Comparison between hard carbon and natural graphite using the thermal safety diagrams of lithium-ion batteries

For the thermal safety of lithium-ion batteries (LIBs), it is important to consider the calorific value (Q), the onset temperature and time of the thermal runaway. Thus far, the onset temperature and time have been estimated for well-known materials using a thermal safety diagram. However, little attention has been paid to the hard carbon (HC) and natural graphite (NG) structures incorporated in a cell. In this study, the calorific values (Q) per unit energy density of cells with LiNi0.8Co0.1Mn0.1O2 (NCM811) as the cathode and HC and NG as the anodes were compared and found to be similar. The structures of these carbon materials were identified using powder X-ray diffraction. The surface on the anode was analyzed using X-ray photoelectron spectroscopy. Open current voltage (OCV) measurements of the cells during heating were also carried out. Furthermore, the obtained diagrams showed that NCM811/HC has a delayed onset time for the thermal runaway compared to NCM811/NG. These results will help in reducing the Q and reaction rates by controlling the OCV of the cell, thereby leading to LIBs with improved thermal safety properties.