Carbon material and preparation method therefor and use thereof

Abstract

A carbon material and a preparation method therefor and a use thereof. The carbon material has pores, and the pore volume of micropores among the pores is greater than or equal to 0.05 cm3 / g. The carbon material contains a doping element, and the doping element includes at least one of nitrogen, phosphorus, arsenic, oxygen, sulfur, selenium, and tellurium. The internal doping concentration of the doping element in the carbon material is Q1 wt%, wherein Q1<8. The surface doping concentration of the doping element in the carbon material is Q2 wt%, wherein 10≤Q2≤25. Q1 wt% and Q2 wt% are obtained by means of the following test method: taking a sample of m1 g from the carbon material, wherein m1≥10; using a scanning electron microscope-energy dispersive spectrometer to test the sample to obtain Q2 wt%; and using a focused ion beam instrument to perform cross-section treatment on the sample, and using the scanning electron microscope-energy dispersive spectrometer to test the cross-section of the sample to obtain Q1 wt%.

Description

Carbon materials and their preparation methods and applications

[0001] The present invention claims priority to the Chinese patent application filed with the State Intellectual Property Office on October 31, 2023, with application number 2023114377036 and application name “Negative electrode material and preparation method thereof, sodium ion battery”, the entire contents of which are incorporated by reference into this application. Technical Field

[0002] The present invention relates to the technical field of negative electrode materials, and in particular to carbon materials and preparation methods and applications thereof. Background Art

[0003] As lithium ore resources become increasingly scarce and the price of raw materials for lithium-ion batteries rises significantly, the development of lithium-ion batteries will inevitably face resource constraints. However, sodium resources are abundant, cheap, and readily available, with low processing costs, making sodium-based batteries significantly cheaper than lithium-ion batteries. Therefore, the development prospects of sodium-based batteries are much broader than those of lithium-ion batteries. Compared to other anode materials, carbon-based anode materials offer advantages such as low cost, large interlayer spacing, low operating voltage, excellent conductivity, and good cycling performance, making them considered the most advantageous anode materials for sodium-based batteries.

[0004] 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.

[0005] Summary of the Invention

[0006] The embodiments of the present application provide a carbon material and a preparation method and application thereof, which solve the technical problem that it is difficult to comprehensively improve the capacity and discharge voltage of negative electrode materials for sodium ion batteries.

[0007] In a first aspect, the present invention provides a carbon material having pores, wherein the pore volume of the micropores is greater than or equal to 0.05 cm 3 / g;

[0008] The carbon material contains doping elements, and the doping elements include nitrogen group elements and / or oxygen group elements;

[0009] The internal doping concentration of the doping element in the carbon material is Q1 wt %, Q1<8, and the surface doping concentration of the doping element in the carbon material is Q2 wt %, 10≤Q2≤25; wherein the Q1 wt % and the Q2 wt % are obtained by the following test method:

[0010] Take m1 g sample from the carbon material, m1 ≥ 10;

[0011] The sample is detected by using a scanning electron microscope energy dispersive spectrometer to obtain the Q2 wt%;

[0012] The sample is subjected to cross-section processing using a focused ion beam instrument, and then the cross-section of the sample is detected using a scanning electron microscope energy dispersive spectrometer to obtain the Q1 wt%.

[0013] In a second aspect, the present invention also provides another carbon material having closed pores, wherein the total pore volume of the closed pores is greater than 0.025 cm 3 / g;

[0014] The carbon material contains doping elements, and the doping elements include nitrogen group elements and / or oxygen group elements;

[0015] The internal doping concentration of the doping element in the carbon material is P1 wt %, P1<0.5; the surface doping concentration of the doping element in the carbon material is P2 wt %, 0.5≤P2≤5; wherein the P1 wt % and the P2 wt % are obtained by the following test method:

[0016] Take m2 g sample from the carbon material, m2 ≥ 10;

[0017] The sample is detected by using a scanning electron microscope energy dispersive spectrometer to obtain the P2 wt%;

[0018] The sample is subjected to cross-section processing using a focused ion beam instrument, and then the cross-section of the sample is detected using a scanning electron microscope energy dispersive spectrometer to obtain the P1 wt%.

[0019] In a third aspect, the present application further proposes a method for preparing a carbon material, comprising the following steps:

[0020] Placing the carbon-based raw material in an atmosphere containing an oxidizing gas for pre-oxidation treatment to obtain a precursor, wherein the pre-oxidation temperature is 200° C. to 350° C., and the oxidizing gas contains at least one of nitrogen, phosphorus, arsenic, oxygen, sulfur, selenium, and tellurium;

[0021] Placing the precursor in an etching gas atmosphere for etching to obtain a carbon material, 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;

[0022] In a fourth aspect, the present application further proposes a negative electrode material, including the carbon material described in the second aspect, or the carbon material prepared by the preparation method described in the third aspect.

[0023] In a fifth aspect, the present application also proposes a sodium ion battery comprising the negative electrode material described in the fourth aspect.

[0024] The technical solution of this application has at least the following beneficial effects:

[0025] The second aspect of the carbon material provided in the present application can be applied to the negative electrode material for sodium ion batteries. The carbon material has abundant closed pores, which is conducive to the formation of filled sodium storage. In addition, the doping elements in the carbon material are mainly distributed in the surface area of ​​the carbon material (specifically the carbon material stack / layer), and a very small amount is distributed in the interior of the carbon material. The doping elements distributed in the surface area of ​​the carbon material can form reversible functional groups with defective carbon atoms on the surface of the carbon material. On the one hand, these reversible functional groups can form reversible sodium storage sites with sodium ions, thereby increasing the sodium storage capacity of the carbon material; on the other hand, these reversible functional groups can increase the voltage platform of the carbon material, thereby reducing the occurrence of sodium precipitation in the battery electrode. In the carbon material of the present application, P1 is less than 0.5wt%, the carbon material is tightly arranged inside, and there are fewer defective carbon atoms, which is conducive to the deintercalation of sodium ions during the charge and discharge process; and 0.5≤P2≤5, while ensuring that the doping elements in the surface area of ​​the carbon material can react with a sufficient amount of sodium ions, the occurrence of side reactions between the doping elements and the electrolyte can be reduced. This application simultaneously controls the closed pore volume of the carbon material, the surface doping concentration of the doping element, and the internal doping concentration, so that the carbon material can have excellent properties such as high capacity, high initial efficiency, and low sodium precipitation risk, thereby improving the electrochemical performance of the battery.

[0026] The carbon material of the first aspect provided in this application can be used to prepare the carbon material of the second aspect. The negative electrode material for sodium ion battery prepared from the carbon material of the first aspect of this application has excellent properties such as high capacity, high initial efficiency and low sodium precipitation risk, and can improve the electrochemical performance of the battery.

[0027] The carbon material obtained by the preparation method of the present application can be used as a negative electrode material for sodium ion batteries, and the sodium ion battery prepared from the carbon material (negative electrode material) has excellent electrochemical properties. In the preparation method of the present application, a large amount of doping elements can be introduced on the surface of the carbon-based raw material by successively subjecting the carbon-based raw material to a pre-oxidation treatment and a vapor-phase etching treatment. Since the temperature of the pre-oxidation treatment and the etching treatment is relatively low, the doping elements can enter the surface of the carbon-based raw material to a large extent, and a very small amount enters the interior of the carbon-based raw material; wherein, the doping elements distributed in the surface area of ​​the carbon material can form reversible functional groups with the defective carbon atoms on the surface of the carbon material, which is beneficial to improving the sodium storage capacity and voltage platform of the carbon material. Furthermore, by successively etching the precursor, a rich pore structure is formed in the carbon material, which can further improve the sodium storage capacity of the carbon material, thereby further improving the capacity performance and the first efficiency of the sodium ion battery. BRIEF DESCRIPTION OF THE DRAWINGS

[0028] FIG1 is a flow chart of the preparation of the carbon material of the present application;

[0029] FIG2 is a schematic diagram of the structure of the etching product prepared in this application;

[0030] FIG3 is a comparison diagram of the cumulative distribution of pore volumes of the etching products in Example 1 and Comparative Example 1 of the present application;

[0031] FIG4 is an energy spectrum distribution diagram of carbon and oxygen elements of the carbon material of Example 1 of the present application;

[0032] FIG5 is an SEM image and EDS-mapping image of a cross-section of the carbon material prepared in Example 1;

[0033] FIG6 is an SEM image and EDS-mapping image of the carbon electrode material prepared in Comparative Example 1;

[0034] FIG7 is a comparison of the first cycle charge and discharge of batteries made from the carbon materials prepared in Example 1 and Comparative Example 1;

[0035] FIG8 is a comparison of the discharge depth and voltage of batteries made from the carbon materials prepared in Example 1 and Comparative Example 1. DETAILED DESCRIPTION

[0036] In order to better understand the technical solutions of this specification, the embodiments of the present application are described in detail below with reference to the accompanying drawings.

[0037] It should be clear that the embodiments described are only part of the embodiments of this specification, not all of the embodiments. Based on the embodiments in this specification, all other embodiments obtained by ordinary technicians in this field without making creative work are within the scope of protection of this specification.

[0038] The terms used in the examples of this application are for the purpose of describing specific embodiments only and are not intended to limit this specification. The singular forms "a," "an," "the," and "the" used in the examples of this application and the appended claims are also intended to include the plural forms unless the context clearly indicates otherwise.

[0039] Biomass carbon materials are widely studied for their application as carbon anode materials for sodium-ion batteries due to their low cost and wide availability. However, current biomass-based hard carbon anode materials have a low capacity, typically only reaching 260Ah / g to 300Ah / g, which cannot meet the capacity requirements of sodium-ion batteries. In addition, biomass-based hard carbon anode materials have a low voltage platform and pose a risk of sodium precipitation.

[0040] In order to solve the above technical problems, in the first aspect, the present application proposes a carbon material.

[0041] In an embodiment of the present application, the carbon material has pores, wherein the pore volume of the micropores is greater than or equal to 0.05 cm 3 / g;

[0042] The carbon material contains doping elements, and the doping elements include nitrogen group elements and / or oxygen group elements;

[0043] The internal doping concentration of the doping element in the carbon material is Q1 wt %, Q1<8, and the surface doping concentration of the doping element in the carbon material is Q2 wt %, 10≤Q2≤25; wherein the Q1 wt % and the Q2 wt % are obtained by the following test method:

[0044] Take m1 g sample from the carbon material, m1 ≥ 10;

[0045] The sample is detected by using a scanning electron microscope energy dispersive spectrometer to obtain the Q2 wt%;

[0046] The sample is subjected to cross-section processing using a focused ion beam instrument, and then the cross-section of the sample is detected using a scanning electron microscope energy dispersive spectrometer to obtain the Q1 wt%.

[0047] Specifically, the step of detecting the sample using a scanning electron microscope spectrometer includes: randomly selecting n1 1 μm*1 μm areas of the sample using a scanning electron microscope spectrometer, measuring the atomic mass percentage of the doping element in each area, and calculating the average atomic mass percentage of the doping element in the n1 areas to obtain the Q2wt%;

[0048] The step of using a scanning electron microscope energy spectrometer to detect the cross-section of the sample includes: randomly selecting n2 1μm*1μm areas of the sample using a scanning electron microscope energy spectrometer, measuring the atomic mass percentage of the doping element in each area, and calculating the average atomic mass percentage of the doping element in these n2 areas to obtain the Q1 wt%; wherein n2=n1, n1 and n2 are natural numbers ≥30.

[0049] The surface doping concentration of the doping element in the carbon material of the first aspect provided by the present application is greater than the internal doping concentration. The doping element is mainly distributed in the surface area of ​​the carbon material (specifically, the carbon material stack / layer), and a very small amount is distributed in the interior of the carbon material. The doping element distributed in the surface area of ​​the carbon material can form a reversible functional group with the defective carbon atoms on the surface of the carbon material, which can increase the sodium storage capacity of the carbon material and reduce the voltage platform of the carbon material. The carbon material includes a pore volume greater than or equal to 0.05 cm 3 / g of micropores, which have abundant pores that are conducive to the filling of sodium ions, forming filled sodium storage, and these abundant pores can be closed into abundant closed pores after post-treatment, thereby further improving its sodium storage capacity. The negative electrode material for sodium ion batteries further prepared from the carbon material of the first aspect of the present application has excellent properties such as high capacity, high initial efficiency and low sodium precipitation risk, and can improve the electrochemical performance of the battery.

[0050] In some embodiments, the average pore size of the carbon material is less than or equal to 3 nm.

[0051] In some embodiments, based on the total pore volume of the carbon material being 100%, the volume proportion of the micropores is greater than or equal to 60%.

[0052] In some embodiments, the total mass percentage of the doping elements in the carbon material is 3% to 15%, specifically 3%, 6%, 9%, 12%, 15% or any value therebetween.

[0053] In some embodiments, the volatile matter of the carbon material is 8% to 18%, specifically 8%, 12%, 18% or any value therebetween.

[0054] In a second aspect, the present application also proposes another carbon material, which can be prepared by post-processing the carbon material in the first aspect.

[0055] In an embodiment of the present application, the carbon material has closed pores, and the total pore volume of the closed pores is greater than 0.025 cm 3 / g;

[0056] The carbon material contains a doping element, the doping element including a nitrogen group element and / or an oxygen group element; the internal doping concentration of the doping element in the carbon material is P1 wt%, P1 < 0.5; the surface doping concentration of the doping element in the carbon material is P2 wt%, 0.5 ≤ P2 ≤ 5; wherein P1 wt% and P2 wt% are obtained by the following test method:

[0057] Take m2 g sample from carbon material, m2 ≥ 10;

[0058] The samples were examined using a scanning electron microscope (SEM) and the P2 wt% was obtained;

[0059] The sample was sectioned using a focused ion beam instrument and then examined using a scanning electron microscope energy dispersive spectrometer to obtain P1 wt%.

[0060] Specifically, the step of detecting the sample using a scanning electron microscope energy spectrometer includes: randomly selecting n3 1 μm*1 μm areas of the sample using the scanning electron microscope energy spectrometer, measuring the atomic mass percentage of the doping element in each area, and calculating the average atomic mass percentage of the doping element in the n3 areas to obtain P2 wt%;

[0061] The steps of using a scanning electron microscope energy spectrometer to detect the cross-section of the sample include: randomly selecting n4 1 μm*1 μm areas of the sample using the scanning electron microscope energy spectrometer, measuring the atomic mass percentage of the doping element in each area, and calculating the average atomic mass percentage of the doping element in these n4 areas to obtain P1 wt%; wherein n4=n3, n3 and n4 are natural numbers ≥30.

[0062] The carbon material of the second aspect provided in this application can be used as a negative electrode material for sodium ions, and the sodium ion battery prepared from the carbon material has excellent electrochemical performance. In this technical solution, the total pore volume of the closed pores on the carbon material is greater than 0.025 cm 3 / g, indicating that the carbon material has abundant closed pores, which are conducive to the filling of sodium ions, forming filled sodium storage, which can improve the sodium storage capacity of the carbon material, thereby improving the capacity performance and initial efficiency of the sodium ion battery made from the carbon material. The surface doping concentration P2 wt% of the doping element in the carbon material is greater than the internal doping concentration P1 wt%, indicating that the doping element is mainly distributed in the surface area of ​​the carbon material (specifically, the carbon material stack / layer, the same below), and a very small amount is distributed in the interior of the carbon material. The doping elements in the surface area of ​​the carbon material can form reversible functional groups with the defective carbon atoms on the surface of the carbon material; on the one hand, these reversible functional groups can form reversible sodium storage sites with sodium ions. In the subsequent charging process, these reversible functional groups are separated from the sodium ions, allowing the sodium ions to be released, so that the surface of the carbon material can adsorb more sodium ions, forming adsorbed sodium storage, and improving the sodium storage capacity of the carbon material; on the other hand, since the potential of these reversible functional groups to react with solvated sodium ions is higher than the platform potential of sodium ion intercalation, these reversible functional groups can react preferentially with solvated sodium ions, which is beneficial to the desolvation of sodium ions, thereby reducing the polarization in the electrochemical process, improving the voltage platform of the carbon material, and reducing the occurrence of sodium precipitation in the negative electrode sheet.

[0063] The present application controls the internal doping concentration of the doping elements in the carbon material to be less than 0.5wt%, so that the carbon material is tightly arranged and has fewer defective carbon atoms. If the internal doping concentration of the doping elements is too high, the doping elements doped into the interior of the carbon material will cause the structure of the carbon material to deform, thereby bringing about changes in the electronic energy band structure. During the charge and discharge process, sodium ions enter the interior of the carbon material to form an electric field. The existence of the electric field hinders the subsequent diffusion of sodium ions into the interior of the carbon material. At the same time, the sodium ions that enter the interior of the carbon material are not easy to escape under the action of the electric field. Therefore, controlling the internal doping concentration of the doping elements in the carbon material to be less than 0.5wt% can reduce the difficulty of sodium ion deintercalation during the charge and discharge process, thereby improving the first coulombic efficiency and capacity of the carbon material.

[0064] If the surface doping concentration of the doping elements in the carbon material is too high, the side reactions between the doping elements and the electrolyte will increase, thereby reducing the initial efficiency of the carbon material; if the surface doping concentration of the doping elements in the carbon material is too low, the doping elements in the surface area of ​​the carbon material will not be able to react with sufficient sodium ions, resulting in a decrease in the desolvation effect, an increase in the degree of polarization, and a decrease in the charging voltage platform.

[0065] This application simultaneously controls the closed pore volume of the carbon material, the surface doping concentration of the doping element, and the internal doping concentration, so that the carbon material can have excellent properties such as high capacity, high initial efficiency, and low sodium precipitation risk, thereby improving the electrochemical performance of the battery.

[0066] In some embodiments, the surface doping concentration P2 wt% is the mass percentage of the doping element in the region d μm measured from the surface of the carbon material to the center of the carbon material, where d ≤ 1. That is, during the actual measurement of the doping concentration P2 wt%, the scanning depth of the scanning electron microscope (EDS) on the sample (carbon material) does not exceed 1 μm.

[0067] In some embodiments, in the carbon material, the average pore size of the closed pores is less than or equal to 5 nm, for example, 0.5 nm, 1 nm, 2 nm, 3 nm, 4 nm, or 5 nm, etc., and of course, other values ​​within the above range can also be used, which is not limited here. Within the above-defined range, it is indicated that the pore size of the closed pores of the carbon material of the present application is small and the number of closed pores is large, which also means that the total area of ​​the inner surface of the closed pores in the carbon material is large, and the carbon material can provide a large adsorption area for sodium ions during the charge and discharge process, thereby improving the capacity performance and initial efficiency of the sodium ion battery (i.e., a sodium ion battery made using the carbon material as the negative electrode material, the same below).

[0068] In some embodiments, the total doping concentration of the doping elements in the carbon material is 0.5 wt% to 4 wt%, specifically 0.5 wt%, 1 wt%, 2 wt%, 3 wt%, 4 wt% or any value therebetween.

[0069] In some embodiments, the doping element includes a nitrogen group element and an oxygen group element. The nitrogen group element includes at least one of nitrogen, phosphorus, and arsenic, and / or the oxygen group element includes at least one of oxygen, sulfur, selenium, and tellurium. Co-doping the carbon material with non-metallic elements from Group V and Group VI can further improve the capacity performance of sodium-ion batteries made from the carbon material and reduce the risk of sodium precipitation.

[0070] In some embodiments, the doping concentration of nitrogen in the carbon material is 0.5wt% to 2wt%, specifically 0.5wt%, 1wt%, 1.5wt%, 2wt% or any value therebetween; and / or the doping concentration of sulfur in the carbon material is 0wt% to 2wt%, specifically 0wt%, 0.5wt%, 1wt%, 1.5wt%, 2wt% or any value therebetween; and / or the doping concentration of oxygen in the carbon material is 0.5wt% to 2wt%, specifically 0.5wt% to 2wt% or any value therebetween.

[0071] In some embodiments, the internal doping concentration of the doping element N in the carbon material is P 1N wt%, the internal doping concentration of doping element S is P 1S wt% and the internal doping concentration of the doping element O is P 1O wt%,P1=P 1N +P1S +P 1O , P 1N 、P 1S 、P 1O ≤0.5.

[0072] In some embodiments, the surface doping concentration of the doping element N in the carbon material is P 2N wt%, the surface doping concentration of doping element S is P 2S wt% and the surface doping concentration of the doping element O is P 2O wt%, P2=P 2N +P 2S +P 2O ; Among them, 0.5≤P 2N ≤5,0≤P 2S ≤5, 0.5≤P 2O ≤5.

[0073] In some embodiments, the carbon material contains at least one of the functional groups C=O, C=S, C=N and C≡N. C=O, C=N, C≡N and C=S are all reversible functional groups. These reversible functional groups can form reversible sodium storage sites with sodium ions. For example, C=O forms C-ONa with sodium. During the subsequent charging process, the sodium ions will be released and then form C=O, so that the surface of the carbon material can adsorb more sodium ions, forming adsorption storage sodium and improving the sodium storage capacity of the carbon material. In addition, the potentials at which these reversible functional groups react with solvated sodium ions (for example, the reaction potential of C=O with sodium ions is ~0.5V) are all higher than the platform potential of sodium ion intercalation (the platform voltage of sodium ion intercalation is generally <0.1V), and can react preferentially with solvated sodium ions, which is beneficial to the desolvation of sodium ions, thereby reducing the polarization in the electrochemical process, improving the voltage platform of the carbon material, and reducing the occurrence of sodium precipitation in the electrode. The role of the C=S functional group is similar to that of the C=O functional group and will not be described in detail. In addition to playing a similar role as the C=S and C=O functional groups, the C=N and C≡N functional groups can also improve the conductivity of carbon materials.

[0074] In some embodiments, the volatile matter of the carbon material is less than 1%, which indicates that the carbon material has a high degree of structural order, is not easily decomposed or volatilized, and has more stable overall performance.

[0075] In some embodiments, the carbon material is an activated carbon material.

[0076] In some embodiments, the carbon material may be selected from any one or more combinations of hard carbon, soft carbon, and graphite.

[0077] In a preferred embodiment, the carbon material is selected from hard carbon. When the carbon material is used as the negative electrode material of a sodium ion battery, sodium ions can be inserted into the carbon layers of the hard carbon to form intercalated sodium storage, thereby increasing the energy storage capacity of the carbon material.

[0078] The present application forms intercalated sodium storage by inserting sodium ions between hard carbon layers, forms adsorbed sodium storage by adsorbing sodium ions on reversible functional groups formed by doping elements on the surface of carbon materials, and forms filled sodium storage by filling sodium ions in closed pores of carbon materials. That is, the present application improves the sodium storage performance of carbon materials through the "adsorption-intercalation-filling" method, thereby improving the capacity performance of sodium ion batteries. At the same time, it can also improve the platform voltage of carbon materials and reduce the occurrence of sodium precipitation in the negative electrode of sodium ion batteries.

[0079] In some embodiments, the carbon material is tested using Raman spectroscopy, and the carbon material is detected at 1350±10 cm -1 There is a characteristic peak D at the position, and the peak intensity of the characteristic peak D is I D , at 1580±10cm -1 There is a characteristic peak G at the position, and the peak intensity of the characteristic peak G is I G , 0.8≤I D / I G ≤1.2. I of carbon materials D / I G The value can reflect the degree of surface defects of carbon materials and control I D / I G Within the above range, the doping elements on the surface of the carbon material are conducive to forming reversible functional groups with defective carbon atoms, which serve as active sites to adsorb more sodium ions and improve the sodium storage capacity of the carbon material.

[0080] In some embodiments, the median particle size of the carbon 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. Of course, it can also be other values ​​within the above range, which is not limited here.

[0081] In some embodiments, the carbon material has a specific surface area of ​​2 m 2 / g~50m 2 / g, specifically 2m 2 / g、10m 2 / g、15m 2 / g, 20m 2 / g, 25m 2 / g、30m 2 / g、35m 2 / g, 40m 2 / g、45m 2 / g or 50m 2 / g, etc., and of course other values ​​within the above range can also be used, which is not limited here. Within the above-defined range, it shows that the specific surface area of ​​the carbon material is appropriate, which is beneficial to improving the initial efficiency of the sodium ion battery.

[0082] In some embodiments, the true density of the carbon material is 1.8 g / cm 3 ~2.2g / cm 3 , 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 other values ​​within the above range can also be used, which is not limited here. Within the above-defined range, it is shown that the true density of the carbon 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 sodium ion battery.

[0083] In some embodiments, the carbon material powder tap density is 0.5 g / cm 3 ~1.5g / cm 3 , specifically 0.5g / cm 3 , 1g / cm 3 , 1.5g / cm 3 or any value in between.

[0084] In some embodiments, the powder conductivity of the carbon material under a pressure of 20 kN is 10 S / cm to 100 S / cm, specifically 10 S / cm, 20 S / cm, 30 S / cm, 40 S / cm, 50 S / cm, 60 S / cm, 70 S / cm, 80 S / cm, 90 S / cm, 100 S / cm or any value therebetween.

[0085] In a third aspect, the present application further proposes a method for preparing a carbon material. Referring to FIG1 , the method comprises the following steps:

[0086] Step S100, placing the carbon-based raw material in an oxidizing gas atmosphere for pre-oxidation treatment to obtain a precursor, wherein the pre-oxidation temperature is 200° C. to 350° C., specifically 200° C., 230° C., 250° C., 280° C., 300° C., 350° C., or any value therebetween, and the oxidizing gas includes at least one of nitrogen, phosphorus, arsenic, oxygen, sulfur, selenium, and tellurium;

[0087] In step S200, the precursor is placed in an etching gas atmosphere for etching to obtain a carbon material. The etching temperature is 400°C to 800°C, specifically 400°C, 500°C, 600°C, 700°C, 800°C or any value therebetween. The etching gas includes at least one of oxygen, ozone, sulfur dioxide, sulfur trioxide, nitrogen dioxide, water vapor and magnesium vapor. The structural schematic diagram of the etching product is shown in Figure 2.

[0088] The carbon material obtained by the preparation method of the present application can be used as a negative electrode material, and the sodium ion battery prepared from the carbon material has excellent electrochemical properties. In the preparation method of the present application, by pre-oxidizing the carbon-based raw material at a temperature of 200°C to 350°C and performing vapor phase etching at 400°C to 800°C, an appropriate amount of doping elements (at least one of nitrogen, phosphorus, arsenic, oxygen, sulfur, selenium, and tellurium) can be introduced on the surface of the carbon-based raw material. Since the temperature of the etching treatment after the pre-oxidation treatment is low, the doping elements can enter the surface of the carbon-based raw material more and hardly enter the interior of the carbon-based raw material, that is, the doping elements are mainly distributed in the surface area of ​​the obtained carbon material (specifically, the carbon material stack / layer, the same below), and are hardly distributed in the interior of the carbon material.

[0089] Among them, the doping elements distributed in the surface area of ​​the carbon material can form reversible functional groups with defective carbon atoms on the surface of the carbon material; on the one hand, these reversible functional groups can form reversible sodium storage sites with sodium ions, and in the subsequent charging process, these reversible functional groups are separated from the sodium ions, so that the sodium ions are released, so that the surface of the carbon material can adsorb more sodium ions, forming adsorption storage sodium, thereby improving the sodium storage capacity of the carbon material, and thus improving the capacity of the sodium ion battery made of the carbon material; on the other hand, since the potential of these reversible functional groups to react with solvated sodium ions is higher than the platform potential of sodium ion intercalation, these reversible functional groups can react preferentially with solvated sodium ions, which is beneficial to the desolvation of sodium ions, thereby reducing the polarization in the electrochemical process, improving the voltage platform of the carbon material, and reducing the occurrence of sodium precipitation in the electrode of the sodium ion battery (that is, the sodium ion battery made of carbon material, the same below).

[0090] By reducing the entry of doping elements into the internal area of ​​the carbon material, it is also beneficial to improve the initial efficiency and capacity of the sodium ion battery. The inventors have found that the doping elements inside the carbon material should not be too much: the carbon material is tightly arranged inside and has fewer defective carbon atoms. The entry of doping elements into the carbon material will cause the carbon material structure to deform and change the electronic band structure. During the charge and discharge process of the carbon material, sodium ions enter the carbon material to form an electric field. The existence of the electric field will hinder the subsequent diffusion of sodium ions into the carbon material. At the same time, it also makes it difficult for sodium ions that have entered the carbon material to escape. Therefore, by reducing the doping concentration of doping elements in the internal area of ​​the carbon material, the difficulty of sodium ions being inserted and removed from the carbon material during the charge and discharge process can be reduced, thereby improving the initial efficiency and capacity of the sodium ion battery.

[0091] In addition, the present application etches the precursor to form a rich pore structure in the carbon material, and the rich pore structure in the carbon material is conducive to the filling of sodium ions, forming a filled sodium storage, and improving the sodium storage capacity of the carbon material, thereby improving the capacity performance and initial efficiency of the sodium ion battery.

[0092] The preparation method of the present application is described in detail below with reference to the examples:

[0093] Step S100: placing the carbon-based raw material in an oxidizing gas atmosphere for pre-oxidation treatment to obtain a precursor, wherein the pre-oxidation treatment temperature is 200° C. to 350° C.

[0094] The temperature of the pre-oxidation treatment is 200°C to 350°C, and specifically can be 200°C, 220°C, 240°C, 260°C, 280°C, 300°C, 320°C, 330°C, 350°C or any value therebetween.

[0095] 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, which will result in a lower content of doping elements on the surface of the carbon material finally obtained, thereby resulting in a lower voltage platform of the carbon material obtained; if the temperature of the pre-oxidation treatment is higher than 350°C, part of the carbon-based raw material will spontaneously combust at the pre-oxidation temperature, greatly reducing the yield of the first precursor, and at the same time allowing more oxidizing gas to enter the interior of the carbon-based raw material, resulting in excessive content of doping elements in the interior of the carbon material finally obtained, and less content of doping elements on the surface of the carbon material, resulting in a lower sodium storage capacity and voltage platform of the carbon material, which will also lead to a decrease in the initial efficiency and capacity of the sodium ion battery and an increased risk of sodium precipitation in the electrode.

[0096] In some embodiments, the carbon-based feedstock includes at least one of biomass shell materials, biomass wood materials, straw, corn cobs, starch, sucrose, asphalt, coal, and coke. For example, the biomass shell materials include apricot shells, walnut shells, coconut shells, etc., and the biomass wood materials include bamboo, pine branches, etc.

[0097] In some embodiments, the oxidizing gas includes at least one of oxygen, ozone, nitrogen dioxide, and sulfur trioxide. By pre-oxidizing the carbon-based raw material, at least one of the doping elements oxygen, nitrogen, and sulfur can be introduced into the carbon-based raw material. These doping elements can combine with carbon atoms in the carbon-based raw material to form reversible energy groups such as C=O, C=N, C≡N, and C=S, which are beneficial to increasing the sodium storage capacity of the carbon material and can increase the voltage platform of the carbon material, thereby reducing the occurrence of sodium precipitation in the negative electrode sheet of the sodium ion battery.

[0098] 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%, 100% or any value therebetween.

[0099] 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, 600 min or any value therebetween.

[0100] In the above step S100, by regulating parameters such as the composition of the oxidizing gas, the oxygen content in the oxidizing gas, and the pre-oxidation treatment time, the content of each doping element on the surface and internal area of ​​the precursor can be controlled within an appropriate range.

[0101] Step S200. Place the precursor in an etching gas atmosphere for etching to obtain a negative electrode material. 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.

[0102] The etching temperature is 400° C. to 800° C., and specifically can be 400° C., 450° C., 500° C., 550° C., 600° C., 650° C., 700° C., 750° C., 800° C. or any value therebetween.

[0103] If the etching temperature is less than 400°C, the etching rate is slow and mostly surface etching, which makes it impossible to form an effective pore distribution inside the carbon material, which is not conducive to the filling of sodium ions in the carbon material and will reduce the capacity performance of the sodium ion battery made from the carbon material; if the etching temperature is greater than 800°C, the etching rate is too fast, which can easily cause the precursor to spontaneously combust, and the pores formed by etching have a larger pore size, which will cause the specific surface area of ​​the carbon material to be finally obtained to be too large, thereby reducing the initial efficiency of the sodium ion battery, and the higher temperature will cause the doping elements to diffuse into the interior of the precursor, which will cause the doping element content in the surface area of ​​the carbon material to be finally obtained to be reduced.

[0104] It is understandable that in some embodiments, the etching gas and the oxidizing gas may be the same, which is not limited in this application, and those skilled in the art may make a selection based on actual conditions.

[0105] More specifically, in some embodiments, the etching process is carried out in an etching gas, and the concentration of the etching gas is 5% to 100%, specifically 5%, 10%, 20%, 30%, 40%, 50%, 60%, 70%, 80%, 90%, 100% or any value therebetween.

[0106] In some embodiments, the etching 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, 600 min or any value therebetween.

[0107] In the above step S200, by regulating various parameters in the etching process such as etching temperature, etching time and etching gas, the porosity of the etching product can be controlled in a relatively ideal state, and the content of each doping element in the surface and internal area of ​​the etching product can be regulated.

[0108] In some embodiments, after the etching process, the preparation method further includes step S300: successively performing a crushing process and a heat treatment on the etching product.

[0109] The carbon material has closed pores, and the total pore volume of the closed pores is greater than 0.025 cm 3 / g. During the precursor etching and pore-forming process, the internal diffusion path of the etching gas in the precursor conforms to the material diffusion trend, resulting in dendritic-shaped pores etched by the etching gas in the precursor. During the subsequent crushing process, the etching product will be crushed along the larger pores in the dendritic pores due to stress, leaving a large number of micropores in the crushed product. During the subsequent heat treatment, these micropores will close to form closed pores, resulting in the prepared carbon material having abundant closed pores. These abundant closed pores can increase the storage sites of sodium ions in the carbon material, that is, can increase the sodium storage capacity of the carbon material.

[0110] In some embodiments, the etching product is a carbon material having pores, wherein the pores include micropores, and the total 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, 0.07cm 3 / g, etc. Within the above-defined range, it is shown that the etching product has formed more effective pores, which is conducive to the formation of more closed pores in the subsequent heat treatment process of the etching product, thereby increasing the sodium storage sites of the carbon material. It can be understood that micropores refer to pores with a pore diameter of less than 2nm. The single-point adsorption total pore volume of micropores can be obtained by nitrogen adsorption test, and the unit is cm 3 / g.

[0111] Furthermore, the average pore size of the carbon material is less than or equal to 3nm. When the average pore size is less than or equal to 3nm, since most of the pores in the carbon material are small in diameter, these pores will be further repaired to form abundant closed pores during the subsequent heat treatment process, which can increase the storage sites of sodium ions in the carbon material, that is, the sodium storage capacity of the carbon material. If the average pore size is greater than 3nm, it is not conducive to the closure of the pores in the subsequent heat treatment of the etching product, which will reduce the sodium storage sites of the obtained carbon material. In addition, the non-closure of the pores will also lead to an increase in the specific surface area of ​​the obtained carbon material, thereby reducing the initial efficiency of the sodium ion battery made of the carbon material.

[0112] In some embodiments, the total volume of micropores in the etched product is greater than or equal to 60%, based on the total pore volume of the etched product being 100%. This indicates that the etched product has a porous structure rich in micropores, which can be filled with more sodium ions, forming filled sodium storage, which is beneficial to improving the sodium storage capacity of the prepared carbon material.

[0113] In some embodiments, the etching product contains a doping element, and the doping element includes at least one of nitrogen, oxygen and sulfur, and the mass proportion of the doping element in the etching product is 3% to 15%, specifically 3%, 5%, 8%, 10%, 12%, 15% or any value therebetween. Wherein, the mass proportion of the doping element in the etching product refers to the mass proportion of at least one of oxygen, nitrogen and sulfur in the etching product, which can be calculated by measuring the content of oxygen, nitrogen and sulfur in the porous carbon-based material using a FlashSmart organic element analyzer (Thermo Fisher Scientific, USA).

[0114] In some embodiments, the volatile content of the etching product is 8% to 18%, specifically 8%, 10%, 12%, 15%, 18%, or any value therebetween. Within the above-defined range, this indicates that sufficient doping elements such as oxygen, nitrogen, and sulfur are present in the etching product, and thus sufficient doping elements such as oxygen, nitrogen, and sulfur are present in the resulting carbon material. This facilitates the formation of reversible functional groups such as C=O, C=N, and C=S in the resulting carbon material, thereby increasing the number of sodium storage sites in the carbon material. It is understood that volatile content refers to the portion of a substance that can evaporate under certain conditions.

[0115] In some embodiments, the pulverization equipment includes at least one of a ball mill pulverization equipment, a jet mill pulverization equipment, and a mechanical pulverization equipment.

[0116] In some embodiments, the median particle size of the pulverized material is between 3 μm and 20 μm, specifically 3 μm, 5 μm, 8 μm, 10 μm, 12 μm, 15 μm, 18 μm, 20 μm, or any value therebetween. Controlling the median particle size of the pulverized material within this range facilitates the formation of closed pores during subsequent heat treatment.

[0117] In some embodiments, after the pulverization process, the method further comprises acid washing the material obtained by the pulverization process to remove impurity elements such as metal and silicon in the material obtained by the pulverization process.

[0118] 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.

[0119] In some embodiments, the concentration of the pickling solution is 10% to 30%, specifically 10%, 15%, 20%, 25%, 30% or any value therebetween.

[0120] In some embodiments, the volume ratio of the material obtained by the pulverization treatment, hydrochloric acid, nitric acid and hydrofluoric acid is 1:(1-6):(0-6):(0-1), and the volume ratio of the material obtained by the pulverization treatment, 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. Of course, it can also be other values ​​within the above range, which is not limited here.

[0121] In some embodiments, the heat treatment temperature is 1000°C to 1600°C, specifically 1000°C, 1100°C, 1200°C, 1300°C, 1400°C, 1500°C, 1600°C, or any value therebetween. Within the above temperature range, reversible functional groups such as C=O, C=N, and C=S in the material obtained by the pulverization process can be stably present, and the pulverized material is conducive to complete carbonization, thereby improving the capacity performance and cycle performance of the sodium ion battery made from the carbon material.

[0122] In some embodiments, the heat treatment time is 1 h to 6 h, specifically 1 h, 2 h, 3 h, 4 h, 5 h, 6 h or any value therebetween.

[0123] In some embodiments, the heat treatment is performed in a protective gas atmosphere, wherein the protective gas includes at least one of nitrogen, argon, neon, helium, xenon, and krypton.

[0124] 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.

[0125] It can be understood that the carbon material described in the first aspect can be prepared through the above steps S100 and S200, and the carbon material described in the second aspect can be prepared by treating the carbon material described in the first aspect through the above step S300.

[0126] In a fourth aspect, the present application further proposes a negative electrode material, which includes the carbon material of the first aspect or the second aspect, or includes the carbon material prepared by the preparation method of the third aspect.

[0127] In a fifth aspect, the present application also proposes a sodium ion battery.

[0128] Specifically, a sodium-ion battery comprises a positive electrode sheet, a negative electrode sheet, and a separator disposed between the positive and negative electrode sheets. The negative electrode sheet comprises a current collector and a negative electrode active material layer. 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, or other common positive electrode current collectors in the art. The negative electrode active material layer comprises a negative electrode active material, a conductive agent, and a binder; wherein the negative electrode active material comprises the carbon material of the second aspect described above.

[0129] The present application will be further described below through specific examples.

[0130] Example 1

[0131] (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 warm for 5 h.

[0132] (2) The material obtained in step (1) (precursor, the same below) is further heated to 500° C. and kept at this temperature for 4 h to obtain an etching product.

[0133] (3) 20 g of the etched product was ball-milled to a D50 of 6 μm. 60 mL of 12% hydrochloric acid was added and heated to 75° C. in a water bath. The mixture was stirred for 2 h, filtered, washed with water until neutral, and dried in an oven at 80° C.

[0134] (4) The material obtained in step (3) (the material obtained by pulverization treatment, the same below) is placed in a high-temperature tube furnace and sintered at 1300° C. for 3 h in a nitrogen atmosphere to obtain a carbon material.

[0135] The carbon material prepared in this example has pores, including closed pores. The test results for the internal doping concentration Q1 wt% and surface doping concentration Q2 wt% of the doping element in the etched product, as well as the specific surface area, average pore diameter, micropore volume, volatile matter, and total doping element content of the etched product are shown in Table 1. The test results for the average pore diameter and pore volume of the closed pores, the internal doping concentration P1 wt% and surface doping concentration P2 wt% of the doping element in the carbon material, the doping element content, specific surface area, true density, and median particle size of the carbon material are shown in Table 2.

[0136] Example 2

[0137] (1) Take 100 g of 100-mesh bamboo powder and place it in a tube furnace. Introduce nitrogen dioxide with an air flow rate of 20 mL / min. The temperature curve is: increase the temperature to 280°C at 5°C / min and keep it warm for 5 h.

[0138] (2) The material obtained in step (1) is further heated to 700° C. and kept at this temperature for 4 hours to obtain an etching product.

[0139] (3) 20 g of the etched product was ball-milled to a D50 of 6 μm. 60 mL of 12% hydrochloric acid was added and heated to 75° C. in a water bath. The mixture was stirred for 2 h, filtered, washed with water until neutral, and dried in an oven at 80° C.

[0140] (4) The material obtained in step (3) was placed in a high-temperature tube furnace and sintered at 1300° C. for 3 h under a nitrogen atmosphere to obtain a negative electrode material.

[0141] The carbon material prepared in this example has pores, including closed pores. The test results for the internal doping concentration Q1 wt% and surface doping concentration Q2 wt% of the doping element in the etched product, as well as the specific surface area, average pore diameter, micropore volume, volatile matter, and total doping element content of the etched product are shown in Table 1. The test results for the average pore diameter and pore volume of the closed pores, the internal doping concentration P1 wt% and surface doping concentration P2 wt% of the doping element in the carbon material, the doping element content, specific surface area, true density, and median particle size of the carbon material are shown in Table 2.

[0142] Example 3

[0143] (1) Take 100 g of 100-mesh bamboo powder and place it in a tube furnace. Sulphur dioxide is introduced with an air 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.

[0144] (2) The material obtained in step (1) is further heated to 600° C. and kept at this temperature for 4 h to obtain an etching product.

[0145] (3) 20 g of the etched product was ball-milled to a D50 of 6 μm. 60 mL of 12% hydrochloric acid was added and heated to 75° C. in a water bath. The mixture was stirred for 2 h, filtered, washed with water until neutral, and dried in an oven at 80° C.

[0146] (4) The material obtained in step (3) was placed in a high-temperature tube furnace and sintered at 1300° C. for 3 h under a nitrogen atmosphere to obtain a negative electrode material.

[0147] The carbon material prepared in this example has pores, including closed pores. The test results for the internal doping concentration Q1 wt% and surface doping concentration Q2 wt% of the doping element in the etched product, as well as the specific surface area, average pore diameter, micropore volume, volatile matter, and total doping element content of the etched product are shown in Table 1. The test results for the average pore diameter and pore volume of the closed pores, the internal doping concentration P1 wt% and surface doping concentration P2 wt% of the doping element in the carbon material, the doping element content, specific surface area, true density, and median particle size of the carbon material are shown in Table 2.

[0148] Example 4

[0149] 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 to 200°C and kept warm for 5 h.

[0150] The carbon material prepared in this example has pores, including closed pores. The test results for the internal doping concentration Q1 wt% and surface doping concentration Q2 wt% of the doping element in the etched product, as well as the specific surface area, average pore diameter, micropore volume, volatile matter, and total doping element content of the etched product are shown in Table 1. The test results for the average pore diameter and pore volume of the closed pores, the internal doping concentration P1 wt% and surface doping concentration P2 wt% of the doping element in the carbon material, the doping element content, specific surface area, true density, and median particle size of the carbon material are shown in Table 2.

[0151] Example 5

[0152] 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 the temperature was kept for 5 h.

[0153] The carbon material prepared in this example has pores, including closed pores. The test results for the internal doping concentration Q1 wt% and surface doping concentration Q2 wt% of the doping element in the etched product, as well as the specific surface area, average pore diameter, micropore volume, volatile matter, and total doping element content of the etched product are shown in Table 1. The test results for the average pore diameter and pore volume of the closed pores, the internal doping concentration P1 wt% and surface doping concentration P2 wt% of the doping element in the carbon material, the doping element content, specific surface area, true density, and median particle size of the carbon material are shown in Table 2.

[0154] Example 6

[0155] 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 the temperature was kept at this temperature for 5 h.

[0156] The carbon material prepared in this example has pores, including closed pores. The test results for the internal doping concentration Q1 wt% and surface doping concentration Q2 wt% of the doping element in the etched product, as well as the specific surface area, average pore diameter, micropore volume, volatile matter, and total doping element content of the etched product are shown in Table 1. The test results for the average pore diameter and pore volume of the closed pores, the internal doping concentration P1 wt% and surface doping concentration P2 wt% of the doping element in the carbon material, the doping element content, specific surface area, true density, and median particle size of the carbon material are shown in Table 2.

[0157] Example 7

[0158] The difference from Example 1 is that: (2) the material obtained in step (1) is further heated to 400° C. and kept warm for 6 hours.

[0159] The carbon material prepared in this example has pores, including closed pores. The test results for the internal doping concentration Q1 wt% and surface doping concentration Q2 wt% of the doping element in the etched product, as well as the specific surface area, average pore diameter, micropore volume, volatile matter, and total doping element content of the etched product are shown in Table 1. The test results for the average pore diameter and pore volume of the closed pores, the internal doping concentration P1 wt% and surface doping concentration P2 wt% of the doping element in the carbon material, the doping element content, specific surface area, true density, and median particle size of the carbon material are shown in Table 2.

[0160] Example 8

[0161] 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.

[0162] The carbon material prepared in this example has pores, including closed pores. The test results for the internal doping concentration Q1 wt% and surface doping concentration Q2 wt% of the doping element in the etched product, as well as the specific surface area, average pore diameter, micropore volume, volatile matter, and total doping element content of the etched product are shown in Table 1. The test results for the average pore diameter and pore volume of the closed pores, the internal doping concentration P1 wt% and surface doping concentration P2 wt% of the doping element in the carbon material, the doping element content, specific surface area, true density, and median particle size of the carbon material are shown in Table 2.

[0163] Example 9

[0164] The difference from Example 1 is that: (2) the material obtained in step (1) is further heated to 700° C. and kept warm for 3 hours.

[0165] The carbon material prepared in this example has pores, including closed pores. The test results for the internal doping concentration Q1 wt% and surface doping concentration Q2 wt% of the doping element in the etched product, as well as the specific surface area, average pore diameter, micropore volume, volatile matter, and total doping element content of the etched product are shown in Table 1. The test results for the average pore diameter and pore volume of the closed pores, the internal doping concentration P1 wt% and surface doping concentration P2 wt% of the doping element in the carbon material, the doping element content, specific surface area, true density, and median particle size of the carbon material are shown in Table 2.

[0166] Example 10

[0167] The difference from Example 1 is that: (2) the material obtained in step (1) is further heated to 800° C. and kept warm for 2 h.

[0168] The carbon material prepared in this example has pores, including closed pores. The test results for the internal doping concentration Q1 wt% and surface doping concentration Q2 wt% of the doping element in the etched product, as well as the specific surface area, average pore diameter, micropore volume, volatile matter, and total doping element content of the etched product are shown in Table 1. The test results for the average pore diameter and pore volume of the closed pores, the internal doping concentration P1 wt% and surface doping concentration P2 wt% of the doping element in the carbon material, the doping element content, specific surface area, true density, and median particle size of the carbon material are shown in Table 2.

[0169] Example 11

[0170] The difference from Example 1 is that in step (1), in addition to the compressed air (oxygen concentration 20%) at a flow rate of 100 mL / min, nitrogen dioxide at a flow rate of 20 mL / min is also introduced.

[0171] The carbon material prepared in this example has pores, including closed pores. The test results for the internal doping concentration Q1 wt% and surface doping concentration Q2 wt% of the doping element in the etched product, as well as the specific surface area, average pore diameter, micropore volume, volatile matter, and total doping element content of the etched product are shown in Table 1. The test results for the average pore diameter and pore volume of the closed pores, the internal doping concentration P1 wt% and surface doping concentration P2 wt% of the doping element in the carbon material, the doping element content, specific surface area, true density, and median particle size of the carbon material are shown in Table 2.

[0172] Comparative Example 1

[0173] (1) Take 100 g of 100-mesh bamboo powder and place it in a tube furnace. Introduce nitrogen 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 warm for 5 h.

[0174] (2) The material obtained in step (1) is further heated to 500° C. and kept at this temperature for 4 h to obtain an etching product.

[0175] (3) 20 g of the etched product was ball-milled to a D50 of 6 μm. 60 mL of 12% hydrochloric acid was added and heated to 75° C. in a water bath. The mixture was stirred for 2 h, filtered, washed with water until neutral, and dried in an oven at 80° C.

[0176] (4) The material obtained in step (3) was placed in a high-temperature tube furnace and sintered at 1300° C. for 3 h under a nitrogen atmosphere to obtain a carbon material.

[0177] Comparative Example 2

[0178] (1) Take 100 g of 100-mesh bamboo powder and place it in a tube furnace. Let nitrogen flow in at a rate of 100 mL / min. Heat the mixture at a rate of 5°C / min to 250°C and keep the temperature for 5 h.

[0179] (2) The material obtained in step (1) was heated to 500° C. in compressed air (oxygen content 20%) and kept warm for 4 h to obtain an etching product.

[0180] (3) 20 g of the etched product was ball-milled to a D50 of 6 μm. 60 mL of 12% hydrochloric acid was added and heated to 75° C. in a water bath. The mixture was stirred for 2 h, filtered, washed with water until neutral, and dried in an oven at 80° C.

[0181] (4) The material obtained in step (3) was placed in a high-temperature tube furnace and sintered at 1300° C. for 3 h under a nitrogen atmosphere to obtain a carbon material.

[0182] Comparative Example 3

[0183] (1) 100 g of 100-mesh bamboo powder was placed in a tube furnace and compressed air (oxygen content 20%) was introduced at a flow rate of 100 mL / min. The temperature was raised at 5°C / min to 250°C and kept at this temperature for 5 h.

[0184] (2) The material obtained in step (1) was placed in nitrogen and heated to 500° C. and kept warm for 4 h to obtain an etched product.

[0185] (3) 20 g of the etched product was ball-milled to a D50 of 6 μm. 60 mL of 12% hydrochloric acid was added and heated to 75° C. in a water bath. The mixture was stirred for 2 h, filtered, washed with water until neutral, and dried in an oven at 80° C.

[0186] (4) The material obtained in step (3) was placed in a high-temperature tube furnace and sintered at 1300° C. for 3 h under a nitrogen atmosphere to obtain a carbon material.

[0187] Comparative Example 4

[0188] 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 the temperature was kept at this temperature for 5 h.

[0189] Comparative Example 5

[0190] 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 to 500°C and kept warm for 5 h.

[0191] Comparative Example 6

[0192] The difference from Example 1 is that: (2) the material obtained in step (1) is further heated to 380° C. and kept warm for 3 h.

[0193] Comparative Example 7

[0194] The difference from Example 1 is that: (2) the material obtained in step (1) is further heated to 900° C. and kept warm for 1 hour.

[0195] Performance Testing

[0196] (1) Material pore size and pore volume test: The pore size and pore volume were obtained using a TriStar II 3030 analyzer (Micromeritics Instruments, Inc., USA).

[0197] (2) Pore volume of closed pores: The pore volume of closed pores was determined using an AccuPyc II 1340 analyzer (Micromeritics Instruments, Inc., USA) using helium as the analytical gas for the true density data.

[0198] The true density of the material is obtained by analytical testing: ρ ture The ideal true density of graphite material is up to 2.26g / cm 3 That is the true density value; from this, the closed pore volume of the carbon material can be calculated as:

[0199] (3) Test of doping element content:

[0200] Testing method for internal doping concentration P1 wt% and surface doping concentration P2 wt% of carbon materials:

[0201] Take 10 g of sample from the carbon material;

[0202] The sample was examined using a scanning electron microscope (SEM) energy dispersive spectrometer. N 1 μm*1 μm areas were randomly selected, and the atomic mass percentage of the doping element in each area was measured. The average atomic mass percentage of the doping element in the n areas was calculated, and the apparent doping concentration of the doping element in the carbon material was obtained as P1 wt%. The measured thickness was equal to 1 μm.

[0203] The sample was sectioned using a focused ion beam instrument and then examined using a scanning electron microscope (SEM) spectrometer. N 1 μm*1 μm areas were randomly selected, and the atomic mass percentage of the doping element in each area was measured. The average atomic mass percentage of the doping element in the n areas was calculated, and the internal doping concentration of the doping element in the carbon material was obtained as P2 wt%; where n is a natural number ≥30.

[0204] Testing method for the content of doping elements in materials: The content of oxygen, nitrogen and sulfur in the materials was determined by FlashSmart organic element analyzer (Thermo Fisher Scientific, USA).

[0205] (4) Volatile matter: The volatile matter of the material is obtained by drying the material sample at 120°C for 2 h, taking 1±0.05 g of the sample and calcining it in a muffle furnace at 900°C for 7 min in an air atmosphere, and weighing it after cooling. The weight loss rate is calculated.

[0206] (5) A nitrogen adsorption test was performed at 77K using a McAb tester (model ASAP2460) to obtain the specific surface area and pore volume of the material, where 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 (unit: cm 3 / g).

[0207] (6) True density ρ: Measured using BEST 3H-2000TD, using helium, unit is cm 3 / g;

[0208] In order to characterize the pore structure characteristics of the negative electrode material provided in this application, the porosity W is defined as: W = V 孔 / (V 孔 +1 / ρ)*100% (II)

[0209] In the above formula, V 孔 is the pore volume, i.e. the total pore volume of single-point adsorption, in cm 3 / g; ρ is true density, unit is cm 3 / g; the physical meaning of porosity is the average value of the ratio between the pore volume contained in the carbon material and the total volume (pore volume + volume of carbon material), and the volume of carbon material = 1 / ρ.

[0210] (7) Functional group testing: X-ray photoelectron spectroscopy is used to analyze and characterize the functional groups contained in the carbon material.

[0211] (8) Electrochemical performance test:

[0212] Preparation of sodium-ion battery button cells: Hard carbon / SP / LA133 / pure water were prepared as an electrode slurry in a ratio of 91:3:6 / 100. The slurry was applied to aluminum foil with a 200μm scraper and then punched into 16mm discs after drying to obtain hard carbon electrode sheets. The hard carbon electrode sheets were assembled into button cells in a glove box for testing. Sodium metal was used as the counter electrode, the separator was glass fiber model GF / D from Whateman, and the electrolyte was 1.0M NaPF6 solution.

[0213] Charge and Discharge Performance Testing: The batteries were subjected to initial charge and discharge tests using a battery testing system (M310A, manufactured by Wuhan Landian Electronics Co., Ltd.) over a voltage range of 0.001 to 2.0 V. The charge and discharge capacity and discharge voltage platform of the negative electrode material were determined. The test results are shown in Tables 1 to 3.

[0214] Table 1: Parameter test results of the etching products obtained in various embodiments and comparative examples

[0215] Table 2: Parameter test of carbon materials prepared in Examples and Comparative Examples

[0216] Table 3. Electrochemical performance test of carbon materials (negative electrode materials) prepared in Examples and Comparative Examples

[0217] As shown in Tables 1 to 3, Examples 1 to 11 of the present application can introduce a large number of microporous structures and at least one doping element of nitrogen, phosphorus, arsenic, oxygen, sulfur, selenium, and tellurium into the etched product 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 make the doping elements enter the carbon surface more and almost not enter the interior of the carbon-based raw material. The doping elements form reversible functional groups C=O, C=N, C=S, etc. with the surface defect carbon atoms of the carbon-based raw material. The reversible functional groups and sodium ions react reversibly to form reversible sodium storage sites, thereby enabling the carbon material surface to adsorb more sodium ions and form adsorbed sodium storage. The above-mentioned reversible functional groups will preferentially react with the solvated sodium ions, which is beneficial to the desolvation of sodium ions, thereby reducing the polarization in the electrochemical process, increasing the voltage platform of the carbon material, and reducing the occurrence of sodium precipitation. Finally, during the further heat treatment of the carbon material, the micropores in the carbon material will close to form closed pores, and the pore volume of the closed pores is greater than 0.025cm 3 / g, which is conducive to the filling of sodium ions and improves the sodium storage capacity of carbon materials, thereby improving the capacity performance and initial efficiency of the battery.

[0218] Compared with Example 1, the carbon material prepared in Example 11 is doped with both nitrogen and oxygen. The carbon material prepared in Example 11 has a higher charge-discharge specific capacity and a higher depth of discharge voltage. This indicates that, compared with single doping with nitrogen or oxygen elements, co-doping with nitrogen and oxygen elements is more conducive to increasing the sodium storage capacity of the carbon material and lowering the voltage platform of the carbon material, thereby further improving the electrochemical performance of the battery.

[0219] Compared with Comparative Example 1, Examples 1, 2, and 3 introduce a large number of microporous structures and appropriate amounts of doping elements into the materials through pre-oxidation with an oxidizing gas and etching with an etching gas. As shown in Table 1 and Figure 3, the micropore volume of the etched products in Examples 1, 2, and 3 is much greater than the micropore volume of the etched product in Comparative Example 1 under nitrogen protection, which is conducive to the formation of closed pores in the carbon material. The total content of oxygen, nitrogen, and sulfur elements in the etched products was tested. The total content of doping elements and the surface doping concentration in the etched products in Examples 1, 2, and 3 were higher, which is conducive to the retention of oxygen / nitrogen / sulfur functional groups in the carbon material obtained after high-temperature carbonization, thereby increasing the voltage platform of the carbon material.

[0220] The true density test was performed on the carbon materials prepared in Example 1, Example 2, Example 3 and Comparative Example 1. The carbon materials prepared in Example 1, Example 2 and Example 3 had a higher closed pore volume, indicating that the carbon materials prepared in Example 1, Example 2 and Example 3 of the present application can provide more sodium storage sites.

[0221] Figure 5 is an energy spectrum distribution diagram of carbon elements and oxygen elements of the carbon material of Example 1 of the present application. It can be seen from Figure 4 that the surface of the carbon 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 carbon material can adsorb more sodium ions, forming adsorbed sodium storage, thereby improving the sodium storage capacity of the carbon material.

[0222] Figure 5 is an SEM image and a cross-sectional EDS-mapping image of the carbon material prepared in Example 1, Figure 6 is an SEM image and a cross-sectional EDS-mapping image of the carbon material prepared in Comparative Example 1, Figure 7 is a comparison of the first cycle charge and discharge of the carbon materials prepared in Example 1 and Comparative Example 1, and Figure 8 is a comparison of the discharge depth and voltage of the carbon materials prepared in Example 1 and Comparative Example 1. It can be seen from Figures 5 to 8 and Table 2 that the carbon 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 carbon 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 carbon material. The functional groups formed by these surface oxygen elements can promote the desolvation of sodium ions and reduce the polarization during the electrochemical reaction, thereby improving the voltage platform of the carbon material and reducing the occurrence of sodium precipitation in the electrode.

[0223] 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 doped elements in the surface area of ​​the prepared carbon material is too small, resulting in a reduction in the reversible functional groups on the surface of the carbon material and a reduction in the volume of the internal closed pores, which in turn leads to a reduction in the voltage platform of the carbon material and a reduction in the capacity of the battery.

[0224] 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 carbon material and a lower voltage platform of the carbon material; in Comparative Example 5, the temperature of the pre-oxidation treatment is too high, and there are fewer doping elements on the surface of the carbon material, more doping elements inside the carbon material, and fewer reversible functional groups formed by the surface doping elements and carbon atoms, resulting in increased side reactions between the carbon material and the electrolyte. The more doping elements inside the carbon material hinder the deintercalation and extraction of sodium ions from the carbon material during the charge and discharge process, thereby reducing the initial efficiency and capacity of the battery.

[0225] 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 carbon material and a reduced capacity of the battery; 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 carbon material, thereby reducing the initial efficiency of the battery. At the same time, the higher temperature allows more doping elements to enter the interior of the carbon material, hindering the deintercalation of sodium ions in the carbon material during the charge and discharge process, thereby reducing the initial efficiency and capacity of the battery.

[0226] The above description is merely a preferred embodiment of the present application and is not intended to limit the present application. Various modifications and variations are possible for those skilled in the art. Any modifications, equivalent substitutions, or improvements made within the spirit and principles of the present application shall be included within the scope of protection of the present application.

Claims

1. A carbon material, characterized in that: The carbon material has pores, wherein the pores include micropores, and the pore volume of the micropores is greater than or equal to 0.05 cm 3 / g; The carbon material contains doping elements, and the doping elements include nitrogen group elements and / or oxygen group elements; The internal doping concentration of the doping element in the carbon material is Q1wt%, Q1<8, and the surface doping concentration of the doping element in the carbon material is Q2wt%, 10≤Q2≤25; wherein the Q1wt% and the Q2wt% are obtained by the following test method: Take m1g of sample from the carbon material, m1≥10; The sample is detected by using a scanning electron microscope energy dispersive spectrometer to obtain the Q2wt%; The sample is subjected to cross-section processing by using a focused ion beam instrument, and then the cross-section of the sample is detected by using a scanning electron microscope energy dispersive spectrometer to obtain the Q1wt%.

2. The carbon material according to claim 1, characterized in that Having at least one of the following characteristics (1)-(3): (1) The average pore size of the carbon material is less than or equal to 3 nm; (2) based on the total pore volume of the carbon material being 100%, the volume proportion of the micropores is greater than or equal to 60%; (3) The total mass proportion of the doping elements in the carbon material is 3% to 15%; (4) The volatile matter of the carbon material is 8% to 18%.

3. A carbon material, characterized in that The carbon material has closed pores, and the total pore volume of the closed pores is greater than 0.025 cm 3 / g; The carbon material contains doping elements, and the doping elements include nitrogen group elements and / or oxygen group elements; The internal doping concentration of the doping element in the carbon material is P1wt%, P1<0.5; the surface doping concentration of the doping element in the carbon material is P2wt%, 0.5≤P2≤5; wherein the P1wt% and the P2wt% are obtained by the following test method: Take m2g sample from the carbon material, m2≥10; The sample was detected by using a scanning electron microscope energy dispersive spectrometer to obtain P2wt%; The sample is subjected to cross-section processing by using a focused ion beam instrument, and then the cross-section of the sample is detected by using a scanning electron microscope energy dispersive spectrometer to obtain P1wt%.

4. The carbon material according to claim 3, characterized in that The average pore size of the closed cells is less than or equal to 5 nm.

5. The carbon material according to claim 1 or 3, characterized in that The nitrogen group element includes at least one of nitrogen, phosphorus and arsenic, and / or the oxygen group element includes at least one of oxygen, sulfur, selenium and tellurium.

6. The carbon material according to claim 3, characterized in that The total doping concentration of the doping elements in the carbon material is 0.5 wt % to 4 wt %.

7. The carbon material according to claim 6, characterized in that The doping concentration of nitrogen in the carbon material is 0.5wt% to 2wt%, and / or the doping concentration of oxygen in the carbon material is 0.5wt% to 2wt%, and / or the doping concentration of sulfur in the carbon material is 0wt% to 2wt%.

8. The carbon material according to claim 3, characterized in that The carbon material contains at least one of the functional groups C=O, C=N, C≡N and C=S.

9. The carbon material according to claim 3, characterized in that The carbon material is selected from at least one of hard carbon, soft carbon and graphite.

10. The carbon material according to claim 3, characterized in that The carbon material was tested by Raman spectroscopy. -1 There is a characteristic peak D at the position, and the peak intensity of the characteristic peak D is I D , at 1580±10cm -1 There is a characteristic peak G at the position, and the peak intensity of the characteristic peak G is I G , 0.8≤I D / I G ≤1.

2.

11. The carbon material according to claim 3, characterized in that The carbon material includes at least one of the following features (1) to (3): (1) The median particle size of the carbon material is 3 μm to 20 μm; (2) The specific surface area of ​​the carbon material is 2 m 2 / g~50m 2 / g; (3) The true density of the carbon material is 1.8 g / cm 3 ~2.2g / cm 3 .

12. A method for preparing a carbon material, characterized in that: The steps include: Placing the carbon-based raw material in an atmosphere containing an oxidizing gas for pre-oxidation treatment to obtain a precursor, wherein the pre-oxidation treatment temperature is 200° C. to 350° C., the oxidizing gas contains nitrogen group elements and / or oxygen group elements, the nitrogen group elements contain at least one of nitrogen, phosphorus and arsenic, and the oxygen group elements contain at least one of oxygen, sulfur, selenium and tellurium; The precursor is placed in an etching gas atmosphere for etching treatment to obtain a carbon material. 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.

13. The preparation method according to claim 12, characterized in that: After the etching process, the preparation method further comprises: performing crushing and heat treatment on the etching product.

14. A negative electrode material, characterized in that: The negative electrode material includes the carbon material according to any one of claims 1 to 11 or the carbon material prepared by the preparation method according to any one of claims 12 to 13.

15. A sodium ion battery, characterized in that: The sodium ion battery comprises the negative electrode material as claimed in claim 14.

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