A starch cellulose composite hard carbon material and its preparation method and application

Abstract

The present invention belongs to the technical field of negative electrode materials for sodium ion batteries, and specifically relates to a starch cellulose composite hard carbon material, a preparation method and application thereof. In this application, pre-oxidation and pre-carbonization are carried out step by step, and the starch cellulose composite hard carbon material is prepared in sequence by low-temperature pre-oxidation, ball milling, low-temperature pre-carbonization and high-temperature pyrolysis. The obtained starch cellulose composite hard carbon material is used to prepare the negative electrode of the sodium ion battery, and then further construct the sodium ion battery. The electrochemical performance test results show that the obtained sodium ion battery has excellent first-cycle coulomb efficiency, cycle performance and rate performance; and the preparation method of the starch cellulose composite hard carbon material is simple to operate, and the pre-oxidation process can be carried out in an air atmosphere. No additional oxidation treatment is required, which is environmentally friendly and can prevent the starch cellulose composite material from melting and foaming during high-temperature pyrolysis, thereby improving the cycle stability of the material and being suitable for industrial production.

Description

Technical Field

[0001] The present invention belongs to the technical field of sodium ion battery negative electrode materials. More specifically, it relates to a starch-cellulose composite hard carbon material and its preparation method and application. Background Art

[0002] Lithium-ion batteries are widely used in mobile smartphones, electric vehicles, etc. due to their high capacity and excellent energy density. However, the reserves of lithium resources on the earth are limited and unevenly distributed, which leads to increased costs and thus limits the further application of lithium-ion batteries in the field of energy storage. Sodium has similar chemical properties to lithium and is one of the most abundant elements on earth. Therefore, sodium-ion batteries have broad research and application prospects in the field of large-scale energy storage. However, graphite, as the most mature negative electrode material in sodium-ion batteries, not only allows a small amount of sodium ions to be embedded in the carbon layer, but the sodium ion intercalation complex in graphite is thermodynamically unstable, which limits the application of graphite in sodium-ion batteries. Therefore, finding a negative electrode material with excellent performance and low price has become an important research direction for sodium-ion batteries.

[0003] Among various negative electrode materials, hard carbon has attracted widespread attention due to its relatively high capacity and excellent cycle stability. In addition, hard carbon has a large interlayer spacing and a disordered structure, which can provide rich defects and pore structures for sodium storage. Hard carbon can be obtained by high-temperature pyrolysis of various precursors. Among them, biomass precursors have attracted the interest of researchers due to their wide availability, low price and environmental friendliness. Starch, as the most common biomass resource on earth, has the advantages of low impurities and high carbon content, making it very suitable as a precursor for hard carbon negative electrode materials. However, starch has poor thermal stability and will melt and foam during high-temperature pyrolysis. To solve this problem, low-temperature pre-oxidation and designed cross-linking reactions have been proposed. Low-temperature pre-oxidation can remove adsorbed and bound water in the precursor. At the same time, the use of pre-oxidation methods and designed cross-linking reactions can maintain the stability of the precursor structure and introduce more oxygen-containing functional groups, providing more sodium storage active sites and improving the electrochemical performance of the material. For example, Chinese patent application CN117163941A discloses the use of biomass to simultaneously pre-oxidize and pre-carbonize at 300-600°C in a low-oxygen atmosphere, and then composite it with starch for high-temperature roasting to obtain a biomass-starch composite-based hard carbon material. The above method can suppress the thermal expansion problem of starch, improve the carbonization structure, pore structure and dual-carbon composite interface, and then synergistically improve the adaptation effect of sodium ion embedding and de-embedding, which can effectively improve the sodium capacitance, rate, cycle stability, fast charge stability and other electrochemical properties of the obtained biomass-starch composite-based hard carbon material. However, the pre-oxidation process of this patent requires the control of a specific oxygen partial pressure and requires a higher pre-oxidation temperature, which increases the difficulty in the preparation process and increases energy consumption. For example, Chinese patent application CN114436237A discloses that a hard carbon precursor is simultaneously pre-oxidized and pre-carbonized at 200-400°C, and then microwave-treated in an oxygen-containing atmosphere and then calcined at high temperature to obtain a hard carbon material suitable for sodium storage. The sodium ion battery prepared based on the hard carbon material has a high initial reversible capacity and good cycle stability. However, this patent requires microwave treatment after pre-oxidation to further introduce oxygen-containing functional groups, which has high requirements for preparation equipment and further increases energy consumption. Summary of the Invention

[0004] The technical problem to be solved by the present invention is to overcome the defects and shortcomings of the existing methods for preparing biomass-based hard carbon materials, such as the need for the pre-oxidation process to be carried out in a specific oxygen atmosphere or the need for microwave treatment after pre-oxidation to enhance the introduction of oxygen-containing functional groups, the relatively complex preparation method, and the high requirements on equipment. A method for preparing starch-cellulose composite hard carbon materials is provided.

[0005] Another object of the present invention is to provide a starch-cellulose composite hard carbon material prepared by the preparation method.

[0006] Another object of the present invention is to provide an application of the starch-cellulose composite hard carbon material in the preparation of a negative electrode material for a sodium ion battery.

[0007] Another object of the present invention is to provide a sodium ion battery negative electrode, wherein the negative electrode active material is the aforementioned starch cellulose composite hard carbon material.

[0008] Another object of the present invention is to provide a sodium ion battery, wherein the negative electrode is the negative electrode of the aforementioned sodium ion battery.

[0009] The above-mentioned purpose of the present invention is achieved through the following technical solutions:

[0010] The present invention provides a method for preparing a starch-cellulose composite hard carbon material, comprising the following steps:

[0011] S1. Take unmodified starch and cellulose, pre-oxidize at least one of them at 200-250 ° C, mix the two, and fully ball-mill to obtain a starch-cellulose composite;

[0012] S2. The starch-cellulose composite obtained in step S1 is heated to 500-600° C. under an inert atmosphere for pre-carbonization to obtain a starch-cellulose composite hard carbon precursor;

[0013] S3. The starch-cellulose composite hard carbon precursor obtained in step S2 is heated to 1300-1500° C. and fully calcined to obtain a starch-cellulose composite hard carbon material.

[0014] To address the problems in existing biomass-based hard carbon material preparation methods, where the pre-oxidation process requires a specific oxygen atmosphere or microwave treatment after pre-oxidation to enhance the introduction of oxygen-containing functional groups, resulting in complex preparation methods and high equipment requirements, the present invention separates pre-oxidation and pre-carbonization into separate steps, sequentially preparing starch-cellulose composite hard carbon materials through low-temperature pre-oxidation, ball milling, low-temperature pre-carbonization, and high-temperature pyrolysis. The low-temperature pre-oxidation improves the thermal stability of the starch and maintains the stability of the precursor; the ball milling ensures the mixing of the raw materials and destroys the starch's original spherical morphology; the low-temperature pre-carbonization process redefines the growth and orientation of the carbon layer, initially forming a carbon layer structure; and the high-temperature pyrolysis process gradually orders the carbon layer structure to form a layered morphology with more pseudo-graphite regions within the hard carbon and a larger interlayer spacing, which is conducive to sodium ion intercalation reactions. The layered morphology also has a smaller specific surface area, which reduces the contact between the hard carbon material and the electrolyte, thereby preventing it from decomposing and forming an excessive solid electrolyte membrane (SEI) due to excessive electrolyte in the electrochemical reaction, thereby improving electrochemical performance.

[0015] Furthermore, the unmodified starch is also often referred to as native starch, which refers to natural starch that has not been treated by any chemical or biological methods. This type of starch is mainly derived from cereals (such as corn, wheat, rice, etc.) or plant tubers (such as potatoes, cassava, etc.), and is produced through simple processes such as soaking, crushing, removing impurities and drying.

[0016] Modified starch obtained by pregelatinization or other modification methods has poor thermal stability and is prone to melting and foaming during the subsequent high-temperature carbonization process.

[0017] Furthermore, in step S1, the unmodified starch includes one or more of corn starch, wheat starch, tapioca starch, potato starch, pea starch, mung bean starch, and rice starch. All of the above starches can be used to prepare hard carbon materials with good performance, which can be further prepared into the negative electrode of a sodium ion battery to construct a battery with good electrochemical performance.

[0018] Furthermore, in step S1, the cellulose comprises one or more of α-cellulose, β-cellulose, and γ-cellulose, preferably α-cellulose. Cellulose with different degrees of polymerization can be used to prepare hard carbon materials with good performance, which can be further prepared into the negative electrode of a sodium ion battery to construct a battery with good electrochemical performance.

[0019] Furthermore, the particle size of the α-cellulose is 15 to 25 μm, preferably 25 μm. Using α-cellulose of different particle sizes can prepare hard carbon materials with good performance, which can be further prepared into the negative electrode of sodium ion batteries to build batteries with good electrochemical performance.

[0020] Furthermore, in step S1, the ball milling speed is 400-500 r / min.

[0021] Furthermore, in step S1, the ball milling time is 6 to 12 hours.

[0022] Furthermore, in step S1, the heating rate of the pre-oxidation is 1-2°C / min.

[0023] Furthermore, in step S1, the pre-oxidation time is 6 to 9 hours.

[0024] Furthermore, in step S1, the mass ratio of the unmodified starch to the cellulose is 1:(0.5-3). If both the unmodified starch and the cellulose are pre-oxidized, the mass ratio here is the mass ratio of the pre-oxidized starch to the pre-oxidized cellulose; if only the unmodified starch is pre-oxidized and the cellulose is not pre-oxidized, the mass ratio here is the mass ratio of the pre-oxidized starch to the non-pre-oxidized cellulose.

[0025] Preferably, in step S1, the mass ratio of the unmodified starch to the cellulose is 1:(0.5-2).

[0026] Furthermore, in step S2, the gas in the inert atmosphere is selected from one or more of nitrogen, argon, and helium.

[0027] Furthermore, in step S2, the heating rate of the pre-carbonization is 1-5°C / min.

[0028] Furthermore, in step S2, the pre-carbonization time is 2 to 3 hours.

[0029] Furthermore, in step S3, the roasting is performed by grinding and crushing and then roasting.

[0030] Furthermore, in step S3, the heating rate of the calcination is 1-2°C / min.

[0031] Furthermore, in step S3, the calcination time is 1 to 2 hours.

[0032] The present invention protects the starch-cellulose composite hard carbon material prepared by the above preparation method.

[0033] The present invention protects the use of the starch-cellulose composite hard carbon material in preparing a negative electrode material for a sodium ion battery.

[0034] The present invention protects a sodium ion battery negative electrode, comprising a current collector and a negative electrode active material loaded on the current collector, wherein the negative electrode active material is the aforementioned starch cellulose composite hard carbon material.

[0035] Preferably, the current collector comprises copper foil or aluminum foil.

[0036] Furthermore, as a preferred manner, the preparation method of the sodium ion battery negative electrode comprises the following steps: dissolving starch cellulose composite hard carbon material, conductive carbon black, binder CMC, and styrene-butadiene rubber emulsion SBR in water to obtain a slurry, coating the obtained slurry on copper foil, and drying it in a blast drying oven at 60-80°C for 12-24 hours, cutting the copper foil loaded with the slurry into discs with a diameter of 10-13 mm, and further vacuum drying it at 60-80°C and a vacuum degree of 0-0.1 MPa for 12-24 hours to obtain the negative electrode.

[0037] Furthermore, the mass ratio of the conductive carbon black, the binder CMC, the styrene-butadiene rubber emulsion SBR, and the starch-cellulose composite hard carbon material is 1: (0.5-6): (0.25-4): (10-48).

[0038] The present invention also protects a sodium ion battery, comprising a positive electrode, a negative electrode, a separator and an electrolyte, wherein the negative electrode comprises the sodium ion battery negative electrode.

[0039] Furthermore, as a preferred embodiment, the negative electrode of the sodium ion battery is the negative electrode of the sodium ion battery described above, the positive electrode is a metallic sodium sheet, the diaphragm is a glass fiber diaphragm, and the electrolyte is NaPF6.

[0040] The sodium ion battery prepared by the present invention can prevent the starch-cellulose composite hard carbon material from melting and foaming during high-temperature pyrolysis, and has excellent first-cycle coulombic efficiency, cycle performance and rate performance.

[0041] Compared with the prior art, the present invention has the following beneficial effects:

[0042] This application conducts pre-oxidation and pre-carbonization in steps, and sequentially prepares starch cellulose composite hard carbon materials through low-temperature pre-oxidation, ball milling, low-temperature pre-carbonization, and high-temperature pyrolysis. The obtained starch cellulose composite hard carbon materials are used to prepare the negative electrode of sodium ion batteries, and then further construct sodium ion batteries. The electrochemical performance test results show that the obtained sodium ion battery has excellent first-cycle coulombic efficiency, cycle performance, and rate performance; moreover, the preparation method of the starch cellulose composite hard carbon material is simple to operate, and the pre-oxidation process can be carried out in an air atmosphere without the need for additional oxidation treatment, which is environmentally friendly and can prevent the starch cellulose composite material from melting and foaming during high-temperature pyrolysis, thereby improving the cyclic stability of the material and being suitable for industrial production. BRIEF DESCRIPTION OF THE DRAWINGS

[0043] Figure 1 This is a field emission scanning electron microscope image of the starch-derived hard carbon material prepared in Comparative Example 1, with a magnification of 1000 times.

[0044] Figure 2 This is a field emission scanning electron microscope image of the starch-cellulose composite hard carbon material prepared in Example 1, with a magnification of 1000 times.

[0045] Figure 3 This is a field emission scanning electron microscope image of the starch-cellulose composite hard carbon material prepared in Example 1, with a magnification of 2500 times.

[0046] Figure 4 These are the XRD patterns of the starch-cellulose composite hard carbon material and the starch-derived hard carbon material prepared in Example 1 and Comparative Example 1, respectively.

[0047] Figure 5 These are the FTIR images of the starch-cellulose composite hard carbon material and the starch-derived hard carbon material prepared in Example 1 and Comparative Example 1, respectively.

[0048] Figure 6 These are the micropore distribution diagrams obtained by BET analysis of the starch-cellulose composite hard carbon material and the starch-derived hard carbon material prepared in Example 1 and Comparative Example 1, respectively.

[0049] Figure 7 The button sodium ion batteries prepared in Example 1 and Comparative Example 1 were heated at 0.1C (i.e. 30mAg -1 )Charge and discharge cycle performance diagram at different current densities.

[0050] Figure 8 This is a charge and discharge rate performance diagram of the button sodium ion battery prepared in Example 1 and Comparative Example 1 at a current density of 0.1 to 5C.

[0051] Figure 9 This is a graph showing the charge and discharge long cycle performance of the button sodium ion battery prepared in Example 1 at a current density of 1C.

[0052] Figure 10 This is a graph showing the long-cycle charge and discharge performance of the button sodium-ion battery prepared in Comparative Example 1 at a current density of 1C. DETAILED DESCRIPTION

[0053] The present invention will be further described below with reference to the accompanying drawings and specific examples, but the examples do not limit the present invention in any way. Unless otherwise specified, the reagents, methods and equipment used in the present invention are conventional reagents, methods and equipment in the art.

[0054] Unless otherwise specified, the reagents and materials used in the following examples were commercially available.

[0055] Example 1 A starch-cellulose composite hard carbon material, its preparation method, and its application in sodium ion batteries

[0056] S1. Weigh 20 g of unmodified, non-pregelatinized industrial-grade corn starch and spread it on several corundum squares. Then place it in a muffle furnace and keep it at 220°C for 24 hours to obtain pre-oxidized starch.

[0057] S2. Weigh 5 g of the pre-oxidized starch obtained in step S1 and then weigh 5 g of 25 μm particle size α-cellulose. Mix the two and transfer them to a polytetrafluoroethylene-lined ball mill and mill at 480 r / min for 6 h to obtain a starch-cellulose composite.

[0058] S3. The starch-cellulose composite obtained in step S2 was heated from 30°C to 600°C at a heating rate of 2°C / min under an inert atmosphere for 3 h, and then cooled naturally to room temperature to obtain a gray-black solid;

[0059] S4. Grind the gray-black solid obtained in step S3 in a mortar, and heat the temperature from 30°C to 1500°C at a heating rate of 2°C / min under a nitrogen atmosphere for 2 h, then cool the temperature from 1500°C to 500°C at a cooling rate of 2°C / min, and naturally cool to room temperature to obtain a starch cellulose composite hard carbon material (CS-HC).

[0060] S5. The CS-HC obtained in step S4 was dissolved with conductive carbon black, binder CMC, and styrene-butadiene rubber emulsion SBR in a mass ratio of 94:2:4:2 in deionized water and mixed evenly to prepare a slurry. The resulting slurry was coated on a copper foil, dried in a forced air drying oven for 24 hours, cut into discs with a diameter of 13 mm, and further vacuum dried for 12 hours to obtain a negative electrode sheet.

[0061] S6. Assemble a button sodium ion battery in an argon-filled glove box, wherein the negative electrode is the negative electrode sheet obtained in step S5, the positive electrode is a metallic sodium sheet, the diaphragm is a glass fiber diaphragm, and the electrolyte is NaPF6.

[0062] Example 2 A starch-cellulose composite hard carbon material, its preparation method, and its application in sodium ion batteries

[0063] The difference from Example 1 is that in step S2, the amount of α-cellulose added is changed to 2.5 g.

[0064] Other steps and conditions are the same as those in Example 1.

[0065] Example 3 A starch-cellulose composite hard carbon material, its preparation method, and its application in sodium ion batteries

[0066] The difference from Example 1 is that in step S2, the amount of α-cellulose added is changed to 10 g.

[0067] Other steps and conditions are the same as those in Example 1.

[0068] Example 4 A starch-cellulose composite hard carbon material, its preparation method, and its application in sodium ion batteries

[0069] The difference from Example 1 is that in step S1, α-cellulose is pre-oxidized according to starch pre-oxidation parameters to obtain pre-oxidized α-cellulose, and in step S2, the obtained pre-oxidized α-cellulose and unmodified, non-pregelatinized industrial-grade corn starch are ball-milled.

[0070] Other steps and conditions are the same as those in Example 1.

[0071] Example 5 A starch-cellulose composite hard carbon material, its preparation method, and its application in sodium ion batteries

[0072] The difference from Example 1 is that in step S1, in addition to pre-oxidizing starch, α-cellulose is also pre-oxidized according to the starch pre-oxidation conditions of step S1, and in step S2, the pre-oxidized starch and pre-oxidized α-cellulose are weighed and mixed and ball-milled.

[0073] Other steps and conditions are the same as those in Example 1.

[0074] Example 6 A starch-cellulose composite hard carbon material, its preparation method, and its application in sodium ion batteries

[0075] The difference from Example 1 is that, in step S1, corn starch is replaced with tapioca starch.

[0076] Other steps and conditions are the same as those in Example 1.

[0077] Example 7 A starch-cellulose composite hard carbon material, its preparation method, and its application in sodium ion batteries

[0078] The difference from Example 1 lies in the changes in temperature and rotation speed, specifically, changing 220°C in step S1 to 200°C, changing the rotation speed of 480r / min in step S2 to 400r / min, changing 600°C in step S3 to 500°C, and changing 1500°C in step S4 to 1300°C.

[0079] Other steps and conditions are the same as those in Example 1.

[0080] Example 8 A starch-cellulose composite hard carbon material, its preparation method, and its application in sodium ion batteries

[0081] The difference from Example 1 lies in the changes in temperature and rotation speed, specifically, changing 220°C in step S1 to 250°C, changing the rotation speed of 480r / min in step S2 to 500r / min, changing 600°C in step S3 to 550°C, and changing 1500°C in step S4 to 1400°C.

[0082] Other steps and conditions are the same as those in Example 1.

[0083] Comparative Example 1 A starch-derived hard carbon material, its preparation method, and its application in sodium ion batteries

[0084] The difference from Example 1 is that in step S2, no cellulose is added and the pre-oxidized starch is directly ball-milled.

[0085] Comparative Example 2 A starch-cellulose composite hard carbon material, its preparation method, and its application in sodium ion batteries

[0086] The difference from Example 1 is that in step S2, the ball milling method is replaced by grinding, and the grinding time is 6 hours.

[0087] Other steps and conditions are the same as those in Example 1.

[0088] Comparative Example 3 A starch-cellulose composite hard carbon material, its preparation method, and its application in sodium ion batteries

[0089] The difference from Example 1 is that step S1, that is, the pre-oxidation step, is not performed, and in step S2, the unmodified, non-pregelatinized industrial-grade corn starch and α-cellulose are directly mixed and ball-milled.

[0090] Other steps and conditions are the same as those in Example 1.

[0091] Comparative Example 4 A starch-cellulose composite hard carbon material, its preparation method, and its application in sodium ion batteries

[0092] The difference from Example 1 is that step S3, that is, no pre-carbonization step, is performed, and the starch-cellulose composite obtained in step S2 is directly subjected to high-temperature pyrolysis.

[0093] Other steps and conditions are the same as those in Example 1.

[0094] Experimental Example 1 Performance Characterization of Hard Carbon Materials

[0095] The performance tests were conducted on the CS-HC and S-HC prepared in Example 1 and Comparative Example 1.

[0096] (1) SEM test

[0097] The CS-HC prepared in Example 1 and the S-HC obtained in Comparative Example 1 were characterized by emission scanning electron microscopy. Figures 1 to 3 As shown in the figure, the starch-derived hard carbon material prepared in Comparative Example 1 exhibits a spherical morphology, a large specific surface area, and a melt foaming phenomenon, that is, there are some holes on the surface of S-HC, and the particles tend to agglomerate ( Figure 1 ), while the CS-HC prepared in the present invention does not show melting and foaming during the high-temperature pyrolysis process, and presents a layered stacking morphology after high-temperature pyrolysis ( Figures 2-3 Compared to other morphologies, such as spherical materials, CS-HC with a layered stacked morphology has a smaller specific surface area, which reduces its contact with the electrolyte solution, thereby preventing excessive electrolyte decomposition and the formation of an excessive solid electrolyte interface (SEI) during the electrochemical reaction. Using CS-HC with this morphology as a negative electrode material for batteries can help improve the battery's first-cycle coulombic efficiency.

[0098] (2) XRD test

[0099] The CS-HC prepared in Example 1 and the S-HC obtained in Comparative Example 1 were characterized by X-ray diffractometer. Figure 4As shown in the figure, both materials exhibit typical diffraction peaks at approximately 24° and 43°, corresponding to the (002) and (100) crystal planes, respectively. For CS-HC, the (002) peak is located at a smaller angle than that of S-HC, indicating that its carbon interlayer spacing is larger. A larger carbon interlayer spacing is conducive to the insertion and extraction of sodium ions.

[0100] (3) FTIR test

[0101] FTIR test was performed on CS-HC obtained in Example 1 and S-HC obtained in Comparative Example 1. The results are as follows: Figure 5 As shown, at a wavelength of about 1074 cm -1 、1722cm -1 and 3400cm -1 Three strong peaks appeared at the α-Hydroxy ...

[0102] (4) Aperture test

[0103] The pore size of CS-HC obtained in Example 1 and S-HC obtained in Comparative Example 1 was measured. The results are as follows: Figure 6 As shown in the figure, S-HC has more micropores distributed in the range of 0.4-0.8 nm, while CS-HC has almost no micropores. Fewer micropores means a lower specific surface area, which can reduce the contact between the material and the electrolyte, preventing excessive decomposition of the electrolyte and the formation of excessive solid electrolyte membrane.

[0104] The results of SEM test, XRD test, FTIR test and pore size test of CS-HC prepared in Experimental Examples 2 to 8 are basically the same as those in Example 1 and are not described again here.

[0105] Experimental Example 2 Performance measurement of sodium ion batteries assembled based on hard carbon materials

[0106] The sodium ion batteries obtained in Examples 1 to 8 and Comparative Examples 1 to 4 were subjected to performance tests.

[0107] (1) Charge and discharge cycle performance test

[0108] Table 1 Performance test results of sodium ion batteries obtained in Examples 1 to 8 and Comparative Examples 1 to 4

[0109]

[0110]

[0111] Note: First cycle coulombic efficiency = charging capacity / discharging capacity × 100%;

[0112] Capacity retention rate = reversible specific capacity of the current cycle / reversible specific capacity of the first cycle × 100%.

[0113] The sodium ion batteries prepared in Examples 1 to 8 and Comparative Examples 1 to 4 were subjected to charge and discharge cycle tests at a voltage range of 0 to 3 V and a current density of 0.1 C. The results are as follows: Figure 7 As shown in Table 1. From the above results, it can be seen that the first cycle reversible specific capacity of the sodium ion batteries prepared based on hard carbon materials in Examples 1 to 8 is greater than 310 mAh g -1 , the first-cycle coulombic efficiency is greater than 90%, and the capacity retention rate is greater than 81%, which are all better than comparative examples 1 to 4, indicating that the present invention utilizes the starch cellulose composite hard carbon material prepared by low-temperature pre-oxidation, ball milling, low-temperature pre-carbonization, and high-temperature pyrolysis to significantly improve energy storage capacity, show better charge transfer efficiency and cycle stability, and extend the service life of the corresponding prepared battery. From comparative examples 1 to 4, it can be seen that the hard carbon material obtained without adding cellulose, using grinding instead of ball milling, and not performing pre-oxidation or pre-carbonization steps, and further using it as the negative electrode material to prepare the sodium ion battery, the performance of the first-cycle reversible specific capacity, first-cycle coulombic efficiency and capacity retention rate are slightly inferior. This shows that only when starch and cellulose are sequentially prepared by low-temperature pre-oxidation, ball milling, low-temperature pre-carbonization, and high-temperature pyrolysis to obtain the starch cellulose composite hard carbon material, and further using it as the negative electrode material to prepare the sodium ion battery has better electrochemical performance.

[0114] (2) Charge and discharge rate performance test

[0115] The sodium ion battery prepared in Example 1 was used as a representative test object to conduct a charge and discharge rate performance test.

[0116] The sodium ion batteries prepared in Example 1 and Comparative Example 1 were tested for charge and discharge rate performance at a current density of 0.1 to 5 C. The results are as follows: Figure 8 As shown, the reversible capacities of the sodium ion battery prepared in Example 1 at current densities of 0.1, 0.3, 0.5, 1, 2, 3 and 5 C were 299.4, 264.9, 248.8, 235.9, 217.1, 197.8 and 154.2 mAh g, respectively. -1 The capacities of the sodium ion battery obtained in Comparative Example 1 at the same current density are 234.7, 168.8, 144.5, 117.3, 69.5, 49.7 and 40.7 mAh g -1When Example 1 experienced a change in current density from 0.1 to 5 C and then returned to a current density of 0.1 C, its reversible specific capacity only decreased by nearly 1 / 8, while the reversible specific capacity of Comparative Example 1 decreased by nearly 1 / 3 after undergoing the above process. From the above results, it can be seen that the sodium ion battery prepared based on CS-HC has better rate performance and excellent reversible specific capacity retention than the sodium ion battery prepared based on S-HC without adding α-cellulose.

[0117] (3) Charge and discharge long cycle performance test

[0118] The sodium ion battery prepared in Example 1 was used as a representative test object to conduct a charge and discharge long cycle performance test.

[0119] The sodium ion batteries prepared in Example 1 and Comparative Example 1 were subjected to a charge and discharge long cycle performance test at a current density of 1C. The results are as follows: Figures 9-10 As shown in FIG1 , the sodium ion battery prepared in Example 1 maintains a capacity of 206.9 mAh g after 500 cycles. -1 The capacity retention rate is 68.5%; the sodium ion battery prepared in Comparative Example 1 maintains a capacity of 63.4 mAhg after 500 cycles. -1 The capacity retention rate was 42.26%. It can be seen that the starch-cellulose composite hard carbon material formed by cross-linking starch and cellulose can effectively improve the long-term cycle stability of the sodium ion battery prepared therefrom.

[0120] The above embodiments are preferred implementation modes of the present invention, but the implementation modes of the present invention are not limited to the above embodiments. Any other changes, modifications, substitutions, combinations, and simplifications that do not deviate from the spirit and principles of the present invention should be considered as equivalent replacement methods and are included in the scope of protection of the present invention.

Claims

1. A method for preparing a starch-cellulose composite hard carbon material, characterized in that: The following steps are involved: S1. Take unmodified starch and cellulose, pre-oxidize at least one of them at 200-250 ° C, mix the two, and fully ball-mill to obtain a starch-cellulose composite; S2. The starch-cellulose composite obtained in step S1 is heated to 500-600° C. under an inert atmosphere for pre-carbonization to obtain a starch-cellulose composite hard carbon precursor; S3. The starch-cellulose composite hard carbon precursor obtained in step S2 is heated to 1300-1500° C. and fully calcined to obtain a starch-cellulose composite hard carbon material.

2. The preparation method according to claim 1, characterized in that In step S1, the non-modified starch includes one or more of corn starch, wheat starch, tapioca starch, potato starch, pea starch, mung bean starch, and rice starch.

3. The preparation method according to claim 1, characterized in that: In step S1, the ball milling speed is 400-500 r / min.

4. The preparation method according to claim 1, characterized in that In step S1, the ball milling time is 6 to 12 hours.

5. The preparation method according to claim 1, characterized in that: In step S1, the cellulose includes one or more of α-cellulose, β-cellulose, and γ-cellulose.

6. The preparation method according to claim 1, characterized in that: In step S1, the mass ratio of the unmodified starch to the cellulose is 1:(0.5-3).

7. The starch-cellulose composite hard carbon material prepared by the preparation method according to any one of claims 1 to 6.

8. Use of the starch-cellulose composite hard carbon material according to claim 7 in the preparation of negative electrode materials for sodium ion batteries.

9. A sodium ion battery negative electrode, characterized in that It comprises a current collector and a negative electrode active material loaded on the current collector, wherein the negative electrode active material comprises the starch-cellulose composite hard carbon material according to claim 7.

10. A sodium ion battery, characterized in that: The invention comprises a positive electrode, a negative electrode, a separator and an electrolyte, wherein the negative electrode comprises the sodium ion battery negative electrode according to claim 9.

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