Porous Sulfur/Polypyrrole/Multiwalled Carbon Nanotube Ternary Composite Cathode for Lithium/Sulfur Batteries

Introduction

      Lithium batteries have the highest energy density among the commercialized secondary batteries and they are leading the path for energy storage and sources for portable applications, including electric vehicle transportation. Commercial lithium batteries are based on oxide cathodes such as LiCoO₂, LiFePO₄ and LiMn₂O₄, which can deliver only up to 200 mAh g^-1 of capacity, limiting their use for energy demanding applications. The elemental sulfur could be a best choice for the cathode due its theoretical capacity of 1672 mAh g^-1, which is one of the highest among all known cathode materials. Combined with its abundant availability, this makes sulfur a most promising cathode for lithium batteries. However, this cathode material suffers from its low conductivity and capacity fading due to the dissolution of electrochemical reaction products [1, 2].

       In this work, we report on the preparation of a novel hierarchical porous sulfur/polypyrrole/multiwalled carbon nanotube composite (S/PPy/MWNT), and on its physical and electrochemical properties as a cathode for lithium secondary batteries. 

Experimental

         The S/PPy/MWNT composite preparation is schematically presented in Fig.1. The porous S/PPy/MWNT composite was synthesized via in situpolymerization of pyrrole monomer with nano-sulfur and MWNT aqueous suspension followed by a low temperature heat-treatment.

      The S/PPy/MWNT composite was characterized using chemical analysis, XRD, SEM and HRTEM. The composite cathode electrode was prepared by mixing S/PPy/MWNT composite with conductive carbon and PVdF binder and coating this mixture on Al foil. The electrochemical properties of the composite cathode were investigated in a lithium half cell (coin-type CR2032) by cyclic voltammetry (CV) and galvanostatic cycling. 

Results and Discussion

       Figure 2a present that SEM result, which reveal that a porous structure with interconnected spherical pores has been formed. While the pore sizes are about 500-1000 nm in diameter, the holes with the diameters of about 20-30 nm connect them with each other in the sample structure. This S/PPy/MWNT composite with hierarchical nano/microstructures can utilize the advantages of both nanometer-sized building blocks and micro- or submicrometer-sized assemblies. While the former could provide negligible lithium ion diffusion time and distances, and possibly a new Li storage mechanism for the favorable kinetics and high capacities, the latter could be beneficial to afford good stability of the electrode materials, which are important for enhancing the cycle performance of the electrodes. The elemental analysis (mapping) of the S/PPy/MWNT composite carried out by EDS is shown in Fig. 2(b and c), demonstrating the existence and homogeneous distribution of  sulfur and MWNT in the composite.

      Further development of this research will be presented at the Meeting.

References

1. X. Ji, K.T. Lee, Linda F. Nazar, Nat. Mater., 2009, 8, 500.

2. X. He, W. Pu, J. Ren, L. Wang, J. Wang, C. Jiang, C. Wan, Electrochim. Acta, 2007, 52, 7372.

Acknowledgements

The authors are grateful of financial support by the National Natural Science Foundation of China (Grant No. 21406052) and financial support by Program for the Outstanding Young Talents of Hebei Province (Grant No. BJ2014010).