The reaction mechanism of a complex intercalation system: in situ X-ray diffraction studies of the chemical and electrochemical lithium intercalation in Cr4TiSe8.

The intercalation reaction between Cr(4)TiSe(8) and Li was investigated from a kinetic and an electrochemical perspective. The structural phase transition from monoclinic to trigonal symmetry was probed by in situ energy-dispersive X-ray diffraction (in situ EDXRD) for chemical intercalation with butyllithium (BuLi). A change in the kinetic mechanism was detected for the reaction at room temperature; this was interpreted in terms of a trend from phase boundary control to diffusion control. A single diffusion-controlled mechanism is obeyed at 60 degrees C. The electrochemical measurements and the corresponding in situ X-ray diffraction (in situ XRD) data revealed that the monoclinic host is intercalated up to the composition Li(x approximately 0.1)Cr(4)TiSe(8) before the characteristic phase transition starts. The monoclinic phase undergoes complex structural changes in the following two-phase regime. Owing to the co-existence of two phases, the cell potential is constant for 0.1<x<0.7 and 0.9<x<3.0. The subsequent intercalation into the trigonal phase leads to a pronounced increase in the cell volume of the trigonal phase that stops at x approximately 0.8. At this point, the complete reduction of Ti(IV) gives rise to a voltage drop of the cell potential. XANES measurements revealed that the reduction of Ti(IV) occurs prior to the reduction of Cr(III).