A coated cathode held 91.2 per cent of its capacity after 300 full-cell cycles against 71.4 for a doped one and 18.6 for untreated — and the authors say scandium is too scarce to use.

Sodium-ion batteries exist because lithium is expensive and unevenly distributed, while sodium is neither. The chemistry is close enough to lithium-ion that much of the manufacturing transfers, the electrolytes are safer, and the energy density is lower — an acceptable trade for stationary storage.

The weak point is the cathode. Layered oxides of the O3 type hold a lot of sodium and degrade quickly, because the large sodium ion rearranges the structure as it moves in and out. A study in Small from Tokyo University of Science tests two ways of adding scandium to one such cathode, and finds that where the scandium sits matters more than that it is there.

The work was led by Professor Shinichi Komaba and Associate Professor Shinichi Kumakura of the Department of Applied Chemistry at TUS, with Kodai Moriya, doctoral student Sho Toriumi, and collaborators Professor Yoshitaka Tateyama and Assistant Professor Hoang Duc Luong.

In the bulk, or on the surface

The cathode is O3-Na[Ni_1/2Mn_1/2]O₂, abbreviated NNMO, and scandium was introduced two ways: doped through the bulk of the crystal, or coated onto its surface.

“Sc ions can be incorporated either into the bulk structure or through an external coating,” Kumakura said. “In this study, we explored how scandium improves battery performance of SIBs through both doping and coating.”

The doped material with 8 per cent scandium, NNMSO8, was the focus of that arm. The coating was applied by a wet process using scandium isopropoxide solution followed by annealing at 800 °C, giving NNMO-SC800.

Running both arms on the same base material is what makes the comparison informative. Doping and coating are usually studied separately in separate papers on separate materials, which leaves no way to tell whether one approach is better or whether one material simply responded better.

The retention figures

In coin-type sodium cells after 100 cycles, undoped NNMO retained 18.6 per cent of its capacity, NNMSO8 retained 67.8 per cent, and NNMO-SC800 retained 75.4 per cent.

An untreated cathode losing more than four fifths of its capacity in 100 cycles is the baseline that explains why this research exists. That is not a battery.

In full cells against a hard carbon anode after 300 cycles, NNMSO8 retained 71.4 per cent and NNMO-SC800 retained 91.2 per cent.

The full-cell results carry more weight than the coin-cell ones, and the reason is the counter-electrode. A half cell tested against sodium metal has an effectively unlimited sodium reservoir, so capacity loss at the cathode can be masked; a full cell against hard carbon has a fixed sodium inventory, which is the situation in a real battery and the one in which degradation shows up honestly. Hard carbon is also the anode sodium-ion batteries actually use.

The coating winning by twenty percentage points over 300 full-cell cycles is the study’s headline result, and it inverts the expectation that fixing a material throughout should beat treating its outside.

Why the two do different things

The mechanisms turn out to be distinct, and the evidence for that is in the shape of the charge–discharge curves.

Doping suppressed sodium/vacancy ordering, producing a smoother curve. Ordering is a phenomenon in which sodium ions and the empty sites they leave behind arrange themselves into regular patterns at particular states of charge, which shows up as steps and plateaus in the voltage curve and as structural strain in the material. Scandium sitting in the lattice disrupts those patterns.

The coated sample showed no such change in curve shape, which indicates that its scandium stayed at the surface rather than entering the structure. Instead, the coating forms an O3-NaScO₂-like surface phase that suppresses side reactions — the parasitic chemistry between a charged cathode and the electrolyte touching it, which consumes both.

So one approach addresses what happens inside the particle and the other addresses what happens at its boundary. Both failure modes are real, and neither treatment fixes the other’s problem: coating improves cycling stability without preventing the loss of crystallinity over long-term cycling, while doping suppresses bulk degradation without fully stopping capacity fade.

Both modified materials also showed improved rate capability — better performance when charged and discharged quickly.

“Our findings show that a synergistic combination of bulk doping and surface coating is a promising strategy,” Komaba said. “This will help extend the lifespan of SIBs and consequently widen their practical application.”

That combination is the logical next experiment and is not reported here, which leaves the central recommendation untested. Whether doping and coating are additive, redundant or interfering is not something the two separate arms can establish.

The admission

The authors say plainly that scandium’s cost and availability limit commercial use, and that more abundant elements are the next step.

This deserves emphasis, because it undercuts the practical reading of the whole study. Scandium is one of the most expensive elements in commercial use — produced in tonnes per year rather than thousands of tonnes, mostly as a by-product of other mining, at prices that have historically run into hundreds of dollars per kilogram for the oxide. The entire argument for sodium-ion batteries is cost, and a cathode treated with scandium is not a cheap cathode.

What the work is for, then, is mechanism rather than product. It establishes that surface side reactions matter more than bulk structural disorder in this material over 300 cycles, and that a surface phase of this type suppresses them — which is a finding that can guide a search for a cheaper element that does the same job. The coating is a demonstration of what to look for, not a specification.

Two smaller notes. The release’s image credit names Komaba twice, once as Professor and once as Associate Professor, which appears to be an error for Kumakura. The authors declare no conflict of interest.

The paper was published on 6 August 2026. Funding came from a MEXT Data Creation and Utilization Type Materials Research programme, JST CREST, ASPIRE and GteX programmes, JSPS KAKENHI, and measurements at the NIMS Battery Research Platform.

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Notes

1. “Scandium doping and coating for improving O3-NaNi1/2Mn1/2O2 electrode in sodium battery.” Small, 2026. DOI: 10.1002/smll.75049.
2. Tokyo University of Science. “How scandium can improve the durability of sodium-ion battery electrodes.” EurekAlert!, 10 August 2026. https://www.eurekalert.org/news-releases/1138986.

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