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Toyota is “not putting a date” on the introduction of lithium ion cells into its hybrid vehicles, insisting they are still unproven. But it could change its series-parallel approach, if it were to develop a diesel Prius.

While GM is committed to a launch date of 2010 for the higher-output Li-ion power packs, Toyota said it wanted first to improve durability, safety, cost and size.

Lithium ion batteries do have safety issues. If cooling systems fail, thermal runaway can cause the packs to overheat or catch fire. Toyota and its battery joint-venture partner Panasonic want to avoid the risk of a high-profile safety scare that could damage their reputation.

“Before you switch, you must be sure of the quality,” said Toyota. “It takes 10 years to know about durability.”

That’s the age of the oldest Prius, and some of these have covered 480,000km without problems. The next Prius is likely to use Ni-MH. Toyota engineers say there is ample development still left in the older-technology battery.

Toyota said it will stick with its series-parallel approach for future gasoline hybrids, but would not rule out another approach for a diesel hybrid.

Adding a hybrid to a diesel would be costly, particularly if Toyota’s complex transmission were included. By using a conventional transmission, perhaps Toyota’s automated manual, some of these could be cut.

But the OEM is not convinced by the concept. It would produce less CO2, but NOx emissions would be far higher than a gasoline hybrid. Address this with exhaust aftertreatment and you’re back to square one with costs, the company said.

Toyota launched a mild diesel hybrid in 2003 in Japan. The Dyna truck costs 10 per cent more than a standard model, but sales have been slow. “Cost is crucial in commercial vehicles,” said Toyota.