Coupling biphasic homojunction interface and oxygen vacancies for enhanced polysulfide capture and catalytic conversion in Li-S batteries

Abstract

Lithium-sulfur (Li-S) batteries promise high energy density but suffer from low conductivity, polysulfide shuttling, and sluggish conversion kinetics. The construction of heterointerfaces is an effective strategy for enhancing both polysulfide adsorption and conversion; however, the poor lattice compatibility in the heterointerface formed by different materials hinders interfacial charge transfer. In response to these challenges, herein, a biphasic homojunction of TiO₂ enriched with oxygen vacancies and decorated with nitrogen-doped carbon nanotubes (B-TiO_2−x@NCNT) was designed to simultaneously enhance adsorption ability and catalytic activity. This homojunction interface composed of rutile (1 1 0) and anatase (1 0 1) plane exhibits excellent compatibility, and density functional theory (DFT) calculations reveal that this biphasic interface possesses a much higher binding energy to polysulfides compared to single-phase TiO₂. Additionally, NCNTs are in situ grown on both interior and exterior surfaces of the hollow TiO₂ nanospheres, facilitating rapid electron transfer for the encapsulated sulfur. The homojunction interface synergistically leverages the oxygen vacancies and highly conductive NCNTs to enhance the bidirectional catalytic activity for polysulfide conversion. Therefore, in this multifunctional sulfur-host, polysulfides are first strongly adsorbed at the homojunction interfaces and subsequently undergo smooth conversion, nucleation, and decomposition, completing a rapid sulfur redox cycle. The assembled Li-S battery delivered a high specific capacity of 1234.3 mAh g⁻¹ at 0.2 C, long cycling stability for over 1000 cycles at 5 C with a low decay rate of 0.035%, and exciting areal capacity at a high sulfur loading of 5.6 mg cm⁻² for 200 cycles.