We have computed the lattice dynamics properties of the archetypal superionic conductor Li 3 N using different levels of phonon theory. We show that the physics of superionic conductors is complex, even in a system as compositionally and structurally simple as Li 3 N. Our results confirm that β-Li 3 N is not involved in the superionic transition of Li 3 N, but that the superionic transition is specific to α-Li 3 N. We have demonstrated that both the harmonic and quasi-harmonic approximations fail to provide a signature of the superionic transition at 678 K. By using AIMD simulations to capture the high-order anharmonicity of the system, we identify a breakdown in the computed phonons in the superionic regime as quantified through the computed vibrational linewidths. This ob-servation mirrors previous experimental observations of phonon breakdown at superionic transitions in other systems . We suggest that the breakdown comes from a combination of short occupation times of the ions at their formal lattice sites and an increase in the occupation of interstitial positions, with both effects potentially contributing to a loss of periodic long-ranged vibrational order, preventing the ion dynamics from being directly mapped onto a set of normal-modes. We anticipate these conclusions to hold for other superionic conductors, and highlight a number of questions that emerge from our findings regarding the nature of the relationship between anharmonic contributions to ion dynamics and superionic phase transitions.