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

Abstract

The Fermi energy E_F of the TiS₂ layers of metallic Li_xTiS₂ 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, E_F lies in a narrow 3d band of the Ti(IV)/Ti(III) mixed-valence state; in the Li-rich x > 1 phase, E_F lies in the 3d band of the Ti(III)/Ti(II) mixed valence. In order to determine how much of the shift of E_F 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 E_F within the Ti(IV)/Ti(III) mixed-valence state of Li_x(Ti_0.9Ni_0.1)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 E_F 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 E_F by Δ_m ≈ 0.5 eV, which indicates a U ≈ 1.3 eV − Δ_m ≈ 0.8 eV in the narrow Ti-3d band of LiTiS₂ having a width W > U.