On the correlation between surface chemistry and performance of graphite negative electrodes for Li ion batteries

This paper discusses some important aspects of the correlation between surf ace chemistry, 3D structure, and the electrochemical behavior of lithiated graphite electrodes. By reviewing results obtained with different electroly te solutions (e.g. ethylene carbonate-based solutions, propylene carbonate solutions, and ether-based systems), we describe the stabilization and capa city fading mechanisms of graphite electrodes. One of the failure mechanism s occurs at potentials >0.5 V Li/Li+, and relates to an increase in the ele ctrode's impedance due to improper passivation and a simultaneous change in the electrode's morphology, probably due to gas formation. At low potentia ls (depending on the electrolyte solution involved), phenomena such as exfo liation and amorphization of the graphite electrodes can be observed. Stabi lization mechanisms are also discussed. In general, surface stabilization o f the graphite is essential for obtaining reversible lithiation and a long electrode cycle life. The latter usually relates to precipitation of highly compact and insoluble surface species, which adhere well, and irreversibly , to the active surface. Hence, the choice of appropriate electrolyte solut ions in terms of solvents, salts and additives is very critical for the use of graphite anodes in Li batteries. The major analytical tools for this st udy included FTIR and impedance spectroscopies, XPS, and in situ and ex sit u XRD in conjunction with standard electrochemical techniques. (C) 1999 Els evier Science Ltd. All rights reserved.