One of the more promising candidates for batteries beyond the current standard of lithium-ion materials is the sodium-ion (Na-ion) battery. Na-ion is particularly attractive because of the greater abundance and lower cost of sodium compared with lithium. In addition, when cycled at high voltage (4.5 V), a sodium-ion battery can greatly increase the amount of energy that can be stored in a given weight or volume. However, its fairly rapid performance decline with charge-discharge cycling has stymied commercialization.

Researchers at the US Department of Energy’s (DOE) Argonne National Laboratory have now discovered a key reason for the performance degradation: the occurrence of defects in the atomic structure that form during the steps involved in synthesizing the cathode material.

These defects eventually lead to a structural earthquake in the cathode, resulting in catastrophic performance decline during battery cycling. With this knowledge, battery developers will now be able to adjust synthesis conditions to fabricate far superior sodium-ion cathodes.

The team published their research in an open-access paper in Nature Communications.

Key to making this discovery was the team’s reliance on the capabilities available at Argonne’s Center for Nanoscale Materials (CNM) and Advanced Photon Source (APS), both of which are DOE Office of Science user facilities. These capabilities allowed the researchers to track changes in the atomic structure of the cathode material in real time while it was being synthesized, said Guiliang Xu, assistant chemist in Argonne’s Chemical Sciences and Engineering division.

During cathode synthesis, material fabricators slowly heat the cathode mixture to a very high temperature in air, hold it there for a set amount of time, then rapidly drop the temperature to room temperature.

To see what was happening during the synthesis, the team used transmission electron microscope in CNM and synchrotron X-ray beams at the APS (at beamlines 11-ID-C and 20-BM).

The data revealed that, upon rapidly dropping the temperature during material synthesis, the cathode particle surface had become less smooth and exhibited large areas indicating strain. The data also showed that a push-pull effect in these areas happens during cathode cycling, causing cracking of the cathode particles and performance decline.

TEM characterization on cycled O3 NaNi_0.4Mn_0.4Co_0.2O₂ cathode particle (2.0–4.4 V for 100 cycles). a Low and b high magnification TEM image. c Zoomed-in view of the dashed square area in b. d, e Zoomed-in views of the dashed rectangle area in a. f Zoomed-in view of dot-circle area in a. g Schematic illustration of the structural earthquake during cycling of strained O3 NaNi0.4Mn0.4Co0.2O2 cathode. R and L in d, e represents rock-salt and layered structure, respectively. Yellow and gray octahedra represents NaO₆ and TMO₆ of a layered structure, respectively. Purple octahedra means rock-salt structure. Xu et al.

Upon further study, the team found that this degradation intensified when cycling cathodes at high temperature (130 degrees Fahrenheit) or with fast charging (one hour instead of 10 hours).

Our insights are extremely important for the large-scale manufacturing of improved sodium-ion cathodes. Because of the large amount of material involved, say, 1000 kilograms, there will be a large temperature variation, which will lead to many defects forming unless appropriate steps are taken.

—Khalil Amine, an Argonne Distinguished Fellow

Earlier research by team members had resulted in a greatly improved anode.

Now, we should be able to match our improved cathode with the anode to attain a 20% - 40% increase in performance. Also important, such batteries will maintain that performance with long-term cycling at high voltage.

—Guiliang Xu

The impact could result in a longer driving range in more affordable electric vehicles and lower cost for energy storage on the electric grid.

This research was supported by DOE’s Vehicle Technologies Office.

Resources

• Xu, GL., Liu, X., Zhou, X. et al. “Native lattice strain induced structural earthquake in sodium layered oxide cathodes.” Nat Commun 13, 436 doi: 10.1038/s41467-022-28052-x