A bioinspired self-healing rechargeable battery

Like the mythological phoenix, which dies in a show of flames and is reborn from the ashes, cell regeneration occurs all the time in organisms to replace dead cells and to restore their biological functions. Properly functioning cell regeneration is important for extending life expectancy.

This self-healing feature is of high interest for increasing the shelf lives of emerging electrochemical energy storage systems that are adopted to enable high-efficiency, carbon-neutral energy utilization and eventually a sustainable society.

"The lithium-ion battery has been brought into our everyday life for more than twenty years. Historically, it changed our life drastically. And at present, it is still under rapid development.," said Dr. Qiang Zhang, a faculty at Department of Chemical Engineering, Tsinghua University, "However, lithium-ion batteries are getting more and more difficult in meeting the demand for high specific energy due to the theoretical limitation of electrode materials. Therefore, we must explore new battery chemistry and high-capacity electrode materials that are adaptive to future development. And lithium–sulfur battery, is what we trust in."

Lithium−sulfur battery use sulfur as the positive electrode (cathode) and lithium metal as the negative electrode (anode). When coupled together, these two electrode materials afford an extremely high specific energy of 2,600 Wh kg−1 theoretically, almost three-fold higher than those of conventional lithium-ion batteries. Sulfur is also a well-known byproduct of petroleum industry and in heavy stock globally. It is cheap, abundant, and nontoxic.

"Like other potentially high-capacity electrode materials such as oxygen, carbon dioxide, and silicon, sulfur shares a common attribute that plagues it. That is the short cycling life.," said Dr. Jia-Qi Huang, who used to be a postdoctoral researcher at Tsinghua University and is now a faculty of Beijing Institute of Technology, a university in the same city.

Jia-Qi further explained, "Improving the capacity of an electrode material is equivalent to increasing the number of lithium ions this material is able to store per unit mass. So almost all the high-capacity electrode materials undergo drastic changes in chemical bond association, molecular structure, physical phase, and/or aggregation status upon uptake of lithium ions. These changes, unfortunately, make the electrode materials and the battery irreversible and unstable, and shorten the battery life, usually shorter than 1,000 cycles."

Aiming at reversibly and stably utilizing sulfur-based cathode materials, Qiang led a long-lasting work, on which his team has spent almost four years on. Qiang told Phys.org, "It is tough but worthy of the time."

This work was recently published on Journal of the American Chemical Society, in which they described a lithium–sulfur battery prototype using commercially viable micron-sized sulfur as cathode materials but with unprecedented cycling life that has never been achieved before.

At a moderate, lab-level areal loading of sulfur, the battery exhibited a 7,500-cycle life expectancy, a record for lithium–sulfur battery research. At a higher, industry-level loading of sulfur, the battery can be reversibly cycled for 2,000 cycles, without any capacity loss. This success in achieving exceptionally long battery life is ascribed to a concept of "self-healing," as indicated at the beginning.

"It is very interesting to learn from nature.," Hong-Jie Peng, a graduate student at Department of Chemical Engineering, Tsinghua University, told Phys.org, "In fact, we initially inspired by other self-healing chemistries for electrochemical energy storage."

"About four years ago, teams led by Prof. Zhenan Bao and Prof. Yi Cui at Stanford University reported a skin-inspired self-healing silicon anode, which showed much promoted cycling stability. At that time, we were actually plagued by the poor cycling life of lithium–sulfur batteries, and were naturally attracted by their exciting results, turning into the implementation of self-healing strategies in lithium–sulfur battery research."

"Exactly in the same year, researchers at Argonne National Laboratory first elucidated the use of polysulfides that were pre-added in electrolytes for self-healing lithium–sulfur batteries. These works motivated us to think about the possibility of mitigating the issue of capacity loss through self-healing. Our primary attempt in employing polysulfides as healing agents suggested such a possibility, and further driven us to think about a couple of questions: Why and how does polysulfides work? How different are these pre-added polysulfides from electrochemically produced ones? And can it be extended to a more general scheme?," said Hong-Jie.

Aforementioned questions are exactly what they attempted to address over the past four years, and they finally answered them in their recent paper after a long "campaign," as the authors mentioned.

"We tried to find a biological analogue for being mimicked. Unlike silicon anodes that only undergo solid-to-solid phase transition, the phase transfer behaviors in sulfur cathode are more complicated, mainly because of the presence of liquid-phase polysulfides.," said Xin-Yan Liu, another author of the paper.

In a typical lithium–sulfur battery using liquid electrolyte, sulfur is firstly reduced to form highly soluble intermediates, polysulfides appearing as sulfur chains with two terminal lithium ions. With the reactions proceed, these soluble intermediates are further reduced, forming insoluble products of lithium disulfides. The whole reaction scheme therefore has a solid–liquid–solid phase transfer feature.

"If the liquid-to-solid phase transition loses the control, the redeposited solid will probably agglomerate and block desired electron and/or ion paths, just like …"

"Just like the thrombus, namely blood clot that obstructs the flow of blood!" Hong-Jie added, "It is just amazing. And then we turned to how the human body deals with the thrombus and make the obstructed blood vessel work again. Luckily we found that it is an enzyme, plasmin, that triggers a healing process called fibrinolysis to release the tightly bound tissue fragments a blood clot consists of."

"Therefore, the key for building a self-healing lithium–sulfur battery is analogously to search for suitable agents being capable of dissolving and reutilizing those irreversibly deposited solid products. The polysulfide, is one of what we confirmed to be effective.," Qiang mentioned, "And that expends a large portion of content in our paper."

"Another challenge we met during the peer-reviewing is to explain the difference between these pre-added polysulfides and electrochemically generated ones."

It is commonly accepted that soluble polysulfides diffuse out and back to sulfur cathode, and reach lithium anode to be reduced, forming an internal redox cycle that releases no electricity to the external circuit. Such a phenomenon is called "shuttle," and has been regarded as the origin of capacity loss for a long time.

"In our study, however, we suggested and proved that polysulfides shuttle not only between the two electrodes but also within the cathode region, because of their concentration variation in routine sulfur cathode.," Hong-Jie explained to Phys.org, "If the distribution of polysulfides is non-uniform, their everywhere deposition rate and amount will also be non-uniform, inevitably rendering large precipitates that is unable to be re-charged electrochemically. As a result, the amount of polysulfides electrochemically produced from remaining active materials is insufficient to restore the deactivated solid phase. A large portion of capacity was lost in this way."

"But when we employed pre-added polysulfide solution as the electrolyte, in which polysulfides had been dispersed homogenously, variations in initial sulfur distribution can be homogenized. As long as the conditions for liquid-to-solid phase transfer remain nearly changed in each cycle, the cycling performance will also remain stable.," said Xin-Yan.

Based on these understandings, this team further explored other healing agents different from pre-added polysulfides. On lithium iodine they demonstrated their universal biomimetic self-healing strategy that is inspired from fibrinolysis.

"In fact, the self-healing process presented in our paper can be analogized to Phoenix Nirvana. Here solid products like sulfur and lithium sulfides are the tangible phoenix, while liquid intermediates that tend to rampantly diffuse are the intangible flame. Solid products are precipitated from liquid intermediates, just like that phoenix is reborn from the flame and ashes.," said Qiang.

"The key is how to reshape the flame into a phoenix instead of a flame lion, a flame monkey, or other things.," Qiang finally concluded, "And that is exactly our work, manipulating the complex phase-transfer-involved reactions and making them constantly the same."

More information:
Hong-Jie Peng et al. Healing High-Loading Sulfur Electrodes with Unprecedented Long Cycling Life: Spatial Heterogeneity Control, Journal of the American Chemical Society (2017). DOI: 10.1021/jacs.6b12358

Provided by Tsinghua University