The Electrochemical Society Interface • Spring 2017 • www.electrochem.org 55 T he choice of electrolyte is literally at the center of lithium and sodium energy storage devices as it provides the pathway for metal ion transport between the electrodes during charge-discharge cycling. However, zoom in on the electrode interface during battery operation and it becomes clear that the electrolyte's role is not always to serve as a passive resistor. The organic solvent molecules used to dissolve metal salts in commercial batteries, namely cyclic and linear carbonates, frequently react with the electrified interfaces to form surface films that in turn effect device performance.1 These films trap metal ions, preventing them from participating in the charge transfer reactions necessary to power an external circuit, and can grow to tens of nanometers in thickness while accumulating over the lifetime of the battery.2 The quest to design a "better" electrolyte is thus complicated by potential degradation at the electrode surface weighed against transport properties tens of nanometers from the interface. 3 In both cases, the manner in which metal ions are coordinated by solvent, co-solvent, additives, and counter-anions at the atomic level has been linked to the chemistry of the breakdown products as well as to trends in ionic conductivity in the bulk phase. 4,5 Connecting the molecular structure of the electrolyte with macroscopic charge cycling performance requires computational strategies that can span several orders of magnitude in spatial and temporal scales (see Fig. 1), from the solvation shell and diffusion of lithium (nanometers and nanoseconds) to the development of surface films (tens of nanometers and microseconds) and the effect of both on the lifetime and performance of the battery on the devicescale. Word-limit constraints on this article do not permit even a cursory examination of the wealth of experimental techniques that have been used to study electrolytes across these disparate spatial and time regimes in both half and full cell configurations (for excellent reviews, see Ref. 6 and 7). Even so, a simplified summary of these efforts points to common challenges in performing such experiments with sufficient spatial and temporal resolution in operando. As a result, many questions remain unanswered concerning the surface chemistry of electrolytes, the process of film formation, and the mechanism for ion transfer from the electrolyte to the electrode. Given the slow progress of various spectroscopic, microscopic, and surface imaging methods across decades of research, a natural question that arises is whether there is another approach that could lend complementary insights to the behavior of electrolytes in electrochemical energy storage devices and provide guidelines for their continued development.In contrast to the early years of lithium-ion battery research, computational modeling is now contributing an important role in studying electrolyte properties and processes across a wide range of length scales. 8 T...