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