Electrochemical lithiation of the ternary silver sulfide Ag 4Hf 3S 8

Abstract

Electrochemical lithiation of cubic Ag 4Hf 3S 8 that possesses both a two-dimensional (2D) array and a 3D channel of octahedral metal vacancy sites has been studied using an organic electrolyte lithium ion cell, Li/1 M LiPF 6 in propylene carbonate/Ag 4Hf 3S 8, under low constant current discharge conditions. Lithiation processes are discussed on the basis of proposed models originating from the inherent structural nature of Ag 4Hf 3S 8. The X-ray powder diffraction (XRD) lines for the lithiated phase shifted to slightly higher angles than those of Ag 4Hf 3S 8, and native Ag peaks were found in the XRD pattern, indicating that the intercalation of a certain amount of Li + induced reduction of Ag + and subsequent removal of Ag atoms from the host. The intercalated Li + became the building constituent instead of the original Ag +, which led to a structural reformation of the host material. The lattice constant of the lithiated phase, Li x Ag y Hf 3S 8 ( x+ y≤7, y≅0), was shortened by about 0.5 Å in comparison with that of Ag 4Hf 3S 8. It was ascertained from Rietveld analyses of XRD patterns at several discharge steps that the structures of lithiated phases could be solved without changing the original space group of Ag 4Hf 3S 8 and that electrochemical lithiation of Ag 4Hf 3S 8 proceeded topochemically conserving the Hf 3S 8 framework structure. The cell showed a long plateau at around 2.2–2.0 V up to the incorporation of about 3 mol Li +.