Sodium-ion batteries are emerging as a promising alternative to the lithium-ion batteries widely used in electronics and electric vehicles.
Now, scientists have found that adding scandium in two different ways could help solve one of sodium-ion batteries’ major problems: declining performance after repeated charging.
Sodium has an important advantage over lithium because it is much more abundant in Earth’s crust.
This could potentially make sodium-ion batteries less expensive and reduce dependence on lithium supplies. They can also offer good safety and performance at low temperatures.
However, developing durable materials for these batteries remains a challenge.
Researchers from Tokyo University of Science in Japan focused on a promising cathode material made from sodium, nickel, manganese and oxygen. The cathode is the part of a battery that receives sodium ions during discharge and releases them during charging.
This material can store a relatively large amount of energy, but repeated charging and discharging causes its structure to expand and contract. Over time, these changes damage the material and dramatically reduce the battery’s capacity.
The researchers investigated whether scandium could make the cathode more stable. Their study, published in Small, examined two approaches: adding scandium directly inside the material, known as doping, and applying it mainly to the surface as a coating.
Both methods produced major improvements, but for different reasons.
After 100 charging and discharging cycles, the original cathode retained only 18.6% of its capacity. A cathode containing scandium within its structure retained 67.8%, while the scandium-coated version retained 75.4%.
The researchers found that scandium added inside the material helped stabilize its layered crystal structure. The scandium also caused some sodium ions to remain in place, where they effectively acted like tiny pillars supporting the layers.
This reduced damaging structural changes as sodium ions repeatedly moved in and out of the cathode. It also produced smoother charging and discharging behavior.
The scandium coating worked differently. Rather than substantially changing the material inside the cathode particles, it formed a protective layer on their surfaces. This helped prevent unwanted chemical reactions where the cathode meets the battery’s electrolyte.
The researchers also tested the materials in more complete sodium-ion battery cells using hard carbon as the anode.
After 300 cycles, the battery containing the scandium-doped cathode retained 71.4% of its original capacity. The battery using the scandium-coated cathode performed even better, retaining 91.2%.
Neither approach was perfect. The coating protected the surface but could not completely prevent structural deterioration inside the material. Doping strengthened the internal structure but could not eliminate all causes of capacity loss.
This suggests that combining the two strategies could be particularly effective, protecting sodium-ion battery electrodes both inside and outside.
Scandium itself is relatively expensive and not abundant enough to be an ideal choice for mass-produced batteries. Instead, the researchers see it as a useful model for understanding how these improvements work.
Their next challenge is to apply the same principles using cheaper and more widely available elements. If successful, the approach could contribute to sodium-ion batteries that last longer and become practical for a wider range of energy-storage applications.
