JOURNAl.OF ELSEVIER Journal of Electroanalytical Chemistry421 (! 997) 79-88 The mechanism of lithium intercalation in graphite film electrodes in aprotic media. Part 1. High resolution slow scan rate cyclic voltammetric studies and modeling Mikhail D. Levi, Doron Aurbach * Department o f Chemistry, Bar-llan University, Ramat Gan 52900, Israel Received 3 January 1996; revised 14 June 1996 Abstract Using slow scan rate (4 to 80 p,Vs-i) cyclic voltammetry for thin graphite electrodes (8 to 10 Ixm thick), two limiting cases for the intercalation mechanism of Li ion in graphite in aprotic solvents have been observed: (i) quasi-equilibrium, capacitive-like step at very slow potential scan rates and (ii) semi-inf'mite diffusion of Li + ions inside the graphite matrix at higher scan rates. Each of these two limiting types of behavior has been appropriately modeled, and from the comparison of experimental and simulated voltammetric curves quantitative information has been extracted, including (a) the effective heterogeneous rate constants for Li* ion transfer through the graphitelsolution interface; (b) the lateral attraction parameter for the intercalated species; (c) half-peak width and peak potential separation; and (d) diffusion coefficients of the intercalated ions. The features of the experimental CV curves are in qualitative agreement with the island model of the staging process proposed in the literature. The diffusion coefficients of Li + ions in graphite evaluated from the voltammetric data were found to be close to those obtained from a potentiostatic intermittent titration technique applied to the same electrodes. Keywords: Intercalation; Lithium; Graphite; Thin film electrodes; Mechanism 1. Introduction It is widely known and well documented [1-9] that graphite and graphitized carbon materials can be consid- ered as excellent anodes for high-energy density 'rocking- chair' Li + insertion rechargeable batteries. Previous stud- ies of electrochemical intercalation of lithium into different kinds of carbon carried out in our laboratory were focused mainly on the influence of solvents, salts and various combinations of additives on the composition of protective surface layer(s) covering the carbon particles of the elec- trode [6,10-12]. Among the carbonaceous materials, graphite has been shown to be the most sensitive to the solution composition which is connected to both the passi- vating properties of SEI-type surface films and the crystal- lographic structure of graphite. * Corresponding author. Formation of effective passivation layers may result in high reversibility of the subsequent Li + intercalation reac- tion (at an ideal capacity, up to LiC6). In a few recent papers the Li-graphite intercalation mechanism was stud- ied by using classical cyclic chronopotentiometric tech- niques in conjunction with ex situ and in situ XRD mea- surements [2,3,8]. Phase transitions during the course of cathodic (intercalation) and anodic (de-intercalation) polar- izafion were investigated, formal reversible potentials and stfindard Gibbs energies of formation of the intercalated compounds were evaluated, and crystallographic models for the staged phases of Li intercalated graphites were proposed to explain the shift of the scattering angle in XRD diffraction patterns with the variation of electrode potential. In addition, the potentiostatic intermittent titra- tion technique (PITI') [13] and electrochemical iiapedance spectroscopy were used to study solid-state diffusion of Li ions in graphite materials [3,14,15]. From these studies diffusion coefficients could be evaluated. Solid-state diffu- sion as well as quasi-reversible charging of graphite parti- 00224)728/97/$17.00 Copyright O 1997Elsevier Science S.A. All fights reserved. PII S0022-0728(96)04832-2