1. the carbon matrix offers mechanical support to accommodate the stress associated with the large volume change of nano-Sn when undergoing lithium insertion and extraction, thus alleviating pulverization;

2. the carbon matrix prevents Sn nanoparticle agglomeration upon prolonged cycling; and

3. carbon network provides continuous path for Li ions and electrons inside the nano-Sn/C composite spheres.

Tin anodes have attracted much attention because it delivers a capacity up to three times higher than that of graphite. Theoretically, one tin atom can maximally react with 4.4 lithium atoms to form Li_4.4Sn alloy, reaching a capacity of 993 mAh/g. However, the large amount of lithium insertion/extraction into/from Sn causes a large volume change (about 300%), which causes pulverization of tin particles and loss of contact with current collector, resulting in poor electrochemical performance.

Extensive efforts have been made to improve the electro- chemical behavior of Sn anodes. The most effective approaches include (1) reducing Sn particle size to nanoscale (<10 nm) to efficiently mitigate the absolute strain induced by the large volume change during lithiation/delithiation, and retard particle pulverization; (2) using nano-Sn with uniform particle size (narrow size distribution) to generate uniform stress/strain over the entire electrode during lithiation/delithiation, preventing local cracking; (3) uniformly dispersing nano-Sn in a conductive matrix (such as carbon) to accommodate volume change and maintain the mechanical integrity of the composite electrode. Clearly, a Sn/C composite with uniform tin nanoparticles (<10 nm) dispersed in a carbon matrix would be an ideal anode for Li-ion batteries.

...In this paper, we introduce aerosol spray pyrolysis to realize the ideal structure with nanograin Sn uniformly dispersed in a spherical conductive carbon matrix.

—Xu et al.

(a) Schematic diagram, (b) TEM image, and (c,d) high- resolution images of the nano-Sn/C composite particles. Insert: SAED image. Credit: ACS, Xu et al. Click to enlarge.

The new process overcomes the difficulties—due to the low melting point of tin and the tendency of grain grow—in creating well-dispersed ultra-small tin nanoparticles within a carbon matrix.

The key to the process is the rapid heating of precursor droplets, allowing the quick formation of tin nanograins and the carbon frame. The short residence time and rapid subsequent cooling enables the freezing of the structure to nonaggregated and uniformly sized nano-Sn grains in a carbon matrix.

The electrochemical performance is superior to the nano-Sn/C composite anodes synthesized using other techniques. Meanwhile, the synthesis of aerosol spray pyrolysis is a widely used technique for commercial nanomaterials and easily scaled up, making nano- Sn/C composite very promising and attractive as an anode material for lithium-ion batteries.

—Xu et al.

Resources

• Yunhua Xu, Qing Liu, Yujie Zhu, Yihang Liu, Alex Langrock, Michael R. Zachariah, and Chunsheng Wang (2013) Uniform Nano-Sn/C Composite Anodes for Lithium Ion Batteries Nano Letters doi: 10.1021/nl303823k