Solvation plays a vital role in determining the thermodynamics¹ and kinetics² of both homogeneous and heterogeneous redox reactions, as well as bulk electrolyte properties such as ionic conductivity. While the enthalpic contribution of aprotic solvents are well understood through empirically measured solvation parameters such as the Gutmann donor and acceptor number³, the contributions of solvation entropy on redox reactions, particularly in nonaqueous solvents, is not well understood. In this work, we consider the effect of solvent, counter anion and salt concentration on the solvation entropy of Li⁺ ions. Through probing the reaction entropy of the Li⁺/Li redox couple in a variety of salt and solvent systems, we find in a fully dissociated electrolyte, such as LiTFSI in DMSO, that the solvation entropy depends mostly on the activity and mobility of Li⁺ ions. Alternatively, in electrolytes with significant ionic association, such as LiTFSI, LiTfO and LiTFA in DME, more complex and non-ideal behavior is observed. The results of this study enable a stronger fundamental understanding of the role of entropy on solvation, promoting rational design of electrolytes in aprotic systems.

References

1. D. G. Kwabi et al., Angewandte Chemie International Edition, 55, 3129–3134 (2016).
2. R. A. Marcus, Reviews of Modern Physics, 65, 599–610 (1993).
3. V. Gutmann, Electrochimica Acta, 21, 661–670 (1976).