Elektirsche Charakterisierung von leitfähigen Ionenspuren in tetraedrisch amorphen Kohlenstoff mit Kupferverunreinigungen

Die Bestrahlung von tetraedrisch amorphen Kohlenstoff (ta-C) mit schnellen schweren Ionen führt zur Bildung von mikroskopischen elektrisch leitfähigen Ionenspuren mit Durchmessern um 10 nm. Dieses Phänomen ist auf das sp² zu sp³ Hybridisierungsverhältnis des amorphen Kohlenstoffes zurückzuführen. Das einschlagende Ion deponiert eine große Menge Energie innerhalb des Spurvolumens, so dass eine Materialtransformation hin zu höheren sp² Hybridisierung stattfindet. Hierdurch wird die elektrische Leitfähigkeit der Ionenspur stark erhöht. Dieser Effekt kann durch die Zugabe von Verunreinigungen wie Kupfer verstärkt werden. Das Ziel dieser Arbeit ist die umfassende Analyse des elektrischen Verhaltens von ta-C mit besonderen Augenmerk auf die Auswirkungen von Kupferverunreinigungen und Ionenspuren. Der Effekt von Kupferverunreinigungen auf das wichtige Hybridisierungsverhältnis vom amorphen Kohlenstoff wird vermessen. Darüber hinaus wurden alle Proben elektrisch mit makroskopischen Kontakten im Temperaturbeireich von 20 K bis 380 K analysiert. Mikroskopisch wurden einzelne leitfähige Ionenspuren mit Hilfe von atomarer Kraftmikroskopie betrachtet. Die statistische Verteilung der Spureigenschaften in Kohlenstofffilmen mit verschiedenen Kupferkonzentrationen werden verglichen, um die Spurbildung besser zu verstehen. Die normalisierten durchschnittlichen Spurleitfähigkeiten aus mikroskopischen und makroskopischen Messungen werden verglichen. Hierbei kann die Zuverlässigkeit der beiden experimentellen Methoden bewertet werden und mögliche Fehlerquellen ausfindig gemacht werden. Schließlich wird ein Konzept für eine Anwendung unterbrochener Ionenspuren gezeigt

Li7La3Zr2O12Li_7La_3Zr_2O_{12} based all solid state thin film batteries

Liquid organic electrolytes cause safety problems due to an insufficient thermal and electrochemical stability. One approach to avoid such disadvantages is the replacement of the liquid electrolyte by a solid one. Next to sulfides and phosphates, current research is focused on Li conducting ceramic oxide materials like Li7La3Zr2O12 (LLZ), a promising garnet-structured material with a Li ion conductivity of about 10-4 S/cm. Li ion conductivity can increased by partial substitution of Li or Zr. Since the ionic conductivity is about two orders of magnitude lower compared to liquid electrolytes, thin electrolyte layers are necessary for a low internal resistance of the cell. Complete thin film batteries based on a current collector substrate, a thin cathode, LLZ electrolyte layer and Li or a Li alloy thin film as anode are deposited by physical vapor deposition techniques. The deposition conditions are optimized for each compound and adjusted to the overall system. The resulting thin film batteries are analyzed with regard to electrochemical behavior, structural and morphological properties and element distribution

Solid-state electrolytes in Lithium-Sulfur Batteries

Lithium ion conductors based on complex oxide materials are considered to be outstanding from their high safety and reasonable Li-ion conductivity. Compared to others solid Li ionic conductors, oxide materials have additional advantages of easier material handling during synthesis, higher chemical stability and wider electrochemical stability window. The high stiffness and electrochemically stability against metallic Li also make oxide-type Li-ion conductors as a perfect Li anode protector when using in Li-S or Li-air batteries. The use of Tantalum-substituted Li7-xLa3Zr2O12 (LLZ:Ta) as solid electrolyte for solid-state battery has been reported in several papers. The reported solid-state batteries were all constructed with a thin film cathode which was made either by physical vapor or sol-gel deposition [1-2]. In order to realize a Li-ion battery based on an oxide conductor as solid electrolyte, LLZ:Ta powder was synthesized via different synthesis routes including solid-state reaction. LLZ:Ta pellets with optimized sintering parameters exhibit a high Li-ion conductivity of 7.8 x 10-4 S cm-1 at 30 oC with a relative density of ~94%. The material was further implanted as a solid electrolyte by using screen printing to put on thick LiCoO2 (> 50 micrometers) as cathode and subsequently tested versus Li metal.Thin-film solid-state batteries allow – on the one hand – a detailed analysis of the compatibility of active storage material and the electrolyte because of well-defined interfaces. On the other hand, thin-film oxide electrolytes might also have the potential for application as thin Li-ion conductive solid-sate separators on porous substrates in Li-S- batteries.[1] M. Kotobuki et al, Ceramics International 39 (2013) 6481[2] Y. Jin et al, J. Power Sources 239 (2013) 32

Time-of-flight secondary ion mass spectrometry study of lithium intercalation process in LiCoO2_2 thin film

A detailed time-of-flight secondary ion mass spectrometry (ToF-SIMS) analysis of the lithium de-/intercalation in thin films of the insertion cathode material lithium cobalt oxide is presented. The LiCoO2 (LCO) thin films are deposited by radio frequency magnetron sputtering at 600 °C, having a (003) preferred orientation after the deposition. The thin electrode films are cycled with liquid electrolyte against lithium metal, showing 80-86% extractable capacities. After disassembling the cells, the depth resolved elemental distribution in the LCO is investigated by ToF-SIMS and glow discharge optical emission spectroscopy. Both techniques show a stepwise lithium distribution in charged state, leading to a lithium depleted layer close to the surface. In combination with the electrochemical results, the qualitative comparison of the different lithium depth profiles yields a reversible lithium extraction in the depleted area below the stability limit for bulk materials of LCO. For bulk LCO, a phase change normally occurs when the lithium concentration in LixCoO2 is lower than x=0.5. As a possible cause for the inhibition of the phase change, the preferred orientation and thus pinning of the crystal structure of the film by the substrate is proposed