Exploration of the dislocation-electrochemistry relation in LiFePO4 cathode materials

Defects, such as dislocations, in electrode materials play a significant role in the performance of lithium-ion batteries. The dislocation-electrochemistry relation has only been observed experimentally and not been fully clarified. Computational studies on this mechanism were also very limited, especially the altered cyclic voltammetry behaviors and associated effective diffusivity. This work focuses on the influences of few characteristics of dislocations on the electrochemical performance of an anisotropic cathode material, lithium iron phosphate (LiFePO ₄ ). Utilizing linear elastic mechanics and the superposition principle, we study stress and displacement fields of a LiFePO ₄ particle containing different densities and orientations of dislocations. With the mechanical-electrochemical coupling effects expressed by the modified Butler-Volmer equation and using the finite different method, the cyclic voltammetry curves for different dislocation configurations in the particle are investigated. Our results show that introducing dislocations can shift and distort the cyclic voltammetry curves, especially at one specific dislocation orientation. It is also found that the Li-ion molar fraction-dependent partial molar volume is an important prerequisite of the distortion in cyclic voltammetry curves. Moreover, the altered cyclic voltammetry curves at different scanning rates indicate the improvements of electrical power, stored electrical energy, and the effective diffusivity of lithium. Our discrete dislocation model indicates that the capacity loss of LiFePO ₄ nanoparticles can be alleviated by introducing tailored dislocations. This study assists the understanding of electrode materials with pre-existing dislocations and provides strategies of using defect engineering to improve the kinetic performance in lithium-ion batteries.