Because lithium is relatively scarce and sodium is abundant in Earth's crust, sodium-ion batteries are being investigated as viable cost-effective alternatives to the widely used lithium-ion batteries. In these batteries, the choice of cathode material primarily influences battery capacity and stability. Layered sodium manganese oxides (Na2/3MnO2) have attracted significant attention in recent years as cathode materials for high-capacity sodium-ion batteries without using any rare-earth metals. However, while these materials exhibit high initial capacity, their rapid capacity fading during charge-discharge cycling remains a significant challenge.
During charge-discharge cycling of NaMnO2 electrodes, Na+ ions are constantly inserted and extracted from the cathode material. This is accompanied by changes in the oxidation states of manganese (Mn) between Mn3+ to Mn4+. When Mn3+ ions form, they distort their surrounding lattice to lower electronic energy, a phenomenon known as Jahn-Teller distortion. Over time, these repeated distortions lead to a buildup of strain at both atomic and particle level in NaMnO2, eventually resulting in the loss of crystallinity and severe capacity degradation. This is the main cause of capacity loss during cycling of Na2/3MnO2 electrodes. Recent studies have attempted to address this issue by substituting metals at Mn sites.
In a recent study, a research team led by Professor Shinichi Komaba, along with Mr. Kodai Moriya and Project Scientist Dr. Shinichi Kumakura, from the Department of Applied Chemistry at Tokyo University of Science, Japan, revealed how scandium (Sc) doping can dramatically improve the cycling stability of P'2 polytype of Na2/3MnO2 electrodes. "Previously, we discovered that Sc doping in P'2 Na2/3[Mn1-xScx]O2 electrodes can improve the battery performance and long-term stability," explains Prof. Komaba. "However, the exact mechanism for this improvement remains unresolved, and it was unclear whether this effect is generally applicable. In this study, we systematically studied P2 and P'2 polytypes of Na2/3[Mn1-xScx]O2 to understand the role of Sc doping." Their study will be published online in the journal Advanced Materials on September 12, 2025.
The crystal structure of Na2/3MnO2 has several polytypes, which differ in several aspects. A key difference between the P2 and P'2 polytypes is that former exhibits localized Jahn-Teller distortions, while the latter features cooperative Jahn-Teller distortion where the distortions are aligned in a long-range order. The researchers conducted a series of experiments on both doped and undoped samples of each polytype containing varying amounts of Sc.
Structural tests revealed that Sc doping in P'2 Na2/3[Mn1-xScx]O2 effectively modulates its structure, resulting in smaller particles and altered crystal growth, while preserving cooperative Jahn-Teller distortion and superstructure. This significantly improves structural stability. In addition, the team found that Sc doping prevents side reactions with liquid electrolytes and enhances moisture stability by forming a cathode-electrolyte interface layer.
As a result, in Na-half-cell tests, the Sc-doped P'2 type Na2/3[Mn1-xScx]O2 electrodes demonstrated a substantial improvement in cycling stability. The sample with 8% Sc doping was found to have optimal performance. The researchers also found that unlike non-doped samples, the crystallinity of the doped samples was remarkably maintained during cycling. Interestingly, Sc doping did not improve the cycling stability of P2 NaMnO2 electrodes, indicating a specific synergy between Sc doping and cooperative Jahn-Teller distortion. Furthermore, doping with other similar metal cations, like ytterbium and aluminum, did not reduce capacity fading, highlighting the unique role of Sc.
They also tested the effect of pre-cycling, a common technique to improve cycle life, which further improved capacity retention in the doped P'2 Na2/3[Mn1-xScx]O2 electrodes. Building upon these results, the researchers fabricated coin-type full cells using the 8% Sc-doped P'2 Na2/3[Mn1-xScx]O2 electrodes, which demonstrated an impressive 60% capacity retention after 300 cycles.
"Since Sc is an expensive metal, our study demonstrates its feasibility in the development of batteries. Our findings can potentially lead to development of high-performance and long-life sodium-ion batteries," says Prof. Komaba, highlighting the importance of their research. "Moreover, beyond sodium-ion batteries, our study illustrates a new strategy to extend the structural stability of layered metal oxides involving the lattice distortion and improve the performance of batteries made using these materials."
Overall, this study demonstrates the unique role of Sc doping for improving cycling stability of sodium-ion batteries, paving the way for their broader adoption.
