Infrared studies of lithium intercalation in the FePS3 and NiPS3 layer-type compounds

Infrared spectra (700-30 cm^-1) of several lithium intercalates (chemically prepared) Li_xMPS₃, with M=Fe, Ni and 0<x<1.5, have been recorded and compared with those known for the corresponding host lattices. These lithium intercalates are mainly characterized by new absorption bands at 336 cm^-1 and 310 cm^-1 for the iron and nickel compounds, respectively. These bands assigned to lithium vibrations increase progressively with lithium content : it is concluded that Li⁺ ions are more likely to occupy the 2d and 4h “octahedral” sites in the gaps. In addition, the spectra of the nickel derivatives reveal some geometric distortion within the layers and a progressive strengthening of the Ni-S interactions. These results are correlated to the best energy yields obtained in NiPS₃/lithium batteries.