K. Hari Prasad, P. Muralidharan, E.S. Srinadhu, N. Satyanarayana
Keywords: nanocrystalline LiMn2O4 thin films, radio-frequency magnetron sputtering, impedance, electrical conductivity, dielectric properties
Summary:Lithium ion batteries (LIB) with high energy density, power density and long charge-discharge life, are considered to be one of the most promising energy storage systems for portable electronic devices as well as electric vehicles applications. Cathode material plays an important role in LIB technology and LiCoO2 is the most widely used cathode materials in LIB technology. Use of LiMn2O4 (LMO) over LiCoO2, as a cathode, has many advantages like natural abundance, environmental friendly, lowest production cost, high reduction potential, good performance, etc. Recently, all-solid-state thin film micro-batteries have attracted more and more interests due to many advantages such as corrosion free, leak proof, ruggedness, long self life, long cycle life, thermal stability, miniaturization, light weight, specific capacity, high power density, etc., and also their potential applications as 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. Radio-frequency magnetron sputtering (RFMS) is the most versatile technique for the fabrication of thin films over other deposition methods such as chemical vapor, spray pyrolysis, dip coating, and pulsed laser deposition, etc. Hence, in the present work, attempts were made to grow the nanocrystalline spinel LiMn2O4 thin films on the Ti/(100) oriented silicon (Si) substrate by RFMS technique. Table-1 shows the summery of the sputtering parameters for the grown LMO thin films. All the grown thin films were post-annealed up to 500 oC, in the presence of oxygen to enrich the crystallinity of the film, and characterized using XRD and Raman techniques. Also, electrical conductivity and dielectric properties of the LiMn2O4 thin films were studied through impedance measurements at different temperature and at different frequencies, to find out their suitability for developing all solid state thin film lithium ion micro batteries. Fig.1, shows the XRD patterns of rf-sputtered LMO thin films as-deposited and post-annealed at 300, 400, and 500 oC, Ti/Si (100) substrate heat treated at 500 oC along with the standard JCPDS data. Fig. 2. (a, c, e, g) and (b, d, f, h), show the two-dimensional (2D), and three-dimensional (3D) AFM images of the as-deposited and post-annealed (300 oC, 400 oC and 500 oC) LMO thin films respectively. XRD and AFM results confirm the formation of nanocrystalline LMO thin films. Fig.3, shows the a.c. conductivity verses frequency plots obtained at room temperature of LMO thin films as deposited and post-annealed at various temperatures. Fig.4, shows the real part of dielectric constant (ε') verses frequency plots obtained at room temperature of LMO thin films as deposited and post-annealed at different temperatures. Fig. 5, shows the imaginary part of electric modulus (M") verses Log (ω) plots obtained at room temperature of LMO thin films as-deposited and post-annealed at different temperatures. Electrical and dielectric properties results confirm the suitability of the prepared LiMn2O4 thin films for developing all solid state thin film lithium ion micro batteries. Detailed results will be presented and discussed.