High Performing Hollow Silicon Nanotube Anodes for Lithium Ion Batteries

Silicon has attracted considerable interest as a as a high energy density anode replacement to graphite for lithium ion batteries due to this high theoretical capacity of 4200 mAh/g. The material however suffers from colossal volume expansion (400%) related stresses during cycling leading to pulverization and cracking of the electrode. Over the past few years different approaches have been developed to address the issue caused by the formation of various zintle phases of Li – Si system during the lithiation and alloying process. These approaches involve the use of silicon nanoparticles[1], composite structures of amorphous silicon[2], in-situ and ex-situ active – inactive matrices[3], electrodeposited thin films and VACNT-Si heterostructures[4-6]. Amongst these, silicon nanoparticles and nanotubes have shown the ability to withstand the large internal stresses and strain mismatch generated during the expansion process and hence, serve as stable architectures for exploration and exploitation as an anode in Li/Li⁺battery.

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 700^oC 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.

Acknowledgement:

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.

References:

[1] I. Kim, P.N. Kumta, G.E. Blomgren, Electrochemical and Solid State Letters 3 (2000) 493-496.

[2] X.H. Liu, L. Zhong, S. Huang, S.X. Mao, T. Zhu, J.Y. Huang, Acs Nano 6 (2012) 1522-1531.

[3] M.K. Datta, J. Maranchi, S.J. Chung, R. Epur, K. Kadakia, P. Jampani, P.N. Kumta, Electrochimica Acta 56 (2011) 4717-4723.

[4] W. Wang, R. Epur, P.N. Kumta, Electrochemistry Communications 13 (2011) 429-432.

[5] W. Wang, P.N. Kumta, Acs Nano 4 (2010) 2233-2241.

[6] R. Epur, M.K. Datta, P.N. Kumta, Electrochimica Acta 85 (2012) 680-684.