Julian Szász

All-solid-state lithium ion batteries may overcome safety issues in future energy storage systems, when a non-flammable solid electrolyte instead of a liquid electrolyte is applied. Garnet-type Li₇La₃Zr₂O₁₂ (LLZO) is an encouraging choice due to its high Li⁺ conductivity and its chemical stability against lithium metal [1]. However, low-resistance LLZO/Li interfaces are deteriorated by incomplete adhesion of lithium metal onto LLZO and by surface contamination [2]. The resulting inhomogeneous distribution of contact areas may be more prone to dendrite formation. Previous studies [3,4] indicate that LLZO/Li interface quality is significantly improved by metallic buffer layers. In this study, we aim for a reliable preparation of low-resistance interfaces between solid electrolyte and lithium metal. At first, magnetron sputtering is applied to deposit an indium buffer layer of 50 to 500 nm thickness onto the polished and cleaned electrolyte surface. For cell assembly, lithium foil is put on both sides of an In-coated LLZO pellet and the test cell is heated to 220°C in argon atmosphere. After that, microstructure and electrochemical properties of the symmetric cells are analyzed. We improved cross-section sample preparation to inspect the quality of LLZO/In/Li interfaces over hundreds of µm. Scanning electron microscopy (SEM, cf. figure 1) confirms excellent adhesion between lithium and LLZO. We use a dry inert atmosphere during preparation, handling and transfer of our samples to avoid artifacts and sample degradation. The FEI Helios G4 FX microscope enables correlative SEM imaging by simultaneous acquisition of up to four different detector signals for interface topography and material contrast. Local chemical composition at the interface is analyzed by energy dispersive X-ray spectroscopy and electron-beam damage is minimized for SEM imaging by selecting optimum electron energies. Electrochemical impedance spectroscopy (EIS) followed by data evaluation via the distribution of relaxation times (DRT) and equivalent circuit modelling [5] deliver the proportional contributions of grain, grain boundaries and interfaces to the total impedance of symmetric cells (Li/In/LLZO/In/Li). Interface microstructures and EIS measurements are correlated for samples with different buffer layer characteristics to allow targeted optimization. We compare the pristine state of the test cells to samples that have been cycled with different current densities. References [1] S. Ramakumar et al., Progress in Materials Science 88 (2017), 325-411 [2] L. Cheng et al., Physical Chemistry Chemical Physics 16 (2014), 18294 [3] W. Luo et al., Advanced Materials 29 (2017), 1606042 [4] C. Tsai et al., ACS Applied Materials & Interfaces 8 (2016), 10617−10626 [5] P. Braun et al., Journal of Electroceramics 38 (2017), 157-167 Figure 1

Interdiffusion phenomena and secondary phase formation at the interface La_0.58Sr_0.4Co_0.2Fe_0.8O_3-δ (LSCF) / Gd_0.2Ce_0.8O_2-δ (GDC) / Y_0.16Zr_0.84O_2-δ (YSZ) are correlated to linear and non-linear losses in symmetrical and full SOFC cells. FIB/SEM (focussed ion beam / scanning electron microscopy) tomography is applied for determining the local distribution of the primary phases LSCF, GDC, and YSZ and elemental analysis via STEM/EDXS (scanning transmission electron microscopy / energy dispersive X-ray spectroscopy) provides information on the secondary phase SrZrO₃ (SZO) and the interdiffusion between GDC and YSZ (ID). This reveals the effect of GDC co-sintering temperature (varied from 1100°C to 1400°C), alongside the sintering of LSCF at 1080°C, on these multi-layered microstructures. Electrochemical impedance spectra on symmetrical cells show that the polarization resistance (ASR_cat) of the cathode/electrolyte interface is pronouncedly affected by three orders of magnitude, changing the overall power density of anode supported SOFC single cells at 0.8V by as much as a factor of 20. In conclusion, the chemical composition of the ID has a tremendous impact on the electrochemical efficiency of the investigated LSCF/GDC/YSZ interface, and processing parameters of anode supported cells with LSCF cathode have to be carefully chosen for individual SOFC cell concepts.

The effect of B-site cation substitution on the phase stability of Ba_0.5Sr_0.5Co_0.8Fe_0.2O_3-δ (BSCF) in O₂- and CO₂-containing atmospheres has been studied thoroughly by our group. In particular, a doping concentration of 10 mol-% yttrium (Y) was found to strongly reduce or suppress secondary phase formation extending the stability of the cubic phase regime. Characterization by electrochemical impedance spectroscopy (EIS) of both, pure and Y-doped BSCF electrodes, is performed in oxygen-nitrogen mixtures as well as in CO₂-containing atmospheres using symmetrical SOFC cells with Ce_0.9Gd_0.1O_2-δ (CGO) as electrolyte from 600 to 900 °C. Analysis of the EIS data in combination with the distribution of relaxation times (DRT) reveals an enhanced tolerance of the Y-doped BSCF cathodes towards the adsorption of CO₂ molecules. The enhanced stability of the cubic phase and reduced poisoning effect on the oxygen reduction reaction in CO₂- containing atmospheres by doping, is further confirmed by scanning electron microscopy (SEM) and analytical (scanning) transmission electron microscopy ((S)TEM) combined with energy dispersive X-ray spectroscopy (EDXS).

High-performance solid oxide fuel cells (SOFC) mostly utilize mixed conducting La_1-xSr_xCo_1-yFe_yO_3-δ (LSCF) cathodes. The drawback with LSCF is the formation of non-conducting secondary phases (SrZrO₃) when it is applied on Y₂O₃ stabilized ZrO₂ (YSZ) electrolytes. This secondary phase reaction can be prevented by applying a dense Gd-doped Ceria (GDC) between LSCF and YSZ. In this study, the characteristics of LSCF/GDC/YSZ interfaces were systematically modified by a variation in GDC sintering temperature. Our findings confirm a heterogeneous cathode/electrolyte interface composed of primary (LSCF, GDC and YSZ) and secondary phases (SrZrO₃ and GDC-YSZ interdiffusion). The performance depends strongly on GDC sintering temperature and interface resistances can lead to performance reduction by ~2000% (1582 to 77mW∙cm² at 800 °C were measured). Regarding technical relevant processes, secondary phases appear and overshadow high-performing electrode properties. A deeper understanding of the interplay between chemical composition, processing and microstructure is provided to improve individual cell concepts.

High-performance oxygen electrodes for solid oxide fuel cells (SOFC) require a highly efficient material selection for the oxygen reduction reaction that takes places near the cathode/electrolyte interface. La_1-xSr_xCo_1-yFe_yO_3-δ (LSCF) delivers fast oxygen-exchange kinetics and is therefore utilized as cathode material for high performing state-of-the-art anode-supported fuel cells. The main disadvantage of LSCF is its instability against Zr-based electrolytes. A diffusion barrier interlayer consisting of Gd doped cerium oxide (GDC) is therefore inserted to prevent the formation of an insulating SrZrO₃ phase. This study presents new results on the importance of the GDC manufacturing conditions. A complex interaction of GDC with LSCF and YSZ during fabrication is found with a tremendous influence on the performance of the cathode/electrolyte interface of SOFCs.