Formation, doping, and lithium incorporation in LiFePO4

Abstract

For over 25 years, lithium iron phosphate (LiFePO4) has been a material of interest for Li-ion batteries as it is environmentally benign, low cost, and structurally stable. Here, we employed density functional theory calculations to examine the formation of LiFePO4 via different reaction routes, intrinsic defect processes, solution of dopants, and impact of doping on its electronic structure. The most thermodynamically favorable process to synthesize LiFePO4 is predicted to be from its constitute elements in their standard states. The Li–Fe anti-site defect is the lowest defect energy process inferring the presence of a small amount of cation intermixing. The most promising isovalent dopants on the Li, Fe, P, and O are the Na, Ca, As, and S, respectively. The substitution of Ru for Fe is energetically favorable. The doping of Ge on the P site is a possible strategy to generate both Li interstitials and holes in this material. The stability of this material upon Li incorporation (up to four atoms per 112-atom supercell) was investigated. Although incorporation is slightly unfavorable, there is a clear enhancement in the incorporation with volume expansion. The insulating nature of this material is affected by the doping and incorporation of Li, which leads to the reduction of the bandgap.