A Catalytic Electrolyte Additive Modulating Molecular Orbital Energy Levels of Lithium Polysulfides for High-Performance Lithium–Sulfur Batteries

Lithium–sulfur (Li–S) batteries have ultrahigh theoretical specific capacity, but the practical application is hindered by the severe shuttle effect and the sluggish redox kinetics of the intermediate lithium polysulfides (LiPSs). Effectively enhancing the conversion kinetics of LiPSs is essential for addressing these issues. Herein, the redox kinetics of LiPSs are effectively improved by introducing 6-azauracil (6-AU) molecules to the organic electrolyte to modulate the molecular orbital energy level of LiPSs. The 6-AU as a soluble catalyst can form complexes with LiPSs via Li–O bonds. These complexes are liable to transform because of the elevated HOMO and the reduced LUMO energy levels as compared to the dissociative LiPSs, resulting in small energy gaps (Egap) and exhibiting stronger redox activity. Benefiting from the rapid conversion kinetics, the shuttling effect of LiPSs is alleviated to a great extent, so that sulfur utilization is improved and the lithium electrode is protected. In addition, the introduction of 6-AU modulates the deposition behavior of Li2S and eases the coverage of the cathode surface by the insulating Li2S layer. The Li–S battery containing 6-AU provides superior capacity retention of 853 mAh g–1 after 150 cycles at 0.2 C and shows remarkable high-rate performance and retains a specific discharge capacity of 855 mAh g–1 at 5 C. This study accelerates the kinetics of Li–S batteries by tuning the HOMO and LUMO energy levels of LiPSs, which opens an avenue for designing functional electrolyte additives.