REPOSITORY > RESULTS

Abstract

All-solid-state batteries are one of the key technologies for future energy storage and the transition to a climate-neutral society. They offer improved safety, stability, and a broader operating temperature range compared to conventional batteries with liquid electrolyte. Perovskite Li0.33La0.56 0.11TiO3 (LLTO) is recognized as one of the most promising solid electrolytes, however, its performance is hindered by the presence of grain boundaries, which exhibit orders of magnitude lower conductivity values than the grains. Microstructures with larger grain sizes are desirable to reduce the impact of grain boundaries and improve overall performance.
In this dissertation, LLTO samples with varying initial La:Ti ratios and excess Li were prepared with solid-state synthesis. In sufficiently Li- and La-rich LLTO compositions, microstructure development starts at around 1100 °C with the formation of plate-like grains composed of layered Ruddlesden-Popper (RP)-type Li2La2Ti3O10 phase with periodic structure and structurally related non-periodic sequences. These grains undergo exaggerated growth and develop into large plate-like grains with thicknesses of up to 10 μm and lengths over 100 μm. Theoretical calculations confirmed that the formation of RP-type sequences is energetically favored and precedes the formation of the LLTO perovskite phase. Microstructure development continues with the crystallization of LLTO perovskite, epitaxially on the plate-like grains and as individual smaller grains with thinner in-grain RP-type lamellae, resulting in bimodal microstructure.
The structural and compositional characteristics of the lamellae were thoroughly investigated by scanning transmission electron microscopy in combination with image simulations. Periodic sequences of the Li2La2Ti3O10 phase consist of pseudo-perovskite blocks separated by a Li-rich layer. Thin in-grain lamellae exhibit a non-periodic structure, with pseudo-perovskite blocks of varying thicknesses. At around 1250 °C, pronounced ion exchange occurs in non-periodic sequences between the Li-rich RP layers and the neighboring Ti and La layers. The degree of ion exchange was estimated by quantitative HAADF-STEM analysis. The thermal instability of the RP-type defects leads to their gradual recrystallization to LLTO.
At higher sintering temperatures (1350 °C), all sequences of RP-type defects completely recrystallize to LLTO. Furthermore, the smaller LLTO grains recrystallize onto the large plate-like seed grains via Ostwald ripening. This process results in the development of dense microstructures with LLTO grains measuring up to 100 μm in diameter. This method represents a novel and straightforward approach for the preparation of coarse-grained LLTO ceramics with total ionic conductivity in the range of 10-4 S/cm.