Construction of built-in electric field in TiO2@TiN heterojunctions toward boosting the polysulfide conversion
Lithium sulfur batteries with merits such as high theoretical specific capacity and energy density have become a great potential power cell for next-generation secondary battery systems.Sulfur cathodes still suffer some problems such as poor conductivity and sluggish redox kinetics of polysulfide conversion reactions,which trigger a serious shuttle effect,ultimately resulting in low sulfur utilization,poor power density,and bad cyclability,thus hindering a further development of lithium sulfur batteries.Heterostructure composites with abundant active sites and superior catalytic activity can effectively catalyze polysulfide conversion.However,catalytic mechanisms of a heterostructure interface on polysulfide conversion remain poorly understood.This study investigates the heterostructure interface and its effects on absorption,the reduction/oxidation of lithium polysulfide with in-situ synthesized sphere TiO2/TiN composites as models to solve the above scientific question.The formation mechanisms of a built-in electric field and the effects of the heterostructure interface on electrochemical performances of lithium sulfur batteries are investigated through absorption experiments,XPS,UV-vis spectroscopy analysis,galvanostatic charge-discharge,and cyclic voltammetry tests.The results showed that a space charge region and built-in electric field were formed at the interface between TiO2 and TiN with electrons flowing from TiN to TiO2.The built-in electric field improved the anchor ability of lithium polysulfides species,rapidly facilitated the Li+transport,and promoted the conversion reaction between lithium polysulfides and Li2S.The step-increased current densities charge-discharge tests revealed that TiO2/TiN heterojunction composite-based lithium sulfur batteries delivered a discharge capacity of 1070 mAh/g at 0.05C,while maintaining a capacity retention of 60.7%at 1C.Cyclic voltammetry tests at various temperatures indicated that the reaction activation energy of lithium polysulfide to Li2S decreases to 2.73 kJ/mol.It provides a new idea for designing composite cathode materials for lithium sulfur batteries and accelerating a further development of lithium sulfur batteries.
lithium-sulfur batterylithium polysulfideshuttle effectheterostructurebuilt-in electric field
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