  Density functional calculations are carried out to understand and tailor the
electrochemical profile diffusivity, band gap and open circuit voltage of
transition metal doped olivine phosphate $LiFe_{1-x}M_{x}PO_{4}$ (M = V, Cr,
Mn, Co and Ni). Diffusion and hence the ionic conductivity is studied by
calculating the activation barrier, $V_{act}$, experienced by the diffusing
$Li^{+}$ ion. We show that the effect of dopants on diffusion is both site
dependent and short ranged and thereby it paves ways for microscopic control of
ionic conductivity via selective dopants in this olivine phosphates. Dopants
with lower valence electrons (LVE) compared to Fe repel the $Li^{+}$ ion to
facilitate its outward diffusion, whereas higher valence electron (HVE) dopants
attracts the $Li^{+}$ ion to facilitate the inward diffusion. From the
electronic structure calculation we establish that irrespective of the dopant
M, except Mn, the band gap is reduced since the M-$d$ states always lie within
the pure band gap. Atomically localized $d$ states of HVE dopants lie above the
Fermi energy and that of LVE lie below it. Half-filled Mn-$d$ states undergo
large spin-exchange split to bury the dopant states in valence and conduction
bands of the pristine system and in turn the band gap remains unchanged in
$LiFe_{1-x}Mn_{x}PO_{4}$. Baring Mn, the open circuit voltage increases with
HVE dopants and decreases with LVE dopants.

Materials Science