Nano-design of metal oxide electrodes for Li- and Na-ion hybrid energy storage

The growing use of portable devices and a global transition to electric vehicles has tremendously increased the demand for energy storage devices such as lithium-ion batteries and supercapacitors. Especially the interest is established for better devices exceeding the energy and power performance of current technology. The hybrid supercapacitor (HSC) concept addresses the limits of each device and utilizes the distinct electrochemical features of lithium-ion batteries and supercapacitors. The focus of this Ph.D. thesis is the nano-design of hybrid materials of metal oxides and carbon for better electrochemical performance in lithium- and sodium-ion hybrid energy storage devices. The hybridization of metal oxide and carbon substrate can be achieved by tailored sol-gel synthesis, yielding a homogeneous distribution of nanosized metal oxide domain in the hybrid material. The performance of the hybrids was superior to the composite concept electrodes, but this is not a statement that can be generalized for all sorts of (nano)composites. In addition to the electrode material, also the electrolyte choice has a strong impact on the device operation and safety. The use of alternative solvents and Li- or Na-containing ionic liquids allows to increase the upper temperature and cell voltage at which Li- and Na-based systems can be safely operated at.Der wachsende Einsatz mobiler elektronischer Geräte und der weltweite Übergang hin zur Elektromobilität hat die Nachfrage nach Energiespeichern wie Lithium-Ionen-Batterien und Superkondensatoren stark erhöht. In diesem Zusammenhang besteht ein besonders großer Bedarf an Technologien, welche die Leistungsmerkmale heutiger Speichermodule in Bezug auf Energie und Leistung deutlich übertreffen können. Ein vielversprechendes Beispiel sind Hybrid-Superkondensatoren (HSC), welche Eigenschaften von Lithium-Ionen-Batterien und Superkondensatoren synergetisch kombinieren. Der Fokus dieser Promotionsarbeit liegt auf dem Nano-Design von Hybridmaterialien aus Metalloxiden und Kohlenstoff für eine verbesserte elektrochemische Leistung in hybriden Lithium- oder Natrium-ionen-Energiespeichern. Die Hybridisierung von Metalloxid- und Kohlenstoffsubstrat kann durch eine angepasste Sol-Gel-Synthese erreicht werden, was zu einer homogenen und nanoskaligen Verteilung des Metalloxids im resultierenden Hybridmaterial führt. Elektroden, welche solche Hybridmaterialien einsetzen, können besser Leistungsmerkmale erreichen, als vergleichbare Komposit-Materialien, welche durch rein physikalisches Mischen zweier Phasen erreicht werden. Dieser Umstand lässt sich allerdings nicht pauschal für alle Arten von (Nano)kompositen anwenden. Neben dem Elektrodenmaterial beeinflusst auch die Wahl des Elektrolyten die Leistungsmerkmale von elektrochemischen Energiespeichern. Der Einsatz alternativer organischer Lösungsmittel oder Li- bzw. Na-haltiger ionischer Flüssigkeiten ermöglicht die betriebssichere Verwendung von Li- und Na-HSC Systemen selbst bei erhöhten Temperaturen bzw. erhöhter Zellspannung

Titanium Niobium Oxide Ti2 Nb10 O29 /Carbon Hybrid Electrodes Derived by Mechanochemically Synthesized Carbide for High-Performance Lithium-Ion Batteries

This work introduces the facile and scalable two‐step synthesis of Ti2Nb10O29 (TNO)/carbon hybrid material as a promising anode for lithium‐ion batteries (LIBs). The first step consisted of a mechanically induced self‐sustaining reaction via ball‐milling at room temperature to produce titanium niobium carbide with a Ti and Nb stoichiometric ratio of 1 to 5. The second step involved the oxidation of as‐synthesized titanium niobium carbide to produce TNO. Synthetic air yielded fully oxidized TNO, while annealing in CO2 resulted in TNO/carbon hybrids. The electrochemical performance for the hybrid and non‐hybrid electrodes was surveyed in a narrow potential window (1.0–2.5 V vs. Li/Li+) and a large potential window (0.05–2.5 V vs. Li/Li+). The best hybrid material displayed a specific capacity of 350 mAh g−1 at a rate of 0.01 A g−1 (144 mAh g−1 at 1 A g−1) in the large potential window regime. The electrochemical performance of hybrid materials was superior compared to non‐hybrid materials for operation within the large potential window. Due to the advantage of carbon in hybrid material, the rate handling was faster than that of the non‐hybrid one. The hybrid materials displayed robust cycling stability and maintained ca. 70 % of their initial capacities after 500 cycles. In contrast, only ca. 26 % of the initial capacity was maintained after the first 40 cycles for non‐hybrid materials. We also applied our hybrid material as an anode in a full‐cell lithium‐ion battery by coupling it with commercial LiMn2O4

Hybrid Anodes of Lithium Titanium Oxide and Carbon Onions for Lithium‐Ion and Sodium‐Ion Energy Storage

This study demonstrates the hybridization of Li4Ti5O12 (LTO) with different types of carbon onions synthesized from nanodiamonds. The carbon onions mixed with a Li4Ti5Ox precursor for sol–gel synthesis. These hybrid materials are tested as anodes for both lithium‐ion battery (LIB) and sodium‐ion battery (SIB). Electrochemical characterization for LIB application is carried out using 1 m LiPF6 in a 1:1 (by volume) ethylene carbonate and dimethyl carbonate as the electrolyte. For lithium‐ion intercalation, LTO hybridized with carbon onions from the inert‐gas route achieves an excellent electrochemical performance of 188 mAh g−1 at 10 mA g−1, which maintains 100 mAh g−1 at 1 A g−1 and has a cycling stability of 96% of initial capacity after 400 cycles, thereby outperforming both neat LTO and LTO with onions obtained via vacuum treatment. The performance of the best‐performing hybrid material (LTO with carbon onions from argon annealing) in an SIB is tested, using 1 m NaClO4 in ethylene/dimethyl/fluoroethylene carbonate (19:19:2 by mass) as the electrolyte. A maximum capacity of 102 mAh g−1 for the SIB system is obtained, with a capacity retention of 96% after 500 cycles

Monitoring the thermally induced transition from sp3-hybridized into sp2-hybridized carbons

The preparation of carbons for technical applications is typically based on a treatment of a precursor, which is transformed into the carbon phase with the desired structural properties. During such treatment the material passes through several different structural stages, for example, starting from precursor molecules via an amorphous phase into crystalline-like phases. While the structure of non-graphitic and graphitic carbon has been well studied, the transformation stages from molecular to amorphous and non-graphitic carbon are still not fully understood. Disordered carbon often contains a mixture of sp3-, sp2-and sp1-hybridized bonds, whose analysis is difficult to interpret. We systematically address this issue by studying the transformation of purely sp3-hybridized carbons, that is, nanodiamond and adamantane, into sp2-hybridized non-graphitic and graphitic carbon. The precursor materials are thermally treated at different temperatures and the transformation stages are monitored. We employ Raman spectroscopy, WAXS and TEM to characterize the structural changes. We correlate the intensities and positions of the Raman bands with the lateral crystallite size La estimated by WAXS analysis. The behavior of the D and G Raman bands characteristic for sp2-type material formed by transforming the sp3-hybridized precursors into non-graphitic and graphitic carbon agrees well with that observed using sp2-structured precursors

Titanium Niobium Oxide Ti2 Nb10 O29 /Carbon Hybrid Electrodes Derived by Mechanochemically Synthesized Carbide for High-Performance Lithium-Ion Batteries

This work introduces the facile and scalable two-step synthesis of Ti2 Nb10 O29 (TNO)/carbon hybrid material as a promising anode for lithium-ion batteries (LIBs). The first step consisted of a mechanically induced self-sustaining reaction via ball-milling at room temperature to produce titanium niobium carbide with a Ti and Nb stoichiometric ratio of 1 to 5. The second step involved the oxidation of as-synthesized titanium niobium carbide to produce TNO. Synthetic air yielded fully oxidized TNO, while annealing in CO2 resulted in TNO/carbon hybrids. The electrochemical performance for the hybrid and non-hybrid electrodes was surveyed in a narrow potential window (1.0-2.5 V vs. Li/Li+ ) and a large potential window (0.05-2.5 V vs. Li/Li+ ). The best hybrid material displayed a specific capacity of 350 mAh g-1 at a rate of 0.01 A g-1 (144 mAh g-1 at 1 A g-1 ) in the large potential window regime. The electrochemical performance of hybrid materials was superior compared to non-hybrid materials for operation within the large potential window. Due to the advantage of carbon in hybrid material, the rate handling was faster than that of the non-hybrid one. The hybrid materials displayed robust cycling stability and maintained ca. 70 % of their initial capacities after 500 cycles. In contrast, only ca. 26 % of the initial capacity was maintained after the first 40 cycles for non-hybrid materials. We also applied our hybrid material as an anode in a full-cell lithium-ion battery by coupling it with commercial LiMn2 O4