Patent Publication Number: US-2021175488-A1

Title: Negative electrode active material, negative electrode including the same, and secondary battery including the negative electrode

Description:
CROSS-REFERENCE TO RELATED APPLICATIONS 
     This application claims the benefit of Korean Patent Application No. 10-2017-0148838, filed on Nov. 9, 2017, in the Korean Intellectual Property Office, the disclosure of which is incorporated herein in its entirety by reference. 
     TECHNICAL FIELD 
     The present invention relates to a negative electrode active material, a negative electrode including the same, and a secondary battery including the negative electrode. Specifically, the negative electrode active material includes a carbonaceous matrix including a first particle and a second particle, wherein the first particle includes a silicon core, an oxide layer disposed on the silicon core and including SiO x  (0&lt;x≤2), and a coating layer covering at least a portion of the surface of the oxide layer and including LiF, and the second particle includes graphene. 
     BACKGROUND ART 
     Demands for the use of alternative energy or clean energy are increasing due to the rapid increase in the use of fossil fuel, and as a part of this trend, the most actively studied field is a field of electricity generation and electricity storage using an electrochemical reaction. 
     Currently, a typical example of an electrochemical device using such electrochemical energy is a secondary battery and the usage areas thereof are increasing more and more. In recent years, as technology development of and demand for portable devices such as portable computers, mobile phones, and cameras have increased, demands for secondary batteries as an energy source have been significantly increased. 
     In general, a secondary battery is composed of a positive electrode, a negative electrode, an electrolyte, and a separator. The negative electrode includes a negative electrode active material for intercalating and de-intercalating lithium ions from the positive electrode, and as the negative electrode active material, a silicon-based particle having high discharge capacity may be used. However, a silicon-based particle such as SiO x  (0≤x&lt;2) has low initial efficiency, and the volume thereof excessively changes during charging and discharging, causing a side reaction with an electrolyte. Therefore, there arises a problem in that the lifespan and safety of a battery are deteriorated. 
     Typically, in order to solve such a problem, techniques for forming a coating layer on the surface of a silicon-based particle have been used. For example, a technique for forming a carbon coating layer on the surface of a silicon-based particle is used (Korean Patent Laid-Open Publication No. 10-2015-0112746). 
     However, the excessive volume expansion of a silicon-based particle is not easily controlled only by the carbon coating layer, and a side reaction of an electrolyte and the silicon-based particle is not effectively controlled. 
     Therefore, there is a demand for a negative electrode active material capable of effectively controlling the volume change during charging and discharging of a secondary battery, and the side reaction with an electrolyte. 
     PRIOR ART DOCUMENT 
     Patent Document 
     (Patent Document 1) Korean Patent Laid-Open Publication No. 10-2015-0112746 
     DISCLOSURE OF THE INVENTION 
     Technical Problem 
     An aspect of the present invention provides a negative electrode active material which is capable of effectively controlling the volume change during charging and discharging of a secondary battery and the side reaction with an electrolyte, a negative electrode including the same, and a secondary battery including the negative electrode. 
     Technical Solution 
     According to an embodiment of the present invention, there is provided a negative electrode active material including a carbonaceous matrix having a first particle and a second particle, wherein the first particle includes a silicon core, an oxide layer disposed on the silicon core and including SiO x  (0&lt;x≤2), and a coating layer covering at least a portion of the surface of the oxide layer and including LiF, and the second particle includes graphene. 
     According to another aspect of the present invention, there are provided a negative electrode including the negative electrode active material, and a secondary battery including the negative electrode. 
     Advantageous Effects 
     According to a negative electrode active material according to an embodiment of the present invention, the initial efficiency and discharge capacity of a battery may be improved by a coating layer including LiF, and the electrode thickness change rate may be small. In addition, the complexation of a first particle and a second particle may be facilitated by graphene, so that the capacity retention rate is improved and the electrode thickness change rate may be even smaller. 
    
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
         FIG. 1  is a schematic view of a negative electrode active material of the present invention; 
         FIG. 2  is a schematic view of a first particle included in a negative electrode active material according to the present invention; and 
         FIG. 3  is a graph of ToF-SIMS results of Example 1 and Comparative Example 1. 
     
    
    
     MODE FOR CARRYING OUT THE INVENTION 
     Hereinafter, the present invention will be described in more detail to facilitate understanding of the present invention. 
     It will be understood that words or terms used in the specification and claims shall not be interpreted as having the meaning defined in commonly used dictionaries. It will be further understood that the words or terms should be interpreted as having a meaning that is consistent with their meaning in the context of the relevant art and the technical idea of the invention, based on the principle that an inventor may properly define the meaning of the words or terms to best explain the invention. 
     The terminology used herein is for the purpose of describing particular exemplary embodiments only and is not intended to be limiting of the present invention. In the specification, the terms of a singular form may include plural forms unless referred to the contrary. 
     It will be further understood that the terms “include,” “comprise,” or “have” when used in this specification, specify the presence of stated features, numbers, steps, elements, or combinations thereof, but do not preclude the presence or addition of one or more other features, numbers, steps, elements, or combinations thereof. 
     Referring to  FIG. 1  and  FIG. 2 , a negative electrode active material ( 100 ) according to an embodiment of the present invention includes a carbonaceous matrix ( 130 ) having a first particle ( 110 ) and a second particle ( 120 ), wherein the first particle ( 110 ) may include a silicon core ( 111 ), an oxide layer ( 112 ) disposed on the silicon core ( 111 ) and including SiO x  (0&lt;x≤2), and a coating layer ( 113 ) covering at least a portion of the surface of the oxide layer ( 112 ) and including LiF, and the second particle ( 120 ) may include graphene. 
     The silicon core ( 111 ) may include Si, and may be specifically made of Si. Accordingly, the capacity of a secondary battery may be increased. 
     The average particle diameter (D 50 ) of the silicon core ( 111 ) may be 40 nm to 400 nm, specifically 60 nm to 200 nm, and more specifically 80 nm to 150 nm. When the above range is satisfied, a silicon core of a nano size is not easily broken during charging and discharging of the battery, and the intercalation and de-intercalation of lithium may be effectively performed. In the present specification, an average particle diameter (D 50 ) may be defined as a particle diameter corresponding to 50% of the volume accumulation in a particle diameter distribution curve of a particle. The average particle diameter (D 50 ) may be measured by using, for example, a laser diffraction method. The laser diffraction method generally enables measurement of a particle diameter of several millimeters from a sub-micron region, so that results of high reproducibility and high resolution may be obtained. 
     The oxide layer ( 112 ) may be disposed on the silicon core ( 111 ). Specifically, the oxide layer ( 112 ) may cover at least a portion of the silicon core ( 111 ). 
     The oxide layer ( 112 ) may include SiO x  (0&lt;x≤2), and may specifically include SiO 2 . Accordingly, during charging and discharging of the secondary battery, the excessive volume change of the silicon core ( 111 ) may be controlled. 
     The thickness of the oxide layer ( 112 ) may be 0.01 nm to 20 nm, specifically 0.05 nm to 15 nm, and more specifically 0.1 nm to 10 nm. When the above range is satisfied, the capacity of the secondary battery is maintained, and the excessive volume change of the silicon core ( 111 ) may be effectively controlled. 
     The coating layer ( 113 ) may cover at least a portion of the surface of the oxide layer ( 112 ). Specifically, the coating layer ( 113 ) may be disposed so as to cover all of the surface of the oxide layer ( 112 ), or disposed so as to cover a portion of the surface. 
     The coating layer ( 113 ) may include LiF, and may be specifically made of LiF. The LiF of the coating layer ( 113 ) may serve as a kind of SEI film so that a side reaction between the silicon core ( 111 ) and an electrolyte may be prevented, and the lithium ion conductivity may be improved. Furthermore, the excessive volume expansion of the silicon core ( 111 ) may be controlled. Accordingly, the initial efficiency of a negative electrode may be improved. Specifically, although not limited thereto, the LiF included in the coating layer ( 113 ) may be made of a crystalline phase and an amorphous phase by a heat treatment applied during the production of a negative electrode active material. At this time, the lithium ion conductivity may be improved by the interface between the crystalline phase and the amorphous phase. 
     The LiF may be included in an amount of 0.05 wt % to 25 wt % based on the total weight of the negative electrode active material ( 100 ), specifically 0.1 wt % to 20 wt %, and more specifically 0.5 wt % to 15 wt %. When the above range is satisfied, a side reaction between the silicon core ( 111 ) and the electrolyte may be effectively prevented, and the lithium ion conductivity may be effectively improved. Furthermore, the excessive volume expansion of the silicon core ( 111 ) may be effectively controlled. As a result, the initial efficiency of the negative electrode may be effectively improved. 
     The thickness of the coating layer ( 113 ) may be 0.01 nm to 50 nm, specifically 0.05 nm to 15 nm, and more specifically 0.1 nm to 10 nm. When the above range is satisfied, the effect of the coating layer ( 113 ) described above may be further improved. 
     The oxide layer ( 112 ) may further include lithium silicate. The lithium silicate may be formed when an appropriate ratio of an oxide layer and a coating layer are heat-treated at a specific heat treatment temperature in the formation of the carbonaceous matrix. That is, the lithium silicate may be a by-product formed by the reaction of the LiF and the oxide layer ( 112 ). Since the initial irreversible capacity of a battery may be reduced by the lithium silicate, the initial efficiency of the battery may be improved. The lithium silicate may include at least any one of Li 2 SiO 3 , Li 4 SiO 4 , and Li 2 Si 2 O 5 , and may specifically include Li 2 SiO 3 . 
     The second particle ( 120 ) may include graphene, and specifically the second particle ( 120 ) may be made of graphene. Since the graphene is included in a negative electrode active material, a conductive path inside the active material may be secured. Furthermore, when compared with typical spherical graphite, graphene has a shape of a very thin plate, so that a first particle and a second particle can be aggregated with higher density during the complexation of the first particle and the second particle. Accordingly, the energy density of a prepared negative electrode may be improved, and a thinner electrode may be prepared. Furthermore, battery resistance may be reduced. In addition, since the conductivity of the negative electrode may be improved by graphene, sufficient conductivity of the negative electrode may be secured even when the content of a conductive material is less than that of a typical case. In the present invention, graphene refers to a thin film-shaped carbonaceous structure having a thickness of 200 nm or less, specifically 10 nm to 200 nm, and more specifically 10 nm to 80 nm. 
     The BET specific surface area value of the graphene may be 50 m 2 /g to 3000 m 2 /g, specifically 75 m 2 /g to 2000 m 2 /g, and more specifically 100 m 2 /g to 1000 m 2 /g. When the above range is satisfied, the first particle ( 110 ) may be effectively disposed among the second particles ( 120 ), so that the complexation of the first particle and the second particle may be facilitated. 
     The thickness of the graphene may be 0.1 μm to 20 μm, specifically 0.2 μm to 15 μm, and more specifically 0.5 μm to 10 μm. When the above range is satisfied, the size of the particles formed by the complexation the first particle ( 110 ) and the second particle ( 120 ) may be uniform. In addition, when the above range is satisfied, particles are easily dispersed during the preparation of a negative electrode, and the agglomeration between particles may be suppressed. At this time, the size of the graphene refers to the longest length of a line assumed to be drawn from one point to another point on a plane of the graphene when the graphene is placed on the plane and viewed in a direction perpendicular to the plane. 
     The weight ratio of the first particle ( 110 ) and the second particle ( 120 ) may be 1:9 to 9:1, specifically 2:8 to 8:2, and more specifically 4:6 to 7:3. When the weight ratio is satisfied, the capacity of the battery may be improved, and at the same time, since the complexation of the first particle ( 110 ) and the second particle ( 120 ) may be stably achieved, the stability of the battery may be improved. 
     Referring to  FIG. 1 , the negative electrode active material ( 100 ) may include a structure in which the first particle ( 110 ) and the second particle ( 120 ) are in contact with each other to form a composite. 
     The carbonaceous matrix ( 130 ) may be present in the form of covering at least a portion of the first particle ( 110 ) and the second particle ( 120 ), and specifically the carbonaceous matrix ( 130 ) may be present in the form of covering all of the first particle ( 110 ) and the second particle ( 120 ). 
     The carbonaceous matrix ( 130 ) may include at least any one of amorphous carbon and crystalline carbon. 
     The crystalline carbon may further improve the conductivity of the negative electrode active material ( 100 ). The crystalline carbon may include at least any one selected from the group consisting of fullerene, carbon nanotube, and graphene. 
     The amorphous carbon may appropriately maintain the strength of the carbonaceous matrix ( 130 ), thereby suppressing the expansion of the silicon core ( 111 ). The amorphous carbon may be at least any one carbide selected from the group consisting of tar, pitch, and other organic materials, or a carbon-based material formed by using hydrocarbon as a source of chemical vapor deposition. 
     The carbide of the other organic materials may be a carbide of an organic material selected from the group consisting of sucrose, glucose, galactose, fructose, lactose, mannose, ribose, aldohexose or ketohexose carbides and combinations thereof. 
     The hydrocarbon may be substituted or unsubstituted aliphatic or alicyclic hydrocarbon, or substituted or unsubstituted aromatic hydrocarbon. Aliphatic or alicyclic hydrocarbon of the substituted or unsubstituted aliphatic or alicyclic hydrocarbon may be methine, ethene, ethylene, acetylene, propene, butane, butene, pentene, isobutene or hexane, and the like. Aromatic hydrocarbon of the substituted or unsubstituted aromatic hydrocarbon may be benzene, toluene, xylene, styrene, ethylbenzene, diphenylmethane, naphthalene, phenol, cresol, nitrobenzene, chlorobenzene, indene, coumarone, pyridine, anthracene, or phenanthrene, and the like. 
     The carbonaceous matrix ( 130 ) may be included in an amount of 5 wt % to 50 wt % based on the total weight of the negative electrode active material ( 100 ), specifically 10 wt % to 45 wt %, and more specifically 12 wt % to 40 wt %. When the above range is satisfied, a conductive path may be effectively secured. At the same time, since the carbonaceous matrix ( 130 ) may effectively maintain the bonding of the first particle ( 110 ) and the second particle ( 120 ), the volume expansion of the negative electrode active material ( 100 ) may be effectively controlled. 
     A negative electrode active material according to another embodiment of the present invention is the same as the negative electrode active material of the embodiment described above except that a second particle further includes flake graphite. Accordingly, the present invention will be described with reference to the differences. 
     The flake graphite may be at least any one selected from the group consisting of natural graphite, artificial graphite, or graphite-based particles produced by crushing artificial graphite, and the like. When a second particle further includes the flake graphite, the complexation of a first particle and the second particle may be facilitated, so that the shape of a prepared negative electrode active material may be uniform. In addition, it is possible to prepare a negative electrode active material having a more uniform size. 
     The BET specific surface area value of the flake graphite may be 1 m 2 /g to 200 m 2 /g, specifically 1.5 m 2 /g to 100 m 2 /g, and more specifically 2 m 2 /g to 50 m 2 /g. When the above range is satisfied, the first particle may be effectively disposed among the second particles, so that the complexation of the first particle and the second particle may be facilitated. 
     The thickness of the flake graphite may be 250 nm to 5 μm, specifically 400 nm to 3 μm, and more specifically 500 nm to 2.5 μm. 
     A method for preparing a negative electrode active material according to another embodiment of the present invention may include preparing a silicon core having an oxide layer including SiO x  (0&lt;x≤2) disposed on the surface thereof; forming a coating layer including LiF on the oxide layer to form a first particle; and forming a carbonaceous matrix including particles in which the first particle and a second particle including graphene are combined. 
     In the preparing of a silicon core having an oxide layer including SiO x  (0&lt;x≤2) disposed on the surface thereof, the oxide layer may be formed by heat treating the silicon core in oxygen or air, or may be formed on the silicon core through a milling process. However, the present invention is not necessarily limited thereto. 
     In the forming of a coating layer including LiF on the oxide layer to form a nano-particle, the coating layer may be formed by the following method. 
     The coating layer may be formed by a method in which the silicon core formed on the oxide layer is milled with the LiF and then pulverized and mixed. Alternatively, the coating layer may be formed by dispersing the silicone core in a solvent, and then mixing with lithium acetate and ammonium fluoride theretogether. Alternatively, the coating layer may be formed by disposing the LiF on the oxide layer through sputtering. However, the present invention is not necessarily limited thereto. 
     The forming of a carbonaceous matrix may include the following method. 
     The first particle is dispersed in a solvent to prepare a mixed solution. Then, the second particle and an organic solution that can be a pitch or a carbon source are dispersed in the mixed solution to prepare a slurry. The slurry is heat treated and then pulverized to form the carbonaceous matrix, and complexation may be simultaneously achieved. Alternatively, the slurry may be subjected to spay drying and pulverized to form the carbonaceous matrix, and complexation may be simultaneously achieved. Alternatively, only the first particle and the second particle may be mixed and heat treated to form a secondary particle, and then either by using a chemical vapor deposition method (CVD) or by mixing an organic material such as a peach and carbonizing the same, the carbonaceous matrix may be formed on the surface of the secondary particle. However, the present invention is not necessarily limited thereto. 
     A negative electrode according to another embodiment of the present invention may include a negative electrode active material, and in this case, the negative electrode active material may be the same as the negative active materials of the embodiments described above. Specifically, the negative electrode may include a current collector and a negative electrode active material layer disposed on the current collector. The negative electrode active material layer may include the negative electrode active material. Furthermore, the negative electrode active material layer may include a binder and/or a conductive material. 
     The current collector is not particularly limited as long as it has conductivity without causing a chemical change in the battery. For example, as the current collector, copper, stainless steel, aluminum, nickel, titanium, fired carbon, or aluminum or stainless steel that is surface-treated with one of carbon, nickel, titanium, silver, and the like may be used. Specifically, a transition metal which adsorbs carbon such as copper and nickel well may be used as the current collector. The thickness of the current collector may be from 6 μm to 20 μm, but the thickness of the current collector is not limited thereto. 
     The binder may include at least any one selected from the group consisting of a polyvinylidene fluoride-hexafluoropropylene copolymer (PVDF-co-HFP), polyvinylidenefluoride, polyacrylonitrile, polymethylmethacrylate, polyvinyl alcohol, carboxymethyl cellulose (CMC), starch, hydroxypropyl cellulose, regenerated cellulose, polyvinylpyrrolidone, tetrafluoroethylene, polyethylene, polypropylene, polyacrylic acid, an ethylene-propylene-diene monomer (EPDM), a sulfonated EPDM, styrene-butadiene rubber (SBR), fluorine rubber, poly acrylic acid, materials having the hydrogen thereof substituted with Li, Na, or Ca, and the like, and a combination thereof. In addition, the binder may include various copolymers thereof. 
     The conductive material is not particularly limited as long as it has conductivity without causing a chemical change in the battery. For example, graphite such as natural graphite or artificial graphite; a carbon-based material such as carbon black, acetylene black, Ketjen black, channel black, furnace black, lamp black, and thermal black; conductive fiber such as carbon fiber and metal fiber; a conductive tube such as a carbon nanotube; metal powder such as fluorocarbon powder, aluminum powder, and nickel powder; a conductive whisker such as zinc oxide and potassium titanate; a conductive metal oxide such as titanium oxide; a conductive material such as a polyphenylene derivative, and the like may be used. 
     A secondary battery according to another embodiment of the present invention may include a negative electrode, a positive electrode, a separator interposed between the positive electrode and the negative electrode, and an electrolyte. The negative electrode is the same as the negative electrode described above. Since the negative electrode has been described above, the detailed description thereof will be omitted. 
     The positive electrode may include a positive electrode current collector, and a positive electrode active material layer formed on the positive electrode current collector and including the positive electrode active material. 
     In the positive electrode, the positive electrode current collector is not particularly limited as long as it has conductivity without causing a chemical change in the battery. For example, stainless steel, aluminum, nickel, titanium, fired carbon, or aluminum or stainless steel that is surface-treated with one of carbon, nickel, titanium, silver, and the like may be used. Also, the positive electrode current collector may typically have a thickness of 3 μm to 500 μm, and microscopic irregularities may be prepared on the surface of the positive electrode current collector to improve the adhesion of the positive electrode active material. The positive electrode current collector may be used in various forms of such as a film, a sheet, a foil, a net, a porous body, a foam, and a non-woven body. 
     The positive electrode active material may be a positive electrode active material commonly used in the art. Specifically, the positive electrode active material may be a layered compound such as lithium cobalt oxide (LiCoO 2 ) and lithium nickel oxide (LiNiO 2 ), or a compound substituted with one or more transition metals; a lithium iron oxide such as LiFe 3 O 4 ; a lithium manganese oxide such as Li 1+c1 Mn 2−c1 O 4  (0≤c1≤0.33), LiMnO 3 , LiMn 2 O 3 , and LiMnO 2 ; lithium copper oxide (Li 2 CuO 2 ); a vanadium oxide such as LiV 3 O 8 , V 2 O 5 , and Cu 2 V 2 O 7 ; a Ni-site type lithium nickel oxide represented by the formula LiNi 1−c2 M c2 O 2  (wherein M is any one of Co, Mn, Al, Cu, Fe, Mg, B or Ga, and 0.01≤c2≤0.3); a lithium manganese composite oxide represented by the formula LiMn 2−c3 M c3 O 2  (wherein, M is any one of Co, Ni, Fe, Cr, Zn, or Ta, and 00.01≤c3≤0.1), or by the formula Li 2 Mn 3 MO 8  (wherein, M is any one of Fe, Co, Ni, Cu, or Zn); LiMn 2 O 4  having a part of Li in the formula substituted with an alkaline earth metal ion, and the like, but is not limited thereto. The positive electrode may be a Li-metal. 
     The positive electrode active material layer may include a positive electrode conductive material and a positive electrode binder, together with the positive electrode active material described above. 
     At this time, the positive electrode conductive material is used to impart conductivity to an electrode, and any positive electrode conductive material may be used without particular limitation as long as it has electronic conductivity without causing a chemical change in a battery to be constituted. Specific examples thereof may include graphite such as natural graphite or artificial graphite; a carbon-based material such as carbon black, acetylene black, Ketjen black, channel black, furnace black, lamp black, thermal black, and carbon fiber; metal powder or metal fiber such as copper, nickel, aluminum, and silver; a conductive whisker such as a zinc oxide whisker and a potassium titanate whisker; a conductive metal oxide such as titanium oxide; or a conductive polymer such as a polyphenylene derivative, and any one thereof or a mixture of two or more thereof may be used. 
     In addition, the binder serves to improve the bonding between positive electrode active material particles and the adhesion between the positive electrode active material and the positive electrode current collector. Specific examples of the binder may include polyvinylidene fluoride (PVDF), a polyvinylidene fluoride-hexafluoropropylene copolymer (PVDF-co-HFP), polyvinyl alcohol, polyacrylonitrile, carboxymethyl cellulose (CMC), starch, hydroxypropyl cellulose, regenerated cellulose, polyvinylpyrrolidone, tetrafluoroethylene, polyethylene, polypropylene, an ethylene-propylene-diene monomer (EPDM), a sulfonated EPDM, styrene-butadiene rubber (SBR), fluorine rubber, or various copolymers thereof, and any one thereof or a mixture of two or more thereof may be used. 
     The separator is to separate the negative electrode and the positive electrode and to provide a movement path for lithium ions. Any separator may be used without particular limitation as long as it is a separator commonly used in a secondary battery. Particularly, a separator having excellent moisture-retention of an electrolyte as well as low resistance to ion movement in the electrolyte is preferable. Specifically, a porous polymer film, for example, a porous polymer film manufactured using a polyolefin-based polymer such as an ethylene homopolymer, a propylene homopolymer, an ethylene/butene copolymer, an ethylene/hexene copolymer, and an ethylene/methacrylate copolymer, or a laminated structure having two or more layers thereof may be used. Also, a typical porous non-woven fabric, for example, a non-woven fabric formed of glass fiber having a high melting point, or polyethylene terephthalate fiber, and the like may be used as the separator. Also, a coated separator including a ceramic component or a polymer material may be used to secure heat resistance or mechanical strength, and may be selectively used having a single layered or a multi-layered structure. 
     The electrolyte may be an organic liquid electrolyte, an inorganic liquid electrolyte, a solid polymer electrolyte, a gel-type polymer electrolyte, a solid inorganic electrolyte, a molten-type inorganic electrolyte, and the like, which may be used in the preparation of a lithium secondary battery, but is not limited thereto. 
     Specifically, the electrolyte may include a non-aqueous organic solvent and a lithium salt. 
     As the non-aqueous organic solvent, for example, an aprotic organic solvent, such as N-methyl-2-pyrrolidone, propylene carbonate, ethylene carbonate, butylene carbonate, dimethyl carbonate, diethyl carbonate, γ-butyrolactone, 1,2-dimethoxy ethane, tetrahydroxy franc, 2-methyl tetrahydrofuran, dimethyl sulfoxide, 1,3-dioxolane, formamide, diemthylformamide, dioxolane, acetonitrile, nitromethane, methyl formate, methyl acetate, phosphate triester, trimethoxy methane, a dioxolane derivative, sulfolane, methyl sulfolane, 1,3-dimethyl-2-imidazolidinone, a propylene carbonate derivative, a tetrahydrofuran derivative, ether, methyl propionate, and ethyl propionate may be used. 
     In particular, among the carbonate-based organic solvents, cyclic carbonates ethylene carbonate and propylene carbonate may be preferably used since they are organic solvents of a high viscosity having high permittivity to dissociate a lithium salt well. Furthermore, such a cyclic carbonate may be more preferably used since the cyclic carbonate may be mixed with a linear carbonate of a low viscosity and low permittivity such as dimethyl carbonate and diethyl carbonate in an appropriate ratio to prepare an electrolyte having a high electric conductivity. 
     As the metal salt, a lithium salt may be used. The lithium salt is a material which is easily dissolved in the non-aqueous electrolyte. For example, as an anion of the lithium salt, one or more selected from the group consisting of F − , Cl − , I − , NO 3   − , N(CN) 2   − , BF 4   − , ClO 4   − , PF 6   − , (CF 3 ) 2 PF 4   − , (CF 3 ) 3 PF 3   − , (CF 3 ) 4 PF 2   − , (CF 3 ) 5 PF − , (CF 3 ) 6 P − , CF 3 SO 3   − , CF 3 CF 2 SO 3   − , (CF 3 SO 2 ) 2 N − , (FSO 2 ) 2 N − , CF 3 CF 2 (CF 3 ) 2 CO − , (CF 3 SO 2 ) 2 CH − , (SF 5 ) 3 C − , (CF 3 SO 2 ) 3 C − , CF 3 (CF 2 ) 7 SO 3   − , CF 3 CO 2   − , CH 3 CO 2   − , SCN − , and (CF 3 CF 2 SO 2 ) 2 N −  may be used. 
     In the electrolyte, in order to improve the lifespan characteristics of a battery, to suppress the decrease in battery capacity, and to improve the discharge capacity of the battery, one or more additives, for example, a halo-alkylene carbonate-based compound such as difluoroethylene carbonate, pyridine, triethylphosphite, triethanolamine, cyclic ether, ethylenediamine, n-glyme, hexaphosphoric triamide, a nitrobenzene derivative, sulfur, a quinone imine dye, N-substituted oxazolidinone, N,N-substituted imidazolidine, ethylene glycol dialkyl ether, an ammonium salt, pyrrole, 2-methoxy ethanol, or aluminum trichloride, and the like may be further included other than the above electrolyte components. 
     According to another embodiment of the present invention, a battery module including the secondary battery as a unit cell, and a battery pack including the same are provided. The battery module and the battery pack include the secondary battery which has high capacity, high rate characteristics, and cycle characteristics, and thus, may be used as a power source of a medium-and-large sized device selected from the group consisting of an electric car, a hybrid electric vehicle, a plug-in hybrid electric vehicle, and a power storage system. 
     Hereinafter, preferred embodiments of the present invention will be described in detail to facilitate understanding of the present invention. However, the embodiments are merely illustrative of the present invention, and thus, it will be apparent to those skilled in the art that various modifications and variations can be made without departing from the scope and spirit of the present invention as disclosed in the accompanying claims. It is obvious that such variations and modifications fall within the scope of the appended claims. 
     EXAMPLES AND COMPARATIVE EXAMPLES 
     Example 1: Preparation of Battery 
     (1) Preparation of a Negative Electrode Active Material 
     10 g of silicon (Si) having a maximum particle diameter (D max ) of 45 μm and 0.2 g of LiF were added to 30 g of isopropanol to prepare a mixed solution. Thereafter, the mixture was pulverized for 30 hours at a bead rotation speed of 1,200 rpm using beads made of zirconia (average particle diameter: 0.3 mm). At this time, the average particle diameter (D 50 ) of the generated silicon was 100 nm, the thickness of SiO 2  formed on the surface of the silicon surface was 10 nm, and the thickness of LiF disposed on the SiO 2  was 0.01 nm to 10 nm. 
     Thereafter, 1.54 g of graphene having a specific surface area of 500 m 2 /g and a size of 5 μm and 3.85 g of a solid phase pitch were added to the mixed solution and dispersed to prepare a slurry. 
     The slurry and ethanol/water (volume ratio=1:9) were mixed at a volume ratio of 1:10 to prepare a dispersion for spray drying. The dispersion was spray dried through a mini spray-dryer (manufacturer: Buchi, Model: B-290 Mini Spray-Dryer) under the conditions of an inlet temperature of 180° C., an aspirator of 95% and a feeding rate of 12. Thereafter, 10 g of the spray-dried mixture (composite) was heat treated at 950° C. under a nitrogen atmosphere to prepare a negative electrode active material. LiF (corresponding to the coating layer of the present invention) in the prepared negative electrode active material was 1.3 wt % based on the total weight of the negative electrode active material. The Li content was measured by ICP and the F content was measured by ion chromatography, and then the sum was calculated. 
     (2) Preparation of Negative Electrode 
     The prepared negative electrode active material, fine graphite as a conductive material, and polyacrylonitrile as a binder were mixed at a weight ratio of 7:2:1 to prepare 0.2 g of a mixture. 3.1 g of N-methyl-2-pyrrolidone (NMP) as a solvent was added to the mixture to prepare a negative electrode slurry. The negative electrode slurry was applied on a copper (Cu) metal thin film having a thickness of 20 μm, which is a negative electrode current collector, and then dried. At this time, the temperature of circulated air was 80° C. Thereafter, the copper (Cu) metal thin film applied with the negative electrode slurry and then dried was roll pressed and dried in a vacuum oven at 130° C. for 12 hours to prepare a negative electrode. 
     (3) Preparation of Secondary Battery 
     A lithium (Li) metal thin film, which was prepared by cutting the prepared negative electrode into a circular shape of 1.7671 cm 2 , was prepared as a positive electrode. A porous polyethylene separator was interposed between the positive electrode and the negative electrode, and then vinylene carbonate dissolved in 0.5 wt % was dissolved in a mixed solution in which methyl ethyl carbonate (EMC) and ethylene carbonate (EC) are mixed in a mixing volume ratio of 7:3. Thereafter, an electrolyte in which LiPF6 of 1.0 M concentration is dissolved was injected to manufacture a lithium coin half-cell. 
     Example 2: Preparation of Battery 
     (1) Preparation of Negative Electrode Active Material 
     10 g of silicon (Si) having a maximum particle diameter (D max ) of 45 μm and 0.2 g of LiF were added to 30 g of isopropanol to prepare a mixed solution. Thereafter, the mixture was pulverized for 30 hours at a bead rotation speed of 1,200 rpm using beads made of zirconia (average particle diameter: 0.3 mm). At this time, the average particle diameter (D 50 ) of the generated silicon was 100 nm, the thickness of SiO 2  formed on the surface of the silicon surface was 10 nm, and the thickness of LiF disposed on the SiO 2  was 0.01 nm to 30 nm. 
     Thereafter, 1.54 g of graphene having a specific surface area of 500 m 2 /g and a size of 5 μm and 3.85 g of a solid phase pitch were added to the mixed solution and dispersed to prepare a slurry. 
     The slurry and ethanol/water (volume ratio=1:9) were mixed at a volume ratio of 1:10 to prepare a dispersion for spray drying. The dispersion was spray dried through a mini spray-dryer (manufacturer: Buchi, Model: B-290 Mini Spray-Dryer) under the conditions of an inlet temperature of 180° C., an aspirator of 95% and a feeding rate of 12. Thereafter, 10 g of the spray-dried mixture (composite) was heat treated at 950° C. under a nitrogen atmosphere to prepare a negative electrode active material. LiF (corresponding to the coating layer of the present invention) in the prepared negative electrode active material was 16 wt % based on the total weight of the negative electrode active material. The Li content was measured by ICP and the F content was measured by ion chromatography, and then the sum was calculated. 
     (2) Preparation of a Negative Electrode and a Secondary Battery 
     A negative electrode and a secondary battery were prepared in the same manner as in Example 1 except that the negative electrode active material was used. 
     Example 3: Preparation of Battery 
     (1) Preparation of Negative Electrode Active Material 
     10 g of silicon (Si) having a maximum particle diameter (D max ) of 45 μm and 0.05 g of LiF were added to 30 g of isopropanol to prepare a mixed solution. Thereafter, the mixture was pulverized for 30 hours at a bead rotation speed of 1,200 rpm using beads made of zirconia (average particle diameter: 0.3 mm). At this time, the average particle diameter (D 50 ) of the generated silicon was 100 nm, the thickness of SiO 2  formed on the surface of the silicon surface was 10 nm, and the thickness of LiF disposed on the SiO 2  was 0.01 nm to 10 nm. 
     Thereafter, 1.54 g of graphene having a specific surface area of 500 m 2 /g and a size of 5 μm and 3.85 g of a solid phase pitch were added to the mixed solution and dispersed to prepare a slurry. 
     The slurry and ethanol/water (volume ratio=1:9) were mixed at a volume ratio of 1:10 to prepare a dispersion for spray drying. The dispersion was spray dried through a mini spray-dryer (manufacturer: Buchi, Model: B-290 Mini Spray-Dryer) under the conditions of an inlet temperature of 180° C., an aspirator of 95% and a feeding rate of 12. Thereafter, 10 g of the spray-dried mixture (composite) was heat treated at 950° C. under a nitrogen atmosphere to prepare a negative electrode active material. LiF (corresponding to the coating layer of the present invention) in the prepared negative electrode active material was 0.3 wt % based on the total weight of the negative electrode active material. The Li content was measured by ICP and the F content was measured by ion chromatography, and then the sum was calculated. 
     (2) Preparation of Negative Electrode and Secondary Battery 
     A negative electrode and a secondary battery were prepared in the same manner as in Example 1 except that the negative electrode active material was used. 
     Example 4: Preparation of Battery 
     (1) Preparation of Negative Electrode Active Material 
     10 g of silicon (Si) having a maximum particle diameter (D max ) of 45 μm and 0.2 g of LiF were added to 30 g of isopropanol to prepare a mixed solution. Thereafter, the mixture was pulverized for 30 hours at a bead rotation speed of 1,200 rpm using beads made of zirconia (average particle diameter: 0.3 mm). At this time, the average particle diameter (D 50 ) of the generated silicon was 100 nm, the thickness of SiO 2  formed on the surface of the silicon surface was 10 nm, and the thickness of LiF disposed on the SiO 2  was 0.01 nm to 10 nm. 
     Thereafter, 0.77 g of graphene having a specific surface area of 500 m 2 /g and a size of 5 μm, 0.77 g of flake graphite having a specific surface area of 45 m 2 /g and a thickness of 2 μm, and 3.85 g of a solid phase pitch were added to the mixed solution and dispersed to prepare a slurry. 
     The slurry and ethanol/water (volume ratio=1:9) were mixed at a volume ratio of 1:10 to prepare a dispersion for spray drying. The dispersion was spray dried through a mini spray-dryer (manufacturer: Buchi, Model: B-290 Mini Spray-Dryer) under the conditions of an inlet temperature of 180° C., an aspirator of 95% and a feeding rate of 12. Thereafter, 10 g of the spray-dried mixture (composite) was heat treated at 950° C. under a nitrogen atmosphere to prepare a negative electrode active material. LiF (corresponding to the coating layer of the present invention) in the prepared negative electrode active material was 1.3 wt % based on the total weight of the negative electrode active material. The Li content was measured by ICP and the F content was measured by ion chromatography, and then the sum was calculated. 
     (2) Preparation of a Negative Electrode and a Secondary Battery 
     A negative electrode and a secondary battery were prepared in the same manner as in Example 1 except that the negative electrode active material was used. 
     Example 5: Preparation of Battery 
     (1) Preparation of Negative Electrode Active Material 
     10 g of silicon (Si) having a maximum particle diameter (D max ) of 45 μm and 0.2 g of LiF were added to 30 g of isopropanol to prepare a mixed solution. Thereafter, the mixture was pulverized for 5 hours at a bead rotation speed of 1,200 rpm using beads made of zirconia (average particle diameter: 0.3 mm). At this time, the average particle diameter (D 50 ) of the generated silicon was 430 nm, the thickness of SiO 2  formed on the surface of the silicon surface was 10 nm, and the thickness of LiF disposed on the SiO 2  was 0.01 nm to 10 nm. 
     Thereafter, 10 g of graphene having a specific surface area of 500 m 2 /g and a size of 5 μm and 5 g of a solid phase pitch were added to the mixed solution and dispersed to prepare a slurry. 
     The slurry and ethanol/water (volume ratio=1:9) were mixed at a volume ratio of 1:10 to prepare a dispersion for spray drying. The dispersion was spray dried through a mini spray-dryer (manufacturer: Buchi, Model: B-290 Mini Spray-Dryer) under the conditions of an inlet temperature of 180° C., an aspirator of 95% and a feeding rate of 12. Thereafter, 10 g of the spray-dried mixture (composite) was heat treated at 950° C. under a nitrogen atmosphere to prepare a negative electrode active material. LiF (corresponding to the coating layer of the present invention) in the prepared negative electrode active material was 0.8 wt % based on the total weight of the negative electrode active material. The Li content was measured by ICP and the F content was measured by ion chromatography, and then the sum was calculated. 
     (2) Preparation of Negative Electrode and Secondary Battery 
     A negative electrode and a secondary battery were prepared in the same manner as in Example 1 using the negative electrode active material. 
     Comparative Example 1 Preparation of Battery 
     (1) Preparation of Negative Electrode Active Material 
     A negative electrode active material was prepared in the same manner as in Example 1 except that LiF was not added when preparing a slurry in the preparation of a negative electrode active material of Example 1. 
     (2) Preparation of Negative Electrode and Secondary Battery 
     A negative electrode and a secondary battery were prepared in the same manner as in Example 1 using the negative electrode active material. 
     Comparative Example 2 Preparation of Battery 
     (1) Preparation of Negative Electrode Active Material 
     10 g of silicon (Si) having a maximum particle diameter (D max ) of 45 μm and 0.2 g of LiF were added to 30 g of isopropanol to prepare a mixed solution. Thereafter, the mixture was pulverized for 30 hours at a bead rotation speed of 1,200 rpm using beads made of zirconia (average particle diameter: 0.3 mm). At this time, the average particle diameter (D 50 ) of the generated silicon was 100 nm, the thickness of SiO 2  formed on the surface of the silicon surface was 10 nm, and the thickness of LiF disposed on the SiO 2  was 0.01 nm to 10 nm. 
     Thereafter, 1.54 g of spherical graphite having a specific surface area of 50 m 2 /g and 3.85 g of a solid phase pitch were added to the mixed solution and dispersed to prepare a slurry. 
     The slurry and ethanol/water (volume ratio=1:9) were mixed at a volume ratio of 1:10 to prepare a dispersion for spray drying. The dispersion was spray dried through a mini spray-dryer (manufacturer: Buchi, Model: B-290 Mini Spray-Dryer) under the conditions of an inlet temperature of 180° C., an aspirator of 95% and a feeding rate of 12. Thereafter, 10 g of the spray-dried mixture (composite) was heat treated at 950° C. under a nitrogen atmosphere to prepare a negative electrode active material. LiF (corresponding to the coating layer of the present invention) in the prepared negative electrode active material was 1.3 wt % based on the total weight of the negative electrode active material. The Li content was measured by ICP and the F content was measured by ion chromatography, and then the sum was calculated. 
     2) Preparation of Negative Electrode and Secondary Battery 
     A negative electrode and a secondary battery were prepared in the same manner as in Example 1 using the negative electrode active material. 
     Comparative Example 3 Preparation of Battery 
     (1) Preparation of a Negative Electrode Active Material 
     10 g of silicon (Si) having a maximum particle diameter (D max ) of 45 μm and 0.2 g of LiF were added to 30 g of isopropanol to prepare a mixed solution. Thereafter, the mixture was pulverized for 30 hours at a bead rotation speed of 1,200 rpm using beads made of zirconia (average particle diameter: 0.3 mm). At this time, the average particle diameter (D 50 ) of the generated silicon was 100 nm, the thickness of SiO 2  formed on the surface of the silicon surface was 10 nm, and the thickness of LiF disposed on the SiO 2  was 0.01 nm to 10 nm. 
     Thereafter, 1.54 g of flake graphite having a specific surface area of 50 m 2 /g and 3.85 g of a solid phase pitch were added to the mixed solution and dispersed to prepare a slurry. 
     The slurry and ethanol/water (volume ratio=1:9) were mixed at a volume ratio of 1:10 to prepare a dispersion for spray drying. The dispersion was spray dried through a mini spray-dryer (manufacturer: Buchi, Model: B-290 Mini Spray-Dryer) under the conditions of an inlet temperature of 180° C., an aspirator of 95% and a feeding rate of 12. Thereafter, 10 g of the spray-dried mixture (composite) was heat treated at 950° C. under a nitrogen atmosphere to prepare a negative electrode active material. LiF (corresponding to the coating layer of the present invention) in the prepared negative electrode active material was 1.3 wt % based on the total weight of the negative electrode active material. The Li content was measured by ICP and the F content was measured by ion chromatography, and then the sum was calculated 
     (2) Preparation of Negative Electrode and Secondary Battery 
     A negative electrode and a secondary battery were prepared in the same manner as in Example 1 using the negative electrode active material. 
     Experimental Example 1: Evaluation of Discharge Capacity, Initial Efficiency, Capacity Retention Rate and Electrode Thickness Change Rate 
     The batteries of Examples 1 to 5 and Comparative Examples 1 to 3 were subjected to charging and discharging to evaluate discharge capacity, initial efficiency, capacity retention rate, and electrode thickness change rate, and the results are shown in Table 1 below. 
     Meanwhile, for the first cycle and the second cycle, charging⋅discharging were performed at 0.1 C, and from the third cycle to the 49th cycle, charging⋅discharging were performed at 0.5 C. The 50th cycle was terminated in the state of charging (the state in which lithium was in the negative electrode), and then the battery was disassembled and the thickness thereof was measured to calculate the electrode thickness change rate. 
     Charging condition: CC (constant current)/CV (constant voltage) (5 mV/0.005 C current cut-off) 
     Discharging condition: CC (constant current) Condition 1.5V 
     The discharge capacity (mAh/g) and the initial efficiency (%) were derived from the result of one charge/discharge. Specifically, the initial efficiency (%) was derived by the following calculation. 
       Initial efficiency (%)=(discharge capacity after 1 discharge/charge capacity of 1 time)×100
 
     The capacity retention rate and the electrode thickness change rate were derived by the following calculations, respectively. 
       Capacity retention rate (%)=(discharge capacity of 49 times/discharge capacity of 1 time)×100
 
       Electrode thickness change rate (%)=(final negative electrode thickness variation/initial negative electrode thickness)×100
 
     
       
         
           
               
               
               
               
               
               
             
               
                   
                 TABLE 1 
               
               
                   
                   
               
               
                   
                   
                 Battery 
                   
                   
                 Electrode 
               
               
                   
                   
                 Discharge 
                 Initial 
                 Capacity 
                 thickness 
               
               
                   
                   
                 capacity 
                 efficiency 
                 retention 
                 change 
               
               
                   
                   
                 (mAh/g) 
                 (%) 
                 rate (%) 
                 rate (%) 
               
               
                   
                   
               
             
            
               
                   
                 Example 1 
                 1750 
                 83 
                 53 
                 160 
               
               
                   
                 Example 2 
                 1700 
                 82 
                 51 
                 167 
               
               
                   
                 Example 3 
                 1650 
                 82 
                 51 
                 166 
               
               
                   
                 Example 4 
                 1750 
                 83 
                 53 
                 161 
               
               
                   
                 Example 5 
                 1750 
                 83 
                 49 
                 169 
               
               
                   
                 Comparative 
                 1500 
                 78 
                 40 
                 180 
               
               
                   
                 Example 1 
                   
                   
                   
                   
               
               
                   
                 Comparative 
                 1300 
                 80 
                 38 
                 190 
               
               
                   
                 Example 2 
                   
                   
                   
                   
               
               
                   
                 Comparative 
                 1400 
                 81 
                 48 
                 170 
               
               
                   
                 Example 3 
               
               
                   
                   
               
            
           
         
       
     
     Referring to Table 1, in the case of Examples 1 to 5, the discharge capacity, the initial efficiency, the capacity retention rate, and the electrode thickness change rate are all good when compared with Comparative Examples 1 to 3. In the case of Comparative Example 1, since the negative electrode active material does not include LiF, a conductive path was not secured, thereby reducing the initial efficiency and discharge capacity. In addition, in the case of Example 1, since lithium silicate (Li 2 SiO 3 ) formed from LiF and SiO 2  may be present in the negative electrode active material, the initial efficiency and discharge capacity may be further improved when compared with Comparative Example 1 in which the LiF and lithium silicate is not present (See  FIG. 3 ). Meanwhile, in Comparative Example 2, as the second particle includes only spherical natural graphite, the complexation of the first particle and the second particle is not stably achieved, and thus the capacity retention rate and the electrode thickness change rate are poor. In the case of Comparative Example 3, as the second particle includes only flake graphite without graphene, the discharge capacity, the initial efficiency, the capacity retention rate, and the electrode thickness change ratio were worse than those of Example 1. Meanwhile, when comparing the examples, it can be seen that Example 1 in which an appropriate content of LiF is satisfied is superior in discharge capacity, initial efficiency, capacity retention rate, and electrode thickness change rate to those of Examples 2 and 3 in which relatively less or more LiF in included. In addition, it can be seen that the capacity retention rate and the electrode thickness change rate of Example 1 in which an appropriate size of the first particle is satisfied are more preferable than those of Example 5 in which the size of the first particle including a silicon core is relatively large.