View source for Zinc–bromine battery

{{short description|Type of electrochemical cell}}
{{Infobox battery
| EtoW = 60–85&nbsp;[[watt-hour|W·h]]/[[kg]]
| EtoS = 15–65&nbsp;W·h/[[Litre|L]] (56–230&nbsp;kJ/L)<ref name="Khor et al 2018">{{cite journal |last1=Khor |first1=A. |last2=Leung |first2=P. |last3=Mohamed |first3=M.R. |last4=Flox |first4=C. |last5=Xu |first5=Q. |last6=An |first6=L. |last7=Wills |first7=R.G.A. |last8=Morante |first8=J.R. |last9=Shah |first9=A.A. |title=Review of zinc-based hybrid flow batteries: From fundamentals to applications |journal=Materials Today Energy |date=June 2018 |volume=8 |pages=80–108 |doi=10.1016/j.mtener.2017.12.012 |hdl=10397/77992 |s2cid=117522227 |hdl-access=free }}</ref>
| CtoDE= 75.9%<ref>{{ cite web | title = Performance Testing of Zinc-Bromine Flow Batteries for Remote Telecom Sites | url = http://www.sandia.gov/ess/publications/SAND2013-2818C.pdf| archive-url=https://web.archive.org/web/20170429065755/http://www.sandia.gov/ess/publications/SAND2013-2818C.pdf | archive-date = 2017-04-29| publisher = Sandia National Laboratories | date = 2013 | access-date = 2015-04-01 | page = 6 }}</ref>
| EtoCP= US$400/kW·h (US$0.11/kJ){{Citation needed|date=December 2010}}
| CD = >6,000&nbsp;[[charge cycle|cycles]]
| NomV = 1.8&nbsp;[[volt|V]]
}}

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 cell]]s. 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 battery|zinc–carbon]] and [[Alkaline battery|alkaline]] primaries.

The leading potential application is stationary energy storage, either for the [[Grid energy storage|grid]], or for domestic or [[Stand-alone power system|stand-alone power systems]]. The aqueous [[electrolyte]] makes the system less prone to [[lithium-ion battery#Safety|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.

There are no longer any companies commercializing flow batteries, Gelion (Australia) have non-flow technology that they are developing and EOS Energy Enterprises (US) are [[rechargeable battery#Other experimental types|commercializing]] their non-flow system.

== Features ==

Zinc–bromine batteries share six advantages over lithium-ion storage systems:

* 100% depth of discharge capability on a daily basis.{{sfn|Rose|Ferreira|p=4}}
* 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.{{sfn|Rose|Ferreira|p=4}}
* 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-the-grid|off-grid]] systems, and [[load management|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 density|Energy densities]] range between 60 and 85&nbsp;W·h/kg.<ref name="Khor et al 2018" />

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, {{clarify-span|text=chemically complexed organic phase|explain=What "chemically complexed organic phase" is it stored in?|date=January 2024}}. Older ZBRFB cells used [[Synthetic membrane|polymer membranes]] (microporous polymers, [[Nafion]], etc.) More recent designs eliminate the membrane.<ref>{{Cite web|url=https://patentscope.wipo.int/search/en/detail.jsf?docId=US282774210&docAn=16337093|title=US20200036046 MEMBRANE-FREE NON-FLOWING SINGLE CELL ZINC BROMINE BATTERY WITH BROMINE-TRAPPING COMPOSITE CARBON FOAM ELECTRODE|website=patentscope.wipo.int}}</ref> 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 ====
Zinc–bromine flow batteries do not enjoy the advantage of scale that other flow-battery technologies enjoy.  Storage capacity cannot be increased by simply adding additional electrolyte tanks (the stack must also be scaled up).   

Zinc-bromine hybrid-flow batteries have many 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.{{sfn|Rose|Ferreira|p=4}}
* Low areal power: (<0.2&nbsp;W/cm<sup>2</sup>) during both charge and discharge, which increases the cost of power.<ref>G.&nbsp;P. Corey, [http://redflow.com/wp-content/uploads/2012/10/Garth-Corey-assessment__zinc_bromine_battery.pdf An Assessment of the State of the Zinc-Bromine Battery Development Effort]. RedFlowLimited Brisbane, Queensland, Australia, 2011.</ref><ref name="Nakatsuji-MatherSaha2012">{{cite book |last1=Nakatsuji-Mather |first1=M. |last2=Saha |first2=T. K. |title=2012 IEEE Power and Energy Society General Meeting |chapter=Zinc-bromine flow batteries in residential electricity supply: Two case studies |year=2012 |pages=1–8 |doi=10.1109/PESGM.2012.6344777|isbn=978-1-4673-2729-9 |s2cid=22810353 }}</ref><ref name="SureshKesavan2014">{{cite journal |last1=Suresh |first1=S. |last2=Kesavan |first2=T. |last3=Munaiah |first3=Y. |last4=Arulraj |first4=I. |last5=Dheenadayalan |first5=S. |last6=Ragupathy |first6=P. |title=Zinc–bromine hybrid flow battery: effect of zinc utilization and performance characteristics |journal=RSC Advances |volume=4 |issue=71 |year=2014 |pages=37947 |issn=2046-2069 |doi=10.1039/C4RA05946H|bibcode=2014RSCAd...437947S }}</ref>
*Low Round Trip Efficiency: <70% on an RTE basis, significantly lower than Li-ion batteries, which typically reach 90% or more.
*Low energy-density: 
*Complex construction with moving parts
*Poor reliability: no manufacturer has yet to produce a reliable Zn-Br flow battery

==== Design ====
The two electrode chambers of each cell are typically divided by a membrane (typically a microporous or [[Ion-exchange membrane|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 {{chem|Br|2}} concentration in the electrolyte.{{Citation needed|date=November 2008}}

==== Developers (all now defunct) ====
* Primus Power – Hayward, California, is a privately held US company. However, as at May 2023, they had had no installations since 2015.<ref>{{Cite web |title=Primus Power |url=https://primuspower.com/ |access-date=2023-05-14 |website=primuspower.com |language=en}}</ref> Primus Power claim 70% efficiency for their 125 kWh unit.<ref>{{Cite web |title=Primus Power |url=https://primuspower.com/ |access-date=2022-02-08 |website=primuspower.com |language=en}}</ref> Although the Primus Power website is still live, the company is not operating.
* RedFlow Limited – went into voluntary administration on the 23rd of August, 2024,<ref>{{cite web |title=Redflow Enters Voluntary Administration | url=https://cdn-api.markitdigital.com/apiman-gateway/ASX/asx-research/1.0/file/2924-02842374-2A1542820 |access-date=2024-10-28 | website=asx.com.au |language=en}}</ref> and was liquidated in December 2024.<ref name="ABCNews1">{{citation|author1-last=Purtill|author1-first=James|title=Redflow was the great hope of Australian manufacturing. Its collapse left customers with broken batteries|website=[[ABC News (Australia)|ABC News]] website|url=https://www.abc.net.au/news/science/2024-12-05/redflow-australian-battery-manufacturer-collapse-defects/104650074|date=2024-12-05|access-date=2024-12-04|archive-url=https://web.archive.org/web/20241204225503/https://www.abc.net.au/news/science/2024-12-05/redflow-australian-battery-manufacturer-collapse-defects/104650074|archive-date=2024-12-04|url-status=live|quote=Redflow's assets are now being liquidated after the administrator was unable to find a buyer for the business.}}</ref> Their ZBM3 battery has very poor reliability and performance.
* EnSync (Formerly ZBB)<ref>{{Cite web|url=http://www.marketwired.com/press-release/zbb-energy-changes-name-to-ensync-inc-nyse-mkt-zbb-2048183.htm|title=ZBB Energy changes name to EnSync|work=Marketwire }}</ref> – Menomonee Falls, Wisconsin, US (defunct).<ref>{{cite web | url = https://www.bizjournals.com/milwaukee/news/2019/03/13/ensync-energy-intends-to-initiate-insolvency-cuts.html | title = EnSync Energy intends to initiate insolvency, cuts nearly all employees | author = Nick Williams | date = 2019-03-13 | publisher = Milwaukee Business Journal}}</ref>

=== 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.<ref>{{cite web|date=2 February 2016|title=Catalyst/BATTERY POWERED HOMES|website=[[Australian Broadcasting Corporation]]|url=http://www.abc.net.au/catalyst/stories/4398364.htm|access-date=15 January 2017}}</ref> In April 2016 Gelion, launched. The company earned an A$11 million investment from UK renewables group [[Armstrong Energy]].<ref>{{cite web|date=13 April 2016|title=Australian gel-based battery technology attracts major UK finance|url=http://reneweconomy.com.au/australian-gel-based-battery-technology-attracts-major-uk-finance-73407/|access-date=15 January 2017}}</ref> Gelion raised further capital with an IPO and listed on the AIM London Stock Exchange 30 November 2021.  However, Gelion is now focussed on its Li-S-Si battery technology and doesn't appear to be investing in its Zn-Br technology.

* EOS Energy Enterprise cathode: {{As of|2023|May}} EOS had announced its Eos Z3 battery and claimed an order backlog of 347MWh and a total 2.2GWh of binding orders.<ref>{{Cite news |date=2023-05-09 |title=Eos Energy Enterprises Reports First Quarter 2023 Financial Results |language=en |work=Bloomberg.com |url=https://www.bloomberg.com/press-releases/2023-05-09/eos-energy-enterprises-reports-first-quarter-2023-financial-results |access-date=2023-05-14}}</ref> 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.<ref>{{Cite web |title=Technology |url=https://www.eose.com/technology/ |access-date=2023-05-14 |website=Eos Energy Enterprises |language=en-US}}</ref>

==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&nbsp;V vs [[standard hydrogen electrode|SHE]]. 

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

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

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

The measured potential difference is around 1.67&nbsp;V per cell (slightly less than that predicted from standard reduction potentials).{{Citation needed|date=November 2008}}

==Applications==
See EOS Energy website.  They are currently the sole commercial supplier of Zn-Br batteries.

== History ==
Many Zn-Br flow battery tech companies have gone broke.  EOS Energy and Gelion are the only two that remain trading, both have non-flow Zn-Br technology.

{{As of|2021|November}} EOS Energy Enterprises had secured a 300 MWh order from Pine Gate Renewables, with installation planned for 2022.<ref>{{Cite web|title=Eos Energy secures order for 300 MWh of battery storage systems|url=https://renewablesnow.com/news/eos-energy-secures-order-for-300-mwh-of-battery-storage-systems-760723/|access-date=2022-02-08|website=Renewablesnow.com|date=10 November 2021 |language=en}}</ref>

{{As of|2022|February}}, Gelion announced an agreement with Acciona Energy to trial Endure batteries for grid-scale applications.<ref name=":1">{{Cite web |last=Vorrath |first=Sophie |date=2022-02-02 |title=Gelion to trial its zinc bromide batteries with Spanish solar farm |url=https://reneweconomy.com.au/gelion-to-trial-its-zinc-bromide-batteries-with-spanish-solar-farm/ |access-date=2022-02-08 |website=RenewEconomy |language=en-AU}}</ref>

==See also==
* [[Electrochemical engineering]]
* [[List of battery types]]

==References==
{{Reflist|30em}}

==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)
* {{cite web |first1=David M. |last1=Rose |first2=Summer R. |last2=Ferreira |url=http://www.sandia.gov/ess/publications/SAND2013-2818C.pdf |title=Performance testing of zinc–bromine flow batteries for remote telecom sites |publisher=Sandia National Laboratory }} [http://www.sandia.gov/ess/publications/SAND12-1352.pdf Update]
* [http://redflow.com/ RedFlow.]

==External links==
* [http://www.electricitystorage.org/technology/storage_technologies/batteries/zinc_bromine_batteries/ ZnBr Batteries] at the Electricity Storage Association

{{Galvanic cells}}

{{DEFAULTSORT:Zinc-bromine battery}}
[[Category:Battery types]]
[[Category:Flow batteries]]