Thermodynamic and Kinetic Limitations for Peroxide and Superoxide Formation in Na-O₂ Batteries

The Na-O₂ system holds great potential as a low-cost, high-energy-density battery, but under normal operating conditions, the discharge is limited to sodium superoxide (NaO₂), whereas the high-capacity peroxide state (Na₂O₂) remains elusive. Here, we apply density functional theory calculations with an improved error-correction scheme to determine equilibrium potentials and free energies as a function of temperature for the different phases of NaO₂ and Na₂O₂, identifying NaO₂ as the thermodynamically preferred discharge product up to ∼120 K, after which Na₂O₂ is thermodynamically preferred. We also investigate the reaction mechanisms and resulting electrochemical overpotentials on stepped surfaces of the NaO₂ and Na₂O₂ systems, showing low overpotentials for NaO₂ formation (η_dis = 0.14 V) and depletion (η_cha = 0.19 V), whereas the overpotentials for Na₂O₂ formation (η_dis = 0.69 V) and depletion (η_cha = 0.68 V) are found to be prohibitively high. These findings are in good agreement with experimental data on the thermodynamic properties of the Na _xO₂ species and provide a kinetic explanation for why NaO₂ is the main discharge product in Na-O₂ batteries under normal operating conditions.