Crystal structures of the new Na2FeS2 positive electrode material. Top left: an iron atom sits at the center of a tetrahedron (brown), surrounded by four sulfur atoms (yellow). Top right: multiple tetrahedra can be combined to form long chains, which can lie parallel to one another. Bottom: sodium atoms (green) enter or leave the tetrahedra, during oxidation and reduction, in a highly reversible way to charge or discharge the battery, while keeping the structure intact.
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.”
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Osaka Metropolitan University is a new public university established by a merger between Osaka City University and Osaka Prefecture University in April 2022. For more science news, see https://www.upc-osaka.ac.jp/new-univ/en-research/, and follow @OsakaMetUniv_en, or search #OMUScience.
