Phase transformations occur in several energy storage and conversion systems which are promising ingredients in the mix of options needed for a more carbon-free energy economy. Conversion batteries featuring alkali metal-oxygen or metal-sulfur reactions are particularly interesting examples of such systems, as they theoretically can deliver specific energy densities 2 - 3 times greater than state-of-the-art intercalation-based lithium-ion systems. However, much remains to be fundamentally understood about the morphological changes and mechanisms involved in the growth and dissolution of solid phases during electrochemical cycling. In-depth understanding of these processes has the potential to bridge the gap between device-level battery performance metrics (rate capability, cycle life, energy density) and atomic-scale interactions among redox-active species and electrified interfaces. Our specific aims under this topic include in situ experimental investigation and modeling of electrochemically induced crystallization and interfacial and electrolyte engineering for high-energy-density systems.