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: LiFe1-xMxPO4 (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 olivine phosphates. Dopants with lower-valence electrons (LVEs) compared to Fe repel the Li+ ion to facilitate its outward diffusion, whereas highervalence-electron (HVE) dopants attract 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 a large spin-exchange split to bury the dopant states in the valence and conduction bands of the pristine system, and, in turn, the band gap remains unchanged in LiFe1-xMnxPO4. Baring Mn, the open-circuit voltage increases with HVE dopants and decreases with LVE dopants.