The properties, insertion mechanisms, and electrochemical performances of the Na1.5Fe0.5Ti1.5(PO4)3/C composite as electrode material for Na-ion batteries are reported. The composite was obtained by solid-state reaction and consists of porous secondary particles of submicron-sized particles coated by carbon. Detailed characterizations were performed by combining theoretical and experimental tools. This includes the determination of the crystal structure of Na1.5Fe0.5Ti1.5(PO4)3 from both first-principles calculations and X-ray diffraction providing Na distribution over M1 and M2 interstitial sites, which is of importance for ionic conductivity. Na1.5Fe0.5Ti1.5(PO4)3/C was used as an electrode material at 2.2 V versus Na+/Na0, exhibiting good Na-storage ability with a specific capacity of 125 mAh g-1, close to the theoretical value, for the first discharge at C/10, good capacity retention, and Coulombic efficiency of 95% and 99.5% at the 60th cycle, respectively, and high power rate with a decrease of the specific capacity of only 14% from C/10 to 2C. These good performances have been related to the morphology of the composite and the substitution of Fe for Ti, leading to an insertion mechanism that differs from that of NaTi2(PO4)3. This mechanism was quantitatively analyzed from operando 57Fe Mössbauer spectroscopy used for the first time in both galvanostatic and GITT modes.