Promote storage of safer lithium energy
Charging our phones has become so routine that we rarely think about the breakthrough that it made possible. Rechargeable lithium-ion batteries, commercially introduced in the nineties, pushed a technological revolution that yielded their makers the Nobel Prize for Chemie 2019. This important innovation supports the functionality of contemporary smartphones, wireless headphones and electric vehicles, making them both financially and environmentally practiced.
As our devices become more advanced, the demand for batteries that pack more power while staying safe continues to rise. Yet the frying of such power sources is far from simple. A promising design is the lithium metal battery, which can supply more stored energy than standard battery types. Unfortunately, the potential of it is limited by a persistent problem: the rise of small threads or dendrites that accumulate with each load. When dendrites accumulate, they can form metal connections that break down the functionality of the battery and form a serious fire hazard. Until recently, researchers had limited approaches to investigate and understand the formation of the dendrite. In a new study led by Dr. Ayan Maity in the Lab of Prof. dr. Michal Leskes of the Molecular Chemistry and Materials Science Department of the Weizmann Institute of Science, scientists developed a new method to identify the factors that cause the growth of the dendrite, as well as quickly evaluate different battery components for improved safety and performance.
Rechargeable batteries function by making positively charged ions migrate between the anode (negative electrode) and the cathode (positive electrode) by an electrolyte. Charging the ions forces back in the anode, against the usual current in a typical chemical reaction, which reaches the battery for another usage cycle. Lithium metal batteries follow a different approach by using a pure lithium metalanode, making higher energy storage possible. However, lithium metal is chemically reactive and quickly forms dendrites when it interacts with the electrolyte. Over time, enough dendrites can increase the battery short and the risk of burning.
A way to prevent fire risks is to replace the volatile liquid electrolyte with a fixed, non-flammable, often consisting of a polymer-ceramic composite. Although changing the ratio of polymer to ceramics can influence the growth of the dendrite, finding the ideal formulation remains a challenge for extending the battery life.
To investigate, the team used nuclear magnetic resonance (NMR) spectroscopy, a standard instrument for determining chemical structures, and followed both dendrite formation and the chemical interplay in the electrolyt. “When we investigated the dendrites in batteries with different proportions of polymer and ceramics, we found a kind of ‘golden ratio’: electrolytes composed of 40 percent ceramics had the longest life,” Leskes explains. “When we went over 40 percent ceramic, we came across structural and functional problems that hinded the battery performance, while less than 40 percent led to a reduced battery life.” Intriguously enough, with that optimum ratio in general, more dendrites showed batteries, but those dendrites were effectively limited in a way that prevented destructive bridging.
These insights have created a greater question: what does the expansion of the dendrites stop? The team assumed that a thin cover on the surface of dendrites, the fixed electrolyte interphage (SEI), could be crucial. This layer, formed when dendrites interact with the electrolyte, can influence how lithium ions travel through the battery, and it can also prevent or accelerate the movement of harmful substances between electrodes. Both factors in turn can, in turn, suppress or promote further development of Dendrite.
Investing the chemical composition of such thin sei films is inherent inherent, because they only measure a few tens of nanometer thick. The researchers have tackled this problem by improving the signals in their NMR data using dynamic nuclear polarization. This specialized technology uses the strong turn of polarized lithium electrons, which strengthens signals from the atomic nuclei in the SEI and exposes the chemical composition. Because of this refined lens, the researchers discovered exactly how lithium metal interacts with polymer or ceramic materials, which shows that certain sei layers can simultaneously improve ion transport and block hazardous substances.
Their findings pave the way to design more energy, safer and more powerful batteries that store more energy with lower environmental and economic costs for a longer duration. Such batteries of the next generation can feed larger devices without increasing the physical size of the battery itself, while also extending the life cycle of the battery.
“One of the things that I like best about this study is that without a deep scientific understanding of fundamental physics we were unable to understand what is happening in a battery. Our process was very typical of the work here at the Weizmann Institute.
Research report:Tracking Dendrites and Fixed electrolyte interpass formation with dynamic nuclear polarization-NMR spectroscopy
