^p
The present invention relates to a rechargeable metal-air accumulator
5 comprising an air electrode, an oxygen-releasing electrode and an automatic device
for protecting the air electrode during the charging and discharging phases of the
accumulator. The invention also relates to a method for storing and releasing
electrical energy using such an accumulator.
Metal-air batteries use a metal-based negative electrode, the metal being such
10 as zinc, iron or lithium, coupled to an air electrode. When discharging such a battery,
molecular oxygen is reduced at the positive electrode and the metal is oxidized at the
negative electrode:
O2 + 2 H2O + 4e- -» 4 OH"
M -^ M"* + ne"
15 Both electrodes most commonly contact an alkaline aqueous electrolyte.
Zinc-air batteries are marketed for use in auditory prostheses for example.
Many studies have been carried out for several decades to develop and
optimize air electrodes, that enable the production of metal-air electrochemical
generators, which are known for their high energy density that can reach several
20 hundreds of Wh/kg.
Air electrodes have the advantage of being able to use oxygen from the air as
the oxidizing agent of the electrochemical reduction, air being available in unlimited
quantities anywhere and at any time. A large reaction surface area is however
necessary, because the density of oxygen in air is low (0.03 mol/L). The porous solid
25 structure of air electrodes provides this large reaction surface area. An air electrode is
a porous solid structure that contacts the liquid electrolyte, generally an alkaline
aqueous solution. The interface between the air electrode and the liquid electrolyte is
a so-called "triple contact" interface where the active material of the electrode, the
gaseous oxidizing agent and the liquid electrolyte are simultaneously present.
30 The reader may refer for example to the article by V. Neburchilov et al. ''A
review on air cathodes for zinc-air fuel cells". Journal of Power Sources 195 (2010),
pages 1271 - 1291 for a description of the various types of air electrodes that can be
used in zinc-air accumulators.
However, air electrodes have a considerable drawback, which is their great
fragility when they are used when recharging the accumulator. Specifically, the air
electrode has a porous structure and operates as a volumetric electrode in which the
electrochemical reaction takes place in the whole volume of the electrode, at the
5 interface between the gaseous oxidizing agent, the liquid electrolyte and the solid
active material (triple contact). In the charging phase, gaseous molecular oxygen is
formed within the structure of the air electrode. This gaseous oxygen rapidly results
in mechanical destruction of the porous structure of the air electrode by breaking the
bonds between the grains of carbon and the used binding polymer.
10 Moreover, the corrosion of the carbon in the presence of oxygen via carbon
oxidation is also accelerated at high potentials (see the article by Kim J., Lee J. Tak
^k Y., ''Relationship between carbon corrosion and positive electrode potential in a
proton-exchange membrane fuel-cell during start/stop operation'" (2009), Journal of
Power Sources, 192 (2), pages 674 - 678, and the chapter "Carbons" by Kinishita, K.
15 (2007), in the Handbook of Battery Materials (ed. J.O. Besenhard), Wiley-VCH
Verlag GmbH, Weinheim, Germany).
The inventors have also observed that oxygen reduction catalysts
incorporated in the air electrode, at the surface of the carbon grains, are not stable at
the high potentials necessary for the reverse oxidation reaction. Some authors have
20 used more resistant oxygen reduction catalysts, coupled to an oxygen-releasing
catalyst in a bi-functional electrode composed of two electrically-coupled layers (see
US 5 306 579). However, this configuration produces electrodes having a short
lifetime and a limited number of cycles.
^ ^ Another known solution to this problem of deterioration of the air electrode
^tf 25 during the recharging phase of the accumulator is the use of an oxygen-releasing
electrode, i.e. a second positive electrode that does not undergo deterioration due to
the formation of molecular oxygen.
Patent US 3 532 548 describes such a rechargeable accumulator comprising a
negative electrode, an air electrode and an oxygen-releasing electrode (called the
30 auxiliary electrode). When charging the accumulator described in this document,
only the negative electrode and the oxygen-releasing electrode (auxiliary electrode)
are connected. When discharging the accumulator two operating modes are used:
^
^
a first "normal" operating mode where only the air electrode operates with the
negative electrode, and
a second "high power" operating mode where the two positive electrodes (air
electrode and auxiliai^ electrode) operate jointly.
5 The Applicant has nonetheless observed that the system as described in
US 3 532 548 does not allow for effective protection of the air electrode.
Specifically, when, at the start of the discharge, the voltage between the two
accumulator terminals is too high, the air electrode may undergo electrochemical
deterioration.
10 According to the Applicant's knowledge this deterioration at the start of the
discharge had not been detected until now and in any case is not taken into
consideration in document US 3 532 548.
The aim of the present invention is to ensure effective protection of the
porous air electrode of a metal-air accumulator not only during the charging phase of
15 the accumulator, but also during discharging.
This aim is achieved in the present invention by virtue of an automatic
protection system that constantly monitors the potential between the two accumulator
terminals, particularly at the start of the discharge, and only connects the air
electrode when this potential is low enough to remove any risk of mechanical or
20 chemical deterioration of the air electrode.
One subject of the present invention is therefore a rechargeable accumulator
comprising one or more metal-air cells, each cell comprising
- a first terminal and a second terminal,
- a metal negative electrode for a metal-air cell, connected to the first terminal,
25 - a first positive oxygen-releasing electrode,
- a second positive electrode, which is a porous air electrode containing at least one
oxygen reduction catalyst, and
- an electrolyte,
- control means adapted to continuously compare the measured voltage (Vm) between
30 the second terminal and the first terminal to a setpoint value (Vc) and to send a
switch signal to switching means when the measured voltage (Vm) falls below the
setpoint voltage (V^),
^p
^B
- the switching means being adapted to receive a switch signal originating from the
control means and to connect and disconnect the air electrode from the second
terminal.
The accumulator of the present invention therefore comprises, in each of its
5 cells, three types of electrode:
a negative electrode permanently connected to the first terminal, i.e. during
the charging phase and the discharging phase,
a first positive electrode that, in one embodiment, is continuously connected
to the second terminal or that, in another embodiment, is connected to the
10 second terminal in alternation with the second positive electrode, i.e. it is
disconnected from the second terminal when the second positive electrode is
being connected, and
an air electrode (second positive electrode) that must be protected from any
deterioration and that is connected to the second terminal only during the
15 second discharging phase when the potential between both terminals is low
enough.
The present invention also encompasses variants where one or more of these
electrodes are present in a plurality of copies or in a plurality of parts. Mention may
be made, by way of example, of a cell with a single negative plate-shaped electrode,
20 two air electrodes situated on either side of the negative electrode, and two metal
positive electrodes (oxygen-releasing electrodes) situated between the plate-shaped
negative electrode and both air electrodes.
The negative electrode may in principle be any metal electrode widely used in
metal-air cells. It may for example be an iron, lithium or zinc electrode, preferably a
25 lithium electrode (Li/LiOH) or a zinc electrode (Zn/Zn(OH)4^"), and
more preferably
a zinc electrode.
The first positive oxygen-releasing electrode may, for example, be a silver
electrode (Ag/AgO) or a nickel electrode (Ni/NiO or NiOH/NiOOH). In the
electrochemical cell, this electrode, preferably a nickel electrode, typically has a grid
30 structure or a perforated plate structure, and it is generally situated between the
negative electrode and the air electrode. The perforated structure of this electrode
serves, as it is known, to ensure the free passage of the constituents of the liquid
electrolyte between the negative electrode and the air electrode during discharging.
The second positive electrode is a porous air electrode designed to allow
triple contact between the electrolyte, the gaseous oxidizing agent and the solid
active material of the electrode. It may be chosen in principle from among all those
usually used in the art, and especially from those described in the article by
5 Neburchiiov mentioned above.
This is preferably an electrode obtained by aggregation of a carbon powder
composed of carbon grains with a high surface area, such as the product Vulcan
XC72 marketed by Cabot. The specific surface area of the carbon may be increased
by reaction with a gas such as CO2, prior to its incorporation into the air electrode.
10 The high specific surface area of the carbon grains is critical. Indeed, the higher it is,
the higher the current density per unit of geometrical surface area of the electrode.
^ The porous elecrod. is ™„ufac.uned by agg.ega.ing .he carbon grains using a
binder which is preferably a hydrophobic fluoropolymer such as FEP marketed by
Dupont. The application WO 2000/036677, for example, gives a detailed description
15 of an air electrode for a metal-air accumulator.
Furthermore, the carbon particle-based air electrode, preferably contains at
least one oxygen reducing catalyst. This oxygen reducing catalyst is preferably
chosen from the group formed by manganese oxide and cobalt oxide. These
catalysts, although very effective, are however deactivated when they are subjected
20 to overly high potentials and the automatic protection device of the present invention,
formed by the combination of a control means and a switching means, described in
more detail below, is therefore particularly important when these oxygen reducing
catalysts are used.
^ ^ The electrolyte used is preferably an alkaline aqueous electrolyte contacting
^ ^ 25 all the electrodes in the cell. In certain embodiments, for example in the case where a
lithium electrode is used as negative electrode, it may be necessary to separate
physically, as is known, the aqueous electrolyte from the negative electrode with a
lithium ion conducting solid electrolyte.
The control means used in the present invention must be adapted to compare
30 continuously the measured voltage (Vm) between the second terminal and the first
terminal to a setpoint value (Vc). This setpoint value is for example the value of a
reference voltage provided by a stabilized voltage source, powered by the battel^ or
by an external batteiy. It is preferably set to a value slightly lower than the open
circuit cell voltage in the charged state when only the air electrode is connected, i.e.
about 1.44 V for a zinc-air battei-y and 3 V for a lithium-air battery.
Preferably, an electromechanical relay driven by an operational amplifier will
be used as a control means. Such an operational amplifier is marketed for example by
5 National Semiconductor under the reference LM324. and will be described in more
detail with reference to figure 2.
Another subject of the present invention is a method for storing and releasing
electrical energy using an accumulator such as described above.
This method comprises the following successive steps:
10 (a) a charging phase during which the negative electrode is connected to the first
terminal, the first positive electrode is connected to the second terminal and the air
^(k electrode is disconnected,
(b) a first discharging phase during which the negative electrode is connected to the
first terminal, the first positive electrode is connected to the second terminal, the air
15 electrode is disconnected, and the control means continuously compare the measured
voltage (Vm) between both terminals to a setpoint value (Vc),
(c) a first switching step during which the second positive electrode, which is the air
electrode, is connected to the second terminal when the value of the voltage between
both terminals falls below the setpoint value (Vc),
20 (d) a second discharging phase during which the negative electrode is connected to
the first terminal and the air electrode is connected to the second terminal, and
(e) a second switching step during which the air electrode is disconnected from the
second terminal.
^ ^ The Applicant considers that the succession of steps (b) and (c), i.e. the non-
^ ^ 25 connection of the air electrode at the start of the discharge, the continuous
monitoring of the decrease in the voltage between both terminals by an appropriate
control means, then the connection of the air electrode only when the absolute value
of the potential difference between the terminals has become sufficiently low,
effectively protects the air electrode and constitutes the contribution of the present
30 invention to the state of the art.
In other words, at the start of the discharge, as long as the absolute value of
the voltage between both terminals is higher than that of the setpoint value (for
m
^a
example V^ = 1.44 V), the air electrode remains disconnected and does not run the
risk of being deteriorated by an overly high potential difference.
In the case where a first positive electrode based on nickel (Ni/NiO) is used,
the voltage at the start of charging is typically equal to 1.7 V.
5 When the value of the voltage between both terminals, measured
continuously by the control means, falls below the setpoint value (1.44 V), the
control device sends a switch signal to the switching means with the aim of finally
connecting the air electrode to the second terminal.
The air electrode may remain connected until the end of the discharging step,
10 i.e. until the start of the next charging phase. The disconnection (step (e)) of the air
electrode before the next charging step (step (a)) may be achieved manually or,
preferably, automatically through control means associated with the switching means
carrying out the first switching step (step (c)) of the method according to the
invention.
15 In the second discharging phase, during which the air electrode is connected
to the second terminal, the first positive electrode may be connected or disconnected.
Whether the first positive electrode is connected or disconnected of course depends
on the structure of the switching means. In one embodiment of the present invention,
the switching means is adapted to disconnect the first positive electrode of the second
20 terminal. In another embodiment, the switching means is adapted to not disconnect
the first positive electrode from the second terminal, which therefore remains
continuously connected to the second terminal, both during charging and
discharging.
The invention will now be described in detail, with reference to the appended
25 figures in which
figure 1 is a schematic diagram of one embodiment of a cell of an
accumulator of the present invention, and
figure 2 is a schematic diagram of a control means and of a switching means,
forming the device for protecting the air electrode of the present invention.
30 The cell of the accumulator comprises a first terminal 1 and a second terminal
2. During discharge of the accumulator, the first terminal is a negative terminal and
the second terminal is a positive terminal.
^
The first terminal I is connected to a first metal electrode 3, the so-called
negative electrode, made of zinc or lithium for example.
The second terminal 2 is coupled, via a switching means 7, to an oxygenreleasing
electrode 4 or to an air electrode 5. During discharge of the accumulator,
5 these two electrodes form positive electrodes.
The oxygen-releasing electrode may for example be made of nickel. The air
electrode is a carbon-based porous electrode containing an oxygen reducing catalyst
at the solid/gas interface. The catalyst may be manganese oxide or cobalt oxide.
The three electrodes 3, 4, 5 contact at least one electrolyte, preferably an
10 alkaline aqueous electrolyte.
The two positive electrodes 4, 5 are coupled to the second terminal 2 through
switching means 7. In the embodiment shown in figure I the switching means 7 are
exclusive, i.e. they connect terminal 2 either to the oxygen-releasing electrode 4, or
to the air electrode 5.
15 The accumulator according to the invention further comprises control means
6, adapted to measure continuously the voltage Vm between both terminals 1 and 2
and to send a switch signal to the switching means 7 and thereby control the
connection or disconnection of one and/or the other positive electrode.
The control means are shown in more detail in figure 2.
20 A reference voltage, equal to the setpoint voltage (Vc), is supplied by a
stabilized voltage source 8 connected between the first terminal 1 and a negative
input of an operational amplifier 9. The second terminal 2 is connected to a positive
input of the operational amplifier 9, which measures a voltage difference between
both inputs. Thus, the setpoint voltage is subtracted from the voltage between the
25 first terminal and the second terminal. When the voltage measured between both
terminals is equal to the setpoint voltage, the difference measured by the operational
amplifier between the voltages applied to both inputs is equal to zero.
In addition, the control means 6 comprise a transistor 10 acting as a switch in
a current loop between two terminals 11 and 12.
30 When the voltage Vm between both terminals 1 and 2 of the accumulator is
higher than the setpoint voltage Vc, the operational amplifier sends a given voltage to
the base B of the transistor 10. The transistor 10 then turns on and allows a current to
pass between its emitter E and its collector C.
In addition, the control means comprise a coil, or an inductor, 13. When
current flows in the current loop, the coil generates a magnetic field.
For example the switching means 7 may comprise a relay containing a
conductive element 14 that can be moved by a magnetic field.
Thus, when the coil 13 generates a magnetic field, the conductive element 14
is moved from a first position to a second position. In the first position, the
conductive element is separated from the air electrode so as to disconnect it from the
second terminal of the accumulator, and in the second position the air electrode is
connected to the second terminal of the accumulator.
10
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n,. V
e
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(.9
lAVe Claim: \ \ ^0 ^'^'^
1. A rechargeable accumulator comprising one or more metal-air cells, each
5 cell comprising
- a first terminal (1) and a second terminal (2),
- a metal negative electrode (3) for a metal-air cell, connected to the first
terminal (1),
- a first positive oxygen-releasing electrode (4),
10 - a second positive electrode (5), which is a porous air electrode containing
at least one oxygen reducing catalyst, and
- an electrolyte,
characterized in that it furthermore comprises
- control means (6) adapted to continuously compare the measured voltage
15 (Vm) between the terminal (2) and the terminal (1) to a setpoint value (Vc)
and to send a switch signal to switching means (7) when the measured
voltage (Vm) falls below the setpoint value (V^),
- the switching means (7) adapted to receive a switch signal originating
from the control means (6) and to connect and disconnect the air electrode
20 (5) from the second terminal (2).
2. The accumulator of claim 1, characterized in that the negative electrode (3)
is a lithium electrode (Li/LiOH) or a zinc electrode (Zn/Zn(OH)4^').
3. The accumulator of claim 1 or claim 2, characterized in that the first
positive oxygen-releasing electrode (4) is a silver electrode (Ag/AgO) or a nickel
25 electrode (Ni/NiO or NiOH/NiOOH).
4. The accumulator as claimed in any one of the preceding claims,
characterized in that the air electrode comprises carbon particles and on at least one
oxygen reducing catalyst.
5. The accumulator as claimed in any one of the preceding claims,
30 characterized in that the oxygen reducing catalyst in the air electrode is chosen from
the group formed by manganese oxide and cobalt oxide.
6. The accumulator as claimed in any one of the preceding claims,
characterized in that the setpoint value (Vc) is set to a value lower than the open
11
t. i
K.- • . . V
circuit voltage of the cell in the charged state when only the air electrode is
connected.
7. The accumulator as claimed in any one of the preceding claims,
characterized in that the control means (6) is an electromechanical relay driven by an
5 operational amplifier.
8. The accumulator as claimed in any one of the preceding claims,
characterized in that the first positive electrode (4) takes the form of a grid or of a
perforated plate and lies between the negative electrode and the air electrode.
9. A method for storing and releasing electrical energy using an accumulator
10 as claimed in any one of the preceding claims, said method comprising the following
successive steps:
^k (a) a charging phase during which the negative electrode (3) is connected to the
first terminal (1), the first positive electrode (4) is connected to the second
terminal (2) and the air electrode (5) is disconnected,
15 (b) a first discharging phase during which the negative electrode (3) is connected
to the first terminal (1), the first positive electrode (4) is connected to the
second terminal (2), the air electrode (5) is disconnected, and the control
means (6) continuously compare the measured voltage (Vm) between the
terminal (2) and the terminal (1) to a setpoint value (Vc),
20 (c) a first switching step during which the second positive electrode (5), which is
the air electrode, is connected to the second terminal (2) when the value of
the voltage between the terminal (2) and the terminal (1) falls below the
setpoint value (Vc),
^^ (d) a second discharging phase during which the negative electrode (3) is
^tf 25 connected to the first terminal (1), and the air electrode (5) is connected to the
second terminal (2), and
(e) a second switching step during which the air electrode (5) is disconnected
from the second terminal (2).
10. The method as claimed in claim 9, characterized in that in the first
30 switching step, the switching means (7) disconnect the first positive electrode (4)
from the second terminal (2).
12
«
ARiGliiAL «B ,-. "7 ,^,^:
'C^
11. The method as claimed in claim 9, characterized in that in the first
switching step, the switching means (7) do not disconnect the first positive electrode
(4) from the second terminal (2).