Solid-Solution Anion-Enhanced Electrochemical Performances of Metal Sulfides/Selenides for Sodium-Ion Capacitors: The Case of FeS2–xSex

Transition-metal sulfides/selenides are explored as advanced electrode materials for nonaqueous sodium-ion capacitors, using FeS2–xSex as an example. A solid solution of S/Se in FeS2–xSex allows it to combine the high capacity of FeS2 and the good diffusion kinetics of FeSe2 together, thereby exhibiting excellent cycle stability (∼220 mA h g–1 after 6000 cycles at 2 A g–1) and superior rate capability (∼210 mA h g–1 at 40 A g–1) within 0.8–3.0 V. These results are much better than those of FeS2 and FeSe2, confirming the advantages of S/Se solid solution, as supported by EIS spectra, DFT calculations, and electronic conductivity. As FeS2–xSex is paired with the activated carbon (AC) as Na-ion capacitors, this device is also better than sodium-ion batteries of FeS2–xSex//Na3V2(PO4)3 and sodium-ion capacitors of metal oxides//AC, particularly at high rates. These results open a new door for the applications of sulfides/selenides in another device of electrochemical energy storage.