Structural and electrochemical properties of nanocrystalline LixMn2O4 thin film cathodes (x = 1.0–1.4)

The process optimization of nanocrystalline lithium manganate thin films (LixMn2O4; x = 1.0–1.4) has been demonstrated by using a cost-effective solution growth technique. Films were first attempted with Pt–Si (Si/SiO2/TiO2/Pt) substrates but because of inter-diffusion of TiO2 buffer layer with Pt at higher annealing temperature, phase impure LiMn2O4 films were obtained. Phase pure films on the basis of XRD analysis were found on Pt substrate at specified growth parameters. The annealing temperature and annealing time were varied, the films annealed at 700 °C for 2 h were found to be the best films. The nanocrystalline nature of the films was revealed by the SEM micrographs and the surface morphology studied using AFM. Finally, the electrochemical properties (cyclic voltammetry and constant current measurements) of these films were analyzed using a home made three-electrode cell and Gamry Battery tester instrumentation. The formation of a prominent layer of fluoride species deposited over the cathode surface during the repeated cycling was revealed by XPS measurements. Further experiments are in progress on identifying the exact composition of these unwanted species. The formation of the Jahn-Teller active Mn3+ during electrochemical cycling was completely ruled out from the XPS analysis. Also the very consistent value of [Mn3+/Mn4+] ratio before and after electrochemical cycling on the surface of the film revealed good quality of the films. Finally, the formation of the fluoride layer was concluded as a passive layer that causes the initial capacity drop during first few cycles of the cell performance.