Integrating our simplified models of sodium insertion with the expansion of C-C bonds, the changes in d-spacing, and DFT calculations, we propose a sequential adsorption mechanism occurring during the slope region. Initially, during the steepest part of the slope region, sodium ions adsorb to high-binding energy defect sites, such as monovacancies, on the surface of HC, as evidenced by DFT calculations (Figure ) and the decrease in d-spacing upon initial sodiation.