Non-aqueous metal-oxygen batteries depend critically on the reversible formation/decomposition of metal oxides on cycling. Irreversible parasitic reactions cause poor rechargeability, efficiency, and cycle life and have predominantly been ascribed to the reactivity of reduced oxygen species with cell components. These species, however, cannot fully explain the side reactions. Here we show that singlet oxygen forms at the cathode of a lithium-oxygen cell during discharge and from the onset of charge, and accounts for the majority of parasitic reaction products. The amount increases during discharge, early stages of charge, and charging at higher voltages, and is enhanced by the presence of trace water. Superoxide and peroxide appear to be involved in singlet oxygen generation. Singlet oxygen traps and quenchers can reduce parasitic reactions effectively. Awareness of the highly reactive singlet oxygen in non-aqueous metal-oxygen batteries gives a rationale for future research towards achieving highly reversible cell operation.

Inverse electron demand Diels-Alder reactions (iEDDA) between 1,2,4,5-tetrazines and olefins have emerged into a state-of-the art concept for the conjugation of biomolecules. Now, this reaction is also increasingly being applied in polymer science and materials science. The orthogonality of this exciting reaction to other well-established click chemistry schemes, its high reaction speed and its biocompatibility are key features of iEDDA making it a powerful alternative to existing ligation chemistries. The intention of this tutorial review is to introduce the reader to the fundamentals of inverse electron demand Diels-Alder additions and to answer the question whether iEDDA chemistry is living up to the criteria for a ''click'' reaction and can serve as a basis for future applications in postsynthetic modification of materials. Key learning points(1) Electron-rich dienophiles and electron-poor dienes (e.g. tetrazines) react in an inverse electron demand Diels-Alder reaction (iEDDA).(2) iEDDA fulfils Sharpless' criteria for a ''click'' reaction while offering advantages over already established ''click'' chemistry schemes.

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Summary: The success of metathesis chemistry techniques has sparked a tremendous interest in polymer and material chemistry. This contribution provides an overview of the state of the art in ring opening metathesis polymerisation (ROMP). It is intended to provide the reader with useful information on the interplay of initiators, monomers, and reaction conditions, thus aiding polymer chemists to utilise the ROMP toolbox. Prominent and illustrative examples from current research are given in the article.The “ROMP toolbox”.magnified imageThe “ROMP toolbox”.

Aprotic alkali metal-oxygen batteries require reversible formation of metal superoxide or peroxide on cycling. Severe parasitic reactions cause poor rechargeability, efficiency, and cycle life and have been shown to be caused by singlet oxygen ( 1 O 2 ) that forms at all stages of cycling. However, its formation mechanism remains unclear. We show that disproportionation of superoxide, the product or intermediate on discharge and charge, to peroxide and oxygen is responsible for 1 O 2 formation. While the overall reaction is driven by the stability of peroxide and thus favored by stronger Lewis acidic cations such as Li + , the 1 O 2 fraction is enhanced by weak Lewis acids such as organic cations. Concurrently, the metal peroxide yield drops with increasing 1 O 2 . The results explain a major parasitic pathway during cell cycling and the growing severity in K-, Na-, and Li-O 2 cells based on the growing propensity for disproportionation. High capacities and rates with peroxides are now realized to require solution processes, which form peroxide or release O 2 via disproportionation. The results therefore establish the central dilemma that disproportionation is required for high capacity but also responsible for irreversible reactions. Highly reversible cell operation requires hence finding reaction routes that avoid disproportionation. Broader contextDecarbonizing the energy system requires energy storage with large capacity but equally low economic and ecological footprint. Alkali metal-O 2 batteries are considered outstanding candidates in this respect. However, they suffer from poor cycle life as a result of cathode degradation. Formation of the highly reactive singlet oxygen has been proposed to cause this degradation, but formation mechanisms have remained unclear. Here, we show that the singlet oxygen source is the disproportionation of thermodynamically unstable superoxide intermediates to the peroxides. The revealed mechanism conclusively explains the strongly growing degree of degradation when going from K-O 2 to Na-O 2 and Li-O 2 cells. A major consequence is that highly reversible cell operation of Li-O 2 and Na-O 2 cells requires them to form and decompose the peroxides without disproportionation. Achieving this requires finding new reaction routes. The work lays the mechanistic foundation to fight singlet oxygen as the predominant source of degradation in metal-O 2 cells.