Invited: Why Na Batteries Could Outperform Li Ones?
Authors: Gaubicher Joël , Boucher Florent , Moreau Philippe , Soudan Patrick , Cuisinier Marine , Guyomard Dominique 
Published: February 2020

Sodium-ion batteries (SIB) are considered as an attractive alternative to lithium-ion batteries (LIB) as they could potentially be much less expensive, safer, and environmentally friendlier[1]. Clearly, one of the major concerns that we are currently faced with consists in determining to what extent the results gathered over the past twenty years in the Li battery field, can be transferred to the Na one. Our research directly addresses this issue by highlighting two facts:
1- The Na intercalation process in FePO4 is significantly different from the Li one with the existence of a stable intermediate composition Na2/3FePO4.[2],[3],[4],[5] By combining electronic and X-Ray synchrotron radiation diffractions as well as Mössbauer and NMR spectroscopies, we identified the intermediate phase as a fully ordered Na2/3FePO4 composition showing a vacancy ordering along the channels coupled with a FeII/FeIII charge ordering. Thanks to ab initio DFT calculations a very good agreement between all analytical methods was found and definitely confirms the good assignment of the superstructure.[6]
2-Contrary to what has hitherto been observed for Li batteries, the thermodynamic phase diagram of FePO4
3, needs thorough reassessment as far as the dynamic intercalation/deintercalation of Na ions within a cycling battery is concerned. Indeed, based on operando synchrotron X-Ray diffraction, we show that structural phase transformation does not proceed at constant composition. Instead, we witnessed the occurrence of vastly extended limits of solubility, which are characterized by continuous variations in the lattice metric mirroring that of the Na occupancy. This striking result[7], hitherto unseen in material science, to our knowledge, results in Na batteries having an enormous advantage over Li ones, since the lattice volume mismatch during phase transformation is reduced by a factor of 30% and 10% on charge and discharge, respectively when compared to what is predicted based on the thermodynamic phase diagram. Kinetically controlled structural behavior such as this could clearly compensate for the less efficient Na-related SEI, as well as the larger size of Na ions compared to Li ones. We anticipate the elucidation of further noteworthy examples pertaining to the influence of dynamics on the structural behavior of positive and especially negative electrode materials of Na batteries in the near future.
In light of these findings the Na intercalation process in FePO4will be discussed.

[1] Ellis, B. L. & Nazar, L. F. Current Opinion in Solid State & Materials Science, 2012, 16, 168-177
[2] Moreau, P.; Guyomard, D.; Gaubicher, J.; Boucher, F. Chem Mater 2010, 22, 4126–4128.
[3] Casas-Cabanas, M.; Roddatis, V. V.; Saurel, D.; Kubiak, P.; Carretero-Gonzalez, J.; Palomares, V.; Serras, P.; Rojo, T. J Mater Chem 2012, 22, 17421–17423.
[4] Lu, J.; Chung, S. C.; Nishimura, S.; Oyama, G.; Yamada, A. Chemistry of Materials 2013, 25, 4557–4565.
[5] Zaghib, K.; Trottier, J.; Hovington, P.; Brochu, F.; Guerfi, A.; Mauger, A.; Julien, C. M. J Power Sources, 2011, 196, 9612-9617
[6] Boucher, F. ; Gaubicher J.; Guyomard D. ; Moreau, P., JACS, submitted

[7] Gaubicher, J.; Boucher, F.; Moreau, P.; Cuisinier, M.; Soudan, P.; Elkaim, E.; Guyomard, D. Electrochem Commun, 2014, 38, 104-106