The association of lithium salts in liquid electrolytes is thought to impact their transport and chemistry. During battery operation, surface films are formed which can also alter the environment seen by lithium ions. Previous simulations done by our group have shown that crystal surfaces encourage ion accumulation and slower dynamics in the electrolyte than seen in the bulk phase. However, it is not clear how these changes are impacted by the degree of cation-anion association. In this work we focus on the role of salt association in changing electrolyte structure in lithium-ion batteries in bulk and at film surfaces. To address this question, an electrolyte comprised of ethylene carbonate (EC) solvent and dissolved LiPF6 salt was simulated at surfaces commonly found in rechargeable batteries during operation (LiF, Li2CO3, and Li2EDC) using classical molecular dynamics. By varying the strength of ion association via charge-scaling, the competition between salt interaction and interface effects were explored in greater detail at LiF, Li2CO3, and Li2EDC surfaces. In all cases, stronger interaction between the salt pair favored greater accumulation of ions at the interface. Regarding changes to solvation structure, the coordination plots showed that the differences between bulk behavior and the salt coordination at the interface were smaller when the solvent charges were scaled. This behavior most likely resulted from the ions already forming contact ion pairs at greater frequency in the bulk and hence experiencing less of a change at the surface. Finally, study of the solvation correlation functions shows that greater ion association does dramatically impact the counterion residence time, but not the solvent coordination. Future work includes expanding these studies to develop general rules for the impact of ion association on electrolyte structure and dynamics.

Ion association and electrolyte structure at surface films in lithium-ion batteries

ACS Fall 2020 Virtual Meeting

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