A modified coal-based hard carbon negative electrode material, its preparation method, a negative electrode sheet, and a sodium ion battery

Abstract

The present invention provides a modified coal-based hard carbon negative electrode material, a preparation method thereof, a negative electrode sheet and a sodium ion battery, belonging to the technical field of battery materials. The preparation method comprises the following steps: carbonizing raw coal to obtain a coal-based hard carbon material with a porous structure; placing the coal-based hard carbon material with a porous structure in a chemical vapor deposition device, introducing a first carbon source gas at a first gas velocity, and simultaneously keeping the temperature constant at a first temperature for T1, so that carbon deposition is carried out in the micropores of the coal-based hard carbon material, then introducing a second carbon source gas at a second gas velocity, and simultaneously keeping the temperature constant at a second temperature for T2, so that carbon deposition is carried out at the micropore orifice of the coal-based hard carbon material, and after completion, a modified coal-based hard carbon negative electrode material is obtained; wherein, the first gas velocity < the second gas velocity. The present invention can shallow the pore depth of the micropores and reduce the pore orifice size of the micropores, and solve the problems of low initial efficiency and low capacity of the coal-based hard carbon negative electrode material.

Description

Technical Field

[0001] The present invention belongs to the technical field of battery materials, and particularly relates to a modified coal-based hard carbon negative electrode material, a preparation method thereof, a negative electrode sheet, and a sodium ion battery. Background Art

[0002] In recent years, with continuous technological breakthroughs, sodium ion batteries have gradually moved towards industrialization.

[0003] Among the numerous negative electrode materials available for sodium ion batteries, carbon negative electrodes have become the preferred negative electrode materials for the commercial development of sodium ion batteries due to their low potential, high capacity, stable physical and chemical properties, and low cost. Hard carbon materials have large interlayer spacings, abundant defects, and nanoporous structures formed by the disordered stacking of graphite microcrystals, which all become active sites available for sodium ion storage, and thus have become the most promising negative electrode materials to promote the industrialization of sodium ions. However, the current hard carbon negative electrode materials still have problems such as uneven pore size distribution, low capacity, and low initial efficiency.

[0004] Therefore, how to solve the problems of uneven pore size distribution, low capacity, and low initial efficiency existing in hard carbon negative electrode materials is an important research direction at present. Summary of the Invention

[0005] Aiming at the deficiencies of the prior art, the purpose of the present invention is to provide a modified coal-based hard carbon negative electrode material, a preparation method thereof, a negative electrode sheet, and a sodium ion battery. The present invention has developed a method for controlling the micropores of coal-based hard carbon materials by a stepwise chemical vapor deposition method, which can not only improve the uniformity of the pore size distribution of coal-based hard carbon negative electrode materials, reduce the pore mouth size of micropores and shallow the pore depth of micropores, so as to reduce the consumption of the formation of the SEI film in the micropores, but also provide space for the deposition of sodium clusters in the plateau region, solving the problems of low initial efficiency and low capacity of coal-based hard carbon negative electrode materials. Moreover, this preparation method has a simple process, is easy to operate, and has good commercial potential.

[0006] To achieve the purpose of this invention, the present invention adopts the following technical solutions:

[0007] In the first aspect, the present invention provides a preparation method of a modified coal-based hard carbon negative electrode material, and the preparation method includes the following steps:

[0008] (1) Carbonize the raw coal to obtain a coal-based hard carbon material with a porous structure.

[0009] (2) Place the coal-based hard carbon material with a porous structure in a chemical vapor deposition device, introduce a first carbon source gas at a first gas velocity, and keep it at a constant temperature of T1 at the first temperature simultaneously, so that carbon deposition occurs inside the micropores of the coal-based hard carbon material. Then introduce a second carbon source gas at a second gas velocity, and keep it at a constant temperature of T2 at the second temperature simultaneously, so that carbon deposition occurs at the orifices of the micropores of the coal-based hard carbon material. After completion, the modified coal-based hard carbon negative electrode material is obtained.

[0010] Among them, the first gas velocity < the second gas velocity.

[0011] The present invention develops a method for controlling the micropores of coal-based hard carbon materials by a stepwise vapor deposition method, which can not only improve the uniformity of the pore size distribution of coal-based hard carbon negative electrode materials, reduce the orifice size of micropores and shallow the pore depth of micropores, so as to reduce the consumption of the formation of the SEI film inside the micropores, but also provide space for the deposition of sodium clusters in the plateau region, solve the problems of low initial efficiency and low capacity of coal-based hard carbon negative electrode materials, and moreover, the preparation method has a simple process, is easy to operate, and has good commercial potential.

[0012] Preferably, the raw coal in step (1) includes any one or a combination of at least two of anthracite, lean coal, coking coal, fat coal, gas coal, weakly caking coal, non-caking coal, long-flame coal or lignite, and is preferably long-flame coal.

[0013] Preferably, the temperature of the carbonization treatment in step (1) is 1100 - 1400 °C, for example, it can be 1100 °C, 1200 °C, 1300 °C or 1400 °C, etc.

[0014] In the present invention, carbonization treatment at an appropriate temperature helps the coal to discharge most of the impurities and volatile components, and at the same time form abundant pores inside it. Under the action of high temperature, the carbon layer lattice is distorted to form a large number of disordered graphite microcrystals and amorphous carbon, and these regions together form the "card house" structure of hard carbon, which is beneficial to the storage of sodium ions.

[0015] Preferably, the time of the carbonization treatment in step (1) is 1 - 10 h, for example, it can be 1 h, 2 h, 3 h, 4 h, 5 h, 6 h, 7 h, 8 h, 9 h or 10 h, etc.

[0016] Preferably, the average pore diameter of the coal-based hard carbon material with a porous structure in step (1) is 1 - 4 nm, for example, it can be 1 nm, 1.5 nm, 2 nm, 3 nm or 4 nm, etc., and the pore volume is 0.1 - 0.3 cm 3 / g, for example, it can be 0.1 cm 3 / g, 0.15 cm 3 / g, 0.2 cm 3 / g, 0.25 cm 3 / g or 0.3 cm 3 / g, etc.

[0017] Preferably, the N 2 specific surface area of the coal-based hard carbon material with a porous structure in step (1) is 300 - 600 m 2 / g, for example, it can be 300 m 2 / g, 350 m 2 / g, 400 m 2 / g, 450 m 2 / g, 500 m 2 / g, 550 m 2 / g or 600 m 2 / g, etc.

[0018] Preferably, the CO 2 specific surface area of the coal-based hard carbon material with a porous structure in step (1) is 100 - 200 m 2 / g, for example, it can be 100 m 2 / g, 120 m 2 / g, 140 m 2 / g, 160 m 2 / g, 180 m 2 / g or 200 m 2 / g, etc.

[0019] Preferably, the first gas velocity in step (2) is 10 - 50 mL / min, for example, it can be 10 mL / min, 20 mL / min, 30 mL / min, 40 mL / min or 50 mL / min, etc.

[0020] In the present invention, an appropriate first gas velocity helps the pyrolytic carbon to be evenly deposited inside the micropores, achieving the purpose of shallowening the pore depth. If the first gas velocity is too small, the carbon deposition rate is too low to fill the inside of the micropores; if the first gas velocity is too large, the carbon deposition rate is too high, the micropores are filled, and the pore structure disappears.

[0021] Preferably, the second gas velocity in step (2) is 100 - 300 mL / min, for example, it can be 100 mL / min, 150 mL / min, 200 mL / min, 250 mL / min or 300 mL / min, etc., preferably 120 - 240 mL / min.

[0022] In the present invention, an appropriate second gas velocity helps to control the contraction rate of the micropore orifice and prevent the deposited carbon from continuing to fill the inside of the micropores. If the second gas velocity is too small, the carbon deposition rate is too low, the orifice contraction is too slow, and the inside of the micropores is filled, and the pore structure disappears; if the second gas velocity is too large, the orifice is quickly narrowed, and then the micropores are directly blocked in diameter under the high-rate deposition, and the pore structure disappears.

[0023] Preferably, the types of the first carbon source gas and the second carbon source gas in step (2) independently include any one or a combination of at least two of methane, acetylene, ethane, propane, or benzene.

[0024] Preferably, the kinetic diameters of the gas molecules of the first carbon source gas and the second carbon source gas in step (2) are independently 0.4 - 0.5 nm, and can be, for example, 0.4 nm, 0.42 nm, 0.44 nm, 0.46 nm, 0.48 nm, or 0.5 nm, etc.

[0025] In the present invention, the kinetic diameter of the gas molecules of the carbon source gas is defined as 0.4 - 0.5 nm. The purpose is that the pyrolytic carbon of the carbon source gas fills the micropores. When the pore orifice of the micropores shrinks to be close to the size of the gas molecule kinetic diameter, the gas molecules cannot enter the micropores and then will modify the periphery of the pore orifice until the micropores are closed. At this time, the size of the pore orifice can be adjusted by adjusting the ventilation flow rate and the pyrolysis time.

[0026] Preferably, the first temperature in step (2) is 600 - 900 °C, and can be, for example, 600 °C, 700 °C, 800 °C, or 900 °C, etc., and T1 is 2 - 4 h, and can be, for example, 2 h, 2.5 h, 3 h, or 4 h, etc.

[0027] In the present invention, heating for T1 time at the first temperature helps the pyrolytic carbon of the carbon source gas to be evenly deposited in the micropores, achieving the purpose of filling the micropores and shallowing the pore depth.

[0028] Preferably, the second temperature in step (2) is 1000 - 1200 °C, and can be, for example, 1000 °C, 1050 °C, 1100 °C, 1150 °C, or 1200 °C, etc., and T2 is 10 - 30 min, and can be, for example, 10 min, 20 min, or 30 min, etc.

[0029] In the present invention, heating for T2 time at the second temperature. At this temperature, the pyrolysis rate of the carbon source gas is fast. Combined with the increased gas flow rate, it can quickly shrink the pore orifice, reduce the continued deposition of carbon in the pores, and prevent the micropores from being blocked.

[0030] Preferably, after carbon deposition in the micropores of the coal-based hard carbon material in step (2), the pore volume of the micropores is reduced to 0.02 - 0.06 cm 3 / g, and can be, for example, 0.02 cm 3 / g, 0.03 cm 3 / g, 0.04 cm 3 / g, 0.05 cm 3 / g, or 0.06 cm 3 / g, etc.

[0031] In the present invention, the micropore volume within the above range is conducive to conveniently and intuitively judging the change in pore depth, thereby controlling the pore depth within a suitable range.

[0032] Preferably, after carbon deposition at the micropore orifice of the coal-based hard carbon material in step (2), the orifice size of the micropores is 0.3 - 0.5 nm, and can be, for example, 0.3 nm, 0.35 nm, 0.4 nm, 0.5 nm, etc.

[0033] The present invention can reduce the orifice size of the micropores of the coal-based hard carbon material to 0.3 - 0.5 nm. The process is controllable, and the pore size distribution is uniform. The micropores with reduced orifice size and micropore depth can prevent the formation of SEI within the pores while providing a certain space for the deposition of sodium clusters in the plateau region, and can solve the problems of low initial efficiency and low capacity of the material.

[0034] In the present invention, after the first gas injection, the pore volume of the micropores decreases significantly, while the specific surface area remains basically unchanged, indicating that the micropores are partially filled and the pore depth is shallowed; after the second gas injection, the pore volume of the micropores decreases slightly, while the specific surface area decreases significantly, indicating that the pore orifice is tightened and the internal space of the micropores remains basically unchanged.

[0035] Preferably, the preparation method includes the following steps:

[0036] (a) Crushing the raw coal to a particle size D50 of 4 - 30 μm (which can be, for example, 4 μm, 5 μm, 10 μm, 15 μm, 20 μm, 25 μm, or 30 μm, etc.), then performing carbonization treatment in an inert atmosphere. The temperature of the carbonization treatment is 1100 - 1400 °C, the heating rate is 1 - 10 °C / min (which can be, for example, 1 °C / min, 2 °C / min, 3 °C / min, 4 °C / min, 5 °C / min, 6 °C / min, 7 °C / min, 8 °C / min, 9 °C / min, or 10 °C / min, etc.), and the time of the carbonization treatment is 1 - 10 h to obtain a coal-based hard carbon material with a porous structure. The average pore diameter of the coal-based hard carbon material with a porous structure is 1 - 4 nm, the pore volume is 0.1 - 0.3 cm 3 / g, N 2 The specific surface area is 300 - 600 m 2 / g, CO 2 The specific surface area is 100 - 200 m 2 / g.

[0037] (b) Place the coal-based hard carbon material with a porous structure in a chemical vapor deposition device, introduce a first carbon source gas with a first gas velocity of 10 - 50 mL / min and an inert protective gas with a gas velocity of 50 - 150 mL / min (such as 50 mL / min, 70 mL / min, 110 mL / min, or 150 mL / min, etc.), and keep it at a constant temperature of T1 at the first temperature simultaneously, so that carbon deposition occurs inside the micropores of the coal-based hard carbon material. The pore volume of the micropores is 0.02 - 0.06 cm 3 / g, then introduce a second carbon source gas with a second gas velocity of 100 - 300 mL / min and an inert protective gas with a gas velocity of 200 - 500 mL / min (such as 200 mL / min, 300 mL / min, 400 mL / min, or 500 mL / min, etc.) (exemplarily, such as nitrogen, argon, etc.), and keep it at a constant temperature of T2 at the second temperature simultaneously, so that carbon deposition occurs at the orifice of the micropores of the coal-based hard carbon material. The pore volume of the micropores is 0.01 - 0.03 cm 3 / g, the orifice size of the micropores is 0.3 - 0.5 nm, and the modified coal-based hard carbon negative electrode material is obtained after completion.

[0038] Among them, the first gas velocity < the second gas velocity, and the kinetic diameters of the gas molecules of the first carbon source gas and the second carbon source gas are each independently 0.4 - 0.5 nm; the first temperature is 600 - 900 °C, and T1 is 2 - 4 h; the second temperature is 1000 - 1200 °C, and T2 is 10 - 30 min.

[0039] In the second aspect, the present invention provides a modified coal-based hard carbon negative electrode material, which is prepared by the preparation method described in the first aspect.

[0040] The N 2 specific surface area of the modified coal-based hard carbon negative electrode material is 2 - 5 m 2 / g, such as 2 m 2 / g, 2.5 m 2 / g, 3 m 2 / g, 4 m 2 / g, or 5 m 2 / g, etc.

[0041] The CO 2 specific surface area of the modified coal-based hard carbon negative electrode material is 0 - 10 m 2 / g, such as 2 m 2 / g, 4 m 2 / g, 6 m 2 / g, 8 m 2 / g, or 10 m 2 / g, etc.

[0042] Through the preparation method provided by the present invention, the N of the modified coal-based hard carbon anode material 2 specific surface area and CO 2 specific surface area are reduced, which is beneficial to improving the structural stability of the coal-based hard carbon anode material, reducing the risk of pore collapse, and making it less likely to suffer structural damage or deformation when subjected to external forces or in a complex environment; at the same time, it is beneficial to reducing electron scattering and promoting the charge transfer rate, enabling the coal-based hard carbon anode material to exhibit better performance in applications. In addition, it is also beneficial to improving the adsorption selectivity and adsorption stability for specific substances.

[0043] In a third aspect, the present invention provides a negative electrode sheet, which includes the modified coal-based hard carbon anode material as described in the second aspect.

[0044] It should be noted that the negative electrode sheet further includes a conductive agent and a binder. Exemplarily, the conductive agent can be, for example, Ketjen black or Super P (conductive carbon black), etc., and the binder can be, for example, PVDF (polyvinylidene fluoride), CMC (sodium carboxymethyl cellulose) or SBR (styrene-butadiene rubber), etc.

[0045] In a fourth aspect, the present invention provides a sodium ion battery, which includes the negative electrode sheet as described in the third aspect.

[0046] The numerical ranges described in the present invention not only include the above-listed point values, but also include any point values between the above numerical ranges not listed. Due to space limitations and for the sake of brevity, the present invention does not exhaustively list the specific point values included in the range.

[0047] Compared with the prior art, the present invention has the following beneficial effects:

[0048] (1) The present invention develops a method for controlling the micropores of coal-based hard carbon materials by a stepwise chemical vapor deposition method, which can not only improve the pore size distribution uniformity of the coal-based hard carbon anode material, reduce the pore mouth size of the micropores and shallow the pore depth of the micropores, so as to reduce the consumption of SEI film formation in the micropores, and at the same time provide space for the deposition of sodium clusters in the plateau region, solving the problems of low initial efficiency and low capacity of the coal-based hard carbon anode material. Moreover, this preparation method has a simple process and convenient operation, and has good commercial potential.

[0049] (2) The modified coal-based hard carbon anode material prepared based on the method provided by the present invention can achieve an initial efficiency of more than 90% and a reversible specific capacity of more than 290 mAh / g. Description of the Drawings

[0050] Figure 1 It is the process flow chart provided in Example 1 of the present invention.

[0051] Figure 2 Scanning electron micrograph of the coal-based hard carbon material with a porous structure provided in Example 1 of the present invention.

[0052] Figure 3 Scanning electron micrograph of the modified coal-based hard carbon negative electrode material provided in Example 1 of the present invention.

[0053] Figure 4 Average pore size distribution curve of the coal-based hard carbon material with a porous structure provided in Example 1 of the present invention.

[0054] Figure 5 Nitrogen adsorption and desorption curve of the coal-based hard carbon material with a porous structure provided in Example 1 of the present invention.

[0055] Figure 6 Nitrogen adsorption and desorption curve of the modified coal-based hard carbon negative electrode material provided in Example 1 of the present invention.

[0056] Figure 7 Small-angle scattering pattern of the modified coal-based hard carbon negative electrode material provided in Example 1 of the present invention.

[0057] Figure 8 First charge-discharge curve of the sodium-ion battery prepared based on the modified coal-based hard carbon negative electrode material provided in Example 1 of the present invention. Detailed implementation manners

[0058] The technical solutions of the present invention will be further described below through specific implementation manners. Those skilled in the art should understand that the embodiments are only for helping to understand the present invention and should not be regarded as specific limitations on the present invention.

[0059] Example 1

[0060] This example provides a preparation method of a modified coal-based hard carbon negative electrode material, and its process flow chart is as Figure 1 shown. The preparation method includes the following steps:

[0061] (1) Crush the raw coal (i.e., long-flame coal) to a particle size D50 of 15 μm, and then carry out carbonization treatment in a nitrogen atmosphere. The carbonization temperature is 1300 °C, the heating rate is 5 °C / min, and the carbonization time is 5 h to obtain a coal-based hard carbon material with a porous structure. The average pore size of the coal-based hard carbon material with a porous structure is 2.5 nm, the pore volume is 0.17 cm 3 / g, N 2 specific surface area is 420 m 2 / g, CO 2 specific surface area is 137 m 2 / g.

[0062] (2) Place the coal-based hard carbon material with a porous structure in a chemical vapor deposition device (rotary tube furnace), introduce a first carbon source gas with a first gas velocity of 30 mL / min and a nitrogen gas with a gas velocity of 100 mL / min, and keep it at a constant temperature of the first temperature T1 at the same time, so that the first carbon deposition occurs in the micropores of the coal-based hard carbon material. The pore volume of the micropores is 0.029 cm 3 / g, then introduce a second carbon source gas with a second gas velocity of 200 mL / min and a nitrogen gas with a gas velocity of 350 mL / min, and keep it at a constant temperature of the second temperature T2 at the same time, so that the second carbon deposition occurs at the orifice of the micropores of the coal-based hard carbon material. The pore volume of the micropores is 0.025 cm 3 / g, and the orifice size of the micropores is 0.4 nm. After that, the modified coal-based hard carbon negative electrode material is obtained.

[0063] Among them, the first gas velocity < the second gas velocity, both the first carbon source gas and the second carbon source gas are methane, and the kinetic diameter of the gas molecules is 0.414 nm; the first temperature is 750 °C, and T1 is 3 h; the second temperature is 1100 °C, and T2 is 20 min.

[0064] Figure 2 and Figure 3 respectively show the scanning electron microscope images of the coal-based hard carbon material with a porous structure and the modified coal-based hard carbon negative electrode material provided in this embodiment. By comparison, it can be seen that a layer of deposited carbon is obviously covered on the surface of the material after vapor deposition.

[0065] Figure 4 shows the average pore size distribution curve of the coal-based hard carbon material with a porous structure provided in this embodiment. As can be seen from the figure, the average pore size distribution of this coal-based hard carbon material is 0 - 4 nm.

[0066] Figure 5 and Figure 6 respectively show the nitrogen adsorption and desorption curves of the coal-based hard carbon material with a porous structure and the modified coal-based hard carbon negative electrode material provided in this embodiment. By comparison, it can be seen that after being modified by vapor deposition, the coal-based hard carbon material changes from the original N 2 with a specific surface area of 420 m 2 / g to 3.5 m 2 / g.

[0067] Figure 7 shows the small-angle scattering pattern of the modified coal-based hard carbon negative electrode material provided in this embodiment. As can be seen from the figure, the orifice size of this coal-based hard carbon material is 0.4 nm, and the internal pore connection structure is still retained, which is beneficial to the storage of sodium ions.

[0068] Figure 8The first charge-discharge curve of a sodium-ion battery prepared from the modified coal-based hard carbon anode material provided in this embodiment is shown. As can be seen from the figure, the sodium-ion battery prepared from this modified coal-based hard carbon material has a first-cycle Coulombic efficiency as high as 90.81%, and the reversible specific capacity reaches 297 mAh / g.

[0069] Example 2

[0070] This embodiment provides a preparation method of a modified coal-based hard carbon anode material. The preparation method includes the following steps:

[0071] (1) The long-flame coal is crushed to a particle size D50 of 4 μm, and then carbonized in a nitrogen atmosphere. The carbonization temperature is 1100 °C, the heating rate is 5 °C / min, and the carbonization time is 10 h to obtain a coal-based hard carbon material with a porous structure. The average pore diameter of the coal-based hard carbon material with a porous structure is 4 nm, the pore volume is 0.30 cm 3 / g, N 2 specific surface area is 556 m 2 / g, CO 2 specific surface area is 177 m 2 / g.

[0072] (2) The coal-based hard carbon material with a porous structure is placed in a chemical vapor deposition device, and a first carbon source gas with a first gas velocity of 10 mL / min and a nitrogen gas of 100 mL / min are introduced. At the same time, it is kept at a constant temperature T1 at the first temperature, so that the first carbon deposition is carried out in the micropores of the coal-based hard carbon material. The pore volume of the micropores is 0.057 cm 3 / g, and then a second carbon source gas with a second gas velocity of 100 mL / min and a nitrogen gas of 200 mL / min are introduced. At the same time, it is kept at a constant temperature T2 at the second temperature, so that the second carbon deposition is carried out at the orifice of the micropores of the coal-based hard carbon material. The pore volume of the micropores is 0.033 cm 3 / g, and the orifice size of the micropores is 0.5 nm. After completion, the modified coal-based hard carbon anode material is obtained.

[0073] Among them, the first gas velocity < the second gas velocity. Both the first carbon source gas and the second carbon source gas are acetylene, and the kinetic diameter of the gas molecules is 0.446 nm; the first temperature is 600 °C, T1 is 2 h; the second temperature is 1000 °C, and T2 is 10 min.

[0074] Example 3

[0075] This embodiment provides a preparation method of a modified coal-based hard carbon anode material. The preparation method includes the following steps:

[0076] (1) Crush the long-flame coal to a particle size D50 of 30 μm, and then carry out carbonization treatment in a nitrogen atmosphere. The carbonization temperature is 1400 °C, the heating rate is 10 °C / min, and the carbonization time is 2 h to obtain a coal-based hard carbon material with a porous structure. The average pore diameter of the coal-based hard carbon material with a porous structure is 3.6 nm, the pore volume is 0.112 cm 3 / g, N 2 The specific surface area is 315 m 2 / g, CO 2 The specific surface area is 113 m 2 / g.

[0077] (2) Place the coal-based hard carbon material with a porous structure in a chemical vapor deposition device, introduce a first carbon source gas with a first gas velocity of 50 mL / min and nitrogen gas of 150 mL / min, and at the same time keep it at a constant temperature T1 at the first temperature, so that the first carbon deposition is carried out in the micropores of the coal-based hard carbon material. The pore volume of the micropores is 0.025 cm 3 / g, and then introduce a second carbon source gas with a second gas velocity of 300 mL / min and nitrogen gas of 500 mL / min, and at the same time keep it at a constant temperature T2 at the second temperature, so that the second carbon deposition is carried out at the orifice of the micropores of the coal-based hard carbon material. The pore volume of the micropores is 0.017 cm 3 / g, and the orifice size of the micropores is 0.3 nm. After completion, the modified coal-based hard carbon negative electrode material is obtained.

[0078] Among them, the first gas velocity < the second gas velocity, both the first carbon source gas and the second carbon source gas are methane, and the kinetic diameter of the gas molecules is 0.414 nm; the first temperature is 900 °C, and T1 is 2 h; the second temperature is 1200 °C, and T2 is 30 min.

[0079] Example 4

[0080] The difference between this example and Example 1 is that the first gas velocity in step (2) is 5 mL / min.

[0081] The remaining preparation methods and parameters are the same as those in Example 1.

[0082] Example 5

[0083] The difference between this example and Example 1 is that the first gas velocity in step (2) is 80 mL / min.

[0084] The remaining preparation methods and parameters are the same as those in Example 1.

[0085] Example 6

[0086] The difference between this example and Example 1 is that the second gas velocity in step (2) is 80 mL / min.

[0087] The remaining preparation methods and parameters are the same as those in Example 1.

[0088] Example 7

[0089] The difference between this example and Example 1 is that the second gas velocity in step (2) is 400 mL / min.

[0090] The remaining preparation methods and parameters are the same as those in Example 1.

[0091] Example 8

[0092] The difference between this example and Example 1 is that the first temperature in step (2) is 500 °C.

[0093] The remaining preparation methods and parameters are the same as those in Example 1.

[0094] Example 9

[0095] The difference between this example and Example 1 is that the first temperature in step (2) is 1000 °C.

[0096] The remaining preparation methods and parameters are the same as those in Example 1.

[0097] Example 10

[0098] The difference between this example and Example 1 is that the second temperature in step (2) is 900 °C.

[0099] The remaining preparation methods and parameters are the same as those in Example 1.

[0100] Example 11

[0101] The difference between this example and Example 1 is that the second temperature in step (2) is 1300 °C.

[0102] The remaining preparation methods and parameters are the same as those in Example 1.

[0103] Comparative Example 1

[0104] The difference between this comparative example and Example 1 is that the first gas velocity in step (2) is 300 mL / min, such that the first gas velocity > the second gas velocity.

[0105] The remaining preparation methods and parameters are the same as those in Example 1.

[0106] Comparative Example 2

[0107] The difference between this comparative example and Example 1 is that in step (2), the first carbon source gas is not introduced, that is, the first carbon deposition is not carried out.

[0108] The remaining preparation methods and parameters are the same as those in Example 1.

[0109] Comparative Example 3

[0110] The difference between this comparative example and Example 1 is that in step (2), the second carbon source gas is not introduced, that is, the second carbon deposition is not carried out.

[0111] The remaining preparation methods and parameters are the same as those in Example 1.

[0112] Performance Test

[0113] The modified coal-based hard carbon negative electrode materials provided in the above examples and comparative examples are made into negative electrode sheets, and then assembled with positive electrode sheets, separators and electrolytes to obtain sodium ion batteries. The specific steps include:

[0114] The modified coal-based hard carbon negative electrode materials, conductive carbon black, sodium carboxymethyl cellulose, and styrene-butadiene rubber are mixed in water as a solvent according to a mass ratio of 93:2:2:3 to obtain a negative electrode slurry, which is then coated on a copper foil and dried to obtain a negative electrode sheet; the negative electrode sheet, a separator (i.e., a polyethylene separator), and a positive electrode sheet (i.e., a sodium sheet) are stacked in sequence, and an electrolyte (i.e., 1 mol / L NaPF 6 (EC:DEC = 1:1)) is injected, and a 2032-type button battery is assembled in a glove box under an argon atmosphere and then sealed to obtain a sodium ion battery.

[0115] The first efficiency and capacity performance of the above sodium ion batteries are tested. The test conditions are: the current density is 50 mA / g, that is, 0.1 C. The first-cycle Coulomb efficiency and reversible specific capacity are recorded during the test, where the reversible specific capacity refers to the first-cycle charging specific capacity, and the first efficiency refers to the first-cycle charging specific capacity / discharge specific capacity.

[0116] The test results are shown in Table 1.

[0117] Table 1

[0118]

[0119] Analysis:

[0120] As can be seen from Table 1, the method developed in the present invention can improve the uniformity of the pore size distribution of the coal-based hard carbon negative electrode material, reduce the pore mouth size of the micropores and shallow the pore depth of the micropores, reduce the consumption of the formation of the SEI film in the micropores, and at the same time provide space for the deposition of sodium clusters in the plateau region, solving the problems of low first efficiency and low capacity of the coal-based hard carbon negative electrode material.

[0121] As can be seen from Example 1 and Examples 4-5, if the first gas velocity is too small, the amount of carbon deposition in the micropores is insufficient, the change in pore volume is not obvious, and the pore depth cannot be improved; if the first gas velocity is too large, too much carbon is deposited in the micropores, resulting in some micropores being blocked and the pore structure disappearing.

[0122] As can be seen from Example 1 and Examples 6-7, if the second gas velocity is too small, the carbon deposition rate at the micropore orifice is slow, and the orifice shrinkage is not obvious; if the second gas velocity is too large, the carbon deposition rate at the micropore orifice is too fast, and the orifice is quickly blocked and closed by deposited carbon, and the pore structure disappears.

[0123] As can be seen from Example 1 and Examples 8-9, if the first temperature is too small, the carbon source gas cannot reach the pyrolysis temperature, resulting in no carbon deposition; if the first temperature is too large, the excessive temperature will cause the orifice to shrink in advance, and the carbon after pyrolysis cannot be deposited into the micropores, failing to achieve the purpose of shallowening the pore depth.

[0124] As can be seen from Example 1 and Examples 10-11, if the second temperature is too small, the temperature is insufficient and the effect of shrinking the orifice is not obvious; if the second temperature is too large, the carbon source gas is pyrolyzed quickly, the orifice shrinks quickly, and the carbon continuously deposited around the orifice will block the orifice.

[0125] As can be seen from Example 1 and Comparative Example 1, if the first gas velocity > the second gas velocity, too much carbon is deposited, causing the micropores to be completely filled with deposited carbon, failing to achieve the purpose of improving the micropore structure.

[0126] The applicant declares that the present invention uses the above examples to illustrate the process method of the present invention, but the present invention is not limited to the above process steps, that is, it does not mean that the present invention must rely on the above process steps to be implemented. Those skilled in the art should understand that any improvement to the present invention, the equivalent substitution of the raw materials selected for the present invention, the addition of auxiliary components, and the selection of specific methods, etc., all fall within the protection scope and the disclosure scope of the present invention.

[0127] As can be seen from Example 1 and Comparative Example 3, if the second carbon source gas is not introduced in step (2), that is, the second carbon deposition is not carried out, the size of the micropore orifice is not adjusted, and the specific surface area is too large, resulting in excessive consumption of electrolyte during the formation of the SEI film during the battery cycle, and the initial Coulomb efficiency of the prepared sodium-ion battery is too low.

[0128] The applicant declares that the present invention uses the above examples to illustrate the process method of the present invention, but the present invention is not limited to the above process steps, that is, it does not mean that the present invention must rely on the above process steps to be implemented. Those skilled in the art should understand that any improvement to the present invention, the equivalent substitution of the raw materials selected for the present invention, the addition of auxiliary components, and the selection of specific methods, etc., all fall within the protection scope and the disclosure scope of the present invention.

Claims

1. A method for preparing a modified coal-based hard carbon negative electrode material, characterized in that: The preparation method comprises the following steps: (a) crushing the raw coal to a particle size D50 of 4-30 μm, and then carbonizing it in an inert atmosphere, wherein the carbonizing temperature is 1100-1400°C, the heating rate is 1-10°C / min, and the carbonizing time is 1-10 h, to obtain a coal-based hard carbon material with a porous structure, wherein the average pore size of the coal-based hard carbon material with a porous structure is 1-4 nm, and the pore volume is 0.1-0.3 cm 3 / g, N2 specific surface area is 300-600m 2 / g, CO2 specific surface area is 100-200m 2 / g; (b) placing the coal-based hard carbon material having a porous structure in a chemical vapor deposition device, introducing a first carbon source gas at a first gas rate of 10-50 mL / min and an inert protective gas at a rate of 50-150 mL / min, and maintaining a constant temperature T1 at a first temperature, so that carbon deposition is performed in the micropores of the coal-based hard carbon material, and the pore volume of the micropores is 0.02-0.06 cm 3 / g, then introduce a second carbon source gas at a second gas velocity of 100-300 mL / min and an inert protective gas at a velocity of 200-500 mL / min, and keep the temperature T2 constant at the second temperature, so that carbon deposition is carried out at the micropore openings of the coal-based hard carbon material, and the pore volume of the micropores is 0.01-0.03 cm 3 / g, the pore size of the micropores is 0.3-0.5nm, and after completion, the modified coal-based hard carbon negative electrode material is obtained; Among them, the first gas velocity is less than the second gas velocity, the kinetic diameters of the gas molecules of the first carbon source gas and the second carbon source gas are each independently 0.4-0.5 nm; the first temperature is 600-900°C, T1 is 2-4h; the second temperature is 1000-1200°C, T2 is 10-30min.

2. The preparation method according to claim 1, characterized in that: The raw coal in step (1) includes any one of anthracite, lean coal, lean coal, coking coal, fat coal, gas coal, slightly sticky coal, non-sticky coal, long flame coal or lignite, or a combination of at least two of them.

3. The preparation method according to claim 1, characterized in that: Step (2) the second gas velocity is 120-240 mL / min; And / or, in step (2), the first carbon source gas and the second carbon source gas each independently include any one of methane, acetylene, ethane, propane or benzene, or a combination of at least two thereof.

4. A modified coal-based hard carbon negative electrode material, characterized in that: The modified coal-based hard carbon negative electrode material is prepared by the preparation method according to any one of claims 1 to 3; The N2 specific surface area of ​​the modified coal-based hard carbon negative electrode material is 2-5 m 2 / g; The CO2 specific surface area of ​​the modified coal-based hard carbon negative electrode material is 0-10m 2 / g.

5. A negative electrode plate, characterized in that: The negative electrode plate includes the modified coal-based hard carbon negative electrode material as described in claim 4.

6. A sodium ion battery, characterized in that: The sodium ion battery comprises the negative electrode sheet as claimed in claim 5.

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