Preparation method and application of modified micro-expansion spherical graphite

Abstract

The invention relates to a preparation method and application of modified micro-expansion spherical graphite, in particular to a preparation method and application of modified micro-expansion spherical graphite. In order to solve the problems of poor rate capability and poor cycle stability of a natural graphite negative electrode material, the surface-modified micro-expansion spherical graphite is obtained by performing oxidation modification treatment on spherical graphite by adopting an electrochemical oxidation technology. The interlayer spacing is enlarged, the diffusion resistance of lithium ions is reduced, and the rate capability is improved. An oxygen-containing functional group is introduced, so that a uniform and stable SEI film is formed, and the cycling stability is improved. The coated asphalt is used as a precursor, and the carbon-coated spherical graphite with a core-shell structure is obtained through a controllable carbonization technology. The carbon coating layer can effectively inhibit graphite stripping, pulverization and volume expansion, and the pore structure can improve the electrolyte compatibility and improve the cycle stability. The method is applied to the field of lithium ion battery negative electrode materials.

Description

Technical Field

[0001] The present invention relates to a preparation method and application of modified micro-expanded spherical graphite. Background Art

[0002] In recent years, emerging industries such as new energy vehicles, artificial intelligence, big data, and 5G communication have developed rapidly, posing higher requirements for the performance of lithium-ion batteries, such as lithium storage capacity, rate performance, cycle stability, etc. The discharge capacity of natural graphite anode materials must be guaranteed under high current, and excellent rate performance and cycle stability are required to better meet market demands. The factors affecting the rate performance of natural graphite anode materials are mainly the following two aspects: (1) The interlayer spacing of the crystal plane of natural graphite (≤0.34 nm) is smaller than that of the graphite intercalation compound Li x C6 (0.37 nm), which causes the interlayer spacing of graphite to change continuously during charge and discharge, leading to the exfoliation and pulverization of the electrode material, and affecting the rate performance and cycle stability of the battery. (2) There are structural defects in natural graphite anode materials, the reaction activity on the outer surface of particles is uneven, the grain size of the crystal grains is relatively large, the surface crystal structure is easily damaged during charge and discharge, and the surface solid electrolyte interface (SEI) film is unevenly covered, resulting in low initial Coulomb efficiency and poor rate performance. Therefore, various methods must be used to improve the rate performance of natural graphite electrodes. Summary of the Invention

[0003] The purpose of the present invention is to solve the problems of poor rate performance and cycle stability of natural graphite anode materials, and provide a preparation method and application of modified micro-expanded spherical graphite.

[0004] A preparation method of modified micro-expanded spherical graphite of the present invention is as follows:

[0005] Using the electrochemical oxidation method, with concentrated HNO3 as the intercalating agent, constant current electrochemical oxidation intercalation is carried out on spherical graphite, and the solid-liquid ratio of spherical graphite to the intercalating agent is adjusted to 1 g: 1-1.5 mL, the current density is 20-60 mA / cm 2 and the reaction time is 10-60 min to obtain a spherical graphite intercalation compound, and then the spherical graphite intercalation compound is expanded by the high-temperature expansion method to obtain modified micro-expanded spherical graphite.

[0006] The present invention uses electrochemical oxidation technology to oxidize and modify spherical graphite to obtain surface-modified micro-expanded spherical graphite. The layer spacing is enlarged, the lithium-ion diffusion resistance is reduced, and the rate performance is improved. Oxygen-containing functional groups are introduced to form a uniform and stable SEI film, improving the cycle stability.

[0007] The application of the modified micro-expanded spherical graphite prepared by the present invention as a lithium-ion battery anode material.

[0008] The present invention uses coated pitch as a precursor and obtains carbon-coated spherical graphite with a "core-shell" structure through a controllable carbonization technique. The carbon coating layer can effectively inhibit graphite peeling, pulverization, and volume expansion, and the pore structure can improve the compatibility with the electrolyte and enhance the cycle stability.

[0009] Beneficial effects of the present invention:

[0010] (1) Electrochemical oxidation intercalation and expansion treatment were carried out on spherical graphite using nitric acid as an intercalating agent to prepare slightly expanded modified spherical graphite. Under constant current conditions, by controlling the electrochemistry reaction time, the slight expansion ratio, pore structure, and surface oxidation degree were regulated. The technical parameters of the slight expansion modification are: material ratio m SG / V HNO3 =1:1, the current density is 40 mA / cm 2 , the reaction time is 20 min; the expansion treatment temperature is 450 °C, and the time is 60 min. The slightly expanded modified spherical graphite expands the graphite layer spacing and introduces oxygen-containing functional groups on the graphite surface.

[0011] (2) Coating modification was carried out on the slightly expanded spherical graphite using petroleum pitch as a precursor to prepare a "core-shell" structure modified natural graphite anode material with slightly expanded spherical graphite as the core and amorphous carbon as the shell. The technical parameters of the coating modification are: using the vacuum impregnation liquid-phase coating method for mixing; under N2 conditions, the pre-carbonization heating rate is 2 °C / min, the temperature is 300 °C, and the time is 2 h; the carbonization heating rate is 10 °C / min, the temperature is 1000 °C, and the time is 4 h. The coating modification reduces the structural defects of the slightly expanded spherical graphite, decreases the specific surface area, and increases the tap density, meeting the product indicators of the natural graphite anode material for lithium-ion batteries.

[0012] (3) Electrochemical slight expansion modification and coating modification can significantly improve the first charge-discharge efficiency, cycle stability, and rate performance of the battery. The lithium-ion button battery assembled with NG@P10 as the anode material has a first discharge specific capacity of 409.83 mAh / g at 0.1C, a first Coulombic efficiency of 93.59%, and a capacity retention rate of 82.21% after 2000 cycles; at 10C, the first discharge specific capacity is 362.24 mAh / g, the first Coulombic efficiency is 92.34%, and the capacity retention rate is 80.36% after 500 cycles. The battery has excellent rate charge-discharge performance, electrochemical reversibility, and low charge transfer resistance. Description of the Drawings

[0013] Figure 1 For the SEM images of SG, and samples;

[0014] Figure 2 For and N2 adsorption - desorption curve of the sample;

[0015] Figure 3 SEM images of SG, ESG10, ESG20, ESG30, ESG40 and ESG50 samples;

[0016] Figure 4 N2 adsorption - desorption curves of SG raw material and ESG samples with different electro - chemical reaction times;

[0017] Figure 5 XRD patterns of SG raw material and ESG samples obtained with different electro - chemical reaction times;

[0018] Figure 6 Raman spectra of SG raw material and ESG samples;

[0019] Figure 7 SEM images of modified anode materials NG@P5, NG@P7.5, NG@P10 and NG@P12.5 prepared with different petroleum asphalt addition amounts;

[0020] Figure 8 N2 adsorption - desorption curves of ESG20 and NG@P samples prepared with different petroleum asphalt addition amounts;

[0021] Figure 9 XRD patterns of ESG20 and NG@P samples with different asphalt addition amounts;

[0022] Figure 10 Raman spectra of ESG20 and NG@P samples with different asphalt addition amounts;

[0023] Figure 11 Initial charge - discharge specific capacity curves of batteries assembled with different anode materials at 0.1C;

[0024] Figure 12 Initial charge - discharge specific capacity curves of NG@P7.5 and NG@P10 batteries at 10C;

[0025] Figure 13 Rate performance of NG@P7.5 and NG@P10 batteries was tested for comparison;

[0026] Figure 14 Cycle retention rate curve of NG@P10 battery at 0.1C;

[0027] Figure 15 Cycle retention rate curve of NG@P10 battery at 10C. Detailed implementation mode

[0028] Specific Embodiment 1: A preparation method of modified slightly expanded spherical graphite in this embodiment is as follows:

[0029] Using the electrochemical oxidation method, with concentrated HNO3 as the intercalating agent, constant current electrochemical oxidation intercalation is carried out on spherical graphite. The solid-liquid ratio of spherical graphite to the intercalating agent is adjusted to 1g:1 - 1.5mL, the current density is 20 - 60mA / cm 2 and the reaction time is 10 - 60min to obtain a spherical graphite intercalation compound, and then the high-temperature expansion method is used to expand the spherical graphite intercalation compound to obtain modified slightly expanded spherical graphite.

[0030] Specific Embodiment 2: The difference between this embodiment and Specific Embodiment 1 is that: the solid-liquid ratio of spherical graphite to the intercalating agent is 1g:1mL, the current density is 40mA / cm 2 and the reaction time is 20min. Others are the same as Specific Embodiment 1.

[0031] Specific Embodiment 3: The difference between this embodiment and Specific Embodiment 1 or 2 is that: the concentration of the concentrated HNO3 is 68wt%. Others are the same as Specific Embodiment 1 or 2.

[0032] Specific Embodiment 4: The difference between this embodiment and one of Specific Embodiments 1 to 3 is that: in the high-temperature expansion method, the high-temperature expansion treatment temperature is 450°C and the time is 30min. Others are the same as one of Specific Embodiments 1 to 3.

[0033] Specific Embodiment 5: The difference between this embodiment and one of Specific Embodiments 1 to 4 is that: the interlayer spacing of the modified slightly expanded spherical graphite is 0.3374 - 0.3376nm. Others are the same as one of Specific Embodiments 1 to 4.

[0034] Specific Embodiment 6: Application of the modified slightly expanded spherical graphite as a negative electrode material for lithium-ion batteries

[0035] Specific Embodiment 7: The difference between this embodiment and Specific Embodiment 6 is that: the specific application method is as follows: Weigh asphalt and dissolve it in tetrahydrofuran to obtain an asphalt solution with a mass concentration of 5% - 12.5%; perform vacuum treatment on the modified slightly expanded spherical graphite, then add the asphalt solution, perform vacuum impregnation, and dry to obtain a mixture; then carry out pre-carbonization treatment on the mixture under the protection of nitrogen, and then raise the temperature for carbonization treatment to obtain a negative electrode material for lithium-ion batteries. Others are the same as Specific Embodiment 6.

[0036] Specific Embodiment 8: The difference between this embodiment and Specific Embodiment 7 is that: the vacuum treatment means evacuating to -0.1MPa and maintaining for 1h. The vacuum treatment means evacuating to -0.1MPa and maintaining for 1h. Others are the same as Specific Embodiment 7.

[0037] Embodiment 9 in detail: The difference between this embodiment and Embodiment 7 in detail is that after vacuum impregnation, it is first transferred to an evaporation device, and tetrahydrofuran is volatilized at a constant temperature of 70 °C, and then dried. Others are the same as Embodiment 7 in detail.

[0038] Embodiment 10 in detail: The difference between this embodiment and Embodiment 7 in detail is that the pre-carbonization mentioned refers to pre-carbonization for 2 h under the conditions of a heating rate of 2 °C / min and 300 °C. Others are the same as Embodiment 7 in detail.

[0039] Embodiment 11 in detail: The difference between this embodiment and Embodiment 7 in detail is that the carbonization mentioned refers to a heating rate of 10 °C / min, a carbonization temperature of 1000 °C, and a carbonization time of 4 h. Others are the same as Embodiment 7 in detail

[0040] The content of the present invention is not limited to the content of the above embodiments, and the combination of one or several of the embodiments in detail can also achieve the purpose of the invention.

[0041] Example 1: Preparation of modified micro-expanded spherical graphite

[0042] Micro-expansion modification of spherical graphite can expand the layer spacing of graphite, reduce the lithium ion diffusion resistance, and improve the rate performance. At the same time, oxygen-containing functional groups are introduced on the surface of spherical graphite to form a uniform and stable SEI film, improving the cycle stability. However, too large specific surface area and irreversible capacity will cause spherical graphite to not become a stable anode material. Therefore, in this example, the spherical graphite (SG) raw material (fixed carbon content ≥ 99.95%, D50 = 18 μm) is subjected to oxidation intercalation and prepared into micro-expanded spherical graphite (ESG) through high-temperature expansion treatment. Using the electrochemistry oxidation technology, without using an oxidant, sulfuric acid (93 wt%), nitric acid (68 wt%) and perchloric acid (70 wt%) are respectively selected as intercalating agents, and the device for electrochemically preparing low-temperature expandable graphite in Patent CN202411315756.5 is used for preparation. Electrochemical oxidation intercalation and micro-expansion modification are carried out on spherical graphite to prepare and micro-expanded spherical graphite. The electrochemistry technical parameters and expansion conditions adopted for different intercalating agents are shown in Table 1.

[0043] Table 1 Technical parameter table for preparing micro-expanded spherical graphite with different intercalating agents

[0044]

[0045] Figure 1 For SG, and SEM images of the samples. From Figure 1(a) It can be seen that the SG raw material has a regular oval spherical structure with a particle size of 15 - 20 μm. Figure 1 (b) This is a high - magnification SEM image of SG. The surface of SG is smooth and the graphite layers are dense. Figure 1 (c, g) After electrochemical oxidation intercalation and high - temperature expansion treatment, and obvious expansion occurred. The edges of the graphite showed a phenomenon of warping ( Figure 1 d); It presented a slightly expanded accordion - like structure ( Figure 1 h). Figure 1 (e) This is the SEM image prepared with a nitric acid (68 wt%) intercalating agent, and no obvious expansion phenomenon occurred. Figure 1 (f) In it, very few edges of the graphite showed a slight warping.

[0046] Figure 2 are and N2 adsorption - desorption curves of the samples. and The width of the hysteresis loop at high pressure of the N2 adsorption - desorption curves of is greater than that of and The hysteresis loop is of type H3, which is the defect brought by the slit on the graphite surface. BET The S and S BET of 2 are 7.0218 m 2 / g and 7.6292 m / g respectively, and the S BET of 2 is relatively low, being 5.5604 m

[0047] / g. This test result is consistent with the SEM morphology analysis.During the electrochemical oxidation intercalation process, under the action of constant current, spherical graphite and the auxiliary anode together form the anode. Carbon atoms lose their relatively active π electrons and are oxidized. The network planar macromolecule of the graphite layer becomes a positively charged planar macromolecule. Due to the repulsive force of like charges, the graphite layer spacing increases. Under the action of the concentration difference driving force and the electric field force, acid root ions are inserted into the graphite layer and diffuse in the layer, forming graphite intercalation compounds. After thermal expansion treatment, the graphite intercalation compounds decompose at high temperature to generate gas, which provides a driving force for the expansion process of graphite and effectively increases the graphite layer spacing. The initial expansion temperature of the spherical graphite intercalation compound prepared with sulfuric acid (93 wt%) intercalating agent is 650 °C. In order to ensure that there is no further deformation due to continued expansion during the subsequent coating modification heating process, high-temperature treatment must be carried out above 650 °C. When the temperature is higher than 600 °C, it will cause the oxidation of spherical graphite, so it is difficult to be applied to the preparation of anode materials. The initial expansion temperatures of the spherical graphite intercalation compounds prepared with perchloric acid (70 wt%) and nitric acid (68 wt%) intercalating agents are both 200 °C, which can meet the requirement of high-temperature treatment below 600 °C. However, obvious expansion occurred, and the excessive expansion volume led to a significant increase in specific surface area and a decrease in tapped density. In addition, the residual sulfur and chlorine in the slightly expanded spherical graphite will affect the electrochemical performance of the anode material. Therefore, subsequently, nitric acid (68 wt%) was selected as the intercalating agent for the research on the electrochemical micro-expansion modification technology of spherical graphite.

[0048] In this embodiment, nitric acid (68 wt%) was used as the intercalating agent to carry out micro-expansion modification treatment on spherical graphite. The material ratio of graphite to nitric acid was selected as The current density was controlled at 40 mA / cm 2 , and a constant current electrochemical oxidation intercalation reaction was carried out. The high-temperature expansion treatment temperature was 450 °C and the time was 60 min. The electrochemical reaction times were 10 min, 20 min, 30 min, 40 min, and 50 min respectively to investigate the influence of the electrochemical reaction time on the slightly expanded spherical graphite. Table 2 is the technical parameter table of the electrochemical micro-expansion modified spherical graphite. The slightly expanded spherical graphite samples prepared with different electrochemical reaction times were named ESG10, ESG20, ESG30, ESG40, and ESG50 respectively.

[0049] Table 2 Technical parameter table of the electrochemical micro-expansion modified spherical graphite

[0050]

[0051]

[0052] Figure 3 SEM images of SG, ESG10, ESG20, ESG30, ESG40, and ESG50 samples. As Figure 3As shown in Figure 2, when the electrochemical reaction time was only 10 min, the particle size and morphology of ESG10 did not change significantly compared with the SG raw material ( Figure 3 b). When the electrochemical reaction time is 20 min, the graphite edge of the ESG20 sample is slightly raised ( Figure 3 c) As the electrochemical reaction time increases, the number of cracks and pores between the ESG layers increases, and expansion becomes increasingly pronounced. More defects appear on the surfaces of ESG40 and ESG50 samples. This indicates that during the galvanostatic reaction, extending the reaction time increases the amount of nitrate intercalation between graphite layers and worsens graphite oxidation. During high-temperature heat treatment, the nitrate between the graphite layers decomposes and escapes as gas, causing the spherical graphite to appear slightly swollen. Therefore, excessively long electrochemical reaction times can increase the interlayer spacing and porosity of spherical graphite.

[0053] Figure 4 The N2 adsorption-desorption curves of SG raw materials and ESG samples with different electrochemical reaction times are shown in the figure. As the reaction time increases, the width of the H3 hysteresis loop in the high-pressure region of the N2 adsorption-desorption curve gradually increases, indicating that micro-expansion becomes increasingly obvious and the number of slits and defects increases. BET The average pore size data are shown in Table 3. BET 5.4682m 2 / g, S of ESG samples BET As the electrochemical reaction time increases, the S BET Increased to 6.2968m 2 / g, and the average pore size increases to 18.68nm. This indicates that electrochemical micro-expansion modification increases the surface pores and surface area of the SG material. This increases the number of active sites during the charge-discharge process, significantly impacting the charge-discharge capacity. However, this increase in defects also affects the material's long-term cycling performance and initial coulombic efficiency.

[0054] Table 3 Comparison of specific surface area, average pore size and tap density of micro-expanded spherical graphite samples

[0055]

[0056] XRD was used to detect the graphite crystal structure of SG raw materials and ESG samples. Figure 5XRD patterns of ESG samples obtained from SG raw materials and different electrochemical reaction times are shown. As can be seen from the figure, the XRD patterns of SG and ESG are in line with the diffraction peak positions corresponding to graphite (PDF#23-0064), indicating that the electrochemical micro-expansion modification does not destroy the original characteristic peaks of graphite. Among them, the three characteristic peaks appearing in the XRD pattern at 2θ = 26.6°, 44.5° and 2θ = 54.7° correspond to the diffraction peaks of the (002), (101) and (004) crystal planes respectively. According to the Bragg equation, the interlayer spacing d of the graphite crystal is calculated to be 0.335 nm. The diffraction peak of the (002) crystal plane of SG is high in intensity, sharp in peak shape, and the graphite sheet layers are arranged regularly, indicating that the crystallinity of graphite is high, the internal crystal lattice is arranged regularly, and there are few defects. With the extension of the electrochemical reaction time, the (002) peak corresponding to 26.6° in the XRD pattern of the ESG sample gradually shifts to a lower angle, the full width at half maximum becomes significantly wider, and the peak value decreases, indicating that after electrochemical oxidation intercalation and thermal expansion, the graphitization degree decreases, the van der Waals force between graphite layers is destroyed, and the graphite interlayer spacing increases. The specific graphite interlayer spacing is shown in Table 3.

[0057] Figure 6 Raman spectra of SG raw materials and ESG samples are shown. As can be seen from the figure, absorption peaks appear at 1350 cm -1 and 1580 cm -1 in the Raman spectra of the samples, which are attributed to the D peak and G peak of carbon materials respectively. Among them, the D peak of the Raman spectrum is due to the irregular scattering of non-graphitized crystals in the graphite sample, while the G peak is generated by the vibration of sp 2 carbon atoms in the two-dimensional hexagonal crystal of graphite. According to research, the relative intensity ratio (I D / I G ) of the two absorption peaks can reflect the disorder degree of the material, usually also representing the defect degree, and the defect degree is proportional to I D / I G . It can be seen from the figure that the I D / I G value of the SG sample is 0.03, and the I D / I G value is very small, indicating that the SG sample has few defects. With the extension of the electrolysis time, the I D / I G value of the ESG sample gradually increases, and the I D / I G value of ESG50 increases to 0.18. This indicates that the defect degree of graphite has increased. The Raman spectrum test results are consistent with the XRD test results.

[0058] Example 2: Application of modified micro-expanded spherical graphite as a negative electrode material for lithium-ion batteries

[0059] Pitch carbon coating of micro-expanded spherical graphite

[0060] In this embodiment, petroleum asphalt produced by Hebei Joyson New Materials Co., Ltd. is used as the carbon source for natural graphite coating modification. 90 The particle size is about 15-20 μm and the softening point is 210°C. First, the Li content of spherical graphite is increased by electrochemical micro-expansion modification. + Embed ion channels and active sites; then, petroleum asphalt is selected to modify the micro-expanded spherical graphite by asphalt carbon coating to obtain a modified natural graphite negative electrode material, thereby improving the structural stability of the graphite negative electrode material, and then improving the cycle efficiency, rate performance and cycle stability of the negative electrode material.

[0061] Using the above-mentioned petroleum asphalt as the coating carbon source, the micro-expanded ESG20 was coated with asphalt carbon to prepare a modified natural graphite negative electrode material (NG@P). The effect of asphalt addition on the structural composition of the negative electrode material was investigated. The asphalt addition amounts were 5%, 7.5%, 10%, and 12.5%, respectively. The fixed experimental conditions were: pre-carbonization temperature of 300°C, time of 2h, and heating rate of 2°C / min; carbonization temperature of 1200°C, time of 4h, and heating rate of 10°C / min. The carbonization process was carried out under N2 protection. The names of the modified coated negative electrode material samples prepared with different asphalt addition amounts are shown in Table 4.

[0062] Figure 7 SEM images of modified anode materials NG@P5, NG@P7.5, NG@P10, and NG@P12.5 prepared with varying amounts of petroleum asphalt addition. The images show that the particle size of the NG@P samples coated with pitch carbon is approximately 15-25 μm, with no apparent particle adhesion, indicating an appropriate asphalt coating ratio and effective results. The surface of the NG@P samples becomes smooth, with the pitch carbon evenly wrapped around the ESG particles, filling numerous surface defects introduced during the micro-expansion modification process. The pitch carbon coating formed after carbonization coats the ESG surface, filling cracks and pores, blunting the surface edges and corners of the particles and effectively reducing the specific surface area of the graphite. With increasing amounts of petroleum asphalt addition, the surface coating of the ESG particles gradually thickens, smoothing the surface and tightly encapsulating the ESG, forming a "core-shell" structure of hard carbon encapsulating soft carbon.

[0063] Table 4 Comparison of physical parameters of ESG20 and NG@P samples

[0064]

[0065] Figure 8 The N2 adsorption-desorption curves of NG@P samples prepared with ESG20 and different amounts of petroleum asphalt are shown in the figure. BETThe specific surface area and average pore diameter data are shown in Table 4. Compared with the ESG20 sample, the S of the NG@P sample after asphalt carbon coating BET significantly decreases, which is consistent with the SEM analysis results. As the asphalt addition amount increases, the S of the NG@P sample BET decreases from 2.9692 m 2 / g to 1.2109 m 2 / g in turn, and the average pore diameter decreases from 15.68 nm to 8.40 nm in turn, indicating that increasing the asphalt addition amount can better fill the pores inside the ESG particles. In Table 4, the tap density of the NG@P sample gradually increases in turn as the asphalt addition amount increases. This is unified with the change law of the specific surface area. When the asphalt addition amount is higher than 7.5%, the specific surface area ≤ 2.5 m 2 / g, and the tap density ≥ 1.0 g / cm 3 , meeting the requirements of GB / T 24533-2019 "Graphite Anode Materials for Lithium-Ion Batteries" for the specific surface area and tap density of natural graphite anode materials.

[0066] Figure 9 Figure 5 is the XRD patterns of ESG20 and NG@P samples with different asphalt addition amounts. As can be seen from the figure, a sharp diffraction peak of the graphite (002) crystal plane appears at 26.6° in the XRD patterns of the NG@P samples with different asphalt addition amounts, indicating that the graphitization degrees of the NG@P samples with different carbon coating amounts are very high, and the coated asphalt carbon does not change the crystal structure of graphite. The peak position of the diffraction peak does not change significantly with the increase of the asphalt addition amount, indicating that although the asphalt coating does not change the layer spacing of SG. However, with the increase of the asphalt addition amount, the intensity of the (002) peak in the XRD pattern gradually decreases, proving that with the increase of the asphalt addition amount, the asphalt coating can form a "core-shell" structure with SG as the core and amorphous asphalt carbon as the shell. The crystal grains of asphalt carbon itself are not fully developed, the carbon network size is small, the layer spacing is large, and the graphitization degree is low, so the graphitization degree of the NG@P sample gradually decreases.

[0067] Figure 10 Figure 6 is the Raman spectra of ESG20 and NG@P samples with different asphalt addition amounts. As can be seen from the figure, a D peak around 1330 cm -1 , a G peak around 1580 cm -1 , and a 2D peak around 2680 cm -1 can be seen in the Raman spectra of ESG20 and NG@P samples. The asphalt carbon coating in the NG@P sample has a great change on the Raman peaks of ESG. With the increase of the asphalt addition amount, the relative intensity of the D peak gradually increases. The I D / I GThe value increases to 0.42, indicating that the pitch carbon is wrapped on the surface of ESG, the amorphous carbon increases, and the disordered structure increases. The Raman test analysis results are consistent with the XRD results.

[0068] Example 3: Electrochemical Performance Study of Modified Natural Graphite Anode Material

[0069] In this example, button cells were assembled with SG, ESG, and NG@P samples prepared with different asphalt addition amounts as the anode materials. Constant current charge-discharge tests were carried out at room temperature to study the effects of asphalt addition amount on the electrochemical performance of NG@P batteries, such as the first charge-discharge efficiency, rate performance, and cycle stability.

[0070] First Charge-Discharge Test

[0071] Figure 11 Figure shows the first charge-discharge specific capacity curves of the batteries assembled with SG, ESG20, NG@P5, NG@P7.5, NG@P10, NG@P12.5, and SG@P10 as the anode materials at 0.1C. It can be seen from the figure that the charge-discharge specific capacity curves all show a "U" shape.

[0072] During the first discharge process, the discharge stage mainly shows four parts:

[0073] (1) The potential rapidly drops from about 2V and there is a small plateau when it drops to around 0.8 - 0.6V. This plateau corresponds to the start of the formation of the SEI film.

[0074] (2) There is an inclined potential drop between 0.7 - 0.25V, corresponding to the continuous formation process of the SEI film. At the same time, higher-order lithium-graphite intercalation compounds are formed during this process.

[0075] (3) The lithium intercalation processes in the first discharge curve are as follows: ① the potential is between 0.25 - 0.18V, ② the 0.18V potential plateau, ③ the potential is between 0.18 - 0.1V, ④ the 0.1V potential plateau, ⑤ the 0.05V potential plateau, corresponding to the process of the step-by-step conversion of higher-order higher-order lithium-graphite intercalation compounds into LiC6 as shown in Equation 1.

[0076]

[0077] (4) The potential drops to 0.01V and the discharge process stops. Among them, voltage plateaus and obvious voltage drops appear in the SG and ESG20 batteries between 0.7 - 0.25V, indicating that more SEI films are formed in the SG and EG20 batteries. While the NG@P batteries basically do not show voltage plateaus and have a higher voltage drop slope.

[0078] Table 5 First charge-discharge data of SG, ESG20, NG@P5, NG@P7.5, NG@P10, NG@P12.5 and SG@P10 batteries at 0.1C

[0079]

[0080]

[0081] Table 5 shows the first charge-discharge test data of the batteries at 0.1C. During the first charge-discharge cycles of the SG and ESG20 batteries, higher voltage plateaus and voltage drops appear, indicating that the formation of the SEI film leads to a higher irreversible capacity and thus a lower charge-discharge efficiency. For the NG@P and SG@P10 batteries, no obvious voltage plateau appears, and the voltage drop slope is higher, indicating that their first charge-discharge irreversible capacity is lower and the charge-discharge efficiency is higher. Among them, the first discharge capacities of the NG@P7.5, NG@P10 and NG@P12.5 batteries are all above 400, and the charge-discharge efficiencies of the NG@P7.5 and NG@P10 batteries reach over 93%, which are higher than those of the SG@P10 battery. This is because the spheroidal graphite after slight swelling is conducive to the pyrolysis of asphalt into the graphite interlayer, forming a uniform and continuous conductive network, which can promote the penetration between the electrolyte and graphite and increase the stable diffusion of Li + and the transport of electrons. As the asphalt addition amount increases from 5% to 12.5%, the first discharge specific capacity and charge-discharge efficiency of the battery first increase to 409.83 mAh / g in sequence and then decrease to 400.52 mAh / g. This shows that although the amorphous carbon-coated graphite can improve the compatibility between graphite and electrolyte, reduce the active specific surface area, and avoid the co-insertion of solvent molecules to form a three-dimensional SEI film. However, when the coating amount is too high, the lithium intercalation process of graphite is restricted, hindering the normal intercalation and deintercalation of Li + . In addition, the excess asphalt carbon reduces the ratio of active materials in graphite, thereby reducing the charge-discharge capacity. According to the test results, NG@P7.5 and NG@P10 meet the technical indicators of the first discharge specific capacity ≥ 370 mAh / g and the first Coulomb efficiency ≥ 93% at 0.1C.

[0082] Figure 12Figure 1 shows the first charge-discharge specific capacity curves of NG@P7.5 and NG@P10 batteries at 10C. The test data are shown in Table 6. As can be seen from the figure, under high current density conditions, the charge-discharge specific capacity curves of the batteries can still maintain the "U" shape, indicating good rate performance of the samples. Due to the too fast charge-discharge rate, the potential drop during the discharge stage between 0.7 - 0.25V in the first discharge process is no longer obvious. The good rate performance of the NG@P battery mainly stems from the slit pore structure in the micro-expanded spherical graphite material, which provides additional ion transport channels. At the same time, the coating of pitch carbon on the surface of graphite particles forms a "core-shell" structure, inhibiting the expansion and peeling of graphite sheets during charge and discharge, improving its compatibility with the electrolyte, and effectively retaining the insertion and extraction channels of Li between the graphite layers. In Table 6, the first discharge specific capacities of NG@P7.5 and NG@P10 batteries at 10C are 345.12 mAh / g and 362.24 mAh / g respectively, and the charge-discharge efficiencies are 90.76% and 92.34% respectively. This indicates that the amorphous carbon coating structure of the NG@P10 sample is more complete, with more Li transport channels, lower defects, and higher compatibility with the electrolyte. The NG@P10 battery at 10C meets the technical indicators of the first discharge specific capacity ≥ 360 mAh / g and the first Coulomb efficiency ≥ 90%. + In Table 6, the first discharge specific capacities of NG@P7.5 and NG@P10 batteries at 10C are 345.12 mAh / g and 362.24 mAh / g respectively, and the charge-discharge efficiencies are 90.76% and 92.34% respectively. This indicates that the amorphous carbon coating structure of the NG@P10 sample is more complete, with more Li transport channels, lower defects, and higher compatibility with the electrolyte. The NG@P10 battery at 10C meets the technical indicators of the first discharge specific capacity ≥ 360 mAh / g and the first Coulomb efficiency ≥ 90%. + transport channels, lower defects, and higher compatibility with the electrolyte. The NG@P10 battery at 10C meets the technical indicators of the first discharge specific capacity ≥ 360 mAh / g and the first Coulomb efficiency ≥ 90%.

[0083] Table 6 First charge-discharge data table of NG@P7.5 and NG@P10 batteries at 10C

[0084]

[0085] Rate performance test

[0086] Figure 13 The rate performance of NG@P7.5 and NG@P10 batteries was compared and tested. As can be seen from the figure, the NG@P10 battery with 10% asphalt addition shows better electrochemical performance both at low current density and high current density, demonstrating excellent rate performance. This is because the NG@P10 sample has the most complete and strongest coating layer, which can provide more additional storage capacity and fast transport channels for Li, and fully shorten the average migration distance of Li during lithium insertion and extraction. However, at a 5C current density, the performance differences between NG@P with different softening point asphalt coatings and NG are not significant, and the discharge capacities are almost the same, which may be due to the inherent structural characteristics of natural graphite materials. + provide more additional storage capacity and fast transport channels for Li, and fully shorten the average migration distance of Li during lithium insertion and extraction. + However, at a 5C current density, the performance differences between NG@P with different softening point asphalt coatings and NG are not significant, and the discharge capacities are almost the same, which may be due to the inherent structural characteristics of natural graphite materials.

[0087] Cycle life test

[0088] As Figure 14As shown, after 2000 cycles at a discharge current density of 0.1C, the capacity retention rate of the NG@P10 battery reaches 82.21%. As Figure 15 shown, after 500 cycles at a discharge current density of 10C, the capacity retention rate of the NG@P10 battery reaches 80.36%, and no battery failure occurs. The cycle life reaches the project technical indicators. The test results show that the carbon coating layer on the surface of NG@P10 has an appropriate thickness and is more orderly distributed, which helps to reduce the electrochemical migration resistance and provides a smoother channel for the migration and diffusion of Li + , thus ensuring the electrochemical performance of the battery during high-rate charge and discharge.

Claims

1. A preparation method of modified slightly expandable spherical graphite, characterized in that The method is as follows: Using the electrochemical oxidation method, with concentrated HNO3 as the intercalating agent, constant current electrochemical oxidation intercalation is carried out on spherical graphite. The solid-liquid ratio of spherical graphite to the intercalating agent is adjusted to 1g:1 - 1.5mL, the current density is 20 - 60mA / cm 2 and the reaction time is 10 - 60min to obtain a spherical graphite intercalation compound. Then, the spherical graphite intercalation compound is subjected to an expansion treatment by the high-temperature expansion method to obtain modified slightly expanded spherical graphite.

2. The preparation method of a modified slightly expandable spherical graphite according to claim 1, characterized in that The solid-liquid ratio of spherical graphite to the intercalating agent is 1 g: 1 mL, the current density is 40 mA / cm 2 and the reaction time is 20 min.

3. The preparation method of a modified slightly expanded spherical graphite according to claim 1, characterized in that The concentration of the concentrated HNO3 is 68 wt%.

4. The preparation method of a modified slightly expandable spherical graphite according to claim 1, characterized in that In the high-temperature expansion method, the high-temperature expansion treatment temperature is 450 °C and the time is 60 min.

5. The preparation method of a modified slightly expandable spherical graphite according to claim 1, characterized in that The interlayer spacing of the modified slightly expanded spherical graphite is 0.3374 - 0.3376 nm.

6. Application of the modified slightly expanded spherical graphite prepared as claimed in claim 1 as a negative electrode material for a lithium-ion battery.

7. The application according to claim 6, wherein The specific application method is as follows: Weigh pitch and dissolve it in tetrahydrofuran to obtain a pitch solution with a mass concentration of 5% - 12.5%; subject the modified slightly expanded spherical graphite to vacuum treatment, then add the pitch solution, carry out vacuum impregnation, and dry it to obtain a mixture; then carry out pre-carbonization treatment on the mixture under the protection of nitrogen, and then raise the temperature for carbonization treatment to obtain a negative electrode material for a lithium-ion battery.

8. The application according to claim 7, wherein The vacuum treatment refers to pumping to -0.1 MPa and maintaining for 1 h.

9. The application according to claim 7, wherein The pre-carbonization mentioned refers to carrying out pre-carbonization for 2 h at a heating rate of 2 °C / min and a temperature of 300 °C.

10. The application according to claim 7, characterized in that, The carbonization mentioned refers to a heating rate of 10 °C / min, a carbonization temperature of 1000 °C, and a carbonization time of 4 h.

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