Today researches in the field of cathode materials for Li-ion batteries are directed towards the development of new materials with high power and energy densities, especially to answer the requirements of large scale applications. In this respect, fluoride-phosphate materials are very attractive as they demonstrate wide structural diversity. Furthermore, they are expected to exploit the advantages of both anion species: the increase in the working potential due to inductive effect of the polyanion group and higher ionicity of the M−F bond, faster kinetics arising from lower affinity of lithium to the F-anion.¹

In the search for new cathode materials for Li-ion batteries we have explored the A₂MPO₄F and AMPO₄F (A=Li, Na, K, and M=Co, Mn, Fe, V) fluorophosphates families. The fluoride-phosphate phases were prepared through different synthetic routes depending on the chemistry of transition metal. Combination of X-ray/neutron diffraction and electron microscopy were applied for a thorough structural characterization. It was shown that in these systems the substitution on the alkali metal site leads to the wide range of adopted structures. Thus, in the Li₂FePO₄F system synthesis of sodium-containing counterpart, NaLiFePO₄F, followed by subsequent electrochemical exchange of Na by Li leads to the cathode material with a 3D “stacked” structure.² In the (Li,Na)₂CoPO₄F system it provides a new low-temperature polymorph adopting a complex layered structure, in contrast to the high-temperature phase with 3D “stacked” arrangement. The introduction of large potassium into the AMPO₄F system gives a raise to a new cathode material with a KTP structure, which exhibits excellent rate capability.³ Reversible electrochemical activity demonstrated by these fluoride-phosphate materials in Li-cell will be discussed. A special attention will be given to the structure peculiarities and their impact on electrochemical properties, which is important for possible practical applications of these materials.

The work was partly supported by Russian Science Foundation (grant 16-19-00190), Skoltech Center for Electrochemical Energy Storage and Moscow State University Development Program up to 2020.

References

1. Antipov, E. V.; Khasanova, N. R.; Fedotov, S. S. International Union of Crystallography Journal 2015, 2, 85-94
2. Khasanova N. R., Drozhzhin O. A., Storozhilova D. A., Delmas C., Antipov E. V. Chem. Mater., 2012, 24, 4271–4273
3. Fedotov, S. S.; Khasanova, N. R.; Samarin, A. Sh.; Drozhzhin, O. A.; Batuk, D.; Karakulina, O. M.; Hadermann, J.; Abakumov, A. M; Antipov, E. V. Chem. Mater. 2016, DOI: 10.1021/acs.chemmater.5b04065