Electrochemical, structural and physical properties of the sodium Chevrel phases Na(x)Mo(6)X(8-y)l(y)(X=S), Se and y=0 to 2.

We report the synthesis, structural and physical properties of the sodium Chevrel phases Na(x)Mo(6)X(8-y)l(y) (X=S, Se). These materials were synthesized by means of electrochemical reactions using Na/Mo(6)X(8-y)l(y) test cells. Structural changes induced by sodium intercalation were followed by in- situ X-ray diffraction measurements. Cycling data indicates that sodium intercalation into Mo(6)X(8-y)l(y) is irreversible and results in compounds of formula Na(x)Mo(6)X(8-y)l(y) with x decreasing from 1 to 0 as y increases from 0 to 2. However, additional Na up to a maximum ranging from x = 4 to 2 as y goes from 0 to 2 is readily reversible over several cycles. This behavior is consistent with the electronic structure of the "host" established from band structure calculations. The anomalies observed in the electrochemical curves (V(x) and dx/dv vs V) correlated perfectly with the observed structural changes.