Pulsed laser deposition and characterization of LiMn 2 O 4 thin Films for applications in Li Ion rechargeable battery sy
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have a good control on crystallinity. The pulsed laser deposition (PLD) technique provided an opportunity to deposit films ranging from highly crystalline to amorphous in nature. The target made with commercial LiMn20 4 powder from MERCK was mounted on an axis, which rotated, between 1-10 rotations per minute. The chamber was pumped down to a base pressure of at least 10-6 Torr. The substrates were heated to a chosen deposition temperature (650 -700 C) and oxygen was bled in the chamber to maintain a background pressure of 200mTorr. All films were deposited with laser energy of 300 mJ per pulse at a frequency of 5-10 Hz and a deposition time of 10 minutes. These films were very dense in nature and were approximately 0.1um - 0.5 um thick. The substrate area was approximately I cm2. Selection of the substrate material was considered to play an important role in determining the crystallinity of the film. Three factors were considered before substrate materials were selected. The first requirement was the crystallinity of the substrate material since highly crystalline films can only be grown on highly crystalline substrates. Secondly, the substrate was
required to be highly conductive so that electrochemical measurements could be performed. The third requirement was the substrate's sensitivity to high temperatures since the substrates were heated to 700 deg C during deposition of these films to obtain a highly crystalline and textured film. Substrates such as silicon, sapphire, highly doped silicon (conductive), stainless steel and nickel were used in this study. Laser Spark Atomizer was another method that was used to deposit thick coatings of nanoparticulate materials in a tri-dimensional mesoporous network forming a web-like structure [7]. These films were deposited on a stainless steel grid and later annealed between (600-700 C). The above films were characterized using materials characterization techniques such as XRD and SEM. These electrodes were then assembled into half-cells consisting of the LiMn204 films as the cathode and Li metal as an anode and a reference. 1 M LiC104 in PC was used as electrolyte in the half-cells. Electrochemical characterization using techniques such as cyclic voltammetery, AC impedance and galvanostatic cycling was conducted.
RESULTS Structural Characterization Thin films with various degrees of crystallinity were grown on the substrates listed in table 1. The films grown using PLD were highly textured in (004) and the (111) directions. SEM micrographs exhibit highly crystalline and dense growth in a certain directions. [8] Polycrystalline films showing cubic crystals of LiMn 20 4 grew on polycrystalline substrates. Temperature of the substrate during deposition also determined the crystallinity of these films. Higher temperatures helped in the growth of highly textured and dense films. Tablel: Effect of Deposition Conditions on LiMn 20 4 Films Energy (mJ) 300
Rep Rate(Hz) 10
Duration (rmin.) 10
Film Quality( XRD, SEM)
Silicon
Temp ( C) 700
Sapphire
700
300
10
10
Texture
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