Preparation method of a biomass-derived hard carbon negative electrode material and a sodium-ion battery

Abstract

The present invention discloses a preparation method of a biomass-derived hard carbon negative electrode material and a sodium-ion battery, relating to the technical field of battery energy storage. The preparation method comprises the following steps: adding a biomass raw material and an oxidant into an alkaline solution to obtain a hydrolysis mixed solution; performing acoustic activation treatment on the hydrolysis mixed solution, and successively performing washing and drying to prepare a biomass precursor; successively performing pre-carbonization and pulverization treatment on the biomass precursor to prepare low-temperature pyrolysis carbon; mixing the low-temperature pyrolysis carbon with an organic acid salt to obtain a coating precursor, and performing gradient calcination on the coating precursor to prepare high-temperature pyrolysis carbon; successively performing pickling treatment, water washing treatment and drying treatment on the high-temperature pyrolysis carbon to prepare a hard carbon negative electrode material; wherein, the gradient calcination comprises a first gradient, a second gradient and a third gradient. The hard carbon negative electrode material prepared by the present invention has high energy storage performance, initial Coulomb efficiency and rate performance.

Description

Technical Field

[0001] The present invention relates to the technical field of battery energy storage, and particularly relates to a preparation method of a biomass-derived hard carbon anode material and a sodium-ion battery. Background Art

[0002] In recent years, due to advantages such as high energy density, lithium-ion batteries have been widely used in the fields of new energy vehicles, portable electronic products, energy storage, etc. However, due to the limited reserves and uneven distribution of lithium resources, their costs remain high, which hinders the development of lithium batteries in large-scale energy storage applications. In this context, sodium-ion batteries have become a potential choice for large-scale energy storage devices due to their similar working principles to lithium-ion batteries, abundant sodium resources, low costs, and good low-temperature performance.

[0003] Currently, the commercially available anode materials for sodium-ion batteries are mainly hard carbon, and their raw materials are mainly divided into resin-based carbon and biomass-derived carbon, etc. For example, phenol formaldehyde resin, polyacrylonitrile, starch, etc. are polymer precursors widely used to prepare hard carbon. However, most polymer precursors are derived from petroleum and its derivatives, with relatively high prices, and both the preparation of precursors and the pyrolysis into carbon will have a negative impact on the environment. In contrast, biomass-based raw materials are widely sourced, sustainable, low-pollution, and inexpensive. Using biomass to prepare hard carbon materials can not only effectively reduce costs but also alleviate the environmental pollution problems caused by the large-scale incineration of waste. The unique microstructure and texture characteristics formed during the growth of biomass materials are retained after carbonization, and their trace impurity elements (such as potassium and silicon) can activate the carbon skeleton during the heat treatment process, enriching the pore structure of the material. These characteristics enable hard carbon to have excellent sodium storage performance and are expected to be widely used in sodium-ion batteries.

[0004] In the existing preparation methods of hard carbon, inorganic elements in biomass may increase the conductivity and layer spacing of the material during carbonization, but they will also introduce too many structural defects, resulting in excessive consumption of the electrolyte. In addition, other non-metal impurities remaining in the hard carbon material will react with sodium ions, reducing the reversible sodium storage capacity and thus affecting the cycle performance of the battery. Generally, the specific surface area of hard carbon materials is too large, greatly increasing the contact area with the electrolyte and generating a relatively thick solid electrolyte interface film, further reducing the reversible sodium storage capacity. At the same time, although a larger layer spacing is beneficial for the insertion of sodium ions, it may limit the improvement of rate performance. Generally speaking, these defects lead to relatively low cycle stability and rate performance of sodium-ion batteries with hard carbon anode materials. Summary of the Invention

[0005] An object of the first aspect of the present invention is to provide a method for preparing a biomass-derived hard carbon anode material, which solves the technical problems in the prior art that the large specific surface area and layer spacing of the biomass-based hard carbon anode material result in poor cycle stability and rate performance of sodium-ion batteries.

[0006] Another object of the first aspect of the present invention is to further improve the hydrolysis rate of biomass raw materials.

[0007] An object of the second aspect of the present invention is to provide a sodium-ion battery including the hard carbon anode material prepared by the method for preparing the hard carbon anode material described above.

[0008] According to the object of the present invention, the present invention provides a method for preparing a biomass-derived hard carbon anode material, comprising the following steps:

[0009] Adding a biomass raw material and an oxidant to an alkaline solution to obtain a hydrolysis mixed solution;

[0010] Performing sonication activation treatment on the hydrolysis mixed solution, and then washing and drying in sequence to prepare a biomass precursor;

[0011] Performing pre-carbonization and pulverization treatment on the biomass precursor in sequence to prepare low-temperature pyrolysis carbon;

[0012] Mixing the low-temperature pyrolysis carbon with an organic acid salt to obtain a coating precursor, and performing gradient calcination on the coating precursor to prepare high-temperature pyrolysis carbon;

[0013] Performing pickling treatment, water washing treatment and drying treatment on the high-temperature pyrolysis carbon in sequence to prepare a hard carbon anode material; wherein:

[0014] The gradient calcination includes a first gradient, a second gradient and a third gradient. The calcination temperature of the first gradient is the melting temperature of the organic acid salt, the calcination temperature of the second gradient is the reduction temperature or decomposition temperature of the metal oxide or metal carbonate, and the calcination temperature of the third gradient is the forming temperature of the hard carbon anode material and the metal gasification temperature.

[0015] Optionally, the mass ratio of the organic acid salt to the low-temperature pyrolysis carbon is any value in the range of 1 wt% - 20 wt%.

[0016] Optionally, the organic acid salt is any one or a mixture of gluconates, acetates, citrates or lactates.

[0017] Optionally, the gluconate is at least one of zinc gluconate, calcium gluconate, potassium gluconate, ferrous gluconate, magnesium gluconate;

[0018] The acetate is at least one of zinc acetate, calcium acetate, potassium acetate, magnesium acetate;

[0019] The citrate is at least one of zinc citrate, calcium citrate, and potassium citrate;

[0020] The lactate is at least one of zinc lactate, calcium lactate, potassium lactate, and magnesium lactate.

[0021] Optionally, the mass ratio of the oxidant to the biomass raw material is any value from 3 wt% to 10 wt%.

[0022] Optionally, the oxidant is any one or a mixture of more than one of hydrogen peroxide, hypochlorite, and persulfate.

[0023] Optionally, the concentration of the alkaline solution is any value from 0.5 mol / L to 3 mol / L, and the temperature of the alkaline solution is any value from 40 °C to 100 °C.

[0024] Optionally, the alkaline solution is an aqueous solution of one or a mixture of more than one of metal hydroxides or metal carbonates.

[0025] Optionally, the biomass raw material is any one or a mixture of more than one of bamboo, wood, reed, coffee grounds, bagasse, wheat straw, rice straw, corn straw, corn cob, coconut shell, durian shell, rice husk, hazelnut shell, starch, lignin, etc.

[0026] According to the purpose of the second aspect of the present invention, the present invention also provides a sodium-ion battery, and the sodium-ion battery includes a hard carbon negative electrode material prepared by any one of the above preparation methods.

[0027] Through the synergistic effects of adding the biomass raw material and the oxidant to the alkaline solution, performing sonication activation treatment on the hydrolyzed mixed solution, and performing gradient calcination on the coated precursor coated with the organic acid salt, the present invention enables the oxidant to form highly oxidizing reactive oxygen species under alkaline conditions. The reactive oxygen species break the chemical bonds in lignin and hemicellulose through oxidation, and the sonication activation treatment can significantly enhance the mass transfer and diffusion performance of the alkaline solution, promote the penetration of the alkaline solution and the reactive oxygen species into the fiber tissue or between carbon layers and carbon pores, promote the hydrolysis reaction rate of the molecular structure in the biomass raw material, reduce the relative contents of lignin and hemicellulose while increasing the relative content of cellulose, increase the graphite domain area after high-temperature carbonization, and while increasing the graphitization degree, the graphite domains shrink and wind at high temperature, promoting the generation of closed pores, thereby increasing the sodium storage capacity in the hard carbon plateau region. Moreover, the gradient calcination of the coated precursor can promote the filling and encapsulation of the pores in the hard carbon and the etching of the nanopores after filling, thereby optimizing the pore characteristics of the hard carbon negative electrode material, improving its sodium storage performance, and enhancing the cycle stability and rate performance.

[0028] Furthermore, the temperature of the alkaline solution in the present invention is any value within the range of 40°C - 100°C. By setting the temperature of the alkaline solution within the above range, the hydrolysis rate of the biomass raw material can be increased, the reaction time can be reduced, and thus the process of the carbon negative electrode material can be shortened.

[0029] The above description is only an overview of the technical solution of the present invention. In order to understand the technical means of the present invention more clearly and implement it according to the content of the specification, the following describes the preferred embodiments of the present invention in detail with reference to the accompanying drawings. Brief Description of the Drawings

[0030] Some specific embodiments of the present invention will be described in detail hereinafter with reference to the accompanying drawings in an exemplary and non-limiting manner. The same reference numerals in the drawings denote the same or similar components or parts. Those skilled in the art should understand that these drawings are not necessarily drawn to scale. In the drawings:

[0031] Figure 1 is a schematic flowchart of a method for preparing a hard carbon negative electrode material according to an embodiment of the present invention;

[0032] Figure 2 is an XRD pattern of a biomass precursor in Example 1 and Comparative Examples 1 - 4 of the present invention;

[0033] Figure 3 is an XRD pattern of the hard carbon negative electrode materials prepared in Example 1 and Comparative Examples 1 - 4 of the present invention;

[0034] Figure 4 is an SEM image of the hard carbon negative electrode material prepared in Example 1 of the present invention;

[0035] Figure 5 is the first charge-discharge curve of the hard carbon negative electrode material prepared in Example 1 of the present invention;

[0036] Figure 6 is the first charge-discharge curve of the hard carbon negative electrode material prepared in Example 2 of the present invention;

[0037] Figure 7 is the first charge-discharge curve of the hard carbon negative electrode material prepared in Example 3 of the present invention;

[0038] Figure 8 is the first charge-discharge curve of the hard carbon negative electrode material prepared in Example 4 of the present invention;

[0039] Figure 9 is the first charge-discharge curve of the hard carbon negative electrode material prepared in Comparative Example 1 of the present invention;

[0040] Figure 10 is the first charge-discharge curve of the hard carbon negative electrode material prepared in Comparative Example 2 of the present invention;

[0041] Figure 11 is the first charge-discharge curve of the hard carbon anode material prepared according to Comparative Example 3 of the present invention;

[0042] Figure 12 is the first charge-discharge curve of the hard carbon anode material prepared according to Comparative Example 4 of the present invention. Detailed Embodiments

[0043] The following will further describe in detail the specific embodiments of the present invention with reference to the accompanying drawings and examples. The following examples are used to illustrate the present invention, but are not used to limit the scope of the present invention.

[0044] To make the above objects, features, and advantages of the present application more obvious and understandable, the following will describe in detail the specific embodiments of the present application with reference to the accompanying drawings. It can be understood that the specific embodiments described herein are only used to explain the present application, rather than limiting the present application. Additionally, it should be noted that for the sake of convenience of description, only parts related to the present application are shown in the accompanying drawings rather than all structures. Based on the embodiments in the present application, all other embodiments obtained by those of ordinary skill in the art without creative efforts fall within the scope of protection of the present application.

[0045] The terms "comprising" and "having" in the present application and any variations thereof are intended to cover non-exclusive inclusion. For example, a process, method, system, product, or device that includes a series of steps or units is not limited to the listed steps or units, but may optionally further include steps or units not listed, or may optionally further include other steps or units inherent to these processes, methods, products, or devices.

[0046] Referring to "embodiments" in this context means that the specific features, structures, or characteristics described in connection with the embodiments can be included in at least one embodiment of the present application. The phrase appears at various positions in the specification does not necessarily refer to the same embodiment, nor is it an independent or alternative embodiment mutually exclusive with other embodiments. Those skilled in the art explicitly and implicitly understand that the embodiments described herein can be combined with other embodiments.

[0047] Figure 1 is a schematic flowchart of a method for preparing a hard carbon anode material according to an embodiment of the present invention, Figure 2 is the XRD pattern of the biomass precursor in Example 1 and Comparative Examples 1-4 of the present invention, Figure 3 is the XRD pattern of the hard carbon anode material prepared in Example 1 and Comparative Examples 1-4 of the present invention, Figure 4 is the SEM image of the hard carbon anode material prepared according to Example 1 of the present invention, Figure 5is the first charge-discharge curve of the hard carbon anode material prepared according to Embodiment 1 of the present invention, Figure 6 is the first charge-discharge curve of the hard carbon anode material prepared according to Embodiment 2 of the present invention, Figure 7 is the first charge-discharge curve of the hard carbon anode material prepared according to Embodiment 3 of the present invention, Figure 8 is the first charge-discharge curve of the hard carbon anode material prepared according to Embodiment 4 of the present invention, Figure 9 is the first charge-discharge curve of the hard carbon anode material prepared according to Comparative Example 1 of the present invention, Figure 10 is the first charge-discharge curve of the hard carbon anode material prepared according to Comparative Example 2 of the present invention, Figure 11 is the first charge-discharge curve of the hard carbon anode material prepared according to Comparative Example 3 of the present invention, Figure 12 is the first charge-discharge curve of the hard carbon anode material prepared according to Comparative Example 4 of the present invention.

[0048] As Figure 1 shown, the present invention provides a method for preparing a biomass-derived hard carbon anode material, comprising the following steps:

[0049] Step S100: Add a biomass raw material and an oxidant to an alkaline solution to obtain a hydrolysis mixed solution;

[0050] Step S200: Perform sonication activation treatment on the hydrolysis mixed solution, and sequentially perform washing and drying to prepare a biomass precursor;

[0051] Step S300: Perform pre-carbonization and pulverization treatment on the biomass precursor in sequence to prepare a low-temperature pyrolysis carbon;

[0052] Step S400: Mix the low-temperature pyrolysis carbon with an organic acid salt to obtain a coated precursor, and perform gradient calcination on the coated precursor to prepare a high-temperature pyrolysis carbon;

[0053] Step S500: Perform pickling treatment, water washing treatment and drying treatment on the high-temperature pyrolysis carbon to prepare a hard carbon anode material; wherein, the gradient calcination includes a first gradient, a second gradient and a third gradient, the calcination temperature of the first gradient is the melting temperature of the organic acid salt, the calcination temperature of the second gradient is the reduction temperature or decomposition temperature of the metal oxide or metal carbonate, and the calcination temperature of the third gradient is the forming temperature of the hard carbon anode material and the metal gasification temperature.

[0054] In this embodiment, in the preparation method of the biomass-derived hard carbon anode material, first, the biomass raw material and the oxidant are added to the alkaline solution, and after mixing evenly, a hydrolysis mixed solution is obtained. The hydrolysis mixed solution is subjected to acoustic activation treatment, and then washed and dried in sequence to prepare a biomass precursor. The biomass precursor is subjected to pre-carbonization and pulverization treatment in sequence to obtain low-temperature pyrolysis carbon. The low-temperature pyrolysis carbon is mixed with the organic acid salt to obtain a coated precursor. The coated precursor is subjected to gradient calcination to obtain high-temperature pyrolysis carbon. The high-temperature pyrolysis carbon is subjected to pickling treatment, water washing treatment and drying treatment to obtain the hard carbon anode material.

[0055] In this embodiment, through the synergistic effect of adding the biomass raw material and the oxidant to the alkaline solution, subjecting the hydrolysis mixed solution to acoustic activation treatment, and subjecting the coated precursor coated with the organic acid salt to gradient calcination, the oxidant forms highly oxidizing reactive oxygen species under alkaline conditions. The reactive oxygen species break the chemical bonds in lignin and hemicellulose through oxidation. And the acoustic activation treatment can significantly enhance the mass transfer and diffusion performance of the alkaline solution, promote the alkaline solution and the reactive oxygen species to penetrate into the fiber tissue or between carbon layers and carbon pores, and promote the hydrolysis reaction rate of the molecular structure in the biomass raw material. While reducing the relative contents of lignin and hemicellulose, the relative content of cellulose is increased, which can increase the graphite domain area after high-temperature carbonization. While increasing the graphitization degree, the graphite domains shrink and wind at high temperature, promoting the generation of closed pores, and then increasing the sodium storage capacity in the hard carbon plateau region. And gradient calcination of the coated precursor can promote the filling and encapsulation of the open pores in the hard carbon and the etching of the nanopores after filling, thereby optimizing the pore characteristics of the hard carbon anode material, improving its sodium storage performance, and enhancing the cycle stability and rate performance.

[0056] In this embodiment, the biomass raw material is hydrolyzed in the mixed solution of the oxidant and the alkaline solution to regulate the composition of the biomass precursor and increase the relative content of cellulose in the biomass precursor, thereby increasing the graphite domain area after high-temperature carbonization, increasing the number of closed pores in the graphite domain area, and then increasing the graphitization degree, so as to improve the electronic and ionic conductivities of the hard carbon anode material. And the higher number of closed pores can increase the filling-type sodium storage amount of the hard carbon, and then increase the sodium storage capacity in the hard carbon plateau region, thereby significantly improving the electrochemical performance of the hard carbon anode material.

[0057] In this embodiment, the biomass is pre-oxidized and carbonized with a strong oxidant to introduce oxygen-rich functional groups on the carbon surface, effectively improving the interfacial compatibility of the hard carbon anode material, enhancing the wettability of the electrolyte to the hard carbon, thereby promoting the transport of electrolyte ions on the electrode surface and optimizing the electrochemistry reaction kinetics. In addition, the oxygen-rich functional groups can serve as active sites to provide reaction sites for subsequent coating, composite or doping, thereby further regulating the physical and chemical properties of the hard carbon anode material to optimize its sodium storage performance and rate performance.

[0058] In this embodiment, the acoustic activation treatment includes multiple intermittent activation treatments and static treatments, with a static treatment after each activation treatment. The activation time is any value from 3 min to 9 min, the static time is any value from 1 min to 7 min, and the total time of the acoustic activation treatment is any value from 15 min to 60 min. By performing the acoustic activation treatment with alternating activation and static treatments on the hydrolysis mixed solution, the alkaline solution and reactive oxygen species can not only continuously penetrate into the molecular structure of the biomass raw material, but also stay in the molecular structures at different depths of the biomass raw material for the static time, so that the alkaline solution and reactive oxygen can act on the molecular structures of the biomass raw material at different depths, further promoting the hydrolysis reaction of the biomass raw material and enabling the complete hydrolysis of the biomass raw material. Here, the activation time can be 3 min, 4 min, 5 min, 6 min, 7 min, 8 min or 9 min, or any value from 3 min to 9 min; the static time can be 1 min, 2 min, 3 min, 4 min, 5 min, 6 min or 7 min, or any value from 1 min to 7 min; the total time of the acoustic activation treatment can be 15 min, 20 min, 25 min, 30 min, 40 min, 50 min or 60 min, or any value from 15 min to 60 min.

[0059] In this embodiment, during the acoustic activation treatment, since the hydrolysis mixed solution can generate microbubbles under the cavitation effect of ultrasonic waves, the bubbles rapidly expand and burst under the action of sound waves, forming a local high-temperature and high-pressure environment, which can promote the destruction of the molecular structure of the biomass raw material and the progress of its decomposition and hydrolysis reaction. In addition, it can also promote an increase in the contact area between the biomass raw material and the alkaline solution, accelerate the reaction rate between the biomass raw material and the alkaline solution, and greatly shorten the hydrolysis reaction time.

[0060] In this embodiment, the sound wave frequency of the acoustic activation treatment in step S200 is any value from 20 kHz to 100 kHz, and the power is any value from 300 W to 1000 W. That is, the sound wave frequency of the acoustic activation treatment can be 20 kHz, 30 kHz, 40 kHz, 70 kHz, 90 kHz or 100 kHz, or any value from 20 kHz to 100 kHz; the power can be 300 W, 400 W, 500 W, 700 W, 900 W or 1000 W, or any value from 300 W to 1000 W. By setting the sound wave frequency and power of the acoustic activation treatment within the above ranges, it is ensured that the alkaline solution and reactive oxygen uniformly penetrate into all parts of the biomass raw material, so that the biomass raw material is completely hydrolyzed, improving the uniformity of the pore structure distribution of the hard carbon after high-temperature carbonization, thereby increasing the specific capacity, reducing structural defects, and improving the first Coulomb efficiency and cycle life.

[0061] In this embodiment, in step S200, the product after acoustic activation treatment is washed with clear water until the pH of the product is less than 9.0, and then dried. By removing the residual alkaline solution in the hydrolysis product of the biomass raw material, the purity of the biomass precursor is improved, thereby increasing the graphitization degree after high-temperature carbonization. At the same time, it prevents the formation of alkaline crystals in the biomass precursor after drying due to excessive alkaline substances, preventing the formation of the pore structure of the hard carbon negative electrode material from being affected. That is, by washing the product after acoustic activation treatment to a pH less than 9.0, it can ensure that there are no high-concentration alkaline salts on the surface of the biomass precursor, so as to form a uniformly distributed pore structure during the carbonization process and optimize the sodium storage channels.

[0062] In this embodiment, the drying temperature is any value between 60°C and 180°C, and the drying time is any value between 1 h and 12 h. That is, the drying temperature can be 60°C, 80°C, 100°C, 120°C, 150°C or 180°C, or any value between 60°C and 180°C, and the drying time can be 1 h, 2 h, 3 h, 5 h, 7 h, 10 h or 12 h, or any value between 1 h and 12 h. By setting the drying temperature and drying time within the above ranges, it prevents the biomass precursor containing a large amount of water from directly entering the carbonization step, where the rapid vaporization of water causes the material to crack, the pores to be uneven, or even local sintering, improving the carbonization uniformity, avoiding the collapse of the pore structure, and increasing the specific capacity. At the same time, it avoids the destruction of the cellulose structure formed by hydrolysis due to too high a drying temperature or too long a drying time, or the failure to achieve the effect of removing the water in the biomass precursor due to too low a drying temperature or too short a drying time, thereby further improving the purity of the biomass precursor.

[0063] In this embodiment, the biomass precursor is pre-carbonized so that the biomass precursor forms a carbon morphology by pre-carbonization at a lower temperature, which is convenient for subsequent pore filling, coating or other modification methods.

[0064] In this embodiment, the calcination temperature of the first gradient is the melting temperature of the organic acid salt, so that the organic acid salt melts and diffuses into the open pore region of the hard carbon to fill the nano-pores of the low-temperature pyrolytic carbon, forming a preliminary pore sealing structure, preliminarily reducing the structural defects and specific surface area of the hard carbon negative electrode material, preventing the material from shrinking excessively or the structure from collapsing during subsequent calcination, enhancing the integrity of the carbon skeleton, and the molten organic acid salt forms a uniform coating layer on the surface of the hard carbon, forming a uniform protective layer on the surface of the low-temperature pyrolytic carbon material to prevent particle agglomeration.

[0065] In this embodiment, the calcination temperature of the first gradient is any value between 150°C and 200°C, the heating rate is between 1°C / min and 10°C / min, and the heat preservation time is any value between 0.5 h and 6 h. That is, the calcination temperature of the first gradient is 150°C, 160°C, 170°C, 180°C, 190°C or 200°C, or it can also be any value between 150°C and 200°C. The heating rate can be 1°C / min, 3°C / min, 5°C / min, 7°C / min, 9°C / min or 10°C / min, or it can also be between 1°C / min and 10°C / min. The heat preservation time can be 0.5 h, 1 h, 3 h, 5 h or 6 h, or it can also be any value between 0.5 h and 6 h. By setting the calcination temperature, heating rate and heat preservation time of the first gradient within the above ranges, it helps the complete melting of the organic acid salt and enables it to fill into the cavities of the low-temperature pyrolytic carbon to reduce structural defects and specific surface area. At the same time, the molten organic acid salt is uniformly coated on the surface of the low-temperature pyrolytic carbon to further fill the pores, thereby improving the structural uniformity of the low-temperature pyrolytic carbon and optimizing the distribution of sodium storage sites. At the same time, it prevents the calcination temperature of the first gradient from being too low to cause the incomplete melting of the organic acid salt, and the too short heat preservation time to cause the molten organic acid salt to not be fully filled into the pores and seal the pores. It can also avoid the situation that the calcination temperature of the first gradient is too high, resulting in the premature decomposition or gasification of the metal in the organic acid salt, leading to the inability to fill the pores.

[0066] In this embodiment, the calcination temperature of the second gradient is the reduction temperature or decomposition temperature of the metal oxide or metal carbonate. That is, under the condition of the calcination temperature of the second gradient, the metal carbonate in the organic acid salt decomposes, releasing carbon dioxide gas and metal oxide. The released carbon dioxide gas forms nano-scale pores inside the hard carbon, optimizing the sodium storage capacity of the material. And part of the metal oxide is reduced or partially transformed into gaseous metal to further optimize and regulate the pore structure, improve the ion diffusion rate and enhance the rate performance. At the same time, under the above temperature conditions, it can also remove some volatile organic compounds, improve the stability of the carbon skeleton, make it more conducive to the insertion of sodium ions, and thus improve the specific capacity of the hard carbon negative electrode material.

[0067] In this embodiment, the calcination temperature of the second gradient is any value in the range of 700°C to 900°C, the heating rate is 1°C / min to 10°C / min, and the holding time is any value in the range of 0.5 h to 6 h. That is, the calcination temperature of the second gradient is 700°C, 750°C, 800°C, 850°C or 900°C, or it can also be any value in the range of 700°C to 900°C. The heating rate can be 1°C / min, 3°C / min, 5°C / min, 7°C / min, 9°C / min or 10°C / min, or it can also be 1°C / min to 10°C / min. The holding time can be 0.5 h, 1 h, 3 h, 5 h or 6 h, or it can also be any value in the range of 0.5 h to 6 h. By setting the calcination temperature, heating rate and holding time of the second gradient within the above ranges, it helps to completely decompose metal carbonates into metal oxides or reduce them to metals, so as to preliminarily optimize the pore structure of low-temperature pyrolytic carbon under the condition of the calcination temperature of the second gradient. While increasing the number of nanopores, it prevents structural defects caused by the formation of large pores, thereby increasing the sodium storage sites of the hard carbon anode material while reducing structural defects. In addition, setting the heating rate within the above range can prevent uneven decomposition of metal carbonates / oxides due to too fast heating of the calcination temperature of the second gradient, thus affecting the stability of the pore structure. It can also prevent the excessive close packing of the structure between carbon layers, affecting the layer spacing, or forming local over-graphitized regions, affecting the electronic conductivity of the overall hard carbon anode material.

[0068] In this embodiment, since the calcination temperature of the third gradient is the key temperature for the final shaping of the hard carbon anode material, it optimizes the disordered arrangement between carbon layers to form a short-range ordered structure or a long-range disordered structure suitable for sodium storage. And the calcination temperature of the third gradient serves as the metal vaporization temperature, and the metal in the organic acid salt vaporizes under the temperature condition of the third gradient, etching out more nanopores to further optimize the pore structure of the hard carbon anode material and ensure the reversible insertion or extraction of sodium ions. Moreover, the calcination temperature of the third gradient can promote the short-range ordering of the hard carbon material, improve its electronic conductivity, optimize the rate performance and enhance the cycle life.

[0069] In this embodiment, the calcination temperature of the third gradient is any value between 1100°C and 1700°C, the heating rate is between 1°C / min and 10°C / min, and the heat preservation time is any value between 1 h and 12 h. That is, the calcination temperature of the third gradient is 1100°C, 1200°C, 1300°C, 1500°C or 1700°C, or it can also be any value between 1100°C and 1700°C. The heating rate can be 1°C / min, 3°C / min, 5°C / min, 7°C / min, 9°C / min or 10°C / min, or it can also be between 1°C / min and 10°C / min. The heat preservation time can be 1 h, 3 h, 5 h, 10 h or 12 h, or it can also be any value between 1 h and 12 h. By setting the calcination temperature, heating rate and heat preservation time of the third gradient within the above ranges, it helps with the disordered arrangement between carbon layers and the final shaping of the hard carbon anode material, preventing local graphitization of the hard carbon anode material caused by too fast heating rate, reducing the electronic conductivity. At the same time, it prevents the structure of the hard carbon anode material from being unstable due to too short heat preservation time or incomplete gasification of metals, which affects the specific capacity and pore structure, thus avoiding the influence on the sodium storage performance of the hard carbon anode material.

[0070] In this embodiment, the pickling treatment in step S500 is to mix and stir the high-temperature pyrolytic carbon with the acid solution to remove the original metal ash of the biomass and the metal impurities remaining from the etching of organic acid salts. Here, the concentration of the acid solution is any value between 1 mol / L and 5 mol / L. The concentration of the acid solution can be 1 mol / L, 2 mol / L, 3 mol / L, 4 mol / L or 5 mol / L, or it can also be any value between 1 mol / L and 5 mol / L. The pickling temperature is any value between 25°C and 80°C. The pickling temperature can be 25°C, 30°C, 40°C, 60°C or 80°C, or it can also be any value between 25°C and 80°C. The pickling time is any value between 0.2 h and 8 h. The pickling time can be 0.2 h, 0.5 h, 1 h, 2 h, 4 h or 8 h, or it can also be any value between 0.2 h and 8 h. The acid solution is any one or several of sulfuric acid, nitric acid, hydrochloric acid, acetic acid, formic acid, citric acid, phosphoric acid.

[0071] In this embodiment, the temperature of the drying treatment is any value between 100°C and 200°C, and the drying time is any value between 1 h and 24 h. By setting the drying temperature higher than the boiling point of water, the moisture in the high-temperature pyrolytic carbon can be completely removed, and by setting the drying time within the above range, the high-temperature pyrolytic carbon can be fully dried. Here, the temperature of the drying treatment can be 100°C, 120°C, 140°C, 160°C, 180°C or 200°C, or it can also be any value between 100°C and 200°C. The drying time can be 1 h, 2 h, 5 h, 10 h, 15 h, 20 h or 24 h, or it can also be any value between 1 h and 24 h.

[0072] In a further embodiment, the mass ratio of the organic acid salt to the low-temperature pyrolytic carbon is any value from 1 wt% to 20 wt%, that is, the mass ratio of the organic acid salt to the low-temperature pyrolytic carbon can be 1 wt%, 2 wt%, 5 wt%, 10 wt%, 12 wt%, 15 wt%, 17 wt% or 20 wt%, or can also be any value from 1 wt% to 20 wt%. By setting the mass ratio of the organic acid salt to the low-temperature pyrolytic carbon within the above range, metal-induced etching can be optimized, the pore structure uniformity can be improved, and the sodium storage active sites can be increased.

[0073] In a preferred embodiment, the mass ratio of the organic acid salt to the low-temperature pyrolytic carbon is any value from 5 wt% to 15 wt% to form a uniform coating layer, optimize the pores, increase the specific capacity, optimize the rate performance, and increase the first Coulombic efficiency. Here, the mass ratio of the organic acid salt to the low-temperature pyrolytic carbon can be 5 wt%, 6 wt%, 7 wt%, 8 wt%, 9 wt%, 10 wt%, 11 wt%, 12 wt%, 13 wt%, 14 wt% or 15 wt%, or can also be any value from 5 wt% to 15 wt%.

[0074] In a further embodiment, the organic acid salt is any one or a mixture of gluconates, acetates, citrates or lactates, that is, the organic acid salt can be a single organic acid salt or a mixture of multiple organic acid salts. Different types of organic acid salts are suitable for calcination and pore size optimization under different temperature conditions, that is, the melting temperature of different organic acid salts, the decomposition temperature and gasification temperature of metal carbonates, and the decomposition temperature of metal oxides or metal gasification temperature are all different, and the organic salts of different components can also regulate the pore sizes differently. A corresponding composition of one or more organic acid salts can be selected according to requirements to coat the low-temperature pyrolytic carbon.

[0075] In a further embodiment, the gluconate is at least one of zinc gluconate, calcium gluconate, potassium gluconate, ferrous gluconate, magnesium gluconate, the acetate is at least one of zinc acetate, calcium acetate, potassium acetate, magnesium acetate, the citrate is at least one of zinc citrate, calcium citrate, potassium citrate, and the lactate is at least one of zinc lactate, calcium lactate, potassium lactate, magnesium lactate. That is, the gluconates, acetates, citrates or lactates in the organic acid salts all include metal carbonates of various different metal types and can be applicable to different calcination temperatures and pore size regulations.

[0076] In a further embodiment, the mass ratio of the oxidant to the biomass raw material is any value in the range of 3 wt% to 10 wt%, that is, the mass ratio of the oxidant to the biomass raw material can be 3 wt%, 4 wt%, 5 wt%, 7 wt% or 10 wt%, or any value in the range of 3 wt% to 10 wt%. By setting the mass ratio of the oxidant to the biomass raw material within the above range, it is possible to effectively promote the full hydrolysis of the biomass raw material, regulate the surface functional groups of the biomass raw material, improve the sodium affinity of the carbon material, while preventing the excessive addition of the oxidant from causing too large pores and an increase in specific surface area, affecting the first Coulombic efficiency, and even causing excessive damage to the carbon skeleton, resulting in a decrease in the mechanical strength of the hard carbon negative electrode material and affecting the cycle life.

[0077] In a further embodiment, the oxidant is any one or a mixture of more than one of hydrogen peroxide, hypochlorite, and persulfate. The oxidation properties of oxidants of different types are different, and they are selected according to the pore size to be regulated in the hard carbon negative electrode material and the cellulose content in the biomass precursor.

[0078] In a further embodiment, the concentration of the alkaline solution is any value in the range of 0.5 mol / L to 3 mol / L, that is, the concentration of the alkaline solution can be 0.5 mol / L, 1 mol / L, 1.5 mol / L, 2 mol / L, 2.5 mol / L or 3 mol / L, or any value in the range of 0.5 mol / L to 3 mol / L. By setting the concentration of the alkaline solution within the above range, the alkaline solution can promote the hydrolysis of hemicellulose and part of lignin in the biomass, improve the reaction activity of cellulose, and can also introduce functional groups such as hydroxyl groups and ether bonds on the surface of the biomass, improve the polarity of the carbon material, contribute to the infiltration of the electrolyte for sodium ion storage, increase the sodium ion migration rate, optimize the rate performance, and reduce the irreversible capacity loss.

[0079] In a further embodiment, the alkaline solution is an aqueous solution of one or a mixture of more than one of metal hydroxides or metal carbonates. In the preparation process of the hard carbon negative electrode material, using an aqueous solution of metal hydroxide or metal carbonate as the alkaline solution can effectively regulate the hydrolysis process, pore structure and final electrochemical performance of the biomass. Moreover, different types of metal hydroxides and metal carbonates have different alkalinities and reaction characteristics. By reasonably mixing and using them, the specific capacity, rate performance and cycle stability of the hard carbon negative electrode material can be further optimized. Here, the metal hydroxide can be sodium hydroxide or potassium hydroxide, and the metal carbonate can be sodium carbonate or potassium carbonate.

[0080] In a further embodiment, the temperature of the alkaline solution is any value between 40°C and 100°C, that is, the temperature of the alkaline solution can be 40°C, 50°C, 60°C, 80°C or 100°C, or any value between 40°C and 100°C. By setting the temperature of the alkaline solution within the above range, the hydrolysis rate of the biomass raw material can be increased, the reaction time can be reduced, and thus the process of the carbon negative electrode material can be shortened.

[0081] In a further embodiment, the biomass raw material is any one or a mixture of more than one of bamboo, wood, reed, coffee grounds, sugarcane bagasse, wheat straw, rice straw, corn straw, corn cob, coconut shell, durian shell, rice husk, hazelnut shell, starch, lignin, etc. The proportions of cellulose, hemicellulose and lignin in the above various biomass raw materials are different, which determine the interlayer structure and pore distribution of the hard carbon during the carbonization process. By selecting different biomass raw materials, the sodium storage sites can be optimized and the specific capacity can be improved.

[0082] The present invention also provides a sodium ion battery. The sodium ion battery includes a biomass-derived hard carbon negative electrode material as the negative electrode active material. The first discharge specific capacity of the sodium ion battery is higher than 358 mAh / g, the first Coulomb efficiency is greater than 92.4%, and the capacity retention rate after 2000 cycles is higher than 91.3%. Regarding the preparation method of the hard carbon negative electrode material, it will not be elaborated here one by one.

[0083] In this embodiment, in the preparation method of the sodium ion battery, using the hard carbon negative electrode material prepared in the above embodiment as the negative electrode active material, after weighing the negative electrode active material, conductive agent, first binder and second binder according to a mass ratio of 95:2:2:1, deionized water is added and stirred for 6 h to obtain a uniform black slurry. The black slurry is in a viscous paste state. The above black slurry is evenly coated on the copper foil, and then vacuum dried at 80°C for 6 h to obtain a pole piece with a uniform thickness. Then the pole piece is punched into a circular piece with a diameter of 14 mm as the negative electrode. Using a metal circular piece with a diameter of 14 mm as the negative electrode, glass fiber as the separator, and 1 mol / L NaPF6 as the electrolyte, a CR2025 type button battery is assembled in a glove box filled with argon. After the battery is left standing for 24 h, the charge-discharge performance test is carried out. Here, the conductive agent is acetylene black, the first binder is sodium carboxymethyl cellulose, and the second binder is styrene-butadiene rubber.

[0084] Hereinafter, the present application will be further described in detail with specific embodiments.

[0085] Example 1

[0086] The bamboo powder passed through a 50-mesh sieve was poured into a 3 mol / L sodium hydroxide solution, and sodium hypochlorite accounting for 5 wt% of the mass of the bamboo powder was added. It was sonicated and infiltrated at 75 °C, with a sonic frequency of 100 kHz and a power of 1000 W. It was allowed to stand for 1 min every 9 min of sonication activation, and the total time of sonication activation and standing was 60 min. Then it was taken out, filtered, washed until neutral, and dried to obtain a biomass precursor. The biomass precursor was heated to 600 °C at a rate of 5 °C / min in a pyrolysis furnace under a nitrogen atmosphere and held for 2 h, then naturally cooled, and pulverized to obtain low-temperature pyrolytic carbon. The low-temperature pyrolytic carbon was fully mixed with zinc lactate with a mass fraction of 20 wt%. Then it was heated to 200 °C at a rate of 5 °C / min in a tubular furnace under a nitrogen atmosphere and held for 1 h, then heated to 800 °C at a rate of 5 °C / min and held for 2 h, and then heated to 1200 °C at a rate of 5 °C / min and held for 2 h. After natural cooling, high-temperature pyrolytic carbon was obtained. The high-temperature pyrolytic carbon was stirred with a 5 mol / L hydrochloric acid solution at 80 °C for 10 min, then washed with water until neutral, and dried at 100 °C for 1 h to prepare a biomass-derived hard carbon anode material.

[0087] Example 2

[0088] The bamboo powder passed through a 50-mesh sieve was poured into a 0.5 mol / L sodium hydroxide solution, and sodium hypochlorite accounting for 3 wt% of the mass of the bamboo powder was added. It was sonicated and infiltrated at 75 °C, with a sonic frequency of 20 kHz and a power of 450 W. It was allowed to stand for 2 min every 3 min of sonication treatment, and the total time of sonication treatment and standing was 15 min. Then it was taken out, filtered, washed until neutral, and dried to obtain a biomass precursor. The biomass precursor was heated to 800 °C at a rate of 20 °C / min in a pyrolysis furnace under a nitrogen atmosphere and held for 1 h, then naturally cooled, and pulverized to obtain low-temperature pyrolytic carbon. The low-temperature pyrolytic carbon was fully mixed with zinc lactate with a mass fraction of 1 wt%. Then it was heated to 200 °C at a rate of 5 °C / min in a tubular furnace under a nitrogen atmosphere and held for 1 h, then heated to 800 °C at a rate of 10 °C / min and held for 2 h, and then heated to 1200 °C at a rate of 5 °C / min and held for 2 h. After natural cooling, high-temperature pyrolytic carbon was obtained. The high-temperature pyrolytic carbon was stirred with a 5 mol / L hydrochloric acid solution at 25 °C for 10 min, then washed with water until neutral, and dried at 200 °C for 1 h to prepare a biomass-derived hard carbon anode material.

[0089] Example 3

[0090] The bamboo powder passed through a 150-mesh sieve was poured into a 3 mol / L sodium hydroxide solution, and sodium hypochlorite accounting for 10 wt% of the mass of the bamboo powder was added. It was sonicated and infiltrated at 40 °C, with a sonic frequency of 20 kHz and a power of 450 W. It was left to stand for 1 minute every 5 minutes of sonication activation, and the total time of sonication activation and standing was 45 minutes. Then it was taken out, filtered, washed until neutral, and dried to obtain a biomass precursor. The biomass precursor was heated to 400 °C at a rate of 5 °C / min in a pyrolysis furnace under a nitrogen atmosphere and held for 24 hours, then naturally cooled, and pulverized to obtain low-temperature pyrolytic carbon. The low-temperature pyrolytic carbon was fully mixed with zinc gluconate with a mass fraction of 20 wt%. Then it was heated to 180 °C at a rate of 1 °C / min in a tubular furnace under a nitrogen atmosphere and held for 1 hour, then heated to 800 °C at a rate of 10 °C / min and held for 2 hours, and then heated to 1100 °C at a rate of 10 °C / min and held for 3 hours. After natural cooling, high-temperature pyrolytic carbon was obtained. The high-temperature pyrolytic carbon was stirred with a 1 mol / L hydrochloric acid solution at 25 °C for 4 hours, then washed with water until neutral, and dried at 120 °C for 6 hours to prepare a biomass-derived hard carbon anode material.

[0091] Example 4

[0092] The bamboo powder passed through a 150-mesh sieve was poured into a 1 mol / L sodium hydroxide solution, and sodium hypochlorite accounting for 10 wt% of the mass of the bamboo powder was added. It was sonicated and infiltrated at 40 °C, with a sonic frequency of 20 kHz and a power of 450 W. It was left to stand for 1 minute every 4 minutes of sonication activation, and the total time of sonication activation and standing was 30 minutes. Then it was taken out, filtered, washed until neutral, and dried to obtain a biomass precursor. The biomass precursor was heated to 400 °C at a rate of 5 °C / min in a pyrolysis furnace under a nitrogen atmosphere and held for 2 hours, then naturally cooled, and pulverized to obtain low-temperature pyrolytic carbon. The low-temperature pyrolytic carbon was fully mixed with zinc gluconate with a mass fraction of 10 wt%. Then it was heated to 180 °C at a rate of 5 °C / min in a tubular furnace under a nitrogen atmosphere and held for 1 hour, then heated to 800 °C at a rate of 5 °C / min and held for 2 hours, and then heated to 1700 °C at a rate of 5 °C / min and held for 1 hour. After natural cooling, high-temperature pyrolytic carbon was obtained. The high-temperature pyrolytic carbon was stirred with a 1 mol / L hydrochloric acid solution at 25 °C for 4 hours, then washed with water until neutral, and dried at 120 °C for 6 hours to prepare a biomass-derived hard carbon anode material.

[0093] Example 5

[0094] The difference between Example 5 and Example 1 is only that it is left to stand for 1 minute every 4 minutes of sonication activation during sonication treatment, the total time of sonication treatment and standing is 45 minutes, and the mass fraction of zinc lactate is 10 wt%.

[0095] Example 6

[0096] Example 6 is only different from Example 1 in that the biomass raw material is waste wood blocks with a size less than 5 mm.

[0097] Example 7

[0098] Example 7 is only different from Example 1 in that the biomass raw material is coconut shell powder with a mesh size of 100.

[0099] Comparative Example 1

[0100] Comparative Example 1 is only different from Example 1 in that the biomass raw material with a mesh size of 50 is directly pre-carbonized.

[0101] Comparative Example 2

[0102] Comparative Example 2 is only different from Example 1 in that the hydrolysis mixed solution is not subjected to sonication activation treatment.

[0103] Comparative Example 3

[0104] Comparative Example 3 is only different from Example 1 in that the hydrolysis mixed solution does not include sodium hypochlorite.

[0105] Comparative Example 4

[0106] Comparative Example 4 is only different from Example 1 in that the low-temperature pyrolytic carbon is not coated with zinc lactate and carbonized and etched.

[0107] Comparative Example 5

[0108] Comparative Example 5 is only different from Example 1 in that the bamboo powder is first mixed and reacted with the alkaline solution for a preset time, and then the oxidant is added to the reaction solution.

[0109] The hard carbon anode materials prepared in Examples 1-7 and Comparative Examples 1-5 are respectively used as the anode active materials of the corresponding sodium-ion batteries, and the electrochemical performance of the sodium-ion batteries prepared based on the hard carbon anode materials prepared in Examples 1-7 and Comparative Examples 1-5 is tested, and the test results of the electrochemical performance shown in Table 1 are obtained. Here, the test temperature in the electrochemical performance test is 25 °C, and the charge and discharge cut-off voltage is 0.01-2.5 V.

[0110] Table 1. Test results of the electrochemical performance of the samples prepared in Examples 1-7 and Comparative Examples 1-5

[0111]

[0112]

[0113] As shown in Table 1, the hard carbon anode materials prepared in Examples 1-7, as the anode active materials of sodium-ion batteries, can all make the first charge specific capacity of the corresponding sodium-ion batteries higher than 358.61 mAh / g, the first Coulombic efficiency higher than 92.4%, the discharge specific capacity in the first cycle at a current density of 1 A / g higher than 330.61 mAh / g, and the capacity retention rate after 2000 cycles at a current density of 1 A / g higher than 91.3%. Compared with the hard carbon anode materials prepared in Comparative Examples 1-5 as the anode active materials of sodium-ion batteries, the sodium-ion batteries prepared from the hard carbon anode materials prepared in Examples 1-7 of the present invention all have significantly improved discharge specific capacity, first Coulombic efficiency, and rate performance.

[0114] As shown in Table 1, the first discharge specific capacity, first Coulombic efficiency, and capacity retention rate in Comparative Example 1 are all significantly lower than the corresponding electrochemical performance parameters of Example 1, indicating that the separate alkaline solution treatment and oxidation treatment of the biomass raw materials in Comparative Example 1 cannot completely hydrolyze the biomass raw materials, resulting in significantly lower electrochemical performance of the sodium-ion batteries prepared from the products prepared in Comparative Example 1 than the relevant electrochemical performance of the hard carbon anode materials prepared in Example 1.

[0115] As Figure 2 shown, Figure 2 is the XRD pattern of the biomass precursor after the treatment in step S200 of Example 1 and Comparative Examples 1-4, including the (002) peak at 2θ = 22.5° and the am peak at 2θ = 18°. The peak of the (002) peak is the diffraction peak of the cellulose crystalline region, and the am peak is the diffuse diffraction peak of the amorphous region. The (002) peaks of Example 1 and Comparative Example 4 have high peak intensity and sharp peak shape, and the (002) peak and the am peak together indicate that the biomass precursor has a high cellulose crystallinity and the components such as lignin and hemicellulose are removed sufficiently. The (002) peak of Comparative Example 1 has low peak intensity and gentle peak shape, indicating that the precursor has not removed the components such as lignin and hemicellulose. The (002) peak intensity and the sharpness of the peak shape of Comparative Examples 2 and 3 exceed those of Comparative Example 1 but do not exceed those of Example 1, indicating that the lack of sonication activation and oxidation treatment cannot achieve a synergistic effect to regulate the biomass precursor.

[0116] As Figure 3 shown, Figure 3 is the XRD pattern of the hard carbon anode material after the treatment in step S500 of Example 1 and Comparative Examples 1-4, including the (002) and (100) diffraction peaks. Among them, the position of the (002) peak appears at 2θ = 22°, and the corresponding interlayer spacing is about (100) The peak position appears at 2θ = 45°, indicating that the material of the product prepared in Example 1 is hard carbon. The (002) peak of Example 1 significantly shifts to the right, and the interlayer spacing decreases, indicating that the graphitization degree of the hard carbon negative electrode material is relatively high and the order increases. This can improve the conductivity of the hard carbon negative electrode material, reduce the excessive growth of the electrolyte solid interface film, and improve the first Coulombic efficiency. Moreover, the peak intensity ratio of the (002) peak to the (100) peak is 2, that is, the intensity of the (002) peak is relatively high and the intensity of the (100) peak is relatively low, indicating that the crystal layer structure of the sample is more layered and neatly stacked, with a strong diffraction signal in the direction perpendicular to the layer plane, indicating that the atomic arrangement in the plane is relatively irregular or disordered. The peak intensity ratio of 2 further indicates the orderliness of the interlayer arrangement of the hard carbon negative electrode material prepared in Example 1, that is, the product prepared in Example 1 has a certain degree of graphitization, and the hard carbon negative electrode material is successfully prepared. The (002) peak of Comparative Example 1 does not shift significantly, and the low graphitization degree is caused by the failure to remove components such as lignin and hemicellulose. The (002) peaks of Comparative Examples 3-4 shift to the right to a certain extent, and the interlayer spacing decreases, indicating that the graphitization degree of the structure of the hard carbon negative electrode material is improved compared with that of Comparative Example 1, but the synergistic effect cannot be achieved to regulate the biomass precursor.

[0117] As Figure 4 shown, Figure 4 Figure 7 is an SEM image of the hard carbon negative electrode material after the treatment in step S500 of Example 1. The prepared hard carbon is mostly 50 μm in size, with a little lactic acid carbon sphere attached and a porous structure etched on the surface. This porous structure can serve as a surface adsorption site and a sodium ion diffusion channel, thereby increasing the sodium storage sites and improving the rate performance.

[0118] As Figures 5 to 8 shown in Figures 8 and 9 are the first charge-discharge curves of the sodium ion batteries with the hard carbon negative electrode materials prepared in Examples 1 to 4. The charge curve and the discharge curve both show a high-voltage ramp region and a low-voltage plateau-like region, and the first discharge specific capacity can reach 358.61 mAh / g, and the 1 A / g discharge specific capacity can reach 330.61 mAh / g. This shows that the pretreatment of the biomass raw material with a mixed solution of an oxidant and an alkaline solution can better hydrolyze to form lignocellulose, reduce the crystallinity of cellulose, and improve the graphitization degree. Combining organic acid salt coating and staged calcination can form more optimized nanoporous structures in the carbon material, increase the specific surface area while reducing structural defects, thereby further etching the hard carbon negative electrode material. More nanoporous structures are more conducive to sodium ion storage.

[0119] As Figure 9 and Figure 12As shown, there is no quasi-plateau region at low voltage in the charge-discharge curves of the products prepared in Comparative Example 1 and Comparative Example 4, indicating that directly pre-carbonizing and subsequent treating the biomass raw materials in Comparative Example 1 cannot completely hydrolyze the biomass raw materials, and the failure to coat organic acid salts and perform gradient calcination on the coating precursor both result in the product after subsequent calcination not being graphitized hard carbon.

[0120] As Figure 10 and Figure 11 shown, the quasi-plateau regions at low voltage in the charge-discharge curves of the products prepared in Comparative Example 2 and Comparative Example 3 are both short, indicating that although sonication activation treatment or alkaline solution treatment can hydrolyze the biomass raw materials to a certain extent, they cannot completely hydrolyze the biomass raw materials, thereby reducing the cellulose content in the biomass precursor, resulting in a decrease in the degree of graphitization, and thus affecting the electrochemical performance of sodium-ion batteries manufactured based on the products of Comparative Example 2 and Comparative Example 3.

[0121] The technical features of the above-described embodiments can be combined arbitrarily. For the sake of brevity of description, not all possible combinations of the technical features in the above embodiments are described. However, as long as there is no contradiction in the combination of these technical features, it should be considered as the scope described in this specification.

[0122] The above-described embodiments only represent several implementation manners of the present invention, and their descriptions are relatively specific and detailed, but they should not be construed as limiting the scope of the invention patent. It should be noted that for those of ordinary skill in the art, without departing from the concept of the present invention, several modifications and improvements can still be made, and these all belong to the protection scope of the present invention. Therefore, the protection scope of the present invention patent shall be subject to the appended claims.

Claims

1. A preparation method of a biomass-derived hard carbon anode material, characterized in that, It includes the following steps: Adding biomass raw materials and an oxidant into an alkaline solution to obtain a hydrolysis mixed solution; Performing sonication activation treatment on the hydrolysis mixed solution, and sequentially performing washing and drying to prepare a biomass precursor; Performing pre-carbonization and crushing treatment on the biomass precursor in sequence to prepare low-temperature pyrolytic carbon; Mixing the low-temperature pyrolytic carbon with an organic acid salt to obtain a coating precursor, and performing gradient calcination on the coating precursor to prepare high-temperature pyrolytic carbon; Performing pickling treatment, water washing treatment and drying treatment on the high-temperature pyrolytic carbon in sequence to prepare the hard carbon negative electrode material; wherein: The gradient calcination includes a first gradient, a second gradient and a third gradient. The calcination temperature of the first gradient is the melting temperature of the organic acid salt, the calcination temperature of the second gradient is the reduction temperature or decomposition temperature of the metal oxide or metal carbonate, and the calcination temperature of the third gradient is the forming temperature of the hard carbon negative electrode material and the metal gasification temperature.

2. The method for preparing a hard carbon negative electrode material according to claim 1, wherein The mass ratio of the organic acid salt to the low-temperature pyrolytic carbon is any value from 1 wt% to 20 wt%.

3. The method for preparing a hard carbon negative electrode material according to claim 2, wherein The organic acid salt is any one or a mixture of gluconates, acetates, citrates or lactates.

4. The method for preparing a hard carbon negative electrode material according to claim 3, wherein The gluconate is at least one of zinc gluconate, calcium gluconate, potassium gluconate, ferrous gluconate, magnesium gluconate; The acetate is at least one of zinc acetate, calcium acetate, potassium acetate, magnesium acetate; The citrate is at least one of zinc citrate, calcium citrate, potassium citrate; The lactate is at least one of zinc lactate, calcium lactate, potassium lactate, magnesium lactate.

5. The method for preparing a hard carbon negative electrode material according to claim 4, wherein The mass ratio of the oxidant to the biomass raw material is any value from 3 wt% to 10 wt%.

6. The method for preparing a hard carbon negative electrode material according to claim 5, wherein The oxidant is any one or a mixture of hydrogen peroxide, hypochlorite, persulfate.

7. The method for preparing a hard carbon negative electrode material according to any one of claims 1-6, wherein The concentration of the alkaline solution is any value from 0.5 mol / L to 3 mol / L, and the temperature of the alkaline solution is any value from 40 °C to 100 °C.

8. The method for preparing a hard carbon negative electrode material according to claim 7, wherein The alkaline solution is an aqueous solution of one or a mixture of metal hydroxides or metal carbonates.

9. The method for preparing a hard carbon negative electrode material according to claim 8, wherein The biomass raw material is any one or a mixture of bamboo, wood, reed, coffee grounds, bagasse, wheat straw, rice straw, corn straw, corn cob, coconut shell, durian shell, rice husk, hazelnut shell, starch, lignin.

10. A sodium-ion battery, characterized in that, The hard carbon negative electrode material obtained by the preparation method of the hard carbon negative electrode material according to any one of claims 1-9.

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