The influence on Fermi energy of Li-site change in LizTi1-yNiyS2 on crossing z=1

Abstract

The Fermi energy EF of the TiS2 layers of metallic LixTiS2 increases by 1.3 eV as x increases through x = 1 to a two-phase state containing a Li-rich phase with Li shifted from octahedral to tetrahedral sites of the space between TiS2 layers. For x < 1, EF lies in a narrow 3d band of the Ti(iv)/Ti(iii) mixed-valence state; in the Li-rich x > 1 phase, EF lies in the 3d band of the Ti(iii)/Ti(ii) mixed valence. In order to determine how much of the shift of EF is due to the on-site electron-electron electrostatic energy U of the narrow band Ti-3d electrons and how much is due to the shift of the Li in the interlayer space from octahedral to tetrahedral sites, we have monitored the change in EF within the Ti(iv)/Ti(iii) mixed-valence state of Lix(Ti0.9Ni0.1)S2 as x is increased through x = 1. Substitution of Ni for Ti changes the sulfur stacking from hexagonal to cubic, but the stacking changes back to hexagonal on insertion of Li to x ≈ 1. A voltage step at 1.45 V with cubic stacking of the sulfide planes and at 1.15 V with hexagonal stacking reveals a raising of EF by 0.3 eV on going from cubic to hexagonal stacking. Shifting of the Li + ions from octahedral sites to a Li-rich phase in which they occupy tetrahedral sites on increasing x through x ≈ 1 raises EF by Δm ≈ 0.5 eV, which indicates a U ≈ 1.3 eV - Δm ≈ 0.8 eV in the narrow Ti-3d band of LiTiS2 having a width W > U.