The Mg-doped LiMnPO4 nanoplates with orthorhombic olivine structure were prepared by a simple solvothermal method. The effects of the Mg dopant on the structure and electrochemical performance of LiMnPO4 are investigated. The X-ray diffraction pattern shows that Mg doping does not change the crystal structure of LiMnPO4. High-resolution transmission electron microscopy images and selected area electron diffraction patterns show that, like the LiMnPO4 nanoplates, the crystallographic facet of the LiMn0.95Mg0.05PO4 nanoplates is the (100) plane, which places the 1D Li+ conduction path, the b axis, in the platelets. Chemical delithiation shows that the volume mismatch across the LiMnPO4/MnPO4 two-phase interface is reduced by Mg doping, and TEM images show that the interface intersects the b axis, thereby controlling the rate and extent of Li+ insertion by the mobility of the interface. Reduction of the mismatch across the interface increases its mobility and therefore the electrochemical performance of the olivine as a cathode material. (C) 2012 The Electrochemical Society. [DOI: 10.1149/2.037207jes] All rights reserved.