Redox Mechanisms, Cation Ordering Thermodynamics and Diffusion Kinetics in Layered Na-Intercalation Compounds

First-principles statistical mechanics approaches have proven invaluable in elucidating how chemistry, electronic structure and crystallography determine electrochemical properties of intercalation compounds. In this talk, I will survey how these approaches have shed light on redox mechanisms, phase stability and ion transport kinetics in layered Na intercalation compounds. The replacement of Li by Na in layered intercalation compounds results in significant changes in electronic, thermodynamic and kinetic properties of the compound. Na-intercalation compounds exhibit many more ordering reactions and structural phase transformations than their Li counterparts, which makes the Na intercalation compounds more prone to chemo-mechanical degradation. The peculiar Na-vacancy ordering tendencies in layered Na-intercalation compounds has important consequences for Na diffusion mechanisms, leading to transport mechanisms that rely on extended defects such as anti-phase boundaries as opposed to point defects such as vacancies and divacancies. The large size of Na+ cations results in prismatic coordination within the intercalation layers, which suppress redox driven migration of transition metal cations such as Mn to the intecalation layer. This offers new opportunities with which to realize true anion-redox that is not convoluted with transition metal migration and structural degradation.