US20130177815A1

US20130177815A1 - Negative active material, lithium secondary battery comprising the negative active material and manufacturing method thereof

Info

Publication number: US20130177815A1
Application number: US13/822,383
Authority: US
Inventors: Young Jun Kim; Yong Nam Jo; Min Sik PARK
Original assignee: Korea Electronics Technology Institute
Current assignee: Korea Electronics Technology Institute
Priority date: 2010-09-16
Filing date: 2011-08-19
Publication date: 2013-07-11
Also published as: KR20120029314A; CN103140968A; CN103140968B; KR101316638B1; KR20120029318A; US20130157139A1; CN103109403B; US8932763B2; CN103109403A; KR101416064B1; KR20130082132A

Abstract

Disclosed are an anode active material, a non-aqueous lithium secondary battery, and a preparation method thereof. The surface of a carbonaceous material is modified without using an electrolyte additive, and the reactivity and structural stability of the surface is improved, thereby obtaining long lifetime characteristics without deteriorating charge/discharge efficiency and rate characteristics when applied as an anode active material of a non-aqueous lithium secondary battery. The anode active material comprises a carbonaceous material, and a coating layer formed on the surface of the carbonaceous material through hetero atom substitution, wherein the hetero atom can be phosphorus (P) or sulfur (S). A side reaction with an electrolyte on the surface of the carbonaceous material is inhibited and the structural stability of the surface is enhanced by forming a coating layer on the surface of the carbonaceous material with a hetero atom such as phosphorus (P) or sulfur (S).

Description

TECHNICAL FIELD

The present invention relates to a non-aqueous lithium secondary battery and a fabricating method thereof, and more particularly, to an anode active material, a non-aqueous lithium secondary battery including the anode active material, and a method for fabricating the anode active material in which the surface of a carbonaceous material used as the anode active material of the lithium secondary battery is treated using hetero elements in order to suppress side reaction of the carbonaceous material with an electrolyte at the surface thereof and to enhance structural stability, thereby improving lifespan characteristics and rate characteristics of the non-aqueous lithium secondary battery.

BACKGROUND ART

As portable small electric/electronic devices are widely propagated, new secondary batteries such as a nickel metal hydride battery and a lithium secondary battery are actively being developed.
The lithium secondary battery uses metal lithium as an anode active material and a non-aqueous solvent as an electrolyte. Lithium can generate a high voltage because it has considerable ionization tendency, and thus a battery having a high energy density using lithium is under development. The lithium secondary battery using metal lithium as an anode active material has been used as a next-generation battery for a long time.
However, the lithium secondary battery has a short life cycle because lithium dendrites grow from the anode and penetrate an insulating membrane as charging and discharging of the lithium secondary battery are repeated, resulting in short-circuit with the cathode, causing battery failure.
To solve the problem that the life cycle of the lithium secondary battery is reduced due to anode deterioration, a method of using a carbon-based material capable of intercalating/deintercalating lithium ions instead of metal lithium as an anode active material was proposed.
In a lithium secondary battery having an anode formed using a carbonaceous material, the lithium ions are intercalated into carbon according to reaction at the cathode during charging/discharging. Electrons are transferred to a carbonaceous material of the anode and thus carbon is negatively charged to deintercalate the lithium ions from the cathode and intercalate the lithium ions into the carbonaceous material of the anode during charging, whereas the lithium ions are deintercalated from the carbonaceous material of the anode and intercalated into the cathode during discharging. Using this mechanism, precipitation of metal lithium at the anode can be prevented to achieve a lithium secondary battery having a considerably long life cycle.
The lithium secondary battery using a carbonaceous material as an anode active material is called a lithium ion secondary battery and has been widely propagated as a battery of portable electronic/communication devices. However, when a carbonaceous material is used as an anode active material, the charge/discharge potential of lithium is lower than the stable range of a conventional non-aqueous electrolyte, and thus decomposition of electrolyte occurs during charging/discharging, causing low initial charging/discharging efficiency of the current lithium secondary battery using a carbonaceous material as an anode material, short battery lifespan, and deterioration of rate characteristics. Accordingly, methods for stabilizing the surface of a carbonaceous anode active material using an electrolyte additive having a decomposition potential higher than that of a carbonaceous electrolyte, such as VC, FEC, etc. are proposed in order to increase the lifespan of a non-aqueous lithium secondary battery using a carbonaceous material.
However, the electrolyte additive cannot solve the problems of rate characteristics and charging/discharging efficiency deterioration although it increases the lifespan of the lithium secondary battery.

DETAILED DESCRIPTION OF THE INVENTION

Technical Problems

An object of the present invention is to provide an anode active material surface-treated using hetero elements, a non-aqueous lithium secondary battery including the anode active material, and a method for fabricating the anode active material by reforming the surface of a carbonaceous material without using an electrolyte additive so as to improve reactivity and structural stability of the surface, thus improving battery lifespan without deteriorating charging/discharging efficiency and rate characteristics when the carbonaceous material is used as an anode active material of the non-aqueous lithium secondary battery.

Technical Solutions

The objects of the present invention can be achieved by providing an anode active material for use in a non-aqueous lithium secondary battery, which includes a carbonaceous material, and a coating layer of hetero elements formed on the surface of the carbonaceous material, wherein the hetero elements include phosphorus (P).
The hetero elements may include sulfur (S).
The carbonaceous material may include at least one of artificial graphite, natural graphite, graphitized carbon fiber, graphitized mesocarbon microbeads, petroleum coke, plastic resins, carbon fiber and pyrocarbon.
The carbonaceous material may have L_a(110)>10 nm and L_c(002)>10 nm L, wherein L_a(110)=0.89λ/[B₁₁₀cos(θ₁₁₀)] and L_s(002)=0.89λ/[B₀₀₂cos(θ₀₀₂)], wherein λ is the wavelength of Cu Kα(λ=0.15418 nm) and B is a full width at half-maximum (FWHM) value with respect to (110) or (002) peak according to Bragg diffraction angle.
The carbonaceous material may have 0.344 nm or less as d₀₀₂with respect to (002) peak.
The carbonaceous material may have a specific surface area of less than 10 m²/g.
The carbonaceous material may have a degree of graphitization in the range of 0.4 to 1.0, and the degree of graphitization is calculated according to (degree of graphitization)=(3.44−d₀₀₂)/(0.086).
The content of the coating layer may be less than 10 wt % with respect to the carbonaceous material.
The coating layer may be formed uniformly on the overall surface of the carbonaceous material or formed on part of the surface of the carbonaceous material.
The present invention provides a lithium secondary battery including an anode formed of the anode active material.
The present invention provides a method for fabricating an anode active material for use in a non-aqueous lithium secondary battery, the method includes preparing a carbonaceous material and a hetero element material, and forming a coating layer of hetero elements on the surface of the carbonaceous material using the hetero element material, wherein the hetero elements include phosphorus (P).
The hetero elements may further include sulfur (S).
The hetero element material may include at least one of NH₄PF₆, (NH₄)₂PO₄, NH₄PO₃, (NH₄)₂SO₃, (NH₄)₂SO₄, NH₄SO₄, and (NH₄)₂S₂O₈.
The forming of the coating layer may include dissolving the hetero element material in a solvent to form a solution; uniformly mixing the carbonaceous material with the solution to form a mixture; vacuum-drying the mixture; and performing heat treatment on the dried material through thermal decomposition to form the coating layer based on the hetero elements on the surface of the carbonaceous material.

Advantageous Effects

According to the present invention, a coating layer can be formed on the surface of a carbonaceous material used as an anode active material of a non-aqueous lithium secondary battery by using hetero elements such as phosphorus (P) or sulfur (S), thereby suppressing a side reaction of the carbonaceous material at the surface thereof according to the coating layer formed on the carbonaceous material and enhancing structural stability.
Furthermore, affinity of the anode active material with the electrolyte can be improved so as to enhance battery lifespan and rate characteristics of the non-aqueous lithium secondary battery.
In addition, fabricating efficiency of the anode active material can be improved according to a simple surface treatment process.

DESCRIPTION OF DRAWINGS

FIG. 1 is a flowchart illustrating a method for fabricating an anode active material surface-treated with hetero elements for a non-aqueous lithium secondary battery according to an embodiment of the present invention.

FIG. 2 shows pictures of anode active materials according to embodiments of the present invention and a comparative example.

FIG. 3 shows EDS (Energy Dispersive Spectroscopy) analysis results regarding an anode active material according to a first embodiment of the present invention.

FIG. 4 shows EDS analysis results regarding an anode active material according to a second embodiment of the present invention.

FIG. 5 shows an XPS (X-ray Photoelectron Spectroscopy) analysis result regarding the anode active material according to the first embodiment of the present invention.

FIG. 6 shows an XPS analysis result regarding the anode active material according to the second embodiment of the present invention.

FIG. 7 shows XRD (X-ray diffraction) analysis results regarding the anode active materials according to the embodiments of the present invention and the comparative example.

FIG. 8 is a graph showing lifespan characteristics of a non-aqueous lithium secondary battery according to surface treatment temperatures of the anode active materials according to the embodiments of the present invention and the comparative example.

FIG. 9 is a graph showing rate characteristics of non-aqueous lithium secondary batteries according to the embodiments of the present invention and the comparative example.

MODE FOR CARRYING OUT THE INVENTION

In describing embodiments of the present invention, detailed descriptions of constructions or processes known in the art may be omitted to avoid obscuring appreciation of the invention by a person of ordinary skill in the art with unnecessary detail regarding such known constructions and functions.
Accordingly, the meaning of specific terms or words used in the specification and claims should not be limited to the literal or commonly employed sense, but should be construed or may be different in accordance with the intention of a user or an operator and customary usages. Therefore, the definition of the specific terms or words should be based on the contents across the specification. It should be understood, however, that there is no intent to limit the invention to the particular forms disclosed, but on the contrary, the invention is to cover all modifications, equivalents, and alternatives falling within the spirit and scope of the invention as defined by the claims.
Embodiments of the present invention will be described in detail with reference to the attached drawings.
An anode active material of a non-aqueous lithium secondary battery according to an embodiment of the present invention includes a carbonaceous material and a coating layer of hetero elements formed on the surface of the carbonaceous material. The hetero elements include phosphorus (P) or sulfur (S).
The carbonaceous material may use at least one of amorphous carbon materials such as artificial graphite, natural graphite, graphitized carbon fiber, graphitized mesocarbon microbeads, petroleum coke, plastic resins, carbon fiber, pyrocarbon, etc.
The following carbonaceous materials are preferably used in order to stably form the coating layer of hetero elements on the surface of the carbonaceous material and also to improve lifespan characteristics and rate characteristics of the non-aqueous lithium secondary battery employing the anode active material.
A carbonaceous material having L_a(110)>10 nm and L_c(002)>10 nm L is preferably used. L_a(110)and L_c(002)can be represented as L_a(110)=0.89λ/[B₁₁₀cos(θ₁₁₀)] and L_c(002)=0.89λ/[B₀₀₂cos(θ₀₀₂)]. Here, λ is the wavelength of Cu Kα(λ=0.15418 nm) and B denotes a full width at half-maximum (FWHM) value with respect to (110) or (002) peak according to Bragg diffraction angle. A carbonaceous material having 0.344 nm or less as d₀₀₂with respect to (002) peak is preferably used.
Furthermore, a carbonaceous material having a degree of graphitization in the range of 0.4 to 1.0 is preferably used. Here, the degree of graphitization can be calculated according to (degree of graphitization)=(3.44−d₀₀₂)/(0.086).
In addition, a carbonaceous material having a specific surface area of 10 m²/g or less is preferably used.
The coating layer may be formed by heat-treating the surface of the carbonaceous material through thermal decomposition using 10% or less by weight of a hetero element material respect to the carbonaceous material. That is, components other than the hetero elements are removed from the hetero element material during heat treatment of the hetero element material through thermal decomposition and the hetero elements forms the coating layer on the surface of the carbonaceous material. The coating layer may be formed uniformly on the overall surface of the carbonaceous material or on only part of the surface of the carbonaceous material according to the quantity of the heat-treated hetero element material. The hetero element material may be present in forms of various compounds including hetero elements. For example, the hetero element material includes NH₄PF₆, (NH₄)₂PO₄, NH₄PO₃, (NH₄)₂SO₃, (NH₄)₂SO₄, NH₄SO₄, (NH₄)₂S₂O₈, etc. However, the hetero element material is not limited thereto.
In this manner, the coating layer of hetero elements such as phosphorus or sulfur is formed on the surface of the carbonaceous material used as the anode active material of the non-aqueous lithium secondary battery, and thus side reaction of the carbonaceous material at the surface thereof can be suppressed and structural stability can be enhanced. Furthermore, affinity of the anode active material with the electrolyte can be improved so as to enhance battery lifespan and rate characteristics of the non-aqueous lithium secondary battery. In addition, production efficiency of the anode active material can be improved through the simple surface treatment process.
A method of forming the anode active material of the non-aqueous lithium secondary battery, which is surface-treated with the hetero element material, according to the present invention will now be described with reference to FIG. 1. FIG. 1 is a flowchart illustrating a method for fabricating an anode active material surface-treated with hetero elements for a non-aqueous lithium secondary battery according to an embodiment of the present invention.
Referring to FIG. 1, the method of fabricating the anode active material according to the present invention includes a step (S11) of preparing the carbonaceous material and the hetero element material and steps (S13 to S19) of forming the coating layer on the surface of the carbonaceous material using the hetero element material.
Specifically, the carbonaceous material and the hetero element material are prepared in step S11. Here, a carbonaceous material having a mean particle size of less than 15 μm may be used as the carbonaceous material. NH₄PF₆, (NH₄)₂PO₄, NH₄PO₃, (NH₄)₂SO₃, (NH₄)₂SO₄, NH₄SO₄, (NH₄)₂S₂O₈, etc. may be used as the hetero element material.
The hetero element material is dissolved in deionized (DI) water to form an aqueous solution in step S13. Here, while ID water is used as a solvent in the present embodiment, an organic solvent such as alcohol can be used.
The carbonaceous material is mixed with the aqueous solution to form a mixture in step S15. Step S15 may be performed for about 15 minutes to uniformly mix the carbonaceous material with the aqueous solution.
The mixture is vacuum-dried in step S17. Vacuum drying may be performed at a temperature in the range of 80 to 150° C. for 1 to 5 hours.
The material dried in step S17 is heat-treated through thermal decomposition in step S19 to form the anode active material corresponding to the carbonaceous material surface-treaded with the hetero element material according to the present invention. That is, during the process of thermally treating the hetero element material through thermal decomposition, components other than the hetero elements are removed from the hetero element material and the hetero elements forms the coating layer on the surface of the carbonaceous material. Heat treatment in step S19 may be performed in an inert gas atmosphere at a temperature in the range of 200 to 3000° C. for 1 hour or longer. For example, heat treatment can be performed in an Ar or N₂atmosphere at a heating rate of 10° C./min.
While the aqueous solution of the carbonaceous material and the hetero element material is formed, vacuum-dried and heat-treated to form the coating layer of the surface of the carbonaceous material through steps S13 to S19 in the present embodiment of the invention, the present invention is not limited thereto. For example, it is possible to dissolve the hetero element material in a solvent to form a solution, inject the solution into the carbonaceous material, and then heat-treat the carbonaceous material into which the solution has been injected to form the coating layer on the surface of the carbonaceous material. Otherwise, it is possible to mix powders of the carbonaceous material and the hetero element material and heat-treat the mixed powders to form the coating layer on the surface of the carbonaceous material. That is, the coating layer is formed on the surface of the carbonaceous material through a dry method. While heat treatment is performed in an inert gas atmosphere in the present embodiment, heat treatment may be carried out in a vacuum or oxidizing atmosphere.
To evaluate the lifespan and rate characteristics of the non-aqueous lithium secondary battery using the anode active material according to the present invention, non-aqueous lithium secondary batteries according to embodiments and a comparative example were manufactured as follows. In the embodiments, a carbonaceous material surface-treated with a hetero element material is used as the anode active material. In the comparative example, a carbonaceous material that is not surface-treated with a hetero element material is used as the anode active material. The non-aqueous lithium secondary batteries according to the embodiments and the comparative example are manufactured in the same manner, excepting the anode active materials, and thus description is focused on the method of fabricating the non-aqueous lithium secondary battery according to the embodiments.
A slurry is formed using 96 wt % of an anode active material, 2 wt % of binding agent SBR and a thickener CMC, and water as a solvent. This slurry is coated on Cu foil having a thickness of 20 μm, dried, consolidated using a press, and then dried in vacuum at 120° C. for 16 hours, to manufacture an electrode in the form of a circular plate having a diameter of 12 mm. Punched lithium metal foil having a diameter of 14 mm is used as a counter electrode, and a PE film is used as a membrane. A mixed solution of LiPF₆of 1M and EC/DMC mixed in a ratio of 3:7 is used as an electrolyte. The electrolyte is impregnated into the membrane, and the membrane is interposed between the electrode and the counter electrode and then is set in a SUS case, achieving a test cell for electrode evaluation, that is, the non-aqueous lithium secondary battery.
The carbonaceous material can be at least one of amorphous carbon materials such as artificial graphite, natural graphite, graphitized carbon fiber, graphitized mesocarbon microbeads, petroleum coke, plastic resins, carbon fiber, pyrocarbon, etc.
The hetero element material can be NH₄PF₆, (NH₄)₂PO₄, NH₄PO₃, (NH₄)₂SO₃, (NH₄)₂SO₄, NH₄SO₄, or (NH₄)₂S₂O₈. However, the hetero element material is not limited thereto.
The carbonaceous material surface-treated with the hetero element material can be used as an anode active material of a non-aqueous lithium secondary battery using a carbonate electrolyte. Furthermore, the carbonaceous anode active material surface-treated with the hetero element material can be applied to a lithium secondary battery having a non-aqueous electrolyte operating in a voltage range of 0V to 5V.
An anode plate is manufactured by adding a conducting material, a binding agent, a filler, a dispersing agent, an ion conducting material, a pressure increasing agent, and one or more generally used additive components to powder of the anode active material surface-treated with the hetero element material as necessary, to form a slurry or paste. The slurry or paste is coated on an electrode support plate using doctor blade method, for example, dried, and then pressed with a rolling roll, to manufacture the anode plate.
Here, graphite, carbon black, acetylene black, Ketjen black, carbon fiber, metal powder, etc. may be used as the conductive material. PVdF, polyethylene, etc. may be used as the binding agent. The anode plate (also referred to as a current collector) may be formed of copper, nickel, stainless steel or aluminum foil or sheet, or carbon fiber, etc.
The lithium secondary battery is manufactured using the anode formed as above. The lithium secondary battery may have any of coin, button, sheet, cylindrical, and rectangular shapes. The anode, electrolyte and membrane of the lithium secondary battery use those of conventional lithium secondary batteries.
A cathode active material includes a material reversibly capable of intercalating and deintercalating lithium ions. A lithium-transition metal oxide such as LiCoO₂, LiNiO₂, LiMnO₂, LiMn₂O₄, or LiNi_1-x-yCo_xM_yO₂(0≦x≦1, 0≦y≦1, 0≦x+y≦1, M being metal such as Al, Sr, Mg, La, etc.) may be used as the cathode active material. Otherwise, one or more of the above cathode active materials can be used. The above-mentioned cathode active material is exemplary and the present invention is not limited thereto.
The electrolyte may use a non-aqueous electrolyte containing lithium carbonate dissolved in an organic solvent, an inorganic solid electrolyte, an inorganic solid electrolyte compound, etc. However, the present invention is not limited thereto.
Here, carbonate, ester, ether or ketone may be used as a solvent of the non-aqueous electrolyte. Dimethyl carbonate (DMC), diethyl carbonate (DEC), dipropyl carbonate (DPC), methylpropyl carbonate (MPC), ethylpropyl carbonate (EPC), ethylmethyl carbonate (EMC), ethylene carbonate (EC), propylene carbonate (PC), butylene carbonate (BC), etc. may be used as the carbonate. Butyrolactone (BL), decanolide, valerolactone, mevalonolactone, caprolactone, n-methyl acetate, n-ethyl acetate, n-propyl acetate, etc. may be used as the ester. Dibutyl ether may be used as the ether. Polymethylvinyl ketone may be used as the ketone. The non-aqueous electrolyte according to the present invention is not limited to non-aqueous organic solvents.
Examples of the lithium carbonate of the non-aqueous electrolyte include one or more of LiPF₆, LiBF₄, LiSbF₆, LiAsF₆, LiClO₄, LiCF₃SO₃, LiN(CF₃SO₂)₂, LiN(C₂F₅SO₂)₂, LiAlO₄, LiAlCl₄, LiN(C_xF_2x+1SO₂)(C_yF_2x+1SO₂) (x and y being natural numbers) and LiSO₃CF₃, or a mixture thereof.
A porous film formed from polyolefin such as PP or PE or a porous material such as non-woven fabric may be used as the membrane.

EMBODIMENTS AND COMPARATIVE EXAMPLE

In the comparative example, natural graphite having a mean particle size of less than 15 μm was used as the carbonaceous material that is not surface-treated with the hetero element material for the anode active material.
In the first embodiment, natural graphite having a mean particle size of less than 15 μm, which has been surface-treated using NH₄PF₆as the hetero element material in order to use phosphorus (P) as the hetero elements, was used as the anode active material.
In the embodiment, natural graphite having a mean particle size of less than 15 μm, which has been surface-treated using (NH₄)₂SO₄as the hetero element material in order to use sulfur (S) as the hetero elements, was used as the anode active material.
The anode active materials according to the first and second embodiments were fabricated as follows. To introduce P and S to the surface of natural graphite as a carbonaceous material, 3 wt % of NH₄PF₆and 3 wt % of (NH₄)₂SO₄were respectively dissolved in DI water, uniformly coated on the surface of the natural graphite, and heat-treated at 800° C., to form anode active materials having P or S contained in the surface thereof, which are used for a non-aqueous lithium secondary battery.
Comparing morphologies of the anode active materials according to the first and second embodiments and the comparative example, no surface structure variation and no impurity generation in the natural graphite were observed, as shown in FIG. 2.
It can be confirmed that P and S are uniformly distributed on the surfaces of the natural graphite from EDS and XPS analysis results regarding the anode active materials according to the first and second embodiments, which are shown in FIGS. 3 and 4. FIGS. 3 and 4 show element mapping results of the surfaces of the natural graphite, to which P and S have been introduced, which are obtained through EDS analysis.
Referring to FIG. 3, 0.59 wt % of P was detected from the natural graphite in the case of the anode active material according to the first embodiment. Referring to FIG. 4, 0.28 wt % of S was detected from the surface of the natural graphite in the case of the anode active material according to the second embodiment.
Results of XPS analysis for analyzing the surface structures of the anode active materials according to the first and second embodiments are shown in FIGS. 5 and 6. Referring to FIG. 5, it can be confirmed that P 2 p peak (131 to 135 eV) is formed on the surface of the anode active material according to the first embodiment. Referring to FIG. 6, it can be confirmed that S 2 p peak (161 to 168 eV) is formed on the surface of the anode active material according to the second embodiment. This means that P and S existing on the surface of the natural graphite form specific combinations with carbon of the natural graphite.
Results of XRD analysis for analyzing the anode active materials according to the comparative example and the first and second embodiments are shown in FIG. 7. Referring to FIG. 7, impurities or second phases are not generated after introduction of the hetero elements. L_a(110)and L_c(002)calculated on the basis of the XRD results are listed in Table 1. It can be confirmed from Table 1 that L_a(110)is hardly varied and L_c(002)is reduced after the hetero elements is introduced.

	TABLE 1

	L_c(002)[nm]	L_a(aa0)[nm]

Comparative example	35.854	71.413
First embodiment	31.512	71.402
Second embodiment	32.268	71.422

Values of d₀₀₂and FWHM values of the anode active materials according to the comparative example and the first and second embodiments, obtained through the XRD data, are shown in Table 2. Referring to Table 2, d₀₀₂hardly varies and FWHM values increase after the hetero elements is introduced. This is regarded as a result of substitution or doping of some of P or S introduced to the surface of the natural graphite for the surface of the natural graphite. Specific surface areas of the anode active materials according to the comparative example and the first and second embodiments were measured as 2.7845 m²/g, 2.7461 m²/g and 2.7199 m²/g, respectively.

TABLE 2

2θ(°)	FWHM*(°)	d**(nm)

Comparative example	26.474	0.225	0.3366(8)
First embodiment	26.453	0.256	0.3369(5)
Second embodiment	26.458	0.250	0.3368(4)

*Full Width at half maximum for (002) peak
**Interlayer spacing for (002) peak

The lifespan and rate characteristics of non-aqueous lithium secondary batteries including the anode active materials according to the comparative example and the first and second embodiments were checked through the following test.
To check the influence of the type of the hetero element material on the lifespan of the non-aqueous lithium secondary battery, the following test was performed using non-aqueous lithium secondary batteries to which the anode active materials according to the comparative example and first and second embodiments are applied. 3 cycles of charging/discharging of the non-aqueous lithium secondary batteries to which the anode active materials according to the comparative example and first and second embodiments are applied were performed using current of 0.2 C (72 mA/g), and then 50 cycles of charging/discharging were carried out using current of 0.5 C (180 mA/g). The test results are shown in FIG. 8. As can be confirmed from FIG. 8, the non-aqueous lithium secondary batteries having the anode active materials surface-treated with the hetero element material according to the first and second embodiments have a longer lifespan than that of the comparative example.
To check the influence of the type of the hetero element material on rate characteristics of the non-aqueous lithium secondary batteries, the following test was performed using the non-aqueous lithium secondary batteries to which the anode active materials according to the comparative example of first and second embodiments were applied. 1-cycle charging/discharging of the non-aqueous lithium secondary batteries to which the anode active materials according to the comparative example and first and second embodiments were applied was performed using current of 0.2 C (72 mA/g). Then, charging is performed with fixed current of 0.5 C (180 mA/g) and discharging cycles are respectively performed for 3 seconds using 0.2 C (72 mA/g), 0.5 C (180 mA/g), 1 C (360 mA/g), 2 C (720 mA/g), 3 C (1080 mA/g) and 5 C (1800 mA/g). Subsequently, 2 cycles of charging/discharging are performed using 0.2 C (72 mA/g). The test results are shown in FIG. 9. As can be confirmed from FIG. 9, rate characteristics are improved after surface treatment.
The above-described test results show that the coating layer formed on the natural graphite by treating the surface of the natural graphite using the hetero element material effectively suppresses side reaction due to direct contact with the electrolyte and enhance structural stability of the surface of the natural graphite, thereby improving battery lifespan and output characteristic of the non-aqueous lithium secondary battery to which the anode active material surface-treated with the hetero element material is applied.
The detailed description of the preferred embodiments of the present invention has been given to enable those skilled in the art to implement and practice the invention. Although the invention has been described with reference to the preferred embodiments, those skilled in the art will appreciate that various modifications and variations can be made in the present invention without departing from the spirit or scope of the invention described in the appended claims.

Claims

1. An anode active material for use in a non-aqueous lithium secondary battery, comprising:

a carbonaceous material; and

a coating layer of hetero elements formed on the surface of the carbonaceous material,

wherein the hetero elements include phosphorus (P).

2. The anode active material of claim 1, wherein the hetero elements include sulfur (S).

3. The anode active material of claim 1, wherein the carbonaceous material includes at least one of artificial graphite, natural graphite, graphitized carbon fiber, graphitized mesocarbon microbeads, petroleum coke, plastic resins, carbon fiber and pyrocarbon.

4. The anode active material of claim 3, wherein the carbonaceous material has L_a(110)>10 nm and L_c(002)>10 nm L,

wherein L_a(110)=0.89λ/[B₁₁₀cos(θ₁₁₀)] and L_s(002)=0.89λ/[B₀₀₂cos(θ₀₀₂)],

wherein λ is the wavelength of Cu Kα(λ=0.15418 nm) and B is a full width at half-maximum (FWHM) value with respect to (110) or (002) peak according to Bragg diffraction angle.

5. The anode active material of claim 4, wherein the carbonaceous material has 0.344 nm or less as d₀₀₂with respect to (002) peak.

6. The anode active material of claim 3, wherein the carbonaceous material has a specific surface area of less than 10 m²/g.

7. The anode active material of claim 3, wherein the carbonaceous material has a degree of graphitization in the range of 0.4 to 1.0, and the degree of graphitization is calculated according to (degree of graphitization)=(3.44−d₀₀₂)/(0.086).

8. The anode active material of claim 1, wherein the content of the coating layer is less than 10 wt % with respect to the carbonaceous material.

9. The anode active material of claim 8, wherein the coating layer is formed uniformly on the overall surface of the carbonaceous material or formed on part of the surface of the carbonaceous material.

10. A lithium secondary battery including an anode formed of an anode active material that includes a carbonaceous material and a coating layer of hetero elements formed on the surface of the carbonaceous material, wherein the hetero elements include phosphorus (P) or sulfur (S).

11. A method for fabricating an anode active material for use in a non-aqueous lithium secondary battery, the method comprising:

preparing a carbonaceous material and a hetero element material; and

forming a coating layer of hetero elements on the surface of the carbonaceous material using the hetero element material,

wherein the hetero elements include phosphorus (P).

12. The method of claim 11, wherein the hetero elements further include sulfur (S).

13. The method of claim 12, wherein the hetero element material includes at least one of NH₄PF₆, (NH₄)₂PO₄, NH₄PO₃, (NH₄)₂SO₃, (NH₄)₂SO₄, NH₄SO₄, and (NH₄)₂S₂O₈.

14. The method of claim 13, wherein the forming of the coating layer comprises:

dissolving the hetero element material in a solvent to form a solution;

uniformly mixing the carbonaceous material with the solution to form a mixture;

vacuum-drying the mixture; and

performing heat treatment on the dried material through thermal decomposition to form the coating layer based on the hetero elements on the surface of the carbonaceous material.