Application of photoacoustic imaging for lithium metal batteries

The problem of lithium (Li) dendrite has been one major obstacle to further improvements of the performance of Li metal batteries. Seeking for possible solutions to the problem demands thorough observations on the dendrite growth process. Despite various imaging techniques implemented hitherto, challenges still exist in direct imaging of Li dendrites with considerable contrast and both high spatial and temporal resolutions, because low electron density of Li element makes Li bulks almost invisible to electrons and X-rays. The very first implementation of photoacoustic (PA) imaging to realize high-contrast direct imaging of three-dimensional (3D) structures of Li electrodepositions inside Li batteries within minutes has been reported by our group previously. In this work, we further utilize PA imaging on top surfaces of electrodes to achieve quantitative studies of Li electrodepositions. Attenuation to PA signal amplitudes by a Celgard separator was calibrated to be less than 50% and thus the feasibility of this approach to observe entire deposited Li within and below the separator was verified. We also computed total mass of deposited Li on the cathode after the battery was discharged with different areal capacities. The results show an overall positive correlation between the computed mass and the applied discharging areal capacity, which conforms with the underlying electrochemical (EC) principles. We notice a discrepancy between the calculated mass by our PA method and by the EC curves. Possible causes of the discrepancy are discussed to provide guidelines on future development of reliable methods for quantifying Li electrodepositions based on PA imaging.