What synchrotron X-rays let us see inside a battery

From 2022 to 2026 I worked as a postdoctoral scholar at the Stanford Synchrotron Radiation Lightsource (SSRL), at SLAC, in the group of Dr. Johanna Nelson Weker. I had already been there during my Ph.D.

People often ask me why battery research needs a facility the size of a synchrotron. This post is my short answer.

Why a synchrotron?

A synchrotron produces X-rays that are far more intense than those of a laboratory instrument, and their energy can be tuned.

Intensity means speed. In our experiments one diffraction pattern took 5 s, and we collected one every 3 minutes. So we can measure a battery while it charges and discharges (operando), and not only after opening it (ex situ).

Tunable energy means that we can choose one element, for example nickel, and look only at it.

Four techniques, four questions

I used mainly four techniques, and each one answers a different question. Most of the examples come from our study of LiNiO₂ cathodes.

X-ray diffraction (XRD): how are the atoms arranged? Diffraction measures the distances between planes of atoms in a crystal. In LiNiO₂ we followed the distance between the layers during the charge and saw it collapse abruptly near 4.3 V. During my Ph.D., the same technique showed that sodium had formed alloys with micrometer-sized silicon.

X-ray absorption spectroscopy (XAS): what is the oxidation state of an element, and what surrounds it? The energy at which nickel absorbs X-rays shifts up when nickel is oxidized, so the absorption edge follows the charge of the battery. The fine structure after the edge gives the Ni–O distance. In LiNiO₂ it moved between about 1.91 and 1.94 Å during the first cycles.

Transmission X-ray microscopy (TXM): where in the particle does it happen? TXM is a microscope that uses X-rays, with a resolution of about 30 nm. We took images at 92 energies across the nickel edge. This gives an absorption spectrum for every pixel, and so a map of the oxidation state inside particles about 12 µm across. This is how we saw that, after 100 cycles, some regions of undoped LiNiO₂ no longer follow the rest of the particle.

X-ray fluorescence (XRF): which elements are present, and where? Each element emits X-rays of a characteristic energy. Scanning a small beam over the sample (µ-XRF) gives a map of the elements. I used it to look for transition metals that leave the cathode and end up on the lithium anode.

What does no technique do alone?

No single technique tells the whole story. XRD gives the average structure of many particles, so a small damaged region can go unnoticed. TXM shows a few particles in detail but not the whole electrode. In our LiNiO₂ study we imaged between one and five particles per sample.

The conclusions are reliable when the different scales agree. That is why most of our papers combine several of these techniques with electrochemistry.

Today I work on the other side of the problem. At Boston University I build new cells, from the synthesis of the active material to the sealed device, and test them for performance and safety. With EXUM Instruments I help battery teams use LALI-TOF-MS, a laboratory technique that maps the chemistry of an electrode without waiting for beamtime. The questions in this post still guide what I measure.