepared from the sample materials of Example 1 and Comparative Example 3 are compared.

[0040] Figure 5 The results of cycle performance tests on button batteries prepared from the materials of Example 1 are shown. Detailed Implementation

[0041] The present invention will be further described below with reference to the accompanying drawings and specific embodiments, so that those skilled in the art can better understand and implement the present invention. However, the embodiments described are not intended to limit the present invention.

[0042] As described in the background section, the synthesis cost of sodium iron pyrophosphate, a polyanionic material for sodium-ion batteries, is currently high. Materials prepared by traditional high-temperature sintering contain many impurities and have uneven crystal structures, resulting in unstable electrical performance and low specific capacity in the assembled sodium-ion batteries.

[0043] To address the above problems, the embodiments of this invention provide a method for preparing sodium iron pyrophosphate (Na4Fe3(PO4)2P2O7), a battery cathode material, comprising the following steps:

[0044] S1. Sodium pyrophosphate, ferric phosphate dihydrate, and iron powder are dry-mixed in a certain proportion to obtain a precursor; the molar ratio of sodium pyrophosphate, ferric phosphate dihydrate, and iron powder is 0.8-1.2:1.8-2.2:0.8-1.2.

[0045] S2. The precursor obtained in step S1 is microwave-heated to 440-460℃ and kept at that temperature to synthesize sodium iron pyrophosphate Na4Fe3(PO4)2P2O7.

[0046] This invention employs a one-step synthesis method, selecting sodium pyrophosphate and ferric phosphate dihydrate with the same theoretical stoichiometric ratio as the target product. Iron powder is uniformly mixed in a solid phase, and then sodium ferric phosphate pyrophosphate is synthesized by microwave heating. This preparation method achieves low-temperature synthesis of sodium ferric phosphate pyrophosphate material. During microwave heating, the polar molecular water of crystallization within the ferric phosphate dihydrate serves as a medium to achieve molecular-level internal heating, which is more efficient, more uniform in heating, and has a faster heating rate compared to conventional external radiation heating. The partial absorption of microwaves by the iron powder results in high temperatures at iron sites and activation of the outer electrons of iron atoms, leading to a redox reaction with the ferric iron in the ferric phosphate dihydrate, spontaneously converting it into the target ferrous iron. This preparation method results in lower sintering temperatures and shorter holding times during the material sintering process, controllable synthesis products, low burn-off rate of the raw material system, no by-product emissions, and low energy consumption.

[0047] Preferably, in step S1, the particle size of sodium pyrophosphate is between 5 and 20 μm. For example, 5 μm, 7 μm, 10 μm, 13 μm, 15 μm, 17 μm, 20 μm, etc., including but not limited to the particle size values ​​listed above.

[0048] Preferably, in step S1, the particle size of ferric phosphate dihydrate is between 5 and 20 μm. For example, 5 μm, 7 μm, 10 μm, 13 μm, 15 μm, 17 μm, 20 μm, etc., including but not limited to the particle size values ​​listed above.

[0049] Preferably, in step S1, the iron powder is micron-sized iron powder, nano-sized iron powder, or a mixture of both.

[0050] Preferably, in step S1, the D50 of the iron powder is 0.5-5 μm. For example, 0.5 μm, 1.0 μm, 1.5 μm, 2.0 μm, 2.5 μm, 3.0 μm, 3.5 μm, 4.0 μm, 4.5 μm, 5.0 μm, etc., including but not limited to the particle size values ​​listed above.

[0051] In this invention, in step S1, the dry mixing is airflow mixing, preferably using a high-speed mixer with high-speed rotating blades to generate airflow, so that different powders evaporate into the air to come into contact and mix.

[0052] In the mixing process, particle size affects the degree of uniform distribution. The more particles there are and the smaller their size, the greater the likelihood of uniform distribution. Furthermore, particles of different sizes exhibit different motion behaviors during mixing. Smaller particles are less affected by gravity, have a larger specific surface area, and are more influenced by particles in contact with them. Excessively small particle sizes increase surface defects, and particle volume expansion reduces the space for particle movement, thus lowering mixing efficiency. Additionally, particle size also affects crystal form, thereby altering the structure and composition of the target product. Therefore, the size of the raw materials and the mixing method both influence the synthesis of the target product. The preferred settings are: airflow velocity of 5-20 m / s, stirrer speed of 500-3000 rpm, mixing time of 30-60 min, and temperature of 20-25℃.

[0053] In this invention, in step S1, the molar ratio of sodium pyrophosphate, ferric phosphate dihydrate, and iron powder is 0.8-1.2:1.8-2.2:0.8-1.2, for example, 0.8:1.8:0.8, 0.9:1.9:0.9, 1:2:1, 1.1:2.1:1.1, 1.2:2.2:1.2, etc., including but not limited to the ratios listed above. The most preferred ratio is 1:2:1, where the stoichiometric ratio of pyrophosphate to phosphate is 1:2. By precisely controlling the raw material ratio, the required pyrophosphate and phosphate ions are provided for the target product, and there are no remaining raw materials for further conversion, thus improving the purity of the product.

[0054] Preferably, in step S2, the microwave power is 1-10 kW, and the temperature is controlled to reach 440-460°C within 2-5 minutes. For example, 440°C, 445°C, 450°C, 455°C, 460°C, etc., including but not limited to the temperature values ​​listed above.

[0055] Preferably, in step S2, the temperature is maintained for 1-3 hours. For example, 1 hour, 1.2 hours, 1.5 hours, 1.7 hours, 2 hours, 2.3 hours, 2.5 hours, 2.8 hours, 3 hours, etc., including but not limited to the time values ​​listed above.

[0056] Microwave heating primarily relies on the process of an object absorbing microwave energy and converting it into heat energy. Under the influence of microwaves, polar molecules (such as water molecules) inside the object rapidly rotate and collide, generating heat. This heating method differs from traditional heat conduction heating because it acts directly on the object's interior, achieving simultaneous heating from the inside out. Ferric phosphate dihydrate, a raw material, serves as a microwave heating aid, achieving uniform and efficient heating through the polar molecular characteristics of water of crystallization. Simultaneously, the microwave absorber in the raw material is iron powder. During the heating process, the nano-iron powder absorbs some microwaves, resulting in a temperature at the iron powder site that is higher than the temperature at adjacent sites. This also activates the external electrons of the iron powder atoms, leading to a redox reaction with the ferric iron in the ferric phosphate, achieving the formation of the target ferrous iron at a macroscopically low temperature.

[0057] Furthermore, heating temperature and heating rate affect the spatial arrangement of molecules and atoms in the material, causing changes in crystal structure and chemical composition, thereby affecting its electrical properties. In this invention, heating to 440-460℃ is controlled for 2-5 minutes. At this temperature, a redox reaction occurs, and molecules rearrange themselves, completely altering the crystal structure of the substance. After a period of time, the heating is stopped, forming the target product with a specific crystal structure. Moreover, the preparation method of this invention uses a single temperature, resulting in low energy consumption and is easier to achieve compared to high-temperature sintering.

[0058] The embodiments of the present invention provide sodium iron pyrophosphate Na4Fe3(PO4)2P2O7 obtained by the above preparation method.

[0059] The embodiments of the present invention also provide a positive electrode sheet, including a positive electrode active layer, wherein the positive electrode active layer includes a positive electrode active material, a positive electrode conductive agent and a positive electrode binder, and the positive electrode active material is sodium iron pyrophosphate Na4Fe3(PO4)2P2O7.

[0060] Preferably, the positive electrode active layer comprises the following components by mass percentage: 75-95% positive electrode active material, 5-15% positive electrode conductive agent and 5-15% positive electrode binder.

[0061] Preferably, the positive electrode conductive agent is selected from one or more of conductive carbon black, conductive graphite, carbon nanotubes, and graphene.

[0062] Preferably, the positive electrode binder is any positive electrode binder well known to those skilled in the art, without any special limitations, such as chitosan, xanthan gum, gellan gum, gum arabic, polyacrylonitrile, polyvinylidene fluoride, polyvinyl alcohol, carboxymethyl cellulose, lithium carboxymethyl cellulose, sodium carboxymethyl cellulose, polymethacrylamide, polyacrylic acid, lithium polyacrylate, sodium polyacrylate, polyacrylamide, styrene-butadiene rubber, sodium alginate, polyethylene glycol, guar gum, guar gum polymers and guar gum copolymers, or one or more of these.

[0063] Preferably, the thickness range of the positive electrode active layer is 100-300μm, such as 150μm, 160μm, 170μm, 180μm, 190μm, 200μm, 210μm, 220μm, 230μm, 240μm, 250μm, etc., including but not limited to the thickness values ​​listed above.

[0064] The embodiments of the present invention also provide a method for preparing the above-described positive electrode sheet, comprising the following steps:

[0065] S01. Add the formulated amount of positive electrode active material, conductive agent, and binder to water and stir evenly to obtain positive electrode active slurry;

[0066] S02. The positive electrode active slurry is coated on at least one side of the current collector along the thickness direction, dried to obtain the positive electrode active layer, and pressed to obtain the positive electrode sheet.

[0067] Preferably, the current collector is aluminum foil, but it can also be other materials that can be used as current collectors; preferably, the thickness of the current collector is in the range of 5-14 μm.

[0068] The present invention also provides a sodium-ion battery comprising a negative electrode, a separator, and an electrolyte, and importantly, comprising the aforementioned positive electrode material or positive electrode.

[0069] Preferably, the negative electrode sheet includes a negative electrode active layer; the negative electrode active layer includes a negative electrode active material, a negative electrode conductive agent, and a negative electrode binder.

[0070] Preferably, the negative electrode active layer comprises the following components by mass percentage: 75-95% negative electrode active material, 5-15% negative electrode conductive agent and 5-15% negative electrode binder.

[0071] Preferably, the negative electrode active material is selected from one or more of artificial graphite, natural graphite, elemental silicon, silicon oxide, and titanium phosphate oxide (including sodium titanium phosphate, sodium titanium manganese phosphate, and titanium phosphate).

[0072] Preferably, the negative electrode conductive agent is selected from one or more of conductive carbon black, conductive graphite, carbon nanotubes, and graphene.

[0073] Preferably, the negative electrode binder is any negative electrode binder well known to those skilled in the art, without any special limitations, such as chitosan, xanthan gum, gellan gum, gum arabic, polyacrylonitrile, polyvinylidene fluoride, polyvinyl alcohol, carboxymethyl cellulose, lithium carboxymethyl cellulose, sodium carboxymethyl cellulose, polymethacrylamide, polyacrylic acid, lithium polyacrylate, sodium polyacrylate, polyacrylamide, styrene-butadiene rubber, sodium alginate, polyethylene glycol, guar gum, guar gum polymers and guar gum copolymers, or one or more of these.

[0074] Preferably, the type of diaphragm can be any diaphragm known to those skilled in the art, and there are no special limitations, such as one or more of polyethylene, polypropylene, polyacrylonitrile, polyacrylonitrile fiber, polyamic acid, polyimide, polyamide, polydopamine, polyarylethersulfone, polyvinylidene fluoride, polyethylene oxide, polyethylene terephthalate, polyester, nonwoven fabric membrane and cellulose paper-based separator membrane.

[0075] Preferably, the electrolyte is any electrolyte well known to those skilled in the art, and there are no special restrictions. The solvent is at least one of methyl ethyl carbonate, diethyl carbonate, propylene carbonate, ethylene carbonate, dimethyl carbonate, and diethylene glycol diethanol ether; the electrolyte salt is at least one of sodium perchlorate, sodium bis(oxalate)borate, and sodium hexafluorophosphate.

[0076] Example 1

[0077] S1, Dry mixing: Weigh 1 mol sodium pyrophosphate (Na4P2O7) (D50 is 12μm), 2 mol ferric phosphate dihydrate (FePO4·2H2O) (D50 is 10μm), and 1 mol iron powder (Fe) with D50:0.5μm. Transfer them to a high-speed airflow mixer and disperse them at high speed for 40 min. The airflow speed is 10 m / s and the stirring speed is 2000 rpm to obtain a uniformly mixed precursor.

[0078] S2, Sintering Synthesis: The obtained precursor was transferred to a microwave heating furnace. Microwave heating was performed using a 2.45 GHz industrial microwave band at a power of 5 kW for 2-5 minutes to rapidly raise the temperature to 440℃. The heating power was controlled to maintain a stable temperature of 440-450℃ for 2 hours to synthesize the target product, sodium iron pyrophosphate. The diffraction pattern of the target product is shown below. Figure 1 a. It conforms to the X-ray diffraction standard card and has no impurity phases of NaFePO4 and Na2FeP2O7.

[0079] Example 2

[0080] S1, Dry mixing: Weigh 1.5 mol sodium pyrophosphate (Na4P2O7) (D50 is 12 μm), 3 mol ferric phosphate dihydrate (FePO4·2H2O) (D50 is 18 μm), and 1.5 mol iron powder (Fe) with D50:0.7 μm. Transfer them to a high-speed airflow mixer and disperse them at high speed for 30 min. The airflow speed is 15 m / s and the stirring speed is 2500 rpm to obtain a uniformly mixed precursor.

[0081] S2, sintering synthesis: The obtained precursor is transferred to a microwave heating furnace, and microwave heating is performed for 2-5 minutes using the 2.45GHz industrial microwave band and a power of 10kW to rapidly raise the temperature to 450℃. The heating power is controlled to maintain the temperature stable at 450-460℃, and the temperature is held for 1 hour to synthesize the target product sodium iron pyrophosphate.

[0082] Example 3

[0083] S1, Dry mixing: Weigh 1.5 mol sodium pyrophosphate (Na4P2O7) (D50 is 10 μm), 3 mol ferric phosphate dihydrate (FePO4·2H2O) (D50 is 15 μm), and 1.5 mol iron powder (Fe) with D50:1.0 μm. Transfer them to a high-speed airflow mixer and disperse them at high speed for 50 min. The airflow speed is 5 m / s and the stirring speed is 1000 rpm to obtain a uniformly mixed precursor.

[0084] S2, sintering synthesis: The obtained precursor is transferred to a microwave heating furnace, and microwave heating is performed for 2-5 minutes using the 2.45GHz industrial microwave band and a power of 5kW to rapidly raise the temperature to 440℃. The heating power is controlled to maintain the temperature at 440℃ and the temperature is held for 3 hours to synthesize the target product sodium iron pyrophosphate.

[0085] Comparative Example 1

[0086] S1, Mixing: Weigh 1 mol sodium dihydrogen phosphate (NaH2PO4), 3 mol ferric phosphate (FePO4), 1.5 mol sodium carbonate, 70 g glucose and 1.5 L water, transfer to a sand mill, wet ball mill for 20-30 minutes to obtain a uniformly mixed precursor slurry;

[0087] S2, Drying: The obtained precursor slurry is transferred to a spray drying device for spray drying to prepare the precursor;

[0088] S3, sintering synthesis: The obtained precursor is transferred to a box furnace, heated at 3℃ / min to 350℃, held for 4 hours, then heated at 3℃ / min to 580℃, held for 10 hours to synthesize the target product sodium ferric pyrophosphate.

[0089] Comparative Example 2

[0090] S1, Mixing: Weigh 1 mol sodium dihydrogen phosphate (NaH2PO4), 3 mol ferric phosphate dihydrate (FePO4·2H2O), 1.5 mol sodium carbonate, 70 g glucose and 1.5 L water, transfer to a sand mill, wet ball mill for 20-30 minutes to obtain a uniformly mixed precursor slurry;

[0091] S2, Drying: The obtained precursor slurry is transferred to a spray drying device for spray drying to prepare the precursor;

[0092] S3, sintering synthesis: The obtained precursor is transferred to a microwave heating furnace, and microwave heating is performed for 2-5 minutes using the 2.45GHz industrial microwave band and a power of 5kW to rapidly raise the temperature to 440℃. The heating power is controlled to maintain the temperature stable at 440-450℃, and the temperature is held for 2 hours to synthesize the target product sodium iron pyrophosphate.

[0093] Comparative Example 3

[0094] S1, Dry mixing: Weigh 1 mol sodium pyrophosphate (Na4P2O7) (D50 is 12μm), 2 mol ferric phosphate dihydrate (FePO4·2H2O) (D50 is 10μm), and 1 mol iron powder (Fe) with D50:0.5μm. Transfer them to a high-speed airflow mixer and disperse them at high speed for 40 min. The airflow speed is 10 m / s and the stirring speed is 2000 rpm to obtain a uniformly mixed precursor.

[0095] S2, sintering synthesis: The obtained precursor is transferred to a box furnace and heated at 3℃ / min to 440℃, and held for 2 hours to synthesize the target product sodium ferric pyrophosphate.

[0096] Comparative Example 4

[0097] S1, Dry mixing: Weigh 1 mol sodium pyrophosphate (Na4P2O7) (D50 is 12μm), 2 mol ferric phosphate dihydrate (FePO4·2H2O) (D50 is 10μm), and 1 mol iron powder (Fe) with D50:0.5μm. Transfer them to a sand mill and wet ball mill for 20-30 minutes to obtain a uniformly mixed precursor slurry.

[0098] S2, Drying: The obtained precursor slurry is transferred to a spray drying device for spray drying to prepare the precursor;

[0099] S3, sintering synthesis: The obtained precursor is transferred to a microwave heating furnace, and microwave heating is performed for 2-5 minutes using the 2.45GHz industrial microwave band and a power of 5kW to rapidly raise the temperature to 440℃. The heating power is controlled to maintain the temperature stable at 440-450℃, and the temperature is held for 2 hours to synthesize the target product sodium iron pyrophosphate.

[0100] Test case

[0101] Sodium-ion batteries were constructed using the cathode materials prepared in the above embodiments and comparative examples, and the electrical performance of each battery was tested.

[0102] (1) Preparation of sodium-ion batteries

[0103] Positive electrode sheet: Na4Fe3(PO4)2P2O7, superP, and polyvinylidene fluoride are mixed in a mass ratio of 8:1:1. NMP is added and the mixture is stirred until homogeneous to obtain a positive electrode active slurry. The positive electrode active slurry is uniformly coated onto the surface of an aluminum foil current collector, and the electrode sheet is prepared by 200µm doctor blade coating. After drying and cold pressing, the positive electrode sheet is obtained. The mass loading of the active material is approximately 2.5 mg / cm³. -2 .

[0104] Negative electrode: Sodium metal sheet.

[0105] Separator: PE is selected as the separator membrane, with a thickness of 10μm.

[0106] Cell assembly: Arrange the positive electrode, separator, negative electrode, and separator in sequence, and use winding as the assembly method.

[0107] Preparation of electrolyte: Sodium hexafluorophosphate was dissolved in ethylene carbonate at a concentration of 1 mol / L.

[0108] Electrolyte is injected into the dry battery cell and soaked for 24 hours. Then, it is formed at 45°C. The formation process is as follows: charge to 3.4V at 0.05C, then charge to 4.0V at 0.2C. After aging at room temperature for 24 hours, the battery cell manufacturing is completed.

[0109] (2) Performance Testing

[0110] The assembled button cells were subjected to conventional 0.1C charge-discharge characterization, with a charge-discharge voltage range of 1.5 to 4.2V.

[0111] The charge-discharge test results of Example 1 and Comparative Examples 1-4 are shown in the table below. Figure 2-4 From this, we can see that:

[0112] During discharge testing, at the same voltage, the battery assembled from the material prepared in Example 1 had a discharge specific capacity close to 110 mAh / g, while Comparative Example 1 was slightly lower, reaching 100 mAh / g. This is because, in preparing the positive electrode active material of this invention, the trivalent iron in ferric phosphate dihydrate undergoes redox reaction with nano-iron powder at a lower temperature. The phase transition temperatures of iron reduction and phosphate conversion to pyrophosphate are close, resulting in a stable conversion ratio. In addition, the control of the raw material ratio leads to fewer impurities in the prepared battery material, resulting in stable sintering capacity. Furthermore, given that the material preparation method in Example 1 is much simpler than that in Comparative Example 1, the material of this invention has significant advantages.

[0113] In the preparation process of Example 1, high-speed airflow mixing is used in the material mixing stage. Compared with the commonly used sand milling and drying process in Comparative Example 1, the consumption of auxiliary materials such as solvents and dispersants in the sand milling step and the energy consumption in the drying step are reduced, resulting in energy savings of more than 60%. In the material sintering stage, the microwave sintering method is used at a temperature of 440-460℃ for 1-3 hours, while the conventional box furnace sintering temperature needs to reach 580℃ and be held for 10 hours to achieve a similar effect. Energy consumption in the sintering stage is reduced by 70-80%.

[0114]

[0115]

[0116] The difference between Comparative Example 2 and Comparative Example 1 is that the sintering method used is microwave sintering. From the test structure, the specific capacity is only 70mAh / g. This is mainly because spray drying is used in the mixing stage. After spray drying, the moisture content of the powder is generally less than 0.5wt%. The amount of polar molecules (water) present during the sintering process is extremely small. Polar molecules assist in the microwave absorption and molecular heating in the preparation process of sodium iron pyrophosphate. However, the absence of water of crystallization in the raw materials will disrupt this process.

[0117] Compared with Example 1, Comparative Example 3 was sintered in a box furnace according to the microwave sintering procedure (ensuring the same preparation process time and energy consumption). The material could not be completely reacted, and the specific capacity was only 47 mAh / g.

[0118] Compared with Example 1, Comparative Example 4 differs in that the mixing process using sand milling results in a material without electrochemical activity. This is mainly because during the spray drying process, Fe metal is oxidized to iron oxide, which cannot achieve the effect of reducing trivalent iron in ferric phosphate dihydrate to divalent iron. Therefore, when using Fe powder as a raw material, only dry mixing can be used, such as the high-speed dry mixing in this invention. Under the same energy consumption, microwave sintering is the preferred solution.

[0119] Based on the above conclusions, the powders prepared using different process parameters in Examples 2-3 show similar specific capacities. Long-cycle testing was conducted on the battery assembled in Example 1, and the cycle performance was as follows: Figure 5 As shown, after 3000 cycles at 1C, the capacity retention rate is above 92%.

[0120] Obviously, the above embodiments are merely illustrative examples for clear explanation and are not intended to limit the implementation. Those skilled in the art will recognize that other variations or modifications can be made based on the above description. It is neither necessary nor possible to exhaustively list all possible implementations here. However, obvious variations or modifications derived therefrom are still within the scope of protection of this invention.

Claims

1. A method for preparing sodium iron pyrophosphate (Na4Fe3(PO4)2P2O7) as a battery cathode material, characterized in that, Includes the following steps: S1. Sodium pyrophosphate, ferric phosphate dihydrate, and iron powder are dry-mixed in a molar ratio of 0.8-1.2:1.8-2.2:0.8-1.2 to obtain a precursor; the particle size of the sodium pyrophosphate is 5-20 μm; the particle size of the ferric phosphate dihydrate is 5-20 μm; the iron powder is micron-sized iron powder, nano-sized iron powder, or a mixture of both; the dry mixing is air-flow mixing, with an airflow velocity of 5-20 m / s, a stirrer speed of 500-3000 rpm, a mixing time of 30-60 min, and a temperature of 20-25 ℃. S2. The precursor obtained in step S1 is microwave-heated to 440-460 °C and kept at that temperature to synthesize sodium iron pyrophosphate Na4Fe3(PO4)2P2O7.

2. The preparation method according to claim 1, characterized in that, In step S1: the D50 of the iron powder is 0.5-5 μm.

3. The preparation method according to claim 1, characterized in that, In step S2: the microwave power is 1-10 kW, and the temperature is controlled to reach 440-460 ℃ in 2-5 minutes, and then kept at 440-460 ℃ for 1-3 hours.

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