The solid electrolyte interphase (SEI) between Li and ionic liquid electrolytes was investigated in 1-methyl-1-propylpyrrolidinium bis(fluorosulfonyl)amide (MPPFSA) and 1-methyl-1-methoxyethylpyrrolidinium bis(fluorosulfonyl)amide (MOEMPFSA) with different LiFSA concentrations. The physicochemical properties and the coordination states of Li+ with FSA– with increasing the LiFSA concentration were similar between xLiFSA-MPPFSA and MOEMPFSA. On the other hand, the SEI resistance obtained by electrochemical impedance spectroscopy using a Li∣Li symmetrical cell increased with the lapse of time in xLiFSA-MPPFSA more significantly than in xLiFSA-MOEMPFSA, suggesting that the SEI formed in xLiFSA-MOEMPFSA was more conductive than that in xLiFSA-MPPFSA. The SEI formed on Li was considered to be composed mainly of the ions in the electrolytes and their decomposed products by X-ray photoelectron spectroscopy. The cyclability of deposition and dissolution of Li was improved with increasing the LiFSA concentration in both xLiFSA-MPPFSA and MOEMPFSA. An increase in the conductivity of the SEI with increasing the LiFSA concentration may lead to the promotion of homogeneous growth of Li rather than dendritic growth of Li.

Thermal stability of electric conductivity of polypyrrole (PPy) was improved using aromatic sulfonate derivatives as dopants having mobile hydrogen in the form of acid groups such as sulfonic acid, carboxylic acid, or hydroxide groups. Particularly, when m-sulfobenzoic acid or 5-sulfosalicylic acid were used as dopants, the PPy showed high thermal stability of electric conductivity and kept 95% electric conductivity even after heating for 8 h at 150 °C in air.

No abstract

In order to improve the thermal stability of the electric conductivity of polypyrrole (PPy), a series of aromatic sulfonate derivatives having acid-type substitute groups, such as –COOH, –OH, and –SO3H, were used as dopants. The PPy doped with these dopants showed excellent thermal stability of their electric conductivity, even at 150 °C in air. Especially, 2-hydroxy-5-sulfobenzoic acid, having one –COOH and one –OH, provided the highest thermal stability; the doped PPy maintained 95% of its initial conductivity, even after heating for 8 h at 150 °C in air. Furthermore, the PPy showed 20-times higher stability against long-term heating at 125 °C for 1000 h in comparison with PPy doped with p-methylbenzenesulfonic acid, which is conventionally used to provide high thermal stability. The thermal-stabilization mechanism has suggested that dopants having acidic substituents should suppress proton dissociation from the N-position of the PPy main chain and keep the π-conjugation structure by supplying a proton from the acidic groups. Such a proton supplement was confirmed by the IR spectroscopy of deuterized PPy.

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