Decoding coupled mechanical–electrochemical responses in multi-layer batteries via generalized ultrasonic dynamics

Abstract

Characterizing and understanding internal battery physics is essential for stability, safety, and recyclability. Ultrasound provides a non-destructive solution by encoding battery dynamics into mechanical waves. However, the complex multi-layer structure and coupled mechanical–electrochemical behaviors of commercial cells hinder standardized and physically interpretable ultrasonic testing. This study presents a unified ultrasonic framework for multi-layer pouch cells, linking wave dynamics to battery structures, materials, and states across frequency and time domains. Inspired by electrochemical impedance spectroscopy, we examine structure- and state–waveform relationships of batteries under various excitation conditions, decoding ultrasonic responses related to mechanical and electrochemical factors in a generalizable manner. Using first-principles modeling and frequency sweep experiments, we identify battery-specific frequency bandstructures and wave modulation signatures tied to cell architecture and cathode chemistry, allowing mechanical discrimination of these factors in electrochemically steady states. In-operando tests demonstrate that changes in localized ultrasonic resonance associated with shifting bandstructure can map variations in battery state of charge, with the evolution of anode material stiffness as a key driving mechanism. This work establishes a physics-grounded foundation for understanding wave–battery interactions and is expected to guide the development of high-sensitivity, task-specific tools and diagnostic strategies across the in-laboratory, post-manufacture, and in-service stages of a battery’s lifecycle.