There is great interest in scaling up Li-ion batteries for use in electric vehicles, requiring cheap and safe cathodes. Li2FeSiO4 shows promising performance as a cathode but shows rather poor rate capability. We have prepared the solid solutions Li2+2xMn1-xSiO4 and Li2+2xFe1-xSiO4, the subject of a previous ISIS proposal. Here we address doping on the Si site ¿ i.e. tuning the polyoxyanion. We have recently succeeded in preparing the solid solutions Li2+xFe1-xSi1-xPxO4 and Li2+yFezSi1-xAlxO4 (for a range of compositions). Although for Fe contents < 1, the theoretical capacity is reduced, replacing the polyoxyanion provides a means of adjusting the operating voltage. Increased Li content should also confer added structural stability on lithium extraction. We wish to establish the structures as a function of dopant concentration and as Li is extracted/re-inserted.