Tuning the local chemistry of SPAN to realize the development of room-temperature sodium-sulfur pouch cells

Sulfurized polyacrylonitrile (SPAN), a nitrogen-rich carbon-sulfur framework, is a promising alternative to the elemental sulfur cathode. However, repetitive formation and breaking of carbon-sulfur bonds causes irreversible capacity loss. The capacity loss of SPAN can be fixed by altering the local environment of the SPAN. This work demonstrates a metal monosulfide, i.e., zinc sulfide (ZnS), as a SPAN cathode additive to fix the capacity loss and boost the overall performance of the cathode. Besides improving the sulfur loading, it alters the local carbon-sulfur environment. The presence of ZnS is expected to shorten the sulfur-sulfur bond of the SPAN matrix, leading to a change in the local nitrogen environment. The ratio of pyridinic-nitrogen to pyrrolic-nitrogen increases sharply upon including ZnS. A coin-cell with ZnS doped SPAN cathode exhibits excellent cycling stability for over 450 cycles, with minimal decay of about 0.07% per cycle. DFT calculations reveal that the addition of ZnS enhances SPAN's sodium migration and electronic conductivity by lowering sodium migration barriers and reducing the HOMO/LUMO gap, improving charge transfer kinetics. Furthermore, a multi-layered pouch cell featuring a ZnS doped SPAN cathode demonstrates the efficiency of the proposed cathode. The pouch cell exhibits excellent cycling stability and coulombic efficiency for over 250 cycles. This study provides a pathway to engineer the local chemistry of the cathode to design innovative cathode materials for a stable and reversible RT-Na/S battery.