A hard carbon negative electrode material optimized and modified by supercritical fluid technology and its preparation method and application

Abstract

The present invention belongs to the technical field of sodium ion batteries, and relates to a hard carbon negative electrode material optimized and modified by supercritical fluid technology, and its preparation method and application. The preparation method is based on a biomass hard carbon material as a matrix and a critical threshold gas as a supercritical medium. The biomass hard carbon material reacts in the supercritical medium for a certain period of time to obtain an optimized and modified hard carbon negative electrode material. The present invention purifies and removes impurities from the hard carbon material and adjusts the pore structure and carbon layer spacing through supercritical fluid technology, thereby preparing a hard carbon negative electrode material with higher purity and excellent electrochemical performance. The hard carbon negative electrode material prepared by the present invention exhibits high specific capacity, good cycle stability, and excellent rate performance and coulombic efficiency. At the same time, the preparation method provided by the present invention is simple, fast, efficient, convenient, and easy to control. It can efficiently remove insoluble impurities in the hard carbon material, improve the sodium storage structure of the hard carbon material, and help promote the development of hard carbon negative electrode materials for sodium ion batteries.

Description

Technical Field

[0001] The present invention relates to the technical field of sodium ion batteries, and in particular to a hard carbon negative electrode material optimized and modified by supercritical fluid technology, and a preparation method and application thereof. Background Art

[0002] With the acceleration of the global industrialization process and the continuous increase in energy consumption, the excessive use of traditional energy has brought about many serious environmental problems. In this context, the development and utilization of clean energy has become a global focus. Ensuring a stable supply of energy and optimizing energy utilization efficiency are of great significance. Sodium-ion batteries have attracted much attention in the field of energy storage due to their abundant raw materials, low cost, high safety and environmental friendliness. Hard carbon materials have gradually become the first choice for commercial negative electrode materials for sodium-ion batteries due to their excellent structural stability, good cycle performance, low cost and abundant resources. However, hard carbon negative electrode materials still have many problems that limit their commercial application, such as low specific capacity, low first coulombic efficiency and poor rate performance.

[0003] Biomass hard carbon has a complex internal structure and contains a variety of impurities such as organic, inorganic and microbial impurities. Therefore, it is usually necessary to purify and remove impurities, as well as adjust the pore structure of the hard carbon. For biomass hard carbon materials, high purity of the material is a prerequisite for excellent electrochemical performance. Usually, it undergoes simple acid or alkaline washing treatment for purification and impurity removal. However, some insoluble impurities will still remain inside the purified hard carbon material, such as inorganic salts such as potassium and sodium, and oxides formed by minerals such as silicon, iron, and calcium at high temperatures. The presence of such impurities will reduce the conductivity of the material and hinder the transmission of sodium ions. On the other hand, it will reduce the reversible specific capacity of the hard carbon material and shorten the battery life. At the same time, the pore structure of the hard carbon material is equally important. A good pore structure is conducive to the transmission and storage of sodium ions in the hard carbon material, alleviates the volume expansion of the material, reduces material stress, and improves the stability of the material.

[0004] Therefore, impurity removal and pore structure adjustment have become key research directions in the field of sodium-ion batteries. For example, Taiyuan University of Technology has adjusted the pore structure of hard carbon by combining hydrothermal pretreatment, freeze-drying, and high-temperature carbonization. During the hydrothermal pretreatment process, the assembly of graphene oxide and starch promotes the formation of open pores, which are then converted into closed pores during the high-temperature carbonization process. Compared with the sample without structural adjustment, the reversible specific capacity of the final prepared hard carbon material increased by 184.21 mAh / g, a growth rate of 73.80%. The University of Science and Technology of China extracted hard carbon materials from a large amount of waste biomass bamboo powder waste through acid-base pretreatment and template carbonization. The hard carbon material has a reversible capacity of 303 mAh / g at a rate of 1 C and good cycling performance, with a capacity retention rate of 92.0% after 100 cycles. Guangdong University of Technology used sodium hydroxide to selectively etch the lignin and hemicellulose components in bamboo fibers at low temperatures, thereby increasing the relative content of cellulose in bamboo fibers. The increase in cellulose content in bamboo fibers increased the order of hard carbon, which was beneficial for the closure of open pores after high-temperature carbonization. The closed pore volume of the optimized hard carbon increased from 0.15 cm 3 / g increased to 0.26 cm 3 Compared to bamboo prepared by direct carbonization without selective etching with sodium hydroxide, the platform capacity of this hard carbon material increased by 88 mAh / g. Although the aforementioned methods for preparing hard carbon materials have achieved certain performance improvements, the impurity removal methods all rely on acid or alkaline washing, which still has certain limitations and cannot effectively remove a small amount of insoluble impurities in the hard carbon material. The purity of the material after acid and alkaline treatment still needs to be improved, and the pore structure still needs further adjustment.

[0005] Based on this, the present invention proposes a hard carbon negative electrode material optimized and modified by supercritical fluid technology, as well as its preparation method and application. The strong solubility of the gas medium in the supercritical state is utilized to prepare high-purity hard carbon negative electrode materials, and the internal microstructure of the hard carbon is regulated to prepare high-performance sodium ion battery negative electrode materials. Summary of the Invention

[0006] The purpose of the present invention is to address the problems in the prior art, such as the residual inorganic salts, metal oxides and other insoluble impurities in the hard carbon negative electrode materials of the sodium ion battery system after the acid-base pretreatment process, resulting in the low purity of the hard carbon material, as well as the initial coulombic efficiency, specific capacity and rate performance of the hard carbon negative electrode materials. A hard carbon negative electrode material optimized and modified by supercritical fluid technology and its preparation method and application are provided. By regulating parameters such as the type of gas medium, gas pressure, supercritical temperature, and reaction time, high-performance hard carbon negative electrode materials can be effectively prepared. The supercritical fluid optimized hard carbon technology provided by the present invention can effectively remove impurities such as inorganic salts and metal oxides remaining in the hard carbon material in a short period of time, improve the hard carbon pore structure, and expand the carbon layer spacing, so that the prepared and processed sodium ion battery hard carbon negative electrode material has high specific capacity, good cycle stability, and excellent rate performance and coulombic efficiency.

[0007] The technical solution adopted by the present invention to solve its technical problem is:

[0008] The present invention provides a method for preparing a hard carbon negative electrode material optimized and modified by supercritical fluid technology. The method uses a biomass hard carbon material as a matrix and a critical threshold gas as a supercritical medium. After the biomass hard carbon material reacts in the supercritical medium for a certain period of time, the pressure is reduced to normal pressure to obtain the optimized modified hard carbon negative electrode material.

[0009] By adopting the above technical solution, using biomass hard carbon as the carbon source matrix, it has the huge advantages of a wide range of raw material sources and low cost. In addition, compared with other carbon materials such as graphite and graphene, the surface of biomass hard carbon materials has many oxygen-containing groups such as hydroxyl and carboxyl groups that can interact with the active groups in the supercritical gas medium. Its rich internal pore structure also facilitates the entry of supercritical fluid into its pore structure. In addition, the crystal structure of biomass hard carbon is disordered, the interlayer spacing is small, and the pore structure is complex. Its sodium storage mechanism is more dependent on the pore structure and surface chemical properties. Compared with carbon materials such as graphite whose lithium / sodium storage performance depends on the interlayer embedding reaction, biomass hard carbon has more special requirements for supercritical fluid technology. At the same time, by reacting the hard carbon material in a supercritical fluid, the remaining impurities such as inorganic salts or metal oxides in the hard carbon material are dissolved in the supercritical fluid, and the gas supercritical fluid enters the pores of the hard carbon material, thereby achieving supercritical fluid technology treatment. It can effectively remove insoluble impurities such as inorganic salts and metal oxides in the hard carbon material, improve the purity and quality of the material, and enhance the material's conductivity and cyclic stability. The contact between the supercritical fluid and the hard carbon material introduces some functional groups such as oxygen, nitrogen, fluorine, and sulfur, which can interact with active oxygen groups such as hydroxyl and carboxyl groups on the hard carbon surface, further optimizing the hard carbon surface structure. During the supercritical decompression process, the supercritical medium instantly transforms from liquid to gas, generating strong stress, which can expand the carbon layer spacing of the hard carbon material, adjust the material's pore structure, increase the active sites for sodium storage in the hard carbon material, increase the sodium storage space, and thus improve the material's reversible specific capacity and cycle life. Under the action of supercritical technology, the pore structure of the hard carbon material is further controlled, accelerating the insertion and extraction of sodium ions, improving the material's structural stability and ensuring the material's high-rate performance and good cyclic performance. Furthermore, the supercritical fluid-assisted process allows the hard carbon materials to react in a relatively mild environment (no higher than 100°C), resulting in low energy consumption and low costs. Compared to conventional modification methods, supercritical fluid technology can effectively reduce the number of reaction steps and time, thereby saving time and material costs and improving production efficiency.

[0010] Preferably, the biomass hard carbon material is obtained by pre-treating a biomass hard carbon precursor; more preferably, the biomass hard carbon precursor is selected from at least one of bamboo powder, straw, cellulose powder, coconut shell, mold silk, rice husk powder, fir powder, corn stalks, sugarcane bagasse, starch, and the like. The present invention uses biomass hard carbon as a matrix, purifies and removes impurities, and optimizes the pore structure and carbon interlayer spacing. After carbonization, the biomass carbon material has oxygen-containing active groups on its surface and a rich internal pore structure that facilitates reaction with a supercritical gas medium. It also has the advantages of low cost and a wide range of raw material sources.

[0011] More preferably, the pretreatment may be a conventional pretreatment, including pickling, drying and high-temperature carbonization; more preferably, after pickling and drying, the product is placed in a tube furnace, subjected to high-temperature carbonization in a protective atmosphere, and annealed to room temperature;

[0012] More preferably, the pickling solution is at least one of hydrochloric acid, sulfuric acid, phosphoric acid, and nitric acid, the pickling solution concentration is 0.5-2 mol / L, and the pickling time is 1-24 h; more preferably, the pickling solution is hydrochloric acid, the pickling solution concentration is 1 mol / L, and the pickling time is 6 h;

[0013] More preferably, the drying temperature range is 50-100°C and the drying time is 2-12 hours; more preferably, the drying temperature is 60°C and the drying time is 8 hours. Drying removes a large amount of water to avoid the phenomenon of large-scale evaporation of water during high-temperature carbonization treatment, which may lead to structural collapse.

[0014] More preferably, the protective atmosphere is at least one of helium, nitrogen, and argon;

[0015] More preferably, the high-temperature carbonization treatment process is: a heating rate of 2-5 ° C / min, a temperature of 1000-1600 ° C, a holding time of 1-3 hours, and then annealing to room temperature at a cooling rate of 2-5 ° C / min to further remove moisture and volatile components in the material and form a stable hard carbon structure; more preferably, a heating rate of 5 ° C / min, a temperature of 1200 ° C, a holding time of 2 hours, and a cooling rate of 5 ° C / min;

[0016] More preferably, the high-temperature carbonization is followed by sieving, and the sieving mesh is preferably 300-800 mesh, more preferably 500 mesh.

[0017] Preferably, the supercritical fluid medium is at least one of carbon dioxide, sulfur hexafluoride, and nitrous oxide, with carbon dioxide being more preferred. By employing this technical solution, the critical threshold gas has a low molecular weight, making it easier to enter the pore structure of the biomass hard carbon material after becoming a supercritical fluid. Furthermore, the critical threshold gas contains functional groups such as oxygen, nitrogen, and fluorine, enabling it to effectively modify the hard carbon material.

[0018] Preferably, the supercritical reaction conditions are: a reaction pressure range of 3.8-8 MPa, a reaction temperature of 35-55°C, and a reaction time of 3-12 hours. More preferably, the reaction time is 6-10 hours. Insufficient reaction time can lead to incomplete removal of impurities within the hard carbon material and insufficient adjustment of the pore structure. Conversely, excessively long reaction times can damage the internal structure of the hard carbon material.

[0019] More preferably, the reaction pressure and reaction temperature should be adjusted based on the conditions for achieving a supercritical state for different supercritical gas media: when the critical threshold gas is carbon dioxide, the reaction temperature should be no less than the critical temperature of 31.1°C, and the reaction pressure should be no less than the critical pressure of 7.38 MPa; when the critical threshold gas is sulfur hexafluoride, the reaction temperature should be no less than the critical temperature of 45.6°C, and the reaction pressure should be no less than the critical pressure of 3.76 MPa; when the critical threshold gas is nitrous oxide, the reaction temperature should be no less than the critical temperature of 36.5°C, and the reaction pressure should be no less than the critical pressure of 7.26 MPa. The optimal reaction pressure and reaction temperature for carbon dioxide are 7.5 MPa and 40°C, for sulfur hexafluoride are 4 MPa and 50°C, and for nitrous oxide are 7.5 MPa and 40°C. Excessively low reaction pressures and reaction temperatures may prevent the critical threshold gas from reaching a supercritical state, while excessively high reaction pressures and reaction temperatures 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 the oven to stand.

[0020] Preferably, the preparation method specifically comprises the following steps:

[0021] (1) Pretreating a biomass hard carbon precursor to obtain a biomass hard carbon material;

[0022] (2) placing the biomass hard carbon material in a critical threshold gas at a specific pressure;

[0023] (3) The reaction system is heated to a supercritical state, and after a certain reaction time, the system is naturally cooled and the pressure is reduced to normal pressure to obtain an optimized and modified hard carbon negative electrode material.

[0024] More preferably, in step (2), the biomass hard carbon material is placed in a sealed supercritical high-pressure reactor, which is evacuated to a vacuum state, and then a critical threshold gas is introduced into the supercritical device until a specific pressure is reached. More preferably, the gas flow rate into the supercritical high-pressure reactor is in the range of 20-70 sccm; more preferably, 40 sccm. More preferably, the specific pressure is the supercritical pressure of the critical threshold gas, more preferably 3.8-8 MPa.

[0025] More preferably, in step (3), the pressure is reduced after cooling to room temperature to further ensure safety. More preferably, the pressure is reduced rapidly by quickly opening the gas valve to release the gas to normal pressure.

[0026] Preferably, the room temperature of the present invention is 15-40°C.

[0027] The present invention also provides a high-performance hard carbon negative electrode material prepared by any of the above preparation methods after optimization and modification using supercritical fluid technology.

[0028] The present invention also provides an application of a high-performance hard carbon negative electrode material prepared by any of the above preparation methods and optimized and modified using supercritical fluid technology in the field of sodium ion batteries.

[0029] The present invention provides a hard carbon negative electrode material optimized and modified using supercritical fluid technology, as well as a preparation method and application thereof. To date, there have been very few reports on the optimization and modification of hard carbon negative electrode materials using supercritical fluid technology, especially regarding the improvement of the purity and structural adjustment of hard carbon materials, as well as their application in hard carbon negative electrodes for sodium ion batteries. The present invention controls conditions such as gas pressure and temperature to transform the gas medium into a supercritical fluid state and penetrate into the interior of a biomass hard carbon material rich in mesopores and micropores, thereby effectively modifying the hard carbon material using supercritical fluid technology. Supercritical fluid technology can effectively remove insoluble impurities such as inorganic salts and metal oxides from the structure of the hard carbon material, reducing the impact of impurities on the battery's charge and discharge reactions, thereby extending the battery's service life. Supercritical fluid technology can be used to adjust the hard carbon pore structure, accelerate sodium ion transport, and optimize battery rate performance. Supercritical fluid technology can also be used to introduce some functional groups to interact with oxygen-containing groups on the surface of the hard carbon material, further optimizing the hard carbon surface structure. In addition, supercritical fluid technology can also expand the carbon layer spacing of the hard carbon material, add more sodium storage sites, and improve the material's specific capacity. Therefore, the hard carbon negative electrode materials prepared by supercritical fluid technology show higher capacity, more superior rate performance and coulombic efficiency.

[0030] Compared with the prior art, the present invention has the following beneficial effects:

[0031] (1) The present invention uses supercritical fluid technology to treat biomass hard carbon materials, and utilizes the strong solubility of the gas medium in the supercritical state to further purify and remove impurities. It can effectively remove insoluble impurities such as inorganic salts and metal oxides in the hard carbon materials, improve the purity and quality of the materials, and enhance the initial coulombic efficiency and cycle performance.

[0032] (2) The contact between supercritical fluid and hard carbon material will introduce some functional groups such as oxygen, nitrogen, fluorine, and sulfur, which can interact with active oxygen groups such as hydroxyl and carboxyl groups on the hard carbon surface, further optimizing the hard carbon surface structure;

[0033] (3) During the supercritical decompression process, the supercritical medium instantly transforms from liquid to gas, generating strong stress to expand the spacing between the carbon layers of the hard carbon material, increase the sodium storage space of the hard carbon material, and improve the discharge specific capacity; under the action of supercritical technology, the pore structure of the hard carbon material is further regulated, which is conducive to the embedding and extraction of sodium ions, improving the structural stability of the material and ensuring the high rate performance and good cycle performance of the material.

[0034] Therefore, the hard carbon negative electrode material of the present invention exhibits good cycle stability, rate capability, and coulombic efficiency, and has excellent electrochemical performance. Furthermore, the preparation method of the hard carbon negative electrode material of the present invention is simple, rapid, efficient, convenient, easy to control, and uses a wide range of raw material sources at low cost. BRIEF DESCRIPTION OF THE DRAWINGS

[0035] Figure 1 1 is a comparison diagram of the XRD patterns of the hard carbon materials of Comparative Example 1 and Example 1;

[0036] Figure 2 1 is a comparison of the SEM morphologies of the hard carbon materials of Comparative Example 1 and Example 1;

[0037] Figure 3 1 is a comparison of Raman spectra of the hard carbon negative electrode materials of Comparative Example 1 and Example 1;

[0038] Figure 4 This is a comparison of high-resolution TEM images of the hard carbon materials of Comparative Example 1 and Example 1;

[0039] Figure 5 1 is a comparison diagram of the adsorption isotherm curves of Comparative Example 1 and Example 1;

[0040] Figure 6 1 is a comparison diagram of the isotherm desorption curves of Comparative Example 1 and Example 1;

[0041] Figure 7 1 is a comparison diagram of the differential distribution curves of adsorption mesopore diameters of the hard carbon materials of Comparative Example 1 and Example 1;

[0042] Figure 8 1 is a comparison chart of the rate performance of the hard carbon negative electrode materials of Comparative Example 1 and Example 1 at current densities of 0.1, 0.2, 0.5, 1, 2, 5, and 10 C;

[0043] Figure 9 This is a comparison chart of the cycle performance of the hard carbon negative electrode materials of Comparative Example 1 and Example 1 at a current density of 50 mA / g. DETAILED DESCRIPTION

[0044] In order to better clarify and understand the purpose, process scheme and advantages of the present invention, the technical scheme and implementation method of the present invention are further clearly, completely and in detail described below through specific examples and in combination with the accompanying drawings. It should be noted that the embodiments described in the present invention are implemented on the premise of the technical scheme of the present invention, and detailed implementation methods and specific operating processes are given, but they are only part of the embodiments of the present invention, not all of the embodiments. The specific implementation methods described are limited to illustrating and explaining the present invention and do not limit the present invention. Based on the embodiments in the present invention, all other implementation methods obtained by ordinary technicians in this field without making creative work are within the scope of protection of the present invention.

[0045] The experimental methods and conditions used in the embodiments of the present invention are conventional methods and conventional conditions unless otherwise specified. The materials, reagents, instruments, devices, etc. used in the embodiments are conventional substances or equipment known to those skilled in the art and can be obtained from commercial channels or prepared by conventional methods unless otherwise specified. The reaction conditions embodied in the summary of the invention of the present invention are all capable of achieving the described reactions and obtaining products with the desired effects. Due to space limitations, some examples are listed below to further illustrate the advantages of the technical solutions of the present invention.

[0046] Example 1

[0047] Bamboo powder was acid-washed with 1 mol / L hydrochloric acid for 6 h and dried at 60°C for 8 h. Subsequently, the mixture was carbonized under an argon atmosphere by heating the mixture to 1200°C at a rate of 5°C / min and holding for 2 h. The mixture was then annealed to room temperature at a rate of 5°C / min and finally screened through a 500-mesh screen to obtain a biomass hard carbon material. The biomass hard carbon material was placed in a sealed supercritical autoclave, which can be a common high-pressure reactor, such as an autoclave or a high-pressure ball mill. The reaction was evacuated to a vacuum state, and carbon dioxide gas was introduced into the supercritical reactor at a rate of 40 sccm to a pressure of 7.5 MPa. The supercritical reactor, after being injected with carbon dioxide, was then transferred to a 40°C oven and allowed to stand until the gas reached a supercritical state. After 6 h of reaction, the mixture was naturally cooled and the gas valve was quickly opened to obtain a high-purity hard carbon anode material optimized for supercritical fluid technology.

[0048] Example 2-11

[0049] On the basis of Example 1, the reaction conditions including biomass hard carbon precursor material, critical threshold gas medium, reaction pressure, reaction temperature, reaction time, etc. were changed. The specific conditions are shown in the following table:

[0050] Table 1 Summary of reaction conditions for each example

[0051]

[0052] Comparative Example 1

[0053] The bamboo powder was pickled with 1 mol / L hydrochloric acid for 6 h and dried at 60 ℃ for 8 h. It was then carbonized in an argon atmosphere at a heating rate of 5 ℃ / min to 1200 ℃ and kept at that temperature for 2 h. It was then annealed to room temperature at a cooling rate of 5 ℃ / min and finally sieved through a 500-mesh screen to obtain a hard carbon material.

[0054] Comparative Example 2

[0055] The hard carbon material from Comparative Example 1 was placed in a sealed supercritical autoclave and evacuated to a vacuum state. Carbon dioxide gas was then introduced into the supercritical chamber at a rate of 40 sccm to a pressure of 7.0 MPa. The supercritical chamber, after the introduction of carbon dioxide, was then transferred to a 40°C oven and allowed to stand for 6 hours. After cooling naturally, the air valve was quickly opened to obtain a hard carbon anode material.

[0056] Comparative Example 3

[0057] The hard carbon material from Comparative Example 1 was placed in a sealed supercritical autoclave and evacuated to a vacuum state. Carbon dioxide gas was then introduced into the supercritical chamber at a rate of 40 sccm to a pressure of 7.5 MPa. The supercritical chamber, after being injected with carbon dioxide, was then transferred to a 30°C oven and allowed to stand for 6 hours. After cooling naturally, the air valve was quickly opened to obtain a hard carbon anode material.

[0058] Comparative Example 4

[0059] The hard carbon material from Comparative Example 1 was placed in a sealed supercritical autoclave and evacuated to a vacuum state. Carbon dioxide gas was then introduced into the supercritical chamber at a rate of 40 sccm to a pressure of 7.5 MPa. The supercritical chamber, after being injected with carbon dioxide, was then transferred to a 40°C oven and allowed to stand for 1 hour. After cooling naturally, the air valve was quickly opened to obtain a hard carbon anode material.

[0060] Comparative Example 5

[0061] The hard carbon material from Comparative Example 1 was placed in a sealed supercritical autoclave and evacuated to a vacuum state. Carbon dioxide gas was then introduced into the supercritical chamber at a rate of 40 sccm to a pressure of 7.5 MPa. The supercritical chamber, after being injected with carbon dioxide, was then transferred to a 40°C oven and allowed to stand for 15 hours. After cooling naturally, the air valve was quickly opened to obtain a hard carbon anode material.

[0062] Comparative Example 6

[0063] The hard carbon material from Comparative Example 1 was placed in a sealed supercritical autoclave and evacuated to a vacuum state. Ethylene gas was then introduced into the supercritical chamber at a rate of 40 sccm to a pressure of 5.5 MPa. The supercritical chamber, after ethylene gas introduction, was then transferred to a 20°C oven and allowed to stand for 6 hours after reaching a supercritical state. After cooling naturally, the gas valve was quickly opened to obtain a hard carbon anode material.

[0064] Comparative Example 7

[0065] The hard carbon material from Comparative Example 1 was placed in a sealed supercritical autoclave and evacuated to a vacuum state. Propane gas was then introduced into the supercritical chamber at a rate of 40 sccm to a pressure of 4.5 MPa. The supercritical chamber, after being injected with propane gas, was then transferred to a 100°C oven and allowed to stand for 6 hours after reaching a supercritical state. After cooling naturally, the gas valve was quickly opened to obtain a hard carbon anode material.

[0066] Comparative Example 8

[0067] The epoxy resin was carbonized under an argon atmosphere by heating to 1200°C at a rate of 5°C / min and holding for 2 h. The product was then annealed to room temperature at a rate of 5°C / min and finally screened through a 500-mesh screen to obtain a hard carbon material. The sintered hard carbon material was placed in a sealed supercritical autoclave and evacuated to a vacuum state. Carbon dioxide gas was introduced into the supercritical chamber at a rate of 40 sccm to a pressure of 7.5 MPa. The supercritical chamber, after being injected with carbon dioxide, was then transferred to a 40°C oven and allowed to stand until the gas reached a supercritical state. After 6 h of reaction, the reaction was allowed to cool naturally, and the gas valve was quickly opened to obtain the hard carbon anode material.

[0068] Comparative Example 9

[0069] Polyacrylonitrile was carbonized under an argon atmosphere by heating to 1200°C at a rate of 5°C / min and holding for 2 hours. The material was then annealed to room temperature at a rate of 5°C / min and finally screened through a 500-mesh screen to obtain a hard carbon material. The sintered hard carbon material was placed in a sealed supercritical autoclave and evacuated to a vacuum state. Carbon dioxide gas was introduced into the supercritical chamber at a rate of 40 sccm to a pressure of 7.5 MPa. The supercritical chamber, after being injected with carbon dioxide, was then transferred to a 40°C oven and allowed to stand until the gas reached a supercritical state. After 6 hours of reaction, the material was naturally cooled and the gas valve was quickly opened to obtain the hard carbon anode material.

[0070] Comparative Example 10

[0071] Graphite sieved through a 500-mesh sieve was placed in a sealed supercritical high-pressure reactor and evacuated to a vacuum state. Carbon dioxide gas was then introduced into the supercritical apparatus at a rate of 40 sccm to a pressure of 7.5 MPa. The supercritical apparatus, after being introduced with carbon dioxide, was then transferred to a 40°C oven and allowed to stand until the gas medium reached a supercritical state. After reacting for 6 hours, the reaction was allowed to cool naturally and the gas valve was quickly opened to obtain a hard carbon anode material.

[0072] Performance Testing

[0073] The hard carbon anode materials prepared in Examples 1-11 and Comparative Examples 1-10 were assembled into button-type 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 membrane. The cells were assembled in the order of positive electrode shell, hard carbon anode sheet, electrolyte, separator, sodium sheet, and negative electrode shell, and sealed using a packaging machine. The hard carbon anode sheet consisted of a mixture of hard carbon anode material, conductive carbon black, and sodium carboxymethyl cellulose in a mass ratio of 8:1:1. Deionized water was added and stirred in a homogenizer for 30 minutes to form a slurry. The slurry was then evenly coated onto bright aluminum foil and vacuum-dried at 80°C for 12 hours. The electrode sheets were finally cut into circular electrodes with a diameter of 12 mm. The active material loading for each electrode was 0.8-1.2 mg. After the cells were allowed to rest for 24 hours, electrochemical testing was performed using a Xinwei test system and a Chenhua electrochemical workstation.

[0074] All electrochemical tests were conducted at a constant temperature of 30°C, primarily constant current charge and discharge tests. The constant current charge and discharge tests primarily measured reversible capacity, rate performance, coulombic efficiency, and cycle life. The rate performance of the battery was tested at current densities of 0.1, 0.2, 0.5, 1, 2, 5, and 10 C. The constant current charge and discharge test process was as follows: 5 min rest – constant current discharge – 5 min rest – constant current charge, with 5 cycles at each current density. The long-term cycle performance of the battery was tested at a current density of 50 mA / g. The constant current charge and discharge test process was as follows: 5 min rest – constant current discharge – 5 min rest – constant current charge, with 500 cycles.

[0075] The test results of rate performance and cycle performance are shown in Table 2 and Table 3 respectively:

[0076] Table 2 Summary of rate performance test results of various embodiments and comparative examples

[0077]

[0078] Table 3 Summary of the cycle performance test results of each embodiment and comparative example

[0079]

[0080] From the rate performance test results of each embodiment and comparative example in Table 2, it can be found that the hard carbon negative electrode materials optimized by the supercritical fluid technology described in the present invention in Examples 1-11 all exhibited a high specific capacity of more than 384 mAh / g and an initial coulombic efficiency of more than 86% at a current density of 0.1 C. In addition, the above materials not only maintained a high reversible specific capacity at low rates of 0.1, 0.2, and 0.5 C, but also were still able to perform reversible charge and discharge at a high rate of 10 C, with excellent rate performance. In contrast, the hard carbon negative electrode materials in Comparative Examples 1-10 had a low initial capacity of less than 355 mAh / g and an initial coulombic efficiency of no more than 85% at a low rate current density of 0.1 C. The reversible specific capacity at rates of 1 and 10 C was much lower than that of Examples 1-11, indicating that supercritical fluid technology is very effective in improving the rate performance of modified hard carbon negative electrode materials.

[0081] The cycling performance test results for the Examples and Comparative Examples in Table 3 show that the hard carbon anode materials optimized using the supercritical fluid technology described in the present invention in Examples 1-11 all exhibited initial discharge capacities higher than those of the Comparative Examples at a current density of 50 mA / g, by 30-40 mAh / g. The initial discharge capacity of the graphite anode material optimized using the supercritical fluid technology in Comparative Example 10 was even lower, at only 263.3 mAh / g. This is partly due to the small interlayer spacing of the graphite material, making it unsuitable for sodium-ion battery systems, and partly due to the lack of the pore structure and functional groups unique to biomass materials. Furthermore, the capacity retention rates of the hard carbon anode material half-cells optimized using the supercritical fluid technology were all above 85% after 500 cycles, compared to the lower capacity retention rates of 27.8-71.5% for the hard carbon anode material half-cells in Comparative Examples 1-10, demonstrating significant improvement.

[0082] Figure 1 is a comparison diagram of the XRD patterns of the hard carbon materials of Comparative Example 1 and Example 1; Figure 1 The XRD pattern shows two obvious peaks at 2θ = 22 ° ~ 25 ° and 2θ = 43 ° ~ 44 °, which correspond to the (002) graphite platelet plane and (100) sp of hard carbon materials. 2The characteristic peaks of the hybrid hexagonal carbon crystal planes of the two materials are both obvious and roughly the same, indicating that the hard carbon material's crystal structure has not been destroyed by the supercritical fluid treatment. In particular, the double diffraction angles of the (002) and (100) crystal planes of the hard carbon material of Example 1 are both smaller than those of the hard carbon material of Comparative Example 1. According to the Bragg equation 2dsinθ=nλ, the lattice spacing of the hard carbon material treated with supercritical fluid technology is larger. This indicates that the strong stress released by the gas medium from liquid to gas during the decompression process of supercritical fluid technology can effectively expand the interlayer spacing of the hard carbon material, thereby facilitating the storage of sodium ions.

[0083] Figure 2 The SEM morphology comparison of the hard carbon negative electrode materials of Comparative Example 1 (left) and Example 1 (right) is shown; Figure 2 The SEM image shows that many impurities other than hard carbon materials are attached to the hard carbon surface that has not been treated with supercritical fluid technology, while the hard carbon surface treated with supercritical fluid technology becomes smooth and tidy, and most of the impurities are obviously removed, indicating that the strong solubility of carbon dioxide in the supercritical state is effective in removing impurities on the surface of hard carbon materials.

[0084] At the same time, Table 4 is an ICP-OES test table of the mass fraction of metal impurity elements in the hard carbon negative electrode materials of Comparative Example 1 and Example 1. It can be found from the table that the Ca, Si, K, Na, Al, and Fe element contents of the hard carbon materials treated with supercritical fluid technology decreased by approximately 90%, 28%, 85%, 80%, 47%, and 48%, respectively. This indicates that the strong solubility of the supercritical fluid can effectively dissolve impurities such as inorganic salts and metal oxides that remain in the hard carbon material after sintering.

[0085] Table 4 Impurity element content of hard carbon materials in Comparative Example 1 and Example 1

[0086]

[0087] Figure 3 is a comparison of the Raman spectra of the hard carbon negative electrode materials of Comparative Example 1 and Example 1; Figure 3 The Raman spectrum of the hard carbon anode material after supercritical fluid treatment showed that the intensity ratio of the D peak to the G peak decreased significantly from 1.076 to 0.909, indicating that the material has a higher degree of graphitization and better conductivity. In addition, the G peak of the hard carbon material modified by supercritical fluid was significantly shifted to the left by 24.6 cm -1 The more the G peak shifts to the left, the larger the carbon interlayer spacing of the hard carbon material, indicating that the modified hard carbon material has a larger carbon interlayer spacing and more sodium storage sites.

[0088] Figure 4Comparison of high-resolution TEM images of the hard carbon negative electrode material of Comparative Example 1 (left) and Example 1 (right); Figure 4 TEM images of the hard carbon material after supercritical fluid treatment reveal clearer carbon lattice fringes, longer graphite crystallites, and a tendency for staggered interlayers rather than slight curvature as in the comparative example. The outer regions of the hard carbon are highly graphitized. Measurements of the interlayer spacings for the hard carbon materials in Comparative Example 1 and Example 1 show that they are 0.339 nm and 0.388 nm, respectively, demonstrating that supercritical fluid treatment can expand the interlayer spacing of hard carbon materials, increase sodium storage sites, and enhance the specific capacity of sodium-ion battery anodes.

[0089] Figure 5 、 Figure 6 The isotherm adsorption and desorption curve comparison diagrams of Comparative Example 1 and Example 1 are shown respectively. The specific surface areas of the materials of Comparative Example 1 and Example 1 are calculated to be 6.132 m 2 / g and 11.175 m 2 / g, the specific surface area of ​​the hard carbon material after supercritical fluid treatment is larger, which is consistent with the result obtained from TEM images that the interlayer spacing of hard carbon is enlarged after supercritical fluid treatment.

[0090] Figure 7 This is a comparison diagram of the differential distribution curves of the adsorbed mesopores of Comparative Example 1 and Example 1; by comparing the pore size distributions of the two hard carbon materials, it can be found that after supercritical treatment, the pore structure in the pore size range of 15-75 nm of the hard carbon material is significantly reduced, while the pore structure in the size of 5 nm is significantly increased, and some large pores on the surface or inside of the material are closed and converted into small pores of about 5 nm, indicating that supercritical fluid treatment has a great influence on the pore structure inside the hard carbon material. Combined with the electrochemical performance data, it is proved that the supercritical fluid optimization technology does have the effect of optimizing and regulating the pore structure of the material.

[0091] Figure 8 This is a comparison chart of the rate performance of the hard carbon negative electrode materials of Comparative Example 1 and Example 1 at current densities of 0.1, 0.2, 0.5, 1, 2, 5, and 10 C. The rate performance comparison chart shows that compared with the untreated hard carbon material, the rate performance of the hard carbon material treated with supercritical fluid technology at different current densities is greatly improved, the initial charge and discharge efficiency remains above 90%, and stable charge and discharge can be achieved at a current density of 10 C, indicating that with the assistance of supercritical fluid technology, the internal structure of the hard carbon material has been successfully changed, the pore structure has been optimized, a large number of sodium storage sites have been added to the hard carbon material, and an ultra-high reversible specific capacity has been exhibited.

[0092] Figure 9This figure compares the cycling performance of the hard carbon anode materials of Comparative Example 1 and Example 1 at a current density of 50 mA / g. Because the supercritical medium penetrates the pores of the hard carbon material and removes insoluble impurities within it, the supercritical fluid-optimized hard carbon material half-cell exhibits an initial capacity of up to 389.4 mAh / g. Furthermore, the carbon dioxide supercritical fluid treatment introduces oxygen groups onto the surface and within the hard carbon material and optimizes its internal pore structure, significantly enhancing its cycling performance. After 500 cycles, the supercritical fluid-treated hard carbon material half-cell maintained a capacity of 87.2%, an increase of nearly 28% compared to the untreated hard carbon material, demonstrating the significant effect of the supercritical fluid modification.

[0093] The hard carbon anode material for sodium-ion batteries, optimized using supercritical fluid technology, exhibits higher purity, larger carbon interlayer spacing, and an optimized pore structure. The hard carbon anode prepared from this material exhibits high initial coulombic efficiency and excellent cycling and rate performance. It holds broad application prospects in small mobile electronic devices, electric vehicles, solar power generation, and aerospace.

[0094] The above-described embodiments are only preferred solutions of the present invention and are not intended to limit the present invention in any form. Other variations and modifications are possible without exceeding the technical solutions described in the claims.

Claims

1. A method for preparing a hard carbon negative electrode material optimized and modified by supercritical fluid technology, characterized in that: The preparation method is based on a biomass hard carbon material as a matrix and a critical threshold gas as a supercritical medium. After the biomass hard carbon material reacts in the supercritical medium for a certain period of time, the pressure is reduced to obtain a hard carbon negative electrode material. The critical threshold gas is selected from at least one of carbon dioxide, sulfur hexafluoride, and nitrous oxide. The reaction conditions are: reaction gas pressure of 3.8-8 MPa, reaction temperature of 35-55°C, and reaction time of 3-12 h.

2. The method for preparing a hard carbon negative electrode material optimized and modified by supercritical fluid technology according to claim 1, characterized in that: The biomass hard carbon material is obtained by pre-treating a biomass hard carbon precursor; the biomass hard carbon precursor is selected from at least one of bamboo powder, straw, cellulose powder, coconut shell, mold silk, rice husk powder, fir powder, corn stalk, sugarcane bagasse, and starch.

3. The method for preparing a hard carbon negative electrode material optimized and modified by supercritical fluid technology according to claim 2, characterized in that: Pretreatment includes pickling, drying and high-temperature carbonization; During the pretreatment, the pickling solution is at least one of hydrochloric acid, sulfuric acid, phosphoric acid, and nitric acid, the concentration of the pickling solution is 0.5-2 mol / L, and the pickling time is 1-24 h; and / or, the drying temperature range is 50-100°C and the drying time is 2-12 h; And / or, the high temperature carbonization process is: heating rate 2-5 ° C / min, temperature 1000-1600 ° C, holding time 1-3 hours, and then annealing to room temperature at a cooling rate of 2-5 ° C / min; And / or, the high-temperature carbonization treatment is further followed by sieving, with the sieving screen having a mesh size of 300-800 meshes.

4. The method for preparing a hard carbon negative electrode material optimized and modified by supercritical fluid technology according to claim 1, characterized in that: When the critical threshold gas is carbon dioxide, the reaction temperature shall not be lower than the critical temperature of 31.1°C, and the reaction gas pressure shall not be lower than the critical pressure of 7.38 MPa; when the critical threshold gas is sulfur hexafluoride, the reaction temperature shall not be lower than the critical temperature of 45.6°C, and the reaction gas pressure shall not be lower than the critical pressure of 3.76 MPa; when the critical threshold gas is nitrous oxide, the reaction temperature shall not be lower than the critical temperature of 36.5°C, and the reaction gas pressure shall not be lower than the critical pressure of 7.26 MPa.

5. The method for preparing a hard carbon negative electrode material optimized and modified by supercritical fluid technology according to claim 4, characterized in that: When the critical threshold gas is carbon dioxide, the reaction gas pressure and reaction temperature are 7.5 MPa and 40 ℃; when the critical threshold gas is sulfur hexafluoride, the reaction gas pressure and reaction temperature are 4 MPa and 50 ℃; when the critical threshold gas is nitrous oxide, the reaction gas pressure and reaction temperature are 7.5 MPa and 40 ℃.

6. The method for preparing a hard carbon negative electrode material optimized and modified by supercritical fluid technology according to claim 1, characterized in that: The preparation method specifically comprises the following steps: (1) Pretreating a biomass hard carbon precursor to obtain a biomass hard carbon material; (2) placing the biomass hard carbon material in a critical threshold gas at a specific pressure, where the specific pressure is the supercritical pressure of the critical threshold gas; (3) The reaction system is heated to make the critical threshold gas reach a supercritical state. After a certain reaction time, it is naturally cooled and the pressure is reduced to normal pressure to obtain a hard carbon negative electrode material.

7. A hard carbon negative electrode material optimized and modified by supercritical fluid technology, prepared according to the preparation method according to any one of claims 1 to 6.

8. Application of the hard carbon negative electrode material optimized and modified by supercritical fluid technology according to claim 7 in the field of sodium ion batteries.

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