WO2009020357A1

WO2009020357A1 - Anode active material for lithium secondary cell

Info

Publication number: WO2009020357A1
Application number: PCT/KR2008/004589
Authority: WO
Inventors: Byung Won Cho; Joong Kee Lee; Kyeong Jik Lee; Hyun Ki Choi
Original assignee: Sodiff Advanced Materials Co., Ltd.; Korea Institute Of Science And Technology
Priority date: 2007-08-07
Filing date: 2008-08-07
Publication date: 2009-02-12
Also published as: KR100817009B1

Abstract

An anode active material for a lithium secondary cell comprising a carbon material which is surface-treated with He, N2, CO2, O2, F2, NF3 or a mixture thereof has a good wetting property for both a water-based binder and a non-aqueous electrolyte, and makes a thin and stable SEI to be formed on its surface, thereby improves the energy density, the reversible capacity and the durability of a lithium secondary cell.

Description

ANODE ACTIVE MATERIAL FOR LITHIUM SECONDARY CELL

FIELD OF THE INVENTION

The present invention relates to an anode active material for a high energy- density lithium secondary cell.

BACKGROUND OF THE INVENTION

A lithium secondary cell generally comprises an anode, a cathode, an electrolyte and a lithium ion exchange membrane disposed between the electrodes. The anode can be manufactured by coating an anode slurry composition comprising an anode active material, e.g., graphite, an electric conductor, e.g., a carbon black, a binder, e.g., a styrene butadiene rubber (SBR), a thickener, e.g., carboxymethylcellulose (CMC), and water, on the surface of a collector, followed by drying and pressing.

As the anode active material, crystalline as well as amorphous carbon have been used, but an anode active material consisting of a single material exhibits a poor packing density and electric capacity. Accordingly, techniques of mixing two or more anode active material having different particle sizes with various coating or surface- treatment have been developed. Surface-modified natural graphite, in particular, has been generally used for manufacturing an anode active material with high capacity, the use of which, however, requires a complicated heating procedure and raises the manufacturing cost. Moreover, in case of using natural graphite alone, the oxygen content at the surface is high, about from 6 to 7 parts by weight based on 100 parts by weight of the graphite, which reacts with the electrolyte during the charge/discharge cycle of the cell to generate an inhomogeneous SEI (Solid Electrolyte Interface), leading to a gradual loss of the capacity. While the high oxygen content enhances the wetting property in preparing an anode slurry composition, it lowers the affinity of the composition toward the electrolyte in the cell.

For improving the cell capacity, Japanese Patent Publication No. H 07-312218 discloses a method for treating a carbon anode active material with fluorine gas. However, such surface-treatment leaves a significant amount of fluorine attached to the surface of the carbon anode active material, which volatilizes during the charging stage and causes the expansion of the carbon lattice. This phenomenon can lower the capacity of the cell, and also, cause a serious safety problem, due to the reduction of the amount of electrolyte.

To solve the above problems, Japanese Patent Publication No. H 09-245793 discloses a method comprising an additional step of heating a fluorinated carbon material with steam to remove the remaining fluorine. However, the anode active material obtained by this method forms a plurality of hydroxyl groups, and its increased specific surface area brings about an irreversible capacity loss of the cell, low durability, and increased manufacturing cost. Korean Patent Publication No. 2005-41474 discloses a method for improving the wetting property of the anode active material, comprising coating or mixing the anode active material with a hydrophilic material. The method can improve the cell performance and shorten the time of the cell production, but it requires an additional step for coating the surface of the electrode material with a hydrophilic material, and while the method can manufacture a slurry composition easily, the bad wetting property thereof toward the electrolyte and the bad property of SEI formed on the anode active material could result in the poor durability of the cell.

Thus, a method for manufacturing a carbon material having good wetting properties toward both the aqueous and electrolytic media has not been disclosed. The conventional method for coating or surface-treating with fluorine has an inherent difficulty due to its liphophile property. Accordingly, there has been a need to develop an anode active material having good wetting properties toward both the aqueous and electrolyte media. SUMMARY OF THE INVENTION

Accordingly, it is an object of the present invention to provide an anode active material for a lithium secondary cell which has good wetting properties toward both the aqueous solution of binder and the non-aqueous electrolyte and improves the cell performance characteristics.

In accordance with an aspect of the present invention, there is provided an anode active material for a lithium secondary cell, comprising a treated carbon material obtained by heat-treating followed by surface-treating a carbon material simultaneously or sequentially with one or more agents selected from the group consisting of He, N₂, CO₂, O₂, F₂ and NF₃, wherein the oxygen content on the surface of the treated carbon material is 5 parts or less by weight based on 100 parts by weight of the carbon material. Preferably, the treated carbon material is a crystalline carbon material having an interplanar distance d(002) of 3.360 A or less and a crystallite size Lc of 500 A or more.

In the present invention, the heat-treatment of the carbon material may be conducted by heating at a temperature of 300 ^°C to 1000⁰C and the surface-treatment may be conducted by treating with the specific agent at a temperature of 100°C to 800⁰C .

The anode active material of the present invention has an electrode density ranging from 1.6 to 2.2, preferably from 1.7 to 2.0 g/cm³, and a tap density ranging from 0.5 to 1.3 g/cm³.

The anode active material of the present invention maximizes the packing density of the anode and prevents the deformation of the anode during intercalation/deintercalation of the lithium ion. Moreover, the present invention improves the wetting properties toward both the water-based binder and the non- aqueous electrolyte, to make it possible to form a thin and stable SEI on the surface of the anode active material, which improves the energy density, the reversible capacity, efficiency and the durability of the cell.

BRIEF DESCRIPTION OF THE DRAWINGS

The above and other objects and features of the present invention will become apparent from the following description of the invention, when taken in conjunction with the accompanying drawings, which respectively show: Fig. 1 shows an example of the process for manufacturing the anode active material of the present invention.

Fig. 2 represents a graph showing the durability of the carbon materials which are untreated or treated, wherein each curves are the results obtained in Example 1 to 3 and Comparative Example 1 to 5, respectively.

DETAILED DESCRIPTION OF THE INVENTION

The anode active material for a lithium secondary cell of the present invention comprises a carbon material which is prepared by heat-treating and surface-treating a starting carbon material.

The heat-treatment is conducted as a pretreatment by placing the starting carbon material powder in a reaction vessel and heating for 30 minutes in the air at a temperature of 300 to 1000°C, preferably 500 to 800 ^°C . By the heat-treatment process, the moisture content of the carbon material decreases and oxidation occurs on the surface of the carbon material, leading to the formation of a plurality of micropores on the surface.

After the heat-treatment, the surface-treatment is conducted by bring the carbon material into contact with the surface-treatment agent in an amount corresponding to a weight% of 0.01 to 10 based on the weight of the carbon material, at a rate of 0.1 to lOL/s, preferably 10L/s, for 1 to 60 minutes at a temperature of 100 to 800 ^°C , preferably 300 to 600 "C , and then, leaving the treated carbon material alone under that temperature for 1 to 60 minutes to allow the surface-treatment agent to diffuse inside of the carbon material through the micropores. For the reaction between the carbon material and the surface-treatment agent to take place evenly a stirring of the reactant in the reaction vessel is preferably conducted. Finally, the treated carbon is cooled in the ambient atmosphere to stabilize the surface of the powder.

The surface-treatment agent may be one or more agents, simultaneously or separatively used, selected from the group consisting of He, N₂, CO₂, O₂, F₂ and NF₃, preferably, O₂, F₂ or NF₃. Particularly, a fluorine-containing surface-treatment agent such as NF₃ can be used together with other surface-treatment agent such as He, N₂, which can tame the initial rapid reaction between the carbon material and the surface- treatment agent to makes the reaction proceed uniforming over the carbon. It also lowers the residual amount of fluorine on the surface of the substrate, thereby maintaining the hydrophilic nature of the carbon material.

An anode active material manufactured by a conventional method has some residual fluorine in the form of C-F bonds on the surface due to the spontaneous reaction between fluorine and the carbon material at room temperature. In case of using a crystalline carbon material, fluorine reacts too rapidly with the surface of the carbon material powder so that the inside remains unreacted, which increases the irreversible capacity of the cell and destroys the surface of the carbon material during charge/discharge cycle of the cell, resulting in poor durability.

In particular, when natural graphite is used without the heat-treatment, a thick and inhomogeneous SEI is generated on the surface due to the severe reactions with the electrolyte. Accordingly, the charge/discharge rate of the carbon material powder and reactivity on the surface-treatment become low. Moreover, the destruction/regeneration of SEI repeated during recharge/discharge lowers the durability of the cell. And if the surface-treatment is not conducted, the repeated intercalation/deintercalation of the lithium ion causes an undesirable lattice expansion of the carbon leading to an irreversible reaction on the interface between the powder and an electrolyte.

However, when the method of the present invention comprising the heat- treatment and the surface-treatment is used, the intercalation or deintercalation of the lithium ion takes place easily, and only a thin and homogeneous SEI is generated on the surface, leading to improve the durability of the cell.

In particular, the anode active carbon material of the present invention has a good electrode density and electric conductivity due to its high packing density obtained by rolling the electrode, and, it does not undergo significant lattice expansion during intercalation/deintercalation of a lithium ion, causing no irreversible reaction.

Moreover, the surface-treatment improves the wetting property thereof through the attainment of hydrophilic property of its surface, which enhances the efficiency of the process. Further, the surface-treatment generates a plurality of micropores on the surface of the carbon material which serve as pathways of the electrolyte, resulting in significant improvement of its wetting property toward the electrolyte.

The surface-treated carbon material of the present invention preferably has an interplanar distance d(002) of 3.360 A or less, a crystallite's size Lc of 500 A or more, and a tap density of 0.5 to 1.3 g/cm³.

The starting carbon material can be a crystalline carbon (such as a natural carbon and a synthetic carbon) or an amorphous carbon doped with a metal (such as Si, Sn, Co, etc.), an alloy (such as Si-alloy, Sn-alloy, etc.), a metal oxide (such as LiCoO₂, LiNiO₂, LiFePO₄, etc.), a fluorine compound (such as LiF, LiCoOF, etc.) or a mixture thereof. Preferably, the carbon material powder has an average particle size ranging from 0.01 to 4Qm. The anode active material of the present invention can further comprise conductive nanoparticles such as carbon black nanoparticles, a graphite powder, and others, in an amount of 0.1 to 10 wt% based on the weight of the anode active material.

The anode active material of the present invention also can be mixed with a conventional conductive nanoparticle, binder, thickener or solvent to manufacture a lithium secondary cell in a conventional way. For example, an anode plate for a lithium secondary cell can be manufactured by a process comprising the steps of: coating the surface of the copper collector with the anode active material composition of the present invention; drying the coat with a hot air flow at a temperature of 80 to 150^°C ; pressing dried coat through a rolling mill; and vacuum-drying the pressed coat for 8 to 12 hours at a temperature of 80 to 150⁰C . The thickness of the anode plate is preferably in the range of 30 to 100 βn. Moreover, a lithium secondary cell can be manufactured by a process comprising the steps of: making a jelly roll by winding an electrode laminate consisting of a cathode, an ion exchange membrane and the anode of the present invention; placing the resulting roll in a cell; partially sealing the cell; filling the cell with an electrolyte composition; and optionally heating the assembly.

The following examples are intended to illustrate the present invention, however these examples are not to be construed to limit the scope of the invention.

EXAMPLES

Example 1

An anode plate was prepared using the procedure comprising the steps of: (a) preparation of a crystalline carbon material - as a crystalline carbon material, a natural graphite powder having an average particle size of 20μm was prepared;

(b) heat-treatment - the natural graphite powder was heated in the air at 700 ^°C for 30 minutes; (c) surface-treatment - the natural graphite powder was treated with F₂ gas flow of O.lL/s at 450 ^°C for 30 minutes;

(d) preparation of an anode active material - a carbon black powder having an average particle size of lOOnm was added as conductive nanoparticles to the surface- treated natural graphite powder in a weight ratio of 2 : 100 to prepare an anode active material;

(e) preparation of an anode active material slurry composition - the anode active material was combined with a styrene butadiene rubber (SBR) binder and a carboxyl methyl cellulose (CMC) thickener in a weight ratio of 100 : 2 : 2. To the resulting mixture, water was added, and stirred thoroughly to prepare an anode active material slurry composition; and

(f) preparation of an anode plate - the surface of a Cu collector was coated with the anode active material slurry composition using a doctor blade, the coat was dried under a hot air flow at 100^°C , roll-pressed under 30kgf/cm², and dried in a vacuum oven for 12 hours, to obtain an anode plate having a thickness of 60 μm.

Comparative Example 1

An anode plate was prepared using the same procedure of Example 1, except that step (b) was not conducted and step (c) was conducted at 300 ^°C .

Comparative Example 2

An anode plate was prepared using the same procedure of Example 1, except that step (c) was not conducted.

Example 2

An anode plate was prepared using the same procedure of Example 1, except that the natural graphite powder was treated with NF₃ gas flow of 0.1 L/s at 450 ^°C for 30 minutes in step (c) and a graphite powder having an average particle size of 4μm was added as conductive nanoparticles to the surface-treated natural graphite powder in a weight ratio of 5 : 100 in step (d).

Comparative Example 3

An anode plate was prepared using the same procedure of Example 2, except that step (b) was not conducted and step (c) was conducted at 450 ^°C . Comparative Example 4

An anode plate was prepared using the same procedure of Example 2, except that step (b) was conducted at 500 °C and step (c) was not conducted.

Example 3

An anode plate was prepared using the same procedure of Example 1, except that the natural graphite powder was treated with NF₃/N₂(100:95, v/v) mixed gas flow of O.lL/s at 550 ^°C for 60 minutes in step (c) and a graphite powder having an average particle size of 4μm was added as conductive nanoparticles to the surface-treated graphite powder in a weight ratio of 5 : 100 in step (d).

Comparative Example 5

An anode plate was prepared using the same procedure of Example 4, except that step (b) was not conducted.

Using each of the anode plates of Examples 1 to 3 and Comparative Examples 1 to 5, a coin-type half cell was prepared by using a conventional method, and the half cell obtained after rolling and vacuum-drying for 12 hours was subjected to measurement to determine its electrode density, reversible capacity, initial efficiency and durability. The results are shown in Table 1 and Fig. 2. The "reversible capacity" was evaluated by measuring the discharging capacity when the cell was charged and discharged to 0.2C, which was shown as the cell capacity per weight of the anode active material (mAh/g). The "initial efficiency" was evaluated by measuring the ratio of the discharging capacity to the recharging capacity observed during the active charge/discharge cycle. The "durability" was evaluated by measuring the reversible capacity when the cell was subjected to 200 cycles of recharge and discharge to l.OC, which was shown in Fig. 2. Table 1

It could be seen from the result of Table 1 that, in comparison with the anode plates prepared in Comparative Examples, trie anode plates prepared in Examples of the present invention each has a lower content of oxygen on the surface, and, if given that the densities of both anode plates are similar, the anode plate of the present invention has a better reversible capacity and a higher efficiency. Moreover, it could be seen from, the result of Fig. 2 that the anode plates prepared in Examples have a better durability.

Accordingly, the cell comprising the anode plate of the present invention has a better charge/discharge property in comparison with those of Comparative Examples.

While the invention has been described with respect to the above specific embodiments, it should be recognized that various modifications and changes may be made to the invention by those skilled in the art which also fall within the scope of the invention as defined by the appended claims.

Claims

WHAT IS CLAIMED IS:

1. An anode active material for a lithium secondary cell, comprising a treated carbon material obtained by heat-treating followed by surface-treating a carbon material simultaneously or sequentially with one or more agents selected from the group consisting of He, N₂, CO₂, O₂, F₂ and NF₃, wherein the oxygen content on the surface of the treated carbon material is 5 parts or less by weight based on 100 parts by weight of the carbon material.

2. The anode active material of claim 1, wherein the treated carbon material is a crystalline carbon material having an interplanar distance d(002) of 3.360 A or less and a crystallite size Lc of 500 A or more.

3. The anode active material of claim 1, wherein the heat-treatment of the carbon material is conducted at a temperature of 300 ^°C to 1000 °C .

4. The anode active material of claim 1, wherein the surface-treatment of the carbon material is conducted at a temperature of 100^°C to 800 ^°C .

5. The anode active material of claim 1, which has an electrode density ranging from 1.7 to 2.0 g/cm³.

6. The anode active material of claim 1, which has a tap density ranging from 0.5 to 1.3 g/cm³.