High Rate Performance for Carbon‐Coated Na 3 V 2 (PO 4 ) 2 F 3 in Na‐Ion Batteries

The electrochemical and structural properties of a series of three different Na 3 V 2 (PO 4 ) 2 F 3 samples (with or without carbon coatings of different natures), obtained via different synthesis methods, are compared through operando high‐resolution synchrotron X‐ray diffraction. The pristine materials all possess negligible quantities of oxygen defects, as probed by the b / a lattice parameters ratios and solid‐state NMR. Operando X‐ray diffraction recorded during charge at C /2 reveals subtle differences between the samples (of different particle size, morphology, and carbon‐coating nature) in the crystallinity of the intermediate compositions formed within the Na 3 V 2 (PO 4 ) 2 F 3 –NaV 2 (PO 4 ) 2 F 3 phase diagram. A new temperature‐controlled operando cell is used to determine this phase diagram at 0 °C, mostly unchanged compared to that of recorded at 25 °C. Very high charging and discharging rates are demonstrated and intermediate compositions can be spotted operando even up to 25 C rate at which the compositional phase diagram is only slightly altered compared to that recorded under equilibrium conditions. Optimized carbon‐coated Na 3 V 2 (PO 4 ) 2 F 3 shows exceptional rate and electrochemical cycling capabilities, as demonstrated by hard carbon//Na 3 V 2 (PO 4 ) 2 F 3 18650 prototypes of 75 Wh kg −1 that can be charged or discharged 4000 times at 1 C rate.