Despite its high reversibility for Li+ intercalation, graphite is known to be electrochemically inactive for Na+ intercalation. On the contrary, recent studies have demonstrated that graphite is active and shows excellent rate and cycle performance for Na+-solvent cointercalation but it exhibits poor performance for Li+-solvent cointercalation. Herein, we elucidate the mechanism of Li+- and Na+-solvent cointercalation into graphite and the origin of the strikingly different electrochemical performance of Li+- and Na+-solvent cointercalation cells. Na+ intercalation into graphite is thermodynamically unfavorable, but Na+-diglyme cointercalation is very favorable. The diglyme–graphene van der Waals interaction reinforces the interlayer coupling strength and thereby improves the resistance of graphite to exfoliation. The transport of solvated Na ions is so fast that the diffusivity of Na+-diglyme complexes is markedly faster (by five orders of magnitude) than that of Li+-diglyme complexes. The very fast Na+-diglyme conductivity is attributed to facile sliding of flat diglyme molecules, which completely solvate Na ions in the interlayer space of graphite. The slow Li+-diglyme conductivity is ascribed to steric hindrance to codiffusion caused by bent diglyme molecules that incompletely solvate Li ions. The bent and flat diglyme molecules surrounding Li and Na ions, respectively, are highly associated with the strong Li+–graphene and weak Na+–graphene interactions, respectively.