A little aluminum improved a sodium battery material—and half as much still worked

What the aluminum changed

The researchers synthesized Na5Fe1−xAlxS4 with aluminum contents ranging from 0% to 100%. Structural analysis found that the sodium iron sulfide and sodium aluminum sulfide end members formed a solid solution across the entire composition range.

Aluminum substitution increased the material’s apparent ionic conductivity, meaning the measured ability of sodium ions to move through the electrode material. The 5% aluminum composition, Na5Fe0.95Al0.05S4, reached an apparent ionic conductivity of 5.8 × 10−7 S cm−1. All-solid-state cells using it delivered an initial capacity of about 500 mAh g−1 and operated reversibly for more than 70 cycles.

The researchers also found that compositions with up to 50% aluminum retained sufficient apparent ion diffusivity throughout charging. Structural measurements showed that the 5% aluminum material became amorphous during its first charge and then cycled reversibly in that state.

Why the composition matters

The results identify a way to modify a sodium iron sulfide electrode while retaining the high capacity associated with sulfur-based reactions. The materials use sodium, iron, sulfur and aluminum, and the study links modest aluminum substitution with improved apparent ion movement and higher capacity in an all-solid-state cell.

The findings also suggest that useful electrode behavior is not limited to very small substitutions: the study reports high performance even when aluminum replaced up to half of the iron. This could broaden the range of compositions considered for sodium iron sulfide-based solid-state batteries, although the experiments do not establish practical battery performance at larger scale.

Evidence and open limits

This was a materials and laboratory-cell study. The researchers synthesized eight compositions, examined their structures and elemental distributions with techniques including X-ray diffraction and electron microscopy, measured apparent ionic conductivity, and tested composite positive electrodes in all-solid-state cells with a sodium-phosphorus-sulfur solid electrolyte and a sodium-tin negative electrode.

Some samples contained secondary phases, including sodium iron sulfide-related and sodium sulfide phases. The researchers said these phases could contribute to the measured capacity, but their individual effects were not separately quantified. Elemental mapping also showed local compositional differences whose chemical identity could not be determined from that analysis alone. The reported cycling result of more than 70 cycles applies to the 5% aluminum composition; the excerpts do not establish comparable cycle counts for every aluminum content or performance in a full-scale battery.