Non-fluorinated ionic liquids and salts from the sweetener acesulfamate for Na and Li battery electrolytes

The development of non-flammable, non-volatile electrolytes is important for safer lithium and sodium batteries, to facilitate our transition to a net zero economy, but the reliance on fluorine-containing anions brings significant environmental concerns. Here, acesulfamate based ionic liquids (ILs), sodium acesulfamate (Na[ace]) and lithium acesulfamate (Li[ace]) are introduced as promising new fluorine free materials as a more sustainable alternative to the perfluorinated anions (BF _4 , PF _6 , FSI and TFSI) currently utilised in energy storage devices. The resulting ammonium, phosphonium and pyrrolidinium acesulfame ILs showed promising physicochemical properties with a relative high conductivity (e.g. 1.4 × 10 ^−4 S cm ^−1 at 30 °C for [N _1222 ][ace]) and a wide electrochemical stability window (∼4 V). Lithium and sodium acesulfamate salts were synthesised from potassium acesulfamate (ACE-K) via an acid-base reaction. The use of acesulfamate ILs as electrolytes was investigated by mixing with sodium or lithium acesulfamate salts and characterisation of their physicochemical and electrochemical properties. Polyethylene oxide based free standing membranes composed of [N _2222 ][ace] with Na or Li [ace] were fabricated and tested in symmetrical Li or Na metal cells, demonstrating good electrochemical performance in both variable current density tests and longer-term cycling at elevated temperatures. Thus, the new Li and acesulfamate salts, the ILs and their mixtures, represent valuable new materials for the development of fluorine free electrolytes for energy storage devices.