Storage is the hard part of renewable energy. In this article, the researches used a molecule that captures sunlight and locks it into a strained, high-energy shape. It can hold that energy for months to years and then release it as heat on demand. The release of that energy makes water boil in about one second. Energy density is 1.65 MJ/kg, beating lithium-ion (~0.9 MJ/kg).
The interesting background is the inspiration of the method by the shape-shift DNA undergoes when UV light damages it. It's the chemistry of sunburn.