Electronic Structure of Sodium Superoxide Bulk, (100) Surface, and Clusters using Hybrid Density Functional: Relevance for Na–O₂ Batteries

Clarifying the electronic structure of sodium superoxide (NaO₂) is a key step in understanding the electrochemical behavior of Na–O₂ batteries. Here we report a density functional theory study to explore the effect of atomic structure and morphology on the electronic properties of different model systems: NaO₂ bulk, (100) surface, and small (NaO₂)_<i>n</i> clusters (<i>n</i> = 3–8). We found that a correct description of the open-shell 2p electrons of O₂^– requires the use of a hybrid functional, which reveals a clear insulating nature of all of the investigated systems. This sheds light onto the capacity limitations of pure NaO₂ as a discharge product and highlights the need for developing new strategies to enhance its electron transport in the optimization of Na–O₂ cells