NaV₆O₁₅: A promising cathode material for insertion/extraction of Mg²⁺ with excellent cycling performance

The rechargeable magnesium batteries (RMBs) are getting more and more attention because of their high-energy density, high-security and low-cost. Nevertheless, the high charge density of Mg²⁺ makes the diffusion of Mg²⁺ in the conventional cathodes very slow, resulting in a lack of appropriate electrode materials for RMBs. In this work, we enlarge the layer spacing of V₂O₅ by introducing Na⁺ in the crystal structure to promote the diffusion kinetics of Mg²⁺. The NaV₆O₁₅ (NVO) synthesized by a facile method is studied as a cathode material for RMBs with the anhydrous pure Mg²⁺ electrolyte. As a result, the NVO not only exhibits high discharge capacity (119.2 mAh·g^-1 after 100 cycles at the current density of 20 mA·g^-1) and working voltage (above 1.6 V vs. Mg²⁺/Mg), but also expresses good rate capability. Besides, the ex-situ characterizations results reveal that the Mg²⁺ storage mechanism in NVO is based on the intercalation and de-intercalation. The density functional theory (DFT) calculation results further indicate that Mg²⁺ tends to occupy the semi-occupied sites of Na⁺ in the NVO. Moreover, the galvanostatic intermittent titration technique (GITT) demonstrates that NVO electrode has the fast diffusion kinetics of Mg²⁺ during discharge process ranging from 7.55 × 10^-13 to 2.41 × 10^-11 cm²·s^-1. Our work proves that the NVO is a potential cathode material for RMBs.