Ab Initio Simulations of Idealized Solid Electrolytes in Lithium Ion Batteries

The general purpose of this work is to develop a detailed understanding of solid state electrolyte materials and to contribute to their development for possible use in Li-Ion batteries using the framework of first-principles computational methods. More specifically, we use different computational methods in the framework of density functional theory to perform an in depth study of the structure, Li ion conductivity, and the stability of recently reported promising inorganic solid electrolyte materials. The structure for some materials was reported from experiment and in some cases was predicted from the simulation and validated to be consistent with the experimental data. Li ion conductivity was studied using the nudged elastic band method and molecular dynamics simulations. The nudged elastic band method was used to analyze the migration barrier of the Li ions. Also, molecular dynamics simulation was used to analyze the migration of the Li ions by visualizing superposed Li positions over the timescale and at various temperatures of the simulation and to calculate the ionic conductivity of the material from the mean square displacement of the Li ions. The stability was studied by analyzing the electronic structure of the interface of the material with metallic Li. Four classes of solid electrolytes identified as promising electrolytes in the recent experimental literature were investigated in this work. The first class of materials studied was the alloy system Li3+xAs 1-xGexS4 (G. Sahu et al., Journal of Materials Chemistry A, 2, 10396 (2014)) where the simulations were able to model the effects of Ge in enhancing the conductivity of pure Li3AsS 4. The second class of materials studied was Li4SnS 4 and Li4SnSe4 (T. Kaib et al., Chemistry of Materials, 24, 2211 (2012), J. A. MacNeil et al., Journal of Alloys and Compounds, 586, 736 (2013), T. Kaib et al., Chemistry of Materials, 25, 2961 (2013)). Our simulations were able to identify the two different crystal structures of the materials and to investigate differences in their conduction properties. The third set of materials studied were two nitrogen rich crystalline lithium oxonitridophosphate materials, Li14P2O3N 6 (D. Baumann et al., European Journal of Inorganic Chemistry, 2015, 617 (2015)) and Li7PN4 (W. Schnick et al., Journal of Solid State Chemistry, 37, 101 (1990)). Our simulations suggest that these materials are promising solid electrolytes due to their ideal interface properties with metallic Li and their promising ionic conductivity. The fourth project is an ongoing study of the newly synthesized electrolyte Li4PS 4I (S. Sedlmaier et al., Chemistry of Materials, 29, 1830 (2017)). The simulations help in the understanding of the structural and ion mobility properties of this material and to study models of interfaces with Li metal.