Surface Enrichment of Redox Mediator for Long-Cyclable Lithium–Air Batteries

Three cobalt porphyrine derivatives, cobalt tetramethoxyphenyl porphyrin (CoTMPP), cobalt mesotetraphenylporphyrin (CoTPP), and cobalt octaethyl-porphyrin (CoOEP), were used as redox mediators (RMs) in the Li–air battery. Through cyclic voltammetry tests and calculation with Randles-Ševčı́k and Brown-Anson equations, it was found that the redox reactions of RMs are controlled by adsorption on the cathode surface rather than diffusion, and the side substituent groups exert critical influences on the RM adsorbability and cell cyclability. Owing to its highest adsorbability as well as its capability to decompose Li2CO3, CoTMPP enables a Li–air cell to operate 200 cycles under 100 mA g–1 carbon and 500 mAh g–1 carbon even without solid-state electrolyte membrane to suppress RM shuttling and deactivation.