Synthesis of f-doped LiFePO4 via precipitation method

Abstract

Fluorine-doped, olivine-structured LiFePO4 suitable for cathode material in rechargeable lithium batteries was prepared by aqueous precipitation followed by high temperature treatment at 700°C under slightly reductive atmosphere (Ar+5%H2). The starting materials were equimolar quantities of (NH4)2HPO4, FeSO4·7H2O and LiF. LiF served as both lithium and fluorine source. Besides the lattice parameters and the primitive cell volume reductions, compared to the undoped sample synthesized under the same conditions using LiNO3 as lithium source, the Rietveld refinement also shows that fluorine ions preferably occupy specific oxygen sites. Particularly, the best refinement is accomplished when fluorine ions occupy O(2) sites exclusively. By means of up-to-date electronic structure and total energy calculations this experimental finding is theoretically confirmed. Such fluorine doping also produces closing of the gap in the electronic structure and consequently better conductivity properties of the doped compound. In addition, the morphological and electrochemical performances of the synthesized powders are fully characterized.