Role of Phonons in superionic Li diffusion in Li1.5Al0.5Ge1.5(PO4)3

NASICON‐type solid electrolytes exhibit high ionic conductivity, stability against air and water, and a wide electrochemical window up to 6 V(vs. Li∣Li+). Li1.5Al0.5Ge1.5(PO4)3 (i.e. LAGP) glass ceramics are potential electrolyte materials in place of Ti-free Li-analogues in the NASICON family because of the high mechanical strength and superior stability of these glass ceramics in contact with Li metal. Moreover, Ti-free LAGP offers relatively high ionic conductivity of 4.2 mS/cm at room temperature, which is comparable with LATP , infers a super-ionic nature. We are interested in exploring the lattice dynamics, Li-diffusion, and their interplay in LAGP using inelastic neutron scattering and quasi-elastic neutron scattering measurements supplemented by large-scale machine-learned molecular dynamics. The reported Li activation energy in LAGP is ~0.37 eV. Hence, it is expected to have higher ionic conductivity manifest in significant QENS broadening (> 20 μeV). Our earlier studies on Na/Li-based super ionic conductors also show the importance of anharmonic lattice dynamics on the onset of diffusion in this system. Still, they all exhibit super ionic behavior at above 500 K, which leads to a sizeable thermal broadening of the spectra. Hence, it was challenging to see the effect of diffusion on phonon explicitly. However, LAGP exhibits super-ionic behavior near room temperature, and hence, changes in the lattice dynamics across the onset of Li diffusion would be better captured.