A zinc–bromine battery is a rechargeable battery system that uses the reaction between zinc metal and bromine to produce electric current, with an electrolyte composed of an aqueous solution of zinc bromide. Zinc has long been used as the negative electrode of primary cells. It is a widely available, relatively inexpensive metal. It is rather stable in contact with neutral and alkaline aqueous solutions. For this reason it is used today in zinc–carbon and alkaline primaries.
The leading potential application is stationary energy storage, either for the grid, or for domestic or stand-alone power systems. The aqueous electrolyte makes the system less prone to overheating and fire compared with lithium-ion battery systems.
Overview
Zinc–bromine batteries can be split into two groups: flow batteries and non-flow batteries.
Redflow (Australia) and Primus Power (US) are active in commercialising flow batteries, while Gelion (Australia) and EOS Energy Enterprises (US) are developing and commercialising non-flow systems.
Features
Zinc–bromine batteries share six advantages over lithium-ion storage systems:
100% depth of discharge capability on a daily basis. Little capacity degradation, enabling 5000+ cycles Low fire risk, since the electrolytes are non-flammable No need for cooling systems Low-cost and readily available battery materials Easy end-of-life recycling using existing processes
They share four disadvantages: Lower energy density Lower round-trip efficiency (partially offset by the energy needed to run cooling systems). The need to be fully discharged every few days to prevent zinc dendrites, which can puncture the separator. Lower charge and discharge rates
These features make zinc–bromine batteries unsuitable for many mobile applications (that typically require high charge/discharge rates and low weight), but suitable for stationary energy storage applications such as daily cycling to support solar power generation, off-grid systems, and load shifting.
Types
Flow
The zinc–bromine flow battery (ZBRFB) is a hybrid flow battery. A solution of zinc bromide is stored in two tanks. When the battery is charged or discharged, the solutions (electrolytes) are pumped through a reactor stack from one tank to the other. One tank is used to store the electrolyte for positive electrode reactions, and the other stores the negative. Energy densities range between 60 and 85 W·h/kg.
The aqueous electrolyte is composed of zinc bromide salt dissolved in water. During charge, metallic zinc is plated from the electrolyte solution onto the negative electrode (carbon felt in older designs, titanium mesh in modern) surfaces in the cell stacks. Bromide is converted to bromine at the positive electrode surface and stored in a safe, chemically complexed organic phase. Older ZBRFB cells used polymer membranes (microporous polymers, Nafion, etc.) More recent designs eliminate the membrane. The battery stack is typically made of carbon-filled plastic bipolar plates (e.g. 60 cells), and is enclosed into a high-density polyethylene (HDPE) container. The battery can be regarded as an electroplating machine. During charging, zinc is electroplated onto conductive electrodes, while bromine is formed. On discharge, the process reverses: the metallic zinc plated on the negative electrodes dissolves in the electrolyte and is available to be plated again at the next charge cycle. It can be left fully discharged indefinitely. Self-discharge does not occur in a fully charged state, when the stack is kept dry.
Features
In addition to the general advantages of the chemistry, zinc–bromine flow batteries have two significant advantages: They are scalable to large storage capacity through larger tanks and stacks. Individual parts can be serviced or replaced – for example the pump, tanks, or electrolyte.
Flow batteries also have specific disadvantages: Reset: Every 1–4 cycles the terminals must be shorted across a low-impedance shunt while running the electrolyte pump, to fully remove zinc from battery plates. Low areal power: (<0.2 W/cm2) during both charge and discharge, which increases the cost of power. Low Round Trip Efficiency: 70-80%, significantly lower than Li-ion batteries, which typically reach 90% or more. Low energy-density: Complex construction with moving parts