(632a) Molecular-Based Modeling Of The Thermodynamic And Kinetic Costs Of Ion Desolvation (Regarding The Commercial Application Of Lithium-Ion Batteries)

Gering, K. L., Idaho National Laboratory

Ion solvation figures centrally to properties of electrolytes tied to natural, industrial, commercial, and biological systems. The existence of ion solvation is a two-edged sword, enabling high salt dissociation, yet producing rate and energy barriers if ions are to be desolvated in the intended application. One such application is rechargeable lithium-ion (Li-ion) batteries, where lithium ions must generally become desolvated prior to intercalation into the electrode host materials. This cycle of ion solvation/desolvation occurs whenever such batteries are operated. This work will demonstrate that lithium desolvation is a process defined by successive removal of ligands from the ion solvation shell. There are time and energy requirements tied to this ligand-wise process, where the time and energy needed to remove successive solvent molecules from a solvated lithium ion become greater as we approach complete desolvation of the ion. It is believed that lithium ion desolvation contributes to poor performance of Li-ion batteries at low temperatures, since both the net time and energy required for desolvation increase at colder conditions. Thermodynamic and kinetic results for conventional battery electrolytes (such as ethylene carbonate + ethylmethyl carbonate + LiPF6) will be shown, as simulated by a contemporary electrolytes code developed for use in the US DOE FreedomCAR program. The model views chemical physics per the Associative Mean Spherical Approximation, and receives other molecular-scale information from an ?equation of state? that provides explicit treatment of ion solvation quantities. This modeling tool can be used to screen and optimize candidate electrolyte systems based on the requirements of minimizing solvent-ion interactions while maintaining favorable transport properties (electrical conductivity, viscosity, ionic diffusivity, etc.). Modeling results indicate that step-wise lithium desolvation is a strong function of lithium solvation number, and becomes more problematic at low temperatures and lower salt concentrations where lithium solvation numbers are higher. Lithium solvation numbers of two to five are common for electrolyte systems found in most Li-ion batteries. Lithium ion desolvation time and energy show similar trends over temperature and salt concentration, although they are obtained through independent approaches. While Li-ion batteries are used as a test case for this modeling capability, other applications abound in areas of industrial and biological systems.