A new olivine composition (i.e., LiFe0.25Mn0.5Co0.25PO4) is proposed as electrode material with increased energy density for application in lithium-ion batteries. The new formulation increases the working voltage and induces different electrochemical behavior with respect to bare olivine materials based on Fe. The study provides deep insight into the features of the Fe3+/Fe2+, Mn3+/Mn2+, and Co3+/Co2+ redox couples within the olivine lattice in terms of electrochemical activity, Li+ transport properties, and Li-cell behavior. The electrochemical characterization clearly reveals the voltage signatures corresponding to the various metals; however, the Mn3+/Mn2+ process has higher intrinsic polarization with respect to Fe3+/Fe2+ and Co3+/Co2+. This issue is efficiently mitigated by carbon coating the material, resulting in enhanced electrochemical performances.