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

The Na-O system holds great potential as a low-cost, high-energy-density battery, but under normal operating conditions, the discharge is limited to sodium superoxide (NaO), whereas the high-capacity peroxide state (NaO) 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 NaO and NaO, identifying NaO as the thermodynamically preferred discharge product up to ∼120 K, after which NaO is thermodynamically preferred. We also investigate the reaction mechanisms and resulting electrochemical overpotentials on stepped surfaces of the NaO and NaO systems, showing low overpotentials for NaO formation (η = 0.14 V) and depletion (η = 0.19 V), whereas the overpotentials for NaO formation (η = 0.69 V) and depletion (η = 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 O species and provide a kinetic explanation for why NaO is the main discharge product in Na-O batteries under normal operating conditions.