Building cheaper, greener batteries is only half the challenge; making them last through hundreds of charge–discharge cycles is equally critical. A team of researchers from Spain and France, led by CIC energiGUNE, has tackled this problem in sodium-ion (Na-ion) batteries by improving Prussian white, an easy-to-synthesize, environmentally friendly material. Using the ALBA Synchrotron, they have discovered how a simple chemical modification can dramatically extend battery lifespan.

For Na-ion batteries to compete with lithium-ion technology —the current prevailing technology— highly cost efficient and more sustainable cathode materials are needed. Prussian white materials containing manganese (Mn) are particularly promising because their performance rivals that of lithium batteries. Their Achilles’ heel, however, is poor long-term durability. During charging, Mn undergoes oxidation, which triggers a local structural distortion. This distortion causes large volume changes, leading to severe structural degradation and rapid capacity loss.

The research team hypothesized that partially replacing Mn with iron (Fe) could stabilize the material over time. The results were striking: the modified material retained 93% of its original charge capacity after 50 cycles, compared to just 62% for the unaltered version. But the real breakthrough was understanding why.

The researchers found that the addition of Fe does not suppress the local structure perturbation but mitigates its damaging effect on the structure. In fact, although the local atomic distortion still occurs, what changes is its overall structural impact: by replacing some Mn with Fe, the number of distortion-prone sites is roughly cut in half. This prevents the local distortions from spreading and building up into a heavily strained form of the material that would damage the cathode material when the battery is charged to high levels. The result is a far more stable structure that holds up well over repeated charging cycles.

To uncover the underlying reason, the researchers examined battery electrodes at different stages of charge using two complementary techniques at the NOTOS beamline of the ALBA Synchrotron. The key is that each technique 'sees' the material at a different length scale. X-ray Absorption Spectroscopy (XAS) zooms in on the very local atomic environment around individual atoms of less than one nanometer. At this close-up, it confirmed that a strong local distortion still occurs in the Fe-partially substituted material. X-ray Diffraction (XRD), by contrast, captures the bigger picture, the ordered repeating structure extending to micrometers . At this scale, it showed the non-substituted original material suffered massive, destructive volume changes, while the Fe-substituted version changed only slightly.

The power of this study lies in bridging these two scales using complementary techniques, demonstrating that a local atomic distortion does not necessarily compromise the overall structure — a paradigm shift in how battery degradation is understood.

This work is the result of a collaboration between CIC energiGUNE, the University of the Basque Country (UPV/EHU), the Institut Charles Gerhardt Montpellier (ICGM), the ALISTORE-European Research Institute, the RS2E, and the ALBA Synchrotron.

Understanding the delicate interplay between atomic structure and electrochemical performance is crucial for scaling up these sustainable materials and bringing Na-ion batteries closer to commercial reality.

The combination of XAS (XANES and EXAFS) and XRD data allows to link the electrochemical profile to the local/long-range structural evolutions and redox reactions, which can explain the differences in cycling profile and stability between the non-substituted and the partially substituted materials.