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

TL;DR

This study uses density functional calculations to explore how transition metal doping affects ionic diffusivity, band gap, and voltage in lithium iron phosphate, revealing site-dependent effects and electronic structure influences.

Contribution

It provides a microscopic understanding of how different dopants modify electrochemical properties in olivine phosphate, enabling targeted material design.

Findings

LVE dopants facilitate outward $Li^{+}$ diffusion by repelling ions.

HVE dopants attract $Li^{+}$ ions, enhancing inward diffusion.

Open circuit voltage increases with HVE dopants and decreases with LVE dopants.

Abstract

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.