No abstract

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.

In situ small-angle X-ray scattering gives new insights into global ion exchange as well as into local re-arrangement of ions across the nanopores of activated carbon supercapacitors.

The historical development of lithium metal batteries is briefly introduced.• General strategies for protection of Li metal anodes are reviewed. • Specific challenges of ASSBs, Li-S and Li-air batteries are extensively discussed.• Current development status is reviewed and compared to the EU SET Plan targets.