Comparison of the Thermal Stability of FeOF Cathode in Na-Ion Batteries and Li-Ion Batteries
Authors: Zhao Liwei , Kitajou Ayuko , Okada Shigeto 
Published: February 2020

Recently ambient temperature sodium-ion batteries have drawn great attention particularly in large-scale electric energy storage applications for renewable energy and smart grid due to the low cost and abundant sodium resources. Up to now, lots of cathode and anode materials as well as electrolytes have been proposed for Na-ion batteries. However, little research about the safety issue of Na-ion batteries has been carried out. It is well known that Li-ion batteries have thermal risk under abuse or severe environment. Because Na-ion batteries have similar working mechanism as Li-ion batteries and sodium shows even higher reactivity against air, the study on thermal stability of Na-ion batteries is indispensable for its practical application.
Iron oxyfluoride (FeOF) is a kind of iron based conversion type cathode material, which had been proposed as a cathode material for Li-ion batteries and obtained a large practical capacity due to conversion reactions [1, 2]. In this study, FeOF was also applied as the cathode material for Na-ion batteries. After electrochemical pretreatment in Na-ion or Li-ion batteries, the charged/discharged FeOF cathodes were mixed with corresponding electrolytes and thermally analyzed. By varying the ratio of cathode/electrolyte in the mixture, the heat generation mechanisms of FeOF cathodes in Na-ion and Li-ion batteries were discussed, and the thermal stability of Na-ion and Li-ion batteries were compared.
FeOF was synthesized by a reaction of 1:2.33 molar ratio of Fe2O3 (Sigma-Aldrich) and FeF3 (Soekawa Chemicals Co., Ltd.) at a temperature over 1000 oC in a roll-quenching machine (Harddays Co. Ltd). The synthesized FeOF had been indexed as a tetragonal structure with P42/mnm space group. The obtained FeOF flake was ground with acetylene black using a planetary ball milling machine at a weight ratio of 70:25. 5 wt.% PVdF and polyacrylate binders were added in the cathodes for Li-ion batteries and Na-ion batteries, respectively. The electrochemical measurements were carried out with 2032 coin-type two-electrode cells. In Li-ion batteries, Li foil was used as a counter electrode and 1 mol/cm3 LiPF6/EC-DMC or 1 mol/cm3 LiClO4/PC as an electrolyte. In Na-ion batteries, Na foil was used as a counter electrode and 1 mol/cm3 NaClO4/PC as an electrolyte. The cells were cycled at constant current densities of 10 mA/g (0.02 mA/cm2). For thermal analysis, the cycled cathodes were taken out from the disassembled cells, soaked in PC or DMC, rinsed by DMC, and then vacuum dried. Finally, the cathode powder was packed into a stainless-steel pan together with some amount of corresponding electrolyte. During TG-DSC analysis, the hermetically sealed pan was heat up to 500 oC with a heating rate of 5 oC/min.
Figure 1 shows the DSC curves of the mixtures of given amount of lithiated or sodiated FeOF cathode and 1 μl corresponding PC-based electrolytes. In both Li- and Na-ions batteries, charged FeOF cathodes showed large exothermic heat with an onset temperature of about 100 oC. This was attributed to reactions between expanded FeOF at the charged state and the electrolyte. Moreover, intercalated Na ions were found to induce an exothermic heat peaked at about 450 oC, while this peak was not observed in Li-ion batteries. Detailed discussion will be presented at the conference.

References

[1] N. Pereira, F. Badway, M. Wartelsky, S. Gunn, and G. G. Amatucci, J. Electrochem. Soc., 156(2009) A407.
[2] A. Kitajou, R. Nagano, and S. Okada, Abstract of PRiME2012, #851 (2012).