Sodium Intercalation Behavior of Layered Na_xNbS₂ (0 ≤ x ≤ 1)

A layered sulfide, Na 0.5 NbS 2 (space group: P 6 3 / mmc ), was synthesized by a conventional solid-state reaction as an electrode material for a Na-ion battery. Galvanostatic Na insertion/extraction was performed to characterize the system Na x NbS 2 (0 ≤ x ≤ 1.0) operating on the Nb(IV)/Nb(III) redox couple. Although the system shows a high specific capacity of 143.6 mAh g –1, the voltage profile is not suitable with a signature of Na/vacancy ordering at x = 0.5. First-principles calculation was applied to reveal possible structures of Na x NbS 2 and describe the corresponding electrochemical properties. The calculated Na binding energies and voltages are in good agreement with experimental charge/discharge voltages. We also found a possible atomic arrangement of Na/vacancy ordering in Na 0.5 NbS 2 . Although layered NaMS 2 systems allow full sodium intercalation, the strong Na + –Na + intralayer interaction induces layer gliding and Na + -ion ordering that create undesirable steps in the voltage profile.