Solid State Ionics 27 (1988) 101-107 North-Holland,Amsterdam LITHIUM INSERTION INTO INDIUM SESQUISELENIDE C. JULIEN and I. SAMARAS Laboratoire de Physique des Solides, associ~ au CNRS, Universit~ Pierre et Marie Curie. 4 Place ]u~siet~ 75252 Paris Cedex 05, France Received25 February 1988; acceptedfor publi~tion 29 February 1988 The effectsof lithium interc~tion in Ir.~Se3 havebeen studiedusingdifferent methods.Electruehemi,:al propert~ of the non. stoicbiometde Li#ln2Se3 phaseare give, in the range 0 ~< x¢ 1 for either quenchedor annealedcompounds.Electronic conductiv- ity has been measured during the inset,ion processin n-butyl lhldum and su~ges'~ weakchargetransfer.The rehlinn between the morphologyofsaraples and insertion mechanism have been discussed. 1. ln~'oducfion It is now widely recognized that intercalation or insertion reactions which occur topotaetteally can provide a mechanism fc, r kinetically fast and re- versible solid state elect~ehermeal reactions [ 1,2]. Several excellent review articles on intercalation chemistry and electrode materials are available [3,4]. Some of the most important properties of the reac- tions and the materials which must be considered in- elude the thermodynamics of the reactions, the chemical diffusion coefficient of the interedant in the host and the electronic properties of the materials. Among the layered materials used as cathode in lithium batteries, the compounds of the III-VI fam- ily can be intercalated by ~ikali guest metals [5,7]. In previous work, we have demonstrated that lith- ium [8] or silver [9] atoms can intercalate In2Se3. Preliminary experiments have shown that in an elec- trochemical non-aqueous lithium ceii~ the diffusivity of the Li ions is 5×10 -~° cm z s-' at room temperature. In2Se3 is a layered se:niconducting material exist- ing in four (a, ~, y, ~) modifications. The room tem- perature phase (a-phase) is a tctrahedrally coordinated compound which crystallizes in a de- fective wiirmite structure ~th rcs~ct to the cationic site occupancy. It is known that while all the bonds are satisfied, there remains one third of the cation sites unoccupied [ 10]. These vacant si~es form the so-called stoichiometric vacancies. When these va- cancies are ordered, the whole stru~ure is ~m-anged in slabs, with two layers per unit ce~ along the c-axis. Each layer is formed in packets of two in and three Se sub-layers, in the sequence Se-In-Se--In-Se. Tke bonding inside the layers is strongly covalem, while the interlayer interaedon (Se-Se) is of the van der Waals type giving a gap of 0.3~7 nm wide. In this paper we discuss the effects of lithium in- tercalation in crystalline In2Se3 ~vith 0<x<2. Our studies have been carried out using electrochemical and chemical intercalation methods. Evidence of phase lm~ormations is seen for quenched and an- nealed compounds as function of littdum eoncenwa- tion. Electrical and electrochemical characterizes of the solid-state-solution electrodes are given and the lithlated compounds Li~.sln2Se3 shows a relatively low free energy. 2. Experimental 2.1. Material In2Se3 crystals were grown by direct fusion of the elements in stoiclfiometric proportion. Details of the er:,.,~ growth were given elsewhere [ 11 ]. Two types of crystals were obtained depending on the cooling rates of the ~own ingots, i.e. quenched (Q) type ~__mmples, after a rapid cooling to liquid nitrogen, and 0 167-2738/88/$ 03.50 © Elsevier Science Publishers B.V. (North-Holland Physics Publishing Division )