Investigation of polypyrrole as cathode material for zinc-ion batteries with bio-ionic liquid-based electrolyte

Zinc-ion batteries (ZIBs) are regarded as promising alternatives to lithium-ion batteries (LIBs). However, the nature of zinc ions limits the development of cathode materials for ZIBs. The most investigated cathode materials for ZIBs are inorganic materials, such as metal oxides and Prussian blue analogues (PBAs). However, phase transition and dissolution of these inorganic materials during cycling can be observed, which causes significant capacity loss. Recently, inert conductive polymers have attracted extensive interest as cathode materials for ZIBs, as they can inhibit the dissolution process and they are inexpensive, environmentally friendly. Furthermore, the dendrite formation on Zn anode and hydrogen evolution reaction in aqueous electrolytes might lead to significant safety issues and poor battery performance. In this thesis, polypyrrole (PPy) and PPy/lignin composite electrodes have been obtained though electrochemical deposition and tested as cathodes in ZIBs with a bio-ionic liquid-water mixture electrolyte. Zn/PPy batteries with aqueous and bio-ionic liquid-water mixture electrolytes showed different initial capacity and cycling stability. In situ Raman spectroscopy, X-ray photoelectron spectroscopy (XPS) have been applied to investigate the Zn storage mechanisms in PPy in aqueous and bio-ionic liquid containing electrolytes. It has been identified that, Zn intercalation/deintercalation process have been changed on addition of bio-ionic liquid into the electrolyte. Moreover, it has been observed that the presence of bio-ionic liquid helps to reduce the strain within the polymer matrix during Zn intercalation. Although Zn/PPy battery with bio-ionic liquid-water mixture electrolyte showed better performance than those with aqueous, significant loss in capacity during cycling still can be observed. Therefore, modification of PPy cathode is required. Lignin has been selected to modify the PPy cathode, as it has low cost and environmental friendliness. The batteries with PPy/lignin composite electrodes showed interesting higher discharge capacity phenomena. According to the IR and XPS results, electrocatalytic oxidation/dissolution process can be identified, which might lead to a secondary reaction, and therefore produce additional energy during cycling. It has been observed that, PPy electrodeposited from aqueous and organic electrolytes showed different initial capacity and cycling stability. Therefore, it is interesting to investigate the influence of electrolytes on the PPy deposition and their performance in ZIBs. PPy films have been successfully electrodeposited from two more ionic liquids (1-ethyl-3- methylimidazolium trifluoromethylsulfonate ([EMIm]TfO) and 1-ethyl-3-methylimidazolium bis(trifluoromethylsulfonyl)imide ([EMIm]TFSI)). The PPy obtained from [EMIm]TFSI showed both the highest initial capacity and the best cycling stability. Based on the in situ Raman spectra, the Zn storage mechanism in PPy prepared in [EMIm]TFSI electrolyte is different from PPy obtained in aqueous, acetonitrile and [EMIm]TfO electrolytes. Moreover, according to the XPS results, an anion exchange process can be observed of PPy obtained from aqueous, acetonitrile and [EMIm]TfO electrolytes. However, during cycling no anion replacement of PPy electrodeposited form [EMIm]TFSI can be observed, which might help to improve the battery performance.