  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 ion and anions is still limited but essential.
Herein, we access the ionic conductivity, the stability and the lattice
dynamics in LiM(SeO3)2 (M =Al, Ga, In, Sc, Y, and La) with two types of oxygen
anions within LiO4 polyhedron, namely edge-shared and corner-shared, the
prototype of which, LiGa(SeO3)2, has been experimentally synthesized. We
studied in detail the anharmonic and harmonic phonon interactions, as well as
couplings between vibrations of edge-bonded or corner-bonded anions in Li
polyanions and Li ion diffusion. As M changing from Sc to La, anharmonic
phonons increase alongside reduced activation energy for Li diffusion. Phonon
modes involving edge-bonded oxygen anions contribute more to Li migration than
corner-bonded oxygen anions, owing to greater atomic interactions between Li
ions and edge-bonded anions. Thus, rather than the overall lattice softness,
attentions shall be paid to reduce the frequency of the critical phonons
contributing to Li ion diffusions as well as to increase the anharmonicity, for
the design of Li ion superionic conductors for all-solid-state-batteries.
