LAUSR

DOI: 10.1002/aenm.201702897 easy to scale in principle.[1] When operated in a flow-through mode, soluble redox couples dissolved in liquid media and stored in large reservoirs can be supplied to the electrochemical cell on demand.[2] Unfortunately, currently available technologies suffer from low energy densities severely restricting their scope of utilization. The poor energy density is attributed to moderate solubility and low potential difference of the redox couples.[3] Higher energy densities may be achieved by adopting tunable design strategies based upon organic or organometallic molecules. The output voltage may be increased by designing catholytes with highly positive redox potentials, while employing molecules with highly negative redox potentials as anolytes. Organometallic complexes represent an underutilized, but potentially powerful foundation.[4] Due to the d-electrons in the frontier orbital space, they can engage in redox events without changing the structure or reactivity notably, in stark contrast to typical organic molecules. First-row metals can access high or low-spin (LS) configurations, providing an additional way for modulating the redox behavior over a wide range.[5] To highlight how transition metals may be used as electrolytes, we examined strategies for rationally modifying the redox properties of a representative Co-complex, namely Redox-active organometallic molecules offer a promising avenue for increasing the energy density and cycling stability of redox flow batteries. The molecular properties change dramatically as the ligands are functionalized and these variations allow for improving the solubility and controlling the redox potentials to optimize their performance when used as electrolytes. Unfortunately, it has been difficult to predict and design the stability of redoxactive molecules to enhance cyclability in a rational manner, in part because the relationship between electronic structure and redox behavior has been neither fully understood nor systematically explored. In this work, rational strategies for exploiting two common principles in organometallic chemistry for enhancing the robustness of pseudo-octahedral cobalt–polypyridyl complexes are developed. Namely, the spin-crossover between low and highspin states and the chelation effect emerging from replacing three bidentate ligands with two tridentate analogues. Quantum chemical models are used to conceptualize the approach and make predictions that are tested against experiments by preparing prototype Co-complexes and profiling them as catholytes and anolytes. In good agreement with the conceptual predictions, very stable cycling performance over 600 cycles is found. Batteries