Title: Lithium secondary battery and positive electrode for lithium secondary battery
Abstract: A lithium secondary battery comprises a negative electrode containing lithium metal or a material previously storing lithium as an active material, a positive electrode containing a positive active material, and an electrolyte containing a non-aqueous electrolyte solution. The positive active material is a thin film formed by depositing on a substrate from vapor phase or liquid phase and including an oxide containing at least iron as a main constituent by a sputtering method, a reactive deposition method, a vacuum deposition method, a chemical vapor deposition method, a spraying method, a plating method, or a method in combination of these methods.
Patent Number: 6,979,516 Issued on 12/27/2005 to Kusumoto, et al.
| Inventors:
|
Kusumoto; Yasuyuki (Moriguchi, JP);
Fujimoto; Masahisa (Osaka, JP);
Fujitani; Shin (Hirakata, JP);
Domoto; Yoichi (Ikoma, JP);
Jito; Daizo (Hirakata, JP);
Tarui; Hisaki (Shijyonawate, JP)
|
| Assignee:
|
Sanyo Electric Co., Ltd. (Osaka, JP)
|
| Appl. No.:
|
056209 |
| Filed:
|
January 28, 2002 |
Foreign Application Priority Data
| Jan 29, 2001[JP] | 2001-019978 |
| Jun 27, 2001[JP] | 2001-194983 |
| Current U.S. Class: |
429/221; 204/192.15 |
| Intern'l Class: |
H01M 004/36; H01M 004/52; H01M 004/58; C23C 014/00; C23C 014/32 |
| Field of Search: |
429/221
204/192.15
|
References Cited [Referenced By]
U.S. Patent Documents
Other References
Abraham, K.M. et al.; "Preparation and Characterization of Some Lithium Insertion
Anodes for Secondary Lithium Batteries"; J. Electrochem. Soc.; vol. 137,
No. 3, pp. 743-749; 1990.
Morzilli, S. et al.; "Iron Oxide Electrodes in Lithium Organic Electrolyte Rechargeable
Batteries"; Electrochimica Acta; vol. 30, No. 10, pp. 1271-1276; 1985.
Pernet, M. et al.; "Structural and Electrochemical Study of Lithium Insertion
into y-Fe2O3"; Solid State Ionics; vol. 66, pp. 259-265; 1993.
Chen, C.J. et al.; "Lithium Insertion into Spinel Ferrites"; Solid State Ioinics;
vol. 18 & 19, pp. 838-846; 1986.
Islam, M.S. et al.; "Lithium Insertion into Fe3O4"; Journal
of Solid State Chemistry; vol. 77, pp. 180-189; 1988.
Ito, S. et al.; "K+ -β-Ferrite as a New Cathode Active Material
for Lithium Secondary Battery"; J. Phys. IV France; vol. 7, pp. C1-161—C1-162; 1997.
Ito, S. et al.; "Lithium Secondary Battery Using Potassium-β-Ferrite as
a New Cathode Active Material"; Solid State Ionics; vol. 113-115, pp. 17-21; 1998.
Ito, S. et al.; "Preparation and Properties of Lithium Inserted K+-β-Ferrite";
Solid State Ionics; vol. 113-115, pp. 23-27; 1998.
Zotti, G. et al.; "Electrodeposition of Amorphous Fe2O3
Films by Reduction of Iron Perchlorate in Acetonitrile" J. Electrochem. Soc.;
vol. 145, No. 2, pp. 385-389; 1998.
Sarradin, J. et al.; "Study of Fe2O3-based thin film electrodes
for lithium-ion batteries"; Solid State Ionics; vol. 112, pp. 35-40; 1998.
Ribes, M. et al.; "Thin films on amorphous electrode materials for Li Microbatteries";
Proceedings of the Symposium on Thin Film Solid Ionic Devices and Materials,
Electrochemical Society Proceedings, vol. 95-22, pp. 164-172; 1996.
|
Primary Examiner: Ryan; Patrick Joseph
Assistant Examiner: Mercado; Julian
Attorney, Agent or Firm: Kubovcik & Kubovcik
Claims
1. A lithium secondary battery comprising a negative electrode containing lithium
metal or a material previously storing lithium as an active material, a positive
electrode containing a positive active material, and an electrolyte containing
a non-aqueous electrolyte solution, wherein said positive active material is a
thin film formed by depositing on a substrate from vapor phase and including an
oxide containing at least iron as a main constituent and wherein said substrate
constituent and a constituent of said thin film are mutually diffused in the interface
of said substrate and said thin film.
2. The lithium secondary battery according to claim 1, wherein said thin film
contains a crystal of Fe
2O
3 or Fe
3O
4.
3. The lithium secondary battery according to claim 1, wherein said thin film
has columnar structure extended substantially in the vertical direction to the substrate.
4. The lithium secondary battery according to claim 1, wherein said film-forming
method is a sputtering method, a reactive deposition method, a vacuum deposition
method, a chemical vapor deposition method, or a method in combination of these methods.
5. The lithium secondary battery according to claim 1, wherein said oxide containing
iron contains potassium.
6. The lithium secondary battery according to claim 5, wherein the concentration
of said potassium is gradually decreased as closer to the surface from the substrate
in the thickness direction.
7. The lithium secondary battery according to claim 5, wherein said oxide containing
iron and potassium further contains carbon.
8. The lithium secondary battery according to claim 1, wherein said substrate
has electron conductivity.
9. The lithium secondary battery according to claim 8, wherein said substrate
is made of a metal or an alloy.
10. The lithium secondary battery according to claim 9, wherein said substrate
is made of aluminum or an aluminum alloy.
11. A positive electrode for a lithium secondary battery formed by depositing
a thin film of an active material on a current collector from vapor phase, wherein
said thin film includes an oxide containing at least iron as a main constituent
and wherein said substrate constituent and a constituent of said thin film are
mutually diffused in the interface of said substrate and said thin film.
12. The positive electrode for the lithium secondary battery according to claim
11, wherein said thin film contains a crystal of Fe
2O
3 or Fe
3O
4.
13. The positive electrode for the lithium secondary battery according to claim
11, wherein said thin film has columnar structure extended approximately in the
vertical direction to said substrate.
14. The positive electrode for the lithium secondary battery according to claim
11, wherein said oxide containing iron contains potassium.
15. The positive electrode for the lithium secondary battery according to claim
14, wherein the concentration of said potassium is gradually decreased as closer
to the surface from the substrate in the thickness direction.
16. The positive electrode for the lithium secondary battery according to claim
14, wherein said oxide containing iron and potassium further contains carbon.
17. The positive electrode for the lithium secondary battery according to claim
11, wherein said film-forming method is a sputtering method, a reactive deposition
method, a vacuum deposition method, a chemical vapor deposition method, or a method
in combination of these methods.
Description
BACKGROUND OF THE INVENTION
1. Field of the Invention
The present invention relates to a lithium secondary battery and a positive electrode
of the lithium secondary battery.
2. Related Art
In recent years, a lithium secondary battery referred to as a lithium ion battery
has widely been used for power sources of cellular phones and personal computers.
Generally, a lithium secondary battery which has recently practically been used
has a weight energy density of about 150 Wh/kg and is required to further densify
weight energy density.
The lithium secondary battery in practical use employs a carbon-based material
such as graphite for the negative electrode, a lithium-containing oxide such as
LiCoO
2 for the positive electrode, and an organic solvent, for example,
cyclic carbonate such as ethylene carbonate, a chain carbonate such as dimethyl
carbonate and the like, in which an electrolyte salt such as LiPF
6 is
dissolved, for the electrolyte solution. In such kind of lithium secondary battery,
since lithium ion moves between the positive electrode and the negative electrode
during charge and discharge, the energy density is determined depending on the
specific capacity of positive electrode, the specific capacity of negative electrode,
and the battery voltage.
The actual specific capacity of the carbon-based material to be employed for
a negative electrode is 370 mAh/g in the case of the graphite which has the highest
specific capacity and the actual specific capacity of LiCoO
2, which
is generally employed for a positive electrode, is about 150 mAh/g. As described
above, comparing actual specific capacity between the positive electrode and the
negative electrode, the capacity of the negative electrode is at least two times
as high as that of the positive electrode and it is found effective to increase
the actual specific capacity of the positive electrode rather than that of the
negative electrode in order to increase the weight energy density of the battery.
However, a lithium-containing oxide such as LiCoO
2 is known as
a material whose crystal structure is broken and whose charge-discharge cycle characteristics
are considerably deteriorated if Li is completely pulled out. Consequently, it
is difficult to increase the actual specific capacity of the positive electrode
using such as Li-containing oxide. Further, since cobalt reserves are in small
amounts and thus cobalt is expensive, a positive electrode material in place of
that is required.
SUMMARY OF THE INVENTION
An object of the present invention is to provide a lithium secondary battery
and
a positive electrode for a lithium secondary battery having a high discharge capacity
and excellent in charge-discharge cycle characteristics.
The lithium secondary battery of the present invention is a lithium secondary
battery provided with a negative electrode containing lithium metal or a material
previously storing lithium as an active material, a positive electrode containing
a positive active material, and an electrolyte containing a non-aqueous electrolyte
solution, wherein the positive active material is a thin film formed by depositing
on a substrate from vapor phase or liquid and including an oxide containing at
least iron as a main constituent.
The oxide containing iron is exemplified by Fe
2O
3, Fe
3O
4
and the like and the thin film preferably contains a crystal thereof. Further,
as the thin film, those having columnar structure extended substantially in the
vertical direction to a substrate are preferable in terms of improvement of the
charge-discharge cycle characteristics. Although the detailed reasons are not made
clear, it is supposed that due to the columnar structure, the expansion and contraction
of the positive active material during charge and discharge take place in the thickness
direction and as a result, even if the charge-discharge cycles are repeated, the
structure of the entire thin film becomes difficult to be broken and decrease of
the capacity becomes slight.
The oxide containing iron may contain other elements. For example, the oxide
containing iron may contain potassium. Examples of such compound containing iron
and potassium are K
1.4Fe
11O
17 and the like. These
ferrite materials are the materials conventionally studied as a positive electrode
material of a lithium secondary battery. Conventionally, these materials have been
used for manufacturing an electrode by mixing the material in powdery state with
a binder and a conductive material and molding or applying the resulting mixture
on a current collector.
In the present invention, the thin film including the above-described oxide containing
at least iron as a main constituent is formed by depositing on a substrate from
vapor phase or liquid phase. Such a film-forming method is exemplified by a sputtering
method, a reactive deposition method, a vacuum deposition method, a chemical vapor
deposition (CVD) method, a spraying method, a plating method, and a method in combination
of these methods.
In the present invention, the oxide containing at least iron to be employed as
an active material may contain potassium as described above. Further, potassium
may have a concentration distributed in the thickness direction and the distribution
is preferable to be gradually decreased toward the thin film surface from the interface
of the substrate and the thin film. It is supposed that insertion and extraction
reaction of lithium ion into and from the iron oxide is affected by the contained
potassium and probably the reacting weight of lithium is decreased, so that the
amounts of expansion and contraction upon the reaction is decreased. As a result,
the crystal structure of the oxide is supposed to become difficult to be broken.
Further, if potassium is distributed in the above-describe manner in the thickness
direction, the thin film structure becomes stable near the substrate, so that the
adhesion to the substrate is increased and the structure of the entire thin film
becomes harder to be broken even if the charge-discharge cycles are repeated. Consequently,
the decrease of the capacity can be suppressed and charge-discharge cycle characteristics
are stable.
Further, the oxide containing iron and potassium may further contain carbon.
The contained carbon improves the conductivity of the iron oxide thin film which
is generally an insulator, so that lithium ion becomes easy to move. From such
a point of view, it is assumed that the same effect is achieved by containing a
conductive element or compound.
In the present invention, it is preferable to employ a substrate having electron
conductivity as a substrate. Employment of the substrate having electron conductivity
makes the substrate possible to function as a current collector. The substrate
is preferably made of a metal or an alloy, further preferably an aluminum or an
aluminum alloy. In the case of using the substrate as a current collector, its
thickness is preferably thin and thus a foil made of a metal or an alloy is preferably
used as the substrate.
Further, in the interface of the above-described substrate and the above-described
iron oxide thin film, it is preferable for the substrate constituent and the thin
film constituent to be diffused each other. Due to the diffusion of the substrate
constituent and the thin film constituent, the adhesion between the thin film and
the substrate is improved and an effect of suppression of the reaction of the iron
oxide and lithium ion is supposed to be caused similarly to the above-described
effect of containing potassium.
Examples of a method for forming an iron oxide thin film of the present
invention are, as described above, various vacuum processes, a spraying method,
a plating method, and a method in combination with these method. Especially, a
sputtering method, a reactive deposition method, a vacuum deposition method, and
a chemical vapor deposition (CVD) method are preferable. That is because the mutual
diffusion of the substrate constituent and the thin film constituent is increased
depending on the effect of the temperature at the time of forming thin film and
because the thin film becomes easy to grow in columnar structure. Further, it makes
easy to control the composition, for example, in the thickness direction by controlling
the film forming conditions. Further, it also makes possible to form a mixed layer
(a mutually diffused region) in the interface between the substrate and the thin
film, control the crystallinity (the orientation) of the thin film, and control
the growth direction (the growth promotion in the columnar direction) by forming
under the conditions where ions are radiated, for example, to the thin film growth
surface. As a practical method for radiating ions, there are a method for radiating
ion beam of such as Ar and oxygen while the deposition materials reach the substrate
surface, and a method for attracting ions such as Ar and oxygen by applying practically
negative voltage to the substrate to collide the ion against the substrate.
The positive electrode for a lithium secondary battery of the present invention
is a positive electrode for a lithium secondary battery formed by depositing a
thin film of the active material on a current collector from vapor phase or liquid
phase and characterized in that the thin film of the active material contains an
oxide containing at least iron as a main constituent.
The positive electrode for a lithium secondary battery of the present invention
is similar to the positive electrode to be employed for the above-described lithium
secondary battery of the present invention.
The negative electrode to be employed for the lithium secondary battery of the
present invention is not particularly restricted if it can be employed as a negative
electrode of a lithium secondary battery and it contains lithium metal or a material
previously storing lithium as an active material. Examples of the material previously
storing lithium are carbon-based materials previously storing lithium and alloys
previously storing lithium. Examples of the alloys are alloys of lithium with silicon,
aluminum, tin, germanium, indium, or magnesium.
The electrolyte to be employed for the lithium secondary battery of the present
invention is an electrolyte containing a non-aqueous electrolyte solution. The
solvent of the non-aqueous electrolyte solution is not particularly restricted
and examples thereof are mixed solvents of cyclic carbonates such as ethylene carbonate,
propylene carbonate, and butylene carbonate and chain carbonates such as dimethyl
carbonate, methyl ethyl carbonate, and diethyl carbonate. Further, examples include
mixed solvents of the above-described cyclic carbonates with ether type solvents
such as 1,2-dimethoxyethane, 1,2-diethoxyethane and the like. Further, examples
of the solute of the non-aqueous electrolyte solution are LiPF
6, LiBF
4,
LiCF
3SO
3, LiN(CF
3SO
2)
2,
LiN(C
2F
5SO
2)
2, LiN(CF
3SO
2)(C
4F
9SO
2),
LiC(CF
3SO
2)
3, LiC(C
2F
5SO
2)
3
or the like and their mixtures. Further, a gel polymer electrolyte may be employed
as the electrolyte, which is prepared by impregnating a polymer electrolyte such
as polyethylene oxide, polyacrylonitrile, and polyfluorovinylidene with an electrolyte solution.
BRIEF DESCRIPTION OF THE DRAWINGS
FIG. 1 is a graph showing the relation between the charge-discharge cycle and
the discharge capacity in the examples of the present invention.
FIG. 2 is a graph showing the relation between the charge-discharge cycle and
the discharge capacity in the examples of the invention.
FIG. 3 is a scanning electron microscopic photograph (30,000 magnifications)
of a cross-section of a thin film of an example of the present invention.
FIG. 4 is a scanning electron microscopic photograph (30,000 magnifications)
of a cross-section of a thin film of an example of the present invention.
FIG. 5 is a scanning electron microscopic photograph (30,000 magnifications)
of a cross-section of a thin film of an example of the present invention.
FIG. 6 is a scanning electron microscopic photograph (30,000 magnifications)
of a surface of a thin film of an example of the present invention.
FIG. 7 is a scanning electron microscopic photograph (30,000 magnifications)
of a surface of a thin film of an example of the present invention.
FIG. 8 is a scanning electron microscopic photograph (30,000 magnifications)
of a surface of a thin film of an example of the present invention.
FIG. 9 is an x-ray diffraction chart of a thin film of an example of the present invention.
FIG. 10 is an x-ray diffraction chart of a thin film of an example of the present invention.
FIG. 11 is an x-ray diffraction chart of a thin film of an example of the present invention.
FIG. 12 is an x-ray diffraction chart of an iron oxide powder of a comparative example.
FIG. 13 is an x-ray diffraction chart of an iron oxide powder of a comparative example.
FIG. 14 is an x-ray diffraction chart of an iron oxide powder of a comparative example.
FIG. 15 shows the SIMS measurement results of a thin film of an example of the
present invention.
FIG. 16 shows the SIMS measurement results of a thin film of an example of the
present invention.
FIG. 17 shows the SIMS measurement results of a thin film of an example of the
present invention.
DESCRIPTION OF PREFERRED EXAMPLES
Hereinafter, the present invention will be described in details based
on examples, however, the invention is not restricted to the following examples
at all and any modifications or proper embodiments are possible within the scope
of the present invention.
(Experiment 1)
[Production of Positive Electrode]
A thin film was formed on an aluminum foil (the thickness of 20 μm) using
a target of K
1.33Fe
11O
17 by RF sputtering method.
The film forming conditions were shown in Table 1.
As the target shown in Table 1, those with a diameter of 10.2 cm (4 inch) and
a thickness of 5 mm were employed. As the carbon chips put on the targets, those
with a diameter of 10 mm and a thickness of 1 mm, and as the iron chips, those
with 10 mm square and a thickness of 1 mm were employed in the number shown in
Table 1, respectively. As the carbon chips, pellets made from powdery graphite
were employed. As the iron chips, iron plates with the above-described shapes were employed.
| |
TABLE 1 |
| |
|
| |
loaded |
sputtering |
sputtering |
| |
electric |
gas flow |
gas flow |
| |
target |
chip |
power |
rate Ar |
rate O2 |
| |
|
| Ex. 1 |
K1.33Fe11O17 |
none |
350 W |
100 sccm |
0 sccm |
| Ex. 2 |
K1.33Fe11O17 |
3 carbon |
350 W |
100 sccm |
0 sccm |
| |
|
chips |
| Ex. 3 |
K1.33Fe11O17 |
8 carbon |
350 W |
100 sccm |
5 sccm |
| |
|
chips |
| Ex. 4 |
K1.33Fe11O17 |
8 iron |
350 W |
100 sccm |
5 sccm |
| |
|
chips |
| |
The resulting aluminum foils on which the above-described thin films were formed
were cut in 20 mm square to obtain electrodes of Examples 1 to 4.
As an electrode of Comparative Example 1, a pellet electrode was produced from
powder of K
1.4Fe
11O
17. Specifically, 40 parts
by weight of K
1.4Fe
11O
17 powder, 40 parts by weight
of acetylene black as a conductive agent, and 20 parts by weight of polytetrafluoroethylene
as a binder were mixed, the mixture was pressured to form into a shape with a diameter
of 16 mm and a thickness of 0.1 mm, and then vacuum drying was conducted at 110°
C. to produce the pellet electrode.
[Preparation of Electrolyte Solution]
To a mixed solvent of ethylene carbonate (EC) and dimethyl carbonate (DMC) by
a volume ratio of 1:1, LiPF
6 was dissolved by a ratio of 1.0 mole/L
to produce an electrolyte solution.
[Production of Beaker Cell]
The above-described positive electrodes were employed as work electrodes, those
produced by molding lithium metal were employed as counter electrodes and reference
electrodes, and the above-described electrolyte solution was employed as an electrolyte,
to produce beaker cells.
[Charge-Discharge Cycle Test]
The charge-discharge cycle test was carried out in the following conditions for
each beaker cell of the above-described Examples 1 to 4 and the Comparative Example 1.
(1) Charge Discharge Conditions of the Examples 1 to 4
The discharge current was set as follows: 2.0 mA at the first cycle; 1.0 mA at
the second cycle; and 0.5 mA at the third cycle and the following cycles. The discharge
end voltage was set 0.5 V (the potential of the work electrode relative to the
reference electrode).
The charge current was set as follows: 2.0 mA at the first cycle; 1.0 mA at the
second cycle; and 0.5 mA at the third cycle and the following cycles. The charge
end voltage was set 4.0 V (the potential of the work electrode relative to the
reference electrode).
(2) Charge-discharge Conditions of the Comparative Example 1
The discharge current was set to be 2.0 mA and the discharge end voltage was
set to be 0.5 V (the potential of the work electrode relative to the reference electrode)
The charge current was set to be 2.0 mA and the charge end voltage was set to
be 4.0 V (the potential of the work electrode relative to the reference electrode).
FIG. 1 shows the relation of the number of cycles and the discharge capacity
of Examples 1 to 4 and Comparative Example 1. As apparent from FIG. 1, in Examples
1 to 4 in which the electrodes were produced by a sputtering method, decrease of
the capacity was slight even if the charge-discharge cycles were repeated. On the
other hand, in Comparative Example 1 in which the electrode was produced from the
powder, it is found that the discharge capacity was greatly decreased due to the
repetition of the charge-discharge cycles.
Table 2 shows the discharge capacity of the third cycle and the 11th cycle
and the capacity retention rate of the 11th cycle. The capacity retention rate
of the 11th cycle were the values calculated from the following formula:
| |
TABLE 2 |
| |
|
| |
discharge |
discharge |
capacity |
| |
capacity of third |
capacity of 11th |
retention rate |
| |
cycle (mAh/g) |
cycle (mAh/g) |
of 11th cycle |
| |
|
| |
| Ex. 1 |
547.4 |
463.5 |
84.7% |
| Ex. 2 |
460.0 |
457.6 |
99.5% |
| Ex. 3 |
417.9 |
409.4 |
98.0% |
| Ex. 4 |
318.5 |
342.2 |
107.4% |
| Comp. |
578.6 |
320.2 |
55.3% |
| Ex. 1 |
| |
As apparent from Table 2, the capacity retention rate of 11th cycle became high
for Examples 2 and 3 in which the electrodes were produced by mounting a carbon
chip or an iron chip on the targets, as compared with that for Example 1 in which
the electrode was produced by mounting no chip on the target.
(Experiment 2)
[Production of Positive Electrode]
A thin film (a ferrite thin film) constituted of Fe and O was produced on an
aluminum
foil (the thickness of 20 μm) by a reactive deposition method. In the reactive
deposition method, O
2 was introduced into a vacuum chamber and the Fe
deposition material was melted and evaporated by an electron beam (EB) gun to form
the ferrite thin film on the aluminum foil, i.e. a substrate (Examples 5-7).
Further, also by an ion-assisting reactive deposition method, in which simultaneously
with the deposition of Fe, ion beam was radiated toward the aluminum foil, an iron
oxide (ferrite) thin film was formed (Examples 8 and 9).
Table 3 shows the respective film forming conditions.
| |
TABLE 3 |
| |
|
| |
film- |
deposition |
reactive |
ion beam |
| |
forming |
deposition |
speed |
gas O2 |
radiation |
| |
method |
material |
(nm/sec.) |
flow rate |
conditions |
| |
|
| Ex. 5 |
reactive |
Fe |
0.25-0.40 |
10 sccm |
no radiation |
| Ex. 6 |
deposition |
|
0.20-0.30 |
20 sccm |
| Ex. 7 |
method |
|
0.30-0.40 |
30 sccm |
| Ex. 8 |
ion beam- |
|
0.40-0.50 |
30 sccm |
Ar 4 sccm |
| |
assisting |
|
|
|
ion |
| |
reactive |
|
|
|
accelerating |
| |
deposition |
|
|
|
voltage |
| |
method |
|
|
|
200 V |
| |
|
|
|
|
ion current |
| |
|
|
|
|
100 mA |
| Ex. 9 |
|
|
0.50-0.60 |
26 sccm |
Ar 4 sccm |
| |
|
|
|
|
O2 4 sccm |
| |
|
|
|
|
ion |
| |
|
|
|
|
accelerating |
| |
|
|
|
|
voltage |
| |
|
|
|
|
200 V |
| |
|
|
|
|
ion current |
| |
|
|
|
|
100 mA |
| |
The resulting aluminum foils on which the above-described thin films were formed
were cut in 20 mm square to obtain electrodes of Examples 5 to 9.
As an electrode of Comparative Example 2, a pellet electrode was produced from
Fe
2O
3 powder. Specifically, 40 parts by weight of Fe
2O
3
powder, 40 parts by weight of acetylene black as a conductive agent, and
20 parts by weight of polytetrafluoroethylene as a binder were mixed, the mixture
was pressured to form into a shape with a diameter of 16 mm and a thickness of
0.1 mm, and then vacuum drying at 110° C. was conducted to produce the pellet electrode.
[Preparation of Electrolyte Solution]
To a mixed solvent of ethylene carbonate (EC) and dimethyl carbonate (DMC) by
a volume ratio of 1:1, LiPF
6 was dissolved by a ratio of 1.0 mole/L
to produce an electrolyte solution.
[Production of Beaker Cell]
The above-described positive electrodes were employed as work electrodes, those
molded by forming lithium metal were employed as counter electrodes and reference
electrodes, and the above-described electrolyte solution was used as an electrolyte,
to produce beaker cells.
[Charge-Discharge Cycle Test]
The charge-discharge cycle test was carried out in the following conditions for
each beaker cell of the above-described Examples 5 to 9 and the Comparative Example 2.
(1) Charge-discharge Conditions of the Examples 5 to 9
The discharge current was set to be 0.5 mA and the discharge end voltage was
set to be 0.5 V (the potential of the work electrode relative to the reference electrode)
The charge current was set to be 0.5 mA and the charge end voltage was set to
be 3.0 V (the potential of the work electrode relative to the reference electrode).
(2) Charge-discharge Conditions of Comparative Example 2
The discharge current was set to be 2.0 mA and the discharge end voltage was
set to be 0.5 V (the potential of the work electrode relative to the reference electrode).
The charge current was set to be 2.0 mA and the charge end voltage was set to
be 4.0 V (the potential of the work electrode relative to the reference electrode).
FIG. 2 shows the relation of the number of charge-discharge cycles and the discharge
capacity of Examples 6 to 9 and Comparative Example 2. As apparent from FIG. 2,
in Examples 6 to 9 in which the electrodes were produced by a reactive deposition
method, decrease of the capacity was slight even if the charge-discharge cycles
were repeated. On the other hand, in Comparative Example 2 in which the electrode
was produced from the powder, the discharge capacity was greatly decreased due
to the repetition of the charge-discharge cycles.
Table 4 shows the discharge capacity of the second cycle and the fifth cycle
and the capacity retention rate of fifth cycle of Examples 5 to 9 and Comparative
Example 2. The capacity retention rate of the fifth cycle was the values calculated
from the following formula:
| |
TABLE 4 |
| |
|
| |
discharge capacity |
discharge capacity |
capacity |
| |
of second cycle |
of fifth cycle |
retention rate |
| |
(mAh/g) |
(mAh/g) |
of fifth cycle |
| |
|
| |
| Ex. 5 |
189.2 |
78.2 |
41.3% |
| Ex. 6 |
276.8 |
403.0 |
145.6% |
| Ex. 7 |
266.4 |
308.3 |
115.7% |
| Ex. 8 |
1071.4 |
425.0 |
39.7% |
| Ex. 9 |
1050.3 |
444.8 |
42.3% |
| Comp. |
1486.5 |
280.6 |
18.9% |
| Ex. 2 |
| |
As apparent from Table 4, the batteries of Examples 5 to 9 show excellent charge-discharge
cycle characteristics as compared with the battery of Comparative Example 2.
Next, the thin films produced in the experiments 1 and 2 were observed with
a scanning electron microscope (SEM). The SEM photographs of the cross-sectional
views of the thin films produced in Examples 1, 7 and 9 were shown in FIG. 3 (Example
1), FIG. 4 (Example 7), and FIG. 5 (Example 9), respectively. The magnification
was 30,000×. In the cross-sectional views, the structure in which a crystal
was grown substantially in the thickness direction was observed, and it is found
that the thin films have columnar structure. Further, it is found that the thin
film formed by the sputtering method (Example 1: FIG. 3) was uniform in the vicinity
of the interface with the substrate and has the columnar structure only in the
vicinity of the surface.
Further, SEM images of the surface of the thin films produced in Examples
1, 7 and 9 were also shown in FIG. 6 (Example 1), FIG. 7 (Example 7) and FIG. 8
(Example 9), respectively. The magnification was 30,000×. It is found that
there is fine irregularity on the surface. Further, in the surface of the films
(Example 7: FIG. 7, Example 9: FIG. 8) formed by the reactive deposition method,
grain boundaries corresponding to the respective columnar structures observed in
the cross-sectional views were clearly observed.
Next, the crystallinity of the respective thin films was evaluated by x-ray
diffraction. The x-ray diffraction results of thin films formed on silicon (Si)
wafers under the same conditions as those of Examples 1, 7 and 9 were shown in
FIG. 9 (Example 1), FIG. 10 (Example 7) and FIG. 11 (Example 9). The x-ray diffraction
results of the K
1.4Fe
11O
17 powder (FIG. 12), the
Fe
2O
3 powder (FIG. 13) and Fe
3O
4 (FIG.
14) employed in Comparative Examples 1 and 2 were also shown together.
In comparison of these figures, although amorphous constituents were also outstanding
in FIG. 9 and FIG. 10, some diffraction peaks were observed to make it clear the
films were crystalline films. Further, from the positions of the peaks, the thin
films of Examples 1, 7 and 9 were supposed to be thin films made of Fe
2O
3
or Fe
3O
4. However, since the peak positions of Fe
2O
3
and Fe
3O
4 were extremely close to each other, they
could not be distinguished between them. Further, from the result (FIG. 9) of the
film produced by sputtering using the potassium-containing target, no peak of KO,
KFeO and the like was observed.
Further, in order to carry out composition evaluation of the respective
thin films, SIMS analysis was carried out. The results of thin films produced in
Examples 1, 7 and 9 were shown in FIG. 15 (Example 1), FIG. 16 (Example 7) and
FIG. 17 (Example 9), respectively. From the results, it was found that the iron
and oxygen concentrations in the respective thin films were approximately constant.
In Example 1 (FIG. 15) in which the film is produced by sputtering using the potassium-containing
target, it is found that potassium exists in the thin film. Further, it is found
that the distribution of potassium is gradually decreasing from the interface toward
the surface. In addition, in Example 9 (FIG. 17), the film formed by the reactive
deposition while oxygen ion beam being radiated, has a widened distribution of
aluminum (the substrate constituent) in the vicinity of the interface, as compared
with the thin film (FIG. 16) of Example 7 formed without ion beam radiation, and
consequently it is found that ion radiation promotes the mutual diffusion.
In the above-described examples, the sputtering method and the reactive deposition
method were described as the film-forming methods, however even in the case where
a positive electrode is produced by forming a thin film of an iron oxide by another
film-forming method, e.g. a vacuum deposition method, a chemical vapor deposition
method, a spraying method, a plating method and the like, it is possible to produce
a lithium secondary battery similarly excellent in charge-discharge cycle characteristics.
According to the present invention, a lithium secondary battery with a
high discharge capacity and excellent in charge-discharge cycle characteristics
can be provided.
*
