Chromium doped NASICON-structured Na3MnTi(PO4)3/C cathode for high-performance sodium-ion batteries

Na3Mn1−xCrxTi(PO4)3/C composites with different chromium doping contents (x = 0.01, 0.03, 0.05, 0.07, 0.10, 0.12, 0.15), a potential cathode material in sodium-ion batteries with its classical 3D NASCION structure, has been synthesized via a feasible sol-gel route. The doping effects on the crystal structure were verified by X-ray diffraction (XRD), X-ray photoelectron spectroscopy (XPS), Raman and Fourier-infrared spectra (FT-IR). The results show that chromium doping does not alter the structure of the material, and chromium is successfully doped for manganese site. Proper chromium doped Na3Mn1−xCrxTi(PO4)3/C composites exhibit a high reversible capacity and structural stability. Among them, Na3Mn0.93Cr0.07Ti(PO4)3/C and Na3Mn0.90Cr0.10Ti(PO4)3/C achieve the highest capacity retention (90.19% after 100 cycles at 0.2 C) and initial discharge specific capacity (119.65 mA h g-1 at 0.2 C), yielding the coulombic efficiency of ~100%. Interestingly, even after cycling at 10 C over 500 cycles, the Na+-storage capacity of 94.37 and 101.51 mA h g-1 with a capacity retention of 79.70% and 73.74% were respectively still obtained with x = 0.07 and 0.10, highlighting their excellent long cycling life and remarkable rate performances. Results from the cyclic voltammetry and impedance spectroscopy studies were combined to verify the enhanced sodium-ion diffusion coefficients.