Electrocatalysis of Polysulfide Redox in Lithium-Sulfur Battery
Authors: Thangavel Naresh kumar , Masurkar Nirul , Sawas Abdul Razzag , Arava Leela 
Published: February 2020

Lithium-sulfur (Li-S) battery offers high theoretical capacity (1673mAh/g) and energy density (2500Wh/kg) which is considered as future energy storage system for extensive use from portable electronics to automobile vehicles. Regardless of gaining high energies with Li-S, technology hampered by numerous challenges such as sluggish kinetics, structural instability due to volume changes, poor reversibility, low coulombic efficiency and limited cycle life primarily due to the dissolution of intermediate species in the electrolyte, commonly labeled as ‘polysulfide shuttle1,2. To alleviate issues, significant progress has been made through designing efficient sulfur host cathode materials with specific properties including polysulfide encapsulation/adsorption, enhanced the reaction kinetics, tolerable to the volume changes, etc. Despite the remarkable success of those strategies, their physical confinement, pore-clogging due to the Li2S deposition, low surface area, and low electronic conductivity hinder them from achieving stable lithium-sulfur cell performance.

Recently, our group has reported a new avenue explaining the electrocatalysis of polysulfides reaction in Li−S by using catalytic materials surfaces like platinum, nickel and transition metal dichalcogenides 3-5. Use of an electrocatalyst in multistep sulfur redox process has shown exciting potential in the trapping of polysulfides, enhancing the specific capacity along with long cycle life, and exhibiting excellent reversibility. To investigate the underlying mechanism, we performed detailed electrochemical characterization including Tafel analysis, adsorption studies, kinetics of Li2S deposition on catalytic (Pt) and non-catalytic surface (carbon). Voltammogram derived parameters evidently reveal that the electrocatalytic surface effect on polysulfide redox reactions. The obtained results corroborate that chemical interaction of liquid polysulfides on catalytic surface and assists the successive reactions rather than dissolution thus reduces the shuttling phenomena. More importantly, catalytic surface assists the early the formation of solid discharge (Li2S) product and provide high capacity. Interfacial charge transfer measurement during charge-discharge exemplified that enhanced reversibility of the polysulfides reaction on the catalytic surface compared to the non-catalytic one.

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