Lithium ion batteries are becoming increasingly prevalent as they exhibit high specific energy, a long cycle-life, a low self-discharge rate and can operate in a wide range of temperatures.¹ The limitation is that carbonaceous anodes do not provide high enough specific capacities for small scale applications of Li-ion batteries.  LiC₆ has a theoretical capacity of 372 mAh/g while Li₁₅Si₄, an alloy formed at room temperature and low voltage, has a theoretical capacity of 3,579 mAh/g.^2,3 Unfortunately, a volume expansion of roughly 270% occurs in silicon upon lithiation and results in fracturing of the electrode particles and rapid loss of capacity with cycling.² Silicon nanowires experience the same volume expansion, but are able to accommodate the expansion better which may improve cyclability.

Commonly, silicon nanowires are grown epitaxial with a metallic impurity and a gas containing the reactive species. This method, known as vapor-liquid-solid mechanism is slow and expensive since it requires a clean room, high temperatures and metallic impurities such as gold.⁴Etching is the other method for producing nanowires. Etching is accomplished by first removing native oxides and then dispersing metallic nanoparticles onto the surface of a silicon wafer.  The silver nanoparticles are oxidized and the cation reacts locally with silicon to remove it from the surface. This results in uniform single crystalline nanowires made from the top down.

This research will discuss the difference in performance of bulk silicon, grown silicon nanowires and etched silicon nanowires as anodes for lithium-ion batteries. The anodes are tested in an electrochemical half-cell and put through cyclic voltammetry and charge-discharge cycling. The cells are disassembled and the physical effects of cycling are examined with a scanning electron microscope (SEM) and energy dispersive X-ray spectroscopy (EDS). Cyclic voltammetry is used to determine the voltages at which insertion and desertion of lithium into silicon occurs. Then charge-discharge cycling is employed to test the performance of the cell over a number of cycles. Since silicon exhibits fracturing as a result of its large volume expansion SEM/EDS is used to examine the damage that cycling has on the electrodes and the overall chemical composition of each component. The chemical composition helps to identify solid electrolyte interphase formation and confirms that silicon is the bulk material in the system.

The bulk silicon wafers experienced rapid deterioration after just a few cycles. Both the etched nanowires and the wafers in Figure 1 exhibited fracturing, but it was more prominent in the bulk wafer than in the nanowires. There was also a clear difference in the capacity of the nanowires versus the bulk wafer. Charge-discharge cycling in Figure 2 revealed that silicon wafers had a capacity around 20 mAh/g while the silicon nanowires were reaching capacities over 2000 mAh/g. While there was a rapid loss in capacity seen in the nanowires the capacity after 20 cycles was still 25 times higher than the bulk silicon. These results clearly show that the silicon nanowires are capable of handling the large volume expansion and maintain their capacity more effectively than bulk silicon wafers. The capacity loss is more drastic, but the nanowires still maintain a higher capacity than the bulk silicon wafers and traditional carbonaceous anodes currently in use. Comparison with grown silicon nanowires will also be discussed, but etched nanowires are the preferred anode since they have economical and production time advantages over the commonly grown nanowires.

¹Dahn, J.; Ehrlich, G.; Reddy, T. McGraw Hill, New York: 2011.

² Beattie, S. D.; Larcher, D.; Morcretty, M.; Simon, B.; Tarascon, J.M.J.; Electrochem. Soc. 2008, 155, A158.

³ Hatchard, T. D.; Dahn, J.R.J.; Electrochem Soc. 2004, 151, A838.

⁴ Wagner, R.S.; Ellis, W.C. Appl. Phys. Lett. 1964, 4, 89.

⁵ Smith, Z.R.; Smith, R.L.; Collins, S.D. Electrochim. Acta 2013, 92,139.

^{Sandia National Laboratories is a multi-program laboratory managed and operated by Sandia Corporation, a wholly owned subsidiary of Lockheed Martin Corporation, for the U.S. Department of Energy’s National Nuclear Security Administration under contract DE-AC04-94AL85000.SAND2015-10771 A}