Fabrication Of Self-supporting Carbon Matrix Composite Membrane And Its Application In Lithium-sulfur Batteries

Lithium-sulfur batteries,due to its high energy density,abundant active material sulfur and environmental friendliness,have gradually become the focus of the industry and one of the most promising energy storage devices.Preparation high sulfur loading electrode is an important way to achieve lithium-sulfur batteries with high energy density.The traditional preparation method of high sulfur loading electrode is usually realized by adjusting the coating thickness of slurry.However,this method may lead to the slurry coating fall off during the drying process and electrode structure failure.Moreover,under high sulfur loading system,“shuttle effect”caused by polysulfides（Li₂S_n（4≤n≤8））leads to rapid decline of capacity,decrease of coulomb efficiency and the serious self-discharge behavior.Based on the previous research work of our group and literature research,the main research contents and results of this paper are as follows:（1）Based on acidified commercial carbon nanotubes（CNTs）,three-dimensional self-supporting CNTs membranes were prepared by vacuum filtration process.Li₂S₆was used as active material.Electrochemical performance of CNTs/Li₂S₆ electrode（sulfur loading:4.74 mg）with different thickness was investigate.The results show that the electrochemical performance of CNTs/Li₂S₆ electrode is effectively improved with the increase of CNTs membrane thickness.When the thickness of CNTs membrane is300μm,CNTs/Li₂S₆ electrode reaches 971.2 mAh g^-1 at 0.2 C in the first cycle,after150 cycles,it still maintained 783.0 mAh g^-1,good electrochemical performance can be attributed to CNTs effectively physically adsorb polysulfides and suppress its“shuttle effect”.（2）Graphene-like carbon nitride（g-C₃N₄）was introduced into the CNTs dispersion to prepare g-C₃N₄/CNTs composite membrane by vacuum filtration process,the electrochemical performance of g-C₃N₄/CNTs/Li₂S₆ composite electrode with high sulfur loading was studied.The results show that the synergistic effect of CNTs and g-C₃N₄ improved the adsorption effect of polysulfides,effectively suppressed"shuttle effect",and improved cycle stability of lithium-sulfur batteries.The g-C₃N₄/CNTs/Li₂S₆ electrode showed a discharge capacity of 876 mAh g^-1 at 0.5 C in the first cycle and 633 mAh g^-1 after 300 cycles.At the same time,comparing to CNTs/Li₂S₆ electrode,the g-C₃N₄/CNTs/Li₂S₆ composite electrode could effectively inhibit self-discharge behavior and improve cycle stability.When the sulfur loading was increased to 7.11 mg,the g-C₃N₄/CNTs/Li₂S₆ electrode also delivered a high specific capacity of 850 mAh g^-1 at 0.2 C in the first cycle,after 200 cycles,the capacity retention rate was 75.5%.（3）Commercial two-dimensional layered graphene oxide（GO）was introduced into the CNTs dispersion to prepare GO/CNTs composite membrane by vacuum filtration process,electrochemical behavior of GO/CNTs/Li₂S₆ composite electrode with high sulfur loading was studied.The results show that the abundant organic functional groups on the GO surface could effectively chemisorb polysulfides,improve the lithium ion migration rate,and reduce interface resistance of electrode.When the sulfur loading was 7.11 mg,GO/CNTs/Li₂S₆ composite electrode showed 731.8 mAh g^-1 at 0.2 C in the first cycle and 564.0 mAh g^-1 after 300 cycles,the capacity retention rate was 77.1%and the average Coulombic efficiency was 97.6%.Besides,GO could also effectively reduce the polarization and self-discharge behavior of high sulfur loading electrodes.