Investigations on Structural and Electrochemical Properties of Layered Composite Cathode Materials for Li Ion Batteries

Abstract

Development of eco-friendly, low cost and high energy density cathode materials for Li ion battery is a challenge for the research community due to the urgent requirements for highly efficient energy storage devices in the fast developing technologies. Layered LiCoO2 was employed as cathode material for commercial Li ion batteries. But low reversible capacity and safety issues due to Co usage forced research world to rethink and find out new materials for better performances. This dissertation explores the chemical and structural factors that control the electrochemical performances of various layered and Li2MnO3 based composite cathode materials for Li ion battery. Layered cathode materials LiNi2/3Co1/6Mn1/6O2 (LNCMO) were synthesized in polycrystalline phase using sol-gel technique and were well characterized by x-ray diffraction (XRD), Raman spectroscopy, energy dispersive x-ray spectroscopy (EDS), scanning electron microscopy (SEM), and x-ray photoemission spectroscopy (XPS). Rietveld refinement of the ambient powder XRD data established that the as synthesized material has layered trigonal crystal structure with a=b=2.8656 Å and c= 14.1812 Å which is further validated by the observed two ( g A1 and g E ) Raman active vibrational modes that belong to space group R3m. In order to achieve improved electrochemical performances, the pristine cathode materials were treated with reduced graphene. Lattice dynamics and XRD studies showed that graphene treated samples also has the same structure without any considerable changes. Additional SEM, TEM and XPS analysis revealed that LNCMO particles are well wrapped with the graphene flakes and the oxidation states of the elements present in the graphene treated compound are intact. Electrochemical analysis provided better performances for the graphene mixed LNCMO cathode materials than the pure LNCMO materials. In the second part high energy density composite cathode 0.3Li2MnO3- 0.7LiNi0.5Mn0.5O2 was prepared using two different wet chemical routes viz. coprecipitation and sol-gel methods and compared their structural and electrochemical properties. Charge-discharge tests indicate the better electrochemical performance of the sol-gel prepared composite in terms of high discharge capacity (~240 mAh/g) and good cycling performance. The improved electrochemical performance was attributed to a decrease in the charge-transfer resistance and high surface area. In the third part of dissertation novel composite cathode materials, xLi2MnO3-(1- x)LiNi2/3Co1/6Mn1/6O2 (where x = 0.3, 0.5, and 0.7), were synthesized by sol-gel route and characterized by advanced techniques for rechargeable Li-ion battery applications. Phase purity of the composites was examined by XRD as well as Raman spectroscopy and the studies revealed good crystallinity and the formation of pure composite phases with monoclinic (C2/m) and hexagonal (R3m) crystal structures for Li2MnO3 and LiNi2/3Co1/6Mn1/6O2, respectively. Valence states of transition metals in the composites were examined by X-ray photoelectron spectroscopy and the analysis suggested predominant oxidation states of Ni, Co, and Mn as 2+, 3+, and 4+, respectively. Galvanostatic charge-discharge tests, performed at different C-rates between 2.0-4.8 V, indicated high discharge capacity (~250 mAh/g), good rate capability, and excellent cycleability of the composite with x = 0.5 compared to the composites with x = 0.3 and 0.7. In-situ Raman spectroscopic studies revealed the activation of Li2MnO3 component in all composite cathode materials during the first cycle charging process with structural stability, thereby enhancing performance of the composite with x = 0.5. Last part of dissertation is devoted to present the preliminary results of composite cathode material xLi2MnO3- (1-x)LiNi0.58Mn0.34Co0.08O2 in which LiNi0.58Mn0.34Co0.08O2 is a combination of LiNi0.5Mn0.5O2 and LiNi0.66Mn0.17Co0.17O2. Our main aim was to reduce the use of cobalt as much as possible and make the cathode material safe and economic. Surface characterization proved the phase formation, crystalanity, size, and presence of constituent particles in the as prepared composite cathode material. Electrochemical characterization shows improved cathode performance in terms of rate capability test and cyclability tests. Cyclic voltametry studies and electrochemical impedance spectroscopy studies confirmed the potential applicability of the composite cathode material for Li ion battery technology.