Electrochemical properties of heterosite FePO₄ in Aqueous Mg²⁺ electrolytes

There is currently much interest in advanced energy storage systems because of their potential relevance to power grid storage media. Both earth abundance and high energy density of divalent Mg²⁺ ion make Mg batteries attractive as alternatives to Li-ion batteries, but the number of host materials for reversible intercalation is very limited due to the strong coulombic interaction induced in solid matrix. In this work, we report the electrochemical properties of FePO₄ in aqueous Mg²⁺ electrolyte. The charge/discharge experiments showed that FePO₄ delivers the first discharge capacity of 90 mAh g^-1 with subsequent partial reversibility. The ex-situ Mössbauer spectroscopy confirmed reversible redox of Fe³⁺/Fe²⁺ during the discharge and charge processes, while little change was observed in the ex-situ X-ray diffraction patterns. Activation energy for Mg²⁺ diffusion in FePO₄ lattice was calculated to over three times larger than that of Li⁺. It is postulated that the electrochemical reaction of FePO₄ in aqueous Mg²⁺ electrolyte proceeds in part by non-topochemical Mg²⁺ (de)intercalation accompanied by the irreversible transformation from the crystalline state to the amorphous state in the vicinity of particle surface.