Lattice dynamics in inorganic solid state electrolytes

As the enabling component of all-solid-state batteries, inorganic solid-state electrolytes govern both energy and safety metrics. However, widely studied families such as sulfides, oxides, and halides still face a fundamental challenge: structure-based, static descriptors often improve one figure of merit at the expense of another, making it difficult to simultaneously maximize room-temperature ionic conductivity and chemical, electrochemical, and thermal stability. Because ion transport is a thermally activated process emerging from collective atomic vibrations, a lattice dynamics perspective that resolves phonon dispersions, lifetimes, and anharmonic couplings offers a direct atomistic route to rational design. In this review, we summarize the recent progress in understanding ion transport mechanisms and stability in inorganic solid electrolytes from the perspective of lattice dynamics and offer actionable design implications. We first outline experimental and computational toolkits that access dynamics across time and length scales, including inelastic and quasi-elastic neutron scattering to probe phonon spectra and jump diffusion, Raman/infrared spectroscopy, pulse-echo ultrasonics to capture optical modes and elastic stiffness, first-principles phonon calculations for harmonic descriptors, and molecular dynamics augmented by machine-learned interatomic potentials for large-scale, long-term simulations and spectral reconstructions. We then discuss the transport mechanisms revealed by these approaches and how vibrational attributes reshape the migration landscape. Lattice softening, achieved with more polarizable anions or flexible frameworks, flattens the potential-energy surface, amplifies cation vibrational amplitudes, and lowers saddle-point barriers. However, instances where global softening fails to predict conductivity motivate mode-resolved descriptors beyond a single band-center metric. The rotational dynamics of polyanions couple to cation motion via two pathways: rare, large-angle rotor-like events and far more prevalent small-amplitude local reconfigurations of coordinating units. The latter continuously relaxes bottlenecks and reduces transition-state energies at operating temperatures, rationalizing why rotation can aid diffusion in some chemistries yet appear ineffective in others. The Meyer-Neldel rule interplay between the activation energy and the pre-factor reflects variations in the vibrational entropy and attempt frequency along the reaction coordinate, and tailoring the relative weight of high- versus low-frequency modes in the transition state enables either leveraging compensation to sustain conductivity or deliberately breaking it to boost pre-factors as barriers are lowered. In parallel, strong anharmonicity, manifested as phonon softening, linewidth broadening, and selective quasiparticle breakdown, dynamically modulates bottlenecks and induces liquid-like sublattice responses that facilitate rapid hopping while preserving overall crystallinity. Finally, a small number of critical phonon modes, often low-frequency framework tilts or specific optical modes at the Brillouin zone center, have been shown to dominate hopping events and can be selectively activated by chemical or external stimuli. We further connect the lattice dynamics descriptors to stability. The anion phonon band center correlates with the oxidation potential in many chemistries, whereas intrinsic anharmonicity can stabilize phases that are harmonically unstable and simultaneously depress thermal conductivity, thereby shaping heat management. Vibrational spectra help decouple ionic and thermal transport and rationalize anomalous temperature trends in thermal conductivity across representative electrolytes. Drawing on these threads, we propose a coherent framework linking phonon characteristics to both conductivity and stability, offering practical guidelines for chemical and structural selection across inorganic solid-state electrolytes. We also underscore the importance of electrode-electrolyte interfaces and the opportunities offered from a lattice dynamics perspective. By consolidating concepts, tools, and case studies, this review reframes solid-electrolyte design around lattice dynamics and charts routes for multi-property optimization in next-generation all-solid-state batteries.