Lithium Ion or more likely LiFePo4. That's why they can cope with such high discharge rates. The downside is that to charge them you either need a complicated (expensive) cell balancing system to take charge quickly or you have to charge at a very low rate, ie the 1-1.5 Amps their chargers come with.
Gradually SLA replacement Lithium's are coming on stream but you're talking well over five times the equivalent price. However they're much lighter and smaller (good for racing applications) and can give and take charge much better than a lead acid can (good for off-grid such as boats and remote houses).
Simple fact is that the next few years are going to see a dramatic change in battery technology as aqueous, graphene, etc. technology develops or better yet, storage systems that are still buried deep in the oil companies vaults somewhere are released as the car manufacturers attempt to clense themselves of the emissions fiasco.
