Surface-controlled deposition of discharge products in Na-O2 batteries using a defect-rich graphene cathode

Sodium-oxygen (Na-O2) batteries are promising high-capacity devices for future energy storage, replacing the unsustainable dependence on fossil fuels. These batteries convert molecular oxygen into sodium superoxide (NaO2) which is deposited during discharge at the cathode. It has been demonstrated that the morphology of the discharged NaO2 is critical for battery performance, as the insulating nature of these solid products leads to premature cell death by passivating the cathode surface at high discharge capacities. These constraints seriously affect the battery rechargeability by hindering the oxidation of NaO2 during charge. In this context, the size and distribution of the discharged solid particles is crucial for the implementation of these batteries. Here, we present a template-assisted electro crystallization of NaO2 in Na-O2 batteries by using a graphene cathode enriched with atomic defects. The high free energy of such atomic defects induces the nucleation of few-micron sized NaO2 cubes strategically localized at dispersed points of the surface. The high dispersion of small superoxide particles, by a surface-controlled crystallization, increases the cyclability of the battery at high discharge capacities, which is the major bottleneck in metal-air battery technology.