Phase boundary propagation mode in nano-sized electrode materials evidenced by potentiostatic current transients analysis: Li-rich LiFePO4 case study

Phase boundary propagation dynamics in phase-transforming battery nanomaterials is widely studied due to high practical relevance of the boundary propagation patterns to the rate performance and degradation of metal-ion batteries. In this work, we decipher the complex interplay between the kinetic limitations in the course of phase boundary movement, which can be controlled by slow diffusion, interfacial charge transfer and nucleation. Employing nanosized Li-rich LiFePO4 materials as model systems, we consistently analyze the effect of each of the possible limiting factors on the evolution of potentiostatic current transients’ shape. Our results conclusively demonstrate that under the experimental conditions all three rate-controlling factors contribute to the rate-limitations of an intercalation material. We used numerical modeling to provide illustrative examples of current transients under different limiting regimes, which can be employed for the rapid and accurate diagnostics of the control factors during phase transformations. The derived conclusions allow rationalizing both the phase-transformation pathways in multiparticle electrodes (particle-by-particle or concurrent intercalation) and phase-growth morphologies (intercalation wave or shrinking core) based on easily accessible experimental estimates of kinetic parameters. These results are of high diagnostic value for the development of physically adequate battery models.