All-solid-state lithium batteries are of great interest scientifically as a next-generation of electrochemical energy storage devices, owing to their superior safety features and their potential to enable new chemistries to improve performance. The properties of the solid state electrolyte are integral to the overall cell capability – to date the most promising group of materials are the garnet-structured oxides, based on Li7La3Zr2O12 (LLZO), with high room temperature ionic conductivity and a wide electrochemical stability window. There are several aspects in the development of this relatively new material which are yet to be fully understood – these are the focus of this thesis.
In this work, processing cubic doped LLZO as a bulk ceramic was investigated and served as a basis for understanding its stability and electrochemical performance; it was optimised to obtain highly dense microstructures under atmosphere-controlled conditions to prevent reaction with moisture. Chemical inhomogeneities in the pellets, especially at the grain boundaries, as investigated by secondary ion mass spectrometry (SIMS) and low energy ion scattering, were shown to be important in determining the transport properties of the electrolyte - in particular the propensity for dendrite formation during cell cycling. It was shown that aluminium-rich grain boundaries in aluminium-doped LLZO favour the formation of inter-granular lithium dendrites (with a 60 % lower critical current density for cell failure) over gallium-doped LLZO.
The use of germanium (Ge4+) as a dopant was studied, and shown to stabilise the cubic LLZO phase through substitution of 0.10 moles of Ge at the lithium sub-lattice (at the tetrahedral 24d sites), giving conductivities of the order 10-4 S cm-1 and redox stability over a 4.5 V range with lithium electrodes. Chemical and electrochemical characterisation of the moisture reactivity of gallium-doped LLZO was also carried out, showing a chemically-altered proton-rich region extending to 1.35 micrometres following 30 minutes immersion in H2O at 100 °C and highly reactive grain boundaries. These chemical changes led to a threefold increase in the resistance of both the electrolyte and the interface with lithium electrodes. Chemical and tracer diffusivity of protons were estimated from the diffusion profiles of H+ and D+ obtained by SIMS depth-profiling.
A new methodology for measuring macroscopic lithium tracer diffusion in LLZO was introduced, using SIMS depth-profiling and isotopic labelling, in which a number of experimental parameters were varied to optimise the technique. The preliminary results for lithium diffusivity in doped LLZO obtained from this method were compared with values from other methods (impedance and nuclear magnetic resonance) and used to comment on the mechanism for lithium diffusion in the materials.Open Acces
