From Atlas of Science:

"...Recent work published in the Chemical Communications, by Wang Hay Kan and Arumugam Manthiram at the University of Texas at Austin and Ashfia Huq at the Neutron Scattering Science Division at Oak Ridge National Laboratory, has shown that a Na-ion cathode Fe3P5SiO19 with two types of XO4n- (X = P and Si; n ≥2) groups exhibit a reversible capacity of ca. 70 mAh g-1i.e., 1.7 Na+ ions per formula can be reversibly inserted/extracted at an average voltage of 2.5 V versus Na+/Na. To understand the Na+-ion conduction pathway, bond valence sum (BVS) mismatch minimization procedure was performed. The low isovalence surfaces are located around the unfilled interstitial sites around the Fe2O9 dimers, phosphates and disilicates. A lower mismatch pathway along z-axis is found, suggesting its anisotropic ionic conductivity.

Overall, this study highlights the importance of cathodes with multiple polyanions and their physical properties and electrochemical performances."

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