In this paper, the influences of the lithium content in the starting materials on the final performances of as-prepared Li_xMnPO₄ (x hereafter represents the starting Li content in the synthesis step which does not necessarily mean that Li_xMnPO₄ is a single phase solid solution in this work.) are systematically investigated. It has been revealed that Mn₂P₂O₇ is the main impurity when Li < 1.0 while Li₃PO₄ begins to form once x > 1.0. The interactions between Mn₂P₂O₇ or Li₃PO₄ impurities and LiMnPO₄ are studied in terms of the structural, electrochemical, and magnetic properties. At a slow rate of C/50, the reversible capacity of both Li_0.5MnPO₄ and Li_0.8MnPO₄ increases with cycling. This indicates a gradual activation of more sites to accommodate a reversible diffusion of Li⁺ ions that may be related to the interaction between Mn₂P₂O₇ and LiMnPO₄ nanoparticles. Among all of the different compositions, Li_1.1MnPO₄ exhibits the most stable cycling ability probably because of the existence of a trace amount of Li₃PO₄ impurity that functions as a solid-state electrolyte on the surface. The magnetic properties and X-ray absorption spectroscopy (XAS) of the MnPO₄·H₂O precursor, pure and carbon-coated Li_xMnPO₄ are also investigated to identify the key steps involved in preparing a high-performance LiMnPO₄.