Anionic Oxygen Redox in the High-Lithium Material Li₈SnO₆

Abstract

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–x SnO6, 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.

Defects, diffusion and dopants in Li8SnO6

“…Local Li hops exhibit a range of activation energies of migration from 0.20 eV to 1.06 eV. In a previous DFT simulation study [27], it has been reported that hopping distances are in the range between 0.43 eV and 1.40 eV. Furthermore, the lowest activation energy of migration is found between two tetrahedral Li sites in agreement with the current simulation though there is an energy difference of 0.23 eV.…”

“…The main drawback of forming Li 2 O is capacity loss together with reduction in Coulombic efficiency [26]. Luo et al [27], performed density functional theory (DFT) simulations to show that the oxygen redox yields a high voltage plateau of over 4.0 V (vs Li/Li þ ). Furthermore, Li-ion diffuses fast in this material with an activation energy of 0.43 eV.…”

Stabilizing the Anionic Redox in 4.6 V LiCoO2 Cathode through Adjusting Oxygen Magnetic Moment

“…also reported that the shorter bond length and nonzero magnetic moments were consistent with the assignment as the O 2 − superoxide anion. [ 11 ] Therefore, the change of oxygen magnetic moment is closely related to the stability of the oxygen, especially at high voltage state. In addition, in Li‐rich cathode materials, Cho et al.…”

Iridate Li8IrO6: An Antiferromagnetic Insulator

“…Li 8 IrO 6 is a ternary iridate reported among the A–T–O (A = alkali or alkali-earth element, T = Ce, Pr, Pb, Tb, Zr, Hf, Sn, Pt, Ir, Ru, Co, Ge, Si) system. − This study is motivated by the fact that while interesting crystal structures and physical properties are reported for many noniridium Li 8 TO 6 compounds, − the physical properties of Li 8 IrO 6 are still unknown. The crystal structure of Li 8 IrO 6 has been reported with a nonpolar space group R 3̅, which is isostructural to most compounds in the Li 8 TO 6 (T = Ce, Pr, Pb, Tb, Zr, Hf, Sn, Pt, Ru) series. − However, it is different from some other Li 8 TO 6 (T = Co, Ge, Si) compounds adopting the polar space group ( P 6 3 mc ). − The Li 8 RuO 6 compound was first reported as a tetragonal or rhombohedral structure, , but it was later determined to be Li-deficient Li 7 RuO 6 in a triclinic structure (space group P 1̅), which transforms into the rhombohedral structure (space group R 3̅) at 450 °C. , Confirming the crystal structure of Li 8 IrO 6 and understanding the reason for forming a nonpolar instead of a polar crystal structure will provide insights for existing and potentially novel Li 8 TO 6 compounds, which hold potentials as high-capacity Li-battery cathode materials, − solid electrolytes, and CO 2 capture materials . Therefore, we launched an investigation to synthesize a phase-pure sample, understand the crystal and electronic structures, and determine the physical properties of Li 8 IrO 6 .…”