Garnet-based all Solid State Battery Design and Operation

Abstract

In the last years, all solid state batteries emerged from an increasing need for safe and high energy-dense storage systems. In the context of a growing demand for electric vehicles and all kinds of portable electronics, such as wearable microchips with sensing and measuring capabilities, soon the energy storage system was identified to bottleneck technological progress. Here, all solid state battery technology may address several key shortcomings, offering non-flammability, reduced packaging needs and potential compatibility with uncommon electrodes of high energy and power density. The replacement of liquid or polymeric electrolytes, up to a point where no more carbon constituents are present, is highly desirable but – so far – broadly unexplored. The lack of experimental efforts towards all-ceramic battery architectures shall be addressed in the following thesis. Specifically, the compatibility of lithium metal oxide anodes, in the form of thin films, have previously never been tested in combination with Li-garnet electrolytes. In a similar manner, it also remained unclear to the field, if Li-garnets are a viable option as an electrolyte for all thin film based microbatteries. Ultimately, the deposition of the highly conductive cubic phase of Li7La3Zr2O12 (LLZO) in the form of thin films was so far never achieved by pulsed laser deposition (PLD). Among aspects such as compatibility with different oxide and nitride electrode materials, structural dependence on ionic transport properties in thin film, and the influence of bulk to thin film transfer, motivated the work performed in the following thesis. Part I, as the general introduction, will cover fundamentals of liquid-based batteries, their history, their working principle as well as current state-of-the-art benchmarks regarding energy and power density. Furthermore, we will review current all solid state battery cells for both, bulk and microbattery performance. Afterwards, main focus will be moved towards lithiated thin films and their application in memory, sensing and energy storage entities, with particular attention toward Li-garnets as a material class. Here, we summarize efforts made in structural phase stabilization and tuning of ionic transport properties by means of doping. To conclude, we will review current attempts on hybrid-based cell designs, and compare them to full ceramic all solid state battery concepts. Part II will cover the first combination of a thin film anode material, namely Li4Ti5O12 (LTO) with Li-garnet LLZO. Thin film deposition through pulsed laser deposition (PLD) will be introduced for the fabrication of thin films of Li4Ti5O12, grown on oriented MgO substrates, as well as on bulk Li-garnet pellets. We extend the study towards structural phase evolution and assess the electrochemical potential in a half-cell pellet-based battery assembly. Galvanostatic cycling at various rates will be performed and capacity retention will be reported and discussed. Following Part III, aspects of the influence of nitrides in combination with LLZO are further elaborated. Here, we investigate the phase evolution of multilayers consisting of Li3N and LLZO deposited by pulsed laser deposition. Structural phase evolution as a function of temperature is measured in-situ as well as ex-situ and ionic transport properties in relation to crystallographic phase are assessed. As a necessity for high Li-ion conductivity, the stabilization of the highly conductive cubic phase of LLZO is required. Nevertheless, up to this date, it was proven to be vastly challenging to achieve the cubic phase and high Li-conductivity in thin films of LLZO, with no report on PLD-deposited thin films of the Li-garnet material class whatsoever. Here, we contribute to the detailed understanding of thin film phase formation in Li6.25Al0.25La3Zr2O12 by employing a novel strategy, embedding a sacrificial Li-reservoir in the form of thin layers of Li3N, which upon a thermal post-annealing step, allows for diffusion into the garnet host material, therefore compensating the occurring Li-deficiency at elevated temperature. Ultimately, we assess ionic transport properties in thin films fabricated by this strategy via electrical impedance spectroscopy (EIS), and discuss the implication of highly lithiated garnet thin films on applications such as stationary or portable energy storage systems and microbattery, sensing and monitoring entities. In Part IV, a thin film ceramic microbattery is constructed consisting of LiNi0.8Co0.15Al0.05O2 (NCA) thin film cathode, Li6.25Al0.25La3Zr2O12 thin film electrolyte and Li7MnN4 (LMN) thin film anode. Particular focus is set on the growth and adhesion of the respective layers and their feasibility to operate as a full microbattery cell of less than 1 µm total thickness. Notably, thin films of Li7MnN4 anode have so far never been deposited or characterized in the form of thin films, neither were they combined with garnet electrolytes. The role of a nitride electrode in combination with an oxide electrolyte was of particular interest, leading over to the interesting role of nitrides in combination with Li-garnets, which were discussed in the previous chapter (Part III). Part V will introduce efforts taken to experimentally gather data for microbattery testing at very low currents. Upon reviewing available commercial systems for battery testing at the time of the experimental process, no suitable systems for galvanostatic cycling at rates below 100 nA were commercially available. Since the testing protocol included pellet-based batteries with thin film anodes and microbatteries of less than 1 µm total thickness, it was necessary to cycle at rates below 1C. Here, we introduce own written software for the purpose of battery testing, memory logic testing as well as pulsed voltage cycling. Customized functions for autonomous cycling testing at different rates for microbattery application was released as an open source software publication, aiding future research on nanoscopic energy storage devices. The thesis concludes with Part VI, in which an overview on achievements in this thesis is critically reflected towards the field, followed by a discussion on potential applications in energy storage devices and portable electronics. Short summaries on each chapter are given as well as a separate section on future challenges in the field of all-solid state batteries. In an outlook section, we propose further experiments and directions, towards which the following thesis could be expanded in a continuation of this work on Li-garnet and its applications.