Carbon-Sulfur Composites as High Capacity Cathode Materials for Lithium-Sulfur Batteries

Bharti, Vikram Kishore and Sharma, Chandra Shekhar (2019) Carbon-Sulfur Composites as High Capacity Cathode Materials for Lithium-Sulfur Batteries. Masters thesis, Indian institute of technology Hyderabad.

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Abstract

Rapid depletion of fossil fuels has led to the extensive research to fabricate electrochemical devices which can serve as a replacement for conventional energy sources. In the recent decades, Lithium-Sulfur (Li-S) batteries has gained paramount position in battery research because of its high theoretical capacity (1675 mAh.g-1 ), high energy density (2600 Wh.kg-1 ) and non-toxic nature of sulfur. Furthermore, the commercialization of Li-S battery has been hurdled by following factors: (a) low conductivity of sulfur (around 5*10-30 S.cm-1 @ NTP); (b) density difference between Li2S and S which causes volume expansion, around 80%, during cycling; and (c) higher order polysulfides dissolve in electrolyte and keep shuttling between cathode and anode (named as “Shuttle effect”). Carbon is considered to be good host for sulfur in Li-S battery because of its high electronic conductivity as well as pore size, pore volume and surface area can be altered easily. High electron conductivity help in the electron mobility whereas the porous structure helps in trapping the higher order polysulfides which are dissolved in electrolyte. In this thesis work, we investigated three carbon precursors as a host for sulfur for Li-S batteries: (a) Coconut husk (because of its fibrous structure); (b) Candle soot (because of its hydrophobic nature) and (c) Soya chunk (because of its porous structure). Different strategies have been followed to improve the electrochemical performance of the used cathodic materials such as introduction of candle soot carbon coating as an interlayer to minimize the shuttling effect produced by the dissolution of higher order polysulfides. Different physical characterization techniques such as X-ray diffraction, RAMAN spectroscopy, Surface area analyzer, Scanning Electron Microscopy etc. has been used to investigate the pysico-chemical characteristic of the resultant composites. Further, Cyclic voltammogram, Galvanostatic charge-discharge, electrochemical impedance spectroscopy, rate performance and cyclic stability have been performed to analyze the electrochemical behavior of different composites. vi As-prepared Carbon-Sulfur (C-S) composite electrodes exhibited impressive electrochemical performance at the different current densities. By further applying carbon coating as an interlayer, the electrochemical performance is greatly enhanced by dislocating the polysulfides produced during electrochemical reaction. The carbon used as interlayer is hydrophobic in nature while the lithium polysulfide is hydrophilic in nature, so there is a physical interaction between the carbon and lithium polysulfides resulting in the suppression in the shuttling effect. Besides the excellent electrochemical performance, considering low cost and ease of fabrication, these C-S composite materials as prepared in this work can further be scaled up and therefore have a great potential as promising cathode candidate for Li-S batteries.

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