KTP-Type (Li,K)VPO4F: A Novel 4V Cathode Material for Li-Ion Batteries
Authors: Fedotov Stanislav S. , Khasanova Nellie R. , Samarin Aleksandr Sh. , Drozhzhin Oleg A. , Batuk Dmitry , Karakulina Olesia M. , Hadermann Joke , Abakumov Artem M. , Antipov Evgeny V. 
Published: February 2020

In a hunt for high-energy cathode materials for Li-ion batteries, different families of fluoride-containing materials such as fluoride-phosphates AxMPO4F (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 K0.15VPO4F formula. The obtained K0.15VPO4F 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. K0.15VPO4F 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%. LixK0.15VPO4F 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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Figure 1