KEY CONCEPTS
• A microfluidic-based electrochemical system was developed to measure the growth of dendrites in batteries by adjusting the rate of ion cross-flow.
• As ion cross-flow increased, dendrite formation declined. 
• The morphology around the electrodes changed with increasing ion cross-flow, leading to the observation of small, uniform crystal bump deposits at high flow rates.

Dendrites are needle-like structures that have been found to develop and grow on the electrodes of metal batteries as they go through charge-discharge cycles. Figure 2 shows the structure of a zinc dendrite above a zinc metal electrode.


Figure 2. Dendrites, such as the structure shown above a zinc metal electrode, have been found to reduce battery performance and eventually lead to catastrophic failure. A new study indicated that increasing the rate of ion cross-flow during electrodeposition can minimize dendrite formation. Figure courtesy of he University of California, Davis.

This phenomenon reduces battery performance and can even lead to catastrophic failure as the dendrites grow from the one electrode to the other electrode. If dendrites connecting the two electrodes are present, an electric current can then flow, leading to a short circuit that may cause the battery to overheat and potentially catch on fire.

In Tech Beat, we’ve detailed several approaches for minimizing dendrite formation. In a previous TLT article,¹ researchers found that depositing multiwalled carbon nanotubes on the lithium anode of a lithium-sulfur battery significantly reduced the presence of dendrites. The mechanism for this effect involves the ability of multiwalled carbon nanotubes to act as an enhanced solid electrolyte phase that typically formed over the surface of the lithium anode. As lithium ions move from the electrolyte to the anode, they lithiate the multiwalled carbon nanotubes, controlling dendrite formation by acting as an ion reservoir.

One issue that has not been carefully examined in trying to determine the source for dendrite formation is ion transport dynamics near the cathode. Jiandi Wan, associate professor of chemical engineering in the College of Engineering at the University of California, Davis, in Davis, Calif., says, “Electroconvection is known to contribute to the growth of dendrites during electrodeposition. This phenomenon enhances ion transport due to the presence of an electric field, which would otherwise be limited by diffusion. When cations (i.e., lithium ions) are reduced on the electrode surface at a greater rate than their mass transfer rate, an electrically neutral environment near the electrode, also known as a space-changer layer, is created. This regime is highly unstable and induces electroconvection, which contributes to the growth of dendrites.”

Past efforts to control electroconvection have included the use of specific additives such as carbonate-based or fluoro-based electrolytes. But limited work has been done to better understand the root cause of how flow affects the formation of dendrites until now.

Microfluidics
In an effort to better understand ion transport dynamics near the cathode, Wan and his colleagues turned to the phenomenon of microfluidics, which is the study of the motion of fluids in small volumes where changes on the electrode surface can be better examined. Wan says, “Our approach was to develop a microfluidic-based electrochemical system to measure the growth of dendrites by adjusting the rate of the cross-flow of ions during electrodeposition. The advantage of using microfluidics is, we are able to observe the formation and evolution of electroconvective vortexes that can convert into dendrites using a two-dimensional structure that minimizes gravitational effects.”

The researchers studied copper and zinc electrode systems. In each case, the electrolyte used is the corresponding sulfate salt (copper sulfate pentahydrate and zinc sulfate heptahydrate). Wan says, “We used copper as the model system and zinc because of the interest in commercializing rechargeable zinc batteries with comparable performance to lithium-metal batteries.”

The researchers used a technique known as chronoamperometry to evaluate electrode morphology and current density. Voltage was maintained at 1.5 V, and each electrolyte was used at a concentration of 0.02 molar.

The cross-flow of ions for the copper system was adjusted from 0 microliters per minute to 200 microliters per minute. As the cross-flow rate increased, dendrite formation declined. In progressing to 200 microliters per minute, the researchers found that the rate of dendrite formation was five times slower than if the flow rate was 0. A similar result was observed for the zinc system. Overall, cross-flow, also known as forced convection, during electrodeposition reduced dendrite growth between 97.7% and 99.4%.

Changes in morphology around the metal electrodes were measured through the use of a scanning electron microscope and energy-dispersive X-ray spectroscopy. Both the copper and zinc systems were evaluated under overlimiting current conditions.

After only 15 seconds and at a low flow rate of 10 microliters per minute, dendrite growth rate was observed with these species exhibiting an average size of 1.5 microns. When the flow rate was increased to 25 microliters per minute, dendrite formation was not detected. The researchers observed small, uniform crystal bumps depositing on the electrodes with diameters of approximately 0.3 microns.

Wan says, “Changing the flow rate has a significant impact on the morphology surrounding the electrodes. We observed similar changes from dendrite formation to the presence of tightly packed crystals as the flow rate increased in both the copper and zinc systems.”

Optical tracking using microparticles also was used to visually follow the motion of fluids near the electrode surface and the formation of dendrites. Initially, vortexes, which Wan characterizes as symmetrical flow streamlines that look like the wings of butterflies, were formed that directed ion flow toward growing dendrite tips. At higher flow rates, the height of vortex particles dropped significantly, leading to minimal dendrite growth and a smoother electrode surface.

While the results show an inverse relationship between flow rate and dendrite growth, the challenge remains to mimic flow rate in a working battery. Wan says, “We recognize that it is challenging to include microfluidics in a real battery with its current architecture. Future work will be devoted to finding an alternative way to simulate the microfluidic effect, or construct batteries with new configurations.”

Additional information can be found in a recent article² or by contacting Wan at jdwan@ucdavis.edu.

REFERENCES
1. Canter, N. (2019), “Minimization of dendrite formation in lithium-sulfur batteries,” TLT, 75 (2), pp. 10-11.
2. Ma, M., Li, G., Chen, X., Archer, L. and Wan, J. (2021), “Suppression of dendrite growth by cross-flow in microfluidics,” Sciences Advances, 7 (8), eabf6941.