Conditions for Reversible Na Intercalation in Graphite: Theoretical Studies on the Interplay Among Guest Ions, Solvent, and Graphite Host

Graphite is the most widely used anode material for Li-ion batteries and is also considered a promising anode for K-ion batteries. However, Na+, a similar alkali ion to Li+ or K+, is incapable of being intercalated into graphite and thus, graphite is not considered a potential electrode for Na-ion batteries. This atypical behavior of Na has drawn considerable attention; however, a clear explanation of its origin has not yet been provided. Herein, through a systematic investigation of alkali metal graphite intercalation compounds (AM-GICs, AM = Li, Na, K, Rb, Cs) in various solvent environments, it is demonstrated that the unfavorable local Na-graphene interaction primarily leads to the instability of Na-GIC formation but can be effectively modulated by screening Na ions with solvent molecules. Moreover, it is shown that the reversible Na intercalation into graphite is possible only for specific conditions of electrolytes with respect to the Na-solvent solvation energy and the lowest unoccupied molecular orbital level of the complexes. It is believed that these conditions are applicable to other electrochemical systems involving guest ions and an intercalation host and hint at a general strategy to tailor the electrochemical intercalation between pure guest ion intercalation and cointercalation.