TiO_(2)/TiN异质结内建电场的构筑及多硫化锂催化转化性能研究

Construction of built-in electric field in TiO_(2)@TiN heterojunctions toward boosting the polysulfide conversion

锂硫电池因具有较高理论容量和能量密度,成为最有前景的下一代电池体系之一。正极材料存在导电性差和多硫化锂转化反应动力学缓慢等问题引起严重的穿梭效应,致使硫利用率低、功率密度小、循环寿命短,严重制约了锂硫电池商业化进程。异质结复合材料具有丰富的活性位点和催化活性,能够催化多硫化锂氧化还原反应,然而其对多硫化锂催化转化机制仍不清晰。本工作以原位构建球形TiO_(2)/TiN异质结复合材料为模型,研究异质界面结构与其对多硫化锂的“吸附-催化-转化”行为,试图解决上述问题。采用多硫化锂吸附实验、X射线光电子能谱分析、紫外可见光谱分析、恒电流充放电测试和循环伏安测试等手段,研究异质界面内建电场形成机理及其对锂硫电池电化学性能的影响。结果表明,Ti O_(2)/TiN异质结界面形成由TiN指向TiO_(2)的空间电荷区并构成内建电场。异质结内建电场显著改善多硫化锂锚定能力,加速Li^(+)快速传输,提高多硫化锂向Li_(2)S转化能力。TiO_(2)/TiN异质结复合材料组装的锂硫电池倍率循环测试结果表明在0.05 C电流密度下初始放电比容量高达1070 mAh/g,多次循环后1C电流密度下容量仍保持60.7%。变温循环伏安测试结果表明TiO_(2)/TiN异质结复合材料将Li_(2)S转化反应活化能降低至2.73 kJ/mol。本工作为锂硫电池正极复合材料的设计和推动锂硫电池进一步发展提供新思路。

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 TiO_(2)/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 TiO_(2) and TiN with electrons flowing from TiN to TiO_(2).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 TiO_(2)/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.