High energy X-rays pave the way for better EV batteries

Tuesday, 13 December, 2022

High energy X-rays pave the way for better EV batteries

A team of scientists led by chemists at the U.S. Department of Energy’s Brookhaven National Laboratory and Pacific Northwest National Laboratory (PNNL) has unravelled the complex chemical mechanisms of a battery component that is crucial for boosting energy density: the interphase. The research was published in Nature Nanotechnology.

Many electronics, including smartphones and electric vehicles, currently rely on conventional lithium-ion batteries. While Li-ion batteries have become common due to their high efficiency and long lifespan, these batteries face challenges in more demanding applications, such as powering electric vehicles over long distances. To help build a better battery for electric vehicles, researchers have formed a consortium called Battery500. Led by PNNL, the consortium aims to make battery cells with an energy density of 500 watt-hours per kilogram — more than double the energy density of current batteries. To do so, the researchers are focusing on lithium-metal batteries. While lithium-ion batteries rely on graphite anodes, these batteries use lithium-metal anodes.

Lithium-metal anodes provide a higher energy density than graphite anodes, but there are trade-offs; it is challenging to find a way to stabilise the anode as the battery charges and discharges. Scientists at Brookhaven Lab and PNNL led a study on lithium-metal batteries’ solid-electrolyte interphase. The interphase is a chemical layer formed between the anode and the electrolyte as the battery charges and discharges. Scientists have learned that the interphase is the key to stabilising lithium-metal batteries, but it is a sensitive sample with convoluted chemistry, making it difficult to study and, therefore, difficult to fully understand.

Brookhaven chemist Enyuan Hu, who led the study, said the interphase influences the cyclability of the whole battery. “It’s a very important, but elusive system. Many techniques can damage this small, sensitive sample, which also has both crystalline and amorphous phases,” Hu said.

The scientific community has conducted many studies using a variety of experimental techniques, including cryo-electron microscopy, to better understand the interphase. “A comprehensive understanding of the interphase provides the foundation for building an effective interphase. The Battery500 Consortium strongly encourages collaborations. We have been collaborating with Brookhaven Lab closely on many scientific projects, especially understanding the interphase,” said Xia Cao, a PNNL scientist who co-led the study.

To analyse the complex and elusive chemistry of the interphase, the researchers turned to a tool called the National Synchrotron Light Source II (NSLS-II). The NSLS-II is a DOE Office of Science User at Brookhaven Lab that generates ultra-bright X-rays for studying the atomic-scale makeup of materials. Hu and colleagues have been leveraging the advanced capabilities of the X-ray powder diffraction (XPD) beamline at NSLA-II to make new discoveries in battery chemistry for many years; the team then turned to XPD to gather precise findings on the interphase.

According to Hu, the researchers had previously discovered that high-energy synchrotron X-rays do not damage the interphase sample — this is important because one of the greatest challenges in characterising the interphase is that the samples are sensitive to other types of radiation, including low-energy X-rays. “We took advantage of two techniques that use high-energy X-rays, X-ray diffraction and pair distribution function analysis to capture the chemistries of both the crystalline and the amorphous phases in the lithium-metal anode interphase,” Hu said.

After cycling a lithium-metal battery 50 times and harvesting enough interphase sample, researchers disassembled the cell, scraped off a trace amount of interphase powder from the surface of the lithium metal and directed XPD’s high-energy X-rays at the sample to reveal its chemistry.

“XPD is one of the few beamlines in the world that is capable of carrying out this research. The beamline provided three advantages for this work: a small absorption cross-section, which damages the sample less; combined techniques, X-ray diffraction to get the phase information and pair distribution function for real space information; and a high-intensity beam for delivering quality data from a trace sample,” said Sanjit Ghose, co-author of the study.

This combination of advanced X-ray techniques provided a detailed chemical map of the interphase components — their origins, functionalities, interactions and evolutions. Sha Tan, first author of the paper, said the researchers focused on three different components of the interphase; first was lithium hydride and its formation mechanism. The researchers had previously found that lithium hydride existed in the interphase, and this time they identified that lithium hydroxide, which can be found natively in the lithium-metal anode, is the likely contributor to lithium hydride. Controlling the composition of this compound will help scientists design an improved interphase with a high performance.

“Second, we studied lithium fluoride, which is very important for electrochemical performance, and found that it can be formed at a large scale in low concentration electrolytes,” Tan said. Previously, scientists believed that lithium fluoride could only be formed in electrolytes using high-concentration electrolytes, which rely on expensive salts. Thus, the work provides evidence that low-concentration electrolytes, which are more cost-effective, can potentially perform well in these battery systems.

“Third, we looked at lithium hydroxide to understand how it is consumed during battery cycling. These are all very new findings and important for understanding the interphase,” Tan said. These findings helped highlight previously overlooked components of the interphase and will enable more accurate and controllable interphase design for lithium-metal batteries.

The team will continue to contribute additional studies to the Battery500 consortium. The consortium is currently in its second phase, which will continue through 2026.

Image credit: iStock.com/Just_Super

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