Nonstoichiometric LiFePO₄: Defects and Related Properties

Abstract

International audienceDeviation 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

Tailoring Native Defects in LiFePO4: Insights from First-Principles Calculations

“…This is consistent with the reported experimental data showing the presence of various native defects in LiFePO 4 samples. [12][13][14][15][16][17][18][19] We note that there are several limitations inherent in our calculations. The first set of limitations comes from standard methodological uncertainties contained in the calculated formation enthalpies and phase diagrams as discussed in Ref.…”

“…[14][15][16][17][18][19] This native defect is believed to be responsible for the loss of electrochemical activity in LiFePO 4 due to the blockage of lithium channels caused by its low mobility. 18,19 Clearly, native defects have strong effects on the material's performance. Experimental reports on the defects have, however, painted different pictures.…”

Multi-Scale Thermo-Electrochemical Modeling of Performance and Aging of a LiFePO4/Graphite Lithium-Ion Cell

“…More sophisticated models that are able to capture the equilibrium-potential hysteresis have been published 75 and may be integrated into the present framework in future, but are out of scope of the present study. As LFP shows a two-phase behavior during charge and discharge, the model of spherical diffusion used for the graphite anode is not applicable here 56,76,77 and requires a more complex mathematical description like the coreshell model used in Dargaville et al 78 or the multi-particle model of Farkhondeh et al 75 Again, these may be integrated into the present framework in future, but are out of scope of the present study. Therefore, bulk diffusion is not modeled at the cathode, recognizing that the model starts to become less reliable for high C-rates (>5 C), as will also be shown below.…”

“…Graphite electrode conductivities are two orders of magnitude higher than that of the electrolyte. [52][53][54] Although pure lithium iron phosphate is a very bad electron conductor, [55][56][57] thanks to carbon coating or conductive additives used in commercial LFP batteries the electrode is likely to have one order of magnitude higher conductivity than the electrolyte. 51,[58][59][60][61][62] Boundary conditions for the species conservation are J i = 0 at both electrode/current collector interfaces.…”

Polaron-ion correlations inLixFePO4studied by x-ray nuclear resonant forward scattering at elevated pressure and temperature

“…Because Li + diffusion in LiFePO 4 is essentially one dimensional, Li + ion mobility suffers as a result of channel blockage by defects. 6,31 Blocked channels for Li + ions then suppress electronic conductivity if polaron-ion interactions are strong. This effect may be common in materials when both ions and electrons are mobile.…”