Li-ion batteries (LIBs) have been used as a power source ranging from microelectronics to electrical vehicles. However, the use of graphite as anode material for next generation LIBs is unsuitable because of the low Li⁺ storage capacity (372 mAh.g^-1). Various alternative, such as Si, Sn, Ge, has been considered as alternative due to their superior specific gravimetric capacity (for example Si = 4200 mAh.g^-1) resulting in high power/energy densities¹. However, these materials are intrinsically prone to large volume change during alloying/dealloying mechanism resulting in pulverization, thick SEI formation, loss of electrical contact and active material². The use of nanostructure has been widely studied to address this problem.

Recently, our group has developed techniques to fabricate ultrathin crystalline silicon nanotube arrays (Si NTAs). In comparison to previous works which focus on sealed SiNTs³ and larger size SiNTs (> 400 nm)⁴, our Si NTAs are ultrathin (10 nm) with porous side walls⁵. Fig.1 a & b, shows the SEM and TEM images of the Si NTAs fabricated by three step template directed method. The first step is the formation of sacrificial ZnO into stainless steel substrates followed by the deposition of Si onto the ZnO nanowires. The last step is the removal of the sacrificial ZnO nanowire templates to obtain the Si NTAs with porous side wall structure.

Fig. 1 (a) SEM and (b) TEM image of Si NTAs.

Fig. 2 shows the gravimetric capacity of Si NTAs as a function of cycle number. The electrochemical tests were performed in two electrode Swagelok cell. The half-cells were assembled by using Si NTAs as a working electrode, Li foil as a counter electrode, and a Whatman glass microfiber soaked in 1 M LiPF₆ in EC:DEC electrolyte as separator. The cell was cycled at C/20 in a potential window of 0.01 – 1.75 V vs Li/Li⁺. It delivers a specific capacity of 1670 mAh.g-1 after 30 cycles. The high capacity values are attributed to the 3D porous structure of the Si NTAs.

During this work, the improved electrochemical performance of Si NTAs will be discussed.

Fig. 2. Gravimetric capacity vs cycle number for Si NTAs at C/20.

Reference

1. Liang, B.; Liu, Y.; Xu, Y., Silicon-based materials as high capacity anodes for next generation lithium ion batteries. Journal of Power Sources 2014, 267, 469-490.

2. Kasavajjula, U.; Wang, C.; Appleby, A. J., Nano- and bulk-silicon-based insertion anodes for lithium-ion secondary cells. Journal of Power Sources 2007, 163 (2), 1003-1039.

3. Song, T.; Xia, J.; Lee, J.-H.; Lee, D. H.; Kwon, M.-S.; Choi, J.-M.; Wu, J.; Doo, S. K.; Chang, H.; Park, W. I., Arrays of sealed silicon nanotubes as anodes for lithium ion batteries. Nano letters 2010, 10 (5), 1710-1716.

4. Wu, H.; Chan, G.; Choi, J. W.; Yao, Y.; McDowell, M. T.; Lee, S. W.; Jackson, A.; Yang, Y.; Hu, L.; Cui, Y., Stable cycling of double-walled silicon nanotube battery anodes through solid-electrolyte interphase control. Nature nanotechnology 2012, 7 (5), 310-315.

5. Huang, X.; Gonzalez-Rodriguez, R.; Rich, R.; Gryczynski, Z.; Coffer, J. L., Fabrication and size dependent properties of porous silicon nanotube arrays. Chemical Communications 2013, 49 (51), 5760-5762.