Synthesis, cathode property and crystal, electronic and local structures of Mg2Mo3O8 as Mg rechargeable battery cathode material

Abstract

Mg2Mo3O8 was synthesized by a solid-phase method in a high vacuum atmosphere. According to the combination of ICP-AES technique and Rietveld analysis for synchrotron X-ray and neutron diffraction patterns, the structure and composition of the synthesized material was identified as single phase of the layered Mg2Mo3O8 of P63mc space-group. Charge and discharge cycle tests showed that Mg2Mo3O8 with carbon composite exhibited 18 and 89 mAh g−1 of initial charge and discharge capacities at 90 °C. Reversible charge/discharge behavior was confirmed up to 5 cycles. Electrochemical demagnesiation using Li anode showed deintercalation of 0.45 Mg2+ pfu. The valence of the Mo in synthesized material was determined by XAFS, whose spectrum at the Mo K-edge confirmed the reductive synthesis of Mg2Mo3O8 because of not hexavalent but reduced tetravalent Mo. Since the Mo K-edge XANES spectrum after first charge was oxidized from tetravalent, it was revealed that the Mg2Mo3O8 could electrochemically demagnesiate in the present cell-condition using [Mg(G4)][TFSA]2/[PYR13][TFSA] electrolyte. The PDF data from synchrotron X-ray total scattering was used to analyze the local structure. The local structure leads to the Mg hopping route in Mg2Mo3O8, where tetrahedrally coordinated Mg hops via octahedral site.