Cesium Tin Iodide Perovskites Engineer the Sn ⁴⁺ /Sn ²⁺ Redox Reaction in Aqueous Zinc‐Ion Batteries

ABSTRACT

Tin displays rich redox chemistry transitioning between metallic tin (Sn ⁰ ), stannous (Sn ²⁺ ), and stannic (Sn ⁴⁺ ) states, among which the Sn ⁴⁺ /Sn ²⁺ couple offers the highest theoretical potential ( >0.14 V vs. SHE) whereas remaining unrealized in practical batteries. Herein, we introduce the in situ and successive growth of CsSnI ₃ single perovskite and Cs ₂ SnI ₆ double perovskite layers on the cathode surface during operation, enabling the first demonstration of the Sn ⁴⁺ /Sn ²⁺ redox reaction in zinc‐ion batteries, with an increased potential to 0.39 V versus SHE due to enhanced tin‐halide coordination. Further integrated into Zn||SnCl ₄ full batteries, the design delivers dual discharge plateaus at 1.18 V (Sn ⁴⁺ /Sn ²⁺ ) and 1.28 V (I ⁰ /I ⁻ ), yielding a high areal capacity of 0.80 mAh cm ⁻² at 1 mA cm ⁻² and high energy density of 0.99 mWh cm ⁻² (660 Wh kg ⁻¹ , per SnCl ₄ ·5H ₂ O mass of the cathode). The three‐electron transfer design exhibits considerable longevity, with Swagelok cells retaining 7000 cycles at 16 mA cm ⁻² and pouch cells sustaining 200 cycles at 4 mA cm ⁻² without capacity loss. By enabling the high‐potential Sn ⁴⁺ /Sn ²⁺ redox reaction, the perovskite interlayer provides a cross‐disciplinary strategy to advancing high‐energy aqueous zinc‐ion batteries.