This thesis aims to provide insight into the fundamental electrochemical processes taking place when cycling SnO2 in lithium-ion batteries (LIBs). Special attention was paid to the partial reversibility of the tin oxide conversion reaction and how to enhance its reversibility. Another main effort was to pinpoint which limitations play a role in tin based electrodes besides the well-known volume change effect in order to develop new strategies for their improvement. In this aspect, Li+ mass transport within the electrode particles and the large first cycle charge transfer resistance were studied. Li+ diffusion was proven to be an important issue regarding the electrochemical cycling of SnO2. It was also shown that it is the Li+ transport inside the SnO2 particles which represents the largest limitation. In addition, the overlap between the potential regions of the tin oxide conversion and the alloying reaction was investigated with photoelectron spectroscopy (PES) to better understand if and how the reactions influence each other`s reversibility. The fundamental insights described above were subsequently used to develop strategies for the improvement of the performance and the cycle life for SnO2 electrodes in LIBs. For instance, elevated temperature cycling at 60 oC was employed to alleviate the Li+ diffusion limitation effects and, thus, significantly improved capacities could be obtained. Furthermore, an ionic liquid electrolyte was tested as an alternative electrolyte to cycle at higher temperatures than 60 oC which is the thermal stability limit for the conventional LP40 electrolyte. In addition, cycled SnO2 nanoparticles were characterized with transmission electron microscopy (TEM) to determine the effects of long term high temperature cycling. Also, the effect of vinylene carbonate (VC) as an electrolyte additive on the cycling behavior of SnO2 nanoparticles was studied in an effort to improve the capacity retention. In this context, a recently introduced intermittent current interruption (ICI) technique was employed to measure and compare the development of internal cell resistances with and without VC additive.

Development of microsized batteries plays an important role in the design of in-situ electrochemical investigation systems and portable/wearable electronics. This emerging field intimately correlates with the topics of rechargeable batteries, nanomaterials, on-chip microfabrication, flexibility with reliable mechanical properties etc. Among the various energy materials, conversion-type materials have been proposed as high-energy-density alternatives to traditional intercalation-based materials. However, these materials usually show complex reaction processes accompanied by multi-reaction intermediates, which poses a great challenge to understand the chemical mechanisms. Benefiting from the merits of microsized battery devices, we develop a novel strategy to investigate and then optimize the electrochemical performance of a specific conversion-type material: nickel-iron oxide (NFO). Subsequently, this kind of materials are employed for flexible minimized energy storage systems. Unlike traditional characterization methods based on slurry-coated electrodes, micro-platforms directly probe the intrinsic electrochemical properties of a single active material in real-time due to the elimination of other additives. In this thesis, we firstly design a micro-lithium batteries (MLBs), based on a single “Swiss-roll” microtubular nanomembrane electrode. This platform enables us to investigate the electrochemical mechanisms of electrode materials in lithium batteries by in-situ Raman spectroscopy, electrical conductivity measurements, and electrochemistry characterization. With this designed MLBs, we systematically studied NFO nanomembranes. Using in-situ Raman spectroscopy during the delithiation/lithiation process, we monitored the transition of the chemical component directly. Guided by our investigations of micro-batteries, composite NFO nanomembrane electrodes were fabricated and tested in coin cells, which showed an excellent rate performance: 440 mAh g-1 at a high rate of 20 A g-1 and a long-term stable cycling performance over 1600 cycles. One step further, a flexible energy storage micro-device is achieved using such optimized materials. We demonstrate a thin, lightweight, and flexible micro-full lithium-ion battery based on nickel-iron oxide with a high-rate performance and energy density that can be repeatedly bent to 180° without structural failure and performance loss. It delivers a stable output capacity of 140 mAh g-1 over 1000 charge/discharge cycles. Meanwhile, the excellent rate performance guarantees high energy output up to 255 W h kg-1 at a high power density of 12000 W kg-1 at the microscale.

Die untersuchten Ausgangsmaterialien α-Fe2U3, ɣ-Fe2O3, Fe3O4, CoO, Co3O4, NiO sowie CuO eignen sich durch ihre hohe theoretische spezifische Kapazität als Elektrodenmaterial für Lithium-Ionen-Batterien. Die zugrundeliegenden Mechanismen zur Speicherung der Li-Ionen konnten mit allen Phasenumwandlungen und der Bildung von Zwischenprodukten im ersten Reduktionsschritt beschrieben werden. In Abhängigkeit von der Kristallstruktur der Ausgangsmaterialien und den Reaktionsgeschwindigkeiten konnten der Gesamtreaktion die einzelnen Mechanismen Interkalation von Li-Ionen, Substitution von Kationen in der Kristallstruktur und Konversionsmechanismus zugeordnet werden. Auf Grund des gemeinsamen kubisch flächenzentrierten Sauerstoffuntergitters der Ausgangsmaterialien und Zwischenprodukte zeigen sich für die Materialsysteme Fe-O, Co-O und Ni-O Orientierungsbeziehungen zwischen den Kristalliten des Ausgangsoxids, des lithiierten Metalloxids und der Li2O-Matrix. Im Gegensatz dazu sind die auf der CuO-Phase basierenden Kristallite regellos orientiert und zeigen eine höhere Zyklenstabilität. Die Orientierungsbeziehung zwischen den lithiierten Metalloxiden und der Li2O-Matrix hindert dagegen den Austausch der Li-Ionen, beeinträchtigt die Zyklenstabilität und trägt so zu einer schnelleren Alterung der Elektrode bei.