Electrochemical Performance of Mg x V2(PO4)3 As Cathode Material for Mg Rechargeable Batteries

Abstract

1. Introduction Magnesium rechargeable batteries are considered as viable alternatives for next generation large-scale energy storage devices. Magnesium provides two electrons per atom (divalent nature of Mg2+), making it an attractive high-energy density battery system over the existing lithium-ion technology.1 However, the specific energy density of rechargeable batteries is low, suffering from the generally low working voltage, which is mainly limited by cathode materials. For the commercial realization of magnesium rechargeable batteries, cathode materials with a high working voltage and good electrochemical performance are highly desirable. In the search for a suitable cathode material, we turned our attention to the design of Mg3V2(PO4)6 (hereafter denoted as MVP). Its analogue in the lithium system is Li3V2(PO4)3 (hereafter LVP), which is a promising high performance cathode due to its high working voltage, three-dimensional framework, good thermal stability and remarkable electrochemical properties.2 - 4 Design of MVP should offer the same electrochemical properties as exhibited in the lithium battery, and therefore it is possible to design the cathode material with high energy density for magnesium battery. Here we report the electrochemical performance of MVP as a high voltage cathode material for magnesium ion battery 2. Experiments Carbon-coated MVP and LVP were successfully synthesized via a wet ball-milling-assisted solid state carbothermal method. V2(PO4)3 (hereafter VP) was prepared by using both electrochemical and chemical oxidation process (hereafter denoted as ED-VP and CD-VP, respectively). Particle morphology, composition, crystal and electronic structure were characterized by using SEM, XRD and XAS, respectively. Galvanostatic charge and discharge measurements were conducted by using three electrode cells. 0.5M Mg(TFSI)2 in acetonitrile was used as the electrolyte. 3. Results and discussions Pure phase of carbon-coated LVP (hereafter C-LVP) was successfully obtained as confirmed by XRD pattern shown in Fig. 1a. Upon delithiation of all three Li+, the crystallinity of both ED-VP and CD-VP decreases. Mg2+ electrochemical insertion into the host structure of VP to synthesize Mg x V2(PO4)3 (M x VP) was successfully conducted. Triclinic Mg3V4(PO4)6 was also successfully synthesized, as shown in Fig. 1b. Analysis of the composition, crystal structure and electronic structure prove that Mg2+ can be reversibly (de) intercalated into the frame structure of VP. It is notable that the average working potential vs. Mg/Mg2+ is around 2.9 V with a potentially high capacity, which surpasses those of reported cathode materials for magnesium rechargeable batteries.5, 6 Further, we will report the effect of synthetic strategies, particle size and temperature on the electrochemical performance of M x VP. Acknowledgement This work was supported by CREST project of the Strategic Basic Research Programs of JST.