Computational insights into the effect of carbon structures at the atomic level for non-aqueous sodium-oxygen batteries

Carbon materials have been widely used to form air cathodes for non-aqueous sodium-oxygen (Na-O₂) batteries due to their large specific surface area, high conductivity and low cost. However, the effect of carbon structures at the atomic level remains poorly understood. In this work, a first-principles study is conducted to investigate how representative carbon structures, including graphite (0001) surface, point defects and fractured edge, influence the discharge and charge processes of non-aqueous Na-O₂ batteries. It is found that the single vacancy (SV) defect has the largest adsorption energy (5.81 eV) to NaO₂ molecule among the structures studied, even larger than that of the NaO₂ molecule on NaO₂ crystal (2.81 eV). Such high adsorption energy is attributed to two factors: the dangling atoms in SV defects decrease the distance from NaO₂ molecules, and the attachment through oxygen atoms increases the electrons transfer. The findings suggest that SV defects can act as the nucleation sites for NaO₂ in the discharge process, and increasing the number of SV defects can facilitate the uniform formation of small-sized particles. The uniformly distributed discharge products lower the possibility for pore clogging, leading to an increased discharge capacity and improved cyclability for non-aqueous Na-O₂ batteries.