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 (ηdis = 0.14 V) and depletion (ηcha = 0.19 V), whereas the overpotentials for Na2O2 formation (ηdis = 0.69 V) and depletion (ηcha = 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.