Electrospun Mns/Porous Carbon Fibers: A Novel Anode Materials for Sodium-Ion Batteries
Authors: Chen Kongyao , Zhang Wuxing , Huang Yunhui 
Published: February 2020

Metal sulfides have attracted much attention as anode for sodium-ion batteries (SIBs) due to their high specific capacity and low price. Among all the sulfides, MnS has shown a high theory capacity of 610 mAh g-1 in SIBs1. However, the sodium storage performance of MnS was rather poor because of its poor conductivity and drastic volume changes during their conversion reaction process. In order to solve this problem, it’s essential to design the MnS material with both electron transportation pathway and solid supporting matrix.
Herein, a MnS/porous carbon fibers (MnS/PCFs) composite was synthesized by the electrospun method. The MnS/PCFs composite has several advantages. Firstly, MnS nanoparitcles were embedded in porous carbon fibers, which can suppress the volume change and materials exfoliation. Secondly, abundant interconnected pores facilitate the electrolyte infiltration and ion transport. Furthermore, 1D nanofibers can form a 3D interconnected conductive network and reduce the resistance. Remarkably, the MnS/PCFs composite exhibits high specific capacity (523.6 mAh g-1), long cycling life (78.4% for 760 cycles) and excellent rate capability (301.4 mAh g-1 at 2 A g-1).
MnS/PCFs with different carbon contents were synthesized via a simple electrospinning method. Firstly, manganese acetate, polyacrylonitrile and polymethyl methacrylate were mixed in DMF and electrospuned. Then the as-spun fibers were carbonized and vulcanized to obtain the MnS/PCFs composites.
The sodium performance of MnS/PCFs with different carbon content is systematically investigated, and the MnS/PCFs-0.8 sample (31.3 wt% carbon) exhibits the best cycling and rate properties, delivering a specific capacity of 523.6 mA h g-1 at 100 mA g-1. After 700 cycles, the capacity still remains 78.4%. At a high current of 2 A g-1, MnS/PCFs-0.8 exhibit the highest capacity of 301.4 mA h g-1. The SEM and TEM images of MnS/PCFs-0.8 proved the uniform distribution of MnS nanoparticles in porous carbon.
The CV curves of MnS/PCFs are shown in Figure 1e. The reduction peaks at 1.2 V may assign to the Na+ insertion into the MnS, and the peaks at 0.6 V corresponds to the conversion reactions to form Na2S and Mn. The oxidation peak at 0.9 V is attributed to the conversion reaction of Na2S and Mn, and the peaks as 1.7 V and 1.95 V indicate a multistep extraction process of Na+from MnS. Detailed electrochemical process are also detected by other test.
In conclusion, MnS/PCFs were prepared by an electrospinning method. The hollow nanofiber structure could not only suppress the volume change of MnS, but also facilitate electrolyte infiltration and ion transport. Furthermore, 3D interconnected conductive network will reduce the resistance and enhance the rate capability of MnS. Remarkably, the MnS/PCFs composite with proper carbon content exhibits high specific capacity (523.6 mAh g-1), long cycle life (78.4% for 760 cycles) and excellent rate capability (301.4 mAh g-1 at 2 A g-1).

References

1 Y. Liu, Y. Qiao, W. X. Zhang, Z. Li, X. L. Hu, L. X. Yuan and Y. H. Huang, J. Mater. Chem., 2012, 22, 24026-24033.
2 X. J. Xu, S. M. Ji, M. Z. Gu and J. Liu, ACS Appl. Mat. Interfaces, 2015, 7, 20957-20964.

Figure 1