Abstract:

Due to the specific working mechanism of liquid metal battery（LMB）， substance change only happens at the cathode interface during charge and discharge process. For better understanding the dynamic property of mass transport process inside LMB noninvasively， the A-scan type ultrasonic pulse echo technology was used to investigate discharge process of 40 ℃ Li-Ga LMB. By comparing the change of ultrasonic echo signal before and after discharge， the acoustic pressure value with specific time of flight was identified as the signal reflected from the liquid Ga/electrolyte interface. The results show that the acoustic pressure value increases proportionally to the discharge capacity. When the solid Li₂Ga₇ interphase forms and entirely covers the liquid Ga cathode， the acoustic pressure value will finally increase about 45% compared with the original state. The quantitative relationship between discharge capacity and ultrasonic pressure value was calculated， which means that ultrasonic pulse echo signal can be used to diagnose discharge state of Li-Ga LMB. Moreover， in order to illustrate the mass transport mechanism at the liquid Ga cathode interface， the formation process of Li₂Ga₇ solid interphase at the initial stage of discharge process was recorded by ultrasonic confocal phased array three-dimensional imaging system. It was found that Li₂Ga₇ solid interphase firstly form at the central part of the liquid Ga cathode interface， which is in accordance with the optical result obtained after 1.4 mA·h capacity discharged. This work provides a novel in?situ and non-destructive interface analysis method for liquid metal battery.

Key words: Liquid metal battery, In?situ analysis, Ultrasonic pulse echo technology, Ultrasonic confocal phased array imaging technology