@phdthesis{Rumpel2024,
  author    = {Rumpel, Matthias},
  title     = {Development of Components for Solid-State Batteries and their Characterization},
  doi       = {10.25972/OPUS-34715},
  url       = {http://nbn-resolving.de/urn:nbn:de:bvb:20-opus-347154},
  school      = {Universit{\"a}t W{\"u}rzburg},
  year      = {2024},
  abstract  = {This Ph.D. thesis has addressed several main issues in current ASSB research within four studies. Ceramic ASSBs are meant to enable the implementation of Li-metal anodes and high voltage cathode materials, which would increase energy density, power density, life time as well as safety aspects in comparison with commercially available liquid electrolyte LiBs. In this thesis, several scientific questions arising on the cathode side of ASSBs have been focused on. With respect to the target system of a ternary composite bulk cathode consisting of ceramic active material, ceramic SSE and an electrically conductive component, studies about the thermal stabilities of these components and their impact on the electrochemical performance have been conducted. Particulate bulk cathode composites have to fulfil electrochemical, chemical, mechanical and structural requirements in order to compete with commercial LiBs. Particularly, the production process requires high-temperature sintering to obtain firmly bonded contacts in order to maximize the electrochemically active area, charge transfer and ionic conduction. However, interdiffusion, intermixing and decomposition of the initial components during sintering result in low-performing ASSBs so far. These side reactions during high-temperature treatment have been investigated in order to gain a better understanding of these mechanisms and to enable a better controlling of the manufacturing process as well as to simplify the choice of material combinations. The first two parts of this thesis deal with the thermal stability of the ceramic SSE LATP in combination with various active materials and with the validation of a probable improvement of the sintering process due to liquid phase sintering of LATP by adding Li3PO4. In the third and fourth parts, the impact of interdiffusion, intermixing and decomposition on the electrochemical performance of TF-SSBs based on the active material LMO and the ceramic SSE Ga-LLZO has been investigated.},
  subject      = {Elektrochemie},
  language  = {en}
}
@phdthesis{Daubinger2024,
  author    = {Daubinger, Philip},
  title     = {Electrochemical and Mechanical Interplay of State-of-the-Art and Next-Generation Lithium-Ion Batteries},
  doi       = {10.25972/OPUS-35125},
  url       = {http://nbn-resolving.de/urn:nbn:de:bvb:20-opus-351253},
  school      = {Universit{\"a}t W{\"u}rzburg},
  year      = {2024},
  abstract  = {The demand for LIB with enhanced energy densities leads to increased utilization of the space within the confinements of the battery housing or to the use of electrode material with increased intrinsic specific energy densities. Both requirements result in more stress on the battery electrodes and separator during cycling or aging. However, the effect of mechanical strain on the cell's electrochemistry and thus the performance of batteries is rather unexplored compared to the impact of current or temperature, for example. The objective of this thesis was to give a better understanding of the electrochemical and mechanical interplay in current- and next-generation lithium based battery cells. Therefore, the thesis was structured into the investigations on SoA and next-generation LIBs. For SoA LIBs, the investigations of the interplay started at laboratory scale. Here, the expansion of various electrodes and also the impact of mechanical pressure and its distribution on the performance of the cells were studied. The investigations at laboratory scale was followed by an examination of the electrochemical and mechanical interactions on large format commercial LIBs which are used in BEVs. Accordingly, the effect of bracing and its effect on the performance was studied in an aging and post-mortem study. To gain a deeper understanding of the mechanical changes in LIBs, an ultrasonic study was performed for pouch cells. Here, the mechanical changes were further investigated in dependence of SoC and SoH. The effects of the mechanical stress on the performance for next-generation batteries were studied at laboratory scale. In the beginning, the expansion of next-generation anode materials such as silicon and lithium was compared with today's anode materials. Furthermore, the effect of mechanical pressure and electrolyte on the irreversible dilation and performance was investigated for lithium metal cells. Overall, it was shown that pressure has a significant effect on the performance of today's and also future LIBs. The interplay of the electrochemical and mechanical effects inside a LIB has a considerable impact on the lifetime, capacity fading and impedance increase of the batteries.},
  subject      = {Lithium-Ionen-Akkumulator},
  language  = {en}
}
@phdthesis{Gold2023,
  author    = {Gold, Lukas},
  title     = {Methods for the state estimation of lithium-ion batteries},
  doi       = {10.25972/OPUS-30618},
  url       = {http://nbn-resolving.de/urn:nbn:de:bvb:20-opus-306180},
  school      = {Universit{\"a}t W{\"u}rzburg},
  year      = {2023},
  abstract  = {This work introduced the reader to all relevant fields to tap into an ultrasound-based state of charge estimation and provides a blueprint for the procedure to achieve and test the fundamentals of such an approach. It spanned from an in-depth electrochemical characterization of the studied battery cells over establishing the measurement technique, digital processing of ultrasonic transmission signals, and characterization of the SoC dependent property changes of those signals to a proof of concept of an ultrasound-based state of charge estimation. The State of the art \& theoretical background chapter focused on the battery section on the mechanical property changes of lithium-ion batteries during operation. The components and the processes involved to manufacture a battery cell were described to establish the fundamentals for later interrogation. A comprehensive summary of methods for state estimation was given and an emphasis was laid on mechanical methods, including a critical review of the most recent research on ultrasound-based state estimation. Afterward, the fundamentals of ultrasonic non-destructive evaluation were introduced, starting with the sound propagation modes in isotropic boundary-free media, followed by the introduction of boundaries and non-isotropic structure to finally approach the class of fluid-saturated porous media, which batteries can be counted to. As the processing of the ultrasonic signals transmitted through lithium-ion battery cells with the aim of feature extraction was one of the main goals of this work, the fundamentals of digital signal processing and methods for the time of flight estimation were reviewed and compared in a separate section. All available information on the interrogated battery cell and the instrumentation was collected in the Experimental methods \& instrumentation chapter, including a detailed step-by-step manual of the process developed in this work to create and attach a sensor stack for ultrasonic interrogation based on low-cost off-the-shelf piezo elements. The Results \& discussion chapter opened with an in-depth electrochemical and post-mortem interrogation to reverse engineer the battery cell design and its internal structure. The combination of inductively coupled plasma-optical emission spectrometry and incremental capacity analysis applied to three-electrode lab cells, constructed from the studied battery cell's materials, allowed to identify the SoC ranges in which phase transitions and staging occur and thereby directly links changes in the ultrasonic signal properties with the state of the active materials, which makes this work stand out among other studies on ultrasound-based state estimation. Additional dilatometer experiments were able to prove that the measured effect in ultrasonic time of flight cannot originate from the thickness increase of the battery cells alone, as this thickness increase is smaller and in opposite direction to the change in time of flight. Therefore, changes in elastic modulus and density have to be responsible for the observed effect. The construction of the sensor stack from off-the-shelf piezo elements, its electromagnetic shielding, and attachment to both sides of the battery cells was treated in a subsequent section. Experiments verified the necessity of shielding and its negligible influence on the ultrasonic signals. A hypothesis describing the metal layer in the pouch foil to be the transport medium of an electrical coupling/distortion between sending and receiving sensor was formulated and tested. Impedance spectroscopy was shown to be a useful tool to characterize the resonant behavior of piezo elements and ensure the mechanical coupling of such to the surface of the battery cells. The excitation of the piezo elements by a raised cosine (RCn) waveform with varied center frequency in the range of 50 kHz to 250 kHz was studied in the frequency domain and the influence of the resonant behavior, as identified prior by impedance spectroscopy, on waveform and frequency content was evaluated to be uncritical. Therefore, the forced oscillation produced by this excitation was assumed to be mechanically coupled as ultrasonic waves into the battery cells. The ultrasonic waves transmitted through the battery cell were recorded by piezo elements on the opposing side. A first inspection of the raw, unprocessed signals identified the transmission of two main wave packages and allowed the identification of two major trends: the time of flight of ultrasonic wave packages decreases with the center frequency of the RCn waveform, and with state of charge. These trends were to be assessed further in the subsequent sections. Therefore, methods for the extraction of features (properties) from the ultrasonic signals were established, compared, and tested in a dedicated section. Several simple and advanced thresholding methods were compared with envelope-based and cross-correlation methods to estimate the time of flight (ToF). It was demonstrated that the envelope-based method yields the most robust estimate for the first and second wave package. This finding is in accordance with the literature stating that an envelope-based method is best suited for dispersive, absorptive media [204], to which lithium-ion batteries are counted. Respective trends were already suggested by the heatmap plots of the raw signals vs. RCn frequency and SoC. To enable such a robust estimate, an FIR filter had to be designed to preprocess the transmitted signals and thereby attenuate frequency components that verifiably lead to a distorted shape of the envelope. With a robust ToF estimation method selected, the characterization of the signal properties ToF and transmitted energy content (EC) was performed in-depth. A study of cycle-to-cycle variations unveiled that the signal properties are affected by a long rest period and the associated relaxation of the multi-particle system "battery cell" to equilibrium. In detail, during cycling, the signal properties don't reach the same value at a given SoC in two subsequent cycles if the first of the two cycles follows a long rest period. In accordance with the literature, a break-in period, making up for more than ten cycles post-formation, was observed. During this break-in period, the mechanical properties of the system are said to change until a steady state is reached [25]. Experiments at different C-rate showed that ultrasonic signal properties can sense the non-equilibrium state of a battery cell, characterized by an increasing area between charge and discharge curve of the respective signal property vs. SoC plot. This non-equilibrium state relaxes in the rest period following the discharge after the cut-off voltage is reached. The relaxation in the rest period following the charge is much smaller and shows little C-rate dependency as the state is prepared by constant voltage charging at the end of charge voltage. For a purely statistical SoC estimation approach, as employed in this work, where only instantaneous measurements are taken into account and the historic course of the measurement is not utilized as a source of information, the presence of hysteresis and relaxation leads to a reduced estimation accuracy. Future research should address this issue or even utilize the relaxation to improve the estimation accuracy, by incorporating historic information, e.g., by using the derivative of a signal property as an additional feature. The signal properties were then tested for their correlation with SoC as a function of RCn frequency. This allowed identifying trends in the behavior of the signal properties as a function of RCn frequency and C-rate in a condensed fashion and thereby enabled to predict the frequency range, about 50 kHz to 125 kHz, in which the course of the signal properties is best suited for SoC estimation. The final section provided a proof of concept of the ultrasound-based SoC estimation, by applying a support vector regression (SVR) to before thoroughly studied ultrasonic signal properties, as well as current and battery cell voltage. The included case study was split into different parts that assessed the ability of an SVR to estimate the SoC in a variety of scenarios. Seven battery cells, prepared with sensor stacks attached to both faces, were used to generate 14 datasets. First, a comparison of self-tests, where a portion of a dataset is used for training and another for testing, and cross-tests, which use the dataset of one cell for training and the dataset of another for testing, was performed. A root mean square error (RMSE) of 3.9\% to 4.8\% SoC and 3.6\% to 10.0\% SoC was achieved, respectively. In general, it was observed that the SVR is prone to overestimation at low SoCs and underestimation at high SoCs, which was attributed to the pronounced hysteresis and relaxation of the ultrasonic signal properties in this SoC ranges. The fact that higher accuracy is achieved, if the exact cell is known to the model, indicates that a variation between cells exists. This variation between cells can originate from differences in mechanical properties as a result of production variations or from differences in manual sensor placement, mechanical coupling, or resonant behavior of the ultrasonic sensors. To mitigate the effect of the cell-to-cell variations, a test was performed, where the datasets of six out of the seven cells were combined as training data, and the dataset of the seventh cell was used for testing. This reduced the spread of the RMSE from (3.6 - 10.0)\% SoC to (5.9 - 8.5)\% SoC, respectively, once again stating that a databased approach for state estimation becomes more reliable with a large data basis. Utilizing self-tests on seven datasets, the effect of additional features on the state estimation result was tested. The involvement of an additional feature did not necessarily improve the estimation accuracy, but it was shown that a combination of ultrasonic and electrical features is superior to the training with these features alone. To test the ability of the model to estimate the SoC in unknown cycling conditions, a test was performed where the C-rate of the test dataset was not included in the training data. The result suggests that for practical applications it might be sufficient to perform training with the boundary of the use cases in a controlled laboratory environment to handle the estimation in a broad spectrum of use cases. In comparison with literature, this study stands out by utilizing and modifying off-the-shelf piezo elements to equip state-of-the-art lithium-ion battery cells with ultrasonic sensors, employing a range of center frequencies for the waveform, transmitted through the battery cell, instead of a fixed frequency and by allowing the SVR to choose the frequency that yields the best result. The characterization of the ultrasonic signal properties as a function of RCn frequency and SoC and the assignment of characteristic changes in the signal properties to electrochemical processes, such as phase transitions and staging, makes this work unique. By studying a range of use cases, it was demonstrated that an improved SoC estimation accuracy can be achieved with the aid of ultrasonic measurements - thanks to the correlation of the mechanical properties of the battery cells with the SoC.},
  subject      = {Lithium-Ionen-Akkumulator},
  language  = {en}
}
@phdthesis{Hofmann2022,
  author    = {Hofmann, Michael},
  title     = {Overcoming Obstacles in the Aqueous Processing of Nickel-rich Layered Oxide Cathode Materials},
  doi       = {10.25972/OPUS-27378},
  url       = {http://nbn-resolving.de/urn:nbn:de:bvb:20-opus-273787},
  school      = {Universit{\"a}t W{\"u}rzburg},
  year      = {2022},
  abstract  = {The implementation of a water-based cathode manufacturing process is attractive, given the prospect of improved sustainability of future lithium-ion batteries. However, the sensitivity of many cathode materials to water poses a huge challenge. Within the scope of this work, a correlation between the water sensitivity of cathode materials from the class of layered oxides and their elemental composition was identified. In particular for the cathode material LiNi0.8Co0.15Al0.05O2 (NCA), the processes taking place in aqueous medium were clarified in detail. Based on this knowledge, the surface of NCA particles could be specifically modified, which led to a reduced water sensitivity. As a result, the electrochemical performance of cells with water-based NCA cathodes was significantly improved and a remarkable long-term cycling performance was achieved. The present work contributes to a deeper understanding of the water sensitivity of cathode materials and at the same time presents a promising approach to overcome this obstacle. Consequently, this work advances the successful widespread realization of water-based cathode manufacturing.},
  subject      = {Elektrochemie},
  language  = {en}
}
@phdthesis{Bach2017,
  author    = {Bach, Tobias},
  title     = {Electromechanical interactions in lithium-ion batteries: Aging effects and analytical use},
  url       = {http://nbn-resolving.de/urn:nbn:de:bvb:20-opus-153325},
  school      = {Universit{\"a}t W{\"u}rzburg},
  year      = {2017},
  abstract  = {In the first part of his work, the causes for the sudden degradation of useable capacity of lithium-ion cells have been studied by means of complementary methods such as computed tomography, Post-Mortem studies and electrochemical analyses. The results obtained point unanimously to heterogeneous aging as a key-factor for the sudden degradation of cell capacity, which in turn is triggered by differences in local compression. At high states of health, the capacity fade rate is moderate but some areas of the graphite electrode degrade faster than others. Still, the localized changes are hardly noticeable on cell level due to averaging effects. Lithium plating occurs first in unevenly compressed areas, creating patterns visible to the human eye. As lithium plating leads to rapid consumption of active lithium, a sudden drop in capacity is observed on cell level. Lithium plating appears to spread out from the initial areas over the whole graphite electrode, quickly consuming the remaining useful lithium and active graphite. It can be hypothesized that a self-amplifying circle of reciprocal acceleration of local lithium loss and material loss causes rapid local degradation. Battery cell designers can improve cycle life by homogeneous pressure distribution in the cell and using negative active materials that are resilient to elevated discharge potentials such as improved carbons or lithium titanate. Also, a sufficiently oversized negative electrode and suitable electrolyte additives can help to avoid lithium plating. When packs are designed, care must be taken not to exert local pressure on parts of cells and to avoid both very high and low states of charge. In the second part of this dissertation the resilience of cylindrical and pouchbag cells to shocks and different vibrations was investigated. Stresses inflicted by vibration and shock tests according to the widely recognized UN38.3 transport test were compared to a long-time test that exposed cells to a 186 days long ordeal of sine sweep vibrations with a profile based on real-world applications. All cells passed visual and electric inspection performed by TU M{\"u}nchen after the vibration tests. Only cylindrical cells subjected to long-term vibrations in axial direction showed an increase in impedance and a loss of capacity that could be recuperated in part. The detailed analyses presented in this thesis gave more details on the damages inflicted by vibrations and shocks and revealed drastic damages in some cases. In cylindrical cells, only movement in axial direction caused damage. Long term vibrations were found to be especially detrimental. No damage whatsoever could be detected for pouch cells, regardless of the test protocol and the direction of movement. The extreme resilience of pouchbag cells shows that the electrode stack of lithium-ion cells is resistant to vibrations, and that damages are caused by design imperfections that can be improved at low cost. The findings of this work, and the general state of research show that it is most crucial to control the lithiation and thus potential of the graphite electrode. In the last part of this work, a new, direct method for charge estimation based on changing transmission is presented. A correlation between transmission of short ultrasonic pulses and state of charge is found. This new technology allows direct measurement of the state of charge. The method is demonstrated for batteries with different positive active materials, showing its versatility. As the observed changes can be traced to the lithiation of graphite, it can be determined without a reference electrode. Already at this early stage of development, the found correlations allow estimation of state of charge. The present hysteresis in the signal height of the slow wave, which is unneglectable especially during discharging at higher currents, will be subject to further investigation. The observed effects can be explained by effects on different length scales. Biot's theory explains the second wave's slowness based on the active material particles size in the range of 0.01 mm and electrolyte-filled pores. Lithiation of graphite changes the porosity of the electrode and thereby the velocity and wavelength of the impulse. When the wavelength approaches the length scale of the layers, 0.1 mm, scattering effects dampen the transmitted signal. Finally, the wavelength of the pulse should be shorter than the transducers diameter to obtain a homogeneous wave front. To conclude, the new method allows the control of each individual cell in a pack independent from the electrical connections of the cells. As the method shows great promise, further studies regarding factors such as long-term behavior, temperature and current rates should be conducted. In this thesis hysteresis was observed and a deeper understanding of the reasons behind it may allow further improvements of measurement precision.},
  subject      = {Lithium-Ionen-Akkumulator},
  language  = {en}
}
@phdthesis{FlatenAndersen2013,
  author    = {Fl{\aa}ten Andersen, Hanne},
  title     = {New Materials for Lithium-Ion Batteries},
  url       = {http://nbn-resolving.de/urn:nbn:de:bvb:20-opus-101434},
  school      = {Universit{\"a}t W{\"u}rzburg},
  year      = {2013},
  abstract  = {Over the last decades, lithium-ion batteries have grown more important and substituted other energy storage systems. Due to advantages such as high energy density and low self-discharge, the lithium-ion battery has taken its part in the rechargeable energy storage market, and it is now found in most laptops, cameras and mobile phones. With the increasing demands for electrical vehicles and stationary energy storage systems, there is a necessity for improved lithium-ion battery materials. In this thesis several alternative electrode materials have been examined with a main focus on the electrochemical characterisation. As an alternative to the commercial cathode LiCoO2, the LiMn2O4 cathode has been suggested due to its reduced toxicity, material abundance, reduced costs and increased specific capacity. On the anode side, several Sn-containing anodes have been investigated and steps to overcome the main challenge, the great volume expansion upon cycling, have been taken. In addition, a novel anode material group was synthesised at the University of Marburg and two substances of the lithium chalcogenidometalate networks were successfully characterised. The cathode material, LiMn2O4, was synthesised via the sol-gel technique and several coating methods such as dip-coating, electrophoretics and infiltration were investigated. The LiMn2O4 material was initially coated on a porous metal foam as a current collector, thus providing new possibilities as the porosity of the substrate increased, mechanical stability and adhesion improved and a 3-dimensional network was obtained. In order to compare the results of the LiMn2O4 cathode material on the novel current collector, the material was also coated on a standard metallic foil and characterised. The analysis followed via X-ray diffraction, electron microscopy, thermogravimetrical analysis and several electrochemical techniques. Tin containing anode materials were chosen due to the doubling of the theoretical capacity compared with the commercially used graphite. However, a great challenge lies with using tin or tin-containing anode materials. Upon lithiation of Sn, the material can expand up to 300 \%, therefore a stabilising effect is necessary to avoid a collapse of the material. This work shows several new concepts and attempts to overcome this challenge, including SnO2 nanowires deposited via chemical vapour deposition on both metallic foam and standard current collectors. A new improvement consisted of the tin - carbon nanofibers where the nanofibers form a stabilising matrix that can partially buffer the volume change of the Sn particles. The synthesis of the Sn-containing anodes took place at the University of Cologne, while characterisation, cell preparation and optimising the electrode system were features of this thesis. In addition, a lithium chalcogenidometalate network proved to be an interesting, new anode material group. Both Li4MnSn2Se7 and Li4MnGe2S7 (synthesised at Philipps-Universit{\"a}t Marburg) were electrochemically examined to better understand the lithiation processes. Both materials obtained very high specific capacities and were found to be possible alternatives to the state-of-the art anodes. All the examined electrode materials were found to have some advantage over the commercially used LiCoO2 and graphite electrodes, and a thorough characterization of the materials was performed to understand the processes that took place.},
  subject      = {Lithium-Ionen-Akkumulator},
  language  = {en}
}