In science and everyday life, the act of observing or measuring something sometimes changes the thing being observed or measured. You may have experienced this "observer effect" when you measured the pressure of a tire and some air escaped, changing the tire pressure. In investigations of materials involved in critical chemical reactions, scientists can hit the materials with an X-ray beam to reveal details about composition and activity, but that measurement can cause chemical reactions that change the materials. Such changes may have significantly hampered scientists learning how to improve - among many other things - rechargeable batteries.
To address this, Stanford University researchers have developed a new twist to an X-ray technique. They applied their new approach by observing key battery chemistries, and it left the observed battery materials unchanged and did not introduce additional chemical reactions. In doing so, they have advanced knowledge for developing rechargeable lithium metal batteries. This type of battery packs a lot of energy and can be recharged very quickly, but it short-circuits and fails after recharging a handful of times. The new study, published today in Nature, also could advance the understanding of other types of batteries and many materials unrelated to batteries.
"Most important perhaps, we think other scientists and engineers may solve many chemical reaction mysteries using this new approach," said a co-senior author on the study, Stacey Bent, the Jagdeep & Roshni Singh Professor of chemical engineering in Stanford's School of Engineering and of energy science and engineering in the Stanford Doerr School of Sustainability.
Protective layer
During the first few use/recharge cycles in lithium metal batteries, a protective film forms on the surface of the lithium anode. This protective layer is as small as a billionth of a meter thick, but it is critical to the battery's performance and durability. This film must allow lithium ions to move back and forth between the opposite electrodes while blocking the negative anode's electrons from doing so.
Battery researchers have used the X-ray beam technique, known as X-ray photoelectron spectroscopy, or XPS, to learn a great deal about this critical protective layer. The standard way of operating an XPS device is at room temperature under ultra-negative pressure, meaning almost no extraneous atoms or molecules float around the observation chamber. Under these conditions, though, the chemical composition of the lithium battery's protective layer changes and it gets thinner, the new study shows.
Thinking that these changes may obscure problems with lithium batteries, researchers in the study tried flash freezing new battery cells just after the protective layer formed at around -325 Fahrenheit (-200 Celsius). Freezing has proven helpful in studies using similar equipment, but its use with XPS is quite new. The researchers hoped their "cryo-XPS" method might maintain the protective layer in its pristine form through XPS observation at somewhat warmer temperatures, around -165 F.
It did.
Stanford researchers placed this sample holder (approximately 1-inch diameter, bottom view) containing an anode after battery operation into the X-ray photoelectron spectroscopy tool for thorough analysis of the anode's flash-frozen, pristine protective layer at cryogenic temperature. | Ajay Ravi
"By comparing the observations using our method, we identified the changes wrought by XPS observation at room temperature, which could lead to overcoming the challenges of lithium metal batteries and improving other lithium-based batteries," said the other co-senior author of the study, Yi Cui, the Fortinet Founders Professor of materials science and engineering in the School of Engineering, of photon science at SLAC National Accelerator Laboratory, and, like Bent, of energy science and engineering.
"Also, cryo-XPS improves performance-based assessments of different electrolyte chemistries used with lithium anodes, which can help researchers working on several new battery architectures," said Cui.
New insights
Using both conventional XPS and their frozen method, the researchers measured how well batteries perform using different chemistries for the electrolyte, through which charged particles travel between the positive and negative electrodes during use and, later, recharging. Electrolytes contain salt and solvent, and salt-based chemicals are considered useful in the protective layer, ensuring stability. They found only a moderate correlation between charge retention and salt-based chemicals in the protective layer using conventional XPS. However, when they used cryo-XPS measurements, the correlation was very strong.
We think other scientists and engineers may solve many chemical reaction mysteries using this new approach.Stacey BentProfessor of Chemical Engineering and of Energy Science & Engineering
"It seems that cryo-XPS delivers more reliable information about which chemical compounds actually improve battery performance," said Sanzeeda Baig Shuchi, the lead student on the research team and a PhD candidate in chemical engineering.
Among other significant differences between room-temperature XPS and cryo-XPS, the research team learned that conventional XPS readings of battery materials increased the amount of lithium fluoride at the protective layer, a compound that has been associated with improved battery performance.
"This may have sent battery design in some wrong directions, because higher lithium fluoride is thought to increase the number of battery discharge/recharge cycles, but standard XPS exaggerates how much lithium fluoride exists in the protective layer," said Shuchi.
Another compound linked to better battery performance, lithium oxide, also showed significant differences at room temperature vs. cryo-XPS. Under frozen conditions, high amounts of lithium oxide were found at the protective layer during battery operation with high-performing electrolytes. This did not happen during conventional XPS observations, likely due to chemical reactions caused by conventional XPS. This outcome, oddly, was reversed when low-performing electrolytes were used, where lithium oxide became more prominent at room temperature XPS measurements.
Lithium metal outlook
The development of cryo-XPS has important implications for designing better batteries. Lithium metal batteries, which use metallic lithium anodes instead of the graphite anodes in lithium-ion batteries, promise substantially higher energy density than today's dominant lithium-ion batteries. However, lithium metal batteries suffer from safety and longevity problems largely related to the anode's protective layer.
"With more accurate insights on the composition of the lithium anode's interface, researchers could design electrolytes or even ultrathin coatings that form more stable interfaces," said Bent. "Knowing which chemicals will actually be present during battery operation is better than characterizing an interface that may not reflect actual conditions."
This work challenges some existing interpretations of the battery interface, the researchers said, but scientists and engineers can move forward with more confidence in their measurements using cryo-XPS.
