High capacity sodium ion batteries, with the capacity to rival mid-range lithium ion batteries, are developing a solid presence in the field of energy storage research. Sodium ion batteries, at a material level, offer a unique opportunity to apply lessons learned in the study of lithium ion battery behavior. Most important of these is the formation of the solid electrolyte interphase layer at the anode, which shields the electrolyte from continuous decomposition. In lithium ion batteries, the formation of the solid electrolyte interphase is highly dependent (though not exclusively) on the solvation condition of the Li⁺ ion as it interacts with the surface of the anode. Previous work has shown that the solution behavior of the Na⁺ ion closely mirrors its alkali metal cousin Li⁺ in a variety of competitive binary solvent systems, such as ethylene carbonate and dimethyl carbonate. We hypothesize that similar rules of interphase formation will be observed in a Na-ion battery system, and the interphase will serve a similar function. This work will describe the connection between the solvation of Na⁺ in binary solvent electrolytes, and how this affects the structure and chemistry of the interface at the anode. We will focus on hard carbon, which has sufficient Na⁺ capacity to support a variety of developing cathode chemistries.