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)n clusters (n = 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.
Keywords: Na-air battery; NaO2; density functional theory; electronic properties of NaO2; energy storage.