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Author: Uhlemann, Margitta × Publisher: Basel : MDPI ×

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    Directly Anodized Sulfur-Doped TiO2 Nanotubes as Improved Anodes for Li-ion Batteries
    (Basel : MDPI, 2020) Sabaghi, Davood; Madian, Mahmoud; Omar, Ahmad; Oswald, Steffen; Uhlemann, Margitta; Maghrebi, Morteza; Baniadam, Majid; Mikhailova, Daria
    TiO2 represents one of the promising anode materials for lithium ion batteries due to its high thermal and chemical stability, relatively high theoretical specific capacity and low cost. However, the electrochemical performance, particularly for mesoporous TiO2, is limited and must be further developed. Elemental doping is a viable route to enhance rate capability and discharge capacity of TiO2 anodes in Li-ion batteries. Usually, elemental doping requires elevated temperatures, which represents a challenge, particularly for sulfur as a dopant. In this work, S-doped TiO2 nanotubes were successfully synthesized in situ during the electrochemical anodization of a titanium substrate at room temperature. The electrochemical anodization bath represented an ethylene glycol-based solution containing NH4F along with Na2S2O5 as the sulfur source. The S-doped TiO2 anodes demonstrated a higher areal discharge capacity of 95 µAh·cm−2 at a current rate of 100 µA·cm−2 after 100 cycles, as compared to the pure TiO2 nanotubes (60 µAh·cm−2). S-TiO2 also exhibited a significantly improved rate capability up to 2500 µA·cm−2 as compared to undoped TiO2. The improved electrochemical performance, as compared to pure TiO2 nanotubes, is attributed to a lower impedance in S-doped TiO2 nanotubes (STNTs). Thus, the direct S-doping during the anodization process is a promising and cost-effective route towards improved TiO2 anodes for Li-ion batteries.
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    Pulse Reverse Plating of Copper Micro-Structures in Magnetic Gradient Fields
    (Basel : MDPI, 2022) Huang, Mengyuan; Uhlemann, Margitta; Eckert, Kerstin; Mutschke, Gerd
    Micro-structured copper layers are obtained from pulse-reverse electrodeposition on a planar gold electrode that is magnetically patterned by magnetized iron wires underneath. 3D numerical simulations of the electrodeposition based on an adapted reaction kinetics are able to nicely reproduce the micro-structure of the deposit layer, despite the height values still remain underestimated. It is shown that the structuring is enabled by the magnetic gradient force, which generates a local flow that supports deposition and hinders dissolution in the regions of high magnetic gradients. The Lorentz force originating from radial magnetic field components near the rim of the electrode causes a circumferential cell flow. The resulting secondary flow, however, is superseded by the local flow driven by the magnetic gradient force in the vicinity of the wires. Finally, the role of solutal buoyancy effects is discussed to better understand the limitations of structured growth in different modes of deposition and cell geometries.