CORRELATIONS BETWEEN SURFACE PROPERTIES OF GRAPHITE AND THE FIRST CYCLE IRREVERSIBLE CAPACITY IN LITHIUM-ION BATTERIES: INFLUENCE OF THE OXYGEN FUNCTIONNEL GROUPS

摘要

During the first electrochemical insertion of lithium-ions into graphite negative electrodes of lithium-ion batteries, a passivation layer called the solid-electrolyte interphase (SEI), composed of electrolyte decomposition products, is formed at the graphite surface. The formation of the SEI layer is of prime importance for highly crystalline graphite since it avoids electrochemical exfoliation of the graphite. On the contrary, the formation of this passivation layer is also responsible for an irreversible capacity "loss". Therefore, the identification of key parameters influencing the SEI formation and its passivation efficiency is a pre-requisite for the development of improved graphite electrode materials. Two surface parameters were studied here: the active surface area (ASA) and the surface chemistry (oxygen functional groups) of the graphite. We show in this study that the ASA is a determining parameter controlling the exfoliation tendency of the graphite electrode during the first electrochemical cycle. For an ASA value lower than 0.2 m~2 g~(-1), the graphite exfoliates no matter what is the ASA value or the corresponding surface chemistry. Above 0.2 m~2g~(-1), the exfoliation phenomenon is avoided and the irreversible capacity seems to be more dependent on the surface chemistry. The removal of the oxygen surface groups is not beneficial for the SEI formation and results in increase of irreversible capacity.