Solvation-mediated adsorption mechanism of solvated lithium ions at a charged solid–liquid interface for electrochemical energy storage: atomic scale investigation and insights

Abstract

Ion encapsulation by solvent molecules significantly impacts ion transport and the adsorption mechanism in energy storage devices. The aim of this investigation is to analyse the adsorption mechanisms associated with the solvation shell of lithium ions near the electrode/electrolyte interface during the charging process. Simulations using molecular dynamics (MD) are conducted for LiPF₆ salt in PC solvent confined in between two flat carbon electrodes. The thermodynamic and physical properties of the simulation show excellent agreement with experimental values. Results indicate that the lithium ion forms a strong tetrahedral solvation structure with PC solvent molecules. Orientation analysis reveals that the polar ends of the solvent molecules in the lithium ion solvation structure are anchored to the positive electrode, which is caused by strong attractive interactions, particularly for high surface charge densities. Meanwhile, the solvation structure and solvent molecules undergo rotation close to the negative electrode at high surface charge densities. These aforementioned phenomena lead to solvation-mediated electrostatic interactions between solvated lithium ions and the electrodes. Finally, the differential capacitance for both positive and negative electrodes decreases under these solvation-mediated electrostatic interactions. This study provides a unique intuitive image of possible implications of the solvation structure on the charging performance of energy storage devices, along with perspectives on developing electrolytes with favorable orientations.