Tailoring Two Polymorphs of LiFePO4 by Efficient Microwave-Assisted Synthesis: A Combined Experimental and Theoretical Study

摘要

LiFePO4 typically crystallizes in the olivine-type phase (denoted as w-phase hereafter). When high pressure (65 kbar) and elevated temperature (900 °C) are applied, the α-LiFePO4 transforms into a high-pressure phase (denoted as β-phase hereafter). Here, we report a facile approach to directly tailor the two polymorphs of LiFePO4 in a controlled way under mild conditions. Employing a microwave-assisted nonaqueous route, highly crystalline LiFePO4 with either α- or β-phase can be efficiently synthesized within 3 min, by simply tuning the ratio of the solvents, benzyl alcohol, and 2-pyrrolidinone. The resulting β-LiFePO4 particles exhibit a hierarchical self-assembled bow-tie-like microstructure, whereas the α-phase consists of nanoplates. In addition, the β-phase irreversibly transforms into the α-phase upon heat treatment without alteration of the morphology. After carbon-coating, α-LiFePO4 and phase-transformed β-LiFePO4, that is, α-LiFePO4 with the hierarchical morphology of the β-phase, exhibit excellent electrochemical performance, whereas pristine β-LiFePO4 displays unfavorable properties. Density functional total energy calculations are performed to get the relative energies and lattice stability of the two phases. A qualitative understanding of the poor electrochemical performance of the β-phase can be deduced from the molecular dynamics of the mobile Li ions in both structures.