Crystallographically Oriented Thin-Film Nanocrystalline Cathode Layers Prepared Without Exceeding 300°C

Abstract

The highest capacity rf sputtered cathode layers created for use in thin-film solid-state batteries have been found to require an annealing step with temperatures in excess of 700°C. Since this high-temperature process step is incompatible with silicon device technology and flexible polymer substrates, the development of a low-process temperature (less than or equal to 300°C) cathode layer has been undertaken. Thin-film cathode layers consisting of LiCoO2LiCoO2 were deposited using planar magnetron rf sputtering and subsequently incorporated into thin-film solid-state cells comprised of a LiPON electrolyte and lithium metal anode. Film composition was examined using Rutherford backscattering spectrometry and inductively coupled plasma mass spectroscopy, while phase content and crystal structure were studied through X-ray diffraction experiments conducted at the Stanford Synchrotron Radiation Laboratory. Microstructure and morphology were examined using transmission and scanning electron microscopy. It was found that LiCoO2LiCoO2 could be deposited at room temperature in a nanocrystalline state with a defined (104) out of plane texture and a high degree of lattice distortion. By heating these layers to 300°C, the average grain size was increased while lattice distortion was minimized. Electrochemical cycling data revealed that the low temperature annealing step increases cell capacity to near theoretical values while significantly improving both the rate capability and discharge voltage. Impedance analysis on test cells showed that the electronic resistance of the cells is decreased after heating to 300°C. © 2001 The Electrochemical Society. All rights reserved.