Transition Metal Phthalocyanines as Redox Mediators in Li-O 2 Batteries: A Combined Experimental and Theoretical Study of the Influence of 3d Electrons in Redox Mediation.

Redox mediation is an innovative strategy for ensuring efficient energy harvesting from metal-oxygen systems. This work presents a systematic exploratory analysis of first-row transition-metal phthalocyanines as solution-state redox mediators for lithium-oxygen batteries. Our findings, based on experiment and theory, convincingly demonstrate that d 5 (Mn), d 7 (Co), and d 8 (Ni) configurations function better compared to d 6 (Fe) and d 9 (Cu) in redox mediation of the discharge step. The d 10 configuration (Zn) and non-d analogues (Mg) do not show any redox mediation because of the inability of binding with oxygen. The solution-state discharge product, transition-metal bound Li 2 O 2 , undergoes dissociation and oxidation in the charging step of the battery, thus confirming a bifunctional redox mediation. Apart from the reaction pathways predicted based on thermodynamic considerations, density functional theory calculations also reveal interesting effects of electrochemical perturbation on the redox mediation mechanisms and the role of the transition-metal center.