X-ray diffraction studies of the electrochemical intercalation of bis(trifluoromethanesulfonyl)imide anions into graphite for dual-ion cells

Abstract “Dual-graphite” cells use graphite as intercalation host at both electrodes, where the energy storage mechanism is based on the intercalation of lithium ions into the negative and the intercalation of electrolyte anions into the positive electrode. In this contribution, the electrochemical intercalation of bis(trifluoromethanesulfonyl)imide (TFSI) anions from an ionic liquid-based electrolyte (Pyr14TFSI) into graphite was studied using ex situ and in situ X-ray diffraction. We prove that the TFSI intercalated graphite exists in a series of staged phases. As for this electrode material the discharge capacity can be strongly influenced by the cut-off potential and temperature, the stage transitions and compositions of the TFSI intercalated graphite were determined in dependency of different temperatures and cut-off potentials. From the experimental data, the periodic repeat distance of the graphite intercalation compound, the TFSI gallery height and the maximum stoichiometries of C n + TFSI− at these different conditions are calculated and discussed.