Mixed chloride–iodide electrolyte lets tin flow-battery anodes use all four electrons

Researchers in China have found a way to make acidic tin electrolytes use all four of the electrons that tin can exchange, rather than the two typically accessed. Adding iodide ions to a chloride-based solution made a sluggish step reversible, and the chemistry was demonstrated in a tin–bromine flow battery and a 20-cell stack that ran for more than 1,000 hours. The work, led by Yue Ao and Congxin Xie at the Dalian Institute of Chemical Physics with Xiaoxiao Wang of Wuhan University, appears in Nature Chemistry.

Why tin's second half-reaction matters

In a flow battery, liquid electrolytes are pumped through cells where reactions store and release energy. The more charge each litre of liquid can hold, the less volume is needed for a given storage capacity. Tin can in principle supply a lot: it moves between dissolved ions and solid metal in two steps, first gaining two electrons and then another two, with the metal depositing on an electrode. Discharge runs the sequence in reverse.

In acidic tin batteries, however, only the two-electron reaction is commonly used, so half of the possible charge goes unused. Earlier work has achieved four-electron tin chemistry in alkaline solutions. The new study addresses a different problem: making the first step readily reversible in an acid electrolyte.

The role of tin's chemical neighbours

Experiments and molecular simulations traced the difficulty to the ions surrounding dissolved tin. Chloride binds strongly around tin, and that arrangement has to reorganise before electrons can transfer. The reorganisation costs energy and slows the reaction.

Iodide forms weaker bonds, so the rearrangement is easier. On its own, though, iodide has a drawback: the tin compound hardly dissolves in it, and an electrolyte with little active material cannot hold much charge, however well the reaction proceeds. Combining the two ions resolved the trade-off. Chloride kept plenty of tin in solution, while iodide created surroundings that eased electron transfer. The proportions mattered, because too much iodide caused material to precipitate.

Performance in cells and a stack

Paired with bromine chemistry on the opposite side of the battery, the mixed electrolyte returned about 89% of the electrical energy supplied during charging. A chloride-only comparison cell returned 53% at the same tested current density.

Separate experiments with a more concentrated formulation reached 205 watt-hours per litre of tin-side electrolyte. The team then built a stack of 20 cells delivering roughly 800 watts, which completed more than 260 charge–discharge cycles over 1,000 hours. Tin deposits showed no obvious sharp, branching growths during extended testing, which is encouraging for a system that repeatedly plates and strips metal.

Limitations and next steps

The tests also exposed a weakness. Iodide and tin ions gradually crossed the membrane separating the two sides of the battery. As iodide was lost from the tin electrolyte, the newly enabled reaction became less reversible and efficiency fell. Some tests also showed declining capacity.

Longer operating times will therefore require ways to keep the electrolyte composition stable, for example through membranes that limit crossover or strategies that restore iodide balance. More broadly, the study illustrates that the ions around an electroactive species can decide how much of its theoretical capacity is usable, a principle that may apply to other multi-electron materials considered for grid-scale storage.

Source: Electrolyte redesign unlocks tin's full charge capacity for flow batteries (Tech Xplore, 31 October 2026).

Image: AI-generated illustration.