Magnesium insertion into Mg 0.5+ y(Fe yTi 1− y) 2(PO 4) 3

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A series of transition metal phosphates, Mg 0.5+ y (Fe y Ti 1− y ) 2(PO 4) 3 (MFTP), modified from Mg 0.5Ti 2(PO 4) 3 (MTP) were prepared by sol–gel method and evaluated as cathode materials for rechargeable magnesium cells. The crystal structure of MFTP at 0≤ y≤0.5 was identical to that of MTP, while their unit cell volumes were smaller than that of MTP due to introduced Mg 2+ ions. Electrochemical magnesium insertion into MFTP was found possible as observed for MTP. However, the limiting extent of magnesium insertion into MFTP at 0.1≤ y≤0.5 was small compared with MTP and remarkably dependent on the current density during discharge, suggesting that the insertion limit is determined not by the number of available sites for Mg 2+ or electrons but by the mobility of Mg 2+ in the host varying with the unit cell volume.