Electric vehicles are positioned to take the lead in automobile sales within the next decade, market researchers are predicting. To meet the demand, manufacturers need new lithium-ion batteries that can satisfy a host of sometimes conflicting demands, including longer lifetimes, lower weight, higher safety and lower prices.
The properties of lithium-ion batteries are inextricably linked to the characteristics of the materials they are made from, including their crystal structures. But it is challenging to observe how these structures change while a battery charges and discharges. Typical X-ray diffraction measurements necessitate dissembling the battery to analyse its components, which may result in missing dynamic processes such as the ferrying of ions between the electrodes.
“X-rays are typically used to identify the crystal phases of a battery’s electrolyte and electrodes,” says Akito Sasaki, a researcher at Rigaku Corporation in Tokyo. “If you can take X-ray diffraction measurements without breaking the product, you can collect data on how charging and discharging impacts the battery’s stability. However, this requires using a more powerful X-ray energy source than usual.”
Getting smart about battery characterization
When using conventional X-ray diffraction systems to analyse crystal structures, crystals have to be milled into powders to ensure that the scattering patterns obtained are as uniform as possible. For characterizing lithium-ion batteries, this sometimes requires breaking apart and grinding up their anodes, cathodes and electrolytes. Until recently, researchers wishing to see rechargeable batteries in action needed to use high-energy synchrotron light to penetrate deep into a battery — a difficult prospect due to the high cost and limited experimental time at synchrotron facilities.
To overcome this problem, Rigaku engineers have developed an X-ray diffraction system that brings synchrotron-like capabilities into a researcher’s own laboratory. Called SmartLab, the new instrument features a rotating anode X-ray source with a power of 9 kilowatts, making it the most powerful commercially available diffractometer.
“We have metals, including silver or molybdenum as sources, which emit higher energy X-rays. They can penetrate thick materials when in transmission mode,” says Sasaki. “Even if you irradiate the battery from the top, you get signals from the electrolyte and electrode layers lower down. Many of our clients tell us they never have to visit a synchrotron now, which saves them a lot of time and money.”
To help customers test under realistic conditions, the Rigaku team designed a setup where standard laminated lithium-ion batteries can be held and powered up directly in the sample holder. A multi-dimensional wide-angle X-ray detector then collects numerous diffraction patterns while the battery undergoes typical operations.
“The wide-angle detector allows us to collect hundreds of X-ray diffraction images a minute and is sensitive to very low intensities” notes Sasaki. “You don’t have to scan for specific peaks. And with SmartLab Studio II software, the measurement process and data analysis is very straightforward. Just press two buttons and you’re done.”
The path less travelled
Transmission-mode measurements on batteries with lithium-based cathodes and graphite anodes revealed that the shrinking or swelling of different crystal lattices could be followed instantly during hundreds of charge cycles. By following certain lattice changes over time, the team could pinpoint a degradation mechanism involving intercalated lithium–carbon species.
The SmartLab instrument can also identify where ions are likely to travel during battery operation, explains Yuji Shiramata, an application scientist at Rigaku. “With the data the system obtains, you can analyse crystal structures and then simulate the lithium diffusion paths based on bond valences,” he says. “The software enables you to plot these paths as two-dimensional maps. If at some point a drastic change occurs, SmartLab will lead you to a link where you can see the X-ray diffraction pattern directly.”
The X-ray source is powerful enough for pair distribution function analysis to give data about the interatomic distances in a sample, Shiramata says. “Sometimes non-crystalline materials such as amorphous solid electrolytes are present in batteries,” he says. “You can perform structural calculations with SmartLab that once would have required an extremely high intensity source like a synchrotron.”
The SmartLab X-ray diffraction system performs as well in conventional reflection geometry as it does in the more advanced transmission setup. In this mode, the instrument can quickly spot impurities in an electrode material, for example, or study the heat-dependent agglomeration of crystallites. Besides temperature control, the instrument also features an airtight sample holder for measuring environmentally sensitive samples, such as experimental lithium–sulfur electrolytes.
Going beyond batteries
Sasaki notes that the in-plane arm on SmartLab is useful for studying thin films, which is valuable given the growing importance of this field of thin-film research. But it is not restricted to thin films. “There is a wide range of sample stages for different size samples, while other stages allow researchers to perform dynamic experiments, so they can observe crystallographic changes as a function of changing variables,” Sasaki adds. “This combination of power and versatility make SmartLab the ultimate system for multi-user facilities and the most futureproof choice, catering to applications from advanced materials and nanomaterials to pharmaceuticals and geological materials, which may require experiments that have yet to be devised to analyse them.”
“The 9 kilowatt Rigaku SmartLab is the most powerful X-ray diffraction system outside of synchrotrons, making it the ideal research tool,” remarks Sasaki.