Solvent-dependent iodide interactions in LiO2 electrolytes – a molecular dynamics study

Abstract

Iodide-based redox mediation in Li–O₂ batteries is regarded as a promising system due to its relatively high round-trip efficiency, compared to alternative systems. Here we explore the role of electrolyte composition in the solvation of I⁻, which has been shown to be critical for the efficient operation of this redox mediator, using a molecular dynamics approach. A combinatorial exploration of I⁻ and H₂O concentrations was performed, for a fixed concentration of Li⁺, across a series of glymes, with increasing chain length (mono- to tetraglyme). The resulting radial distribution functions show that shorter glymes allow for a closer packing of the I⁻ redox mediator. Furthermore, increasing the I⁻ concentration also reduces the solvation of Li⁺ in the glymes, especially in G2. The presence of water further pulls the I⁻ and Li⁺ together. With increasing water content, its presence in the iodide's coordination shell increases markedly – an effect most pronounced for monoglyme. Competition between Li⁺ and I⁻ for the coordination of water is modulated by the different solvents as they perturb the local coordination shell of these important complexes, with longer chain lengths being less affected by increases in water concentrations.