User-side energy storage is critical to low-carbon energy deployment across residential, commercial, industrial and transportation sectors, driving rising demand for energy-storage technologies featuring high energy density and safety.

Tin-based anodes represent promising candidates for high-energy-density flow batteries, owing to their high theoretical specific capacity, high solubility, dendrite-free deposition behaviour, and the potential for four-electron transfer. Nevertheless, the conversion of Sn⁴⁺ to metallic Sn proceeds via two sequential electrochemical reactions: Sn⁴⁺/Sn²⁺ and Sn²⁺/Sn⁰. The Sn⁴⁺/Sn²⁺ conversion suffers from high reorganization energy, sluggish reaction kinetics and poor reversibility, which restricts access to the full four-electron capacity.

To tackle this challenge, a research team led by Prof. LI Xianfeng from the Dalian Institute of Chemical Sciences (DICP) of the Chinese Academy of Sciences (CAS), in collaboration with Prof. CHEN Shengli from Wuhan University, has developed a coordination-structure modulation strategy to boost the kinetics and reversibility of tin redox reactions, realizing highly reversible four-electron transfer.

The study was published in Nature Chemistry on Oct. 9.

The researchers incorporated iodide ions into a highly soluble tin tetrachloride (SnCl₄) electrolyte to build a coordination environment comprising both SnCl₆^2- and SnI₆^2- species. SnI₆^2- reduces the reorganization energy for Sn⁴⁺/Sn²⁺ electron transfer, enabling faster and more reversible redox reactions, whereas SnCl₆^2- preserves the high solubility of tin salts within the electrolyte. This mechanism sustains continuous four-electron transfer from Sn⁴⁺ to Sn⁰ via the Sn²⁺ intermediate state.

Using this strategy, the potential gap between the Sn⁴⁺/Sn²⁺ redox peaks narrowed from 978 mV under a chloride-dominated coordination environment to 97 mV in the iodide-containing system. The team also detected reduced charge-transfer impedance, indicating improved electrochemical kinetics.

To assess the strategy in practical battery hardware, the researchers paired the highly reversible Sn⁴⁺/Sn⁰ anode with a Br^-/Br₂ cathode to construct a Sn-Br flow battery. At a current density of 40 mA cm^-2, the battery delivered an energy efficiency of 89 %. The anode electrolyte attained a volumetric capacity density of 227 Ah L^-1, corresponding to an energy density of 205 Wh L^-1.

The researchers further scaled up the technology and demonstrated its performance in a kilowatt-scale stack. The stack maintained stable operation for over 1 000 hours, highlighting the promise of this coordination-engineering approach for developing high-energy-density flow batteries.