On the behavior of different types of graphite anodes Doron Aurbach * , Hanan Teller, Maxim Koltypin, Elena Levi Department of Chemistry, Bar-Ilan University, Ramat-Gan 52900, Israel Abstract Different types of graphite materials, i.e. synthetic graphite flakes and natural graphite flakes which are used as anode materials in Li-ion batteries were studied. Differences in the electrochemical behavior of electrodes comprised of these materials, mainly in their irreversible capacity, could be correlated to the differences in the particle morphologies and their crystal structure. We propose that graphite particles with a large amount of crevices in their edge planes can crack, due to a build-up of internal pressure as a result of reduction of solution species on the carbon surfaces during the first cathodic polarization on the electrodes. Another important factor determining the electrodes’ stability is the existence of some disorder in the particles’ structure. # 2003 Elsevier Science B.V. All rights reserved. Keywords: Graphite; Particle morphology; Irreversible capacity; AFM; XRD; PC; EC 1. Introduction Graphite electrodes are widely used as anodes in Li-ion batteries [1]. Graphite intercalates reversibly with lithium in a four-stage process, which involves phase transitions [2]. Since the lithium insertion process into graphite occurs at very low potentials (below 0.3 V versus Li/Li þ ), all the relevant solvents and salts in which Li intercalation into graphite may proceed reversibly, are reduced on the graphite electrodes during their cathodic polarization at potentials higher than Li insertion potentials [3], forming passivating surface films on the anodes [4,5]. Graphite electrodes fail in a number of commonly used electrolyte solutions, especially those based on propylene carbonate (PC). A failure mechan- ism suggested for graphite electrodes in PC solutions relates to an insufficient passivation, which enables co-intercalation of PC molecules with Li-ions into the graphite lattice, causing the exfoliation of the graphene planes (i.e. amor- phization of the graphite particles) [6,7]. We have found in recent studies that this mechanism applies to ethereal Li salt solutions [8], whereas in PC solutions the basic 3D structure of the electrodes’ active mass remained graphitic (measured by XRD). Our suggestion was that in PC solutions, graphite particles crack due to gas formation. The graphite anodes are thus deactivated due to electrical isolation of the cracked particles by surface films [9]. In this study we tested different kinds of graphite materials in LiPF 6 /EC–DMC and LiClO 4 /EC–PC solutions. The differences in behavior of these graphite materials in the latter solution can be explained by the differences in their morphology and structure. The experimental tools for this study included chronopotentiometry (galvanostatic lithiation–delithiation), atomic force microscopy (in situ), electron microscopy, and XRD. 2. Experimental In this study we used commercial 1 M LiPF 6 in an EC:DMC ¼ 1:1 solution from Merck KGaA, Germany. In addition, a solution of 1 M LiClO 4 in an EC:PC ¼ 2:3 solution (Tomiyama) was prepared. The water content of these solutions was less than 15 ppm (monitored by Karl Fisher titration, a Metrohm Inc., 562 CF coloumeter). We tested synthetic graphite flakes, KS6, KS15, KS25, and KS44 powders from Timcal Inc., Switzerland, natural gra- phite flakes (NGF) and another type of natural graphite flake (TNGF) from Chuetso Graphite Works Co. Ltd., Japan. The electrodes were prepared by mixing the active material powder and 10% PVdF binder and adding 1-methyl-2- pyrrolydonen to obtain an homogeneous slurry. The slurry was then spread on a 12 mm diameter pre-rubbed copper disk. The electrodes’ mass was usually 4 mg. The electrodes were dried under vacuum for 12 h. A T-shaped cell was used for the galvanostatic measurements, in which a lithium disk was used as a counter electrode. Galvanostatic measurements Journal of Power Sources 119–121 (2003) 2–7 * Corresponding author. Fax: þ972-3-5351250. E-mail address: aurbach@mail.biu.ac.il (D. Aurbach). 0378-7753/03/$ – see front matter # 2003 Elsevier Science B.V. All rights reserved. doi:10.1016/S0378-7753(03)00115-0