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Optimizing Li diffusion in LiFePO4: The key role of Fe antisite defect clustering

 

J. Lee, W. Zhou. J.-C. Idrobo, S.J. Pennycook, and S.T. Pantelides, Vacancy-driven anisotropic defect distribution in the novel battery-cathode material LiFePO4. Phys. Rev. Lett. (2011).


Text Box:
(Left) EELS data from FeLi (Fe occupying a Li site, shown in the red circle in the inset) compared to Fe in bulk positions in LiFePO4. The inset shows an atomically-resolved Z-contrast image of LiFePO4 oriented along the b-axis. Scale bar is 0.5 nm.

(Right) Fe L3/L2 ratios for FeLi (red triangle) and Febulk (black triangle) sites in LiFePO4 compared with different iron compounds.

We show why crystal lattice defects in the battery material LiFePO4 cluster in particular channels, leaving others open to Li ion diffusion1. The result is important since maintaining open channels is necessary to allow charge and discharge of the battery. First-principles calculations and statistical mechanics show that the clustering of Fe antisite defects (FeLi) is the result of an unusual energy-lowering mechanism that is a feature of the one-dimensional nature of the Li diffusion paths. We also obtain atomically-resolved electron energy-loss spectra of the FeLi defects which reveals a slightly higher oxidation state for the defects relative to the nominal value of +2 for Fe in LiFePO4, a result that is consistent with our theoretical predictions.  Our study suggests temperatures for growth and/or annealing to optimize segregation and maximize Li diffusion.


 Oak Ridge National Laboratory