From what I've read toyota has already released an improved prius battery and lowered the cost recently - right now it's about $3k according to edmunds. Plus their warranty is 8 years or 100k miles. Not bad.
I wrote a letter to the editor of the now long-defunct Los Angeles Herald Examiner in 1979 suggesting that the key to low-emission fuel-efficient cars lay in diesel-locomotive technology even without batteries because the engine could be run at a low, constant speed while traction motors did the real propulsion work.
As I said, this is old technology, the real inertia is in the politics and the infrastructure.