CN107248570B - Core-shell structure Si/C material, preparation method and application - Google Patents
Core-shell structure Si/C material, preparation method and application Download PDFInfo
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Abstract
The invention belongs to the field of lithium ion batteries, and particularly relates to a preparation method of a Si/C material with a core-shell structure, which comprises the following steps: (1) preparation of Fe (NO)3)3A solution; (2) putting Si powder into the solution and stirring the solution evenly to form turbid liquid; (3) pouring liquid nitrogen into the suspension to form ice state Si/Fe (NO)3)3(ii) a (4) Vacuum freeze-drying to obtain Si/Fe (NO)3)3A sample; (5) mixing Si/Fe (NO)3)3Carrying out high-temperature carbon coating process on the sample to obtain the product containing Fe3O4Si/C powder of (1); (6) and (4) carrying out acid washing and drying to obtain the core-shell structure Si/C material. The invention also discloses a core-shell structure Si/C material and application thereof. The Si/C material prepared by the method has excellent electrochemical performance, does not need high-concentration acetylene and a special CVD device, and has higher operability and safety.
Description
Technical Field
The invention belongs to the field of lithium ion batteries, and particularly relates to a core-shell structure Si/C material, and a preparation method and application thereof.
Background
As early as the Si-based negative electrode material appears, the Si/C composite material is considered as the most potential Si-based material, the carbon material has good conductivity and cycling stability, and carbon in the Si/C material can play a role in improving the conductivity of the silicon-based material, stabilizing an SEI film and buffering the volume effect.
The earliest preparation of Si/C composite materials was mainly the simple compounding of Si and a carbon source, and Kumta and Kim et al used Polystyrene (PS) as a C source and Si powder as a Si source, and the Si/C composite was prepared by high-energy ball milling in an inert atmosphere. Energy dispersive X-ray spectroscopy (EDX) shows that the Si content of the material is 66 percent, and the reversible capacity is 850mA h g–1And the capacity retention rate after 30 cycles was 98.9%. Liu et al obtain PVA/Si material by similar method, and the reversible capacity of the prepared material reaches 754mA h g–1The first coulombic efficiency reaches 80.3 percent. However, it is difficult to further increase Si-based negative charge by simple Si/C recombinationThe electrochemical performance of the electrode material shows a rapid development of the Si/C composite material with the appearance of a structural design concept. Si/C materials such as mesoporous Si/C, colk-shell structures, hollow Si/C composite structures and the like appear. Hollow core-shell structure Si/C mainly synthesized SiO2And carbon layer sequence, and etching SiO with HF solution2The carbon layer being mainly derived from decomposition of polymers or acetylene (C)2H2) And (3) pyrolyzing the carbon.
According to the above studies, the core-shell structure Si/C composite has excellent electrochemical properties, however, when preparing the core-shell structure Si/C composite, the decomposition of the polymer to form a carbon layer requires a special polymer coating method, and the following problems are encountered in the preparation of the core-shell structure Si/C composite by using acetylene-coated carbon instead of acetylene-coated carbon: (1) the preparation process requires acetylene gas with higher concentration, so the preparation method is very complicated and unsafe, (2) the low concentration C2H2Gas deposition directly on Si surface is difficult, therefore, C2H2The material prepared when the gas is low has the performance which is difficult to meet the use requirement (3), the preparation method has long synthesis process, and needs a professional CVD device, thus improving the cost and difficulty of preparation.
Due to the defects and shortcomings, further improvement and improvement are needed in the art, and a preparation method of the core-shell structure Si/C composite is designed to avoid the problems so as to meet the production requirement of the core-shell structure Si/C composite.
Disclosure of Invention
Aiming at the defects or the improvement requirements of the prior art, the invention provides a core-shell structure Si/C material, a preparation method and application thereof, wherein Fe (NO) is utilized3)3As a precursor, Fe (NO) in a high-temperature carbon coating process3)3Decomposition to Fe3O4The catalyst has strong catalytic action on the growth of carbon, can promote acetylene decomposition, forms a carbon layer by using low-concentration acetylene gas, and finally removes Fe element by using dilute acid to obtain the core-shell structure Si/C material. The Si/C material with the core-shell structure prepared by the method has excellent electrochemical performance, and is particularly suitable for a negative electrode material of a lithium ion battery. And the sameThe method can successfully coat the carbon layer without high-concentration acetylene and a special CVD device, has higher operability and safety, and also has the advantages of simple preparation flow, low manufacturing cost and the like.
In order to achieve the above object, according to one aspect of the present invention, there is provided a method for preparing a core-shell Si/C material, comprising the steps of:
(1) fe (NO) to be prepared3)3Dissolving in deionized water to obtain Fe (NO)3)3Solution, and weighing Si powder for later use;
(2) adding the weighed Si powder into Fe (NO)3)3Adding a dispersing agent into the solution, and ultrasonically stirring to uniformly disperse the Si powder to form a suspension;
(3) pouring the suspension obtained in the step (2) into a low temperature resistant container, pouring clean liquid nitrogen into the suspension for freezing to form ice state Si/Fe (NO)3)3；
(4) Mixing the ice state Si/Fe (NO) obtained in the step (3)3)3Lyophilizing under vacuum to obtain lyophilized Si/Fe (NO)3)3A sample;
(5) taking out the lyophilized Si/Fe (NO)3)3Heating the sample, firstly raising the temperature under an inert atmosphere, then changing the inert atmosphere into an organic gas and preserving the temperature, wherein Fe (NO) is used3)3In situ generation of Fe3O4The nano particles catalyze organic gas to form a carbon layer on the surface of Si, and then the carbon layer is replaced by inert atmosphere and cooled to room temperature to obtain the Fe-containing nano particles3O4Si/C powder of (1);
(6) the Fe content obtained in the step (5)3O4Washing the Si/C powder with excessive acid to remove Fe element, and separating and drying to obtain the Si/C material with the core-shell structure.
The principle of the preparation method of the invention is as follows: first, using Fe (NO)3)3As a precursor, ice Si/Fe (NO) is formed3)3Adding Si/Fe (NO) in an ice state3)3Freeze drying to obtain Si/Fe (NO)3)3The sample is then subjected to a high-temperature carbon coating process,in this process, Fe (NO)3)3Decomposition to Fe3O4The catalyst has strong catalytic action on the growth of carbon, promotes the decomposition of organic gas, forms a carbon layer by using low-concentration organic gas, and finally removes Fe element by using dilute hydrochloric acid to obtain the Si/C material with the core-shell structure. The preparation method can successfully coat the carbon layer on the surface of the Si without high-concentration acetylene and a special CVD device, simplifies the preparation process, solves the problem that low-concentration acetylene gas is difficult to directly deposit on the surface of the Si, and the obtained core-shell structure Si/C material has excellent electrochemical performance because of Fe3O4The nanoparticles are easily removed, and the safety and operability of the preparation process are improved.
Further preferably, in the step (1), the Si powder is nano Si powder or micro Si powder, and the mass ratio of Fe to Si is 5: 1-1: 1.
Preferably, in step (2), the dispersant comprises a non-polar liquid, preferably anhydrous ethanol.
Preferably, in step (3), the low temperature resistant container used is a plastic container.
Preferably, in the step (4), freeze-drying is carried out in a freeze-drying machine after precooling, wherein the freeze-drying temperature is less than or equal to 30 ℃, the freeze-drying time is 10-100 h, and the vacuum degree is<50 Pa. More comparative experiments show that the ice state Si/Fe (NO) can be rapidly adjusted by controlling the condition parameters of the freeze-drying process within the range3)3The water in the sample is drained and completely dried to obtain a freeze-dried sample.
Preferably, the heating process in the step (5) is performed in a tubular furnace, the heating temperature is 300-100 ℃, the heat preservation time is 10-80 min, and the organic gas is acetylene, methane or ethylene. Many comparative tests have shown that Fe (NO) can be made by controlling the heating temperature and the holding time within the above ranges in the high-temperature carbon coating3)3Successfully decomposed into Fe3O4Thereby playing a strong catalytic role, promoting the acetylene to decompose, forming a carbon layer on the surface of the Si and leading the carbon coating process to be carried out smoothly. Acetylene, methane and ethylene are all organic gases with high carbon content, and the acetylene, methane and ethylene are usedThe carbon-coated Si/C material is used as a carbon-coated atmosphere, so that the carbon coating efficiency can be improved, and the finally obtained core-shell structure Si/C material has better electrochemical performance.
Preferably, the acid used in the step (6) is preferably an acid such as dilute hydrochloric acid, dilute nitric acid or dilute sulfuric acid, and the separation method is centrifugation, suction filtration or other solid-liquid separation methods; the drying process is carried out under the conditions of air, inert atmosphere or vacuum and the like, and the drying temperature is 30-150 ℃, preferably 80 ℃. Many comparative tests show that excessive dilute hydrochloric acid, dilute nitric acid or dilute sulfuric acid can quickly and effectively react with ferric oxide to remove the iron element in the powder, residual iron element in the powder can be effectively removed by adopting methods such as centrifugation and suction filtration, the drying temperature is limited in the range, the powder can be thoroughly dried, the structure and the performance of the Si/C material cannot be damaged, and thus the pure Si/C material is finally obtained.
According to another aspect of the invention, the invention provides a preparation method of the Si/C material with the core-shell structure, which is characterized in that Fe (NO) is adopted3)3And replaced by manganese salt, nickel salt or other iron salt. Because the metals of iron, manganese or nickel have good catalytic performance and strong catalytic action on the growth of carbon, acetylene can be promoted to decompose, and a coating carbon layer is formed on the surface of Si, Fe (NO) is added3)3The material is replaced by manganese salt, nickel salt or other iron salts, and the core-shell structure Si/C material with good electrochemical performance can also be prepared.
According to another aspect of the invention, the invention provides a core-shell structure Si/C material prepared by the preparation method of the core-shell structure Si/C material.
According to another aspect of the invention, the invention provides an application of the core-shell structure Si/C material prepared by the preparation method of the core-shell structure Si/C material as a negative electrode material. Preferably, the anode material is for a secondary battery.
Further tests show that when the Si/C material with the core-shell structure prepared by the invention is used as a lithium ion battery cathode, the Si/C electrode material shows excellent electrochemical performance which is 200 DEGmA g–1The specific capacity is stabilized at 1000mA h g after charging and discharging for 100 times under the current density–1In contrast, the specific capacity of the pure Si powder electrode is rapidly attenuated to 700mA h g–1The following.
Generally, compared with the prior art, the technical scheme of the invention has the following advantages and beneficial effects:
(1) the method of the invention uses Fe generated in situ3O4Nanoparticles as template, Fe3O4Nanoparticle catalytic decomposition of low concentration acetylene (C)2H2) Gas, the Si/C cathode material with a core-shell structure is prepared, and the problem of low concentration C is solved2H2The gas is difficult to deposit on the surface of Si directly. In addition, since it is not necessary to use acetylene gas at a high concentration, and Fe3O4The nanoparticles are easily removed, improving the operability and safety of the experiment. As one of key points of the invention, the Si/C material with the core-shell structure is successfully prepared by optimally designing specific reaction conditions such as Fe/Si ratio, reaction time of acetylene gas, decomposition temperature and the like.
(2) The Si/C material with the core-shell structure prepared by the invention has excellent electrochemical performance. The cycle curve of the Si/C material is basically a horizontal straight line, and compared with the slope-shaped cycle curve of pure Si powder, the cycle performance is greatly improved. The Si/C electrode material obtained by using the composite material as the negative electrode of the lithium ion battery has excellent electrochemical performance, and the electrochemical performance is 200mA g–1The specific capacity is stabilized at 1000mA h g after charging and discharging for 100 times under the current density–1While the specific capacity of the pure Si powder electrode is rapidly attenuated to 700mA h g–1The following.
(3) The invention utilizes the Fe generated in situ3O4Synthesis of Si/C composite material, Fe, using nano particles as template3O4The particles play a role in catalyzing the growth of carbon, and the low-concentration acetylene gas can still be decomposed into a carbon layer which is uniformly coated on the surface of the Si particles. Avoids using high-concentration acetylene gas, and improves the operability and safety of the experiment. The invention adopts the Fe/Si ratio, the acetylene gas type concentration, the high-temperature carbon-coating reaction condition and the freeze-drying stripParameters such as the piece, centrifugal drying conditions and the like are controlled within a certain range, so that the carbon coating efficiency can be improved, and the finally obtained core-shell structure Si/C material has better electrochemical performance. The invention provides a novel metal template with a catalytic effect, and provides a novel thought for designing a Si/C nano structure.
(4) The method has the advantages of simple and common instruments, no need of complex synthesis flow, realization of preparation of the core-shell structure Si/C material by using a common tube furnace, no need of a professional CVD device, simplification of experimental conditions and reduction of manufacturing cost.
(5) The invention generates Fe in situ3O4The particle serving as a template for catalyzing the growth of the carbon layer can be expanded to other metal elements with catalytic action, and the metal template thought with catalytic carbon growth activity has value and significance for continuous research.
Drawings
FIG. 1 is a schematic diagram of a method for preparing a Si/C material with a core-shell structure according to the present invention;
FIG. 2 is an equivalent circuit diagram of a cell prepared from the Si/C material of the invention when the cell is subjected to electrochemical performance tests.
FIGS. 3(a) - (d) are representations of core-shell structured Si/C materials prepared at different temperatures in examples 1-4, where: (a) is an X-ray diffraction (XRD) pattern; (b) the cycle performance of the Si/C material is shown; (c) is the charge-discharge curve of the Si/C material; (d) is an alternating current impedance spectroscopy (EIS).
FIGS. 4(a) - (f) are Scanning Electron Microscope (SEM) pictures of Si/C materials prepared at different temperatures in examples 2-4, wherein (a) and (b): 500 ℃; (c) and (d): 600 ℃; (e) and (f): 700 ℃.
FIGS. 5(a) - (f) are Transmission Electron Microscope (TEM) pictures of Si/C materials prepared at different temperatures in examples 2-4, wherein (a) and (b): 500 ℃; (c) and (d): 600 ℃; (e) and (f): 700 ℃.
FIGS. 6(a) - (d) are comparative graphs of electrochemical performances of core-shell structure Si/C material and pure Si powder, wherein (a) is cycle performance; (b) is a charge-discharge curve; (c) is an alternating current impedance spectrum; (d) the rate capability.
Detailed Description
In order to make the objects, technical solutions and advantages of the present invention more apparent, the present invention is described in further detail below with reference to the accompanying drawings and embodiments. It should be understood that the specific embodiments described herein are merely illustrative of the invention and are not intended to limit the invention. In addition, the technical features involved in the embodiments of the present invention described below may be combined with each other as long as they do not conflict with each other.
FIG. 1 is a schematic diagram of a method for preparing a Si/C material with a core-shell structure, and as shown in the figure, the method for preparing the Si/C material with the core-shell structure specifically comprises the following steps:
(1) fe (NO) to be prepared3)3Dissolving in deionized water to obtain Fe (NO)3)3Weighing Si powder for later use, wherein the Si powder is nano Si powder or micron Si powder, and the mass ratio of Fe to Si is 5: 1-1: 1;
(2) adding the weighed Si powder into Fe (NO)3)3Adding a dispersing agent into the solution, and ultrasonically stirring to uniformly disperse the Si powder to form a suspension, wherein the dispersing agent is a non-polar liquid, and preferably absolute ethyl alcohol;
(3) pouring the suspension obtained in the step (2) into a low temperature resistant container, pouring clean liquid nitrogen into the suspension for freezing to form ice state Si/Fe (NO)3)3Preferably, the low temperature resistant container used is a plastic container;
(4) mixing the ice state Si/Fe (NO) obtained in the step (3)3)3Lyophilizing under vacuum to obtain lyophilized Si/Fe (NO)3)3Freeze-drying the sample in a pre-cooled freeze dryer at the freeze-drying temperature of less than or equal to 30 ℃ for 10-100 h and under the vacuum degree<50 Pa; by controlling the condition parameters of the freeze-drying process within the above range, the ice state Si/Fe (NO) can be rapidly converted3)3The water in the sample is drained and completely dried to obtain a freeze-dried sample.
(5) Taking out the lyophilized Si/Fe (NO)3)3The sample is heated, first, in an inert atmosphereRaising the temperature in the atmosphere of the nature, then changing the inert atmosphere into the organic gas and preserving the temperature, wherein Fe (NO) is used3)3In situ generation of Fe3O4The nano particles catalyze organic gas to form a carbon layer on the surface of Si, and then the carbon layer is replaced by inert atmosphere and cooled to room temperature to obtain the Fe-containing nano particles3O4Si/C powder of (1);
specifically, the heating process is carried out in a tubular furnace, the heating temperature is 300-1000 ℃, the heat preservation time is 10-80 min, and the organic gas is acetylene, methane or ethylene. When high-temperature carbon coating is performed, the heating temperature and the holding time are controlled within the above ranges, so that the carbon coating process can be smoothly performed. Acetylene, methane and ethylene are all organic gases with high carbon content, and the acetylene, methane and ethylene are used as carbon-coated gas atmosphere, so that the carbon coating efficiency can be improved, and the finally obtained core-shell structure Si/C material has good electrochemical performance.
(6) The Fe content obtained in the step (5)3O4Washing the Si/C powder with excessive acid to remove Fe element, separating and drying to obtain the Si/C material with the core-shell structure, wherein the acid is preferably acid such as dilute hydrochloric acid, dilute nitric acid or dilute sulfuric acid, and the separation method is centrifugation, suction filtration or other solid-liquid separation methods; the drying process is carried out under the conditions of air, inert atmosphere or vacuum and the like, and the drying temperature is 30-150 ℃, preferably 80 ℃. Excessive dilute hydrochloric acid, dilute nitric acid or dilute sulfuric acid can quickly and effectively react with ferric oxide, and the drying temperature is limited in the range by combining methods such as centrifugation, suction filtration and the like, so that the iron element in the powder can be removed, and the pure and dry core-shell structure Si/C material can be obtained.
The general carbon coating process needs high temperature and complex professional CVD setting, and the experiment mainly depends on Fe3O4The carbon layer, Fe, can be successfully coated on the Si surface without high-concentration acetylene and a special CVD device3O4The carbon layer is formed more easily by the catalytic property of (C) and the low concentration of C is solved2H2The gas is difficult to deposit on the surface of Si directly, and the obtained Si/C material with the core-shell structure has excellent electrochemical performance due to Fe3O4Nano meterThe particles are easy to remove, the safety and operability of the preparation process are improved, and the test is simplified.
In another specific embodiment of the present invention, the invention in step (6) further provides a preparation method of the above-mentioned Si/C material with core-shell structure, where Fe (NO) is used3)3And replaced by manganese salt, nickel salt or other iron salt. Because the metals of iron, manganese or nickel have good catalytic performance and strong catalytic action on the growth of carbon, acetylene can be promoted to decompose, and a coating carbon layer is formed on the surface of Si, Fe (NO) is added3)3The material is replaced by manganese salt, nickel salt or other iron salts, and the core-shell structure Si/C material with good electrochemical performance can also be prepared.
The invention also provides the core-shell structure Si/C material prepared by the preparation method of the core-shell structure Si/C material.
The invention also provides an application of the core-shell structure Si/C material prepared by the preparation method of the core-shell structure Si/C material as a negative electrode material. Wherein the negative electrode material is for a secondary battery.
To better explain the invention, several specific examples are given below:
example 1
Weighing 100mg of nano Si powder for later use, and weighing proper amount of Fe (NO)3)3Dissolving in 20mL of deionized water; adding the weighed nano Si powder into Fe (NO)3)3In solution, and Fe: adding 3mL of absolute ethyl alcohol to help the nano Si powder to disperse, and ultrasonically stirring and uniformly dispersing to form a suspension; pouring the suspension into a plastic container, adding clean liquid nitrogen, and freezing to obtain ice state Si/Fe (NO)3)3(ii) a Mixing the above ice state Si/Fe (NO)3)3Freeze-drying in a pre-cooled freeze dryer, starting a vacuum pump, and freeze-drying at-70 deg.C under vacuum degree of 1Pa for 48 hr; taking out the freeze-dried sample, and putting the sample into a tube furnace, N2Heating to 400 ℃ under the atmosphere, and N2Atmosphere is changed to C2H2The atmosphere is kept warm for 40min and then N is changed2Is cooled downCooling to room temperature; and taking out the obtained powder, washing the powder with excessive dilute hydrochloric acid, centrifuging, and drying at 80 ℃ to obtain the Si/C material with the core-shell structure.
Example 2
Weighing 100mg of nano Si powder for later use, and weighing proper amount of Fe (NO)3)3Dissolving in 20mL of deionized water; adding the weighed nano Si powder into Fe (NO)3)3In solution, and Fe: adding 3mL of absolute ethyl alcohol to help the nano Si powder to disperse, and ultrasonically stirring and uniformly dispersing to form a suspension; pouring the suspension into a plastic container, adding clean liquid nitrogen, and freezing to obtain ice state Si/Fe (NO)3)3(ii) a Mixing the above ice state Si/Fe (NO)3)3Freeze-drying in a pre-cooled freeze dryer, starting a vacuum pump, and freeze-drying at-70 deg.C under vacuum degree of 1Pa for 48 hr; taking out the freeze-dried sample, and putting the sample into a tube furnace, N2Heating to 500 ℃ under the atmosphere, N2Atmosphere is changed to C2H2The atmosphere is kept warm for 40min and then N is changed2Cooling to room temperature; and taking out the obtained powder, washing the powder with excessive dilute hydrochloric acid, centrifuging, and drying at 80 ℃ to obtain the Si/C material with the core-shell structure.
Example 3
Weighing 100mg of nano Si powder for later use, and weighing proper amount of Fe (NO)3)3Dissolving in 20mL of deionized water; adding the weighed nano Si powder into Fe (NO)3)3In solution, and Fe: adding 3mL of absolute ethyl alcohol to help the nano Si powder to disperse, and ultrasonically stirring and uniformly dispersing to form a suspension; pouring the suspension into a plastic container, adding clean liquid nitrogen, and freezing to obtain ice state Si/Fe (NO)3)3(ii) a Mixing the above ice state Si/Fe (NO)3)3Freeze-drying in a pre-cooled freeze dryer, starting a vacuum pump, and freeze-drying at-70 deg.C under vacuum degree of 1Pa for 48 hr; taking out the freeze-dried sample, and putting the sample into a tube furnace, N2Raising the temperature to 600 ℃ under the atmosphere, and N2Atmosphere is changed to C2H2The atmosphere is kept warm for 40min and then N is changed2Cooling to room temperature; taking out the obtained powderWashing with excessive dilute hydrochloric acid, centrifuging, and drying at 80 ℃ to obtain the core-shell structure Si/C material.
Example 4
Weighing 100mg of nano Si powder for later use, and weighing proper amount of Fe (NO)3)3Dissolving in 20mL of deionized water; adding the weighed nano Si powder into Fe (NO)3)3In solution, and Fe: adding 3mL of absolute ethyl alcohol to help the nano Si powder to disperse, and ultrasonically stirring and uniformly dispersing to form a suspension; pouring the suspension into a plastic container, adding clean liquid nitrogen, and freezing to obtain ice state Si/Fe (NO)3)3(ii) a Mixing the above ice state Si/Fe (NO)3)3Freeze-drying in a pre-cooled freeze dryer, starting a vacuum pump, and freeze-drying at-70 deg.C under vacuum degree of 1Pa for 48 hr; taking out the freeze-dried sample, and putting the sample into a tube furnace, N2Heating to 700 ℃ under the atmosphere, and N2Atmosphere is changed to C2H2The atmosphere is kept warm for 40min and then N is changed2Cooling to room temperature; and taking out the obtained powder, washing the powder with excessive dilute hydrochloric acid, centrifuging, and drying at 80 ℃ to obtain the Si/C material with the core-shell structure.
Example 5
Weighing 100mg of nano Si powder for later use, and weighing proper amount of Fe (NO)3)3Dissolving in 20mL of deionized water; adding the weighed nano Si powder into Fe (NO)3)3In solution, and Fe: adding 3mL of absolute ethyl alcohol into the Si powder at a mass ratio of 1:1 to help the nano Si powder to disperse, and ultrasonically stirring and uniformly dispersing to form a suspension; pouring the suspension into a plastic container, adding clean liquid nitrogen, and freezing to obtain ice state Si/Fe (NO)3)3(ii) a Mixing the above ice state Si/Fe (NO)3)3Freeze-drying in a pre-cooled freeze dryer, starting a vacuum pump, and freeze-drying at-20 deg.C under 50Pa for 10 hr; taking out the freeze-dried sample, and putting the sample into a tube furnace, N2Raising the temperature to 1000 ℃ under the atmosphere, and N2The atmosphere is changed into methane atmosphere, the temperature is kept for 10min, and then N is changed back2Cooling to room temperature; taking out the obtained powder, washing with excessive dilute hydrochloric acid, centrifuging, and drying at 150 deg.CAnd drying to obtain the Si/C material with the core-shell structure.
Example 6
Weighing 100mg of nano Si powder for later use, and weighing proper amount of Fe (NO)3)3Dissolving in 20mL of deionized water; adding the weighed nano Si powder into Fe (NO)3)3In solution, and Fe: adding 3mL of absolute ethyl alcohol into the Si powder at a mass ratio of 5:1 to help the nano Si powder to disperse, and ultrasonically stirring and uniformly dispersing to form a suspension; pouring the suspension into a plastic container, adding clean liquid nitrogen, and freezing to obtain ice state Si/Fe (NO)3)3(ii) a Mixing the above ice state Si/Fe (NO)3)3Freeze-drying in a pre-cooled freeze dryer, starting a vacuum pump, and freeze-drying at the well cooling temperature of 30 ℃ and the vacuum degree of 5Pa for 100 h; taking out the freeze-dried sample, and putting the sample into a tube furnace, N2Heating to 300 ℃ under the atmosphere, and N2The atmosphere is changed into the ethylene atmosphere, the temperature is kept for 10min, and then N is changed back2Cooling to room temperature; and taking out the obtained powder, washing the powder with excessive dilute sulfuric acid, centrifuging, and drying at 30 ℃ to obtain the Si/C material with the core-shell structure.
60% of the prepared core-shell structure Si/C material, 20% of superconducting carbon black (super P) and 20% of binder (sodium alginate) are sequentially weighed, placed in agate grinding, fully ground and uniformly dispersed, and a proper amount of water is added in the grinding process to form slurry which is moderate in viscosity and easy to coat. And uniformly coating the slurry on the treated clean copper foil, moving to 80 ℃, standing for 12 hours, drying, and cutting into a wafer with the diameter of 8mm by using a slicing machine for later use.
In order to test the performance of the electrode material in the above embodiment, a secondary battery was prepared by using the above electrode material as a positive electrode plate of a half-battery (the electrode material is a negative electrode material in actual use, and the battery used for the test is a half-battery in the test stage of preparing the secondary battery, so it is called a positive electrode material), and the performance of the electrode plate was judged by testing the cycle performance and rate performance of the secondary battery.
The electrode material pole piece in the embodiment is made into a CR2032 type button secondary battery, and the secondary battery comprises a positive pole piece, a negative pole piece, a diaphragm, a non-aqueous electrolyte and a battery case; positive electrode baseThe sheet was the electrode sheet prepared in the above example; the negative pole piece is metal lithium; the diaphragm comprises one of an aramid diaphragm, a non-woven fabric diaphragm, a polyethylene microporous film, a polypropylene-polyethylene double-layer or three-layer composite film and a ceramic coating diaphragm thereof; the electrolyte comprises an electrolyte and a solvent; the electrolyte is LiPF6、LiBF4、LiClO4、LiAsF6、LiCF3SO3、LiN(CF3SO2) At least one of LiBOB, LiCl, LiBr and LiI or a mixture thereof; the solvent comprises at least one or more of Propylene Carbonate (PC), dimethyl carbonate (DMC), Ethyl Methyl Carbonate (EMC), 1, 2-Dimethoxyethane (DME), ethylene carbonate, propylene carbonate, butylene carbonate, diethyl carbonate, methyl propyl carbonate, acetonitrile, ethyl acetate and ethylene sulfite.
Analysis of Experimental results
FIG. 3(a) shows XRD patterns of Si/C materials prepared at different temperatures in examples 1-4, the diffraction peak positions of the Si/C materials correspond to crystal faces (111), (220), (311), (400) and (311), respectively, the diffraction patterns correspond to cubic system of Si (JCPDS card number: 00-005-) -0565) after being compared with JCPDS card (XRD standard card), and 2 theta 28 in the diffraction peak curves of samples at 600 ℃ and 700 DEG CoHas a diffraction peak of one carbon, which shows that C is formed at a temperature of 600 ℃ or higher2H2The decomposition product begins to generate a carbon diffraction peak, the carbon content of the Si/C material increases along with the increase of the temperature, and the crystallinity is enhanced. The carbon diffraction peak is weak, and most of the carbon in the Si/C material is amorphous carbon. The XRD patterns of the Si/C materials prepared at different temperatures are compared, and the comparison is known along with the C2H2The decomposition temperature is increased, the position and the intensity of the diffraction peak of Si in the Si/C material are not greatly changed, the diffraction peak of carbon gradually appears, and the content and the crystallinity of the carbon in the Si/C sample are gradually increased.
In order to investigate the effect of different temperatures on the electrochemical performance of the prepared Si/C materials, this experiment performed electrochemical tests on the materials obtained at the respective temperatures corresponding to examples 1-4. FIG. 3(a) shows the cycling performance of Si/C materials prepared at different temperatures with a current density of 500mA g–1The voltage window is0.01-2V. For convenience of labeling, the temperature of 400 ℃, 500 ℃, 600 ℃ and 700 ℃ are respectively abbreviated as Si/C-400, Si/C-500, Si/C-600 and Si/C-700 in the invention. FIG. 3(b) shows that the first-turn discharge capacity of Si/C-400 is 879.2mA h g–1The first charge capacity is 365.6mA h g–1The first coulombic efficiency was 42%, and the first irreversible capacity was mainly attributed to the formation of the SEI film. The first-turn discharge capacity of Si/C-500 is 1329.3mA hg–1The first charge capacity is 769.8mA h g–1The first coulombic efficiency was 58%. The first-turn discharge capacity of Si/C-600 is 1486.2mA hg–1The first charge capacity is 816.7mA h g–1The first coulombic efficiency was 55%. The first-turn discharge capacity of Si/C-700 is 1646mA h g–1The first charge capacity is 973.1mA h g–1The first coulombic efficiency was 59%. It can be known that the reversible capacity of the Si/C sample is very low at a lower temperature, the reversible capacity of the Si/C material is gradually increased along with the increase of the temperature, the reversible capacity of the Si/C material has little difference between 500 ℃ and 600 ℃, the temperature is further increased to 700 ℃, and the reversible capacity is further increased.
Si/C-700 sample at 500mA g–1Discharging capacity 1006mA h g after 150 circles of charging and discharging circulation under current density–1Charge capacity 998.9mA h g–1Coulombic efficiency 99%. The black curve in the graph corresponds to the coulombic efficiency of the Si/C-700 material, and it can be seen that after the first efficiency is removed, the coulombic efficiency rapidly increases to nearly 100% as the charge-discharge cycle progresses, and remains stable. The Si/C-400, Si/C-500 and Si/C-600 samples were measured at 500mA g–1The discharge capacity after 150 cycles of circulation under the current density is 332.6mA h g–1、784mA h g–1And 785.5mA h g–1The charging capacity is 329.1mA h g–1、778.2mA h g–1And 778.2mA h g–1Coulombic efficiency is also close to 100%. It is obvious from the cycle curve in the figure that the cycle performances of Si/C-400, Si/C-500 and Si/C-600 are not much different, the cycle curve shows that the stability is slightly poor due to the higher initial capacity of Si/C-700, and the reversible capacity tends to be stable after 60 charge-discharge cycles. The Si/C-500 and Si/C-700 samples have an electrochemical activation process in the early stage of the charge-discharge cycle and all capacitiesAnd (4) increasing. In conclusion, Si/C-500 has similar reversible capacity and cycle performance to Si/C-600, but the Si/C-500 material is unstable in the early stage of charge-discharge cycle, needs an electrochemical activation process, and Si/C-700 has higher capacity but is unstable in the early stage of charge-discharge cycle, and moreover, for C2H2700 ℃ is slightly too high.
FIG. 3(C) shows the first-turn charge-discharge curves of four samples of Si/C-400, Si/C-500, Si/C-600 and Si/C-700. the Si/C-400 sample has a capacity of 0.1V or more which is not much different from that of the Si/C-500 and Si/C-600 samples, but the voltage interval of 0.1V or less is much lower than that of the former two samples, which indicates that the Si capacity of Si/C-400 is not completely released, and that C at 400 ℃ is not completely released, indicating that C at 400 DEG C2H2The decomposed carbon has an inhibitory effect on the activity of Si. The voltage interval capacity of the Si/C-700 sample of 0.1V or less is not much different from that of the Si/C-500 and Si/C-600 samples, but the voltage interval capacity of 0.1V or more is higher than that of the two samples, and it is found that the increased capacity of the Si/C-700 sample is mainly derived from the voltage interval of 0.1V or more, does not belong to the main contribution voltage interval of Si, is mainly derived from pyrolytic carbon, and it is found that C at 700 ℃ is a C2H2The carbon produced by the decomposition has a higher capacity contribution.
The electrochemical performance of the Si/C-400 sample is much different from the other three samples, and the next discussion is presented herein for the three samples Si/C-500, Si/C-600, and Si/C-700. FIG. 3(d) is an alternating current impedance spectrum (EIS) of three materials, Si/C-500, Si/C-600 and Si/C-700, whose equivalent circuits were obtained by fitting the data (solid squares in the figure represent raw data and solid circles represent fitted data), as shown in FIG. 2, Re represents the internal resistance of the cell, Rct is from the interface charge reaction, CPE represents the non-ideal electric double layer behavior of the surface film region during lithium ion migration, and W represents the Warburg impedance controlled by lithium ion diffusion. As shown in Table 1, the three samples showed similar Re values, and the Si/C-10 sample had a relatively large Rct value of 100. omega. cm–2The Rct values for both the Si/C-500 and Si/C-600 samples were 65. omega. cm–2It is known that the large electrochemical resistance of Si/C-700 is a cause of poor early cycle stability.
TABLE 1 fitting impedance parameters before charging and discharging of Si/C materials prepared at different temperatures
FIGS. 4(a) - (f) are Scanning Electron Microscope (SEM) pictures of core-shell Si/C materials prepared at different temperatures, and it can be seen that the particle sizes of the Si/C materials obtained at 500-700 deg.C are similar and are between 50-200 nm. SEM pictures (fig. 4(a) and (b)) of the Si/C-500 sample compared to pure Si powder, the particles were slightly larger and there was carbon connectivity between the particles, indicating that the carbon layer of the outer layer of the nano Si powder was successfully coated, but at a lower temperature, with a larger amount of carbon between the particles. When the reaction temperature is raised to 600 ℃, the particle Si/C particle size does not change much, however the carbon between the particles disappears, becoming the Si/C material of the individual particles (see fig. 4(C) and (d)). FIGS. 4(e) and (f) are SEM pictures of Si/C-700 samples, and it can be seen that the particle size is comparable to that of Si/C-600, but Si/C particles are partially aggregated together, and the free Si/C particle structure is partially destroyed due to the excessive temperature and the too fast carbon growth.
In order to further clarify the internal appearance of the Si/C material with the core-shell structure, the invention respectively carries out transmission electron microscope tests on three samples of Si/C-500, Si/C-600 and Si/C-700. FIG. 5(a) is a TEM image of a Si/C-500 sample, which shows that the core-shell structure of the Si/C material has been successfully prepared, and graphene layers are connected among Si/C particles, thereby illustrating that iron catalyzes C at 500 ℃2H2The decomposition of the graphene generates a part of graphene which is coated on the Si/C structure, and the cyclic stability of the Si/C material is improved. The presence of carbon between Si/C particles in TEM was consistent with SEM testing. FIG. 5(b) shows a High Resolution Transmission Electron Microscope (HRTEM) image of the Si/C-500 sample, in which a carbon layer is clearly visible on the Si surface, further illustrating the presence of a core-shell structure, in which the lattice fringe spacing of Si is 0.31nm, corresponding to the (111) crystal plane.
FIG. 5(C) shows TEM image of Si/C-600, core-shell structure of Si/C material is more obvious compared with Si/C-500, however, graphene between Si/C particles disappears, but illustrates that temperature increase is favorable for formation of core-shell structure of Si/C, C2H2Increased decomposition rate and difficulty in shape formationInto a thin graphene layer. FIG. 5(d) is an HRTEM image of Si/C-600 showing a uniform carbon layer on the surface of the Si particle, in which the Si has a lattice fringe spacing of 0.31nm, corresponding to the (111) plane. As the temperature was increased further to 700 ℃, the structure of the Si/C material remained (FIG. 5 (e)). FIG. 5(f) is an HRTEM image of Si/C-700 in which the Si lattice fringe spacing is 0.31nm and corresponds to the (111) plane, and the carbon layer is still clearly visible.
FIG. 6(a) shows Si/C-600 at 200mA g with pure Si powder–1The first discharge capacity of Si/C-600 is 1872.9mA h g–1The first charge capacity is 1060.3mA h g–1The first coulombic efficiency was 57%, and the first turn of irreversible capacity was mainly attributed to the formation of the SEI film. The first discharge capacity of the pure Si powder is 1483.1mA h g–1The first charge capacity is 895mA h g–1The first coulombic efficiency was 60%. As can be seen, the reversible capacity of Si/C-600 is higher than that of pure Si powder, and the difference of the first coulombic efficiency is not much. The discharge capacity of the Si/C-600 after 100 charge-discharge cycles is 1020.6mA hr g–1The charging capacity is 1007.9mA h g–1Coulombic efficiency 99%. The discharge capacity of the Si/C-600 after 100 charge-discharge cycles is 670.2mA hr g–1The charging capacity is 663mA h g–1Coulombic efficiency 99%. As can be seen, Si/C-600 is much better than pure Si powder in the aspects of reversible capacity and cycle performance, and the core-shell structure designed by the experiment plays a role in increasing the cycle stability of the Si-based material. The black curve in fig. 6(a) corresponds to the coulombic efficiency of the Si/C-600 material, and it can be seen that the coulombic efficiency rapidly increases by nearly 100% after the first cycle of charge-discharge.
Fig. 6(b) is a first-turn charge-discharge curve of the Si/C material and the pure Si material, and it can be seen that the main capacity contribution is in the same voltage interval, which proves that the content source in the Si/C material is mainly Si component, and the carbon layer in the core-shell structure mainly functions to stabilize the SEI film and buffer the volume effect of the Si-based negative electrode material.
Fig. 6(C) is an alternating current impedance spectrum (EIS) of three materials of Si/C-600 and pure Si powder, an equivalent circuit thereof is obtained by fitting data (solid squares in the figure represent original data, solid circles represent fitted data), as shown in fig. 2, Re represents the internal resistance of the battery, Rct is derived from the interface charge reaction, CPE represents the non-ideal electric double layer behavior of the surface film region during lithium ion migration, and W represents Warburg impedance controlled by lithium ion diffusion. Table 2 shows the fitting result of the alternating current impedance spectrum, and the Si/C-600 and pure Si powder electrodes have similar Re and Rct, which shows that the internal resistance and the electrochemical impedance of the electrodes are similar, and the improvement of the cycle performance mainly comes from the advantages of the core-shell structure.
TABLE 2 AC impedance fitting results for Si/C materials and pure Si powder electrodes
FIG. 6(d) shows the rate capability of Si/C-600 material, the present invention is at 200mA g–1、500mA g–1And 1000mAg–1The current densities of (A) and (B) were subjected to charge-discharge cycle tests, and it was found that the reversible capacity gradually decreased with the increase of the current densities, with 200mA g–1、500mA g–1And 1000mAg–1The first discharge capacity of Si/C-600 at the current density of (A) was 1872.9mA h g–1、1486.2mA h g–1And 853.3mA g–1. After circulating for 100 times under three multiplying powers, the reversible capacity is stabilized at 1020.6mA h g–1、850.2mA h g–1And 517.2mA g–1. The cycling curves of Si/C-600 at all three rates are stable, but the capacity at high rate is smaller, and further improvement is needed.
It will be understood by those skilled in the art that the foregoing is only a preferred embodiment of the present invention, and is not intended to limit the invention, and that any modification, equivalent replacement, or improvement made within the spirit and principle of the present invention should be included in the scope of the present invention.
Claims (9)
1. A preparation method of a core-shell structure Si/C material is characterized by comprising the following steps:
(1) fe (NO) to be prepared3)3Dissolving in deionized water to obtain Fe (NO)3)3Solution, and weighing Si powder for later use; the Si powder is nano Si powder or micron Si powder, and the mass ratio of Fe to Si is 5: 1-1: 1;
(2) adding the weighed Si powder into Fe (NO)3)3Adding a dispersing agent into the solution, and ultrasonically stirring to uniformly disperse the Si powder to form a suspension;
(3) pouring the suspension obtained in the step (2) into a low temperature resistant container, pouring clean liquid nitrogen into the suspension for freezing to form ice state Si/Fe (NO)3)3；
(4) Mixing the ice state Si/Fe (NO) obtained in the step (3)3)3Lyophilizing under vacuum to obtain lyophilized Si/Fe (NO)3)3A sample;
(5) taking out the lyophilized Si/Fe (NO)3)3Heating the sample, firstly raising the temperature under an inert atmosphere, then changing the inert atmosphere into an organic gas and preserving the temperature, wherein Fe (NO) is used3)3In situ generation of Fe3O4The nano particles catalyze organic gas to form a carbon layer on the surface of Si, and then the carbon layer is replaced by inert atmosphere and cooled to room temperature to obtain the Fe-containing nano particles3O4Si/C powder of (1);
(6) the Fe content obtained in the step (5)3O4Washing the Si/C powder with excessive acid to remove Fe element, and separating and drying to obtain the Si/C material with the core-shell structure.
2. The preparation method of the core-shell structure Si/C material according to claim 1, wherein in the step (2), the dispersing agent is a nonpolar liquid which is absolute ethyl alcohol.
3. The method for preparing the Si/C material with the core-shell structure as recited in claim 1, wherein in the step (3), the low temperature resistant container is a plastic container.
4. The preparation method of the core-shell structure Si/C material according to claim 1, wherein in the step (4), the pre-cooled freeze-drying is performed in a freeze-dryer, the freeze-drying temperature is less than or equal to 30 ℃, the freeze-drying time is 10h to 100h, and the vacuum degree is less than 50 Pa.
5. The preparation method of the core-shell structure Si/C material according to claim 1, wherein the heating process in the step (5) is performed in a tubular furnace, the heating temperature is 300-1000 ℃, the holding time is 10-80 min, the organic gas is acetylene, methane or ethylene, and the concentration of the organic gas is less than 20%.
6. The preparation method of the core-shell structure Si/C material according to claim 1, wherein the acid used in the step (6) is dilute hydrochloric acid, dilute nitric acid or dilute sulfuric acid, and the separation method is centrifugation, suction filtration or other solid-liquid separation methods; the drying process is carried out in air, inert atmosphere or vacuum, and the drying temperature is 30-150 ℃.
7. Preparation method of the core-shell structure Si/C material according to any one of claims 1 to 6, characterized in that Fe (NO) is used3)3And replaced by manganese salt, nickel salt or other iron salt.
8. The core-shell structure Si/C material is prepared by the preparation method of the core-shell structure Si/C material according to any one of claims 1 to 7.
9. The core-shell structure Si/C material prepared by the preparation method of the core-shell structure Si/C material according to any one of claims 1 to 7 is applied as a negative electrode material.
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