Note: Descriptions are shown in the official language in which they were submitted.
CA 02556201 2001-04-24
AN ELECTRODE FOR A RECHARGEABLE LITHIUM BATTERY
AND A RECHARGEABLE LITHIUM BATTERY
This application is a division of Canadian Patent
Application Serial No. 2,407,414, filed on April 24, 2001.
In view of division enforced by the Canadian
Intellectual Property Office the claims of this application
are directed to an electrode for a rechargeable lithium
battery having three layers made from Cu, Sn, and Cu6Sn5.
However, for the purpose of facilitating an understanding of
all objects and features of the development which are
inextricably bound-up in one and the same inventive concept as
taught and claimed in the parent application, the objects and
teachings of those features claimed in the parent Canadian
Application Serial No. 2,407,414 are retained herein.
Accordingly, in view of enforced division required
by the Examiner in the prosecution of the aforesaid parent
application, object clauses and features have been retained
for the purposes of facilitating and understanding of the
overall development. However, the retention of any clauses or
features which may be more particularly related to the parent
application or a separate divisional thereof should not be
regarded as rendering the teachings and claiming ambiguous or
inconsistent with the subject matter defined in the claims of
the divisional application presented herein when seeking to
interpret the scope thereof and the basis in this disclosure
for the claims recited herein.
1
CA 02556201 2001-04-24
FIELD OF THE INVENTION
The present invention relates to a novel electrode for a
rechargeable lithium battery and a rechargeable lithium
battery utilizing the same.
BACKGROUND OF THE INVENTION
Rechargeable lithium batteries, recently under extensive
development and research, exhibit battery characteristics,
such as charge-discharge voltage, charge-discharge cycle life
characteristics and storage characteristics, which depend
largely upon the types of the electrodes used. This has led
to the various attempts to better battery characteristics by
improving active electrode materials.
The use of metallic lithium as the negative active
material enables construction of batteries which exhibit high
energy densities, both gravimetric and volumetric. However,
the lithium deposited on charge grows into dendrites, which
could cause problematic internal short-circuiting.
On the other hand, rechargeable lithium batteries are
reported using an electrode composed of aluminum, silicon, tin
or the like which alloys electrochemically with lithium during
charge (Solid State Ionics, 113-115, p57(1998)).
However, the use of these metals that alloy with lithium
(Li) for the negative electrode material has problematically
resulted in the failure to obtain sufficient cycle
characteristics because such active electrode materials
undergo large expansion and shrinkage in volume as they store
and release lithium and are pulverized and separated from the
current collector.
2
CA 02556201 2001-04-24
SUMMARY OF THE INVENTION
It is an object of the present invention to provide an
electrode for a rechargeable lithium battery, which solves
these conventional problems and exhibits a high discharge
capacity and excellent cycle characteristics and also provide
a rechargeable lithium battery using the same.
The electrode for a rechargeable lithium battery, in
accordance with the present invention, comprises a layer
composed of a metal that does not alloy with Li, a layer
composed of a metal that alloys with Li, and a mixed layer
interposed between these layers and composed of the metal that
does not alloy with Li and the metal that alloys with Li.
According to the invention, a layer composed of a metal
that alloys with Li is provided on a substrate composed of a
metal that does not alloy with Li, and a mixed layer composed
of these metals is interposed between the layer and the
substrate.
In the present invention, a layer composed of a metal
that does not alloy with Li is provided on a substrate
composed of a metal that alloys with Li, and a mixed layer
composed of these metals is interposed between the layer and
the substrate.
A layer composed of a metal that alloys with Li is
provided on a substrate composed of a metal that does not
alloy with Li, and the metal in the substrate that does not
alloy with Li is diffused into the layer so that a mixed layer
is formed containing the metal that does not alloy with Li and
the metal that alloys with Li. In this case, the layer
provided on the substrate and composed of a metal that alloys
3
CA 02556201 2001-04-24
with Li may be rendered into the mixed layer substantially in
its entirety.
In the present invention, the mixed layer preferably has
a thickness of 0.5 pm or above. If the mixed layer becomes
excessively thin, the cycle characteristics improving effect
as expected from formation of the mixed layer may not be
obtained sufficiently.
3a
CA 02556201 2001-04-24
In the present invention, the mixed layer is
preferably divided into islands by gaps formed therein in a
manner to extend in its thickness direction. Preferably,
these gaps are formed on or after the first charge-discharge
cycle. That is, the mixed layer, on or after the first
charge-discharge cycle, preferably undergoes volumetric
expansion and shrinkage to form therein such gaps whereby
the mixed layer is divided into islands. Where a layer
composed of a metal that alloys with Li is located to
overlie the mixed layer, it is preferred that the layer is
also divided into islands by gaps formed therein in a manner
to extend in its thickness direction.
Also preferably, the mixed layer has irregularities on
its surface. The above-described gaps are.preferably formed
along valleys of the irregularities. In the case where the
mixed layer is formed to overlie the substrate, it is
preferred that the substrate has irregularities on its
surface and the corresponding irregularities are formed on a
surface of the mixed layer.
When the mixed layer is divided into islands by the
gaps formed therein in a manner to extend in its thickness
direction, spaces are defined to surround the island
portions. These spaces can accommodate changes in volume of
the mixed layer containing active material as it expands and
shrinks during charge and discharge, and thus suppress
4
CA 02556201 2001-04-24
production of a stress in the mixed layer. This prevents
fall-off or separation of the mixed layer from the substrate
to result in obtaining satisfactory cycle characteristics.
The metal that does not alloy with Li, as used herein,
refers to a metal that does not form a solid solution with
Li, and specifically is a metal that does not show the
presence of an alloy state in a metal-Li binary phase
diagram. Examples of metals that do not alloy with Li
include Cu, Fe, Ni, Co, Mo, W, Ta and the like.
The metal that alloys with Li, as used herein, refers
to a metal that forms a solid solution or intermetallic
compound with Li. Examples of metals that alloy with Li
include Sn, Ge, A1, In, Mg and the like. Such metals that
alloy with Li may be incorporated alone or in combination.
That is, the layer or substrate composed of a metal that
alloys with Li may be composed of an alloy containing two or
more of the metals that alloy with Li. Preferably, these
two or more alloy-constituting metals do not form an
intermetallic compound with each other.
Where one of the metals that constitute the alloy is
Sn, another metal may preferably be Pb, Zn, Bi, A1 or the
like. Accordingly, examples of such alloys include an alloy
of Sn and Pb, an alloy of Sn and Zn, an alloy of Sn and Bi,
an alloy of Sn and A1, and the like.
Preferably, a concentration of one metal in the alloy
5
CA 02556201 2001-04-24
does not exceed 40 % by weight. Where the alloy is an alloy
of Sn and Pb or an alloy of Sn and Zn, the aforementioned
one metal may preferably be Pb or Zn. That is, a Pb or Zn
content of the alloy is preferably up to 40 o by weight.
In the present invention, the mixed layer can be
formed by providing a layer composed of a metal that alloys
with Li on a layer or substrate composed of a metal that
does not alloy with Li and then subjecting the stack to a
heat treatment whereby the metal that does not alloy with Li
is caused to diffuse into the layer or substrate composed of
the metal that alloys with Li. For example, a Cu-Sn mixed
layer can be formed by providing an Sn layer on a Cu
substrate and then subjecting the stack to a heat treatment
which causes Cu to diffuse into the Sn layer.
Alternatively, the mixed layer may be formed by
providing a layer composed of a metal that does not alloy
with Li on a layer or substrate composed of a metal that
alloys with Li and there subjecting the resulting stack to a
heat treatment. For example, a sequence of providing a Cu
layer on an Sn substrate and subjecting the stack to a heat
treatment results in the formation of an Sn-Cu mixed layer
at an interface between the Sn substrate and the Cu layer.
The aforementioned heat treatment is preferably
effected at a temperature lower than a melting point of the
metal that does not alloy with Li or the metal that alloys
6
CA 02556201 2001-04-24
with Li, whichever is lower. Specifically, the heat
treatment may preferably be carried out at a temperature
which is approximately 50 0 - 95 ~ of a melting point of
either one of the metals, whichever is lower. In the case
where Cu is used as the metal that does not alloy with Li
and Sn is used as the metal that alloys with Li, since a
melting point of Cu is 1085 °C and that of Sn is 232 °C, the
heat treatment may preferably be carried out at a
temperature that is 50 0 - 95 0 of the lower melting point,
232 °C, i.e., at a temperature within the range of 116 -
220 °C.
As will be described later, in the case where Cu is
used as the metal that does not alloy with Li and Sn is used
as the metal that alloys with Li, it has been demonstrated
that the heat treatment is preferably effected at a
temperature of 160 - 240 °C, more preferably 180 - 240 °C.
As described above, the heat treatment causes
diffusion of the metal that does not alloy with Li and/or
the metal that alloys with Li and results in the formation
of the mixed layer which has a concentration gradient. In
this invention, the mixed layer preferably has such a
concentration gradient that the metal that alloys with Li
increases its concentration and the metal that does not
alloy with Li decreases its concentration toward the layer
or substrate composed of the metal that alloys with Li. The
7
CA 02556201 2001-04-24
concentration gradient in the mixed layer may vary either
stepwise or continuously.
In the case of stepwise varying concentration gradient,
the mixed layer comprises at least two superimposed layers
5- which contain the metal that alloys with Li and the metal
that does not alloy with Li at different concentration
ratios. That is, if the metal that does not alloy with Li
is represented by A and the metal that alloys with Li by B,
the mixed layer comprises at least two layers having
different A:B concentration ratios. For example, a first
layer with a concentration ratio A:B = 10:1 - 2:1 is
provided on a layer or substrate composed of a metal that
does not alloy with Li. A second layer with a concentration
ratio A:B = 4:1 - 1:1 is then provided on the first layer.
These first and second layers constitute the mixed layer.
In the present invention, preferably, the mixed layer
is positively formed to a thickness of 0.5 um or greater, as
described earlier. For example, when a thin film is vapor-
or liquid-phase deposited on a substrate, a very thin mixed
layer is occasionally formed between the substrate and the
thin film. The present invention prefers the mixed layer
formed positively such as by heat treatment to the such-
formed mixed layer.
8
CA 02556201 2001-04-24
In an aspect of the invention, there is provided an
electrode for a rechargeable lithium battery, which comprises
a layer composed of a metal that does not alloy with Li, a
layer composed of a metal that alloys with Li and a mixed
layer interposed between the layers and composed of these
metals, the metal that does not alloy with Li is Cu, the metal
that alloys with Li is Sn, and the mixed layer comprises at
least one layer composed of Cu6Sn5.
In another aspect of the invention, there is provided an
electrode for a rechargeable lithium battery, which comprises
a substrate composed of a metal that does not alloy with Li, a
layer provided on the substrate and composed of a metal that
alloys with Li and a mixed layer interposed between the
substrate and the layer and composed of these metals, the
metal that does not alloy with Li is Cu, the metal that alloys
with Li is Sn, and the mixed layer comprises at least one
layer composed of Cu6Sn5.
In a further aspect of the invention, there is provided
an electrode for a rechargeable lithium battery, which
comprises a substrate composed of a metal that alloys with Li,
a layer provided on the substrate and composed of a metal that
does not alloy with Li and a mixed layer interposed between
the substrate and the layer and composed of these metals, the
metal that does not alloy with Li is Cu, the metal that alloys
with Li is Sn, and the mixed layer comprises at least one
layer composed of Cu6Sn5.
In yet a further aspect of the invention, there is
provided an electrode for a rechargeable lithium battery,
which comprises a substrate composed of a metal that does not
9
CA 02556201 2001-04-24
alloy with Li, a layer provided on the substrate and composed
of a metal that alloys with Li and a mixed layer of the metals
formed by allowing the metal that does not alloy with Li to
diffuse into the layer from the substrate, the metal that does
not alloy with Li is Cu, the metal that alloys with Li is Sn,
and the mixed layer comprises at least one layer composed of
Cu6Sn5 .
Alternatively, the mixed layer in the present invention
may comprise a solid solution of the metal that does not alloy
with Li and the metal that alloys with Li. Whether the mixed
layer comprises an intermetallic compound or a solid solution
is determined by the types and proportions in composition of
the metals mixed, the forming conditions of the mixed layer or
the like. The mixed layer may also have a preferred
orientation. The presence of orientation in the mixed layer
conceivably increases cycle characteristics. Also, the mixed
layer may have nano-order voids.
In the present invention, electrochemical processes such
as electroplating and electroless plating, and physical thin
film forming processes such as CVD, sputtering, vapor
evaporation and spraying can be utilized to deposit, in the
form of a layer on the substrate, a metal that alloys with Li
or a metal that does not alloy with Li. Subsequent to
deposition of the layer of a metal that alloys with Li or a
9a
CA 02556201 2001-04-24
metal that does not alloy with Li, the resulting stack is
subjected to a heat treatment or other process so that the
mixed layer can be formed at an interface between the
substrate and the layer.
In the present invention, the layer or substrate
composed of a metal that does not alloy with Li is not
particularly specified in thickness, nor is the layer or
substrate composed of a metal that alloys with Li. However,
the increase in thickness of these layers or substrates
increases an overall thickness of an electrode and lowers an
energy density of a battery, either gravimetric or
volumetric. It is accordingly preferred that they have a
thickness of up to about 50 um.
In the present invention, the layer may be deposited
on the substrate and then subjected to a heat treatment or
the like so that the mixed layer is formed at an interface
between the substrate and the layer. In this case, the
substrate preferably has irregularities on its surface. The
provision of such surface irregularities improves adhesion
between the substrate and the active material and thus
prevents separation of the active material during charge and
discharge. Specifically, the surface roughness Ra of the
substrate is preferably in the approximate range of 0.01 - 2
um, more preferably 0.1 - 2 Vim. The surface roughness Ra is
defined in Japan Industrial Standards (JIS B 0601-1994) and
CA 02556201 2001-04-24
can be determined as by a surface roughness meter.
In an exemplary case where a layer of a metal that
alloys with Li, such as an Sn layer, is deposited on a Cu
substrate, the use of an electrolytic copper foil which is a
copper foil having a large surface roughness Ra is preferred.
In the present invention, provided between the layer
or substrate composed of a metal that does not alloy with Li
and the layer or substrate composed of a metal that alloys
with Li is the mixed layer which contains a mixture of these
metals. Due to the presence of such a mixed layer, the
layer or substrate composed of a metal that alloys with Li
can be kept well adhered to the layer or substrate composed
of a metal that does not alloy with Li, even when the former
layer or substrate expands and shrinks in volume as it
stores and releases lithium during a charge-discharge
reaction. This prevents fall-off, separation or the like of
the layer or substrate composed of a metal that alloys with
Li, which is an active material, and permits the electrode
to undergo a charge-discharge reaction while collecting
current in a satisfactory fashion. As a result,
satisfactory cycle characteristics can be obtained.
In the present invention, a thin film which does not
react with Li ions, permits passage of Li ions and has no
ionic conductivity for Li ions may be provided on the layer
or substrate composed of a metal that alloys with Li. That
' 11
CA 02556201 2001-04-24
is, such a thin film may be provided on the layer or
substrate which serves as active material. Specifically,
the layer or substrate composed of a metal that alloys with
Li carries on its one surface the layer or substrate
composed of a metal that does not alloy with Li and on its
other surface the thin film. The provision of the thin film
prevents the layer or substrate composed of a metal that
alloys with Li and serving as active material from growing
into dendrites or being pulverized during charge and
discharge.
As described above, the thin film is a thin film which
does not react with Li ions, permits passage of Li ions and
has no ionic conductivity for Li ions. Because the thin
film does not react with Li ions, the thin film itself is
not alloyed and thus undergoes neither expansion nor
shrinkage. Also because the thin film permits passage of Li
ions, a battery reaction in the active material is not
hindered. Also because the thin film does not have ionic
conductivity for Li ions, unlike a solid electrolyte thin
film, the thin film itself is not deformed during charge and
discharge.
In the present invention, the thin film preferably has
a volume resistivity of 101° ~~cm or below. The thin film
having such good electronic conductivity can also serve as a
current collector.
12
CA 02556201 2001-04-24
In the present invention, the thin film can be
deposited as by CVD, sputtering or vapor evaporation
techniques.
The thin film in accordance with the present invention
is illustrated by a diamond-like carbon thin film or other
hard carbon thin films. Such hard carbon thin films show no
tendency to react with Li, permit passage of Li ions and
have no ionic conductivity for Li ions. The preferred hard
carbon thin film shows a Rarnan scattering spectrum in which
a ratio (Id/Ig) of a peak intensity (Id) around 1,400 cm~i to
a peak intensity (Ig) around 1,550 cm-1 is 0.5 - 3Ø
As stated earlier, such a hard carbon thin film
preferably has a volume resistivity of 101° ~~cm or below. A
typical example of the hard carbon thin film having such
good conductivity is a hard carbon thin film containing COZ
molecules. The hard carbon thin film containing C02
molecules can be deposited by a CVD process using a mixed
gas of COz and hydrocarbon as a source gas. Also, the
deposited hard carbon thin film may be subjected to a
surface treatment with a COz-containing gas so that an
electrically conductive property is imparted to a surface of
the hard carbon thin film.
While not particularly limited, the thickness of the
thin film is preferably in the approximate range of 50 -
1,000 nm, more preferably in the approximate range of 100 -
13
CA 02556201 2001-04-24
500 nm. If the thin film is excessively, thin, its effect
that prevents the active material from being separated from
the current collector or growing into dendrites may become
insufficient. On the other hand, if the thin film is
excessively thick, it may restrict the passage of hi ions
during charge and discharge to result occasionally in an
insufficient reaction between the active material and the Li
ions.
In the present invention, an interlayer may be
provided between the thin film and the layer or substrate
composed of a metal that alloys with Li, i.e., between the
thin film and the active material. One purpose in providing
such an interlayer is to improve adhesion between the thin
film and the active material. The interlayer may comprise
at least one selected from Si, Ti, Zr, Ge, Ru, Mo, W and
their oxides, nitrides and carbides. The interlayer
preferably has a thickness in the approximate range of 10 -
500 nm. The interlayer can be formed by a CVD, sputtering,
vacuum deposition or plating process, for example.
The rechargeable lithium battery of the present
invention includes a negative electrode comprised of
the above-described electrode of the present invention,
a positive electrode and a nonaqueous electrolyte.
An electrolyte solvent for use in the rechargeable
lithium battery of the present invention is not particularly
14
CA 02556201 2001-04-24
specified in type but can be illustrated by a mixed solvent
which contains cyclic carbonate such as ethylene carbonate,
propylene carbonate, butylene carbonate or vinylene
carbonate and also contains chain carbonate such as dimethyl
carbonate, methyl ethyl carbonate or diethyl carbonate.
Also applicable is a mixed solvent of the above-listed
cyclic carbonate and an ether solvent such as 1,2-
dimethoxyethane or 1,2-diethoxyethane. Examples of
electrolyte solutes include LiPF6, LiBF4, LiCF3S03,
LiN (CF3S02) z, LiN (CZFSSOZ) 2, LiN (CF3S0z) (C4F9SOz) , LiC (CF3S02) s~
LiC(CZFSSOZ)3 and mixtures thereof. Other applicable
electrolytes include gelled polymer electrolytes comprised
of an electrolyte solution impregnated into polymer
electrolytes such as polyethylene oxide, polyacrylonitrile
and polyvinylidene fluoride; and inorganic solid
electrolytes such as LiI and Li3N, for example. The
electrolyte for the rechargeable lithium battery of the
present invention can be used without limitation, so long as
an Li compound as its solute that imparts ionic conductivity,
together with its solvent that dissolves and retains the Li
compound, remain undecomposed at voltages during charge,
discharge and storage of the battery.
Examples of useful active materials for the positive
electrode of the rechargeable lithium battery of the present
invention include lithium-containing transition metal oxides
CA 02556201 2001-04-24
such as LiCoOz, LiNiO~, LiMnz09, LiMnOz, LiCoo.SNio.502 and
LiNio.,Coo_zMno.~~z% and lithium-free metal oxides such as MnOz.
Other substances can also be used, without limitation, if
they are capable of electrochemical lithium insertion and
deinsertion.
BRIEF DESCRIPTION OF THE DRAWINGS
Figure 1 is a photomicrograph taken using a scanning
electron microscope, showing a section of the electrode al
of Example in accordance with the present invention;
Figure 2 is a photomicrograph taken using a scanning
electron microscope, showing a cross-section of the
electrode b1 of Comparative Example;
Figure 3 is a photomicrograph taken using a scanning
ion microscope, showing a cross-section of the electrode al
of Example in accordance with the present invention;
Figure 4 is a photomicrograph taken using a scanning
ion microscope, showing a cross-section of the electrode b1
of Comparative Example;
Figure 5 is a chart, showing the X-ray microanalysis
result of the A layer shown in Figure 3;
Figure 6 is a chart, showing the X-ray microanalysis
result of the B layer shown in Figure 3;
Figure 7 is a chart, showing the X-ray microanalysis
result of the C layer shown in Figure 3;
16
CA 02556201 2001-04-24
Figure 8 is a chart, showing the X-ray microanalysis
result of the D layer shown in Figure 4;
Figure 9 is a chart, showing the X-ray microanalysis
result of the E layer shown in Figure 4;
Figure 10 is a schematic cross-sectional view, showing
the beaker cell constructed in Example;
Figure 11 is a plan view, showing the rechargeable
lithium battery constructed in Example;
Figure 12 is a cross-sectional view, showing a
construction by which the electrodes are assembled for the
rechargeable lithium battery shown in Figure 11;
Figure 13 is a view, showing an X-ray diffraction
pattern of the electrode al of Example in accordance with
the present invention
Figure 14 is a view, showing an X-ray diffraction
pattern of the electrode b1 of Comparative Example;
Figure 15 is a photomicrograph taken using a scanning
electron microscope, showing a cross-section of the
electrode al after charge and discharge (1st cycle);
Figure 16 is a photomicrograph taken using a scanning
electron microscope, showing a surface of the electrode al
after charge and discharge (1st cycle);
Figure 17 is view, schematically showing the electron
photomicrograph of Figure 15; and
Figure 18 is a photomicrograph taken using a scanning
17
CA 02556201 2001-04-24
electron microscope, showing a cross-section of the
electrode f5 after charge and discharge (first cycle).
BEST MODE FOR CARRYING OUT THE INVENTION
The present invention is below described in more
detail by way of Examples. It will be recognized that the
following examples merely illustrate the practice of the
present invention but are not intended to be limiting
thereof. Suitable changes and modifications can be effected
without departing from the scope of the present invention.
EXPERIMENT 1
(Fabrication of Electrodes)
A 2 pm thick Sn layer was deposited on a 18 um thick
electrolytic copper foil (surface roughness = 0.188 ~zm) by
an electrolytic plating process. Sn was used for an anode.
A plating bath having the composition specified in the
following Table 1 was used.
TABLE 1
Component Concentration
Stannous Sulfate 40g dm'3
Sulfuric Acid 1508 dm'3
Formalin 5cm3 dm 3
Additive 3 _3
40cm dm
(Product of UYEMURA & Co.,Ltd.)
After deposition of the Sn layer, the resulting stack
was cut into a 2 cm x 2 cm size and then heat-treated at
18
CA 02556201 2001-04-24
200 °C for 24 hours to obtain an electrode al.
For a comparative purpose, the above procedure was
repeated, except that the heat treatment was not carried out,
to fabricate an electrode b1.
Figure 1 is a photomicrograph taken using a scanning
electron microscope, showing a cross-section of the
electrode al. Figure 2 is a photomicrograph taken using a
scanning electron microscope, showing a cross-section of the
electrode b1. Both were taken at a magnification of 2,500X.
In Figure 2 which shows the electrode b1 without heat
treatment, an upper portion which appears lightened is the
Sn layer and a lower portion which appears slightly dark is
the Cu substrate. As apparent from Figure 1 which shows the
heat-treated electrode al, the application of heat treatment
confines the portion of the Sn layer that appears lightened
in a thinner surface region. This demonstrates that Cu in
the Cu substrate is diffused into the Sn layer to form a
mixed layer of Cu and Sn. It is also observed in the mixed
layer that an Sn concentration increases and a Cu
concentration decreases toward the surface Sn layer.
Figure 3 is a photomicrograph taken using a scanning
ion microscope, showing the cross-section of the electrode
a1. Figure 4 is a photomicrograph taken using a scanning
ion microscope, showing the cross-section of the electrode
b1. Both were taken at a magnification of 13,500X. As shown
19
CA 02556201 2001-04-24
in Figure 3, the electrode al has A and B layers formed on a
C layer. Observed on the A layer is a layer of a coating
resin applied onto an electrode surface for fabrication of
an observation sample. As shown in Figure 4, the electrode
b1 has a D layer formed on an E layer. Observed on the D
layer is a thin layer of a coating resin as similar to the
above.
Figures 5, 6 and 7 are charts indicating respective
compositions of the A, B and C layers in the electrode al
shown in Figure 3 when analyzed by an X-ray microanalyzer
(XMA). Figures 8 and 9 are charts indicating respective
compositions of the D and E layers in the electrode b1 shown
in Figure 4 when analyzed by an X-ray microanalyzer (XMA).
As can be appreciated from the results shown in
Figures 8 and 9, the D layer observed as being a dark
portion in Figure 4 is the Sn layer and the E layer observed
as being a light portion is the Cu foil substrate.
From the results shown in Figure 7, the C layer in
Figure 3 is identified as the Cu foil substrate. Also from
the results shown in Figures 5 and 6, each of the A and B
layers shown in Figure 3 is identified a~ a mixed layer of
Sn and Cu. The peak intensities in the chart shown in
Figure 5 indicate the A layer as having a composition with
Sn:Cu = 1:2. The peak intensities in the chart shown in
Figure 6 indicate the B layer as having a composition with
CA 02556201 2001-04-24
Sn:Cu = 1:4.5. The A and B layers each has a specific
proportion of the components and a specific crystal
structure throughout the layer. This suggests that each
layer is an intermetallic compound of Sn and Cu. From the
photograph shown in Figure 3, the A layer is found to be 1.7
um thick and the B layer to be 2.2 um thick. The Ni peaks
observed in the charts shown in Figures 5 - 9 are those from
sample holders.
As indicated by P in Figure 3, a number of voids
exists in the A layer which is the mixed layer. These voids
have sizes ranging from several tens to several hundreds
manometers, i.e., have namo-order sizes. The presence of
such voids relaxes volumetric expansion and shrinkage during
charge and discharge and is thus considered to contribute to
the improvement in cycle characteristics.
In Figure 3, the A layer is about 1.8 um thick and the
B layer is about 2.2 um thick.
Figure 13 is an X-ray diffraction pattern obtained for
the electrode al. Figure 14 is an X-ray diffraction pattern
obtained for the electrode b1.
As apparent from Figure 14, an intermetallic compound
of tin (Sn) and copper (Cu), probably Cu6Sn5, is found to be
present in a slight amount even in the electrode b1 without
heat treatment. This suggests that the mere plating of tin
on the copper foil results in the formation of a slight
21
CA 02556201 2001-04-24
amount of an tin-copper intermetallic compound. Although
supposed to be present at an interface between the plated
tin layer and the copper foil as a current collector, such
an intermetallic compound is not perceived in the electron
photomicrograph of Figure 4. It is accordingly considered
that a thickness of this intermetallic compound is very
small, probably below 0.5 um.
As shown in Figure 13, an intermetallic compound of
Cu6Sn5 and CujSn is identified as being present in the
electrode al. By the comparison thereof to the results from
the preceding X-ray microanalysis, the A layer having a
composition with Cu:Sn = 2:1 is considered as corresponding
to Cu6Sn5 and the B layer having a composition with Cu:Sn =
4.5:1 to Cu3Sn. It is accordingly considered the A layer,
i.e., Cu6Sn5, as being a main active material in a reaction
with lithium. Since the presence of simple Sn is identified
in Figure 13, the simple Sn is supposed to exist on a
surface the A layer. However, it is not perceived in the
electron photomicrograph of Figure 3. Its thickness is thus
considered to be 0.5 um or below.
From the comparison of peak intensities shown in
Figure 13 to the intensity ratio given in the JCPDS card, Sn
and Cu6Sn5 appear to be oriented in a specific direction.
Such orientation of the active material is considered to be
another contributor to the improved cycle characteristics.
22
CA 02556201 2001-04-24
(Preparation of Electrolyte Solution)
1 mole/liter of LiPF6 was dissolved in a mixed solvent
containing ethylene carbonate and diethyl carbonate at a 1:1
ratio by volume to prepare an electrolyte solution.
(Construction of Beaker Cell)
Using each of the above-fabricated electrodes al and
b1 as a working electrode, a beaker cell as shown in Figure
was constructed. As shown in Figure 10, the beaker cell
includes a counter electrode 3, a working electrode 4 and a
10 reference electrode 5, which are all immersed in an
electrolyte solution contained in a container 1. The above-
prepared electrolyte solution was used as the electrolyte
solution 2. Metallic lithium was used for both the counter
electrode 3 and the reference electrode 5.
(Evaluation of Cycle Characteristics)
Each of the above-constructed beaker cells was charged
at 25 °C at a constant current of 0.2 mA to 0 V (vs. Li/Li~)
and then discharged at a constant current of 0.2 mA to 2 V
(vs. Li/Li+). This unit cycle was repeated 10 times to
determine a cycle efficiency which is defined by the
following equation. The results are given in Table 2. In
this beaker cell, reduction of the working electrode takes
place during charge and oxidation thereof takes place during
discharge.
Cycle Efficiency (o) - (10th-cycle discharge capacity
23
CA 02556201 2001-04-24
/ 1st-cycle discharge capacity) X 100
TABLE 2
Electrode Cycle Efficiency (o)
al 80
b1 17
As can be clearly seen from the results shown in Table
2, the electrode al including the mixed layer formed at an
interface between Cu and Sn by heat treatment exhibits the
increased cycle efficiency compared to the electrode b1
without heat treatment. This is considered due probably to
the presence of the mixed layer which prevents separation of
the Sn layer from the Cu substrate during charge-discharge
cycles.
The electrode al after the first cycle in the above-
described charge-discharge cycle test was observed using a
scanning electron microscope. Figure 15 is a
photomicrograph taken using a scanning electron microscope,
showing a cross-section of the electrode al after charge and
discharge. Figure 16 is a photomicrograph taken using a
scanning electron microscope, showing the electrode after
the charge-discharge cycle test when viewed from above.
Figure 15 is taken at a magnification of 500X. Figure 16 is
taken at a magnification of 1,000X.
Figure 17 schematically illustrates the
photomicrograph taken using a scanning electron microscope
24
CA 02556201 2001-04-24
and shown in Figure 15. As can be clearly seen from Figures
l5 and 17, the B layer 22, which is the mixed layer provided
on the C layer 21 in the form of the copper foil, has gaps
22a formed therein in a manner to extend in its thickness
direction. As apparent from Figure 16, the A and B layers,
which are both the mixed layers, are divided into islands by
such-formed gaps. The A and B layers when divided into
islands are surrounded by spaces. These surrounding spaces
can accommodate changes in volume of such layers as they
expand and shrink during a charge-discharge reaction and
suppress production of a stress. As a result, pulverization
of the active material, located close to the current
collector and above there, can be prevented. Further,
separation of the active material from the current collector
can be prevented, resulting in the successful improvement of
cycle characteristics.
EXPERIMENT 2
(Fabrication of Electrode)
A 2 um thick, Sn-Pb alloy layer was deposited on an
electrolytic copper foil, similar to that used in Experiment
1, by an electrolytic plating process. An Sn-Pb alloy was
used for an anode. A plating bath similar to that used in
Experiment 1 was used.
After deposition of the Sn-Pb layer, the resulting
stack was cut into a 2 cm x 2 cm size and then heat-treated
CA 02556201 2001-04-24
at 200 °C for 24 hours to obtain an electrode c1.
As analogous to Experiment 1, the fabricated electrode
c1 was observed using a scanning electron microscope, a
scanning ion microscope and an X-ray microanalyzer (XMA).
The results confirmed the formation of a mixed layer of Cu,
Sn and Pb at an interface between the Cu substrate and the
Sn-Pb alloy layer.
(Measurement of Cycle Performance)
Using the electrode c1 as a working electrode, a
beaker cell was constructed in a similar manner to
Experiment 1. The beaker cell thus constructed was charged
and discharged in the same manner as in Experiment 1. The
cycle efficiency was determined to be 88 %. The improved
cycle efficiency relative to the beaker cell al
incorporating the Sn layer was obtained for the beaker cell
in this Experiment.
EXPERIMENT 3
Rechargeable lithium batteries were constructed using
the electrodes al and b1 obtained in Experiment 1 and the
electrode c1 obtained in Experiment 2 as their respective
negative electrodes, and then evaluated for charge-discharge
cycle characteristics.
(Fabrication of Fositive Electrode)
85 o by weight of LiCo02 powder with a mean particle
diameter of 10 um, 10 % by weight of carbon powder as an
26
CA 02556201 2001-04-24
electrically conducting agent and 5 o by weight of
polyvinylidene fluoride powder were mixed. After addition
of N-methylpyrrolidone thereto, the mixture was kneaded to
prepare a slurry. The slurry was coated onto one surface of
a 20 um thick aluminum foil by a doctor blade technique and
then dried. A 2 cm x 2 cm piece was cut out from the coated
aluminum foil to fabricate a positive electrode.
Construction of Battery)
The above-fabricated positive electrode and the
electrode al, b1 or c1 were combined together through a
polyethylene microporous membrane and then inserted into a
casing comprised of laminated aluminum material.
Subsequently, 500 u1 of an electrolyte solution similar to
that prepared in Experiment 1 was introduced into the casing
to thereby construct a rechargeable lithium battery.
Figure 11 is a plan view of the rechargeable lithium
battery such constructed. As shown in Figure 11, the
positive electrode 11 and the negative electrode 13 are
assembled with a separator 12 comprised of a polyethylene
microporous membrane between them, and then inserted into a
casing 14. After insertion of the assembly into the casing
14, the electrolyte is poured. Subsequently, the casing 14
is sealed at its region 14a to complete construction of the
rechargeable lithium battery.
Figure 12 is a cross-sectional view, showing a
27
CA 02556201 2001-04-24
construction by which the electrodes are assembled within
the battery. As shown in Figure 12, the positive electrode
11 and the negative electrode 13 are assembled to locate in
opposite sides of the separator 12. The positive electrode
11 has a layer lla of positive active material provided on a
positive current collector llb made of aluminum. This layer
lla of positive active material is located in contact with
the separator 12. Likewise, the negative electrode 13 has a
layer 13a of negative active material provided on a negative
current collector 13b made of copper. This layer 13a of
negative active material is located in contact with the
separator 12. In this embodiment, the layer 13a of negative
active material is a layer composed of Sn or an Sn-Pb alloy.
As shown in Figure 11, a positive tab llc made of
aluminum is attached to the positive current collector llb
for extension toward an exterior of the casing. Likewise, a
negative tab 13c made of nickel is attached to the negative
current collector 13b for extension toward an exterior of
the casing.
The rechargeable lithium batteries constructed using
the electrodes al, b1 and c1 as their respective negative
electrodes were designated as the batteries Al, B1 and C1,
respectively. A designed capacity of each battery was 5.0
mAh.
(Charge-Discharge Test)
28
CA 02556201 2001-04-24
The above-constructed batteries A1, Bl and Cl were
subjected to a charge-discharge test. Each battery was
charged at a constant current of 1.0 mA to 4.0 V and then
discharged at a constant current of 1.0 mA to 2.0 V. This
unit cycle was repeated and a cycle efficiency after 10
cycles was determined from the equation defined in
Experiment 1. Measurement was carried out at 25 °C.
TABLE 3
Battery Cycle Efficiency (o)
Battery A1 86
(Sn La er Heat-Treated)
Comp.Battery B1 33
(Sn Layer . Not Heat-Treated)
Battery Cl
91
(Sn-Pb La er Heat-Treated)
As shown in Table 3, the batteries A1 and Cl in
accordance with the present invention are proved to exhibit
improved cycle characteristics compared to the comparative
battery B1.
EXPERIMENT 4
As analogous to Experiment 1, a 2 um thick Sn layer
was deposited on an electrolytic copper foil by an
electrolytic plating process. A 2 cm x 2 cm piece was cut
out from the resulting stack and then subjected to a heat
treatment. The heat treatment was carried out at
temperatures of 160 °C, 180 °C, 200 °C, 220 °C and
240 °C. The
heat treatment at each temperature was continued for a
29
CA 02556201 2001-04-24
duration of 24 hours. As a result, an electrode dl (heat-
treated at 160 °C), an electrode d2 (heat-treated at 180 °C),
an electrode d3 (heat-treated at 200 °C), an electrode d4
(heat-treated at 220 °C) and an electrode d5 (heat-treated at
240 °C) were obtained. The electrode d3 was identical to the
electrode al in Experiment 1.
A beaker cell was constructed using each electrode and
then subjected to 10 cycles of charges and discharges to
determine a cycle efficiency in the same manner as in
Experiment 1. The results are shown in Table 4. In Table 4,
the result for the comparative electrode b1 in Experiment 1
is also shown.
TABLE 4
Electrode Cycle Efficiency (%)
Electrode dl 50
(Heat-Treated at 160C)
Electrode d2
(Heat-Treated at 180C)
Electrode d3 80
(Heat-Treated at 200C)
Electrode d4
(Heat-Treated at 220C)
Electrode d5 ~1
(Heat-Treated at 240C)
Comp.Electrode b1 1~
(Not Heat-Treated)
As can be appreciated from the results shown in Table
4, in the case where Cu is used as a metal that does not
alloy with Li and Sn is used as a metal that alloys with Li,
the electrodes fabricated with heat treatment at
CA 02556201 2001-04-24
temperatures within the range of 160 °C - 240 °C, preferably
within the range of 180 °C - 240 °C, provide satisfactory
results. The heat treatment, if applied in such a
temperature range, successfully increases a cycle efficiency.
EXPERIMENT 5
As analogous to Experiment l, a 2 um thick Sn layer
was deposited on an electrolytic copper foil and then heat-
treated at 200 °C for 24 hours. Subsequently, an Si thin
film (20 nm thick), as an interlayer, was deposited on the
Sn layer by an RF sputtering technique. This thin film was
deposited at a target RF voltage of 200 W and a substrate
bias voltage of -100 V.
Thereafter, a hard carbon thin film (diamond-like
carbon thin film: DLC thin film) was deposited on the Si
thin film by a CVD process. A thickness of the thin film
was controlled at 200 nm. The hard carbon thin film was
deposited by an ion beam CVD process under the conditions of
a CH4 gas flow rate of 40 sccm, a COZ gas flow rate of 10
sccm, 330 W microwave power from an ECR plasma source and an
ion beam acceleration voltage of 200 V.
The resulting hard carbon thin film was found to have
a volume resistivity of about 109 S2~cm. Also, measurement of
a Raman scattering spectrum of the hard carbon thin film
revealed a ratio (Id/Ig) of a peak intensity Id around 1,400
cm-1 to a peak intensity Ig around 1, 550 cm-1 as being about
31
CA 02556201 2001-04-24
1.1.
The above-fabricated stack was cut into a 2 cm x 2 cm
size to obtain an electrode e1. A beaker cell was
constructed using the electrode e1 obtained and then
subjected to 10 cycles of charges and discharges to
determine a cycle efficiency in the same manner as in
Experiment 1. The results are shown in Table 5. In Table 5,
the results for the electrode al of the present invention as
obtained in Experimen~ 1 and the comparative electrode b1
are also shown.
TABLE 5
Electrode DLC Film Thickness Cycle Efficiency
(nm) (o)
Electrode a1 0 80
Comp.Electrode 0 17
b1
Electrode e1 200 89
As apparent from Table 5, the provision of the hard
carbon thin film on the layer of Sn active material improves
a cycle efficiency.
After 10 cycles, the electrode e1 of the present
invention was observed. While the layer of Sn active
material was slightly pulverized at its surface, a shape of
the electrode before charges and discharges was
substantially sustained. Furthered pulverization of the Sn
layer surface was observed in the electrode al of the
32
CA 02556201 2001-04-24
present invention than in the electrode e1 of the present
invention. On the other hand, pulverization of the Sn layer
in its entirety was observed in the comparative electrode b.
It has been confirmed from these observations that the
S provision of the hard carbon thin film on a surface of the
layer of Sn active material further prevents pulverization
of active material and thereby further improves cycle
characteristics.
EXPERIMENT 6
Using the electrode e1 obtained in the above Experiment
5, a rechargeable lithium battery was constructed in the
same manner as in Experiment 3. The battery E1 obtained was
subjected to a charge-discharge test in the same manner as
in Experiment 3 to determine a cycle efficiency. The
results are shown in Table 6. In Table 6, the results for
the battery Al of the present invention as constructed in
Experiment 3 and the comparative battery B1 are also shown.
TABLE 6
Batter DLC Film Thickness Cycle Efficiency
y (nm) (o)
Battery A1 0 86
Comp.Battery 0 33
B1
Battery El 200 88
As can be clearly seen from the results shown in Table
6, the provision of the hard carbon thin film on the layer
33
CA 02556201 2001-04-24
of Sn active material prevents pulverization of active
material and further improves cycle characteristics.
EXPERIMENT 7
As analogous to Experiment 1, a 2 um thick Sn layer
was deposited on a rolled copper foil having a surface
roughness = 0.04 um or an electrolytic copper foil having a
surface roughness = 0.60 or 1.19 um by an electrolytic
plating process. After deposition of the Sn layer, each
stack was cut into pieces each measuring 2 cm x 2 cm in size.
One piece was heat-treated at 200 °C for 24 hours, while the
other was not subjected to a heat treatment.
Out of the pieces fabricated via deposition of Sn on
the rolled copper foil having a surface roughness = 0.04 um,
the piece which was subsequently subjected to heat treatment
was designated as an electrode fl and the piece which was
not subjected to heat treatment was designated as an
electrode f2. Out of the pieces fabricated via deposition
of Sn on the electrolytic copper foil having a surface
roughness = 0.60 um, the piece which was subsequently
subjected to heat treatment was designated as an electrode
f3 and the piece which was not subjected to heat treatment
was designated as an electrode f4. Out of the pieces
fabricated via deposition of Sn on the electrolytic copper
foil having a surface roughness = 1.19 um, the piece which
was subsequently subjected to heat treatment was designated
34
CA 02556201 2001-04-24
as an electrode f5 and the piece which was not subjected to
heat treatment was designated as an electrode f6.
Each of the above-fabricated electrodes fl - f6 was
subjected to cycle characteristics measurement in the same
manner as in Experiment 1 to determine a cycle efficiency.
The results are given in Table 7. In Table 7, the results
for the electrode a1 and the electrode b1 in Experiment 1
are also shown.
TABLE 7
Surface Heat Cycle
Electrode Rou hness Ra( m) Treatment Efficienc (o)
fl Present 33
0.04
f2 Absent 1
a1 Present 80
0.188
b1 Absent 17
f3 Present 82
0
60
.
f4 Absent 18
f5 Present 89
1
19
.
f6 Absent 20
As can be clearly seen from the results shown in Table
7, the electrodes fl, al, f3 and f5 fabricated by a process
that includes heat treatment exhibit improved cycle
efficiencies compared to the electrodes f2, b1, f4 and f6
fabricated by a process that excludes heat treatment. Also,
when viewed from the surface roughness Ra of the copper foil
used, the electrodes al, f3 and f5 exhibit improved cycle
efficiencies relative to the electrode fl. This
CA 02556201 2001-04-24
demonstrates that the preferred surface roughness Ra of the
copper foil is 0.1 ~m or above.
The electrode f5 after the first cycle in the charge-
discharge cycle test described above was observed using a
scanning electron microscope. Figure 18 is a
photomicrograph taken at a magnification of 2,500X using a
scanning electron microscope, showing a cross-section of the
electrode f after charge and discharge. Since this
electrode includes a current collector having a surface
roughness larger than the surface roughness Ra = 0.188 um of
the electrode a1 (Figure 15), it is observed more clearly in
Figure 18 than in Figure 15 that the layer of active
material is divided into islands by gaps formed therein in a
manner to extend upwardly in its thickness direction from
valleys of irregularities defined on a surface of the
current collector.
EXPERIMENT 8
A 2 ~m thick Sn-Pb or Sn-Zn alloy layer was deposited
on an electrolytic copper foil similar in type to that used
in Experiment 1 by an electrolytic plating process.
In the deposition of the Sn-Pb alloy layer, an Sn-Pb plating
bath was used containing a mixture of tin borofluoride, lead
borofluoride, fluoroboric acid, boric acid and peptone. In
the deposition of the Sn-Zn alloy layer, an Sn-Zn plating
bath was used containing a mixture of organotin, organic
36
CA 02556201 2001-04-24
acid zinc and a complexing agent. As shown in Table 8, the
Sn-Pb alloy layers comprising different ratios of Sn and Pb
were deposited.
After deposition of each alloy layer, the resulting
stack was cut to a 2 cm x 2 cm size and subsequently heat-
treated at 200 °C for 24 hours. As a result, the electrodes
g1 - g9 listed in Table 8 were fabricated. Compositions of
the alloy layers were determined by ICP emission spectral
analysis. In Table 8, the compositions are listed in
accordance with a particular form which, in the case of the
electrode g1, means to contain 99.5 o by weight of Sn and
0.5 o by weight of Pb. The varied compositions of the Sn-Pb
alloy layers result primarily from the use of the plating
baths having varied compositions. In the deposition of the
Sn-Pb alloy layers having the compositions 99.5Sn-0.5Pb -
82Sn-l8.Pb (electrodes g1 - g5), a 90 wt.o Sn-10 wt.o Pb
alloy (94 atomic o Sn-6 atomic o Pb alloy) is used as an
anode. In the deposition of the Sn-Pb alloy layers having
the compositions 78Sn-22Pb - 62Sn-38Pb (electrodes g6 - g8),
a 60 wt.o Sn-40 wt.o Pb alloy (74 atomic o Sn-26 atomic o Pb
alloy) is used as an anode. In the deposition of the Sn-Zn
alloy layer having the composition 85Sn-l4Zn (electrode g9),
an Sn metal is used as an anode. In this case, plating is
carried out while providing the plating bath with a fresh
supply of Zn (organic acid zinc) that decreases during
37
CA 02556201 2001-04-24
formation of a deposit film.
As also shown in Table 8, the electrode c1 fabricated
in Experiment 2 includes the Sn-Pb alloy layer of 90Sn-lOPb.
The Sn-Pb alloy used as an anode in Experiment 2 has the
composition of 90 wt.° Sn-10 wt.o Pb.
Using the electrodes g1 - g9 and c1, a charge-
discharge cycle test was performed to determine their cycle
efficiencies. The results are shown in Table 8.
TABLE 8
Electrode Composition (WT.o) Cycle Efficiency (o)
g1 99.5Sn - 0.5Pb 83
g2 99Sn - 1Pb 87
g3 98Sn - 2Pb 88
g4 95Sn - 5Pb 94
g5 82Sn - l8Pb 90
g6 78Sn - 22Pb 80
g7 68Sn - 32Pb 72
g8 62Sn - 38Pb 62
g9 86Sn - l4Zn 95
c1 90Sn - lOPb 88
As apparent from Table 8, the electrodes g1 - g9 and
c1 fabricated via a sequence of deposition of an Sn-Pb or
Sn-Zn alloy layer and subjection to a heat treatment all
exhibit satisfactory cycle characteristics. Although Sn, Pb
and Zn are metals that alloy with Li, neither Sn and Pb nor
Sn and Zn enter into an intermetallic compound with each
other.
38
CA 02556201 2001-04-24
In the above Examples, the Sn or Sn alloy layer is
deposited on the Cu substrate by an electrolytic plating
process. However, they may be deposited by an electroless
plating process. Other thin film-forming processes, such as
sputtering, vapor evaporation and spraying, can also be
utilized to deposit the Sn or Sn alloy layer.
UTILITY IN INDUSTRY
In accordance with the present invention, a
rechargeable lithium battery can be provided which has a
high discharge capacity and exhibits excellent cycle
characteristics.
39
