High rate and cyclic performance of Na3–2xMgxV2(PO4)3/C cathode for sodium-ion batteries
Na 3–2 x Mg x V 2 (PO 4 ) 3 /C (0 ≤ x ≤ 0.2) were synthesized by a sol–gel method. According to X-ray diffraction (XRD) Rietveld refinement and energy dispersive spectroscopy (EDS) mapping results, Mg ions were approved to occupy the Na2 (18e) site in Na 3–2 x Mg x V 2 (PO 4 ) 3 /C for the first time. The amount of Mg substitution in Na 3–2 x Mg x V 2 (PO 4 ) 3 /C was optimized. The specific discharge capacities of Na 2.9 Mg 0.05 V 2 (PO 4 ) 3 /C at 10 C, 20 C, and 30 C were 98.9%, 91.5%, and 76.2% of the value obtained at 1 C, respectively. Na 2.9 Mg 0.05 V 2 (PO 4 ) 3 /C exhibited the highest capacity retention of 80.3% with a discharge capacity of 77.3 mAh·g −1 after 1200 cycles at 10 C. Na 2.9 Mg 0.05 V 2 (PO 4 ) 3 /C exhibited superior rate performance, resulting from a high sodium diffusion coefficient ( D Na ) of 3.91 × 10 –12 cm 2 ·s −1 and the excellent electrical conductivity (5.7 × 10 –2 s·cm −1 ). Combining the X-ray photoelectron spectroscopy (XPS) and Electrochemical impedance spectroscopy (EIS) results, the mechanism of Mg substitution was investigated. Since two Na + were replaced by one Mg 2+ , \({\mathrm{V}}_{\text{Na}}^{^{\prime}}\) was produced to maintain valence equilibrium. This phenomenon benefited Na + migration by broadening the diffusion channel in the structure of Na 3 V 2 (PO 4 ) 3 . According to the EPR results, the signal intensity of Na 2.9 Mg 0.05 V 2 (PO 4 ) 3 /C was much stronger than that of Na 3 V 2 (PO 4 ) 2 /C, thus confirming the existence of \({\mathrm{V}}_{\text{Na}}^{\mathrm{^{\prime}}}\) after Mg substitution. When the amount of Mg substitution was further increased to 0.1, the D Na decreased because of the small radius of Mg 2+ , which led to severe lattice distortion and volumetric shrinkage of the unit cell.
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