Metal-air battery having a device for controlling
the potential of the negative electrode
The present invention relates to a method for storing and releasing electrical energy
using a battery of the metal-air type, as well as the battery specially designed for
implementing this method.
Metal-air batteries use a negative electrode based on a metal such as zinc, iron or
lithium, coupled to an air electrode. The electrolyte used most often is an alkaline aqueous
electrolyte.
During discharging of such a battery, oxygen is reduced at the positive electrode and
the metal is oxidized at the negative electrode:
Discharge at the negative electrode: M  Mn+ + n e-
Discharge at the positive electrode: O2 + 2 H2O + 4 e-  4 OHWhen
a metal-air battery has to be recharged electrically, the direction of the current
is reversed. Oxygen is produced at the positive electrode and the metal is redeposited by
reduction at the positive electrode:
Recharge at the negative electrode: Mn+ + n e-  M
Recharge at the positive electrode: 4 OH-  O2 + 2 H2O + 4 e-
Metal-air systems have the advantage of using a positive electrode of infinite
capacity. Electrochemical generators of the metal-air type are therefore known for their
high specific energies, which can reach several hundred Wh/kg. The oxygen consumed at
the positive electrode does not need to be stored in the electrode but can be taken from the
surrounding air. Air electrodes are also used in alkaline fuel cells, which are particularly
advantageous compared with other systems owing to the high reaction kinetics at the level
of the electrodes and owing to the absence of noble metals such as platinum.
Problems during the recharging of batteries of the metal-air type have yet to be
solved. In particular, the air electrode, which is the positive electrode of the battery during
discharging, is not designed to be used in the recharging direction.
An air electrode is a porous solid structure in contact with the liquid electrolyte. The
interface between the air electrode and the liquid electrolyte is a so-called "triple-contact"
interface, where the active solid material of the electrode, the gaseous oxidant, i.e. air, and
the liquid electrolyte are present simultaneously. A description of the different types of air
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electrodes for zinc-air batteries is presented for example in the bibliographic article by V.
Neburchilov et al., with the title "A review on air cathodes for zinc-air fuel cells", Journal
of Power Sources 195 (2010) pp. 1271-1291.
The air electrode is usually composed of carbon particles with a large surface area
such as Vulcan® XC72 marketed by Cabot. The surface area of the carbon can be
increased by reaction with a gas, such as CO, prior to its integration in the air electrode. A
porous electrode is then produced by agglomeration of the carbon particles using a
fluorinated hydrophobic polymer such as FEP (fluorinated ethylene propylene) marketed
by the company DuPont. Patent WO 2000/036677 describes such an electrode for a metalair
battery.
It is preferable to have a reaction surface area on the air electrode that is as large as
possible, in order to have a current density relative to the geometric surface area of the
electrode that is as high as possible. A large reaction surface area is also useful because the
density of gaseous oxygen is low compared with a liquid. The large surface area of the
electrode allows the reaction sites to be multiplied. Conversely, this large reaction surface
area is no longer necessary for the reverse reaction of oxidation during recharging since the
concentration of active material is much higher.
The use of an air electrode during charging to bring about an oxidation reaction and
evolution of oxygen presents many drawbacks. The porous structure of the air electrode is
fragile. It was observed by the inventors that this structure was destroyed mechanically by
the evolution of gas when it was used to produce oxygen by oxidation of a liquid
electrolyte. The hydraulic pressure generated within the electrode by the production of gas
is sufficient to cause the bonds between the carbon particles constituting the air electrode
to rupture.
It was also observed by the inventors that the catalyst added to the air electrode to
improve the energy yield of the reaction of reduction of oxygen, such as manganese oxide
or cobalt oxide, is not stable at the potential required for the reverse oxidation reaction.
The corrosion of carbon in the presence of oxygen by oxidation of carbon is also
accelerated at higher potentials.
Some use a more resistant oxygen reduction catalyst coupled to an oxygen evolution
catalyst in a bifunctional electrode composed of two electrically coupled layers, as
described in patent US 5 306 579. However, this configuration produces electrodes that
nevertheless have a short service life and a limited number of cycles.
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The degradation of the air electrode, when it is used to recharge the metal-air battery,
greatly reduces the battery’s service life. This is one of the main reasons for the low level
of commercial development of electrically rechargeable metal-air accumulators.
Faced with these problems, one of the means that has been adopted for protecting the
air electrode against degradation consists of using a second positive electrode, which is
used for the oxygen evolution reaction. The air electrode is then decoupled from the
oxygen evolution electrode and only the latter is used during the charging phase. For
example, patent US 3 532 548 of Z. Starchurski describes a zinc-air battery with a second
auxiliary electrode used for the charging phase. During the charging phase, the air
electrode is therefore inactive. As far as the inventors know, it has never been suggested
that this air electrode could be used for anything during the step of charging the battery.
Moreover, as with all batteries, it is important to monitor and control the voltage at
the terminals of a battery of the metal-air type while it is being discharged and recharged.
The voltage at the terminals of a battery is generally measured without difficulty directly
between the negative terminal and the positive terminal of the battery. The voltage at the
terminals of the battery represents the potential difference of the positive electrode and
negative electrode.
In the case of conventional batteries, the voltage can be controlled by electronic
control systems or a BMS (battery management system). These devices are well known to
a person skilled in the art. The aim of a BMS is to monitor the state of the various elements
of the battery, as well as to protect it from the degradation that could be caused by
improper use, for example overvoltage or undervoltage. The BMS therefore also has the
function of increasing the battery’s service life.
It was found by the inventors that simply controlling the voltage at the terminals of
batteries of the metal-air type could be insufficient for optimum protection of the battery
against certain types of degradation occurring on the negative electrode side during
electrical recharging of a metal-air battery.
For example, in a zinc-air battery, during recharging, the Zn2+ metal ions are reduced
at the negative electrode and are deposited in their metallic form Zn once the potential at
the level of this electrode is sufficiently negative. A uniform and homogeneous deposit of
metal on the electrode is desired for ensuring good durability during the cycles of charging
and discharging of this battery.
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It was found that, under certain conditions, the metal was deposited in the form of
foam with little adherence to the surface of the electrode, and this foam could then become
detached from the electrode, causing a loss of active material and consequently a loss of
specific capacity of the battery. In other cases, it was found that the metal could also be
deposited in the form of dendrites. These dendrites can grow until they reach the positive
electrode during charging, causing an internal short-circuit, preventing recharging.
It was observed by the inventors that controlling the potential of the negative
electrode during charging to prevent it becoming too high makes it possible to limit the
formation of zinc deposits in the form of foam or dendrites.
However, in the case of metal-air batteries, it is known that, during charging, the
potential of the positive electrode increases much more quickly than the potential of the
negative electrode. Because of this, the control of the voltage at the terminals of the battery
is not sufficiently precise to provide control of the potential of the negative electrode.
Therefore there is at present a need for an accurate means of measuring and
controlling the potential of the negative electrode of a battery of the metal-air type while it
is being charged.
One of the aims of the present invention is therefore to devise a method for charging
and discharging a battery of the metal-air type during which precise control of the potential
of the negative electrode is provided. A battery having a means capable of performing this
function is also desired. However, it would be advantageous not to increase the weight of
the battery, as this would have the effect of decreasing its specific capacity.
In view of these findings, the inventors had the idea of taking advantage, during the
discharging phase, of the presence of the air electrode, which is then inactive.
A subject of the present invention is a method for storing and releasing electrical
energy using a metal-air battery comprising:
- a negative terminal,
- a positive terminal,
- a negative electrode, connected to the negative terminal,
- a first positive air electrode, and
- a second positive oxygen evolution electrode,
comprising the following steps:
(a) a discharging phase during which the first positive air electrode is connected to
the positive terminal of the battery and the second positive oxygen evolution electrode is
disconnected from the positive terminal of the battery;
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(b) a recharging phase during which the second positive oxygen evolution electrode
is connected to the positive terminal of the battery and the first positive air electrode is
disconnected from the positive terminal of the battery, and during which the potential of
the negative electrode is measured relative to the first positive air electrode.
Moreover, another subject of the invention is the device specially designed for
implementing this method, i.e. a metal-air battery comprising:
- a negative terminal,
- a positive terminal,
- a negative electrode, connected to the negative terminal,
- a first positive air electrode,
- a second positive oxygen evolution electrode,
- a switching means allowing either the first positive air electrode or the second
positive oxygen evolution electrode to be connected to the positive terminal, and
- a means for measuring the potential of the negative electrode, said measurement
being performed relative to the first positive air electrode.
Figure 1, appended to the present application, is a diagrammatic representation of an
embodiment of a battery forming the subject of the present invention, in a recharging
configuration.
In the present application, the terms "charging" and "recharging" are used as
synonyms, and are interchangeable.
The method for storing and releasing electrical energy according to the invention is
implemented with a battery of the metal-air type. This metal-air battery comprises,
conventionally, a negative terminal and a positive terminal. These two terminals allow the
battery to be connected in order to form a power circuit: either a charging circuit in which
the battery is connected to a charging means which supplies energy to the battery or a
discharging circuit in which the battery is connected to any device to which it supplies
energy.
The terminals of the battery are connected, in the battery, to electrodes.
The metal-air battery according to the invention comprises at least three electrodes:
- a negative electrode,
- a first positive air electrode, and
- a second positive oxygen evolution electrode.
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The negative electrode is connected permanently, i.e. during charging and during
discharging, to the negative terminal of the battery. The negative electrode can in principle
be any metal electrode commonly used in a metal-air cell. It can for example be an iron
electrode, a lithium electrode or a zinc electrode, preferably a lithium electrode (Li/Li+) or
a zinc electrode (Zn/Zn2+).
The first positive electrode of the battery according to the invention is an air
electrode. This type of electrode has been described in general above. Any type of air
electrode can be used in the battery according to the present invention. In particular, the
first positive air electrode of the battery can be an electrode obtained by agglomeration of a
carbon powder constituted by carbon particles with a large specific surface area, as
described in patent application WO 2000/036677. The air electrode, based on carbon
particles, can further contain at least one oxygen reduction catalyst. This oxygen reduction
catalyst is preferably selected from the group constituted by manganese oxide and cobalt
oxide.
The second positive electrode of the battery according to the invention is an oxygen
evolution electrode. Any type of electrode performing this function known to a person
skilled in the art can be used in the battery according to the present invention. The second
positive oxygen evolution electrode can for example be a metal electrode that is stable in
the electrolyte of the battery, such as an electrode made of silver, of nickel or of stainless
steel.
The method for storing and releasing electrical energy according to the invention
comprises at least one discharging phase and one recharging phase.
During the discharging phase (a), the first positive air electrode is connected to the
positive terminal of the battery and the second positive oxygen evolution electrode is
disconnected from the positive terminal of the battery. The first positive air electrode is
intended to be used as the working electrode during discharging of the battery, i.e. as the
active positive electrode on which the electrochemical reaction occurring during
discharging of the battery takes place.
During the recharging phase (b), the second positive oxygen evolution electrode is
connected to the positive terminal of the battery and the first positive air electrode is
disconnected from the positive terminal of the battery. The second positive oxygen
evolution electrode is intended to be used as the working electrode during recharging of
the battery, i.e. as the active positive electrode on which the electrochemical reaction
occurring during recharging of the battery takes place.
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That is why the battery specially designed for implementing this method also
comprises a switching means allowing either the first positive air electrode or the second
positive oxygen evolution electrode to be connected to the positive terminal.
According to an embodiment, switching of the connection of the positive terminal
between the first and the second positive electrode can be operated manually. However,
advantageously, the switching means can be connected to a switching control means. This
means can be electronic, and it can advantageously be an element of an electronic control
system or a BMS. The switching control means can operate the switching means so that it
is the first positive air electrode that is connected to the positive terminal of the battery
when the latter is being discharged and it is the second positive oxygen evolution electrode
that is connected to the positive terminal of the battery when the latter is being recharged.
The switching control means can be adapted for measuring the voltage between the
positive and negative terminals of the battery. This involves measuring the potential
difference between the two working electrodes, namely between the negative electrode and
the first positive air electrode during discharging and between the negative electrode and
the second positive oxygen evolution electrode during recharging.
However, this measurement of the voltage at the terminals of the battery does not
make it possible to provide precise control of the potential of the negative electrode during
charging as the potential of the positive electrode then varies more quickly than the
potential of the negative electrode. The potential difference measured between the positive
terminal and the negative terminal of the battery therefore does not accurately reflect the
potential of the negative terminal instantaneously.
So that measurement of the potential of the negative electrode does not depend on
the potential of the second positive oxygen evolution electrode, the inventors propose that
measurement is performed between the negative electrode and a reference electrode. A
reference electrode is an electrode the potential of which is set during measurement. A
working electrode, i.e. an electrode that is active during the electrochemical reaction,
cannot be a reference electrode as its potential varies owing to the passage of the current.
That is why the recharging phase (b) of the method according to the present
invention advantageously comprises a step consisting of measuring the potential of the
negative electrode relative to the first positive air electrode. During recharging of the
battery, the first air electrode is disconnected from the positive terminal of the battery. It is
no longer the working electrode, and no current circulates through it. It can therefore
advantageously be used as reference electrode for measuring the potential of the negative
electrode during recharging of the battery.
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The battery according to the invention, specially designed for implementing this
method, advantageously comprises a means for measuring the potential of the negative
electrode, said measurement being performed relative to the first positive air electrode.
Use of the positive air electrode as reference electrode for measuring the potential of
the negative electrode during the discharging phase of the battery is particularly
advantageous as it is not necessary to add to the device an electrode which would be
dedicated to this function alone. The present invention therefore has the advantage of being
simple and inexpensive as it does not require critical structural modifications to already
existing batteries in order to be implemented. Moreover, as the metal-air batteries specially
designed for implementing the method according to the invention do not comprise an
additional electrode, their weight and their overall dimensions are unaffected.
Having a precise measurement of the value of the potential of the negative electrode
is interesting for a person skilled in the art as it allows better management of the battery.
For example, controlling the potential of the negative electrode during charging in batteries
of the zinc-air type to prevent it being too high makes it possible to limit the formation of
zinc deposits in the form of foam or dendrites.
In an advantageous embodiment, the absolute value of the potential of the negative
electrode can be controlled during the recharging phase of the battery, so that it is not
above a set value. The value of the potential of the negative electrode during recharging
can preferably be controlled automatically by an electronic charge control means. The
electronic control means can be an element of an electronic control system or a BMS. The
charge control means can be adapted for continuously comparing, during recharging, the
value of the potential of the negative electrode measured according to the invention against
a set value, and for sending a regulation signal to the battery’s charging means so that the
absolute value of the measured potential remains below the set value.
In another advantageous embodiment, the absolute value of the potential of the
negative electrode measured relative to the first positive air electrode can also be
controlled during the discharging phase of the battery, so that it is not below a second set
value. During the discharging phase, the first positive air electrode is connected to the
positive terminal of the battery. Measuring the potential of the negative electrode relative
to this first positive air electrode is therefore equivalent to measuring the voltage between
the positive and negative terminals of the battery. Preferably, the discharging phase (a) of
the battery is therefore stopped before the absolute value of the measured voltage is below
the second fixed set value. The value of the potential of the negative electrode during
recharging can be controlled automatically by an electronic discharge control means. This
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means can be combined with the electronic charge control means, and it can be an element
of an electronic control system or a BMS. The discharge control means can be adapted for
continuously comparing, during discharging, the value of the potential of the negative
electrode measured according to the invention against the second set value, and for
stopping the discharge of the battery if the absolute value of the measured potential falls
below the set value.
So as to be able to implement these preferred embodiments, the metal-air battery
according to the invention can comprise a charge control means adapted for keeping the
absolute value of the voltage measured between the negative electrode and the first
positive air electrode below a set value, during charging of the battery. Alternatively, or in
addition, the metal-air battery according to the invention can comprise a discharge control
means adapted for keeping the absolute value of the voltage measured between the
negative electrode and the first positive air electrode above a second set value, during
discharging of the battery. The charge control means and the discharge control means can
optionally be one and the same means, performing both of these functions.
A battery according to the invention can further comprise an electronic battery
control system, also called a BMS. The charge control means and/or discharge control
means can form part of said electronic control system.
The invention will now be described in more detail with reference to the appended
Figure 1, showing diagrammatically an embodiment of a battery forming the subject of the
present invention, in a recharging configuration.
The battery 1 comprises a negative terminal 2, a positive terminal 3, a negative
electrode 4, connected to the negative terminal 2, a first positive air electrode 5 and a
second positive oxygen evolution electrode 6. In the configuration shown in Figure 1,
which is a configuration that the battery can assume during a charging phase, it is the
second positive oxygen evolution electrode 6 that is connected to the positive terminal 3 of
the cell. However, the battery 1 also comprises a switching means 7 that makes it possible
to disconnect the second positive oxygen evolution electrode 6 from the positive terminal 3
in order to connect the first positive air electrode 5 to it during the discharging phases. As
the battery 1 is shown in the charging phase, a charging means 11 was shown in Figure 1,
connected to the negative 2 and positive 3 terminals of the battery. However, it does not
form part of the battery 1.
The battery 1 further comprises a means for measuring the potential of the negative
electrode 8. This potential is measured by measuring the voltage V between the negative
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electrode 4 and the first positive air electrode 5. During the recharging phase of the battery
shown, the first positive air electrode 5 performs the role of a reference electrode as it is
not connected to the positive terminal 3 of the battery.
The battery 1 further comprises a charge control means 9. This charge control means
9 continuously compares, during recharging, the value V measured by the measuring
means 8 against a set value Vc, and the charge control means 9 sends a regulation signal
10 to the battery’s charging means 11 so that the absolute value of the measured potential
remains below the set value.

We claim:-
1. Method for storing and releasing electrical energy using a metal-air battery
comprising:
- a negative terminal (2),
- a positive terminal (3),
- a negative electrode (4), connected to the negative terminal,
- a first positive air electrode (5), and
- a second positive oxygen evolution electrode (6),
comprising the following steps:
(a) a discharging phase during which the first positive air electrode (5) is connected
to the positive terminal (3) of the battery and the second positive oxygen evolution
electrode (6) is disconnected from the positive terminal (3) of the battery;
(b) a recharging phase during which the second positive oxygen evolution electrode
(6) is connected to the positive terminal (3) of the battery and the first positive air electrode
(5) is disconnected from the positive terminal (3) of the battery, and during which the
potential of the negative electrode (4) is measured relative to the first positive air electrode
(5).
2. Method according to claim 1, characterized in that the absolute value of the
potential of the negative electrode (4) is controlled during the recharging phase of the
battery, so that it is not above a set value.
3. Method according to claim 2, characterized in that the value of the potential
of the negative electrode (4) during recharging is controlled automatically by an electronic
charge control means (9).
4. Method according to any one of claims 1 to 3, characterized in that, during
the discharging phase, the absolute value of the potential of the negative electrode (4)
measured relative to the first positive air electrode (5) connected to the positive terminal
(3) of the battery is controlled so that it is not below a set value.
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5. Method according to claim 4, characterized in that the value of the potential
of the negative electrode (4) during discharging is controlled automatically by an
electronic discharge control means.
6. Metal-air battery (1) comprising:
- a negative terminal (2),
- a positive terminal (3),
- a negative electrode (4), connected to the negative terminal (2),
- a first positive air electrode (5),
- a second positive oxygen evolution electrode (6),
- a switching means (7) allowing either the first positive air electrode (5) or the
second positive oxygen evolution electrode (6) to be connected to the positive terminal (3),
and
- a means for measuring the potential (8) of the negative electrode (4), adapted for
measuring the potential of the negative electrode during recharging of the battery, said
measurement being performed relative to the first positive air electrode (5).
7. Battery according to claim 6, characterized in that it further comprises a
charge control means (9) adapted for keeping the absolute value of the voltage measured
between the negative electrode (4) and the first positive air electrode (5) below a set value,
during charging of the battery.
8. Battery according to either of claims 6 or 7, characterized in that it further
comprises a discharge control means adapted for keeping the absolute value of the voltage
measured between the negative electrode (4) and the first positive air electrode (5) above a
second set value, during discharging of the battery.
9. Battery according to either of claims 7 or 8, characterized in that the battery
further comprises an electronic battery control system, and said charge control means
and/or discharge control means form part of the electronic control system.