Na-Salt Eutectic Dihydrate Melt for High-Voltage Aqueous Batteries

Abstract

Aqueous battery electrolytes have attracted research attention owing to their safety and cost benefits. However, the narrow electrochemical window of water (1.23 V) has hindered steady operation at high voltages. The concept of highly concentrated aqueous electrolytes has recently led to the discovery of several Li-and K-salt hydrate melts with low hydration numbers (n) by mixing two or more salts, that is, eutectic hydrate melts. However, the lowest hydration number for Na-salt eutectic systems remained at n = 3.0, limiting the steady operation voltage. Herein, we report a room-temperature Na-salt eutectic dihydrate melt (n = 2.0) with a wide potential window of over 3 V, achieved by mixing two asymmetric imide salts, Na[N(SO 2 C 2 F 5 )(SO 2 CF 3 )] (NaPTFSI) and Na[N(SO 2 C 3 F 7 )(SO 2 CF 3 )] (NaHTFSI). The dihydrate melt enables reversible insertion/desertion of Na-ions into/from NaTiOPO 4 (1.6 V vs Na/Na + ) and Na 4 Co 3 (PO 4 ) 2 P 2 O 7 (4.4 V vs Na/Na + ), both of which are located far beyond the stable potential windows of the conventional aqueous electrolytes.

Issues and challenges facing aqueous sodium‐ion batteries toward practical applications

“…In light of this, Kitada et al designed a room-temperature eutectic dihydrate melt, 27.8 mol kg −1 Na(PTFSI) 0.55 (HTF-SI) 0.45 •2H 2 O, with the operating potential exceeding 3 V (Figure 6E). 63 Unfortunately, although WiSE and its derived electrolytes broaden the ESW of aqueous electrolytes, highly concentrated NaClO 4 is prone to explode at high 63 Copyright 2023, ACS. HER, hydrogen evolution reaction; OER, oxygen evolution reaction.…”

“…Copyright 2017, ACS. (E) ESW of 1 mol L −1 NaFSI and 27.8 mol L −1 Na(PTFSI) 0.55 (HTFSI) 0.45 ·2H 2 O. Reproduced with permission 63 . Copyright 2023, ACS.…”

“…In light of this, Kitada et al designed a room‐temperature eutectic dihydrate melt, 27.8 mol kg −1 Na(PTFSI) 0.55 (HTFSI) 0.45 ·2H 2 O, with the operating potential exceeding 3 V (Figure 6E). 63 …”

Are NaTFSI and NaFSI Salt-Based Water-in-Salt Electrolytes Structurally Similar or Different?

“…Further, they compared the solubility and electrochemical performance of different sodium salts dissolved in water and concluded that, apart from having higher salt solubility in water, the anions must involve in the SEI formation to achieve better battery performance. Later on, several research studies on WiSEs emphasized the role of higher salt solubility and the ability of anions to interact with cations in order to enhance the SEI formation as well as suppress the water activity. ,− In addition to this, in recent studies, various approaches have been adapted to increase the solubility limit of sodium salts in water. ,, The sodium salt having a bis(trifluoromethylsulfonyl)imide (TFSI) anion has a limited solubility of around 7 m in water, but it shows a dense SEI formation at a lower salt concentration as compared to LiTFSI salt . Also, sodium bis(fluorosulfonyl)imide (NaFSI) based aqueous electrolyte renders its electrochemical potential window up to 2.6 V at a high salt concentration of 35 m .…”

Research and Development of Novel Secondary Batteries in Japan

“…In addition to these batteries, the VSI also illustrates novel materials development, theoretical calculations, and characterization for other secondary batteries, such as Li-ion and Na-ion batteries. − …”