Morphological and orientational diversity of LiFePO4 crystallites: remarkable reaction path dependence in hydrothermal/solvothermal syntheses

Abstract

We design four-group experiments to understand the morphological and orientational diversity of LiFePO₄ crystallites in hydrothermal and/or solvothermal syntheses. In the solvothermal synthesis, in which water is highly deficient, the starting Li₃PO₄ nanoparticle likely directly evolves into LiFePO₄ through an in situ transition mechanism. In contrast, under the other three conditions, i.e., hydrothermal synthesis with stoichiometric H₃PO₄, hydrothermal and solvothermal syntheses in the presence of excess H₃PO₄, the starting Li₃PO₄ nanoparticle follows three diverse paths, generating different precursors and/or intermediates whose compositions and dissolution properties are remarkably divergent. Such divergences in reaction paths dramatically influence the colloidal stability of the small, primary nanosheets participating in oriented-attachment aggregation growth, resulting in the diversity of the resultant LiFePO₄ in crystallite size from nanometer to micrometer shape (rod-like platelet, slab, and flake), and orientation ([010], [100] and [211]) as well as point defect concentration. Electrochemical performances of the diverse LiFePO₄ crystallites synthesized in this study correlate well with the morphological and orientational diversity, shedding light on the tailored synthesis of LiFePO₄ crystallites for high-performance lithium-ion batteries.