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

Abstract

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.

Advances in studying interfacial reactions in rechargeable batteries by photoelectron spectroscopy

“…131,132 XPS has been found to be a valuable tool in studying both SIBs and KIBs. [133][134][135][136][137][138] Going even further into novel systems, we nd multivalent-ion batteries based on magnesium and calcium that present further unique challenges and require some unique solutions. 139,140 Fig.…”

Fundamental Understanding and Optimization Strategies for Dual-Ion Batteries: A Review

“…In DIBs, the electrolyte system not only serves as the ion transport medium but also acts as the active materials to provide both the cations and anions to be stored in the electrode [ 91 , 92 ]. As a result, the effects of separate electrolyte components (salts, solvents, and additives), their oxidation/reduction at the cathode/anode sides, the “solvation effects” of anions, the formation and instability of SEI layer, the co-intercalation of solvent molecules into cathode materials are key issues to be addressed [ 93 ].…”

Spreading the Landscape of Dual Ion Batteries: from Electrode to Electrolyte

“…However, the low working potential and limited energy density are the biggest obstacles to the further development and widespread use of water-based electrolytes. [200] To solve this problem, several highly concentrated aqueous electrolytes or "water-in-salt" electrolytes (WiBS) have been developed. [201] WiBS has been shown to extend the oxidative stability of aqueous electrolyte beyond 1.65 V vs. Ag/AgCl (2.60 V vs. Zn/ Zn 2 + ) and to promote reversible plating/stripping of Zn 2 + .…”

“…However, the low working potential and limited energy density are the biggest obstacles to the further development and widespread use of water‐based electrolytes [200] . To solve this problem, several highly concentrated aqueous electrolytes or “water‐in‐salt” electrolytes (WiBS) have been developed [201] .…”