Pair distribution function analysis and Mössbauer study of defects in microwave-hydrothermal LiFePO4

Abstract

Olivine-type LiFePO₄ is nowadays one of the most important cathode materials of choice for high-energy lithium ion batteries. Its intrinsic defectivity, and chiefly the so-called lithium ironanti-site, is one of the most critical issues when envisaging electrochemical applications. This paper reports a combined diffractometric (Synchrotron Radiation XRD with Rietveld and PDF analyses) and spectroscopic (Mössbauer) approach able to give a thorough characterization of the material defectivity. Such analytical procedure has been applied to a sample prepared following an innovative microwave-assisted hydrothermal synthesis route that, in a few minutes, allowed us to obtain a well crystallized material. PDF analysis, which is applied for the first time to this type of battery material, reveals the presence of disorder possibly due to Li/Fe exchange or to a local symmetry lowering. A 5% amount of iron on the lithium site has been detected both by PDF as well as by Mössbauer spectroscopy, which revealed a small percentage of Fe³⁺ on the regular sites.