Researchers from the Weizmann Institute of Science have developed innovative approaches to study dendrites in lithium-metal batteries, with the potential to improve battery life and reduce safety risks
Every night we not only "charge" ourselves while we sleep, but we also make sure to plug our mobile phones into an electrical outlet and fill them with good energy for a day of scrolling, texting and notifications. But we haven't always been able to do this: rechargeable lithium-ion batteries first appeared commercially in the 90s and marked a real technological revolution, which also won its developers the Nobel Prize for Chemistry in 2019. Without them, smart devices such as mobile phones, wireless headphones and electric cars, would simply not be possible from an environmental and economic point of view.
The pace of technology development requires stronger and safer batteries, but developing such batteries is not a simple task. Lithium-metallic batteries, for example, promise to increase the energy capacity several times in the future than the batteries commonly used today, but also pose a significant challenge: with each charge, tiny root-like wires called dendrites are formed in them. The dendrites may pile up and form metallic "bridges" inside the battery, over which there is an uncontrolled passage of electrons - something that threatens to destroy the battery and may even lead to fires. To date, methods to characterize dendrite formation have been limited. In a new study from the laboratory of Prof. A sex container From the Department of Molecular Chemistry and Materials Science at the Weizmann Institute of Science, the researchers, led by Dr. Iain Mighty, developed an innovative approach that allows both to identify the factors in the battery that influence the formation of the dendrites, and also to quickly test the efficiency and safety of alternative battery assemblies.
During the use of rechargeable batteries, positively charged ions move between the negative electrode (anode) and the positive electrode (cathode) through a conductive material called an electrolyte. When the battery is charged, the ions return to the negative electrode - contrary to the natural course of the chemical reaction - and thus it is ready for reuse. The innovation in lithium-metal batteries lies in the anode, which is made of pure lithium-metal. This material allows for the storage of a lot of energy, but it is very active, chemically speaking, and interacts with any material in its path. Thus, in its meeting with the electrolyte, dendrites are formed quickly and in a high quantity, which is dangerous for the user and the health of the battery.
It is possible to prevent the batteries from igniting by changing the electrolyte from a liquid and flammable material to a solid and non-flammable material, for example a combination of polymers and ceramic particles. The balance between these two components has a significant effect on the formation of the dendrites and the longevity of the battery, but the main challenge remains: how do we find the ideal composition for long-lasting batteries?
The researchers used NMR
The research team chose to answer the question using nuclear magnetic resonance spectroscopy (NMR) - an accepted method for identifying the chemical structures of materials, which allowed the researchers to follow the development of the dendrites and identify chemical interactions in the electrolyte. "When we looked at the dendrites in the batteries with different ratios of ceramic material and polymers, we discovered a sort of 'golden ratio': electrolytes consisting of 40% ceramic material exhibited the longest lifespan," explains Dr. Michal Lasaks. "When we increased above 40% ceramic material , we encountered structural and functional problems that impaired battery performance, while less than that resulted in shorter battery life." But surprisingly, In the batteries that produced the best results, an increase in the number of dendrites was actually observed, but the growth of the wires was inhibited and they produced fewer dangerous bridges.
These findings gave rise to the million dollar question, which may be worth much more in terms of commercial applications: What inhibits dendritic growth? The researchers estimated that the answer lies in a thin layer on top of the dendrites known as solid electrolyte interphase, or for short - SEI. The SEI layer is formed when the dendrites react with the electrolyte, and it can be composed of different materials that have a positive or negative effect on the battery. For example, the chemical composition of the SEI layer can improve or inhibit the passage of lithium ions along the battery, block or allow the passage of harmful substances from the anode to the cathode, and slow down or accelerate the development of dendrites.
To characterize the thin SEI layer the team had to think outside the battery. The layers are made of a few tens of individual nanometers of atoms, and the signals obtained from them in NMR are too weak. In an attempt to amplify the signals, the researchers made use of a technique not used in battery research until now: amplifying the NMR through dynamic nuclear polarization.
Using the strong spin of polarized electrons in lithium
The method uses the strong spin of electrons polarized in lithium, which have powerful signals that allow to amplify the signals produced by the nuclei of the atoms in the SEI layer. When the researchers did this, they were able to decode with great precision the chemical composition of the SEI layers, and through them learn about the interactions that existed between the lithium and the various structures in the electrolyte. For example, they could understand whether the dendrite developed at the junction between the lithium and the polymer or the ceramic particles. Thus they surprisingly discovered that the SEI layers formed in the dendrites sometimes optimize the passage of ions in the electrolyte while blocking harmful substances.
The research findings provide new insights that can help develop more durable, stronger and safer batteries that will provide more energy at a lower environmental and economic cost. These future batteries will make it possible to operate smarter and larger devices without increasing the battery volume and while increasing their lifespan.
"One of my favorite things about this research is that without a deep scientific understanding of basic physics, it was impossible to decipher what happens inside the batteries. We went through a process that is very characteristic of the work here at the Weizmann Institute - we started with a pure scientific question, which did not deal with dendrites at all, and from that developed research with applications Practical ones that can improve the lives of all of us", concludes Prof. Lasaks.
Dr. Asia Sabrinovsky-Arveli, Yehuda Boganim and Chen Oppenheim from the department of molecular chemistry and materials science at the institute also participated in the study.
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