Hye Ryung Byon1

1, Korea Advanced Institute of Science and Technology, Daejeon, , Korea (the Republic of)

Redox flow batteries continue to attract much attention as a scalable solution for the ever increasing demand of storing energy from naturally intermittent energy sources such as solar and wind energy farms. Organometallic complexes are attractive candidates for serving as electrolytes, as they provide a rich and versatile electronic structure that can be controlled conveniently by varying the composition of the ligands, which in principle allows for engineering effective solutions. Here we examined strategies for rationally modifying the redox properties of a common and representative Co-complex, namely Co(II) bearing bidentate or tridentate pyridyl and pyrazolyl ligands to optimize their redox behavior. We profiled galvanostatic performances of Co-complexes as catholytes and anolytes in prototype battery cells over 600 cycles and demonstrated enhanced cell voltage and stability through tuning of Co metal-ligand bonding strength and denticity. Unlike organic or single-atom systems, these complexes allow for dissipating the electronic stress associated with the excess electron by spin-crossover. In addition, the chelation effect can be used to enhance the stability of the complex: By increasing the denticity of the ligands, the number of ligands on the metal can be reduced to two from three while maintaining the core structure of the metal complex. As a consequence, the entropic penalty associated with the complexation is lowered by nearly 10 kcal/mol, contributing to the stability of the catholyte during the charge-discharge cycles. This approach allowed for identifying specific guiding principles for designing efficient and robust electrochemical materials that display promising cycling stabilities.