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Abstract
The object of the present work is the development and modification of the solid oxide electrodes such as α-MnO2 and LiCo1-yMyO2 (M= Ni or Mn) used as cathodes in the alkaline cells and in Lithium secondary batteries respectively by enhancing their structural stability and improving their electrochemical performance. Thus the thermal stability, local structure, and electrical properties of the α-MnO2 phase doped with Sn and Co were studied. It is found that doping prevents the transformation from α-MnO2 to α-Mn2O3 that occurred in the temperature range of 500–600°C. The samples have been synthesized in an acidic medium using the reduction of potassium permanganate by fumaric acid. X-ray diffraction patterns (XRD) of pure and doped α-MnO2 prepared at 450°C do not show new peaks related to dopant species. Thermogravimetric analysis (TGA) of the Sn and Co doped MnO2 reveals that transformation from MnO2 to α-Mn2O3 starts above 700°C. The increase in the thermal stability is attributed to the presence of Sn or Co ions incorporated inside the large 2×2 tunnels as revealed by Fourier transform infrared (FTIR) spectra measurements. An increase in the electrical conductivity with the presence of Sn or Co ions is observed. Electrochemical features of the doped MnO2 samples in alkaline cells are reported and compared with that of the pristine α-MnO2 phase.
The layered LiCo1-yMyO2 (M= Ni or Mn) Microcrystalline powders were synthesized by a sol–gel method using succenic and citric acid respectively as a chelating agent. Submicron-sized particles of the precursors were obtained at temperature below 400˚C and microcrystalline powders were grown by thermal treatment at 800˚C for 24 h in air.
The synthesized products were characterized by structural and spectroscopic analyses. XRD and FTIR measurements were carried out for the prepared samples. Layered structure of α- NaFeO2 was estimated from these measurements. The valence state of the cations in LiCo1−yNiyO2 and LiCo1−yMnyO2 were determined by the magnetic measurements using superconducting quantum interference devices (SQUID).
The electrochemical performance of the synthesized products LiCo1−yNiyO2 and LiCo1−yMnyO2 in rechargeable Li cells was evaluated using non-aqueous solution 1M LiPF6 in EC-DMC as electrolyte. LiCo0.6Ni0.4O2 gave specific capacity 134mAh/g while the sample of high content of Ni (LiCo0.5Ni0.5O2) gave lower specific capacity 122mAh/g. This is mainly due to the presence of high amount of Ni increasing the degree of cation mixing and affects the electrochemical properties.
Specific capacity of LiCo0.8Mn0.2O2 compound has been found to be about 129.58mAh/g with a discharge efficiency of 93.59% in the potential domain 2.7–4.3V.