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.
The processes at the interface between ionic liquids (ILs) and metals are a key factor for understanding especially in electrochemical deposition, nanoscale tribology applications and batteries. In the present work, the interfaces of 1-butyl-1-methylpyrrolidinium bis(trifluoromethylsulfonyl)imide ([Py1,4]TFSI) and 1-ethyl-3-methylimidazolium bis(trifluoromethylsulfonyl)imide ([EMIm]TFSI) and platinum and aluminum were investigated by depositing thin IL films and studying them with X-ray photoelectron spectroscopy (XPS) in ultrahigh vacuum. It is found that there is no evidence of a decomposition reaction of either IL on platinum; however, the imidazolium cation of [EMIm]TFSI shows a strong interaction with the surface in the monolayer regime. In contrast, [Py1,4]TFSI and [EMIm]TFSI show massive decomposition on the aluminum surface without applying any electrochemical potential. The spectra for the [TFSI]− anion components show cleavage of C-F or N-S bonds in both cases. Both cleavage of a single fluorine atom and complete cleavage were observed, leading to further decomposition reactions of the anion. Consequently, new components such as AlOOH, Al(OH)3, Al2S3, Al2(SO4)3 and AlF3 appear at the interface. In addition, there is also evidence of decomposition of the cation by the splitting off hydrogen atoms or parts of the alkyl chain in both ILs.
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