Sodium iron pyrophosphate positive electrode membrane, its dry preparation method and application

Abstract

The present invention provides a sodium iron pyrophosphate positive electrode film, a dry preparation method thereof, and an application. The dry preparation method includes: dividing a conductive agent into conductive agent A and conductive agent B; mixing conductive agent A with a sodium iron pyrophosphate positive electrode material and obtaining a first material through a first stirring; mixing conductive agent B with sodium trifluoromethanesulfonate and obtaining a second material through a second stirring; mixing the first material with the second material and obtaining a third material after a third stirring; mixing the third material with a binder and obtaining a fourth material after a fourth stirring; and subjecting the fourth material to fibrillation treatment and rolling in sequence to obtain a sodium iron pyrophosphate positive electrode film. By designing the contents and ratios of the sodium iron pyrophosphate positive electrode material, the conductive agent, the binder, and sodium trifluoromethanesulfonate, and simultaneously cooperating with the control of the mixing method, sequence, and steps, the present invention prepares a sodium iron pyrophosphate positive electrode film with a high tap density, high conductivity, and excellent ion transport channels.

Description

Technical Field

[0001] The present invention relates to the field of sodium ion batteries, and in particular to a sodium iron pyrophosphate positive electrode membrane, a dry preparation method thereof and applications thereof. Background Art

[0002] At present, the production of sodium iron pyrophosphate electrode sheets for sodium ion batteries is usually as follows: active material particles are mixed with adhesives to form a slurry with a certain viscosity, the slurry is then coated on the metal surface of the current collector, the solvent in the adhesive is dried, and then it is rolled to obtain the electrode sheet. The defects of this coating process are very obvious, mainly reflected in the following points: (1) Na in sodium iron pyrophosphate + The contradiction between the solid-phase diffusion transmission rate and the particle size. In order to ensure the effective electrochemical performance, the primary and secondary particles of sodium iron pyrophosphate cannot be made too large, but too small particles will affect the processing performance and compaction density; (2) Strong water absorption and high requirements for environmental humidity. The residual alkali content of sodium iron pyrophosphate is high. The material itself inevitably contains a certain amount of alkaline substances, which are easy to absorb water, increase the viscosity of the slurry and form a gel. Therefore, the current electrode slurry process still requires acid neutralization and has high requirements for environmental humidity, which increases production costs; (3) Low compaction density and poor conductivity. In the wet slurry coating process, the adhesive wraps the active material. The adhesive itself is not conductive, which inevitably causes a decrease in conductivity; (4) Low energy density. The wet slurry coating process cannot further increase the thickness of the electrode, so it is difficult to increase the proportion of active materials in the entire battery, resulting in low energy density; (5) The electrode is brittle and easy to crack. After coating and drying, due to the material itself, the electrode is relatively brittle, which leads to easy cracking and easy powder loss during subsequent process operations; (6) The final sodium iron pyrophosphate battery has a low initial efficiency, resulting in poor battery capacity.

[0003] The dry process of sodium iron pyrophosphate pole pieces is designed to solve the above problems. Dry electrode technology includes powder mixing, fiberization, roll-pressing film formation and compounding with current collector. The above process features simple process, greater mechanical strength of pole pieces, pure dry process, avoiding the hygroscopicity problem of sodium iron pyrophosphate, higher active material content, and the ability to increase the compaction density and thickness of pole pieces. However, the porosity of the current dry diaphragm is inevitably low, which cannot suppress the electrochemical capacity decay of sodium ion batteries, thereby affecting battery performance. At the same time, in the process of dry preparation of diaphragms, high-speed fiberization and hot rolling may produce uneven stress distribution inside the diaphragm. When the diaphragm is bonded to the current collector, this uneven stress may cause a decrease in local bonding strength, affecting the overall structural stability, and ultimately leading to poor electrical performance of the resulting battery.

[0004] Based on this, how to design a preparation method of a dry film using sodium iron pyrophosphate as the cathode material, so that the obtained dry cathode film shows higher bonding strength after being laminated with the cathode current collector, is less likely to peel off, is less likely to gel during the subsequent battery application process, and ultimately improves the initial efficiency of the corresponding sodium-ion battery, is one of the important technical problems to be solved in this field. Summary of the Invention

[0005] The main object of the present invention is to provide a sodium iron pyrophosphate cathode film, its dry preparation method and application, to solve the problem that the sodium iron pyrophosphate cathode film in the prior art has poor electrical performance of the corresponding sodium-ion battery due to low porosity, unstable structure, poor bonding with the cathode current collector, and easy gelation during the subsequent application process.

[0006] To achieve the above object, the first aspect of the present invention provides a dry preparation method of a sodium iron pyrophosphate cathode film, including: Step S1, dividing the conductive agent into conductive agent A and conductive agent B; the weight ratio of conductive agent A to conductive agent B is (2~3):1; Step S2, mixing conductive agent A with the sodium iron pyrophosphate cathode material, and obtaining the first material through the first stirring; mixing conductive agent B with sodium trifluoromethanesulfonate, and obtaining the second material through the second stirring; Step S3, mixing the first material with the second material, and obtaining the third material after the third stirring; Step S4, mixing the third material with the binder, and obtaining the fourth material after the fourth stirring; Step S5, subjecting the fourth material to fibrillation treatment to obtain the fifth material; Step S6, rolling the fifth material to obtain the sodium iron pyrophosphate cathode film; the weight ratio of the sodium iron pyrophosphate cathode material, the conductive agent and the binder is (45~50):1:1; based on the total weight of the sodium iron pyrophosphate cathode material, the conductive agent and the binder being 100%, the content of sodium trifluoromethanesulfonate is 1%~3%.

[0007] Further, the rotation speeds of the first stirring and the second stirring are each independently 1000 rpm~1500 rpm, the times are each independently 20 min~30 min, and the stirring temperatures of the first stirring and the second stirring are each independently 20°C~30°C.

[0008] Further, in Step S3, the rotation speed of the third stirring is 500 rpm~1000 rpm, the time is 30 min~40 min, and the third stirring is carried out at 10°C~20°C.

[0009] Further, in Step S4, the rotation speed of the fourth stirring is 1500 rpm~2000 rpm, and the time is 10 min~15 min.

[0010] Further, in step S5, the fibrillation treatment is achieved by air jet milling. During the air jet milling process, the feeding speed is 1 g / min to 500 g / min, and the air pressure is 0.5 MPa to 1 MPa.

[0011] Further, in step S6, the rolling speed of the rolling is 1 rpm / min to 30 rpm / min, the rolling pressure is 0.1 T to 30 T, and the rolling is performed at 160°C to 180°C.

[0012] Further, the conductive agent is selected from one or more of SP, VGCF, and CNT; the binder is selected from one or more of polytetrafluoroethylene, PVDF, and CMC.

[0013] The second aspect of the present invention provides a sodium iron pyrophosphate positive electrode film. The sodium iron pyrophosphate positive electrode film is prepared by the dry method for preparing the sodium iron pyrophosphate positive electrode film described above, and the thickness of the sodium iron pyrophosphate positive electrode film is 100 μm to 250 μm, and the areal density is 150 mg / cm 2 ~400 mg / cm 2 .

[0014] The third aspect of the present invention provides an application of the above sodium iron pyrophosphate positive electrode film as a positive electrode active layer in the battery field to prepare a sodium ion battery positive electrode sheet. The sodium iron pyrophosphate positive electrode film is adhered to at least one surface of the positive electrode current collector, and heat lamination is performed at 110°C to 120°C to obtain a sodium ion battery positive electrode sheet.

[0015] The fourth aspect of the present invention provides a sodium ion battery, including a positive electrode sheet, a negative electrode sheet, and an electrolyte, wherein the positive electrode sheet is the above sodium ion battery positive electrode sheet.

[0016] By applying the technical solution of the present invention, through precise design of the contents and ratios of the sodium iron pyrophosphate positive electrode material, the conductive agent, the binder, and sodium trifluoromethanesulfonate, and at the same time cooperating with strict control of the mixing method, sequence, and steps, finally, through fibrillation and rolling treatments, a sodium iron pyrophosphate positive electrode film with high tap density, high conductivity, and excellent ion transport channels is prepared. Detailed Embodiments

[0017] It should be noted that, without conflict, the embodiments in the present application and the features in the embodiments can be combined with each other. The present invention will be described in detail below with reference to the embodiments.

[0018] As described in the background art, the sodium iron pyrophosphate positive electrode film in the prior art has problems such as low porosity, unstable structure, poor bonding with the positive electrode current collector, and poor electrochemical performance of the corresponding sodium-ion battery caused by easy gelation during subsequent applications. To solve the above technical problems, the first aspect of the present invention provides a dry preparation method of a sodium iron pyrophosphate positive electrode film, including: Step S1, dividing the conductive agent into conductive agent A and conductive agent B; the weight ratio of conductive agent A to conductive agent B is (2~3):1; Step S2, mixing conductive agent A with the sodium iron pyrophosphate positive electrode material and obtaining the first material through the first stirring; mixing conductive agent B with sodium trifluoromethanesulfonate and obtaining the second material through the second stirring; Step S3, mixing the first material and the second material and obtaining the third material after the third stirring; Step S4, mixing the third material with the binder and obtaining the fourth material after the fourth stirring; Step S5, subjecting the fourth material to fibrillation treatment to obtain the fifth material; Step S6, rolling the fifth material to obtain the sodium iron pyrophosphate positive electrode film; the weight ratio of the sodium iron pyrophosphate positive electrode material, the conductive agent, and the binder is (45~50):1:1; based on the total weight of the sodium iron pyrophosphate positive electrode material, the conductive agent, and the binder being 100%, the content of sodium trifluoromethanesulfonate is 1%~3%.

[0019] In the above preparation method, first, by dividing the conductive agent into two parts, A and B, with a specific weight ratio, and mixing them with the positive electrode material and sodium trifluoromethanesulfonate respectively, while achieving more accurate control of the distribution of the conductive agent and avoiding problems such as local excess or deficiency of the conductive agent that may occur in traditional mixing, better dispersion of sodium trifluoromethanesulfonate is achieved, thereby optimizing the performance of sodium trifluoromethanesulfonate, a sodium supplementing agent and pore-forming agent, in the obtained film, and enabling the obtained film to have more excellent electrochemical performance. More specifically, by pre-setting 1%~3% sodium trifluoromethanesulfonate in the positive electrode material and premixing and activating it with a conductive agent in a specific weight ratio, during the electrolyte infiltration and formation processes, it can promote the dissolution of sodium trifluoromethanesulfonate into the electrolyte to form pores, construct ion transport channels, and decompose to supplement sodium at a certain voltage, significantly improving the first efficiency of the battery. And for the fifth material obtained by mixing and preparing through the above steps and process sequence, after fibrillation treatment, the bonding strength and mechanical properties of each component material in it are significantly improved, avoiding problems such as pole piece powder falling and cracking commonly seen in traditional wet processes. At the same time, the fibrillated binder forms a network structure, further optimizing the ion transport channels and improving the electrochemical performance of the obtained positive electrode film. Moreover, by precisely controlling the weight ratio of sodium iron pyrophosphate, the conductive agent, and the binder to be (45~50):1:1, a film with both high energy density and good electrochemical stability can be prepared, because the increase in the content of the active substance is directly related to the improvement of the battery energy density, and the reasonable ratio of the conductive agent and the binder ensures good conductivity and mechanical strength of the film.

[0020] In summary, the present invention prepares a sodium iron pyrophosphate positive electrode membrane with high compaction density, high conductivity and excellent ion transmission channel by accurately designing the content and proportion of sodium iron pyrophosphate positive electrode material, conductive agent, binder and sodium trifluoromethanesulfonate, and strictly controlling the mixing method, sequence and steps.

[0021] Furthermore, by optimizing the specific conditions of the first stirring, i.e., a rotation speed of 1000 rpm to 1500 rpm, a time of 20 min to 30 min, and a stirring temperature of 20°C to 30°C, the conductive agent A can be uniformly mixed with the sodium iron pyrophosphate positive electrode material, reducing the discontinuity of the conductive network inside the membrane caused by insufficient mixing, thereby improving the uniformity and stability of the electrode membrane, and making the sodium ion battery in which it is located have a higher first efficiency. By optimizing the second stirring speed of 1000 rpm to 1500 rpm, the time of 20 min to 30 min, and the temperature of 20°C to 30°C, it is helpful to form a more stable intermediate mixture, reduce the agglomeration and stratification that may occur during the entire mixing process, and promote a more uniform distribution of sodium trifluoromethanesulfonate in the resulting membrane. At the same time, the conductive agent B and sodium trifluoromethanesulfonate are uniformly mixed according to the above conditions, which can promote the physical and chemical reaction between the two, so that in the subsequent battery assembly and formation process, the presence of the conductive agent can better assist the decomposition of sodium trifluoromethanesulfonate, so that it can release sodium ions more efficiently when a specific voltage is reached, replenish the sodium in the electrode, and thus more significantly improve the initial efficiency of the corresponding sodium ion battery.

[0022] In step S3, by preferably having a third stirring speed of 500 rpm to 1000 rpm, a time of 30 min to 40 min, and the third stirring is carried out at 10°C to 20°C, a more thorough mixing of the first material and the second material obtained above can be promoted. Under the above more suitable rotation speed and temperature conditions, a more stable composite system is formed, the conductivity and mechanical properties of the final diaphragm, as well as the uniform distribution of sodium trifluoromethanesulfonate therein are optimized, and ultimately the first efficiency of the sodium ion battery is made higher in the actual application process. In order to make the third material and the binder more evenly mixed, and then promote the formation of a more stable and conductive dry diaphragm, further in step S4, it is preferred that the fourth stirring speed is 1500 rpm to 2000 rpm, and the time is 10 min to 15 min.

[0023] In several typical embodiments, in step S5, the fibrillation treatment is achieved by air jet milling. During the air jet milling process, the feeding speed is 1 g / min to 500 g / min, and the air pressure is 0.5 MPa to 1 MPa. In order to obtain materials with more suitable length and diameter during the fibrillation process, so as to form a more stable network structure in the subsequent preparation process of the diaphragm, improve the mechanical strength and compaction density, and at the same time maintain good electrical conductivity and ion transport characteristics, further preferably, during the air jet milling process, the feeding speed is 100 g / min to 200 g / min. In order to cooperate with the components in the material, reduce unnecessary thermal reactions, and at the same time provide sufficient temperature to soften the binder and form fibers, so as to more effectively improve the stability and electrical conductivity of the subsequent obtained diaphragm, it is preferred that the air jet milling is carried out at 50°C to 70°C. In practical applications, it is preferred that the gas used for air jet milling is one or more of air, nitrogen, and argon. For the formed fifth material, it is preferred that it has a D50 particle size range of 2 μm to 10 μm, so as to form a diaphragm structure with a certain compaction density and good ion transport channels. A more preferred D50 particle size range of 5 μm to 8 μm can further improve the mechanical strength, electrical conductivity, and ion transport efficiency of the diaphragm, while reducing the instability caused by over-dense or over-sparse structures.

[0024] In several typical embodiments, the rolling speed in step S6 is 1 rpm / min to 30 rpm / min, the rolling pressure is 0.1 T to 30 T, and the rolling is carried out at 160°C to 180°C. More preferably, the rolling speed of the rolling is 7 rpm / min to 14 rpm / min, and the rolling pressure is 10 T to 30 T. Through the optimization and more optimization of the parameters during the above rolling process, the sodium iron pyrophosphate positive electrode diaphragm prepared by the dry method can have higher uniformity and consistency. The selected rolling speed and pressure can promote each component material to have enough time to melt and redistribute at high temperature while being under pressure, so as to form a more uniform diaphragm structure, which is convenient for subsequent pore formation and sodium supplementation by sodium trifluoromethanesulfonate, and optimize the performance of the sodium-ion battery. The selected rolling temperature can soften the binder and form a more stable three-dimensional network structure, enhance the mechanical strength of the obtained diaphragm and the subsequent electrode sheet obtained by laminating the diaphragm with the current collector, optimize its electrical conductivity and ion transport path, and improve the electrical performance of the finally obtained sodium-ion battery.

[0025] In several particularly typical embodiments, step S6 further includes: step S6-1, subjecting the fifth material to roll pressing to obtain a prefabricated film; step S6-2, subjecting the prefabricated film to activation treatment to obtain a sodium iron pyrophosphate positive electrode film. That is to say, after roll pressing, the present invention preferably further performs activation treatment on the film obtained by roll pressing. The purpose is to promote the microscopic rearrangement of each component in the film obtained by roll pressing, form a structure more conducive to electrochemical reactions, and further improve the initial efficiency and cycling performance of the corresponding sodium-ion battery. In addition, the activation treatment also helps to eliminate the mechanical stress that may be generated during the preparation of the above-mentioned dry film, thereby making its structure more stable, reducing the structural changes during the subsequent charge and discharge of the battery, and ultimately enhancing its performance. On this basis, it is further preferred that the activation treatment is achieved by heating the prefabricated film at 60°C to 80°C for 10 minutes to 20 minutes. The optimization of the above temperature range and time window is based on the thermal properties of the sodium iron pyrophosphate positive electrode material, the conductive agent, and sodium trifluoromethanesulfonate. The specific heating temperature can promote more effective optimization of the internal structure of the film, reduce the decomposition of its components or the excessive thermalization of the binder; while the control of the heating time improves the sufficiency of the activation treatment and reduces the waste of energy consumption. Ultimately, it more effectively and significantly improves the microscopic structure of the obtained sodium iron pyrophosphate positive electrode film, enhances its electrochemical activity and structural stability, and enables it to exhibit more excellent performance in the corresponding sodium-ion battery.

[0026] Furthermore, in order to better form a fibrous network structure, improve the mechanical strength of the obtained film, and improve the overall performance and reliability of the electrode and the battery where it is located, it is preferred that the conductive agent is selected from one or more of SP, VGCF, and CNT; the binder is selected from one or more of polytetrafluoroethylene, PVDF, and CMC.

[0027] The second aspect of the present invention provides a sodium iron pyrophosphate positive electrode film, which is prepared by the dry preparation method of the above-mentioned sodium iron pyrophosphate positive electrode film, and the thickness of the sodium iron pyrophosphate positive electrode film is 100μm to 250μm, and the areal density is 150mg / cm 2 ~400mg / cm 2 . The obtained positive electrode film has excellent microscopic structure, stability, and electrochemical performance. The optimization of the above areal density and thickness can provide sufficient active materials, improve the energy density, while enhancing the rationality of the internal structure of the film and its electrochemical performance, as well as enhancing the mechanical properties and structural integrity of the electrode after it is laminated with the positive electrode current collector.

[0028] It should be noted that due to the particularity of the electrode material field and the limitations of existing testing and characterization methods, it is difficult to comprehensively and quantitatively characterize the complex microstructure of the obtained sodium iron pyrophosphate positive electrode film. However, the performance test results show that the obtained sodium iron pyrophosphate positive electrode film has a higher bonding strength and is less likely to peel off after being combined with the positive electrode current collector. At the same time, it is also less likely to gel during the subsequent application of the battery, thus providing particularly strong support for the good performance of sodium-ion batteries.

[0029] The third aspect of the present invention provides an application of the above-mentioned sodium iron pyrophosphate positive electrode film as a positive electrode active layer in the battery field to prepare a positive electrode sheet of a sodium-ion battery. The sodium iron pyrophosphate positive electrode film is attached to at least one surface of the positive electrode current collector and thermocompounded at 110°C to 120°C to obtain a positive electrode sheet of a sodium-ion battery. Compounding at this temperature condition to obtain a positive electrode sheet of a sodium-ion battery can better optimize the microstructure of the obtained film, so that the bonding force between the sodium iron pyrophosphate positive electrode film and the positive electrode current collector in the obtained dry-process positive electrode sheet structure is higher, and it is less likely to peel off during subsequent use, and the electrical performance and cycle stability of the sodium-ion battery in which it is located are also higher.

[0030] The fourth aspect of the present invention provides a sodium-ion battery, including a positive electrode sheet, a negative electrode sheet, and an electrolyte, wherein the positive electrode sheet is the above-mentioned positive electrode sheet of a sodium-ion battery. Because the dry-process film prepared by the special dry-process technology provided by the present invention has stronger bonding with the positive electrode current collector after being compounded, when it is applied to a sodium-ion battery, the corresponding battery also shows a higher initial efficiency.

[0031] The following further describes the present application in detail with specific embodiments, and these embodiments should not be construed as limiting the scope claimed by the present application.

[0032] Unless otherwise defined, all professional terms used hereinafter have the same meaning as commonly understood by those skilled in the art. The professional terms used herein are only for the purpose of describing specific embodiments and are not intended to limit the protection scope of the present invention.

[0033] Example 1

[0034] A dry-process preparation method of a sodium iron pyrophosphate positive electrode film:

[0035] Weigh 4 g of dispersed polytetrafluoroethylene powder as a binder, 4 g of conductive carbon black SP as a conductive agent, and 192 g of sodium iron pyrophosphate positive electrode material (i.e., the weight ratio of sodium iron pyrophosphate positive electrode material, conductive agent, and binder is 48:1:1). Then, according to the total weight of the above three being 100%, weigh 1%, that is, 2 g of sodium trifluoromethanesulfonate.

[0036] Step (1): Divide the conductive agent into conductive agent A and conductive agent B, with a weight ratio of conductive agent A: conductive agent B = 2:1.

[0037] Step (2): Mix conductive agent A with the sodium iron pyrophosphate cathode material and conduct the first stirring at 25°C with a rotation speed of 1000 revolutions per minute for 30 minutes to obtain the first material; mix conductive agent B with sodium trifluoromethanesulfonate and conduct the second stirring at 25°C with a rotation speed of 1500 revolutions per minute for 20 minutes to obtain the second material.

[0038] Step (3): Mix the obtained first material and second material at 15°C and conduct the third stirring at a rotation speed of 800 revolutions per minute for 30 minutes to obtain the third material.

[0039] Step (4): Add a binder to the obtained third material and conduct the fourth stirring at a rotation speed of 1500 revolutions per minute for 15 minutes to obtain the fourth material.

[0040] Step (5): Use nitrogen, pass it through a jet mill, set its feeding speed to 100 g / min, the air pressure to 0.6 MPa, and conduct fiberization treatment on the obtained fourth material at 60°C to obtain the fifth material with a D50 of 8 μm.

[0041] Step (6-1): Screen and load the obtained fifth material, and conduct rolling at a rolling temperature of 160°C, a rolling pressure of 10 T, and a rolling speed of 7 rpm / min to obtain a prefabricated membrane sheet.

[0042] Step (6-2): Heat the obtained prefabricated membrane sheet at 60°C for 20 minutes to activate it, and then obtain the sodium iron pyrophosphate cathode membrane sheet through multiple rollings.

[0043] Preparation of a dry sodium-ion battery cathode sheet: Bond one side surface of the obtained sodium iron pyrophosphate cathode membrane sheet to the positive current collector aluminum foil, and obtain the dry cathode sheet after thermal lamination, with a lamination temperature of 120°C.

[0044] Example 2

[0045] A dry preparation method of a sodium iron pyrophosphate cathode membrane sheet:

[0046] Weigh 4 g of dispersed polytetrafluoroethylene powder as the binder, 4 g of conductive carbon black SP as the conductive agent, and 192 g of sodium iron pyrophosphate cathode material (i.e., the weight ratio of sodium iron pyrophosphate cathode material, conductive agent, and binder is 48:1:1). Then, according to the total weight of the above three being 100%, weigh 2%, that is, 4 g of sodium trifluoromethanesulfonate.

[0047] Step (1): Divide the conductive agent into conductive agent A and conductive agent B, with the weight ratio of conductive agent A:conductive agent B = 3:1.

[0048] Step (2): Mix conductive agent A with the sodium iron pyrophosphate cathode material and conduct the first stirring at 30°C with a rotation speed of 1500 revolutions per minute for 20 minutes to obtain the first material; mix conductive agent B with sodium trifluoromethanesulfonate and conduct the second stirring at 30°C with a rotation speed of 1500 revolutions per minute for 20 minutes to obtain the second material.

[0049] Step (3): Mix the obtained first material and second material at 20°C and conduct the third stirring at a rotation speed of 500 revolutions per minute for 40 minutes to obtain the third material.

[0050] Step (4): Add a binder to the obtained third material and conduct the fourth stirring at a rotation speed of 1500 revolutions per minute for 10 minutes to obtain the fourth material.

[0051] Step (5): Use air, pass it through a jet mill, set its feeding speed to 150 g / min, the air pressure to 0.8 MPa, and conduct fiberization treatment on the obtained fourth material at 50°C to obtain the fifth material with a D50 of 6 μm.

[0052] Step (6-1): Screen and load the obtained fifth material, and conduct rolling at a rolling temperature of 160°C, a rolling pressure of 20 T, and a rolling speed of 11 rpm / min to obtain a prefabricated film.

[0053] Step (6-2): Heat the obtained prefabricated film at 80°C for 10 minutes to activate it, and then obtain the sodium iron pyrophosphate cathode film through multiple rollings.

[0054] Preparation of a dry sodium-ion battery cathode sheet: Bond one side surface of the obtained sodium iron pyrophosphate cathode film to the positive current collector aluminum foil, and obtain the dry cathode sheet after thermal lamination, with the lamination temperature being 120°C.

[0055] Example 3

[0056] A dry preparation method of a sodium iron pyrophosphate cathode film:

[0057] Weigh 4 g of dispersed polytetrafluoroethylene powder as the binder, 4 g of conductive carbon black SP as the conductive agent, and 192 g of sodium iron pyrophosphate cathode material (i.e., the weight ratio of the sodium iron pyrophosphate cathode material, conductive agent, and binder is 48:1:1). Then, according to the total weight of the above three being 100%, weigh 3%, that is, 6 g of sodium trifluoromethanesulfonate.

[0058] Step (1): Divide the conductive agent into conductive agent A and conductive agent B, with a weight ratio of conductive agent A: conductive agent B = 2:1.

[0059] Step (2): Mix conductive agent A with the sodium iron pyrophosphate cathode material and conduct a first stirring at 20°C with a rotation speed of 1000 revolutions per minute for 30 minutes to obtain a first material; mix conductive agent B with sodium trifluoromethanesulfonate and conduct a second stirring at 20°C with a rotation speed of 1000 revolutions per minute for 30 minutes to obtain a second material.

[0060] Step (3): Mix the obtained first material and second material at 10°C and conduct a third stirring at a rotation speed of 1000 revolutions per minute for 30 minutes to obtain a third material.

[0061] Step (4): Add a binder to the obtained third material and conduct a fourth stirring at a rotation speed of 2000 revolutions per minute for 10 minutes to obtain a fourth material.

[0062] Step (5): Use argon, pass it through an air jet mill, set its feeding speed to 200 g / min, the air pressure to 1 Mpa, and conduct fiberization treatment on the obtained fourth material at 70°C to obtain a fifth material with a D50 of 5 μm.

[0063] Step (6-1): Screen and load the obtained fifth material, and conduct rolling at a rolling temperature of 180°C, a rolling pressure of 30 T, and a rolling speed of 13 rpm / min to obtain a prefabricated film.

[0064] Step (6-2): Heat the obtained prefabricated film at 70°C for 20 minutes to activate it, and then obtain a sodium iron pyrophosphate cathode film through multiple rollings.

[0065] Preparation of a dry sodium-ion battery cathode sheet: Attach one side surface of the obtained sodium iron pyrophosphate cathode film to the positive current collector aluminum foil, and obtain a dry cathode sheet after thermal lamination, with a lamination temperature of 110°C.

[0066] Example 4

[0067] A dry preparation method of a sodium iron pyrophosphate cathode film:

[0068] The difference between this example and Example 1 is only that: change the rotation speed of the first stirring in Step (2) to 500 revolutions per minute, the time to 20 minutes, and the temperature to 40°C.

[0069] Preparation of a dry sodium-ion battery cathode sheet: The same as in Example 1.

[0070] Example 5

[0071] A dry preparation method of sodium iron pyrophosphate cathode film

[0072] The difference between this example and Example 1 is only that: the rotation speed of the second stirring in step (2) is changed to 2000 revolutions per minute, the time is changed to 5 minutes, and the temperature is changed to 5 °C.

[0073] The preparation of a dry sodium-ion battery cathode sheet: It is the same as that in Example 1.

[0074] Example 6

[0075] A dry preparation method of sodium iron pyrophosphate cathode film

[0076] The difference between this example and Example 1 is only that: the rotation speed of the third stirring in step (3) is changed to 1500 revolutions per minute, the time is changed to 20 minutes, and the temperature is changed to 30 °C.

[0077] The preparation of a dry sodium-ion battery cathode sheet: It is the same as that in Example 1.

[0078] Example 7

[0079] A dry preparation method of sodium iron pyrophosphate cathode film

[0080] The difference between this example and Example 1 is only that: the rotation speed of the fourth stirring in step (4) is changed to 3000 revolutions per minute, and the time is changed to 20 minutes.

[0081] The preparation of a dry sodium-ion battery cathode sheet: It is the same as that in Example 1.

[0082] Example 8

[0083] A dry preparation method of sodium iron pyrophosphate cathode film

[0084] The difference between this example and Example 1 is only that the parameters of the air jet mill in step (5) are changed to: the feeding speed is 1 g / min, the air pressure is 1.0 MPa, and the obtained fourth material is fibrillated at 80 °C, and finally the fifth material with a D50 of 2 μm is obtained.

[0085] The preparation of a dry sodium-ion battery cathode sheet: It is the same as that in Example 1.

[0086] Example 9

[0087] A dry preparation method of sodium iron pyrophosphate cathode film

[0088] The difference between this embodiment and Embodiment 1 is only that the parameters of the jet mill in step (5) are changed to: the feeding speed is 500 g / min, the air pressure is 0.5 MPa, and the obtained fourth material is fibrillated at 30 °C, and finally the fifth material with a D50 of 10 μm is obtained.

[0089] The preparation of a dry-process sodium-ion battery positive electrode sheet: The same as Embodiment 1.

[0090] Embodiment 10

[0091] A dry preparation method of a sodium iron pyrophosphate positive electrode film:

[0092] The difference between this embodiment and Embodiment 1 is only that the temperature of the rolling in step (6-1) is changed to 120 °C, the rolling pressure is changed to 30 T, and the rolling speed is changed to 1 rpm / min.

[0093] The preparation of a dry-process sodium-ion battery positive electrode sheet: The same as Embodiment 1.

[0094] Embodiment 11

[0095] A dry preparation method of a sodium iron pyrophosphate positive electrode film:

[0096] The difference between this embodiment and Embodiment 1 is only that the temperature of the rolling in step (6-1) is changed to 200 °C, the rolling pressure is changed to 0.1 T, and the rolling speed is changed to 30 rpm / min.

[0097] The preparation of a dry-process sodium-ion battery positive electrode sheet: The same as Embodiment 1.

[0098] Embodiment 12

[0099] A dry preparation method of a sodium iron pyrophosphate positive electrode film:

[0100] The difference between this embodiment and Embodiment 1 is only that the temperature of the activation in step (6-2) is changed to 100 °C and the time is changed to 5 min.

[0101] The preparation of a dry-process sodium-ion battery positive electrode sheet: The same as Embodiment 1.

[0102] Embodiment 13

[0103] A dry preparation method of a sodium iron pyrophosphate positive electrode film:

[0104] The difference between this embodiment and Embodiment 1 is only that the temperature of the activation in step (6-2) is changed to 50 °C and the time is changed to 30 min.

[0105] The preparation of a dry-process sodium-ion battery positive electrode sheet: The same as Embodiment 1.

[0106] Example 14

[0107] A dry method for preparing a sodium iron pyrophosphate positive electrode film: Consistent with Example 1.

[0108] Preparation of a dry sodium-ion battery positive electrode sheet:

[0109] The difference between this example and Example 1 is only that: the temperature used for thermal lamination is changed to 150 °C.

[0110] Comparative Example 1

[0111] Preparation of a wet sodium-ion battery positive electrode sheet:

[0112] Weigh 4 g of PVDF powder as a binder, 4 g of conductive carbon black SP as a conductive agent, and 192 g of sodium iron pyrophosphate positive electrode material, and then take NMP as a dispersant.

[0113] Step (1): Mix and beat the binder and the dispersant for 4 h to obtain a first mixture.

[0114] Step (2): Add the conductive agent to the first mixture and mix for 4 h to obtain a second mixture.

[0115] Step (3): Add the sodium iron pyrophosphate positive electrode material to the second mixture, mix at a low speed of 800 revolutions per minute for 5 h, and then mix at a high speed of 2500 revolutions per minute for 3 h to obtain an electrode slurry with a solid content of 50%.

[0116] Step (4): Coat the electrode slurry on the positive electrode current collector aluminum foil through a coating and drying machine to obtain a wet electrode sheet, and the surface density and thickness of the positive electrode active layer thereon are consistent with those in Example 1.

[0117] Comparative Example 2

[0118] Preparation of a wet sodium-ion battery positive electrode sheet:

[0119] The difference between this comparative example and Comparative Example 1 is only that the solid content of the electrode slurry in step (3) is changed to 55%, and at the same time, the surface density and thickness of the formed positive electrode active layer are changed. See Table 1 for details.

[0120] Comparative Example 3

[0121] A dry method for preparing a sodium iron pyrophosphate positive electrode film:

[0122] The difference between this comparative example and Example 1 is only that steps (1) to (4) were not carried out. Instead, the dispersed polytetrafluoroethylene powder, conductive carbon black, sodium iron pyrophosphate cathode material, and sodium trifluoromethanesulfonate were directly mixed at a rotation speed of 15 revolutions per minute at 25 °C for 15 minutes to obtain a fourth material, and the subsequent steps were directly carried out.

[0123] Preparation of a dry-process sodium-ion battery cathode sheet: The same as in Example 1.

[0124] Comparative Example 4

[0125] A dry-process preparation method for a sodium iron pyrophosphate cathode film:

[0126] The difference between this comparative example and Example 1 is only that the conductive agent was not divided into conductive agent A and conductive agent B. Instead, the conductive agent and the sodium iron pyrophosphate cathode material were directly mixed under the conditions in step (2) to obtain a mixed material. Then, sodium trifluoromethanesulfonate was added to the mixed material and mixed under the conditions in step (3). The resulting mixed material was used as the third material, and the subsequent steps were carried out.

[0127] Preparation of a dry-process sodium-ion battery cathode sheet: The same as in Example 1.

[0128] Comparative Example 5

[0129] A dry-process preparation method for a sodium iron pyrophosphate cathode film:

[0130] The difference between this comparative example and Example 1 is only that in step (1), the ratio of conductive agent A: conductive agent B was changed to 1:1 by weight. The subsequent steps were the same as those in Example 1.

[0131] Preparation of a dry-process sodium-ion battery cathode sheet: The same as in Example 1.

[0132] Comparative Example 6

[0133] A dry-process preparation method for a sodium iron pyrophosphate cathode film:

[0134] The difference between this comparative example and Example 1 is only that the amounts of the dispersed polytetrafluoroethylene powder, conductive carbon black, sodium iron pyrophosphate cathode material, and sodium trifluoromethanesulfonate are different. Specifically:

[0135] 8 g of dispersed polytetrafluoroethylene powder was weighed as the binder, 8 g of conductive carbon black SP was weighed as the conductive agent, and 160 g of sodium iron pyrophosphate cathode material. Then, according to the total weight of the above three being 100%, 5%, that is, 8.8 g of sodium trifluoromethanesulfonate was weighed.

[0136] Preparation of a dry-process sodium-ion battery cathode sheet: The same as in Example 1.

[0137] Comparative Example 7

[0138] A dry method for preparing a sodium iron pyrophosphate positive electrode film:

[0139] The difference between this comparative example and Example 1 is only that the dosages of dispersed polytetrafluoroethylene powder, conductive carbon black, sodium iron pyrophosphate positive electrode material, and sodium trifluoromethanesulfonate are different. Specifically:

[0140] Weigh 2 g of dispersed polytetrafluoroethylene powder as the binder, 2 g of conductive carbon black SP as the conductive agent, and 200 g of sodium iron pyrophosphate positive electrode material. Then, according to the total weight of the above three being 100%, weigh 0.5%, that is, 1.02 g of sodium trifluoromethanesulfonate.

[0141] Preparation of a dry sodium-ion battery positive electrode sheet: The same as in Example 1.

[0142] Testing method

[0143] Peelability: Obtained according to GB / T 2790.

[0144] Assembly of battery samples and first-efficiency test: Use the dry sodium-ion battery positive electrode sheets obtained in each example and comparative example as the positive electrode, graphite as the negative electrode, and lithium hexafluorophosphate as the electrolyte solute to prepare battery samples with a capacity of 5 Ah. Test at 25 °C to obtain the first efficiency.

[0145] The above test results are all shown in Table 1. The thickness and surface density of the sodium iron pyrophosphate positive electrode films obtained in each example and comparative example are also shown in Table 1.

[0146] Table 1

[0147]

[0148] From the above description, it can be seen that the above embodiments of the present invention achieve the preparation of a dry positive electrode film with excellent performance. After the obtained sodium iron pyrophosphate positive electrode film is laminated with the positive electrode current collector, the binding property is stronger, it is less likely to peel and gel, and the first efficiency of the corresponding sodium-ion battery is also higher. At the same time, in Comparative Examples 1 and 2, although sodium iron pyrophosphate is used as the positive electrode active material and the thickness and surface density are similar to those of the examples, the performance of the corresponding batteries is poor. In particular, the positive electrode slurry will undergo serious gelation during use and storage, which is not conducive to practical applications.

[0149] It should be noted that the terms "first", "second", etc. in the description and claims of this application are used to distinguish similar objects and do not necessarily have to be used to describe a specific order or sequence. It should be understood that such terms can be interchanged under appropriate circumstances so that the embodiments of the present application described herein can be implemented, for example, in an order other than those described herein.

[0150] The above are only the preferred embodiments of the present invention and are not intended to limit the present invention. For those skilled in the art, the present invention can have various modifications and changes. Any modifications, equivalent replacements, improvements, etc. made within the spirit and principle of the present invention shall be included within the protection scope of the present invention.

Claims

1. A dry preparation method of sodium iron pyrophosphate positive electrode film, characterized in that, Including: Step S1: Divide the conductive agent into conductive agent A and conductive agent B; the weight ratio of the conductive agent A to the conductive agent B is (2 - 3):

1. Step S2: Mix the conductive agent A with the sodium iron pyrophosphate cathode material and obtain a first material through a first stirring; mix the conductive agent B with sodium trifluoromethanesulfonate and obtain a second material through a second stirring. Step S3: Mix the first material with the second material and obtain a third material after a third stirring. Step S4: Mix the third material with a binder and obtain a fourth material after a fourth stirring. Step S5: Subject the fourth material to fibrillation treatment to obtain a fifth material; the fibrillation treatment is achieved by air jet milling, and during the air jet milling process, the feeding speed is 100 g / min - 200 g / min, and the air pressure is 0.5 MPa - 1 MPa; the D50 particle size range of the fifth material is 5 μm - 8 μm. Step S6: Subject the fifth material to roll pressing to obtain the sodium iron pyrophosphate cathode film. The weight ratio of the sodium iron pyrophosphate cathode material, the conductive agent, and the binder is (45 - 50):1:

1. Based on the total weight of the sodium iron pyrophosphate cathode material, the conductive agent, and the binder being 100%, the content of sodium trifluoromethanesulfonate is 1% - 3%.

2. The dry preparation method of the sodium iron pyrophosphate cathode film according to claim 1, characterized in that The rotation speeds of the first stirring and the second stirring are each independently 1000 revolutions per minute - 1500 revolutions per minute, the times are each independently 20 min - 30 min, and the stirring temperatures of the first stirring and the second stirring are each independently 20°C - 30°C.

3. The dry preparation method of the sodium iron pyrophosphate positive electrode membrane according to claim 1, characterized in that, In step S3, the rotation speed of the third stirring is 500 revolutions per minute - 1000 revolutions per minute, the time is 30 min - 40 min, and the third stirring is carried out at 10°C - 20°C.

4. The dry preparation method of the sodium iron pyrophosphate positive electrode membrane according to claim 1, characterized in that, In step S4, the rotation speed of the fourth stirring is 1500 revolutions per minute - 2000 revolutions per minute, and the time is 10 min - 15 min.

5. The dry preparation method of the sodium iron pyrophosphate positive electrode film according to any one of claims 1 to 4, characterized in that, In step S6, the roll pressing speed of the roll pressing is 1 rpm / min - 30 rpm / min, the roll pressing pressure is 0.1 T - 30 T, and the roll pressing is carried out at 160°C - 180°C.

6. The dry preparation method of the sodium iron pyrophosphate cathode film according to any one of claims 1 to 4, characterized in that The conductive agent is selected from one or more of SP, VGCF, and CNT. The binder is selected from one or more of polytetrafluoroethylene, PVDF, and CMC.

7. A sodium iron pyrophosphate positive electrode membrane, characterized in that, The sodium iron pyrophosphate positive electrode film is prepared by the dry preparation method of the sodium iron pyrophosphate positive electrode film according to any one of claims 1 to 6, and the thickness of the sodium iron pyrophosphate positive electrode film is 100 μm to 250 μm, and the areal density is 150 mg / cm 2 ~400 mg / cm 2 .

8. Use of the sodium iron pyrophosphate positive electrode film described in claim 7 as a positive electrode active layer in the battery field to prepare a positive electrode sheet of a sodium ion battery, characterized in that, Bond at least one side surface of the sodium iron pyrophosphate cathode film according to claim 7 with a positive electrode current collector and perform thermal lamination at 110°C - 120°C to obtain the sodium ion battery positive electrode sheet.

9. A sodium-ion battery, comprising a positive electrode sheet, a negative electrode sheet and an electrolyte, characterized in that, The positive electrode sheet is the sodium ion battery positive electrode sheet according to claim 8.

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