Anionic Oxygen Redox in the High-Lithium Material Li₈SnO₆

Lithiated transition metal oxide cathode materials that combine oxygen-anion redox [O2-/(O2)n-] with cation redox offer substantial capacities and higher voltage than cathodes without oxygen redox, but oxygen-anion redox is usually accompanied by the formation of peroxo or superoxo species that may cause oxygen release. To clarify the relationship between the oxygen release and peroxide and superoxide dimers without the complication of transition-metal redox, we performed density functional theory calculations to study the layered Li8SnO6 cathode because it has only oxygen-anion redox during delithiation. Features with O-O distances of ∼1.5 Å (corresponding to the formation of a peroxide) were observed at 75% lithium concentration, and further delithiation to 62.5% lithium concentration shortened the O-O bond distance to ∼1.3 Å (corresponding to the formation of superoxide). The solely anionic oxygen redox stabilizes the delithiated structure of Li8-xSnO6, and it does not cause structural disorder or release oxygen gas. Ab initio molecular dynamics calculations in the supercell at 300 K indicate that oxygen dimerization may occur at 75% lithium concentration. On the (0001) surface with a high lithium concentration (98%), oxygen dimerization is unfavorable. The calculations by HSE06 show that oxygen redox in Li8SnO6 offers an initial voltage (vs Li/Li+) greater than 4.2 V. The GGA+U calculations indicate that the Li+-polaron in Li95Sn12O72 migrates with a migration barrier of only 0.43 eV. This work also quantifies a solely anionic oxygen redox mechanism in Li8SnO6, and it provides a deeper understanding of anionic oxygen redox in Li-excess oxide cathode materials.