WO2008126968A1

WO2008126968A1 - Anode material for secondary battery, method for preparing the same, and secondary battery containing the same for anode

Info

Publication number: WO2008126968A1
Application number: PCT/KR2007/005436
Authority: WO
Inventors: Jeong-Min Han; Jeong-Hun Oh; Jong-Sung Kim; Chul Youm; Kyung-Hee Han
Original assignee: Ls Mtron, Ltd.
Priority date: 2007-04-16
Filing date: 2007-10-31
Publication date: 2008-10-23
Also published as: KR20080093242A

Abstract

The present invention relates to an anode material for a secondary battery, a method for preparing the same and a secondary battery containing the same for an anode. The anode material for a secondary battery according to the present invention comprises an anode active material and a fluorine-based compound. The present invention stabilizes the surface of the anode active material to reduce the influence caused by a decomposition reaction of an organic electrolyte liquid that results in an irreversible capacity, and reduces the influence on an acid generated by oxidation of an electrolyte during charging/discharging, thereby improving efficie ncy and cycleability of the battery.

Description

Description ANODE MATERIAL FOR SECONDARY BATTERY,

METHOD FOR PREPARING THE SAME, AND SECONDARY BATTERY CONTAINING THE SAME FOR

ANODE Technical Field

[1] The present invention relates to an anode material for a secondary battery, a method for preparing the same and a secondary battery containing the same for an anode, and in particular, to an anode material for a secondary battery, in which a fluorine-based compound is added to an anode active material when preparing a slurry for manufacturing an electrode plate to stabilize the surface of the anode active material, thereby reducing the influence caused by a decomposition reaction of an organic electrolyte liquid that results in an irreversible capacity and reducing the influence on an acid generated by oxidation of an electrolyte during charging/discharging, consequently improving efficiency and cycleability of the battery, and to a method for preparing the same and a secondary battery comprising an anode made of 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 research and development of the lithium secondary battery has been made lively at the inside and outside of the country, and currently the lithium secondary battery is used more widely than other secondary batteries.

[3] The lithium secondary battery comprises 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, but low efficiency and cycleability. It is known that this drawback results from a decomposition reaction of an electrolyte liquid occurring at an edge portion of the natural graphite of high crystallinity.

[5] The drawback may be overcome by surface-treating (coating) the natural graphite with a low crystallinity carbon and thermally treating them at 1,000⁰C or more, so that the surface of the natural graphite is coated with a low crystallinity carbide. In this case, an anode active material can be obtained, in which an initial capacity is reduced a little, but efficiency and cycleability are improved. In particular, the low crystallinity carbon used as a coating material is thermally treated at a high temperature, and thus it makes the natural graphite an artificial graphite, so that the reduction of an initial capacity can be minimized and the decomposition reaction of an electrolyte liquid can be prevented.

[6] And, in the carbon material used as the anode material of the lithium secondary battery, during reduction (charge), lithium (Li) atoms go between carbon layers or nano-scale clusters are formed on the surface and micropore of carbon, and thus dendrite occurring in lithium-metal is not formed. Further, the carbon material used as the anode material of the lithium secondary battery has relatively excellent rechargeability, safety and high energy density, and thus can be prepared at a general air atmosphere. However, the carbon material has a relatively low energy density (3.9 Ah/g) per weight of a lithium metal, and thus has limitations in capacity.

[7] A material for a carbon electrode has various kinds of crystal structures depending on a raw material, an organic precursor or a thermodecomposition process, and lithium- intercalation may vary depending on the kind of crystal structure, thereby leading to various capacities and reversible characteristics. Generally, it is known that one kind of carbon material has excellent characteristics as an anode material, but has relatively poor characteristics in other fields. For example, in the case that an anode is manufactured using mesocarbon microbeads (MCMB) obtained from a phenol resin and a carbon treated thermally at a low temperature such as poly-p-phenylene (PPP), lithium ions are inserted into an interlayer of a graphite layer or a battery comprising the anode has a larger capacity 2.5 times than a battery using an intercalated graphite (LiC ),

6 however the carbon treated thermally at a low temperature shows a larger loss of irreversible capacity caused by charge/discharge.

[8] Generally, because an electrode material is in contact with an electrolyte liquid, an electrolyte is decomposed on the surface of a portion of an electrode, and as a result, charge/discharge characteristics of a battery may be deteriorated. Similarly, in the case of a carbon electrode, a solid-electrolyte interface film is formed by a sub-reaction with an electrolyte liquid, an initial irreversible capacity gets large, due to a large change in volume when charging/discharging, and a cycle life is reduced. These problems may be solved by changing a crystal structure of the surface of carbon through surface modification of the electrode material to minimize an irreversible capacity and prevent reduction of a cycle life, thereby improving the performance of a lithium ion battery. Recently, studies have been made lively to change a crystal structure of the surface of carbon through surface modification including surface coating and surface oxidation by thermal treatment or to adsorb gas such as hjdrogen fluoride (HF) or carbon dioxide (CO ) on the surface of carbon, thereby minimizing the irreversible capacity.

[9] Therefore, attempts have been continuously 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

[10] An object of the present invention is to provide an anode material for a secondary battery, which stabilizes the surface of an anode active material to reduce the influence of a decomposition reaction of an organic electrolyte liquid that results in an irreversible capacity, and reduces the influence on an acid generated by oxidation of an electrolyte during charging/discharging, thereby improving efficiency and cycleability of the battery, and to a method for preparing the same and a secondary battery comprising an anode made of the same. Technical Solution

[11] In order to achieve the above-mentioned object, an anode material for a secondary battery comprises an anode active material; and a fluorine-based compound.

[12] In order to achieve the above-mentioned object, a method for preparing an anode material for a secondary battery comprises preparing an anode active material and a fluorine-based compound; mixing the anode active material with the fluorine-based compound to prepare a slurry for manufacturing an electrode plate; coating the slurry for manufacturing an electrode plate on an electrode collector; and drying the slurry for manufacturing an electrode plate coated on the electrode collector.

[13] In order to achieve the above-mentioned object, a secondary battery comprises an anode made of the anode material prepared by the above-mentioned method. 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] The present invention coats a natural graphite with a low crystallinity carbon to prepare an anode active material and mixes the anode active material with a fluorine- based compound to prepare a slurry for manufacturing an electrode plate, thereby improving efficiency and cycleability of a battery.

[16] The fluorine-based compound means all compounds containing fluorine, and may be, for example, CsF, KF, LiF, NaF, RbF, TiF, AgF, AgF , BaF , CaF , CuF , CdF , FeF ,

2 2 2 2 2 2

HgF , Hg F , MnF , MgF , NiF , PbF , SnF , SrF , XeF , ZnF , AlF , BF , BiF , CeF ,

2 2 2 2 2 2 2 2 2 2 2 3 3 3 3

CrF , DyF , EuF , GaF , GdF , FeF , HoF , InF , LaF , LuF , MnF , NdF , VOF , PrF ,

3 3 3 3 3 3 3 3 3 3 3 3 3 3

SbF , ScF , SmF , TbF , TiF , TmF , YF , YbF , TiF , CeF , GeF , HfF , SiF , SnF ,

3 3 3 3 3 3 3 3 3 4 4 4 4 4

TiF , VF , ZrF , NbF , SbF , TaF , BiF , MoF , ReF , SF or WF , singularly or in

4 4 4 5 5 5 5 6 6 6 6 combination.

[17] Preferably, the fluorine-based compound is used with a content of at least 0.1 weight% based on the anode active material, more preferably 0.5 to 20 weight%. In the case that the content of the fluorine-based compound meets the above-mentioned numerical range, it is preferable because the influence on an acid generated by a sub- reaction with an electrolyte liquid can be reduced sufficiently.

[18] The anode material for a secondary battery according to the present invention, in which the above-mentioned fluorine -based compound is added as an additive, may be prepared by a typical method known in the prior art. Specifically, the anode active material and the fluorine-based compound were mixed to prepare a slurry for manufacturing an electrode plate, and the slurry for manufacturing an electrode plate was coated on an electrode collector and dried to remove a solvent or dispersion medium. At this time, active materials are stuck to the electrode collector, and the active materials are stuck together. And, a conductive agent and/or a binder may be selectively added when preparing the slurry for manufacturing an electrode plate.

[19] The anode active material may be prepared by coating a core carbon material with a low crystallinity carbon using a typical method.

[20] The core carbon material may be a natural graphite, an artificial graphite or a mixture thereof, and in particular, it is preferable to use a natural graphite.

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

[22] The fluorine-based compound is mixed with a content of at least 0.1 weight%, preferably 0.5 to 20 weight% based on the anode active material prepared by coating the core carbon material with the low crystallinity carbon, so that the slurry for manufacturing an electrode plate is prepared.

[23] At this time, the fluorine-based compound mixed with the anode active material may be pulverized to a desired size before mixing, and thus the particle size of the fluorine- based compound may vary depending on purpose of use.

[24] And, when preparing the slurry for manufacturing an electrode plate, the conductive agent or the binder may be selectively added according to necessity. A content of the conductive agent or binder may be adjusted properly to a typical range used in the prior art, and the range does not influence the present invention.

[25] 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 battery. For example, the electronically conductive material 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, the conductive agent may be carbon black, graphite powder or a carbon fiber.

[26] 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.

[27] The slurry for manufacturing an electrode plate including the anode active material, the fluorine-based compound 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.

[28] 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.

[29] And, in a secondary battery of the present invention comprising a cathode, an anode, and 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. [30] The secondary battery of the present invention may be manufactured by a typical method, i.e. interposing a porous separator between a cathode and an anode and adding an electrolyte.

[31] 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 selected from the group consisting of ethylene carbonate (EC), propylene carbonate (PC), γ-butyrolactone (CBL), diethyl carbonate (DEC), dimethyl carbonate (DMC), ethyl methyl carbonate (EMC) and methyl propyl carbonate (MPC).

[32] Preferably, the separator used to manufacture the secondary battery of the present invention is a porous separator, for an unlimited example a polypropylene-based, polyethylene-based or polyolefin-based porous separator.

[33] The secondary battery of the present invention is not limited to a specifical shape, however it may be cylindrical, angular, pouch-shaped or coin-shaped using a can. Mode for the Invention

[34] Hereinafter, a preferred embodiment of the present invention will be described in detail with reference to the accompanying drawings.

[35] Example 1

[36] A pitch dissolved in tetrahydrofuran was added to a spherical natural graphite with a weight ratio of 100:10, and wet-mixed at an atmospheric pressure for 2 hours or more and dried to produce a mixture. The mixture was sintered at 1,100⁰C and 1,500⁰C for 1 hour, respectively, and classified to remove fine powder, thereby preparing an anode active material.

[37] 100 g of the prepared anode active material and 0.5 weight% of a silicon fluorine compound (SiF , Aldrich Chemical, USA) were put into a reactor of 500 m-6. N-

4 methylpyrrolidone (NMP) and polyvinylidene fluoride (PVDF) as a binder were added with a small amount and mixed using a mixer to prepare a slurry for manufacturing an electrode plate. Next, the prepared slurry for manufacturing an electrode plate was compressionsdried on a copper foil and used as an electrode. At this time, the density of the electrode was 1.5 g/cnf and the thickness of the electrode was 70 /M.

[38] Example 2

[39] This example 2 was carried out by the same method as the example 1 except that a silicon fluorine compound (SiF ) was used with a content of 1 weight%.

4

[40] Example 3 [41] This example 3 was carried out by the same method as the example 1 except that a silicon fluorine compound (SiF ) was used with a content of 2 weight%.

4

[42] Example 4

[43] This example 4 was carried out by the same method as the example 1 except that a silicon fluorine compound (SiF ) was used with a content of 3 weight%.

4

[44] Example 5

[45] This example 5 was carried out by the same method as the example 1 except that a silicon fluorine compound (SiF ) was used with a content of 4 weight%.

4

[46] Example 6

[47] This example 6 was carried out by the same method as the example 1 except that a tin fluorine compound (SnF ) was used instead of the silicon fluorine compound (SiF ).

4 4

[48] Comparative example 1

[49] A pitch dissolved in tetrahydrofuran was added to a spherical natural graphite with a weight ratio of 100:10, and wet-mixed at an atmospheric pressure for 2 hours or more and dried to produce a mixture. The mixture was sintered at 1,100⁰C and 1,500⁰C for 1 hour, respectively, and classified to remove fine powder, thereby preparing an anode active material.

[50] 100 g of the prepared anode active material was put into a reactor of 500 m#, and N- methylpyrrolidone (NMP) and polyvinylidene fluoride (PVDF) as a binder were added with a small amount and mixed using a mixer to prepare a slurry for manufacturing an electrode plate. Next, the prepared slurry for manufacturing an electrode plate was compressionsdried on a copper foil and used as an electrode. At this time, the density of the electrode was 1.5 g/cnf and the thickness of the electrode was 70 μm.

[51] And, coin cells were manufactured using the electrodes of the above examples 1 to 6 and the comparative example 1. Next, charge and discharge characteristics were tested by the following method, and results of the test are shown in the following Table 1.

[52] First, 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, the coin cell was discharged with a discharge current of 0.5 mA/cnf until the voltage is 1.5 V. In the following Table 1, the charge/discharge efficiency is a ratio of a discharged electrical capacity to a charged electrical capacity.

[53] Table 1 [Table 1] [Table ]

[54] As shown in the above Table 1, it is found that the examples 1 to 6 each showed a little better 1st cycle discharge capacity and 1st cycleability than the comparative example 1. In particular, it is found that the examples 1 to 6 each showed remarkably higher 50th cycle capacity maintenance rate than the comparative example 1.

[55] 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

[56] The present invention adds a fluorine-based compound to an anode active material when preparing a slurry for manufacturing an electrode plate, so that the surface of the anode active material is stabilized to reduce the influence caused by a decomposition reaction of an organic electrolyte liquid that results in an irreversible capacity. And, the present invention can reduce the influence on an acid generated by oxidation of an electrolyte during charging/discharging, thereby improving efficiency and cycleability of a battery.

Claims

[1] An anode material for a secondary battery, comprising: an anode active material; and a fluorine -based compound. [2] The anode material for a secondary battery according to claim 1, wherein the fluorine-based compound is any one selected from the group consisting of CsF, KF, LiF, NaF, RbF, TiF, AgF, AgF , BaF , CaF , CuF , CdF ,

2 2 2 2 2

FeF , HgF , Hg F , MnF , MgF , NiF , PbF , SnF , SrF , XeF , ZnF , AlF , BF ,

2 2 2 2 2 2 2 2 2 2 2 2 3 3

BiF , CeF , CrF , DyF , EuF , GaF , GdF , FeF , HoF , InF , LaF , LuF , MnF ,

3 3 3 3 3 3 3 3 3 3 3 3 3

NdF , VOF , PrF , SbF , ScF , SmF , TbF , TiF , TmF , YF , YbF , TiF , CeF ,

3 3 3 3 3 3 3 3 3 3 3 3 4

GeF , HfF , SiF , SnF , TiF , VF , ZrF , NbF , SbF , TaF , BiF , MoF , ReF , SF

4 4 4 4 4 4 4 5 5 5 5 6 6 6 and WF , or mixtures thereof.

6

[3] The anode material for a secondary battery according to claim 1, wherein the fluorine-based compound is used with a content of at least 0.1 weight% based on the anode active material. [4] The anode material for a secondary battery according to claim 1, wherein the anode active material is any one selected from the group consisting of a natural graphite and an artificial graphite, or a mixture thereof. [5] The anode material for a secondary battery according to claim 1, wherein the anode active material is coated with a low crystallinity carbon. [6] The anode material for a secondary battery according to claim 5, wherein the low crystallinity carbon is any one selected from the group consisting of pitch, tar, a phenol resin, a furan resin and a furfurly alcohol, or mixtures thereof. [7] A method for preparing an anode material for a secondary battery, comprising: preparing an anode active material and a fluorine-based compound; mixing the anode active material with the fluorine-based compound to prepare a slurry for manufacturing an electrode plate; coating the slurry on an electrode collector; and drying the slurry coated on the electrode collector. [8] The method for preparing an anode material for a secondary battery according to claim 7, further comprising: pulverizing the fluorine -based compound before mixing the anode active material with the fluorine-based compound. [9] The method for preparing an anode material for a secondary battery according to claim 7, wherein the fluorine-based compound is used with a content of at least 0.1 weight% based on the anode active material. [10] The method for preparing an anode material for a secondary battery according to claim 7, wherein the electrode collector is any one selected from the group consisting of copper, gold, nickel and a copper alloy, or mixtures thereof. [11] A secondary battery, comprising an anode made of the anode material prepared by the method defined in any one of claims 7 to 10.