Unlocking the Electrochemistry and the Activation Mechanism in the Iron‐Rich Na_0.6Fe_1.2PO₄ Phase for High‐Performance Sodium‐Ion Storage

Abstract

Sodium‐ion batteries are considered as the immediate sustainable alternative to lithium‐ion systems. To reduce the competitiveness gap, improved performances and better understanding of sodium storage, especially of new phases based on sustainable materials, are further required. In this work, we provide advanced investigation of the structure and the electrochemistry of a peculiar off‐stoichiometric iron‐rich phase (Na0.6Fe1.2PO4) for sodium storage. An interesting electrochemical activation phenomenon is described and contrary to conventional ageing processes it is found to significantly enhance the energy and power rate performances.

New Cathode Materials in the Fe‐PO4‐F Chemical Space for High‐Performance Sodium‐Ion Storage

“…The Na 0.6 Fe 1.2 PO 4 ( x = 0) phase made in this work shows similarity to the one reported by Yamada et al., indicating the good reproducibility of the method. [ 10 ] As the x increases, the PXRD pattern gradually evolves, while also maintaining similarities for specific diffraction regions (marked with #1–6 in Figure 3A ). The two groups of materials ( x = 0–0.2 and x = 0.4–0.8) share similar PXRD patterns separately, attributed to possible mixed phases existing in the respective x ranges, as discussed in the following.…”

“…C) Energy density comparison (performances considered at the material level) of various iron phosphate and iron fluorophosphate cathode materials for SIBs. [ 9 , 10 , 24 ] The energy density guidelines can be considered as the product of average discharge redox potential as measured in a half‐cell configuration multiplied by the discharge capacity.…”

“…Inspired by the knowledge on reported archetypal Na 1 + x FePO 4 F x (with x = 0 or 1) and NaFeF 3 chemistries, as well as the recently reported Na 0.6 Fe 1.2 PO 4 , we developed a series of linear combinations of these with the empirical formula of Na 0.6 + x Fe 1.2 PO 4 F x (with analyzed values of x = 0, 0.2, 0.4, 0.6, 0.8, and 1). [ 10 , 15 ] Likewise the compositional building units (the Na 0.6 + x Fe 1.2 PO 4 F x could be regarded as a mixture of NaFePO 4 and NaFeF 3 ), we found the thermodynamically stable phases to be electrochemically less active and focused our interest on the transient synthesis states. These are suggested to exist as mixed‐phase compositions.…”

“…With this respect, Yamada et al recently reported on a peculiar Fe‐rich Na 0.6 Fe 1.2 PO 4 phase, which further broadened the off‐stoichiometric positive electrode materials family. [ 10 ] The Na 0.6 Fe 1.2 PO 4 phase can be regarded as an Fe‐rich phase (0.4 Na + being replaced with 0.2 Fe 2+ ) with potentially 1.2 redox electrons being available, while practically being limited to only 0.6 Na + contained. Fluorine doping, bringing along higher Na + content, could thus be regarded as a means to further increase the specific capacity of this material.…”

Progress on Fe‐Based Polyanionic Oxide Cathodes Materials toward Grid‐Scale Energy Storage for Sodium‐Ion Batteries

“…[57] Iron-rich Na 0.6 Fe 1.2 PO 4 and Na 0.71 Fe 1.07 PO 4 cathodes with off-stoichiometry also showed superior electrochemical performance, which riches the Na-Fe-PO 4 systems. [58][59][60] Brief summary, the synthesis methods of olivine-type NaFePO 4 are complex and expensive by Li-Na exchange from LiFePO 4 in ionic liquid or organic electrolyte solution, although it has an excellent theoretical capacity of 154 mAh g −1 . As for maricitetype NaFePO 4 , they may perform an excellent electrochemical activity only if the particles are reduced to nanoscale and gradually transformed to amorphous FePO 4 .…”