Exploration of Relationship Between Phonons and LiFePO₄ Cathode Behavior using Density Functional Theory

LiFePO₄ is a widely used battery cathode material. During electrochemical cycling of LiFePO₄, energy is generated via reversible delithiation and lithium intercalation, forming an intermediate mixture of LiFePO₄ and FePO₄ during charging and discharging. While it is known that lithium ions migrate in and out of the material along the b-axis, the effect of lattice vibrations on ion migration is not understood. Ab initio Density Functional Theory calculations have been used to determine the phonon dispersions and density of states of LiFePO₄, FePO₄, and an intermediate phase. We compare these calculations to inelastic neutron scattering measurements of single crystalline LiFePO₄ to gain insight into the influence of lattice dynamics on ion mobility. Thermal effects including the phonon energy evolution and temperature dependent lifetime information will also be discussed.