Preparation method of hard carbon composite material, hard carbon composite material, negative electrode sheet

Abstract

The application provides a preparation method of a hard carbon composite material, the hard carbon composite material and a negative electrode sheet, and the preparation method comprises the following steps: adding nanocellulose into a dimethylformamide alkaline solution containing bromopyrrole, and obtaining pyrrole grafted nanocellulose after reaction, wherein the nanocellulose is prepared from bamboo fibers; adding the pyrrole grafted nanocellulose and a linear high molecular polymer into a solvent for stirring, and then adding a silver nitrate solution, and obtaining a nanocellulose-linear high molecular polymer spherical composite material loaded with silver ions after spray drying; performing first carbonization treatment on the spherical composite material in an inert gas atmosphere to obtain a precursor material, and then mixing the precursor material with a soft carbon material, and performing second carbonization treatment on the mixture in the inert gas atmosphere to obtain the hard carbon composite material.

Description

Technical Field

[0001] This application relates to the field of electrochemical technology, and in particular to a method for preparing a hard carbon composite material, the hard carbon composite material, and a negative electrode sheet. Background Technology

[0002] Sodium-ion batteries, with their advantages of low cost, abundant sodium resources, and relatively high energy density, are expected to replace traditional lithium-ion batteries in the field of energy storage.

[0003] Hard carbon materials, due to their low degree of graphitization and underdeveloped layered structure, have larger interlayer spacing than ordinary graphite, providing sufficient space for sodium ion storage and deintercalation, and maintaining good stability during sodium ion insertion and extraction. Therefore, compared to other graphitic carbon materials, hard carbon materials are more suitable as anode materials for sodium-ion batteries. However, in practical applications, hard carbon materials still suffer from problems such as low initial coulombic efficiency, poor rate performance, and poor cycle stability when used as anode materials for sodium-ion batteries. Summary of the Invention

[0004] To address the aforementioned technical problems, this application discloses a method for preparing a hard carbon composite material, the hard carbon composite material, and a negative electrode sheet, in order to at least solve the problems of poor rate performance and poor cycle stability of existing hard carbon materials.

[0005] In the first aspect, this application provides a method for preparing a hard carbon composite material, comprising the following steps:

[0006] Preparation of pyrrole-grafted nanocellulose: Nanocellulose is added to an alkaline solution of dimethylformamide containing bromopyrrole, and after reaction, pyrrole-grafted nanocellulose is obtained, wherein the nanocellulose is made from bamboo fiber;

[0007] Preparation of spherical composite material: The pyrrole-grafted nanocellulose and linear polymer were added to a solvent and stirred, then silver nitrate solution was added, and after spray drying, a nanocellulose-linear polymer spherical composite material loaded with silver ions was obtained.

[0008] Preparation of hard carbon composite material: The spherical composite material is subjected to a first carbonization treatment under an inert gas atmosphere to obtain a precursor material. The precursor material is then mixed with soft carbon material and subjected to a second carbonization treatment under an inert gas atmosphere to obtain the hard carbon composite material.

[0009] In one optional embodiment of this application, the stirring temperature in the preparation step of the pyrrole-grafted nanocellulose is 20°C to 40°C.

[0010] In one optional embodiment of this application, in the preparation step of the spherical composite material, the mass ratio of pyrrole-grafted nanocellulose to linear polymer is 1:0.4 to 0.7, the stirring temperature is 40℃ to 120℃, and the reaction time is 2h to 4h.

[0011] In one optional embodiment of this application, in the preparation steps of the hard carbon composite material, the temperature of the first carbonization treatment is T1, where 1000℃≤T1<1200℃, and the temperature of the second carbonization treatment is T2, where 1200℃≤T2≤1400℃.

[0012] In one optional embodiment of this application, in the preparation step of the hard carbon composite material, the mass ratio of soft carbon material to precursor material is 1:10 to 20.

[0013] In one optional embodiment of this application, the preparation process of the nanocellulose is as follows:

[0014] Bamboo fiber is added to a sulfuric acid solution with a mass concentration of 50%–70% and reacted at 40℃–60℃ for 2–4 hours to obtain nanocellulose.

[0015] In one optional embodiment of this application, after the second carbonization treatment, the mass percentage of silver in the hard carbon composite material formed is a, where 0.5% ≤ a ≤ 0.8%.

[0016] In one alternative embodiment of this application, the linear polymer is selected from at least one of polyvinylpyrrolidone, polyacrylamide, polycaprolactone, polycaprolactam, and polymethyl methacrylate.

[0017] In one alternative embodiment of this application, the solvent is selected from at least one of deionized water, ethanol, acetone, ethyl acetate, tetrahydrofuran, and dimethylformamide.

[0018] Secondly, this application provides a hard carbon composite material having a core-shell structure. The core of the core-shell structure includes a three-dimensional cross-linked conductive network carbon skeleton, in which carbon layers and elemental silver are distributed. The shell of the core-shell structure includes a coating layer formed of soft carbon material.

[0019] In one alternative embodiment of this application, the hard carbon composite material satisfies at least one of the following characteristics:

[0020] a) The electrical conductivity of the hard carbon composite material is σ, 45S / cm≤σ≤60S / cm;

[0021] b) The specific surface area of ​​the hard carbon composite material is BET, 2m². 2 / g≤BET≤4m 2 / g;

[0022] c) The tap density of the hard carbon composite material is ρ r 0.85g / cm 3 ≤ρ r ≤0.95g / cm 3 ;

[0023] d) The interlayer spacing of the hard carbon composite material is D1, 0.380nm≤D1≤0.385nm;

[0024] e) The D50 of the hard carbon composite material is D2, where 5μm≤D2≤7μm.

[0025] Thirdly, this application provides a negative electrode sheet, including a current collector and a negative electrode active material layer disposed on at least one surface of the current collector, wherein the negative electrode active material layer includes a hard carbon composite material prepared by the preparation method described in the first aspect, or includes a hard carbon composite material described in the second aspect.

[0026] Fourthly, this application provides a sodium-ion battery, the sodium-ion battery comprising the negative electrode sheet described in the third aspect.

[0027] Fifthly, this application provides a battery pack including a housing and at least one sodium-ion battery as described in the fourth aspect, the sodium-ion battery being housed within the housing.

[0028] In a sixth aspect, this application provides an electrical device including the sodium-ion battery described in the fourth aspect or the battery pack described in the fifth aspect.

[0029] Compared with the prior art, this application has at least the following beneficial effects:

[0030] This application provides a method for preparing a hard carbon composite material, the hard carbon composite material itself, and a negative electrode sheet. The method for preparing the hard carbon composite material provided in this application is based on nanocellulose biomass materials, which have a wide range of raw material sources. Nanocellulose, as a raw material, can realize high-value utilization of various waste fibers (such as bamboo fiber) and avoid environmental pollution. Furthermore, the preparation method of this application is simple, and the prepared hard carbon composite material has excellent performance and low cost, thereby improving the reversible capacity, initial coulombic efficiency, rate performance, and cycle stability of sodium-ion batteries while reducing the production cost of sodium-ion batteries. Attached Figure Description

[0031] To more clearly illustrate the technical solutions in the embodiments of this application, the drawings used in the embodiments will be briefly introduced below. Obviously, the drawings described below are only some embodiments of this application. For those skilled in the art, other drawings can be obtained based on these drawings without creative effort.

[0032] Figure 1 This is a schematic cross-sectional view of a hard carbon composite material in one embodiment of this application;

[0033] Figure 2 This is a schematic diagram of the structure of a residential energy storage system according to one embodiment of this application;

[0034] Figure 3 This is a schematic diagram of the structure of an energy storage system according to one embodiment of this application;

[0035] Figure 4 This is a scanning electron microscope (SEM) image of the hard carbon composite material prepared in Example 1.

[0036] Explanation of reference numerals in the attached figures: 1-Energy storage device, 2-Electric power conversion device, 3-First user load, 4-Second user load, 11-Three-dimensional cross-linked conductive network carbon skeleton, 12-Carbon layer, 13-Nano silver element, 20-Coating layer, 400-Energy storage system, 410-High voltage cable, 420-First electric power conversion device, 430-Second electric power conversion device. Detailed Implementation

[0037] The technical solutions of the embodiments of this application will be clearly and completely described below with reference to the accompanying drawings. Obviously, the described embodiments are only some embodiments of this application, and not all embodiments. Based on the embodiments of this application, all other embodiments obtained by those skilled in the art without creative effort are within the scope of protection of this application.

[0038] In this application, the terms "upper," "lower," "left," "right," "front," "rear," "top," "bottom," "inner," "outer," "vertical," "horizontal," "lateral," and "longitudinal" indicate the orientation or positional relationship based on the orientation or positional relationship shown in the accompanying drawings. These terms are primarily for the purpose of better describing this application and its embodiments, and are not intended to limit the indicated device, element, or component to having a specific orientation, or to be constructed and operated in a specific orientation.

[0039] Furthermore, in addition to indicating location or positional relationship, some of the aforementioned terms may also have other meanings. For example, the term "above" may also be used in some cases to indicate a certain dependency or connection relationship. Those skilled in the art can understand the specific meaning of these terms in this application based on the specific circumstances.

[0040] Furthermore, the terms "installation," "setup," "equipped with," "connection," and "linked" should be interpreted broadly. For example, they can refer to a fixed connection, a detachable connection, or an integral structure; they can refer to a mechanical connection or an electrical connection; they can refer to a direct connection or an indirect connection through an intermediate medium, or an internal connection between two devices, components, or parts. Those skilled in the art can understand the specific meaning of these terms in this application based on the specific circumstances.

[0041] Furthermore, the terms "first," "second," etc., are primarily used to distinguish different devices, elements, or components (which may be the same or different in specific type and construction), and are not intended to indicate or imply the relative importance or quantity of the indicated devices, elements, or components. Unless otherwise stated, "a plurality of" means two or more.

[0042] This application provides a method for preparing a hard carbon composite material, comprising the following steps:

[0043] Preparation of pyrrole-grafted nanocellulose: Nanocellulose is added to an alkaline solution of dimethylformamide containing bromopyrrole, and after reaction, pyrrole-grafted nanocellulose is obtained. The nanocellulose is made from bamboo fiber.

[0044] Preparation of spherical composite materials: Pyrrole-grafted nanocellulose and linear polymer were added to a solvent and stirred, then silver nitrate solution was added, and after spray drying, spherical composite materials of nanocellulose-linear polymer loaded with silver ions were obtained.

[0045] Preparation of hard carbon composite material: The spherical composite material is subjected to a first carbonization treatment under an inert gas atmosphere to obtain a precursor material. The precursor material is then mixed with soft carbon material and subjected to a second carbonization treatment under an inert gas atmosphere to obtain the hard carbon composite material.

[0046] In the preparation steps of pyrrole-grafted nanocellulose, the concentration of bromopyrrole in the dimethylformamide (DMF) alkaline solution is 20wt%–35wt%, for example, 20wt%, 25wt%, 30wt%, and 35wt%. This application does not impose any particular limitation on the amount of nanocellulose added, as long as it can react with bromopyrrole to form pyrrole-grafted nanocellulose. The reaction product can be purified by centrifugation, washing, and drying. Nanocellulose can be prepared in advance from waste bamboo fiber, which is widely available and inexpensive, thus reducing the preparation cost of hard carbon composite materials. This application does not impose any particular limitation on centrifugation, washing, and drying processes, as long as they can purify the reaction product. The DMF alkaline solution can be a DMF solution containing NaOH, for example, the NaOH content in the DMF solution is 10wt%.

[0047] In the preparation of spherical composite materials, through stirring, linear polymers and pyrrole-grafted nanocellulose can self-assemble to form a three-dimensional multi-hydrogen-bonded cross-linked network structure, thus forming a nanocellulose-linear polymer composite material. After adding silver nitrate solution, hydroxyl and amino groups on the pyrrole-grafted nanocellulose and linear polymer form coordination bonds with silver ions in silver nitrate, thereby forming a silver ion complex. This allows the silver ions to be uniformly dispersed and firmly loaded onto the nanocellulose-linear polymer composite material. In this silver ion complex, the mass percentage of nanocellulose is 49.5%–79.5%, the mass percentage of linear polymer is 20%–50%, and the mass percentage of silver nitrate is 0.2%–0.5%. Spray drying is used to prepare spherical composite materials, which is beneficial for reducing the specific surface area of ​​hard carbon composite materials and increasing the tap density of the material.

[0048] In the preparation of hard carbon composite materials, a first carbonization treatment causes condensation, cyclization, and aromatization reactions between adjacent nanocellulose nanoparticles in the spherical composite material, forming a precursor material with a spherical three-dimensional cross-linked conductive network carbon skeleton structure. Furthermore, silver ion complexes are reduced to nanoscale elemental silver during carbonization, and this nanoscale elemental silver can be uniformly and firmly loaded onto the surface and pores of the three-dimensional cross-linked conductive network carbon skeleton structure. A second carbonization treatment forms a coating layer on the surface of the precursor material, thus forming a core-shell structure. This application does not have specific limitations on the inert gas used; for example, argon can be used.

[0049] In some embodiments of this application, the inlet air temperature of the spray dryer is 100℃~180℃, for example, 100℃, 150℃, 180℃; the feed rate is 20mL / min~50mL / min, for example, 20mL / min, 30mL / min, 40mL / min, 50mL / min; and the outlet air temperature is 70℃~90℃, for example, 70℃, 80℃, 90℃. By controlling the inlet air temperature, feed rate, and outlet air temperature of the spray dryer within the above ranges, it is beneficial to obtain hard carbon composite materials with the desired specific surface area and tap density.

[0050] In some embodiments of this application, the preparation method further includes: ball milling and depolymerizing the precursor material, sieving, ball milling and mixing the treated precursor material, asphalt, and ethanol, and then drying. The above process enables the precursor material to be dispersed and less prone to agglomeration, and facilitates obtaining precursor material with the desired particle size.

[0051] In some embodiments of this application, the stirring temperature in the preparation step of pyrrole-grafted nanocellulose is 20°C to 40°C, for example, 20°C, 30°C, or 40°C. By controlling the stirring temperature within the above range, it is beneficial to ensure that the nanocellulose reacts fully with bromopyrrole to form pyrrole-grafted nanocellulose.

[0052] In some embodiments of this application, in the preparation step of the spherical composite material, the mass ratio of pyrrole-grafted nanocellulose to linear polymer is 1:0.4 to 0.7, for example, 1:0.4, 1:0.5, 1:0.6, or 1:0.7; the stirring temperature is 40℃ to 120℃, for example, 40℃, 50℃, 70℃, 90℃, 100℃, or 120℃; and the reaction time is 2h to 4h, for example, 2h, 3h, or 4h. The inventors have found that excessively low stirring temperatures or short reaction times are detrimental to the formation of a three-dimensional cross-linked conductive network structure between pyrrole-grafted nanocellulose and linear polymer; excessively high stirring temperatures or long reaction times are detrimental to reducing preparation costs. This application, by controlling the stirring temperature and stirring time within the above ranges, facilitates the formation of a three-dimensional multi-hydrogen-bonded cross-linked network structure between the linear polymer and pyrrole-grafted nanocellulose through self-assembly, thereby facilitating the subsequent formation of a three-dimensional cross-linked conductive network carbon skeleton.

[0053] In some embodiments of this application, in the preparation steps of the hard carbon composite material, the temperature of the first carbonization treatment is T1, where 1000℃≤T1<1200℃, for example, T1=1000℃, T1=1150℃, T1=1180℃; the first carbonization treatment is used to remove volatiles and form a preliminary carbon skeleton structure, therefore the temperature should not be too high. The temperature of the second carbonization treatment is T2, where 1200℃≤T2≤1400℃, for example, T2=1200℃, T2=1300℃, T2=1400℃. The inventors discovered that when the temperature of the second carbonization treatment is too high (e.g., above 1400℃), the interlayer spacing of the soft carbon components in the shell structure decreases, leading to a reduction in the sodium storage and kinetic properties of the hard carbon composite material. Conversely, when the temperature of the second carbonization treatment is too low (e.g., below 1200℃), the sodium storage performance of the hard carbon components in the core structure decreases, and the surface functional groups and defects of the soft carbon components in the shell structure are not completely removed, resulting in a decrease in initial efficiency. By controlling the temperatures of the two carbonization treatments within the aforementioned range, with the second carbonization temperature being higher than the first, a preliminary carbon skeleton structure can be formed through the first carbonization treatment, and then the pores in the material can be closed through the second carbonization treatment, thereby reducing the number of open pores and improving the sodium storage capacity of the hard carbon composite material. Through the synergistic effect of the two carbonization treatments, a hard carbon composite material with high electrical conductivity and high sodium storage performance is formed.

[0054] In some embodiments of this application, in the preparation steps of the hard carbon composite material, the mass ratio of soft carbon material to precursor material is 1:10 to 20, for example, 1:10, 1:15, or 1:20, which is beneficial to improving the electrical conductivity of the hard carbon composite material.

[0055] In some embodiments of this application, the preparation process of nanocellulose is as follows:

[0056] Bamboo fiber is added to a 50%–70% sulfuric acid solution and reacted at 40℃–60℃ for 2–4 hours to obtain nanocellulose. The average length of the nanocellulose is no greater than 350 nm, and the average diameter is no greater than 15 nm. Compared with ordinary cellulose, the nanocellulose of this application is more likely to complex with silver ions, thereby enabling more uniform dispersion of silver ions, which is beneficial to improving the conductivity of hard carbon composite materials.

[0057] In some embodiments of this application, the hard carbon composite material formed after the second carbonization treatment has closed pores, mesopores, and micropores, which is beneficial to increasing sodium storage capacity, thereby improving the sodium storage performance of sodium-ion batteries and improving the capacity of sodium-ion batteries.

[0058] In this application, closed pores refer to pores in hard carbon composite materials that are not connected to the outside world, mesopores refer to pores in hard carbon composite materials with a pore size of 2nm to 50nm, and micropores refer to pores in hard carbon composite materials with a pore size of less than 2nm.

[0059] In some embodiments of this application, after the second carbonization treatment, the mass percentage of silver in the hard carbon composite material is 'a', where 0.5% ≤ a ≤ 0.8%. By controlling 'a' within the above range, it is beneficial to maintain excellent sodium storage performance while improving the conductivity of the hard carbon composite material, thereby improving the rate performance and capacity of the sodium-ion battery.

[0060] The inventors discovered that the nanocellulose prepared by the above method, due to the introduction of sulfur, and the subsequent doping effect of sulfur and nitrogen introduced by grafting pyrrole, enhances the conductivity of the material through the interaction of lone pair electrons and π electrons. This alters the electronic state of the microstructure of the hard carbon composite material. Furthermore, heteroatom doping can improve the surface functional groups of the hard carbon composite material, modify the structure of the carbon material, promote electron transfer, and enhance conductivity. Under the combined effect of sulfur and nitrogen atoms, the interlayer spacing, electronic conductivity, and number of reversible defects of the hard carbon composite material are improved, thereby improving the sodium storage performance of the hard carbon composite material.

[0061] In some embodiments of this application, the linear polymer is selected from at least one of polyvinylpyrrolidone (PVP), polyacrylamide (PAM), polycaprolactone, polycaprolactam, and polymethyl methacrylate. By selecting at least one of the above linear polymers, during the carbonization process, the small gas molecules such as CO, CO2, and H2O released by the linear polymer and nanocellulose can change the microstructure of the carbon material, forming abundant micropores, mesopores, and closed-pore structures; furthermore, the carbonized linear polymer can seal the open-pore structures formed between the carbonized nanocellulose particles. Under the synergistic effect of the linear polymer and nanocellulose, the sodium storage performance of the hard carbon composite material is improved.

[0062] In some embodiments of this application, the solvent is selected from at least one of deionized water, ethanol, acetone, ethyl acetate, tetrahydrofuran, and dimethylformamide. These solvents facilitate the self-assembly of linear polymers and pyrrole-grafted cellulose nanoparticles through stirring to form a three-dimensional, multi-hydrogen-bonded cross-linked network structure.

[0063] The method for preparing hard carbon composite materials provided in this application is based on nanocellulose biomass materials. The raw materials are widely available, and nanocellulose can be used to achieve high-value utilization of various waste fibers (such as bamboo fiber) while avoiding environmental pollution. Furthermore, the preparation method of this application is simple, and the prepared hard carbon composite material exhibits excellent performance and low cost. This improves the reversible capacity, initial coulombic efficiency, rate performance, and cycle stability of sodium-ion batteries while reducing their production cost.

[0064] This application also provides a hard carbon composite material, which is prepared by the hard carbon composite material preparation method described in any of the above embodiments. The hard carbon composite material has a core-shell structure, wherein the core of the core-shell structure includes a three-dimensional cross-linked conductive network carbon skeleton, in which carbon layers and elemental silver are distributed, and the shell of the core-shell structure includes a coating layer formed of soft carbon material.

[0065] Figure 1 This is a schematic cross-sectional view of a hard carbon composite material according to one embodiment of this application. (Reference) Figure 1 This hard carbon composite material has a core-shell structure. The core of the core-shell structure includes a three-dimensional cross-linked conductive network carbon skeleton 11. This three-dimensional cross-linked conductive network carbon skeleton can form a fast electron transport pathway and also serve as a stable mechanical skeleton, thereby improving the conductivity and cycle performance of the hard carbon material. Carbon layers 12 and elemental silver 13 are distributed within the three-dimensional cross-linked conductive network carbon skeleton. The particle size of the elemental silver ranges from 5 nm to 25 nm, and the nanoscale silver particles can be uniformly and firmly loaded on the surface and pores of the three-dimensional cross-linked conductive network carbon skeleton structure, further improving the conductivity of the hard carbon material. The shell of the core-shell structure includes a coating layer 20 formed of soft carbon material, which can reduce the specific surface area and surface defects of the hard carbon composite material, improving its first-pass efficiency. Furthermore, the conductivity of soft carbon material is superior to that of hard carbon material, and the coating layer formed by soft carbon material is beneficial to improving the conductivity of the hard carbon composite material.

[0066] The hard carbon composite material of this application exhibits excellent electrical conductivity and sodium storage performance, resulting in significant improvements in the reversible capacity, initial coulombic efficiency, rate performance, and cycle stability of sodium-ion batteries incorporating this hard carbon composite material. In this application, the soft carbon material may include, but is not limited to, asphalt.

[0067] In some embodiments of this application, the electrical conductivity σ of the hard carbon composite material is 45 S / cm ≤ σ ≤ 60 S / cm. For example, σ is 45 S / cm, 47 S / cm, 50 S / cm, 55 S / cm, and 60 S / cm. By adjusting σ within the above range, it is beneficial to improve the electrical conductivity of the hard carbon composite material, thereby improving the rate performance of the sodium-ion battery.

[0068] In some embodiments of this application, the specific surface area of ​​the hard carbon composite material is BET, 2m². 2 / g≤BET≤4m 2 / g. For example, BET is 2m. 2 / g、3m 2 / g, 3.5m 2 / g、4m 2 / g. By adjusting BET within the above range, it is beneficial to provide more active sites and reaction area, improve the utilization rate of negative electrode active material, and improve the stability of hard carbon composite material, thereby enabling sodium-ion batteries to exhibit high capacity, high rate capability, and high reversible capacity performance.

[0069] In some embodiments of this application, the tap density of the hard carbon composite material is ρ r 0.85g / cm 3 ≤ρ r ≤0.95g / cm 3 For example, ρ r 0.85 g / cm 3 0.88g / cm 3 0.9g / cm 3 0.95g / cm 3 By adjusting ρ r Within the aforementioned range, it is beneficial to improve the volumetric energy density of sodium-ion batteries.

[0070] In some embodiments of this application, the interlayer spacing of the hard carbon composite material is D1, where 0.380 nm ≤ D1 ≤ 0.385 nm. For example, D1 is 0.380 nm, 0.381 nm, 0.383 nm, or 0.385 nm. By adjusting D1 within the above range, sufficient space can be provided for the storage and deintercalation of sodium ions, thereby maintaining good stability during the sodium ion deintercalation process and improving the cycle performance of the sodium-ion battery.

[0071] In this application, interlayer spacing refers to the distance between the atomic layers of carbon in a hard carbon composite material. The interlayer spacing can be determined by analyzing the diffraction pattern of the hard carbon composite material using X-ray diffraction.

[0072] In some embodiments of this application, the D50 of the hard carbon composite material is D2, where 5μm ≤ D2 ≤ 7μm. For example, D2 can be 5μm, 5.5μm, 6μm, 6.5μm, or 7μm. By controlling D2 within the above range, it is beneficial to reduce irreversible capacity loss during the first charge and discharge. A slightly wider particle size distribution allows for some small powder filling between larger particles, increasing the contact area between particles, thereby reducing internal resistance and increasing electron conductivity. This is beneficial for improving the overall performance of the negative electrode, including conductivity, electrolyte wettability, and flexibility.

[0073] This application also provides a negative electrode sheet, including a current collector and a negative electrode active material layer disposed on at least one surface of the current collector. The negative electrode active material layer includes the hard carbon composite material described in any of the above embodiments, or includes the hard carbon composite material prepared by the preparation method described in any of the above embodiments.

[0074] The negative electrode active material layer of this application can be disposed on one or both surfaces of the negative electrode current collector in the thickness direction. In this application, the negative electrode active material layer is disposed on the surface of the negative electrode current collector; that is, the negative electrode active material layer can be disposed on a portion of one surface of the negative electrode current collector, or it can be disposed on the entire surface of one surface of the negative electrode current collector. This application does not have any particular limitation on the negative electrode current collector, as long as it can achieve the purpose of this application; for example, it can include, but is not limited to, copper foil, copper alloy foil, or aluminum foil. In this application, there is no particular limitation on the thickness of the negative electrode current collector, as long as it can achieve the purpose of this application; for example, the thickness can be 4 μm to 12 μm. The thickness of the negative electrode material layer in this application can be 70 μm to 200 μm.

[0075] In this application, the negative electrode active material layer may also include a negative electrode binder. This application does not impose any particular limitation on the negative electrode binder, as long as it can achieve the purpose of this application. For example, it may include at least one of acrylate, polyamide, polyimide, polyamide-imide, polyvinylidene fluoride, styrene-butadiene rubber, sodium alginate, polyvinyl alcohol, polytetrafluoroethylene, and sodium carboxymethyl cellulose.

[0076] This application also provides a sodium-ion battery, including the negative electrode sheet described in any of the above embodiments.

[0077] The sodium-ion battery of this application also includes a positive electrode, a separator, and an electrolyte, wherein the separator is located between the positive electrode and the negative electrode and plays a role in isolation.

[0078] This application does not impose any particular limitation on the positive electrode sheet, as long as it achieves the purpose of this application. For example, the positive electrode sheet typically includes a positive current collector and a positive active material layer. The positive active material layer can be disposed on one surface or two surfaces in the thickness direction of the positive current collector. In this application, the positive active material layer is disposed on the surface of the positive current collector, that is, the positive active material layer can be disposed on a portion of a surface of the positive current collector or on the entire surface of a surface of the positive current collector. This application does not impose any particular limitation on the positive current collector, as long as it achieves the purpose of this application, such as including but not limited to aluminum foil, aluminum alloy foil, or copper foil. In this application, there is no particular limitation on the thickness of the positive current collector, as long as it achieves the purpose of this application, such as a thickness of 8 μm to 13 μm. The thickness of the positive active material layer in this application can be 150 μm to 400 μm.

[0079] In this application, the positive electrode active material layer includes a positive electrode active material. This application does not have any particular restrictions on the positive electrode active material, as long as it can achieve the purpose of this application. For example, it may include at least one of transition metal oxides, Prussian blue compounds and polyanionic compounds.

[0080] In this application, the positive electrode active material layer may further include a positive electrode conductive agent. This application does not impose any particular limitation on the positive electrode conductive agent, as long as it can achieve the purpose of this application. For example, it may include, but is not limited to, at least one of conductive carbon black (Super P), carbon nanotubes (CNTs), carbon fibers, acetylene black, and graphene. The aforementioned carbon nanotubes may include, but are not limited to, single-walled carbon nanotubes and / or multi-walled carbon nanotubes. In this application, the positive electrode active material layer may further include a positive electrode binder. This application does not impose any particular limitation on the positive electrode binder, as long as it can achieve the purpose of this application. For example, it may include, but is not limited to, at least one of fluorinated resins, polypropylene resins, fiber-type binders, rubber-type binders, or polyimide-type binders.

[0081] This application does not impose any particular restrictions on the diaphragm; those skilled in the art can choose according to actual needs, as long as the purpose of this application can be achieved. For example, the diaphragm material can be selected from at least one of glass fiber, non-woven fabric, polyethylene, polypropylene, and polyvinylidene fluoride. The diaphragm can be a single-layer film or a multi-layer composite film, without particular restrictions. When the diaphragm is a multi-layer composite film, the materials of each layer can be the same or different, without particular restrictions.

[0082] The sodium-ion battery of this application also includes an electrolyte. This application does not impose any particular limitation on the electrolyte; those skilled in the art can choose according to actual needs, as long as the purpose of this application is achieved. For example, at least one of ethylene carbonate (EC), propylene carbonate (PC), diethyl carbonate (DEC), ethyl propionate (EP), propyl propionate (PP), ethyl methyl carbonate (EMC), dimethyl carbonate (DMC), vinylene carbonate (VC), or fluoroethylene carbonate (FEC) can be mixed in a certain mass or volume ratio to obtain a non-aqueous organic solvent, and then a sodium salt can be added to dissolve and mix evenly. This application does not limit the type of sodium salt, as long as the purpose of this application is achieved. For example, the sodium salt may include at least one of sodium hexafluorophosphate, sodium perchlorate, sodium tetrafluoroborate, sodium trifluoromethanesulfonate, and sodium p-toluenesulfonate. This application does not impose any particular limitation on the concentration of the sodium salt in the electrolyte, as long as the purpose of this application is achieved. For example, the concentration of the sodium salt is 1.0 mol / L to 2.0 mol / L.

[0083] The sodium-ion battery of this application also includes a casing. This application does not impose any particular limitations on the casing, and those skilled in the art can choose one according to actual needs, as long as it can achieve the purpose of this application. For example, the casing may include an aluminum-plastic film.

[0084] This application does not impose any particular limitation on the preparation method of sodium-ion batteries. Any preparation method known in the art can be used, as long as it can achieve the purpose of this application. For example, the preparation method of sodium-ion batteries includes, but is not limited to, the following steps: stacking the positive electrode, separator, and negative electrode in sequence, and winding and folding them as needed to obtain a bare cell with a wound structure; placing the bare cell in a packaging bag; injecting electrolyte into the packaging bag and sealing it to obtain a sodium-ion battery.

[0085] This application also provides a battery pack, including a housing and at least one sodium-ion battery according to any of the above embodiments, wherein the sodium-ion battery is housed within the housing. The battery pack with these two batteries exhibits excellent performance, which is beneficial for its use. Housed within the housing, the batteries are secured and protected, thus extending the battery pack's lifespan. It is understood that the battery pack may contain one or more sodium-ion batteries, and when the battery pack contains multiple sodium-ion batteries, these batteries can be connected in at least one manner, such as in parallel or in series.

[0086] This application also provides an electrical device including a sodium-ion battery or battery pack as described in any of the above embodiments, which is beneficial for improving the product competitiveness and performance of the electrical device. In an optional embodiment, the electrical device includes a device body, and the sodium-ion battery or battery pack is used to power the device body. In an optional embodiment, the device body includes a positive electrode and a negative electrode, the positive electrode of the sodium-ion battery or battery pack is used to electrically connect to the positive electrode of the device body, and the negative electrode of the sodium-ion battery or battery pack is used to electrically connect to the negative electrode of the device body, so as to power the electrical device.

[0087] The electrical equipment covered by this application may include, but is not limited to: containers, household energy storage systems, electric vehicles, electric cars, ships, spacecraft, electric toys, and power tools, etc. Among them, spacecraft include, for example, airplanes, rockets, space shuttles, and spacecraft, etc. Electric toys include, for example, stationary or mobile electric toys, specifically, electric car toys, electric ship toys, and electric airplane toys, etc. Power tools include, for example, metal cutting power tools, grinding power tools, assembly power tools, and railway power tools, specifically, electric drills, electric grinders, electric wrenches, electric screwdrivers, electric hammers, impact drills, concrete vibrators, and electric planers.

[0088] Please see Figure 2 , Figure 2 This is a schematic diagram of the structure of a residential energy storage system according to one embodiment of this application, and this application... Figure 2 The implementation plan is illustrated using the residential energy storage scenario in user-side energy storage as an example. The energy storage device in this application is not limited to the residential energy storage scenario.

[0089] This application provides a residential energy storage system, which includes a power conversion device 2 (photovoltaic panel), a first user load 3 (streetlight), a second user load 4 (e.g., household appliances such as air conditioners), and an energy storage device 1. The energy storage device 1 is a small energy storage box that can be wall-mounted to an outdoor wall. Specifically, the photovoltaic panel can convert solar energy into electrical energy during periods of low electricity prices, and the energy storage device 1 is used to store this electrical energy and supply it to streetlights and household appliances during periods of high electricity prices, or to provide power during power outages / power failures.

[0090] Please see Figure 3 , Figure 3 This is a schematic diagram of the structure of an energy storage system 400 according to one embodiment of this application, and this application Figure 3 The implementation plan is illustrated using the shared energy storage scenario on the power generation / distribution side as an example. The energy storage device 1 in this application is not limited to the power generation / distribution side energy storage scenario.

[0091] This application provides an energy storage system 400, which includes a high-voltage cable 410, a first power conversion device 420, a second power conversion device 430, and the energy storage device 1 provided in this application. During power generation, the first power conversion device 420 and the second power conversion device 430 convert other forms of energy into electrical energy, which is then connected to the high-voltage cable 410 and supplied to the power consumption side of the distribution network. When the power load is low and the first power conversion device 420 and the second power conversion device 430 generate excess power, the excess power is stored in the energy storage device 1, reducing wind and solar curtailment rates and improving the absorption of new energy power generation. When the power load is high, the power grid issues an instruction to transmit the power stored in the energy storage device 1, along with the high-voltage cable 410, in a grid-connected mode to supply power to the power consumption side. This provides various services such as peak shaving, frequency regulation, and backup for the power grid operation, fully leveraging the peak shaving function of the power grid, promoting peak shaving and valley filling, and alleviating the power supply pressure on the power grid.

[0092] Optionally, the first power conversion device 420 and the second power conversion device 430 can convert at least one of solar energy, light energy, wind energy, thermal energy, tidal energy, biomass energy and mechanical energy into electrical energy.

[0093] The number of energy storage devices 1 can be multiple, and these devices can be connected in series or in parallel. The multiple energy storage devices 1 are supported and electrically connected by an isolation plate (not shown). In this embodiment, "multiple" refers to two or more. An energy storage box can also be provided outside the energy storage device 1 to house it.

[0094] Optionally, the energy storage device 1 may include, but is not limited to, a single battery cell, a battery module, a battery pack, or a battery system. The actual application form of the energy storage device 1 provided in this application embodiment may be, but is not limited to, the listed products, and may also be other application forms. This application embodiment does not strictly limit the application form of the energy storage device 1. This application embodiment only uses a multi-cell battery as an example for illustration. When the energy storage device 1 is a single battery cell, the energy storage device 1 may be at least one of cylindrical batteries, prismatic batteries, etc.

[0095] Example

[0096] The embodiments and comparative examples provided below illustrate the implementation of this application in more detail. Various tests and evaluations were conducted according to the methods described below.

[0097] Example 1

[0098] <Preparation of Nanocellulose>

[0099] Bamboo fiber was added to a 68% sulfuric acid solution at a mass ratio of 1:65. The solution was then ultrasonically treated at 60°C for 4 hours. After washing and drying, nanocellulose with an average length of about 210 nm and an average diameter of about 12 nm was obtained.

[0100] Preparation of pyrrole-grafted nanocellulose

[0101] 100g of nanocellulose was placed in 1000g of DMF alkaline solution containing bromopyrrole and stirred at 30℃ for 2h. After centrifugation, washing and drying, pyrrole-grafted nanocellulose was obtained.

[0102] <Preparation of spherical composite materials>

[0103] 70g of pyrrole-grafted nanocellulose was first dissolved in deionized water and heated to 50℃. Then, 29.7g of polyvinylpyrrolidone (PVP) was added (the mass ratio of pyrrole-grafted nanocellulose to PVP was 1:0.42). The mixture was kept at 50℃ and stirred for 3 hours. Then, 1.5g of 20wt% silver nitrate solution was added to the mixture and stirred evenly. After spray drying, the inlet air temperature was 150℃, the feed rate was 30mL / min, and the outlet air temperature was 70℃, resulting in a nanocellulose-linear polymer spherical composite material loaded with nanosilver ions.

[0104] <Preparation of Hard Carbon Composite Materials>

[0105] Under an argon atmosphere, the prepared spherical composite material was placed in a tube furnace for the first carbonization treatment: the temperature was increased to T1 = 1000℃ at a heating rate of 5℃ / min, and then held for 2 hours to obtain the precursor material; the precursor material was then ball-milled to depolymerize and passed through a 400-mesh sieve. 2g of asphalt, 20g of the obtained precursor material, and 10g of ethanol were ball-milled and mixed (the mass ratio of asphalt to precursor material was 1:10). After drying, the mixture was placed in a tube furnace for the second carbonization treatment: the temperature was increased to T2 = 1300℃ at a heating rate of 2℃ / min, and then held for 4 hours to obtain the hard carbon composite material.

[0106] Example 2

[0107] Except for the addition of pyrrole-grafted nanocellulose to 60g and PVP to 37.9g ​​(i.e., the mass ratio of pyrrole-grafted nanocellulose to PVP is 1:0.63) in the <Preparation of Spherical Composite Materials>, the rest is the same as in Example 1.

[0108] Example 3

[0109] Except for the addition of silver nitrate, which was adjusted to 2g in the <Preparation of Spherical Composite Material>, the rest is the same as in Example 1.

[0110] Example 4

[0111] Except for the fact that in the <Preparation of Hard Carbon Composite Material>, the amount of asphalt added was adjusted to 1g and the amount of precursor material added was adjusted to 20g (i.e., the mass ratio of asphalt to precursor material was 1:20), the rest was the same as in Example 1.

[0112] Example 5

[0113] Except for the second carbonization temperature T2 being adjusted to 1200°C in the <Preparation of Hard Carbon Composite Material>, the rest is the same as in Example 1.

[0114] Example 6

[0115] Except for replacing PVP with PAM in the <Preparation of Spherical Composite Materials> section, the rest is the same as in Example 1.

[0116] Example 7

[0117] Except for adjusting the sulfuric acid concentration to 52%, the reaction temperature to 50°C, and the reaction time to 3 hours in the <Preparation of Nanocellulose> section, the rest was the same as in Example 1. The average length of the nanocellulose was approximately 320 nm, and the average diameter was approximately 14 nm.

[0118] Example 8

[0119] Except for adjusting the stirring temperature to 20°C in <Preparation of pyrrole-grafted nanocellulose>, adjusting the stirring temperature to 40°C and the reaction time to 4h in <Preparation of spherical composite materials>, and adjusting the temperature T1 of the first carbonization treatment to 1100°C and the temperature T2 of the second carbonization treatment to 1250°C in <Preparation of hard carbon composite materials>, the rest are the same as in Example 1.

[0120] Example 9

[0121] Except for adjusting the stirring temperature to 40°C in <Preparation of pyrrole-grafted nanocellulose>, adjusting the stirring temperature to 120°C and the reaction time to 2h in <Preparation of spherical composite materials>, and adjusting the temperature T1 of the first carbonization treatment to 1190°C and the temperature T2 of the second carbonization treatment to 1400°C in <Preparation of hard carbon composite materials>, the rest are the same as in Example 1.

[0122] Comparative Example 1

[0123] Except for omitting the <Preparation of pyrrole-grafted nanocellulose> step, that is, in the <Preparation of spherical composite material>, the nanocellulose prepared in the <Preparation of nanocellulose> step is directly mixed with PVP, the rest is the same as in Example 1.

[0124] Comparative Example 2

[0125] Except for the difference in the preparation of hard carbon composite materials compared to Example 1, the rest is the same as Example 1.

[0126] <Preparation of Hard Carbon Composite Materials>

[0127] Under an argon atmosphere, the prepared spherical composite material was placed in a tube furnace for the first carbonization treatment: the temperature was increased to T1 = 1000℃ at a heating rate of 5℃ / min, and then held for 2 hours to obtain the precursor material; the precursor material was then ball-milled to depolymerize, passed through a 400-mesh sieve, and placed in a tube furnace for the second carbonization treatment: the temperature was increased to T2 = 1300℃ at a heating rate of 2℃ / min, and then held for 4 hours to obtain the hard carbon composite material.

[0128] Table 1. Preparation parameters of hard carbon composite materials for each embodiment and comparative example.

[0129]

[0130] In Table 1, " / " indicates that the relevant preparation parameters are not included.

[0131] Test methods and equipment:

[0132] Tests on silver content, specific surface area, tap density, interlayer spacing, and particle size of hard carbon composite materials:

[0133] The test was conducted according to the test methods in the national standard GB / T 24533 2019 "Graphite Anode Materials for Lithium-ion Batteries".

[0134] Electrical conductivity testing of hard carbon composite materials:

[0135] The four-probe method was used, with the four probes of the resistivity meter in contact with the hard carbon composite powder sample. Current and voltage were measured, and the resistivity of the powder under different pressures was calculated. Conductivity = 1 / resistivity, with units of S / cm. The conductivity data in Table 2 are all test results at a pressure of 25 MPa.

[0136] Table 2. Physicochemical parameters of hard carbon composite materials in each embodiment and comparative example.

[0137]

[0138] Button cell battery performance test:

[0139] To test the effectiveness of the hard carbon composite material described in the above embodiments and comparative examples in sodium-ion battery materials, this application prepared a 2032-type coin cell by using the hard carbon composite material as the negative electrode of a half-cell, and tested and evaluated its effectiveness. In practical applications, this hard carbon composite material can be used as the negative electrode active material for sodium-ion batteries.

[0140] The method for manufacturing coin cells includes the following steps:

[0141] Preparation of the negative electrode sheet: The hard carbon composite material, carboxymethyl cellulose (CMC) thickener, conductive carbon black (Super-P) conductive agent, and styrene-butadiene rubber (SBR) binder prepared in the various examples and comparative examples were mixed in a mass ratio of 95:2:1.5:1.5. Deionized water was added and the mixture was stirred evenly to obtain a negative electrode slurry with a solid content of 30%. The negative electrode slurry was uniformly coated on one surface of a copper foil, and then vacuum dried. After rolling and stamping, a circular negative electrode sheet was obtained.

[0142] Electrolyte preparation: In an argon-atmosphere glove box with a moisture content ≤1ppm, ethylene carbonate (EC) and diethyl carbonate (DEC) were mixed at a volume ratio of 1:1. Then, sodium salt NaPF6 was added and dissolved in the solvent. After thorough mixing, the electrolyte was obtained. The molar concentration of NaPF6 in the electrolyte was 1 mol / L.

[0143] Assembly of coin cell: The circular sodium sheet, the separator (glass fiber membrane with a thickness of 260μm), and the circular negative electrode sheet prepared above are stacked in sequence, with the separator positioned between the circular sodium sheet and the negative electrode sheet to act as a separator. Then, the prepared electrolyte is injected to assemble the coin cell.

[0144] After the assembled button cells were left to stand for 12 hours, the following charge and discharge tests were performed using a Blue Battery testing system (model CT2001A):

[0145] Reversible capacity, discharge capacity and first-stage efficiency testing:

[0146] The test temperature was 25℃. After standing for 10 minutes, the battery was discharged at a constant current of 0.1 times (C) to 0.005V, and then discharged at a constant current of 0.05C to 0.005V. After standing for 10 minutes, this was the discharge stage. The battery was then charged at a constant current of 0.1C to 2V, which was the charging stage. The initial efficiency was calculated as: (charge capacity during charging stage / discharge capacity during discharging stage) × 100%.

[0147] Ratio performance test:

[0148] The test temperature was 25℃. The coin cell was discharged at 0.1C to 0.005V; then discharged at 0.05C to 0.005V; after resting for 10 minutes, it was charged at a constant current of 0.1C to 2V; after resting for 10 minutes, the coin cell was discharged at 0.1C to 0.005V; after resting for 10 minutes, it was charged at a constant current of 0.1C to 2V. The capacity obtained in this step is the 0.1C discharge capacity, denoted as C1. After resting for 10 minutes, the coin cell was discharged at 3C to 0.005V; after resting for 10 minutes, it was charged at a constant current of 0.1C to 2V. The capacity obtained in this step is the 3C discharge capacity, denoted as C2. Rate performance = (discharge capacity C2 / discharge capacity C1) × 100%.

[0149] Cyclic performance test:

[0150] The test temperature was 25℃. The coin cell battery was discharged at 0.2C to 0.005V, allowed to stand for 10 minutes, and then charged at a constant current of 0.2C to 2V. The capacity obtained in this step was taken as the initial discharge capacity C0. A 0.2C charge / discharge cycle test was performed 200 times, and the discharge capacity of the 200th cycle was recorded. Cycle capacity retention rate = (Discharge capacity of the 200th cycle / Initial discharge capacity C0) × 100%.

[0151] Table 3 Performance parameters of the button cells in each embodiment and comparative example

[0152]

[0153] Referring to Tables 2 and 3, and comparing Examples 1 to 9 with Comparative Examples 1 to 2, it can be seen that when nanocellulose is directly mixed with linear polymers (e.g., Comparative Example 1), the lack of pyrrole grafting results in a weak complexation ability for silver ions. This makes it difficult for silver ions to be uniformly dispersed and firmly loaded onto the spherical three-dimensional cross-linked conductive network framework of the nanocellulose-linear polymer during carbonization. Consequently, the nanosilver cannot synergistically interact with the three-dimensional cross-linked conductive network framework, affecting the conductivity of the hard carbon composite material and thus the performance of the sodium-ion battery. When the hard carbon composite material is not coated with soft carbon material (e.g., Comparative Example 2), the lack of a core-shell structure and the unclosed pores in the hard carbon composite material lead to excessive electrolyte consumption during cycling, resulting in lower initial efficiency and poorer cycle performance. Therefore, the absence of the synergistic effect of the soft carbon component also affects the conductivity of the hard carbon composite material, thereby impacting the performance of the sodium-ion battery. The hard carbon composite material of this application has the advantage that the nano-silver can be uniformly and firmly loaded on the surface and pores of the three-dimensional cross-linked conductive network carbon skeleton structure, and it has a soft carbon material coating layer. Under the combined effect of the two, the conductivity and other properties of the hard carbon composite material are significantly improved, which significantly improves the reversible capacity, initial coulombic efficiency, rate performance and cycle stability of sodium-ion batteries.

[0154] Figure 4 This is a SEM image of the hard carbon composite material prepared in Example 1. From... Figure 4 It can be seen that hard carbon composite materials have a distinct spherical structure.

[0155] The preparation method of a hard carbon composite material, the hard carbon composite material, and the negative electrode sheet disclosed in this application have been described in detail above. Specific examples have been used to illustrate the principles and implementation methods of this application. The description of the above embodiments is only for the purpose of helping to understand the technical solutions and core inventive points of the embodiments of this application. At the same time, for those skilled in the art, there will be changes in the specific implementation methods and application scope based on the ideas of this application. Therefore, the content of this specification should not be construed as a limitation of this application.

Claims

1. A method for preparing a hard carbon composite material, characterized in that, Includes the following steps: Preparation of pyrrole-grafted nanocellulose: Nanocellulose is added to an alkaline solution of dimethylformamide containing bromopyrrole, and after reaction, pyrrole-grafted nanocellulose is obtained, wherein the nanocellulose is made from bamboo fiber; Preparation of spherical composite material: The pyrrole-grafted nanocellulose and linear polymer were added to a solvent and stirred, then silver nitrate solution was added, and after spray drying, a nanocellulose-linear polymer spherical composite material loaded with silver ions was obtained. Preparation of hard carbon composite material: The spherical composite material is subjected to a first carbonization treatment under an inert gas atmosphere to obtain a precursor material. The precursor material is then mixed with soft carbon material and subjected to a second carbonization treatment under an inert gas atmosphere to obtain the hard carbon composite material. The hard carbon composite material has a core-shell structure. The core of the core-shell structure includes a three-dimensional cross-linked conductive network carbon skeleton. Carbon layers and elemental silver are distributed in the three-dimensional cross-linked conductive network carbon skeleton. The shell of the core-shell structure includes a coating layer formed by soft carbon material.

2. The preparation method according to claim 1, characterized in that, In the preparation steps of the pyrrole-grafted nanocellulose, the stirring temperature is 20℃~40℃.

3. The preparation method according to claim 1, characterized in that, In the preparation steps of the spherical composite material, the mass ratio of pyrrole-grafted nanocellulose to linear polymer is 1:0.4~0.7, the stirring temperature is 40℃~120℃, and the reaction time is 2h~4h.

4. The preparation method according to claim 1, characterized in that, In the preparation steps of the hard carbon composite material, the temperature of the first carbonization treatment is T1, where 1000℃≤T1<1200℃, and the temperature of the second carbonization treatment is T2, where 1200℃≤T2≤1400℃.

5. The preparation method according to claim 1, characterized in that, In the preparation steps of the hard carbon composite material, the mass ratio of soft carbon material to precursor material is 1:10~20.

6. The preparation method according to claim 1, characterized in that, The preparation process of the nanocellulose is as follows: Bamboo fiber is added to a sulfuric acid solution with a mass concentration of 50%~70% and reacted at 40℃~60℃ for 2h~4h to obtain nanocellulose.

7. The preparation method according to claim 1, characterized in that, After the second carbonization treatment, the mass percentage of silver in the hard carbon composite material is a, where 0.5% ≤ a ≤ 0.8%.

8. The preparation method according to any one of claims 1 to 7, characterized in that, The linear polymer is selected from at least one of polyvinylpyrrolidone, polyacrylamide, polycaprolactone, polycaprolactam, and polymethyl methacrylate.

9. The preparation method according to any one of claims 1 to 7, characterized in that, The solvent is selected from at least one of deionized water, ethanol, acetone, ethyl acetate, tetrahydrofuran, and dimethylformamide.

10. A hard carbon composite material, characterized in that, The hard carbon composite material has a core-shell structure. The core of the core-shell structure includes a three-dimensional cross-linked conductive network carbon skeleton. Carbon layers and elemental silver are distributed in the three-dimensional cross-linked conductive network carbon skeleton. The shell of the core-shell structure includes a coating layer formed of soft carbon material.

11. The hard carbon composite material according to claim 10, characterized in that, The hard carbon composite material satisfies at least one of the following characteristics: a) The electrical conductivity of the hard carbon composite material is σ, 45 S / cm≤σ≤60 S / cm; b) The specific surface area of ​​the hard carbon composite material is BET, 2 m². 2 / g≤BET≤4 m 2 / g; c) The tap density of the hard carbon composite material is ρ r 0.85 g / cm 3 ≤ρ r ≤0.95 g / cm 3 ; d) The interlayer spacing of the hard carbon composite material is D1, 0.380 nm ≤ D1 ≤ 0.385 nm; e) The D50 of the hard carbon composite material is D2, where 5 μm ≤ D2 ≤ 7 μm.

12. A negative electrode sheet, characterized in that, The present invention includes a current collector and a negative electrode active material layer disposed on at least one surface of the current collector, wherein the negative electrode active material layer comprises a hard carbon composite material prepared by the preparation method according to any one of claims 1 to 9, or comprises a hard carbon composite material according to any one of claims 10 to 11.

13. A sodium-ion battery, characterized in that, Includes the negative electrode sheet as described in claim 12.

14. A battery pack, characterized in that, It includes a housing and at least one sodium-ion battery as described in claim 13, the sodium-ion battery being housed within the housing.

15. An electrical appliance, characterized in that, Includes the sodium-ion battery of claim 13, or includes the battery pack of claim 14.

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