First-Principles Study of the Reaction Mechanism in
Sodium–Oxygen Batteries
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Abstract
Li/O<sub>2</sub> battery has the
highest theoretical energy density
among any battery systems reported to date. However, its poor cycle
life and unacceptable energy efficiency from a high charging overpotential
have been major limitations. Recently, much higher energy efficiency
with low overpotential was reported for a new metal/oxygen system,
Na/O<sub>2</sub> battery. This finding was unexpected since the general
battery mechanism of the Na/O<sub>2</sub> system was assumed to be
analogous to that of the Li/O<sub>2</sub> cell. Furthermore, it implies
that fundamentally different kinetics are at work in the two systems.
Here, we investigated the reaction mechanisms in the Na/O<sub>2</sub> cell using first-principles calculations. In comparative study with
the Li/O<sub>2</sub> cell, we constructed the phase stability maps
of the reaction products of Na/O<sub>2</sub> and Li/O<sub>2</sub> batteries
based on the oxygen partial pressure, which explained why certain
phases should be the main discharge products under different operating
conditions. From surface calculations of NaO<sub>2</sub>, Na<sub>2</sub>O<sub>2</sub>, and Li<sub>2</sub>O<sub>2</sub> during the oxygen
evolution reaction, we also found that the minimum energy barrier
for the NaO<sub>2</sub> decomposition was substantially lower than
that of Li<sub>2</sub>O<sub>2</sub> decomposition on major surfaces
providing a hint for low charging overpotential of Na/O<sub>2</sub> battery