Sodium-ion battery positive electrode material with high cycling stability and preparation method of sodium-ion battery positive electrode material

Abstract

The invention discloses a sodium-ion battery positive electrode material with high cycling stability and a preparation method thereof, and belongs to the technical field of sodium-ion batteries, the structural formula of the positive electrode material is NaCrO2atAl2O3, and NaCrO2 is uniformly coated with Al2O3. The preparation method comprises the following steps: firstly, adopting a fractional precipitation method, and controlling the pH value of a solution, the reaction temperature and the aging time to prepare a sodium-ion battery positive electrode material with high cycling stability; preparing a precursor Cr (OH) 3-Al (OH) 3 with chromium hydroxide uniformly coated with aluminum hydroxide; then the precursor and a sodium source are mixed according to a certain molar ratio, sintering treatment is carried out, the positive electrode material NaCrO2-coated Al2O3 is obtained, a shell layer of the positive electrode material Al2O3 can isolate direct contact of NaCrO2 and electrolyte, overgrowth of a CEI film is reduced, the volume change of material crystals is inhibited, the stability of a surface lattice structure is enhanced, and the service life of the positive electrode material is prolonged. Therefore, the material is endowed with good electrochemical performance and cycle performance.

Description

Technical Field

[0001] This invention relates to the field of sodium-ion battery technology, and in particular to a sodium-ion battery cathode material with high cycle stability and its preparation method. Background Technology

[0002] Since its commercialization in 1991, lithium-ion batteries have achieved great success in electric vehicles and portable electronic products. However, due to the small reserves and uneven global distribution of lithium resources, the cost of lithium-ion batteries has continued to rise, which has limited their development in large-scale energy storage to some extent. Sodium, on the other hand, is abundant in the Earth's crust, widely distributed, and inexpensive, providing strong support for the widespread application of sodium-ion batteries in energy storage.

[0003] NaCrO2 has a theoretical capacity of up to 250 mAh / g, and it also has a high charge-discharge voltage plateau (~3V), a smooth charge-discharge curve, and excellent thermal stability, making it a highly sought-after O3-type layered oxide.

[0004] Patent CN119370888A uses reducing sodium source and hexavalent chromium source as raw materials, which are directly mixed and prepared through solid-phase redox reaction. The sodium chromite cathode material prepared by this method is prone to side reactions with the electrolyte during charging and discharging, resulting in rapid decay of discharge capacity.

[0005] Patent CNA118619343A describes a physical ball milling process to mix a chromium source and a carbon source to obtain a solid mixture. This solid mixture is then heated and reduced to obtain a primary modified material. After crushing, it is further heated and kept at a certain temperature to obtain a carbon-intercalated sodium chromite material. However, because the chromium and carbon sources are physically blended, the carbon cannot uniformly and completely coat the sodium chromite material.

[0006] Patent CN115863609A describes the preparation of carbon-coated sodium chromite materials using a sol-gel and solid-state sintering method, which improves the electrochemical stability and cycle performance of the materials. However, the sol-gel method is only suitable for small-batch production and cannot be applied to large-scale industrial production.

[0007] Patent CN118598187A describes a process where a chromium source and aluminum additive are ground and blended, followed by multiple sintering cycles to obtain an aluminum-doped sodium chromite-coated material. However, this patent employs a multiple sintering method and uses hydrogen to reduce sodium chromite, resulting in high energy consumption and a significant safety risk. The hexavalent chromium salt used as the chromium source is carcinogenic, listed as a Group 1 carcinogen, and causes severe environmental pollution, being included in the list of toxic and hazardous pollutants. Furthermore, the method uses ball milling to mix the materials, leading to relatively poor material distribution uniformity and an inability to form an effective, continuous, and dense coating layer. During battery operation, the sodium chromite comes into contact with more electrolyte, exacerbating side reactions and deteriorating the material's cycle performance. In an inert water test, the surface coating of the sodium chromite was not continuous and dense, causing the core sodium chromite to precipitate in the water, forming alkaline sodium hydroxide and sodium carbonate, further increasing the pH of the aqueous solution after 24 hours.

[0008] Based on the above research, there is a need to develop a method for preparing sodium chromite cathode materials that is safe, environmentally friendly, suitable for large-scale industrial production, and has high cycle stability. Summary of the Invention

[0009] The purpose of this invention is to provide a sodium-ion battery cathode material with high cycle stability and its preparation method.

[0010] To achieve the above objectives, the technical solution adopted by the present invention is as follows: The first objective of this invention is to provide a sodium-ion battery cathode material with high cycle stability, having the structural formula NaCrO2@Al2O3, wherein Al2O3 is uniformly coated on NaCrO2.

[0011] A second aspect of this invention aims to provide a method for preparing a sodium-ion battery cathode material with high cycle stability, comprising the following steps: (1) Preparation of aluminum hydroxide coated chromium hydroxide Cr(OH)3@Al(OH)3 A Cr-containing compound was dissolved in water to obtain solution A, and an Al-containing compound was dissolved in water to obtain solution B. NaOH solution was added to solution A, and the pH was controlled within a certain range. The mixture was stirred to allow Cr(OH)3 to precipitate completely. After precipitation, the mixture was aged for a certain period of time. Then, solution B and NaOH solution were added, and the pH was controlled within a certain range. After the reaction was completed, the mixture was aged for a period of time to finally obtain Cr(OH)3@Al(OH)3. (2) Preparation of sodium chromite cathode material coated with alumina NaCrO2@Al2O3 The Cr(OH)3@Al(OH)3 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 sodium chromite cathode material NaCrO2@Al2O3 coated with aluminum oxide.

[0012] This invention employs stepwise precipitation to achieve heterogeneous nucleation and epitaxial growth of the shell material on the core surface, constructing a continuous and robust coating layer. Then, combined with solid-state sintering, a uniform Al2O3 coating of NaCrO2 is achieved. The Al2O3 shell acts as a physical barrier, buffering the volume expansion of NaCrO2 during charge and discharge, inhibiting particle breakage and Cr dissolution. The Al2O3 shell also isolates NaCrO2 from direct contact with the electrolyte, reducing excessive CEI film growth and suppressing crystal volume changes, thus enhancing the stability of the surface lattice structure and endowing the material with excellent electrochemical and cycling performance.

[0013] Further settings are as follows: 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.

[0014] The Al-containing compound is one or a mixture of two or more of aluminum nitrate, aluminum sulfate, aluminum trichloride, potassium aluminum sulfate dodecahydrate, and aluminum isopropoxide in any proportion.

[0015] NaOH solution was added to solution A, and the pH of the reaction was controlled at 6-8. The reaction was carried out at 40-80℃ with continuous stirring at 400-600 rpm for 1-3 hours. After the reaction was completed, the mixture was aged for 2-3 hours.

[0016] The solution B and NaOH solution are added, and the pH of the reaction is controlled at 6-8. After the reaction is completed, the mixture is aged for 1-3 hours.

[0017] 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.

[0018] The molar ratio of Cr(OH)3@Al(OH)3 to the sodium source is 1:1.01-1.05.

[0019] The protective gas is one of nitrogen and argon, or a mixture of two or more of them in any proportion.

[0020] The sintering process is as follows: first sinter at 500-600℃ for 2-3 hours, then sinter at 800-1000℃ for 8-10 hours, with a heating rate of 5℃ / min.

[0021] Particularly preferred: A method for preparing a sodium-ion battery cathode material with high cycle stability includes the following steps: (1) Dissolve the Cr-containing compound in water to prepare a 0.1-0.3 mol / L solution A, and dissolve the Al-containing compound in water to prepare a 0.01-0.05 mol / L solution B. The molar ratio of Al to Cr is 1:50-150. Add solution A to the reaction vessel, add 1-3 mol / L NaOH solution to maintain the pH of the solution in the reaction vessel at 6-8, and stir continuously at 400-600 rpm at 40-80℃ for 1-3 h to completely precipitate Cr(OH)3. After precipitation, continue aging for 2-3 h. Then slowly add solution B and 0.1-0.5 mol / L NaOH solution to the reaction vessel, maintain the pH at 6-8, and continue aging for 1-3 h after solution B is added to finally obtain Cr(OH)3@Al(OH)3. (2) The Cr(OH)3@Al(OH)3 prepared in step (1) is mixed with sodium source at a molar ratio of 1:1.01-1.05. The mixture is sintered under a protective gas. First, it is sintered at 500-600℃ for 2-3h, and then heated to 800-1000℃ for 8-10h. The heating rate is 5℃ / min. Then, it is ground and sieved to obtain alumina-coated sodium chromite cathode material NaCrO2@Al2O3.

[0022] The third objective of this invention is to provide an application of the sodium-ion battery cathode material with high cycle stability prepared by the above method in the preparation of sodium-ion batteries.

[0023] Compared with the prior art, the present invention has the following beneficial effects: (1) This invention achieves heterogeneous nucleation and epitaxial growth of the shell material on the core surface through stepwise precipitation, constructing a continuous and robust coating layer. Then, combined with solid-state sintering, it achieves uniform coating of NaCrO2 with Al2O3 to obtain the cathode material NaCrO2@Al2O3. The Al2O3 shell acts as a physical barrier, buffering the volume expansion of NaCrO2 during charging and discharging, and inhibiting particle breakage and Cr element dissolution. The Al2O3 shell can isolate NaCrO2 from direct contact with the electrolyte, reducing side reactions (such as Na...). + (Solvation effect and interface corrosion), thereby improving cycle life, and the capacity retention rate can still reach 80% after 1136 cycles at 1C rate.

[0024] (2) The process of this invention is simple, safe and environmentally friendly, and suitable for continuous industrial production. Attached Figure Description

[0025] Figure 1 XRD pattern of the cathode material prepared in Example 1.

[0026] Figure 2 SEM image of the cathode material prepared in Example 1.

[0027] Figure 3 Capacity performance diagram of the cathode material prepared in Example 1.

[0028] Figure 4 Cyclic performance diagram of the cathode material prepared in Example 1. Specific implementation methods

[0029] 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 raw materials and reagents used in the embodiments are prior art or commercially available products.

[0030] Example 1

[0031] (1) Dissolve chromium chloride in water to prepare a 0.2 mol / L solution A, and dissolve aluminum sulfate in water to prepare a 0.03 mol / L solution B, controlling the molar ratio of Al to Cr to be 1:100. Add solution A to the reaction vessel, add 2 mol / L NaOH solution, control the pH of the solution in the reaction vessel to be 6, and stir continuously at 500 rpm at 60℃ for 2 h to allow Cr(OH)3 to precipitate completely. After precipitation is complete, continue aging for 2.5 h. Then slowly add solution B and 0.3 mol / L NaOH solution to the reaction vessel, maintain pH at 7, and continue aging for 2 h after solution B is completely added, finally obtaining Cr(OH)3@Al(OH)3.

[0032] (2) Cr(OH)3@Al(OH)3 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 950℃ for 9h. The heating rate was 5℃ / min. After the reaction, it was ground and sieved to obtain sodium chromite cathode material NaCrO2@Al2O3 coated with aluminum oxide.

[0033] Product characterization: Figure 1 The image shows the XRD pattern of the cathode material prepared in Example 1. As can be seen from the image, the coating of NaCrO2 with a layer of Al2O3 did not change the crystal form of the material, which remained in the O3 form. Figure 2The image shows a SEM image of the cathode material prepared in Example 1. The particle surface is coated with a dense and continuous layer of Al2O3. During charge and discharge, Al2O3 can prevent direct contact between NaCrO2 and the electrolyte, reduce excessive growth of the CEI film, inhibit changes in the material's crystal volume, and enhance the stability of the surface lattice structure, thereby improving the material's cycle performance. Furthermore, the Al2O3 coating layer can effectively isolate moisture, reducing the precipitation of Na in the cathode material. Figure 3 , Figure 4 The figure shows the electrochemical performance of the cathode material prepared in Example 1. As can be seen from the figure, the material has a first-cycle discharge specific capacity of 113.76 mAh / g at 2-3.6V and 0.2C current, and a capacity retention rate of 80% after 1136 cycles at 1C current.

[0034] Example 2

[0035] (1) Dissolve chromium sulfate in water to prepare a 0.2 mol / L solution A, and dissolve aluminum sulfate in water to prepare a 0.03 mol / L solution B. The molar ratio of Al to Cr is 1:100. Add solution A to the reaction vessel, add 2 mol / L NaOH solution, and control the pH of the solution in the reaction vessel to 6. Stir continuously at 500 rpm at 60℃ for 2 h to completely precipitate Cr(OH)3. After precipitation is complete, continue aging for 2.5 h. Then slowly add solution B and 0.3 mol / L NaOH solution to the reaction vessel, maintain pH at 7, and continue aging for 2 h after solution B is completely added to finally obtain Cr(OH)3@Al(OH)3.

[0036] (2) Cr(OH)3@Al(OH)3 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 950℃ for 9h. The heating rate was 5℃ / min. After the reaction, it was ground and sieved to obtain sodium chromite cathode material NaCrO2@Al2O3 coated with aluminum oxide.

[0037] Example 3

[0038] (1) Chromium chloride was dissolved in water to prepare a 0.2 mol / L solution A, and aluminum nitrate was dissolved in water to prepare a 0.03 mol / L solution B. The molar ratio of Al to Cr was 1:100. Solution A was added to a reaction vessel, and 2 mol / L NaOH solution was added to control the pH of the solution in the reaction vessel to 6. The reaction was carried out at 60°C with continuous stirring at 500 rpm for 2 h to completely precipitate Cr(OH)3. After precipitation, the mixture was aged for 2.5 h. Then, solution B and 0.3 mol / L NaOH solution were slowly added to the reaction vessel, maintaining the pH at 7. After solution B was added, the mixture was aged for another 2 h to finally obtain Cr(OH)3@Al(OH)3.

[0039] (2) Cr(OH)3@Al(OH)3 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 950℃ for 9h. The heating rate was 5℃ / min. After the reaction, it was ground and sieved to obtain sodium chromite cathode material NaCrO2@Al2O3 coated with aluminum oxide.

[0040] Example 4

[0041] (1) Dissolve chromium chloride in water to prepare a 0.2 mol / L solution A, and dissolve aluminum sulfate in water to prepare a 0.03 mol / L solution B. The molar ratio of Al to Cr is 1:50. Add solution A to the reaction vessel, add 2 mol / L NaOH solution, and control the pH of the solution in the reaction vessel to 6. Stir continuously at 500 rpm at 60℃ for 2 h to completely precipitate Cr(OH)3. After precipitation is complete, continue aging for 2.5 h. Then slowly add solution B and 0.3 mol / L NaOH solution to the reaction vessel, maintain pH at 7, and continue aging for 2 h after solution B is completely added to finally obtain Cr(OH)3@Al(OH)3.

[0042] (2) Cr(OH)3@Al(OH)3 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 950℃ for 9h. The heating rate was 5℃ / min. After the reaction, it was ground and sieved to obtain sodium chromite cathode material NaCrO2@Al2O3 coated with aluminum oxide.

[0043] Example 5

[0044] (1) Dissolve chromium chloride in water to prepare a 0.2 mol / L solution A, and dissolve aluminum sulfate in water to prepare a 0.03 mol / L solution B. The molar ratio of Al to Cr is 1:150. Add solution A to the reaction vessel, add 2 mol / L NaOH solution, and control the pH of the solution in the reaction vessel to 6. Stir continuously at 500 rpm at 60℃ for 2 h to allow Cr(OH)3 to precipitate completely. After precipitation, continue aging for 2.5 h. Then slowly add solution B and 0.3 mol / L NaOH solution to the reaction vessel, maintaining the pH at 7. After solution B is added, continue aging for 2 h to finally obtain Cr(OH)3@Al(OH)3.

[0045] (2) Cr(OH)3@Al(OH)3 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 950℃ for 9h. The heating rate was 5℃ / min. After the reaction, it was ground and sieved to obtain sodium chromite cathode material NaCrO2@Al2O3 coated with aluminum oxide.

[0046] Example 6

[0047] (1) Dissolve chromium chloride in water to prepare a 0.2 mol / L solution A, and dissolve aluminum sulfate in water to prepare a 0.03 mol / L solution B. The molar ratio of Al to Cr is 1:100. Add solution A to the reaction vessel, add 2 mol / L NaOH solution, and control the pH of the solution in the reaction vessel to 7. Stir continuously at 600 rpm at 80℃ for 1 h to completely precipitate Cr(OH)3. After precipitation is complete, continue aging for 3 h. Then slowly add solution B and 0.3 mol / L NaOH solution to the reaction vessel, maintaining the pH at 7. After solution B is completely added, continue aging for 2 h to finally obtain Cr(OH)3@Al(OH)3.

[0048] (2) Cr(OH)3@Al(OH)3 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 950℃ for 9h. The heating rate was 5℃ / min. After the reaction, it was ground and sieved to obtain sodium chromite cathode material NaCrO2@Al2O3 coated with aluminum oxide.

[0049] Example 7

[0050] (1) Dissolve chromium chloride in water to prepare a 0.2 mol / L solution A, and dissolve aluminum sulfate in water to prepare a 0.03 mol / L solution B. The molar ratio of Al to Cr is 1:100. Add solution A to the reaction vessel, add 2 mol / L NaOH solution, and control the pH of the solution in the reaction vessel to 8. Stir continuously at 400 rpm for 3 h at 40 °C to completely precipitate Cr(OH)3. After precipitation is complete, continue aging for 2 h. Then slowly add solution B and 0.3 mol / L NaOH solution to the reaction vessel, maintain pH at 7, and continue aging for 2 h after solution B is completely added to finally obtain Cr(OH)3@Al(OH)3.

[0051] (2) Cr(OH)3@Al(OH)3 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 950℃ for 9h. The heating rate was 5℃ / min. After the reaction, it was ground and sieved to obtain sodium chromite cathode material NaCrO2@Al2O3 coated with aluminum oxide.

[0052] Example 8

[0053] (1) Chromium chloride was dissolved in water to prepare a 0.2 mol / L solution A, and aluminum sulfate was dissolved in water to prepare a 0.03 mol / L solution B. The molar ratio of Al to Cr was 1:100. Solution A was added to a reaction vessel, and 2 mol / L NaOH solution was added to maintain the pH of the solution in the reaction vessel at pH 6. The reaction was carried out at 60°C with continuous stirring at 500 rpm for 2 h to completely precipitate Cr(OH)3. After precipitation, the mixture was aged for 2.5 h. Then, solution B and 0.3 mol / L NaOH solution were slowly added to the reaction vessel, maintaining the pH at 6. After solution B was added, the mixture was aged for another 3 h to finally obtain Cr(OH)3@Al(OH)3.

[0054] (2) Cr(OH)3@Al(OH)3 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 950℃ for 9h. The heating rate was 5℃ / min. After the reaction, it was ground and sieved to obtain sodium chromite cathode material NaCrO2@Al2O3 coated with aluminum oxide.

[0055] Example 9

[0056] (1) Chromium chloride was dissolved in water to prepare a 0.2 mol / L solution A, and aluminum sulfate was dissolved in water to prepare a 0.03 mol / L solution B. The molar ratio of Al to Cr was 1:100. Solution A was added to the reactor, and 2 mol / L NaOH solution was added to maintain the pH of the solution in the reactor at pH 6. The reactor was stirred continuously at 500 rpm at 60°C for 2 h to completely precipitate Cr(OH)3. After precipitation, the reactor was aged for 2.5 h. Then, solution B and 0.3 mol / L NaOH solution were slowly added to the reactor to maintain the pH at 8. After solution B was added, the reactor was aged for another 1 h to finally obtain Cr(OH)3@Al(OH)3.

[0057] (2) Cr(OH)3@Al(OH)3 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 950℃ for 9h. The heating rate was 5℃ / min. After the reaction, it was ground and sieved to obtain sodium chromite cathode material NaCrO2@Al2O3 coated with aluminum oxide.

[0058] Example 10

[0059] (1) Dissolve chromium chloride in water to prepare a 0.2 mol / L solution A, and dissolve aluminum sulfate in water to prepare a 0.03 mol / L solution B. The molar ratio of Al to Cr is 1:100. Add solution A to the reactor, add 2 mol / L NaOH solution to maintain the pH of the solution in the reactor at pH 6, and stir continuously at 500 rpm at 60℃ for 2 h to completely precipitate Cr(OH)3. After precipitation is complete, continue aging for 2.5 h. Then slowly add solution B and 0.3 mol / L NaOH solution to the reactor, maintain pH at 7, and continue aging for 2 h after solution B is completely added to finally obtain Cr(OH)3@Al(OH)3.

[0060] (2) Cr(OH)3@Al(OH)3 and sodium carbonate were mixed in a molar ratio of 1:1.01. The mixture was sintered under nitrogen. First, it was sintered at 550℃ for 2.5h, and then heated to 950℃ for 9h. The heating rate was 5℃ / min. After the reaction, it was ground and sieved to obtain sodium chromite cathode material NaCrO2@Al2O3 coated with aluminum oxide.

[0061] Example 11

[0062] (1) Dissolve chromium chloride in water to prepare a 0.2 mol / L solution A, and dissolve aluminum sulfate in water to prepare a 0.03 mol / L solution B. The molar ratio of Al to Cr is 1:100. Add solution A to the reactor, add 2 mol / L NaOH solution to maintain the pH of the solution in the reactor at pH 6, and stir continuously at 500 rpm at 60℃ for 2 h to completely precipitate Cr(OH)3. After precipitation is complete, continue aging for 2.5 h. Then slowly add solution B and 0.3 mol / L NaOH solution to the reactor, maintain pH at 7, and continue aging for 2 h after solution B is completely added to finally obtain Cr(OH)3@Al(OH)3.

[0063] (2) Cr(OH)3@Al(OH)3 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 950℃ for 9h. The heating rate was 5℃ / min. After the reaction, it was ground and sieved to obtain sodium chromite cathode material NaCrO2@Al2O3 coated with aluminum oxide.

[0064] Example 12

[0065] (1) Dissolve chromium chloride in water to prepare a 0.2 mol / L solution A, and dissolve aluminum sulfate in water to prepare a 0.03 mol / L solution B. The molar ratio of Al to Cr is 1:100. Add solution A to the reactor, add 2 mol / L NaOH solution to maintain the pH of the solution in the reactor at pH 6, and stir continuously at 500 rpm at 60℃ for 2 h to completely precipitate Cr(OH)3. After precipitation is complete, continue aging for 2.5 h. Then slowly add solution B and 0.3 mol / L NaOH solution to the reactor, maintain pH at 7, and continue aging for 2 h after solution B is completely added to finally obtain Cr(OH)3@Al(OH)3.

[0066] (2) Cr(OH)3@Al(OH)3 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 1000℃ for 8h. The heating rate was 5℃ / min. After the reaction, it was ground and sieved to obtain sodium chromite cathode material NaCrO2@Al2O3 coated with aluminum oxide.

[0067] Example 13

[0068] (1) Dissolve chromium chloride in water to prepare a 0.2 mol / L solution A, and dissolve aluminum sulfate in water to prepare a 0.03 mol / L solution B. The molar ratio of Al to Cr is 1:100. Add solution A to the reactor, add 2 mol / L NaOH solution to maintain the pH of the solution in the reactor at pH 6, and stir continuously at 500 rpm at 60℃ for 2 h to completely precipitate Cr(OH)3. After precipitation is complete, continue aging for 2.5 h. Then slowly add solution B and 0.3 mol / L NaOH solution to the reactor, maintain pH at 7, and continue aging for 2 h after solution B is completely added to finally obtain Cr(OH)3@Al(OH)3.

[0069] (2) Cr(OH)3@Al(OH)3 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 2h, and then heated to 800℃ for 10h. The heating rate was 5℃ / min. After the reaction, it was ground and sieved to obtain sodium chromite cathode material NaCrO2@Al2O3 coated with aluminum oxide.

[0070] Comparative Example 1 Chromium trioxide and sodium carbonate were blended in a molar ratio of 1:1.03. The blend was then sintered under nitrogen protection. The sintering process was 550℃ for 2.5 h and 950℃ 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.

[0071] Comparative Example 2 Chromium hydroxide monohydrate, aluminum hydroxide, and sodium carbonate were blended at a molar ratio of Cr, Al, and Na of 100:1:103. The blend was sintered under nitrogen protection in the following order: 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.

[0072] Comparative Example 3 (1) Chromium trioxide and sodium carbonate were blended in a molar ratio of 1:1.03. The blend was sintered under nitrogen protection. The sintering program was 550℃ for 2.5h and 950℃ for 9h. The heating rate was 5℃ / min. After the reaction, the mixture was ground and sieved to obtain sodium chromite material.

[0073] (2) Sodium chromite and aluminum hydroxide were ground and mixed at a molar ratio of Cr to Al of 100:1, and then sintered. The sintering procedure was 300℃ for 3h and 800℃ for 5h, with a heating rate of 5℃ / min. After the reaction, the mixture was ground and sieved to obtain sodium chromite cathode material. The cathode materials prepared in the aforementioned examples and comparative examples were subjected to electrochemical performance and water inertness tests, respectively. The test methods are as follows:

[0074] (1) Electrochemical performance The powdered layered positive electrode material for sodium-ion batteries was ground through a 200-mesh sieve and 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 to form a slurry, which was then coated onto aluminum foil. After drying, stamping, and pressing, the sodium-ion battery positive electrode material was formed. Using metallic sodium as the negative electrode, glass fiber (Whatman GF / D brand) as the separator, and NaPF6 (sodium hexafluorophosphate) and 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 conditions of a current density of 100 mAh / g and a voltage range of 2.0-3.6V, the capacity and cycle performance were tested first at 0.2C for 2 cycles, and then at 1C.

[0075] (2) Inertness test in water Place 0.5g of the positive electrode material into a small bottle containing 50mL of deionized water, seal the bottle and store it for 0h and 24h, and then measure the pH value of the solution using a pH meter.

[0076] The test results for electrochemical performance and inertness in water are shown in Table 1.

[0077] Table 1 .

[0078] analyze: As shown in Table 1, compared with Comparative Examples 1-3, the cathode material prepared by this invention exhibits superior electrochemical and cycling performance. Specifically, Example 1 achieved a discharge specific capacity of 113.76 mAh / g at 2-3.6V and 0.2C, a discharge specific capacity of 110.38 mAh / g at 1C, and 1136 cycles with 80% capacity retention at 1C. After 24 hours in water, the pH of the aqueous solution was 7.31, demonstrating the best overall performance.

[0079] Compared to Example 1, the sodium chromite cathode material prepared in Comparative Example 1 exhibited poorer cycle performance due to the lack of Al coating on its surface, achieving only 201 cycles with 80% capacity retention at 1C. In an inert water test, the pH rose to 12.31 after 24 hours.

[0080] Compared with Example 1, the sodium chromite cathode material prepared in Comparative Example 2 had Al element added during the first sintering, resulting in some Al being doped in the bulk sodium chromite phase rather than coated on the material surface, which caused the cycle performance of the material to be significantly different from that of Example 1.

[0081] Compared with Example 1, the sodium chromite cathode material prepared in Comparative Example 3 was obtained by secondary sintering and coating. However, Comparative Example 3 only used grinding and blending, which could not make Al2O3 uniformly coat the surface of NaCrO2 to form a continuous phase. This further resulted in a significant difference in the cycling performance and water stability of the material compared with Example 1.

[0082] Compared with Example 1, Examples 2-3 changed the raw materials. The selection of different raw materials has a certain impact on the capacity and cycle performance of the cathode material.

[0083] Compared to Example 1, Examples 4-5 changed the molar ratio of Al to Cr. The increase in Al content leads to an increase in the thickness of the NaCrO2 surface coating, which seriously affects the movement of sodium ions and reduces the discharge specific capacity of the material. The decrease in Al content is not conducive to the formation of a continuous coating layer, resulting in poor suppression of lattice phase transitions during charge and discharge, further reducing the cycling performance of the material.

[0084] Compared to Example 1, Examples 6-7 altered the reaction conditions of Cr(OH)3. Excessive pH caused some chromium hydroxide to redissolve in the solution, reducing the yield. Furthermore, the resulting chromium hydroxide particles became charge-neutralized, accelerating agglomeration and tending to form larger particles. These larger particles were more prone to internal cracking during long cycles, leading to decreased material cycling performance. The reduced stirring speed and reaction time resulted in incomplete precipitation and poor particle uniformity during the reaction.

[0085] Compared with Example 1, Examples 8-9 changed the reaction conditions of Al(OH)3. If the aging time is too short, the shell of the final sintered material will be loose and porous, with weak bonding ability with the core. If the aging time is too long, Al(OH)3 will agglomerate and fail to form an effective shell.

[0086] Compared with Example 1, Examples 10-11 changed the molar ratio of Cr(OH)3@Al(OH)3 to sodium carbonate. The increase in sodium content not only failed to improve the discharge capacity of the material, but also reduced the cycling performance of the material and increased the alkalinity of the material.

[0087] Compared to Example 1, Examples 12-13 altered the sintering conditions of the mixture. The results showed that increasing the temperature and reaction time led to a faster volatilization rate of Na₂O, the decomposition product of sodium carbonate, further reducing the electrochemical performance of the material. Furthermore, higher energy consumption increased the cost. Decreasing the reaction temperature and reducing the reaction time resulted in a lower crystallinity of the material.

[0088] 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 sodium-ion battery cathode material with high cycle stability, characterized in that: The cathode material has the structural formula NaCrO2@Al2O3, with Al2O3 uniformly coated on NaCrO2.

2. A method for preparing the sodium-ion battery cathode material with high cycle stability as described in claim 1, characterized in that: Includes the following steps: (1) Preparation of aluminum hydroxide coated chromium hydroxide Cr(OH)3@Al(OH)3 A Cr-containing compound was dissolved in water to obtain solution A, and an Al-containing compound was dissolved in water to obtain solution B. NaOH solution was added to solution A, and the pH was controlled within a certain range. The mixture was stirred to allow Cr(OH)3 to precipitate completely. After precipitation, the mixture was aged for a certain period of time. Then, solution B and NaOH solution were added, and the pH was controlled within a certain range. After the reaction was completed, the mixture was aged for a period of time to finally obtain Cr(OH)3@Al(OH)3. (2) Preparation of sodium chromite cathode material coated with alumina NaCrO2@Al2O3 The Cr(OH)3@Al(OH)3 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 sodium chromite cathode material NaCrO2@Al2O3 coated with aluminum oxide.

3. The method for preparing a sodium-ion battery cathode material with high cycle stability according to claim 2, 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.

4. The method for preparing a sodium-ion battery cathode material with high cycle stability according to claim 2, characterized in that: In step (1): the Al-containing compound is one or a mixture of two or more of aluminum nitrate, aluminum sulfate, aluminum trichloride, potassium aluminum sulfate dodecahydrate, and aluminum isopropoxide in any proportion.

5. The method for preparing a sodium-ion battery cathode material with high cycle stability according to claim 2, characterized in that: In step (1): NaOH solution is added to solution A, the pH of the reaction is controlled to be 6-8, and the reaction is continuously stirred at 400-600 rpm for 1-3 hours at 40-80℃. After the reaction is completed, the mixture is aged for 2-3 hours.

6. The method for preparing a sodium-ion battery cathode material with high cycle stability according to claim 2, characterized in that: In step (1): the added solution B and NaOH solution are controlled to a pH of 6-8. After the reaction is completed, the mixture is aged for 1-3 hours.

7. The method for preparing a sodium-ion battery cathode material with high cycle stability according to claim 2, 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; the molar ratio of Cr(OH)3@Al(OH)3 to the sodium source is 1:1.01-1.

05.

8. The method for preparing a sodium-ion battery cathode material with high cycle stability according to claim 2, 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.

9. The method for preparing a sodium-ion battery cathode material with high cycle stability according to claim 2, characterized in that: In step (2): the sintering process is as follows: first sinter at 500-600℃ for 2-3 hours, then sinter at 800-1000℃ for 8-10 hours, with a heating rate of 5℃ / min.

10. The application of the sodium-ion battery cathode material with high cycle stability as described in claim 1 in the preparation of sodium-ion batteries.

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