Abstract:
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 sustainable. 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 an intermediate Na2/3FePO4 phase. By combining TEM, synchrotron XRD as well as Mossbauer and NMR, we proved Na2/3FePO4 as being a fully ordered composition showing vacancy ordering. ab initio DFT calculations confirmed a very good agreement between all analytical methods.
2- Contrary to what has hitherto been observed for Li batteries, the thermodynamic phase diagram of FePO4 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. This striking result, 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 greatly reduced. Such kinetically controlled structural behavior could clearly compensate for the less efficient Na-related SEI, as well as the larger size of Na ions compared to Li ones.
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