A study of Li intercalation into Cr3Ti2Se8 using electrochemistry, in-situ energy dispersive X-ray diffractometry and NMR spectroscopy

Lithium has been inserted into Cr3Ti2Se8 by chemical and electrochemical methods. Rietveld refinements were used to analyze the X-ray diffraction patterns of the lithiated phase. Upon intercalation, the monoclinic symmetry of the genuine host material Cr3Ti2Se8 changes to trigonal symmetry. The structure of the intercalated phase LixCr0.75Ti0.5Se2 is reminiscent of the well known transition metal dichalcogenides with the guests residing in the van der Waals gaps. The results of the electrochemical intercalation and in situ X-ray diffraction data revealed that the genuine material is intercalated up to a critical composition Li0.06Cr0.75Ti0.5Se2 before the phase transition occurs. The lattice parameters of the new phase increase with the Li concentration. A maximum Li content of x ≈ 0.68 was obtained. The electrochemical discharge curve exhibits two constant cell potentials at EMF ≈ 1.8 V and 0.7 V. The Li insertion is reversible and treating fully intercalated material with water yields Li0.18Cr0.75Ti0.5Se2 as final product. The symmetry remains trigonal indicating that the structural phase transition is not reversible. 7Li magic angle spinning (MAS) NMR measurements reveal only one unique Li position. The results are compared with the structurally related Cr4TiSe8 / LixCrTi0.25Se2 system and similarities for the intercalation kinetics are found, but also pronounced differences concerning the electrochemistry are observed reflecting the different electronic structures of the two materials.