|Fig. 1: Schematic of lithiation of double-walled Si-SiOx nanotube anode. (Source: J. Zhao)|
The emerging markets of electric vehicles have stimulated intensive research on low-cost lithium-ion (Li-ion) batteries with high energy densities and long cycle life.  Conventional anodes, primarily based on synthetic graphites with a theoretical capacity of 370 mAh/g, do not completely fulfill the demand.  Silicon (Si) anode materials with an unparalleled theoretical capacity of 4200 mAh/g, on the other hand, have been widely recognized as a promising alternative for high energy density applications. Nevertheless, Si anode is still far from commercialization. Despite its high capacity, Si anodes undergo significant volume expansion and contraction during Li insertion/extraction. This volume change (300% for Si) can result in pulverization of the initial particle morphology and causes the loss of electrical contact between active materials and the electrode framework. Therefore, various nanostructures have already explored to minimize the effect of large volume change.
The double-walled Si-SiOx nanotube anode (an active silicon nanotube surrounded by an ion-permeable silicon oxide shell) have demonstrated excellent electrochemical performance, cycling more than 6000 times without obvious capacity decay.  Fig. 1 shows TEM image of Double-walled Si-SiOx nanotube anode, indicating the uniform hollow structure with smooth tube walls.The outer surface of the silicon nanotube is prevented from expansion by the oxide shell, and the expanding inner surface is not exposed to the electrolyte, resulting in a stable electrode surface.
|Fig. 2: Schematic of lithiation of an individual Si-C yolk-shell nanoparticles. (Source: J. Zhao)|
Fig. 2 indicates commercially available Si nanoparticles are completely sealed inside conformal, thin, self-supporting carbon shells, with rationally designed void space in between the particles and the shell.  Rationally designed void space in between the shell and the particles allows for the expansion of Si without deforming the carbon shell or disrupting the structure of the whole electrodes. This structure shows excellent capacity and long cycle life.
Si) anode materials with an unparalleled theoretical capacity have been widely recognized as a promising alternative to realize high energy density lithium ion batteries. Different nanostructures have already deployed to address to the problems arising from the large volume change. To realize the commercialization of Si anodes, large scale synthesis should be realized and the cost should be further decreased.
© Jie Zhao. The author grants permission to copy, distribute and display this work in unaltered form, with attribution to the author, for noncommercial purposes only. All other rights, including commercial rights, are reserved to the author.
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