High Performing Hollow Silicon Nanotube Anodes for Lithium Ion Batteries
Previously we have demonstrated a simple and large scale synthesis approach for the generation of hollow silicon nanotubes (h-SiNTs) using Inorganic nanowire (I-NW) templates and chemical vapor deposition of amorphous Si which show excellent and stable cycling capacities (2420mAh/g @ 300mA/g) for the Li/Li+system. However, the system shows a high first cycle irreversible (FIR) loss (24%-27%) due to the SEI formation and relatively low columbic efficiency of 98.0%
In the present work, we have studied the effect of optimizing the time of deposition of Si during the CVD on the FIR loss and loading density. These hollow Si nanotubes were then coated with a thin layer of carbon by gaseous thermal decomposition at 700oC and further washing with suitable reagents to remove the unwanted phases on the surface. Slurry based electrodes of hollow nanostructures (carbon coated and reagent washed) mixed with binder and conductive additives were then fabricated and tested in a half cell configuration against lithium foil between the voltage range 0.01V – 1V vs. Li/Li+in 1M LiPF6 (dissolved in EC:DEC:FEC=45:45:10) electrolyte.
We have demonstrated the increase in loading density of the active material in the electrode by increasing the time of deposition thus, leading to increase in the thickness of the hollow Si nanotube wall. Subsequently, the FIR loss decreases to 15% with the increase in the time of Si – CVD deposition due to the reduction in the specific surface area of the hollow SiNTs however, without affecting the stability of the nanostructured electrodes. The carbon coated hollow SiNTs (Figure 1) showed a first cycle capacity of 1780 mAh/g (at 300mA/g) with FIR loss of only 14.2% and a columbic efficiency of 99.9%. The reagent washed carbon coated hollow SiNTs further exhibited a first cycle capacity of 1800mAh/g (at 300mAh/g) with an FIR loss of only 13% and a columbic efficiency of 99.92%.
Materials phase, morphology and surface analysis was conducted on the electrodes before and after electrochemical testing to confirm the stability of the nanostructures on long term cycling. SEM analysis was conducted on the electrode in lithiated state to study the effect of volume expansion on the morphology and configuration of the electrodes. Results of these studies will be presented and discussed.
The authors gratefully acknowledge financial support of the DOE-BATT (DE-AC02-05CHl1231) and NSF (CBET-0933141) programs. The authors also acknowledge the Edward R. Weidlein Chair Professorship funds and the Center for Complex Engineered Multifunctional Materials (CCEMM) for support of this research.
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