Among the iron-based positive electrode materials for Li- and Na-ion batteries, FeF₃ with high electronegativity of fluorine exhibits high voltage Fe(II/III+) redox couple with excellent electrochemical performances such as high capacity and good cycle stability.[1]  However, FeF₃ does not contain an alkali metal in the structure and can be used with limited negative electrode materials containing adequate amount of the alkali metal.  In this study, perovskite-type transition metal fluorides containing potassium, KFeF₃, were synthesized and their electrochemical performance was examined.  Furthermore, combined characterization with X-ray diffraction (XRD), scanning electron microscopy (SEM), energy dispersive X-ray spectroscopy (EDS), and X-ray absorption spectroscopy were carried out for the perovskite-type KFeF₃ to understand the insertion mechanism in Li and Na cells.

  KFeF₃ samples were prepared by liquid-phase reaction from starting materials of FeF₂ and KF with ethylene glycol.  The mixed dispersion was heated at 120 ºC for 7 days in a Teflon-lined stainless steel autoclave.  The products were filtered and then dried in air at 80 ºC under vacuum.  Obtained KFeF₃ was mixed with acetylene black (AB) at a weight ratio of 7:2 by ball-milling.  Electrochemical properties were tested using a coin-type cell.  Positive electrodes consisted of the active material, AB and polyvinylidenefluoride(PVdF) with a gravimetric ratio of 70:20:10.  Metallic lithium and sodium were used as a negative electrode.  The electrolyte used was 1.0 mol dm^-3 LiPF₆ / EC:DMC (= 1:1 v/v) and 1.0 mol dm^-3 NaPF₆ / PC.

  Figure 1 shows charge and discharge curves of KFeF₃/AB composite electrodes in Li and Na cells.  Similar potential profiles showing a plateau at ca. 3 V were obtained and reversible capacities of 100 mAh g^-1 were delivered in both cells after an initial charge process in which potassium would be partially extracted from the structure.  The partial potassium extraction in the Na cell upon the initial charge was confirmed by SEM-EDS analysis and remained potassium was gradually extracted as a function of cycle number.  The reaction mechanism on the charge/discharge in Li and Na cells will be discussed in detail.

Reference

[1] N. Yabuuchi, S. Komaba et al., J. Mater. Chem., 21, 10035 (2011).