Alumni

Title of thesis:

Diskrete elektrochemische Modellierung für Elektrodendesign und Laderegelung von Lithium-Ionen-Batterien

2022

Supervising Chair:

Electrical Energy Systems

Electrifying the energy market and the transport sector based on renewable energies is indispensable to meet the objective of the Paris Agreement: Limiting the human-caused climate change to 1,5 °C. In both sectors, efficient energy storage at a low cost is required to fulfill the demand for mobility and to overcome the volatility in power generation. For this purpose, lithium-ion batteries are increasingly used in manifold applications. Alongside many advantages, this technology also comes with challenges. One of those is the limitation of its fast-charging capability, which defines the time required to recharge a vehicle in analogy to the re-fueling of a combustion vehicle. This thesis aims to develop methods and tools in the field of engineering to precisely determine those limitations and to extend those limits through design modifications.
After discussing the structure and the working principle of lithium-ion batteries, the characterization method Distribution of Relaxation Times is described, refined and the impact of its meta parameters is analyzed. The method allows for the separation, identification, and quantification of processes in arbitrary electrochemical systems without any a priori knowledge, assumptions, or models and is based on standard measurement procedures. The characterization of lithium-ion batteries is typically used to develop and parameterize battery models. These models can describe and predict the behavior at various load conditions and can furthermore be used for fast-charging algorithms. However, the state-of-the-art models do not meet the requirements of a charge controller based on local states inside the battery. Furthermore, zero-dimensional models which lack spatial information are not capable of characterizing and quantifying the behavior of porous electrodes. Therefore, the aim of this thesis is to adapt a frequency domain model originating from energy transmission and communications engineering as an electrode model for use in the time domain with a real-time capable solution. Finally, the model shall be applied for fast charging.
This transmission line model characterizes single electrodes and enables a spatial resolution alongside the thickness of the electrode. As a mixed conducting network, the discrete electrochemical model structure characterizes ionic as well as electronic transport processes. Based on the processes identified by the distribution of relaxation times, the model structure is developed using concentrated electrical network elements. The model is described mathematically and implemented for the time and frequency domain, respectively. Spatial discretization is introduced to allow for the transformation into the time domain. The model structure leads to a classification between zero-dimensional, phenomenological models and physical-chemical models.
The frequency domain variant can be calculated and solved iteratively with an analytic transfer function. In contrast, the time domain variant is solved by a sophisticated numerical algorithm since the resulting differential-algebraic equation system demands high stability of the solver. Through an efficient implementation, the model can be executed on a real-time system at a 10 ms cycle. The parameterization is carried out in the frequency domain using electrochemical impedance spectra of half-cells made from commercial electrode material which is brought into a three-electrode experimental cell setup. The achieved model precision and parameters are strongly dependent on the discretization in the frequency domain. For a well-interpretable result with a low deviation between measurement and model, at least several hundred discrete elements are required. Caused by the small number of model parameters, the parameterization is efficient and unambiguous. In the time domain, validation measurements show a high agreement with the model for static and dynamic load profiles at a significantly coarser discretization. Several model extensions, developed by students during their theses, demonstrate a further reduction of the computational effort as well as combinations of an anode and a cathodemodel to a full-cell model.
The frequency domain model is applied for the analysis of the electrode design of the anode. Besides the effect of varied model parameters on the impedance, various design properties are investigated through simulations. These include the electrode thickness, the porosity, and the active material particle size. Summarizing the findings, the commercial electrode is designed well considering its purpose in a high-energy battery. Furthermore, the model is proven to be capable of characterizing varying electrode properties. Therefore, it is suitable for the efficient model-based design and optimization of electrodes. This includes identifying rate-limiting factors during charging, allowing design changes to improve the fast-charging capability.
Finally, the time-domain model variant is used for model-predictive fast charging control using different cost functions. One of the proposed methods controls the potential at the electrode surface to a pre-defined value to avoid damage to the electrode by lithium deposition. The charging capability can be fully used without causing significant aging. A safety margin, which is used in state-of-the-art algorithms, can be omitted as local, internal states are used instead of the clamp behavior. A second approach allows for a user-defined trade-off between an even shorter charging time and the induced, increased aging. This allows for an adequate charging of electric vehicles at a minimum duration. The charge algorithm is proven stable against parameter uncertainties. State estimation is required for the application in a battery management system. At the time of submission related research is ongoing.
The proposed model copes with the requirements regarding fast charging for both the electrode development process and the battery operation. This is an enhancement compared to the state of the art. Unlike established models, the approach proposed in this work combines distinct parameterability and real-time capability based on the electrical network structure with spatially resolved, electrochemically-physically well-interpretable states inside the electrode. However, the model is not limited to the mentioned applications. Subsequent research based on this thesis is working on including a thermal sub-model. Detailed aging studies can be carried out, resulting in an aging sub-model. Furthermore, the model will be used and adapted to evaluate novel, solid electrolytes and their application within porous electrodes.

Titel der Dissertation:

Characterizing and Manipulating by Local Electrochemical Techniques

2024

Betreuender Lehrstuhl:

Physical Chemistry II

New challenges in battery research demand a more fundamental understanding of electrochemical processes on a local scale that play a key role in various battery components. While several analytical techniques have been developed for macroscopic characterization, only a few methods allow to locally resolve the electrochemical behavior on a molecular level. One highly promising method for the measurement of local electrochemistry is the atomic force microscopy (AFM), which allows for high resolution imaging of various interfaces and manipulation of objects. While some combinations of the AFM technique with electrochemical methods already exist, there is still plenty of room for improvements of existing techniques and the development of new applications. The aim of this work is the development of new electrochemical AFM methods for analytics and manipulation. This newly developed methods are used to study interfacial interactions between electrochemical active AFM-probes and different substrates.

Title of thesis:

Elektrochemische Charakterisierung und systemtechnische Diagnose von Lithium-Ionen-Batterien

2025

Supervising Chair:

Electrical Energy Systems

In light of the growing prevalence of lithium-ion batteries, it is imperative to investigate alternative approaches to recycling and to enhance the longevity and performance of these batteries. To develop appropriate solutions, it's essential to have a thorough understanding of the electrical characteristics of both individual cells and modules. For this reason, a detailed method for process characterization, the Löwner-method, is used in this thesis. It is validated through the utilization of both equivalent circuit models and an established method for process characterization. It offers a promising and innovative approach for the analysis of a wide variety of electrochemical systems. Moreover, the Löwner-method is integrated with alternative techniques to enable a thorough characterization of 92 lithium-ion batteries, surpassing the limitations of traditional approaches like equivalent circuit parameter analysis or direct variable measurement.
The identified processes are examined for their distribution function, where the general assumption of a normal distribution can be declined for some processes. In addition, a correlation analysis reveals three discrete process correlation groups that are assigned to cell winding, surface processes, and diffusion processes. The results of the cell-to-cell-variation study are then used to model the energy and pulse power capability of battery modules in series and parallel connection. To account for parameter variation and correlation, multivariate normal distributions are used. A simulation is then employed to examine the impact of cell sorting and an inhomogeneously aged cell on the performance variables of energy and pulse power capability. The results show that cell sorting, despite its significant sorting effort, only marginally enhances performance.
In contrast, the performance exhibits a significant dependence on the inhomogeneity, particularly in the serial connection. Consequently, a method for detecting inhomogeneities in serially connected battery modules is being developed. For this purpose, the impedance of the entire module is simulated and analyzed using various impedance characteristics. It turns out that the low-frequency minimum is the most suitable feature for this purpose. It is therefore validated by measurements. This feature enables the detection of inhomogeneities in a series connection of up to ten cells.
This work therefore makes a significant contribution to the understanding of the electrical properties of battery modules and their performance. Moreover, it serves as a foundation for the future development of decision support algorithms in the field of circular economy.

Titel der Dissertation:

3d - Transition Metal Chalcogenides for Applications in Photo- and Electrocatalysis

2023

Betreuender Lehrstuhl:

Physical Chemistry III


Photo- and electrocatalysis represent sustainable alternatives to conventional fossil-fuel reliant industrial processes for the production of green fuels and commodity chemicals. However, they are still not cost-competitive, amongst others requiring the development of new, earth-abundant catalysts.
The first part of my PhD work focused on the development of a low-temperature microwave-assisted solvothermal synthesis of CuFe2O4 nanoparticles in only a couple of minutes. An optimisation of synthesis parameters, including the solvent mixture and the pH value allowed for the preparation of CuFe2O4 particles with a narrow size distribution over a wide range of different synthesis times and temperatures. The CuFe2O4 particles were employed in the electrochemical reduction of CO2 to CO. The synthesis time was shown to have a significant influence on both CO yield and selectivity, which could be explained by a combination of different material properties.
Compared to electrocatalysis, photocatalysis combines light absorption and charge carrier excitation with the subsequent target reactions into one system, without the need for an external driving force. To improve the efficiency, charge separation is commonly promoted by the addition of a cocatalyst. Charges are transferred to these cocatalysts and they provide active sites for the reaction – thus they share many similarities with electrocatalysts. The second work of my PhD thesis therefore targeted the synthesis and application of Ni2FeS4 as an earth-abundant cocatalyst substitute for noble metals in the H2 evolution from water. Phase-pure, crystalline Ni2FeS4 nanosheets could be prepared in only 1 min using a microwave assisted approach and benzyl mercaptan as a sulphur source. Ni2FeS4 was used as a cocatalyst on TiO2 (P25), which efficiently promoted its photocatalytic activity for H2 production under both simulated sunlight and intense UV-rich light. Exceptionally low mass-ratios of 0.5 wt.% of Ni2FeS4 could be realised, without a loss of activity.
While H2 is an ideal green fuel, its storage is complicated and large portions of the produced H2 are actually required as feedstock for industrial processes. One such process is the Haber-Bosch process for the synthesis of NH3. Photocatalytic nitrogen reduction offers a sustainable alternative, but it requires the development of efficient N2 activation catalysts. Carbon nitrides (CN) are among the most widely investigated catalysts for these nitrogen fixation reactions. Therefore, vacancy rich carbon nitride (VN-CN) was synthesised in the third work of my thesis and combined with biomimetic FeS2 (pyrite). This combination resulted in an increased ammonia yield by approx. 400% compared to unmodified carbon nitride, even at low loadings of FeS2. However, a set of material characterisation and control experiments revealed that ammonia is not generated via the reduction of N2 gas, but instead by a decomposition of cyano-groups at the defect sites in VN-CN. FeS2 is further promoting this light-induced decomposition reaction by coordinating to the defect sites and activating the cyano-groups via π-back-donation. It was thus shown that although comparatively high ammonia yields can be achieved by this system, it is not via photocatalytic NRR, for which VN-CN is therefore unsuitable.