Lithium-ion batteries (LIBs) have dominated the energy storage for the past 20 years [1]. However, due to the limited abundance and uneven distribution of lithium deposits, the cost will be a major issue for the LIBs to take a further application. As an alternative to LIBs, sodium-ion batteries (SIBs) have gained a considerable attention in view of their greater natural abundance and lower cost of sodium compared with lithium [2]. Extensive researches have been conducted to explore suitable electrode materials with high rate capacity and good cycling stability. Unfortunately, the larger radius of Na⁺ (102 pm) compared to that of Li⁺ (76 pm) leads to a series of severe problems for the electrodes of NIBs, such as large volume changes, structural pulverization and unstable solid electrolyte interphase (SEI) layer, which consequently gives rise to electrical contact loss and eventually results in poor reversibility, decreased rate capacity and low columbic efficiency. Substantial efforts have been made to design anode materials for NIBs [3-4]. Among the anode material candidates, Sb-based materials have attracted increasing attention and have been investigated as anode materials for SIBs in recent years due to their high theoretical capacities. Antimony has a theoretical capacity of 660 mAh·g^-1  upon full sodiation to Na₃Sb.Considering the additional capacity contribution from the conversion reaction, antimony oxides and sulfide as anode material for SIBs has been reported, however, the columbic efficiency, rate capability and cycling stability results are still unsatisfactory. Thus, it is important to develop new antimony based anode materials to enhance the sodium storage capability.

Herein, we designed a new method for the facile synthesis of novel Sb₂S₃/Sb-C nanostructure for the first time, in which uniform Sb and Sb₂S₃ nanoparticles are distributed onto the porous carbon matrix. Such architecture synergistically combines the advantages of a porous and interconnected network and of smaller sized nanoparticles. Firstly, good dispersion of Sb and Sb₂S₃ nanoparticles(less than 30nm in diameters) onto the one-dimensional (1D) carbon NWs as well as confinement in the carbon shell avoids the aggregation of Sb and Sb₂S₃ NPs and retains the structural stability during repeated cycling. Secondly, the carbon shell is helpful to enhance the electrode conductivity and buffer the volume change. Lastly, the overall porous structure contributes to large surface area for the better electrode and electrolyte contact, giving rise to substantially improved electrochemical reactions. When evaluated as anode material for sodium-ion battery, the reversible capacity of the Sb₂S₃/Sb-C electrode keeps very stable even cycled after 100 cycles, displaying a good capacity retention. Significantly, the Sb₂S₃/Sb-C electrode still delivers an obviously improved reversible capacity as high as 491mAh·g^-1 at 3A·g^-1, representing the 63% utilization of the theoretical value of 774 mAh·g^-1, exhibiting an extraordinary rate capability. Particularly, when the current decreases to 100 mA·g^-1 after 70 cycles, the capacity recovers completely to 613mAh·g^-1. As far as we know, the electrochemical properties of Sb₂S₃/Sb-C are more distinguished than the most Sb-based anode materials reported previously. More importantly, by tuning the cut-off voltage to the range of 0.05-1.0V, the composites show an excellent rate capability with enhanced cycling performance (a capacity nearly 227 mAh·g^-1 over 1000 cycles with an exceptional columbic efficiency of approaching 100% at a current density of 5A·g^-1). Moreover, an excellent reversible capacity (220 mAh·g^-1) is achieved at high rate (10 A·g^-1). It is worthwhile to note that when the current is switched from 10 A·g^-1 to 0.1 A·g^-1, the reversible capacity rebound successfully to the initial capacity at 0.1 A·g^-1, and then, a rather high reversible capacity of 320 mAh·g^-1 over 200 cycles can still be restored at a current density of 1A·g^-1, demonstrating the structure integrity of electrodes. As we know, such rate performances of Sb₂S₃/Sb-C outperform the Sb-based anodes for Na-ion batteries published to date, indicating its superiority in commercial applications.

References

1.   Armand, M.; Tarascon, J. M., Building better batteries. Nature 2008, 451 (7179), 652-657.

2.   Kim, S.-W.; Seo, D.-H.; Ma, X.; Ceder, G.; Kang, K., Electrode Materials for Rechargeable Sodium-Ion Batteries: Potential Alternatives to Current Lithium-Ion Batteries. Advanced Energy Materials 2012, 2 (7), 710-721.

3.   Qian, J.; Xiong, Y.; Cao, Y.; Ai, X.; Yang, H., Synergistic Na-Storage Reactions in Sn4P3 as a High-Capacity, Cycle-stable Anode of Na-Ion Batteries. Nano Letters 2014, 14 (4), 1865-1869.

4. Zhu, Z.; Cheng, F.; Hu, Z.; Niu, Z.; Chen, J., Highly stable and ultrafast electrode reaction of graphite for sodium ion batteries. Journal of Power Sources 2015, 293, 626-634.