context: Acoustic wave velocity is determined by the speed of wave propagation through a medium with defined thickness. While the wave arrival time can be determined from the measured acoustic signal, thickness must be measured independently to verify the wave velocity. The expansion and contraction of a pouch cell during cycling can be imaged with transmission X-ray microscopy (TXM), which has sufficient range and pixel resolution to measure both the total cell thickness and the average layer thicknesses. TXM parameters were optimized with exposure time of 20 seconds, beam voltage of 140 kV, a 0.4× objective lens and 90° projection angle (Table 1). Fig. 1 illustrates the experimental configuration and example radiographs of the mechanical expansion of a pouch cell upon cycling. The commercial pouch cell chosen (LiCoO2/graphite, 210 mA h nominal capacity) has a total thickness of approximately 5.6 mm when fully charged. The measured thickness varies between 5.4 and 5.6 mm (∼4% change) at a rate of 1C. There is less variation (∼0.5%) at a rate of 3C because of less attainable capacity before hitting the 4.5 V voltage cutoff on charge. The thickness changes are dominated by the ∼10% volume expansion and contraction of graphite anodes upon lithiation/delithiation. With 15 double-sided anodes and cathodes, each of the 30 cell layers (one layer is defined as an anode, a cathode, with a separator layer in between each electrode) is approximately 170 μm in thickness as measured by average peak-to-peak spacing (additional information on pixel thresholds for thickness measurements can be found in Fig. S1–S5†).