Thermal cycling induced capacity enhancement of graphite anodes in lithium-ion cells

Graphite electrodes were electrochemically cycled in Li-ion cells at 50 and 60 °C in order to determine the changes in their surface properties in comparison to the electrodes tested at 25 °C. A 17% drop in planar capacity occurred during the first cycle at 60 °C compared to a 40% at 25 °C and reduced the amount of damage that occurred to graphite due to a rapidly formed solid electrolyte interphase (SEI). During the following cycles, a planar capacity of 3.11 ± 0.12 mAh cm−2 was attained at 60 °C rather than 0.53 ± 0.03 mAh cm−2 at 25 °C. The SEI layer formed at 60 °C predominantly consisted of Li2CO3 and was devoid of residual LiClO4 detected at 25 °C. At 25 °C, the diffusion coefficient of Li+ (DLi+) was calculated as 1.07 × 10−8 cm2 s−1, whereas at 60 °C, DLi+ increased to 3.25 × 10−8 cm2 s−1. A pre-treatment conducted at 60 °C enhanced the cyclic performance of graphite subsequently cycled at 25 °C; a Li2CO3-enriched SEI, generated during the 60 °C pre-treatment, covered the graphite surface uniformly and resulted in a 28% increase in battery capacity at 25 °C.