Sodium diffusion and redox properties of alluaudite Na2+2xM2−x(MoO4)3 (M = Fe, Co, Ni) from DFT+U study

Abstract

The alluaudite-type oxides have recently been proposed as perspective cathode materials for rechargeable sodium-ion batteries. Here we present ab initio insights into the sodium diffusion in alluaudite molybdates Na2+2xM2−x(MoO4)3 (M = Fe, Co, Ni), as well as their structural, electronic, and redox properties. Among the various compositions of Na2+2xM2−x(MoO4)3, the most stable configuration turned out to be Na3M1.5(MoO4)3. The sequence of sodium extraction from nonequivalent sites, vacancy formation energy and cell volume shrinkage were studied in Na3M1.5(MoO4)3. Our calculations predict the high redox voltages of 3.9, 4.6, and 4.9 V for M = Fe, Co, and Ni, respectively. Desodiation reduces the band gap in these molybdates that suggests an enhancement of electronic conductivity, which along with fast sodium diffusion and high voltage, is important for good electrochemical performance. Our results demonstrate that alluaudite molybdates have diffusion and redox properties as sulphates, and can also be promising high-voltage cathode materials for sodium-ion batteries.