Iron chlorides are potentially some of the most inexpensive energy-dense positive electrodes for non-aqueous redox flow batteries. So far, solutions of FeCl_x in ethylene glycol have achieved concentrations of over 4 mol/liter, but experience significant capacity fade upon cycling [1,2]. We study FeCl_x solutions in carbonate solvents as positive electrolytes in non-aqueous potassium-ion redox flow batteries [3]. While FeCl_x species alone do not yield reversible cycling, the addition of formally redox-inactive AlCl₃ enables in model solutions Coulombic efficiencies over 99%, and capacity fade under 20% over 200 cycles. We further discuss the evolving speciation of Fe^2+/3+ in solution from electron spin resonance and cycling data, the role of common-ion and supporting salts, and the mechanism by which Al species enhance the cycle life of FeCl_x positive electrode solutions.

1. A. Miller, J. S. Wainriht, R. F. Savinell, Journal of the Electrochemical Society, 164 (2017) A796–A803.
2. A. Miller, J. S. Wainright, R. F. Savinell, Journal of the Electrochemical Society, 163 (2016) A578–A579.
3. Xue, H. Gao, W. Zhou, S. Xin, K. Park, Y. Li, J. B. Goodenough, Advanced Materials, 28 (2016) 9608-9612.