Utilization of cobalt bis(terpyridine) metal complex as soluble redox mediator in Li-O2 batteries

Abstract

Redox mediators hold significant promise in reducing the large overpotentials pervasive upon charging of lithium-oxygen (Li-O2) cells. Cobalt bis(terpyridine) (Co(Terp)2) was investigated as mediator of the Li2O2 oxidation reaction using electrochemical, XRD and mass spectrometry measurements and benchmarked against tetrathiafulvalene (TTF). Significant reductions in reversible potential versus Li+/Li are measured for Co(Terp)2 and TTF from diglyme to Pyr14TFSI:diglyme to Pyr14TFSI, attributable to upward shift in the Li+/Li, due to weakening Li+ solvation in this solvent order. The lowering of the reversible potentials has noticeable gains on the kinetics of the charge reaction and greater reduction in charge overpotential are observed with the cobalt complex. However, probing the efficacy of the discharge and charge processes using differential electrochemical mass spectrometry reveal that Co(Terp)2 undergoes CoII to CoI reduction on cell discharge which interferes with the desired O2 reduction; furthermore less than 25% of the O2 consumed on discharge is recovered on charge. On the other hand, TTF allows the ideal 2 e-/O2 on discharge and enables up to 32% O2 recovery on charge. CO2 is a significant charging product as voltages becomes greater than 4.0 V vs. Li+/Li because of electrolyte decomposition. The methodology here developed is valuable to critically evaluate the effectiveness of redox agents in Li-O2 batteries.

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Identifiers

ORCID for John R. Owen: orcid.org/0000-0002-4938-3693

ORCID for Nuria Garcia-Araez: orcid.org/0000-0001-9095-2379

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