Mathias Widmaier

Tuning pseudocapacitive and battery-like lithium intercalation in vanadium dioxide/carbon onion hybrids for asymmetric supercapacitor anodes

The study presents the synthesis of vanadium oxide/carbon onion hybrid materials. Flower-like vanadium oxide nanostructures nucleate on carbon onion nanoparticles under hydrothermal conditions, forming a highly intertwined network. By varying the amount of added carbon onions during the synthesis, the number of possible nucleation sites can be adjusted, resulting in the preferential growth of vanadium dioxide in either P21/c or C2/m space group. When employed as a lithium intercalation electrode, P21/c VO2 exhibits capacitor-like (pseudocapacitive) lithium intercalation, whereas C2/m VO2 shows battery-like intercalation peaks with a maximum capacity of 127 mA h g−1. By selecting an optimum ratio and thereby combining both intercalation mechanisms, enhanced kinetics with discharge capacities of 45 mA h g−1 and 29 mA h g−1 at high rates of 50 A g−1 and 100 A g−1 (equal to 394C and 788C) are obtained. This behavior can be translated to a device level by using the material as anodes in asymmetric supercapacitors with activated carbon cathodes, yielding a maximum specific energy of 45 W h kg−1 and a high power of 58 kW kg−1, while longevity over 5000 charge/discharge cycles is demonstrated.

Continuous silicon oxycarbide fiber mats with tin nanoparticles as a high capacity anode for lithium-ion batteries

Continuous fiber mats are attractive electrodes for lithium-ion batteries, because they allow operation at high charge/discharge rates in addition to being free of polymer binders and conductive additives. In this work, we synthesize and characterize continuous Sn/SiOC fibers (diameter ca. 0.95 μm), as a Li-ion battery anode. Our synthesis employs electrospinning of a low-cost silicone resin, using tin acetate in a dual role both as a polymer crosslinker and as a tin precursor (6–22 mass%). The hybrid electrodes present very high initial reversible capacities (840–994 mA h g−1) at 35 mA g−1, and retain 280–310 mA h g−1 at 350 mA g−1. After 100 cycles at 70 mA g−1, the hybrid fibers maintained 400–509 mA h g−1. Adding low amounts of Sn is beneficial not just for the crosslinking of the polymer precursor, but also to decrease the presence of electrochemically inactive silicon carbide domains within the SiOC fibers. Also, the metallic tin clusters contribute to a higher Li+ insertion in the first cycles. However, high amounts of Sn decrease the electrochemical performance stability. In SiOC fibers synthesized at high temperatures (1200 °C), the Cfree phase has a significant influence on the stability of the system, by compensating for the volume expansion from the alloying systems (Sn and SiO2), and improving the conductivity of the hybrid system. Therefore, a high amount of carbon and a high graphitization degree are crucial for a high conductivity and a stable electrochemical performance.