Cheaper positive electrode material improves all-solid-state sodium batteries

Osaka Metropolitan University scientists have successfully developed a new positive electrode material Na₂FeS₂, consisting of sodium, iron, and sulfur. During testing, batteries using the Na₂FeS₂ positive electrode had a high energy storage capacity and could be charged and discharged for more than 300 cycles. Because the Na₂FeS₂ is made of abundant inexpensive elements, it is expected to be used in all-solid-state sodium batteries with higher capacity and lower costs.

The demand for high energy density rechargeable batteries, such as lithium-ion batteries, increases every year, as society shifts toward becoming carbon neutral. Sodium-ion batteries—which have a resource advantage over lithium-ion batteries—are attracting more attention, as cheap new high-performance materials continue to be developed.

A research group led by Associate Professor Atsushi Sakuda, President Masahiro Tatsumisago, and Professor Akitoshi Hayashi, at the Graduate School of Engineering, Osaka Metropolitan University, has successfully developed a new positive electrode, made of Na₂FeS₂, for all-solid-state sodium batteries. The batteries have a high energy storage capacity, high reversibility and use inexpensive elements that are readily available.

Furthermore, the batteries using the Na₂FeS₂ as a positive electrode can be charged and discharged for more than 300 cycles, due to the unique crystal structure of the Na₂FeS₂ that gives the electrode a long lifespan. Most high-capacity metal sulfide electrodes rely on conversion-type reactions, during which large rearrangements—during charging and discharging—are associated with inhomogeneous reactions and degradation. The Na₂FeS₂, on the other hand, achieves a high degree of reversibility during charging and discharging, by undergoing insertion-type reactions, which allow the electrode to retain its crystal structure over many cycles.

"The new Na₂FeS₂ positive electrodes are well balanced in terms of materials, cost, and lifetime; we expect them to be put to practical use in all-solid-state sodium batteries," Professor Sakuda concluded. "In the future, we will continue our research to develop cheaper all-solid-state sodium batteries with even higher performance, by examining high input and output for rapid recharging, as well as making and testing of superior anode materials."