Preparation method for sodium iron sulfate polyanion positive electrode material, and device

Abstract

A preparation method for a sodium iron sulfate polyanion positive electrode material, and a device, relating to the technical field of secondary battery material manufacturing. The provided preparation method comprises: performing wet ball milling on sodium sulfate and ferrous sulfate and sintering the resulting mixture. The sintering is performed in a protective atmosphere having a flow rate of 0.024 to 20 mm / s; a moisture absorbent is present in the sintering environment, and the moisture absorbent is located downstream of the protective atmosphere; and the sintering comprises: sequentially performing constant-temperature treatment at 60-120°C, 150-250°C, and 300-380°C. By means of the provided preparation method, the number of instruments used can be effectively reduced, the number of steps of the preparation method is reduced, and finally, the preparation costs are reduced. Also provided is a device for implementing the preparation method.

Description

A method and device for preparing sodium ferric sulfate polyanion positive electrode material Technical Field

[0001] The present invention relates to the technical field of secondary battery material manufacturing, and in particular to a method and equipment for preparing a sodium ferric sulfate polyanion positive electrode material. Background Art

[0002] Due to the low redox potential, low molar mass, and compact size of metallic lithium, lithium-based secondary batteries (i.e., lithium-ion batteries) offer high energy density, excellent stability, and long cycle life. In recent years, they have been widely used in mobile electronic devices, electric vehicles, and other fields. However, due to the limited and uneven geographical distribution of lithium resources, the price of lithium-ion batteries has gradually increased in recent years. Sodium and lithium are in the same main group and adjacent periods, and therefore share many similar physical and chemical properties. Sodium-ion batteries operate on a similar principle to lithium-ion batteries. Furthermore, sodium resources are abundant and widely distributed, unrestricted by reserves and geographic location. Therefore, as a supplement to lithium-ion batteries, sodium-ion batteries hold great promise for application in low-speed electric vehicles and large-scale distributed energy storage. Currently, the cost and electrochemical performance of cathode materials limit the further development of sodium-ion batteries. Therefore, finding low-cost, high-performance cathode materials is key to the development of sodium-ion batteries.

[0003] Currently, the three most studied cathode materials for sodium-ion batteries are oxides, Prussian blue analogs, and polyanionic compounds. While oxide-based cathode materials offer high theoretical specific capacities, structural changes during cycling result in poor cycling stability. Prussian blue analogs are environmentally unfriendly due to the cyanide groups in their structures, and the difficulty in removing the crystalline water within them compromises the material's sodium storage performance. Compared to these two types of materials, polyanionic compounds are considered ideal cathode materials for future sodium-ion batteries due to their stable crystal structure and three-dimensional sodium ion diffusion channels, as well as the strong inductive effect of polyanions such as sulfate (SO42-). This results in higher operating voltages and improved cycling stability.

[0004] Compared to other typical polyanion cathode materials, such as phosphates, borates, and silicates, sulfate-based polyanion cathode materials have been extensively studied due to their strong sulfate induction effect. Furthermore, Fe is abundant and inexpensive in the Earth's crust. In 2014, Yamada et al. (Nat. Comm. 2014, 5, 4358) first discovered sodium ferric sulfate (Na2Fe2(SO4)3) and used it as a cathode material for sodium-ion batteries. This material, under Fe2+ / Fe3+ redox reaction conditions, achieved an operating voltage of 3.8V relative to a metallic sodium anode, attracting widespread attention.

[0005] Sodium iron sulfate positive electrode material stands out because of its low raw material cost and good electrochemical stability. Technical issues

[0006] In traditional technology, the preparation methods of sodium ferric sulfate include liquid phase method and solid phase method, but the processes of these two methods are relatively complicated. For example, Janina Molenda et al. reported that a relatively pure sodium ferric sulfate positive electrode material was synthesized by liquid phase method, which includes four steps of raw material dissolution, low temperature drying, granulation and sintering in sequence. This process requires a long time of stirring, drying, grinding and sintering. The process is complicated and consumes a lot of manpower, material resources and time. For another example, the preparation process of the typical solid phase method includes four steps in sequence: dehydration of raw materials, wet ball milling, drying and sintering. It can be seen that the four processes of the solid phase method use different devices respectively, and the connection between the front and back processes requires manual operation, which has problems of energy consumption and time waste.

[0007] Therefore, simplifying the synthesis process and reducing the synthesis cost are the key to the commercialization of sodium iron sulfate positive electrode materials for sodium ion batteries. Technical Solutions

[0008] The present invention aims to solve at least one of the technical problems existing in the prior art. To this end, the present invention provides a method for preparing a sodium ferric sulfate polyanion cathode material, which can effectively save the number of instruments used, simplify the number of steps in the preparation method, and ultimately reduce the preparation cost.

[0009] The present invention also provides equipment for implementing the preparation method.

[0010] According to an embodiment of the first aspect of the present invention, there is provided a method for preparing a sodium ferric sulfate polyanion cathode material, the method comprising wet ball milling sodium sulfate and ferrous sulfate, and sintering the resulting mixture;

[0011] The sintering is carried out in a flowing protective atmosphere; the flow rate of the protective atmosphere is 0.024-20 mm / s;

[0012] The sintering is carried out in the presence of a moisture absorbent, and compared with the mixture, the moisture absorbent is located downstream of the protective atmosphere;

[0013] The sintering includes a first stage sintering, a second stage sintering and a third stage sintering performed sequentially;

[0014] The temperature of the first sintering stage is 60-120°C; the temperature of the second sintering stage is 150-250°C; and the temperature of the third sintering stage is 300-380°C.

[0015] The preparation method according to the embodiment of the present invention has at least the following beneficial effects:

[0016] Since moisture or organic solvents have a significant negative impact on the sintering process of sodium ferric sulfate polyanion positive electrode materials, the traditional solid-phase preparation process requires that the raw materials be fully dehumidified before sintering; for example, the crystallization water in sodium sulfate and ferric sulfate is removed before mixing (which takes about 12 hours), and the mixed mixture is dried twice to remove the solvent in the mixing process and the water absorbed from the environment (which takes about 2 to 6 hours).

[0017] In the preparation method provided by this invention, the first sintering stage corresponds to the volatilization temperature of the solvent used in wet ball milling, which also corresponds to the evaporation temperature of adsorbed water in the raw materials; the second sintering stage corresponds to the dehydration temperature of bound water; and the third sintering stage corresponds to the temperature at which the sodium ferric sulfate polyanion cathode material crystallizes and reshapes. Therefore, the present invention utilizes a single sintering device to achieve the multiple functions of multi-step drying and calcination.

[0018] If temperature is simply controlled, the volatilized solvent and bound water will still adversely affect the performance of the sodium ferric sulfate polyanion cathode material. The present invention creatively adjusts the flow rate of the protective atmosphere and incorporates a desiccant. As a result, the solvent vapor and bound water vapor generated during the sintering process flow downstream with the protective atmosphere and are temporarily fixed by the desiccant. This prevents backflow of the solvent vapor and ultimately mitigates any impact on the sodium ferric sulfate sintering atmosphere.

[0019] Furthermore, the fixed solvent vapor or combined water vapor in the desiccant will evaporate under the residual heat generated during the sintering process (equivalent to the rebirth of the desiccant), and under the flow rate control of the protective atmosphere, the vapor evaporated from the desiccant will only be discharged from the sintering system, and will not reflux to affect the preparation environment of the sodium ferric sulfate polyanion positive electrode material.

[0020] In summary, the preparation method provided by the present invention, by controlling the sintering temperature, placing a moisture absorbent at a specific location, and controlling the flow rate of the protective atmosphere, can utilize raw materials containing various crystalline waters, even those that are damp, and omits steps such as drying the raw materials and intermediate mixtures. This broadens the range of raw material options, simplifies the preparation steps of the sodium ferric sulfate polyanion cathode material, and reduces preparation costs. Furthermore, the sodium ferric sulfate polyanion cathode material prepared by the present invention has high purity, high operating voltage, and high discharge specific capacity.

[0021] In addition, compared with other types of mixing methods, wet ball milling can obtain a more uniform mixed phase, which provides a guarantee for the subsequent preparation of sodium iron sulfate polyanion positive electrode materials with more stable performance.

[0022] According to some embodiments of the present invention, the sodium sulfate includes at least one of anhydrous sodium sulfate and hydrated sodium sulfate.

[0023] The hydrated sodium sulfate includes sodium sulfate decahydrate.

[0024] According to some embodiments of the present invention, the ferrous sulfate includes at least one of anhydrous ferrous sulfate and hydrated ferrous sulfate.

[0025] The hydrated ferrous sulfate includes at least one of ferrous sulfate heptahydrate, ferrous sulfate monohydrate and ferrous sulfate tetrahydrate.

[0026] According to some embodiments of the present invention, the molar ratio of ferrous sulfate to sodium sulfate is (2-x):(1+x), wherein 0≤x≤0.35.

[0027] According to some embodiments of the present invention, the value range of x is 0.05~0.3.

[0028] According to some embodiments of the present invention, the value range of x is 0.1 to 0.25, for example, 0.15 to 0.2.

[0029] According to some embodiments of the present invention, the dispersant used in the wet ball milling includes at least one of anhydrous ethanol, acetone, isobutanol and ethylene glycol.

[0030] According to some embodiments of the present invention, the dispersant used in the wet ball milling includes anhydrous ethanol.

[0031] According to some embodiments of the present invention, the solid content in the wet ball milling is 40-50%. It should be noted that the purpose of wet ball milling is to improve the mixing effect. In actual production, the solid content is not strictly limited as long as the dispersant can submerge the solid matter in the wet ball milling.

[0032] According to some embodiments of the present invention, the rotation speed in the wet ball milling is 1-600 r / min.

[0033] According to some embodiments of the present invention, the rotation speed during the wet ball milling is 100-500 rpm, for example, about 300 rpm.

[0034] According to some embodiments of the present invention, the duration of the wet ball milling is 1 to 600 minutes.

[0035] According to some embodiments of the present invention, the duration of the wet ball milling is 10 to 500 minutes, for example, 200 to 300 minutes.

[0036] According to some embodiments of the present invention, the heating rate of the sintering is 1-10° C. / min, for example, 2-5° C. / min.

[0037] The heating rates of the first stage sintering, the second stage sintering and the third stage sintering may be the same or different.

[0038] According to some embodiments of the present invention, the temperature of the first sintering stage is 80-100°C.

[0039] According to some embodiments of the present invention, the first sintering period is 2 to 6 hours.

[0040] According to some embodiments of the present invention, the first sintering stage lasts for 3 to 5 hours, for example, about 4 hours.

[0041] According to some embodiments of the present invention, the heating rate of the first sintering stage is 1-7° C. / min, for example, 2-5° C. / min.

[0042] According to some embodiments of the present invention, the temperature of the second sintering stage is 200-230°C.

[0043] According to some embodiments of the present invention, the second sintering period is 2 to 10 hours.

[0044] According to some embodiments of the present invention, the second sintering period is 4 to 6 hours.

[0045] According to some embodiments of the present invention, the heating rate of the second sintering stage is 3-10° C. / min, for example, 5-7° C. / min.

[0046] According to some embodiments of the present invention, the temperature of the third sintering stage is 320-350°C.

[0047] According to some embodiments of the present invention, the duration of the third sintering stage is 12 to 36 hours.

[0048] According to some embodiments of the present invention, the third sintering period is 24 to 30 hours.

[0049] According to some embodiments of the present invention, the heating rate of the third sintering stage is 3-8° C. / min, for example, 5-7° C. / min.

[0050] Unless otherwise specified, the sintering time and temperature in the present invention refer to the parameters of the constant temperature stage, excluding the heating stage and the cooling stage; and constant temperature does not mean a stage in which the temperature remains completely unchanged, but refers to a stage in which the temperature varies within the range of ±5°C of the set temperature; slight temperature changes in the constant temperature stage are usually inevitable for sintering equipment.

[0051] Based on the above-mentioned limitation on sintering time, it can be seen that when the present invention omits the drying step and proceeds directly to sintering, the sintering time required is shorter than the time required for two drying steps and one calcination step in the conventional technique. Therefore, it can be seen that the preparation method provided by the present invention significantly reduces the total time required for the preparation method.

[0052] According to some embodiments of the present invention, the protective atmosphere includes at least one of nitrogen, argon and hydrogen.

[0053] According to some embodiments of the present invention, the protective atmosphere is a mixture of hydrogen and argon, wherein the volume ratio of hydrogen to argon is 1:8 to 10, for example, specifically about 1:9.

[0054] According to some embodiments of the present invention, the flow rate of the protective atmosphere is 0.2-5 mm / s.

[0055] According to some embodiments of the present invention, the moisture absorbent includes at least one of calcium chloride, quicklime, mineral desiccant, silica gel, molecular sieve and fiber desiccant.

[0056] According to some embodiments of the present invention, the mineral desiccant comprises a montmorillonite desiccant.

[0057] According to some embodiments of the present invention, the moisture absorbent comprises calcium chloride.

[0058] According to some embodiments of the present invention, the moisture absorbent comprises molecular sieves.

[0059] According to some embodiments of the present invention, the sintering is performed in the presence of a moisture-sensitive color-changing substance. The moisture-sensitive color-changing substance changes color when wet, where wetness can be water or other non-aqueous solvents, thereby enabling intuitive determination of wetness.

[0060] According to some embodiments of the present invention, the humidity-sensitive color-changing material includes at least one of color-changing silica gel, copper chloride, copper sulfate, cobalt chloride, water-writing cloth, and water-writing paper. The material is recyclable. Specifically, after being moistened, it can be restored by heating and drying, thereby improving material utilization efficiency and avoiding resource waste.

[0061] The color-changing silica gel appears blue and pink in dry and wet states respectively;

[0062] The copper chloride appears brown and blue-green in dry and wet states respectively;

[0063] The copper sulfate appears white and blue in dry and wet states respectively;

[0064] The cobalt chloride appears blue and red in dry and wet states respectively;

[0065] The water writing cloth and water writing paper appear white and black in dry and wet states respectively.

[0066] According to some embodiments of the present invention, the moisture-sensitive color-changing substance is located on a side of the moisture absorbent close to the mixture, and may be, for example, color-changing silica gel.

[0067] According to some embodiments of the present invention, the moisture-sensitive color-changing substance is located on a side of the moisture absorbent away from the mixture, for example, it may be water writing paper.

[0068] According to some embodiments of the present invention, the hygroscopic material is provided on both sides of the hygroscopic agent, close to one side of the mixture, and the hygroscopic material is located between the mixture and the hygroscopic agent. Indicators are provided on both sides in order to verify whether the flow rate of the protective atmosphere, the sintering temperature and the sintering time are appropriate. For example, if the indicator close to the side of the mixture changes color, it means that the temperature is appropriate and the solvent, crystal water, etc. can be removed; but if only the indicator close to the side of the mixture changes color, it means that the atmosphere that has reached the hygroscopic agent will return to the sintering space of the mixture, affecting the sintering process of the sodium iron sulfate polyanion positive electrode material. Furthermore, after the indicator close to the side of the mixture changes color and then recovers, it means that the next stage of calcination can be carried out. The present invention can more accurately control the sintering conditions by providing a hygroscopic material. In traditional technology, one can only rely on experience or increase the sintering time.

[0069] It should be noted that the humidity-sensitive color-changing substance is not necessarily required. It can be used to confirm the sintering parameters when changing the preparation raw materials and sintering equipment. However, if the sintering parameters are already mature, the use of the humidity-sensitive color-changing substance can be omitted.

[0070] There is no particular requirement for the distance between the moisture-sensitive color-changing substance and the moisture absorbent, and they may be, for example, in direct contact or spaced apart arrangement.

[0071] According to an embodiment of the second aspect of the present invention, a device for implementing the preparation method is provided, the device comprising a sintering furnace; the sintering furnace is provided with an air inlet and an air outlet; the desiccant is provided near the air outlet in the sintering furnace.

[0072] Since the desiccant is provided in the device, the application range thereof is broadened.

[0073] According to some embodiments of the present invention, the sintering furnace includes at least one of a tube furnace, a roller kiln, a pusher kiln, a rotary kiln and a box furnace.

[0074] According to some embodiments of the present invention, the desiccant covers all transmission channels of the protective atmosphere at a certain cross-section of the sintering furnace. For example, when the sintering furnace is a tubular furnace, the desiccant covers a certain cross-section of the quartz tube downstream of the mixture. This prevents backflow of humid gas downstream of the desiccant.

[0075] According to some embodiments of the present invention, the sintering furnace further includes a moisture-sensitive color-changing substance, and the moisture-sensitive color-changing location is located on at least one side of the moisture absorbent. When the sintering furnace further includes the moisture-sensitive color-changing substance, the location where the moisture-sensitive color-changing substance is located should be transparent to facilitate observation. The transparent location may be made of high-temperature glass.

[0076] Unless otherwise specified, the term “about” in the present invention means that the error is allowed to be within the range of ±2%. For example, about 100 is actually 100±2%×100.

[0077] Unless otherwise specified, “between” in the present invention includes the number itself, for example, “between 2 and 3” includes the endpoint values ​​2 and 3. Beneficial effects

[0078] The sodium ferric sulfate polyanion positive electrode material prepared by the sodium ferric sulfate preparation method and equipment provided by the present invention has a high discharge median voltage, a high discharge specific capacity and good cycle performance. Therefore, the sodium ion battery prepared therefrom is expected to be widely used in the field of energy storage, the field of power batteries or the field of 3C. Furthermore, compared with the traditional solid-phase preparation method, the preparation method and equipment provided by the present invention reduce the original four-step method to a two-step method, and the effect achieved is significantly better than the traditional technology. Therefore, the preparation method and equipment provided by the present invention have many advantages such as saving equipment cost, energy consumption cost, and reaction time. BRIEF DESCRIPTION OF THE DRAWINGS

[0079] The above and / or additional aspects and advantages of the present invention will become apparent and readily understood from the following description of the embodiments with reference to the accompanying drawings, in which:

[0080] FIG1 is a partial schematic diagram of a device provided in Example 1 of the present invention;

[0081] FIG2 is a schematic structural diagram of a device provided in Example 1 of the present invention;

[0082] FIG3 is a comparison of the XRD spectrum of the material obtained in Example 1 of the present invention and the standard spectrum of sodium ferric sulfate.

[0083] FIG4 is a 0.1C first-cycle charge-discharge curve of the sodium ferric sulfate polyanion cathode material obtained in Example 1 of the present invention.

[0084] FIG5 is a cycle performance curve of the sodium iron sulfate polyanion cathode material obtained in Example 1 of the present invention after 1000 cycles at a 5C rate.

[0085] Reference numerals:

[0086] Furnace tube 110, furnace tube support 111, flange 120, air outlet pipe 130, air inlet pipe 140, electric heating element 150, switch 160, flow meter 170, high-temperature insulation layer 180, desiccant 210, moisture-sensitive color-changing substance 220, place for placing material to be sintered 300, panel switch 410, temperature controller 420, instrument 430. Modes for Carrying Out the Invention

[0087] The following will clearly and completely describe the concept of the present invention and the technical effects produced by it in conjunction with the embodiments, so as to fully understand the purpose, features and effects of the present invention. Obviously, the embodiments described are only some embodiments of the present invention, not all embodiments. Based on the embodiments of the present invention, other embodiments obtained by those skilled in the art without paying creative work shall fall within the scope of protection of the present invention.

[0088] In the description of the present invention, reference to terms such as "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 the present invention. In this specification, the exemplary expressions of the above terms do not necessarily refer to the same embodiment or example. Moreover, the specific features, structures, materials, or characteristics described may be combined in any suitable manner in any one or more embodiments or examples.

[0089] Equipment Examples

[0090] Referring to Figures 1 and 2, this example provides a device, which is a modified tube furnace, specifically comprising:

[0091] The furnace tube 110 is made of high-temperature resistant quartz;

[0092] Furnace tube supports 111, there are two furnace tube supports 111, used to fix the furnace tube 110;

[0093] The electric heating element 150 wraps a local area of ​​the furnace tube 110;

[0094] A high-temperature heat-insulating layer 180 , wherein the high-temperature heat-insulating layer 180 wraps the electric heating element 150 ;

[0095] There are two flanges 120, which are respectively provided at both ends of the furnace tube 110;

[0096] An air outlet pipe 130 is provided on a flange 120 and communicates with the interior of the furnace tube 110;

[0097] An air inlet pipe 140 is provided on another flange 120 and communicates with the interior of the furnace tube 110;

[0098] Switch 160, there are two switches 160, which are respectively provided on the air inlet pipe 140 and the air outlet pipe 130;

[0099] Flow meter 170, the flow meter 170 is provided on the intake pipe 140;

[0100] The moisture absorbent 210 is disposed inside the furnace tube 110 and is distributed over a certain cross section of the furnace tube 110 in an area not covered by the electric heating element 150 ;

[0101] The moisture-sensitive color-changing substance 220 is disposed on both sides of the moisture absorbent 210;

[0102] The place 300 for placing the material to be sintered is located inside the furnace tube 110 and in an area surrounded by the electric heating element 150 . The moisture absorbent 210 is close to one side of the air inlet pipe 140 .

[0103] Panel switch 410, which is provided on the modified tube furnace and is used to control whether the modified tube furnace is working;

[0104] A temperature controller 420 is provided on the modified tube furnace to improve the temperature control accuracy of the modified tube furnace;

[0105] Instrument 430, instrument 430 is arranged on the modified tube furnace and is used to display the parameters of the modified tube furnace.

[0106] Example 1

[0107] This example uses the equipment provided in the equipment example to prepare a sodium iron sulfate polyanion positive electrode material. The specific steps are as follows:

[0108] S1. Weigh 0.35 mol of anhydrous sodium sulfate and 0.49 mol of ferrous sulfate heptahydrate respectively; place in a ball mill, add anhydrous ethanol dispersant to obtain a mixture with a solid content of 45%, and ball mill at 300 r / min for 300 min to obtain a precursor mixture;

[0109] S2. The precursor mixture obtained in step S1 is transferred to a porcelain boat and placed in a tube furnace, specifically at 300 where the material to be sintered is placed;

[0110] Nitrogen was introduced from the air inlet pipe 140 to maintain the gas flow rate in the furnace tube 110 at 0.2 mm / s; then the temperature was raised to 80°C at 5°C / min and kept warm for 3 hours; then the temperature was raised to 200°C at 5°C / min and kept warm for 4 hours; then the temperature was raised to 350°C at 5°C / min and kept warm for 24 hours, and then cooled to room temperature to obtain a sodium iron sulfate polyanion positive electrode material.

[0111] The desiccant used in this example is molecular sieve.

[0112] Examples 2 to 34 respectively prepared a sodium iron sulfate polyanion positive electrode material, which differed from Example 2 in that some parameters were different. The specific parameters with differences are shown in Table 1, and the others were the same as Example 1.

[0113]

[0114] Comparative Example 1

[0115] This example prepares a sodium iron sulfate polyanion positive electrode material, which differs from Example 1 in that:

[0116] In step S2, a one-step heating method is adopted, specifically:

[0117] The temperature was directly raised to 350° C. at a rate of 5° C. / min, and the heating time was the total heating time of Example 1, i.e., 31 h.

[0118] Comparative Example 2

[0119] This example prepares a sodium iron sulfate polyanion positive electrode material, which differs from Example 1 in that:

[0120] The device used does not include the desiccant 210 and the moisture-sensitive color-changing substance 220 .

[0121] Comparative Example 3

[0122] This example prepares a sodium iron sulfate polyanion positive electrode material, which differs from Example 1 in that:

[0123] The device used does not include the desiccant 210 .

[0124] Comparative Example 4

[0125] This example prepares a sodium iron sulfate polyanion positive electrode material, which differs from Example 1 in that:

[0126] The gas flow rate in the furnace tube 110 is 0.015 mm / s.

[0127] Comparative Example 5

[0128] This example prepares a sodium iron sulfate polyanion positive electrode material, which differs from Example 1 in that:

[0129] The gas flow rate in the furnace tube 110 is 25 mm / s.

[0130] Comparative Example 6

[0131] In this example, a sodium iron sulfate polyanion cathode material was prepared. The specific steps are as follows:

[0132] D1. Take the same mass of ferrous sulfate heptahydrate as in Example 1 and dehydrate it at 200 ° C for 12h;

[0133] D2. The ferrous sulfate and sodium sulfate obtained in step D1 were placed in a ball mill and milled at a speed of 300r / min for 300min; wherein the amount and source of sodium sulfate were the same as in Example 1;

[0134] D3. The mixture obtained in step D2 was dried in an oven at 80°C for 4h;

[0135] D4. Using a conventional tube furnace (i.e., without the desiccant and the hygroscopic material), heat the mixture obtained in step D3 to 350°C for 24 hours. The heating rate, type of protective atmosphere, and flow rate during the heating process are the same as those in Example 1.

[0136] Comparative Example 7

[0137] This example prepares a sodium iron sulfate polyanion positive electrode material, which differs from Example 1 in that:

[0138] In step S2, a two-step heating method is adopted, specifically: first, the temperature is increased to 200°C at a heating rate of 5°C / min and kept at that temperature for 4 hours, and then the temperature is continued to be increased to 350°C at a heating rate of 5°C / min and kept at that temperature for 24 hours.

[0139] Application Examples

[0140] In this example, the sodium ferric sulfate polyanion positive electrode material obtained in the examples and comparative examples was used as the positive electrode active material to prepare a sodium ion button battery. Specifically:

[0141] Prepare the battery positive electrode: Add 1.4g of sodium ferric sulfate polyanion cathode material powder, 0.4g of acetylene black, and 0.2g of polyvinylidene fluoride to 2mL of nitrogen-methyl pyrrolidone and grind thoroughly to obtain a uniform slurry. Apply the slurry evenly to the surface of aluminum foil (positive electrode current collector) and vacuum dry. Cut the dried electrode sheet into 10mm diameter discs, compact them, and use them as battery positive electrode sheets. The loading of the battery positive electrode sheet is 2-4mg / cm2.

[0142] Prepare the diaphragm: Cut the glass fiber film into 16 mm diameter discs and use them as diaphragms.

[0143] Prepare the electrolyte: Weigh 3.0313 g of sodium bis(fluorosulfonyl)imide and dissolve it in a mixed solvent of 6 mL of ethyl acetate and 4 mL of fluoroethylene carbonate. Mix thoroughly and use as the electrolyte.

[0144] Prepare the battery negative electrode: Press the sodium metal into thin sheets and cut into 12mm diameter discs to serve as the battery negative electrode.

[0145] Battery assembly: In an inert gas-protected glove box, the prepared positive electrode, separator, and negative electrode are tightly stacked in sequence, and electrolyte is added to completely soak the separator. The stacked portion is then encapsulated into a button-type battery casing to complete the battery assembly.

[0146] The obtained button battery is named according to the specific embodiment of the source of its positive electrode active material.

[0147] Test Case

[0148] The first aspect of this example confirms whether the material obtained in the embodiment is sodium ferric sulfate. The specific test method is to obtain the XRD spectrum of the obtained material and compare it with the standard spectrum. The results show that in the XRD spectrum, the characteristic peaks of the embodiment material of the present invention (experimental results) and the sodium ferric sulfate standard spectrum (theoretical results) are in good agreement, indicating that the sodium ferric sulfate polyanion positive electrode material is successfully prepared by the preparation method of the present invention, and the obtained positive electrode material has good crystallinity. The comparison of the XRD spectrum of the material obtained in Example 1 and the standard spectrum is shown in Figure 3. The XRD results of the materials obtained in other embodiments are similar to those obtained in Example 1, and are not listed here one by one. The chemical formula of the materials obtained in each embodiment is calculated from the feed ratio of sodium sulfate and ferrous sulfate in the preparation process, as shown in Table 2.

[0149] The second aspect of this example involved testing the sodium-ion half-cell obtained from the corresponding example. Constant-current charge-discharge tests were conducted using a BTSDA test system from Shenzhen Xinwei Electronics Co., Ltd. in a constant temperature and humidity chamber (25°C, 35%). The voltage range was 2-4.5V, and the charge-discharge rate for the first cycle was 0.1C. The discharge capacity for the first cycle is shown in Table 2. Cycling performance testing began with the second cycle at a rate of 5C. The capacity retention was the ratio of the discharge capacity at the 1000th cycle to the discharge capacity at the second cycle. The test results are shown in Table 2.

[0150] Table 2 Properties of sodium ferric sulfate polyanion cathode materials obtained in Examples 1 to 34 and Comparative Examples 1 to 7

[0151]

[0152] This example also records the first-cycle charge-discharge curves of the half-cell of Example 1, where the discharge specific capacity and operating voltage correspond to the results in Table 1. Furthermore, the charge-discharge curves show that the sodium ferric sulfate polyanion cathode material provided by the present invention has a discharge median voltage of approximately 3.75V. Due to this high median voltage, the cathode material provided by the present invention also has a high energy density. Specific test results are shown in Figure 4.

[0153] This example also records the cycling performance of the half-cell of Example 1, where the capacity retention corresponds to the results in Table 2. As can be seen from the figure, the sodium iron sulfate polyanion cathode material provided by the present invention has excellent cycling stability. Specific test results are shown in Figure 5.

[0154] The results in Table 2, Figures 4 and 5 illustrate that when the sodium ferric sulfate provided by the one-step solid-phase method is used as the positive electrode material for sodium ion batteries, a higher operating voltage, a higher discharge specific capacity and excellent cycle stability are obtained.

[0155] Examples 1-4 investigate sodium ferric sulfate polyanion cathode materials obtained using different raw materials and ratios. The results show that varying the raw material ratios slightly influences the material's specific discharge capacity, but has minimal impact on cycling performance. This is primarily due to the similar structure of the resulting sodium ferric sulfates, which all possess good structural stability and, therefore, exhibit good cycling performance. The specific discharge capacity is positively correlated with the iron content; higher iron content results in higher specific discharge capacity.

[0156] Examples 1, 5-7 are sodium iron sulfate polyanion cathode materials obtained by using different ball milling dispersants. The results show that different dispersants have little effect on the electrochemical properties of the materials. Therefore, non-toxic and low-cost anhydrous ethanol is selected as the dispersant for ball milling.

[0157] Examples 1, 8, and 12 illustrate sodium ferric sulfate polyanion cathode materials obtained using different desiccant agents. The results show that the different desiccant agents have minimal electrochemical effects on the materials, likely due to the strong moisture absorption capacity of the selected desiccant agents. Within the experimental scope of the present invention, the desiccant agents used resulted in sodium ferric sulfate cathode materials with good purity and electrochemical performance, and the materials are recyclable, in line with the concept of green chemistry.

[0158] Examples 1, 13 to 34 are Na2.5Fe1.75(SO4)3 sodium ferric sulfate polyanion positive electrode materials obtained using different preparation processes. By comparison, it was found that different synthesis conditions have a certain influence on the electrochemical properties of the materials; within the scope verified by the examples, the more excellent synthesis conditions are as follows: the ball milling speed is 300r / min, and the ball milling time is 300min. The nitrogen flow rate is 0.2mm / s. The heating rate of the first stage is 5℃ / min, the holding temperature is 80℃, and the holding time is 3h. The heating rate of the second stage is 5℃ / min, the holding temperature is 200℃, and the holding time is 4h. The calcination heating rate of the third stage is 5℃ / min, the holding temperature is 350℃, and the holding time is 24h.

[0159] In Comparative Example 1, a one-step heating method was used, that is, within a temperature range suitable for the crystallization reaction of sodium ferric sulfate, the environment contained a large amount of steam (dispersant or water), thereby affecting the obtained sodium ferric sulfate product and its performance as a positive electrode active material.

[0160] In Comparative Examples 2 and 3, no desiccant is included, or no moisture-sensitive color-changing substance is included at the same time. Therefore, the dispersant vapor or water vapor in the area where the mixture of ferric sulfate and sodium sulfate is located cannot be absorbed in time, and thus the electrochemical performance of the obtained sodium ferric sulfate is not ideal.

[0161] In Comparative Examples 4-5, using either too low or too high airflow velocities, respectively, it was found that the electrochemical performance of the resulting sodium ferric sulfate was poor. This may be because too low an airflow velocity was insufficient to remove the moisture generated during the heating process in the first and second stages, thereby affecting the sintering process of the sodium ferric sulfate in the third stage and thus affecting product quality; while too high an airflow velocity may sweep the raw materials used in the preparation, resulting in insufficient contact between the raw material particles. Furthermore, too high an airflow velocity removes a large amount of heat, causing the temperature in the sintering equipment to fall below the set temperature, thereby affecting the sintering process and product quality.

[0162] In Comparative Example 6, the electrochemical performance of the sodium ferric sulfate obtained using conventional step-by-step sintering was slightly inferior to that of Example 1 of the present invention. This may be due to the inevitable moisture absorption (or water absorption, or moisture absorption) of the sample during the sample transfer between preparation steps D3 and D4, which affected the sintering reaction in step D4. Furthermore, the synthesis process was cumbersome and time-consuming.

[0163] Comparative Example 7 employed a simple two-step sintering process. Without the initial low-temperature heat preservation to remove the dispersant from the ball mill, the volatilization of the organic dispersant had a certain impact on the subsequent sintering of the material, resulting in unsatisfactory electrochemical performance. Furthermore, the absence of a hygroscopic agent or a color-changing agent prevented the reaction from being effectively indicated.

[0164] In summary, the sodium ferric sulfate polyanion positive electrode material prepared by the sodium ferric sulfate preparation method and equipment provided by the present invention has a high discharge median voltage, a high discharge specific capacity and good cycle performance, so the sodium ion battery prepared therefrom is expected to be widely used in the field of energy storage, the field of power batteries or the field of 3C. Furthermore, compared with the traditional solid-phase preparation method, the preparation method and equipment provided by the present invention reduce the original four-step method to a two-step method, and the effect achieved is significantly better than the traditional technology. Therefore, the preparation method and equipment provided by the present invention have many advantages such as saving equipment cost, energy consumption cost, and reaction time.

[0165] While the embodiments of the present invention have been described in detail above with reference to the accompanying drawings, the present invention is not limited to the embodiments described above. Various modifications may be made within the scope of knowledge possessed by a person skilled in the art without departing from the spirit of the present invention. Furthermore, the embodiments of the present invention and the features thereof may be combined with one another unless there is a conflict.

Claims

1. A method for preparing a sodium iron sulfate polyanion positive electrode material, characterized in that: The preparation method comprises wet ball milling sodium sulfate and ferrous sulfate, and sintering the obtained mixture; The sintering is carried out in a flowing protective atmosphere; the flow rate of the protective atmosphere is 0.024-20 mm / s; The sintering is carried out in the presence of a moisture absorbent, which is located downstream of the protective atmosphere compared to the mixture; The sintering includes a first stage sintering, a second stage sintering and a third stage sintering which are performed sequentially; The temperature of the first sintering stage is 60-120°C; the temperature of the second sintering stage is 150-250°C; the temperature of the third sintering stage is 300-380°C.

2. The preparation method according to claim 1, characterized in that: The sintering is carried out in the presence of a hygroscopic color-changing substance, wherein the hygroscopic color-changing substance is located on a side of the hygroscopic agent close to the mixture; and / or the hygroscopic color-changing substance is located on a side of the hygroscopic agent away from the mixture.

3. The preparation method according to claim 1, characterized in that: The desiccant comprises at least one of calcium chloride, quicklime, mineral desiccant, silica gel, molecular sieve and fiber desiccant.

4. The preparation method according to claim 1, characterized in that: The first sintering stage lasts for 2 to 6 hours; the second sintering stage lasts for 2 to 10 hours; and the third sintering stage lasts for 12 to 36 hours.

5. The preparation method according to any one of claims 1 to 4, characterized in that The molar ratio of the ferrous sulfate to the sodium sulfate is (2-x):(1+x), wherein 0≤x≤0.

35.

6. The preparation method according to any one of claims 1 to 4, characterized in that The sodium sulfate includes at least one of anhydrous sodium sulfate and hydrated sodium sulfate; and / or, the ferrous sulfate includes at least one of anhydrous ferrous sulfate and hydrated ferrous sulfate.

7. The preparation method according to any one of claims 1 to 4, characterized in that The dispersant used in the wet ball milling includes at least one of anhydrous ethanol, acetone, isobutanol and ethylene glycol.

8. The preparation method according to any one of claims 1 to 4, characterized in that The protective atmosphere includes at least one of nitrogen, argon and hydrogen.

9. An apparatus for implementing the preparation method according to any one of claims 1 to 8, characterized in that: The equipment comprises a sintering furnace; the sintering furnace is provided with an air inlet and an air outlet; the moisture absorbent is arranged near the air outlet in the sintering furnace.

10. The device according to claim 9, characterized in that The sintering furnace includes at least one of a tubular furnace, a roller kiln, a pusher kiln, a rotary kiln and a box furnace.

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