Equilibrium Potential of Li/Li + in Pyrrolidinium-Based Ionic Liquids

Equilibrium potential, a fundamental parameter in electrochemical systems, governs reaction thermodynamics and kinetics across diverse applications, including energy storage and other electrochemical processes. In rechargeable batteries, the equilibrium potential of Li/Li+ reflects the chemical potential of Li+ in the electrolyte and plays an essential role in the reversibility of Li metal deposition and dissolution. This highlights the importance of understanding equilibrium potentials in alternative systems such as ionic liquids, which are promising electrolytes because of their wide electrochemical windows and tunable properties. However, systematic studies on how subtle cation structural variations in ionic liquids influence the Li/Li+ potential remain scarce. Herein we investigate the equilibrium potential of Li/Li+ in a series of ionic liquids composed of pyrrolidinium cations and bis(trifluoromethanesulfonyl)amide (TFSA) anions. We show that the measured Li/Li+ quasi-equilibrium potential exhibits a clear structure–property relationship: longer alkyl side chains shift the potential positively, whereas ether-functionalized side chains shift it negatively. Molecular dynamics simulations suggest a correlation between the fraction of LiTFSA clusters formed in the ionic liquid and the observed potential. The Li/Li+ equilibrium potentials were also analyzed by using the Nernst equation incorporating the activity of free TFSA–, offering a thermodynamic rationale for the potential difference arising from cation structural variations. These findings underscore the importance of cation design in modulating the Li/Li+ equilibrium potential, offering insights for tailoring ionic-liquid-based electrolytes for batteries and other electrochemical applications.