Identifying Reactive Sites and Transport Limitations of Oxygen Reactions in Aprotic Lithium-O2 Batteries at the Stage of Sudden Death

Surface-Enhanced Raman Spectroscopy and Electrochemistry: The Ultimate Chemical Sensing and Manipulation Combination

One of the lessons we learned from the COVID-19 pandemic is that the need for ultrasensitive detection systems is now more critical than ever. While sensors' sensitivity, portability, selectivity, and low cost are crucial, new ways to couple synergistic methods enable the highest performance levels. This review article critically discusses the synergetic combinations of optical and electrochemical methods. We also discuss three key application fields-energy, biomedicine, and environment. Finally, we selected the most promising approaches and examples, the open challenges in sensing, and ways to overcome them. We expect this work to set a clear reference for developing and understanding strategies, pros and cons of different combinations of electrochemical and optical sensors integrated into a single device.

K-O2 electrochemistry at the Au/DMSO interface probed by in situ spectroscopy and theoretical calculations

The reaction mechanism underpinning the operation of K-O2 batteries, particularly the O2 reactions at the positive electrode, is still not completely understood. In this work, by combining in situ Raman spectroelectrochemistry and density functional theory calculations, we report on a fundamental study of K-O2 electrochemistry at a model interface of Au electrode/DMSO electrolyte. The key products and intermediates (O2-, KO2 and K2O2) are identified and their dependency on the electrode potential is revealed. At high potentials, the first reduction intermediate of O2-* radical anions (* denotes the adsorbed state) can desorb from the Au electrode surface and combine with K+ cations in the electrolyte producing KO2via a solution-mediated pathway. At low potentials, O2 can be directly reduced to on the Au electrode surface, which can be further reduced to at extremely low potentials. The fact that K2O2 has only been detected in the very high overpotential regime indicates a lack of KO2 disproportionation reaction both on the Au electrode surface and in the electrolyte solution. This work addresses the fundamental mechanism and origin of the high reversibility of the aprotic K-O2 batteries.

Utilizing a Photocatalysis Process to Achieve a Cathode with Low Charging Overpotential and High Cycling Durability for a Li-O2 Battery

The practical applications of non-aqueous lithium-oxygen batteries are impeded by large overpotentials and unsatisfactory cycling durability. Reported here is that commonly encountered fatal problems can be efficiently solved by using a carbon- and binder-free electrode of titanium coated with TiO₂ nanotube arrays (TNAs) and gold nanoparticles (AuNPs). Ultraviolet irradiation of the TNAs generates positively charged holes, which efficiently decompose Li₂ O₂ and Li₂ CO₃ during recharging, thereby reducing the overpotential to one that is near the equilibrium potential for Li₂ O₂ formation. The AuNPs promote Li₂ O₂ formation, resulting in a large discharge capacity. The electrode exhibits excellent stability with about 100 % coulombic efficiency during continuous cycling of up to 200 cycles, which is due to the carbon- and binder-free composition. This work reveals a new strategy towards the development of highly efficient oxygen electrode materials for lithium-oxygen batteries.

High-Capacity and Stable Li-O2 Batteries Enabled by a Trifunctional Soluble Redox Mediator

Li-O₂ batteries with ultrahigh theoretical energy densities usually suffer from low practical discharge capacities and inferior cycling stability owing to the cathode passivation caused by insulating discharge products and by-products. Here, a trifunctional ether-based redox mediator, 2,5-di-tert-butyl-1,4-dimethoxybenzene (DBDMB), is introduced into the electrolyte to capture reactive O₂ ^- and alleviate the rigorous oxidative environment of Li-O₂ batteries. Thanks to the strong solvation effect of DBDMB towards Li⁺ and O₂ ^- , it not only reduces the formation of by-products (a high Li₂ O₂ yield of 96.6 %), but also promotes the solution growth of large-sized Li₂ O₂ particles, avoiding the passivation of cathode as well as enabling a large discharge capacity. Moreover, DBDMB makes the oxidization of Li₂ O₂ and the decomposition of main by-products (Li₂ CO₃ and LiOH) proceed in a highly effective manner, prolonging the stability of Li-O₂ batteries (243 cycles at 1000 mAh g^-1 and 1000 mA g^-1 ).

Highly Reversible Cuprous Mediated Cathode Chemistry for Magnesium Batteries

Sluggish kinetics and poor reversibility of cathode chemistry is the major challenge for magnesium batteries to achieve high volumetric capacity. Introduction of the cuprous ion (Cu⁺ ) as a charge carrier can decouple the magnesiation related energy storage from the cathode electrochemistry. Cu⁺ is generated from a fast equilibrium between copper selenide electrode and Mg electrolyte during standing time, rather than in the electrochemical process. A reversible chemical magnesiation/de-magnesiation can be driven by this solid/liquid equilibrium. During a typical discharge process, Cu⁺ is reduced to Cu and drives the equilibrium to promote the magnesiation process. The reversible Cu to Cu⁺ redox promotes the recharge process. This novel Cu⁺ mediated cathode chemistry of Mg battery leads to a high reversible areal capacity of 12.5 mAh cm^-2 with high mass loading (49.1 mg cm^-2 ) of the electrode. 80 % capacity retention can be achieved for 200 cycles after a conditioning process.

DABCOnium: An Efficient and High-Voltage Stable Singlet Oxygen Quencher for Metal-O2 Cells

Singlet oxygen (1 O2 ) causes a major fraction of the parasitic chemistry during the cycling of non-aqueous alkali metal-O2 batteries and also contributes to interfacial reactivity of transition-metal oxide intercalation compounds. We introduce DABCOnium, the mono alkylated form of 1,4-diazabicyclo[2.2.2]octane (DABCO), as an efficient 1 O2 quencher with an unusually high oxidative stability of ca. 4.2 V vs. Li/Li+ . Previous quenchers are strongly Lewis basic amines with too low oxidative stability. DABCOnium is an ionic liquid, non-volatile, highly soluble in the electrolyte, stable against superoxide and peroxide, and compatible with lithium metal. The electrochemical stability covers the required range for metal-O2 batteries and greatly reduces 1 O2 related parasitic chemistry as demonstrated for the Li-O2 cell.

Isotopic Labeling Reveals Active Reaction Interfaces for Electrochemical Oxidation of Lithium Peroxide

The unresolved debate on the active reaction interface of electrochemical oxidation of lithium peroxide (Li₂ O₂ ) prevents rational electrode and catalyst design for lithium-oxygen (Li-O₂ ) batteries. The reaction interface is studied by using isotope-labeling techniques combined with time-of-flight secondary ion mass spectrometry (ToF-SIMS) and on-line electrochemical mass spectroscopy (OEMS) under practical cell operation conditions. Isotopically labelled microsized Li₂ O₂ particles with an Li₂ ¹⁶ O₂ /electrode interface and an Li₂ ¹⁸ O₂ /electrolyte interface were fabricated. Upon oxidation, ¹⁸ O₂ was evolved for the first quarter of the charge capacity followed by ¹⁶ O₂ . These observations unambiguously demonstrate that oxygen loss starts from the Li₂ O₂ /electrolyte interface instead of the Li₂ O₂ /electrode interface. The Li₂ O₂ particles are in continuous contact with the catalyst/electrode, explaining why the solid catalyst is effective in oxidizing solid Li₂ O₂ without losing contact.

High-Capacity and High-Rate Discharging of a Coenzyme Q10 -Catalyzed Li-O2 Battery

Discharging of the aprotic Li-O₂ battery relies on O₂ reduction to insulating solid Li₂ O₂ , which can either deposit as thin films on the cathode surface or precipitate as large particles in the electrolyte solution. Toward realizing Li-O₂ batteries with high capacity and high rate capability, it is crucially important to discharge Li₂ O₂ in the electrolyte solution rather than on the cathode surface. Here, a soluble electrocatalyst of coenzyme Q₁₀ (CoQ₁₀ ) that can efficaciously drive solution phase formation of Li₂ O₂ in current benchmark ether-based Li-O₂ batteries is reported, which would otherwise lead to Li₂ O₂ surface-film growth and premature cell death. In the range of current densities of 0.1-0.5 mA cm^-2_areal , the CoQ₁₀ -catalyzed Li-O₂ battery can deliver a discharge capacity that is ≈40-100 times what the pristine Li-O₂ battery could achieve. The drastically enhanced electrochemical performance is attributed to the CoQ₁₀ that not only efficiently mediates the electron transfer from the cathode to dissolve O₂ but also strongly interacts with the newly formed Li₂ O₂ in solution retarding its precipitation on the cathode surface. The mediated oxygen reduction reaction and the bonding mechanism between CoQ₁₀ and Li₂ O₂ are understood with density functional theory calculations.