Invited Presentation: Fluorophosphates As High Energy Density Positive Electrodes for Li and Na Batteries

Development of new materials for high energy density batteries is essential as they power a wide range of devices ranging from portable electronics to transport and storage applications. Since they are still facing limitations (energy and power density, safety …), intense academic and industrial research activities are devoted to the development of better performing materials with in particular high voltage and safety. Beside the extensively studied layered and spinel oxides, polyanionic materials have aroused huge interest in the past ten years [1]. Among them, the so-called fluorophosphates, actually fluoride phosphates combining phosphate and fluoride anions in the same framework, are very attractive positive electrode materials with increased voltage versus Li and Na as compared to regular (oxide) phosphates for a given M⁽ⁿ⁺¹⁾⁺/Mⁿ⁺ redox couple. We will discuss results obtained recently in our groups for Tavorite-like phosphate materials LiMPO₄X (M = V, Fe, Ti; X = F, OH, O) used as electrodes in Li-ion batteries, but also for Na₃V₂(PO₄)₂F₃ in Li and Na-ion batteries. The vanadium-rich compounds LiVPO₄F and LiVPO₄O have been shown to be of particular interest as V is stable at different oxidation states in similar environments (V^II, V^III, V^IV and V^V) in these Tavorite-type structures, leading to a possible exchange of two electrons per vanadium [2-5]. Very distinct values are interestingly observed for the “same” V³⁺/V⁴⁺ redox couple in LiVPO₄F and LiVPO₄O: ~ 4.25 V vs. Li upon Li⁺ extraction from LiVPO₄F versus ~ 2.3 V vs. Li upon Li⁺ insertion into LiVPO₄O, showing that the potential can be tuned over a wide range moving from a V³⁺-rich material to a vanadyl (VO)²⁺-rich material. Na₃V₂(PO₄)₂F₃ is also undoubtedly of high interest for the development of Na-ion batteries: it was shown recently to allow fast sodium diffusion, even at high rates (up to 20C) and despite the formation of successive intermediate phases with Na⁺/vacancy ordering [6]. The average potential observed for the V³⁺/V⁴⁺ redox couple in Na₃V₂(PO₄)₂F₃ is around 3.9 V vs. Na⁺/Na, i.e. (as expected) significantly higher than that of the Nasicon phase Na₃V₂(PO₄)₃.

References

[1] C. Masquelier and L. Croguennec, Chem. Rev., 113, 6552 (2013)

[2] J. Barker, R.K.B. Gover, P. Burns, A. Bryan, M.Y. Saidi and J.L. Swoyer, J. Power Sources, 146, 516 (2005)

[3] J.M. Ateba Mba, C. Masquelier, E. Suard and L. Croguennec, Chem. Mater., 24, 1223 (2012)

[4] J.M. Ateba Mba, L. Croguennec, N.I. Basir, J. Barker and C. Masquelier, J. Electrochem. Soc., 159, A1171 (2012)

[5] M. Bianchini, J.M. Ateba Mba, P. Dagault, E. Bogdan, D. Carlier, E. Suard, C. Masquelier and L. Croguennec, submitted

[6] A. Ponrouch, R. Dedryvère, D. Monti, A.E. Demet, J.M. Ateba Mba, L. Croguennec, C. Masquelier, P. Johansson and M.R. Palacin, Energy and Environmental Science, 6, 2361 (2013)