Batteries

Low profile: Ultra thin like a credit card but adaptable batteries are feasible.

Flexible form factor: Production are not bound by standard cell formats.  Any reasonable size or capacity is possible. 

Light weight: Lipo battery electrolytes are gelled rather than in liquid form. This helps to simplify packaging, even eliminate the metal outer shell. 

More safety: Electrolyte leakage situation is less likely and the unit is more resistant to overcharge conditions.