The First Fe-based sodium-ion cathode with two distinct type of polyanions
The current lithium-ion battery technology finds wide applications, powering cell phones, laptops, and electric vehicles. In 1991, Sony announced the first-generation of commercial Li-ion batteries, which composed of LiCoO2 cathode and graphite anode. Such cells have the drawback of safety concerns due to the liberation of oxygen from LiCoO2 at deep charge. Due to safety concerns, only half of the theoretical capacity of LiCoO2 can be used in practical cells.
To overcome the above difficulties, various alternative cathodes have been explored during the past three decades. Among them, cathodes consisting of poly-anions (XO4n-; X = S, P, and Si; n ≥ 2) offer superior thermal stability. An excellent example is LiFePO4, reported in 1997. For most of the polyanion cathodes reported, the structure is composed of only one type of XO4n- (X = S, P, or Si) groups. Na-ion batteries are attracting much attention recently due to the high abundance and low cost of sodium as an alternative to lithium-ion batteries.
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-1, i.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.
The first Fe-based Na(+)-ion cathode with two distinct types of polyanions: Fe3P5SiO19.
Kan WH, Huq A, Manthiram A
Chem Commun (Camb). 2015 Jul 4