High rate and cyclic performance of Na3–2xMgxV2(PO4)3/C cathode for sodium-ion batteries

Abstract

Na3–2xMgxV2(PO4)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 Na3–2xMgxV2(PO4)3/C for the first time. The amount of Mg substitution in Na3–2xMgxV2(PO4)3/C was optimized. The specific discharge capacities of Na2.9Mg0.05V2(PO4)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. Na2.9Mg0.05V2(PO4)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. Na2.9Mg0.05V2(PO4)3/C exhibited superior rate performance, resulting from a high sodium diffusion coefficient (DNa) of 3.91 × 10–12 cm2·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 Mg2+, $${\mathrm{V}}_{\text{Na}}^{^{\prime}}$$ was produced to maintain valence equilibrium. This phenomenon benefited Na+ migration by broadening the diffusion channel in the structure of Na3V2(PO4)3. According to the EPR results, the signal intensity of Na2.9Mg0.05V2(PO4)3/C was much stronger than that of Na3V2(PO4)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 DNa decreased because of the small radius of Mg2+, which led to severe lattice distortion and volumetric shrinkage of the unit cell.