Ab Initio Study of the Sodium Intercalation and Intermediate Phases in Na0.44MnO2 for Sodium-Ion Battery

The Na0.44MnO2 structure is a promising cathode material for sodium ion batteries due to a high capacity (similar to 130 mAh/g) and good cycle performance. In this work, we present the results of density functional theory (DFT) calculations on the structural and electrochemical properties of Na0.44MnO2, combined with experiments. Seven intermediate phases and the two-phase reactions among them were found, where the calculated voltage profile agreed well with experiments. We found that the S-shaped tunnel is not empty in the deintercalated Na0.22MnO2 structure but has a partial occupancy of sodium ions. The new sodium sites were found in a limited sodium composition range (x = 0.44-0.55) which is attributed to the electrostatic interactions between sodium ions and manganese atoms. The asymmetric lattice evolution in Na0.44MnO2 as a function of sodium insertion/deinsertion is shown to be due to the Jahn-Teller effects. On the basis of this interpretation, we suggest that the Cr substitution will reduce the volume change significantly.