First-Order or Second-Order? Disproportionation of Lithium Superoxide in Li-O2 Batteries

Atomically Dispersed Ta-O-Co Sites Capable of Mitigating Side Reaction Occurrence for Stable Lithium-Oxygen Batteries

The side reactions accompanying the charging and discharging process, as well as the difficulty in decomposing the discharge product lithium peroxide, have been important issues in the research field of lithium-oxygen batteries for a long time. Here, single atom Ta supported by Co3O4 hollow sphere was designed and synthesized as a cathode catalyst. The single atom Ta forms an electron transport channel through the Ta-O-Co structure to stabilize octahedral Co sites, forming strong adsorption with reaction intermediates and ultimately forming a film-like lithium peroxide that is highly dispersed. More importantly, the formation of the Ta-O-Co structure can reduce the vacancy formation energy on the catalyst surface, accelerate oxygen activation and conversion into superoxide anions, promote the rapid conversion of strong oxidizing intermediate lithium superoxide into lithium peroxide, avoid the oxidation of lithium superoxide to the electrode and electrolyte, reduce the occurrence of side reactions, and mitigate the production of byproduct lithium carbonate. The overpotential of the battery is reduced significantly, and the reversibility and cycling stability of the battery are improved. This study provides a practical and feasible direction for mitigating the side reaction and improving the performance of the battery.

To DISP or Not? The Far-Reaching Reaction Mechanisms Underpinning Lithium-Air Batteries

The short history of research on Li-O2 batteries has seen a remarkable number of mechanistic U-turns over the years. From the initial use of carbonate electrolytes, that were then found to be entirely unsuitable, to the belief that (su)peroxide was solely responsible for degradation, before the more reactive singlet oxygen was found to form, to the hypothesis that capacity depends on a competing surface/solution mechanism before a practically exclusive solution mechanism was identified. Herein, we argue for an ever-fresh look at the reported data without bias towards supposedly established explanations. We explain how the latest findings on rate and capacity limits, as well as the origin of side reactions, are connected via the disproportionation (DISP) step in the (dis)charge mechanism. Therefrom, directions emerge for the design of electrolytes and mediators on how to suppress side reactions and to enable high rate and high reversible capacity.

New Reaction Pathway of Superoxide Disproportionation Induced by a Soluble Catalyst in Li-O2 Batteries

Aprotic Li-O2 battery has attracted considerable interest for high theoretical energy density, however the disproportionation of the intermediate of superoxide (O2 - ) during discharge and charge leads to slow reaction kinetics and large voltage hysteresis. Herein, the chemically stable ruthenium tris(bipyridine) (RB) cations are employed as a soluble catalyst to alternate the pathway of O2 - disproportionation and its kinetics in both the discharge and charge processes. RB captures O2 - dimer and promotes their intramolecular charge transfer, and it decreases the energy barrier of the disproportionation reaction from 7.70 to 0.70 kcal mol-1 . This facilitates the discharge and charge processes and simultaneously mitigates O2 - and singlet oxygen related side reactions. These endow the Li-O2 battery with reduced discharge/charge voltage gap of 0.72 V and prolonged lifespan for over 230 cycles when coupled with RuO2 catalyst. This work highlights the vital role of superoxide disproportionation for Li-O2 battery.

Metal-air batteries: progress and perspective

The metal-air batteries with the largest theoretical energy densities have been paid much more attention. However, metal-air batteries including Li-air/O₂, Li-CO₂, Na-air/O_2, and Zn-air/O₂ batteries, are complex systems that have their respective scientific problems, such as metal dendrite forming/deforming, the kinetics of redox mediators for oxygen reduction/evolution reactions, high overpotentials, desolution of CO₂, H₂O, etc. from the air and related side reactions on both anode and cathode. It should be the main direction to address these shortages to improve performance. Here, we summarized recently research progress in these metal-air/O₂ batteries. Some perspectives are also provided for these research fields.

Exclusive Solution Discharge in Li-O2 Batteries?

Capacity, rate performance, and cycle life of aprotic Li-O₂ batteries critically depend on reversible electrodeposition of Li₂O₂. Current understanding states surface-adsorbed versus solvated LiO₂ controls Li₂O₂ growth as surface film or as large particles. Herein, we show that Li₂O₂ forms across a wide range of electrolytes, carbons, and current densities as particles via solution-mediated LiO₂ disproportionation, bringing into question the prevalence of any surface growth under practical conditions. We describe a unified O₂ reduction mechanism, which can explain all found capacity relations and Li₂O₂ morphologies with exclusive solution discharge. Determining particle morphology and achievable capacities are species mobilities, true areal rate, and the degree of LiO₂ association in solution. Capacity is conclusively limited by mass transport through the tortuous Li₂O₂ rather than electron transport through a passivating Li₂O₂ film. Provided that species mobilities and surface growth are high, high capacities are also achieved with weakly solvating electrolytes, which were previously considered prototypical for low capacity via surface growth.

Suppressing Redox Shuttling with Lithiated Nafion-Modified Separators for Li-O2 Batteries

Although the employment of redox mediator (RM) is an effective strategy to reduce the overpotential by avoiding the direct electrochemical oxidization of Li₂ O₂ during charging, an unexpected redox shuttling in Li-O₂ system leads to RM degradation and continuous deterioration of Li anode, finally resulting in a limited cycling stability. Here, a functional lithiated Nafion-modified separator was firstly introduced to inhibit the shuttle effect by coulombic/electrostatic interactions in RM-involved Li-O₂ batteries. The fabrication of the separator involved easily accessible raw materials and an easy-to-operate process, which made it suitable for large-scale production. The enhancement of lithiated process on electrochemical properties was systematically investigated. In addition, the influence of decorated amount on cycling stability was also studied. Furthermore, the functional contribution of lithiated Nafion on inhibition of redox shuttling and the working mechanism for cells with and without as-prepared separators were proposed. This work can give an insight into the development of functional separator (i. e., activity issue) and the suppression of parasitic reactions (i. e., selectivity issue) in Li-O₂ batteries.