Material Insights · Understanding the Material World

Flow batteries: Science and technology

How liquid vanadium avoids the wear that kills solid batteries

What this tab covers The physics and chemistry: how the energy is stored, which materials it needs (scarcity, toxicity, recyclability) and what makes it age.

Flow batteries hold energy in liquids pumped past a cell stack, so they age slowly but store little energy per litre. Vanadium is the mature, China-led variant; zinc-bromine, iron and organic chemistries trade lower material risk for weaker commercial track records.

Mechanism

Mixed

Two electrolyte tanks feed a stack where dissolved ions are oxidised on one side of a membrane and reduced on the other (a redox reaction). In vanadium systems both tanks hold vanadium in sulphuric acid at different oxidation states, so ions that leak across the membrane do not poison the other side; full-cell voltage is about 1.25 V at 1 M acid. Zinc-bromine is a hybrid design: zinc plates onto an electrode during charging, so part of the energy sits on a solid surface. Iron flow uses iron salts in water, and organic flow uses carbon-based molecules such as quinones. Water-based chemistries are limited to about 1.5 to 1.8 V by water splitting, and the liquids are dilute, which keeps energy density low; DOE lists raising electrolyte energy density as a key research need.41014

Vanadium full-cell voltage About 1.25 V14 PNNL mixed-acid vanadium electrolyte (laboratory claim) 70% higher energy density than standard sulphate electrolyte10

Material scarcity and toxicity

Mixed

Vanadium supply is concentrated: China mined about 82,000 t of the world's 110,000 t in 2025, roughly 75% (calculated), with Russia second at 21,000 t. USGS flags high capital costs and limited high-purity vanadium feedstock as the main barriers, and DOE says vanadium's volatile price impedes market growth. Bromine is toxic and corrosive, and Israel and Jordan together produced about 72% (calculated) of world supply in 2025. Iron flow largely avoids both problems, because iron salts in water are cheap, abundant and far less hazardous than vanadium compounds in sulphuric acid. Organic molecules can be made from abundant feedstocks, so they are not limited by mining.248

China share of vanadium mine output, 2025 About 75% (calculated from 82,000 of 110,000 t)2 Vanadium pentoxide price, China average 2025 USD 5.02 per lb2 Israel plus Jordan share of bromine output, 2025 About 72% (calculated from 310,000 of 430,000 t)8

Degradation and ageing

Mixed

Because the active material is dissolved rather than built into a solid electrode, it does not crack or swell, and vanadium species are chemically stable indefinitely, so capacity lost to crossover can largely be recovered by remixing or rebalancing the tanks. DOE assumes 10,000 cycles for vanadium electrolyte but only 12 years for stacks and pumps, which wear out first. Organic electrolytes are the weak point of that variant: a 2020 review found that most reported organic chemistries fade faster than 0.1% of capacity a day, too fast for decades of service; the record at the time was 0.014% a day. Zinc-bromine units face zinc dendrite growth and need periodic full discharge.414

Vanadium electrolyte cycle life (DOE 2030 basis) 10,000 cycles4 Stack and pump calendar life (DOE) 12 years4 Organic flow capacity fade Most above 0.1% a day; best reported 0.014% a day14

Flow battery variants compared34567141623

VariantActive materialCommercial status (2022 to 2026)Main weakness
Vanadium redoxVanadium in sulphuric acidLargest plant 200 MW / 1,000 MWh at Jimusar, Xinjiang (Rongke Power supplier), built mid-2025 and reported connected Jan 2026Cost, vanadium price and supply concentration
Zinc-bromineZinc metal and bromineRedflow (Australia) entered voluntary administration Aug 2024 after failing to raise equityBromine handling, zinc dendrites, company failures
Iron flowIron salts in waterESS Inc (US) had nearly 800 MWh a year of factory capacity at end 2022 but warned of going-concern doubt in Aug 2025; FY2025 revenue USD 1.6 millionSupplier finances, large footprint
Organic flowCarbon-based molecules (e.g. quinones)Start-ups such as Quino Energy (USD 10 million Series A, Nov 2025); CMBlu conditional framework to supply Uniper with at least 5 GWh from 2027 (signed Jan 2026)Molecule stability: capacity fade

Sources

  1. Mineral Commodity Summaries 2026: Vanadium US Geological Survey, 2026
  2. China connects world's largest vanadium flow battery project ESS News (pv magazine), 2026
  3. Technology Strategy Assessment: Flow Batteries (Energy Storage Grand Challenge), Table 2 p. 5 US Department of Energy, 2023
  4. Iron flow battery maker ESS Inc warns of survival battle through 'operational reset' period Energy-Storage.news, 2025
  5. Flow battery startup ESS Tech Inc looks to acquisitions, big-name US partnerships for hope in future profitability Energy-Storage.news, 2026
  6. Australian battery manufacturer Redflow enters voluntary administration ESS News (pv magazine), 2024
  7. Mineral Commodity Summaries 2026: Bromine US Geological Survey, 2026
  8. Redox flow batteries for grid-scale energy storage (available technologies) Pacific Northwest National Laboratory, 2024
  9. Electrolyte lifetime in aqueous organic redox flow batteries: a critical review (Kwabi, Ji and Aziz, Chemical Reviews 120) American Chemical Society / Harvard University, 2020
  10. Quino Energy secures funding to scale organic flow batteries (11 Nov 2025) ESS News (pv magazine), 2025
  11. CMBlu Energy and Uniper sign long-term framework agreement for 5 GWh of SolidFlow large-scale battery storage (press release, 3 Feb 2026) CMBlu Energy, 2026