In-Situ Studies on the Electrochemical Intercalation of Hexafluorophosphate Anion in Graphite with Selective Cointercalation of Solvent

Electrochemical cells utilizing graphite intercalation compounds at both electrodes have been proposed as an energy storage technology where the electrolyte salt is split and stored in the electrodes on charge and reformed on discharge. The anion intercalation compounds of graphite proposed as cathodes in these systems have been studied in electrolytes that are resistant to oxidation at 5 V but that are incompatible with graphite anodes. Recent work has demonstrated that electrolytes based on monofluoroethylene carbonate (FEC) and ethylmethyl carbonate (EMC) have superior oxidative stability on graphite cathodes over previously studied electrolytes and form a stable solid electrolyte interphase (SEI) on graphite anodes that allow for full dual-graphite cells to be evaluated for energy storage applications. There is still a limited understanding as to structure of the anion intercalate formed in these electrolyte systems and the effect of solvent cointercalation on cathode performance. This effort was undertaken using a number of in situ techniques to better characterize the fully intercalated composition as well as to investigate the process of solvent cointercalation. It was shown that a series of stages based on the C24PF6 composition are formed until, upon reaching full charge, the structure approaches a C20PF6 stage I composition with PF6– anion in close contact with the graphite layers and 0.7 molecules of cointercalated solvent. For the first time, we have shown that solvent molecules move with anion during the intercalation/deintercalation process while analysis of fully intercalated crystals demonstrated that there is an unusually strong preference for EMC over FEC to cointercalate in this anion intercalation compound.