Solvation Sheath of Li+ in Nonaqueous Electrolytes and Its Implication of Graphite/Electrolyte Interface Chemistry

The formation chemistry of graphite/electrolyte interface and its dependence on electrolyte bulk composition were investigated by conducting electrochemical impedance analyses on interfaces systematically formed in various electrolytes and NMR identification of surface species harvested therefrom. The interpretation of these analyses strongly suggests that Li+ solvation sheath structure is central in defining the anode surface chemistry, because solvent molecules preferentially recruited by Li+ into the solvation sheath would be preferentially reduced on graphene surface upon initial charge of the electrode. Due to the preference of Li+ in binding the more polar molecules from the electrolyte solvent mixture, the contributions from cyclic and linear carbonates to the interface chemistry are unsymmetrical, and ethylene carbonate, the universal cosolvent in all electrolyte formulations, consequently becomes the favored chemical source for the interfacial ingredients. Since the chemical composition of the interface dictates Li+ transport kinetics at low temperatures, the understandings about how the electrolyte cosolvents share the responsibility for the graphite/electrolyte interfacial chemistry will benefit the efforts to tailor an interface that is more tolerant toward the operation of Li ion devices at sub-zero temperatures.