Formation of Nanosized Defective Lithium Peroxides through Si-Coated Carbon Nanotube Cathodes for High Energy Efficiency Li-O2 Batteries

Reversible Discharge Products in Li-Air Batteries

Lithium-air (Li-air) batteries stand out among the post-Li-ion batteries due to their high energy density, which has rapidly progressed in the past years. Regarding the fundamental mechanism of Li-air batteries that discharge products produced and decomposed during charging and recharging progress, the reversibility of products closely affects the battery performance. Along with the upsurge of the mainstream discharge products lithium peroxide, with devoted efforts to screening electrolytes, constructing high-efficiency cathodes, and optimizing anodes, much progress is made in the fundamental understanding and performance. However, the limited advancement is insufficient. In this case, the investigations of other discharge products, including lithium hydroxide, lithium superoxide, lithium oxide, and lithium carbonate, emerge and bring breakthroughs for the Li-air battery technologies. To deepen the understanding of the electrochemical reactions and conversions of discharge products in the battery, recent advances in the various discharge products, mainly focusing on the growth and decomposition mechanisms and the determining factors are systematically reviewed. The perspectives for Li-air batteries on the fundamental development of discharge products and future applications are also provided.

Tuning the oxygen vacancy of mixed multiple oxidation states nanowires for improving Li-air battery performance

Mixed multiple oxidation states CoMoO₄ nanowires (electrocatalysts) with tunable intrinsic oxygen vacancies were fabricated. CoMoO₄ with proper oxygen vacancy can be employed to construct a Li-air battery with a high capacity and stable cyclability. This is possible because CoMoO₄ contains surface oxygen vacancies, which result in the unit of CoMo bond, that is important for electrocatalysts used in Li-air batteries. Both the experimental and theoretical results demonstrate that the surface oxygen vacancies containing CoMoO₄ nanowires have a higher electrocatalytic activity. This shows that the highly efficient electrocatalysts used for Li-air batteries were designed to modify the redox properties of the mixed metal oxide in the catalytic active sites. This successful material design led to an improved strategy for high oxygen reduction reaction (ORR) and oxygen evolution reaction (OER) activities based on the fast formation and extinction of ORR products.

Li2 O2 Formation Electrochemistry and Its Influence on Oxygen Reduction/Evolution Reaction Kinetics in Aprotic Li-O2 Batteries

Aprotic Li-O₂ batteries are regarded as the most promising technology to resolve the energy crisis in the near future because of its high theoretical specific energy. The key electrochemistry of a nonaqueous Li-O₂ battery highly relies on the formation of Li₂ O₂ during discharge and its reversible decomposition during charge. The properties of Li₂ O₂ and its formation mechanisms are of high significance in influencing the battery performance. This review article demonstrates the latest progress in understanding the Li₂ O₂ electrochemistry and the recent advances in regulating the Li₂ O₂ growth pathway. The first part of this review elaborates the Li₂ O₂ formation mechanism and its relationship with the oxygen reduction reaction/oxygen evolution reaction electrochemistry. The following part discusses how the cycling parameters, e.g., current density and discharge depth, influence the Li₂ O₂ morphology. A comprehensive summary of recent strategies in tailoring Li₂ O₂ formation including rational design of cathode structure, certain catalyst, and surface engineering is demonstrated. The influence resulted from the electrolyte, e.g., salt, solvent, and some additives on Li₂ O₂ growth pathway, is finally discussed. Further prospects of the ways in making advanced Li-O₂ batteries by control of favorable Li₂ O₂ formation are highlighted, which are valuable for practical construction of aprotic lithium-oxygen batteries.

Composite Cathode Architecture with Improved Oxidation Kinetics in Polymer-Based Li-O2 Batteries

The Li-O₂ battery based on the polymer electrolyte has been considered as the feasible solution to the safety issue derived from the liquid electrolyte. However, the practical application of the polymer electrolyte-based Li-O₂ battery is impeded by the poor cyclability and unsatisfactory energy efficiency caused by the structure of the porous cathode. Herein, an architecture of a composite cathode with improved oxidation kinetics of discharge products was designed by an in situ method through the polymerization of the electrolyte precursor for the polymer-based Li-O₂ battery. The composite cathode can provide sufficient gas diffusion channels, abundant reaction active sites, and continuous pathways for ion diffusion and electron transport. Furthermore, the oxidation kinetics of nanosized discharge products formed in the composite cathode can be improved by hexamethylphosphoramide during the recharge process. The polymer-based Li-O₂ batteries with the composite cathode demonstrate highly reversible capacity when fully charged and a long cycle lifetime under a fixed capacity with low overpotentials. Moreover, the interface contact between hexamethylphosphoramide and the Li metal can be stabilized simultaneously. Therefore, the composite cathode architecture designed in this work shows a promising application in high-performance polymer-based Li-O₂ batteries.

Cathode Local Curvature Affects Lithium Peroxide Growth in Li-O2 Batteries

The growth of lithium peroxide (Li₂O₂) in cathodes determines the performance of lithium-oxygen batteries (LOBs). The factor affecting the Li₂O₂ growth position is of great importance. Here, three hollow carbon spheres with diameters of 200 nm, 500 nm, and 2 μm, corresponding to different curvatures of 10, 4, and 1, respectively, are prepared as LOB cathodes. It is found that the larger the curvature, the more difficult it is for Li₂O₂ to grow inside the hollow sphere. Increasing the discharge current density can promote the growth of Li₂O₂ onto a highly curved concave substrate. Therefore, to maximize the battery performance, the applied current density and the local curvature of the porous cathode need to match to optimize the pore space utilization and meanwhile to enhance the interface charge transfer between Li₂O₂ and electrode. The revealed relationship among the local curvature of the porous electrode, Li₂O₂ deposition position, and battery performance is valuable to the topography design of the LOB cathode.

Recent Progress on Catalysts for the Positive Electrode of Aprotic Lithium-Oxygen Batteries †

Rechargeable aprotic lithium-oxygen (Li-O2) batteries have attracted significant interest in recent years owing to their ultrahigh theoretical capacity, low cost, and environmental friendliness. However, the further development of Li-O2 batteries is hindered by some ineluctable issues, such as severe parasitic reactions, low energy efficiency, poor rate capability, short cycling life and potential safety hazards, which mainly stem from the high charging overpotential in the positive electrode side. Thus, it is of great significance to develop high-performance catalysts for the positive electrode in order to address these issues and to boost the commercialization of Li-O2 batteries. In this review, three main categories of catalyst for the positive electrode of Li-O2 batteries, including carbon materials, noble metals and their oxides, and transition metals and their oxides, are systematically summarized and discussed. We not only focus on the electrochemical performance of batteries, but also pay more attention to understanding the catalytic mechanism of these catalysts for the positive electrode. In closing, opportunities for the design of better catalysts for the positive electrode of high-performance Li-O2 batteries are discussed.

Easily Decomposed Discharge Products Induced by Cathode Construction for Highly Energy-Efficient Lithium-Oxygen Batteries

The lithium-oxygen (Li-O₂) battery is deemed as a promising candidate for the next-generation of energy storage system due to its ultrahigh theoretical energy density. However, low energy efficiency and inferior cycle stability induced by the sluggish kinetics of charge transfer in discharge products limit its further development in practical application. In this work, tin dioxide (SnO₂) nanoparticles decorated carbon nanotubes (SnO₂/CNTs) have been constructed as composite cathodes to manipulate the morphology and component of discharge products in Li-O₂ batteries. Owing to the strong oxygen adsorption of SnO₂, oxygen-reduction reactions tend to occur on composite cathode surfaces, resulting in the formation of flake-like discharge products of Li_{2- x}O₂ less than 10 nm in thickness rather than toroidal particles of several hundred nanometers. Such homogeneous nanosized discharge products with lithium vacancies markedly enhance the electrode kinetics and charge transfer in discharge products. Consequently, the Li-O₂ batteries based on the SnO₂/CNT cathodes show a small polarization voltage gap, which leads to superior energy efficiency (80%) compared with that based on pristine CNT cathodes. The results demonstrate that optimizing the discharge products with nanosized morphology and defective component by cathode construction is an effective strategy to realize the Li-O₂ batteries with increased energy efficiency and improved cycle stability.