The present patent application relates to metal-air alkaline battery cells for the use in electrically rechargeable batteries. More particularly, the invention relates to battery cells comprising a first galvanic cell formed from a first reversible metal electrode and an air electrode, and also comprising a second reversible electrode (i) forming a second galvanic cell with the said first reversible metal electrode, and (ii) acting as second cathode during discharging and anode during charging, thereafter "bi-cathode discharging cells", thus providing better specific energy and energy usage efficiency (the ratio of discharging vs. charging energies) characteristics and resulting in an increased number of charge/discharge cycles owing to reduced dendritic propagation, lesser electrode passivation and shape change.

The present patent application also relates to electrically rechargeable batteries incorporating such bi-cathode discharging cells.

BACKGROUND

The issue is the storage of energy with electric batteries.

This issue becomes exacerbated especially with the trend of replacing gasoline powered vehicles by electric ones.

Metal air battery cells are well-known for high specific energy and are widely used as primary batteries, for example for hearing aid or military and aero-space equipments.

However metal air secondary battery cells characteristics suffer from well-known shortcomings: dendritic propagation, passivation, zincate ion aging, formation of inactive islets on the electrode, active mass redistribution (shape change), active mass relocation, etc.

These problems are well-known and described for example in the PCT Application PCT/AM2010/000001 from the present applicant. Up to now there are no commercially available reversible air electrodes so they cannot be used to recharge the cell. Hence the classical approach to electrically recharge the first reversible electrode of cells incorporating an air electrode, is through the use of an auxiliary electrode which is indifferent, i.e. it only provides electrons and the material itself does not participate in the electrochemical processes. This auxiliary electrode works only during battery charging and is disconnected during battery discharge. Thus, the presence of an auxiliary electrode diminishes important battery characteristics, such as specific energy value because of extra weight and extra occupied volume of auxiliary electrode, separator and extra electrolyte, see for example United States Patent 4,039,729 describing such auxiliary electrodes.

With the presence of an auxiliary electrode, two battery cell setups are possible:

• first setup: the auxiliary electrode is placed in between the first reversible electrode and the air electrode, or,

• second setup: the first reversible electrode is placed in between the auxiliary electrode and the air electrode.

Comparison of the first and second setups:

1. Short-circuiting: the first setup eliminates Zn-to-air electrode short- circuiting, however, it does not eliminate Zn-to-auxiliary electrode short-circuiting; the second setup does not eliminate both the Zn-to- air and Zn-to auxiliary electrode short-circuiting, yet it considerably diminishes Zn-to-air electrode short-circuiting.

2. Ions concentration gradients: the first setup keeps the concentration gradients of ions over both surfaces of Zn electrode comparatively even; in the second setup the concentration profiles of ionic species vary considerably during charge-discharge cycle, which increases Zn-electrode polarization with ensuing Zn electrode problems such as dendrite propagation, shape change, mass relocation...

3. Cell inner-resistance at discharge: in the first setup it is very high, so the first setup is usable for mild discharge rates; in the second setup it is low as there is no extra resistance conditioned by the volume occupied by auxiliary electrode being in between the first reversible metal electrode and the air electrode.

Because of Point 3, the first setup offers poor overall characteristics and almost only the second setup is used in practice despite drawbacks from points 1 and 2.

Classical approaches for reducing dendritic propagation, active mass redistribution "shape change" and active mass relocation issues

Different approaches have been tried in order to diminish dendritic propagation, active mass redistribution "shape change" and active mass relocation. For example in case of zinc-air cells, these issues and some solutions have been described in the PCT Application PCT/AM2010/000001 published as WO/2010/118442). Classical approaches for increasing specific energy values

The classical solution for increasing specific energy of metal-air battery cells is to increase the active mass of the metal electrode relatively to the cell's total mass, in practice to make the metal- electrode thicker. However this thicker metal electrode tends to increase other problems such as pore plugging, passivation, formation of inactive islets and increasing electrode polarization related to unit active mass.

Also the solution of doubling the air-electrode cathode is well-known as described in US Patent 5,569,551 , or US Patent 7,807,304 (primary zinc-air battery cell), but in each case the reversible metal electrode forms a single galvanic couple type having one discharging curve plateau.

As described above in the Background presentation, metal-air electrically rechargeable battery cells still need improvement to compete with other types of rechargeable batteries such as Ni-MH or Li-ion ones, especially with regard to service life, i.e. cyclability.

The aim of the present patent application is to provide a solution to improve the characteristics of such battery cells.

Surprisingly, the applicant has found that by including a second reversible cathode electrode such bi-cathode discharging cells exhibit better specific energy values, and cyclability than the standard cells not including a second reversible cathode electrode.

The first object of the invention is an electrically rechargeable metal- air battery cell with alkaline aqueous electrolyte, comprising a first galvanic cell formed from a first reversible metal electrode and an air electrode, wherein said battery cell comprises also a second reversible electrode (i) forming a second galvanic cell with the first reversible metal electrode, and (ii) acting as second cathode during discharging and anode during charging of the said battery second galvanic cell - the discharging curve thus presenting two distinct plateaus -, whereby the total specific energy is increased, and dendrites formation are lowered. As announced in the introduction at the beginning of this patent application these said battery cells will be referred thereafter as "bi-cathode discharging cells".

In preferred embodiments the discharging of a said bi-cathode discharging cell comprises a first phase concerning the second galvanic cell solely and a second phase concerning the first galvanic cell solely or concomitantly with the first galvanic cell; the second phase starting only after the second galvanic cell voltage has dropped to a specific value so that no anodic process takes place on air electrode upon connection with second reversible electrode.

In another preferred embodiment, the said first reversible electrode of the bi-cathode discharging cell is a zinc electrode.

In yet another preferred embodiment the second reversible electrode is a nickel oxide electrode.

More preferably the first reversible electrode is placed between the air electrode and the second reversible electrode whereby the dendrites being formed during charging on the surface of the first reversible electrode are mostly dissolved during the first discharge phase via the second reversible electrode. Yet more preferably the said bi-cathode discharging cell comprises an auxiliary electrode between the first reversible electrode and the air electrode, such auxiliary electrode permitting easy migration and diffusion of ions through it and sharing the charging current with the second reversible electrode while charging the said battery cell., whereby the charging of the first reversible electrode is balanced on its two faces.

In preferred embodiments the said auxiliary electrode is a mesh coated or made from indifferent electrode material such as nickel.

More preferably during charging the said auxiliary electrode is connected directly or by a resistor to the second reversible electrode, or is connected to the first reversible electrode through an another external power source in order to adjust the charge amount going to this auxiliary electrode.

In another preferred embodiments the battery cell is doubled, i.e. composed by two said bi-cathode discharging cells as described above, wherein the said two bi-cathode discharging cells have in common their second reversible electrode. These battery cells are referred thereafter as "doubled cells"

The second object of the invention is a method for the production of bi-cathode discharging cells.

In this method a recommended way is to place the first reversible electrode between the air electrode and the second reversible electrode. Another recommended way of this method is to prepare battery cells composed by two said bi-cathode discharging cells, wherein the said two bi-cathode discharging cells have in common their second reversible electrode whereby the second reversible electrode works symmetrically which improves the overall and specific characteristics.

In a further aspect, the present patent application describes an electrically rechargeable metal-air battery comprising at least one bi- cathode discharging cell.

More particularly, the said electrically rechargeable metal-air battery may include a doubled cell.

The mechanism of the bi-cathode discharging cell given below is the presumed mechanism but this explanation must be considered as a hypothesis and cannot limit this invention in any points.

The use of bi-cathode discharging cells object of the present invention mollify the problem stated in point 2 of the "Comparison between first and second setups" by using the second setup with a second reversible electrode instead of auxiliary electrode with the following consequences.

• Consequence 1 : a most important consequence is the reduction of ions concentration gradient within the first reversible electrode between its two faces during cell discharge.

For example during charging on the first reversible electrode the following reactions take place via so-called solid-state mechanism: on Zn electrode: Zn(OH) _2s0 iid + 2e ^" => Zn + 20H ^" (1 ) on auxiliary electrode: 20H ^" - 2e ^" => H ₂ 0 + 0.5 0 ₂ (2) while Zn dendrites are initiating via reduction of zincate ions (so- called soluble mechanism):

Zn(OH) ₄ ^2" + 2e => Zn + 40H ^" (3) The front of the reaction (1) initiates on the side of Zn electrode that faces auxiliary electrode, while during discharge - when the reverse reaction (1 ) occurs - the reaction proceeds on the side of Zn electrode that faces to air electrode

During charging, high alkalinity (high concentration of OH ^" ions) electrolyte surrounds the side of the Zn electrode facing to auxiliary electrode according to reaction (1). Again during discharge this process further deepens when the surrounding electrolyte of the side of Zn electrode facing now to air electrodes is depleted of OH ^" ions according to the reverse of reaction (1), and this process is further exacerbated with each new cycle. Hence, a considerable concentration gradient develops between the two faces of Zn electrode, in other words, inside the Zn electrode.

This unwanted process creates uneven overvoltage distribution on the electrode which for example of Zn electrode, shifts Zn electrode active mass, aggravates passivation, pore plugging, etc. Substitution of auxiliary electrode (usually made of metallic nickel or Ni-coated iron grid) with a second reversible electrode (such as Ni-oxide electrode conventionally used in Zn-Ni, Cd-Ni, Fe-Ni or metal hydride secondary batteries) lessens this unwanted effect because the side of the Zn electrode facing to second reversible Ni-oxide electrode works during the first discharge phase . • Consequence 2: consequence also very important is the reduction of dendritic propagation. With each new cell charging, the resultant Zn forms dendrites which grow further on top of the dendrites already developed during former charging period with gradual penetration through the separator between the first reversible metal electrode and the auxiliary electrode. After several cycles, dendrites pierce the separator and short-circuit the cell with as understandable consequence a cell failure.

However, when we substitute the auxiliary electrode with a second reversible electrode, dendrites grown on the side of the Zn electrode facing to auxiliary electrode during charging mostly dissolve during the first discharge phase thanks to electrochemical process held on the second reversible electrode (for example Ni-oxide electrode) according to reverse of reaction (3).

Thus this bi-cathode discharging cell design dramatically reduces dendritic propagation on the first reversible electrode.

• Consequence 3: consequence of including a second reversible cathode is that in replacing an auxiliary electrode it increases the average discharge voltage and charging/discharging efficiency. These gains are even more pronounced in case of a doubled cell for which it also globally increases the specific energy and energy density,

Compared with regular metal-air battery cells, the bi-cathode discharging cells - object of the present invention - possess higher energy density, specific energy, charging/discharging efficiency and offer better cyclability. According to present invention, high energy density, high specific energy, long service-life metal-air cells are possible with the use of a second reversible electrode.

Detailed Description of the Invention

The invention will be further understood from the following detailed description of specific embodiments with reference to accompanying drawings wherein:

Figure 1 shows a cross section view of a zinc-air battery cell with second reversible electrode and without auxiliary electrode, where (11) is an air electrode; (12) is a first reversible zinc electrode; (13) is a second reversible nickel-oxide electrode; (14) (15) are separators placed in between the neighboring electrodes; (16) is the casing of the battery.

Figure 2 shows a cross section view of a zinc-air battery cell with second reversible electrode and with an auxiliary electrode, where (21) is an air electrode; (22) is a first reversible zinc electrode; (23) is a second reversible nickel-oxide electrode; (24) is an indifferent auxiliary electrode; (25) (26) (27) are separators placed in between the neighboring electrodes; (28) is the casing of the battery.

Figure 3 shows a cross section view of a zinc-air battery doubled cell with second reversible electrode and with auxiliary electrodes, where (31) (31 ') are air electrodes; (32) (32') are first reversible zinc electrodes; (33) is a unique second reversible nickel-oxide electrode positioned in the center and facing to the zinc electrodes; (34) (34') are indifferent auxiliary electrodes; (35) (36) (37) (35') (36') (37') are separators placed in between the neighboring electrodes; (38) is the casing of the battery.

Figure 4 shows a cross section view of a classical zinc-air battery cell, i.e. including an indifferent auxiliary electrode instead of a the second reversible electrode of Fig. 1 , where (41) is an air electrode; (42) is a first reversible zinc electrode; (43) is an indifferent auxiliary electrode; (44) (45) are separators placed in between the neighboring electrodes; (46) is the casing of the battery.

Figure 5 shows comparative discharge curves of battery cells corresponding to Fig. 1 to Fig. 4 configurations.

The products and methods of the invention are illustrated but not limited to the following examples.

EXAMPLE 1

This example describes an embodiment of a battery cell including second reversible electrode but without auxiliary electrodes, as illustrated in Fig. 1 where:

(11) is an air electrode produced by the company MEET (Korea, www.mee-t.com); (12) is a zinc electrode, having Ca 5 mm thickness and 5.5 Ah nominal capacity, prepared for example as described in PCT Application PCT/AM2010/000001 ; (13) is a nickel-oxide electrode possessing 1.1 Ah nominal capacity such as one from NKBN 11 D Ni-Cd accumulator of Lugansk accumulator production plant; all these electrodes having the same visible surface area of 40x80 mm ² ; (14) (15) separators could be of any type of separator being described for alkaline batteries containing Zn electrode, for example of rayon type or microporous type, or any other type, such as provided in USSR Certificate of Authorship #1391401.

The basic component of the electrolyte is NaOH or KOH at 100-500 g.r ¹ concentration. In other embodiments it could also contain additives, such as F ^~ , quaternary alkyl-ammonia salts, calcium zincate, etc., including LHOS.

The casing of the battery (16) is a conventional one used in alkaline batteries and is made of plastic.

This cell is charged through connecting the zinc electrodes to the negative pole of an external DC power source and the nickel oxide electrode to the positive pole of external DC power source to provide charging capacity amounting 120% of the nominal capacity value of the zinc electrode. Charging current density is 20 mA cm ^"2 and discharging is 30 mA cm ^"2 with regard to Zn electrode surface area. Discharge of this cell is done in the following manner:

In a first phase the load is connected to the zinc electrode and nickel-oxide electrode leads.

In a second phase, when the discharge voltage between the zinc and nickel-oxide electrodes has become equal or lower than the potential difference between the zinc electrode and the air electrode (ca 1.25V), the air-electrode is electrically connected to the nickel- oxide electrode. In another example, during phase 2 the nickel oxide electrode could be disconnected while air electrode is connected to load through the zinc electrode. The transition between the first and second phases is normally done automatically by the use of electronic comparators and suitable switching systems (such as relays or transistors). The discharge curve is provided in Fig. 5 and has a form with two distinct discharge plateaus, the first plateau where voltage is approximately from 1.8 V till 1.25 V corresponds to Zinc-Nickel- Oxide galvanic couple's voltage. The second plateau where voltage is approximately from 1.25 V till 0.8 V corresponds to Zn-air galvanic couple's voltage.

Characteristics of this battery cell are indicated in Table 1.

This cell can be cycled 15 times and then fails further cycling because of wetting of air electrode's surface with alkaline electrolyte. Also it can happen that during these 15 cycles small dendrites start to penetrate the separators placed between the zinc electrodes and the nickel oxide electrode.

Table 1 and Fig. 5 allow comparing the characteristics of the embodiment of Example 1 with the characteristics of a classical battery cell - illustrated in Fig. 4 - including an auxiliary electrode instead of the second reversible electrode.

This said classical battery cell is prepared with the same components as for the embodiment of Example 1 except that the Ni- oxide electrode is replaced by an auxiliary electrode made of fine nickel grid of ca 6 Tyler mesh.

Typically this type of classical battery cell fails within the first 8-12 cycles due to problems with zinc or/and air electrode. Post mortem examination of the zinc electrode tested within classical battery shows more dendrites and zinc electrode's active mass shifting towards the air electrode than is the case with the zinc electrode of example 1. In Table 1 , the comparison of the characteristics of the embodiment of Example 1 (illustrated with Fig. 1) with those of the said classical battery cell (illustrated with Fig. 4) shows that the cell including a second reversible electrode has a higher average discharge voltage and better energy density, specific energy and cyclability than the one not including said second reversible electrode.

Table 1

EXAMPLE 2

This example, illustrated by Fig. 2, describes an embodiment of a battery cell according to the invention. The battery cell of this example contains the components described in Example 1 illustrated by Fig. 1 with an additional indifferent auxiliary electrode made of fine nickel mesh (such as 6 Tyler mesh), it could also be an Ni- coated iron grid, for this example electrically connected to the nickel- oxide electrode during cell charge. This embodiment is tested the same way as described in Example 1 except that during charging the auxiliary electrode is directly connected to the nickel oxide electrode (it could also be connected via a resistor, or independently connected to an external power source).

The characteristics of this battery cell are indicated in Table 1 and Figure 5.

As in example 1 this embodiment permits to cycle the battery cell for 15 cycles. Then the battery failed further cycling because of wetting of air electrode's surface with alkaline electrolyte.

No dendrites appeared within the separators being between the zinc electrodes and nickel oxide electrode. However, the provided energy is ca 10% lesser than for Example 1 embodiment because of the additional internal resistance consequent to the presence of the auxiliary electrode.

The comparison in Table 1 and Fig. 5 of this example with example 1 demonstrates that in case of a bi-cathode discharging cell, the use of an additional auxiliary electrode prevents dendrites formation between the zinc electrodes and nickel oxide electrode. This should lead to an extended life time when the air electrode leakage will be overcome.

EXAMPLE 3

This example, illustrated by Fig. 3, describes an embodiment of a battery cell according to the invention. The battery cell of this example contains the components described in Example 2 illustrated by Fig 2 with the addition of the components (31') (32') (34') (35') (36') and (37') which are respectively the symmetrical components of (31) (32) (34) (35) (36) and (37).

This embodiment is tested the same way as described in Example 2 with the two auxiliary electrodes directly connected to the nickel oxide electrode during charging. The discharge current density for this embodiment is 25 mA/cm ² , while the magnitude of the current is twice higher than for the embodiments of example 1 , 2 and 3.

As in example 2 this embodiment permits to cycle the battery cell for 5 cycles, then the battery failed further cycling because of wetting of air electrode's surface with alkaline electrolyte.

No dendrites appeared within the separators being between the zinc electrodes and nickel oxide electrode. The characteristics of this battery cell are indicated in Table 1 and Figure 5.

The comparison in Table 1 and Figure 5 of this example with example 2 demonstrates that in case of a bi-cathode discharging cell, the doubled (symmetric) design leads to enhanced overall characteristics.