Investigation of Li0.6FePO4 Phase Separation Kinetics Starting from Solid Solution

Abstract

The major contribution of lithium ion batteries towards sustainable energy originates in their use as energy vectors. In this context, lithium iron phosphate represents a safe, stable and environmental benign cathode material. The phase transition process during charge and discharge leads to questions regarding reactions rates, as the deintercalation/intercalation process via a phase transition mechanism is expected to strongly affect ionic and electronic transport. In order to study intrinsic properties of partially lithiated lithium iron phosphate, this study reports on factors that affect the transformation of the metastable Li0.6FePO4 solid solution prepared using both physical and chemical means. The stability of the solid solution was examined using a series of diverse experimental conditions. Crystal structure effects were examined via X-ray diffraction (XRD) while more localized changes were revealed by attenuated total reflectance infrared spectroscopy (ATR-IR). Specifically, the internal vibrations of the phosphate group were used with the aim of detecting lithium rich and lithium poor phases. These two techniques corroborate to distinguish the metastable solid solution phase relative to the olivine and heterosite phases which emerge over time. Particles morphologies and size distribution were analysed by field-emission gun scanning electron microscopy (FEG-SEM) and dynamic light scattering (DLS) respectively. In addition, lithium content was confirmed by flame emission spectrometry (AES). The developed techniques and results relative to the solid solution metastability will serve as the basis for upcoming kinetics studies. These studies will rely on imposing a uniform reaction environment on the entire particle population, in contrast to kinetics derived from electrochemical analysis using composite electrodes.