Interrogation of the Reaction Mechanism in a Na-O2 Battery Using In Situ Transmission Electron Microscopy

Critical factors that govern the composition and morphology of discharge products are largely unknown for Na-O₂ batteries. Here we report a reversible oxygen reduction reaction (ORR) and oxygen evolution reaction (OER) process in a sodium-oxygen battery observed using in situ environmental-transmission electron microscopy (TEM) experiment. The reaction mechanism and phase evolution are probed using in situ electron diffraction and TEM imaging. The reversible ORR and OER cycling lies upon the nanosized copper clusters that were formed in situ by sodiation of CuS. In situ electron diffraction revealed the formation of NaO₂ initially, which then disproportionated into orthorhombic and hexagonal Na₂O₂ and O₂. Na₂O₂ was the major final ORR product that uniformly covered the whole wire-shape cathode. This uniform product morphology largely increased the application feasibility of Na-O₂ batteries in industry. In the following OER process, the Na₂O₂ transformed to NaO₂, which resulted in volume expansion at first, and then the NaO₂ decomposed to sodium ions and O₂ gas. Galvanostatic charge/discharge profiles of CuS in real Na-O₂ cells revealed a maximum capacity over 3 mAh cm^-2 with a discharge cutoff voltage of 1.8 V and high cycling stability. The nanosized copper catalyst plays a dominating role in controlling the morphology, chemical composition of discharge products, and reversibility of this Na-O₂ battery. Our finding shines light on the exploration of effective catalysts for the Na-O₂ battery.

Can Hybrid Na-Air Batteries Outperform Nonaqueous Na-O2 Batteries?

In recent years, there has been an upsurge in the study of novel and alternative energy storage devices beyond lithium-based systems due to the exponential increase in price of lithium. Sodium (Na) metal-based batteries can be a possible alternative to lithium-based batteries due to the similar electrochemical voltage of Na and Li together with the thousand times higher natural abundance of Na compared to Li. Though two different kinds of Na-O2 batteries have been studied specifically based on electrolytes until now, very recently, a hybrid Na-air cell has shown distinctive advantage over nonaqueous cell systems. Hybrid Na-air batteries provide a fundamental advantage due to the formation of highly soluble discharge product (sodium hydroxide) which leads to low overpotentials for charge and discharge processes, high electrical energy efficiency, and good cyclic stability. Herein, the current status and challenges associated with hybrid Na-air batteries are reported. Also, a brief description of nonaqueous Na-O2 batteries and its close competition with hybrid Na-air batteries are provided.

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.

Sodium Batteries: A Review on Sodium-Sulfur and Sodium-Air Batteries

Lithium-ion batteries are currently used for various applications since they are lightweight, stable, and flexible. With the increased demand for portable electronics and electric vehicles, it has become necessary to develop newer, smaller, and lighter batteries with increased cycle life, high energy density, and overall better battery performance. Since the sources of lithium are limited and also because of the high cost of the metal, it is necessary to find alternatives. Sodium batteries have shown great potential, and hence several researchers are working on improving the battery performance of the various sodium batteries. This paper is a brief review of the current research in sodium-sulfur and sodium-air batteries.

A titanium dioxide nanoparticle sandwiched separator for Na–O2 batteries with suppressed dendrites and extended cycle life

A new TiO₂ sandwiched separator is used in Na–O₂ batteries, effectively inhibiting dendrites and extending their cycle life.

High-Energy-Density Metal-Oxygen Batteries: Lithium-Oxygen Batteries vs Sodium-Oxygen Batteries

The development of next-generation energy-storage devices with high power, high energy density, and safety is critical for the success of large-scale energy-storage systems (ESSs), such as electric vehicles. Rechargeable sodium-oxygen (Na-O₂ ) batteries offer a new and promising opportunity for low-cost, high-energy-density, and relatively efficient electrochemical systems. Although the specific energy density of the Na-O₂ battery is lower than that of the lithium-oxygen (Li-O₂ ) battery, the abundance and low cost of sodium resources offer major advantages for its practical application in the near future. However, little has so far been reported regarding the cell chemistry, to explain the rate-limiting parameters and the corresponding low round-trip efficiency and cycle degradation. Consequently, an elucidation of the reaction mechanism is needed for both lithium-oxygen and sodium-oxygen cells. An in-depth understanding of the differences and similarities between Li-O₂ and Na-O₂ battery systems, in terms of thermodynamics and a structural viewpoint, will be meaningful to promote the development of advanced metal-oxygen batteries. State-of-the-art battery design principles for high-energy-density lithium-oxygen and sodium-oxygen batteries are thus reviewed in depth here. Major drawbacks, reaction mechanisms, and recent strategies to improve performance are also summarized.

Improving Na–O2 batteries with redox mediators

Enhanced capacity and decreased overpotential are achieved due to the promotion of the solution-based mechanism in mediated Na–O₂ cells.

Mechanistic origin of low polarization in aprotic Na–O2 batteries

The mechanistic difference between Li–O₂ and Na–O₂ batteries has been revealed by in situ spectroscopy coupled with theory calculations.

A highly efficient bifunctional heterogeneous catalyst for morphological control of discharged products in Na–air batteries

Heterogeneous compounds of Co₃O₄ and liquid ferrocene as an air catalyst show an excellent cycle performance of up to 570 cycles.

Sodium-Oxygen Batteries: A Comparative Review from Chemical and Electrochemical Fundamentals to Future Perspective

Alkali metal-oxygen (Li-O2 , Na-O2 ) batteries have attracted a great deal of attention recently due to their high theoretical energy densities, comparable to gasoline, making them attractive candidates for application in electrical vehicles. However, the limited cycling life and low energy efficiency (high charging overpotential) of these cells hinder their commercialization. The Li-O2 battery system has been extensively studied in this regard during the past decade. Compared to the numerous reports of Li-O2 batteries, the research on Na-O2 batteries is still in its infancy. Although, Na-O2 batteries show a number of attractive properties such as low charging overpotential and high round-trip energy efficiency, their cycling life is currently limited to a few tens of cycles. Therefore, understanding the chemistry behind Na-O2 cells is critical towards enhancing their performance and advancing their development. Chemical and electrochemical reactions of Na-O2 batteries are reviewed and compared with those of Li-O2 batteries in the present review, as well as recent works on the chemical composition and morphology of the discharge products in these batteries. Furthermore, the determining kinetics factors for controlling the chemical composition of the discharge products in Na-O2 cells are discussed and the potential research directions toward improving Na-O2 cells are proposed.

Visualizing Current-Dependent Morphology and Distribution of Discharge Products in Sodium-Oxygen Battery Cathodes

Synchrotron X-ray tomography and scanning electron microscopy were applied to elucidate the spatial distribution of discharge product (NaO2) in the carbon cathode of sodium-oxygen batteries. Various batteries were discharged galvanostatically and their cathodes were analyzed. We observe a particle density gradient along the cathode that scales with the current density applied. Besides, we show that the particle size and shape of discharge product strongly depend on current density, and on whether the particles are deposited close to the oxygen reservoir or near the separator. We correlate our findings to transport limitations for the supplied oxygen and gain crucial information for optimal operation of sodium-oxygen batteries. Our findings imply that for low current densities pore clogging might occur, and that for elevated current densities small high surface area particles with limited electric conductivity form; both phenomena can decrease the available discharge and charge capacity significantly.

Sodium-Oxygen Battery: Steps Toward Reality

Rechargeable metal-oxygen batteries are receiving significant interest as a possible alternative to current state of the art lithium ion batteries due to their potential to provide higher gravimetric energies, giving significantly lighter or longer-lasting batteries. Recent advances suggest that the Na-O2 battery, in many ways analogous to Li-O2 yet based on the reversible formation of sodium superoxide (NaO2), has many advantages such as a low charge overpotential (∼100 mV) resulting in improved efficiency. In this Perspective, we discuss the current state of knowledge in Na-O2 battery technology, with an emphasis on the latest experimental studies, as well as theoretical models. We offer special focus on the principle outstanding challenges and issues and address the advantages/disadvantages of the technology when compared with Li-O2 batteries as well as other state-of-the-art battery technologies. We finish by detailing the direction required to make Na-O2 batteries both commercially and technologically viable.