Sodium-ion diffusion studies of the cathode–electrolyte interfaces (NaxO2@Na2CO3, x=1 and 2) and discharge products of non-aqueous rechargeable sodium–air batteries

Abstract

Sodium–air batteries have superior theoretical specific energy to existing secondary batteries, even compared to state-of-the-art secondary lithium-ion batteries (LIBs). They exhibit less than 200 mV discharge and recharge overpotentials and above 90% electrical energy efficiency drawn at fairly excessive current densities. However, like lithium–air batteries, sodium–air batteries also suffer from poor rechargeability, low capacity, and dendrite formation. In this study, charge transport (ionic conductivity) studies of the cathode–electrolyte interfaces (Na_xO₂@Na₂CO₃, x = 1 and 2) and discharge products (NaO₂, Na₂O₂, and Na₂CO₃) of non-aqueous secondary sodium–air/O₂ batteries are presented using the density functional theory framework. The results revealed that the sodium-ion diffusion is taking place too rapidly with an activation barrier of less than half an eV despite the bandgaps of these materials exceeding 4 eV. The ionic conductivity study is mediated by negative sodium vacancies (V_Na⁻). Interfaces revealed magnificent ionic conduction; the sodium-ion diffuses with little or no activation or thermodynamic barrier. Thus, the interfaces may provide an alternative pathway for rapid ion transfer during the battery operation, which might enhance the conductivity and eventually boost the accessible capacity and performances of secondary non-aqueous sodium–air batteries.