Toward Fluorine-Free Electrolytes: Physicochemical Properties and Ionic Interactions in Acesulfame-Based Ionic Electrolytes

Fluorine-free ionic liquids and salts are attracting growing interest as safer and more sustainable electrolyte components for Li-ion batteries, addressing both the flammability of conventional carbonate-based electrolytes and the environmental persistence of fluorinated anions. This motivates the use of the nonfluorinated acesulfame (ACE–) anion, derived from potassium acesulfame, a toxicologically evaluated tabletop sweetener. Further, ether-functionalized cations may facilitate lithium-ion transport by promoting favorable lithium coordination environments. Here, two ionic electrolytes were prepared by combining the ACE– anion with either the cyclic-ether N-ethyl-N-methyloxazolidinium (C2moxa+) cation or the N-methoxy-N,N,N-trimethylammonium (N111(1O1)+) cation, where the ether is functionalized on the side chain, and lithium acesulfame (Li-ACE) to evaluate their promise as battery electrolytes. The thermal and transport properties and the ion dynamics were studied and compared at various Li-ACE concentrations. Among all the lithium-based electrolyte systems, [N111(1O1)][ACE] containing 10 mol % Li-ACE exhibited the highest conductivity, reaching 5.4 × 10–4 S cm–1 at 50 °C. However, [C2moxa][ACE] exhibited a higher Li+ diffusivity ratio at both 20 and 50 mol % Li-ACE content, indicating that the cyclic ether oxygen may create a distinct local chemical environment that facilitates greater relative mobility of Li+. Additionally, the Li+ solvation environment in acesulfame-based electrolytes was elucidated using combined experimental and simulation approaches. In both electrolytes, Li+ was found to coordinate with ACE- through the nitrogen atom as well as the sulfonyl and carbonyl oxygens. Moreover, molecular dynamics simulations suggest that smaller Li–Li clusters were formed in [C2moxa][ACE] electrolytes compared to [N111(1O1)][ACE], which may contribute to the enhanced Li+ mobility compared with ACE– in [C2moxa][ACE] electrolytes at 20 and 50 mol %. These results indicate that cation chemistry plays an important role in tuning Li+ solvation structure, ion clustering, and ion transport.