A thermodynamic study of sodium-intercalated TaS2 and TiS2

The variation of the sodium chemical potential (μNa) with composition x in NaxTaS2 and NaxTiS2 has been measured electrochemically using propylene carbonate-based and β-alumina electrolytes. At 300°K the sodium chemical potential in NaxTaS2 varies from −63 to −25 kcal/mole as x increases from 0.003 to 0.92, respectively. Within experimental uncertainty, the compositional variation of μNa was linear. In NaxTiS2, the sodium chemical potential varies from −60 to −36 kcal/mole as x increases from 0.001 to 1.0, respectively. The compositional variation of μNa in NaxTiS2 exhibits two plateaus indicative of two-phase regions for 0.2 ≲ x ≲ 0.35 and 0.6 ≲ x ≲ 0.7. The standard free energies of intercalation for sodium-intercalated TaS2 and TiS2 are less negative than those reported for the respective lithium-intercalated compounds. The standard free energy of intercalation becomes more negative for both the sodium- and lithium-intercalated compounds as the dichalcogenide changes from TaS2 to TiS2.