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

Abstract

The Na0.44MnO2 structure is a promising cathode material for sodium ion batteries since it shows several outstanding properties: (1) It has a capacity of 120~130 mAh/g in experiments which is one of the highest capacity among the existing sodium ion batteries. (2) The nanosized Na0.44MnO2 shows a good cycle performance with 77% capacity retention after 1000 cycles. (3) It is applicable in aqueous electrolyte which means that a substantial reduction of cost is additionally possible. In this work, we present the results of density functional theory (DFT) calculations on the structural and electrochemical properties of Na0.44MnO2 that are not easily addressable in experiments. Seven intermediate phases and the two-phase reactions among them were found, where the calculated voltage profile agreed well with experiments. The new sodium sites were found in a limited sodium composition range (x=0.44 ~0.55) which is attributed to the electrostatic interations between sodium ions and manganese atoms. The asymmetric lattice evolution in Na0.44MnO2 as a function of sodium insertion and deinsertion is shown to be due to the Jahn-Teller effects. Based on this interpretation, we suggest that the Cr substitution will reduce the volume change significantly.