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

In this work, we proposed a new strategy to obtain hierarchically structured olivine type LiFePO4 (space group Pnma, denoted as alpha-LiFePO4) composed of nanosheets with largely exposed (010) facets. To this end, a high-pressure phase LiFePO4 (space group Cmcm, denoted as beta-LiFePO4) with a hierarchical structure was firstly prepared through solvothermal reactions, and then the as-prepared beta-LiFePO4 was converted directly to alpha-LiFePO4 via 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 beta- to alpha-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 alpha-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.