The two-dimensional polymer structure and lattice dynamics of the superionic conductor Li 4 C 60 are investigated by neutron diffraction and spectroscopy. The peculiar bonding architecture of this compound is confirmed through the precise localization of the carbon atoms involved in the intermolecular bonds. The spectral features of this phase are revealed through a combination of ab initio lattice dynamics calculations and inelastic neutron scattering experiments. The neutron scattering observables are found to be in very good agreement with the simulations which predict a partial charge transfer from the Li atoms to the C 60 cage. The absence of a well-defined band associated with the Li atoms in the experimental spectrum suggests that this species is not ordered even at the lowest temperatures. The calculations predict an unstable Li sublattice at a temperature of ∼200 K, which we relate to the large ionic diffusivity of this system: Low-frequency optic modes of the Li ions couple to the soft structure of the polymer.