Prediction of Quaternary Spinel Oxides as Li-Battery Cathodes: Cation Site Preference, Metal Mixing, Voltage and Phase Stability

Abstract

Using first-principles calculations, we explore the quaternary spinel oxides of the 3d transition metal series (Ti, V, Cr, Mn, Fe, Co, Ni) having pairs of transition metals in equal concentrations. We examine several properties of these materials relevant to Li-ion battery cathode applications. By examining cation site preference, delithiation voltage and thermodynamic stability, we predict five new quaternary spinel compounds which show promise as novel cathode materials: LiTiFeO4, LiCrFeO4, LiMnFeO4 and LiFeCoO4 and LiCoNiO4. The predicted compounds have tetrahedral Li/vacancy sublattice for efficient Li-transport, average voltage greater than 4.4 V, and are predicted to be stable ground states on a T = 0 K computed phase diagram. These predictions should be tested experimentally by synthesis and electrochemical measurements of these five compounds. For the lithiated oxides, we find predominant preference for the normal spinel structure (with Li in tetrahedrally coordinated sites), and for most of the delithiated oxides, transition metal cations spontaneously relax to non-spinel octahedral sites. We also find that metal mixing in these spinels is a strong function of charge state: most of the pairs of ternary transition metal oxide spinels tend to mix when lithiated and phase separate when delithiated.