Iron trifluoride (FeF₃) has been attracted as one of the promising candidates for the electrode material of lithium ion batteries due to their high theoretical capacity (712 mA g^–1), high average potential and abundance of Fe. However, various problems such as low conductivity of FeF₃, large overpotential and poor cyclability still remain with regard to overcoming for commercialization. Improvement of the cycling performance of FeF₃-based lithium ion battery have been achieved by addition of anion receptor, nano-sizing of FeF₃ and tuning of electrolyte composition so far [1-3], but the suppression of degradation and the elucidation of its mechanism have not accomplished completely. In the present study, we investigated charge and discharge behavior of FeF₃ positive electrode using designed two-compartment type cell to analyze the degradation mechanism of FeF₃ positive electrode.

FeF₃/C composite electrode and coin cell were prepared by following our previous literature. [4] The coin cell using 1.0 mol dm^–3 LiPF₆ in 1:1 ratio of ethylene carbonate (EC) and dimethyl carbonate (DMC) as an electrolyte showed over 600 mAh g^–1 of discharge capacity at 2nd cycle. However, the discharge capacity decreased to 200 mAh g^–1 at 30th cycle. Although several studies on the effects of additives to the electrolyte were conducted to improve the cycle performance, charge and discharge behavior did not changed drastically. Then, we designed the two-compartment cell which has lithium ion conducting ceramics as a separator to prevent the crossover of the electrolyte between the positive and negative electrode. In the case the electrolyte with a certain additive was used only for the positive electrode, the discharge capacity exhibited over 400 mAh g^–1 at 30th cycle. This result indicates that crossover of the decomposed product of the electrolyte is one of the reasons for the degradation of the coin cell using additives. After cycling test, various analyses such as, electrochemical impedance, X-ray photoelectron spectroscopy and scanning electron microscopy, were also conducted for the electrode with and without additves in the electrolyte to examine the degradation mechanism of FeF₃ positive electrode. The obtained results were considered useful for the improvement of the cycling performance of FeF₃-based lithium ion batteries.

References

[1] K. Kumagae, K.-I. Okazaki, K. Matsui, H. Horino, T. Hirai, J.-I. Yamaki and Z. Ogumi, J. Electrochem. Soc., 163, A1633 (2016).

[2] W. Fu, E. Zhao, Z. Sun, X. Ren, A. Magasinski and G. Yushin, Adv. Func. Mater., 28 1801711 (2018).

[3] E. Zhao, O. Borodin, X. Gao, D. Lei, Y. Xiao, X. Ren, W. Fu, A. Magasinski, K. Turcheniuk and G. Yushin, Adv. Energy Mater., 8, 1800721(2018).

[4] T. Takami, K. Matsui, H. Senoh, N. Taguchi, M. Shikano, H. Sakaebe and T. Fukunaga, J. Alloy. Comp., 769, 539 (2018).

Acknowledgement

This work was supported by the New Energy and Industrial Technology Development Organization (NEDO) under the “Research and Development Initiative for Scientific Innovation of New Generation Batteries 2 (RISING2)”