Improving Fluorophosphates Na3V2(PO4)2F3 As Na-Ion Cathodes Beyond the 2 Sodium Limit
Authors: Bianchini Matteo , Xiao Penghao , Wang Yan Eric , Ceder Gerbrand 
Published: February 2020

In the last 30 years the world of portable electronics has been powered by Ni-Cd, Ni-MH and then Li-ion batteries, which rapidly became the technology of choice for such applications. More recently, great interest has been given to transport and storage applications. Since concerns have been raised about the future availability of lithium resources, much research has been addressed to alternatives such as Na-ion batteries. Materials for such technology could develop rapidly because of the essential similarity between Li+ and Na+ intercalation chemistry. A significant fraction of recently proposed electrodes are phosphate-based polyanionic materials which possess stable structural frameworks and whose potential can be tailored thanks to the inductive effect. Among them, Na3V2(PO4)2F3 is a material attracting great interest. Its crystal structure had been determined from single crystal X-Ray diffraction in 1999 by Le Meins et al. in the tetragonal space group P42/mnm at 298K [1]. Very recently high resolution diffraction techniques led to new insight on Na3V2(PO4)2F3, both from the crystal structure and electrochemical points of view, and explaining the significant amount of discrepancies found throughout the literature [2-6], which we believe to be the result of a partial substitution of oxygen for fluorine. Synchrotron radiation powder diffraction data revealed a small but significant orthorhombic distortion in Na3V2(PO4)2F3 (b/a=1.002). This led to an indexation in the Amam space group and to a new structure preserving the framework, but strongly impacting the sodium distribution [7]. Moreover, a subsequent operando synchrotron radiation XRD allowed to determine the complicated phase diagram observed upon cycling of Na3V2(PO4)2F3, consisting of several phase transition between intermediate phases, leading to the fully charged state NaV2(PO4)2F3 [8].
Na3V2(PO4)2F3 has the potential to deintercalate 3 sodium ions, but only two have been experimentally observed to be electrochemically active. The extraction of the third one would require a potential too high, where common electrolytes are not stable. Moreover, Na3V2(PO4)2F3 actually belongs to a full solid solution Na3V2(PO4)2F3-2xO2x [3], where the whole family delivers good performances as positive electrodes for Na-ion batteries, thanks to the active redox couple shifting from V3+/V4 to V4+/V5+. This means a very high degree of flexibility is present in this class of materials. To improve their performances even further, we considered a doping strategy with different transition metals to replace part of the vanadium into the structure, in the attempt at lowering such potential; in this presentation we will show the results of our doping from the predictions of ab initio calculations to the actual synthesis. The best strategies will be presented and the improvement they bring in terms of energy density and/or Na ion kinetics highlighted.

References

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