An electrostatic potential study of LiFePO4 cathode material for lithium-ion battery

Abstract

The topological features of the dynamic electron density and the electrostatic potential (ESP) of the dynamic charge density of LiFePO4 have been analyzed by using the structure factors calculated accurately from the independent atom model (IAM). The integrated charges of ions at nuclear critical points (NCPs) have been evaluated and the topological properties at bond critical points (BCPs) of P–O and Fe–O bonds in the PO4 tetrahedron and FeO6 octahedron have been considered. The values and variations of the ESP have been analyzed through two-dimensional contour maps in the planes perpendicular to the a and b axes. We have found four spiral channels with negative ESP that Li+ ion migration is possible along the [010] direction in a unit cell. Each channel reveals extremely low values (less than −0.9e/Å) of the ESP and variations undistinguished within 0.04e/Å interval. The average width of each channel is more than known radius of Li+ ion. In a unit cell, two channels show right hand twist and other two channels do left hand twist. Since passing through the gaps between adjacent FeO6 octahedrons, these channels can be considered as the possible pathways of Li+ ion migration that oxidation/reduction of the Fe2+ ion can be expected during charge/discharge. Although there exist also four zigzag channels with negative ESP along the [100] direction, because of the average width less than ionic radius, Li+ ion migration is impossible. The ESP channels along the [001] and [101] directions have been not found. Possible three-dimensional curved aspects of the pathways of Li+ ion migration in charge/discharge process in lithium-ion battery have been presented.