Ab Initio Study of the
Sodium Intercalation and Intermediate
Phases in Na<sub>0.44</sub>MnO<sub>2</sub> for Sodium-Ion Battery
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Abstract
The Na<sub>0.44</sub>MnO<sub>2</sub> 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<sub>0.44</sub>MnO<sub>2</sub>, 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<sub>0.22</sub>MnO<sub>2</sub> structure
but has a partial occupancy of sodium ions. The new sodium sites were
found in a limited sodium composition range (<i>x</i> =
0.44–0.55) which is attributed to the electrostatic interactions
between sodium ions and manganese atoms. The asymmetric lattice evolution
in Na<sub>0.44</sub>MnO<sub>2</sub> 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