Supercritical fluid technology optimized modified layered oxide positive electrode material and preparation method and application thereof

Abstract

The application belongs to the technical field of sodium ion batteries, and relates to a supercritical fluid technology optimized modified layered oxide positive electrode material and a preparation method and application thereof. x The preparation method is to take layered oxide (general formula Na [TMO2]) as a matrix, react in a supercritical fluid medium for a certain time, and obtain the optimized modified layered oxide positive electrode material. The layered oxide material is expanded in layer spacing, the surface is purified, and a sodium salt coating layer is constructed in situ through the supercritical fluid technology, so that the layered oxide positive electrode material with high air stability and excellent electrochemical performance is prepared. The layered oxide positive electrode material prepared by the application has the characteristics of large layer spacing, excellent air stability and excellent cycle performance. Meanwhile, the preparation method provided by the application does not need to introduce exogenous elements, the process is simple and efficient, and the parameters are easy to control, so that the surface side reaction of the material can be significantly inhibited, and key technical support is provided for promoting the commercialization of the sodium ion battery positive electrode material.

Description

Technical Field

[0001] This invention belongs to the field of sodium-ion battery cathode material modification technology, specifically relating to a layered oxide cathode material optimized and modified by supercritical fluid technology, its preparation method, and its application. Background Technology

[0002] With the development of the new energy industry, the demand for lithium resources has increased dramatically, posing risks to the safe and sustainable supply of lithium resources. Therefore, the development of sodium-ion batteries has become a current research hotspot. Compared with lithium-ion batteries, sodium-ion batteries have advantages such as high safety and low cost. Electrode materials are crucial in determining battery performance. In sodium-ion battery cathode materials, layered transition metal (TM) oxide materials (Na...) are... x TMO2 has become one of the most promising cathode materials for sodium-ion batteries due to its very high theoretical capacity, suitable electrochemical window, and synthesis process similar to that of lithium-ion ternary materials.

[0003] Layered transition metal oxide cathode materials exposed to air are affected by factors such as elemental composition, interlayer spacing, and surface morphology. The composition of alkali metals and transition metals in the material has a significant impact on air stability. Compared to lithium, sodium is more alkaline and has a larger ionic radius, resulting in lower space utilization. Correspondingly, sodium hydroxide is more hydrophilic than lithium hydroxide, readily absorbing large amounts of water, and... x More sodium salts form on the surface of TMO2 particles. For transition metal elements, the active elements in the material in a low valence state (such as Mn) form more sodium salts. 2+ Ni 2+ Fe 2+ and V 3+ Sodium ions (Na+, etc.) readily react with oxygen in the air, thus exhibiting air sensitivity and leading to changes in the surface composition and structure of the material. Furthermore, sodium ions (Na+, etc.) are particularly reactive with oxygen in the air, exhibiting air sensitivity and causing changes in the surface composition and structure of the material. + During charging and discharging, repeated de-intercalation and intercalation are required between the layers of the positive electrode material. The interlayer spacing of this material is a layered transition metal oxide (Na₂O₃). x The core structural parameters of [TMO2] directly determine the thermodynamic stability and kinetic barriers of the material, thus dominating its environmental sensitivity. Smaller interlayer spacing induces a series of unfavorable structural chemical properties, making it susceptible to irreversible side reactions with H2O and CO2 in the environment, ultimately affecting electrochemical performance, increasing the production, transportation, and storage costs of the material, and hindering commercialization. Therefore, improving the air stability of layered transition metal oxide cathode materials is one of the key focuses of current sodium-ion battery research. Consequently, more and more researchers are paying attention to this problem and have proposed many solutions, such as improvement methods through doping and coating.

[0004] Doping introduces external impurity atoms (additives) into the bulk structure of a material, stabilizing its structure and suppressing irreversible phase transitions in air, thereby enhancing the performance of layered transition metal oxides such as Na. x The stability of [TMO2] in air. Currently, ion doping mainly includes cation doping, anion doping, and co-doping with multiple additives. The team led by Zhao Junmei at the Institute of Process Engineering, Chinese Academy of Sciences, successfully synthesized NaFe with excellent air stability using Al doping. （1 / 3-0.01) Ni 1 / 3 Mn 1 / 3 A1 0.01 O2 (NFNMA) layered material. After exposing it to air for 30 days, the Al-doped NFNMA sample showed a smooth surface and retained its initial structure via XRD, demonstrating excellent air stability. The team led by Luo Shaohua at Northeastern University studied Cu... 2+ The doping amount of Na 0.67 Ni 0.33-x Cu x Mn 0.67 The effect of O2 on air stability was found to be related to Cu. 2+ Increasing the doping concentration enhances the material's air stability, reduces irreversible phase transitions, and smooths the charge-discharge curve. However, the reversible specific capacity gradually decreases; at a doping concentration of 0.33%, the reversible specific capacity drops by 50%. The team led by Yan Guochun at Central South University used Cu... 2+ and Ti 4+ Co-doped NaNi 0.45 Cu 0.05 Mn 0.4 Ti 0.1 O2, research found that co-doping can suppress the embedding of H2O and CO2 into the positive electrode layered structure, Cu 2+ and Ti 4+ It can also improve the ionic conductivity of materials, Mg 2+ and Ti 4+ Doping can also improve Na xTMO2 exhibits good air stability; however, multi-element co-doping requires the introduction of numerous exogenous elements, which can easily introduce uncontrollable factors into the process and results, further increasing costs. In contrast, coating modification is an effective strategy to improve electrochemical performance. Coatings are formed through ball milling or sol-gel methods, followed by high-temperature calcination to obtain coated products such as ZrO2, B2O3, and AlF3. This prevents the dissolution of transition metal ions, improves the formation of residual alkali on the material surface, and enhances the cycling stability of the material. The ArumugamManthiram team used coating and heat treatment to uniformly coat the surface of layered oxide materials with ZrO2, effectively suppressing the reaction of the material with H2O and CO2 in the air. Electrochemical tests on the coated material after 7 days of exposure to air showed that it still provided 76% of the initial capacity. However, the coating layer may have issues such as unevenness and insufficient density, indicating room for further improvement. Chinese patent CN117334863A, concerning a carbon-coated sodium ferric sulfate cathode material and its preparation method and application, mentions using supercritical fluid to assist in the carbon coating of sodium ferric sulfate materials with halogenated carboxylic acids, but it still requires the introduction of exogenous substances, and the supercritical fluid only acts as a solvent. Chinese patent CN115011897A, concerning a method for surface modification of metallic lithium by exciting supercritical fluid plasma, provides a method to modify the surface of metallic lithium by exciting plasma in a supercritical fluid, thereby forming a uniform and dense protective layer in situ on its surface, significantly improving its air stability and electrochemical performance. However, using supercritical fluid to assist in the carbon coating of exogenous substances is a complex process and introduces impurities. In the above methods for modifying materials, although the materials all achieve certain performance improvements, the modification methods all introduce new elements or structures, increasing costs during industrial production. Furthermore, the thickness and properties of the conductive layer of the treated material still need further adjustment, or even further sintering steps are required. It is also difficult to generate a uniform coating layer, and the influence of the interlayer spacing of layered oxides on the material is not addressed; therefore, there is still room for further improvement.

[0005] Based on this, the present invention proposes a layered oxide cathode material optimized and modified by supercritical fluid technology, its preparation method and application, which utilizes a selected gaseous medium to permeate into Na under supercritical conditions. + The interlayer spacing of the material is expanded, and the high activity of the supercritical medium and the hydrophilic sodium-based residual alkali on the surface are combined to transform it into a dense, stable, and hydrophobic protective layer, thereby preparing a sodium-ion battery cathode material with high air stability. Summary of the Invention

[0006] The purpose of this invention is to address a series of problems in existing sodium-ion battery systems, such as uneven and insufficiently dense surface coatings, closed interlayer spacing, and layer collapse, which lead to issues with specific capacity, cycle stability, and air stability. This invention provides a layered oxide cathode material optimized and modified using supercritical fluid technology, along with its preparation method and applications. Based on this, this paper selects synthesized Na... x Using [TMO2] as the research object, layered oxide cathode materials with wide interlayer spacing and a dense, uniform coating can be effectively prepared by selecting parameters such as sodium salt type, precursor element ratio, gas medium type, gas pressure, supercritical temperature, and reaction time. The supercritical fluid-optimized layered oxide material technology provided by this invention can effectively expand the interlayer spacing of layered oxides in a short time, while forming a uniform and dense coating layer on the surface of the layered oxide cathode material. This results in sodium-ion battery layered oxide cathode materials with high specific capacity, good cycle stability, and excellent air stability, effectively promoting the commercialization of layered oxide cathode materials.

[0007] The technical solution adopted by this invention to solve its technical problem is as follows:

[0008] A method for preparing a layered oxide cathode material optimized and modified using supercritical fluid technology includes the following steps:

[0009] (1) The sodium ion layered oxide is sealed in a supercritical high-pressure reactor, and the atmosphere in the reactor is replaced with a supercritical fluid atmosphere, and the pressure is increased to reach the supercritical pressure condition of the atmosphere.

[0010] (2) The sealed supercritical high-pressure reactor is heated to the supercritical temperature of the atmosphere and kept at that temperature for a period of time to obtain the modified sample.

[0011] This invention provides a supercritical stabilization treatment method for layered oxide cathode materials for sodium-ion batteries. The method uses sodium-ion layered oxide materials, such as sodium nickel iron manganese oxide (NFM111), as the matrix, and a gaseous medium that reaches a supercritical state under specific temperature and pressure as the supercritical medium. After the sodium-ion layered oxide material reacts in the supercritical medium for a certain time, the pressure is reduced to atmospheric pressure to obtain an optimized and modified layered oxide cathode material.

[0012] The following are preferred technical solutions of the present invention:

[0013] Preferably, in step (1), there are no special requirements for sodium ion layered oxides, which can be prepared by the following method: sodium salt and precursor material are mixed and sintered in an oxygen atmosphere to obtain sodium ion layered oxides, which are then ground, sieved, and transferred to a supercritical high-pressure reaction vessel, and the vacuum vessel is sealed.

[0014] More preferably, in the above method, the sodium salt is at least one of sodium carbonate, sodium nitrate, sodium hydroxide, etc., and more preferably one of sodium carbonate and sodium nitrate; the precursor has the chemical formula Na. x [TM(OH)2], where TM is one or more of Fe, Mn, Ni, Cu, etc., and the chemical formula of the sodium ion layered oxide material obtained by sintering is Na. x [TMO2]; The mass of the sodium-ion layered oxide ternary cathode material in the reaction vessel is between 1 and 10 g. More preferably, the sodium salt is sodium carbonate, and the precursor element ratio is NaNi. 1 / 3Fe 1 / / 3 Mn 1 / 3 (OH)2, more preferably 2 g of sodium nickel iron manganate ternary cathode material;

[0015] More preferably, the sodium ion layered oxide material is formed by segmental sintering of a mixture of sodium salt and precursor under an oxygen atmosphere. To compensate for the loss caused by sodium volatilization during the calcination stage, the amount of sodium salt should be 5% excess molar ratio. Segmental sintering is adopted, and the low-temperature sintering process does not significantly affect the results. Preferably, the temperature range and time are controlled within 350 ℃~550 ℃ and 2~8 h. The temperature range and time during the high-temperature sintering process are within 600 ℃~950 ℃ and 6~15 h. For the obtained sodium ion layered oxide, excessively high sintering temperatures and excessively long sintering times will lead to excessive crystal growth, exacerbating the mixing and significantly deteriorating the material's structure, composition, and electrochemical performance. Conversely, excessively low sintering temperatures or short sintering times will prevent the material from forming a stable crystal structure. More preferably, the sintering temperature is 900 ℃ and the sintering time is 12 h. The obtained sodium ion layered oxide should be sieved and then subjected to supercritical treatment. The preferred sieve mesh size is 100-800 mesh; more preferably 200 mesh.

[0016] Preferably, in step (1), the supercritical fluid atmosphere includes at least one of carbon dioxide, ethane, propane, ethylene, and 1,1,1,2-tetrafluoroethane (R134a) as a supercritical gas source; more preferably, it is carbon dioxide. By employing the above technical solution, the gas source has the characteristics of small molecular weight, high diffusivity and uniform surface coverage in the supercritical state, and can quickly penetrate into the nanopores on the surface of layered oxide particles, penetrating into Na under high pressure. +The interlayer structure expands the material's interlayer spacing and utilizes its high activity and surface hydrophilic sodium-based residual alkali to transform it into a dense, stable, hydrophobic, and uniform sodium-based protective layer.

[0017] Preferably, the supercritical reaction conditions in the supercritical fluid atmosphere are: a critical pressure range of 3.5-8 MPa, a reaction temperature of 35-55 °C, and a reaction time of 0.5-12 h; more preferably, the reaction time is 1-6 h. Insufficient reaction time will lead to uneven coating on the surface of the layered oxide material. Conversely, excessive reaction time may result in an excessively thick coating on the surface of the layered oxide material.

[0018] More preferably, the selection of reaction pressure and reaction temperature needs to be adjusted according to the conditions for different supercritical gas media to reach the supercritical state: when the supercritical fluid atmosphere is carbon dioxide, the reaction temperature is not lower than the critical temperature of 31.1℃ and the reaction pressure is not lower than the critical pressure of 6.3 MPa; when the supercritical fluid atmosphere is ethane, the reaction temperature is not lower than the critical temperature of 32.2℃ and the reaction pressure is not lower than 3.5 MPa; when the supercritical fluid atmosphere is propane, the reaction temperature is not lower than the critical temperature of 95℃ and the reaction pressure is not lower than the critical pressure of 4.25 MPa; when the supercritical fluid atmosphere is ethylene, the reaction temperature is not lower than the critical temperature of 10℃ and the reaction pressure is not lower than the critical pressure of 5.0 MPa; when the supercritical fluid atmosphere is tetrafluoroethane, the reaction temperature is not lower than the critical temperature of 80℃ and the reaction pressure is not lower than the critical pressure of 4.0 MPa.

[0019] More preferably, the optimal reaction pressure and temperature are 7.5 MPa and 43 °C for carbon dioxide, 4.8 MPa and 35 °C for ethane, 4.25 MPa and 96 °C for propane, 5.2 MPa and 15 °C for ethylene, and 4.06 MPa and 100 °C for tetrafluoroethane. Too low a reaction pressure and temperature may prevent the supercritical fluid atmosphere from reaching the supercritical state, while too high a temperature may cause the supercritical device to explode. More preferably, the reaction system is heated by transferring the supercritical device to an oven at a certain temperature and allowing it to stand, selecting the optimal reaction conditions based on the introduced gas.

[0020] Preferably, in step (1), the atmosphere inside the reaction vessel is replaced with a supercritical fluid atmosphere, and its pressure is made to reach the supercritical pressure condition of the atmosphere by the following steps: turn on the vacuum pump, maintain the vacuum environment inside the reaction vessel, fill it with a gas source, reach the supercritical pressure condition of the gas medium, and stop filling.

[0021] Preferably, in step (2), the temperature is 30~100 ℃, the heating rate is 2~8 ℃ / min, and the holding time is 0.5~12h. More preferably, the optimal reaction conditions are selected according to the gas introduced, and the heating rate is 5 The temperature is set at ℃ / min, and the holding time is 1 h. More preferably, the sealed reaction vessel is placed in an oven for heating.

[0022] Preferably, in step (2), a coating layer is formed in situ in the sodium ion layered oxide after the reaction, with a thickness of 3~8 nm, more preferably 5 nm.

[0023] Preferably, after the reaction in step (2) is completed, the gas is allowed to cool naturally to room temperature, and then the valve is slowly opened to release the pressure and obtain the modified sample. More preferably, the gas flow rate when the valve is opened should not exceed 30 sccms. If the flow rate is too high, the powder will be blown out.

[0024] Preferably, the composition of the in-situ formed sodium salt coating layer is determined by the gas medium: a carbon dioxide medium generates a sodium carbonate coating layer; an ethane medium generates an ethyl sodium coating layer; a propane medium generates an isopropyl sodium coating layer; an ethylene medium generates a polymerized organic polymer layer; and a tetrafluoroethane medium generates a sodium fluoride and sodium carbonate coating layer.

[0025] Preferably, the resulting optimized modified layered oxide cathode material has an increased interlayer spacing of d. (003) Not less than 5.50 Å; more preferably 5.50-5.58 Å.

[0026] Preferably, the preparation method specifically includes the following steps:

[0027] (1) The layered oxide precursor is combined with sodium salt and then sintered to obtain sodium ion layered oxide material;

[0028] (2) Place the above-mentioned layered oxide material in a supercritical medium gas at a specific pressure;

[0029] (3) The above reaction system is heated to make the gas medium reach the supercritical state, and after a certain reaction time, it is naturally cooled and the pressure is reduced to normal pressure to obtain a layered oxide material with a coating layer.

[0030] This invention also provides a supercritical fluid technology prepared by any of the above preparation methods, which expands the interlayer spacing and simultaneously forms an optimized modified layered oxide cathode material by in-situ conversion to form a protective layer.

[0031] This invention also provides an application of the optimized and modified layered oxide cathode material prepared by any of the above-mentioned methods in the field of sodium-ion batteries. Particularly, it is applicable to large-scale energy storage systems, electric vehicles, or portable electronic devices.

[0032] The supercritical method for constructing sodium salt-coated layered oxide cathode materials provided by this invention achieves multiple technological breakthroughs through innovative process design: The highly efficient mass transfer characteristics generated under supercritical gas conditions, combined with the high diffusivity and solubility of the gas, react with the added sodium ions in the layered oxide, allowing them to penetrate into the Na+ layer. + The interlayer spacing of the material is expanded, and the supercritical gas medium combines with the hydrophilic sodium-based residual alkali on the material surface, transforming it into a dense, stable, and hydrophobic protective layer. This allows for the preparation of high-performance sodium-ion battery cathode materials with good air stability using supercritical fluids under high pressure. This provides an innovative solution for the industrial production and modification of cathode materials.

[0033] This invention provides a sodium-ion layered oxide material optimized and modified using supercritical fluid technology, its preparation method, and its applications. To date, there are very few reports on supercritical fluid technology-modified layered oxide materials, particularly regarding the control of interlayer spacing, surface coating composition and thickness, and applications in sodium-ion batteries. This invention utilizes supercritical fluid technology to allow a gaseous medium to permeate into the sodium-ion battery. + Interlayer modification expands the interlayer spacing of the material, enabling in-situ transformation on the material surface to improve its electrochemical performance and air stability. Specifically, by controlling gas pressure and reaction temperature, the gaseous medium is transformed into a supercritical fluid state, utilizing its efficient mass transfer characteristics, high diffusivity, and high solubility to penetrate into the surface of the layered oxide material. Supercritical fluid technology effectively modifies the material. It allows the hydrophilic sodium-based residual alkali on the surface to react with the supercritical gaseous medium, transforming in-situ into a dense, stable, and hydrophobic sodium salt coating. This improves the material's stability in air and its impact on battery charge-discharge reactions, thereby extending battery life.

[0034] Compared with existing technologies, this invention utilizes supercritical fluid technology to modify sodium ion layered oxide materials, which has the following significant advantages: supercritical fluid permeates into the Na+ ions under high pressure. + The interlayer spacing is increased, and the hydrophilic sodium-based residual alkali on the surface combines with it to transform it into a dense, stable, and hydrophobic protective layer, thus enabling the preparation of sodium-ion battery cathode materials with high air stability.

[0035] (1) Due to the large interlayer spacing of sodium ion layered oxides, supercritical fluids can more easily penetrate into Na under high pressure. + This further expands the interlayer spacing of the material, which means that the channels for sodium ion insertion and extraction are wider, resulting in better kinetic performance and effectively improving the electrochemical performance of the material.

[0036] (2) The method of expanding the interlayer spacing of layered oxides in this invention can avoid the possibility of destroying the lattice integrity and making the material structure fragile due to the traditional physical or chemical "expansion" of the interlayer spacing. At the same time, compared with the potential problem that simply expanding the interlayer spacing in the traditional layer expansion method will increase the side reactions generated by the material and air, this invention realizes the in-situ preparation of the coating layer while expanding the layer, effectively avoiding the harm caused by the change of interlayer spacing.

[0037] (3) The high diffusivity, high permeability, and high solubility of supercritical fluids enable them to penetrate uniformly into the surface and subsurface regions of materials without dead zones. By precisely controlling the gas pressure, reaction temperature, and time, it is possible to achieve fine, in-situ control over the composition (depending on the gas medium) and thickness (nanometer to submicron level) of the generated sodium salt coating. This level of uniformity and precision is difficult to achieve with traditional wet chemical coating or physical vapor deposition methods.

[0038] (4) The inherent sodium-based residual alkali on the material surface reacts directly with the supercritical gas medium to generate a protective sodium salt coating in situ. The uniform, dense, and stable sodium salt coating formed in situ can effectively isolate the layered oxide material from moisture (H2O) and carbon dioxide (CO2) in the air. This fundamentally inhibits the phase change and structural collapse caused by water absorption on the material surface, as well as the side reactions such as the formation of sodium carbonate / sodium bicarbonate due to the reaction with CO2, greatly reducing the performance degradation of the material during storage and electrode processing. At the same time, a strong chemical bond is formed between the coating and the substrate material, and the interfacial bonding force is extremely strong, avoiding the problems of poor interfacial compatibility, weak bonding, and easy peeling that may exist in traditional exogenous coatings.

[0039] Therefore, the layered oxide cathode material of this invention exhibits good air stability and excellent electrochemical performance. Furthermore, the preparation method of the layered oxide cathode material described in this invention is simple, rapid, efficient, convenient, easy to control, low-cost, and pollution-free, which helps to promote the commercialization of sodium-ion battery cathode materials. Attached Figure Description

[0040] Figure 1 This is a comparison of the XRD patterns of the layered oxide NFM111 materials in Comparative Example 1 and Example 1.

[0041] Figure 2 The image shows a comparison of the scanning electron microscope (SEM) morphology of the layered oxide NFM111 in Comparative Example 1 (left) and Example 1 (right).

[0042] Figure 3 Comparative Example 1 (left) and Example 1 (right) are SEM images comparing the morphology of the layered oxide NFM111 after one week of exposure to air.

[0043] Figure 4The graph shows a comparison of the cycling performance of the layered oxide NFM111 material in Comparative Example 1 and Example 1 at a current density of 180 mA / g.

[0044] Figure 5 This is a comparison graph showing the cycling performance of the layered oxide NFM111 materials of Comparative Example 1 and Example 1 after one week of exposure to air at a current density of 180 mA / g. Detailed Implementation

[0045] To better clarify and understand the objectives, process solutions, and advantages of this invention, the technical solutions and implementation methods of this invention will be further described clearly, completely, and in detail below through specific embodiments and in conjunction with the accompanying drawings. It should be understood that the embodiments described in this invention are implemented under the premise of the technical solutions of this invention, providing detailed implementation methods and specific operating procedures, but are only some embodiments of this invention, not all embodiments. The specific implementation methods described are limited to illustrating and explaining this invention and do not limit this invention. Based on the embodiments of this invention, all other implementation methods obtained by those skilled in the art without creative effort are within the scope of protection of this invention.

[0046] Unless otherwise specified, the experimental methods and conditions used in the embodiments of this invention are conventional methods and conditions. The materials, reagents, instruments, and equipment used in the embodiments, unless otherwise specified, are all conventional substances or equipment known to those skilled in the art and can be obtained commercially or prepared by conventional methods. The reaction conditions described in the invention can all achieve the reactions and obtain the desired products. In this invention, the room temperature is 10-40℃, and the exposure conditions for air stability testing are 25℃ and 60% RH. Due to space limitations, some embodiments are listed below to further illustrate the advantages of the technical solution of this invention.

[0047] Example 1

[0048] To compensate for the loss caused by sodium volatilization during the calcination stage, sodium carbonate with a molar excess of 5% is mixed with the precursor [Ni]. 1 / 3 Fe 1 / / 3 Mn 1 / 3After mixing with [OH)2] and successfully sintering under oxygen conditions (specific conditions refer to Comparative Example 1), the mixture was sieved through a 200-mesh sieve to obtain a layered oxide material. This material was then transferred to a sealed supercritical high-pressure reactor, such as a high-pressure reactor or a high-pressure resistant ball mill jar, and evacuated to a vacuum state. Carbon dioxide gas was then introduced into the supercritical device to achieve a pressure of 7.5 MPa. Subsequently, the supercritical device with carbon dioxide introduced was transferred to an oven at 43 °C and allowed to stand to allow the gas medium to reach a supercritical state. After reacting for 1 h, it was allowed to cool naturally. Once the tank temperature had cooled to room temperature, the gas valve was slowly opened to obtain the layered oxide material optimized by supercritical fluid technology. The coating layer was Na2CO3, with a thickness of approximately 5 nm and an interlayer spacing of ~5.52 Å.

[0049] Example 2-11

[0050] Based on Example 1, the reaction conditions were changed, including the layered oxide precursor material, gas medium, reaction pressure, reaction temperature, and reaction time. The specific conditions are shown in Table 1 below.

[0051]

[0052] Example 12

[0053] Excess 5% sodium nitrate and Ni 1 / 3 Fe 1 / 3 Mn 1 / 3 (OH)2 was pre-sintered in an oxygen atmosphere at a heating rate of 5 °C / min to 500 °C and held for 6 h. Then, it was sintered at a heating rate of 5 °C / min to 900 °C and held for 12 h. After sintering, it was annealed to room temperature and sieved to obtain layered oxide cathode material.

[0054] The layered oxide was transferred to a sealed supercritical high-pressure reactor and evacuated to a vacuum state. Carbon dioxide gas was then introduced into the supercritical device to achieve a pressure of 7.5 MPa. Subsequently, the supercritical device with carbon dioxide introduced was transferred to an oven at 43 °C and allowed to stand to allow the gas medium to reach a supercritical state. After reacting for 1 h, it was allowed to cool naturally. Once the tank temperature had cooled to room temperature, the gas valve was slowly opened to obtain the layered oxide material optimized by supercritical fluid technology, with a coating thickness of approximately 5 nm.

[0055] Example 13

[0056] Add excess 5% sodium hydroxide and Ni 1 / 3 Fe 1 / 3 Mn 1 / 3(OH)2 was pre-sintered in an oxygen atmosphere at a heating rate of 5 °C / min to 500 °C and held for 6 h. Then, it was sintered at a heating rate of 5 °C / min to 900 °C and held for 12 h. After sintering, it was annealed to room temperature and sieved to obtain layered oxide cathode material.

[0057] The layered oxide was transferred to a sealed supercritical high-pressure reactor and evacuated to a vacuum state. Carbon dioxide gas was then introduced into the supercritical device to achieve a pressure of 7.5 MPa. Subsequently, the supercritical device with carbon dioxide introduced was transferred to an oven at 43 °C and allowed to stand to allow the gas medium to reach a supercritical state. After reacting for 1 h, it was allowed to cool naturally. Once the tank temperature had cooled to room temperature, the gas valve was slowly opened to obtain the layered oxide material optimized by supercritical fluid technology, with a coating thickness of approximately 5 nm.

[0058] Example 14

[0059] Add excess 5% sodium carbonate and Ni 2 / 3 Fe 1 / 6 Mn 1 / 6 (OH)2 was pre-sintered in an oxygen atmosphere at a heating rate of 5 °C / min to 500 °C and held for 6 h. Then, it was sintered at a heating rate of 5 °C / min to 900 °C and held for 12 h. After sintering, it was annealed to room temperature and sieved to obtain layered oxide cathode material.

[0060] The layered oxide was transferred to a sealed supercritical high-pressure reactor and evacuated to a vacuum state. Carbon dioxide gas was then introduced into the supercritical device to achieve a pressure of 7.5 MPa. Subsequently, the supercritical device with carbon dioxide introduced was transferred to an oven at 43 °C and allowed to stand to allow the gas medium to reach a supercritical state. After reacting for 1 h, it was allowed to cool naturally. Once the tank temperature had cooled to room temperature, the gas valve was slowly opened to obtain the layered oxide material optimized by supercritical fluid technology, with a coating thickness of approximately 5 nm.

[0061] Example 15

[0062] Add excess 5% sodium carbonate and Ni 1 / 6 Fe 2 / 3 Mn 1 / 6 (OH)2 was pre-sintered in an oxygen atmosphere at a heating rate of 5 °C / min to 500 °C and held for 6 h. Then, it was sintered at a heating rate of 5 °C / min to 900 °C and held for 12 h. After sintering, it was annealed to room temperature and sieved to obtain layered oxide cathode material.

[0063] The layered oxide was transferred to a sealed supercritical high-pressure reactor and evacuated to a vacuum state. Carbon dioxide gas was then introduced into the supercritical device to achieve a pressure of 7.5 MPa. Subsequently, the supercritical device with carbon dioxide introduced was transferred to an oven at 43 °C and allowed to stand to allow the gas medium to reach a supercritical state. After reacting for 1 h, it was allowed to cool naturally. Once the tank temperature had cooled to room temperature, the gas valve was slowly opened to obtain the layered oxide material optimized by supercritical fluid technology, with a coating thickness of approximately 5 nm.

[0064] Example 16

[0065] Add excess 5% sodium carbonate and Ni 1 / 6 Fe 1 / 6 Mn 2 / 3 (OH)2 was pre-sintered in an oxygen atmosphere at a heating rate of 5 °C / min to 500 °C and held for 6 h. Then, it was sintered at a heating rate of 5 °C / min to 900 °C and held for 12 h. After sintering, it was annealed to room temperature and sieved to obtain layered oxide cathode material.

[0066] The layered oxide was transferred to a sealed supercritical high-pressure reactor and evacuated to a vacuum state. Carbon dioxide gas was then introduced into the supercritical device to achieve a pressure of 7.5 MPa. Subsequently, the supercritical device with carbon dioxide introduced was transferred to an oven at 43 °C and allowed to stand to allow the gas medium to reach a supercritical state. After reacting for 1 h, it was allowed to cool naturally. Once the tank temperature had cooled to room temperature, the gas valve was slowly opened to obtain the layered oxide material optimized by supercritical fluid technology, with a coating thickness of approximately 5 nm.

[0067] Comparative Example 1

[0068] To compensate for the loss caused by sodium volatilization during the calcination stage, an excess of 5% sodium carbonate is added to the precursor [Ni]. 1 / 3Fe 1 / 3 Mn 1 / 3 The mixture of (OH)₂ was heated to 500 °C for 6 h under oxygen conditions at a heating rate of 5 °C / min, then heated to 900 °C for 12 h at a heating rate of 5 °C / min, and finally cooled naturally to room temperature. After successful sintering, the layered oxide material was obtained by sieving through a 200-mesh sieve.

[0069] Comparative Example 2

[0070] The layered oxide material from Comparative Example 1 was placed in a sealed supercritical high-pressure reactor and evacuated to a vacuum state. Carbon dioxide gas was then introduced into the supercritical device to achieve a pressure of 7.5 MPa. Subsequently, the supercritical device after the introduction of carbon dioxide was transferred to an oven at 43 °C and allowed to stand for 15 h. After natural cooling, the gas valve was quickly opened to obtain the layered oxide cathode material with a coating thickness of approximately 10 nm.

[0071] Comparative Example 3

[0072] The layered oxide material from Comparative Example 1 was placed in a sealed supercritical high-pressure reactor and evacuated to a vacuum state. Carbon dioxide gas was introduced into the supercritical device to achieve a pressure of 7.5 MPa. Subsequently, the supercritical device after the introduction of carbon dioxide was transferred to an oven at 43 °C and allowed to stand for 20 min. After natural cooling, the gas valve was quickly opened to obtain the layered oxide cathode material, which did not form a uniform coating layer.

[0073] Comparative Example 4

[0074] The layered oxide material from Comparative Example 1 was placed in a sealed supercritical high-pressure reactor and evacuated to a vacuum state. Carbon dioxide gas was introduced into the supercritical device to achieve a pressure of 7.5 MPa. Subsequently, the supercritical device after the introduction of carbon dioxide was transferred to an oven at 30 °C and allowed to stand for 1 h. After natural cooling, the gas valve was quickly opened to obtain the layered oxide cathode material, which did not form a uniform coating layer.

[0075] Comparative Example 5

[0076] The layered oxide material from Comparative Example 1 was placed in a sealed supercritical high-pressure reactor and evacuated to a vacuum state. Carbon dioxide gas was introduced into the supercritical device to achieve a pressure of 7.5 MPa. Subsequently, the supercritical device after the introduction of carbon dioxide was transferred to an oven at 23 °C and allowed to stand for 1 h. After natural cooling, the gas valve was quickly opened to obtain the layered oxide cathode material, which did not form a uniform coating layer.

[0077] Comparative Example 6

[0078] The layered oxide material was wet-coated by placing the layered oxide material from Comparative Example 1 in a beaker, adding 5% sodium carbonate powder according to the mass ratio, and adding ethanol to cover the powder. The mixture was stirred at 70 °C and 500 r / min on an electronic stirring table until the ethanol was dried, thus obtaining the coating layer formed by ordinary sodium carbonate coating of the layered oxide material, but the coating layer was uneven.

[0079] Performance testing

[0080] The layered oxide cathode materials prepared in the above embodiments and comparative examples were assembled into coin half-cells for electrochemical testing. The electrolyte was 1 mol / L NaPF6 in DME = 100 vol % (DME: ethylene glycol dimethyl ether), and the separator was a glass fiber separator. The battery was assembled in the order of positive electrode shell, layered oxide cathode sheet, electrolyte, separator, sodium sheet, and negative electrode shell, and then pressed and sealed with a sealing machine. The layered oxide cathode sheet was prepared by adding an appropriate amount of N-methylpyrrolidone to the layered oxide cathode material, conductive carbon black, and polyvinylidene fluoride in a mass ratio of 8:1:1, and then stirring in a homogenizer for 40 min to form a slurry. The slurry was then uniformly coated onto bright aluminum foil and vacuum dried at 80 ℃ for 12 h. Finally, the electrode sheet was cut into circular electrodes with a diameter of 12 mm. The active material loading of each electrode was 2.0-3.0 mg. After the battery was left to stand for 24 hours, electrochemical tests were performed using the Xinwei testing system and the Chenhua electrochemical workstation.

[0081] Electrochemical tests were conducted under a constant temperature condition of 30 °C, primarily consisting of constant current charge-discharge tests. These tests mainly assessed reversible capacity, air stability, and cycle life. The constant current charge-discharge test procedure was as follows: rest for 5 min – constant current charging – rest for 5 min – constant current discharging, with the battery's long-cycle performance tested at a current density of 180 mA / g. The cycle length was 100 times.

[0082] The results of material interlayer spacing, coating type, and thickness are shown in Table 2 below:

[0083]

[0084] The results of the air stability and cycling performance tests are shown in Tables 3 and 4 below:

[0085]

[0086]

[0087] Based on the interlayer spacing and coating results of the examples and comparative examples in Table 2, it can be seen that the layered oxide materials modified by the supercritical fluid technology described in this invention (Examples 1-11) exhibit significant optimization in both crystal structure and surface properties. The (003) diffraction peak position (2θ) of all the materials in these examples shifts to a lower angle (~16.0°-16.3°), and the calculated interlayer spacing d... (003)The thickness increased to 5.50-5.58 Å, significantly greater than the 5.36 Å of the untreated material in Comparative Example 1. This indicates that the supercritical fluid successfully penetrated into the sodium ion interlayer under high pressure, effectively expanding the sodium ion insertion / extraction channels and improving ion transport kinetics. Simultaneously, after supercritical treatment, a uniform coating layer with controllable composition and thickness was formed in situ on the material surface. Examples 1-11 all formed Na₂CO₃ or organic sodium salt coating layers with thicknesses between 3-7 nm, with Example 9 also generating a composite coating layer (6 nm) containing Na₂CO₃ and NaF. The formed coating layers are dense and stable, effectively isolating the material from contact with water vapor and carbon dioxide in the air, thereby suppressing surface side reactions.

[0088] Meanwhile, Examples 12-16 also successfully formed corresponding coating layers for the application of other sodium salts and precursor materials with other nickel-iron-manganese ratios, and also obtained the modification result of increased interlayer spacing, which improved the air stability compared with the corresponding unmodified materials and the corresponding electrochemical performance. However, it is still slightly inferior to Example 1 with sodium carbonate as sodium salt. This is mainly because different sodium sources and different element ratios have an impact on the synthesized materials themselves.

[0089] Comparative Examples 2-6 investigated the effects of other conditions on layer expansion, coating, and electrochemical performance. It can be seen that the coating in Comparative Example 2 was too thick (10 nm), which may have hindered ion migration. Comparative Examples 3-6 did not form an effective coating due to insufficient process conditions. The conventional coating in Comparative Example 6 could not achieve the layer expansion effect on the layered oxide, and the coating was uneven, which limited the electrochemical performance.

[0090] According to the air exposure performance test results (25 °C, 60 RH%) of the examples and comparative examples in Table 3, the layered oxide materials modified by the supercritical fluid technology described in this invention (Examples 1-11) all exhibit excellent initial capacity retention at a current density of 1 C, with the initial specific capacity of all examples remaining above 115 mAh / g. Notably, even after one week of air exposure, Example 1 still maintains a reversible capacity of 100 mAh / g, significantly better than the comparative materials (which generally decreased to 40-60 mAh / g).

[0091] Regarding long-term stability, the materials in the examples exhibited significant resistance to degradation: after 3 days of exposure, the capacity retention rates (based on the initial capacity) of Examples 1-11 were concentrated in the range of 69%-85%, while those of Comparative Examples 1-6 generally dropped to 60%-75%. In particular, the capacity retention rates of Example 1 after 1 day, 3 days, and 1 week of air exposure were 91.1%, 85.2%, and 74.1%, respectively, far exceeding those of Comparative Example 1 (79.4%, 58.5%, and 42.5%, respectively). The increased interlayer spacing and uniformity of the coating layer resulted in higher electrochemical performance than materials coated by ordinary wet processes.

[0092] As can be seen from the cycle performance test results of the examples and comparative examples in Table 4, the layered oxide cathode materials modified with the supercritical fluid technology described in this invention (Examples 1-11) exhibit significantly better electrochemical performance than traditional materials. Regarding the initial capacity, all examples maintained above 120 mAh / g. Due to the formation of the coating layer, conductivity was slightly limited, resulting in a slight decrease compared to the initial capacity of 141 mAh / g in Comparative Example 1. Notably, Example 1, with an initial capacity of 135 mAh / g and an initial coulombic efficiency of 85.6%, became the best performing example, far exceeding the best comparative example (Comparative Example 1, 141 mAh / g but with a coulombic efficiency of only 78.6%).

[0093] Regarding long-term cycling stability, the materials in the examples exhibited outstanding capacity retention: after 100 cycles, the capacity retention rates of Examples 1-11 were concentrated in the range of 72%-84%, significantly higher than the 55%-79% of Comparative Examples 1-6. In particular, Example 1 led with a retention rate of 83.8%, while in the control group, except for Comparative Example 1 (78.7%), the remaining samples rapidly declined to around 60%. Based on Tables 3 and 4, it can be concluded that materials optimized using supercritical fluid technology not only have higher initial capacity but also significantly improved environmental stability and capacity retention.

[0094] Figure 1 The images show a comparison of the XRD patterns of the modified layered oxide NFM111 materials from Comparative Example 1 and Example 1, as well as the PDF standard card of the NFM111 material in the ICDD PDF database, labeled PDF#25-0819. Figure 1 As can be seen, two characteristic peaks appear at 2θ = 16°-17° and 2θ = 44°-45° in the spectrum, corresponding to the (003) crystal plane (Na) of the layered structure. +Interlayer spacing) and (104) crystal plane (order of transition metal layer). The characteristic peak positions of the two materials are consistent and the peak shape is sharp, indicating that the supercritical fluid treatment did not destroy the crystal framework structure of the materials. The (003) peak position of Example 1 (e.g., 2 θ = 16.2°) is shifted to a lower angle compared with Comparative Example 1 (2 θ = 16.5°). According to the Bragg equation (2dsinθ = nλ), its interlayer spacing (d (003) The interlayer spacing increased from 5.36 Å to 5.52 Å. This change stems from the permeation of supercritical CO2 into the Na layer under high pressure. Upon depressurization, the stress generated by the gas expansion expands the crystal lattice, forming a stable extended layer structure. The larger interlayer spacing means more spacious channels for sodium ion insertion and extraction, resulting in better kinetic performance and effectively improving the electrochemical performance of the material.

[0095] Figure 2 The image shows a comparison of the scanning electron microscope (SEM) morphology of the layered oxide (NFMI11) cathode material in Comparative Example 1 (left) and Example 1 (right). As can be seen from the SEM images, the surface of the NFMI11 material without supercritical fluid treatment has obvious impurity particles attached, and its layered structure is somewhat obscured. However, the surface of the NFMI11 material after supercritical fluid treatment becomes clean, and the original layered structure is clearly revealed, indicating that most of the surface impurities have been effectively removed, and the surface impurity content has decreased by ≥90%. This fully demonstrates that supercritical carbon dioxide fluid has a significant effect on purifying the surface of layered oxide materials due to its strong solubility.

[0096] at the same time, Figure 3 The images show a comparison of the SEM morphology of the layered oxide (NFM111) cathode material in Comparative Example 1 (left) and Example 1 (right) after being exposed to air for one week. The SEM images clearly show that the NFM111 material without supercritical fluid pretreatment exhibits significant corrosion or contaminant deposition on its surface, affecting its structural integrity. In contrast, the NFM111 material after supercritical fluid pretreatment maintains a relatively clean and smooth surface, with its structural features still clearly discernible. This indicates that supercritical pretreatment significantly enhances the material's resistance to surface degradation caused by air exposure. This fully demonstrates that supercritical carbon dioxide treatment can not only effectively remove initial surface impurities but also form or strengthen a protective layer, thereby significantly improving the stability of the layered oxide material in air.

[0097] Figure 4This is a comparison chart of the cycling performance of the layered oxide (NFM111) cathode material in Comparative Example 1 and Example 1 at a voltage range of 2.0–4.2 V and a current density of 180 mA / g. The chart shows that, compared to the NFM111 material without supercritical fluid treatment, the NFM111 material treated with supercritical fluid technology did not show a significant impact on its original electrochemical performance due to the formation of the coating layer. The degradation rate of the NFM111 material after supercritical fluid treatment was slowed down. This indicates that supercritical fluid treatment effectively optimizes the structure of the NFM111 material (e.g., reducing surface impurities, improving interfacial stability, or enhancing structural integrity), thereby improving its long-term cycling performance as a cathode material.

[0098] Figure 5 This is a comparison of the cycling performance of the layered oxide (NFM111) cathode materials of Comparative Example 1 and Example 1 after one week of exposure to air, at a voltage range of 2.0–4.2 V and a current density of 180 mA / g. The figure clearly shows that the NFM111 material pretreated with supercritical fluid technology (Example 1) exhibits significantly better cycling stability than the untreated material (Comparative Example 1), with a higher initial discharge capacity. After one week of air exposure, Example 1 still maintains a reversible capacity of 100 mAh / g, while Comparative Example 1 only has a reversible capacity of 60 mAh / g. This fully demonstrates that supercritical fluid pretreatment effectively removes unstable impurities on the surface of the NFM111 material and / or forms a protective layer, significantly suppressing surface side reactions caused by air exposure (such as carbonate formation and structural degradation), thereby greatly improving the structural integrity and electrochemical cycle retention rate of the material after air exposure, demonstrating a very significant modification effect.

[0099] The sodium-ion battery layered oxide cathode material (NFM111), optimized using supercritical fluid technology, exhibits enhanced air stability and maintains high performance even after air exposure. It shows great promise for applications in large-scale energy storage systems, electric vehicles, smart grids, and portable electronic devices.

[0100] The embodiments described above are merely preferred embodiments of the present invention and are not intended to limit the present invention in any way. Other variations and modifications may be made without departing from the technical solutions described in the claims.

Claims

1. A method for preparing a layered oxide cathode material optimized and modified using supercritical fluid technology, characterized in that, Includes the following steps: (1) The sodium ion layered oxide is sealed in a supercritical high-pressure reactor, and the atmosphere in the reactor is replaced with a supercritical fluid atmosphere, and the pressure is made to reach the supercritical pressure condition of the atmosphere; the supercritical fluid atmosphere includes at least one of carbon dioxide, ethane, propane, ethylene, and 1,1,1,2-tetrafluoroethane. (2) The sealed supercritical high-pressure reactor is heated to the supercritical temperature of the atmosphere and subjected to a heat preservation reaction to obtain the modified material. The resulting modified layered oxide cathode material has an increased interlayer spacing of d. (003) Not less than 5.50 Å; after the reaction, a coating layer with a thickness of 3~8 nm is formed in situ in the sodium ion layered oxide.

2. The method for preparing a layered oxide cathode material optimized and modified by supercritical fluid technology according to claim 1, characterized in that, In step (1), the sodium ion layered oxide is prepared by the following method: sodium salt and precursor material are mixed and sintered in an oxygen atmosphere to obtain the sodium ion layered oxide; the general chemical formula of the sodium ion layered oxide is Na. x [TMO2], wherein TM is at least one of Fe, Mn, Ni, and Cu.

3. The method for preparing a layered oxide cathode material optimized and modified by supercritical fluid technology according to claim 1, characterized in that, The supercritical reaction conditions for the supercritical fluid atmosphere are as follows: when the supercritical fluid atmosphere is carbon dioxide, the reaction temperature is not lower than the critical temperature of 31.1℃ and the reaction pressure is not lower than the critical pressure of 6.3 MPa; when the supercritical fluid atmosphere is ethane, the reaction temperature is not lower than the critical temperature of 32.2℃ and the reaction pressure is not lower than the critical pressure of 3.5 MPa; when the supercritical fluid atmosphere is propane, the reaction temperature is not lower than the critical temperature of 95℃ and the reaction pressure is not lower than the critical pressure of 4.25 MPa; when the supercritical fluid atmosphere is ethylene, the reaction temperature is not lower than the critical temperature of 10℃ and the reaction pressure is not lower than the critical pressure of 5.0 MPa; when the supercritical fluid atmosphere is 1,1,1,2-tetrafluoroethane, the reaction temperature is not lower than the critical temperature of 80℃ and the reaction pressure is not lower than the critical pressure of 4.0 MPa.

4. The method for preparing a layered oxide cathode material optimized and modified by supercritical fluid technology according to claim 1, characterized in that, The supercritical reaction conditions are as follows: when the supercritical fluid atmosphere is carbon dioxide, the pressure is 7.5 MPa and the temperature is 43 °C; when the supercritical fluid atmosphere is ethane, the pressure is 4.8 MPa and the temperature is 35 °C; when the supercritical fluid atmosphere is propane, the pressure is 4.25 MPa and the temperature is 96 °C; when the supercritical fluid atmosphere is ethylene, the pressure is 5.2 MPa and the temperature is 15 °C; and when the supercritical fluid atmosphere is 1,1,1,2-tetrafluoroethane, the pressure is 4.06 MPa and the temperature is 100 °C.

5. The method for preparing a layered oxide cathode material optimized and modified by supercritical fluid technology according to claim 1, characterized in that, In step (2), the temperature is 30~100 ℃, the heating rate is 2~8 ℃ / min, and the holding time is 0.5~12h.

6. The method for preparing a layered oxide cathode material optimized and modified by supercritical fluid technology according to claim 1, characterized in that, After the reaction in step (2) is completed, the temperature is naturally cooled to room temperature, and then the valve is slowly opened to release the pressure to atmospheric pressure.

7. A layered oxide cathode material optimized and modified by supercritical fluid technology, prepared by the preparation method according to any one of claims 1-6.

8. The application of a layered oxide cathode material optimized and modified by supercritical fluid technology as described in claim 7 in the field of sodium-ion batteries.

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