Revealing the Chemical Mechanism of NaO₂ Decomposition by In Situ Raman Imaging

Sodium–oxygen (Na–O₂) batteries exhibit a low charging overpotential owing to the reversible formation and decomposition of sodium superoxide (NaO₂) on discharge and charge cycles. However, the cycling performance of the battery system is compromised by the side reactions occurring between the reactive NaO₂ discharge product with the other components of the cell including the air electrode and the organic electrolyte. In the present study, we employ a Raman imaging technique to reveal the chemical mechanism behind the decomposition reaction of NaO₂ in the presence of diglyme-based electrolyte. Our results illustrate the formation of oxalate-based side products resulting from prolonged exposure of NaO₂ to the cell electrolyte. Moreover, we show that Na₂O₂·2H₂O is not the thermodynamically favorable side product for decomposition of NaO₂ and may only be formed under the high-energy beam used by the measuring probe. The findings of this study help to better understand the underlying chemical reaction mechanisms of Na–O₂ cells

Detection of Electrochemical Reaction Products from the Sodium–Oxygen Cell with Solid-State ²³Na NMR Spectroscopy

²³Na MAS NMR spectra of sodium–oxygen (Na–O₂) cathodes reveals a combination of degradation species: newly observed sodium fluoride (NaF) and the expected sodium carbonate (Na₂CO₃), as well as the desired reaction product sodium peroxide (Na₂O₂). The initial reaction product, sodium superoxide (NaO₂), is not present in a measurable quantity in the ²³Na NMR spectra of the cycled electrodes. The reactivity of solid NaO₂ is probed further, and NaF is found to be formed through a reaction between the electrochemically generated NaO₂ and the electrode binder, polyvinylidene fluoride (PVDF). The instability of cell components in the presence of desired electrochemical reaction products is clearly problematic and bears further investigation

How to Control the Discharge Products in Na–O₂ Cells: Direct Evidence toward the Role of Functional Groups at the Air Electrode Surface

Sodium–oxygen batteries have received a significant amount of research attention as a low-overpotential alternative to lithium–oxygen. However, the critical factors governing the composition and morphology of the discharge products in Na–O₂ cells are not thoroughly understood. Here we show that oxygen containing functional groups at the air electrode surface have a substantial role in the electrochemical reaction mechanisms in Na–O₂ cells. Our results show that the presence of functional groups at the air–electrode surface conducts the growth mechanism of discharge products toward a surface-mediated mechanism, forming a conformal film of products at the electrode surface. In addition, oxygen reduction reaction at hydrophilic surfaces more likely passes through a peroxide pathway, which results in the formation of peroxide-based discharge products. Moreover, in-line X-ray diffraction combined with solid state ²³Na NMR results indicate the instability of discharge products against carbonaceous electrodes. The findings of this study help to explain the inconsistency among various reports on composition and morphology of the discharge products in Na–O₂ cells and allow the precise control over the discharge products

Toward a Sodium–“Air” Battery: Revealing the Critical Role of Humidity

Room temperature sodium–air batteries have a similar design and concept as lithium–air batteries. Using ambient air instead of pure oxygen as oxygen source is challenging because the minor components in air could lead to various side reactions and influence the electrochemical reaction route. Although water is an innegligible component in air, its impact on Li– and Na–air batteries is often underestimated. In this study, the electrochemical behavior of Na–air batteries under different relative humidity (RH) has been systemically investigated by galvanic cycling and cyclic voltammetry tests, as well as the identification of corresponding discharge products by physical characterizations such as XRD, FT-IR, and SEM. The reaction mechanisms of Na–air batteries under humid conditions are revealed and discussed. Na–air batteries suffer from more severe impact from the water content in air than Li–air batteries. NaOH and its derivatives are found to form and are proven to be fatal to the cells under humid ambience. Understanding the reaction mechanisms occurred in sodium air batteries under dry and humid ambient is critical to design and develop sodium–air batteries of high performance and long durability