The influence of preparation conditions on electrochemical properties of LiNi0.5Mn1.5O4 thin film electrodes by PLD
Introduction
Layered LiCoO2, LiNO2 and spinel LiMn2O4 are the most important cathode materials for Li-ion batteries because of their high operating voltage (4 V versus Li/Li+) and good capacity retention. Among them, the spinel LiMn2O4 is the most favored one, especially for large sized batteries for electric vehicle (EV) applications, due to its lower cost and nontoxicity [1], [2], [3]. However, LiMn2O4 is not stable during cycling especially at elevated temperature, which results in a rapid capacity fade and hence limits its practical use [4], [5], [6]. Therefore, much research has been carried out to improve its charge and discharge cycle performance.
One excellent method for improving the cycle performance is the substitution of other transition metals for Mn to make LiMxMn2−xO4 (M = Co [7], Cr [8], Ni [9], Fe [10], Cu [11], etc.). Interestingly, it has been found that this approach is also accompanied by a higher voltage plateau at about 5 V as a result of the redox system provided by the transition metal substituted. Materials of this group are now attractive candidates as cathode materials for lithium-ion batteries because they can increase the cell voltage to 5 V from the present 4 V. Within this group of materials, LiNi0.5Mn1.5O4 is the most attractive material for the practical preparation of 5 V cathodes due to its high capacity and good stability on repeated Li-ion extraction and insertion [12], [13], [14].
Owing to the fact that the development in microelectronics industries has reduced the current and power requirements of electronic devices to an extremely low level, it is therefore possible the use of thin film batteries (TFBs) as power sources for these devices. Further more, thin film electrodes are ideal samples for studying the intrinsic properties of electrode materials because there are no binder and conductive additive in the films. Thin film electrodes have been successfully prepared by various techniques such as radio frequency (rf) sputtering [15], pulsed laser deposition [16], chemical vapor deposition [17], spin coating [18] and electrostatic spray deposition [19]. Among them, PLD is a powerful and easy method for producing high quality and dense films without post-deposition annealing. Studies on thin film electrodes have been mainly applied to LiCoO2 and LiMn2O4. Very limited research [20], [21] has been done on spinel LiNi0.5Mn1.5O4 thin film electrodes.
In this work, we first report on the preparation and characterization of LiNi0.5Mn1.5O4 thin films by PLD. Influences of both substrate temperature and oxygen partial pressure on the microstructure and morphology of LiNi0.5Mn1.5O4 thin films are investigated. Electrochemical properties of LiNi0.5Mn1.5O4 thin films are further characterized as a function of deposition conditions.
Section snippets
Experimental
Starting materials, MnO2 99.9% (Alfa Aeser), NiO 99% (Alfa Aeser) and LiOH 98% (Merck) with appropriate molar ratios were mixed using ball mill for 2 h. The mixture was placed in an Al2O3 crucible and heated in air at 750 °C for 24 h. After grinding, the powder was cold-pressed into a pellet and subsequently fired in air at 900 °C for 2 h to densify the target. The LiNi0.5Mn1.5O4 thin films were deposited on stainless steels (SS) and SiO2/Si (SOS) substrates by PLD in a vacuum chamber at a base
Structure characterization
The microstructure and crystallinity of LiNi0.5Mn1.5O4 thin films are dependent on particular deposition conditions. One major factor is the substrate temperature, TS. Fig. 1 shows the XRD spectra obtained from films deposited on SS substrates at different temperatures ranging from 300 to 600 °C with a constant oxygen partial pressure of 200 mTorr. All diffraction peaks attributed to the films can be indexed based on a spinel structure with space group . In this spinel-framework structure,
Conclusions
Spinel LiNi0.5Mn1.5O4 thin films were successfully prepared on the SS substrates by PLD. The microstructure and surface morphology of thin films are highly affected by the substrate temperature and oxygen partial pressure. A high degree of crystallinity and smooth surface morphology can only be achieved at high substrate temperatures. Impurity formed during deposition and a small amount of Mn3+ ions in the film can be reduced by increasing the oxygen partial pressure. Thin films with better
Acknowledgements
This research was supported by Advanced Materials for Micro- and Nano-System (AMM&NS) programme under Singapore-MIT Alliance (SMA) and by National University of Singapore.
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