Sven Uhlenbruck

Li7La3Zr2O12 Interface Modification for Li Dendrite Prevention

Al-contaminated Ta-substituted Li7La3Zr2O12 (LLZ:Ta), synthesized via solid-state reaction, and Al-free Ta-substituted Li7La3Zr2O12, fabricated by hot-press sintering (HP-LLZ:Ta), have relative densities of 92.7% and 99.0%, respectively. Impedance spectra show the total conductivity of LLZ:Ta to be 0.71 mS cm(-1) at 30 °C and that of HP-LLZ:Ta to be 1.18 mS cm(-1). The lower total conductivity for LLZ:Ta than HP-LLZ:Ta was attributed to the higher grain boundary resistance and lower relative density of LLZ:Ta, as confirmed by their microstructures. Constant direct current measurements of HP-LLZ:Ta with a current density of 0.5 mA cm(-2) suggest that the short circuit formation was neither due to the low relative density of the samples nor the reduction of Li-Al glassy phase at grain boundaries. TEM, EELS, and MAS NMR were used to prove that the short circuit was from Li dendrite formation inside HP-LLZ:Ta, which took place along the grain boundaries. The Li dendrite formation was found to be mostly due to the inhomogeneous contact between LLZ solid electrolyte and Li electrodes. By flatting the surface of the LLZ:Ta pellets and using thin layers of Au buffer to improve the contact between LLZ:Ta and Li electrodes, the interface resistance could be dramatically reduced, which results in short-circuit-free cells when running a current density of 0.5 mA cm(-2) through the pellets. Temperature-dependent stepped current density galvanostatic cyclings were also carried out to determine the critical current densities for the short circuit formation. The short circuit that still occurred at higher current density is due to the inhomogeneous dissolution and deposition of metallic Li at the interfaces of Li electrodes and LLZ solid electrolyte when cycling the cell at large current densities.

Oxidation behaviour of ferrous alloys used as interconnecting material in solid oxide fuel cells

Under operating conditions in the solid oxide fuel cell (SOFC), metallic interconnect plates form electrically insulating or poor-conducting oxide scales (e.g. Cr2O3, Al2O3) at their surface which increase the contact resistance from one fuel cell membrane to the next. In order to minimize electric losses in a fuel cell stack, the formation of oxide scales on the interconnect surface must either be prevented or the oxide scale formed must have sufficient electrical conductivity. In the present work, investigations were carried out on the corrosion behaviour of different FeCrAl and FeCrMn alloys, some of which were coated with nickel (Ni). Information about ageing of these alloys on the anode side of the fuel cell was obtained by means of contact resistance measurements and scanning electron microscopy. The results reveal that FeCrMn(LaTi) alloys and Ni-coated interconnects exhibit low ageing rates and are thus suitable for use on the anode side of SOFCs.

Advances in Research, Development, and Testing of Single Cells at Forschungszentrum Jülich

This paper presents an overview of the main advances in solid oxide fuel cells (SOFCs) research and development (R&D), measurement standardization, and quality assurance in SOFC testing at the Forschungszentrum Julich. These activities have resulted in both a significant improvement of the electrochemical performance and a better understanding of the electrochemical behavior of SOFCs. Research and development of SOFCs was mainly focused on two types of anode-supported cells, namely, those employing either La 0.65 Sr 0.3 MnO 3 (LSM) or La 0.58 Sr 0.4 Co 0.2 fe 0.8 O 3-δ (LSCF) cathode materials. In both cases the optimization of processing and microstructural parameters resulted in satisfactory power output and long-term stability at reduced operation temperatures. Standardization and quality assurance in SOFC testing was also addressed with the goal of producing consistent and reliable tests and measurement results. At present, under optimized experimental conditions, SOFCs with LSM or LSCF cathodes can deliver a power output of about 1.0 W/cm 2 and 1.9 W/cm 2 at 800°C (700 mV), respectively.

Suppression of Aluminum Current Collector Dissolution by Protective Ceramic Coatings for Better High-Voltage Battery Performance

Batteries based on cathode materials that operate at high cathode potentials, such as LiNi0.5 Mn1.5 O4 (LNMO), in lithium-ion batteries or graphitic carbons in dual-ion batteries suffer from anodic dissolution of the aluminum (Al) current collector in organic solvent-based electrolytes based on imide salts, such as lithium bis(trifluoromethanesulfonyl) imide (LiTFSI). In this work, we developed a protective surface modification for the Al current collector by applying ceramic coatings of chromium nitride (Crx N) and studied the anodic Al dissolution behavior. By magnetron sputter deposition, two different coating types, which differ in their composition according to the CrN and Cr2 N phases, were prepared and characterized by X-ray diffraction, X-ray photoelectron spectroscopy (XPS), scanning electron microscopy (SEM), and their electronic conductivity. Furthermore, the anodic dissolution behavior was studied by cyclic voltammetry and chronocoulometry measurements in two different electrolyte mixtures, that is, LiTFSI in ethyl methyl sulfone and LiTFSI in ethylene carbonate/dimethyl carbonate 1:1 (by weight). These measurements showed a remarkably reduced current density or cumulative charge during the charge process, indicating an improved anodic stability of the protected Al current collector. The coating surfaces after electrochemical treatment were characterized by means of SEM and XPS, and the presence or lack of pit formation, as well as electrolyte degradation products could be well correlated to the electrochemical results.

Three-Dimensional, Fibrous Lithium Iron Phosphate Structures Deposited by Magnetron Sputtering

Crystalline, three-dimensional (3D) structured lithium iron phosphate (LiFePO4) thin films with additional carbon are fabricated by a radio frequency (RF) magnetron-sputtering process in a single step. The 3D structured thin films are obtained at deposition temperatures of 600 °C and deposition times longer than 60 min by using a conventional sputtering setup. In contrast to glancing angle deposition (GLAD) techniques, no tilting of the substrate is required. Thin films are characterized by X-ray diffraction (XRD), Raman spectrospcopy, scanning electron microscopy (SEM), cyclic voltammetry (CV), and galvanostatic charging and discharging. The structured LiFePO4+C thin films consist of fibers that grow perpendicular to the substrate surface. The fibers have diameters up to 500 nm and crystallize in the desired olivine structure. The 3D structured thin films have superior electrochemical properties compared with dense two-dimensional (2D) LiFePO4 thin films and are, hence, very promising for application in 3D microbatteries.

Reactions of garnet-based solid-state lithium electrolytes with water — A depth-resolved study

Abstract Garnet Li6.4La3Zr1.6Ta0.4O12 thin films prepared by magnetron sputtering were analysed by secondary ion mass spectrometry, nuclear reaction analysis and Rutherford backscattering to identify, localize and quantify the reactions associated with the presence of low amounts of water and carbon dioxide. Samples in a pristine state and after storage in an Argon-filled glove box for months were compared. Both, lithium hydroxide and lithium carbonate were detected, with carbon-containing species and hydrogen-containing having surprisingly different depth profiles.

High Capacity Garnet-Based All-Solid-State Lithium Batteries: Fabrication and 3D-Microstructure Resolved Modeling

The development of high-capacity, high-performance all-solid-state batteries requires the specific design and optimization of its components, especially on the positive electrode side. For the first time, we were able to produce a completely inorganic mixed positive electrode consisting only of LiCoO2 and Ta-substituted Li7La3Zr2O12 (LLZ:Ta) without the use of additional sintering aids or conducting additives, which has a high theoretical capacity density of 1 mAh/cm2. A true all-solid-state cell composed of a Li metal negative electrode, a LLZ:Ta garnet electrolyte, and a 25 μm thick LLZ:Ta + LiCoO2 mixed positive electrode was manufactured and characterized. The cell shows 81% utilization of theoretical capacity upon discharging at elevated temperatures and rather high discharge rates of 0.1 mA (0.1 C). However, even though the room temperature performance is also among the highest reported so far for similar cells, it still falls far short of the theoretical values. Therefore, a 3D reconstruction of the manufactured mixed positive electrode was used for the first time as input for microstructure-resolved continuum simulations. The simulations are able to reproduce the electrochemical behavior at elevated temperature favorably, however fail completely to predict the performance loss at room temperature. Extensive parameter studies were performed to identify the limiting processes, and as a result, interface phenomena occurring at the cathode active material/solid-electrolyte interface were found to be the most probable cause for the low performance at room temperature. Furthermore, the simulations are used for a sound estimation of the optimization potential that can be realized with this type of cell, which provides important guidelines for future oxide based all-solid-state battery research and fabrication.

Development of Thin-Film Manufacturing Technologies for Solid Oxide Fuel Cells and Gas Separation Membranes

The development of solid oxide fuel cells (SOFCs) and gas separation membranes for fossil (fuel?) power plants has previously suffered from cost issues like the manufacturing of the core components including i) the ceramic fuel cell and ii) the ceramic membrane, and from insufficient power density (current density or flow rate) on the stack, module or system level. Forschungszentrum Julich has been working on SOFC development for 20 years, and on membrane development for 6 years. Both energy-related applications are based on similar materials systems, similar micro-structural features (porous-dense, coarse-fine), comparable application parameters (e.g. high temperature) and are manufactured with similar technologies. In the past the focus laid mostly on basic materials research and proving the functionality of the membranes or fuel cells. Meanwhile, one key topic has been the application of low-cost thin-film high-throughput manufacturing technologies. This includes the fabrication of the supports (mostly...

Bias-Assisted Sputtering of Gadolinia-Doped Ceria Interlayers for Solid Oxide Fuel Cells

The effects of both temperature and applied bias power during the sputtering of gadolinia-doped ceria (GDC) interlayers used as diffusion barriers in anode-supported solid oxide fuel cells (SOFC) were investigated. Scanning electron microscopy analysis revealed that increasing the applied bias power progressively inhibits the columnar structure typically observed in sputtered films, favoring the deposition of dense interlayers. Such feature was mirrored in the electrochemical tests of single cells that demonstrated enhanced power density for SOFC. The presented results evidenced that bias-assisted sputtering is an effective technique for the fabrication of high-performance anode supported SOFC. Diffusion barrier interlayers, deposited onto the electrolyte, have demonstrated to be an effective way to avoid undesired reactions and preventing the degradation of the SOFC. Ceria-based oxides have been reported as convenient materials for such barriers due to good transport properties and reasonable compatibility with both yttria-stabilized zirconia (YSZ) and LSCF cathode. One of the main challenges is the fabrication of cost effective and reproducible protective interlayers, at sufficiently low temperatures in order to avoid undesired reactions. The effectiveness of the diffusion barrier is closely related to its microstructural properties, and homogeneous interlayers with high density are advantageous. Previous studies, concerning deposition techniques, evidenced that single cells with ceria-based interlayers fabricated by reactive magnetron sputtering (RMS) exhibited enhanced performance when compared to wet ceramic depositions. Such an improved performance is possibly associated with the higher density of such layers. Although sputtering is a scalable industrial process, further investigation is required for the deposition of optimized SOFC protective interlayers. In the present study, the effects of applied bias voltage during sputtering of gadolinia-doped ceria (GDC) diffusion barriers over YSZ electrolytes were studied. Half cells comprised of Ni / YSZ anode support and functional layer, and YSZ electrolyte were used as substrates for the deposition of GDC interlayers. Coatings were carried out in a physical vapor deposition system CS 400ES (Von Ardenne Anlagentechnik). High-frequency bias voltage was applied to the metallic sample holder by controlling a fixed bias power, ranging from 0 to 300 W. Depositions were carried out at different temperatures in the 400-800°C range. Screen-printed LSCF cathodes were applied to the half cells, followed by heat treatment. The microstructure of deposited interlayers was studied by scanning electron microscopy (SEM). The electrochemical properties of single cells were investigated by I-Vcurves under H2/air flow in the 600-900 °C range. Microstructural analyses (Fig. 1) of GDC barriers revealed that increasing the applied bias power progressively inhibits the usual columnar structure of sputtered interlayers, resulting in dense microstructures. Such a feature was reflected in the I-V curves, which showed that the performance of solid oxide fuel cell having bias sputtered interlayers is increased in the whole temperature range studied, as shown in the Fig. 2. 650 700 750 800 850 0.8 1.2 1.6 2.0 2.4