LiCoO 2 Thin Films Grown by Pulsed Laser Deposition on Low Cost Substrates
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Abstract We report on the use of pulsed laser depositon (PLD) to grow thin films of LiCoO 2 on a number of low cost substrates including SnO 2 coated Upilex, stainless steel and SnO 2 coated glass. Highly textured (001) films grown on CVD deposited SnO 2 films on 7059 glass, were obtained at 200 to 500 mTorr 02 and a temperature of 500 C. Similar texture was not obtained on the stainless or Upilex however dense films from crystalline to amorphous were obtained. The films were characterized by x-ray diffraction and Raman spectroscopy. Introduction LiCoO 2 is one of the key materials being looked at as a cathode for rechargable Li secondary batteries. These materials are key for 4 V and 3 V batteries. LiCoO 2 is also of interest for electrochromic devices [1,2]. It is part of the general familiy of materials including LiCoO 2, LiNiO 2 and LiMnO 2 . These materials all have layered rock salt structure and need to be synthesized in the lithiated state unlike the vanadium oxide materials which are classic bronze formers and can be lithiated after synthesis. It is key to understanding these materials to have good control of the crystallinity and morphology (porousity) on the substrates of interest. Thin films of LiCoO 2 have been synthesized by a variety of techniques including sputtering [3-6], spray deposition [7-12], reaction of metals [13] sol gel synthesis [14,15] and pulsed laser deposition [16-19]. Typically the films were deposited on stainless steel or Ta substrates. These techniques produce a wide spread in crystal quality from amorphous to highly crystalline and of porousity from very porous aerogel and sol gel structures to highly dense films. In this work we report on the pulsed laser deposition of thin films of LiCoO 2 on a variety of potentially low cost substrates including Upilex (a polyimide), stainless steel and tin oxide coated 7059 glass. In all cases dense films were grown. In the case of the SnO 2 coated glass substrates the highly textured films were obtained. On the other substrates the films were predominately amorphous but some polycrystalline films were grown. Films were characterized by a variety of techniques includeing x-ray diffraction, Raman spectroscopy and inductively coupled plasma analysis. Experimental Synthesis: The films are grown by laser ablation of a stoichiometric 2.54 cm ceramic LiCoO 2 target obtained from Target Materials or synthesized in-house (most of the results) in a controlled atmosphere vacuum chamber. A schematic of the apparatus is shown in Figure 1. The target and heater are on-axis with each other at a distance of approximately 8.5 cm. The target is rotated about its axis at between 1-10 rotations per minute. Typically three 1 cm x 4 cm substrates are mounted on the heater for each run, thus creating three sister samples at each set of deposition conditions. The typical substrates included SnO 2 -coated glass (11 I -cm) obtained by CVD of tetramethyl tin on 7059 glass substrates, stainless steel (430) foil, and 293
Mat. Res. Soc. Symp. Proc. Vol. 496 ©1998 Materi
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