The current commercial anode material, graphite has low energy density (375 mAh/g) and safety problems due to lithium deposition^[1]. Alloy anodes (Li_xSn or Li_xSi) are found to be interesting in terms of high energy-density, enhanced safety and long cycle life. The theoretical specific capacities of alloy anodes are 2-10 times higher than that of graphite and 4-20 times higher than that of lithium titanium oxide (Li₄Ti₅O₁₂). Though the cell potential of alloy anode materials (electrode potential is 0.3-0.4 V vs. Li/Li+) is lower than that of graphite (electrode potential is 0.05V vs. Li/Li+), the alloy anodes are safer than graphite anodes due to the high reactivity of graphite which has much lower standard reduction potential.  However, this moderate operation voltage is much better than that of lithium titanate anodes (1.5V vs. Li/Li+)^[2]. In essence, these alloy based anodes are eligible candidates in terms of high energy density and safety. However, the main hurdle faced in commercializing these alloy anodes is the huge volume variation (up to 300%) during lithium insertion and extraction, which leads to the rupture of the active alloy particles and poor cycling, caused by the poor electrical contact^[3]. In order to overcome the poor contact in the electrode due to the pulverization, we apply a yolk-shell nano-rattle structure of Sn/C . The carbon shell will act as a cushion to avoid the pulverization of Sn nanoparticles due to volume expansion after several cycles of lithium insertion and extraction. The conductive and fair mechanically strong carbon shell will further prevent the aggregation of Sn particles and contribute to enhanced electronic conductivity.

Obtaining nano-rattle setup:

To attain this, a reverse micelle formation method, previously formulated in our group has been applied to form the first step of forming yolk-shell type Sn/SiO₂^[4]. We obtained Sn nanoparticles encapsulated onto SiO₂ shells. The SiO₂ has been used as the substrate for the carbon shell formation. Accordingly, the carbon is formed by hydrothermal synthesis and the substrate is removed finally to form the nano-rattle Sn/C with a void in between to accommodate the volume expansion.

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Conclusion:

Though the desired nano-rattles have been achieved, we are still concerned about the Sn nanoparticles that are scattered outside the carbon shell. We have to effectively fill the silica shell with Sn nanoparticles in the preliminary step of reverse micelle microemulsion. We are also looking forward to measure high resolution TEM pictures to have much clear idea about the metal core nanoparticles. In addition, the composite material without silica will be fabricated as an electrode to check the electrochemical performance in The near future. Further, ICP, XPS measurements will be done in future to elucidate the exact Sn material in the composite. Once the system is successful with the lithium system, the same will be applied to Na ion batteries.

References:

[1] M. Winter, J. O. Besenhard, M. E. Spahr and P. Novák, Advanced Materials 1998, 10, 725-763.

[2] W.-J. Zhang, Journal of Power Sources 2011, 196, 13-24.

[3] M. Winter and J. O. Besenhard, Electrochimica Acta 1999, 45, 31-50.

[4] M. Priebe and K. M. Fromm, Particle & Particle Systems Characterization 2014, 31, 645-651.