Scientists have found a path to safe and durable lithium metal batteries

For a quarter of a century, humanity has been enjoying the benefits that have blossomed with the introduction of lithium-ion batteries. It is not for nothing that the Nobel Prize in Chemistry was awarded for their discovery in 2019. But we need to go further – to create more advanced batteries. And here everything came up against a whole set of problems, which only fundamental science can help overcome. And she helps with this.

An example of dendrite growth in a battery. Image source: Weizmann

One of the promising ways to increase the capacity of lithium-containing batteries is to switch to lithium metal anodes. The anode releases ions during a chemical reaction when the battery is discharged and returns them to itself during the charging process for use in the next cycle of operation (discharge).

As has been repeatedly reported, lithium metal is an extremely reactive substance. Its use in the anode of batteries provokes the growth of dendrites – thin filaments of lithium, capable of growing to the opposite electrode over several hundred and even tens of charge/discharge cycles and causing a short circuit, followed by ignition of the battery and the risk of fire. The liquid and usually flammable electrolyte in the battery – needed there as a conductor of ions – only increases this danger.

The problem with dendritic growth is partly solved by switching to solid electrolytes. It is usually a mixture of ceramic and polymer. While remaining a conductor of ions, the solid electrolyte slows down and even stops the growth of lithium metal needles from the anode. The task is to select the best ratio of ceramics and polymer, as well as the materials themselves, which would not impair the cycling of batteries and their performance characteristics – capacity, stored energy density, charging speed, and others.

The difficulty with choosing a material for solid electrolytes is that at the interface between the anode and electrolyte, chemical and physical processes occur in a very thin space – from 5 to 50 nm wide. Meanwhile, this is a critical area that determines the characteristics of the battery as a whole. To continue moving towards better batteries, it is important to understand exactly what is happening there. Scientists usually use nuclear magnetic resonance (NMR) to study the chemical (atomic) composition of a material, but not in this case. To study the interface using NMR would require years of measurements, which is simply not beneficial for anyone.

Researchers from Israel’s Weizmann Institute put batteries aside for a while and set themselves a fundamental task – to develop a technique for analyzing the boundary layers of batteries.

«One of the things I like most about this research is that without a deep scientific understanding of fundamental physics, we wouldn’t be able to understand what’s going on inside a battery. Our process was very typical of the work here at the Weizmann Institute. We started with a purely scientific question that had nothing to do with dendrites, and this led us to research with a practical solution that could improve everyone’s lives,” say the participants in the work.

Ultimately, the scientists enhanced the material’s response by combining NMR with dynamic nuclear polarization, where the spins of lithium’s electrons were driven by a radio frequency field. This greatly enhanced the response and made it possible to determine the exact chemical composition of the layer literally in hours, not years. The analysis showed that the most optimal ratio of ceramics and polymer in a solid electrolyte will be if the ceramics retains 40% in the mixture. At the same time, the cyclicity of the battery and its characteristics are preserved. Scientists hope that the results of their research will spur the creation of more advanced lithium batteries, and this will happen soon enough.