U.S. patent application number 13/597399 was filed with the patent office on 2012-12-20 for method for producing porous metal body, porous aluminum body, battery electrode material including porous metal body or porous aluminum body, and electrode material for electrical double layer capacitor.
This patent application is currently assigned to SUMITOMO ELECTRIC INDUSTRIES, LTD.. Invention is credited to Tomoyuki AWAZU, Kengo GOTO, Akihisa HOSOE, Shinji INAZAWA, Koutarou KIMURA, Masatoshi MAJIMA, Koji NITTA, Kazuki OKUNO, Hajime OTA, Shoichiro SAKAI.
Application Number | 20120321951 13/597399 |
Document ID | / |
Family ID | 44673017 |
Filed Date | 2012-12-20 |
United States Patent
Application |
20120321951 |
Kind Code |
A1 |
KIMURA; Koutarou ; et
al. |
December 20, 2012 |
METHOD FOR PRODUCING POROUS METAL BODY, POROUS ALUMINUM BODY,
BATTERY ELECTRODE MATERIAL INCLUDING POROUS METAL BODY OR POROUS
ALUMINUM BODY, AND ELECTRODE MATERIAL FOR ELECTRICAL DOUBLE LAYER
CAPACITOR
Abstract
A porous metal body containing continuous pores and having a low
oxygen content is provided by decomposing a porous resin body that
contains continuous pores and has a layer of a metal thereon by
heating the porous resin body at a temperature equal to or less
than the melting point of the metal while the porous resin body is
immersed in a first molten salt and a negative potential is applied
to the metal layer; and a method for producing the porous metal
body is provided.
Inventors: |
KIMURA; Koutarou; (Osaka,
JP) ; NITTA; Koji; (Osaka, JP) ; HOSOE;
Akihisa; (Osaka, JP) ; INAZAWA; Shinji;
(Osaka, JP) ; OKUNO; Kazuki; (Itami-shi, JP)
; MAJIMA; Masatoshi; (Osaka, JP) ; OTA;
Hajime; (Osaka, JP) ; SAKAI; Shoichiro;
(Osaka, JP) ; GOTO; Kengo; (Osaka, JP) ;
AWAZU; Tomoyuki; (Itami-shi, JP) |
Assignee: |
SUMITOMO ELECTRIC INDUSTRIES,
LTD.
|
Family ID: |
44673017 |
Appl. No.: |
13/597399 |
Filed: |
August 29, 2012 |
Related U.S. Patent Documents
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Application
Number |
Filing Date |
Patent Number |
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13236041 |
Sep 19, 2011 |
8277535 |
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13597399 |
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PCT/JP2011/056152 |
Mar 16, 2011 |
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13236041 |
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Current U.S.
Class: |
429/218.1 ;
205/78; 428/613 |
Current CPC
Class: |
H01G 11/86 20130101;
Y02E 60/13 20130101; H01M 4/661 20130101; H01M 4/80 20130101; H01M
4/131 20130101; H01G 11/70 20130101; Y10T 428/12479 20150115; H01G
11/44 20130101; H01G 11/50 20130101; H01G 11/28 20130101; H01G
11/46 20130101; Y02E 60/10 20130101 |
Class at
Publication: |
429/218.1 ;
428/613; 205/78 |
International
Class: |
B32B 5/18 20060101
B32B005/18; C25D 1/00 20060101 C25D001/00; B32B 15/00 20060101
B32B015/00; H01M 4/38 20060101 H01M004/38 |
Foreign Application Data
Date |
Code |
Application Number |
Mar 26, 2010 |
JP |
2010-072348 |
Dec 17, 2010 |
JP |
2010-281938 |
Claims
1-9. (canceled)
10. A porous aluminum body comprising continuous pores, wherein an
oxygen content of a surface of the porous aluminum body is 3.1 mass
% or less, the oxygen content being determined by energy dispersive
X-ray spectroscopy (EDX) at an accelerating voltage of 15 kV.
11. A battery electrode material comprising the porous metal body
produced by the production method according to claim 1 and an
active material held on the porous body.
12. A battery comprising the battery electrode material according
to claim 11 for one or both of a positive electrode and a negative
electrode.
13. An electrode material for an electrical double layer capacitor,
comprising the porous metal body produced by the production method
according to claim 1 and an electrode active material that contains
activated carbon as a main component and is held on the porous
body.
14. An electrical double layer capacitor comprising the electrode
material for an electrical double layer capacitor according to
claim 13.
15. A method for producing a porous metal body, comprising
decomposing a porous resin body that contains continuous pores and
has a metal layer thereon by immersing the porous resin body in
supercritical water.
Description
CROSS-REFERENCE TO RELATED APPLICATION
[0001] This application is a continuation of International
Application PCT/JP2011/056152 filed on Mar. 16, 2011, claiming
priority to Japanese patent applications No. 2010-072348 filed on
Mar. 26, 2010, and No. 2010-281938 filed on Dec. 17, 2010, the
entireties of which are incorporated herein by reference.
TECHNICAL FIELD
[0002] The present invention relates to a porous metal body that
can be suitably used as the collector of a battery electrode and an
electrode for an electrical double layer capacitor.
BACKGROUND ART
[0003] Aluminum has an excellent conductive property and is used as
an electrode material of a battery such as a lithium-ion battery.
For example, the positive electrode of a lithium-ion battery is
constituted by an aluminum foil to the surfaces of which an active
material such as lithium cobalt oxide is applied.
[0004] To increase the capacity of such a positive electrode, a
porous aluminum body can be used so that the surface area of the
positive electrode is increased and the aluminum body is filled
with an active material; in this case, the active material is
available even when the thickness of the electrode is large, and
hence the availability ratio of the active material per unit area
increases.
[0005] Such a porous aluminum includes an aluminum nonwoven fabric
formed by entanglement of fibrous aluminum and an aluminum foam
formed by foaming of aluminum. Patent Literature 1 discloses a
method for producing a metal foam containing a large number of
independent bubbles by adding a foaming agent and a thickening
agent to a molten metal and stirring the resultant mixture.
[0006] As a porous metal, there is a porous nickel body that is
commercially available under the trade name Celmet (registered
trademark). Celmet (registered trademark) is a porous metal body
that has continuous pores and has a high porosity (90% or more).
This is obtained by forming a nickel layer on the surface of the
skeleton of a porous resin body containing continuous pores such as
a urethane foam, subsequently decomposing the porous resin body by
a heat treatment, and subjecting the nickel to a reduction
treatment. The nickel layer is formed in the following manner: the
porous resin body is subjected to a conductive treatment by the
application of carbon powder or the like to the surface of the
skeleton of the porous resin body, and nickel is subsequently
deposited on the porous resin body by electroplating.
[0007] Patent Literature 2 discloses a method for producing a
porous metal body in which the production method of Celmet is
applied to aluminum. Specifically, a film of a metal (copper or the
like) that forms a eutectic alloy with aluminum at a temperature
equal to or less than the melting point of aluminum is formed on
the skeleton of a porous resin body having a three-dimensional
network structure; the porous resin body is subsequently coated
with aluminum paste; the resultant body is subjected to a heat
treatment at a temperature of 550.degree. C. or more and
750.degree. C. or less in a non-oxidizing atmosphere to evaporate
the organic constituent (porous resin body) and to sinter the
aluminum powder to thereby provide the porous metal body. Patent
Literature 2 states that, although aluminum forms a strong oxide
film and hence has sintering resistance, aluminum powder applied on
a film of a metal that forms a eutectic alloy with aluminum causes
a eutectic reaction at the surface boundary between the aluminum
powder and the metal film as a base in the process of a heat
treatment to produce liquidus surfaces at a temperature equal to or
less than the melting point of aluminum; and the partially produced
liquidus surfaces breach the oxide film of aluminum so that
sintering of the aluminum powder proceeds while the
three-dimensional network skeleton structure is maintained.
CITATION LIST
Patent Literature
[0008] PTL 1: Japanese Patent No. 4176975
[0009] PTL 2: Japanese Unexamined Patent Application Publication
No. 8-170126
SUMMARY OF INVENTION
Technical Problem
[0010] An aluminum nonwoven fabric and an aluminum foam tend to
have an oxide film thereon because aluminum is heated to a
temperature equal to or more than the melting point thereof in the
production process and oxidation tends to proceed until the
aluminum is cooled. Aluminum is susceptible to oxidation and it is
difficult to reduce oxidized aluminum at a temperature equal to or
less than the melting point. Accordingly, an aluminum nonwoven
fabric and an aluminum foam that have a low oxygen content are not
obtained. Although an aluminum foam containing independent bubbles
(closed bubbles) has a large surface area as a result of foaming,
effective use of the entire surface of the aluminum foam cannot be
achieved. Accordingly, when such an aluminum foam is used as a
battery electrode material (collector), it is difficult to increase
the use efficiency of the active material.
[0011] The porous metal body of PTL 2 contains continuous pores and
can be used as a battery electrode material. However, the resultant
porous metal body is not composed of elemental aluminum but
contains a metal element in addition to aluminum, and hence may
have poor properties in terms of corrosion resistance and the like.
The heat treatment needs to be performed at a temperature close to
the melting point of aluminum to sinter aluminum and an oxide film
may be formed on the surface of aluminum in spite of the
non-oxidizing atmosphere.
[0012] Even when a metal other than aluminum is used, for example,
in the production of a porous nickel body, the surface of nickel is
oxidized in the step of decomposing a porous resin body by a heat
treatment and hence a reduction treatment needs to be subsequently
performed.
[0013] Accordingly, an object of the present invention is to
provide a porous metal body that contains continuous pores and has
a small amount of an oxide in the surface thereof (the thickness of
an oxide layer is small), and a method for producing the porous
metal body.
Solution to Problem
[0014] The present invention provides a method for producing a
porous metal body, the method including a step of decomposing a
porous resin body that contains continuous pores and has a layer of
a metal thereon by heating the porous resin body at a temperature
equal to or less than the melting point of the metal while the
porous resin body is immersed in a first molten salt and a negative
potential is applied to the metal layer (the first invention of the
present application).
[0015] FIG. 1 is a schematic view illustrating a production method
according to the present invention. The part (a) of FIG. 1 is an
enlarged schematic view illustrating a portion of a section of a
porous resin body containing continuous pores and illustrates a
state in which the pores are formed in a porous resin body 1
serving as a skeleton. The porous resin body 1 containing
continuous pores is provided; a layer 2 of a metal such as aluminum
is formed on the surface of the porous resin body 1 to provide a
metal-coated porous resin body (part (b) of FIG. 1); and the porous
resin body 1 is subsequently decomposed and evaporated to thereby
provide a porous metal body 3 constituted by the remaining metal
layer (part (c) of FIG. 1).
[0016] The porous resin body is decomposed in a molten salt. As
illustrated in FIG. 2, a porous resin body 11 having a metal layer
thereon and a positive electrode 12 are immersed in a first molten
salt 13, and a negative potential is applied to the metal layer. By
applying a negative potential to the metal layer immersed in a
molten salt, the oxidation of the metal can be suppressed. In such
a state, by heating the porous resin body 11 having a metal layer
thereon to a temperature equal to or more than the decomposition
temperature of the porous resin body, the porous resin body is
decomposed to provide a porous metal body constituted by the
remaining metal. To prevent the metal from melting, the heating
temperature is a temperature equal to or less than the melting
point of the metal. When aluminum is selected as the metal, the
heating is performed at a temperature equal to or less than the
melting point (660.degree. C.) of aluminum. In this way, a porous
metal body having a thin oxide layer (low oxygen content) in the
surface thereof can be obtained.
[0017] The first molten salt may be a halide or a nitrate of an
alkaline metal or an alkaline earth metal with respect to which the
electrode potential of the metal layer is less-noble. Specifically,
the first molten salt preferably contains one or more selected from
the group consisting of lithium chloride (LiCl), potassium chloride
(KCl), sodium chloride (NaCl), aluminum chloride (AlCl.sub.3),
lithium nitrate (LiNO.sub.3), lithium nitrite (LiNO.sub.2),
potassium nitrate (KNO.sub.3), potassium nitrite (KNO.sub.2),
sodium nitrate (NaNO.sub.3), and sodium nitrite (NaNO.sub.2) (the
second invention of the present application). Since the temperature
of the molten salt is made to be a temperature equal to or less
than the melting point of the metal, the molten salt is preferably
a eutectic salt made to have a low melting point by mixing two or
more salts. Specifically, the heating is preferably performed at a
temperature of 380.degree. C. or more and 600.degree. C. or less
(the ninth invention of the present application). In particular,
such a method is advantageous when aluminum whose surface is
susceptible to oxidation and is less likely to be reduced is used
(the third invention of the present application).
[0018] In the step of decomposing the porous resin body, an
antioxidant measure that suppresses oxidation of the metal layer is
preferably provided (the fourth invention of the present
application). When the porous resin body 11 that is to be treated
and has a metal layer thereon is immersed in the first molten salt,
oxidation of the metal layer can be suppressed by the application
of a negative potential. However, the negative potential cannot be
applied immediately before the porous resin body 11 is immersed in
the molten salt. Since the first molten salt has a high temperature
as described above, a region near a first molten salt bath, for
example, an upper space of the molten salt bath has a
high-temperature atmosphere and hence the metal layer may be
oxidized immediately before the immersion in the first molten salt
or immediately after the immersion in the first molten salt. In
particular, when the porous resin body that is to be treated and
has a metal layer thereon has a large area, such a problem tends to
occur. Accordingly, an antioxidant measure is preferably
provided.
[0019] The antioxidant measure is preferably configured to make an
inert gas flow in the first molten salt (the fifth invention of the
present application). By bubbling the first molten salt by making
an inert gas flow in the first molten salt, the first molten salt
becomes full of bubbles of the inert gas that are generated in the
first molten salt; the bubbles rise from the liquid surface of the
first molten salt to the space above the first molten salt so that
the space above the first molten salt also becomes full of the
inert gas. Accordingly, the oxidation of the metal layer can be
suppressed before and after the immersion in the first molten salt.
In the step of decomposing the porous resin body in the first
molten salt, oxygen is generated from organic matter (porous resin
body) decomposed; when this oxygen remains in the porous metal
body, the metal may be oxidized. By making an inert gas flow in the
first molten salt to cause bubbles of the inert gas to bump against
a body to be treated (a porous resin body having a metal layer
thereon or a porous metal body), the generated oxygen is expelled.
In addition, by making an inert gas flow in the first molten salt,
the first molten salt is stirred with the bubbles of the inert gas
so that the first molten salt can be uniformly brought into contact
with the interior of the porous body and the porous resin body can
be efficiently decomposed.
[0020] The metal layer can be formed by a method, for example, a
gas phase method such as vapor deposition, sputtering, or plasma
chemical vacuum deposition (CVD); coating with a metal paste; or
plating. When aluminum is selected as the metal, plating with
aluminum in an aqueous solution is substantially impossible in
terms of practicality and hence molten salt electrolytic plating of
plating with aluminum in a molten salt is preferably performed. In
a preferred embodiment of this plating, after the surface of the
porous resin body is subjected to a conductive treatment, the
porous resin body is plated with the metal in a second molten salt
to form the metal layer (the sixth invention of the present
application). The second molten salt may be aluminum chloride,
potassium chloride, sodium chloride, or the like. When two or more
salt components are used as a eutectic salt, the melting
temperature becomes low, which is preferable. The second molten
salt needs to contain at least one metal-ion component to be made
to adhere.
[0021] Alternatively, the surface of the porous resin body may be
coated with a metal paste to form the metal layer (the seventh
invention of the present application). The metal paste is a mixture
of a metal powder, a binder (binder resin), and an organic solvent.
After the surface of the porous resin body is coated with the metal
paste, the resultant body is heated to evaporate the organic
solvent and the binder resin and to sinter the metal paste. The
heating may be performed by a single process or divided into
processes. For example, the following processes may be performed:
the porous resin body is coated with the metal paste and the
resultant body is then heated at a low temperature to evaporate the
organic solvent; and the resultant body is subsequently immersed in
the first molten salt and heated to decompose the body and sinter
the metal paste. By such processes, the metal layer can be readily
formed.
[0022] The material of the porous resin body can be selected from
resins that can be decomposed at a temperature equal to or less
than the melting point of the metal. Examples of the material of
the porous resin body include polyurethane, polypropylene, and
polyethylene. An urethane foam, which is a material that has a high
porosity and is susceptible to pyrolysis, is preferred as the
porous resin body (the eighth invention of the present
application). The porosity of the porous resin body is preferably
80% to 98%; and the pore size of the porous resin body is
preferably about 50 to 500 .mu.m.
[0023] An invention described in the tenth invention of the present
application is a porous aluminum body including continuous pores,
wherein an oxygen content of a surface of the porous aluminum body
is 3.1 mass % or less, the oxygen content being determined by
energy dispersive X-ray spectroscopy (EDX) at an accelerating
voltage of 15 kV. Since the porous aluminum body contains
continuous pores and has a small amount of oxide in the surface
thereof (thin oxide layer), when the porous aluminum body is used
as a battery electrode material or an electrode material for an
electrical double layer capacitor, the amount of an active material
held thereon can be made large and the contact resistance between
the active material and the porous aluminum body can be made low.
As a result, use efficiency of the active material can be
enhanced.
[0024] An invention described in the eleventh invention of the
present application is a battery electrode material including the
porous metal body produced by the production method or the porous
aluminum body; and an active material held on the porous body. FIG.
3 is an enlarged schematic view illustrating a section of a battery
electrode material. In a battery electrode material 5, an active
material 4 is held on the surface of an aluminum skeleton part
(aluminum layer) 2 of the porous aluminum body. The porous aluminum
body can be made to have a high porosity and a large surface area
and hence the amount of the active material held thereon can be
increased. In addition, even when the active material is applied to
a small thickness, a large amount of the active material can be
held. Thus, the distance between the active material and the
collector (porous aluminum body) can be shortened and hence use
efficiency of the active material can be enhanced.
[0025] An invention described in the twelfth invention of the
present application is a battery including the battery electrode
material for one or both of a positive electrode and a negative
electrode. By using the battery electrode material, a battery can
be made to have a high capacity.
[0026] An invention described in the thirteenth invention of the
present application is an electrode material for an electrical
double layer capacitor, the electrode material including the porous
metal body produced by the production method or the porous aluminum
body; and an electrode active material that contains activated
carbon as a main component and is held on the porous body. As in
the battery electrode material, in the electrode material for an
electrical double layer capacitor, an electrode active material is
held on the surface of an aluminum skeleton part (aluminum layer)
of the porous aluminum body. As in the battery electrode material,
the amount of the electrode active material held can be increased
and use efficiency of the electrode active material can be
enhanced.
[0027] An invention described in the fourteenth invention of the
present application is an electrical double layer capacitor
including the electrode material for an electrical double layer
capacitor. By using the electrode material for an electrical double
layer capacitor, a capacitor can be made to have a high output and
a high capacitance.
[0028] An invention described in the fifteenth invention of the
present application is a method for producing a porous metal body,
the method including decomposing a porous resin body that contains
continuous pores and has a metal layer thereon by immersing the
porous resin body in supercritical water. Supercritical water at a
high temperature and a high pressure beyond the critical point of
water (critical temperature: 374.degree. C., and critical pressure:
22.1 MPa) is excellent in the capability of degrading organic
matter and can decompose a porous resin body without oxidizing
metal. Use of the production method can provide a porous metal body
having a small amount of an oxide layer (with a small thickness) in
the surface thereof.
Advantageous Effects of Invention
[0029] According to the present invention, a porous metal body that
contains continuous pores and has a thin oxide layer (low oxygen
content) in the surface thereof can be obtained. Use of the porous
metal body can provide an electrode material in which use
efficiency of an active material is enhanced so that the capacity
of a battery can be increased. A battery including the electrode
material can be obtained.
BRIEF DESCRIPTION OF DRAWINGS
[0030] FIG. 1 is a schematic view illustrating steps of producing a
porous metal body: the part (a) of FIG. 1 illustrates a portion of
a section of a porous resin body containing continuous pores; the
part (b) of FIG. 1 illustrates a state in which a metal layer is
formed on the porous resin body; and the part (c) of FIG. 1
illustrates a porous metal body after evaporation of the porous
resin body.
[0031] FIG. 2 is a schematic explanatory view of a step of
decomposing a porous resin body in a molten salt.
[0032] FIG. 3 is an enlarged schematic view illustrating a portion
of a section of a battery electrode material.
[0033] FIG. 4 is a schematic view illustrating an example of a
molten salt battery according to the present invention.
[0034] FIG. 5 is a schematic view illustrating an example of an
electrical double layer capacitor according to the present
invention.
[0035] FIG. 6 is a scanning electron microscope (SEM) photograph of
a section of a porous aluminum body.
[0036] FIG. 7 illustrates an EDX result of a porous aluminum
body.
[0037] FIG. 8 is a schematic explanatory view of an apparatus for
decomposing a porous resin body in a molten salt.
DESCRIPTION OF EMBODIMENTS
[0038] Hereinafter, embodiments of the present invention will be
described. In the explanation of the drawings, like elements are
denoted by like reference signs and redundant explanations are
omitted. The dimensional proportions in the drawings do not
necessarily match those of what are described.
[0039] A method for producing a porous aluminum body will be
described. A porous resin body containing continuous pores is first
provided. The material of the porous resin body can be selected
from resins that can be decomposed at a temperature equal to or
less than the melting point of aluminum. Examples of the material
of the porous resin body include polyurethane, polypropylene, and
polyethylene. Although the term "porous resin body" is used, a
resin having any shape can be selected as long as it contains
continuous pores. For example, fibrous resins entangled in the form
of nonwoven fabric may be used instead of the porous resin body.
The porosity of the porous resin body is preferably 80% to 98%; and
the pore size of the porous resin body is preferably about 50 to
500 .mu.m. An urethane foam has a high porosity, continuity of
pores, and uniformity of the pore size, and is also excellent in a
pyrolysis property, and hence can be preferably used as the porous
resin body.
[0040] An aluminum layer is formed on the surface of the porous
resin body. The aluminum layer can be formed by a method, for
example, a gas phase method such as vapor deposition, sputtering,
or plasma CVD; coating with aluminum paste; or plating. Plating
with aluminum in an aqueous solution is substantially impossible in
terms of practicality and hence molten salt electrolytic plating of
plating with aluminum in a molten salt is preferably performed. In
molten salt electrolytic plating, for example, a two-component
system salt of AlCl.sub.3--XCl (X: alkaline metal) or a
multicomponent system salt is used; the porous resin body is
immersed in such a salt being molten and electrolytic plating is
performed while a potential is applied to the aluminum layer. The
molten salt may be a eutectic salt of an organic halide and an
aluminum halide. The organic halide may be an imidazolium salt, a
pyridinium salt, or the like. In particular,
1-ethyl-3-methylimidazolium chloride (EMIC) and butylpyridinium
chloride (BPC) are preferred. To perform electrolytic plating, the
surface of the porous resin body is subjected to a conductive
treatment in advance. The conductive treatment can be selected from
methods including, for example, electroless plating with a
conducting metal such as nickel, vapor deposition and sputtering of
aluminum or the like, and application of an electrically-conductive
coating containing conducting particles such as carbon
particles.
[0041] Alternatively, the aluminum layer may be formed by coating
with aluminum paste. The aluminum paste is a mixture of aluminum
powder, a binder (binder resin), and an organic solvent. The
aluminum paste is preferably sintered in a non-oxidizing
atmosphere.
[0042] The porous resin body on the surface of which the aluminum
layer is formed is immersed in a first molten salt and heated while
a negative potential is applied to the aluminum layer, to thereby
decompose the porous resin body. The application of a negative
potential to the aluminum layer immersed in a molten salt
suppresses the oxidation reaction of aluminum. Heating in such a
state results in decomposition of the porous resin body without
oxidizing aluminum. Although the heating temperature may be
appropriately selected in accordance with the type of the porous
resin body, in order not to melt aluminum, the heating should be
performed at a temperature equal to or less than the melting point
(660.degree. C.) of aluminum, preferably 600.degree. C. or less.
When urethane is selected for the porous resin body, since urethane
decomposes in a molten salt at 380.degree. C. or more, the heating
temperature is preferably made 380.degree. C. or more, more
preferably in the temperature range of 500.degree. C. or more and
600.degree. C. or less. The magnitude of the negative potential
applied is on the negative side with respect to the reduction
potential of aluminum and on the positive side with respect to the
reduction potential of a cation in the molten salt. By such a
method, a porous aluminum body containing continuous pores and
having a thin oxide layer and a low oxygen content in the surface
thereof can be provided.
[0043] Examples of a salt constituting the first molten salt
include lithium chloride (LiCl), potassium chloride (KCl), sodium
chloride (NaCl), aluminum chloride (AlCl.sub.3), lithium nitrate
(LiNO.sub.3), lithium nitrite (LiNO.sub.2), potassium nitrate
(KNO.sub.3), potassium nitrite (KNO.sub.2), sodium nitrate
(NaNO.sub.3), and sodium nitrite (NaNO.sub.2). To decrease the
melting point, two or more of such salts are preferably mixed to
form a eutectic salt. When ions of a metal with respect to which
the electrode potential of the layer of a metal such as aluminum is
noble, that is, ions of a metal having a low ionization tendency
are contained in the molten salt, the metal precipitates in the
metal layer and becomes impurities, which is not preferable. When a
urethane foam is used as the porous resin body, the heating
temperature of the first molten salt is preferably made 380.degree.
C. or more. Urethane can be properly pyrolyzed at 380.degree. C. or
more. Examples of a eutectic salt that melts at 380.degree. C. or
more include LiCl--KCl, CaCl.sub.2--LiCl, CaCl.sub.2--NaCl,
LiNO.sub.3--NaNO.sub.3, Ca(NO.sub.3).sub.2--NaNO.sub.3, and
NaNO.sub.2--KNO.sub.3.
[0044] FIG. 8 is a schematic explanatory view illustrating a step
of decomposing a porous resin body in a molten salt in further
detail. A molten salt 35 is placed in a molten salt bath 31 and
heated. To keep the molten salt at a high temperature, the molten
salt bath 31 is disposed within a container 32. A treatment sample
33 (porous resin body having a metal layer thereon) enters the
container 32 from the left side in the figure and is routed along
guide rollers 36 and immersed in the molten salt 35. To immerse the
treatment sample 33 in the molten salt, a press plate or the like
(not shown) is disposed above the treatment sample 33. The
treatment sample 33 in which the porous resin body has been
decomposed in the molten salt 35 (porous metal body) is withdrawn
from the molten salt 35. A positive electrode (not shown) is
disposed in the molten salt bath 31.
[0045] The electrode (not shown) applies a negative potential to
the treatment sample 33 and oxidation of the metal layer can be
suppressed while the treatment sample 33 is immersed in the molten
salt 35. However, since the heating temperature of the molten salt
is a very high temperature of 380.degree. C. to 600.degree. C., an
upper space 38 of the container 32 has a high-temperature
atmosphere. When the upper space 38 contains oxygen, the metal
layer of the treatment sample 33 to be immersed in the molten salt
may be oxidized. Accordingly, an antioxidant measure that
suppresses oxidation of the metal layer is preferably disposed. As
the antioxidant measure, an inert-gas bubbling measure 34 may be
disposed in the molten salt bath 31 to make an inert gas flow in
the molten salt. Bubbles 39 of the inert gas generated from the
inert-gas bubbling measure 34 fill the molten salt 35, pass through
cavities (porous portion) of the treatment sample 33, and fill the
space 38 above the molten salt. As a result, the entirety of the
container 32 is filled with the inert-gas atmosphere and oxidation
of the metal layer can be suppressed. In addition, as has been
described, the advantages can be achieved in which oxygen derived
from the decomposed resin is expelled from the molten salt 35 and
the molten salt can be sufficiently stirred with the bubbles 39 of
the inert gas.
[0046] As another antioxidant measure, an inert-gas ejection
measure 37 may be disposed outside the container 32 so that an
inert gas is sprayed onto the treatment sample 33 prior to entry
into the container 32 to remove oxygen remaining within the porous
body of the treatment sample 33. Such an inert-gas ejection measure
may be disposed within the container 32. A measure that ejects an
inert gas may be simply disposed such that the container 32 is
filled with the inert-gas atmosphere.
(Battery)
[0047] Hereinafter, a battery electrode material and a battery that
include a porous aluminum body will be described. For example, when
the porous aluminum body is used for the positive electrode of a
lithium-ion battery, examples of an active material used include
lithium cobalt oxide (LiCoO.sub.2), lithium manganese oxide
(LiMn.sub.2O.sub.4), and lithium nickel dioxide (LiNiO.sub.2). Such
an active material is used in combination with a conductive
assistant and a binder. Existing positive-electrode materials for
lithium-ion batteries are obtained by applying an active material
to the surface of an aluminum foil; to increase a battery capacity
per unit area, the thickness of the active material applied is made
large; and, to effectively use the active material, the aluminum
foil and the active material need to be in electrical contact with
each other and hence the active material is used as a mixture with
a conductive assistant. In contrast, a porous aluminum body
according to the present invention has a high porosity and a large
surface area per unit area. Accordingly, even when an active
material is held with a small thickness over the surface of the
porous body, the active material can be effectively used. Thus, the
battery capacity can be increased and the amount of a conductive
assistant mixed can be reduced.
[0048] A lithium-ion battery includes a positive electrode
constituted by the above-described positive-electrode material; a
negative electrode composed of graphite; and an electrolyte
constituted by an organic electrolyte. In such a lithium-ion
battery, the capacity can be increased even when the electrode area
is small. Accordingly, the energy density of the battery can be
made high, compared with existing lithium-ion batteries.
[0049] A porous aluminum body may be used as an electrode material
for a molten salt battery. When a porous aluminum body is used as a
positive-electrode material, a metal compound into which a cation
of a molten salt serving as the electrolyte can intercalate, such
as sodium chromite (NaCrO.sub.2) or titanium disulfide (Ti
S.sub.2), is used as the active material. Such an active material
is used in combination with a conductive assistant and a binder.
The conductive assistant may be acetylene black or the like. The
binder may be polytetrafluoroethylene (PTFE) or the like. When
sodium chromite is used as an active material and acetylene black
is used as a conductive assistant, they are strongly bound with
PTFE, which is preferable.
[0050] A porous aluminum body can also be used as a
negative-electrode material for a molten salt battery. When a
porous aluminum body is used as a negative-electrode material,
elemental sodium, an alloy of sodium and another metal, carbon, or
the like may be used as the active material. Since sodium has a
melting point of about 98.degree. C. and the metal softens as the
temperature increases, sodium and another metal (Si, Sn, In, or the
like) is preferably alloyed. Of these, an alloy of sodium and Sn is
particularly preferred because of ease of handling. Sodium or a
sodium alloy can be held on the surface of a porous aluminum body
by a method such as electrolytic plating or hot dipping. A sodium
alloy can be formed by making a metal (Si or the like) to be
alloyed with sodium adhere to a porous aluminum body by a method
such as plating, and subsequently charging a molten salt battery
including the porous aluminum body.
[0051] FIG. 4 is a schematic sectional view illustrating an example
of a molten salt battery including the battery electrode material.
In the molten salt battery, a positive electrode 21 in which a
positive-electrode active material is held on the surface of the
aluminum skeleton part of a porous aluminum body; a negative
electrode 22 in which a negative-electrode active material is held
on the surface of the aluminum skeleton part of a porous aluminum
body; and a separator 23 impregnated with a molten salt serving as
an electrolyte, are contained in a case 27. A presser member 26
constituted by a presser plate 24 and a spring 25 pressing the
presser plate is disposed between the upper surface of the case 27
and the negative electrode. By disposing the presser member, even
when the volumes of the positive electrode 21, the negative
electrode 22, and the separator 23 change, the presser member
uniformly presses these members so that the members are in contact
with each other. The collector (porous aluminum body) of the
positive electrode 21 and the collector (porous aluminum body) of
the negative electrode 22 are respectively connected to a positive
terminal 28 and a negative terminal 29 through lead wires 30.
[0052] Examples of a molten salt serving as an electrolyte include
various inorganic salts that melt at an operation temperature. For
example, a molten salt containing an anion represented by the
following formula (1) and at least one metal cation from alkaline
metals and alkaline earth metals, is preferably used.
##STR00001##
[0053] In the formula (1), R.sup.1 and R.sup.2 each independently
represent a fluorine atom or a fluoroalkyl group. In particular, a
bis(fluorosulfonyl)amide ion (hereafter, TFSA ion) in which R.sup.1
and R.sup.2 each represent F and a
bis(trifluoromethylsulfonyl)amide ion (hereafter, TFSA ion) in
which R.sup.1 and R.sup.2 each represent CF.sub.3 are preferably
used because the melting point of the molten salt can be decreased
and the operation temperature of the battery can be decreased.
[0054] The cation of the molten salt may be one or more selected
from alkaline metals such as lithium (Li), sodium (Na), potassium
(K), rubidium (Rb), and cesium (Cs) and alkaline earth metals such
as beryllium (Be), magnesium (Mg), calcium (Ca), strontium (Sr),
and barium (Ba).
[0055] To decrease the melting point of the molten salt, two or
more salts are preferably used as a mixture. For example, when KFSA
and NaFSA are used in combination, the operation temperature of the
battery can be made 90.degree. C. or less.
[0056] The molten salt is used by impregnating a separator with the
molten salt. The separator prevents the positive electrode and the
negative electrode from coming into contact with each other and may
be composed of a glass nonwoven fabric, a porous resin, or the
like. The positive electrode, the negative electrode, and the
separator impregnated with the molten salt are laminated, contained
in a case, and used as a battery.
(Electrical Double Layer Capacitor)
[0057] A porous aluminum body can also be used as an electrode
material for an electrical double layer capacitor. When a porous
aluminum body is used as an electrode material for an electrical
double layer capacitor, activated carbon or the like is used as the
electrode active material. The activated carbon is used in
combination with a conductive assistant and a binder. The
conductive assistant may be graphite, carbon nano-tubes, or the
like. The binder may be polytetrafluoroethylene (PTFE),
styrene-butadiene rubber, or the like.
[0058] FIG. 5 is a schematic sectional view illustrating an example
of an electrical double layer capacitor including the electrode
material for an electrical double layer capacitor. Electrode
materials in which an electrode active material is held on porous
aluminum bodies are disposed as polarizable electrodes 41 in an
organic electrolyte 43 divided with a separator 42. The electrode
materials 41 are connected to lead wires 44 and the entire
structure including the electrode materials 41 is contained in a
case 45. By using porous aluminum bodies as collectors, the surface
area of the collectors is increased; and, even when activated
carbon serving as the active material is applied to a small
thickness to the collectors, an electrical double layer capacitor
having a high output and a high capacitance can be obtained.
[0059] The embodiments where aluminum is used as the metal have
been described so far. However, the present invention is not
limited to aluminum but is useful as a method for producing a
porous metal body in which oxidation is suppressed (the oxygen
content is low) in the surface thereof. Specifically, nickel,
copper, silver, or the like may be used.
EXAMPLE 1
(Production of Porous Aluminum Body: Formation of Aluminum Layer by
Vapor Deposition)
[0060] Hereinafter, an example of producing a porous aluminum body
will be specifically described. A polyurethane foam (thickness: 1
mm) having a porosity of 97% and a pore size of about 300 .mu.m was
provided as a porous resin body and cut into a square having 20 mm
sides. Aluminum was vapor-deposited onto the surface of the
polyurethane foam to form an aluminum layer having a thickness of
15 .mu.m.
(Production of Porous Aluminum Body: Decomposition of Porous Resin
Body)
[0061] The porous resin body having the aluminum layer was immersed
in LiCl--KCl eutectic molten salt at 500.degree. C. and a negative
potential of -1 V was applied thereto for 30 minutes. Bubbles were
generated in the molten salt, which probably showed the occurrence
of the decomposition reaction of polyurethane. The molten salt was
then cooled in the air to room temperature and a porous aluminum
body was cleaned with water to remove the molten salt. Thus, the
porous aluminum body was obtained. A SEM photograph of the obtained
porous aluminum body is illustrated in FIG. 6. FIG. 6 shows that
the obtained porous aluminum body contains continuous pores and has
a high porosity. The result of subjecting the surface of the
obtained porous aluminum body to EDX at an accelerating voltage of
15 kV is illustrated in FIG. 7. Substantially no peaks of oxygen
were observed and hence the oxygen content of the porous aluminum
body was equal to or less than the detection limit (3.1 mass %) of
EDX. A battery including the obtained porous aluminum body was
evaluated and it worked properly.
(EXAMPLE 2
[0062] A polyurethane foam (thickness: 1 mm) having a porosity of
97% and a pore size of about 300 .mu.m was provided as a porous
resin body and cut so as to have a width of 100 mm and a length of
200 mm. Aluminum was vapor-deposited onto the surface of the
polyurethane foam to form an aluminum layer having a thickness of
15 LiCl--KCl eutectic molten salt was placed in an aluminum bath
having a width of 160 mm, a length of 430 mm, a depth of 80 mm, and
a thickness of 10 mm and heated at 500.degree. C. While nitrogen
gas was made to flow in the LiCl--KCl eutectic molten salt at a
flow rate of 3.times.10.sup.-4 m.sup.3/s, the porous resin body
having the aluminum layer was immersed in the molten salt for 5
minutes. A negative potential of 1.1 V was applied to the aluminum
layer. Bubbles were generated in the molten salt, which probably
showed the occurrence of the decomposition reaction of
polyurethane. The molten salt was then cooled in the air to room
temperature and a porous aluminum body was cleaned with water to
remove the molten salt. Thus, the porous aluminum body was
obtained. The surface of the obtained porous aluminum body was
subjected to EDX at an accelerating voltage of 15 kV, and it was
found that the oxygen content was 2.9 mass % and the carbon content
was 1.54 mass %.
EXAMPLE 3
[0063] A porous aluminum body was produced and evaluated by the
same procedures as in EXAMPLE 2 except that nitrogen gas was not
made to flow in the molten salt. The oxygen content was 7.61 mass %
and the carbon content was 1.74 mass %. The oxygen content and the
carbon content were slightly high, compared with EXAMPLE 2 in which
the resin was decomposed while nitrogen gas was made to flow in the
molten salt. Embodiments and Examples disclosed herein are given by
way of illustration in all the respects, and not by way of
limitation. The scope of the present invention is indicated not by
the above descriptions but by the Claims and embraces all the
modifications within the meaning and range of equivalency of the
Claims.
INDUSTRIAL APPLICABILITY
[0064] According to the present invention, a porous metal body
containing continuous pores and having a thin oxide film (low
oxygen content) in the surface thereof can be obtained. Use of such
a porous metal body can provide an electrode material in which use
efficiency of an active material can be enhanced so that the
capacity of a battery can be increased; and the porous metal body
can be suitably applied to a battery including the electrode
material.
REFERENCE SIGNS LIST
[0065] 1 porous resin body
[0066] 2 metal layer
[0067] 3 porous metal body
[0068] 4 active material
[0069] 5 battery electrode material
[0070] 11 porous resin body having metal layer thereon
[0071] 12 positive electrode
[0072] 13 first molten salt
[0073] 21 positive electrode
[0074] 22 negative electrode
[0075] 23 separator
[0076] 24 presser plate
[0077] 25 spring
[0078] 26 presser member
[0079] 27 case
[0080] 28 positive terminal
[0081] 29 negative terminal
[0082] 30 lead wire
[0083] 31 molten salt bath
[0084] 32 container
[0085] 33 treatment sample
[0086] 34 inert-gas bubbling measure
[0087] 35 molten salt
[0088] 36 guide roller
[0089] 37 inert-gas ejection measure
[0090] 38 upper space
[0091] 39 bubble
[0092] 41 polarizable electrode
[0093] 42 separator
[0094] 43 organic electrolyte
[0095] 44 lead wire
[0096] 45 case
* * * * *