Silicon is among the most promising building blocks for negative electrode active materials used in the fabrication of lithium ion batteries. It has been identified that silicon has a theoretical capacity of approximately 3578 mAh/g and this is much more than 372 mAh/g for graphite currently applied in standard lithium ion batteries. Silicon powder prepared by expensive approaches including laser ablation as well as plasma-enhanced chemical vapor deposition has been evaluated to mitigate the stress of silicon caused by the change in size in the course of delithiation and lithiation.
Despite the outstanding attributes of silicon, silicon nanopowder still suffers several problems to use as lithium ion battery electrode active materials. Silicon nanopowder experiences volume expansion of about four times during lithiation, and considerable shrinkage during delithiation. This results in the formation of cracks in the nanopowder. Electrical isolation of the silicon nanopowder is another problem, which leads to an increase in internal resistance, low columbic efficiency and poor cyclability.
In a bid to suppress the volume change of the silicon powder, the lithiation capacity has been limited at 1500 mAh/g and 1200 mAh/g after considerable delithiation. Nevertheless, the impact of limitation of the delithiation capacity after deep lithiation on cyclability has not been studied.
Researchers led by Professor Taketoshi Matsumoto from Osaka University fabricated lithium ion battery half cells using silicon nanopowder generated from silicon swarf and investigated the impact of delithiation and lithiation capacity after deep lithiation at 0.01V on the performance of the cell. They found that the limitation of the delithiation capacity at 1500mAh/g improved the cyclability. Their work is published in Journal of The Electrochemical Society.
Silicon swarf was annealed in hydrogen atmosphere at 1000°C and later at 1000°C in ethylene environment to coat the silicon nanopowder with a 10nm carbon layer. The carbon coated silicon nanopowder was then mixed with polymer binders. The authors coated copper foil with the resulting slurry and the sample was dried where it was then packed as working electrode in a coin cell with a lithium foil counter electrode. The coin cell was also equipped with a polyethylene separator and an electrolyte.
The authors then cycled the cells in the cell voltage range of 0.01-1.5V in the course of 300 cycles using a battery charge-discharge unit. They set delithiation and lithiation current densities at 180mA/g for the first 5 cycles and 1800mA/g for the next 295 subsequent cycles.
The authors observed that limitation of delithiation capacity at 1500mAh/g resulted in the best cyclability. This capacity remained constant at 1500mAh/g until the 290th cycle, where it reduced slightly to 1480mAh/g at the 300th cycle. The overvoltage for delithiation-limited case was observed to be lower than that for lithiation-limited case. The low overvoltage as well as excellent cyclability was referenced to suppression of electrical isolation of silicon nanopowder owing to limited shrinkage of the silicon powder in the high lithium concentration zone.
Limitation of lithiation capacity at 1500mAh/g caused the delithiation capacity to remain at 1470mAh/g until the 137th cycle and then decreased to 860mAh/g at the 300th cycle. Electrical isolation, high overvoltage, and peeling-off of the silicon nanopowder resulted from low inter-particle contact reference to large size change of the silicon powder.
Katsuya Kimura, Taketoshi Matsumoto, Hirotomo Nishihara, Takatoshi Kasukabe, Takashi Kyotani, and Hikaru Kobayashi. Improvement of Cyclability of Li-Ion Batteries Using C-Coated Si Nanopowder Electrode Fabricated from Si Swarf with Limitation of Delithiation Capacity. Journal of the Electrochemical Society, 164 (6) A995-A1001 (2017).Go To Journal of the Electrochemical Society