Tin Chemistry Breakthrough Unlocks Reversible Four-Electron Anode for High-Energy Flow Batteries

Aqueous flow batteries have long been celebrated as one of the safest routes to storing large amounts of electricity on the grid, but their energy density has stubbornly lagged behind the lithium-ion cells that power our phones and cars. Now a team of researchers in China reports a chemistry-level breakthrough that could change that calculus. Writing in Nature Chemistry, a collaboration led by the Dalian Institute of Chemical Physics of the Chinese Academy of Sciences, together with colleagues at Wuhan University, Jilin University and the Institute of Chemistry in Beijing, demonstrates a reversible tin anode that exploits four electrons per tin atom, delivering an anolyte energy density of 205 watt-hours per litre and powering a kilowatt-scale stack that ran stably for more than 1,000 hours. The trick, remarkably, lies in a dash of ordinary iodide added to a tin chloride solution.

The appeal of tin as a negative electrode material in water-based batteries is easy to understand. Unlike zinc, the workhorse metal of many emerging aqueous systems, tin does not grow dendrites, the spiky metal deposits that can pierce separators and short-circuit cells. More importantly, tin can in principle shuttle between the zero oxidation state of metallic tin and the plus-four state of tin(IV), a swing that involves four electrons per atom. In electrochemistry, the number of electrons transferred per active species is a direct multiplier on capacity, so a four-electron couple promises far more stored charge per litre of electrolyte than the one- or two-electron reactions that dominate most aqueous flow batteries today.

There has, however, been a catch that has stymied researchers for decades. The upper half of that four-electron staircase, the conversion between tin(IV) and tin(II), behaves irreversibly in conventional chloride-based electrolytes. When the team measured the cyclic voltammetry of tin(IV) in chloride solution, they found a redox peak potential difference of a punishing 978 millivolts between the reduction and oxidation waves, a signature of sluggish, lossy electron transfer. In practical terms, charging the tin(IV) to tin(II) couple wasted enormous energy and the reaction refused to run backwards cleanly, capping any practical anode at the two-electron tin(II)/tin(0) couple and forfeiting half the theoretical capacity.

The Dalian-led team traced the problem to a concept from classical electron transfer theory: the reorganization energy. According to the framework developed by Rudolph Marcus, an electron transfer reaction requires the surrounding solvent and ligand shell to rearrange itself before and after the electron hops. The larger the structural mismatch between the oxidized and reduced forms of a redox couple, the greater the energy barrier and the slower and less reversible the reaction. Tin(IV) and tin(II) ions coordinated by chloride adopt very different coordination geometries, so every electron transfer forced a costly restructuring of the ion’s immediate environment. The team reasoned that if they could find a ligand that holds the tin coordination sphere more nearly constant across the two oxidation states, the barrier would collapse and reversibility would follow.

Their solution drew on a classic principle of inorganic chemistry, the hard and soft acids and bases concept articulated by Ralph Pearson. Tin(IV) is a relatively hard Lewis acid, and chloride is a hard ligand, but iodide is soft and polarizable. By adding iodide to highly soluble tin tetrachloride solutions at carefully chosen ratios, the researchers steered the tin ions into two distinct hexahalide complexes that each do a specialized job. The tin hexachloride anion, SnCl6(2-), keeps the electrolyte highly concentrated and soluble, providing the volumetric capacity that flow batteries need. The tin hexaiodide anion, SnI6(2-), meanwhile, cushions the tin(IV)-to-tin(II) transition, dramatically reducing the reorganization energy so that the electron transfer becomes fast and reversible.

The electrochemical results were striking. Substituting iodide into the tin coordination sphere slashed the redox peak potential difference from 978 millivolts in the all-chloride system to just 97 millivolts, roughly a tenfold improvement, alongside a substantial decrease in electron transfer impedance. Spectroscopic and structural analyses, including X-ray absorption measurements performed at the BL16U beamline of the Shanghai Synchrotron Radiation Facility, supported the picture of a mixed chloride-iodide coordination environment whose anionic composition tunes both the solubility and the kinetics of the tin redox chemistry. In effect, the team decoupled two properties that normally trade off against each other: you can have a concentrated electrolyte and a fast, reversible electrode reaction at the same time.

To turn this anolyte chemistry into a working battery, the researchers paired the tin negative electrolyte with a bromine-based positive electrolyte built on the well-established bromide/bromine couple. The resulting tin-bromine aqueous flow battery delivered an energy efficiency of 89 percent at a current density of 40 milliamperes per square centimetre, a combination of efficiency and power that few aqueous systems can match. The anolyte itself achieved a capacity of 227 ampere-hours per litre and an energy density of 205 watt-hours per litre, figures that approach the territory of lithium-ion batteries while retaining the intrinsic safety, scalability and decoupled energy-power design of flow systems, in which the energy-storing electrolytes sit in external tanks and the power-determining stack is sized independently.

Crucially, the team did not stop at coin-cell-scale demonstrations, a stage at which many promising battery chemistries stall. They engineered a kilowatt-scale stack from the new chemistry and ran it continuously for more than 1,000 hours with stable operation, an early but meaningful signal that the mixed-halide tin electrolyte can withstand the mechanical and chemical stresses of real hardware: pumping, membrane transport, electrode cycling and long-term exposure to concentrated halide solutions. Stack-level testing is where vanadium flow batteries, the current commercial standard, built their credibility, and the performance comparison presented in the study suggests the tin chemistry is competitive on the metrics that matter for grid storage.

The broader significance of the work may lie in its design philosophy as much as in its numbers. Rather than searching for new elements or exotic electrode materials, the researchers treated reversibility itself as an engineering target, manipulating the coordination chemistry of a solution to lower the reorganization energy of a multielectron redox couple. That approach echoes recent successes in the same group’s orbit, including a reversible iodate-iodide cathode enabled by hetero-halogen electrolytes reported in Nature Energy in 2024, and suggests a generalizable playbook: pick a cheap, abundant, multielectron element, then use ligand design to make its electron transfer reversible. Tin is inexpensive and widely available, and bromine and iodine chemistries are already familiar to the flow battery industry, which eases the path toward commercialization.

Challenges remain before tin-bromine flow batteries can challenge vanadium systems in the field, including the management of bromine crossover and the long-term stability of mixed-halide electrolytes at scale, questions that thousand-hour tests only begin to answer. But the demonstration that a once-irreversible four-electron metal couple can be tamed in water, at high concentration, in a stack that runs for weeks, marks a genuine milestone for aqueous energy storage. If the chemistry scales as its laboratory performance suggests, the humble tin can, reimagined as a four-electron workhorse dissolved in a cleverly seasoned brine, could become a cornerstone of the renewable-powered grid.

Subject of Research: Reversible tin-based aqueous redox flow battery anode chemistry

Article Title: Reversible Sn4+/Sn0 anode for high-energy-density flow batteries

Article References: Ao, Y., Wang, X., Zhao, C., Zhang, Y., Peng, Z., Wang, F., Chen, S., Xie, C., & Li, X. (2026). Reversible Sn4+/Sn0 anode for high-energy-density flow batteries. Nature Chemistry. https://doi.org/10.1038/s41557-026-02275-7

Image Credits: AI Generated

DOI: 10.1038/s41557-026-02275-7

Keywords: flow batteries, tin anode, aqueous batteries, redox chemistry, energy storage, electron transfer, reorganization energy, halide complexes, grid-scale batteries, electrolyte design, tin-bromine battery, Nature Chemistry