A battery structure changes as the cell is charged and discharged.
During charge-and-discharge, the electrons, or current, go in and out of the electrode, leading to changes in chemical state. When electrons are pulled out, the electrode is oxidized; when electrons are injected, the electrode is reduced.
Researchers like Berkeley Lab's Wanli Yang want a deeper understanding of this reduction and oxidation, or "redox," reaction. The change of a battery electrode's chemical state is one of the most important chemistries to understand in battery operations, according to the staff scientist.
"It defines whether a battery could work in a stable way, so it could be charged and discharged safely for many cycles without triggering safety issues," said Yang. "The type and the amount of the redox reaction determines directly the capacity, the voltage, and the stability of a battery cell."
A change in oxygen states, for example, has been found to hamper battery performance in studies of lithium-rich electrodes.
Wanli Yang, a staff scientist at Lawrence Berkeley National Laboratory ’s Advanced Light Source (ALS), and his team at the facility adapted an X-ray technique known as RIXS (resonant inelastic X-ray scattering) for use in battery experiments.
The updated technology: mRIXS, or mapping of RIXS.
The high-efficiency system samples battery electrodes to measure the chemical states of different elements at a specific point in the battery’s charge or discharge cycle.
The mRIXS method slowly X-ray-scans across a sample that chemically preserves a point in the battery charge or discharge cycle.
Such a full-map scan would have taken days before the high-efficiency RIXS system was introduced at the ALS. A map scan now takes about three hours to complete per sample.
The process can tell researchers whether, and how fully, battery materials are gaining and losing electrons and ions – in a stable way, so teams can learn how quickly and why a battery is degrading, for example.
"Our goal here is to 'see' not only the surface signals, but also the chemistry in the bulk material," Yang told Tech Briefs.
The mRIXS technique spots metal states in the electrode and detects whether the oxygen redox states are reversible– a valuable capability for studies of the high-voltage, high-capacity battery materials that have become a growing focus for battery R&D.
“The uniqueness of the system here is not only on the data collection time, but its ability to look at unconventional chemical states that typically are not very stable under X-rays,” said Yang in a press release last week .
Yang and his team at the ALS are further optimizing RIXS systems to hopefully achieve a higher efficiency for battery research and energy-material needs.
In an edited interview below, Yang tells Tech Briefs how the tool will unlock some of the most "mysterious" inner workings of a charging battery.
Tech Briefs: What are the drawbacks with using conventional spectroscopy to inspect a battery?
Wanli Yang: I need to first clarify that there is no "good" or "bad" regarding a research technique; it all depends on what we want to get!
Techniques like X-Ray Photoelectron Spectroscopy (XPS ) and (XAS ) have been broadly used and will still be used. But we want get more and more capacity, and higher and higher voltage for the power of a battery cell. What this triggers for the battery electrode is that the chemical/oxidation state of the battery material can no longer stay in its conventional range like before; it will be reduced/oxidized more than before. Whether those conventional techniques are sensitive enough to see these unconventional states becomes a question now.
Indeed, with the technique called RIXS, we have found in more and more cases that conventional tools do not have enough sensitivity to see the states at very high or very low voltages beyond the conventional operation range for batteries.
Tech Briefs: What is better about RIXS compared with conventional techniques, including conventional spectroscopic tools?
Wanli Yang: The beauty of mapping of RIXS (mRIXS) is that it is much more sensitive to the unconventional states in batteries that are triggered by the push towards higher capacity and larger voltages range. So to put it in a simple way, RIXS (or mapping of RIXS) provides a much more sensitive probe of the chemical state of battery electrodes, especially when the battery is operated under high capacity mode and when conventional techniques cannot sense the changes. We have found in several cases now that conventional techniques cannot sense the change of chemical states of either the transition-metals or oxygen when the battery electrode is operated in an unconventional range, but mRIXS could detect them.
Tech Briefs: Why is it so important to detect the change of oxygen state in a battery?
Wanli Yang: This is very important not only because we could tell the changing state of oxygen while the electrode is charged to high voltage, but also, oxygen itself has been almost a "mystery element" in battery electrodes. If oxygen is too active, the battery can catch fire. If oxygen is controlled well, the oxygen redox could enable the extra capacity.
Detecting and controlling oxygen chemistry have been one of the hot topics in battery researches in the last 5 years, but lots of questions and debates remain, and controlling oxygen chemistry in batteries remains a dream. But as you could tell, we are able to tackle this problem now with a very powerful tool, and we can start to clarify many misunderstandings on the oxygen activities in batteries.
Tech Briefs: The RIXS system looks massive. Does the complex nature of the RIXS system create any challenges?
Wanli Yang: Oh, yes, absolutely. RIXS systems have been realized more than two decades ago; however, employing this tool for battery research is a nontrivial issue because these "unconventional" chemical states, as explained above, are often not stable under X-rays.
Also, doing research on practical materials is often very different from that on fundamental physics: We need to measure a very large amount of samples at each different charge/discharge states, while we often focus on one or couple specific samples for fundamental physics research.
So, the detection efficiency becomes a more important parameter and we even give up the experimental resolution somewhat to make such experiments feasible. Back in 2015-2016, we completely rebuilt our RIXS experimental system to achieve the high detection efficiency.
Tech Briefs: How does the testing work exactly?
Wanli Yang: This has become fairly straightforward these days. We load the battery electrode (cycled to the different voltages) into our experimental vacuum chamber through a protected sample transfer kit to avoid air exposure. Then, all we need is to locate the electrode, so the X-ray can hit it, and turn on our RIXS system to collect data.
Tech Briefs: What is most exciting to you about your system and what it can provide?
Wanli Yang: Many spectroscopic tools evolve from fundamental physics research fields. These tools have all made significant contributions to our fundamental researches. I myself was trained and came from the physics field, and physicists are still exploring new techniques and/or improving the technical parameters especially the resolution.
The power of RIXS has been continuously explored when this technique is employed for the energy material researches. In this aspect, the vast ground of energy materials helps the advances of RIXS technique because the many different systems provide opportunities to uncover the true power of RIXS, which has never been even realized.
Also, the push of this technique is no longer based solely on the need of fundamental physics research, but also on the broad material and chemistry sciences. For example, a very recent finding with collaborators in Xiamen University shows that RIXS could detect some subtle oxygen state change that is affected by only an inductive effect from a nearby proton in battery electrodes .
In the meantime, our society encounters very practical problems on sustainable energy applications. Energy storage is actually just one of the issues. Energy science is a vast field that requires collaborations of not only people all over the world, but also expertise from different fields. Developing and introducing the high-efficiency RIXS technique to the energy material field not only answer the call for detecting the unusual chemistry in high capacity batteries, from a broader view; it will inspire more overlapping and bridging between the two fields of fundamental physics and practical materials, and probably others too.
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