Lithium-ion batteries have two to three times the energy density of the nickel-metal hydride batteries currently used in hybrids, and they are cheaper to produce. Unfortunately, in automotive applications they aren't very safe - yet. (Remember the exploding cell phones?)
Sandia National Labs, funded by the U.S. Department of Energy's FreedomCAR program, is actively researching ways to make lithium-ion batteries safe and cost-effective for automotive use. (A prototype Sandia battery is shown at right.) Although their first application is likely to be in gas/electric hybrids, lithium-ion batteries will be a key component of fuel cell vehicles.