Unveil the Chemistry of Olivine FePO4 as Magnesium Battery Cathode

Despite growing interest in magnesium batteries, it is still a challenge to find a cathode that fulfills requirements such as high capacity and good cyclability. Because of their positions in the periodic table and the similar ionic sizes of lithium and magnesium, it was naturally postulated that a classical intercalation-type Li-ion battery cathode may also accommodate the intercalation of Mg. On the contrary, many Li-ion battery cathodes performed very poorly in Mg cells, although the mechanism behind such phenomena is still unclear. Here we provide first-hand evidence about the chemistry of olivine FePO4 as Mg battery cathode using a combined theoretical and experimental approach. Although LiFePO4 is a commercial cathode with extraordinary good performance in Li-ion batteries, the measured capacity of FePO4 in nonaqueous Mg cell was only similar to 13 mAh/g. Density functional theory calculations predicted sufficient mobility of Mg2+ in FePO4 lattice to support the insertion of Mg at a reasonable rate, suggesting the poor performance cannot be simply attributed to the limitation of Mg2+ diffusion. Instead, the recorded low capacity was the result of surface amorphorization that prohibited the electrochemical reaction from penetrating deeply into the bulk phase. The amorphorization had a thermodynamic origin from the instability of intercalated product, which was predicted from DFT calculations and supported by the failure to synthesize magnesiated FePO4 in the solid state reaction route. These results highlighted the importance of a thermodynamically preferred intercalation in order to achieve successful Mg battery cathode.