LiMn2O4 thin films are deposited on gold coated polyimide flexible substrates using RF magnetron sputtering technique maintained at a moderate substrate temperature of 300℃. The films exhibited characteristic peaks with predominant (111) orientation representing cubic spinel structure of Fd3m symmetry with an evaluated lattice parameter of 8.199 ?. The surface topography of films exhibited pyramidal shaped grains oriented vertical to the substrate surface with root mean square surface roughness of 90 nm. The Pt/LiMn2O4 electrochemical cell in aqueous region exhibited two step de-insertion and insertion kinetics of Li ion during oxidation and reduction reaction with an initial discharge capacity of 36 μAh?cm_2?μm_1.
In the existing prompt developing science and technology decade, much attention has been devoted to the development of all solid state thin film microbatteries to power the miniaturized micro and nano electronic devices such as MEMS and NEMS [1,2]. The realization of such microbatteries is originated from the synthesis and properties of thin film cathode materials with high energy density and specific capacity [3,4]. Lithium manganese oxide (LiMn2O4) with spinel structure is one of the most extensively studied cathode material for Li/Li+ rechargeable batteries due to its low cost, non-toxicity and relatively high energy density [5-13]. Most of the researchers prepared LiMn2O4 thin films on solid substrates using various physical [14-19] and chemical [20,21] vapour deposition techniques and studied their electrochemical performance and durability at device level [
Among the various physical vapour deposition techniques, RF magnetron sputtering technique is observed to be one of the most favorable and industrially viable technique since it enables the formation of homogeneous films with definite thickness along with good adhesion. The chief advantage is that RF magnetron sputtering activates broad ionization even at low sputtering powers and allow the film to crystallize at moderate substrate temperatures, especially during long sputtering times. Also, the microstructural properties can be altered by properly controlling the deposition parameters. Hence, in the present investigation, thin films of LiMn2O4 are deposited on Au/polyimide flexible substrates using RF magnetron sputtering technique at a moderate substrate temperature and studied the growth and microstructural properties. The electrochemical properties are studied in aqueous electrolyte media by investigating cyclic voltametry and chronopotentiometry measurements.
LiMn2O4 thin films are deposited from a three inch diameter cold pressed and sintered lithium rich (10%) LiMn2O4 target using RF magnetron sputtering technique on gold coated polyimide (Kapton) substrates (obtained from M/s Aarthai engineers). During the depositions, the substrate temperature was kept at 300˚C and the sputtered gas (O2/Ar) composition of 1:6 was maintained to minimize the loss of lithium [
sites [
The corresponding strain observed for the films deposited at a moderate substrate temperature of 573 K was 4.5 × 10−3. From XRD studies the average grain size for the films was calculated by considering FWHM values and is observed to be as 190 nm.
The Raman scattering measurements were carried out for the LiMn2O4 film deposited on Au/Flexible substrates and the spectrum is shown in
The scanning electron micrograph of LiMn2O4 thin films is shown in
The image processing of the SEM images were carried out in two steps: 1) denoising using a median filter of radius 9.3 nm (2 pixels); 2) Quantification of grain surface area fraction and surface roughness using the “3D” and “roughness calculation” Java pug-ins [
The surface nucleation and film formation density on flexible polymer is quite critical because of poor rate of adatom mobility induced by the lower aggregation energy of the sputter ejected particles on the substrate surface [
prove the sputtering yield the RF power was maintained at 140 W where the observed deposition rate was 150 Å/min. The ejected large particle from the target at this power processes to have higher kinetic energy and impinges onto the substrate surface and initiates the growth. At constant substrate temperature, the adatom mobility is constant on the surface of the substrate and favors the formation of greater number of crystallite centers rather than the coalescence of islands. The enhancement of crystallite size and the surface roughness of the films grown on flexible substrate is a positive observation for electrochemical research. Generally this type of surface topographical features of LiMn2O4 films is more favorable for obtaining improved electrochemical response of positive electrode films.
sets of well-separated peaks are clearly seen, which correspond to the potential plateaus. The peaks located at 0.789 V and 0.961 V during cathodic scan corresponding to the Li ion deintercalation from LiMn2O4 host matrix to form λ-MnO2, while the peaks located at 0.590 V and 0.773 V during anodic scan correspond to Li ion intercalation in to λ-MnO2 to form LiMn2O4. This two step deinserted and inserted kinetics of Li ion during oxidation and reduction reactions indicates characteristic property of the spinel LiMn2O4 [
The first oxidation peak (O1) at 0.789 V is attributed to the removal of lithium ions from half of the tetrahedral sites, whereas the second oxidation peak (O2) at 0.961 V is due to the removal of lithium ions from the remaining tetrahedral sites.
sed to 32.8 mAh/cm2. mm for 10 cycles. The platinum counter electrode couldn’t act as a perfect reversible source and sink of lithium ions and may be one of the reason for the low cyclic retention of the Pt/LiMn2O4 cells. However the results are seem to be encouraging and further investigations are in progress to understand detailed electrochemical behavior of the films.
LiMn2O4 films are deposited successfully on metallized polyimide flexible substrates at a moderate temperature of 300˚C using RF magnetron sputtering technique. The films exhibited predominant (111) orientation along with the characteristic peaks representing cubic spinel structure with Fd3m symmetry. The calculated lattice parameter, Mn-Mn and Mn-O interatomic distances are observed to be 8.199 Å, 2.898 Å and 1.918 Å respectively. From “imageJ” analysis of SEM data the root mean square roughness (RMS) are observed to be 90 nm with an average grain size of 164 nm. From slow scan cyclic voltametry (SSCV) studies, the presence of two well separated electrochemically active redox peaks during oxidation and reduction reactions indicate the characteristic property of the spinel LiMn2O4 structure in the films. The Pt/LiMn2O4 electrochemical cell with LiMn2O4 film coated on metallized flexible Kapton substrates exhibited an initial discharge capacity of about 36 μAh∙cm−2∙μm−1 for the first cycle which is an encouraging result. Further investigations are in progress to improve the electrochemical properties such as capacity and cycling life.
This research work is supported by DRDO and one of the authors Mr. Jayanth is highly thankful to DST for providing necessary financial assistance to carry out this work under Promotion of University Research and Scientific Excellence (PURSE) programme.