Functionalized spirogyra biowaste as a sustainable cathode material for high performance aqueous rechargeable Zn-I battery with dual mechanism.

Zn-ion batteries have been explored rigorously in the last decade owing to its high abundance, ease of availability, and theoretical specific capacity, which makes it a viable alternative to Li-ion batteries. Carbonizing waste biomass for electrode material can make the battery sustainable and cost-effective but the involvement of difficult cleaning and high temperature calcination process negates the cost-effectiveness. As a viable solution, a simple low temperature chemical treatment of spirogyra biomass is being proposed as an alternative to carbon-based cathode material in this work. The treatment of dried spirogyra biomass with orthophosphoric acid led to enhancement in specific surface area, thermal stability, and chemical functional groups. After treatment, the treated material is utilized as an iodine host in aqueous rechargeable Zn-I battery. The battery delivers a high specific capacity of 440 mAh g at 3C rate with an excellent coulombic efficiency of 97 %, owing to a dual charge storage offered by the treated spirogyra biomass and I-catholyte. Additionally, the Zn-treated spirogyra battery without I-catholyte delivers a specific capacity of 116 mAh g at 3C rate. The carboxyl group present in the treated spirogyra biomass cathode contributes with I-catholyte in better retention of Zn ions which is identified by the ex-situ measurements and computational study, thereby displaying higher capacity performance.