In a hunt for high-energy cathode materials for Li-ion batteries, different families of fluoride-containing materials such as fluoride-phosphates A_xMPO₄F (A = Li, Na; M = V, Mn, Fe, Co, Ni; x = 1, 2)^1,2,3have been extensively examined. Introduced to the lattice fluorine brings about not only an increase in operating voltages due to its highest electronegativity, but also a richer structural variety. In these systems the nature of alkali and transition metals strongly affects the adopting crystal structure, which in turn determines electrochemical properties. Due to a larger ionic radius compared to Li and Na and somewhat different crystal chemistry, K provides ample opportunities in searching and stabilizing new frameworks for reversible ion de/intercalation.

Here we report on a novel vanadium-based AVPO₄F (A = Li, K) cathode material adopting a KTP-type framework for high power rechargeable batteries with enhanced specific energy and excellent rate capability⁴. KVPO₄F was prepared via freeze-drying assisted solid-state route in two steps. The composition of the material was studied by TEM-EDX, ICP-AES and FTIR spectroscopy. The structure of KVPO₄F was refined from powder XRD data [S.G. Pna2₁, a = 12.8200(3) Å, b = 6.3952(1) Å, c = 10.6115(2)] using the KFeSO₄F structure as an initial model⁵. The refined structure was confirmed with HAADF-STEM imaging and electron diffraction. Vanadium +3 oxidation state was confirmed with EELS. For the electrochemical evaluation the initial KVPO₄F material was oxidized by charging up to 4.8 V vs Li/Li⁺ at C/20 rate and holding at this potential for 5 hours. According to the EDX data, potassium in not completely removed and about 15% remains in the structure giving rise to a K_0.15VPO₄F formula. The obtained K_0.15VPO₄F material was tested within the 2.0–4.7 V vs Li/Li⁺ potential range at different rates from C/5 to 40C showing an uptake of 0.7 Li per formula unit at the average voltage of ~4 V. K_0.15VPO₄F exhibits a sloping voltage profile indicating a solid-solution-like de/intercalation mechanism. Ex-situ XRD and electron diffraction tomography were applied to examine the crystal structure of the recovered oxidized and lithiated electrode materials. In contrast to the noncentrosymmetric parent KVPO₄F phase (Pna2₁), the crystal structures of both oxidized and lithiated electrode materials possess a centrosymmetric space group (Pnan); however, KTP-type structure is preserved. The difference in the unit cell volume of all three phases is small and does not exceed 2.2%. Li_xK_0.15VPO₄F shows remarkable capacity retention at 40C maintaining more than 50% of theoretical (156 mAh/g) or 75% of initial specific capacity (111 mAh/g at C/5). Rapid kinetics of the Li de/intercalation is also evidenced by rate capability measurements (figure 1).

Synthesis, structure and electrochemical properties of the obtained cathode material will be discussed in detail. A special focus will be given on structural peculiarities and substitution in the metal sublattice and their effect on the electrochemical performance in a Li anode cell.

Figure 1. Rate capability measurements of (K,Li)VPO₄F.

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