Ionic and electronic contributions to the Li chemical potential in ${\mathrm{Li}}_{\mathrm{x}}$${\mathrm{Ru}}_{\mathrm{z}}$${\mathrm{Mo}}_{6\mathrm{-}\mathrm{z}}$${\mathrm{Se}}_8$

We studied Li interactions in ${\mathrm{Li}}_{\mathrm{x}}$ ${\mathrm{Ru}}_{\mathrm{z}}$ ${\mathrm{Mo}}_{6\mathrm{-}\mathrm{z}}$ ${\mathrm{Se}}_8$ for 0<z<2 using electrochemical cells. Near x=4-2z the chemical potential of Li rises and the resistance of the compound, measured in situ, has a sharp maximum, as expected if Li transfers its 2s electron to the host. Nevertheless, Li intercalation is reversible to x=4 and the triclinic distortion previously observed at x=3.7 for z=0 is unchanged for z≤1. Hence, ionic rather than electronic factors must cause the limit x=4 and this distortion. We also show that if a rapid-band model holds here, the bands must shift down in energy as electrons are added, as expected from electronic screening.