Improved electrochemical performance of Li-excessive LiMn₂O₄ cathode for secondary batteries

Li-excessive Li_1+xMn_2-xO_4-δ (x = 0, 0.05 and 0.1) thin films were prepared by a sol-gel method where the gel deposit on Pt/Ti/SiO₂/Si substrate was annealed at 700 °C for 6 h in air. The physical properties of the samples were characterized by X-ray diffraction (XRD) and Raman spectroscopy. The electrochemical properties of the thin-film Li_1+xMn_2-xO_4-δ cathodes were investigated by cyclic voltammetry and galvanostatic cycling test. As a result of XRD analysis, all samples showed a spinel structure without any secondary phase, and the lattice parameter of the Li-excessive samples was reduced compared to the pristine LiMn₂O₄. The observed high-energy shift of the Raman-active A_1g mode for the Li-excessive samples compared to the pristine LiMn₂O₄ supports the reduction of the lattice parameter. The electrochemical data revealed that the Li-excessive cathodes exhibited better cycling stability than the pristine LiMn₂O₄ in the 3.5−4.5 V range. The Li_1.1Mn_1.9O_4-δ cathode showed a capacity retention of about 51.1% after 700 cycles for the secondary battery, which is about 26% higher than that of the pristine LiMn₂O₄.