The pH effect of
poly(3,4-ethylenedioxythiophene):poly(4-styrenesulfonate) (PEDOT:PSS)
water dispersion on colloidal particle size (D
50), zeta potential (ζ), and electrical conductivity
was investigated. An increase in the pH of the PEDOT:PSS water dispersion
from 2 to 11 increased the D
50 from 10
to 100 nm owing to the aggregation of the colloidal particles and
decreased the electrical conductivity from 750 to 62 S cm–1 by dedoping, respectively, while the ζ remained almost constant
at ca. – 50 mV. Furthermore, aluminum solid electrolytic capacitors
were fabricated using PEDOT:PSS as a cathode material. It was found
that the electrical characteristics of the PEDOT:PSS aluminum solid
electrolytic capacitors were optimized at pH 3, where D
50 and electrical conductivity played an important role
for low equivalent series resistance (ESR) and high capacitance (Cap).
Furthermore, the ESR decreased and Cap increased by repeating the
fabrication process, where the Cap usable rate reached as high as
92% because of the increase in the surface coverage of the etched
aluminum foil with the PEDOT:PSS.
Li metal has a high specific capacity, and films formed on the surface of Li metal must thus be stabilized in some way. We used a nonionic surfactant resembling poly(ethylene glycol) dimethyl ether (PEGDME) to achieve this stabilization. The success of this stabilization as a function of the molecular weight of the polyether (Mw 90 to 2000) was investigated with microelectrode voltammetry (MEV) and the in situ quartz crystal microbalance method. In 1 mol dm Ϫ3 LiClO 4 /PC, the activation energy (⌬G*) for charge transfer of the Li/Li ϩ couple (as obtained from MEV measurements) increased by 11 to 60 kJ mol Ϫ1 as the molecular weight (g mol Ϫ1 ) was increased from 90 to 400, and remained almost unchanged as it was increased from 400 to 2000. The variation of ⌬G* indicates that Li ions were preferentially coordinated with ethylene oxide (EO) chains and were taken into the helix structures of the EO chains when PEGDME (which possesses repeated EO units in the range of 180 to 2000 Mw) was added to electrolytes. As for the stoichiometry, the surface film formed in the presence of PEGDME with Mw Ն 180 causes no accumulation during the Li deposition and dissolution cycles. Such a stable film including a few EO units eventually functioned as a uniform path for Li ions in both deposition and dissolution processes. The extent of inactivation of the deposited Li for various molecular weights of the PEGDME was consistent with the change of solvation state and surface chemistry. Namely, the inactivation for the PEGDME-added (with Mw Ն 180) systems was diminished markedly (by ca. 40 to 65%) as compared with surfactant-free and monomer (Mw 90)-added systems.
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