WO2008146995A1

WO2008146995A1 - Anode material for secondary battery and secondary battery using the same

Info

Publication number: WO2008146995A1
Application number: PCT/KR2007/005450
Authority: WO
Inventors: Jong-Sung Kim; Jeong-Hun Oh; Chul Youm; Kyung-Hee Han; Jeong-Min Han
Original assignee: Ls Mtron, Ltd.
Priority date: 2007-05-29
Filing date: 2007-10-31
Publication date: 2008-12-04
Also published as: KR20080104833A; JP2009533835A; KR100884431B1

Abstract

The present invention relates to an anode material for a secondary battery and a secondary battery using the same. The anode material for a secondary battery according to the present invention is prepared by coating a core carbon material with a low crystallinity carbon material and sintering the materials, wherein the anode material has a specific surface area ratio of 1.4 or less. The present invention can improve a protection function against a decomposition reaction of an electrolyte liquid on its surface to improve efficiency and cycleability of the secondary battery.

Description

ANODE MATERIAL FOR SECONDARY BATTERY AND SECONDARY BATTERY USING THE SAME

Technical Field

[1] The present invention relates to an anode material for a secondary battery and a secondary battery using the same, and in particular, to an anode material for a secondary battery, of which a specific surface area ratio is adjusted to 1.4 or less to improve a protection function against a decomposition reaction of an electrolyte liquid on its surface, thereby improving efficiency and cycleability of the secondary battery, and a secondary battery using the same. Background Art

[2] As various portable electronic equipments such as video cameras, wireless phones, mobile phones or notebook computers spread into daily life rapidly, the demand for a secondary battery as a power source increased considerably. Among the secondary battery, a lithium secondary battery has excellent battery characteristics such as large capacity and high energy density, and thus currently is used the most widely.

[3] The lithium secondary battery includes basically a cathode, an anode and an electrolyte, and accordingly research and development of the lithium secondary battery includes largely studies about a cathode material, an anode material and an electrolyte.

[4] A natural graphite used as an anode material of the lithium secondary battery has an excellent initial capacity, however it has low efficiency and cycleability. It is known that this phenomenon is caused by a decomposition reaction of an electrolyte liquid occurring at an edge portion of the natural graphite of high crystallinity.

[5] To solve the problem, a method was suggested to surface-treat (coat) the natural graphite with a low crystallinity carbon material and thermally treat them at 1,000 ⁰C or more, so that the surface of the natural graphite is coated with the low crystallinity carbide. Through this method, an anode active material can be obtained, in which an initial capacity of a battery reduces a little, but efficiency and cycleability of the battery are improved.

[6] For example, Japanese Patent Application No. 2002-084836 discloses characteristics of a graphite, in which an edge portion of a crystal of a core carbon material is partially or wholly coated with a carbon material for coating.

[7] However, the coating carbide is broken when a resultant anode active material is coated on an electrode collector such as a copper foil and compressed for use as an electrode. And, an edge portion of a natural graphite of high crystallinity is reacted with an electrolyte liquid through the broken portion of the carbide, and in practice, a carbide coating effect is reduced.

[8] The above-mentioned Japanese patent application teaches an amount of the carbon material for coating the natural graphite, temperature of thermal treatment, and X-ray diffraction analysis and Raman analysis of the natural graphite coated with the carbon material for coating. However, the above-mentioned Japanese patent application does not disclose the influence that the coating carbide is broken during a compression process when the anode active material is applied to an electrode in practice.

[9] And, in the case that a graphite is used as an active material of a lithium secondary battery, the graphite is broken due to a change in volume while charge and discharge are performed repetitively, however the above-mentioned Japanese patent application does not mention any influence caused by such a phenomenon.

[10] Therefore, attempts have been made in the related industry to solve the above- mentioned conventional problems, and the present invention was devised under this technical background. Disclosure of Invention Technical Problem

[11] An object of the present invention is to provide an anode material for a secondary battery, which can prevent a decomposition reaction of an electrolyte liquid at an edge portion of a natural graphite, caused by breakage of a coating carbide during a compression process in a practical application to an electrode, and a secondary battery using the same. Technical Solution

[12] In order to achieve the above-mentioned object, an anode material for a secondary battery is prepared by coating a core carbon material with a low crystallinity carbon material and sintering the materials, wherein the anode material has a specific surface area ratio of 1.4 or less.

[13] In order to achieve the above-mentioned object, a secondary battery comprises an anode made of the above-mentioned anode material. Best Mode for Carrying Out the Invention

[14] Hereinafter, preferred embodiments of the present invention will be described in detail with reference to the accompanying drawings. Prior to the description, it should be understood that the terms used in the specification and the appended claims should not be construed as limited to general and dictionary meanings, but interpreted based on the meanings and concepts corresponding to technical aspects of the present invention on the basis of the principle that the inventor is allowed to define terms appropriately for the best explanation. Therefore, the description proposed herein is just a preferable example for the purpose of illustrations only, not intended to limit the scope of the invention, so it should be understood that other equivalents and modifications could be made thereto without departing from the spirit and scope of the invention.

[15] In an anode material for a secondary battery according to the present invention, an extent that a coating of the anode material used practically to the secondary battery is maintained to an electrode state varies depending on a specific surface area ratio obtained by measuring specific surface areas before and after compression when manufacturing an electrode using a coated natural graphite, and charge/discharge characteristics of the battery is influenced accordingly.

[16] The specific surface area ratio of the anode material is obtained by measuring specific surface areas before and after compression of a core carbon material coated with a low crystallinity carbon material and applying the measured specific surface areas to the following Math Figure 1.

[17] MathFigure 1

[Math.l]

SPECIFIC SURFACE AREA AFTER COMPRESSION (Sa)

AREA RATIO (SP)

SPECIFIC SURFACE AREA BEFORE COMPRESSION (Sf)

[18] Preferably, the specific surface area ratio of the anode material for the secondary battery as obtained above is 1.4 or less.

[19] In the case that the specific surface area ratio of the anode material is 1.4 or less according to the present invention, an initial efficiency is 94 % or more, and 25th cycle discharge capacity (retention capacity) is maintained at 95 % or more. However, in the case that the specific surface area ratio is more than 1.4, it is not preferable because an initial efficiency is less than 94 %, and 25th cycle discharge capacity is less than 95 %, thereby resulting in poor cycleability (capability to provide repetitive charge and discharge cycles).

[20] And, the anode material of the secondary battery according to the present invention may be prepared by coating a core carbon material with a low crystallinity carbon material and sintering the materials using a typical method used in the prior art. [21] The core carbon material may be a natural graphite, an artificial graphite or a mixture thereof, and it is preferable to use a natural graphite.

[22] The low crystallinity carbon material may be pitch, tar, a phenol resin a furan resin or furfuryl alcohol.

[23] That is, the present invention adjusts the specific surface area ratio of the anode material prepared by partially or wholly coating an edge portion of the core carbon material with the low crystallinity carbon material to 1.4 or less. The specific surface area ratio may be adjusted by selecting the kind and process conditions of the low crystallinity carbon material.

[24] A conductive agent or binder may be selectively added with a small amount to a slurry for manufacturing an electrode plate including the above-mentioned anode material according to necessity.

[25] The content of the conductive agent or binder may be adjusted properly to a typical level used in the prior art, and the content range does not influence the present inv ention.

[26] The conductive agent is not limited to a specific material if it is an electronically conductive material that does not bring about a chemical change in the resultant battery. For example, the conductive agent may be carbon black such as acetylene black, Ketjen black, furnace black or thermal black; a natural graphite; an artificial graphite; or a conductive carbon fiber, and in particular, it is preferable to use carbon black, graphite powder or carbon fiber.

[27] The binder may be a thermoplastic resin, a thermosetting resin or a mixture thereof.

In particular, preferably the binder may be polyvinylidene fluoride (PVDF) or polyte- trafluoroethylene (PTFE), more preferably polyvinylidene fluoride.

[28] The slurry for manufacturing an electrode plate including the anode active material and selectively at least one of the conductive agent and the binder is coated on an electrode collector, and dried to remove a solvent or dispersion medium, so that active materials are stuck to the electrode collector and the active materials are stuck together.

[29] The electrode collector is not limited to a specific material if it is made of a conductive material, however in particular, it is preferable to use a foil made of copper, gold, nickel, a copper alloy or combination thereof.

[30] And, in a secondary battery of the present invention comprising a cathode, an anode, a separator interposed between the cathode and the anode, and an electrolyte, the anode is made of the anode material prepared by the above-mentioned preparing method. [31] The secondary battery of the present invention may be manufactured by a typical method used in the prior art, for example interposing a porous separator between a cathode and an anode, and a±frng an electrolyte. [32] The electrolyte is a non-aqueous electrolyte liquid including a lithium salt and an electrolyte liquid compound, and the lithium salt may be at least one compound selected from the group consisting of LiClO , LiCF SO , LiPF , LiBF , LiAsF and

4 3 3 6 4 6

LiN(CF SO ) . And, the electrolyte liquid compound may be at least one compound

3 2 2 selected from the group consisting of ethylene carbonate (EC), propylene carbonate

(PC), γ-butyrolactone (GBL), diethyl carbonate (DEC), dimethyl carbonate (DMC), ethyl methyl carbonate (EMC) and methyl propyl carbonate (MPC). [33] Preferably, the separator of the present invention is a porous separator, for example polypropylene-based, polyethylene-based or polyolefin-based porous separator. [34] The secondary battery of the present invention is not limited to a specific shape, however may be manufactured in various shapes without limitation, for example cylindrical, angular, pouch-shaped or coin-shaped using a can. [35] The secondary battery of the present invention has a charge/discharge efficiency of

94 % or more and 25th cycle discharge capacity of 95 % or more.

Mode for the Invention

[36] Hereinafter, for understanding of the present invention, the present invention will be described in detail through examples and comparative examples.

[37]

[38] Example 1

[39] A carbon material of spherical natural graphite and pitch were prepared.

[40] First, the pitch dissolved in tetrahydrofuran was added to the spherical natural graphite with a content of 3 weight% based on weight of the natural graphite, wet- mixed at an atmospheric pressure for 2 hours or more, and dried to produce a mixture. The mixture was sintered at 1,100⁰C for 1 hour with temperature being increased with a temperature increase rate of 1 °C/min, and classified to remove fine powder, thereby preparing an anode active material.

[41] 100 g of the prepared anode active material was put into a reactor of 500 mH, and N- methylpyrrolidone (NMP) and polyvinylidene fluoride (PVD) as a binder were added with a small amount. Next, the reactant was mixed with a mixer to prepare an anode material having a specific surface area ratio of 1.21.

[42] [43] Example 2

[44] An anode material having a specific surface area ratio of 1.38 was prepared in the same method as that of the Example 1, except that pitch was 5 weight% based on weight of a natural graphite and a temperature increase rate is 3 °C/min.

[45]

[46] Comparative example 1

[47] An anode material having a specific surface area ratio of 1.45 was prepared in the same method as that of the Example 1, except that pitch was 10 weight% based on weight of a natural graphite and a temperature increase rate is 3 °C/min.

[48]

[49] Comparative example 2

[50] An anode material having a specific surface area ratio of 1.53 was prepared in the same method as that of the Example 1, except that pitch was 10 weight% based on weight of a natural graphite and a temperature increase rate is 5 °C/min.

[51]

[52] Comparative example 3

[53] An anode material having a specific surface area ratio of 1.61 was prepared in the same method as that of the Example 1, except that pitch was 10 weight% based on weight of a natural graphite and a temperature increase rate is 10 °C/min.

[54]

[55] Comparative example 4

[56] An anode material having a specific surface area ratio of 1.67 was prepared in the same method as that of the Example 1, except that pitch was 10 weight% based on weight of a natural graphite and a temperature increase rate is 15 °C/min.

[57]

[58] A test was made on a specific surface area ratio and charge/discharge characteristics using the anode materials for secondary batteries prepared in the examples 1 and 2 and the comparative examples 1 to 4 as follows, and test results are shown in the following Table 1.

[59] First, measurement of a specific surface area before compression - specific surface areas of the anode materials for secondary batteries of the examples 1 and 2 and the comparative examples 1 to 4 were measured using a specific surface area analyzer, TriStar 3000 Mxiel (Mcromeritics).

[60] Second, compression - 2 g of each anode material for secondary batteries of the examples 1 and 2 and the comparative examples 1 to 4 was put into a hole of Φ 1.4 cm, and pressure of 2t was applied to an area of Φ 1.4 cm for 2 seconds using a press machine (i.e. a pressure f 1.3 t/cnf was applied for 2 seconds).

[61] Third, measurement of a specific surface area after compression - specific surface areas of the anode materials for secondary batteries of the examples 1 and 2 and the comparative examples 1 to 4, compressed in the compression step were measured using a specific surface area analyzer, TriStar 3000 Mxiel (Mcromeritics).

[62] Fourth, calculation of a specific surface area ratio - the measured specific surface areas before and after compression were applied to the above Equation 1 to obtain a specific surface area ratio.

[63] Fifth, manufacture of an electrode - each anode material for secondary batteries of the examples 1 and 2 and the comparative examples 1 to 4 was coated on a copper foil, compressed and dried to manufacture an electrode. At this time, all of the electrodes have a density after compression of 1.65 g/cnf uniformly.

[64] Sixth, charge/discharge characteristics - a coin cell was manufactured to evaluate a charge/discharge efficiency of the electrode manufactured in the previous step. The charge/discharge test was performed such that the coin cell was charged with a charge current of 0.5 mA/cnf until a voltage is 0.01 V while an electrical potential was limited to the range of 0 to 1.5 V, and was continuously charged until the charge current is 0.02 mA/cnf while maintaining the voltage at 0.01 V. And, discharge was performed with a discharge current of 0.5 mA/cnf until the voltage is 1.5 V.

[65] Table 1

[Table 1] [Table ]

[66] As shown in the above Table 1, it is found that while the anode materials of the comparative examples 1 to 4 had specific surface area ratios of more than 1.4, the anode materials of the examples 1 and 2 had specific surface area ratios adjusted to 1.4 or less according to the present invention, and thus showed initial efficiencies of 94.7 % and 94.0 %, respectively, i.e. 94 % or more. And, it is found that the anode materials of the examples 1 and 2 showed 25th cycle discharge capacity (retention capacity) of 98.3 % and 97.5 %, respectively, i.e. 95 % or more.

[67] And, it is found through the above Table 1 that a specific surface area ratio is not related to an initial efficiency, but as the specific surface area ratio is larger, efficiency and cycleability are reduced. Such results tell a large specific surface area after compression means that the surface of the natural graphite coated with the carbon layer is broken, exposed to an electrolyte liquid and decomposed in the electrolyte liquid during a compression process.

[68] As such, the preferred embodiments of the present invention are described in detail with reference to the accompanying drawings. However, it should be understood that the detailed description and specific examples, while indicating preferred embodiments of the invention, are given by way of illustration only, since various changes and modifications within the spirit and scope of the invention will become apparent to those skilled in the art from this detailed description. Industrial Applicability

[69] The present invention improves a protection function against a decomposition reaction of an electrolyte liquid on the surface of an anode material to improve efficiency and cycleability of a secondary battery so as to solve a problem that a coating carbide is broken during a compression process in a practical application to an electrode.

Claims

[1] An anode material for a secondary battery, prepared by coating a core carbon material with a low crystallinity carbon material and sintering the materials, wherein the anode material has a specific surface area ratio of 1.4 or less.

[2] The anode material for a secondary battery according to claim 1, wherein the core carbon material is any one selected from the group consisting of a natural graphite and an artificial graphite, or a mixture thereof.

[3] The anode material for a secondary battery according to claim 1, wherein the low crystallinity carbon material is any one selected from the group consisting of pitch, tar, a phenol resin, a furan resin and furfuryl alcohol, or mixtures thereof.

[4] A secondary battery, comprising an anode made of the anode material defined in any one of claims 1 to 3.

[5] The secondary battery according to claim 4, wherein the secondary battery has a charge/discharge efficiency of 94 % or more, and 25th cycle discharge capacity of 95 % or more.