A Phonon-Interaction Perspective on Ionic Flux in Disordered Solid Electrolytes

The advancement of oxide-based solid-state electrolytes (SSEs) is essential for realizing next-generation all-solid-state batteries (ASSBs) with improved safety and energy density. Unlike liquid electrolytes, SSEs offer intrinsic thermal and mechanical stability. However, their performance remains limited by low Li ion conductivity, interfacial incompatibility, and electronic inhomogeneity, often originating from structural or chemical disorder. These limitations arise not only from static lattice defects but also from complex vibrational and electronic dynamics. To address this, we propose a perspective from quantum physics, which helps to correlate lattice vibration, particularly phonon dynamics, with Li ion migration and electrochemical stability in oxide-based SSEs. First of all, we focus on Ta-doped Li₇La₃Zr₂O₁₂ (LLZTO), where Ta substitution at Zr sites induces a non-linear vibrational environment that enables Li ion migration. In this environment, correlated Li ion hopping events are governed by the dynamic reshaping of local potential energy landscapes. This phenomenon enhances phonon–phonon coupling and activates low-frequency anharmonic phonons, leading to temporal desynchronization of Li hopping and thereby increasing Li migration entropy. This behavior facilitates a transition from conventional single ion migration to multi-ion migration, highlighting the role of phonon engineering in improving Li ion conductivity. In addition to this change in the bulk, we found that grain boundaries (GBs) in Li_0.33La_0.57TiO₃ (LLTO) act as electron conduction pathways due to oxygen deficiency, inducing a space charge layer accompanied by Li accumulation and inhomogeneous Ti⁴⁺ reduction. This environment facilitates polaron formation and inhomogeneous Li ion flux, leading to Li dendritic growth. However, aliovalent doping introduces controlled anharmonic phonon that suppresses polaron formation and enables homogeneous Li ion flux. These results demonstrate that phonon–phonon and phonon–electron interactions serve as key physical levers for modulating Li ion flux. Collectively, these insights offer a mechanism-based design strategy for high-performance SSEs.