Co/Co2P heterojunction embedded in N, P and S co-doped carbon nanocages for enhancing polysulfide conversion in lithium sulfur batteries

Abstract

High energy density and low cost are great advantages of lithium-sulfur batteries (LSBs), but its practical application is severely hindered by the lithium polysulfides (LiPSs) shuttling and its slow transformation kinetics. Herein, by adjusting the coating thickness of polyphosphazene (PZS) on the surface of cobalt-based organic frameworks (ZIF-67) and combining with the subsequent pyrolysis process, we fabricated Co/Co₂P heterojunction nanoparticles modified N, P and S co-doped carbon nanocages (Co/Co₂P-NPSC) for catalysis of LiPSs conversion. Unlike conventional sodium hypophosphite-based phosphidation, this approach offers a simpler and greener synthetic route. The theoretical simulations and valence-band X-ray photoelectron analysis reveal that Co as an electron donor can form a built-in electric field with Co₂P and upshift the d-band center of Co/Co₂P heterojunction, thus significantly improving its anchoring and catalytic ability for LiPSs. Moreover, the Li₂S₈ adsorption experiment, Li₂S deposition analysis as well as in-situ Raman characterization effectively verifiy that Co/Co₂P-NPSC host possesses the optimal adsorption capacity and catalytic effect of LiPSs. The electrochemical analysis results show that the initial discharge capacity of S@Co/Co₂P-NPSC cathode is 1291 mAh g^-1 at 0.1C with an E/S of about 10 μL/mg. Even the E/S is reduced to 5.2 μL/mg, the discharge capacity at 0.1C is still remain 880 mAh g^-1. More importantly, the discharge can hold 465 mAh g^-1 after 500 cycles at 1.0C and the attenuation per cycle is only 0.087 % with the Coulomb efficiency of about 95 %. This work provides atomic-level insights into heterojunction engineering and proposes a scalable synthesis strategy for high-efficiency LSBs.

Keywords: Co/Co(2)P heterojunction; Heteroatom co-doped carbon nanocages; Lithium-sulfur batteries; Transformation kinetics; d-Band center.