Investigating the Thermal Stability and Cation Ordering in Layered Cathode LixMO2 (x ≤ 1; M = Co, Mn, Ni) Materials for Li-ion Rechargeable Batteries and Studying the Ferroelectric Properties of LiNbO3 Nanoparticles
In recent years, transition metal oxides have drawn extensive attention because of their wide application in electronic, memory, battery, informatics, and optoelectronics devices. In this dissertation, we have studied two different types of oxide materials which are technologically important: LiMO 2 (M = Co, Mn, Ni), has served as cathode materials in the rechargeable battery, and LiNbO3, has wide application in ferroelectric devices such as electronics, non-volatile memories, and thin film capacitors. LiMO2 was synthesized and characterized to understand the correlation between capacity fading and thermal stability relating to the microstructures. Our results showed that delithiated (charged) LiCoO2, forms a metastable LiCo2O4 spinel phase during the ageing process, and eventually decomposes to Co3O4 over time. These two phases were identified from their magnetic responses. Paramagnetic behavior is observed in the starting material without indication of any magnetic ordering prior to heat treatment. Heat treatment of delithiated materials progressively changes the magnetic nature of the compounds. After short term heat treatment of delithiated LiCoO2, spin-glass-like or geometrically frustrated behavior is observed that suggests the formation of metastable spinel phase LiCo2O4 in the lattice. After long-term annealing, pronounced strong antiferromagnetic (AFM) ordering is observed consistent with the formation of Co3O4. The thermal stability of LiCoO2 was compared with LiMn1/3Ni1/3Co1/3O2. The result showed that, unlike LiCoO2, LiMn1/3Ni1/3Co1/3O2 does not decompose. However, selected area electron diffraction (SAED) and the bright field images from transmission electron microscopy studies revealed significant microstructure changes in the delithiated material and thermally aged products. In another system, Li[Ni1−xMnx]O2 (x = 0.3, 0.5, 0.7), the cation ordering was successfully monitored to understand the Li/Ni disorder for different compositions. This eventually determines the electrochemical capacity of these cathodes. The results on the starting materials revealed that the manganese-rich composition has more Li/Ni disorder compared to the other compositions. The Li/Ni disorder was detected by powder X-ray Diffraction, magnetic studies, as well as SAED studies. From the SAED studies, it was found that Li/Ni disorder creates x R30º type cation ordering in the transition metal layers. For delithiated materials this ordering was found to be suppressed indicating that the extraction of lithium occurs from the transition metal layer rather from the lithium layer. In another study, the ferroelectric properties of LiNbO 3 nanoparticles were studied as a function of shape. By employing a solvothermal method, cube- and sphere-like ferroelectric LiNbO3 nanoparticles were prepared by decomposition of the single-source precursor, LiNb(O-Et)6, in the absence of surfactants. X -ray diffraction showed that the LiNbO3 nanoparticles were rhombohedral (R3c) with a = 5.145(3) Å, c = 13.867(3) Å for nanocubes and a = 5.139(3) Å, c = 13.855(3) Å for nanospheres. Ferroelectric properties for these nanoparticles were also confirmed by piezoresponse force microscopy (PFM) and Raman spectroscopy. From PFM measurements, it was observed that both sets of particles exhibited polarization switching at room temperature with static d33 coefficient values of 17 pm/V for cube-like and 12 pm/V for spherical LN nanoparticles.
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