  Conventional lithium-ion batteries, and many next-generation technologies,
rely on organic electrolytes with multiple solvents to achieve the desired
physicochemical and interfacial properties. The complex interplay between these
properties can often be elucidated via the coordination environment of the
cation. We develop a theory for the coordination shell of cations in
non-aqueous solvent mixtures that can be applied with high fidelity, up to
super-concentrated electrolytes. Our theory can naturally explain simulation
and experimental values of cation solvation in ''classical'' non-aqueous
electrolytes. Moreover, we utilise our theory to understand general design
principles of emerging classes of non-aqueous electrolyte mixtures, such as
high entropy electrolytes. It is hoped that this theory provides a systematic
framework to understand simulations and experiments which engineer the
solvation structure and ionic associations of concentrated non-aqueous
electrolytes.
