CN113644250A - Nitrogen-phosphorus-doped Si/CNTs/C composite lithium ion battery cathode material and preparation method thereof - Google Patents
Nitrogen-phosphorus-doped Si/CNTs/C composite lithium ion battery cathode material and preparation method thereof Download PDFInfo
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- H—ELECTRICITY
- H01—ELECTRIC ELEMENTS
- H01M—PROCESSES OR MEANS, e.g. BATTERIES, FOR THE DIRECT CONVERSION OF CHEMICAL ENERGY INTO ELECTRICAL ENERGY
- H01M4/00—Electrodes
- H01M4/02—Electrodes composed of, or comprising, active material
- H01M4/36—Selection of substances as active materials, active masses, active liquids
- H01M4/38—Selection of substances as active materials, active masses, active liquids of elements or alloys
- H01M4/386—Silicon or alloys based on silicon
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- H—ELECTRICITY
- H01—ELECTRIC ELEMENTS
- H01M—PROCESSES OR MEANS, e.g. BATTERIES, FOR THE DIRECT CONVERSION OF CHEMICAL ENERGY INTO ELECTRICAL ENERGY
- H01M10/00—Secondary cells; Manufacture thereof
- H01M10/05—Accumulators with non-aqueous electrolyte
- H01M10/052—Li-accumulators
- H01M10/0525—Rocking-chair batteries, i.e. batteries with lithium insertion or intercalation in both electrodes; Lithium-ion batteries
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- H—ELECTRICITY
- H01—ELECTRIC ELEMENTS
- H01M—PROCESSES OR MEANS, e.g. BATTERIES, FOR THE DIRECT CONVERSION OF CHEMICAL ENERGY INTO ELECTRICAL ENERGY
- H01M4/00—Electrodes
- H01M4/02—Electrodes composed of, or comprising, active material
- H01M4/62—Selection of inactive substances as ingredients for active masses, e.g. binders, fillers
- H01M4/624—Electric conductive fillers
- H01M4/625—Carbon or graphite
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- H—ELECTRICITY
- H01—ELECTRIC ELEMENTS
- H01M—PROCESSES OR MEANS, e.g. BATTERIES, FOR THE DIRECT CONVERSION OF CHEMICAL ENERGY INTO ELECTRICAL ENERGY
- H01M4/00—Electrodes
- H01M4/02—Electrodes composed of, or comprising, active material
- H01M4/62—Selection of inactive substances as ingredients for active masses, e.g. binders, fillers
- H01M4/628—Inhibitors, e.g. gassing inhibitors, corrosion inhibitors
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- H—ELECTRICITY
- H01—ELECTRIC ELEMENTS
- H01M—PROCESSES OR MEANS, e.g. BATTERIES, FOR THE DIRECT CONVERSION OF CHEMICAL ENERGY INTO ELECTRICAL ENERGY
- H01M4/00—Electrodes
- H01M4/02—Electrodes composed of, or comprising, active material
- H01M2004/026—Electrodes composed of, or comprising, active material characterised by the polarity
- H01M2004/027—Negative electrodes
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- Y—GENERAL TAGGING OF NEW TECHNOLOGICAL DEVELOPMENTS; GENERAL TAGGING OF CROSS-SECTIONAL TECHNOLOGIES SPANNING OVER SEVERAL SECTIONS OF THE IPC; TECHNICAL SUBJECTS COVERED BY FORMER USPC CROSS-REFERENCE ART COLLECTIONS [XRACs] AND DIGESTS
- Y02—TECHNOLOGIES OR APPLICATIONS FOR MITIGATION OR ADAPTATION AGAINST CLIMATE CHANGE
- Y02E—REDUCTION OF GREENHOUSE GAS [GHG] EMISSIONS, RELATED TO ENERGY GENERATION, TRANSMISSION OR DISTRIBUTION
- Y02E60/00—Enabling technologies; Technologies with a potential or indirect contribution to GHG emissions mitigation
- Y02E60/10—Energy storage using batteries
Abstract
The invention discloses a nitrogen-phosphorus doped Si/CNTs/C composite lithium ion battery cathode material and a preparation method thereof, wherein the nitrogen-phosphorus doped Si/CNTs/C composite lithium ion battery cathode material comprises the following components: silicon nanoparticles, carbon nanotubes, pitch derived carbon, and doped nitrogen and phosphorous. The silicon nano-particles and the carbon nano-tubes are twisted together and coated by the asphalt derived carbon layer; the doping of nitrogen and phosphorus is realized through the thermal decomposition of hexachlorocyclotriphosphazene. The preparation method comprises the following steps: and (3) uniformly mixing the silicon nanoparticles, the asphalt powder, the carbon nanotubes and the hexachlorocyclotriphosphazene in a planetary ball mill. Then melting, pyrolyzing and carbonizing in nitrogen atmosphere to obtain the N, P-Si/CNTs/C composite material. The carbon nano tube can improve the conductivity of the silicon material, the volume expansion of silicon nano particles can be relieved by coating the asphalt-derived carbon layer, and the active sites and the lithium storage performance of the material can be improved by doping nitrogen and phosphorus.
Description
Technical Field
The invention relates to a lithium ion battery cathode material, in particular to a nitrogen-phosphorus doped Si/CNTs/C composite lithium ion battery cathode material and a preparation method thereof, belonging to the technical field of new materials.
Background
Lithium Ion Batteries (LIBs), a classic commercial rechargeable battery, have been rapidly developed and widely used in human daily life. However, the lithium ion battery industry is eagerly looking to explore the replacement of the traditional graphite (372mAh g)-1) The anode material can meet the increasing energy density and power density requirements of energy storage devices such as portable digital devices and electric automobiles. Of these, silicon-based negative electrode materials are considered to be the most commercially viable materials due to their extremely high theoretical specific capacity (4200mAh g)-1) Relatively low intercalation potential (<0.5V vs Li+/Li). However, the following technical problems still exist in the existing silicon-based negative electrode: (1) lower Li+Diffusion rate and electronic conductivity; (2) the occurrence of a large volume expansion during multiple lithiation and delithiation>300%); (3) consuming active lithium and repeatedly destroying and re-forming a solid electrolyte interface layer (SEI), resulting in poor conductivity and poor cycling stability.
Disclosure of Invention
The invention aims to provide a nitrogen-phosphorus-doped Si/CNTs/C composite lithium ion battery cathode material and a preparation method thereof, which solve the problem of Li existing in the conventional silicon-based cathode material+Low diffusion rate, volume expansion of silicon nanoparticles, poor conductivity and poor cycle stability.
The purpose of the invention is realized by the following technical scheme:
a nitrogen-phosphorus-doped Si/CNTs/C composite lithium ion battery cathode material comprises silicon nanoparticles, carbon nanotubes and pitch-derived carbon, wherein the silicon nanoparticles are doped with nitrogen and phosphorus, the silicon nanoparticles are wound by the carbon nanotubes, the dispersed silicon nanoparticles are connected through the carbon nanotubes, and molten pitch is pyrolyzed on the surfaces of the silicon nanoparticles to form carbon layers, wherein the content of the silicon nanoparticles is 43.0-70.2 wt%, the content of the carbon nanotubes is 21.0-51.6 wt%, and the content of the nitrogen-phosphorus-doped carbon is 3.8-13.6 wt%.
A preparation method of a nitrogen-phosphorus-doped Si/CNTs/C composite lithium ion battery cathode material comprises the following steps of (1), (0.3-1.2) and 6% by mass of silicon nanoparticles, carbon nanotubes, asphalt powder and hexachlorocyclotriphosphazene: firstly, adding silicon nanoparticles, carbon nanotubes, asphalt powder and hexachlorocyclotriphosphazene into a ball milling tank, then ball milling in a planetary ball mill, uniformly mixing to obtain a silicon nanoparticle, carbon nanotubes, hexachlorocyclotriphosphazene and asphalt composite precursor, and then melting, pyrolyzing and carbonizing the silicon nanoparticles, carbon nanotubes, hexachlorocyclotriphosphazene and asphalt composite precursor in a nitrogen atmosphere to obtain the nitrogen-phosphorus-doped Si/CNTs/C composite lithium ion battery cathode material.
The object of the invention can be further achieved by the following technical measures:
preferably, the preparation method of the nitrogen-phosphorus-doped Si/CNTs/C composite lithium ion battery cathode material comprises the steps of melting silicon nanoparticles, carbon nanotubes, hexachlorocyclotriphosphazene and a pitch composite precursor in a tubular furnace under the nitrogen atmosphere, wherein the temperature is 300-400 ℃, the heating rate is 1-5 ℃/min, and the melting time is 1-5 h after the set temperature is reached; and then carbonizing in the same gas atmosphere, wherein the temperature is 800-1000 ℃, the heating rate is 6-10 ℃/min, and the carbonizing time is 1-4 h after the set temperature is reached.
Preferably, in the preparation method of the nitrogen-phosphorus-doped Si/CNTs/C composite lithium ion battery anode material, the mass ratio of the material to be subjected to ball milling to balls is 1: (20-50), wherein the rotating speed of the planetary ball mill is 300-500 r/min, and the ball milling time is 1-5 h; the ball milling mode is bidirectional alternate operation, the unidirectional operation time each time is 20-40 min, and the ball milling is carried out in an argon atmosphere.
Preferably, in the preparation method of the nitrogen-phosphorus-doped Si/CNTs/C composite lithium ion battery anode material, the mass ratio of the silicon nanoparticles to the carbon nanotubes to the asphalt powder to the hexachlorocyclotriphosphazene is 1: 0.6: 6; when the mass ratio of ball milling materials to balls is 1: 30, the ball milling tank is an agate ball milling tank, the rotating speed of the planetary ball mill is 400r/min, the ball milling time is 2 hours, the ball milling mode is bidirectional alternate operation, and the unidirectional operation time is 30min each time; melting the silicon nanoparticles, the carbon nanotubes, the hexachlorocyclotriphosphazene and the asphalt composite precursor in a tubular furnace in a nitrogen atmosphere at the temperature of 350 ℃, the heating rate of 3 ℃/min, the melting time of 3h after reaching the set temperature, and then carbonizing in the same gas atmosphere at the temperature of 900 ℃, the heating rate of 10 ℃/min, and the carbonizing time of 2h after reaching the set temperature.
Compared with the prior art, the invention has the beneficial effects that:
the invention adopts mechanical ball milling and high-temperature carbonization methods to prepare the nitrogen-phosphorus doped Si/CNTs/C (N, P-Si/CNTs/C) composite material. The silicon nanoparticles and the Carbon Nanotubes (CNTs) are intertwined together and coated by an asphalt derived carbon layer, and the doping of nitrogen and phosphorus is realized through the thermal decomposition of hexachlorocyclotriphosphazene. The preparation method has the advantages of simple synthesis, easy operation, suitability for industrial production and the like. The carbon nano tube can improve the conductivity of the silicon cathode, and the molten asphalt derived carbon layer can relieve the volume expansion of the silicon nano particles; the nitrogen and phosphorus doping can increase the active sites and lithium storage performance of the material. The N, P-Si/CNTs/C composite material as the negative electrode of the lithium ion battery shows excellent cycle performance and rate capability; at 0.2A g-1After 30 cycles under the current density, the discharge specific capacity can still reach 1474.8mAh g-1(ii) a At 0.1, 0.2, 0.5, 1.0, 2.0A g-1Under different current densities, the reversible specific capacity distribution is 1636.0, 1493.0, 1153.6/633.3 and 255.3mAh g-1。
Drawings
FIG. 1 is an X-ray diffraction spectrum of the negative electrode material of the lithium ion battery of example 1 and the control group; in the figure, the example 1 is an N, P-Si/CNTs/C composite material, the comparison group 1 is a silicon, carbon nano tube and carbon (Si/CNTs/C) composite material, the comparison group 2 is a silicon and carbon nano tube (Si/CNTs) composite material, and the comparison group 3 is silicon nano particles;
FIG. 2 is an SEM image (scanning electron micrograph) of the negative electrode material of the N, P-Si/CNTs/C composite lithium ion battery prepared in example 1 of the present invention;
FIG. 3 is an SEM image of the Si/CNTs/C composite lithium ion battery anode material prepared by the comparison group 1 of the invention;
FIG. 4 is an SEM image of the Si/CNTs composite lithium ion battery anode material prepared by the comparison group 2 of the invention;
FIG. 5 is an SEM image of a silicon nanoparticle lithium ion battery anode material prepared by comparison group 3 of the present invention;
fig. 6 is an SEM image of the negative electrode material of the lithium ion battery prepared in example 2 of the present invention;
fig. 7 is an SEM image of the negative electrode material of the lithium ion battery prepared in example 3 of the present invention;
fig. 8 is an SEM image of the negative electrode material of the lithium ion battery prepared in example 4 of the present invention;
FIG. 9 is a TEM image (transmission electron micrograph) of the N, P-Si/CNTs/C composite lithium ion battery anode material prepared in example 1 of the present invention;
FIG. 10 is a TG diagram (thermogravimetric plot) of the N, P-Si/CNTs/C composite lithium ion battery anode material prepared in example 1 of the present invention;
FIG. 11 is an XPS spectrum (X-ray photoelectron spectrum) of the negative electrode material of the N, P-Si/CNTs/C composite lithium ion battery prepared in example 1 of the present invention;
FIG. 12 is a Si 2P high resolution XPS spectrum of the N, P-Si/CNTs/C composite lithium ion battery anode material prepared in example 1 of the present invention;
FIG. 13 is an N1s high resolution XPS spectrum of the N, P-Si/CNTs/C composite lithium ion battery anode material prepared in example 1 of the present invention;
FIG. 14 is a P2P high resolution XPS spectrum of the N, P-Si/CNTs/C composite lithium ion battery anode material prepared in example 1 of the present invention;
FIG. 15 is a cyclic voltammetry curve of the N, P-Si/CNTs/C composite lithium ion battery negative electrode material prepared in example 1 of the present invention;
FIG. 16 shows that the negative electrode material of the N, P-Si/CNTs/C composite lithium ion battery prepared in the embodiment 1 of the invention is 200mA g-1A charge-discharge curve at current density;
FIG. 17 shows that the negative electrode material for lithium ion battery prepared in example 1 and the control group of the present invention has a voltage of 200mA g-1Long cycle performance at current density;
fig. 18 shows the rate capability of the negative electrode material for lithium ion batteries prepared in example 1 of the present invention and the control group at different current densities.
Detailed Description
The following examples are presented to enable one of ordinary skill in the art to more fully understand the present invention and are not intended to limit the scope of the embodiments described herein.
Example 1:
preparing an N, P-Si/CNTs/C composite material:
respectively weighing 0.1g of silicon nanoparticles, 0.06g of carbon nanotubes, 0.06g of asphalt powder and 0.6g of hexachlorocyclotriphosphazene, adding the materials into an agate ball milling tank (the mass ratio of agate balls to materials is 30:1), and ensuring that the ball milling tank is filled with argon through a transition bin of a glove box. And performing positive and negative alternate ball milling for 2 hours under a planetary ball mill at the ball milling rotation speed of 400r/min, and performing positive and negative alternate operation for 30min each time. And finally, scraping the ball-milled material from the inner wall of the agate tank by using a small medicine spoon to obtain the silicon, carbon nano tube, hexachlorocyclotriphosphazene and asphalt composite precursor. The obtained nitrogen-phosphorus doped silicon, carbon nano tube and asphalt composite precursor is put into a porcelain boat, and an alumina plate is covered on the porcelain boat, so that the sample is prevented from being polluted in the calcining process. Keeping at 350 deg.C for 3h at a heating rate of 3 deg.C/min under nitrogen atmosphere to make pitch molten, and coating on the surface of silicon and carbon nanotube. And then rapidly heating to 900 ℃ at the heating rate of 10 ℃/min, and preserving the heat for 2h to obtain the N, P-Si/CNTs/C composite material.
FIG. 1 is an X-ray diffraction (XRD) spectrum of the N, P-Si/CNTs/C composite material prepared in example 1. The result shows that sharp diffraction characteristic peaks can be clearly seen at the 2 theta values of 28.4 degrees, 47.3 degrees, 56.1 degrees, 69.1 degrees and 76.4 degrees for the N, P-Si/CNTs/C composite material and the silicon nanoparticles. These characteristic peaks correspond to the (111), (220), (311), (400) and (331) crystal planes (JCPDS card numbers 27-1402) of crystalline silicon, respectively, indicating that the crystal structure of Si nanoparticles remains good during high temperature calcination. In addition, the N, P-Si/CNTs/C composite material has a weak peak around 23 degrees, which corresponds to the carbon nanotubes in the composite material and the carbon derived from pitch carbonization.
FIG. 2 is an SEM photograph of the N, P-Si/CNTs/C composite prepared in example 1, and shows that silicon nanoparticles and carbon nanotubes are entangled and the surface becomes rough. This is because pitch in a molten state adheres to the surfaces of silicon nanoparticles and carbon nanotubes, and forms a carbon layer on the surface of the silicon particles through high-temperature carbonization.
Fig. 9 is a TEM image of the N, P-Si/CNTs/C composite material prepared in example 1, and it can be seen that the surface of the silicon nanoparticle becomes rough, which is attributed to the carbon layer formed by pyrolysis of the molten asphalt on the surface of the silicon nanoparticle, and thus the volume expansion problem of silicon in the circulation process can be effectively alleviated, which is beneficial to improving the cycle performance of the silicon negative electrode. Meanwhile, the silicon nanoparticles are wound by the carbon nanotubes, and the dispersed silicon nanoparticles are connected through the carbon nanotubes, which is beneficial to improving the conductivity of the silicon cathode and accelerating the transmission of electrons.
FIG. 10 is a TG diagram of the N, P-Si/CNTs/C composite prepared in example 1. The lithium storage performance of the N, P-Si/CNTs/C composite material is related to the content of silicon, and the content of silicon in the composite material prepared by thermogravimetric analysis shows that the mass weight loss of about 1 wt% is generated in the range of the temperature lower than 100 ℃, which is caused by the evaporation of water absorbed by a sample in the storage process; the thermal weight loss of the N, P-Si/CNTs/C composite material at 530-836 ℃ corresponds to the decomposition of the carbon nano tube and the asphalt derived carbon. The content of silicon in the N, P-Si/CNTs/C composite material is 58.0 wt%, the content of the carbon nano tube is 34.7 wt%, and the content of nitrogen-phosphorus doped carbon is 7.3 wt%.
FIG. 11 is an XPS survey of the N, P-Si/CNTs/C composite prepared in example 1. The result shows that the N, P-Si/CNTs/C composite material contains Si, C, N and P elements.
FIG. 12 is a high resolution XPS spectrum of Si 2P for the N, P-Si/CNTs/C composite prepared in example 1. The results show that the peak appearing at 100.4eV corresponds to Si of elemental Si0The existence of the Si simple substance in the N, P-Si/CNTs/C composite material is proved, and the result is consistent with the XRD result.
FIG. 13 is a high resolution XPS spectrum of N1s for the N, P-Si/CNTs/C composite prepared in example 1. The results show that the N atom forms three configurations as a dopant: pyridine N (398.4eV), pyrrole N (400.4eV), and graphite N (402.0 eV). The doping of N generates defects in the carbon crystal lattice and generates more electrochemically active sites, which is beneficial to the adsorption of lithium ions.
FIG. 14 is a high resolution XPS spectrum of P2P for the N, P-Si/CNTs/C composite prepared in example 1. The results show that three peaks appear at 132.8, 133.8 and 134.7eV, corresponding to P-C, P-N and P-O, respectively. The presence of N and P elements is mainly derived from Cl6N3P3Thermal decomposition of (3). The N and P codoping can change the edge or integral atomic structure of the carbon material, so that more defects are induced, and the active sites and lithium storage performance of the material are improved.
FIG. 15 is a cyclic voltammogram of the N, P-Si/CNTs/C composite prepared in example 1. During the first cathodic scan, a distinct reduction peak at 0.65V appeared, which disappeared in subsequent cycles, indicating stable SEI layer formation. While the strong reduction peak at 0.01V forms amorphous Li with lithiation of crystalline SixSi alloy. Two oxidation peaks were observed at 0.36 and 0.54V, which are attributed to LixThe Si is delithiated to form amorphous Si. After the first cycle, a new reduction peak occurs around 0.21V due to reversible lithiation of the amorphous silicon. In addition, the peak currents of the reduction peak and the oxidation peak gradually increased with the increase of the number of cycles, indicating that the electrolyte penetrated into the N, P-Si/CNTs/C composite, resulting in more Si being activated with Li+The reaction takes place.
FIG. 16 is a graph of N, P-Si/CNTs prepared in example 1the/C composite material is 0.2A g-1Charge and discharge curves at current density. As can be seen from the first-turn discharge curve, a very long discharge platform is arranged between 0.01 and 0.10V, and the very long discharge platform and Si and Li are subjected to alloying reaction to form LixThe Si alloy corresponds to. Charging plateaus occurring between 0.25-0.50V, corresponding to LixDelithiation process of Si. As can be seen from the figure, the specific charge-discharge capacity is increased along with the increase of the number of cycles, and more Si in the N, P-Si/CNTs/C composite material is activated and Li is activated+The reaction occurred, substantially coinciding with the CV curve.
The N, P-Si/CNTs/C composite prepared from example 1 in FIG. 17 was at 0.2A g-1The long-cycle chart under the current density can be seen, and the initial discharge/charge specific capacity is 1494.0/1243.6mAh g-1The initial coulombic efficiency was 83.24% higher than that of Si/CNTs/C composite (83.14%) and Si/CNTs composite (79.65%), indicating that the carbon-coated Si/CNTs composite derived from asphalt can improve the coulombic efficiency of the first cycle. After 30 cycles, the specific discharge capacity of the N, P-Si/CNTs/C composite material is 1474.8mAh g-1And the specific capacities of the Si/CNTs/C and Si/CNTs composite materials are 1057.1mAh g respectively-1And 1053.9mAh g-1. The nitrogen and phosphorus doping improves the cycle stability of the Si/CNTs/C composite material. As the number of cycles increases, the specific capacity of the N, P-Si/CNTs/C composite also begins to exhibit a similar decay tendency as Si/CNTs/C and Si/CNTs composites. After 100 cycles, the specific capacity of the N, P-Si/CNTs/C composite material is 603.9mAh g-1Higher than that of Si/CNTs/C composite material (419.4mAh g)-1) And Si/CNTs composite (363.2mAh g-1). The specific volume of the N, P-Si/CNTs/C composite material is reduced due to the agglomeration of silicon nano particles and the uneven coating of a carbon coating. However, under the same experimental conditions, the cycle performance of the composite material is obviously improved by the doping of nitrogen and phosphorus.
FIG. 18 shows a graph of the rate capability of the N, P-Si/CNTs/C composite prepared in example 1 at different current densities. N, P-Si/CNTs/C composite materials are in the ranges of 0.1, 0.2, 0.5, 1.0 and 2.0A g-1The specific discharge capacities of the materials are 1636.0, 1493.0, 1153.6 and 633.3 respectivelyAnd 255.3mAh g-1. As can be seen from the figure, the specific discharge capacity of the N, P-Si/CNTs/C composite material is higher than that of the Si/CNTs/C and Si/CNTs composite materials under different multiplying factors. Especially at 0.5A g-1At current density, the specific discharge capacity of the N, P-Si/CNTs/C composite material is 1153.6mAh g-1Much higher than that of Si/CNTs/C composite material (844.6mAh g)-1) And Si/CNTs composite (826.2mAh g-1). When the current density is restored to 0.2A g-1In time, the specific discharge capacity of the N, P-Si/CNTs/C composite material is recovered to 899.7mAh g-1And the result shows that the N, P-Si/CNTs/C composite material has good rate capability.
Control group 1:
preparation of Si/CNTs/C composite material:
0.1g of silicon nanoparticles, 0.06g of asphalt powder and 0.06g of carbon nanotubes are respectively weighed, then added into an agate ball milling pot (the mass ratio of agate balls to materials is 30:1), and the ball milling pot is ensured to be filled with argon through a transition bin of a glove box. Performing positive and negative alternate ball milling for 2 hours under a planetary ball mill, wherein the ball milling speed is 400r/min, the positive and negative alternate ball milling runs for 30min each time, and the time interval is 10 min. And finally, scraping the ball-milled material from the inner wall of the agate tank by using a small medicine spoon to obtain the silicon/carbon nano tube/asphalt composite precursor. The obtained precursor is put into a porcelain boat, and an alumina plate is covered on the porcelain boat, so that the sample is prevented from being polluted in the calcining process. The surface of Si/CNTs was coated with asphalt in order to change the asphalt into a molten state by keeping the temperature at 350 ℃ for 3 hours at a heating rate of 3 ℃/min under a nitrogen atmosphere. And then rapidly heating to 900 ℃ at the heating rate of 10 ℃/min, and preserving the heat for 2h to obtain the Si/CNTs/C composite material.
As can be seen from the X-ray diffraction (XRD) pattern of the Si/CNTs/C composite material prepared by the control group 1 in the figure 1, sharp diffraction characteristic peaks are clearly seen at the 2 theta values of 28.4 degrees, 47.3 degrees, 56.1 degrees, 69.1 degrees and 76.4 degrees. These characteristic peaks correspond to the (111), (220), (311), (400) and (331) crystal planes (JCPDS card numbers 27-1402) of crystalline silicon, respectively, indicating that the crystal structure of Si nanoparticles remains good during high temperature calcination. In addition, the Si/CNTs/C composite has a weak peak around 23 °, corresponding to carbon nanotubes and pitch-derived carbon in the composite.
FIG. 3 is an SEM image of the Si/CNTs/C composite prepared in control 1, and it can be seen that the surface of the silicon nanoparticles becomes rough and the carbon nanotubes are slightly broken. This is because pitch in a molten state adheres to the surfaces of silicon nanoparticles and carbon nanotubes, and forms a carbon layer on the surfaces thereof by high-temperature carbonization.
The Si/CNTs/C composite prepared from control 1 in FIG. 17 was used at 200mA g-1The long cycle chart under the current density can be seen, and the initial discharge/charge specific capacity is 1699.6/1413.0mAh g-1The initial coulombic efficiency was 83.14%, and after 100 cycles, only 419.4mAh g was provided-1The specific capacity of (A).
FIG. 18 shows the rate capability of the Si/CNTs/C composite prepared by the control 1 at different current densities. The Si/CNTs/C composite material is in the range of 0.1, 0.2, 0.5, 1.0 and 2.0A g-1Specific discharge capacities at 1537.5, 1265.9, 844.6, 481.8 and 198.1mAh g-1. When the current density is restored to 0.2A g-1When the specific discharge capacity of the Si/CNTs/C composite material is recovered to 727.7mAh g-1。
Control group 2:
preparing a Si/CNTs negative electrode material:
0.1g of silicon nanoparticles and 0.1g of carbon nanotubes are respectively weighed, then added into an agate ball milling pot (the mass ratio of agate balls to materials is 30:1), and the ball milling pot is ensured to be filled with argon through a transition bin of a glove box. Performing positive and negative alternate ball milling for 2 hours under a planetary ball mill, wherein the ball milling speed is 400r/min, the positive and negative alternate ball milling runs for 30min each time, and the time interval is 10 min. And finally, scraping the ball-milled material from the inner wall of the agate tank by using a small medicine spoon to obtain the Si/CNTs composite material.
As can be seen from the X-ray diffraction (XRD) spectrum of the Si/CNTs composite material prepared by the control group 2 in FIG. 1, sharp diffraction characteristic peaks are clearly seen at the 2 theta values of 28.4 degrees, 47.3 degrees, 56.1 degrees, 69.1 degrees and 76.4 degrees. These characteristic peaks correspond to the (111), (220), (311), (400) and (331) crystal planes (JCPDS card numbers 27-1402) of crystalline silicon, respectively. In addition, the Si/CNTs/C composite material has a weak peak around 23 degrees, which corresponds to the carbon nanotubes in the composite material.
FIG. 4 is an SEM image of the Si/CNTs composite prepared in control 2, in which silicon nanoparticles and carbon nanotubes are entangled by mechanical ball milling.
The Si/CNTs composite prepared from control 2 in FIG. 17 was used at 200mA g-1The long cycle chart under the current density can be seen, and the initial discharge/charge specific capacity is 1541.7/1228.0mAh g-1Initial coulombic efficiency of 79.65%, after 100 cycles, only 363.2mAh g can be provided-1The specific capacity of (A).
FIG. 18 shows a graph of the rate capability of the Si/CNTs composite prepared by the control 2 at different current densities. Si/CNTs composites at 0.1, 0.2, 0.5, 1.0 and 2.0A g-1Specific discharge capacities of 1414.5, 1186.0, 826.2, 548.0 and 275.8mAh g-1. When the current density is restored to 0.2A g-1When the specific discharge capacity of the Si/CNTs/C composite material is recovered to 878.9mAh g-1。
Control group 3:
the control group 3 is commercial nano silicon particles (Si NPs) as the negative electrode material of the lithium ion battery.
Fig. 5 is an SEM image of comparative group 3 commercial Si nanoparticles, and it can be seen that silicon particles having non-uniform size are aggregated together, and the surface thereof is smooth.
The Si nano-particle anode material in the control group 3 in FIG. 17 is 200mA g-1The long cycle chart under the current density can be seen, and the initial discharge/charge specific capacity is 2765.3/2427.4mAh g-1The initial coulomb efficiency is 87.78%, after 20 cycles, the discharge specific capacity is quickly attenuated to 98mAh g-1。
Fig. 18 shows a graph of rate performance of the silicon nanoparticle anode material in control 3 at different current densities. The silicon nano-particle negative electrode material is 0.1A g-1After 10 cycles at current density, the specific capacity is directly from the initial 1864.4mAh g-1Attenuation to 20.8mAh g-1. At greater than 0.2A g-1The specific capacity of the silicon nanoparticles is almost zero at the multiplying power of (2). This is due to the silicon nanoparticlesLow electrical conductivity and large volume expansion.
In conclusion, the indexes of the N, P-Si/CNTs/C composite material of the embodiment 1 are better than those of the Si/CNTs/C composite material of the comparison group 1, the Si/CNTs composite material of the comparison group 2 and the silicon nano-particles of the comparison group 3.
Example 2:
the difference from example 1 is that N, P-Si/CNTs/C composite material was prepared by using silicon nanoparticles, carbon nanotubes, pitch powder and hexachlorocyclotriphosphazene in a mass ratio of 1: 0.8: 0.4: 6, with a ball milling time of 1h and a carbonization temperature of 900 ℃, wherein the silicon nanoparticles content was 53.1 wt%, the carbon nanotubes content was 42.4 wt%, and the nitrogen-phosphorus-doped carbon content was 4.5 wt%. As can be seen from the SEM image of the lithium ion battery anode material prepared in example 2 of fig. 6, the silicon nanoparticles become coarse and intertwined with the carbon nanotubes. The ball milling time is short, the carbon nano tube is not broken, and the good fiber shape is still kept.
Example 3:
the difference from example 1 is that the mass ratio of silicon nanoparticles, carbon nanotubes, pitch powder and hexachlorocyclotriphosphazene was 1: 0.4: 0.8: 6, the ball milling time was 3 hours, and the carbonization temperature was 800 ℃. As can be seen from the SEM image of the negative electrode material for lithium ion battery prepared in example 3 of fig. 7, the silicon nanoparticles become coarse, and the carbon layer generated by pyrolysis of the pitch coats the surface of the silicon cluster. Since less carbon nanotubes were added, no excess carbon nanotubes were present.
Example 4:
the difference from example 1 is that the mass ratio of silicon nanoparticles, carbon nanotubes, pitch powder and hexachlorocyclotriphosphazene was 1: 0.4: 0.8: 6, the ball milling time was 2h, and the carbonization temperature was 1000 ℃. As can be seen from the SEM image of the negative electrode material for lithium ion battery prepared in example 4 of fig. 8, due to the excessive carbonization temperature, the pitch-derived carbon and the carbon nanotube are sintered together and coated on the surface of the silicon nanoparticle.
Example 5:
the difference from example 1 was that the mass ratio of silicon nanoparticles, carbon nanotubes, pitch powder and hexachlorocyclotriphosphazene was 1: 0.3: 0.6: 6, and the ball milling time was 1: 0.3: 0.6: 6The reaction time is 1h, the carbonization temperature is 900 ℃, and the N, P-Si/CNTs/C composite material is prepared, wherein the content of silicon nanoparticles is 70.2 wt%, the content of carbon nanotubes is 21.0 wt%, and the content of nitrogen-phosphorus doped carbon is 8.8 wt%. The initial capacity of the N, P-Si/CNTs/C composite material is higher due to the high content of the silicon nano particles, 0.2A g-1The initial specific discharge capacity is 1653.2mAh g under the current density-1(ii) a However, since the amount of carbon nanotubes is small, the conductivity is poor, and the cycle performance of the N, P-Si/CNTs/C composite material is poor.
Example 6:
the difference from example 1 is that the mass ratio of silicon nanoparticles, carbon nanotubes, pitch powder and hexachlorocyclotriphosphazene was 1: 1.2: 0.6: 6, the ball milling time was 1h, and the carbonization temperature was 900 ℃, thereby obtaining N, P-Si/CNTs/C composite material, in which the content of silicon nanoparticles was 43.0 wt%, the content of carbon nanotubes was 51.6 wt%, and the content of nitrogen-phosphorus-doped carbon was 5.4 wt%. The initial capacity of the N, P-Si/CNTs/C composite is reduced due to the reduced content of silicon nanoparticles, 0.2A g-1The initial specific discharge capacity is 1216.3mAh g under the current density-1(ii) a As the amount of the carbon nano-tube is increased, the conductivity is good, and the cycle performance of the N, P-Si/CNTs/C composite material is slightly better than that of the N, P-Si/CNTs/C composite material in the embodiment 1.
Example 7:
the difference from example 1 is that N, P-Si/CNTs/C composite material was prepared by using silicon nanoparticles, carbon nanotubes, pitch powder and hexachlorocyclotriphosphazene in a mass ratio of 1: 0.6: 0.3: 6, with a ball milling time of 1h and a carbonization temperature of 900 ℃, wherein the silicon nanoparticles content was 60.1 wt%, the carbon nanotubes content was 36.1 wt%, and the nitrogen-phosphorus-doped carbon content was 3.8 wt%. As the content of the carbon derived from asphalt carbonization is reduced by about half, the buffer capacity is reduced, the rate capability is poorer than that of the embodiment 1, and the N, P-Si/CNTs/C composite materials prepared in the embodiment are 0.1, 0.2, 0.5, 1.0 and 2.0A g-1Specific discharge capacities of 1687.5, 1471.3, 975.6, 568.1 and 204.8mAh g-1。
Example 8:
the difference from example 1 is that the mass ratio of the silicon nanoparticles, the carbon nanotubes, the asphalt powder and the hexachlorocyclotriphosphazene is 1: 0.6: 1.2: 6, the ball milling time is 1h, the carbonization temperature is 900 ℃, and the N, P-Si/CNTs/C composite material is prepared, wherein the silicon nanoparticles content is 54.0 wt%, the carbon nanotubes content is 32.4 wt%, and the nitrogen-phosphorus-doped carbon content is 13.6 wt%. Because the content of the silicon nano-particles and the content of the carbon nano-tubes are reduced to a certain degree, the initial discharge specific capacity and the cycle performance of the N, P-Si/CNTs/C composite material prepared by the embodiment are weaker than those of the N, P-Si/CNTs/C composite material in the embodiment 1.
The foregoing shows and describes the general principles and features of the present invention, together with the advantages thereof. It will be understood by those skilled in the art that the present invention is not limited to the embodiments described above, which are described in the specification and illustrated only to illustrate the principle of the present invention, but that various changes and modifications may be made therein without departing from the spirit and scope of the present invention, which fall within the scope of the invention as claimed. The scope of the invention is defined by the appended claims and equivalents thereof.
Claims (5)
1. The nitrogen-phosphorus-doped Si/CNTs/C composite lithium ion battery cathode material is characterized by comprising silicon nanoparticles, carbon nanotubes and asphalt-derived carbon, wherein the silicon nanoparticles are doped with nitrogen and phosphorus, the silicon nanoparticles are wound by the carbon nanotubes, the dispersed silicon nanoparticles are connected through the carbon nanotubes, and molten asphalt is pyrolyzed on the surfaces of the silicon nanoparticles to form carbon layers, wherein the content of the silicon nanoparticles is 43.0-70.2 wt%, the content of the carbon nanotubes is 21.0-51.6 wt%, and the content of the nitrogen-phosphorus-doped carbon is 3.8-13.6 wt%.
2. The preparation method of the nitrogen-phosphorus-doped Si/CNTs/C composite lithium ion battery negative electrode material as claimed in claim 1, wherein the mass ratio of the silicon nanoparticles to the carbon nanotubes to the asphalt powder to the hexachlorocyclotriphosphazene is 1 (0.3-1.2) to 6, and the preparation method comprises the following steps: firstly, adding silicon nanoparticles, carbon nanotubes, asphalt powder and hexachlorocyclotriphosphazene into a ball milling tank, then ball milling in a planetary ball mill, uniformly mixing to obtain a silicon nanoparticle, carbon nanotubes, hexachlorocyclotriphosphazene and asphalt composite precursor, and then melting, pyrolyzing and carbonizing the silicon nanoparticles, carbon nanotubes, hexachlorocyclotriphosphazene and asphalt composite precursor in a nitrogen atmosphere to obtain the nitrogen-phosphorus-doped Si/CNTs/C composite lithium ion battery cathode material.
3. The preparation method of the nitrogen-phosphorus-doped Si/CNTs/C composite lithium ion battery anode material according to claim 2, characterized in that the melting of the silicon nanoparticles, the carbon nanotubes, the hexachlorocyclotriphosphazene and the asphalt composite precursor is carried out in a tubular furnace under a nitrogen atmosphere, the temperature is 300-400 ℃, the temperature rise rate is 1-5 ℃/min, and the melting time is 1-5 h after the set temperature is reached; and then carbonizing in the same gas atmosphere, wherein the temperature is 800-1000 ℃, the heating rate is 6-10 ℃/min, and the carbonizing time is 1-4 h after the set temperature is reached.
4. The preparation method of the nitrogen-phosphorus-doped Si/CNTs/C composite lithium ion battery anode material according to claim 3, wherein the mass ratio of the material to be ball-milled to the balls is 1: (20-50), wherein the rotating speed of the planetary ball mill is 300-500 r/min, and the ball milling time is 1-5 h; the ball milling mode is bidirectional alternate operation, the unidirectional operation time each time is 20-40 min, and the ball milling is carried out in an argon atmosphere.
5. The preparation method of the nitrogen-phosphorus-doped Si/CNTs/C composite lithium ion battery anode material according to claim 4, wherein the mass ratio of the silicon nanoparticles to the carbon nanotubes to the asphalt powder to the hexachlorocyclotriphosphazene is 1: 0.6: 6; when the mass ratio of ball milling materials to balls is 1: 30, the ball milling tank is an agate ball milling tank, the rotating speed of the planetary ball mill is 400r/min, the ball milling time is 2 hours, the ball milling mode is bidirectional alternate operation, and the unidirectional operation time is 30min each time; melting the silicon nanoparticles, the carbon nanotubes, the hexachlorocyclotriphosphazene and the asphalt composite precursor in a tubular furnace in a nitrogen atmosphere at the temperature of 350 ℃, the heating rate of 3 ℃/min, the melting time of 3h after reaching the set temperature, and then carbonizing in the same gas atmosphere at the temperature of 900 ℃, the heating rate of 10 ℃/min, and the carbonizing time of 2h after reaching the set temperature.
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