Thionin as a Bipolar Organic Cathode Material for Aqueous Rechargeable Zinc Batteries

Graphical Bipolar ion storage : Thionin (Thn-CH 3 COO), a biological dye, can be also used as a high-performance bipolar-type organic cathode material for aqueous rechargeable zinc batteries. In the Zn(CF 3 SO 3 ) 2 electrolyte, both CH 3 COO − and CF 3 SO 3 − anions participate the p-type reaction, while H + cation dominate the n-type reaction. Organic cathode materials (OCMs) have been widely applied in aqueous rechargeable zinc batteries (ARZBs), but there are still many novel structures to be explored for better electrochemical performance and clearer mechanism. Herein, we have studied an organic biological dye, namely thionin (Thn-CH 3 COO), as a bipolar-type OCM for ARZBs based on its electroactive phenothiazine unit. In the optimal electrolyte of 2 M Zn(CF 3 SO 3 ) 2 , Thn-CH 3 COO exhibited the lowest solubility and thus the highest cycling stability (94 % capacity retention after 100 cycles), with a reversible capacity of 162 mAh g −1 (the theoretical value is 186 mAh g −1 ) and a discharge plateau at ∼0.8 V vs. Zn 2+ /Zn. Various characterization and DFT calculations have revealed that both CH 3 COO − and CF 3 SO 3 − anions and H + cations participated the p-type and n-type reactions, respectively. In brief, the novel structure, competitive performance, and exhaustive mechanism investigation will facilitate the further development of ARZBs based on OCMs.