Professor John Goodenough, the inventor of the lithium-ion battery, and his team at the University of Texas at Austin have identified a new cathode material made of the nontoxic and inexpensive mineral eldfellite (NaFe(SO4)2), presenting a significant advancement in the quest for a commercially viable sodium-ion battery. (Edfellite was first found among fumarolic encrustations collected in 1990 on the Eldfell volcano, Heimaey Island, Iceland.) The researchers reported their findings in the RSC journal Energy & Environmental Science.
Sodium-ion intercalation batteries—i.e., using the same process of ion insertion and removal as Li-ion batteries—have emerged as a promising new type of rechargeable battery and a potentially attractive alternative to lithium-ion batteries because sodium is abundant and inexpensive, and the sodium batteries would be safer. (Earlier post.) In contrast, lithium-ion batteries are limited by high production costs and availability of lithium.
However, sodium-ion batteries face issues related to performance, weight and instability of materials. Sodium ion intercalation and storage is challenging because Na ions are about 70% larger in radius than Li ions. This makes it difficult to find a suitable host material to accommodate the Na ions and to allow reversible and rapid ion insertion and extraction.
|Crystal structure of the eldfellite cathode for a sodium-ion battery. Click to enlarge.
The team’s proposed cathode material addresses instability. Its structure consists of fixed sodium and iron layers that allow for sodium to be inserted and removed while retaining the integrity of the structure.
Its 3 V discharge versus sodium for reversible Na+ intercalation has a better capacity, but lower insertion rate than Li+ intercalation, the researchers showed. The theoretical specific capacity for Na+ insertion is 99 mAh g−1. After 80 cycles at 0.1C versus a Na anode, the specific capacity was 78 mA h g−1 with a coulombic efficiency approaching 100%.
At the core of this discovery is a basic structure for the material that we hope will encourage researchers to come up with better materials for the further development of sodium-ion batteries.—Preetam Singh, a postdoctoral fellow and researcher in Goodenough’s lab
One challenge the team is currently working through is that their cathode would result in a battery that is less energy dense than today’s lithium-ion batteries. The UT Austin cathode achieved a specific capacity that is only about two-thirds of that of commercial lithium-ion batteries.
There are many more possibilities for this material, and we plan to continue our research. We believe our cathode material provides a good baseline structure for the development of new materials that could eventually make the sodium-ion battery a commercial reality.—Preetam Singh
Preetam Singh, Konda Shiva, Hugo Celio and John B. Goodenough (2015) “Eldfellite, NaFe(SO4)2: an intercalation cathode host for low-cost Na-ion batteries” Energy Environ. Sci. doi: 10.1039/C5EE02274F