Negative electrode material and preparation method thereof, sodium ion battery

Abstract

The present application relates to a negative electrode material and a preparation method thereof, and a sodium ion battery. The negative electrode material comprises a carbon matrix, wherein the carbon matrix has pores, wherein the pores comprise closed pores, and the pore volume of the closed pores is greater than 0.025 cm 3 / g; the negative electrode material contains doping elements, the doping elements include at least one of nitrogen, sulfur and oxygen, the negative electrode material has a first region located in the central region of the negative electrode material and a second region distributed on the surface of the first region, the thickness of the second region is less than or equal to 1μm, and the negative electrode material is tested by using an EDS (Energy Dispersive Spectrometer) spectrometer. The mass proportion of the doping element in the second region is 0.5% to 5%, and the mass proportion of the doping element in the first region is less than 0.5%.

Description

Technical Field

[0001] The present application belongs to the technical field of negative electrode materials, and more specifically, relates to negative electrode materials and preparation methods, and sodium ion batteries. Background Art

[0002] As lithium ore resources become increasingly scarce, the price of raw materials for lithium-ion batteries has risen significantly, and the development of lithium-ion batteries will inevitably face resource constraints. Sodium resources are abundant, cheap and easy to obtain, and have low processing costs. The cost of sodium-ion batteries is significantly lower than that of lithium-ion batteries. Therefore, the development prospects of sodium-ion batteries are broader than those of lithium-ion batteries. Compared with other negative electrode materials, carbon negative electrode materials have the advantages of low cost, large carbon layer spacing, low operating voltage, good conductivity, and good cycle performance. They are considered to be the most advantageous sodium-ion battery negative electrode materials.

[0003] The raw materials of existing carbon negative electrode materials for sodium ion batteries are generally divided into the following categories: resin carbon materials (such as phenolic resin, epoxy resin, polyfurfuryl alcohol resin, etc.), organic polymer carbon materials (such as polyvinyl alcohol PVA, polyvinyl chloride PVC, polyvinylidene fluoride PVDF, polyacrylonitrile PAN, etc.), biomass carbon materials, biomass extracts (starch, sucrose, etc.), asphalt and coal-based, etc. Among them, biomass carbon materials are widely studied and applied to carbon negative electrode materials for sodium ion batteries due to their low cost and wide sources. However, the capacity of current biomass-based hard carbon negative electrode materials is relatively low, usually only reaching 260Ah / g to 300Ah / g, which cannot meet the capacity requirements of sodium ion batteries, and the voltage platform of biomass-based hard carbon negative electrode materials is relatively low, and there is a risk of sodium precipitation. Summary of the invention

[0004] The present application provides a negative electrode material and a preparation method thereof, and a sodium ion battery, which solves the technical problem that the capacity, initial efficiency and discharge voltage of the negative electrode material for the sodium ion battery are difficult to improve.

[0005] In a first aspect, an embodiment of the present application provides a negative electrode material, the negative electrode material comprising a carbon matrix, the carbon matrix having pores, the pores comprising closed pores, and the pore volume of the closed pores being greater than 0.025 cm 3 / g;

[0006] The negative electrode material contains doping elements, and the doping elements include at least one of nitrogen, sulfur and oxygen. The negative electrode material has a first region located in the central region of the negative electrode material and a second region distributed on the surface of the first region. The thickness of the second region is less than or equal to 1 μm. The negative electrode material is tested by an EDS (Energy Dispersive Spectrometer) spectrometer. The mass proportion of the doping element in the second region is 0.5% to 5%, and the mass proportion of the doping element in the first region is less than 0.5%.

[0007] In some embodiments, the negative electrode material includes at least one of the following features (1) to (6):

[0008] (1) The carbon matrix comprises hard carbon;

[0009] (2) The mass percentage of the doping element in the negative electrode material is 0.5% to 2%;

[0010] (3) The average pore size of the closed pores is less than or equal to 5 nm;

[0011] (4) The specific surface area of ​​the negative electrode material is 2 m 2 / g~50m 2 / g;

[0012] (5) The true density of the negative electrode material is 1.8 g / cm 3 ~2.2g / cm 3 ;

[0013] (6) The median particle size of the negative electrode material is 3 μm to 20 μm.

[0014] In a second aspect, an embodiment of the present application provides a method for preparing a negative electrode material, comprising the following steps:

[0015] Placing the carbon-based raw material in an oxidizing gas atmosphere for pre-oxidation treatment to obtain a first precursor, wherein the pre-oxidation treatment temperature is 200° C. to 350° C.;

[0016] Placing the first precursor in an etching gas atmosphere for etching to obtain a second precursor, wherein the etching temperature is 400° C. to 800° C., and the etching gas includes at least one of oxygen, ozone, sulfur dioxide, sulfur trioxide, nitrogen dioxide, water vapor and magnesium vapor;

[0017] The second precursor is crushed to obtain a third precursor;

[0018] The third precursor is subjected to heat treatment to obtain a negative electrode material.

[0019] In some embodiments, the preparation method includes at least one of the following features (1) to (4):

[0020] (1) The oxidizing gas includes at least one of oxygen, ozone, nitrogen dioxide and sulfur trioxide;

[0021] (2) The carbon-based raw material includes at least one of biomass shell materials, biomass wood materials, straw, corn cobs, starch, sucrose, asphalt, coal and coke;

[0022] (3) The oxygen content in the oxidizing gas is 5% to 100%;

[0023] (4) The pre-oxidation treatment time is 2 min to 600 min.

[0024] In some embodiments, the preparation method includes at least one of the following features (1) to (3):

[0025] (1) The concentration of the etching gas is 5% to 100%;

[0026] (2) The etching treatment time is 2 min to 600 min;

[0027] (3) The median particle size of the third precursor is 3 μm to 20 μm.

[0028] In some embodiments, the second precursor includes a porous carbon-based material, and the second precursor includes at least one of the following features (1) to (5):

[0029] (1) The average pore size of the porous carbon-based material is less than or equal to 3 nm;

[0030] (2) The porous carbon-based material includes micropores, and the pore volume of the micropores is greater than or equal to 0.05 cm 3 / g;

[0031] (3) The porous carbon-based material includes micropores, and the volume of the micropores accounts for greater than or equal to 60% based on the total volume of the pores as 100%;

[0032] (4) The volatile matter of the porous carbon-based material is 8% to 18%;

[0033] (5) The porous carbon-based material contains a doping element, and the doping element includes at least one of nitrogen, oxygen and sulfur. The mass proportion of the doping element in the porous carbon-based material is 3% to 15%.

[0034] In some embodiments, before subjecting the third precursor to heat treatment, the process further includes subjecting the third precursor to acid washing.

[0035] In some embodiments, the preparation method includes at least one of the following features (1) to (3):

[0036] (1) The pickling solution includes at least one of hydrochloric acid, nitric acid and hydrofluoric acid;

[0037] (2) The concentration of the pickling solution is 10% to 30%;

[0038] (3) The pickling solution includes at least one of hydrochloric acid, nitric acid and hydrofluoric acid, and the volume ratio of the third precursor, hydrochloric acid, nitric acid and hydrofluoric acid is 1:(1-6):(0-6):(0-1).

[0039] In some embodiments, the preparation method includes at least one of the following features (1) to (4):

[0040] (1) The temperature of the heat treatment is 1000° C. to 1600° C.;

[0041] (2) the heat treatment is carried out in a protective gas atmosphere, wherein the protective gas comprises at least one of nitrogen, argon, neon, helium, xenon and krypton;

[0042] (3) The heat treatment time is 1 h to 6 h;

[0043] (4) The heat treatment equipment includes at least one of a tubular furnace, a box furnace, a push plate kiln, a graphitization furnace and a roasting kiln.

[0044] In a third aspect, the present application provides a sodium ion battery, wherein the sodium ion battery comprises the negative electrode material described in the first aspect or comprises the negative electrode material described in the second aspect.

[0045] The technical solution of this application has at least the following beneficial effects:

[0046] The negative electrode material of the present application comprises a carbon matrix having pores, the pores comprising closed pores, and the pore volume of the closed pores is greater than 0.025 cm 3 / g, indicating that the carbon matrix of the present application has abundant closed pores, which is conducive to the filling of sodium ions, forming filled sodium storage, and improving the sodium storage capacity of the negative electrode material, thereby improving the capacity performance and first efficiency of the negative electrode material. At the same time, the negative electrode material of the present application contains doping elements, and the doping elements include at least one of nitrogen, sulfur and oxygen. The mass proportion of the doping elements in the second region is 0.5% to 5%, and the mass proportion of the doping elements in the first region is less than 0.5%, indicating that the doping element 3 of the present application is mainly distributed in the surface area of ​​the negative electrode material, and is almost not distributed in the interior of the negative electrode material. There are many defective carbon atoms on the surface of the negative electrode material. The doping elements with more surface areas can form reversible functional groups such as C=O, C=N, and C=S with the defective carbon atoms on the surface of the negative electrode material. The above reversible functional groups can form reversible sodium storage sites with sodium ions, such as C=O and sodium to form C-ONa. In the subsequent charging process, Na will be released and then form C=O, so that the surface of the negative electrode material can adsorb more sodium ions, forming adsorbed sodium storage, and improving the sodium storage capacity of the negative electrode material. In addition, the potential at which the above-mentioned reversible functional group reacts with the solvated sodium ions (for example, the reaction potential of C=O with sodium ions is ~0.5V) is higher than the platform potential of the sodium ion intercalation (the platform voltage of the sodium ion intercalation is generally <0.1V), so the above-mentioned reversible functional group can react with the solvated sodium ions preferentially, which is beneficial to the desolvation of the sodium ions, thereby reducing the polarization in the electrochemical process, improving the voltage platform of the negative electrode material, and reducing the occurrence of sodium precipitation in the pole piece. The carbon matrix of the negative electrode material is tightly arranged inside, with fewer defective carbon atoms, and the doping elements entering the negative electrode material inevitably cause the carbon matrix structure to deform and change the electronic energy band structure. During the charge and discharge process of the negative electrode material, the sodium ions enter the carbon matrix to form an electric field. The existence of the electric field hinders the subsequent diffusion of sodium ions into the carbon matrix. At the same time, the sodium ions entering the carbon matrix are not easy to escape under the action of the electric field. Therefore, the present application can reduce the difficulty of sodium ion deintercalation in the negative electrode material during the charge and discharge process by controlling the mass proportion of the doping elements in the first region to be less than 0.5%, thereby improving the initial efficiency and capacity of the negative electrode material. BRIEF DESCRIPTION OF THE DRAWINGS

[0047] The present application is further described below in conjunction with the accompanying drawings and embodiments.

[0048] Figure 1 This is a schematic diagram of the structure of the negative electrode material of this application;

[0049] Figure 2 This is a flow chart for preparing the negative electrode material of this application;

[0050] Figure 3 A schematic diagram of the structure of the second precursor prepared for this application;

[0051] Figure 4This is a comparison diagram of the cumulative distribution of the pore volume of the second precursor in Example 1 of the present application and Comparative Example 1;

[0052] Figure 5 The energy spectrum distribution diagram of carbon and oxygen elements of the negative electrode material of Example 1 of the present application;

[0053] Figure 6 The SEM image and cross-sectional EDS-mapping image of the negative electrode material prepared in Example 1;

[0054] Figure 7 The SEM image and cross-sectional EDS-mapping image of the negative electrode material prepared in Comparative Example 1;

[0055] Figure 8 Comparison of the first cycle charge and discharge of the negative electrode materials prepared in Example 1 and Comparative Example 1;

[0056] Fig. 9 Comparison of discharge depth and voltage of the negative electrode materials prepared in Example 1 and Comparative Example 1.

[0057] Figure 1 middle:

[0058] 1-Carbon matrix;

[0059] 11- First area;

[0060] 12-Second area;

[0061] 2- Porosity;

[0062] 3- Doping elements. DETAILED DESCRIPTION

[0063] In order to better understand the technical solution of the present application, the embodiments of the present application are described in detail below with reference to the accompanying drawings.

[0064] It should be clear that the described embodiments are only part of the embodiments of the present application, rather than all the embodiments. Based on the embodiments in the present application, all other embodiments obtained by ordinary technicians in the field without creative work are within the scope of protection of the present application.

[0065] The terms used in the embodiments of the present application are only for the purpose of describing specific embodiments, and are not intended to limit the present application. The singular forms "a", "said" and "the" used in the embodiments of the present application and the appended claims are also intended to include plural forms, unless the context clearly indicates other meanings.

[0066] It should be understood that the term "and / or" used in this article is only a description of the association relationship of associated objects, indicating that there can be three relationships. For example, A and / or B can represent: A exists alone, A and B exist at the same time, and B exists alone. In addition, the character " / " in this article generally indicates that the associated objects before and after are in an "or" relationship.

[0067] In the prior art, for the preparation of carbon negative electrode materials for sodium ion batteries from biomass raw materials, a technical route is usually adopted in which the biomass material precursor is pre-carbonized, crushed, purified, and high-temperature carbonized. The pre-carbonization process is usually carried out under a protective atmosphere such as nitrogen and argon, or the capacity of the negative electrode material is increased by pore formation through alkaline activation. However, the cost of the above process is relatively high, and the alkaline-activated pores are mostly mesopores (mesopores). During the high-temperature carbonization process, closed pores cannot be formed, thereby increasing the specific surface area of ​​the negative electrode material, resulting in an increase in the specific surface area of ​​the negative electrode material and a decrease in the initial efficiency and capacity.

[0068] The present application provides a negative electrode material, such as Figure 1 As shown, the negative electrode material includes a carbon matrix 1, the carbon matrix 1 has pores 2, the pores 2 include closed pores, and the pore volume of the closed pores is greater than 0.025 cm 3 / g;

[0069] The negative electrode material contains a doping element 3, and the doping element 3 includes at least one of nitrogen, sulfur and oxygen. The negative electrode material has a first region 11 located in the central area of ​​the negative electrode material and a second region 12 distributed on the surface of the first region 11. The negative electrode material is tested by an EDS (Energy Dispersive Spectrometer) spectrometer. The mass proportion of the doping element in the second region is 0.5% to 5%, and the mass proportion of the doping element in the first region is less than 0.5%.

[0070] In the above scheme, the negative electrode material of the present application includes a carbon matrix 1, the carbon matrix 1 has pores 2, the pores 2 include closed pores, and the pore volume of the closed pores is greater than 0.025 cm 3 / g, within the above-defined range, it is shown that the carbon matrix 1 of the present application has abundant closed pores, which is conducive to the filling of sodium ions, forming filled sodium storage, and improving the sodium storage capacity of the negative electrode material, thereby improving the capacity performance and first efficiency of the negative electrode material. At the same time, the negative electrode material of the present application contains doping elements 3, and the doping elements 3 include at least one of nitrogen, sulfur and oxygen. The mass proportion of the doping elements in the second region is 0.5% to 5%, and the mass proportion of the doping elements in the first region is less than 0.5%, indicating that the doping elements 3 of the present application are mainly distributed in the surface area of ​​the negative electrode material, and are almost not distributed in the interior of the negative electrode material. There are many defective carbon atoms on the surface of the negative electrode material. The doping elements with more surface areas can form reversible functional groups such as C=O, C=N, and C=S with the defective carbon atoms on the surface of the negative electrode material. The above reversible functional groups can form reversible sodium storage sites with sodium ions, such as C=O and sodium to form C-ONa, and Na will be released in the subsequent charging process to form C=O, so that the surface of the negative electrode material can adsorb more sodium ions, forming adsorbed sodium storage, and improving the sodium storage capacity of the negative electrode material. In addition, the potential at which the above-mentioned reversible functional group reacts with the solvated sodium ions (for example, the reaction potential of C=O with sodium ions is ~0.5V) is higher than the platform potential of the sodium ion intercalation (the platform voltage of the sodium ion intercalation is generally <0.1V), so the above-mentioned reversible functional group can react with the solvated sodium ions preferentially, which is beneficial to the desolvation of the sodium ions, thereby reducing the polarization in the electrochemical process, improving the voltage platform of the negative electrode material, and reducing the occurrence of sodium precipitation in the pole piece. The carbon matrix 1 of the negative electrode material is tightly arranged inside, with fewer defective carbon atoms, and the doping elements entering the negative electrode material inevitably cause the carbon matrix 1 structure to deform and change the electronic energy band structure. During the charge and discharge process of the negative electrode material, the sodium ions enter the carbon matrix 1 to form an electric field. The existence of the electric field hinders the subsequent diffusion of the sodium ions into the carbon matrix 1. At the same time, the sodium ions entering the carbon matrix 1 are not easy to escape under the action of the electric field. Therefore, the present application can reduce the difficulty of sodium ion deintercalation in the negative electrode material during the charge and discharge process by controlling the mass proportion of the doping elements in the first region to be less than 0.5%, thereby improving the initial efficiency and capacity of the negative electrode material.

[0071] The negative electrode material particles of the present application can be divided into two regions, such as Figure 1As shown, the negative electrode material particles of the present application have a first region 11 and a second region 12, and the second region 12 is a region extending from the surface of the negative electrode material to the central region inside the negative electrode material with a thickness of less than or equal to 1 μm. Specifically, the thickness of the second region can be 1nm, 5nm, 10nm, 100nm, 300nm, 500nm, 800nm ​​or 1μm, etc., and of course it can also be other values ​​within the above range, and the present application does not limit it here. In the second region 12, sodium ions can react with doping elements to form reversible functional groups, thereby forming reversible sodium storage sites, so that the functional groups of the negative electrode material can remove sodium ions during the charging process, and reversibly react again, which is conducive to the adsorption of more sodium ions on the surface of the negative electrode material, thereby increasing the capacity of the negative electrode material. Preferably, the thickness of the second region 12 is 10 nm to 100 nm, and the first region 11 is the other region of the negative electrode material except the second region 12. The present application does not impose specific restrictions on the specific boundaries of the first region 11 and the second region 12. It can be understood that the contact area between the first region 11 and the second region 12 can be a situation where there is an obvious interface at the edge of the coating layer, or a situation where there is no obvious interface and a smooth transition. Figure 1 In, through Figure 1 The dashed line shown in FIG. 1 distinguishes the first region 11 from the second region 12 .

[0072] In some embodiments, the pore volume of closed cells is greater than 0.025 cm 3 / g, specifically 0.028cm 3 / g, 0.030cm 3 / g, 0.035cm 3 / g, 0.038cm 3 / g, 0.040cm 3 / g, 0.045cm 3 / g or 0.050cm 3 / g, etc., and of course, it can also be other values ​​within the above range, which is not limited here. Within the above limited range, it is shown that the carbon matrix of the present application has abundant closed pores, which is conducive to the filling of sodium ions and is conducive to improving the sodium storage performance of the negative electrode material.

[0073] The method for testing the mass proportion of the doping element in the first region and the mass proportion of the doping element in the second region of the present application specifically includes the following steps:

[0074] The negative electrode material particles are sectioned by a focused ion beam FIB (The Vion PFIB, FEI Titan, manufactured by FEI Company) instrument, and the Ultim 170 spectrometer (UK, Oxford Instruments Ltd.) is used to test and select the section area of ​​the material to obtain the distribution of nitrogen, oxygen and sulfur elements inside the material (first area). The mass proportion of nitrogen in the first area, the mass proportion of oxygen in the first area, and the mass proportion of sulfur in the first area can be obtained by calculation. The mass proportions of the above three mass proportions are added together to obtain the mass proportions of nitrogen, oxygen and sulfur elements in the negative electrode material in the first area.

[0075] The surface of the negative electrode material particles was directly measured by an Ultim 170 spectrometer (Oxford Instruments Ltd., UK). According to the test conditions of the equipment, the actual measured thickness was less than or equal to 1 μm, and the distribution of nitrogen, oxygen and sulfur elements in the surface area of ​​the material (i.e., the second area) was obtained. The mass proportion of nitrogen in the second area, the mass proportion of sulfur in the second area, and the mass proportion of oxygen in the second area were obtained by calculation. The mass proportions of the above three mass proportions were added together to obtain the mass proportions of nitrogen, oxygen and sulfur elements in the negative electrode material in the second area.

[0076] In some embodiments, the mass proportion of the doping element 3 in the second region 12 is 0.5% to 5%, specifically 0.5%, 1%, 2%, 3%, 4% or 5%, etc., and of course, it can also be other values ​​within the above range, which are not limited here. If the mass proportion of the doping element 3 in the second region 12 is less than 0.5%, the doping element accounts for too little, and the doping element cannot react with a sufficient amount of sodium ions, resulting in a reduced desolvation effect, an increased degree of polarization, and a reduced charging voltage platform; if the mass proportion of the doping element 3 in the second region 12 is greater than 5%, the side reaction of the doping element and the electrolyte increases, resulting in a reduced first efficiency of the negative electrode material.

[0077] In some embodiments, the mass proportion of the doping element 3 in the first region 11 is less than 0.5%, and can be specifically 0%, 0.1%, 0.2%, 0.3%, 0.4% or 0.45%, etc., and of course, it can also be other values ​​within the above range, which are not limited here. If the mass proportion of the doping element 3 in the first region 11 is greater than or equal to 0.5%, the doping of the doping element in the carbon matrix 1 causes the structure of the carbon matrix 1 to deform and change the electronic band structure, thereby reducing the initial efficiency and capacity of the negative electrode material.

[0078] In some embodiments, the carbon matrix 1 includes hard carbon, that is, the present application is a hard carbon negative electrode material. During the use of the negative electrode material, sodium ions can be inserted into the hard carbon carbon layers to form intercalation sodium storage, thereby improving the energy storage capacity of the negative electrode material.

[0079] In the present application, sodium ions are inserted into the carbon layers of hard carbon to form intercalated sodium storage, and doping elements adsorbed on the surface of the negative electrode material by sodium ions can form reversible functional groups. The reversible functional groups form adsorbed sodium storage, and sodium ions are filled in the closed pores of the negative electrode material to form filled sodium storage. That is, the present application improves the sodium storage performance of the negative electrode material through the "adsorption-intercalation-filling" method, improves the capacity performance of the negative electrode material, and increases the platform voltage, thereby reducing the occurrence of sodium precipitation in the electrode sheet.

[0080] In some embodiments, the mass percentage of the doping element in the negative electrode material is 0.5% to 2%, specifically 0.5%, 1%, 1.5% or 2%, etc., and of course, it can also be other values ​​within the above range, which is not limited here.

[0081] In some embodiments, the average pore size of the closed pores is less than or equal to 5 nm, for example, it can be 0.5 nm, 1 nm, 2 nm, 3 nm, 4 nm or 5 nm, etc., and of course it can be other values ​​within the above range, which is not limited here. Within the above-defined range, it indicates that the closed pore size of the present application is smaller, then the number of closed pores in the negative electrode material is larger, and the total area of ​​the inner surface of the closed pores in the material is larger. The closed pores with a larger total inner surface area are beneficial to increase the adsorption surface area of ​​sodium ions in the negative electrode material during the charge and discharge process, thereby improving the capacity performance and initial efficiency of the negative electrode material.

[0082] In some embodiments, the specific surface area of ​​the negative electrode material is 2 m 2 / g~20m 2 / g, specifically 2m 2 / g, 5m 2 / g, 8m 2 / g, 10m 2 / g, 12m 2 / g, 15m 2 / g, 18m 2 / g or 20m 2 / g, etc., and of course, it can also be other values ​​within the above range, which is not limited here. Within the above limited range, it indicates that the specific surface area of ​​the negative electrode material is appropriate, which is conducive to improving the initial efficiency of the negative electrode material.

[0083] In some embodiments, the true density of the negative electrode material is 1.8 cm 3 / g~2.2cm 3 / g, specifically 1.8cm 3 / g, 1.9cm 3 / g, 2.0cm 3 / g, 2.1cm 3 / g and 2.2cm 3 / g, etc., and of course, it can be other values ​​within the above range, which is not limited here. Within the above limited range, it shows that the true density of the negative electrode material of the present application is relatively high, which is beneficial to improving the initial charge and discharge specific capacity and initial efficiency of the negative electrode material.

[0084] In some embodiments, the median particle size of the negative electrode material is 3 μm to 20 μm, specifically 3 μm, 5 μm, 8 μm, 10 μm, 13 μm, 15 μm, 18 μm and 20 μm, etc., and of course it can also be other values ​​within the above range, which is not limited here.

[0085] The present application also provides a method for preparing a negative electrode material, such as Figure 2 As shown, the following steps are included:

[0086] Step S100, placing the carbon-based raw material in an oxidizing gas atmosphere for pre-oxidation treatment to obtain a first precursor, the pre-oxidation treatment temperature is 200°C to 350°C.

[0087] Step S200, placing the first precursor in an etching gas atmosphere for etching treatment to obtain a second precursor, the etching treatment temperature is 400°C to 800°C, and the etching gas includes at least one of oxygen, ozone, sulfur dioxide, sulfur trioxide, nitrogen dioxide, water vapor and magnesium vapor.

[0088] Step S300: crushing the second precursor to obtain a third precursor.

[0089] Step S400: heat-treating the third precursor to obtain a negative electrode material.

[0090] In the above scheme, the present application can introduce a large amount of doping elements (at least one of oxygen, nitrogen and sulfur) on the surface of the carbon-based raw material by pre-oxidizing the carbon-based raw material at a temperature of 200°C to 350°C and performing medium-temperature vapor phase etching at a temperature of 400°C to 800°C. Since the temperature of the pre-oxidation and etching treatment is relatively low, the doping elements can enter the surface of the carbon-based raw material more and almost not enter the interior of the carbon-based raw material, thereby making the negative electrode material meet the following conditions: the mass proportion of the doping elements in the second region (the surface region of the negative electrode material) is 0.5% to 5%, and the mass proportion of the doping elements in the first region (the internal region of the negative electrode material) is less than 0.5%. The doping elements can form reversible functional groups C=O, C=N, C=S, etc. with carbon atoms on the surface of the carbon-based raw material, and the reversible functional groups and sodium ions react reversibly to form reversible sodium storage sites, so that the surface of the negative electrode material can adsorb more sodium ions, forming adsorption storage sodium, and improving the sodium storage capacity of the negative electrode material. In addition, the above-mentioned reversible functional groups react preferentially with solvated sodium ions, which is beneficial to the desolvation of sodium ions, thereby reducing polarization in the electrochemical process, increasing the voltage platform of the negative electrode material, and reducing the occurrence of sodium precipitation. During the etching process, the etching gas can also etch and form pores in the carbon-based raw materials. The internal diffusion path of the gas-phase etching gas in the carbon-based raw materials conforms to the diffusion trend of the material. The etching gas forms dendritic pores in the carbon-based raw materials. In the subsequent crushing process, the carbon-based raw materials will be crushed along the larger pores in the dendritic pores due to stress, thereby leaving a large number of micropores in the negative electrode material. Finally, during the heat treatment process, the micropores are closed to form closed pores, and the pore volume of the closed pores is greater than 0.025cm 3 / g, which is beneficial to the filling of sodium ions and improves the sodium storage capacity of the negative electrode material, thereby improving the capacity performance and initial efficiency of the negative electrode material.

[0091] The preparation method of the present application is described in detail below with reference to the examples:

[0092] Step S100, placing the carbon-based raw material in an oxidizing gas atmosphere for pre-oxidation treatment to obtain a first precursor, the pre-oxidation treatment temperature is 200°C to 350°C.

[0093] In some embodiments, the temperature of the pre-oxidation treatment is 200°C to 350°C, specifically 200°C, 230°C, 250°C, 280°C, 300°C or 350°C, etc., and of course other values ​​within the above range may also be used, which is not limited here. Within the above-defined range, the temperature of the pre-oxidation treatment of the present application is relatively low, so that the oxidizing gas is mainly on the surface of the carbon-based raw material, and reacts with the carbon atoms in the carbon-based raw material to generate reversible functional groups. If the temperature of the pre-oxidation treatment is lower than 200°C, the reaction process is mainly the dehydration of the carbon-based raw material, and the pre-oxidation effect of the carbon-based raw material cannot be achieved, resulting in a lower oxygen content on the surface of the negative electrode material, thereby resulting in a lower voltage platform; if the temperature of the pre-oxidation treatment is higher than 350°C, the temperature is too high, resulting in spontaneous combustion of part of the carbon-based raw material at the pre-oxidation temperature, and the yield of the carbon-based raw material is greatly reduced. At the same time, the oxidizing gas can enter the interior of the carbon-based raw material more, so that the content of doping elements on the surface of the negative electrode material is less, resulting in a lower initial efficiency, capacity and voltage platform of the negative electrode material.

[0094] In some embodiments, the oxidizing gas includes at least one of oxygen, ozone, nitrogen dioxide and sulfur trioxide. The above-mentioned oxidizing gas can introduce at least one of the doping elements oxygen, nitrogen and sulfur into the carbon-based raw material during the pre-oxidation process. The above-mentioned doping elements can react with the carbon-based raw material at 200°C to 350°C to generate reversible energy groups such as C=O, C=N, C=S, etc.

[0095] In some embodiments, the carbon-based raw material includes at least one of biomass shell materials, biomass wood materials, straw, corn cobs, starch, sucrose, asphalt, coal and coke. Exemplarily, the biomass shell materials include apricot shells, walnut shells, coconut shells, etc., and the biomass wood materials include bamboo, pine branches, etc.

[0096] In some embodiments, the oxygen content in the oxidizing gas is 5% to 100%, specifically 5%, 10%, 20%, 30%, 40%, 50%, 60%, 70%, 80%, 90% or 100%, etc. Of course, it can also be other values ​​within the above range, which is not limited here.

[0097] In some embodiments, the pre-oxidation treatment time is 2 min to 600 min, specifically 2 min, 10 min, 30 min, 60 min, 90 min, 120 min, 180 min, 240 min, 360 min, 480 min or 600 min, etc. Of course, it can also be other values ​​within the above range, which is not limited here.

[0098] Step S200, placing the first precursor in an etching gas atmosphere for etching treatment to obtain a second precursor, the etching treatment temperature is 400°C to 800°C, and the etching gas includes at least one of oxygen, ozone, sulfur dioxide, sulfur trioxide, nitrogen dioxide, water vapor and magnesium vapor.

[0099] In this step, the first precursor is placed in an etching gas atmosphere for etching treatment. The etching gas can enter the interior of the carbon-based raw material and diffuse from the surface of the carbon-based raw material to the interior of the carbon-based raw material to form dendritic pores. Under the medium-temperature etching treatment conditions of 400°C to 800°C, according to the diffusion law, at least one of the nitrogen, sulfur and oxygen elements contained in the etching gas is mainly introduced into the second region of the surface of the first precursor, and less or almost does not enter the first region of the negative electrode material, further increasing the content of doping elements in the second region of the negative electrode material. The structural schematic diagram of the second precursor is shown in FIG. Figure 3 shown.

[0100] In some embodiments, the temperature of the etching treatment is 400°C to 800°C, specifically 400°C, 500°C, 600°C, 700°C or 800°C, etc., and of course other values ​​within the above range, which are not limited here. Within the above-defined range, the etching gas can enter the carbon matrix and react with the carbon elements in the carbon matrix, thereby etching and forming pores. If the temperature of the etching treatment is less than 400°C, the etching rate is slow, and it is mostly surface etching, and it is impossible to form an effective pore distribution inside the negative electrode material, reducing the filling of sodium ions in the negative electrode material, thereby reducing the capacity performance of the negative electrode material; if the temperature of the etching treatment is greater than 800°C, the etching rate is too fast, which is easy to cause the first precursor to spontaneously combust, and the pore size is large, and it is difficult to form closed pores during the subsequent heat treatment process, resulting in an excessively large specific surface area of ​​the negative electrode material, reducing the initial efficiency of the negative electrode material, and the higher temperature will cause the doping elements of the negative electrode material to diffuse inward, reducing the doping element content in the surface area of ​​the negative electrode material.

[0101] In some embodiments, the etching gas includes at least one of oxygen, ozone, sulfur dioxide, sulfur trioxide, nitrogen dioxide, water vapor and magnesium vapor. It is understood that in some embodiments, the oxidizing gas and the etching gas can be the same. In the pre-oxidation step, due to the low pre-oxidation temperature, only a small amount of doping elements exist in the surface area of ​​the carbon-based raw material. In the etching step, the etching temperature is high, and the doping elements carried in the etching gas can enter the carbon-based raw material to react, further increasing the mass proportion of the doping elements in the second region.

[0102] In some embodiments, the concentration of the etching gas is 5% to 100%, specifically 5%, 10%, 20%, 30%, 40%, 50%, 60%, 70%, 80%, 90% or 100%, etc. Of course, it can also be other values ​​within the above range, which is not limited here.

[0103] In some embodiments, the etching treatment time is 2 min to 600 min, which can be 2 min, 10 min, 30 min, 60 min, 90 min, 120 min, 180 min, 240 min, 360 min, 480 min or 600 min, etc. Of course, it can also be other values ​​within the above range, which is not limited here.

[0104] In some embodiments, the second precursor obtained by the etching process includes a porous carbon-based material.

[0105] In some embodiments, the average pore size of the pores is less than or equal to 3 nm, and can be specifically 0.5 nm, 1 nm, 1.5 nm, 2 nm, 2.5 nm or 3 nm, etc., and of course, it can also be other values ​​within the above range, which are not limited here. Within the above-mentioned limited range, it is beneficial to increase the storage sites of sodium ions and improve the sodium storage capacity of the porous carbon-based material. If the average pore size of the pores is greater than 3 nm, it is not conducive to the closure of the pores of the second precursor during the subsequent heat treatment process, reducing the sodium storage sites of the porous carbon-based material, and the non-closure of the pores leads to an increase in the specific surface area of ​​the porous carbon-based material, which in turn leads to a decrease in the first efficiency of the porous carbon-based material.

[0106] In some embodiments, the pores include micropores, and the pore volume of the micropores is greater than or equal to 0.05 cm 3 / g, specifically 0.05cm 3 / g, 0.051cm 3 / g, 0.055cm 3 / g, 0.058cm 3 / g, 0.061cm 3 / g, 0.065cm 3 / g or 0.07cm 3 / g, etc., of course, it can also be other values ​​within the above range, which is not limited here. Within the above-defined range, it shows that more effective pores can be formed inside the porous carbon-based material through etching treatment, which is beneficial for the porous carbon-based material to form more closed pores during the subsequent heat treatment process, thereby increasing the sodium storage sites of the porous carbon-based material. It can be understood that micropores refer to pores with a pore size of less than 2nm. The single-point adsorption total pore volume can be obtained by nitrogen adsorption test, and the unit is cm 3 / g.

[0107] In some embodiments, the volume of the micropores is greater than or equal to 60% based on the total volume of the pores as 100%. The volume of the micropores may be, for example, 60%, 65%, 70%, 75%, 80%, 85%, 90% or 95%, etc. Of course, it may be other values ​​within the above range, which are not limited here. Within the above-defined range, it is shown that the porous carbon-based material of the present application is a porous structure containing abundant micropores. The abundant micropores can fill more sodium ions to form filled sodium storage, which is beneficial to improve the sodium storage capacity of the porous carbon-based material.

[0108] In some embodiments, the volatile matter of the porous carbon-based material is 8% to 18%, specifically 8%, 10%, 12%, 15% or 18%, etc., and of course, it can also be other values ​​within the above range, which is not limited here. Within the above-mentioned limited range, it indicates that there are sufficient doping elements such as oxygen, nitrogen, and sulfur in the porous carbon-based material, which is conducive to the formation of reversible functional groups between the doping elements and the carbon atoms in the porous carbon-based material, which is conducive to increasing the sodium storage sites. It can be understood that the volatile matter refers to the part of a substance that can volatilize under certain conditions.

[0109] In some embodiments, the porous carbon-based material contains a doping element, and the doping element includes at least one of nitrogen, oxygen and sulfur. The mass proportion of the doping element in the porous carbon-based material is 3% to 15%, specifically 3%, 5%, 8%, 10%, 12% or 15%, etc., and of course, it can also be other values ​​within the above range, which are not limited here. It can be understood that the mass proportion of the doping element in the porous carbon-based material refers to the mass proportion of at least one of oxygen, nitrogen and sulfur in the second precursor, which can be calculated by measuring the content of oxygen, nitrogen and sulfur in the porous carbon-based material using FlashSmart organic element analyzer (Thermo Fisher Scientific, USA).

[0110] Step S300: crushing the second precursor to obtain a third precursor.

[0111] In this step, the second precursor is crushed. During the crushing process, due to stress, the larger pores in the dendritic pores are crushed, thereby leaving a large number of pores with smaller pore sizes in the third precursor.

[0112] In some embodiments, the pulverizing equipment includes at least one of a ball mill pulverizing equipment, a jet mill pulverizing equipment, and a mechanical pulverizing equipment.

[0113] In some embodiments, the median particle size of the third precursor is 3 μm to 20 μm, for example, 3 μm, 5 μm, 8 μm, 10 μm, 12 μm, 15 μm, 18 μm or 20 μm, etc., and of course, it can also be other values ​​within the above range, which is not limited here.

[0114] In some embodiments, the crushing process further includes: a step of pickling the crushed material, the purpose of which is to remove impurity elements such as metal and silicon in the crushed material.

[0115] In some embodiments, the pickling solution includes at least one of hydrochloric acid, nitric acid and hydrofluoric acid. It is understood that the pickling solution can be a single hydrochloric acid, nitric acid or hydrofluoric acid, or a combination of any two of hydrochloric acid, nitric acid and hydrofluoric acid, or a mixed solution of hydrochloric acid, nitric acid and hydrofluoric acid.

[0116] In some embodiments, the concentration of the pickling solution is 10% to 30%, specifically 10%, 15%, 20%, 25% or 30%, etc. Of course, it can also be other values ​​within the above range, which is not limited here.

[0117] In some embodiments, the volume ratio of the third precursor, hydrochloric acid, nitric acid and hydrofluoric acid is 1:(1-6):(0-6):(0-1), and the volume ratio of the third precursor, hydrochloric acid, nitric acid and hydrofluoric acid can be 1:5:0:0, 1:3:2:0.5, 1:6:1:0.3, 1:1:6:1 or 1:3:3:0.5, etc., and of course it can also be other values ​​within the above range, which is not limited here.

[0118] Step S400: heat-treating the third precursor to obtain a negative electrode material.

[0119] In some embodiments, the temperature of the heat treatment is 1000°C to 1600°C, specifically 1000°C, 1100°C, 1200°C, 1300°C, 1400°C, 1500°C or 1600°C, etc., and of course other values ​​within the above range, which are not limited here. Within the above-defined range, it is beneficial to completely carbonize the carbon-based raw materials, improve the capacity performance and cycle performance of the negative electrode material, and at the same time, the reversible functional groups C=O, C=N, C=S and other double bond functional groups can be stably present.

[0120] In some embodiments, the heat treatment is performed in a protective gas atmosphere, and the protective gas includes at least one of nitrogen, argon, neon, helium, xenon, and krypton.

[0121] In some embodiments, the heat treatment time is 1 h to 6 h, specifically 1 h, 2 h, 3 h, 4 h, 5 h or 6 h, etc. Of course, it can also be other values ​​within the above range, which is not limited here.

[0122] In some embodiments, the heat treatment equipment includes at least one of a tube furnace, a box furnace, a push plate kiln, a graphitization furnace, and a roasting kiln.

[0123] The embodiment of the present application also provides a sodium ion battery, which comprises a positive electrode sheet, a negative electrode sheet and a separator disposed between the positive electrode sheet and the negative electrode sheet. The negative electrode sheet comprises a current collector and a negative electrode active material layer. The negative electrode active material layer comprises a negative electrode active material, a conductive agent and a binder. The positive electrode active material layer may comprise a positive electrode active material, a conductive agent and a binder. The positive electrode current collector may be Al foil, and similarly, other positive electrode current collectors commonly used in the art may also be used. Among them, the negative electrode active material comprises the above-mentioned negative electrode material.

[0124] The present application is further described below through specific embodiments.

[0125] Example 1

[0126] (1) Take 100 g of 100-mesh bamboo powder and place it in a tube furnace. Pass compressed air (oxygen concentration 20%) at an air flow rate of 100 mL / min. The temperature curve is: increase the temperature to 250°C at 5°C / min and keep it for 5 h.

[0127] (2) The material obtained in step (1) is further heated to 500° C. and kept at this temperature for 4 h to obtain a second precursor.

[0128] (3) Take 20g of the second precursor and perform ball milling. The material after ball milling has a D50 of 6μm. Then add 60mL of 12% hydrochloric acid and heat to 75°C in a water bath. Stir for 2h, filter and wash with water until neutral, and dry in an oven at 80°C.

[0129] (4) The material obtained in step (3) was placed in a high-temperature tube furnace and sintered at 1300° C. for 3 h in a nitrogen atmosphere to obtain a negative electrode material.

[0130] The negative electrode material prepared in this embodiment includes a carbon matrix, and the carbon matrix has pores, and the pores include closed pores. The test results of the specific surface area, average pore size, micropore volume, volatile matter and total content of oxygen, nitrogen and sulfur of the second precursor are shown in Table 1. The test results of the average pore size, pore volume, mass proportion of the doping element in the second region in the second region, mass proportion of the doping element in the first region, oxygen content, specific surface area, true density and median particle size of the negative electrode material are shown in Table 2.

[0131] Example 2

[0132] (1) Take 100 g of 100-mesh bamboo powder and place it in a tube furnace. Pass nitrogen dioxide and air at a flow rate of 20 mL / min. The temperature curve is: increase the temperature to 280°C at 5°C / min and keep it at this temperature for 5 h.

[0133] (2) The material obtained in step (1) is further heated to 700° C. and kept at this temperature for 4 h to obtain a second precursor.

[0134] (3) Take 20g of the second precursor and perform ball milling. The material after ball milling has a D50 of 6μm. Then add 60mL of 12% hydrochloric acid and heat to 75°C in a water bath. Stir for 2h, filter and wash with water until neutral, and dry in an oven at 80°C.

[0135] (4) The material obtained in step (3) was placed in a high-temperature tube furnace and sintered at 1300° C. for 3 h in a nitrogen atmosphere to obtain a negative electrode material.

[0136] The negative electrode material prepared in this embodiment includes a carbon matrix having pores, and the pores include closed pores. The test results of the specific surface area, average pore size, micropore volume, volatile matter and total content of oxygen, nitrogen and sulfur of the second precursor are shown in Table 1. The test results of the average pore size, pore volume, mass proportion of the doping element in the second region, mass proportion of the doping element in the first region, oxygen content, specific surface area, true density and median particle size of the closed pores are shown in Table 2.

[0137] Example 3

[0138] (1) Take 100 g of 100-mesh bamboo powder and place it in a tube furnace. Pass sulfur dioxide and air at a flow rate of 20 mL / min. The temperature curve is: increase the temperature to 320°C at 5°C / min and keep it warm for 5 h.

[0139] (2) The material obtained in step (1) is further heated to 600° C. and kept at this temperature for 4 h to obtain a second precursor.

[0140] (3) Take 20g of the second precursor and perform ball milling. The material after ball milling has a D50 of 6μm. Then add 60mL of 12% hydrochloric acid and heat to 75°C in a water bath. Stir for 2h, filter and wash with water until neutral, and dry in an oven at 80°C.

[0141] (4) The material obtained in step (3) was placed in a high-temperature tube furnace and sintered at 1300° C. for 3 h in a nitrogen atmosphere to obtain a negative electrode material.

[0142] The negative electrode material prepared in this embodiment includes a carbon matrix, and the carbon matrix has pores, and the pores include closed pores. The test results of the specific surface area, average pore size, micropore volume, volatile matter and total content of oxygen, nitrogen and sulfur of the second precursor are shown in Table 1. The test results of the average pore size, pore volume, mass proportion of the doping element in the second region in the second region, mass proportion of the doping element in the first region, oxygen content, specific surface area, true density and median particle size of the negative electrode material are shown in Table 2.

[0143] Example 4

[0144] The difference from Example 1 is that: (1) 100 g of 100-mesh bamboo powder was placed in a tube furnace, and compressed air (oxygen concentration 20%) was introduced at an air flow rate of 100 mL / min. The temperature curve was: 3°C / min, the temperature was raised to 200°C and kept warm for 5 h.

[0145] The negative electrode material prepared in this embodiment includes a carbon matrix, and the carbon matrix has pores, and the pores include closed pores. The test results of the specific surface area, average pore size, micropore volume, volatile matter and total content of oxygen, nitrogen and sulfur of the second precursor are shown in Table 1. The test results of the average pore size, pore volume, mass proportion of the doping element in the second region in the second region, mass proportion of the doping element in the first region, oxygen content, specific surface area, true density and median particle size of the negative electrode material are shown in Table 2.

[0146] Example 5

[0147] The difference from Example 1 is that: (1) 100 g of 100-mesh bamboo powder was placed in a tube furnace, and compressed air (oxygen concentration 20%) was introduced at an air flow rate of 100 mL / min. The temperature curve was: 7°C / min, the temperature was raised to 300°C and kept warm for 5 h.

[0148] The negative electrode material prepared in this embodiment includes a carbon matrix, and the carbon matrix has pores, and the pores include closed pores. The test results of the specific surface area, average pore size, micropore volume, volatile matter and total content of oxygen, nitrogen and sulfur of the second precursor are shown in Table 1. The test results of the average pore size, pore volume, mass proportion of the doping element in the second region in the second region, mass proportion of the doping element in the first region, oxygen content, specific surface area, true density and median particle size of the negative electrode material are shown in Table 2.

[0149] Example 6

[0150] The difference from Example 1 is that: (1) 100 g of 100-mesh bamboo powder was placed in a tube furnace, and compressed air (oxygen concentration 20%) was introduced at an air flow rate of 100 mL / min. The temperature curve was: 7°C / min, the temperature was raised to 350°C and kept for 5 h.

[0151] The negative electrode material prepared in this embodiment includes a carbon matrix, and the carbon matrix has pores, and the pores include closed pores. The test results of the specific surface area, average pore size, micropore volume, volatile matter and total content of oxygen, nitrogen and sulfur of the second precursor are shown in Table 1. The test results of the average pore size, pore volume, mass proportion of the doping element in the second region in the second region, mass proportion of the doping element in the first region, oxygen content, specific surface area, true density and median particle size of the negative electrode material are shown in Table 2.

[0152] Example 7

[0153] The difference from Example 1 is that: (2) the material obtained in step (1) is further heated to 400° C. and kept at this temperature for 6 hours.

[0154] The negative electrode material prepared in this embodiment includes a carbon matrix, and the carbon matrix has pores, and the pores include closed pores. The test results of the specific surface area, average pore size, micropore volume, volatile matter and total content of oxygen, nitrogen and sulfur of the second precursor are shown in Table 1. The test results of the average pore size, pore volume, mass proportion of the doping element in the second region in the second region, mass proportion of the doping element in the first region, oxygen content, specific surface area, true density and median particle size of the negative electrode material are shown in Table 2.

[0155] Example 8

[0156] The difference from Example 1 is that: (2) the material obtained in step (1) is further heated to 600°C and kept warm for 4 hours. The negative electrode material prepared in this embodiment includes a carbon matrix, and the carbon matrix has pores, and the pores include closed pores. The test results of the specific surface area, average pore size, micropore volume, volatile matter and total content of oxygen, nitrogen and sulfur of the second precursor are shown in Table 1. The test results of the average pore size, pore volume, mass proportion of the doping element in the second region in the second region, mass proportion of the doping element in the first region in the first region, oxygen content, specific surface area, true density and median particle size of the closed pores are shown in Table 2.

[0157] Example 9

[0158] The difference from Example 1 is that: (2) the material obtained in step (1) is further heated to 700° C. and kept at this temperature for 3 h.

[0159] The negative electrode material prepared in this embodiment includes a carbon matrix, and the carbon matrix has pores, and the pores include closed pores. The test results of the specific surface area, average pore size, micropore volume, volatile matter and total content of oxygen, nitrogen and sulfur of the second precursor are shown in Table 1. The test results of the average pore size, pore volume, mass proportion of the doping element in the second region in the second region, mass proportion of the doping element in the first region, oxygen content, specific surface area, true density and median particle size of the negative electrode material are shown in Table 2.

[0160] Example 10

[0161] The difference from Example 1 is that: (2) the material obtained in step (1) is further heated to 800° C. and kept at this temperature for 2 h.

[0162] The negative electrode material prepared in this embodiment includes a carbon matrix, and the carbon matrix has pores, and the pores include closed pores. The test results of the specific surface area, average pore size, micropore volume, volatile matter and total content of oxygen, nitrogen and sulfur of the second precursor are shown in Table 1. The test results of the average pore size, pore volume, mass proportion of the doping element in the second region in the second region, mass proportion of the doping element in the first region, oxygen content, specific surface area, true density and median particle size of the negative electrode material are shown in Table 2.

[0163] Comparative Example 1

[0164] (1) Take 100 g of 100-mesh bamboo powder and place it in a tube furnace. Pass nitrogen gas at a nitrogen flow rate of 100 mL / min. The temperature curve is: increase the temperature to 250°C at 5°C / min and keep it for 5 h.

[0165] (2) The material obtained in step (1) is further heated to 500° C. and kept at this temperature for 4 h to obtain a second precursor.

[0166] (3) Take 20g of the second precursor and perform ball milling. The material after ball milling has a D50 of 6μm. Then add 60mL of 12% hydrochloric acid and heat to 75°C in a water bath. Stir for 2h, filter and wash with water until neutral, and dry in an oven at 80°C.

[0167] (4) The material obtained in step (3) was placed in a high-temperature tube furnace and sintered at 1300° C. for 3 h in a nitrogen atmosphere to obtain a negative electrode material.

[0168] Comparative Example 2

[0169] (1) Take 100 g of 100-mesh bamboo powder and place it in a tube furnace. Pass nitrogen gas at a nitrogen flow rate of 100 mL / min. The temperature curve is: increase the temperature to 250°C at 5°C / min and keep it for 5 h.

[0170] (2) The material obtained in step (1) was heated to 500° C. in compressed air (oxygen content 20%) and kept at this temperature for 4 h to obtain a second precursor.

[0171] (3) Take 20g of the second precursor and perform ball milling. The material after ball milling has a D50 of 6μm. Then add 60mL of 12% hydrochloric acid and heat to 75°C in a water bath. Stir for 2h, filter and wash with water until neutral, and dry in an oven at 80°C.

[0172] (4) The material obtained in step (3) was placed in a high-temperature tube furnace and sintered at 1300° C. for 3 h in a nitrogen atmosphere to obtain a negative electrode material.

[0173] Comparative Example 3

[0174] (1) Take 100 g of 100-mesh bamboo powder and place it in a tube furnace. Pass compressed air (oxygen content 20%) at a flow rate of 100 mL / min. The temperature curve is: increase the temperature to 250°C at 5°C / min and keep it at this temperature for 5 h.

[0175] (2) The material obtained in step (1) is placed in nitrogen and heated to 500° C. and kept at this temperature for 4 hours to obtain a second precursor.

[0176] (3) Take 20g of the second precursor and perform ball milling. The material after ball milling has a D50 of 6μm. Then add 60mL of 12% hydrochloric acid and heat to 75°C in a water bath. Stir for 2h, filter and wash with water until neutral, and dry in an oven at 80°C.

[0177] (4) The material obtained in step (3) is placed in a high-temperature tube furnace and sintered at 1300° C. for 3 h in a nitrogen atmosphere to obtain a negative electrode material.

[0178] Comparative Example 4

[0179] The difference from Example 1 is that: (1) 100 g of 100-mesh bamboo powder was placed in a tube furnace, and compressed air (oxygen concentration 20%) was introduced at an air flow rate of 100 mL / min. The temperature curve was: 7°C / min, the temperature was raised to 150°C and kept warm for 5 h.

[0180] Comparative Example 5

[0181] The difference from Example 1 is that: (1) 100 g of 100-mesh bamboo powder was placed in a tube furnace, and compressed air (oxygen concentration 20%) was introduced at an air flow rate of 100 mL / min. The temperature curve was: 7°C / min, the temperature was raised to 400°C and kept at this temperature for 5 h.

[0182] Comparative Example 6

[0183] The difference from Example 1 is that: (2) the material obtained in step (1) is further heated to 350° C. and kept warm for 3 h.

[0184] Comparative Example 7

[0185] The difference from Example 1 is that: (2) the material obtained in step (1) is further heated to 850° C. and kept at this temperature for 1 hour.

[0186] Performance Testing

[0187] (1) The pore size distribution and pore volume of the material were obtained using a TriStar II 3030 analyzer (Micromeritics Instruments, Inc., USA).

[0188] (2) AccuPyc II 1340 analyzer (Mike Instruments, Inc., USA) was used to analyze the true density of the material using helium as the analytical gas: ρ ture The ideal graphite material is the most perfect layered material and the most compressed carbon material without closed pores. Its true density is up to 2.26g / cm 3 ; From this, the closed pore volume of the negative electrode material can be calculated as:

[0189]

[0190] (3) The negative electrode material particles are subjected to cross-section processing by a focused ion beam FIB (The Vion PFIB, FEI Titan, manufactured by FEI Company) instrument, and the Ultim 170 spectrometer (UK, Oxford Instruments Ltd.) is used to test and select the cross-section area of ​​the material to obtain the distribution of nitrogen, oxygen and sulfur elements inside the material (first area). The mass proportion of nitrogen in the first area, the mass proportion of oxygen in the first area, and the mass proportion of oxygen in the first area can be calculated. The above three mass proportions are added to obtain the mass proportion of nitrogen, oxygen and sulfur in the first area of ​​the negative electrode material.

[0191] The surface of the negative electrode material particles is directly measured by an Ultim 170 spectrometer (Oxford Instruments Ltd., UK) with a measured thickness of less than or equal to 1 μm, and the distribution of nitrogen, oxygen and sulfur elements in the surface area of ​​the material (i.e., the second area) can be obtained. The mass proportion of the nitrogen element in the second area, the mass proportion of the oxygen element in the second area, and the mass proportion of the oxygen element in the second area can be obtained by calculation. The above three mass proportions are added to obtain the mass proportion of nitrogen, oxygen and sulfur elements in the second area of ​​the negative electrode material.

[0192] The contents of oxygen, nitrogen and sulfur in the second precursor were measured by FlashSmart organic element analyzer (Thermo Fisher Scientific, USA) for calculation.

[0193] (4) The material sample was dried at 120°C for 2 h, 1±0.05 g of the sample was taken and calcined in a muffle furnace at 900°C for 7 min in an air atmosphere. The sample was cooled and weighed, and the weight loss rate was calculated to obtain the volatile matter of the material.

[0194] (5) The specific surface area and pore volume were obtained by nitrogen adsorption test at 77K using a Microbead tester (model ASAP2460). The specific surface area was calculated using the BET formula (unit: m 2 / g), pore volume V 孔 is the total pore volume of single-point adsorption (in cm3 / g).

[0195] (6) True density ρ is measured by BEST 3H-2000TD using helium gas, and the unit is cm 3 / g; To characterize the pore structure characteristics of the negative electrode material provided in this application, the porosity W is defined as:

[0196] W=V 孔 / (V 孔 +1 / ρ)*100%(II)

[0197] In the above formula, V孔 is the pore volume, i.e. the total pore volume of single-point adsorption, in cm3 / g; ρ is the true density, in cm 3 / g; The physical meaning of porosity is the average value of the ratio between the pore volume contained in the negative electrode material particles and the total volume of the particles (pore volume + volume of the carbon matrix), and the volume of the carbon matrix = 1 / ρ.

[0198] (7) Electrochemical performance test:

[0199] Preparation of sodium ion battery button cells: hard carbon / SP / LA133 / pure water were prepared as electrode slurry in a ratio of 91:3:6 / 100, applied on aluminum foil with a 200μm scraper, and then punched into 16mm discs after drying to obtain hard carbon electrode sheets, which were assembled into button cells in a glove box for testing. Sodium metal was used as the counter electrode, the diaphragm was glass fiber model GF / D from Whateman, and the electrolyte was 1.0M NaPF6 solution.

[0200] Charge and discharge performance test: The battery was tested for the first cycle of charge and discharge using a battery test system (M310A, manufactured by Wuhan Blue Electric Electronics Co., Ltd.) with a voltage range of 0.001 to 2.0 V to obtain the charge and discharge capacity and discharge voltage platform of the negative electrode material. The test results are shown in Tables 1 to 3.

[0201] Table 1. Parameter test results of the second precursor obtained by pre-oxidation and etching treatment in various embodiments and comparative examples

[0202]

[0203] Table 2. Parameter test of negative electrode materials prepared in Examples and Comparative Examples

[0204]

[0205]

[0206] In Table 2, the detection limit of the doping element content detected by the Ultim 170 spectrometer is 0.5%, and non-detection means that the mass proportion of the doping element in the first region is less than 0.5%, or the mass proportion of the doping element in the second region is less than 0.5%.

[0207] Table 3. Electrochemical performance test of negative electrode materials prepared in Examples and Comparative Examples

[0208]

[0209]

[0210] As shown in Tables 1 to 3, Examples 1 to 10 of the present application can introduce a large number of microporous structures and oxygen, nitrogen and sulfur doping elements into the second precursor by low-temperature pre-oxidation treatment at 200°C to 350°C and medium-temperature vapor phase etching treatment at 400°C to 800°C, and the doping elements enter the surface of the carbon-based raw material more, while the doping elements hardly enter the interior of the carbon-based raw material. The doping elements and the defective carbon atoms on the surface of the carbon-based raw material form reversible functional groups C=O, C=N, C=S, etc. The reversible functional groups and sodium ions react reversibly to form reversible sodium storage sites, so that the surface of the negative electrode material can adsorb more sodium ions, forming adsorption storage sodium and improving the sodium storage capacity of the negative electrode material. In addition, the above-mentioned reversible functional groups preferentially react with the solvated sodium ions, which is beneficial to the desolvation of the sodium ions, thereby reducing the polarization in the electrochemical process, increasing the voltage platform of the negative electrode material, and reducing the occurrence of sodium precipitation. Finally, during the heat treatment process, the micropores are closed to form closed pores, and the pore volume of the closed pores is greater than 0.025cm 3 / g, which is beneficial to the filling of sodium ions and improves the sodium storage capacity of the negative electrode material, thereby improving the capacity performance and initial efficiency of the negative electrode material.

[0211] Compared with Comparative Example 1, Examples 1, 2 and 3 introduce a large number of microporous structures into the material by pre-oxidizing with an oxidizing gas and etching with an etching gas. Figure 4 It can be seen that the micropore volume of the second precursor in Example 1, Example 2, and Example 3 is much larger than the micropore volume of the second precursor under nitrogen protection in Comparative Example 1, which is conducive to the formation of closed pores in the negative electrode material. The total content of oxygen, nitrogen, and sulfur elements in the second precursor was tested. The second precursor in Example 1, Example 2, and Example 3 has a higher total content of oxygen, nitrogen, and sulfur elements, which is also conducive to the retention of oxygen / nitrogen / sulfur functional groups of the negative electrode material obtained after high-temperature carbonization, thereby improving the voltage platform of the negative electrode material.

[0212] The true density test was performed on the negative electrode materials prepared in Example 1, Example 2, Example 3 and Comparative Example 1. The negative electrode materials prepared in Example 1, Example 2 and Example 3 had a higher closed pore volume, indicating that the negative electrode materials prepared in Example 1, Example 2 and Example 3 of the present application can provide more sodium storage sites.

[0213] Figure 5 is the energy spectrum distribution diagram of carbon element and oxygen element of the negative electrode material of Example 1 of the present application, Figure 5It can be seen that the surface of the negative electrode material prepared in Example 1 is evenly distributed with oxygen elements, and the oxygen elements can form C=O functional groups with carbon atoms on the surface of the material. The C=O functional groups and sodium ions can form reversible sodium storage sites C-ONa. In the subsequent charging process, Na will be released and then form C=O, so that the surface of the negative electrode material can adsorb more sodium ions, forming adsorbed sodium storage, thereby improving the sodium storage capacity of the negative electrode material.

[0214] Figure 6 The SEM image and cross-sectional EDS-mapping image of the negative electrode material prepared in Example 1 are shown in FIG. Figure 7 The SEM image and cross-sectional EDS-mapping image of the negative electrode material prepared in Comparative Example 1 are shown in FIG. Figure 8 Comparison of the first cycle charge and discharge of the negative electrode materials prepared in Example 1 and Comparative Example 1. Fig. 9 The discharge depth and voltage comparison of the negative electrode materials prepared in Example 1 and Comparative Example 1 are shown in FIG. Figure 6 to Figure 9 As shown in Table 2, the negative electrode material prepared in Example 1 contains more oxygen than the negative electrode material prepared in Comparative Example 1, and through EDS-mapping analysis of the cross-section of the material, it can be seen that the oxygen content inside the negative electrode material particles of Example 1 is low, which is lower than the detection limit of EDS, indicating that the oxygen elements are mostly distributed on the surface of the negative electrode material. The functional groups formed by the oxygen elements on the surface can promote the desolvation of sodium ions and reduce the polarization during the electrochemical reaction, thereby increasing the voltage platform of the negative electrode material and reducing the occurrence of sodium precipitation in the electrode sheet.

[0215] In comparative example 2, step (1) uses nitrogen for low-temperature treatment, and in comparative example 3, step (2) uses nitrogen for medium-temperature etching treatment. The mass proportion of the doping element in the second region of the prepared negative electrode material is too small, resulting in a reduction in the reversible functional groups on the surface of the negative electrode material and a reduction in the volume of the internal closed pores, which in turn leads to a reduction in the voltage platform and a reduction in the capacity of the negative electrode material.

[0216] In Comparative Example 4, the temperature of the pre-oxidation treatment is too low, and the pre-oxidation effect is reduced, resulting in a lower oxygen content on the surface of the negative electrode material and a lower voltage platform of the negative electrode material; in Comparative Example 5, the temperature of the low-temperature pre-oxidation treatment is too high, then there are fewer doping elements on the surface of the negative electrode material, and more doping elements inside the negative electrode material, and there are fewer reversible functional groups formed by the surface doping elements and carbon atoms, resulting in increased side reactions between the negative electrode material and the electrolyte, and more doping elements inside the negative electrode material hinder the deintercalation of sodium ions in the negative electrode material during the charge and discharge process, thereby resulting in a decrease in the initial efficiency and capacity of the negative electrode material.

[0217] In Comparative Example 6, the temperature of the etching treatment is too low, and an effective pore distribution cannot be formed, resulting in a small closed pore volume of the negative electrode material and a reduced capacity of the negative electrode material. In Comparative Example 7, the temperature of the etching treatment is too high, then larger pores are formed during the etching process, and the pores cannot form closed pores during the heat treatment process, resulting in a larger specific surface area of ​​the negative electrode material, thereby resulting in a reduction in the first efficiency of the negative electrode material. At the same time, the higher temperature allows more doping elements to enter the interior of the negative electrode material, hindering the deintercalation of sodium ions in the negative electrode material during the charge and discharge process, thereby resulting in a reduction in the first efficiency and capacity of the negative electrode material.

[0218] The above description is only the preferred embodiment of the present application and is not intended to limit the present application. For those skilled in the art, the present application may have various modifications and variations. Any modification, equivalent replacement, improvement, etc. made within the spirit and principle of the present application shall be included in the protection scope of the present application.

Claims

1. A negative electrode material, It is characterized in that The negative electrode material includes a carbon matrix, the carbon matrix has pores, the pores include closed pores, the pore volume of the closed pores is greater than 0.025 cm³ / g, and the average pore diameter of the closed pores is less than or equal to 5 nm; The negative electrode material contains a doping element, the doping element includes at least one of nitrogen, sulfur and oxygen, the negative electrode material has a reversible functional group of at least one of C=N and C=S, the negative electrode material has a first region located in the central region of the negative electrode material and a second region distributed on the surface of the first region, the thickness of the second region is less than or equal to 1 μm, and the negative electrode material is tested by an EDS (Energy Dispersive Spectrometer) spectrometer. The mass proportion of the doping element in the second region is 0.5%~5%, and the mass proportion of the doping element in the first region is less than 0.5%.

2. The negative electrode material according to claim 1, It is characterized in that The negative electrode material includes at least one of the following features (1) to (6): (1) The carbon matrix comprises hard carbon; (2) The mass proportion of the doping element in the negative electrode material is 0.5% to 2%; (3) The negative electrode material has a C=O reversible functional group; (4) The specific surface area of ​​the negative electrode material is 2 m² / g ~50 m² / g; (5) The true density of the negative electrode material is 1.8 g / cm³~2.2 g / cm³; (6) The median particle size of the negative electrode material is 3 μm to 20 μm.

3. A method for preparing a negative electrode material, It is characterized in that The steps include: Placing the carbon-based raw material in an oxidizing gas atmosphere for pre-oxidation treatment to obtain a first precursor, wherein the first precursor has at least one reversible functional group of C=N and C=S, and the pre-oxidation treatment temperature is 200° C. to 350° C.; Placing the first precursor in an etching gas atmosphere for etching treatment to obtain a second precursor, wherein the second precursor includes a porous carbon-based material, the pores in the porous carbon-based material include micropores, and the pore volume of the micropores is greater than or equal to 0.05 cm³ / g, the temperature of the etching treatment is 400°C to 800°C, and the etching gas includes at least one of oxygen, ozone, sulfur dioxide, sulfur trioxide, nitrogen dioxide, water vapor and magnesium vapor; The second precursor is crushed to obtain a third precursor; The third precursor is subjected to heat treatment at a temperature of 1000° C. to 1600° C. in a protective gas atmosphere to obtain a negative electrode material.

4. The preparation method according to claim 3, It is characterized in that The preparation method comprises at least one of the following features (1) to (4): (1) The oxidizing gas includes at least one of oxygen, ozone, nitrogen dioxide and sulfur trioxide; (2) The carbon-based raw material includes at least one of biomass shell materials, biomass wood materials, straw, corn cobs, starch, sucrose, asphalt, coal and coke; (3) The oxygen content in the oxidizing gas is 5% to 100%; (4) The pre-oxidation treatment time is 2 min to 600 min.

5. The preparation method according to claim 3, It is characterized in that The preparation method comprises at least one of the following features (1) to (3): (1) The concentration of the etching gas is 5% to 100%; (2) The etching treatment time is 2 min to 600 min; (3) The median particle size of the third precursor is 3 μm to 20 μm.

6. The preparation method according to claim 3, It is characterized in that The second precursor includes at least one of the following features (1) to (4): (1) The average pore size of the pores of the porous carbon-based material is less than or equal to 3 nm; (2) The volume of the micropores is greater than or equal to 60% based on the total volume of the pores as 100%; (3) The volatile matter of the porous carbon-based material is 8% to 18%; (4) The porous carbon-based material contains a doping element, wherein the doping element includes at least one of nitrogen, oxygen and sulfur, and the mass proportion of the doping element in the porous carbon-based material is 3% to 15%.

7. The preparation method according to claim 3, It is characterized in that Before subjecting the third precursor to heat treatment, the method further includes subjecting the third precursor to acid washing.

8. The preparation method according to claim 7, It is characterized in that The preparation method comprises at least one of the following features (1) to (3): (1) The pickling solution includes at least one of hydrochloric acid, nitric acid and hydrofluoric acid; (2) The concentration of the pickling solution is 10% to 30%; (3) The pickling solution includes at least one of hydrochloric acid, nitric acid and hydrofluoric acid, and the volume ratio of the third precursor, hydrochloric acid, nitric acid and hydrofluoric acid is 1:(1-6):(0-6):(0-1).

9. The preparation method according to claim 3, It is characterized in that The preparation method comprises at least one of the following features (1) to (3): (1) The protective gas includes at least one of nitrogen, argon, neon, helium, xenon and krypton; (2) The heat treatment time is 1h to 6h; (3) The heat treatment equipment includes at least one of a tube furnace, a box furnace, a push plate kiln, a graphitization furnace and a roasting kiln.

10. A sodium ion battery, It is characterized in that The sodium ion battery comprises the negative electrode material according to any one of claims 1 to 2 or the negative electrode material prepared by the preparation method according to any one of claims 3 to 9.

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