Enhancing battery longevity by regulating the solvation chemistry of organic iodide

For rechargeable zinc-iodine batteries, the low electrical conductivity of iodine and the easy dissolution of polyiodide in the electrolyte need to be carefully managed to ensure efficient operation. Herein, we introduce an organic iodized salt, formamidinium iodide (CH₅N₂I), to modulate the solvation structure of iodide ion, aimed to improve the reaction kinetics of iodine for reversible redox conversion. The participation of formamidinium ion (FA⁺) into solvation structure leads to the formation of the favorable FAIZn(H₂O)₄ ²⁺ complex, facilitating easier desolvation for redox conversion with iodine. Consequently, the inhibited formation of soluble polyiodide efficiently suppresses self-discharging and improves reversible redox conversion. Specifically, the fabricated battery demonstrates extraordinary cycling stability, with 90.1 % capacity retention after 10,000 cycles and high coulombic efficiency of 99.6 %. These results provide inspiring principles to develop organohalide materials for high-performance rechargeable batteries through the solvation modification and interface regulation strategies.