A cathode material for improving the first charge-discharge efficiency of sodium batteries and its preparation method

Abstract

This invention relates to the field of sodium battery technology and discloses a cathode material for improving the first charge-discharge efficiency of sodium batteries and its preparation method; including the following steps: Step 1: NaFePO4 material and Na4SiO4 material are uniformly mixed in pure water, and then baked, washed, filtered and dried in sequence to obtain NaFePO4-Si; Step 2: NaFePO4-Si is uniformly mixed with MXene material and sintered at high temperature under a nitrogen atmosphere to obtain modified NaFePO4 material, which is used as the cathode material.

Description

Technical Field

[0001] This invention relates to the field of sodium battery technology, specifically to a cathode material for improving the first charge-discharge efficiency of sodium batteries and its preparation method. Background Technology

[0002] In recent years, the new energy industry has gradually replaced traditional energy in many fields. Lithium batteries, with their high energy density and excellent cycle performance, have been developed by many battery manufacturers. However, the abundance of lithium resources in the Earth's crust is only 0.0017%, and their distribution is uneven. Most mineral resources are concentrated in South America. The reserves of elements such as nickel and cobalt, which are commonly used in commercially successful lithium-ion batteries, are also not abundant in the Earth's crust. Sodium batteries make up for this deficiency. Sodium resources are abundant in the Earth's crust and are also widely distributed in the ocean. The iron, manganese, and aluminum required for sodium-ion batteries are also abundant in the Earth's crust. These factors make it easier to reduce the overall cost of sodium-ion battery manufacturing.

[0003] However, NaFePO4 is a commonly used cathode material for sodium batteries. It has two distinct phases: olivine and maricite. The olivine phase provides sufficient space for sodium ions to stably insert and extract during charge and discharge, ensuring stable cycling and high battery efficiency and long lifespan. The maricite phase, unlike the olivine phase, is a type of sodium phosphate mineral. In this structure, the insertion sites and structure of sodium ions are relatively loose, making its structure more prone to change than the olivine phase. This results in NaFePO4 having good sodium storage performance in sodium batteries, but its poor native electronic conductivity and low rate performance limit its application in many fields. Therefore, improving the initial charge-discharge efficiency and rate performance of NaFePO4 cathode materials is of great significance. Summary of the Invention

[0004] The purpose of this invention is to provide a cathode material and its preparation method for improving the first charge-discharge efficiency of sodium batteries, so as to solve the problems raised in the prior art.

[0005] To achieve the above objectives, the present invention provides the following technical solution:

[0006] A method for preparing a positive electrode material to improve the initial charge-discharge efficiency of sodium batteries includes the following steps:

[0007] Step 1: Mix NaFePO4 material and Na4SiO4 material evenly in pure water, and then bake, wash, filter and dry in sequence to obtain NaFePO4-Si.

[0008] Step 2: After uniformly mixing NaFePO4-Si with MXene material, high-temperature sintering is performed to obtain modified NaFePO4 material, which is then used as the cathode material.

[0009] Ideally, the molar ratio of NaFePO4 material to Na4SiO4 material is (100~105):1.

[0010] An optimal mass ratio of NaFePO4 material to MXene material is (40~45):1.

[0011] The optimized high-temperature sintering process conditions are as follows: the gas atmosphere includes nitrogen, the high-temperature sintering temperature is 800~850℃, the high-temperature sintering time is 10~12 hours, the heating rate is 25~30℃ / min, and the annealing rate is 45~50℃ / min; the baking and bonding conditions are as follows: the baking and bonding temperature is 160~200℃, and the baking time is 15~18 hours.

[0012] A more optimized method for preparing the modified MXene material is as follows: (1) Add Ti3AlC2 to a hydrofluoric acid aqueous solution, stir at 22~25℃ for 20~24 hours, wash with deionized water until pH=6~7, freeze dry to obtain Ti3C2; (2) Add Ti3C2 to a hexadecyltrimethylammonium bromide (CTAB) aqueous solution and ultrasonically disperse for 5~10 minutes, add a nitrogen source, stir for 4~5 hours, freeze dry; heat to 400~450℃ under an ammonia atmosphere and hold for 3~4 hours, cool to room temperature to obtain the modified MXene material.

[0013] More optimally, the concentration of the hydrofluoric acid aqueous solution is 30-40 wt%; the mass ratio of Ti3AlC2 to the hydrofluoric acid aqueous solution is 1:(10-15); and the concentration of the hexadecyltrimethylammonium bromide (CTAB) aqueous solution is 0.4-1.5 wt%.

[0014] In a more optimized form, the raw materials of the MXene material include the following components: by mass, 2-3 parts Ti3C2, 17-20 parts hexadecyltrimethylammonium bromide aqueous solution, and 0.08-0.12 parts nitrogen source; wherein the nitrogen source is triammonium citrate and metal-based melamine in a mass ratio of 1:(0.04-0.07).

[0015] A more optimized method for preparing the metal-based melamine is as follows: (1) Melamine is added to glacial acetic acid and mixed uniformly at 120~130℃ to obtain a melamine mixed solution; polyphosphoric acid is added to glacial acetic acid and mixed uniformly at 60~70℃ to obtain a polyphosphoric acid mixed solution; (2) polyphosphoric acid mixed solution is added dropwise to the melamine mixed solution, and the addition is completed in 1~2 hours. The temperature is raised to 120~125℃ and magnesium dihydrogen phosphate is added. The mixture is mixed uniformly for 12~15 hours, filtered, washed, dried and ground to obtain metal-based melamine.

[0016] In a more optimized form, the raw materials for the metal-based melamine include the following components: by mass, 6-8 parts melamine, 4-5 parts polyphosphoric acid, 70-85 parts glacial acetic acid, and 2-3.5 parts magnesium dihydrogen phosphate.

[0017] Ideally, the ammonia gas introduction rate is 0.05~0.07 L / min; the heating rate is 4~5 °C / min.

[0018] Compared with the prior art, the beneficial effects of the present invention are:

[0019] This application uses a hydrothermal synthesis and calcination method to prepare modified NaFePO4 materials; in the scheme, SiO4 4- and Na + Ion doping was performed via a hydrothermal synthesis reaction to obtain the intermediate NaFePO4-Si; subsequently, a small amount of MXene was added to the intermediate NaFePO4-Si by calcination to obtain modified NaFePO4; Na + Doping allows for pre-sodium treatment at the positive end of conventional NaFePO4, thereby improving the initial charge-discharge efficiency; the introduction of Si-O tetrahedra expands the ion diffusion channels, facilitating Na... + The transmission provides a fast path, accelerating Na... + The diffusion kinetics were improved, and the electrochemical performance was enhanced; meanwhile, SiO4 4- The introduction of this also increases the contact area between the material and the electrolyte, which is beneficial for ion transport during charging and discharging and improves the rate performance of the battery.

[0020] When the temperature exceeds 500~600℃, the olivine phase will transform into the sodium phosphate phase, which will affect the performance of sodium batteries. Therefore, in this solution, it is combined with MXene material to improve its electrochemical performance.

[0021] MXene is a two-dimensional material. The larger interlayer spacing means smaller volume changes and a lower ion insertion barrier, providing shorter transport paths for ions and electrons. This can improve the conductivity of the original cathode material. The large specific surface area of ​​MXene can also be used to improve ion transport capability and further enhance rate performance.

[0022] In this scheme, MXene is modified by CTAB, metal-based melamine, triammonium citrate, and ammonia gas. This reduces the damage to the two-dimensional structure of MXene during high-temperature sintering with NaFePO4 material, thereby improving ion transport capacity and thus enhancing the performance of sodium batteries.

[0023] Among them, CTAB can act as a surfactant. The cationic part of CTAB interacts with the negative charge on the surface of MXene material, preventing MXene material from re-aggregating or clustering in the solution and promoting the uniform dispersion of Ti3C2 material. CTAB and triammonium citrate have the effect of intercalation agents, increasing the interlayer spacing of MXene material, so that the material surface can better interact with other materials, thus further improving the stability of MXene.

[0024] In this scheme, triammonium citrate and metal-based melamine are used as nitrogen sources for doping MXene. The synergistic effect of the two can not only change the surface chemical properties of MXene, but also improve its electronic conductivity and electrochemical stability. Among them, the metal-based melamine is prepared by melamine, polyphosphoric acid and magnesium dihydrogen phosphate.

[0025] In this scheme, the presence of melamine promotes the surface nitridation of MXene, which helps improve the stability of the material. Furthermore, heat treatment in an ammonia atmosphere further promotes nitrogen doping, ensuring that nitrogen atoms are effectively incorporated into the MXene structure. The introduction of polyphosphoric acid reduces interfacial impedance and enhances electron transport between electrodes, thereby improving the fast-charging performance of sodium batteries. Magnesium dihydrogen phosphate, as an active material, helps improve ion transport rate and cycle life in sodium batteries, enhancing the structural stability of the material. Moreover, an ammonia atmosphere not only promotes nitrogen source doping but also prevents the presence of oxygen, avoiding the oxidation of Ti3C2 at high temperatures. This, in turn, improves the performance of modified NaFePO4 materials in sodium battery applications. Detailed Implementation

[0026] The technical solutions of the present invention will be clearly and completely described below with reference to the embodiments of the present invention. Obviously, the described embodiments are only some embodiments of the present invention, and not all embodiments. Based on the embodiments of the present invention, all other embodiments obtained by those of ordinary skill in the art without creative effort are within the scope of protection of the present invention.

[0027] In this embodiment, it should be noted that there are no special restrictions on the manufacturers of the raw materials involved in this invention. Exemplary examples include: tetrasodium silicate (Na4SiO4) CAS number 13472-30-5, purchased from Hubei Chenghai Chemical Co., Ltd.; Ti3AlC2 CAS number 196506-01-1, purchased from Beijing Huawirui Chemical Technology Co., Ltd.; hydrofluoric acid CAS number 7664-39-3; CTAB (hexadecyltrimethylammonium bromide) CAS number 57-09-0; triammonium citrate CAS number 3458-72-8; and melamine CAS number 108-78-1.

[0028] The CAS number for (NH4)2Fe(SO4)2·6H2O is 7783-85-9; the CAS number for Na3PO4 is 7601-54-9; the CAS number for ascorbic acid is 299-36-5; and the EINECS number for polyphosphoric acid is 232-417-0, with the molecular formula H6P4O. 13 Purchased from Shandong Yukang Chemical Co., Ltd.; CAS number for melamine is 108-78-1; CAS number for magnesium dihydrogen phosphate is 13092-66-5.

[0029] The preparation method of NaFePO4 material is as follows: (NH4)2Fe(SO4)2·6H2O and Na3PO4 are weighed at a mass ratio of 1:0.4; 1.5 mmol of ascorbic acid aqueous solution is heated to 70℃, Na3PO4 and (NH4)2Fe(SO4)2·6H2O are added and mixed evenly. When the solution turns dark green, it is transferred to a reaction vessel and kept at 145℃ for 6 hours. After cooling to room temperature, it is washed with deionized water and dried. Under a nitrogen atmosphere, it is heat-treated at 650℃ for 6.5 hours to obtain NaFePO4 material.

[0030] The preparation method of metal-based melamine is as follows: (1) 6.3 parts of melamine are added to 40 parts of glacial acetic acid and mixed evenly at 125°C to obtain a melamine mixed solution; 4.2 parts of polyphosphoric acid are added to 45 parts of glacial acetic acid and mixed evenly at 65°C to obtain a polyphosphoric acid mixed solution; (2) polyphosphoric acid mixed solution is added dropwise to the melamine mixed solution and the addition is completed in 1.5 hours. The temperature is raised to 125°C and 3 parts of magnesium dihydrogen phosphate are added and mixed evenly for 14 hours. The mixture is then filtered, washed, dried and ground to obtain metal-based melamine.

[0031] Example 1: A method for preparing a cathode material to improve the initial charge-discharge efficiency of a sodium battery;

[0032] Step 1: Place NaFePO4 material and Na4SiO4 material in pure water, mix them evenly, put them into a reaction vessel, and bake them in a forced-air drying oven at 160℃ for 18 hours. After washing, filtering and drying, NaFePO4-Si is obtained.

[0033] Step 2: Mix NaFePO4-Si and MXene material uniformly, place in a tube furnace, and sinter at 800℃ for 10 hours under nitrogen atmosphere at a heating rate of 30℃ / min. Then anneal at a rate of 50℃ / min to obtain modified NaFePO4 material, which is used as the cathode material.

[0034] The molar ratio of NaFePO4 to Na4SiO4 is 100:1, and the mass ratio of NaFePO4 to MXene is 40:1.

[0035] Example 2 is based on Example 1, except that the molar ratio of NaFePO4 material to Na4SiO4 is 90:1;

[0036] Step 1: Place NaFePO4 material and Na4SiO4 material in pure water, mix them evenly, put them into a reaction vessel, and bake them in a forced-air drying oven at 160℃ for 18 hours. After washing, filtering and drying, NaFePO4-Si is obtained.

[0037] Step 2: Mix NaFePO4-Si and MXene material uniformly, place in a tube furnace, and sinter at 800℃ for 10 hours under nitrogen atmosphere at a heating rate of 30℃ / min. Then anneal at a rate of 50℃ / min to obtain modified NaFePO4 material, which is used as the cathode material.

[0038] The molar ratio of NaFePO4 to Na4SiO4 is 90:1, and the mass ratio of NaFePO4 to MXene is 40:1.

[0039] Example 3 is based on Example 1, except that the molar ratio of NaFePO4 material to Na4SiO4 is 95:1;

[0040] Step 1: Place NaFePO4 material and Na4SiO4 material in pure water, mix them evenly, put them into a reaction vessel, and bake them in a forced-air drying oven at 160℃ for 18 hours. After washing, filtering and drying, NaFePO4-Si is obtained.

[0041] Step 2: Mix NaFePO4-Si and MXene material uniformly, place in a tube furnace, and sinter at 800℃ for 10 hours under nitrogen atmosphere at a heating rate of 30℃ / min. Then anneal at a rate of 50℃ / min to obtain modified NaFePO4 material, which is used as the cathode material.

[0042] The molar ratio of NaFePO4 to Na4SiO4 is 95:1, and the mass ratio of NaFePO4 to MXene is 40:1.

[0043] Example 4 is based on Example 1, except that the molar ratio of NaFePO4 material to Na4SiO4 material is 105:1;

[0044] Step 1: Place NaFePO4 material and Na4SiO4 material in pure water, mix them evenly, put them into a reaction vessel, and bake them in a forced-air drying oven at 160℃ for 18 hours. After washing, filtering and drying, NaFePO4-Si is obtained.

[0045] Step 2: Mix NaFePO4-Si and MXene material uniformly, place in a tube furnace, and sinter at 800℃ for 10 hours under nitrogen atmosphere at a heating rate of 30℃ / min. Then anneal at a rate of 50℃ / min to obtain modified NaFePO4 material, which is used as the cathode material.

[0046] The molar ratio of NaFePO4 to Na4SiO4 is 105:1, and the mass ratio of NaFePO4 to MXene is 40:1.

[0047] Example 5 is based on Example 1, except that the molar ratio of NaFePO4 material to Na4SiO4 is 110:1;

[0048] Step 1: Place NaFePO4 material and Na4SiO4 material in pure water, mix them evenly, put them into a reaction vessel, and bake them in a forced-air drying oven at 160℃ for 18 hours. After washing, filtering and drying, NaFePO4-Si is obtained.

[0049] Step 2: Mix NaFePO4-Si and MXene material uniformly, place in a tube furnace, and sinter at 800℃ for 10 hours under nitrogen atmosphere at a heating rate of 30℃ / min. Then anneal at a rate of 50℃ / min to obtain modified NaFePO4 material, which is used as the cathode material.

[0050] The molar ratio of NaFePO4 to Na4SiO4 is 110:1, and the mass ratio of NaFePO4 to MXene is 40:1.

[0051] Example 6 is based on Example 1, except that the mass ratio of NaFePO4 material to MXene material is 30:1;

[0052] Step 1: Place NaFePO4 material and Na4SiO4 material in pure water, mix them evenly, put them into a reaction vessel, and bake them in a forced-air drying oven at 160℃ for 18 hours. After washing, filtering and drying, NaFePO4-Si is obtained.

[0053] Step 2: Mix NaFePO4-Si and MXene material uniformly, place in a tube furnace, and sinter at 800℃ for 10 hours under nitrogen atmosphere at a heating rate of 30℃ / min. Then anneal at a rate of 50℃ / min to obtain modified NaFePO4 material, which is used as the cathode material.

[0054] The molar ratio of NaFePO4 to Na4SiO4 is 100:1, and the mass ratio of NaFePO4 to MXene is 30:1.

[0055] Example 7 is based on Example 1, except that the mass ratio of NaFePO4 material to MXene material is 35:1;

[0056] Step 1: Place NaFePO4 material and Na4SiO4 material in pure water, mix them evenly, put them into a reaction vessel, and bake them in a forced-air drying oven at 160℃ for 18 hours. After washing, filtering and drying, NaFePO4-Si is obtained.

[0057] Step 2: Mix NaFePO4-Si and MXene material uniformly, place in a tube furnace, and sinter at 800℃ for 10 hours under nitrogen atmosphere at a heating rate of 30℃ / min. Then anneal at a rate of 50℃ / min to obtain modified NaFePO4 material, which is used as the cathode material.

[0058] The molar ratio of NaFePO4 to Na4SiO4 is 100:1, and the mass ratio of NaFePO4 to MXene is 35:1.

[0059] Example 8 is based on Example 1, except that the mass ratio of NaFePO4 material to MXene material is 45:1;

[0060] Step 1: Place NaFePO4 material and Na4SiO4 material in pure water, mix them evenly, put them into a reaction vessel, and bake them in a forced-air drying oven at 160℃ for 18 hours. After washing, filtering and drying, NaFePO4-Si is obtained.

[0061] Step 2: Mix NaFePO4-Si and MXene material uniformly, place in a tube furnace, and sinter at 800℃ for 10 hours under nitrogen atmosphere at a heating rate of 30℃ / min. Then anneal at a rate of 50℃ / min to obtain modified NaFePO4 material, which is used as the cathode material.

[0062] The molar ratio of NaFePO4 to Na4SiO4 is 100:1, and the mass ratio of NaFePO4 to MXene is 45:1.

[0063] Example 9 is based on Example 1, except that the mass ratio of NaFePO4 material to MXene material is 50:1;

[0064] Step 1: Place NaFePO4 material and Na4SiO4 material in pure water, mix them evenly, put them into a reaction vessel, and bake them in a forced-air drying oven at 160℃ for 18 hours. After washing, filtering and drying, NaFePO4-Si is obtained.

[0065] Step 2: Mix NaFePO4-Si and MXene material uniformly, place in a tube furnace, and sinter at 800℃ for 10 hours under nitrogen atmosphere at a heating rate of 30℃ / min. Then anneal at a rate of 50℃ / min to obtain modified NaFePO4 material, which is used as the cathode material.

[0066] The molar ratio of NaFePO4 to Na4SiO4 is 100:1, and the mass ratio of NaFePO4 to MXene is 50:1.

[0067] Example 10 is based on Example 1, modifying MXene material;

[0068] Step 1: Place NaFePO4 material and Na4SiO4 material in pure water, mix them evenly, bake at 160℃ for 18 hours, wash, filter and dry to obtain NaFePO4-Si;

[0069] Step 2: (1) Add Ti3AlC2 to a 35wt% hydrofluoric acid aqueous solution, stir at 23℃ for 24 hours, wash with deionized water until pH=6.8, freeze dry to obtain Ti3C2 (MXene material); (2) Add 2.5 parts of Ti3C2 (MXene material) to 17 parts of CTAB aqueous solution (1.2wt%) and ultrasonically disperse for 10 minutes, add 0.1 parts of nitrogen source, stir for 5 hours, freeze dry; pass ammonia gas at a rate of 0.06L / min, heat to 400℃ at a rate of 5℃ / min and hold for 3 hours, cool to room temperature to obtain modified MXene material; wherein, the mass ratio of Ti3AlC2 to hydrofluoric acid aqueous solution is 1:12; the nitrogen source is triammonium citrate and melamine at a mass ratio of 1:0.06;

[0070] (3) Mix NaFePO4-Si with modified MXene material uniformly, place it in a tube furnace, and sinter it at 800℃ for 10 hours under nitrogen atmosphere at a heating rate of 30℃ / min. Then anneal it at a rate of 50℃ / min to obtain modified NaFePO4 material, which is used as positive electrode material.

[0071] The molar ratio of NaFePO4 to Na4SiO4 is 100:1, and the mass ratio of NaFePO4 to MXene is 40:1.

[0072] Example 11 is based on Example 10, except that melamine is replaced with metal-based melamine.

[0073] Step 1: Place NaFePO4 material and Na4SiO4 material in pure water, mix them evenly, bake at 160℃ for 18 hours, wash, filter and dry to obtain NaFePO4-Si;

[0074] Step 2: (1) Add Ti3AlC2 to a 35wt% hydrofluoric acid aqueous solution, stir at 23℃ for 24 hours, wash with deionized water until pH=6.8, freeze dry to obtain Ti3C2 (MXene material); (2) Add 2.5 parts of Ti3C2 (MXene material) to 17 parts of CTAB aqueous solution (1.2wt%) and ultrasonically disperse for 10 minutes, add 0.1 parts of nitrogen source, stir for 5 hours, freeze dry; pass ammonia gas at a rate of 0.06L / min, heat to 400℃ at a rate of 5℃ / min and hold for 3 hours, cool to room temperature to obtain modified MXene material; wherein, the mass ratio of Ti3AlC2 to hydrofluoric acid aqueous solution is 1:12; the nitrogen source is triammonium citrate and metal-based melamine at a mass ratio of 1:0.06;

[0075] (3) Mix NaFePO4-Si with modified MXene material uniformly, place it in a tube furnace, and sinter it at 800℃ for 10 hours under nitrogen atmosphere at a heating rate of 30℃ / min. Then anneal it at a rate of 50℃ / min to obtain modified NaFePO4 material, which is used as positive electrode material.

[0076] The molar ratio of NaFePO4 to Na4SiO4 is 100:1, and the mass ratio of NaFePO4 to MXene is 40:1.

[0077] Comparative Example 1 is based on Example 1, except that Na4SiO4 was not introduced;

[0078] NaFePO4 material and MXene material were uniformly mixed and placed in a tube furnace. Under a nitrogen atmosphere, the mixture was heated to 800℃ at a heating rate of 30℃ / min and sintered for 10 hours. The mixture was then annealed at a rate of 50℃ / min to obtain modified NaFePO4 material, which was used as the cathode material. The mass ratio of NaFePO4 material to MXene material was 40:1.

[0079] Comparative Example 2 is based on Example 1, except that MXene material was not introduced;

[0080] Step 1: Place NaFePO4 material and Na4SiO4 material in pure water, mix them evenly, put them into a reaction vessel, and bake them in a forced-air drying oven at 160℃ for 18 hours. After washing, filtering and drying, NaFePO4-Si is obtained and used as the positive electrode material.

[0081] The molar ratio of NaFePO4 to Na4SiO4 is 100:1.

[0082] Comparative Example 3 is based on Example 1, except that NaFePO4 material is used directly as the positive electrode material.

[0083] Comparative Example 4 is based on Example 10, but introduces triammonium citrate alone;

[0084] Step 1: Place NaFePO4 material and Na4SiO4 material in pure water, mix them evenly, bake at 160℃ for 18 hours, wash, filter and dry to obtain NaFePO4-Si;

[0085] Step 2: (1) Add Ti3AlC2 to 35wt% hydrofluoric acid aqueous solution, stir at 23℃ for 24 hours, wash with deionized water until pH=6.8, freeze dry to obtain Ti3C2 (MXene material); (2) Add 2.5 parts of Ti3C2 (MXene material) to 17 parts of CTAB aqueous solution (1.2wt%) and ultrasonically disperse for 10 minutes, add 0.1 parts of triammonium citrate (nitrogen source), stir for 5 hours, freeze dry; pass ammonia gas at a rate of 0.06L / min, heat to 400℃ at a rate of 5℃ / min and hold for 3 hours, cool to room temperature to obtain modified MXene material; wherein, the mass ratio of Ti3AlC2 to hydrofluoric acid aqueous solution is 1:12;

[0086] (3) Mix NaFePO4-Si with modified MXene material uniformly, place it in a tube furnace, and sinter it at 800℃ for 10 hours under nitrogen atmosphere at a heating rate of 30℃ / min. Then anneal it at a rate of 50℃ / min to obtain modified NaFePO4 material, which is used as positive electrode material.

[0087] The molar ratio of NaFePO4 to Na4SiO4 is 100:1, and the mass ratio of NaFePO4 to modified MXene is 40:1.

[0088] Comparative Example 5 is based on Example 10, but without the introduction of CTAB and ammonia;

[0089] Step 1: Place NaFePO4 material and Na4SiO4 material in pure water, mix them evenly, bake at 160℃ for 18 hours, wash, filter and dry to obtain NaFePO4-Si;

[0090] Step 2: (1) Add Ti3AlC2 to a 35wt% hydrofluoric acid aqueous solution, stir at 23℃ for 24 hours, wash with deionized water until pH=6.8, freeze dry to obtain Ti3C2 (MXene material); (2) Add 2.5 parts of Ti3C2 (MXene material) to 17 parts of aqueous solution and ultrasonically disperse for 10 minutes, add 0.1 parts of nitrogen source, stir for 5 hours, freeze dry; heat to 400℃ at a rate of 5℃ / min and hold for 3 hours, cool to room temperature to obtain modified MXene material; wherein, the mass ratio of Ti3AlC2 to hydrofluoric acid aqueous solution is 1:12; the nitrogen source is triammonium citrate and melamine in a mass ratio of 1:0.06;

[0091] (3) Mix NaFePO4-Si with modified MXene material uniformly, place it in a tube furnace, and sinter it at 800℃ for 10 hours under nitrogen atmosphere at a heating rate of 30℃ / min. Then anneal it at a rate of 50℃ / min to obtain modified NaFePO4 material, which is used as positive electrode material.

[0092] The molar ratio of NaFePO4 to Na4SiO4 is 100:1, and the mass ratio of NaFePO4 to modified MXene is 40:1.

[0093] Test: (1) Preparation of positive electrode slurry: The positive electrode materials of Examples 1-11 and Comparative Examples 1-5 were mixed with binder (PVDF, KF9700) and conductive agent (Super P, Swiss TMEGO) in a ratio of 8:1:1. After being mixed evenly, the mixture was coated on 12μm aluminum foil and dried in a vacuum drying oven at 60℃ for 8h to obtain positive electrode sheet.

[0094] (2) Assembly of button batteries: The button battery casing is of model CR2032, the separator is 20um separator, and the electrode is the positive electrode prepared by Examples 1-11 and Comparative Examples 1-5. In a glove box filled with argon gas, the button batteries are assembled in the following order: battery casing - placing positive electrode - adding electrolyte - placing separator - adding electrolyte - placing sodium sheet - placing spacer spring - battery casing.

[0095] (3) Cyclic performance test: The button batteries in the above test (2) were tested at 0.5C, 1C and 3C currents using a button battery charge and discharge tester (Wuhan Landian, CT3002A) with a voltage range of 2.0-3.8V; the test results are shown in Table 1.

[0096] (4) Test of initial charge and discharge efficiency: The button battery in the above test (2) was tested at 0.2C using a button battery charge and discharge tester (Wuhan Landian, CT3002A) with a voltage range of 2.0-3.8V; the test results are shown in Table 1.

[0097] Table 1

[0098]

[0099] Conclusion: As can be seen from Table 1, in Examples 1-9, the sodium battery exhibits the best electrochemical performance when the molar ratio of NaFePO4 material to Na4SiO4 is (100-105):1 and the mass ratio of NaFePO4 material to MXene material is (40-45):1.

[0100] Comparative Example 1 is based on Example 1, except that Na₄SiO₄ was not introduced. The study found that the electrochemical performance of the sodium battery made using Comparative Example 1 as the cathode material was reduced. This is because the introduction of Si-O tetrahedra in the scheme expanded the ion diffusion channels, allowing Na₄SiO₄ to pass through. + The transmission provides a fast path, accelerating Na... +The diffusion kinetics were improved, and the electrochemical performance was enhanced, while SiO4 4- The introduction of MXene also increases the contact area between the material and the electrolyte, which is beneficial for ion transport during charging and discharging and improves the rate performance of the battery. Comparative Example 2 is based on Example 1, except that MXene material was not introduced. Therefore, the performance of the sodium battery is reduced. This is because MXene can not only improve the conductivity of the raw materials, but also improve the ion transport capability by utilizing the large specific surface area of ​​MXene material, thereby improving the application of sodium batteries. Comparative Example 3 is based on Example 1, except that NaFePO4 material is directly used as the positive electrode material. Therefore, the performance of the sodium battery is reduced.

[0101] Comparative Example 4, based on Example 11, introduced only triammonium citrate; this led to a decrease in the performance of the sodium battery because the presence of melamine can promote the surface nitridation of MXene, which helps to improve the stability of the material. Comparative Example 5, based on Example 11, did not introduce CTAB and ammonia; in this scheme, CTAB can act as a surfactant. The cationic part of CTAB interacts with the negative charge on the surface of the MXene material, preventing the MXene material from re-aggregating or clustering in the solution, and promoting the uniform dispersion of the Ti3C2 material (MXene material). Furthermore, CTAB also has the effect of an intercalating agent, increasing the interlayer spacing of the MXene material, so that the material surface can better interact with other materials. The ammonia atmosphere can also prevent the presence of oxygen and avoid the oxidation of Ti3C2 at high temperatures, thus further improving the stability of MXene. This is beneficial to improving the electrochemical performance of the sodium battery when the modified NaFePO4 material is used as the cathode material.

[0102] It will be apparent to those skilled in the art that the present invention is not limited to the details of the exemplary embodiments described above, and that the invention can be implemented in other specific forms without departing from the spirit or essential characteristics of the invention. Therefore, the embodiments should be considered in all respects as exemplary and non-limiting, and the scope of the invention is defined by the appended claims rather than the foregoing description. Thus, it is intended that all variations falling within the meaning and scope of equivalents of the claims be included within the present invention.

Claims

1. A method for preparing a positive electrode material to improve the initial charge-discharge efficiency of a sodium battery, characterized in that: The following steps are included: Step 1: Mix NaFePO4 material and Na4SiO4 material evenly in pure water, and then bake, wash, filter and dry in sequence to obtain NaFePO4-Si. Step 2: After uniformly mixing NaFePO4-Si with MXene material, high-temperature sintering is performed to obtain modified NaFePO4 material, which is then used as the cathode material.

2. The method for preparing a positive electrode material for improving the first charge-discharge efficiency of a sodium battery according to claim 1, characterized in that: The molar ratio of NaFePO4 material to Na4SiO4 material is (100~105):

1.

3. The method for preparing a positive electrode material for improving the first charge-discharge efficiency of a sodium battery according to claim 1, characterized in that: The mass ratio of NaFePO4 material to MXene material is (40~45):

1.

4. The method for preparing a positive electrode material for improving the first charge-discharge efficiency of a sodium battery according to claim 1, characterized in that: The high-temperature sintering process conditions are as follows: the gas atmosphere includes nitrogen, the high-temperature sintering temperature is 800~850℃, the high-temperature sintering time is 10~12 hours, the heating rate is 25~30℃ / min, and the annealing rate is 45~50℃ / min; the baking and bonding conditions are as follows: the baking and bonding temperature is 160~200℃, and the baking time is 15~18 hours.

5. The method for preparing a positive electrode material for improving the first charge-discharge efficiency of a sodium battery according to claim 1, characterized in that: The MXene material is prepared by the following method: (1) Ti3AlC2 is added to hydrofluoric acid aqueous solution, stirred at 22~25℃ for 20~24 hours, washed with deionized water until pH=6~7, and freeze-dried to obtain Ti3C2; (2) Ti3C2 is added to hexadecyltrimethylammonium bromide aqueous solution and ultrasonically dispersed for 5~10 minutes, nitrogen source is added, stirred for 4~5 hours, and freeze-dried; the temperature is raised to 400~450℃ under ammonia atmosphere and held for 3~4 hours, and then cooled to room temperature to obtain the modified MXene material.

6. The method for preparing a positive electrode material for improving the first charge-discharge efficiency of a sodium battery according to claim 5, characterized in that: The concentration of the hydrofluoric acid aqueous solution is 30-40 wt%; the mass ratio of Ti3AlC2 to the hydrofluoric acid aqueous solution is 1:(10-15); and the concentration of the hexadecyltrimethylammonium bromide aqueous solution is 0.4-1.5 wt%.

7. The method for preparing a positive electrode material for improving the first charge-discharge efficiency of a sodium battery according to claim 5, characterized in that: The raw materials of the MXene material include the following components: by mass, 2-3 parts Ti3C2, 17-20 parts hexadecyltrimethylammonium bromide aqueous solution, and 0.08-0.12 parts nitrogen source; the nitrogen source is triammonium citrate and metal-based melamine in a mass ratio of 1:(0.04-0.07).

8. The method for preparing a positive electrode material for improving the first charge-discharge efficiency of a sodium battery according to claim 7, characterized in that: The preparation method of the metal-based melamine is as follows: (1) Melamine is added to glacial acetic acid and mixed uniformly at 120~130℃ to obtain a melamine mixed solution; polyphosphoric acid is added to glacial acetic acid and mixed uniformly at 60~70℃ to obtain a polyphosphoric acid mixed solution; (2) polyphosphoric acid mixed solution is added dropwise to the melamine mixed solution, and the addition is completed in 1~2 hours. The temperature is raised to 120~125℃ and magnesium dihydrogen phosphate is added. The mixture is mixed uniformly for 12~15 hours, filtered, washed, dried and ground to obtain metal-based melamine.

9. The method for preparing a positive electrode material for improving the first charge-discharge efficiency of a sodium battery according to claim 8, characterized in that: The raw materials for the metal-based melamine include the following components: by mass, 6-8 parts melamine, 4-5 parts polyphosphoric acid, 70-85 parts glacial acetic acid, and 2-3.5 parts magnesium dihydrogen phosphate.

10. The method for preparing a positive electrode material for improving the first charge-discharge efficiency of a sodium battery according to claim 5, characterized in that: The ammonia gas is introduced at a rate of 0.05~0.07 L / min; the heating rate is 4~5 °C / min.

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