Researchers at the University of Texas at Austin have developed a non-woven fabric of silicon nanowires; the material has the look and feel of tissue paper, yet is composed entirely of crystalline silicon (Si).
Thermal annealing of the nanowire fabric in a reducing environment results in good performance of the resulting material as an anode in a Li-ion battery without the addition of conductive carbon or binder. In a paper published in the Journal of the American Chemical Society, the team reports achieving anode capacities of more than 800 mAh g–1.
The work represents another approach to devising a commercially viable solution for practically applying silicon, with its much higher theoretical storage capacity than the graphitic anode materials commonly used, in Li-ion batteries (LIB).
The LIB capacity is limited in part by the intercalation of Li+ by the graphitic anode material; thus, higher capacity batteries require anode materials that can accommodate more Li+. The theoretical capacity of the graphite anode is 372 mAh g-1. Lithium-alloying materials have the potential for significantly higher storage capacities. For instance, Si alloys with Li+ at room temperature to form Li15Si4, which corresponds to a substantially higher theoretical capacity of 3579 mAh g-1. However, Si undergoes an enormous volume expansion of nearly 300% when fully lithiated.
Bulk crystalline Si cannot tolerate the stresses associated with these lithiation/delithiation cycles and crumbles, resulting in battery failure.
On the other hand, Si nanostructures have been found to tolerate extreme changes in volume with cycling. Thin Si films have realized capacities above 2000 mAh g-1. Si nanoparticles have also been explored, but with overall specific capacities that have been limited by the need for a conductive carbon matrix to ensure electrical contact with the electrode. Si nanowires have been promising. For example, Cui and co-workers achieved a capacity of 2725 mAh g-1 with very good stability, retaining more than 1400 mAh g-1 after 700 cycles using an interconnected amorphous Si hollow nanosphere thin-ﬁlm electrode. Unfortunately, these electrode materials are very expensive at present and cannot be produced in the significant quantities needed for commercial LIB applications.
Solvent-based processes for nanowire synthesis, such as supercritical-fluid-liquid-solid (SFLS) and solution-liquid-solid (SLS) growth, can produce large amounts of Si nanowires (SiNWs) at relatively low cost...Here we demonstrate the creation of a Si nanowire fabric and show that it can function as a standalone anode material without the need for additional conductive fillers (activated carbon) or polymeric binders.—Chockla et al.
The nanowires, made by the supercritical-fluid–liquid–solid process, are crystalline, range in diameter from 10 to 50 nm with an average length of >100 μm, and are coated with a thin chemisorbed polyphenylsilane shell. About 90% of the nanowire fabric volume is void space.
The findings show that the surface composition of Si nanowires plays a critical role in their effectiveness as an anode material in an LIB, the researchers conclude.
Future research needs to focus on understanding the cycling durability and the factors that limit performance, including the formation and stability of the solidelectrolyte interface layer, they suggest.
Another issue is that the since the nanowire fabric has 90% void volume, the volumetric capacity of the nanowire fabric is relatively low. The specific capacity of 800 mAh g-1 corresponds to 186 mAh cm-3, which is significantly lower than the volumetric capacity of 777–867 mAh cm-3 for graphite.However, they suggest, it should be possible to increase the volumetric capacities of the nanowire fabric by densifying the films with pressure. They estimate that the maximum volumetric capacity of a densified Si nanowire fabric would be 1864 mAh cm-3—more than a factor of 2 higher than that of graphite. However, the influence of volumetric expansion and contraction upon lithiation and delithiation on cycling stability would still need to be understood.
Aaron M. Chockla, Justin T. Harris, Vahid A. Akhavan, Timothy D. Bogart, Vincent C. Holmberg, Chet Steinhagen, C. Buddie Mullins, Keith J. Stevenson, and Brian A. Korgel (2011) Silicon Nanowire Fabric as a Lithium Ion Battery Electrode Material. Journal of the American Chemical Society doi: 10.1021/ja208232h