As good as lithium-ion batteries are, they have their limitations and challenges, but there’s also plenty of battery alternatives. Flow batteries alone have enough variations in chemistry to make your head spin. Zinc-bromine batteries are one up-and-coming contender … and calling them up and coming sounds funny when you consider that they’ve existed for 137 years … but they might hold the future for energy storage. And for such an old idea, why now?

It’s no question that developing efficient energy storage systems will make or break our progress in the use of intermittent renewable energy. When it isn’t sunny enough to rely on solar or windy enough to turn turbines, batteries full of previously-generated renewable energy can keep grids going. When it’s bright and sunny or the wind is blowing, the opposite case, it can store excess energy for when it’s needed most. Lithium-ion batteries are the reigning champion in the chemical battery arena, but a 137-year old concept might be what knocks them out of the ring.

That’s because the use of lithium-based batteries is somewhat paradoxical in the quest for sustainable energy. They’re great for electric vehicles, but recycling li-ion batteries has historically been difficult and dangerous, though there are several companies working on that problem. The process of mining lithium is also hugely disruptive to the environment, requiring about 500,000 gallons of water for each metric ton produced.1
Despite this, li-ion batteries continue to dominate the market due to their low cost and high energy density. Yet the price of lithium is rising. In China, one of the world’s largest producers of lithium,2 the cost of battery-grade carbonate lithium increased by more than 700% between early 2021 and February 2022.3

That doesn’t look good in light of the ubiquity of li-ion. Rising lithium prices also adds to the challenge of producing even more energy-dense variations like lithium metal and solid-state electrolyte batteries, the latter not even having hit the market yet. There’s a clear need to mitigate our reliance upon lithium. And as the demand for EVs and renewable energy storage increases, alternative types of batteries are gaining traction. It feels like there’s a new one every week, but in this case … it’s an old one.

Two Australian companies, Gelion and Redflow, have stepped up to the plate with zinc-bromine batteries that promise safer, more reliable, and more robust designs. In July, Redflow began production of the third generation of its zinc-bromine flow battery, the ZBM3, at its manufacturer in Thailand.4 In September, the company officially teamed up with Empower Energies to bring their 10 kWh battery to North America.5 The same month, Gelion began producing Endure, its non-flow zinc-bromide battery, using an existing lead-acid battery factory in Sydney.6

So, what is it that gives zinc-bromine an edge over other types of battery chemistries?

Before we discuss the value of zinc-bromine batteries compared to their competitors, let’s break down how they work. Zinc-bromine batteries are a form of redox-flow battery or RFB. Like any kind of battery, RFBs contain a cathode, an anode, and a separator.

What distinguishes flow batteries from others is their use of tanks full of liquid electrolyte on either side of the battery: one for catholyte, and one for anolyte, instead of the solid components found in the batteries used currently. Each side contains a chemical in a different oxidation state. A chamber between the two tanks holds a cell stack, which is divided by an ion-selective membrane.

When you discharge a flow battery, the chemical energy stored inside it is converted into electrical energy through reduction-oxidation (AKA “redox”). The ions in the anolyte oxidate, or lose electrons. These electrons then move through the battery’s circuit and past the membrane. On the other side, ions in the catholyte take these electrons, reducing their oxidation state. As the battery charges, this process is reversed.

The typical RFB structure of tanks and pumps translates into a lot of bulk, so they’re most appropriate for stationary applications. Think storage for an EV charging station, not EV battery. They’re highly useful at the home or grid scale because of the flexibility of their design. If you want more power, just add more stacks; if you want more storage capacity, add more electrolytes and storage tanks.7

Gelion takes things a step further, because their battery, the Endure, isn’t a RFB. Instead, it uses the same plate format and style of casing as a lead-acid battery.8 The difference is that it swaps lead and sulphuric acid for a bromide-positive plate and a zinc-negative plate, with a layer of gel acting as the electrode in the middle.9 The “gel” in “Gelion” refers to the gel-based, rather than liquid-based, electrolytes.
This means that unlike RFBs, the Endure doesn’t use pumps or tanks, allowing for a much more compact design. The plate design not only saves space, but eliminates the need for specialized maintenance, an auxiliary power source, and a secondary backup system.10

The chemical reaction inside, though, remains the same. When the battery charges, zinc ions move to the negative electrode, accept electrons, and reduce to zinc. Bromide ions move to the positive electrode, lose electrons, and oxidizes to bromine. The reverse happens upon discharge.11

A major benefit of flow batteries (and the Endure) is their depth of discharge. It’s possible to discharge flow batteries, including those produced by Redflow, all the way down to zero without damaging them.129 In contrast, some forms of lithium-ion batteries have an inherent limitation on their capacity, because they tend to degrade when discharged to below 20%.13

RFBs also have crucial advantages over li-ion in the realm of safety. Lithium-ion batteries require careful management to avoid failures that can spell disaster. When mishandled, damaged, or overheated, they’re prone to thermal runaway, which can cause fires. They’re also, well, known for exploding. Fires caused by lithium-ion batteries are especially dangerous because they’re difficult to put out and can even re-ignite.14

To put this into perspective, last year a Tesla lithium-ion battery pack caught fire during testing at the Victoria Big Battery in Australia. It burned for three days.15 Another Tesla Megapack energy storage system, this time in California, caught fire last September. Like the fire in Australia, nearby residents were warned to shut their windows to protect themselves from the smoke, which is full of toxic gases.16

While these events are rare and lithium-ion batteries are generally very safe, when things go wrong there are serious safety concerns. Flow batteries solve this problem by using non-flammable elements and water-based electrolytes rather than organic ones. This greatly reduces the chances of fire.

As of right now, vanadium RFBs are the most popular, but there’s no shortage of chemistries to choose from, like all-iron, uranium, or even copper. That said, zinc-bromine is probably the oldest of the bunch by a wide margin. The concept of a zinc-bromine battery was first patented by a New Yorker named C.S. Bradley all the way back in 1885.17 So, why are they relevant now?
For one, the incredible simplicity. Another, bromine is a natural fire retardant − an appealing quality when considering the instability of some lithium chemistries.18 Gelion claims that its bromine gel can “substantially moderate or even extinguish a fire.”19
Still, other flow batteries can also provide a low fire risk, with vanadium also having a leg up in its unique qualities. Vanadium can exist in multiple oxidation states, and a study published by the U.S. Department of Energy last month notes that VRFBs have a virtually unlimited cycle life.20 We actually covered a new vanadium redox flow battery chemistry in a recent video.

Even so, for all its usefulness, vanadium comes with its own set of weaknesses.21 Cost is a big one. The metal itself, which is mostly used to strengthen and toughen steel alloys, is known for its price volatility.22 In contrast, the components of zinc-bromine batteries are significantly cheaper. Most of the earth’s bromine is found in ocean water, so we’re literally swimming in it.23

It’s easy to “sea” how big of a difference that makes. A 2017 study estimated that the chemical cost of storage for a vanadium RFB is about $124.4/kWh. That’s about 15 and a half times more expensive than the cost of a zinc-bromine system at $8/kWh.24

More recently, a 2021 study examined the materials cost associated with vanadium, zinc-bromine, and all-iron batteries. Among the three, zinc-bromine batteries won out as the cheapest at $153/kWh, far lower than vanadium’s $491/kWh.25

Researchers also investigated the impacts of each type of battery on human and environmental health, revealing another issue with VRFBs: toxicity. Vanadium oxides are highly toxic, and as a result, VRFB production involves a higher potential for human health hazards relative to zinc-bromine. Vanadium is also dissolved into sulphuric acid, which is corrosive and hazardous to both people and the environment.26 But just to call this out, all those issues can and are addressed through proper engineering and design.

These results don’t mean zinc-bromine gets off scot-free from a safety perspective. Bromine still poses a risk to human health in electrolyte form due to its carcinogenic nature. Nevertheless, Sandia National Laboratories argues that the chemical reactivity and evaporation rate of the bromine contained in electrolyte is much lower than pure bromine – and the chemical smells so bad that a spill would be noticed quickly.27

Sure, zinc-bromine may beat vanadium in terms of cost and relative environmental impact. But does it stink in comparison to li-ion?

To be clear, zinc-bromine batteries are definitely not as energy-dense. On a grid scale, Gelion’s batteries’ have an energy density of 120 Wh/kg.28 And Redflow’s Systems Integration Architect, Simon Hackett, explained in a 2021 presentation that the company’s batteries are “more energy-dense than lead-acid, but much less energy-dense than lithium.”29

When it comes to round-trip efficiency, though, zinc-bromine batteries can potentially compete with li-ion. Gelion’s Endure battery has a RTE of about 85 to 90%.30 This is comparable to the 82 to 90% RTE for li-ion batteries reported in a U.S. Department of Energy study published in September.20

That study, which assessed the cost and performance of grid energy storage tech, also points to the ways in which the capital cost of zinc-bromine batteries can be less expensive than either li-ion or vanadium. In the case of a 10 MW battery storing energy for four hours, li-ion batteries are the cheapest when considering things like the storage block itself, which includes the cost of battery modules, racks, flow battery stacks, electrolyte, and tanks. Their HVAC and pipe systems are also cheaper.
However, both Gelion and Redflow claim that their zinc-bromine batteries don’t require air conditioning in the first place.31 32 Gelion’s founder, Thomas Maschmeyer, told The Guardian in 2021 that he predicted that their costs of operation would be 25% less than lithium’s precisely because their batteries don’t require air conditioning or fire-suppression systems.31 On top of that, the Endure battery doesn’t use tanks, either. There’s no fluid to hold.

As for the cost of systems integration, which involves shipping and installing the batteries, the Department of Energy figures for lithium and vanadium range between about $46 to $52/kWh. The cost of zinc-bromine is between $10 to $18/kWh. And in other aspects, like the cost of engineering, construction equipment, connecting to the grid, and transformers, zinc-bromine is also considerably cheaper.

Gelion claims that its approach to manufacturing drives down costs further. Typically, a RFB requires a new production line. But the Endure’s casing is the same kind used in lead-acid batteries, and 18 out of the 22 steps of its manufacturing process can be completed within an existing lead-acid battery factory.10

In a presentation given to investors earlier this year, Gelion estimated that it would cost about $16 million to retrofit a lead-acid battery facility to produce 1Gwh of their batteries, while building a new 1Gwh li-ion battery facility from scratch would cost about $130 million.33
Zinc-bromine batteries last longer than lithium ones, too. This is partly because of their durability and their depth of discharge, as with other flow batteries. It’s also the case in the context of their cycle life. Department of Energy statistics indicate that lithium phosphate batteries last for roughly 2,400 cycles, with lithium nickel manganese cobalt batteries lasting for about 1,500 cycles. In contrast, zinc-bromine flow batteries last for closer to 4,500 cycles, and zinc-bromine non-flow batteries last about 5,000.20

Both Gelion and Redflow also emphasize the toughness of their batteries. Gelion calls the Endure “abuse-tolerant” and, in one test, managed to keep it operating even as it was “smoking and charring” atop a hotplate heated to 700 C.34 The two products boast similar levels of tolerance to high temperatures, with the Endure’s upper limit being 50 C, and the ZBM3’s being 45 C.9 32
It seems that as far as design improvements go, zinc-bromine batteries are on fire. And considering Australia’s bushfires and overall hot climate, the country’s relationship to zinc-bromine batteries makes perfect sense. In fact, the blackouts that Australians experience as a consequence of bushfires are what prompted Australian state and federal governments to invest in using rechargeable batteries for residential battery energy storage systems and at remote telecommunications stations. 3536 Redflow’s battery is especially poised for use at telecom stations because of its small size relative to other flow batteries, according to the company’s Systems Integration Architect.37

Once zinc-bromine batteries reach the end of their lives, they’re easier to recycle than lithium-based batteries, as well. Because zinc-bromine electrolyte is dense, it’s often recycled by the oil and gas industry.20 And because the Endure’s casing is the same as a lead-acid battery’s casing, it can be recycled in the same way, but with the added advantage of what the company calls more “benign” components − as in, not toxic lead or corrosive sulfuric acid.19

Zinc-bromine batteries aren’t perfect. In general, zinc-bromine batteries face the risk of zinc dendrite formation, which can threaten to poke through the battery’s separator.38 As a result, zinc-bromine RFBs usually require maintenance in the form “strip cycles” to remove zinc buildup.3940 It’s worth mentioning, though, that Redflow claims that completely discharging its batteries strips the zinc away.29 And Gelion’s answer to dendrites is a porous membrane separating the zinc and bromide that evidently inhibits their growth.9

But like any other technology, zinc-bromine batteries are not the end-all be-all. They’re tailored to specific purposes: large-scale grids, long-term storage, rural areas, and extreme temperatures. It isn’t sensical to compare them to solid-state batteries, for example, because they’re meant to solve different problems. Solid-state batteries are thin and small; zinc-bromine batteries are (relatively) big and heavy. Solid-states are generating buzz for their potential use in EVs.41 Zinc-bromine batteries, on the other hand, are meant for providing electricity to your home, solar or wind farms, or remote areas. Overall, zinc-bromine batteries may work well for fixed locations, but will be far too bulky for mobile or portable uses.

Perhaps the most critical difference, though, is that the production of solid-state batteries is currently too expensive to adapt to applications in EVs and consumer electronics.41 zinc-bromine batteries are already here.

  1. “The spiraling environmental cost of our lithium battery addiction” 
  2. Periodic Table – Lithium 
  3. “Chinese EVs get squeezed by rising lithium price and falling subsidies” 
  4. “ROUNDUP: Fluence’s India tech centre, Redflow launches Gen3 flow battery, Zinc backup at Wyoming data centre” 
  5. “Redflow targets US market with lithium-ion battery alternative” 
  6. “Australian zinc bromide batteries start rolling off production line in Sydney” 
  7. “2020 Grid Energy Storage Technology Cost and Performance Assessment” 
  8. “Manufacturing Synegies with Lead Acid Batteries” 
  9. Gelion – “The Sustainable Energy-Storage Solution” 
  10. Just Have a Think – “Zinc Bromide GEL batteries. Cheaper, greener, simpler & safer than lithium -ion!” 
  11. Gelion – “Zinc Bromide Battery Basics” 
  12. RMI – “Breakthrough Batteries” 
  13. “Vanadium fuels growing demand for VRFBs” 
  14. “How Do You Put Out A Lithium-Ion Battery Fire?” 
  15. “Tesla’s lithium-ion megapack causes three-day fire during test at Australian 300MW ESS” 
  16. “A Tesla Megapack Caught Fire, Causing Californians to ‘Shelter in Place’” 
  17. Patent – No. 312,802. Patented Feb. 24, 1885. 
  18. Bromine-based flame Retardants 
  19. Gelion – “Revolutionary Zinc Bromide Batteries” 
  20. “2022 Grid Energy Storage Technology Cost and Performance Assessment” 
  21. “Vanadium fuels growing demand for VRFBs” 
  22. “Redflow boss reveals company’s plan to refine manufacturing, bolstering next leg up” 
  23. Science Direct – “Bromine” 
  24. “Air-Breathing Aqueous Sulfur Flow Battery for Ultralow-Cost Long-Duration Electrical Storage” 
  25. “Life Cycle Assessment of Environmental and Human Health Impacts of Flow Battery Energy Storage Production and Use” 
  26. “Salt and a battery – smashing the limits of power storage” 
  27. Sandia – “Zinc/Bromine Batteries” 
  28. “Life after Lithium-Ion” 
  29. “Deploying Redflow Zinc Bromine Batteries from Houses up to Grid Scale” 
  30. “Zinc bromide’s transformative role in the future of stationary energy storage” 
  31. “‘Real beacon’: Australian company developing zinc-bromide battery technology lists for $285m” 
  32. Redflow – “ZBM3 flow battery” 
  33. “GELION PLC – Investor Presentation” 
  34. “This Australian-developed stationary battery is safer and greener than its competitors” 
  35. “After bushfires, batteries seen as wise investment in Australia” 
  36. “Redflow zinc-bromine flow batteries to ensure resilient telecoms in Australian government initiative” 
  37. “The Reflow Gen3 ZBM 
  38. Sandia – “Performance Testing of Zinc-Bromine Flow Batteries for Remote Telecom Sites” 
  39. “South Africa Energy Storage Technology and Market Assessment” 
  40. “Initial Test Results from the RedFlow 5 kW, 10 kWh Zinc-Bromide Module, Phase 1” 
  41. “The state of solid-state batteries” 

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