Hard carbon negative electrode material, preparation method of hard carbon negative electrode material and sodium ion battery

Abstract

The invention relates to a hard carbon negative electrode material, a preparation method of the hard carbon negative electrode material and a sodium ion battery. The closed pore volume Vcp of the hard carbon negative electrode material is more than 0.01 cm < 3 > / g and less than 0.5 cm < 3 > / g; the condition that Vcp is equal to 1 / rho true-1 / (2.26 * 0.3354 / d002act) is met; wherein rho true is the true density of the hard carbon negative electrode material, d002act is the interlayer spacing of X-ray diffraction 002 crystal lattices, and the true density of the hard carbon negative electrode material is 0.5 g / cm < 3 > < rho true < 2.26 g / cm < 3 >. By controlling the temperature and the pressure, the true density of the negative electrode material is directly controlled, and the closed pore structure proportion of the negative electrode material is indirectly controlled, so that the negative electrode material has relatively high reversible specific capacity and first coulombic efficiency, and the assembled sodium ion battery has relatively high energy density and excellent rate capability.

Description

Technical Field

[0001] The present invention relates to the technical field of negative electrode materials and battery preparation, and in particular to a hard carbon negative electrode material, a method for preparing the hard carbon negative electrode material, and a sodium ion battery prepared using the hard carbon negative electrode material. Background Art

[0002] Hard carbon is an amorphous carbon material with low platform potential, good structural stability and low cost. It is considered to be one of the most likely negative electrode materials for commercial application in sodium-ion batteries. However, hard carbon also has some disadvantages, such as low first-cycle coulombic efficiency, low specific capacity and poor rate performance.

[0003] In order to improve the sodium storage performance of hard carbon, a commonly used method is to optimize the multi-scale structure and regulate the function of hard carbon by creating closed pores, thereby improving its application performance in sodium ion batteries.

[0004] The Chinese patent application with application number CN202311744491.6 discloses “a negative electrode material and a sodium ion battery”, wherein the negative electrode material comprises a carbon matrix with a three-dimensional porous structure, and a doping element loaded in the pores of the carbon matrix; the relationship between the doping element content of the negative electrode material and the gas adsorption amount is as follows: 0<α / β×10 3 ≤1; wherein α is the content of the doping element in the negative electrode material, in ppm; β is the gas adsorption capacity of the negative electrode material, in cm 3 / g. The doping element includes any one or a combination of at least two of P, S, N, O, H, B, Mg, Ti, Zn or Fe; the carbon matrix includes hard carbon. By controlling the doping element content and gas adsorption amount of the negative electrode material, the pore structure of the negative electrode material is regulated, so that the negative electrode material has a higher reversible specific capacity and first coulombic efficiency.

[0005] The present invention is different from the method in the above-mentioned public documents. The present invention directly controls the true density of the negative electrode material and indirectly controls the closed-pore structure ratio of the negative electrode material by controlling temperature and pressure, so that the negative electrode material has a higher reversible specific capacity and first coulombic efficiency. Summary of the invention

[0006] The present invention provides a hard carbon negative electrode material, a method for preparing the hard carbon negative electrode material and a sodium ion battery. By controlling the temperature and atmosphere, the true density of the negative electrode material is directly controlled, and the closed-pore structure ratio of the negative electrode material is indirectly controlled, so that the negative electrode material has a higher reversible specific capacity and first coulombic efficiency, and the assembled sodium ion battery has a higher energy density and excellent rate performance.

[0007] In order to achieve the above object, the present invention adopts the following technical solutions:

[0008] A hard carbon negative electrode material, which forms a three-dimensional carbon matrix with a large number of closed-pore structures by regulating the reaction conditions under a set atmosphere; the reaction conditions include carbonization temperature and system pressure; the closed-pore volume of the hard carbon negative electrode material is 0.01cm 3 / g<V cp <0.5cm 3 / g; and satisfy V cp =1 / ρ 真 -1 / (2.26×0.3354 / d002 act ), where ρ 真 is the true density of the hard carbon negative electrode material, in g / cm 3 ;d002 act is the interlayer spacing of the X-ray diffraction 002 lattice, in nm; the true density of the hard carbon negative electrode material is 0.5 g / cm 3 <ρ 真 <2.26g / cm 3 .

[0009] Furthermore, the true density of the hard carbon negative electrode material is 真 =(0.75T / 620)ρ0-0.25(T / 1000)(P / P0) 0.3 ; Wherein, ρ0 is the density of the initial carbon source; T is the carbonization temperature, and T = 200 ~ 1500 ° C; P0 is the atmospheric pressure; P is the system pressure, and P = 0.1 ~ 1.5MPa.

[0010] Furthermore, the set atmosphere is one or a combination of nitrogen, argon, helium, carbon dioxide, chlorine, water vapor, and air.

[0011] Furthermore, the hard carbon forming the three-dimensional carbon matrix is ​​formed by high-temperature carbonization of an organic thermosetting precursor; the regulation of the reaction conditions under the set atmosphere refers to adjusting the pore structure and the ratio of open and closed pores of the carbonized product by controlling the temperature rise curve, system pressure and system atmosphere during the carbonization process of the organic thermosetting precursor, thereby controlling the true density of the carbonized product.

[0012] Furthermore, the hard carbon includes one or more of asphalt, cellulose, sucrose, glucose, phenolic resin, starch, coal, and heavy oil.

[0013] Furthermore, in the X-ray diffraction spectrum of the hard carbon negative electrode material, there are characteristic peaks unique to hard carbon in the ranges of 20° to 30° and 35° to 50°.

[0014] A method for preparing a hard carbon negative electrode material comprises the following steps:

[0015] (1) crushing the carbon matrix raw material to a set particle size;

[0016] (2) After mixing the carbon matrix raw materials, pre-carbonization is performed at a temperature of 200 to 1000° C. in an atmosphere of inert gas / air / carbon dioxide / water vapor;

[0017] (3) breaking up the pre-carbonized carbon matrix into pieces of 3 to 20 microns to obtain a negative electrode material precursor;

[0018] (4) The negative electrode material precursor is carbonized at a temperature of 1000-1800°C. During this process, the pore structure of different negative electrode material precursors is precisely controlled by controlling the temperature, pressure and reaction atmosphere. Finally, the closed pore volume is 0.01cm 3 / g<V cp <0.5cm 3 / g of hard carbon negative electrode material.

[0019] Furthermore, the carbon matrix raw material includes one or more of biomass and its modified products, coal and its modified products, asphalt and its modified products, resin and its modified products.

[0020] A sodium ion battery, wherein the negative electrode of the sodium ion battery comprises the hard carbon negative electrode material.

[0021] Compared with the prior art, the present invention has the following beneficial effects:

[0022] The present invention mainly provides a hard carbon negative electrode material, a preparation method of the hard carbon negative electrode material and a sodium ion battery. The carbon matrix of the hard carbon negative electrode material has a three-dimensional porous structure. The true density of the negative electrode material is regulated by controlling the temperature and pressure. The relationship between the closed pore volume and the true density of the negative electrode material is: V cp =1 / ρ 真 -1 / 2.26(d002 gr / d002 hard ), d002 gr is the interlayer spacing of X-ray diffraction graphite 002 lattice, d002 hard is the interlayer spacing of the X-ray diffraction hard carbon 002 lattice. The characteristic pore range of the negative electrode material is expressed as the gas adsorption amount; when the closed pore volume and true density satisfy the above relationship, the closed pores can not only support the structure of the carbon material, but also form certain characteristic pores in the negative electrode material. During the charge and discharge process, sodium ions can form sodium clusters in the characteristic pores, thereby providing a high specific capacity and affecting the intercalation and filling process of sodium ions, ultimately improving the reversible specific capacity and first coulomb efficiency of the negative electrode material, thereby making the assembled sodium ion battery have a higher energy density and excellent rate performance. BRIEF DESCRIPTION OF THE DRAWINGS

[0023] Figure 1This is the XRD diagram of the hard carbon negative electrode material prepared in Example 1 of the present invention.

[0024] Figure 2 This is a SEM image of the hard carbon negative electrode material prepared in Example 1 of the present invention.

[0025] Figure 3 It is a charge and discharge curve diagram of the hard carbon negative electrode material prepared in Example 1 of the present invention. DETAILED DESCRIPTION

[0026] The hard carbon negative electrode material of the present invention forms a three-dimensional carbon matrix with a large number of closed-pore structures by regulating the reaction conditions under a set atmosphere; the reaction conditions include carbonization temperature and system pressure; the closed-pore volume of the hard carbon negative electrode material is 0.01cm 3 / g<V cp <0.5cm 3 / g; and satisfy V cp =1 / ρ 真 -1 / (2.26×0.3354 / d002 act ), where ρ 真 is the true density of the hard carbon negative electrode material, in g / cm 3 ;d002 act is the interlayer spacing of the X-ray diffraction 002 lattice, in nm; the true density of the hard carbon negative electrode material is 0.5 g / cm 3 <ρ 真 <2.26g / cm 3 .

[0027] Furthermore, the true density of the hard carbon negative electrode material is 真 =(0.75T / 620)ρ0-0.25(T / 1000)(P / P0) 0.3 ; Wherein, ρ0 is the density of the initial carbon source; T is the carbonization temperature, and T = 200 ~ 1500 ° C; P0 is the atmospheric pressure; P is the system pressure, and P = 0.1 ~ 1.5MPa.

[0028] Furthermore, the set atmosphere is one or a combination of nitrogen, argon, helium, carbon dioxide, chlorine, water vapor, and air.

[0029] Furthermore, the hard carbon forming the three-dimensional carbon matrix is ​​formed by high-temperature carbonization of an organic thermosetting precursor; the regulation of the reaction conditions under the set atmosphere refers to adjusting the pore structure and the ratio of open and closed pores of the carbonized product by controlling the temperature rise curve, system pressure and system atmosphere during the carbonization process of the organic thermosetting precursor, thereby controlling the true density of the carbonized product.

[0030] Furthermore, the hard carbon includes one or more of asphalt, cellulose, sucrose, glucose, phenolic resin, starch, coal, and heavy oil.

[0031] Furthermore, in the X-ray diffraction spectrum of the hard carbon negative electrode material, there are characteristic peaks unique to hard carbon in the ranges of 20° to 30° and 35° to 50°.

[0032] The method for preparing a hard carbon negative electrode material of the present invention comprises the following steps:

[0033] (1) crushing the carbon matrix raw material to a set particle size;

[0034] (2) After mixing the carbon matrix raw materials, pre-carbonization is performed at a temperature of 200 to 1000° C. in an atmosphere of inert gas / air / carbon dioxide / water vapor;

[0035] (3) breaking up the pre-carbonized carbon matrix into pieces of 3 to 20 microns to obtain a negative electrode material precursor;

[0036] (4) The negative electrode material precursor is carbonized at a temperature of 1000-1800°C. During this process, the pore structure of different negative electrode material precursors is precisely controlled by controlling the temperature, pressure and reaction atmosphere. Finally, the closed pore volume is 0.01cm 3 / g<V cp <0.5cm 3 / g of hard carbon negative electrode material.

[0037] Furthermore, the carbon matrix raw material includes one or more of biomass and its modified products, coal and its modified products, asphalt and its modified products, resin and its modified products.

[0038] The present invention provides a sodium ion battery, wherein the negative electrode of the sodium ion battery comprises the hard carbon negative electrode material.

[0039] The hard carbon negative electrode material of the present invention has a three-dimensional porous carbon matrix with a controllable closed-pore structure, and the closed-pore volume ratio of the hard carbon negative electrode material is: 0.5cm 3 / g<V cp / V OP+CP <1cm 3 / g; where V cp is the closed pore volume of the hard carbon negative electrode material, in cm 3 / g; V OP+CP is the total pore volume of the open and closed pores of the hard carbon negative electrode material, in cm 3 / g.

[0040] Furthermore, V cp The range is 0.01~0.5cm 3 / g; VOP+CP The range is 0.01~2cm 3 / g.

[0041] Preferably, the closed pore volume V of the hard carbon negative electrode material is cp Meet: 0.1cm 3 / g<V cp ≤0.5cm 3 / g, for example, 0.1 cm 3 / g, 0.15cm 3 / g, 0.2cm 3 / g, 0.25cm 3 / g or 0.3cm 3 / g, etc., but are not limited to the listed values, and other unlisted values ​​within the numerical range are also applicable.

[0042] Generally speaking, hard carbon is synthesized by high-temperature carbonization of organic thermosetting precursors (such as cellulose, sucrose, glucose, phenolic resin, starch, coal, heavy oil, etc.). During the pyrolysis process, the precursor carbon layer easily forms vortex layers and disordered microstructure graphite clusters composed of surface defects, nanopores or voids. In this process, sodium ions first appear in the hard carbon defect site in the oblique region (0.1~1V vs Na / Na + ), followed by the formation of sodium clusters in the platform region (0-0.1 V vs Na / Na + ). Closed pores are mainly responsible for sodium storage in the low-pressure platform. Increasing the number of closed pores and the size of nanopores can maximize the capacity of the platform to generate hard carbon, while the number and proportion of closed pores are characterized and measured by the true density of the negative electrode material.

[0043] In the present invention, the empirical relationship between the closed pore volume and true density of the hard carbon negative electrode material is as follows:

[0044] V cp =1 / ρ 真 -1 / (2.26×0.3354 / d002 act )

[0045] In the formula, ρ 真 is the true density of the hard carbon negative electrode material, in g / cm 3 .

[0046] In the present invention, the true density of the hard carbon negative electrode material is 真 The value of can be 0.5, 0.6, 0.7, 0.8, 0.9, 1.0, 1.1, 1.2, 1.3, 1.4, 1.5, 1.6, 1.7, 1.8, 1.9, 2.0, 2.1 or 2.2, etc.

[0047] The organic thermosetting precursor undergoes molecular rearrangement and gas escape during the carbonization process. The present invention adjusts the pore structure and open-closed pore ratio of the carbonized product by controlling the heating curve, system pressure and system atmosphere, thereby controlling the true density of the carbonized product, i.e., ρ 真 =(0.75T / 620)ρ0-0.25(T / 1000)(P / P0) 0.3 .

[0048] When the true density and the closed pore volume satisfy the relationship of the present invention, the capacity and coulombic efficiency of the negative electrode material can be improved. When the true density is lower than the limit range of the relationship, the closed pores will cause the characteristic pores to collapse due to insufficient pore wall strength, thereby reducing the specific capacity; and when the true density is higher than the limit range of the relationship, no suitable characteristic pores will be generated, or the generated characteristic pores will be insufficient, thereby reducing the specific capacity.

[0049] Preferably, the carbon matrix of the present invention comprises hard carbon.

[0050] Preferably, if Figure 1 As shown, in the X-ray diffraction (XRD) spectrum of the hard carbon negative electrode material of the present invention, there is a characteristic peak in the range of 20° to 30°, and the half-peak width of the characteristic peak is 5 to 20, for example, it can be 5, 7, 10, 12, 15, 17 or 20, but it is not limited to the listed values, and other unlisted values ​​in the numerical range are also applicable. The characteristic peak in the range of 20° to 30° in the present invention reflects the carbon layer spacing and the degree of interlayer order.

[0051] Preferably, in the X-ray diffraction spectrum of the hard carbon negative electrode material of the present invention, there is a characteristic peak in the range of 35° to 50°, and the characteristic peak in the range of 35° to 50° reflects the order and grain size in the carbon layer.

[0052] Preferably, the preparation method of the hard carbon negative electrode material of the present invention is as follows:

[0053] (1) crushing the carbon matrix raw material to a set particle size;

[0054] (2) After mixing the carbon matrix raw materials, pre-carbonization is performed at a temperature of 200 to 600° C. in an atmosphere of inert gas / air / carbon dioxide;

[0055] (3) breaking up the pre-carbonized carbon matrix into pieces of 3 to 20 microns to obtain a negative electrode material precursor;

[0056] (4) The negative electrode material precursor is carbonized at a temperature of 1000-1800°C to obtain a hard carbon negative electrode material.

[0057] When the present invention adopts the above-mentioned preparation method of hard carbon negative electrode material, the pore structure of different negative electrode material precursors can be accurately controlled by controlling the temperature, pressure and reaction atmosphere; the prepared hard carbon negative electrode material has a suitable pore structure inside and obtains a closed pore volume of 0.01cm 3 / g<V cp <0.5cm 3 / g of hard carbon negative electrode material.

[0058] Preferably, the carbon matrix raw material includes any one or a combination of any two or more of biomass and its modified products, coal and its modified products, asphalt and its modified products, resin and its modified products.

[0059] The present invention also provides a sodium ion battery, wherein the negative electrode of the sodium ion battery comprises the hard carbon negative electrode material of the present invention.

[0060] The following embodiments are implemented on the premise of the technical solution of the present invention, and provide detailed implementation methods and specific operation processes, but the protection scope of the present invention is not limited to the following embodiments.

[0061] [Example 1]

[0062] This embodiment prepares a hard carbon negative electrode material.

[0063] (1) crushing coal-based asphalt to a particle size D50 of less than 20 μm to obtain asphalt powder;

[0064] (2) placing the asphalt powder in a mixed atmosphere of air and water vapor with an absolute humidity of 40% and a system pressure of 0.3 MPa, and heating it to 900° C. at a rate of 0.5° C. / min to obtain pre-carbonized powder;

[0065] (3) breaking up the pre-carbonized powder to D50 = 6 μm to obtain a hard carbon negative electrode material precursor;

[0066] (4) The hard carbon negative electrode material precursor is subjected to high temperature carbonization at a temperature of 1400° C. to obtain the hard carbon negative electrode material.

[0067] In this embodiment, the hard carbon negative electrode material includes a carbon matrix with a three-dimensional porous structure, and the carbon matrix is ​​hard carbon. The true density of the hard carbon negative electrode material is 1.62 g / cm 3 .

[0068] In this embodiment, the closed pore volume of the hard carbon negative electrode material is 0.185 cm 3 / g.

[0069] Figure 1The XRD diagram of the hard carbon negative electrode material in this embodiment is shown, which has a characteristic peak in the range of 20° to 30°, and the characteristic peak is the (002) peak, which reflects the carbon layer spacing and the degree of interlayer order, and its d002 is 0.372nm; it also has a characteristic peak in the range of 35° to 50°, and the characteristic peak is the (100) peak, which reflects the order within the carbon layer and the grain size.

[0070] Figure 2 This is the SEM image of the hard carbon negative electrode material in this embodiment.

[0071] The performance test process of hard carbon negative electrode material is as follows:

[0072] (i) The negative electrode material, polyvinylidene fluoride (PVDF) and conductive agent (SP) provided in this embodiment and comparative example are dispersed and dissolved in N-methylpyrrolidone (NMP) at a mass ratio of 90:5:5, and a slurry with a solid mass content of 55% is obtained after stirring and dispersion. After coating, drying and rolling, the slurry is punched into button battery original sheets; a metal sodium sheet is used as a counter electrode, polypropylene (PP) is used as a diaphragm material, and a small amount of sodium ion battery electrolyte is dripped to prepare a negative electrode material button battery, wherein the solute of the electrolyte is 1M NaPF, and the solvent is an ethylene carbonate-dimethyl carbonate (EC-DMC) system.

[0073] In the voltage range of 0-2V, 1) stand for 2h; 2) rate discharge (0.1C, 0V); 3) stand for 10min; 4) rate discharge (0.02C, 0V); 5) stand for 30s; 6) rate charge (0.1C, 2.0V), record the first charge specific capacity and first coulomb efficiency, the test results are as follows: Figure 3 shown.

[0074] [Example 2]

[0075] This embodiment prepares a hard carbon negative electrode material, and the preparation process is as follows:

[0076] (1) crushing the phenolic resin to a particle size D50 of less than 18 μm, to obtain phenolic resin powder;

[0077] (2) heating the phenolic resin powder to 900° C. at a rate of 0.25° C. / min in air and carbon dioxide at a system pressure of 0.1 MPa to obtain pre-carbonized powder;

[0078] (3) breaking up the pre-carbonized powder to D50 = 5 μm to obtain a hard carbon negative electrode material precursor;

[0079] (4) The hard carbon negative electrode material precursor is subjected to high temperature carbonization at a temperature of 1300° C. to obtain the hard carbon negative electrode material.

[0080] In this embodiment, the hard carbon negative electrode material includes a carbon matrix with a three-dimensional porous structure, and the carbon matrix is ​​hard carbon. The true density of the hard carbon negative electrode material is 1.58 g / cm 3 .

[0081] In this embodiment, the closed pore volume of the hard carbon negative electrode material is 0.167 cm 3 / g.

[0082] The performance testing process of the hard carbon negative electrode material is the same as that of Example 1.

[0083] [Example 3]

[0084] This embodiment prepares a hard carbon negative electrode material, and the preparation process is as follows:

[0085] (1) grinding starch to a particle size D50 of less than 18 μm to obtain starch powder;

[0086] (2) raising the temperature of starch powder to 900° C. at a rate of 0.3° C. / min in air at a system pressure of 0.15 MPa to obtain pre-carbonized powder;

[0087] (3) breaking up the pre-carbonized powder to a size of D50 = 5.5 μm to obtain a hard carbon negative electrode material precursor;

[0088] (4) The hard carbon negative electrode material precursor is subjected to high temperature carbonization at a temperature of 1200° C. to obtain the hard carbon negative electrode material.

[0089] In this embodiment, the hard carbon negative electrode material includes a carbon matrix with a three-dimensional porous structure, and the carbon matrix is ​​hard carbon. The true density of the hard carbon negative electrode material is 1.36 g / cm 3 .

[0090] In this embodiment, the closed pore volume of the hard carbon negative electrode material is 0.158 cm 3 / g.

[0091] The performance test process of the hard carbon negative electrode material is the same as that of Example 1

[0092] [Comparative Example]

[0093] The process of preparing the negative electrode material in this comparative example is as follows:

[0094] (1) crushing coal-based asphalt to a particle size D50 of less than 20 μm to obtain asphalt powder;

[0095] (2) placing the asphalt powder in a mixed atmosphere of air and water vapor with an absolute humidity of 5% and a system pressure of 0.2 MPa, and heating it to 900° C. at a rate of 0.5° C. / min to obtain pre-carbonized powder;

[0096] (3) breaking up the pre-carbonized powder to D50=6 μm to obtain a hard carbon negative electrode material precursor;

[0097] (4) The hard carbon negative electrode material precursor is subjected to high temperature carbonization at a temperature of 1400° C. to obtain the hard carbon negative electrode material.

[0098] In the comparative example, the hard carbon negative electrode material includes a carbon matrix with a three-dimensional porous structure, and the carbon matrix is ​​hard carbon. The true density of the hard carbon negative electrode material is 1.71 g / cm 3 .

[0099] The XRD diagram of the hard carbon negative electrode material in the comparative example also has a characteristic peak in the range of 20° to 30°, and the characteristic peak is the (002) peak, which reflects the carbon layer spacing and the degree of interlayer order, and its d002 is also 0.372nm; it also has a characteristic peak in the range of 35° to 50°, and the characteristic peak is the (100) peak, which reflects the order within the carbon layer and the grain size.

[0100] In the comparative example, the closed pore volume of the hard carbon negative electrode material is 0.153 cm 3 / g.

[0101] The negative electrode materials of Examples 1-3 and the comparative example were respectively made into negative electrode sheets, and the negative electrode sheets, positive electrode sheets and separators were respectively assembled into sodium ion batteries. The preparation method of the positive electrode sheet is as follows: the active material, PVDF and SP are mixed in a mass ratio of 90:5:5 to prepare a positive electrode slurry, and the positive electrode sheet is obtained after coating; the solute of the electrolyte is 1M NaPF, and the solvent is an ethylene carbonate-6 dimethyl carbonate (EC-DMC) system.

[0102] The charge and discharge test was carried out at a voltage of 2.5-4.05 V and a rate of 0.2C, and the first discharge capacity was recorded; the results are shown in Table 1.

[0103] Table 1 First charge and discharge results of negative electrode materials

[0104]

[0105] The above description is only a preferred specific implementation manner of the present invention, but the protection scope of the present invention is not limited thereto. Any technician familiar with the technical field can make equivalent replacements or changes according to the technical scheme and inventive concept of the present invention within the technical scope disclosed by the present invention, which should be covered by the protection scope of the present invention.

Claims

1. A hard carbon negative electrode material, characterized in that: By adjusting the reaction conditions under a set atmosphere, a three-dimensional carbon matrix with a large number of closed-pore structures is formed; the reaction conditions include carbonization temperature and system pressure; the closed-pore volume of the hard carbon negative electrode material is 0.01cm 3 / g<V cp <0.5cm 3 / g; and satisfy V cp =1 / ρ 真 -1 / (2.26×0.3354 / d002 act ); Among them, ρ 真 is the true density of the hard carbon negative electrode material, in g / cm 3 ;d002 act is the interlayer spacing of the X-ray diffraction 002 lattice, in nm; the true density of the hard carbon negative electrode material is 0.5 g / cm 3 <ρ 真 <2.26g / cm 3 .

2. A hard carbon negative electrode material according to claim 1, characterized in that: The true density ρ of the hard carbon negative electrode material 真 =(0.75T / 620)ρ0-0.25(T / 1000)(P / P0) 0.3 ; Wherein, ρ0 is the density of the initial carbon source; T is the carbonization temperature, and T=200-1500°C; P0 is the atmospheric pressure; P is the system pressure, and P=0.1-1.5MPa.

3. A hard carbon negative electrode material according to claim 1, characterized in that: The set atmosphere is one or a combination of nitrogen, argon, helium, carbon dioxide, chlorine, water vapor, and air.

4. The hard carbon negative electrode material according to claim 1, characterized in that: The hard carbon forming the three-dimensional carbon matrix is ​​formed by high-temperature carbonization of an organic thermosetting precursor; the regulation of the reaction conditions under the set atmosphere refers to adjusting the pore structure and the ratio of open and closed pores of the carbonized product by controlling the temperature rise curve, system pressure and system atmosphere during the carbonization process of the organic thermosetting precursor, thereby controlling the true density of the carbonized product.

5. A hard carbon negative electrode material according to claim 4, characterized in that: The hard carbon includes one or more of asphalt, cellulose, sucrose, glucose, phenolic resin, starch, coal, and heavy oil.

6. The hard carbon negative electrode material according to claim 1, characterized in that: In the X-ray diffraction pattern of the hard carbon negative electrode material, there are characteristic peaks unique to hard carbon in the ranges of 20° to 30° and 35° to 50°.

7. The method for preparing a hard carbon negative electrode material according to any one of claims 1 to 6, characterized in that: The steps include: (1) crushing the carbon matrix raw material to a set particle size; (2) After mixing the carbon matrix raw materials, pre-carbonization is performed at a temperature of 200 to 1000° C. in an atmosphere of inert gas / air / carbon dioxide / water vapor; (3) breaking up the pre-carbonized carbon matrix into pieces of 3 to 20 microns to obtain a negative electrode material precursor; (4) The negative electrode material precursor is carbonized at a temperature of 1000-1800°C. During this process, the pore structure of different negative electrode material precursors is precisely controlled by controlling the temperature, pressure and reaction atmosphere. Finally, the closed pore volume is 0.01cm 3 / g<V cp <0.5cm 3 / g of hard carbon negative electrode material.

8. The method for preparing a hard carbon negative electrode material according to claim 7, characterized in that: The carbon matrix raw material includes one or more of biomass and its modified products, coal and its modified products, asphalt and its modified products, resin and its modified products.

9. A sodium ion battery, characterized in that: The negative electrode of the sodium ion battery comprises the hard carbon negative electrode material according to any one of claims 1 to 6.

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