Some experts predict that fuel cells will power the cars of the future. But with fuel-cell vehicles and infrastructure still early in their development, hybrid electric vehicles, or HEVs, are more likely to emerge as the auto industry's near-term response to demands for greater fuel efficiency and lower emissions. With a few HEV models already available, many in the electronics industry are looking to develop components for this market.
Battery developers are among them. That's not surprising as HEVs are expected to require more powerful electrical systems with greater battery capacity. Although typical lead acid boasts low cost, just 10 cents/Wh, its energy density ratings are rather paltry—90 Wh/l and 30 Wh/kg. So, boosting ca-pacity makes batteries heavier and bulkier.
One of the candidates to replace lead acid is nickel-metal hydride (NiMH), which offers energy densities of around 280 Wh/l and 80 Wh/kg. Despite its higher cost, NiMH has been adopted in current HEVs. But will future HEVs use NiMH?
That's uncertain. We might see a whole generation of "mild" hybrids in the next several years, as car makers roll out their dual 12/42-V electrical systems. While some of these hybrids may get by with lead acid batteries, the more advanced 42-V cars with hybrid features, like motor power assist and regenerative braking, could demand something more powerful. NiMH is one option, but lithium-ion (Li-ion) could be even better.
Despite its higher cost (at least double per watt-hour versus NiMH) and safety concerns, Li-ion's performance makes it a good candidate for powering HEVs. Cells developed for portable devices already achieve nearly 200 Wh/kg and 500 Wh/l. These levels would be welcome in automotive designs.
Working in that direction, Valence Technology of Austin, Texas, is developing Saphion. This Li-ion variation replaces the traditional lithium-cobalt-oxide cathodes with a phosphate-based material that's more thermally stable than cobalt. The greater stability eases safety concerns, allowing the creation of larger cells that reduce requirements for battery pack electronics.
The switch to phosphate-based cathodes sacrifices some of the energy density of standard Li-ion cells. (Saphion claims 371 Wh/l and 124 Wh/kg.) But raw phosphate is less expensive and more environmentally friendly than cobalt. Plus, the Saphion cells could cut battery pack costs by 45% over cobalt-based Li-ion cells.
Meanwhile, Electrovaya of Mississauga, Ontario, is continuing to develop its Li-polymer (Li-ion with gelled electrolyte) batteries with an eye toward the automotive area. The company, which produces high-energy-density battery packs for powering laptops, has been working with the U.S. Ad-vanced Battery Consortium over the past two years to determine the feasibility of using its cells in electric vehicles.
But these developments aside, the value of existing, standard Li-ion technology for future HEVs shouldn't be discounted. Hideo Takeshita, from the Institute of Information Technology, predicts that the cost of both Li-ion and NiMH battery packs for HEVs will steadily drop this decade until they sell for under $1/Wh, with NiMH still costing about half the price of Li-ion.
That forecast assumes continued use of existing cell designs and materials with price reductions following from falling material costs and rising yields. Given the ongoing R&D efforts to improve Li-ion cell chemistries and packaging, the value of Li-ion will probably increase even faster, making its use in HEV designs even more likely.