Ultra-Small Ferromagnetic Fe3O4 Nanoparticles Modified Separator for High-Rate and Long Cycling Lithium-Sulfur Batteries

Graphical Dual-function separator for LiS batteries : We decorated carbon nanotubes with ferromagnetic Fe 3 O 4 nanoparticles and then coated the composite (Fe 3 O 4 @CNTs) on a commercial separator. Carbon nanotubes can provide a network for electron transport, and ferromagnetic Fe 3 O 4 nanoparticles can serve as polysulfide trapping centers to anchor polysulfides. The cell with FC-6/PP can provide a high initial discharge capacity of 585 mAh g −1 after activation and a reversible specific capacity of 468 mAh g −1 after 700 cycles at 5 C. As a promising energy storage system with high specific capacity, lithium-sulfur (Li−S) battery is striving for a high-rate and longer lifetime. Whereas the slow redox kinetics and the shuttle effect of polysulfides (PSs) still severely restrict the performance of Li−S batteries. Here, a rational designed ferromagnetic Fe 3 O 4 nanoparticles (FM-Fe 3 O 4 NPs) modified separator is engineered for Li−S batteries. Moreover, the particle size of FM-Fe 3 O 4 NPs was tuned to enhance the energy storage performance. Consequently, the cells with FM-Fe 3 O 4 NPs modified separator achieved a high-rate and long cycling performance, the specific capacity could reach 672.8 mAh g −1 and a low fading rate (0.028 % per cycle over 700 cycles) at a high rate of 5 C. Experimental results reveals that FM-Fe 3 O 4 NPs could mitigate shuttle effect, enable efficient redox reactions, and protect the Li anode from severe corrosion. The performance-enhanced mechanism is further discovered by DFT calculations, the results show that FM-Fe 3 O 4 NPs act as mediators of redox reaction to block PSs shuttle and dynamically facilitate conversion via changing the molecular structure of PSs and reducing reaction barrier. This work provides a new pathway to improving Li−S batteries based on ferromagnetic materials, which may be generalized to other advanced batteries.