2, KAIST Institute NanoCentrury, Daejeon, Yuseong-gu, Korea (the Republic of)
3, Byon Initiative Research Unit (IRU), Wako, Hirosawa, Japan
Metal-oxygen batteries have been paid attention to one of the promising next-generation batteries owing to their higher energy densities than current lithium-ion batteries. In particular, sodium-oxygen (Na-O2) batteries are a promising candidate applied for electric vehicles, because they provide far higher round-trip efficiency than lithium-oxygen (Li-O2) batteries. The critical problems in Li-O2 cells typically arise from the positive electrode side where O2 gas is injected. The oxygen reduction reaction occurring in discharging produces the superoxide (O2–) species, which is chemically reactive and is susceptible to degradation on non-aqueous electrolyte solution and carbonaceous electrode in the presence of Li+ ion. By contrast, the O2– may be more stable with Na+ ion caused by favorable soft base and soft acid interaction. The main discharge product of sodium superoxide (NaO2) and insignificant amount of side product in Na-O2 cells demonstrate highly stable superoxide species at the positive electrode side. Nevertheless, cycling stability has still underperformed in Na-O2 cells, which have not as yet been fully understood.
Here we improve cycling performance of Na-O2 cells by increasing stability of negative electrode, Na metal. Na metal is highly reactive and easily forms solid electrolyte interphase (SEI) layer through chemical reduction of electrolyte solution. The sodium triflate (NaOTf) salt in diglyme (G2) has been widely used as the electrolyte solution in Na-O2 cells, while dead Na powders and electrical short have been often observed after a couple of cycles due to instability of SEI and negative electrode. In addition, the observations of color change of electrolyte solution and severe precipitates that cover the positive carbon electrode at even open circuit voltage (OCV) state allow us to hypothesize that soluble SEI causes continuous decomposition of electrolyte solution from Na metal. To mitigate this problem and establish stable SEI layer protecting Na metal, we have examined various sodium salts to Na-O2 cells. Among them, sodium hexafluorophosphate (NaPF6)/G2 exhibits the preeminent results such as negligible color change of electrolyte solution and insignificant precipitates on positive electrode surface. It indicates the formation of highly stable and uniform SEI layer, and X-ray photoelectron spectroscopy (XPS) reveals the main species of layer to sodium fluoride (NaF). The stable SEI layer improves cycling performance in Na-O2 cells. The capacity retention with NaPF6/G2 is superior to others, which is attributed to trivial dead Na and dendrite at separator after 10 cycles. In the presentation, I will discuss detailed chemical and electrochemical analyses of SEI layers with different electrolytes and the correlated cycling performance in Na-O2 cells using online-electrochemical mass spectroscopy and XPS analysis.
 Nature Materials, 2013, 12, 228–232
 Phys. Chem. Chem. Phys., 2014, 16, 15646-15652