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Ion Conduction in Soft-Solid Alkali Metal Battery Electrolytes


SEM image of cocrystals of (ADN)3NaClO4, enlarged to show the surface model from molecular dynamics simulations, and the mechanism of ion conduction from quantum calculations (plane-wave DFT). Chemistry of MaterialsJust Accepted

Solid-state lithium-ion batteries and the alternative sodium-ion batteries are investigated as electrolytes to develop safer and mechanically stronger batteries. In liquid electrolytes, the volatility of organic solvents have issues related to safety, stability and stiffness; solid electrolytes like ceramics and garnets do not possess conductivity as good as liquid electrolytes and also suffer from poor electrode-electrolyte interface. This leads to the pursuit of alternative safe electrolytes with low boundary resistance and high conductivity. The experimental work from our collaborators at Temple University, USA have shown that organic solvents like N,N-dimethylformamide and adiponitrile co-crystalize with salts such as NaClO4 and LiPF6 leading to ion channels and networks which facilitate ion transport. The objective of our research program is to offer a molecular level understanding of the experimental observations. To achieve this, we deploy quantum chemistry calculations and MD simulations to elucidate the structural complexity and thermal stability of the electrolyte, and pathways and activation energy barriers of Li+ and Na+ ion hopping in these electrolytes. The MD simulations model ion conduction in a liquid-like layer that exists around several co-crystalline solids as reported from DSC, X-ray scattering and SEM experiments. The simulations reveal a complex interplay between solvent-ionic/inter-ionic interactions and also show the process of decomposition/melting of these electrolytes. To examine the influence of Schottky defects, supercells with ionic defects are also simulated using MD simulations. The activation energy barrier for ion conduction is calculated and compared with barriers observed from Impedance Spectroscopy measurements. To understand ion migration, we use Plane-wave DFT calculations to ascertain the minimum energy paths, which can assist in understanding the role of the anion and solvents during the migration of the cations. The insights from the calculations/MD simulations validates experimental observations and can motivate the synthesis of efficient electrolytes for higher ion conduction. The focus of future activities is a complete computational investigation of the electrolyte-electrode interface and dendrite formation in batteries.