A detailed-balance model for thermionic emission from polyanions has been developed and applied to fullerene dianions. The specificity of this delayed decay process is electron tunneling through the repulsive Coulomb barrier (RCB). An analytical expression of the RCB is derived from electrostatic modeling of the fullerene cage. The reverse process, namely, electron attachment to the singly charged anion, is described by a hard sphere cross section weighted by the Wentzel-Kramers-Brillouin tunneling probability. This simple expression leads to a very good agreement with a measured time-resolved kinetic energy distribution of C$_{84}$$^{2−}$. Electron binding energy is reduced when the fullerene cage size decreases, leading to an almost zero one for C$_{70}$$^{2−}$ and a negative one for C$_{60}$$^{2−}$. Extension of the model to these systems of interest is discussed, and model outputs are compared with the experimental data from the literature.