Hard carbon, preparation method thereof, negative electrode sheet and secondary battery

Abstract

The application provides a kind of hard carbon and its preparation method, negative pole piece and secondary battery, belong to battery negative material technical field, wherein, the preparation method of the hard carbon includes: vacuum drying treatment is carried out to plant cellulose, and nanofiber precursor is obtained;The nanofiber precursor and the plant cellulose without vacuum drying treatment are mixed to obtain a mixed fiber material;The mixed fiber material is subjected to tabletting treatment to obtain a fiber sheet;The fiber sheet is subjected to calcination treatment to obtain the hard carbon.The application is beneficial to obtain raw material source, low cost, the purity of the product prepared is high, and the electrochemical performance of the hard carbon material is excellent, and the electrochemical performance of the secondary battery prepared by using the hard carbon material is excellent.

Description

TECHNICAL FIELD

[0001] The present application relates to the technical field of battery negative electrode materials, and particularly relates to a hard carbon, a preparation method thereof, a negative electrode sheet and a secondary battery. BACKGROUND

[0002] The working principle of a sodium ion battery is similar to that of a lithium ion battery, but due to the fact that the radius of sodium ions (0.102 nm) is much larger than that of lithium ions (0.076 nm), sodium ions are more sluggish than lithium ions in dynamics, and the energy density is lower, so the commercialization is slow. Sodium metal and lithium metal both belong to alkali metal elements, and they have similar chemical properties and applications in many aspects. Compared with lithium metal, sodium metal is abundant in the earth's crust, easy to exploit and use, and the price is also relatively low, so sodium ion batteries have the potential to become a substitute for lithium ion batteries.

[0003] The application of a sodium ion battery is closely related to the positive and negative electrode materials of the battery, and the performance of the positive and negative electrode materials directly affects the performance of the battery. At present, hard carbon material is a good sodium ion battery negative electrode material with good application prospect. The hard carbon material has the advantages of large interlayer spacing, small sodium insertion expansion coefficient, good sodium ion diffusion, good compatibility with electrolyte, long cycle life, good rate performance and the like. However, the hard carbon prepared by the existing preparation method has the disadvantages of low charge specific capacity, low first cycle coulombic efficiency and poor rate performance when used as a sodium ion battery negative electrode material, which seriously limits the development of sodium ion battery negative electrode materials.

[0004] Therefore, there is an urgent need for a hard carbon, a preparation method thereof, a negative electrode sheet and a secondary battery to solve the above problems. SUMMARY

[0005] In view of the technical problems in the background art, the present application provides a hard carbon, a preparation method thereof, a negative electrode sheet and a secondary battery, aiming to solve the technical problem that the prepared sodium ion battery has poor electrochemical performance when the hard carbon prepared by the existing preparation method is used as a sodium ion battery negative electrode material.

[0006] In a first aspect, the present application provides a preparation method of a hard carbon, comprising the following steps:

[0007] vacuum drying the plant cellulose to obtain a nanofiber precursor;

[0008] mixing the nanofiber precursor and the plant cellulose which has not been subjected to vacuum drying treatment to obtain a mixed fiber material;

[0009] tabletting the mixed fiber material to obtain a fiber sheet;

[0010] The fiber sheet is calcined to prepare hard carbon.

[0011] In the technical scheme of the embodiment, the moisture in the plant cellulose is removed by heating and drying the plant cellulose; then, the mixed fiber material formed by the nanofiber precursor and the plant cellulose without vacuum drying treatment is pressed to form a fiber sheet with a sheet structure. Compared with the case where the mixed fiber material is not pressed, the gas released in the subsequent heating and calcination process passes through the sheet structure, so that more pores are formed on the surface of the fiber. On the one hand, the specific surface area of the material is increased, so that the electrolyte is more fully infiltrated. On the other hand, a large-pore mesopore coupled pore structure can be formed, which can serve as a sodium ion buffer pool to ultimately improve the rate performance of the material. Furthermore, the increased pores can fill more sodium ions, thereby improving the capacity of the platform region of the material and ultimately improving the electrochemical performance of the material. In addition, the hard carbon prepared by heating and calcining the fiber sheet has high purity and high compaction density (carbon content ≥ 74.93%, compaction density ≥ 1.01 g / cm 3 ), and the battery prepared from the hard carbon has excellent specific capacity, coulombic efficiency and rate performance.

[0012] In some embodiments, before the step of vacuum drying the plant cellulose to obtain the nanofiber precursor, the method further comprises: sequentially washing the plant cellulose with water and an organic solvent, wherein the organic solvent is ethanol or acetone.

[0013] In the embodiment, washing the plant cellulose with water and an organic solvent can remove impurities on the surface of the plant cellulose.

[0014] In some embodiments, in the step of vacuum drying the plant cellulose to obtain the nanofiber precursor, the plant cellulose is any one of cotton, bamboo, wheat straw and sponge gourd pulp, the temperature for vacuum drying is 80-120°C, and the time for vacuum drying is 10-24h.

[0015] In the embodiment, the selected plant cellulose has a wide source and low cost, which is conducive to mass production in industry. In addition, the heating and drying of the plant cellulose can effectively remove the residual washing solvent on the surface of the plant cellulose.

[0016] In some embodiments, the nanofiber precursor and the plant cellulose without vacuum drying treatment are mixed to form a mixed fiber material at a mass ratio of (5-10):1.

[0017] In this embodiment, the addition of plant cellulose without vacuum drying treatment in the nanofiber precursor can easily form fiber flakes during the subsequent tabletting process.

[0018] In some embodiments, during the step of tabletting the mixed fiber material, the pressure range of the tabletting is 29.5-30 tons, and the pressure time is greater than or equal to 60 s.

[0019] In this embodiment, there is more air between the mixed fiber materials, which occupies more space. After the tabletting process, the air between the fibers is discharged, and the fibers are rearranged, so the mixed fiber material occupies less space. Specifically, when the pressure range is 9.5-30 tons and the pressure time is greater than or equal to 60 s, a better tabletting effect can be achieved, and the fiber flakes formed by tabletting are not easily broken when moving.

[0020] In some embodiments, the step of calcining the fiber flakes to prepare hard carbon includes:

[0021] In an inert gas atmosphere, the fiber flakes are subjected to a first calcination treatment at 300-400°C, and then heated to 1200-1600°C for a second calcination treatment to prepare hard carbon; wherein the time of the first calcination treatment and the time of the second calcination treatment are both 2-3 h.

[0022] In this embodiment, the hard carbon prepared by the first and second calcination treatments of the fiber flakes has high purity and high compaction density. Through experiments, it is found that the fiber flakes begin to accelerate decomposition at 300-400°C, and a large amount of gas and small molecular organic matter is generated during the accelerated decomposition process, which can easily lead to instability of the prepared hard carbon material. If the above gas and small molecular organic matter cannot escape in time, it will form pressure inside, resulting in the formation of pore structures. Based on the above experimental findings, the first calcination treatment of the fiber flakes is set to calcine at 300-400°C for 2-3 h, so that the fiber flakes are in the initial accelerated decomposition state, and the decomposition rate is relatively low, which can improve the stability and internal density of the hard carbon material during the initial preparation. Subsequently, the second calcination treatment of the fiber flakes is set to calcine at 1200-1600°C for 2-3 h, so that the fiber flakes maintain a faster decomposition rate at a higher temperature, thereby forming a hard carbon material with more pore structures, and further forming a sodium ion buffer pool on the surface of the hard carbon material, thereby improving the rate performance of the prepared hard carbon material. That is, the above two-stage sintering treatment can simultaneously improve the stability, compaction density, and electrical properties of the prepared hard carbon material.

[0023] In a second aspect, the embodiments of the present application provide a hard carbon prepared by the method for preparing hard carbon according to any one of the above, wherein the hard carbon has a charge specific capacity of ≥285.9 mAh / g at 0.1C; and / or a discharge specific capacity of ≥321.3 mAh / g at 0.1C; and / or a capacity retention rate of ≥90% after 100 cycles at 1C; and / or a first-cycle coulombic efficiency of ≥84.7%.

[0024] In some embodiments, the hard carbon has a carbon content of ≥74.93%, a compaction density of ≥1.01 g / cm 3 .

[0025] In this embodiment, the hard carbon is prepared by the method described above, and thus has the advantages of high purity and large compaction density.

[0026] In a third aspect, the embodiments of the present application provide a negative electrode sheet mainly prepared from the hard carbon prepared by the method for preparing hard carbon according to any one of the above or prepared from the hard carbon prepared according to the above.

[0027] In this embodiment, the negative electrode sheet contains the hard carbon material described above, and thus has the advantages of excellent charge specific capacity, first-cycle coulombic efficiency and rate performance.

[0028] In a fourth aspect, the embodiments of the present application provide a secondary battery comprising the negative electrode sheet described above.

[0029] In this embodiment, the secondary battery contains the negative electrode sheet described above, and thus has the advantages of excellent charge specific capacity, first-cycle coulombic efficiency and rate performance.

[0030] The above description is only a summary of the technical solutions of the present application, in order to enable the technical means of the present application to be implemented according to the content of the description, and in order to make the above and other purposes, features and advantages of the present application more obvious and easy to understand, the following specific embodiments of the present application are described. BRIEF DESCRIPTION OF DRAWINGS

[0031] In order to more clearly illustrate the technical solutions of the present application, the following will briefly introduce the drawings used in the present application. Obviously, the drawings described below are only some embodiments of the present application, and other drawings can also be obtained according to these drawings without creative labor for those skilled in the art.

[0032] Figure 1 The process flow diagram of the method for preparing hard carbon provided in the embodiments of the present application. DETAILED DESCRIPTION

[0033] The embodiments of the technical solutions of the present application will be described in detail below with reference to the drawings. The following embodiments are only used to more clearly illustrate the technical solutions of the present application, and therefore only serve as examples, but cannot limit the protection scope of the present application.

[0034] Unless otherwise defined, all technical and scientific terms used herein have the same meaning as commonly understood by one of ordinary skill in the art to which this application belongs; the terminology used herein is for the purpose of describing particular embodiments only and is not intended to be limiting of the present application; the terms "comprising" and "having," and any variations thereof, as used in the specification and claims and the aforementioned description of the drawings, are intended to cover not exclusive inclusions.

[0035] Reference herein to "an embodiment" means that a particular feature, structure, or characteristic described in connection with the embodiment can be included in at least one embodiment of the application. The appearances of the phrase in various places in the specification are not necessarily all referring to the same embodiment, nor are they necessarily mutually exclusive of one another. It is expressly understood that the embodiments described herein are merely examples from a whole class of embodiments that can be claimed as falling within the scope of the application.

[0036] In the description of the embodiments of the present application, the term "and / or" is only a description of the association relationship of the associated objects, which means that there can be three relationships, for example, A and / or B, which can represent the existence of A alone, the existence of A and B at the same time, and the existence of B alone. In addition, the character " / " in this paper generally represents that the front and rear associated objects are a "or" relationship.

[0037] At present, the hard carbon materials obtained by the existing preparation methods of hard carbon are generally resin carbon, organic polymer pyrolysis carbon and carbon black, etc. When the above hard carbon is used as a negative electrode material of a sodium ion battery, there are disadvantages such as low charging specific capacity, low first cycle coulombic efficiency and poor rate performance, which seriously limit the development of the negative electrode material of the sodium ion battery.

[0038] In order to solve the technical problems of the existing hard carbon used as a negative electrode material of a sodium ion battery, such as low charging specific capacity, low first cycle coulombic efficiency and poor rate performance, the present application provides a hard carbon and a preparation method thereof, a negative electrode sheet and a secondary battery, wherein the above preparation method of the hard carbon is used, so that a hard carbon material with wide raw material sources, low cost, high product purity and excellent electrochemical performance can be obtained, and the charging specific capacity, the first cycle coulombic efficiency and the rate performance of the negative electrode sheet and the secondary battery prepared by using the hard carbon material are also improved.

[0039] The secondary battery provided by the embodiments of the present application can provide power and power supply for mobile phones, tablets, notebook computers, electric toys, electric tools, electric vehicles, electric cars, ships, spacecraft, etc. The electric toys can include fixed or mobile electric toys, such as game consoles, electric car toys, electric ship toys, and electric plane toys. The spacecraft can include airplanes, rockets, space shuttles, and spaceships.

[0040] Please refer to Figure 1 In a first aspect, the embodiments of the present application provide a preparation method of hard carbon, comprising the following steps:

[0041] S10, vacuum drying treatment is performed on the plant cellulose to obtain a nanofiber precursor;

[0042] S20, the nanofiber precursor and the plant cellulose without vacuum drying treatment are mixed to obtain a mixed fiber material;

[0043] S30, tabletting treatment is performed on the mixed fiber material to obtain a fiber sheet;

[0044] S40, calcination treatment is performed on the fiber sheet to obtain the hard carbon.

[0045] In the technical solution of the embodiments of the present application, the water in the plant cellulose is removed by heating and drying treatment on the plant cellulose. Then, tabletting treatment is performed on the mixed fiber material formed by the nanofiber precursor and the plant cellulose without vacuum drying treatment, so that the mixed fiber material forms a fiber sheet with a sheet structure. Compared with the way of not performing tabletting treatment on the mixed fiber material, the gas released in the subsequent heating and calcination treatment process passes through the sheet structure in the present application by tabletting, so that more pores are formed on the surface of the fiber, thereby improving the electrochemical performance of the material. Finally, the hard carbon obtained by heating and calcination treatment on the fiber sheet has high purity and high compaction density, and the secondary battery prepared by the above hard carbon has excellent specific charge capacity, first circle coulomb efficiency, and rate performance.

[0046] In some embodiments, the plant cellulose is a kind of organic compound, and its chemical formula is (C6H 10 O5)n, which is a polysaccharide composed of hundreds to thousands of linear chains of β(1→4) linked D-glucose units (glycosidic bonds), including kapok, flax, bamboo, wheat straw, loofah, sugarcane, and cotton, preferably cotton.

[0047] In some embodiments, before the step of vacuum drying treatment on the plant cellulose to obtain a nanofiber precursor, the method further comprises: sequentially washing the plant cellulose with water and an organic solvent, wherein the organic solvent is ethanol or acetone.

[0048] The water and the organic solvent can remove plant gum impurities or other impurities in the plant cellulose, and then solid-liquid separation is performed to obtain the cleaned plant cellulose.

[0049] Further, the plant cellulose is any one of cotton, bamboo, wheat straw, and sponge gourd pulp; the temperature of vacuum drying is 80-120 DEG C; and the time of vacuum drying is 10-24 h; the residual washing solvent on the surface of the plant cellulose can be effectively removed within the temperature range and the heating time range.

[0050] The application utilizes the characteristics that the plant cellulose is almost insoluble in water and ethanol, and ensures the purity of the hard carbon product.

[0051] In some embodiments, the nanofiber precursor is mixed with the plant cellulose at a mass ratio of (5-10):1 to form a mixed fiber material.

[0052] In this embodiment, the plant cellulose without vacuum drying treatment is added to the nanofiber precursor, so that the fiber sheet can be easily formed in the subsequent tabletting process.

[0053] In some embodiments, in the step of tabletting the mixed fiber material, the pressure range of the tabletting is 9.5-30 tons, and the pressure time is greater than or equal to 60 s. In this embodiment, the tabletting of the mixed fiber material can reduce the volume of the mixed fiber material, rearrange the fibers, and reduce the use area to avoid space waste; secondly, the tabletting of the mixed fiber material forms the fiber sheet with a sheet structure, so that the gas released by the fiber sheet in the subsequent heating and calcination process needs to pass through the sheet structure, and more pores are formed on the surface of the fiber, thereby improving the electrochemical performance of the material.

[0054] Further, the appropriate tabletting parameters can make the mixed fiber material form the fiber sheet with a sheet structure, so that the fiber sheet is not easy to break when moving.

[0055] In some embodiments, the step of calcining the fiber sheet to prepare the hard carbon comprises the following steps.

[0056] The fiber sheet is subjected to first calcination treatment at 300-400 DEG C in an inert gas atmosphere, and then the temperature is raised to 1200-1600 DEG C to perform second calcination treatment on the fiber sheet to prepare the hard carbon; wherein the time of the first calcination treatment and the time of the second calcination treatment are both 2-3 h.

[0057] The hard carbon prepared by the first calcination treatment and the second calcination treatment of the fiber sheet has high purity and high compaction density. It is found through experiments that the fiber sheet starts to accelerate decomposition at 300-400℃, and a large amount of gas and small-molecule organic matter is generated in the process of accelerated decomposition, which is easy to cause the prepared hard carbon material to be unstable. If the gas and small-molecule organic matter cannot escape in time, pressure will be formed inside, thereby causing the generation of pore structures. Based on the above experimental findings, the first calcination treatment of the fiber sheet is set to calcination at 300-400℃ for 2-3h, so that the fiber sheet is in the state of just starting to accelerate decomposition, the decomposition rate is relatively low, and the stability and internal density of the hard carbon material in the initial stage of preparation can be improved. Subsequently, the second calcination treatment of the fiber sheet is set to calcination at 1200-1600℃ for 2-3h, so that the fiber sheet maintains a faster decomposition rate at a higher temperature, thereby forming a hard carbon material with more pore structures, and then forming a sodium ion buffer pool on the surface of the hard carbon material, and improving the rate performance of the prepared hard carbon material. That is, the above two-stage sintering treatment method can improve the stability, compaction density and electrical properties of the prepared hard carbon material at the same time.

[0058] Further, the inert gas atmosphere is preferably an argon atmosphere, and the heating rate for the first calcination treatment and the second calcination treatment of the fiber sheet is 2-3℃ / min.

[0059] In this embodiment, the inert gas can prevent the fiber sheet from reacting with components such as oxygen in the air during calcination, thereby ensuring the accuracy of the experiment and the quality of the product. The heating rate is controlled at 2-3℃ / min to ensure that the fiber sheet can be uniformly heated, and to avoid local overheating or uneven temperature due to rapid heating; in this way, the physical and chemical changes of the fiber sheet during calcination can be more uniform and controllable, which is beneficial to improve the performance and quality of the hard carbon.

[0060] In the embodiments of the present application, the fiber sheet after calcination also needs to be subjected to wall breaking, crushing, screening and iron removal treatment to obtain a hard carbon negative material in the form of nanofiber.

[0061] Specifically, the crushing method is preferably an air jet crusher or a mechanical crusher; and the material passes through at least two levels of screen meshes during screening, the mesh number of the first level of screen meshes is preferably 80-100 meshes, and the mesh number of the second level of screen meshes is preferably 120-320 meshes.

[0062] Crushing and screening can improve the uniformity of the finished product, and avoid the adverse effects of large particles in the finished product on the electrochemical performance; the environmental humidity during crushing and screening is controlled to be less than 15%, so as to avoid sample water absorption causing material deterioration, production of impurities and reduction of electrochemical performance.

[0063] In one embodiment of the present application, the processes of crushing, screening and removing iron of the fibrous sheet are continuous operations, connected by pipes, transported by negative pressure, the finished product is deaerated and then enters the crushing equipment, dry nitrogen is introduced into the processes of crushing, screening and removing iron to protect the material from contacting water and carbon dioxide, and production impurities are avoided.

[0064] In a second aspect, the embodiments of the present application provide a hard carbon prepared by the preparation method of the hard carbon according to any one of the above; wherein the hard carbon has a charge specific capacity of ≥285.9 mAh / g at 0.1C; and / or a discharge specific capacity of ≥321.3 mAh / g at 0.1C; and / or a capacity retention rate of ≥90% after 100 cycles at 1C; and / or a first cycle coulombic efficiency of ≥84.7%, and / or a carbon content of ≥74.93%, and a compacted density of ≥1.01 g / cm 3 .

[0065] In this embodiment, the hard carbon is prepared by the above method, and thus has the advantages of high purity and large compacted density.

[0066] In a third aspect, the embodiments of the present application provide a negative electrode sheet mainly prepared from the hard carbon prepared by the preparation method of the hard carbon according to any one of the above or the hard carbon prepared according to the above.

[0067] In this embodiment, the negative electrode sheet contains the hard carbon material described above, and thus has excellent electrochemical performance.

[0068] In a fourth aspect, the embodiments of the present application provide a secondary battery comprising the negative electrode sheet described above.

[0069] In this embodiment, the secondary battery contains the negative electrode sheet described above, and thus has the advantages of excellent charge specific capacity, first cycle coulombic efficiency and rate capability.

[0070] Some specific embodiments are listed below. It should be noted that the embodiments described below are exemplary and are only used to explain the present application, and cannot be understood as a limitation on the present application. If the specific technology or condition is not specified in the embodiments, the technology or condition described in the literature in the art or according to the product manual is used. If the reagent or instrument is not specified by the manufacturer, it is a conventional product that can be obtained from the market.

[0071] I. Preparation method

[0072] Embodiment 1:

[0073] Step one, 100g of cotton is washed with pure water for 3 times to wash off the impurity particles attached to the surface, and then washed with ethanol for 3 times to wash off the oil stains and the like attached to the surface, and then placed into a vacuum oven at 105℃ for drying for 12 hours.

[0074] Step two, the above material is placed in the die of the tablet press (the die diameter is about 45mm) for tabletting treatment. During the tabletting process, 10g of cotton is added in the die with a diameter of about 45mm, and after assembly, it is placed in the tablet press, and the tabletting parameters are set as follows: the upper limit of pressure is 30 tons, the lower limit of pressure is 29.5 tons, and the pressure time is 60 seconds. After the parameter setting is completed, the sample is prepared, and the prepared cotton sheet precursor is obtained by demolding.

[0075] Step three, the above cotton sheet precursor material is calcined in a high temperature furnace, and the calcination temperature curve is as follows: the temperature is increased to 360℃ at a rate of 2℃ / min from room temperature, and then kept for 2h, and then increased to 1400℃ at a rate of 2℃ / min and kept for 2h, and the inert gas in the high temperature furnace is argon, and then quickly cooled to room temperature to obtain the precursor hard carbon nanofiber.

[0076] Step four, the above precursor hard carbon nanofiber is broken, crushed, screened and de-ironed to obtain a hard carbon material in the form of nanofiber, and the environmental humidity of the crushing and screening is controlled to be less than 15%, the C content of the obtained hard carbon material is 75.42%, the compacted density is 1.09g / cm 3 , the particle size range is: D50 is 7.9μm, D100 is 35.5μm, the content of magnetic foreign matter is 88ppm, and the water content is 910ppm.

[0077] Example 2:

[0078] Step one, 100g of cotton is washed with pure water for 3 times to wash away the impurity particles attached to the surface, and then washed with ethanol for 3 times to wash away the oil stains and the like attached to the surface, and then placed in a vacuum oven at 105℃ for drying for 12 hours.

[0079] Step two, the above material is placed in the die of the tablet press (the die diameter is about 45mm) for tabletting treatment. During the tabletting process, 10g of cotton is added in the die with a diameter of about 45mm, and after assembly, it is placed in the tablet press, and the tabletting parameters are set as follows: the upper limit of pressure is 20 tons, the lower limit of pressure is 19.5 tons, and the pressure time is 60 seconds. After the parameter setting is completed, the sample is prepared, and the prepared cotton sheet precursor is obtained by demolding.

[0080] Step three, the above cotton sheet precursor material is calcined in a high temperature furnace, and the calcination temperature curve is as follows: the temperature is increased to 360℃ at a rate of 2℃ / min from room temperature, and then kept for 2h, and then increased to 1400℃ at a rate of 2℃ / min and kept for 2h, and the inert gas in the high temperature furnace is argon, and then quickly cooled to room temperature to obtain the precursor hard carbon nanofiber.

[0081] Step four, after breaking the wall, crushing, screening (control the environment humidity of crushing and screening is lower than 15%) and removing iron of the precursor hard carbon nanofiber, the hard carbon material in nanofiber shape is obtained, at this time, the C content of the hard carbon material is 75.00%, the compacted density is 1.07g / cm 3 , the particle size range is: D50 is 13.464μm, D100 is 40.069μm, the content of magnetic foreign matter is 45.090ppm, and the moisture content is 1997.20ppm.

[0082] Example 3:

[0083] Step one, 100g of cotton is washed with pure water for 3 times to wash off the impurity particles attached to the surface, and then washed with ethanol for 3 times to wash off the oil stains and the like attached to the surface, and then placed into a vacuum oven at 105℃ for drying for 12 hours.

[0084] Step two, the above material is placed in the die of the tablet machine (the die diameter is about 45mm) for tablet processing. During the tablet processing, 10g of cotton is added into the die with a diameter of about 45mm, and after the assembly is completed, it is placed into the tablet machine, and the tablet parameters are set as follows: the upper limit of the pressing is 10 tons, the lower limit of the pressing is 9.5 tons, and the pressing time is 60 seconds. After the parameter setting is completed, the sample preparation is carried out, and the prepared cotton sheet precursor is obtained after demolding.

[0085] Step three, the above cotton sheet precursor material is calcined in a high temperature furnace, and the calcination temperature curve is as follows: the temperature is increased to 360℃ at a rate of 2℃ / min from room temperature, and then kept for 2h, and then increased to 1400℃ at a rate of 2℃ / min and kept for 2h, the inert gas in the high temperature furnace is argon, and then quickly cooled to room temperature to obtain the precursor hard carbon nanofiber.

[0086] Step four, after breaking the wall, crushing, screening (control the environment humidity of crushing and screening is lower than 15%) and removing iron of the precursor hard carbon nanofiber, the hard carbon material in nanofiber shape is obtained, at this time, the C content of the hard carbon material is 75.20%, the compacted density is 1.06g / cm 3 , the particle size range is: D50 is 12.742μm, D100 is 40.14μm, the content of magnetic foreign matter is 43.540ppm, and the moisture content is 2471.9ppm.

[0087] Example 4:

[0088] Step one, 100g of cotton is washed with pure water for 3 times to wash off the impurity particles attached to the surface, and then washed with ethanol for 3 times to wash off the oil stains and the like attached to the surface, and then placed into a vacuum oven at 105℃ for drying for 12 hours.

[0089] Step two, the above material is placed in the die of the tablet press (the die diameter is about 45mm) for tabletting treatment. During the tabletting process, 10g of cotton is added in the die with a diameter of about 45mm, and after assembly, it is placed in the tablet press, and the tabletting parameters are set as follows: the upper limit of pressure is 10 tons, the lower limit of pressure is 9.5 tons, and the pressure time is 60 seconds. After the parameter setting is completed, the sample is prepared, and the prepared cotton sheet precursor is obtained by demolding.

[0090] Step three, the above cotton sheet precursor material is calcined in a high temperature furnace, and the calcination temperature curve is as follows: the temperature is increased to 380℃ at a rate of 2℃ / min from room temperature, and kept for 2h, then increased to 1400℃ at a rate of 2℃ / min and kept for 2h, the inert gas in the high temperature furnace is argon, and then quickly cooled to room temperature to obtain the precursor hard carbon nanofiber.

[0091] Step four, the above precursor hard carbon nanofiber is broken, crushed, screened (the environmental humidity of crushing and screening is controlled to be less than 15%) and de-ironed to obtain a hard carbon material in the form of nanofiber. At this time, the C content of the hard carbon material is 74.93%, the compacted density is 1.01g / cm 3 , the particle size range is: D50 is 16.387μm, D100 is 51.119μm, the content of magnetic foreign matter is 38.18ppm, and the water content is 1955.20ppm.

[0092] Example 5:

[0093] Step one, 100g of cotton is washed with pure water for 3 times to wash away the impurity particles attached to the surface, and then washed with ethanol for 3 times to wash away the oil stains and the like attached to the surface, and then placed in a vacuum oven at 105℃ for drying for 12 hours.

[0094] Step two, the above material is placed in the die of the tablet press (the die diameter is about 45mm) for tabletting treatment. During the tabletting process, 10g of cotton is added in the die with a diameter of about 45mm, and after assembly, it is placed in the tablet press, and the tabletting parameters are set as follows: the upper limit of pressure is 10 tons, the lower limit of pressure is 9.5 tons, and the pressure time is 60 seconds. After the parameter setting is completed, the sample is prepared, and the prepared cotton sheet precursor is obtained by demolding.

[0095] Step three, the above cotton sheet precursor material is calcined in a high temperature furnace, and the calcination temperature curve is as follows: the temperature is increased to 380℃ at a rate of 2℃ / min from room temperature, and kept for 2h, then increased to 1400℃ at a rate of 2℃ / min and kept for 2h, the inert gas in the high temperature furnace is argon, and then quickly cooled to room temperature to obtain the precursor hard carbon nanofiber.

[0096] Step four, after breaking the wall, crushing, screening (control the environment humidity of crushing and screening is lower than 15%) and removing iron of the precursor hard carbon nanofiber, the hard carbon material in the form of nanofiber is obtained, at this time, the C content of the hard carbon material is 75.34%, the compacted density is 1.02 g / cm 3 , the particle size range is: D50 is 6.695 μm, D100 is 30.810 μm, the content of magnetic foreign matter is 48.27 ppm, and the water content is 2383.40 ppm.

[0097] Comparative example 1:

[0098] Step one, 100 g of cotton is washed with pure water for 3 times to wash off the impurity particles attached to the surface, and then washed with ethanol for 3 times to wash off the oil stains and the like attached to the surface, and then placed into a vacuum oven at 105℃ for drying for 12 hours.

[0099] Step two, the dried cotton material is placed in a high-temperature furnace for calcination, the calcination temperature curve is: the temperature is raised from room temperature to 1400℃ at a rate of 2℃ / min, and then kept for 2 hours, the inert gas in the high-temperature furnace is argon, and then quickly cooled to room temperature to obtain the precursor hard carbon nanofiber.

[0100] Step three, after breaking the wall, crushing, screening (control the environment humidity of crushing and screening is lower than 15%) and removing iron of the precursor hard carbon nanofiber, the hard carbon material is obtained. The C content of the obtained hard carbon material is 75.12%, the compacted density is 0.9 g / cm 3 , the particle size range is: D50 is 8.5 μm, D100 is 100 μm, the content of magnetic foreign matter is 100 ppm, and the water content is 1040 ppm.

[0101] Comparative example 2:

[0102] Step one, 100 g of cotton is washed with pure water for 3 times to wash off the impurity particles attached to the surface, and then washed with ethanol for 3 times to wash off the oil stains and the like attached to the surface, and then placed into a vacuum oven at 105℃ for drying for 12 hours.

[0103] Step two, the material is placed in the die of the tablet machine (the die diameter is about 45 mm) for tablet processing. During the tablet processing, 10 g of cotton is added into the die with a diameter of about 45 mm, and after the assembly is completed, it is placed into the tablet machine, and the tablet parameters are set as follows: the upper limit of the pressing is 30 tons, the lower limit of the pressing is 29.5 tons, and the pressing time is 60 seconds. After the parameter setting is completed, the sample is prepared, and the mold is removed to obtain the prepared cotton flake precursor.

[0104] Step three, the above-mentioned cotton flake precursor material is placed in a high-temperature furnace for calcination, the calcination temperature curve is: the temperature is raised from room temperature to 1400℃ at a rate of 2℃ / min, and the temperature is kept for 2h, the inert gas in the high-temperature furnace is argon, and then the temperature is quickly cooled to room temperature to obtain the precursor hard carbon nanofiber.

[0105] Step four, the above-mentioned precursor hard carbon nanofiber is subjected to wall breaking, crushing, screening (the environmental humidity of crushing and screening is controlled to be less than 15%) and iron removal to obtain a hard carbon material, at this time, the C content of the hard carbon material is 74.96%, the compaction density is 0.92g / cm 3 , the particle size range is: D50 is 8.501μm, D100 is 35.220μm, the content of magnetic foreign matter is 38.68ppm, and the water content is 2607.70ppm.

[0106] Comparative example 3:

[0107] Step one, 100g of cotton is washed with pure water for 3 times to wash away the impurity particles attached to the surface, and then washed with ethanol for 3 times to wash away the oil stains and the like attached to the surface, and then placed into a vacuum oven at 105℃ for drying for 12 hours.

[0108] Step two, the above-mentioned dried cotton material is placed in a high-temperature furnace for calcination, the calcination temperature curve is: the temperature is raised from room temperature to 360℃ at a rate of 2℃ / min, and then raised to 1400℃ at a rate of 2℃ / min and kept for 2h, the inert gas in the high-temperature furnace is argon, and then the temperature is quickly cooled to room temperature to obtain the precursor hard carbon nanofiber.

[0109] Step four, the above-mentioned precursor hard carbon nanofiber is subjected to wall breaking, crushing, screening (the environmental humidity of crushing and screening is controlled to be less than 15%) and iron removal to obtain a hard carbon material, at this time, the C content of the hard carbon material is 74.97%, the compaction density is 0.96g / cm 3 , the particle size range is: D50 is 15.090μm, D100 is 40.118μm, the content of magnetic foreign matter is 25.150ppm, and the water content is 2569.00ppm.

[0110] II. Test method

[0111] The hard carbon materials obtained in examples 1 to 5 and comparative examples 1 to 3 are used to prepare button cells, and the electrical properties of the obtained button cells are detected, wherein the preparation process of the button cells comprises:

[0112] The hard carbon material provided by the embodiments 1 to 5 and the comparative examples 1 to 3 of the present application is mixed with acetylene black conductive agent and binder at a mass ratio of 80:10:10, the obtained mixture is coated on a copper foil current collector, and after drying at 80℃, an electrode sheet with a diameter of 1cm is prepared by a punching machine; the obtained electrode sheet is used as a negative electrode, the positive electrode is a sodium nickel copper manganese magnesium layered oxide, and the separator is Celgard 2400, and a button cell is assembled in an inert gas glove box (O2 and H2O content is less than 1ppm) produced by Braun Company in Germany with a model of UNlab.

[0113] III. Analysis of test results of each embodiment and comparative example

[0114] The battery test system produced by Wuhan Blue Electric Company with a model of CT 2001A is used to test the electrochemical performance of the button cell prepared, the voltage range of the test condition is 0.01-2.0V, and the results are shown in Table 1 below:

[0115] Table 1: Electrochemical performance test results of each group of button cells

[0116] Item Charge specific capacity at 0.1 C (mAh / g) Discharge specific capacity at 0.1 C (mAh / g) Capacity retention after 100 cycles at 1 C (%) Coulombic efficiency of the first cycle (%) Example 1 320.8 360 / 89.1 Example 2 294 333.6 / 88.1 Example 3 285.9 321.3 / 89 Example 4 301.3 352.8 92 85.4 Example 5 291.8 335.1 / 87.1 Comparative Example 1 227.6 304.2 90 74.7 Comparative Example 2 238.7 295.8 92 80.6 Comparative Example 3 243.4 297.5 92 81.8

[0117] Specifically, by comparing the test data of the hard carbon material prepared by the embodiments 1-5 with the test data of the hard carbon material prepared by the comparative examples 1-3, it can be known that the compacted density of the hard carbon material prepared by the present application is obviously higher.

[0118] Further, the hard carbon material obtained by the embodiments 1-5 and the hard carbon material prepared by the comparative examples 1-3 are used to prepare button cells, the 0.1C charge specific capacity, 0.1C discharge specific capacity, capacity retention rate after 100 cycles at 1C, and first cycle coulombic efficiency of the button cell prepared by using the hard carbon material prepared by the embodiments 1-5 as a negative electrode material are all higher.

[0119] It should be noted that the present application is not limited to the above-mentioned embodiments. The above-mentioned embodiments are only examples, and embodiments having the same technical idea and playing the same role and effect within the scope of the technical solution of the present application are all included in the technical scope of the present application. In addition, within the scope of the main idea of the present application, various modifications that can be thought of by those skilled in the art are applied to the embodiments, and other ways are constructed by combining part of the constituent elements in the embodiments, which are also included in the scope of the present application.

Claims

1. A method for producing hard carbon, characterized by, It comprises the following steps: Step one: sequentially washing the cotton with water and ethanol, and vacuum drying the cotton to obtain a nanofiber precursor; the temperature of the vacuum drying is 80-120℃, and the time of the vacuum drying is 10-24h; Step two: mixing the nanofiber precursor and cotton that has not been vacuum dried at a mass ratio of (5-10):1 to obtain a mixed fiber material; tabletting the mixed fiber material to obtain a fiber sheet; the tabletting pressure is 9.5-30 tons, and the tabletting time is greater than or equal to 60s; Step three: calcining the fiber sheet to obtain a precursor hard carbon nanofiber; Step four: breaking the wall of the precursor hard carbon nanofiber, crushing, screening, and removing iron to obtain a hard carbon material in the form of nanofibers; Step three specifically comprises: under an inert gas atmosphere, heating at a rate of 2℃ / min to 300-400℃, first calcining the fiber sheet for 2-3h, then heating at a rate of 2℃ / min to 1200-1600℃, second calcining the fiber sheet for 2-3h, and then rapidly cooling to room temperature.

2. A hard carbon, characterized by, The hard carbon is prepared by the method of claim 1; The hard carbon has a charge specific capacity of ≥285.9mAh / g at 0.1C; and / or The hard carbon has a discharge specific capacity of ≥321.3mAh / g at 0.1C; and / or The hard carbon has a capacity retention rate of ≥90% after 100 cycles at 1C; and / or The hard carbon has a first-cycle coulombic efficiency of ≥84.7%.

3. The hard carbon of claim 2, wherein, The hard carbon has a carbon content of ≥ 74.93% and a compaction density of ≥ 1.01 g / cm 3 .

4. A negative electrode sheet characterized by comprising: The hard carbon is prepared by the method of claim 1, or comprises the hard carbon of claims 2-3.

5. A secondary battery characterized by comprising: The negative electrode sheet comprises the hard carbon of claim 4.

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