Exploiting Zinc Dendrite Formation and Bromine Transport Properties to Build a Safer, More Reliable Zn-Br2 Battery
Authors: Hodson Thomas 
Published: February 2020

In order to address inefficiencies that exist in our electrical grid, low cost grid scale electrochemical energy storage (EES) systems, such as zinc bromine batteries, are being researched extensively. Zinc bromine batteries, however, have issues at both the anode and the cathode that affect the reliability of the cell as well as present hazardous conditions of operation. At the anode, current density localization from the inhomogeneity of the zinc surface leads to the formation of dendritic structures. These dendritic structures can grow to the point of touching the other electrode, causing shorting of the cell. At the cathode, bromine contamination into the electrolyte can lead to issues of bromine corrosion at the anode as well as safety concerns of corrosive, vapor phase bromine escaping from the system.
Standard Zn-Br2 flow-cell designs alleviate these limitations with bromine-complexing agents to improve Br2(l) solubility, separation membranes to prevent crossover and shorting, and flowing electrolyte to remove bromine and to minimize dendrite formation. These approaches are implemented at the expense of cell resistance, performance efficiency, system size, and especially capital costs. However, if constructed properly, the cell design could be used to address the issues at the anode and cathode without the added cost and complexity of complexing agents, membranes, or flow systems. Here, we show a simple and scalable, low-cost, membrane-free, non-flowing single-chamber zinc-bromine (SC-Zn-Br2) secondary battery design that utilizes the physical properties of liquid bromine and a highly-porous carbon foam electrode, and also allows zinc dendrites to form freely. We demonstrate the local containment of Br2 in the carbon foam electrode, and discuss a color tracking and feedback monitoring scheme to actively control the reactive species transport and improve reliability.
To take advantage of the density and low miscibility of Br2, an anodic carbon current collector is placed on top of the cell. During charging, metallic Zn gets plated onto the carbon cloth electrode, while Br2(l) is generated at a carbon foam cathode. If dendrites form and creep towards this carbon foam electrode (CFE), Zn(s) will react with Br2(l) on the surface of the foam and dissolve back into the electrolyte as Zn2+ and Br- ions. This spontaneous process can prevent short circuiting, as the Zn(s) + Br2(l) reaction occurs faster than zinc deposition at low charging currents. Thus, the Br2(l) loosely held on the CFE surface acts as a natural protection from shorting, and can be exploited for ‘self-maintenance’, removing the need for separation membranes or protective coatings.
We also take advantage of the distinct colors of the ZnBr2(aq) electrolyte, Br2(l), and dissolved Br2(aq): clear, red, and yellow, respectively. By calibrating the solution color with Br2(l,aq) concentration, we can track the generation, consumption, and transport of bromine in real time to improve battery performance and prevent unwanted processes (i.e., crossover). In addition, we can use this color tracking technique to prevent bromine escape from the system and thus alleivate safety concerns of operating with voliltite bromine. To optimize performance, a feedback loop is written into the cycling control algorithm to stop the charge step at this point. This optical visualization, tracking, and in operando feedback technique has never been used for any bromine-based electrochemical cells, to the best of our knowledge.