|Fig. 1: Electrochemical data for Si NW anodes. (Data from .)|
Despite the success of Lithium ion (Li-ion) batteries with graphite anodes, these batteries are unable to meet the higher charge storage and longer battery life requirements of new technologies. Current lithium ion battery capacity is primarily limited by the low theoretical capacity of graphite as the anode material.  Research has shown that when three different nanowire (NW) materials, silicon, germanium, and carbon-silicon core-shell, each with their own advantages and applications, are substituted for the battery anode, they increase the capacity of Li-ion batteries. [1-3.
Silicon is an attractive anode material for lithium batteries because it has a low discharge potential and the highest known theoretical charge capacity at 4,200 milli-Ampere hours per gram (mAh/g), which is more than ten times higher than that of existing graphite anodes.  However, using conventional bulk silicon by itself has limited applications due to silicon's significant volume changes upon insertion and extraction of lithium, which leads to pulverization and capacity fading. To minimize the issues related with bulk silicon, a nanowire form of silicon is considered. Nanowires are structures that have a thickness or diameter constrained to tens of nanometers (10-9 meters) or less, and an unconstrained length. Thus, utilizing a nanowire form of silicon allows the strands of each nanowire to swell up in volume, without adversely affecting capacity.  The usage of silicon nanowires as battery anodes circumvent the issues of bulk silicon as they are able to accommodate large strain without pulverization, provide good electronic contact and conduction, and display short lithium insertion distances.  The capacity observed during the first charging operation is 4,277 mAh/g; this is essentially the same as the theoretical capacity of conventional Si, within experimental error. Also, experiments have shown that Silicon nanowire-based anodes have a first discharge capacity of 3,124 mAh/g, indicating a coulombic efficiency of 73%.  As can be seen in Fig. 1, both the charge and discharge capacities remain nearly constant for subsequent cycles, with little fading for up to 10 cycles. To compare, the charge and discharge data for currently used graphite anodes are also shown within the same figure. Using Si nanowires as the anode material for Li-ion batteries helps to achieve the theoretical charge capacity for silicon anodes, while maintaining a discharge capacity close to 75% of this maximum. The improved capacity and cycle life, resulting from the usage of Si NWs, demonstrates the advantages of this type of anode design.
|Fig. 2: Comparison between the fabrication of traditional and nanowire based anode design. |
The analogous LixGe system also has similar advantages of improved capacity and cycle life, along with additional benefits. The fully lithiated Li4.4Ge has a high theoretical capacity of 1600 mAh/g. Although this value is less than that of Si, the room-temperature diffusivity of Li in Ge is 400 times higher than that in Si; this indicates that Ge is an attractive anode material for high-power-rate anodes.  In this method, Ge NWs are directly synthesized onto metal current collector substrates for use as Li-ion battery anodes; this method, when compared with conventional Li-on batteries, has the advantage of also being easier to fabricate. Fabrication of the traditional battery anode, shown schematically in Fig. 2a, often involves mixing of the active material with conducting carbon and a nonconducting polymeric binder (such as polyvinylidene fluoride) and then casting it onto the current collector as a slurry followed by annealing for several hours.  In comparison, this type of NW anode design (Fig. 2b) is fabricated in one step during the NW synthesis without any post-growth processing needed. This fabrication of the NW anode design has several advantages linked to the overall charge capacity. First, there is adequate electrical contact between the current collector and every NW so that more active material can contribute to the capacity. Furthermore, no binders or conducting carbon are needed, which add extra weight and lower the overall specific capacity of the battery.  As a result, GeNW anodes have a high specific capacity and excellent cycling performance. To summarize, this type of GeNW anode design is also easier to fabricate and has substantial electronic contact between each NW and the current collector.  Thus, GeNWs are a promising, higher-capacity alternative for the existing graphite anode in Li ion batteries in higher power applications.
The crystalline-amorphous core-shell structure, developed as Carbon-Silicon core-shell in this case, further improves the high power performance and the cycle life of Li-ion batteries. These C-Si core-shell NWs are synthesized by chemical vapor deposition (CVD) of amorphous Si (a-Si) onto carbon nanofibers (CNFs) as illustrated in Fig. 3 below. 
|Fig. 3: Illustration of Si coating onto carbon nanofibers. |
Not only do these types of NWs allow for high-power and long-life anodes, these CNFs are commercially available in a large quantity; this allows for the mass production of C-Si core-shell NWs. In addition, since carbon has a much smaller capacity compared to silicon, the carbon experiences less structural stress or damage during lithium cycling and can function both as a mechanical support and an efficient electron conducting pathway.  These types of nanowires have a high charge storage capacity of 2000 mAh/g as well as a good cycling life. They also have a high Coulombic efficiency of 90% for the first cycle and 98-99.6% for the subsequent cycles. 
High capacity, high charging rate and long lifetime lithium ion batteries can be used in many different types of applications, ranging from household appliances all the way to electric vehicles. Currently, though, the limits in the performance of Li-ion batteries are being reached due to the use of the most conventional anode material, graphite. Nanostructured materials such as Si, Ge, and C-Si core-shell NWs have great potential to increase energy and power densities, charging rate and structure integrity. Through the control of these properties, the charging and discharging cycle life of Li-ion batteries can be increased. As a result, the lithium ion battery charge storage will increase while still maintaining a longer battery life.
© Soham Chowdhury. The author grants permission to copy, distribute and display this work in unaltered form, with attribution to the author, for noncommerical purposes only. All other rights, including commerical rights, are reserved to the author.
 C. K. Chan et al., "High-Performance Lithium Battery Anodes Using Silicon Nanowires," Nature Nanotechnol. 3, 31 (2008).
 C. K. Chan, X. F. Zhang, and Yi Cui, "High Capacity Li Ion Battery Anodes Using Ge Nanowires," Nano Lett. 8, 307 (2008).
 L.-F. Cui et al., "Carbon-Silicon Core-Shell Nanowires as High Capacity Electrode for Lithium Ion Batteries," Nano Letters, 9, 3370 (2009).