"Three-in-one" strategy: Heat regulation and conversion enhancement of a multifunctional separator for safer lithium-sulfur batteries

The safety problems encountered with lithium-sulfur batteries (LSBs) hinder their development for practical applications. Herein, a highly thermally conductive separator was constructed by cross-weaving super-aligned carbon nanotubes (SA-C) on super-aligned boron nitride@carbon nanotubes (SA-BC) to create a composite film (SA-BC/SA-C). This separator was used to fabricate safe LSBs with improved electrochemical performance. The highly aligned separator structure created a uniform thermal field that could rapidly dissipate heat accumulated during continuous operation due to internal resistance, which prevented the development of extremely high temperatures. The array of boron nitride nanosheets endowed the composite separator with a large number of adsorption sites, while the highly graphitized carbon nanotube skeleton accelerated the catalytic conversion of high-valence polysulfides into low-valence polysulfides. The arrayed molecular brush design enabled the regulation of local current density and ion flux, and considerably alleviated the growth of lithium dendrites, thus promoting the smooth deposition of Li metal. Consequently, a battery constructed with the SA-BC/SA-C separator showed a good discharge capacity of 685.2 mAh g^-1 over 300 cycles (a capacity decay of 0.026% per cycle) at 2 C and 60°C. This "three-in-one" multifunctional separator design strategy constitutes a new path forward for overcoming the safety problems of LSBs.