Ni-free high-entropy rock salt oxides with Li superionic conductivity

Abstract

High-entropy oxides with rock salt structure represent an attractive class of materials with intriguing functional properties. Nevertheless, some elements in their composition, like Ni, represent a relevant threat to human health including also possible mutagenic effects. Herein, we develop the first Ni-free high entropy rock salt by replacing nickel with manganese. (Mg, Co, Mn, Cu, Zn)O powders were produced by co-precipitation and doped with different amounts of Li₂O. Lithium stabilizes the rock salt, causing a substantial reduction of secondary phases, like Mn-rich spinel that forms during the thermal treatment. Samples with high density and good phase purity can be successfully obtained by annealing at 1200 °C for 0.5 h, longer treatments leading to Li sublimation and formation of secondary phases. We show that the Mn substitution is effective in preserving the electrical properties of the high entropy solution. The materials present fast Li-ion conduction with conductivity above 10⁻³ S cm⁻¹ and stable properties up to 80 °C.