Hard carbon negative electrode material, preparation method and sodium ion battery

Abstract

The invention discloses a hard carbon negative electrode material, a preparation method and a sodium ion battery. In the sodium embedding process of the hard carbon negative electrode material, the intercalation capacity is greater than or equal to 220mAh / g, the true density is greater than or equal to 2.1 g / cm < 3 >, and the compaction density is greater than or equal to 0.95 g / cm < 3 >; wherein the intercalation capacity in the sodium intercalation process refers to the capacity of the voltage between 30mV and 100mV in the electrochemical test discharge process. The hard carbon negative electrode material provided by the invention has relatively high intercalation capacity, fewer defects exist in the carbon layer, and the structure is more complete; and meanwhile, relatively high compaction density and true density are kept, so that relatively high energy density is kept.

Description

Technical Field

[0001] This application relates to the technical field of anode materials for sodium-ion batteries, and more specifically, to hard carbon anode materials, preparation methods, and sodium-ion batteries. Background Art

[0002] Sodium-ion batteries have the same working principle as lithium-ion batteries. Compared with lithium resources, sodium elements have a more abundant reserve and a relatively lower price. Therefore, sodium-ion batteries can be used as a supplementary application to lithium-ion batteries. However, due to the larger radius of sodium ions, they cannot intercalate into the graphite interlayer. The traditional graphite anode in lithium-ion batteries cannot be used as the anode material for sodium-ion batteries. Hard carbon materials are considered to be the first anode materials for sodium-ion batteries to be industrialized due to their larger graphene layer spacing (≥0.37 nm), rich pore structure, and lower cost.

[0003] The sodium storage mechanism of "adsorption-intercalation-filling" in hard carbon materials is widely recognized. Sodium ions are adsorbed at the edges, defects, and oxygen-containing functional groups of hard carbon materials to form adsorbed sodium storage; sodium ions intercalate into the carbon layers of hard carbon to form intercalated sodium storage; closed pores are formed by the stacking of curved and turbulent carbon layers, and sodium ions fill into these closed pores to form filled sodium storage. The current technical solutions usually aim to increase the closed pore structure of hard carbon materials to increase the sodium storage capacity of the materials. However, when the closed pore structure in hard carbon materials increases, the true density of the materials will decrease. For example, Yuqi Li et al. prepared hard carbon materials with rich closed pores using waste cork. When the capacity was increased to ~330 mAh / g of the hard carbon material capacity, the true density of the material was <1.6 g / cm 3 (much less than 2.26 g / cm of graphite 3 ), resulting in a lower compaction density of the electrode sheet, which is not conducive to the improvement of the energy density of sodium-ion batteries. Summary of the Invention

[0004] The purpose of this application is to provide a hard carbon anode material, a preparation method, and a sodium-ion battery, which can maintain a certain energy density while increasing the capacity of the hard carbon material.

[0005] This application is implemented as follows:

[0006] In a first aspect, this application provides a hard carbon anode material with an intercalation capacity ≥220 mAh / g and a true density ≥2.1 g / cm 3 during the sodiation process, and a compaction density ≥0.95 g / cm 3 ;

[0007] Among them, the intercalation capacity during the sodiation process refers to the capacity between 30 mV and 100 mV of the voltage during the discharge process of electrochemical testing.

[0008] In an alternative embodiment, I D / IG <1.1> and / or, the conductivity of the hard carbon negative electrode material powder tested at a density of 1.0 g / cm 3 is ≥ 3×10 3 S / m.

[0009] In an alternative embodiment, the C1s spectrum is measured by X-ray photoelectron spectroscopy and analyzed to obtain that, in the carbon element of the hard carbon negative electrode material, the mass fraction of carbon present in the form of C═C is ≥ 60%; and / or, the mass fraction of carbon present in the form of C-C is < 30%; and / or, the mass fraction of carbon present in the forms of C-O and C═O is ≤ 1%.

[0010] In a second aspect, the present application provides a method for preparing the hard carbon negative electrode material according to any one of the foregoing embodiments, including: first performing low-temperature carbonization on a hard carbon precursor at 800 °C to 950 °C, and then performing high-temperature carbonization at 1000 °C to 1600 °C to obtain the hard carbon negative electrode material.

[0011] In an alternative embodiment, the time for the low-temperature carbonization is 1 h to 6 h;

[0012] and / or, the time for the high-temperature carbonization is 1 h to 6 h.

[0013] In an alternative embodiment, the low-temperature carbonization is carried out under an inert gas atmosphere and / or a repair gas atmosphere; the atmosphere for the low-temperature carbonization satisfies at least one of the following items ①-③;

[0014] ① The inert gas includes nitrogen and / or argon;

[0015] ② The repair gas includes hydrogen and / or an organic gas;

[0016] ③ The repair gas includes an organic gas, and the organic gas includes at least one of methane, acetylene or ethanol.

[0017] In an alternative embodiment, the preparation method includes:

[0018] compounding the hard carbon precursor with a component containing a transition metal element to obtain a composite intermediate product;

[0019] performing low-temperature carbonization on the composite intermediate product to obtain a low-temperature carbonization intermediate product;

[0020] performing acid washing on the low-temperature carbonization intermediate product to remove the transition metal element therein to obtain an acid-washed intermediate product, and then performing high-temperature carbonization on the acid-washed intermediate product;

[0021] The content or composition of the transition metal element satisfies at least one of the following items ④-⑥:

[0022] ④ The content of the transition metal element in the composite intermediate product is 100 ppm to 50,000 ppm;

[0023] ⑤ The content of the transition metal element in the pickling intermediate product is less than 100 ppm;

[0024] ⑥ The component containing the transition metal element includes at least one of transition metal simple substance, transition metal salt and transition metal oxide.

[0025] In an alternative embodiment, the compounding includes: mixing the hard carbon precursor with a solution containing a transition metal element, followed by solid-liquid separation and drying; the compounding step satisfies at least one of the following items ⑦-⑧:

[0026] ⑦ The concentration of the transition metal element in the solution containing the transition metal element is 0.01 mol / L to 1 mol / L;

[0027] ⑧ The mixing time is 10 min to 120 min.

[0028] In an alternative embodiment, the fixed carbon content in the hard carbon precursor is > 70 wt%, the volatile content is 5 wt% to 25 wt%, and the ash content is ≤ 1 wt%;

[0029] And / or, the hard carbon precursor is obtained by subjecting a carbon source to low-temperature pre-carbonization at 200 °C to 700 °C, crushing it to 4 μm to 20 μm, and then removing impurities by pickling.

[0030] In a third aspect, the present application provides a sodium-ion battery, including the hard carbon negative electrode material described in any one of the foregoing embodiments or the hard carbon negative electrode material prepared by the method described in any one of the foregoing embodiments.

[0031] The present application has the following beneficial effects:

[0032] The hard carbon negative electrode material in the present application has a higher intercalation capacity, fewer defects in the carbon layer, and a more complete structure; at the same time, it maintains a higher tap density and true density, thereby maintaining a higher energy density. Description of the Drawings

[0033] In order to more clearly illustrate the technical solutions of the embodiments of the present application, the following will briefly introduce the drawings required for the embodiments. It should be understood that the following drawings only show some embodiments of the present application, and therefore should not be regarded as limiting the scope. For those of ordinary skill in the art, other related drawings can be obtained based on these drawings without creative efforts.

[0034] Figure 1 It is a schematic diagram of the oxygen element escaping during the low-temperature carbonization process;

[0035] Figure 2 Schematic diagram of oxygen element removal during high-temperature carbonization without low-temperature carbonization

[0036] Figure 3 Schematic diagram of the hard carbon structure and carbon layer structure before and after repair Detailed implementation manners

[0037] To make the objectives, technical solutions, and advantages of the embodiments of the present application clearer, the technical solutions in the embodiments of the present application will be clearly and completely described below. For those not specified in the embodiments, the conventional conditions or the conditions recommended by the manufacturer are followed. For the reagents or instruments without indicating the manufacturer, they are all conventional products that can be obtained through commercial purchase.

[0038] An embodiment of the present application provides a hard carbon negative electrode material, with an intercalation capacity during sodium intercalation ≥ 220 mAh / g, a true density ≥ 2.1 g / cm 3 , and a tap density ≥ 0.95 g / cm 3 ;

[0039] Among them, the intercalation capacity during sodium intercalation refers to the capacity between 30 mV and 100 mV during the discharge process of the electrochemical test.

[0040] Currently, the common idea for preparing high-capacity hard carbon is to increase the closed pore volume of the material. However, during the process of increasing the closed pores, the tap density of the material will decrease. And during the pore-forming process, methods such as oxidative etching and adding pore-forming agents, although improving the sodium storage capacity of the hard carbon material, will inevitably lead to a decrease in the tap density of the material. And during the etching process, pore defects that are difficult to repair will inevitably be formed inside the carbon layer, resulting in a decrease in the true density of the material, thereby leading to a decrease in the tap density of the electrode sheet and a decrease in the energy density of the battery. The hard carbon negative electrode material in this embodiment mainly improves the ability of intercalation sodium storage, so it can maintain a relatively high true density and tap density.

[0041] In an alternative embodiment, I D / I G <1.1 and / or, the conductivity of the hard carbon negative electrode material powder tested at a density of 1.0 g / cm 3 ≥ 3×10 3 S / m.

[0042] Perform Raman test on the hard carbon negative electrode material. The D peak and G peak are two independent typical carbon material characteristic broad peaks at about 1350 cm -1 (D band) and 1600 cm -1 (G band) in the Raman spectrum respectively. The peak intensities of the D peak and G peak are represented by the intensity ratio I D / IG It is shown that I D / I G The larger the value, the higher the degree of disorder and the lower the degree of graphitization of the hard carbon material. In the existing preparation process of hard carbon, the graphene layers formed during the high-temperature carbonization of the carbon source often have many defects, and the I D / I G ratio of the obtained hard carbon is often greater than 1.2, indicating that there are many defects in the graphene layers. In this embodiment, reducing the I D / I G to below 1.1 is beneficial to reducing the defects of the carbon layer, thereby reducing the formation of irreversible sodium sites at the defects, and further improving the initial efficiency.

[0043] In the prior art, the conductivity of the hard carbon powder tested at a density of 1.0 g / cm 3 is usually (1 - 2)×10 3 S / m. Under the same test conditions, the conductivity of the graphite material is usually (2 - 5)×10 4 S / m, and the conductivity of carbon black is usually (2 - 3)×10 3 S / m. The conductivity of hard carbon is one order of magnitude lower than that of graphite and is close to that of amorphous carbon black, which also indicates that the material has large defects. The defects of the material will lead to a decline in the rate performance. In this embodiment, by reducing the I D / I G to below 1.1, while increasing the conductivity of hard carbon and reducing the defects of the hard carbon material, the rate performance of the material is improved.

[0044] In an alternative embodiment, the C1s spectrum is measured by X-ray photoelectron spectroscopy, and the C1s spectrum is analyzed. In the carbon element of the hard carbon negative electrode material, the mass fraction of carbon in the form of C═C is ≥60%; and / or, the mass fraction of carbon in the form of C-C is <30%; and / or, the mass fraction of carbon in the form of C-O and C═O is ≤1%.

[0045] In this embodiment, the proportion of C═C bonds can indicate the degree of proximity of the hard carbon negative electrode material to the graphene material or the degree of remoteness from the amorphous carbon. The increase in the proportion of C═C in the hard carbon negative electrode material is beneficial to reducing the defects of the hard carbon material; the increase in the proportions of C-C, C-O, and C═O increases the defects in the hard carbon material.

[0046] In some embodiments, a Thermo Scientific K-Alpha+ X-ray photoelectron spectrometer produced by Thermo Scientific of the United States is used to analyze the relative elemental content and functional groups of hard carbon. The monochromatic Al target Kα line is adopted, the pass energy of the full spectrum is 100 eV, and the pass energy of the fine spectrum is 30 eV. The C1s of the XPS fine spectrum of the material is subjected to peak fitting analysis. The peaks are respectively assigned to C═C bond (284.0 eV), C-C bond (284.8 eV), C-O bond (286.6 eV), and C═O bond (287.8 eV). And calculate the ratio of the peak area ratio of the C═C bond, that is, the contents of the C═C bond, C-C bond, C-O bond, and C═O; 10 different test sites are taken for each sample, the contents of different bonds are analyzed, and the average value is taken to obtain the contents of each carbon-containing functional group.

[0047] Another embodiment of the present application provides a method for preparing the hard carbon negative electrode material according to any one of the foregoing embodiments, including: first performing low-temperature carbonization on the hard carbon precursor at 800 °C to 950 °C, and then performing high-temperature carbonization at 1000 °C to 1600 °C to obtain the hard carbon negative electrode material.

[0048] Specifically, the time for low-temperature carbonization can be 800 °C, 830 °C, 860 °C, 890 °C, 920 °C, 950 °C or any value between 800 °C and 950 °C; the time for high-temperature carbonization can be 1000 °C, 1100 °C, 1200 °C, 1300 °C, 1400 °C, 1500 °C, 1600 °C or any value between 1000 °C and 1600 °C.

[0049] In the range of 800 °C to 950 °C, the hard carbon precursor is mainly in the deoxidation process. Compared with directly heating to above 1000 °C, holding in the range of 800 °C to 950 °C can enable the oxygen atoms in the precursor to slowly escape, tending to form carbon dioxide, as Figure 1 shown; while directly heating to above 1000 °C, oxygen is more inclined to form carbon monoxide, as Figure 2 shown, which leads to more pore defects formed inside the material, and low temperature can make the rate of gas formation slower, reducing the rapid formation of gas and causing new defects in the carbon matrix, making the material diffuse.

[0050] In an alternative embodiment, the time for low-temperature carbonization is 1 h to 6 h, specifically it can be 1 h, 2 h, 3 h, 4 h, 5 h, 6 h or any value between 1 h and 6 h.

[0051] In an alternative embodiment, the time for high-temperature carbonization is 1 h to 6 h, specifically it can be 1 h, 2 h, 3 h, 4 h, 5 h, 6 h or any value between 1 h and 6 h.

[0052] In an alternative embodiment, the low-temperature carbonization is carried out under an inert gas atmosphere and / or a repair gas atmosphere;

[0053] Preferably, the inert gas includes nitrogen and / or argon;

[0054] Preferably, the repair gas includes hydrogen and / or an organic gas;

[0055] Preferably, the repair gas includes an organic gas, and the organic gas includes at least one of methane, acetylene or ethanol.

[0056] During the low-temperature carbonization process, maintaining an inert gas atmosphere can reduce the oxidation of carbon; if hydrogen and / or an organic gas is introduced, gases such as hydrogen, methane, acetylene and ethanol can react with the oxygen element in the hard carbon precursor, reducing the reaction between the oxygen element in the precursor and the carbon in the precursor during the heating and removal process to produce carbon monoxide or carbon dioxide, which takes away the carbon element in the precursor, thereby increasing the vacancy defects. The introduced carbon-containing organic gases such as methane, acetylene and ethanol can also crack within the temperature range of low-temperature carbonization to repair the carbon matrix defects. Under the same conditions, the carbon-containing organic gas has a better repair effect than hydrogen.

[0057] Specifically, in some embodiments, a pure repair gas atmosphere can be adopted. For example, when the repair gas atmosphere is methane, but for some repair gas atmospheres, such as hydrogen, due to certain safety hazards of pure hydrogen, when the repair gas atmosphere is hydrogen, the volume fraction of hydrogen in the atmosphere is usually 5%-10%, and the rest is an inert gas, such as nitrogen or argon, etc.

[0058] In an alternative embodiment, the preparation method includes:

[0059] Composite the hard carbon precursor with a component containing a transition metal element to obtain a composite intermediate product;

[0060] Carry out low-temperature carbonization on the composite intermediate product to obtain a low-temperature carbonization intermediate product;

[0061] Carry out pickling on the low-temperature carbonization intermediate product to remove the transition metal element therein to obtain a pickling intermediate product, and then carry out high-temperature carbonization on the pickling intermediate product.

[0062] During the low-temperature carbonization process, the transition metal element can catalyze the transformation of the amorphous carbon atoms with sp3 hybridization in the hard carbon precursor into graphene materials with sp2 hybridization, and to a certain extent catalyze the repair of the carbon matrix defects. Schematic diagrams of the hard carbon structure and carbon layer structure before and after repair are as Figure 3As shown; however, during the high-temperature carbonization process, the presence of transition metal elements will lead to an increase in the number of closed pores in the material. At the same time, when the temperature > 1000 °C, the transition metal elements will catalyze an increase in the graphitization degree of the material, resulting in a decrease in the interlayer spacing, and further leading to a decrease in the material capacity. Therefore, before high-temperature carbonization, the material is pickled first to remove the existing transition metal elements and reduce the increase in the number of closed pores and the capacity reduction caused by the increase in the graphitization degree.

[0063] In an alternative embodiment, the content of transition metal elements in the composite intermediate product is 100 ppm to 50000 ppm. Specifically, it can be 100 ppm, 500 ppm, 1000 ppm, 2000 ppm, 5000 ppm, 10000 ppm, 20000 ppm, 30000 ppm, 40000 ppm, 50000 ppm or any value between 100 ppm and 50000 ppm; if the content of transition metal elements is too low, the catalytic effect is not obvious. If the content of transition metal elements is too high, it is beneficial to increase the graphitization degree of the hard carbon anode material, reduce the interlayer spacing of the material, and reduce the intercalation capacity.

[0064] In an alternative embodiment, the content of transition metal elements in the pickled intermediate product is less than 100 ppm. For some transition metal elements with better catalytic effects, the content after pickling should be even smaller. For example, when the transition metal element is iron, pickling needs to be stopped until the iron element content is less than 10 ppm.

[0065] In an alternative embodiment, the component containing transition metal elements includes at least one of transition metal simple substances, transition metal salts, and transition metal oxides.

[0066] Specifically, transition metal elements can include elements such as scandium, titanium, vanadium, chromium, manganese, iron, cobalt, nickel, copper, zinc, etc. The existence form of transition metals can be chloride salts, nitrate salts of transition metals, such as ferric chloride, nickel chloride, etc., or oxides, such as iron oxide, etc.

[0067] In an alternative embodiment, the composite includes: mixing the hard carbon precursor with a solution containing transition metal elements, followed by solid-liquid separation and drying.

[0068] Under normal circumstances, the transition metal element and the hard carbon precursor can also be mixed by ball milling or other methods. However, in order to enable the transition metal element to penetrate into the interior of the hard carbon precursor, the transition metal element is usually selected in a water-soluble form and configured into a solution. After the transition metal element is relatively evenly distributed inside the hard carbon precursor, the water can be removed by drying. Configuring the transition metal element into an aqueous solution is more conducive to the uniform mixing of the transition metal element and the hard carbon precursor. The drying temperature can be selected as needed. For example, in some embodiments, the drying temperature can be 60°C - 100°C; in some embodiments, other solvents can also be selected to configure the solution containing the transition metal element, and solvents with relatively low boiling points are usually selected to facilitate the removal of the solvent.

[0069] In an alternative embodiment, the concentration of the transition metal element in the solution containing the transition metal element is 0.01 mol / L to 1 mol / L, which is beneficial to obtaining a hard carbon precursor with an appropriate content of the transition metal element.

[0070] In an alternative embodiment, the mixing time is 10 min to 120 min to enable the transition metal element to fully penetrate into the hard carbon precursor.

[0071] In an alternative embodiment, the fixed carbon content in the hard carbon precursor is > 70 wt%, the volatile matter content is 5 wt% - 25 wt%, and the ash content is ≤ 1 wt%.

[0072] Preferably, the hard carbon precursor is obtained by subjecting a carbon source to low-temperature pre-carbonization at 200°C - 700°C, crushing it to 4 μm - 20 μm, and then removing impurities by pickling. Specifically, the carbon source can include biomass, resin, starch, etc.

[0073] In some embodiments, in the pickling step, it is usually soaked in a hydrochloric acid solution with a concentration of 0.001 mol / L - 1 mol / L.

[0074] Another embodiment of the present application provides a sodium-ion battery, including the hard carbon negative electrode material of any one of the foregoing embodiments or the hard carbon negative electrode material prepared by the method of any one of the foregoing embodiments.

[0075] In this embodiment, by adding a low-temperature carbonization process and combining a repair atmosphere or a transition metal element used for catalyzing the repair process, the defects of the hard carbon material can be reduced, and I D / I G can be reduced to below 1.1, while improving the conductivity of the hard carbon to improve the rate performance of the material.

[0076] The features and properties of the present application will be further described in detail below in conjunction with embodiments.

[0077] Example 1:

[0078] Take 100 g of bamboo-based carbonized, crushed, pickled and purified materials as the hard carbon precursor. The fixed carbon content in the hard carbon precursor is 83.2 wt%, the volatile content is 16.5 wt%, and the ash content is 0.3 wt%. The hard carbon precursor is placed in 200 ml of 0.01 mol / L iron chloride solution, stirred for 30 min and then filtered by suction. After drying at 80 °C, a composite intermediate product with an iron element content of 1248 ppm is obtained. It is heated to 850 °C in a tubular furnace and held for 3 h under a nitrogen protective atmosphere. After taking out, it is washed with 1 mol / L hydrochloric acid until the content of transition metal elements is less than 50 ppm, and then heated to 1300 °C in the tubular furnace and held for 3 h under a nitrogen protective atmosphere to obtain the hard carbon material.

[0079] Example 2:

[0080] It is different from Example 2 in that hydrogen is introduced into the atmosphere during the heat preservation stage at 850 °C until the volume fraction of hydrogen in the atmosphere is 10%.

[0081] Example 3:

[0082] It is different from Example 2 in that the hydrogen introduced during the heat preservation stage at 850 °C is replaced with methane gas until the volume fraction of methane in the atmosphere is 100%.

[0083] Example 4

[0084] It is different from Example 2 in that the hydrogen introduced during the heat preservation stage at 850 °C is replaced with ethanol gas until the volume fraction of ethanol in the atmosphere is 10%.

[0085] Example 5

[0086] Take 100 g of bamboo-based carbonized, crushed, pickled and purified materials as the hard carbon precursor. The fixed carbon content in the hard carbon precursor is 83.2 wt%, the volatile content is 16.5 wt%, and the ash content is 0.3 wt%. The hard carbon precursor is placed in 200 ml of 0.2 mol / L iron chloride solution, stirred for 30 min and then filtered by suction. After drying at 80 °C, a composite intermediate product with an iron element content of 3076 ppm is obtained. It is heated to 850 °C in a tubular furnace and held for 3 h under a nitrogen protective atmosphere. After taking out, it is washed with 1 mol / L hydrochloric acid until the content of transition metal elements is less than 50 ppm, and then heated to 1300 °C in the tubular furnace and held for 3 h under a nitrogen protective atmosphere to obtain the hard carbon material.

[0087] Example 6

[0088] It is different from Example 5 in that hydrogen gas is introduced during the heat preservation stage at 850 °C until the volume fraction of hydrogen in the atmosphere is 10%.

[0089] Example 7

[0090] It is different from Example 2 in that hydrogen is continuously introduced after the heat preservation at 850 °C until the end of the heat preservation at 1300 °C.

[0091] Example VIII

[0092] It is different from Example V in that the concentration of the ferric chloride solution is 2 mol / L; the iron element content is: 63475 ppm.

[0093] Example IX

[0094] It is different from Example V in that the 850 °C heat preservation stage is cancelled. After mixing the catalysts, it is directly heated to 1300 °C and kept warm for 3 h. The protective atmosphere is nitrogen, and then it is washed with hydrochloric acid with a concentration of 1 mol / L to obtain the hard carbon material.

[0095] Comparative Example I:

[0096] Take the hard carbon precursor in Example 1 and heat it to 1300 °C in a tube furnace and keep it warm for 3 h. The protective atmosphere is nitrogen to obtain the hard carbon material.

[0097] The materials obtained in the above examples and comparative examples were tested. The specific test methods are as follows, and the test results are shown in Table 1 and Table 2.

[0098] 1 Powder conductivity:

[0099] Use the MCP-PD51 powder conductivity meter of Mitsubishi Chemical of Japan. The system uses the four-probe method to measure the volume resistivity of the sample. Use this instrument to measure the resistance of the powder, and then the computer automatically calculates the conductivity of the negative electrode material at 1.0 g / cm 3 Under the condition. Through the powder conductivity tester, the conductivity of the negative electrode material at 1.0 g / cm 3 Under the condition is tested.

[0100] 2 Powder compaction:

[0101] Through the automatic compaction density meter UTM7305, the compaction density measured under a pressure of 3 T is the compaction density measured in this application.

[0102] 3 Raman test:

[0103] The Raman spectrum of the negative electrode material was measured by a Raman spectrometer (trade name: Renishaw in Via, laser wavelength 532 nm, manufactured by Renishaw Co., Ltd., UK). The peak intensity I of the negative electrode material at 1350 cm-1 was measured D And the peak intensity I at 1580 cm-1 G The ratio I of D / I G I D / I G Can represent the defect degree of the hard carbon material. The larger the value, the more defects.

[0104] 4 True density test:

[0105] The AccuPyc II 1340 analyzer (Micromeritics Instrument Corporation, USA) uses helium as the analysis gas for true density data, and the true density of the material is obtained through analysis and testing.

[0106] Characterization of C=C bond content:

[0107] In this paper, a Thermo Scientific K-Alpha+ type X-ray photoelectron spectrometer produced by Thermo Scientific Company in the United States was used to analyze the relative elemental content and functional groups of hard carbon. The monochromatic Al target Kα line was used, the pass energy of the full spectrum was 100 eV, and the pass energy of the fine spectrum was 30 eV. The C1s of the XPS fine spectrum of the material was subjected to peak fitting analysis. The peaks were respectively fitted into C=C bond (284.0 eV), C-C bond (284.8 eV), C-O bond (286.6 eV), and C=O bond (287.8 eV). And the ratio of the peak area of the C=C bond was calculated, that is, the contents of C=C bond, C-C bond, C-O bond, and C=O; 10 different test sites were taken for each sample, the contents of different bonds were analyzed, and the average value was taken.

[0108] 6 Content of transition metal element catalysts:

[0109] Weigh 0.4 - 0.5 g of the sample, add 9 mL of hydrochloric acid and 3 mL of nitric acid, heat on a graphite digestion instrument for about 30 minutes to completely digest the trace elements in the sample. After filtration and volume fixation, the content of trace elements was detected by an inductively coupled plasma optical emission spectrometer (ICP-OES).

[0110] 7 Interlayer spacing test

[0111] In this paper, an X-ray diffractometer - 7000 (XRD-7000) produced by Shimadzu Corporation in Japan was used to test the hard carbon material. After obtaining the X-ray diffraction pattern of the material, the interlayer spacing was calculated using the Bragg equation where θ is the angle between the incident X-ray and the corresponding crystal plane, which can be obtained from the diffraction pattern, and λ is the wavelength of the X-ray.

[0112] 8 Electrochemical performance test:

[0113] Fabrication of sodium-ion battery coin cells: The hard carbon / SP / LA133 / pure water was configured into an electrode slurry in a ratio of 91:3:6 / 100, coated on aluminum foil with a 200 μm doctor blade, dried and punched into a 16 mm round sheet to obtain a hard carbon electrode sheet, which was assembled into a coin cell in a glove box for testing. Among them, the counter electrode used sodium metal, the separator was a glass fiber of Whateman model GF / D, and the electrolyte was a 1.0 M NaPF6 solution.

[0114] Charge and discharge performance test:

[0115] The first charge and discharge test of the battery was carried out using a battery test system (M310A, manufactured by Wuhan Blue Electronic Co., Ltd.), with a voltage range of 0.001 - 2.0V and a temperature of 25 ± 5°C.

[0116] Rate performance test: At a temperature of 25 ± 5°C, charge and discharge cycles were carried out at 0.3A / g, 0.6A / g, 0.9A / g, 1.5A / g, 2.0A / g, and 0.3A / g for 5 cycles each. Capacity retention rate at 2.0A / g: That is, the ratio of the capacity of the battery discharged at 2.0A / g to the sodium intercalation capacity of the battery discharged at 0.3A / g * 100%.

[0117] Table 1 Material physical property statistics

[0118]

[0119] Table 2 Material electrochemical performance

[0120]

[0121]

[0122] 1. Comparing Examples 1 - 6 and Comparative Example 1: Compared with directly carbonizing at >1100°C, due to holding at 800 - 1000°C, it is beneficial for oxygen atoms to diffuse on the carbon matrix, beneficial for the slow removal of oxygen atoms. Oxygen elements tend to form carbon dioxide or oxygen and escape, reducing the formation of carbon monoxide during rapid carbonization and taking out carbon atoms, and reducing the increase in carbon matrix defects caused by vacancy defects. Therefore, it is beneficial to improve the true density of the hard carbon negative electrode material and further reduce the decrease in the compaction density of the material. In Comparative Example 1, the content of C = C bonds in the material is low, and the Raman I D / I G is high, and the powder conductivity is low, which verifies this conclusion. In the electrochemical test, the process of 30 - 100mV during discharge is considered as the intercalation sodium storage process during sodium intercalation. All the examples are higher than Comparative Example 1, which also proves that the carbon layer structure of the material is relatively more complete;

[0123] 2. Comparing Example 2 and Example 7, the physical and chemical properties and electrochemical properties of the materials are not much different, indicating that in the heat preservation section at 800 - 1000°C, the structure repair of the material has been completed. When the temperature rises, the layer spacing of the material decreases, the openings close, and it is difficult for gas to enter the material interior to repair the carbon layer. Therefore, the physical properties of the materials are not much different.

[0124] 3. Comparing Example 5, Example 8, and Example 9, when the content of transition metal is relatively high or when the temperature rises above 1100 °C, the presence of transition metal elements increases the graphitization degree of the catalytic material and the true density of the material. Additionally, XRD testing of the material reveals that the interlayer spacing d002 of the hard carbon negative electrode material obtained in Example 9 is less than 0.37 nm. At this time, it is difficult for sodium atoms to insert into the carbon layer. Electrochemical testing shows that the intercalation sodium storage capacity during the sodiation process decreases severely, which also demonstrates this conclusion.

[0125] The hard carbon negative electrode material in this application has fewer defects and a more complete structure in the carbon layer, which is beneficial for: ① improving the intercalation capacity of the material; ② reducing the pore defect concentration in the carbon layer of the material, thereby increasing the compaction density of the material; ③ reducing the defects in the carbon layer, thereby reducing the formation of irreversible sodium sites at the defects, thus improving the first cycle efficiency; ④ reducing irreversible sodium intercalation, being beneficial for the diffusion of sodium ions within the carbon layer, thereby enhancing the rate performance of the material; ⑤ increasing the conductivity of the material, being beneficial for enhancing the rate performance of the material.

[0126] The above are only the preferred embodiments of this application and are not used to limit this application. For those skilled in the art, various changes and modifications can be made to this application. Any modification, equivalent replacement, improvement, etc. made within the spirit and principle of this application shall be included within the protection scope of this application.

Claims

1. A hard carbon negative electrode material, characterized in that, The intercalation capacity of the sodium insertion process is ≥220mAh / g, and the true density is ≥2.1g / cm 3 , compacted density ≥ 0.95g / cm 3 ; Among them, the intercalation capacity during the sodiation process refers to the capacity between 30 mV and 100 mV of the voltage during the discharge process of the electrochemical test.

2. The hard carbon negative electrode material according to claim 1, wherein I D / I G <1.1; and / or, the electrical conductivity of the hard carbon negative electrode material powder tested at a density of 1.0 g / cm 3 is ≥ 3×10 3 S / m.

3. The hard carbon negative electrode material according to claim 1, characterized in that, The C1s spectrum is measured by X-ray photoelectron spectroscopy, and the C1s spectrum is analyzed to obtain that in the carbon element of the hard carbon negative electrode material, the mass fraction of carbon existing in the form of C═C ≥ 60%; and / or, the mass fraction of carbon existing in the form of C-C < 30%; and / or, the mass fraction of carbon existing in the forms of C-O and C═O ≤ 1%.

4. A method for preparing the hard carbon negative electrode material according to any one of claims 1-3, characterized in that, Including: First, the hard carbon precursor is subjected to low-temperature carbonization at 800 °C to 950 °C, and then high-temperature carbonization at 1000 °C to 1600 °C to obtain the hard carbon negative electrode material.

5. The preparation method of the hard carbon negative electrode material according to claim 4, wherein, The time of the low-temperature carbonization is 1 h to 6 h; and / or, the time of the high-temperature carbonization is 1 h to 6 h.

6. The preparation method of the hard carbon negative electrode material according to claim 4, characterized in that, The low-temperature carbonization is carried out under an inert gas atmosphere and / or a repair gas atmosphere, and the atmosphere of the low-temperature carbonization satisfies at least one of the following items ①-③; ① The inert gas includes nitrogen and / or argon; ② The repair gas includes hydrogen and / or organic gas; ③ The repair gas includes organic gas, and the organic gas includes at least one of methane, acetylene or ethanol.

7. The preparation method of the hard carbon negative electrode material according to claim 4, characterized in that, The preparation method includes: The hard carbon precursor is compounded with a component containing a transition metal element to obtain a composite intermediate product; The composite intermediate product is subjected to low-temperature carbonization to obtain a low-temperature carbonization intermediate product; The low-temperature carbonization intermediate product is pickled to remove the transition metal element therein to obtain a pickled intermediate product, and then the pickled intermediate product is subjected to high-temperature carbonization; The content or composition of the transition metal element satisfies at least one of the following items ④-⑥: ④ The content of the transition metal element in the composite intermediate product is 100 ppm to 50000 ppm; ⑤ The content of the transition metal element in the pickled intermediate product is less than 100 ppm; ⑥ The component containing a transition metal element includes at least one of a transition metal simple substance, a transition metal salt and a transition metal oxide.

8. The preparation method of the hard carbon negative electrode material according to claim 7, characterized in that, The compounding includes: mixing the hard carbon precursor with a solution containing a transition metal element and then performing solid-liquid separation and drying; the compounding step satisfies at least one of the following items ⑦-⑧: ⑦ The concentration of the transition metal element in the solution containing a transition metal element is 0.01 mol / L to 1 mol / L; ⑧ The mixing time is 10 min to 120 min.

9. The preparation method of the hard carbon negative electrode material according to claim 4, wherein The fixed carbon content in the hard carbon precursor > 70 wt%, the volatile content is 5 wt% to 25 wt%, and the ash content ≤ 1 wt%; and / or, the hard carbon precursor is obtained by subjecting a carbon source to low-temperature pre-carbonization at 200 °C to 700 °C, crushing it to 4 μm to 20 μm, and then pickling to remove impurities.

10. A sodium-ion battery, characterized in that, Including the hard carbon negative electrode material according to any one of claims 1-3 or the hard carbon negative electrode material prepared by the method according to any one of claims 4-9.

Images