Preparation method of hard carbon negative electrode material and hard carbon negative electrode pole piece and sodium ion battery of hard carbon negative electrode material and hard carbon negative electrode pole piece

Abstract

The invention discloses a preparation method of a hard carbon negative electrode material and a hard carbon negative electrode pole piece and a sodium ion battery thereof, and the preparation method comprises the following steps: firstly, pretreating a hard carbon precursor, including crushing, screening and purifying to obtain purified hard carbon precursor powder; then carrying out glow discharge plasma treatment on the purified hard carbon precursor powder; carrying out high-temperature carbonization in a high-temperature tube furnace under the protection of an inert atmosphere to obtain a hard carbon negative electrode material; the hard carbon negative electrode material is prepared into a pole piece, the prepared pole piece is subjected to surface treatment through glow discharge plasma under different gases, and the treated hard carbon negative electrode piece is obtained. An electrochemical performance test proves that the hard carbon negative electrode material after plasma treatment shows high sodium storage capacity and first coulombic efficiency in a sodium ion battery. The used raw materials are cheap and easy to obtain, the carbon content is high, the preparation process is simple and easy to operate, no catalyst needs to be introduced, and the method is environmentally friendly, is an effective way for efficiently preparing the low-cost negative electrode material and is suitable for industrial production.

Description

Technical Field

[0001] The present invention belongs to the fields of plasma and new energy, and particularly relates to a preparation method of a hard carbon negative electrode material and a hard carbon negative electrode sheet, and a sodium ion battery thereof. Background Art

[0002] There is a van der Waals force between the carbon layers inside hard carbon, which causes partial local stacking of some graphene sheets to form a multi-layer short-range ordered graphite microcrystalline region. These graphite microcrystalline regions are rearranged and stacked at high temperatures to form pores of different sizes and morphologies. Hard carbon is widely used as the main material for the negative electrode of sodium ion batteries due to its suitable interlayer distance (0.37 - 0.42 nm), rich graphite microcrystalline domains, etc., and its low cost. In addition, due to the disordered structure of hard carbon, it has more defects and can provide more sodium storage active sites. Therefore, hard carbon materials are considered to be the most promising negative electrode materials for sodium ion batteries in commercial applications. However, the first Coulomb efficiency of hard carbon materials is low, and existing reports have shown that only when the interlayer distance of the graphite microcrystalline domains in hard carbon materials reaches more than 0.37 nm, sodium ions can be embedded into the graphite interlayer for storage, and the graphite layer is also the only way for sodium ions to fill the closed pores. Therefore, developing a method that can significantly improve the first Coulomb efficiency while maintaining its high capacity and high rate performance is of great significance for the commercialization and wide application of battery technology.

[0003] To solve the above problems, researchers have explored various modification methods, such as morphology control, heteroatom doping, and pore formation. Although these methods have improved the electrochemical performance of hard carbon materials to a certain extent, they have also caused new challenges such as high cost, complex processes, and capacity loss. Summary of the Invention

[0004] The purpose of the present invention is to provide a preparation method of a hard carbon negative electrode material and a hard carbon negative electrode sheet, and a sodium ion battery thereof.

[0005] To achieve the above purpose, the present invention adopts the following technical solutions: A preparation method of a hard carbon negative electrode material, a preparation method of a hard carbon negative electrode sheet, and a sodium ion battery, comprising the following steps:

[0006] Step 1: Crush and screen the hard carbon precursor with a pulverizer to obtain a hard carbon precursor powder, heat and wash the hard carbon precursor powder with an alkali solution and an acid solution in sequence to remove ash, and then cool to room temperature, filter, wash, centrifuge, and dry to obtain a purified powder.

[0007] Step 2: Pretreat the purified powder obtained in Step 1 with glow discharge plasma to obtain a pretreated powder, and then place the pretreated powder in a tube furnace and carbonize it under the protection of an inert gas to obtain a hard carbon negative electrode material.

[0008] Step 3: Prepare the hard carbon negative electrode material obtained in Step 2 into a pole piece, and grind and mix the pole piece with a conductive material and a binder to form a slurry, coat and dry it to obtain a negative electrode pole piece; and

[0009] Step 4: Surface-treat the negative electrode pole piece obtained in Step 3 with glow discharge plasma to obtain a treated hard carbon negative electrode pole piece.

[0010] Further, the hard carbon precursor in Step 1 is coal, and the coal is one or more of lignite, bituminous coal, sub-bituminous coal, anthracite, and coal tar pitch;

[0011] Further, the alkali solution in Step 1 is one or more of sodium hydroxide, potassium hydroxide, ammonia water, ammonium fluoride, sodium carbonate, and sodium bicarbonate; the acid solution is one or two of hydrochloric acid, nitric acid, sulfuric acid, phosphoric acid, oxalic acid, acetic acid, perchloric acid, and hydrofluoric acid, the heating temperature is 50 - 120°C, and the heating time is 2 - 6 h;

[0012] Further, the washing in Step 1 uses one or two of deionized water or absolute ethanol, the number of centrifugation times is 3 - 6 times, and the rotation speed is 3000 - 6500 r / min -1 , the drying temperature is 60 - 120°C, and the time is 8 - 24 h;

[0013] Further, the glow discharge plasma pretreatment in Step 2 is carried out in one or more gases of helium, argon, oxygen, carbon dioxide, air, and nitrogen, and the gas flow rate is 100 - 600 ml / min -1 , the power is 50 - 500 W, and the treatment time is 5 - 60 min.

[0014] Further, in the carbonization under inert gas protection in Step 2, the inert gas is one of nitrogen and argon, and the heating rate is 0.5 - 15°C / min -1 , the carbonization temperature is 800 - 1600°C, and then the heat preservation time is 1 - 6 h.

[0015] Further, the glow discharge plasma surface treatment in Step 4 is carried out in one or more gases of argon-hydrogen mixture, nitrogen-hydrogen mixture, argon, and nitrogen, and the gas flow rate is 100 - 600 ml / min -1 , the power is 50 - 500 W, and the treatment time is 2 - 15 min.

[0016] Further, a preparation method of a hard carbon negative electrode material is characterized by including the following steps:

[0017] (1) The hard carbon precursor is pulverized by a pulverizer, and the sieved powder is successively heated and washed with an alkali solution and an acid solution to remove ash, and then cooled to room temperature, filtered by suction, washed, centrifuged, and dried to obtain a purified powder; and

[0018] (2) The purified powder obtained in step (1) is pretreated by glow discharge plasma to obtain a pretreated powder, and then the pretreated powder is placed in a tubular furnace and carbonized under the protection of an inert gas to obtain a hard carbon negative electrode material.

[0019] Compared with the prior art, the present invention has the following beneficial effects:

[0020] The functions played by plasma treatment in the present invention include: (1) etching the material surface without changing the overall structure, increasing the surface roughness and specific surface area, and increasing the number of electrochemically active sites of the material, thereby improving the capacity and rate performance of the material; (2) generating free radicals to activate the material surface and improve the wettability of the material surface; (3) performing surface cleaning treatment on the material to avoid interference from impurities; (4) introducing the required functional groups and charges on the material surface through grafting, polymerization, and gas regulation, that is, using glow discharge plasma to pretreat the hard carbon precursor and combining high-temperature carbonization to optimize the sodium storage sites and carbon microcrystal structure, thereby improving the sodium storage performance of the hard carbon negative electrode material; the prepared electrode sheet is treated by glow discharge plasma to improve its wettability and facilitate ion transport, and a higher sodium storage specific capacity and first Coulombic efficiency are obtained; the process is simple, the conditions are mild, and the controllability is strong, which is more conducive to industrial promotion and application. The hard carbon material has a large carbon layer spacing, a large number of closed pores, and rich oxygen-containing functional groups, which are beneficial to the storage and rapid deintercalation / insertion of sodium ions.

[0021] In addition, the present invention also has the following effects: 1. The raw materials of the present invention are cheap and easily available, which is beneficial to realizing the efficient and large-scale preparation of hard carbon materials; 2. The present invention uses glow discharge plasma technology to prepare hard carbon negative electrodes, which has a simple process, is easy to operate, does not require the introduction of catalysts, is environmentally friendly, and has low cost and high efficiency, which is beneficial to promoting the large-scale production of hard carbon negative electrodes; 3. The method for preparing hard carbon negative electrode materials in the present invention is compared with other preparation methods, and this method has controllable conditions, high efficiency, and simple steps, and the prepared hard carbon negative electrode material has high purity and low cost. Brief Description of the Drawings

[0022] Figure 1 It is a schematic flow chart of the preparation method of coal-based hard carbon provided by the embodiment of the present invention;

[0023] Figure 2 It is a high-magnification transmission electron microscope (HRTEM) image of the hard carbon negative electrode material prepared in Example 3;

[0024] Figure 3 Electrochemical performance graphs of the hard carbon anode material prepared in Example 3 at different current rates from 0.05 A g -1 to 5 A g -1 ;

[0025] Figure 4 Initial Coulombic efficiency (ICE) graph of the hard carbon anode material prepared in Example 3;

[0026] Figure 5 Long cycle graph of the hard carbon anode material prepared in Example 5 at the specified current rate of 3 A g -1 ;

[0027] Figure 6 ICE comparison graph of the hard carbon anode material prepared in Example 3 and the hard carbon anode materials prepared in Comparative Example 1 and Comparative Example 2. Detailed implementation manners

[0028] The present invention will be further described in detail below with reference to specific examples and the accompanying drawings. In the following examples, where the experimental methods are not specifically described, they are all conventional operations, and the reagents used are commercially available.

[0029] Example 1

[0030] Preparation of hard carbon anode material:

[0031] 1) First, coal blocks used as hard carbon precursors are crushed by a pulverizer, and the coal powder is sieved through a 200-mesh sieve. The sieved coal powder is uniformly mixed with 3 mol L -1 NaOH solution in a beaker at a solid-liquid ratio of 1:10, treated in an oil bath at 120 °C for 3 h, and finally dried in an oven at 80 °C to obtain the coal powder after preliminary impurity removal by washing. Subsequently, it is uniformly mixed with 3 mol L -1 HCl at the same ratio, and treated in an oil bath at 100 °C for 5 h to obtain the purified coal powder;

[0032] Since coal contains inorganic ash, which includes one or more of silicon dioxide, metal oxides, etc., it is necessary to purify it to obtain a hard carbon anode material with higher purity. Among them, the alkaline solution is conducive to the removal of silicon dioxide, and the acidic solution is easy to react with metal oxides to become a solution and be leached out.

[0033] 2) 6 g of the purified coal powder obtained in step 1) is evenly spread in a plasma mold, placed in a plasma chamber, the pressure is set to 80 Pa, and it is treated under helium conditions for 30 min, and the gas flow rate is 100 ml min -1 . After pressure relief and cooling, the pretreated powder is obtained;

[0034] Helium has a small molecular weight. Under the action of plasma, it can form microporous structures on the surface of coal, and the improvement of micropores is beneficial to the increase of platform dosage.

[0035] 3) Perform high-temperature carbonization treatment on the pretreated coal powder obtained in step 2). The high-temperature carbonization treatment includes heating to 1200 °C at a heating rate of 2 °C / min in an argon atmosphere, holding for 3 h, and then cooling to room temperature to obtain a coal-based hard carbon anode material; -1 The heating rate is selected because the corresponding carbonization temperature needs to be coordinated with an appropriate heating rate and carbonization temperature. If the heating rate is too high, it is difficult to form a closed pore structure. If the heating rate is too low, it is easy to be graphitized, with a small interlayer spacing, which is not conducive to the transmission of sodium ions. Similarly, if the temperature is too low, it is not easy to form a microcrystalline structure, and if the temperature is too high, the interlayer spacing decreases.

[0036] The selection of the parameters in step 3) above is because the corresponding carbonization temperature needs to be coordinated with an appropriate heating rate and carbonization temperature. If the heating rate is too high, it is difficult to form a closed pore structure. If the heating rate is too low, it is easy to be graphitized, with a small interlayer spacing, which is not conducive to the transmission of sodium ions. Similarly, if the temperature is too low, it is not easy to form a microcrystalline structure, and if the temperature is too high, the interlayer spacing decreases.

[0037] Example 2

[0038] Prepare a hard carbon anode material:

[0039] 1) First, crush coal blocks with a pulverizer. The coal powder is sieved through a 200-mesh sieve. The sieved coal powder is uniformly mixed with a 2 mol / L NaOH solution in a beaker according to a solid-liquid ratio of 1:10, treated in an oil bath at 120 °C for 5 h, and finally dried in an oven at 80 °C to obtain the preliminarily impurity-removed coal powder after cleaning; Subsequently, it is uniformly mixed with a 9 mol / L HNO₃ in the same proportion, treated in an oil bath at 80 °C for 2 h to obtain the purified coal powder; -1 NaOH solution is uniformly mixed in a beaker according to a solid-liquid ratio of 1:10, treated in an oil bath at 120 °C for 5 h, and finally dried in an oven at 80 °C to obtain the preliminarily impurity-removed coal powder after cleaning; Subsequently, it is uniformly mixed with 9 mol / L HNO₃ in the same proportion, treated in an oil bath at 80 °C for 2 h to obtain the purified coal powder; -1 HNO 3 is uniformly mixed, and treated in an oil bath at 80 °C for 2 h to obtain the purified coal powder;

[0040] 2) Spread 6 g of the purified coal powder obtained in step 1) evenly in a plasma mold, place it in a plasma chamber, set the pressure to 80 Pa, treat it in air for 20 min, and the gas flow rate is 200 ml / min -1 , and after depressurization and cooling, obtain the pretreated powder;

[0041] Oxygen in the air has a linking effect to enhance the cross-linking degree of coal molecules, and it will also inhibit the directional growth of carbon layers during subsequent carbonization, making it form a layered graphite-like domain structure.

[0042] 3) Perform high-temperature carbonization treatment on the pretreated coal powder obtained in step 2). The high-temperature carbonization treatment includes heating to 1200 °C at a heating rate of 3 °C / min in an argon atmosphere, holding for 3 h, and then cooling to room temperature to obtain a coal-based hard carbon anode material. -1 The heating rate is selected because the corresponding carbonization temperature needs to be coordinated with an appropriate heating rate and carbonization temperature. If the heating rate is too high, it is difficult to form a closed pore structure. If the heating rate is too low, it is easy to be graphitized, with a small interlayer spacing, which is not conducive to the transmission of sodium ions. Similarly, if the temperature is too low, it is not easy to form a microcrystalline structure, and if the temperature is too high, the interlayer spacing decreases.

[0043] Example 3

[0044] Prepare a hard carbon anode material:

[0045] 1) First, crush the coal blocks with a pulverizer. Sieve the coal powder through a 200-mesh sieve. The sieved coal powder is uniformly mixed with 3 mol / L -1 NaOH solution in a beaker at a solid-liquid ratio of 1:10, and treat it in an oil bath at 120 °C for 3 h. Finally, dry it in an oven at 80 °C to obtain the preliminarily impurity-removed coal powder after cleaning; then mix it with 3 mol / L -1 HCl uniformly at the same ratio and treat it in an oil bath at 100 °C for 5 h to obtain the purified coal powder;

[0046] 2) Spread 6 g of the purified coal powder obtained in step 1) evenly in the plasma mold, put it into the plasma chamber, set the pressure to 80 Pa, and treat it under oxygen conditions for 15 min. The gas flow rate is 300 ml / min -1 , and after depressurization and cooling, obtain the pretreated powder;

[0047] 3) Perform high-temperature carbonization treatment on the pretreated coal powder obtained in step 2). The high-temperature carbonization treatment includes heating to 1300 °C at a heating rate of 5 °C / min -1 in an argon atmosphere, holding for 3 h, and then cooling to room temperature to obtain the coal-based hard carbon anode material;

[0048] In step 2) above, oxygen is preferably used because the crosslinking degree of air is limited and the air composition is complex, including nitrogen, oxygen, carbon dioxide, water vapor, and rare gases, etc. To avoid the interference of other gases, oxygen is used to improve the deficiency. Oxygen can enhance the crosslinking degree of coal molecules, prevent the graphitization of the carbon layer during subsequent carbonization, and the introduction of oxygen in the early stage makes the carbon layer show a bent and randomly entangled structure during the subsequent high-temperature carbonization process, which is beneficial to increasing the interlayer spacing of the coal-based hard carbon and facilitating the transmission of sodium, and the formed closed pores improve the sodium storage. Experiments have proved that if nitrogen doping treatment is carried out in step 2), a larger carbon layer spacing can be obtained, but the effect of improving the closed pore structure is not achieved.

[0049] Figure 2 Figure [ID number] is the HRTEM image of the coal-based hard carbon obtained after oxygen plasma treatment. A clear short-range ordered vortex-layer graphite microcrystalline structure is observed, and no long-range highly graphitized structure is seen, indicating that the prepared sample is a typical hard carbon, which confirms that the introduction of oxygen can effectively inhibit the rearrangement of the carbon layer during high-temperature carbonization, thereby increasing the carbon layer spacing and forming a closed pore structure, which is beneficial to the transmission and storage of sodium ions.

[0050] Figure 3 Figure [ID number] is for the coal-based hard carbon obtained after oxygen plasma treatment at 0.05 A / g -1 ~5 A / g -1Electrochemical performance graphs at different magnification factors, with reversible specific capacities of 302.5, 293.2, 287.3, 274.9, 265.2, 249.1, 239.3 mAh g -1 , when the current density is restored to 0.05 A g -1 , the reversible capacity is 305.2 mAh g -1 . The above data shows that the oxygen plasma-treated coal-based hard carbon has a high sodium storage capacity. By etching the surface of the carbon material with oxygen, defects are generated. After high-temperature carbonization, some open pores are transformed into closed pores, and the oxygen retained in the carbon skeleton expands the interlayer spacing of the carbon layers, which is beneficial to the diffusion and transport of Na + , further improving the rate performance.

[0051] Figure 4 The first Coulombic efficiency (ICE) of the coal-based hard carbon obtained after oxygen plasma treatment at different magnification factors. Due to the generation of microporous structures by oxygen plasma etching, the oxygen functional groups introduced by oxygen plasma treatment not only increase the interlayer spacing of the carbon layers but also help to enhance the wettability of the electrode surface, thus improving the ICE.

[0052] Example 4

[0053] Preparation of a hard carbon negative electrode material:

[0054] 1) First, crush coal lumps with a pulverizer. Sieve the coal powder through a 200-mesh sieve. The sieved coal powder is uniformly mixed with 3 mol L -1 NaOH solution in a beaker at a solid-liquid ratio of 1:10, and treat it in an oil bath at 120 °C for 3 h. Finally, dry it in an oven at 80 °C to obtain the preliminarily purified coal powder after cleaning; then mix it with 3 mol L -1 HCl uniformly at the same ratio, and treat it in an oil bath at 100 °C for 5 h to obtain the purified coal powder;

[0055] 2) Spread 6 g of the purified coal powder obtained in step 1 evenly in a plasma mold, place it in a plasma chamber, set the pressure to 80 Pa, and treat it under oxygen conditions for 15 min. The gas flow rate is 300 ml min -1 . After depressurization and cooling, obtain the pretreated powder;

[0056] 3) Perform high-temperature carbonization treatment on the pretreated coal powder obtained in step 2. The high-temperature carbonization treatment includes heating to 1200 °C at a heating rate of 5 °C min -1 in an argon atmosphere, holding for 3 h, and then cooling to room temperature to obtain the coal-based hard carbon negative electrode material;

[0057] 4) The electrode sheet obtained in step 3) is subjected to glow discharge plasma surface reduction. Set the pressure to 80 Pa, and treat it in a nitrogen-hydrogen (1:1) mixed gas atmosphere for 8 min. Then relieve the pressure and cool it to room temperature to obtain the reduced coal-based hard carbon negative electrode material;

[0058] In the above step (2), the introduction of oxygen can increase the cross-linked structure and thus improve the closed pore rate. However, too much oxygen will consume the electrolyte and cause a decrease in irreversible capacity. Therefore, a nitrogen-hydrogen mixed gas is used to reduce the excess oxygen to improve the first efficiency.

[0059] Example 5

[0060] Preparation of hard carbon negative electrode sheet:

[0061] 1) First, crush the coal blocks with a pulverizer. Sieve the coal powder through a 200-mesh sieve. The sieved coal powder is evenly mixed with 3 mol / L -1 NaOH solution in a beaker according to a solid-liquid ratio of 1:10, treat it in an oil bath at 120 °C for 3 h, and finally dry it in an oven at 80 °C to obtain the coal powder after preliminary impurity removal by washing; Subsequently, mix it evenly with 3 mol / L -1 HCl in the same proportion, and treat it in an oil bath at 100 °C for 5 h to obtain the purified coal powder;

[0062] 2) Take 3 g of the coal powder obtained in step 1) and place it evenly in a quartz boat. Transfer it to a tube furnace (non-plasma chamber). Under an oxygen atmosphere, increase the temperature to 300 °C at a heating rate of 5 °C / min -1 and keep it at this temperature for 2 h. After cooling, collect the pre-oxidized coal powder;

[0063] 3) Perform high-temperature carbonization treatment on the pretreated coal powder obtained in step 2). Under an argon atmosphere, increase the temperature to 1300 °C at a heating rate of 5 °C

[0064] / min -1 and keep it at this temperature for 3 h. Then cool it to room temperature to obtain the coal-based hard carbon negative electrode material;

[0065] 4) Prepare the pretreated coal-based hard carbon negative electrode material obtained in step 3) into an electrode sheet. Mix the electrode sheet with a conductive material (carbon black) and a binder (sodium carboxymethyl cellulose) in a mass ratio of 8:1:1 respectively in deionized water, grind and mix them to form a slurry, coat and dry it to obtain the negative electrode sheet;

[0066] 5) Perform glow discharge plasma surface treatment on the negative electrode sheet obtained in step 4). Set the program, set the pressure to 80 Pa, and treat it in a nitrogen atmosphere for 6 min. Then relieve the pressure and cool it to room temperature to obtain the surface-treated negative electrode sheet.

[0067] The hard carbon anode material prepared under nitrogen conditions incorporates nitrogen. Nitrogen has a high structural similarity to carbon atoms, so it easily replaces the positions of carbon atoms and forms chemical bonds with surrounding carbon atoms. Moreover, the electronegativity of nitrogen is higher than that of carbon, which improves the electrical conductivity of the hard carbon material. Pyrrolic nitrogen can introduce active sites and defects, enhancing sodium storage.

[0068] Figure 5 This is the electrochemical performance graph of the electrode sheet treated by nitrogen plasma. At 3 A g -1 After 300 cycles, the reversible capacity can be maintained at 175.8 mAh g -1 , Therefore, the introduction of nitrogen can effectively improve the sodium storage performance of coal-based hard carbon.

[0069] Example 6

[0070] 1) First, crush the coal blocks with a pulverizer. Sieve the coal powder through a 200-mesh sieve. Mix the sieved coal powder with 3 molL -1 NaOH solution in a beaker at a solid-liquid ratio of 1:10 and mix evenly. Treat it in an oil bath at 120 °C for 3 h, and finally dry it in an oven at 80 °C to obtain the preliminarily purified coal powder after cleaning; Subsequently, mix it with 3 mol L -1 HCl evenly at the same ratio and treat it in an oil bath at 100 °C for 5 h to obtain the purified coal powder;

[0071] 2) Take 3 g of the coal powder obtained in step 1) and place it evenly in a quartz boat. Transfer it to a tube furnace (non-plasma chamber). Under an oxygen atmosphere, heat it at a heating rate of 5 °C min -1 to 300 °C and hold for 2 h. After cooling, collect the pre-oxidized coal powder;

[0072] 3) Perform high-temperature carbonization treatment on the coal powder obtained in step 2). The high-temperature carbonization treatment includes heating it to 1300 °C at a heating rate of 5 °C min -1 in an argon atmosphere, holding for 3 h, and then cooling to room temperature to obtain the coal-based hard carbon anode material;

[0073] 4) Prepare the coal-based hard carbon anode material obtained in step 3) into an electrode sheet. Grind and mix it with a conductive material (carbon black) and a binder (sodium carboxymethyl cellulose) in a mass ratio of 8:1:1 respectively in deionized water to make a slurry for coating, and dry it to obtain the negative electrode sheet;

[0074] 5) Perform glow discharge plasma surface treatment on the electrode sheet obtained in step 4). Set the pressure to 80 Pa and treat it in a nitrogen-hydrogen (1:1) mixed gas atmosphere for 8 min. Release the pressure and then cool to room temperature to obtain the treated negative electrode sheet;

[0075] The hard carbon negative electrode sheet prepared in the above embodiment is combined with a positive electrode to obtain a sodium-ion full battery.

[0076] Comparative Example 1

[0077] 1) First, the coal blocks are crushed by a pulverizer. The pulverized coal after sieving is uniformly mixed with 3 mol / L -1 NaOH solution according to a solid-liquid ratio of 1:10, treated in an oil bath at 100 °C for 3 h, and finally dried in an oven at 80 °C to obtain the preliminarily impurity-removed pulverized coal after cleaning; then, it is uniformly mixed with 3 mol / L -1 HCl in the same proportion, treated in an oil bath at 100 °C for 3 h to obtain the purified pulverized coal;

[0078] 2) Take 2 g of the purified coal powder obtained in step 1) and place it evenly in a quartz boat, transfer it to a tube furnace (non-plasma chamber) for direct heating;

[0079] 3) The coal powder obtained in step 2) is subjected to high-temperature carbonization treatment. The high-temperature carbonization treatment includes heating the temperature to 1300 °C at a heating rate of 5 °C / min in an argon atmosphere, holding for 3 h, and after the reaction ends, cooling to room temperature in an air atmosphere, taking out the sample to obtain a coal-based hard carbon negative electrode material; -1 4) The pretreated coal-based hard carbon negative electrode material obtained in step 3) is made into an electrode sheet, and it is ground and mixed with a conductive material and a binder in a mass ratio of 8:1:1 in deionized water, made into a slurry for coating, and dried to obtain a negative electrode sheet.

[0080] 4) The pretreated coal-based hard carbon negative electrode material obtained in step 3) is made into an electrode sheet, and it is ground and mixed with a conductive material and a binder in a mass ratio of 8:1:1 in deionized water, made into a slurry for coating, and dried to obtain a negative electrode sheet.

[0081] Comparative Example 2

[0082] 1) First, the coal blocks are crushed by a pulverizer. The pulverized coal after sieving is uniformly mixed with 3 mol / L -1 NaOH solution according to a solid-liquid ratio of 1:10, treated in an oil bath at 100 °C for 3 h, and finally dried in an oven at 80 °C to obtain the preliminarily impurity-removed pulverized coal after cleaning; then, it is uniformly mixed with 3 mol / L -1 HCl in the same proportion, treated in an oil bath at 100 °C for 3 h to obtain the purified pulverized coal;

[0083] 2) Take 3 g of the coal powder obtained in step 1) and place it evenly in a quartz boat, transfer it into a tube furnace (non-plasma chamber), and in an oxygen atmosphere, heat the temperature to 300 °C at a heating rate of 5 °C / min -1 and hold for 2 h, and collect the pre-oxidized pulverized coal after cooling;

[0084] 3) Subject the coal powder obtained in step 2) to high-temperature carbonization. The high-temperature carbonization treatment includes heating the temperature to 1300 °C at a heating rate of 2 °C / min in an argon atmosphere and holding the temperature for 2 h. After the reaction is completed, cool it to room temperature in an air atmosphere, take out the sample, and obtain the coal-based hard carbon anode material. -1 After the reaction is completed, cool it to room temperature in an air atmosphere, take out the sample, and obtain the coal-based hard carbon anode material.

[0085] 4) Prepare the pretreated coal-based hard carbon anode material obtained in step 3) into an electrode sheet. Mix it with a conductive material (carbon black) and a binder (sodium carboxymethyl cellulose) in a mass ratio of 8:1:1 respectively in deionized water, grind and mix them to form a slurry, coat the slurry, and dry it to obtain the negative electrode sheet.

[0086] The electrochemical performance of the coal-based carbon anode materials prepared in Examples 1-6 of the present invention is compared with that of the carbon anode materials in Comparative Examples 1-2. The specific results are shown in Table 1 below, and the current density is 0.05 A / g. -1 .

[0087] Table 1 Comparison table of electrochemical performance of negative electrode materials

[0088]

[0089] Comparing the present invention with Comparative Example 1 and Comparative Example 2, it is found that using the glow discharge plasma technology to treat coal to prepare the hard carbon anode has a high first Coulombic efficiency, excellent rate performance, a simple preparation process, easy operation, no need to introduce a catalyst, environmental friendliness, low cost, short time consumption, high efficiency, no subsequent treatment process, which is beneficial to promoting the large-scale production of the hard carbon anode, has important significance for realizing the clean, efficient and high-value utilization of coal resources, and at the same time can promote the commercialization process of the coal-based hard carbon anode.

[0090] Figure 6 Shows the first discharge curves of Example 3 and Comparative Example 1 and Comparative Example 2 at 0.05 A / g. Obviously, Example 3 shows a higher charging specific capacity and excellent ICE. Compared with Comparative Example 1, the reversible capacity of Example 3 increased from 248.2 mAh / g -1 to 302.5 mAh / g -1 , and the ICE is as high as 89.3%. Using O -1 as the working gas, oxygen in different excited states (O, O 2 , O 2 , O 3 ), oxygen ions (O + , O 2+ , O - , O 2-etc.) and electrons can react with carbon atoms at the reaction sites generated by ion bombardment. These substances chemically react with the material surface to form functional groups (-OH, C-O, C=O, O-C=O), thereby generating a cross-linked structure, which is beneficial to the generation of closed pores and pore structures during the subsequent high-temperature carbonization process. Therefore, Example 3 exhibits a relatively high charge specific capacity and ICE.

[0091] In summary, the functions played by the plasma treatment of the present invention include: (1) etching the material surface without changing the overall structure, increasing the surface roughness and specific surface area, and increasing the number of electrochemically active sites of the material, thereby improving the capacity and rate performance of the material; (2) generating free radicals to activate the material surface and improving the wettability of the material surface; (3) performing surface cleaning treatment on the material to avoid the interference of impurities; (4) introducing the required functional groups and charges on the material surface through grafting, polymerization, and gas regulation, that is, using glow discharge plasma to pretreat the hard carbon precursor and combining it with high-temperature carbonization to optimize the sodium storage sites and carbon microcrystal structure, thereby improving the sodium storage performance of the hard carbon negative electrode material; the prepared electrode sheet is treated by glow discharge plasma to improve its wettability and facilitate the transport of ions, obtaining a higher sodium storage specific capacity and first Coulombic efficiency; this process is simple, the conditions are mild, and the controllability is strong, which is more conducive to industrial promotion and application. The hard carbon material has a large carbon layer spacing, a large number of closed pores, and rich oxygen-containing functional groups, which are beneficial to the storage and rapid deintercalation / insertion of sodium ions. In addition, the raw materials of the present invention are cheap and easily available, which is conducive to the efficient and large-scale preparation of hard carbon materials; using the glow discharge plasma technology to prepare the hard carbon negative electrode, the process is simple, easy to operate, does not require the introduction of catalysts, is environmentally friendly, and has low cost and high efficiency, which is conducive to promoting the large-scale production of the hard carbon negative electrode; compared with other preparation methods, the method for preparing the hard carbon negative electrode material of the present invention has controllable conditions, high efficiency, and simple steps, and the prepared hard carbon negative electrode material has high purity and low cost.

[0092] Finally, it should be noted that the above embodiments are only used to illustrate the technical solutions of the present invention and not to limit them. Although the present invention has been described in detail with reference to the preferred embodiments, those of ordinary skill in the art should understand that the technical solutions of the present invention can be modified or equivalently replaced without departing from the spirit and scope of the technical solutions of the present invention, and they should all be covered by the scope of the claims of the present invention.

Claims

1. A method for preparing a hard carbon negative electrode sheet, characterized in that: The following steps are involved: (1) crushing a hard carbon precursor with a crusher, sieving to obtain a hard carbon precursor powder, heating and washing the hard carbon precursor powder with an alkaline solution and an acid solution in sequence to remove ash, cooling to room temperature, filtering, washing, centrifuging, and drying to obtain a purified powder; (2) pre-treating the purified powder obtained in step (1) by glow discharge plasma to obtain a pre-treated powder, and then carbonizing the pre-treated powder in a tube furnace under the protection of an inert gas to obtain a hard carbon negative electrode material; (3) preparing the hard carbon negative electrode material obtained in step (2) into a pole piece, grinding and mixing the pole piece with a conductive material and a bonding material to prepare a slurry, coating the slurry, and drying the slurry to obtain a negative electrode piece; as well as (4) The negative electrode plate obtained in step (3) is surface treated by glow discharge plasma to obtain a treated hard carbon negative electrode plate.

2. The preparation method according to claim 1, characterized in that: The hard carbon precursor is coal, and the coal is one or more of lignite, bituminous coal, sub-bituminous coal, anthracite, and coal tar.

3. The preparation method according to claim 1, characterized in that: In the step (1), the alkaline solution is one or more of sodium hydroxide, potassium hydroxide, ammonia water, ammonium fluoride, sodium carbonate, and sodium bicarbonate; the acid solution is one or two of hydrochloric acid, nitric acid, sulfuric acid, phosphoric acid, oxalic acid, acetic acid, perchloric acid, and hydrofluoric acid; the heating temperature is 50 to 120° C., and the heating time is 2 to 6 hours.

4. The preparation method according to claim 1, characterized in that: In the step (1), the washing is carried out using one or both of deionized water and anhydrous ethanol, the number of centrifugation is 3 to 6 times, and the speed is 3000 to 6500 rmin. -1 The drying temperature is 60-120°C and the drying time is 8-24 hours.

5. The preparation method according to claim 1, characterized in that: The glow discharge plasma pretreatment in step (2) is carried out in one or more gases such as helium, argon, oxygen, carbon dioxide, air, and nitrogen, with a gas flow rate of 100 to 600 ml min -1 The equipment power is 50-500W and the processing time is 5-60min.

6. The preparation method according to claim 1, characterized in that: In the carbonization under the protection of an inert gas in step (2), the inert gas is one of nitrogen and argon, and the heating rate is 0.5-15°C min -1 , the carbonization temperature is 800-1600℃, and then the insulation time is 1-6h.

7. The preparation method according to claim 1, characterized in that: The glow discharge plasma surface treatment in step (4) is carried out in one or more gases selected from argon-hydrogen mixed gas, nitrogen-hydrogen mixed gas, argon gas, and nitrogen gas, with a gas flow rate of 100 to 600 ml min -1 The equipment power is 50-500W and the processing time is 2-15min.

8. The preparation method according to claim 1, characterized in that: The hard carbon precursor is bio-based, resin-based, or asphalt, the bio-based is selected from one or a combination of nut shells, coconut shells, bamboo or starch, the resin-based is selected from one or a combination of phenolic resin or epoxy resin, and the asphalt is selected from one or a combination of coal tar or petroleum asphalt.

9. A sodium ion battery comprising a positive electrode and a hard carbon negative electrode sheet prepared by the preparation method according to any one of claims 1 to 8.

10. A method for preparing a hard carbon negative electrode material, characterized in that: The following steps are involved: (1) crushing a hard carbon precursor with a crusher, and washing the sieved powder with an alkaline solution and an acid solution in turn to remove ash, and cooling to room temperature, filtering, washing, centrifuging, and drying to obtain a purified powder; as well as (2) Pre-treating the purified powder obtained in step (1) with glow discharge plasma to obtain pre-treated powder, and then carbonizing the pre-treated powder in a tubular furnace under the protection of an inert gas to obtain a hard carbon negative electrode material.

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