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

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(SO2C2F5)(SO2CF3)] (NaPTFSI) and Na[N(SO2C3F7)(SO2CF3)] (NaHTFSI). The dihydrate melt enables reversible insertion/desertion of Na-ions into/from NaTiOPO4 (1.6 V vs Na/Na+) and Na4Co3(PO4)2P2O7 (4.4 V vs Na/Na+), both of which are located far beyond the stable potential windows of the conventional aqueous electrolytes.

Automated summary

Highly concentrated aqueous electrolytes have been developed by mixing two or more salts, leading to the discovery of a room-temperature Na-salt eutectic dihydrate melt with a wide potential window. The dihydrate melt enables reversible insertion/desertion of Na-ions into/from compounds located far beyond the stable potential windows of conventional aqueous electrolytes.