Crystallinity Regulated Functional Separator Based on Bimetallic NixFey Alloy Nanoparticles for Facilitated Redox Kinetics of Lithium-Sulfur Batteries

The practical application of lithium-sulfur batteries (LSBs) is limited by the shuttle effect of lithium polysulfides (LiPSs), large volume expansion, and sluggish conversion kinetics of sulfur. Herein, the crystallinity regulation of NixFey alloy anchored on oxidized carbon nanotube/nitrogen-doped graphene (NixFey@OCNT/NG) for application of a functional separator into LSBs is demonstrated. A low crystalline NixFey@OCNT/NG (LC-NixFey@OCNT/NG) modified polypropylene separator is systematically compared with its highly crystalline counterpart (HC-NixFey@OCNT/NG), demonstrating superior LiPS absorbability, redox mediating capability into facilitated conversion kinetics, and uniform flux of Li+ into the anode. Furthermore, theoretical calculations confirm that the LC-NixFey alloy features high adsorption energies and low diffusion energy barriers toward LiPSs, as well as a decreased energy gap and larger electron density near Fermi level. Accordingly, the LSB cells with LC-NixFey@OCNT/NG modified separators deliver a high specific capacity of 1379.13 mA h g(-1) at 0.1 C and a low decay ratio of 0.04%/cycle over 600 cycles at 5.0 C with a high capacity of 410 mA h g(-1). Even under high sulfur loading (5.37 mg cm(-2)) and lean electrolyte (E/S = 4.9 mu L mg(-1)) conditions, the LSB cells with LC-NixFey@OCNT/NG/PP deliver a high areal capacity of 4.1 mAh cm(-2) at 0.2 C.