Engineering Diffusivity and Operating Voltage in Lithium Iron Phosphate through Transition Metal Doping

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.