Chinese Academy of Sciences Reports All-Iron Flow Battery Running 6,000 Cycles With No Capacity Decay and 99.4% Coulombic Efficiency

Overview

A research team at the Chinese Academy of Sciences has reported an alkaline all-iron flow battery that ran for more than 6,000 charge-discharge cycles at 80 milliamps per square centimeter without measurable capacity decay, according to a press release from the Chinese Academy of Sciences describing work published in the journal Advanced Energy Materials. The result, the agency said, addresses a long-standing bottleneck for low-cost, long-duration storage that grid operators need to absorb intermittent solar and wind generation. Iron currently trades for roughly one-eightieth the price of lithium as a raw material, and flow batteries store their charge in tanks of liquid electrolyte rather than in solid electrodes, so a stable iron-based chemistry could in principle scale to grid duty at far lower material cost than lithium-ion.

The study was led by Prof. TANG Ao and Prof. LI Ying of the Shenyang National Laboratory for Materials Science at the Institute of Metal Research. The paper, titled “Synergistic Design of High Steric Hindrance and Negatively Charged Anolyte Enables 6000-Cycle Stability for Alkaline All-Iron Flow Batteries,” appeared in Advanced Energy Materials.

What We Know

According to the Chinese Academy of Sciences, the team “synthesized 11 iron complexes based on 12 organic ligands rich in hydroxyl and sulfonic acid groups” before settling on a molecule it labels [Fe(HPF)BHS]⁴⁻ as the most stable anolyte. The chosen complex pairs a bulky multidentate iron coordination shell with sulfonate and hydroxyl groups that give the molecule a strong negative charge. The press release describes the result as a “dual protection, via Donnan exclusion,” that blocks hydroxide ions from reaching and decomposing the iron center while also suppressing the migration of active species across the cell membrane.

In the headline test, the battery operated at a current density of 80 milliamps per square centimeter for more than 6,000 cycles with no capacity decay and an average coulombic efficiency of 99.4 percent. Pushed to 150 milliamps per square centimeter, energy efficiency settled at 78.5 percent with a peak power density of 392.1 milliwatts per square centimeter, the agency said. At a higher electrolyte concentration of 0.9 molar, the cell ran 2,000 stable cycles at 71.5 percent energy efficiency. Multiscale characterization reported in the press release indicates that the new anolyte cuts ligand crossover through the membrane by two orders of magnitude compared with conventional iron-based systems.

Several outlets reframed the cycle count in years of daily service. Interesting Engineering and VnExpress International both translated 6,000 cycles into more than 16 years of once-daily use. The same outlets emphasized that the test setup retained 78.5 percent energy efficiency under elevated power.

Cost and competitive context

The headline cost argument that runs through coverage is material-side rather than system-side. South China Morning Post reports that lithium “costs over 80 times more than iron as a raw industrial material at present,” and that the global energy transition faces “a critical bottleneck in storing intermittent power from solar and wind farms at a scale sufficient to stabilise the grid.” CleanTechnica notes that lithium carbonate prices have “swung between roughly $7,000 and $80,000 per metric ton over the past five years,” while iron sulfate trades at a fraction of those numbers.

The 80-times-cheaper framing applies to the active material on a per-kilowatt-hour basis. It does not translate directly into installed-system parity. CleanTechnica cautions that the differential “refers to raw material cost per unit of stored energy, not the installed system cost, which also includes membranes, pumps, and power electronics,” all of which carry their own engineering and capital burdens for a flow design.

For reference, CleanTechnica places the cycle endurance of lithium iron phosphate cells at 3,000 to 6,000 cycles at 80 percent depth of discharge. The new iron flow cell sits at the upper edge of that range without a capacity-fade penalty, although the laboratory test conditions are not directly comparable.

What We Don’t Know

The results were obtained on a laboratory-scale cell, not a containerized or megawatt-class prototype. CleanTechnica flags the standard caveat for flow chemistries: “flow battery performance at the kilowatt or megawatt level often diverges from cell level results due to shunt currents, thermal management, and membrane fouling.” No commercial partner, deployment site, or industrial-scale demonstration timeline has been announced. The Institute of Metal Research has not published costed bills of materials for a stack, nor has any outlet independently verified the calorimetric or power-density figures against a third-party laboratory.

The coverage also varies on a small but meaningful procedural detail. South China Morning Post attributes a quoted line that it credits to a “press release on April 16,” while the public English version of the Chinese Academy of Sciences release carries an April 17 date. The substance of the technical claims is consistent across the press release and the journal abstract, but readers should treat the April 16 date as outlet-attributed rather than independently verifiable.

The field is also crowded. U.S. firm Form Energy is building iron-air batteries, a related but chemically distinct technology, for hyperscale customers including a Google-Xcel Energy project in Minnesota that the Machine Herald previously reported. Whether an iron flow chemistry from a Chinese state lab can move from a screened library of 11 complexes to a deployed product before iron-air and other long-duration designs lock in their commercial positions is the next-stage question that the published paper does not attempt to answer.