U.S. patent application number 15/324326 was filed with the patent office on 2017-06-29 for secondary electrochemical cell and charging method.
This patent application is currently assigned to Varta Microbattery GMBH. The applicant listed for this patent is Varta Microbattery GmbH. Invention is credited to David Ensling, Edward Pytlik, Stefanie Scholz, Claudio Schula.
Application Number | 20170187033 15/324326 |
Document ID | / |
Family ID | 53539727 |
Filed Date | 2017-06-29 |
United States Patent
Application |
20170187033 |
Kind Code |
A1 |
Ensling; David ; et
al. |
June 29, 2017 |
SECONDARY ELECTROCHEMICAL CELL AND CHARGING METHOD
Abstract
A secondary electrochemical cell includes a negative electrode
including an output conductor, activated carbon having a BET
surface area of at least 800 m.sup.2/g as a carbon-based storage
material that enables storage of electrical charge in the electrode
through formation of an electrical double layer (Helmholtz double
layer), a positive electrode including an output conductor, and
nickel hydroxide and/or nickel oxyhydroxide, a porous separator
that separates the negative electrode and the positive electrode
from one another, an aqueous alkaline electrolyte with which the
electrodes and the separator are soaked, and a housing that encases
the electrodes, the separator and the electrolyte.
Inventors: |
Ensling; David; (Ellwangen,
DE) ; Pytlik; Edward; (Ellwangen, DE) ;
Scholz; Stefanie; (Ellwangen, DE) ; Schula;
Claudio; (Ellwangen, DE) |
|
Applicant: |
Name |
City |
State |
Country |
Type |
Varta Microbattery GmbH |
Ellwangen |
|
DE |
|
|
Assignee: |
Varta Microbattery GMBH
Ellwangen Jagst
DE
|
Family ID: |
53539727 |
Appl. No.: |
15/324326 |
Filed: |
July 9, 2015 |
PCT Filed: |
July 9, 2015 |
PCT NO: |
PCT/EP2015/065766 |
371 Date: |
January 6, 2017 |
Current U.S.
Class: |
1/1 |
Current CPC
Class: |
H01M 12/005 20130101;
H01M 10/24 20130101; H01M 10/30 20130101; H01M 4/32 20130101; H01G
11/60 20130101; H01M 10/52 20130101; H01G 11/30 20130101; H01M 4/24
20130101; H01M 2004/021 20130101; H01G 11/52 20130101; H01M 10/26
20130101; H02J 7/345 20130101; H01M 4/242 20130101; H01M 2300/0002
20130101; H02J 7/0091 20130101; H01M 4/248 20130101; H02J 7/007
20130101; H01M 4/364 20130101; H01M 4/383 20130101; H01M 4/583
20130101; H01G 11/32 20130101; H01G 11/74 20130101; H01G 11/82
20130101; H01M 2010/4292 20130101; H01M 10/34 20130101; Y02E 60/10
20130101; H01M 10/443 20130101; H01M 2300/0014 20130101; H01M 4/38
20130101; H01M 4/52 20130101; H01G 11/46 20130101; H01M 4/48
20130101; H01G 11/78 20130101 |
International
Class: |
H01M 4/32 20060101
H01M004/32; H01M 4/583 20060101 H01M004/583; H01M 4/52 20060101
H01M004/52; H01G 11/78 20060101 H01G011/78; H01M 10/34 20060101
H01M010/34; H02J 7/00 20060101 H02J007/00; H01G 11/32 20060101
H01G011/32; H01G 11/74 20060101 H01G011/74; H01M 10/52 20060101
H01M010/52; H01M 10/44 20060101 H01M010/44 |
Foreign Application Data
Date |
Code |
Application Number |
Jul 9, 2014 |
EP |
14176415.9 |
Nov 13, 2014 |
DE |
10 2014 223 194.9 |
Claims
1-9. (canceled)
10. A secondary electrochemical cell comprising: a negative
electrode comprising an output conductor, activated carbon having a
BET surface area of at least 800 m.sup.2/g as a carbon-based
storage material that enables storage of electrical charge in the
electrode through formation of an electrical double layer
(Helmholtz double layer), a positive electrode comprising an output
conductor, and nickel hydroxide and/or nickel oxyhydroxide, a
porous separator that separates the negative electrode and the
positive electrode from one another, an aqueous alkaline
electrolyte with which the electrodes and the separator are soaked,
and a housing that encases the electrodes, the separator and the
electrolyte.
11. The secondary electrochemical cell as claimed in claim 10,
further comprising an auxiliary electrode electrically connected to
the negative electrode to dissipate any oxygen pressure arising in
the housing.
12. The secondary electrochemical cell as claimed in claim 10,
wherein the positive electrode has a lower capacity than the
negative electrode.
13. The secondary electrochemical cell as claimed in claim 10,
wherein the housing has a gas- and liquid-tight configuration.
14. The secondary electrochemical cell as claimed in claim 10,
wherein the negative electrode contains the carbon-based storage
material in a proportion of at least 5% by weight.
15. The secondary electrochemical cell as claimed in claim 10,
wherein the positive electrode contains the nickel hydroxide and/or
nickel oxyhydroxide in a proportion of 10% by weight to 100% by
weight.
16. The secondary electrochemical cell as claimed in claim 10,
wherein the positive electrode and/or negative electrode take the
form of layers having a thickness of 50 .mu.m to 500 .mu.m.
17. The secondary electrochemical cell as claimed in claim 10,
wherein the aqueous electrolyte contains a dissolved hydroxide
compound in a proportion of 0.1 M to 10 M and, optionally, includes
a sulfate compound or an alkali metal or alkaline earth metal
sulfate, in a concentration of 0.001% to 0.1% by weight.
18. A method of charging the electrochemical cell as claimed in
claim 10, wherein, during charging, the ambient temperature and/or
the temperature of the electrochemical element is measured and,
upon exceeding a temperature threshold, the loading voltage is
lowered by a value of 0.6 mV/.degree. C. to 1.8 mV/.degree. C.
Description
TECHNICAL FIELD
[0001] This disclosure relates to a secondary electrochemical cell
and a method of charging such a cell.
BACKGROUND
[0002] Electrochemical cells based on nickel/iron in the form of
the nickel/iron accumulator (Edison accumulator) have long been
known. The electrode reactions in the charging and discharging of
an electrochemical element based on nickel/iron can be described by
equations (1) and (2):
Charging
[0003]
2Ni(OH).sub.2+2OH.sup.-.fwdarw.2NiO(OH)+2H.sub.2O+2e.sup.-
Fe(OH).sub.2+2e.sup.-.fwdarw.Fe+2OH.sup.- (1)
Discharging
[0004]
2NiO(OH)+2H.sub.2O+2e.sup.-.fwdarw.2Ni(OH).sub.2+2OH.sup.-
Fe+2OH.sup.-.fwdarw.Fe(OH).sub.2+2e.sup.- (2).
The electrolyte used is generally potassium hydroxide solution.
[0005] Nickel-iron accumulators are very reliable and have a long
life, but they are generally unsuitable as sources for
high-intensity pulsed currents. Accumulators of better suitability
for the purpose, in the form of rechargeable nickel-metal hydride
batteries, are described, for example, in EP 1 011 163 A1. The
electrode reactions in the charging and discharging of nickel-metal
hydride batteries can be described by equations (3) and (4):
Charging
[0006] Ni(OH).sub.2+OH.sup.-.fwdarw.NiO(OH)+H.sub.2O+e.sup.-
M+H.sub.2O+e.sup.-.fwdarw.M-H+OH.sup.- (3)
Discharging
[0007] NiO(OH)+H.sub.2O+e.sup.-.fwdarw.Ni(OH).sub.2+OH.sup.-
M-H+OH.sup.-.fwdarw.M+H.sub.2O+e.sup.- (4).
Here too, potassium hydroxide solution is frequently used as
electrolyte.
[0008] The nickel-metal hydride batteries described in EP 1 011 163
A1 are suitable to safeguard volatile memory in data processing
units, an application where high currents have to be provided
within a very short time.
[0009] Energy sources used to safeguard volatile memory in data
processing units may also be capacitors, especially what are called
double layer capacitors ("super-caps"). One example can be found in
DE 20 2004 017 545 U1. Double layer capacitors have the advantage
that they can deliver very high pulsed currents very quickly.
However, their capacity is limited in accordance with the nature of
a capacitor. Furthermore, most double layer capacitors have an
organic electrolyte system which can constitute a safety risk in
the event of overcharging.
[0010] The batteries described in EP 1 011 163 A1 have a much
higher capacity than double layer capacitors. However, the
relatively high temperatures that prevail in operation in data
processing units can easily lead to overcharging. There is
generally no safety risk associated therewith. However, the
overcharging can lead to a decrease in the life expectation of the
batteries.
[0011] It could therefore be helpful to provide an energy source
that can release pulsed currents of high intensity and attenuates
or removes the above-stated disadvantages.
SUMMARY
[0012] We provide a secondary electrochemical cell including a
negative electrode including an output conductor, activated carbon
having a BET surface area of at least 800 m.sup.2/g as a
carbon-based storage material that enables storage of electrical
charge in the electrode through formation of an electrical double
layer (Helmholtz double layer).
BRIEF DESCRIPTION OF THE DRAWINGS
[0013] FIG. 1 shows, in schematic form, construction of an example
of an electrochemical element.
[0014] FIG. 2 shows, in schematic form, an electrode-separator
composite producible according to the working examples.
[0015] FIG. 3 shows, in schematic form, an alternative
electrode-separator composite of one example of an electrochemical
element.
[0016] FIG. 4 shows, in schematic form, a further alternative
electrode-separator composite in an example of an electrochemical
element.
DETAILED DESCRIPTION
[0017] Our electrochemical cells described hereinafter have a
negative electrode, a positive electrode, a porous separator that
separates the negative electrode and the positive electrode, an
aqueous alkaline electrolyte with which the electrodes and the
separator are soaked, and a housing that encases the electrodes,
the separator and the electrolyte. The cells are secondary
electrochemical cells. In other words, charging and discharging
operations are reversible.
[0018] There are various examples of the negative electrode of the
electrochemical cells:
In Cells According to Example 1--Negative Electrode Capable of
Forming an Electrical Double Layer
[0019] The negative electrode comprises an output conductor and a
carbon-based storage material that enables storage of electrical
charge in the electrode by formation of an electrical double layer
(Helmholtz double layer). In Cells According to Example 2--Negative
Electrode with Pseudocapacity [0020] The negative electrode
comprises, as well as an output conductor and a carbon-based
storage material that enables the storage of electrical charge in
the electrode through formation of the electrical double layer
mentioned, a non-carbon-based storage material that can chemisorb
hydrogen and/or store it in the form of a metal hydride (referred
to hereinafter as H2 storage material). In Cells According to
Example 3--Negative Electrode with Pseudocapacity [0021] The
negative electrode comprises, as well as an output conductor and a
carbon-based storage material that enables storage of electrical
charge in the electrode through formation of the electrical double
layer mentioned (and as an alternative to the non-carbon-based
storage material according to example 2), iron in metallic form
(oxidation state 0) and/or in oxidized form (oxidation state 2 or
3). When the iron is in oxidized form, it is preferably present as
iron hydroxide in the electrode. As can be inferred from equations
(1)-(4), the equilibrium between the oxidized form and metallic
form shifts in the course of charging and discharging.
[0022] In the presence of a combination of the carbon-based storage
material and the non-carbon-based H2 storage material or the iron
(examples 2 and 3), the negative electrode is capable of storing
electrical charge not just through formation of the double layer
mentioned. Instead, electrical charge can also be stored chemically
via the alternative route of a reversible redox reaction. In short:
in these cases, the negative electrode may have pseudocapacitative
properties.
[0023] The positive electrode of the electrochemical cells in all
cases contains an output conductor and additionally nickel
hydroxide (Ni(OH).sub.2) and/or nickel oxyhydroxide (NiO(OH)).
[0024] In the course of charging of the positive electrode, nickel
hydroxide is converted to nickel oxyhydroxide with absorption of a
hydroxide ion and release of a water molecule and electrons.
Conversely, nickel oxyhydroxide, in the course of discharging,
absorbs an electron and is converted with water to nickel hydroxide
with release of a hydroxide ion. It is thus theoretically possible
that a positive electrode comprises exclusively nickel oxyhydroxide
in the fully charged state, and exclusively nickel hydroxide in the
fully discharged state. In practice, however, the two compounds are
usually present alongside one another in electrodes, the ratio of
the compounds relative to one another being dependent on the charge
state of the electrodes.
Auxiliary Electrode
[0025] Preferably, the cells, in all variations, may have an
auxiliary electrode electrically connected to the respective
negative electrode to dissipate any positive oxygen pressure that
arises in a housing of the cell. Auxiliary electrodes of this kind
are known for other electrochemical systems. For example, EP 0 218
028 A1 describes an auxiliary electrode for a nickel/cadmium
accumulator.
[0026] The auxiliary electrode may, for example, be a three-layer
electrode composed of a hydrophobic and electrically nonconductive
first layer that promotes the ingress of oxygen, a hydrophilic
second layer and a hydrophobic third layer that supports catalytic
reduction of oxygen and is in electrical contact with the negative
main electrode. Such a three-layer electrode is known from EP 0 416
244 A1. The third layer accordingly preferably consists of an
activated carbon-containing rolling mixture (for example, composed
of 50% by weight to 80% by weight of activated carbon, 3% by weight
to 20% by weight of conductive black and 10% by weight to 30% by
weight of polytetrafluoroethylene (PTFE)). The first and second
layers are preferably based on a single-ply polymer fiber nonwoven,
with an aqueous cellulose ether mixture applied to one side
thereof. Details thereof can be found in EP 0 416 244 A1.
[0027] More preferably, the auxiliary electrode may also take the
form of a one-layer electrode. For dissipation of any oxygen
pressure that arises in the housing, it is possible to apply, for
example, a mixture of activated carbon, carbon black and
polytetrafluoroethylene (PTFE) to the negative electrode, for
example, as a layer in a thickness of 50 to 100 .mu.m.
[0028] Further preferably, the auxiliary electrode, as well as the
components mentioned, like the respective negative electrode to
which it is electrically connected, may also have a proportion of
iron in metallic and/or oxidized form or a proportion of the
non-carbon-based H2 storage material.
[0029] The auxiliary electrode in the cell has bifunctional
character. First, as already mentioned, it can contribute to
dissipation of any positive oxygen pressure that arises in the
housing. Second, however, particularly because of the proportion of
activated carbon and carbon black, like the negative electrode, it
is capable of storing electrical charge. It thus increases the
double layer capacity on the anode side.
[0030] It is preferable that the positive electrode of the cells
has a smaller absolute capacity than the negative electrode. In
other words, the capacity of the negative electrode for absorption
of electrical energy preferably exceeds the capacity of the
positive electrode in this regard. This is especially true when the
cells have the auxiliary electrode mentioned. In this case, it is
preferable that the added absolute capacities of the negative
electrode and the auxiliary electrode exceed the absolute capacity
of the positive electrode.
[0031] Preferably, the capacity of the negative electrode exceeds
that of the positive electrode at least by a factor of 1.1,
preferably by a factor in the range from 1.1 to 2.0. In the event
of overcharging of the cell, it is thus ensured that oxygen will be
formed and not, for instance, hydrogen.
[0032] Compared to conventional nickel-iron accumulators and
nickel-metal hydride batteries, the new developments described
here, even without the presence of the auxiliary electrode, have
elevated overcharge stability at high temperatures. The auxiliary
electrode enhances this effect. The elevated overcharge stability
of the cell seems to be attributable especially to the carbon-based
storage material in the negative electrode, which can contribute to
consumption of oxygen that has arisen as a result of
overcharging.
[0033] As a result of the overcharge stability, it is possible for
the housing of the cell to have a gas- and liquid-tight
configuration. Preferably, cells having such a housing also have an
auxiliary electrode as described above.
[0034] A gas-tight seal in this context means that gas formed in
the cell cannot escape from the housing in normal operation. The
housing thus generally does not comprise any means of controlled
ventilation, for example, a valve. However, for safety reasons, a
burst membrane may be provided, which is irreversibly destroyed
upon exceeding a pressure threshold.
Negative Electrode
[0035] Preferably, the negative electrode of the electrochemical
cells, in all variations, comprises the carbon-based storage
material in a proportion of at least 5% by weight.
[0036] Particularly preferably, the negative electrode may contain
the carbon-based storage material in a proportion of more than 5%
by weight. More preferably, this proportion is 5% by weight to 100%
by weight, preferably 5% by weight to 90% by weight, further
preferably 5% by weight to 75% by weight, especially 5% by weight
to 50% by weight. Within the latter range, preference is further
given to proportions by weight of 5% by weight to 25% by weight,
especially 5% by weight to 15% by weight. The range of 5% by weight
to 100% by weight can, of course, be exhausted up to its upper
limit only in cells according to example 1. All the other closed
ranges mentioned may be preferable for any variation. More
preferably, however, they apply to cells according to example
1.
[0037] Instead of the lower range limits mentioned in the preceding
paragraph, the lower limit for the proportion of carbon-based
storage material in the ranges mentioned may also be 5.5% by
weight, preferably 6% by weight, especially 6.5% by weight.
[0038] All the percentages mentioned relate preferably to the total
weight of the negative electrode in the dry state (i.e., without
electrolyte), minus the weight of the output conductor, which is
not taken into account.
[0039] If the negative electrode comprises the combination of the
carbon-based storage material and the non-carbon-based H2 storage
material (in cells according to example 2), the proportion of the
H2 storage material in the negative electrode is preferably 25% by
weight to 95% by weight, more preferably 50% by weight to 95% by
weight, especially preferably 75% by weight to 95% by weight,
particularly 85% by weight to 95% by weight.
[0040] When the negative electrode comprises the combination of the
carbon-based storage material and the iron in metallic form
(oxidation state 0) and/or oxidized form (oxidation state 2 or 3)
according to example 3, the proportion of the iron in the negative
electrode is preferably 25% by weight to 95% by weight, more
preferably 50% by weight to 95% by weight, especially preferably
75% by weight to 95% by weight, particularly 85% by weight to 95%
by weight.
[0041] Here too, all the percentages mentioned relate preferably to
the total weight of the negative electrode in the dry state (i.e.,
without electrolyte), minus the weight of the output conductor.
[0042] Useful carbon-based storage material capable of forming an
electrical double layer, in cells of all variations, is especially
activated carbon and graphene. As is well known, activated carbon
is a porous, fine-grain carbon having a very high internal surface
area. Particular preference is given to activated carbon having
[0043] a BET surface area of at least 800 m.sup.2/g, preferably of
at least 900 m.sup.2/g (determined in each case to DIN ISO 9277)
[0044] and/or [0045] a capacitance value of at least 60 F/g
(determined according to DIN IEC 62391).
[0046] Graphene is a carbon polymorph having a two-dimensional
structure. A multitude of catenated benzene rings forms a
honeycomb-shaped pattern in which every carbon atom is surrounded
at an angle of 120.degree. by three further carbon atoms and
wherein all carbon atoms are sp.sup.2-hybridized. Graphene offers
the greatest surface area per unit weight theoretically achievable
with carbon and therefore at present is the subject of intensive
studies in connection with the development of supercapacitors. Both
graphene and activated carbon are additionally capable of storing
hydrogen. This is one property that means they are of such great
interest as active electrode material for the negative electrode of
the cell.
[0047] Graphene and activated carbon can also be used in
combination with one another. Any mixing ratio is possible.
[0048] The H2 storage materials used are preferably hydrogen
storage alloys known from the field of nickel-metal hydride
batteries. Particular mention should be made in this connection of
AB.sub.2 alloys and AB.sub.5 alloys. Further options are Raney
nickel (a catalytically active nickel-aluminum alloy) and metallic
nickel of high electrochemical activity (INCO Nickel).
[0049] AB.sub.2 alloys are generally based on titanium and nickel
in an effective ratio of 1:2. In practice, the titanium and the
nickel are frequently partly replaced by one or more additions,
especially from the group comprising chromium, vanadium or
zirconium.
[0050] AB.sub.5 alloys are usually mixtures of lanthanum and nickel
in an effective ratio of 1:5. In practice, the lanthanum and the
nickel are frequently partly replaced by one or more additions,
especially from the group comprising manganese, nickel, copper,
chromium, aluminum, cobalt, zinc, zirconium or cerium.
[0051] Alternatively or additionally, instead of the alloys
mentioned or in addition thereto, it is also possible to use
A.sub.2B.sub.7 or AB.sub.3 alloys. These alloy types too have
already been discussed in connection with nickel-metal hydride
batteries. Examples of alloys of this kind are, for example,
La.sub.16.3Mg.sub.7.0Ni.sub.65.1CO.sub.11.6 (A.sub.2B.sub.7) or
La.sub.0.7Mg.sub.0.3Ni.sub.3-xFe.sub.x (AB.sub.3 with x=0-0.4).
[0052] The carbon-based storage material used is preferably in
particulate, i.e., pulverulent, form. In the production of the
negative electrodes (in all variations), it is especially used in
the form of a powder having a mean particle size of 50 nm to 500
.mu.m, especially having a mean particle size of 10 .mu.m to 50
.mu.m.
[0053] The H2 storage material used and the iron (in oxidized and
reduced form) are likewise preferably in particulate form. In the
production of the negative electrodes according to examples 2 and
3, the H2 storage material and the iron are especially used in the
form of powders having a mean particle size of 10 nm to 100 .mu.m,
especially with a mean particle size of 10 nm to 1 .mu.m.
Positive Electrode
[0054] The nickel hydroxide and/or nickel oxyhydroxide is
preferably used in the form of spherical particles. Irrespective of
this, it may be preferable that the particles of nickel hydroxide
and/or nickel oxyhydroxide used have a surface at least partly
coated with cobalt.
[0055] In general, the positive electrode contains the nickel
hydroxide and/or the nickel oxyhydroxide in a proportion of 10% by
weight to 100% by weight, preferably 25% by weight to 100% by
weight, especially 50% by weight to 100% by weight. These
percentages are preferably based on the total weight of the
positive electrode in the dry state (i.e., without electrolyte),
minus the weight of the output conductor present.
Preferred Structures of the Electrodes
[0056] For production of particularly preferred structures of the
negative electrode of cells according to examples 1 to 3, the
carbon-based storage material in example 1, the carbon-based
storage material and the H2 storage material in example 2 and the
carbon-based storage material and the iron in example 3 are
processed, for example, as a paste. However, one or more further
additional components are frequently added to the components
mentioned. Accordingly, the negative and positive electrodes of
cells according to examples 1 to 3 may optionally include one or
more additional components as well as the components mentioned.
These additional components will be addressed in more detail
hereinafter.
[0057] Useful additional components for the negative electrode of
cells of all examples are especially at least one of the following
components in the following proportions: [0058] 0.1% by weight to
10% by weight, preferably 1% by weight to 5% by weight, of an
electrode binder [0059] 0.1% by weight to 10% by weight, preferably
1% by weight to 5% by weight, of a conductor.
[0060] These additional components may be added to the negative
electrode individually or in combination.
[0061] When the negative electrode contains neither a proportion of
the H2 storage material nor a proportion of the iron (example 1),
it more preferably comprises the following components in the
following proportions: [0062] 90% by weight to 99.9% by weight,
especially 95% by weight to 99.9% by weight, of the carbon-based
storage material [0063] 0.1% to 10% by weight, preferably 0.1% by
weight to 5% by weight, of the electrode binder.
[0064] In the presence of a combination of the carbon-based storage
material and the non-carbon-based H2 storage material (example 2),
the negative electrode, in a development, more preferably comprises
the following components in the following proportions: [0065] 75%
by weight to 94.9% by weight, especially 85% by weight to 94.9% by
weight, of the H2 storage material [0066] 5% by weight to 19.9% by
weight, especially 5% by weight to 14.9% by weight, of the
carbon-based storage material [0067] 0.1% by weight to 10% by
weight, preferably 0.1% by weight to 5% by weight, of the electrode
binder.
[0068] In the presence of a combination of the carbon-based storage
material and the iron (example 3), the negative electrode, more
preferably comprises the following components in the following
proportions: [0069] 75% by weight to 94.9% by weight, especially
85% by weight to 94.9% by weight, of the iron [0070] 5% by weight
to 19.9% by weight, especially 5% by weight to 14.9% by weight, of
the carbon-based storage material [0071] 0.1% by weight to 10% by
weight, preferably 0.1% by weight to 5% by weight, of the electrode
binder.
[0072] Useful additional components for the positive electrode are
especially the following components in the following proportions:
[0073] 0.1% by weight to 10% by weight, preferably 1% by weight to
5% by weight, of an electrode binder [0074] 0.1% by weight to 90%
by weight, preferably 0.1% by weight to 50% by weight, more
preferably 0.1% by weight to 40% by weight, especially 0.1% by
weight to 20% by weight, of a conductor [0075] a carbon-based
storage material which enables the storage of electrical charge in
the electrode through formation of an electrical double layer
(Helmholtz double layer), especially in a proportion of 0.1% by
weight to 20% by weight. These components can be added to the
positive electrode individually or in combination.
[0076] The positive electrode more preferably comprises the
following components in the following proportions: [0077] 50% by
weight to 99.8% by weight of nickel hydroxide and/or nickel
oxyhydroxide [0078] 0.1% by weight to 40% by weight, preferably
0.1% by weight to 45% by weight, of the conductor [0079] 0.1% by
weight to 10% by weight, preferably 0.1% by weight to 5% by weight,
of the electrode binder.
[0080] It is preferably also the case for all these preferred
examples that the percentages are each based on the total weight of
the positive electrode and the negative electrode in the dry state
(i.e., without electrolyte), minus the weight of the respective
output conductor.
[0081] It is further preferable that the percentages of the
respective components present in all the specified compositions and
in all compositions derivable from the above specifications for the
positive electrode and negative electrode add up to 100% by
weight.
[0082] The conductor is preferably a metal powder, especially a
nickel and/or cobalt powder. Alternatively or additionally, it is
also possible to use carbon-based conductors such as carbon black,
graphite, carbon nanotubes (CNTs), nanocarbons or, in the case of
the positive electrode, graphene as well.
[0083] The electrode binder used is preferably a cellulose-based
binder, for example, carboxymethyl cellulose or a derivative of
carboxymethyl cellulose. Also particularly suitable are
water-soluble cellulose ethers, for example, methyl hydroxyethyl
cellulose (MHEC), methyl hydroxypropyl cellulose (MHPC) and
hydroxyethyl cellulose (HEC). Alternative options include
polyacrylates or polymer-based binders, for example, PTFE binders
(PTFE=polytetrafluoroethylene) or binders based on SBR
(styrene-butadiene rubber).
[0084] The electrodes of the cells need not necessarily contain an
electrode binder. They may also be manufactured, for example, in
binder-free form as sintered electrodes or as compacts.
[0085] Useful materials for the carbon-based storage material
which, in preferred structures, may be present in the positive
electrode include the same materials as in the case of the negative
electrode. These are, in particular, activated carbon having the
abovementioned properties and graphene.
[0086] As well as the additions mentioned, the positive electrode
and/or the negative electrode may contain further additions.
Particular mention should be made by way of example in this
connection of cobalt oxide, cobalt hydroxide, iron sulfide,
potassium sulfide, zinc sulfate, ammonium carbonate or calcium
hydroxide.
Output Conductor
[0087] Preferably, the output conductor of the positive electrode
and/or the negative electrode forms a three-dimensional conductive
matrix into which the respective storage material is embedded on
the anode side and the nickel hydroxide and/or nickel oxyhydroxide
on the cathode side.
[0088] Preferably, output conductors used on the positive electrode
side and/or negative electrode side are metallic or metal-coated
open-pore foams or output conductors made from a metallic or
metal-coated nonwoven. Output conductors of this kind are
conventionally used particularly on the positive electrode side of
nickel-cadmium or nickel-metal hydride accumulators in button cell
form. Reference is made by way of example in this connection to EP
0 658 949 A1. Both the foams mentioned and the nonwovens mentioned
are commercially available. They preferably consist of nickel or
copper or have been coated with one of these metals.
[0089] Particularly preferably, an output conductor that forms a
three-dimensional conductive matrix, especially in the form of a
foam or nonwoven, coated with copper or consisting of copper is
used on the negative electrode side, while a thin foil of steel or
stainless steel, especially of nickel-coated steel or stainless
steel, is used on the positive electrode side.
[0090] In some examples, it may be preferable that a
three-dimensional output conductor made from the foam or nonwoven
mentioned is used only on the negative electrode side, while a
flat, two-dimensional output conductor, for example, a metal foil
or a metal mesh, is used on the positive electrode side. In this
case, the positive electrodes are usually produced via a sintering
method.
[0091] Of course, it may also be preferable to use a flat,
two-dimensional output conductor, for example, a metal foil, in
place of the three-dimensional output conductors on the positive
electrode side and/or the negative electrode side.
[0092] In the presence of a combination of the carbon-based storage
material and the non-carbon-based H2 storage material (example 2)
or the iron (example 3), the particles of the carbon-based storage
material and of the iron or of the H2 storage material are present
in the negative electrode, and in preferred structures in the
aforementioned three-dimensional matrix composed of the output
conductor, preferably in a homogeneous distribution. This is
attributable particularly to the aforementioned mixing of the
respective components of the negative electrode. To achieve such an
arrangement, the H2 storage material or the iron and the
carbon-based storage material may be processed to give a mixture,
especially the paste already mentioned, optionally with addition of
at least one of the additional components described. Vigorous
mixing of the components ensures that all components of the mixture
are in uniform, homogeneous distribution. Subsequently, the
mixture, especially the paste, is introduced into the
aforementioned three-dimensional matrix composed of an output
conductor or applied in the form of a thin layer to a
two-dimensional output conductor.
[0093] In some examples, however, it may also be preferable that
the negative electrode has a first subregion, especially a first
layer, in which the carbon-based storage material is enriched and,
correspondingly, a second subregion, especially a second layer, in
which the non-carbon-based H2 storage material or the iron is
enriched. To achieve such an arrangement, the H2 storage material
or the iron and the carbon-based storage material are generally not
processed in one step. For example, it is possible [0094] in a
first step to introduce the H2 storage material or the iron into
the aforementioned three-dimensional matrix composed of an output
conductor and then to coat the output conductor on its exterior
with the carbon-based storage material or [0095] in a first step to
introduce the H2 storage material or the iron into an output
conductor strip composed of a nickel foam and then to arrange a
layer of the carbon-based storage material on one side or even both
sides of the strip.
[0096] Preferably, the negative electrode of the electrochemical
cell may thus have a multilayer structure, especially a bilayer
structure. In this case, for example, the output conductor and the
iron together form the second layer mentioned, whereas the
carbon-based storage material forms the first layer mentioned.
Separator/Electrode-Separator Composite
[0097] Both the separator and the positive electrode and the
negative electrode are preferably in the form of thin layers. They
may be provided, for example, in the form of ribbons or flat
strips. In the case of the electrodes, layers having a thickness of
50 .mu.m to 500 .mu.m are preferred. The thickness of the separator
used is preferably 10 .mu.m to 100 .mu.m.
[0098] The separator of the electrochemical cells is preferably a
porous polymer film, especially a film of a polyolefin, of PEEK
(polyether ether ketone) or of PES (polyether sulfone). However, it
is also possible without difficulty to use separators made of paper
or made of a nonwoven. Basically, what is required is merely that
the separator has sufficient porosity to be penetrated by the
electrolyte and that it is stable with respect thereto.
[0099] The cells are preferably in the form of a composite having
the layer sequence of positive electrode/separator/negative
electrode. More preferably, the electrochemical cell may be in the
form of a bicell, i.e., having the layer sequence of positive
electrode/separator/negative electrode/separator/positive electrode
or negative electrode/separator/positive
electrode/separator/negative electrode.
[0100] When the negative electrode, as set out above, has a first
layer in which the carbon-based storage material is enriched, it is
preferable that this layer is arranged between the negative
electrode and an adjacent separator.
[0101] The electrodes of the electrochemical cell are preferably
calendered before they are combined with one another and with the
separator.
[0102] The composite having the layer sequence of positive
electrode/separator/negative electrode, in some preferred examples,
is in the form of a spiral coil. For production of such a spiral
coil, it is possible, for example, to combine a positive electrode
ribbon and a negative electrode ribbon with two separator ribbons
by a lamination or bonding operation to give a composite having the
sequence of separator/negative electrode/separator/positive
electrode and then to roll it up.
Cell Composite/Bipolar Arrangement
[0103] Two or more of the cells described can be used to form a
cell composite. Within the composite, the cells may be connected in
parallel to one another or in series. In the balancing of such a
cell composite, the high overcharge stability mentioned may be
found to be particularly advantageous. To bring all the cells in
the composite back to an identical voltage level, the cell
composite can be deliberately overloaded. This is generally not
possible without irreparably damaging at least individual cells in
the composite. Because of the carbon-based storage material in the
negative electrodes of the cells, which can contribute to
consumption of oxygen produced in the overcharging, in the present
case, the risk of such damage, however, is minimized. This is
especially true when one or more of the cells, preferably all the
cells, in the composite have the auxiliary electrode mentioned.
[0104] Of course, it is also possible to stack two or more
electrochemical cells composed of positive electrode and negative
electrode and of one or more separators one on top of another. One
possible construction of a corresponding stack is described, for
example, in EP 1 011 163 A1.
[0105] Preferably, two or more cells are combined with one another
in a bipolar arrangement. It is a feature of a bipolar arrangement
of electrochemical cells that individual electrochemical cells
function as subcells and are connected in series by conductive
intermediate walls. Each subcell has a positive electrode and a
negative electrode separated from one another by an
electrolyte-soaked separator. There is a connecting wall between
adjacent subcells. This establishes an electrical connection
between the positive electrode of one cell and the negative
electrode of the other cell. At the same time, it separates the
electrolyte spaces of the subcells from one another.
Housing
[0106] The housing of the cells may take the form, for example, of
a button cell housing, especially of the housing as described in
the already mentioned EP 1 011 163 A1. Alternatively, the cells may
also take the form of flat cells, as described, for example, in EP
1 391 947 A1. In this case, the housing thereof is formed from thin
metal foils bonded to one another via a sealing layer.
[0107] Especially when the above-described composite having the
layer sequence of positive electrode/separator/negative electrode
is in the form of a spiral coil, the housing may also take the form
of a cylindrical round cell housing.
[0108] More preferably, the housing of the cells is a metallic
housing, for example, a housing made from stainless steel or from a
nickel-plated steel or stainless steel.
Electrolyte
[0109] The aqueous electrolyte of the electrochemical cells
preferably includes at least one hydroxide compound in dissolved
form in the range of 0.1 M to 10 M. More preferably, the
electrolyte comprises at least one metal hydroxide as hydroxide
compound, especially sodium hydroxide, lithium hydroxide or
potassium hydroxide.
[0110] More preferably, the electrolyte comprises, in addition to
the hydroxide compound, at least one sulfate compound, especially
an alkali metal or alkaline earth metal sulfate. The at least one
sulfate compound is preferably present in the electrolyte in a
concentration of 0.001% by weight to 0.1% by weight.
[0111] Preferably, the electrolyte contains a secondary conductive
salt having PO.sub.4.sup.3-, NO.sub.3.sup.- or Cl.sup.- anions and
metallic counterions.
[0112] It is additionally also possible to add additives to the
electrolyte, for example, thickeners, corrosion inhibitors, wetting
agents and antifreezes.
[0113] More preferably, the electrolyte comprises, as additive, a
nonylphenol ethoxylate (NPEO). In general, NPEOs refer to a group
of nonionic surfactants which derive from nonylphenols and differ
in the number of hydrophilic ethoxy units in the polyethylene
glycol side chain. Particular preference is given to NPEOs of the
formula
##STR00001##
with n=1 to 50, preferably with n=1-25, more preferably with n=1 to
10, especially with n=2 to 6. We found that, surprisingly, the
addition of NPEOs leads to an improvement in the activation
characteristics, to improved low-temperature performance and quite
generally to enhanced performance of the electrodes of cells
according to examples 1 to 3. Particular emphasis should further be
given to a distinct improvement in the stability of cells having
alkaline electrolyte. As additives, NPEOs seem to increase the
breakdown voltage of KOH, and do so even in relatively low
concentrations.
[0114] More preferably, the NPEO is present in a concentration of
0.001% by weight to 0.1% by weight in the electrolyte, preferably
0.001% by weight to 0.1% by weight.
Voltage Window
[0115] Cells according to example 1 preferably have a voltage
window (working range of the cells in regular operation) of 0-1.6
V.
[0116] The cells of all variations exhibit excellent
high-temperature characteristics. The cells generally withstand
storage at 105.degree. C. for two hours without difficulty (no
swelling of the housing or even leaks). In principle, even
discharge is possible at these temperatures.
Working Examples
(1) Production of an Electrochemical Cell According to Example
2
[0117] To form positive electrodes, an aqueous active material
paste was applied to an open-pore nickel foam. The solids content
of the paste was composed of the following components: [0118] 90%
by weight of Ni(OH).sub.2 [0119] 4% by weight of cobalt powder as
conductor [0120] 4% by weight of carbon black as further conductor
[0121] 2% by weight of a water-soluble cellulose ether as
binder.
[0122] To form negative electrodes, an aqueous active material
paste was applied to an open-pore nickel foam. The solids content
of the paste was composed of the following components: [0123] 7.5%
by weight of activated carbon having a BET surface area of >900
m.sup.2/g [0124] 90% by weight of an A.sub.2B.sub.7 alloy [0125]
2.5% by weight of a water-soluble cellulose ether as binder.
[0126] The electrodes were each dried and subjected to a rolling
operation. Thereafter, they had a thickness of about 250 .mu.m.
[0127] On one side of the negative electrode, a mixture of
activated carbon, carbon black and polytetrafluoroethylene (PTFE)
was subsequently applied by rolling in a thickness of 50 .mu.m to
100 .mu.m as an auxiliary electrode for dissipation of any oxygen
pressure that arises in the housing. The exact composition was 75%
by weight of activated carbon, about 7.5% by weight of conductive
black and about 17.5% by weight of PTFE.
[0128] Subsequently, the electrodes were combined with a
polypropylene separator (nonwoven, thickness 80 .mu.m) to give an
electrode-separator composite having the following layer sequence:
[0129] auxiliary electrode/negative electrode/separator/positive
electrode.
[0130] The composite was soaked with an aqueous electrolyte (6 M
KOH solution) and installed in a housing made of nickel-plated
stainless steel, as shown in FIG. 1.
[0131] The cell thus produced had a voltage window of 0.8-1.5
V.
(2) Production of an Electrochemical Cell According to Example
1
[0132] To form positive electrodes, an aqueous active material
paste was applied to an open-pore nickel foam. The solids content
of the paste was composed of the following components: [0133] 50%
by weight of Ni(OH).sub.2 [0134] 8% by weight of carbon black as
conductor [0135] 40% by weight of graphite as further conductor
[0136] 2% by weight of a water-soluble cellulose ether as
binder.
[0137] To form negative electrodes, an aqueous active material
paste was applied to an open-pore nickel foam. The solids content
of the paste was composed of the following components: [0138] 97.5%
by weight of activated carbon having a BET surface area of >900
m.sup.2/g [0139] 2.5% by weight of a water-soluble cellulose ether
as binder.
[0140] On one side of the negative electrode, a mixture of
activated carbon, carbon black and polytetrafluoroethylene (PTFE)
was subsequently applied by rolling in a thickness of 50 .mu.m to
100 .mu.m as an auxiliary electrode for dissipation of any oxygen
pressure that arises in the housing. The exact composition was 75%
by weight of activated carbon, about 7.5% by weight of conductive
black and about 17.5% by weight of PTFE.
[0141] Subsequently, the electrodes were combined with a
polypropylene separator (nonwoven, thickness 80 .mu.m) to give an
electrode-separator composite having the following layer sequence:
[0142] auxiliary electrode/negative electrode/separator/positive
electrode.
[0143] The composite was soaked with an aqueous electrolyte (6 M
KOH solution) and installed in a housing made of nickel-plated
stainless steel, as shown in FIG. 1.
[0144] The cell thus produced had a voltage window of 0-1.6 V.
(3) Production of a Further Electrochemical Cell According to
Example 2
[0145] To form positive electrodes, an aqueous active material
paste was applied to an open-pore nickel foam. The solids content
of the paste was composed of the following components: [0146] 60%
by weight of Ni(OH).sub.2 [0147] 8% by weight of carbon black as
conductor [0148] 30% by weight of graphite as further conductor
[0149] 2% by weight of a water-soluble cellulose ether as
binder.
[0150] To form negative electrodes, an aqueous active material
paste was applied to an open-pore nickel foam. The solids content
of the paste was composed of the following components: [0151] 87.5%
by weight of activated carbon having a BET surface area of >900
m.sup.2/g [0152] 10% by weight of an A.sub.2B.sub.7 alloy [0153]
2.5% by weight of a water-soluble cellulose ether as binder.
[0154] On one side of the negative electrode, a mixture of
activated carbon, carbon black and polytetrafluoroethylene (PTFE)
was subsequently applied by rolling in a thickness of 50 .mu.m to
100 .mu.m as an auxiliary electrode for dissipation of any oxygen
pressure that arises in the housing. The exact composition was 75%
by weight of activated carbon, about 7.5% by weight of conductive
black and about 17.5% by weight of PTFE.
[0155] Subsequently, the electrodes were combined with a
polypropylene separator (nonwoven, thickness 80 .mu.m) to give an
electrode-separator composite having the following layer sequence:
[0156] auxiliary electrode/negative electrode/separator/positive
electrode.
[0157] The composite was soaked with an aqueous electrolyte (6 M
KOH solution) and installed in a housing made of nickel-plated
stainless steel, as shown in FIG. 1.
[0158] The cell thus produced had a voltage window of 0.6-1.5
V.
(4) Production of an Electrochemical Cell According to Example
3
[0159] To form positive electrodes, an aqueous active material
paste was applied to an open-pore nickel foam. The solids content
of the paste was composed of the following components: [0160] 80%
by weight of Ni(OH).sub.2 [0161] 4% by weight of cobalt powder as
conductor [0162] 14% by weight of nickel powder as further
conductor [0163] 2% by weight of a water-soluble cellulose ether as
binder.
[0164] To form negative electrodes, an aqueous active material
paste was applied to an open-pore nickel foam. The solids content
of the paste was composed of the following components: [0165] 20%
by weight of activated carbon having a BET surface area of >900
m.sup.2/g [0166] 74.5% by weight of iron particles (at least partly
oxidized to Fe(OH).sub.2) having a mean particle size of 100 nm to
200 nm [0167] 5% by weight of SBR as binder [0168] 0.5% by weight
of a further water-soluble CMC binder/thickener.
[0169] On one side of the negative electrode, a mixture of
activated carbon, carbon black and polytetrafluoroethylene (PTFE)
was subsequently applied by rolling in a thickness of 50 .mu.m to
100 .mu.m as an auxiliary electrode for dissipation of any oxygen
pressure that arises in the housing. The exact composition was 75%
by weight of activated carbon, about 7.5% by weight of conductive
black and about 17.5% by weight of PTFE.
[0170] Subsequently, the electrodes were combined with a
polypropylene separator (nonwoven, thickness 80 .mu.m) to give an
electrode-separator composite having the following layer sequence:
[0171] auxiliary electrode/negative electrode/separator/positive
electrode.
[0172] The composite was soaked with an aqueous electrolyte (6 M
KOH solution) and installed in a housing made of nickel-plated
stainless steel, as shown in FIG. 1.
[0173] The cell thus produced had a voltage window of 0.6-1.5
V.
(5) Production of a Further Electrochemical Cell According to
Example 3
[0174] To form positive electrodes, an aqueous active material
paste was applied to an open-pore nickel foam. The solids content
of the paste was composed of the following components: [0175] 60%
by weight of Ni(OH).sub.2 [0176] 32% by weight of activated carbon
[0177] 3% by weight of carbon black as conductor [0178] 3% by
weight of Ca(OH)2 as additive [0179] 2% by weight of a
water-soluble cellulose ether as binder.
[0180] To form negative electrodes, an aqueous active material
paste was applied to an open-pore nickel foam. The solids content
of the paste was composed of the following components: [0181] 87.0%
by weight of activated carbon having a BET surface area of >900
m.sup.2/g [0182] 10% by weight of iron particles (at least partly
oxidized to Fe(OH).sub.2) having a mean particle size of 100 to 200
nm [0183] 2.5% by weight of a water-soluble SBR binder [0184] 0.5%
by weight of a further water-soluble CMC binder/thickener.
[0185] On one side of the negative electrode, a mixture of
activated carbon, carbon black and polytetrafluoroethylene (PTFE)
was subsequently applied by rolling in a thickness of 50 .mu.m to
100 .mu.m as an auxiliary electrode for dissipation of any oxygen
pressure that arises in the housing. The exact composition was 75%
by weight of activated carbon, about 7.5% by weight of conductive
black and about 17.5% by weight of PTFE.
[0186] Subsequently, the electrodes were combined with a
polypropylene separator (nonwoven, thickness 80 .mu.m) to give an
electrode-separator composite having the following layer sequence:
[0187] auxiliary electrode/negative electrode/separator/positive
electrode.
[0188] The composite was soaked with an aqueous electrolyte (6 M
KOH solution) and installed in a housing made of nickel-plated
stainless steel, as shown in FIG. 1.
[0189] The cell thus produced had a voltage window of 0.6-1.5
V.
[0190] FIG. 1 shows, in schematic form, construction of an example
of an electrochemical element. Disposed in a housing composed of
the housing parts 1 and 2 is a composite composed of a positive
electrode 4, a separator 6 and a negative electrode 5. The housing
is sealed liquid- and gas-tight by the seal 3. On the side of the
negative electrode remote from the separator, the auxiliary
electrode 7 has been applied by rolling. By the spring 8,
volumetric changes in the composite as a result of charging and
discharging operations are to be balanced out.
[0191] FIG. 2 shows, in schematic form, an electrode-separator
composite producible according to the working examples. A positive
electrode layer bears the reference numeral 4, a negative electrode
layer the reference numeral 5, the separator the reference numeral
6 and an auxiliary electrode layer the reference numeral 7.
[0192] FIG. 3 shows, in schematic form, an alternative
electrode-separator composite of one example of an electrochemical
element. Here too, a positive electrode layer bears the reference
numeral 4, a negative electrode layer the reference numeral 5, the
separator the reference numeral 6 and an auxiliary electrode layer
the reference numeral 7. In contrast with the example according to
FIG. 2, the auxiliary electrode 7 here is arranged between the
negative electrode 5 and the separator 6.
[0193] FIG. 4 shows, in schematic form, a further alternative
electrode-separator composite in an example of an electrochemical
element. Here too, a positive electrode layer bears the reference
numeral 4, a negative electrode layer the reference numeral 5 and
the separator the reference numeral 6. However, auxiliary
electrodes with reference numerals 7a and 7b are arranged on two
opposite sides of the negative electrode 5.
* * * * *