Microstructure-property relation in Li$_{1.3}$Al$_{0.3}$Ti$_{1.7}$(PO$_{4}$)$_{3}$ superionic Li-conducting ceramics

  • Mikrostruktur-Eigenschaften-Beziehung in Li$_{1.3}$Al$_{0.3}$Ti$_{1.7}$(PO$_{4}$)$_{3}$ Li-leitenden Superionischen Keramiken

Gündüz, Deniz Cihan; Eichel, Rüdiger-Albert (Thesis advisor); Mayer, Joachim (Thesis advisor); Englert, Ullrich (Thesis advisor)

Aachen (2019)
Dissertation / PhD Thesis

Dissertation, RWTH Aachen University, 2019

Abstract

Lithium aluminium titanium phosphate Li$_{1+x}$Al$_{x}$Ti$_{2-x}$(PO$_{4}$)$_{3}$ is one of the materials under consideration as a solid electrolyte in future all solid-state lithium-ion batteries. In this work, I study the evolution of the microstructure of LATP ceramic samples, sintered between 900 and 1100 $^{o}$C from Li$_{1.3}$Al$_{0.3}$Ti$_{1.7}$(PO$_{4}$)$_{3}$ powders prepared via a sol-gel route, by scanning electron microscopy (SEM) and confocal laser scanning microscopy (CLSM). LATP ceramics are highly brittle and they degrade under water; therefore, an all-oil-based grinding and polishing route was developed that does not alter the microstructure during sample preparation that allows differentiation of different microstructural components. Sintering temperature dependent evolution of grain size, morphology, and connectedness as well as secondary phase content are tracked by CLSM and SEM. In addition to that, transmission electron microscopy (TEM) was used for the determination of grain boundary width and to identify the amorphous structure of the secondary phase. Goal of the work is to correlate the microstructure with total, grain and grain boundary resistance as extracted from electrochemical impedance spectroscopy via means of distribution of relaxation times (DRT). The latter showed a grain conductivity almost three order of magnitude larger than that of the grain boundaries. An increase of total conductivity with grain size is observed, which correlates with the grain size. However, at 1100 $^{o}$C, total resistance increases with excess amount of secondary phase and crack formation comes into account. In ceramic processing and for final macroscopic conductivity, the presence of secondary phases and porosity play an important role. In presence of more than one secondary phase and pores, image analysis must tackle the difficulties about distinguishing between these microstructural features. Therefore, I come up with a novel image segmentation and reconstruction method. In this work, we also study the phase evolution of LATP ceramic samples by the image segmentation method based on elemental maps acquired in the scanning electron microscope combined with quantitative analysis of LATP grains. We find aluminium phosphate AlPO$_{4}$ and another phosphate phase containing an only little amount of aluminium. It may contain lithium, which is not detectable by energy dispersive X-ray spectroscopy (EDS). The amount of these phases changes with sintering temperature which may influence the ionic conductivity of the whole material. First: As the grains act as an aluminium source for AlPO$_{4}$ formation, the aluminium content decreases decreasing also the intragranular conductivity. Second: Also, the amount of secondary phase changes from more (Li$_{x}$)P$_{y}$)O$_{z}$ at 950°C to mainly AlPO$_{4}$ at 1100°C sintering temperature, which in addition may influence the grain boundary conductivity.

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