Hard carbon negative electrode material optimized and modified by supercritical fluid technology as well as preparation method and application of hard carbon negative electrode material

Abstract

The invention belongs to the technical field of sodium ion batteries, and relates to a hard carbon negative electrode material optimized and modified by a supercritical fluid technology as well as a preparation method and application of the hard carbon negative electrode material. According to the preparation method, a biomass hard carbon material is taken as a matrix, critical threshold gas is taken as a supercritical medium, the biomass hard carbon material reacts in the supercritical medium for a certain time, and the optimized and modified hard carbon negative electrode material is obtained. According to the invention, the hard carbon material is subjected to purification and impurity removal through a supercritical fluid technology, and the pore structure and the carbon layer spacing are adjusted, so that the hard carbon negative electrode material with higher purity and excellent electrochemical performance is prepared. The hard carbon negative electrode material prepared by the invention shows high specific capacity, good cycling stability and excellent rate capability and coulombic efficiency. Meanwhile, the preparation method provided by the invention is simple, rapid, efficient, convenient and easy to control, non-soluble impurities in the hard carbon material can be efficiently removed, the sodium storage structure of the hard carbon material can be improved, and the development of the hard carbon negative electrode material of the sodium-ion battery can be promoted.

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 the focus of global attention. It is extremely important to ensure the stable supply of energy and optimize energy utilization efficiency. 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, there are still many problems with hard carbon negative electrode materials 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 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 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 one hand, and 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 hard carbon materials is equally important. A good pore structure is conducive to the transmission and storage of sodium ions in hard carbon materials, alleviates the volume expansion of the material, reduces material stress, and improves material stability.

[0004] Therefore, impurity removal and adjustment of pore structure have become key research directions in the field of sodium ion batteries. For example, Taiyuan University of Technology 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 promoted the formation of open pores, which were then converted into closed pores during the high-temperature carbonization process. The hard carbon material finally prepared had a reversible specific capacity increase of 184.21 mAh / g compared with the sample without structural regulation, with 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 by acid-base pretreatment and template carbonization. The reversible capacity of the hard carbon material at 1 C rate was 303 mAh / g, and the cycle performance was good. The capacity retention rate after 100 cycles was 92.0%. Guangdong University of Technology used sodium hydroxide to selectively etch the lignin and hemicellulose components in bamboo fibers at low temperature, thereby increasing the relative content of cellulose in bamboo fibers. The increase in the cellulose content in bamboo fibers increased the order of hard carbon, which was beneficial to 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 increases to 0.26 cm 3 / g. Compared with bamboo prepared by direct carbonization without selective etching with sodium hydroxide, the platform capacity of the hard carbon material is increased by 88 mAh / g. In the above methods for preparing hard carbon materials, although the materials have achieved certain performance improvements, the impurity removal methods are all acid or alkali washing. This method still has certain limitations and cannot effectively remove a small number of insoluble impurities in the hard carbon material. The purity of the material still has room for improvement after acid and alkali treatment and the pore structure still needs to be further adjusted.

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

[0006] The purpose of the present invention is to provide a hard carbon negative electrode material optimized and modified by supercritical fluid technology and its preparation method and application, and to effectively prepare high-performance hard carbon negative electrode materials by regulating parameters such as the type of gas medium, gas pressure, supercritical temperature, reaction time, etc. 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 time, improve the pore structure of hard carbon, and expand the distance between carbon layers, so that the prepared hard carbon negative electrode material for sodium ion batteries 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: 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 and modified hard carbon negative electrode material.

[0008] By adopting the above technical solution, biomass hard carbon is used as the carbon source matrix, which has the huge advantages of wide 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, and its rich internal pore structure is also conducive to the supercritical fluid entering 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 a more special demand for supercritical fluid technology. At the same time, by reacting the hard carbon material in a supercritical fluid, the impurities such as inorganic salts or metal oxides in the hard carbon material are dissolved into the supercritical fluid, and the gas supercritical fluid enters the pores of the hard carbon material, so that the supercritical fluid technology treatment can effectively remove non-soluble impurities such as inorganic salts and metal oxides in the hard carbon material, improve the purity and quality of the material, and improve the conductivity and cycle stability of the material; the supercritical fluid contacts the hard carbon material and introduces some functional groups such as oxygen, nitrogen, fluorine, and sulfur, which can interact with the active hydroxyl, carboxyl and other oxygen groups on the hard carbon surface, further optimizing the surface structure of the hard carbon; in the supercritical decompression process, the supercritical medium is instantly converted from liquid to gas to generate strong stress, which can expand the spacing between the carbon layers of the hard carbon material, adjust the pore structure of the material, increase the sodium storage active sites of the hard carbon material, increase the sodium storage space, and thus improve the reversible specific capacity and cycle life of the material; under the action of supercritical technology, the pore structure of the hard carbon material is further regulated, accelerating the insertion and removal of sodium ions, improving the structural stability of the material and ensuring the high rate performance and good cycle performance of the material. In addition, in the process assisted by supercritical fluid technology, hard carbon materials react in a relatively mild environment (no higher than 100°C), with low energy consumption and low cost. Compared with ordinary modification methods, supercritical fluid technology can effectively reduce the reaction steps and time, thereby saving time and material costs and improving production efficiency.

[0009] 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 stalk, sugarcane bagasse, starch and other biomass hard carbon precursors. The present invention uses biomass hard carbon as a matrix, purifies and removes impurities, and optimizes the pore structure and carbon layer spacing. After carbonization, the biomass carbon material has oxygen-containing active groups on the surface and rich pore structures inside, which are conducive to reaction with supercritical gas medium and have the advantages of low cost and wide source of raw materials; More preferably, the pretreatment may be a conventional pretreatment, including pickling, drying and high-temperature carbonization; more preferably, after pickling and drying, the pretreatment is placed in a tube furnace, subjected to high-temperature carbonization in a protective atmosphere, and annealed to room temperature; 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; More preferably, the drying temperature ranges from 50 to 100 °C and the drying time is 2 to 12 h; more preferably, the drying temperature is 60 °C and the drying time is 8 h. A large amount of water is removed by drying to avoid the phenomenon of large amount of water evaporation during high temperature carbonization treatment leading to structural collapse; More preferably, the protective atmosphere is at least one of helium, nitrogen and argon; More preferably, the high temperature carbonization treatment process is: heating rate 2-5 ° C / min, temperature 1000-1600 ° C, holding time 1-3 h 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, the heating rate is 5 ° C / min, the temperature is 1200 ° C, the holding time is 2h, and the cooling rate is 5 ° C / min; More preferably, the high-temperature carbonization is further subjected to sieving treatment, and the sieving mesh number is preferably 300-800 mesh; more preferably 500 mesh.

[0010] Preferably, the critical threshold gas is selected from at least one of carbon dioxide, sulfur hexafluoride, and nitrous oxide as the supercritical fluid medium; more preferably, carbon dioxide. By adopting the technical solution, the critical threshold gas has the characteristics of small molecular weight, and it is easier to enter the pore structure of the biomass hard carbon material after becoming a supercritical fluid; at the same time, the critical threshold gas has functional groups such as oxygen, nitrogen, and fluorine, which can effectively modify the hard carbon material at the same time.

[0011] Preferably, the supercritical reaction conditions are: reaction pressure range of 3.8-8 MPa, reaction temperature of 35-55°C, reaction time of 3-12 h; more preferably, the reaction time is 6-10 h. Insufficient reaction time will lead to incomplete removal of impurities inside the hard carbon material and failure to fully adjust the pore structure. On the contrary, too long a reaction time may destroy the internal structure of the hard carbon material.

[0012] More preferably, the selection of reaction pressure and reaction temperature needs to be adjusted according to the conditions for different supercritical gas media to reach the supercritical state: when the critical threshold gas is carbon dioxide, the reaction temperature is not less than the critical temperature of 31.1 ° C, and the reaction pressure is not less than the critical pressure of 7.38 MPa; when the critical threshold gas is sulfur hexafluoride, the reaction temperature is not less than the critical temperature of 45.6 ° C, and the reaction pressure is not less than the critical pressure of 3.76 MPa; when the critical threshold gas is nitrous oxide, the reaction temperature is not less than the critical temperature of 36.5 ° C, and the reaction pressure is not less than the critical pressure of 7.26 MPa. Among them, the more optimal reaction pressure and reaction temperature of carbon dioxide are 7.5 MPa and 40 ° C, the more optimal reaction pressure and reaction temperature of sulfur hexafluoride are 4 MPa and 50 ° C, and the more optimal reaction pressure and reaction temperature of nitrous oxide are 7.5 MPa and 40 ° C. Too low reaction pressure and reaction temperature may cause the critical threshold gas to fail to reach the supercritical state, and too high 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 leaving it to stand.

[0013] Preferably, 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; (3) The reaction system is heated to make the gas medium reach a supercritical state. 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.

[0014] More preferably, in step (2), the biomass hard carbon material is placed in a sealed supercritical high-pressure reactor, and after being evacuated to a vacuum state, a critical threshold gas is introduced into the supercritical device until a certain specific pressure is reached. More preferably, the gas flow rate of 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.

[0015] More preferably, in step (3), the pressure is reduced after cooling to room temperature, thereby further ensuring safety. More preferably, the rapid pressure reduction is achieved by quickly opening the gas valve to release the gas to normal pressure.

[0016] Preferably, the room temperature of the present invention is 15-40°C.

[0017] The present invention also provides a high-performance hard carbon negative electrode material prepared by any of the above preparation methods after being optimized and modified by supercritical fluid technology.

[0018] The present invention also provides an application of a high-performance hard carbon negative electrode material prepared by any of the above-mentioned preparation methods after optimization and modification using supercritical fluid technology in the field of sodium ion batteries.

[0019] The present invention provides a hard carbon negative electrode material optimized and modified by supercritical fluid technology, and a preparation method and application thereof. So far, there are very few reports on the optimization and modification of hard carbon negative electrode materials by supercritical fluid technology, especially for the improvement of the purity of hard carbon materials and the adjustment of the structure, as well as the application of hard carbon negative electrodes in sodium ion batteries. The present invention controls the conditions such as air pressure and temperature to transform the gas medium into a supercritical fluid state and penetrate into the interior of the biomass hard carbon material rich in mesopores and micropores, and effectively modifies the hard carbon material by supercritical fluid technology. Supercritical fluid technology can effectively remove insoluble impurities such as inorganic salts and metal oxides in the structure of hard carbon materials, reduce the influence of impurities on the charge and discharge reaction of the battery, and thus extend the service life of the battery. Supercritical fluid technology can adjust the pore structure of hard carbon, accelerate the transmission of sodium ions, and optimize the battery rate performance. Supercritical fluid technology can also introduce some functional groups to interact with the oxygen-containing groups on the surface of hard carbon materials to further optimize the surface structure of hard carbon. In addition, supercritical fluid technology can also expand the carbon layer spacing of hard carbon materials, add more sodium storage sites, and improve the specific capacity of materials. Therefore, the hard carbon negative electrode material prepared by supercritical fluid technology shows higher capacity, more superior rate performance and coulombic efficiency.

[0020] Compared with the prior art, the present invention has the following beneficial effects: (1) The present invention uses supercritical fluid technology to treat biomass hard carbon materials, and uses the strong dissolving ability of gas medium in a supercritical state to further purify and remove impurities, which 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 first coulomb efficiency and cycle performance; (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 hydroxyl, carboxyl and other oxygen groups on the surface of hard carbon, further optimizing the surface structure of hard carbon; (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.

[0021] Therefore, the hard carbon negative electrode material of the present invention exhibits good cycle stability, rate performance and coulombic efficiency, and has excellent electrochemical performance. At the same time, the preparation method of the hard carbon negative electrode material of the present invention is simple, fast, efficient, convenient, easy to control, and has a wide range of raw material sources and low cost. BRIEF DESCRIPTION OF THE DRAWINGS

[0022] Figure 1 This is a comparison diagram of the XRD spectra of the hard carbon materials of Comparative Example 1 and Example 1; Figure 2 This is a SEM morphology comparison diagram of the hard carbon materials of Comparative Example 1 and Example 1; Figure 3 This is a comparison diagram of the Raman spectra of the hard carbon negative electrode materials of Comparative Example 1 and Example 1; Figure 4 This is a comparison of high-resolution TEM images of the hard carbon materials of Comparative Example 1 and Example 1; Figure 5 It is a comparison diagram of the isotherm adsorption curves of Comparative Example 1 and Example 1; Figure 6 It is a comparison diagram of the isotherm desorption curves of Comparative Example 1 and Example 1; Figure 7 It 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; 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; Fig. 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

[0023] 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 described clearly, completely and in detail 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 explaining and interpreting 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.

[0024] 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 or 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 sources or prepared by conventional methods unless otherwise specified. The reaction conditions embodied in the content of the invention of the present invention can achieve the reaction and obtain the product of the expected effect. Due to space limitations, some embodiments are listed below to further illustrate the advantages of the technical solution of the present invention.

[0025] Example 1

[0026] The bamboo powder was pickled with 1 mol / L hydrochloric acid for 6 h and dried at 60 °C for 8 h. Then, it was heated to 1200 °C for 2 h in an argon atmosphere at a heating rate of 5 °C / min for high-temperature carbonization. Then, it was annealed to room temperature at a cooling rate of 5 °C / min. Finally, it was sieved through a 500-mesh net to obtain the biomass hard carbon material. The biomass hard carbon material was placed in a sealed supercritical high-pressure reactor, which can be a common high-pressure resistant reaction device such as a high-pressure reactor or a high-pressure resistant ball mill, and evacuated to a vacuum state. Carbon dioxide gas was introduced into the supercritical device at a gas rate of 40 sccm to make the gas pressure reach 7.5 MPa. Then, the supercritical device after the introduction of carbon dioxide was transferred to a 40 °C oven and allowed to stand to allow the gas medium to reach a supercritical state. After 6 h of reaction, it was naturally cooled, and the gas valve was quickly opened to obtain a high-purity hard carbon negative electrode material optimized by supercritical fluid technology.

[0027] Example 2-11

[0028] 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. are changed. The specific conditions are shown in the following table: Table 1 Summary of reaction conditions of various examples

[0029] Comparative Example 1

[0030] 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 at 1200 ℃ in an argon atmosphere at a heating rate of 5 ℃ / min and kept for 2 h. It was then annealed to room temperature at a cooling rate of 5 ℃ / min and finally sieved through a 500-mesh net to obtain the hard carbon material.

[0031] Comparative Example 2

[0032] The hard carbon material in Comparative Example 1 was placed in a sealed supercritical high-pressure reactor and evacuated to a vacuum state, and carbon dioxide gas was introduced into the supercritical device at a gas rate of 40 sccm to make the gas pressure reach 7.0 MPa. The supercritical device after the introduction of carbon dioxide was then transferred to a 40°C oven for standing, kept warm for 6 hours, and then cooled naturally, and the gas valve was quickly opened to obtain a hard carbon negative electrode material.

[0033] Comparative Example 3

[0034] The hard carbon material in Comparative Example 1 was placed in a sealed supercritical high-pressure reactor and evacuated to a vacuum state, and carbon dioxide gas was introduced into the supercritical device at a gas rate of 40 sccm to make the gas pressure reach 7.5 MPa. The supercritical device after the introduction of carbon dioxide was then transferred to a 30°C oven for standing, kept warm for 6 hours, and then cooled naturally, and the gas valve was quickly opened to obtain a hard carbon negative electrode material.

[0035] Comparative Example 4

[0036] The hard carbon material in Comparative Example 1 was placed in a sealed supercritical high-pressure reactor and evacuated to a vacuum state, and carbon dioxide gas was introduced into the supercritical device at a gas rate of 40 sccm to make the gas pressure reach 7.5 MPa. The supercritical device after the introduction of carbon dioxide was then transferred to a 40°C oven for standing, kept warm for 1 hour, and then cooled naturally, and the gas valve was quickly opened to obtain a hard carbon negative electrode material.

[0037] Comparative Example 5

[0038] The hard carbon material in Comparative Example 1 was placed in a sealed supercritical high-pressure reactor and evacuated to a vacuum state, and carbon dioxide gas was introduced into the supercritical device at a gas rate of 40 sccm to make the gas pressure reach 7.5 MPa. The supercritical device after the introduction of carbon dioxide was then transferred to a 40°C oven for standing, kept warm for 15 hours, and then cooled naturally, and the gas valve was quickly opened to obtain a hard carbon negative electrode material.

[0039] Comparative Example 6

[0040] The hard carbon material in Comparative Example 1 was placed in a sealed supercritical high-pressure reactor and evacuated to a vacuum state. Ethylene gas was introduced into the supercritical device at a gas rate of 40 sccm to make the gas pressure reach 5.5 MPa. The supercritical device after the ethylene gas was then transferred to a 20°C oven and allowed to stand until the supercritical state was reached. After being kept warm for 6 hours, it was naturally cooled and the gas valve was quickly opened to obtain a hard carbon negative electrode material.

[0041] Comparative Example 7

[0042] The hard carbon material in Comparative Example 1 was placed in a sealed supercritical high-pressure reactor and evacuated to a vacuum state. Propane gas was introduced into the supercritical device at a gas rate of 40 sccm to make the gas pressure reach 4.5 MPa. The supercritical device after the propane gas was then transferred to a 100°C oven and allowed to stand until the supercritical state was reached. After being kept warm for 6 hours, it was naturally cooled, and the gas valve was quickly opened to obtain a hard carbon negative electrode material.

[0043] Comparative Example 8

[0044] The epoxy resin was heated to 1200 °C at a heating rate of 5 °C / min and kept at this temperature for 2 h under an argon atmosphere for high-temperature carbonization treatment, and then annealed to room temperature at a cooling rate of 5 °C / min, and finally sieved through a 500-mesh screen to obtain a hard carbon material. The sintered hard carbon material was placed in a sealed supercritical high-pressure reactor and evacuated to a vacuum state. Carbon dioxide gas was introduced into the supercritical device at a gas rate of 40 sccm to make the gas pressure reach 7.5 MPa. Subsequently, the supercritical device after the introduction of carbon dioxide was transferred to a 40 °C oven and allowed to stand to allow the gas medium to reach a supercritical state. After the reaction for 6 h, it was naturally cooled, and the gas valve was quickly opened to obtain a hard carbon negative electrode material.

[0045] Comparative Example 9

[0046] Polyacrylonitrile was heated to 1200 °C at a heating rate of 5 °C / min and kept at this temperature for 2 h in an argon atmosphere for high-temperature carbonization treatment, and then annealed to room temperature at a cooling rate of 5 °C / min, and finally sieved through a 500-mesh screen to obtain a hard carbon material. The sintered hard carbon material was placed in a sealed supercritical high-pressure reactor and evacuated to a vacuum state. Carbon dioxide gas was introduced into the supercritical device at a gas rate of 40 sccm to make the gas pressure reach 7.5 MPa. Subsequently, the supercritical device after the introduction of carbon dioxide was transferred to a 40 °C oven and allowed to stand to allow the gas medium to reach a supercritical state. After the reaction for 6 h, it was naturally cooled, and the gas valve was quickly opened to obtain a hard carbon negative electrode material.

[0047] Comparative Example 10

[0048] The 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 introduced into the supercritical device at a gas rate of 40 sccm to make the gas pressure reach 7.5 MPa. The supercritical device after the introduction of carbon dioxide was then transferred to a 40°C oven and allowed to stand to allow the gas medium to reach a supercritical state. After reacting for 6 hours, it was naturally cooled and the gas valve was quickly opened to obtain a hard carbon negative electrode material.

[0049] Performance Testing

[0050] The hard carbon negative electrode materials prepared in the above Examples 1-11 and Comparative Examples 1-10 were assembled into button-type half-cells for electrochemical testing. The electrolyte was 1 mol / L NaPF 6 in DME=100 vol% (DME: ethylene glycol dimethyl ether), the diaphragm is a glass fiber diaphragm. The battery is assembled in the order of positive electrode shell, hard carbon negative electrode sheet, electrolyte, diaphragm, sodium sheet, and negative electrode shell and sealed with a packaging machine. The hard carbon negative electrode sheet is made of hard carbon negative electrode material, conductive carbon black, and sodium carboxymethyl cellulose in a mass ratio of 8:1:1. An appropriate amount of deionized water is added and stirred by a homogenizer for 30 min to form a slurry. The slurry is then evenly coated on a bright aluminum foil and vacuum dried at 80 °C for 12 h. Finally, the electrode sheet is cut into a circular electrode with a diameter of 12 mm. For each electrode, the loading amount of active material is 0.8-1.2 mg. After the battery was left to stand for 24 hours, electrochemical tests were performed using the Xinwei test system and Chenhua electrochemical workstation.

[0051] The electrochemical tests were all carried out at a constant temperature of 30 °C, mainly constant current charge and discharge tests. In the constant current charge and discharge test, the main indicators include reversible capacity, rate performance, coulomb efficiency, cycle life and so on. 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 process of constant current charge and discharge test is: standby for 5 min-constant current discharge-standby for 5 min-constant current charge, and 5 cycles for each different current density. The long cycle performance of the battery was tested at a current density of 50 mA / g. The process of constant current charge and discharge test is: standby for 5 min-constant current discharge-standby for 5 min-constant current charge, and 500 cycles.

[0052] The test results of rate performance and cycle performance are shown in Table 2 and Table 3 respectively: Table 2 Summary of rate performance test results of various embodiments and comparative examples

[0053] Table 3 Summary of the cycle performance test results of each embodiment and comparative example

[0054] 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 show a high specific capacity of more than 384 mAh / g and a first coulombic efficiency of more than 86% at a current density of 0.1 C. In addition, the above-mentioned materials not only maintain a high reversible specific capacity at small rates of 0.1, 0.2, and 0.5 C, but also can still be reversibly charged and discharged at a high rate of 10 C, and the rate performance is very excellent. On the contrary, the hard carbon negative electrode materials in Comparative Examples 1-10 have a low initial capacity of less than 355 mAh / g and a first coulombic efficiency of no more than 85% at a small rate current density of 0.1 C, and the reversible specific capacity at 1 and 10 C rates is much lower than that of Examples 1-11, which shows that supercritical fluid technology is very effective for modifying the rate performance of hard carbon negative electrode materials.

[0055] From the cycle performance test results of each embodiment and comparative example in Table 3, it can be found that the initial discharge capacity of the hard carbon negative electrode material optimized by the supercritical fluid technology described in the present invention at a current density of 50 mA / g in Examples 1-11 is higher than that of the comparative example by 30-40 mAh / g. The initial discharge capacity of the graphite negative electrode material optimized by the supercritical fluid technology in Comparative Example 10 is even only 263.3 mAh / g. On the one hand, it is because the interlayer spacing of the graphite material is small and not suitable for the sodium ion battery system. On the other hand, the graphite material lacks the unique pore structure and some functional functional groups of the biomass material. In addition, the capacity retention rate of the hard carbon negative electrode material half-cell optimized by the supercritical fluid technology is not less than 85% after 500 cycles. Compared with the lower capacity retention rate of 27.8-71.5% of the hard carbon negative electrode material half-cell of Comparative Examples 1-10, the modification effect is significant.

[0056] Figure 1 is a comparison diagram of the XRD spectra 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) graphene crystal plane and (100) sp 2Hybrid hexagonal carbon crystal plane, the characteristic peaks of the two materials are very obvious and roughly the same, indicating that the hard carbon material has not been destroyed in the crystal structure due to the supercritical fluid treatment. In particular, the double diffraction angles of the (002) and (100) crystal planes of the hard carbon material in Example 1 are both smaller than those of the hard carbon material in Comparative Example 1. From the Bragg equation 2dsinθ=nλ, it can be seen that the lattice spacing of the hard carbon material treated with supercritical fluid technology is larger, indicating 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.

[0057] Figure 2 The SEM morphology comparison diagram of the hard carbon negative electrode material of Comparative Example 1 (left) and Example 1 (right); 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.

[0058] At the same time, Table 4 is a test table of the mass fraction of metal impurity elements of the hard carbon negative electrode materials of Comparative Example 1 and Example 1 by ICP-OES; from the table, it can be found that the Ca, Si, K, Na, Al, and Fe element contents of the hard carbon materials after treatment with supercritical fluid technology decrease by about 90%, 28%, 85%, 80%, 47%, and 48%, respectively, indicating that the strong solubility of the supercritical fluid can effectively dissolve impurities such as inorganic salts and metal oxides that still remain after sintering of the hard carbon material.

[0059] Table 4 Impurity element contents of hard carbon materials in Comparative Example 1 and Example 1

[0060] Figure 3 is a comparison diagram 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 negative electrode material treated with supercritical fluid technology found that the intensity ratio of the D peak to the G peak was significantly reduced 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 is shifted to the left, the larger the carbon layer spacing of the hard carbon material is, indicating that the modified hard carbon material has a larger carbon layer spacing and more sodium storage sites.

[0061] Figure 4 Comparison of high-resolution TEM images of the hard carbon negative electrode material of Comparative Example 1 (left) and Example 1 (right); Figure 4 The TEM image shows that the carbon lattice stripes of the hard carbon material treated with supercritical fluid are clearer, the graphite crystallites have a longer length, the graphite layers tend to be staggered rather than slightly curved in the comparative example, and the outer layer of the hard carbon has a high degree of graphitization. The carbon interlayer spacing of the hard carbon material of comparative example 1 and example 1 was measured to be 0.339 nm and 0.388 nm, respectively, proving that supercritical fluid treatment can expand the carbon interlayer spacing of the hard carbon material, increase the sodium storage sites, and improve the specific capacity of the negative electrode of the sodium ion battery.

[0062] Figure 5 , Figure 6 They are the isotherm adsorption and desorption curve comparison diagrams of Comparative Example 1 and Example 1; 2 The specific surface areas of the materials in Comparative Example 1 and Example 1 were calculated to be 6.132 m 2 / g and 11.175 m 2 / g, the hard carbon material treated with supercritical fluid has a larger specific surface area, which is consistent with the result obtained from TEM image that the interlayer spacing of hard carbon is enlarged after supercritical fluid treatment.

[0063] Figure 7 It is a comparison diagram of the differential distribution curves of the adsorbed mesopores between 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 an optimization and regulation effect on the pore structure of the material.

[0064] Figure 8 This is a rate performance comparison chart 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. From the rate performance comparison chart, it can be found 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 first charge and discharge efficiency is maintained at more than 90%, and it can be stably charged and discharged 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.

[0065] Fig. 9This 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; because the supercritical medium penetrates into the pores of the hard carbon material and removes the insoluble impurities inside the hard carbon material, the hard carbon material half-cell optimized by the supercritical fluid shows an initial capacity of up to 389.4 mAh / g. In addition, the carbon dioxide supercritical fluid treatment introduces oxygen groups on the surface and inside of the hard carbon material and optimizes the pore structure inside the hard carbon material, which greatly improves the cycle performance of the hard carbon material. After 500 cycles, the capacity retention rate of the hard carbon material half-cell treated with the supercritical fluid is as high as 87.2%, which is nearly 28% higher than that of the hard carbon material that has not been treated with the supercritical fluid. The supercritical fluid modification effect is significant.

[0066] The hard carbon negative electrode material of sodium ion battery after optimization by supercritical fluid technology has higher material purity, larger carbon layer spacing and optimized pore structure. The hard carbon negative electrode prepared by this material has higher first coulomb efficiency and excellent cycle performance and rate performance. It has broad application prospects in small mobile electronic devices, electric vehicles, solar power generation and aerospace.

[0067] 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 uses biomass hard carbon material as a matrix and 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.

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, 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; In 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 ranges from 50 to 100 °C and the drying time ranges from 2 to 12 h; And / or, the high temperature carbonization treatment process is: heating rate 2-5 °C / min, temperature 1000-1600 °C, holding time 1-3h, and then annealing to room temperature at a cooling rate of 2-5 °C / min; And / or, after the high-temperature carbonization treatment, the material is further sieved, and the mesh number of the sieve is 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: The critical threshold gas is selected from at least one of carbon dioxide, sulfur hexafluoride, and nitrous oxide.

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: The reaction conditions are: reaction pressure of 3.8-8 MPa, reaction temperature of 35-55°C, and reaction time of 3-12h.

6. The method for preparing a hard carbon negative electrode material optimized and modified by supercritical fluid technology according to claim 5, 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.

7. The method for preparing a hard carbon negative electrode material optimized and modified by supercritical fluid technology according to claim 6, 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 ℃.

8. 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, wherein the specific pressure is a 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, the system is naturally cooled and the pressure is reduced to normal pressure to obtain a hard carbon negative electrode material.

9. 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 8.

10. Application of the hard carbon negative electrode material optimized and modified by supercritical fluid technology as claimed in claim 9 in the field of sodium ion batteries.

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