Solid-Solution Anion-Enhanced Electrochemical Performances of Metal Sulfides/Selenides for Sodium-Ion Capacitors: The Case of FeSSe .

Transition-metal sulfides/selenides are explored as advanced electrode materials for nonaqueous sodium-ion capacitors, using FeSSe as an example. A solid solution of S/Se in FeSSe allows it to combine the high capacity of FeS and the good diffusion kinetics of FeSe together, thereby exhibiting excellent cycle stability (∼220 mA h g after 6000 cycles at 2 A g) and superior rate capability (∼210 mA h g at 40 A g) within 0.8-3.