Probing the Chemical Action of M-N4 Motif in Metal Phthalocyanine-Based Redox Mediators in Li-O2 Batteries.

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Despite the exceptionally high theoretical specific energy of Li-O2 rechargeable batteries, their practical realization remains elusive, primarily due to the sluggish oxygen reduction/evolution kinetics and the underlying nontrivial mechanisms. Here, we systematically investigate by operando spectroscopy and DFT calculations the anchoring characteristics of various discharge (viz. metal-superoxide, metal-peroxide) and side (viz. Li2CO3, LiOH) products on the M-N4 motif of first-row transition metal phthalocyanines redox mediators (RMs) and their impact on Li-O2 battery performance. The unsaturated d-orbital and discharge products oriented between the two M-N bonds lead to stability of the Mn, Fe, and Co-based RM, low charge polarization, and superior battery performance. On the contrary, strong anchoring with the phthalocyanine ring leads to a loss of RM activity. While discharge and parasitic products coordinate more strongly with the metal center for unsaturated d-orbital RMs, for filled d-orbitals, the products coordinate with the porphyrin ring. The findings on the orientation of discharge/side products on the M-N4 motif catalyst clearly account for the polarization during the charging process. This fundamental study will aid in comprehensive molecular designs for liquid-based RM for next-generation battery systems for stationary applications.