Lithium insertion into transition-metal monosulfides: Tuning the position of the metal 4s band

Concern for safety with carbon anodes has motivated a search for an alternative anode material for high-power lithium-ion batteries. Although sulfides cannot provide the high voltages versus lithium required of a cathode material, whether they can provide the low voltages required of an anode material remains to be determined. Investigation of the displacement reaction 2Li + MS = Li2S + M for M = Ti, V, Cr, Fe, Co, Ni reveals that, with increasing atomic number, the bottom of the M 4s band falls systematically to lower energy and thereby increases the voltage of the displacement reaction. In addition, the reversibility of the displacement reaction is observed to be increased on going to heavier transition-metal atoms. Ex-situ X-ray diffraction (XRD) of the insertion of Li into FeS provides evidence that the reversibility is associated with formation of an amorphous phase rather than Li2S. It is suggested from this investigation that the Ti3+/Ti2+, V3+/V2+, and Cr3+/Cr2+ couples in an insertion sulfide can be below the 4s band at V <= 1.0 V versus Li+/Li-0.