Facile synthesis of hierarchical β-LiFePO4 and its phase transformation to electrochemically active α-LiFePO4 for Li-ion batteries

Abstract

In this work, we proposed a new strategy to obtain hierarchically structured olivine type LiFePO₄ (space group Pnma, denoted as α-LiFePO₄) composed of nanosheets with largely exposed (010) facets. To this end, a high-pressure phase LiFePO₄ (space group Cmcm, denoted as β-LiFePO₄) with a hierarchical structure was firstly prepared through solvothermal reactions, and then the as-prepared β-LiFePO₄ was converted directly to α-LiFePO₄via thermal annealing. The original hierarchical structures were mostly preserved during the phase transformation process. Furthermore, the carbon coating process could also be accomplished simultaneously along with the phase transformation. Careful structural analyses including X-ray diffraction and high-resolution transmission electron microscopy were carried out to confirm the β- to α-LiFePO₄ transformation. A tentative mechanism was proposed, that is, the reconstruction of PO₄ tetrahedra and FeO₆ octahedra might be attributed to atomic structural origins responsible for the phase transformation. Preliminary tests proved that the as-prepared hierarchical α-LiFePO₄ materials possess excellent electrochemical performance toward lithium storage, and therefore can be used for designing novel lithium ion batteries with higher tap densities and volumetric energy densities.