Zinc Storage Mechanism in Polypyrrole Electrodeposited from Aqueous, Organic, and Ionic Liquid Electrolytes: An In Situ Raman Spectroelectrochemical Study

Zinc-ion batteries (ZIBs) are emerging as cheap and safe alternatives to lithium-ion batteries (LIBs). However, due to the divalent nature of Zn ions, it is a challenge to choose suitable cathode materials for ZIBs. Conductive polymers are regarded as promising materials for zinc storage. However, little has been understood regarding the Zn storage mechanism, thereby making it difficult to rationally modify the polymer to improve its performance and stability. In the present study, polypyrrole (PPy) was electrochemically deposited from aqueous, organic, and ionic liquid electrolytes, and the Zn storage mechanism was investigated in PPy using electrochemical methods, X-ray photoelectron spectroscopy (XPS), and in situ Raman spectroscopy. From in situ Raman spectroscopy, it was observed that Zn storage in PPy electrodeposited from ionic liquid took place by a shrinking and stretching mechanism (structural change), whereas a phase transformation mechanism was observed for PPy electrodeposited from aqueous and organic electrolytes. The change in Zn storage mechanisms led to different initial capacities of PPy deposited from different electrolytes. The initial capacity of PPy deposited from 1-ethyl-3-methylimidazolium bis(trifluoromethylsulfonyl)amide ([EMIm]TFSI) can achieve 126 mAh g-1 and the PPy deposited from 1-ethyl-3-methylimidazolium trifluoromethylsulfonate ([EMIm]TfO) can reach 90 mAh g-1, which is higher than 75 mAh g-1 for the PPy deposited from acetonitrile and 20 mAh g-1 for the PPy deposited from aqueous solution. Furthermore, XPS results showed that an insignificant amount of zinc was trapped in the PPy deposited from [EMIm]TFSI after 100 charge-discharge cycles. Therefore, the PPy deposited from [EMIm]TFSI showed better cycling stability, after 100 cycles. The different zinc storage mechanisms of PPy from different electrolytes offer possibilities to improve the cycling stability for a Zn/PPy battery by modulating the electrode and electrolyte compositions.