Supporting Information

Experimental Section, detailed structural information from Rietveld refinements, comparison of Na storage performance, redox potentials and corresponding peak potential separations obtained from CV curves, specifications of the hard carbon (Kuraray Type 2) used in full-cell tests, the formula for calculating the sodium-ion diffusion coefficient based on Galvanostatic Intermittent Titration Technique data, XRD pattern, SEM image and TEM images of the NaFePO4 intermediate, comparison of discharge capacity after 50 cycles at 0.1C for samples with different PVP contents via two different processes, rietveld refinement curves of samples, SEM image, TEM images and HAADF image and corresponding EDS elemental mappings of NFPF-T, nitrogen adsorption-desorption isotherms and pore-size distributions: NFPF-T and NFPF-N, high-resolution XPS spectra of Fe 2p of NFPF-T and NFPF-N, second charge/discharge curves of NFPF-T and NFPF-N samples at different rates, voltage gap between the charge and discharge plateaus of NFPF-T and NFPF-N at various rates, cycle performance of two samples at 0.1C and 5C, GITT analysis and Na+ diffusion determination, CV profiles of NFPF-T and NFPF-N at various scan rates from 0.1 to 0.5 mV⋅s-1.