Recent advances in nanocrystalline intermetallic tin compounds for the negative electrode of lithium ion batteries

Intermetallic compounds of tin and first-row transition metals have been considered as potential anode materials for Li-ion batteries that could improve the performance of pure tin. Particularly, the solids dispersed at the nano scale provide interesting behavior. Thus CoSn, FeSn2 and CoSn3 nanocrystalline samples have been obtained at low temperatures. As compared with micrometric particles of CoSn, significantly higher reversible capacities are found for nanocrystalline CoSn. For nanocrystalline CoSn3 maximum reversible capacities of 690 mAh g-1 were observed in lithium test cells. Nanocrystalline products in the series CoSn2-FeSn2 could be prepared by chemical reduction in polyol solvents using a "one-pot" method. Superparamagnetic nanocrystalline FeSn2 delivers reversible capacities of ca. 600 mAhg-1 by the formation of LixSn phases and superparamagnetic iron nanoparticles. A comparison between the properties of nano- FeSn2 and micro-FeSn2 shows a significantly better electrochemical behavior and electrode stability for the nanocrystalline material. For Fe1-xCoxSn2 solid solutions with x= 0.25, 0.3, 0.5, 0.6 and 0.8, particle diameters of about 20 nm and different morphologies were obtained. The substitution of iron by cobalt induces a contraction of the unit cell volume and the hyperfine parameters of the 57Fe Mössbauer spectra reveal a superparamagnetic behavior. The intermediate compositions exhibit better electrochemical performance than the limit compositions CoSn2 and FeSn2. To improve the performance of CoSnx intermetallics, composites in which the nanocrystalline intermetallic material is embedded in an amorphous layer based on the polyacrylonitrile (PAN) polymer were used. The PAN shell contributes to stabilize the intermetallic phases upon electrochemical cycling.