Structural properties and electrochemical performance of different polymorphs of FePO4 as raw materials for lithium ion electrodes

Abstract

In this study, three different polymorphs of FePO₄ (amorphous, monoclinic, and trigonal) are synthesized and their crystal phase structures are investigated using X-ray diffraction (XRD). The morphological characteristics are studied through scanning electron microscopy (SEM). Both amorphous FePO₄ and monoclinic FePO₄ exhibit pebble-shaped aggregates formed by microsheets, and have a rough surface. Trigonal FePO₄, which can be formed from amorphous FePO₄ or monoclinic FePO₄ under high-temperature conditions, is produced through simple sintering and exhibits higher crystallinity. However, excessive sintering leads to severe particle aggregation and undesirable particle size growth. To explore their electrochemical activity, we prepared various LiFePO₄ cathode materials (denoted as A-LFP, M-LFP, and T-LFP), with different polymorphs of iron phosphate as the source materials. The results showed that the M-LFP demonstrated exceptional performance, with a specific capacity of 115.8 mA h g⁻¹ at 5C and a capacity retention rate of 94% over 300 cycles at 1C.