High energy density zinc-air technology, although offering high energy density—about twice the gravimetric density (Wh/kg) and three times the volumetric density (Wh/L) of Li-ion technology—has been generally limited to low-power, non-rechargeable (primary batteries) applications up to now.

ReVolt patent portfolio. Click to enlarge.

Barriers to successful commercialization have included low-power, rapid loss of power, and a lack of a satisfactory mechanism for recharging them. Secondary batteries that have used zinc as an anode active material can suffer from short cycle life due to dendrite formation during the charging of the battery, resulting in short circuits; and shape changes of the electrode, resulting in a loss of active surface area.

Revolt’s rechargeable zinc-air technology, in an inherently safe and cost-effective package, is working to address those issues and open up a variety of future markets, such as consumer electronics, specialty markets, and transportation.

Revolt had been focusing its development efforts initially on consumer electronics; RWE is interested in the larger-scale applications such as grid storage and electric vehicles.

Due to their high energy density, zinc-air batteries could be used not only for electricity storage from wind and solar power, but also for electric vehicles. In contrast to lithium-ion rechargeable batteries commonly used today, zinc-air batteries are more powerful, cheaper to produce, safer and more benign to the environment.

——Crispin Leick, RWE Innogy Ventures

ReVolt was spun-off from SINTEF in Norway to commercialize six years of R&D on a rechargeable Zn-air system.

Metal-air electrochemical cells use an anode made from metals such as zinc (Zn), aluminium (Al), magnesium (Mg), iron (Fe), lithium (Li) or vanadium (V) (earlier post) and a cathode made from a porous structure with catalytic properties for the oxygen reaction. An alkaline electrolyte maintains high ionic conductivity between the two electrodes. A separator between the anode and cathode prevents short circuits.

Discharging the metal-air cells entails the conversion of oxygen from the atmosphere to hydroxyl ions in the air electrode. The hydroxyl ions then migrate to the metal electrode, where they cause the metal contained in the electrode to oxidize.

Charging of metal-air cells converts hydroxyl ions to oxygen in the air electrode, releasing electrons. On the metal electrode the metal oxides or ions are reduced to form the metal while electrons are consumed.

Discharge curve for one prototype battery (7.16 Ah). Estimated capacity for cell: 1.140 kWh/l. Click to enlarge.

ReVolt is focusing on the areas of power, battery life, rechargeability and compact size. Some issues remain to be addressed prior to successful market introduction, the company says.

ReVolt’s technology developments include placement of the zinc (microscopic localization) on the anode; humidity management in the cell; and a bi-functional air-electrode. In a bi-functional air electrode, both the oxygen reduction and oxygen evolution reactions occur.

RWE is among Europe’s five largest utilities and is active in the generation and transmission as well as the sale and trading of electricity and gas. RWE is also active in the water business in Continental Europe.

The investment in Revolt Technology AS is RWE Innogy’s third venture capital deal. Last year, the company bought shares in a UK developer of micro-wind turbines as well as in a Dutch company developing a process for the production of biocoal pellets.

Resources

• Rechargeable Zn-air Battery for Mobility

• ReVolt Portable Battery Technology Brief