On the Stability of NaO₂ in Na–O₂ Batteries

Abstract

Na-O batteries are regarded as promising candidates for energy storage. They have higher energy efficiency, rate capability, and chemical reversibility than Li-O batteries; in addition, sodium is cheaper and more abundant compared to lithium. However, inconsistent observations and instability of discharge products have inhibited the understanding of the working mechanism of this technology. In this work, we have investigated a number of factors that influence the stability of the discharge products. By means of in operando powder X-ray diffraction study, the influence of oxygen, sodium anode, salt, solvent, and carbon cathode were investigated. The Na metal anode and an ether-based solvent are the main factors that lead to the instability and decomposition of NaO in the cell environment. This fundamental insight brings new information on the working mechanism of Na-O batteries.

Recent Development of Aprotic Na−O2 Batteries

“…Moreover, when discharged electrode containing NaO 2 is exposed to flowing O 2 for 1 hour, Na 2 O 2 ⋅ 2H 2 O immediately appears as identified by XRD. In comparation, NaO 2 that does not experience flowing O 2 can be stored in a vacuum pouch for 110 days with a very slow decomposition rate, and no Na 2 O 2 ⋅ 2H 2 O is detected . Stainless steel and glass testing setup generate NaO 2 and Na 2 O 2 ⋅ 2H 2 O as the major discharge product, respectively.…”

“…Recent research indicates that Na 2 O 2 ⋅ 2H 2 O is possibly originated from NaO 2 , which is initially generated in ether‐based Na−O 2 batteries and undergoes decomposition to Na 2 O 2 ⋅ 2H 2 O. As shown in Figure , the key factor that drives the conversion of NaO 2 into Na 2 O 2 ⋅ 2H 2 O is water, which can be introduced from feeding gas, electrolyte, and leakage of the test setup and characterization holder .…”

“…Glymes with different chain length, including DME, DEGDME, triethylene glycol dimethyl ether (TRIDME), TEGDME and polyethylene glycol dimethyl ether (PEGDME) are extensively investigated in Na−O 2 batteries . The solvent‐solute association among different glymes can lead to distinguish battery performances.…”

Alkali‐Oxygen Batteries Based on Reversible Superoxide Chemistry

“…Additionally, the decomposition of NaO 2 was accompanied by an unknown phase, which can transform into Na 2 O 2 ·2H 2 Oa fter prolonged exposure to solvent or moisture. [38] In another in operando study employing electrochemical STEM, Lutz et al observed the formation and dissolution of the NaO 2 product. They observed that the NaO 2 cubes formed are more complex,c ontaining shells of different composition surrounding the NaO 2 ,a nd that both the reduction and oxidation of the NaO 2 occurs through as olution-mediated process.…”

“…They found that the decomposition rate is over 40 times slower in the absence of Na metal. Additionally, the decomposition of NaO 2 was accompanied by an unknown phase, which can transform into Na 2 O 2 ⋅ 2 H 2 O after prolonged exposure to solvent or moisture …”