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 recently prepared the solid solutions Li2+2xMn1-xSiO4 and Li2+2xFe1-xSiO4 which exhibit enhancement of the rate capability compared with the stoichiometric compounds. We wish to determine the detailed structure, including Li positions, for several Li2+2xFe1-xSiO4 and Li2+2xMn1-xSiO4 phases both as-prepared and as a function of electrochemical cycling. In order to optimise these potentially important materials it is vital to understand the processes which occur on cycling, particularly phase changes.