|Schematic illustration for the structure change of hollow porous SiO2 nanocubes during the discharge/charge process. Yan et al. Click to enlarge.|
A team at the University of Science and Technology of China (Hefei) has developed hollow porous SiO2 (silicon dioxide, or silica—e.g., sand, or quartz) nanocubes as an anode material for Li-ion batteries (LIBs). In a paper in Scientific Reports, the Nature Publishing Group’s open access journal, the team reported that the nanocubes exhibited a reversible capacity of 919 mAhg−1 over 30 cycles.
They attributed the electrochemical performance to the unique hollow nanostructure with large volume interior and numerous crevices in the shell, which could accommodate the volume change and alleviate the structural strain during Li ions’ insertion and extraction, as well as allow rapid access of Li ions during charge/discharge cycling.
Silicon is known to have the highest theoretical specific capacity (4200 mAhg-1) and considered to be an optimal anode material for the next generation LIBs. However, the drastic volume variation (around 300%) during repeated insertion and extraction of lithium ions leads to its remarkable capacity fading. Some novel silicon-based nanomaterials such as nanowire, hollow nanoparticle, nanotube, Si-C nanocomposite and york-shell nanoparticle have shown improved cycling performance, however, they are usually prepared by a multi-step and advanced fabrication process, making the product costly.
As an alternative, silica (SiO2) has been considered to be the anode material of LIBs because of the analogous advantage of storing a large quantity of lithium and low discharge potentials. Besides, SiO2 is one of the most abundant materials on Earth and the major constituent of sand and therefore, the cost is cheaper than other metal-based materials.
In the past decades, silica is not generally considered to be electrochemically active for lithium storage until Gao et al.  reported that commercial SiO2 nanoparticles could react with Li between 0.0 and 1.0 V (vs. Li/Li+) with a reversible capacity of 400 mAhg-1. After that, some investigations on SiO2 materials with different structures have been reported for application as LIBs anodes, such as film, carbon-coated nanoparticles, hollow nanospheres and so on.
Although the theoretical specific capacity of SiO2 was calculated to be 1965 mAhg-1, the electrochemical performance was not obviously improved for the reasons of volume expansion effect and generating irreversible lithium silicate particles via the electrochemical reaction during cycling....the capacity of silica anode material has a large potential to be enhanced, which relies on the precise designing of nanostructures to achieve this unique functionality.—Yan et al.
The team prepared hollow porous SiO2 nanocubes (HPSNCs) with numerous crevices in the shell via a two-step hard-template process. To investigate the electrochemical performance of HPSNCs, they fabricated two-electrode 2032 coin cells with HPSNCs anodes with Li metal as the counter electrode.
|Galvanostatic discharge/charge voltage profiles of hollow porous SiO2 nanocubes at a rate of 100 mAg-1. Yan et al. Click to enlarge.|
They found that the discharge and charge capacities of the first cycle are 3084 and 1457 mAhg-1, respectively, with a low initial Coulombic efficiency of 47%. The discharge capacity of the second cycle is 1807 mAhg-1. They attributed the large irreversible capacity (1277 mAhg-1) to the formation of SEI layer and irreversible electrochemical reactions between lithium ions and SiO2. The electrochemical performance of HPSNCs subsequently becomes stable.
It is considered that the unique hollow porous nanostructure with numerous crevices in the shell can adapt to the volume expansion during cycling. Moreover, the structure could also reduce the diffusion path length of lithium ions and supply enough Li ions to react with SiO2, which is necessary for increasing the formation ratio of Li2O and Si and improving the performance of SiO2 electrodes.—Yan et al.
Nan Yan, Fang Wang, Hao Zhong, Yan Li, Yu Wang, Lin Hu & Qianwang Chen (2013) Hollow Porous SiO2 Nanocubes Towards High-performance Anodes for Lithium-ion Batteries. Scientific Reports 3, Article 1568 doi: 10.1038/srep01568