Lithium was electroplated on a fresh magnesium surface. The deposit was tested as a negative electrode in a cell containing LiFePO4 as a positive electrode. The reactions that took place on charging the cell were total conversion of phospholivine into a heterosite structure and lithium deposition onto Li(Mg). On discharging, the heterosite phase was partially converted to LiFePO4 and lithium extracted from the Li1+x(Mg) electrode. The performance of this cell surpassed that of lithium foil as an electrode, providing higher capacities under C/10 and C/5 regimes. Several factors associated with the Li1+x(Mg) surface and the Li/Mg interface account for this good behavior.