#Japan #Tokyo #NFM #Calcium Ion #Sodium Ion

Enhancing Air Stability in Sodium-Ion Batteries with Calcium Ion Substitution

In a significant step forward for sodium-ion battery technology, researchers from the Tokyo University of Science have developed a new method to enhance the air stability of a promising cathode material, Na2/3[Fe1/2Mn1/2]O2 (often abbreviated as NFM). This innovative approach involves substituting sodium ions with a small percentage of calcium ions, which helps the battery achieve superior performance while maintaining durability against atmospheric conditions.

Key Findings

The study reveals that by replacing just 1 wt% of sodium ions with calcium ions, the NFM cathode material retains a high discharge capacity of 190 mAh/g. Remarkably, even after being exposed to the atmosphere for two days, the material showed minimal degradation, showcasing its potential for commercial viability. This enhancement in stability stems from calcium ions migrating to the surface of the particles upon exposure to air, forming a protective layer that suppresses harmful exchange reactions between sodium ions and hydrogen ions.

The Research Team

A collaborative research group comprising Monalisha Mahapatra, a PhD student at the Graduate School of Science, along with assistant professors Zachary T. Gossage and Changhee Lee, and professor Shinichi Komaba, has led this groundbreaking research. They explored methods to improve the atmospheric stability of cathodes extensively, as the inherent instability in air has been a significant impediment to the practical application of sodium-ion batteries, which are seen as a viable alternative to lithium-ion batteries.

Challenges in Sodium-Ion Batteries

Sodium-ion batteries utilizing transition metal layered oxides like NFM offer performances comparable to lithium-ion batteries, yet their atmospheric stability has been a hurdle. The exposure to moisture and air results in the gradual performance decay of these batteries, often due to the formation of impurities and the replacement of sodium ions with hydrogen ions at the ion exchange sites.

To tackle this challenge, the researchers proposed substituting a small portion of sodium ions in the NFM structure with calcium ions. It was hypothesized that calcium's similar ionic size to sodium would allow it to effectively integrate into the crystal structure without compromising its lattice integrity, thus maintaining performance characteristics while enhancing air stability.

Research Methodology and Results

The research team synthesized the modified compound (NCFM) through a meticulous process involving ball milling various raw materials, including calcium hydroxide, iron oxide, manganese oxide, and sodium carbonate, followed by calcination at high temperatures.

The resulting NCFM showed promising results when tested under various conditions. Key observations included:

• - Excellent Discharge Capacity: The NCFM maintained a discharge capacity of approximately 190 mAh/g over 50 cycles, showcasing a capacity retention rate of 72%.
• - Improved Rate Performance: Compared to NFM, NCFM exhibited enhanced rate characteristics, delivering 110 mAh/g at 1C and 67 mAh/g at 2C.
• - Superior Atmospheric Stability: When subjected to 65% relative humidity, the NCFM material demonstrated negligible performance loss after two days, contrasting sharply with NFM, which experienced significant degradation.

Mechanism Behind Enhanced Stability

The researchers employed several analytical techniques to investigate the observed improvements. They uncovered that the calcium ions migrate to form a dense, protective layer on the particle surface, which effectively prevents the degradation reactions typically triggered by atmospheric exposure. This layer substantially mitigates the risk of decomposition and ion exchange reactions that could threaten battery performance.

Implications for the Future

As the demand for sustainable and cost-effective energy storage solutions rises, these findings could herald a new era in sodium-ion battery technology. The calcium ion substitution method stands to enhance the commercial viability of sodium-ion batteries, accelerating their practical applications in various energy storage systems.

Professor Shinichi Komaba commented on the research's implications, stating, "This study not only contributes to the practical realization of sodium-ion batteries but also addresses cost-effective production, essential for wider adoption. The remarkable achievements of our students, particularly in the context of international collaboration, highlight the research community's capabilities in tackling pressing energy challenges."

This research was published online on August 29, 2025, in the international scientific journal Journal of Materials Chemistry A, providing significant insights and potential pathways for further inquiry into enhancing battery technologies to meet evolving energy demands.

Conclusion

The journey towards efficient and durable sodium-ion batteries has taken a promising turn with calcium ion substitution emerging as a dependable solution for improving air stability. As research continues to evolve, the prospects for practical applications in this field look increasingly positive and encouraging.