Thermodynamic Evaluation of the system Ta–O and Preliminary Assessment of the Systems Al–Nb–O and Al–Ta–O

The binary tantalum–oxygen system is assessed using the CALculation of PHase Diagrams (CALPHAD) method with experimental data from the literature. The oxygen solubility in the Ta solid-solution phase is discussed and modeled. The low- and high-temperature modifications of Ta₂O₅ are described as stoichiometric compounds. This dataset is extended into the ternary Al–Ta–O system by complementing it with binary datasets for Al–O and Al–Ta from the literature and adding mixed-oxide AlTaO₄. The dataset for the ternary system Al–Nb–O is created by combining the three corresponding binary datasets from the literature and by assessing the quasibinary section Al₂O₃–Nb₂O₅. The ternary aluminum niobates are described as stoichiometric compounds. Phase equilibria between refractory metals and alumina at high temperature are discussed

Laser-assisted post-processing of additive manufactured metallic parts

Laser-assisted additive manufacturing (AM) is the process of successively melting thin layers of material using a laser source to produce a three dimensional device or product. From the many technologies available, only a few can produce metallic parts that fulfil the requirements of industrial applications. Ultrafast laser machining is a new and promising technical approach for post-processing AM parts since laser ablation and surface modification processes could be applied with high accuracy for trimming shape and functionality, i.e., edge quality and wettability. The impact of different ultrafast laser parameters is evaluated for AM samples, which are examined for surface roughness before and after the laser-assisted post-processes. For all the parameters tested, the use of ultrafast laser resulted in a homogeneous material ablation of the samples’ surfaces. For the investigated parameter range, the AM building tracks were still maintained even after ultrafast laser post-processing. The achieved results showed the formation of self-organized porous structures at low laser scan velocities leading to an enhanced surface roughness. For higher scan velocities characteristic nano ripples might be induced having no significant impact on the measured surface roughness

Modeling and Simulation the Thermal Runaway Behavior of Cylindrical Li-Ion Cells—Computing of Critical Parameter

The thermal behavior of Li-ion cells is an important safety issue and has to be known under varying thermal conditions. The main objectives of this work is to gain a better understanding of the temperature increase within the cell considering different heat sources under specified working conditions. With respect to the governing physical parameters, the major aim is to find out under which thermal conditions a so called Thermal Runaway occurs. Therefore, a mathematical electrochemical-thermal model based on the Newman model has been extended with a simple combustion model from reaction kinetics including various types of heat sources assumed to be based on an Arrhenius law. This model was realized in COMSOL Multiphysics modeling software. First simulations were performed for a cylindrical 1860 cell with a -cathode to calculate the temperature increase under two various simple electric load profiles and to compute critical system parameters. It has been found that the critical cell temperature [Math Processing Error] , above which a thermal runaway may occur is approximately [Math Processing Error] , which is near the starting temperature of the decomposition of the Solid-Electrolyte-Interface in the anode at [Math Processing Error] . Furthermore, it has been found that a thermal runaway can be described in three main stages

Investigation of Fast-Charging and Degradation Processes in 3D Silicon–Graphite Anodes

The 3D battery concept applied on silicon–graphite electrodes (Si/C) has revealed a significant improvement of battery performances, including high-rate capability, cycle stability, and cell lifetime. 3D architectures provide free spaces for volume expansion as well as additional lithium diffusion pathways into the electrodes. Therefore, the cell degradation induced by the volume change of silicon as active material can be significantly reduced, and the high-rate capability can be achieved. In order to better understand the impact of 3D electrode architectures on rate capability and degradation process of the thick film silicon–graphite electrodes, we applied laser-induced breakdown spectroscopy (LIBS). A calibration curve was established that enables the quantitative determination of the elemental concentrations in the electrodes. The structured silicon–graphite electrode, which was lithiated by 1C, revealed a homogeneous lithium distribution within the entire electrode. In contrast, a lithium concentration gradient was observed on the unstructured electrode. The lithium concentration was reduced gradually from the top to the button of the electrode, which indicated an inhibited diffusion kinetic at high C-rates. In addition, the LIBS applied on a model electrode with micropillars revealed that the lithium-ions principally diffused along the contour of laser-generated structures into the electrodes at elevated C-rates. The rate capability and electrochemical degradation observed in lithium-ion cells can be correlated to lithium concentration profiles in the electrodes measured by LIBS

Thermophysical Properties of Lithium Aluminum Germanium Phosphate with Different Compositions

The NASICON system LAGP (Li$_{1+x}$Al$_{x}$Ge$_{2-x}$ (PO$_{4}$)$_{3}$ was studied, which is a candidate material for solid state electrolytes. LAGP substrates with different compositions (x = 0.3–0.7) were prepared using a melt quenching route with subsequent heat treatment. In order to develop a better understanding of the relationships between the structure and the ionic as well as the thermal conductivity, respectively, the samples were characterized by X-ray diffraction. The ionic conductivity was measured using impedance spectroscopy while the thermal diffusivity and the specific heat were determined by Laser Flash technique and differential scanning calorimetry, respectively. Additionally, thermal analysis was performed in order to evaluate the thermal stability a higher temperatures and, also to identify the optimum temperature range of the thermal post-processing. The measured values of the ionic conductivities were in the range of 10$^{-4}$Ω$^{-1}$·cm$^{-1}$ to 10$^{-3}$ Ω$^{-1}$·cm$^{-1}$ at room temperature, but exhibited an increasing behavior as a function of temperature reaching a level of the order 10$^{-2}$ Ω$^{-1}$· cm$^{-1}$ above 200 °C. The thermal conductivity varies only slowly as a function of temperature but its level depends on the composition. The apparent specific heat depends also on the composition and exhibits enthalpy changes due to phase transitions at higher temperatures for LAGP samples with x > 0.5. The compositional dependencies of the ionic and thermal transport properties are not simply correlated. However, the compound with the highest Li-doping level shows the highest ionic conductivity but the lowest thermal conductivity, while the lowest doping level is associated with highest thermal conductivity but the lowest ionic conductivity

Progress in thermal management and safety of cells and packs by testing in battery calorimeters

With increasing energy density the safety and the thermal management of Li-ion batteries is becoming more and more important, because the thermal runaway can cause an ignition or even explosion of the battery with simultaneous release of toxic gases. In the last nine years we have established battery calorimetry as a powerful and versatile electrochemical-thermal characterization technique, which allows both advancements for the thermal management and the safety of batteries. With six adiabatic Accelerating Rate Calorimeters (ARC) of different sizes and two sensitive Tian-Calvet calorimeters, all of them combined with cyclers, the IAM-AWP now operates Europe’s largest battery calorimeter center, which enables the evaluation of thermodynamic, thermal and safety data on material, cell and pack level under quasiadiabatic and isoperibolic environments for both normal and abuse conditions (thermal, electrical, mechanical). It will be shown how sophisticated battery calorimetry allows finding new and quantitative correlations between different critical thermally and safety related parameters that will help to design safer systems. For an optimized design with regard to safety cells and packs have to be characterized not only for their temperature behavior but also for the heat dissipation and the pressure development in a quantitative manner. Calorimetry allows the collection of quantitative data required for optimum battery performance and safety. This information can then be used to define the requirements for cooling and thermal management and adapt them accordingly. The battery calorimeters can be used for studies on heat generation and dissipation of Li-ion cells and are coupled to a battery cycler in order to perform the measurements during charging and discharging of the cells under defined thermal conditions. Isoperibolic (constant temperature of the calori-meter) or quasiadiabatic (no heat exchange with the calorimeter) ambient conditions are adjusted by heaters and thermocouples that are located in lid, bottom and side walls of the calorimeter chamber, in which the cell is inserted. For improving the thermal management system, the measured temperature data are converted into generated and dissipated heat data [1] by determination of specific heat capacity and heat transfer coefficient using heat flux sensors. Concerning safety aspects it will be presented how battery calorimeters provide thermal stability data on materials level, e.g. of anodes, cathodes or electrolytes or there combinations and to perform safety tests on cell and pack level by applying thermal [2], mechanical or electrical [3] abuse conditions. The studies on materials level are especially important for Post-Li cells, which make use of more abundant materials, such as sodium or magnesium instead of Li, nickel and cobalt, because these data help to develop safe cells from the beginning all along the value chain. For the advanced Li-ion technology, a holistic safety assessment is in the focus, because the thermal runaway can have multiple interacting causes and effects. A test in the calorimeter is much more sensitive than a hotbox test and reveals the entire process of the thermal runaway with the different stages of exothermic reactions. Self-heating, thermal stability and thermal runaway are characterized and the critical parameters and their thresholds for safe cell operation are determined. As a result of the different tests quantitative and system relevant data for temperature, heat and pressure development of materials and cells are provided. In addition it will be explained how calorimeters allow studying the thermal runaway propagation in order to develop and qualify suitable countermeasures, such as heat protection barriers, which is currently becoming a very hot topic, because a global technical regulation (GTR) on electric vehicle safety is being developed, which includes thermal propagation. There is still the open question, which is the best initialization method to become a standard. We hope that the research in the Calorimeter Center will help to make progress in this field as well. References: [1] C. Ziebert et al., in: L.M. Rodriguez, N. Omar, eds., EMERGING NANOTECHNOLOGIES IN RECHARGABLE ENERGY STORAGE SYSTEMS, Elsevier Inc., ISBN 978032342977, 195-229. 2017. [2] B. Lei, W. Zhao, C. Ziebert, et al., Experimental analysis of thermal runaway in 18650 cylindrical cells using an accelerating rate calorimeter, Batteries 3 (2017) 14, doi:10.3390/batteries3020014.. [3] A. Hofmann, N. Uhlmann, C. Ziebert, O. Wiegand, A. Schmidt, Th. Hanemann, Preventing Li-ion cell explosion during thermal runaway with reduced pressure, Appl. Thermal Eng. 124 (2017) 539-544