Metal-air batteries are used as storage devices for relatively high demand emergency power supplies, for example to serve buildings in the event of power failure. Such batteries typically consist of a container for an electrolyte a first air porous electrode serving as a cathode which keeps electrolyte on one side and is in contact with the air on the other side, and a second metal electrode, for example of steel or other iron- based alloy or an alloy of aluminium, zinc or magnesium, serving as an anode, immersed in the electrolyte in the vicinity of the porous electrode. Typical cells comprise a metal anode sandwiched between a pair of air breathing cathodes.

Systems are provided comprising a plurality of cells to make up a battery of the required nominal voltage (eg 24V or 48V). <BR> <BR> <BR> <BR> <BR> <BR> <P>The electrolyte is typically a common alkali such as NaCl or KOH. In the case of an aluminium electrode (often a preferred choice), the battery generates energy as aluminium reacts with OH-ions to form A1 (OH) 3 and three electrons (giving the battery a particularly high power density), as follows: Al + 30H-< A1 (OH) 3 + 3e~ At the porous electrode water in the electrolyte reacts with oxygen from air and absorbs the electrons produced at the aluminium electrode, as follows: OZ + 2H20 + 4e- 40H- If the porous electrode is covered during operation and 02 cannot reach it electrons<BR> <BR> <BR> <BR> <BR> <BR> are still obsorbed but 02 is not used,<BR> <BR> <BR> <BR> <BR> <BR> <BR> <BR> <BR> <BR> 2H20 + 2e~ X H2 + 20H Ultimately, the aluminium anode is completely consumed by this process, and the system then requires mechanically recharging by replacement of the anode and electrolyte.

The choice of electrolyte is in conventional metal-air batteries is fairly flexible but the choice of metal for the second electrode is more critical. The most suitable metals when in the pure form or as common alloys tend to become coated with a protective passivating layer which will ultimately prevent the battery from functioning. As a result only special alloys can be used which overcome this passivation problem. These alloys or"superalloys"often contain toxic and expensive metals (e. g. Ga, La, Nd, Sn, Ta, Te, Ti, and Tl etc.).

The use of such alloys gives rise to a further problem. The alloy will spontaneously corrode in the continued presence of the electrolyte, so that when the battery is not in use the electrolyte must be removed and stored separately to prevent corrosion of the electrode. This means that a pumping system and tank will be required to store and circulate the electrolyte when the battery is needed adding weight and complexity to the system.

In accordance with the invention a metal-air battery cell comprises a container describing a space for containing an electrolyte, a first gas permeable electrode configured in use to have electrolyte in contact with a first surface and the <BR> <BR> <BR> atmosphere external to the battery in contact with a second surface, and a second metal electrode located in the space for the electrolyte in the vicinity of the first electrode, wherein the second electrode is functionally coupled to an ultrasonic transducer. The coupling between the transducer and the second electrode may be a direct mechanical coupling. Alternatively, a like effect is achieved by locating the transducer near to the electrode so that the latter lies within the caviatation region of the former, and the electrode is then functionally coupled via the electrolyte.

The basic chemistry of a battery in accordance with the invention is essentially the same as in a conventional metal-air battery. However, the battery construction is different and the removal of passivating layers is physical rather than chemical.

During use, ultrasonic energy is applied to the second electrode via the ultrasonic transducer. The ultrasound generates cavitation bubbles within the electrolyte.

Shortly after formation the cavitation bubbles collapse and produce high pressure shock waves and/or microjects directed at the solid surface which erode the oxide layer that would otherwise form on the electrode surface. This process allows the aluminium to react again and generate energy.

The invention offers the advantage that since a physical route is taken to prevent formation of an excessive surface film on the second electrode, it is not necessary to <BR> <BR> <BR> <BR> use alloys with a particular non-passivating chemistry, so that a broader range of alloys is made available. This is likely to reduce the cost of the system and be more environmentally friendly. As in conventional batteries, systems based on aluminium, zinc, magnesium and iron alloys are particularly useful materials for the second electrode, with aluminium being especially preferred. However, the invention allows use of simple alloys, or even the metal in substantially pure form.

Indeed, it can be seen that it will be not just possible but advantageous to select an alloy which forms a passive protective layer in the presence of the chosen electrolyte, since in such circumstances it will not be necessary to remove the electrolyte when the battery is no longer in use. Instead, once the ultrasonic transducer is switched off the second electrode will rapidly passivate, and the battery can then be stored in a non-active state with the electrolyte in situ. The system is therefore simpler, and the lag time associated with the time required to get the electrolyte into the cell and heat the electrolyte to the operating temperature in conventional aluminium/air cells, which can be typically two minutes for 50 % operation, can be reduced.

To facilitate this operation, the ultrasonic transducer is preferably provided with control means for controlled activation and deactivation of the ultrasonic signal. To improve the efficiency of the system the ultrasonic transducer is preferably capable of producing a pulsed signal. Piezoelectric or magnetostrictive materials are particularly suitable for the ultrasonic transducer.

For operation, the cell may be charged with any suitable electrolyte. The electrolyte in conventional metal/air batteries is usually alkali in order to provide conditions in which the A1 oxide layer is less stable. This reduces the potential of the oxygen reduction couple, and places a limit on cell voltage; for example, the cell voltage of the conventional aluminium/air system is around 1.3V. A further advantage of the present invention is that it more readily allows the use of an acidic electrolyte.

Because H+ is good for 02/H20 couple, this improves the efficiency of the air electrode and offers increased voltage; in the case of an aluminium/air system the cell voltage is theoretically 2.9V under acidic conditions. The presence of H+ tends to lead to passivation of the Al in absence of ultrasound (eg in conventional batteries, where 02+H++e-o H20/H202) but where ultrasound is applied acid electrolytes become a practical proposition.

The choice of elctrolyte is not critical, and any standard metal/air electrolytes are likely to be suitable, as are a range of acidic electrolytes. Examples include species comprising OH, Cl-and H+.

Increased rates of erosion of the erosion and hence current output can be obtained by incorporation of chemically inert hard particles, for example ceramic particles, into the electrolyte which erode the oxide layer more rapidly. After a period of operation, a similar effect can be expected from the formation of particulate matter from the oxide layer and other precipitates. This is in marked contrast to conventional batteries where implies solubility problems leading to precipitate formation is usually viewed as deleterious to battery performance.

A battery providing a power supply of suitable nominal voltage can be readily constructed by combination of a plurality of cells, suitable output power supply means, and control means for activation of the ultrasonic transducers.

The invention will now be described by way of example only with reference to figures 1 and 2, in which: <BR> <BR> <BR> figure 1 is a schematic cross-section of a battery cell constructed in accordance with the invention; figure 2 is a graphical representation of current time transients for the battery cell of figure 1 in operation under a variety of conditions.

Figure 1 illustrates a battery cell consisting of a pure aluminium electrode 1, an air porous electrode 2 and an electrolyte 3 comprising 0.5 mol dm-3 Na2SO4 in a container 4. Ultrasonic energy is applied to the aluminium electrode 1 via a piezoelectric magnetostrictive transducer 5 (alternatively a magnetostrictive transducer is used). In use, the piezoelectric transducer 5 generates a pulsed ultrasound signal which produces cavitation bubbles 6 within the electrolyte. These bubbles then collapse producing high pressure waves which erode the oxide layer that would otherwise form on the surface of the electrode 1 so that the aluminium can continue to react.

Fig. 2. shows a current output from the battery of figure 1, plotting current (i) in mA cm against time (t) in seconds over a period of operation covering a range of operating circumstances. The cell consisted of a 2mm diameter electrode in 0.5 mol dm-3 Na2SO4. The electrode was held at 0.33V vs Ag and the experiment performed at 25°C under anaerobic conditions.

Prior to point A no ultrasound was applied. Between points A and B ultrasound was applied causing break up of the oxide layer on the electrode 1 and so increasing the current output of the cell. At point B alumina particles of nominal 25pm diameter were added to the electrolyte, and it can be seen from the increased current output that these assist in the erosion of any oxide layer on the electrode 1 which might otherwise form during operation of the cell. At points C and E the ultrasound was turned off, and at D the ultrasound was reapplied. The rapid fall off in current output as the electrode 1 passivates at C and E, and the rapid increase in output as the ultrasound was reapplied at D, shows that the battery can be readily activated and deactivated without the need to separate the electrolyte from the aluminium metal electrode.

The above compositions and operating parameters are given by way of example, and other suitable materials for the electrodes, electrolyte, and hard particles will readily suggest themselves to those skilled in the metal-air batteries art.