Enabling bis(fluorosulfonyl)imide-based ionic liquid electrolytes for application in dual-ion batteries

In this work, we present a comprehensive study on the effect of adding different conductive salt additives including LiPF6, LiBF4 and LiDFOB, as well as the fluorinated solvent additive methyl difluoroacetate (MDFA) to a bis(fluorosulfonyl)imide (FSI)-based ionic liquid (IL) electrolyte, i.e. Pyr(14)FSI/LiFSI, to protect the Al current collector (ACC) from anodic dissolution and, thus, enable reversible charge/discharge cycling in a high performance dual-ion cell. Chronocoulometry and scanning electron microscopy measurements were conducted to evaluate the specific ACC passivation ability of each electrolyte. Furthermore, the influence of these additives on anion intercalation behavior into the graphite positive electrode with special emphasis on the Coulombic efficiency (CE), reversible capacity, as well as capacity retention is presented. Overall, we can show that the addition of small amounts of LiPF6, LiBF4 and MDFA (0.5 wt%) into the FSI-based IL electrolyte significantly increases the overall cell performance, whereas LiDFOB as electrolyte additive deteriorates the dual-ion cell performance. In addition, an excellent cycling performance for 1000 cycles is obtained for the Pyr(14)FSI electrolyte having 5 wt% LiPF6, displaying an average reversible capacity of 40 mAh g(-1), a CE exceeding 98% and a capacity retention of 91%, which has not been reported so far.