U.S. patent application number 11/923920 was filed with the patent office on 2008-03-13 for cell electrode and electrochemical cell therewith.
This patent application is currently assigned to NEC TOKIN CORPORATION. Invention is credited to Hiroyuki Kamisuki, Shinako Kaneko, Masato Kurosaki, Masaya Mitani, Yuji Nakagawa, Toshihiko Nishiyama, Tomoki Nobuta.
Application Number | 20080063940 11/923920 |
Document ID | / |
Family ID | 30437722 |
Filed Date | 2008-03-13 |
United States Patent
Application |
20080063940 |
Kind Code |
A1 |
Nobuta; Tomoki ; et
al. |
March 13, 2008 |
CELL ELECTRODE AND ELECTROCHEMICAL CELL THEREWITH
Abstract
This invention provides an electrode for an electrochemical cell
in which an active material in an electrode material is a
proton-conducting compound, wherein the electrode material
comprises a nitrogen-containing heterocyclic compound or a polymer
having a unit containing a nitrogen-containing heterocyclic
moiety.
Inventors: |
Nobuta; Tomoki; (Miyagi,
JP) ; Nishiyama; Toshihiko; (Miyagi, JP) ;
Kamisuki; Hiroyuki; (Miyagi, JP) ; Kaneko;
Shinako; (Miyagi, JP) ; Kurosaki; Masato;
(Tokyo, JP) ; Nakagawa; Yuji; (Tokyo, JP) ;
Mitani; Masaya; (Miyagi, JP) |
Correspondence
Address: |
KATTEN MUCHIN ROSENMAN LLP
575 MADISON AVENUE
NEW YORK
NY
10022-2585
US
|
Assignee: |
NEC TOKIN CORPORATION
Miyagi
JP
|
Family ID: |
30437722 |
Appl. No.: |
11/923920 |
Filed: |
October 25, 2007 |
Related U.S. Patent Documents
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Application
Number |
Filing Date |
Patent Number |
|
|
10634607 |
Aug 5, 2003 |
7309544 |
|
|
11923920 |
Oct 25, 2007 |
|
|
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Current U.S.
Class: |
429/213 ;
429/212; 524/612; 528/423 |
Current CPC
Class: |
H01M 4/608 20130101;
H01M 4/60 20130101; Y02E 60/13 20130101; H01M 4/137 20130101; H01M
10/05 20130101; Y02E 60/10 20130101; H01G 9/155 20130101 |
Class at
Publication: |
429/213 ;
429/212; 524/612; 528/423 |
International
Class: |
H01M 4/60 20060101
H01M004/60; C08G 73/06 20060101 C08G073/06; C08G 73/18 20060101
C08G073/18 |
Foreign Application Data
Date |
Code |
Application Number |
Aug 5, 2002 |
JP |
2002-227160 |
Claims
1. An electrode for an electrochemical cell, which comprises: a
proton-conducting compound as an active material; and a polymer
comprising a unit containing a nitrogen-containing heterocyclic
moiety.
2. The electrode of claim 1, wherein the nitrogen-containing
heterocyclic moiety is a moiety selected from the group consisting
of a benzimidazole moiety, a benzbisimidazole moiety and an
imidazole moiety.
3. The electrode of claim 1, wherein the polymer is a
polybenzimidazole represented by formula (6) or a
polyvinylimidazole represented by formula (7); ##STR9## wherein n
represents a positive integer.
4. The electrode of claim 1, wherein the polymer further comprises
a unit containing a proton-conducting moiety.
5. The electrode of claim 1, wherein a content of the unit
containing the nitrogen-containing heterocyclic moiety in the
polymer is at least 5 mol %.
6. The electrode of claim 1, further comprising a
nitrogen-containing heterocyclic compound.
7. The electrode of claim 6, wherein the nitrogen-containing
heterocyclic compound is one or more compounds selected from the
group consisting of imidazole, triazole, pyrazole, benzimidazole
and their derivatives.
8. The electrode of claim 6, wherein the nitrogen-containing
heterocyclic compound is one or more compounds selected from the
group consisting of imidazole or its derivatives represented by
formula (1), trizole or its derivatives represented by formula (2)
or (3), pyrazole or its derivatives represented by formula (4) and
benzimidazole or its derivatives represented by formula (5):
##STR10## wherein R independently represents hydrogen, alkyl having
1 to 4 carbon atoms, amino, carboxyl, nitro, phenyl, vinyl,
halogen, acyl, cyano, trifluoromethyl, alkylsulfonyl or
trifluoromethylthio.
9. The electrode of claim 1, comprising 1 to 80 parts by weight of
the polymer to 100 parts by weight of the active material.
10. The electrode of claim 6, comprising 1 to 80 parts by weight of
the nitrogen-containing heterocyclic compound and the polymer to
100 parts by weight of the active material.
11. An electrochemical cell, comprising: a positive electrode
comprising a proton-conducting compound as an active material; and
a negative electrode comprising a proton-conducting compound as an
active material; wherein at least one of the electrodes is the
electrode as claimed in claim 1.
12. An electrochemical cell of claim 11, comprising an electrolyte
containing a proton source wherein only protons act as a charge
carrier in a redox reaction in both electrodes associated with
charge and discharge.
13. A secondary battery comprising the electrochemical cell of
claim 11.
14. An electrode for an electrochemical cell, comprising: a
proton-conducting polymer comprising a unit containing a
proton-conducting moiety and a unit containing a
nitrogen-containing heterocyclic moiety, as an active material.
15. The electrode of claim 14, wherein the nitrogen-containing
heterocyclic moiety is a moiety selected from the group consisting
of: a benzimidazole moiety, a benzbisimidazole moiety, and an
imidazole moiety.
16. The electrode of claim 14, wherein the unit containing the
proton-conducting moiety is a unit selected from the group
consisting of: a quinoxaline moiety and a phenylquinoxaline
moiety.
17. The electrode of claim 14, wherein the proton-conducting
polymer is a polymer comprising a unit represented by formula (18):
##STR11##
18. An electrochemical cell, comprising: a first electrode
containing a polymer comprising a unit containing a
nitrogen-containing heterocyclic moiety; a second electrode; and a
separator separating the first electrode and the second
electrode.
19. The electrochemical cell of claim 18, wherein the
nitrogen-containing heterocyclic moiety is a moiety selected from
the group consisting of: a benzimidazole moiety, a benzbisimidazole
moiety and an imidazole moiety.
20. The electrochemical cell of claim 18, wherein the first
electrode further comprises a proton-conducting compound as an
active material.
21. The electrochemical cell of claim 18, wherein the polymer is a
proton conducting polymer comprising a unit containing a
proton-conducting moiety and a unit containing a
nitrogen-containing heterocyclic moiety, the polymer being an
active material.
22. The electrochemical cell of claim 18, wherein the second
electrode comprises a proton-conducting compound as an active
material.
23. The electrochemical cell of claim 18, wherein the second
electrode comprises a proton-conducting compound as an active
material, and a nitrogen-containing heterocyclic compound.
24. The electrochemical cell of claim 18, wherein the second
electrode comprises a proton-conducing compound as an active
material, and a polymer comprising a unit containing a
nitrogen-containing heterocyclic moiety.
25. The electrochemical cell of claim 18 further comprising a
proton-ionizing electrolyte.
Description
BACKGROUND OF THE INVENTION
[0001] 1. Field of the Invention
[0002] This invention relates to an electrode used in an
electrochemical cell such as a secondary battery and an electric
double-layer capacitor and an electrochemical cell using the
electrode. In particular, it relates to an electrode having
improved cycle properties without reduction in an appearance
capacity, and an electrochemical cell using the electrode.
[0003] 2. Description of the Related Art
[0004] There have been suggested and practically used
electrochemical cells (hereinafter, referred to as "cell") such as
secondary batteries and electric double-layer capacitors in which a
proton-conducting compound is used as an electrode active material.
Such a cell is illustrated in a cross-sectional view of FIG. 1.
[0005] Specifically, FIG. 1 shows a cell where a positive electrode
2 containing a proton-conducting compound as an active material is
formed on a positive current collector 1 while a negative electrode
3 is formed on a negative current collector 4, and these electrodes
are combined via a separator 5 and where only protons are involved
in an electrode reaction as a charge carrier. Also, the cell is
filled with an aqueous or non-aqueous solution containing a proton
source as an electrolytic solution, and is sealed by a gasket
6.
[0006] The electrodes 2, 3 are formed as follows. A powdery doped
or undoped proton-conducting compound is blended with a conductive
auxiliary and a binder to prepare a slurry, which is then placed in
a mold and molded by a hot press to form an electrode having a
desired electrode density and a desired film thickness.
Alternatively, the slurry is screen-printed on a conductive
base-material and dried to form an electrode. Then, a positive
electrode and a negative electrode thus formed are mutually faced
via a separator to give a cell.
[0007] Examples of a proton-conducting compound used as an
electrode active material include .pi.-conjugated polymers such as
polyaniline, polythiophene, polypyrrole, polyacetylene,
poly-p-phenylene, polyphenylene-vinylene, polyperinaphthalene,
polyfuran, polyflurane, polythienylene, polypyridinediyl,
polyisothianaphthene, polyquinoxaline, polypyridine,
polypyrimidine, polyindole, polyaminoanthraquinone and their
derivatives; indole-based compounds such as indole trimer; and
hydroxyl-containing polymers such as polyanthraquinone and
polybenzoquinone where a quinone oxygen is converted into a
hydroxyl group by conjugation). These compounds may be doped to
form a redox pair exhibiting conductivity. These compounds are
appropriately selected as a positive active material and a negative
active material, taking a redox potential difference into
account.
[0008] Known electrolytic solutions include an aqueous electrolytic
solution consisting of an aqueous acid solution and a non-aqueous
electrolytic solution based on an organic solvent. When using a
proton-conducting compound, the former aqueous electrolytic
solution is preferentially used because it can give a high-capacity
cell. The acid used may be an organic or inorganic acid; for
example, inorganic acids such as sulfuric acid, nitric acid,
hydrochloric acid, phosphoric acid, tetrafluoroboric acid,
hexafluorophosphoric acid and hexafluorosilicic acid and organic
acids such as saturated monocarboxylic acids, aliphatic carboxylic
acids, oxycarboxylic acids, p-toluenesulfonic acid,
polyvinylsulfonic acid and lauric acid.
[0009] A cell using such a proton-conducting compound as an
electrode active material has a short cycle life due to increase in
an internal resistance, and the tendency becomes more prominent as
a temperature is elevated. Furthermore, it has a drawback of
insufficient long term stability under a high temperature
atmosphere.
[0010] These problems are caused by aggravated deterioration
atmosphere due to deceleration of proton adsorption-desorption
reaction as a charge/discharge mechanism of an electrode active
material. In particular, at an elevated temperature, peroxidation
of a material is much more accelerated, resulting in accelerated
deterioration.
[0011] An electrode active material is susceptible to deterioration
in an oxidized state. It is probably because a proton (H.sup.+)
adsorption-desorption reaction for the active material is
deteriorated over time in the charge/discharge mechanism as
described below. Such deterioration proceeds because
doping/dedoping activity of the active material is reduced under an
excess proton atmosphere rather than an optimal proton atmosphere
which depends on the identity of the active material and the number
of reaction electrons, in a proton adsorption-desorption reaction
between the active material and an electrolyte. Thus,
charge/discharge power of the cell is deteriorated. It is called
"peroxidation-perreduction deterioration"; specifically,
peroxidation deterioration for an active material of positive
electrode and perreduction deterioration for an active material of
negative electrode.
[0012] This phenomenon will be described for a case where an active
material of positive electrode is an indole derivative (indole
trimer) while an active material of negative electrode is a
quinoxaline polymer. Herein, charge/discharge mechanisms for a
positive and a negative electrode materials are as indicated in
chemical formulas (8) and (9), respectively, wherein R represent
appropriate substituents and X.sup.- represents an anion.
##STR1##
[0013] Under a high-level acid atmosphere (low pH), the phenomenon
particularly tends to occur so that deterioration in cycle
properties is accelerated. For polyphenylquinoxaline which can be
used as a material of negative electrode, tetraprotonation may be
caused whereas a normal doped state is represented by a
diprotonated derivative in a charge/discharge mechanism. Thus, the
active material is dissolved, leading to reduction in a
charge/discharge power. An excessively higher electrolyte
concentration (proton concentration) may further accelerate
oxidation deterioration.
[0014] FIG. 6 is a graph showing variation in cycle properties to
an electrolyte concentration (sulfuric acid concentration). As seen
in this graph, as an electrolytic solution concentration increases,
a capacity decreases according to the cycle number so that cycle
properties are deteriorated. In addition, under a low concentration
atmosphere, cycle properties are improved while an appearance
capacity tends to be reduced. FIG. 7 is a graph illustrating
variation in an appearance capacity to an electrolyte concentration
(sulfuric acid concentration). As seen in this graph, as an
electrolyte concentration is reduced, an appearance capacity is
reduced.
[0015] Electrolytic solutions comprising a nitrogen-containing
heterocyclic compound as a non-aqueous electrolytic solution in the
prior art have been described in Japanese Laid-open Patent
Publication Nos. 2000-156329 (Prior art 1) and 2001-143748 (Prior
art 2). Japanese Laid-open Patent Publication No. 7-320780 (Prior
art 3) has described a solid-electrolyte secondary battery
comprising a polymer gel electrolyte consisting of, for example, an
aprotic solvent and polyimidazole. Japanese Laid-open Patent
Publication No. 10-321232 (Prior art 4) has described an electrode
comprising a benzimidazole derivative although an electrolytic
solution used therein is different from that in this invention.
[0016] In Prior art 1, there has been disclosed an electrolytic
solution for an aluminum electrolysis capacitor comprising a
quaternary salt having of a quaternary cation from a compound
containing N,N,N'-substituted amidine group and an organic acid
anion, and an organic solvent. There has been described that
although a conventional electrolytic solution comprising a
quaternary ammonium carboxylate has a drawback that degradation of
a rubber packing is accelerated so that sealing performance is
significantly deteriorated, an additive having a cationic,
quaternary amidine group may improve thermal stability of the
electrolytic solution and a specific conductivity, and that in
particular, a compound in which electrons in the amidine group are
delocalized and a cation is stabilized by resonance gives an
improved specific conductivity because of accelerated ion
dissociation. There has been further described that when excess
hydroxide ions are generated after electrolysis in the electrolytic
solution, the hydroxide ions may rapidly disappear by reaction of
the hydroxide ions and the amidine group so that unlike a
conventional quaternary ammonium salt, effects of the electrolysis
can be reduced and thus degradation of a packing in a capacitor can
be minimized, resulting in improved sealing performance.
[0017] Prior art 2 has disclosed an electrolytic solution for a
non-aqueous electrolyte lithium secondary battery, comprising a
lithium salt of a perfluoroalkylsulfonic acid dissolved in an
organic solvent and at least one selected from heterocyclic
compounds containing at least one fluorine atom and a nitrogen or
oxygen atom. According to Prior art 2, the heterocyclic compound
added to the electrolytic solution can form a strongly adsorptive
and antioxidative film on a positive current collector, resulting
in preventing oxidation deterioration of the positive current
collector and thus improvement in cycle properties.
[0018] Prior art 3 has disclosed a solid electrolyte secondary
battery comprising a positive electrode, a negative electrode
containing lithium as an active material, and a polymer solid
electrolyte consisting of a complex of an electrolyte salt with a
polymer or a polymer gel electrolyte prepared by impregnating an
electrolytic solution of an electrolyte salt dissolved in an
aprotic solvent into a polymer, wherein the polymer is selected
from the group consisting of a polyamide, polyimidazole, a
polyimide, polyoxazole, polytetrafluoroethylene,
polymelamineformamide, a polycarbonate and polypropylene. There is
described that cycle properties are improved because the
electrolyte is unreactive to the negative electrode and thus an
internal resistance is unlikely to be increased even after
repeating charge/discharge cycles.
[0019] For solving the problems of a reduced appearance capacity
and deteriorated cycle properties seen in FIGS. 6 and 7, it is
necessary to provide an optimal electrolyte composition (H.sup.+,
X.sup.-), or to improve an electrode for preventing
peroxidation-perreduction deterioration of an electrode active
material in the reaction between an electrolyte and the active
material.
[0020] In both Prior arts 1 and 2, a nitrogen-containing
heterocyclic compound is added to a non-aqueous electrolytic
solution. In Prior art 3, a polymer gel electrolyte consisting of,
for example, an aprotic solvent and a polyimidazole is used to make
the electrolyte unreactive to lithium in the negative electrode so
that increase of an internal resistance can be minimized and thus
cycle properties can be improved. In any of Prior arts 1, 2 and 3,
a nitrogen-containing heterocyclic compound or its polymer is added
to an electrolyte, which is different from this invention where a
particular substance is added and blended in an electrode.
[0021] Since Prior art 4 relates to a lithium battery in which an
electrolytic solution contains an organic solvent, a proton
concentration is not taken into consideration. Thus, a mechanism of
proton conductivity or deterioration as characteristics of an
active material is considerably different. Prior art 4 is different
from this invention in which an electrolytic solution contains a
proton source and a proton-conducting compound is used as an active
material.
SUMMARY OF THE INVENTION
[0022] An objective of this invention is to improve an electrode
for preventing peroxidation-perreduction deterioration of an
electrode active material and to provide a cell electrode
exhibiting improved cycle properties and an electrochemical cell
comprising the electrode.
[0023] This invention provides an electrode for an electrochemical
cell in which an active material in an electrode material is a
proton-conducting compound, wherein the electrode material
comprises a nitrogen-containing heterocyclic compound or a polymer
having a unit containing a nitrogen-containing heterocyclic
moiety.
[0024] This cell electrode may be suitably used in an
electrochemical cell in which only protons act as a charge carrier
in a redox reaction in both electrodes associated with charge and
discharge.
[0025] This invention also provides an electrochemical cell wherein
the above cell electrode according to this invention is used for at
least one of the electrodes and both electrodes comprise a
proton-conducting compound as an active material.
[0026] This invention also relates to the above electrochemical
cell wherein only protons can act as a charge carrier in a redox
reaction in both electrodes associated with charge and discharge.
More specifically, this invention relates to the electrochemical
cell comprising an electrolyte containing a proton source wherein
only adsorption and desorption of protons in the electrode active
material can be involved in electron transfer in a redox reaction
in both electrodes associated with charge and discharge.
[0027] In this invention, the nitrogen-containing heterocyclic
compound may be one or more selected from the group consisting of
imidazole, triazole, pyrazole, benzimidazole and their
derivatives.
[0028] The above polymer having a unit containing a
nitrogen-containing heterocyclic moiety may be a polymer having a
unit containing a benzimidazole, benzbisimidazole or imidazole
moiety.
[0029] This invention can improve cycle properties while inhibiting
reduction in an appearance capacity. This is because a
nitrogen-containing heterocyclic compound or polymer having a unit
containing a nitrogen-containing heterocyclic moiety added to an
electrode interacts with protons in an electrolyte so that only a
proton concentration can be controlled without reducing a
concentration of anions acting as dopant in an
adsorption-desorption reaction between the active material and
protons in the electrolyte. It is also because an optimal
proton-concentration atmosphere for the reaction can be created,
resulting in inhibition of deterioration due to peroxidation.
[0030] The polymer in this invention implies the compound having
two or more recurring unit, or includes so-called oligomers.
BRIEF DESCRIPTION OF THE DRAWINGS
[0031] FIG. 1 is a cross section of an electrochemical cell
according to an embodiment of this invention.
[0032] FIG. 2 is a graph showing CV measurement results for a
positive electrode in an aqueous sulfuric acid solution using
electrodes according to this invention and according to the prior
art.
[0033] FIG. 3 is a graph showing CV measurement results for a
negative electrode in an aqueous sulfuric acid solution using
electrodes according to this invention and according to the prior
art.
[0034] FIG. 4 is a graph showing variation in cycle properties for
batteries according to this invention (Examples 1, 3, 5, 7, 14 and
19 and according to the prior art (Comparative Examples 1 and
2).
[0035] FIG. 5 is a graph showing variation in a cell internal
resistance vs the cycle number for batteries according to this
invention (Examples 1, 3, 7, 14 and 19) and according to the prior
art (Comparative Examples 1 and 2).
[0036] FIG. 6 is a graph showing variation in cycle properties for
different sulfuric acid concentrations.
[0037] FIG. 7 is a graph showing variation in an appearance
capacity for different sulfuric acid concentrations.
DETAILED DESCRIPTION OF THE INVENTION
[0038] Preferred embodiments of this invention will be
detailed.
[0039] A cell electrode according to this invention is made of an
electrode material comprising a proton-conducting compound as an
active material and a nitrogen-containing heterocyclic compound or
a polymer having a unit containing a nitrogen-containing
heterocyclic moiety. Another cell electrode according to this
invention is made of an electrode material comprising, as an active
material, a polymer having a unit constituting a proton-conducting
polymer and a unit having a nitrogen-containing heterocyclic moiety
(the both units may constitute one unit).
[0040] An electrochemical cell according to this invention employs
the above electrode according to this invention as at least one
electrode and otherwise may be as with a conventional cell. An
electrochemical cell according to this invention is preferably that
wherein only protons act as a charge carrier in a redox reaction
associated with charge and discharge in both electrodes; more
specifically that comprising an electrolyte containing a proton
source wherein only adsorption and desorption of protons in the
electrode active material can be involved in electron transfer in a
redox reaction in both electrodes associated with charge and
discharge.
[0041] An electrochemical cell may have a basic configuration as
shown in, for example, FIG. 1, where a positive electrode 2
comprising a proton-conducting compound as an active material and a
negative electrode 3 are formed on a positive current collector 1
and a negative current collector 4, respectively, and these
electrodes are laminated via a separator 5. The cell is filled with
an aqueous or non-aqueous solution containing a proton source as an
electrolytic solution and is sealed by a gasket 6.
[0042] The electrodes 2, 3 can be, for example, formed as follows.
A powdery doped or undoped proton-conducting compound is blended
with a conductive auxiliary, a binder and a nitrogen-containing
heterocyclic compound or a polymer having a unit containing a
nitrogen-containing heterocyclic moiety to prepare a slurry, which
is then placed in a mold with a desired size and molded by a hot
press to form an electrode having a desired electrode density and a
desired film thickness. Then, a positive electrode and a negative
electrode thus formed are mutually faced via a separator to give a
cell.
[0043] A nitrogen-containing heterocyclic compound used in this
invention may be preferably one or more selected from the group
consisting of imidazole, triazole, pyrazole, benzimidazole and
their derivatives. Specifically, the nitrogen-containing
heterocyclic compound represented by chemical formulas (1) to (5)
may be used. ##STR2##
[0044] wherein R independently represent hydrogen, alkyl having 1
to 4 carbon atoms, amino, carboxyl, nitro, phenyl, vinyl, halogen,
acyl, cyano, trifluoromethyl, alkylsulfonyl and
trifluoromethylthio.
[0045] Examples of a halogen atom include fluorine, chlorine,
bromine and iodine. Examples of an alkyl group having 1 to 4 carbon
atoms include methyl, ethyl, propyl, isopropyl, n-butyl, s-butyl,
isobutyl and t-butyl. An acyl group may be selected from those
having an alkyl having 1 to 4 carbon atoms as described above. An
alkylsulfonyl group may be selected from those having an alkyl
having 1 to 4 carbon atoms as described above.
[0046] A polymer having a unit containing a nitrogen-containing
heterocyclic moiety may be a polymer having a unit containing a
benzimidazole, benzbisimidazole or imidazole moiety; for example, a
nitrogen-containing basic polymer such as a benzimidazole-based
polymer represented by chemical formula (6) or (10) and a
polyvinylimidazole represented by chemical formula (7),
polybenzbisimidazole, benzbisimidazole-based polymer represented by
chemical formula (11) or polyimidazole represented by chemical
formula (12). ##STR3##
[0047] wherein n represents a positive integer, and H bonded to N
may be independently replaced with a substituent selected from the
above-described R. ##STR4##
[0048] wherein n represents a positive integer, H bonded to N may
be independently replaced with a substituent selected from the
above-described R, and R1 represents a divalent group such as an
alkylene having 1 to 4 carbon atoms and a substituted or
non-substituted phenylene.
[0049] Using such an electrode, a reaction described below may
occur with ions in an electrolytic solution containing a proton
source. When the nitrogen-containing heterocyclic compound is
imidazole, imidazole adsorbs a proton as shown in chemical formula
(13). ##STR5##
[0050] wherein n represents a positive integer and m represents an
integer larger than n.
[0051] Such proton adsorption by imidazole results in prevention of
peroxidation or perreduction of an active material of positive and
thus a longer cycle life of the cell. As described above, a
concentration of protons involved in a reaction with an active
material may be appropriately adjusted by controlling the amount of
a nitrogen-containing heterocyclic compound or polymer having a
unit containing a nitrogen-containing heterocyclic moiety to be
added and blended, without varying a concentration of an anion to
be a dopant. Thus, a higher appearance capacity of the cell can be
maintained and cycle properties can be improved.
[0052] A polymer having a unit containing a nitrogen-containing
heterocyclic moiety may be a polymer having a unit constituting a
conventional proton-conducting polymer and a unit of a
nitrogen-containing heterocyclic compound or of a monomer compound
having a nitrogen-containing heterocyclic moiety. The polymer acts
as a proton-conducting active material as well as an inhibitor of
peroxidation-perreduction deterioration for the above
nitrogen-containing heterocyclic compound. An electrode comprising
the polymer as an electrode active material can, therefore, exhibit
improvement equivalent to that achieved by an electrode comprising
the above nitrogen-containing heterocyclic compound or the polymer
having a unit containing a nitrogen-containing heterocyclic moiety.
In other words, there may be provided a cell where
peroxidation-perreduction deterioration is much more reduced in
comparison with a conventional electrode as described later even
under a high proton concentration atmosphere.
[0053] In the light of inhibition of peroxidation-perreduction
deterioration, a copolymerization composition for a polymer having
a unit containing a nitrogen-containing heterocyclic moiety
according to this invention may be such that a unit containing a
nitrogen-containing heterocyclic moiety is preferably at least 5
mol %, more preferably at least 10 mol %. On the other hand, in the
light of its function as an active material such as a capacity
appearance rate, the unit may be contained in an amount of
preferably 90 mol % or less, more preferably 80 mol % or less. The
polymer having the weight-average molecular weight of 1000 to
50000, preferably 3000 to 15000 measured with GPC may be used in
this invention.
[0054] For determining the effects of this invention, a positive
electrode (comprising an indole trimer as an active material) was
evaluated by cyclic voltammetry (CV-measurement). In this
measurement, a working electrode was an electrode formed by
depositing a mixture of an active material of positive electrode
with imidazole on a carbon sheet; a counter electrode was a
platinum electrode; and a reference electrode was an Ag/AgCl
electrode. A measuring temperature was 25.degree. C., a scan
voltage ranged from 600 to 1100 mV, and a scan speed was 1 mV/sec.
An electrolytic solution was a 20 wt % aqueous solution of sulfuric
acid, and a composition of a positive electrode material as the
working electrode was that described in Example 3 (containing 20 wt
% of imidazole). An electrode without imidazole (Comparative
Example 1 described later) was also evaluated as a reference
example. The results are shown in a graph in FIG. 2.
[0055] The results show that reduction in a discharge capacity in
Example 3 is less than that in Comparative Example 1. In relation
to Comparative Example 1, a redox potential in Example 3 was
shifted to lower potential side by several tens of mV. That is,
shift to a stable potential at which oxidation deterioration is
reduced was observed. It may be concluded that a cycle life was
prolonged.
[0056] An active material of negative electrode
(polyphenylquinoxaline) was also evaluated by CV measurement using
the negative electrodes described in Example 3 and Comparative
Example 1. FIG. 3 shows the results of variation in their discharge
capacity. The results show that deterioration in a capacity due to
excessive protonation of the active material of negative electrode
was inhibited.
[0057] It was, therefore, shown that this invention can prevent
deterioration in both electrodes, a positive electrode and a
negative electrode.
[0058] In the above examples, an aqueous electrolytic solution has
been described. However, in this invention, an electrolyte may be
any electrolyte containing a proton source, and reduction in a
capacity may be similarly inhibited for a different type of
electrolyte such as a non-aqueous electrolytic solution, a gel
electrolyte and a solid electrolyte. In both cases using a cell
electrode comprising the above nitrogen-containing heterocyclic
compound and using a cell electrode comprising the polymer having a
unit containing a nitrogen-containing heterocyclic moiety,
inhibition in capacity reduction (inhibition of active material
deterioration) can be obtained.
[0059] An electrode active material constituting a cell electrode
of this invention exhibits conductivity by being doped to form a
redox pair, and thus may be a proton-conducting compound known in
the art. A proton-conducting compound means a compound which can
generate an electrochemical reaction involving only adsorption and
desorption of protons in electron transfer associated with a redox
reaction. Examples of such a compound include n-conjugated polymers
such as polyaniline polymers (e.g., polyaniline), polythiophene,
polypyrrole, polyacetylene, poly-p-phenylene,
polyphenylene-vinylene, polyperinaphthalene, polyfuran,
polyflurane, polythienylene, polypyridinediyl,
polyisothianaphthene, polyquinoxaline, polypyridine,
polypyrimidine, polyindole, polyaminoanthraquinone and their
derivatives; indole-based compounds such as indole trimer and its
derivative; and hydroxyl-containing polymers such as
polyanthraquinone and polybenzoquinone where a quinone oxygen is
converted into a hydroxyl group by conjugation. These compounds are
appropriately selected as an active material of positive electrode
or negative electrode, taking a redox potential difference into
account.
[0060] Among these, an active material of positive electrode is
preferably selected from the group consisting of polyaniline,
polydianiline, polydiaminoanthraquinone, polybiphenylaniline,
polynaphthylaniline, polyindole and indole-based compounds. An
active material of negative electrode is preferably selected from
the group consisting of polypyridine, polypyrimidine,
polyquinoxaline and their derivatives. In particular, preferred is
a combination of an indole-based compound as an active material of
positive electrode and a quinoxaline-based polymer as an active
material of negative electrode. The indole-based compound is
preferably one or more of an indole trimer and its derivatives (an
indole trimer compound) while the quinoxaline-based polymer is
preferably polyphenylquinoxaline.
[0061] An indole trimer compound has a fused polycyclic structure
comprising a six-membered ring formed by atoms at the second and
the third positions in three indole rings. The indole trimer
compound can be prepared from one or more compounds selected from
indole or indole derivatives or alternatively indoline or its
derivatives, by a known electrochemical or chemical process.
[0062] Examples of such indole trimer compound include those
represented by the following chemical formulas: ##STR6##
[0063] wherein R independently represent hydrogen, halogen,
hydroxyl, carboxyl, sulfone, sulfate, nitro, cyano, alkyl, aryl,
alkoxyl, amino, alkylthio or arylthio.
[0064] In these formulas, examples of halogen in R include
fluorine, chlorine, bromine and iodine. Examples of alkyl in R in
these formulas include methyl, ethyl, propyl, isopropyl, n-butyl,
s-butyl, isobutyl, t-butyl, n-pentyl, n-hexyl, n-heptyl and
n-octyl. Alkoxy in R in these formulas is a substituent represented
by --OX, wherein X may be alkyl as described above. Examples of
aryl in R in these formulas include phenyl, naphthyl and anthryl.
The alkyl moiety in alkylthio in R in these formulas may be
selected from those described above. The aryl moiety in arylthio in
R in these formulas may be selected from those described above.
[0065] A quinoxaline-based polymer is a polymer having a unit
containing a quinoxaline moiety which may be represented by any of
the following formulas (16) and (17). A preferable
quinoxaline-based polymer is a polymer having a unit containing
2,2-(p-phenylene)diquinoxaline moiety represented by the formula
(17). ##STR7##
[0066] wherein n represents a positive integer.
[0067] An electrolyte in this invention may be any electrolyte
containing a proton source, preferably an electrolytic solution
containing a proton source, particularly an aqueous solution of
sulfuric acid. A proton source may be an inorganic or organic acid.
Examples of an inorganic acid include sulfuric acid, nitric acid,
hydrochloric acid, phosphoric acid, tetrafluoroboric acid,
hexafluorophosphoric acid and hexafluorosilicic acid. Examples of
an organic acid include saturated monocarboxylic acids, aliphatic
carboxylic acids, oxycarboxylic acids, p-toluenesulfonic acid,
polyvinylsulfonic acid and lauric acid.
[0068] A proton concentration in an electrolytic solution
containing a proton source is preferably 10.sup.-3 mol/L or more,
more preferably 10.sup.-1 mol/L or more in the light of reactivity
of the electrode materials while being preferably 18 mol/L or less,
more preferably 7 mol/L or less in the light of deterioration in
activity of the electrode materials and prevention of
dissolution.
[0069] A content of a nitrogen-containing heterocyclic compound or
a polymer having a unit containing a nitrogen-containing
heterocyclic moiety in a cell electrode may be appropriately
selected depending on the type of the compound or polymer and the
type and a concentration of the electrolyte. If it is too low,
oxidation deterioration of an active material may be inadequately
inhibited. If the content is too high, an appearance capacity may
be reduced, leading to deterioration in other properties. The
content is, therefore, preferably 1 to 80 parts by weight to 100
parts by weight of the active material.
EXAMPLES
[0070] This invention will be described with reference to, but not
limited to, examples, and variations may be acceptable in this
invention without departing from the gist of this invention. There
will be described examples of application to a secondary battery,
but this invention may be suitably applied to another
electrochemical cell such as an electric double layer capacitor by
properly adjusting parameters such as a capacity and a
charge/discharge rate.
Example 1
[0071] A positive electrode used was prepared as follows. To indole
trimer 69 wt % as an active material were added 23 wt % of vapor
growth carbon (VGCF) as a conductive auxiliary and 8 wt % of a
polyfluorovinylidene (average molecular weight: 1100) as an
electrode molding component. To 100 wt % of the mixture was added 5
wt % of imidazole. The resultant mixture was stirred and blended in
a blender and then molded by a hot press into a solid electrode
having a desired size, which was used as a positive electrode
2.
[0072] A negative electrode used was prepared as follows. To
polyphenylquinoxaline 75 wt % as an active material were added 25
wt % of carbon black (K.B.600) as a conductive auxiliary. To 100 wt
% of the mixture was then added 5 wt % of imidazole. The resultant
mixture was stirred and blended in a blender and then molded by a
hot press into a solid electrode having a desired size, which was
used as a negative electrode 3.
[0073] An electrolytic solution used was a 20 wt % aqueous solution
of sulfuric acid.
[0074] A separator 5 used was a cation-exchange membrane with a
thickness of 10 to 50 .mu.m.
[0075] The positive electrode and the negative electrode were
laminated together via a separator such that their electrode
surfaces mutually faced, and a gasket was mounted to form a battery
as shown in FIG. 1.
Example 2
[0076] A positive electrode was prepared as described in Example 1
without adding imidazole. A negative electrode was prepared as
described in Example 1 except adding 20 wt % of imidazole. A
battery was formed as described in Example 1, except these
electrodes were used.
Example 3
[0077] A positive electrode was prepared as described in Example 1
except adding 20 wt % of imidazole. A negative electrode was
prepared as described in Example 1 except adding 20 wt % of
imidazole. A battery was formed as described in Example 1, except
these electrodes were used.
Example 4
[0078] A positive electrode was prepared as described in Example 1
except adding 50 wt % of imidazole. A negative electrode was
prepared as described in Example 1 except adding 50 wt % of
imidazole. A battery was formed as described in Example 1, except
these electrodes were used.
Example 5
[0079] A positive electrode was prepared as described in Example 1
except adding 20 wt % of imidazole. A negative electrode was
prepared as described in Example 1 except adding 20 wt % of
1,2,4-triazole. A battery was formed as described in Example 1,
except these electrodes were used.
Example 6
[0080] A positive electrode was prepared as described in Example 1
except adding 20 wt % of 2-phenylimidazole instead of the
imidazole. A negative electrode was prepared as described in
Example 1 except adding 20 wt % of 2-phenylimidazole. A battery was
formed as described in Example 1, except these electrodes were
used.
Example 7
[0081] A positive electrode was prepared as described in Example 1
except adding 20 wt % of 3-trifluoromethylbenzimidazole instead of
the imidazole. A negative electrode was prepared as described in
Example 1 except adding 20 wt % of 3-trifluoromethylbenzimidazole
instead of the imidazole. A battery was formed as described in
Example 1, except these electrodes were used.
Example 8
[0082] A positive electrode was prepared as described in Example 1
except adding 20 wt % of imidazole. A negative electrode was
prepared as described in Example 1 except adding 20 wt % of
3-trifluoromethylbenzimidazole instead of the imidazole. A battery
was formed as described in Example 1, except these electrodes were
used.
Example 9
[0083] A positive electrode was prepared as described in Example 1
except adding 10 wt % of imidazole and 10 wt % of 1,2,4-triazole. A
negative electrode was prepared as described in Example 1 except
adding 20 wt % of 1,2,4-triazole instead of the imidazole. A
battery was formed as described in Example 1, except these
electrodes were used.
Example 10
[0084] A positive electrode was prepared as described in Example 1
except adding 10 wt % of imidazole and 10 wt % of
3-trifluoromethylbenzimidazole. A negative electrode was prepared
as described in Example 1 except adding 10 wt % of 1,2,4-triazole
and 10 wt % of 3-trifluoromethylpyrazole instead of the imidazole.
A battery was formed as described in Example 1, except these
electrodes were used.
Example 11
[0085] A positive electrode was prepared as described in Example 1
except adding 60 wt % of imidazole. A negative electrode was
prepared as described in Example 1 except adding 60 wt % of
imidazole. An electrolytic solution used was a 30 wt % aqueous
solution of sulfuric acid. A battery was formed as described in
Example 1, except these electrodes and the electrolytic solution
were used.
Example 12
[0086] A positive electrode was prepared as described in Example 1
without adding imidazole. A negative electrode was prepared as
described in Example 1 except adding 5 wt % of polybenzimidazole
instead of the imidazole. A battery was formed as described in
Example 1, except these electrodes were used.
Example 13
[0087] A positive electrode was prepared as described in Example 1
except adding 5 wt % of polybenzimidazole instead of the imidazole.
A negative electrode was prepared as described in Example 1 except
adding 5 wt % of polybenzimidazole instead of the imidazole. A
battery was formed as described in Example 1, except these
electrodes were used.
Example 14
[0088] A positive electrode was prepared as described in Example 1
adding 20 wt % of polybenzimidazole instead of the imidazole. A
negative electrode was prepared as described in Example 1 adding 20
wt % of polybenzimidazole instead of the imidazole. A battery was
formed as described in Example 1, except these electrodes were
used.
Example 15
[0089] A positive electrode was prepared as described in Example 1
except adding 20 wt % of polyvinylimidazole instead of the
imidazole. A negative electrode was prepared as described in
Example 1 except adding 20 wt % of polyvinylimidazole instead of
the imidazole. A battery was formed as described in Example 1,
except these electrodes were used.
Example 16
[0090] A positive electrode was prepared as described in Example 1
except adding 10 wt % of polybenzimidazole and 10 wt % of
polyvinylimidazole instead of the imidazole. A negative electrode
was prepared as described in Example 1 except adding 20 wt % of
polyvinylimidazole instead of the imidazole. A battery was formed
as described in Example 1, except these electrodes were used.
Example 17
[0091] A positive electrode was prepared as described in Example 1
except adding 20 wt % of imidazole. A negative electrode was
prepared as described in Example 1 except adding 10 wt % of
polybenzimidazole and 10 wt % of polyvinylimidazole instead of the
imidazole. A battery was formed as described in Example 1, except
these electrodes were used.
Example 18
[0092] A positive electrode was prepared as described in Example 1
except adding 20 wt % of 3-trifluoromethylpyrazole instead of the
imidazole. A negative electrode was prepared as described in
Example 1 except adding 10 wt % of polybenzimidazole and 10 wt % of
polyvinylimidazole instead of the imidazole. A battery was formed
as described in Example 1, except these electrodes were used.
Example 19
[0093] A positive electrode was prepared as described in Example 1
without adding imidazole. As an active material of a negative
electrode, a proton-conducting polymer (Mw: 10000) having the unit
represented by the formula (18) was prepared by condensation
polymerization of 3,3-diaminobenzidine (DABZ) and 1,4-bisbenzil
(BBZ) in the presence of terephthalaldehyde using a platinum
catalyst in DMF solvent. A negative electrode comprising the
polymer having the units containing the phenylqinoxaline moiety and
the benzimidazole moiety (75 wt %) and a conductive auxiliary (25
wt %) was prepared. A battery was formed as described in Example 1,
except these electrodes were used. ##STR8##
Example 20
[0094] A positive electrode was prepared as described in Example 1
except adding 1,2,4-triazole. A negative electrode was prepared as
described in Example 19. A battery was formed as described in
Example 1, except these electrodes were used.
Example 21
[0095] A positive electrode was prepared as described in Example 1
except adding 1,2,4-triazole instead of the imidazole. A negative
electrode was prepared as described in Example 1 except adding 10
wt % (to the content of polyphenylquinoxaline) of a
proton-conducting polymer having a unit containing a
nitrogen-containing heterocyclic moiety as described in Example 19
instead of the imidazole. A battery was formed as described in
Example 1, except these electrodes were used.
Example 22
[0096] A positive electrode was prepared as described in Example 1
without adding imidazole. A negative electrode was prepared as
described in Example 1 except adding 10 wt % (to the content of
polyphenylquinoxaline) of a proton-conducting polymer having a unit
containing a nitrogen-containing heterocyclic moiety as described
in Example 19 instead of the imidazole. A battery was formed as
described in Example 1, except these electrodes were used.
Example 23
[0097] A positive electrode was prepared as described in Example 1
without adding imidazole. A negative electrode was prepared as
described in Example 1 except adding 10 wt % (to the content of
polyphenylquinoxaline) of a proton-conducting polymer having a unit
containing a nitrogen-containing heterocyclic moiety as described
in Example 19 and 10 wt % (to the content of polyphenylquinoxaline)
of polybenzimidazole instead of the imidazole. A battery was formed
as described in Example 1, except these electrodes were used.
Example 24
[0098] A positive electrode was prepared as described in Example 1
without adding imidazole. A negative electrode was prepared as
described in Example 1 except adding 50 wt % (to the content of
polyphenylquinoxaline) of a proton-conducting polymer having a unit
containing a nitrogen-containing heterocyclic moiety as described
in Example 19 and 10 wt % (to the content of polyphenylquinoxaline)
of polybenzimidazole instead of the imidazole. A battery was formed
as described in Example 1, except these electrodes were used.
Comparative Example 1
[0099] Electrodes were prepared as described in Example 1 without
adding imidazole in either electrode. A battery was formed as
described in Example 1, except these electrodes were used.
Comparative Example 2
[0100] Electrodes were prepared as described in Example 1 without
adding imidazole in either electrode. An electrolytic solution used
was a 30 wt % aqueous solution of sulfuric acid. A battery was
formed as described in Example 1, except these electrodes and the
electrolytic solution were used.
[0101] The batteries prepared in Examples 1 to 24 and Comparative
Examples 1 and 2 were evaluated for an appearance capacity and
cycle properties. The results are shown in Table 1. TABLE-US-00001
TABLE 1 Cell internal Appearance resistance capacity Cycle
properties variation (%) (%) ratio(%) Exam. 1 98.4 83.4 118 Exam. 2
99.9 82.6 119 Exam. 3 97.2 88.3 111 Exam. 4 85.8 90.5 107 Exam. 5
100.1 84.6 115 Exam. 6 99.9 85.7 115 Exam. 7 100.1 86.8 115 Exam. 8
97.8 82.1 119 Exam. 9 102.3 86.7 115 Exam. 10 99.4 85.2 115 Exam.
11 99.9 82.9 119 Exam. 12 100.9 85.6 114 Exam. 13 102.1 88.8 111
Exam. 14 101.5 93.4 106 Exam. 15 100.6 93.2 106 Exam. 16 100.1 90.8
109 Exam. 17 100.1 86.7 112 Exam. 18 99.8 86.4 113 Exam. 19 104.2
94.9 105 Exam. 20 102.8 96.4 104 Exam. 21 101.3 95.2 102 Exam. 22
102.1 92.6 105 Exam. 23 100.5 91.9 107 Exam. 24 101.9 94.1 105
Comp. Ex. 1 100.0 80.1 121 Comp. Ex. 2 102.6 65.0 138
[0102] In Table 1, an appearance capacity is a relative value (%)
calculated to an appearance capacity in Comparative Example 1
(100%). Cycle properties is expressed as a relative discharge
capacity (%) (measured at 25.degree. C.) to a discharge capacity at
the initiation of the cycles. Cell internal resistance variation
ratio is a relative value (%) of a direct-current resistance after
10,000 cycles to a direct-current resistance at the initiation of
the cycles. Cycle conditions were as follows; charging: CCCV
discharge at 1 A and 1.2 V for 10 min, discharging: CC discharge at
0.2 A (equivalent to 1 C), and final voltage: 0.8 V.
[0103] FIGS. 4 and 5 show the evaluation results of Examples 1, 3,
5, 7, 14 and 19 and Comparative Examples 1 and 2 for cycle
properties and cell internal resistance variation ratio. As seen
from discharge capacity variation in FIG. 4, as the cycle number
increased, a discharge capacity was reduced to 80% and 65% in
Comparative Examples 1 and 2, respectively, while discharge
capacities in Examples were less reduced to 83% to 96%. It
indicates that a discharge capacity is less varied in Examples.
[0104] As seen from cell internal resistance variation ratio in
FIG. 5, cell internal resistance variation ratios in Examples 1, 3,
7, 14 and 19 were 105 to 118%, while cell internal resistance
variation ratios in Comparative Examples 1 and 2 were 121% and
138%, respectively. It indicates that cell internal resistance
variation in Examples is less than that in Comparative Example 1 or
2.
[0105] These results show that this invention can improve cycle
properties while inhibiting reduction in an appearance
capacity.
[0106] Although these Examples employ indole trimer or
polyphenylquinoxaline as an active material, an active material is
not limited to those, but any active material having proton
conductivity may be suitably used.
* * * * *