Sure, we have heard word of lithium ion batteries being possibly implemented in a Prius before, but Mercedes-Benz is going beyond concept speculation with its S400 BlueHybrid, which promises Li-ion HEV action to be hitting the production lines by 2009. At present, that makes the S400 BlueHybrid the first Li-ion HEV with a set mass-production time frame.

Current hybrid vehicles use nickel-metal hydride batteries, but the lithium-ion alternatives are known to offer greater efficiency, as well as a better overall weight-to-power ratio. The problem with integrating the Li-ion technology was in securing their stability—they became far too hot to be a viable option in a hybrid vehicle. Fortunately, Mercedes-Benz has 25 patents it is confident solves the dilemma of battery overheating, whether they stay on target for 2009 is another matter, but we certainly wouldn't doubt them. (N.B. The model pictured is the standard petrol engine S400. Images of the BlueHybrid have not been released as yet.)[CNET]