Single crystals as a model system for materials for advanced energy storage technologies

Flux grown single crystals of NASICON-based electrode materials
Orientated single Crystals of Ga-stabilized LLZO made by Czochralski method

One of the main research areas of my group is the design of ceramics with tunable functionality by crystal chemical engineering. In particular the understanding of the underlying mechanism of dynamic processes in solid electrolytes for future Li- and Na-ion battery technologies. This information is also important for industry to provide guidelines to improve their materials to enable future micro-electronic devices. In order to investigate those materials, we use a wide spectrum of methods, such as powder and single crystal (X-ray and neutron) diffraction, Raman/IR spectroscopy, impedance spectroscopy, and electrochemistry in general to study the material properties over different length scales.

We use of single crystals grown by different methods, such as flux growth (by our group and Prof. G√ľnther Redhammer, University of Salzburg), floating zone (Prof. Junji Akiomot, AIST, Japan) and Czochralski method (Dr. Steffen Ganschow, IKZ, Berlin) in combination with impedance spectroscopy and electrochemical methods for battery testing.

In order to study small single crystals grown by flux (50-100 micron) I deposit microelectrodes (15-30 micron) on top polished surfaces. The electrodes are contacted by a metal tip using a micromanipulator and an optical microscope to investigate the Li-ion dynamic as a function of temperature using impedance spectroscopy. The data are correlated afterwards with single-crystal diffraction (SCXRD) data and DSC data to understand the interplay of Li-ion dynamics, phase behavior and structure. The main benefit of this method is that chemical, structural, and microstructural inhomogeneities can be excluded, which provides the most accurate description of structure-property relations.

Moreover, we study single crystalline electrode materials, in particular the structural changes which occur during charge and discharge as well as their impact on kinetics (e.g., exchange current densities, diffusion). Therefore, I glue small single crystals (10 - 100 micron) on top of metal tip (see Figure 2), put it into a liquid electrolyte with counter and reference electrodes (Na or Li) and perform electrochemical tests. The advantage is compared to standard methods that volume, and surface area is very well known makes it the most accurate way to determine the material properties.

Both techniques can be applied to all battery materials of interest. This makes it a huge research area for potential new discoveries with high impact.

Despite small single crystals large single crystals provide even more benefits. It is obvious, if large single crystals are available many techniques can be applied on one and the same sample. This providing a great chance to gain a fundamental understanding of the interplay of structure and properties. Unfortunately, such single crystals are rarely available. As an example we use Li6La3ZrTaO12 single crystal, one of the most promising solid electrolytes, grown by Czochralski method (see Figure 3). Afterwards a broad portfolio is applied to understand structural and mechanical properties, as well as electrochemical properties giving new insights into materials, which clearly push the field forward.