Ultrafast cathode characteristics of a nano-V2(PO4)3 carbon composite for rechargeable magnesium batteries

Magnesium rechargeable batteries (Mg batteries) are currently attracting attention as high-energy and low-cost energy storage devices that can replace lithium-ion batteries. Within this context, V2(PO4)3 is a promising material for high-voltage and high-rate cathodes for Mg batteries. However, the strong electrostatic attraction between Mg2+and anions degrades the Mg2+ diffusion in the solid-state of the cathode, which hinders the room-temperature operation. Furthermore, the detailed charge–discharge mechanism of V2(PO4)3 during Mg2+ insertion/extraction is not yet fully understood. Here, we synthesized V2(PO4)3 nanocrystals (50 nm), which are highly dispersed and directly embedded in conductive carbon, for realizing ultrafast cathode reaction for Mg batteries. The V2(PO4)3/carbon composite exhibited a high capacity of 210 mA h g−1 (at 1C-rate) and 110 mA h g−1 (at 10C-rate, i.e., under ultrafast conditions) during Mg2+ insertion/extraction even at room temperature. The phase transition and valence change of vanadium in MgxV2(PO4)3 were evaluated by in situ X-ray diffraction (XRD) and X-ray absorption fine structure (XAFS) analyses. Both in situ measurements revealed that Mg2+ insertion/extraction of MgxV2(PO4)3 (0.5 ≤ x ≤ 1.3) proceeds reversibly with a valence change of vanadium through a solid-solution reaction, unlike the Li+ insertion/extraction of LixV2(PO4)3 (1 ≤ x ≤ 3) via a two-phase reaction. Our findings provide a promising synthesis method of V2(PO4)3 for ultrafast and high-voltage cathodes for practical Mg batteries and experimental evidence for a unique charge–discharge mechanism in MgxV2(PO4)3.