(Digital Presentation) Monitoring Dendrite Growth in Aqueous Zinc Ion Batteries with Guided Ultrasonic Waves
Authors: Zhang Yifeng , Dong Haobo , Wang Tianlei , He Guanjie , Parkin Ivan P. , Cegla Frederic 
Published: July 2022

Dendrites form on the surface of metal electrodes as a battery undergoes repeated charging and discharging. The formation of dendrites not only affects the electrochemical performances of a battery but may also cause internal short-circuiting and thermal runaway events. Hence, there is a clear need to understand the formation mechanisms and growth kinetics of dendrites. Currently available technologies such as (in-situ) X-ray computed tomography and scanning electron microscopy can be used in a laboratory environment to study dendrite formation. However, they are often prohibitively expensive or unsuitable for long-term, large-scale, and in-operando monitoring.
In this study we present an ultrasound-based technique which is cheap, with potential to scale up, and suitable in both laboratory and industrial environments. Different from conventional ultrasonic approaches which excite ultrasonic waves that propagating through layers of cathode, anode and electrolyte, the method presented in this study utilises miniature transducers which excite fundamental shear-horizontal (SH0*) guided ultrasonic waves that propagate along the cathode and anode. Hence, the presented method can individually monitor the condition of multiple electrodes which may be undergoing different ion-insertion mechanisms.
A proof-of-concept study was performed on a symmetric aqueous zinc ion cell to explore the sensitivity of SH0* mode guided ultrasonic waves to zinc metal plating/stripping and dendrite formation at the electrode-electrolyte interface. We observed that the attenuation and velocity of waves show cyclical variations when the cell was in a chronopotentiometric cycling test. Variations of wave attenuation and velocity on the opposing working and counter electrode are observed to be in the opposite senses and the increasing/decreasing trends depend on the states (i.e., charging/discharging) of the cell. The cyclical variations of the ultrasound properties are only partially reversible and a monotonic drift can be observed which increases with increasing number of cycles.
Coupling the ultrasonic measurements with images acquired by an optical microscope, clear correlations can be observed between changes in ultrasonic wave properties and zinc dendrite thickness. The partially reversible cyclical variations can be attributed to zinc plating/stripping that occur at the electrode-electrolyte interface. The irreversible, monotonic drifting can be attributed to the growth of zinc dendrites on the electrode surface. Hence, these findings demonstrate that the SH0* mode guided ultrasonic waves can offer valuable insights on physical changes at the electrode-electrolyte interface. It is therefore possible to quantitatively model and predict dendrite growth based on ultrasonic measurements. In conclusion, we believe the presented method can inspire more quantitative research on dendrite formation and in-operando monitoring of battery health.

Figure 1