Process for preparing flaky microcrack hard carbon by rapid cooling and hard carbon, sodium ion battery

Abstract

The application provides a process for rapidly cooling and preparing flaky micro-crack hard carbon and the hard carbon. The process for rapidly cooling and preparing flaky micro-crack hard carbon comprises the following steps: obtaining precursor particles; performing first-stage pyrolysis treatment on the precursor particles; performing second-stage pyrolysis treatment on the porous carbon material to obtain hard carbon material; and performing cooling treatment on the hard carbon material to obtain flaky micro-crack hard carbon with rich defect sites. Through the method, while ensuring that the flaky micro-crack hard carbon has high conductivity, new channels are provided for effective diffusion and embedding of sodium ions, so as to help more sodium ions to be quickly de-embedded in the negative electrode material. Meanwhile, the flaky micro-crack hard carbon can increase the contact area of the negative electrode material and the electrolyte, effectively improve the permeability of the electrolyte, and improve the initial efficiency, reversible capacity and cycle performance of the sodium ion battery.

Description

TECHNICAL FIELD

[0001] The present application relates to the technical field of negative electrode materials, in particular to a process for preparing flaky microcrack hard carbon by rapid cooling and the hard carbon. BACKGROUND

[0002] Sodium-ion batteries have the advantages of high energy density, good rate performance and long cycle life, and have become one of the main electrochemical energy storage devices in recent years, and are widely used in mobile devices, electric vehicles and renewable resource energy storage fields. However, with the development of sodium-ion batteries, lithium resources are becoming increasingly scarce, and the cost of lithium is also increasing day by day, so the development and application of sodium-ion batteries in large-scale electrochemical energy storage systems are hindered.

[0003] Sodium element is in the same group as lithium element, has similar physical and chemical properties as lithium, and has much more abundant reserves on earth than lithium, and is also much cheaper, so sodium-ion batteries are attracting more and more attention. However, due to the larger radius of sodium ions than lithium ions, and the smaller interlayer spacing of traditional graphite (0.34 nm), it is difficult for sodium ions to enter the graphite layer, so the sodium storage capacity of graphite materials in sodium-ion batteries is not high, which causes sodium-ion batteries to be not widely used.

[0004] At present, many scholars believe that hard carbon will be the most promising negative electrode material in sodium-ion batteries, mainly because hard carbon is difficult to graphitize, and its carbon layer spacing is larger than that of graphite, so it can accommodate more sodium ions with larger radius.

[0005] Because biomass materials have natural microstructure and are rich in carbon elements, the derived carbon materials have large interlayer spacing, high degree of disorder, rich active sites, low cost, renewability, green environmental protection and other advantages, so they are the preferred raw materials for preparing hard carbon.

[0006] In the preparation process of traditional hard carbon, peanut shells, rice husks, straw and the like are usually used as biomass precursors to prepare hard carbon by one-step pyrolysis. Although the hard carbon prepared after pyrolysis can significantly improve the electrochemical properties of the material, the initial efficiency and reversible capacity are low, and the cycle performance is poor, which is difficult to meet the commercialization requirements of sodium-ion batteries. SUMMARY

[0007] The purpose of the present application is to overcome the deficiencies in the prior art, and to provide a process for preparing flaky microcrack hard carbon by rapid cooling, which has high initial efficiency, reversible capacity and cycle performance, and the hard carbon and sodium-ion battery.

[0008] The purpose of the present application is achieved by the following technical solutions:

[0009] A process for preparing flaky micro-crack hard carbon by rapid cooling, comprising the following steps:

[0010] Obtaining precursor particles;

[0011] Performing first-stage pyrolysis treatment on the precursor particles to obtain graphitic microcrystalline carbon material;

[0012] Performing second-stage pyrolysis treatment on the graphitic microcrystalline carbon material to obtain pore-rich carbon material, wherein the temperature of the second-stage pyrolysis is higher than that of the first-stage pyrolysis;

[0013] Performing cooling treatment on the pore-rich carbon material to obtain flaky micro-crack hard carbon.

[0014] In one of the embodiments, when performing the first-stage pyrolysis treatment on the precursor particles, heating at a heating rate of 3C / min to 8C / min to 400C to 800C, and maintaining the temperature for 1h to 3h.

[0015] In one of the embodiments, when performing the second-stage pyrolysis treatment on the graphitic microcrystalline carbon material, heating at a heating rate of 2C / min to 5C / min to 600C to 1200C, and maintaining the temperature for 2h to 4h.

[0016] In one of the embodiments, when performing the cooling treatment on the pore-rich carbon material, passing a cooling agent into the pore-rich carbon material to cool it.

[0017] In one of the embodiments, the cooling agent comprises at least one of propane, liquid ammonia and liquid nitrogen.

[0018] In one of the embodiments, the first-stage pyrolysis treatment and the second-stage pyrolysis treatment are performed under the condition of liquid nitrogen or argon gas, and the flow rate of the liquid nitrogen or the argon gas is controlled to be 1L / min to 3L / min.

[0019] In one of the embodiments, in the step of obtaining precursor particles, the following specific steps are included:

[0020] Performing oscillation cleaning operation on the biomass raw material by an ultrasonic cleaning device;

[0021] Performing drying operation on the biomass raw material after the oscillation cleaning operation;

[0022] Performing crushing operation on the biomass raw material after the drying operation to obtain precursor particles.

[0023] In one of the embodiments, after the step of performing crushing operation on the biomass raw material after the drying operation, and before the step of performing first-stage pyrolysis treatment on the precursor particles, the following steps are further included:

[0024] Screening the precursor particles to obtain the precursor particles with a particle size of 44-74 mu m.

[0025] A hard carbon prepared by the process for preparing laminar microcrack hard carbon by rapid cooling in any of the above embodiments.

[0026] A sodium ion battery comprising a positive electrode and a negative electrode, characterized in that the active material of the negative electrode comprises the hard carbon in any of the above embodiments.

[0027] Compared with the prior art, the present application has at least the following advantages:

[0028] 1) The process for preparing laminar microcrack hard carbon by rapid cooling, first, the precursor particles are subjected to a first-stage pyrolysis treatment to enable the organic macromolecules in the biomass to be rapidly released to obtain a graphite microcrystalline carbon material; then the graphite microcrystalline carbon material is subjected to a second-stage pyrolysis treatment, and the temperature of the second-stage pyrolysis is higher than that of the first-stage pyrolysis, so that the graphite microcrystalline is gradually carbonized to obtain a porous carbon material with large pores, stable structure and good electrical conductivity; finally, the porous carbon material is subjected to a cooling treatment, so that many microcracks are formed on the surface of the porous carbon material to obtain laminar microcrack hard carbon, thus ensuring that the laminar microcrack hard carbon has high electrical conductivity, while providing new channels for the effective diffusion and intercalation of sodium ions, thereby helping more sodium ions to be rapidly deintercalated in the negative electrode material, at the same time, the laminar microcrack hard carbon can also increase the contact area of the negative electrode material with the electrolyte, thereby effectively improving the permeability of the electrolyte, and further improving the liquid injection efficiency of the electrolyte, and also helping the electrolyte to better infiltrate the negative electrode material during the cycle process of the sodium ion battery, thereby greatly improving the initial efficiency, reversible capacity and cycle performance of the sodium ion battery.

[0029] 2) The process for preparing laminar microcrack hard carbon by rapid cooling uses precursor particles as raw materials, which can utilize agricultural waste such as peanut shells, straws, leaves and dead branches, and is not only widely available and inexpensive, but also more environmentally friendly. BRIEF DESCRIPTION OF DRAWINGS

[0030] In order to more clearly illustrate the technical solutions of the embodiments of the present application, the following will briefly introduce the drawings needed to be used in the embodiments, and it should be understood that the following drawings only show some embodiments of the present application, and therefore should not be regarded as a limitation on the scope, and for those skilled in the art, other related drawings can also be obtained without creative labor on the basis of these drawings.

[0031] Figure 1 Flow chart of the process for preparing laminar microcrack hard carbon by rapid cooling in an embodiment of the present application;

[0032] Figure 2 SEM image of hard carbon according to an embodiment of the present application;

[0033] Figure 3 Graph of initial charge-discharge specific capacity according to Example 2;

[0034] Figure 4 Graph of cycle performance according to Examples 1 to 3 and Comparative Examples 1 to 3. DETAILED DESCRIPTION

[0035] For the purposes of promoting an understanding of the principles of the application, reference will now be made to the embodiments illustrated in the drawings. It is expressly understood that the drawings are for illustration only and that the application can be embodied in many different forms. In the drawings:

[0036] It is to be understood that the singular forms "a," "an," and "the" include plural referents unless the context clearly dictates otherwise. It is to be understood that the term "or" as used herein is to be interpreted as inclusive or meaning at least one, unless the context clearly indicates otherwise. It is also to be understood that the terms "comprise," "comprises" and "comprising" are to be interpreted inclusively rather than exclusively.

[0037] 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 in the description of the application herein is for the purpose of describing particular embodiments only and is not intended to be limiting of the application. As used in this description, the terms "may" and "might" include any one or all of possible combinations of the constituent terms.

[0038] The application provides a process for preparing laminar microcrack hard carbon by rapid cooling, comprising the following steps: obtaining precursor particles; performing first-stage pyrolysis on the precursor particles to obtain graphitic microcrystalline carbon material; performing second-stage pyrolysis on the graphitic microcrystalline carbon material to obtain porous carbon material, wherein the temperature of the second-stage pyrolysis is higher than that of the first-stage pyrolysis; and performing cooling treatment on the porous carbon material to obtain laminar microcrack hard carbon.

[0039] The aforementioned rapid cooling process for preparing lamellar microcracked hard carbon involves several steps. First, the precursor particles undergo a first-stage pyrolysis treatment to rapidly release the organic macromolecules from the biomass, yielding graphite microcrystalline carbon material. Then, the graphite microcrystalline carbon material undergoes a second-stage pyrolysis treatment at a higher temperature than the first stage, allowing the graphite microcrystals to gradually carbonize, resulting in a porous carbon material with large pores, stable structure, and good electrical conductivity. Finally, the porous carbon material is cooled, which forms numerous microcracks on its surface, resulting in a lamellar microcracked hard carbon material. The lamellar microcracked hard carbon, while ensuring high conductivity, also provides a new channel for the effective diffusion and insertion of sodium ions, thus facilitating the rapid insertion and extraction of more sodium ions in the negative electrode material. At the same time, the lamellar microcracked hard carbon can also increase the contact area between the negative electrode material and the electrolyte, thereby effectively improving the electrolyte permeability and thus improving the electrolyte injection efficiency. Furthermore, it helps the electrolyte to better wet the negative electrode material during the cycle of the sodium-ion battery, thereby greatly improving the initial efficiency, reversible capacity and cycle performance of the sodium-ion battery.

[0040] The above-mentioned rapid cooling process for preparing lamellar microcracked hard carbon utilizes precursor particles as raw materials, which can utilize agricultural waste such as peanut shells, straw, leaves and dead branches. It is not only widely available and inexpensive, but also more environmentally friendly.

[0041] Please see Figure 1 To better understand the technical solution and beneficial effects of this application, the following detailed description is provided in conjunction with specific embodiments. One embodiment of rapid cooling preparation of lamellar microcracked hard carbon includes some or all of the following steps:

[0042] S101, Obtain precursor particles.

[0043] In this embodiment, firstly, biomass raw materials are obtained. Specifically, in one embodiment, the biomass raw materials include at least one of peanut shells, straw, and dead branches. Then, the biomass raw materials are subjected to an ultrasonic cleaning operation to loosen the internal structure of the biomass raw materials and effectively remove dust and soluble contaminants from the surface of the biomass raw materials. Next, the biomass raw materials after the ultrasonic cleaning operation are dried to effectively remove moisture. Then, the dried biomass raw materials are pulverized to obtain precursor particles. This increases the surface area of ​​the biomass raw materials, which helps the precursor particles to undergo rapid pyrolysis and cooling treatment to ensure the production of lamellar microcracked hard carbon.

[0044] It should be noted that when peanut shells are used as biomass raw materials, the surface of the peanut shells is formed with longitudinal protrusions and transverse protrusions connected to each other in a network shape, so that the peanut shells contain a network of dense fiber bundles. If the biomass raw materials are not subjected to the oscillation cleaning operation by the ultrasonic cleaning device, the network of dense fiber bundles in the peanut shells cannot be well relaxed, causing the phenomenon of serious fiber drawing on the edge of the precursor particles during subsequent crushing, and the precursor particles with good network protrusion structure cannot be ensured, thereby being not conducive to the preparation of the laminated microcrack hard carbon with stable structure and high conductivity. Therefore, in one of the embodiments, the biomass raw materials are subjected to the oscillation cleaning operation by the ultrasonic cleaning device, so that the ultrasonic cleaning device can perform ultrasonic oscillation and beating on the biomass raw materials. In this way, the internal structure of the biomass raw materials can be relaxed, thereby effectively avoiding the phenomenon of serious fiber drawing on the edge of the precursor particles due to the difficulty in crushing the dense fiber bundles, and the impurities, dust and soluble pollutants on the surface of the biomass raw materials can be effectively removed, so as to ensure the preparation of the precursor particles with good network protrusion structure, thereby ensuring that the precursor particles still maintain good network protrusion structure during the subsequent segmented pyrolysis process. In this way, when the porous carbon material after the second pyrolysis process is immediately subjected to the cooling process, the surface layer and the inside of the porous carbon material are not uniformly cooled during the rapid cooling process, causing inconsistent shrinkage, so that many microcracks are formed on the network protrusion structure on the surface of the porous carbon material, thereby preparing the laminated microcrack hard carbon with good conductivity and stable structure. It is worth mentioning that the precursor particles maintain good network protrusion structure, which is conducive to the formation of good network microcracks of the porous carbon material during cooling.

[0045] Further, in one of the embodiments, when the biomass raw materials are subjected to the oscillation cleaning operation by the ultrasonic cleaning device, the following specific steps are included: the biomass raw materials are poured into the cleaning tank of the ultrasonic cleaning device containing the mixed solution of water and alcohol for ultrasonic cleaning for 20-30 min, so that the ultrasonic cleaning device can perform ultrasonic oscillation and beating on the biomass raw materials. In this way, the internal structure of the biomass raw materials can be better relaxed, and the impurities, dust and soluble pollutants on the surface of the biomass raw materials can be more effectively removed, so as to ensure the subsequent preparation of the laminated microcrack hard carbon with good conductivity and stable structure. Further, in one of the embodiments, the volume ratio of water to alcohol is (3-4): 1. In a more preferred embodiment, the water is deionized water.

[0046] It should be noted that when the volume ratio of water and alcohol is (3-4):1 as the ultrasonic cleaning agent, the alcohol is impacted and beaten on the biomass raw material by the cavitation effect of ultrasonic waves, which can better relax the internal structure of the biomass raw material, and can also effectively strip or dissolve the impurities, dust and soluble pollutants on the surface of the biomass raw material, thereby achieving better cleaning and relaxation effects.

[0047] In one of the embodiments, the biomass raw material after the oscillation cleaning operation is subjected to a drying operation, which includes the following specific steps: using a forced air drying oven to dry the biomass raw material to achieve the drying operation of the biomass raw material.

[0048] In one of the embodiments, after the step of crushing the biomass raw material after the drying operation, and before the step of subjecting the precursor particles to the first-stage pyrolysis treatment, the following step is further included: sieving the precursor particles with a mesh size of 200-325 meshes to obtain the precursor particles with a particle size of 44-74 μm, so as to ensure that the particle size of the precursor particles is appropriate, which is conducive to the subsequent preparation of hard carbon with stable structure and good microcracks, thereby ensuring the preparation of the lamellar microcrack hard carbon with high conductivity, and effectively avoiding the phenomenon that the structure of the lamellar microcrack hard carbon is unstable and the conductivity is poor due to too small or too large particle size.

[0049] S102, subjecting the precursor particles to a first-stage pyrolysis treatment to obtain a graphite microcrystalline carbon material.

[0050] In the present embodiment, when the precursor particles are subjected to the first-stage pyrolysis treatment, heating is performed at a heating rate of 3-8℃ / min to 400-800℃, and the temperature is kept constant for 1-3h.

[0051] It can be understood that the precursor particles are first transferred to the sagger, and then the sagger and the precursor particles are placed in the pyrolysis furnace and heated at a heating rate of 3-8℃ / min to 400-800℃, and the temperature is kept constant for 1-3h. Since the heating rate of the first stage is relatively fast, the organic macromolecules in the precursor particles can be quickly released, so as to obtain the graphite microcrystalline carbon material, so that the graphite microcrystalline carbon material can obtain the porous carbon material with large pores and stable structure in the subsequent second-stage pyrolysis.

[0052] S103, subjecting the graphite microcrystalline carbon material to a second-stage pyrolysis treatment to obtain a porous carbon material, wherein the temperature of the second-stage pyrolysis is higher than that of the first-stage pyrolysis.

[0053] It can be understood that, since the temperature of the second stage pyrolysis is higher than that of the first stage pyrolysis, the graphite crystallites can be gradually carbonized, so that the porous carbon material with larger pores and more stable structure can be obtained, so as to ensure that the flaky microcrack hard carbon with stable structure, larger pores and good conductivity can be prepared subsequently.

[0054] In the embodiment, the graphite crystalline carbon material is subjected to the second stage pyrolysis treatment, and is heated to 600-1200℃ at a heating rate of 2-5℃ / min, and is pyrolyzed at constant temperature for 2-4h.

[0055] It can be understood that, since the heating rate of the second stage is slower than that of the first stage, it is ensured that the pore structure of the graphite crystalline carbon material after the first stage pyrolysis treatment does not collapse, that is, the heating rate of the second stage is slower than that of the first stage, so that the graphite crystalline carbon material can continue to be carbonized to form the porous carbon material with larger pores, stable structure and good conductivity.

[0056] S104, the porous carbon material is subjected to cooling treatment to obtain flaky microcrack hard carbon.

[0057] In the embodiment, when the porous carbon material is subjected to cooling treatment, the coolant is introduced into the porous carbon material for cooling, so that many microcracks can be formed on the surface of the porous carbon material, so that the flaky microcrack hard carbon is obtained, and details can be referred to the electron microscope image of Figure 2 , so that the flaky microcrack hard carbon has higher conductivity, and provides a new channel for effective diffusion and embedding of sodium ions, so as to help more sodium ions to be quickly de-embedded in the negative electrode material, at the same time, the flaky microcrack hard carbon can also increase the contact area of the negative electrode material and the electrolyte, so as to effectively improve the permeability of the electrolyte, and further improve the liquid injection efficiency of the electrolyte, and also helps the electrolyte to better wet the negative electrode material in the cycle process of the sodium ion battery, and further improves the initial efficiency, reversible capacity and cycle performance of the sodium ion battery.

[0058] In order to realize the cooling treatment of the porous carbon material, in one of the embodiments, the coolant includes at least one of liquid propane, liquid ammonia and liquid nitrogen.

[0059] It can be understood that, since the boiling points of propane, ammonia and liquid nitrogen are very low, when the propane, ammonia and liquid nitrogen are pressurized or cooled, they will be in liquid state, so as to be used as coolants. When the liquid coolants are introduced into the high-temperature porous carbon material, the liquid coolants can be converted into gaseous propane, ammonia and liquid nitrogen by absorbing a large amount of heat in the porous carbon material, on the one hand, the rapid cooling of the porous carbon material can be ensured, so as to ensure that the porous carbon material is easily formed with many micro-cracks on the surface of the porous carbon material under the condition of thermal expansion and contraction, so as to prepare the flaky micro-crack hard carbon with good electrical conductivity and stable structure; on the other hand, the cooling treatment of the porous carbon material under the condition of propane, ammonia and liquid nitrogen can be ensured, and the step of filling inert gas is effectively saved. In this way, the operation process is more compact and simple, so as to improve the production efficiency.

[0060] In one of the embodiments, the pyrolysis furnace comprises a pyrolysis furnace body and a cooling assembly, the cooling assembly comprises a cooling pipe, an on-off control valve, a liquid pumping pump and a liquid storage tank, the pyrolysis furnace body is formed with a pyrolysis cavity for placing the retort, the cooling pipe is at least partially located in the pyrolysis cavity, the cooling pipe is connected with the liquid storage tank through the liquid pumping pump, the liquid storage tank is used for storing the coolant, the on-off control valve is arranged on the cooling pipe, and the on-off control valve is used for controlling the flow of the coolant in the cooling pipe.

[0061] It can be understood that, since the pyrolysis furnace is additionally provided with the cooling assembly, after the drive body particles complete the second-stage pyrolysis process, the on-off control valve is opened, the liquid pumping pump is connected with the external power supply and starts to work, so that the coolant in the liquid storage tank can be pumped into the cooling pipe by the liquid pumping pump, and then the liquid coolant in the cooling pipe enters the pyrolysis cavity. Since the heat in the pyrolysis cavity is high, the introduced liquid coolant can be converted into gas, so as to realize the rapid cooling of the pyrolysis furnace and the porous carbon material in the retort, so that the surface layer and the inside of the porous carbon material are not uniformly cooled during the rapid cooling process, and the shrinkage is inconsistent, and then many micro-cracks can be formed on the surface of the porous carbon material.

[0062] It should be noted that if the biomass raw material is not subjected to the oscillation cleaning operation by the ultrasonic cleaning device, the internal structure of the biomass raw material is relatively strong and impurities are left on the surface of the biomass raw material. On the one hand, this is not conducive to subsequent pyrolysis treatment, so that the porous carbon material with large pores and stable structure cannot be ensured. On the other hand, the impurities left on the surface of the biomass raw material will affect the formation of the microcracks during the subsequent cooling treatment, so that better microcracks cannot be ensured. Therefore, when the biomass raw material is pretreated, the oscillation cleaning operation is performed on the biomass raw material by the ultrasonic cleaning device, so that the ultrasonic cleaning device can perform ultrasonic oscillation and beating on the biomass raw material. In this way, the internal structure of the biomass raw material can be loosened, which is conducive to the formation of the porous carbon material with large pores, large interlayer spacing and stable structure during the pyrolysis process of the precursor particles, so that the porous carbon material with relatively regular internal structure can be prepared, and the surface of the porous carbon material is relatively uniform when the porous carbon material is subjected to the cooling treatment, thereby ensuring the formation of better microcracks during the rapid cooling stage of the porous carbon material.

[0063] It should also be noted that the microcracks formed on the surface of the laminar microcrack hard carbon can improve the injection efficiency of the electrolyte of the sodium ion battery, thereby improving the production efficiency of the sodium battery. On the other hand, the microcracks can ensure that the electrolyte can better penetrate the laminar microcrack hard carbon during the cycle process of the sodium ion battery, thereby helping the electrolyte to better wet the negative electrode material during the cycle process of the sodium ion battery, and thereby improving the initial efficiency, reversible capacity and cycle performance of the sodium ion battery.

[0064] In order to ensure that the ultrasonic cleaning device can better loosen the internal structure of the biomass raw material, in one embodiment, the vibration frequency of the ultrasonic cleaning device is 20Hz-50Hz, so that the intensity of the oscillation and beating of the ultrasonic cleaning device on the biomass raw material is more appropriate, and the original structure of the internal structure of the biomass raw material will not be broken. That is, when the vibration frequency of the ultrasonic cleaning device is 20Hz-50Hz, especially when the volume ratio of water and alcohol is (3-4):1, the internal structure of the biomass raw material can be more effectively expanded and loosened, which is conducive to subsequent pyrolysis carbonization to obtain the porous carbon material with large pores, large interlayer spacing and stable structure.

[0065] In one embodiment, the flow rate of the coolant is controlled to be 15-25 L / min. It can be understood that if the flow rate of the coolant is greater than 25 L / min, the flow rate of the coolant is too fast, which causes a large amount of coolant to enter the pyrolysis cavity to cause the temperature in the pyrolysis cavity to drop rapidly, and thus the temperature difference between the surface and the inside of the porous carbon material during the rapid cooling process is larger, causing the surface of the porous carbon material to shrink significantly inconsistently, thereby forming larger gap microcracks on the surface of the porous carbon material to reduce the structural strength of the laminar microcracked hard carbon. If the flow rate of the coolant is less than 15 L / min, the flow rate of the coolant is too small, and the coolant cannot ensure that the coolant can rapidly cool in the pyrolysis cavity, thereby failing to form microcracks on the surface of the porous carbon material. Therefore, in the present application, the flow rate of the coolant is controlled to be 15-25 L / min, so that the flow rate of the coolant entering the pyrolysis cavity is appropriate, thereby ensuring that the surface and the inside of the porous carbon material are unevenly cooled during the rapid cooling process, causing inconsistent shrinkage, and thus forming better microcracks on the surface of the porous carbon material.

[0066] In one embodiment, the gap of the microcracks of the laminar microcracked hard carbon is 0.01-2 nm. It can be understood that by controlling the flow rate of the coolant to be 15-25 L / min, the gap of the microcracks of the laminar microcracked hard carbon is 0.01-2 nm. In this way, the laminar microcracked hard carbon prepared has many microcracks on the surface, and the structure is stable and the electrical conductivity is good, thereby ensuring that the electrolyte can better infiltrate the laminar microcracked hard carbon, and thus improving the initial efficiency, reversible capacity and cycle performance of the sodium ion battery. It is worth mentioning that the stable structure and good electrical conductivity of the laminar microcracked hard carbon mainly reflect that the capacity retention rate can still be maintained after 1000 cycles.

[0067] In a more preferred embodiment, when the flow rate of the coolant is controlled to be 18-25 L / min, the gap of the microcracks is 0.4-2 nm, which can also provide new channels for the effective diffusion and embedding of sodium ions, thereby helping more sodium ions to quickly de-embed in the negative electrode material. At the same time, the laminar microcracked hard carbon can also increase the contact area of the negative electrode material and the electrolyte, thereby effectively improving the permeability of the electrolyte, and thus improving the liquid injection efficiency of the electrolyte. Moreover, it is also helpful for the electrolyte to better infiltrate the negative electrode material during the cycle process of the sodium ion battery, thereby greatly improving the initial efficiency, reversible capacity and cycle performance of the sodium ion battery.

[0068] In one embodiment, the cooling pipe is provided with a plurality of nozzles, and the cooling pipe is located above the pyrolysis furnace, and each nozzle is arranged towards the pyrolysis cavity.

[0069] It can be understood that, since the cooling pipe is located above the pyrolysis furnace, the plurality of nozzles are arranged towards the pyrolysis cavity, the saggar is placed in the pyrolysis cavity and is located below the cooling pipe, so that the liquid coolant sprayed from the plurality of nozzles can better cool the saggar area, so that the porous carbon material in the saggar is easily formed with many micro-cracks on the surface of the porous carbon material under the condition of thermal expansion and cold shrinkage, and at the same time, the liquid coolant absorbs a large amount of heat to become gaseous propane, ammonia and liquid nitrogen, thereby creating an inert atmosphere condition in the pyrolysis furnace. In this way, not only can the inert gas filling process be effectively saved, but also the operation process is more compact and simple, thereby improving the production efficiency, and the prepared flaky micro-crack hard carbon with good electrical conductivity and stable structure can be ensured.

[0070] In one embodiment, the total projection area of the plurality of nozzles is equal to the flat area of the porous carbon material. It can be understood that, if the projection area of each nozzle is less than the flat area of the porous carbon material, part of the high-temperature porous carbon material cannot form micro-cracks, thereby generating more defective products and increasing the defective rate; if the projection area of each nozzle is greater than the flat area of the porous carbon material, the cooling agent area is larger, thereby causing the surface of the porous carbon material in the saggar to shrink more obviously, so that micro-cracks with larger gaps are formed on the surface of the porous carbon material to reduce the structural strength of the flaky micro-crack hard carbon. Therefore, in the present application, the total projection area of the plurality of nozzles is equal to the flat area of the porous carbon material, so that the liquid coolant sprayed from each nozzle can better quickly cool the porous carbon material in the saggar to form many micro-cracks, and at the same time, the yield of single production can be ensured.

[0071] In one embodiment, the first-stage pyrolysis treatment and the second-stage pyrolysis treatment are carried out under the condition of liquid nitrogen or argon, and the ventilation amount of the liquid nitrogen or the argon is controlled to be 1 L / min to 3 L / min, so as to ensure that the flaky micro-crack hard carbon with stable structure, high electrical conductivity and porous structure is obtained.

[0072] It should be noted that in the preparation process of the traditional hard carbon negative material, the biomass precursor needs to be washed with alkali and acid respectively after calcination, such as patent CN 106299365 B discloses a biomass hard carbon negative material for sodium ion battery, and specifically discloses that after calcination, alkali washing and acid washing are needed respectively, which is more troublesome, resulting in low production efficiency. It is worth mentioning that although alkali washing or acid washing can more completely remove impurities on the surface of the precursor to obtain hard carbon with lower ash content and other impurities, in actual application, the traditional alkali washing or acid washing does not have a very obvious effect on improving the specific capacity, cycle and rate performance of sodium ion battery, that is, the first charge-discharge efficiency of the traditional sodium ion battery is only 90%, and the reversible specific capacity is 300 mAh / g, which still has the phenomenon of low first charge-discharge efficiency and reversible specific capacity, and cannot meet the requirements of commercialization of sodium ion battery.

[0073] Therefore, in the present application, since the biomass raw material is subjected to oscillation cleaning operation by the ultrasonic cleaning device, not only can the impurities, dust and soluble pollutants on the surface of the biomass raw material be effectively removed, but also the biomass raw material can be subjected to ultrasonic oscillation and beating, which is beneficial to the formation of porous carbon material with large pore size, large interlayer spacing and stable structure in the pyrolysis process of the precursor particles, so as to ensure the preparation of porous carbon material with relatively regular internal structure, thereby ensuring that the surface of the porous carbon material is relatively well heated during cooling treatment, and further ensuring that the porous carbon material can form microcracks in the rapid cooling stage; and the impurities, dust and soluble pollutants on the surface of the biomass raw material can be effectively removed to ensure that the precursor particles obtained are fluffy and have no impurities on the surface, which is beneficial to the formation of microcracks during subsequent cooling treatment. The biomass raw material subjected to oscillation cleaning operation is subjected to drying operation to effectively remove the moisture of the biomass raw material, and then the dried biomass raw material is subjected to crushing operation to obtain precursor particles. In this way, the surface area of the biomass raw material is increased, which is helpful for the subsequent rapid pyrolysis and cooling treatment of the precursor particles. At the same time, the subsequent first-stage pyrolysis, second-stage pyrolysis and cooling treatment are carried out, so as to ensure the preparation of lamellar microcrack hard carbon. While ensuring that the lamellar microcrack hard carbon has high electrical conductivity, it also provides a new channel for the effective diffusion and embedding of sodium ions, thereby helping more sodium ions to be rapidly de-embedded in the negative material. At the same time, the lamellar microcrack hard carbon can also increase the contact area between the negative material and the electrolyte, thereby effectively improving the permeability of the electrolyte and further improving the electrolyte injection efficiency. Moreover, it is also helpful for the electrolyte to better infiltrate the negative material during the cycle of the sodium ion battery, thereby greatly improving the initial efficiency, reversible capacity and cycle performance of the sodium ion battery. Moreover, the operation is relatively simple and the production efficiency is relatively high.

[0074] It is worth mentioning that, compared with traditional hard carbon, the alkali washing and acid washing operations are not required in the present application, not only simplifying the production process, but also improving the production efficiency, and the first charge-discharge efficiency reaches more than 96%, the reversible specific capacity is more than 350 mAh / g, and the capacity retention rate is maintained at more than 91% after 1000 cycles, thereby greatly improving the initial efficiency, reversible capacity and cycle performance of the sodium ion battery.

[0075] The above process for preparing the laminar microcrack hard carbon by rapid cooling utilizes the precursor particles as raw materials, and can utilize agricultural waste peanut shells, straws, leaves and dead branches, etc., which are not only widely available and cheap, but also beneficial to environmental protection.

[0076] The present application also provides a hard carbon prepared by the process for preparing the laminar microcrack hard carbon by rapid cooling described in any of the above embodiments.

[0077] It can be understood that the laminar microcrack hard carbon prepared by the process for preparing the laminar microcrack hard carbon by rapid cooling of the present application can provide new channels for the effective diffusion and embedding of sodium ions while ensuring that the laminar microcrack hard carbon has high electrical conductivity, thereby helping more sodium ions to be quickly de-embedded in the negative electrode material, and the laminar microcrack hard carbon can also increase the contact area of the negative electrode material and the electrolyte, thereby effectively improving the permeability of the electrolyte and the liquid injection efficiency of the electrolyte, and also helping the electrolyte to better infiltrate the negative electrode material during the cycle process of the sodium ion battery, thereby greatly improving the initial efficiency, reversible capacity and cycle performance of the sodium ion battery.

[0078] The present application also provides a sodium ion battery comprising a positive electrode and a negative electrode, and the active material of the negative electrode comprises the hard carbon described in any of the above embodiments. It can be understood that by applying the hard carbon prepared by the present application to the sodium ion battery, the laminar microcrack hard carbon can provide new channels for the effective diffusion and embedding of sodium ions, thereby helping more sodium ions to be quickly de-embedded in the negative electrode material, and the laminar microcrack hard carbon can also increase the contact area of the negative electrode material and the electrolyte, thereby effectively improving the permeability of the electrolyte and the liquid injection efficiency of the electrolyte, and also helping the electrolyte to better infiltrate the negative electrode material during the cycle process of the sodium ion battery, thereby greatly improving the initial efficiency, reversible capacity and cycle performance of the sodium ion battery.

[0079] Compared with the prior art, the present application has at least the following advantages:

[0080] 1) The process for preparing the laminar microcrack hard carbon by rapid cooling above, first, the precursor particles are subjected to a first pyrolysis treatment, so that the organic macromolecules in the biomass can be quickly released, to obtain a graphite crystalline carbon material; then the graphite crystalline carbon material is subjected to a second pyrolysis treatment, and the temperature of the second pyrolysis is higher than that of the first pyrolysis, so that the graphite crystalline can be gradually carbonized, to obtain a porous carbon material with large pores, stable structure and good electrical conductivity; then, the porous carbon material is subjected to a cooling treatment, so that many microcracks can be formed on the surface of the porous carbon material, to obtain a laminar microcrack hard carbon, so that while ensuring that the laminar microcrack hard carbon has high electrical conductivity, it also provides a new channel for the effective diffusion and embedding of sodium ions, thereby helping more sodium ions to be quickly de-embedded in the negative electrode material, at the same time, the laminar microcrack hard carbon can also increase the contact area of the negative electrode material with the electrolyte, thereby effectively improving the permeability of the electrolyte, and in turn improving the liquid injection efficiency of the electrolyte, and also helping the electrolyte to better wet the negative electrode material during the cycle process of the sodium ion battery, thereby greatly improving the initial efficiency, reversible capacity and cycle performance of the sodium ion battery.

[0081] 2) The process for preparing the laminar microcrack hard carbon by rapid cooling above, using precursor particles as raw materials, can utilize agricultural waste such as peanut shells, straw, leaves and dead branches, which are not only widely available and inexpensive, but also more environmentally friendly.

[0082] The following examples are given to illustrate some specific embodiments. If % is mentioned, it means percentage by weight. It should be noted that the following examples do not exhaust all possible cases, and the materials used in the following examples, unless otherwise specified, can be obtained from commercial sources.

[0083] Example 1

[0084] Pre-treatment of biomass raw material: the straw is washed clean by an ultrasonic cleaning device (20 min) with deionized water and alcohol (volume ratio of deionized water to alcohol is 4:1), and then is dried in a forced air drying oven (70°C, 4h), wherein the vibration frequency of the ultrasonic cleaning device is 20 Hz;

[0085] Preparation of precursor particles: the dried straw is crushed by a crusher and passed through a 200-mesh sieve to obtain precursor particles;

[0086] Preparation of the laminar microcrack hard carbon: 5 kg of the precursor particles were transferred into a crucible and placed in a pyrolysis furnace, and two-stage pyrolysis and rapid cooling were carried out under the protection of an argon atmosphere according to the following procedures: the first-stage pyrolysis treatment, the precursor particles were heated from room temperature to 600 °C at a heating rate of 4 °C / min, and kept at 600 °C for 2.5 h; the second-stage pyrolysis treatment, the temperature was increased to 1000 °C at a heating rate of 3 °C / min, and kept at 1000 °C for 2 h; rapid cooling: the pyrolysis furnace was rapidly cooled to room temperature with liquid ammonia as the cooling agent, and the first-stage pyrolysis and the second-stage pyrolysis treatment were continuously supplied with argon at a flow rate of 2 L / min, and the cooling agent was supplied at a flow rate of 18 L / min.

[0087] Example 2

[0088] Pre-treatment of the biomass raw material: the peanut shells were washed with deionized water and alcohol (the volume ratio of deionized water to alcohol was 4:1) by an ultrasonic cleaning device (30 min), and then dried in a blast drying oven (70 °C, 4 h), wherein the vibration frequency of the ultrasonic cleaning device was 40 Hz;

[0089] Preparation of the precursor particles: the dried peanut shells were crushed by a pulverizer and passed through a 325-mesh sieve to obtain the precursor particles;

[0090] Preparation of the laminar microcrack hard carbon: 5 kg of the precursor particles were transferred into a crucible and placed in a pyrolysis furnace, and two-stage pyrolysis and rapid cooling were carried out under the protection of an argon atmosphere according to the following procedures: the first-stage pyrolysis treatment, the precursor particles were heated from room temperature to 600 °C at a heating rate of 4 °C / min, and kept at 600 °C for 2.5 h; the second-stage pyrolysis treatment, the temperature was increased to 1000 °C at a heating rate of 3 °C / min, and kept at 1000 °C for 2 h; rapid cooling: the pyrolysis furnace was rapidly cooled to room temperature with liquid ammonia as the cooling agent, and the first-stage pyrolysis and the second-stage pyrolysis treatment were continuously supplied with argon at a flow rate of 2 L / min, and the cooling agent was supplied at a flow rate of 18 L / min.

[0091] Example 3

[0092] Pre-treatment of the biomass raw material: the peanut shells were washed with deionized water and alcohol (the volume ratio of deionized water to alcohol was 4:1) by an ultrasonic cleaning device (30 min), and then dried in a blast drying oven (70 °C, 4 h), wherein the vibration frequency of the ultrasonic cleaning device was 40 Hz;

[0093] Preparation of the precursor particles: the dried peanut shells were crushed by a pulverizer and passed through a 325-mesh sieve to obtain the precursor particles;

[0094] Preparation of laminar microcrack hard carbon: 5 kg of precursor particles were transferred into a crucible and placed in a pyrolysis furnace, and two-stage pyrolysis and rapid cooling were carried out under the protection of argon atmosphere according to the following procedures: the first stage pyrolysis treatment, the precursor particles were heated from room temperature to 600℃ at a heating rate of 4℃ / min, and kept for 2h; the second stage pyrolysis treatment, the temperature was increased to 1000℃ at a heating rate of 3℃ / min, and kept for 2h; rapid cooling: the pyrolysis furnace was rapidly cooled to room temperature with propane as the cooling agent, and the whole process of the first stage pyrolysis and the second stage pyrolysis treatment was continuously passed with argon, the argon flow rate was 2.5L / min, and the cooling agent flow rate was 20L / min.

[0095] Comparative Example 1

[0096] The difference between Example 2 and Comparative Example 1 is that the second stage pyrolysis treatment and the cooling step are not carried out in the step of preparing laminar microcrack hard carbon, and the rest remains unchanged.

[0097] Comparative Example 2

[0098] The difference between Example 2 and Comparative Example 2 is that the cooling step is not carried out in the step of preparing laminar microcrack hard carbon, and the rest remains unchanged.

[0099] Comparative Example 3

[0100] The difference between Example 2 and Comparative Example 3 is that the ultrasonic cleaning device is not used in the step of pretreating the biomass raw material, i.e., the peanut shell is washed with deionized water and alcohol (volume ratio of deionized water to alcohol is 4:1), and then dried in a forced air drying oven (70℃, 4h); the rest remains unchanged.

[0101] The laminar microcrack hard carbon prepared in Examples 1-3 and Comparative Examples 1-3 was mixed with acetylene black and polyvinylidene fluoride in a mass ratio of 8:1:1, and then dissolved in nitrogen methyl pyrrolidone, and then coated on a copper foil current collector to prepare a sodium battery negative electrode sheet; assembled into a button type sodium battery in a glove box, with a sodium sheet as the positive electrode and NaPF6 as the electrolyte (non-aqueous solvent is a mixture of EC, DMC and DEC in a volume ratio of 1:1:1); the sodium ion battery was tested for electrochemical performance using a new Wei CT-4008, and the cycle period was set to 1000 cycles, wherein the current during the cycle period was 0.1C, and the data in Table 1 below were obtained:

[0102]

[0103] As can be seen from the examples 1-3 and the comparative examples 1-3 in Table 1, the laminar microcrack hard carbon with large pores, stable structure and good conductivity can be quickly obtained by ultrasonic cleaning, first pyrolysis, second pyrolysis and cooling of the biomass raw material in the examples 1-3, respectively, so that the first efficiency of the laminar microcrack hard carbon in the examples 1-3 is all above 96%, and the first discharge gram capacity is all ≥356 mAh / g, which is obviously higher than the first efficiency and the first discharge gram capacity of the comparative examples 1-3, and the capacity retention rate of the examples 1-3 is still above 91% after 1000 cycles, so that the cycle performance of the laminar microcrack hard carbon in the examples 1-3 is obviously better than that of the comparative examples 1-3, for details, please refer to Figure 4 , and the indicators of the example 2 are the best. It is worth mentioning that the capacity retention rate of the example 2 can be maintained at 102% after 1000 cycles.

[0104] The above examples only express several embodiments of the present application, and the description is more specific and detailed, but it cannot be understood as the restriction of the scope of the patent. It should be pointed out that for ordinary skilled in the art, without departing from the concept of the present application, a number of modifications and improvements can be made, which all belong to the protection scope of the present application. Therefore, the protection scope of the patent of the present application should be subject to the appended claims.

Claims

1. A process for rapidly cooling and preparing lamellar microcracked hard carbon, characterized in that, Includes the following steps: The biomass raw material, peanut shells, is subjected to an ultrasonic cleaning process involving vibration and cleaning. The biomass raw material is poured into the cleaning tank of an ultrasonic cleaning device containing a mixture of water and alcohol and ultrasonically cleaned for 20 to 30 minutes; wherein the volume ratio of water to alcohol is (3 to 4): 1; the vibration frequency of the ultrasonic cleaning device is 20 Hz to 50 Hz. The biomass raw material, after being subjected to a shaking and washing operation, is then dried. The dried biomass raw material is pulverized to obtain precursor particles. The precursor particles were sieved to obtain precursor particles with a particle size of 44 μm to 74 μm. When the precursor particles are subjected to the first stage of pyrolysis treatment, they are heated to 400°C to 800°C at a heating rate of 3°C / min to 8°C / min to obtain graphite microcrystalline carbon material; wherein, during the first stage of pyrolysis treatment of the precursor particles, the temperature is kept constant for 1h to 3h. The graphite microcrystalline carbon material is subjected to a second-stage pyrolysis treatment to obtain a porous carbon material, wherein the temperature of the second-stage pyrolysis is higher than that of the first-stage pyrolysis; the graphite microcrystalline carbon material is subjected to a second-stage pyrolysis treatment by heating to 600~1200°C at a heating rate of 2~5°C / min and pyrolyzing at a constant temperature for 2h~4h. Coolant is introduced into the porous carbon material to cool it, resulting in lamellar microcracked hard carbon, wherein the flow rate of the coolant is controlled at 15L / min to 25L / min.

2. The process for rapidly cooling and preparing lamellar microcracked hard carbon according to claim 1, characterized in that, The coolant includes at least one of propane, liquid ammonia, and liquid nitrogen.

3. The process for rapidly cooling and preparing lamellar microcracked hard carbon according to claim 1, characterized in that, The first and second stages of pyrolysis are performed under liquid nitrogen or argon conditions, and the flow rate of the liquid nitrogen or argon is controlled to be 1 L / min to 3 L / min.

4. A type of hard carbon, characterized in that, It is prepared using the rapid cooling process described in any one of claims 1 to 3 to prepare lamellar microcracked hard carbon.

5. A sodium-ion battery, comprising a positive electrode and a negative electrode, characterized in that, The active material of the negative electrode includes the hard carbon as described in claim 4.

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