Real-Time Imaging of the Electrochemical Process in Na-O2 Nanobatteries Using Pt@CNT and Pt0.8Ir0.2@CNT Air Cathodes

Compared to lithium-oxygen batteries, sodium-oxygen (Na-O₂) batteries exhibit a number of advantages: extremely low cost, low charging overpotential, and stability under nitrogen. However, accumulation of insoluble discharge products and failure of catalysts often result in poor performance of Na-O₂ batteries and limit their cycling life. In this work, electrochemical reactions of Na-O₂ batteries were directly investigated in situ by assembling a solid-state Na-O₂ nanobattery in an aberration-corrected environmental transmission electron microscope. During discharge, NaO₂ hollow spheres formed and expanded continuously, accompanying their partial decomposition into Na₂O₂. These spheres shrank and collapsed into Na₂O₂ nanoparticles during the charging process. Carbon nanotubes doped with Pt and bimetallic Pt/Ir nanoscale catalyst can promote product formation and reversible evolution. In-depth investigation of the electrochemical reaction mechanism in Na-O₂ cells helps to accelerate the development of metal-air devices.

On the Stability of NaO2 in Na-O2 Batteries

Na-O₂ batteries are regarded as promising candidates for energy storage. They have higher energy efficiency, rate capability, and chemical reversibility than Li-O₂ 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 NaO₂ in the cell environment. This fundamental insight brings new information on the working mechanism of Na-O₂ batteries.

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)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.