Nanostructured electrode materials for electrochemical energy storage systems have been shown
to improve both rate performance and capacity retention, while allowing considerably longer
cycling lifetime. The nano-architectures provide enhanced kinetics by means of larger surface
area, higher porosity, better material interconnectivity, shorter diffusion lengths, and overall
mechanical stability. Meanwhile, active materials that once were excluded from use due to bulk
property issues are now being examined in new nanoarchitecture.
Silicon was such a material, desired for its large lithium-ion storage capacity of 4,200
mAh g[superscript]-1 and low redox potential of 0.4 V vs. Li/Li[superscript]+; however, a ~300% volume expansion and
increased resistivity upon lithiation limited its broader applications. In the first study, the
silicon-coated vertically aligned carbon nanofiber (VACNF) array presents a unique core-shell
nanowire (NW) architecture that demonstrates both good capacity and high rate performance. In
follow-up, the Si-VACNFs NW electrode demonstrates enhanced power rate capabilities as it
shows excellent storage capacity at high rates, attributed to the unique nanoneedle structure that
high vacuum sputtering produces on the three-dimensional array.
Following silicon’s success, titanium dioxide has been explored as an alternative highrate
electrode material by utilizing the dual storage mechanisms of Li+ insertion and
pseudocapacitance. The TiO[subscript]2-coated VACNFs shows improved electrochemical activity that
delivers near theoretical capacity at larger currents due to shorter Li[superscript]+ diffusion lengths and highly
effective electron transport. A unique cell is formed with the Si-coated and TiO[subscript]2-coated
electrodes place counter to one another, creating the hybrid of lithium ion battery-pseudocapacitor
that demonstrated both high power and high energy densities. The hybrid cell
operates like a battery at lower current rates, achieving larger discharge capacity, while retaining
one-third of that capacity as the current is raised by 100-fold. This showcases the VACNF
arrays as a solid platform capable of assisting lithium active compounds to achieve high capacity
at very high rates, comparable to modern supercapacitors.
Lastly, manganese oxide is explored to demonstrate the high power rate performance that
the VACNF array can provide by creating a supercapacitor that is highly effective in cycling at
various high current rates, maintaining high-capacity and good cycling performance for
thousands of cycles.
