The new insight into the lithium migration mechanism of LiFePO4 by atomic simulation method

In this work, atomistic simulation method based on a simple force filed model is applied to observe the concerted motion of lithium ions in the perfect lattice of LiFePO4 cathode material. The simulation is carried out at a series of increasingly elevated temperature in a super cell containing 64 unit cells. The superimposed Li+ trajectory at all timeframes offers an intuitive, reliable image of the Li+ migration in crystal lattice, which gives us new insight into the migration mechanism of lithium ion in the lattice of LiFePO4. It reveals that at lower temperatures, lithium ion propagates along the b-axis and follows zigzag channels consisting of alternating 4a-4c hopping and 8d-8d stretching paths. At the temperature higher than 1050 K, Li-Fe collaborative movement is responsible for the lithium ion flow along the a-axis and the c-axis, namely, the alternating 4a Li -4c tetra-c-4d Fe site-4a Li site route and the alternating 4a Li site -4c Fe site-4a Li site route. This Li-Fe collaborative movement, normally referred to as anti-site, is better to be described as site disordering. At the same time, the wandering Fe ion at the 4c unoccupied tetrahedral position will also generate blocking effect that deteriorates the lithium ions migration along the b-axis. The minimum energy barrier of LiFePO4 cathode material is about 0.55 eV along the b-axis, and the estimated lithium ion diffusion coefficient at room temperature is estimated at ca. 3.67 × 10-12 cm2·s-1.