There probably is. I'm no chemical engineer, but this is a question they would be best suited to answer. From my experience observing the technology from an R/C

There probably is. I'm no chemical engineer, but this is a question they would be best suited to answer. From my experience observing the technology from an R/C standpoint, it seems appearant that the maximum capacity you can achieve with a NiCd (nickel cadmium) battery in a 1.2v "Sub-C" form factor is about 2,000 MaH. The max capacity for a NiMH (nickel metal-hydride) is about 6,000 Mah. Anything lithium is capable of anything nickel can do, but with approximately 50% the weight. Lithium cells are where the technology currently lies and it seems that there is plenty of potential, although graphene is also promising. We haven't hit the limit for lithium batteries yet (or nickel for that matter, but the remaining potential is no longer relevant) Li-Ion (lithium ion) are less stable than Li-Po (lithium-polymer). Also, since Lithium cells are usually 3.4v rectangles and Nickel cells are 1.2v cylinders there are some form-factor differences to consider.