Tin phosphides (SnP_x) has been considered as one of the most promising anode materials for sodium-ion batteries (SIBs) due to the high theoretical volumetric and gravimetric capacities. Nevertheless, SnP_x has a low electrical conductivity and undergoes the severe pulverization resulted from a large volume change during sodiation/desodiation process, which leads to rapid decline in the performance. These problems can be solved by tuning nanostructure of the composite materials composed of SnP_x and conductive carbon materials. On the other hand, SnP_x has three different forms depending on phosphorus (P) content, Sn₄P₃, SnP, and SnP₃, the higher the P content provide the higher theoretical capacity because the theoretical capacity of P is about three times higher than metallic tin. However, only a method using ball milling has been reported for the synthesis of SnP_x having a high P content, and the nanostructure can’t be precisely controlled by this method [1]. Additionally, solvothermal method between metallic tin and red P allows to prepare thenanostructured SnP_x/C composite, however, it gives only Sn₄P₃ phase which has the lowest P content [2].

In this work, to suggest the appropriate nanostructure and P content to obtain the high reversible capacity of SnP_x as well as improved rate capability in SIBs, we synthesized hybrid nanocomposites composed of size-controlled SnP_x (x = 3/4, 1, and 3) nanoparticles and low-dimensional carbon material by a facile direct reaction method between liquid phase tin and gas phase P. Through this method, we could control the Sn/P composition by regulating of cooling rate after reaction and P partial pressure in a reactor, which is confirmed by XRD analysis of Fig. 1(a). Using the three types of as-prepared SnP_x/C composites with different P contents and nanostructures., we investigated their electrochemical properties for sodium ion battery anodes The obtained SnP_x/C nanocomposite with x=3/4, 1, and 2 showed 500, 610, and 810 mAh g^-1 of reversible capacities, respectively, as anode active materials of SIBs. Although SnP_x with the higher the P content gave the enhanced reversible capacity, the poor rate capability and cycle durability were obtained presumably due to the increased internal resistance components and volume expansion ratio. For this reason, SnP_x/C anode requires the empty space, where the volume expansion of sodiated SnP_x can be accommodated without pulverization. Specifically, by preparing the Yolk-shell structured SnP/C (Y-SnP/C) composite (Fig. 1(b)), we achieved 3 times larger capacity retention at 3.2 A g^-1 of current density as shown in Fig. 1(c).

References

[1] X. Fan, J. Mao, Y. Zhu, C. Luo, L. Suo, T. Gao, F. Han, S-C. Liou, and C. Wang, Adv. Energy Mater., 2015, 5, 1500174

[2] J. Liu, P. Kopold, C. Wu, P. A. van Aken, J. Maier, and Y. Yu, Energy Environ. Sci., 2015, 8, 3531-3538