In Operando Electrochemical-Acoustic Time-of-Flight Analysis: Correlating Physical Dynamics within Batteries to States of Charge and Health

Recently, advances in the mechanical understanding of electrochemical energy storage devices, based investigations of stress/strain, have yielded significant improvements in battery materials, designs, and management systems. To date, however, the field has lacked a non-invasive method for monitoring these complex mechanics within in practical cells. Here, we demonstrate a simple electrochemical-acoustic model and experiment as the basis for a potentially universal in operando, field-deployable tool for determining the mechanical evolution, state-of-charge (SOC) and state-of-health (SOH) of any closed battery. This technique, which we call Electrochemical Acoustic Time of Flight (EAToF) analysis, was tested against several off-the-shelf lithium ion and alkaline batteries and a general phenomenon was observed: that the behavior of sound passing through the cells correlated strongly to its SOC and SOH. Regardless of the chemistry, the density distribution within a battery must change as a function of its SOC, and the bulk moduli of the anode and cathode change as well. These shifts, in turn, directly influence the behavior of sound passing through the cells. Overall, a one- or two-point acoustic measurement can be related to the interaction of a pressure wave at multiple discrete interfaces within a battery, which in turn provides insights into state of charge, state of health, and mechanical evolution/degradation. This work, which is unique to the literature, provides new physical insights into the physical dynamics within batteries. With this EAToF technique, we propose a framework relating changes in sound speed within the cell, via changes in electrode densities and moduli, to SOC and SOH measurements in a manner that can be readily embedded into battery management systems.