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

Abstract

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.