Expendable thermal batteries are often used in applications (like rockets or tactical missiles) where a large power demand is required in a short period of time. These are basically closed galvanic reaction cells in which the salts are isolated until a barrier is broken by an electrical impulse or squib, and then the battery produces a large amount of energy until the working salts are all neutralized. This often produces a more even amp supply than lead-acid or dry cell batteries, but most thermal batteries are use-once-and-dispose (or at least use-once-and-refurbish), and so are limited to the above military and rocket launch applications.
Another option, albeit one that is still more in the realm of speculation rather than practical application, are high energy metastable systems like excimers, stabilized amorphous or crystalline matricies (like solid ozone, cyclic O[sub]4[/sub] and O[sub]6[/sub], and C[sub]6[/sub] cubane) where the energy is stored in internal strains, and highly energetic neutral free radicals. You could make a fuel cell with an energy density ability two or three orders of magnitude greater than normal dry or liquid galvanic reaction cells. The trick is keeping the above systems stabilized. Something like metastable helium has a duration of around two hours before degredation via spin-orbit coupling in a vacuum; however, interaction with the walls of a real storage container or reactor vessel gives a lifetime on the order of a tenth of a second.
In any case, you’d probably need to also carry a bank of supercapacitors as described by Jurph to get the actually power supply that you need to operate this thing. That alone is a lot of mass (though the actual capacitors can be pretty compact). It’s also a pretty hazardous thing to carry around in an aircraft; burn one of those babies and you’ll need to be venting the fuselage quickly before the crew starts melting.