Lithium-metal batteries are among the most promising candidates for high-density energy storage technology, but uncontrolled lithium dendrite growth, which results in poor recharging capability and safety hazards, currently is hindering their commercial potential.

Now, researchers at Arizona State University, with colleagues at Rice University, have used a 3-dimensional layer of Polydimethylsiloxane (PDMS), or silicone, as the substrate of lithium metal anode to mitigate dendrite formation. The approach, described in a paper in Nature Energy could extend battery life and diminish safety risks for both lithium-ion and lithium-air batteries, as well as for other metal-anode-based batteries, said Hanqing Jiang, a professor in Arizona State University’s School for Engineering of Matter, Transport and Energy and one of the corresponding authors of the paper.

Almost all metals used as battery anodes tend to develop dendrites For example, these findings have implications for zinc, sodium and aluminum batteries as well.

—Hanqing Jiang

Jiang said he and the research team, rather than approaching the problem from a materials or electrochemical perspective, looked for solutions as mechanical engineers; specifically, they looked at the possibility of stress as a factor in lithium dendrite growth.

Despite the efforts from all of these different aspects to tackle the dendrite growth problem, one critical and fundamental aspect has not been widely explored and appreciated, namely, the presence of plating-induced stress in Li metal and its effect on Li growth morphology. Previously, interfacial stress generated at the deformed solid–solid interface between the electrode and the solid electrolyte and its role in the interface stability and dendrite initiation have been studied in seminal work.

Many microstructural evolution phenomena in materials are stress-driven. For example, it has long been known that whiskers can grow from tin films under compressive stress as a stress relief mechanism. Since residual stress is ubiquitous in the metal plating process, it is logical to ask whether significant stress exists during Li electrodeposition, whether it is the cause of filamentary Li dendrite growth and whether this undesirable phenomenon can be suppressed through effective control of stress in plated Li.

Here, we aim to provide answers to the above fundamental questions by designing experiments for plating Li on a thin copper (Cu) current collector supported by soft substrates.

—Wang et al.

The first round of research involved adding a layer of PDMS to the bottom of battery anode. This resulted in “remarkable reductions” in dendrite growth, said Jiang. The researchers discovered that this was directly related to the fact that stress accumulated inside the lithium metal is relieved by the deformation of the PDMS substrate in the form of “wrinkles.”

Cu thin film wrinkling during Li plating and dendrite mitigation by using soft substrate. a, Cu thin film wrinkles due to the compressive stress generated during Li plating. b, Upon Li electroplating, compressive stress is generated, and soft substrate releases compressive stress, and thus mitigates Li dendrite growth. c, Li-plating-induced compressive stress cannot be relaxed on hard Cu foil, and causes the formation of Li dendrites. Orange, blue, green and grey represent Cu, Li, SEI and PDMS, respectively. Source: Wang et al. Click to enlarge.

This is the first time convincing evidence shows that residual stress plays a key role in the initiation of lithium dendrites.

—Hanqing Jiang

In addition to obtaining a fundamental understanding of the lithium dendrite growth mechanism, Jiang’s group also devised a way to utilize the phenomenon to extend the life of lithium-metal batteries while maintaining their high energy density. The solution is to give PDMS substrate a three-dimensional form with a lot of surface.

Envision sugar cubes that contain a lot of small internal pores. Inside these cubes, the PDMS forms a continuous network as the substrate, covered by a thin copper layer to conduct electrons. Finally, lithium fills the pores. The PDMS, which serves as a porous, sponge-like layer, relieves the stress and effectively inhibits dendrite growth.

—Hanqing Jiang

Our findings here provide mechanistic insights into the Li dendrite growth phenomenon. It is believed that this will inspire many further studies on stress relaxation during electrochemical plating and open up an unexplored front in the extensive pursuit of Li dendrite suppression strategies with potential implications for other metallic electrode systems. Processing optimizations, such as thinning down the 3D scaffold, better control of the pore sizes and distributions, and the amount of Li pre-deposited into the 3D scaffold, need to be thoroughly conducted in the future to harness this Li dendrite mitigation methodology towards practical battery applications.

—Wang et al.

Resources

• Xu Wang, Wei Zeng, Liang Hong, Wenwen Xu, Haokai Yang, Fan Wang, Huigao Duan, Ming Tang & Hanqing Jiang (2018) “Stress-driven lithium dendrite growth mechanism and dendrite mitigation by electroplating on soft substrates” Nature Energy volume 3, pages 227–235 doi: 10.1038/s41560-018-0104-5