Fluorosulfonamide-type electrolyte additives for long-life K-ion batteries

Graphite is a promising negative electrode material for emerging potassium ion batteries (KIBs), offering a good capacity and a low-potential discharge plateau. To date, achieving long cycle life KIBs with graphite remains limited due to the formation of an unstable solid electrolyte interphase (SEI), especially in common potassium hexafluorophosphate (KPF6) electrolytes. Herein, we show that fluorosulfonamide-type additives, such as dimethyl sulfamoyl fluoride (DMSF) or potassium bis(fluorosulfonyl)amide (KFSA), can be incorporated into KPF6 electrolytes to improve the cycling performance of graphite. In half-cells, 1 and 10 wt% added DMSF showed significant improvements in the charge/discharge coulombic efficiency (CE) and a low cell polarization. Adding DMSF to full cells (graphite||K2Mn[Fe(CN)(6)]) further showed a high capacity retention of 68% after 500 cycles compared with 37% for the additive-free electrolyte. Even higher performance was observed when combining DMSF with KFSA in the same electrolyte, demonstrating a capacity retention of 85% after 500 cycles. Electrochemical impedance measurements suggested an improved charge transfer resistance in DMSF containing electrolytes. Ex situ analyses of the graphite surface indicated differences in the SEI composition for the similar structured additives. Notably, N incorporation was not observed with DMSF, but only when using KFSA, suggesting differences in degradation products and incorporation into the SEI. At the same time, computational analyses suggested similar HOMO and LUMO energy levels for the additives. This presents an opportunity to tune a portion of the fluorosulfonamide structure for further improving the SEI and KIB performance.