Hard carbon and preparation method and application thereof

Abstract

The invention discloses hard carbon as well as a preparation method and application thereof. The preparation method of the hard carbon comprises the following steps: 1) pre-carbonizing and crushing a biomass raw material to obtain carbon powder; 2) soaking carbon powder in alkali liquor to obtain alkali-treated carbon powder; 3) carbonizing the alkali-treated carbon powder in a protective atmosphere at the temperature of 900-1200 DEG C, and then performing acid washing to obtain acid-washed carbon powder; and 4) carbonizing the acid-washed carbon powder in a protective atmosphere at the temperature of 1300-1500 DEG C to obtain the hard carbon. The hard carbon disclosed by the invention is simple in preparation method and low in production cost, and the sodium ion battery prepared by taking the hard carbon as a negative electrode material is large in reversible specific capacity, high in platform capacity, high in first coulombic efficiency, excellent in cycling stability and suitable for large-scale industrial production and application.

Description

Technical Field

[0001] The present invention relates to the technical field of biomass materials, and particularly relates to a hard carbon and its preparation method and application. Background Art

[0002] Sodium-ion batteries have the characteristics of rich raw material resources, low cost, environmental friendliness, etc., and are considered to be one of the important technologies to meet the future large-scale energy storage needs. Hard carbon has a unique amorphous and graphitized microstructure, a low working voltage, and excellent cycle stability, and is a sodium-ion battery negative electrode material that has received much attention. However, existing sodium-ion batteries based on hard carbon generally have problems such as low specific capacity, low initial Coulombic efficiency (ICE), and unclear sodium-ion storage mechanism, resulting in great limitations in its practical application. At present, researchers mainly improve the electrochemical performance of hard carbon by doping heteroatoms (such as sulfur, nitrogen, oxygen, and phosphorus, etc.). By doping heteroatoms, a reasonable pore structure is constructed to create rich active sites, improve the surface adsorption performance of hard carbon, and expand the carbon layer spacing to enhance the sodium-ion storage capacity, so that the sodium-ion battery based on hard carbon has high-rate performance and high reversible capacity. However, doping heteroatoms into hard carbon cannot improve the initial Coulombic efficiency of sodium-ion batteries and still cannot fully meet the actual application requirements.

[0003] Therefore, it is of great significance to develop a hard carbon with low cost, large reversible specific capacity / high plateau capacity / high initial Coulombic efficiency / excellent cycle stability as a negative electrode material for sodium-ion batteries. Summary of the Invention

[0004] The purpose of the present invention is to provide a hard carbon and its preparation method and application.

[0005] The technical solution adopted by the present invention is as follows:

[0006] A preparation method of a hard carbon, comprising the following steps:

[0007] 1) Pre-carbonize and pulverize the biomass raw material to obtain carbon powder;

[0008] 2) Immerse the carbon powder in an alkali solution for soaking to obtain alkali-treated carbon powder;

[0009] 3) Carbonize the alkali-treated carbon powder under a protective atmosphere at a temperature of 900°C to 1200°C, and then perform acid washing to obtain acid-washed carbon powder;

[0010] 4) Carbonize the acid-washed carbon powder under a protective atmosphere at a temperature of 1300°C to 1500°C to obtain the hard carbon.

[0011] Preferably, the biomass raw material in step 1) is at least one of balsa wood, birch wood, oak wood, beech wood, bamboo, pulp, lignin, walnut shell, oil-tea camellia shell, peanut shell, coconut shell, xylose residue, fruit pit, corncob, straw, and cellulose powder.

[0012] Preferably, the pre-carbonization in step 1) includes the following operations: heating from room temperature to 100°C - 800°C at a heating rate of 1°C / min - 10°C / min and holding for 0.1 h - 24 h.

[0013] Preferably, the particle size of the carbon powder in step 1) is 120 μm - 180 μm.

[0014] Preferably, the mass ratio of the carbon powder to the alkali solution in step 2) is 1:1 - 50.

[0015] Preferably, the concentration of the alkali solution in step 2) is 0.1 mol / L - 10 mol / L.

[0016] Preferably, the alkali in the alkali solution in step 2) is at least one of lithium hydroxide, sodium hydroxide, potassium hydroxide, and calcium hydroxide.

[0017] Preferably, the soaking time in step 2) is 0.1 h - 24 h.

[0018] Preferably, the protective atmosphere in step 3) is one of a nitrogen atmosphere, an argon atmosphere, and a nitrogen-argon mixed atmosphere.

[0019] Preferably, the carbonization in step 3) includes the following operations: heating from room temperature to 900°C - 1200°C at a heating rate of 1°C / min - 20°C / min and holding for 0.1 h - 6 h.

[0020] Preferably, the mass ratio of the product obtained by carbonization in step 3) to the acid solution used for pickling is 1:1 - 50.

[0021] Preferably, the acid solution used for pickling in step 3) is hydrochloric acid with a concentration of 0.1 mol / L - 5 mol / L.

[0022] Preferably, the protective atmosphere in step 4) is one of a nitrogen atmosphere, an argon atmosphere, and a nitrogen-argon mixed atmosphere.

[0023] Preferably, the carbonization in step 4) includes the following operations: heating from room temperature to 1300°C - 1500°C at a heating rate of 1°C / min - 20°C / min and holding for 0.1 h - 6 h.

[0024] A hard carbon is made by the above preparation method.

[0025] A negative electrode contains the above hard carbon.

[0026] A sodium-ion battery comprising the above-mentioned negative electrode.

[0027] The beneficial effects of the present invention are as follows: The preparation method of the hard carbon of the present invention is simple and the production cost is low. The sodium-ion battery made with it as the negative electrode material has a large reversible specific capacity, a high plateau capacity, a high initial Coulombic efficiency, and excellent cycle stability, and is suitable for large-scale industrial production and application.

[0028] Specifically:

[0029] 1) The hard carbon of the present invention has a suitable specific surface area, a large tap density, excellent electrochemical performance, and good electrical conductivity. The sodium-ion battery made with it as the negative electrode material has a large reversible specific capacity (the initial charging specific capacity can reach 390 mAh / g), a high plateau capacity (greater than 260 mAh / g), a high initial Coulombic efficiency (up to 90%), and excellent cycle stability;

[0030] 2) The hard carbon of the present invention is made from renewable, abundant in reserves, and low-cost biomass. Most biomass materials are applicable, and the universality of the raw materials is excellent;

[0031] 3) The preparation method of the hard carbon of the present invention is simple and the production cost is low, and it is suitable for large-scale industrial production and application. Description of the Drawings

[0032] Figure 1 SEM image of the hard carbon in Example 1.

[0033] Figure 2 First charge-discharge curve of the sodium-ion battery in Example 1.

[0034] Figure 3 Graph of the cycle performance test results of the sodium-ion battery in Example 1. Detailed Embodiments

[0035] The present invention will be further explained and illustrated below with specific embodiments.

[0036] Example 1:

[0037] A kind of hard carbon, and its preparation method is as follows:

[0038] 1) Add waste balsa wood into an atmosphere furnace, control the heating rate to be 5 °C / min, heat from room temperature to 300 °C, keep warm for 3 h (pre-carbonization), cool naturally to room temperature, then add the pre-carbonized product into a ball mill for pulverization, and pass through a 100-mesh sieve to obtain carbon powder;

[0039] 2) Immerse the carbon powder in a potassium hydroxide solution with a concentration of 0.4 mol / L. The mass ratio of the carbon powder to the potassium hydroxide solution is 1:40. Stir for 12 h, filter, and take the solid for drying to obtain the alkali-treated carbon powder.

[0040] 3) Add the alkali-treated carbon powder to an atmosphere furnace, fill it with nitrogen, control the heating rate at 5 °C / min, heat from room temperature to 1100 °C, hold for 2 h, and naturally cool to room temperature. Then immerse the carbonization product in hydrochloric acid with a concentration of 1 mol / L. The mass ratio of the carbonization product to the hydrochloric acid is 1:10. Stir for 12 h, filter, and take the solid for drying to obtain the acid-washed carbon powder.

[0041] 4) Add the acid-washed carbon powder to an atmosphere furnace, fill it with nitrogen, control the heating rate at 5 °C / min, heat from room temperature to 1300 °C, hold for 4 h, and naturally cool to room temperature to obtain the hard carbon.

[0042] A sodium-ion battery is prepared as follows:

[0043] Add 94 mg of the above hard carbon, 3 mg of the conductive agent Super P, 3 mg of sodium carboxymethyl cellulose (CMC; binder), and 0.45 mL of deionized water to a homogenizer and homogenize for 15 min to prepare an electrode slurry (black paste). Then coat the electrode slurry on an aluminum foil (current collector) and dry it to make a negative electrode sheet. Then, use a sodium sheet as the positive electrode sheet, a NaPF6 solution with a concentration of 1.0 mol / L (the solvent is EC, DMC, and EMC, and the volume ratio of EC, DMC, and EMC is 1:1:1) as the electrolyte, a glass fiber membrane as the separator, and stainless steel as the outer shell to assemble a CR2032 type button battery to obtain the sodium-ion battery.

[0044] Performance test:

[0045] 1) The scanning electron microscope (SEM) image of the hard carbon in this example is as Figure 1 shown.

[0046] It can be seen from Figure 1 that the surface of the hard carbon is relatively smooth, the particles are irregular in shape, and the overall particle size distribution is relatively uniform.

[0047] 2) Perform an adsorption-desorption isothermal experiment on the hard carbon in this example, and then calculate the specific surface area of the hard carbon to be 10.2 m 2 / g according to the nitrogen adsorption-desorption isothermal curve.

[0048] 3) Perform a charge-discharge performance test on the sodium-ion battery in this example under the condition of a current density of 20 mA / g. The first charge-discharge curve of the sodium-ion battery obtained is as Figure 2 shown.

[0049] It can be seen fromFigure 2 It can be known that the initial discharge specific capacity of the sodium-ion battery is 433 mAh / g, the initial charge specific capacity is 390 mAh / g, the initial Coulombic efficiency is as high as 90%, and there is still a large amount of capacity at the position where the discharge curve is close to 0 V on the voltage plateau, showing typical charge and discharge characteristics of hard carbon, indicating that the electrochemical performance of the hard carbon of the present invention is very excellent.

[0050] Note: Initial Coulombic efficiency (%) = (Initial charge specific capacity / Initial discharge specific capacity) × 100%.

[0051] 4) Under the condition of a current density of 100 mA / g, the cycle performance of the sodium-ion battery in this example was tested, and the cycle performance test results of the sodium-ion battery are as Figure 3 shown.

[0052] It can be Figure 3 known that the sodium-ion battery still has a capacity retention rate of 95% after 300 cycles, showing excellent cycle stability, indicating that the sodium-ion battery of the present invention has excellent cycle stability.

[0053] Example 2:

[0054] A kind of hard carbon, and its preparation method is as follows:

[0055] 1) Add waste balsa wood into an atmosphere furnace, control the heating rate to rise from room temperature to 300 °C at 5 °C / min, keep warm for 3 h, cool naturally to room temperature, then add the pre-carbonized product into a ball mill for pulverization, and pass through a 100-mesh sieve to obtain carbon powder;

[0056] 2) Immerse the carbon powder in a sodium hydroxide solution with a concentration of 0.4 mol / L, and the mass ratio of the carbon powder to the sodium hydroxide solution is 1:40, stir for 12 h, filter, and take the solid for drying to obtain alkali-treated carbon powder;

[0057] 3) Add the alkali-treated carbon powder into an atmosphere furnace, fill it with nitrogen, control the heating rate to rise from room temperature to 1100 °C at 5 °C / min, keep warm for 2 h, cool naturally to room temperature, then immerse the carbonized product in hydrochloric acid with a concentration of 1 mol / L, and the mass ratio of the carbonized product to the hydrochloric acid is 1:10, stir for 12 h, filter, and take the solid for drying to obtain acid-washed carbon powder;

[0058] 4) Add the acid-washed carbon powder into an atmosphere furnace, fill it with nitrogen, control the heating rate to rise from room temperature to 1300 °C at 5 °C / min, keep warm for 4 h, cool naturally to room temperature, and then the hard carbon is obtained.

[0059] A sodium-ion battery, except that the hard carbon in this example is used during preparation, the rest is exactly the same as in Example 1.

[0060] After testing (the testing method is the same as that in Example 1), the specific surface area of the hard carbon in this example is 7.5 m 2 / g.

[0061] After testing (the testing method is the same as that in Example 1), the initial discharge specific capacity of the sodium-ion battery in this example is 389 mAh / g under the condition of a current density of 20 mA / g, the initial charge specific capacity is 350 mAh / g, the initial Coulomb efficiency is as high as 90%, and there is still a large amount of capacity at the position where the discharge curve is close to 0 V on the voltage plateau, showing typical charge and discharge characteristics of hard carbon, indicating that the electrochemical performance of the hard carbon of the present invention is very excellent.

[0062] Example 3:

[0063] A kind of hard carbon, and its preparation method is as follows:

[0064] 1) Add waste balsa wood into an atmosphere furnace, control the heating rate to be 5 °C / min, heat from room temperature to 300 °C, keep warm for 3 h, cool naturally to room temperature, then add the pre-carbonized product into a ball mill for pulverization, and pass through a 100-mesh sieve to obtain carbon powder;

[0065] 2) Immerse the carbon powder into a lithium hydroxide solution with a concentration of 0.4 mol / L, and the mass ratio of the carbon powder to the lithium hydroxide solution is 1:40, stir for 12 h, filter, and take the solid for drying to obtain alkali-treated carbon powder;

[0066] 3) Add the alkali-treated carbon powder into an atmosphere furnace, fill with nitrogen, control the heating rate to be 5 °C / min, heat from room temperature to 1100 °C, keep warm for 2 h, cool naturally to room temperature, then immerse the carbonized product into hydrochloric acid with a concentration of 1 mol / L, and the mass ratio of the carbonized product to the hydrochloric acid is 1:10, stir for 12 h, filter, and take the solid for drying to obtain acid-washed carbon powder;

[0067] 4) Add the acid-washed carbon powder into an atmosphere furnace, fill with nitrogen, control the heating rate to be 5 °C / min, heat from room temperature to 1300 °C, keep warm for 4 h, cool naturally to room temperature, and then the hard carbon is obtained.

[0068] A sodium-ion battery, except that the hard carbon in this example is used during preparation, the rest is exactly the same as in Example 1.

[0069] After testing (the testing method is the same as that in Example 1), the specific surface area of the hard carbon in this example is 6.8 m 2 / g.

[0070] After testing (the testing method is the same as that in Example 1), the first discharge specific capacity of the sodium-ion battery in this example is 400 mAh / g under the condition of a current density of 20 mA / g, the first charge specific capacity is 360 mAh / g, the first Coulomb efficiency is as high as 90%, and there is still a large amount of capacity at the position where the discharge curve is close to 0 V on the voltage plateau, showing typical hard carbon charge-discharge characteristics, indicating that the electrochemical performance of the hard carbon of the present invention is very excellent.

[0071] Example 4:

[0072] A kind of hard carbon, and its preparation method is as follows:

[0073] 1) Add waste bamboo into an atmosphere furnace, control the heating rate to be 5 °C / min, heat from room temperature to 300 °C, keep warm for 3 h, cool naturally to room temperature, then add the pre-carbonized product into a ball mill for pulverization, and pass through a 100-mesh sieve to obtain carbon powder;

[0074] 2) Immerse the carbon powder in a potassium hydroxide solution with a concentration of 0.4 mol / L, and the mass ratio of the carbon powder to the potassium hydroxide solution is 1:40, stir for 12 h, filter, and take the solid for drying to obtain alkali-treated carbon powder;

[0075] 3) Add the alkali-treated carbon powder into an atmosphere furnace, fill it with nitrogen, control the heating rate to be 5 °C / min, heat from room temperature to 1100 °C, keep warm for 2 h, cool naturally to room temperature, then immerse the carbonized product in hydrochloric acid with a concentration of 1 mol / L, and the mass ratio of the carbonized product to the hydrochloric acid is 1:10, stir for 12 h, filter, and take the solid for drying to obtain acid-washed carbon powder;

[0076] 4) Add the acid-washed carbon powder into an atmosphere furnace, fill it with nitrogen, control the heating rate to be 5 °C / min, heat from room temperature to 1300 °C, keep warm for 4 h, cool naturally to room temperature, and then the hard carbon is obtained.

[0077] A sodium-ion battery, except that the hard carbon in this example is used during preparation, the rest is exactly the same as that in Example 1.

[0078] After testing (the testing method is the same as that in Example 1), the specific surface area of the hard carbon in this example is 8.1 m 2 / g.

[0079] After testing (the testing method is the same as that in Example 1), the first discharge specific capacity of the sodium-ion battery in this example is 389 mAh / g under the condition of a current density of 20 mA / g, the first charge specific capacity is 350 mAh / g, the first Coulomb efficiency is as high as 90%, and there is still a large amount of capacity at the position where the discharge curve is close to 0 V on the voltage plateau, showing typical hard carbon charge-discharge characteristics, indicating that the electrochemical performance of the hard carbon of the present invention is very excellent.

[0080] Example 5:

[0081] A kind of hard carbon, and its preparation method is as follows:

[0082] 1) Add waste birch wood into an atmosphere furnace, control the heating rate to be 5°C / min, heat up from room temperature to 300°C, keep warm for 3 h, naturally cool to room temperature, then add the pre-carbonized product into a ball mill for pulverization, pass through a 100-mesh sieve to obtain carbon powder;

[0083] 2) Immerse the carbon powder in a potassium hydroxide solution with a concentration of 0.4 mol / L, the mass ratio of the carbon powder to the potassium hydroxide solution is 1:40, stir for 12 h, filter, take the solid for drying to obtain alkali-treated carbon powder;

[0084] 3) Add the alkali-treated carbon powder into an atmosphere furnace, fill with nitrogen, control the heating rate to be 5°C / min, heat up from room temperature to 1100°C, keep warm for 2 h, naturally cool to room temperature, then immerse the carbonized product in hydrochloric acid with a concentration of 1 mol / L, the mass ratio of the carbonized product to the hydrochloric acid is 1:10, stir for 12 h, filter, take the solid for drying to obtain acid-washed carbon powder;

[0085] 4) Add the acid-washed carbon powder into an atmosphere furnace, fill with nitrogen, control the heating rate to be 5°C / min, heat up from room temperature to 1300°C, keep warm for 4 h, naturally cool to room temperature to obtain hard carbon.

[0086] A sodium-ion battery, except that the hard carbon in this example is used during preparation, the rest is exactly the same as in Example 1.

[0087] After testing (the testing method is the same as in Example 1), the specific surface area of the hard carbon in this example is 7.8 m 2 / g.

[0088] After testing (the testing method is the same as in Example 1), the first discharge specific capacity of the sodium-ion battery in this example under the condition of a current density of 20 mA / g is 411 mAh / g, the first charge specific capacity is 370 mAh / g, the first Coulomb efficiency is as high as 90%, and there is still a large amount of capacity at the position where the discharge curve is close to 0 V on the voltage platform, showing typical charge and discharge characteristics of hard carbon, indicating that the electrochemical performance of the hard carbon of the present invention is very excellent.

[0089] Comparative Example 1:

[0090] A kind of hard carbon, and its preparation method is as follows:

[0091] 1) Add waste balsa wood into an atmosphere furnace, control the heating rate to be 5°C / min, heat up from room temperature to 300°C, keep warm for 3 h, naturally cool to room temperature, then add the pre-carbonized product into a ball mill for pulverization, pass through a 100-mesh sieve to obtain carbon powder;

[0092] 2) Add carbon powder into an atmosphere furnace, fill it with nitrogen, control the heating rate at 5 °C / min, heat from room temperature to 1100 °C, hold for 2 h, cool naturally to room temperature, then immerse the carbonized product in hydrochloric acid with a concentration of 1 mol / L. The mass ratio of the carbonized product to hydrochloric acid is 1:10. Stir for 12 h, filter, take the solid and dry it to obtain acid-washed carbon powder;

[0093] 3) Add the acid-washed carbon powder into an atmosphere furnace, fill it with nitrogen, control the heating rate at 5 °C / min, heat from room temperature to 1300 °C, hold for 4 h, cool naturally to room temperature to obtain hard carbon.

[0094] A sodium-ion battery, except that the hard carbon in this comparative example is used during preparation, the rest is exactly the same as in Example 1.

[0095] After testing (the testing method is the same as in Example 1), the specific surface area of the hard carbon in this comparative example is 16 m 2 / g.

[0096] After testing (the testing method is the same as in Example 1), the first discharge specific capacity of the sodium-ion battery in this comparative example under the condition of a current density of 20 mA / g is 375 mAh / g, the first charge specific capacity is 300 mAh / g, and the first Coulomb efficiency is 80%. Comparative Example 2:

[0097] A kind of hard carbon, and its preparation method is as follows:

[0098] 1) Add waste balsa wood into a ball mill for pulverization, pass through a 100-mesh sieve to obtain balsa wood powder;

[0099] 2) Immerse the balsa wood powder in potassium hydroxide solution with a concentration of 0.4 mol / L. The mass ratio of the balsa wood powder to the potassium hydroxide solution is 1:40. Stir for 12 h, filter, take the solid and dry it to obtain alkali-treated balsa wood powder;

[0100] 3) Add the alkali-treated balsa wood powder into an atmosphere furnace, fill it with nitrogen, control the heating rate at 5 °C / min, heat from room temperature to 1100 °C, hold for 2 h, cool naturally to room temperature, then immerse the carbonized product in hydrochloric acid with a concentration of 1 mol / L. The mass ratio of the carbonized product to hydrochloric acid is 1:10. Stir for 12 h, filter, take the solid and dry it to obtain acid-washed carbon powder;

[0101] 4) Add the acid-washed carbon powder into an atmosphere furnace, fill it with nitrogen, control the heating rate at 5 °C / min, heat from room temperature to 1300 °C, hold for 4 h, cool naturally to room temperature to obtain hard carbon.

[0102] A sodium-ion battery, except that the hard carbon in this comparative example is used during preparation, the rest is exactly the same as in Example 1.

[0103] After testing (the testing method is the same as that in Example 1), the specific surface area of the hard carbon in this comparative example is 86 m 2 / g.

[0104] After testing (the testing method is the same as that in Example 1), the initial discharge specific capacity of the sodium-ion battery in this comparative example under the condition of a current density of 20 mA / g is 360 mAh / g, the initial charge specific capacity is 270 mAh / g, and the initial Coulombic efficiency is 75%. Comparative Example 3:

[0105] A kind of hard carbon, and its preparation method is as follows:

[0106] 1) Add waste balsa wood into an atmosphere furnace, control the heating rate to be 5 °C / min, heat from room temperature to 300 °C, keep warm for 3 h, naturally cool to room temperature, then add the pre-carbonized product into a ball mill for pulverization, pass through a 100-mesh sieve to obtain carbon powder;

[0107] 2) Immerse the carbon powder in a potassium hydroxide solution with a concentration of 0.4 mol / L, and the mass ratio of the carbon powder to the potassium hydroxide solution is 1:40, stir for 12 h, filter, and take the solid for drying to obtain alkali-treated carbon powder;

[0108] 3) Add the alkali-treated carbon powder into an atmosphere furnace, fill it with nitrogen, control the heating rate to be 5 °C / min, heat from room temperature to 1100 °C, keep warm for 2 h, naturally cool to room temperature, then immerse the carbonized product in a hydrochloric acid solution with a concentration of 1 mol / L, and the mass ratio of the carbonized product to the hydrochloric acid is 1:10, stir for 12 h, filter, and take the solid for drying to obtain the hard carbon.

[0109] A sodium-ion battery, except that the hard carbon in this comparative example is used during preparation, the rest is exactly the same as that in Example 1.

[0110] After testing (the testing method is the same as that in Example 1), the specific surface area of the hard carbon in this comparative example is 980 m 2 / g.

[0111] After testing (the testing method is the same as that in Example 1), the initial discharge specific capacity of the sodium-ion battery in this comparative example under the condition of a current density of 20 mA / g is 400 mAh / g, the initial charge specific capacity is 180 mAh / g, and the initial Coulombic efficiency is 45%. Comparative Example 4:

[0112] A kind of hard carbon, and its preparation method is as follows:

[0113] 1) Add waste balsa wood into an atmosphere furnace, control the heating rate to be 5 °C / min, heat from room temperature to 300 °C, keep warm for 3 h, naturally cool to room temperature, then add the pre-carbonized product into a ball mill for pulverization, pass through a 100-mesh sieve to obtain carbon powder;

[0114] 2) Immerse the carbon powder in a potassium hydroxide solution with a concentration of 0.4 mol / L. The mass ratio of the carbon powder to the potassium hydroxide solution is 1:40. Stir for 12 h, filter, and take the solid for drying to obtain the alkali-treated carbon powder.

[0115] 3) Add the alkali-treated carbon powder into an atmosphere furnace, fill it with nitrogen, control the heating rate at 5 °C / min, heat from room temperature to 1300 °C, keep it warm for 2 h, and then cool it naturally to room temperature. Then immerse the carbonized product in hydrochloric acid with a concentration of 1 mol / L. The mass ratio of the carbonized product to the hydrochloric acid is 1:10. Stir for 12 h, filter, and take the solid for drying to obtain the hard carbon.

[0116] A sodium-ion battery, except that the hard carbon in this comparative example is used during preparation, the rest is exactly the same as in Example 1.

[0117] After testing (the testing method is the same as in Example 1), the specific surface area of the hard carbon in this comparative example is 1880 m 2 / g.

[0118] After testing (the testing method is the same as in Example 1), the initial discharge specific capacity of the sodium-ion battery in this comparative example under the condition of a current density of 20 mA / g is 239 mAh / g, the initial charge specific capacity is 105 mAh / g, and the initial Coulomb efficiency is 44%.

[0119] The performance summary tables of the hard carbon and sodium-ion batteries in Examples 1-5 and Comparative Examples 1-4 are shown in the following table:

[0120] Table 1 Performance summary table of hard carbon and sodium-ion batteries

[0121] Test Items <![CDATA[Specific surface area (m 2 / g)]]> Initial Charge Specific Capacity (mAh / g) Initial Coulombic Efficiency (%) Example 1 10.2 390 90 Example 2 7.5 350 90 Example 3 6.8 360 90 Example 4 8.1 350 90 Example 5 7.8 370 90 Comparative Example 1 16 300 80 Comparative Example 2 86 270 75 Comparative Example 3 980 180 45 Comparative Example 4 1880 105 44

[0122] As can be seen from Table 1:

[0123] 1) The specific surface areas of the hard carbon in Examples 1-5 are all lower than that of the hard carbon in Comparative Example 1. The reason is that: the alkali treatment in Examples 1-5 optimized the microstructure of the precursor material, forming more ultramicroporous structures, while there are more mesopores and macropores in the hard carbon of Comparative Example 1;

[0124] 2) The specific surface areas of the hard carbon in Examples 1-5 are all lower than that of the hard carbon in Comparative Example 2. The reason is that: the pre-carbonization in Examples 1-5 weakened the dissolution and etching effect of the alkali on the precursor;

[0125] 3) The specific surface areas of the hard carbon in Examples 1-5 are all lower than that of the hard carbon in Comparative Example 3. The reason is that: the secondary high-temperature carbonization in Examples 1-5 converted most of the open pores into closed pores;

[0126] 4) The specific surface areas of the hard carbon in Examples 1 to 5 are all lower than that in Comparative Example 4. The reason is that pickling after primary carbonization in Examples 1 to 5 can remove the alkaline substances in the carbon powder treated with alkali, preventing the continuous etching of the carbon material during secondary high-temperature carbonization;

[0127] 5) The electrochemical properties of the sodium-ion batteries in Examples 1 to 5 are superior to those in Comparative Examples 1 to 4. The reason is that targeted component removal and chemical etching strategies are carried out in Examples 1 to 5, which can precisely regulate the porous structure of the hard carbon, thereby significantly improving the plateau capacity of the sodium-ion battery. The specific mechanism is as follows: Alkaline substances can dissolve the amorphous components to form closed pore cores with adjustable sizes. Subsequently, the alkaline substances in-situ occupy the amorphous regions and modify the pore structure through chemical corrosion. The optimized hard carbon not only has a short-range disordered graphite domain but also can promote the intercalation and deintercalation of sodium ions (Na + ). In addition, the hard carbon also has a closed pore structure with abundant micropores, appropriate pore sizes, and ultra-thin carbon layers (1 to 3 layers), significantly increasing the storage sites for sodium ions.

[0128] The above embodiments are preferred embodiments of the present invention, but the embodiments of the present invention are not limited to the above embodiments. Any other changes, modifications, substitutions, combinations, and simplifications made without departing from the spirit and principle of the present invention shall be equivalent replacement methods and are all included in the protection scope of the present invention.

Claims

1. A method for preparing hard carbon, characterized in that, It includes the following steps: 1) Pre-carbonize and pulverize the biomass raw material to obtain carbon powder; 2) Immerse the carbon powder in an alkali solution for soaking to obtain alkali-treated carbon powder; 3) Carbonize the alkali-treated carbon powder under a protective atmosphere at a temperature of 900°C to 1200°C, and then perform acid washing to obtain acid-washed carbon powder; 4) Carbonize the acid-washed carbon powder under a protective atmosphere at a temperature of 1300°C to 1500°C to obtain hard carbon.

2. The preparation method according to claim 1, wherein: The biomass raw material described in step 1) is at least one of balsa wood, birch, oak, beech, bamboo, pulp, lignin, walnut shell, oil-tea camellia shell, peanut shell, coconut shell, xylose residue, fruit pit, corncob, straw, and cellulose powder.

3. The preparation method according to claim 1 or 2, characterized in that: The pre-carbonization described in step 1) includes the following operations: Heat from room temperature to 100°C to 800°C at a heating rate of 1°C / min to 10°C / min, and keep warm for 0.1 h to 24 h.

4. The preparation method according to claim 1, characterized in that: In step 2), the mass ratio of the carbon powder to the alkali solution is 1:1 to 50; the concentration of the alkali solution in step 2) is 0.1 mol / L to 10 mol / L; the alkali in the alkali solution in step 2) is at least one of lithium hydroxide, sodium hydroxide, potassium hydroxide, and calcium hydroxide; the soaking time in step 2) is 0.1 h to 24 h.

5. The preparation method according to claim 1, wherein: The carbonization described in step 3) includes the following operations: Heat from room temperature to 900°C to 1200°C at a heating rate of 1°C / min to 20°C / min, and keep warm for 0.1 h to 6 h.

6. The preparation method according to claim 1 or 5, characterized in that: In step 3), the mass ratio of the product obtained by carbonization to the acid solution used for acid washing is 1:1 to 50; the acid solution used for acid washing in step 3) is hydrochloric acid with a concentration of 0.1 mol / L to 5 mol / L.

7. The preparation method according to claim 1, wherein: The carbonization described in step 4) includes the following operations: Heat from room temperature to 1300°C to 1500°C at a heating rate of 1°C / min to 20°C / min, and keep warm for 0.1 h to 6 h.

8. A hard carbon, characterized in that, It is made by the preparation method described in any one of claims 1 to 7.

9. A negative electrode, characterized in that, It contains the hard carbon described in claim 8.

10. A sodium-ion battery, characterized in that, It contains the negative electrode described in claim 9.

Images