Fluoroborate ionic liquids as sodium battery electrolytes†

High-voltage sodium batteries are an appealing solution for economical energy storage applications. Currently available electrolyte materials have seen limited success in such applications therefore the identification of high-performing and safer alternatives is urgently required. Herein we synthesise six novel ionic liquids derived from two fluoroborate anions which have shown great promise in recent battery literature. This study reports for the first time the electrochemically applicable room-temperature ionic liquid (RTIL) N-ethyl-N,N,N-tris(2-(2-methoxyethoxy)ethyl)ammonium (tetrakis)hexafluoroisopropoxy borate ([N_2(2O2O1)3][B(hfip)₄]). The RTIL shows promising physical properties with a very low glass-transition at −73 °C and low viscosity. The RTIL exhibits an electrochemical window of 5.3 V on a glassy carbon substrate which enables high stability electrochemical cycling of sodium in a 3-electrode system. Of particular note is the strong passivation behaviour of [N_2(2O2O1)3][B(hfip)₄] on aluminium current-collector foil at potentials as high as 7 V (vs. Na⁺/Na) which is further improved with the addition of 50 mol% Na[FSI]. This study shows [B(hfip)₄]⁻ ionic liquids have the desired physical and electrochemical properties for high-voltage sodium electrolytes.