Enhanced Performance of Li-S Battery with Self-Assembled Polypyrrole Film As a Functional Interlayer

Lithium sulfur batteries are attracting increasing attentions in recent years due to their high theoretical capacity of 1675 mAh.g^-1, low cost, natural abundance and environmental friendliness. However, a rapid capacity fading hinders their practical application, which is attributed to the high insulating nature of sulfur, the dissolution of lithium polysulfides and volume expansion[1, 2]. In this work, as schematically illustrated in Fig.1a, a self-assembled polypyrrole (PPy) films composing of PPy nano-wires and PPy nano-tubes are sandwiched between sulfur cathode and separator of Li-S cell to act as functional interlayer in attempt to alleviate the polarization of sulfur cathode and to suppress the shuttle effect.

Fig.1 TEM images of PPy nano-wires and PPy nano-tubes, and a schematic cell configuration of rechargeable Li-S batteries with PPy interlayer (a). Initial charge/discharge profiles (b), Nyquist plots measured before cycling (above) and after 100cycles at 0.2C (below) (c), cycle performance and coulombic efficiency of Li-S batteries with PPy nano-wire interlayer, PPy nanotube interlayer and without interlayer(d).

As shown in Fig.1b, the Li-S battery with PPy interlayers shows higher discharge plateaus and lower charge plateaus than the cell without interlayer, indicating a decreased polarization. Moreover, both the typical valley at the end of the first discharge plateaus and a peak at the beginning of the charge profile for Li-S cell disappear for the cell with PPy interlayers. Fig.1c shows evidently decreased resistance of the cells with PPy interlayer, indicating that the interlayer acts as a pseudo-upper current collector which reduces the effective resistance of the highly insulating sulfur cathodes and enhances the utilization rate of the active material. Furthermore, the cycle performance and coulombic efficiency of the cells with PPy nano-wire and PPy nano-tube interlayers are much better than the cell without interlayer. The positive effect of PPy interlayer could be ascribed to the strong adsorption effect between PPy and lithium polysulfides[3] which can inhibit the migration of lithium polysulfides effectively and thus improve the coulombic efficiency and cycling stability. As seen in Fig.1d, the discharge capacities of Li-S batteries with PPy nano-wires interlayer and PPy nano-tube interlayer are maintained at 782mAh.g^-1 and 687.5mAh.g^-1 after 200 cycles respectively, which are much higher than the cell without interlayer (476mAh.g^-1after 100 cycles).

 

References

[1] S.A.F. Peter G. Bruce, Laurence J. Hardwick and Jean-Marie Tarascon, Nat Mater, (2012) 11,19-29.

[2] Y. Yang, G. Zheng, Y. Cui, Chem Soc Rev, 42 (2013) 3018-3032.

[3] W. Li, Q. Zhang, G. Zheng, Z.W. Seh, H. Yao, Y. Cui, Nano Lett, 13 (2013) 5534-5540.