A study of Li intercalation into Cr 3 Ti 2 Se 8 using electrochemistry, in-situ energy dispersive X-ray diffractometry and NMR spectroscopy Joseph Wontcheu a , Malte Behrens a , Wolfgang Bensch a, ⁎ , Sylvio Indris b , Martin Wilkening b , Paul Heitjans b a Institut für Anorganische Chemie, University of Kiel, Germany b Institut für Physikalische Chemie und Elektrochemie, Leibniz Universität Hannover, Callinstraβe 3-3a, D-30167 Hannover, Germany Received 2 October 2006; accepted 20 February 2007 Abstract Lithium has been inserted into Cr 3 Ti 2 Se 8 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 Cr 3 Ti 2 Se 8 changes to trigonal symmetry. The structure of the intercalated phase Li x Cr 0.75 Ti 0.5 Se 2 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 Li 0.06 Cr 0.75 Ti 0.5 Se 2 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 Li 0.18 Cr 0.75 Ti 0.5 Se 2 as final product. The symmetry remains trigonal indicating that the structural phase transition is not reversible. 7 Li magic angle spinning (MAS) NMR measurements reveal only one unique Li position. The results are compared with the structurally related Cr 4 TiSe 8 / Li x CrTi 0.25 Se 2 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. © 2007 Elsevier B.V. All rights reserved. Keywords: Chromium selenides; Lithium intercalation; Structural phase change; NMR spectroscopy 1. Introduction One fascinating property of transition metal dichalcogenides (TMDC) is the formation of intercalation complexes with guest species ranging from simple atoms to large molecules. During the intercalation process, electrons are transferred from the guest species to the transition-metal based d states of the host altering the physical properties of the host–guest complex. Intercalation with alkali metals A is particularly interesting due to the strong tendency to donate the single valence electron, and the possibility to form in-situ intercalation compounds. The intercalation reaction is done by deposition of the alkali metals onto the TMDC surface from where it spontaneously inter- calates [1,2]. From a chemical point of view, the intercalation process may be described as a reversible topotactic reaction accompanied by a simultaneous cation–electron transfer [3]. The alkali metal intercalation reactions of TMDC (TQ 2 with T = elements of group 4–6, Q = S, Se) were extensively studied in the past [3–6]. Until now, only few studies were conducted with compounds exhibiting more complex crystal structures and an electronic structure which may be altered by changing the constituents of the material. In a previous study we reported surprising results of the chemical and electrochemical intercalation of Li in Cr 4 TiSe 8 [7]. The crystal structure of Cr 4 TiSe 8 can be derived from the CdI 2 type [8] reducing the formula to M 1.25 Se 2 (M = Cr, Ti). Edge sharing MSe 6 (M = Cr, Ti) octahedra form layers Solid State Ionics 178 (2007) 759 – 768 www.elsevier.com/locate/ssi ⁎ Corresponding author. Insitut für Anorganische Chemie, Universität Kiel, Olshausenstraβe 40-60, 24098 Kiel, Germany. Tel.: +49 431 880 2419; fax: +49 431 880 1520. E-mail address: bensch@ac.uni-kiel-de (W. Bensch). 0167-2738/$ - see front matter © 2007 Elsevier B.V. All rights reserved. doi:10.1016/j.ssi.2007.02.026