Biomass-starch composite-based hard carbon material, its preparation and application in sodium-ion batteries

Abstract

The present invention belongs to the field of carbon-based composite materials, and particularly relates to a preparation method of a biomass-starch composite-based hard carbon material, which is obtained by pre-treating biomass under a low-oxygen atmosphere to obtain a heat-treated material, and then compounding the heat-treated material with starch and performing a roasting treatment; in the heat treatment stage, the oxygen partial pressure in the low-oxygen atmosphere is 1-15 v%, and the heat treatment temperature is 300-600 °C. The present invention also includes the material obtained by the above preparation method and its application in sodium batteries. The process of the present invention can solve the problem of starch thermal expansion, construct special physical and chemical structural characteristics, and can also improve the interfacial compatibility of carbons from different sources, thereby synergistically improving the sodium battery electrochemical performance of the material.

Description

Technical Field

[0001] The present invention relates to the field of electrode materials, and particularly to the field of anode materials for sodium-ion batteries. Technical Field

[0003] Since lithium-ion batteries were commercially applied in the 1990s of the last century, they have been widely used in various portable electronic devices one after another. In recent years, with the rapid development of new energy vehicles and large-scale energy storage technologies, the demand for batteries has been increasing day by day. However, due to the uneven distribution of lithium resources reserves and the relatively high price, it is difficult for lithium-ion batteries to support the development of both the electric vehicle and grid energy storage industries at the same time. Sodium-ion batteries have attracted much attention due to their advantages such as high safety, rich raw material reserves, wide resource distribution, and low price. Although the principles of sodium-ion batteries and lithium ions are similar, the radius of sodium ions is larger than that of lithium ions, and most of the materials suitable for lithium ion insertion and extraction are difficult to adapt to the insertion and extraction of sodium ions. This is the main reason why lithium-ion batteries have been widely used while sodium-ion batteries are far from reaching the commercial situation.

[0004] Among various anodes of sodium-ion batteries, hard carbon materials have many pores in the internal crystal arrangement, which are disordered and there are various types of reversible sodium storage sites, with a high sodium storage capacity. In addition, hard carbon also has the advantages of small volume expansion after sodium insertion, good safety, stable structure, good electrical conductivity, environmental friendliness, etc., and is the preferred anode material for commercial application of sodium-ion batteries at present.

[0005] Hard carbon is mainly prepared by high-temperature carbonization of raw materials such as macromolecular compounds, coal, asphalt or biomass. Biomass raw materials have a wide source and low price cost, but due to the diversity of biomass structures, the structures and properties of hard carbon materials obtained by direct carbonization treatment are not ideal enough; raw materials such as coal and asphalt contain more volatile substances, and additional waste gas treatment is required during the production process, which will increase the production cost; starch, as a precursor material, has the advantages of rich yield, environmental friendliness, low impurity content, biodegradability, etc., and is also gradually used to synthesize hard carbon materials, but starch will foam and fuse during the heating process, thus affecting the performance of carbon materials.

[0006] In order to improve the sodium storage capacity and sodium battery performance, the most reported solutions in the industry are to use starch as the carbon source raw material. However, during the preparation process of starch carbon source, phenomena such as foaming, melting, and drastic volume expansion will occur, which brings difficulties to material preparation, and the morphology of the material particles is irregular after foaming and fusing, which will also affect the final performance. To solve this problem, the existing reported solutions have the following improvement ideas, for example:

[0007] (1) CN115536002A discloses heating and reacting starch after mixing it with polyhydroxy organic acids or organic acid anhydrides, and using the esterification of organic acids / acid anhydrides to solve the problem of starch foaming when heated. However, this type of method involves the use of organic acids (or organic acid anhydrides), which will significantly increase the material cost. At the same time, the generation and treatment of waste liquid will also pose environmental protection hazards.

[0008] (2) CN115207350A discloses using dry esterification reaction to solve the problem of foaming and swelling during the direct carbonization of starch, and obtaining a hard carbon anode material by reducing the oxygen content in starch at low temperature. However, the dry esterification link in this method includes steps such as mixing, heating reaction, washing, and drying, with complicated operations. Moreover, organic acid anhydrides, alkali solutions / organic solvents, etc. are still required for the esterification reaction and subsequent washing, and there are still problems with the generation and treatment of harmful waste liquid.

[0009] (3) CN114988391A discloses mixing starch with nano-silica, performing low-temperature heat treatment in an inert and oxygen atmosphere in sequence, separating and removing the silica after that, and then obtaining a hard carbon material through pyrolysis. This method uses the barrier of silica to inhibit the coalescence of starch particles, but an additional separation process is required to remove the nano-silica. At the same time, there will still be a small amount of silica residue in the separated material, which has an adverse effect on the performance of the hard carbon material.

[0010] In summary, the existing technologies have not yet effectively solved the problems of poor thermal stability, easy thermal expansion during the preparation process, and unsatisfactory electrochemical performance of starch-based carbon anode materials under conditions without acids, alkalis, etc. Summary of the Invention

[0011] Aiming at the problems of unsatisfactory thermal stability, easy thermal expansion of existing starch-based hard carbon materials, difficult effective compounding with biomass carbon, and unsatisfactory sodium battery performance, the first object of the present invention is to provide a preparation method for a biomass-starch composite-based hard carbon material, aiming to solve the problems of starch thermal expansion and difficult effective compounding with biomass carbon.

[0012] The second object of the present invention is to provide the biomass-starch composite-based hard carbon material prepared by the above-mentioned preparation method and its application in sodium ion batteries.

[0013] The third object of the present invention is to provide a sodium ion battery containing the biomass-starch composite-based hard carbon material and its anode.

[0014] Aiming at the problems of easy thermal expansion of starch and difficult high-quality compounding and unsatisfactory sodium battery performance caused by the mismatch of the interface between starch and biomass carbon, the present invention provides the following solutions:

[0015] A preparation method of a biomass-starch composite-based hard carbon material, which comprises pre-treating biomass in a low-oxygen atmosphere to obtain a heat-treated material, and then compounding the heat-treated material with starch and performing a calcination treatment to obtain the material;

[0016] In the heat treatment stage, the oxygen partial pressure in the low-oxygen atmosphere is 1-15 v%, and the heat treatment temperature is 300-600 °C.

[0017] In the present invention, the biomass is innovatively pre-treated in the low-oxygen system. Based on the combination of the oxygen partial pressure and temperature, the controllable pre-oxidation and synchronous pre-carbonization of the biomass can be achieved in advance. Then, it is further compounded with starch and calcined. In this way, a local physical barrier and confinement effect can be realized on the starch, effectively inhibiting the foaming, fusion and cross-linking of starch granules during the heating preparation process. In addition, it is also beneficial to inhibit the directional growth of graphite microcrystals, construct more disordered graphite microstructures, and induce the formation of closed pores. Moreover, it further induces the interfacial adaptability between the starch-based and biomass-based carbon, improving the interfacial composite effect. Thus, through the combination of the process and parameters of the present invention, the problem of starch thermal expansion can be effectively solved, and it is beneficial to construct a physical and chemical structure suitable for sodium ion insertion and extraction. Moreover, it is also beneficial to improve the interfacial adaptability between starch and biomass-based materials, improve the sodium storage capacity, improve the sodium ion and electron conduction modes, and contribute to improving the electrochemical properties such as sodium capacitance, initial efficiency, rate performance, and fast charge stability.

[0018] In the present invention, the biomass is at least one of plant fruit shells and rhizomes, preferably any one of coconut shells, peanut shells, walnut shells, bamboo, and hazelnut shells, and further preferably coconut shells. The present invention has found through research that using coconut shells as raw materials, their original substances and structural characteristics have better adaptability and synergy with the process of the present invention, and can further synergistically improve sodium capacitance, initial efficiency, and fast charge stability.

[0019] In the present invention, the biomass is innovatively pre-treated in a low-oxygen environment. Combining with the oxygen partial pressure and heat treatment temperature in the low-oxygen atmosphere helps to unexpectedly solve the problems of expansion and difficult high-quality compounding in the subsequent starch heat treatment process, and can further improve the sodium electrical properties of the prepared composite material.

[0020] In the present invention, the low-oxygen atmosphere includes an oxygen-containing atmosphere and a diluent gas. The oxygen-containing atmosphere is at least one of oxygen and air, and the diluent gas is at least one of nitrogen, inert gas, and carbon dioxide.

[0021] Preferably, the oxygen content in the low-oxygen atmosphere is 1.5-10 v%.

[0022] Preferably, the heat treatment temperature is 400-500 °C;

[0023] Preferably, the heating rate for the stage of heating up to the heat treatment temperature is 2 - 10 °C / min;

[0024] Preferably, the heat preservation time for the heat treatment stage is 1 - 5 h.

[0025] In the present invention, the starch is at least one of wheat starch, corn starch, tapioca starch, potato starch, pea starch, mung bean starch, and rice starch, further preferably a mixture of wheat starch and corn starch; more preferably a mixture of wheat starch and corn starch with a ratio of 1 - 3:1. The research of the present invention also finds that by using the preferred starch raw materials, especially the combined materials, based on their adapted branched / linear structures, they can be further combined synergistically with the process of the present invention, which is not only beneficial for further solving the problem of starch thermal expansion, but also can improve the microstructure and the interfacial adaptation relationship, and can further improve the sodium capacitance, initial efficiency, and fast charging stability.

[0026] Preferably, the mass ratio of starch to the heat treatment material is 10:1 - 10.

[0027] In the present invention, the roasting process includes two heat preservation stages. Among them, the temperature of the first heat preservation stage is 200 - 400 °C, and the time is 1 - 3 h;

[0028] Preferably, the temperature of the second heat preservation stage is 1000 - 1500 °C, and the time is 2 - 5 h;

[0029] Preferably, the roasting stage is carried out in a protective atmosphere.

[0030] The present invention also provides a biomass-starch composite-based hard carbon material prepared by the described preparation method.

[0031] In the present invention, the described preparation method can endow the prepared product with special microstructures and physical and chemical characteristics, and the materials with the characteristics endowed by the preparation method can exhibit better sodium electrochemical performance.

[0032] The present invention also provides an application of a biomass-starch composite-based hard carbon material prepared by the described preparation method, using it as the negative electrode active material of a secondary battery.

[0033] In the present invention, the biomass-starch composite-based hard carbon material of the present invention can be used to prepare the required battery based on the conventional application idea of the negative electrode active material and the known process operations. For example, as the negative electrode active material, it is used to prepare sodium ion batteries.

[0034] The present invention also provides a negative electrode of a sodium ion battery, including a current collector and a negative electrode active material loaded on the current collector, and the negative electrode active material includes the biomass-starch composite-based hard carbon material prepared by the described preparation method of the present invention.

[0035] In the present invention, in the negative electrode, except for the biomass-starch composite-based hard carbon material described in the present invention, other components, the negative electrode sheet structure, and the preparation method can all be well-known.

[0036] In the present invention, in the negative electrode, in the negative electrode active material, the content of the biomass-starch composite-based hard carbon material can be greater than 1 wt.%, further greater than 50 wt.%, and even further can be 100 wt.%.

[0037] The present invention also provides a sodium-ion battery, including a negative electrode, a separator, and a positive electrode, wherein the negative electrode contains the biomass-starch composite-based hard carbon material prepared by the preparation method described in the present invention.

[0038] Beneficial effects

[0039] In the present invention, the biomass is innovatively pre-treated by heat treatment in a low-oxygen atmosphere, and further combined with the joint control of the oxygen partial pressure and temperature in this process, the biomass can be pre-synchronously pre-oxidized and pre-carbonized to construct a microscopic physical and chemical structure suitable for sodium-ion batteries. Then, it is innovatively compounded with starch and carbonized, which can inhibit the problem of starch thermal expansion. Moreover, it can also improve the carbonized structure, pore structure, and dual-carbon composite interface, and further can synergistically improve the adaptation effect of sodium ion insertion and extraction, and can effectively improve the electrochemical properties such as sodium capacitance, rate performance, cycle stability, and fast charge stability of the composite material.

[0040] The whole process of the present invention does not require the use of acids, alkalis, and other organic reagents, effectively avoiding the problems of generation, emission, and treatment of waste acids and organic waste liquids in the preparation process of the previously reported starch-based hard carbon materials.

[0041] The biomass raw materials are widely sourced and are cheaper than starch. Compared with the existing preparation processes of starch-based hard carbon materials, this method further reduces the raw material cost. The preparation method of the sodium-ion battery composite hard carbon negative electrode material provided by the present invention has a short process flow, few operation steps, is simple in process, low in cost, safe, efficient, and environmentally friendly, and is suitable for large-scale production. Description of the drawings

[0042] Figure 1 SEM image of the hard carbon negative electrode material prepared in Example 1;

[0043] Figure 2 Physical image of the hard carbon negative electrode material prepared in Comparative Example 2;

[0044] Figure 3 SEM image of the hard carbon negative electrode material prepared in Comparative Example 2. Detailed implementation manners

[0045] The following are examples to further illustrate the present invention, but the protection scope of the present invention is not limited to the examples. Other changes and modifications made by those skilled in the art without departing from the spirit and protection scope of the present invention are still included within the protection scope of the present invention.

[0046] In order to achieve the above invention object, the specific technical solution of the present invention is as follows:

[0047] The preparation method of the biomass-starch composite-based hard carbon material of the present invention is to pre-treat the biomass in a low-oxygen atmosphere to obtain a heat-treated material, and then compound the heat-treated material with starch and perform a roasting treatment to obtain it.

[0048] In the heat treatment stage, the oxygen partial pressure in the low-oxygen atmosphere is 1-15 v%, and the heat treatment temperature is 300-600 °C.

[0049] A more convenient and economical preparable method of the present invention includes the following steps:

[0050] Pretreatment: After drying the biomass raw material, it is crushed, and the crushed biomass raw material powder is placed in a tube furnace and treated in an air + inert protective atmosphere to obtain biomass carbon powder.

[0051] Mixing: The starch and the biomass carbon powder obtained by the pre-carbonization treatment in step (1) are added to a mixer according to a certain mass ratio, and the mixing time is, for example, more than 10 min, and further can be 20-60 min.

[0052] Pyrolysis: The mixed powder obtained in step (2) is subjected to a segmented pyrolysis treatment in an inert atmosphere to obtain the composite hard carbon negative electrode material for the sodium ion battery.

[0053] In the present invention, first, the biomass raw material is crushed, and the crushed biomass powder is heat-treated in a low-oxygen atmosphere, which can realize controllable pre-carbonization and pre-oxidation, is beneficial to inhibiting the directional growth of graphite microcrystals in the subsequent carbonization process, constructing more disordered graphite microstructures to accommodate more sodium ion filling, and at the same time can also induce the formation of closed pores in the carbonization process, which helps to improve the capacity and first Coulomb efficiency of the negative electrode material. Then, the biomass carbon powder obtained after pretreatment is mixed with starch, and the physical barrier and constraint effect of the carbon powder on the expansion of starch in a local area is utilized to inhibit the foaming, melting and cross-linking of starch particles during the heating process, and a hard carbon material with excellent performance is prepared.

[0054] As an exemplifiable scheme, in step (1), the biomass raw material is at least one of coconut shell, peanut shell, walnut shell, bamboo, and hazelnut shell.

[0055] As an exemplary embodiment, the pretreatment in step (1) is to heat up to 300-600°C at a heating rate of 2-10°C / min, and the holding time is 1-5 h.

[0056] As an exemplary embodiment, the starch in step (2) is at least one of wheat starch, corn starch, cassava starch, potato starch, pea starch, mung bean starch, and rice starch, and the mass ratio of starch to biomass carbon powder is 10:1-10.

[0057] As an exemplary embodiment, the staged pyrolysis treatment in step (3) is to heat up to 200-400°C and 1000-1500°C in sequence at a heating rate of 1-5°C / min, hold for 1-3 h and 2-5 h respectively, cool to room temperature, and pulverize to obtain the product.

[0058] As an exemplary embodiment, the inert atmosphere in step (1) is at least one of nitrogen and argon, and the volume flow ratio of the inert gas to air is 10:1-5.

[0059] As an exemplary embodiment, the inert atmosphere in step (3) is at least one of nitrogen and argon.

[0060] The following are more specific exemplary embodiments:

[0061] Example 1

[0062] (1) Pretreatment: Crush the biomass raw material walnut shell to obtain walnut shell powder, and then place the walnut shell powder in a tubular furnace for pretreatment. The volume flow ratio of argon to air is 6:1. Heat up to 600°C at a heating rate of 10°C / min and hold for 3 h to obtain walnut shell carbon powder;

[0063] (2) Mixing: Add potato starch and the walnut shell carbon powder obtained in step (1) to a mixer at a mass ratio of 3:1 and mix for 30 min to obtain a mixed powder of starch and walnut shell carbon powder;

[0064] (3) Pyrolysis: Place the mixed powder obtained in step (2) in a carbonization furnace, heat up to 400°C at a heating rate of 2°C / min, hold for 1 h, then heat up to 1100°C at a heating rate of 2°C / min, hold for 4 h, and after cooling to room temperature, the hard carbon negative electrode material can be obtained. The scanning electron microscope image is as shown in Figure 1 shown.

[0065] The active material, SBR, CMC, and SP with a mass ratio of 94.5:1.5:1.5:2.5 were made into a slurry with deionized water, coated on a copper foil, and dried in a vacuum drying oven for 12 hours to make a pole piece. Using metallic sodium as the counter electrode, 1mol / L NaPF6 in EC / DEC (volume ratio 1:1) as the electrolyte, and glass fiber as the separator, a CR2025 button cell was assembled in a dry glove box filled with argon to prepare a button half cell. Charge-discharge tests were carried out at room temperature (25°C) in the voltage range of 0.01 - 3.0V, with a current density of 30mA / g. The first-cycle reversible specific capacity was 344.5mAh / g, the first Coulombic efficiency was 88.9%, and it was rapidly charged and discharged 200 times under the condition of 2C, with a capacity retention rate of 98%.

[0066] Example 2

[0067] Compared with Example 1, the only difference is that the type of biomass raw material is changed. The experimental groups are as follows:

[0068] Group A: The biomass raw material is coconut shell;

[0069] Group B: The biomass raw material is peanut shell;

[0070] Group C: The biomass raw material is bamboo;

[0071] Other operations and parameters are the same as those in Example 1, and performance tests were carried out according to the method of Example 1. The results are as follows:

[0072] Group A: The first-cycle reversible specific capacity was 347.2mAh / g, the first Coulombic efficiency was 91.4%, and it was rapidly charged and discharged 200 times under the condition of 2C, with a capacity retention rate of 98.8%.

[0073] Group B: The first-cycle reversible specific capacity was 341.5mAh / g, the first Coulombic efficiency was 88.2%, and it was rapidly charged and discharged 200 times under the condition of 2C, with a capacity retention rate of 97%.

[0074] Group C: The first-cycle reversible specific capacity was 343.6mAh / g, the first Coulombic efficiency was 89.1%, and it was rapidly charged and discharged 200 times under the condition of 2C, with a capacity retention rate of 98.3%.

[0075] It can be seen from Examples 1 and 2 that using the preferred coconut shell as the raw material and combining with the process described in the present invention can further facilitate the original structure of the coconut shell and the subsequent process, construct a physicochemical structure more suitable for the insertion and extraction of sodium ions, and can further improve the capacity, first efficiency, and fast charging stability.

[0076] Example 3

[0077] Compared with Example 1, the only difference is that the type of starch is changed. The experimental groups are as follows:

[0078] Group A: The starch is wheat starch;

[0079] Group B: The starch is wheat starch and corn starch with a weight ratio of 2:1;

[0080] Group C: The starch is corn starch;

[0081] Other operations and parameters are the same as those in Example 1, and the performance tests are carried out according to the method of Example 1. The results are as follows:

[0082] Group A: The first-cycle reversible specific capacity is 350.1 mAh / g, the first Coulombic efficiency is 90%, and it is rapidly charged and discharged 200 times under the condition of 2C, and its capacity retention rate is 98.2%.

[0083] Group B: The first-cycle reversible specific capacity is 357.5 mAh / g, the first Coulombic efficiency is 92.1%, and it is rapidly charged and discharged 200 times under the condition of 2C, and its capacity retention rate is 99.1%.

[0084] Group C: The first-cycle reversible specific capacity is 346.2 mAh / g, the first Coulombic efficiency is 88.4%, and it is rapidly charged and discharged 200 times under the condition of 2C, and its capacity retention rate is 98%.

[0085] It was unexpectedly found in the research of the present invention that the combination of wheat starch and corn starch as starch raw materials, through their more suitable branched / linear structure and the combination with the process of the present invention, can further improve the problem of thermal expansion, improve the composite interface and microstructure between carbons, and can further facilitate the construction of a physicochemical structure suitable for sodium ion adaptation, thereby further improving the capacity, initial efficiency and fast charge stability.

[0086] Example 4

[0087] (1) Pretreatment: The biomass raw material coconut shell is crushed to obtain coconut shell powder, and then the coconut shell powder is placed in a tubular furnace for pretreatment. The volume flow ratio of argon to air is 3:1, and it is heated to 300 °C at a heating rate of 2 °C / min and kept warm for 5 h to obtain coconut shell carbon powder;

[0088] (2) Mixing: Wheat starch and the coconut shell carbon powder obtained in step (1) are added to a mixer in a mass ratio of 5:1 and mixed for 30 min to obtain a mixed powder of starch and coconut shell carbon powder;

[0089] (3) Pyrolysis: The mixed powder obtained in step (2) is placed in a carbonization furnace, heated to 200 °C at a heating rate of 1 °C / min, kept warm for 3 h, and then heated to 1200 °C at a heating rate of 2 °C / min and kept warm for 2 h. After cooling to room temperature, the hard carbon negative electrode material can be obtained.

[0090] The battery assembly and electrochemical tests were carried out using the method of Example 1. The first-cycle reversible specific capacity was 352.7 mAh / g, the first Coulombic efficiency was 91.2%, and it was rapidly charged and discharged 200 times under the condition of 2C, and the capacity retention rate was 98.7%.

[0091] Example 5

[0092] (1) Pretreatment: The biomass raw material peanut shell was crushed to obtain peanut shell powder. Subsequently, the peanut shell powder was placed in a tubular furnace for pretreatment. The volume flow ratio of argon to air was 8:1, and it was heated to 500 °C at a heating rate of 5 °C / min and held for 2 h to obtain peanut shell carbon powder;

[0093] (2) Mixing: Rice starch and the peanut shell carbon powder obtained in step (1) were added to a mixer in a mass ratio of 3:1 and mixed for 30 min to obtain a mixed powder of starch and peanut shell carbon powder;

[0094] (3) Pyrolysis: The mixed powder obtained in step (2) was placed in a carbonization furnace, heated to 300 °C at a heating rate of 3 °C / min, held for 2 h, and then heated to 1200 °C at a heating rate of 3 °C / min and held for 3 h. After cooling to room temperature, a hard carbon negative electrode material could be obtained.

[0095] The battery assembly and electrochemical tests were carried out using the method of Example 1. The first-cycle reversible specific capacity was 349.5 mAh / g, the first Coulombic efficiency was 89.5%, and it was rapidly charged and discharged 200 times under the condition of 2C, and the capacity retention rate was 98.4%.

[0096] Example 6

[0097] (1) Pretreatment: The biomass raw material bamboo was crushed to obtain bamboo powder. Subsequently, the bamboo powder was placed in a tubular furnace for pretreatment. The volume flow ratio of argon to air was 4:1, and it was heated to 400 °C at a heating rate of 2 °C / min and held for 2 h to obtain bamboo carbon powder;

[0098] (2) Mixing: Corn starch and the bamboo carbon powder obtained in step (1) were added to a mixer in a mass ratio of 8:1 and mixed for 30 min to obtain a mixed powder of starch and bamboo carbon powder;

[0099] (3) Pyrolysis: The mixed powder obtained in step (2) was placed in a carbonization furnace, heated to 300 °C at a heating rate of 2 °C / min, held for 1 h, and then heated to 1500 °C at a heating rate of 5 °C / min and held for 2 h. After cooling to room temperature, a hard carbon negative electrode material could be obtained.

[0100] The method of Example 1 was used for battery assembly and electrochemical testing. The first-cycle reversible specific capacity was 340.2 mAh / g, the first Coulombic efficiency was 90.6%, and it was rapidly charged and discharged 200 times under 2C conditions, with a capacity retention rate of 98.2%.

[0101] Example 7

[0102] (1) Pretreatment: The biomass raw material, hazelnut shells, was crushed to obtain hazelnut shell powder. Subsequently, the hazelnut shell powder was placed in a tube furnace for pretreatment. The volume flow ratio of argon to air was 10:1, and it was heated to 600 °C at a heating rate of 3 °C / min and held for 1 h for pre-carbonization treatment to obtain hazelnut shell carbon powder;

[0103] (2) Mixing: Potato starch and the hazelnut shell carbon powder obtained in step (1) were added to a mixer in a mass ratio of 1:1 and mixed for 30 min to obtain a mixed powder of starch and hazelnut shell carbon powder;

[0104] (3) Pyrolysis: The mixed powder obtained in step (2) was placed in a carbonization furnace, heated to 400 °C at a heating rate of 5 °C / min, held for 3 h, then heated to 1000 °C at a heating rate of 2 °C / min, and held for 5 h. After cooling to room temperature, a hard carbon anode material could be obtained.

[0105] The method of Example 1 was used for battery assembly and electrochemical testing. The first-cycle reversible specific capacity was 335.1 mAh / g, the first Coulombic efficiency was 89.3%, and it was rapidly charged and discharged 200 times under 2C conditions, with a capacity retention rate of 98.1%.

[0106] Example 8

[0107] (1) Pretreatment: The biomass raw material, coconut shells, was crushed to obtain coconut shell powder. Subsequently, the coconut shell powder was placed in a tube furnace for pretreatment. The volume flow ratio of argon to air was 2:1, and it was heated to 300 °C at a heating rate of 2 °C / min and held for 4 h for pre-carbonization treatment to obtain coconut shell carbon powder;

[0108] (2) Mixing: Corn starch and the coconut shell carbon powder obtained in step (1) were added to a mixer in a mass ratio of 8:1 and mixed for 30 min to obtain a mixed powder of starch and hazelnut shell carbon powder;

[0109] (3) Pyrolysis: The mixed powder obtained in step (2) was placed in a carbonization furnace, heated to 200 °C at a heating rate of 1 °C / min, held for 2 h, then heated to 1100 °C at a heating rate of 2 °C / min, and held for 4 h. After cooling to room temperature, a hard carbon anode material could be obtained.

[0110] The battery assembly and electrochemical tests were carried out using the method of Example 1. The first-cycle reversible specific capacity was 356.4 mAh / g, the first Coulombic efficiency was 92.4%, and it was rapidly charged and discharged 200 times under 2C conditions, with a capacity retention rate of 98.9%.

[0111] Example 9

[0112] (1) Pretreatment: The biomass raw material bamboo was crushed to obtain bamboo powder, and then the bamboo powder was placed in a tube furnace for pretreatment. The volume flow ratio of argon to air was 5:1, and it was heated to 500 °C at a heating rate of 5 °C / min and held for 3 h for pre-carbonization treatment to obtain bamboo charcoal powder;

[0113] (2) Mixing: Rice starch and the bamboo charcoal powder obtained in step (1) were added to a mixer in a mass ratio of 4:1 and mixed for 30 min to obtain a mixed powder material of starch and hazelnut shell carbon powder;

[0114] (3) Pyrolysis: The mixed powder obtained in step (2) was placed in a carbonization furnace, heated to 400 °C at a heating rate of 1 °C / min, held for 1 h, and then heated to 1300 °C at a heating rate of 3 °C / min and held for 1 h. After cooling to room temperature, a hard carbon anode material could be obtained.

[0115] The battery assembly and electrochemical tests were carried out using the method of Example 1. The first-cycle reversible specific capacity was 338.3 mAh / g, the first Coulombic efficiency was 89.7%, and it was rapidly charged and discharged 200 times under 2C conditions, with a capacity retention rate of 98%.

[0116] Comparative Example 1

[0117] Compared with Example 1, the difference was only that the biomass raw material was not pre-oxidized. That is, in step 1, the atmosphere in the pretreatment stage was only argon, and other operations and parameters were the same as those in Example 1.

[0118] The prepared hard carbon anode material was subjected to battery assembly and electrochemical tests using the method of Example 1. The first-cycle reversible specific capacity was 317.8 mAh / g, the first Coulombic efficiency was 76.4%, and it was rapidly charged and discharged 200 times under 2C conditions, with a capacity retention rate of 79.1%.

[0119] It can be seen from Example 1 and the comparative example that by using the described process, the starch swelling and the interfacial compatibility between carbons from different sources can be effectively improved, and the performance of the prepared material can be further synergistically improved.

[0120] Comparative Example 2: No biomass carbon powder was mixed in

[0121] Compared with Example 1, the difference is only that no biomass carbon powder is added, that is, steps (1) and (2) in Example 1 are removed, and the starch is directly pyrolyzed according to step (3) in Example 1. After pyrolysis, the volume of wheat starch expands significantly, and a hard carbon material is obtained, such as Figure 2 shown; the hard carbon material prepared by pyrolyzing wheat starch is crushed, and the morphology of the crushed material is as Figure 3 shown. It can be seen that the shapes and sizes of the material particles are irregular, and the particle sizes are relatively large.

[0122] The prepared hard carbon negative electrode material is assembled into a battery and electrochemically tested by the method of Example 1. The first-cycle reversible specific capacity is 323.5 mAh / g, the first Coulombic efficiency is 75.2%, and it is rapidly charged and discharged 200 times under the condition of 2C, and its capacity retention rate is 80.3%.

[0123] It can be seen from Example 1 and Comparative Example 2 that through the process of the present invention, the violent expansion during the pyrolysis process can be effectively avoided, the shapes and sizes of the material particles can be made more regular, and the first reversible specific capacity, the first-cycle Coulombic efficiency and the fast charge and discharge stability of the material can be significantly improved.

[0124] Comparative Example 3

[0125] Compared with Example 1, the difference is only that step (2) of Example 1 is removed. After the biomass raw material is pretreated in step (1), it is directly pyrolyzed in step (3) without mixing starch.

[0126] The prepared hard carbon material is assembled into a battery and electrochemically tested by the method of Example 1. The first-cycle reversible specific capacity is 312.6 mAh / g, the first Coulombic efficiency is 85.7%, and it is rapidly charged and discharged 200 times under the condition of 2C, and its capacity retention rate is 84.6%.

[0127] Comparative Example 4

[0128] Compared with Example 1, the difference is only that in step 1, the atmosphere in the pretreatment stage is air, and the total gas flow rate and other operations and parameters are the same as those in Example 1.

[0129] The electrochemical performance is tested by the method of Example 1. The results are as follows: the first-cycle reversible specific capacity is 304 mAh / g, the first Coulombic efficiency is 86.5%, and it is rapidly charged and discharged 200 times under the condition of 2C, and its capacity retention rate is 83.9%.

[0130] Comparative Example 5

[0131] Compared with Example 1, the difference is only that in step 1, the temperature in the pretreatment stage is 250 °C, and other operations and parameters are the same as those in Example 1.

[0132] The electrochemical performance was tested according to the method of Example 1, and the results were as follows: the reversible specific capacity in the first cycle was 293 mAh / g, the first Coulombic efficiency was 82.5%, and it was rapidly charged and discharged 200 times under the condition of 2C, and the capacity retention rate was 81.4%.

[0133] Comparative Example 6

[0134] Compared with Example 1, the difference was only that in Step 1, the temperature in the pretreatment stage was 650 °C, and other operations and parameters were the same as those in Example 1.

[0135] The electrochemical performance was tested according to the method of Example 1, and the results were as follows: the reversible specific capacity in the first cycle was 328.4 mAh / g, the first Coulombic efficiency was 91.6%, and it was rapidly charged and discharged 200 times under the condition of 2C, and the capacity retention rate was 89.2%.

[0136] Comparative Example 7

[0137] Compared with Example 1, the difference was only that the pre-carbonization and pre-oxidation were not synchronized. That is, in Step 1, the biomass raw material was pre-calcined in a pure Ar atmosphere at 600 °C for 1.5 h, and then calcined in an atmosphere with a volume flow ratio of argon to air of 6:1 at 600 °C for 1.5 h. The total gas flow rate and other operations and parameters were the same as those in Example 1.

[0138] The electrochemical performance was tested according to the method of Example 1, and the results were as follows: the reversible specific capacity in the first cycle was 310.8 mAh / g, the first Coulombic efficiency was 90.6%, and it was rapidly charged and discharged 200 times under the condition of 2C, and the capacity retention rate was 90.6%.

[0139] Comparative Example 8

[0140] Compared with Example 1, the difference was only that the starch and the biomass were not processed step by step. That is, the starch required in Step 2 was directly mixed with the biomass raw material in Step 1, and Step 1 was carried out together, and then Step 3 was carried out. The process operations were the same as those in Example 1.

[0141] The electrochemical performance was tested according to the method of Example 1, and the results were as follows: the reversible specific capacity in the first cycle was 323.1 mAh / g, the first Coulombic efficiency was 91.1%, and it was rapidly charged and discharged 200 times under the condition of 2C, and the capacity retention rate was 90.5%.

Claims

1. A preparation method of a biomass-starch composite-based hard carbon material, characterized in that, The biomass is pre-treated by heat treatment in a low-oxygen atmosphere to obtain a heat-treated material, and the heat-treated material and starch are compounded and then subjected to roasting treatment to obtain the product; In the heat treatment stage, the oxygen partial pressure in the low-oxygen atmosphere is 1-15 v%, and the heat treatment temperature is 300-600 °C; The biomass is at least one of plant fruit shells and rhizomes.

2. The preparation method of the biomass-starch composite-based hard carbon material according to claim 1, characterized in that, The biomass is any one of coconut shells, peanut shells, walnut shells, bamboo, and hazelnut shells.

3. The preparation method of the biomass-starch composite-based hard carbon material according to claim 1, characterized in that, The low-oxygen atmosphere includes an oxygen-containing atmosphere and a diluent gas. The oxygen-containing atmosphere is at least one of oxygen and air, and the diluent gas is at least one of nitrogen, inert gas, and carbon dioxide.

4. The preparation method of the biomass-starch composite-based hard carbon material according to claim 1, characterized in that, The heat treatment temperature is 400-500 °C.

5. The preparation method of the biomass-starch composite-based hard carbon material according to claim 1, wherein, The heating rate in the stage of heating to the heat treatment temperature is 2-10 °C / min.

6. The preparation method of the biomass-starch composite-based hard carbon material according to claim 1, wherein, The heat preservation time in the heat treatment stage is 1-5 h.

7. The preparation method of the biomass-starch composite-based hard carbon material according to claim 1, characterized in that The starch is at least one of wheat starch, corn starch, cassava starch, potato starch, pea starch, mung bean starch, and rice starch.

8. The preparation method of the biomass-starch composite-based hard carbon material according to claim 1, characterized in that, The mass ratio of starch to the heat-treated material is 10:1-10.

9. The preparation method of the biomass-starch composite-based hard carbon material according to claim 1, characterized in that, The roasting process includes two heat preservation stages. Among them, the temperature of the first heat preservation stage is 200-400 °C, and the time is 1-3 h; The temperature of the second heat preservation stage is 1000-1500 °C, and the time is 2-5 h.

10. The preparation method of the biomass-starch composite-based hard carbon material according to claim 1, characterized in that, The roasting stage is carried out in a protective atmosphere.

11. A biomass-starch composite-based hard carbon material prepared by the preparation method according to any one of claims 1-10.

12. Use of the biomass-starch composite-based hard carbon material prepared by the preparation method according to any one of claims 1 to 10, characterized in that, Use it as the negative electrode active material of a secondary battery.

13. The application according to claim 12, wherein Use it as the negative electrode active material for preparing a sodium ion battery.

14. A negative electrode of a sodium ion battery, comprising a current collector and a negative electrode active material loaded on the current collector, and the negative electrode active material includes a biomass-starch composite-based hard carbon material prepared by the preparation method according to any one of claims 1-10.

15. A sodium-ion battery, comprising a negative electrode, a separator, and a positive electrode, characterized in that, The negative electrode is a biomass-starch composite-based hard carbon material prepared by the preparation method according to any one of claims 1-10.

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