Investigation of Electrical and Acoustic Impedance with Volumetric Deformation of a Li-ion Cell Under Varying State of Charge

A Lithium-polymer pouch cell was characterized over one charge and discharge using three different characterization methods. The set comprised electrochemical impedance spectroscopy (EIS), strain-gauge measurement, and pulse-echo ultrasound. The results show that the combined approach provides a picture of SOC-dependent changes: the strain gauge reliably tracked external volume changes, pulse-echo ultrasound detected systematic Time of Flight (ToF) shifts indicative of internal structural, and EIS reported concurrent changes in charge-transfer resistance (R₁).

The results show that individual methods - ultrasound (ToF), EIS (changes in R1) and strain gauges were able to track changes in volume (strain gauge) and internal structure (ultrasound and EIS) of the cell depending on state of charge (SOC). The verification of these conclusions were the specific signatures of each method: the systematic shortening of ToF without new internal reflections, the sensitivity of R1 to EIS interface modifications, and the direct mechanical shifts of the measured strain gauges.