WO2012067298A1 - Anode active material for a lithium secondary battery with silicon nanoparticles and lithium secondary battery comprising the same - Google Patents

Anode active material for a lithium secondary battery with silicon nanoparticles and lithium secondary battery comprising the same Download PDF

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WO2012067298A1
WO2012067298A1 PCT/KR2010/008931 KR2010008931W WO2012067298A1 WO 2012067298 A1 WO2012067298 A1 WO 2012067298A1 KR 2010008931 W KR2010008931 W KR 2010008931W WO 2012067298 A1 WO2012067298 A1 WO 2012067298A1
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lithium secondary
secondary battery
active material
anode active
anode
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PCT/KR2010/008931
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French (fr)
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Jung Hoon Lee
Byung Sun Gong
Dae Jin Kim
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Kcc Corporation
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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
    • H01M4/362—Composites
    • H01M4/364—Composites as mixtures
    • B—PERFORMING OPERATIONS; TRANSPORTING
    • B82—NANOTECHNOLOGY
    • B82Y—SPECIFIC USES OR APPLICATIONS OF NANOSTRUCTURES; MEASUREMENT OR ANALYSIS OF NANOSTRUCTURES; MANUFACTURE OR TREATMENT OF NANOSTRUCTURES
    • B82Y30/00—Nanotechnology for materials or surface science, e.g. nanocomposites
    • C—CHEMISTRY; METALLURGY
    • C01—INORGANIC CHEMISTRY
    • C01B—NON-METALLIC ELEMENTS; COMPOUNDS THEREOF; METALLOIDS OR COMPOUNDS THEREOF NOT COVERED BY SUBCLASS C01C
    • C01B33/00—Silicon; Compounds thereof
    • C01B33/02—Silicon
    • C01B33/021—Preparation
    • C—CHEMISTRY; METALLURGY
    • C01—INORGANIC CHEMISTRY
    • C01B—NON-METALLIC ELEMENTS; COMPOUNDS THEREOF; METALLOIDS OR COMPOUNDS THEREOF NOT COVERED BY SUBCLASS C01C
    • C01B33/00—Silicon; Compounds thereof
    • C01B33/02—Silicon
    • C01B33/021—Preparation
    • C01B33/027—Preparation by decomposition or reduction of gaseous or vaporised silicon compounds other than silica or silica-containing material
    • C01B33/035—Preparation by decomposition or reduction of gaseous or vaporised silicon compounds other than silica or silica-containing material by decomposition or reduction of gaseous or vaporised silicon compounds in the presence of heated filaments of silicon, carbon or a refractory metal, e.g. tantalum or tungsten, or in the presence of heated silicon rods on which the formed silicon is deposited, a silicon rod being obtained, e.g. Siemens process
    • 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
    • H01M4/04—Processes of manufacture in general
    • H01M4/0402—Methods of deposition of the material
    • H01M4/0404—Methods of deposition of the material by coating on electrode collectors
    • 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
    • H01M4/13—Electrodes for accumulators with non-aqueous electrolyte, e.g. for lithium-accumulators; Processes of manufacture thereof
    • H01M4/139—Processes of manufacture
    • H01M4/1395—Processes of manufacture of electrodes based on metals, Si or alloys
    • 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
    • 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
    • H01M4/58—Selection of substances as active materials, active masses, active liquids of inorganic compounds other than oxides or hydroxides, e.g. sulfides, selenides, tellurides, halogenides or LiCoFy; of polyanionic structures, e.g. phosphates, silicates or borates
    • H01M4/583—Carbonaceous material, e.g. graphite-intercalation compounds or CFx
    • H01M4/587—Carbonaceous material, e.g. graphite-intercalation compounds or CFx for inserting or intercalating light metals
    • C—CHEMISTRY; METALLURGY
    • C01—INORGANIC CHEMISTRY
    • C01P—INDEXING SCHEME RELATING TO STRUCTURAL AND PHYSICAL ASPECTS OF SOLID INORGANIC COMPOUNDS
    • C01P2004/00—Particle morphology
    • C01P2004/60—Particles characterised by their size
    • C01P2004/64—Nanometer sized, i.e. from 1-100 nanometer
    • H—ELECTRICITY
    • H01—ELECTRIC ELEMENTS
    • H01M—PROCESSES OR MEANS, e.g. BATTERIES, FOR THE DIRECT CONVERSION OF CHEMICAL ENERGY INTO ELECTRICAL ENERGY
    • H01M10/00—Secondary cells; Manufacture thereof
    • H01M10/05—Accumulators with non-aqueous electrolyte
    • H01M10/052—Li-accumulators
    • H—ELECTRICITY
    • H01—ELECTRIC ELEMENTS
    • H01M—PROCESSES OR MEANS, e.g. BATTERIES, FOR THE DIRECT CONVERSION OF CHEMICAL ENERGY INTO ELECTRICAL ENERGY
    • H01M4/00—Electrodes
    • H01M4/02—Electrodes composed of, or comprising, active material
    • H01M2004/021—Physical characteristics, e.g. porosity, surface area
    • Y—GENERAL TAGGING OF NEW TECHNOLOGICAL DEVELOPMENTS; GENERAL TAGGING OF CROSS-SECTIONAL TECHNOLOGIES SPANNING OVER SEVERAL SECTIONS OF THE IPC; TECHNICAL SUBJECTS COVERED BY FORMER USPC CROSS-REFERENCE ART COLLECTIONS [XRACs] AND DIGESTS
    • Y02—TECHNOLOGIES OR APPLICATIONS FOR MITIGATION OR ADAPTATION AGAINST CLIMATE CHANGE
    • Y02E—REDUCTION OF GREENHOUSE GAS [GHG] EMISSIONS, RELATED TO ENERGY GENERATION, TRANSMISSION OR DISTRIBUTION
    • Y02E60/00—Enabling technologies; Technologies with a potential or indirect contribution to GHG emissions mitigation
    • Y02E60/10—Energy storage using batteries

Definitions

  • the present invention relates to an anode active material for a lithium secondary battery with silicon nanoparticles and a lithium secondary battery comprising the same. More specifically, the present invention relates to an anode active material for a lithium secondary battery with silicon nanoparticles which are preferably obtained on the wall surface of a Siemens reactor as a byproduct of a reaction by the Siemens process and have a diameter ranging from 5 to 100 nm, by which high capacity and cycle property can be achieved, and a lithium secondary battery comprising the same.
  • the lithium secondary battery is used through the cycle of charging and discharging by which lithium ions are intercalated and released, respectively.
  • the lithium secondary battery also is an essential power source for large devices such as electric vehicles, as well as for portable information/telecommunication electronic devices.
  • the performance of these devices depends on that of the battery which is a key component as a power source of the devices, there has been a constant need for a lithium secondary battery which can become smaller and lighter while achieving high capacity, high energy density, stability and life characteristics.
  • the improvement in the performance of a lithium secondary battery is principally based on the improvement of four (4) key components: anode, cathode, separation membrane and liquid electrolyte.
  • the improvement in the performance of the anode has been focused on increasing its capacity through the development of anode material.
  • Lithium metal was conventionally used as an anode active material for a lithium secondary battery, in which case, however, there is a danger of explosion caused by generation of a short circuit in the battery due to the formation of dendrite.
  • carbon-based anode active materials are now widely used instead of lithium metal.
  • Carbon-based anode active materials include crystalline carbons such as graphite and artificial black carbon, and non-crystalline carbons such as soft carbon and hard carbon.
  • the one most often used one is graphite among crystalline carbons.
  • the carbon-based anode active material such as graphite
  • its maximum theoretical capacity is limited to about 372 mAh/g, and thus its application for a high-capacity lithium secondary battery is limited.
  • metal-based anode active materials are actively being studied.
  • lithium secondary batteries utilizing, as an anode active material, metals or half-metals such as silicon (Si), tin (Sn), aluminum (Al), germanium (Ge), lead (Pb), zinc (Zn).
  • silicon is a material having a high theoretical capacity up to about 4200 mAh/g.
  • silicon is inferior to carbon-based anode active materials in terms of cycle property. This has been an obstacle in its commercialization because when silicon is used as a material for intercalating and releasing lithium ions, the volume is changed in the course of charging and discharging, by which the electrical contact property is reduced or the peeling-off phenomenon of active material from the current collector occurs. That is, the volume of silicon contained in the anode active material expands more than 300% by charging and a mechanical stress applied at such a time causes cracks in the inner part and on the surface of the electrode.
  • Japanese Patent Application Publication No. 1994-318454 A discloses an anode prepared by simply mixing metal or alloy particles with a carbon-based active material capable of intercalating and releasing lithium ions.
  • the excessive volume expansion and contraction of the metal-based active material during the charging/discharging cycle pulverize the metal-based active material, and the pulverized particles are peeled off from the current collector, resulting in a shortened battery life time.
  • the present invention has an object of providing an anode active material for a lithium secondary battery which can achieve high capacity and cycle property by minimizing the electrode deterioration phenomenon due to the volume change of silicon and improving the electrical contact property, and a lithium secondary battery comprising the same.
  • the present invention provides an anode active material for a lithium secondary battery comprising silicon nanoparticles having a diameter ranging from 5 to 100 nm, which are preferably obtained on the wall surface of a Siemens reactor as a byproduct of a reaction by the Siemens process.
  • the silicon nanoparticles are obtained through the steps of: a) reacting metal silicon with hydrochloric acid to obtain monosilane or trichlorosilane in gas phase; b) reacting the monosilane or trichlorosilane with a ⁇ -shaped silicon rod in a Siemens reactor; and c) obtaining silicon nanoparticles formed on the wall surface of the Siemens reactor.
  • the present invention also provides an anode material for a lithium secondary battery comprising the anode active material, a conductive material and a binder; an anode for a lithium secondary battery comprising an anode current collector, and the anode material which is coated on the anode current collector; and a lithium secondary battery comprising the anode, a cathode, a separation membrane and a liquid electrolyte.
  • the electrode deterioration phenomenon due to the volume change of silicon can be minimized and the electrical contact property can be improved, by which high capacity, high energy density, cycle property and life characteristics can be achieved in lithium secondary batteries.
  • Figure 1 is a schematic figure of a device for producing silicon nanoparticles according to the Siemens process.
  • Figure 2 is a graph showing the cycle property change of the lithium secondary battery according to the Example.
  • Figure 3 is a graph showing the cycle property change of the lithium secondary battery according to the Comparative Example.
  • Figure 4 is the TEM analysis result of the silicon nanoparticles produced by the Siemens process, in an embodiment of the present invention.
  • Figure 5 is the XRD analysis result of the silicon nanoparticles produced by the Siemens process, in an embodiment of the present invention.
  • the silicon nanoparticles contained in the anode active material for a lithium secondary battery of the present invention have a diameter ranging from 5 to 100 nm. If the diameter of the silicon particles is smaller than 5 nm, coagulation between the silicon particles may occur and thus it may be difficult to disperse them in the active material. If the diameter of the silicon particles is greater than 100 nm, the volume may change more during the charging/discharging and thus the electrical contact property may be deteriorated or the particles may be peeled off from the current collector.
  • the silicon nanoparticles contained in the anode active material for a lithium secondary battery of the present invention are obtained on the wall surface of a Siemens reactor as a byproduct of a reaction by the Siemens process.
  • the Siemens process was developed by Siemens (Germany) for producing polysilicon, and it is currently employed in about 90% of global polysilicon production.
  • Polysilicon is a highly pure compound having a polycrystalline molecular structure.
  • quartz extracted from silicon is mixed with a carbon compound and heated (i.e., carbon melt reduction of silicon) to produce metallurgical silicon (MG-Si).
  • metallurgical silicon is then put into a melting pot and chemically reacted therein with monosilane or trichlorosilane to obtain a purified polysilicon.
  • this process produces polysilicon by using the device as shown in Fig. 1 according to the following procedure.
  • a ⁇ -shaped silicon rod heated to about 1100°C is reacted with the monosilane or trichlorosilane to produce polysilicon.
  • the silicon powder deposited on the ⁇ -shaped silicon rod has a diameter of tens of micrometer ( ⁇ m) whereas the silicon powder existing on the wall surface of the Siemens reactor has a diameter of tens of nanometer (nm).
  • the present invention utilizes, in the Siemens process, the silicon particles of nm size obtained on the wall surface of the Siemens reactor as a reaction byproduct and not the silicon particles of ⁇ m size deposited on the ⁇ -shaped silicon rod. In such a way, the size of silicon particles to be used as a silicon-based anode active material can be reduced to nm scale and thus its absolute volume change can be minimized, by which the problems caused by the silicon volume change, such as cycle property deterioration and battery life time shortening, can be resolved.
  • the anode active material of the present invention comprises the silicon particles of nm size (for example, 5 ⁇ 100 nm) and can maintain high capacity even as the cycle proceeds.
  • the anode active material of the present invention can show an initial capacity ranging from about 1500 to about 2000 mAh/g.
  • the anode active material of the present invention can further comprise a metal-based anode active material in addition to the silicon nanoparticles.
  • a metal-based anode active material one or more metals selected from the group consisting of Sn, Al, Ge, Co, Cu, Ti, Ni, Li, Pb, Zn, Ag and Au, or alloys thereof can be used, but it is not limited thereto.
  • the anode active material of the present invention can further comprise a carbon-based anode active material in addition to the silicon nanoparticles.
  • a carbon-based anode active material those known in this field can be used without limitation ⁇ for example, crystalline carbons such as graphite, natural black carbon and artificial black carbon, and non-crystalline carbons such as soft carbon and hard carbon can be used alone or in combination of two or more thereof.
  • the silicon nanoparticles and metal-based anode active material and/or carbon-based anode active material can be mixed by a mechanical treatment method such as ball milling, or dispersed and mixed in a solvent together with a dispersing agent under agitation, ultrasonic wave or the like, but it is not limited thereto.
  • the silicon nanoparticles according to the present invention can be prepared by using a conventional The Siemens process known in this field.
  • the silicon nanoparticles having a diameter ranging from 5 to 100 nm can be obtained through the steps of: a) reacting metal silicon with hydrochloric acid to obtain monosilane or trichlorosilane in gas phase; b) reacting the monosilane or trichlorosilane with a heated ⁇ -shaped silicon rod in a Siemens reactor; and c) obtaining silicon nanoparticles formed on the wall surface of the Siemens reactor.
  • an anode material for a lithium secondary battery comprising the anode active material of the present invention, a conductive material and a binder; and an anode for a lithium secondary battery comprising an anode current collector, and the anode material which is coated on the anode current collector; are provided.
  • the conductive material contained in the anode material increases the overall conductivity of the anode material and improves the output characteristics of the battery.
  • the conductive material any of those having a good electrical conductivity and not causing a side reaction in the environment inside the lithium secondary battery can be used without limitation.
  • carbon-based materials having high conductivity such as black carbon, conductive carbon and the like are used.
  • conductive polymers having high conductivity can also be used.
  • the black carbon is not limited to natural black carbon or artificial black carbon.
  • the conductive carbon particularly preferred are carbon-based materials having a high conductivity, and concretely, a material selected from the group consisting of carbon blacks such as carbon black, acetylene black, Ketjen black, channel black, furnace black, lamp black, thermal black etc., or materials having a crystal structure comprising grapheme or graphite can be used alone or in combination of two or more thereof.
  • carbon blacks such as carbon black, acetylene black, Ketjen black, channel black, furnace black, lamp black, thermal black etc.
  • materials having a crystal structure comprising grapheme or graphite can be used alone or in combination of two or more thereof.
  • the method of adding the conductive material is also not particularly limited, and a conventional method known in this field such as coating on the anode active material can be employed.
  • binder conventional ones known in this field can be used without limitation.
  • PVDF polyvinylidene fluoride
  • polyacrylonitrile polyacrylonitrile
  • polymethylmethacrylate polymethylmethacrylate
  • vinylidene fluoride/hexafluoropropylene copolymer or the like can be used alone or in combination of two or more thereof.
  • the anode of the present invention can be prepared by using a conventional method known in this field.
  • the anode of the present invention can be prepared by mixing the anode active material, a conductive material, a binder and a solvent to prepare slurry, and applying the slurry on an anode current collector such as copper and then drying. Filler can be added to the mixture if needed.
  • a lithium secondary battery comprising the aforesaid anode, a cathode, a separation membrane and a liquid electrolyte.
  • a lithium secondary battery is constituted with an anode comprising an anode material and an anode current collector, a cathode comprising a cathode material and a cathode current collector, and a separation membrane which blocks electron conduction between the anode and cathode, and conducts lithium ions.
  • a liquid electrolyte is included in the empty space between the anode, cathode and separation membrane for conducting lithium ions. There is no particular limitation to the method for preparing the cathode.
  • the cathode can be prepared by using a conventional method known in this field.
  • the cathode can be prepared by mixing a cathode active material, a conductive material, a binder and a solvent to prepare slurry, and applying the slurry on a cathode current collector such as aluminum and then drying. Filler can be added to the mixture if needed.
  • the lithium secondary battery can be prepared by using a conventional method known in this field.
  • the lithium secondary battery can be prepared by placing a porous separation membrane between the anode and cathode, and adding a non-aqueous liquid electrolyte thereto.
  • the lithium secondary battery of the present invention can be preferably used as a unit cell of a medium- or large-scale battery module comprising plural battery cells, as well as a battery cell used as a power source of small devices such as cellular phones.
  • Applicable medium- or large-scale devices include power tools; electric vehicles (EV) including hybrid electric vehicles (HEV) and plug-in hybrid electric vehicles (PHEV); electric bicycles including E-bikes and E-scooters; electric golf carts; electric trucks; electric commercial vehicles; power storage systems and the like.
  • the anode active material was prepared by using the device as shown in Fig. 1 according to the following procedure.
  • Metallurgical silicon was reacted with hydrochloric acid to prepare trichlorosilane in gas phase. Then, an electric current was supplied from a power supply to a ⁇ -shaped silicon rod in a Siemens reactor in which an alternating current power controller was connected to an output terminal. The current intensity and voltage were determined by the alternating current power controller, and the electrical power converted to heat in the silicon rod was set.
  • the Siemens reactor was filled with trichlorosilane, and a high-pressure condition was formed.
  • the silicon rod was heated so that the surface temperature of the silicon rod was elevated up to about 1,100°C by using the loss of electrical power converted to heat due to the electric current conduction at the silicon rod, and hydrogen was then added in the Siemens reactor, by which trichlorosilane was decomposed into silicon and hydrogen chloride, and the decomposed silicon was deposited on the silicon rod in the Siemens reactor.
  • silicon nanoparticles anode active material which had a diameter of tens of nm and were formed on the wall surface of the Siemens reactor, were obtained.
  • Figs. 4 and 5 The TEM analysis result and XRD analysis result of the prepared particles are shown in Figs. 4 and 5, respectively. According to Fig. 4, it can be confirmed that the anode active material as prepared above was spherical particles having a diameter of 100 nm or smaller. According to Fig. 5, it can be confirmed that the prepared particles were silicon.
  • the anode active material as prepared above, a conductive material (super p black, SPB) and a binder (polyvinylidene fluoride, PVDF) were provided with a weight ratio of 75 : 15 : 10.
  • the charging/discharging capacity hereinafter is a calculated value on the basis of 75% anode active material.
  • the binder was dissolved in an NMP solvent (N-methylpyrrolidone, 99%, Aldrich Co.) for 10 minutes by using a mini mill, and then the anode active material and conductive material were added thereto, and agitated for 30 minutes to obtain a homogeneous slurry.
  • the prepared slurry was applied on a copper foil by using a blade and dried in an oven at 110°C for 2 hours to evaporate the solvent, and then compressed by using a hot press roll.
  • the prepared anode was dried in a vacuum oven at 120°C for 12 hours.
  • a cathode active material of lithium metal, a conductive material (super p black, SPB) and a binder (polyvinylidene fluoride, PVDF) were provided with a weight ratio of 75 : 15 : 10.
  • the binder was dissolved in an NMP solvent (N-methylpyrrolidone, 99%, Aldrich Co.) for 10 minutes by using a mini mill, and then the cathode active material and conductive material were added thereto, and agitated for 30 minutes to obtain a homogeneous slurry.
  • the prepared slurry was applied on an aluminum foil by using a blade and dried in an oven at 110°C for 2 hours to evaporate the solvent, and then compressed by using a hot press roll.
  • the prepared cathode was dried in a vacuum oven at 120°C for 12 hours.
  • the anode, cathode and lithium secondary battery were prepared according to the same procedures as described in the Example, except that a commercial silicon powder (633097, 98%, Aldrich Co.) having a diameter of tens of ⁇ m was used as the anode active material.
  • a commercial silicon powder (633097, 98%, Aldrich Co.) having a diameter of tens of ⁇ m was used as the anode active material.
  • the lithium secondary batteries prepared in the Example and Comparative Example were kept for 24 hours in order to stabilize the batteries. Then, the charging/discharging test was conducted by using WBCS3000L charging/discharging apparatus (Won-A Tech). The charging/discharging was carried out with a current of 0.14 mA (1C/20) in a voltage range of 0.0 to 1.5 V.
  • the cycle property changes of the lithium secondary batteries according to the Example and Comparative Example are shown in Figs. 2 and 3, respectively.
  • the battery of the Example showed an initial anode capacity of 1750 mAh/g while the battery of the Comparative Example showed an initial anode capacity of 1050 mAh/g, by which it can be known that the battery of the Example showed a higher initial anode capacity than that of the Comparative Example. Furthermore, according to the result of 5 cycles of charging/discharging, the battery of the Example maintained higher initial anode capacity than that of the Comparative Example, by which it can be known that the cycle property and life time characteristics of the battery of the Example were better than those of the Comparative Example.

Abstract

The present invention relates to an anode active material for a lithium secondary battery with silicon nanoparticles and a lithium secondary battery comprising the same. More specifically, the present invention relates to an anode active material for a lithium secondary battery with silicon nanoparticles which are preferably obtained on the wall surface of a Siemens reactor as a byproduct of a reaction by the Siemens process and have a diameter ranging from 5 to 100 nm, by which high capacity and cycle property can be achieved, and a lithium secondary battery comprising the same.

Description

ANODE ACTIVE MATERIAL FOR A LITHIUM SECONDARY BATTERY WITH SILICON NANOPARTICLES AND LITHIUM SECONDARY BATTERY COMPRISING THE SAME
The present invention relates to an anode active material for a lithium secondary battery with silicon nanoparticles and a lithium secondary battery comprising the same. More specifically, the present invention relates to an anode active material for a lithium secondary battery with silicon nanoparticles which are preferably obtained on the wall surface of a Siemens reactor as a byproduct of a reaction by the Siemens process and have a diameter ranging from 5 to 100 nm, by which high capacity and cycle property can be achieved, and a lithium secondary battery comprising the same.
Electronic and information/telecommunication industries have been rapidly developed through the development of electronic devices in terms of their portability, size, weight and performance. Accordingly, as a power source for electronic devices, there is a rapidly increasing need for a lithium secondary battery which can provide high capacity and high performance. The lithium secondary battery is used through the cycle of charging and discharging by which lithium ions are intercalated and released, respectively. The lithium secondary battery also is an essential power source for large devices such as electric vehicles, as well as for portable information/telecommunication electronic devices. In particular, since the performance of these devices depends on that of the battery which is a key component as a power source of the devices, there has been a constant need for a lithium secondary battery which can become smaller and lighter while achieving high capacity, high energy density, stability and life characteristics.
The improvement in the performance of a lithium secondary battery is principally based on the improvement of four (4) key components: anode, cathode, separation membrane and liquid electrolyte. Among them, the improvement in the performance of the anode has been focused on increasing its capacity through the development of anode material. Lithium metal was conventionally used as an anode active material for a lithium secondary battery, in which case, however, there is a danger of explosion caused by generation of a short circuit in the battery due to the formation of dendrite. Accordingly, carbon-based anode active materials are now widely used instead of lithium metal. Carbon-based anode active materials include crystalline carbons such as graphite and artificial black carbon, and non-crystalline carbons such as soft carbon and hard carbon. The one most often used one is graphite among crystalline carbons. However, in the carbon-based anode active material such as graphite, its maximum theoretical capacity is limited to about 372 mAh/g, and thus its application for a high-capacity lithium secondary battery is limited.
In order to resolve such a problem, metal-based anode active materials are actively being studied. Examples include lithium secondary batteries utilizing, as an anode active material, metals or half-metals such as silicon (Si), tin (Sn), aluminum (Al), germanium (Ge), lead (Pb), zinc (Zn). Such materials can intercalate and release more lithium ions than carbon-based anode active materials, and thus are suitable for manufacturing batteries having high capacity and high energy density. Especially, silicon is a material having a high theoretical capacity up to about 4200 mAh/g.
However, silicon is inferior to carbon-based anode active materials in terms of cycle property. This has been an obstacle in its commercialization because when silicon is used as a material for intercalating and releasing lithium ions, the volume is changed in the course of charging and discharging, by which the electrical contact property is reduced or the peeling-off phenomenon of active material from the current collector occurs. That is, the volume of silicon contained in the anode active material expands more than 300% by charging and a mechanical stress applied at such a time causes cracks in the inner part and on the surface of the electrode. In addition, when lithium ions are released by discharging, the volume of silicon is in turn reduced, and the reiteration of such charging/discharging causes the peeling-off of active material from the current collector and the formation of void space between silicon particles and active material, by which an electrical insulation may occur and the battery life time is shortened.
In this regard, Japanese Patent Application Publication No. 1994-318454 A discloses an anode prepared by simply mixing metal or alloy particles with a carbon-based active material capable of intercalating and releasing lithium ions. However, in this case the excessive volume expansion and contraction of the metal-based active material during the charging/discharging cycle pulverize the metal-based active material, and the pulverized particles are peeled off from the current collector, resulting in a shortened battery life time.
Accordingly, there is an urgent need to develop a silicon-based anode active material which can achieve high capacity and cycle property by minimizing the volume change of silicon and improving the electrical contact property.
[PRIOR ART PUBLICATIONS]
[PATENT PUBLICATIONS]
Japanese Patent Application Publication No. 1994-318454 A
To resolve the problems of prior arts as explained above, the present invention has an object of providing an anode active material for a lithium secondary battery which can achieve high capacity and cycle property by minimizing the electrode deterioration phenomenon due to the volume change of silicon and improving the electrical contact property, and a lithium secondary battery comprising the same.
To achieve the object as explained above, the present invention provides an anode active material for a lithium secondary battery comprising silicon nanoparticles having a diameter ranging from 5 to 100 nm, which are preferably obtained on the wall surface of a Siemens reactor as a byproduct of a reaction by the Siemens process.
According to an embodiment of the present invention, the silicon nanoparticles are obtained through the steps of: a) reacting metal silicon with hydrochloric acid to obtain monosilane or trichlorosilane in gas phase; b) reacting the monosilane or trichlorosilane with a ∩-shaped silicon rod in a Siemens reactor; and c) obtaining silicon nanoparticles formed on the wall surface of the Siemens reactor.
According to other aspects, the present invention also provides an anode material for a lithium secondary battery comprising the anode active material, a conductive material and a binder; an anode for a lithium secondary battery comprising an anode current collector, and the anode material which is coated on the anode current collector; and a lithium secondary battery comprising the anode, a cathode, a separation membrane and a liquid electrolyte.
By using the anode active material for a lithium secondary battery of the present invention, the electrode deterioration phenomenon due to the volume change of silicon can be minimized and the electrical contact property can be improved, by which high capacity, high energy density, cycle property and life characteristics can be achieved in lithium secondary batteries.
Figure 1 is a schematic figure of a device for producing silicon nanoparticles according to the Siemens process.
Figure 2 is a graph showing the cycle property change of the lithium secondary battery according to the Example.
Figure 3 is a graph showing the cycle property change of the lithium secondary battery according to the Comparative Example.
Figure 4 is the TEM analysis result of the silicon nanoparticles produced by the Siemens process, in an embodiment of the present invention.
Figure 5 is the XRD analysis result of the silicon nanoparticles produced by the Siemens process, in an embodiment of the present invention.
The present invention is explained in detail below.
The silicon nanoparticles contained in the anode active material for a lithium secondary battery of the present invention have a diameter ranging from 5 to 100 nm. If the diameter of the silicon particles is smaller than 5 nm, coagulation between the silicon particles may occur and thus it may be difficult to disperse them in the active material. If the diameter of the silicon particles is greater than 100 nm, the volume may change more during the charging/discharging and thus the electrical contact property may be deteriorated or the particles may be peeled off from the current collector.
Preferably, the silicon nanoparticles contained in the anode active material for a lithium secondary battery of the present invention are obtained on the wall surface of a Siemens reactor as a byproduct of a reaction by the Siemens process. The Siemens process was developed by Siemens (Germany) for producing polysilicon, and it is currently employed in about 90% of global polysilicon production. Polysilicon is a highly pure compound having a polycrystalline molecular structure. In the Siemens process, quartz extracted from silicon is mixed with a carbon compound and heated (i.e., carbon melt reduction of silicon) to produce metallurgical silicon (MG-Si). The metallurgical silicon is then put into a melting pot and chemically reacted therein with monosilane or trichlorosilane to obtain a purified polysilicon. Concretely, this process produces polysilicon by using the device as shown in Fig. 1 according to the following procedure. First, metallurgical silicon is reacted with hydrochloric acid (HCl) to produce monosilane (SiH4) in gas phase, or an alloy of metallurgical silicon with 5% copper (Cu) is chlorinated with hydrochloric acid at 240℃ or lower to produce trichlorosilane (TCS) gas (Si + 3HCl = H2 + SiHCl3). Then, a ∩-shaped silicon rod heated to about 1100℃ is reacted with the monosilane or trichlorosilane to produce polysilicon.
At this time, the silicon powder deposited on the ∩-shaped silicon rod has a diameter of tens of micrometer (㎛) whereas the silicon powder existing on the wall surface of the Siemens reactor has a diameter of tens of nanometer (nm). The present invention utilizes, in the Siemens process, the silicon particles of nm size obtained on the wall surface of the Siemens reactor as a reaction byproduct and not the silicon particles of ㎛ size deposited on the ∩-shaped silicon rod. In such a way, the size of silicon particles to be used as a silicon-based anode active material can be reduced to nm scale and thus its absolute volume change can be minimized, by which the problems caused by the silicon volume change, such as cycle property deterioration and battery life time shortening, can be resolved.
The anode active material of the present invention comprises the silicon particles of nm size (for example, 5~100 nm) and can maintain high capacity even as the cycle proceeds. As an embodiment, the anode active material of the present invention can show an initial capacity ranging from about 1500 to about 2000 mAh/g.
In an embodiment, the anode active material of the present invention can further comprise a metal-based anode active material in addition to the silicon nanoparticles. As the metal-based anode active material, one or more metals selected from the group consisting of Sn, Al, Ge, Co, Cu, Ti, Ni, Li, Pb, Zn, Ag and Au, or alloys thereof can be used, but it is not limited thereto.
In another embodiment, the anode active material of the present invention can further comprise a carbon-based anode active material in addition to the silicon nanoparticles. As the carbon-based anode active material, those known in this field can be used without limitation―for example, crystalline carbons such as graphite, natural black carbon and artificial black carbon, and non-crystalline carbons such as soft carbon and hard carbon can be used alone or in combination of two or more thereof.
The silicon nanoparticles and metal-based anode active material and/or carbon-based anode active material can be mixed by a mechanical treatment method such as ball milling, or dispersed and mixed in a solvent together with a dispersing agent under agitation, ultrasonic wave or the like, but it is not limited thereto.
There is no particular limitation to the method for producing the anode active material of the present invention. The silicon nanoparticles according to the present invention can be prepared by using a conventional The Siemens process known in this field. As an embodiment, the silicon nanoparticles having a diameter ranging from 5 to 100 nm can be obtained through the steps of: a) reacting metal silicon with hydrochloric acid to obtain monosilane or trichlorosilane in gas phase; b) reacting the monosilane or trichlorosilane with a heated ∩-shaped silicon rod in a Siemens reactor; and c) obtaining silicon nanoparticles formed on the wall surface of the Siemens reactor.
According to other aspects of the present invention, an anode material for a lithium secondary battery comprising the anode active material of the present invention, a conductive material and a binder; and an anode for a lithium secondary battery comprising an anode current collector, and the anode material which is coated on the anode current collector; are provided.
The conductive material contained in the anode material increases the overall conductivity of the anode material and improves the output characteristics of the battery. As the conductive material, any of those having a good electrical conductivity and not causing a side reaction in the environment inside the lithium secondary battery can be used without limitation. Preferably, carbon-based materials having high conductivity such as black carbon, conductive carbon and the like are used. In some cases, conductive polymers having high conductivity can also be used. Concretely, the black carbon is not limited to natural black carbon or artificial black carbon. As the conductive carbon, particularly preferred are carbon-based materials having a high conductivity, and concretely, a material selected from the group consisting of carbon blacks such as carbon black, acetylene black, Ketjen black, channel black, furnace black, lamp black, thermal black etc., or materials having a crystal structure comprising grapheme or graphite can be used alone or in combination of two or more thereof. In addition, as a precursor of the above conductive material, any material which is converted into a conductive material in the procedure of calcinations at a relatively low temperature under an oxygen-containing atmosphere―for example, air atmosphere―can be used without particular limitation. The method of adding the conductive material is also not particularly limited, and a conventional method known in this field such as coating on the anode active material can be employed.
As the binder, conventional ones known in this field can be used without limitation. For example, polyvinylidene fluoride (PVDF), polyacrylonitrile, polymethylmethacrylate, vinylidene fluoride/hexafluoropropylene copolymer or the like can be used alone or in combination of two or more thereof.
There is no particular limitation to the method for producing the anode of the present invention. The anode of the present invention can be prepared by using a conventional method known in this field. In an embodiment, the anode of the present invention can be prepared by mixing the anode active material, a conductive material, a binder and a solvent to prepare slurry, and applying the slurry on an anode current collector such as copper and then drying. Filler can be added to the mixture if needed.
According to another aspect of the present invention, a lithium secondary battery comprising the aforesaid anode, a cathode, a separation membrane and a liquid electrolyte is provided. In general, a lithium secondary battery is constituted with an anode comprising an anode material and an anode current collector, a cathode comprising a cathode material and a cathode current collector, and a separation membrane which blocks electron conduction between the anode and cathode, and conducts lithium ions. A liquid electrolyte is included in the empty space between the anode, cathode and separation membrane for conducting lithium ions. There is no particular limitation to the method for preparing the cathode. The cathode can be prepared by using a conventional method known in this field. In an embodiment, the cathode can be prepared by mixing a cathode active material, a conductive material, a binder and a solvent to prepare slurry, and applying the slurry on a cathode current collector such as aluminum and then drying. Filler can be added to the mixture if needed.
There is no particular limitation to the method for preparing the lithium secondary battery of the present invention. The lithium secondary battery can be prepared by using a conventional method known in this field. In an embodiment, the lithium secondary battery can be prepared by placing a porous separation membrane between the anode and cathode, and adding a non-aqueous liquid electrolyte thereto.
The lithium secondary battery of the present invention can be preferably used as a unit cell of a medium- or large-scale battery module comprising plural battery cells, as well as a battery cell used as a power source of small devices such as cellular phones. Applicable medium- or large-scale devices include power tools; electric vehicles (EV) including hybrid electric vehicles (HEV) and plug-in hybrid electric vehicles (PHEV); electric bicycles including E-bikes and E-scooters; electric golf carts; electric trucks; electric commercial vehicles; power storage systems and the like.
The present invention is explained in more detail by the following Example, Comparative Example and Experimental Example. However, the scope of the present invention is not limited by them.
EXAMPLE
Preparation of anode active material
The anode active material was prepared by using the device as shown in Fig. 1 according to the following procedure.
Metallurgical silicon was reacted with hydrochloric acid to prepare trichlorosilane in gas phase. Then, an electric current was supplied from a power supply to a ∩-shaped silicon rod in a Siemens reactor in which an alternating current power controller was connected to an output terminal. The current intensity and voltage were determined by the alternating current power controller, and the electrical power converted to heat in the silicon rod was set. The Siemens reactor was filled with trichlorosilane, and a high-pressure condition was formed. The silicon rod was heated so that the surface temperature of the silicon rod was elevated up to about 1,100℃ by using the loss of electrical power converted to heat due to the electric current conduction at the silicon rod, and hydrogen was then added in the Siemens reactor, by which trichlorosilane was decomposed into silicon and hydrogen chloride, and the decomposed silicon was deposited on the silicon rod in the Siemens reactor. As a byproduct, silicon nanoparticles (anode active material) which had a diameter of tens of nm and were formed on the wall surface of the Siemens reactor, were obtained.
The TEM analysis result and XRD analysis result of the prepared particles are shown in Figs. 4 and 5, respectively. According to Fig. 4, it can be confirmed that the anode active material as prepared above was spherical particles having a diameter of 100 nm or smaller. According to Fig. 5, it can be confirmed that the prepared particles were silicon.
Preparation of anode and cathode
The anode active material as prepared above, a conductive material (super p black, SPB) and a binder (polyvinylidene fluoride, PVDF) were provided with a weight ratio of 75 : 15 : 10. (The charging/discharging capacity hereinafter is a calculated value on the basis of 75% anode active material.) First, the binder was dissolved in an NMP solvent (N-methylpyrrolidone, 99%, Aldrich Co.) for 10 minutes by using a mini mill, and then the anode active material and conductive material were added thereto, and agitated for 30 minutes to obtain a homogeneous slurry. The prepared slurry was applied on a copper foil by using a blade and dried in an oven at 110℃ for 2 hours to evaporate the solvent, and then compressed by using a hot press roll. The prepared anode was dried in a vacuum oven at 120℃ for 12 hours.
A cathode active material of lithium metal, a conductive material (super p black, SPB) and a binder (polyvinylidene fluoride, PVDF) were provided with a weight ratio of 75 : 15 : 10. First, the binder was dissolved in an NMP solvent (N-methylpyrrolidone, 99%, Aldrich Co.) for 10 minutes by using a mini mill, and then the cathode active material and conductive material were added thereto, and agitated for 30 minutes to obtain a homogeneous slurry. The prepared slurry was applied on an aluminum foil by using a blade and dried in an oven at 110℃ for 2 hours to evaporate the solvent, and then compressed by using a hot press roll. The prepared cathode was dried in a vacuum oven at 120℃ for 12 hours.
Preparation of lithium secondary battery
The dried anode prepared as above was cut in a size of 1.4 cm in diameter, and used with the above-prepared cathode to prepare a 2016 type coin cell by using, as an electrolyte, a solution in which 1M LiPF6 was dissolved in ethylene carbonate (EC)/ethyl methyl carbonate (EMC)(v/v = 1/1) and vinylene carbonate (VC, 2 wt%). All procedures of preparing the battery were carried out in a glove box under an argon atmosphere having inner moisture content of 10 ppm or less.
COMPARATIVE EXAMPLE
The anode, cathode and lithium secondary battery were prepared according to the same procedures as described in the Example, except that a commercial silicon powder (633097, 98%, Aldrich Co.) having a diameter of tens of ㎛ was used as the anode active material.
EXPERIMENTAL EXAMPLE
The lithium secondary batteries prepared in the Example and Comparative Example were kept for 24 hours in order to stabilize the batteries. Then, the charging/discharging test was conducted by using WBCS3000L charging/discharging apparatus (Won-A Tech). The charging/discharging was carried out with a current of 0.14 mA (1C/20) in a voltage range of 0.0 to 1.5 V. The cycle property changes of the lithium secondary batteries according to the Example and Comparative Example are shown in Figs. 2 and 3, respectively.
As shown in Figs. 2 and 3, the battery of the Example showed an initial anode capacity of 1750 mAh/g while the battery of the Comparative Example showed an initial anode capacity of 1050 mAh/g, by which it can be known that the battery of the Example showed a higher initial anode capacity than that of the Comparative Example. Furthermore, according to the result of 5 cycles of charging/discharging, the battery of the Example maintained higher initial anode capacity than that of the Comparative Example, by which it can be known that the cycle property and life time characteristics of the battery of the Example were better than those of the Comparative Example.

Claims (10)

  1. An anode active material for a lithium secondary battery comprising silicon nanoparticles having a diameter ranging from 5 to 100 nm.
  2. The anode active material for a lithium secondary battery according to claim 1, wherein the silicon nanoparticles are obtained on the wall surface of a Siemens reactor as a byproduct of a reaction by the Siemens process.
  3. The anode active material for a lithium secondary battery according to claim 2, wherein the silicon nanoparticles are obtained through the steps of:
    a) reacting metal silicon with hydrochloric acid to obtain monosilane or trichlorosilane in gas phase;
    b) reacting the monosilane or trichlorosilane with a ∩-shaped silicon rod in a Siemens reactor; and
    c) obtaining silicon nanoparticles formed on the wall surface of the Siemens reactor.
  4. The anode active material for a lithium secondary battery according to claim 1, which shows an initial capacity ranging from 1500 to 2000 mAh/g.
  5. The anode active material for a lithium secondary battery according to claim 1, further comprising one or more metals selected from the group consisting of Sn, Al, Ge, Co, Cu, Ti, Ni, Li, Pb, Zn, Ag and Au, or alloys thereof.
  6. The anode active material for a lithium secondary battery according to claim 1, further comprising a carbon-based anode active material.
  7. The anode active material for a lithium secondary battery according to claim 6, wherein the carbon-based anode active material is one or more selected from the group consisting of graphite, natural black carbon, artificial black carbon, soft carbon and hard carbon.
  8. An anode material for a lithium secondary battery comprising the anode active material according to any one of claims 1 to 7, a conductive material and a binder.
  9. An anode for a lithium secondary battery comprising an anode current collector, and the anode material according to claim 8 which is coated on the anode current collector.
  10. A lithium secondary battery comprising the anode according to claim 9, a cathode, a separation membrane and a liquid electrolyte.
PCT/KR2010/008931 2010-11-15 2010-12-14 Anode active material for a lithium secondary battery with silicon nanoparticles and lithium secondary battery comprising the same WO2012067298A1 (en)

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