Preferred orientation of polycrystalline LiCoO2 films

Polycrystalline films of LiCoO2 deposited by radio frequency magnetron sputtering exhibited a strong preferred orientation or texturing after annealing at 700 degrees C. For films thicker than about 1 mu m, more than 90% of the grains were oriented with their (101) and (101) planes parallel to the substrate and less than 10% with their (003) planes parallel to the substrate. As the film thickness decreased below 1 mu m, the percentage of (003)-oriented grains increased until at a thickness of about 0.05 mu m, 100% of the grains were (003) oriented. These extremes in texturing were caused by the tendency to minimize volume strain energy for the thicker films or the surface energy for the very thin films. Films were deposited using different process gas mixtures and pressures, deposition rates, substrate temperatures: and substrate bias. Of these variables, only changes in substrate temperature could cause large changes in texturing of thick films from predominately (101)-(101) to (003). Although lithium ion diffusion should be much faster through cathodes with a high percentage of(101)- and (104)-oriented grains than through cathodes with predominately (003)-oriented grains, it was not possible to verify this expectation because the resistance of most cells was dominated by the electrolyte and electrolyte-cathode interface. Nonetheless, cells with cathodes thicker than about 2 mu m could deliver more than 50% of their maximum energies at discharge rates of 5 mA/cm(2) or higher. (C) 2000 The Electrochemical Society. S0013-4651(99)04-087-2. All rights reserved.