Mixed-metals olivine cathode materials are one of the most promising systems in the field of lithium batteries. Their main drawback is the low electronic conductivity, which limits the current rate at which they can be cycled. Here, the effect on the electrochemical properties of vicariant high-valence transition metals (HVTMs) inserted in the olivine crystal structure is studied. The strategy here adopted consists in doping a LiFe1/3Ni1/15Co3/5PO4 cathode by substituting part of the cobalt(II) ions with vanadium(IV), niobium(V), or tantalum(V) HVTMs. WAXD studies reveal that small HVTMs molar quantities (HVTM/P = 0.04) are vicariant into the olivine crystal lattice. Single-cell tests show for Nb-doped cathode a high-voltage (ca. 4.5 V) and a specific capacity of 149 mAh g-1 (656 mWh g-1). Thus, the insertion in the cathode structure of HVTMs exceptionally rises the electric conductivity of the material and yields an …