In the race to develop safer, faster-charging solid-state batteries and more efficient thermoelectric conversion technologies, engineers and scientists have long faced a fundamental challenge: how to ensure ions move through hard, solid materials as quickly as they do in liquids? Recently, a team led by Prof. ZHOU Yanguang, Associate Professor in the Department of Mechanical and Aerospace Engineering (MAE) at The Hong Kong University of Science and Technology (HKUST), discovered a novel mechanism for rapid ion transport in solids, opening new avenues for materials design.
The study shows that the ionic transport is governed by collective dynamics. The results were published in the journal Physical Review Letters, titled "Fast Ionic Transport Governed by Collective Vibrational Dynamics". Under the leadership of Prof. Zhou, the research team comprises Postdoctoral Fellows Dr. XU Yixin and Dr. XIANG Xing, Prof. LI Zhigang, Professor, and Prof. LU Yanglong, Assistant Professor, all from the MAE Department of HKUST.
Historically, ion transport in solids has been described using the classic diffusion model, in which ions are thought to expend energy to overcome static energy barriers as they move from one site to another. However, using advanced machine-learning molecular dynamics simulations, the researchers found that this static perspective is overly simplistic and fails to capture the true microscopic dynamics. In superionic conductors (solid materials with extremely high ion diffusivity), ions are never stationary; instead, they are always vibrating.
Using superionic silver telluride (α-Ag2Te) as a case study, the team found that rapid ion migration arises from the synergy of two different types of collective vibrational modes. First, unstable collective vibration modes generate irreversible ionic displacements, directly breaking the equilibrium and triggering ions to depart from their original positions, thereby initiating the hopping process. Meanwhile, atoms continue to vibrate under the stable modes. The synergistic action of these two modes can further increase the separation between cations and anions, facilitating ionic diffusion. The team explains that ionic transport is therefore not driven by a single mechanism, but by the coordinated action of both vibrational modes: unstable modes initiate ionic jumps through irreversible displacement, while stable modes cooperate to sustain effective cation-anion separation.
Building on this discovery, the team proposed a strategy to design the ion diffusion using defects. Simulations show that introducing Te2- vacancies into α-Ag2Te significantly increased the proportion of unstable collective vibration modes. These modes provided more jump opportunities and better cooperation with stable modes, nearly doubling the silver ion diffusion rate at 500 K (from 0.84×10-5 cm2/s to 1.54×10-5 cm2/s when 10% Te2- vacancies are introduced), indicating strong potential for practical applications.
Furthermore, this study establishes a more general physical model to predict ion diffusion. While the conventional reaction-diffusion model (Arrhenius equation) focuses primarily on how temperature influences the ability of ions to overcome energy barriers, the research team proposed a new model based on the "ratio of unstable modes". This model can directly predict ion diffusion coefficients across different defect concentrations and thermodynamic conditions, providing a more universal theoretical framework for understanding ion transport in complex material systems.
Prof. Zhou noted, "This research successfully bridges the gap between microscopic atomic dynamics and macroscopic ion transport, holding significant engineering value. It offers a clear path for material scientists: the key to designing solid-state batteries that charge faster or thermoelectric materials with higher efficiency lies in tuning the material's vibrational spectrum. By exciting specific collective vibrational modes through carefully designed material engineering, this discovery promises to accelerate the development of all-solid-state lithium batteries, sodium batteries, and novel thermoelectric conversion devices."
