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 surface chemistry, 3D structure, and the electrochemical behavior of lithiated graphite electrodes. By reviewing results obtained with different electrolyte solutions (e.g. ethylene carbonate-based solutions, propylene carbonate solutions, and ether-based systems), we describe the stabilization and capacity fading mechanisms of graphite electrodes. One of the failure mechanisms occurs at potentials > 0.5 V Li/Li⁺, and relates to an increase in the electrode's impedance due to improper passivation and a simultaneous change in the electrode's morphology, probably due to gas formation. At low potentials (depending on the electrolyte solution involved), phenomena such as exfoliation and amorphization of the graphite electrodes can be observed. Stabilization mechanisms are also discussed. In general, surface stabilization of 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 solutions 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 study included FTIR and impedance spectroscopies, XPS, and in situ and ex situ XRD in conjunction with standard electrochemical techniques.