Ion mobility and solvation complexes at liquid–solid interfaces in dilute, high concentration, and localized high concentration electrolytes

Abstract

The underlying mechanisms of the solvated lithium cation diffusion and deposition on the Li metal surface occurring at electrochemical interfaces are still not fully understood. In this work, density functional theory and a thermodynamic integration method implemented in constrained-ab initio molecular dynamics are used to calculate the free energy profile for the lithium cation transport pathway in the absence of an external field. The trajectory and evolution of the solvation complex surrounding the lithium cation, alongside the effect of salt concentration and diluent presence are studied in carbonate-based electrolytes including low concentration electrolytes (LCEs), high concentration electrolytes (HCEs), and localized high concentration electrolytes (LHCEs). Energy barriers for transport and desolvation are obtained with the thermodynamic integration method and discussed in relation to the solvation shell surrounding the Li-ion. In dilute electrolytes, the energy barriers for cation diffusion in the electrolyte phase are relatively low and the final deposition is guided mostly by solvent reduction. In HCEs, the high connectivity between the primary solvation complex and the rest of the electrolyte leads to a significant increase in the energy barriers for diffusion, and the ion can get trapped in the electrolyte slowing down the deposition, while the early development of SEI formation shows a thick and compact SEI structure built by anion decomposition. In the LHCE the diluent helps in reducing the barriers found in HCEs and breaking the high connectivity thus facilitating cation diffusion and the simultaneous SEI formation process.