Dopant-induced stabilization of rhombohedralLiMnO2against Jahn-Teller distortion

Abstract

Dopants introduced into layered $\mathrm{Li}\mathrm{Mn}{\mathrm{O}}_{2}$, a candidate cathode material for lithium batteries, tend to suppress the Jahn-Teller-effect-driven monoclinic distortion (symmetry $C2∕m$) in favor of the layered rhombohedral structure $(R\overline{3}m)$, which has superior Li insertion/extraction cycling properties. First-principles calculations, within the Local-Spin-Density-Approximation Generalized-Gradient-Approximation (LSDA-GGA) framework, implemented in the VASP code, are performed for $3d$ transition-metal, as well as Mg, Zn, Al, and N dopants, in order to assess their relative effectiveness in stabilizing the $R\overline{3}m$ symmetry. At the concentration $x=0.25$, the selected cation dopants (with the exception of Co and Fe) are found to adopt the same oxidation state (divalent or trivalent) in both monoclinic and rhombohedral structures. Transition metals earlier than Fe are trivalent, whereas those later than Co are divalent. Co in monoclinic $\mathrm{Li}\mathrm{Mn}{\mathrm{O}}_{2}$ exhibits (stable) trivalent and (metastable) divalent states that are only narrowly different in energy. Fe is trivalent in the monoclinic structure, but is mixed valent in the rhombohedral structure. Divalent dopants (which oxidize a neighboring Mn to the [non-Jahn-Teller-active] $4+$ oxidation state) promote rhombohedral structure stabilization to a greater extent than trivalent dopants. Within the class of divalent dopants, the filled shell systems Mg and Zn are more effective than those with partially filled ${e}_{g}$ shells. Within the class of trivalent dopants, those with partially filled ${t}_{2g}$ shells are more effective than those with filled or empty shells. Breathing mode $({Q}_{1})$ and Jahn-Teller active (${Q}_{2}$, ${Q}_{3}$) phonon coordinates of transition metal octahedra are analyzed.