Electrochemical energy storage holds the promise to transform modern society's relationship with energy in ways comparable to previous revolutions in agriculture, transportation, and information. However, significant advances in performance and lifecycle are required in order to utilize several of the most promising chemical systems. Addressing the present limitations requires answering fundamental questions about the how the materials store charge, and the conditions in which they fail. These questions are most directly answered through the study of the materials in their electrochemical environment, and ideally, during the electrochemical reactions (inoperando). This study demonstrates the utility of several characterization techniques to probe chemical reactivity within the electrochemical environment, including synchrotron x-ray diffraction, synchrotron x-ray absorption spectroscopy, and confocal Raman spectroscopy. By correlating results from electrochemistry, ex -situ and in-operando spectroscopy and diffraction, and computational modeling, reaction mechanisms can be clarified and failure modes identified. Of particular interest is the application of this approach to the lithium-sulfur system for secondary batteries, where spectroscopy reveals the main reaction pathways and identifies promising new designs for lithium-sulfur energy storage.