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Posted: Apr 19, 2017
A self-healing rechargeable battery with ultralong cycling life
(Nanowerk Spotlight) Cell regeneration occurs all the time in organisms, to replace died cells and restore their biological functions. In broader perspective, organisms need to possess such a ubiquitous ability of repairing damage spontaneously to extend their life expectancy.
The ability is so-called 'self-healing', which not only helps biological systems to maintain their survivability and extend the lifespan, but also inspires many growing applications such as smart electronics, artificial skin, and energy storage devices. In particular, a self-healing feature is of exceptional interest for increasing the shelf live 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 part of our everyday life for more than twenty years. Historically, it changed our life drastically. And at present, it is still under rapid development," said Prof. Qiang Zhang, a faculty at Department of Chemical Engineering, Tsinghua University. "However, lithium-ion batteries are having more and more difficulies in meeting the demand for high specific energy due to the theoretical limitation of its electrode materials. Therefore, we must explore new battery chemistry and high-capacity electrode materials that are adaptive to future developments. And the lithium–sulfur battery is what we focus on."
Lithium-sulfur batteries 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 by-product of the petroleum industry; 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 that "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 an almost 4-year long research effort. Qiang told Nanowerk, "It has been tough but worth the time."
By mimicking a biological self-healing process, fibrinolysis, we introduced an extrinsic healing agent, polysulfide, to enable the stable operation of sulfur microparticle (SMiP) cathodes. This can guide the design of novel healing agents (e.g., lithium iodine) toward high-performance rechargeable batteries.
At a moderate, lab-level 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 above.
"It is very interesting to learn from nature," said Hong-Jie Peng, a graduate student at Department of Chemical Engineering, Tsinghua University. "In fact, initially we were 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," he continues. "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.
“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. As 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!" added Hong-Jie. "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-review of our paper is to explain the difference between these pre-added polysulfides and electrochemically generated ones."
It is commonly accepted that soluble polysulfides diffuse out of and back into the sulfur cathode, and reach the 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. "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 homogeneously, 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.