Sulfurized Polyacrylonitrile Cathodes with High Compatibility in Both Ether and Carbonate Electrolytes for Ultrastable Lithium–Sulfur Batteries

Sulfurized polyacrylonitrile (SPAN) is a promising material capable of suppressing polysulfide dissolution in lithium–sulfur (Li–S) batteries with carbonate electrolyte. However, undesirable spontaneous formation of soluble polysulfides may arise in the ether electrolyte, and the conversion of sulfur in SPAN during the lithiation/delithiation processes is yet to be understood. Here, a highly reliable Li–S system using a freestanding fibrous SPAN cathode, as well as the sulfur conversion mechanism involved, is demonstrated. The SPAN shows high compatibility in both ether and carbonate electrolytes. The sulfur atoms existing in the form of short S₂ and S₃ chains are covalently bonded to the pyrolyzed PAN backbone. The electrochemical reduction of the SPAN by Li⁺ is a single-phase solid–solid reaction with Li₂S as the sole discharge product. Meanwhile, the parasitic reaction between Li⁺ and CN bonds exists upon the first discharge, and the residual Li⁺ enhances the conductivity of the backbone. The recharge ability and rate capability are kinetically dominated by the activation of Li₂S nanoflakes generated during discharge. At 800 mA g⁻¹, a specific capacity of 1180 mAh g⁻¹ is realized without capacity fading in the measured 1000 cycles, which makes SPAN promising for practical application.