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The Na_0.44MnO₂ structure is a promising cathode material for sodium ion batteries due to a high capacity (∼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 Na_0.44MnO₂, 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 Na_0.22MnO₂ 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 Na_0.44MnO₂ as a function of sodium insertion/deinsertion is shown to be due to the Jahn …