Lithium insertion mechanism in tin-based spinel sulfides

Cu2MSn3S8 (M = Fe, Co) spinel compounds have been studied as cathode materials in Li/LiPF6(1M)-PC-EC/spinel sulfide cells. To understand the lithium insertion mechanism, both pristine compounds and chemically inserted products were characterized by X-ray Powder Diffraction (XPD), Sn-119 Mossbauer Spectroscopy (MS), and X-ray Absorption Spectroscopy (XAS) at the sulfur K-edge. To accommodate the inserted lithium ions, two steps of reduction have been observed. First, for a low lithium content (up to 2 Li per formula), a copper reduction is occurring. Moreover, for the iron-based compound the copper atoms extraction is associated with a migration of iron atoms from the 16c octahedral sites toward the 8a tetrahedral sites. Second, for a high lithium content (above 2 Li per formula) a complex and simultaneous reduction of tin(IV) toward tin(II) and tin(0) has been clearly observed as well as a distortion of sulfur coordination polyhedra during the insertion process. From an electrochemical point of view, these spinels are rather interesting as model compounds regarding the lithium insertion study.