Polypyrrole polymerized by iodine oxidation for zinc iodine batteries with high stability

Currently, aqueous rechargeable zinc-iodine (ZnI2) batteries are recognized as promising new energy storage devices due to their significant advantages, including large theoretical capacity, rapid charging and discharging capabilities and high safety. However, the shuttle effect caused by the soluble polyiodide ions leads to low utilization of active iodine and coulombic efficiency (CE). Herein, we use a high-concentration iodine solution as the oxidizing agent to synthesize polypyrrole (PPy/I2) for the first time. Compared with the PPy polymerized by using FeCl3 as oxidant (PPy/FeCl3), the ZnI2 battery based on PPy/I2 cathode demonstrates an extended cycle life (84.2 % after 3800 cycles at 2.0 A g−1), high CE (>98 % at 1.0 A g−1), and minimal self-discharge (75 % after rest 12 h). The results based on electrochemical testing and characterization indicate that the exceptional performance of PPy/I2 is primarily attributed to the early insertion of iodine species during the preparation process. This insertion creates reserved space for polyiodide ions, and the positive charge on the main chains of PPy effectively restricts their shuttle during battery operating. Furthermore, due to the ideal shuttle inhibition of PPy/I2, the Zn corrosion caused by polyiodide ions is effectively inhibited. The detailed characterizations explore the charge storage mechanism of PPy/I2. This research has guiding significance for improving the performance of PPy-based ZnI2 batteries.