Preparation method and application of a sodium-ion battery cathode material

Abstract

The application discloses a preparation method and application of a sodium ion battery positive electrode material, relates to the technical field of sodium ion batteries, and comprises the following steps: (1) dissolving a compound containing Cr elements in water, adding the solution into a reaction kettle, gradually adding NaOH solution, adding a chitosan hydrochloride aqueous solution into the reaction kettle in proportion after reaction is completed, filtering, washing and drying after reaction for a certain time to obtain chitosan-coated chromium hydroxide Cr(OH)3@CS; (2) blending Cr(OH)3@CS prepared in step (1) with a sodium source in a certain molar ratio, sintering the mixture under a protective gas, and then grinding and screening to obtain a carbon-coated sodium chromite positive electrode material NaCrO2@C; the application has good electrochemical performance and cycle performance, and promotes sustainable development of the sodium ion battery industry.

Description

Technical Field

[0001] This invention relates to the field of sodium-ion battery technology, and in particular to a method for preparing and applying a sodium-ion battery cathode material. Background Technology

[0002] Currently, electrochemical energy storage technology is widely used to provide continuous and stable energy. Lithium-ion batteries, with their long cycle life, high energy density, and high operating voltage, have become an important electrochemical energy storage technology. However, due to the limited availability of lithium resources, lithium-ion batteries cannot simultaneously support the development of electric vehicles and large-scale energy storage. Sodium-ion batteries share the same working principle and similar battery components as lithium-ion batteries. Furthermore, sodium resources are abundant, inexpensive, and offer good overall performance, making them a suitable energy storage material with better safety features.

[0003] Cathode materials are key components for the energy density and cost of sodium-ion batteries. Developing sodium-ion battery materials with high safety, low cost, long lifespan, and high specific capacity is of great significance. Currently, there are many research reports on sodium-ion battery cathode materials, with layered transition metal oxides such as Na... x MO2 (M = Ni, Fe, Mn, Co, and Cr, etc.) has become a research hotspot. Among them, NaCrO2 has a high theoretical capacity (250 mAh / g), a high charge-discharge voltage plateau (~3V), a smooth charge-discharge curve, and excellent thermal stability, thus showing great potential in the safe batteries required for energy storage technology.

[0004] Although NaCrO2 shows great potential, it also faces the same problems as other O3-type layered cathode materials. (1) Due to the large ionic radius of Na ions, the insertion and extraction of Na ions during charging and discharging cause drastic volume changes, and the continuous phase transitions result in poor long-cycle capacity retention of the material; (2) Only 0.5 Na ions in the material can be reversibly inserted and extracted. When the number of Na ions inserted and extracted is greater than 0.5, the Cr... 4+ Ions slide into the Na layer, causing an irreversible phase transition, which leads to an irreversible and rapid decay of the material's capacity; (3) It is highly sensitive to air and cannot be exposed to air for a long time. Therefore, how to improve the structural stability of NaCrO2 under high voltage, suppress irreversible phase transition, and improve the material's energy density is an urgent challenge to overcome.

[0005] Patent 109052474A describes the sintering of sodium dichromate in a reducing gas, hydrogen, to obtain sodium chromite cathode material. However, the sodium chromite prepared by this method is prone to side reactions with the electrolyte during charge-discharge processes, leading to capacity decay. Patent CN115818716A describes the mixing of dopant and sodium chromate through ball milling, followed by solid-state sintering to obtain sodium chromite material. However, ball milling is a physical mixing method; while it can reduce particle size, the significant differences in hardness between different materials can result in delamination or localized unevenness. Patent CN115863609A describes the preparation of carbon-coated sodium chromite material through a sol-gel and solid-state sintering method, thereby improving the material's electrochemical stability and cycle performance. However, the sol-gel method is only suitable for small-batch production and is not applicable to large-scale industrial production. Patent CN118745001A dissolves a chromium source, a sodium source, and a dopant element in water, and uses a drying method to obtain a blend with uniform element distribution. After sintering, a modified sodium chromite cathode material is obtained. This method effectively achieves uniform element distribution, but requires a large amount of heat to dry a significant amount of moisture, greatly increasing the preparation cost and hindering industrial production. Patent CN118619343A ball-mills and blends a carbon source and a chromium source, then adds it to a reduction process to obtain a primary modified product. This primary modified product is then crushed, heated, and kept at a specific temperature to obtain a carbon-intercalated sodium chromite material. However, this method cannot form a dense and continuous carbon layer on the surface of the sodium chromite.

[0006] Based on the above research, there is a need to develop a method for preparing sodium chromite cathode materials with high discharge capacity, excellent cycle performance, low cost, and uniform elemental distribution. Summary of the Invention

[0007] The first aspect of this invention is to provide a method for preparing a cathode material for sodium-ion batteries. This invention achieves uniform carbon layer coating of NaCrO2 through a precipitation method combined with an in-situ coating process using a bio-based carbon source, endowing the material with excellent electrochemical and cycle performance, and promoting the sustainable development of the sodium-ion battery industry.

[0008] To achieve the above objectives, the technical solution adopted by the present invention is as follows:

[0009] A method for preparing a sodium-ion battery cathode material includes the following steps:

[0010] (1) Preparation of chitosan-coated chromium hydroxide Cr(OH)3@CS

[0011] A Cr-containing compound was dissolved in water, and NaOH solution was gradually added dropwise to maintain the pH of the solution in the reactor within a certain range. After the reaction was completed, an aqueous solution of chitosan hydrochloride was gradually added to the reactor, and the reaction was continuously stirred at a certain temperature and pH to obtain chitosan-coated chromium hydroxide Cr(OH)3@CS.

[0012] (2) Preparation of carbon-coated sodium chromite cathode material NaCrO2@C

[0013] The Cr(OH)3@CS prepared in step (1) was mixed with a sodium source at a certain molar ratio, and the mixture was sintered under a protective gas to obtain the carbon-coated sodium chromite cathode material NaCrO2@C.

[0014] Further settings include:

[0015] In step (1), the Cr-containing compound is one or a mixture of two or more of chromium trioxide, chromium nitrate, chromium sulfate, chromium chloride, and potassium chromium sulfate in any proportion.

[0016] In step (1), the degree of deacetylation of the chitosan hydrochloride is 80.0-90.0%.

[0017] In step (1), the chitosan hydrochloride aqueous solution is gradually added to the reactor at a mass ratio of chromium hydroxide to chitosan of 3:1 to 8:1. Preferably, the mass ratio of chromium hydroxide to chitosan is 5:1.

[0018] In step (1), the Cr-containing compound is dissolved in water to prepare a 0.2-0.5 mol / L solution, which is then added to the reaction vessel. 1-3 mol / L NaOH solution is gradually added dropwise to make the pH of the solution in the reaction vessel 6-8. The reaction is continued for 1-3 hours at a stirring speed of 400-600 rpm. After the reaction is completed, 1-3 wt% of chitosan hydrochloride aqueous solution is gradually added to the reaction vessel at a mass ratio of 5:1 (chromium hydroxide to chitosan). The reaction is continued at a temperature of 40-60℃ and a pH of 6-8. After the reaction is completed for 2-3 hours, the mixture is filtered, washed, and vacuum dried to obtain chitosan-coated chromium hydroxide Cr(OH)3@CS.

[0019] In step (2), the sodium source is one or a mixture of two or more of sodium carbonate, sodium bicarbonate, sodium hydroxide, sodium citrate, sodium nitrate, and sodium oxalate in any proportion.

[0020] In step (2), the protective gas is one of nitrogen and argon or a mixture of two or more of them in any proportion.

[0021] In step (2), Cr(OH)3@CS is mixed with sodium carbonate at a molar ratio of 1:1.0-1.05. The mixture is sintered under a protective gas, first at 500-600℃ for 2-3 hours, and then at 800-900℃ for 8-10 hours. The heating rate is 5℃ / min. After the reaction, the mixture is ground and sieved to obtain carbon-coated sodium chromite cathode material NaCrO2@C.

[0022] Particularly preferred, a method for preparing a sodium-ion battery cathode material includes the following steps:

[0023] (1) Dissolve chromium chloride in water to prepare a 0.3 mol / L solution, then add the solution to the reaction vessel, and gradually add 2 mol / L NaOH solution to make the pH of the solution in the reaction vessel 7. Continue the reaction at a stirring speed of 500 rpm for 2 h. After the reaction is completed, add 2 wt% chitosan hydrochloride aqueous solution to the reaction vessel at a mass ratio of chromium hydroxide to chitosan of 5:1. Continue the reaction at a temperature of 50℃ and pH=7. After the reaction is completed for 2.5 h, filter and wash, and vacuum dry to obtain chitosan-coated chromium hydroxide Cr(OH)3@CS.

[0024] (2) Cr(OH)3@CS and sodium carbonate were mixed in a molar ratio of 1:1.03. The mixture was sintered under nitrogen. First, it was sintered at 550℃ for 2.5h, and then heated to 850℃ for 9h. The heating rate was 5℃ / min. After the reaction, it was ground and sieved to obtain carbon-coated sodium chromite cathode material NaCrO2@C.

[0025] A second objective of this invention is to provide an application of the sodium-ion battery cathode material prepared by the aforementioned method in sodium-ion batteries.

[0026] Compared with the prior art, the present invention has the following advantages:

[0027] (1) The Cr(OH)3@CS prepared by precipitation method in this invention can better coat chitosan on Cr(OH)3. In the subsequent sintering process, the carbon layer formed by the carbonization of chitosan at high temperature has nanoscale pores, which provides a direct diffusion channel for Na2O molecules, so that Na2O can enter the carbon layer and react with Cr2O3 to generate NaCrO2.

[0028] (2) The carbon layer acts as a physical barrier, buffering the volume expansion of NaCrO2 during charging and discharging, and inhibiting particle breakage and Cr dissolution. The high conductivity of the carbon layer can reduce electrode polarization and significantly improve the electron transport capability of NaCrO2. Nitrogen-doped carbon can enhance the contribution of pseudocapacitance on the material surface, thus Cr(OH)3@CS has ultra-high rate performance. The carbon layer can isolate NaCrO2 from direct contact with the electrolyte, reducing side reactions (such as Na+, Na+, and Cr+). + (Solvation effect and interface corrosion) enable it to maintain excellent stability at high rates, thereby improving cycle life.

[0029] (3) This method has a simple process flow and is suitable for continuous industrial production.

[0030] (4) As a bio-based material, chitosan emits no harmful gases during carbonization, and the coated cathode material can recover metals through acid hydrolysis, which meets the requirements of green manufacturing.

[0031] The present invention will be further described below with reference to the accompanying drawings and specific embodiments. Attached Figure Description

[0032] Figure 1 SEM images of the cathode materials prepared in Example 1 and Comparative Examples 1-3 are shown for comparison.

[0033] Figure 2 The XRD patterns of the cathode materials prepared in Example 1 and Comparative Examples 1-3 are shown for comparison. Detailed Implementation

[0034] The present invention will now be explained in more detail through specific embodiments. However, it should be understood that the specific functional details disclosed in this specification should not be construed as limiting, but rather as the basis for the claims and as a representative basis for teaching those skilled in the art to employ the invention in different ways in any suitable detailed embodiment. Unless otherwise specified, the experimental or testing methods described in the following embodiments are conventional methods; the reagents and materials, unless otherwise specified, are obtained from conventional commercial sources or prepared using conventional methods.

[0035] Example 1

[0036] A method for preparing a sodium-ion battery cathode material includes the following steps:

[0037] (1) Chromium chloride was dissolved in water to prepare a 0.3 mol / L solution, which was then added to the reaction vessel. 2 mol / L NaOH solution was gradually added dropwise to make the pH of the solution in the reaction vessel 7. The reaction was continued for 2 hours with stirring at 500 rpm. After the reaction was completed, 2 wt% chitosan hydrochloride aqueous solution was gradually added to the reaction vessel at a mass ratio of 5:1 (chromium hydroxide to chitosan). The reaction was continued with stirring at 50℃ and pH=7. After 2.5 hours of reaction, the mixture was filtered, washed, and vacuum dried to obtain chitosan-coated chromium hydroxide Cr(OH)3@CS.

[0038] (2) Cr(OH)3@CS and sodium carbonate were mixed in a molar ratio of 1:1.03. The mixture was sintered under nitrogen. First, it was sintered at 550℃ for 2.5h, and then heated to 850℃ for 9h. The heating rate was 5℃ / min. After the reaction, it was ground and sieved to obtain the carbon-coated sodium chromite cathode material NaCrO2@C.

[0039] Example 2

[0040] This embodiment mainly examines the effect of pH value changes on materials.

[0041] (1) Chromium chloride was dissolved in water to prepare a 0.3 mol / L solution, which was then added to a reaction vessel. 2 mol / L NaOH solution was gradually added dropwise to bring the pH of the solution in the reaction vessel to 8. The reaction was continued for 2 hours with stirring at 500 rpm. After the reaction was complete, 2 wt% chitosan hydrochloride aqueous solution was gradually added to the reaction vessel at a mass ratio of 5:1 (chromium hydroxide to chitosan). The reaction was continued with stirring at 50°C and pH 7. After 2.5 hours of reaction, the mixture was filtered, washed, and vacuum dried to obtain chitosan-coated chromium hydroxide Cr(OH)3@CS.

[0042] (2) Cr(OH)3@CS and sodium carbonate were mixed in a molar ratio of 1:1.03. The mixture was sintered under nitrogen. First, it was sintered at 550℃ for 2.5h, and then heated to 850℃ for 9h. The heating rate was 5℃ / min. After the reaction, it was ground and sieved to obtain the carbon-coated sodium chromite cathode material NaCrO2@C.

[0043] Example 3

[0044] This embodiment mainly examines the effects of different chromium-containing compounds on the material.

[0045] (1) Dissolve the chromium nitrate compound in water to prepare a 0.5 mol / L solution, then add the solution to a reaction vessel. Gradually add 1 mol / L NaOH solution to adjust the pH of the solution in the reaction vessel to 8, and continue the reaction at a stirring speed of 600 rpm for 3 h. After the reaction is complete, add 2 wt% chitosan hydrochloride aqueous solution to the reaction vessel at a mass ratio of chromium hydroxide to chitosan of 5:1, and continue the reaction at a temperature of 50℃ and pH=6. After the reaction is completed for 2.5 h, filter, wash, and vacuum dry to obtain chitosan-coated chromium hydroxide Cr(OH)3@CS.

[0046] (2) Cr(OH)3@CS and sodium bicarbonate were mixed in a molar ratio of 1:1.03. The mixture was sintered under nitrogen. First, it was sintered at 550℃ for 2.5h, and then heated to 850℃ for 9h. The heating rate was 5℃ / min. After the reaction, it was ground and sieved to obtain the carbon-coated sodium chromite cathode material NaCrO2@C.

[0047] Example 4

[0048] This embodiment mainly examines the effects of stirring speed and reaction time on the materials.

[0049] (1) Chromium chloride was dissolved in water to prepare a 0.3 mol / L solution, which was then added to a reaction vessel. 2 mol / L NaOH solution was gradually added dropwise to bring the pH of the solution in the reaction vessel to 7. The reaction was continued for 1 hour with stirring at 400 rpm. After the reaction was complete, 2 wt% chitosan hydrochloride aqueous solution was gradually added to the reaction vessel at a mass ratio of 5:1 (chromium hydroxide to chitosan). The reaction was continued with stirring at 50°C and pH=7. After 2.5 hours of reaction, the mixture was filtered, washed, and vacuum dried to obtain chitosan-coated chromium hydroxide Cr(OH)3@CS.

[0050] (2) Cr(OH)3@CS and sodium nitrate were mixed in a molar ratio of 1:1.03. The mixture was sintered under nitrogen. First, it was sintered at 550℃ for 2.5h, and then heated to 850℃ for 9h. The heating rate was 5℃ / min. After the reaction, it was ground and sieved to obtain the carbon-coated sodium chromite cathode material NaCrO2@C.

[0051] Example 5

[0052] This embodiment mainly investigates the effect of chitosan hydrochloride aqueous solution concentration on the material.

[0053] (1) Chromium chloride was dissolved in water to prepare a 0.3 mol / L solution, which was then added to a reaction vessel. 2 mol / L NaOH solution was gradually added dropwise to bring the pH of the solution in the reaction vessel to 7. The reaction was continued for 2 hours with stirring at 500 rpm. After the reaction was complete, 3 wt% chitosan hydrochloride aqueous solution was gradually added to the reaction vessel at a mass ratio of 5:1 (chromium hydroxide to chitosan). The reaction was continued with stirring at 50°C and pH=7. After 2 hours of reaction, the mixture was filtered, washed, and vacuum dried to obtain chitosan-coated chromium hydroxide Cr(OH)3@CS.

[0054] (2) Cr(OH)3@CS was mixed with sodium carbonate at a molar ratio of 1:1.03. The mixture was sintered under argon gas. First, it was sintered at 550℃ for 2.5h, and then heated to 850℃ for 9h. The heating rate was 5℃ / min. After the reaction, it was ground and sieved to obtain the carbon-coated sodium chromite cathode material NaCrO2@C.

[0055] Example 6

[0056] This embodiment mainly examines the impact of the chitosan-coated chromium hydroxide process on the material.

[0057] (1) Chromium chloride was dissolved in water to prepare a 0.3 mol / L solution, which was then added to a reaction vessel. 2 mol / L NaOH solution was gradually added dropwise to bring the pH of the solution in the reaction vessel to 7. The reaction was continued for 2 hours with stirring at 500 rpm. After the reaction was complete, 1 wt% chitosan hydrochloride aqueous solution was gradually added to the reaction vessel at a mass ratio of 5:1 (chromium hydroxide to chitosan). The reaction was continued with stirring at 60℃ and pH=8. After 3 hours of reaction, the mixture was filtered, washed, and vacuum dried to obtain chitosan-coated chromium hydroxide Cr(OH)3@CS.

[0058] (2) Cr(OH)3@CS and sodium carbonate were mixed in a molar ratio of 1:1.03. The mixture was sintered under nitrogen. First, it was sintered at 550℃ for 2.5h, and then heated to 850℃ for 9h. The heating rate was 5℃ / min. After the reaction, it was ground and sieved to obtain the carbon-coated sodium chromite cathode material NaCrO2@C.

[0059] Example 7

[0060] This embodiment mainly examines the effect of sodium carbonate dosage on the materials.

[0061] (1) Chromium chloride was dissolved in water to prepare a 0.3 mol / L solution, which was then added to a reaction vessel. 2 mol / L NaOH solution was gradually added dropwise to bring the pH of the solution in the reaction vessel to 7. The reaction was continued for 2 hours with stirring at 500 rpm. After the reaction was complete, 2 wt% chitosan hydrochloride aqueous solution was gradually added to the reaction vessel at a mass ratio of 5:1 (chromium hydroxide to chitosan). The reaction was continued with stirring at 50°C and pH=7. After 2.5 hours of reaction, the mixture was filtered, washed, and vacuum dried to obtain chitosan-coated chromium hydroxide Cr(OH)3@CS.

[0062] (2) Cr(OH)3@CS and sodium carbonate were mixed in a molar ratio of 1:1.05. The mixture was sintered under nitrogen. First, it was sintered at 550℃ for 2.5h, and then heated to 850℃ for 9h. The heating rate was 5℃ / min. After the reaction, it was ground and sieved to obtain the carbon-coated sodium chromite cathode material NaCrO2@C.

[0063] Example 8

[0064] This embodiment mainly examines the effects of different sodium sources on the material.

[0065] (1) Chromium chloride was dissolved in water to prepare a 0.3 mol / L solution, which was then added to a reaction vessel. 2 mol / L NaOH solution was gradually added dropwise to bring the pH of the solution in the reaction vessel to 7. The reaction was continued for 2 hours with stirring at 500 rpm. After the reaction was complete, 2 wt% chitosan hydrochloride aqueous solution was gradually added to the reaction vessel at a mass ratio of 5:1 (chromium hydroxide to chitosan). The reaction was continued with stirring at 50°C and pH=7. After 2.5 hours of reaction, the mixture was filtered, washed, and vacuum dried to obtain chitosan-coated chromium hydroxide Cr(OH)3@CS.

[0066] (2) Cr(OH)3@CS and sodium citrate were mixed in a molar ratio of 1:1.05. The mixture was sintered under argon gas. First, it was sintered at 550℃ for 2.5h, and then heated to 850℃ for 9h. The heating rate was 5℃ / min. After the reaction, it was ground and sieved to obtain the carbon-coated sodium chromite cathode material NaCrO2@C.

[0067] Examples 9-10 mainly investigate the effects of sintering temperature and time on the materials.

[0068] Example 9

[0069] (1) Chromium chloride was dissolved in water to prepare a 0.3 mol / L solution, which was then added to a reaction vessel. A 2 mol / L NaOH solution was gradually added dropwise to bring the pH of the solution in the reaction vessel to 7. The reaction was continued for 2 hours with stirring at 500 rpm. After the reaction was complete, 2 wt% of chitosan hydrochloride aqueous solution was gradually added to the reaction vessel at a mass ratio of 5:1 (chromium hydroxide to chitosan). The reaction was continued with stirring at 50°C and pH=7. After 2.5 hours of reaction, the chitosan-coated chromium hydroxide Cr(OH)3@CS was obtained by vacuum drying.

[0070] (2) Cr(OH)3@CS and sodium carbonate were mixed in a molar ratio of 1:1.03. The mixture was sintered under nitrogen. First, it was sintered at 500℃ for 3h, and then heated to 800℃ for 8h. The heating rate was 5℃ / min. After the reaction, it was ground and sieved to obtain the carbon-coated sodium chromite cathode material NaCrO2@C.

[0071] Example 10

[0072] (1) Chromium chloride was dissolved in water to prepare a 0.3 mol / L solution, which was then added to a reaction vessel. 2 mol / L NaOH solution was gradually added dropwise to bring the pH of the solution in the reaction vessel to 7. The reaction was continued for 2 hours with stirring at 500 rpm. After the reaction was complete, 2 wt% chitosan hydrochloride aqueous solution was gradually added to the reaction vessel at a mass ratio of 5:1 (chromium hydroxide to chitosan). The reaction was continued with stirring at 50°C and pH=7. After 2.5 hours of reaction, the mixture was filtered, washed, and vacuum dried to obtain chitosan-coated chromium hydroxide Cr(OH)3@CS.

[0073] (2) Cr(OH)3@CS and sodium carbonate were mixed in a molar ratio of 1:1.03. The mixture was sintered under nitrogen. First, it was sintered at 600℃ for 2.5h, and then heated to 900℃ for 10h. The heating rate was 5℃ / min. After the reaction, it was ground and sieved to obtain the carbon-coated sodium chromite cathode material NaCrO2@C.

[0074] Comparative Example 1

[0075] Chromium trioxide and sodium carbonate were ball-milled and blended in a molar ratio of 1:1. The blend was then sintered under nitrogen protection. The sintering program was 550℃ for 2.5 h and 850℃ for 9 h, with a heating rate of 5℃ / min. After the reaction, the mixture was ground and sieved to obtain sodium chromite cathode material.

[0076] Comparative Example 2

[0077] Chromium hydroxide monohydrate and sodium carbonate were ball-milled and blended at a molar ratio of 2:1. The blend was then sintered under nitrogen protection. The sintering program was 550℃ for 2.5 h and 850℃ for 9 h, with a heating rate of 5℃ / min. After the reaction, the mixture was ground and sieved to obtain sodium chromite cathode material.

[0078] Comparative Example 3

[0079] Chromium hydroxide monohydrate and sodium carbonate were ball-milled and blended at a molar ratio of 2:1. The blend was then sintered under nitrogen protection. The sintering program was 550℃ for 2.5 h and 850℃ for 9 h, with a heating rate of 5℃ / min. After the reaction, the mixture was ground and sieved to obtain sodium chromite cathode material.

[0080] Sodium chromite and chitosan were blended at a mass ratio of 5:1 and heated to 900℃ at a rate of 5℃ / min under nitrogen protection and held for 5 hours. After the reaction, the mixture was ground and sieved to obtain carbon-coated sodium chromite cathode material.

[0081] Product characterization:

[0082] Figure 1 For comparison, here are SEM images of the cathode materials prepared in Example 1 and Comparative Examples 1-3. Figure 2 The XRD patterns of the cathode materials prepared in Example 1 and Comparative Examples 1-3 are shown for comparison.

[0083] like Figure 1 As shown: The SEM image of Comparative Example 3 contains a large number of needle-like substances, while the needle-like substances in Example 1 are less numerous, indicating that the carbon layer coating in Example 1 is better.

[0084] like Figure 2 As shown, the XRD curves of Example 1 and Comparative Example 3 show a peak at around 13°, further proving the presence of a carbon layer in the material.

[0085] Performance testing:

[0086] The sodium-ion battery cathode materials prepared in the aforementioned examples and comparative examples were subjected to electrochemical performance testing according to the following methods.

[0087] Test Method: The positive electrode material powder was ground through a 200-mesh sieve, then mixed with SP (conductive carbon black) and PVDF (polyvinylidene fluoride) in a mass ratio of 8:1:1. NMP (N-methylpyrrolidone) was added and stirred into a slurry, which was then coated onto aluminum foil. After drying, stamping, and pressing, a sodium-ion battery positive electrode material sheet was formed. Using metallic sodium as the negative electrode, glass fiber (Whatman GF / D brand) as the separator, and a NaPF6 (sodium hexafluorophosphate) / PC (propylene carbonate) / EMC (ethyl methyl carbonate) solution as the electrolyte, CR2025 button batteries were assembled in an argon-filled glove box for charge-discharge testing. Under a current density of 100 mAh / g and a voltage range of 2.0-3.6V, the battery was first cycled twice at 0.2C, and then its capacity and cycle performance were tested at equal rates of 1C / 5C.

[0088] The test results are shown in Table 1:

[0089] Table 1

[0090] .

[0091] analyze:

[0092] As shown in Table 1, compared with Comparative Examples 1-3, the cathode materials prepared in Examples 1-10 exhibit superior electrochemical and cycling performance. Among them, Example 1 achieves a discharge specific capacity of 126.5 mAh / g at 2-3.6V and 0.2C, a discharge specific capacity of 125.2 mAh / g at 1C, and a discharge specific capacity of 123.4 mAh / g at 5C. Furthermore, it retains 82.3% of its capacity after 1000 cycles at 5C, demonstrating the best overall performance.

[0093] Combination Figure 1 , Figure 2It can be seen that, compared with Comparative Examples 1-2, Example 1 has a nitrogen-doped carbon layer on the positive electrode material NaCrO2. The presence of the carbon layer can buffer the volume expansion of the positive electrode material during charge and discharge, inhibit particle breakage and Cr element dissolution, thereby improving the material's cycle performance. It can also improve the material's electron transport capability, enabling the material to maintain excellent performance at high rates. Compared with Comparative Example 3, Comparative Example 3 only mixes chitosan and sodium chromite through physical ball milling, while Example 1 uses a precipitation method to attach chitosan to chromium hydroxide, which allows for a more uniform carbon layer coating on the sodium chromite during subsequent sintering. Figure 1 The SEM image of Comparative Example 3 shows a large number of needle-like substances, while Example 1 shows fewer needle-like substances, indicating that the carbon layer coating in Example 1 is better. Furthermore, the 5C cycling performance in Table 1 further demonstrates that the precipitation method can achieve better uniform carbon layer coating.

[0094] Different process conditions can also lead to differences in performance, as follows:

[0095] Compared to Example 1, Example 2 adjusted the precipitation conditions in step 1. An excessively high pH caused some chromium hydroxide to redissolve in the solution, reducing the yield. Furthermore, the formed chromium hydroxide particles became charge-neutralized, accelerating agglomeration and tending to form large particles. These large particles were more prone to internal cracking during long cycles, leading to decreased material cycling performance. In Example 3, the chromium-containing compound was replaced with chromium nitrate in step 1. To synthesize Cr(OH)3 precipitate, the pH was adjusted to 8. However, in the subsequent synthesis of Cr(OH)3@CS, the pH was only 6, indicating a significant difference in the required pH for precipitation between the two substances. In Example 4, the stirring speed and reaction time for Cr(OH)3 precipitation were adjusted in step 1. The reduced stirring speed and reaction time resulted in incomplete precipitation and poor particle uniformity during the reaction. In Example 5, the concentration of the chitosan hydrochloride solution was increased in step 1. This increased concentration reduced the solution's fluidity, causing the precipitated chitosan to tend to self-aggregate and fail to disperse uniformly on the chromium hydroxide. In Example 6, the process parameters for chitosan-coated chromium hydroxide were adjusted in step 1. Excessive temperature caused rapid formation of chitosan precipitate during the reaction, accompanied by particle aggregation and uneven distribution. This prevented the formation of a continuous carbon layer on the material surface during sintering in step 2, resulting in a decrease in the electrochemical performance of carbon-coated sodium chromite. Compared to Example 1, Example 7 increased the sodium carbonate content in step 2. However, as shown in Table 1, the increase in sodium content not only failed to improve the discharge specific capacity of the material but also decreased its cycle performance. In Example 8, sodium carbonate was replaced with sodium citrate in step 2. The addition of sodium citrate did not improve the discharge specific capacity of the material. Examples 9-10 investigated the effects of reaction temperature and reaction time on material properties in step 2. The results showed that increased temperature and longer reaction time densified the carbon layer on the material surface, leading to increased ion transport resistance and a faster volatilization rate of Na₂O, the decomposition product of sodium carbonate, further reducing the electrochemical performance of the material. Furthermore, the energy consumption was higher, increasing the cost. Decreasing the reaction temperature and reducing the reaction time will reduce the crystallinity of the material and result in incomplete carbon layer coating, which will exacerbate the side reactions between the material and the electrolyte.

[0096] Those skilled in the art will readily understand that the above description is merely an embodiment of the present invention and is not intended to limit the present invention. Any modifications, equivalent substitutions, and improvements made within the spirit and principles of the present invention should be included within the scope of protection of the present invention.

Claims

1. A method for preparing a sodium-ion battery cathode material, characterized in that, Includes the following steps: (1) Preparation of chitosan-coated chromium hydroxide Cr(OH)3@CS A Cr-containing compound was dissolved in water, and NaOH solution was gradually added dropwise to maintain the pH of the solution in the reactor within a certain range. After the reaction was completed, an aqueous solution of chitosan hydrochloride was gradually added to the reactor, and the reaction was continuously stirred at a certain temperature and pH to obtain chitosan-coated chromium hydroxide Cr(OH)3@CS. (2) Preparation of carbon-coated sodium chromite cathode material NaCrO2@C The chitosan-coated chromium hydroxide Cr(OH)3@CS prepared in step (1) was mixed with a sodium source at a certain molar ratio, and the mixture was sintered under a protective gas to obtain the carbon-coated sodium chromite cathode material NaCrO2@C.

2. The method for preparing a sodium-ion battery cathode material according to claim 1, characterized in that: In step (1), the Cr-containing compound is one or a mixture of two or more of chromium trioxide, chromium nitrate, chromium sulfate, chromium chloride, and potassium chromium sulfate in any proportion.

3. The method for preparing a sodium-ion battery cathode material according to claim 1, characterized in that: In step (1), the chitosan hydrochloride aqueous solution is gradually added to the reactor at a mass ratio of chromium hydroxide to chitosan of 3:1 to 8:

1.

4. The method for preparing a sodium-ion battery cathode material according to claim 1, characterized in that: In step (1), the Cr-containing compound is dissolved in water to prepare a 0.2-0.5 mol / L solution, which is then added to the reaction vessel. 1-3 mol / L NaOH solution is gradually added dropwise to make the pH of the solution in the reaction vessel 6-8. The reaction is continued for 1-3 hours at a stirring speed of 400-600 rpm. After the reaction is completed, 1-3 wt% of chitosan hydrochloride aqueous solution is gradually added to the reaction vessel at a mass ratio of 5:1 (chromium hydroxide to chitosan). The reaction is continued at a temperature of 40-60℃ and a pH of 6-8. After the reaction is completed for 2-3 hours, the mixture is filtered, washed, and vacuum dried to obtain chitosan-coated chromium hydroxide Cr(OH)3@CS.

5. The method for preparing a sodium-ion battery cathode material according to claim 1, characterized in that: In step (2), the sodium source is one or a mixture of two or more of sodium carbonate, sodium bicarbonate, sodium hydroxide, sodium citrate, sodium nitrate, and sodium oxalate in any proportion.

6. The method for preparing a sodium-ion battery cathode material according to claim 1, characterized in that: In step (2), the protective gas is one of nitrogen and argon or a mixture of two or more of them in any proportion.

7. The method for preparing a sodium-ion battery cathode material according to claim 1, characterized in that: In step (2), Cr(OH)3@CS is mixed with sodium carbonate at a molar ratio of 1:1.0-1.

05. The mixture is sintered under a protective gas, first at 500-600℃ for 2-3 hours, and then at 800-900℃ for 8-10 hours. The heating rate is 5℃ / min. After the reaction, the mixture is ground and sieved to obtain the carbon-coated sodium chromite cathode material NaCrO2@C.

8. The method for preparing a sodium-ion battery cathode material according to claim 7, characterized in that: In step (2), Cr(OH)3@CS is mixed with sodium carbonate at a molar ratio of 1:1.

03. The mixture is sintered under nitrogen, first at 550℃ for 2.5h, and then at 850℃ for 9h. The heating rate is 5℃ / min. After the reaction, the mixture is ground and sieved to obtain the carbon-coated sodium chromite cathode material NaCrO2@C.

9. The method for preparing a sodium-ion battery cathode material according to claim 1, characterized in that: (1) Dissolve chromium chloride in water to prepare a 0.3 mol / L solution, then add the solution to the reaction vessel, and gradually add 2 mol / L NaOH solution to make the pH of the solution in the reaction vessel 7. Continue the reaction at a stirring speed of 500 rpm for 2 h. After the reaction is completed, add 2 wt% chitosan hydrochloride aqueous solution to the reaction vessel at a mass ratio of chromium hydroxide to chitosan of 5:

1. Continue the reaction at a temperature of 50℃ and pH=7. After the reaction is completed for 2.5 h, filter and wash, and vacuum dry to obtain chitosan-coated chromium hydroxide Cr(OH)3@CS. (2) Cr(OH)3@CS was mixed with sodium carbonate at a molar ratio of 1:1.

03. The mixture was sintered under nitrogen. First, it was sintered at 550℃ for 2.5h, and then heated to 850℃ for 9h. The heating rate was 5℃ / min. After the reaction, it was ground and sieved to obtain the carbon-coated sodium chromite cathode material NaCrO2@C.

10. The application of a sodium-ion battery cathode material prepared by the method according to any one of claims 1-9 in a sodium-ion battery.

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