CN111668474A - Negative electrode material, preparation method thereof and secondary battery - Google Patents
Negative electrode material, preparation method thereof and secondary battery Download PDFInfo
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- CN111668474A CN111668474A CN202010374125.6A CN202010374125A CN111668474A CN 111668474 A CN111668474 A CN 111668474A CN 202010374125 A CN202010374125 A CN 202010374125A CN 111668474 A CN111668474 A CN 111668474A
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- H—ELECTRICITY
- H01—ELECTRIC ELEMENTS
- H01M—PROCESSES OR MEANS, e.g. BATTERIES, FOR THE DIRECT CONVERSION OF CHEMICAL ENERGY INTO ELECTRICAL ENERGY
- H01M4/00—Electrodes
- H01M4/02—Electrodes composed of, or comprising, active material
- H01M4/36—Selection of substances as active materials, active masses, active liquids
- H01M4/38—Selection of substances as active materials, active masses, active liquids of elements or alloys
- H01M4/386—Silicon or alloys based on silicon
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- B—PERFORMING OPERATIONS; TRANSPORTING
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- H—ELECTRICITY
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- H01M10/00—Secondary cells; Manufacture thereof
- H01M10/05—Accumulators with non-aqueous electrolyte
- H01M10/052—Li-accumulators
- H01M10/0525—Rocking-chair batteries, i.e. batteries with lithium insertion or intercalation in both electrodes; Lithium-ion batteries
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- H01M4/62—Selection of inactive substances as ingredients for active masses, e.g. binders, fillers
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- H01M4/625—Carbon or graphite
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- H—ELECTRICITY
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- H01M—PROCESSES OR MEANS, e.g. BATTERIES, FOR THE DIRECT CONVERSION OF CHEMICAL ENERGY INTO ELECTRICAL ENERGY
- H01M4/00—Electrodes
- H01M4/02—Electrodes composed of, or comprising, active material
- H01M2004/026—Electrodes composed of, or comprising, active material characterised by the polarity
- H01M2004/027—Negative electrodes
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- Y02E60/00—Enabling technologies; Technologies with a potential or indirect contribution to GHG emissions mitigation
- Y02E60/10—Energy storage using batteries
Abstract
The invention belongs to the technical field of synthesis of battery materials, and particularly relates to a negative electrode material, a preparation method thereof and a secondary battery. The cathode material of the invention takes graphite as a core structure, a silicon layer, a carbon coating layer and carbon nano tubes are sequentially arranged on the surface of the core structure along the direction far away from the core structure, and the length-diameter ratio of the carbon nano tubes is more than 3000. According to the cathode material, the carbon nano tube with the high length-diameter ratio is arranged at the outermost part, so that the conductivity can be improved, and a 'cage' structure can be formed during mixing and stirring so as to buffer the volume change of the silicon layer. Therefore, the cathode material has the advantages of high coulombic efficiency and specific capacity for the first time, low irreversible capacity, good cycle stability and good conductivity.
Description
Technical Field
The invention belongs to the technical field of synthesis of battery materials, and particularly relates to a negative electrode material and a preparation method thereof, and a secondary battery.
Background
At present, graphite carbon materials are mainly used as the negative electrode materials of commercial lithium ion batteries, and the graphite carbon materials have the advantages of low lithium intercalation/deintercalation potential, proper reversible capacity, rich resources, low price and the like, so the graphite carbon materials are relatively ideal negative electrode materials of the lithium ion batteries. However, the theoretical specific capacity of the graphite is only 372mAh/g, which limits the further improvement of the energy density of the lithium ion battery and is difficult to meet the increasingly improved requirements of power batteries and the like on the energy density of the lithium ion battery.
Silicon is one of the most promising materials for the next generation of lithium ion battery negative electrode materials because it has the advantages of a theoretical specific capacity as high as 4200mAh/g, a smooth discharge platform similar to graphite, and a wide source. However, the volume of the silicon material is changed greatly in the lithium ion intercalation/deintercalation process, and the drastic change of the volume easily causes the collapse of the material structure, even the electrode material is peeled off from the current collector, so that the electrode material loses electric contact, and the cycle performance of the electrode is reduced sharply or even fails. Research shows that when the grain size of the silicon material is reduced to the nanometer level, the volume effect in the charging and discharging process is greatly weakened, the electrochemical performance is improved, but the nanometer material has larger surface energy and is easy to agglomerate, so that the charging and discharging efficiency is reduced, the capacity attenuation is accelerated, and the advantages of the nanometer particles are weakened. In addition, silicon and silicon monoxide have poor conductivity, cannot be directly used, and are often required to be mixed with a conductive material for use. The current trend in the research for improving the performance of silicon negative electrodes is to prepare composite materials or alloys of silicon and other materials, wherein silicon/graphite composite materials prepared by combining the good conductivity of graphite, the structural cycling stability and the high specific capacity characteristic of silicon show great application prospects.
Disclosure of Invention
The invention aims to provide a negative electrode material, a preparation method thereof and a secondary battery, and aims to solve the technical problems of low specific capacity, high capacity attenuation, poor cycle stability and the like of the conventional negative electrode material.
In order to achieve the purpose of the invention, the technical scheme adopted by the invention is as follows:
the invention provides a preparation method of a negative electrode material, which comprises the following steps:
providing graphite and silicon powder;
mixing the graphite and the silicon powder, and coating the silicon powder on the surface of the graphite to obtain a silicon/graphite composite material;
carrying out carbon coating treatment on the silicon/graphite composite material, and preparing a carbon coating layer on the surface of the silicon/graphite composite material to obtain a carbon-coated silicon/graphite composite material;
and growing a carbon nano tube with the length-diameter ratio of more than 3000 on the surface of the carbon-coated silicon/graphite composite material in situ to obtain the cathode material.
As a preferred technical scheme of the invention, the method for in-situ growing the carbon nano tube with the length-diameter ratio more than 3000 on the surface of the carbon-coated silicon/graphite composite material comprises the following steps: mixing the carbon-coated silicon/graphite composite material with a catalyst, heating to 200-1800 ℃ at the speed of 1-20 ℃/min under a protective atmosphere, preserving the temperature for 0.5-8 h, adding a carbon source, depositing for 1-20 h, and introducing inert gas for 1-10 h.
As a further preferable technical scheme of the invention, the mass ratio of the carbon-coated silicon/graphite composite material to the catalyst is 1000 (1-10).
In a further preferred embodiment of the present invention, the carbon source is at least one selected from the group consisting of methane, acetylene, ethylene, propylene and benzene.
As a preferable technical solution of the present invention, the catalyst is at least one selected from elemental iron, elemental cobalt, and elemental nickel.
As a preferable technical scheme of the invention, the median particle size of the catalyst is less than or equal to 300 nm.
In a preferred embodiment of the present invention, the graphite is at least one selected from natural graphite, artificial graphite, and mesocarbon microbeads.
In a preferred embodiment of the present invention, the graphite has an average particle diameter of 5 to 30 μm.
As a preferred technical scheme of the invention, the median particle size of the silicon powder is less than or equal to 300 nm.
In a preferred embodiment of the present invention, in the step of mixing the graphite and the silicon powder, the mass ratio of the graphite to the silicon powder is 100 (1-10).
In a preferred embodiment of the present invention, a surfactant is added in the step of mixing the graphite and the silicon powder.
As a further preferable technical scheme of the invention, the mass ratio of the graphite, the silicon powder and the surfactant is 100 (1-10) to (1-5).
In a further preferred embodiment of the present invention, the surfactant is at least one selected from polyvinylpyrrolidone, sodium dodecylbenzenesulfonate and polyethylene glycol.
In a preferred embodiment of the present invention, in the step of carbon-coating the silicon/graphite composite material, the carbon source for carbon-coating is at least one selected from pitch, phenol resin, glucose, citric acid, and starch.
As a preferred technical solution of the present invention, in the step of performing carbon coating treatment on the silicon/graphite composite material, the temperature of the carbon coating treatment is 600 ℃ to 1200 ℃.
As a preferable technical solution of the present invention, in the step of performing carbon coating treatment on the silicon/graphite composite material, the time of the carbon coating treatment is 0.5h to 6 h.
As a preferred technical scheme of the invention, in the step of carrying out carbon coating treatment on the silicon/graphite composite material, the heating rate of the carbon coating treatment is 1-20 ℃/min.
As a preferred technical solution of the present invention, in the step of performing carbon coating treatment on the silicon/graphite composite material, the carbon coating amount of the carbon coating treatment is 0.5% to 10%.
The invention provides a cathode material, which takes graphite as a core structure, wherein a silicon layer, a carbon coating layer and carbon nanotubes are sequentially arranged on the surface of the core structure along the direction far away from the core structure, and the length-diameter ratio of the carbon nanotubes is more than 3000.
In a preferred embodiment of the present invention, the graphite is at least one selected from natural graphite, artificial graphite, and mesocarbon microbeads.
In a preferred embodiment of the present invention, the graphite has an average particle diameter of 5 μm to 30 μm.
In a preferred embodiment of the present invention, the mass ratio of the graphite to the silicon layer is 100 (1-10).
As a preferable technical scheme of the invention, the thickness of the carbon coating layer is 1nm-20 nm.
The invention further provides a secondary battery, which comprises the negative electrode material prepared by the preparation method of the negative electrode material or the negative electrode material.
According to the preparation method of the cathode material, firstly, silicon powder is wrapped on the surface of graphite, and the graphite can support the silicon powder, so that the volume expansion effect of the silicon powder in the charging and discharging processes is relieved, the stability of the obtained cathode material in the charging and discharging processes is ensured, and the defect of low capacity of a single graphite cathode material can be overcome; secondly, the carbon coating layer is formed by performing carbon coating treatment on the silicon/graphite composite material, so that direct contact between silicon powder and electrolyte can be avoided, lithium consumption in the electrolyte is reduced, the problem of poor circulation caused by introduction of silicon is solved, and meanwhile, the volume expansion effect of the silicon powder is further limited by limiting the silicon powder in the carbon coating layer, so that the improvement of the circulation performance of the silicon is facilitated; finally, by growing the carbon nano tube with the length-diameter ratio larger than 3000 on the surface of the carbon-coated silicon/graphite composite material in situ, the conductivity of the obtained cathode material is obviously improved, and the defect of weak conductivity of silicon is overcome, meanwhile, the carbon nano tube has a large length-diameter ratio and is easy to intertwine, so that line-to-line contact can be formed among cathode material particles, electrical connection is increased, a flexible cage structure can be formed on the outermost layer of the cathode material particles in the slurry mixing and stirring process, the buffer is provided for the volume expansion effect of silicon, the stress caused by the volume expansion effect of silicon powder in the charging and discharging process to the cathode material particles is further reduced, and the cycle performance of the obtained cathode material is ensured.
The cathode material provided by the invention is a composite material of silicon powder and graphite, so that the specific capacity is higher; in addition, in the structure of the cathode material, graphite is used as a core structure, and the graphite and the carbon coating layer together sandwich the silicon layer, so that the volume expansion effect of the silicon layer in the charging and discharging processes can be limited, and the direct contact between the silicon layer and electrolyte can be avoided; in addition, the high length-diameter ratio carbon nano tube positioned at the outermost part of the cathode material can improve the conductivity and further reduce the volume expansion effect of the silicon layer. Therefore, the cathode material has the advantages of good conductivity, high first coulombic efficiency and specific capacity, low irreversible capacity and good cycling stability.
The secondary battery provided by the invention comprises the anode material, and has excellent charge-discharge efficiency and cycle performance.
Drawings
Fig. 1 is a schematic structural diagram of an anode material in one embodiment of the present invention.
Detailed Description
In order to make the objects, technical solutions and technical effects of the embodiments of the present invention clearer, the technical solutions in the embodiments of the present invention are clearly and completely described, and the embodiments described below are a part of the embodiments of the present invention, but not all of the embodiments. All other embodiments obtained by a person of ordinary skill in the art without any inventive step in connection with the embodiments of the present invention shall fall within the scope of protection of the present invention. Those whose specific conditions are not specified in the examples are carried out according to conventional conditions or conditions recommended by the manufacturer; the reagents or instruments used are not indicated by the manufacturer, and are all conventional products available commercially.
In the description of the present invention, it should be understood that the weight of the related components mentioned in the embodiments of the present invention may not only refer to the specific content of each component, but also represent the proportional relationship of the weight among the components, and therefore, it is within the scope of the disclosure that the content of the related components is scaled up or down according to the embodiments of the present invention. Specifically, the weight described in the embodiments of the present invention may be a unit of mass known in the chemical field such as μ g, mg, g, kg, etc.
In addition, unless the context clearly uses otherwise, an expression of a word in the singular is to be understood as including the plural of the word. The terms "comprises" or "comprising" are intended to specify the presence of stated features, quantities, steps, operations, elements, portions, or combinations thereof, but are not intended to preclude the presence or addition of one or more other features, quantities, steps, operations, elements, portions, or combinations thereof.
The embodiment of the invention provides a preparation method of a negative electrode material, which comprises the following steps:
s1, providing graphite and silicon powder;
s2, mixing graphite and silicon powder, and coating the silicon powder on the surface of the graphite to obtain a silicon/graphite composite material;
s3, performing carbon coating treatment on the silicon/graphite composite material, and preparing a carbon coating layer on the surface of the silicon/graphite composite material to obtain a carbon-coated silicon/graphite composite material;
s4, growing the carbon nano tube with the length-diameter ratio larger than 3000 on the surface of the carbon-coated silicon/graphite composite material in situ to obtain the cathode material.
According to the preparation method of the cathode material provided by the embodiment of the invention, firstly, silicon powder is wrapped on the surface of graphite, and the graphite can support the silicon powder, so that the volume expansion effect of the silicon powder in the charging and discharging processes is relieved, the stability of the obtained cathode material in the charging and discharging processes is ensured, and the defect of low capacity of a single graphite cathode material can be overcome; secondly, the carbon coating layer is formed by performing carbon coating treatment on the silicon/graphite composite material, so that direct contact between silicon powder and electrolyte can be avoided, lithium consumption in the electrolyte is reduced, the problem of poor circulation caused by introduction of silicon is solved, and meanwhile, the volume expansion effect of the silicon powder is further limited by limiting the silicon powder in the carbon coating layer, so that the improvement of the circulation performance of the silicon is facilitated; finally, by growing the carbon nano tube with the length-diameter ratio larger than 3000 on the surface of the carbon-coated silicon/graphite composite material in situ, the conductivity of the obtained cathode material is obviously improved, and the defect of weak conductivity of silicon is overcome, meanwhile, the carbon nano tube has a large length-diameter ratio and is easy to intertwine, so that line-to-line contact can be formed among cathode material particles, electrical connection is increased, a flexible cage structure can be formed on the outermost layer of the cathode material particles in the slurry mixing and stirring process, the buffer is provided for the volume expansion effect of silicon, the stress caused by the volume expansion effect of silicon powder in the charging and discharging process to the cathode material particles is further reduced, and the cycle performance of the obtained cathode material is ensured.
In some embodiments, the graphite in S1 is selected from at least one of natural graphite, artificial graphite, mesocarbon microbeads. The natural graphite, the artificial graphite and the mesocarbon microbeads have good conductivity and excellent cycle performance, but the specific capacity is relatively low, while the specific capacity of silicon is relatively high but the conductivity and the cycle performance are poor, so that the graphite material and the silicon are compounded in the embodiment of the invention, the advantages of the graphite material and the silicon can be played, the conductivity of the obtained negative electrode material can be improved, and the capacity and the cycle performance of the negative electrode material can be improved.
In some embodiments, graphite having an average particle size of 5 μm to 30 μm is selected as the starting material. The graphite in the particle size range is beneficial to enabling silicon powder to be better attached to the surface of the silicon layer to form the silicon layer, so that the supporting effect of the graphite material on the silicon layer is improved, and the volume expansion effect of the silicon layer is limited; and when the graphite particles in the particle size range are used for loading nano-sized silicon powder, the silicon powder can be prevented from agglomerating due to large size difference between the graphite particles and the silicon powder. Specifically, typical, but not limiting, graphite starting materials have an average particle size of 5 μm, 6 μm, 7 μm, 8 μm, 9 μm, 10 μm, 11 μm, 12 μm, 13 μm, 14 μm, 15 μm, 16 μm, 17 μm, 18 μm, 19 μm, 20 μm, 21 μm, 22 μm, 23 μm, 24 μm, 25 μm, 26 μm, 27 μm, 28 μm, 29 μm, 30 μm.
In some embodiments, silicon powder having a median particle size of 300nm or less is selected as the starting material. The silicon powder is nanoscale and uniform in size, and the volume effect is small; meanwhile, the silicon powder particles in the particle size range and the graphite particles with the average particle size of 5-30 mu m have large size difference, so the nano-scale silicon powder can be uniformly attached to the surface of the graphite and is not easy to agglomerate.
In S2, the graphite and the silicon powder are mixed to enable the silicon powder to be attached to the surface of the graphite to form the silicon/graphite composite material, the graphite can provide supporting force for the silicon powder to relieve the volume expansion effect of the silicon powder in the charging and discharging processes, and meanwhile, due to the existence of the silicon powder, the defect that the capacity of a single graphite cathode material is low is overcome. In some embodiments, the mass ratio of graphite to silicon powder is controlled to be 100 (1-10). Too high a proportion of silica fume can lead to serious particle agglomeration problems. Meanwhile, too many silicon powder particles can lead to too many silicon powder layers on the surface of the graphite, so that the graphite and the carbon coating layer are not favorable for effectively buffering the volume expansion of the silicon layer; if the proportion of the silicon powder is too low, the capacity of the obtained graphite cathode material is improved a little, and the effect is relatively unobvious. Specifically, typical, but not limiting, mass ratios between graphite and silicon powder are 100:1, 100:2, 100:3, 100:4, 100:5, 100:6, 100:7, 100:8, 100:9, 100: 10.
Preferably, when the graphite and the silicon powder are mixed, a surfactant is further added into the mixing system to improve the dispersibility of the silicon powder and the graphite.
Furthermore, the mass ratio of the graphite, the silicon powder and the surfactant is controlled to be 100 (1-10) to (1-5). The proportion of the surfactant is too high, so that too many impurities or inactive substances are easily introduced into a mixed system, and the capacity of the material is influenced; the ratio of the surfactant is too low, so that the nanoparticles are easy to agglomerate, and a uniformly dispersed mixed system is difficult to form.
Furthermore, at least one of polyvinylpyrrolidone, sodium dodecyl benzene sulfonate and polyethylene glycol with low cost and good dispersibility is used as a surfactant to further improve the dispersibility of the silicon powder and the graphite.
In one embodiment, the method for mixing graphite and silicon powder comprises the following steps: firstly, dissolving or dispersing a surfactant in an organic solvent, adding silicon powder, ultrasonically stirring and dispersing, adding graphite, stirring and mixing to form uniform slurry with the solid content of 20-40%, and finally, carrying out spray drying to obtain the graphite material with the silicon coated on the surface. Wherein the organic solvent is at least one of absolute ethyl alcohol, methanol, N-dimethylformamide, dimethyl sulfoxide and N-methylpyrrolidone. The conditions of spray drying were: the inlet temperature of the hot air is 200-300 ℃, and the outlet temperature is 40-110 ℃.
In S3, the silicon/graphite composite material is subjected to carbon coating treatment, so that the surface of the silicon/graphite composite material is coated with a carbon layer, and the carbon-coated silicon/graphite composite material is obtained. The carbon coating layer can avoid direct contact of the silicon powder and the electrolyte, so that lithium consumption of the electrolyte in the battery circulation process is reduced, the silicon powder can be limited in the carbon coating layer, and the carbon coating layer and the graphite together provide buffer for the volume expansion effect of the silicon powder. In some embodiments, at least one of asphalt, phenolic resin, glucose, citric acid and starch is selected as a carbon source for carbon coating treatment, and the carbon coating has the advantages of easily available raw materials, low cost, easy obtaining of a uniform and compact carbon coating layer after thermal cracking, and good conductivity.
Further, petroleum coke asphalt with the softening point of 200-280 ℃ and the median particle size of 1-5 μm is preferred because the asphalt with the high softening point has higher carbon content and smaller relative dosage, and is easier to form a compact carbon coating layer with good conductivity.
The conditions of the carbon coating treatment have a significant influence on the structure and performance of the formed carbon coating layer. In some embodiments, the temperature of the carbon coating process is controlled between 600 ℃ and 1200 ℃ to sufficiently decompose the carbon source into amorphous or graphitized carbon. If the temperature is too low, the carbon source is difficult to completely decompose; when the temperature is too high, the problem of serious oxidation of silicon is easily caused, and simultaneously, the energy consumption is higher, so that the material cost is increased. Specifically, typical, but not limiting, carbon coating treatment temperatures are 600 ℃, 650 ℃, 700 ℃, 750 ℃, 800 ℃, 850 ℃, 900 ℃, 950 ℃, 1000 ℃, 1050 ℃, 1100 ℃, 1150 ℃, 1200 ℃.
In some embodiments, the time for the carbon coating treatment is controlled to be 0.5h to 6 h. The treatment time is too short, the carbon source can not be completely decomposed into carbon, and the formed carbon coating layer has the problem of being not compact and uniform enough; the treatment time is too long, which not only increases the energy consumption, but also easily causes other side reactions. Specifically, typical, but not limiting, carbon coating treatment times are 0.5h, 1h, 1.5h, 2h, 2.5h, 3h, 3.5h, 4h, 4.5h, 5h, 5.5h, 6 h.
In some embodiments, the temperature rise rate of the carbon coating process is controlled to be between 1 ℃/min and 20 ℃/min. The heating rate can be specifically adjusted according to the heating capacity of the used equipment, and in addition, under the condition of equipment with the same heating capacity, the temperature in the furnace chamber is easily uneven due to too fast heating, so that the service life of the equipment is negatively influenced; too slow temperature rise can lead to problems of too long time consumption and the like. Specifically, typical but non-limiting ramp rates are 1 deg.C/min, 2 deg.C/min, 3 deg.C/min, 4 deg.C/min, 5 deg.C/min, 6 deg.C/min, 7 deg.C/min, 8 deg.C/min, 9 deg.C/min, 10 deg.C/min, 11 deg.C/min, 12 deg.C/min, 13 deg.C/min, 14 deg.C/min, 15 deg.C/min, 16 deg.C/min, 17 deg.C/min, 18 deg.C/min, 19 deg.C/min.
In some embodiments, the amount of carbon coating is 0.5% to 10%, preferably 1% to 6%. The carbon coating amount is too much, so that the transmission distance of lithium ions is easily lengthened, and the electrical property is further influenced; meanwhile, the tap density and compaction of the obtained anode material are influenced due to excessive carbon coating amount, and the specific capacity is reduced; when the carbon coating amount is too low, the carbon coating layer cannot completely coat the substrate material, so that silicon is in direct contact with the electrolyte, and the cycle performance is seriously affected.
In one embodiment, when pitch is used as the carbon source, the method of carbon coating the silicon/graphite composite material is as follows: dissolving asphalt in an organic solvent, filtering to remove insoluble parts to obtain filtrate, adding the silicon/graphite composite material into the filtrate, uniformly stirring, carrying out vacuum drying, crushing and sieving the obtained mixture, transferring the obtained mixture to a tubular furnace, and carrying out carbon coating under the protection of inert gas to obtain the carbon-coated silicon/graphite composite material. Wherein the organic solvent can be at least one of tetrahydrofuran, toluene, carbon tetrachloride and chloroform; the inert gas may be selected from nitrogen and/or argon.
In S3, the carbon nanotubes with the length-diameter ratio larger than 3000 are grown in situ on the surface of the carbon-coated silicon/graphite composite material, so that the conductivity of the obtained negative electrode material can be improved, and the defect of weak conductivity of the silicon material can be overcome. In some embodiments, the method for in-situ growth of the carbon nanotube with the length-diameter ratio of more than 3000 is to mix the carbon-coated silicon/graphite composite material with a catalyst, heat up to 200 ℃ -1800 ℃ at a rate of 1 ℃/min-20 ℃/min under a protective atmosphere, keep the temperature for 0.5h-8h, then add a carbon source, deposit for 1h-20h, introduce an inert gas, keep for 1h-10h, and finally naturally cool to room temperature. The carbon nano tube grows in situ by the method, so that the carbon nano tube has higher length-diameter ratio.
The catalyst can catalyze the carbon source to crack and change into carbon atoms, and the carbon atoms are deposited on the surface of the carbon-coated silicon/graphite composite material particles to form the carbon nano tube. In some embodiments, at least one of the simple substance of iron, the simple substance of cobalt and the simple substance of nickel with the single powder purity of more than or equal to 99.99% is selected as a catalyst, which is beneficial to growing the carbon nano tube with high length-diameter ratio.
Furthermore, the median particle size of the catalyst is controlled to be less than or equal to 300nm, which is beneficial to the growth of the carbon nano tube with high length-diameter ratio. If the particle size of the catalyst is too large, the catalyst is difficult to be well dispersed on the surface of the carbon-coated silicon/graphite composite material; if the particle size of the catalyst is too small, the problems of difficult dispersion and easy agglomeration can occur, and the problem of uneven distribution of the carbon nanotubes is finally caused.
In some embodiments, the selection of at least one of methane, acetylene, ethylene, propylene, benzene as a carbon source facilitates the growth of high aspect carbon nanotubes.
In some embodiments, the mass ratio of the carbon-coated silicon/graphite composite material to the catalyst is controlled to be 1000 (1-10), which facilitates the growth of carbon nanotubes with high aspect ratio. The problem that the generated carbon nanotubes are too much and the final material is difficult to disperse is easily caused by too high content of the catalyst; if the content of the catalyst is too low, the catalyst cannot be uniformly attached to the surface of the carbon-coated silicon/graphite composite material, and the amount of the generated carbon nanotubes is small, so that the conductivity of the obtained cathode material is not obviously improved.
Correspondingly, an embodiment of the present invention further provides a negative electrode material, as shown in fig. 1, the negative electrode material is a graphite core structure, a silicon layer, a carbon coating layer and a carbon nanotube are sequentially disposed on a surface of the core structure along a direction away from the core structure, and an aspect ratio of the carbon nanotube is greater than 3000.
The negative electrode material provided by the embodiment of the invention is a composite material of silicon powder and graphite, so that the specific capacity is higher; in addition, in the structure of the cathode material provided by the embodiment of the invention, graphite is used as a core structure, and the graphite and the carbon coating layer together sandwich the silicon layer, so that the volume expansion effect of the silicon layer in the charge and discharge process can be limited, and the direct contact between the silicon layer and electrolyte can be avoided; in addition, the high length-diameter ratio carbon nano tube positioned at the outermost part of the cathode material can improve the conductivity and further reduce the volume expansion effect of the silicon layer. Therefore, the negative electrode material provided by the embodiment of the invention has the advantages of good conductivity, high first coulombic efficiency and specific capacity, low irreversible capacity and good cycle stability.
In some embodiments, graphite having an average particle size of 5 μm to 30 μm is selected as the starting material. The graphite in the particle size range is used as a core structure, so that silicon powder is favorably attached to the surface of the graphite to form a silicon layer, the supporting effect of the core structure on the silicon layer is improved, and the volume expansion effect of the silicon layer is limited; and when the graphite particles in the particle size range are used as a core structure for loading nano-sized silicon powder, the graphite particles and the nano-sized silicon powder have large size difference, so that the silicon powder can be prevented from agglomerating, and a silicon layer with uniform thickness can be formed on the surface of the core structure.
In some embodiments, the ratio of the mass content of the graphite core structure to the mass content of the silicon layer in the anode material is 100 (1-10) based on the total weight of the anode material. By optimizing the content ratio of the graphite core structure to the silicon layer, the problem that excessive silicon materials are difficult to completely attach to the surface of graphite particles can be avoided on the basis of increasing the capacity of the obtained cathode material by adding the silicon layer, and the volume expansion effect caused by forming an excessively thick silicon layer can also be prevented.
In some embodiments, the carbon coating has a thickness of 1nm to 20 nm. The carbon coating layer is too thick, so that the transmission distance of lithium ions is easily prolonged, and the electrical property is further influenced; meanwhile, the tap density and compaction of the obtained anode material are influenced due to the fact that the carbon coating layer is too thick, and the specific capacity is reduced; when the carbon coating layer is too thin, the carbon coating layer cannot completely coat the substrate material, so that silicon is in direct contact with electrolyte, and the cycle performance is seriously affected.
Correspondingly, the embodiment of the invention also provides a secondary battery, which comprises the anode material prepared by the preparation method of the anode material or the anode material.
The secondary battery provided by the embodiment of the invention comprises the negative electrode material prepared by the preparation method of the negative electrode material or the negative electrode material, and the negative electrode material has the advantages of good conductivity, high first coulombic efficiency and specific capacity, low irreversible capacity and good cycle stability, so that the secondary battery provided by the embodiment of the invention also has excellent charge-discharge efficiency and cycle performance.
In order to make the above implementation details and operations of the present invention clearly understood by those skilled in the art and to make the advanced performance of the negative electrode material, the preparation method thereof, and the secondary battery of the embodiments of the present invention remarkably manifest, the above technical solutions are exemplified by a plurality of embodiments below.
Example 1
A preparation method of the anode material comprises the following steps:
(11) weighing 20g of polyvinylpyrrolidone, dissolving the polyvinylpyrrolidone in 2500g of absolute ethyl alcohol, adding 50g of silicon powder with the average particle size of 100nm, performing ultrasonic dispersion, adding 1000g of artificial graphite with the average particle size of 5-20 mu m, and stirring and mixing to obtain uniform slurry with the solid content of 30%; then spray drying is carried out (the inlet temperature of hot air is 200 ℃, the outlet temperature is 40 ℃) to obtain the silicon/graphite composite material;
(12) dissolving 35g of high-temperature petroleum asphalt in tetrahydrofuran, performing suction filtration to remove filter residues, adding the silicon/graphite composite material into the filtrate, stirring for 2 hours, removing the solvent in a vacuum drying oven at 70 ℃, performing ball milling crushing, sieving with a 200-mesh sieve, heating the obtained powder to 1000 ℃ at the speed of 5 ℃/min under the nitrogen atmosphere, preserving heat for 2 hours, naturally cooling, and cooling to obtain a carbon-coated silicon/graphite composite material;
(13) adding the carbon-coated silicon/graphite composite material and the catalyst nano nickel powder into a fusion machine according to the mass ratio of 1000:1.5 for solid-phase mixing, wherein the rotating speed is 4500rpm, and the fusion time is 10 min; placing the obtained mixture in a tubular furnace for growing carbon nanotubes, introducing argon, heating to 700 ℃ at a heating rate of 1 ℃/min, carrying out heat treatment for 1h, introducing carbon source gas ethylene at a flow rate of 0.05L/min for carrying out vapor deposition, keeping for 4h, stopping introducing the carbon source gas, continuously introducing argon at a flow rate of 0.1L/min, keeping for 6h, stopping introducing the argon, and naturally cooling to below 100 ℃; and screening, acid washing and demagnetizing the obtained product to obtain the negative electrode material.
Example 2
A preparation method of the anode material comprises the following steps:
(21) weighing 20g of polyvinylpyrrolidone, dissolving the polyvinylpyrrolidone in 2450g of absolute ethyl alcohol, adding 30g of silicon powder with the average particle size of 100nm, performing ultrasonic dispersion, adding 1000g of natural graphite with the average particle size of 5-20 mu m, and stirring and mixing to obtain uniform slurry with the solid content of 30%; then spray drying is carried out (the inlet temperature of hot air is 250 ℃, the outlet temperature is 60 ℃) to obtain the silicon/graphite composite material;
(22) dissolving 50g of phenolic resin in absolute ethyl alcohol, adding a silicon/graphite composite material, stirring for 2 hours, removing the solvent in a vacuum drying oven at 85 ℃, and sieving with a 200-mesh sieve after ball milling and crushing; heating the obtained powder to 900 ℃ at the speed of 2 ℃/min under the nitrogen atmosphere, preserving the heat for 5 hours, naturally cooling, and cooling to obtain the carbon-coated silicon/graphite composite material;
(23) adding the carbon-coated silicon/graphite composite material and catalyst nano iron powder into a fusion machine according to the mass ratio of 1000:3 for solid-phase mixing, wherein the rotating speed is 4500rpm, and the fusion time is 5 min; placing the obtained mixture in a tubular furnace for growing carbon nano tubes, introducing nitrogen, heating to 600 ℃ at a heating rate of 10 ℃/min, carrying out heat treatment for 2h, introducing carbon source gas acetylene at a flow rate of 0.05L/min for carrying out vapor deposition, keeping for 4h, stopping introducing the carbon source gas, continuously introducing the nitrogen at a flow rate of 0.1L/min, keeping for 6h, stopping introducing the nitrogen, and naturally cooling to below 100 ℃; and screening, acid washing and demagnetizing the obtained product to obtain the negative electrode material.
Example 3
A preparation method of the anode material comprises the following steps:
(31) weighing 20g of polyethylene glycol-2000, dispersing in 2415g of absolute ethyl alcohol, adding 15g of silicon powder with the average particle size of 100nm, performing ultrasonic dispersion, adding 1000g of mesocarbon microbeads with the average particle size of 5-20 mu m, and stirring and mixing to obtain uniform slurry with the solid content of 30%; then spray drying is carried out (the inlet temperature of hot air is 300 ℃, the outlet temperature is 90 ℃) to obtain the silicon/graphite composite material;
(32) dissolving 40g of citric acid in pure water, adding the silicon/graphite composite material, stirring for 2 hours, removing the solvent in a vacuum drying oven at 85 ℃, performing ball milling and crushing, sieving with a 200-mesh sieve, heating the obtained powder to 750 ℃ at the speed of 1 ℃/min under the atmosphere of nitrogen, preserving the temperature for 4 hours, naturally cooling, and cooling to obtain the carbon-coated silicon/graphite composite material;
(33) adding the carbon-coated silicon/graphite composite material and catalyst nano nickel-iron powder (the mass ratio is 3:97) into a fusion machine according to the mass ratio of 100:1 for solid-phase mixing, wherein the rotating speed is 4500rpm, and the mixing time is 5 min; placing the mixture in a tubular furnace for growing carbon nanotubes, introducing nitrogen, heating to 800 ℃ at a heating rate of 1 ℃/min, carrying out heat treatment for 0.5h, introducing a carbon source gas propylene at a flow rate of 0.05L/min for carrying out vapor deposition, keeping for 4h, stopping introducing the carbon source gas, continuously introducing the nitrogen at a flow rate of 0.1L/min, keeping for 6h, stopping introducing the nitrogen, and naturally cooling to below 100 ℃; and screening, acid washing and demagnetizing the obtained product to obtain the negative electrode material. Comparative example 1
Comparative example 1 is substantially the same as example 1 except that the added silicon powder is micron-sized silicon powder, to obtain a negative electrode material.
Comparative example 2
Comparative example 2 is substantially the same as example 1 except that graphite was used instead of the silicon powder to obtain a negative electrode material.
Comparative example 3
Comparative example 3 is substantially the same as example 1 except that there is no step of in-situ growing carbon nanotubes, resulting in a carbon-coated silicon/graphite composite material.
Examples 1-3 and comparative examples 1-3 half cells were prepared under the same conditions as for the preparation of the negative electrode material (or the carbon-coated silicon/graphite composite material of comparative example 3): the mass ratio of SP-Li to LA133 is 94.5:2:3.5, water is added for dilution to enable the slurry to be uniformly mixed, the mixture is coated on the surface of copper foil, after the copper foil is rolled to a certain thickness, vacuum drying is carried out for 8 hours at the temperature of 110 ℃, a negative pole piece is manufactured, a metal lithium piece is used as a counter electrode, and electrolyte is 1mol/L LiPF6Ethylene carbonate and methyl ethyl carbonate (volume ratio) is 1:1, and a polypropylene microporous membrane is used as a diaphragm to assemble the battery. The assembled battery was left at room temperature for 6 hours and then subjected to charge and discharge tests at a charge and discharge voltage range of 0.01V to 1.5V and a charge and discharge rate of 0.1C, with the test results shown in table 1.
Table 1 charge and discharge performance test results of the anode materials obtained in examples 1 to 3 and comparative examples 1 to 3
As can be seen from table 1, in examples 1 to 3 of the present invention, the carbon nanotubes having an aspect ratio greater than 3000 are grown in situ on the surface of the carbon-coated silicon/graphite composite material, so that the obtained negative electrode material has high specific capacity and cycle performance. In example 2 and example 3, the content of silicon is lower than that in example 1, so the specific capacity is correspondingly lower than that in example 1, and the first coulomb pinning rate and the cycling performance are higher than those in example 1. The quality of the silicon powder added in the comparative example 1 is consistent with that of the silicon powder added in the example 1, so the first-time discharge capacity is similar theoretically, but the silicon powder added in the comparative example 1 is micron-sized, the effect of attaching the silicon powder to the surface of graphite and the coating effect are poorer than those of the nanoscale silicon powder, and the problems of incomplete coating and non-uniformity are easily caused, so the conductivity is poorer, and the first-time charge and discharge efficiency is lower (although the first-time charge and discharge efficiency is lower, the first-time discharge capacity is relatively larger, so the first-time charge capacity is higher than that of the examples 2 and 3; in addition, the volume effect of the micron-sized silicon powder is more serious than that of the nanometer-sized silicon powder, so that the problems of cracking and pulverization are easily caused in the circulation process, and even the materials and the current collectors are separated to cause failure, so that the circulation capacity retention rate of the comparative example 1 is poor. In comparative example 2, since silicon powder was not contained, the first charge-discharge efficiency and the capacity retention rate were high, but the capacity exertion was limited. In comparative example 3, since no carbon nanotube was grown, the relative content of active materials (silicon and graphite) in the obtained material was high, and the first discharge capacity was high; however, the lack of the carbon nanotubes can obviously weaken the conductivity of the material and influence the exertion of the electrochemical performance of the material, so that the first charge-discharge efficiency and the cycle performance of the comparative example 3 are poor.
The above-mentioned embodiments only express several embodiments of the present invention, and the description thereof is more specific and detailed, but not construed as limiting the scope of the present invention. It should be noted that, for a person skilled in the art, several variations and modifications can be made without departing from the inventive concept, which falls within the scope of the present invention. Therefore, the protection scope of the present patent shall be subject to the appended claims.
Claims (10)
1. The preparation method of the anode material is characterized by comprising the following steps of:
providing graphite and silicon powder;
mixing the graphite and the silicon powder, and coating the silicon powder on the surface of the graphite to obtain a silicon/graphite composite material;
carrying out carbon coating treatment on the silicon/graphite composite material, and preparing a carbon coating layer on the surface of the silicon/graphite composite material to obtain a carbon-coated silicon/graphite composite material;
and growing a carbon nano tube with the length-diameter ratio of more than 3000 on the surface of the carbon-coated silicon/graphite composite material in situ to obtain the cathode material.
2. The method for preparing the cathode material according to claim 1, wherein the method for growing the carbon nanotubes with the aspect ratio of more than 3000 on the surface of the carbon-coated silicon/graphite composite material in situ comprises the following steps: mixing the carbon-coated silicon/graphite composite material with a catalyst, heating to 200-1800 ℃ at the speed of 1-20 ℃/min under a protective atmosphere, preserving the temperature for 0.5-8 h, adding a carbon source, depositing for 1-20 h, and introducing inert gas for 1-10 h.
3. The preparation method of the anode material of claim 2, wherein the mass ratio of the carbon-coated silicon/graphite composite material to the catalyst is 1000 (1-10); and/or
The carbon source is at least one selected from methane, acetylene, ethylene, propylene and benzene; and/or
The catalyst is selected from at least one of elementary iron, elementary cobalt and elementary nickel; and/or
The median particle size of the catalyst is less than or equal to 300 nm.
4. The method for preparing the negative electrode material according to claim 1, wherein the graphite is at least one selected from natural graphite, artificial graphite and mesocarbon microbeads; and/or
The average particle size of the graphite is 5-30 μm; and/or
The median particle size of the silicon powder is less than or equal to 300 nm.
5. The preparation method of the anode material according to claim 1, wherein in the step of mixing the graphite and the silicon powder, the mass ratio of the graphite to the silicon powder is 100 (1-10); and/or
And adding a surfactant in the step of mixing the graphite and the silicon powder.
6. The preparation method of the negative electrode material as claimed in claim 5, wherein the mass ratio of the graphite to the silicon powder to the surfactant is 100 (1-10) to (1-5); and/or
The surfactant is at least one selected from polyvinylpyrrolidone, sodium dodecyl benzene sulfonate and polyethylene glycol.
7. The method for preparing the anode material according to any one of claims 1 to 6, wherein in the step of carbon-coating the silicon/graphite composite material, the carbon source for the carbon-coating is selected from at least one of pitch, phenolic resin, glucose, citric acid and starch; and/or
In the step of carrying out carbon coating treatment on the silicon/graphite composite material, the temperature of the carbon coating treatment is 600-1200 ℃; and/or
In the step of carrying out carbon coating treatment on the silicon/graphite composite material, the carbon coating treatment time is 0.5h-6 h; and/or
In the step of carrying out carbon coating treatment on the silicon/graphite composite material, the temperature rise rate of the carbon coating treatment is 1-20 ℃/min; and/or
In the step of performing carbon coating treatment on the silicon/graphite composite material, the carbon coating amount of the carbon coating treatment is 0.5-10%.
8. The negative electrode material is characterized in that graphite is used as a core structure, a silicon layer, a carbon coating layer and carbon nanotubes are sequentially arranged on the surface of the core structure along the direction far away from the core structure, and the length-diameter ratio of the carbon nanotubes is larger than 3000.
9. The negative electrode material of claim 8, wherein the graphite is selected from at least one of natural graphite, artificial graphite, mesocarbon microbeads; and/or
The average particle size of the graphite is 5-30 μm; and/or
The mass ratio of the graphite to the silicon layer is 100 (1-10); and/or
The thickness of the carbon coating layer is 1nm-20 nm.
10. A secondary battery comprising the negative electrode material produced by the method for producing a negative electrode material according to any one of claims 1 to 7 or the negative electrode material according to any one of claims 8 to 9.
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