There is great interest in scaling up Li-ion batteries for use in electric vehicles, requiring cheap andsafe cathodes. Recently, silicate-based cathodes have been described, especially Li2FeSiO4. Recently we studied the structure of Li2FeSiO4 and Li2MnSiO4 as a function of temperature on Polaris. We have succeeded in preparing the solid solutions Li2+2xM1-xSiO4 (M =Fe,Mn). Although by replacing the transition metal by Li the capacity is reduced, the rate capability is enhanced compared with the stoichiometric compounds. The increased Li content should increase stability on Li extraction ¿ a problem associated with Li2MnSiO4. Furthermore in Li2+2xMn1-xSiO4 a drop in the beta to gamma transition temperature is observed for increasing x. Accordingly we wish to study the structures of Li2+2xMnSiO4, and Li2+2xFeSiO4 as a function of temperature from 25 oC to 1000 oC.