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In the OPINCHARGE Scientific Publications series, we highlight research that helps us better understand what happens inside batteries during operation.
The publication “X-ray Coulomb Counting to Understand Electrochemical Systems” introduces the concept of X-ray Coulomb Counting – an approach that uses X-ray techniques to determine how much electrical charge is associated with individual reactions taking place inside an electrochemical system.
The publication was authored by Chuntian Cao from Brookhaven National Laboratory and Hans-Georg Steinrück from Forschungszentrum Jülich and RWTH Aachen University.
Read the publication: https://zenodo.org/records/19909498
Electrochemical measurements can tell researchers very precisely how much current flows through a battery. But in a complex system such as a Li-ion battery, several reactions can occur at the same time.
This makes it difficult to determine exactly which reaction is responsible for which part of the measured current. Even small side reactions matter: although they may account for only a fraction of the charge during each cycle, over time they can contribute significantly to battery degradation and capacity loss.
The idea behind X-ray Coulomb Counting is to combine electrochemical measurements with X-ray techniques that can selectively observe individual processes inside the system.
The approach aims to answer three key questions:
Which reactions are taking place? How much of each reaction occurs? And what happens to the electrode during these reactions?
By translating X-ray measurements into the amount of charge transferred in a particular reaction, researchers can effectively assign parts of the measured electrical charge to specific processes occurring inside the battery.
X-rays are particularly useful because they can penetrate electrochemical cells and investigate materials in situ or operando – while the electrochemical processes are taking place.
Different techniques reveal different aspects of the system. X-ray diffraction (XRD) can track changes within electrode materials, X-ray reflectivity (XRR) can investigate surface layers and interfaces, while X-ray absorption can provide information about ion concentrations.
Together, these techniques can help researchers connect the electrical signal measured by the battery with the actual physical and chemical changes occurring inside it.
The publication discusses several examples of X-ray Coulomb Counting in Li-ion battery research.
One important application is Extreme Fast Charging (XFC). Using high-energy X-ray diffraction, researchers can quantify processes such as unwanted lithium plating, changes in active electrode materials and other sources of capacity loss.
Because these measurements can be performed without dismantling the battery, they can also reveal where different reactions occur across the electrode and how unevenly a battery responds during fast charging.
Another important application concerns the solid electrolyte interphase (SEI) – a thin layer that forms on battery anodes and strongly influences battery performance and lifetime.
Using operando X-ray reflectivity, researchers can follow the formation, thickness and composition of this layer and connect these structural changes directly with the electrochemical charge consumed during its formation.
This creates a quantitative bridge between what the battery electrically measures and what is physically happening at its interfaces.
X-ray Coulomb Counting provides researchers with a way to go beyond measuring whether a battery performs well or poorly and investigate why it behaves the way it does.
By quantitatively distinguishing individual electrochemical reactions, the approach can support a deeper understanding of degradation, interfaces and ion transport – knowledge that is essential for the rational development of improved battery materials and technologies.
While the publication focuses strongly on Li-ion batteries, the concept could also be extended to other battery chemistries and electrochemical technologies such as electrolysers, supercapacitors and fuel cells.
