Effects of lithium intercalation on the electronic properties of FePS3 single crystals

The reaction of FePS3 single crystals with a 1.6M n-butyl lithium solution in n-hexane has been followed through de electrical conductivity measurements as a function of the intercalation time. In order to investigate the influence of lithium insertion on the FePS3 electronic properties we have also measured as a function of both temperature and intercalation time the de conductivity of equilibrated LixFePS3 single crystals. Upon lithium intercalation we have observed the following main effects: (i) an increase in the electrical conductivity, (ii) a simultaneous decrease in the activation energy, and (iii) a degenerate semiconductor behavior at the highest lithium content. In analogy to the LixNiPS3 systems, these results, discussed in terms of both the rigid band model and the so-called transition-metal weakly interacting one, seem to indicate that in the LixFePS3 complexes a new conduction mechanism appears at a different energy level from the beginning of the intercalation process with respect to the pure FePS3. Aiming at a better understanding of the still uncertain nature of reduction sites in the FePS3 lattice during lithium intercalation, we have also carried out Fourier transform infrared absorption measurements on single crystals of FePS3 and their lithium intercalation compounds at room temperature and in the frequency region from 800 to 3000 cm(-1). The results agree well with both the literature and the above conductivity data allowing a better identification of the lithium 2s electron accepting levels.