Study On Electrochemical Properties Of Polypyrrole Cathode Composites For Lithium-sulfur Batteries

Lithium-sulfur（Li-S）batteries have attracted more attentions of many researchers due to their theoretical energy density and specific capacity up to 2400 Wh kg^-1 and 1675 mAhg^-1,respectively.In addition,sulfur also has other noticeable characteristics such as abundant storage,non-toxicity and eco-friendly,etc.However,some natural drawbacks of sulfur cathodes have still obstructed their market applications in large scale of Li-S batteries,including the well-known shuttle effect origin from polysulfides dissolution,the poor conductivity of sulfur and its end oxidizing products（Li₂S₂ and Li2S）,as well as the volumetric expansion of sulfur,etc.These issues can lead to poor utilization of sulfur,disappointing cyclic lifespan and rapid capacity weakening of Li-S batteries.In order to improve the electrochemical performance of lithium-sulfur batteries,the S/Polypyrrole（PPy）,S/PPy/metal oxide particles（Co₃O₄,Ni O,Al2O3）,S@Co₃O₄ and S@Co₃O₄/PPy composites were prepared in this work.We also investigated the effects of composites with different morphology and compositions on the electrochemical performance of lithium-sulfur batteries.The primary research details and results are presented as following:（1）In order to improve the poor conductivity of sulfur and its end oxidizing products（Li₂S₂ and Li2S）,the tubular polypyrrole was firstly prepared by chemical oxidation method and mixted with sulfur by melt diffusion method.The effects of different experimental factors on the morphology of PPy were studied.The optimized experimental conditions for the preparation of PPy nanotubes with diameter ranging from 100 to 150 nm are5 m M of MO solution,reaction time of 24 h,and the molar mass ratio of MO,Fe Cl3.6H2 O and pyrrole（Py）of 1:10:10.Then,we futher investigated the effect of sulfur content in S/PPy composite on the elextrochemical properties of batteries.The results showed that the S/PPy composites with sulfur content of 70 wt% possessed the initial discharge capacity of 1210.8 mAhg^-1 at 0.2 C,and still retaining 392.4 mAhg^-1 after 200 cycles.（2）Based on the above experiments,in order to further improve the cycle stability of the S/PPy composite electrodes,some metal oxide particles（Co₃O₄,Ni O,Al2O3）were mixed respectively with PPy nanotubes to prepare PPy/metal oxide composite materials.The results showed that different metal oxide additives presented different performance enhancing ability.The overall result was S/PPy/Co₃O₄ > S/PPy/Ni O > S/PPy/Al2O3 > S/PPy.At the same 0.5 C current density,the S/PPy/Co₃O₄ cathodes possessed the initial discharge capacity of 975.2 mAhg^-1,and still retained 51.5 % capacity after 200 cycles,which showed better cycle performance and rate ability than others.In addition,the influences of morphology of Co₃O₄ on its electrochemical performance was also studied.For comparison with particles,Co₃O₄ hollow microspheres with a diameter of 2-2.5 μm were prepared by the self-assembled solvothermal reaction method.The results showed that S@Co₃O₄ composite with core-shell structure retained the discharge specific capacity of 41.3% after 200 cycles at 0.2 C,which is better than those of Co₃O₄ particles.（3）Finally,we proposed anoptimized S@Co₃O₄/PPy cathode for Li-S batteries,in which Co₃O₄ hollow microspheres was embedded into conductive network of polypyrrole nanotubes,then sulfur was permeated into the interior of Co₃O₄ hollow microspheres and coated on surface of PPy nanotubes.The electrochemical performance testing results showed that when the mass ratio of Co₃O₄ hollow microspheres and PPy nanotubes was 4:1,S@Co₃O₄/PPy cathodes showed a significant long-cycling property preserving the discharge capacity of 370.6 mAhg^-1 after 1000 cycles under a 0.5 C current density.A super long lifespan within 1900 cycles at 1 C was also observed with an as low as decay rate in capacity of 0.034 % per cycle.Moreover,after 1000 cycles at 3 C and 5 C rates,the S@Co₃O₄/PPy composite also guaranteed an enough capacity output of 180.2 and 135.9 mAhg^-1.The synergism between Co₃O₄ hollow microspheres and PPy conductive nanotubes network with sulfur will contribute an excellent long-term cyclability of S@Co₃O₄/PPy composite.Based on the investigation of this work,lithium-sulfur batteries with PPy composite cathode showed promising performances,which will provide a design route for preparing a lithium-sulfur battery with better electrochemical performances.