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

The Na–O 2 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 2 ), whereas the high-capacity peroxide state (Na 2 O 2 ) 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 2 and Na 2 O 2, identifying NaO 2 as the thermodynamically preferred discharge product up to ∼120 K, after which Na 2 O 2 is thermodynamically preferred. We also investigate the reaction mechanisms and resulting electrochemical overpotentials on stepped surfaces of the NaO 2 and Na 2 O 2 systems, showing low overpotentials for NaO 2 formation (η dis = 0.14 V) and depletion (η cha = 0.19 V), whereas the overpotentials for Na 2 O 2 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 x O 2 species and provide a kinetic explanation for why NaO 2 is the main discharge product in Na–O 2 batteries under normal operating conditions.