Abstract
Li and Na intercalation compounds exhibit a wide variety of electronic, structural and phase transformation phenomena. Very different phases and electronic properties can be realized within the same layered transition metal oxide by simply changing the guest cation from Li to Na. For example, Na intercalation compounds exhibit many more ordering reactions and structural phase transformations than their Li counterparts. In this talk we will focus on layered Li and Na intercalation compounds that start out as O3. The larger size of Na leads to the stabilization of P3 upon Na extraction. Our first-principles statistical mechanics studies have revealed that the P3 host tends to stabilize an intriguing family of Na-vacancy orderings that consist of well-ordered domains separated by anti-phase boundaries. The family of ordered phases forms a devil's staircase whereby the concentration can be varied by changing the density of anti-phase boundaries. A systematic study of diffusion mechanisms in the devil's staircase of ordered P3 phases shows that Na transport is mediated by the migration of anti-phase boundaries rather than conventional cation-vacancy exchanges as occurs in Li-intercalation compounds.
We will also discuss a variety of intriguing redox mechanisms in Li and Na layered intercalation compounds that arise either from p-bonded complexes involving oxygen and transition metals or complexes made of metal-metal bonds. An important example is Na2Mn3O7, which can be charged in a Na-ion battery in spite of the fact that Mn has a formal oxidation state of 4+. The 'excess' capacity is enabled by a p-redox centered that is delocalized over a ring of six Mn p-bonded to six O surrounding a Mn vacancy. Similar molecular-like complexes that can host redox centers are also present in Mo containing layered Li-intercalation compounds.