Alumni

Title of the thesis:

Machine Learning Driven Materials Design from the Perspective of Charge Transport

2025

Supervising chair:

Theoretical Physics VII

Title of the thesis:

Characterizing and Manipulating by Local Electrochemical Techniques

2024

Supervising Chair:

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 the thesis:

Polymethacrylate and Polyacrylate based Polymer and Nanocomposite Electrolytes for Lithium Metal Batteries 

2023

Supervising chair:

Applied Functional Polymers

This thesis addresses the design of ion-conducting polymers, their application as host material in solid polymer electrolytes (SPEs) and the detailed characterization in the context of materials and at the level of solid-state battery cells involving lithium metal anodes and lithium iron phosphate cathodes. In particular, the focus is on structure-property relationships between chemistry and physico-chemical as well as electrochemical performance, in-depth analysis of electrochemical processes in various cell geometries and the behavior of the SPEs in a realistic battery environment. One central theme throughout the thesis is the functional polyacrylate (PA) or polymethacrylate (PMA) backbone featuring a variable ester function. Decorated with functional ether- or ester-based pendant groups, these polymers can conduct lithium-ions (Li+) via coordination to the free electron pairs of their oxygens. Solid electrolytes, especially polymer-based systems are considered a key technology for the successful implementation of high energy and power density conversion electrodes on the way to next generation all-solid-state lithium metal batteries (ASSLMBs) with improved safety. Thereby, the SPE plays the role of both ion transport medium and electrical separator between the electrodes. This work is separated into two major parts with respect to the chemistry of ion-conducting moieties in the polymer architecture. In chapter 4, five new bottlebrush copolymers bearing different lengths of poly(ethylene glycol) (PEG) side chains attached to two different backbones were synthesized. SPEs obtained from these bottlebrushes mixed with LiTFSI conducting salt were examined and the results were compared to the respective linear polymer systems. The occurring processes in the cell were identified by the Distribution of Relaxation Times (DRT) or extended DRT (eDRT) method. It could be shown that the crystallinity of PEG can be suppressed by the polymer architecture without any considerable loss in ionic conductivity (σ) regardless of the investigated backbone. Among different low-Tg materials, the one with the highest σ shows the lowest Tg and simultaneously the lowest crystallinity. The lithium transport number (LTN) and the electrochemical stability of the bottlebrush SPEs are comparable to linear PEG SPEs. Unresolved questions of interfacial as well as bulk processes are addressed by comparing bottlebrushes with linear poly(ethylene oxide) (PEO) in chapter 5. Besides using a bottlebrush host polymer, the role of the Li salt anion was investigated in a detailed comparative analysis using LiTFSI and the borate salts LiBOB and LiDFOB in these SPEs. Irrespective of the chemical nature of the anion these SPEs possess one major, resistive-capacitive bulk conductivity mechanism. For the borate-containing SPEs, the time-dependent formation of a solid electrolyte interphase layer (SEI) was identified, which could be separated from the slower charge transfer process via eDRT method. Furthermore, the contribution with the highest resistance was identified as the diffusion of Li+ in the electrolyte. Besides a more beneficial SEI for the LiBOB-SPE compared to LiDFOB, the LTNs of both systems are comparable to those observed in LiTFSI systems. To further improve the SPEs for the application in ASSLMBs at ambient temperature in chapter 6, solid polymer nanocomposite electrolytes (SPNEs) based on TiO2 nanoparticles in a bottlebrush-LiTFSI matrix were prepared and analyzed while the results were interpreted with eDRT. The SPNEs were referenced against linear PEO-LiTFSI SPEs and nanoparticle-free bottlebrush-LiTFSI to understand the influence of nanoparticles on the electrochemical, interfacial, and mechanical properties. Compared to the respective SPEs, an optimum nanoparticle concentration was found at 10 wt% in terms of increased σ-values as well as mechanical and electrochemical stability. While exhibiting a similar LTN, the compatibility of the SPNEs with Li metal was significantly improved due to the formation of a stable low-resistive SEI. In ASSLMBs, the bottlebrush SPNEs performed efficiently even at 30 °C, while the PEO systems suffered from crystallization. When the current rate was increased at 70 °C, the cell with the bottlebrush SPNE maintained a high discharge capacity with enhanced stability compared to the linear analogue. Chapter 7 constitutes the second part of this thesis, focusing on functional amorphous polyesters in “Beyond PEO” SPEs. By systematic structural variation of the alkyl spacer, the diester groups as well as the backbone chemistry (PA vs. PMA), seven new graft copolymers were synthesized, characterized, analyzed, and consequently studied as host polymers in SPEs as well as SPNEs. The maximum σ was found at 25 wt% salt concentration for PA-based polyesters with the longest alkyl spacers showing the lowest Tg. According to FT-IR spectroscopy studies both uncoordinated and Li+-influenced ester groups were identified to be present in roughly equal molar ratio to achieve high σ. The measured LTNs where significantly higher than those for PEO-based SPEs. For SPNEs, cell failure caused by Li dendrite short-circuiting could be fully avoided and stable and reliable long-term Li plating and stripping with a low overpotential was realized. In ASSLMBs, stable cycling with a high capacity retention at 70 and even at 40 °C was obtained.

Title of the thesis:

Application of 2D Clay Materials for Sustainable Energy Storage

Supervising chair: 

Inorganic Colloids for Electrochemical Energy Storage

Together with energy harvesting and distribution, energy storage technologies are essential to broadly establish renewable energy sources in a power grid. Chemical energy storage, e.g., through hydrogen, requires two conversion steps. Step one resembles the storage of electrical energy in chemical bonds (electrolyzer), and step two reverses this process (fuel cells). This work will introduce the technology of green hydrogen generation via water electrolysis and illustrate why efficient and sustainable catalyst systems based on non-toxic, abundant, and cost-effective materials are required. For this purpose, the investigations focus on 2D layered materials, which have proven to be a versatile material class to facilitate the oxidative half-reaction of electrochemical water splitting, which is the oxygen evolution reaction (OER). This work will focus on the structure-property relationship in such materials while also paying attention to the ecological aspects of the technology. The aim is to tailor catalytical systems, further improving their capabilities and scalability. In the scope of this work, the influence of composition, specifically iron-content, in layered double hydroxides is investigated concerning the triggering of grafting. That is the chemical bonding of interlayer anions to brucite-like layers. Due to the high importance of bimetallic iron-containing layered double hydroxides, it is crucial to understand which implication its incorporation bears for the structure and, ultimately, the catalytic performance. The systematic variation of Co/Fe composition within the layers showed that the presence of iron favors grafting, thereby inducing structural disorder in the form of random interstratification and planar defects. At the same time, having a minimum amount of Co in the structure is essential to ensure high catalytic activity. From the perspective of the structure-properties relationship, the question remains open as to what kind of effect the extent of grafting has. For this purpose, different brucite-type materials are compared. Three classes are chosen that feature inherently different interlayer constitutions, while the layers have similar compositions. These include metal hydroxides M(OH)2 without interlayer anions, layered double hydroxides with free interlayer anions, and hydroxynitrates with fully grafted interlayer anions. This work shows that an interlayer anion's chemical bonding can alter the metal centers' electronic structure. This is decisive for their oxidation potential, i.e., the potential at which the electrocatalytically active center forms. In the last part of this work, the previously gained knowledge is combined to achieve control of grafting within the same material. The all-iron electrocatalyst mössbauerite is known to exhibit extensive grafting of interlayer anions. By employing a corrosion engineering approach, it is possible to obtain its precursor green rust on a steel plate as a large area electrode and control the ratio of grafted to ungrafted interlayer anions by choice of the oxidation method. This control of grafting in the same material allows for the first systematic study on the influence of grafting in mössbauerite.

Title of the thesis:

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

2023

Supervising chair:

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.

Title of the thesis:

Defektreiches α-MnO₂ und Co₃O₄ als bifunktionale Sauerstoffelektrokatalysatoren für elektrisch wiederaufladbare Zink-Luft-Batterien

2024

Supervising chair:

Electrochemical Process Engineering

Electrically rechargeable zinc-air batteries, which stand out from other battery types due to their higher energy density, better operational safety and good environmental and climate benignity, represent a promising battery system to implement the increasing electrification of road transportation and storage of fluctuating renewable energies, though especially in a context of the energy transition. However, given the low reversibility and lifetime of both the zinc-anode and the air-cathode, the so-called gas diffusion electrode (GDE), zinc-air batteries are yet to be relevant for these field of application. While there are promising attempts at solutions for the zinc-anode, the challenges on the air-cathode side are more manifold. This is where the present work intervenes. On the cathode side, in addition to the development of noble metal-free, bifunctional hybrid-/composite-catalysts and GDEs based on defect-rich α-MnO₂, the work also focuses on a mechanistic understanding of material properties acting as catalytically active sites, which favour the oxygen reactions taking place during discharging and charging of zinc-air batteries, the oxygen reduction reaction (ORR) and oxygen evolution reaction (OER). Based on as-synthesised α-MnO₂ catalysts and various reference manganese oxides, a clear relationship between Mn³⁺ cation defects, which are supposed to be the catalytically active sites of manganese oxides, and intrinsic ORR activities, i.e., normalised by the electrochemically active surface areas (ECSAs), could be demonstrated for the first time in a comprehensive structure-activity-correlation. Accordingly, a volcano-like trend revealed ~50 mol-% Mn³⁺ to be the optimal concentration for highest ORR activities. Besides extensive structural, physicochemical and electrochemical characterisation, X-ray photoelectron spectroscopy and the thin-film rotating disk electrode (TF-RDE) were used for a quantitative evidence of Mn³⁺ cation defects, but also to assess electrocatalytic properties and ECSAs required for the determination of intrinsic activities. By preparing hybrid-/composite-catalysts based on the most active Mn³⁺ and OER-active Co₃O₄ nanoparticles, the present work describes two generic approaches to obtain a catalyst with improved bifunctional properties. i) By introducing α-MnO₂ to the reactant solution of the partner, a α-MnO₂-Co₃O₄ core-corona composite-catalyst consisting of a α-MnO₂ core with a thin Co₃O₄-rich shell (corona) could be prepared straight from the synthesis of Co₃O₄. ii) At less expenditure, the α-MnO₂-Co₃O₄ tandem-catalyst was obtained by means of a physical (powder) mixture of both catalysts. While the composite-catalyst offers optimisation opportunities, the α-MnO₂-Co₃O₄ tandem-catalyst was able to surpass the catalytic properties of the individual catalysts, owing to synergistic effects as well. Moreover, improvements over state-of-the-art carbon supported noble metal catalysts could be achieved, comprising 1.5-fold higher kinetic current densities within the ORR and significantly increased OER long-term stability as compared to commercial Pt/C and Ir/C, respectively. For evaluation in electrically rechargeable zinc-air batteries, the catalysts together with the binders styrene-butadiene rubber (SBR), sodium carboxy-methylcellulose (Na-CMC) and conductive supports were successfully processed to GDEs via a practicable doctor blade technique. In addition to the optimal binder compositions facilitating efficient discharging and charging of zinc-air cells, attention was also paid to the performance associated with the wetting properties of α-MnO₂ and Co₃O₄ GDEs. The wetting properties, as characterized by means of contact angle measurements and electrochemically via the ECSA, were found to be complex indicating a strong dependence on the physicochemical properties (i.e., particle size, porosity) of the catalysts themselves, in addition to the portion of the hydrophobic binder. Drawing from these findings, a bifunctional α-MnO₂-Co₃O₄ dual-layer GDE featuring superimposed α-MnO₂ and Co₃O₄ layers, each with optimal binder compositions, was found to be superior to single-layer GDEs. Even more important, compared to a commercial, state-of-the-art Pt/C GDE, the α-MnO₂-Co₃O₄ dual-layer GDE was able to provide more efficient charging of zinc-air cells. Finally, long-term cycling in a zinc-air cell lasting up to 1000 charge/discharge cycles revealed excellent stability of the α-MnO₂-Co₃O₄ dual-layer GDE developed as part of this work. Defect-rich α-MnO₂ and Co₃O₄ as bifunctional oxygen electrocatalysts and GDEs aim not only to provide a substitute for commercial, state-of-the-art noble metal-based catalysts and GDEs, but the results and insights from this work should also pave the way for electrically rechargeable zinc-air batteries as a future battery system to meet the challenges posed by the energy transition.

Title of the 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 of the thesis:

Operando-Methoden der Diagnose und Laderegelung von Lithium-Ionen-Batterien

2023

Supervising chair:

Electrical Energy Systems

The electrification of private transport is crucial for achieving the climate goals of the Paris Agreement and the associated reduction of greenhouse gases. In order to increase the attractiveness of batteryelectric vehicles, it is necessary to increase the life expectancy of lithium-ion batteries, reduce their costs and improve their fast-charging capability. The latter is mainly limited by the degradation mechanism of lithium deposition, which occurs at the graphitic electrode. In this process, cyclable lithium is lost, leading to accelerated ageing and should therefore be prevented. This work aims to extend the understanding of this mechanism and to develop operando detection methods suitable for use in a battery management system. For this purpose, these methods must be sensitive, reliable, robust and must work automatically. To demonstrate the possibility of use in a technical application, one detection method will be implemented in a charging control system and tested in a long-term study. Detection is based on anomalies in voltage and impedance. In extensive experiments, a large number of cells are charged critically and non-critically, accordingly with and without induced lithium deposition. The aim is to study the polarisation behaviour for both cases and to derive methods for diagnosis. Introspective and retrospective detection methods are developed to identify lithium deposition during and after the charging phase. First, the behaviour during non-critical charging phases is characterised and anomalies are identified based on a significant deviation from this behaviour. For reference, the coulomb-counting method is used. Here, the amount of charged and discharged charge is measured with high accuracy. An excess of the charged charge quantity diagnoses irreversible charge losses, which can be primarily attributed to LM due to the experimental conditions in this work. The focus lies on the analysis of impedance as the targeted frequency excitation allows the investigation of individual transport processes, and thus i) simplifies the physicochemical interpretation of cell behaviour and ii) increases sensitivity. Therefore, the used cells will be investigated by characterisation methods in the frequency domain to identify the excitation frequencies of relevant transport processes. All methods will be applied on both commercial full cells and graphitic half cells. The experiments on the latter ensure that the anomalies occur at the electrode where the degradation mechanism takes place. The impedance-based retrospective method is used for adaptive charge control, which controls the charge current as a function of the detection result in a long-term experiment. The degradation analysis of the cells is carried out using capacity measurement, electrochemical impedance spectroscopy and differential voltage analysis. Two retrospective methods and one introspective detection method are developed, which are suitable for use in a battery management system. The sensitivity of the state-of-the-art, retrospective method of differential voltage analysis is increased by means of model support, so that with an irreversible charge loss of only 0.064 % of the nominal capacity, lithium deposition is detected with a probability of 97 %. The advantage of this method is its simplicity as no additional sensors are needed. With retro- and introspective impedance analysis, the processes most affected by lithium deposition are identified. The impedance relaxation method is successfully implemented in a charging control system, resulting in a significant reduction in charging time compared to conventional charging methods, with marginally accelerated ageing. The long-term study shows that the sensitivity of the method is increased to such an extent that lithium metal deposition is detected before irreversible ageing occurs. For introspective impedance analysis, the sensitivity is increased even further. Since this method enables detection during the charging phase, it can also be used for online charging controls.

Title of the thesis:

Aerosolbasierte Kaltabscheidung zur industriellen Produktion von oxidkeramischen Festelektrolyten für metallische Lithiumakkumulatoren

2022

Supervising Chair:

Functional Materials

As electrochemical energy storage devices, primary batteries and accumulators enable the direct conversion of chemically stored energy into electrical energy and vice versa with high levels of efficiency. Due to the high scalability and no emission of pollutants, sales markets for mobile and stationary applications are constantly increasing.
Rechargeable solid-state batteries with a metallic lithium electrode are seen as the next generation battery type, whereby the higher achievable energy densities are due to the high specific capacity of lithium. However, due to safety concerns, metallic lithium electrodes cannot be used in current commercial lithium-ion batteries with a liquid electrolyte. The reasons for this are the high reactivity of lithium with the liquid electrolytes and the formation of dendrites during operation, which lead to short-circuiting of the battery. The hope for the development of competitive all solid state batteries (ASSB) lies in the suppression of dendrite growth by means of an electrochemically and thermally stable solid electrolyte. The main challenges for the commercial introduction of this technology are insufficient solid-state contacts between the electrodes and the electrolyte and the production of thin and dense solid electrolyte layers.
Despite a great deal of research and development work over the past ten years, there is still a lack of process technologies for the industrial production of solid-state batteries and, in particular, solid electrolyte layers based on oxide ceramics.
This is where this work comes in. With the aerosol-based cold deposition method (Powder Aerosol Deposition Method, PAD for short), a novel coating process for the production of cathode and solid electrolyte layers at room temperature in the thickness range of a few micrometres is being tested with regard to its functional and economic suitability.
Al_0.02Li_6.025La₃Zr_1.625Ta_0.375O₁₂ powder, as a representative of the highly Li-ion conductive oxide ceramic solid electrolytes, is produced for processing by means of aerosol-based cold deposition via the mixed oxide route. The coating process is evaluated with regard to the achievable coating quality and process-related influencing variables. The functional suitability of the PAD coatings was demonstrated by means of electrochemical measurements on symmetrical cells with metallic lithium electrodes. Due to the coating mechanism, the deposited layers exhibit a high degree of lattice deformation, which presumably results in reduced mobility of the charge carriers, leading to reduced electrical conductivity. A moderate thermal post-treatment of the deposited layers at 400°C restores layer conductivities almost to the level of sintered solids. Novel radiation-induced post-treatment of the solid electrolyte layers enables very localised energy input and a targeted influence on the morphological and functional (cover) layer properties. The results differed with regard to the radiation source. Both a frequency-tripled Nd:YAG laser and low-cost high-power light-emitting diodes were used. By using these methods, the layer properties can be adjusted locally within a few seconds.
A fundamental economic analysis of the process identifies key influencing variables with regard to the costs of coating on a laboratory scale. This work is intended to serve readers from industry as a decision-making basis for adapting the process and to provide researchers and developers with starting points for further developing the process.

Title of the thesis:

Potenzial der additiven Fertigung für die Ersatzteil-Supply-Chain

Supervising chair:

Manufacturing and Remanufacturing Technology

Title of the thesis:

Zink-Glas-Kompositelektroden für wiederaufladbare Zink-Luft-Batterien

Supervising chair:

Electrochemical Process Engineering

Title of the 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.

Title of the thesis:

Entwicklung glasbasierter Separatoren für Lithium-Ionen-Batterien

Supervising chair:

Keylab Glass Technology

As part of this work, the development of a glass-based separator for lithium-ion batteries is described, which is considerably more temperature-stable than commercial polymer-based separators. In addition, this separator is a chemically active cell component due to an interaction with the electrolyte conducting salt and contributes to the improvement of electrochemical properties of the battery cell. The separator consists of glass, whereby mainly micrometer-sized glass platelets are used. Different glass morphologies and material compositions were investigated and optimized for their electrochemical properties in organic battery electrolytes as well as in battery experimental cells using a full cell configuration. For the practical use of the glass platelets as separators, they were either integrated into a glass fiber nonwoven, which acts as a support material, or coated directly onto a battery electrode. Thus, self-supporting separators and separator/battery electrode-composites were available for characterization. A major aspect of this work was the electrochemical characterization of the glass separators, whereby different galvanostatic analyses with diverse charge/discharge profiles as well as their influence on cell aging, predominantly in the graphite|lithium iron phosphate cell configuration, were investigated. In addition, a combination of the electrochemical impedance spectroscopy (EIS) and the analysis of the distribution of relaxation time constants (DRT) was used, since these methods can be used to assess some specific ageing processes and these are still mostly unknown, at least for the use of glass-based separators. The interaction of separator glass/battery electrolyte conducting salt or separator glass/electrode active materials was evaluated by means of accompanying analytical characterizations in addition to the electrochemical evaluation. Defined aged cell components were investigated by energy dispersive X-ray spectroscopy (EDX) and X-ray photoelectron spectroscopy (XPS) in post-mortem-analyses. In addition to the determination of electrochemical properties, some important physical properties of the separators such as their thermal and mechanical stability as well as the wetting behaviour with battery electrolytes were investigated. These measurements were always performed in comparison to commercially available polymer-based separators. The practically applicable separator concepts implemented in this work are showing a dimensional stability up to temperatures of at least 600 °C and are non-combustible due to the extremely low proportion of organic components such as binder materials. They are also mechanically flexible due to the use of glass platelets and show a low average weight of just 85 g/m² in case of the self-supporting glass-separator design. By modifying its morphology and material composition, the electrochemical profile of used basic separator glass can be optimized for a wide range of applications. For an optimized long-term battery cycling stability with a maximum Coulomb efficiency of greater than 99.9 % (1C, target depth of discharge 100 %), unporous and untreated sodium borosilicate glass platelets are used as separators. With this configuration, a modification of the graphite anode/electrolyte interphase (SEI) occurs due to an interaction of the glass with the electrolyte conducting salt. This improves electrochemical processes such as the fast charging capability with progressing cell ageing and shows an increase of more than 25 % of the chargeable or dischargeable capacity (10C, target depth of discharge 100 %) in the observation period of 500 charges and discharges. If fast charge/discharge rates are required, the glass platelets can be prepared with a percolating network of pores via the Vycor route. This doubles the rate capability compared to untreated separator glass platelets, after which a graphite|lithium iron phosphate experimental cell can be charged at up to 60 % of the nominal capacity at 10C. The achieved specific performance data of the tested glass separators can be transferred to a wide range of electrode/active material combinations such as graphite|Lithium cobalt oxide or graphite|Lithium nickel manganese cobalt oxide. The glass separators are comparable with commercially available, polymer-based separators in terms of charge/discharge efficiency. They also enable a significantly increased rate capability with a simultaneous optimized wettability with organic battery electrolytes, as well as a significantly increased temperature stability.