A sucrose-based hard carbon with a high proportion of long plateau region capacity and its preparation method

Abstract

This invention discloses a sucrose-based hard carbon with a high capacity ratio in the long plateau region and its preparation method, belonging to the field of sodium-ion battery anode materials. The preparation method includes using sucrose as a raw material, first preparing a precursor, ball milling followed by low-temperature pre-carbonization, and then high-temperature carbonization. The material of this invention has a significantly reduced specific surface area, increased carbon interlayer spacing, decreased proportion of micropores and mesopores, and increased packing density, thereby improving the energy density of the material. The reversible specific capacity of the hard carbon material of this invention can be increased from an initial 270 mAh.g. ‑1 Upgraded to 370mAh.g ‑1 The capacity share of the low-voltage platform region can be increased from 54% to 63%, 100mA.g ‑1 It still retains 280mAh after 150 cycles. ‑1 Its capacity is much higher than that of directly calcined sucrose-based hard carbon materials (180 mAh.g). ‑1 The material exhibits good cycle stability and rate performance.

Description

Technical Field

[0001] This invention relates to the field of sodium-ion battery anode materials, and more particularly to a sucrose-based hard carbon with a high capacity ratio in the long plateau region and its preparation method. Background Technology

[0002] In recent years, the consumption of fossil fuels has led to a series of chain reactions, including carbon emissions, the greenhouse effect, and the energy crisis. The large-scale development of renewable energy sources such as solar, wind, and tidal power, and the acceleration of the energy consumption structure towards a low-carbon and clean energy direction, has become a global consensus, with new energy playing a crucial role. Since the commercialization of lithium-ion batteries by Sony in the 1990s, lithium-ion batteries have been widely used in mobile electronic devices and electric vehicles. However, the scarce lithium resources in the Earth's crust are insufficient to meet the ever-increasing energy demand. The abundant sodium resources on Earth make sodium-ion batteries a viable alternative to lithium-ion batteries. However, providing suitable anode materials is the main constraint on the development of sodium-ion batteries. In 2000, Dahn et al. discovered that the reversible specific capacity of hard carbon anodes could reach 300 mAh·g. -1 The lithium storage capacity is close to that of graphite, which has sparked a research boom in hard carbon anode materials for sodium-ion batteries.

[0003] Hard carbon, also known as "difficult-to-graphitize carbon," is composed of randomly stacked graphene sheets with a lateral dimension of 40 Å that are twisted. It can maintain this disordered state even at a high temperature of 2800 °C. The (002) interlayer spacing of hard carbon is usually 0.37~0.40 nm, which is larger than the interlayer spacing of graphite (0.335 nm). Furthermore, hard carbon contains nanopores and defects formed due to the different orientations of the graphene sheets. These pores and defects are the key factors that determine the electrochemical performance of hard carbon materials.

[0004] Currently, there are two main categories of raw materials for hard carbon materials: petroleum and coal chemical materials and biomass materials. Petroleum and coal chemical materials are widely available, and the hard carbon produced from them has stable performance and high carbon yield. However, due to their structural characteristics, their specific capacity is relatively low compared to other materials. Biomass materials are widely available and inexpensive, and the hard carbon produced from them has a higher specific capacity. However, due to the instability of crop growth, the raw materials can experience significant batch-to-batch fluctuations.

[0005] This method uses sucrose as the raw material for hard carbon materials. Sucrose is abundant in sugarcane, sugar beets, and fruits, and is renewable. Sucrose is a disaccharide formed by the condensation and dehydration of the hemiacetal hydroxyl groups of one molecule of glucose and one molecule of fructose. Furthermore, it is a small molecule compound with a stable structure, which is beneficial for mass production process debugging. In particular, sucrose molecules have a high carbon content and a simple composition, making it an inexpensive and renewable biomass carbon source. Hard carbon prepared by sucrose carbonization has been proven to be applicable to sodium-ion batteries.

[0006] However, sucrose-based hard carbon suffers from problems such as low reversible specific capacity, low first-cycle coulombic efficiency, and poor rate performance. The main solutions to these problems generally include sintering sucrose with other materials such as phenolic resins, graphene oxide, and lignite; adding trace amounts of surfactants during the hydrothermal carbonization stage to regulate the morphology of the carbon spheres, ultimately altering their microstructure; and improving the electrochemical performance of sucrose-based hard carbon through interfacial construction and electrolyte optimization.

[0007] For example, Chinese patent application CN108682858A discloses a method for preparing hard carbon by "dissolving coal tar pitch and mixing it with biomass polyhydroxy alcohol (starch, cellulose or sucrose), then pre-carbonizing it at low temperature, and then treating it at 1100℃~1200℃". This patent is to prepare a composite material by combining asphalt and biomass materials. It removes a large amount of special solvents from the system through pre-carbonization. This patent has at least the following problems: complex raw materials, lengthy preparation process, environmental pollution caused by the addition and removal of special solvents, and increased costs. For example, Chinese patent application CN115304052A discloses a method for preparing a wood-based composite material, in which sucrose is used as a material to modify lignin-based hard carbon and plays a role in filling the spaces between lignin cells. The problems with this patent include at least the following: the wood-based carbon source and the sugar-based carbon source are combined, but the wood-based carbon source raw material used is unstable, uncertain and non-uniform. In addition, Chinese patent application CN107437613A discloses a method for preparing a microcrystalline graphite-hard carbon composite anode material. The modification method used in this patent is actually a way of constructing a heterojunction, that is, mixing the two materials. In other words, the composite material is the same as the method used in the previous two patents. This patent only compares the performance of the composite material microcrystalline graphite, and does not fully utilize the electrochemical performance of biomass hard carbon itself.

[0008] In summary, the above-mentioned patents are all methods for preparing hard carbon composite materials, and they have problems such as unstable, uncertain and non-uniform carbon source raw material composition, complex raw materials, lengthy preparation process, and environmental pollution and increased costs caused by the addition and removal of special solvents. There is an urgent need to solve the above problems in this field. Summary of the Invention

[0009] One of the objectives of this invention is to provide a method for preparing sucrose-based hard carbon with a high proportion of long plateau region capacity, so as to solve the above-mentioned problems.

[0010] To achieve the above objectives, the technical solution adopted by the present invention is as follows: a method for preparing sucrose-based hard carbon with a high proportion of long plateau region capacity, comprising the following steps: (1) The sucrose solution was subjected to hydrothermal carbonization, and after hydrothermal treatment, it was washed and dried to obtain the sucrose-based hard carbon precursor M-SC; (2) After the precursor M-SC obtained in step (1) is ball-milled into powder, it is pre-carbonized at low temperature. The temperature of the pre-carbonization is 400℃~600℃, the heating rate is 1~7℃ / min, and an inert gas is introduced for protection during the heating and holding process. (3) The material that has undergone low-temperature pre-carbonization in step (2) is carbonized at high temperature in a tube furnace; (4) After high-temperature carbonization, after the material cools down, the black material is taken out, washed, filtered and dried to obtain sucrose-based hard carbon anode material with a capacity ratio of long platform region.

[0011] Temperature, heating rate, and calcination method significantly influence the final electrochemical performance of hard carbon during its formation. This invention addresses this issue by simply controlling the calcination process to affect the carbon layer arrangement, pore distribution, interlayer distance, and disorder of hard carbon. This method is currently the simplest and most cost-effective. Furthermore, recent research has shown that sodium storage in the plateau region of hard carbon is an effective way to improve its electrochemical performance. Sodium storage in the plateau region has been proven to be related to the interlayer spacing, closed pores, and pore size within the hard carbon. The method of this invention can increase the capacity ratio of the plateau region. Based on the influence of electrochemical performance, especially the capacity ratio of the plateau region during the first discharge cycle, this invention sets the pre-carbonization temperature according to the temperature range of secondary molecular structure changes in sucrose and demonstrates that low-temperature pre-carbonization improves carbon yield, thus solving the problem of low carbonization yield for biomass-based hard carbon.

[0012] As a preferred technical solution, in step (1), the hydrothermal carbonization is carried out in a hydrothermal kettle at a hydrothermal temperature of 160-200°C, more preferably 180°C; the hydrothermal heat preservation time is 8-24 hours, more preferably 8 hours; and the washing solvent is distilled water or ethanol.

[0013] As a preferred technical solution, in step (2), the low-temperature pre-carbonization is carried out in a tube furnace, and the low-temperature pre-carbonization time is 1 to 5 hours, more preferably 4 hours, and the inert gas is high-purity argon or nitrogen.

[0014] As a preferred technical solution, in step (3), the high-temperature carbonization is carried out at a heating rate of 1 to 7°C / min to 1000°C to 1300°C and held for 8 to 12 hours. Inert gas is introduced for protection during the heating and holding process.

[0015] As a preferred technical solution, in step (4), water or ethanol is used for washing.

[0016] This invention optimizes the microstructure of hard carbon by appropriately adjusting a low-temperature pre-carbonization process, creating more reversible plateau regions for sodium storage, thereby improving the electrochemical performance of sucrose-based hard carbon in sodium-ion batteries.

[0017] Specifically, the inventors of this application discovered through extensive experiments that adding a low-temperature pre-carbonization process during sintering provides sufficient reaction time for the second degradation of sucrose, delaying the overflow of volatile small molecules from the system. The reduced overflow of small molecule gases in the system leads to a decrease in the porosity induced by the vapor generated by the overflow of small molecules in the resulting hard carbon material. The low-temperature pre-carbonization stage allows small molecules to become active within the carbon layers, participating in the arrangement and movement of the carbon layers, resulting in a more disordered carbon layer orientation, providing conditions for the formation of closed pores. Increased closed pores are beneficial for increasing the reversible capacity of the hard carbon material; the more closed pores, the greater the possibility of obtaining a higher reversible capacity. Simultaneously, the active small molecules within the carbon layers cause some micropores to aggregate into mesopores, further improving material performance. By controlling the pre-carbonization temperature and heating rate, more closed micropores are introduced into the sucrose-based hard carbon material, optimizing the ratio of micropores to mesopores in the material. After low-temperature pre-carbonization, the specific surface area of ​​the material is significantly reduced, the carbon layer spacing increases, the ratio of micropores to mesopores decreases, the packing density of the material increases, and the energy density of the material is improved.

[0018] The method of this invention prepares sucrose-based hard carbon anode materials by primarily controlling the amount and rate of volatile small molecules generated within the carbon material at low temperatures, thus positively intervening in the growth process of sucrose-based hard carbon. Precise control of the hard carbon microstructure is achieved by regulating the temperature and heating rate of the low-temperature holding section. Specifically, the hydrothermal precursor M-SC undergoes a second carbonization process. When the temperature is below 600℃, its structure undergoes a second change. During this process, some volatile small molecules leave the carbon material bulk, causing significant mass loss. When the temperature rises above 600℃, its structure remains essentially unchanged, and the mass change is negligible. Therefore, this invention incorporates a low-temperature insulation stage during the M-SC carbonization process, providing sufficient reaction time for the second structural change and delaying the overflow of small molecules. Simultaneously, the active small molecules in the system interfere with the arrangement of the carbon layers, making the carbon layer arrangement more random and resulting in the formation of more closed pores. Furthermore, some micropores are merged into mesopores, increasing the carbon layer spacing. These structural changes are all beneficial for sodium storage in the plateau region, and sodium storage in the long, low-voltage plateau region will contribute more reversible specific capacity, which is beneficial for the application of sucrose-based hard carbon in sodium-ion batteries.

[0019] The second objective of this invention is to provide a sucrose-based hard carbon with a high capacity ratio in the long plateau region obtained by the above-described method.

[0020] Compared with the prior art, the advantages of the present invention are: the reversible specific capacity of the hard carbon material of the present invention can be increased from the initial 270 mAh·g -1 Upgraded to 370mAh.g -1 The capacity share of the low-voltage platform region can be increased from 54% to 63%, 100mA.g -1 It still retains 280mAh after 150 cycles. -1 Its capacity is much higher than that of directly calcined sucrose-based hard carbon materials (180 mAh.g). -1 The material exhibits good cycle stability and rate performance; moreover, the preparation method of this invention uses a stable and uniform carbon source, simple raw materials, a simple preparation process, and low cost, and solves the problem of environmental pollution, thus having good social and economic benefits. Attached Figure Description

[0021] Figure 1 This is a comparison graph showing the cycle performance of negative electrode sheets made of sucrose-based hard carbon in different embodiments and comparative examples of the present invention. Figure 2 This is a TEM image of sucrose-based hard carbon from Comparative Example 1 of the present invention. Figure 3 This is a TEM image of the sucrose-based hard carbon of Example 4 of the present invention. Detailed Implementation

[0022] The invention will now be further described with reference to the accompanying drawings. Example 1:

[0023] A sucrose-based hard carbon with a high proportion of long plateau region capacity is prepared by the following method: (1) Prepare a 0.2 mol / L sucrose solution, measure 70 mL of the above sucrose solution into a 100 mL polytetrafluoroethylene hydrothermal reactor, place it in a vacuum oven at 180 °C and keep it warm for 8 h. After the hydrothermal treatment is completed, wash with water, filter and dry to obtain reddish-brown blocky sucrose-based hard carbon precursor M-SC. (2) The precursor M-SC obtained in step (1) and zirconium beads are ground into powder in a ball mill jar at a mass ratio of 1:1.7, and then pre-carbonized at low temperature in a tube furnace. The specific method is: heating to 400°C in an argon atmosphere at a heating rate of 5°C / min and holding for 4 hours. (3) After pre-carbonization, heat to 1300℃ at a heating rate of 1~5℃ / min, sinter for 8 hours, cool to room temperature, and pass high-purity argon gas for protection during the heating and holding process. (4) After the material cools to room temperature, the black material is taken out from the tube furnace, washed with water, filtered and dried to obtain sucrose-based hard carbon material with a capacity ratio of long platform region. Example 2:

[0024] The difference between this embodiment and embodiment 1 is that the low-temperature pre-carbonization temperature in step (2) is 450°C, while the rest is the same as in embodiment 1. Example 3:

[0025] The difference between this embodiment and embodiment 1 is that the low-temperature pre-carbonization temperature in step (2) is 600°C, while the rest is the same as in embodiment 1. Example 4:

[0026] The difference between this embodiment and embodiment 2 is that the heating rate in step (2) is 3℃ / min, while the rest is the same as in embodiment 2. Example 5:

[0027] The difference between this embodiment and embodiment 2 is that the heating rate in step (2) is 1℃ / min, while the rest is the same as in embodiment 2.

[0028] Comparative Example 1: The difference between this comparative example and Example 4 is that the precursor M-SC was ground into powder and then directly heated to 1300°C for 8 hours in an argon atmosphere at a heating rate of 5°C / min, without low-temperature pre-carbonization. The heating rate was 5°C / min, and the rest was the same as in Example 4.

[0029] Performance test example: The sucrose-based hard carbon anode material prepared in the above examples and comparative examples was mixed with conductive agent carbon black (SP) and binder (PVDF) at a mass ratio of 8:1:1, and an appropriate amount of N-methylpyrrolidone was added. The mixture was then prepared into a slurry in a planetary ball mill. The slurry was then coated onto copper foil with a 125 μm doctor blade, dried in a forced-air oven at 60°C for 2 hours, and then dried in a vacuum at 105°C for 12 hours to prepare a negative electrode sheet for button cells. Assemble the aforementioned negative electrode for button cells inside a glove box, allow it to stand at room temperature for 12 hours, and then apply it at 20 mA / g under a voltage range of 0.01–2.5V. -1 After three cycles at a current density of 100 mA.g -1 Current density was used for charge-discharge cycle testing, and the results are shown in Table 1. Table 1: Specific capacity of sodium-ion batteries during the first charge / discharge cycle (20 mA g) -1 (current density)

[0030] By comparing the first-cycle charge-discharge specific capacity of each embodiment and comparative example, it can be seen that low-temperature pre-carbonization significantly increases the first-cycle charge specific capacity of sucrose-based hard carbon, from 281 mAh.g. -1 Increased to 374mAh.g -1The first-cycle coulombic efficiency increased from 66% to 85%; in particular, the sodium storage contribution capacity in the plateau region of the first-cycle discharge curve increased from 54% to 63%. The inventors compared the changes in the internal microstructure of the materials in Examples 2, 4, and Comparative Example 1, and tested the specific surface area and corresponding pore size distribution of the materials using nitrogen isothermal adsorption-desorption experiments; X-ray diffraction was used to test the interlayer spacing (d) of the materials. 002 ), and L, representing the size of graphite crystallites, was calculated according to the Sher formula. a L c Raman spectroscopy was used to characterize the carbon layer, obtaining the ratio A of the D peak to the G peak, which represents the degree of disorder. D / A G The specific data results are shown in Table 2: Table 2. Effects of low-temperature precarbonization on the microstructural parameters of sucrose-based hard carbon

[0031] In Table 2, Example 1 represents "Comparative Example 1", Example 2 represents "Example 2", and Example 3 represents "Example 4"; As can be seen from Table 2, the specific surface area of ​​the material is significantly reduced after low-temperature pre-carbonization, the micro-mesoporous ratio is significantly reduced, and the large pore size is beneficial for sodium storage in the platform region.

[0032] The transmission electron microscope image of Comparative Example 1 is shown below. Figure 2 As shown, the transmission electron microscope image of the material in Example 4 is as follows. Figure 3 As shown, comparison Figure 2 and Figure 3 The TEM image clearly shows an increase in closed pores inside the hard carbon of Example 4 due to the random orientation of the carbon layers, and a further increase in the disorder of the carbon layers. This is consistent with the A shown in Table 2. D / A G Increase, L a L c Reduced information consistency and increased carbon interlayer spacing after low-temperature pre-carbonization affected the plateau capacity of hard carbon during the first discharge cycle. It was found that a higher proportion of micropores, a larger carbon interlayer spacing, a lower degree of graphitization, and more closed pore structures are more conducive to sodium storage in the plateau region. Higher sodium storage in the plateau region will contribute more reversible specific capacity and be more conducive to improving cycle performance.

[0033] In addition, the hydrothermal intermediate before and after calcination and the prepared hard carbon material were weighed using a balance, and the carbonization yield of the material in the high-temperature carbonization stage was calculated. The results are shown in Table 3. Table 3 Comparison of carbon yield in the high-temperature carbonization stage for each embodiment and comparative example.

[0034] As can be seen from Table 3, the method of the present invention can control the microstructure of sucrose-based hard carbon simply by low-temperature pre-carbonization, significantly improve sodium storage in the sucrose-based hard carbon platform region, significantly increase the reversible specific capacity, and improve both cycle performance and rate performance. Furthermore, it improves the yield of hard carbon prepared from sucrose, with the carbon yield in the high-temperature carbonization stage increasing from 44.64% to 51.17%. It has significant advantages such as simple synthesis process, short process, and low cost.

[0035] The above description is only a preferred embodiment of the present invention and is not intended to limit the present invention. Any modifications, equivalent substitutions, and improvements made within the spirit and principles of the present invention should be included within the protection scope of the present invention.

Claims

1. A method for preparing sucrose-based hard carbon with a high proportion of long plateau region capacity, characterized in that, The method consists of the following steps: (1) The sucrose solution was subjected to hydrothermal carbonization, and after hydrothermal treatment, it was washed and dried to obtain the sucrose-based hard carbon precursor M-SC; (2) After the precursor M-SC obtained in step (1) is ball-milled into powder, it is pre-carbonized at low temperature. The temperature of the pre-carbonization is 450℃, the pre-carbonization time is 1 to 5 hours, the heating rate is 3℃ / min, and inert gas is introduced for protection during the heating and holding process. (3) The material that has undergone low-temperature pre-carbonization in step (2) is carbonized at high temperature in a tube furnace; (4) After high-temperature carbonization, after the material cools down, the black material is taken out, washed, filtered and dried to obtain sucrose-based hard carbon anode material with a capacity ratio of long platform region.

2. The method according to claim 1, characterized in that, In step (1), the hydrothermal carbonization is carried out in a hydrothermal reactor at a temperature of 160-200°C and a holding time of 8-24 hours. The washing solvent is distilled water or ethanol.

3. The method according to claim 1, characterized in that, In step (2), the low-temperature pre-carbonization is carried out in a tube furnace, and the inert gas is high-purity argon or nitrogen.

4. The method according to claim 1, characterized in that, In step (3), the high-temperature carbonization is carried out at a heating rate of 1 to 7°C / min to 1000°C to 1300°C and held for 8 to 12 hours. Inert gas is introduced for protection during the heating and holding process.

5. The method according to claim 1, characterized in that, In step (4), water or ethanol is used for washing.

6. Sucrose-based hard carbon with a high capacity ratio in the long plateau region obtained by the method according to any one of claims 1 to 5.

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