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

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.