High-voltage sodium-ion batteries with up to 15% higher energy density

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From pv magazine Global

Sodium-ion batteries continue to edge toward commercialisation as a viable alternative to lithium-ion technology. And researchers from Russia’s Skolkovo Institute of Science and Technology (Skoltech) and Lomonosov Moscow State University have developed a new cathode material that promises higher energy storage capacity in sodium-ion batteries.

They developed a powder of sodium-vanadium phosphate fluoride (NaVPO4F). It has a particular crystal structure that could ensure 10% to 15% higher energy density than current sodium-ion batteries.

“Our material also compares well with the class of layered materials for cathodes. It provides roughly the same battery capacity and greater stability, which translates into longer life and higher cost-efficiency of the battery,” said Stanislav Fedotov, an assistant professor at Skoltech. “Remarkably, even the theoretical predictions for the competing materials fall short of the practical performance of ours, and this is far from trivial, because the theoretical potential is never fully realised.”

The researchers confirmed the material’s superior characteristics through an experiment, after determining that the crystal structure could unlock the high energy storage potential of NaVPO4F. The one they chose is known as the KTP-type framework and comes from nonlinear optics, which is not common in battery engineering. By combining a NaVPO4F composition and KTiOPO4-type framework via a low-temperature (e.g., 190 C) ion-exchange synthesis approach, the researchers managed to develop a high-capacity and high-voltage positive electrode active material.

When tested in a coin cell configuration in combination with a Na metal negative electrode and a NaPF6-based non-aqueous electrolyte solution, the cathode active material enabled a discharge capacity of 136 mAh g−1 at 14.3 mA g−1, with an average cell discharge voltage of about 4.0 V. The scientists also reported a specific discharge capacity of 123 mAh g−1 at 5.7 A g−1 for the same cell configuration.

They described their research findings in “Development of vanadium-based polyanion positive electrode active materials for high-voltage sodium-based batteries,” which was recently published in Nature Communications.

“Higher energy storage capacity is just one of the advantages of this material,” said Fedotov. “It also enables the cathode to operate at lower ambient temperatures, which is particularly relevant for Russia.”

As lithium prices continue to surge, the need for batteries based on cheap and abundant materials, such as alkali metal sodium, is pressing. The Russian scientists said that efficient materials for sodium-ion batteries could eventually supersede lithium-ion batteries in heavy electric vehicles, such as buses and trucks, as well as in stationary energy storage applications.

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