Sodium-ion batteries (SIBs) are increasingly becoming popular as alternatives to lithium-ion batteries. This is due to the much greater abundance of sodium (Na) in Earth’s crust compared to lithium, presenting an opportunity for lower costs. In addition, SIBs also offer better safety and low-temperature performance. Indeed, SIBs are already being commercialized for applications ranging from portable electronics to electric vehicles.
Among the various cathode materials under development, layered sodium nickel manganese oxides (Nax[Ni,Mn]O2) have emerged as particularly promising candidates. Specifically, the O3 polytype of Na[Ni1/2Mn1/2]O2 is particularly attractive as it contains stoichiometric Na+ in the pristine state and delivers a relatively large reversible capacity. However, it suffers from severe capacity fading due to large volume changes that occur during charging and discharging processes. To address this issue, previous studies have explored hetero-metal substitution. Among the available candidates, scandium ions (Sc3+) have shown great promise in improving cycling performance. Yet, the exact mechanisms behind these improvements remain underexplored.
Against this backdrop, a research team led by Professor Shinichi Komaba and Associate Professor Shinichi Kumakura from the Department of Applied Chemistry at Tokyo University of Science (TUS), Japan, investigated how the incorporation of Sc3+ into O3-Na[Ni1/2Mn1/2]O2 (NNMO) affects its crystal structure and electrode properties. The team also included Mr. Kodai Moriya and doctoral student Sho Toriumi from TUS, in collaboration with Professor Yoshitaka Tateyama and Assistant Professor Hoang Duc Luong. Their study was published in the journal Small on August 06, 2026. The researchers utilized two approaches to study the effects of incorporating Sc3+: doping and surface coating.
“Sc ions can be incorporated either into the bulk structure or through an external coating. Both can improve the cycling performance, but the underlying mechanisms have not been elucidated,” explains Dr. Kumakura. “In this study, we explored how scandium improves battery performance of SIBs through both doping and coating, clarifying their distinct mechanisms.”
The researchers synthesized Sc-doped samples of NNMO, termed NNMSOx, where ‘x’ denotes the percentage of Sc doping, through a bulk doping method. In particular, the study focused on NNMSO8 due to its superior cycling performance. In addition, the team also prepared samples with Sc coating, where NNMO particles underwent a wet process using scandium isopropoxide solution, followed by annealing at 800 °C, creating NNMO-SC800.
The team then evaluated the electrochemical properties of the synthesized samples using coin-type aprotic Na cells. The results showed that both doping and coating significantly improved charging/discharging durability of the cells, improving capacity retention after 100 cycles from 18.6% for undoped NNMO to 67.8% for NNMSO8 and 75.4% for NNMO-SC800. This improved performance was attributed to suppression of side reactions and bulk stabilization of structure due to Sc3+ doping. In the case of NNMSO8, the Sc3+-doped structure stabilized the bulk phase, while for NNMO-SC800, the Sc3+-coated phase acted as a protective layer that suppressed interfacial degradation.
In addition, NNMSO8 demonstrated a smoother charging/discharging curve compared to NNMO. The team found that this effect was due to the suppression of Na+/vacancy ordering due to Sc3+ substitution. In contrast, NNMO-SC800 did not show any noticeable change in charging/discharging curve shape, suggesting that Sc3+ is present mainly at the surface. Both modified materials also demonstrated improved rate capability.
The researchers further evaluated Na-ion full cells using NNMSO8 or NNMO-SC800 as cathodes and hard carbon as the anode. Both full cells exhibited substantially improved long-term cycling performance. After 300 cycles, the full cell employing NNMSO8 retained 71.4% of its initial capacity, while the cell using NNMO-SC800 retained 91.2%. While direct comparisons across different battery chemistries are complex, this level of durability enhancement represents a significant advancement over current leading Na-ion cathodes.
To understand the origin of these improvements, the team performed detailed structural and mechanistic analyses. They found that doping and coating improved cycling performance through distinct mechanisms. When incorporated into the crystal lattice, electrochemically inactive Sc3+ ions, whose ionic radius is comparable to those of the transition metals, immobilize a fraction of Na+ ions, which then act as pillars stabilizing the layered structure. This smooths charge/discharge profiles and mitigates large volume changes. In contrast, Sc3+ coating leads to the formation of an O3-NaScO2-like phase on the surface of NNMO particles, without any change in the bulk structure. The coating effectively suppresses side reactions, improving cycling performance.
Overall, coating enhances cycling stability but does not prevent loss of crystallinity during long-term cycling, while doping suppresses bulk degradation but does not fully mitigate capacity fading.
“Our findings show that a synergistic combination of bulk doping and surface coating is a promising strategy to improve performance of SIBs,” notes Prof. Komaba. “This will help extend the lifespan of SIBs and consequently widen their practical application.”
While Sc serves as an ideal model system to demonstrate these distinct structural mechanisms, its cost and availability mean that the team’s next crucial step will be applying these insights to more abundant, cost-effective elements suitable for commercial scaling.
This study offers valuable design principles for developing longer-lasting, high-performance SIBs.
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