Polypeptide Radical Cathode for Aqueous Zn-Ion Battery with Two-Electron Storage and Faster Charging Rate

The rapidly growing demand for batteries has led to a lack of global mineral resources and rechargeable organic batteries are paid extensive attention, owing to the abundance resources, light weight, and high flexibility of organic electrodes. However, most organic electrodes that use aliphatic backbones are nondegradable, leading to unsustainability when active sites fail. In this study, a poly(aspartic acid) polypeptide (PASP) with amide links in the backbone and nitroxide radical pendant groups in the side chains is synthesized by modifying the polypeptides with 4-amino-2,2,6,6-tetramethylpiperidine. In combination with a Zn anode, the PASP-TEMPO composite electrode exhibits rapid charge-discharge and superior cycling stability with reversible two-electron redox reaction in aqueous electrolyte. The Zn/PASP-TEMPO organic radical battery delivers a discharge capacity of around 80 mAh g^-1 by two-electron reaction and charge-discharge rates of up to 18 A g^-1 . Because the redox reaction process of the nitroxyl radical turning into oxoammonium follows a p-type mechanism that interacts with an anion, three electrolytes with different anions are tested in the Zn/PASP-TEMPO organic radical battery. Experimental results indicate that discharge plateau voltage is tunable by choosing different zinc salts as electrolytes. Capacity retention of up to 97.4 % after 500 cycles is realized in 1 m ZnClO₄ electrolyte, which can be attributed to the adjacent reaction potentials of the two-step one-electron reaction.