Source: http://io9.com/5580592/new-ultra+battery-is-the-most-powerful-non+nuclear-energy-storage-ever
The material used to make the "battery" is xenon difluoride (XeF2), a white crystal primarily used to etch silicon conductors. The crystal was placed in a diamond anvil cell, a tiny device that measures only two inches by three inches. The cell uses two tiny diamond anvils (as you might expect, considering its name) to produce incredibly high pressures in tiny, contained spaces.
Normally, the molecules in xenon difluoride stay relatively far apart. The squeezing process the crystals underwent in the diamond anvil cell forced the molecules closer and closer together. At first, the squeezing caused the crystal to become a two-dimensional semiconductor, but then something even more remarkable happened. When the pressure reached a million atmospheres, similar to the pressure found halfway to the center of the Earth, the molecules formed 3D metallic "network structures", which forced all the mechanical energy of the compression process to be stored as chemical energy within the molecular bonds. At a million atmospheres, that's a whole lot of stored energy.
Heading up this research is Washington State chemistry professor Choong-Shik Yoo, who says this "is the most condensed form of energy storage outside of nuclear energy." The possible applications of the material pretty much all include the word "super": superconductors, super-oxidizing materials that can destroy chemical and biological agents, not to mention new fuels and, most obviously, an energy storage device.