Deciphering the interaction mechanism between copper and polysulfides for enhanced performance and cost-efficiency in quasi-sodium-sulfur batteries

Room-temperature Na-S batteries with Al current collectors face the long-standing challenges of poor cycling performance and rate capability due to the serious sodium polysulfides (NaPSs) shuttling and sluggish reaction kinetics. Here, we demonstrate that a high-performance and low-cost quasi-Na-S battery can be realized by using the Cu@carbon nanotube (CNT) cathode host and unconventional Cu current collector (Cu-CC) in an ether electrolyte. Detailed ex-situ characterizations reveal that the Cu@CNT/S was transformed to NaPSs anchoring on Cu7S4 nanocrystals (~11 nm) during the cycling, forming the NaPS@Cu7S4/CNT cathode with robust sulfur immobilization and enhanced Na+ reaction kinetics. Moreover, the chemical interaction between NaPSs and Cu-CC in situ creates a robust Cu-CC/electrode interface with ultra-small resistance (< 4 Ω), greatly improving the electrode stability and charge transfer kinetics. The synergy of efficient NaPSs trapping and favorable Cu/electrode interface endows the quasi-Na-S battery with a moderate discharge plateau (around 1.2~1.75 V), excellent rate capability (396.9 mAh g-1 at 10 A g-1), and ultra-stable cycling performance (nearly no capacity decay over 1190 cycles). These features make it quite suitable for stable and cost-sensitive grid-scale applications.