Single atom catalysts for triiodide adsorption and fast conversion to boost the performance of aqueous zinc-iodine batteries

Zinc-iodine (Zn-I2) batteries are promising for energy storage because of their low cost, environmental friendliness, and attractive energy density. However, triiodide dissolution and poor conversion kinetics hinder their application. Herein, we demonstrated that the ‘shuttle effect’ in Zn-I2 batteries can be suppressed via single atom catalyst (SAC) cathodes because of efficient catalytic activity in I2/I3−/I− reactions and their ability to adsorb I3−. Based on DFT computations, an I− poisoning mechanism was proposed for SAC selection to suppress the shuttle effect in Zn-I2 batteries. I− formation and desorption are crucial to maintaining the catalytic and adsorption role of metallic elements. SACu favours the reduction of I2 to I and exhibits a low energy barrier to release I− from the surface, thus allowing more rapid conversion kinetics, while at the same time suppressing the shuttle effect of I3− in Zn-I2 batteries. In contrast, without sufficient energy, the final product of I− will remain adsorbed at the metal site of SAFe, SAMn, SAV, and SATi, thus killing the catalytic activity of SACs to facilitate the iodine reduction reaction (IRR). To confirm practicality, single-atom Cu-embedded nitrogen-doped Ketjen black (SACu@NKB), together with SACo@NKB and NKB, were synthesized and electrochemically assessed. The as-prepared SACu@NKB outperformed the SACo@NKB and NKB cathodes in terms of reversible capacity and cycle life. In addition, a rate-limiting step in these redox reactions was identified, and overpotential was estimated, and these were found to be dependent on the d-band centre of SACs. A lower d-band centre can be associated with more optimal catalytic performance in SACs. This work reveals that the superior cycle life of Zn-I2 batteries is underpinned by the catalytic and adsorption role of metallic catalysts, and we report an in-depth understanding of how this boosts the performance of Zn-I2 batteries, with implications for future long-life battery design.