Phonon-Lithium Ion Interactions: A Case Study of LiM(SeO₃)₂ (M = Al, Ga, In, Sc, Y, and La)

Li ion diffusion is fundamentally a thermally activated ion hopping process. Recently, soft lattice, anharmonic phonon, and paddlewheel mechanism have been proposed to potentially benefit the ion transport, while the understanding of vibrational couplings of mobile ions and anions is still very limited but essential. Herein, we accessed the ionic conductivity, stability, and especially, lattice dynamics in LiM(SeO₃)₂ (M = Al, Ga, In, Sc, Y, and La) with two different types of oxygen anions within a LiO₄ polyhedron, namely, edge-shared and corner-shared with MO₆ polyhedra, the prototype of which, LiGa(SeO₃)₂, has been theoretically reported before with the similar structural features to NASICON and later experimentally synthesized with the room temperature conductivity ∼0.11 mS cm^-1. It is interesting to note that LiM(SeO₃)₂ with a higher Li phonon band center shows higher Li conductivity, which is in contradiction to the conventional understanding of the importance for soft lattice to superionic conductors. The anharmonic and harmonic phonon interactions as well as the couplings between the vibration of the edge-bonded or corner-bonded anion in Li polyanions and the Li ion diffusion have been studied in detail. With transition metal M changing from La, Y, In, Ga, Al, and Sc, anharmonic phonons increase with reduced activation energy for Li diffusion. The phonon modes dominated by the edge-bonded oxygen anions contribute more to the migration of the Li ion than those dominated by the corner-bonded oxygen anions because of the greater atomic interaction between the Li ion and the edge-bonded anions. Thus, rather than the overall lattice softness, attention shall be given to reduce the frequency of the critical phonons contributing to Li ion diffusion as well as to increase the anharmonicity, i.e., through asymmetric Li polyhedra, for the design of Li ion superionic conductors for all-solid-state batteries.