I know its kind of a ofshot question- but is there research in non-chemical batterys? As in storing energy mechanically in nano-structures? Goggle did not reveal much there.
In general storing mechanical energy at the nanoscale is not efficient. As you get smaller mechanical properties become less relevant due to the increase in relative surface area (aka square-cube law). That makes electrical and chemical properties much more powerful.
Muscles are also excellent at storing energy in spring-like fibres that stretch/relax. But I can't think of any manufactured materials at that scale.