Volts

Metal-air batteries have been studied for more than 40 years, with most emphasis having been placed on primary cells. During the same time-frame Li-ion batteries based on intercalation reactions have been commercialized and now dominate the small portable market for rechargeables. They are now finding application in several larger configurations such as HEV buses with more than 20 million miles completed, and are also finding use for grid and renewable energy smoothing with systems as large as 32 MW. Intercalation based batteries are inherently limited in capacity because of the need for a host compound to contain the lithium. It is unlikely that intercalation reactions will much exceed 1 kWh/kg theoretical, and around 200-400 Wh/kg in a packaged system. The present status is indicated in Table 1, where it can be seen that today the volumetric capacity is less than 20 % of the theoretical value. This has doubled over the last decade, and when the Panasonic cell is released will increase by another 50 % approaching 1 kWh/L. Thus, there is much interest in studying more energy intensive systems. These might include conversion reactions, for example iron fluorides, or elemental systems such as oxygen or sulfur. In addition, electropositive elements other than lithium might be considered. Table 2 shows the theoretical energy density of several metal oxide couples