Atomistic Simulation Studies of Alkali Ion Conducting Superionic Conductors

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Designing of all-solid-state-battery by replacing the currently used liquid/gel electrolytes with solid ones is the prospect of next generation energy storage devices. This advancement promises higher energy density, safety and longer operating cycles. However, the progress towards the commercial realization of such devices demands microscopic insights on various factors, including the mechanism of ion transport in solids. This calls for computational investigations complementing experimental studies. In this thesis, computational studies of some of the promising classes of Li/Na-ion conducting solids are presented. The first chapter of the thesis is devoted to the review of the ongoing research on Li/Na-ion conducting inorganic solids. In the second chapter, the theoretical background of the computational methods, such as classical and ab initio molecular dynamics, metadynamics, nudged elastic band method, etc. employed in the studies are discussed. The third chapter presents fresh atomic-scale insights on the role of framework dynamics on ion transport in Li-substituted NASICONs (LiM2P3O12 where M = Zr, Hf, Sn, Ti) based on classical molecular dynamics study. The fourth chapter of the thesis addresses the ‘time scale’ issue of standard molecular dynamics simulation in the study of slow diffusing systems. Plugged in with classical molecular dynamics, the utility of metadynamics technique has been demonstrated by taking the low diffusing phosphate and silicate end members of the true-NASICON family Na1+xZr2SixP3– xO12 (0 · x · 3) as the prototype systems. This utility is further explored in the fifth chapter by using metadynamics interfaced with ab initio molecular dynamics, to understand the Li migration mechanism in °-Li3PS4. The sixth chapter summarizes the results.
Supervisor: Padma Kumar Padmanabhan