CN111900411A - Self-supporting silicon-carbon negative electrode material and preparation method thereof - Google Patents
Self-supporting silicon-carbon negative electrode material and preparation method thereof Download PDFInfo
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Abstract
The invention discloses a self-supporting silicon-carbon negative electrode material and a preparation method thereof. The prepared self-supporting silicon-carbon cathode material is a composite material of composite nanofibers and carbon, has a three-dimensional network structure, can improve the dispersibility of nano silicon, effectively relieves the volume change of silicon, can form a conductive network, and is beneficial to transfer of electrons and ions. The method has the advantages of simple operation, stability, continuity, controllability, low energy consumption and low cost. The prepared self-supporting silicon-carbon cathode material can be directly used as a lithium ion battery, shows better cycle performance in a Si/C battery cathode, and has good application prospect.
Description
Technical Field
The invention belongs to the technical field of lithium ion batteries, and particularly relates to a self-supporting silicon-carbon negative electrode material and a preparation method thereof.
Background
The lithium ion battery has the advantages of high energy density, long service life, environmental friendliness, wide allowable working temperature range, greenness, environmental friendliness, no memory effect and the like, and is used as a green energy source in the new century. At present, the energy storage system is widely applied to energy storage systems of smart phones, small airplanes and new energy automobile tools. However, when graphite is used as a negative electrode material of a lithium ion battery, the theoretical specific capacity of the graphite is low (372 mA · h/g). And the poor rate performance limits the long-term application of the electrochemical energy storage system in electric automobiles and energy storage systems, and cannot meet the increasing requirement of high-capacity electrochemical energy storage. Therefore, the research on the lithium ion negative electrode material is focused on finding a material having excellent theoretical specific capacity, suitable charge/discharge voltage, longer cycle life, good safety characteristics, and lower production cost.
At present, various carbon materials are adopted as the battery negative electrode in the commercialized lithium ion battery negative electrode. The carbon material has the advantages of high capacity, low discharge plateau and the like. However, carbon negative electrode materials also have significant disadvantages, such as lithium insertion potentials less than 0.1V (vs. Li/Li)+) BatteriesIf overcharged, lithium metal can be caused to form lithium dendrites on the surface of the carbon electrode, and the lithium dendrites can pierce through a diaphragm to cause internal short circuit; on the other hand, the carbon electrode can generate 10% volume deformation after lithium intercalation, so that the discontinuity among particles is caused, the electrode material is easy to fall off from the surface of a current collector, the service life of the battery is shortened, and the like. At present, a great deal of attempts have been made to modify and modify carbon electrodes, for example, by using graphite coke, carbon fibers, and cracking polymers as novel carbon negative electrode materials. However, these modified carbon negative electrodes have significant problems such as poor cycle life and voltage hysteresis. In addition, researchers also develop and research other various negative electrode materials, such as tin-based materials, silicon-based materials, alloy materials and the like, and although the materials have relatively large lithium intercalation capacity, the cycle stability is poor, and various performance requirements of people on the negative electrode materials cannot be met. These have prompted researchers to continue to investigate new anode materials.
The silicon-based negative electrode material is a recognized next-generation negative electrode material due to the extremely high specific capacity (4200 mA · h/g), the lower lithium-intercalation and deintercalation potential, the abundant reserves, the low price, the natural abundance and the environmental friendliness. However, in practical applications, silicon-based negative electrode materials are also plagued by problems such as significant Si volume expansion (about 400%) during repeated lithiation/delithiation processes, resulting in problems such as instability of SEI films, pulverization of electrode materials and detachment from current collectors, resulting in low coulombic efficiency, poor cycle performance and rate capability, and thus the progress of commercialization of silicon-based materials is greatly limited. In addition, when the silicon powder is used for assembling the battery, inactive components such as a binder and a current collector are added, which not only increases the weight of the battery, but also increases the internal resistance of the battery, which is not beneficial to improving the performance of the battery.
In order to solve the problems, the invention patent CN111244410A discloses a lithium battery cathode material and a preparation method thereof, the silicon-based cathode material is of a core-shell structure, the core contains a silicon-oxygen compound, the shell contains 3 layers, the inner layer is a carbon coating layer, which can effectively buffer the volume expansion of the core and improve the electronic conductivity, and the middle layer is an isolated HF layer which can effectively prevent F-Passing without blocking Li+The transmission of (a) is performed,the outermost layer is Li with the function of an artificial SEI film+The conductor layer can effectively improve the conductivity of the lithium ion and stabilize the SEI film. The invention patent CN108390049A discloses a silicon @ silicon carbide @ carbon core-shell structure composite material, which comprises an inner layer, a middle layer and an outer layer, wherein the inner layer is a silicon Si matrix layer, the middle layer is a silicon carbide SiC matrix layer, and the outer layer is a carbon C matrix layer. The invention patent CN109728259A discloses a silicon-based composite cathode material, a preparation method thereof and an energy storage device, wherein a silicon substrate is coated by a fast ion conductor layer and a fluorocarbon-containing material layer, the fast ion conductor is positioned at an inner layer, and a carbon material is positioned at an outer layer, so that the corrosion of a silicon-based material core by HF can be prevented, meanwhile, an artificial SEI film is expected to be generated in situ, and the transmission of lithium ions between electrolyte and the silicon-based material core is accelerated. Although the above-mentioned solutions solve the problems of HF corrosion, electrical conductivity and volume expansion of silicon to some extent, it is inevitable to add inactive components such as a binder and a current collector during battery assembly, which is not favorable for improving battery performance.
Disclosure of Invention
Aiming at the defects in the prior art, the invention aims to provide a self-supporting silicon-carbon negative electrode material and a preparation method thereof, and solves the problems that the existing carbon negative electrode material is low in coulombic efficiency and poor in cycle performance, needs to add inactive ingredients such as a binder and a current collector, and causes poor electrochemical performance of a battery.
In order to achieve the purpose, the invention adopts the following technical scheme: a preparation method of a self-supporting silicon-carbon negative electrode material comprises the following steps:
1) dropwise adding a proper amount of surfactant into the nano silicon powder to soak the nano silicon powder, adding a solvent to perform ultrasonic dispersion, then placing the mixture in an oil bath kettle to stir at a constant temperature, and obtaining a spinning solution after the solute is completely dissolved;
2) placing the spinning solution obtained in the step 1) in a container loading device of an electrostatic spinning machine, setting electrospinning process parameters, and then carrying out electrospinning to obtain composite nanofibers;
3) placing the composite nanofiber membrane obtained in the step 2) in a drying oven to remove a solvent, then pressing the composite nanofiber membrane into a fiber membrane by using a graphite sheet with a smooth surface, placing the fiber membrane in a quartz tube furnace to perform pre-oxidation treatment and carbonization treatment, cooling the fiber membrane along with the furnace to room temperature after the reaction is finished, and taking out a sample to obtain the self-supporting composite electrode material.
Further, the dosage of the surfactant is 0.5 wt% of the nano silicon powder.
Further, the surfactant is KH550, KH560, KH570, KH580 or CTAB.
Further, the solvent is DMF or DMAC; the dosage of the nano silicon powder is 0-1.5 wt% of the solvent.
Further, the constant-temperature stirring temperature in the step 1) is 40-80 ℃, and the time is 5-20 hours.
Further, the electrospinning process parameters are as follows: the spinning temperature is 35-50 ℃, the voltage is 18-28 Kv, the rotating speed is 150-300 r/min, the flow speed is 1-3 ul/min, and the distance between a needle head and a receiving disc is 15-25 cm.
Further, the temperature of the drying oven is 40-80 ℃.
Further, the pre-oxidation treatment is that under the air atmosphere, the temperature is increased to 130-150 ℃ at the temperature increase rate of 1-5 ℃/min, the temperature is maintained for 1-3 h, then the temperature is increased to 220-230 ℃ at the temperature increase rate of 1-5 ℃/min, and the temperature is maintained for 1-3 h; then heating to 260-280 ℃ at a heating rate of 1-5 ℃/min, and preserving heat for 1-2 h;
and in the carbonization treatment, in an argon atmosphere, firstly heating to 350 ℃ at a heating rate of 1-5 ℃/min, preserving heat for 30-60 min, then heating to 430 ℃ at a heating rate of 1-5 ℃/min, preserving heat for 1-2 h, finally heating to 600-800 ℃ at a heating rate of 1-5 ℃/min, and preserving heat for 1-2 h.
The purpose of multi-stage temperature rise in the pre-oxidation stage is as follows: during the pre-oxidation process of the PAN fiber, the-C [ identical to ] N in the structure is opened into a double bond to form a ring with adjacent carbon atoms, and the double bond is further dehydrogenated in subsequent high-temperature carbonization to form a heat-resistant graphite ring structure. PAN fiber pre-oxidation involves a series of severe processes such as cyclization, dehydrogenation and cracking. Therefore, the pre-oxidation is carried out in two stages, wherein the pre-oxidation temperature in the first stage is higher than 100 ℃; the pre-oxidation temperature of the second stage is lower than 280 ℃.
The purpose of multistage temperature rise in the carbonization stage is as follows: at 290-450 ℃, the fiber can generate violent thermal decomposition reaction and solid-phase crosslinking reaction, a series of substances such as acrylonitrile dimer, acrylonitrile trimer and a large amount of gas are released in cracking reaction, so that the polymer is obviously lost in quality, and a loose network of linear conjugated systems separated from each other is obtained. In this stage, mainly the PAN fibers gradually carbonize to form pyrolytic carbon, which gives off a large amount of heat and small molecule gases. When the temperature is higher than 450 ℃, the material is further dehydrogenated to form an aromatic ladder-shaped polymer, and a solid-phase polycondensation reaction is carried out along with the mutual combination of the separated conjugated systems to form a conductive network structure, so that the conductivity of the fiber is increased. When the temperature exceeded 800 ℃ and weight loss again occurred, it was likely that the residual N, H and O in the fiber decreased further with further increase in temperature and shrinkage occurred.
The invention also provides the self-supporting silicon-carbon cathode material prepared by the method.
The invention also provides a lithium ion battery which comprises the self-supporting silicon-carbon negative electrode material.
Compared with the prior art, the invention has the following beneficial effects:
1. the self-supporting silicon-carbon composite electrode prepared by the electrostatic spinning technology has good conductivity, flexibility and the like, and the used nano silicon powder can be used without dipping treatment, so that the self-supporting silicon-carbon composite electrode is convenient and time-saving; only a small amount of surfactant is needed to be added, high-temperature treatment and high-temperature dissolution are not needed, the cost is relatively low, and the DMF has a boiling point of 152.8 ℃ and is relatively high in boiling point, so that the DMF is not easy to volatilize and causes inhalation poisoning. The method is stable, continuous, controllable, simple, convenient and economical, and can improve the dispersibility of the nano-silicon. The prepared electrode material has certain self-supporting property, can be directly used as a battery cathode material, can avoid using non-active ingredients such as a binder, a conductive agent and the like, saves the preparation cost, reduces the weight and the internal resistance of the battery, and is beneficial to improving the cycle performance of the battery performance.
2. The self-supporting silicon-carbon cathode material prepared by the invention has a three-dimensional network structure, can improve the dispersibility of nano silicon, effectively relieves the volume change of the silicon, can form a conductive network, and is beneficial to the transfer of electrons and ions. The material is a composite material of composite nanofiber and carbon, the nanofiber filaments prepared by the electrostatic spinning method have higher specific surface area and active sites, can be beneficial to infiltration of electrolyte and desorption of lithium ions in the charging and discharging processes, and are also Li+More positions for storing/storing lithium are provided, which is beneficial to improving the specific discharge capacity, the multiplying power and the cycling stability. Carbon can form a conductive network, is beneficial to the transfer of electrons and ions, and can effectively relieve the volume expansion effect of silicon. The synergistic effect of the two can improve the electrochemical performance of the composite material.
3. The self-supporting silicon-carbon cathode material prepared by the invention can be directly used as a lithium ion battery, the structural characterization and the electrochemical characteristics of the lithium ion battery are tested, the self-supporting silicon-carbon cathode material has better cycle performance in a Si/C battery cathode, and the self-supporting silicon-carbon cathode material has good application prospect.
Drawings
FIG. 1 is a composite nanofiber membrane prepared in the example; a is a photograph of the macro topography at different degrees of folding and b is a SEM image at 5 Kx.
FIG. 2 is an electrode of the self-supporting Si-C negative electrode material prepared in example 1; a is a photograph of the macro-topography at different degrees of folding and b is a SEM image at 5 Kx.
FIG. 3 is an electrode of the self-supporting silicon carbon anode material of example 1 after cycling at a current density of 50 mA/g; a is a macro morphology picture, and b is an SEM picture of the self-supporting silicon carbon anode material CNFs after circulation under 5 Kx.
FIG. 4 is an XRD spectrum of the self-supporting silicon-carbon anode material prepared by the invention;
a is example 1, b is example 2, c is example 3, and d is example 4.
FIG. 5 is a Raman diagram of the self-supporting silicon-carbon anode material prepared by the invention;
a is example 1, b is example 2, c is example 3, and d is example 4.
FIG. 6 is an SEM image of a self-supporting Si-C negative electrode material prepared by the invention;
a is example 1, b is example 2, c is example 3, and d is example 4.
FIG. 7 is a graph of rate capability of a self-supporting silicon carbon anode material prepared by the present invention;
a is example 1, b is example 2, c is example 3, and d is example 4.
FIG. 8 shows the electrochemical performance of the self-supporting Si-C negative electrode material prepared in example 1; a is a charge-discharge curve diagram and b is a cycle performance diagram.
FIG. 9 shows the electrochemical performance of the self-supporting Si-C negative electrode material obtained in example 2; a is a charge-discharge curve diagram and b is a cycle performance diagram.
FIG. 10 shows the electrochemical performance of the self-supporting Si-C negative electrode material obtained in example 3; a is a charge-discharge curve diagram and b is a cycle performance diagram.
FIG. 11 shows the electrochemical performance of the self-supporting Si-C negative electrode material obtained in example 4; a is a charge-discharge curve diagram and b is a cycle performance diagram.
Detailed Description
The present invention will be described in further detail with reference to the following specific embodiments and the accompanying drawings. The experimental procedures are not specifically described in the following examples, and are carried out in a conventional manner using reagents which are generally commercially available.
Preparation method of self-supporting silicon-carbon negative electrode material
Example 1
1) Adding 8.7 g of DMF (dimethyl formamide) into a beaker, then adding 1.3g of PAN (polyacrylonitrile) for ultrasonic dispersion, then placing the mixture into a 60 ℃ oil bath kettle for stirring at constant temperature for 6 hours, and obtaining a spinning solution after the PAN is completely dissolved;
2) sucking the spinning solution obtained in the step 1) by using a 20 ml syringe, placing the spinning solution in a container loading device of an electrostatic spinning machine, fixing the container loading device on an injection flow rate controller, opening electrostatic spinning equipment, and setting electrospinning process parameters: spinning at 45 deg.c and 36% humidity with 22G needle, voltage 23 Kv, rotation speed 200r/min, flow rate 2 ul/min and needle distance 21 cm to the receiving disc, and electrospinning to obtain the composite nanometer fiber.
3) Placing the composite nanofiber membrane obtained in the step 2) in a 60 ℃ blast drying oven to remove a solvent for 6 hours, cutting the composite nanofiber membrane into square sheets of 5 x 9 cm, then placing graphite sheets with the same size and the same size as that of the square sheets with the square surfaces of 5 x 9 cm on the composite nanofiber membrane to press the composite nanofiber membrane into a fiber membrane, then placing the fiber membrane in a quartz tube furnace, adopting program temperature control, heating to 150 ℃ at the heating rate of 2 ℃/min in the Air (Air) atmosphere, and preserving heat for 3 hours; then the temperature is raised to 230 ℃ at the heating rate of 1 ℃/min, and the temperature is kept for 3 h; heating to 280 ℃ at the heating rate of 1 ℃/min, and keeping the temperature for 1 h. Then heating to 350 ℃ at the heating rate of 2 ℃/min under the argon (Ar) atmosphere, and preserving the heat for 30 min; heating to 430 ℃ at the heating rate of 1 ℃/min, and keeping the temperature for 1 h; and finally, respectively heating to 800 ℃ at the heating rate of 2 ℃/min, preserving the heat for 1.5 h for carbonization, cooling to room temperature, and taking out the sample to obtain the self-supporting silicon-carbon negative electrode materials CNFs.
Example 2
1) Accurately weighing 0.025 g of nano silicon powder in a 50 ml small beaker, dropwise adding a proper amount of KH580 to soak the nano silicon powder, dropwise adding 8.7 g of DMF, ultrasonically dispersing for 20 min, supplementing 1.3g of PAN, placing in a 60 ℃ oil bath pot, stirring at constant temperature for 6 h, and after the PAN is completely dissolved, preparing a coffee emulsion to obtain a spinning solution;
2) sucking the spinning solution obtained in the step 1) by using a 20 ml syringe, placing the spinning solution in a container loading device of an electrostatic spinning machine, fixing the container loading device on an injection flow rate controller, opening electrostatic spinning equipment, and setting electrospinning process parameters: spinning at 45 deg.C and 36% humidity with 22G needle, voltage 23 Kv, rotation speed 200r/min, flow rate 2 ul/min, needle distance 21 cm from receiving disc, and electrospinning to obtain Si0.025A/PAN composite nanofiber.
3) Si obtained in the step 2)0.025The PAN composite nanofiber membrane is placed in a 60 ℃ forced air drying oven to remove the solvent for 6 hours, then cut into square sheets of 5 multiplied by 9 cm, then a graphite sheet with a smooth surface is placed on the composite nanofiber membrane to be pressed into a fiber membrane, and the fiber membrane is placed in a quartz tube furnace, temperature is controlled by adopting a program in the air (A)ir) is heated to 150 ℃ at the heating rate of 2 ℃/min, the temperature is maintained for 3 h, then is heated to 230 ℃ at the heating rate of 1 ℃/min, the temperature is maintained for 3 h, and then is heated to 280 ℃ at the heating rate of 1 ℃/min, and the temperature is maintained for 1 h. Then heating to 350 ℃ at the heating rate of 2 ℃/min in the argon (Ar) atmosphere, preserving heat for 30 min, heating to 430 ℃ at the heating rate of 1 ℃/min, preserving heat for 1 h, finally heating to 800 ℃ at the heating rate of 2 ℃/min, preserving heat for 1.5 h, cooling to room temperature, taking out a sample, and obtaining the self-supporting silicon-carbon negative electrode material Si0.025/CNFs。
Example 3
1) Accurately weighing 0.05 g of nano silicon powder in a 50 ml small beaker, dropwise adding a proper amount of KH580 to soak the nano silicon powder, dropwise adding 8.7 g of DMF, ultrasonically dispersing for 20 min, supplementing 1.3g of PAN, placing in a 60 ℃ oil bath pot, stirring at constant temperature for 6 h, and after the PAN is completely dissolved, preparing a coffee emulsion to obtain a spinning solution;
2) sucking the spinning solution obtained in the step 1) by using a 20 ml syringe, placing the spinning solution in a container loading device of an electrostatic spinning machine, fixing the container loading device on an injection flow rate controller, opening electrostatic spinning equipment, and setting electrospinning process parameters: spinning at 45 deg.C and 36% humidity with 22G needle, voltage 23 Kv, rotation speed 200r/min, flow rate 2 ul/min, needle distance 21 cm from receiving disc, and electrospinning to obtain Si0.05A/PAN composite nanofiber.
3) Si obtained in the step 2)0.025The method comprises the following steps of placing a PAN composite nanofiber membrane in a 60 ℃ forced Air drying oven to remove a solvent for 6 hours, cutting the PAN composite nanofiber membrane into square sheets of 5 x 9 cm, placing graphite sheets with the same size and the same size as that of the square sheets of 5 x 9 cm on the right side of the composite nanofiber membrane to be pressed into a fiber membrane, placing the fiber membrane in a quartz tube furnace, adopting program temperature control, raising the temperature to 150 ℃ at a temperature rise rate of 2 ℃/min in the Air (Air) atmosphere, preserving the temperature for 3 hours, raising the temperature to 230 ℃ at a temperature rise rate of 1 ℃/min, preserving the temperature for 3 hours, raising the temperature to 280 ℃ at a temperature rise rate of 1 ℃/min, and preserving the temperature for 1 hour. Then heating to 350 deg.C at a rate of 2 deg.C/min under argon (Ar) atmosphere, maintaining for 30 min, heating to 430 deg.C at a rate of 1 deg.C/min, maintaining for 1 h, and finally maintaining at the same temperatureHeating to 800 ℃ at a heating rate of 2 ℃/min, preserving heat for 1.5 hours, cooling to room temperature, taking out a sample, and obtaining the self-supporting silicon-carbon negative electrode material Si0.05/CNFs。
Example 4
1) Accurately weighing 0.1g of nano silicon powder in a 50 ml small beaker, dropwise adding a proper amount of KH580 to soak the nano silicon powder, dropwise adding 8.7 g of DMF, ultrasonically dispersing for 20 min, supplementing 1.3g of PAN, placing in a 60 ℃ oil bath pot, stirring at constant temperature for 6 h, and after the PAN is completely dissolved, preparing a coffee emulsion to obtain a spinning solution;
2) sucking the spinning solution obtained in the step 1) by using a 20 ml syringe, placing the spinning solution in a container loading device of an electrostatic spinning machine, fixing the container loading device on an injection flow rate controller, opening electrostatic spinning equipment, and setting electrospinning process parameters: spinning at 45 deg.C and 36% humidity with 22G needle, voltage 23 Kv, rotation speed 200r/min, flow rate 2 ul/min, needle distance 21 cm from receiving disc, and electrospinning to obtain Si0.1A/PAN composite nanofiber.
3) Si obtained in the step 2)0.025The method comprises the following steps of placing a PAN composite nanofiber membrane in a 60 ℃ forced Air drying oven to remove a solvent for 6 hours, cutting the PAN composite nanofiber membrane into square sheets of 5 x 9 cm, placing graphite sheets with the same size and the same size as that of the square sheets of 5 x 9 cm on the right side of the composite nanofiber membrane to be pressed into a fiber membrane, placing the fiber membrane in a quartz tube furnace, adopting program temperature control, raising the temperature to 150 ℃ at a temperature rise rate of 2 ℃/min in the Air (Air) atmosphere, preserving the temperature for 3 hours, raising the temperature to 230 ℃ at a temperature rise rate of 1 ℃/min, preserving the temperature for 3 hours, raising the temperature to 280 ℃ at a temperature rise rate of 1 ℃/min, and preserving the temperature for 1 hour. Then heating to 350 ℃ at the heating rate of 2 ℃/min in the argon (Ar) atmosphere, preserving heat for 30 min, heating to 430 ℃ at the heating rate of 1 ℃/min, preserving heat for 1 h, finally heating to 800 ℃ at the heating rate of 2 ℃/min, preserving heat for 1.5 h, cooling to room temperature, taking out a sample, and obtaining the self-supporting silicon-carbon negative electrode material Si0.1/CNFs。
Second, performance verification
1. Scanning electron microscopy is used for observing the nanofiber membrane prepared in example 1, the self-supporting silicon-carbon negative electrode material and the morphology of the self-supporting silicon-carbon negative electrode material after circulation under the current density of 50mA/g, and the results are shown in figures 1-3.
As can be seen from fig. 1a, the composite nanofiber membrane (PAN nanofiber membrane) is milky white in appearance, brittle cracking does not occur after the composite nanofiber membrane is folded by tweezers, and the fibers show strong static electricity. From fig. 1b, it can be clearly seen that the fiber surface in the composite nanofiber membrane is smooth, the diameter is uniform, the composite nanofiber membrane is in cross lamination, the average diameter distribution of the fiber is about 350 nm, and the phenomena of dropping liquid, beading, mutual dissolution between fibers, adhesion and dissolution, and the like do not occur in the whole spinning process.
Fig. 2 a is a macro-topographic photograph of a self-supporting CNFs membrane at the degree of folding. The self-supporting CNFs electrode with smooth surface and no burrs can be prepared by the gradient pre-oxidation and high-temperature carbonization processes. After the carbon film is folded to different degrees, brittle fracture does not occur, the original shape is still kept, the self-supporting CNFs can be well restored to the original shape after the tweezers are loosened, and certain flexibility is still shown. Indicating that the self-supporting CNFs membrane has certain strength and flexibility. Compared with PAN nanofibers, the surface of the self-supporting CNFs fiber becomes rough and slightly bent, which is probably the process of heat shrinkage of the PAN fiber in the heat treatment process, and a stable annular structure is not completely formed at a low temperature stage. Fig. 2 b is an SEM image of the self-supporting CNFs membrane at 5 Kx, from which it can be seen that the morphology of the self-supporting CNFs remains substantially unchanged, there is cross-linking between fibers, and there is no significant fracture and granulation. Compared with the composite nanofiber membrane (figure 1), the average diameter of the fiber filaments is obviously reduced, the insertion and extraction paths of lithium ions in the material are favorably shortened in the charging and discharging process, the migration rate of charges on the surface and inside of the material is accelerated, and the adsorption sites of the lithium ions are increased, so that the battery capacity is increased. The average diameter range is about 300 nm or so.
FIG. 3 a is a photograph of the macro-topography of self-supporting CNFs electrodes after cycling at a current density of 50 mA/g. It can be seen from the figure that the self-supporting silicon carbon negative electrode material CNFs electrode after circulation keeps good in appearance shape, size, structural integrity and the like, no crack appears on the surface, and traces left after the electrolyte is soaked can be seen, which indicates that the appearance structure of the CNFs electrode keeps good after circulation. Fig. 3 b is an SEM image at 5 Kx of the cycled self-supporting CNFs film, from which it can be seen that the carbon fibers after cycling are rough in surface and slightly larger in diameter, possibly associated with SEI formation and some residual electrolyte.
2. The crystal structures of the self-supporting silicon carbon anode materials obtained in examples 1 to 4 were analyzed using an X-ray diffraction analyzer, as shown in fig. 4.
With the help of jades 9.0 analysis software, in PDF #27-1402 card number, silicon particles have peaks at 2 θ =28.442 °, 2 θ =47.302 °, 2 θ =56.121 °, 2 θ =69.130 ° and 2 θ =76.377 ° corresponding to the (111), (220), (311), (400) and (331) crystal planes, respectively. As can be seen from the figure, in each of the four samples, there is a widely distributed "steamed bread peak" at 2 θ =25.5 °, corresponding to the (002) crystal plane. In samples (c) and (d), a crystalline diffraction peak of weaker silicon was observed, and in sample (a), a crystalline diffraction peak of silicon was not observed, and in sample (b), it was possible that the crystalline diffraction peak of silicon was masked by a carbon peak because the content of amorphous carbon was too low.
3. The results of raman spectroscopy analysis of the self-supporting silicon carbon negative electrode materials obtained in examples 1 to 4 are shown in fig. 5.
Peak D in the figure (1300 cm)-1Left and right) represents a lattice defect of a carbon atom, and a G peak (1580 cm)-1Left and right) represents a carbon atom in sp2Orbital hybrid telescopic vibration, IDAnd IGThe relative ratio of (A) to (B) is indicative of the degree of disorder and defects in the sample. As can be seen, the four samples are at 1340 cm-1And 1580 cm-1There are characteristic peaks corresponding to the D peak and G peak of graphite. No characteristic peak of silicon was found in sample (b), while samples (c) and (d) were at 518 cm-1A characteristic peak of silicon appears, and the intensity of the raman peak increases with the increase of the silicon content. When the content of silicon is small and the content of graphite is high, the raman peak of silicon has too low an intensity compared to the D peak or G peak of the self-supporting CNFs electrode, resulting in difficulty in being characterized in the fitting of raman spectra.
4. The shapes of the self-supporting silicon-carbon negative electrode materials obtained in examples 1 to 4 were observed by using a scanning electron microscope, as shown in fig. 6.
As can be seen from the figure, the appearance of the fiber is not damaged after carbonization, the phenomenon of fiber breakage does not occur, the obvious contraction phenomenon does not occur, the crossing and stacking between adjacent fiber yarns can be clearly seen, the nano fiber yarns with smooth surfaces are provided, and the average diameter distribution is about 200-350 nm. Fig. 6 b to d clearly show that with the increase of nano silicon particles, the bulk particle substances on the fiber surface increase continuously, and part of the positions break the carbon fibers and protrude in a bulge shape, which indicates that with the increase of silicon content, the agglomeration is serious, and the nano silicon is mainly dispersed in the carbon fibers and on the surface. Therefore, the nano silicon particles wrapped by the carbon fibers can effectively inhibit the volume expansion of silicon in the process of lithium intercalation/deintercalation, the nano silicon particles distributed on the fiber surface are not easy to fall off due to the pinning effect, the carbon fibers which are mutually crossed and laminated can also relieve the volume expansion effect to a certain extent, and the self-supporting Si is improvedχThe electrochemical performance of the/CNFs composite electrode. Furthermore, the compact fibers may avoid the formation of unstable Solid Electrolyte Interphase (SEI) layers on the surface of the Si nanoparticles.
5. The self-supporting silicon-carbon negative electrode material prepared in the embodiment 1-4 is directly cut into thin wafers with the diameter of 12 mm by a cutting machine, and then assembled into a CR2032 button cell in a glove box. The assembled CR2032 button cells were cycled 10 times at current densities of 50mA/g, 100mA/g, 200 mA/g, 500mA/g, 1000 mA/g, 2000 mA/g, and 50mA/g, respectively, and the results were compared to the rate performance and shown in FIG. 7.
FIG. 7 a is a graph showing rate capability of self-supporting CNFs anodes, cycling 10 times at current densities of 50mA/g, 100mA/g, 200 mA/g, 500mA/g, 1000 mA/g, 2000 mA/g and 50mA/g, respectively, with specific discharge capacities of 348mAh/g, 325.4 mAh/g, 283.7 mAh/g, 235.9 mAh/g, 198.1 mAh/g, 160.1 mAh/g and 322.6 mAh/g, respectively. Except for the fact that the loss of large irreversible capacity is caused during initial charging and discharging, the initial coulombic efficiency is 59%, the charging and discharging specific capacity in the subsequent circulation process is basically kept stable, and the coulombic efficiency is high and is close to 100%. Under the high current density of 2000 mA/g, the discharge specific capacity of the self-supporting CNFs anode is 160.1 mAh/g after 10 times of charge and discharge, and the current of 61 st charge and dischargeThe density is recovered to 50mA/g at the beginning, the discharge specific capacity after 10 times of circulation is 322.6 mAh/g, and the recovery rate reaches 92.7 percent. It can be seen that the self-supporting CNFs electrode forms a stable SEI film in the initial charge-discharge process, has a more stable structure and higher capacity, and has certain advantages in the high-rate charge-discharge process. FIG. 7 b is a self-supporting Si0.025The multiplying power performance of the/CNFs composite anode is respectively circulated for 10 times under current densities of 50mA/g, 100mA/g, 200 mA/g, 500mA/g, 1000 mA/g, 2000 mA/g and 50mA/g, and the specific discharge capacities of the/CNFs composite anode are respectively 771.7 mAh/g, 610.1 mAh/g, 462.9 mAh/g, 268.2 mAh/g, 143.0 mAh/g, 85.0mAh/g and 408 mAh/g. Except for slightly large irreversible capacity loss during initial charging and discharging, the initial coulombic efficiency is 76%, the charging and discharging specific capacity in the subsequent circulation process is basically kept stable, and the coulombic efficiency is higher and is more than 95%. Self-supporting Si0.025Under the large current density of 2000 mA/g, the specific discharge capacity of the/CNFs composite anode is 85.0mAh/g after 10 times of charging and discharging, the current density of 61 st charging and discharging is recovered to the initial 50mA/g, the specific discharge capacity is 408 mAh/g after 10 times of circulation, and the recovery rate is 59.8%. FIG. 7 c is a self-supporting Si0.05The multiplying power performance of the/CNFs composite anode is respectively cycled for 10 times under current densities of 50mA/g, 100mA/g, 200 mA/g, 500mA/g, 1000 mA/g, 2000 mA/g and 50mA/g, the discharge specific capacities of the/CNFs composite anode are respectively 557.6mAh/g, 497.6 mAh/g, 416.9 mAh/g, 302.4 mAh/g, 193 mAh/g, 86 mAh/g and 409 mAh/g, except for the loss of slightly larger irreversible capacity during initial charge and discharge, the initial coulombic efficiency is 71%, the charge and discharge specific capacity in the subsequent cycling process is basically kept stable, and the coulombic efficiency is higher and is more than 95%. FIG. 7 d is a self-supporting Si0.1The multiplying power performance of the/CNFs composite anode is respectively circulated for 10 times under current densities of 50mA/g, 100mA/g, 200 mA/g, 500mA/g, 1000 mA/g, 2000 mA/g and 50mA/g, and the discharge specific capacities of the/CNFs composite anode are respectively 801.9 mAh/g, 652.2 mAh/g, 490.5 mAh/g, 257.6 mAh/g, 117.1 mAh/g, 69.4 mAh/g and 453.8 mAh/g. Except for slightly large irreversible capacity loss during initial charging and discharging, the initial coulombic efficiency is 75%, the stability of the electrode under different multiplying power is poor, the charging and discharging cyclic coulombic efficiency is more than 95%, and large fluctuation does not occur.
6. The CR2032 button cell assembled from the negative electrode materials obtained in examples 1 to 4 was subjected to initial charge/discharge performance tests at a current density of 100mA/g, and the results are shown in fig. 8 to 11.
Fig. 8 is a charge-discharge curve and a cycle performance diagram of a self-supporting CNFs electrode sample, and as can be seen from fig. 8 a and 8b, the initial discharge/charge specific capacity of the self-supporting CNF negative electrode is 808/462.3mAh/g, and the initial coulombic efficiency is 57.21%. The self-supporting CNFs electrode has two longer discharge voltage platforms in the primary discharge process of 0.01-0.50V and 0.5-0.95V respectively, and similar curves do not appear in the subsequent cycle discharge process, possibly caused by the fact that an electrolyte forms a solid electrolyte interface film (SEI film) on the surface of the CNFs and the electrolyte is decomposed, and possibly caused by the fact that the SEI film is formed by the decomposition of the electrolyte when lithium is inserted for the first time. The discharge specific capacities after 3 times of circulation and 30 times of circulation are 373.5 mAh/g and 287.1 mAh/g respectively, the coulombic efficiency is more than 95%, and the capacity loss of the self-supporting CNFs electrode is mainly generated in the first 5 times.
FIG. 9 is a view of self-supporting Si0.025FIG. 9a and FIG. 9b show the charge/discharge curves and cycle performance of the/CNFs electrode samples, showing that the self-supporting Si0.025The initial discharge specific capacity of the/CNFs electrode is 923.4 mAh/g, the initial charge specific capacity is 585.5mAh/g, and the initial coulombic efficiency is 63.41%. The discharge specific capacity is 481.3 mAh/g after 3 times of circulation, the capacity begins to slowly decrease after 25 times of circulation, the discharge specific capacity is 419 mAh/g after 30 times of circulation, and the coulombic efficiency is more than 95%.
FIG. 10 is a view of self-supporting Si0.05FIG. 10a and FIG. 10b show the charge/discharge curves and cycle performance of the CNFs electrode samples, showing that the self-supporting Si0.05The initial discharge specific capacity of the/CNFs electrode is 946.9 mAh/g, the initial charge specific capacity is 605.8mAh/g, and the initial coulombic efficiency is 63.97%. Self-supporting Si0.05The CNFs electrode has a discharge voltage platform around 0.01-0.50V and 0.5-0.95V respectively, and a similar curve does not appear in the subsequent cyclic charge-discharge process. The discharge specific capacity is 627.6 mAh/g after 3 times of circulation, the capacity begins to slowly decrease after 20 times of circulation, the discharge specific capacity is 487 mAh/g after 30 times of circulation, and the coulomb efficiency is more than 94%.
FIG. 11 is a view of self-supporting Si0.1/CNAs can be seen from FIGS. 11 a and 11b, the charge/discharge curve and cycle performance of the Fs electrode sample are self-supporting Si0.1The initial discharge specific capacity of the/CNFs electrode is 1056.2 mAh/g, the initial charge specific capacity is 739.2mAh/g, the initial coulombic efficiency is 69.66%, and the initial coulombic efficiency is higher than that of self-supporting Si0.05a/CNFs electrode. Self-supporting Si0.1The specific discharge capacity of the CNFs is 780.3 mAh/g after the CNFs are cycled for 3 times, the capacity begins to slowly decrease after the CNFs are cycled for 20 times, the specific discharge capacity is 598.7 mAh/g after the CNFs are cycled for 30 times, and the coulomb efficiency is more than 95%.
In summary, from self-supporting CNFs negative electrodes and SiχIn the cycle test of the/CNFs composite negative electrode sample, the fact that the electrode is reduced to different degrees in the charging and discharging process due to the fact that the self-supporting structure can effectively inhibit the volume expansion effect of the nano-silicon to a certain extent in the charging and discharging process can be seen when nano-silicon particles are introduced into the system. The cycle performance test was carried out at a current density of 100mA/g, with free-standing Si0.1The initial discharge specific capacity of the/CNFs can reach 1056.2 mAh/g, the initial charge specific capacity can reach 739.2mAh/g, and the coulombic efficiency is more than 95% after 30 times of circulation. Therefore, the electrochemical performance is well maintained, the loss of large irreversible capacity is avoided when the battery is charged and discharged for the first time, the charging and discharging specific capacity is basically kept stable in the subsequent cycle process, and the coulombic efficiency is high. At the same time, self-supporting Siχthe/CNFs show better cycling stability, and the capacity of the/CNFs is continuously increased along with the increase of the content of nano-silicon. This not only indicates self-supporting SiχThe nano silicon particles precipitated on the surface of the CNFs electrode are less, the self-supporting CNFs electrode can effectively relieve the volume change of silicon, and the addition of the nano silicon can increase the charge and discharge capacity to a certain extent. The composite negative electrode material can provide good conductivity, and a large number of gaps among the nano fibers provide enough space to adapt to volume expansion in the charging/discharging process, so that the diffusion and migration path of lithium ions is shortened, the charge and discharge resistance is reduced, and the electrochemical performance is improved. The composite anode material of the invention does not need a current collector, a conductive agent and a binder as a self-supporting electrode, which effectively reduces the total weight and cost of the lithium ion battery anode.
Finally, the above embodiments are only for illustrating the technical solutions of the present invention and not for limiting, although the present invention has been described in detail with reference to the preferred embodiments, it should be understood by those skilled in the art that modifications or equivalent substitutions may be made to the technical solutions of the present invention without departing from the spirit and scope of the technical solutions of the present invention, and all of them should be covered in the claims of the present invention.
Claims (10)
1. A preparation method of a self-supporting silicon-carbon negative electrode material is characterized by comprising the following steps:
1) dropwise adding a proper amount of surfactant into the nano silicon powder to soak the nano silicon powder, adding a solvent to perform ultrasonic dispersion, then placing the mixture in an oil bath kettle to stir at a constant temperature, and obtaining a spinning solution after the solute is completely dissolved;
2) placing the spinning solution obtained in the step 1) in a container loading device of an electrostatic spinning machine, setting electrospinning process parameters, and then carrying out electrospinning to obtain composite nanofibers;
3) placing the composite nanofiber membrane obtained in the step 2) in a drying oven to remove a solvent, then pressing the composite nanofiber membrane into a fiber membrane by using a graphite sheet with a smooth surface, placing the fiber membrane in a quartz tube furnace to perform pre-oxidation treatment and carbonization treatment, cooling the fiber membrane along with the furnace to room temperature after the reaction is finished, and taking out a sample to obtain the self-supporting composite electrode material.
2. The method for preparing the self-supporting silicon-carbon anode material as claimed in claim 1, wherein the surfactant is used in an amount of 0.5 wt.% based on the nano silicon powder.
3. The method for preparing the self-supporting silicon-carbon negative electrode material as claimed in claim 1, wherein the surfactant is KH550, KH560, KH570, KH580 or CTAB.
4. The method for preparing the self-supporting silicon-carbon anode material according to claim 1, wherein the solvent is DMF or DMAC; the dosage of the nano silicon powder is 0-1.5 wt% of the solvent.
5. The preparation method of the self-supporting silicon-carbon anode material as claimed in claim 1, wherein the constant-temperature stirring temperature in the step 1) is 40-80 ℃ and the time is 5-20 h.
6. The preparation method of the self-supporting silicon-carbon anode material according to claim 1, wherein the electrospinning process parameters are as follows: the spinning temperature is 35-50 ℃, the voltage is 18-28 Kv, the rotating speed is 150-300 r/min, the flow speed is 1-3 ul/min, and the distance between a needle head and a receiving disc is 15-25 cm.
7. The preparation method of the self-supporting silicon-carbon negative electrode material as claimed in claim 1, wherein the temperature of the drying oven is 40-80 ℃.
8. The preparation method of the self-supporting silicon-carbon negative electrode material according to claim 1, wherein the pre-oxidation treatment comprises heating to 130-150 ℃ at a heating rate of 1-5 ℃/min in an air atmosphere, and then maintaining the temperature for 1-3 h, and heating to 220-230 ℃ at a heating rate of 1-5 ℃/min, and maintaining the temperature for 1-3 h; then heating to 260-280 ℃ at a heating rate of 1-5 ℃/min, and preserving heat for 1-2 h;
and in the carbonization treatment, in an argon atmosphere, firstly heating to 350 ℃ at a heating rate of 1-5 ℃/min, preserving heat for 30-60 min, then heating to 430 ℃ at a heating rate of 1-5 ℃/min, preserving heat for 1-2 h, finally heating to 600-800 ℃ at a heating rate of 1-5 ℃/min, and preserving heat for 1-2 h.
9. A self-supporting silicon carbon anode material prepared by the method of any one of claims 1 to 8.
10. A lithium ion battery comprising the self-supporting silicon carbon anode material of claim 9.
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