Batteries haven't advanced all that much in their "base technology" since Volta set those frog legs twitching. You still get 1.something volts out of a cell, and all of the improvements since that discovery have been in longevity and electrolyte chemistry (which is pretty much the same thing).
Aspire to more than 1.5 volts, and you have to start talking creation of a "battery" (which is nothing more than a bank of cells, ranged next to each other in a similar fashion as the guns in an artillery battery), thin cell technology, and alternate electrolytes. Then there's all of that hydrogen (and heat) to get rid of...