In the quest to develop sustainable and cost-effective energy storage solutions, sodium-ion batteries have recently emerged as a promising alternative to the well-established lithium-ion technology. Unlike lithium, which has limited abundance and uneven geological distribution, sodium is plentiful and widely available in the Earth’s crust, making it an attractive candidate for large-scale applications. However, despite sodium-ion batteries’ potential, the challenge of maintaining long-term cycling stability and high capacity retention has hindered their widespread adoption. In particular, the cathode materials—critical components that largely dictate the battery’s capacity, voltage, and overall stability—have faced significant performance issues due to structural degradation during repeated charge-discharge cycles.
Layered sodium manganese oxides, especially those with a chemical formula near Na₂/₃MnO₂, have attracted considerable attention as cathodes for sodium-ion batteries. These materials stand out because they do not rely on rare-earth metals, thus offering a more sustainable and cost-effective pathway. Initially, sodium manganese oxides deliver high capacities, but they suffer from rapid capacity fading when subjected to the mechanical and chemical stresses of cycling. This fading is fundamentally linked to changes in the crystal structure caused by the sodium ions moving in and out of the lattice, which triggers complex oxidation state changes and distortions in the manganese ions themselves.
During battery operation, the Mn ions in Na₂/₃MnO₂ toggle between oxidation states Mn³⁺ and Mn⁴⁺ as sodium ions are inserted or extracted. Particularly, the presence of Mn³⁺ leads to a well-known structural effect called the Jahn-Teller distortion, where the Mn-centered octahedra become distorted to reduce their electronic energy. This structural distortion can be localized or cooperative, but in either case, these repeated lattice distortions generate cumulative strain. Such mechanical stress undermines the crystallinity of the cathode material, promotes microstructural defects, and accelerates capacity degradation, posing a persistent challenge to the advancement of high-performance sodium-ion batteries.
In pioneering research conducted by a team led by Professor Shinichi Komaba at the Tokyo University of Science, significant progress has been made in understanding and mitigating these issues through selective doping. Their recent study focused on the effects of scandium (Sc) doping on different polytypes of Na₂/₃MnO₂, specifically the P2 and P’2 structural variants. Each polytype exhibits distinct behaviors: while the P2 variant is characterized by localized Jahn-Teller distortions, the P’2 polytype features a cooperative distortion where the distorted MnO₆ units align in a long-range order, with different implications for material stability.
Through detailed experimental analyses, the research revealed that Sc doping has a transformative impact specifically on the P’2 polytype structure. By incorporating scandium ions modestly—approximately 8% substitution for manganese—the team demonstrated that the cathode material undergoes significant modulation in particle size distribution and crystal growth processes. More importantly, scandium doping preserves the cooperative Jahn-Teller distortion inherent in the P’2 structure while enhancing its overall structural integrity, thereby stabilizing the electrode at an atomic level during cycling. This delicate balance leads to remarkable improvements in capacity retention and resistance to mechanical degradation.
Beyond structural effects, Sc doping also influences the interfacial chemistry between the cathode and electrolyte. The researchers observed that the scandium-doped cathodes exhibited suppressed side reactions with liquid electrolytes and increased resistance to moisture-induced damage. This was attributed to the formation of a more stable cathode-electrolyte interface layer, which acts as a protective barrier preventing deleterious degradation processes commonly associated with long-term battery operation. Such interface engineering is crucial for enhancing practical battery lifetimes and performance consistency.
Electrochemical testing in sodium half-cells further substantiated the benefits of scandium doping. The 8% Sc-doped P’2 Na₂/₃[Mn₁₋ₓScₓ]O₂ electrodes demonstrated a drastic improvement in cycling stability compared to undoped counterparts, maintaining much of their initial capacity over extended cycling periods. Intriguingly, this enhancement was not observed in the P2 polytype, suggesting that the synergistic effect between Sc doping and cooperative Jahn-Teller distortion is fundamental to the observed performance gains. Additionally, doping with other rare-earth or trivalent metal ions such as ytterbium and aluminum failed to replicate these beneficial effects, underscoring the unique role of scandium in this system.
The team also explored the impact of pre-cycling—the practice of conditioning electrode materials through initial cycles to stabilize their structures and interfaces. This method further boosted the capacity retention of the Sc-doped P’2 electrodes, demonstrating that combining doping strategies with electrochemical conditioning could be a powerful approach to prolong battery life. Building on these findings, full coin-cell sodium-ion batteries were fabricated using the optimized Sc-doped cathode. These cells exhibited an impressive 60% capacity retention after 300 charge-discharge cycles, marking a significant step toward the practical viability of sodium-ion battery technology.
Professor Komaba emphasizes the broader implications of their work: “While scandium is a relatively costly element, our study validates its utility in advancing sodium-ion batteries. Importantly, the mechanistic insights we have uncovered open avenues for designing longer-lasting and higher-performance energy storage devices.” Beyond sodium-ion batteries, their findings propose a novel strategy to enhance the structural robustness of layered metal oxide materials where lattice distortions often limit performance. This could influence the development of various battery chemistries reliant on similar cathode architectures.
Overall, this breakthrough highlights the power of precise chemical modification—in this case, using Sc doping—to contend with intrinsic material challenges in sodium-ion battery electrodes. It represents a leap forward in overcoming structural degradation mechanisms that have long stifled the practical deployment of these promising batteries. As global energy demands intensify and resource sustainability takes center stage, innovations like these bring sodium-ion batteries closer to commercial reality, offering an alternative that balances cost, performance, and environmental impact.
The study’s findings are set to be published in the prestigious journal Advanced Materials on September 12, 2025, offering the scientific community both a detailed experimental framework and new perspectives on electrode design. As researchers worldwide pursue energy storage breakthroughs, the work from Tokyo University of Science underscores the importance of fundamental materials chemistry and interfacial engineering in creating the next generation of safe, efficient, and durable batteries.
It is clear that through targeted doping strategies and a deep understanding of the interplay between crystal structure and electrochemical behavior, the limitations of sodium-ion batteries can be addressed. Scandium’s unique ability to maintain cooperative Jahn-Teller distortions while modulating crystal growth and stabilizing interfaces exemplifies how subtle atomic-level changes can lead to substantial performance enhancements. Such advances echo the ongoing evolution of battery science toward ever more sophisticated materials tailored to meet tomorrow’s energy needs.
Subject of Research:
Not applicable
Article Title:
Unique Impacts of Scandium Doping on Electrode Performance of P’2- and P2-type Na₂/₃MnO₂
News Publication Date:
12-Sep-2025
References:
DOI: 10.1002/adma.202511719
Image Credits:
Professor Shinichi Komaba from Tokyo University of Science, Japan
Keywords:
Batteries, Electrochemistry, Electrochemical cells, Physical sciences, Earth sciences, Materials science, Chemical engineering
