Phase Behaviors and Ion Transport Properties of LiN(SO<sub>2</sub>CF<sub>3</sub>)<sub>2</sub>/Sulfone Binary Mixtures

Highly concentrated Li salt/aprotic solvent solutions are promising electrolytes for next-generation batteries. Understanding the Li+ ion transport process is crucial for designing novel battery electrolytes. In this study, we systematically investigated the phase behavior, solvate structures, and Li+ transport properties of binary mixtures comprising lithium bis(trifluoromethanesulfonyl)amide (LiTFSA) and various sulfones, such as sulfolane (SL), 3-methyl sulfolane (MSL), dimethyl sulfone (DMS), ethyl methyl sulfone (EMS), and ethyl isopropyl sulfone (EiPS). Except for the MSL system, the [LiTFSA]/[sulfone] = 1/2 mixtures remained in a liquid state at room temperature, thus enabling a systematic comparison of the Li+ transport properties in the highly concentrated electrolytes. In highly concentrated liquid electrolytes, Li+ ions diffuse by exchanging ligands (sulfone and TFSA). Li+ ions diffuse faster than TFSA in all electrolytes except the EiPS-based electrolyte at a composition of [LiTFSA]/[sulfone] = 1/2, resulting in high Li+ transference numbers. SL-based electrolytes show higher ionic conductivity and Li+ transference numbers than other sulfone-based electrolytes. Consequently, sulfone solvents with compact molecular sizes and low energy barriers of conformational change are favorable for enhancing the Li+ ion transport in the electrolytes.