Superconcentrated NaFSA-KFSA Aqueous Electrolytes for 2 V-Class Dual-Ion Batteries.

Superconcentrated aqueous electrolytes containing NaN(SO 2 F) 2 and KN(SO 2 F) 2 (for which sodium and potassium bis(fluorosulfonyl)amides (FSA), respectively, are abbreviated) have been developed for 2 V-class aqueous batteries. Based on the eutectic composition of the NaFSA-KFSA (56:44 mol/mol) binary system, the superconcentrated solutions of 35 mol kg -1 Na 0.55 K 0.45 FSA/H 2 O and 33 mol kg -1 Na 0.45 K 0.55 FSA/H 2 O are found to form at 25 °C. As both electrolytes demonstrate a wider potential window of ∼3.5 V compared to that of either saturated 20 mol kg -1 NaFSA or 31 mol kg -1 KFSA solution, we applied the 33 mol kg -1 Na 0.45 K 0.55 FSA/H 2 O to two different battery configurations, carbon-coated Na 2 Ti 2 (PO 4 ) 3 ∥K 2 Mn[Fe(CN) 6 ] and carbon-coated Na 3 V 2 (PO 4 ) 3 ∥K 2 Mn[Fe(CN) 6 ]. The former cell shows highly reversible charge/discharge curves with a mean discharge voltage of 1.4 V. Although the latter cell exhibits capacity degradation, it demonstrates 2 V-class operations. Analysis data of the two cells confirmed that Na + ions were mainly inserted into the negative electrodes passivated by a Na-rich solid electrolyte interphase, and both Na + and K + ions were inserted into the positive electrode. Based upon the observation, we propose new sodium-/potassium-ion batteries using the superconcentrated NaFSA-KFSA aqueous electrolytes.