At base, creating better lithium-ion batteries is all about understanding the diffusion of Li${}^{+}$, particularly its reversible intercalation in materials like olivine phosphates. This study shows that for a given diffusion path for Li${}^{+}$, the forward and backward activation barriers can be manipulated simply by choosing the level of doping $x$ in LiFe${}_{1-x}{M}_{x}$PO${}_{4}$. This simple knob can tune three of the most important electrochemical parameters: diffusivity, operating voltage, and band gap. The insight from this study should also apply to other transition-metal oxide cathode materials.