Abstract:

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.

Key words: lithium-sulfur battery, lithium polysulfide, shuttle effect, heterostructure, built-in electric field