Batteries are complex systems based on heterogeneous structures that operate far from equilibrium. Pursuit of fundamental understanding over multiple length scales remains critically important to effectively design and control materials, interfaces, and their functional properties for energy storage systems to meet the needs of the future with high performance, long lifetimes, and reliable, safe operation. At the atomic level, the active materials and their structures determine the open circuit potential of a battery. Yet, kinetics influence how much of the contained energy is realizable for practical application where localized resistances influence the polarization of the battery while under load. Examples of particle and aggregate size influence on realizable electrochemical energy will be highlighted. Further, the role and control of dynamic interphases will be discussed. Insight into reaction mechanisms and their transport mechanisms determined by operando and in-situ methods will be featured.