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An Anode Material And Method Of Preparation Thereof
Technical Field
The present invention relates to an anode material for
a lithium secondary cell and a lithium secondary cell using
the same.
Background Art
Currently, carbonaceous materials are used as anode
materials for lithium secondary cells. However, it is
necessary to use an anode material with a higher capacity in
order to further improve the capacity of a lithium secondary
cell.
For the purpose of satisfying such demands, metals
capable of forming alloys electrochemically with lithium, for
example Si, Al, etc., which have a higher charge/discharge
capacity, may be considered for use as anode materials.
However, . such metal-based anode materials undergo extreme
changes in volume, as lithium intercalation/deintercalation
progresses, and thus the active materials are finely divided
and the lithium cells have poor cycle life characteristics.
Japanese Patent Application Laid-Open No. 2001-297757
discloses an anode material essentially comprising an a-phase
(e.g. Si) consisting of at least' one element capable of
lithium intercalation/ deintercalation and a ß-phase that is
an intermetallic compound or solid solution of the element
with another element (b).
However, the anode material according to the prior art
cannot provide sufficient and acceptable cycle life
characteristics, and thus it may not be used as a practical
anode material for a lithium secondary cell.
Brief Description of the Accompanying Drawings
FIG. 1 is a sectional view of an anode material
according to a preferred embodiment of the present invention.
FIG. 2 is a graph showing the cycle life
characteristics of the cells obtained from Example 1 and
Comparative Example 1.
FIG. 3 is a graph showing the cycle life
characteristics of the cells obtained from Example 2 and
Comparative Example 2.
FIG. 4 is an SEM (scanning electron microscope) photo
showing the particle surface of the anode material obtained
from Example 2, before charge/discharge (A) and after three
cycles of charge/discharge (B).
FIG. 5 is an SEM photo showing the particle surface of
the anode material obtained from Comparative Example 2,
before charge/discharge (A) and after three cycles of
charge/discharge (B).
FIG. 6 is a TEM (transmission electron microscope)
photo of the anode material obtained from Example 1.
FIG. 7 is a graph showing the cycle life
characteristics of the cells obtained from Example 1 and
Comparative Examples 3 and 4.
Disclosure of the Invention
Therefore, the present invention has been made in view
of the above-mentioned problems, and it is an object of the
present invention to provide an anode material for a lithium
secondary cell having a high charge/discharge capacity and
excellent cycle life characteristics.
It is another object of the present invention to
provide an anode material for a lithium secondary cell, the
anode material comprising a metal layer (core layer) capable
of repetitive lithium intercalation/ deintercalation, the
surface of which is partially or totally coated with
amorphous carbonaceous materials and crystalline carbonaceous
materials, successively. By using the aforesaid anode
material, it is possible to inhibit changes in the volume of
a metal caused by the progress of lithium
intercalation/deintercalation and to maintain a high electron
conductivity among anode material particles, thereby
providing a high charge/discharge capacity and excellent
cycle life characteristics.
It is still another object of the present invention to
provide a lithium secondary cell using the aforementioned
anode material.
According to an aspect of the present invention, there
is provided an anode material comprising: a metal core layer
capable of repetitive lithium, intercalation/deintercalation;
an amorphous carbon layer coated on the surface of the metal
core layer; and a crystalline carbon layer coated on the
amorphous carbon layer. According to another aspect of the
present invention, there is provided a lithium secondary cell
using the above-described anode material.
According to the present invention, the metal core
layer can provide a high charge/discharge capacity.
Additionally, the amorphous carbon layer and the
crystalline carbon layer can inhibit changes in the volume of
a metal caused by the progress of lithium
intercalation/deintercalation, thereby improving the cycle
life characteristics.
Even if a metal layer, for example a metal layer formed
of Si, has electron conductivity and lithium ion conductivity
to permit lithium intercalation/ deintercalation, the
electron conductivity, in this case, is too low to allow
smooth progress of lithium intercalation/ deintercalation.
Therefore, the lithium intercalation/deintercalation property
can be improved by forming a crystalline carbon layer so as
to reduce contact resistance between an active material layer
and a current collector, and contact resistance among active
material particles.
The coating layers including the amorphous carbon layer
and the crystalline carbon layer may partially or totally
cover the surface of the metal core layer.
Meanwhile, the anode material preferably comprises the
metal core layer, the amorphous carbon layer and the
crystalline carbon layer, from core to surface, successively.
Hereinafter, the present invention will be explained in
detail.
FIG. 1 is a sectional view of an anode material
according to a preferred embodiment of the present invention.
As can be seen from FIG. 1, the surface of a metal capable of
electromechanical charge/discharge is coated with a surface
layer consisting of an amorphous carbon layer and a
crystalline carbon layer.
Metals for forming the metal core layer may include at
least one metal selected from the group consisting of Si, Al,
Sn, Sb, Bi, As, Ge and Pb or alloys thereof. However, there
is no particular limitation in the metals, as long as they
are capable of electrochemical and reversible lithium
intercalation/ deintercalation.
The amorphous carbon may include carbonaceous materials
obtained by the heat-treatment of coal tar pitch, petroleum,
pitch and various organic materials.
The crystalline carbon may include natural graphite,
artificial graphite, etc. having a high degree of
graphitization, and such graphite-based materials may include
MCMB (MesoCarbon MicroBead), carbon fiber and natural
graphite.
Preferably, the ratio of the metal core layer to the
amorphous carbon layer to, the crystalline carbon layer is 90-
10 wt% : 0.1-50 wt% : 9-90 wt%. If the core layer is present
in an amount less than 10 wt%, reversible capacity is low,
and thus it is not pcasible to provide an anode material
having a high capacity. If the crystalline carbon layer is
present in an amount less than 9 wt%, it is not possible to
ensure sufficient conductivity. Further, the amorphous carbon
layer is present in an amount less than 0.1 wt%, it is not
possible to inhibit the expansion of a metal sufficiently,
while it is present in an amount greater than 50 wt%, there
is a possibility for the reduction of capacity and
conductivity.
The anode material according to the present invention
may be prepared as follows. The amorphous carbon layer may be
directly coated on the metal forming the core layer by a thin
film deposition process such as CVD (chemical vapor
deposition), PVD (physical vapor deposition), etc. Otherwise,
the metal core layer is coated with various organic material
precursors such as petroleum pitch, coal tar pitch, phenolic
resins, PVC (polyvinyl chloride), PVA (polyvinyl alcohol),
etc., and then the precursors are heat treated under inert
atmosphere, at 500-1300°C for 30 minutes to 3 hours so as to
be carbonized, thereby coating the amorphous carbon layer on
the metal core layer. Next, to a mixture containing 90-98 wt%
of crystalline carbonaceous materials and 2-10 wt% of a
binder optionally with 5 wt% or less of a conducting agent,
an adequate amount of a solvent is added, and the resultant
mixture is homogeneously mixed to form slurry. The slurry is
coated on the amorphous carbon layer and then dried to form
the crystalline carbonaceous layer.
In a variant, a metal forming the core layer is mixed
with crystalline carbon in a predetermined ratio, for
example, in the ratio of 10-90 wt%:90-10wt% of the metal to
the crystalline carbon. Then, the amorphous carbon layer and
the crystalline carbon layer may be simultaneously formed by
using a technique such as a ball mill method, a mechano-
fusion method and other mechanical alloying methods.
Mechanical alloying methods provide alloys having
uniform composition by applying mechanical forces.
Preferably, in the amorphous carbon layer, the
interlayer distance (d002) of carbon is 0.34 nm or more and
the thickness is 5 nm or more. If the thickness is less than
5 nm, it is not possible to inhibit changes in the volume of
the metal core layer sufficiently. If the interlayer distance
is less than 0.34 nm, t13 coating layer itself may undergo an
extreme change in volume as the result of repetitive
charge/discharge cycles, and thus it is not possible to
inhibit changes in the volume of the metal core layer
sufficiently, thereby detracting from cycle life
characteristics.
Preferably, in the crystalline carbon layer, the
interlayer distance (d002) of carbon ranges from 0.3354 run to
0.35 nm. The lower limit value is the theoretically smallest
interlayer distance of graphite and a value less than the
lower limit value does not exist. Additionally, carbon having
an interlayer distance greater than the upper limit value is
poor in conductivity, so that the coating layer has low
conductivity, and thus it is not possible to obtain excellent
lithium intercalation/deintercalation property.
Further, although there is no particular limitation in
the thickness of the crystalline carbon layer, the thickness
preferably ranges from 1 micron to 10 microns. If the
thickness is less than 1 micron, it is difficult to ensure
sufficient conductivity among particles. On the other hand,
the thickness is greater than 15 microns, carbonaceous
materials occupy a major proportion of the anode material,
and thus it is not possible to obtain a high charge/discharge
capacity.
The lithium secondary cell according to the present
invention utilizes the above-described anode material
according to the present invention.
In one embodiment, to prepare an anode by using the
anode material according to the present invention, the anode
material powder according to the present invention is mixed
with a binder and a solvent, and optionally with a conducting
agent and a dispersant, and the resultant mixture is agitated
to form paste (slurry). Then, the paste is coated on a
collector made of a metal, and the coated collector is
compressed and dried to provide an anode having a laminated
structure.
The binder and the conducting agent are suitably used
in an amount of 1-10 wt% and 1-30 wt%, respectively, based on
the total weight of the anode material according to the
present invention.
Typical examples of the binder include
polytetrafluoroethylene (PTFE), polyvinylidene fluoride
(PVdF) or copopymers thereof, cellulose, SBR (styrene-
butadiene rubber), etc. Further, the solvent may be an
organic solvent such as NMP (N-methylpyrrolidone), CMF
(dimethylformamide), etc., or water depending on the
selection of the binder.
Generally, carbon black may be used as a conducting
agent, and commercially available products of carbon black
include Acetylene Black series from Chevron Chemical Company
or Gulf Oil Company; Ketjen Black EC series from Armak
Company; Vulcan XC-72 from Cabot Company; and Super P from
MMM Company, or the like.
The collector made of a metal comprises a high-
conductivity metal to which the anode material paste is
easily adhered. Any metal having no reactivity in the range
of drive voltage of the cell may be used. Typical examples
for the current collector include copper, gold, nickel,
copper alloys, or the combination of them, in the shape of
mesh, foil, etc.
In order to coat the paste of anode material to the
metal collector, conventional methods or other suitable
methods may be used depending on the properties of the used
materials. For example, the paste is distributed on the
collector and dispersed uniformly with a doctor blade, etc.
If desired, the distribution step and the dispersion steps
may be performed in one step. In addition to these methods, a
die casting method, a comma coating methods and a screen
printing method may be selected. Otherwise, the paste is
formed on a separate substrate and then pressed or laminated
together with the collector.
The coated paste may be dried in a vacuum oven at 50-
200°C for 0.5-3 days, but the drying method is merely
illustrative.
Meanwhile, the lithium secondary cell according to the
present invention may be prepared with an anode obtained
according to the present invention by using a method
generally known to one skilled in the art. There is no
particular limitation in the preparation method. For example,
a separator is inserted between a cathode and an anode, and a
non-aqueous electrolyte is introduced. Further, as the
cathode, separator, non-aqueous electrolyte, or other
additives, if desired, materials known to one skilled in the
art may be used, respectively.
Cathode active materials that may be used in the
cathode of the lithium secondary cell according to the
present invention include lithium-containing transition metal
oxides. For example, at least one oxide selected from the
group consisting of LiCoO2, LiNiO2, LiMnO2, LiMnO4,
Li(NiaCobMnc)O2 (wherein 0
yCoyO2, LiCo1-yMnyO2, LiNi1-yMnYO2 (wherein ObMnc) O4
(wherein 0
zCOzO4 (wherein 0
In order to prepare the cell according to the present
invention, a porous separator may be used. Particularly, the
porous separator may be polyproplene-based, polyethylene-
based and polyolef in-based porous separators, but is not
limited thereto.
Non-aqueous electrolyte that may be used in the lithium
secondary cell according to the present invention may include
cyclic carbonates and linear carbonates. Typical examples of
cyclic carbonates include ethylene carbonate (EC), propylene
carbonate (PC), Y-butyrolactone (GBL) or the like. Typical
examples of linear carbonates include diethyl carbonate
(DEC), dimethyl carbonate (DMC), ethylmethyl carbonate (EMC),
and methylpropyl carbonate (MPC). If desired, electrolyte
additives, such as VC(Vinylene Carbonate), PS (1,3-Propane
Sultone), ES(Ethylene Sulfite), CHB(Cyclohexyl Benzene),
etc., can be used. Further, the non-aqueous electrolyte of
the lithium secondary cell according to the present invention
further comprises lithium salts in addition to the carbonate
compounds. Particular examples of the lithium salts include
LiClO4, LiCF3SO3, LiPF6, LiBF4, LiAsF6, LiN(CF3SO2)2, or the
like.
A lithium ion secondary cell is a typical example of
non-aqueous electrolyte-based secondary cells. Therefore, as
long as the anode material according to the present invention
is used, the spirit and concept of the present invention may
be applied to a non-aqueous electrolyte-based secondary cell
that permits reversible intercalation/deintercalation of an
alkali metal such as Li, besides a lithium secondary cell.
This is also included in the scope of the present invention.
Best Mode for Carrying Out the Invention
Reference will now be made in detail to the preferred
embodiments of the present invention. The following examples
are illustrative only, and the scope of the present invention
is not limited thereto.
Example 1
Natural graphite was mixed with Si in a ratio of 50
wt%:50 wt%. Then, mechanical alloying of the mixture was
performed by using a Mechano Fusion device available from
Hosokawa Micron Company 'aider a rotation speed of 600 rpm for
30 minutes to obtain an anode material. As shown in FIG. 6r
the resultant anode material was composed of a Si metal
layer, an amorphous carbon layer and a crystalline carbon
layer.
In order to evaluate the anode material, the anode
material powder was mixed with 10 wt% of PVDF as a binder, 10
wt% of acetylene black as a conducting agent and NMP as a
solvent to form homogeneous slurry. The slurry was coated on
a copper foil, dried, rolled and then punched into a desired
size to obtain an anode. A coin type cell was formed by using
the anode, a lithium metal electrode as a counter electrode
and an electrolyte containing 1 mole of LiPF6 dissolved in EC
and EMC.
Example 2
Example 1 was repeated to obtain an anode material and
a coin type cell, except that Si was substituted with an
alloy having the composition of Si 62 wt% + Co 38% and
obtained by a gas atomization method.
Comparative Example 1
Example 1 was repeated to obtain a coin type cell,
except using an anode material obtained by carrying out
mechanical alloying of Si for 30 minutes by using a mechano
fusion device.
Comparative Example 2
Example 1 was repeated to obtain a coin type cell,
except that an alloy having the composition of Si 62 wt% + Co
38% and obtained by a gas atomization method was used as an
anode material.
Comparative Example 3
Example 1 was repeated to obtain an anode material and
a coin type cell, except that Si and graphite were
substituted with Si and inherently amorphous hard carbon. The
resultant anode material in this case was composed of a Si
metal layer and an amorphous carbon layer.
Comparative Example 4
A Si-Co alloy was mixed with graphite micropowder
having an average particle diameter of 5 microns or less, and
the mixture was treated with a hybridization system for 3
minutes to form an anode material, which was composed of a
metal layer and an crystalline carbon layer. Example 1 was
repeated to obtain a coin, type cell, except that the anode
material obtained as described above was used.
Experimental Results
As shown in FIG. 2, the cell obtained by using an anode
material according to Example 1 maintained its initial
capacity until 50 cycles. On the other hand, the capacity of
the cell obtained by using an anode material according to
Comparative Example 1 reduced rapidly in several cycles from
the initial point. Such a trend can be seen also from FIG. 3
illustrating the cycle life characteristics of the cells
obtained from Example 2 and Comparative Example 2.
It seems that the anode materials according to Examples
1 and 2 substantially have no changes in their particles,
before and after charge/discharge, and thus can provide
excellent cycle life characteristics (See, (A) and (B) in
FIG. 4). On the other hand, it seems that the anode materials
according to Comparative Examples 1 and 2 undergo changes in
volume as a result of repetitive charge/discharge, and thus
their particles were transformed into porous particles so
that their availability was reduced, thereby rapidly
detracting from cycle life characteristics (See, (A) and (B)
in FIG. 5).
Meanwhile, after completion of 3 cycles of
charge/discharge, coin cells were decomposed and thickness of
each electrode was measured. In case of using the anode
material according to Comparative Example 2, the electrode
thickness increased by about 300%, i.e., from 28 µm to 83µm.
On the other hand, in the case of using the anode material
according to Example 2, the electrode thickness increased by
about 50%, i.e., from 33µm to 50pm.. Therefore, it can be seen
that the anode material according to Example 2 inhibits the
volume expansion.
FIG. 6 is a TEM photo of the anode material according
to Example 1. By observing the section of the anode material
having excellent properties as described above, it can be
seen that an amorphous carbon layer is present on the surface
of a metal core layer. In Fig. 6, the left side is a part
corresponding to Si and the right side is a part
corresponding to carbon. As can be seen from FIG. 6, Si
retains an excellent crystalline property by the interface
between Si and carbon, while carbon loses its inherent
crystalline property and provides an amorphous layer in a
thickness of about 30 nm.
Further, as can be seen from FIG. 7, excellent cycle
life characteristics can be obtained in the case of
coexistence of amorphous and crystalline carbon layers. This
can be demonstrated by comparing Comparative Example 3 (black
line) including a metal layer coated only with an amorphous
carbon layer, Comparative Exanple 4 (green line) including a
metal layer coated only with a crystalline carbon layer,, and
Example 1 (red line) including a metal layer coated with an
amorphous carbon layer and a crystalline carbon layer,
successively.
Industrial Applicability
As can be seen from the foregoing, the anode material
according to the present invention not only maintains a high
charge/discharge capacity, which is an advantage of a metal-
based anode material, but also inhibits changes in the volume
of a metal core layer caused by repetitive lithium
intercalation/ deintercalation in virtue of an amorphous
carbon layer and a crystalline carbon layer, thereby
improving the cycle life characteristics of cells.
While this invention, has been described in connection
with what is presently considered to be the most practical
and preferred embodiment, it is to be understood that the
invention is not limited to the disclosed embodiment and the
drawings, but, on the contrary, it is intended to cover
various modifications and variations within the spirit and
scope of the appended claims.
WE CLAIM :
1. An anode material comprising:
a metal core layer capable of repetitive lithium intercalation/
deintercalation;
an amorphous carbon layer coated on the surface of the metal core layer;
and
a crystalline carbon layer coated on the amorphous carbon layer.
2. The anode material as claimed in claim 1, wherein the metal core layer is
composed of a metal or an alloy comprising at least one metal selected from the
group consisting of Si, Al, Sn, Sb, Bi, As, Ge and Pb.
3. The anode material as claimed in claim 1, wherein the surface of the
metal core layer is partially or totally coated with a coating layer comprising the
amorphous carbon layer and the crystalline carbon layer.
4. The anode material as claimed in claim 1, wherein the ratio of the metal
core layer to the amorphous carbon layer to the crystalline carbon layer is 90-10
wt% : 0.1-50 wt% : 9-90 wt%.
5. The anode material as claimed in claim 1, wherein the amorphous carbon
layer has an interlayer distance (d002) of carbon atom of 0.34 nm or more, and a
thickness of 5 nm or more.
6. The anode material as claimed in claim 1, wherein the crystalline carbon
layer has an interlayer distance (d002) of carbon atom ranged from 0.3354 nm to
0.35 nm, and a thickness ranged from 1 micron to 10 microns.
7. A secondary cell using the anode material as claimed in any one of claims
1 to 6.
8. A method for preparing the anode material as claimed in any one of
claims 1 to 6, comprising the steps of:
coating an amorphous carbon layer on a metal core layer by a thin film
deposition process, or coating pitch or organic material precursors on a metal
core layer and heat treating to perform carbonization, thereby coating an
amorphous carbon layer on the metal core layer; and
coating slurry containing crystalline carbonaceous materials on the
surface of the amorphous carbon layer and drying to form a crystalline carbon
layer.
9. A method for preparing the anode material as claimed in any one of
claims 1 to 6, comprising the steps of:
mixing a metal forming a core layer with crystalline carbon; and
carrying out a mechanical alloying process to form an amorphous carbon
layer and a crystalline carbon layer simultaneously on the metal core layer.
10. The method as claimed in claim 9, wherein the mixing ratio of the metal to
the crystalline carbon is 10-90:90-10.
Disclosed is an anode material comprising a metal core layer capable of repetitive
lithium intercalation/deintercalation; an amorphous carbon layer coated on the
surface of the metal core layer; and a crystalline carbon layer coated on the
amorphous carbon layer. The anode material not only maintains a high
charge/discharge capacity, which is an advantage of a metal-based anode material,
but also inhibits changes in the volume of a metal core layer caused by repetitive
lithium intercalation/deintercalation in virtue of an amorphous carbon layer and a
crystalline carbon layer, thereby improving the cycle life characteristics of cells.
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