This is an update to a post that in December 2007 noted the potential of silicon nanowires. Green Car Congress relays a report from an international team of researchers. As Cui fan boys know, such nanostructured silicon arrays are capable of accommodating the large volume changes associated with lithiation in Li-ion battery applications.
A professor of material science and engineering at Stanford University, Yi Cui developed a silicon nanowire material for use in battery electrodes. The beauty of the tiny wire bundles is that they have exponentially more surface area than a conventional flat surface electrode. That allows the electrodes to absorb and release far more electrons for greater energy density.
Researchers from Samsung, Hanyang University, University of Illinois at Urbana-Champaign, Northwestern University and Tsinghua University report on how they have been able to fabricate arrays of sealed, tubular nanowires. They also claim that robust performance in silicon-based, lithium ion batteries is possible by structuring the silicon into arrays of nano-tubes.
As has been noted many times, silicon is a promising candidate for electrodes in lithium ion batteries due to its large theoretical energy density of about 4,200 mAhg-1—10x higher than graphite (372 mAhg-1)—and relatively low working potential (~0.5 V vs Li/Li+). However, the pulverization of silicon caused by volume expansion with lithium ion insertion results in rapid capacity fading with cycling.
A number of different nanophase forms of silicon show promise in addressing that problem, including nanocrystals; nanocomposites with either carbon or other phases inactive to lithium; nanoporous materials; nanowires; bundled Si nanotubes; and thin films.
In their study, Song et al. used arrays of Si nanotubes to comprise anode material. the purely silicon anodes exhibit high initial Coulombic efficiencies (i.e., >85%) and stable capacity-retention (>80% after 50 cycles). The process, performed on a stainless steel substrate, involves three steps:
- Growth of arrays of dense ZnO nanorods as a sacrificial template
using a hydrothermal process;
- Chemical vapor deposition (CVD) of Si onto these nanorods; and
- Selective removal of ZnO via a high temperature reduction process.
The Si NT arrays showed stable performance over prolonged cycles without restricted voltage ranges to limit the extent of lithiation. The researchers found that the capacity retention of their array after 50 cycles is ~81 ±0.5% and 82 ±2% at a rate of 0.05 and 0.2 C, respectively, significantly better than the corresponding Si NWs system (~62%). The team suggests that the void spaces in the Si NTs improve the mechanics associated with cycling.
Sealed nanotube (NT) layouts, in particular, can be expected to improve electrochemical performance and reversible morphological changes via more favorable mechanics during lithiation. In this paper we demonstrate that this intuition is accurate, as supported by detailed experimental results and theoretical analysis of the mechanics.
Despite tremendous stresses and volume changes associated with intercalation of lithium, Si NT arrays exhibit robust performances such as stable cycle retention and reversible morphology change. In particular, the axial void spaces of the Si NTs provide additional free surfaces to benefit the mechanics, without creating additional surfaces for accelerated SEI formation. These results suggest a general capacity to use nanostructures with theoretically guided layouts to improve the performance of anodes in lithium ion batteries.
The key contributions presented in the following include (1) the optimized use of nanostructure geometry for silicon anodes, in the form of organized arrays of nanotubes, (2) the experimental demonstration of performance in such systems that exceeds that of other reported approaches for using silicon, and (3) theoretical analysis of the mechanics,
to reveal aspects of the underlying physics, to account for experimental observations and to derive optimized dimensions in the tubes.
—Song et al.
GCC Recommended Resource
Taeseup Song, Jianliang Xia, Jin-Hyon Lee, Dong Hyun Lee, Moon-Seok Kwon, Jae-Man Choi, Jian Wu, Seok Kwang Doo, Hyuk Chang, Won Il Park, Dong Sik Zang, Hansu Kim, Yonggang Huang, Keh-Chih Hwang, John A. Rogers and Ungyu Paik (2010) Arrays of Sealed Silicon Nanotubes As Anodes for Lithium Ion Batteries. Nano Lett., Article ASAP