Length‐Scale‐Dependent Phase Transformation of LiFePO4: An In situ and Operando Study Using Micro‐Raman Spectroscopy and XRD

The charge and discharge of lithium ion batteries are often accompanied by electrochemically driven phase‐transformation processes. In this work, two in situ and operando methods, that is, micro‐Raman spectroscopy and X‐ray diffraction (XRD), have been combined to study the phase‐transformation process in LiFePO₄ at two distinct length scales, namely, particle‐level scale (～1 μm) and macroscopic scale (～several cm). In situ Raman studies revealed a discrete mode of phase transformation at the particle level. Besides, the preferred electrochemical transport network, particularly the carbon content, was found to govern the sequence of phase transformation among particles. In contrast, at the macroscopic level, studies conducted at four different discharge rates showed a continuous but delayed phase transformation. These findings uncovered the intricate phase transformation in LiFePO₄ and potentially offer valuable insights into optimizing the length‐scale‐dependent properties of battery materials.