A so-called electrochemical dilatometer can be used to record the macroscopic expansion (dilatation) and contraction of samples which undergo insertion/deinsertion reactions. Such dilatations and contractions are found in crystals having a layered lattice when guest ions undergo intercalation/deintercalation. These macroscopic changes of crystal dimensions (thickness) can be monitored in situ, simultaneously with electrochemical parameters such as the electrode potential, when the dilatometry is performed in combination with some electrochemical technique (here, cyclic voltammetry).Apparatus.-The dilatometric measurements are based on the resonance frequency shift occurring in an oscillatory circuit containing two ferrite shells, with one of the ferrite shells mounted so as to be mobile. This approach is schematically illustrated by Fig. 1. The sample used in the dilatometer (graphite, in this work) is made to function as the working electrode (WE) and is placed between two pistons. The lower piston, made of nickel, is stationary and represents the current collector. The upper piston, made of polypropylene (PP), is mobile and transmits changes in sample thickness to the upper ferrite shell of the oscillatory circuit. Close contact of the nickel current collector with the sample and the upper piston is assured by a small copper weight (10.5 g) positioned on top of the upper piston. The resulting resonance frequency shift of this circuit is transformed into a voltage value by a frequency transducer, and then recorded simultaneously with the electrochemical data (voltammetric signals).Previous work.-Dilatometry has been used since the late 1970s to monitor the "swelling" of layered host compounds due to intercalation of guest ions, atoms, and/or molecules between the layers of the host. 1-7 However, the dilatometer records obtained in those studies suffered from a relatively poor resolution of the dilatometer apparatus. Biberacher et al. 8 developed a high resolution electrochemical dilatometer able to resolve staging during intercalation of guests into the host. In the present work, we used a dilatometer that has the same operating principle and resolution (cf. Fig. 1).Most previous work has involved protic electrolytes such as HSO 4Ϫ /H 2 SO 4 , 8 M xϩ /H 2 O (M ϭ Na, K, Ca, Zn, Mn, Co, La, Gd, Pr; x ϭ 1, 2, 3), 8-10 and C 4 F 9 SO 3 H. 11 Layered samples, not necessarily single crystals, such as highly oriented pyrolytic graphite (HOPG), 8,11 natural graphite flakes, 11 and 2H-TaS 2 8-10 have been the host materials.Electrochemical dilatometry in nonaqueous aprotic electrolytes was first reported in 1994. 12 The intercalation of solvated lithium ions, Li ϩ (solv) y , also known as solvated lithium intercalation, was studied in HOPG using electrolytes such as 1 M LiClO 4 in ethylene carbonate (EC)/dimethoxyethane (1:1 by volume), 12-15 where a large relative expansion of ⌬L/L 0 Ϸ 150% occurs (L 0 is the thickness of the sample prior to reaction). The macroscopic dilations and contractions associated with the alloy...
