Ab initio insights into Na-ion diffusion and intercalation mechanism in alluaudite NaxMn2(MoO4)3 as cathode material for sodium-ion batteries

Abstract

Alluaudite-type compounds have been recently proposed as new promising cathode materials for sodium batteries due to their high operating voltage, high capacity and good cyclability. In this work, we present the GGA and GGA + U studies of sodium diffusion and (de)intercalation mechanism in alluaudite NaxMn2(MoO4)3, as well as its electronic structure and magnetic properties. We predict that, unlike the known alluaudite sulphates, the Na-ion migration in this molybdate should occur not only through one-dimensional channels along the c axis, but also due to their cross-linking, which is responsible for two-dimensional diffusion. These cross-channel sodium hops with the low-energy barriers may reduce the negative influence of defects and improve rate. They can also provide additional mobile ions to the main channels and play an important role in (de)intercalation. The charging process involves the Na+ extraction in a specific sequence from different Na sites, which is accompanied by a Mn3+/Mn2+ redox reaction and has an average redox potential of 3.89 V. A large volume shrinkage during the last desodiation stages narrows the Na migration channels that impedes the diffusion and removal of all Na from NaxMn2(MoO4)3.