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 0bMnc) O4
(wherein 0