Method for managing the electrical energy passing through a metal-air battery and associated cell

Abstract

A method for managing the electrical energy passing through a metal-air battery comprising a cell and the associated cell comprising a negative electrode, a first positive electrode referred to as the air electrode, and a second positive electrode referred to as the power electrode. The cell further comprises a third positive electrode. In a first charging phase, a charging voltage is applied to the cell, this voltage causing current to travel between the negative electrode and the second positive electrode, the first and third positive electrodes being electrically inactive. In a second charging phase, the charging voltage causes current to travel between the negative electrode and said third positive electrode, the first and second positive electrode being electrically inactive.

Claims

1. A method for managing the electrical energy passing through a metal-air battery comprising at least one cell comprising: a negative electrode; a first positive electrode forming an air electrode of the cell; and a second positive electrode forming a power electrode of the cell; wherein, the cell further comprising an oxygen-evolution third positive electrode, the method comprises: applying a charging voltage to the cell during a first cell-charging phase, the charging voltage during the first cell-charging phase causing current to travel between the negative electrode and the second positive electrode, the first and third positive electrodes being electrically inactive; applying the charging voltage to the cell during a second cell-charging phase, the charging voltage during the second cell-charging phase causing current to travel between the negative electrode and said third positive electrode, the first and second positive electrodes being electrically inactive; and connecting the third positive electrode during the second charging phase, before oxygen is formed on the second positive electrode.

2. The method according to claim 1, wherein, the second positive electrode comprising an oxide of a metal in a given oxidation state, the method further comprises: switching from the first phase to the second phase upon detection of a change in the charging voltage.

3. The method according to claim 1, further comprising: at least during the first charging phase, measuring a voltage amplitude between the negative electrode and the second positive electrode, upon detecting a measurement of said voltage amplitude greater than a predetermined voltage threshold, disconnecting the second positive electrode and connecting the third positive electrode, in order to switch from the first charging phase to the second charging phase.

4. The method according to claim 3, wherein the voltage threshold is a voltage amplitude above which oxygen evolution occurs on the second positive electrode.

5. The method according to claim 1, wherein, for a discharging phase of the cell in which the negative electrode is connected to a negative terminal of an electric circuit in order to supply electrical energy to this electric circuit, the method further comprises: obtaining information on the demand for electrical energy of said circuit, and based on the demand of the circuit, applying one of the following: a first operating mode for discharging the cell in which the first positive electrode is connected to a positive terminal of the electric circuit, and a second operating mode for discharging the cell in which the second positive electrode is connected to the positive terminal of the electric circuit.

6. The method according to claim 5, wherein the first operating mode corresponds to a supply of electrical power below a demand threshold of the circuit, and the second operating mode corresponds to a supply of electrical power above the demand threshold.

7. The method according to claim 6, further comprising: measuring a voltage amplitude between the negative electrode and an electrode among the first positive electrode and the second positive electrode which is connected to the positive terminal of the circuit; and when the voltage amplitude is above a predetermined threshold voltage, the threshold voltage being representative of a demand threshold of the circuit: selecting the second operating mode, and when the voltage is below the threshold voltage: selecting the first operating mode.

8. The method according to claim 7, wherein the predetermined threshold voltage is estimated at regular time intervals based on a comparison between the voltage amplitude measured between the negative electrode and the first positive electrode and the voltage amplitude measured between the negative electrode and the second positive electrode.

9. A non-transitory computer program product comprising a series of instructions stored on a storage medium for execution by a computer or a dedicated device, said program being configured to execute the method according to claim 1.

Description

BRIEF DESCRIPTION OF THE DRAWINGS

(1) The method which is the object of the invention will be better understood by reading the following description of some exemplary embodiments presented for illustrative purposes and in no way limiting, and by examining the following drawings in which:

(2) FIG. 1A is a schematic representation of a metal-air battery cell comprising a third positive electrode according to the invention, and FIG. 1B is a schematic representation of a symmetrical metal-air battery cell comprising two sets of positive electrodes according to FIG. 1A which share their negative electrode;

(3) FIG. 2 is a schematic representation of an electrical connection which allows managing the electrical energy passing through a metal-air battery cell according to the invention;

(4) FIG. 3 is a flowchart representing a method for charging a metal-air battery, comprising two charging phases according to the invention;

(5) FIG. 4 is a flowchart representing a method for discharging a metal-air battery; making it possible to select a power electrode or a high-energy-density electrode according to the demand for power;

(6) FIG. 5 is a graph showing the voltage measured over time across a metal-air battery according to the invention, during several charging and discharging cycles;

(7) FIG. 6 is a schematic representation of a computer system that can be used to implement the method of the invention.

(8) For clarity, the dimensions of the various elements shown in these figures are not necessarily in proportion to their actual dimensions. In the figures, identical references correspond to identical elements.

DETAILED DESCRIPTION

(9) The present invention proposes a novel cell architecture for a metal-air battery which makes it possible to increase their life as well as their electrical performance. The invention also proposes a method for managing the electrical energy passing through a metal-air batter composed of at least one cell that protects the positive electrodes of the battery and extends its life.

(10) In order to protect a positive electrode used during the recharging of a metal-air battery, the invention proposes adding a third positive electrode used preferably when oxygen is produced during Charging of the battery.

(11) Metal-air batteries typically consist of a negative electrode of a metal such as zinc, lithium, or iron. The metal-air batteries described for example in document WO 2014/083267 use, in addition to a first positive electrode called an air electrode, a second positive electrode which is used during charging of the battery.

(12) The second positive electrode may typically be of a metal such as nickel, silver, or stainless steel. This electrode could also be composed of a material capable of storing a limited amount of electrical energy without oxygen evolution. An example of such a positive electrode is an electrode composed of nickel oxide hydroxide, NiO(OH). This electrode is used for the charging phase so that oxygen evolution does not occur within the fragile structure of the air electrode made of carbon powder.

(13) However, to prevent the oxygen evolution from progressively deteriorating the second positive electrode, which may also be subject to a disintegration phenomenon leading to lower electrical performance of the battery over time, the invention proposes an original battery architecture according for example to the structure shown in FIG. 1A or 1B.

(14) FIG. 1A represents a cell 100 of a metal-air battery comprising a negative electrode 10, an electrolyte 50 of strongly basic pH, typically of a value greater than or equal to 14, and a first positive electrode 20 forming an air electrode. The air electrode comprises carbon grains 21 in its structure. In addition, the cell 100 comprises a second positive electrode 30 forming a power electrode of the battery comprising the cell 100 and a third positive electrode 40 forming an oxygen-evolution electrode of the cell 100.

(15) The negative electrode is intended to be connected to a negative terminal 101 of the battery, and the air electrode is intended, at least in the discharging phase, to be connected to a positive terminal 102 of the battery. Ions M.sup.n+ of the metal M constituting the negative electrode flow in the electrolyte between the electrodes connected at the terminals. The electrolyte also comprises a high concentration of hydroxyl ions OH.sup.−.

(16) The inventors have noticed that the second positive electrode 30 also undergoes degradation during the charging phases, although this is slower than that which the air electrode would undergo if the first positive electrode 20 (the air electrode) was used during charging.

(17) To limit wear and degradation of the second positive electrode 30 over time, and to increase the battery life, the invention proposes providing two distinct charging phases: a first charging phase during which the second positive electrode 30 is connected to the positive terminal 102 of the battery, and a second charging phase during which the third positive electrode 40 is connected to the positive terminal 102 of the battery. During the first charging phase, the charge current passing through the battery results in oxidation of the second positive electrode 30 but for a shorter duration than what would result in a degradation of said electrode. The second charging phase gives rise to oxygen evolution on the third positive electrode 40, which preserves the second positive electrode 30 from the harmful consequences of this significant production of oxygen during charging.

(18) It has been observed that the use of metal oxides in the second positive electrode 30 could be of interest for further increasing the electrical performance of a metal-air battery.

(19) This second positive electrode 30, generally made of a metal such as nickel, is sometimes composed of an oxide such as, for example, nickel oxide hydroxide (NiOOH) using the Ni(II)Ni(III) pair.

(20) It has been found that the use of nickel oxide hydroxide (NiOOH) using the Ni(II)/Ni(III) pair in the material of the second positive electrode 30 has advantages from an electrical standpoint. Indeed, the nickel oxide hydroxide provides more power than the air electrode. The air electrode offers higher energy densities than those accessible with a nickel oxide hydroxide electrode but is limited by the rate of diffusion of air in the electrode structure which reduces the power that can be provided by a battery using only the first positive electrode 20 in the discharging phase.

(21) The use of a second positive electrode 30 of nickel oxide hydroxide offers the possibility of combining the advantages of the discharging cycle duration of metal-air batteries (accessible due to the high energy density of air electrodes) with the power performances of metal-nickel batteries. A metal-air battery cell using a metal oxide such as nickel oxide hydroxide as a second positive electrode forms a “metal-nickel-air” hybrid cell.

(22) In the discharging phase, the following reaction is observed on the second positive electrode 30:
NiO(OH)+H.sub.2O+e.sup.−.fwdarw.Ni(OH).sub.2+OH.sup.−  (reaction a)

(23) Such a battery can allow more refined management of the power and electrical energy passing through the battery. When a standard demand for power reaches the battery in the discharging phase, the first positive electrode 20 can be connected to the positive terminal. For larger power draws, it is possible to connect the second positive electrode of nickel oxide hydroxide.

(24) The electrical capacity (expressed in mAh/cm.sup.2) of the second positive electrode of nickel oxide hydroxide turns out to be lower than that of the negative electrode 10. One consequence of this difference in capacity is that recharging the cell requires two oxidation reactions at the second positive electrode 30. In a first oxidation reaction, the nickel in oxidation state (II) is converted into nickel in oxidation state (III) according to the reaction:
Ni(OH).sub.2+OH.sup.−.fwdarw.NiO(OH)+H.sub.2O+e.sup.−  (reaction b)

(25) When the nickel has changed its oxidation state, a second oxidation reaction takes over to continue charging the negative electrode 10 (this negative electrode typically being of zinc, iron, or lithium), until the battery is completely charged. This second reaction converts the hydroxyl ions of the electrolyte 50 into oxygen according to the conventional oxygen evolution reaction:
4OH.sup.−.fwdarw.O.sub.2+2H.sub.2O+4e.sup.−

(26) Separating the charging of a metal-air battery into two phases as described above, by providing a first phase using the second positive electrode 30 then a second phase using a third positive electrode 40, is of particular interest when the second positive electrode 30 is composed of an oxide which undergoes two successive oxidation reactions as described above. It is then relevant to provide a switch from the first charging phase to the second charging phase when the first oxidation reaction (reaction b) has converted most of the metal of the oxide from a first oxidation state to a second oxidation state.

(27) The example described above can typically concern a second positive electrode 30 made of nickel oxide hydroxide. However, other compounds may be used, for example such as silver oxide or manganese oxide. The second positive electrode 30 is typically a compound having a more positive oxidation-reduction potential than the oxidation-reduction potential of the metal of the negative electrode (typically zinc, iron, or lithium). It is further advantageous to provide a material for the second positive electrode 30 having a more positive oxidation-reduction potential than the oxidation-reduction potential of the air electrode.

(28) The implementation of the charging and discharging method may be based on the use of a system of relays or any type of switch such as those shown in FIG. 2.

(29) FIG. 1B represents a symmetrical cell (100) of a metal-air battery comprising two sets of positive electrodes according to FIG. 1A which share their negative electrode. The symmetrical cell comprises a negative electrode 10, an electrolyte 50 of strongly basic pH, typically of a value greater than or equal to 14, and, on each side of the negative electrode 10, a set of positive electrodes comprising a first positive electrode 20 forming an air electrode, a second positive electrode 30 forming a power electrode of the battery comprising the cell 100, and a third positive electrode 40 forming an oxygen-evolution electrode of the cell 100.

(30) Thus, according to one particular embodiment, the first, second, and third positive electrodes are positioned symmetrically in the cell (100) around the negative electrode 10.

(31) FIG. 2 schematically represents a negative electrode 10, a first positive electrode 20 (air electrode), a second positive electrode 30 (called the power electrode), and a third positive electrode 40 (typically a metal grid made of a metal, preferably a pure metal such as nickel or silver. The electrodes of FIG. 2 are of materials that are stable in the highly basic medium of a metal-air battery electrolyte.

(32) The battery 200 is connected by its negative 101 and positive 102 terminals to a circuit that supplies power to the battery or consumes the energy supplied by the battery. In FIG. 2, the circuit 201 is supplying power to charge the battery. The negative terminal 202 of the circuit 201 is connected to the negative electrode 10, while the positive terminal 203 of the circuit 201 is connected to a positive electrode of the battery 200. A first switch 210 enables selecting a connection between the positive terminal 203 of the circuit 201 and the first positive electrode 20 or one among the second positive electrode 30 and third positive electrode 40.

(33) This first switch 210 comprises three connection points 211, 212, 213, with a member 214 enabling the connection of connection point 211 to either of points 212 or 213.

(34) A first measuring device makes it possible to monitor an electrical parameter of the battery such as a current, an electric potential, or a voltage, upstream of the first switch 210, in order to compare it with a threshold value to enable determining when the first switch must switch from the air electrode to one of the other two positive electrodes. The first measuring device may be a current sensor or a voltmeter 204, for example.

(35) A second switch 220 placed downstream of connection point 212 makes it possible to select an electrode among the second positive electrode 30 and the third positive electrode 40. This second switch 220 also comprises three connection points 221, 222, 223 and a member 224 enabling the connection of connection point 221 to one among the second positive electrode 30 connected to connection point 223 and the third positive electrode 40 connected to connection point 222.

(36) A second measuring device makes it possible to monitor an electrical parameter of the battery, for example such as a current, an electric potential, or a voltage, upstream of the second switch 220 and downstream of the first switch 210. This electrical parameter is compared to a threshold value to enable determining when the first switch must switch from one electrode to another. The second measuring device may for example be a current sensor or a voltmeter 205.

(37) The use of a first switch 210 and a second switch 220 makes it possible to implement the two-phase charging method of the invention and also allows more relevant management of the delivery of electrical energy by the battery 200 during the discharging phase.

(38) FIG. 3 schematically represents a flowchart in three steps for charging a cell 100 of a metal-air battery 200 according to the invention.

(39) Initially, the first 210 and second 230 switches are configured so that the second positive electrode 30 is connected to the positive terminal 203 of a circuit 201 providing electrical energy to the battery. The first step S301 therefore corresponds to beginning to charge the battery, which preferably first makes use of the second positive electrode 30. This second positive electrode 30 may in particular be recharged by the oxidation reaction b) mentioned above when the second positive electrode 30 comprises nickel oxide hydroxide. Thus, in the first charging phase S301, when the second positive electrode 30 comprises an oxide such as nickel oxide, no oxygen evolution takes place on the electrode, which protects it from premature deterioration.

(40) As indicated by step S302, a voltage amplitude measured by the second measuring device, for example the voltmeter 205 of FIG. 2, is compared with a voltage threshold Vth2, 300. This voltage threshold is the voltage amplitude above which oxygen evolution occurs on the second positive electrode. One way of determining this voltage threshold consists in particular of observing a variation in the rate of increase of the voltage amplitude measured by the voltmeter 205. Such a voltage variation is the sign indicating that most of the metal of the electrode has been converted from a first oxidation state to a second oxidation state (according to reaction b) and that the second oxidation reaction with oxygen evolution is about to begin.

(41) This variation is usually expressed as a sudden variation in the voltage across the cell or the current flowing through the cell.

(42) For a zinc-air battery cell with a second positive electrode of nickel oxide hydroxide and a third positive electrode of nickel metal, the voltage threshold 300 can typically be comprised between 1.5V and 2.5V, and preferably is set at 1.9V. The value of this voltage threshold 300 depends in particular on the metal of the negative electrode 10 (for example zinc, iron, or lithium) and on the composition of the second positive electrode 30.

(43) As long as the value of the voltage V2 measured by the second measuring device (voltmeter 224 in FIG. 2) corresponds to an amplitude lower than that of the voltage threshold 300, the charging occurs on the second positive electrode 30. When the measured voltage amplitude becomes greater than the voltage threshold 300, the second switch 220 disconnects the second positive electrode 30 from the positive terminal 203 of the circuit 201 and connects the third positive electrode 40 to this positive terminal 203, as illustrated in step S303 of FIG. 3. This step S303 corresponds to the second charging phase of the method according to the invention.

(44) As the electric power supplied to the battery during charging can undergo fluctuations, for example a sudden supply of high electrical power, it may be advantageous to provide a continuous comparison of the voltage V2 to the voltage threshold 300.

(45) In some exemplary applications of the present invention, the circuit 201 may comprise a photovoltaic panel or an electric vehicle accumulator, for which the charging electrical power fluctuates over time. The level of sunshine can generate power spikes in the circuit 201. Similarly, in an electric vehicle, braking can be an opportunity for recovering the high power generated during a brief period of time. When such events occur, it is advantageous to give preference to charging the second positive electrode 30 which is able, particularly when composed of a metal oxide, to offer faster charging than the third positive electrode 40. Proceeding in this manner during brief periods of high power being supplied during charging limits the negative effects of oxygen evolution on the second positive electrode 30, but reduces the charging time.

(46) In addition, a variant in the charging method can be envisaged in which the third positive electrode 40 or the first positive electrode is connected to the positive terminal 203 of the circuit 201 when charging begins, so that the second positive electrode 30 is only connected when the electrical power supplied to the battery exceeds a certain threshold. This threshold of supplied electrical power can be detected in particular by a current sensor, a hall effect sensor, a voltmeter, or an ammeter, which measure the current or voltage across the battery. A switchover to the second positive electrode 30 during charging is performed at the switches when a sudden upward variation in the current amplitude or voltage amplitude across the battery is detected.

(47) Once the battery 200 is fully or partially Charged, with a charging method comprising two phases as described above, it can be used in the discharging phase.

(48) FIG. 4 illustrates a flowchart in which the circuit 201 consumes electrical energy delivered by the battery 200.

(49) When the battery 200 is discharging, the third positive electrode 40 is rarely used and the second switch 220 connects the second positive electrode 30 to the positive terminal 203 of the circuit 201.

(50) When discharging begins, it is possible to place the first switch 210 in a position which connects the first positive electrode 20 to the positive terminal 203 of the circuit 201. This is represented in step S401 in FIG. 4. The first positive electrode 20 consumes oxygen from the air to provide electrical power to the circuit. However, the rate of diffusion of air into the porous structure of the air electrode limits the maximum electrical power that the battery can deliver in this configuration.

(51) To respond to larger demands for power, the invention proposes, in the discharging phase, determining an electrical power demand threshold of the circuit 201. When a power demand greater than the demand threshold is identified, the first switch 210 is activated in order to connect the second positive electrode 30 to the positive terminal 203 of the circuit 201 while disconnecting the air electrode.

(52) As indicated in the flowchart of FIG. 4, the demand threshold may be a threshold voltage 400, Vth1. When the amplitude of the voltage V1 measured by the voltmeter 204 of FIG. 2 is greater than the threshold voltage 400, the discharging follows a first discharge operating mode using the air electrode on the positive terminal 203 of the circuit 201. When the amplitude of the voltage V1 is below the threshold voltage 400, the first switch connects the second positive electrode 30 to the positive terminal 203 of the circuit 201. This is represented by step S403 and S404 of FIG. 4.

(53) In addition, FIG. 4 indicates that the threshold voltage is estimated in an intermediate step S402, and this occurs continuously. Indeed, the capacity of the battery to respond to a circuit's demand for power decreases as the battery discharges. The threshold voltage 400 may in particular be estimated from a comparison between the amplitude of a voltage measured between the negative electrode 10 and the first positive electrode 20, and the amplitude of a voltage measured between the negative electrode 10 and the second positive electrode 30.

(54) Alternatively, the threshold voltage 400 may be estimated on the basis of measurements made during a first cycle of use during battery discharge, or else from numerical simulations carried out while taking into account the materials used in the electrodes of the battery.

(55) The method described above may be implemented in particular by a control unit such as a BMS (for “battery management system”).

(56) In particular, it has been found that the electrical performance of the positive electrodes depends on their sizes. A thicker electrode offers a higher energy density due to the presence of a larger amount of material that can be converted in the oxidation-reduction reactions taking place in the battery, but offers lower power due to a greater electrical resistance. To find a rood balance between available power and power density, it is advantageous to provide a second positive electrode 30 whose energy capacity (expressed in mAh/cm.sup.2) substantially corresponds to one third of the energy capacity of the negative electrode 10. This ratio offers good power and energy performance in a metal-air battery having a second positive electrode 30 of nickel oxide hydroxide in particular.

(57) The charging and discharging cycles described above are repeated in a metal-air battery throughout its use. In this vein, FIG. 5 illustrates the voltage measured across a metal air battery composed of a cell as described above, during two successive charging/discharging cycles.

(58) The vertical axis 501 of FIG. 5 represents the voltage measured at the terminals of the cell, the horizontal axis 502 represents the time in hours.

(59) FIG. 5 comprises in particular a first cycle 512 at constant current during which the measured voltage amplitude progressively decreases. This first cycle 512 corresponds to a discharging cycle of the battery. The first cycle 512 is continuous and does not include any repeated switching between the air electrode and power electrode in order to respond to demands for power over time. It is quite possible, however, to discharge the battery with a very different voltage profile, when a demand for more power occurs at one or more moments in the discharging cycle. The first cycle 512 is subdivided into two operating modes: a second operating mode 510 during which the second positive electrode 30 is used, and a first operating mode 511 during which the first positive electrode 20 is used. The transition from the first operating mode to the second operating mode is marked by a significant and rapid decrease in the voltage amplitude 504. As shown by the curve of the changing voltage measured over time during the second operating mode 510, the power available on the second positive electrode 30 decreases as the battery discharges.

(60) When the battery is discharged, a second cycle 522 begins. This is a charging cycle of the battery. In a first phase 520, the second positive electrode 30 is connected while the air electrode and the third positive electrode 40 are electrically inactive. When the measured voltage amplitude 503 reaches a threshold value (for example 1.9 V), the second oxidation reaction with oxygen production is about to begin on the second positive electrode 30. The second switch 220 then connects the third positive electrode 40 to the positive terminal of the circuit 201, while the air electrode and the second positive electrode are electrically inactive. This second charging phase 521 using the third positive electrode 40 protects the second positive electrode 30 from the negative effects of oxygen evolution.

(61) It is possible that the battery remains inactive between two charging/discharging cycles, which is represented in FIG. 5 by a period 523 during which the measured voltage is the open circuit voltage at the end of charging the battery. The charging and discharging cycles then repeat as many times as necessary, until the battery needs to be replaced.

(62) The invention also relates to a computer program product comprising a series of instructions stored on a storage medium for execution by a computer or a dedicated device, the program being configured to execute the method described above.

(63) FIG. 6 shows an example of a computer system which allows running a computer program product comprising instructions implementing the method of the present invention.

(64) In this embodiment, the device comprises a computer 600, comprising a memory 605 for storing instructions for implementing the method, the received measurement data, and temporary data for carrying out the various steps of the method as described above.

(65) The computer further comprises a circuit 604. This circuit may be, for example: a processor capable of interpreting instructions in the form of a computer program, or a circuit board in which the steps of the method of the invention are defined in the silicon, or a programmable electronic chip such as an FPGA chip (for “Field-Programmable Gate Array”).

(66) This computer has an input interface 603 for receiving measurement data, and an output interface 606 for providing commands controlling the evacuation device 607. Finally, the computer may comprise a screen 601 and a keyboard 602, to enable easy interaction with a user. Of course, the keyboard is optional, particularly in the context of a computer having the form of a touchscreen tablet for example.

(67) The invention is not limited exclusively to the exemplary embodiments presented above which served to illustrate the invention. In particular, the materials used in the various electrodes are given for illustrative purposes only. A third positive electrode may in particular be used even when the second positive electrode is itself a metal grid and not a material comprising an oxide undergoing two different and successive oxidation reactions in the charging phase.

(68) The invention finds applications in rechargeable metal-air batteries, and allows increasing their service life and their electrical performance. The improved management of the electrical energy passing through a metal-air battery according to the invention makes them usable in many systems, for example such as photovoltaic devices subject to voltage variations related to sunlight, or electric vehicles that consume and store variable electrical power related to the use being made of the vehicle and to the braking or acceleration conditions.