Spectroscopic Analysis of LixMnyFe1-YPO4 Cathodes Materials to Revel the Phase Transition Mechanism.

Abstract

Olivine-type mixed-metal phosphate cathodes have demonstrated their ability to efficiently store energy in a wide range of applications. Moreover, ultrafast charging rates (seconds order) are observed for the LiFePO4 system. Nonetheless, charge storage proceeds via a phase transition mechanisms where fully charged or discharged particles act as electronic insulators. Considering olivine as a model structure with its outstanding stability, a higher energy density material is achieved by the incorporation of high operating potential transition metals. The challenge here is to understand the exact Li+ transport mechanism engendered by the modified crystalline structure, as it impacts on the dynamics of de/lithiation process.For the purpose of identifying the encountered phase transition mechanism is impacted by the nature of the redox active sites, a range of LixMnyFe1-yPO4 (0 ≤ y ≤ 0.9) has been delithiated by a chemical route resulting in materials at various state of charge. Powder X-ray diffraction (PXRD) and Rietveld refinement was used to describe the bulk crystalline structure. 6Li nuclear magnetic resonance (MAS NMR) and attenuated total reflectance infrared spectroscopy (ATR-IR) where employed to detect the Li+ and PO4 local environments, respectively. The information obtained by spectroscopic characterizations, e.g. local organization of transitions metals and their oxidations state, phosphate backbone distortion and presence of defects, provide an exhaustive image of the phases changes, including the crucially the Li+ transport mechanism in high energy density cathodes. Figure 1