|Rate capability (top) and cycle life performance (bottom) of the Si nanotubes anodes in pouch-type Li-ion cells between 2.75 and 4.3 V to 200 cycles. C rate for the cycle test was 1C. Credit, ACS, Park et al. 2009. Click to enlarge.
Researchers from South Korea and Stanford University, led by Dr. Jaephil Cho at Ulsan National Institute of Science & Technology (UNIST) and Dr. Yi Cui at Stanford, have developed another approach to silicon-based anodes for Li-ion batteries that show promise for capacity and efficiency retention over cycling.
Prepared by reductive decomposition of a silicon precursor in an alumina template and etching, the Si nanotubes show a very high reversible charge capacity of 3,247 mAh/g with Coulombic efficiency of 89%, and also demonstrate a superior capacity retention even at a 5C rate (=15 A/g). A Li-ion full cell consisting of a LiCoO2 cathode and Si nanotube anode demonstrated 10 times higher capacity than commercially available cells with graphite anodes even after 200 cycles.
A paper on the work was published online 11 September in the ACS journal Nano Letters. In addition to Drs. Cho and Cui, authors included Mi-Hee Park of UNIST; Min Gyu Kim of Pohang Accelerator Laboratory; Jaebum Joo of Hanyang University; and Kitae Kim, Jeyoung Kim and Soonho Ahn of Battery R&D, LG Chem, Ltd.
|USABC Cycle Life Goals for EV and PHEV Batteries
Silicon is a very promising material for use as an anode material in Li-ion batteries, due to its potential for increasing the capacity of the resulting cells. The obstacle researchers worldwide have been working to clear, however, is the rapid loss of reversible capacity upon cycling of the Si-anode batteries, which is associated with the large volume expansion of the Si anode. Researchers have tried a variety of approaches, some of them showing promise, including the use of Si nanowires and nanoporous silicon. Both Dr. Cho (earlier post) and Dr. Cui (earlier post) have published recent efforts in this area.
When reacting with Li, Si is known to incorporate 4.4 Li atoms per Si atom. In Li-ion batteries, this results in the extremely high specific capacity of 4200 mAh/g, which is 10 times higher than the capacity of graphitic carbon (372 mAh/g). However, the 300% volume change upon lithium insertion commonly causes pulverization and thus a loss of electrical contact between Si and the current collector.—Park et al. 2009
However, the researchers noted, even some of the promising approaches showing improved capacity still suffer loss of capacity and efficiency after cycling.
In the current approach described in the Nano Letters paper, the researchers fabricated novel Si nanotube structures to increase the surface area accessible to the electrolyte, which allows the Li ions to intercalate at the interior and exterior of the nanotubes. In addition, they deposited a carbon coating on the surface of the nanotubes, which stabilized the Si-electrolyte interface and promoted stable SEI formation for long cycle life.
This work was supported by the World Class University (WCU) program supported by National Research Foundation (NRF) and Ministry of Education, Science and Technology (MEST) of Korea.