Steven A. Klankowski

A high-performance lithium-ion battery anode based on the core–shell heterostructure of silicon-coated vertically aligned carbon nanofibers

This study reports a high-performance hybrid lithium-ion anode material using coaxially coated silicon shells on vertically aligned carbon nanofiber (VACNF) cores. The unique “cup-stacking” graphitic microstructure makes VACNFs a good lithium-ion intercalation medium and, more importantly, a robust bush-like conductive core to effectively connect high-capacity silicon shells for lithium-ion storage. The vertical core–shell nanowires remain well separated from each other even after coating with bulk quantities of silicon (equivalent to 1.5 μm thick solid films). This open structure allows the silicon shells to freely expand/contract in the radial direction during lithium-ion insertion/extraction. A high specific capacity of 3000–3650 mA h (gSi)−1, comparable to the maximum value of amorphous silicon, has been achieved. About 89% of the capacity is retained after 100 charge–discharge cycles at the C/1 rate. After long cycling, the electrode material becomes even more stable, showing the invariant lithium-ion storage capacity as the charge–discharge rate is increased by 20 times from C/10 to C/0.5 (or 2C). The ability to obtain high capacity at significantly improved power rates while maintaining the extraordinary cycle stability demonstrates that this novel structure could be a promising anode material for high-performance lithium-ion batteries.

Tin dioxide@carbon core-shell nanoarchitectures anchored on wrinkled graphene for ultrafast and stable lithium storage.

The SnO2@C@GS composites as a new type of 3D nanoarchitecture have been successfully synthesized by a facile hydrothermal process followed by a sintering strategy. Such a 3D nanoarchitecture is made up of SnO2@C core-shell nanospheres and nanochains anchored on wrinkled graphene sheets (GSs). Transmission electron microscopy shows that these core-shell nanoparticles consist of 3-9 nm diameter secondary SnO2 nanoparticles embedded in about 50 nm diameter primary carbon nanospheres. Large quantities of core-shell nanoparticles are uniformly attached to the surface of wrinkled graphene nanosheets, with a portion of them further connected into nanochains. This new 3D nanoarchitecture consists of two different kinds of carbon-buffering matrixes, i.e., the carbon layer produced by glucose carbonization and the added GS template, leading to enhanced lithium storage properties. The lithium-cycling properties of the SnO2@C@GS composite have been evaluated by galvanostatic discharge-charge cycling and electrochemical impedance spectroscopy. Results show that the SnO2@C@GS composite has discharge capacities of 883.5, 845.7, and 830.5 mA h g(-1) in the 20th, 50th and 100th cycles, respectively, at a current density of 200 mA g(-1) and delivers a desirable discharge capacity of 645.2 mA h g(-1) at a rate of 1680 mA g(-1). This new 3D nanoarchitecture exhibits a high capability and excellent cycling and rate performance, holding great potential as a high-rate and stable anode material for lithium storage.

Preparation and Characterization of TiO2 Barrier Layers for Dye-Sensitized Solar Cells

A TiO2 barrier layer is critical in enhancing the performance of dye-sensitized solar cells (DSSCs). Two methods to prepare the TiO2 barrier layer on fluorine-doped tin dioxide (FTO) surface were systematically studied in order to minimize electron-hole recombination and electron backflow during photovoltaic processes of DSSCs. The film structure and materials properties were correlated with the photovoltaic characteristics and electrochemical properties. In the first approach, a porous TiO2 layer was deposited by wet chemical treatment of the sample with TiCl4 solution for time periods varying from 0 to 60 min. The N719 dye molecules were found to be able to insert into the porous barrier layers. The 20 min treatment formed a nonuniform but intact TiO2 layer of ∼100-300 nm in thickness, which gave the highest open-circuit voltage VOC, short-circuit photocurrent density JSC, and energy conversion efficiency. But thicker TiO2 barrier layers by this method caused a decrease in JSC, possibly limited by lower electrical conductance. In the second approach, a compact TiO2 barrier layer was created by sputter-coating 0-15 nm Ti metal films on FTO/glass and then oxidizing them into TiO2 with thermal treatment at 500 °C in the air for 30 min. The dye molecules were found to only attach at the outer surface of the barrier layer and slightly increased with the layer thickness. These two kinds of barrier layer showed different characteristics and may be tailored for different DSSC studies.

Effective Infiltration of Gel Polymer Electrolyte into Silicon-Coated Vertically Aligned Carbon Nanofibers as Anodes for Solid-State Lithium-Ion Batteries

This study demonstrates the full infiltration of gel polymer electrolyte into silicon-coated vertically aligned carbon nanofibers (Si-VACNFs), a high-capacity 3D nanostructured anode, and the electrochemical characterization of its properties as an effective electrolyte/separator for future all-solid-state lithium-ion batteries. Two fabrication methods have been employed to form a stable interface between the gel polymer electrolyte and the Si-VACNF anode. In the first method, the drop-casted gel polymer electrolyte is able to fully infiltrate into the open space between the vertically aligned core-shell nanofibers and encapsulate/stabilize each individual nanofiber in the polymer matrix. The 3D nanostructured Si-VACNF anode shows a very high capacity of 3450 mAh g(-1) at C/10.5 (or 0.36 A g(-1)) rate and 1732 mAh g(-1) at 1C (or 3.8 A g(-1)) rate. In the second method, a preformed gel electrolyte film is sandwiched between an Si-VACNF electrode and a Li foil to form a half-cell. Most of the vertical core-shell nanofibers of the Si-VACNF anode are able to penetrate into the gel polymer film while retaining their structural integrity. The slightly lower capacity of 2800 mAh g(-1) at C/11 rate and ∼1070 mAh g(-1) at C/1.5 (or 2.6 A g(-1)) rate have been obtained, with almost no capacity fade for up to 100 cycles. Electrochemical impedance spectroscopy does not show noticeable changes after 110 cycles, further revealing the stable interface between the gel polymer electrolyte and the Si-VACNFs anode. These results show that the infiltrated flexible gel polymer electrolyte can effectively accommodate the stress/strain of the Si shell due to the large volume expansion/contraction during the charge-discharge processes, which is particularly useful for developing future flexible solid-state lithium-ion batteries incorporating Si-anodes.

Atomic layer deposition of Al-doped ZnO/Al2O3 double layers on vertically aligned carbon nanofiber arrays.

High-aspect-ratio, vertically aligned carbon nanofibers (VACNFs) were conformally coated with aluminum oxide (Al2O3) and aluminum-doped zinc oxide (AZO) using atomic layer deposition (ALD) in order to produce a three-dimensional array of metal-insulator-metal core-shell nanostructures. Prefunctionalization before ALD, as required for initiating covalent bonding on a carbon nanotube surface, was eliminated on VACNFs due to the graphitic edges along the surface of each CNF. The graphitic edges provided ideal nucleation sites under sequential exposures of H2O and trimethylaluminum to form an Al2O3 coating up to 20 nm in thickness. High-resolution transmission electron microscopy (HRTEM) and scanning electron microscopy images confirmed the conformal core-shell AZO/Al2O3/CNF structures while energy-dispersive X-ray spectroscopy verified the elemental composition of the different layers. HRTEM selected area electron diffraction revealed that the as-made Al2O3 by ALD at 200 °C was amorphous, and then, after annealing in air at 450 °C for 30 min, was converted to polycrystalline form. Nevertheless, comparable dielectric constants of 9.3 were obtained in both cases by cyclic voltammetry at a scan rate of 1000 V/s. The conformal core-shell AZO/Al2O3/VACNF array structure demonstrated in this work provides a promising three-dimensional architecture toward applications of solid-state capacitors with large surface area having a thin, leak-free dielectric.

High-rate lithium-ion battery anodes based on silicon-coated vertically aligned carbon nanofibers

A multiscale hierarchical lithium-ion battery (LIB) anode composed of Si shells coaxially coated on vertically aligned carbon nanofibers has been explored. A high Li storage capacity of ~3,000-3,500 mAh (gSi)-1 and > 99% Coulombic efficiency have been obtained. Remarkable stability over 500 charge-discharge cycles have been demonstrated. Particularly, this electrode present a high-rate capability that the capacity remains within ~7% as the C-rate was increased from ~C/10 to ~8C. Electron microscopy, Raman spectroscopy and electrochemical impedance spectroscopy revealed that the electrode structure remains stable during long cycling. This high-rate property is likely associated with the unique nanocolumnar microstructure of Si in the shell. It reveals an exciting potential to develop high-performance LIBs.