Analysis of Intercalation/De-Intercalation of Li Ions Into/From Graphite at 0 °C via Operando Synchrotron X-ray Diffraction

Abstract

The charge/discharge reaction mechanisms of graphite negative electrodes in Li ion batteries were investigated via operando synchrotron X-ray diffraction at 0 °C and 25 °C. The intercalation of Li ions at 25 °C formed the stage 1 compound with an in-plane structure of LiC₆; while intercalation at 0 °C only formed stage 2, with in-plane structures of LiC₉ and LiC₆. The degree of graphite expansion in the a, b-axes and c-axis directions by intercalation at 0 °C was less than that at 25 °C. Hence, it was difficult to form the stage 1 structure by further increases in the Li ion concentration, and the charging reaction at low temperature became difficult. De-intercalation at 0 °C did not follow the Daumas–Hérold model and proceeded discretely in the order: stage 1 → stage 2 → stage 4 → graphite, without going through stages 3, 5–8 and dilute stage 1.