Correlations between surface properties of graphite and the first cycle specific charge loss in lithium-ion batteries

We report the influence of two surface parameters, the active surface area (ASA) and the surface chemistry (oxygen functional groups) on the first electrochemical reduction of graphite. The experimental results highlight two important points. One, the ASA is the determining parameter which controls the exfoliation during the first electrochemical cycle. An ASA limit (ca. 0.2 m² g⁻¹) above which the exfoliation is suppressed was experimentally found. Below or above this limit, the specific charge loss remains almost constant even if the ASA changes. Two, for a sample having an ASA value higher than 0.2 m² g⁻¹, it is shown that the presence of oxygen groups at the surface is critical for the formation of an efficient solid electrolyte interphase (SEI) layer. The lack of oxygen groups during the first electrochemical reduction cycle hinders the electrolyte reduction process, and consequently increases the specific charge loss via exfoliation of the graphite electrode. This was confirmed by TPD measurement where significant release of CO gas occured above 400 °C, suggesting the presence of high-thermal-stable surface oxygen-containing groups of different natures in the as-received - SLX50 sample. Finally, it was found that H₂ treatment avoids the formation of oxygen-containing groups during air contact leading to exfoliation of the graphite sample.