First-Principles Study of the Reaction Mechanism in Sodium–Oxygen Batteries

Li/O₂ 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/O₂ battery. This finding was unexpected since the general battery mechanism of the Na/O₂ system was assumed to be analogous to that of the Li/O₂ cell. Furthermore, it implies that fundamentally different kinetics are at work in the two systems. Here, we investigated the reaction mechanisms in the Na/O₂ cell using first-principles calculations. In comparative study with the Li/O₂ cell, we constructed the phase stability maps of the reaction products of Na/O₂ and Li/O₂ 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 NaO₂, Na₂O₂, and Li₂O₂ during the oxygen evolution reaction, we also found that the minimum energy barrier for the NaO₂ decomposition was substantially lower than that of Li₂O₂ decomposition on major surfaces providing a hint for low charging overpotential of Na/O₂ battery.