Investigations on LiM0.5Mn1.5O4 (M = Fe, Co, Ni) Spinels as High-Volt Cathode Materials for Rechargeable Lithium-Ion Batteries

Development of Lithium-ion batteries for high power applications is one of the active research fields in renewable energies. The work presented in this thesis focuses on the synthesis, structural and electrochemical characterization, thermal stabilities and metal ion dissolution (in the electrolyte) of a class of high-voltage materials, LiM0.5Mn1.5O4 (M = Fe, Co, Ni), with a spinel structure. The materials were synthesized via modified sol-gel synthesis and post-annealed at higher temperature to increase the crystallinity. They are candidates for high-power applications due to their spinel structure which facilitates a three dimensional Li diffusion. Detailed structural analyses were conducted using X-ray and neutron diffraction techniques. The LiNi0.5Mn1.5O4 annealed at 600°C showed a partial cation ordering as revealed from the neutron diffraction analysis whereas all the other materials exhibited a cation disordered structure. Morphology and particle size of the materials were analyzed by scanning electron microscopic studies and the influence of these parameters on the electrochemical cycling performance will be discussed. The LiNi0.5Mn1.5O4-1000 material was found to exhibit a superior cycling performance at RT and at 55°C in comparison with the Co- and Fe-doped materials. An optimization of the electrode composition and preparation was found to bring an improvement in the rate-capability of the electrodes. Investigations were conducted on the electrochemical mechanism of LiM0.5Mn1.5O4-1000 (M = Fe, Co) through in situ synchrotron diffraction and both the materials were found to have a solid-solution mechanism of electrochemical reaction even though a pseudo two-phase behavior was observed in the case of Fe-doped spinel. Metal dissolution studies in electrolyte were conducted with ICP-OES analysis and the results showed that in the delithiated state and at higher temperatures, metal dissolution is more pronounced in comparison with the lithiated state and RT. Thermal stabilities of the LiM0.5Mn1.5O4-1000 (M = Fe, Co, Ni) electrodes were analyzed using TG-DSC experiments and in situ synchrotron diffraction and the relation of onset temperature of structural degradation and the amount of lithium extracted will be discussed