Calculation of solvation free energies of Li⁺ and O₂^- ions and neutral lithium-oxygen compounds in acetonitrile using mixed cluster/continuum models

Solvation effects play a major role in determining the cycling characteristics of the non-aqueous rechargeable Li-air battery. We use a mixed cluster/continuum solvent model with varying number of explicit solvent molecules (n = 4-10) to calculate the solvation free energies (ΔG_solv^*) of Li⁺ and O₂^- ions and neutral LiO₂, Li₂O₂, LiO, and Li₂O species in acetonitrile solvent. Calculations for complexes with the full first solvation shell around Li⁺ (n = 4) and O₂^- (n = 8) show excellent agreement with the solvation free energies obtained using the cluster pair approximation (the error is below 2.0 kcal/ mol). The use of the pure continuum model fitted to reproduce the experimental values of ΔG_solv^*(Li⁺) and ΔG_solv^*(O₂^-) gives the solvation free energies of various lithium-oxygen species (LixOy; x, y = 1, 2) that are in excellent agreement with the results obtained using mixed cluster/continuum models (n ≥ 8). This provides a theoretical framework for including solvent effects in the theoretical models of oxygen reduction and evolution reactions in the aprotic Li-air battery.