  Lithium-ion batteries (LIBs) are becoming an important energy storage
solution to achieve carbon neutrality, but it remains challenging to
characterise their internal states for the assurance of performance, durability
and safety. This work reports a simple but powerful non-destructive
characterisation technique, based on the formation of ultrasonic resonance from
the repetitive layers within LIBs. A physical model is developed from the
ground up, to interpret the results from standard experimental ultrasonic
measurement setups. As output, the method delivers a range of critical pieces
of information about the inner structure of LIBs, such as the number of layers,
the average thicknesses of electrodes, the image of internal layers, and the
states of charge variations across individual layers. This enables the
quantitative tracking of internal cell properties, potentially providing new
means of quality control during production processes, and tracking the states
of health and charge during operation.
