Nonstoichiometric LiFePO4: Defects and Related Properties
Résumé
Deviation from ideal stoichiometry of LiFePO4 has been investigated. Any attempt to increase the Li concentration of samples prepared either by the precursor precipitation route or by the continuous aqueous precursor synthesis route results in the formation of lithium phosphate impurity, in addition to stoichiometric LiFePO4 free of any Li vacancy. On the other hand, Li-deficient homogeneous solid solutions of composition Li1−2xFexFePO4 could be obtained. For x ≥ 0.06, however, a sarcopside impurity phase is formed. Investigations of structural properties allow us to define the defect responsible for the solid solution as Fe•Li + V′Li in the Krger−Vink notation. Because the chemical formula of the sarcopside is obtained by writing x = 1/2 in the chemical formula of the solid solution, this impurity phase can be viewed as a condensation of the Fe•Li + V′Li defects. Magnetic measurements show that isolated lithium vacancies V′Li are also diluted in the Li1−2xFexFePO4 matrix. The negative charge of the isolated V′Li is compensated by the valence change Fe2+ → Fe3+ of an iron ion in its vicinity, forming a small magnetic polaron that is detected by magnetic measurements. The concentration of such polarons, however, remains very small as it saturates to a concentration of 0.2−0.3 mol %, much smaller than the concentration x in V′Li bound to Fe•Li. The electrochemical features are significantly damaged by the Fe•Li defects that block the diffusion of lithium along the corresponding channel, while the Li3PO4 only acts as an inert mass.