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

Abstract

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 Mg_xV₂(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 Mg_xV₂(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 Li_xV₂(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 Mg_xV₂(PO₄)₃.