Patent Publication Number: US-2022216462-A1

Title: Anode active material comprising metal phosphide coating on surface of carbon material, preparation method therefor, nonaqueous lithium secondary battery comprising anode active material, and manufacturing method therefor

Description:
TECHNICAL FIELD 
     The present invention relates to a quick charging nonaqueous lithium secondary battery and a manufacturing method therefor and, more specifically, to an anode active material in which a functional coating layer is formed on the surface of a carbon-based material applied as an anode active material of a lithium secondary battery, a nonaqueous lithium secondary battery having the same, and a manufacturing method therefor. 
     BACKGROUND ART 
     With the increasing spread of portable small electric and electronic devices, new secondary batteries, such as nickel-hydrogen batteries and lithium secondary batteries, have been being actively developed. 
     Out of these, lithium secondary batteries use metal lithium as an anode active material and a nonaqueous solvent as an electrolyte. Since lithium is a metal having a high ionization tendency to thereby enable high voltage expression, batteries having a high energy density have been developed. Lithium secondary batteries using lithium metal as an anode active material have been used as next-generation batteries over a long period of time. 
     When a carbon-based material is applied as an anode active material in such lithium secondary batteries, the decomposition reaction of an electrolyte occurs during charging and discharging since the charge and discharge potential of lithium is lower than the stable range of an existing nonaqueous electrolyte. As a result, a film is formed on the surface of the carbon-based anode active material. That is, the electrolyte is decomposed to form a film on the surface of an electrode before lithium ions are intercalated into the carbon-based material, and this film has a property of passing the lithium ions therethrough but has a property of blocking the movement of electrons. Therefore, once the film is formed, the decomposition of the electrolyte due to the movement of electrons between the electrode and the electrolyte is suppressed and only intercalation and de-intercalation of lithium ions can be selectively conducted. Such a film is called a solid electrolyte interface or solid electrolyte interphase (SEI). 
     For this reason, the resistance generated on the surface of the carbon-based material in the procedure of intercalating lithium ions into the carbon-based material during charging is very high, so that during high-rate charging, precipitation of lithium metal occurs, which is indicated as a fundamental cause for low charging and discharging efficiency and the deterioration of lifespan characteristics during high-rate charging of a current lithium secondary battery in which a carbon-based material is applied as an anode active material. 
     In order to solve these problems, methods of improving the mobility of lithium ions through physical or chemical surface modification of a carbon-based anode active material have been proposed so as to secure high-rate charging characteristics of a nonaqueous lithium secondary battery employing a carbon-based material. However, such surface modification can improve the lifespan characteristics, but cannot solve the problems of precipitation of lithium metal and reductions in capacity, high-rate characteristics, and charging and discharging efficiency. 
     Therefore, studies have been conducted on the formation of a functional coating layer, for reducing the generation of resistance and suppressing the precipitation of lithium metal during high-rate charging, on the surface of a carbon-based material by various methods, but no technology of suppressing the precipitation of lithium metal during high-rate charging has yet been developed. 
     SUMMARY 
     Technical Problem 
     Therefore, the present disclosure has been made in view of the above-mentioned problems, and an aspect of the present invention is to provide an anode active material, which has improved high-rate charging characteristics by forming a coating layer containing a phosphide on the surface of a carbon-based material to reduce the resistance generated during the intercalation of lithium and to improve high-rate charging characteristics through suppression of precipitation of lithium metal and which, when applied as an anode active material for a nonaqueous lithium secondary battery, enables secure high-rate charging characteristics without the deterioration of charging and discharging efficiency and lifespan characteristics. 
     Another aspect of the present invention is to provide a nonaqueous lithium secondary battery including the aforementioned anode active material. 
     Still another aspect of the present invention is to provide a method for preparing the aforementioned anode active material. 
     Solution to Problem 
     In accordance with an aspect of the present disclosure, there is provided a method for preparing an anode active material for a nonaqueous lithium secondary battery, the method including: preparing a carbon-based material; forming a precursor coating layer containing Me and A (A is O or S) on the surface of the carbon-based material; supplying a P precursor to the precursor coating layer on the carbon-based material; and reacting the precursor coating layer and the P precursor to convert at least a portion of the precursor coating layer into a compound represented by chemical formula Me x1 P y1  (x1&gt;0 and y1&gt;0), thereby forming a phosphide coating layer, wherein Me is at least one same metal element selected from Mo, Ni, Fe, Co, Ti, V, Cr, Nb, and Mn. 
     In the present invention, the phosphide coating layer may contain a composite of a compound represented by chemical formula Me x1 P y1  (x1&gt;0 and y1&gt;0) and a compound represented by chemical formula Me x2 A y2  (A is O or S; and x2&gt;0 and y2&gt;0). 
     In the present invention, in the supplying of the P precursor, the P precursor may be supplied as a gas source. Alternatively or additionally, in the supplying of the P precursor, the P precursor may be mixed, as a solid or liquid source, with the carbon-based material. 
     In the present invention, the P precursor may be at least one type selected from the group consisting of sodium hypophosphite (NaH 2 PO 2 ), phosphoric acid (H 3 PO 4 ), phosphorous trichloride (PCl 3 ), phosphorous, red (P), Phosphorous, black (P), and triphenyl phosphine (C 18 H 15 P). 
     In addition, in the forming of the coating layer, heat treatment may be conducted for reaction of the precursor coating layer and the P precursor. 
     In addition, in the converting, P of the P precursor may be substituted for A in the precursor coating layer. 
     In addition, the P precursor may contain no metal element Me. 
     In addition, the forming of the phosphide coating layer may be performed in an inert gas atmosphere at 500-1000° C. for 1-10 hours. 
     In accordance with another aspect of the present invention, there is provided an anode active material for a nonaqueous lithium secondary battery, the anode active material including: a carbon-based material; and a coating layer formed on the surface of the carbon-based material and containing a compound represented by chemical formula Me x1 P y1  (x1&gt;0 and y1&gt;0), wherein Me is at least one same metal element selected from the group consisting of Mo, Ni, Fe, Co, Ti, V, Cr, Nb, and Mn. 
     In the present invention, Me may include Mo; Me x1 P y1  may include at least one type from the group consisting of MoP, MoP 2 , Mo 3 P, MoP 4 , Mo 4 P 3 , and Mo 8 P 5 ; and Me x2 A y2  may include at least one type selected from the group consisting of MoO, MoO 2 , and MoO 3 . In addition, the anode active material may have distinctive peaks in the vicinity of 2θ=32.0° and 43.0° in an X-ray diffraction pattern. Also, the anode active material may have distinctive peaks in the vicinity of 2θ=23.9°, 29.5°, and 41.7° in an X-ray diffraction pattern. 
     In addition, the coating layer may further contain a compound represented by chemical formula Me x2 A y2  (A is O or S; and x2&gt;0 and y2&gt;0). 
     In addition, Me may include Ni, and Me x1 P y1  may include at least one type selected from the group consisting of Ni 5 P 2 , Ni 4 P 2 , Ni 3 P, Ni 12 P 5 , Ni 2 P, Ni 5 P 4 , NiP, NiP 2 , and NiP 3 . 
     In addition, Me may include Fe, and Me x1 P y1  may include at least one type from the group consisting of FeP, Fe 2 P, Fe 3 P, and Mo 8 P 5 . 
     In addition, Me may include Co, and Me x1 P y1  may include at least one type from the group consisting of CoP and Co 2 P. 
     In the present invention, the coating layer may be uniformly or partially formed on the surface of the carbon-based material. 
     In addition, the carbon-based material may include at least one type selected from the group consisting of artificial graphite, natural graphite, graphitized carbon fibers, graphitized mesocarbon microbeads, petroleum coke, resin burned bodies, carbon fibers, and pyrolytic carbon. 
     In the present invention, the carbon-based material may have a particle diameter of 20 μm or less. 
     Advantageous Effects of Invention 
     According to the present invention, a MeP x  phase is formed on the surface of a carbon-based material used as an anode active material for a nonaqueous lithium secondary battery, thereby inducing more stable high-rate charging characteristics in the surface of the carbon-based material. 
     Furthermore, the resistance generated in the surface of the anode active material can be reduced by the surface coating layer, and thus the anode active material, when applied as an anode active material for a nonaqueous lithium secondary battery, can improve high-rate charging characteristics without the deterioration of lifespan characteristics. 
    
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
         FIG. 1  is a flowchart schematically illustrating a process of preparing an anode active material for a nonaqueous lithium secondary battery, the anode active material having a MoO x  functional coating layer containing molybdenum phosphides (MoP x ), such as MoP and MoP 2 , according to an embodiment of the present invention. 
         FIG. 2  illustrates images showing SEM analysis results of anode active materials prepared according to a first exemplary embodiment of the present invention. 
         FIGS. 3A and 3B  illustrate XRD patterns of anode active materials according to a first exemplary embodiment of the present invention. 
         FIG. 4  illustrates images of TEM analysis of the anode active material according to a first exemplary embodiment of the present invention. 
         FIG. 5  illustrates images of EDS analysis of the anode active material according to a first exemplary embodiment of the present invention. 
         FIGS. 6A and 6B  illustrate graphs showing half-cell charge and discharge characteristics of nonaqueous lithium secondary batteries using the anode active materials according to a first exemplary embodiment of the present invention. 
         FIGS. 7A, 7B and 7C  illustrate graphs comparing full cell charge and discharge characteristics of nonaqueous lithium secondary batteries using the anode active materials according to a first exemplary embodiment of the present invention. 
         FIG. 8  illustrates graphs showing full cell lifespan characteristics of nonaqueous lithium secondary batteries using the anode active materials according to a first exemplary embodiment of the present invention. 
         FIG. 9  illustrates graphs showing full cell charging and discharging efficiency characteristics of nonaqueous lithium secondary batteries using the anode active materials according to a first exemplary embodiment of the present invention. 
         FIG. 10  illustrates XRD analysis results of Co—P-coated graphite powders prepared according to a second exemplary embodiment of the present invention. 
         FIG. 11  illustrates XRD analysis results of a Fe—P-coated graphite powder prepared according to a third exemplary embodiment of the present invention. 
         FIG. 12  illustrates XRD analysis results of a Ni—P-coated graphite powder prepared according to a fourth exemplary embodiment of the present invention. 
         FIG. 13  illustrates a graph showing initial charge and discharge characteristics of half-cells according to Examples 2 to 4 of the present invention. 
         FIG. 14  illustrates a graph showing long-term lifespan characteristics of full cells according to Examples 2 to 4 of the present invention. 
     
    
    
     DETAILED DESCRIPTION 
     It is to be noted that in the following description, only the parts necessary for understanding exemplary embodiments of the present invention will be described, and descriptions of the other parts will be omitted so as not to deviate from the gist of the present invention. 
     Hereinafter, exemplary embodiments of the present invention will be described in more detail with reference to the accompanying drawings. 
     An anode active material for a nonaqueous-based lithium secondary battery according to the present invention includes: a carbon-based material; and a coating layer formed on the surface of the carbon-based material and containing a compound represented by chemical formula Me x1 P y1  (integers of x1&gt;0 and y1&gt;0, hereinafter MeP x ). In the present invention, Me is a transition metal and may preferably include at least one type selected from the group consisting of Mo, Ni, Fe, Co, Ti, V, Cr, Nb, and Mn. 
     Herein, in some cases, chemical formula MeA x  is expressed as MeO x  for an oxide and MeS x  for a sulfide. 
     In the present invention, the Me x1 P y1  compound is derived from a Me-A compound precursor. The Me x1 P y1  compound reacts with the precursor compound Me-A to convert the precursor into a Me-P compound, and for example, this reaction may result in a form in which A is substituted with P in the Me-A compound. 
     In such a case, a portion of the Me-A compound precursor may be converted into the Me-P compound or the substantially entire portion of the Me-A compound precursor may be converted into the Me-P compound. The remaining Me-A compound may be present as a compound of Me x2 A y2  (integers of x2&gt;0 and y2&gt;0; and A is O or S, hereinafter MeA x ). 
     In the present invention, the fractions of the MeA x  compound and the MeP x  compound in the coating layer may be determined by a thermodynamic equilibrium state. However, of course, the reaction rate may not be sufficient to reach a thermodynamic parallel state during the preparation procedure. For example, the thicker the Me-A precursor and the lower the temperature, the less likely the coating layer may reach a thermodynamic parallel state. 
     Therefore, the final coating layer in the present invention may be present in a composite form of the MeP x  compound and the MeA x  compound. As will be described later, the MeP x  compound and the MeA x  compound in the present invention are distinguished from a simple mixture form. In the present invention, the compounds constituting the coating layer are physically and chemically self-bound and require no binder for binding. In the present invention, the MeP x  compound and the MeA x  compound may form a solid solution. 
     In the present invention, the metal elements (Me) of the respective compounds are the same type of metal element. According to the present invention, the metal element may be derived from a starting material or a precursor. 
     In the present invention, the MeP x  compound may be a binary compound, such as M1-P (M1 is a transition metal) or a ternary compound, such as M1-M2-P (M1 and M2 are transition metals), or other higher-order compound. In the present invention, M1-P and M1-A may be present as a ternary compound such as M1-A-P. 
     In the present invention, the MeP x  compound may contain two types of phases having different valences. For example, in cases of Co, at least two types of phases selected from CoP, Co 2 P, and CoP 2  may be present in the Co—P compound. The MeP x  compound may be present in a thermodynamically stable or metastable form. 
     Similarly, in the present invention, the MeA x  compound is an oxide or sulfide of the same metal as in the MeP x  compound. The MeA x  compound may be a binary compound, a ternary compound, or other higher-order compound. The MeA x  compound may also contain two types of phases having different valences. 
     In the present invention, exemplary compounds which can be present for an anode active material are shown in the following table. 
     
       
         
           
               
               
             
               
                 TABLE 1 
               
               
                   
               
               
                 Metal element 
                 Compound 
               
               
                   
               
             
            
               
                 Mo 
                 MoP, MoP 2 , Mo 3 P, MoP 4 , Mo 4 P 3 , Mo 8 P 5 MoO, MoO 2 , MoO 3 , MoS 2   
               
               
                 Ni 
                 Ni 5 P 2 , Ni 4 P 2 , Ni 3 P, Ni 12 P 5 , Ni 2 P, Ni 5 P 4 , NiP, NiP 2 , NiP 3 NiO, NiO 2 , 
               
               
                   
                 Ni 3 S 4 , Ni 7 S 6 , NiS 2   
               
               
                 Fe 
                 FeP, Fe 2 P, Fe 3 PFeO, Fe 2 O 3 , FeS, Fe 3 S, FeS 2   
               
               
                 Co 
                 CoP, Co 2 P, CoP 2 CoO, Co 3 O 4 , CoO 2 , CoS 2 , Co 4 S 3 , Co 3 S 4 , CoS 
               
               
                 Ti 
                 Ti 3 P, Ti 2 P, Ti 7 P, Ti 4 P 3 , TiP, Ti 5 P, Ti 7 P 4 TiO, Ti 2 O 3 , Ti 3 O 4 , TiO 2 , Ti 3 O 2 , 
               
               
                   
                 Ti 8 S 10 , Ti 8 S 9 , Ti 16 S 21 , Ti 2 S, Ti 3 S, Ti 6 S, TiS 3 , TiS 2 , TiS 
               
               
                 V 
                 VPV 2 O 3 , V 3 O 5 , V 4 O 7 , V 5 O 9 , V6O 11 , V 7 O 13 , V 8 O 15 , VO 2 , V 2 O 5 , VS, VS 2   
               
               
                 Cr 
                 Cr 2 P, Cr 3 P, CrP, CrP 4 , Cr 12 P 7 , CrP 2 , CrPCr 2 O 3 , Cr 3 O 4 , Cr 2 S 3   
               
               
                 Mn 
                 Mn 3 P 2 , Mn 2 P, MnP, MnO, Mn 2 O 3 , MnO 2 , Mn 3 O 4   
               
               
                   
                 Mn 2 S 7 , MnS 2 , MnS 
               
               
                 Nb 
                 NbPNbO, NbO 2 , Nb 2 O 5 , NbS 2   
               
               
                   
               
            
           
         
       
     
     For example, as for Co, a phosphide constituting a composite coating layer may include at least one type selected from compounds including CoP, CoP 2 , and Co 2 P. Preferably, the phosphide may include CoP or CoP 2 . In addition, the oxide contained in the composite coating layer may be CoO, Co 3 O 4 , or CoO 2 . Preferably, the oxide includes Co 3 O 4 . For example, the phosphide of the composite coating layer in the present invention may include two types of CoP x  phases composed of CoP and CoP 2 . Herein, the MoP x  phase composed of CoP and CoP 2  is chemically or physically bound to the surface of the carbon-based material together with the MoA x  phase to be present in the form of a compound. Similarly, Ni, Fe, Co, V, Cr, Mn, and Nb in the present invention may form a composite of the listed phosphides, oxides, and sulfides, and description thereof is omitted. 
     The composite coating layer may be uniformly coated on the surface of the carbon-based material. In the present invention, the coating layer may also be locally coated on a partial surface of the carbon-based material. 
     The content of the phosphide, such as CoP and CoP 2 , in the composite coating layer may be 5 wt % or more, 10 wt % or more, 20 wt % or more, 30 wt % or more, 40 wt % or more, 50 wt % or more, 60 wt % or more, 70 wt % or more, 80 wt % or more, 90 wt % or more, or 100 wt % or more. Preferably, the content of the phosphide is 50% or more. 
     In the present invention, the carbon-based material may employ at least one from materials composed of crystalline or amorphous carbon, such as artificial graphite, natural graphite, graphitized carbon fibers, graphitized mesocarbon microbeads, petroleum coke, resin burned bodies, carbon fibers, pyrolytic carbon, and the like. The carbon-based material preferably has a particle diameter of 20 μm. 
     Hereinafter, a method for preparing an anode active material of a nonaqueous lithium secondary battery according to an embodiment of the present invention will be described. 
       FIG. 1  is a flowchart schematically illustrating a process of preparing an anode active material for a nonaqueous lithium secondary battery, the anode active material having a functional coating layer containing metal phosphide particles, such as MoP, MoP 2 , CoP, or CoP 2 , according to an embodiment of the present invention. 
     Referring to  FIG. 1 , a carbon-based material, a MeA x  precursor, and a material, such as a precursor containing phosphorus (P) for forming a metal phosphide (MeP x ) are prepared. In the present invention, the MeA x  precursor may be a mixture of Me and A or a Me-A compound. 
     In the present invention, the MeA x  precursor may employ an oxide or a phosphide listed on Table 1. For example, Co 3 O 4 , which is an oxide, and NiS 2 , which is a nickel sulfide. The present invention is not limited thereto, and various precursors, such as (NH 4 ) 6 Mo 7 O2 4 .4H 2 O, may be used. 
     According to a preferable embodiment of the present invention, Me contained in the MeA x  precursor may act as a metal source of a phosphide. That is, the MeA x  precursor reacts with the P precursor to form a MeP x  compound. 
     In the present invention, various forms of precursors containing phosphorus (P) may be used as P precursors. For example, sodium hypophosphite (NaH 2 PO 2 ), phosphoric acid (H 3 PO 4 ), or phosphorous trichloride (PCl 3 ) may be used as a liquid source or a solid source. As a gas source, sodium hypophosphite (NaH 2 PO 2 ), phosphorous, red (P), phosphorous, black (P), or triphenyl phosphine (C 18 H 15 P) may be used. One or more types of these sources may be used in combination. 
     Besides, various sources may be used, such as (R)-1,1′-binaphthyl phosphorochloridate, (S)-1,1′-binaphthyl phosphorochloridate, bis(3,5-di-tert-butyl-4-methoxyphenyl)chlorophosphine, bis(3,5-di-tert-butyl-4-methoxyphenyl)phosphine, bis(diisopropylamino)chlorophosphine, bis(3,5-di(trifluoromethyl)phenyl)phosphine, bis(2-isopropoxyphenyl)chlorophosphine, bis(2-isopropoxyphenyl)phosphine, bis(2-methoxyphenyl)chlorophosphine, bis(4-methoxyphenyl)chlorophosphine, bis(4-methoxyphenyl)phosphine, bis(2,4,6-trimethylphenyl)phosphine, bis(2,4,6-trimethylphenyl)phosphorus chloride, borane diphenylphosphine complex, tert-butyldichlorophosphine, tert-butyldimethylphosphine borane, tert-butylphosphonic dichloride, 2-chloro-1,3,2-benzodioxaphosphorin-4-one, chlorodi(o-tolyl)phosphine, di(1-adamantyl)chlorophosphine, 5,5-dimethyl-1,3,2-dioxaphosphorinan-2-one, diphenylphosphine oxide, 3-(diphenylphosphino)propionic acid, di(o-tolyl)phosphine Iodo[4,5-bis(diphenylphosphino)-9,9-dimethylxanthene]copper(I), o-phenylene phosphorochloridite, phenyl N-phenylphosphoramidochloridate, and (2,4,6-tri-tert-butylphenyl)phosphine. 
     In the present invention, the carbon-based material preferably has an average particle size (particle diameter) of 20 μm or less. Various materials may be used as carbon-based materials, but a graphite-based material is preferably used considering the formation of a composite coating layer containing a metal phosphide, such as CoP or CoP 2 . 
     Hereinafter, a method for forming a MeP x  coating layer by using a carbon-based material, a MeA x  precursor, and a P precursor, which have been described above, and specifically, a method for forming a CoP x  coating layer by using Co as a metal element, will be described. 
     Referring to  FIG. 1 , for example, Co 3 O 4  as a precursor containing Co is dissolved in a mixture solvent composed of H 2 O 2  and HNO 3  (S 110 ) to prepare a MeA x  precursor solution (S 120 ). In this situation, H 2 O 2  and HNO 3  are mixed at a volume ratio of 1:1, and then Co 3 O 4  is dissolved at a weight ratio of 2 wt % relative to graphite in the prepared mixture solution to prepare a precursor solution. 
     Then, a carbon-based material and the coating solution are stirred to conduct coating on the surface of the carbon-based material. The stirring may be conducted at a temperature of, for example, 50-85° C. Then, the coated carbon-based material is dried (S 130 ). The drying may be conducted at room temperature to a temperature of 100° C., and for example, the drying may be conducted at 100° C. for 12 hours. 
     Next, a precursor containing P is supplied to the dried carbon-based material to form a MeP x  coating layer on the surface of the carbon-based material. At least a portion or the entire portion of the MeP x  coating layer is converted into a phosphide, such as MeP x , by a reaction of the MeA x  precursor and the P precursor. 
     For example, the P precursor may be provided by each of the following processes. 
     &lt;Process 1&gt; 
     A P precursor (e.g., NaH 2 PO 4 ) is inserted into a first heat treatment furnace, and a graphite powder coated with a MeA x  precursor is inserted into a second heat treatment furnace. The first and second heat treatment furnaces thus set are connected to attain gas communication therebetween, and the P precursor in the first heat treatment furnace is heated at a high temperature (e.g., 300° C.) to supply a gas-phase P source to the second heat treatment furnace. In the second heat treatment furnace, heat treatment is conducted at, for example, 800° C. to induce a reaction in which the MeA x  precursor is converted into MeP x . In the heat treatment processes, the heat treatment is conducted in an inert gas atmosphere by continuously allowing gas, for example, argon gas, to flow. 
     &lt;Process 2&gt; 
     A P precursor (NaH 2 PO 4 ) is mixed with a graphite powder coated with a MeA x  precursor. The mixing may be conducted in a mortar mix. The mixed powder is inserted into a heat treatment furnace, and subjected to heat treatment at, for example, 800° C. In this situation, argon gas is allowed to continuously flow while the heat treatment is conducted. The heat-treated powder is washed using distilled water. The washed powder is filtered and then dried at a temperature of 70° C. for 12 hours. 
     For example, the CoP x  coating layer in the present invention may be formed by heat treatment in an inert gas atmosphere of 500-1000° C. for 1-10 hours, and for example, the heat treatment may be performed in an argon atmosphere of 800° C. for 3 hours. 
     In the present invention, Me contained in the MeA x  precursor acts as a source of the MeP x  compound. In cases where a Co 3 O 4  precursor is used, Co 3 O 4  is in a thermodynamically unstable state at a heat treatment temperature, and in this procedure, reacts with a P source to generate a compound, such as CoP or CoP 2 , as a reaction product. Of course, whether unconverted CoO x  remains in the coating layer compound to form a composite may depend on kinetics. 
     As such, in the anode active material according to the present invention, the CoP x  phase, such as CoP or CoP 2 , is formed on the surface of the carbon-based material, leading to the more stable movement of lithium ions without precipitation of lithium metal in the surface of the carbon-based material during high-rate charging. 
     It is not easy to introduce the MeP x  compound coating layer on the surface of the carbon-based material without using a binder. If a separate binder is used, the binder will have a negative effect on a function of an anode active material. According to the present invention, a MeP x  coating layer self-bound on the surface of the carbon-based material can be provided through the conversion of at least a portion of the MeA x  precursor coating layer into the MeP x  compound, without the introduction of a separate binder. 
     Furthermore, in the present invention, such a functional coating layer is introduced to improve the reactivity and structural stability of the surface of the anode active material, and thus such an anode active material, when applied as an anode active material for a nonaqueous lithium secondary battery, can suppress the precipitation of lithium metal during high-rate charging and secure high-rate charging characteristics without deterioration of lifespan characteristics. 
     MODE FOR CARRYING OUT THE INVENTION 
     Example 1: Preparation of Mo—P-Based Anode Active Materials and Manufacture of Secondary Batteries 
     In order to evaluate lifespan characteristics and high-rate charging characteristics of a nonaqueous lithium secondary battery employing an anode active material according to an exemplary embodiment of the present invention, an anode active material was prepared and nonaqueous lithium secondary batteries using the same were manufactured. Artificial graphite having an average particle diameter of 17 μm was used as a carbon-based material. As for a comparative example, artificial graphite without a coating layer was used as an anode active material. Nonaqueous lithium secondary batteries according to examples and a comparative example are manufactured in substantially the same manner except for an anode active material, and thus a method for manufacturing a nonaqueous lithium secondary battery according to the examples will be mainly described hereinafter. 
     First, in order to prepare a coating layer composed of a MoP x  and MoO x  composite on the surface of artificial graphite among carbon-based materials, a coating solution was prepared by dissolving MoO 3  as a MoO x  precursor at weight ratio of 2 wt % or 5 wt % in H 2 O 2  solution. 
     Then, the coating solution was uniformly coated on the surface of the artificial graphite, and sufficient drying after stirring was conducted 100° C. until the solvent was evaporated. Thereafter, the dried carbon-based material was mixed with NaH 2 PO 2 , followed by heat treatment at 600° C. 
     As shown in Table 2, among carbon-based materials, artificial graphite having a particle diameter of 20 μm or less without a coating layer was used for Comparative Example 1-1. For Example 1-1, 2 wt % of a MoO 3  precursor was coated on the surface of artificial graphite, and for Example 1-2, about 5 wt % of a MoO 3  precursor was coated on the surface of artificial graphite. 
     
       
         
           
               
               
               
               
               
               
               
             
               
                 TABLE 2 
               
               
                   
               
               
                   
                   
                 Content of 
                 Content of 
                   
                   
                 Heat 
               
               
                   
                 Base 
                 base 
                 MoO 3   
                   
                 Drying 
                 treatment 
               
               
                 Classification 
                 material 
                 material 
                 coating 
                 Solvent 
                 temperature 
                 temperature 
               
               
                   
               
             
            
               
                 Comparative 
                 Artificial 
                 100 wt % 
                 — 
                 — 
                 — 
                 — 
               
               
                 Example 1-1 
                 graphite 
                   
                   
                   
                   
                   
               
               
                 Example 1-1 
                 Artificial 
                  98 wt % 
                 2 wt % 
                 H 2 O 2   
                 100° C. 
                 600° C. 
               
               
                   
                 graphite 
                   
                   
                   
                   
                   
               
               
                 Example 1-2 
                 Artificial 
                  95 wt % 
                 5 wt % 
                 H 2 O 2   
                 100° C. 
                 600° C. 
               
               
                   
                 graphite 
               
               
                   
               
            
           
         
       
     
       FIG. 2  illustrates scanning electron microscope images of anode active materials according to the examples of the present invention, and  FIGS. 3A and 3B  illustrate XRD patterns of anode active materials of the examples and the comparative example. 
     Referring to  FIGS. 2 and 3 , a coating layer containing a MoP x  compound as a phosphide was formed on the surface of artificial graphite in the examples, unlike in the comparative example. 
     As shown in  FIGS. 3A and 3B , as a result of XRD analyses, Comparative Example 1-1 and Examples 1-1 and 1-2 showed substantially different peak patterns. That is, when compared with the artificial graphite according to Comparative Example 1, patterns corresponding to MoP and MoP 2  were observed in the active materials prepared according to the examples of the present invention. In the examples, as summarized in the following table, corresponding peaks could be observed in the vicinity of 2θ=32.0° and 43.0°, which are characteristic peaks of MoP, and corresponding peaks can be observed in the vicinity of 23.9°, 29.5°, 41.7°, 47.2°, and 50.0°, which are characteristic peaks of MoP 2 . Additionally, in the examples of the present invention, corresponding peaks can be observed in the vicinity of 2θ=37.0° and 53.5°, which are characteristic peaks of MoO 2 , and corresponding peaks can be observed in the vicinity of 2θ=9.7° and 29.4°, which are characteristic peaks of MoO 3 . 
     
       
         
           
               
               
               
               
             
               
                   
                 TABLE 3 
               
               
                   
                   
               
               
                   
                 Classification 
                 Example 1 
                 Example 2 
               
               
                   
                   
               
             
            
               
                   
               
            
           
           
               
               
               
               
            
               
                   
                 MoP 
                 31.91° 
                 31.93° 
               
               
                   
                   
                 43.15° 
                 43.15° 
               
               
                   
                 MoP 2   
                 23.84° 
                 23.94° 
               
               
                   
                   
                 29.41° 
                 29.47° 
               
               
                   
                   
                 41.66° 
                 41.74° 
               
               
                   
                   
                 47.19° 
                 47.27° 
               
               
                   
                   
                 50.04° 
                 50.02° 
               
               
                   
                 MoO 2   
                 37.0 
                 37.0 
               
               
                   
                   
                 53.6 
                 53.5 
               
               
                   
                 MoO 3   
                 9.6 
                 9.7 
               
               
                   
                   
                 29.2 
                 29.2 
               
               
                   
                   
               
            
           
         
       
     
     Meanwhile, the other properties than the above phase analysis results, such as the formation of impurities or changes in average particle diameter and specific surface area, were insignificant between the examples and the comparative example. 
     As described above, in the present examples, the MoO 3  precursor and the P precursor were coated on the surface of artificial graphite, thereby obtaining a coating layer containing MoP and MoP 2  as MoP compounds and composites thereof with MoO x  compounds. This can be confirmed through transmission electron microscope (TEM) and energy dispersive spectroscopy (EDS) analysis results in  FIG. 4 .  FIGS. 4 and 5  illustrate transmission electron microscope (TEM) images and energy dispersive spectroscopy (EDS) analysis images of anode active materials according to Example 1 of the present invention, respectively, and confirming that MoO x  coating layer containing MoP and MoP 2  particles was formed on the surface of artificial graphite. This indicates that MoP and MoP 2  were physically or chemically bound with MoO x  in the surface of artificial graphite. 
     The anode active materials thus prepared were used to manufacture nonaqueous lithium secondary batteries. 
     First, a slurry was prepared using 96 wt % of an anode active material, 4 wt % of polyvinylidene fluoride (PVDF) as a binder, and N-methyl-2-pyrrolidone (NMP) as a solvent. The slurry was coated on a copper foil and dried, thereby manufacturing an electrode. In this situation, the loading level and the mixture density of the electrode were 5 mg/cm 2  and 1.5 g/cc, respectively. Electrochemical characteristics were evaluated after a half cell was manufactured using a lithium metal counter electrode, and 1 M LiPF 6  in EC/EMC was used as an electrolyte. 
     The results of evaluating the charge and discharge characteristics of the nonaqueous lithium secondary batteries according to the examples and comparative example of the present invention are shown in  FIGS. 6A and 6B .  FIGS. 6A and 6B  illustrate graphs showing rate-specific charging characteristics of nonaqueous lithium secondary batteries using the anode active materials according to the examples and the comparative example of the present invention. 
     For the lifespan evaluation of the comparative example and the examples, charging and discharging was performed three times with a constant current of 0.2 C (70 mA/g) in the 0.01-2.0 V vs. Li/Li+ potential region, and the results are shown in  FIGS. 6A and 6B .  FIG. 6B  is an enlarged view of the initial section in  FIG. 6A . 
     Referring to  FIGS. 6A and 6B , Examples 1-1 and 1-2 with MoO x  coating layer containing MoP and MoP 2  particles showed improved charging capacity compared with Comparative Example 1-1. It is considered that the introduction of the MoO x  coating layer containing MoP composed of MoP and MoP 2  into the surface of artificial graphite effectively reduces the resistance during the intercalation of lithium ions into the surface of artificial graphite and induces more stable movement of lithium ions during high-rate charging, thereby improving high-rate charging characteristics. However, it was confirmed through the comparison of Examples 1 and 2 that the more the amount of the MoO x  coating layer containing MoP composed of MoP and MoP 2 , the larger the reversible capacity reduction due to a decrease in the content of graphite. 
     The results of evaluating the full cell lifespan characteristics of the nonaqueous lithium secondary batteries according to the examples and the comparative example are shown in  FIGS. 7A to 7C .  FIG. 7A  shows the result for Comparative Example 1-1,  FIG. 7B  is the result for Example 1-1, and  FIG. 7C  is the result for Example 1-2. The NCM622 material was used as cathodes. Comparative Example 1-1 and Examples 1-1 and 1-2 were manufactured as counter anodes, and charging and discharging was performed three times with a constant current of 0.2 C (70 mA/g) in in a 2.5 to 4.3 V vs. Li/Li+ potential region. Thereafter, charging with a constant current of 6 C (2100 mA/g) and then discharging with a constant current of 1 C (350 mA/g) were performed 100 times, and the results are shown in  FIG. 7 .  FIG. 8  illustrates graphs comparing full cell lifespan characteristics of nonaqueous lithium secondary batteries using the anode active materials according to the examples and the comparative example of the present invention. 
     Referring to  FIGS. 7 and 8 , Examples 1-1 and 1-2 showed relatively excellent capacity retention rates during high-rate charging and discharging, and the charging and discharging efficiency results shown in  FIG. 9  also show improved characteristics. This means that the introduction of a MoO x  coating layer containing MoP composed of MoP and MoP 2  improves high-rate charging characteristics. 
     Example 2: Preparation of Co—P-Based Anode Active Materials and Manufacture of Secondary Batteries 
     A coating solution was prepared by dissolving Co 3 O 4  as a CoO x  precursor at weight ratio of 2 wt % in nitric acid. 
     Then, artificial graphite was placed in the coating solution, followed by stirring for 12 hours, and then stirring was conducted with application of heat until the solvent was completely evaporated, and then drying was conducted at 100° C. in an oven for 12 hours. 
     The Co—P coated graphite powders were prepared by process 1 (gas-phase reaction) and process 2 (solid-phase reaction), respectively. 
     (Process 1) 
     NaH 2 PO 4  as a P precursor was set in a first heat treatment furnace, and a graphite powder coated with a CoO x  precursor was set in a second heat treatment furnace. The first and second heat treatment furnaces thus set are allowed communicate with each other as a gas passage, and the first heat treatment furnace was maintained at 300° C. and the second heat treatment furnace was maintained at 800° C. In this situation, argon gas was allowed to continuously flow during heat treatment. 
     (Process 2) 
     A graphite powder coated with a CoO x  precursor was mixed with NaH 2 PO 4  as a P precursor in a mortar. The thus mixed powder was set in a heat treatment furnace, and subjected to heat treatment at 800° C. In this situation, argon gas was allowed to continuously flow while the heat treatment was conducted. Then, the heat-treated powder was washed using distilled water, and the washed powder was filtered and then dried at a temperature of 70° C. for 12 hours. 
       FIG. 10  illustrates XRD analysis results of Co—P-coated graphite powders manufactured by Process 1 and Process 2. 
     Referring to  FIG. 10 , strong CoP 2  peaks were observed in the surface of the powder subjected to Process 1, and most of the Co—P compounds were present as a CoP2 phase. However, CoP was present as a main phase in the surface of the graphite powder subjected to Process 2, and CoP 2  was hardly observed. These results show that a desired phosphide state can be controlled by a reaction mechanism. As shown in  FIG. 10 , no oxide peak was observed in the final coating layer. This shows that the unreacted CoO x  phase is hardly present under heat treatment at approximately 800° C. 
     An anode was prepared with a composition of anode active material prepared according to each of Processes 1 and 2/PVDF=96/4 (wt %). A cathode was prepared with a composition of NCM622/SPB/PVDF=95/2.5/2.5 (wt %). The electrodes thus prepared were applied to manufacture a 2032 coin type cell. In the half-cell, lithium metal was used as a counter electrode instead of the cathode. In this situation, the loading level of the anode was set to 2.2 mAh/cm 2  and the mixture density of the anode was set to a range of 1.5-1.6 g/cc. 
     The evaluation conditions of charge and discharge characteristics of the manufactured cells were as follows. 
     Initialization of half-cell: charge—0.2 C, CC/CV to 0.02 C, 0.005 V cut off//discharge—0.2 C, CC, 1.5 V cut off 
     Initialization of full cell: charge—0.2 C, CC/CV to 0.02 C, 4.2 V cut off//discharge—0.2 C, CC, 2.5 V cut off 
     long-term lifespan of full cell: charge—6 C, CC/CV to 0.01 C, 4.2 V cut off//discharge—1 C, CC, 2.5 V cut off 
     Example 3: Preparation of Fe—P-Based Anode Active Material and Manufacture of Secondary Battery 
     A coating solution was prepared by dissolving Fe 3 O 4  as a FeO x  precursor at weight ratio of 2 wt % in nitric acid. 
     Then, artificial graphite was placed in the coating solution, followed by stirring for 12 hours, and then stirring was conducted with application of heat until the solvent was completely evaporated, and then drying was conducted at 100° C. in an oven for 12 hours, thereby preparing a FeO x  precursor-coated graphite powder. 
     A Fe—P-coated graphite powder was prepared by the following solid-phase reaction process. First, a graphite powder coated with a FeO x  precursor was mixed with NaH 2 PO 4  as a P precursor in a mortar. The thus mixed powder was set in a heat treatment furnace, and subjected to heat treatment at 800° C. In this situation, argon gas was allowed to continuously flow while the heat treatment was conducted. Then, the heat-treated powder was washed using distilled water, and the washed powder was filtered and then dried at a temperature of 70° C. for 12 hours. 
       FIG. 11  illustrates XRD analysis results of the prepared Fe—P-coated graphite powder. 
       FIG. 11  shows that most of the Fe—P compounds were present as a FeP phase. In addition, no oxide peak was observed in the final coating layer. 
     An anode was prepared with a composition of prepared anode active material/PVDF=96/4 (wt %). A cathode was prepared with a composition of NCM622/SPB/PVDF=95/2.5/2.5 (wt %). The electrodes thus prepared were applied to manufacture the 2032-coin-type cell. In the half-cell, lithium metal was used as a counter electrode instead of the cathode. The loading level of the anode was set to 2.2 mAh/cm 2  and the mixture density of the anode was set to a range of 1.5-1.6 g/cc. 
     The charge and discharge characteristics of the cells were evaluated under the same conditions as in Example 2. 
     Example 4: Preparation of Ni—P-Based Anode Active Material and Manufacture of Secondary Battery 
     A coating solution was prepared by dissolving NiO as a NiO x  precursor at weight ratio of 2 wt % in nitric acid. 
     Then, artificial graphite was placed in the coating solution, followed by stirring for 12 hours, and then stirring was conducted with application of heat until the solvent was completely evaporated, and then drying was conducted at 100° C. in an oven for 12 hours, thereby preparing a NiO x  precursor-coated graphite powder. 
     A Ni—P-coated graphite powder was prepared by the following solid-phase reaction process. First, a graphite powder coated with a NiO x  precursor was mixed with NaH 2 PO 4  as a P precursor in a mortar. The thus mixed powder was set in a heat treatment furnace, and subjected to heat treatment at 800° C. for 3 hours. In this situation, argon gas was allowed to continuously flow while the heat treatment was conducted. Then, the heat-treated powder was washed using distilled water, and the washed powder was filtered and then dried at a temperature of 70° C. for 12 hours. 
       FIG. 12  illustrates XRD analysis results of the prepared Fe—P-coated graphite powder. 
       FIG. 12  shows that most of the Fe—P compounds were present as a NiP2 phase. In addition, no oxide peak was observed in the final coating layer. 
     An anode was prepared with a composition of the prepared anode active material/PVDF=96/4 (wt %). A cathode was prepared with a composition of NCM622/SPB/PVDF=95/2.5/2.5 (wt %). The electrodes thus prepared were applied to manufacture a 2032 coin-type cell. In the half-cell, lithium metal was used as a counter electrode instead of the cathode. The loading level of the anode was set to 2.2 mAh/cm 2  and the mixture density of the anode was set to a range of 1.5-1.6 g/cc. 
     The charge and discharge characteristics of the cells were evaluated under the same conditions as in Example 2. 
       FIG. 13  illustrates a graph showing initial charge and discharge characteristics of half-cells of Examples 2 to 4, and Table 4 illustrates a table showing the summary of charge and discharge characteristics. 
     
       
         
           
               
               
               
               
               
             
               
                   
                 TABLE 4 
               
               
                   
                   
               
               
                   
                   
                 Charge 
                 Discharge 
                 CE 
               
               
                   
                 Classification 
                 (mAh/g) 
                 (mAh/g) 
                 (%) 
               
               
                   
                   
               
             
            
               
                   
               
            
           
           
               
               
               
               
               
            
               
                   
                 Ref. Gr 
                 372.8 
                 345.4 
                 92.7 
               
               
                   
                 CoP@Gr 
                 387.4 
                 343.0 
                 88.5 
               
               
                   
                 FeP@Gr 
                 379.4 
                 342.2 
                 90.2 
               
               
                   
                 Ni 2 P@Gr 
                 394.6 
                 344.6 
                 87.3 
               
               
                   
                 CoP 2 2@Gr 
                 406.9 
                 356.6 
                 87.6 
               
               
                   
                   
               
            
           
         
       
     
     For evaluation of discharge characteristics of Comparative Examples (Ref. Gr) having graphite as an active material, Example 2 (CoP@Gr, CoP2@Gr), Example 3 (FeP@Gr), and Example 4 (NiP2@GR), charging and discharging was performed with a constant current of 0.2 C (70 mA/g) in the 0.01-1.5 V vs. Li/Li+ potential region. According to  FIG. 13 , after the introduction of a lithium insertion promoting material into the surface of graphite, an increase in the initial charge capacity was observed, and the change in the initial discharge amount was insignificant. This is considered to be the capacity contribution by a reversible reaction of the lithium insertion promoting material and lithium, and a side reaction due to the introduction of the lithium insertion promoting material was not observed in the shown charge and discharge curves. However, in Example 2, the CoP 2  and Li reaction was observed in 1.1 V vs. Li/Li+ during discharging by the CoP2 phase. 
       FIG. 14  illustrates a graph showing the results of evaluating lifespan characteristics of full cells of Examples 2 to 4. 
     NCM622 material was used as cathodes, and the active materials of the examples were manufactured into counter electrodes. Charging and discharging was performed three times with a constant current of 0.2 C (70 mA/g) in in a 2.5 to 4.2 V vs. Li/Li+ potential region, and thereafter, charging with a constant current of 6 C (2100 mA/g) and then discharging with a constant current of 1 C (350 mA/g) were performed 100 times. Referring to  FIG. 14 , after 100 cycles, the examples showed relatively superior capacity retention compared to the comparative example during high-rate charging and discharging, and this means that the high-rate charging characteristics of the full cell were improved after the introduction of various lithium insertion promoting materials into the surface of graphite. 
     Meanwhile, the examples described in the present specification and drawings are merely the representation of specific examples to aid the understanding, and are not intended to limit the scope of the present invention. It would be obvious to a person skilled in the art to which the present invention pertains that other modifications based on the technical spirit of the present invention in addition to the examples disclosed herein can be implemented. 
     INDUSTRIAL APPLICABILITY 
     The secondary batteries of the present invention can be applied.