Fabrication Nano-Structured Snsb Alloy on the Polypyrrole Fibre and Its Application in Sodium-Ion Batteries

The sodium (Na)-ion battery has attracted widespread attention for energy-storage applications because Na⁺ is much cheaper and safer, and it is more environmentally benign compared to the conventional Li⁺ battery. Most research works on negative electrode for the Na⁺ battery are focused on hard carbon, which can deliver a capacity of about 300 mAh g^-1[1]. Recently, many metallic elements such as Sn, Sb, and Ge have been proved to be a viable alternative to hard carbon as negative electrode material in the sodium-ion battery due to their high theoretical capacity. For instance, the theoretical capacity of SnSb alloy is assumed to reach 753 mAh g^-1, based on 50 % Na₁₅Sn₄ (847 mAh g^-1) plus 50 % Na₃Sb (660 mAh g^-1) [2], which is almost two times higher than the best results obtained from carbon-based anode materials. Nevertheless, up to now, there have been very limited experimental studies on the SnSb alloy reaction for Na ion insertion. The SnSb nanoparticles are usually prepared by high-energy mechanical milling under an argon atmosphere, but this process may lead to common Sn and Sb impurities [3]. In this work, the SnSb nanoparticles are synthesized by a simple and fast one-pot method in aqueous solution. In addition, we also try fabricated SnSb nanoparticles on conductive polypyrrole (PPy) fibre and investigated their physico-chemical and electrochemical properties.

The X-ray diffraction patterns suggest that a pure, single-phase rhombohedral SnSb alloy was produced. High-magnification SEM images (Fig. 1a) reveal the average diameter of bare SnSb particles is only around 20-30 nm. SEM images of SnSb/PPy (Fig. 1b) show a rough surface with some small particles attached to the surface of the PPy fibre. As shown in Fig. 2,  the initial discharge capacity of SnSb gives an overall capacity of 620 mAh g^-1 at constant current of 100 mA g^-1 between 0.01 V and 1.5 V vs. Na⁺/Na. The first charge recovers a 550 mAh g^-1 capacity, with an initial coulombic efficiency of 88.7%. The discharge capacity decreases to 359 mAh g^-1 in fifteen cycles. In contrast, the discharge capacity of SnSb/PPy in the first cycle reaches 780 mAh g^-1, and after 15 cycles it still remains around 665 mAh g^-1. In order to explore the reasons for such high discharge capacity of the SnSb/PPy composite, electrochemical impedance spectroscopy (EIS) and morphological study of the electrodes were conducted. The results show that the PPy fibre in the composite can act as a conductive host matrix to prevent cracking and pulverization of the SnSb electrode due to phase transitions. At the same time, the introduction of PPy effectively enlarges the specific surface area of the material, which can effectively buffer the agglomeration of nano-SnSb particles.

[1]        Stevens, D. A.; Dahn, J. R. Journal of The Electrochemical Society 2000, 147, 1271.

[2]        Xiao, L.; Cao, Y.; Xiao, J.; Wang, W.; Kovarik, L.; Nie, Z.; Liu, J. Chemical Communications 2012, 48, 3321.

[3]       Darwiche, A.; Sougrati, M. T.; Fraisse, B.; Stievano, L.; Monconduit, L. Electrochemistry Communications 2013, 32, 18.