Figure 1: The of electronic-band models of molten salt in two cases: (A) Fermi level, εF, (red line) is slightly above the impurity level, εMiKi+ (blue line segments); (B) the former is essentially below the latter; here “0” denotes a free-electron energy level, εc is the electron energy at the bottom of conduction band, εv is the electron energy at the top of valence band, εg is the energy band-gap of pure molten salt, solid line segments are the local energy levels occupied by electrons and the dotted ones are the vacant local levels (cations).