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Utilization of Cobalt Bis(terpyridine) Metal Complex as Soluble Redox Mediator in Li–O2 Batteries

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journal contribution
posted on 29.06.2016 by Koffi P. C. Yao, James T. Frith, Sayed Youssef Sayed, Fanny Bardé, John R. Owen, Yang Shao-Horn, Nuria Garcia-Araez
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 a mediator of the Li–O2 charging 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 electrode, due to weakening Li+ solvation in this solvent order. 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, using differential electrochemical mass spectrometry reveals that less than 25% of the O2 consumed on discharge is recovered on charge in the presence of Co­(Terp)2, while TTF enables up to 32% O2 recovery on charge. CO2 is a significant charging product at voltages greater than 4.0 V vs Li+/Li because of electrolyte decomposition. Further work is required in order to develop mediators with 100% of oxygen evolution efficiency, and the present finding of the possibility to tune the reversible redox potential of the mediator by changing the solvent will be very useful to achieve this formidable task.