To characterize such complex environment, we analyze the Li ion coordination shell as a function of the distance to the electrode surface and electrode potential. We use density functional theory (DFT) and ab initio molecular dynamics (AIMD), to identify structural properties, as well as reactions and reaction mechanisms. Among the various “additional” species we consider the presence of long-chain polysulfides, that migrate in the electrolyte towards the Li metal anode in the Li-S chemistry. In this talk we will discuss physical, chemical, and electrochemical phenomena occurring at solid/electrolyte interfaces of Li-ion batteries and how the methods of first-principles computational analysis may be used to understand them and suggest strategies for practical solutions. In particular we will focus on the solid electrolyte interphase (SEI) occurring due to electrolyte reduction on or near Li anode surfaces and the role of the nature of the solvents, salts, and additives on the resultant SEI properties as well as on dendrite formation and battery performance.