Research Group Prof. Christian Kübel

The morphology and microstructure of electrodes and solid electrolytes and, in particular, the interface within and between materials play a crucial role for battery performance and long term stability. The aim of the research unit 'Advanced Electron Microscopy in Materials Research' is to use and develop advanced electron microscopy techniques ranging from classical TEM and STEM imaging in combination with EELS and EDX spectroscopy to tomography and further to 4D-STEM techniques such as crystal orientation (ACOM) or field mapping (DPC) to provide structural and functional information from the atomic scale to the micron level and relate it to transport and degradation processes in batteries using both ex situ and in situ approaches. 1. POLiS - Impact of microstructural features on sodium transport & filament formation in all solid-state sodium batteries 2. POLiS - Electron Beam Effects in Solid Electrolytes 3. POLiS - Hard Carbons as Electrodes for Sodium Batteries

Electron Microscopic Characterization of Batteries and Materials

The development of advanced materials for new batteries increasingly focuses on nanostructured composites to combine high electron and ion conductivity for new cathode materials. These nano composites typically exhibit a complex morphology geared to maintain high capacity over extended charging/discharging cycles. The aim of the electron microscopy & spectroscopy group is to characterize the morphology and composition of the active battery materials at the atomic to nanometer scale in 2D and also 3D using a combination of high-resolution imaging (TEM, HAADF-STEM), spectroscopy (EELS, EDX) and tomographic techniques to correlate the structure and morphology at different charging states with the cyclic stability. To some extent, this characterization is possible using well-developed ex-situ analytical (S)TEM techniques that only need to be optimized to deal with the electron beam sensitivity of most battery materials. However,for a more direct analysis of the structural changes during electrochemical cycling, we are also actively developing new techniques for in-situ imaging of micron/nano-sized battery models inside the TEM.

Metal fluorides as conversion electrodes in lithium ion batteries

In collaboration with Prof. Max Fichtner, we have characterized the structure of Fe/Lif/C conversion electrodes used as cathode materials in lithium-ion batteries. The conversion materials prepared pyrolysis of ferrocene with LiF initially consist of iron (and some iron carbide) nanoparticles with a thin graphitic shell, interconnected by MWCNTs. Charging leads to the formation of FeF₂/C or FeF₃/C nanoparticles with the graphitic shell still present. Both, the graphitic shell and the MWCNTs are crucial for a good cyclic behavior with the graphitic shell presumably responsible forthe cyclicstability of the iron nanoparticles and the MWCNTs ensuring electrical contact through the electrode.

Development of in-situ TEM characterization of the cyclic behaviour of solid state batteries

In addition to the ex-situ analysis shown above, we are developing new preparation and analysis approaches to follow the structural changes in micron sized solid state batteries and half cells in-situ using TEM techniques. The basic idea is to use FIB preparation to cut a thin cross-section from a solid state battery that is then transferred under (close to) inert conditions onto an electro-contacting TEM holder for in-situ analysis using low-dose and low voltage TEM and STEM techniques. The materials currently under investigation are solid state lithium ion batteries with Garnet type electrolytes in collaboration with Prof. Jürgen Janek group at the University Giessen  and new lithium-free fluoride based solid state batteries in collaboration with Prof. Max Fichtner.