Locating redox couples in the layered sulfides with application to Cu[Cr2]S4

Use of LiPF6 in EC:DEC as electrolyte has allowed electrochemical extraction of Li from LiV1−yMyS2 and LiTi1−yMyS2 (M=Cr or Fe). The data show access not only to the Ti(IV)/Ti(III) and V(IV)/V(III) redox couples, but also to the V(V)/V(IV) and Fe(III)/Fe(II) couples in these layered sulfides. However, the Cr(IV)/Cr(III) couple could not be accessed. The concept of redox-couple pinning is outlined and applied to the V(V)/V(IV) and Fe(III)/Fe(II) couples, which are pinned at the top of the S-3p bands. Holes associated with the “pinned” couples occupy orbitals of dominant S-3p character, but they have sufficient cation-3d character to prevent condensation of the holes into p–p antibonding states of disulfide bonds. Strong covalent bonding in the pinned couples creates itinerant-electron states in the partially occupied couples. Application to the metallic, ferromagnetic thiospinel Cu[Cr2]S4 favors location of the itinerant holes in states of a pinned Cu(II)/Cu(I) couple having primarily S-3p character.

Graphical AbstractSchematic representation of a slightly oxidized redox couple as it passes through the top of the anion-p bands: (a) itinerant versus polaronic character of hole states of couple on the approach to the top of anion-p band, (b) pinned couple with predominantly antibonding (a.b.) anion-p hole states and predominantly cation-d bonding (b.) states, (c) couple too far below top of anion-p band for significant cation-d character in hole states.Figure optionsDownload full-size imageDownload as PowerPoint slide