A number of theoretical and practical advances enable humans in the future to be much more energy rich, allowing the practicality of such things as directed energy weapons.
Muon-catalyzed cold fusion
This is a major source of power in the far distant future, a form of cold fusion enabled by extending the half-life of Muons by either a) accelerating them to relativistic speeds (via plasma wakefield acceleration) and thus inducing time dilation, or b) preventing the Weak Interaction (and thus Weak Decay) by preventing the formation of W bosons, either by Neutrino flooding, or producing a local Weakless Universe.
The Antimatter Battery/Fuel Cell
These “batteries” are in fact specialized encapsulated containers for controlled matter/antimatter reactions. The matter/antimatter fuel requires almost exactly as much energy to create as will be released by the battery, and has the highest known energy density of any fuel system. The battery constantly drains energy in order to maintain containment, but does so at a high enough efficiency that it would take thousands of years to run out, and at that point, all the matter/anti-matter will have been consumed maintaining the encapsulation, leaving only an inert capsule. On the other hand, deliberately damaging the capsule has been known to cause massive explosions on par with smaller antimatter bombs.
Microsingularity Planck Particle Generator
Basically a micro black hole kept alive using temporal stasis technology, the Planck Particle absorbs photons and emits random particle matter, including the desired Muons and highly desired anti-matter. Though often called a Planck particle, its actual size is around 2.25 mm and has a mass (4.5 × 1022 kg) approximately equal to the moon.
A simpler variation is a micro black hole that has slightly less mass than 4.5 × 1022 kg, and as such is a net emitter of blackbody radiation, which is used as a powerful battery/energy source. It also works as a matter-to-energy converter, by being fed matter to maintain its mass while emitting energy.
The handheld version of this keeps the micro black hole inside a special self-contained pocket dimension.
Betavoltaic Tritium Batteries
These batteries emit electrons over many decades and are the staple battery for military and space applications. They are often recharged by converting the helium-3 by-product back into tritium through an infusion of neutrons, most likely produced through Muon-catalyzed fusion.
The Muon-Black Hole Hypothesis
Muons = Black Holes of Mass: 105.658369(9) MEV/c2 and Lifetimes: 2.19703(4)×10−6 s?