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This study reports the preparation of Al2O3 and TiO2 coatings on the as-prepared LiCoO2 electrodes using atomic layer deposition (ALD). A thin Al2O3 ALD coating was shown to eliminate capacity fading effectively during repeated charging and discharging, whereas a TiO2 coating led to significant improvement only at high cycle numbers. An analysis of the differential capacity versus potential curves suggests that this poorer cycling performance could be related to the participation of the TiO2 thin film in the redox reaction. Graphical representation of the energy levels of the various ALD coatings on LiCoO2 during charging and discharging indicated that the redox current is impeded at the Al2O3–LiCoO2 junction, whereas electrons and holes were energetic enough to flow into the TiO2 because of the smaller band gap energy. The barrier between the valence band maxima of TiO2 and LiCoO2 expands as the charge–discharge cycle number increases, eventually making TiO2 redox-inactive. These conclusions are supported by both XPS spectra and the cycle performance in the established literature references. Our results suggest that large band gap materials should be considered to be potentially useful ALD coatings on cathode materials.

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Conducting polymers have attracted considerable attention due to their varied potential applications in batteries, 1,2 electrochromic devices, 3 sensors, 4,5 and electrocatalysis. 6 Among all conducting polymers, polypyrrole (PPy) is the most studied polymer due to its high stability and simple preparation. 7 The kinetics of dopingdedoping processes and electrochemical behavior of PPy are mainly dependent on the surface properties. An understanding of the microstructure of PPy is very important for development of future applications. In the literature, generally the nucleation and growth mechanisms of conducting polymers have been investigated either by means of current-time transient or microscopic/spectroscopic measurements. [8][9][10][11][12][13][14][15][16][17][18][19][20] It is known that the nucleation and growth mechanism of conducting polymers are very similar to those of metals. Thus the mechanism for conducting polymer growth can be proved by using the theory model of metal growth. 21,22 The mechanism of nucleation includes instantaneous and progressive nucleation, and the direction of nucleation includes two-dimensional (2D) and three-dimensional (3D) growth. Many reports indicated that the nucleation and growth mechanisms are instantaneous 2D and progressive 3D. 22,23 A few reports revealed that the nucleation mechanism is 2D layer by layer growth. 21,[24][25][26] The nucleation and growth mechanisms are complicated because they involve so many parameters, including temperature, pH value, concentration, amount of charge passed, and nature of the substrate. Hence, in addition to microscopic studies, it is necessary to study the nucleation and growth mechanisms by some other techniques also. Usually in the literature the nucleation and growth mechanisms were reported by ellipsometry 10,27 or scanning tunneling microscopy (STM) [28][29][30] or atomic force microscopy (AFM) 31,32 measurements. Recent reports also explained the morphological changes during the electropolymerization of pyrrole using microscopic techniques. 33,37 The aim of the present work is to explore the nucleation and growth mechanisms of electropolymerization of pyrrole on a gold/highly oriented pyrolytic graphite (Au/HOPG) substrate for different amounts of charge passed. The mechanism is proposed from current-time transient measurements and compared with the theoretical model. The proposed mechanism is also verified with AFM images. ExperimentalAll chemicals used were of analytical reagent grade from Aldrich Chemicals and were used without further purification. The HOPG (Union Carbide, USA) used in this investigation had a surface area of 0.2375 cm 2 and was cleaved before use. In our experiment the gold was deposited on HOPG by evaporation. The thickness of the deposit was about 1-2 nm. The electrochemical cell consisted of Au/HOPG, Pt/Ti mesh, and Ag/AgCl electrodes as the working,

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