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Abstract:
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The main challenges in the deposition of cathode materials in thin film form are
the reproduction of stoichiometry close to the bulk material and attaining higher
rates of deposition and excellent crystallinity at comparatively lower annealing
temperatures. There are several methods available to develop stoichiometric thin
film cathode materials including pulsed laser deposition; plasma enhanced
chemical vapor deposition, electron beam evaporation, electrostatic spray
deposition and RF magnetron sputtering. Among them the most versatile
method is the sputtering technique, owing to its suitability for micro-fabricating
the thin film batteries directly on chips in any shape or size, and on flexible
substrates, with good capacity and cycle life. The main drawback of the
conventional sputtering technique using RF frequency of 13.56MHz is its lower
rate of deposition, compared to other deposition techniques A typical cathode
layer for a thin film battery requires a thickness around one micron. To deposit
such thick layers using convention RF sputtering, longer time of deposition is
required, since the deposition rate is very low, which is typically 10-20 Å/min.
This makes the conventional RF sputtering technique a less viable option for
mass production in an economical way.
There exists a host of theoretical and experimental evidences and results that
higher excitation frequency can be efficiently used to deposit good quality films
at higher deposition rates with glow discharge plasma. The effect of frequencies
higher than the conventional one (13.56MHz) on the RF magnetron sputtering
process has not been subjected to detailed investigations. Attempts have been
made in the present work, to sputter deposit spinel oxide cathode films, using
high frequency RF excitation source. Most importantly, the major challenge faced by the thin film battery based on the LiMn2O4 cathode material is the poor capacity retention during charge discharge cycling. The major causes for the capacity fading reported in LiMn2O4cathode materials are due to, Jahn-Teller distortion, Mn2+ dissolution into the electrolyte and oxygen loss in cathode material during cycling. The work discussed in this thesis is an attempt on overcoming the above said challenges and developing a high capacity thin film cathode material. |