The effect of CO₂ contamination in rechargeable non-aqueous sodium-air batteries

Metal-air batteries have higher theoretical specific energies than existing rechargeable batteries including Li-ion batteries. Among metal-air batteries, the Na-O₂ battery has gained much attention due to its low discharge/charge overpotentials (∼100 mV) at relatively high current densities (0.2 mA/cm²), high electrical energy efficiency (90%), high theoretical energy density, and low cost. However, there is no information reported regarding the effect of CO₂ contamination in non-aqueous Na-air batteries. Density functional theory has, here, been applied to study the effect of low concentrations of CO₂ contamination on NaO₂ and Na₂O₂ growth/depletion reaction pathways and overpotentials. This was done on step surfaces of discharge products in non-aqueous Na-air batteries. Adsorption energies of CO₂ at various nucleation sites for both step surfaces were determined, and results revealed that CO₂ preferentially binds at the step valley sites of (001) NaO₂ and 11¯00 Na₂O₂ surfaces with binding energies of -0.65 eV and -2.67 eV, respectively. CO₂ blocks the step nucleation site and influences the reaction pathways and overpotentials due to carbonate formation. The discharge electrochemical overpotential increases remarkably from 0.14 V to 0.30 V and from 0.69 V to 1.26 V for NaO₂ and Na₂O₂ surfaces, respectively. CO₂ contamination is thus drastically impeding the growth/depletion mechanism pathways and increases the overpotentials of the surface reaction mechanism, hampering the performance of the battery. Avoiding CO₂ contamination from intake of gas and electrolyte decomposition is thus critical in development of Na-air batteries.