Many of the most important technological and social developments of the 21st century would not have been possible without lithium-ion batteries. Since they were launched on the market in 1991, these batteries, with their comparatively high energy density and durability, have become vital for operating mobile devices, for electric vehicles of all kinds and, increasingly, for use in stationary energy storage units. It was therefore only fitting that the 2019 Nobel Prize in Chemistry was awarded to three lithium battery researchers. Lithium also plays an important role in other technological fields such as ultralight structural alloys for the aviation and aerospace industry.

However, taking development of lithium materials for the various applications further forward presents researchers with a problem: To date there has been no straightforward method in practice for spatially resolved determination of lithium content at microscopic level. This information is important for developing or further optimizing materials with the desired properties. But in the case of lithium, researchers are still in the dark, as it were.

Lithium is hard to detect

Normally, an electron microscope fitted with a detector for energy-dispersive X-ray spectroscopy (EDS) is used to determine distribution profiles and maps of elements. While the electron beam scans the specimen in nanometric steps, a chemical spectrum is captured for every point, allowing conclusions to be drawn about the specimen’s elemental composition. This makes it possible to generate images of element distribution, known as maps. The problem with lithium is that, as the least dense of all elements that are solid at room temperature (atomic number 3), it is undetectable with conventional EDS detectors. Although lithium atoms emit characteristic X-rays when excited, they have such low energy levels that they cannot be detected. What this means is that an EDS map of a specimen containing lithium will only show the other elements – lithium itself is “invisible”. Alternative methods for quantifying lithium have been proposed in the scientific literature, but they require special equipment and are consequently extremely labour-intensive and costly.

Combination of two measuring techniques

A team led by Johannes Österreicher, senior scientist at AIT Austrian Institute of Technology’s LKR Leichtmetallkompetenzzentrum Ranshofen, has now found a way around these difficulties: a new method of mapping lithium at microscopic level has been developed that can be performed using a standard scanning electron microscope and has no need of any additional and exotic analysers. This technique combines the EDS method with so-called quantitative backscattered electron microscopy. Backscattered electrons are the electrons in the electron beam in an electron microscope that penetrate the specimen and are deflected from the atomic nucleus and reflected (backscattered). Backscattered electrons can be detected and are often used for imaging because areas with a different chemical composition appear as lighter or darker.

The AIT team has exploited this effect: with the aid of different element standards, a calibration of lightness was created on the basis of the atomic number to ascertain the median atomic number of each point on a specimen. If this information is combined with a conventional EDS measurement the lithium content of each point can be calculated – even if only tiny amounts are present. “The huge importance of lithium-ion batteries has made lithium the Holy Grail, so to speak, of spatially resolved chemical analysis in electron microscopes,” says Johannes Österreicher, pleased that “Our new method has taken us a decisive step forward.”

Cooperation with US industry partner

An international patent was filed for the new method and published in the respected journal Scripta Materialia. This immediately triggered enormous interest from electron microscope manufacturers. The AIT subsequently entered into strategic cooperation with the firm of Gatan in California (USA) with a view to refining the method and marketing it. Gatan is a leading electron microscopy company and part of the AMETEK concern which, with its market capitalization of almost USD 32 billion, is nearly twice as big as Austria’s largest company, OMV. Initial joint undertakings confirmed the new method’s potential, leading to creation of a joint poster for presentation at the Microscopy & Microanalysis meeting in 2021. “We at Gatan are delighted about this collaboration and the prospect of establishing an extensive cooperation programme,” says Oleg Lourie, Director of Product Management/SEM at Gatan. “The partnership with the AIT team is a privilege for us and an opportunity to support top-level research.”

Valuable information for research & development at the AIT

At LKR, the new method for detecting lithium is proving very valuable in the ongoing development of high-performance materials such as magnesium-aluminium-lithium alloys that have potential for many uses in the mobility sector. The new measuring technique is also very important for the work being done at the AIT Battery Lab where lithium-ion batteries are being optimized and the solid-state batteries of the future are being developed. “This new method greatly helps us to implement fundamental innovations for the next generation of sustainable transport technologies,” explains Christian Chimani, head of the AIT Center for Low-Emission Transport and CEO of LKR.