Structure and properties of garnet-type lithium ion conductors Li7-xLa3Zr2-xTaxO12 and their applicability in lithium-ion batteries

Li-ion batteries have served as the primary energy storage units in portable electronic devices and will continue to play an important role in the sustainable energy future. Despite decades of developments, the energy density and safety of Li-ion batteries still cannot live up to the increasing demands of modern society. Many issues are related to the state-of-art liquid electrolyte due to its intrinsic volatility and flammability. Solid-state lithium ionic conductors have received much attention recently and the garnet-type lithium ion conductors have emerged as the most promising candidate in the oxide system owing to their high conductivity and good chemical/thermal stability. This dissertation describes my PhD research work on the structure and properties of the garnet materials and their applicability in lithium-ion batteries.;In Chapter 3, a series of garnet compounds Li7-xLa3Zr 2-xTaxO12 (LLZTx, x= 0 -- 2) were synthesized and their phases and transport properties were characterized. The highest bulk conductivity of 9.6 x 10-4 S/cm is achieved for the composition Li6.7La3Zr1.7Ta 0.3O12 which marks the highest value reported in the literature at the time of publication. Ta5+ doping stabilizes the cubic garnet structure and the cation Ta5+ and Zr4+ form complete solid solutions for the cubic garnet compounds. A carefully conducted study of the LLZTx series reveals that pure cubic garnet phase forms only when x ≥ 0.6.;To understand the transport properties of the LLZTx series, the structures of selected compositions in this series were studied by diffraction techniques and computer simulation. Neutron diffraction study provided information about the average structure whereas the pair distribution function (PDF) analysis shed light on the local structure. Atomistic simulation based on interatomic potentials was carried out and a statistical approach was employed to extract useful structural information. We found that both tetrahedral and octahedral Li display strong position disorders as visualized via nuclear density maps. The first-neighbor Li-Li interactions strongly influence the lithium distribution. PDF analysis and computer simulation results indicate that mobile Li ions hop through the tetrahedral-octahedral shared faces.;Finally, the applicability of the LLZTx compounds in lithium-ion batteries was evaluated. It was found that the LLZTx compounds were stable against Li metal but unstable against moisture and CO2. The composition LLZT0.3 is more compatible with the cathode material LiCO2 than the LiFePO 4. Partial success was achieved using sol-gel method. The pulsed electric current sintering was successfully applied to the densification of LLZT0.6 powder.