The feasibility of zinc-iron flow batteries using mixed metal ions in mildly acidic chloride electrolytes was investigated. Iron electrodeposition is strongly inhibited in the presence of Zn2+ and so the deposition and stripping processes at the negative electrode approximate those of normal zinc electrodes. In addition, the zinc ions have no significant effect on the Fe(II/III) couple at the positive electrode. This enables the use of mixed Zn-Fe electrolytes and microporous separators in place of expensive ion-exchange membranes. Considering the low-cost materials and simple design, zinc-iron chloride flow batteries represent a promising new approach in grid-scale energy storage. The preferential deposition of zinc occurs with similar behavior on titanium, graphite and glassy carbon substrates. A proof-of-concept zinc-iron chloride battery starting with mixed electrolytes was demonstrated and maintains a consistent open-circuit voltage of about 1.5 V and stable performance during over 10 days and 100 cycles of continuous charge-discharge cycling. (C) The Author(s) 2017. Published by ECS.