Apr 17, 2008
Quasi-2D structures make better batteries
"Sugar-coated" silicon-carbon nanotube (Si-CNT) composites have hit the headlines promising increased battery life, but this isn't the only electrode material bidding to boost the performance of lithium-ion cells. Hui-Ming Cheng and his colleagues from Shenyang National Laboratory for Materials Science, People's Republic of China, are also backing the use of nickel silicide nanobelts and sheets.
"Each system has its own advantages and disadvantages. Silicon has a very high theoretical capacity, which contributes a lot to the capacity of the corresponding composite material, but there are practical difficulties in implementing the system due to the large volume change of silicon during charging and discharging of the cell," Cheng told nanotechweb.org. "Nickel silicide nanobelts and sheets are new promising materials for lithium-ion battery applications. Because they are grown in situ on the surface of the current collector they can be employed directly as a working electrode without the need for binders and conductive additives, which will make cell manufacturing more convenient."
The stumbling block for nickel-based materials is that their first coulombic efficiency is quite low. The group has formulated Si-CNT composites with an electrochemical capacity of more than 720 mAh per gram, whereas the team's nickel silicide nanosheets are closer to 540 mAh per gram. However, interest remains in these nickel silicide materials. The picture becomes clearer when you consider that today's graphite anodes have an average capacity of just 350 mAh per gram.
Cheng explains that nickel silicide nanobelts and sheets have a favourable morphology, especially compared with previous nanowire formulations. "The structures' nanoscale thicknesses provide a short diffusion path for lithium ions, while the flat side facets help to reduce the surface strain energy," he said. "As a result, the nanobelts and sheets are more powerful in maintaining structural stability during lithium ion insertion (charging) and removal (discharging), which ensures excellent cyclic performance."
The group's nanosheets maintained their capacity after 20 electrochemical charge and discharge cycles. Upon dismantling the lithium cell, the researchers discovered that there was almost no difference in the pattern of the pristine and cycled nanosheets, which supports the team's claim that its nickel silicide nanomaterials are highly stable.
The researchers presented their work in Nanotechnology.
About the author
James Tyrrell is editor of nanotechweb.org.