Lithium-ion batteries excel at short-duration discharges; two to four hours is their sweet spot. However, the power grid sometimes requires energy storage that can last for most of the day, and that’s when another type of technology comes into play: flow batteries.

I. It’s based on an entirely different concept from lithium-ion batteries.

In a flow battery, the electrolyte is housed in an external tank; the stack is responsible for discharging, while the tank stores energy, resulting in a decoupling of power and capacity. The most typical example is the all-vanadium flow battery, in which both the anode and cathode consist of vanadium ions in different oxidation states, and a pump is used to circulate the electrolyte through the stack for the reaction.

II. Its strengths are well-suited for long-term energy storage

Aqueous electrolytes are inherently non-flammable, making them naturally safe; their cycle life easily exceeds 10,000 cycles, which is several times that of lithium-ion batteries; to increase capacity, simply enlarge the tank and add more electrolyte—capacity and power are calculated separately; they can also be deeply discharged to 100 percent without damage. Efficiency typically ranges from 70 to 85 percent.

Dimension All-Vanadium Flow Battery Lithium-ion battery
Discharge Duration 4 hours or more 2–4 hours
Cycle Life 10,000+ times 3,000–6,000 times
Safety Intrinsically Safe (Water-Based) Thermal management required
Initial Installation Costs High Chinese
Energy Density Low, takes up a lot of space Tall, Compact

III. We Must Also Be Clear About Our Weaknesses

Low energy density means a larger footprint and higher initial installation costs. However, when spread over the entire life cycle—especially under conditions involving frequent deep and shallow charge-discharge cycles—its cost per kilowatt-hour may actually be more favorable. Form Energy in the U.S. is also promoting iron-air technology, a cheaper long-duration storage solution, further broadening the scope of this market segment.

The rise in popularity of long-duration energy storage essentially stems from the fact that, as the share of renewable energy increases, the power grid needs a type of “slow-cooking” power reservoir that can store electricity for several hours.

Flow batteries may not necessarily replace lithium-ion batteries, but they are likely to be one of the most reliable contenders in the market for battery life of four hours or more.