Generally speaking lithium
Generally speaking lithium ion batteries start at twice the energy density of nickel chemistry batteries.
Yes I know quite a bit about the first generation RAV4 EV, and the patent issues concerning NiMH batteries. But if you do the math, lithium batteries will go quite a bit further than nickel batteries because lithium batteries are in a different realm of energy density.
Yes it is corporate policy - because they're targeting a desired price range and do not want the Leaf or Volt or Focus Electric price to be unaffordably high.
It also has to do with science and technology because lithium batteries are still being developed.