Impact of temperature on short-range charge ordering in LiFePO4/FePO4

Energy is stored in a LiFePO4 battery electrode through the intercalation of Li. As Li incorporate into the crystal lattice of Fe(III)PO4, electrons reduce Fe(III) into Fe(II). The interactions of Li and its vacant site (Va) with these localized electrons (holes), so-called polarons, cause phase separation during battery operation. These fundamental interactions are however difficult to quantify using standard electronic structure calculations. In this paper, we utilize DFT+U with occupation matrix control to compute interaction energies at varying Li-Fe(II) and Va-Fe(III) pair separations. The increased energy with separation warrants the use of an electrostatic description. The DFT+U data are fitted to a Coulombic potential with two-body corrections and used in a Monte Carlo scheme. The coordination of the species determines their short-range ordering, showing that the Li and Va create chains bridged by their associated polarons which dissociate into dimers at higher temperatures. This dissociation happens at higher temperatures for Va than for Li, indicating a more pronounced clustering behavior during the formation of FePO4. Notably, a significant amount of uncoordinated Li exists at elevated temperatures, challenging the simplified picture of complete Li-Fe(II) pairing. Published by the American Physical Society 2024