Preparation method for and use of hard carbon negative electrode material

Abstract

A preparation method for a hard carbon negative electrode material. The method comprises the following steps: uniformly mixing a polymer material and a cross-linking agent, so as to obtain a mixture; placing the mixture in an oxygen-free environment and performing primary calcination, so as to obtain a precursor; placing the precursor in an oxygen-containing environment and performing secondary calcination, so as to obtain a modified precursor; and placing the modified precursor in an oxygen-free environment and performing third calcination, so as to obtain a hard carbon negative electrode material. By mixing the polymer material and the cross-linking agent, carrying out primary calcination to achieve cross-linking among polymers, carrying out secondary calcination to introduce an oxygen-containing functional group, and carrying out tertiary calcination and a pore closing treatment, the prepared hard carbon has a disordered interlayer structure, which is beneficial for the intercalation / deintercalation of sodium ions, thereby exhibiting a relatively high reversible capacity and first charge-discharge efficiency. The prepared hard carbon negative electrode material is further applied to a negative electrode of a sodium-ion battery.

Description

Preparation method and application of hard carbon negative electrode material Technical Field

[0001] The present disclosure relates to the field of sodium ion battery negative electrode materials, and in particular to a preparation method and application of a hard carbon negative electrode material. Background Art

[0002] As secondary batteries, sodium-ion batteries (SIBs) share an electrochemical mechanism similar to lithium-ion batteries. However, compared to lithium-ion batteries, SIBs offer advantages: abundant raw material reserves, low cost, and an operating voltage as low as 0V. Their energy density exceeds 100Wh / kg, comparable to lithium iron phosphate batteries.

[0003] Compared to graphite, a common negative electrode material for lithium-ion batteries, hard carbon offers superior specific capacity and cycling stability as a negative electrode material for sodium-ion batteries. This is because the radius of a sodium ion (0.103 nm) is much larger than that of a lithium ion (0.071 nm), and the smaller interlayer spacing in graphite hinders the intercalation and deintercalation of sodium ions. However, the long-range disorder of hard carbon's structure, along with its numerous defects and micropores, facilitates the adsorption and storage of sodium ions.

[0004] Polymer materials such as starch and resin are the raw materials for sintering hard carbon negative electrodes, but these materials are sensitive to temperature and are very prone to expansion at high temperatures, which increases the difficulty of processing. In addition, the sintering temperature during the processing is often higher than the decomposition temperature of the material, causing the raw materials to decompose violently. If the sintering temperature is lowered, the cross-linking between the polymer materials will take a lot of time, increasing the processing cost.

[0005] Summary of the Invention

[0006] Based on this, the purpose of the present disclosure is to provide a method for preparing hard carbon negative electrode materials, introduce high molecular polymers as cross-linking agents, promote cross-linking between high molecular materials such as starch and resin, shorten the cross-linking time, reduce processing difficulty, and reduce processing costs.

[0007] Part 1:

[0008] A method for preparing a hard carbon negative electrode material comprises the following steps:

[0009] uniformly mixing the polymer material and the cross-linking agent to obtain a mixture;

[0010] placing the mixture in an oxygen-free environment and calcining it once to obtain a precursor;

[0011] placing the precursor in an oxygen-containing environment for secondary calcination to obtain a modified precursor;

[0012] calcining the modified precursor three times in an oxygen-free environment to obtain a hard carbon negative electrode material;

[0013] The polymer material is at least one of corn starch, wheat starch, sweet potato starch, potato starch, phenolic resin and epoxy resin; and the cross-linking agent is a thermally unstable polymer.

[0014] The present invention discloses a method of mixing a polymer material with a cross-linking agent, calcining once to cross-link the polymer molecules, calcining twice to introduce oxygen-containing functional groups, and calcining three times to close the pores. The hard carbon prepared has a disordered interlayer structure, which is conducive to the insertion / extraction of sodium ions and exhibits a high reversible capacity and initial charge and discharge efficiency.

[0015] During the first calcination, the hydrogen bonds between the segments of the polymer material are broken, and the segments react and cross-link under the promotion of the cross-linking agent, and the segments are reorganized to form new connecting ether bonds. The cross-linking of normal polymer materials themselves is a relatively slow process, and they are prone to violent decomposition at high temperatures. The present disclosure solves the above problems by introducing a thermally unstable cross-linking agent. During calcination, the cross-linking agent decomposes to produce new polymer segments, and the new polymer segments promote the cross-linking of the polymer material and shorten the cross-linking time; at the same time, the polymer segments are cross-linked with the polymer material, which increases the stability of the polymer material and avoids violent decomposition under high temperature conditions. After cross-linking, the cross-linking points between the material chains inhibit the sliding between the polymer chains, which is beneficial to the subsequent skeleton as a hard carbon material, increases the stability of the material skeleton, and can retain more carbon atoms after high-temperature closed-cell treatment, greatly improving the yield of the material.

[0016] During secondary calcination in an oxygen-containing environment, a portion of the oxygen molecules react with the carbon in the polymer material to form oxygen-containing functional groups as active sites. At the same time, another portion of the oxygen molecules react with part of the carbon in the polymer material to generate CO and CO2, forming pores on the surface and inside the precursor. These pores help store sodium ions and thus improve the electrochemical properties of the material.

[0017] During the three calcinations, the temperature continues to rise, and the tension and stress of the modified precursor itself exist, causing the material to continuously shrink, achieving a "self-repairing" effect, "repairing" the pores on the surface, thereby forming a closed-pore structure. During the first cycle of charge and discharge of the negative electrode material, sodium ions react with the negative electrode and the electrolyte, forming a solid electrolyte interface film (SEI film) on the negative electrode surface. The sodium ions involved in the reaction are irreversible, resulting in irreversible capacity. The larger the SEI film area, the more sodium ions are consumed, the higher the irreversible capacity, and the lower the initial charge and discharge efficiency. The closed-pore structure can effectively reduce the specific surface area of ​​the material, thereby reducing the sodium ion consumption of the SEI film, thereby improving the initial charge and discharge efficiency.

[0018] The hard carbon negative electrode material prepared in the present disclosure can be applied to different polymer-based hard carbon systems, has a wide range of applicability and low cost, and therefore has practical significance for the subsequent industrial application of hard carbon negative electrodes.

[0019] As a preferred embodiment, the cross-linking agent is at least one of polyvinyl alcohol, sodium carboxymethyl cellulose, nitrogen methyl pyrrolidone, and polyethylene glycol with a molecular weight of 1000 to 6000. The polymer can effectively undergo a cross-linking reaction with the polymer material, can decompose at 300 to 600°C, and can polymerize with the polymer material.

[0020] As a preferred solution, the mass ratio of the polymer material to the crosslinking agent is 5:1 to 10:1. If the amount of crosslinking agent is too much, the active substance content of the precursor obtained after sintering is low; if the amount of crosslinking agent is too little, the crosslinking effect cannot be achieved, and the polymer material is prone to violent decomposition.

[0021] As a preferred solution, the primary calcination is placed in an inert gas environment with an oxygen concentration lower than 1000 ppm, wherein the inert gas is at least one of nitrogen, argon and helium. Calcination in an oxygen-free environment can avoid oxidation of the crosslinking agent and the polymer material.

[0022] As a preferred solution, the temperature of the primary calcination is 300-600°C and the time is 2-6 hours. If the temperature is too low, the polymers cannot be cross-linked or the cross-linking time is very long; if the temperature is too high, the polymers will still decompose violently; if the time is too short, the degree of cross-linking between the polymers is low; if the time is too long, there is no obvious improvement in the degree of cross-linking, and energy is wasted.

[0023] As a preferred solution, the precursor needs to be crushed to a median particle size of 4 to 10 μm. Reducing the precursor particle size can effectively improve the oxidation modification effect and reduce the secondary calcination time.

[0024] As a preferred solution, the secondary calcination is placed in an oxygen-containing environment with an oxygen concentration of 2 to 8%. If the oxygen concentration is too low, fewer oxygen-containing functional groups will be introduced, and the effect of oxidative modification will not be achieved. If the oxygen concentration is too high, more pores will be formed, consuming more carbon in the polymer material.

[0025] As a preferred solution, the temperature of the secondary calcination is 400-600°C and the time is 1-5 hours. If the calcination temperature is too low and the time is too short, fewer oxygen-containing functional groups will be introduced and the effect of oxidative modification will not be achieved; if the calcination temperature is too high and the time is too long, the oxidative modification effect will not be significantly improved and energy will be wasted.

[0026] As a preferred solution, the three calcinations are carried out in an argon environment. Since the temperature of the three calcinations is relatively high, an argon environment is required to ensure that there is no reaction with the ambient gas.

[0027] As a preferred solution, the temperature of the three calcinations is 1500-1800°C, the time is 3-30h, and the heating rate is 0.5-3°C / min. If the temperature is too low, the sintering time needs to be extended to 40-50h to achieve the purpose of closed pores. If the temperature is too high, the closed pore effect can be achieved, but the internal result tends to be regularized, the interlayer spacing of the material is reduced, resulting in poor kinetic properties of the material, which is not conducive to the shuttle of sodium ions in the negative electrode material.

[0028] As a preferred solution, the specific surface area of ​​the hard carbon negative electrode material is less than 5g / m 2 By controlling the specific surface area of ​​the hard carbon material, the area of ​​the SEI film can be reduced, thereby improving the initial charge and discharge efficiency.

[0029] Part II:

[0030] A hard carbon negative electrode material is prepared by the preparation method of a hard carbon negative electrode material described in the first part.

[0031] Part III:

[0032] A hard carbon negative electrode comprises the hard carbon negative electrode material described in the second part.

[0033] Part 4:

[0034] A sodium ion battery comprises the hard carbon negative electrode described in the third part.

[0035] Part 5:

[0036] An application of a hard carbon negative electrode material comprises the following steps:

[0037] The hard carbon negative electrode material described in the second part, sodium carboxymethyl cellulose, super P conductive agent, and polymer binder were dissolved in deionized water at a mass ratio of 95:2:1:2 to obtain a slurry;

[0038] Applying the slurry on copper foil to obtain a pole piece;

[0039] The electrode piece is placed in a drying oven and dried at 80-100° C. for 4-10 hours to obtain a hard carbon negative electrode.

[0040] Furthermore, the hard carbon negative electrode is placed in a glove box with an argon atmosphere to assemble a button battery, wherein the counter electrode is a sodium metal foil; the electrolyte is a mixed solution of NaClO4, ethylene carbonate and propylene carbonate, and the volume ratio of ethylene carbonate to propylene carbonate is 1:1.

[0041] The hard carbon negative electrode material is prepared into a negative electrode and assembled into a whole battery to facilitate testing the electrochemical performance of the hard carbon negative electrode material disclosed in the present disclosure. BRIEF DESCRIPTION OF THE DRAWINGS

[0042] FIG1 is a SEM image of the mixture after primary calcination in Example 1;

[0043] FIG2 is a SEM image of the hard carbon negative electrode material described in Example 1;

[0044] FIG3 is an XRD pattern of the hard carbon negative electrode material described in Example 1;

[0045] FIG4 is a charge and discharge curve diagram of the hard carbon negative electrode material described in Example 1 applied to a sodium ion battery;

[0046] FIG5 is a material state diagram of the mixed material after primary calcination in Example 2;

[0047] FIG6 is a material state diagram of the corn starch after one calcination in Comparative Example 2. DETAILED DESCRIPTION

[0048] The present invention discloses a method for preparing a hard carbon negative electrode material, comprising the following steps:

[0049] The polymer material and the cross-linking agent are uniformly mixed in a mass ratio of 5:1 to 10:1 to obtain a mixture;

[0050] The mixed material is placed in an oxygen-free environment for a primary calcination, wherein the oxygen concentration is less than 1000 ppm, the calcination temperature is 300-600° C., the calcination time is 2-6 hours, and the calcined material particle size is crushed to a median particle size of 4-10 μm to obtain a precursor;

[0051] The precursor is placed in an oxygen-containing environment for secondary calcination, wherein the oxygen concentration is 2-8%, the calcination temperature is 400-600° C., and the calcination time is 1-5 hours to obtain a modified precursor;

[0052] The modified precursor is placed in an oxygen-free environment and calcined three times, wherein the calcination temperature is 1500-1800° C., the calcination time is 3-30 hours, and the heating rate is 0.5-3° C. / min to obtain a hard carbon negative electrode material.

[0053] The application of a hard carbon negative electrode material disclosed in the present disclosure comprises the following steps:

[0054] The hard carbon negative electrode material, sodium carboxymethyl cellulose, super P conductive agent, and polymer binder are dissolved in deionized water at a mass ratio of 95:2:1:2 to prepare a slurry;

[0055] Applying the slurry on copper foil to obtain a pole piece;

[0056] The electrode piece is placed in a drying oven and dried at 80-100° C. for 4-10 hours to obtain a hard carbon negative electrode;

[0057] The hard carbon negative electrode is placed in an oxygen-free environment to assemble a button battery, wherein the counter electrode is a sodium metal foil; the electrolyte is a mixed solution of NaClO4, ethylene carbonate and propylene carbonate, and the volume ratio of the ethylene carbonate to the propylene carbonate is 1:1.

[0058] Example 1

[0059] The present invention discloses a method for preparing a hard carbon negative electrode material, comprising the following steps:

[0060] 500 g of phenolic resin and 100 g of polyethylene glycol with a molecular weight of 4000 were mixed uniformly by a high-speed mixer to obtain a mixture;

[0061] The mixture was calcined once in a nitrogen atmosphere with an oxygen concentration below 200 ppm at a temperature of 350°C for 6 hours. The calcined material was cooled to below 50°C. FIG1 is a SEM image of the calcined material. The calcined material was crushed to a median particle size of 5 μm using a jaw crusher and a jet mill to obtain a precursor.

[0062] The precursor is placed in a nitrogen atmosphere for secondary calcination, wherein the oxygen concentration is controlled at 8%, the temperature is increased to 600° C. at 5° C. / min, and calcined for 4 hours to introduce oxygen-containing functional groups to obtain a modified precursor;

[0063] The modified precursor was placed in an argon atmosphere and calcined three times, rising to 1600°C at 2°C / min, calcined for 8 hours, and cooled to room temperature after calcination to complete the high-temperature closed-pore treatment to obtain a hard carbon negative electrode material with an internal porous structure. Figure 2 is the SEM image of the hard carbon negative electrode material, and Figure 3 is the XRD image of the hard carbon negative electrode material.

[0064] The application of a hard carbon negative electrode material disclosed in the present disclosure comprises the following steps:

[0065] The hard carbon negative electrode material, sodium carboxymethyl cellulose, super P conductive agent, and polymer binder are dissolved in deionized water at a mass ratio of 95:2:1:2 to prepare a slurry;

[0066] Applying the slurry on copper foil to obtain a pole piece;

[0067] The electrode piece is placed in a drying oven and dried at 80° C. for 4 to 10 hours to obtain a hard carbon negative electrode;

[0068] The hard carbon anode was placed in an argon-filled glove box to assemble a button-type battery. The counter electrode was a sodium metal foil; the electrolyte was a mixed solution of NaClO₄, ethylene carbonate, and propylene carbonate, with a volume ratio of 1:1. Figure 4 shows the charge-discharge curves of the hard carbon anode material used in a sodium-ion battery.

[0069] Example 2

[0070] The present invention discloses a method for preparing a hard carbon negative electrode material, comprising the following steps:

[0071] 500 g of corn starch and 250 g of polyethylene glycol with a molecular weight of 6000 were mixed uniformly in a high-speed mixer to obtain a mixture;

[0072] The mixture is placed in a nitrogen atmosphere for primary calcination, wherein the oxygen concentration is less than 200 ppm, the calcination temperature is 300° C., and the calcination time is 6 hours. The calcined material is cooled to below 50° C. and crushed to a median particle size of 5 μm using a jaw crusher and a jet mill to obtain a precursor;

[0073] The precursor is placed in a nitrogen atmosphere for secondary calcination, wherein the oxygen concentration is controlled at 2%, the temperature is increased to 400° C. at 5° C. / min, and calcined for 5 hours to introduce oxygen-containing functional groups to obtain a modified precursor;

[0074] The modified precursor was placed in an argon atmosphere and calcined three times, rising to 1500° C. at 2° C. / min, calcined for 3 hours, and cooled to room temperature after calcination to complete the high-temperature closed-pore treatment to obtain a hard carbon negative electrode material with an internal porous structure.

[0075] The application of a hard carbon negative electrode material disclosed in the present disclosure comprises the following steps:

[0076] The hard carbon negative electrode material, sodium carboxymethyl cellulose, super P conductive agent, and polymer binder are dissolved in deionized water at a mass ratio of 95:2:1:2 to prepare a slurry;

[0077] Applying the slurry on copper foil to obtain a pole piece;

[0078] The electrode piece is placed in a drying oven and dried at 80° C. for 4 to 10 hours to obtain a hard carbon negative electrode;

[0079] The hard carbon negative electrode is placed in a glove box with an argon atmosphere to assemble a button battery, wherein the counter electrode is a sodium metal foil; the electrolyte is a mixed solution of NaClO4, ethylene carbonate and propylene carbonate, and the volume ratio of ethylene carbonate to propylene carbonate is 1:1.

[0080] Example 3

[0081] The present invention discloses a method for preparing a hard carbon negative electrode material, comprising the following steps:

[0082] 500 g of potato starch and 50 g of polyvinyl alcohol were mixed uniformly by a high-speed mixer to obtain a mixture;

[0083] The mixture is placed in a nitrogen atmosphere for primary calcination, wherein the oxygen concentration is less than 200 ppm, the calcination temperature is 300° C., and the calcination time is 6 hours. The calcined material is cooled to below 50° C. and crushed to a median particle size of 5 μm using a jaw crusher and a jet mill to obtain a precursor;

[0084] The precursor is placed in a nitrogen atmosphere for secondary calcination, wherein the oxygen concentration is controlled at 2%, the temperature is increased to 400° C. at 5° C. / min, and calcined for 4 hours to introduce oxygen-containing functional groups to obtain a modified precursor;

[0085] The modified precursor was placed in an argon atmosphere and calcined three times, rising to 1600° C. at 2° C. / min, calcined for 8 hours, and cooled to room temperature after calcination to complete the high-temperature closed-pore treatment to obtain a hard carbon negative electrode material with an internal porous structure.

[0086] The application of a hard carbon negative electrode material disclosed in the present disclosure comprises the following steps:

[0087] The hard carbon negative electrode material, sodium carboxymethyl cellulose, super P conductive agent, and polymer binder are dissolved in deionized water at a mass ratio of 95:2:1:2 to prepare a slurry;

[0088] Applying the slurry on copper foil to obtain a pole piece;

[0089] The electrode piece is placed in a drying oven and dried at 80° C. for 4 to 10 hours to obtain a hard carbon negative electrode;

[0090] The hard carbon negative electrode is placed in a glove box with an argon atmosphere to assemble a button battery, wherein the counter electrode is a sodium metal foil; the electrolyte is a mixed solution of NaClO4, ethylene carbonate and propylene carbonate, and the volume ratio of ethylene carbonate to propylene carbonate is 1:1.

[0091] Comparative Example 1

[0092] 500 g of phenolic resin was calcined once in a nitrogen atmosphere, wherein the oxygen concentration was less than 200 ppm, the calcination temperature was 300° C., and the calcination time was 6 h. The calcined material was cooled to below 50° C. and crushed to a median particle size of 5 μm using a jaw crusher and a jet mill to obtain a precursor.

[0093] The precursor is placed in a nitrogen atmosphere for secondary calcination, wherein the oxygen concentration is controlled at 8%, the temperature is increased to 600° C. at 5° C. / min, and calcined for 4 hours to introduce oxygen-containing functional groups to obtain a modified precursor;

[0094] The modified precursor was placed in an argon atmosphere and calcined three times, rising to 1600° C. at 2° C. / min, calcined for 8 hours, and cooled to room temperature after calcination to complete the high-temperature closed-pore treatment to obtain a hard carbon negative electrode material with an internal porous structure.

[0095] The hard carbon negative electrode material, sodium carboxymethyl cellulose, super P conductive agent, and polymer binder are dissolved in deionized water at a mass ratio of 95:2:1:2 to prepare a slurry;

[0096] Applying the slurry on copper foil to obtain a pole piece;

[0097] The electrode piece is placed in a drying oven and dried at 80° C. for 4 to 10 hours to obtain a hard carbon negative electrode;

[0098] The hard carbon negative electrode is placed in a glove box with an argon atmosphere to assemble a button battery, wherein the counter electrode is a sodium metal foil; the electrolyte is a mixed solution of NaClO4, ethylene carbonate and propylene carbonate, and the volume ratio of ethylene carbonate to propylene carbonate is 1:1.

[0099] Comparative Example 2

[0100] 500 g of corn starch was calcined once in a nitrogen atmosphere, wherein the oxygen concentration was less than 200 ppm, the calcination temperature was 300° C., and the calcination time was 6 h. The calcined material was cooled to below 50° C. and crushed to a median particle size of 5 μm using a jaw crusher and a jet mill to obtain a precursor;

[0101] The precursor is placed in a nitrogen atmosphere for secondary calcination, wherein the oxygen concentration is controlled at 8%, the temperature is increased to 600° C. at 5° C. / min, and calcined for 5 hours to introduce oxygen-containing functional groups to obtain a modified precursor;

[0102] The modified precursor was placed in an argon atmosphere and calcined three times, rising to 1500° C. at 2° C. / min, calcined for 2 hours, and cooled to room temperature after calcination to complete the high-temperature closed-pore treatment to obtain a hard carbon negative electrode material with an internal porous structure.

[0103] The hard carbon negative electrode material, sodium carboxymethyl cellulose, super P conductive agent, and polymer binder are dissolved in deionized water at a mass ratio of 95:2:1:2 to prepare a slurry;

[0104] Applying the slurry on copper foil to obtain a pole piece;

[0105] The electrode piece is placed in a drying oven and dried at 80° C. for 4 to 10 hours to obtain a hard carbon negative electrode;

[0106] The hard carbon negative electrode is placed in a glove box with an argon atmosphere to assemble a button battery, wherein the counter electrode is a sodium metal foil; the electrolyte is a mixed solution of NaClO4, ethylene carbonate and propylene carbonate, and the volume ratio of ethylene carbonate to propylene carbonate is 1:1.

[0107] Comparative Example 3

[0108] 500 g of corn starch was calcined once in a nitrogen atmosphere, wherein the oxygen concentration was less than 200 ppm, the calcination temperature was 300° C., and the calcination time was 6 h. The calcined material was cooled to below 50° C. and crushed to a median particle size of 5 μm using a jaw crusher and a jet mill to obtain a precursor;

[0109] The precursor is placed in an argon atmosphere for secondary calcination, raised to 1500° C. at 2° C. / min, calcined for 2 hours, and cooled to room temperature after calcination to complete the high-temperature closed-pore treatment to obtain a hard carbon negative electrode material with an internal porous structure.

[0110] The hard carbon negative electrode material, sodium carboxymethyl cellulose, super P conductive agent, and polymer binder are dissolved in deionized water at a mass ratio of 95:2:1:2 to prepare a slurry;

[0111] Applying the slurry on copper foil to obtain a pole piece;

[0112] The electrode piece is placed in a drying oven and dried at 80° C. for 4 to 10 hours to obtain a hard carbon negative electrode;

[0113] The hard carbon negative electrode is placed in a glove box with an argon atmosphere to assemble a button battery, wherein the counter electrode is a sodium metal foil; the electrolyte is a mixed solution of NaClO4, ethylene carbonate and propylene carbonate, and the volume ratio of ethylene carbonate to propylene carbonate is 1:1.

[0114] Hard carbon product quality

[0115] From the comparison of FIG5 and FIG6, it can be seen that the corn starch in Example 2 was added with a cross-linking agent during the first calcination, which avoided the rapid decomposition of the polymer material.

[0116] As can be seen in Table 1, the precursors of the examples, when cross-linked by a cross-linking agent, exhibit a smaller specific surface area after single calcination than the comparative examples, and exhibit no expansion. This is because the polymer material of the comparative examples undergoes drastic decomposition, disrupting and rearranging the chain segments within the structure, leading to the collapse of the segment structure. The H₂O produced by the decomposition rapidly evaporates, forming a large number of pores and a new structure, ultimately manifesting as expansion of the precursor and a larger specific surface area.

[0117] The hard carbon negative electrode material of the embodiment has a smaller specific surface area than the comparative example. This is because after high-temperature closed-pore treatment, the material undergoes structural rearrangement and can repair surface defects by itself through high temperature and its own tension. However, this repair ability is limited. For materials with rich pores, it is difficult to achieve the closed-pore effect. Therefore, the specific surface area of ​​the hard carbon negative electrode materials in the comparative example is relatively large.

[0118] Through the formula: hard carbon negative electrode material yield = hard carbon negative electrode material mass / polymer material mass × 100%, it is obtained that the hard carbon negative electrode material yield of the embodiment is larger than that of the comparison example. This is because the cross-linking points between the material chains after cross-linking inhibit the sliding between the polymer chains, which is beneficial to the subsequent use as the skeleton of the hard carbon material, increases the stability of the material skeleton, and can retain more carbon atoms after high-temperature closed-pore treatment, greatly improving the yield of the material.

[0119] Table 1 Some characteristics of precursors and hard carbon anode materials

[0120] Electrochemical performance

[0121] The electrochemical performance of the hard carbon negative electrode material for sodium ion batteries was obtained by electrochemical workstation testing. As can be seen from Table 2, the first charge specific capacity of the hard carbon negative electrode material prepared in the embodiment is not much different from that of the comparative example, but the first charge and discharge efficiency of the hard carbon negative electrode material prepared in the embodiment is better than that of the comparative example. This is because the precursor obtained in the comparative example expanded during the first calcination, and the specific surface area was too large. Although the material underwent structural rearrangement during the subsequent high-temperature closed-pore treatment process and repaired the surface defects by itself through high temperature and its own tension, this repair ability is limited. For materials with rich pores, it is difficult to achieve the closed-pore effect, resulting in a higher specific surface area and more sodium ions consumed to form a solid electrolyte interface film (SEI), which leads to a higher first charge specific capacity of the material, but poor first charge and discharge efficiency performance.

[0122] Table 2 Comparison of electrochemical performance of button batteries

Claims

1. A method for preparing a hard carbon negative electrode material, The following steps are involved: Mixing the polymer material and the cross-linking agent uniformly to obtain a mixture; The mixed material is placed in an oxygen-free environment and calcined once to obtain a precursor; placing the precursor in an oxygen-containing environment for secondary calcination to obtain a modified precursor; The modified precursor is placed in an oxygen-free environment and calcined three times to obtain a hard carbon negative electrode material; The polymer material is at least one of corn starch, wheat starch, sweet potato starch, potato starch, phenolic resin and epoxy resin; and the cross-linking agent is a thermally unstable polymer.

2. The method for preparing a hard carbon negative electrode material according to claim 1, characterized in that: The cross-linking agent is at least one of polyvinyl alcohol, sodium carboxymethyl cellulose, nitrogen methyl pyrrolidone, and polyethylene glycol with a molecular weight of 1000-6000.

3. The method for preparing a hard carbon negative electrode material according to claim 2, characterized in that: The mass ratio of the polymer material to the cross-linking agent is 5:1 to 10:

1.

4. The method for preparing a hard carbon negative electrode material according to claim 1, characterized in that: The primary calcination is placed in an inert gas environment with an oxygen concentration lower than 1000 ppm, and the inert gas is at least one of nitrogen, argon and helium.

5. The method for preparing a hard carbon negative electrode material according to claim 1, characterized in that: The primary calcination is carried out at a temperature of 300 to 600° C. and for a time of 2 to 6 hours.

6. The method for preparing a hard carbon negative electrode material according to claim 1, characterized in that: The precursor needs to be crushed to a median particle size of 4 to 10 μm.

7. The method for preparing a hard carbon negative electrode material according to claim 1, characterized in that: The secondary calcination is placed in an oxygen-containing environment with an oxygen concentration of 2-8%.

8. The method for preparing a hard carbon negative electrode material according to claim 7, characterized in that: The secondary calcination is carried out at a temperature of 400 to 600° C. and for a time of 1 to 5 hours.

9. The method for preparing a hard carbon negative electrode material according to claim 1, characterized in that: The three calcinations were carried out in an argon environment.

10. The method for preparing a hard carbon negative electrode material according to claim 1, characterized in that: The temperature of the three calcinations is 1500-1800° C., the time is 3-30 hours, and the heating rate is 0.5-3° C. / min.

11. A method for preparing a hard carbon negative electrode material according to any one of claims 1 to 10, characterized in that: The specific surface area of ​​the hard carbon negative electrode material is less than 5 g / m 2 .

12. A hard carbon negative electrode material, characterized in that: Prepared by the method for preparing a hard carbon negative electrode material according to any one of claims 1 to 11.

13. A hard carbon negative electrode, characterized in that: Comprising the hard carbon negative electrode material as described in claim 12.

14. A sodium ion battery, characterized in that: Comprising the hard carbon negative electrode as claimed in claim 13.

15. An application of a hard carbon negative electrode material, comprising the following steps: The hard carbon negative electrode material according to claim 12, sodium carboxymethyl cellulose, super P conductive agent, and polymer binder are dissolved in deionized water at a mass ratio of 95:2:1:2 to obtain a slurry; The slurry is coated on a copper foil to obtain a pole piece; The electrode piece is placed in a drying oven and dried at 80-100° C. for 4-10 hours to obtain a hard carbon negative electrode.

16. The use of a hard carbon negative electrode material according to claim 15, further comprising the following steps: The hard carbon negative electrode is placed in a glove box with an argon atmosphere to assemble a button battery, wherein the counter electrode is a sodium metal foil; the electrolyte is a mixed solution of NaClO4, ethylene carbonate and propylene carbonate, and the volume ratio of ethylene carbonate to propylene carbonate is 1:1.

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