Modulating Chemical State of Cation Site for Fast and Reversible Na Ion Storage of Fluorophosphate Cathodes

An ionic bond by modulating chemical states is proposed to facilitate the Na + diffusion kinetics of fluorophosphate cathodes for high-power-density sodium-ion batteries. Herein, the model of Na 3–2 x V 2– x Mo x (PO 4 ) 2 O 2 F cathodes is designed to form strong ionic bonds for adjusting the Na + diffusion environment. High-valent Mo 6+ substitution at V 4+ sites leads to the expansion of Na + diffusion channels along the ab plane due to the contraction of (V/Mo)O 5 F octahedra and the formation of Na + vacancies. Furthermore, the Na + diffusion was facilitated due to the lowered Na + migration energy barrier and the emergence of activated ion diffusion pathways. Consequently, the optimal Na 2.6 V 1.8 Mo 0.2 (PO 4 ) 2 O 2 F cathode achieved a discharge capacity of 72.8 mAh g –1 even at 5C, delivering a capacity retention of 95.2% after 500 cycles and a Coulombic efficiency (∼99.9%). Therefore, this study offers insights into the relationship among the chemical states of cations, crystal lattices, and ion diffusion kinetics within intercalation compounds.