In situ iodine-grafted SPAN via a decoupled additive for durable and high-rate Na-S batteries.

Localized high-concentration electrolytes (LHCEs) alleviate polysulfide shuttling and sodium dendrite growth in room-temperature sodium-sulfur (RT Na-S) batteries, yet they remain incapable of resolving the high nucleation barrier for sodium and sluggish sulfur conversion kinetics. Herein, we introduce antimony triiodide (SbI3) as a decoupled additive into an LHCE for RT Na-S batteries. The Sb ions promote the formation of a sodiophilic Na3Sb interphase, effectively lowering the nucleation overpotential and enabling uniform Na deposition on the anode. Meanwhile, the iodine species are in situ grafted onto sulfurized polyacrylonitrile (SPAN) skeletons through their attack on the C[triple bond, length as m-dash]N bonds, which perturbs the adjacent C-S bonds and thereby weakens the S-S bonds within the covalent -C-S x -C- chains. The iodine-grafted SPAN cathode is shown to achieve fast redox reaction kinetics and high reversibility. As a result, RT Na-SPAN batteries employing the SbI3-modified LHCE deliver a low capacity decay rate of 0.09% per cycle over 2500 cycles at 1.0C as well as a superb rate capability of 685 mA h g-1 at 4.0C. This work establishes a new paradigm of in situ covalent grafting via a bifunctional additive and offers a new approach for the simultaneous engineering of cathode redox kinetics and anode interfacial stability in high-performance metal-sulfur batteries.