Potassium Vanadium Fluorides as Positive Electrode Materials for K-Ion Batteries

Abstract

Potassium-ion batteries (KIBs) have attracted much attention as a potential high-voltage and high-power secondary batteries due to a low standard electrode potential of K+/K in carbonate ester-based electrolytes and weak Lewis acidity of K+ (1). We reported the K+ ion extraction/insertion reactions in polyanionic compounds at a high average working potential of > 4 V vs. K+/K, which should be attributed to the inductive effect of anion(2)(3). However, the specific capacity of polyanionic compounds is often limited by the high mass of the redox-inactive polyanion framework. In this study, we focused on potassium-containing vanadium fluorides, which have high theoretical capacity.We first created a triangle phase diagram of K-V-F systems considering with their theoretical capacities, as shown in Fig. 1a. Based on the theoretical capacity and valence of V, K3VF6 was selected and synthesized, and its electrode behavior and structural changes associated with potassium extraction were investigated as a positive electrode material for KIBs.K3VF6 was synthesized through a mechanochemical method after mixing stoichiometric ratios of KF and VF3 with 10 wt% Ketjen black (KB) and calcined to enhance crystallinity. The calcination was performed in an argon atmosphere at 300 °C. The working electrodes were prepared by mixing the synthesized sample with conductive carbon (KB) and binder (PVdF) at a weight ratio of 8 : 1 : 1 and applying it to an Al foil. The charge-discharge tests were conducted using coin cells with the prepared positive electrode, K metal counter electrode, and 1.0 mol dm-3 KPF6/EC : PC = 1 : 1 (v/v) or 5.6 mol kg-1 KFSA/G3 electrolyte (4).The Rietveld refinement of the synchrotron XRD pattern of K3VF6 is shown in Figure 1b. All of the diffraction peaks, except for VO peaks, could be indexed to the tetragonal space group I4/m. The refinement showed good fitting with a low R wp value of 6.18 %. The diffusion barrier of K+ ions in K3VF6 was calculated to 0.77 and 0.89 eV along b-axis and c-axis, respectively, by the BVEL method.Figure 1c shows the initial charge/discharge curve of K3VF6 electrode. The electrode exhibited a plateau at about 3.7 V and an initial discharge capacity of about 95 mAh g-1, which corresponds to a one-electron reaction of V4+/3+ (Figure 1c).Figure 1d shows ex-situ XAFS measurements to investigate the variation of V valence state during charge/discharge. After being charged to 4.3 V, the white line peak shifted to the higher energy and the pre-edge peak became stronger than the pristine spectra, indicating oxidation of V from trivalent to tetravalent during charge. The white line and the pre-edge peaks of K3VF6 discharged to 2.0 V are almost identical to that of the pristine, showing reversible V3+/4+ redox. We will further discuss the structural changes during the K+ ion extraction/insertion process and the redox behavior of V in the presentation.