A scheme for the generation of Fe–P networks to search for low-energy LiFePO₄ crystal structures

Abstract

Herein we present a network generation scheme to explore the structural diversity of LiFePO4, which is an important cathode material in Li-ion batteries.

Fe–Si networks and charge/discharge-induced phase transitions in Li2FeSiO4 cathode materials

“…This pool represents structures which can return to the fully lithiated compound (x=2) by inserting back the Li atoms. The other source is based on the low-energy LiFePO4 crystal structures generated using Fe-P networks 19 . Low-energy structures in the LiFePO4 pool are substituted with Si on the P sites to get the LiFeSiO4 structures, which may or may not be fully recovered to Li2FeSiO4.…”

“…The other source of LiFeSiO4 structures is from the substitution of LiFePO4, where we were able to identify a new group of structures which do not have well-defined vacancy sites to recover to a low-energy Li2FeSiO4 state. As discussed in our previous work 19 , low-energy LiFePO4 structures are constructed initially focusing on building Fe-P networks. Afterwards, O atoms are added around each P atom to form PO4 tetrahedrons whose orientations are then optimized by force-field methods.…”

“…Li atoms are finally added into the remaining vacancy sites of FePO4 configurations. Details of this method can be find in the literature 19 . The previously searched LiFePO4 structures are substituted with Si on P sites to generate low-energy LiFeSiO4 structures.…”

“…We discover this mechanism through an extensive investigation of structural transitions that can occur in the LixFeSiO4 system at x=1. To efficiently explore the lowenergy structural landscape of LiFeSiO4, we utilized structural databases we previously developed, including the one for Li2FeSiO4 using Motif-network scheme 18 and that for LiFePO4 using Fe-P network-generation scheme 19 . We show that substitution of P atoms in low-energy LiFePO4 structures with Si produce low-energy structural candidates for LiFeSiO4.…”

“…The electrochemical properties are also related to the type of Fe-Si networks: crystal structures with common Fe-Si networks have similar electrochemical behaviour from first principles calculations 21 . The concept has been generalized to LiFePO4 systems by Lv et al where Fe-P networks are used to predict new low-energy structures with remarkable success 19 . Following previous work 21 , the Fe-Si networks in low-energy LiFeSiO4 structures are classified into 2D-ladder-like, 3D-diamond-like and 3D*-4-8-membered-rings types (For simplicity, we use name "2D", "3D" and "3D*" in the following discussion) according to the FeSi4 and SiFe4 polyhedra which are considered as building blocks of the Fe-Si networks ( fig.1).…”