Superconcentrated NaFSA–KFSA Aqueous Electrolytes for 2 V-Class Dual-Ion Batteries
Tomooki Hosaka, Ayumi Noda, Kei Kubota, Kento Chiguchi, Yuki Matsuda, Kazuhiko Ida, Satoshi Yasuno, Shinichi Komaba
Abstract
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 Na0.55K0.45FSA/H2O and 33 mol kg–1 Na0.45K0.55FSA/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 Na0.45K0.55FSA/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.