Singlet oxygen formation in non-aqueous oxygen redox chemistry: direct spectroscopic evidence for formation pathways and reliability of chemical probes.

Singlet oxygen (1O2) formation is now recognised as a key aspect of non-aqueous oxygen redox chemistry. For identifying 1O2, chemical trapping via 9,10-dimethylanthracene (DMA) to form the endoperoxide (DMA-O2) has become the main method due to its sensitivity, selectivity, and ease of use. While DMA has been shown to be selective for 1O2, rather than forming DMA-O2 with a wide variety of potentially reactive O-containing species, false positives might hypothetically be obtained in the presence of previously overlooked species. Here, we first provide unequivocal direct spectroscopic proof via the 1O2-specific near-infrared (NIR) emission at 1270 nm for the previously proposed 1O2 formation pathways, which centre around superoxide disproportionation. We then show that peroxocarbonates, common intermediates in metal-O2 and metal carbonate electrochemistry, do not produce false-positive DMA-O2. Moreover, we identify a previously unreported 1O2-forming pathway through the reaction of CO2 with superoxide. Overall, we provide unequivocal proof for 1O2 formation in non-aqueous oxygen redox chemistry and show that chemical trapping with DMA is a reliable method to assess 1O2 formation.

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.

On the nanoscale structural evolution of solid discharge products in lithium-sulfur batteries using operando scattering.

The inadequate understanding of the mechanisms that reversibly convert molecular sulfur (S) into lithium sulfide (Li2S) via soluble polysulfides (PSs) formation impedes the development of high-performance lithium-sulfur (Li-S) batteries with non-aqueous electrolyte solutions. Here, we use operando small and wide angle X-ray scattering and operando small angle neutron scattering (SANS) measurements to track the nucleation, growth and dissolution of solid deposits from atomic to sub-micron scales during real-time Li-S cell operation. In particular, stochastic modelling based on the SANS data allows quantifying the nanoscale phase evolution during battery cycling. We show that next to nano-crystalline Li2S the deposit comprises solid short-chain PSs particles. The analysis of the experimental data suggests that initially, Li2S2 precipitates from the solution and then is partially converted via solid-state electroreduction to Li2S. We further demonstrate that mass transport, rather than electron transport through a thin passivating film, limits the discharge capacity and rate performance in Li-S cells.

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.

In situ small-angle X-ray scattering reveals solution phase discharge of Li-O2 batteries with weakly solvating electrolytes.

Electrodepositing insulating lithium peroxide (Li2O2) is the key process during discharge of aprotic Li-O2 batteries and determines rate, capacity, and reversibility. Current understanding states that the partition between surface adsorbed and dissolved lithium superoxide governs whether Li2O2 grows as a conformal surface film or larger particles, leading to low or high capacities, respectively. However, better understanding governing factors for Li2O2 packing density and capacity requires structural sensitive in situ metrologies. Here, we establish in situ small- and wide-angle X-ray scattering (SAXS/WAXS) as a suitable method to record the Li2O2 phase evolution with atomic to submicrometer resolution during cycling a custom-built in situ Li-O2 cell. Combined with sophisticated data analysis, SAXS allows retrieving rich quantitative structural information from complex multiphase systems. Surprisingly, we find that features are absent that would point at a Li2O2 surface film formed via two consecutive electron transfers, even in poorly solvating electrolytes thought to be prototypical for surface growth. All scattering data can be modeled by stacks of thin Li2O2 platelets potentially forming large toroidal particles. Li2O2 solution growth is further justified by rotating ring-disk electrode measurements and electron microscopy. Higher discharge overpotentials lead to smaller Li2O2 particles, but there is no transition to an electronically passivating, conformal Li2O2 coating. Hence, mass transport of reactive species rather than electronic transport through a Li2O2 film limits the discharge capacity. Provided that species mobilities and carbon surface areas are high, this allows for high discharge capacities even in weakly solvating electrolytes. The currently accepted Li-O2 reaction mechanism ought to be reconsidered.

Mechanism of mediated alkali peroxide oxidation and triplet versus singlet oxygen formation.

Aprotic alkali metal-O2 batteries face two major obstacles to their chemistry occurring efficiently, the insulating nature of the formed alkali superoxides/peroxides and parasitic reactions that are caused by the highly reactive singlet oxygen (1O2). Redox mediators are recognized to be key for improving rechargeability. However, it is unclear how they affect 1O2 formation, which hinders strategies for their improvement. Here we clarify the mechanism of mediated peroxide and superoxide oxidation and thus explain how redox mediators either enhance or suppress 1O2 formation. We show that charging commences with peroxide oxidation to a superoxide intermediate and that redox potentials above ~3.5 V versus Li/Li+ drive 1O2 evolution from superoxide oxidation, while disproportionation always generates some 1O2. We find that 1O2 suppression requires oxidation to be faster than the generation of 1O2 from disproportionation. Oxidation rates decrease with growing driving force following Marcus inverted-region behaviour, establishing a region of maximum rate.

Author Correction: Persistent and reversible solid iodine electrodeposition in nanoporous carbons.

An amendment to this paper has been published and can be accessed via a link at the top of the paper.

Persistent and reversible solid iodine electrodeposition in nanoporous carbons.

Aqueous iodine based electrochemical energy storage is considered a potential candidate to improve sustainability and performance of current battery and supercapacitor technology. It harnesses the redox activity of iodide, iodine, and polyiodide species in the confined geometry of nanoporous carbon electrodes. However, current descriptions of the electrochemical reaction mechanism to interconvert these species are elusive. Here we show that electrochemical oxidation of iodide in nanoporous carbons forms persistent solid iodine deposits. Confinement slows down dissolution into triiodide and pentaiodide, responsible for otherwise significant self-discharge via shuttling. The main tools for these insights are in situ Raman spectroscopy and in situ small and wide-angle X-ray scattering (in situ SAXS/WAXS). In situ Raman confirms the reversible formation of triiodide and pentaiodide. In situ SAXS/WAXS indicates remarkable amounts of solid iodine deposited in the carbon nanopores. Combined with stochastic modeling, in situ SAXS allows quantifying the solid iodine volume fraction and visualizing the iodine structure on 3D lattice models at the sub-nanometer scale. Based on the derived mechanism, we demonstrate strategies for improved iodine pore filling capacity and prevention of self-discharge, applicable to hybrid supercapacitors and batteries.

2-Methoxyhydroquinone from Vanillin for Aqueous Redox-Flow Batteries.

We show the synthesis of a redox-active quinone, 2-methoxy-1,4-hydroquinone (MHQ), from a bio-based feedstock and its suitability as electrolyte in aqueous redox flow batteries. We identified semiquinone intermediates at insufficiently low pH and quinoid radicals as responsible for decomposition of MHQ under electrochemical conditions. Both can be avoided and/or stabilized, respectively, using H3 PO4 electrolyte, allowing for reversible cycling in a redox flow battery for hundreds of cycles.

Competitive Salt Precipitation/Dissolution During Free-Water Reduction in Water-in-Salt Electrolyte.

Water-in-salt electrolytes based on highly concentrated bis(trifluoromethyl)sulfonimide (TFSI) promise aqueous electrolytes with stabilities nearing 3 V. However, especially with an electrode approaching the cathodic (reductive) stability, cycling stability is insufficient. While stability critically relies on a solid electrolyte interphase (SEI), the mechanism behind the cathodic stability limit remains unclear. Now, two distinct reduction potentials are revealed for the chemical environments of free and bound water and that both contribute to SEI formation. Free water is reduced about 1 V above bound water in a hydrogen evolution reaction (HER) and is responsible for SEI formation via reactive intermediates of the HER; concurrent LiTFSI precipitation/dissolution establishes a dynamic interface. The free-water population emerges, therefore, as the handle to extend the cathodic limit of aqueous electrolytes and the battery cycling stability.

Lithium-Oxygen Batteries and Related Systems: Potential, Status, and Future.

The goal of limiting global warming to 1.5 °C requires a drastic reduction in CO2 emissions across many sectors of the world economy. Batteries are vital to this endeavor, whether used in electric vehicles, to store renewable electricity, or in aviation. Present lithium-ion technologies are preparing the public for this inevitable change, but their maximum theoretical specific capacity presents a limitation. Their high cost is another concern for commercial viability. Metal-air batteries have the highest theoretical energy density of all possible secondary battery technologies and could yield step changes in energy storage, if their practical difficulties could be overcome. The scope of this review is to provide an objective, comprehensive, and authoritative assessment of the intensive work invested in nonaqueous rechargeable metal-air batteries over the past few years, which identified the key problems and guides directions to solve them. We focus primarily on the challenges and outlook for Li-O2 cells but include Na-O2, K-O2, and Mg-O2 cells for comparison. Our review highlights the interdisciplinary nature of this field that involves a combination of materials chemistry, electrochemistry, computation, microscopy, spectroscopy, and surface science. The mechanisms of O2 reduction and evolution are considered in the light of recent findings, along with developments in positive and negative electrodes, electrolytes, electrocatalysis on surfaces and in solution, and the degradative effect of singlet oxygen, which is typically formed in Li-O2 cells.

Deactivation of redox mediators in lithium-oxygen batteries by singlet oxygen.

Non-aqueous lithium-oxygen batteries cycle by forming lithium peroxide during discharge and oxidizing it during recharge. The significant problem of oxidizing the solid insulating lithium peroxide can greatly be facilitated by incorporating redox mediators that shuttle electron-holes between the porous substrate and lithium peroxide. Redox mediator stability is thus key for energy efficiency, reversibility, and cycle life. However, the gradual deactivation of redox mediators during repeated cycling has not conclusively been explained. Here, we show that organic redox mediators are predominantly decomposed by singlet oxygen that forms during cycling. Their reaction with superoxide, previously assumed to mainly trigger their degradation, peroxide, and dioxygen, is orders of magnitude slower in comparison. The reduced form of the mediator is markedly more reactive towards singlet oxygen than the oxidized form, from which we derive reaction mechanisms supported by density functional theory calculations. Redox mediators must thus be designed for stability against singlet oxygen.

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.

Inter-Backbone Charge Transfer as Prerequisite for Long-Range Conductivity in Perylene Bisimide Hydrogels.

Hydrogelation, the self-assembly of molecules into soft, water-loaded networks, is one way to bridge the structural gap between single molecules and functional materials. The potential of hydrogels, such as those based on perylene bisimides, lies in their chemical, physical, optical, and electronic properties, which are governed by the supramolecular structure of the gel. However, the structural motifs and their precise role for long-range conductivity are yet to be explored. Here, we present a comprehensive structural picture of a perylene bisimide hydrogel, suggesting that its long-range conductivity is limited by charge transfer between electronic backbones. We reveal nanocrystalline ribbon-like structures as the electronic and structural backbone units between which charge transfer is mediated by polar solvent bridges. We exemplify this effect with sensing, where exposure to polar vapor enhances conductivity by 5 orders of magnitude, emphasizing the crucial role of the interplay between structural motif and surrounding medium for the rational design of devices based on nanocrystalline hydrogels.

Electrochemical Oxidation of Lithium Carbonate Generates Singlet Oxygen.

Solid alkali metal carbonates are universal passivation layer components of intercalation battery materials and common side products in metal-O2 batteries, and are believed to form and decompose reversibly in metal-O2 /CO2 cells. In these cathodes, Li2 CO3 decomposes to CO2 when exposed to potentials above 3.8 V vs. Li/Li+ . However, O2 evolution, as would be expected according to the decomposition reaction 2 Li2 CO3 →4 Li+ +4 e- +2 CO2 +O2 , is not detected. O atoms are thus unaccounted for, which was previously ascribed to unidentified parasitic reactions. Here, we show that highly reactive singlet oxygen (1 O2 ) forms upon oxidizing Li2 CO3 in an aprotic electrolyte and therefore does not evolve as O2 . These results have substantial implications for the long-term cyclability of batteries: they underpin the importance of avoiding 1 O2 in metal-O2 batteries, question the possibility of a reversible metal-O2 /CO2 battery based on a carbonate discharge product, and help explain the interfacial reactivity of transition-metal cathodes with residual Li2 CO3 .

Mechanism and performance of lithium-oxygen batteries - a perspective.

Rechargeable Li-O2 batteries have amongst the highest formal energy and could store significantly more energy than other rechargeable batteries in practice if at least a large part of their promise could be realized. Realization, however, still faces many challenges than can only be overcome by fundamental understanding of the processes taking place. Here, we review recent advances in understanding the chemistry of the Li-O2 cathode and provide a perspective on dominant research needs. We put particular emphasis on issues that are often grossly misunderstood: realistic performance metrics and their reporting as well as identifying reversibility and quantitative measures to do so. Parasitic reactions are the prime obstacle for reversible cell operation and have recently been identified to be predominantly caused by singlet oxygen and not by reduced oxygen species as thought before. We discuss the far reaching implications of this finding on electrolyte and cathode stability, electrocatalysis, and future research needs.