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

[Image: see text] 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.