Fluorination of Vanadium Oxy-Phosphate By Lif: Electrochemical Behavior in Li-Ion Battery
Authors: Semsari Parapari Sorour , Ateba Mba Jean-Marcel , Tchernychova Elena , Gulgun Mehmet Ali , Dominko Robert 
Published: February 2020

Polyanionic materials display a wide range of possible available compositions leading to a great amount of different properties. Therefore, they are attracting interest in the field of Li-ion battery research. Tavorite type compositions offer a rich crystal chemistry, among which LiVPO4F delivers the highest theoretical energy density ([1]–[3]).
These materials benefit from the inductive effect of both phosphate and F- anions, which lowers the energy of a given Mn/Mn-1 redox couple (with M being the transition metal). This property induces the formation of a more ionic metal-ligand bond, a lower antibonding orbital level energy and therefore a higher potential difference vs. Li. Therefore, lithium transition metal fluoro-phosphates are an appealing class of materials for Li-ion batteries since they are capable to operate at very high potential compared to other phosphates (Li3V2(PO4)3, LiVPO4O, LiFePO4 etc.). One of the possible methods to synthesize such compounds is by incorporation of LiF in the phosphate materials, such as the method used to produce LiVPO4F, in which LiF is reacted with VPO4 to obtain the fluorinated phase LiVPO4F [1],[3]. In this work, a new class of fluoro-phosphates (Li1+

x
VPO4OF
x
) is synthesized by incorporation of LiF into vanadium oxy-phosphates. It is worthwhile to note that Li1+

x
VPO4OF
x
is different than LiVPO4F1-

x
O
x
for which fluorine is substituted by oxygen through a simple oxidation of LiVPO4F upon air [4].
The new fluorinated materials were obtained through a two-step ceramic synthesis. In the first step, vanadium oxy-phosphates with the general formula Li
x
VPO4O (with x = 0, 1 and 2) were synthesized. In the second step, the oxy-phosphate materials are mixed with LiF and annealed at high temperatures. The powders obtained through the incorporation of LiF in LiVPO4O (Li1+

x
VPO4OF
x
) crystallized in Tavorite type structure with a unit cell slightly bigger than LiVPO4O (V/Z of 86 A3
vs. 85.5 A3), as was calculated from XRD patterns (Figure 1a). The obtained material was further studied by NMR and x-ray absorption spectroscopy.
The electrochemical behavior (Figure 1b) of Li1+

x
VPO4OF
x
exhibited an operating potential higher than LiVPO4O (4.15 V vs. 3.95 V) when the same redox potential is involved (V3+/V4+). The mechanism of Li+ extraction and/or insertion is being tackled by in situ XRD during electrochemical cycling.

Acknowledgements: This work has received funding from the European Union‘s Horizon 2020 research and innovation program under Grant Agreement No 711792 (LiRichFCC) and from the Slovenian Research Agency research program P2-0393.
[1] J. Barker, M. Y. Saidi, and J. and Swoyer, “Lithium Metal Fluorophosphate and Preparation Thereof, US Patent,” 2005.
[2] J. Barker, M. Y. Saidi, and J. L. Swoyer, “Electrochemical Insertion Properties of the Novel Lithium Vanadium Fluorophosphate, LiVPO[sub 4]F,” J. Electrochem. Soc., vol. 150, no. 10, p. A1394, 2003.
[3] J.-M. Ateba Mba, C. Masquelier, E. Suard, and L. Croguennec, “Synthesis and Crystallographic Study of Homeotypic LiVPO4F and LiVPO4O,” Chem. Mater., vol. 24, no. 6, pp. 1223–1234, 2012.
[4] E. Boivin, R. David, J. N. Chotard, T. Bamine, A. Iadecola, L. Bourgeois, E. Suard, F. Fauth, D. Carlier, C. Masquelier, and L. Croguennec, “LiVPO4F1- yOyTavorite-Type Compositions: Influence of the Concentration of Vanadyl-Type Defects on the Structure and Electrochemical Performance,” Chem. Mater., vol. 30, no. 16, pp. 5682–5693, 2018.
Figure1. (a) XRD patterns and (b) Electrochemical cycling (3-4.5 V) of LiVPO4O, Li1+

x
VPO4OF
x
and LiVPO4F for comparison.

Figure 1