Molecular insights into an ether-functionalised ionic liquid electrolyte with hydrogen-modified anions at electrode interfaces

Abstract

The chemistry of cations and anions in an ionic liquid (IL) significantly affects its bulk phase and interfacial properties, requiring an in-depth understanding to advance its application in energy storage, including batteries and supercapacitors. In this work, a novel ionic liquid electrolyte featuring an ether-functionalised cation, N-methyl-N-methoxyethylpyrrolidinium [C_2O1mpyr], and a hydrogen-modified anion, (difluoromethanesulfonyl) (trifluoromethanesulfonyl)imide [DFTFSI], was studied computationally as a sodium battery electrolyte. The impact of the ether oxygen in the cation and hydrogen in the anion on the bulk phase structure, ion diffusion and interfacial chemistry was examined. The presence of the ether oxygen enhances ion diffusion and disrupts Na-anion aggregation through Na–O_cation coordination in both the bulk phase and at the electrode interface. The Na–O_cation coordination slightly affects the number of cations and anions in the innermost electrolyte layer but significantly influences the redox stability of the cation. The decomposition of the DFTFSI anion was explored, and two reduction routes and potential products were identified. This work provides new insights into the role of functionalised cations and anions in ionic liquid electrolytes, contributing to the development of advanced sodium batteries.