WO2007133005A1

WO2007133005A1 - Electrode active material with high capacity

Info

Publication number: WO2007133005A1
Application number: PCT/KR2007/002288
Authority: WO
Inventors: Ou-Jung Kwon; Won-Seok Chang; Ki-Young Lee; Seo-Jae Lee; Ki-Tae Kim; Yong-Ju Lee
Original assignee: Lg Chem, Ltd.
Priority date: 2006-05-12
Filing date: 2007-05-10
Publication date: 2007-11-22
Also published as: KR20070109634A; TW200816547A; TWI431841B; EP2018677A4; EP2018677B1; EP2018677A1

Abstract

Disclosed is an electrode active material comprising: (a) a core layer containing (i) a metal and/or metalloid capable of repeating lithium intercalation/ deintercalation, and (ii) a metal oxide and/or metalloid oxide capable of repeating lithium intercalation/ deintercalation; (b) an amorphous carbon layer coated on a surface of the core layer; and (c) a crystalline carbon layer coated on the amorphous carbon layer. An electrochemical device comprising the electrode active material is also disclosed.

Description

Description ELECTRODE ACTIVE MATERIAL WITH HIGH CAPACITY

Technical Field

[1] The present invention relates to an electrode active material with high capacity, a method for preparing the same, and an electrochemical device comprising the same. Background Art

[2] In general, a lithium secondary battery, a typical example of electrochemical devices, is obtained by using electrode active materials capable of lithium ion intercalation/ deintercalation for a cathode and an anode, and by injecting an organic electrolyte or a polymer electrolyte between the cathode and the anode. Such lithium secondary batteries produce electric energy via redox reactions upon the lithium ion intercalation/ deintercalation at the cathode and the anode.

[3] Currently, carbonaceous materials are widely used as electrode active materials forming anodes for lithium secondary batteries. However, it is required to use an electrode active material with high capacity in order to further improve the capacity of a lithium secondary battery.

[4] To satisfy the above requirement, many studies have been conducted to develop novel materials other than carbonaceous materials. Among such materials, most recent studies have focused on: (1) silicon-carbon composite materials; (2) oxide materials including SiO or SnO ; and (3) composite materials comprising mixtures of oxide materials (2) with carbonaceous materials.

[5] Materials (1) provide high discharge capacity and high initial efficiency. However, materials (1) are problematic in that they have a low charge/discharge efficiency and poor cycle life characteristics. Materials (2) have a high charge/discharge efficiency. However, materials (2) are problematic in that they have relatively low discharge capacity as compared to materials (1), and they show significantly high irreversible capacity due to the bonding between lithium ions and oxygen atoms upon charge, resulting in an extremely low initial efficiency. Further, materials (3) partially solve the problems occurring in materials (1) and (2). However, they have a disadvantage in that carbonaceous materials mixed therein cause a drop in capacity.

[6] However, the above electrode active materials according to the prior art still provide unsatisfactory cycle life characteristics, and thus cannot be applied to practical lithium secondary batteries as electrode active materials. Disclosure of Invention Technical Problem

[7] Therefore, the present invention has been made in view of the above-mentioned problems. It is an object of the present invention to provide an electrode active material having high charge/discharge capacity and excellent cycle life characteristics, the electrode active material comprising an amorphous carbon layer and a crystalline carbon layer successively on a surface of a core layer containing a metal (metalloid)/a metal (metalloid) oxide. The electrode active material according to the present invention inhibits variations in volume of the core layer that may occur during charge/ discharge cycles, and maintains a high electric conductivity and conduction paths among electrode active material particles. It is another object of the present invention to provide an electrochemical device comprising the above electrode active material. It is still another object of the present invention to provide a method for preparing the above electrode active material. Technical Solution

[8] In order to achieve the above-mentioned object, there is provided an electrode active material comprising:

[9] (a) a core layer containing (i) a metal and/or metalloid capable of repeating lithium intercalation/ deintercalation, and (ii) a metal oxide and/or metalloid oxide capable of repeating lithium intercalation/ deintercalation;

[10] (b) an amorphous carbon layer coated on a surface of the core layer; and

[11] (c) a crystalline carbon layer coated on the amorphous carbon layer. There is also provided an electrochemical device comprising the above electrode active material.

[12] Additionally, there is provided a method for preparing the above electrode active material, the method comprising the steps of:

[13] (1) mixing (i) a metal and/or metalloid with (ii) a metal oxide and/or metalloid oxide to provide a first mixture, and further mixing the first mixture with crystalline carbon to provide a second mixture; and

[14] (2) carrying out mechanical alloying of the second mixture in the presence of balls.

[15] Further, there is provided another method for preparing the above electrode active material, the method comprising the steps of:

[16] (1) coating an amorphous carbon layer onto a core layer formed of (i) a metal and/or metalloid and (ii) a metal oxide and/or metalloid oxide via a thin film vapor deposition process, or coating pitch or an organic precursor onto a core layer formed of (i) a metal and/or metalloid and (ii) a metal oxide and/or metalloid oxide, and heat treating and carbonizing the coating layer to allow amorphous carbon to be coated on the core layer; and

[17] (2) coating slurry containing a crystalline carbonaceous material onto the resultant product of step (1), followed by drying, to form a crystalline carbon layer. Brief Description of the Drawings [18] The foregoing and other objects, features and advantages of the present invention will become more apparent from the following detailed description when taken in conjunction with the accompanying drawings in which:

[19] FlG. 1 is a sectional view of the electrode active material that may be prepared according to a preferred embodiment of the present invention;

[20] FlG. 2 is a photographic view of the electrode active material according to Example

1, taken by SEM (scanning electron microscopy);

[21] FlG. 3 is a graph showing the ratio of residual discharge capacity of each of the batteries according to Example 1 and Comparative Example 1 as a function of cycle number; and

[22] FlG. 4 is a graph showing the charge/discharge efficiency of each of the batteries according to Example 1 and Comparative Example 1 as a function of cycle number. Mode for the Invention

[23] The electrode active material according to the present invention comprises a core layer containing (i) a metal and/or metalloid capable of repeating lithium intercalation/ deintercalation, and (ii) a metal oxide and/or metalloid oxide capable of repeating lithium intercalation/ deintercalation.

[24] As used herein, the expression "a metal and/or metalloid" is an abbreviation for a metal or metalloid, or a metal and metalloid. Also, the expression "a metal oxide and/ or metalloid oxide" is an abbreviation for a metal oxide or metalloid oxide, or a metal oxide and metalloid oxide.

[25] The electrode active material according to the present invention can accomplish high charge/discharge capacity by virtue of the core layer containing (i) a metal and/or metalloid, and (ii) a metal oxide and/or metalloid oxide.

[26] Additionally, the electrode active material according to the present invention can inhibit variations in volume of the core layer that may occur during repeated lithium intercalation/deintercalation by virtue of the amorphous carbon layer coated on the surface of the core layer and the crystalline carbon layer coated on the amorphous carbon layer, and thus can provide improved cycle life characteristics.

[27] Further, a composite material of (i) a metal and/or metalloid with (ii) a metal oxide and/or metalloid oxide, such as Si/SiO (wherein 0<x<2), is capable of repeating charge/discharge cycles due to its electron conductivity and lithium ion conductivity. However, such composite materials have a low electron conductivity and cannot allow smooth lithium intercalation/ deintercalation. Therefore, a crystalline carbon layer is formed on an electrode active material layer to reduce the contact resistance between the electrode active material layer and a collector, as well as among the electrode active material particles, and thus to improve lithium intercalation/deintercalation char- acteristics.

[28] Hereinafter, the present invention will be explained in more detail.

[29] The coating layer including the amorphous carbon layer and the crystalline carbon layer may be partially or totally coated on the surface of the core layer. [30] FlG. 1 is a sectional view of the electrode active material that may be prepared according to a preferred embodiment of the present invention. As shown in FlG. 1, the surface of the core layer 11 formed of an electrochemically rechargeable Si and SiO

(wherein 0<x<2) is coated with a surface layer including an amorphous carbon layer 12 and a crystalline carbon layer 13.

[31] Particular examples of the metal and/or metalloid include at least one selected from the group consisting of Si, Al, Sn, Sb, Bi, As, Ge, Pb, Zn, Cd, In, Tl and Ga, or an alloy thereof. However, any metal and/or metalloid capable of electrochemical and reversible lithium intercalation/ deintercalation can be used with no particular limitation.

[32] Additionally, particular examples of the metal oxide and/or metalloid oxide include at least one selected from the group consisting of SiO , AlO , SnO , SbO , BiO , AsO , GeO , PbO , ZnO , CdO , InO , TlO and GaO (wherein 0<x<2). However, any metal oxide and/or metalloid oxide capable of electrochemical and reversible lithium in- tercalation/deintercalation can be used with no particular limitation.

[33] In the core-forming materials, i.e. (i) a metal and/or metalloid, and (ii) a metal oxide and/or metalloid oxide, the metals in the metal and the metal oxide may be the same or different. This also applies to the metalloids in the metalloid and the metalloid oxide. For example, combinations including Si/SiO , Sn/SnO , Si/SnO or Sn/SiO (wherein O

<x<2) may be used in the core layer.

[34] Particular examples of the crystalline carbon include natural graphite, artificial graphite, etc., which have a high degree of graphitization. Particular examples of the graphite-based material include MCMB (MesoCarbon MicroBead), carbon fiber, natural graphite, or the like.

[35] Particular examples of the amorphous carbon include coal tar pitch, petroleum pitch, and carbonaceous materials obtained by heat treatment of various organic materials.

[36] In a preferred embodiment of the electrode active material according to the present invention, the crystalline carbon layer comprises sheet-like carbon layer units connected with each other, wherein each sheet-like carbon layer unit is formed of at least two carbon layers having the same c-axis direction. In other words, as shown in FlG. 1, the term sheet-like carbon layer unit 14 refers to a plurality of sheet-like carbon layers having the same c-axis direction in each crystal as the concept of a unit. The sheet-like carbon layer units are connected with each other at the edge portions thereof to form the crystalline carbon layer. Additionally, the above crystalline carbon layer structure can inhibit variations in volume of the core layer that may occur during repeated lithium intercalation/deintercalation.

[37] Preferably, as shown in FlG. 1, the c-axis direction 20 of the sheet-like carbon layer units is perpendicular to the tangent direction 21 of the electrode active material particles according to the present invention. Because the c-axis direction of the sheet- like carbon layer units is perpendicular to the tangent direction of the electrode active material particles, edge portions of one sheet-like carbon layer unit are connected to those of another sheet-like carbon layer unit, and no edge portions are exposed to the exterior. If any edge portion of each sheet-like carbon layer unit is exposed, such exposed portions may contact with an electrolyte to form a coating film and generate an irreversible reaction.

[38] According to the present invention, the core layer comprises (i) a metal and/or metalloid and (ii) a metal oxide and/or metalloid oxide in a ratio of 5—95 parts by weight: 95 -5 parts by weight.

[39] If the metal and/or metalloid are/is used in an amount of less than 5 parts by weight, it is not possible to obtain high discharge capacity and high initial efficiency. On the other hand, if the metal oxide and/or metalloid oxide are/is used in an amount of less than 5 parts by weight, it is not possible to sufficiently improve charge/discharge efficiency.

[40] According to the present invention, the electrode active material comprises the core layer, the amorphous carbon layer and the crystalline carbon layer, successively, in a ratio of [core layer : amorphous carbon layer : crystalline carbon layer] of 70—30 parts by weight : 0.1-50 parts by weight : 29.9-70 parts by weight.

[41] If the core layer capable of repeating lithium intercalation/deintercalation is present in an amount of less than 30 parts by weight, the electrode active material cannot serve as a high-capacity electrode active material due to its low reversible capacity. If the crystalline carbon layer is present in an amount of less than 29.9 parts by weight, it is not possible to ensure a sufficient degree of conductivity. Additionally, if the amorphous carbon layer is present in an amount of less than 0.1 parts by weight, it is not possible to inhibit a so-called swelling phenomenon sufficiently. On the other hand, if the amorphous carbon layer is present in an amount of greater than 50 parts by weight, there is a possibility of a drop in capacity and conductivity.

[42] Preferably, the amorphous carbon layer has an interlayer spacing d002 of 0.34nm or more and a thickness of 5nm or more. If the amorphous carbon layer has a thickness of less than 5nm, it is not possible to sufficiently inhibit variations in volume of the core layer. If the interlayer spacing is less than 0.34nm, the amorphous carbon layer itself undergoes severe variations in volume during repeated charge/discharge cycles. Thus, it is not possible to sufficiently inhibit variations in volume of the core layer, resulting in degradation of cycle life characteristics.

[43] Preferably, the crystalline carbon layer has an interlayer spacing d002 of

0.3354~0.35nm. The lowest critical value is the theoretical minimum interlayer spacing of graphite, and thus any value smaller than the lowest critical value does not exist. Carbon having an interlayer spacing greater than the highest critical value has poor conductivity, so that the crystalline carbon layer using the same shows low conductivity. Thus, in this case, lithium intercalation/ deintercalation cannot proceed smoothly.

[44] Although there is no particular limitation in thickness of the crystalline carbon layer, the crystalline carbon layer preferably has a thickness of l~10 microns. If the crystalline carbon layer has a thickness of less than 1 micron, it is difficult to ensure sufficient conductivity among electrode active material particles. On the other hand, if the crystalline carbon layer has a thickness greater than 10 microns, proportion of the carbonaceous materials to the electrode active material is too high to obtain high charge/discharge capacity.

[45] The electrode active material according to the present invention can be obtained by the method comprising the steps of:

[46] (1) mixing (i) a metal and/or metalloid with (ii) a metal oxide and/or metalloid oxide to provide a first mixture, and further mixing the first mixture with crystalline carbon to provide a second mixture; and

[47] (2) carrying out mechanical alloying of the second mixture in the presence of balls.

[48] Herein, the term "mechanical alloying" refers to a process for forming an alloy having a uniform composition by applying a mechanical force. Such mechanical alloying is carried out preferably in a high energy ball mill system.

[49] In step (1), the first mixture may be obtained by mixing the metal and/or metalloid with the metal oxide and/or metalloid oxide in a ratio of 5~95 parts by weight: 95 ~5 parts by weight (metal and/or metalloid: metal oxide and/or metalloid oxide). Additionally, in step (1), the second mixture may be obtained by mixing the first mixture with crystalline carbon in a ratio of 70~30 parts by weight: 30~70 parts by weight (first mixture: crystalline carbon).

[50] In step (2), the balls may be mixed with the second mixture obtained from the first step in a ratio of [balls: second mixture of the first step] of 50—98 parts by weight: 50~2 parts by weight. If the ratio is less than 50:50, it is not possible to transfer compression stress to the mixture. On the other hand, if the ratio is greater than 98:2, the balls are used in an excessive amount, resulting in a drop in productivity.

[51] Further, the balls that may be used in the second step include stainless steel balls or zirconia balls.

[52] In a variant, the electrode active material according to the present invention can be obtained by the method comprising the steps of:

[53] (1) coating an amorphous carbon layer onto a core layer formed of (i) a metal and/or metalloid and (ii) a metal oxide and/or metalloid oxide via a thin film vapor deposition process, or coating pitch or an organic precursor onto a core layer formed of (i) a metal and/or metalloid and (ii) a metal oxide and/or metalloid oxide, and heat treating and carbonizing the coating layer to allow amorphous carbon to be coated on the core layer; and

[54] (2) coating slurry containing a crystalline carbonaceous material onto the resultant product of step (1), followed by drying, to form a crystalline carbon layer.

[55] In a preferred embodiment of the method, an amorphous carbon layer may be directly coated onto the metal/metal oxide forming the core layer via a thin film vapor deposition process, such as CVD (chemical vapor deposition) or PVD (physical vapor deposition). Otherwise, various organic precursors, such as petroleum-based pitch, coal tar pitch, phenolic resins, PVC (polyvinyl chloride) or PVA (polyvinyl alcohol), are coated onto the metal/metal oxide forming the core layer, and then carbonized via heat treatment under an inert atmosphere at a temperature of 500~1300°C for 30 minutes~3 hours, so that an amorphous carbon layer can be coated on the core layer. Next, an adequate amount of solvent is added to a mixture containing 90~98 parts by weight of a crystalline carbonaceous material and 2~10 parts by weight of a binder optionally with at most 5 parts by weight of a conductive agent, and the materials are mixed uniformly to provide slurry. Then, the slurry is coated onto the amorphous carbon layer, followed by drying, to form a crystalline carbon layer.

[56] Further, the present invention provides an electrochemical device comprising the electrode active material according to the present invention. Such electrochemical devices include any devices in which electrochemical reactions occur, and particular examples thereof include all kinds of primary batteries, secondary batteries, fuel cells, solar cells or capacitors. Preferably, the electrochemical device is a secondary battery. A lithium secondary battery including a lithium metal secondary battery, lithium ion secondary battery, lithium polymer secondary battery or lithium ion polymer secondary battery is more preferred.

[57] The electrochemical device may be manufactured by using the electrode active material according to the present invention via a conventional method known to one skilled in the art. In one embodiment of the method for manufacturing the electrochemical device, an electrode assembly having a cathode, an anode and a separator interposed between both electrodes is formed, and then an electrolyte is injected into the electrode assembly.

[58] The electrode using the electrode active material according to the present invention may be manufactured by a conventional method known to those skilled in the art. For example, the electrode active material according to the present invention may be mixed with a binder and a solvent, and optionally with a conductive agent and a dispersant, and the mixture is agitated to provide slurry. Then, the slurry is applied onto a metal collector, and the collector coated with the slurry is compressed and dried to provide an electrode.

[59] The binder and the conductive agent may be used in an amount of l~10 parts by weight and l~30 parts by weight, respectively, based on the weight of the electrode active material.

[60] Particular examples of the binder that may be used in the present invention include polytertrafluoro ethylene (PTFE), polyvinylidene fluoride (PVdF), or the like.

[61] There is no particular limitation in the conductive agent, as long as the conductive agent is an electron conductive material that causes no chemical change in a battery. In general, the conductive agent that may be used in the present invention includes carbon black, graphite, carbon fibers, carbon nanotubes, metal powder, conductive metal oxides, organic conductive agents, or the like. Commercially available conductive agents include acetylene black-based conductive agents (available from Chevron Chemical Company or Gulf Oil Company), Ketjen Black EC series (available from Armak Company), Vulcan XC-72 (available from Cabot Company) and Super P (available from MMM Co.)

[62] The metal collector includes a metal with high conductivity. Any metal to which the electrode active material slurry can be adhered with ease can be used as long as it shows no reactivity in the drive voltage range of a battery using the same. Typical examples of the collector include mesh, foil, etc., obtained from aluminum, copper, gold, nickel, aluminum alloy or a combination thereof.

[63] Also, there is no particular limitation in methods of applying the slurry onto the collector. For example, the slurry may be applied onto the collector via a doctor blade coating, dip coating or brush coating process. There is no particular limitation in amount of the slurry applied onto the collector. However, it is preferred that the slurry is applied in such an amount that the active material layer formed after removing a solvent or a dispersant can have a thickness of generally 0.005~5mm, and preferably 0.05~2mm.

[64] Further, there is no particular limitation in methods of removing the solvent or the dispersant. However, it is preferred that the solvent or the dispersant is allowed to evaporate as quickly as possible, provided that no cracking occurs in the active material layer due to the stress concentration and no separation occurs between the active material layer and the collector. For example, the collector coated with the active material slurry may be dried in a vacuum oven at 50~200°C for 0.5~3 days.

[65] The electrolyte may comprise a non-aqueous solvent and an electrolyte salt. [66] Any non-aqueous solvent currently used for a non-aqueous electrolyte may be used with no particular limitation. Particular examples of such non-aqueous solvents include cyclic carbonates, linear carbonates, lactones, ethers, esters, and/or ketones.

[67] Particular examples of the cyclic carbonates include ethylene carbonate (EC), propylene carbonate (PC), butylene carbonate (BC), or the like. Particular examples of the linear carbonates include diethyl carbonate (DEC), dimethyl carbonate (DMC), dipropyl carbonate (DPC), ethylmethyl carbonate (EMC), methyl propyl carbonate (MPC), or the like. Particular examples of the lactone include gamma-butyrolactone (GBL). Particular examples of the ether include dibutyl ether, tetrahydrofuran, 2-methyltetrahydrofuran, 1,4-dioxane, 1,2-dimethoxyethane, or the like. Additionally, particular examples of the ester include methyl acetate, ethyl acetate, methyl propionate, methyl pivalate, or the like. Further, particular examples of the ketone include polymethylvinyl ketone. Such non-aqueous solvents may be used alone or in combination.

[68] Any electrolyte salt currently used for a non-aqueous electrolyte may be used in the present invention with no particular limitation. Non-limiting examples of the electrolyte salt include a salt represented by the formula of A⁺B^", wherein A⁺ represents an alkali metal cation selected from the group consisting of Li⁺, Na⁺, K⁺ and combinations thereof, and B^" represents an anion selected from the group consisting of PF , BF₄ , Cl^", Br , I , ClO₄ , AsF₆ , CH₃CO₂ , CF₃SO₃ , N(CF₃SOp₂ , C(CF₂SO₂)₃ and combinations thereof. A lithium salt is particularly preferred. Such electrolyte salts may be used alone or in combination.

[69] The electrochemical device according to the present invention may further comprise a separator. There is no particular limitation in the separator that may be used in the present invention, as long as the separator is a porous material that serves to interrupt an internal short circuit in an electrode and is impregnated with an electrolyte. Non- limiting examples of the separator that may be used include a polypropylene-based, polyethylene-based, polyolefin-based porous separator, or composite porous separators including the above porous separators to which inorganic materials are added.

[70] There is no particular limitation in the outer shape of the secondary battery according to the present invention. The secondary battery may be a cylindrical battery using a can, a prismatic battery, a pouch-type battery or a coin-type battery.

[71] Reference will now be made in detail to the preferred embodiments of the present invention. It is to be understood that the following examples are illustrative only and the present invention is not limited thereto.

[72] Example 1

[73] Si was mixed with SiO in a ratio of 50 parts by weight:50 parts by weight to provide a first mixture (Si/SiO), which, in turn, was mixed with natural graphite in a ratio of 50 parts by weight:50 parts by weight to provide a second mixture.

[74] Next, stainless steel balls having a diameter of 3 mm and the second mixture were introduced into a high energy ball mill system in a ratio of 80 parts by weight:20 parts by weight. The resultant mixture was subjected to mechanical alloying at a rotation speed of 600 rpm for 30 minutes to provide an electrode active material. FlG. 2 is a photographic view of the electrode active material taken by SEM.

[75] Then, 100 parts by weight of the electrode active material powder obtained as described above, 10 parts by weight of PVDF as a binder and 10 parts by weight of acetylene black as a conductive agent were mixed, NMP was further added to the above mixture as a solvent, and then the resultant mixture was mixed thoroughly to provide uniform slurry. The slurry was coated onto copper foil with a thickness of 20 micron, followed by drying and rolling. The coated foil was cut into a desired size via punching to provide an electrode.

[76] As an electrolyte, a non-aqueous solvent comprising ethylene carbonate (EC) and ethyl methyl carbonate (EMC) in a ratio of 1:2 (v:v) and containing IM LiPF 6 dissolved therein was used. [77] The electrode obtained as described above was used as an anode and lithium metal was used as a counter electrode. Then, a polyolefin-based separator was interposed between both electrodes and the electrolyte was injected thereto to provide a coin-type battery according to the present invention. [78] Example 2

[79] An electrode active material and a battery were provided in the same manner as described in Example 1, except that Sn and SnO were used instead of Si and SiO. [80] Comparative Example 1

[81] An electrode active material and a battery were provided in the same manner as described in Example 1, except that SiO was not used and Si was mixed with natural graphite in a ratio of 50 parts by weight:50 parts by weight. [82] Comparative Example 2

[83] An electrode active material and a battery were provided in the same manner as described in Example 1, except that natural graphite was not used and Si was mixed with SiO in a ratio of 50 parts by weight:50 parts by weight. [84] Experimental Example

[85] (Measurement of Residual Discharge Capacity)

[86] The residual discharge capacity of each of the batteries according to Example 1 and

Comparative Example 1 was measured up to 50 charge/discharge cycles. The results are shown in FlG. 3. [87] As shown in FlG. 3, the battery obtained by using the electrode active material according to Example 1 maintains the initial capacity to a ratio of 91% or more even after fifty charge/discharge cycles. On the contrary, the battery obtained by using the electrode active material according to Comparative Example 1 maintains the initial capacity to a ratio of merely about 74% after fifty charge/discharge cycles.

[88] Therefore, it can be seen from the above results that the electrode active material according to the present invention comprising a core layer formed of Si and oxide thereof (SiO) can improve the residual discharge capacity ratio.

[89] (Measurement of Charge/Discharge Efficiency) [90] The charge/discharge efficiency of each of the batteries according to Example 1 and Comparative Example 1 was measured up to 50 charge/discharge cycles. The results are shown in FIG. 4.

[91] As shown in FIG. 4, the battery obtained by using the electrode active material according to Example 1 maintains the charge/discharge efficiency to a ratio of about 99.2—99.6%, even after fifty charge/discharge cycles. On the contrary, the battery obtained by using the electrode active material according to Comparative Example 1 maintains the charge/discharge efficiency to a ratio of about 98.3—99% after fifty charge/discharge cycles.

[92] Therefore, it can be seen from the above results that the electrode active material according to the present invention comprising a core layer formed of Si and oxide thereof (SiO) can improve the charge/discharge efficiency as well as the residual discharge capacity ratio.

[93] (Measurement of Change in Thickness of Electrode) [94] Each of the batteries according to Examples 1 and 2 and Comparative Examples 1 and 2 was subjected to two charge/discharge cycles, and measured for variations in volume. The results are shown in the following Table 1.

[95] Table 1 [Table 1] [Table ]

[96] As shown in Table 1, the battery according to Example 1 shows a variation in volume of about 40% (33D→46D), while the battery according to Comparative Example 1 shows a variation in volume of about 180% (25D→70D) and the battery according to Comparative Example 2 shows a variation in volume of about 132% (22D→51D). This indicates that the electrode active material according to the present invention has an effect of inhibiting a volume expansion.

Industrial Applicability

[97] As can be seen from the foregoing, the electrode active material according to the present invention, comprising a core layer formed of (i) a metal and/or metalloid and (ii) a metal oxide and/or metalloid oxide, maintains high charge/discharge capacity. Also, due to the presence of the amorphous carbon layer and the crystalline carbon layer, the electrode active material according to the present invention inhibits variations in volume of the core layer that may occur during repeated lithium in- tercalation/deintercalation. Therefore, the electrode active material according to the present invention can provide a battery with improved cycle life characteristics.

[98] Although several preferred embodiments of the present invention have been described for illustrative purposes, those skilled in the art will appreciate that various modifications, additions and substitutions are possible, without departing from the scope and spirit of the invention as disclosed in the accompanying claims.

Claims

[ 1 ] An electrode active material comprising :

(a) a core layer containing (i) a metal and/or metalloid capable of repeating lithium intercalation/ deintercalation, and (ii) a metal oxide and/or metalloid oxide capable of repeating lithium intercalation/ deintercalation;

(b) an amorphous carbon layer coated on a surface of the core layer; and

(c) a crystalline carbon layer coated on the amorphous carbon layer.

[2] The electrode active material as claimed in claim 1, wherein the metal and/or metalloid is at least one selected from the group consisting of Si, Al, Sn, Sb, Bi, As, Ge, Pb, Zn, Cd, In, Η and Ga, or an alloy thereof.

[3] The electrode active material as claimed in claim 1, wherein the metal oxide and/ or metalloid oxide is at least one selected from the group consisting of SiO , AlO , SnO , SbO , BiO , AsO , GeO , PbO , ZnO , CdO , InO , TlO and GaO

(wherein 0<x<2). [4] The electrode active material as claimed in claim 1, wherein the crystalline carbon layer comprises sheet-like carbon layer units connected with each other, each sheet-like carbon layer unit being formed of at least two carbon layers having the same c-axis direction. [5] The electrode active material as claimed in claim 4, wherein the c-axis direction of the sheet-like carbon layer units is perpendicular to the tangent direction of the electrode active material particles. [6] The electrode active material as claimed in claim 1, wherein the metal and/or metalloid and the metal oxide and/or metalloid oxide are used in a ratio of [metal and/or metalloid: metal oxide and/or metalloid oxide] of 5—95 parts by weight:

95 -5 parts by weight. [7] The electrode active material as claimed in claim 1, which comprises the core layer, the amorphous carbon layer and the crystalline carbon layer, in a ratio of

[core layer : amorphous carbon layer : crystalline carbon layer] of 70—30 parts by weight : 0.1-50 parts by weight : 29.9-70 parts by weight. [8] The electrode active material as claimed in claim 1, wherein the crystalline carbon layer has an interlayer spacing d002 of 0.3354— 0.35nm and a thickness of

1—10 microns. [9] The electrode active material as claimed in claim 1, wherein the amorphous carbon layer has an interlayer spacing d002 of 0.34nm or more and a thickness of

5 nm or more. [10] An electrochemical device comprising the electrode active material as defined in any one of claims 1 to 9. [11] The electrochemical device as claimed in claim 10, which is a secondary battery.

[12] A method for preparing the electrode active material as defined in any one of claims 1 to 9, the method comprising the steps of:

(1) mixing (i) a metal and/or metalloid with (ii) a metal oxide and/or metalloid oxide to provide a first mixture, and further mixing the first mixture with crystalline carbon to provide a second mixture; and

(2) carrying out mechanical alloying of the second mixture in the presence of balls.

[13] The method as claimed in claim 12, wherein the first mixture in step (1) comprises the metal and/or metalloid and the metal oxide and/or metalloid oxide, in a ratio of [metal and/or metalloid: metal oxide and/or metalloid oxide] of 5~95 parts by weight: 95~5 parts by weight.

[14] The method as claimed in claim 12, wherein the second mixture in step (1) comprises the first mixture and the crystalline carbon, in a ratio of [first mixture: crystalline carbon] of 70—30 parts by weight: 30—70 parts by weight.

[15] The method as claimed in claim 12, wherein step (2) is carried out by using the balls and the second mixture in a ratio of [balls: second mixture of step (I)] of 50~98 parts by weight: 50~2 parts by weight.

[16] The method as claimed in claim 12, wherein the balls used in step (2) are stainless steel balls or zirconia balls.

[17] A method for preparing the electrode active material as defined in any one of claims 1 to 9, the method comprising the steps of:

(1) coating an amorphous carbon layer onto a core layer formed of (i) a metal and/or metalloid and (ii) a metal oxide and/or metalloid oxide via a thin film vapor deposition process, or coating pitch or an organic precursor onto a core layer formed of (i) a metal and/or metalloid and (ii) a metal oxide and/or metalloid oxide, and heat treating and carbonizing the coating layer to allow amorphous carbon to be coated on the core layer; and

(2) coating slurry containing a crystalline carbonaceous material onto the resultant product of step (1), followed by drying, to form a crystalline carbon layer.