Fig 1

X-ray diffraction diagrams of bare and Nb2O5-coated Na3V2(PO4)3 samples (black dots). Fitted patterns (red), and differential curves (grey) are also included. DIF pattern for trigonal Na3V2(PO4)3 (blue) is depicted below."

Table 1

Unit cell parameters in the trigonal R-3c space group, along with carbon and Nb2O5 contents, were determined for bare-coated Na3V2(PO4)3 samples."

Fig 2

Raman spectra recorded for bare and 5% Nb2O5-coated Na3V2(PO4)3 samples recorded at selected Raman shift ranges a) 950–1910 cm−1 and b) 150–900 cm−1."

Fig 3

HRTEM images of selected regions of samples (a, b) NVP@Nb1 and (c, d) NVP@Nb5. The FT of image a) is ascribed to the < 111 > zone axis. In b), two FT of different regions are shown; the one inside the NVP particle corresponds to the < 3 −5 2 > zone axis, while the FT of the surface shows a poorly crystalline coating."

Fig 4

Galvanostatic experiment on sodium half-cells cycled between 2.0–4.3 V. a) Rate capability test at several rates from 0.1C to 20C and then 1C until the 150th cycle; b) Charge and discharge curves recorded at different rates; c) Charge-discharge hysteresis versus current density. Inset: Direct-current resistance versus % Nb2O5 plot and d) Nyquist representations of the impedance spectra for the bare and coated samples after one cycle at 0.1C and the 150th cycle at 1C."

Table 2

Resistance values calculated from the impedance spectra of the studied bare and coated electrodes after 1st cycle at 0.1C and 100 cycles at 1C."

Fig 5

Ex situ XRD patterns of partially charged and discharged NVP@Nb3 electrodes. Reflections coming from the Al support are marked."

Fig 6

Cyclic voltammograms of sodium half-cells assembled with bare and Nb2O5-coated Na3V2(PO4)3 samples recorded at several rates after a) a pre-activation cycle at 0.1C; b) the 150th cycle at 1C."

Fig 7

a) Cyclic voltammograms recorded at 0.05 mV s−1, remarking the capacitive contribution as a shaded area after a) a pre-activation cycle at 0.1C; b) the 150th cycle at 1C. Column representation of the capacitive contributions versus % Nb2O5 at each sweep rate after c) a pre-activation cycle at 0.1C; d) the 150th cycle at 1C. Plots of the apparent sodium diffusion coefficients determined along a charge and discharge cycle after e) a pre-activation cycle at 0.1C; f) the 150th cycle at 1C."

Fig 8

Extended galvanostatic cycling of a) Sodium half-cells assembled with bare and NVP@Nb3 coated samples at 25 ℃ and cycled at 10C rate; b) a hard carbon/NVP@Nb3 full cell at 25 ºC and 1C rate. Sodium half-cells assembled with NVP@Nb3 and cycled at 2C rate at c) 60 ℃, and d) −15 ℃."

Fig 9

XPS spectra of a) NVP@Nb3 and NVP@Nb1 raw samples at the Nb 3d core level; b) fully charged and discharged NVP@Nb3 electrodes at the F 1s core level; and c) raw NVP@Nb3 and the corresponding fully charged and discharged electrodes at the Nb3d core level."