Direct Observations of the Formation and Redox-Mediator-Assisted Decomposition of Li₂O₂ in a Liquid-Cell Li–O₂ Microbattery by Scanning Transmission Electron Microscopy

Operando scanning transmission electron microscopy observations of cathodic reactions in a liquid-cell Li–O₂ microbattery in the presence of the redox mediator tetrathiafulvalene (TTF) in 1.0 m LiClO₄ dissolved dimethyl sulfoxide electrolyte are reported. It is found that the TTF addition does not obviously affect the discharge reaction for the formation of a solid Li₂O₂ phase. The coarsening of Li₂O₂ nanoparticles occurs via both conventional Ostwald ripening and nonclassical crystallization by particle attachment. During charging, the oxidation reaction at significantly reduced charge potentials mainly takes place at Li₂O₂/electrolyte interfaces and has obvious correspondence with the oxidized TTF⁺ distributions in the electric fields of the charged electrode. This study provides direct evidence that TTF truly plays a role in promoting the decomposition of Li₂O₂ as a soluble charge-transfer agent between the electrode and the Li₂O₂.