Zinc–bromine battery

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

Design 
The two electrode chambers of each cell are typically divided by a membrane (typically a microporous or ion-exchange variety). This helps to prevent bromine from reaching the negative electrode, where it would react with the zinc, causing self-discharge. To further reduce self-discharge and to reduce bromine vapor pressure, complexing agents are added to the positive electrolyte. These react reversibly with the bromine to form an oily red liquid and reduce the  concentration in the electrolyte.

Developers 
 Primus Power – Hayward, California, is a privately held US company. However, as at May 2023, they had had no installations since 2015. Primus Power claim 70% efficiency for their 125 kWh unit.
 RedFlow Limited – Brisbane, Australia, is a publicly listed company on the ASX. Their ZBM3 battery claimed to supply 12 hours of continuous power. They claimed DC–DC "stack energy efficiency" of up to 80% for their ZBM3 battery and  42 Wh/kg for the ZBM3, a 10 kWh unit. 
 EnSync (Formerly ZBB) – Menomonee Falls, Wisconsin, US (defunct).

Non-flow 
Non-flow batteries do not pass battery materials between two tanks.

Developers 

 Gelion: Thomas Maschmeyer at the University of Sydney replaced the liquid with a gel. Ions can move more quickly, decreasing charging time. The gel is fire-retardant. In April 2016 Gelion, launched. The company earned an A$11 million investment from UK renewables group Armstrong Energy. Gelion raised further capital with an IPO and listed on the AIM London Stock Exchange 30 November 2021.

Gelion plans to commercialise a 1.2 kWh monoblock battery for use in commercial and grid applications.  Gelion claimed its monoblocks will have higher energy density (120 Wh/kg), higher round-trip efficiency (>87%), no moving parts, and manufacturing scalability to gigawatt capacity by adapting existing lead–acid battery factories., Gelion planned to test-deploy a system for Acciona Energy in 2023. Gelion announced a fast discharge mode, lower cost electrodes (to replace titanium) and improvements for dendrite management and prevention.

 EOS Energy Enterprise cathode:  EOS had announced its Eos Z3 battery and claimed an order backlog of 347MWh and a total 2.2GWh of binding orders. EOS claimed its battery has an RTE "in the mid 80s" (with reduced depth of discharge) and a lifetime of 6,000 cycles/20 years.

Electrochemistry
Flow and non-flow configuration share the same electrochemistry.

At the negative electrode zinc is the electroactive species. It is electropositive, with a standard reduction potential E° = −0.76 V vs SHE. 

The negative electrode reaction is the reversible dissolution/plating of zinc:
 Zn_{(s)} <=> {Zn^2+}_{(aq)} + 2e^-

At the positive electrode bromine is reversibly reduced to bromide (with a standard reduction potential of +1.087 V vs SHE):
 {Br2_{(aq)}} + 2e^- <=> {2Br^-}_{(aq)} 

So the overall cell reaction is
 {Zn_{(s)}} + Br2_{(aq)} <=> {2Br^-}_{(aq)} + {Zn^2+}_{(aq)}

The measured potential difference is around 1.67 V per cell (slightly less than that predicted from standard reduction potentials).

Applications

Remote telecom sites
Significant diesel-generator fuel savings are possible at remote telecom sites operating under conditions of low electrical load and large installed generation by using multiple systems in parallel to maximise the benefits and minimise the drawbacks of the technology.

History 
In December 2021 Redflow completed a 2 MWh installation for Aneargia to support a 2.0 MW biogas-fuelled cogeneration unit, and a microgrid control system in California.

 EOS Energy Enterprises had secured a 300 MWh order from Pine Gate Renewables, with installation planned for 2022.

, Gelion announced an agreement with Acciona Energy to trial Endure batteries for grid-scale applications.

In June 2023, Redflow announced an agreement to supply a 20 MWh system to help power California's Rolling Hills Casino.

See also
 Electrochemical engineering
 List of battery types

References

Further reading
 Bromine complexation in zinc–bromine circulating batteries D. J. Eustace, J. Electrochem. Soc. 127(3), 528–32 (1980)
 Handbook of batteries, 3rd edition. D. Linden, T. B. Reddy. 39.1–39.8 (2002)
  Update
 RedFlow.

External links
 ZnBr Batteries at the Electricity Storage Association

Battery types
Flow batteries