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

Li/O 2 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 2 battery. This finding was unexpected since the general battery mechanism of the Na/O 2 system was assumed to be analogous to that of the Li/O 2 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 2 cell using first-principles calculations. In comparative study with the Li/O 2 cell, we constructed the phase stability maps of the reaction products of Na/O 2 and Li/O 2 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 2, Na 2 O 2, and Li 2 O 2 during the oxygen evolution reaction, we also found that the minimum energy barrier for the NaO 2 decomposition was substantially lower than that of Li 2 O 2 decomposition on major surfaces providing a hint for low charging overpotential of Na/O 2 battery.