Fig. 1. (a) Schematic of the fiber optic FPI ultrasound sensor tip. (b) Photograph of the sensor tip. (c) The reflected spectrum and its derivation of the fiber optic FP ultrasound sensor.

Fig. 2. (a) The schematic diagram of the ultrasound sensing demodulation system. Detected signals when the voltage frequency is 500 kHz in (b) time domain and (c) frequency domain. (d) The output voltages of the sensor when ultrasound with different pressure is applied. (e) The frequency response of the sensor.

Fig. 3. (a) The simplified experiment setup of the fixed-point detection. Signals from the LFP||Gr batteries which are (b) healthy and (c) faulty. (d, e) Signals from different parts of the battery which cycles for 2000 times, respectively.

Fig. 4. (a) The schematic diagram of the fiber optic ultrasound BHM system setup. (b) The photograph of the acrylic plate. (c) The imaging result of the acrylic plate.

Fig. 5. (a) Photograph of the LFP||Gr battery. The ultrasound image of the (b) healthy battery, (c) battery cycled for 2000 times and (d) faulty battery. (e) The photo of the AFLMB. (f) The ultrasound image of the dry AFLMB. The ultrasound images of the AFLMB when it is injected with electrolyte after (g) 24 hours and (h) 48 hours.

Fig. 6. The galvanostatic curves and gas ratio variation of the pouch cell in the formation process.