What is a flow battery?

By Kathie Zipp | December 4, 2017

While solid-state batteries such as lithium ion store energy in solid electrode material like metal, flow batteries store energy in electrolyte liquids.

Most conventional flow batteries use two electrolyte liquids: one with a negatively charged cathode, and one with a positively charged anode. The cathode and anode are separated into two tanks by a membrane, because if they come into contact with each other the battery will short and require replacement. This is often what happens with lithium-ion batteries; the membrane degrades over time. But the exchange of negatively and positively charged fluids in flow batteries produces electrical current without degradation, providing a longer cycle life and quick response times.

While lithium-ion batteries are great for mobile, high-power applications like laptops and cell phones, flow batteries are able to store power for longer periods (four hours or more), and last for decades before needing replacement. This makes them good for providing power to large utility applications, military bases, data centers, microgrids and off-grid projects that are not space-constrained.

Flow battery manufacturers offer a variety of chemistries including vanadium, iron chromium, zinc bromine, zinc iron and more. Flow batteries can also be redox, hybrid and membraneless.

Redox flow batteries employ reduction (a gain of electrons) and oxidation (a loss of electrons) reactions as electrons are transferred in the electrolyte. Energy is stored in the electrolyte, which flows through the battery during charge and discharge.

In true redox flow batteries, energy is stored in the liquid at all times. However, hybrid redox flow batteries store at least some energy in solid metal during charge.

In a membraneless flow battery, the liquids self-separate in one tank.

Though it depends on the chemistry, flow batteries tend to be less reactive and easy to dispose, with no fire risk. They are also often recyclable.

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