Dual-Ion Cells based on the Electrochemical Intercalation of Asymmetric Fluorosulfonyl-(trifluoromethanesulfonyl) imide Anions into Graphite

• Study of fluorosulfonyl-(trifluoromethanesulfonyl) imide (FTFSI)-based electrolyte.
• Comparative study of the intercalation of TFSI and FTFSI anions into graphite.
• Enhanced discharge capacity is achieved with a new FTFSI based electrolyte.
• Comparative X-ray diffraction study of FTFSI and TFSI intercalation.
• Calculation of the intercalant gallery height and gallery expansion.

In this work, we investigate the electrochemical intercalation of the asymmetric fluorosulfonyl-(trifluoromethanesulfonyl) imide (FTFSI−) anion into a graphite-based cathode for application in dual-ion cells. Since FTFSI− anions are smaller than bis(trifluoromethanesulfonyl) imide (TFSI−) anions, a higher specific capacity can be expected as the decreased anion size should lead to an enhanced receptivity of the anions between the graphene sheets of graphite. The discharge capacity and the coulombic efficiency are studied at varying upper charging end potentials ranging from 4.8 V to 5.2 V vs. Li/Li+. At these varying conditions a discharge capacity of 43 mAh g−1 to 99 mAh g−1 is obtained, respectively. However, the increase of the upper cut-off potential leads also to a decrease of the coulombic efficiency.To gain a better insight into the influence of the anion size on the performance, FTFSI− and TFSI− intercalation is discussed on a comparative basis, with respect to the potential range for anion intercalation/de-intercalation, the discharge capacity and efficiency. We observed a lower coulombic efficiency, caused by the lower electrochemical stability, as well as an enhanced discharge capacity for all investigated upper cut-off potentials for the intercalation/de-intercalation of FTFSI− compared to TFSI− anion uptake/release.