Layered oxide positive electrode material optimized and modified by supercritical fluid technology as well as preparation method and application of layered oxide positive electrode material

Abstract

The invention belongs to the technical field of sodium ion batteries, and relates to a layered oxide positive electrode material optimized and modified by a supercritical fluid technology, and a preparation method and application thereof. According to the preparation method, a layered oxide (with a general formula of Na < x > [TMO2]) is taken as a matrix and reacts in a supercritical fluid medium for a certain time, and the optimized and modified layered oxide positive electrode material is obtained. According to the preparation method, interlayer spacing expansion, surface purification and in-situ construction of the sodium salt coating layer are performed on the layered oxide material through a 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 invention has the characteristics of relatively large interlayer spacing, excellent air stability and excellent cycle performance; meanwhile, the preparation method provided by the invention does not need to introduce exogenous elements, the process is simple and efficient, parameters are easy to control, the side reaction on the surface of the material can be remarkably inhibited, and a key technical support is provided for promoting commercialization of the sodium-ion battery positive electrode material.

Description

TECHNICAL FIELD

[0001] The application belongs to the technical field of modification of sodium ion battery cathode materials, and particularly relates to a layered oxide cathode material modified by supercritical fluid technology and a preparation method and application thereof. BACKGROUND

[0002] With the development of new energy industry, the demand for lithium resources has increased dramatically, and the safe and sustainable supply of lithium resources is at risk. Therefore, the development of sodium ion batteries has become a research hotspot. Compared with lithium ion batteries, sodium ion batteries have the advantages of high safety and low cost. Electrode materials are the key to determine the performance of battery materials. In the cathode material of sodium ion battery, layered transition metal (TM) oxide material (Na x TMO2) exhibits very high theoretical capacity, suitable electrochemical window, and similar synthesis process to lithium battery ternary materials, and has become one of the most promising cathode materials in sodium ion batteries.

[0003] The layered transition metal oxide cathode material is exposed to air, and the factors affecting stability include element composition, interlayer spacing, and surface morphology. The composition of alkali metal and transition metal in the material has a great influence on air stability. Compared with lithium, sodium is more alkaline and has a larger ionic radius, resulting in lower space volume utilization. Correspondingly, sodium hydroxide is more hydrophilic than lithium hydroxide, easily absorbs a large amount of water, and forms more sodium salts on the surface of Na x TMO2particles. For transition metal elements, active elements in low valence state (such as Mn 2+ , Ni 2+ , Fe 2+ , and V 3+ , etc.) in the material are prone to react with oxygen in the air, thereby exhibiting air sensitivity, leading to changes in the composition and structure of the material surface. In addition, sodium ions (Na + ) need to be repeatedly extracted and embedded in the interlayer of the cathode material during charging and discharging, and the interlayer spacing of the material is a core structural parameter of the layered transition metal oxide (Na x [TMO2]), which directly determines the thermodynamic stability and kinetic barrier of the material, and further dominates its environmental sensitivity. Smaller interlayer spacing will induce a series of adverse structural chemical properties, making it easy to react with H2O and CO2 in the environment, ultimately affecting the electrochemical performance, increasing the production, transportation and storage cost of the material, and hindering the commercialization process. Therefore, improving the air stability of the layered transition metal oxide cathode material is one of the focuses of current sodium ion battery research. Therefore, more and more researchers have begun to pay attention to this problem and have given many solutions, such as improving methods such as doping and coating to solve this problem.

[0004] Doping stabilizes the bulk structure of materials, suppresses the irreversible phase transition of materials in air, and thus improves the air stability of layered transition metal oxide Na x [TMO2] in air. Currently, ion doping mainly includes cation doping, anion doping, and co-doping of multiple additives. The team of Zhao Junmei from the Institute of Process Engineering of the Chinese Academy of Sciences successfully synthesized Na （1 / 3-0.01) Ni 1 / 3 Mn 1 / 3 A1 0.01 O2(NFNMA) layered material with excellent air stability by Al doping. After being exposed to air for 30 days, the sample NFNMA doped with Al still had a smooth surface and XRD remained the initial structure, showing excellent air stability. The team of Luo Shaohua from Northeastern University studied the effect of Cu 2+ doping amount on the air stability of Na 0.67 Ni 0.33-x Cu x Mn 0.67 O2, and found that with the increase of Cu 2+ doping amount, the air stability of the material was enhanced, the irreversible phase transition was reduced, and the charge-discharge curve became smooth, but the reversible specific capacity gradually decreased, and when the doping amount was 0.33%, the reversible specific capacity decreased by 50%. The team of Yin Guochun from Central South University co-doped NaNi 2+ Cu 4+ Mn 0.45 Ti 0.05 O2 with Cu 0.4 and Ti 0.1 , and found that co-doping could inhibit the embedding of H2O and CO2 into the positive electrode layered structure, Cu 2+ and Ti 4+ could also improve the ionic conductivity of the material, and Mg 2+ and Ti 4+ doping could also improve the Na xThe air stability of TMO2; but multi-element co-doping needs to introduce more exogenous elements, which is easy to cause more uncontrollability to the process and result, and further increase the cost. In contrast, coating modification is an effective strategy to improve the electrochemical performance. The coating layer is formed by ball milling or sol-gel method, and then high-temperature sintering is performed to obtain coating products such as ZrO2, B2O3 and AlF3, so as to prevent the dissolution of transition metal ions and improve the formation of material surface residual alkali and the cycle stability of the material. The Arumugam Manthiram team uses coating and heat treatment to uniformly coat the surface of layered oxide material with ZrO2, which effectively inhibits the reaction of the material with H2O and CO2 in the air. Electrochemical tests on the coated material exposed to air for 7 days show that it can still provide 76% of the initial capacity. However, the coating layer may have problems such as uneven coating and insufficient density, so there is still room for further improvement. Chinese patent CN117334863A A kind of carbon-coated sodium iron sulfate positive electrode material and its preparation method and application mentioned that supercritical fluid assisted halogenated carboxylic acid and other substances realize the carbon coating of sodium iron sulfate material, but it still needs to introduce exogenous substances, and the supercritical fluid is only used as a solvent. Chinese patent CN115011897A A method for exciting supercritical fluid plasma surface modification of metal lithium provides a method for modifying the surface of metal lithium in situ by exciting plasma in supercritical fluid to form a uniform and dense protective layer on the surface, thereby significantly improving its air stability and electrochemical performance. However, using supercritical fluid to assist exogenous substances to realize carbon coating is complex and introduces impurities. In the above method of modifying the material, although the material has been improved to a certain extent, the modification method introduces new element substances or structures. This method increases the cost in the process of industrial production, and the thickness and properties of the conductive layer of the treated material still need to be further adjusted, or even a further sintering step is needed. At the same time, it is difficult to form a uniform coating layer, and the influence of the interlayer spacing of the layered oxide on the material has not been discussed, so there is still room for further improvement.

[0005] Based on this, the present application proposes a kind of supercritical fluid technology optimization modified layered oxide positive electrode material and its preparation method and application, using selected gas medium penetrates into Na + Interlayer, expand the material interlayer spacing, and use the high activity and sodium-based residual alkali of the surface hydrophilic of supercritical medium to convert it into a dense, stable and hydrophobic protective layer, thereby preparing a sodium ion battery positive electrode material with high air stability. SUMMARY

[0006] The application aims at solving the problems of the sodium ion battery system layered oxide positive electrode material, such as the non-uniform and non-dense coating layer on the structure surface, the non-opened layer spacing, the collapse between layers, the specific capacity, the cycle stability and the air stability of the layered oxide positive electrode material after the modification process in the prior art. x [TMO2] as the research object, by selecting the type of sodium salt, the ratio of precursor elements, the type of gas medium, the size of gas pressure, the supercritical temperature and the reaction time, the layered oxide positive electrode material with wide layer spacing and dense and uniform coating layer can be effectively prepared. The supercritical fluid optimization layered oxide material technology provided by the application can effectively expand the layer spacing of the layered oxide in a short time, and form a uniform and dense coating layer on the surface of the layered oxide positive electrode material, so that the treated sodium ion battery layered oxide positive electrode material has high specific capacity, good cycle stability and excellent air stability, which can effectively promote the commercialization process of the layered oxide positive electrode material.

[0007] The technical scheme adopted by the application to solve the technical problems is as follows: A preparation method of a layered oxide positive electrode material optimized and modified by a supercritical fluid technology, comprising the following steps: (1) sealing the sodium ion layered oxide in a supercritical high-pressure reaction tank, replacing the atmosphere in the reaction tank with a supercritical fluid atmosphere, and increasing the pressure in the tank to reach the supercritical pressure condition of the atmosphere; (2) heating the sealed supercritical high-pressure reaction tank to the supercritical temperature condition of the atmosphere, and keeping the temperature for a period of time to obtain a modified sample.

[0008] The application provides a supercritical stabilization treatment method for a layered oxide positive electrode material for a sodium ion battery, which takes a sodium ion layered oxide such as sodium iron manganese oxide (NFM111) material as a matrix, takes a gas medium reaching a supercritical state at a specific temperature and pressure as a supercritical medium, and obtains an optimized and modified layered oxide positive electrode material after the sodium ion layered oxide material is reacted in the supercritical medium for a certain time and then the pressure is reduced to normal pressure.

[0009] The following is a preferred technical scheme of the application: Preferably, in step (1), there is no special requirement for the sodium ion layered oxide, which can be prepared by the following method: mixing sodium salt and precursor material, sintering under oxygen atmosphere to obtain sodium ion layered oxide, grinding and sieving, and then transferring to a supercritical high-pressure reaction tank, and sealing the vacuum tank.

[0010] More preferably, the sodium salt in the above method is at least one of sodium carbonate, sodium nitrate, sodium hydroxide, and the like, and more preferably one of sodium carbonate and sodium nitrate; the chemical formula of the precursor is Na x [TM(OH)2], TM is one or more of Fe, Mn, Ni, Cu, and the like, 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 positive electrode material in the reaction tank is between 1 and 10 g, and more preferably, the sodium salt selected is sodium carbonate, and the element ratio of the precursor is NaNi 1 / 3Fe 1 / / 3 Mn 1 / 3 (OH)2, and more preferably, the mass of the sodium nickel manganese ternary positive electrode material is 2 g; More preferably, the sodium ion layered oxide material is formed by mixing a sodium salt and a precursor and then sintering in stages under an oxygen atmosphere, wherein to compensate for the loss of sodium elements caused by volatilization during the calcination stage, the amount of sodium salt should be in excess of 5% molar ratio; the low-temperature sintering process does not have a significant impact on the result, and the temperature range and time are preferably controlled to be between 350°C and 550°C and 2 and 8 hours; the temperature range and time during the high-temperature sintering process are between 600°C and 950°C and 6 and 15 hours, and the resulting sodium ion layered oxide material will cause the crystal to grow excessively, the disordering will be intensified, and the material structure, composition, and electrochemical performance will be significantly degraded, and the sintering temperature is too low or the sintering time is too short, which will result in the material being unable to form a stable crystal structure, and more preferably, the sintering temperature is 900°C, the sintering time is 12 hours, and the resulting sodium ion layered oxide material is subjected to supercritical treatment after being sieved. Preferably, the mesh size of the sieve is 100-800 mesh; and more preferably, it is 200 mesh.

[0011] 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; and more preferably, it is carbon dioxide. By using the technical solution, the gas source has the characteristics of small molecular weight, high diffusivity in a supercritical state, and uniform surface coverage, can quickly penetrate into the nanometer pores on the surface of the layered oxide particles, and can penetrate into the Na + interlayer under high pressure, expand the material interlayer spacing, and convert it into a dense, stable, hydrophobic, and uniform sodium-based protective layer by using the high activity and surface hydrophilicity of the sodium-based residual alkali.

[0012] Preferably, the supercritical reaction conditions of the supercritical fluid atmosphere are: critical gas pressure ranging from 3.5 to 8 MPa, reaction temperature ranging from 35 to 55 ℃, and reaction time ranging from 0.5 to 12 h; more preferably, the reaction time ranges from 1 to 6 h, and a reaction time less than 0.5 h can result in uneven coating of the layered oxide material surface. Conversely, a reaction time longer than 12 h can result in an excessively thick coating of the layered oxide material surface.

[0013] More preferably, the reaction gas pressure and reaction temperature are selected according to the conditions for achieving a supercritical state of different supercritical gas media: when the supercritical fluid atmosphere is carbon dioxide, the reaction temperature is not lower than the critical temperature 31.1 ℃, and the reaction pressure is not lower than the critical pressure 6.3 MPa; when the supercritical fluid atmosphere is ethane, the reaction temperature is not lower than the critical temperature 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 95 ℃, and the reaction pressure is not lower than the critical pressure 4.25 MPa; when the supercritical fluid atmosphere is ethylene, the reaction temperature is not lower than the critical temperature 10 ℃, and the reaction pressure is not lower than the critical pressure 5.0 MPa; and when the supercritical fluid atmosphere is tetrafluoroethane, the reaction temperature is not lower than the critical temperature 80 ℃, and the reaction pressure is not lower than the critical pressure 4.0 MPa.

[0014] More preferably, the more optimal reaction gas pressure and reaction temperature of carbon dioxide are 7.5 MPa and 43 ℃, the more optimal reaction gas pressure and reaction temperature of ethane are 4.8 MPa and 35 ℃, the more optimal reaction gas pressure and reaction temperature of propane are 4.25 MPa and 96 ℃, the more optimal reaction gas pressure and reaction temperature of ethylene are 5.2 MPa and 15 ℃, and the more optimal reaction gas pressure and reaction temperature of tetrafluoroethane are 4.06 MPa and 100 ℃. A reaction gas pressure and reaction temperature that is too low can result in the supercritical fluid atmosphere not reaching a supercritical state, and a reaction gas pressure and reaction temperature that is too high can result in an explosion of the supercritical device. More preferably, the reaction system is heated by transferring the supercritical device to an oven at a certain temperature and allowing it to stand, and the optimal reaction conditions are selected according to the gas being introduced.

[0015] Preferably, in step (1), the supercritical fluid atmosphere is replaced by opening the vacuum pump to maintain a vacuum environment in the reaction tank, introducing a gas source into the reaction tank, and stopping the introduction when the gas medium reaches the supercritical pressure condition.

[0016] Preferably, in step (2), the temperature is 30 to 100 ℃, the heating rate is 2 to 8 ℃ / min, and the holding time is 0.5 to 12 h; more preferably, the optimal reaction conditions are selected according to the gas being introduced, the heating rate is 5 ℃ / min, and the holding time is 1 h, and more preferably, the sealed reaction tank is placed in an oven for heating.

[0017] Preferably, the coating layer is formed in-situ in the sodium ion layered oxide after the reaction in step (2), and the thickness is 3-8 nm, and more preferably, 5 nm.

[0018] Preferably, after the reaction in step (2) is completed, the valve is slowly opened to release the gas after natural cooling to room temperature, and the modified sample is obtained. More preferably, the gas flow when the valve is opened is not higher than 30 sccms, and if the flow is too large, the powder will be blown out.

[0019] Preferably, the composition of the in-situ formed sodium salt coating layer is determined by the gas medium: a sodium carbonate coating layer is formed in a carbon dioxide medium; an ethyl sodium coating layer is formed in an ethane medium; an isopropyl sodium coating layer is formed in a propane medium; a polymerized organic polymer layer is formed in an ethylene medium; and a sodium fluoride, sodium carbonate coating layer is formed in a tetrafluoroethane medium.

[0020] Preferably, the obtained optimized modified layered oxide positive electrode material has an expanded interlayer spacing of d (003) not less than 5.50 Å; and more preferably, 5.50-5.58 Å.

[0021] Preferably, the preparation method specifically comprises the following steps: (1) combining a layered oxide precursor with a sodium salt to obtain a sodium ion layered oxide material through sintering; (2) placing the above layered oxide material in a supercritical medium gas with a specific pressure; (3) heating the above reaction system to make the gas medium reach a supercritical state, naturally cooling and reducing the pressure to normal pressure after a certain reaction time, and obtaining a layered oxide material with a coating layer.

[0022] The application also provides an optimized modified layered oxide positive electrode material prepared by any one of the above preparation methods, which has an expanded interlayer spacing and an in-situ converted protective layer.

[0023] The application also provides an application of the optimized modified layered oxide positive electrode material prepared by any one of the above preparation methods in the field of sodium ion batteries, especially in large-scale energy storage systems, electric vehicles, or portable electronic devices.

[0024] The method for constructing a sodium salt coating layer to coat a layered oxide positive electrode material provided by the application realizes multiple technical breakthroughs through innovative process design: under the action of a gas supercritical state, the high-efficiency mass transfer characteristics are used to react with the added sodium ion layered oxide, penetrate into the Na +The interlayer is expanded, and the supercritical gas medium is combined with the sodium-based residual alkali on the material surface to be converted into a dense, stable and hydrophobic protective layer, so that the sodium ion battery positive electrode material with high performance and good air stability is prepared by using the supercritical fluid under high pressure.

[0025] The application provides a sodium ion layered oxide material modified by a supercritical fluid technology and a preparation method and application thereof. + The interlayer is expanded, and the material surface is in-situ converted to improve the electrochemical performance and air stability of the material. Specifically, the gas medium is converted into a supercritical fluid state by controlling the gas pressure, reaction temperature and other conditions, and the high-efficiency mass transfer characteristics, high diffusivity and high solubility of the supercritical fluid are used to penetrate into the surface of the layered oxide material, so that the material is effectively modified by the supercritical fluid technology. The supercritical fluid technology can make the hydrophilic sodium-based residual alkali on the surface react with the gas medium in the supercritical state to be in-situ converted into a dense, stable and hydrophobic sodium salt coating, improve the air stability of the material, and reduce the influence of the battery charging and discharging reaction, thereby prolonging the service life of the battery.

[0026] Compared with the prior art, the sodium ion layered oxide material is modified by the supercritical fluid technology, and the following remarkable beneficial effects are achieved: the supercritical fluid penetrates into the Na + The interlayer is expanded, and the material surface is in-situ converted to improve the electrochemical performance and air stability of the material. Specifically, the gas medium is converted into a supercritical fluid state by controlling the gas pressure, reaction temperature and other conditions, and the high-efficiency mass transfer characteristics, high diffusivity and high solubility of the supercritical fluid are used to penetrate into the surface of the layered oxide material, so that the material is effectively modified by the supercritical fluid technology. The supercritical fluid technology can make the hydrophilic sodium-based residual alkali on the surface react with the gas medium in the supercritical state to be in-situ converted into a dense, stable and hydrophobic sodium salt coating, improve the air stability of the material, and reduce the influence of the battery charging and discharging reaction, thereby prolonging the service life of the battery.

[0027] (1) The interlayer spacing of the sodium ion layered oxide is large, and the supercritical fluid is more easily penetrated into the Na + The interlayer is expanded, and the material surface is in-situ converted to improve the electrochemical performance and air stability of the material. Specifically, the gas medium is converted into a supercritical fluid state by controlling the gas pressure, reaction temperature and other conditions, and the high-efficiency mass transfer characteristics, high diffusivity and high solubility of the supercritical fluid are used to penetrate into the surface of the layered oxide material, so that the material is effectively modified by the supercritical fluid technology. The supercritical fluid technology can make the hydrophilic sodium-based residual alkali on the surface react with the gas medium in the supercritical state to be in-situ converted into a dense, stable and hydrophobic sodium salt coating, improve the air stability of the material, and reduce the influence of the battery charging and discharging reaction, thereby prolonging the service life of the battery.

[0028] (2) The method for expanding the interlayer spacing of the layered oxide layer of the present application can avoid the damage to the lattice integrity and the weakening of the material structure caused by the traditional physical or chemical "expansion" of the interlayer spacing; at the same time, compared with the potential problem of the traditional expansion method that simply expanding the interlayer spacing will increase the side reaction between the material and air, the present application realizes the in-situ preparation of the coating layer while expanding the layer, effectively avoiding the harm caused by the change of the interlayer spacing.

[0029] (3) The high diffusivity, high permeability and high solubility of the supercritical fluid enable it to uniformly penetrate into the surface and subsurface regions of the material without dead angles. By accurately controlling the gas pressure, reaction temperature and time, the composition (depending on the gas medium) and thickness (nanometer to submicron) of the generated sodium salt coating can be precisely and in-situ controlled. This uniformity and precision is difficult to achieve by traditional wet chemical coating or physical vapor deposition methods.

[0030] (4) The inherent sodium-based residual alkali on the surface of the material directly reacts 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 transition and structural collapse of the material surface due to water absorption, as well as the side reactions such as the generation of sodium carbonate / sodium bicarbonate by 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, with extremely strong interfacial bonding force, avoiding the problems of poor interfacial compatibility, weak bonding and easy peeling of traditional exogenous coatings.

[0031] Therefore, the layered oxide positive electrode material of the present application exhibits good air stability and excellent electrochemical performance. At the same time, the preparation method of the layered oxide positive electrode material described in the present application is simple, fast, efficient and convenient, easy to control, low cost and pollution-free, which is helpful to promote the commercialization development of sodium ion battery positive electrode materials. BRIEF DESCRIPTION OF DRAWINGS

[0032] Figure 1 is the XRD pattern comparison diagram of the layered oxide NFM111 material of Comparative Example 1 and Example 1; Figure 2 is the scanning electron microscope SEM morphology comparison diagram of the layered oxide NFM111 of Comparative Example 1 (left) and Example 1 (right); Figure 3 is the scanning electron microscope SEM morphology comparison diagram of the layered oxide NFM111 of Comparative Example 1 (left) and Example 1 (right) exposed to air for one week; Figure 4Cycle performance comparison chart of the layered oxide NFM111 material of Comparative Example 1 and Example 1 at a current density of 180 mA / g; Figure 5 Cycle performance comparison chart of the layered oxide NFM111 material of Comparative Example 1 and Example 1 after exposure to air for one week at a current density of 180 mA / g. DETAILED DESCRIPTION

[0033] In order to make the objects, technical schemes and advantages of the present application clearer, better understood, the technical schemes and embodiments of the present application are further described and explained in detail below with reference to the drawings, it should be understood that the embodiments described in the present application are implemented on the premise of the technical schemes of the present application, and give detailed embodiments and specific operation processes, but only part of the embodiments of the present application, not all the embodiments, the specific embodiments described are limited to the description and explanation of the present application, and do not limit the present application. Based on the embodiments in the present application, all other embodiments obtained by those skilled in the art without creative labor belong to the scope of protection of the present application.

[0034] The experimental methods and conditions used in the embodiments of the present application are conventional methods and conventional conditions, and the materials, reagents or instrument devices used in the embodiments, unless otherwise specified, are conventional substances or devices known to those skilled in the art and can be obtained commercially or prepared by conventional methods. The reaction conditions embodied in the summary of the present application can achieve the described reactions and obtain the expected product. In the present application, room temperature is 10-40℃, and the exposure condition for air stability test is 25℃, 60 RH%. Due to the limitation of the length, some of the following examples are listed to further illustrate the advantages of the technical schemes of the present application.

[0035] Example 1

[0036] In order to make up for the loss caused by the volatilization of sodium element in the calcination stage, 5% excess of sodium carbonate in molar ratio is added to the precursor [Ni 1 / 3 Fe 1 / / 3 Mn 1 / 3The layered oxide material is obtained by mixing (OH)2, sintering under oxygen conditions (for specific conditions, refer to Comparative Example 1), and sieving through a 200-mesh screen. The layered oxide material is transferred to a sealed supercritical high-pressure reaction kettle, which can be a high-pressure reaction kettle, a high-pressure resistant ball mill tank, or other common high-pressure resistant reaction devices, and is pumped to a vacuum state. Carbon dioxide gas is introduced into the supercritical device to reach a gas pressure of 7.5 MPa. Then, the supercritical device after the introduction of carbon dioxide is transferred to an oven at 43°C for standing, so that the gas medium reaches a supercritical state. After 1 hour of reaction, natural cooling is performed. After the tank temperature cools to room temperature, the gas valve is slowly opened to obtain a layered oxide material optimized by supercritical fluid technology. The coating layer is Na2CO3, and the coating layer thickness is about 5 nm, and the interlayer spacing is ~ 5.52Å.

[0037] Example 2-11

[0038] On the basis of Example 1, the reaction conditions are changed, including layered oxide precursor material, gas medium, reaction gas pressure, reaction temperature, reaction time, etc. The specific conditions are shown in Table 1 below:

[0039] Example 12

[0040] An excess of 5% sodium nitrate and Ni 1 / 3 Fe 1 / 3 Mn 1 / 3 The layered oxide positive electrode material is obtained by pre-sintering (OH)2 under an oxygen atmosphere at a temperature increasing rate of 5°C / min to 500°C for 6 hours, sintering at a temperature increasing rate of 5°C / min to 900°C for 12 hours, and annealing to room temperature after sintering, and sieving.

[0041] The layered oxide is transferred to a sealed supercritical high-pressure reaction kettle, which is pumped to a vacuum state. Carbon dioxide gas is introduced into the supercritical device to reach a gas pressure of 7.5 MPa. Then, the supercritical device after the introduction of carbon dioxide is transferred to an oven at 43°C for standing, so that the gas medium reaches a supercritical state. After 1 hour of reaction, natural cooling is performed. After the tank temperature cools to room temperature, the gas valve is slowly opened to obtain a layered oxide material optimized by supercritical fluid technology. The coating layer thickness is about 5 nm.

[0042] Example 13

[0043] An excess of 5% sodium nitrate and Ni 1 / 3 Fe 1 / 3 Mn 1 / 3The layered oxide positive electrode material is obtained by pre-sintering treatment of the layered oxide (OH)2 under an oxygen atmosphere at a temperature rising rate of 5 ℃ / min to 500 ℃ for 6 h, sintering at a temperature rising rate of 5 ℃ / min to 900 ℃ for 12 h, annealing to room temperature after sintering, and sieving.

[0044] The layered oxide is transferred into a sealed supercritical high-pressure reaction kettle and is pumped to a vacuum state, carbon dioxide gas is introduced into the supercritical device to reach a gas pressure of 7.5 MPa. Subsequently, the supercritical device after introduction of carbon dioxide is transferred to an oven at 43 ℃ and is left to stand, so that the gas medium reaches a supercritical state, and after reaction for 1 h, it is naturally cooled. After the tank temperature is cooled to room temperature, the gas valve is slowly opened, and the layered oxide material optimized by supercritical fluid technology is obtained, and the thickness of the coating layer is about 5 nm.

[0045] Example 14

[0046] An excess of 5% sodium carbonate and Ni 2 / 3 Fe 1 / 6 Mn 1 / 6 The layered oxide positive electrode material is obtained by pre-sintering treatment of the layered oxide (OH)2 under an oxygen atmosphere at a temperature rising rate of 5 ℃ / min to 500 ℃ for 6 h, sintering at a temperature rising rate of 5 ℃ / min to 900 ℃ for 12 h, annealing to room temperature after sintering, and sieving.

[0047] The layered oxide is transferred into a sealed supercritical high-pressure reaction kettle and is pumped to a vacuum state, carbon dioxide gas is introduced into the supercritical device to reach a gas pressure of 7.5 MPa. Subsequently, the supercritical device after introduction of carbon dioxide is transferred to an oven at 43 ℃ and is left to stand, so that the gas medium reaches a supercritical state, and after reaction for 1 h, it is naturally cooled. After the tank temperature is cooled to room temperature, the gas valve is slowly opened, and the layered oxide material optimized by supercritical fluid technology is obtained, and the thickness of the coating layer is about 5 nm.

[0048] Example 15

[0049] An excess of 5% sodium carbonate and Ni 1 / 6 Fe 2 / 3 Mn 1 / 6 The layered oxide positive electrode material is obtained by pre-sintering treatment of the layered oxide (OH)2 under an oxygen atmosphere at a temperature rising rate of 5 ℃ / min to 500 ℃ for 6 h, sintering at a temperature rising rate of 5 ℃ / min to 900 ℃ for 12 h, annealing to room temperature after sintering, and sieving.

[0050] The layered oxide is transferred into a sealed supercritical high-pressure reaction kettle and is pumped to a vacuum state, carbon dioxide gas is introduced into the supercritical device to make the gas pressure reach 7.5 MPa. Then the supercritical device after the introduction of carbon dioxide is transferred to a 43 ℃ oven and is left to stand, so that the gas medium reaches the supercritical state, and after reaction for 1 h, it is naturally cooled. After the tank temperature is cooled to room temperature, the gas valve is slowly opened, and the layered oxide material optimized by supercritical fluid technology is obtained, and the coating layer thickness is about 5 nm.

[0051] Example 16

[0052] In order to make up for the loss caused by the volatilization of sodium element in the calcination stage, 5% excess sodium carbonate is mixed with the precursor [Ni 1 / 6 Fe 1 / 6 Mn 2 / 3 (OH)2], and is sintered at a temperature increasing rate of 5 ℃ / min to 500 ℃ under oxygen for 6 h, and then sintered at a temperature increasing rate of 5 ℃ / min to 900 ℃ for 12 h and naturally cooled to room temperature. After successful sintering, the layered oxide material is obtained by sieving with a 200 mesh sieve.

[0053] The layered oxide is transferred into a sealed supercritical high-pressure reaction kettle and is pumped to a vacuum state, carbon dioxide gas is introduced into the supercritical device to make the gas pressure reach 7.5 MPa. Then the supercritical device after the introduction of carbon dioxide is transferred to a 43 ℃ oven and is left to stand, so that the gas medium reaches the supercritical state, and after reaction for 1 h, it is naturally cooled. After the tank temperature is cooled to room temperature, the gas valve is slowly opened, and the layered oxide material optimized by supercritical fluid technology is obtained, and the coating layer thickness is about 5 nm.

[0054] Comparative Example 1

[0055] In order to make up for the loss caused by the volatilization of sodium element in the calcination stage, 5% excess sodium carbonate is mixed with the precursor [Ni 1 / 3Fe 1 / 3 Mn 1 / 3 (OH)2], and is sintered at a temperature increasing rate of 5 ℃ / min to 500 ℃ under oxygen for 6 h, and then sintered at a temperature increasing rate of 5 ℃ / min to 900 ℃ for 12 h and naturally cooled to room temperature. After successful sintering, the layered oxide material is obtained by sieving with a 200 mesh sieve.

[0056] Comparative Example 2

[0057] The layered oxide material in Comparative Example 1 was placed in a sealed supercritical high-pressure reaction kettle and was pumped to a vacuum state, carbon dioxide gas was introduced into the supercritical device to make the gas pressure reach 7.5 MPa. Subsequently, the supercritical device after the introduction of carbon dioxide was transferred to an oven at 43 °C and was left to stand, after 15 h of heat preservation, it was naturally cooled, the gas valve was quickly opened, and a layered oxide positive electrode material was obtained, and the coating layer had a thickness of about 10 nm.

[0058] Comparative Example 3

[0059] The layered oxide material in Comparative Example 1 was placed in a sealed supercritical high-pressure reaction kettle and was pumped to a vacuum state, carbon dioxide gas was introduced into the supercritical device to make the gas pressure reach 7.5 MPa. Subsequently, the supercritical device after the introduction of carbon dioxide was transferred to an oven at 43 °C and was left to stand, after 15 h of heat preservation, it was naturally cooled, the gas valve was quickly opened, and a layered oxide positive electrode material was obtained, and the coating layer had a thickness of about 10 nm.

[0060] Comparative Example 4

[0061] The layered oxide material in Comparative Example 1 was placed in a sealed supercritical high-pressure reaction kettle and was pumped to a vacuum state, carbon dioxide gas was introduced into the supercritical device to make the gas pressure reach 7.5 MPa. Subsequently, the supercritical device after the introduction of carbon dioxide was transferred to an oven at 43 °C and was left to stand, after 15 h of heat preservation, it was naturally cooled, the gas valve was quickly opened, and a layered oxide positive electrode material was obtained, and the coating layer had a thickness of about 10 nm.

[0062] Comparative Example 5

[0063] The layered oxide material in Comparative Example 1 was placed in a sealed supercritical high-pressure reaction kettle and was pumped to a vacuum state, carbon dioxide gas was introduced into the supercritical device to make the gas pressure reach 7.5 MPa. Subsequently, the supercritical device after the introduction of carbon dioxide was transferred to an oven at 43 °C and was left to stand, after 15 h of heat preservation, it was naturally cooled, the gas valve was quickly opened, and a layered oxide positive electrode material was obtained, and the coating layer had a thickness of about 10 nm.

[0064] Comparative Example 6

[0065] The layered oxide material was wet coated, the layered oxide material in Comparative Example 1 was placed in a beaker, 5% of sodium carbonate powder was added according to the mass ratio, ethanol was added to cover the powder, and the electronic stirring table was stirred at 70 °C and 500 r / min until the ethanol was stirred dry, thereby obtaining a layered oxide material coated with ordinary sodium carbonate, and the formed coating layer was not uniform.

[0066] Performance test

[0067] The layered oxide positive electrode materials prepared in the above examples and comparative examples were assembled into button half-cells for electrochemical tests. 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 cells were assembled in the order of positive electrode shell, layered oxide positive electrode sheet, electrolyte, separator, sodium sheet, and negative electrode shell, and were tightly sealed with a packaging machine, wherein the layered oxide positive electrode sheet was prepared by adding a proper amount of N-methyl pyrrolidone to the layered oxide positive electrode material, conductive carbon black, and polyvinylidene fluoride in a mass ratio of 8:1:1, stirring for 40 min with a homogenizer, then uniformly coating the slurry on a bright aluminum foil, vacuum drying at 80 °C for 12 h, and finally cutting the electrode sheet into a circular electrode with a diameter of 12 mm. The loading amount of active material for each electrode was 2.0-3.0 mg. After the cells were left to stand for 24 hours, electrochemical tests were performed using a Neware test system and a Chenhua electrochemical workstation.

[0068] The electrochemical tests were all carried out under constant temperature conditions at 30 °C, mainly constant current charge-discharge tests. In the constant current charge-discharge tests, the main indicators included reversible capacity, air stability, and cycle life. The process of the constant current charge-discharge tests was: standing for 5 min-constant current charging-standing for 5 min-constant current discharging, and the long cycle performance of the cells was tested at a current density of 180 mA / g. The process of the constant current charge-discharge tests was: standing for 5 min-constant current charging-standing for 5 min-constant current discharging, and the cells were cycled for 100 cycles.

[0069] The results of the material layer spacing, the type and thickness of the coating layer are shown in Table 2 below:

[0070] The results of the air stability and cycle performance tests are shown in Tables 3 and 4, respectively:

[0071]

[0072] According to the results of the layer spacing and the coating layer of the examples and comparative examples in Table 2, it can be seen that the layered oxide materials (Examples 1-11) modified by the supercritical fluid technology described in the present application exhibit significant optimization in crystal structure and surface properties. The (003) crystal face diffraction peak position (2 θ) of all the example materials moves to a low angle direction (~ 16.0°-16.3°), and the calculated layer spacing d (003)Expanded to 5.50-5.58 Å, significantly larger than 5.36 Å of the untreated material of Comparative Example 1. This indicates that the supercritical fluid successfully penetrated into the interlayer of sodium ions at high pressure, effectively expanding the sodium ion deintercalation channel, which is conducive to improving the ion transport kinetics. At the same time, after supercritical treatment, a uniform coating layer with controllable composition and thickness is formed in situ on the surface of the material. Examples 1-11 all form Na2CO3 or organic sodium salt coating layers with a thickness of 3-7 nm, of which Example 9 also generates a composite coating layer containing Na2CO3 and NaF (6 nm). The coating layers formed are dense and stable, which can effectively isolate the material from contact with water vapor and carbon dioxide in the air, thereby inhibiting surface side reactions.

[0073] At the same time, Examples 12-16 also successfully form corresponding coating layers and obtain modified interlayer spacing expansion results, which improve the air stability and electrochemical performance of the corresponding unmodified materials, but still slightly insufficient compared to Example 1 using sodium carbonate as the sodium salt. This is mainly because different sodium sources and different element ratios have an impact on the material synthesized.

[0074] Comparative Examples 2-6 investigate the effects of other conditions on layer expansion, coating layer, and electrochemical performance. It can be seen that the coating layer of Comparative Example 2 is too thick (10 nm) and may hinder ion migration; Comparative Examples 3-6 do not form effective coating layers due to insufficient process conditions; and the conventional coating of Comparative Example 6 cannot achieve layer expansion for the layered oxide, and the coating layer is uneven, limiting the electrochemical performance.

[0075] According to the air exposure performance test (25 °C, 60 RH%) results of Examples and Comparative Examples in Table 3, it can be seen that the layered oxide materials modified by the supercritical fluid technology described in the present application (Examples 1-11) all exhibit excellent initial capacity retention ability at a current density of 1 C. The initial specific capacity of all examples is maintained at 115 mAh / g or more. It is worth noting that even after 1 week of air exposure, Example 1 can still maintain a reversible capacity of 100 mAh / g, which is significantly better than the comparative material (generally decreased to 40-60 mAh / g).

[0076] In terms of long-term stability, the example materials exhibit obvious anti-fading properties: after 3 days of exposure, the capacity retention rates (based on initial capacity) of Examples 1-11 are concentrated in the 69%-85% range, while Comparative Examples 1-6 have generally fallen to 60%-75%. In particular, Example 1 has a capacity retention rate of 91.1%, 85.2%, and 74.1% after 1 day / 3 days / 1 week of air exposure, far exceeding Comparative Example 1 (79.4%, 58.5%, and 42.5%, respectively). Due to the expansion of the interlayer spacing and the uniformity of the coating layer, the electrochemical performance of the material is higher than that of ordinary wet-coated materials.

[0077] As can be seen from the cycle performance test results of the examples and comparative examples in Table 4, the layered oxide positive electrode material modified using the supercritical fluid technology of the present application (Examples 1-11) exhibits significantly better electrochemical performance than conventional materials. In terms of first-cycle specific capacity, all examples maintain above 120 mAh / g, and due to the formation of the coating layer, the conductivity is slightly limited, with a slight decrease in initial capacity of 141 mAh / g for Comparative Example 1. Notably, Example 1 has an initial capacity of 135 mAh / g and a first-cycle coulombic efficiency of 85.6%, making it the best-performing sample, far exceeding Comparative Example 1 (141 mAh / g but a coulombic efficiency of only 78.6%) in the comparative group.

[0078] In terms of long cycle stability, the example materials exhibit outstanding capacity retention capability: after 100 cycles, the capacity retention rates of Examples 1-11 are concentrated in the 72%-84% range, significantly higher than the 55%-79% of Comparative Examples 1-6. In particular, Example 1 leads with a retention rate of 83.8%, while the remaining samples in the comparative group rapidly decay to around 60% except for Comparative Example 1 (78.7%). According to Tables 3 and 4, it can be concluded that the material optimized by the supercritical fluid technology not only has a higher initial capacity, but also has significantly improved environmental stability and capacity retention rate.

[0079] Figure 1 The XRD patterns of the modified layered oxide NFM111 materials of Comparative Example 1 and Example 1 are compared, and the PDF standard card of NFM111 material in the ICDD PDF database is labeled as PDF#25-0819. From the XRD patterns of Comparative Example 1 and Example 1, it can be seen that two characteristic peaks appear at 2θ = 16°-17° and 2θ = 44°-45°, respectively, corresponding to the (003) crystal plane (Na Figure 1 It can be seen that two characteristic peaks appear at 2θ = 16°-17° and 2θ = 44°-45° in the pattern, corresponding to the (003) crystal plane (Na +d-spacing) and (104) plane (ordering of transition metal layers). The characteristic peak positions of both materials are consistent and sharp, indicating that the supercritical fluid treatment did not destroy the crystal framework structure of the material. The (003) peak position (e.g. 2 theta = 16.2°) of Example 1 shifted to lower angles compared to Comparative Example 1 (2 theta = 16.5°), according to Bragg's equation (2dsin theta = n lambda), the d-spacing (d (003) ) of Example 1 expanded from 5.36 A to 5.52 A. This change resulted from the penetration of supercritical CO2 into the Na layers at high pressure, and the stress created by the expansion of the gas upon depressurization to push apart the lattice and form a stable expanded structure. The larger d-spacing means that the channels for sodium ion deintercalation are wider and the kinetics are better, effectively improving the electrochemical performance of the material.

[0080] Figure 2 Figure 1 is a comparison of the SEM images of the layered oxide (NFM111) cathode material of Comparative Example 1 (left) and Example 1 (right). As can be seen from the SEM images, the surface of the NFM111 material that has not been treated by supercritical fluid technology is attached to obvious impurity particles, and its layered structure is somewhat obscured. The surface of the NFM111 material that has been treated by supercritical fluid technology is clean, and the original layered structure is clearly visible, indicating that most of the surface impurities have been effectively removed, and the content of surface impurities has decreased by > 90%, which fully demonstrates that carbon dioxide supercritical fluid, with its strong solubility, has a significant effect on purifying the surface of layered oxide materials.

[0081] At the same time, Figure 3 Figure 2 is a comparison of the SEM images of the layered oxide (NFM111) cathode material of Comparative Example 1 (left) and Example 1 (right) after exposure to air for 1 week. As can be clearly seen from the SEM images, the surface of the NFM111 material that has not been pretreated by supercritical fluid technology has obvious corrosion or deposition of pollutants, and the structural integrity has been somewhat affected. The surface of the NFM111 material that has been pretreated by supercritical fluid technology remains relatively clean and flat, and the structural features are still clearly visible, indicating that supercritical pretreatment significantly enhances the ability of the material to resist surface degradation caused by air exposure, which fully demonstrates that treatment by carbon dioxide supercritical fluid not only effectively removes initial surface impurities, but also forms or strengthens a protective layer, thereby significantly improving the stability of the layered oxide material in air.

[0082] Figure 4The cycle performance comparison chart of the layered oxide (NFM111) positive electrode material of Comparative Example 1 and Example 1 in the voltage range of 2.0-4.2 V and at the current density of 180 mA / g. It can be observed from the cycle performance comparison chart that, compared with the NFM111 material not treated by the supercritical fluid technology, the NFM111 material treated by the supercritical fluid technology does not have a great influence on the original electrochemical performance due to the formation of the coating layer. The attenuation rate of the NFM111 material treated by the supercritical fluid technology is slowed down. It is indicated that, by the treatment of the supercritical fluid technology, the structure of the NFM111 material is effectively optimized (such as reducing the surface impurities, improving the interface stability or enhancing the structural integrity), so that the long-term cycle performance of the NFM111 material as the positive electrode material is improved.

[0083] Figure 5 The cycle performance comparison chart of the layered oxide (NFM111) positive electrode material of Comparative Example 1 and Example 1 after being exposed in air for 1 week in the voltage range of 2.0-4.2 V and at the current density of 180 mA / g. It is clearly shown in the chart that the NFM111 material treated by the supercritical fluid technology (Example 1) exhibits a significantly better cycle stability than the untreated material (Comparative Example 1), and has a higher initial discharge capacity. After being exposed in air for 1 week, the Example 1 can still maintain a reversible capacity of 100 mAh / g, while the Comparative Example 1 only has a reversible capacity of 60 mAh / g. It is fully indicated that the supercritical fluid pretreatment effectively removes the unstable impurities on the surface of the NFM111 material and / or forms a protective layer, significantly inhibits the surface side reactions (such as the generation of carbonate, structural degradation, etc.) caused by the air exposure, so that the structural integrity and the electrochemical cycle retention rate of the material after the air exposure are greatly improved, and the modification effect is very significant.

[0084] The layered oxide positive electrode material (NFM111) of the sodium ion battery treated by the supercritical fluid technology has higher air stability and can still maintain high performance after being exposed in air. It has a broad application prospect in the fields of large-scale energy storage systems, electric vehicles, smart grids and portable electronic devices.

[0085] The above-described examples are only the preferred schemes of the present application, and do not limit the present application in any form. Other variants and modifications can be made without exceeding the technical scheme recited in the claims.

Claims

1. A method for preparing a layered-oxide cathode material modified by supercritical fluid technology optimization, characterized in that, The method comprises the following steps: (1) sealing a sodium-ion layered oxide in a supercritical high-pressure reaction tank, replacing the atmosphere in the reaction tank with a supercritical fluid atmosphere, and bringing the pressure to the supercritical pressure condition of the atmosphere; (2) heating the sealed supercritical high-pressure reaction tank to the supercritical temperature condition of the atmosphere, and carrying out a heat preservation reaction to obtain a modified material.

2. The method for preparing a layered-oxide cathode material modified by supercritical fluid technology optimization according to claim 1, characterized in that, In step (1), the sodium-ion layered oxide can be prepared by mixing a sodium salt and a precursor material and sintering under an oxygen atmosphere to obtain a sodium-ion layered oxide; the sodium-ion layered oxide has a general chemical formula of Na x [TM02], wherein TM is at least one of Fe, Mn, Ni, and Cu.

3. The method for preparing a layered-oxide cathode material modified by supercritical fluid technology optimization according to claim 1, characterized in that, In step (1), the supercritical fluid atmosphere comprises at least one of carbon dioxide, ethane, propane, ethylene, and 1,1,1,2-tetrafluoroethane.

4. The method for preparing a layered oxide cathode material optimized and modified by supercritical fluid technology according to claim 3, characterized in that, The supercritical reaction condition of the supercritical fluid atmosphere is that the critical gas pressure ranges from 3.5 MPa to 8 MPa, the reaction temperature is from 35 ℃ to 55 ℃, and the reaction time is from 0.5 h to 12 h.

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

6. The method for preparing a layered oxide cathode material optimized and modified by supercritical fluid technology according to claim 3, characterized in that, The supercritical reaction condition is that when the supercritical fluid atmosphere is carbon dioxide, the pressure is 7.5 MPa and the temperature is 43 ℃; when the supercritical fluid atmosphere is ethane, the pressure is 4.8 MPa and the temperature is 35 ℃; when the supercritical fluid atmosphere is propane, the pressure is 4.25 MPa and the temperature is 96 ℃; when the supercritical fluid atmosphere is ethylene, the pressure is 5.2 MPa and the temperature is 15 ℃; and when the supercritical fluid atmosphere is tetrafluoroethane, the pressure is 4.06 MPa and the temperature is 100 ℃.

7. 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 from 30 ℃ to 100 ℃, the heating rate is from 2 ℃ / min to 8 ℃ / min, and the heat preservation time is from 0.5 h to 12 h.

8. 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 valve is slowly opened to release the pressure to the normal pressure after natural cooling to room temperature.

9. A supercritical fluid technology-optimized modified layered oxide cathode material prepared by the preparation method according to any one of claims 1-8.

10. Application of the supercritical fluid technology-optimized modified layered oxide cathode material according to claim 9 in the field of sodium-ion batteries.

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