Lithium Cobalt Oxide (LiCoO 2 ) Nanocoatings by Sol-Gel Methods Forest T. Quinlan, † Ruxandra Vidu, † Luminita Predoana, ‡ Maria Zaharescu, ‡ Mariuca Gartrner, ‡ Joanna Groza, † and Pieter Stroeve* ,† Institute of Physical Chemistry, “I. G. Murgulescu” Romanian Academy, 202 Splaiul Independentei, 77208 Bucharest, Romania, and Department of Chemical Engineering and Materials Science, University of California, Davis, 1 Shields Avenue, Davis, California 95616 Lithium cobalt oxide (LiCoO 2 ) was synthesized using two different sol-gel processes. The first method used stoichiometric molar ratios of cobalt 2-methoxyethoxide and lithium nitrate in alcoholic solutions; the second method used aqueous equimolar ratios of precursors of lithium nitrate and cobalt acetate with poly(ethylene glycol) (PEG) 200 as the chelating agent. Lithium cobalt oxide films were deposited on Si wafers and silica-soda-lime glass using a dipping technique. Based on differential thermal analysis/thermogravimetric analysis results, densifi- cation of the films was achieved by thermal treatment at both 500 and 800 °C in the case of silicon wafer substrates and at 500 °C for the silica-soda-lime glass. X-ray diffraction, spectroellipsometry, and atomic force microscopy were used to characterize the films. The correlation between the preparation procedure and the type of support on the structure and morphology of LiCoO 2 compound films is discussed. Uniform ultrathin films were obtained on the glass (heat treated at 500 °C) and Si wafer (heat treated at 800 °C). The alcoholic synthetic procedure was also used to coat particles of LiMn 2 O 4 with films of LiCoO 2 . The sol-gel method resulted in partial coverage of LiMn 2 O 4 by LiCoO 2 as determined by elemental mapping with scanning electron microscopy. 1. Introduction The use of LiCoO 2 as cathodes for rechargeable lithium batteries is due to its high theoretical specific capacity, ease of preparation, and excellent stability. Because both oxidation and reduction reactions occur during the charge/discharge process at the positive material of LiCoO 2 or LiMn 2 O 4 in a Li battery, it is more appropriate to refer to the cathode as “positive active material”. It is anticipated that lithium ion batteries will become one of the most prevalent types of energy storage batteries of the future. The preparation of LiCoO 2 powders has been studied extensively with regard to the application toward microbatteries. 1 How- ever, few studies have been done on LiCoO 2 film coatings. Because of a recent trend to construct microbatteries, interest in thin film coatings has increased. Novel fabrication techniques, based on physical methods for the preparation of thin layers of solid electrolytes and positive active materials for lithium batteries, have been developed: electrostatic spray deposition, 2 electrostatic spray pyrolysis, 3,4 chemical vapor deposition, 4 plasma chemical vapor deposition, 5 and pulsed laser deposi- tion. 6,7 Lithium cobalt oxide films have also been deposited by radio-frequency (RF) magnetron sputtering of a LiCoO 2 target. 8-10 The electrical conductivity of a sputter-deposited LiCoO 2 thin film was studied in situ during electrochemical lithium insertion/extraction by Nishizawa et al. 11 Wet chemical methods have also been used for the preparation of these films. Han et al. 12,13 and Song et al. 14,15 prepared LiCoO 2 films by hydrothermal treat- ment of a cobalt metal plate in a concentrated LiOH solution at fixed temperatures between 20 and 200 °C without any postsynthetic annealing. Thin films of LiCoO 2 (0.22-1.15 μm) were prepared also by Sato et al. 16,17 by oxidation of metallic cobalt in molten alkali carbonates containing Li + ions. Svegl et al. 18 prepared layered Li x CoO 2 thin films (x similar to 1) by a peroxo wet chemistry procedure from lithium(I) and cobalt(II) acetate precursors and the addition of H 2 O 2 . Among wet chemical methods of obtaining thin films, the sol-gel procedure is the most widely used. The advantages of the sol-gel method are homogeneity at the molecular level, control of stoichiometry, ease of doping, high deposition rate, and low cost. Chang et al. 19 prepared lithium-cobalt(III) oxide films by a spray- coating process using a series of sols as precursors and different reaction conditions. Their procedure changed the microstructure of the films and affected the elec- trochemical performance of lithium ion batteries. Chen et al. 20 also prepared LiCoO 2 films by spraying alcohol solutions of nitrates and acetates of cobalt. To enhance the performance of LiMn 2 O 4 positive active materials, Park et al. 21,22 and Kim et al. 23 coated the surface of LiMn 2 O 4 particles with a LiCoO 2 film by a sol-gel method. The present work presents a systematic study on the correlation between the sol-gel preparation procedure, type of substrate, and influence of these factors on the structure and morphology of LiCoO 2 films. Two syn- thetic procedures of film preparation, alcoholic and aqueous, were employed. Further, we explore the use of sol-gel methods to coat LiMn 2 O 4 nanoparticles via the alcoholic synthesis procedure. Compared to previous work, an advantage of our work is that the sol-gel reactions are carried out at room temperature. * To whom correspondence should be addressed. † University of California, Davis. ‡ “I. G. Murgulescu” Romanian Academy. 2468 Ind. Eng. Chem. Res. 2004, 43, 2468-2477 10.1021/ie034086r CCC: $27.50 © 2004 American Chemical Society Published on Web 04/17/2004