|SEM images of the 3D porous c-Si particles after etching. Click to enlarge. Credit: Angewandte Chemie|
A research team led by Dr. Jaephil Cho at Hanyang University in Korea has developed a new silicon material for lithium-ion battery anodes—three-dimensional porous bulk silicon particles—that can accommodate large strains without pulverization after 100 cycles and maintain a charge capacity of greater than 2,800 mAh g-1 at a rate of 1 C. A report on their work is a “hot paper” published online in the journal Angewandte Chemie International Edition.
The use of silicon as a next-generation high capacity anode material has attracted a great deal of research interest. (Earlier post, earlier post.) Silicon has a theoretical lithium capacity of approximately 4,200 mAh g-1 corresponding to Li4.4Si. This is 10-11 times greater than that of graphite (ca. 372 mAh g-1) and much larger than various nitride and oxide materials. However, practical application of silicon anode materials has been problematic.
Lithium-ion batteries produce current by moving lithium ions. While the battery is being charged, lithium ions migrate into the anode. When the battery is being discharged, these ions migrate back to the cathode. An anode material with significantly greater capacity than the commonly used graphite could thus significantly improve the capacity of the cells.
Although silicon offers that greater capacity, its volume changes by up to 400% upon the insertion and extraction of lithium ions during charge/discharge cycles (the alloying and de-alloying process to form LixSi and reform Si, respectively). This results in pulverization, in turn resulting in electrically disconnected smaller particles and rapid capacity fading.
Numerous studies have tackled reducing this volume change by using a variety of approaches, including developing composites with an inactive carbon phase and the development of silicon nanowires.
Cho, for example, also published a paper in October in the ACS journal Nanoletters (Kim 2008b) reporting on the development of mesoporous Si@carbon core-shell nanowires for use as an anode material in Li-ion batteries. This effort resulted in an initial capacity of 3,163 mAh g-1 and capacity retention after 80 cycles of 87%. Other nanowire efforts have returned somewhat similar results in terms of capacity, but have been hampered by capacity retention.
Cho’s team has now developed a new method for the production of a porous silicon anode that can withstand the strain. They annealed silicon dioxide nanoparticles with silicon particles whose outermost silicon atoms have short hydrocarbon chains attached to them at 900° C under an argon atmosphere. The silicon dioxide particles were removed from the resulting mass by etching. What remained were carbon-coated silicon crystals in a continuous, three-dimensional, highly porous structure.
Anodes made of this highly porous silicon have a high charge capacity for lithium ions. In addition, the lithium ions are rapidly transported and stored, making rapid charging and discharging possible. A high specific capacity is also attained with high current. The changes in volume that occur upon charging and discharging cause only a small degree of swelling and shrinking of the pore walls, which have a thickness of less than 70 nm.
In addition, the first charging cycle results in an amorphous (noncrystalline) silicon mass around residual nanocrystals in the pore walls. Consequently, even after 100 cycles, the stress in the pore wall is not noticeable in the material.
The present work demonstrates that 3D, porous Si particles that consist of bulk sizes greater than 20 µm can be prepared by simple thermal annealing of SiO2 and butyl-capped Si particles at 900° C under an Ar stream. Since this method does not require the use of a sealed ampoule, the reduction is easy to scale up. These particles facilitate faster transport and better intercalation kinetics of lithium ions; the ordered arrangement guarantees that a rapid charge–discharge process can be completed in a very short time, which results in a high specific capacity even with a high charge–discharge current.—Kim et al. (2008b)
Hyunjung Kim, Byunghee Han, Jaebum Choo, Jaephil Cho (2008a) Three-Dimensional Porous Silicon Particles for Use in High-Performance Lithium Secondary Batteries. Angew. Chem. Int. Ed. 47, 1 doi: 10.1002/anie.200804355
Hyesun Kim and Jaephil Cho (2008b) Superior Lithium Electroactive Mesoporous Si@Carbon Core-Shell Nanowires for Lithium Battery Anode Material. Nanoletters Vol. 8, No. 11 3688-3691 doi: 10.1021/nl801853x