05 Fakultät Informatik, Elektrotechnik und Informationstechnik
Permanent URI for this collectionhttps://elib.uni-stuttgart.de/handle/11682/6
Browse
67 results
Search Results
Item Open Access Analytic free-energy expression for the 2D-Ising model and perspectives for battery modeling(2023) Markthaler, Daniel; Birke, Kai PeterAlthough originally developed to describe the magnetic behavior of matter, the Ising model represents one of the most widely used physical models, with applications in almost all scientific areas. Even after 100 years, the model still poses challenges and is the subject of active research. In this work, we address the question of whether it is possible to describe the free energy A of a finite-size 2D-Ising model of arbitrary size, based on a couple of analytically solvable 1D-Ising chains. The presented novel approach is based on rigorous statistical-thermodynamic principles and involves modeling the free energy contribution of an added inter-chain bond DAbond(b, N) as function of inverse temperature b and lattice size N. The identified simple analytic expression for DAbond is fitted to exact results of a series of finite-size quadratic N N-systems and enables straightforward and instantaneous calculation of thermodynamic quantities of interest, such as free energy and heat capacity for systems of an arbitrary size. This approach is not only interesting from a fundamental perspective with respect to the possible transfer to a 3D-Ising model, but also from an application-driven viewpoint in the context of (Li-ion) batteries where it could be applied to describe intercalation mechanisms.Item Open Access Modeling and experimental investigation of the interaction between pressure-dependent aging and pressure development due to the aging of lithium-ion cells(2023) Avdyli, Arber; Fill, Alexander; Birke, Kai PeterIn order to meet the increasing demands of the battery in terms of range, safety and performance, it is necessary to ensure optimal operation conditions of a lithium-ion cell. In this thesis, the influence of mechanical boundary conditions on the cell is investigated theoretically and experimentally. First, fundamental equations are derived that lead to coupled models that can be parameterized based on specific cell measurements and predict the pressure evolution due to capacity aging and vice versa. The model is used to derive optimal operating points of the cell, which can be considered in the module design.Item Open Access Top‐down approach to study chemical and electronic properties of perovskite solar cells : sputtered depth profiling versus tapered cross‐sectional photoelectron spectroscopies(2021) Das, Chittaranjan; Zia, Waqas; Mortan, Claudiu; Hussain, Navid; Saliba, Michael; Ingo Flege, Jan; Kot, MałgorzataA study of the chemical and electronic properties of various layers across perovskite solar cell (PSC) stacks is challenging. Depth‐profiling photoemission spectroscopy can be used to study the surface, interface, and bulk properties of different layers in PSCs, which influence the overall performance of these devices. Herein, sputter depth profiling (SDP) and tapered cross‐sectional (TCS) photoelectron spectroscopies (PESs) are used to study highly efficient mixed halide PSCs. It is found that the most used SDP‐PES technique degrades the organic and deforms the inorganic materials during sputtering of the PSCs while the TCS‐PES method is less destructive and can determine the chemical and electronic properties of all layers precisely. The SDP‐PES dissociates the chemical bonding in the spiro‐MeOTAD and perovskite layer and reduces the TiO2, which causes the chemical analysis to be unreliable. The TCS‐PES revealed a band bending only at the spiro‐MeOTAD/perovskite interface of about 0.7 eV. Both the TCS and SDP‐PES show that the perovskite layer is inhomogeneous and has a higher amount of bromine at the perovskite/TiO2 interface.Item Open Access Surface charge density and induced currents by self-charging sliding drops(2024) Bista, Pravash; Ratschow, Aaron D.; Stetten, Amy Z.; Butt, Hans-Jürgen; Weber, Stefan A. L.Spontaneous charge separation in drops sliding over a hydrophobized insulator surface is a well-known phenomenon and lots of efforts have been made to utilize this effect for energy harvesting. For maximizing the efficiency of such devices, a comprehensive understanding of the dewetted surface charge would be required to quantitatively predict the electric current signals, in particular for drop sequences. Here, we use a method based on mirror charge detection to locally measure the surface charge density after drops move over a hydrophobic surface. For this purpose, we position a metal electrode beneath the hydrophobic substrate to measure the capacitive current induced by the moving drop. Furthermore, we investigate drop-induced charging on different dielectric surfaces together with the surface neutralization processes. The surface neutralizes over a characteristic time, which is influenced by the substrate and the surrounding environment. We present an analytical model that describes the slide electrification using measurable parameters such as the surface charge density and its neutralization time. Understanding the model parameters and refining them will enable a targeted optimization of the efficiency in solid–liquid charge separation.Item Open Access Impedance based temperature estimation of lithium ion cells using artificial neural networks(2021) Ströbel, Marco; Pross-Brakhage, Julia; Kopp, Mike; Birke, Kai PeterItem Open Access Pressure characteristics and chemical potentials of constrained LiFePO4/C6 cells(2018) Singer, Jan Patrick; Kropp, Timo; Kuehnemund, Martin; Birke, Kai PeterConstraining lithium-ion cells increases the cyclic lifetime. However, depending on an expected volume expansion during charge and discharge cycling, defining the optimal constraining pressure range is not straightforward. In this study, we investigate a lithium iron phosphate/graphite pouch cell at four initial constraining pressure levels. As a function of C-Rate, the thermodynamic principle of the non-monotonic pressure curve during full charge and discharge cycles is evaluated. Using the rubber balloon model to calculate the chemical potential of lithium iron phosphate and discussing the relationship between the chemical potential and pressure, we illustrate the pressure curve qualitatively. By applying differential pressure analysis, we evaluate the resulting pressure curves of a single graphite stage. Approaching a fundamental understanding of reduced cycling lifetime of full cells with unknown material composition, we allocate the stages and stage transitions of graphite as well as the phase transition of lithium iron phosphate. Local extreme values in the differential pressure analysis indicate phase and stage transitions. These values can identify critical operating conditions that should be considered for defining the optimum initial constraining pressure range.Item Open Access Non-uniform circumferential expansion of cylindrical Li-ion cells - the potato effect(2021) Hemmerling, Jessica; Guhathakurta, Jajnabalkya; Dettinger, Falk; Fill, Alexander; Birke, Kai PeterThis paper presents the non-uniform change in cell thickness of cylindrical Lithium (Li)-ion cells due to the change of State of Charge (SoC). Using optical measurement methods, with the aid of a laser light band micrometer, the expansion and contraction are determined over a complete charge and discharge cycle. The cell is rotated around its own axis by an angle of α=10° in each step, so that the different positions can be compared with each other over the circumference. The experimental data show that, contrary to the assumption based on the physical properties of electrode growth due to lithium intercalation in the graphite, the cell does not expand uniformly. Depending on the position and orientation of the cell coil, there are different zones of expansion and contraction. In order to confirm the non-uniform expansion around the circumference of the cell in 3D, X-ray computed tomography (CT) scans of the cells are performed at low and at high SoC. Comparison of the high resolution 3D reconstructed volumes clearly visualizes a sinusoidal pattern for non-uniform expansion. From the 3D volume, it can be confirmed that the thickness variation does not vary significantly over the height of the battery cell due to the observed mechanisms. However, a slight decrease in the volume change towards the poles of the battery cells due to the higher stiffness can be monitored.Item Open Access Cycling of double-layered graphite anodes in pouch-cells(2022) Müller, Daniel; Fill, Alexander; Birke, Kai PeterIncremental improvement to the current state-of-the-art lithium-ion technology, for example regarding the physical or electrochemical design, can bridge the gap until the next generation of cells are ready to take Li-ions place. Previously designed two-layered porosity-graded graphite anodes, together with LixNi0.6Mn0.2Co0.2O2 cathodes, were analysed in small pouch-cells with a capacity of around 1 Ah. For comparison, custom-made reference cells with the average properties of two-layered anodes were tested. Ten cells of each type were examined in total. Each cell pair, consisting of one double-layer and one single-layer (reference) cell, underwent the same test procedure. Besides regular charge and discharge cycles, electrochemical impedance spectroscopy, incremental capacity analysis, differential voltage analysis and current-pulse measurement are used to identify the differences in ageing behaviour between the two cell types. The results show similar behaviour and properties at beginning-of-life, but an astonishing improvement in capacity retention for the double-layer cells regardless of the cycling conditions. Additionally, the lifetime of the single-layer cells was strongly influenced by the cycling conditions, and the double-layer cells showed less difference in ageing behaviour.Item Open Access Unified model for laser doping of silicon from precursors(2021) Hassan, Mohamed; Dahlinger, Morris; Köhler, Jürgen R.; Zapf-Gottwick, Renate; Werner, Jürgen H.Laser doping of silicon with the help of precursors is well established in photovoltaics. Upon illumination with the constant or pulsed laser beam, the silicon melts and doping atoms from the doping precursor diffuse into the melted silicon. With the proper laser parameters, after resolidification, the silicon is doped without any lattice defects. Depending on laser energy and on the kind of precursor, the precursor either melts or evaporates during the laser process. For high enough laser energies, even parts of the silicon’s surface evaporate. Here, we present a unified model and simulation program, which considers all these cases. We exemplify our model with experiments and simulations of laser doping from a boron oxide precursor layer. In contrast to previous models, we are able to predict not only the width and depth of the patterns on the deformed silicon surface but also the doping profiles over a wide range of laser energies. In addition, we also show that the diffusion of the boron atoms in the molten Si is boosted by a thermally induced convection in the silicon melt: the Gaussian intensity distribution of the laser beam increases the temperature-gradient-induced surface tension gradient, causing the molten Si to circulate by Marangoni convection. Laser pulse energy densities above H > 2.8 J/cm2 lead not only to evaporation of the precursor, but also to a partial evaporation of the molten silicon. Without considering the evaporation of Si, it is not possible to correctly predict the doping profiles for high laser energies. About 50% of the evaporated materials recondense and resolidify on the wafer surface. The recondensed material from each laser pulse forms a dopant source for the subsequent laser pulses.Item Open Access Introducing a concept for designing an aqueous electrolyte with pH buffer properties for Zn-MnO2 batteries with Mn2+/MnO2 deposition/dissolution(2023) Fitz, Oliver; Wagner, Florian; Pross-Brakhage, Julia; Bauer, Manuel; Gentischer, Harald; Birke, Kai Peter; Biro, DanielFor large-scale energy-storage systems, the aqueous rechargeable zinc–manganese dioxide battery (ARZMB) attracts increasing attention due to its excellent advantages such as high energy density, high safety, low material cost, and environmental friendliness. Still, the reaction mechanism and its influence on the electrolyte's pH are under debate. Herein, a pH buffer concept for ARZMB electrolytes is introduced. Selection criteria for pH buffer substances are defined. Different buffered electrolytes based on a zinc salt (ZnSO4, Zn(CH3COO)2, Zn(CHOO)2), and pH buffer substances (acetic acid, propionic acid, formic acid, citric acid, 4-hydrobenzoic acid, potassium bisulfate, potassium dihydrogen citrate, and potassium hydrogen phthalate) are selected and compared to an unbuffered 2 m ZnSO4 reference electrolyte using titration, galvanostatic cycling with pH tracking, and cyclic voltammetry. By adding buffer substances, the pH changes can be reduced and controlled within the defined operating window, supporting the Mn2+/MnO2 deposition/dissolution mechanism. Furthermore, the potential plateau during discharge can be increased from ≈1.3 V (ZnSO4) to ≈1.7 V (ZnSO4 + AA) versus Zn/Zn2+ and the energy retention from ≈30% after 268 cycles (ZnSO4) to ≈86% after 494 cycles (ZnSO4 + AA). Herein, this work can serve as a basis for the targeted design of long-term stable ARZMB electrolytes.