Competitive lithium-ion solvation structures in linear and cyclic carbonate electrolytes: insights from OPLS-AA...

The solvation structure of lithium ions is an important factor in determining the performance of lithium-ion batteries. However, the detailed study of solvation structures and coordination behavior of Li⁺ ions in pure and mixed carbonate (ethylene carbonate (EC) and diethyl carbonate (DEC)) electrolytes has not been thoroughly discussed. Moreover, there are some conflicting views regarding the preferential coordination of Li⁺ with linear or cyclic carbonates in mixed carbonate electrolytes. Using molecular dynamics (MD) simulations, we investigated Li⁺ solvation in pure and mixed carbonate electrolytes with lithium hexafluorophosphate (LiPF₆). Our results indicate a strong preference of Li⁺ to interact with DEC over EC in the primary solvation shell of binary mixtures, especially at low concentrations. Increasing LiPF₆ concentration in 1 : 1 EC/DEC mixtures shifts the solvation structure from solvent separated ion pairs (SSIPs) to aggregates (AGGs), with SSIPs decreasing from 68% at 1 M to 3.99% at 4 M, while AGGs increase from 5.33% to 78.74%. Pure EC electrolytes exhibit fewer SSIPs and more AGGs than pure DEC electrolytes. We also observe that the primary species in solution vary with the concentration of LiPF₆. In 1 : 1 EC/DEC mixed electrolyte, at low salt concentrations (1 M), the solution is primarily composed of SSIPs, while at higher salt concentrations (4 M), AGGs become the predominant species. This study contributes to the ongoing debate about Li⁺ coordination preferences and provides a thorough understanding of Li⁺ solvation structures in linear and cyclic carbonate electrolytes.