Future technology requires energy storage systems to have much larger storage capability, rapid charge/discharge cycling and improved endurance. Progress in these areas demands a more complete understanding of energy storage processes from atomic through micron-length scales. Because these complex processes can change significantly as the system is charged and discharged, researchers have increasingly focused on how to look inside an operating energy storage system. While computational approaches have advanced over the last few decades, the development of experimental approaches has been very challenging, particularly for studying the light elements that are prevalent in energy storage materials.