Mechanism of Zn-Ion Intercalation/Deintercalation in a Zn–Polypyrrole Secondary Battery in Aqueous and Bio-Ionic liquid Electrolytes

Zn-ion batteries(ZIB) have recently emerged as a promising and rather cheap alternative to Li-ion batteries. However, the divalent charge of Zn limits the choice of cathode materials, whereas the choice of electrolyte is limited by hydrogen-evolution reaction. Polymer cathodes have been shown to be a promising material for ZIB. In this paper, we have studied in detail a Zn/polypyrrole battery in both aqueous and bio-ionic liquid-water mixture electrolytes. From in situ Raman spectroelectrochemistry, it was observed that in aqueous solution, Zn intercalation/deintercalation takes place by a two-step mechanism, whereas a single-step mechanism for Zn storage was involved in bio-ionic liquid-water mixture electrolytes. The charge-discharge measurements showed a higher Zn-storage capacity in the mixture of bio-ionic liquid-water electrolyte compared to the aqueous electrolyte. However, with cycling, a capacity loss was observed. Post analysis of the polymer after cycling showed that a phase transformation has taken place in the polymer with Zn ions trapped in the polymer matrix that decreased the Zn-storage capacity. Furthermore, the Zn anode showed the formation of Zn nanoflakes from aqueous electrolytes that might lead to dendritic growth, whereas dendrite-free Zn nanoparticles were observed on using the bio-ionic liquid-water electrolyte.