Sucrose-based hard carbon nanosheet as well as preparation method and application thereof

Abstract

The invention relates to the technical field of sodium ion batteries, in particular to a sucrose-based hard carbon nanosheet as well as a preparation method and application thereof. The method comprises the following steps: dissolving cane sugar and sodium chloride in water to obtain a mixed solution; carrying out thickening treatment on the mixed solution to form jelly, and then carrying out ball milling treatment to obtain a pasty mixture; and calcining the pasty mixture in an inert atmosphere to obtain the sucrose-based hard carbon nanosheet. Compared with the prior art, the preparation method disclosed by the invention is simpler and safer to operate, not only reduces the difficulty and danger of experimental operation at high temperature and high pressure, but also omits a high-pressure hydrothermal reaction link required when the hard carbon is prepared by adopting cane sugar conventionally; the adopted template agent is sodium chloride which can be obtained through distillation again, so that the template agent can be repeatedly used, and the cost is saved.

Description

TECHNICAL FIELD

[0001] The application relates to the technical field of sodium ion batteries, in particular to a sucrose-based hard carbon nanosheet and a preparation method and application thereof. BACKGROUND

[0002] In recent years, the demand for renewable energy storage has risen sharply, making the electrochemical storage technology of secondary batteries and supercapacitors develop unprecedentedly. Especially, lithium ion batteries, benefiting from high specific capacity, excellent cycle stability and long service life, occupy the majority of the market share. However, the reserves of lithium resources are not abundant and are extremely unevenly distributed. With the surge in demand for lithium ion batteries, lithium is becoming increasingly rare and expensive like gold, and it is only a matter of time. Sodium and lithium belong to the same group of elements, have similar electrochemical properties, and are abundant in resources, widely distributed and low in price, with reserves about 430 times that of lithium. Therefore, sodium ion batteries have become one of the next-generation energy storage battery technologies with great prospects and suitable for large-scale renewable energy applications.

[0003] Carbon-based materials are considered to be the most promising negative electrode materials for sodium ion batteries due to their low cost, abundant resources, non-toxicity and good electrical conductivity. Due to the large radius of sodium ions and the low binding energy with graphite, it is difficult for sodium ions to be intercalated and deintercalated between the layers of graphite, which makes it almost impossible for graphite to have reversible sodium intercalation capacity in sodium ion batteries. Hard carbon has a unique structure, i.e. microcrystalline, defects and nanopores, and has a unique sodium storage mechanism. In addition, it has the advantages of abundant raw materials, low cost, good electrical conductivity, environmental friendliness and high specific capacity, and is higher than commercial graphite, which is a potential negative electrode material for sodium ion batteries.

[0004] Currently, the core method for preparing hard carbon based on sucrose is hydrothermal-high temperature pyrolysis. This method usually needs to be carried out under high temperature and high pressure conditions, which has strict requirements on experimental environment conditions and significantly increases the risk of experimental operation. In addition, amine-based high-cost and toxic organic reagents need to be added during the reaction process, which further increases the preparation cost and also increases the difficulty of safety control.

[0005] It is worth noting that the hard carbon material prepared based on the existing process with sucrose is mostly in the form of spherical particles. Although such spherical particles have a very high specific surface area, which can improve the capacity of the hard carbon material, the side reactions caused by the spherical particles are also significantly enhanced during the first charge and discharge cycle, which has a significant adverse effect on the first charge and discharge coulombic efficiency. At the same time, the spherical particles have significant disadvantages in terms of ohmic contact characteristics between particles and ion transport efficiency. SUMMARY

[0006] In view of the problems in the prior art, the present application provides a sucrose-based hard carbon nanosheet and a preparation method and application thereof. The present application uses sucrose as a precursor, sodium chloride as a template agent and a peeling promoter, dissolves the sucrose and the sodium chloride in water together, and sequentially performs thickening, ball milling and calcination treatment. With the sodium chloride as the template agent and the peeling promoter, the sucrose bulk is peeled layer by layer, and finally the sucrose-based hard carbon nanosheet is obtained. Compared with the prior art, the present application realizes the directional conversion of the sucrose bulk to a two-dimensional nanosheet structure by using sodium chloride as the template agent and the peeling promoter, and successfully prepares the sucrose-based hard carbon nanosheet. The preparation method of the present application is simpler and safer in operation, reduces the difficulty and danger of experimental operation under high temperature and high pressure, and eliminates the preparation step of high-pressure hydrothermal reaction required when sucrose is used to prepare hard carbon in the conventional way. In addition, the auxiliary agent used in the present application is sodium chloride, which can be obtained again by distillation, and therefore can be reused, saving cost.

[0007] In order to achieve the above-mentioned purpose, the technical scheme adopted by the present application is: The first object of the present application is to provide a preparation method of a sucrose-based hard carbon nanosheet, comprising the following steps: S1. Using sucrose as a precursor and sodium chloride as a template agent, dissolving the sucrose and the sodium chloride in water together to obtain a mixed solution.

[0008] S2. Performing thickening treatment on the mixed solution to remove part of the water in the mixed solution and form a gelatinous substance, and then performing ball milling treatment on the gelatinous substance. The cubic crystal structure of the sodium chloride is embedded in the interior and surface of the gelatinous substance to form a composite gelatinous substance in which the sodium chloride crystals are embedded, and the mechanical force generated by the ball milling effectively reduces the thickness of the internal structural units of the gelatinous substance to obtain a paste-like mixture.

[0009] S3. Performing calcination treatment on the paste-like mixture in an inert atmosphere. In the process of increasing the calcination temperature, the sucrose is converted from an organic molecule to hard carbon through dehydration and carbonization, and is shaped under the space template action of the solid sodium chloride to limit the growth dimension of the hard carbon and guide it to form a hard carbon-sodium chloride composite with a cubic pore structure around the sodium chloride template. When the temperature increases and exceeds the melting point of the sodium chloride, the solid sodium chloride melts into a liquid state, and the cubic pore structure of the hard carbon collapses to form a nanosheet. After cooling to room temperature, the liquid sodium chloride completes the reversible phase change process and returns to a solid state. After removing the sodium chloride, the sucrose-based hard carbon nanosheet is obtained.

[0010] Preferably, the mass ratio of the sucrose to the sodium chloride is 0.1-10:1-100, and more preferably, the mass ratio of the sucrose to the sodium chloride is 1:20.

[0011] Preferably, the thickening treatment is performed at 45-120 DEG C for 30-150 min to volatilize part of the water until the mixed solution becomes a white gelatinous substance.

[0012] Preferably, the calcination treatment is performed at a temperature rising rate of 1℃ / min~10℃ / min to 900℃~1700℃ for 1h~8h.

[0013] Preferably, the ball milling treatment in step S2 is performed at a speed of 200rpm~2500rpm for 30min~120min to make sodium chloride and sucrose fully contact and fully play the role of template.

[0014] Preferably, the sodium chloride is further subjected to ball milling treatment before being dissolved to obtain fine sodium chloride crystals, so that the dissolution is faster and more complete. The ball milling treatment is performed at a speed of 200rpm~2500rpm for 5min~120min.

[0015] Preferably, the content of water in the gel is less than 5%.

[0016] Preferably, the inert atmosphere is at least one selected from nitrogen and argon.

[0017] Preferably, the sucrose-based hard carbon nanosheet prepared in step S3 is further subjected to post-treatment, and the post-treatment is performed as follows: The sucrose-based hard carbon nanosheet is sequentially subjected to ultrasonic cleaning, filtration and drying treatment to obtain the hard carbon nanosheet.

[0018] Preferably, the drying treatment is performed at a temperature of 45℃~80℃ for 1h~6h.

[0019] Preferably, the ultrasonic treatment is performed at a power of 15W~400W for 5min~120min.

[0020] The second object of the present application is to provide the sucrose-based hard carbon nanosheet prepared by the preparation method.

[0021] Preferably, the sucrose-based hard carbon nanosheet has a two-dimensional nanosheet structure.

[0022] The third object of the present application is to provide the application of the sucrose-based hard carbon nanosheet in preparing a negative electrode material for sodium ion batteries.

[0023] Compared with the prior art, the present application has the following advantages: 1. The application provides a preparation method of sucrose-based hard carbon nanosheets, sucrose and sodium chloride are dissolved in water to obtain a mixed solution; the mixed solution is thickened to remove part of the water in the mixed solution and form a colloidal substance, and then the colloidal substance is subjected to ball milling treatment, at this time the cubic crystal structure of sodium chloride is embedded in the inside and surface of the colloidal substance to form a sodium chloride crystal-embedded composite colloid, and the mechanical force generated by ball milling extrudes the colloidal substance, effectively reducing the thickness of the internal structure unit, and a paste-like mixture is obtained; in an inert atmosphere, the paste-like mixture is calcined, during the calcination process, sucrose is converted into hard carbon from an organic molecule through dehydration and carbonization, and is shaped by the space template effect of sodium chloride to form a hard carbon-sodium chloride composite with a cubic pore structure; as the temperature is further increased and exceeds the melting point of sodium chloride, the sodium chloride melts into a liquid state, causing the cubic pore structure of the hard carbon to collapse to form nanosheets, and after cooling to room temperature, the sodium chloride returns to a solid state, and after removing the sodium chloride, sucrose-based hard carbon nanosheets are obtained. The preparation method of the application is significantly different from the existing high-temperature hydrothermal process, and the dependence on high temperature and high pressure conditions in the traditional method is abandoned, effectively reducing the safety risk in the production process. At the same time, through process optimization, the operation steps are simplified, and under the premise of ensuring the performance of sucrose-based hard carbon nanosheets, higher process feasibility and cost advantage are shown.

[0024] 2. The application adopts a thickening process combined with ball milling technology, wherein the thickening process realizes the uniform dispersion of sodium chloride and sucrose in a colloidal system through a low-temperature thickening process to form a stable colloidal substance; the subsequent ball milling treatment extrudes the colloidal substance to reduce the thickness of the structure unit through mechanical force, further strengthens the mixing uniformity of sucrose and sodium chloride, and provides necessary energy input for the phase transition of sucrose to a two-dimensional nanosheet structure, thereby realizing the effective peeling and preparation of sucrose-based hard carbon nanosheets.

[0025] 3. Compared with the spherical particle structure prepared by the traditional hydrothermal method, the sucrose-based hard carbon nanosheets prepared by the application present a two-dimensional nanosheet structure, which completely retains the typical advantages of two-dimensional materials. Specifically, the two-dimensional nanosheet structure endows the sucrose-based hard carbon nanosheets with excellent ion conductivity, effectively promotes the charge transfer dynamics process by shortening the ion and electron transport path, and further improves the electrochemical performance of the sodium ion battery. Especially in terms of rate performance, compared with the sucrose-based hard carbon nanosphere material reported in the existing literature, the two-dimensional nanosheet structure of the application enhances the ion diffusion dynamics behavior, so that the sodium ion battery can still maintain a relatively high specific capacity under high current density charging and discharging conditions.

[0026] 4、The sodium chloride is selected as the template agent in the application, which can be fully separated from the sucrose-based hard carbon nanosheet after the calcination process is completed. After distillation treatment, the sodium chloride can be recycled and reused, thereby significantly reducing the consumption cost of raw materials. In addition, the sucrose raw material used in the application is derived from natural biomass resources such as sugarcane or sugar beet, which is not only low in price, widely available and easy to obtain. At the same time, sucrose has a high carbon content and exhibits excellent conversion efficiency in the carbonization process, which can effectively reduce the occurrence of side reactions and thereby improve the overall efficiency of the carbonization process. BRIEF DESCRIPTION OF DRAWINGS

[0027] Figure 1 The scanning electron microscope graph of the sucrose-based hard carbon nanosheet prepared in Example 1, wherein a is a low magnification graph, and b is a high magnification graph.

[0028] Figure 2 The scanning electron microscope graph of the sucrose-based hard carbon nanosheet prepared in Example 2, wherein a is a low magnification graph, and b is a high magnification graph.

[0029] Figure 3 The scanning electron microscope graph of the sucrose-based hard carbon prepared in Comparative Example 1, wherein a is a low magnification graph, and b is a high magnification graph.

[0030] Figure 4 The first charge-discharge curve graph of the battery using the sucrose-based hard carbon nanosheet of Application Example 1 at a current density of 250 mA / g, and the break of the discharge process curve has no effect on the test results.

[0031] Figure 5 The discharge specific capacity graph of the battery using the sucrose-based hard carbon nanosheet of Application Example 1 at different current densities. DETAILED DESCRIPTION

[0032] The technical solutions of the application will be described in detail below with reference to the data in the embodiments of the application. Obviously, the described embodiments are only a part of the embodiments of the application, rather than all the embodiments. Based on the embodiments in the application, all other embodiments obtained by those skilled in the art without creative labor fall within the scope of protection of the application.

[0033] It should be noted that the professional terms used in the application are only for the purpose of describing the specific embodiments and are not intended to limit the scope of protection of the application. Unless otherwise specified, the various raw materials, reagents, instruments and equipment used in the following embodiments of the application can be purchased from the market or prepared by existing methods.

[0034] In the prior art, although hydrothermal-high temperature pyrolysis method has been widely used for the preparation of sucrose-based hard carbon materials, this method usually needs to be carried out under extreme conditions of high temperature and high pressure, which not only puts strict requirements on experimental equipment and environment, but also significantly increases the safety risk of experimental operation. In addition, this method needs to introduce amine, a high-cost and toxic organic reagent, during the reaction process, which further increases the preparation cost and makes the safety control more difficult. More importantly, the sucrose-based hard carbon materials prepared based on the existing process are mostly in spherical particle morphology, which although improves the specific surface area and capacity of the hard carbon materials, but in the first charge-discharge cycle process, the spherical particles easily cause significant side reactions, which adversely affects the first charge-discharge coulombic efficiency. At the same time, the spherical particles also have obvious disadvantages in the ohmic contact characteristics between particles and ion transmission efficiency, which limits their performance in sodium ion batteries.

[0035] In view of the problems existing in the prior art, the present application provides a preparation method of sucrose-based hard carbon nanosheet, comprising the following steps: dissolving sucrose and sodium chloride in water to obtain a mixed solution; performing thickening treatment on the mixed solution to remove part of the water in the mixed solution and form a colloidal substance, then performing ball milling treatment on the colloidal substance, the cubic crystal structure of sodium chloride is embedded in the inside and surface of the colloidal substance to form a composite colloid with sodium chloride crystals embedded, and a paste-like mixture is obtained; performing calcination treatment on the paste-like mixture in an inert atmosphere, in the process of increasing the calcination temperature, the sucrose is first dehydrated and carbonized to convert from an organic molecule to a hard carbon, and is shaped under the space template action of the solid sodium chloride to form a hard carbon-sodium chloride composite with a cubic pore structure; when the temperature increases and exceeds the melting point of sodium chloride, the solid sodium chloride melts into a liquid state, and the cubic pore structure of the hard carbon collapses to form a nanosheet; after cooling to room temperature, the liquid sodium chloride returns to a solid state, and after removing the sodium chloride, a sucrose-based hard carbon nanosheet is obtained.

[0036] In view of the problem that the high-pressure hydrothermal-high temperature pyrolysis method in the prior art needs to be carried out under extreme conditions of high temperature and high pressure, the present application overcomes this problem by using normal pressure low temperature thickening combined with ball milling process. Specifically, the high-pressure hydrothermal reactor is abandoned, and 45℃-120℃ oil bath normal pressure heating is used to realize the colloidalization of the sucrose and sodium chloride mixed solution to obtain a colloidal substance, and ball milling treatment is combined to realize the uniform dispersion and sheet layer pre-peeling of the colloidal substance, which significantly reduces the equipment requirements and operation safety risk.

[0037] In view of the problem that the prior art needs to introduce high-cost organic reagents, the present application uses sodium chloride as a template and peeling promoter to overcome this problem. Sodium chloride is non-toxic, inexpensive and can be recycled by water washing, which not only reduces the raw material cost, but also avoids the environmental pollution and health hazards to the operators caused by organic reagents.

[0038] The sucrose-based hard carbon nanosheet prepared by the method has a two-dimensional nanosheet structure, in the application of a sodium ion battery, the structure shortens the diffusion path of sodium ions, reduces the inter-particle contact resistance, and relatively reduces the specific surface area, thereby inhibiting the side reaction, and the first charge-discharge coulombic efficiency is improved to more than 92%.

[0039] The technical solutions of the present application are further explained and described below by examples, as follows: Example 1 A preparation method of sucrose-based hard carbon nanosheets, comprising the following steps: S1, sodium chloride is put into a ball mill tank, ball milling at 1500 r / min for 30 min to obtain fine sodium chloride crystals; 20 g of fine sodium chloride crystals and 1 g of sucrose are added to 20 mL of deionized water, and after strong magnetic stirring for 30 min, a mixed solution is obtained.

[0040] S2, the mixed solution is placed in an oil bath pot and heated and stirred at 98 DEG C for 150 min until it thickens into a honey-like colloidal state, i.e. a mixed colloid; the mixed colloid is transferred to a stainless steel ball mill tank for ball milling treatment, ball milling at 1500 rpm for 90 min to obtain a paste-like mixture.

[0041] S3, the paste-like mixture is transferred to a porcelain boat and placed in a tube furnace for calcination treatment, heated to 1300 DEG C at a heating rate of 5 DEG C / min under an argon atmosphere, and then kept for 4 h, and after the holding is completed, it is naturally cooled to obtain a black powder.

[0042] S4, the black powder is ultrasonically cleaned with deionized water, filtered after cleaning, and then placed in an oven and dried at 60 DEG C for 8 h to obtain sucrose-based hard carbon nanosheets.

[0043] Example 2 A preparation method of sucrose-based hard carbon nanosheets, which is the same as the preparation method of Example 1, except that the amount of fine sodium chloride crystals in S1 is replaced by 10 g, comprising the following steps: S1, sodium chloride is put into a ball mill tank, ball milling at 1500 r / min for 30 min to obtain fine sodium chloride crystals; 20 g of fine sodium chloride crystals and 1 g of sucrose are added to 20 mL of deionized water, and after strong magnetic stirring for 30 min, a mixed solution is obtained.

[0044] S2, the mixed solution is placed in an oil bath pot and heated and stirred at 98 DEG C for 150 min until it thickens into a honey-like colloidal state, i.e. a mixed colloid; the mixed colloid is transferred to a stainless steel ball mill tank for ball milling treatment, ball milling at 1500 rpm for 90 min to obtain a paste-like mixture.

[0045] S3, the paste mixture is transferred to a porcelain boat and placed in a tube furnace for calcination treatment, heated to 1300℃ at a heating rate of 5℃ / min under an argon atmosphere, and then kept for 4h, and then naturally cooled to obtain a black powder.

[0046] S4, the black powder is ultrasonically cleaned with deionized water, filtered after cleaning, and then placed in an oven and dried at 60℃ for 8h to obtain sucrose-based hard carbon nanosheets.

[0047] Example 3 A method for preparing sucrose-based hard carbon nanosheets, which is the same as the preparation method of Example 1, except that the amount of fine sodium chloride crystals in S1 is replaced by 1000g instead of 20g, comprising the following steps: S1, sodium chloride is placed in a ball mill jar and ball milled at 1500r / min for 30min to obtain fine sodium chloride crystals; 1000g of fine sodium chloride crystals and 1g of sucrose are added to 20mL of deionized water, and then stirred with strong magnetic force for 30min to obtain a mixed solution.

[0048] S2, the mixed solution is placed in an oil bath and heated and stirred at 98℃ for 150min until it becomes a honey-like colloidal gel, i.e., a mixed colloid; the mixed colloid is transferred to a stainless steel ball mill jar for ball milling treatment at 1500rpm for 90min to obtain a paste mixture.

[0049] S3, the paste mixture is transferred to a porcelain boat and placed in a tube furnace for calcination treatment, heated to 1300℃ at a heating rate of 5℃ / min under an argon atmosphere, and then kept for 4h, and then naturally cooled to obtain a black powder.

[0050] S4, the black powder is ultrasonically cleaned with deionized water, filtered after cleaning, and then placed in an oven and dried at 60℃ for 8h to obtain sucrose-based hard carbon nanosheets.

[0051] Example 4 A method for preparing sucrose-based hard carbon nanosheets, which is the same as the preparation method of Example 1, except that the amount of fine sodium chloride crystals in S1 is replaced by 0.1g instead of 20g, comprising the following steps: S1, sodium chloride is placed in a ball mill jar and ball milled at 1500r / min for 30min to obtain fine sodium chloride crystals; 0.1g of fine sodium chloride crystals and 1g of sucrose are added to 20mL of deionized water, and then stirred with strong magnetic force for 30min to obtain a mixed solution.

[0052] S2, the mixed solution is placed in an oil bath pot and heated and stirred at 98℃ for 150 min until thickened into a honey-like colloid, i.e. a mixed colloid; the mixed colloid is transferred to a stainless steel ball mill pot for ball milling treatment at 1500 rpm for 90 min to obtain a paste-like mixture.

[0053] S3, the paste-like mixture is transferred to a porcelain boat and placed in a tube furnace for calcination treatment; under an argon atmosphere, the temperature is raised to 1300℃ at a rate of 5℃ / min, and then held for 4 h; after the holding period, the temperature is naturally lowered to obtain black powder.

[0054] S4, the black powder is ultrasonically cleaned with deionized water, filtered after cleaning, and then placed in an oven for drying at 60℃ for 8 h to obtain sucrose-based hard carbon nanosheets.

[0055] Example 5 A method for preparing sucrose-based hard carbon nanosheets, which is the same as the preparation method of Example 1, comprises the following steps: S1, sodium chloride is placed in a ball mill pot and ball milled at 1500 r / min for 30 min to obtain fine sodium chloride crystals; 20 g of fine sodium chloride crystals and 1 g of sucrose are added to 20 mL of deionized water, and then subjected to strong magnetic stirring for 30 min to obtain a mixed solution.

[0056] S2, the mixed solution is placed in an oil bath pot and heated and stirred at 45℃ for 90 min until thickened into a honey-like colloid, i.e. a mixed colloid; the mixed colloid is transferred to a stainless steel ball mill pot for ball milling treatment at 2500 rpm for 30 min to obtain a paste-like mixture.

[0057] S3, the paste-like mixture is transferred to a porcelain boat and placed in a tube furnace for calcination treatment; under an argon atmosphere, the temperature is raised to 1700℃ at a rate of 10℃ / min, and then held for 1 h; after the holding period, the temperature is naturally lowered to obtain black powder.

[0058] S4, the black powder is ultrasonically cleaned with deionized water, filtered after cleaning, and then placed in an oven for drying at 60℃ for 8 h to obtain sucrose-based hard carbon nanosheets.

[0059] Example 6 A method for preparing sucrose-based hard carbon nanosheets, which is the same as the preparation method of Example 1, comprises the following steps: S1, sodium chloride is placed in a ball mill pot and ball milled at 1500 r / min for 30 min to obtain fine sodium chloride crystals; 20 g of fine sodium chloride crystals and 1 g of sucrose are added to 20 mL of deionized water, and then subjected to strong magnetic stirring for 30 min to obtain a mixed solution.

[0060] S2, the mixed solution was placed in an oil bath pot and heated and stirred at 120°C for 30 min until thickened to a honey-like colloidal state, i.e., a mixed colloid; the mixed colloid was transferred to a stainless steel ball mill jar for ball milling treatment at 200 rpm for 120 min to obtain a paste-like mixture.

[0061] S3, the paste-like mixture was transferred to a porcelain boat and placed in a tube furnace for calcination treatment; under an argon atmosphere, the temperature was raised to 900°C at a rate of 1°C / min and then held for 8 h; after the holding period, the temperature was allowed to decrease naturally to obtain a black powder.

[0062] S4, the black powder was cleaned with ultrasonic deionized water, filtered after the cleaning, and then placed in an oven for drying at 60°C for 8 h to obtain sucrose-based hard carbon nanosheets.

[0063] Comparative Example 1 A method for preparing sucrose-based hard carbon was the same as that of Example 1, except that the amount of fine sodium chloride crystals in S1 was changed from 20 g to 0 g, including the following steps: S1, 1 g of sucrose was added to 20 mL of deionized water, and after strong magnetic stirring for 30 min, a sucrose solution was obtained.

[0064] S2, the sucrose solution was placed in an oil bath pot and heated and stirred at 98°C until thickened to a honey-like colloidal state, i.e., a sucrose colloid; the sucrose colloid was transferred to a stainless steel ball mill jar for ball milling treatment at 1500 rpm for 90 min to obtain a paste-like sucrose.

[0065] S3, the paste-like sucrose was transferred to a porcelain boat and placed in a tube furnace for calcination treatment; under an argon atmosphere, the temperature was raised to 1300°C at a rate of 5°C / min and then held for 4 h; after the holding period, the temperature was allowed to decrease naturally to obtain a black powder.

[0066] S4, the black powder was cleaned with ultrasonic deionized water, filtered after the cleaning, and then placed in an oven for drying at 60°C for 8 h to obtain sucrose-based hard carbon.

[0067] Figure 1 The results show that sucrose-based hard carbon nanosheets were successfully prepared using sucrose as a precursor and sodium chloride as a template; the sucrose-based hard carbon nanosheets have a two-dimensional sheet structure, an average thickness of 90 nm, and a size of 3 μm to 15 μm. It can also be seen from the high-magnification SEM image that the sucrose-based hard carbon nanosheets are assembled from fine particles, further indicating that the sucrose-based hard carbon nanosheets have a large specific surface area, which is conducive to the storage of electrolyte and provides more active sites.

[0068] Figure 2The results show that the mass ratio of sodium chloride to sucrose is reduced from 20:1 to 10:1, which has a relatively obvious effect on the two-dimensional nanosheet structure of sucrose-based hard carbon nanosheets. Macroscopically, the two-dimensional nanosheet structure is no longer dominant in the product, and most of them are block particles. The results show that sodium chloride as a template plays an important role in the formation of nanosheets, and the ratio of sodium chloride to sucrose also plays a key role in the formation of sucrose-based hard carbon nanosheets.

[0069] Figure 3 The results show that the sucrose-based hard carbon prepared without adding sodium chloride in Comparative Example 1 has a surface of 40 microns to hundreds of microns of block particles, compared with the results in Comparative Example 2, further verifying the role of sodium chloride as a template in the preparation of two-dimensional nanosheet sucrose-based hard carbon nanosheets. Figure 2

[0070] Examples 1 to 3 of the present application all prepared sucrose-based hard carbon nanosheets, and the effects are parallel, and the sucrose-based hard carbon nanosheets prepared in Example 1 are taken as an example to study their electrochemical properties: The method for preparing the sodium ion battery negative electrode and assembling the battery is as follows: The sucrose-based hard carbon nanosheets and sodium alginate are mixed in a mass ratio of 95:5 to form a slurry; the slurry is uniformly coated on a copper foil by a doctor blade method, and dried at 120°C under vacuum for 12h to obtain a sodium ion battery negative electrode. The loading amount of the sucrose-based hard carbon nanosheets is 2.3mg / cm 2 ;

[0071] A button-type half battery is assembled in an argon-filled glove box, and the specific process is as follows: the sodium ion battery negative electrode material is used as a working electrode, placed in a CR2032 battery negative electrode shell, 1mol / L NaClO4 is used as an electrolyte, a Whatman GF / F glass fiber diaphragm is covered, a sodium metal sheet is placed as a counter electrode, a gasket and a spring are added, and finally a positive electrode shell is covered. Under the condition that the humidity and oxygen concentration in the glove box are strictly controlled below 0.1ppm, the battery is packaged into a CR2032 button-type half battery using a battery sealing machine.

[0072] The test method is as follows: a. First coulombic efficiency: the charge and discharge cut-off voltage of the battery is set to 0.01V~3V, the current density is 50mA / g, only the first cycle of charge and discharge test is carried out, and the first coulombic efficiency of the sucrose-based hard carbon nanosheets used as a sodium ion battery negative electrode is calculated according to the charge specific capacity and discharge specific capacity.

[0073] ​b. Rate performance: The battery was tested at seven current densities (20 mA / g, 50 mA / g, 100 mA / g, 150 mA / g, 200 mA / g, 250 mA / g, and 300 mA / g) for constant current charge and discharge cycling. The battery was cycled for 10 cycles at each current density before switching to the next density.

[0074] Depend on Figure 4 It was found that the battery using the sucrose-based hard carbon nanosheets of Example 1 had an initial discharge capacity of 217.1 mAh / g, a charge capacity of 201.5 mAh / g, and an initial charge and discharge coulombic efficiency of 92.9% at a current density of 250 mA / g.

[0075] The results of the cyclic stability test are as follows Figure 5 As shown, the battery using the sucrose-based hard carbon nanosheets of Example 1 has a first discharge specific capacity of 332.5 mAh / g and a coulombic efficiency of 91.8% at an initial current density of 20 mA / g. As the current density gradually increases to 300 mA / g, the sucrose-based hard carbon nanosheet negative electrode can still maintain a reversible specific capacity of 194.6 mAh / g, showing excellent rate performance. More importantly, when the current density drops from 300 mA / g to 20 mA / g again, the specific capacity of the sucrose-based hard carbon nanosheet negative electrode quickly recovers to 98% of the initial value, showing excellent structural stability and cycle reversibility. The sucrose-based hard carbon nanosheet negative electrode exhibits high specific capacity, excellent rate performance and cycle stability over a wide current density range, fully verifying its feasibility and application potential as a negative electrode material for sodium ion batteries. The results of the present invention provide new research ideas for the development of high-performance sodium ion battery electrode materials.

[0076] It should be noted that when the present invention involves numerical ranges, it should be understood that both endpoints of each numerical range and any value between the two endpoints can be selected. Since the steps and methods used are the same as those in the embodiments, in order to avoid redundancy, the present invention describes preferred embodiments. Although preferred embodiments of the present invention have been described, those skilled in the art will be able to make additional changes and modifications to these embodiments once they understand the basic inventive concepts. Therefore, the appended claims are intended to be interpreted as including the preferred embodiments and all changes and modifications that fall within the scope of the present invention.

Claims

1. A method for preparing sucrose-based hard carbon nanosheets, characterized in that: The following steps are involved: Dissolve sucrose and sodium chloride in water to obtain a mixed solution; The mixed solution is subjected to a thickening treatment to remove part of the water in the mixed solution and form a colloid, and then the colloid is subjected to a ball milling treatment, so that the cubic crystal structure of sodium chloride is embedded in the interior and surface of the colloid to form a composite colloid embedded with sodium chloride crystals, thereby obtaining a paste-like mixture; The paste mixture is calcined in an inert atmosphere. As the calcination temperature increases, sucrose is first dehydrated and carbonized, converted from organic molecules into hard carbon, and shaped under the spatial template of solid sodium chloride to form a hard carbon-sodium chloride complex with a cubic pore structure; when the temperature rises and exceeds the melting point of sodium chloride, the solid sodium chloride melts into liquid, and the cubic pore structure of the hard carbon collapses to form nanosheets; after cooling to room temperature, the liquid sodium chloride returns to a solid state, and after removing the sodium chloride, sucrose-based hard carbon nanosheets are obtained.

2. The method for preparing a sucrose-based hard carbon nanosheet according to claim 1, wherein: The mass ratio of sucrose to sodium chloride is 0.1~10:1~100.

3. The method for preparing a sucrose-based hard carbon nanosheet according to claim 1, wherein: The calcination treatment conditions are: calcination at 900°C~1700°C for 1h~8h.

4. The method for preparing a sucrose-based hard carbon nanosheet according to claim 1, wherein: The thickening treatment conditions are: stirring at 45°C~120°C for 30min~150min.

5. The method for preparing sucrose-based hard carbon nanosheets according to claim 1, wherein: The ball milling conditions are: ball milling at 200 rpm ~ 2500 rpm for 30 min ~ 120 min.

6. The method for preparing sucrose-based hard carbon nanosheets according to claim 1, wherein: The water content in the jelly is less than 5%.

7. A sucrose-based hard carbon nanosheet, characterized in that: Prepared by the preparation method according to any one of claims 1 to 6.

8. The sucrose-based hard carbon nanosheet according to claim 7, characterized in that: Sucrose-based hard carbon nanosheets present a two-dimensional nanosheet structure.

9. Use of the sucrose-based hard carbon nanosheets according to claim 7 in preparing a negative electrode material for sodium ion batteries.

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