The ªmetallicº state of conducting polymers continues to be a topic of interest and controversy. [1] Although disorder is generally recognized to play an important role in the physics of ªmetallicº polymers, the length scale of the disorder and the nature of the metal±insulator (M-I) transition are the central unresolved issues. [1±3] In particular, the question of whether disorder is present over a wide range of length scales or whether the properties are dominated by more macroscopic inhomogeneities has been a subject of considerable discussion. In the former case, [2] the M-I transition would be described by conventional localization physics (e.g., the Anderson transition), while in the latter case, the M-I transition would be better described in terms of percolation between metallic islands. [3] Recent progress in the processing of conducting polymers has significantly improved the quality of the materials with corresponding improvements in the electrical conductivity. An example is polypyrrole doped with PF 6 , PPy-PF 6 . [4] Transport studies [5] demonstrated that the improved material is more highly conducting and more homogeneous than that studied earlier. As is typical of conducting polymers, PPy-PF 6 is partially crystalline. The structural coherence length, x, is, however, only »20±50 , less than any length used to characterize the electronic properties near the M-I transition, i.e., less than the inelastic scattering length (L in » 300 ) in the metallic regime, and less than the localization length (L c » 200±300 ) in the insulating regime. [2,5] The corresponding transport data in the critical regime and the crossover from metal to insulator have been successfully analyzed in terms of conventional disorderinduced localization. [5] In spite of the evidence for the disorder-induced M-I transition as inferred from the transport [5] and optical measurements, [6] the metallic state of PPy-PF 6 remains a subject of controversy. Kohlman et al. [7] reported infrared (IR) reflectance measurements, R(o), which they analyzed in terms of the frequency-(o-) dependent optical constants. They reported a zero-crossing in the dielectric function, e 1 (o), at o » 250 cm ±1 (well below the p-electron plasma frequency at 1.2 eV). At frequencies below the zero-crossing, they reported e 1 (o) becoming increasingly negative. This low-frequency zero-crossing is not consistent with a disordered metal near the M-I transition; Kohlman et al. attributed the zero-crossing to the plasma resonance of a low density of ªdelocalized carriersº with a long scattering time (t » 10 ±11 s). They concluded that metallic PPy-PF 6 is inhomogeneous, consisting of a composite of metallic islands (crystalline regions) embedded in an amorphous matrix and interpreted the M-I transition in terms of percolation between the metallic islands. The inference of a small fraction of carriers with long relaxation time was used to predict ultra-high conductivity polymers in which all the carriers were delocalized with similarly long scattering times. [7]...

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Electrochemical impedance spectroscopy has been applied to investigate the formation of insulating layers at the surfaces of microscopic particles of mesocarbon microbeads (MCMB), graphite, and hard carbon during the first Li-intercalation into these materials at ambient temperature as well as at -20°C. Investigations were carried out in a three-electrode sandwich cell, designed for impedance measurements in the frequency range 64 kHz to 5 mHz. The impedance spectra, obtained in the potential range 1.5 and 0.02 V during the first charge, were analyzed by complex nonlinear least square fits. A new model, taking into account the porous structure of the intercalation material, electrochemical processes at the interface, as well as spherical diffusion of Li ions toward the centers of the particles, has been used for this analysis. The first intercalation at -20°C results in formation of an insulating layer, which is about 90 times thinner than in the room-temperature case, as concluded from an analysis of experimental results. The irreversible capacity loss, which is 1.3 times larger at -20°C than at room temperature, is ascribed to the formation of a porous precipitate of electrolyte decomposition products on the particle surface. Additional Li intercalation at room temperature results in an irreversible capacity loss of 26% from the initial value, and formation of a composite layer, including low-temperature and room-tempekature deposited components * Electrochemical Society Active Member.by Jacobsen and West.13 The impedance of the insulating layers on the surfaces of the particles has been included in the model as proposed by authors in Ref. 7. The results of the analysis of impedance spectra, measured during the first Li intercalation into several carbon-based materials, are compared for conventional and low-temperature cases.

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