Tesla's CEO is reportedly unhappy with its current Li-Ion battery technology. He announced plans to develop a battery with almost double the current capacity. A Chinese media report claims that these batteries use graphene.

This makes sense as we've seen graphene-based battery prototypes achieving over four times the capacity of commercial batteries, and it seems that the technology is ready for commercialization:

http://www.graphene-info.com/tesla-developing-graphene-enhanced-li-ion-battery