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

Superconcentrated aqueous electrolytes containing NaN(SO₂F)₂ and KN(SO₂F)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_0.55K_0.45FSA/H₂O and 33 mol kg^-1 Na_0.45K_0.55FSA/H₂O 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_0.45K_0.55FSA/H₂O to two different battery configurations, carbon-coated Na₂Ti₂(PO₄)₃∥K₂Mn[Fe(CN)₆] and carbon-coated Na₃V₂(PO₄)₃∥K₂Mn[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.