Scientific interest is strongly drawn towards all-solid-state lithium batteries, considered
the future generation of electrochemical energy storage solutions. Their appeal lies in
their enhanced safety characteristics and their capacity to open up fresh avenues in chemistry,
thereby improving performance. The features of the solid-state electrolyte play a
crucial role in the battery’s overall cell competence. So far, the materials showing the most
promise are garnet-structured oxides, notably those based on LLZO, which demonstrate
high ionic conductivity at room temperature and an extensive electrochemical stability
range. However, lithium-stuffed garnets remain hindered from the onset of metallic dendrites
that permeate the electrolyte upon cycling.
This research aimed to explore whether elemental segregation and electronic conductivity
are key contributors to dendrite-induced garnet failure, utilizing Ga-doped LLZO
and Ta-doped LLZO as case studies. The initial sections (Chapters 5 and 6) present a
thorough evaluation of each solid electrolyte’s properties, including their composition, microstructure,
purity, and electrochemical performance, as determined by Electrochemical
Impedance Spectroscopy (EIS) and Galvanostatic cycling.
Chapter 5 focuses on Ga-LLZO prepared using alumina, magnesia, or zirconia
crucibles in either argon or oxygen atmospheres. EPR spectroscopy was employed to detect
free electrons in the garnet, revealing signals for unpaired electrons in Ga-LLZO treated
under a reducing atmosphere. However, a conclusive analysis of the nuclear environment
around the free electron could not be drawn from the hyperfine splitting data. Chapter 6
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mirrors the trajectory of Chapter 5 but centres on Ta-LLZO SE. Chapter 7 delves into the
surface chemistry and elemental segregation of both doped LLZO compositions using XPS,
ToF-SIMS, and LEIS. The findings showed that Ga tends to segregate towards the LLZO
surface and grain boundaries, while Ta behaves similarly to matrix cations in LLZO,
residing predominantly in the bulk of the electrolyte. The impact of the crucible was
examined, with magnesia serving as a relatively new sample support for LLZO sintering.
It was confirmed that MgO− contamination is prevalent in the LLZO, with ToF-SIMS
imaging revealing MgO in the grain boundary of Ga-LLZO, similar to observations with
LLZO sintered using alumina crucibles.
Chapter 8 explores oxygen diffusion in LLZO using IEDP on ToF-SIMS to obtain
secondary ion images and diffusion profiles of the 18O isotope. Preliminary findings suggested
18O segregation in grain boundaries of Ga-LLZO that underwent chemical diffusion
with 18O. Tracer diffusivity was obtained from self-exchanged LLZO with 18O, showing a
D* of 10−17-10−19 cm2/s. These results were compared with previous literature values of
D* for 18O diffusion in garnets such as YIG and YAG which were lower. Thus indicating
a higher degree of disorder due to possible oxygen vacancies in LLZO. The initial attempt
at determining the activation energy for oxygen diffusion in LLZO showed an Ea of 1.7
eV, though more data collection is necessary to minimize potential errors. Finally, future
work encompasses contributions from in-operando SIMS using the bespoke Hi5 instrument,
which will facilitate SIMS analysis alongside battery operation. This system has
also demonstrated the ability to collect both polarities of SI with a highly focused plasma
primary beam, a challenge with previous SIMS-based instruments.