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Batteries/Metal-air 060706
From the 42nd Power Sources Conference...
Metal Air Batteries, Half a Fuel Cell?
by Donald Georgi

With an air cathode constantly utilizing incoming air, the metal-air battery has a lot in common with fuel cells. Since the anode is a pure metal and the cathode an inexhaustible supply of air, the possibility of having a large energy density is only design limited. The power density however is low, so a system powered by a metal air battery should require low currents over a long period of time, or have a hybrid supply such as an ultracapcitor for power requirements. The most prevalent metal-air battery is the ubiquitous hearing aid Zinc-air cell.

But other anode materials hold great prospects for having very high energy density such as lithium with a theoretical energy density of 13,000 Wh/kg. Although limited by problems such as dry-out, limited power, limited temperature operation, corrosion and carbonation, the reward-for-risk possibilities keeps federally funded R&D alive.

Large Prototype Lithium Air Batteries (Session 8.1) With a target of a large (up to 5 inch square) case, a prototype cell built by Yardney has a lithium metal anode, liquid electrolyte (Li PF6 in EC/DEC/DMC) and carbon composite with metal cathode current collector. The cell is assembled using accepted manufacturing processes plus a proprietary oxygen transport additive which increases specific capacity. Testing showed that at low currents of 0.05 mA/cm2 the specific capacity was 2500 mAh/g, but it reduced to just 200 mAh/g at current density of 2 mA/cm2.

High Energy density Lithium-Air... with No Self-Discharge (Session 8.2) Polyplus has approached the challenge of the Lithium metal electrode with a coating of a glass-ceramic membrane, sealing the Lithium from an aqueous catholyte. The  resultant structure exhibits very small self discharge, ordinarily a large contributor to cell failure. Test cells have produced 0.5 mAh/cm2 for 230 hours exhibiting approximately 100% Coulombic efficiency.

A production oriented cell construction with double sided lithium anode, solid electrolyte and double sided air/cathode is anticipated to have 600 to 1000 Wh/kg energy density.

To reduce fears of  mechanical safety hazards, the cells have been subjected to crush tests which have fractured the glass-ceramic membrane with only a 2-30 C temperature rise, followed by a gradual decline in open circuit potential over several hours. An operational sample of the water-activated cell was shown at the conference.

Carbon-air Battery... (Session 8.3) This is the focus of a program being performed by St. Andrews University, (UK) and funded by the UK Ministry of Defense. The goal is to produce a 50 Watt, 500 Wh battery for recharging man-portable  devices used in the field. The investigative approach is to start with carbon-air technology which can be pursued both as a battery and as a fuel cell system. The free energy of carbon oxidation is 9100 Wh kg and a fuel-only specific energy of 7200 Wh/kg is possible. The final system is anticipated to have a device specific energy of 2000-3000 Wh/kg.

A major consideration is to maintain the operational temperature of the electrolyte at 7000 C or greater. This limits the carry-it-around-in-your-pocket and turn-it-on-in-a-moment possibilities. Obviously, the high energy density is worth pursuing in the light of the operational complexity. The conceptual design chosen uses  a tubular solid oxide cathode, molten carbonate slurry electrolyte and carbon as a fuel. Experimentation with various materials and configurations led the researchers to propose limiting temperatures to 8000 C. Activated carbon provided the highest current density.

For more details of allied Direct Carbon Fuel Cells, See BD #117, pp 2-6.

... Advanced Design Zinc-Air Cell ( Session 8.4) Zinc-air is a proven chemistry, but it has not reached its zenith in performance. Electric Fuel Battery Corporation has delivered  the Zinc-air BA 8180, a size equivalent to the BA 5590 to the military. The zinc-air battery has about twice the energy performance, giving the Warfighter greater  operational power for a given weight of gear. For some applications, the extra capacity of the BA 8180 is more than what is needed, so a half size BA-8140 Zinc-air battery with half the weight and size is available. These batteries are designated as the third generation of military Zinc-air batteries.

Now a fourth generation is being pursued. These are physically smaller cells to meet other requirements such as Unmanned Air Vehicles where size and weight are critical, but power densities up to 200 W/kg are necessary. Another group of batteries needing greater energy density pertains to the body-worn electronics such as computers and cameras. Other items such as surveillance equipment require flat form factor batteries. The work performed has resulted in Gen4 batteries which have up to 20% improvements in energy density and 80 % improvement in power density.

...Metal-Air Battery/Fuel Cell... (Session 8.5) By improving characteristics of each part of the metal-air battery, eVionyx
has been able to overcome many limitations of self corrosion and passivation while increasing specific energy, specific power and Coulombic efficiency.

Air cathodes provide up to 10 times the current of conventional air cathodes with proprietary electrolytes and patented processes. Solid polymer electrolyte membranes have reduced the problems of electrolyte loss due to dry out. Zinc-air cells produce up to 450 Wh/kg while Aluminum-air cells can produce a specific energy of more than 550 Wh/kg, and specific energies up to 650 Wh/kg are expected. Ordinarily, Magnesium-air fuel cells utilize only up to 60%, but with the electrolyte additives, can utilize up to 94% of the alloy.