Abstract:

Lithium sulfur batteries with the merits such as high theoretical specific capacity and energy density, has become a great potential power cell during the next generation secondary battery systems. Sulfur cathode 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, bad cyclability and hinder further development of lithium sulfur batteries. Heterostructure composites with abundant active sites and superior catalytic activity, can catalyze polysulfide conversion effectively. However, catalytic mechanisms of heterostructure interface on polysulfide conversion remain poorly understood. This paper is devoted to study heterostructure interface and its effects on absorption, the reduction/oxidation of lithium polysulfide within situ synthesized sphere TiO₂/TiN composite as models to solve the above scientific question. The formation mechanisms of built-in electric field and the effects of heterostructure interface on electrochemical performances of lithium sulfur batterie 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 formed at the interface between TiO₂ and TiN with electrons flow from TiN to TiO₂. The built-in electric field improve anchor ability of lithium polysulfides species, facilitate the Li⁺ transport rapidly and promote the conversion reaction between lithium polysulfides and Li₂S. The step-increased current densities charge-discharge tests present that TiO₂/TiN heterojunction composites based lithium sulfur batteries deliver the discharge capacity of 1070 mAh/g at 0.05C, and keep the capacity retention of 60.7% at 1C. Cyclic voltammetry tests at various temperatures indicate the reaction activation energy of lithium polysulfide to Li₂S decreases to 2.73 kJ/mol. It provides a new idea for designing the composite cathode materials for lithium sulfur batteries and accelerating further development of lithium sulfur batteries.

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