Abstract
Climate change is a major global concern which can potentially be addressed by integrating renewable energy sources and low cost grid scale electrochemical energy storage (EES) systems. Most research efforts to improve energy storage have been directed to redox flow, lithium-ion and next generation sodium and multivalent batteries. The focus has been on incremental improvements in cost, and efficiency of electrode active materials and electrolytes. Most of these batteries still employ traditional cell designs with expensive passive components. Unfortunately, this approach results in the balance-of-plant costs far outweighing the cost of electrochemically active materials. Techno-economic analyses indicate that the cost of energy storage must be <$100/kWh and power cost <$600/kW. Commercially available EES systems today do not meet these targets. In addition, battery life and energy efficiencies must be over 10,000 cycles and 70%, respectively, to be a sustainable option. Based on these targets, a more holistic approach is needed when considering new energy storage system materials and designs.
Here, we discuss the design and working principle of a membrane free, non-flowing single-chamber zinc-bromine (SC-Zn-Br2) battery that utilizes the physical properties of liquid bromine, a porous carbon foam electrode, and also allows zinc dendrites to form freely. With this design, we eliminate the need to use failure prone and expensive passive components such as membrane separators, bromine complexing agents, flow-rated hardware, and control software as employed in traditional Zn-Br2 redox flow batteries (RFB). The ultimate figure of merit for feasibility in grid-scale EES system is $/kWh over its lifetime (cycles) at a given energy efficiency, or levelized cost of energy stored (LCOES). We demonstrate an SC-Zn-Br2 system with a maximum specific capacity of 89 mAh/g and energy density >150 Wh/kg (normalized to the mass of ZnBr2 salt in the electrolyte) and cell cost of $94/kWh including passive components. We achieve coulombic and energy efficiencies of up to 95% and 75%, respectively, for over 1000 cycles. The LCOES of SC-Zn-Br2 is shown to be an order of magnitude better than Zn-Br2 and vanadium RFB, as well as grid-scale lead-acid batteries and Li-ion batteries.