Structural Origin of the Thermal and Diffusion Behaviors of Lithium Aluminosilicate Crystal Polymorphs and Glasses

Abstract

Lithium aluminosilicate polymorphs α–LiAlSi2O6, β–LiAlSi2O6, and the LiAlSi2O6 glass have been studied comparatively using classical molecular dynamics (MD) simulations with an aim to better understand the structural origin of the different thermomechanical behaviors and lithium ion diffusion properties. The melting behaviors and structural evolution were investigated for the three phases using MD simulations. The structural features of the three simulated samples were analyzed using coordination number, pair and bond angle distributions. The results showed that β‐LiAlSi2O6 and the LiAlSi2O6 glass had similar melting behavior, had more random short‐range atomic structures, and lower densities as compared to the α‐LiAlSi2O6 phase, which has a more ordered and compact structure. The lithium ion diffusion behavior in α–LiAlSi2O6, β–LiAlSi2O6, and LiAlSi2O6 glass and their melts are determined and compared by calculating the mean square displacements. It was found that at high temperatures, the melts of α–LiAlSi2O6, β–LiAlSi2O6, and LiAlSi2O6 glass had similar diffusion properties. While at low temperatures, α–LiAlSi2O6 had the lowest diffusion coefficient and highest diffusion energy barrier due to its more close‐packed structure and lacking of defects to facilitate lithium ion diffusion. Both the β–LiAlSi2O6 and glass show high ionic conductivity even at low temperatures. This originates from their lower density and thus relatively open structures, but slightly different diffusion mechanisms. Lithium ion diffusion in β–LiAlSi2O6 is through the large available interstitial sites while that in the glass is through vacancies due to high free volume. The glass phase had slightly lower lithium ion diffusion energy barrier and higher lithium ion diffusion coefficients as compared to the β–LiAlSi2O6 phase, indicating the glass phase can achieve high ionic diffusion and, in some cases, even higher than the crystalline phases with similar densities and short‐range structures.