Sodium-ion batteries are moving closer to the center of the global energy-storage race, and a new study from Japan has revealed why a small amount of scandium can make a major difference. Researchers at Tokyo University of Science have shown that scandium can extend the life of promising sodium-ion battery cathodes through two fundamentally different strategies: strengthening the material from within or shielding it from damaging reactions at its surface.
Sodium-ion batteries are attracting intense interest because sodium is far more abundant than lithium and is widely distributed across Earth’s crust. That abundance could help reduce raw-material costs and ease pressure on lithium supplies. Sodium-based cells also offer safety and low-temperature advantages, making them attractive for applications ranging from stationary energy storage to electric vehicles. However, their commercial progress depends on solving a major problem: many sodium-ion cathodes lose capacity rapidly after repeated charging and discharging.
The Tokyo University of Science team focused on O3-type sodium nickel manganese oxide, written chemically as O3-Na[Ni1/2Mn1/2]O2, or NNMO. This layered material begins with a favorable, stoichiometric sodium arrangement and can deliver relatively high reversible capacity. Yet sodium ions move in and out of its crystal structure during battery operation, causing large changes in the spacing and volume of the layered lattice. Over time, these repeated structural shifts can trigger cracking, phase transformations, loss of crystallinity and severe capacity fading.
To investigate how scandium works, the researchers introduced Sc3+ ions into NNMO in two ways. In the first approach, scandium was incorporated directly into the bulk crystal structure through a doping process. The resulting materials were labeled NNMSOx, with the number representing the scandium content. The researchers paid particular attention to NNMSO8, which demonstrated the strongest cycling performance among the doped compositions. In the second approach, they treated NNMO particles with a scandium isopropoxide solution and then annealed them at 800 degrees Celsius, producing a surface-modified material known as NNMO-SC800.
The difference between the two approaches became strikingly clear when the materials were tested in coin-type sodium cells. After 100 charge-discharge cycles, undoped NNMO retained only 18.6 percent of its original capacity. By comparison, NNMSO8 retained 67.8 percent, while NNMO-SC800 retained 75.4 percent. These results show that both bulk doping and surface coating can dramatically improve durability, although they do so through different chemical and structural mechanisms.
Inside the doped material, electrochemically inactive Sc3+ ions occupy positions normally associated with transition metals. Their ionic size is comparable to that of the nickel and manganese ions in the host lattice, allowing them to become part of the layered framework without simply forming a separate phase. Because scandium does not participate in the same redox reactions as the active transition metals, it helps immobilize nearby sodium ions. These relatively fixed sodium ions function like structural pillars, supporting the layers as sodium is extracted and reinserted during operation.
This internal stabilization also changes the battery’s electrochemical signature. NNMSO8 displayed a smoother charging and discharging profile than the undoped cathode. The researchers attributed this behavior to suppression of sodium-ion and vacancy ordering, a process in which sodium ions and empty sites arrange themselves into ordered patterns during cycling. Such ordering can promote abrupt structural changes and voltage plateaus. By disrupting it, scandium doping allows sodium ions to move through the cathode more smoothly while reducing the size of harmful volume fluctuations.
The coated material followed a different path. In NNMO-SC800, scandium was found mainly at the particle surface, where it formed a phase resembling O3-NaScO2. This protective layer did not substantially alter the crystal structure inside the cathode. Instead, it acted as a barrier between the active electrode and the electrolyte, suppressing parasitic reactions that gradually consume active sodium, damage the surface and accelerate interfacial degradation. The coating therefore improved cycling stability without producing the smoother voltage profile observed in the bulk-doped material.
Tests in full sodium-ion cells further demonstrated the practical significance of the findings. The researchers paired the modified cathodes with hard-carbon anodes and operated the cells for 300 cycles. The full cell using NNMSO8 retained 71.4 percent of its initial capacity, while the cell using NNMO-SC800 retained an impressive 91.2 percent. The results suggest that surface protection is especially powerful for preserving capacity over extended operation, while bulk doping provides important resistance to structural collapse. Neither strategy alone solved every degradation pathway: coating could not fully prevent long-term loss of crystallinity, and doping did not completely eliminate capacity fading.
The researchers say the most promising future direction may be to combine both approaches, creating cathodes that are reinforced internally and protected externally. Scandium provides an exceptionally clear model for understanding how these mechanisms operate, but its cost and limited availability make it unlikely to be the final commercial solution. The next challenge will be to identify more abundant elements that can reproduce scandium’s ability to stabilize sodium-ion battery structures and protect their surfaces. If successful, this design principle could help transform sodium-ion batteries into longer-lasting, lower-cost alternatives for the rapidly expanding energy-storage market.
Subject of Research: Not applicable
Article Title: Scandium doping and coating for improving O3-NaNi1/2Mn1/2O2 electrode in sodium battery
News Publication Date: 8 August 2026
Web References: https://www.tus.ac.jp/en/mediarelations/
References: Small, DOI: 10.1002/smll.75049
Image Credits: Professor Shinichi Komaba and Associate Professor Shinichi Kumakura, Tokyo University of Science, Japan
Keywords
Sodium-ion batteries, scandium doping, surface coating, cathode materials, energy storage, battery technology, electrochemistry, electric vehicles, sustainable energy, materials science
