Electrochemical Performance and Thermal Stability of Iron Oxyfluoride (FeOF) for Sodium-Ion Batteries

Ayuko Kitajou,Eiji Kobayashi,Shigeto Okada,Atsushi Inoishi,Liwei Zhao,Rintaro Nagano

doi:10.3390/batteries4040068

Ayuko Kitajou, Eiji Kobayashi + Show 4 more

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https://doi.org/10.3390/batteries4040068

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Journal: Batteries	Publication Date: Dec 7, 2018
Citations: 4	License type: CC BY 4.0

Affiliation: Kyushu University, Yamaguchi University

Abstract

Self-synthesized rutile iron oxyfluoride (FeOF) was studied as a cathode material for Na-ion batteries. The highly crystalline FeOF provided an initial discharge capacity of 246 mAh g−1 in a voltage range of 1.0–4.0 V, followed by 88% of capacity retention after 20 cycles. This discharge-charge reaction of FeOF between 0.8 and 4.0 V are advanced by the Fe2+/Fe3+ redox reaction. That is, no conversion reaction was involved in the application of FeOF as a cathode material for Na-ion batteries because of the low potential of Na-insertion. In addition, the structure change of FeOF from rutile to cubic during Na ion insertion, which was similar to that in Li-ion batteries. No remarkable HF release was detected even up to 700 °C, indicating a low toxic risk of the FeOF cathode. The thermal properties of sodiated and desodiated FeOF electrodes in the associated electrolyte were investigated by DSC (Differential scanning calorimetry) up to 500 °C. Sodiated FeOF electrodes showed larger exothermic heat generation than desodiated ones, especially at a temperature higher than 380 °C. Finally, the thermal stability of FeOF cathodes in the associated Li- and Na-ion battery electrolytes was quantitatively compared with variations of the electrode/electrolyte ratio.

Highlights

Na-ion batteries are an attractive candidate for next-generation secondary batteries because of the low cost and the abundance of sodium
From the X-ray diffraction (XRD) profile, the obtained FeOF could be indexed to a tetragonal structure of the space group P42 /mnm, in agreement with our previous report [7], a small amount of the starting material, FeF3 and Fe2 O3, remained as an impurity
A discharge capacity of 210 mAh g1 was obtained after 20 cycles down to 1.0 V, and the cycling efficiency of FeOF between 1.0 and 4.0 V was found to be 88%

Summary

Introduction

Na-ion batteries are an attractive candidate for next-generation secondary batteries because of the low cost and the abundance of sodium. Numerous cathode active materials have been proposed for Na-ion batteries. Conversion-type cathode materials, such as FeS2 [1], are of special interest to battery researchers for the use in the fabrication of large capacity Na-ion batteries due to their ability to utilize the entire valence change between the ionic and metallic state of the cation in the active material [2]. Three ferric conversion-type active materials, Fe2 O3 , FeF3 , and iron oxyfluoride (FeOF), have been investigated as potential electrode materials for Li-ion batteries. Fe2 O3 generates the largest theoretical capacity but the lowest operating potential of 0.8 V versus Li metal [3], and it would be applicable only as an anode material. FeF3 gives the highest average voltage due to its highly ionic metal-ligand bonds, its electronic conductivity is very poor due to Batteries 2018, 4, 68; doi:10.3390/batteries4040068 www.mdpi.com/journal/batteries

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