K. Hari Prasad, P. Muralidharan, E.S. Srinadhu, N. Satyanarayana
Keywords: nanocrystalline LiCoO2 thin films, radio-frequency magnetron sputtering, a.c. conductivity, dielectric properties
Summary:Lithium-ion batteries have emerged as one of the most important power sources for portable electronic devices due to their higher energy density, high discharge capacity and good reversibility. In order to reduce the weight and size for portable electronic devices, the demand for developing lighter and thinner miniaturized batteries is increasing. So, developing the thin-film rechargeable lithium-ion battery technology is the is the best solution, since these are suitable as a micro power sources for micro- and nano-devices such as smart cards, implantable medical devices, complementary metal oxide semiconductors (CMOS), memory chips, micro-electro-mechanical systems (MEMS) and nano-electro-mechanical systems (NEMS), etc. Additionally, their large surface area allows high current rates and hence, these can be useful for high power electronic device applications. Cathode, anode and electrolyte materials are the major parts of the lithum batteries, of which, cathode material plays an important role in lithum battery technology. Among the available, LiCoO2 (LCO) is the most widely used cathode materials in lithum battery technology, due to its advantages of exhibiting high energy density, high discharge capacity, good reversibility during the oxidation and reduction process, etc. Hence, in the present investigation, the effect of annealing on the morphological, a.c. conductivity and dielectric properties of LiCoO2 cathode thin films grown by rf magnetron sputtering were studied to find out its suitability for developing all solid state thin film lithium ion micro batteries. Table-1 gives the summery of the sputtering parameters for the grown LCO thin films. Fig.1, shows the X-ray diffraction (XRD) patterns of as deposited and annealed at various temperatures (400 oC, 500 oC, and 600 oC) of LCO thin films along with the standard JCPDS data. Fig. 2. [a, c, e, g] and [b, d, f, h], respectively show the two-dimensional (2D), and three-dimensional (3D) AFM images of the as-grown as well as post-annealed (at 400, 500, and 600 oC) LCO cathode thin films. XRD and AFM confirms the formation of nanocrystalline rhombohedral layer structured LCO thin films and the grain size of the LCO thin film increases with increasing post-annealing temperature. Fig. 3, shows the σac vs. log (ω) plots obtained as-grown as well as post-annealed (at 400, 500, and 600 oC) LCO thin films. Fig. 4, shows the real part of dielectric permittivity (ε') vs. Log (ω) obtained as-grown as well as post-annealed (at 400, 500, and 600 oC) LCO cathode thin films. The observed a.c. conductivity and dielectric results indicate that the developed LiCoO2 thin film grown by rf magnetron sputtering may be suitable for developing all solid state thin film lithium ion micro batteries. Detailed results will be presented and discussed.