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UT Austin team uses polypyrrole-MnO2 coaxial nanotubes as sulfur host to improve performance of Li−sulfur battery
5 December 2016
Researchers at the University of Texas at Austin have developed a novel electrode for lithium-sulfur batteries that improves cyclic stability and rate capability significantly. In a paper published in the ACS journal Nano Letters, they report using polypyrrole-MnO2 coaxial nanotubes to encapsulate sulfur. MnO2 restrains the shuttle effect of polysulfides greatly through chemisorption and the polypyrrole serves as conductive frameworks.
They report a stable Coulombic efficiency of ∼98.6% and a decay rate of 0.07% per cycle with 500 cycles at 1C-rate with the S/PPy-MnO2 ternary electrodes with 70 wt % sulfur and 5 wt % of MnO2. The ternary electrodes have an initial high rate of 1420 milliampere-hours per gram (mAh/g) at 0.2 C and deliver 985 mAh/g after 200 cycles.
Despite their theoretical ultrahigh energy density, low cost, and nontoxicity, Lithium-sulfur batteries suffer poor performance due to several issues, including poor conductivity of sulfur and the dissolution of redox intermediates, resulting in low Coulombic efficiency and poor cyclic stability (the shuttle effect).
Researchers have taken a number of different approaches to addressing these issues, none completely satisfactory, challenged by lower conductivity, the need for additives, or complicated synthetic strategies, the authors note.
In this work, we designed PPy-MnO2 coaxial nanotubes to accommodate sulfur. … In this ternary system, the polar material MnO2 provides strong adsorption to polysulfides while PPy serves as a flexible yet conductive network to tolerate volume change and facilitate electron transport. Therefore, significantly improved cyclic stability, Coulombic efficiency and rate capability are achieved when compared with pure PPy nanotubes encapsulated sulfur. An optimized ratio of MnO2 in the binary PPy-MnO2 is found to be 17% (5% in ternary composite of S/PPy-MnO2), which balances the adsorption ability and the conductivity.—Zhang et al.
|Illustration of the synthesis of S/PPy-MnO2 ternary composites and the advantages during charge/discharge process. Credit: ACS, Zhang et al. Click to enlarge.|
The team synthesized the PPy-MnO2 nanotubes through a simple polymerization of pyrrole using MnO2 nanowires as both template and oxidation initiator. The amount of MnO2 in the composite can be controlled by adjusting the ratio of oxidant and pyrrole monomer. The team then infiltrated sulfur into the nanotubes through a well-established melt-diffusion method.
Jun Zhang, Ye Shi, Yu Ding, Wenkui Zhang, and Guihua Yu (2016) “In Situ Reactive Synthesis of Polypyrrole-MnO2 Coaxial Nanotubes as Sulfur Hosts for High-Performance Lithium–Sulfur Battery” Nano Letters 16 (11), 7276-7281 doi: 10.1021/acs.nanolett.6b03849