Multi-functional TiO2 nanosheets/carbon nanotubes modified separator enhanced cycling performance for lithium-sulfur batteries

TiO₂ nanosheets (TiO₂NSs) have been investigated for lithium-sulfur (Li-S) batteries as strategically designed TiO₂ nanosheet/carbon nanotube (TiO₂NS/CNT) composite modified polypropylene (PP) separator to inhibit the shuttling of the intermediate polysulfides. The modified separator was fabricated by the vacuum filtration method using the exfoliation TiO₂NSs and untreated carbon nanotube (CNT) composites. The multi-functional TiO₂NS/CNT coating not only reduced the electrochemical resistance but also localized the migrating polysulfides by the cooperative effect of physical adsorption and chemical binding. Specifically, the composition ratio of TiO₂NSs/CNTs and the interface character have been studied. It was found that the optimum ratio and perfect electrolyte wettability of the TiO₂NS/CNT layers were all the critical reasons to achieve good battery performance. The high initial discharge capacity of 1247 mA h g⁻¹ at 0.2 C rate, which was 75% of the theoretical capacity of sulfur, 98% average coulombic efficiency, and 627 mA h g⁻¹ discharge capacity retention after 100 cycles were obtained with the TiO₂NS/CNT coating separator.     

Rational design and fabrication of frame-structured doped bismuth vanadate nanoarchitectures as a polysulfide shield to boost the performance of lithium-sulfur batteries

Lithium-sulfur (Li-S) battery has been evoked increasing attention due to its creditable energy density and the abundant sulfur in nature. Nevertheless, its commercialization is still hampered in virtue of the poor conductivity of sulfur and grievous polysulfide shuttling. To address these issues, in this work, we report a cost-efficient and facile tactic to fabricate frame-structured neodymium-doped bismuth vanadate (BiVO₄) nanoarchitectures, which are reasonably designed as a polysulfide shield to alleviate the shuttling effects. The synthesized nanoarchitectures exhibit typical tetragonal phase with a unique frame structure. The frame structure of BiVO₄ impedes polysulfide shuttling, and supplies electronic and ionic transmission routes. Furthermore, the rational doping of neodymium within the BiVO₄ nanoarchitectures can improve electric conductivity, as well as efficiently alleviate the loss of sulfur and improve activity for sulfur reduction. Benefitting from the structural feature and doping strategy, the Li-S battery performance with a neodymium-doped BiVO₄ nanoarchitecture is significantly improved. This strategy is easily adopted for fabricating other nanostructures, providing a feasible way to design cheap and efficient battery materials. This work is further available for synergistically uniting the virtues of the frame nanoarchitectures and doping protocol to develop advanced Li-S batteries.     

Molybdenum carbide nanocrystals modified carbon nanofibers as electrocatalyst for enhancing polysulfides redox reactions in lithium-sulfur batteries

The high performance of lithium sulfur (Li S) batteries is the focus of research in recent years. However, the low sulfur loading, shuttling effect in electrolyte, and poor cycling stability limit their applications. Herein, molybdenum carbide nanocrystals embedded carbon nanofibers (Mo₂C@CFs: MCCFs) hybrid membrane was prepared in situ on CFs membrane based on carbonthermal reduction of ammonium molybdate. The fibrous MCCFs network is used as the current collector with Li₂S₆ catholyte solution for Li S batteries, which inhibits the shuttle effect and accelerates kinetics redox reaction. In addition, Mo₂C, as electrocatalyst, promotes nucleation of Li₂S of the MCCFs substance, which can reduce polarization and increase the specific capacity. As a result, the free-standing MCCFs@Li₂S₆ electrode (sulfur loading: 4.74 mg) shows a capacity of 977 mAh g⁻¹ and maintains at 828 mAh g⁻¹ at 0.2 C over 250 cycles, and indicates excellent reversibility and cycling stability. Even with sulfur loading as high as 7.11 mg, the MCCF@Li₂S₆ electrode exhibits an extremely high capacity of 5.75 mAh. Meanwhile, the Mo₂C modified CFs can be effectively retarding the self-discharge behavior by trapping the polysulfides. Furthermore, the stability improvement of lithium anode state by effectively suppressing the shuttle effect of polysulfide, played an important role in enhancing the electrochemical performance.