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

Superconcentrated aqueous electrolytes containing NaN(SOF) and KN(SOF) (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 NaKFSA/HO and 33 mol kg NaKFSA/HO 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 NaFSA or 31 mol kg KFSA solution, we applied the 33 mol kg NaKFSA/HO to two different battery configurations, carbon-coated NaTi(PO)∥KMn[Fe(CN)] and carbon-coated NaV(PO)∥KMn[Fe(CN)]. 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.