Structure-Property Relationships in Fluorine-Free Ionic Liquids and Salts Utilising the Acesulfame Anion

Ionic liquids and salts are of interest as safer next-generation electrolytes for energy storage applications due to beneficial properties that can include nonvolatility and nonflammability. However, to overcome environmental concerns associated with perfluorinated anions prevalent in these materials, it is important to develop alternative, fluorine-free anions. Here, we report the synthesis and characterization of five nonfluorinated salts by pairing the acesulfame anion (ACE^-) with a range of cations: N-methoxymethyl-N,N,N-trimethylammonium (N_111(1O1) ⁺), N-methoxyethyl-N,N,N-trimethylammonium (N_111(2O1) ⁺), N-ethyl-N-methyloxazolidinium (C₂moxa⁺), N-ethyl-N-methylmorpholinium (C₂mmor⁺), and N-methyl-N-propylpyrrolidinium (C₃mpyr⁺). The thermal, structural, transport and electrical properties of these materials were studied using techniques including differential scanning calorimetry, single crystal X-ray diffraction, electrochemical impedance spectroscopy, pulsed-field gradient nuclear magnetic resonance and cyclic voltammetry. Discussion of the structure-property relationships in acesulfame-based salts and their comparison with fluorinated salts with the same cations is reported for the first time, revealing distinct trends that are concluded to be largely a result of the size, conformational freedom, charge delocalisation and the intermolecular interactions of the ACE^- anion. All acesulfame-based salts displayed thermal stabilities above battery operational range (>180°C) and wide electrochemical stability windows exceeding 4.9 V, demonstrating their potential as electrolytes for lithium and sodium batteries.