Structural and chemical characterisation of LiCoO2 and Li7La3Zr2O12 from the micron to the atomic scales for solid-state Li-ion energy storage materials

Solid-state devices, as the next generation of lithium-ion batteries, have the potential to improve performance characteristics, meeting growing demands for energy storage infrastructure. Electrode materials of importance focus on layered lithium metal oxides, while many candidates for solid-state electrolytes have emerged. Lithium lanthanum zirconium oxide (Li7La3Zr2O12) is an electrolyte of significant interest, based on a competitive lithium conductivity. Electron microscopy and associated spectroscopic techniques, from micron to atomic scales, can develop understanding of bulk and interfacial characteristics regarding features fundamental to battery performance, such as lithium intercalation routes and structural failure mechanisms. Atomic resolution microscopy has the potential to develop upon current imaging and spectroscopic studies of solid-state materials. In his project, GB spanning particle cross-sections and complex defect structures were revealed within both LiCoO2 and Li7La3Zr2O12. Grain boundaries and defects exhibited variable potential impacts on Li mobility both beneficial and detrimental to performance. Significant interfacial decomposition occurs between LiCoO2 and Li7La3Zr2O12 particles during annealing, forming Li2CO3 or LaCoO3 depending on temperature. Pulsed laser deposition of LiCoO2 onto Al2O3, SrTiO3 and Gd5Ga3O12 substrates, at low temperature formed multi-phase systems, with nanograins of rhombohedral LiCoO2, rock salt CoO and spinel Co3O4. Li7La3Zr2O12 grown on SrTiO3 and Gd5Ga3O12 crystallised at 600 °C, either during deposition or through annealing of amorphous films. Lithium loss during deposition and annealing temperatures above 600 °C was a limitation for the studies conducted, forming large grains of La2Zr2O7. Deposition of LiCoO2/Li7La3Zr2O12 heterostructures provide samples where growth of crystalline interfaces and the influence of annealing are assessed down to atomic resolutions. Annealing improved the quantity and orientation consistency of rhombohedral LiCoO2 within the electrode, although detrimental intermixing and crystallite formation occurred at the LiCoO2/Li7La3Zr2O12 interface. LiCoO2 crystallites growing into the Li7La3Zr2O12 layer may have a significant role to play in Li mobility through annealed solid electrode-electrolyte interfaces.