U.S. patent application number 15/526707 was filed with the patent office on 2019-01-24 for negative electrode composition for electric storage device, negative electrode including the composition, electric storage device, and method for producing negative electrode for electric storage device.
This patent application is currently assigned to Sumitomo Electric Industries, Ltd.. The applicant listed for this patent is Atsushi Fukunaga, Eiko Imazaki, Hideaki Ito, Hitoshi Kobayashi, Shinichi Maruyama, Koji Nitta, Koma Numata, Shoichiro Sakai, Toshiaki Yamashita. Invention is credited to Atsushi Fukunaga, Eiko Imazaki, Hideaki Ito, Hitoshi Kobayashi, Shinichi Maruyama, Koji Nitta, Koma Numata, Shoichiro Sakai, Toshiaki Yamashita.
Application Number | 20190027744 15/526707 |
Document ID | / |
Family ID | 55954464 |
Filed Date | 2019-01-24 |
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United States Patent
Application |
20190027744 |
Kind Code |
A2 |
Nitta; Koji ; et
al. |
January 24, 2019 |
NEGATIVE ELECTRODE COMPOSITION FOR ELECTRIC STORAGE DEVICE,
NEGATIVE ELECTRODE INCLUDING THE COMPOSITION, ELECTRIC STORAGE
DEVICE, AND METHOD FOR PRODUCING NEGATIVE ELECTRODE FOR ELECTRIC
STORAGE DEVICE
Abstract
Provided is a method for producing a negative electrode for an
electric storage device, the method comprising the steps of:
preparing a negative electrode composition comprising a negative
electrode active material that reversibly carries a sodium ion,
metal sodium, and a liquid dispersion medium for dispersing them;
allowing a negative electrode current collector to hold the
negative electrode composition; evaporating at least part of the
liquid dispersion medium from the negative electrode composition
held by the negative electrode current collector, thereby giving a
negative electrode precursor comprising the negative electrode
active material, the metal sodium, and the negative electrode
current collector; and bringing the negative electrode precursor
into contact with an electrolyte having sodium ion conductivity,
thereby doping the negative electrode active material with sodium
eluted from the metal sodium.
Inventors: |
Nitta; Koji; (Osaka-shi,
JP) ; Sakai; Shoichiro; (Osaka-shi, JP) ;
Fukunaga; Atsushi; (Osaka-shi, JP) ; Imazaki;
Eiko; (Osaka-shi, JP) ; Numata; Koma;
(Osaka-shi, JP) ; Ito; Hideaki; (Joetsu-shi,
JP) ; Kobayashi; Hitoshi; (Joetsu-shi, JP) ;
Yamashita; Toshiaki; (Joetsu-shi, JP) ; Maruyama;
Shinichi; (Joetsu-shi, JP) |
|
Applicant: |
Name |
City |
State |
Country |
Type |
Maruyama; Shinichi
Nitta; Koji
Sakai; Shoichiro
Fukunaga; Atsushi
Imazaki; Eiko
Numata; Koma
Ito; Hideaki
Kobayashi; Hitoshi
Yamashita; Toshiaki
Maruyama; Shinichi |
Joetsu-shi
Osaka-shi
Osaka-shi
Osaka-shi
Osaka-shi
Osaka-shi
Joetsu-shi
Joetsu-shi
Joetsu-shi
Joetsu-shi |
|
JP
JP
JP
JP
JP
JP
JP
JP
JP
JP |
|
|
Assignee: |
Sumitomo Electric Industries,
Ltd.
Osaka-shi
JP
Nippon Soda Co., Ltd.
Tokyo
JP
|
Prior
Publication: |
|
Document Identifier |
Publication Date |
|
US 20170324086 A1 |
|
|
US 20180151877 A2 |
May 31, 2018 |
|
|
Family ID: |
55954464 |
Appl. No.: |
15/526707 |
Filed: |
November 12, 2015 |
PCT Filed: |
November 12, 2015 |
PCT NO: |
PCT/JP2015/081834 PCKC 00 |
371 Date: |
May 12, 2017 |
Current U.S.
Class: |
1/1 |
Current CPC
Class: |
H01M 4/362 20130101;
H01M 10/0564 20130101; Y02E 60/13 20130101; H01M 10/054 20130101;
H01M 10/0566 20130101; H01G 11/30 20130101; H01G 11/86 20130101;
H01G 11/62 20130101; H01M 4/0459 20130101; Y02E 60/10 20130101;
H01G 11/04 20130101; H01M 4/134 20130101; H01M 4/1395 20130101;
H01M 4/381 20130101 |
International
Class: |
H01M 4/38 20060101
H01M004/38; H01M 10/0564 20100101 H01M010/0564 |
Foreign Application Data
Date |
Code |
Application Number |
Nov 13, 2014 |
JP |
2014-230862 |
Claims
1. A negative electrode composition for an electric storage device
comprising: a negative electrode active material that reversibly
carries a sodium ion; and metal sodium.
2. The negative electrode composition for an electric storage
device according to claim 1, wherein the metal sodium is in a form
of a metal sodium particle.
3. The negative electrode composition for an electric storage
device according to claim 1, wherein the negative electrode
composition contains the metal sodium in an amount corresponding to
10 to 200% of an irreversible capacity of the negative electrode
active material.
4. The negative electrode composition for an electric storage
device according to claim 1, wherein at least part of the negative
electrode active material and at least part of the metal sodium are
mixed.
5. The negative electrode composition for an electric storage
device according to claim 1, wherein the metal sodium has an
average particle diameter D of 10 to 60 .mu.m.
6. The negative electrode composition for an electric storage
device according to claim 1, wherein part of the metal sodium is
ionized and occluded in the negative electrode active material.
7. A negative electrode for an electric storage device comprising:
the negative electrode composition according to claim 6; and a
negative electrode current collector for holding the negative
electrode composition.
8. An electric storage device comprising: the negative electrode
according to claim 7; a positive electrode comprising a positive
electrode active material; a separator interposed between the
negative electrode and the positive electrode; and an electrolyte
having sodium ion conductivity.
9. The electric storage device according to claim 8, wherein the
electrolyte comprises an ionic liquid composed of an anion and a
cation.
10. The electric storage device according to claim 9, wherein 90
mol % or more of the anion is a fluorine-containing
bis(sulfonyl)amide anion.
11. A method for producing a negative electrode for an electric
storage device, the method comprising the steps of: preparing a
negative electrode composition comprising a negative electrode
active material that reversibly carries a sodium ion, metal sodium,
and a liquid dispersion medium for dispersing the negative
electrode active material and the metal sodium; allowing a negative
electrode current collector to hold the negative electrode
composition; evaporating at least part of the liquid dispersion
medium from the negative electrode composition held by the negative
electrode current collector, thereby giving a negative electrode
precursor comprising the negative electrode active material, the
metal sodium, and the negative electrode current collector; and
bringing the negative electrode precursor into contact with an
electrolyte having sodium ion conductivity to ionize the metal
sodium, thereby doping the negative electrode active material with
sodium.
Description
TECHNICAL FIELD
[0001] The present invention relates to an electric storage device
utilizing a faradaic reaction of a sodium ion, and particularly
relates to improvement in a negative electrode composition for use
in producing a negative electrode for an electric storage
device.
BACKGROUND ART
[0002] In recent years, attention has been directed to techniques
for converting natural energy, such as solar light or wind power,
into electric energy. Further, there is a growing demand for an
electric storage device that can storage much electric energy, such
as a lithium ion secondary battery and a lithium ion capacitor.
However, the prices of lithium resources have also increased due to
the growing market of such electric storage devices.
[0003] For this reason, a electric storage device including a
sodium ion as a carrier ion have been studied. Patent Literature 1
discloses a sodium ion capacitor including a polarizable positive
electrode and a negative electrode including hard carbon or the
like in combination. In Patent Literature 1, a negative electrode
active material is pre-doped with a sodium ion from the viewpoint
of enhancing a discharge capacity or a cycle characteristic.
[0004] The pre-doping of a negative electrode active material with
a sodium ion is performed by putting a metal sodium foil in the
electric storage device and bringing the negative electrode and the
metal sodium foil into contact with an electrolyte inside the
electric storage device to allow the negative electrode to occlude
a sodium ion eluted from the metal sodium foil into the
electrolyte.
CITATION LIST
Patent Literature
[0005] Patent Literature 1: Japanese Unexamined Patent Publication
No. 2012-69894
SUMMARY OF INVENTION
Technical Problem
[0006] However, since the metal sodium foil is usually localized in
one area in the electric storage device, it difficult to allow the
pre-doping to uniformly and quickly proceed.
[0007] It is therefore an object of the present invention to
provide means for uniformly and quickly pre-doping with sodium a
negative electrode active material for an electric storage device,
the negative electrode active material carrying reversibly a sodium
ion.
Solution to Problem
[0008] One aspect of the present invention relates to a negative
electrode composition for an electric storage device including: a
negative electrode active material that reversibly carries a sodium
ion; and metal sodium.
[0009] Another aspect of the present invention relates to a
negative electrode for an electric storage device having: the
above-described negative electrode composition; and a negative
electrode current collector for holding the negative electrode
composition.
[0010] Yet another aspect of the present invention relates to an
electric storage device including: the above-described negative
electrode; a positive electrode including a positive electrode
active material; a separator interposed between the negative
electrode and the positive electrode; and an electrolyte having
sodium ion conductivity.
[0011] Yet another aspect of the present invention relates to a
method for producing a negative electrode for an electric storage
device, the method including the steps of preparing a negative
electrode composition including a negative electrode active
material that reversibly carries a sodium ion, metal sodium, and a
liquid dispersion medium for dispersing the negative electrode
active material and the metal sodium; allowing a negative electrode
current collector to hold the negative electrode composition;
evaporating at least part of the liquid dispersion medium from the
negative electrode composition held by the negative electrode
current collector, thereby giving a negative electrode precursor
having the negative electrode active material, the metal sodium,
and the negative electrode current collector; and bringing the
negative electrode precursor into contact with an electrolyte
having sodium ion conductivity to ionize the metal sodium, thereby
doping the negative electrode active material with sodium.
Advantageous Effects of Invention
[0012] According to the above aspects of the present invention, it
is possible to uniformly and quickly pre-dope with sodium a
negative electrode active material for an electric storage device,
the negative electrode active material carrying reversibly a sodium
ion.
BRIEF DESCRIPTION OF DRAWING
[0013] FIG. 1 is a longitudinal sectional view that schematically
shows a sodium ion battery according to one embodiment of the
present invention.
DESCRIPTION OF EMBODIMENTS
Description of Embodiments of Invention
[0014] First, embodiments of the present invention listed below
will be described.
[0015] A negative electrode composition for an electric storage
device according to one embodiment of the present invention
includes: a negative electrode active material that reversibly
carries a sodium ion; and metal sodium.
[0016] The negative electrode composition including metal sodium
constitutes a negative electrode active material layer precursor of
a negative electrode having a negative electrode active material
layer and a negative electrode current collector. The negative
electrode active material is doped with a sodium ion eluted from
the metal sodium so that a negative electrode is formed from the
precursor.
[0017] In the negative electrode composition, it is preferable that
at least part of the negative electrode active material and at
least part of the metal sodium are mixed. That is, it is preferable
that the negative electrode composition is a mixture including at
least a negative electrode active material and metal sodium. The
negative electrode composition can be a negative electrode slurry
further containing a liquid dispersion medium for dispersing the
negative electrode active material and the metal sodium. That is,
the term "negative electrode composition" is used as a concept
encompassing a negative electrode slurry before being held by a
negative electrode current collector, and a negative electrode
mixture layer held by a negative electrode current collector.
[0018] Inside the negative electrode active material layer
precursor, the metal sodium is present in the form of, for example,
a particle. Therefore, most of the negative electrode active
material in the layer is close to the metal sodium. As a result, it
possible to uniformly and quickly dope the negative electrode
active material with sodium. In the negative electrode composition,
part of the sodium mixed with the negative electrode active
material can be ionized to be occluded in the negative electrode
active material.
[0019] It is preferable that the negative electrode composition
contains the metal sodium in an amount corresponding to 10 to 200%
of an irreversible capacity of the negative electrode active
material. This makes it possible to suppress a reduction in the
capacity of an electric storage device caused by the irreversible
capacity of the negative electrode active material.
[0020] It is preferable that the metal sodium particle preferably
has an average particle diameter D of 10 to 60 .mu.m. This allows
quick elution (ionization) of sodium from the metal sodium.
Further, since adequate contact between particles of the negative
electrode active material is maintained after the metal sodium is
completely dissolved, a strength of a negative electrode active
material layer is easily maintained.
[0021] When the doping of the negative electrode active material
with sodium in the metal sodium is completed, the negative
electrode composition constitutes a negative electrode active
material layer. The negative electrode composition, in which the
negative electrode active material is doped with part of sodium in
the metal sodium, constitutes an intermediate between the negative
electrode active material layer precursor and the negative
electrode active material layer. However, the negative electrode
composition, in which the negative electrode active material is
doped with at least part of sodium in the metal sodium, is herein
regarded to constitute a negative electrode active material
layer.
[0022] A negative electrode for an electric storage device
according to another embodiment of the present invention includes:
the above-described negative electrode composition constituting a
negative electrode active material layer; and a negative electrode
current collector for holding the negative electrode composition.
Since the above-described negative electrode is uniformly doped
with sodium, the above-described negative electrode is excellent in
a capacitance characteristic from the initial stage of charge and
discharge.
[0023] An electric storage device according to yet another
embodiment of the present invention includes: the above-described
negative electrode; a positive electrode including a positive
electrode active material; a separator interposed between the
negative electrode and the positive electrode; and an electrolyte
having sodium ion conductivity. It is preferable that the
electrolyte contains an ionic liquid.
[0024] The term "ionic liquid" refers to an ionic liquid material
composed of an anion and a cation. A salt in a molten state can be
referred to as an ionic liquid.
[0025] When an ionic liquid is used for the electrolyte, the amount
of the ionic liquid contained in the electrolyte is preferably 80
mass % or more, more preferably 90 mass % or more, particularly
preferably 98 mass % or more. When the amount of the ionic liquid
contained in the electrolyte is within the above range, it is easy
to enhance the heat resistance and/or flame resistance of the
electrolyte.
[0026] Since the ionic liquid is highly stable toward the metal
sodium, the ionic liquid makes it possible to minimize a side
reaction with the metal sodium. Therefore, pre-doping can be more
efficiently performed as compared to a case where an electrolyte
mainly containing an organic solvent (organic electrolyte) is
used.
[0027] For the above reason, the above-described negative electrode
is suitable for an electric storage device including an ionic
liquid as an electrolyte (hereinafter, referred to as an ionic
liquid-type electric storage device). Examples of the ionic
liquid-type electric storage device include a sodium ion battery
and a sodium ion capacitor, each containing, as an electrolyte, an
ionic liquid containing a sodium ion. However, the above-described
negative electrode can be applied not only to such an ionic
liquid-type electric storage device but also to a sodium battery or
a sodium ion capacitor, the sodium battery or sodium ion capacitor
containing an organic electrolyte.
[0028] A method for producing a negative electrode for an electric
storage device according to yet another embodiment of the present
invention includes the steps of preparing a negative electrode
composition (first step); allowing a negative electrode current
collector to hold the negative electrode composition (second step);
then giving a negative electrode precursor (third step); and doping
with sodium a negative electrode active material of the negative
electrode precursor (fourth step).
[0029] In the first step, a negative electrode composition
including a negative electrode active material that reversibly
carries a sodium ion, metal sodium, and a liquid dispersion medium
for dispersing them is prepared. In the third step, at least part
of the liquid dispersion medium is evaporated from the negative
electrode composition held by the negative electrode current
collector, thereby forming a negative electrode precursor including
the negative electrode active material, the metal sodium, and the
negative electrode current collector. In the fourth step, the
negative electrode precursor is brought into contact with an
electrolyte having sodium ion conductivity, thereby doping the
negative electrode active material with sodium eluted from the
metal sodium.
[0030] The negative electrode precursor includes a negative
electrode active material layer precursor. A negative electrode is
formed from the negative electrode precursor through the fourth
step (pre-doping with sodium). The above-described method allows
the negative electrode active material layer precursor itself to
contain metal sodium. This makes it possible to incorporate the
step of introducing metal sodium for pre-doping into an electric
storage device into the process of producing a negative electrode.
Therefore, a step of separately putting a metal sodium foil inside
an electric storage device is not required.
[0031] In the fourth step, in order to enhance the contact
efficiency between the negative electrode precursor and the
electrolyte, it is preferable to use the negative electrode
precursor containing no liquid dispersion medium or the negative
electrode precursor containing a minute amount of the remaining
liquid dispersion medium that is to be replaced with the
electrolyte.
Details of Embodiments of Invention
[0032] Herein below, the embodiments of the present invention will
be described with reference to the drawings. The present invention
is not limited to the following embodiments but is defined by the
appended claims, and equivalents to the claims and all
modifications within the scope of the claims are intended to be
embraced by the claims.
Negative Electrode Composition
[0033] First, the negative electrode composition as a negative
electrode active material layer precursor will be described.
[0034] The negative electrode composition includes: a negative
electrode active material that reversibly carries a sodium ion; and
metal sodium. The negative electrode composition can further
include, as an optional component, a binder, a conductive auxiliary
agent, a liquid dispersion medium (first dispersion medium), or the
like.
[0035] The negative electrode composition is prepared by, for
example, adding metal sodium and an optional component to an
appropriate dispersion medium and mixing them. At this time, the
preparation of the negative electrode composition can be performed
by adding both metal sodium and an optional component to a
dispersion medium, or by preparing a dispersion of metal sodium and
a solution or dispersion containing an optional component
separately from each other and then mixing both of them.
[0036] The dispersion of metal sodium can be obtained by putting
solid metal sodium pieces having surfaces with metallic luster in a
dispersion medium and then heating the resulting mixture to a
temperature equal to or higher than the melting point of sodium
(97.8.degree. C.) with stirring. Alternatively, previously-molten
metal sodium can be added to a dispersion medium with stirring.
This makes it possible to uniformly disperse the metal sodium
(e.g., particulate metal sodium) in the dispersion medium while
suppressing the generation of an oxide or the like. A method for
mixing the metal sodium and the dispersion medium is not
particularly limited. In the mixing of the metal sodium and the
dispersion medium, there can be used a disperser such as a
homogenizer, a ball mill, a sand mill, or a planetary mixer.
[0037] It is preferable that an impurity, such as moisture, be
removed from the dispersion medium and another optional component
before mixing with the metal sodium. For example, when a solution
or dispersion containing an optional component such as a binder and
a dispersion of metal sodium are mixed together, it is preferable
that a dehydration reaction is performed previously by separately
adding a small amount of metal sodium to the solution or dispersion
containing an optional component.
[0038] The dispersion medium for dispersing the metal sodium is not
particularly limited as long as it is a liquid medium that does not
react with the metal sodium. Such a liquid medium is preferably a
hydrocarbon-based solvent. Examples of the liquid medium include a
paraffin-based hydrocarbon having 6 to 20 carbon atoms and having
flowability, an aromatic hydrocarbon such as toluene or xylene, an
animal or plant oil such as coconut oil or castor oil, a synthetic
oil such as silicone oil, and the like. The paraffin-based
hydrocarbon can be linear normal paraffin or branched isoparaffin.
Among these paraffin-based hydrocarbons, normal paraffin is more
preferred. These dispersion media can be used alone or used in
admixture of two or more of them.
[0039] The content of the metal sodium in the dispersion of metal
sodium can be, for example, 1 to 15 mass %, but is preferably 10 to
15 mass %.
[0040] The metal sodium is preferably particulate. When the metal
sodium is particulate, the shape of its particles is not
particularly limited. The particles can have any shape such as a
spherical, rod-like, needle-like, plate-like, columnar, indefinite,
scale-like, or spindle-like shape. Alternatively, the metal sodium
can be in a molten state (i.e., liquid) or in the form of clusters
composed of a few sodium atoms. When the metal sodium is in a
molten state, the metal sodium shall be present in the dispersion
of metal sodium so as to be distinguishable from the dispersion
medium.
[0041] In the negative electrode composition, the average particle
diameter D of the metal sodium is not particularly limited, but is
preferably 10 to 60 .mu.m, more preferably 10 to 55 .mu.m,
particularly preferably 10 to 50 .mu.m. When the average particle
diameter D of the metal sodium is within the above range, the metal
sodium is further easily handled, and pre-doping is likely to
quickly proceed. The average particle diameter means a median value
in a volume-based particle size distribution. The particle size
distribution is measured by a laser diffraction particle size
distribution analyzer. The same applies hereinafter.
[0042] The negative electrode composition can be, for example, a
mixture of a dispersion of sodium metal, a negative electrode
active material, and an optional component used as necessary. In
this case, it is preferable that the negative electrode composition
contains the metal sodium in an amount corresponding to 10 to 200%,
more preferably 50 to 150% of the irreversible capacity of the
negative electrode active material.
[0043] Further, when a positive electrode active material of an
electric storage device has an irreversible capacity, additional
metal sodium can be added to the negative electrode composition.
Further, when a synthesized positive electrode active material has
a composition poor in Na, metal sodium can be added to the negative
electrode composition to cover a shortfall (e.g., in an amount
corresponding to 10 to 67% of the capacity of a positive
electrode).
[0044] Examples of the negative electrode active material that
reversibly carries a sodium ion include hard carbon
(non-graphitizable carbon), a sodium-containing titanium oxide, a
lithium-containing titanium oxide, and the like. The
sodium-containing titanium oxide and the lithium-containing
titanium oxide each have a spinel-type crystal structure.
[0045] The hard carbon is different from graphite having a
graphite-type crystal structure, in which carbon layer planes are
stacked in layers, in that the hard carbon has a turbostratic
structure in which carbon layer planes are stacked to be
three-dimensionally misaligned. Even when the hard carbon is
subjected to heat treatment at a high temperature (e.g., at
3000.degree. C.), conversion from the turbostratic structure to the
graphite structure does not occur and graphite crystallites do not
grow. For this reason, the hard carbon is also referred to as
non-graphitizable carbon.
[0046] As an indicator of the degree of growth of a graphite-type
crystal structure in a carbonaceous material, an average
interplanar spacing d.sub.002 between (002) planes of the
carbonaceous material measured from an X-ray diffraction (XRD)
spectrum is used. A carbonaceous material classified as graphite
generally has an average interplanar spacing don as small as less
than 0.337 nm, but the hard carbon having a turbostratic structure
has a large average interplanar spacing d.sub.002 . The average
interplanar spacing don of the hard carbon is, for example, 0.37 nm
or more, preferably 0.38 nm or more. The upper limit of the average
interplanar spacing d.sub.002 of the hard carbon is not
particularly limited. The average interplanar spacing don of the
hard carbon can be, for example, 0.42 nm or less. The average
interplanar spacing don of the hard carbon can be, for example,
0.37 to 0.42 nm, preferably 0.38 to 0.4 nm.
[0047] The hard carbon has a turbostratic structure. The proportion
of a graphite-type crystal structure in the hard carbon is low.
When a sodium ion is occluded in the hard carbon, the sodium ion
enters the turbostratic structure (more specifically, portions
other than portions between layers of the graphite-type crystal
structure) of the hard carbon or are adsorbed to the hard carbon to
be occluded in the hard carbon. The portions other than portions
between layers of the graphite-type crystal structure include, for
example, voids (or pores) formed in the turbostratic structure.
[0048] Since the hard carbon has such voids (or pores) as described
above, the hard carbon has a lower average specific gravity than
graphite having a crystal structure in which carbon layer planes
are densely stacked in layers. The average specific gravity of the
hard carbon is, for example, 1.7 g/cm.sup.3 or less, preferably 1.4
to 1.7 g/cm.sup.3, more preferably 1.5 to 1.7 g/cm.sup.3. When the
hard carbon has such an average specific gravity, a volume change
caused by occlusion and release of a sodium ion during charge and
discharge can be made small, which effectively suppresses
deterioration of the active material.
[0049] The average particle diameter (particle diameter at a
cumulative volume of 50% in a volume particle size distribution) of
the hard carbon is, for example, 3 to 20 .mu.m, preferably 5 to 15
.mu.m. When the average particle diameter is within such a range,
pre-doping with sodium is likely to quickly proceed, and the
filling property of the negative electrode active material in a
negative electrode is easily improved.
[0050] An example of the hard carbon includes a carbonaceous
material obtained by carbonizing a raw material that is carbonized
in a solid state. The raw material that is carbonized in a solid
state is a solid organic substance. Specific examples of such a
solid organic substance include saccharides, resins (e.g.,
thermosetting resins such as phenol resins and thermoplastic resins
such as polyvinylidene chloride), and the like. Examples of the
saccharides include a saccharide having a relatively short sugar
chain (monosaccharides or oligosaccharides such as sugar) and a
polysaccharide such as cellulose (e.g., cellulose or derivatives
thereof (e.g., cellulose esters and cellulose ethers) and materials
containing cellulose such as wood and fruit shells (e.g., coconut
shells)). The hard carbon includes also glassy carbon. These hard
carbons can be used alone or used in admixture of two or more of
them.
[0051] Among the sodium-containing titanium oxides, sodium titanate
is preferred. More specifically, it is preferable that at least one
selected from the group consisting of Na.sub.2Ti.sub.3O.sub.7 and
Na.sub.4Ti.sub.5O.sub.12 is used. The sodium titanate can contain
at least one of another atom or ion replaced with part of Ti or Na
in its crystal structure and a lattice defect. The
sodium-containing titanium oxide to be used can be, for example,
Na.sub.2-xM.sup.1.sub.xTi.sub.3-yM.sup.2.sub.yO.sub.7,
(0.ltoreq.x.ltoreq.3/2, 0.ltoreq.y.ltoreq.8/3, and M.sup.1 and
M.sup.2 are each independently a metal element (atom or ion) other
than Na and Ti, for example, at least one selected from the group
consisting of Ni, Co, Mn, Fe, Al, and Cr),
Na.sub.4-xM.sup.3.sub.xTi.sub.5-yM.sup.4.sub.yO.sub.12(0.ltoreq.x.ltoreq.-
11/3, 0.ltoreq.y.ltoreq.14/3, and M.sup.3 and M.sup.4 are each
independently a metal element (atom or ion) other than Na and Ti,
for example, at least one selected from the group consisting of Ni,
Co, Mn, Fe, Al, and Cr), or the like. These sodium-containing
titanium compounds can be used alone or used in admixture of two or
more of them. The M.sup.1 and M.sup.3 are elements (atoms or ions)
that occupy the Na site in the crystal structure. Further, the
M.sup.2 and M.sup.4 are elements (atoms or ions) that occupy the Ti
site in the crystal structure.
[0052] Among the lithium-containing titanium compounds, lithium
titanate is preferred. More specifically, it is preferable that at
least one selected from the group consisting of
Li.sub.2Ti.sub.3O.sub.7 and Li.sub.4Ti.sub.5O.sub.12 is used. The
lithium titanate can contain at least one of another atom or ion
replaced with part of Ti or Li in its crystal structure and a
lattice defect. The lithium-containing titanium compound to be used
can be, for example,
Li.sub.2-xM.sup.5.sub.xTi.sub.3-yM.sup.6.sub.yO.sub.7
(0.ltoreq.x.ltoreq.3/2, 0.ltoreq.y.ltoreq.8/3, and M.sup.5 and
M.sup.6 are each independently a metal element (atom or ion) other
than Li and Ti, for example, at least one selected from the group
consisting of Ni, Co, Mn, Fe, Al, and Cr),
Li.sub.4-xM.sup.7.sub.xTi.sub.5-yM.sup.8.sub.yO.sub.12(0.ltoreq.x.ltoreq.-
11/3, 0.ltoreq.y.ltoreq.14/3,and M.sup.7 and M.sup.8 are each
independently a metal element (atom or ion) other than Li and Ti,
for example, at least one selected from the group consisting of Ni,
Co, Mn, Fe, Al, and Cr), or the like. These lithium-containing
titanium compounds can be used alone or used in admixture of two or
more of them. The M.sup.5 and M.sup.7 are elements (atoms or ions)
that occupy the Li site in the crystal structure. Further, M.sup.6
and M.sup.8 are elements (atoms or ions) that occupy the Ti site in
the crystal structure.
[0053] The sodium-containing titanium compound and the
lithium-containing titanium compound are each usually particulate.
In this case, the average particle diameter (particle diameter at a
cumulative volume of 50% in a volume particle size distribution) of
each of the sodium-containing titanium compound and the
lithium-containing titanium compound is, for example, 3 to 20
.mu.m, preferably 5 to 15 .mu.m. When the average particle diameter
is within such a range, pre-doping with sodium is likely to quickly
proceed, and the filling property of the negative electrode active
material in a negative electrode is easily improved.
[0054] The binder plays a role in binding particles of the negative
electrode active material together and fixing the negative
electrode active material to a current collector. Examples of the
binder include: a fluorocarbon resin such as
polytetrafluoroethylene, a tetrafluoroethylene-hexafluoropropylene
copolymer, or polyvinylidene fluoride; a polyamide resin such as an
aromatic polyamide; a polyimide resin such as a polyimide (e.g., an
aromatic polyimide) or a polyamideimide; styrene rubber such as
styrene-butadiene rubber (SBR); a rubbery polymer such as butadiene
rubber; a cellulose derivative (e.g., a cellulose ether) such as
carboxymethyl cellulose (CMC) or a salt (e.g., a Na salt) thereof,
and the like. The amount of the binder is preferably 1 to 10 parts
by mass, more preferably 3 to 5 parts by mass per 100 parts by mass
of the negative electrode active material.
[0055] Examples of the conductive auxiliary agent include: a
carbonaceous conductive auxiliary agent such as carbon black or
carbon fiber; and metal fiber. The amount of the conductive
auxiliary agent can be appropriately selected from the range of,
for example, 0.1 to 15 parts by mass, and can be 0.3 to 10 parts by
mass per 100 parts by mass of the active material.
Negative Electrode
[0056] A negative electrode precursor can be obtained by fixing the
negative electrode composition to the surface of a negative
electrode current collector. The negative electrode precursor can
be formed by, for example, applying a negative electrode slurry as
the negative electrode composition to the surface of a negative
electrode current collector, drying the negative electrode slurry,
and, if necessary, rolling the negative electrode current collector
having the negative electrode composition.
[0057] The negative electrode slurry can be prepared by, for
example, mixing a negative electrode active material, metal sodium,
and a liquid dispersion medium (e.g., a second dispersion medium)
for dispersing them (first step). The negative electrode
composition further contains, for example, a second dispersion
medium, a binder, a conductive auxiliary agent, or the like in
addition to the negative electrode active material and the metal
sodium. The second dispersion medium is used to adjust the solid
content of the negative electrode slurry to a level within a range
suitable for application.
[0058] Examples of the second dispersion medium include, but are
not particularly limited to a ketone such as acetone; an ether such
as tetrahydrofuran; a nitrile such as acetonitrile; an amide such
as dimethylacetamide; N-methyl-2-pyrrolidone, and the like. These
second dispersion media can be used alone or used in admixture of
two or more of them.
[0059] By applying the negative electrode slurry to the current
collector, the negative electrode composition can be held by a
negative electrode current collector (second step). A method for
applying the negative electrode slurry is not particularly limited.
The negative electrode slurry can be applied using, for example, an
applicator such as a die coater.
[0060] Examples of the negative electrode current collector
include, but are not limited to, a metal foil, a non-woven fabric
made of metal fiber, a metal porous body, and the like. A metal
constituting the negative electrode current collector is preferably
copper, a copper alloy, nickel, a nickel alloy, aluminum, an
aluminum alloy, or the like, since the metal does not form an alloy
with sodium and is stable at a negative electrode electric
potential. It is preferable that the copper alloy contains an
element other than copper in an amount of less than 50 mass %. It
is preferable that the nickel alloy contains an element other than
nickel in an amount of less than 50 mass %. It is preferable that
the aluminum alloy contains an element other than aluminum in an
amount of less than 50 mass %.
[0061] When the negative electrode current collector is a metal
foil, the metal foil has a thickness of, for example, 10 to 50
.mu.m. When the negative electrode current collector is a non-woven
fabric made of metal fiber or a metal porous body, the non-woven
fabric made of metal fiber and the metal porous body each have a
thickness of, for example, 100 to 1000 .mu.m.
[0062] After the second step, at least part of the liquid
dispersion medium (first and second dispersion media) is evaporated
from the negative electrode slurry held by the negative electrode
current collector, thereby giving a negative electrode precursor
(third step). In this step, pre-doping with sodium does not usually
proceed, and therefore the resulting negative electrode active
material layer is in the form of a precursor. The negative
electrode composition is deposited on the negative electrode
current collector through the third step, which makes it easy to
perform subsequent processing.
[0063] When the second dispersion medium is completely evaporated,
there is a possibility that the metal sodium is brought into a dry
state so that a side reaction proceeds. In this case, subsequent
processes need to be performed in a reduced-pressure atmosphere or
an inert gas atmosphere. From the viewpoint of eliminating such a
need, only part of the liquid dispersion medium is preferably
evaporated in the third step. Then, the negative electrode current
collector for holding the negative electrode composition containing
the liquid dispersion medium can be rolled to improve the strength
of the negative electrode active material layer precursor.
[0064] After the third step, the negative electrode precursor is
brought into contact with an electrolyte having sodium ion
conductivity, thereby doping the negative electrode active material
with a sodium ion eluted from the metal sodium (fourth step). The
fourth step can be usually performed in a device case, after the
assembly of an electric storage device is almost completed.
However, the fourth step can be previously performed before the
assembly of an electrode group including a positive electrode, a
negative electrode, and a separator interposed between them. For
example, a negative electrode is formed from the negative electrode
precursor by immersing the negative electrode precursor in an
electrolyte having sodium ion conductivity to allow at least part
of pre-doping to proceed, thereafter the assembly of an electrode
group can be performed by making use of the resulting negative
electrode.
[0065] In the fourth step, it is preferable that the negative
electrode precursor containing no liquid dispersion medium, from
the viewpoint of enhancing the efficiency of contact between the
negative electrode precursor and the electrolyte. Therefore, when
only part of the second dispersion medium is evaporated in the
third step, it is preferable that the operation of substantially
completely removing the liquid dispersion medium (e.g., drying by
heating, drying under reduced pressure, or drying by heating under
reduced pressure) is performed just before (e.g., within 10 minutes
before) contact between the negative electrode precursor and the
electrolyte. Alternatively, in the fourth step, there can be used
the negative electrode precursor containing only a minute amount
(e.g., an amount corresponding to 0.1 mass % or less of the
negative electrode active material contained in the negative
electrode precursor) of the remaining liquid dispersion medium.
This is because a minute amount of the liquid dispersion medium is
easily replaced with the electrolyte by introducing the
electrolyte.
[0066] The negative electrode preferably has a thickness of 50 to
600 .mu.m. When the negative electrode has a thickness within the
above range, pre-doping with sodium is likely to be more uniformly
performed. This is because the electrolyte can uniformly penetrate
the entire of the negative electrode, including the inside of the
negative electrode, and therefore a sodium ion also can be smoothly
moved inside the negative electrode.
Positive Electrode
[0067] The positive electrode includes a positive electrode active
material. The positive electrode active material preferably
electrochemically occludes and releases a sodium ion. The positive
electrode includes a positive electrode current collector and a
positive electrode active material fixed to the surface of the
positive electrode current collector. The positive electrode can
include, as an optional component, a binder, a conductive auxiliary
agent, or the like.
[0068] Similarly to the negative electrode current collector,
examples of the positive electrode current collector include, but
are not limited to, a metal foil, a non-woven fabric made of metal
fiber, a metal porous sheet, and the like. A metal constituting the
positive electrode current collector is not particularly limited,
but is preferably aluminum or an aluminum alloy, since the aluminum
or aluminum alloy is stable at a positive electrode electric
potential. The thickness of the positive electrode current
collector can be selected from the same range as that of the
negative electrode current collector. As the positive electrode
current collector, the metal porous body that has been described
above as the negative electrode current collector can also be
used.
[0069] From the viewpoint of thermal stability and electrochemical
stability, there can be preferably used as the positive electrode
active material a transition metal compound such as a compound
containing a sodium atom and a transition metal atom (e.g., a
transition metal atom in the fourth period of the periodic table,
such as a chromium atom, a manganese atom, an iron atom, a cobalt
atom, or a nickel atom) as a constituent atom; another compound
containing the above-described transition metal atom as a
constituent atom, or the like. Such a compound can contain one or
two or more kinds of transition metal atoms as a constituent atom.
The compound can contain at least one of an atom or ion of a
typical metal, such as aluminum, replaced with part of at least one
of Na and the transition metal in its crystal structure and a
lattice defect.
[0070] Examples of the transition metal compound include a sulfide
an oxide, and the like. Examples of the sulfide include: a
transition metal sulfide such as TiS.sub.2 or FeS.sub.2; a
sodium-containing transition metal sulfide such as NaTiS.sub.2; and
the other transition metal sulfide. Examples of the oxide include a
sodium-containing transition metal oxide such as NaCrO.sub.2,
NaNi.sub.0.5Mn.sub.0.5O.sub.2, NaMn.sub.1.5Ni.sub.0.5O.sub.4,
NaFeO.sub.2, NaFe.sub.x1 (Ni.sub.0.5Mn.sub.0.5).sub.1-x1O.sub.2
(0<x1<1), Na.sub.2/3Fe.sub.1/3Mn.sub.2/3O.sub.2, NaMnO.sub.2,
NaNiO.sub.2, NaCoO.sub.2, or Na.sub.0.44MnO.sub.2. These positive
electrode active materials can be used alone or used in admixture
of two or more of them. Among these transition metal compounds,
sodium-containing transition metal compounds, for example, at least
one selected from the group consisting of sodium chromite
(NaCrO.sub.2) and sodium iron manganate
(Na.sub.2/3Fe.sub.1/3Mn.sub.2/3O.sub.2).
[0071] The binder and the conductive auxiliary agent can each be
appropriately selected from those exemplified above with reference
to the negative electrode. The amount of each of the binder and the
conductive auxiliary agent with respect to the amount of the active
material can also be appropriately selected from the range
exemplified above with reference to the negative electrode.
[0072] As in the case of the negative electrode, the positive
electrode can be formed by applying, to the surface of a positive
electrode current collector, a positive electrode slurry obtained
by dispersing a positive electrode active material and a binder and
a conductive auxiliary agent, which are used if necessary, in a
dispersion medium, drying the positive electrode slurry, and, if
necessary, rolling the positive electrode current collector having
the positive electrode active material. The dispersion medium can
be appropriately selected from those exemplified above with
reference to the negative electrode.
Separator
[0073] The separator plays a role in physically isolating the
positive electrode and the negative electrode from each other to
prevent an internal short-circuit. The separator is made of a
porous material, and pores in the porous material are filled with
an electrolyte so that the separator has sodium ion permeability to
ensure a battery reaction. As the separator, for example, a
microporous film made of a resin or a non-woven fabric can be used.
The thickness of the separator is not particularly limited, but can
be selected from the range of, for example, about 10 to 300
.mu.m.
Electrolyte
[0074] The electrolyte having sodium ion conductivity contains at
least a sodium salt. As the electrolyte there can be used an ionic
liquid containing a sodium salt or an organic solvent in which a
sodium salt is dissolved (organic electrolyte). The concentration
of the sodium salt contained in the organic electrolyte can be, for
example, 0.3 to 3 mol/L.
[0075] An anion (first anion) constituting the sodium salt is not
particularly limited. Examples of the anion include: an anion of a
fluorine-containing acid (e.g., an anion of fluorine-containing
phosphoric acid such as a hexafluorophosphate ion (PF.sub.6.sup.-);
an anion of a fluorine-containing boric acid such as a
tetrafluoroborate ion (BF.sub.4.sup.-)); an anion of a
chlorine-containing acid (e.g., a perchlorate ion
(ClO.sub.4.sup.-)); an anion of a fluoroalkane sulfonic acid (e.g.,
a trifluoromethanesulfonate ion (CF.sub.3SO.sub.3.sup.-)); and a
bis(sulfonyl)amide anion. The sodium salt can be used alone or used
in admixture of two or more kinds of the sodium salts different in
the kind of first anion.
[0076] The anion of the sodium salt is preferably a
fluorine-containing bis(sulfonyl)amide anion. Examples of the
fluorine-containing bis(sulfonyl)amide anion include: a
bis(fluorosulfonyl) amide anion (N(SO.sub.2F).sub.2.sup.-); a
(fluorosulfonyl)(perfluoroalkylsulfonyl)amide anion (e.g., a
(fluorosulfonyl)(trifluoromethylsulfonyl)amide anion
((FSO.sub.2)(CF.sub.3SO.sub.2)N.sup.-)); a
bis(perfluoroalkylsulfonyl)amide anion (e.g., a
bis(trifluoromethylsulfonyl)amide anion
(N(SO.sub.2CF.sub.3).sub.2.sup.-), a
bis(pentafluoroethylsulfonyl)amide anion
(N(SO.sub.2C.sub.2F.sub.5).sub.2.sup.-)), and the like. The
perfluoroalkyl group has, for example, 1 to 4 carbon atoms,
preferably 1 to 3 carbon atoms. The sodium salt is particularly
preferably sodium bisfluorosulfonylamide
(NaN(SO.sub.2F).sub.2).
[0077] The ionic liquid can further contain, in addition to a
sodium ion (first cation), a second cation. The second cation can
be an inorganic cation other than sodium (e.g., a potassium ion, a
magnesium ion, a calcium ion, an ammonium cation), but is
preferably an organic cation. These second cations can be used
alone or used in admixture of two or more of them.
[0078] Examples of the organic cation include: a cation derived
from an aliphatic amine, an alicyclic amine, or an aromatic amine
(e.g., a quaternary ammonium cation); a nitrogen-containing onium
cation such as a cation having a nitrogen-containing heterocycle (a
cation derived from a cyclic amine); a sulfur-containing onium
cation; a phosphorus-containing onium cation, and the like. The
second cation is particularly preferably an organic onium cation
having a pyrrolidine skeleton or an imidazoline skeleton. When the
second cation is an organic cation, it is easy to reduce the
melting point of a molten salt electrolyte.
[0079] A second anion as a counter anion of the second cation is
preferably a bis(sulfonyl)amide anion (particularly, a
fluorine-containing bis(sulfonyl)amide anion). Further, 90 mol % or
more of the anion constituting the ionic liquid is preferably a
fluorine-containing bis(sulfonyl)amide anion.
[0080] The mole ratio between sodium ion and organic cation (sodium
ion/organic cation) of the ionic liquid is preferably, for example,
1/99 to 60/40, more preferably 5/95 to 50/50.
Sodium Ion Battery
[0081] Hereinbelow, the structure of a sodium ion battery as one
example of the electric storage device will be described.
[0082] FIG. 1 is a longitudinal sectional view schematically
showing the structure of a sodium ion battery. A sodium ion battery
100 includes a stack-type electrode group, an electrolyte (not
shown), and a rectangular aluminum battery case 10 that houses
them. The battery case 10 includes a case main body 12 having an
upper opening and a closed bottom and a lid 13 that closes the
upper opening.
[0083] The assembly of the sodium ion battery is performed by first
stacking negative electrodes 2 and positive electrodes 3 with
separators 1 being interposed between them to form an electrode
group. The formed electrode group is inserted into the case main
body 12 of the battery case 10. The negative electrodes 2 can be
negative electrode precursors containing metal sodium.
Alternatively, negative electrodes in which a negative electrode
active material has already been pre-doped with sodium can be used.
Then, the step of pouring an electrolyte into the case main body 12
is performed to fill gaps between the separators 1 and the negative
and positive electrodes 2 and 3 constituting the electrode group
with the electrolyte. At this time, a sodium ion is eluted from the
metal sodium contained in the negative electrode precursors so that
the negative electrode active material is doped with sodium.
[0084] In the center of the lid 13, a safety valve 16 is provided
to release gas generated inside the battery case 10 when the inner
pressure of the battery case 10 is increased. At a position close
to one side of the lid 13 having the safety valve 16 in the center
thereof, an external negative electrode terminal 14 is provided so
as to pass through the lid 13, and at a position close to the other
side of the lid 13, an external positive electrode terminal is
provided so as to pass through the lid 13.
[0085] The stack-type electrode group is constituted from a
plurality of the negative electrodes 2 and a plurality of the
positive electrodes 3, each of which has a rectangular sheet shape,
and a plurality of the separators 1 interposed between them. In
FIG. 1, each of the separators 1 is bag-shaped to envelop the
positive electrode 3, but the form of the separator is not
particularly limited. A plurality of the negative electrodes 2 and
a plurality of the positive electrodes 3 are alternately arranged
in their stacking direction in the electrode group.
[0086] At one end of each of the negative electrodes 2, a negative
electrode lead piece 2a can be formed. The negative electrode lead
pieces 2a of a plurality of the negative electrodes 2 are tied
together and connected to the external negative electrode terminal
14 provided in the lid 13 of the battery case 10 so that a
plurality of the negative electrodes 2 are connected in parallel.
Similarly, at one end of each of the positive electrodes 3, a
positive electrode lead piece 3a can be formed. The positive
electrode lead pieces 3a of a plurality of the positive electrodes
3 are tied together and connected to the external positive
electrode terminal provided in the lid 13 of the battery case 10 so
that a plurality of the positive electrodes 3 are connected in
parallel.
[0087] Each of the external negative electrode terminal 14 and the
external positive electrode terminal is columnar, and has a thread
groove in at least a portion exposed to the outside. A nut 7 is
engaged with the thread groove of each of the terminals. The nut 7
is fixed to the lid 13 by rotating the nut 7. Each of the terminals
has a flange 8 provided in its portion to be housed inside the
battery case so that the flange 8 is fixed to the inner surface of
the lid 13 via a washer 9 by rotating the nut 7.
[0088] Hereinbelow, the present invention will be described based
on examples and comparative examples, but is not limited to the
following examples.
Example 1
(1) Preparation of Positive Electrode
[0089] A positive electrode slurry is prepared by mixing
NaCrO.sub.2 (positive electrode active material), acetylene black
(conductive auxiliary agent), polyvinylidene fluoride (PVDF)
(binder) and N-methyl-2-pyrrolidone (NMP) so that a ratio (mass
ratio) of positive electrode active material/conductive auxiliary
agent/binder was 85/10/5. A disk-shaped positive electrode
(diameter: 12 mm, thickness of positive electrode active material
layer: 85 .mu.m) was obtained by applying the resulting positive
electrode slurry to one of the surfaces of an aluminum foil as a
positive electrode current collector, drying the resulting product,
compressing dried product, drying in vacuum the compressed product
at 150.degree. C., and stamping the dried product into a circular
shape. The mass of the positive electrode active material per unit
area of the resulting positive electrode was 13.3 mg/cm.sup.2.
(2) Preparation of Negative Electrode
First Step
[0090] Molten metal sodium was solidified in a mold and then taken
out of the mold. The resulting solid metal sodium was transferred
into a 4N grade nitrogen atmosphere having a dew point of
-10.degree. C. or below and an oxygen concentration of 0.01% or
less (into a glove box). In the glove box, 10.2 g of the solid
metal sodium and 90 g of normal paraffin were put into a
four-necked flask. Then, the mixture was stirred while heated to a
temperature equal to or higher than the melting point of the metal
sodium to disperse the sodium. As a result, a dispersion containing
10 mass % of metal sodium particles was obtained.
[0091] The surface of the metal sodium particle was black. The
metal sodium particle had an almost spherical shape and an average
particle diameter D of 10 .mu.m as measured by a particle size
analyzer (SYMPATEC HELOS Laser Diffraction Analyser manufactured by
SYMPATEC).
[0092] At the same time, 9 g of a polymer containing a styrene unit
and a butadiene unit, 90 g of normal paraffin, and 170 g of hard
carbon as a negative electrode active material (average particle
diameter: 10 .mu.m) were mixed to prepare a negative electrode
mixture containing a binder. The dispersion of metal sodium (in an
amount corresponding to 100% of the irreversible capacity of the
negative electrode active material) and N-methyl-2-pyrrolidone
(NMP) as a second dispersion medium were added to and mixed with
the negative electrode mixture, thereby giving a negative electrode
composition (negative electrode slurry).
Second Step
[0093] The resulting negative electrode composition was applied to
an aluminum foil as a negative electrode current collector.
Third Step
[0094] The negative electrode composition applied to the negative
electrode current collector was dried at 130.degree. C. to
evaporate the first dispersion medium and the second dispersion
medium. In the negative electrode composition after drying, the
first dispersion medium and the second dispersion medium were
allowed to remain in a total amount of 0.01 parts by mass per 100
parts by mass of the negative electrode active material. Then, a
disk-shaped negative electrode precursor (diameter: 12 mm,
thickness of negative electrode active material layer precursor: 70
.mu.m) was obtained by rolling the negative electrode composition,
and stamping the resulting product into a circular shape. The mass
of the negative electrode active material per unit area of the
negative electrode precursor was 5.4 mg/cm.sup.2.
Fourth Step
[0095] A coin-type battery was produced using the resulting
positive electrode and the negative electrode precursor. Here, the
negative electrode precursor was placed at the inner bottom of a
case of a coin-type battery, and a separator was placed on the
negative electrode precursor. Then, the positive electrode was
placed so as to be opposed to the negative electrode with the
separator being interposed between them. Then, an electrolyte was
poured into the case, and a lid having an insulating gasket at its
edge was fitted to the opening of the battery case to produce a
coin-type sodium ion battery (battery A1). As the separator, a
microporous film made of a heat-resistant polyolefin (thickness: 50
.mu.m) was used. The ratio of the reversible capacity of the
negative electrode (C.sub.n ) to the reversible capacity of the
positive electrode (C.sub.p) (C.sub.n/C.sub.p) was 1.
[0096] After the completion of the battery A1, the battery A1was
allowed to stand at 60.degree. C. for 72 hours. This allowed a
sodium ion to elute from the metal sodium into the electrolyte
within the battery so that doping of the negative electrode active
material with sodium proceeded.
[0097] As the electrolyte, a molten salt electrolyte was used which
was composed of 100% ionic liquid containing sodium
bisfluorosulfonylamide (NaFSA) and 1-methyl- 1-propylpyrrolidinium
bisfluorosulfonylamide (P.sub.13FSA) in a mole ratio of 40/60
(NaFSA/P.sub.13FSA).
Comparative Example 1
[0098] A battery B1 was produced in the same manner as in Example 1
except for the following points.
First Step
[0099] A negative electrode composition (negative electrode slurry)
was prepared in the same manner as in Example 1 except that the
dispersion of metal sodium was not used.
Second Step
[0100] The resulting negative electrode composition was applied to
an aluminum foil as a negative electrode current collector.
Third Step
[0101] The negative electrode composition applied to the negative
electrode current collector was dried at 130.degree. C. to
evaporate the first dispersion medium and the second dispersion
medium. From the viewpoint of matching drying conditions, in the
negative electrode composition after drying, the first dispersion
medium and the second dispersion medium were allowed to remain in a
total amount of 0.01 parts by mass per 100 parts by mass of the
negative electrode active material. Then, a disk-shaped negative
electrode (diameter: 12 mm, thickness of negative electrode active
material layer: 70 .mu.m) was obtained by rolling the negative
electrode composition, and stamping the resulting product into a
circular shape. The mass of the negative electrode active material
per unit area of the negative electrode was 5.4 mg/cm.sup.2.
Fourth Step
[0102] A coin-type battery was produced using the resulting
positive electrode and the negative electrode precursor. Here, a
metal sodium foil having a thickness of 50 .mu.m was attached to
the inner bottom of a case of a coin-type battery. Then, the
negative electrode was placed on the metal sodium foil, and a
separator was placed on the negative electrode. Then, the positive
electrode was placed so as to be opposed to the negative electrode
with the separator being interposed between them. Then, an
electrolyte was poured into the case, and a lid having an
insulating gasket at its edge was fitted to the opening of the
battery case to produce a coin-type sodium ion battery (battery
B1).
[0103] After the completion of the battery B1, the battery B1 was
allowed to stand at 60.degree. C. for 72 hours. This allowed a
sodium ion to elute from the metal sodium foil into the electrolyte
within the battery so that doping of the negative electrode active
material with sodium proceeded.
Evaluation
[0104] The sodium ion battery was heated to 60.degree. C., and was
subjected to constant-current charge at a current rate of 1C up to
3.3 V and then subjected to constant-voltage charge at 3.3 V
(initial charge). Then, the sodium ion battery was discharged to
1.8 V at a current rate of 1C (initial discharge) to measure the
discharge capacity of the battery during initial discharge (first
cycle discharge capacity).
[0105] The results of measuring the initial discharge capacity per
gram of the positive electrode active material in Example and
Comparative Example are shown in Table 1.
TABLE-US-00001 TABLE 1 Initial discharge capacity (mAh/g) Example 1
110 mAh/g Comparative 85 mAh/g Example 1
[0106] As shown in Table 1, the initial discharge capacity of
Example 1 is larger than that of Comparative Example 1. It is
considered that a sufficient utilization rate of the active
material was achieved from the initial stage, since the negative
electrode active material in the negative electrode was uniformly
doped with sodium. Further, the cycle characteristic of Example 1
is superior to that of Comparative Example 1.
INDUSTRIAL APPLICABILITY
[0107] The present invention is useful in the field of electric
storage devices utilizing a faradaic reaction of a sodium ion, and
is particularly suitable for promoting the efficiency of a sodium
ion battery production process.
REFERENCE SIGNS LIST
[0108] 1: SEPARATOR [0109] 2: NEGATIVE ELECTRODE [0110] 2a:
NEGATIVE ELECTRODE LEAD PIECE [0111] 3: POSITIVE ELECTRODE [0112]
3a: POSITIVE ELECTRODE LEAD PIECE [0113] 7: NUT [0114] 8: FLANGE
[0115] 9: WASHER [0116] 10: BATTERY CASE [0117] 12: CASE MAIN BODY
[0118] 13: LID [0119] 14: EXTERNAL NEGATIVE ELECTRODE TERMINAL
[0120] 16: SAFETY VALVE [0121] 100: SODIUM ION SECONDARY
BATTERY
* * * * *