In Simple Terms
Some sodium-battery materials can become damaged when charged to high voltages. Researchers found that balancing how different ingredients affect the material may help it hold its shape as sodium moves in and out. The idea could guide the design of longer-lasting batteries, but it still needs to be tested in practical cells.
Why Does a Battery Cathode Wear Out?
A sodium-ion battery’s performance depends in part on whether its cathode can take in and release sodium ions again and again without changing its internal structure in ways that interfere with operation. In a study published online on 20 October 2026 in the Journal of the American Chemical Society, researchers tested a way to guide the choice of ingredients in these materials. Rather than relying only on average ion size or charge, their approach considers how much the ions vary in their local electrical effects within the crystal.
Why High-Voltage Charging Strains the Cathode
Layered O3-type oxides are an important option for sodium-ion battery cathodes. They combine a high theoretical capacity with a large initial supply of sodium, and their composition can be varied by changing which transition metals are included. This flexibility gives researchers plenty of room to tune a material’s properties, but it does not, by itself, ensure that the structure will remain stable during charging and discharging.
The problem becomes clear when a battery operates above about 4.0 volts. As more sodium is removed from the material during charging, the electrostatic shielding between neighboring layers of transition-metal oxides weakens. The layers may then slide relative to one another, changing their crystal arrangement. The oxygen atoms and surrounding metal atoms can also become distorted in units known as TMO6.
These changes are not merely microscopic curiosities with no effect on battery performance. As they build up, they can speed the deterioration of the crystal lattice and make it harder for the material to return fully to its previous state during discharge. The material may then become less able to repeatedly take in and release sodium efficiently. A cathode must do more than store charge when it is new: during repeated use, it also needs to follow a structural pathway that it can reverse.
From Trial-and-Error Additives to a Calculable Measure
Cathode designers sometimes add electrochemically inactive ions—ions that do not directly take part in charge-storage reactions—to reinforce a material’s structure. But this approach can come at a cost. Adding more ingredients that do not contribute to redox reactions can reduce the share of active components and, in turn, lower the reversible capacity. Choosing the right element has also often relied on average properties, such as ionic radius, oxidation state, or overall composition.
Those averages can hide important differences among the ions in a material. The ions do not all affect their local bonds with oxygen to the same degree. This variation can go hand in hand with differences in bond shape, uneven strain across the lattice, and regions of the crystal changing at different rates during charging. The researchers began with a question: Could this variation be described by a quantitative measure that helps predict cathode stability, rather than relying only on broad compositional properties?
The team focused on what it calls variation in ionic field strength among cations. According to the study, the measure is calculated using each ion’s formal valence, ionic radius, and proportion in the composition. The idea is not that a single value can reveal everything that will happen inside a battery. Rather, the measure could add a local perspective to the design process by drawing attention to differences in the effects of individual cations, not just to their average properties.
Two Materials, One Telling Difference
The researchers tested a cathode with a low variation value of 0.34 and compared it with a counterpart with a higher value of 0.395. Although both materials shared the same general O3 structure, they did not behave identically as sodium was removed. The material with the lower value followed a more reversible structural pathway, changing from O3 to P3 and then back to O3. The sample with greater variation underwent additional transitions at high voltages.
This comparison matters because it links differences at the level of individual ions to changes that can be observed across an entire crystal. The team used operando X-ray diffraction to track changes in the lattice while the material was operating. The measurements showed that the crystal’s dimensions changed less in the lower-variation cathode during deep sodium extraction, suggesting that it resisted distortion more effectively under conditions that challenge its structure.
Returning to the O3–P3–O3 pathway does not mean that every atom in the material returns to its ideal position in every cycle. It does, however, provide evidence that the overall structural changes can be more orderly and less complicated. By contrast, extra transitions at high voltage may cause different parts of the material to respond out of sync. Local differences can then accumulate, making it harder for the material to return to its previous structure.
Signs of Less Chemical Damage and Strain
The team also looked for gaseous products that could indicate side reactions or oxygen-related degradation. Using differential electrochemical mass spectrometry, the researchers detected no oxygen gas (O2) within the voltage range they tested. They also recorded a clear decrease in carbon dioxide (CO2) production. These observations are consistent with the possibility that some oxygen-related damage pathways and reactions at the interface between the cathode and the cell’s other components were reduced.
Other measurements added to the picture. X-ray absorption analysis using a synchrotron light source allowed the researchers to examine the local environment around atoms, while atomic-scale strain maps revealed spatial differences within the material. According to the results, lower variation was associated with weaker local disruptions in atomic coordination, better structural recovery, and less buildup of uneven strain. This evidence matters because X-ray diffraction alone generally describes structural features on a broader scale, while the other techniques help reveal what is happening around individual atoms.
The researchers also found a difference in how much the variation measure changed as sodium was removed. It increased by 7.35% in the lower-variation cathode, compared with 17.72% in its higher-variation counterpart. This comparison suggests that maintaining a more stable distribution of ionic field strengths may go along with smaller changes during charging. The finding supports the researchers’ interpretation, but it does not mean that the measure alone determines the performance of every material or that the mechanism has been settled for every possible composition.
How Can Inactive Ions Help?
The researchers propose a mechanism in which ionic field strengths compensate for one another. Adding inactive cations with an intermediate effect could reduce the gap between the effects of different active cations. Bringing those effects closer together may make metal–oxygen bonds more uniform, so that different regions of the material do not change in sharply contrasting ways as sodium is removed. The structure may then move along a more coordinated pathway instead of leaving some regions under greater strain than others.
This idea is not a call to add as many elements as possible. Increasing compositional complexity is not an end in itself, and excess inactive ions may reduce reversible capacity as well as make the material harder to prepare. The design lesson suggested by the results is that keeping differences in ionic field strengths moderate may matter more than pursuing an extremely complex or high-entropy composition simply because it contains many components.
The team did not limit its test of the idea to the main pair of materials. It also reported using the measure to guide the examination of two additional cathode compositions, both of which showed improved electrochemical performance. The researchers also compared two materials with similar average ionic radii and similar valence states but different values for the variation measure. This comparison helped show that the difference could not easily be reduced to average ionic size or valence alone.
What the Measure Can—and Cannot—Tell Us
The measure could be useful before moving on to making cells and large-scale testing because it may help narrow down the list of candidate additives and compositions. It is calculated from compositional information, including formal valence, ionic radius, and the proportion of each cation. The team therefore suggests that the calculation could be adapted for a range of multicomponent O3-type layered oxides and provide a more systematic starting point for designing cathodes that operate at high voltages.
Still, the results do not yet show that this measure is enough on its own to predict a battery’s commercial lifetime. The work needs to be extended to more compositions, and the materials’ performance must be verified in full cells under practical operating conditions. The researchers also cautioned against applying the idea directly to cathodes that rely on oxygen redox. Those materials can face additional problems, including metals moving within the lattice and oxygen escaping from the structure—processes that cannot be addressed simply by managing variation in ionic field strength.
The work was carried out in collaboration among Nankai University, Nanjing University of Science and Technology, the China Spallation Neutron Source, the Institute of High Energy Physics of the Chinese Academy of Sciences, and the Haihe Laboratory of Sustainable Chemical Transformations. The study was published in the Journal of the American Chemical Society with DOI 10.1021/jacs.6c11691, after being made available online on 20 October 2026. The researchers combined material synthesis, structural analysis, electrochemical testing, and calculations to investigate the link between cathode composition and stability.
The Bottom Line
The study offers a different way to think about additives for sodium-battery cathodes. Instead of considering only average ion size or valence, designers can also look at how much the ions’ local effects differ within a material. In the comparison reported, a value of 0.34 was associated with a more reversible structural pathway and smaller changes in lattice dimensions, while a value of 0.395 was associated with extra transitions at high voltage. Lower variation also went along with reduced indicators of disruption and strain, and with an increase of 7.35% rather than 17.72% in the measure during sodium removal. This is a promising design principle, not a ready-made recipe: it must be tested across more compositions and in practical cells. Researchers will also need to account for the possibility that too many additives reduce capacity, and that cathodes relying on oxygen redox face separate challenges.