Abstract Due to the important similarities between olivine-LiFePO4 and olivine-LiMnPO4, manganese doping has drawn a lot of attention since it can improve the electronic and ionic conductivity of LiFePO4, hence enhance the electrochemical properties. The thermodynamic behavior of Mn-doped-LiFePO4 cathodes has been examined through the thermodynamic investigation of Li(MnyFe1-y)PO4-(MnyFe1-y)PO4 olivine joins. New sublattice thermodynamic models are proposed to describe the para-equilibrium in Li(MnyFe1-y)PO4-(MnyFe1-y)PO4 joins. Moreover, the elastic Gibbs energy approach extended in the large deformation regime is used to calculate the coherent miscibility gaps corresponding to (100) habit plane. The para-equilibrium and coherent miscibility gaps are calculated providing our prior estimated enthalpy of formation and the elastic constants of olivine compounds and enthalpy of mixing of binary sub-systems from first principles simulations based on Density Functional Theory (DFT). The experimental data on the para-equilibrium join is successfully reproduced, and the system is likely to experience the (100) coherent phase transformation. Our thermodynamic models of the Li(MnyFe1-y)PO4-(MnyFe1-y)PO4 join are able to describe most of the features of the electrochemical behavior of Li(MnyFe1-y)PO4 cathodes including the electrochemically driven phase diagram, open circuit voltage (OCV), asymmetry of charging/discharging processes, potential shift and favorable coherent phase transformation.